energy harvesting through sound

Energy Harvesting Through Sound For Nano-devices

1.INTRODUCTION

Energy harvesting[1] (also known as power harvesting or energy scavenging) is the

process by which energy is derived from external sources (e.g., solar power, thermal

energy, wind energy, salinity gradients , and kinetic energy), captured, and stored for

small, wireless autonomous devices, like those used in wearable electronics and wireless

sensor networks.

Energy harvesters provide a very small amount of power for low-energy electronics.

While the input fuel to some large-scale generation costs money (oil, coal, etc.), the

energy source for energy harvesters is present as ambient background and is free.

In the search for alternating energy sources there is one form of energy we don’t hear

much about, which is ironic because I am referring to sound energy[2]. Sound energy is the

energy produced by sound vibrations as they travel through a specific medium. Speakers

use electricity to generate sound waves and here we use zinc oxide to do the reverse –

convert sound waves into electricity. The sound power can be used for various novel

applications including mobile phones that can be charged during conversations and

sound-insulating walls near highways that generate electricity from sound of passing

vehicles

Fig. 1 shows the most desirable thing that everyone has been waiting for is “Yell at your

cell phone to charge”. Just imagine what an argument could the phrase “Keep it quite,

your battery will explode..” make.

Fig. 1. A lady yelling at cellphone.

Dept of ECE, CEC 2011-12 Page | 1


1.1. Piezoelectricity

Piezoelectricity[3] is the charge that accumulates in certain solid materials

(notably crystals, certain ceramics, and biological matter such as bone, DNA and

various proteins) in response to applied mechanical stress. The

word piezoelectricity means electricity resulting from pressure. It is derived from

the Greek piezo or piezein , which means to squeeze or press, and electric orelectron ,

which stands for amber, an ancient source of electric charge. Piezoelectricity is the direct

result of the piezoelectric effect.

The piezoelectric effect is understood as the linear electromechanical interaction between

the mechanical and the electrical state in crystalline materials with no inversion

symmetry. The piezoelectric effect is a reversible process in that materials exhibiting the

direct piezoelectric effect (the internal generation of electrical charge resulting from an

applied mechanical force) also exhibit the reverse piezoelectric effect (the internal

generation of a mechanical strain resulting from an applied electrical field).

Piezoelectricity is found in useful applications such as the production and detection of

sound, generation of high voltages, electronic frequency generation, microbalances, and

ultrafine focusing of optical assemblies. Figures below shows a typical phenomenona of

Piezoelectricity.

Fig. 1.1 a. Fig. 1.1 b. Fig. 1.1 c.

No compression Small compression High compression


http://en.wikipedia.org/wiki/Microbalance

http://en.wikipedia.org/wiki/Force

http://en.wikipedia.org/wiki/Reversible_process

http://en.wikipedia.org/wiki/Centrosymmetry

http://en.wikipedia.org/wiki/Centrosymmetry

http://en.wikipedia.org/wiki/Piezoelectricity#Mechanism

http://en.wikipedia.org/wiki/Amber

http://en.wikipedia.org/wiki/Greek_language

http://en.wikipedia.org/wiki/Stress_(physics)

http://en.wikipedia.org/wiki/Protein

http://en.wikipedia.org/wiki/DNA

http://en.wikipedia.org/wiki/Ceramic

http://en.wikipedia.org/wiki/Crystal


1.2. Motivation

The history of energy harvesting dates back to the windmill and the waterwheel[1]. People

have searched for ways to store the energy from heat and vibrations for many decades.

One driving force behind the search for new energy harvesting devices is the desire to

power sensor networks and mobile devices without batteries. Energy harvesting is also

motivated by a desire to address the issue of climate change and global warming. The

sound that always exists in our everyday life and environments has been overlooked as a

source[4]. This motivated us to realise power generation by turning sound energy from

speech, music or noise into electrical power.

There has been a lot of interest in making nanodevices, but we have tended not to think

about how to power them. This nanogenerator allows us to harvest or recycle energy from

many sources yo power these nanodevices.

1.3. Nanogenerator

This sound-driven nanogenerator is based on piezoelecrtic ZnO nanowires[5]. This

nanogenerator is integrated with piezoelectric ZnO nanowires and a cross-sectional field-

emission scanning electron microscopy image of vertically well-aligned ZnO nanowire

arrays (acting as a piezoelectric active layer), respectively. The average length and

diameter of ZnO nanowires were ~10 um and ~150 nm, respectively. This nanogenerator

has the capability to generate 50 millivolts from sound with 100 decibles strenght or

about the noise from heavy street traffic.

Fig. 1.3 shows a typical schematic diagram of an integrated nanogenerator.

Fig. 1.3 Sound-driven nanogenerator with piezoelectric ZnO nanowires.



1.4. Sapphire

Sapphire[6] is a gemstone variety of the mineral corundum, an aluminium oxide (α-

Al2O3). Trace amounts of other elements such as iron,titanium, or chromium can give

corundum blue, yellow, pink, purple, orange, or greenish color. Chromium impurities in

corundum yield a red tint, and the resultant gemstone is called a ruby.

Sapphires are commonly worn in jewelry. Sapphires can be found naturally, by searching

through certain sediments (due to their resistance to being eroded compared to softer

stones), or rock formations, or they can be manufactured for industrial or decorative

purposes in large crystal boules.

Because of the remarkable hardness of sapphires (and of aluminum oxide in general),

sapphires are used in some non-ornamental applications,

including infrared opticalcomponents, such as in scientific instruments; high-

durability windows (also used in scientific instruments); wristwatch crystals and

movement bearings; and very thin electronic wafers, which are used as

the insulating substrates of very special-purpose solid-state electronics (most of which

are integrated circuits).

2. APPROACHES AND DEVELOPMENT PROCESS

As there are a vast array of contending methods for harvesting energy, our aim is to

harvest the energy from sound. And the idea is actually pretty simple, atleast on the

paper[7]. When the sound waves hit an absorbing pad, they make the pad vibrate, and this

is translated to tiny zinc oxide nanowires that are placed between two electrodes.

The back and forth movement creates electricity, which in turn can be used for charging

or supply for nanodevices, body implants.


http://en.wikipedia.org/wiki/Integrated_circuit

http://en.wikipedia.org/wiki/Solid-state_electronics

http://en.wikipedia.org/wiki/Substrate_(materials_science)

http://en.wikipedia.org/wiki/Insulator_(electrical)

http://en.wikipedia.org/wiki/Wafer_(electronics)

http://en.wikipedia.org/wiki/Watch

http://en.wikipedia.org/wiki/Window

http://en.wikipedia.org/wiki/Scientific_instrument

http://en.wikipedia.org/wiki/Optics

http://en.wikipedia.org/wiki/Infrared

http://en.wikipedia.org/wiki/Hardness

http://en.wikipedia.org/wiki/Boule_(crystal)

http://en.wikipedia.org/wiki/Sediment

http://en.wikipedia.org/wiki/Jewellery

http://en.wikipedia.org/wiki/Ruby

http://en.wikipedia.org/wiki/Chromium

http://en.wikipedia.org/wiki/Titanium

http://en.wikipedia.org/wiki/Iron

http://en.wikipedia.org/wiki/Oxygen

http://en.wikipedia.org/wiki/Aluminium

http://en.wikipedia.org/wiki/Aluminium_oxide

http://en.wikipedia.org/wiki/Corundum

http://en.wikipedia.org/wiki/Gemstone


2.1. Approach to Development

Figure 2.1 a and 2.1 b show a schematic diagram of an integrated nanogenerator with

piezoelectric ZnO nanowires and a cross-sectional field-emission scanning electron

microscopy (FE-SEM) image of vertically well-aligned ZnO nanowire arrays (acting as a

piezoelectric active layer), respectively. Figure 2.2 shows the step by step process of

development of nanogenerator. The nanowire were grown using a thermal chemical vapor

deposition (CVD) system via a vapor-liquid-solid mechanism on an n-type GaN thin film

(acting as a bottom electrode) deposited sapphire substrate[8].

Fig. 2.1 a. Schematic diagram of Fig 2.1 b. FE-SEM image of ZnO

an integrated naogerator nanowire arrays on

a GaN/Sapphire substrate

A palladium gold (PdAu) coated polethersulfone (PES) substarte were used as both a top

and a vibration plate and placed above the ZnO naowire arrays. The integrated device was

then scaled at the edges to prevent physical and chemical damage. The average lenght and

diameter of ZnO naowires were ~10 um and ~150 nm respectively. The integrated

nanogenerator was then connected to a measurement system.



2.2. Step by step process of Development

Fig. 2.2 a. A sapphire substarate is taken Fig. 2.2 b. An n-type GaN thin film

(acting as the base substrate deposited.

as bottom electrode)

Fig. 2.2 c. ZnO nanowires grown on Fig. 2.2 d. A polyethersulfone (PES) n-

type GaN thin film. The average length of substrate is coated with palladium

ZnO nanowires ~ 10 gold (PdAu) to be used as both a top

electrode and vibration plate.



Fig. 2.2 e. The coated PES substrate is placed Fig. 2.2.f. The integrated

Above ZnO nanowire arrays and Nanogenerator subjecte

scaled at the edges to prevent a sound of ~ 100decible

physical & chemical damages (10 -2 W m-2 at 100 Hz)

Fig. 2.2 Step by step process of Developement

2.3. Growth Mechanism of Vertically Aligned ZnO Nanowire Arrays

Semiconducing nanowires possess many of the unique features required for potential

applications in electrons and optoelectronics[9]. The controlled growth of one-

dimensional (1-D) semiconductor nanostructures has recently been the focus of much

attention.

Researchers have employed various techniques to grow one-dimensional nano-structures,

including bottom-up techniques such as the va-por-solid (VS) process and vapor-liquid-

solid (VLS) process and top-down techniques, i.e., lithographic tech-niques. Other

deposition techniques support the growth of nanostructures using grated substrates or

embedded matri-

ces. Still other techniques confine material to 1-D tubular structures or arrange granular

material in a linear fash-ion.

The VLS process, one of the methods frequently applied to grow 1-D ZnO

nanostructures, was first described in 1965 for the Au-assisted growth of Si whiskers, and



is currently a well-understood system. In this process, a metal liquid droplet serves as a

preferential site for absorbing the vapor reactant. Nanostructure growth begins when the

droplet is supersaturated with the source material, and continues as long as the droplet

remains in the liquid state and

vapor is supplied. However, several investigations on this process have shown

unexpected growth behaviors. The dynamic reshaping of catalyst particles during

nanowire growth

determines the length and shape of Si nanowires.

We synthesized ZnO nanowires in a low pressure vapor phase system consisting of a

horizontal quartz tube in a conventional furnace, as schematically depicted in Fig. 2.3 a.

Fig. 2.3 a. Schematic illustration of the horizontal furnace system for growing

ZnO nanowire arrays. (1)Zn metal source; (2) substrate; (3) porous gas

shower head distributor; (4) furnace; (5) quartz tube.

Commercially available high purity metallic zinc powder (Zn, 99.999%) and

commercially available oxygen gas (O2,99.999%) were used as source materials for zinc

and oxyen, respectively. The nanowires were grown on Si(100) substrates coated with Au

thin film, which was deposited by electron beamevaporationmethod.

When the Au coated Si(100) substrates were loaded into the furnace, the distance between

the Zn source at the center and substrates located downstreamof the Ar ranged from2 to 5

cm. In a typical reaction process, approximately 2 g of Zn powder was placed in an

alumina boat in the center of the quartz tube furnace. We used high purity argon gas (Ar,

99.999%) as the carrier gas, running it through a shower head distributor throughout the

reaction process.



After loading the samples, the chamber pressure was pumped down to 10-3 Torr using

a rotary pump, and Ar gas was added at the desired growth temperature to drive out any

remaining oxygen. The growth conditions were kept constant at 550 ± 10 °C and 100 Torr

in the center of the furnace for the duration of the reaction process. Ar gas was used to

carry Zn vapor onto the Au coated Si(100) substrates. When the temperature reached the

processing temperature, O2 was introduced as the reactive gas at the flow rate of 10 sccm

to grow ZnO nanowire arrays. After the desired growth was obtained, we allowed the

furnace to cool down to room temperature and then characterized the products on the

substrates in terms of their structural and optical properties.

The morphologies of ZnO nanowire arrays grown on Au coated Si(100) substrate were

examined using a FE-SEM, as Fig. 2.3 b indicates.

Fig 2.3 b. Tilted 45 degree view and cross-sectional SEM images of synthesized ZnO

nanowires grown at 550 degree C for 1 hr on Au(5 nm)/Si(100) substrates with Ar flow

rates of (a)-(b) 300 sccm and (c)-(d) 100 sccm. O2 flow rates were 10 sccm in both cases.

The white arrow indicates the Au catalyst on the tip of ZnO nanowires (scale bar:(a)-(b)

are 1m and (c)-(d) are 500 nm)



3. Operation and Observation

3.1. Operation Mechanism

Sound is a regular mechanical vibration that travels through matter as a waveform [10].

Longitudinal sound waves (compression waves) transmitted through the ambient air are

made up of waves of alternating pressure deviations from the equilibrium pressure,

causing local regions of compression and rarefraction.

When a sound wave strikes the PdAu-caoted flexible PES substrate acting as the top

electrode, it causes the flexible substrate to vibrate. This mechanical vibration of the

flexible substrate generated by the sound wave is directly transferred to the vertically

aligned ZnO nanowires, causing compression and releasing of the nanowires.

The power output mechanism of the sound-driven nanogenerator is based on the coupling

of the piezoelectric and semiconducting properties of ZnO. The preferred c-axis

alignment of ZnO nanowires leads to strong piezoelectric alignment with regard to the

external force. The sound wave is used to vibrate the top contact electrode, which

generates electric potential through the vertically well aligned ZnO nanowires in the

direct compression mode.

With a force applied in the direction parallel to the vertically aligned ZnO nanowires, the

centers of mass of the positive and negative ions are shifted , resulting in polarization

along this direction. The created piezoelectric potentials along individual nanowires have

the same tendency of distribution because the ZnO nanowires are grown on the substrate

with the preffered c-axis orientation.

Under direct compression by the sound wave to the vertically aligned ZnO nanowires, a

negative piezoelectric potential is generated on the top side while a positive piezoelectric

potential is generated on the bottom side.



3.2. Observation

Figure 3.1 show plots of the input signal generating the sound wave and the output

voltage obtained from the integrated nanogenerator.

Fig. 3.1 The input signal for the generation of a sound wave and output voltage from the

piezoeletric ZnO nanowire arrays due to the sound wave

The converted electrical energy from the sound wave was displayed on the oscilloscope

as a voltage in the alternating current (AC) mode, following the frequency of the sound

wave with the sinusodial mode electrical input with a small phase difference. This phase

difference between the signals is due to the impedance of the intrinsic capacitance and

reactance within the piezoelectric circuit. The intensity of the input sound was ~ 100

decible (dB) (10-2 W m-2 at 100 Hz), and the amplitude of the output voltage was ~50 mv.

3.3. AC output Mode

As we have observed that the converted electrical energy from the sound wave was

displayed on the oscilloscope as a voltage in the alternating current (AC) mode [11]. The

AC output mode from the nanogenerator is due to the Schottky contact formation

between the nanowires and the elkectrode, which acts as a capacitor.



Schottky contact is the rectifying contact that occurs between a metal and a lightly doped

semiconductor.

The negative piezoelectric potential generated by the wave increases the conduction band

and the fermi level at the top of the nanowires. Electrons then flow from the top electrode

to the bottom side through the external circuit and the positive potential is generated

around the top PdAu electrode (Figure 3.2 b)

Fig. 3.2 a. The generator with no sound wave application

Fig. 3.2 b. Electrons flow from the top electrode to the bottom side through the external

circuit by the negative piezoelectric potential generated at the top side of the ZnO

nanowires under direct compression by the sound wave. At this time, the positive

potential is generated around the top PdAu electrode.

The potential is kept since the Schottky contact hinders the electrons from being

transported through the interface. The piezoelectric potential dissipates when the external

pressure on the top PES substrate is momentrily removed in the rarefaction mode of the

sound wave (compressive strain is released from the nanowires) (figure 3.2 c).



Fig. 3.2 c. The piezoelectric potential dissipates when the external pressure on the top

PES substrate is momentrily removed in the rarefraction mode of the sound wave.

Electrons flow back via the external circuit till neutralizing the positive potential around

the top electrode.

The electrons flow back via the external circuit till neutralizing the positive potential

around the top electrode. These repeated cycles then result in the AC voltage pulses

through the generator.

The functional relationship of the parameters used in the sound-driven piezoelectric

nanogenerator can be described as

Where, P,V and f are the power (which is defined as the input intensity of the sound wave

multiplied by the surface area), the output voltage obtained from the nanogenerator, and

the frequency of the applied sound wave, respectively. Under a fixed frequency (100

Hz), the V2 generated from the nanogenerator is propotional to the input power applied to

the nanogenerator (Figure ). The V2 value is increased linearly within the intensity range

of 0-3 m W m-2, which can be converted to a power ranging from 0-0.3 u W considering

a device size of 1cm2. Above this range, V2 was saturated, possible due to limitation of

energy conversion.

When the input intensity of the sound wave was held constant at ~ 100 dB, V2 was

linearly dependenet on the frequency of the applied sound wave (figure 3.3).



Fig. 3.3 Output voltage vs. the input power of the applied

sound wave under a fixed freuency of 100 Hz

These two relationships clearly supports the evidance that the piezoelectric output

generated by the acoustic system follows piezoelectric-mechanical-acoustic energy

conversion (i.e., piezoelectirc energy is transformed into the acoustic one via the

mechanical vibration)[12].



3.4. Superposition

Further we performed a linear superposition test in which the two nanogenerators, Cell 1

and Cell 2, showed output voltage of 24 mv and 26 mv, respectively (Figure 3.4).

Fig. 3.4 Results of the “Linear superposition “ tests.

In the linear superposition test, the output voltages of the nanogenerator were enhanced

by connecting them in series. The output voltage of 52 mv is the approximate sum of the

output voltage of individual nanogenerators. Therefore, these experimental results clearly

support the hypothesis that the measured signal originated sound-driven piezoelectric

nanogenerator rather than from the measurement system.



3.5. Photoluminescence

Figure 3.5 a. shows the room temperature photoluminescence (PL) of the ZnO nanowires

in the nanogenerator used in this work. The predominant peak seen at 375 nm is usually

attributed to the recombination of free exicition, while a broad deep level emission band

with the relatively weak emission intensity was also observed at about 510 nm.

Fig. 3.5 a. Room-temperature PL spectrum of the ZnO nanowires

In general, braod deep-level emission prevails over free exicition-related emission in the

ZnO nanostructures. It is wellk known that deep-level emission is mainly due to states

inthe band gap, which originates from defects such as zinc interstitials, oxygen vacancies,

and their complexes in the nanostructures. It may be proposed that the realtively deep-

level emission from our ZnO nanowires is due to formation of small number of point

defect in the nanowires.



Because the charged point defects acting as actiove carriers in ZnO nanowires speed up

the rate at which the piezoelectric potential is screened [13]. It was concluded that our ZnO

nanowires with a small number of point defects grown on GaN with a negligibly low

lattice mismatch to ZnO is very promissing for effectively generating piezoelectric

potential in the nanowires due the mechnical stimuli of sound waves.

It has been reported that the surface of the ZnO nanowires are apt to be covered with

oxygen (O2) molecules, creating a depletion region. As a result, this not only decreaes the

carrier density (due to the trapping of free electrons in the n-type ZnO nanowires), but

also decreases the conductivity.

Fig. 3.5 b. Behavior of the output voltage from the UV irradiated piezoelectric

nanogenerators driven by sound waves

Under ultraviolet (UV) light irradiation, however, oxygen is able to detached, which

results in increased conductance through nanowires. This is because the holes resulting

from the electrons-holes pair generated by the UV light irradiation can migrate to the

surface and recombination with O2- trapped electrons, causing their release. These

electrons then contribute to the current, enhancing the conductivity of the nanowires. In

addtion UV light irradiation generates photoexcited carries into ZnO nanowires with a

direct band gap, resulting in increased free carrier concentration in the conductance band.

Therefore, it is expected that it will be difficult to effectively generate piezoelectric

potential in very conductive nanowires, which then cause a decrease in the output voltage

of the piezoelectric nanogenerators driven by the sound waves when the ZnO nanowires



are exposed to UV light irradiation (figure 3.5 b ). This result is additional evidence that

the output voltage is due the piezoelectric potential generation under sound wave

irradiation.

4. Experimental

Growth of ZnO nanowires on the GaN/Sapphire substrate : ZnO nanowires were

thermally grown using a horizontal quartz tube by vaporizing mixed ZnO and Graphite

(1:1) powder. The growth substrate used was GaN (2 um)/Sapphire on which 3 nm of an

Au thin film catalist was deposited using thermal evaporator. The source powder and the

Au-coated substrates were placed on the ceramic boat and then loaded at the center of the

tube. The ZnO nanowires were synthesized at a temperature of 950 degree C for 20 min

under Argon gas (50 sccm).

Characterization: The morphological properties of the grown ZnO nanowires were

examined by FE-SEM. The sound wave was generated by speaker and function generator

and the intensity of the generated sound was measured by dB meter. The nanogenerator

perfomance in the presence of sound wave was evaluated using an oscillioscope. PL

measurements were conducted using He-Cd laser (ʎ) for excitation.

5. Advantages

Sound that always exists in our everyday life and environments can be a great resource

for energy, which can be harvested from the proposed nanogenerator.

The main advantages of this proposed system are:

1. There has been a lot of interest in making nanodevices, but we have tended not to

think about how to power them. This nanogenerator allows to harvest or recycle

energy from many sources to power these devices.

2. This would be specially convinient for body implants, such as pacemaker, since

surgeries are now required to replace the dead batteries. It could replace those

batteries with power directly harvested from the continual motion of the lungs.



3. This system can be used to harvest energy from our body motion such as walking,

finger typing, or breathing.

6. Future Scopes

Since, at the present scenario the device (nanogenerator) is able to generate ~ 50 mv at

~100 dB of sound level, while normal conversation takes place at 60-70 dB. We know

that 50 mv is not enough to charge a mobile phone, but it can surely support the life of

battery. The advancement in the device can increase the output levels and hence can be

used to charge mobile phones when you talk.



7. Conclusion

In summary, we demonstrated the sound-driven power generation using nanogenerators

based on piezoelectric ZnO nanowires. The sound wave was used to vibrate the top

contact electrode, which generated electric potential throught the vertical well-aligned

ZnO nanowires in the direct compression mode. When sound with an intensity of ~ 100

dB applied to the nanogenerator, an AC output voltage of about 50 mv was obtained from

the nanogenerator. Systematic investigation of the generated output voltage as a function

of the input power and frequency of the applied sound wave, linear superposition tests,

and the observation of output voltage drop with UV light irradiation applied to the

nanogenerator clearly support the notion that the measured output voltage originated from

the sound-driven nanogenerators. Therfore, our result suggest that sound can be one of

promissing energy source with highly efficient generator based on piezoelectric

nanowires.



8. REFERENCES

[1]http://en.wikipedia.org/wiki/energy_harvesting

[2] http://www.gizmag.com/mobiles-powered-by-conversation

[3] http://en.wikipedia.org/wiki/Piezoelectricity

[4] http://www.phonearena.com/news/scientist-harvesting-energy-from-sound

[5] Seung Nam Cha, Ju-Seok Seo, Seong Min Kim, Hyun Jin Kim, Young Jun Park, Sang-woo Kim, and Jong Min Kim, “Sound-driven Piezoelectric Nanowires-based Nanogenerator [6] http://en.wikipedia.org/wiki/Sapphire[7] http:/www.nanowerk.com/spotlight/spotid=14653.php

[8] S-D. Lee, Y.-S. Kim, M.-S. Yi, J.-Y. Choi, S.W. Kim, J. Phys. Chem. C 2009

[9] Yu-Tung Yin,Yen-Zhi Chen,Ching-Hsiang Chenb and Liang-Yih Chen, “The Growth Mechanism of Vertically Aligned ZnO Nanowire Arrays on Non-epitaxial Si(100) Substrates”, Journal of the Chinese Chemical Society, 2011, 58, 817-821

[10] Ju-Seok Seo, Seong Min Kim, Hyun Jin Kim, Young Jun Park, Sang-woo Kim, and Jong Min Kim, “Sound-driven Piezoelectric Nanowires-based Nanogenerator

[11] R. Yang, Y. Qin, C. Li, G. Zhu, X. D. Wang, Nano lett. 2009, 9, 1201

[12] S. Priya, D. J. Inman, Energy harvesting Technologies, Springer, New York 2009, p. 25.

[13] J.Liu, P.Fei, J.H.Song, X. D. Wang, Nano lett. 2008,8, 328.


energy harvesting through sound

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