Gerhard Fischerauer

SOUND VELOCITY OF ALXGA1-XN THIN FILMS OBTAINED BY SURFACE ACOUSTIC-WAVE MEASUREMENTS

To determine the sound velocity in wurtzite AlxGa1−xN, we have used surface acoustic-wave (SAW) delay lines on AlxGa1−xN/c-Al2O3. AlxGa1−xN films with compositions from x=0 to x=1 were grown by plasma-induced molecular beam epitaxy. Starting from published data, we fine tuned the values of the elastic moduli used in numerical calculations such that the simulated and measured dispersion of the SAW were in good agreement. Based on these values, the surface and bulk acoustic-wave velocities of single-crystal AlxGa1−xN were determined as functions of the composition. The resulting SAW velocities ranged from 3700 to 5760 m/s for GaN and AlN, respectively.

Surface acoustic wave sensors for high-temperature applications

Results of investigations of surface acoustic wave (SAW) devices working at high temperatures are presented in this paper. The considered substrate materials are lithium niobate (LiNbO/sub 3/), quartz and langasite (La/sub 3/Ga/sub 5/SiO/sub 14/). In addition, electrode materials and assembly, interconnect and packaging strategies are discussed or examined. Delay lines on LiNbO/sub 3/ had low SAW attenuation up to approx. 550/spl deg/C. The frequency response disappeared completely at about 800/spl deg/C. Delay lines on quartz had low SAW attenuation up to 500/spl deg/C, a temperature being significantly below the phase transition from /spl alpha/- to /spl beta/-quartz at 573/spl deg/C. Delay lines on La/sub 3/Ga/sub 5/SiO/sub 14/ withstand extreme temperatures: a SAW response was observed up to destruction of the packages used in these tests, however, resulted in a degradation of the device performance starting at about 600/spl deg/C. An identification (ID) tag on La/sub 3/Ga/sub 5/SiO/sub 14/, could be used as a wireless passive thermometer for measuring high temperatures in industrial process control applications.

State of the art in wireless sensing with surface acoustic waves

Surface acoustic wave (SAW) devices can be turned into novel identification and sensor elements (transponders) that do not need any power supply and may be interrogated wirelessly. Such a transponder picks up an electromagnetic request signal and stores it until all echoes caused by multipath propagation have died away. Then, a characteristic response signal is beamed back to the interrogator unit. In radio-link sensors, a physical or chemical quantity influences the propagation properties of the SAW and consequently changes the response pattern of the device. This contribution surveys the operating principle of such sensors and their state-of-the-art performance. The discussion is supported by illustrative examples such as temperature sensors and sensors for mechatronic applications.

SAW devices for consumer communication applications

An overview of surface acoustic wave (SAW) filter techniques available for different applications is given. Techniques for TV IF applications are outlined, and typical structures are presented. This is followed by a discussion of applications for SAW resonators. Low-loss devices for mobile communication systems and pager applications are examined. Tapped delay lines (matched filters) and convolvers for code-division multiaccess (CDMA) systems are also covered. Although simulation procedures are not considered, for many devices the theoretical frequency response is presented along with the measurement curve.<<ETX>>

SAW sensors for harsh environments

Surface acoustic wave (SAW) sensors are rugged components made on highly stable substrate materials. In addi- tion, by their operating principle they lend themselves to wireless readout by radio signals. For these reasons, they are a first choice for sensing in harsh environments. A review is given on SAW device design, instrumentation of sensor systems, and on the physical interactions underlying sensing mechanisms. Recent progress in chemical as well as physical sensing toward various applications, e.g., in pollution control, biomedicine, and industry, is highlighted. Index Terms—Chemosensors, coatings, detection limit, high temperature, instrumentation, resolution, selectivity, sensing mechanisms, sensitivity, sensor materials, surface acoustic waves, torque.

SAW devices : sensitivity enhancement in going from 80 MHz to 1 GHz

Abstract SAW-devices with the highest resonance frequency (1 GHz) ever reported in chemical sensor applications have been tested. The sensor response increases in a parabolic manner with the resonance frequency, as is shown, in going from 80 MHz to 1 GHz, whereas the noise level only increases approximately linearly. If a linked cyclodextrin in a height of 60 nm is used as a sensitive coating, a sensor response of 61 kHz/1000 ppm of toluene is obtained. This makes the detection of aromatic hydrocarbons, such as toluene or xylene, down to 1 ppm easily possible.

Sensing the soot load in automotive diesel particulate filters by microwave methods

Modern vehicles with diesel engines need to be equipped with particulate filters (DPFs) to meet todays and tomorrows stringent emission regulations. Such filters must be regenerated on a regular basis to burn off the soot adsorbed in the course of time. As the regeneration processes consume fuel, they must be kept to the bare minimum which requires a detailed knowledge of the actual soot load in the filter. We have investigated if the soot load can be determined in situ by the cavity perturbation method at operating frequencies in the low GHz range. We will show that, indeed, current microwave technology as used, for instance, in cellular phones is capable of detecting the soot load in a diesel particulate filter. Experimentally observed sensitivities of cavity resonance frequencies to soot load (adsorbed mass) were as high as 3 MHz g−1. This contribution reports on the measurement approach, experimental results obtained in industrial dynamometer test benches and the conclusions to be drawn from the results.

Catalyst State Observation via the Perturbation of a Microwave Cavity Resonator

The state of the three-way catalyst (TWC) of a gasoline engine is commonly estimated by measuring the air-to-fuel ratio in the exhaust gas. This indirect method suffers from several disadvantages that one could avoid by directly observing the catalyst state and in particular its oxygen loading. We have investigated an RF approach to this direct measurement problem which is based on the fact that the TWC and its stainless steel canning form a filled cavity resonator. It will be shown that the resonator is perturbed when the TWC changes its conductivity as a function of oxygen loading and that the characteristics of the resonator S-parameters mirror the oxidation/reduction state of the catalyst. Index Terms – Three-way catalyst, TWC, state observation, in situ, cavity resonator, perturbation

Maxwell's grid equations

A numerical approach for the solution of Maxwells equation is presented. Based on a finite difference Yee lattice the method transforms each of the four Maxwell equations into an equivalent matrix expression that can be subsequently treated by matrix mathematics and suitable numerical methods for solving matrix problems. The algorithm, although derived from integral equations, can be considered to be a special case of finite difference formalisms

Determination of the NOx Loading of an Automotive Lean NOx Trap by Directly Monitoring the Electrical Properties of the Catalyst Material Itself

Recently, it has been shown that the degree of loading of several types of automotive exhaust aftertreatment devices can be directly monitored in situ and in a contactless way by a microwave-based method. The goal of this study was to clarify whether this method can also be applied to NOx storage and reduction catalysts (lean NOx traps) in order to obtain further knowledge about the reactions occurring in the catalyst and to compare the results with those obtained by wirebound NOx loading sensors. It is shown that both methods are able to detect the different catalyst loading states. However, the sensitivity of the microwave-based method turned out to be small compared to that previously observed for other exhaust aftertreatment devices. This may limit the practical applicability of the microwave-based NOx loading detection in lean NOx traps.