Thick-film Strain Research Articles

A novel thick-film strain transducer using piezoelectric paint

This paper describes the manufacture and testing of a novel strain transducer for use in structural vibration monitoring, based on piezoelectric paint. The materials selection and development of application techniques are outlined for the three principal components of the transducer (paint binder, piezoelectric material and electrode). The final design comprised a suspension of milled lead zirconate/lead titanate (PZT) ceramic powder in lacquer with a gold electrode formed by vapour deposition. The transducers were tested for variability and for dynamic properties (bandwidth and dynamic range). It was found that there is considerable scatter between the output sensitivities of individual transducers, but that the dynamic properties are very consistent. It is shown that the techniques developed form the basis for a viable vibration transducer in the range 20–600 Hz.

Thick-film strain transducers made using piezoelectric paint

This paper describes the manufacture and testing of a novel strain transducer based on piezoelectric paint for use in structural vibration monitoring. This is a form of thick-film sensor, similar in structure to conventional thick-film sensors based on piezoelectric ink. However, the use of paint avoids two problems associated with the ink based manufacturing process: screen printing and high temperature curing. The materials selection and application techniques development are outlined for the three principal components of the transducer: the resin binder, the piezoelectric material and the electrode. The final design comprised a sprayable suspension of milled PZT powder in lacquer (the piezoelectric paint) with a gold electrode formed by physical vapour deposition. The transducers were tested for variability and for dynamic properties (bandwidth and dynamic range). It was found that there is considerable scatter between the output sensitivities of individual transducers, but that the dynamic properties were very consistent. It is shown that the techniques developed form the basis for a viable low cost vibration transducer in the range 20 - 600Hz.

Development of a novel thick-film strain gauge sensor system

In this paper a novel strain gauge system on stainless steel and 96% alumina substrates is described incorporating a Wheatstone bridge network, thick-film piezoresistive strain gauges, instrumentation, and a novel thermal and resistor mismatch tolerance correction software. The resistors in the Wheatstone bridge are fabricated with a novel mixture of ruthenium, bismuth and indium oxides and three different thick-film layouts are used. Mechanical measurements are performed using the cantilever beam set-up. The linearity, hysteresis, repeatability, reproducibility, creep, stability and temperature effects are measured for the novel thick-film pastes. The linearity and temperature effect on the output of the different bridge layouts are also examined. The printing of resistors using thick-film technology can result in resistor tolerances of %. Also, thick-film piezoresistors are intrinsically cross sensitive to temperature. These two properties cause an offset voltage to appear at the output of the bridge under no-load/load conditions and a subsequent error in the calculated applied strain. In order to compensate for the offset error voltage, novel correction software has been developed and implemented using a microcontroller. The error in the predicted strain was measured before and after application of the correction software under different bridge layouts and temperature conditions. The error was reduced from a maximum of 45% to approximately 1%. The effectiveness of the correction software in reducing the error suggests that this technique can be used without the need for trimming or bridge balancing.

Thick-film weldable strain gauges

Abstract In this work the development of a new thick-film strain gauge specially designed for spot welding to metallic structures and components is presented. The weldable strain gauge consists of a thick-film resistor screen-printed on 0.125 mm stainless-steel and titanium carriers. Typical values have been found for the tensile strain: GFL = 8.7, GFT = 5.2, hysteresis 0.3% FS and linearity 0.1% FS for stainless-steel carrier.

A novel model for the temperature characteristic of a thick-film piezoresistive sensor

Abstract The resistance of a thick-film piezoresistor as a function of temperature has previously been shown to exhibit a parabolic characteristic. Through experimental research, a new model for this characteristic has been derived, which better explains the underlying mechanisms. The temperature characteristics of resistivity and resistance of z-plane thick-film resistors have been determined, and are shown to be exponential. The temperature characteristic of resistance of a conventional x-plane thick-film resistor is shown to result from a combination of this characteristic and a positive linear response. This linear response is due to the change in resistance caused by a strain resulting from the temperature coefficient of expansion mismatch between the resistor and substrate. These results will facilitate accurate temperature compensation of thick-film strain sensors, and further promote the adoption of a z-plane thick-film resistor as an ideal strain-sensor configuration.

Analysis of thick film strain resistors on stainless steel and ceramic substrates

Abstract Strain sensors printed on ceramic substrates and bonded to steel, are compared with those printed directly on steel. Various strain gauge characteristics are analysed, including linearity, hysteresis, creep and temperature effects. It has been observed that strain sensors printed directly on steel perform much belter than those printed on ceramics (and bonded to steel). The degraded performance of the ceramic sensor is attributed to the adhesive and the inferior mechanical properties of the ceramic itself.

The effect of co-firing on the characteristics of thick-film strain gauges on stainless steel substrates

The characteristics of thick-film strain gauges fabricated using conventional techniques are compared with those produced with co-fired layers. Three layers have been co-fired — two dielectric and one conductor. Various strain gauge parameters are analysed, including linearity, hysteresis, repeatability, creep and temperature effects. It has been observed that the strain sensor with co-fired layers exhibits very similar characteristics to the one printed with no co-firing. However, the co-fired gauge exhibits slightly lower sensitivity to strain. The minor degradation in performance of the co-fired sensor is attributed to weak bonding between the co-fired layers.

A novel accelerometer using thick-film technology

Abstract This paper explores the use of a thick-film strain gauge for the sensing element within an accelerometer, suitable for use at low frequencies. Theoretical and practical considerations for the use of a thick-film strain gauge in which the applied strain is perpendicular to the substrate are given. A mechanism for the transmission of an acceleration into a strain perpendicular to the substrate is given, along with preliminary results for the device.

Residual strain measurements in thick InxGa1−xAs layers grown on GaAs (100) by molecular beam epitaxy

The final stages of strain relief in the lattice mismatched InxGa1−xAs/GaAs(100) system are addressed by the examination of the residual strain in thick films (∼3 μm), grown by molecular beam epitaxy, across the entire compositional range. These results are compared with the observed variations in both the growth mode and material quality, and related to available theories. It is found that measurements are not consistent with a degradation of material quality that is simply misfit dependent, or to an abrupt change from two-dimensional (2-D) to three-dimensional (3-D) growth in the relaxation stage. Instead, the results seem to be more consistent with a continuous change from 2-D to 3-D growth between x=0 and x=0.4 (it is wholly 3-D above x=0.4). In addition, the large residual strains observed around x=0.5 are related to the poor material quality (possibly through work hardening) at this composition, which is in turn due to problems peculiar to the growth of mismatched alloys.

Performance of thick film strain gauges at cryogenic temperatures

Thick film resistors have been prepared on enamelled steel substrates and their extensometric properties have been investigated at 300, 77 and 4.2 K. The measurements made included the longitudinal and transverse gauge factors, the temperature coefficients of resistance and of the gauge factor, and current noise. Repeatability and apparent strain have also been evaluated. The characteristics of strain gauges prepared from two different resistive systems, based on RuO 2 and ruthenate particles, respectively, have been compared. The results show that thick film resistors on enamelled steel are adequate for the development of sensors and transducers of strain and strain-related physical quantities in cryogenic environments.

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

An investigation has been carried out on the correlations between composition/morphology of RuO 2-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, RuO 2 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.

New thick-film strain gauge

Thick-film strain gauges having strain characteristics with small temperature dependency have been developed. Their gauge factor is more than ten times higher than that of a thin-film strain gauge and nearly half the value of a semiconductor gauge. This strain gauge is a thick film composed of 15 wt. % WO3⋅15 wt. % RuO2⋅70 wt. % glass (weight percent ratio). Its gauge factor is 40 to 55, its temperature coefficient of resistance is 0 to −200 ppm/°C, and its temperature coefficient of gauge factor is about −400 ppm/°C. The change of gauge factor in a repeat test after a strain had been applied to the gauge was smaller than 1%. The gauge was also very stable.

The correlation between strain and electronic structure in monolayer thick films

We explote and empirical relationship between strain (and lattice constant) and the valence electronic structure for mercury, bromine and iron overlayers. These overlayers have a range of lattice constants. For the Hg overlayers, all share a common cubic crystallography. We generally observe that the smaller the overlayer lattice constant, the greater the energy separation betweend-bands (Hg and Fe) orp-bands (Br). These results have important implications in relating electronic structure to fundamental properties such as magnetism. In addition, the film thickness limits for pseudomorphic growth calculated from the bulk properties are consistent with the experimental studies of Hg pseudomorphic growth on Ag (100) at 90 K.

THE THICK-FILM STRAIN GAGE

The history and current status of thick-film strain gage technology are briefly reviewed, and data are presented on the performance characteristics of six different types of bonded piezoresistive strain gages. It is shown that the gage factor/thermal sensitivity characteristics make the thick-film strain gage technology attractive for general purpose applications. It is further noted that the low cost of a high-volume production process and a low-cost capsule packaging associated with the thick-film technology can result in low installed cost for measurement of pressure with very acceptable combined errors. 9 references.

Assessment of thick-film fabrication methods for force (pressure) sensors

New types of force sensors have been made using thick-film technology to achieve minimal dimensions and highest sensitivity. Using the well-known expressions for longitudinal and transverse sensitvity, a theoretical study of the perpendicular sensitivity is carried out. Two new sensors are designed based on this principle. Instead of measuring the bending stress of the ceramic carrier by the attached thick-film strain gauges, the resistors themselves are exposed to the entire load. This has led to the realization of prototypes with excellent characteristics. One sensor shows a sensitivity of 1.5%/kg applied force for total dimensions 5 mm × 5 mm × 1.2 mm. Linearity remains better than 1% even after thermal cycling between 20 and 120 °C. A low cost, highly sensitive force transducer could thus be achieved.

Thick-film strain gauges and pressure transducers

Experiments have shown that thick-film strain gauges have gauge factors in excess of 7. The nominal resistance values of the gauges were between 300 and 700 Ω.