G. De Cicco

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.

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.

A thick-film resonant sensor for humidity measurements

Abstract The paper describes the implementation and performance of a resonant humidity sensor relying on the piezoelectric properties of PZT-based thick films on an alumina diaphragm and the reversible hygroscopic behaviour of a cellulose acetate coating on the substrate. A shift of about 100 Hz of the resonance frequency o = 56.675 Hz (22 °C, 35% RH) is obtained by changing the relative humidity from 10 to 95%, with a linear response over the whole range. In a time of 0.5 min the sensor changes its output when the relative humidity is varied from 10 to 40%. The dynamics of interaction between the water molecules and the organic film are discussed and related to the prompt response of the sensor to changes of environmental conditions. It is suggested that the joint exploitation of resonant techniques and thick-film technology can be a powerful strategy for manufacturing other inherently digital sensors of chemical species and biosensors.

Elastic surface wave devices based on piezoelectric thick-films

A study has been performed for the development of elastic-waves structures, involving screen printed and fired piezoelectric layers on nonpiezoelectric substrates, intended for new families of thick film sensors. This study approached the electroacoustic effects, i.e., the coupling between electromechanical parameters of the layers and elastic properties of the substrate, related to the wave propagation. In this work, particular attention is devoted to devices implemented on alumina substrates where SAW as well as (symmetric and antisymmetric) Lambs waves are generated. The realization processes and the performance characteristics of these structures are presented.

A novel torque sensor based on elastic waves generated and detected by piezoelectric thick films

Abstract Screen-printed and fired PZT-based films have been used as the source and detectors of elastic waves near the surface of a torsional shaft. The change in wave propagation velocity due to applied torques has been taken as the operating principle of a torque sensor. The new opportunities opened by piezoelectric thick-film materials in the implementation of this novel sensor are demonstrated.

Magnetoresistive properties of Ni-based thick films

Investigations have been carried out on the electrical properties of nickel-based films prepared from air-fireable pastes on alumina substrates. In particular, magnetoresistive effects have been studied as a function of temperature, strength and direction of the magnetic field. These properties were determined in films at different stages of formation, i.e. prepared at various peak firing temperatures and dwell times, and compared with those displayed by nickel foils. These results provide the basis for the development of magnetoresistive thick-film sensors.

A new ultrasonic composite transducer implemented with thick film technology

A new ultrasonic transducer operating in air and fabricated by thick film technology is presented. It consists of a composite mechanical structure in which three active elements, made with planar alumina substrates supporting PZT-based piezoelectric films, behave as elastic guides. The active elements have a constant rectangular cross-section and are radially placed, 120/spl deg/ apart. They are mounted between a rigid base (operating as the acoustic backing) and an alumina disk. On both sides of each substrate a piezoelectric layer has interdigital thick-film electrodes embedded in layer itself, so that it can be polarized parallel to the long side of the active element; consequently, it allows the creation of periodic surface stresses responsible for symmetric extensional strains in the alumina plates. The elements, synchronously driven in phase, form, with the disk, a vibrating resonant structure, which radiates acoustic energy perpendicular to the disk surface. The assembly of active elements, acoustic backing and radiating disk forms a composite ultrasonic transducer operating in air without the need of any acoustic matching layer. Among the prominent features offered by the transducer design are: a transduction efficiency of 25% at an exiting voltage of 100 V/sub p/ and a high directivity of the emitted beam.

Thick-film magnetoresistors and related sensors

This paper deals with ferromagnetic thick-film materials and with sensors relying on the resistance change due to an external magnetic field, usually in the range from to . The research for the development of thick-film magnetoresistors (TF-MRs) is briefly summarized from Ni-based air-fireable pastes to -based films fireable in atmosphere. The different materials are compared, regarding their magnetoresistive performances (mainly in terms of magnetoresistive anisotropy and temperature sensitivity). Some applications are described, covering the area of position sensing (displacement, proximity switch, angle) and the measurement of dynamic quantities (angular rotation) either for digital or analogue control systems.

Hybrid thick-film magnetoresistive sensors

This paper describes a first prototype of a thick-film sensor using Ni-based pastes, whose resistance changes with the applied magnetic field. The magnetoresistive properties of thick films (Ital. Patent No. 9477/A86 (1986)) have been exploited for the development of hybrid sensors for proximity detection and rotational speed. The conditioning electronic circuits are specifically designed to compensate the relevant temperature effects, which otherwise would overwhelm the weak magnetoresistive effects. The electronic solution, though simple, proves to be very effective. On this basis integrated magnetoresistive sensors, with standard trimmable hybrid circuits and Ni-based films on the same substrate, have been developed in the framework of a collaboration aimed at looking for reliable integrated sensors for various applications. The design and operation of a prototype are described. Some areas of application of hybrid magnetoresistive sensors will be analysed.

Preparation and properties of new thick-film magnetoresistive materials

Abstract Ni x Co y -based thick-film magnetoresistors (TF-MRs) have been prepared with two different approaches: either the in situ alloy formation (and sintering) from fine powders of Ni and Co or the sintering of a Ni 70 Co 30 powder obtained by co-reduction of Ni(II) and Co(II) acetates in diethyleneglycol. The pastes, printed on properly glazed alumina substrates in meander configuration, have been fired in N 2 . The microstructure, electrical and magnetoresistive properties of these films have been studied. Both materials appear of interest for the implementation of contactless sensors.