H. Neff

Modeling and performance of vanadium–oxide transition edge microbolometers

The performance of a VO2 thin-film microbolometer has been investigated. The device is operated within 35°C<T<60°C, in the hysteretic metal-insulator transition region. An algebraic hysteresis model has been used to model the resistance-temperature characteristic of the sensor. It accurately describes the resistance versus temperature characteristics of the material. Employing this model, and in conjunction with established bolometer theory, the responsivity of a VO2 film is calculated and compared with experimental data. Superior performance of the device is achievable under conditions of single pulse incident radiation where the operating point remains on the major hysteresis loop. This results in a pronounced responsivity peak within the center of the metal-insulator transition. Continuous periodic excitation, in contrast, leads to a steadily decreasing and much lower sensitivity at higher temperature, due to the formation of minor hysteresis loops and the loop accommodation process.

Optimization strategy for a small-scale reverse osmosis water desalination system based on solar energy

Abstract An optimization strategy for the design and operation of a small-scale solar-powered reverse osmosis desalination system is presented. It has been analyzed and optimized with regard to power needs and energy consumption. Both quantities scale linearly with the concentration of the total dissolved salt (TDS) concentration in the feed solution. The desalination of brackish water at a TDS value of 3000 mg/L requires energy of approximately 0.84 kWh/m 3 . For seawater at a TDS value of 35,000 mg/L, this value increases to 7.0 kWh/m 3 . The selected type of membrane, the membrane configuration, the recovery rate, the area of membrane and the efficiency of the high pressure unit crucially affect energy consumption. The minimum desalination cost has been estimated for a small-scale system. It approximately scales linearly with the TDS value from US

Sensitive high‐Tc transition edge bolometer on a micromachined silicon membrane

3.3/m 3 for TDS of 3000 mg/L to US

Thermal dynamics of VO2 films within the metal–insulator transition: Evidence for chaos near percolation threshold

7.8/m 3 for seawater. Preliminary experimental results are presented and compared with the model calculations.

A hysteresis model for a vanadium dioxide transition-edge microbolometer

Superconducting transition edge bolometers on micromachined silicon membranes have been fabricated. The optical response is 580 V/W at a time constant of 0.4 ms. The detectivity D* is 3.8×109 (cm Hz1/2 W−1) at a temperature of 84.5 K and within the frequency regime 100<f<300 Hz. This is one of the fastest composite type bolometers ever reported. Upon thermal optimization, this type of detector should be competitive with state‐of‐the‐art quantum detectors.

Modeling of the hysteretic metal-insulator transition in a vanadium dioxide infrared detector

The thermal dynamics of thin vanadium dioxide films at the martensitic metal–insulator phase transition has been evaluated experimentally by thermal excitation spectroscopy. Over the transition region, the device becomes highly nonlinear, and its bolometric performance is affected. At low thermal cycling rates for a temperature around the percolation threshold, the device stochastically switches into an unusual pattern. The originally smooth and monotonic shape of the R(T) curve for minor loops suddenly becomes unstable and unpredictable. By direct observation of at least two strange attractors, the phenomenon clearly has been identified as chaotic. Bolometric performance of VO2 based devices in the transition region may suffer strong degradation for low thermal cycling rates. In this region, sensor responsivity for periodic thermal excitation is significantly reduced. Resistance noise is 1/f-type and self-generated oscillations were observed at frequency <10−2 Hz.

Modeling a magnetostrictive transducer using genetic algorithm

This paper presents the adaptation of the Preisach model, originally developed for magnetic hysteresis, to describe mathematically the hysteresis in the resistance-temperature characteristics of vanadium-dioxide (VO/sub 2/) thin film radiation sensors. The necessary and sufficient conditions for the applicability of the Preisach model to a VO/sub 2/ film sensor are experimentally verified. Experimentally measured characteristics are compared with those given by the model for minor and major loops.

Limiting loop proximity hysteresis model

The vanadium dioxide (VO2) thin film, usually employed as an infrared detector, exhibits hysteresis in its resistance-temperature characteristic. Considering a polycrystalline VO2 thin film as a composite medium, containing semiconducting and metallic microcrystals, the well- known effective-medium approximation theory is employed to relate the volume fraction of the semiconducting microcrystals to the effective film resistance. A phenomenological model is first proposed for describing the hysteretic dependence of volume fraction on temperature. From this, a model for hysteresis in the resistance-temperature characteristic is then derived, and a procedure for estimating the model parameters is outlined. The model reproduces the more important hysteretic character- istics such as the major, minor, and nested loops, in good agreement with the experimental characteristics.

Stability conditions, nonlinear dynamics, and thermal runaway in microbolometers

Abstract This work reports on the applicability of the genetic algorithm (GA) to the problem of parameter determination of magnetostrictive transducers. A combination of the Jiles–Atherton hysteresis model with a quadratic moment rotation model is simulated using known parameters of a sensor. The simulated sensor data are then used as input data for the GA parameter calculation method. Taking the previously known parameters, the accuracy of the GA parameter calculation method can be evaluated.

Figures of merit and optimization of a VO 2 microbolometer with strong electrothermal feedback

This paper introduces a simple algebraic limiting loop proximity ((L/sup 2/P)) model to describe magnetic hysteresis. With only four parameters, it has low computational cost and reduced mathematical complexity, thus permitting a fast numerical implementation and simple parameter estimation procedure. The paper presents simulation results and discusses the model performance in terms of its capacity to represent common nonlinearities of the magnetic hysteresis phenomenon. The model avoids the heavy computational burden that results when magnetic hysteresis is included in computer-aided tools for electromagnetic devices and circuit analysis - for example, when hysteresis is combined with the finite-element method. The model also avoids the need to use repeated numerical approximation steps to adjust traditional models to experimental data.