This paper aims at the transformation of mechanical energy, sound energy and various kinds of disturbances to electrical energy using piezoelectric materials. Using piezoelectric materials it is easily possible to convert any kind of mechanical stress to electrical energy and vice versa .Thus the sound produced during the usage of devices like cellphones can be converted to electrical energy and thus can be utilized for recharging the device. This paper presents a model of piezoelectric ZnO nanowire transducer for a gadget like mobile phone
prototype of the power scavenging circuit, and the overall circuit for charging the mobile battery using the generated energy.
INDEX TERMS :
Charge generation, Diaphragm type , PVDF copolymer ,piezoelectric polymer , zinc oxide nanoparticle , Key depressions, Piezoelectric effect.
A mobile phone (also known as a cellular phone, cell phone and a hand phone) is a device that can make and receive telephone calls over a radio link while moving around a wide geographic area. It does so by connecting to a cellular network provided by a mobile phone operator, allowing access to the public telephone network.A mobile phone uses a battery which serves as a power source for phone functions. The battery basically has one of the most important considerations when choosing a mobile phone.The basic cellphone batteries used are Lithium ion batteries which are of pouch format .These batteries are basically soft type with flat body.The talk-time battery time for a basic cellphone is considered to be six hours. Thus this short lifetime causes problem in case of battery rundown when there is no charging point available or during an important phone call.
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This problem of battery life in a mobile phone can be solved by using the unused power existing in form of sound ,vibration ,human activities, structures and environmental sources. Among these, one of the promising sources of recovering energy is from the vibrations generated by the key depressions of any keypad integrated device such as a mobile phone. Primarily, the selection of energy harvester as compared to other alternatives such as battery depends on two main factors, cost effectiveness and reliability. Conversion of mechanical low frequency stress into electrical energy is obtained through the direct piezoelectric effect, using a rectifier and DC-DC converter circuit to store the generated electrical energy. There are three primary steps in power generation:
(a) trapping mechanical AC stress from available source.
(b) Converting the mechanical energy to electrical energy using piezoelectric transducer.
(c) Processing and storing the generated electrical energy.
The mechanical output can be in the form of burst or continuous signal depending on the cyclic mechanical amplifier assembly. Depending on the frequency and amplitude of mechanical stress, one can design the required transducer, its dimensions, vibration mode and desired piezoelectric material. The energy generated is proportional to frequency and strain and higher energy can be obtained by operating at the resonance of the system.
The idea of this paper is to trap the vibrations of sound and other mechanical vibrations in a cellphone and use it to recharge the cellphone. Thus this creates a easier energy harvesting method. A conventional piezoelectric ceramic that is composed of a perovskite ceramic crystals, which consist of a small, tetravalent metal ion, usually of titanium or zirconium mineral, in an arrangement of lattice of larger, divalent lead or barium metal ions, and O2- ions (Figure 1). Under such arrangements they attain tetragonal or rhombohedral symmetry on the crystals. Individual crystal has a dipole moment. Refer Figure 1 b.
Preparation of piezoelectric ceramicinvolves the following steps.
The required proportions of Fine PZT powders of the component metal oxides are mixed.
Then, it is heated to form a uniform powder.
Now the piezo powder is mixed with an organic binder to get structural elements of desired shape (discs, rods, plates, etc.).
The mixture is exposed to fire for a specific time and temperature program. It shapes the piezo powder particles and attains a dense crystalline structure.
Finally the elements are cooled, then shaped or trimmed to various specifications, and electrodes are applied to the appropriate surfaces.
Above the Curie point which is the critical temperature, the perovskite crystal in the fired ceramic element acquires a simple cubic symmetry with no dipole moment (Figure 1a).Below the Curie point, each crystal exihibits tetragonal or rhombohedral symmetry with a dipole moment (Figure 1b). Adjacent dipoles that are connected form regions of local alignment are called domains. This gives a net dipole moment to the domain, and thus a net polarization. The direction of polarization among all the adjacent domains is random, so the ceramic element has zero overall polarization (Figure 1a).
Figure(1.b) CRYSTAL STRUCRURE – PIEZOELECTRIC CERAMIC
The alignment of the domains is done by exposing them to a stronger and direct current electric field,at a temperature slightly below the Curie point (Figure:2b). Through this special polarizing treatment, domains that are aligned closer with the electric field expand at the expense of unaligned domains, and the element lengthens in the direction of the field. When the electric field is withdrawn, nearly all the dipoles are locked into a configuration of closer alignment (Figure:2c). The element now is said to have permanent polarization, the permanent polarization, and is permanently elongated.
Fig(2) POLARIZING OR POLLING A PIEZO CRYSTAL
The tension produced by the mechanical compression on a poled piezoelectric ceramic element alters the dipole moment due to which a voltage is created. voltage of the same polarity as that of the poling voltageis generated by the compression produced along the direction of polarization, or tension perpendicular to the direction of polarization. Tension opposite to the direction of polarization, or compression parallel to the direction of polarization, generates a voltage with polarity opposite that of the poling voltage i.e., Disk is stretched. These actions are called generator actions .The ceramic element converts the mechanical energy (due to compression / tension) into electrical energy. This action is used in fuel-igniting devices, solid state batteries, force-sensing devices, and many other products. Values for compressive stress and the voltage (or field strength) generated by applying stress to a piezoelectric ceramic element are linearly proportional up to a material-specific stress. And also true for applied voltage and generated strain.
The piezoelectric materials are advantageous in that they do not rely on external power sources (e.g., batteries or alternating current (AC) power) for continued operations, unlike strain gages ¬ber optics , wireless sensor nodes, micro-electromechanical systems (MEMS) devices , and other types of sensing systems. Unfortunately, PZT and PVDF suffer from fundamental limitations intrinsic to their material. Although piezo-ceramic PZT transducers possess high piezoelectricity and d33 piezoelectric constants approximately 200-400 pC N-1 , they are extremely brittle, have high loss factors, and are characterized by highly hysteretic behavior . On the other hand, piezoelectric polymers such as PVDF and PVDF-copolymers are ¬‚exible, conformable, and can be fabricated to different sizes and thicknesses . However, they possess considerably lower piezoelectric constants are compared to PZTs (10 pC N-1) and require intricate mechanical stretching to enhance their bulk ¬lm piezo-electricity. Furthermore, both PVDF ¬lms and PZTs require high-voltage poling so as to enhance their piezo-electricity. Thus, in order to use piezoelectric transducers for sensing applications in complex laboratory and ¬eld environments, it is desirable for them to simultaneously possess high piezoelectricity and excellent mechanical properties. On the other hand, the nanotechnology domain offers a diverse suite of new materials and composite fabrication methodologies for high-performance piezoelectrics . Among the plethora of nanomaterials, zinc oxide (ZnO) nanostructures (e.g., nanowires, nanosprings, and nanoparticles, among others) can be readily synthesized and exhibit inherent piezoelectricity .
The basic block diagram of the proposed model is shown in Fig. below. It consists of 3 main blocks,
(a) piezoelectric power generation
(c) storage of DC voltage.
AC voltage is generated from the piezoelectric material which is rectified by the rectification block and then it is stored in a storage device such as a battery.
Fig (4):BLOCK DIAGRAM
(PROTOTYPE MODEL OF PIEZOELECTRIC MATERIAL FOR MOBILE PHONES )
A diaphragm assembly comprises at least two piezoelectric diaphragm members arranged in a stacking direction. An interface layer is situated between adjacent
piezoelectric diaphragm members. The interface layer in the stacking direction is displaceable and incompressible or resilient. The interface layer permits lateral movement of the adjacent piezoelectric diaphragm members relative to the interface layer in a direction perpendicular to the stacking direction.
The interface layer can comprise, for example, an incompressible liquid or a semi fluid or a compressible gas. A gasket can be used to seal the substance in the interface layer if necessary.
Fig. 5. Basic model of diaphragm type piezoelectricity
2. Piezoelectric material (PVDF)
5. Hole for the central part of film to deform itself
The figure illustrates the overall circuit diagram of the entire process. The rectifier shown in the Fig maybe either a full wave rectification circuit or a half wave rectification circuit based on the combination of diodes or a voltage double rectifier. Since a diode is being used in the rectifier, a p-n junction diode or a Schottky diode can be used. The Schottky diode has a threshold voltage which is lesser than that of a p-n junction diode. For example, if the diode is formed on a silicon substrate, a p-n diode may have a threshold voltage of approximately 0.065 volts while the threshold voltage of a Schottky diode is approximately 0.30 volts. Accordingly, the uses of Schottky diode instead of p-n diode will reduce the power consumption required for rectification and will effectively increase the electrical charge available for accumulation by the capacitor. When the electromotive force in the piezoelectricity generation section is small, a Schottky diode having a low rising voltage is more preferable. The bridge rectifier section provides rectification of the AC voltage generated by the piezoelectric section. By arranging the rectification section on a monolithic n-Si substrate, it is possible to form a very compact rectification section. A typical diode can rectify an alternating current-that is, it is able to block part of the current so that it will pass through the diode in only one direction. However, in blocking part of the current, the diode reduces the amount of electric power the current can provide. A full-wave rectifier is able to rectify an alternating current without blocking any part of it. The voltage between
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two points in an AC circuit regularly changes from positive to negative and back again. In the full-wave rectifier shown in Fig the positive and negative halves of the current are handled by different pairs of diodes. The output signal produced by the full-wave rectifier is a DC voltage, but it pulsates. To be useful, this signal must be smoothed out to produce a constant voltage at the output. A simple circuit for filtering the signal is one in which a capacitor is in parallel with the output. With this arrangement, the capacitor becomes charged as the voltage of the signal produced by the rectifier increases. As soon as the voltage begins to drop, the capacitor begins to discharge, maintaining the current in the output. This discharge continues until the increasing voltage of the next pulse again equals the voltage across the capacitor. The rectified voltage is stored into a storage capacitor as shown in Fig., which gets charged upto a pre-decided value, at which the switch closes and the capacitor discharges through the storage device or the battery. In this way the energy can be stored in the capacitor, and can be discharged when required .
Fig 6:Circuit diagram of the whole process.
The material used for the current application is a PZT with 1.5 Mpa lateral stress operating at 15Hz. The output power produced is 1.2W. The energy/power density is 6mW/cm3.The output voltage is 9V . The volume of the material used is 0.2cm3. This voltage can be used to produce the required amount of charge after being processed.
The design of the proposed energy conservation system for mobile phones and has been presented in this paper. The design helps to provide easy energy harvesting technique for mobile phones.The design presented here will be quite effective in providing an alternate means of power supply for the mentioned devices during emergency.The design is improved by implementing nanowires .The implementation of nanowires reduces the size of the system and also improves the efficiency on the other hand increases the cost of the system.Thus this design converts the vibrartion due to sound and key depression into electrical energy to recharge the mobile phones.
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