Chapter 1Introduction1.1 General Low-power wireless distributed sensor networks are becoming attractive 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. As this can be a costly and time consuming procedure for networks with many nodes, a means of powering the devices indefinitely would be a more practical solution.Solar power provides a considerable amount of energy per area and volume, but unfortunately is limited to applications that are reliably sunlit. A promising alternative takes advantage of the energy in ambient vibrations and converts it to electrical power. This approach compares very favorably with batteries, providing equal or greater power per unit volume. [1]There are multiple techniques for converting vibrational energy to electrical energy. The most prevalent three are electrostatic, electromagnetic, and piezoelectric conversion. A majority of current research has been done on piezoelectric conversion due to the low complexity of its analysis and fabrication.

1.2 The Piezoelectric EffectThe piezoelectric effect, in essence, is the separation of charge within a material as a result of an applied strain. This charge separation effectively creates an electric field within the material and is known as the direct piezoelectric effect. The converse piezoelectric effect is the same process in reverse: the formation of stresses and strains in a material as a result of an applied electric field. The IEEE standard on piezoelectricity lists several different forms for the piezoelectric constitutive equations [2]. The form used here is known as the d-form, and the equations are as follows:

S = sET + dE

D = dT +T E

These equations, known as the coupled equations, reduce to the well-known stress-strain relationship at zero electric field, and the electric field and charge displacement relationship at zero stress.

Figure1.1: piezoelectric effect cause crystal materials like quartz to generate electric charge when the crystal material is compressed, twisted, or pulled. The reverse is also true (www.cosmic-energy.org).

1.3 Piezoelectric MaterialZnO is the most attractive since it exhibits the coupling of piezoelectric and semiconducting properties as well as the formation of a barrier at the electrode contact to draw higher power from the ambience. The ZnO has a non-Centro symmetric structure, which naturally exhibits a piezoelectric effect when subjected to a strain due to the displacement of Zn+2 cations and O2 anions in its tetrahedral configuration. Self-powered MEMS systems demand sustainable low voltage, high current characteristic inputs, and easy integration with the MEMS device, and the ability to be controlled with reliable transduction mechanisms. As miniaturized piezoelectric transducers have a high resonant frequency, they cannot be used for harvesting ambient vibrations. Low resonance frequency structures have macro dimensions that impose integration challenges on MEMS systems.

1.4 Photosensitive polymerSU-8 photosensitive cross-linking polymer that can be patterned for high aspect ratio structures. Since SU-8 is highly flexible and has a very low Youngs modulus (27 GPa), it is commonly used as a structural layer in MEMS. Though this polymer is inherently an electrical insulator with no additional functionalities. The chemical resistance of SU-8 is quite excellent however most chemical etches it, albeit at a very slow rate. This very slow rate creates diverging report for the etching resistance of SU8. Adhesion of SU-8 is usually good, but depends on the material. The adhesion is worst with gold (Au), average with silicon with native oxide, and best with silicon nitride (SiN) and ZnO. The adhesion of SU-8 seems to be affected by the chemical and SU-8 lift-off with immersion in KOH. [5].

1.5 Photoplastic piezoelectric NanocompositeCurrently, there has been a growing interest in scientific community on developing microelectromechanical-systems (MEMS)-based piezoelectric sensors and energy-harvesting devices. The most common piezoelectric materials have been the piezoceramics, with the leading candidates being lead zirconate titanate (PZT), barium titanate, strontium titanate, and quartz-based structures, which possess high strain response. There has also been an interest in developing alternative materials, and recently, piezoelectric ZnO has attracted a lot of attention. It finds applications in MEMS due to its unique combination of electrical, optical, and piezoelectric properties [6]. However, ZnO is a sensitive material for wet etching and treatment by temperature, acids, bases, and even water. Thus, for the successful fabrication of ZnO-based MEMS, a dry etching technique is needed [6], [5]. Moreover, most of the piezoelectric materials are ceramics, which are brittle and have low fracture toughness, posing challenges in fabrication. The mixing of nonmaterials into polymer matrices offers the promise of developing new polymer composite materials with extraordinary properties. Nevertheless, 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 (NPs) to agglomerate. Youngs modulus and hardness values are measured for ZnO NWs using the nanoindentation technique. Also, the same experiment was performed on a ZnO NW array after embedding these NWs inside the SU-8 matrix. The elastic modulus of vertical NWs as measured by nanoindentation comes in the range 7293 GPa, which is close to the ZnO thin-film data. After embedding these NWs inside an SU-8 matrix, the Youngs modulus values are seen to be in the range of 519 GPa.[7]

Figure 1.2: Youngs modulus measurements of Nanocomposite thin films with different ZnO concentration estimated using a nano-indentation method. [7]

1.7 problem statement To increase the sensitivity (i.e. output voltage) of the energy harvesting material by selecting proper material and geometric structure to get maximum power output.

1.8 Objectives and scope of the project To analyze the different structure for the material for which we get maximum voltage for the minimum deflection by using COMSOL Multiphysics. Analysis of the SU 8/ZnO Nanocomposite material for the lower resonant frequencies i.e. for the ambient vibrations by altering the structure of the material and adding the proof mass.

1.9 Limitations Fabrication will be difficult. Cannot used for large power production.

Chapter 2Literature reviewThis chapter addresses a literature review is presented in which previous works in fields of vibration energy harvesting are highlighted.2.1 S Roudy and P K Write (2004)The focus of this is to discuss the modeling, design, and optimization of a piezoelectric 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 optimization. Furthermore, an analytical model of the generator has been developed and validated. In addition to providing intuitive design insight, designs of 1 cm3 in size generated using the model have demonstrated a power output of 375w from a vibration source of 120Hz.2.2 M. Guizzetti et al., (2009)This paper shows the FEM simulations of piezoelectric cantilever as a microgenarator. Piezoelectric energy converters realized in a cantilever configuration are the most studied for this purpose, In order to improve the performances of the converter, the geometry has to be properly designed. In this context FEM simulations have been used in order optimize the piezoelectric mode. The electrical energy generated by the converted under an applied acceleration is computed, finding the optimal thickness for the piezoelectric layer. Different geometries were considered verifying that they do not affect the optimal thickness. Geometries with different dimensions were considered, verifying that the optimal thickness ratio tpzt/t substrate is independent from the converter dimensions, and it is influenced only by the mechanical properties of the piezoelectric layer and the substrate.2.3 Suyog N Jagtap et al., (2011)This paper gives investigation of design and modeling of piezoelectric cantilever for energy harvesting. MEMS based energy harvesting device is designed to convert mechanical vibration energy via piezoelectric effect. In order to improve the performance of the device, the geometry has been optimized by using moving mesh ALE model available in COMSOL Multiphysics. The proposed device is suitable for vibration energy harvesting and can be uses as potential micro generator.2.4 XiaotongGao (2009)Traditional piezoelectric Cantilever use piezoelectric and no piezoelectric layers of the same length. This paper shows the investigation of unequal piezoelectric and no piezoelectric lengths namely two section piezoelectric cantilever. For step-wise tip forces the results showed that longer non piezoelectric layer is preferred for generation a higher induced voltage while a longer piezoelectric layer reduced the induced voltage due to charge cancellation. With harmonic base vibrations, the results showed that there exists an optimal no piezoelectric piezoelectric length ratio at which output voltage, current, and power can be maximized. Theoretical analysis of two-section PCs was performed within the framework of beam theory. The results were in good agreement with experiments.2.5 K. Prashanthi et al., (2012)This letter reports a Photoplastic (SU-8) piezoelectric (ZnO) Nanocomposite route for realization of simple and low cost piezoelectric microelectromechanical systems (MEMS). Integrating the ZnO nanoparticles into a photosensitive SU-8 polymer matrix not only retains the highly desired piezoelectric properties of ZnO but also combines the photopatternability and the optical transparency of the SU-8 polymer. These two aspects, therefore, lead to exciting MEMS applications with simple photolithography-based microfabrication. This approach opens up many new applications in the field of both sensor and energy harvesting.2.6 Adel Campo and CGreiner (2007)SU-8 has become the favorite photoresist for high-aspect-ratio (HAR) and three-dimensional (3D) lithographic patterning due to its excellent coating, planarization and processing properties as well as its mechanical and chemical stability. Simple SU-8 structures (pillars and walls) with aspect ratio above 100 but a maximum lateral resolution of 8m have been reported after UV exposure. Better lateral resolution (