B. Morten

Resonant pressure sensor based on piezoelectric properties of ferroelectric thick films

Abstract The paper describes the design, implementation and performance of a resonant sensor for gas-pressure measurement realized with screen-printed and fired PZT-based layers on an alumina diaphragm. The piezoelectric properties of these layers have been exploited to excite (with low voltage signals) and detect oscillations of the diaphragm. The layer configurations and associated vibrational modes of the diaphragm have been designed and a phase-locked loop (PLL) technique implemented in electronic circuits able to keep the resonant condition, despite the change of resonance frequency due to variation of the applied differential pressure. It is shown that for an adequate choice of operating frequency (i.e. of the vibrational mode) a good sensitivity and thermal stability of the sensor can be obtained; in this device, operating at frequency around 57.8 kHz, corresponding to the third-mode frequency ⨍ 0.3 , a shift Δ⨍ 0.3 = 650 Hz is found by changing the pressure from 0 to 900 mmHg. We found also that ⨍ 0.3 changes less than 6 × 10 −3 by changing the temperature from 23 to 100 °C. Moreover the sensor allows long-term measurements of static pressures without significant hysteresis effects.

Piezoresistive effects in thick‐film resistors

Piezoresistive properties of thick‐film resistors obtained with ink series supplied by different manufacturers have been investigated as a function of composition, structure, sheet resistivity, and applied strain between 0 and ±1000 μ strain. The strain sensitivity of thick‐film resistors appears to be a strong function of the nature of the conductive grains and of the sheet resistivity of the paste; the results obtained suggest a dominant role of the tunneling effect in the conduction mechanism and in the strain sensitivity.

Piezoresistivity effects in MOS-FET useful for pressure transducers

The piezoresistivity coefficients in p- and n-channel MOS transistors manufactured on silicon wafers cut parallel to the (111) and (100) planes as a function of VD, VG and of the temperature, between -20 and 120 degrees C, have been measured with the particular aim of investigating the possible application of MOS transistors for strain measurements, by comparing the advantages and the disadvantages with bulk and diffused resistors. The characteristics of the MOS devices can be consistently better than the corresponding ones of bulk semiconductor strain gauges, particularly as regards the temperature stability of the resistivity and of the electrical characteristics.

A Temperature Compensated Ultrasonic Sensor Operating in Air for Distance and Proximity Measurements

An ultrasonic sensor operating in air was developed to measure distance (up to 100 cm) or to operate as a proximity sensor in a presettable distance range. The error in the measured distance is about ±1 mm while the accuracy is maintained within ±1 percent of the reading in the whole temperature range from -20°C to +110°C, being the temperature dependence of the sound velocity electronically compensated. As a proximity sensor it can overcome most of the drawbacks and limitations of inductive, capacitive, optical, and Hall-effect commercial proximity sensors.

Thick-film sensors: an overview

Abstract Sensors form a vital part of any measurement and control system. Demands for sensors generally are increasing, but particularly for rugged, reliable, integrated sensors for computer controlled systems. Among the various sensor technologies, thick-film technology (TFT) does not really offer conceptually new or sophisticated solutions, but does offer several appreciable capabilities, e.g. , flexibility in choice of materials and design, easy integration with electronic circuits and in packaging. This paper illustrates some specific features of TFT for sensor manufacture and overviews most of the types of devices used or developed at present. Moreover, some new sensors, which might be conceived and develoved in the near future, are suggested.

Excess noise and refiring processes in thick-film resistors

Excess noise in thick-film resistors (TFRs) fired from one to ten times under the same firing profile has been investigated. The changes in structure and composition, which are responsible for variations in sheet resistivity and TCR, affect also the noise index of TFRS, sometimes in a substantial way, and the changes in noise index depend both on the resistor composition and the nature of the substrate. Moreover, the excess noise appears to be correlated to physico-chemical phenomena induced by refiring in the bulk of the resistors, much more than to changes in the density of microdefects (or microvoids) in the resistors investigated.

Pyroelectricity of PZT-based thick-films

Abstract We studied the pyroelectric properties of lead titanate-zirconate (PZT)-based layers prepared with standard procedures of thick film technology (950°C peak temperature) on alumina substrate and with buried interdigital electrodes. The investigation was aimed at setting adequate procedures, and at identifying sample configurations, for measuring the relevant pyroelectric quantities rather than to optimize the material pyroelectric performances. The results show that the pyroelectric coefficient of our porous layers (∼1.2±0.1×10 −4 C/m 2 K) is lower than that of ceramic (∼3.8×10 −4 C/m 2 K) and dense thick-films (∼2×10 −4 C/m 2 K) of comparable composition, but the thermal properties of our samples (low heat capacity and thermal conductivity) as well as the low relative dielectric constant result in figures of merit better than the corresponding values either for ceramic PZT or other pyroelectric materials (including TGS and LiTaO 3 ). Hence, porous ferroelectric thick-films are good candidate for new pyroelectric devices.

Evolution of ruthenate-based thick film cermet resistors

An experimental investigation has been carried out to study variations of microstructure, composition and electrical properties (sheet resistance and its temperature coefficient) occurring in thick film (cermet) resistors during the annealing process which transforms the initial ink into a resistive layer. Resistors based on Bi2Ru2O7 and a lead silica glass have been studied. Analyses have been performed on the inks using thermogravimetric measurements, and on annealed (fired) layers by means of X-ray diffraction, atomic absorption, electron microscopy, microprobe analysis and other complementary techniques. The results point out the role of exchange reactions and redox reactions inside the resistor body and emphasise the complexity of the phenomena which concur to define the final electrical properties of these resistive systems. Interrelations between microstructure, composition and electrical properties have been found and tentative explanations proposed.

THe state of the art in thick-film sensors

Abstract The speed of evolution of thick-film technology as an advanced technique for solid state sensors is illustrated. Recent innovations are mentioned with regard to modern techniques for the design, realisation and test of hybrid circuits for signal processing, use of new materials both for substrates and transducing elements, new concept designs and processing techniques. Current research activities and programmes, which promise new interesting materials for, and emerging applications of, thick-film sensors, are described. However, several problems remain unsolved; some topics which should improve our understanding and performance of thick-film sensors are mentioned.

Very high strain sensitivity in thick-film resistors: real and false super gauge factors

Abstract An investigation has been carried out on the correlations between composition/morphology of RuO2-based thick-film resistors and their strain sensitivity. Very high gauge factors (GFs), as well as satisfactory performance in terms of TCR (temperature coefficient of resistance), excess noise and reliability, are obtained with an appropriate choice of glassy matrix, RuO2 grain size and concentrations. Resistive systems modified with metal and metal oxide additions are also analysed. In some cases notable changes of electrical properties are observed, related to defective structures. Methods for a simple and accurate diagnosis of reliable or fictitious performance of thick-film strain gauges are identified.