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A TERM PAPER ON HIGH VOLTAGE ENGINEERING II

SUBMITTED BY:

NDUBUISI CHINONSO DAVID

050403074

COURSE CODE: EEG508

COURSE LECTURER: T. O AKINBULIRE

CO-LECTURER: ENGR P.O OLUSEYI

DATE: 12 TH JULY, 2010

IN PARTIAL FULFILMENT OF THE REQUIREMENT FOR AN A IN

EEG508



CHAPTER 1

1.0 INTRODUCTION TO CONDUCTING MATERIALSIn some materials, electrons move easily from atom to atom. In others, the electrons move withdifficulty. And in some materials, it is almost impossible to get them to move. An electrical conductor isa substance in which the electrons are mobile. The best conductor at room temperature is pureelemental silver. Copper and aluminum are also excellent electrical conductors. Iron, steel, and variousother metals are fair to good conductors of electricity. In most electrical circuits and systems, copper oraluminum wire is used. Silver is impractical because of its high cost. Some liquids are good electricalconductors. Mercury is one example. Salt water is a fair conductor. Gases are, in general, poorconductors of electricity. This is because the atoms or molecules are usually too far apart to allow a free

exchange of electrons. But if a gas becomes ionized, it is a fair conductor of electricity. Electrons in aconductor do not move in a steady stream, like molecules of water through a garden hose. Instead, theyare passed from one atom to another right next to it. This happens to countless atoms all the time. As aresult, literally trillions of electrons pass a given point each second in a typical electrical circuit. Youmight imagine a long line of people, each one constantly passing a ball to the neighbor on the right. If there are plenty of balls all along the line, and if everyone keeps passing balls along as they come, theresult will be a steady stream of balls moving along the line. This represents a good conductor. If thepeople become tired or lazy, and do not feel much like passing the balls along, the rate of flow willdecrease. The conductor is no longer very good.

ELECTRICAL CONDUCTIVITY:Electrical conductivity is a measure of how well a material accommodatesthe movement of an electric charge. It is the ratio of the current density to the electric field strength. ItsSI derived unit is the Siemens per meter , but conductivity values are often reported as percent IACS.IACS is an acronym for International Annealed Copper Standard or the material that was used to maketraditional copper-wire. The conductivity of the annealed copper (5.8108 x 10 7S/m) is defined to be100% IACS at 20°C. All other conductivity values are related back to this conductivity of annealedcopper. Therefore, iron with a conductivity value of 1.044 x 10 7 S/m, has a conductivity of approximately18% of that of annealed copper and this is reported as 18% IACS. An interesting side note is thatcommercially pure copper products now often have IACS conductivity values greater than 100% becauseprocessing techniques have improved since the adoption of the standard in 1913 and more impuritiescan now be removed from the metal. Conductivity values in Siemens/meter can be converted to % IACS

by multiplying the conductivity value by 1.7241 x10-6

. When conductivity values are reported inmicroSiemens/centimeter, the conductivity value is multiplied by 172.41 to convert to the % IACS value

Electrical conductivity is a very useful property since values are affected by such things as a substanceschemical composition and the stress state of crystalline structures. Therefore, electrical conductivityinformation can be used for measuring the purity of water, sorting materials, checking for proper heattreatment of metals, and inspecting for heat damage in some materials.



1.1 INTRODUCTION TO INSULATING MATERIALSAn insulator , also called a dielectric , is a material that resists the flow of electric current. An insulating

material has atoms with tightly bonded valence electrons. These materials are used in parts of electrical

equipment, also called insulators or insulation , intended to support or separate

electrical conductors without passing current through themselves. The term is also used more

specifically to refer to insulating supports that attach electric power transmission wires to utility

poles or pylons. Some materials such as glass, paper or Teflon are very good electrical insulators. A much

larger class of materials, for example rubber-like polymers and most plastics are still good enough to

insulate electrical wiring and cables even though they may have lower bulk resistivity. These materials

can serve as practical and safe insulators for low to moderate voltages (hundreds, or even thousands,

of volts). Most (though not all see Mott insulator) insulators are characterized by having a large band

gap. This occurs because the valence band containing the highest energy electrons is full, and a large

energy gap separates this band from the next band above it. There is always some voltage (called

the breakdown voltage) that will give the electrons enough energy to be excited into this band. Once

this voltage is exceeded, the material ceases being an insulator, and charge will begin to pass through it.

However, it is usually accompanied by physical or chemical changes that permanently degrade the

material s insulating properties. Materials that lack electron conduction are insulators if they lack other

mobile charges as well. For example, if a liquid or gas contains ions, then the ions can be made to flow

as an electric current, and the material is a conductor. Electrolytes and plasmas contain ions and will act

as conductors whether or not electron flow is involved.

BREAKDOWN: Insulators suffer from the phenomenon of electrical breakdown. When the electric field

applied across an insulating substance exceeds in any location the threshold breakdown field for that

substance, which is proportional to the band gap energy, the insulator suddenly turns into a resistor,

sometimes with catastrophic results. During electrical breakdown, any free charge carrier being

accelerated by the strong e-field will have enough velocity to knock electrons from (ionize) any atom it

strikes. These freed electrons and ions are in turn accelerated and strike other atoms, creating more

charge carriers, in a chain reaction. Rapidly the insulator becomes filled with mobile carriers, and its

resistance drops to a low level. In air, corona discharge is normal current near a high-voltage

conductor; an arc is an unusual and undesired flow of current. Similar breakdown can occur within any

insulator, even within the bulk solid of a material. Even a vacuum can suffer a sort of break down, but in



this case the breakdown or vacuum arc involves charges ejected from the surface of metal electrodes

rather than produced by the vacuum itself.

MATERIAL: Insulators used for high-voltage power transmission are made from glass, porcelain,

or composite polymer materials. Porcelain insulators are made

from clay, quartz or alumina and feldspar, and are covered with a smooth glaze to shed water. Insulators

made from porcelain rich in alumina are used where high mechanical strength is a criterion. Porcelain

has a dielectric strength of about 4 10 kV/mm. [1] Glass has a higher dielectric strength, but it attracts

condensation and the thick irregular shapes needed for insulators are difficult to cast without internal

strains. [2]Some insulator manufacturers stopped making glass insulators in the late 1960s, switching to

ceramic materials.

Recently, some electric utilities have begun converting to polymer composite materials for some types

of insulators. These are typically composed of a central rod made of fiber reinforced plastic and an outer

weather shed made of silicone rubber or EPDM. Composite insulators are less costly, lighter in weight,

and have excellent hydrophobic capability. This combination makes them ideal for service in polluted

areas. However, these materials do not yet have the long-term proven service life of glass and porcelain.

The electrical breakdown of an insulator due to excessive voltage can occur in one of two ways:

Puncture voltage is the voltage across the insulator (when installed in its normal manner) which

causes a breakdown and conduction through the interior of the insulator. The heat resulting from

the puncture arc usually damages the insulator irreparably.

F lashover voltage is the voltage which causes the air around or along the surface of the insulator to

break down and conduct, causing a flashover arc along the outside of the insulator. They are

usually designed to withstand this without damage.

Most high voltage insulators are designed with a lower flashover voltage than puncture voltage, so they

will flashover before they puncture, to avoid damage

USES: Insulators are commonly used as a flexible coating on electric wire and cable. Since air is an

insulator, no other substance is needed to keep power where it should be. High-voltage power lines

commonly use just air, since a solid (e.g., plastic) coating would be impractical. However, wires which

touch each other will produce cross connections, short circuits, and fire hazards. In coaxial cable thecenter conductor must be supported exactly in the middle of the hollow shield in order to prevent EM

wave reflections. And any wires which present voltages higher than 60V can cause human shock

and electrocution hazards. Insulating coatings prevent all of these problems.

In electronic systems, printed circuit boards are made from epoxy plastic and fiberglass. The

nonconductive boards support layers of copper foil conductors. In electronic devices, the tiny and



delicate active components are embedded within nonconductive epoxy or phenolic plastics, or within

baked glass or ceramic coatings.

In microelectronic components such as transistors and ICs, the silicon material is normally a conductor

because of doping, but it can easily be selectively transformed into a good insulator by the application of

heat and oxygen. Oxidized silicon is quartz, i.e. silicon dioxide.

In high voltage systems containing transformers and capacitors, liquid insulator oil is the typical method

used for preventing arcs. The oil replaces the air in any spaces which must support significant voltage

without electrical breakdown. Other methods of insulating high voltage systems are ceramic or glass

wire holders and simply placing the wires with a large separation, using the air as insulation

1.2 INTRODUCTION TO MAGNETIC MATERIALSNever before has our daily life and environment been so significantly dependent on materials withoutstanding magnetic properties. Modern life is today in many aspects an automated world which usesferro- and ferromagnetic materials in nearly all important technical fields as, e.g., electrical power,mechanical power, high-power electromotors, miniature motors, computer technique, magnetic high-density recording, telecommunication, navigation, aviation and space operations, automationmicromechanics, medicine, sensor techniques, magnetocaloric refrigeration, materials testing andhousehold applications

The term magnet is typically reserved for objects that produce their own persistent magnetic field evenin the absence of an applied magnetic field. Only certain classes of materials can do this. Most materials,however, produce a magnetic field in response to an applied magnetic field; a phenomenon known as

magnetism. There are several types of magnetism, and all materials exhibit at least one of them. Theoverall magnetic behavior of a material can vary widely, depending on the structure of the material, andparticularly on its electron configuration. Several forms of magnetic behavior have been observed indifferent materials, including:

Ferromagnetic and ferromagnetic: materials are the ones normally thought of as agnetic ; they are

attracted to a magnet strongly enough that the attraction can be felt. These materials are the only ones

that can retain magnetization and become magnets; a common example is a traditional refrigerator

magnet. Ferrimagnetic materials, which include ferrites and the oldest magnetic

materials magnetite and lodestone, are similar to but weaker than ferromagnetics. The differencebetween ferro- and ferrimagnetic materials is related to their microscopic structure, as explained below.

Paramagnetic substances such as platinum, aluminum, and oxygen are weakly attracted to a magnet.

This effect is hundreds of thousands of times weaker than ferromagnetic materials attraction, so it can

only be detected by using sensitive instruments, or using extremely strong magnets.



Magnetic ferrofluids, although they are made of tiny ferromagnetic particles suspended in liquid, are

sometimes considered paramagnetic since they cannot be magnetized.

Diamagnetic : means repelled by both poles. Compared to paramagnetic and ferromagnetic

substances, diamagnetic substances such as carbon , copper , water , and plastic are even more weakly

repelled by a magnet. The permeability of diamagnetic materials is less than the permeability of a

vacuum . All substances not possessing one of the other types of magnetism are diamagnetic; this

includes most substances. Although force on a diamagnetic object from an ordinary magnet is far too

weak to be felt, using extremely strong superconducting magnets diamagnetic objects such as pieces

of lead and even mice can be levitated so they float in mid-air. Superconductors repel magnetic fields

from their interior and are strongly diamagnetic.

CHAPTER 2

1 THIN FILMSIn recent years, thin film science has grown world-wide into a major research area. The importance of coatings and the synthesis of new materials for industry have resulted in a tremendous increase of innovative thin film processing technologies. Currently, this development goes hand-in-hand with theexplosion of scientific and technological breakthroughs in microelectronics, optics and nanotechnology

[1]. A second major field comprises process technologies for films with thicknesses ranging from one toseveral microns. These films are essential for a multitude of production areas, such as thermal barriercoatings and wear protections, enhancing service life of tools and to protect materials against thermaland atmospheric influences [2, 3]. Presently, rapidly changing needs for thin film materials and devicesare creating new opportunities for the development of new processes, materials and technologies



Therefore, basic research activities will be necessary in the future, to increase knowledge,understanding, and to develop predictive capabilities for relating fundamental physical and chemicalproperties to the microstructure and performance of thin films in various applications. In basic research,special model systems are needed for quantitative investigations of the relevant and fundamentalprocesses in thin film materials science. In particular, these model systems enable the investigation of i.e. nucleation and growth processes, solid state reactions, the thermal and mechanical stability of thinfilm systems and phase boundaries. Results of combined experimental and theoretical investigations are

a prerequisite for the development of new thin film systems and the tailoring of their microstructureand performance.

The major exploitation of thin film science is still in the field of microelectronics. However, there aregrowing applications in other areas like thin films for optical and magnetic devices, electrochemistry,protective and decorative coatings and catalysis. Most features of these thin film activities arerepresented by a relatively new research area, called surface engineering [2]. Surface engineering hasbeen one of the most expanding scientific areas in the last 10 years and includes the design and



processing of surface layers and coatings, internal interfaces and their characterization. Surfaceengineering is directed by the demands of thin film and surface characteristics of materials.

2.1 CHARACTERISTICS OF MATERIALS IN THEIR THIN FILM STATE

Physical and Chemical Durability

Among factors that limit the mechanical and chemical durability of thin film materials are the layerpacking density and morphology. These structural factors are influenced by the deposition technique.Layer composition is, of course, of high importance if the desired index and transparency are to beobtained. Oxide-compound deposition requires the presence of excess oxygen, often in activatedspecies, to compensate for oxygen loss or to reactively compose the final desired oxidation state.Fluoride compounds require high vacuum to achieve high purity and low absorption. Thus the production of a coating design that includes oxide and non-oxide compounds without creating crosscontamination requires a more complicated coating procedure. It should be apparent even from this

limited discussion that all the parameters involved in growing thin film layers are interrelated, and thatthe development of a coating for a particular application is a complex multi-step process.

The mechanical properties of a coating, namely adhesion, hardness, and abrasive wear resistance, aredetermined by the microstructure morphology and the nature of chemical bonds. Films with lowpacking density are structurally weak; exhibit poor wear resistance, and absorb and desorb water andother gaseous materials. Packing density is determined by the energetics of the deposition process andto some degree the nature of the chemical bonding that is established within the layer. Sputtering, ionplating, and the inclusion of a supplemental ion source are high-energy deposition techniques thatproduce compact layers

Mechanical Stress Issues

Mechanical stress effects can assume a variety of forms, including increased optical scatter frommicroscopic crazing, macroscopic cracking, and partial or complete adhesion failure between layers or atthe substrate. Stress associated with a thin film layer can originate within the layer or at the layerinterface with another material. In the later case, differences in thermal expansion coefficient need tobe considered; similarly, differences in mutual chemical bonding can impose a structural mismatch.Therefore, it is important to select materials with compatible thermal expansion coefficients whenpossible, especially when the finished product is to be exposed to large or rapid temperature excursions.Because of this requirement, the available selection of stable glasses provides more suitable substrate

choices than fused silica, whose TCE is the smallest existing. Deposition temperature is a parameter thatinfluences the film morphology, particularly grain size. Temperature also affects the relative stressbetween coating and substrate, as noted above. Many oxide and fluoride coating materials require asubstrate temperature near or greater than 200° C to achieve the desirable mechanical and opticalproperties we have been discussing. Their intrinsic layer stresses can be either compressive or tensile,depending on material and microstructure and substrate temperature. Metals generally are undertensile stress, while compounds can be of either nature. For example, titania, alumina, and fluoride-



compound films are generally in tension, while silica films are in compression. Thus the pair TiO2 / SiO2can produce a low stress multi-layer within a certain substrate temperature range, generally lower thanthat required for single layers. Control of the stress nature can be exerted by the deposition process tosome degree, with sputtering having the most versatility in controlling the sign of the stress throughvariation of pressure and power and at lower substrate temperature than E-beam or thermal

evaporation

2.3 THIN FILM PROCESSING TECHNIQUESThere exists a huge variety of thin film deposition processes and technologies which originate frompurely physical or purely chemical processes. The more important thin film processes are based onliquid phase chemical techniques, gas phase chemical processes, glow discharge processes andevaporation methods [4]. Recently, a considerable number of novel processes that utilize a combinationof different processes have been developed. This combination allows a more defined control andtailoring of the microstructure and properties of thin fi lms. Typical processes are e.g. ion beam assisted

deposition (IBAD) and plasma enhanced CVD (PECVD). Examples for novel thin film processingtechniques, which are still under development, are pulsed laser ablation (PLD) and chemical solutiondeposition (CSD). Both techniques enable the synthesis of complex thin film materials (complex oxides,carbides, and nitrides). Presently, experimental efforts are increasingly supported by computationalapproaches that address complex growth processes, saving time and money. These approaches enablee.g. the description of the evolution of thin film microstructures as a function of processing parameters

The thin film process equipment can be categorized into production equipment for devicemanufacturing, equipment for research and development, and prototype apparatus for fundamentalinvestigations of new or established deposition processes. One reason for the world-wide rapid growth

of deposition technology is that equipment manufacturers have successfully met the demands for moresophisticated deposition systems including in situ characterization (e.g. reflection high-energy electrondiffraction (RHEED), scanning probe microscopy (SPM)) and process monitoring techniques formeasuring process parameters and film properties (e. g. ellipsometry , plasma analysis techniques).Novel experimental tools have enabled discoveries of a variety of new phenomena at the nanoscalewhich have in turn opened unexpected opportunities for the development of thin film systems, andtremendous progress regarding a fundamental understanding of the respective technological processeshas been made.

2.4 ELECTRONIC TRANSPORTElectrons in the conduction band and holes in the valence band are able to move upon thermalactivation, a gradient or an applied electric field. In the following the concepts of electronic transport incrystalline materials will be described.

2.4.1 Thermal movement of carriers



Electrons in the conduction or holes in the valence band can essentially be treated as free carriers orfree particles. Even in the absence of an electric field the carriers follow a thermally activated randommotion. In thermal equilibrium the average thermal energy of a particle (electron or hole) can be

obtained from the theorem for equipartition of Ethermal = (Average thermal energy of an electron /

hole). The thermal energy of the particle is equal to the kinetic energy of the electron, so that thevelocity of the particle can be calculated. The mass of the electron is equal to the effective mass of theelectron.

Furthermore, the velocity of the electron corresponds to the thermal velocity of the electron, so that the

thermal velocity can be determined by: Ekin = eff vth2 (Kinetic energy of an electron / hole). At room

temperature the average thermal velocity of an electron is about 10 5m/s in silicon and GaAs.

Vth = (Thermal velocity of an electron). Thermal motion of free carriers can be seen as random

collision (scattering) of the free carriers with the crystal lattice. A random motion of an electron or hole

leads to zero net displacement of the free carrier over a sufficient long distance / period of time. Theaverage distance between two collisions within the crystal lattice is called mean free path. Associated tothe mean free path we can introduce a mean free time . A typical mean free path is in the range of 100nm and the mean free time is in the range of 1ps.

BAND-LIKE TRANSPORT

When a small electric field is applied to the semiconductor material each free carrier will experience anelectro static force (force= -qF) . So that the carrier is accelerated along the field (in opposite direction of the field). An additional velocity component will be superimposed upon the thermal motion of theelectron. The additional velocity is caused by an applied electric field F. The additional component iscalled drift velocity. The drift of the electrons can be described by a steady state motion since the gainedmomentum is lost due to collisions of the electrons and the lattice.

P = m nvn = -q.F. c .Based on momentum conservation the drift velocity can be calculated. The driftvelocity is proportional to the applied electric field F ( vn = F electron drift velocity).

(Electron and hole mobility)The mobility is an important electronic transport parameter. The mobility directly related to thematerial properties. Rewriting of the expression for the drift velocity leads to



( Electron and hole mobility). The mobility

is directly related to the mean free time between two collisions, which is determined by variousscattering mechanisms. The most important scattering mechanisms are lattice scattering and impurityscattering. Lattice scattering is caused by thermal vibrations of the lattice atoms at any temperatureabove 0K. Due to the vibrations energy can be transferred from the carriers and the lattice

CHAPTER 3

3.0 SOLID DIELECTRIC MATERIALSSolid dielectric materials are used in all kinds of electrical circuits and devices to insulate one currentcarrying part from another when they operate at different voltages. A good dielectric should have low

dielectric loss, high mechanical strength, should be free from gaseous inclusions, and moisture, and beresistant to thermal and chemical deterioration. Solid dielectrics have higher breakdown strengthcompared to liquids and gases. Studies of the breakdown of solid dielectrics are of extreme importancein insulation studies. When breakdown occurs, solids get permanently damaged while gases fully andliquids partly recover their dielectric strength after the applied electric field is removed.

The mechanism of breakdown is a complex phenomena in the case of solids, and varies depending onthe time of application of voltage as shown in Fig. 4.1. The various breakdown mechanisms can beclassified as follows:

intrinsic or ionic breakdown, electromechanical breakdown, failure due to treeing and tracking, thermal breakdown, electrochemical breakdown, and Breakdown due to internal discharges.



Fig. 3.1 Variation of breakdown strength with time after application of voltage

3.1 BREAKDOWN MECHANISMSINTRINSIC BREAKDOWN: When voltages are applied only for short durations of the order of 10 8S thedielectric strength of a solid dielectric increases very rapidly to an upper limit called the intrinsic electricstrength. Experimentally, this highest dielectric strength can be obtained only under the bestexperimental conditions when all extraneous influences have been isolated and the value depends onlyon the structure of the material and the temperature. The maximum electrical strength recorded is 15MV/cm for poly vinyl-alcohol at - 1960C. The maximum strength usually obtainable ranges from 5MV/cm to 10MV/cm. Intrinsic breakdown depends upon the presence of free electrons which arecapable of migration through the lattice of the dielectric. Usually, a small number of conductionelections are present in solid dielectrics, along with some structural imperfections and small amounts of impurities. The impurity atoms, or molecules or both act as traps for the conduction electrons up tocertain ranges of electric fields and temperatures. When these ranges are exceeded, additionalelectrons in addition to trapped electrons are released, and these electrons participate in theconduction process. Based on this principle, two types of intrinsic breakdown mechanisms have beenproposed.

ELECTRONIC BREAKDOWN: As mentioned earlier, intrinsic breakdown occurs in time of the order of 108s and therefore is assumed to be electronic in nature. The initial density of conduction (free)electrons is also assumed to be large, and electron-electron collisions occur. When an electric field isapplied, electrons gain energy from the electric field and cross the forbidden energy gap from thevalency to the conduction band. When this process is repeated, more and more electrons becomeavailable in the conduction band, eventually leading to breakdown.

AVALANCHE OR STREAMER BREAKDOWN: This is similar to breakdown in gases due to cumulativeionization. Conduction electrons gain sufficient energy above a certain critical electric field and causeliberation of electrons from the lattice atoms by collisions. Under uniform field conditions, if theelectrodes are embedded in the specimen, breakdown will occur when an electron avalanche bridgesthe electrode gap. An electron within the dielectric, starting from the cathode will drift towards theanode and during this motion gains energy from the field and loses it during collisions. When the energy



gained by an electron exceeds the lattice ionization potential, an additional electron will be liberateddue to collision of the first electron. This process repeats itself resulting in the formation of an electronavalanche. Breakdown will occur, when the avalanche exceeds a certain critical size.

In practice, breakdown does not occur by the formation of a single avalanche itself, but occurs as a

result of many avalanches formed within the dielectric and extending step by step through the entirethickness of the material as shown in Fig. 4.2. This can be readily demonstrated in a laboratory byapplying an impulse voltage between point-plane electrodes with point embedded in a transparent soliddielectric such as perspex.

FIG 3.2 Breakdown channels in perspex between point-plane electrodes

ELECTROMECHANICAL BREAKDOWN:

When solid dielectrics are subjected to high electric fields, failure occurs due to electrostaticcompressive forces which can exceed the mechanical compressive strength. If the thickness of thespecimen is 4$ and is compressed to a thickness d under an applied voltageV, then the electrically

developed compresive stress is in equilibrium, if

THERMAL BREAKDOWN

In general, the breakdown voltage of a solid dielectric should increase with its thickness. But this is trueonly up to a certain thickness above which the heat generated in the dielectric due to the flow of currentdetermines the conduction. When an electric field is applied to a dielectric, conduction current, howeversmall it may be, flows through the material. The current heats up the specimen and the temperature



rises. The heat generated is transferred to the surrounding medium by conduction through the soliddielectric and by radiation from its outer surfaces. Equilibrium is reached when the heat used to raisethe temperature of the dielectric, plus the heat radiated out, equals the heat generated. The heatgenerated under d.c. stress E is given as

W dc = E2

W/cm3

This is of great importance to practising engineers, as most of the insulation failures in high voltagepower apparatus occur due to thermal breakdown. Thermal breakdown sets up an upper limit forincreasing the breakdown voltage when the thickness of the insulation is increased. For a given lossangle and applied stress, the heat generated is proportional to the frequency and hence thermalbreakdown is more serious at high frequencies. Table 4.1 gives the thermal breakdown voltages of various materials under d.c. and a.c. fields. It can be seen from this table that since the power loss undera.c. fields is higher, the heat generation is also high, and hence the thermal breakdown stresses arelower under a.c. conditions than under d.c. conditions.

BREAKDOWN OF SOLID DIELECTRICS IN PRACTICE

There are certain types of breakdown which do not come under either intrinsic breakdown or thermalbreakdown, but actually occur after prolonged operation. These are, for example, breakdown due totracking in which dry conducting tracks are formed on the surface of the insulation. These tracks act asconducting paths on the insulator surfaces leading to gradual breakdown along the surface of theinsulator. Another type of breakdown in this category is the electrochemical breakdown causedbychemical transformations such as electrolysis, formation of ozone, etc. In addition, failure also occursdue to partial discharges which are brought about in the air pockets inside the insulation. This type of breakdown is very important in the impregnated paper insulation used in high voltage cables and

capacitors.

CHEMICAL AND ELECTROCHEMICAL DETERIORATION AND BREAKDOWN

In the presence of air and other gases some dielectric materials undergo chemical changes whensubjected to continuous electrical stresses. Some of the important chemical reactions that occur are:

Oxidation : In the presence of air or oxygen, materials such as rubber and polyethylene undergooxidation giving rise to surface cracks.

Hydrolysis : When moisture or water vapour is present on the surface of a solid dielectric, hydrolysis

occurs and the materials lose their electrical and mechanical properties. Electrical properties of materials such as paper, cotton tape, and other cellulose materials deteriorate very rapidly due tohydrolysis. Plastics like polyethylene undergo changes, and their service life considerably reduces.

Chemical Action : Even in the absence of electric fields, progressive chemical degradation of insulatingmaterials can occur due to a variety of processes such as chemical instability at high temperatures,oxidation and cracking in the presence of air and ozone, and hydrolysis due to moisture and heat. Sincedifferent insulating materials come into contact with each other in any practical apparatus, chemical



reactions occur between these various materials leading to reduction in electrical and mechanicalstrengths resulting in failure. The effects of electrochemical and chemical deterioration could beminimized by carefully studying and examining the materials. High soda content glass insulation shouldbe avoided in moist and damp conditions, because sodium, being very mobile, leaches to the surfacegiving rise to the formation of a strong alkali which will cause deterioration. It was observed that this

type of material will lose its mechanical strength within 24 hrs, when it is explosed to atmosphereshaving 100% relative humidity at 7O0C. In paper insulation, even if partial discharges are preventedcompletely, breakdown can occur due to chemical degradation. The chemical and electrochemicaldeterioration increases very rapidly with temperature, and hence high temperatures should be avoided.

SOLID DIELECTRICS USED IN PRACTICE

The majority of the insulating systems used in practice are solids. They can be broadly classified intothree groups: organic materials, inorganic materials and synthetic polymers. Some of these materials arelisted in Table 3.2 below.

Table 3.2 Classification of Solid Insulating Materials

Organic materials are those which are produced from vegetable or animal matter and all of them havesimilar characteristics. They are good insulators and can be easily adopted for practical applications.However, their mechanical and electrical properties always deteriorate rapidly when the temperatureexceeds 100C. Therefore, they are generally used after treating with a varnish or impregnation with an

oil. Examples are paper and press board used in cables, capacitors and transformers.

Inorganic materials, unlike the organic materials, do not show any appreciable reduction (< 10%) in theirelectrical and mechanical properties almost up to 250C. Important inorganic materials used for electricapplications are glasses and ceramics. They are widely used for the manufacture of insulators, bushingsetc., because of their resistance to atmospheric pollutants and their excellent performance undervarying conditions of temperature and pressure.



Synthetic polymers are the polymeric materials which possess excellent insulating properties and can beeasily fabricated and applied to the apparatus. These are generally divided into two groups, thethermoplastic and the thermosetting plastic types. Although they have low melting temperatures in therange 100-120C, they are very flexible and can be moulded and extruded at temperatures below theirmelting points. They are widely used in bushings, insulators etc. Their electrical use depends on their

ability to prevent the absorption of moisture.

CHAPTER 4

4.0 INTRODUCTION TO SUPERCONDUCTIVITY

Superconductivity is an electrical resistance of exactly zero which occurs in certain materials below a

characteristic temperature. It was discovered by Heike Kamerlingh Onnes in 1911.

Like ferromagnetism and atomic spectral lines, superconductivity is a quantum mechanical

phenomenon. It is also characterized by a phenomenon called the Meissner effect, the ejection of any

sufficiently weak magnetic field from the interior of the superconductor as it transitions into the

superconducting state. The occurrence of the Meissner effect indicates that superconductivity cannot

be understood simply as the idealization of "perfect conductivity" in classical physics.

The electrical resistivity of a metallic conductor decreases gradually as the temperature is lowered.

However, in ordinary conductors such as copper and silver, this decrease is limited by impurities and

other defects. Even near absolute zero, a real sample of copper shows some resistance. In a

superconductor however, despite these imperfections, the resistance drops abruptly to zero when the

material is cooled below its critical temperature. An electric current flowing in a loop of superconducting

wire can persist indefinitely with no power source [1]. Superconductivity occurs in many materials: simple

elements like tin and aluminum, various metallic alloys and some heavily-doped semiconductors.

Superconductivity does not occur in noble metals like gold and silver, nor in pure samples

of ferromagnetic metals. In 1986, it was discovered that some cuprate-perovskite ceramic materials

have critical temperatures above 90 Kelvins (-183.15 degrees Celsius). These high-temperature

superconductors renewed interest in the topic because of the prospects for improvement and potential

room-temperature superconductivity. From a practical perspective, even 90 kelvins is relatively easy to

reach with the readily available liquid nitrogen (boiling point 77 kelvins), resulting in more experiments

and applications.



4.1 HISTORIC BACKGROUND

Superconductivity was first discovered in 1911 by the Dutch physicist, Heike Kammerlingh Onnes.Onnes dedicated his scientific career to exploring extremely cold refrigeration. On July 10, 1908, hesuccessfully liquefied helium by cooling it to 452 degrees below zero Fahrenheit (4 Kelvin or 4 K). Onnes

produced only a few milliliters of liquid helium that day, but this was to be the new beginnings of hisexplorations in temperature regions previously unreachable. Liquid helium enabled him to cool othermaterials closer to absolute zero (0 Kelvin), the coldest temperature imaginable. Absolute zero is thetemperature at which the energy of material becomes as small as possible.

In 1911, Onnes began to investigate the electrical properties of metals in extremely cold temperatures.It had been known for many years that the resistance of metals fell when cooled below roomtemperature, but it was not known what limiting value the resistance would approach, if thetemperature were reduced to very close to 0 K. Some scientists, such as William Kelvin, believed thatelectrons flowing through a conductor would come to a complete halt as the temperature approached

absolute zero. Other scientists, including Onnes, felt that a cold wire's resistance would dissipate. Thissuggested that there would be a steady decrease in electrical resistance, allowing for better conductionof electricity. At some very low temperature point, scientists felt that there would be a leveling off asthe resistance reached some ill-defined minimum value allowing the current to flow with little or noresistance. Onnes passed a current through a very pure mercury wire and measured its resistance as hesteadily lowered the temperature. Much to his surprise there was no leveling off of resistance, let alonethe stopping of electrons as suggested by Kelvin. At 4.2 K the resistance suddenly vanished. Current wasflowing through the mercury wire and nothing was stopping it, the resistance was zero. (See Figure1)According to Onnes, "Mercury has passed into a new state, which on account of its extraordinaryelectrical properties may be called the superconductive state". The experiment left no doubt about the

disappearance of the resistance of a mercury wire. Kamerlingh Onnes called this newly discoveredstate, Superconductivity.



FIGURE 1

Onnes recognized the importance of his discovery to the scientific community aswell as its commercialpotential. An electrical conductor with no resistance could carry current any distance with no losses. Inone of Onnes experiments he started a current flowing through a loop of lead wire cooled to 4 K. A yearlater the current was still flowing without significant current loss. Onnes found that the superconductorexhibited what he called persistent currents, electric currents that continued to flow without an electricpotential driving them. Onnes had discovered superconductivity, and was awarded theNobel Prize in1913.

4.2 ELEMENTARY PROPERTIES OF SUPERCONDUCTIVE MATERIALS

Most of the physical properties of superconductors vary from material to material, such as the heat

capacity and the critical temperature, critical field, and critical current density at which

superconductivity is destroyed. On the other hand, there is a class of properties that are independent of

the underlying material. For instance, all superconductors have exactly zero resistivity to low applied

currents when there is no magnetic field present or if the applied field does not exceed a critical value.The existence of these "universal" properties implies that superconductivity is a thermodynamic phase,

and thus possesses certain distinguishing properties which are largely independent of microscopic

details.

Z ero electrical DC resistance



The simplest method to measure the electrical resistance of a sample of some material is to place it in

an electrical circuit in series with a current source I and measure the resulting voltage V across the

sample. The resistance of the sample is given by Ohm's la as R = V/I . If the voltage is zero, this means

that the resistance is zero and that the sample is in the superconducting state.Superconductors are also

able to maintain a current with no applied voltage whatsoever, a property exploited in superconducting

electromagnets such as those found in MRI machines. Experiments have demonstrated that currents in

superconducting coils can persist for years without any measurable degradation. Experimental evidence

points to a current lifetime of at least 100,000 years. Theoretical estimates for the lifetime of a persistent

current can exceed the estimated lifetime of the universe, depending on the wire geometry and the

temperature. [1]

In a normal conductor, an electrical current may be visualized as a fluid of electrons moving across a

heavy ionic lattice. The electrons are constantly colliding with the ions in the lattice, and during each

collision some of the energy carried by the current is absorbed by the lattice and converted into heat,which is essentially the vibrational kinetic energy of the lattice ions. As a result, the energy carried by the

current is constantly being dissipated. This is the phenomenon of electrical resistance.The situation is

different in a superconductor. In a conventional superconductor, the electronic fluid cannot be resolved

into individual electrons. Instead, it consists of bound pairs of electrons known as Cooper pairs. This

pairing is caused by an attractive force between electrons from the exchange of phonons. Due

to quantum mechanics, the energy spectrum of this Cooper pair fluid possesses an energy gap , meaning

there is a minimum amount of energy E that must be supplied in order to excite the fluid. Therefore, if

E is larger than the thermal energy of the lattice, given by kT , where k is Boltzmann's constant and T is

the temperature, the fluid will not be scattered by the lattice. The Cooper pair fluid is thus a superfluid,

meaning it can flow without energy dissipation. In a class of superconductors known as type II

superconductors, including all known high-temperature superconductors, an extremely small amount of

resistivity appears at temperatures not too far below the nominal superconducting transition when an

electrical current is applied in conjunction with a strong magnetic field, which may be caused by the

electrical current. This is due to the motion of vortices in the electronic superfluid, which dissipates some

of the energy carried by the current. If the current is sufficiently small, the vortices are stationary, and the

resistivity vanishes. The resistance due to this effect is tiny compared with that of non-superconducting

materials, but must be taken into account in sensitive experiments. However, as the temperaturedecreases far enough below the nominal superconducting transition, these vortices can become frozen

into a disordered but stationary phase known as a "vortex glass". Below this vortex glass transition

temperature, the resistance of the material becomes truly zero.

Superconducting phase transition



FIGURE 2: Behavior of heat capacity (c v, blue) and resistivity ( , green) at the superconducting phase transition

In superconducting materials, the characteristics of superconductivity appear when the temperature T islowered below a critical temperature T c. The value of this critical temperature varies from material tomaterial. Conventional superconductors usually have critical temperatures ranging from around 20 K toless than 1 K. Solid mercury, for example, has a critical temperature of 4.2 K. As of 2009, the highestcritical temperature found for a conventional superconductor is 39 K for magnesiumdiboride (MgB 2),[2][3] although this material displays enough exotic properties that there is some doubtabout classifying it as a "conventional" superconductor. [4] Cuprate superconductors can have muchhigher critical temperatures: YBa2Cu3O7, one of the first cuprate superconductors to be discovered, has a

critical temperature of 92 K, and mercury-based cuprates have been found with critical temperatures inexcess of 130 K. The explanation for these high critical temperatures remains unknown. Electron pairingdue to phonon exchanges explains superconductivity in conventional superconductors, but it does notexplain superconductivity in the newer superconductors that have a very high critical temperature.Similarly, at a fixed temperature below the critical temperature, superconducting materials cease tosuperconduct when an external magnetic field is applied which is greater than the critical magnetic field .This is because the Gibbs free energy of the superconducting phase increases quadratically with themagnetic field while the free energy of the normal phase is roughly independent of the magnetic field. If the material superconducts in the absence of a field, then the superconducting phase free energy islower than that of the normal phase and so for some finite value of the magnetic field (proportional to

the square root of the difference of the free energies at zero magnetic field) the two free energies willbe equal and a phase transition to the normal phase will occur. More generally, a higher temperatureand a stronger magnetic field lead to a smaller fraction of the electrons in the superconducting band andconsequently a longer London penetration depth of external magnetic fields and currents. Thepenetration depth becomes infinite at the phase transition. The onset of superconductivity isaccompanied by abrupt changes in various physical properties, which is the hallmark of a phasetransition. For example, the electronic heat capacity is proportional to the temperature in the normal



(non-superconducting) regime. At the superconducting transition, it suffers a discontinuous jump andthereafter ceases to be linear. At low temperatures, it varies instead as e /T for some constant . Thisexponential behavior is one of the pieces of evidence for the existence of the energy gap.

The order of the superconducting phase transition was long a matter of debate. Experiments indicate

that the transition is second-order, meaning there is no latent heat. However in the presence of anexternal magnetic field there is latent heat, as a result of the fact that the superconducting phase haslower entropy below the critical temperature than the normal phase. It has been experimentallydemonstrated [5] that, as a consequence, when the magnetic field is increased beyond the critical field,the resulting phase transition leads to a decrease in the temperature of the superconducting material.Calculations in the 1970s suggested that it may actually be weakly first-order due to the effect of long-range fluctuations in the electromagnetic field. In the 1980s it was shown theoretically with the help of a disorder field theory, in which the vortex lines of the superconductor play a major role, that thetransition is of second order within the type II regime and of first order (i.e., latent heat) within the typeI regime, and that the two regions are separated by a tricritical point.[6] The results were confirmed by

Monte Carlo computer simulations.[7]

Meissner effect

When a superconductor is placed in a weak external magnetic field H, and cooled below its transitiontemperature, the magnetic field is ejected. The Meissner effect does not cause the field to becompletely ejected but instead the field penetrates the superconductor but only to a very smalldistance, characterized by a parameter , called the London penetration depth, decaying exponentiallyto zero within the bulk of the material. The Meissner effect, is a defining characteristic of superconductivity. For most superconductors, the London penetration depth is on the order of 100 nm.

The Meissner effect is sometimes confused with the kind of diamagnetism one would expect in a perfectelectrical conductor: according to Lenz's law, when a changing magnetic field is applied to a conductor,it will induce an electrical current in the conductor that creates an opposing magnetic field. In a perfectconductor, an arbitrarily large current can be induced, and the resulting magnetic field exactly cancelsthe applied field. The Meissner effect is distinct from this it is the spontaneous expulsion which occursduring transition to superconductivity. Suppose we have a material in its normal state, containing aconstant internal magnetic field. When the material is cooled below the critical temperature, we wouldobserve the abrupt expulsion of the internal magnetic field, which we would not expect based on Lenz'slaw.The Meissner effect was given a phenomenological explanation by the brothers Fritz and HeinzLondon, who showed that the electromagnetic free energy in a superconductor is minimized provided

Where H is the magnetic field and is the London penetration depth.

This equation, which is known as the London equation, predicts that the magnetic field in asuperconductor decays exponentially from whatever value it possesses at the surface.



A superconductor with little or no magnetic field within it is said to be in the Meissner state. TheMeissner state breaks down when the applied magnetic field is too large. Superconductors can bedivided into two classes according to how this breakdown occurs. In Type I superconductors,superconductivity is abruptly destroyed when the strength of the applied field rises above a criticalvalue H c. Depending on the geometry of the sample, one may obtain an intermediate state[8] consisting

of a baroque pattern [9] of regions of normal material carrying a magnetic field mixed with regions of superconducting material containing no field. In Type II superconductors, raising the applied field past acritical value H c1 leads to a mixed state (also known as the vortex state) in which an increasing amountof magnetic flux penetrates the material, but there remains no resistance to the flow of electricalcurrent as long as the current is not too large. At a second critical field strength Hc2, superconductivity isdestroyed. The mixed state is actually caused by vortices in the electronic superfluid, sometimescalled fluxons because the flux carried by these vortices is quantized. Mostpure elemental superconductors, except niobium, technetium, vanadium and carbon nanotubes, areType I, while almost all impure and compound superconductors are Type II.

London moment

Conversely, a spinning superconductor generates a magnetic field, precisely aligned with the spin axis.The effect, the London moment, was put to good use in Gravity Probe B. This experiment measured themagnetic fields of four superconducting gyroscopes to determine their spin axes. This was critical to theexperiment since it is one of the few ways to accurately determine the spin axis of an otherwisefeatureless sphere.

4.3 APPLICATIONS

Soon after Kamerlingh Onnes discovered superconductivity, scientists began dreaming up practical

applications for this strange new phenomenon. Powerful new superconducting magnets could be mademuch smaller than a resistive magnet, because the windings could carry large currents with no energyloss. Generators wound with superconductors could generate the same amount of electricity withsmaller equipment and less energy. Once the electricity was generated it could be distributed throughsuperconducting wires. Energy could be stored in superconducting coils for long periods of time withoutsignificant loss.

The recent discovery of high temperature superconductors brings us a giant step closer to the dream of early scientists. Applications currently being explored are mostly extensions of current technology usedwith the low temperature superconductors. Current applications of high temperature superconductors

include; magnetic shielding devices, medical imaging systems, superconducting quantum interferencedevices (SQUIDS) infrared sensors, analog signal processing devices, and microwave devices. As ourunderstanding of the properties of superconducting material increases, applications such as; powertransmission, superconducting magnets in generators, energy storage devices, particle accelerators,levitated vehicle transportation, rotating machinery, and magnetic separators will become morepractical



The recent discovery of high temperature superconductors brings us a giant step closer to the dream of early scientists. Applications currently being explored are mostly extensions of current technology usedwith the low temperature superconductors. Current applications of high temperature superconductorsinclude; magnetic shielding devices, medical imaging systems, superconducting quantum interferencedevices (SQUIDS) infrared sensors, analog signal processing devices, and microwave devices. As our

understanding of the properties of superconducting material increases, applications such as; powertransmission, superconducting magnets in generators, energy storage devices, particle accelerators,levitated vehicle transportation, rotating machinery, and magnetic separators will become morepractical

The use of superconductors for transportation has already been established using liquid helium as arefrigerant. Prototype levitated trains have been constructed in Japan by using superconductingmagnets. Superconducting magnets are already crucial components of several technologies. Magneticresonance imaging (MRI) is playing an ever increasing role in diagnosticmedicine. The intense magneticfields that are needed for these instruments are a perfect application of superconductors. Similarly,

particle accelerators used in high-energy physics studies are very dependent on high-fieldsuperconducting magnets. The recent controversy surrounding the continued funding for theSuperconducting Super Collider (SSC) illustrates the political ramifications of the applications of newtechnologies.

P romising future applications include high-performance smart grid, electric power transmission, transformers, power storage devices, electric motors (e.g. for vehicle propulsion, as invactrains or maglev trains), magnetic levitation devices, fault current limiters, nanoscopic materials suchas buckyballs, nanotubes, composite materials and superconducting magnetic refrigeration. However,superconductivity is sensitive to moving magnetic fields so applications that use alternating current (e.g.transformers) will be more difficult to develop than those that rely upon direct current.

CHAPTER 5

5 .0 MAGNETISM

The term magnetism is used to describe how materials respond on the microscopic level to an

applied magnetic field; to categorize the magnetic phase of a material. For example, the most well known

form of magnetism is ferromagnetism such that some ferromagnetic materials produce their ownpersistent magnetic field. However, all materials are influenced to greater or lesser degree by the

presence of a magnetic field. Some are attracted to a magnetic field (paramagnetism); others are

repulsed by a magnetic field (diamagnetism); others have a much more complex relationship with an

applied magnetic field. Substances that are negligibly affected by magnetic fields are known as non-

magnetic substances. They include copper, aluminum, water, gases, and plastic. The magnetic state (or



phase) of a material depends on temperature (and other variables such as pressure and applied

magnetic field) so that a material may exhibit more than one form of magnetism depending on its

temperature, etc.

5 .1 PROPERTIES OF MAGNETIC MATERIALSParamagnetism

In a paramagnetic material there are unpaired electrons , i.e. atomic or molecular orbitals with exactlyone electron in them. While paired electrons are required by the Pauli Exclusion Principle to have theirintrinsic ('spin') magnetic moments pointing in opposite directions, causing their magnetic fields tocancel out, an unpaired electron is free to align its magnetic moment in any direction. When an externalmagnetic field is applied, these magnetic moments will tend to align themselves in the same direction asthe applied field, thus reinforcing it

Ferromagnetism

A ferromagnet, like a paramagnetic substance, has unpaired electrons. However, in addition to the

electrons' intrinsic magnetic moments tendency to be parallel to an applied field , there is also in these

materials a tendency for these magnetic moments to orient parallel to each other to maintain a lowered

energy state. Thus, even when the applied field is removed, the electrons in the material maintain a

parallel orientation. Every ferromagnetic substance has its own individual temperature, called the Curie

temperature, or Curie point, above which it loses its ferromagnetic properties. This is because the

thermal tendency to disorder overwhelms the energy-lowering due to ferromagnetic order.

Some well-known ferromagnetic materials that exhibit easily detectable magnetic properties (to

form magnets) are nickel, iron, cobalt, gadolinium and their alloys.

Diamagnetism

Diamagnetism appears in all materials, and is the tendency of a material to oppose an applied magnetic

field, and therefore, to be repelled by a magnetic field. However, in a material with paramagnetic

properties (that is, with a tendency to enhance an external magnetic field), the paramagnetic behavior

dominates. [9]Thus, despite its universal occurrence, diamagnetic behavior is observed only in a purely

diamagnetic material. In a diamagnetic material, there are no unpaired electrons, so the intrinsic

electron magnetic moments cannot produce any bulk effect. In these cases, the magnetization arises

from the electrons' orbital motions, which can be understood classically as follows:

When a material is put in a magnetic field, the electrons circling the nucleus will experience, in addition

to their Coulomb attraction to the nucleus, a Lorentz force from the magnetic field. Depending on which

direction the electron is orbiting, this force may increase the centripetal force on the electrons, pulling

them in towards the nucleus, or it may decrease the force, pulling them away from the nucleus. This



effect systematically increases the orbital magnetic moments that were aligned opposite the field, and

decreases the ones aligned parallel to the field (in accordance with Lenz's law). This results in a small

bulk magnetic moment, with an opposite direction to the applied field.

Note that this description is meant only as a heuristic; a proper understanding requires a quantum-

mechanical description. Note that all materials undergo this orbital response. However, in paramagnetic

and ferromagnetic substances, the diamagnetic effect is overwhelmed by the much stronger effects

caused by the unpaired electrons.

5 .2 THEORECTICAL AND EXPERIMENTAL MAGNETISMTheoretical magnetism was dominated by Weiss (1907) molecular field theory. According to thistheory, below their Curie temperatures, ferromagnetic materials can be considered as paramagnetswhose magnetic moments are strongly coupled by a fictitious magnetic field proportional to thespontaneous magnetization of the substance. Above the Curie point, the substance becomesparamagnetic, the inverse of the susceptibility thus being constrained to obey the linear Curie-Weisslaw. The molecular field is enormous, of the order of several hundred Tesla. Why then is it easy tochange the magnetism, or even demagnetize a piece of ordinary iron using quite small magnetic fields?To explain this puzzling observation, Weiss hypothesized that magnetic matter is subdivided intoelementary domains. The directions of magnetization within domains and the boundaries betweenthem could be changed by weak or moderate fields.

Many questions remained unanswered, however. A large number of metals have a positivesusceptibility that is independent of temperature «paramagnétisme constant». In the ferrites, of which magnetite is the type example, inverse susceptibility varies with temperature in hyperbolicfashion. Above all, even for archetypal ferromagnets like iron and nickel, the variation of inversesusceptibility with temperature only becomes linear about a hundred degrees above the Curie point.Extrapolating this linear segment gives a second Curie point, called the paramagnetic Curie point, ten orso degrees higher than the first. Louis Néel while a student at Strasbourg, measured and pointed outsome of these experimental discrepancies, which had been glossed over by previous researchers in theirdesire to «verify» Weiss theory



CHAPTER 6

6 .0 PROPERTIES OF MATERIALSA material undergoes transition under the influence of temperature and pressure, and these changesare physical in nature, because their molecules remain intact. During our school days, we were asked todistinguish physical and chemical changes. At that stage, we began to think in more details than whatour senses have detected. Having the ability to distinguish physical properties from chemical propertiesis indeed a good beginning in the study of materials.

Effects of Temperature on Substances : When temperature rises, a typical substance changes from solidto liquid and then to vapor, at a constant pressure. Some substance has several crystal forms in the solidstate. The glassy state is also considered a solid. Transitions from one solid to another solid form, fromsolid to liquid, from liquid to vapor, from vapor to solid etc. are called phase transitions .

Phase transitions from solid to liquid, and from liquid to vapor absorb heat. The temperature of asystem usually does not change as long as two phases are present. The (phase) transition temperaturefrom solid to liquid is called the melting point whereas the temperature at which the vapor pressure of a liquid equals 1 atm (101.3 kPa) is called the boiling point . Thus, the measured boiling point dependson the atmosphere pressure. For compounds that decompose at high temperature, boiling point can beeither specified at lower pressure or be replaced by the decomposition temperature. Thus, conditions aswell as the value of boiling point listed in literature must be taken into account for applicationconsiderations. Boiling points of mixtures change with composition. The boiling points of some commonmixtures are listed in handbooks, and boiling points can be used to assess the composition of a mixture

or the purity of a compound. However, a glassy material becomes soft in a wide range of temperatures.The temperature at which the material becomes soft (behave molten like) is called glassy temperature ,but it may be a range of temperatures. Behavior of a substance as the temperature changes must becarefully considered in its applications. Behavior of a mixture as temperature rises is different from itscomponents. There is no theoretical way to predict the behavior of a mixture from its components, evenif its exact composition is known. Addition of one or more materials usually changes the melting orglassy temperature of a substance. Thus, we often employ a blend (mixture) of materials whosebehavior is acceptable within the desirable range of temperatures. Antifreeze for automobile radiatorand deicing liquid for airplanes are examples of this application.

Thermal expansion coefficient:

A substance expands on heating. For a rod, the lengthening of a unit length per degree Kelvin is thelinear thermal expansion coefficient. This factor affects the substance performance in machines orstructural assemblies. Thermal expansion causes tight fitted parts to break and moving part to jam, inany machine. The problem is serious if different material is used. When a large body of glass is subject tolocal heating or cooling, it breaks up due to expansion or shrinkage. Thermal expansion also causesdistortion, and some thermometers are made of two strips of different metals. Thermal properties must



be considered in any engineering constructions such as railroad, bridges, pipelines, and buildings,especially in areas where temperatures go to extreme values.

Heat and Electric Conductance

Transmissions of energy and electric charge across a body of material give rise to heat and electricconductance respectively. The rate of flow across a unit-area section when the temperature or electricpotential difference applied to the wire of unit length is called the thermal or electric conductancecoefficient. Metals are usually good conductors of both, and their conductance coefficients are high.Insulation material for heat and electricity should have low conductance, whereas metals have highconductance.The reciprocal of electric conductance is called electric resistance; thus, the higher theconductance, the lower the resistance. Electric resistance for some familiar materials are given in thetable here. Note the large range of 10 15 among these substances. Aluminium and copper are very goodconductors , and their resistances are very low, in the order of 10 -8, almost 100 times smaller than thatof tungsten, W. Germanium, Ge, and silicon, Si, are typical semiconductors , whereas sulfur andphosphorous are insulation material .

Magnetic Properties

A magnetic field strength is measured in Tesla (T) and gauss (G, 1 T = 10,000 G). The Earth magnetic fieldis 0.5 G. When a material is placed into a magnetic field H, a magnetic field of different intensity B isproduced inside the material. The ratio B/H is called the magnetic susceptibility. The higher themagnetic susceptibility, the easier the material is magnetized. Most substances are diamagnetic . Themagnetic fields (B) within the bodies of these substances when they are placed in a magnetic field (H)are less than that of an empty space (vacuum).; thus their magnetic susceptibilities (B/H ratio) are lessthan 1. When a body of paramagnetic substance is placed in a magnetic field, the intensity of the fieldwithin the body is slightly larger than that of the applied field. The magnetic susceptibilities of paramagnetic substances are slightly greater than 1.Iron, cobalt and nickel are

some ferromagnetic substances, there are some other alloys and oxides that behave this way. Theypossess a spontaneous magnetic moment. A magnetic field is present in these materials even in theabsence of an external magnetic field. However, ferromagnetism is temperature dependent, and abovethe so called Curie temperatures of the substances, magnetism vanishes. The Curie temperature orCurie point of a substance is unique. The Curie points for Fe, Co, and Ni are 1043, 1400, and 630 Krespectively. Ferromagnetism are due to the presence of magnetic domains in the substance, and whenthese domains line up parallel to each other, they give a net magnetic field. If the domains line upantiparallel to each other at the Curie point, the substance is said to be antiferromagnetic. The magneticsusceptibility reaches a maximum at Curie temperature for antferromagnetic material. For example,FeO, MnO, CoO, NiO, FeF 2, FeCl2, a-Mn, Cr 2O3 etc. are some of the antiferromagnetic substances.

Ferromagnetic substances play important roles in recording tapes and disks for audio, video, andcomputer signals. Furthermore, ferromagnetic materials are used in permanent magnets, which areused in motors, antenna, and speakers. Recent development in strong magnets enables communicationequipment and computers to be miniaturized.

Density : The mass per unit volume (cm 3 = mL, m3 etc.) of is called density, an intensive property. Often,specific gravity is given. Specific gravity is the ratio of density of a substance compared to that of water.As a ratio, it has no units. Since density of water is 1.00 g/mL, specific gravity is the density in g/mL.Other units to use are kg/L or 10 3 kg m-3. Specific gravity for a few common substances are given here:



Au, 19.3; mercury, 13.6; alcohol, 0.7893; benzene, 0.8786. Do you know which element has the highestdensity?

Dielectric Constant

The dielectric constant e of a medium is its ability to reduce the force F of attraction of charged(q 1 and q 2) particles separated at distance r , compared to vacuum. It is usually defined by the equation, F = q 1q 2 / (e r ). A substance with large dielectric constant placed between two plates to which an electricvoltage has been applied will result in a weak electric field within it. Water, due to its polar nature, has arather large dielectric constant, 80.4. At the atomic scale, water molecules weaken the attractionbetween Na+ and Cl- ions, resulting in dissolving it. Dielectric constants for some familiar substancesare: H 2O, 80.4; methanol, 33.6; benzene, 2.3; H 2 at 20 K, 1.23.

Heat capacity

The amount of energy required to raise the temperature of a substance by 1 K is the heat capacity. If thesubstance has a unit mass, the amount is referred to as specific heat capacity, or specific heat. Forexample, it takes 1 cal (4.184 J) to raise the temperature of 1 g water by 1 K. Thus, the specific heat forwater is 1 cal g -1 K-1 (75 J mol -1 K-1). Specific heat of water is large compared to most other substances,for example: Cu, 24.4 J mol -1 K-1. This large heat capacity of water affect the weather, makingtemperatures in areas close to large bodies of water more steadier than large dry land.

Refractive Index

The ratio of light speed in vacuum to its speed in the medium is refractive index. Light travel slower inany medium than in vacuum. Thus, refractive index is always greater than unity (1), and light beamusually bents when entering from air to another medium. This value depends on the wavelength of thelight used, and the property is important for material used in optics instrument such as eye glasses. The

higher the refractive index, the thinner the glasses to achieve the same power. The difference inrefractive indexes between two liquid gives rise to the visible boundary between layers.

Difference in refractive indexes of lights of different wavelengths can be separated using a prism.Refractive indexes for some familiar substances are given in a box. It should also be kept in mind thatindex of refraction changes with dissolved substance and concentration.



REFERENCES

1.0) www.wikipedia.com

2.0) WTEC Panel Report on R & D Status and Trends in Nanoparticles, Nano-structured Materials, and Nanodevices, R. W. Siegel, E. H. Hu, M. C. Roco,Workshop 1997 ( http://itri.loyola.edu/nano/us_r_n_d/toc.htm )

3.0) Of Dwyer, J.J., Theory of Dielectric Breakdown in Solids, Clarendon Press, Oxford(1964).

4.0) Surface Engineering, Science and Technology I, Editors : A. Kumar , Y.-W.Chung, J. J. Moore, J. E.Smugeresky, The Minerals, Metals &MaterialsSociety, Warrendale, 1999.

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