Tim Schwebel

CO-Sensor for domestic use based on high temperature stable Ga2O3 thin films

Abstract Gas sensors based on high temperature operated metal oxides, like Ga2O3 thin films show promising properties in terms of reproducibility, long-term stability against interfering gases and low cross sensitivity to humidity. In this paper a surface modification of Ga2O3 is presented which allows the set up of a sensor suitable for indoor CO monitoring. The modification based on Au-clusters on the Ga2O3 surface yields high sensitivity to CO and a distinct reduction of the cross sensitivity towards organic solvents. With the specimens, a resistance change of approx. factor 4 to 100 ppm CO in wet air is attained. By employing catalytic filters of ceramic material, cross sensitivities to organic solvents are virtually completely eliminated.

A selective, temperature compensated O2 sensor based on Ga2O3 thin films

Abstract Pure semiconducting Ga2O3 thin films show a reaction to reducing gases as well to oxygen variations at operation temperatures between 600°C–900°C. By applying surface modifications with catalytically active oxides like La2O3 or CeO2, a complete suppression of the reaction to reducing gases in oxygen-rich atmospheres could be achieved, yielding devices that only respond to the oxygen content. By an analysis of the desorbing gases with NIR-Spectroscopy, varying production rates of carbon oxides and unsaturated carbohydrates were observed. A modification with manganese oxide yielded complete gas-insensitive devices, which still show a thermal-activated conductivity. This effect can be used for temperature compensation purposes.

Accurate Load Detection Based on a New Piezoelectric Drive Principle Employing Phase-Shift Measurement

This paper introduces a technique for measuring the torque applied to a piezoelectric motor while it is operating. The technique utilizes phase measurement rather than amplitude measurement and takes advantage of the kinematical principle of a piezoelectric actuator drive (PAD) later described in the paper. Piezoelectric actuators are bidirectional converters of electrical energy into mechanical energy and vice versa. Due to the special kinematical principle of the PAD, an applied external torque and internal torque lead to a phase-shift between the driving signals (a voltage applied to actuators) and the corresponding mechanical feedback (a modulated force). The actuator acts as a sensor to the feedback force converting it into a charge signal. The charge signal is measured and converted into a voltage by a simplified Sawyer-Tower-Circuit. The dc bias of both signals the driving actuator voltage, and measured charge signal is removed by a high-pass filter. The signals are then amplified and limited to form digital signals out of the sinusoidal input signals. The phase-shift between both signals is analyzed by a phase detector based on a zero-crossing time difference measurement. By employing the theory of electromechanic conversion of the piezoelectric actuators under the marginal conditions of the drive setup, the torque value is calculated, based on the measured phase-shift. The described technique offers highly accurate real-time torque measurement and additional information that can be used for an on-line diagnosis of the piezoelectric ring motor. The theory was validated in an experiment showing typical errors of 5% and 312 to 1248 measurements per 360 deg turn of the motor shaft. The piezomotor used during the experiment offered a maximum torque of 5 Nm

A new type of gas sensor based on thermionic charge carrier emission

Abstract This paper presents investigations on DC emission currents from heated surfaces at ambient pressure for the purpose of gas sensing. Since applied gases alter the work function (WF) of a material, the emitted electron current changes and becomes gas sensitive. Gas sensitive effects were identified as being dependent upon the emission material used. In contrast to vacuum conditions, additional effects arise from interactions among electrons, ions and atoms. Consequently, the electrical behaviour at atmospheric pressure is completely different. This paper presents preliminary results on this new gas sensing principle with materials such as barium nitrate and scandium oxide.

CO-sensor for domestic use based on high temperature stable Ga/sub 2/O/sub 3/ thin films

Gas sensors based on high temperature operated metal oxides, like Ga/sub 2/O/sub 3/ thin films show promising properties in terms of reproducibility, long-term stability against interfering gases and low cross sensitivity to humidity. In this paper a surface modification of Ga/sub 2/O/sub 3/ is presented which allows the setup of a sensor suitable for indoor CO monitoring. The modification based on Au-clusters on the Ga/sub 2/O/sub 3/ surface yields high sensitivity towards CO and a distinct reduction of the cross sensitivity towards organic solvents. With the specimens, a resistance change of about a factor of 4 to 100 ppm CO in wet air is attained. By employing catalytic filters of ceramic material, cross sensitivities to organic solvents are nearly complete eliminated.