Chapter 1 Introduction 1.1 General Low-pow Low-power er wireles wirelesss distribu distributed ted sensor sensor networks networks are becomin becoming g attract attractive ive for monitoring different variables – such as temperature, strain in a material, or air pressure over a wide area. However, one drawback of these networks is the power each node draws, though recent work has shown this can be lowered considerably. Batteries can be used to power nodes for extended periods of time, but they have a limited life cycle and eventually need to be replaced. s this can be a costly and time consuming procedure procedure for networks with many nodes, a means of powering the devices indefinitely would be a more practical solution. !olar power provides a considerable amount of energy per area and volume, but unfortunately is limited to applications that are reliably sunlit. promising alternative takes advantage of the energy in ambient vibrations and converts it to electrical power. "his approach compares very favorably with batteries, providing e#ual or greater power per unit volume. $%& "here "here are multipl multiplee techni#u techni#ues es for converti converting ng vibratio vibrational nal energy energy to electric electrical al energy energy.. "he most prevalen prevalentt three three are electros electrostati tatic, c, electrom electromagne agnetic, tic, and pie'oel pie'oelect ectric ric conversion. ma(ority of current research has been done on pie'oelectric conversion due to the low complexity of its analysis and fabrication.
1.2 The Piezoelectric Effect "he pie'oelectric effect, in essence, is the separation of charge within a material as a result of an applied strain. "his charge separation effectively creates an electric field within the material and is known as the direct pie'oelectric effect. "he converse pie'oelectric effect is the th e same process in reverse) reverse ) the formation of stresses and a nd strains in a material as a result of an applied electric field. "he *+++ standard on pie'oelectricity lists several different forms for the pie'oelectric constitutive e#uations $&. "he form used here is known as the d-form, and the e#uations are as follows) S s E T dE D dT /T E
"hese e#uations, known as the 0coupled1 e#uations, reduce to the well-known stress-strain relationship at 'ero electric field, and the electric field and charge displacement relationship at 'ero stress.
2igure% 2igure%.%) .%) pie'oele pie'oelectri ctricc effect effect cause cause crystal crystal materi materials als like like #uart' #uart' to generate generate electric charge when the crystal material is compressed, twisted, or pulled. "he reverse is also true 3www.cosmic-energy.org4. 3www.cosmic-energy.org4.
1.3 Piezoelectric Material 5n6 is the most attractive since it exhibits the coupling of pie'oelectric and semiconducting semiconducting properties as well as the formation of a barrier at the electrode contact to draw higher power from the ambience. "he 5n6 has a non-7entro symmetric structure, which naturally exhibits a pie'oelectric effect when sub(ected to a strain due to the displacement of 5n cations and 68 anions in its tetrahedral configuration. !elf-powered 9+9! systems demand sustainable low voltage, high current characteristic inputs, and easy integration with the 9+9! device, and the ability to be controlled with reliable transduction transduction mechanisms. mechanisms. s miniaturi'ed miniaturi'ed pie'oelectric transducers have a high resonant fre#ue fre#uency ncy,, they they canno cannott be used used for for harve harvesti sting ng ambie ambient nt vibra vibrati tions ons.. Lo Low w resona resonance nce fre#uenc fre#uency y structur structures es have macro macro dimensi dimensions ons that impose impose integrat integration ion challeng challenges es on 9+9! systems.
1.4 Photosensitive polyer !:-; photosensitive cross-linking polymer that can be patterned for high aspect ratio structures. !ince !:-; is highly flexible and has a very low
?@a4, it is commonly used as a structural layer in 9+9!. "hough this polymer is inhere inherent ntly ly an elect electric rical al insul insulato atorr with with no additi additiona onall funct function ionali aliti ties. es. "h "hee chem chemica icall resistance of !:-; is #uite excellent however most chemical etches it, albeit at a very slow rate. "his very slow rate creates diverging report for the etching resistance of !:;. dhesion of !:-; is usually good, but depends on the material. "he adhesion is worst with gold 3u4, average with silicon with native oxide, and best with silicon nitride 3!iA4 and 5n6. "he adhesion of !:-; seems to be affected by the chemical and !:-; lift-off with immersion in 6H. $C&.
1.! Photoplastic piezoelectric "anocoposite 7urr 7urren entl tly y, ther theree has has been been a grow growin ing g inte intere rest st in scie scient ntif ific ic comm commun unit ity y on developing microelectromechanical-sy microelectromechanical-systems stems 39+9!4-based 39+9!4-based pie'oelectric pie'oelectric sensors and energy-h energy-harve arvesti sting ng devices. devices. "he most common common pie'oele pie'oelectr ctric ic materia materials ls have been the pie'oceramics, with the leading candidates being lead 'irconate titanate 3@5"4, barium titanate titanate,, strontiu strontium m titanat titanate, e, and #uart'-b #uart'-base ased d structu structures, res, which which poss possess ess high strain strain response. "here has also been an interest in developing alternative materials, and recently, pie'oelectric 5n6 has attracted a lot of attention. attent ion. *t finds applications ap plications in 9+9! due du e to its uni#ue combination of electrical, electrical, optical, and pie'oelectric pie'oelectric properties $D&. However, However, 5n6 is a sensitive material for wet etching and treatment by temperature, acids, bases, and even water. "hus, for the successful fabrication of 5n6-based 9+9!, a dry etching techni#ue is needed $D&, $C&. 9oreover, most of the pie'oelectric materials are ceramics, which are brittle and have low fracture toughness, posing challenges in fabrication. "he mixing of nonmaterials into polymer matrices offers the promise of developing new polymer composite materials with extraordinary properties. Aevertheless, the development of these materials has been constrained by difficulties in incorporation and dispersion of these materials into polymer media because of the strong tendency of the nanoparticles 3A@s4 to agglomerate. –FG ?@a, which is close to
the 5n6 thin-film data. fter embedding these AEs inside an !:-; matrix, the &
2igure %.) &
1.# pro$le stateent •
"o increase the sensitivity 3i.e. output voltage4 of the energy harvesting material by selecting proper material and geometric structure to get maximum power output.
1.% &$'ectives and scope of the pro'ect •
•
"o analy'e the different structure for the material for which we get maximum voltage for the minimum deflection by using 769!6L 9ultiphysics. nalysis of the !: ;5n6 Aanocomposite material for the lower resonant fre#uencies i.e. for the ambient vibrations by altering the structure of the material and adding the proof mass.
1.( )iitations • •
2abrication will be difficult. 7annot used for large power production.
Chapter 2 )iterature revie* "his chapter addresses a literature review is presented in which previous works in fields of vibration energy harvesting are highlighted.
2.1 + ,oudy and P - rite /2004 "he focus of this is to discuss the modeling, design, and optimi'ation of a pie'oelectric generator based on a two layer vending element, the model has been validated and use not only to estimate power output under a given set of conditions, but also as the basis for generator design optimi'ation. 2urthermore, an analytical model of the generator has been developed and validated. *n addition to providing intuitive design insight, designs of % cmG in si'e generated using the model have demonstrated a power output of G>C
μ
w from a vibration source of %IH'.
2.2 M. Guizzetti et al. /200( "his paper shows the 2+9 simulations of pie'oelectric cantilever as a microgenarator. @ie'oelectric energy converters reali'ed in a cantilever configuration are the most studied for this purpose, *n order to improve the performances of the converter, the geometry has to be properly designed. *n this context 2+9 simulations have been used in order optimi'e the pie'oelectric mode. "he electrical energy generated by the converted under an applied acceleration is computed, finding the optimal thickness for the pie'oelectric layer. Jifferent geometries were considered verifying that they do not affect the optimal thickness. ?eometries with different dimensions were considered, verifying that the optimal thickness ratio t p'tt substrate is independent from the converter dimensions, and it is influenced only by the mechanical properties of the pie'oelectric layer and the substrate.
2.3 +uyo " atap et al. /2011 "his paper gives investigation of design and modeling of pie'oelectric cantilever for energy harvesting. 9+9! based energy harvesting device is designed to convert
mechanical vibration energy via pie'oelectric effect. *n order to improve the performance of the device, the geometry has been optimi'ed by using moving mesh L+ model available in 769!6L 9ultiphysics. "he proposed device is suitable for vibration energy harvesting and can be uses as potential micro generator.
2.4 5iaotonGao /200( "raditional pie'oelectric 7antilever use pie'oelectric and no pie'oelectric layers of the same length. "his paper shows the investigation of une#ual pie'oelectric and no pie'oelectric lengths namely two section pie'oelectric cantilever. 2or step-wise tip forces the results showed that longer non pie'oelectric layer is preferred for generation a higher induced voltage while a longer pie'oelectric layer reduced the induced voltage due to charge cancellation. Eith harmonic base vibrations, the results showed that there exists an optimal no pie'oelectric pie'oelectric length ratio at which output voltage, current, and power can be maximi'ed. "heoretical analysis of two-section @7=s was performed within the framework of beam theory. "he results were in good agreement with experiments.
2.! -. Prashanthi et al. /2012 "his letter reports a @hotoplastic 3!:-;4 pie'oelectric 35n64 Aanocomposite route for reali'ation of simple and low cost pie'oelectric microelectromechanical systems 39+9!4. *ntegrating the 5n6 nanoparticles into a photosensitive !:-; polymer matrix not only retains the highly desired pie'oelectric properties of 5n6 but also combines the photopatternability and the optical transparency of the !:-; polymer. "hese two aspects, therefore, lead to exciting 9+9! applications with simple photolithography-based microfabrication. "his approach opens up many new applications in the field of both sensor and energy harvesting.
2.6 7del Capo and CGreiner /200# !:-; has become the favorite photoresist for high-aspect-ratio 3HK4 and threedimensional 3GJ4 lithographic patterning due to its excellent coating, planari'ation and processing properties as well as its mechanical and chemical stability. !imple !:-; structures 3pillars and walls4 with aspect ratio above %II but a maximum lateral resolution of ; μm have been reported after : exposure. Better lateral resolution 3<% μm4 maintaining high aspect ratios has been achieved using x-rays. High-energy beams may render even smaller structures, but with limited aspect ratio.
2.# -haled ,aadan Ian G. 8oulds 92011: %CIIMm x%CIIMm x %CIMm out-of-plane, gap closing, electrostatic energy harvester is designed and fabricated to harvest low-fre#uency ambient vibrations. !:-; is used to fabricate the proof mass 3%IIMm x %IIMm x %CIMm4 and the CMm springs. Jifferent harvesters were designed to harvest at CI, >C and %%I H'. t %%I H', !imulations show that with an input vibration of %IMm amplitude at the fre#uency of resonance of the structure, the energy harvester should generate an average output power density of I.IGMEmmG.
2.% Matthe* ;opcroft To$ias -raer /2003 "he mechanical properties of these cantilevers were investigated using two microscale mechanical testing techni#ues) contact surface profilometer deflection, known as 9"-"est, and static load deflection using a specially designed test machine, the 92"III. "he
2.( Mano' -andpal et al./2012 @hoto-curable Aanocomposite material was formulated by embedding 5n6 nanoparticles into a !:-; matrix and studied for its pie'oelectric properties for low cost fabrication of self-powered nanodevices. "he pie'oelectric coefficient of 5n6 nanoparticles was observed to be ranging between %C and G pm, which is the highest reported. "hese experimental studies support the recent theoretical predictions where the pie'oelectric coefficients in 5n6 nanoparticles were found to be higher compared to the thin films because of the surface relaxation induced volume reductions in the nanometer scale.
2.10 Prasen'it ,ay et al./2014 "he mechanical properties of pie'oelectric 5n6 nanowire 3AE4 films have been measured using a nano indentation techni#ue. Ee demonstrated a fabrication process to embed this AE film inside a polymeric 3!:-;4 cantilever. 9echanical properties of this
!:-;5n6 AE film and cantilever have been measured. "he 5n6 AEs have been grown vertically using a low temperature chemical synthesis. "he observed value of –FG?@a. fter embedding this AE film inside a polymer matrix, the
2.11 -. Prashanthi et al./2013 *ntegrating pie'oelectric 'inc oxide 35n64 nanoparticles 3A@s4 with optimum weight fraction into a photosensitive !:-; polymer matrix not only retains the highly desired pie'oelectric and semiconducting properties of the 5n6, but also combines the photopatternability and the optical transparency of the!:-;. "he maximum output power produced by Aanocomposite cantilevers was I.ICME across a resistive load of %IIkO with peak to peak voltage of ∼%NIm at a resonance fre#uency of N kH'.
2.12 Mr. ,avi pra
Chapter 3 E=periental Methodoloy 3.1 Introduction 3.1.17nisotropic effect and couplin odes @ie'oelectric materials have a built-in polari'ation, and therefore respond differently to stresses depending on the direction. "here are two primary modes of electromechanical coupling for pie'oelectric materials) the G-% mode and the G-G mode. *n the G-% mode 32igure .%a4, the electric field is produced on an axis orthogonal to the axis of applied strain, but in the G-G mode 32igure .%b4, the electric field produced is on the same axis as the applied strain.
2igure G.%) 3-1 mode, the electric feld is produced on an axis orthogonal to the axis o applied strain [13]
Figure 3.2: 3-3 modes, the electric feld produced is on the same axis as the applied strain. [13] hile the pie!oelectric coe"cient is higher in the 3-3 mode or most materials, ta#ing ad$antage o the larger coe"cient re%uires a much more complex design. &nstead o simple planar electrodes, a series o interdigitated electrodes '&()* can +e used to ta#e ad$antage o the 3-3 coupling mode [].
3.1.2 Device Confguration "he vast ma(ority of pie'oelectric energy harvesting devices uses a cantilever beam structure. cantilever beam, by definition, is a beam with a support only one end, and is often referred to as a 0fixed-free1 beam. Ehen the generator is sub(ected to vibrations in the vertical direction, the support structure will move up and down in sync with the external acceleration. "he vibration of the beam is induced by its own inertiaP since the beam is not perfectly rigid, it tends to deflect when the base support is moving up and down 3see 2igure .G4. "ypically, a proof mass is added to the free end of the beam to increase that deflection amount. "his lowers the resonant fre#uency of the beam and increases the deflection of the beam as it vibrates. "he larger deflection leads to more stress, strain, and conse#uently a higher output voltage and power. )lectrodes co$ering a portion o the cantile$er +eam are used to conduct the electric charges produced to an electrical circuit, where they can be utili'ed to charge a capacitor or drive a load. Jifferent electrode lengths or shapes have been shown to affect the output voltage, since strain is not uniform across the beam. $;&
Figure 3.3: note that strain is generated along the length o the +eam, hence the use o the 3-1 mode [13]
3.1.3 Modes of >i$ration and ,esonance cantilever beam can have many different modes of vibration, each with a different resonant fre#uency. "he first mode of vibration has the lowest resonant fre#uency, and typically provides the most deflection and therefore electrical energy. lower resonant fre#uency is desirable, since it is closer in re%uenc to phsical $i+ration sources and generall more poer is produced at loer re%uencies. /hereore, energ har$esters are generall designed to operate in the frst resonant mode.
2igure G.N) Jifferent mode shapes of a vibrating beam.$%G&
3.2 Modelin MEM+ and Piezoelectric ?evices usin C&M+&) Multiphysics. 9+9! and @ie'oelectric devices and transducers are growing in use as their technology becomes more sophisticated. !imulation is an important part in understanding, designing, and optimi'ing them due to their si'e and complexity. @ie'oelectric devices are used in a wide range of applications such as actuators, sensors and ultrasonic devices. *n electronic devices, they are often deployed as precision fre#uency resonators in, for example, #uart' wrist watches. Jirect pie'oelectric behavior occurs in certain materials where an applied mechanical stress results in the build-up of electric charge or a voltage, while the inverse pie'oelectric effect occurs when an electric potential induces mechanical deformations. Kelated to this are devices based on pie'oresistive and magnetostrictive effects. $%N& +nergy harvester is designed and modeled in the 769!6L 9ultiphysics for the anisotropic material and coupling modes, resonance fre#uency and voltage for the deflection of the cantilever beam are obtained by solving it.
Chapter 4 ,esult and ?iscussion 9odeling and simulation of cantilever beam is been done in 769!6L 9ultiphysics using the electrostatic analysis, Bimorph cantilever +nergy harvester consists of two layers, upper layer consist of the 5n6 and lower layer is of !: ; material is operated in d-GG mode.
4.1 $iorph cantilever $ea of @n& and +A % 4.1.2 +iulation B "he first six model resonance fre#uency analysis is carried out to find the lower resonance fre#uency. "he results related to first six fre#uencies are as shown in figure 3%D4. "he first fre#uency gives the lower resonance fre#uency.
2igure N.%) %st mode of resonance fre#uency
2igure N.) nd mode resonance fre#uency
2igure N.G) Grd mode of resonance fre#uency
2igure N.N) Nth mode of resonance fre#uency
4.1.3 +tatic analysis force of CIA per unit area applied on the upper surface of the bimorph for the dG% mode is selected by making electric potential for the upper surface and grounding lower face of the 5n6 layer, while all other face of pie'oelectric layer are kept as 'ero charge constrained.
2igure N.C) bimorph cantilever beam
2igure N.D) meshed model of bimorph
2igure N.>) fre#uency and voltage "he transducer cantilever depends upon the length, width, thickness and the various properties of the material used make the structure. "he geometric shape of the structure as well as the material used to build the cantilever determines the cantilever stiffness. arious dimensions of cantilever beam simulations are carried out to get the optimum dimensions for getting high voltage for lower resonance fre#uency of the structure. "ables show the different dimensions and its fre#uencies, displacement and voltage. ?iensions/
8reuency/<;z
?isplaceent/
μm ¿
>oltae/>
μm ¿
G
F.>%D
I.I;
I.CCD%%
II × I × G
.FFG
I.NC;
.%CN
G
%I.%;>
.GGN
N.>;G
NII × I × G
C.>C
>.GF%
;.NN
G.DDI
%;.I>;
%G.%>
%II
GII
CII
×
×
×
I
I
I
×
×
×
G
"able N.) varying length of the cantilever beam and its fre#uencies, displacement, voltage.
?iensions/
8reuency/<;z
μm ¿
CII
×
NI
?isplaceent/
>oltae/>
μm ¿
×
G
G.D>;G
%>.FI
%.%D
CII × DI × G
G.DFC;
%>.>NN
%%.D>G
"able N.G) varying breadth of the cantilever beam and its fre#uencies, displacement, voltage. 2rom table 3N.%4 as length increases the fre#uency decreases and voltage increases. fter the length CII
μm
the edges and the face is much smaller than the
specified element si'e. nd keeping CII μ m constant, as the breadth increases the fre#uency increase and voltage decreases, so for lower operating fre#uency and higher voltage we assume optimum dimension as CII μ m
μ m×
I
μm
. 2or the thickness of N
, fre#uency is D.>I% kH' and voltage is %%.%G. i.e. the fre#uency increases and
voltage decreases, so the dimension we chose for the higher fre#uency is CII μ m ×
μ m×
I
G μm .
4.2 cantilever of +A %D@n& nano coposite aterial 2or the IQ of 5n6 in the !: ; matrix the strength of the !: ;5n6 nano composite material increases and the mechanical properties are as tabulated bellow.
μ m×
I
μ m×
G
Aanocomposite material, resonance fre#uencies are as shown in the bellow
figures, from that the lower resonance fre#uency is chosen to be %. kE.
2igure N.;) %st-Dth resonance fre#uencies of Aanocomposite material
4.2.1 +tatic analysis force of %IA per unit area applied on the upper surface of the cantilever for the dG% mode is selected by making electric potential for the upper surface and grounding lower face of the !: ;5n6 Aanocomposite layer, while all other face of pie'oelectric layer are kept as 'ero charge constrained. 9aximum displacement observed as shown in the figure bellow, i.e. total displacement is F.>Ix%I; μ m .
2igure N.F) total displacement of !: ;5n6 Aanocomposite cantilever beam
4.2.2 Proof ass ttaching the proof mass to the Aanocomposite material the resonance fre#uency will get lowered and deflection increase for that voltage is going to increase. 2or the optimi'ed model, the proof mass of si'e %II μ m × I μ m × C μ m is attached to the Aanocomposite cantilever beam, by that the resonance fre#uency going to reduce to a value >D.>GH' i.e. it will deflect at lower fre#uency and produce the more voltage.
2igure N.%I) Kesonance fre#uencies from %st to Dth node fre#uencies.
2igure N.%%) total displacement of the Aanocomposite with proof mass.
! Conclusions Jesign and simulation of bimorph energy harvest is designed for an applied load of %IA per unit area, static displacement and voltage as shown in table N. and N.G. the cantilever dimension should be chose for maximum energy harvesting for lower μm ×
fre#uency3G.DDIkH'4 and high voltage, for that dimension found out to be CII μm ×
G
μm
I
. Because of the brittleness of the 5n6 material, it is added into the !: ;
matrix making Aanocomposite material results in good pie'oelectric and @hotopatternable material. *ts resonance fre#uency for the optimi'ed dimensions is %. kH', to reduce the resonance fre#uency proof mass of %II
μ m×
I
μm×
C
μm
is
attached to the Aanocomposite material, by that the resonance fre#uency changes to value of >D.>GH'.
,eference $%& ! Koudy and @ write 0 pie'oelectric vibration based generator for wireless electronic1, *nstitute of physics @ublishing, !mart 9ater. !truct. %G 3IIN4 %%G%-%%N. $& 9. ?ui''etti et al.,1 "hickness optimai'ation of a pie'oelectric 7onverter for energy Harvesting1 +xcerpt from the proceeding of the 769!6L conference IIF milan. $G& !uyog A Ragtap and Koy @aily 0?eometry optimi'ation of a 9+9! based +nergy Harvesting device1 proceeding of the I%% *+++ students "echnology !ymposium %N-%D Ranuary, I%%. **" kharagur. $N&Siaotong?ao 0ibration and 2low +nergy Harvesting using @ie'oelectric1 thesis submitted to the 2aculty of Jrexel :niversity. $C& . @rashanthi, 9. Aaresh et al.,1 Aovel @hotoplastic @ie'oelectric Aanocomposite for 9+9! pplications1 (ournal of microelectromechanical systems, vol. %, no. , april I%.
$D& del 7ampo and 7?reiner, 0!:-;) a photoresist for high-aspect-ratio and GJ submicron lithography1 R. 9icromech. 9icroeng. %> 3II>4 K;%–KFC. $>& haled Kamadan, *an ?. 2oulds,1 2abrication of !:-; Low 2re#uency +lectrostatic +nergy Harvester1. F>;-%-ND>G-INDC-;%%TD.II UI%% *+++. $;& 9atthew Hopcroft, "obias ramer,1 9icromechanical testing of !:-; cantilevers1 "+9VIG, R!9+-99J, !ep. %I-%, IIG. $F& 9ano( andpal, et al., 0@hotopatternable nano-composite 3!:-;5n64 thin films for pie'oelectric applications1 applied physics letters %I%, %IN%I 3I%4. $%I& . @rashanthi et al.,0ibtrational energy harvesting using @hotopatternable pie'oelectric Aanocomposite cantilevers1 Aano +nergy 3I%G4 , FG–FG. $%%& . @rashanthi et al.,1 @lastic Jeformation !tudy of ertical 5inc 6xide Aanowire for @olymer 7antilever-Based !ensor pplications1 *+++ transactions on nanotechnology, vol. %G, no. N, Ruly I%N. $%& 9r. Kavi prakash at al.,1 !tudy of @ie'oelectric 9+9! "ransducers1. $%G& ndrew "ownley1 ibrational energy harvesting using 9+9! pie'oelectric generators1 +lectrical +ngineering, :niversity of @ennsylvania $%N& http)www.comsol.co.inshowcasememsWctaframe