| dc.description.abstract | Recent advancements in the field of wireless sensor networks have resulted in increasing demand for self-powering techniques that reduces the dependence on batteries. In order to address this problem, there has been significant effort on generating small electrical power locally by harvesting energy from freely available environmental sources such as mechanical vibrations, wind, and stray magnetic field. Further, there is need for inventing new sensing techniques to reduce the overall power consumption. The road for reaching the destination of self-powered sensor networks requires cooperative progress in reduction in sensor power consumption by developing new sensing mechanisms and local generation of power by developing high efficiency energy harvesters.The sensing techniques investigated in this thesis utilize piezoelectric materials, piezoresistive materials, and magnetoelectric composites. Prior studies on structural health monitoring have demonstrated the use and promise of piezoelectric sensors. In this research, impedance spectroscopy based sensing technique was investigated with respect to two parameters (i) effect of the piezoelectric vibration mode on damage index metric, and (ii) selection of frequency band through manipulation of the electrode size and shape. These results were then used to determine sensor geometry and dimensions for detecting surface defects, fatigue and corrosion. Based upon these results, power requirement for structural health monitoring sensors was determined. Next, piezoelectric materials were coupled with magnetostrictive material for novel magnetic field gradient sensing. Novel ME composite materials were synthesized to achieve passive sensing of magnetic field. ME particulate composites in system [Pb(Zr0.52Ti0.48)O3] - 0.2[Pb(Zn1/3Nb2/3)O3] (PZT) - NiFe2O4 (NFO) were synthesized using (i) core-shell particles followed by high pressure compaction sintering, (ii) organic slurry in a sacrificial matrix followed by high temperature annealing, and (iii) co-firing. A pressure assisted co-firing technique was developed to achieve 3D pillar composite structure.The ceramic - ceramic (CC) gradiometer resembles in functionality a magnetoelectric transformer. It measures the magnetic field gradient and sensitivity with respect to a reference value. The CC gradiometer designed in this study was based upon the magnetoelectric (ME) composites and utilizes the ring-dot piezoelectric transformer structure working near resonance as the basis. This study investigated the gradiometer design and characterized the performance of gradiometer based upon Terfenol-D - PZT composites. The generated magnetic field due to converse ME effect interacts with the external applied magnetic field producing flux gradient which is detected through the frequency shift and output voltage change of the gradiometer structure. The performed measurements of voltage dependence on applied magnetic field clearly illustrate that the proposed design has extremely high level of magnetic field sensitivity and it can be used for measurement of magnetic field gradient.Based upon these results, next a metal - ceramic gradiometer consisting of PZT and nickel was designed and characterized. In this thesis, two different designs of gradiometer with nickel and PZT laminate composites were fabricated. Nickel was chosen over other materials considering its co-firing ability with PZT. It can give a better control over dimensional parameters of the gradiometer sample and further size reduction is possible with tape casting technique. Detailed theoretical analysis was conducted in order to understand the experimental results. The fabricated metal - ceramic gradiometer showed high sensitivity for a wide range of frequency (247 - 251 kHz for gradiometer design A and 234 - 239 kHz for gradiometer design B) and can be utilized for broadband magnetic field gradient sensing. Power analysis was conducted for the magnetic field gradient sensors based on the performance curves.In order to significantly reduce the power consumption of health monitoring and magnetic field sensors, bottom - up design of structural health monitoring and magnetic field sensors was investigated. A MWCNT/SiCN nanotube template was developed that exhibits piezoresistive effect. Next, a novel nanotube morphology "nanoNecklace" was synthesized that consists of BaTiO3 (BTO) nanoparticles decorated along the surface of SiCN. Monolayer coating of SiCN on MWCNT serves two purposes: (i) modifies the surface wetting characteristics, and (ii) enhances the piezoresistive effect. Investigation of the mechanisms that provide periodic arrangement of BTO on nanotube surface was conducted using HRTEM and contact angle measurements. Next, we tried to modify the surface wetting characteristics of MWCNTs in order to get a full coating of BTO nanoparticles. We synthesized acid treated functionalized MWCNTs and coated them with BTO. Acid concentration, coating temperature and coating time were optimized in order to obtain fully coated MWCNTs with BTO. Microstructural characterization was performed using various techniques such as Scanning Electron Microscopy (SEM), Transmission Electron Microscopy (TEM), and Energy Dispersive Spectrum (EDS). Coating thickness of 5-15 nm was confirmed with High Resolution - TEM imaging. X-ray Photoelectron Spectroscopy was performed to confirm perovskite phase of BTO. The average BTO nanoparticle size was found to be 4-8 nm. Contact angle measurements were carried out and were correlated with the percentage of coating on the surfaces of nanotubes. The SiCN/MWCNT approach was further extended to fabricate magnetoelectric nanowire based sensors designs. In this approach a SiCN-NT template was coated with BTO and CoFe2O4 (CFO) nanoparticles. Microstructural studies indicated the presence of piezoelectric (BTO) as well as magnetic (CFO) material on the nanotube surface. In order to power the sensors from mechanical vibrations, we investigated two different techniques, (i) piezoelectric and (ii) inductive. For piezoelectric energy harvesting, high energy density piezoelectric compositions corresponding to 0.9Pb(Zr0.56Ti0.44)O3 - 0.1Pb[(Zn0.8/3Ni0.2/3)Nb2/3]O3 + 2 mol% MnO2 (PZTZNN) and 0.8[Pb(Zr0.52Ti0.48)O3]-0.2[Pb(Zn1/3Nb2/3)O3] (PZTPZN) were synthesized by conventional ceramic processing technique using different sintering profiles. Plates of the sintered samples were used to fabricate the piezoelectric bimorphs with optimized dimensions to exhibit resonance in the loaded condition in the range of ~200 Hz. An analytical model for energy harvesting from bimorph transducer was developed which was confirmed by experimental measurements. The results show that power density of bimorph transducer can be enhanced by increasing the magnitude of product (d.g), where d is the piezoelectric strain constant and g is the piezoelectric voltage constant.Under inductive energy harvesting, we designed and fabricated a small scale harvester that was integrated inside a pen commonly carried by humans to harvest vibration energy. Inductive energy harvesting was selected in order to achieve high power at lower frequencies. The prototype cylindrical harvester was found to generate 3mW at 5 Hz and 1mW at 3.5 Hz operating under displacement amplitude of 16mm (corresponding to an acceleration of approximately 1.14 grms at 5Hz and 0.56 grms at 3.5 Hz, respectively). A comprehensive mathematical modeling and simulations were performed in order to optimize the performance of the vibration energy harvester. The integrated cylindrical harvester prototype was found to generate continuous power of 0.46-0.66mW under normal human actions such as jogging and jumping which is enough for a small scale sensor. | en_US |
