Bulletin of the Transilvania University of BraşovCIBv 2015 • Vol. 8 (57) Special Issue No. 1 - 2015

THE PIEZOELECTRICITY AND THENANOTECHNOLOGIES – A POSSIBLE

FUTURE

D.T. BABOR1 D. COVATARIU2

Abstract: Starting by observing the mechanical phenomenon of tensioning– relaxing that repeatedly takes place within the surrounding environment(the example of a leave that moves due to the slightest breeze thus creating atension into its tail) it has been analyzed the possibility of harvesting thosetensions and turning them into electricity. As the idea was new but thephenomenon known for the last several decades (the transformation ofmechanical work into electricity = the piezoelectricity and the piezoelectriceffect) it has been identified the possibility of designing devices made ofmaterials with known piezoelectric proprieties and adapting them to theoriginal desired applications. The nanogenerators own characteristics sooffer new opportunities having multiple field applications: medical,communications, auto industry, electronics, research and lab applicationsand last but not least the development of new materials, having usage withinthe construction industry.

Key words: nanotechnologies, nanogenerators.

1 “Gheorghe Asachi” Technical University from Iaşi, Faculty of Civil Engineering and Building Services2 “Gheorghe Asachi” Technical University from Iaşi, Faculty of Civil Engineering and Building Services

1. Introduction

By observing the behaviour of naturallyoccurring mechanical systems, we noticedthe phenomenon of tensioning – relaxingthat repeatedly takes place within thesurrounding environment, so the questionarose whether the surface occurringtensions could somehow produceelectricity – the methods that could be usedto make use of the abundance on tensionsand to have them converted to electricpower. The phenomenon of tensioning –relaxing mostly observed was themovement of leaves due to the slightestbreeze – followed by the appearance of thetensions in their tails. [1], [4]

2. A New Idea with not Such a Shy Past

Following the research was proven that,although original – transforming intoelectricity the tensions that appear withinthe tail of a moving leaf and the harvestingof such produced electricity – the notion of“piezoelectricity” or the manifestation ofelectricity within materials subjected tomechanical work has been observed to takeplace both within naturally occurringmaterials and the manmade once (Fig. 1,Fig. 2, Fig. 3) , [2].

The naturally occurring materialscomprise:

- Quartz, Berlinite (a rare phosphatemineral structurally identical to quartz),

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- Sugarcane, Rochelle Salt, Topaz (afluoride and aluminium silicate mineral),Tourmaline (a boron silicate mineralcrystal),

The man made piezo materials include:- Artificial crystals – quartz analogue

crystals (GaPO₄ si LaGaSIO₁₄)- Artificial ceramics – ceramics’ family

having either perovskite structure orBronze – Tungsten structure,[3].

- Barium titanate (BaTiO₃); Lead titanate(PbTiO₃); Lead zirconate titanate – named“PZT”

Fig. 1. Atomic structure - Barium Titanate

Fig. 2. Unit cell of Tetragonal LeadTitanate

- Ceramics without lead- Potassium sodium niobate (NaKNb);

Bismuth ferrite (BiFeO₃); Sodium niobate(NaNbO₃).

- Polymers - Polyvinylidene fluoride orPVDF

We refer to all the above listedsubstances as materials displayingpiezoelectric characteristic.

Fig. 3. PVDF is a high-molecular weightpolymer with repeat unit [CH₂–CF₂]

2.1. The Piezoelectricity[6],[9]

The piezoelectricity refers to theelectricity resulted from pressure. Theterm’s origin comes from the greek“Piezo” = to squeeze, to press and‘electron’ which refers to amber, anancient source of electric charge.

The Piezoelectricity is the direct result ofthe Piezoelectric effect.

Although initially discovered by Jacquesand Pierre Currie in 1880, the piezoelectriceffect displayed by the Quartz has beenwidely used with applications in sonartechnology since the First World War.

During the Second World War theQuartz had ceased its place in favour ofBarium Titanate (BaTiO₃) for usage insonar technology, the latter subsequentlybecoming the most used piezoelectricmaterial.

Currently both the Quartz (SiO2) and theBarium Titanate are frequently used inmany applications.

2.1.1. The Piezoelectric Effect [10], [12]

2.1.1.a. The Description of the Process:When in a perfect equilibrium, the

constituent positive protons, negativeelectrons and neutral neutrons of anymaterial, do not make possible ameasurable electric charge within thematerial despite the fact that their atomshave a high electric potential.

A separation of electric charges through

D.T. BABOR et al.: The Piezoelectricity and The Nanotechnologies – A Possible Future 3

the removal of atoms from theirequilibrium (result of an external appliedforce – the bending of the material) willcause net electrical charges to appear (thecase only applies to crystals having theirunit cells asymmetrically arranged), Fig. 4.

Fig. 4. An upset of the balance of atomswill cause a spatial separation of positiveand negative charges. It will appear on

opposite, outer faces of the crystal

2.1.1.b. Mechanism:The nature of the piezoelectric effect is

closely related to the appearance of strongelectric dipole moments in solids. Theelectric dipole moments can be induced forions on crystal lattice sites withasymmetric charge surroundings (as inBaTiO3 and PZTs).

For crystals, the dipole density(polarization) “P” may be calculated bysumming up the dipole moments pervolume of the crystallographic unit cell.

Of crucial importance for thepiezoelectric effect is the alteration ofpolarization “P” when a mechanical stressis being applied.

Fig. 5. Stress-induced phase transition inPZT. Blue = Pb atoms. Green = O atoms.

Orange = either Ti or Zr atoms

In Fig. 5 the displacements of positivelycharged metal atoms compared to thenegatively charged oxygen give a largespontaneous polarization in the PZTcrystal at room temperature.

The changing of the polarization ‘P’ mayhave one of the following causes:

- A shifting of the dipole molecularmoments under the influence of an externaleffort (the shape changing of the ceramicdisc when pressed – see Fig 7); [11]

- A reconfiguring of the polarisablesurroundings due to exposure to an electricfield.

Fig. 6. Piezoelectric ceramic disc attachedto a thin flexible metallic disc

The piezoelectric effect is a reversibleprocess. If an electric current is applied tothe same piezoelectric material, theelectricity generates an internal mechanicalforce leading to the deformation of thematerial, such characteristic leading toextremely useful applications (Fig 6)

3. The Nanotechnologies – The NewWave Overcoming the OldTechnological Barriers [5], [8], [13]

3.1. The Nanomaterials

The development of technologies thatallowed creation of nano dimensionalmaterials, by atom manipulation, hasopened a new perspective for thepiezoelectric materials’ domain, Fig. 8.

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Fig. 7. Comparison of diferent sizes

Fig. 8. The synthesizing of materials from molecular components (metal oxides inparticular)

Out of the new nanomaterials category ofparticular interest for piezoelectricity areboth those obtained from Gallium nitride(GaN – used in making of the blu-raytechnology) and those synthesized fromdifferent metal oxides – TiO₂, Al₂O₃,ZnO, SnO₂, CuO – all of them presentingnotifiable piezoelectric properties. [7]

Fig. 9. ZnO nanostructures (nanocombs,nanorings, nanohelixes/nanosprings,

nanobows, nanobelts, nanowires,nanocages)

In Fig. 9 are shown ZnO nanostructuressynthesized under specific growthconditions using a solid-vapor phasethermal sublimation technique. Most of thepictured structures can be obtained having100% purity.[11]

These nanomaterials having controlledcomposition, surface endings and crystalstructures are of importance in therealization of innovative devices – theNANOGENERATORS.

3.2. The Nanogenerators[

Are integrated nanosystems obtainedfrom metal oxides nanowires (Zn, Ti)having particular piezoelectric properties,assembled (grown or printed) on flexiblesubstrates and allowing electricitygeneration by harvesting and transformingmechanical energy, a process that becomesmuch more efficient no longer beingrestricted by the limitations imposed by thetraditional piezoelectric materials, Fig. 10a, b. [14], [16].

D.T. BABOR et al.: The Piezoelectricity and The Nanotechnologies – A Possible Future 5

Fig. 10. a) A PZT layer having thesubstrate, the top and base electrodes

indicated;b) The piezoelectric field (FET) producedacross a nanowire by an external force F.

In Fig. 11, the key innovation leading tothe emergence of the nanogenerator wasthe designing of a new zigzag shapedelectrode coated with Pt, having parallelpeaks and valleys alternately arranged.[13]

Fig. 11. The top zigzag electrodeinteracting with vertically aligned array of

ZnO nanowires

The top zigzag electrode, having beneaththe vertically arranged arrays of ZnOnanowires, causes the compression andbending of a large number of nanowireswhen moving, thus generating the electriccurrent that the conductive bottomelectrode collects simultaneously,[15].

3.3. AdvantagesThe piezoelectric coefficient displayed

by the nanowires having a diameter of 2.4nanometers is 20 times higher for ZnOnanowires and 100 times higher for GaNnanowires when compared to thecoefficient displayed by the same materialsat macro scale, Fig. 12.[17]

Fig. 12. Arrays of ZnO nanowires growneither on a GaN substrate or sapphire

substrates that are covered by a thin layerof ZnO film, serving as a common

electrode for directly connecting thenanowires with an external circuit.

As the entire nanosystem is totallyencapsulated, the nanogenerators can beused in a variety of environments, Fig. 13.

a) Graphic sketch

b) The experimental built deviceFig. 13. A totally encapsulated

nanogenerator

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The entire nanowires system can begrown both on flexible and on stretchablesubstrates at relatively small temperatures( 100º), making it economical, reliable,with low manufacturing costs, and suitableto a multitude of substrates, Fig. 14. Thelack of mobile, constitutive partssignificantly increases the usage life ofnanogenerators, depending only on theageing of the used substrates.

Fig 14. Flexible nanogenerators thin filmtype, without moveable, quick wearable

constitutive parts[19]

4. Remarks

The excellent performance of ribbontype piezoelectric nanodevices coupledwith biocompatible, flexible substratessuggests various applications starting withtheir introduction in both our everydayactivities for the purpose of supplying withpower electric mobile devices (phones,MP3’s) and medical usages where thebiomechanical energy offered by thehuman body’s regular movement (organs)could be harvested and transformed inuseful electric energy supplying a range ofimplantable medical devices (pacemakers,insulin delivery pumps, magnetic valvesserving as urinary sphincters).

The generator’s small dimensions are

opening the road for the designing of newdevices functioning without batteries,devices supplied by the surrounding’sharvested mechanical energy, therefore thenanogenerators are emerging as a new,viable alternative of today’s energysources. Of the total possible usages werefer to the following:

- The nanogenerators are functioning atfrequencies lower than 10Hz, in the rangeof the conventional mechanical vibrations(such as stepping, heart beating) thereforesubstantially expanding the areas of thepiezoelectric devices’ applications:

a) The naturally occurring energysources (wind, vibrations, sound, waves)and the biomechanical forces produced bythe human body (the heart beating,swinging arms, bending knees, lung andcavity expansion during respiration) couldproduce inexhaustible, clean energy;

Fig. 15. The total available power forevery day bodily activities[18]

b) By making use of ZnO nanowiresgrown on textile fibers it has becomepossible the designing of pliable, bendable,wearable, robust energy sources made oftextile materials in any shape – as anexample a shirt or power generating pair oftrousers, Fig. 15 ;

- In Nano Letters in a publisheddocument doctor Zhong Lin Wangdemonstrated the ability of five stampsized nanogenerators stacked together,when squeezed between fingers, toproduce about 1 μAmpere output current at3 volts – similar to the voltage of 2 regularbatteries of 1.5 V each, Fig. 16.

D.T. BABOR et al.: The Piezoelectricity and The Nanotechnologies – A Possible Future 7

Fig. 16. ZnO nanowires grown on textilefibers

- The possibility of creating independentnanosystems made of nanosensors coupledto ZnO nanogenerators, having thefollowing usages:

a) The detection of pH and UV;

Fig. 17. A ZnO based UV nanosensorcoupled with a nanogenerator in anenergetic independent nanosystem

b) Creating environment sensorspowered by nanogenerators motioned bywind;

c) Ultrasound sensors;d) Wireless hermetically encapsulated

sensors, powered by bridges’ vibrations,that could collect data for long periods oftime without the need for maintenance,offering a continuous monitoring ofdifferent parameters such as icy conditions,cracks or corrosion, traffic flow etc;

e) Creating chemical sensors withoutbatteries, sensors that take advantage of thedynamic interaction between moleculesand the semiconducting nanowires’surfaces (interaction that induces anelectrical charge) Fig. 18.

Fig. 18. A chemical sensor withoutbattery. The nanosensor responds

selectively for different types of chemicalcategories and concentration levels.

Due to their extreme precision and fastresponse, the piezoelectric nanomaterialshave found applications in the field ofnanopositioning, designing of mechanismsand control systems for accuratemovement, domain that requires thecontrol of vibrations, positioning errorsand shifts to less than 0.1 nm;

Fig. 19. As the conventional positioningsystems do not offer the required stabilityand accuracy, a fast response and highprecision is offered by the nanosensors

with usage in measuring motion asaccurate as the sub-nanometer range.

The inherent properties of the newnanogenerators are offering newopportunities with applications in multiplefields: medical, - auto, - electronics, -research and laboratory applications andnot least in the development of newmaterials for construction purposes, Fig.19.

The potential offered by thenanogenerators is therefore limited only byeveryone’s imagination.

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References

1. Dave Mosher, Engineers Create FirstMotion-Powered Nanodevice, 2011

2. C.N.R. Rao, S.R.C. Vivekchand, A.Govindaraj, Inorganic Nanowires,2011

3. I.M. Tiginyanu, O. Lupan, V.V.Ursaki, L. Chow, M. Enachi,Nanostructures of Metal Oxides, 2010

4. Katherine Bourzac, NanogeneratorPowers Up - A device containingpiezoelectric nanowires can nowscavenge enough energy to powersmall electronic devices, 2010

5. Phil Berardelli, So Long, EnergizerBunny, 2011

6. Prachi Patel, Nanogenerator Fueled byVibrations, 2011

7. Pratima R. Solanki, Ajeet Kaushik,Ved V. Agrawal, Bansi D. Malhotra,Nanostructured metal oxide-basedbiosensors, 2011

8. S. Öztürk, N. Kılınc, N. Taşaltin, Z.Z.Öztürk, A comparative study on theNO2 gas sensing properties of ZnOthin films and nanowires, 2011

9. Pu Xian Gao, Zhong L. Wang,

Nanoarchitectures of semiconductingand piezoelectric zinc oxide, 2010

10. Zhong Lin Wang, Self-assemblednanoarchitectures of polarnanobelts/nanowires, 2011

11. *** Atomic basis of non-spontaneouslypolarised piezoelectrics, University ofCambridge, 2011

12. *** New Vibration PoweredGenerator For Wireless Systems, 2011

13. *** Nanogenerator ProvidesContinuous Power By HarvestingEnergy From The Environment, 2011

14. *** Nanowires exhibit giantpiezoelectricity, 2011

15. *** New Forms of Highly Efficient,Flexible Nanogenerator Technology,2011

16. *** New Electronic Devices Createdfrom Bent Nanowires, 2011

17. *** Power Shirt: Nanotechnology InClothing Could Harvest Energy FromBody Movement, 2011

18. Zhong Lin, Wang’s Nano ResearchGroup, 2011

19. *** Some new perspectives onemerging nanotechnology, 2011