Laser Pulses Characterization with Pyroelectric Sensors

There are many industrial and medical applications of CO2 (λ=10.6 μm) and Nd:YAG (λ=1.06 μm) infrared lasers for which the quality of the process are tightly connected to the characteristic of the laser pulse. These two types of lasers deliver pulses with duration, repetition frequency and power that can be controlled by means of a programmable electronic control unit. An open-loop control generally optimize the process performances by availing of a laser system model. However, this method cannot control that during the operation the laser source and the optical delivering system could deteriorate; moreover the laser beam characteristics and laser pulse temporal envelope could change by several factors like power supply variations, optical beam misalignments, dirty deposits on mirrors, changes in laser efficiency and many others

Pyroelectric Materials for Uncooled Infrared Detectors: Processing, Properties, and Applications

Uncooled pyroelectric detectors find applications in diverse and wide areas such as industrial production; automotive; aerospace applications for satellite-borne ozone sensors assembled with an infrared spectrometer; health care; space exploration; imaging systems for ships, cars, and aircraft; and military and security surveillance systems. These detectors are the prime candidates for NASA s thermal infrared detector requirements. In this Technical Memorandum, the physical phenomena underlying the operation and advantages of pyroelectric infrared detectors is introduced. A list and applications of important ferroelectrics is given, which is a subclass of pyroelectrics. The basic concepts of processing of important pyroelectrics in various forms are described: single crystal growth, ceramic processing, polymer-composites preparation, and thin- and thick-film fabrications. The present status of materials and their characteristics and detectors figures-of-merit are presented in detail. In the end, the unique techniques demonstrated for improving/enhancing the performance of pyroelectric detectors are illustrated. Emphasis is placed on recent advances and emerging technologies such as thin-film array devices and novel single crystal sensors

Graphene/P(VDF-TrFE) Heterojunction Based Wearable Sensors with Integrated Piezoelectric Energy Harvester

Graphene, with its outstanding material properties, including high carrier mobility, atomically thin nature, and ability to tolerate mechanical deformation related strain up to 20% before breaking, make it very attractive for developing highly sensitive and conformable strain/pressure sensor for wearable electronics. Unfortunately, graphene by itself is not piezoresistive, so developing a strain sensor utilizing just graphene is challenging. Fortunately, graphene synthesized on Cu foil can be transferred to arbitrary substrates (preserving its high quality), including flexible polymer substrates, which will allow the overall flexibility and conformability of the sensing element, to be maintained. Furthermore, a graphene/polymer based sensor devices can be easily patterned into an array over dimensions reaching several feet, taking advantage of large area synthesis of graphene, which will make the ultimate sensor very inexpensive. If a piezo-electric polymer, such as P(VDF-TrFE), is chosen to form a heterojunction with graphene, it will strongly affect the carrier density in graphene, due to the fixed charge developing on its surface under strain or pressure. Taking advantage of the high carrier mobility in graphene, such a charge change can result in very high sensitivity to pressure and strain. Hence, these features, coupled with the flexible nature of the device and ease of fabrication, make it a very attractive candidate for use in the growing wearable technology market, especially biomedical applications and smart health monitoring system as well as virtual reality sensors. In this dissertation, various unique properties of graphene and P(VDF-TrFE), and their current applications and trends are discussed in chapter 1. Additionally, synthesis of graphene and P(VDF-TrFE) and their characterizations by various techniques are investigated in chapter 2. Based on piezoelectric property of P(VDF-TrFE), a highly flexible energy harvesters on PDMS as well as PET substrates have been developed and demonstrated their performances in chapter 3. As follow-up research, graphene/P(VDF-TrFE) heterojunction based wearable sensors with integrated piezoelectric energy harvester on flexible substrates have also been fabricated and demonstrated for practical wearable application in chapter 4. Finally, major findings and future directions of the project are discussed in chapter 5

Flexible electronics : materials and sensor fabrication

This dissertation demonstrates how to fabricate piezoelectric/pyroelectric thin films by using different printing techniques. These techniques could replace vacuum techniques for manufacturing piezoelectric/pyroelectric sensors. Ink-jet, screen and stencil printing techniques were developed to print these devices. This work outlines attempts to develop a solution processable conductive ink for ink-jet printing. It then details the printing of commercial conductive ink on flexible substrates employing the three printing methods. Raman spectroscopy and Fourier transform infrared spectroscopy, are both used to investigate the structure of the P(VDF-TrFE) films. Optical microscopy is used to investigate the thickness and uniformity of the deposited films. The formulation of P(VDF-TrFE) for printing is also described for the three printing methods. Piezoelectric accelerometers have been developed and demonstrated. The sensors are axial compression piezoelectric accelerometers which measure impacts in the direction perpendicular to the sensors themselves. When the sensors are moved downward the top electrode tends to move upward, inducing charge via the piezoelectric effect. The sensors were mounted on an electrodynamic shaker and tested with an input vibration up to 1.5 g s at 100 Hz. The test data show that the accelerometers track the frequency of the input vibration; the output increases with increasing input acceleration. A comparison of the three printing methods to fabricate sensors on flexible substrates with commercial conductive inks and formulated P(VDF-TrFE) ink specific to the print method with similar geometries produces the following conclusions: Excellent adhesion of the commercial silver ink for screen and stencil printing has been achieved. The stencil printed silver films are smoother and more uniform than the screen printed films. Adhesion of the commercial PEDOT/PSS ink-jettable was successful. However, smoothness and uniformity were issues that need to be resolved. Also, when the ink-jetted PDOT/PSS films were exposed to high temperatures the films tended to crack and adhesion was lost. Functional devices were fabricated with screen and stencil printing quickly. In a one day period, multiple sheets of functional devices were obtained with both printing methods. Ink-jet printing, on the other hand, required greater then twenty four hours to fabricate one sheet of sensors even when the sensor size was reduced. The cost of masks/cartridges was $0.75,$1.68 and $59 per layer for stencil, screen and ink-jet printing respectively. The ink-jet print system cartridges were manufactured for one time use, whereas the masks were reusable for both screen and stencil printing. The best stencil and screen printed accelerometers demonstrated a voltage sensitivity of 145 mV/g. It is believed that the performance of these sensors can be enhanced with an automated printing system that is equipped with optical vision and automated alignment systems. The successful development of printed devices demonstrates that these print methods will be beneficial to the future of flexible electronics

Design and fabrication of optical filters for long wavelength spectroscopy application

The design and fabrication of thin film Fabry-Perot interferometer (FPI) for long wavelength spectroscopy application is demonstrated. The system is designed to be integrated in a small portable spectrometer for the measurement of molecular absorption or emission as well as substance that has an infrared signature. A Fabry-Perot interferometer with dielectric mirrors was fabricated using fabrication process on a silicon substrate. The FPI was made of multi thin layers, deposited on silicon (Si) substrate, alternating between high and low refractive-index (n) layers. Si was used as a substrate due to the high precision of etching achievable using conventional VLSI fabrication techniques. Since the wavelength of interest was in the far infrared (5 to 15 micrometers), the layers were selected carefully to minimize the thickness required to meet the quarter-wave optical-thickness criteria for the interferometer. Another criterion that had to be met is the ratio of the refractive indices (n) between the layers. In this study, we have utilized germanium (Ge), which has n value of ~ 4 in the wavelength range of interest, and zinc oxide (ZnO), which has n value average of ~1.8 in the range of interest. Deposition of the layers was carried out using electron beam deposition for Ge and sputtering for ZnO. First the Si substrate was etched precisely to provide the gap needed for the wavelengths on interest and then the dielectric layers were deposited. For example, using Ge thickness of 0.576 µm, ZnO thickness of 1.22 µm, and a gap of 4.77 µm, we have demonstrated a filter transmitting a wavelength of 9.2 micrometers with a full width at half maximum of ~ 0.5 microns using one stack of Ge/ZnO layers. Simulations, using Freesnell software, were consistent with the experimental results. The tuning of the FPI with different cavity distances was demonstrated by measuring the transmission spectrum of the FPI. The transmission measurement was carried out using Fourier Transform Infrared Spectroscopy (FTIR) while the thickness of the layers was confirmed by scanning electron microscopy (SEM)

Modeling And Development Of A MEMS Device For Pyroelectric Energy Scavenging

As the world faces an energy crisis with depleting fossil fuel reserves, alternate energy sources are being researched ever more seriously. In addition to renewable energy sources, energy recycling and energy scavenging technologies are also gaining importance. Technologies are being developed to scavenge energy from ambient sources such as vibration, radio frequency and low grade waste heat, etc. Waste heat is the most common form of wasted energy and is the greatest potential source of energy scavenging. Pyroelectricity is the property of some materials to change the surface charge distribution with the change in temperature. These materials produce current as temperature varies in them and can be utilized to convert thermal energy to electrical energy. In this work a novel approach to vary temperature in pyroelectric material to convert energy has been investigated. Microelectromechanical Systems or MEMS is the new technology trend that takes advantage of unique physical properties at micro scale to create mechanical systems with electrical interface using available microelectronic fabrication techniques. MEMS can accomplish functionalities that are otherwise impossible or inefficient with macroscale technologies. The energy harvesting device modeled and developed for this work takes full benefit of MEMS technology to cycle temperature in an embedded pyroelectric material to convert thermal energy from low grade waste heat to electrical energy. Use of MEMS enables improved performance and efficiency and overcomes problems plaguing previous attempts at pyroelectric energy conversion. A Numerical model provides accurate prediction of MEMS performance and sets design criteria, while physics based analytical model simplifies design steps. A SPICE model of the MEMS device incorporates electrical conversion and enables electrical interfacing for current extraction and energy storage. Experimental results provide practical implementation steps towards of the modeled device. Under ideal condition the proposed device promises to generate energy density of 400 W/L

Design of a Solid-State Electrochemical Methane Sensor Based on Laser-Induced Graphene

Methane is a potent greenhouse gas with significant, yet largely unknown, emissions occurring across gas distribution networks and mining/extraction infrastructure. The development of low-cost, low-power electrochemical sensors could provide an inexpensive means to carry out distributed and easy sensing over the entire network and to identify leaks for rapid mitigation. In this work, a simple and cost-effective approach is proposed for developing electrochemical methane sensors which operate at room temperature with the highest reported sensitivity and response time. Laser-induced graphene (LIG) technology, which selectively carbonizes commercial polyimide films using a low-cost CO₂ laser cutting and patterning system is utilized. Interdigitated LIG electrodes are infiltrated with a dilute palladium (Pd) nanoparticle dispersion which distributes within and coats the high surface area LIG electrode. A pseudo-solid state electrolyte ionic liquid (IL)/polyvinylidene fluoride was painted onto the flexible cell resulting in a porous electrolyte structure which allows for rapid gas transport and improved three-phase contact between methane, IL and Pd. By subjecting the resulting sensors to methane in a gas flow cell, with off-gas analysis analyzed by Fourier transform infrared spectroscopy, the performance of the sensor over a wide range of operating conditions can be determined and the methane oxidation mechanism can be investigated. The optimized system provides a rapid response (less than 50 s) and high sensitivity (0.55 μA/ppm/cm²) enabling a ppb-level detection limit