Dynamic response analysis of pyroelectric sensitive element for thermal imaging

Temperature distributions under periodic thermal excitations and the responsivity of a pyroelectric device consisting of a cover layer, infrared absorber, metal contact, sensitive pyroelectric element, interconnecting column, and bulk silicon are found. Some results of numerical thermal modeling and analysis of exact expressions for a few extreme cases are presented. Pyroelectric responses of real structures are compared with the response of a single pyroelectric element in air as a limiting case of maximum sensitivity. The analytical approximations and numerical simulation show that the frequency response of the multilayered structure consists of different parts with simple frequency dependencies. In the region of high frequencies of light modulation, the responsivity is proportional to /spl omega//sup -1/, at low frequencies /spl sim/ /spl omega//sup -0.5/, and, in the region of intermediate frequencies, the voltage responsivity is independent of frequency.     

Surface-micromachined pyroelectric infrared imaging array withvertically integrated signal processing circuitry

Surface-micromachining techniques have been used in the fabrication of a 64×64 element PbTiO₃ pyroelectric infrared imager. Polysilicon microbridges of 1.2 μm-thickness have been formed 0.8 μm above the surface of a silicon wafer. Each of the 4096 polysilicon microbridges measures 50×50 μm² and forms a low thermal mass support for a 30×30 μm² PbTiO₃ pyroelectric capacitor with a thickness of 0.36 μm. The air-bridge formed reduces the thermal conduction path between the detector element and substrate. An NMOS preamplifier cell is located directly beneath each microbridge element. The measured blackbody voltage responsivity at 30 Hz is 1.2×10⁴ V/W. The corresponding measured normalized detectivity (unamplified) D* is 2×10⁸ cm-Hz^1/2W at 30 Hz. The test chip fabricated measures 1×1 cm² and contains more than ten thousand transistors and 4096 micromechanical structures with integrated ferroelectric microsensors. The technique of stacking of microsensors and integrated circuits represents a new approach for achieving high-density and high-performance integrated pyroelectric microsensors through minimization of circuit to sensor interconnection with extremely small thermal crosstalk     

Frequency response of multilayer pyroelectric sensors

The pyroelectric response of the multilayer system consisting of the front surroundings, the pyroelectric element, the intermediate layer, and the back surroundings is calculated. The general expression and simple relations are obtained between main geometric and thermophysical parameters of the system under consideration and characteristics of the pyroelectric response in a large variety of configurations of pyroelectric sensors, such as detectors with the pyroelectric element in a gas, detectors without air gap as well as detectors with air gap between the element and the substrate, and detectors in immersion systems, etc. In particular, it is shown that, for the detectors with air gap, the wide flat part of the frequency response appears with any properties of substrate if the double thermal resistance of the air gap exceed that of the pyroelectric element, whereas for the detectors without air gap it appears only if the substrate has relatively large complex thermal conductivity.     

热释电红外探测材料的研究进展

热释电红外探测材料主要分为热电堆型、热敏电阻型和热释电型，本文综述了热释电红外热探测材料的种类、性能和研究进展。     

多孔硅基热敏薄膜的制备与性能

对MEMS用具有绝热性能的多孔硅基底上沉积的热敏感薄膜进行了研究．首先用电化学方法制备多孔硅,分别在多孔硅基底和硅基底上通过溅射镀膜方法沉积氧化钒、Cu、Au热敏薄膜,测试多孔硅基底和硅基底上的氧化钒及金属薄膜电阻的热敏特性．结果表明,在多孔硅基底表面沉积的热敏薄膜具有与硅基表面热敏薄膜同样的热敏特性且表现出更高的灵敏度;此外,对沉积在不同制备条件得到的多孔硅上的氧化钒薄膜电阻热敏特性进行比较,发现随着孔隙率和厚度的增加,多孔硅的绝热性能提高,其上沉积的氧化钒薄膜电阻热敏特性增强．     

Optimization and improvement of thermal energy harvesting by using pyroelectric materials

We deal with the thermal energy which is one of the ambient energy sources surely exploitable, but it has not been much interest as the mechanical energy. In this paper, our aim is to use thermal energy and show that it’s an important source for producing the electrical energy through pyroelectric effect which is the property of some dielectric materials to show a spontaneous electrical polarization versus temperature. In this context, we present a concept to harvest a thermal energy using infrared rays and pyroelectric effect.The pyroelectric material used in this work can generate an electrical voltage when it subjected to a temperature change which will be ensured by the use of infrared lamp. Our experimental results show that the electrical voltage, current and harvested power increased significantly when increasing the area of the pyroelectric element. The experimental results show also that with this simple concept we harvested a heavy amount value of power which will certainly be useful in an extensive range of applications, including sensors and infrared detection. These results shed light on the thermoelectric energy conversion by Ceramic lead zirconate titanate (PZT) buzzer having the pyroelectric property.     

PCLT/P (VDF-TrFE) nanocomposite pyroelectric sensors

Thin composite films consisting of 12 vol% of nanosized lanthanum and calcium-modified lead titanate (PCLT) powder embedded in a vinylidene fluoride-trifluoroethylene copolymer [P(VDF-TrFE)] matrix were deposited on silicon (Si) substrates to form pyroelectric sensors with three different configurations. The influences of a thermal buffer layer and back etching of the silicon substrate on the current and voltage responsivities of the sensors were investigated theoretically and experimentally. The specific detectivity of the sensors was also calculated from the measured voltage responsivity and noise     

Thermal behavior of freestanding microstructures fabricated by silicon frontside processing using porous silicon as sacrificial layer

Freestanding microstructures are essential elements in thermal and mechanical microsensors. In this paper, microbridges were fabricated by silicon surface micromachining using porous silicon as sacrificial layer. Two different approaches were considered. In first approach, n-Si was used as anodization masking material and n-Si/SiO/sub 2/ as microstructure material. In the second approach, silicon nitride and SiO/sub 2/ /Si/sub 3/ N/sub 4/ bilayer has constituted masking and microstructure materials, respectively. In order to characterize their thermal behavior, platinum heating elements were defined on developed microbridges. Microstructures fabrication process was described, insisting specially on silicon anodization step. The process parameters (HF-electrolyte concentration, current density, and process duration) were established for both approaches. Thermal behavior of developed microbridges was studied in relation to anodization masking materials and freestanding microstructure materials. Microbridge temperature versus applied power to heating element was analyzed. Additionally, entire sample thermal behavior and microbridge dynamic thermal behavior was characterized. The obtained results suggest developing a third approach where n-Si will be used as masking material and SiO/sub 2/ / Si/sub 3/ N/sub 4/ bilayer as freestanding microstructure material.     

Adaptive 2D finite element simulation of metal forming processes

The development and integration of available current methods and the development of new methods for an adaptive finite element analysis of metal forming processes are presented. The analysis includes large-strain, elastic–plastic, and thermal effects. Many numerical methods such as mesh generation, simulation of the contact between the workpiece and tool and die, and optimization of the finite element mesh are integrated and incorporated. In addition, an algorithm is developed which can detect certain severely distorted elements where the area of integration is approaching zero. The advantage of correcting these regions of locally distorted elements is demonstrated. These numerical methods are implemented in a finite element program developed for simulating metal forming processes, with the emphasis on automating the analysis. Examples include an axisymmetric stress simulation of a coldheading process, a plane strain simulation of an extrusion process and a plane strain simulation of orthogonal metal cutting, all with noticeable thermal effects. The orthogonal cutting forces and feed forces calculated are compared with two sets of experimental data, with good agreement.     

Ultrathin tunnel insulator films on silicon for electrochemiluminescence studies

Thin insulating films on conductive substrates have been used for tunnel emission of hot electrons into aqueous solution. The hydrated electrons thus formed induce electrochemiluminescence (ECL) in various luminophores, e.g. rare-earth metal chelates, which can be detected in sub-nanomolar concentrations. The luminophores can be used as labels for antibodies, enabling simple and highly sensitive immunoassays. This paper compares thermal silicon dioxide, low pressure chemical vapor deposited silicon nitride, plasma enhanced chemical vapor deposited silicon dioxide and nitride, atomic layer deposited alumina, and liquid phase deposited silicon dioxide for electrodes in ECL applications.     

Metal-decorated multi-wall carbon nanotubes for low temperature gas sensing

We investigate the possibility of using, as a gas sensitive material, multi-walled carbon nanotubes (MWCNT) covered with gold or silver nanoclusters deposited by thermal evaporation. Metal-decorated MWCNT were dispersed in an organic vehicle, micro-deposited onto silicon micro-machined sensor substrates and subsequently annealed to remove the organic vehicle. The resulting sensors are shown to be sensitive to NO₂ when operated at room temperature and significantly more selective than sensors based on MWCNT without metal nanoclusters attached to their surface.     

Deposited dielectrics on metal thin films using silicon and glass substrates for hot electron-induced electrochemiluminescence

Electrochemiluminescence by tunnel emission of hot electrons into aqueous solution is a sensitive method for detection of luminophores e.g. rare-earth chelates, which may be used as labels in bioassays. Electrons are injected into solution from an insulating film-coated working electrode, working against a platinum counter electrode. Conductive silicon electrodes with various tunnel dielectric materials e.g. thermal oxide have been used in previous work. In this paper we explore the use of metal thin film electrodes on silicon and glass substrates, using tunneling dielectrics of aluminum oxide and silicon dioxide made by the low-temperature processes of atomic layer deposition or plasma-enhanced chemical vapor deposition.     

Polarization processes in pyroelectric Langmuir-Blodgett films

We report on an investigation into the origins of the pyroelectric effect in some alternate layer Langmuir-Blodgett (LB) films. The pyroelectric coefficient is measured as a function of temperature and of the thermal expansion coefficient of the substrate on which the film is deposited; these experiments enable a partial separation of the primary and secondary pyroelectric effects. The results indicate that thermal expansion of the substrate tends to reduce the total pyroelectric coefficient through a piezoelectrically induced effect. At low temperatures, the film behaves like a “free crystal”, and the pyroelectric coefficient shows a power law dependence on temperature. However, at temperatures above 240 K, the substrate- induced effect becomes dominant. The results are used to deduce methods by which the pyroelectric response of the LB film devices might be improved.     

Evaluation of a pyroelectric detector with a carbon multiwalled nanotube black coating in the infrared

The performance of a pyroelectric detector with a carbon multiwalled nanotube coating was evaluated in the 0.9-14 microm wavelength range. The relative spectral responsivity of this detector was shown to be flat over most of the wavelength range examined, and the spectral flatness was shown to be comparable to the best infrared black coatings currently available. This finding is promising because black coatings with spectrally flat absorbance profiles are usually associated with the highest absorbance values. The performance of the detector (in terms of noise equivalent power and specific detectivity) was limited by the very thick (250 microm thick) LiNbO3 pyroelectric crystal onto which the coating was deposited. The responsivity of this detector was shown to be linear in the 0.06-2.8 mW radiant power range, and its spatial uniformity was comparable to that of other pyroelectric detectors that use different types of black coating. The carbon nanotube coatings were reported to be much more durable than other infrared black coatings, such as metal blacks, that are commonly used to coat thermal detectors in the infrared. This, in combination with their excellent spectral flatness, suggests that carbon nanotube coatings appear extremely promising for thermal detection applications in the infrared.     

Experimental study on effect of flux composition on element transfer during submerged arc welding

The effects of flux composition on transfer of the elements have been studied through developed agglomerated fluxes on mild steel plates. The elements transferred to the welds have been shown in terms of a delta (?) quantity, which may be positive, negative or zero depending upon the composition of flux, wire and base plate. Carbon and manganese contents both show negative ? quantity for most of the welds, indicating that both have been transferred from the weld metal to the slag. The results of this study show that for most of the welds, desulphurization and removal of phosphorus have been reported. The amount of element transferred is different for different welds depending upon the flux composition, dilution and slag metal reactions. Response surface methodology has been used for developing models for element transfer. The suggested model has been given for sulphur transfer to the weld. In this study the transfers of carbon, manganese, sulphur, phosphorus, silicon and nickel have been studied.     

Pyroelectric behaviour of laser-evaporated poly(vinyl fluoride) films

Laser-evaporated poly(vinyl fluoride) films were prepared with a view to their use as pyroelectric sensing elements. A high power (60 W) CO₂ laser was employed to prepare these films (0.1–1 microm thick) in a vacuum of better than 1 × 10^?5Torr. The films were characterized by determining their structure and molecular weight using IR spectroscopy and mass spectrometry. The pyroelectric currents developed in these films by poling them in electric fields of strength (1–5) × 10⁵ V cm^?1 and at poling temperatures of 303–450 K were measured. Various metal electrodes were used and the effect of the electrode material on the pyroelectric activity was also studied. The results are used to interpret the origin of the pyroelectricity in these films. The experimental details of the preparation of the films, their characterization and the pyroelectric current measurements are discussed.     

Calculating the response of sensitive elements based on wave delay devices

A method of calculating the response of sensitive elements based on wave delay devices is described using as an example a gauge for measuring the diameter of holes in metal objects. The efficiency of a sensitive element consisting a cylindrical helix with a longitudinally conducting shield is demonstrated. Translated from Izmeritel'naya Tekhnika, No. 9, pp. 46–50, September, 1996.     

Photothermal gas analyzer for simultaneous measurements of thermal diffusivity and thermal effusivity

In this paper, we describe a new, simple, and fast photothermal method for simultaneous measurements of two important gas thermal properties: thermal diffusivity and thermal effusivity. The method consists essentially in combining a photoacoustic cell and a thermal wave pyroelectric cell enclosed in a single compact gas analyzer. The photoacoustic cell is kept filled with synthetic air and sealed. The pyroelectric cell is also filled with synthetic air, and after some warm up time, the synthetic air is exchanged to the gas of interest. It is shown that the analysis of the transient and saturation signals of both photoacoustic and pyroelectric cells is capable of measuring the thermal properties with an accuracy of 3%. This particular capability of performing simultaneously the measurements of thermal diffusivity and thermal effusivity allows us to carry on the complete characterization of the thermal properties of gases.     

Stability of high temperature chemical vapor deposited silicon based structures on metals for solar conversion

Highly crystallized silicon layers were grown on metal sheets at high temperature (950 degrees C) by thermal CVD from silane. An intermediate buffer layer was mandatory to prevent interdiffusion and silicide formation but also to compensate lattice parameters and thermal expansion coefficients mismatches between metal and silicon and ideally transfer some crystalline properties (grain size, texture) from the substrate to the silicon layer. After a thermodynamic study, aluminum nitride or titanium nitride diffusion barrier layers were selected and processed by CVD. The structure and the interfaces stabilities of these silicon/nitride/metal stacks were studied by field effect gun scanning and transmission electron microscopy, X-ray diffraction, Raman and energy dispersive X-ray spectroscopy. As a result, TiN deposited by CVD appears to be an efficient material as a buffer layer between steel and silicon.