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TRANSCRIPT
i
On the polymer-based nanocomposites for electrical
switching applications
Venkatesh Doddapaneni
PhD Thesis School of Engineering Sciences
KTH Royal Institute of Technology Stockholm, Sweden, 2017
ii
TRITA-FYS 2017:08
ISSN 0280-316X KTH School of Engineering Sciences
ISRN KTH/FYS/–17:08–SE SE-100 44 Stockholm
ISBN 978-91-7729-288-3 SWEDEN
Akademisk avhandling som med tillstånd av Kungl Tekniska högskolan framlägges till
offentlig granskning för avläggande av teknologie doktorsexamen i fysik fredagen den 24
mars klockan 14:00 i FA31, Roslagstullsbacken 21, Albanova Universitetscentrum,
Stockholm.
© Venkatesh Doddapaneni, Mars 2017
Tryck: Universitetsservice US AB
Frontpage: Photograph of the MCB and its internal components, (a, b, c) recorded images
of the electric arc at different times. (Courtesy of ABB, www.abb.com.)
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"Keep going even when there are obstacles."
- Oliver Hart, awarded The Sveriges Riksbank Prize in Economic
Sciences in Memory of Alfred Nobel 2016.
TO
My dear princess
My beloved mother & father
v
PREFACE
This thesis is submitted as a compilation of scientific papers for fulfilling the requirements
of the doctoral thesis (PhD) in the physics department at KTH Royal Institute of
Technology. The research has been supported by the Department of Electromagnetic
Engineering at KTH and ABB.
ACKNOWLEDGEMENTS
I would like to thank my supervisor Prof. Muhammet S. Toprak for all the support and
encouragement during this thesis work. He has given very object oriented support, which
led to get this thesis done as per our time bound.
My sincere thanks go to Prof. Hans Edin for being my co-supervisor, Prof. Rajeev
Thottappillil and Prof. Joydeep Dutta for their constructive and continuous support to move
ahead smoothly during my PhD studies.
I am very grateful to my industrial supervisor Dr. Rudolf Gati, ABB corporate
research, Switzerland for sharing his experimental and theoretical insights on the switching
electrical arcs. Such insights helped me to understand experimental results with
nanocomposites leading to appropriate scientific reasoning. I will always remember his
interest towards engineering of present polymeric materials for different electrical
switching applications.
I would like to thank KIC InnoEnergy PhD School for giving an opportunity to learn
entrepreneurial skills towards a startup from PhD studies.
In the first part of my thesis, I worked for fabrication of PNCs at the Nano-
characterization Laboratory in KTH. I would like to thank Dr. Mohsin Saleemi, Dr. Fei Ye
and Dr. Carmen Vogt for sharing their knowledge on the synthesis of nanoparticles and
nanocomposites, helping me to fabricate PNCs quickly. In the second part, fabricated PNCs
are tested their impact on electrical arcs generate in a real circuit breaker system setup at
Electromagnetic Engineering Lab, KTH. I would like to thank my colleagues Dr. Ara
Bissal and Jesper Magnusson for their assistance during the experimental tests with the
electric arcs.
My special thanks go to Dr. Mykailo Gnybida and Dr. Christian Rumpler for allowing
me to work on theoretical models to better understand the outgassing process at Eaton
European Innovation Center, Prague. This period was good to start from basic to advanced
models and detailed knowledge on different high voltage breakers.
Many thanks to all my colleagues from Applied Physics, Electromagnetic Engineering
departments for being so friendly during this period. I must admit that I really enjoyed
being a PhD student at KTH.
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Finally yet importantly, I would like to thank my family members for all the encouragement
and support they have given me.
Venkatesh Doddapaneni Stockholm, March 2017
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ABSTRACT
Recent research demonstrated that polymer based nanocomposites (PNCs) have been
engineered in order to improve the arc interruption capability of the circuit breakers.
PNCs are the combination of nano-sized inorganic nanoparticles (NPs) and polymers,
opened up new developments in materials science and engineering applications.
Inorganic NPs are selected based on their physical and chemical properties which
could make multifunctional PNCs in order to interrupt the electrical arcs effectively.
In particular, we presented the PNCs fabricated by using CuO, Fe3O4, ZnO and Au
NPs in a poly (methyl methacrylate) (PMMA) matrix via in-situ polymerization
method, recently developed method to avoid NPs agglomeration, leading to good
spatial distribution in the polymer matrix. Thus, several samples with various wt% of
NPs in PMMA matrix have been fabricated. These PNCs have been characterized in
detail for the morphology of NPs, interaction between NPs and polymer matrix, and
radiative/thermal energy absorption properties. In the next stage, PNCs are tested to
determine their arc interruption performance and impact on the electrical arcs of
current 1.6 kA generated using a specially designed test set-up. When PNCs interact
with the electrical arcs, they generate ablation of chemical species towards core of the
electrical arc, resulting in cooling-down the arc due to strong temperature and pressure
gradient in the arc quenching domain. This thesis demonstrates for the first time that
these engineered PNCs are easily processed, reproducible, and can be used to improve
the arc interruption process in electrical switching applications.
Keywords: Polymer-based nanocomposites, Inorganic nanoparticles, Electrical arcs,
Circuit breakers, Ablation/Outgassing, Arc interruption capability, PMMA, CuO,
Fe3O4, ZnO, Au, Radiative energy, Electric power, Arc temperature
viii
SAMMANFATTNING
Ny forskning har visat att polymerbaserade nanokompositer (PNCs) har utformats för
att förbättra strömbrytares förmåga att undvika ljusbågar vid överslag. PNCs är en
kombination av nanostora oorganiska nanopartiklar (NP) och polymerer, som har
öppnat upp för ny utveckling inom materialvetenskap och tekniska tillämpningar.
Oorganiska NP väljs baserat på deras fysikaliska och kemiska egenskaper som kan
hjälpa PNCs att motverka elektriska ljusbågar effektivt. I synnerhet, presenterade vi
PNCs tillverkade genom användning av CuO, Fe3O4, ZnO och Au NP i en poly
(metylmetakrylat) (PMMA)-matris via in situ-polymerisationsmetod, nyligen
utvecklad för att undvika NP-agglomerering, vilket leder till god rumslig fördelning i
polymermatrisen. Därför har flera prover med olika vikt% av NP i PMMA-matris
tillverkats. Dessa PNCs har utvärderats i detalj för NP-morfologi, interaktion mellan
NP och polymermatris, och strålnings- och värmeenergiabsorption. I nästa skede
testas PNCs för att bestämma deras förmåga att undvika ljusbågar och påverkan på de
elektriska ljusbågarna av 1,6 kA strömstyrka, genererade med hjälp av en
specialdesignad test-set-up. När PNCs interagerar med de elektriska ljusbågarna,
genererar de ablation av kemiska ämnen mot kärnan i den elektriska ljusbågen, vilket
resulterar i nedkylning av ljusbågen på grund av starka temperatur- och
tryckgradienter i området. Denna avhandling visar för första gången att dessa
konstruerade PNCs är lätta att framställa, reproducerbara, och kan användas för att
förbättra avbrottsprocessen för ljusbågen i elektriska kopplingstillämpningar.
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LIST OF PUBLICATIONS
This PhD thesis is based on the compilation of the following six papers, referred in the text
by using the serial number assigned below. All these publications are appended at the end
of the thesis with permission from the respective journal.
I Doddapaneni, V., Magnusson, J., Bissal, A., Gati, R., Edin, H., & Toprak, M. S. (2015).
Spectroscopic investigations of the ablated species from the polymers exposed to electric
arcs in air. International Conference on Electric Power Equipment – Switching
Technology, 3, 337.
II Doddapaneni, V., Zhao, Y., Ye, F., Gati, R., Edin, H., & Toprak, M. S. (2015).
Improvement of UV radiation absorption by cupric oxide NPs/PMMA
nanocomposites for electrical switching applications. Journal of Powder Metallurgy
and Metal Ceramics, 54(7), 397.
III Doddapaneni, V., Gati, R., & Toprak, M. S. Engineered PMMA-CuO nanocompositesfor improving the electric arc interruption process in electrical switching
applications. Manuscript.
IV Doddapaneni, V., Saleemi, M., Ye, F., Gati, R., & Toprak, M. S. (2016). On the
electrical arc interruption by using PMMA/iron oxide nanocomposites. Materials
Research Express, 3(10), 105043.
V Doddapaneni, V., Saleemi, M., Ye, F., Gati, R., & Toprak, M. S. (2017). Engineered
PMMA-ZnO nanocomposites for improving the electric arc interruption process in
electrical switching applications: Unprecedented experimental insights. Composites
Science and Technology, 141, 113.
VI Doddapaneni, V., Ye, F., Saleemi, M., Gati, R., & Toprak, M. S. (2017). New
experimental insights for controoling the electrival arcs in electrical switching
applications: a comparitive study on PMMA nanocomposites of Au and ZnO.
Under review, Composites Science and Technology.
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CONTRIBUTIONS OF THE AUTHOR
Paper I. Literature survey, planning, performing the experiments, characterizations,
results analysis and writing the complete article.
Paper II. Literature review on the composites, planning, performing the experiments,
evaluation of results and writing the complete article.
Paper III. Literature survey, experimental design, performing the experiments, analyzing
the results and writing the manuscript.
Paper IV. Literature survey, fabrication of nanocomposites, experimental design, results
analysis and writing the complete article.
Paper V. Literature survey, fabrication of nanocomposites, experimental design, results
analysis and writing the complete article.
Paper VI. Literature survey, fabrication of nanocomposites, experimental design, results
analysis and writing the complete article.
xi
TABLE OF CONTENTS
Preface..................................................................................................................................... v
Acknowledgements ................................................................................................................. v
Abstract .................................................................................................................................vii
Sammanfattning .................................................................................................................. viii
List of publications ................................................................................................................ ix
Contributions of the author ..................................................................................................... x
List of Abbreviations .......................................................................................................... xiii
1. Introduction ................................................................................................................... 1 1.1 Electrical switching applications .............................................................................. 1
1.1.1 Modern air based circuit breakers and materials limitations ............................ 2
1.2 Engineered nanocomposites ..................................................................................... 5
1.2.1 Potential advantages and challenges ................................................................. 5
1.3 Integration of PNCs in electrical switching applications ......................................... 7
1.4 Scope of the thesis .................................................................................................... 9
2. Electrical switching applications ................................................................................ 11 2.1 The electric arc in air .............................................................................................. 11
2.1.1 Introduction to electric arc .............................................................................. 11
2.1.2 Different types of electrodes ........................................................................... 13
2.1.3 Different types of electrical arcs ..................................................................... 14
2.1.4 Electric arc roots and arc column .................................................................... 15
2.1.5 Dissipation of arc energy ................................................................................ 16
2.2 Arc interruption process ......................................................................................... 16
3. Polymer-based Nanocomposites ................................................................................. 19 3.1 Polymers ................................................................................................................. 19
3.1.1 Why thermoplastic polymer? .......................................................................... 19
3.1.2 PMMA ............................................................................................................ 20
3.2 Nanoparticles .......................................................................................................... 20
3.2.1 Surface modification of NPs ........................................................................... 21
3.3 Fabrication of PNCs ............................................................................................... 22
4. Experimental ................................................................................................................ 25
4.1 Materials and Methods ........................................................................................... 25
4.1.1 Synthesis of NPs ............................................................................................. 25
4.1.2 Fabrication of PNCs ........................................................................................ 27
4.2 Characterization techniques ................................................................................... 27
4.2.1 X-ray Diffraction, XRD .................................................................................. 28
4.2.2 Transmission Electron Microscopy, TEM ...................................................... 28
4.2.3 Fourier Transform Infrared Spectroscopy, FTIR ............................................ 28
4.2.4 UV-Vis Absorption Spectroscopy, UV-Vis .................................................... 29
4.2.5 Thermal Gravimetric Analysis, TGA ............................................................. 29
4.2.6 Differential Scanning Calorimetry, DSC ........................................................ 29
xii
4.2.7 Scanning Electron Microscopy, SEM ............................................................. 29
4.3 Experimental Tests with the electrical arcs ............................................................ 30
4.3.1 Electrical arc discharge circuit ........................................................................ 30
4.3.2 Experimental test set-up .................................................................................. 30
5. Results and discussion ................................................................................................. 33 5.1 Polymers ................................................................................................................. 33
5.2 PNCs with Cupric oxide NPs ................................................................................. 34
5.3 PNCs with Iron oxide NPs ..................................................................................... 37
5.4 PNCs with Zinc oxide QDs .................................................................................... 41
5.5 PNCs with Gold NPs .............................................................................................. 44
6. Conclusions................................................................................................................... 47
7. Future work.................................................................................................................. 49
Appendix A: ......................................................................................................................... 51 A: Drawing of the experimental test set-up .......................................................................... 51
Appendix B: ......................................................................................................................... 53 B: TGA-FTIR spectroscopy – PA66, PMMA ...................................................................... 53
Bibliography ........................................................................................................................ 55
xiii
LIST OF ABBREVIATIONS
PNCs Polymer-based NanoComposites
MCB Miniature Circuit Breakers
PMMA Poly (methyl methacrylate)
NPs Nanoparticles
Fe3O4 Iron oxide
CuO Cupric oxide
ZnO Zinc oxide
Au Gold
TEM Transmission electron microscopy
SEM Scanning electron microscopy
FTIR Fourier transform infrared spectroscopy
TGA Thermogravimetric analysis
DSC Differential scanning calorimetry
UV-Vis Ultraviolet-visible spectroscopy
LTE Local thermodynamic equilibrium
MEA Monoethanolamine
1
1. INTRODUCTION
Engineered polymer-based nanocomposites (PNCs) are attracting great interest due to their
multifunctional properties[1], chosen alternative to the ceramic walls and traditional polymers
such as PMMA, PA66 etc. in power switching devices (such as fuses, circuit breakers and
contactors). All these materials are intended to protect the switching devices from high
energetic fault currents, improve the reliability and efficiency. Basically a power switching
device is a switch, which has been used for protecting the electric grid from over and short
circuit currents by automatic interruption[2-6].
1.1 ELECTRICAL SWITCHING APPLICATIONS
A Fuse - Every one of us have it in our homes. It is used to protect people and electrical
appliances from high fault currents that could lead to electric shock and fire[7].
In general, an electric power system is a set of electrical components used during
generation, transmission and distribution of electric power [8, 9]. Fuse is the basic key
component in the system in order to protect from over-current and short circuit currents [10,
11]. By definition, fuses are designed to open the circuit or creating infinite resistance when
high currents are present due to the presence of unintended disturbances in the electric power
system[12]. According to the IEC standard i.e. IEC 60269-1, fuses are designed with metal
wire, it melts when it receives either heat or rated fault current. It is clearly mentioned in the
standard is that, “a device that by fusing of one or more of its specially designed and
proportioned components opens the circuit in which it is inserted by breaking the current when
this exceeds a given value for a sufficient time. The fuse comprises all the parts that form the
complete device”[13]. One of the main limitations of the fuse is that it has to be replaced after
interruption of a fault current, wouldn’t be able to provide complete protection due to its
limitations like materials used for fuse wire and frequency of unintentional faults generated in
the system. If an arc fault was not quickly extinguished, that can cause great damage such as
electric shock, fire and personal injuries[14].
In order to overcome the limitations of the fuses, advanced version of fuses i.e. Miniature
Circuit Breakers (MCBs) are introduced since early1920’s[15]. MCBs are combined with
thermal and magnetic trips due to over and short-circuit currents [16, 17], more importantly
reusability without need of replacing repeatedly. MCBs are mainly aimed for current limiting,
reducing electric arc generation [17, 18], improved response time compared to standard zero
2
crossing circuit breakers, significantly reducing let-through energy (i2t), damage to the
connected electrical appliances[19], improved tripping characteristics.
Early version of MCBs were described by Thomas Edison is his patent application in
1879. The present version shown in Figure 1.1 is invented and patented in Germany by Hugo
Stotz and Heinrich Schachtner in 1924 [15]. This version increased safer and efficient
electrification of private homes. Today, billions of MCBs are produced, and nearly all homes
are using MCBs to control the flow of electric power, safeguarding people and electrical
appliances from high fault currents. It looks like a traffic police, who controls traffic at an
intersection when there’s a heavy snow fall in winter, the MCBs interrupt the electric
power reaching home when something inside or outside of homes goes wrong.
Figure 1.1. Evolution of different MCBs in time, a) First MCB in 1924, b) improved MCB with K-characteristics for
motors in 1928, c) High-efficiency MCB S201-K4 in 1957, d) MCB S161 in 1961, e) MCB from System pro M compact
S200 in 2012 (Courtesy of ABB, www.abb.com.)[15].
According to the standards IEC/EN 60898-1, IEC/EN 60947-2 and UL 1077, MCB is
defined as “a mechanical switching device, capable of making, carrying and breaking currents
under normal circuit conditions and also making, carrying for a specified time, and
automatically breaking currents under specified abnormal circuit conditions such as those of
short-circuit”.
1.1.1 MODERN AIR BASED CIRCUIT BREAKERS AND MATERIALS LIMITATIONS
Modern MCBs shows an improved reliability and safety of operation in terms of trip-
free mechanism operation compared to the early stages of fuse shown in Figure 1.1[17, 20].
Designed for domestic and commercial applications like data centers, high speed trains, etc.
they work up-to breaking currents of 15 kA according to the requirement[21]. MCBs lifetime
is approximately 30 years but it depends more on the number of operations, overloads
currents, short-circuits, operating environment, etc.
(a)
(b) (c)
(d) (e)
3
In general, overloading currents result when the devices draw more current than expected from
the source. Short circuit currents result when the current takes a low resistance path due to a
defect instead of the device being connected in load. In both cases, MCBs work through either
thermal or magnetic tripping mechanisms[22]. In case of overcurrent’s, bimetallic strip triggers
tripping mechanism due to heat generated from high currents while short circuit currents
inductance coil triggers, the key components are shown in Figure 1.2.
Figure 1.2. The cross section of a low-voltage MCB and its components (Courtesy of ABB, www.abb.com.).
Some known facts about MCBs are[23]:
1. The triggering speed of MCB is 10 ms, which is about 10 times faster than a normal
human eye can blink
2. MCB can disconnect up to 50 kA short circuit current, equivalent to the same amount
of current drawn from 50,000 smart phone batteries
3. When it breaks currents, it can go up to 10 000 oC, at which even rocks can melt
4. The pressure inside of MCB can go from 1 to 10 bar, which is similar to the pressure
levels used in water jet cutter to cut materials such as metals and granites
5. There are more than 4500 MCBs available in market, same as many different types of
mammals on the earth
Basically, when MCB triggers due to highly energetic fault currents, it will take little
electric field to ionize air and form an electric arc with highly intense radiative[24] and thermal
energy between two electrodes[25]. Illustrations of detailed working MCB in time domain are
shown in Figure 1.3[26]. Traditionally, ceramics and polymeric materials have been used to
improve the performance of MCBs. These materials outgas (ablate) due to the strong energy
generated by an electric arc[27, 28]. The outgassed chemical species move towards the core of
the electric arc and draws energy from the electric arc, helping effectively to cool down the
temperature of the arc[29]. Besides that, the outgassed chemical species create more pressure
Fixed / moving electrodes PNC / outgassing
polymers
The inductance coil
Extinguishing chamber
Bimetallic strip
4
on the electric arc, which helps to rise the current density of the electric arc by creating Lorenz
forces, helping to push the arc towards arc quenching blades. Under these conditions, it would
be possible to control and interrupt/extinguish the electric arcs quickly and effectively[30].
Based on the detailed understanding of the MCBs, we have noticed the limitation of
materials while controlling the electric arc in order to improve the efficiency. We have
identified the scientific need of improving the controlled outgassing process of polymeric
materials by using the radiative and thermal energy emitted from the electric arc. The aim of
this thesis work is to design composite wall materials (PNCs) by embedding appropriate
nanoparticles into the polymer matrix in order to absorb more energy from the electric arc and
will be utilized in outgassing process to improve the arc interruption process.
Figure 1.3. The engineering schematic of typical MCB shown with working times (Courtesy of ABB, www.abb.com.)
[26].
< 0.5 ms
At 0.5 ms
At 1 ms
At 1.5 ms
At 2 ms
At 10 ms
(a) (b) (c)
(d) (e) (f)
5
1.2 ENGINEERED NANOCOMPOSITES
According to the Oxford English Dictionary, The term “nano” is introduced from the Greek
etonym nanos meaning “dwarf” and mathematically it means either “billionth part” or “10-9
”.
The term “Nano” is used to refer the fraction/orders of nanometer, nanoscale, nanosecond, or
even nanomaterial (material with dimensions in the order of < 100 nanometers) and etc. In
1959, the physicist Richard Feynman started exploring about Nanotechnology and it
technological advantages when he was working at Caltech. One of his famous lectures titled as
“There’s Plenty of Room at the Bottom” gave a kick start/boost in nano-science and nano-
technology[31, 32].
PNCs can be defined as a combination of a polymer matrix and nanoparticles of a size
where at least one dimension i.e. length, width, or thickness is below 100 nm. In the last
decade, engineering of PNCs are becoming the innovative and smart solutions due to their
ability to tailor or enhance the specific properties of the polymers/materials such as improving
the thermal conductivity, increasing radiative and thermal energy absorption for different
applications. Some key developments are shown in Figure 1.4.
Figure 1.4. The schematic representation of different applications for PNCs.
1.2.1 POTENTIAL ADVANTAGES AND CHALLENGES
There are four generations of PNCs according to the developments in the field of
composites[33]
1st generation: Since 1940’s; Glass fiber reinforced composites fabricated for airplane
wings, helicopter rotors, nose cones
2nd
generation: Since 1960’s; High performance composites is developed after the
launch of the Soviet Sputnik satellite
3rd
generation: Between 1970s and 1980s; The quest for new markets by synergizing
the properties apart from space applications
4th
generation: Since 1990’s; the era of hybrid materials, nanocomposites for
interdisciplinary applications like Automobiles
PNCs
Electrical switching applications
Electrode materials for Li ion batteries
Aerospace and marine applications
6
According to the above division of generations, now we are in the fourth generation where one
can integrate the great developments in PNCs for different applications such as electrical,
automobile applications. Several studies have been performed to develop and optimize the
PNCs in the areas of photonics[34], energy storage[35, 36], polymer LED, solar energy
harvesting[37], optics (or optical applications)[38, 39], automobile[40], food packaging[41],
cosmetics[42], environment[43], military[44], imaging, drug delivery, bio-sensing[45, 46],
cancer therapy[47].
There are several well established methods for the fabrication of PNCs, among which can be
listed the followings[48]:
1. Solution induced intercalation: by solubilizing polymer in a solvent and adding NPs
2. In-situ polymerization: mixing of NPs with liquid monomer and then polymerization
3. Polymer melt intercalation: NPs are directly dispersed into the molten polymers and the
mixture is quenched
There are also several challenges during the fabrication of PNCs, which can be listed as
follows[38]:
1. Homogeneity of dispersion and distribution of NPs
2. Agglomeration
3. Viscosity increase
When inorganic nanoparticles come to the market for fabricating PNCs, there are obvious
questions to answer: “Why nano”? Why not micro?
Nanoparticles exhibit relatively different chemical and physical properties compared to the
micrometer sized counterparts and bulk materials[49]. When materials scale down from bulk
towards nanoparticles, changes in physical and chemical properties are clearly noticed due to
increase of surface to volume ratio shown in Figure 1.5. Recently, NPs attracted significant
research interest for improving the specific properties of composite/hybrid materials. In our
applications, ultra-low quantities (wt %) of NPs in PMMA matrix exhibited variation and
improvement in broad range UV radiation and thermal energy absorption properties.
According to the merits and challenges for engineering of PNCs, the PNCs in this thesis
are fabricated through in-situ polymerization method by combining the polymer i.e. PMMA
monomer solution and inorganic nanoparticles. This approach has been proven successful for
dispersing NPs in PMMA matrix and is also applicable at an industrial scale.
7
1.3 INTEGRATION OF PNCS IN ELECTRICAL SWITCHING
APPLICATIONS
Material science development in the last decades is broadening the great opportunities to
improve electrical switching applications (e .g. breakers, switchgear, furnaces, etc.)[50, 51].
Therefore, it is possible to design new materials with profoundly improved specific properties,
which could be used to optimize the breakers to become more reliable, environmentally
friendly and possibly cheaper. During these studies, we focused mainly on improving the
radiative and thermal energy absorption of PNC.
Figure 1.5. Specific surface area (SSA) as a function of NPs diameter[49].
Our first experimental tests/research demonstrated that PNC can be rather promising
candidates for improving the reliability of the circuit breakers, by controlling the generated
electric arc. Therefore, this thesis is aimed at developing PNCs, which will control the specific
physical process i.e. outgassing (from our experimental tests shown in Figure 1.6) by
integrating carefully designed nanoparticles (various morphology, size and architecture) into
the polymeric matrices to influence the absorption of broadband radiation and thermal energy
emitted by an electric arc. New materials design, absorption and outgassing are at the interface
8
between materials science and engineering of devices, the understanding of which is critical for
further improvements of power devices specifically switching to use newly developed PNCs.
Figure 1.6. Schematic of an experimental test set-up depicting the interaction between the PNCs and electric arcs
generated between the two contacts (a) and a flowchart of energy transfer between them (b).
Outgassing is a spontaneous evolution of chemical species from the PNC due to intense
radiative and thermal energy emitted from an electrical arc. It is also called as ablation. In
Figure 1.6, schematic part is explained as follows. The energy from the electric arc (red
arrows) impinges into the bulk polymer network through NPs where it causes outgassing of the
polymers. The outgassed vapour is injected towards the arc (thin sky blue arrows). In our
experimental test set-up itself, there is a possibility of radiation and outgassed vapour could be
exhausted radially. The vapor is subjected into the arc core, where they have to be heated up in
order to remove the energy from the electric arc, thus improving the arc interruption process. In
our experimental test set-up, quartz and glass slides are placed next to each other to have a
better impact of an electric arc on the PNC.
9
1.4 SCOPE OF THE THESIS
This thesis has three-fold objectives:
1. The fabrication of novel engineered PNCs with appropriate nanoparticles: The
objective includes identifying appropriate NPs (CuO/Fe3O4/ZnO/Au) based on the
physical and chemical properties. It starts with NPs synthesis and surface modification
by using earlier developed energy and resource effective methods. Most effective
fabrication method will be selected and used for PNCs fabrication by embedding NPs
into the PMMA matrix. The PNCs are intended to improve electric arc interruption
process.
2. Comprehensive structural and physicochemical characterization of PNCs: The
synthesized NPs and fabricated PNCs will be characterized in detail to understand their
thermal and radiative energy absorption process.
3. Investigating the effects of the fabricated PNCs on the electric arcs: This involves
designing a test set-up to generate an electric arc of prospective current 1.6 kA. The
electrical measurements unveil the impact of PNCs on the electric arcs, turn to improve
the electric arc interruption process. In addition, the investigation of the surface
morphology of the PNCs will be performed to correlate the outgassing process.
11
2. ELECTRICAL SWITCHING APPLICATIONS
2.1 THE ELECTRIC ARC IN AIR
In electrical switching applications, a low voltage circuit breaker is an automatic switch, which
has been used since 1920’s for protecting the electric grid from fault currents by automatic
interruption. When they interrupt the current, especially high fault currents on the order of
several kA, an electrical arc is generated. This arc produces extreme conditions with
temperatures up to 20 000 oC and pressure nearly of several 10 bars[2]. In order to interrupt the
electrical arc quickly and protect the devices from such extreme conditions, conventionally,
ceramics have been the material of choice due to their resistance to thermal shock, ability to
withstand higher operating temperatures. Afterwards, polymers are used due to their
mechanical and outgassing properties in addition to their influence on the arc interruption
performance of the circuit breakers. In general, when polymers are placed near the electrical
arcs they ablate (is also called as outgassing) due to the interaction of high radiative and
thermal energy releases from the electrical arcs. The outgassed chemical species help to cool
the electrical arcs to improve the arc interruption process effectively[52].
2.1.1 INTRODUCTION TO ELECTRIC ARC
An electric arc is a plasma discharge produced due to ionization/electrical breakdown of
air in between two electrical contacts of a circuit breaker[53]. Such electric arcs are similar to
lightning strikes, northern lights, discharge lamps, welding, and arc furnace.
Generation of an electric arc mainly depends on temperature of the surrounding medium
like SF6, vacuum, air and applied current. Electric arc is generated under a complex process,
which contains highly dense free electrons and ions and continuous ionization and de-
ionization takes place. The generation process of ions and electrons are due to field and
thermionic emission, thermal ionization, photon absorption and secondary emissions from the
surfaces. The nature of an electric arc generation depends on the above processes of supplying
electrons at the cathode, and then how it is confined in air[3, 54-56].
Typical potential between two contacts in MCBs, the higher potential is at the anode and
negative potential is at the cathode side as shown in Figure 2.1. Generally, air gets a small
conductivity continuously due to few electrons generations from the cosmic rays and natural
radioactivity. Such electrons can ionize air molecules by collision, leading to generate a new
12
electron and a positive ion pair moving towards the electrodes due to the potential applied
between them. The applied electric field will accumulate kinetic energy (KE) to move in
between the electrodes i.e.
KE = mv2/2 = eEx,
where E is the applied electric field and x is the distance travelled. In general, energy (U) of the
electron is in electron-volts (eV), where
eU=eEx
and U is the potential applied between the distance (x) of two electrodes. It is noticed that the
electron needs larger electric field (E) in order to accelerate between the two electrodes.
Furthermore, the presence of magnetic field (increases the current density and) radially
constrains the electrons towards center and the arc becomes less diffusive.
Figure 2.1. Typical potential distribution in an electric arc [57].
Other sources of electrons are photon absorption due to photoelectric effect, when light of
sufficiently high energy/short wavelength (hν) falls on electrode surface and releases a photo-
electron. Besides that, when a photon is absorbed by air molecule, an electron and ion pair is
generated. The photon energy needed to release the photo-electron is called the work function.
In case of thermionic emission, the release of electrons due to a heated electrode, can lead to
electric arc discharges. But, the electrode must be heated to incandescence and having lower
work function leads to efficient emission of electrons.
In our case, copper metal surface is knocked by the primary electron to give secondary
electrons leading to the discharge of higher currents. It is noticed in any electric arc discharge,
all the above mentioned processes are dominating one among the other and often explanations
and theories are in subjects of dispute. Therefore, we attempted to explain our tests with
electrical arcs using quantitative results such as I-V measurements.
(II)
(I)
(III)
13
2.1.2 DIFFERENT TYPES OF ELECTRODES
There are two types of electrodes classified as refractory and non-refractory electrodes[58]. A
refractory electrode has the ability to withstand high amount of heat when it gets heated,
therefore emission of electrons are not so easy. It weakens the thermionic emission process
from cathode. They have high melting points and are also called as hot electrodes (e.g.
Tungsten).
A non-refractory electrode has opposite characteristics to refractory electrode. They have
very little thermionic emission when it is heated and also called as cold electrode (e.g. Copper,
Silver). In this case, thermionic ionization is elevated by secondary ionization. Thermionic
emission at cold cathode is a finite value as shown in Figure 2.2.
Figure 2.2 shows the variation of temperature vs. the ratio of vaporized atom to thermionic
electrons of different electrodes. The thermionic electrons emission varies with respect to the
properties of the hot and cold electrode materials studied for electrical switching applications.
Figure 2.2. Computed ratio of the flux of vaporized atoms to the flux of thermionic electrons as a function of cathode
surface temperature of different electrode materials [59].
14
2.1.3 DIFFERENT TYPES OF ELECTRICAL ARCS
In the electric grid, types of electrical arc have been divided based on the electrical power
switching applications as shown in Figure 2.3. Mainly, low pressure arcs are generated and
utilized at the distribution level and high pressure arcs are utilized at the transmission level
breakers[60].
In furtherance, high-pressure arcs are divided into two categories as axisymmetric and
non-axisymmetric. Axisymmetric arcs are produced due to convective-controlled by the gases
produced by the outgassing polymers or gaseous environment such as SF6 or air. Such arcs can
be observed in different (medium and low voltage) circuit breakers. Non-axisymmetric arcs are
also called as cross-flow arcs and such arcs are controlled under Lorentz forces. One can notice
such arcs in auro-puffer and self-pressurizing interruptors, and combination of rotating-arc and
auto-explansion breaker types are under developing stage.
Figure 2.3. Classification of the electric arc [61].
Electric arc
Low perssure arcs
High pressure arcs (> 1 bar)
Axisymmetric
Wall or Outgassing
stabilized/Free burning
Non-Axisymmetric
Rotating/Helical arc
15
2.1.4 ELECTRIC ARC ROOTS AND ARC COLUMN
An electric arc between the two electrodes can be divided into 3 parts i.e. an arc column (I),
cathode region (II) and anode region (III) as shown in Figure 2.4.
Arc column is sandwiched between anode and cathode in a breaker [62]. Arc column has
been characterized by the temperature of ~ 20, 000 K, and current density of ~ 15 kA/mm2 and
pressure of ~ 10 bar. Under these conditions, the air molecules start dissociating into atoms and
ions in air. The atoms and electrons released due to process of recombination and dissociation
of atoms, ions and electrons. Such arc column is either in stable mode i.e. local thermodynamic
equilibrium (LTE) state, the rate of ionization is equal to rate of recombination or unstable
mode when it reaches to non-LTE state. Arc column reaches high conductivity, because most
of the current conduction is due to the electrons as compared to positive ions. In general
electrons tend to have higher mobility than positive ions. In addition, there will be a space
charge layer accumulating near cathode and anode due to the applied voltage between them.
The shape of the arc column is mainly depending on the supplied energy between the
electrodes and the voltage distribution near the electrodes is shown in Figure 2.4. In case of a
high current, pressure and short arc, a large voltage build-up at the anode side and vice versa.
The voltage distribution is sum of the voltages of the cathode, anode and arc column. Arc
voltage gradient depends on the efficiency of cooling in arc column by the outgassed chemical
species and the ambient atmosphere. In general, the arc voltage gradient of an electric arc
depends on several factors such as arc length, arc current magnitude, the ambient atmosphere
and contact materials.
Figure 2.4. Illustration of several regions of typical electric arc[57].
In our experiments, we have used copper electrodes of hemispherical shape. Copper
electrodes have lower evaporation temperature and induced electron emission due to the
applied electric field. Specific shape of the electrode is chosen to initiate electrons from the
center of curvature of hemispherical tip of the cathode towards anode in axisymmetric way and
to confine between electrodes and PNCs.
(I) (II) (III)
16
2.1.5 DISSIPATION OF ARC ENERGY
An electric arc dissipates the supplied energy in several ways such as broad range optical
radiation, conduction and convection [56, 60, 63]. The radiation is in a broad wavelength
regimes of 250-700 nm due to line and continuum radiation, including bremsstrahlung and
excitation/de-excitation processes[28] utilized for outgassing of surrounding PNCs and other
lower wavelengths are absorbed by air. Energy dissipation at higher wavelengths isn’t a
considerable percentile. In case of conduction, generated joule energy is dissipated[7]. It is a
known fact that, infrared wavelength range contributes towards thermal energy transfer.
Besides that, thermal conductivity is mainly due to translation of positive ions, electrons and
availability of high thermal conductivity species such as H2 and CO2. When H2 and CO2 mix
with the electric arc, thermal conductivity will be increased to reach LTE. Such gases are
possible to transfer the energy in the convective mode.
Few researchers published on the dissipated energy distribution in low voltage electric
arcs, 80% of energy in the form of radiation and rest is thermal energy. There are some other
reports on the dissipated energy where 65% of energy is in the form of radiation and 35% in
the form of thermal energy[64].
2.2 ARC INTERRUPTION PROCESS
Since last decade, circuit breaker’s performance has been improved using arc interruption
by outgassing of polymers in air. Nevertheless, such arc interruption process is very complex
and not fully understood till now.
In electric arc interruption process, PNCs start outgassing due to the energy dissipated by
the electrical arcs. During this process, PNCs could have better radiative and thermal energy
absorption via NPs lead to strong outgassing features. Outgassing process can be due to several
physical mechanisms, photochemical, photothermal and thermal processes or combination of
them[65, 66]. All these mechanisms induce outgassed chemical species helping to improve the
arc interruption process.
In the literature, there are different processes proposed to explain the performance
improvement of breakers using the above mechanisms. First, the arc is convectively cooled by
the flow of outgassed chemical species from the polymers[52]. Secondly, outgassed chemical
species change the thermodynamic and transport properties in the arcing environment, which
help to interrupt the arcs effectively[67]. Third, outgassed chemical species could increase the
current density of arc, induced Lorentz force caused to move the arc quickly to arc
splitters[68]. But, all these processes are not well understood due to the complexity involved in
the outgassing process. Our research is motivated to improve the polymers using nanoparticles,
PNCs due to all the above processes.
17
It is worth to mention about few difficulties involved in understanding the complex process are
splitting the broad range of radiation wavelengths ranges contributing for outgassing process,
mix of electrode material erosion, intense joule heating effects, outgassing material properties,
outgassed gases, 3D nature of the process, lack of reliable material database and of course, cost
incurred for these experiments.
19
3. POLYMER-BASED NANOCOMPOSITES
In particular, the combination of polymer and nanosized inorganic particles into a single
material has opened up a new area in materials science that has extraordinary implications in
the development of multifunctional materials. These are considered as innovative advanced
materials, with promising applications expected in many fields, including optics, electronics,
protective coatings, catalysis, sensors, biology, and others.
Among inorganic-organic
composites materials, PNCs are of particular interest since they inherit the properties of the
bulk polymer such as being easily processable, and suitable for cost efficient high-volume
manufacturing, while at the same time introducing new properties of nanoparticles. Significant
scientific and technological interest has therefore focused on polymer based nanocomposites
over the last two decades. [38, 47, 69, 70]
In the last decade, PNCs unveils more inter-disciplinary applications such as
automobiles[40], packaging[41] and bio-medicals[45, 46]. Recent developments in composite
materials science broaden the opportunities to improve electrical power switching components
(e.g. breakers, transformers, contractors, etc.) with optimum performance. Therefore, today it is
possible to design novel materials with profoundly improved specific properties, which could
be used for improving the reliability and efficiency of electrical switching applications. The
generated knowledge form this thesis will be useful for effective design and implementation of
new “tailor-made” materials for the next generation MCBs.
3.1 POLYMERS
Different polymers have been used in several technological applications such as circuit
breakers, arc furnaces, arc heaters, pulsed plasma thrusters, high-density plasma sources[52,
71]. In the last decade, polymers have been widely used to improve the electrical arc
interruption process in electrical switching applications.
3.1.1 WHY THERMOPLASTIC POLYMER?
Several extensive studies are reported on the polymeric materials, screened for arc-
quenching capabilities in breakers and fuses of high breaking fault currents[29, 52, 71-74]. All
such studies show that cellulosic and polyacetal groups are useful for quenching electrical arcs.
These groups are able to generate cooling gases such as H2 and CO for improving the
20
efficiency of breakers[72]. Polytetrafluoroethylene (PTFE) and quartz are known for being an
almost inactive polymer when exposed to electric arcs, used as a reference material[75]. In our
first experimental studies, it is noticed that improvement is achieved by the outgassing
(ablation) of the polymers due to the highly energetic radiation and thermal energy generated
by the electrical arcs. Such electrical arcs are generated between a 5 mm air gap with
prospective current of 1.4 kA in the designed experimental setup. During the early stage of the
project, two commercial polymers i.e. poly (methyl methacrylate) - PMMA (–C5H8O2–) and
polyamide 66 – PA 66 (–C12H22O2N2–) shown in Figure 3.1 were used to investigate the
outgassed chemical species and the arc interruption process.
Figure 3.1. Chemical structure of PMMA and PA66 [76].
3.1.2 PMMA
Since electrical switching application requires the controlled amount of outgassed cooling
vapour, we have done several tests with the polymers with the atomic compositions of C, H, O
and N. Based on our experimental tests reported[77], we have chosen to use poly(methyl
methacrylate) (PMMA) for fabricating PNCs. The release of cooling gases is highly
endothermic, due to outgassing of polymeric material and more energy released prior to an
electric arc interruption.
3.2 NANOPARTICLES
Nanoparticles (NPs) are the most important components in the design of PNCs[78] for the
power switching devices as they can significantly influence the absorption of broadband
radiative energy (from ultraviolet to infrared) and thermal energy from electrical arcs,
improving the outgassing performance of the polymer for extinguishing the arc. In particular,
specific functionality of PNCs can be improved by using NPs[79]. For example magnetic NPs-
polymer nanocomposites are investigated for magnetic and optical applications [38, 79]. In our
work, we are in a need of enhancing the absorption by proper NPs design from families of
dielectric ceramics, magnetic materials, metallic materials and their combinations. These
materials will have different types of energy absorption characteristics based on their particle
size, shape and the NP architecture[80]. Therefore different materials in different absorption
PMMA PA66
21
regions, with various morphologies and in various configurations have been chosen for the
investigation[81].
Several morphologies of NPs including spherical, rod-like, core-shell, Janus
nanostructures shown in Figure 3.2 can be fabricated. It is well known that electromagnetic
absorption[82] of the materials is influenced by their size and morphology, especially in the
case of metallic nanostructures when they are anisotropic. For that reason, nanoparticles of the
same family with various size and agglomeration characteristics can be developed and used for
the fabrication of PNCs.
Figure 3.2. Morphology of NPs (a,b) and architectures (c-e): (a) spherical, (b) rod-like, (c,d) core-shell, (e) janus type.
Methodologies of colloidal synthesis can be adapted from more energy and resource
effective methods to scale up industrial scale, including microwave assisted synthesis, and
solvothermal synthesis, considering the environmental impact of the precursor and waste
materials. Spherical NPs from different materials families (metallic, dielectric, magnetic
compositions as well as their combinations) with a different size population and extent of
agglomeration can be fabricated.
In particular, proposed NPs in this work used for improving the specific functionality of PNCs,
are listed below:
1. CuO NPs: Enhanced thermal conductivity and radiation absorption
2. Fe3O4 NPs: Magnetic, higher thermal stability and controlled outgassing of PMMA
3. ZnO QDs: Enhanced UV radiation absorption
4. Au NPs: Higher thermal conductivity, and visible-infrared radiation absorption
(plasmon absorption)
3.2.1 SURFACE MODIFICATION OF NPS
In order to fabricate high-quality PNCs, surface modification of NPs has to be done[83]
with respect to chosen polymeric matrix i.e. PMMA.
Poly (methyl methacrylate) (PMMA) is a common commercial polymer that fits best for the
function of the matrix for PNCs. To facilitate strong interactions and energy transfer between
the fabricated NPs and the polymeric matrix, the NPs surfaces need to be tailored as shown in
(a) (b) (c) (d) (e)
(a) (b) (c) (d) (e)
22
Figure 3.3. Ideally, surface treatment of the NPs should (i) passivate any dangling bonds to
minimize contributions of unwanted surface states for charge traps; (ii) allow the particles to
form stable dispersions in MMA monomer solvents without particle agglomeration; and (iii)
allow for thermal coupling with a polymer matrix[48, 84, 85]. Thiol, amine, and carboxylic
acid groups coordinate strongly to the surface of the NPs surfaces to be prepared during this
work. Thus, a variety of surface modification schemes are considered based on combining
these different head groups that have affinity for the particle surface with tail groups that are
compatible with PMMA. In this way both the dispersability of NPs in the PMMA matrix and
effective energy transfer of PMMA are possible to tailor.
3.3 FABRICATION OF PNCS
In the literature, there are mainly three methods of polymerization methods discussed and
classification is shown in Figure 3.4. These methods were explained by top-down and bottom-
up approach[38, 47, 48, 69, 86].
Figure 3.3. Illustration of PNCs fabrication followed in this work[87].
In-situ Intercalative Polymerization: This method is first used by Toyota Research Group
to produce polyimide based nanocomposites[88]. Actually, this method is initiated by the
polymer dissolved in a solvent and then NPs are added to it[89, 90]. The solvents such as
hexane, chloroform and toluene have been used according to the polymer and surface modified
NPs. Afterwards, the solvent is evaporated when the NPs are adsorbed by the PMMA matrix
completely.
In-situ polymerization: This method is processed by mixing of surface modified NPs in the
liquid monomer by using ultra-sonication. When it is stabilized, the polymerization is started
either by heat or UV radiation. This method has been successfully tested using nylon–
montmorrillonite nanocomposite, and then extended to thermoplastics [38, 86, 91]. The
Surface modifier
Monomer chains
NPs
(a) (b) (c)
23
advantage of this method is the stabilization/dispersion, covalently bonded between NPs and
PMMA matrix[85].
Melt Intercalation: This method is started at the molten state of polymer matrix and then
NPs are mixed into it[92, 93]. This method has been mechanically processed by using injection
molding and extrusion technique[93, 94]. It is very popular in industrial scale and aimed for
fabricating thermoplastic based nanocomposites. This technique is alternatively used with
respect to other two methods explained above.
Figure 3.4. Classification of fabrication methods of PNCs[61].
PNCs Fabrication methods
Top-down approach
Polymer intercalation in a
solution Melt Intercalation
Bottom-up approach
In-situ polymerization
[Hybrid sol-gel solution ]
Organic precursor [PMMA - MMA
solution ]
Inorganic precursor
[Suface modified NPs]
25
4. EXPERIMENTAL
A wide variety of fabrication methods have been used for NPs and PNCs during the first phase
of the thesis. In the second phase, PNCs have been tested with the electrical arcs to study the
arc interruption process. In both phases, different physiochemical properties have been
analyzed using several characterization techniques on the NPs and PNCs. All of these
techniques were performed at state of the art Nano-Characterization center at Electrum
laboratory, KTH. In the second phase, tests for arc interruption process were carried out at the
Department of Electromagnetic Engineering at KTH. In this chapter, different fabrication
methods of NPs and PNCs, characterization techniques and experimental test set-up used for
arc interruption process are described in detail.
4.1 MATERIALS AND METHODS
In the first phase of the thesis, all the raw materials used in the fabrication of PNCs are
commercially available from Sigma-Aldrich. Specific details are given on the synthesis
methods of the NPs are presented below, followed by the fabrication methods of PNCs.
4.1.1 SYNTHESIS OF NPS
Due to the broad range of our studies, we have synthesized CuO, Fe3O4, ZnO and Au
nanoparticles. The procedures followed for their fabrication is described below.
4.1.1.1 CUPRIC OXIDE NPS SYNTHESIS
Copper(II) acetate monohydrate (1.2g) dissolved in aqueous solution of 300 ml. Acetic
acid (1ml) was added to prevent the hydrolysis and calcination of the above solution until
100°C with reflux, followed by the addition of NaOH (800mg) at 100 °C till pH value reached
6-7. The mixture was then cooled down to room temperature and the black precipitated powder
was filtred, washed once with distilled water and three times with acetone before final drying
at room temperature.
Oleic acid (OA) is used for surface modification of cupric oxide powders and the
procedure is explained as follows: Initially, 35 mg of NaOH was dissolved in 10 ml of distilled
water with mechanical stirring for 10 min. Then, OA (0.3M) is added drop wise to the above
26
solution in order to prepare a solution with pH of 11. Afterwards, 1 mg of CuO nanoparticles
were added to 10 ml of the above solution and mixing was continued at 85 oC for 60 min. After
cooled to room temperature, HCl (1M) was added to the above suspension to remove unreacted
oleic acid and change the pH of the suspension from 11 to 6.
The surface modified cupric oxide nanoparticles were centrifuged, filtered and washed
with acetone for several times. The prepared OA modified cupric oxide powders were
dispersed in hexane before using for fabrication of PNCs[91].
4.1.1.2 IRON OXIDE NPS SYNTHESIS
All the chemicals used in this synthesis are procured from Sigma-Aldrich Chemie GmbH
(Germany). Iron oxide (-Fe3O4-) nanoparticles are synthesized using thermal decomposition
method[85]. This is a two-step method; the first step, iron-oleate is produced using previously
reported method. Afterwards, in the second step, thermal decomposition of 5 mL of iron-oleate
complex was performed in 20 mL of dioctyl ether in the presence of oleic acid. This
decomposition method was initiated in a dual necked flask by refluxing the complex at ca. 297 oC. This method obtained the black color powder, separated with the help of an external
magnet. The black colored powder containing iron oxide nanoparticles are washed three times
by centrifugation with ethanol to remove the byproducts, and then re-dispersing in hexane for
getting a homogeneous distribution of nanoparticles and stored at 4 oC for fabricating PNCs.
4.1.1.3 ZINC OXIDE QDS SYNTHESIS
The synthesis of ZnO QDs[95] were initiated by hydrolyzing zinc acetate dihydrate
(Zn(CH3COO)2·2H2O, ZAD, > 98% ACS reagent, Sigma-Aldrich) with Ethanolamine (MEA,
≥99% ReagentPlus, Sigma-Aldrich) as described by Znaidi et al[96]. The synthesis process
was followed in two steps. In the first step, Zn(II) solution (0.01–0.1 M) was prepared by
refluxing ZAD in ethanol (Solveco) at 85 °C for two hours under continuous stirring. In the
second step, MEA was added to these solutions with the ratio i.e. [MEA]/[Zn(II)] = 3 to obtain
ZnO QDs. The resulting color of the ZnO QDs suspensions varied form transparent to bluish
and yellow depending on the particular concentrations of [Zn(II)] and [MEA].
4.1.1.4 GOLD NPS SYNTHESIS
The typical synthesis process for Au NPs is as follows[97]: 15 mL of hydrochloroauric
acid (10 mg/mL) (HAuCl4, > 99.99%, Sigma-Aldrich) was added into 40 mL of chloroform
(CHCl3, ≥99.9%, Sigma-Aldrich) containing 1.1 g of tetraoctylammonium bromide (TOAB,
[CH3(CH2)7]4N(Br), 98%, Sigma-Aldrich). After 30 minutes of continuous stirring, 120 mL of
1-dodecanethiol (NDM, CH3(CH2)11SH, ≥98%, Sigma-Aldrich) was added to the
aforementioned mixture. Then, 192 mg of sodium borohydride (NaBH4, ≥98%, Sigma-
27
Aldrich) was added to this mixture of solutions in aqueous form to obtain a desired size of Au
NPs in chloroform. Afterwards, the mixture containing synthesized Au NPs was flushed by DI
water three times in separation funnel and the chloroform suspension of Au NP was
concentrated using rotary evaporator and then re-dispersed in hexane for fabricating PNCs.
4.1.2 FABRICATION OF PNCS
In this work, several PNCs are fabricated with different weight percentages (wt %) of
synthesized NPs in methyl methacrylate (MMA) monomer (99 %, Sigma-Aldrich) by in-situ
polymerization. Different wt% of nanoparticles is added to the glass vials containing MMA
monomer (5 mL) separately and wt % of NPs is detailed in chapters of Paper II, IV, V and VI.
Recrystallized 2 2′-Azobis (2-methylpropionitrile) (AIBN, 98 %, Sigma-Aldrich) was added as
an initiator to final monomer solution and well-dispersed by sonication. Afterwards, the
polymerization is initiated in the glass vials at 75 oC in a temperature controlled oil bath for 12
hours. Polymerization process is continued under mechanical stirring, transparent PNCs were
obtained. The schematic flow of the process is shown in Figure 4.1.
Figure 4.1. General schematic of the experimental process followed in the fabrication of PNCs.
4.2 CHARACTERIZATION TECHNIQUES
Several material characterization techniques have been used for the evaluation of NPs and
PNCs at different processing and testing stages. Crystalline structure of the as-synthesized
nanoparticles were studied by x-ray powder diffraction (XRD); their morphology and size was
investigated by transmission electron microscopy (TEM), while surface functionality has been
evaluated by Fourier-transform infrared spectroscopy (FT-IR); Absorption characteristics of
colloidal NP solutions and PNCs have been studied by UV-Vis spectroscopy, while the thermal
characteristics have been investigated using thermal gravimetric analysis (TGA) and
differential scanning Calorimetry (DSC); decomposition products of PNCs have been
identified using TGA system coupled with FTIR. Morphology of the PNCs after exposure to
electric arcs has been studied by using Scanning Electron Microscopy (SEM). Details of each
technique are presented below.
NPs dispersed in a PMMA monomer
Ultrasonication Add initator in-situ
polymerization PNCs
28
4.2.1 X-RAY DIFFRACTION, XRD
The crystalline structures of the NPs before fabricating PNCs were analyzed by using
PANalytical Empyrian X-ray diffractometer (using the source of Cu-kα line wavelength of
1.5405 Å)[98].
Monochromatic X-rays of wavelength of 1.5405 Å are generated by a cathode ray tube
using high voltage source and then incident on the NPs. When electrons in inner shell of NPs
receive sufficient energy, generates characteristic X-ray radiation and collected by the detector
in broad range of angles from 2o to 60
o. The main physics/principle behind this process is the
Bragg’s Law i.e. nλ = 2d sinθ, when the wavelength of incident X-rays with the sample
produces constructive interference with the diffraction angle. One should be able to find lattice
spacing in a crystalline sample, and its crystal structure by using this principle.
4.2.2 TRANSMISSION ELECTRON MICROSCOPY, TEM
The high-resolution microstructure/morphology of the NPs was analyzed by using
Transmission electron microscopy[99], model JEOL FEG JEM 2100F. Most of our TEM
samples were prepared by diluting the NPs in hexane and then drop-casted homogeneously on
the copper grid, later dried under ambient conditions.
The TEM operates based on the same basic principles as the normal light microscope, but
uses electrons instead of light. Electrons are emitted from the electrode using the high voltage
source of 200 kV and passing through vacuum in the column of the microscope. The
electromagnetic lenses are used to confine the electrons into a very thin beam and then passing
through the copper grid. According to sample in the copper grid, some electrons will be
scattered and some disappear from the source beam. The un-scattered electrons are collected
by the fluorescent screen (or nowadays CCD camera), which reveals the morphology of the
NPs.
4.2.3 FOURIER TRANSFORM INFRARED SPECTROSCOPY, FTIR
The surface functionality of the NPs are investigated by using FTIR spectroscopy[100],
(Perkin Elmer Spectrum-One) in the attenuated total reflectance (ATR) mode, by placing the
NPs on the crystal window.
In principle, FT-IR spectrometer analyze and record the interaction between infrared
radiation (wavenumber ranges~500-4000 cm-1
) and a sample i.e. NPs surface, by measuring
the frequencies and their intensity of radiation absorbed by the NPs surface. The absorption
spectrum evidences the number of absorption bands corresponding to functional groups within
the sample, which is then used for the identification of fingerprint regions of molecules and
crystalline materials.
29
4.2.4 UV-VIS ABSORPTION SPECTROSCOPY, UV-VIS
The broad range radiation absorption spectra of nanoparticles and PNCs were recorded using
Cary 100 Bio UV–Vis spectrometer.
Ultraviolet or visible light is absorbed by different molecules in the sample. The
absorbance is directly proportional to the path length travelled with-in the sample (b), and the
concentration of the sample (C).
According to Beer's Law, the absorptivity of the sample i.e. A = ε b c, where ε is molar
absorptivity of the sample. The spectrometer has dual beam-path, passing through the “sample”
cuvettes and background as the ‘Reference’, thus the influence of the background is eliminated
from the resultant spectrum [101].
4.2.5 THERMAL GRAVIMETRIC ANALYSIS, TGA
Thermal degradation of the PNCs was studied using TGA Q500 system (TA Instruments).
The measurements are performed under synthetic air atmosphere with a heating rate of 10 oC/min from 25 to 600
oC [102].
TGA measures % weight change (loss/evaporation) w.r.to. temperature of the PNCs, while
temperature is increased at a controlled rate. These measurements can be done under different
atmospheric conditions such as nitrogen, hydrogen, air or in vacuum. TGA can be interfaced
with FTIR to identify and measure the vapors generated from the PNCs; this is called evolved
gas analysis (EGA).
4.2.6 DIFFERENTIAL SCANNING CALORIMETRY, DSC
Thermal stability and material properties such as melting point, heat capacities are
measured using DSC (Q2000 system from TA Instruments). The measurements are performed
under synthetic air atmosphere with a heating rate of 10 oC/min from 25 to 600
oC [103].
The advantage of using DSC is to observe the physical transformations, or changes, within
the materials, which are not necessarily accompanied by any weight changes. In case of PNCs,
glass transition temperature variation, exothermic and endothermic peaks are important to
evaluate the PNCs.
4.2.7 SCANNING ELECTRON MICROSCOPY, SEM
The morphology of the NPs and PNCs is imaged using scanning electron microscopy
(SEM)[104] (model Zeiss Ultra 55) equipped with field emission gun (FEG) and energy
30
dispersive x-ray analyser (EDX). Imaging can be done without any post processing of NPs as
mentioned for TEM.
The SEM uses intense beam of electrons to interact with the NPs surface, a wide range of signals from electron-NPs interactions. The generated signals will reveal the
information about the sample such as morphology, and chemical composition. In case of
PNCs, the insulating samples were coated with a thin layer of gold/copper by sputtering
process in order to minimize the charging effects during the interactions with the electron
beam.
4.3 EXPERIMENTAL TESTS WITH THE ELECTRICAL ARCS
During the second phase of the thesis, we have tested the PNCs for understanding the arc
interruption process. The experimental setup comprises of the electrical arc discharge circuit
and the test set-up.
4.3.1 ELECTRICAL ARC DISCHARGE CIRCUIT
The discharge circuit as shown in Figure 4.2 is an RLC circuit consisting of a capacitor (16
mF), an inductor (496 μH) for creating a damped oscillation at a frequency of 50 Hz. In
addition, a thyristor switch is connected in series for switching on and off the current [102].
The capacitor is charged to 450 V for all the discharges in order to generate reproducible
electrical arcs with an equivalent peak current of 1.6 kA, approximately. Electrical
measurements such as arc current and voltages are recorded in order to understand the electric
arc interruption process.
4.3.2 EXPERIMENTAL TEST SET-UP
The experimental test set-up is connected in parallel to the discharged circuit and then, the
electric arcs are generated between the two semi spherically shaped contacts (electrodes)
separated by a distance of 5 mm. A thin copper wire of diameter of 20 µm is used for igniting
Vc
Rc Rstray L
Figure 4.2. The high voltage arc discharge circuit containing the power source and test set-up.
Test
se
t-u
p
31
the electrical arc. PNCs and non-ablating quartz slide are placed next to the electrodes in this
set-up in order to test the arc interruption process by the PNCs. Detailed image of the test set-
up is shown in Figure 4.3 and in Appendix A.
Figure 4.3. Schematic illustration of the designed test set-up and enlarged view of the specific details of PNC location
and electrode geometry[95].
300mm
10mm
Thick glass slide
Thin quartz slide
PNCs
5mm
33
5. RESULTS AND DISCUSSION
This section is aimed to discuss summary of all the results obtained and presented in the
appended articles. This part has been divided into four sections (as shown in Figure 5.1) based
on the NPs chosen for different PNCs i.e. CuO NPs, Fe3O4 NPs, ZnO QDs and Au NPs. In
each section, we will discuss the results from physiochemical and arc resistant material
characterization of NPs, PNCs and the impact of PNCs on the electrical properties of electrical
arcs (i.e. current and voltage) generated by using designed test set-up at KTH laboratory is
described in detail.
Figure 5.1. General schematic of the experimental process followed in this thesis.
5.1 POLYMERS
It is found that the two polymers showed a clear influence on the arc voltage and current
due to the monomer chemical structure. Two different techniques namely FTIR spectroscopy
and time resolved TGA connected to FTIR spectroscopy are used to identify the dominant
outgassed species such as H2, CO, CO2 produced during the arc interruption process[77]. 3D
and 2D plots of these results are shown in Appendix B: TGA-FTIR spectroscopy. Surprisingly,
the outgassed species were also observed from modelling and simulation works. Furthermore,
the analysis of morphological changes on the outgassed surface of the PMMA and PA66
showed the differences in the outgassing effect. All these results are reported in Paper I.
Synthesis of NPs Suface modification of
NPs Fabrication of PNCs
Validate PNCs impact on the electrical arcs
34
5.2 PNCS WITH CUPRIC OXIDE NPS
CuO NPs are synthesized using precipitation method[91] as detailed in Chapter 2 and Paper
II. TEM image of the as-synthesized CuO NPs shown in Figure 5.2 displaying near spherical
morphology. The average diameter of the NPs was obtained from TEM micrograph as 6±1.3
nm. The SAED pattern of the as-synthesized CuO NPs shows that NPs are crystalline in nature
and the diffraction rings observed matched with the XRD pattern of CuO (Paper II).
Before fabricating PNCs, surface modified CuO NPs were investigated by FT-IR and the
spectra is shown in Figure 5.3. The surface modified NPs with oleic acid (CuO_OA) shows the
peaks of aliphatic monocarboxylates for asymmetric COO-- stretch frequencies at 1545 cm
-1
and symmetric stretch frequencies at 1375 cm-1
instead of –COOH vibration peak at 1710 cm-1
.
Long alkyl chains (–CH, –CH2, and –CH3) peaks were located in between 2750–3000 cm-1
. So,
all these peaks indicate the possible interactions between a carboxylate group and the metal
centre (Cu) in the NPs.
After the surface modification of NPs using OA, PNCs were fabricated with two different
wt% of CuO NPs. A well-dispersed CuO suspension was dispersed in MMA monomer solution
of 5 mL using sonication. Afterwards, the initiator 2,2-azobisisobutyronitrile (AIBN) is added
to the final solution in order to start the polymerization. The polymerization was carried out at
70 oC for 12 h in order to reach the final bulk transparent PMMA/CuO nanocomposites shown
in Figure 5.4. The method of fabrication of such PNCs is also called as in-situ polymerization.
(111)
(a) (b)
Figure 5.2. High resolution TEM image of (a) CuO NPs and (b) SAED pattern[91].
2 1/nm
35
Figure 5.3. FTIR spectra of as synthesized CuO NPs, OA and Oleate modified CuO (CuO_OA)[91].
UV–Vis absorption analysis has been performed on the as-synthesized CuO NPs dispersed in
water, PMMA, PNC_CuO_1 and PNC_CuO_2 detailed in Paper II. Absorption edges of the
PNCs with different wt% of CuO NPs in UV-Vis absorption spectra evidence the improved UV
radiation absorption in PNCs in comparison to bulk PMMA.
In order to understand the impact of PNCs on the electrical arcs, the designed experimental
test set-up was used. The photograph of the test set-up is shown in Paper I in detail such as
electrode shapes, configuration and how the PNCs are placed in the system. Mass loss of the
PNCs are recorded during the tests with the electric arcs, where PNC_CuO_2 has the highest
values of mass loss due to the highest values of arc voltage. The highest mass loss values also
lead to strongest depositions on PNC_CuO_2 compared to PNC_CuO_1 and PMMA as shown
in Figure 5.5.
The electrical signals shown in Figure 5.6 and the arcing time and arc voltage correlates
with the UV radiation absorbance of the PNCs and PMMA. Fresh samples and electrodes are
used during the electrical tests with the electrical arcs. It is noticed that ignition wire is
evaporated at 1 ms and PNC_CuO_2 exhibits relatively high arc voltage which leads to the
limited current and the arc is extinguished about 0.37ms faster than PNC_CuO_1 and PMMA.
There is no significant difference between the electrical signals for PMMA and
PNC_CuO_1[105].
Figure 5.4. Photograph of the PNCs with two different wt% of cupric oxide NPs[91].
PMMA PNC_CuO_1 PNC_CuO_2
0.0017 wt% 0.024 wt%
1 cm
36
Figure 5.5. Mass losses of the CuO-PMMA PNCs during three consecutive tests[105].
Figure 5.6. Electrical signals measured using CuO-PMMA PNCs and PMMA[105].
PMMA
PNC_CuO_1
PNC_CuO_2
Arc
volt
age
(V
)C
urr
ent
(k
A)
37
PMMA PNC_Fe3O4_1 PNC_ Fe3O4_2
1 cm
0.0017 wt% 0.024 wt%
5.3 PNCS WITH IRON OXIDE NPS
Figure 5.7. TEM image of the surface modified Fe3O4 NPs and enlarged image of a single NP in the inset [85].
Iron oxide, Fe3O4, NPs are synthesized using thermal decomposition method as detailed in the
experimental section and Paper IV. High resolution TEM image of surface modified Fe3O4
NPs is shown in Figure 5.7. It is shown that Fe3O4 NPs display nearly spherical morphology,
with average diameter of 11 ± 1 nm. The high-resolution TEM image reveals that NPs are
single crystalline and complemented by the XRD diffraction spectrum shown in Paper IV.
Figure 5.8. Photograph of the PNCs with two different wt% of iron oxide NPs[85].
It is observed that the Fe3O4 NPs are well separated due to their surface functional groups,
allowing homogenous distribution in the PMMA matrix. Afterwards, in-situ polymerization is
used to fabricate PNCs with different wt% of Fe3O4 NPs as shown in Figure 5.8.
10 nm
5 nm
38
TGA and DSC are used to evaluate the thermal properties such as thermal stability, and
thermal energy absorption characteristics of PMMA and PNCs. It is observed that
PNC_Fe3O4_1 has better thermal stability in comparison to PMMA and PNC_Fe3O4_2 as
shown in Figure 5.9. It is also confirmed that the surface modified Fe3O4 NPs and their
homogeneous distribution in the PNC_ Fe3O4_ l lead to enhance its thermal stability,
leading to controlled outgassing process in arc interruption process.
Figure 5.9.TGA (a) and DSC (b) thermograms of PMMA and Fe3O4-PMMA PNCs[85].
(a)
(b)
Wei
gh
t (%
)
Exo up
39
Figure 5.10. Electrical signals recorded during the tests with Fe3O4-PMMA PNCs frontside and backside[85].
The electric arc interruption process of the PNCs is observed using the recorded electrical
signals shown in Figure 5.10. The weights of the samples were recorded by using micro
balance in order to estimate the ablated mass values during the tests. According to the
recorded electrical signals, voltage signals show two different arcing states such as high
voltage and low voltage state. In case of high voltage state, the electric arc is controlled due
to outgassed chemical species lead to buildup voltage, while in case of low voltage state,
the electric arc is much more stable and impact of outgassed chemical species were not
observed. Furthermore, effect of the PNCs and PMMA on the electrical arc is noticed using
the ablated mass values coupled with total dissipated energy as shown in Figure 5.11. In
case of PNC_Fe3O4_1, observed a difference of 61.7 % in ablated mass values between the
front and back sides. It is surprised to notice that front side of the PNC show high
dissipated energy and quick arc interruption, while the amount of outgassed chemical
species is lower. Besides that, trend of these results implies that with increasing wt % of
NPs in PNCs switch the electric arc from high voltage state to low/ more stable voltage
state. More detailed discussion is followed in Paper IV.
40
Figure 5.11.The total dissipated energy (bar charts) and Average ablated mass (solid and dashed lines) measured
during the electric arc and Fe3O4-PMMA PNCs interaction[85].
PMMA PNC_Fe3O4_1 PNC_Fe3O4_2
41
5.4 PNCS WITH ZINC OXIDE QDS
ZnO QDs are synthesized using thermal decomposition method [31]. TEM micrograph
shown in Figure 5.12 reveals nearly spherical NPs with particle diameter of 3.5±0.6 nm,
detailed in Paper V.
Figure 5.12. TEM micrograph of ZnO QDs and inset high resolution image of a single crystalline ZnO QD[95].
In the process of fabricating PNCs, FT-IR spectrum shows the surface functional groups of
ZnO QDs. It is noticed that MEA helps to avoid agglomeration between the ZnO QDs due
to the functional groups revealed by respective absorption regimes, such as C=O stretching
(1748 cm-1
), -CH, -CH2 and –CH3 stretching (2750-3000, 1382 cm-1
) and -NH (3300 cm-1
),
respectively.
Afterwards, six PNCs are fabricated by using in-situ polymerization process with
different wt% of QDs as shown in Figure 5.13, named based on the increasing NP content
as PNC_ZnO_1, PNC_ZnO_2, PNC_ZnO_3, PNC_ZnO_4, PNC_ZnO_5 and
PNC_ZnO_6.
The optical absorption spectrum of PMMA and PNCs are demonstrated by using UV-
Vis spectroscopy. The absorption edges of PNCs shown in Figure 5.14 are increased
systematically and stepwise from PMMA till PNC_ZnO_5, while decreased for
PNC_ZnO_6. The absorption edge values are recorded for PMMA and all PNCs are in
order of 270 nm, 278 nm, 279 nm, 284 nm, 288 nm, 338 nm and 306 nm respectively. It is
noticed that the inversion in the absorption edge for PNC_6 due to the increased wt % of
QDs over percolation threshold level in PMMA matrix. Besides that, the trend of
fluorescence intensity of the samples shown in Figure 5.13 following the UV absorption
edges revealed from UV-Vis spectrum.
Figure 5.13. Photograph of the fabricated PMMA and the ZnO-PMMA PNCs under low intensity UV lamp with
different wt % of ZnO QDs[95].
5 nm
2 nm
0.01 wt %
0.025 wt % 0.05 wt % 0.075 wt % 0.1 wt % 0.25 wt %
5mm
PMMA PNC_ZnO_1 PNC_ZnO_2 PNC_ZnO_3 PNC_ZnO_4 PNC_ZnO_5 PNC_ZnO_6
Figure 5.14. UV-vis absorption spectra of all the ZnO - PMMA PNCs and PMMA[95].
The electric arc interruption process of PNCs is analyzed using the experimental test set-up
detailed in Paper V. It is understood that the arc interruption process is statistical in its
nature and few conclusions are drawn from the electrical signals shown in Figure 5.15 and
5.16. The electrical signals show peaks at 1 ms due to the ignition wire evaporation. As we
observed in iron oxide based PNCs, the electrical signals clearly depict the two states of the
electrical arc such as high voltage (HV) and low voltage (LV) state. In case of HV state, the
voltage starts to building up in the electrical arc due to the presence of ablated chemical
species. It is noticed that for low wt % of QDs in PMMA got the improved UV radiation
absorption lead to reach high voltage state, a further increase of QDs wt% turn to reduce
UV radiation absorption and the electric arc moved from HV state to LV state. Such
transition is observed during the tests with both front and backside.
Figure 5.15. Electrical signals during the tests with the ZnO-PMMA PNCs backside[95].
42
43
Figure 5.16. Electrical signals during the tests with the ZnO-PMMA PNCs frontside[95].
The total dissipated energy coupled to the ablation rate of the electric arc as shown in
Figure 5.17 in order to understand the electric arc interruption process of the PNCs and
PMMA. It is shown that pure PMMA outgassed low amount of chemical species towards
the electrical arc, revealing a number of fast varying voltage spikes in the electrical signals
shown in Figure 5.15 and 5.16. In case of frontside and backside tests, PNCs up to PNC_5
show higher dissipated energy and better arc interruption process due to lower ablation
rates. The variation of ablation rates varies in a non-monotonous way and correlated to the
UV absorption edge w.r.to wt% of ZnO QDs in PMMA matrix. It is also observed that
variation of ablation rates is same for both sides of PNCs and PMMA. Detailed information
is provided in Paper V.
Figure 5.17. Total dissipated energy of the electric arc (bar plot) and Ablation rate (solid and dashed lines) of the
ZnO QDs –PMMA PNCs [95].
PMMA PNC_ZnO_1 PNC_ZnO_2 PNC_ZnO_3 PNC_ZnO_4 PNC_ZnO_5 PNC_ZnO_6
44
5 nm 2 nm
(a) (b)
5.5 PNCS WITH GOLD NPS
Figure 5.18. TEM images of the Au NPs at low (a) and high (b) magnification[105].
Au NPs are chosen to improve the thermal conductivity of PNCs and to see the arc
interruption process. In general, these PNCs are aimed to withdraw more thermal energy
from the electrical arcs, leading to improve the outgassing nature of PNCs using Au NPs.
Morphology of synthesized Au NPs is studied by using the TEM image shown in
Figure 5.18. NPs are spherical, crystalline and have an average diameter of about 2.75 ± 0.4
nm. Homogeneous distribution of NPs in the PMMA matrix is critical for fabrication of
PNCs, surface modification of NPs is necessary to have better interfacial interaction with
the polymeric monomer chains. The surface modified Au NPs are analyzed using FT-IR
spectroscopy, several functional groups such as methyl and carboxyl groups are exhibited
on the Au NPs. More detailed explanation about FTIR spectrum is presented in Paper VI.
PNCs using Au NPs are fabricated using in-situ polymerization process. The PNCs are
named with respect to increased wt% of the Au NPs as PNC_Au_1, PNC_Au_2,
PNC_Au_3, PNC_Au_4, PNC_Au_5 and PNC_Au_6, as shown in Figure 5.19. Besides
that, PMMA is fabricated without NPs for the reference sample by the same polymerization
process.
Thermogravimetric analysis of the PNCs is done to understand the
degradation/decomposition process w.r.to wt% of NPs. The TGA thermograms reported in
the Paper VI and discussed about thermal stability and extent of outgassing of the PNCs.
Figure 5.19. Photograph of the PNCs with different wt% of Au NPs[105].
5 mm
PMMA PNC_Au_1 PNC_Au_2 PNC_Au_3 PNC_Au_4 PNC_Au_5 PNC_Au_6
45
The electrical signals presented in Figure 5.20 show only one state of the electric arc using
PNCs i.e. high-voltage state and no other state is observed. Nevertheless, it is noticed that
PNCs using ZnO QDs and Fe3O4 NPs observed the two states i.e. high-voltage and low-
voltage state. It could be due to wt% of Au NPs and high molecular weight of Au NPs
compared to ZnO QDs in the PNCs. More details are reported in Paper VI.
Figure 5.20. Electrical signals recorded during the tests with the Au-PMMA PNCs frontside (a); backside (b) [105].
(a)
(b)(b)
PMMA
PNC_Au_1
PNC_Au_2
PNC_Au_3
PNC_Au_4
PNC_Au_5
PNC_Au_6
46
The electric arc interruption process of the PNCs using Au NPs and PMMA is explained
using total dissipated energy and ablated mass values, shown in Figure 5.21. PMMA
outgassed lower amount of chemical species and evidenced by fast varying voltage spikes.
In case of frontside and backside tests, all PNCs with ultra-low wt% of Au NPs showed
higher dissipated energy turned to improve arc interruption process. The ablated mass
values followed similar trend on the both sides of PNCs except for PNC_Au_3. The
comparison between Au and ZnO based PNCs, detailed information is reported in Paper
VI.
Figure 5.21. Total dissipated energy of the electric arc (bar plot) and Ablated mass (solid and dashed lines) of the
PNCs made of Au NPs[105].
PMMA PNC_Au_1 PNC_Au_2 PNC_Au_3 PNC_Au_4 PNC_Au_5 PNC_Au_6
47
6. Conclusions
In conclusion, few series of bulk homogeneous and transparent PNCs are successfully
fabricated with different low wt % NPs loadings by using an in-situ polymerization process.
Different loading of wt% of NPs in PMMA matrix is chosen based on their chemical and
physical properties. Our preliminary experimental tests were started with CuO NPs, then
followed towards Fe3O4 NPs, ZnO QDs and Au NPs.
The PNCs are fabricated, characterized in detail for the morphology of NPs, interaction
between NPs and polymer matrix monomer chains. Several physiochemical
characterizations were performed on the PNCs and NPs have been performed to assess their
radiative absorption and thermal properties. Such material properties are essential to
understand the electric arc interruption process.
The electric arc interruption process of the PNCs is observed using the designed test
set-up, electrical signals, ablation rate are recorded. Based on the observations with all the
PNCs, the electrical signals showed two states of electrical arc such as high voltage and low
voltage state. In the high voltage state, the voltage of the electric arc starts to build-up due
to the presence of outgassed chemical species. According to our understanding, the
outgassed chemical species move towards core of the electric arc causes a drop-down the
temperature; turn to reduction of the electrical conductivity and rise in arc voltage. This
analogy is characterized by current limited/controlled electric arc, less back-ignition and
spikes in voltage signals. In low voltage state, the outgassed chemical species don’t lead to
build-up the arc voltage and vice versa. For example, as the wt% of QDs is increased in the
PMMA matrix (shown in Paper V), electric arc undergoes a smooth transition from a high
voltage to a low voltage state.
In addition to that, the ablated mass values of PNCs vary in a non-monotonous way
and are correlated to the trend of UV absorption edges, thermal degradation properties.
Furthermore, the total dissipated energy i.e. ∫ 𝐈(𝐭)𝐕(𝐭)𝐝𝐭 values and ablation mass values
are coupled to see the trend of tests with PNCs frontside and backside. Electron microscope
images of the outgassed signature presented for all the PNCs, the variation in blister size
and erosion density observed w.r.to different wt% of NPs addition in the PMMA matrix.
Specific conclusions on different PNC compositions achieved in various articles are
summarized below:
48
Paper I was designed to understand basic polymer-electric arc interactions using two
polymers of PMMA and PA66, which demonstrates the effect on the electric arcs of current
1.6kA using electrical measurements such as the arc voltage and current. We have
understood the effect of chemical composition of the monomer on the electrical arcs.
Furthermore, surface morphology of the outgassed polymers is imaged and interpreted,
revealing differences of outgassing process.
Paper II demonstrated a successful route to the synthesis of CuO NPs and fabrication of
CuO-PMMA PNCs. The range of UV absorption of the PMMA has been broadened due to
the presence of CuO NPs. Paper III demonstrated electric arc interruption measurements,
and correlation of recorded electrical signals of arc with UV radiation absorption of PNCs.
Paper IV was designed for investigating the interactions of magnetic nanoparticle
inclusions with the arc. A highly thermally stable Fe3O4–PMMA PNCs were fabricated
and observed their impact on the electric arc. The result showed a significant influence of
ultra-low wt% of Fe3O4 NPs on the state of electric arc interruption process.
Paper V focused on strong UV absorbing PNCs fabricated by the inclusion of ZnO QDs in
PMMA matrix. The UV radiation absorption edge of PNCs showed red-shift by increasing
content of ZnO QDs. The experimental findings from the tests with both sides of PNCs
show two states of arc interruption process, which are reliable and reproducible.
Paper VI focused on enhanced radiation and thermal energy absorption by introducing a
metallic component in PMMA matrix. Therefore, Au-PMMA PNCs were fabricated to
enhance thermal energy transfer and visible radiation absorption of PNCs. We have
observed important experimental insights during the electric arc interruption process using
Au-PMMA PNCs. Furthermore, a critical comparison of PNCs containing ZnO QDs and
Au NPs revealed a superior electric arc interruption process with enhanced radiative
transfer in case of ZnO QDs-PMMA PNCs.
These experimental insights gained from our industrial collaborators suggested that
these PNCs can be used to replace the traditional polymers in the circuit breakers,
which will reduce the arc interruption time and improve the efficiency and safety of
the high power electrical switching applications.
These are the first experimental investigations with PNCs used for interrupting the
electrical arcs generated in breakers. The electric arc interruption process of the
nanocomposites is investigated using the specially designed test set-up with electrical
measurements. In particular, a clear influence of the nanoparticles loading on the state
of the electric arc interruption in breakers. In conclusion, the reported experimental
insights and results showed that the nanocomposites with tunable nanoparticle
loadings are useful for low power electrical switching applications.
49
7. FUTURE WORK
The purpose of this thesis work was to engineer present polymeric materials by using
developments in material science, in particular towards polymer based nano-composites
(PNCs). The developed PNCs should be able to replace the present insulating, ceramic
materials in power switching devices (such as circuit breakers, contactors, fuses) in order to
protect from the electrical arcs and improve the efficiency and reliability.
This work can be further developed in number ways and possible ideas are mentioned
below:
1. According to current advancements in computational power, developing a
theoretical model to broaden the knowledge on PNCs and electric arc interactions is
necessary to improve predictive power of arc interruption process
2. Theoretical and experimental studies on developing PNCs properties such as UV
radiative absorption and thermal properties
3. Engineering of insulating materials using newly developed core-shell NPs and other
morphology of NPs and their impact on the electric arc interruption process
4. Statistical analysis of electric arc interruption process using PNCs can be studied
5. Perform optical emission spectroscopy and high speed imaging studies to better
understand PNCs and electric arc interactions
6. Integrate mass spectroscopic studies to analyze and quantify outgassed chemical
species during electric arc and PNCs interaction
51
APPENDIX A:
A: DRAWING OF THE EXPERIMENTAL TEST SET-UP
Figure A.1. Drawing of an experimental test set-up connected to arc discharge circuit to generate the electrical arcs
between the two contacts.
53
APPENDIX B:
B: TGA-FTIR SPECTROSCOPY – PA66, PMMA
Figure B.1. TGA-FTIR spectrum of PA66 sample in 2D (a) and temperature resolved 3D (b).
(a)
(b)
54
9
Figure B.2. TGA-FTIR spectrum of PMMA sample in 2D (a) and temperature resolved 3D (b).
(a)
(b)
55
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