Thin-film Gauges Research Articles

TiB₂/SiCN Thin-Film Strain Gauges Fabricated by Direct Writing for High-Temperature Application

The in situ strain/stress monitoring of hot components in harsh environments remains a challenging task. In this study, TiB <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> /SiCN thin-film strain gauges were fabricated on nickel base alloy substrates via direct writing. The static and dynamic strain responses were investigated from 25 °C up to 800 °C. The results show that this thin-film strain gauge fabricated by ceramic-based materials exhibits excellent thermal stability and strain response. Without any anti-oxidative protective layer deposited, its operating temperature is as high as <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$700^{\circ }\text{C}$ </tex-math></inline-formula> , which is 200 °C higher than that of the high-temperature piezoresistive thin-film strain gauges that have been developed and comparable to thin-film strain gauges with protective layers. The gauge factor of theTiB <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> /SiCN high-temperature thin-film strain gauge is 7.12, which is higher than that of most high-temperature thin-film strain gauges. In addition, the strain gauge exhibits excellent resistance stability with a mechanical hysteresis of <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$3~\mu \varepsilon $ </tex-math></inline-formula> and a resistance drift of 0.0008/h at room temperature. Therefore, TiB <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> /SiCN thin-film strain gauges provide an effective approach for the measurement of in-situ static and dynamic strain of hot components in harsh environments.

Magnetohydrodynamic Experiments of Total Heat Flux Mitigation for Superorbital Earth Reentry

A set of experiments was conducted with the aim of evaluating total heat flux mitigation using magnetohydrodynamic flow control over ionized shock layers surrounding planetary entry vehicles. The University of Queensland’s X2 expansion tube was used to generate a flow condition representative of a Mars return trajectory. Spherical permanent magnets were fitted into a scaled test model based on NASA’s Stardust capsule. Thermocouples located at the stagnation point and shoulder of the test model, as well as thin-film heat transfer gauges located in the aft section, measured total heat flux for both magnetic and nonmagnetic experiments. High-speed imaging of the shock layer showed a significant increase in shock standoff when using the magnet, and polynomial fits were derived, describing standoff distance along the model’s forebody. Despite significant noise affecting the heat flux gauges signals for the magnetic experiments, a simple methodology allowed to compare heat flux during the useful test time. It was found that the applied magnetic field significantly reduces total heat flux at the stagnation point and shoulder of the test model, although heat flux to the aft section was found to significantly increase. Reasonable agreement with the numerical predictions was found for the aft section of the test model, but discrepancies were observed at the other locations.

Investigation of textured sensing skin for monitoring fatigue cracks on fillet welds

Load-induced fatigue cracking in welds is a critical safety concern for steel transportation infrastructure, and the automation of their detection using commercial sensing technologies remains challenging due to the randomness in crack initiation and propagation. The authors have previously proposed a corrugated soft elastomeric capacitor (cSEC), which is a flexible and ultra-compliant thin-film strain gauge that transduces strain into a measurable change in capacitance. The cSEC technology has been successfully demonstrated for measuring bending strain as well as angular rotation in a folded configuration. This study builds on prior discoveries to characterize the sensor’s capability at monitoring fatigue cracks in corner welds, for which the sensor needs to be installed in a folded configuration. A crack monitoring algorithm is developed to fuse the cSEC data into actionable information. Experimental work is conducted on an orthogonal welded connection, mimicking a plate-to-web joint in steel bridges, with cSECs folded over the fillet welds. The sensor’s electromechanical behavior is characterized, and results confirm that the cSEC is capable of fatigue crack detection and quantification. In particular, results show that the cSEC can detect a minimum crack length of 0.48 mm and that its overall sensing performance, including signal linearity, resolution, and accuracy, is adequate under no damage, yet decreases with increasing crack size, likely attributable to the simplification of the electromechanical model and higher noise produced by the loading equipment under smaller applied displacement.

Experimental Study on Cooling Efficiency of Hypersonic Optical Window

The cooling effectiveness of optical window influences the imaging quality of hypersonic vehicles. This study focuses on the supersonic film cooling efficiency on the optical window of a blunt cone in hypersonic flow. The experiments were conducted in a gun tunnel equipped with a Mach 8 nozzle providing a total pressure and temperature of 9 MPa and 900 K respectively. Three tangential 2D nozzles with different combinations of slot heights and Mach numbers were designed to detect the film cooling length under different injection pressures. The heat flux on window surface was measured by Thin Film Gauges and the flow field was monitored by schlieren technique. When the jet pressure matched the mainstream, the window was completely cooled, and the minimum mass flow rate was achieved when the slot height was 5mm and the jet Mach number was 2.5. If the pressure ratio of jet continues to increase, the heat flux density could be furtherly reduced, but the cooling efficiency of unit coolant mass flow decreased significantly. The data correlation results showed that the cooling efficiency presented a nonlinear relationship of second order polynomial with (x/Sh)λ-0.8 , and the effective cooling length of film was positively correlated with the cooling mass flow ratio (λ) and slot height of the nozzle. Besides, the increase of jet pressure resulted in thickening the mixing layer, which enhanced the heat insulation effect and reduced the heat flux as a result.

A New Calibration Technique for Thin-Film Gauges and Coaxial Thermocouples Used to Measure the Transient Heat Flux

A New Calibration Technique for Thin-Film Gauges and Coaxial Thermocouples Used to Measure the Transient Heat Flux

Microstrain-range giant piezoresistivity of silicon oxycarbide thin films under mechanical cyclic loads

In the present study, thin-film strain gauge element arrays were prepared based on large-area silicon oxycarbide thin films and lithographic deposition of structured electrodes. The individual strain gauge elements were systematically investigated concerning their piezoresistive behavior at ambient temperature and shown to possess giant piezoresistivity with gauge factors in the range of 3–5 × 103. This has been correlated with the large charge carrier mobility in the silicon oxycarbide thin films (i.e., 186 cm2 V−1 s−1) as well as with a unique phase composition and morphology thereof, consisting of high-conductivity carbon-rich segregations homogeneously dispersed within a silicon oxycarbide-based matrix. The studied strain gauge elements were evaluated in both cyclic tensile and compression load modes and showed excellent reversibility and short response times. The process capability of the strain gauge elements has been statistically assessed and revealed good robustness and replicability which may be further improved. The present work provides a robust and highly reproducible manufacturing process for an ultrasensitive strain gauge prototype and thus points towards a great potential concerning the use of silicon oxycarbides in MEMS-related applications.

Analysis of wall partial pressure-dependence on oxygen surface catalytic recombination with shock-heated flow

Wall partial pressure dependence analysis of oxygen surface catalytic recombination efficiency on copper-coated surface is experimentally performed in a shock tube at room temperature. Three flow conditions with different flow enthalpy in a gas mixture that contains 21% oxygen and 79% argon are considered. The recombination efficiency is determined based on the stagnation heat transfer measurement at the end-wall of the shock tube. A copper-coated platinum thin-film gauge is used to determine the total stagnation heat transfer. An additional silicon dioxide-coated platinum thin film gauge is applied to measure the conductive heat transfer. Catalytic heat transfer theories based on binary and tertiary gas mixtures are applied to obtain the oxygen catalytic recombination efficiency and the corresponding atomic wall fraction. The oxygen atomic wall partial pressure is computed based on the oxygen atomic wall mass fraction which varies according to the surface catalysis. By combining theoretical and stagnation heat transfer measurement, the oxygen recombination efficiency on copper material is found to be between 0.0022 and 0.0213 as the oxygen atomic wall partial pressure decreases from 25.62 kPa to 13.41 kPa. The results observed in the present work confirm that the wall partial pressure influences the oxygen recombination efficiency.

Application of 3-omega method for thin-film heat flux gauge calibration

Double-sided thin-film resistance temperature detector (RTD) heat flux gauges (HFGs) are commonly used to characterize heat transfer rates in high-heat flux environments with complex flow features. These gauges comprise two thin-film RTDs on opposing sides of a dielectric. To deduce accurate heat flux, the RTDs must be properly calibrated and the material properties of the dielectric must be characterized. This study presents a complete gauge characterization method for sensors of this type by applying standard calibration procedures with specially-designed RTDs capable of utilizing the 3-omega method. The 3-omega method quantifies the thermal conductivity and thermal product of a material by measuring the response of a specially designed heater/thermometer deposited on the substrate. This study shows the 3-omega method enables RTD calibrations and thermal property determination over a range of temperatures for individual gauges, reducing the uncertainty in calculated heat flux. Although the method is quite general, this study utilized platinum RTDs with a polyimide dielectric, which is common in turbomachinery applications. The thermal properties obtained through this method agree with previous characterization efforts; however, discrete characterization of seven gauges shows that gauge-to-gauge variation in the dielectric could influence measured heat flux by as much as 30%. This study also builds the framework to characterize the thermal conductivity of the adhesive layer beneath the gauge which is necessary to mount the sensors to the test article. Although often uncharacterized, the adhesive thermal conductivity has a significant impact on matching experimental measurements to simulations. Additionally, this study found that if the thermal conductivity of the dielectric is constant (an assumption that holds for the present study), an in-situ RTD calibration can be performed. In-situ RTD calibration and traditional method RTD calibration agreed to within 0.1%. Overall, this work has practical implications in obtaining high quality measurements from HFGs of this type.

Fast response transient behaviour of a coaxial thermal probe and recovery of surface heat flux for shock tube flows

The magnitude of surface heat flux and its accurate estimation are very crucial owing to their applicability in impulsive thermal loads for high-speed flights. The correctness of such heat flux is associated with the implications from highly responsive thermal sensors in ground-based aerodynamic studies. In the present study, the performance effectiveness of an indigenous, in-house-made coaxial thermal probe (CTP) is validated against a standard silver thin film gauge (TFG) counterpart. The thermal probe has been fabricated in-house from two metal alloys; chromel (3.25 mm diameter and 10 mm length) and constantan (0.91 mm diameter and 15 mm length), by disposing one in the annulus of another coaxially, subsequently clubbed by a thin layer of epoxy in between them. Similarly, the thin film gauge is fabricated by putting silver paste (~1 µm thickness) on the pyrex substrate (3.25 mm diameter, 10 mm length). The fabrication process is followed by different calibration activities to determine the characteristics constants for sensors (sensitivity, thermal product value for CTP, and temperature coefficient of resistance for TFG). Both the probes are tested in a highly transient environment in a shock tube that generates high density, high pressure, and high enthalpy flows. The response time of the CTP and TFG are found to be 265 µs and 240 µs, respectively. The heat flux values estimated through temperature responses are having a nice match for CTP as well as TFG with a deviation in peak heat flux value within a range of ±2%. A similar trend and magnitude are also re-assured through numerical simulation within an uncertainty band of ±1.5%. The exhaustive study carried out in a shock tube has assured the performance and reliability of a CTP. Hence, it is recommended as an effective heat flux sensor for routine measurements in impulsive heat loadings.

Creep adjustment of strain gauges based on granular NiCr-carbon thin films

Abstract. An important property of high-precision mechanical sensors such as force transducers or torque sensors is the so-called creep error. It is defined as the signal deviation over time at a constant load. Since this signal deviation results in a reduced accuracy of the sensor, it is beneficial to minimize the creep error. Many of these sensors consist of a metallic spring element and strain gauges. In order to realize a sensor with a creep error of almost zero, it is necessary to compensate for the creep behavior of the metallic spring element. This can be achieved by creep adjustment of the used strain gauges. Unlike standard metal foil strain gauges with a gauge factor of 2, a type of strain gauges based on sputter-deposited NiCr-carbon thin films on polymer substrates offers the advantage of an improved gauge factor of about 10. However, for this type of strain gauge, creep adjustment by customary methods is not possible. In order to remedy this disadvantage, a thorough creep analysis is carried out. Five major influences on the creep error of force transducers equipped with NiCr-carbon thin-film strain gauges are examined, namely, the material creep of the metallic spring element (1), the creep (relaxation) of the polymer substrate (2), the composition of the thin film (3), the strain transfer to the thin film (4), and the kind of strain field on the surface of the transducer (5). Consequently, we present two applicable methods for creep adjustment of NiCr-carbon thin- film strain gauges. The first method addresses the intrinsic creep behavior of the thin film by a modification of the film composition. With increasing Cr content (at the expense of Ni, the intrinsic negative creep error can be shifted towards zero. The second method is not based on the thin film itself but rather on a modification of the strain transfer from the polyimide carrier to the thin film. This is achieved by controlled cutting of well-defined deep trenches into the polymer substrate via a picosecond laser.

A review on thin film fast response heat transfer gauges

In this paper, an effort has been made to compile various research investigations done so far on fast response gauges for the short duration measurement in the transient state. The review paper is not only about summarization of all-important research work; rather, a detailed review on the development of the fast response gauges, its application, and various methods of heat estimation from the measured transient temperature history is also presented. Before discussing the development of fast response gauges, various heat flux measurement techniques and newer trends in heat transfer gauges are presented. The focus of this Review is mainly on the thin-film gauge, a robust fast response temperature detector best suited for the short duration measurement. The literature on the application of thin-film gauges for various engineering systems such as turbomachinery and aerodynamic heating systems is broadly categorized as single-layered and multi-layered gauges. The applications for the internal combustion engine and low heat flux measurement are also discussed later. The summary tables with studies arranged in chronological order are also provided. Finally, the estimation of the heat transfer rate from the measured transient temperature data is presented.

Investigation of the Properties of Thin-Film Baroresistors based on Sm1-xGdxS Solid Solutions

The effect of Gd concentration on the physical properties of thin polycrystalline films of Sm1-xGdxS solid solutions obtained by explosive evaporation in vacuum of powders of the starting compounds and deposition of the latter from the gas phase on glass substrates was studied. Analysis of dependences of temperature α and baric β resistance coefficients on the Gd concentration in SmS-GdS system, as well as their ratio γ, was used as a basis to determine optimal compositions for manufacturing thin-film strain gauges and baroresistors.

Gas-Surface Energy Exchange Characterization Around a Cone in the Free-Piston-Driven Shock Tunnel

This work describes numerical and experimental research in the Indian Hypersonic Shock Tunnel 3 on a blunt cone model and related computation of the facility parameters: nozzle reservoir conditions, nozzle transit time, and freestream conditions. Specialized codes accounting for real gas effects and nonequilibrium are used to obtain facility parameters. Starting at freestream conditions, a physicochemical model is developed in Fluent to simulate the dissociating environment within the shock layer over the model, assuming thermodynamic equilibrium. The model is validated, then invoked with a surface reaction of exothermic chromium oxidation, to study model erosion and gas-surface energy exchange. A mathematical analysis is performed to estimate surface oxidation rate, heat release, and its effect on shock layer temperature and aerodynamic heating. Both the quantities show considerable increase. Experimental measurement of surface heat flux and temperature using thin film gauges and two-color ratio pyrometry, respectively, also show an increase. The Fluent model with an iterative technique is used to show that the actual temperature at stagnation point is about 90 K higher than its counterpart based on line-of-sight averaged measurements. Finally, analytical calculations are performed to obtain the reaction rate parameter as a measure of the degree of dissociation in the shock layer.

The influence of spatial arrangement of endpoints on output characteristics of ITO/In2O3 heat flux gauge

Thermopile is a common test structure, in which the distribution of hot and cold endpoints has a high impact on its performance especially for applications in thin-film heat flux gauge. In this work, the influence of spatial distribution of endpoints is investigated on the output characteristics of the thin film heat flux gauge based on ITO/In2O3 thermopile. Three different structures of the heat flux gauges, which are horizontal, vertical and stepped, are designed and fabricated via sputtering according to different spatial arrangement of endpoints. The results show that the heat flux gauges based on vertical distributed structures have higher sensitivities than the gauges based on the other two structures on a convection heat source calibration platform. The output sensitivities of the gauges based on stepped distributed structure is the highest on laser radiation calibration platform, which reach 74.9 μV/(kW/m2). The thin-film heat flux gauge based on horizontal distribution structure has the best dynamic characteristic on a laser heat source calibration system. The operating temperature of the gauge is close to 800 °C, and the dynamic frequency reaches 5.2 kHz.

Catalytic recombination assessment on carbon in dissociated shock tube flow

Owing to its superior chemical stability at high temperatures, carbon is attractive as a candidate for ablation thermal protection system (TPS) materials for hypersonic re-entry vehicles. The catalytic recombination behavior is crucial to the ablation process during atmospheric entry because it provokes considerable heat flux at the surface. In the present study, catalytic efficiency values for oxygen and nitrogen recombination on carbon material have been determined by reflected shock experiments that can replicate the flow behavior of a hypersonic stagnation-point blunt body. Dissociated oxygen and nitrogen atoms were adequately produced adjacent to the shock tube endwall. Typical thin-film gauges coated with carbon are used for the stagnation heat-transfer measurements at the endwall. Before and after the tests, the properties of carbon were investigated by scanning electron microscopy (SEM), atomic force microscopy (AFM), and X-ray photoelectron spectroscopy (XPS). Surface heat-transfer data were analyzed with catalytic heat-transfer theories. The role of recombination efficiency on the species mass fraction is also investigated by solving the governing equation for species conservation. The recombination efficiencies of carbon are found to be 0.0023 and 0.00075 for oxygen and nitrogen, respectively. It has been shown that the carbon surface is more catalytically active towards oxygen atoms than nitrogen atoms.

Numerical modelling of thin film gauge for short duration thermal measurement

Abstract The numerical study of direct heat conduction through thin film heat flux gauge is carried out to see how closely it resembles with short duration experimental thermal data. The thermal data obtained from transient impulse facilities at Von Karman Institute, Belgium (case1) and Oxford University (case2) is taken from literature as a reference for validation purpose. The numerically simulated temperature data is compared with experimental data. A good agreement is observed. Further, heat flux is recovered from temperature data using the cubic spline technique for comparative study. The peak deviation from experimental heat flux data for case 1 and 2 is found to be 6% and 7.2% respectively. All these deviations are in the acceptable range of 8%. This deviation is also accounted by the fact that direct heat condition modelling is used instead of numerical modelling of impulse facilities. Thus, direct heat conduction modelling is acceptable approach to predict the transient thermal data with admissible errors.

Heated thin film gauge arrangements to reduce uncertainty in transient heat transfer measurements

This paper describes the development of heated double-sided thin film gauge configurations for transient heat transfer measurements. By heating the substrate it is possible to measure the heat flux over a range of surface temperatures and deduce the adiabatic wall temperature and the external heat transfer coefficient. The accuracy of the measurement depends on the stability of the regression of heat flux against wall temperature and can be improved by extending the range of wall temperature over which the regression is performed. In this paper we compare two methods of local heating: double-sided gauges with an underside thin film heater and self-heating double-sided gauges. Both arrangements have been used in the Oxford Turbine Research Facility to measure the heat transfer on the uncooled turbine shroud of the MT1 high-pressure turbine stage at engine-representative conditions. These measurements yield improved regressions compared to conventional techniques to determine the adiabatic wall temperature and the heat transfer coefficient.

Evaluation of response characteristics of thin film gauge for conductive heat transfer mode

Heat transfer analysis is the one of the most important designing aspects for many engineering systems. The design prospect in the preview of heat transfer focuses on the prediction of heat flux with the help of measured transient temperature data. Thin film gauges are one of the most predominant method for the heat flux prediction especially for short duration transient temperature measurement. Thin film gauges are usually exposed to the heated environment for the measurement purpose. However, there are some prominent research areas like ablation phenomenon met to spacecraft thermal shields during re-entry to the atmosphere, for which direct exposure of the thin film gauge to the heated environment causes the functional and working difficulties associated with the gauge. In the present study, it is aimed to investigate the suitability of thin film gauge for the conduction-based short duration measurement. An experimental set up is fabricated, which is used to supply the heat load to the hand-made thin film gauge using platinum as sensing element and quartz as a substrate. The transient temperature data is recorded during experiment is further compared with the simulated temperature histories obtained through finite element analysis. The heat flux estimation for both the analysis is made using measured transient temperature data by convolute integral of one- dimensional heat conduction equation. The estimated heat flux value for the experimental and numerical result is found to be in excellent agreement.

Analysis of nitrogen recombination activity on silicon dioxide with stagnation heat-transfer

Investigation of surface recombination activity of nitrogen on certain materials is crucial in the design of hypersonic re-entry vehicles. In the present work, an experimental study using a conventional shock tube facility has been conducted to investigate the phenomena of nitrogen recombination. Stagnation heat-transfer measurements were conducted behind reflected shock waves in a dissociated shock-tube flow for a mixture of 20% nitrogen and 80% argon. A platinum thin-film gauge was used to measure the stagnation heat-transfer in a shock heated gas mixture under a well-defined test condition. The surface of the heat-transfer gauge was coated with silicon dioxide (SiO2), the principal material used in thermal protection systems (TPS) for the hypersonic vehicles. Spectroscopic analyses were performed for in-depth characterization of the initial surface condition of the test specimen. Recombination activity behavior of nitrogen was evaluated by combining the heat-transfer data with catalytic heat transfer theories. The nitrogen recombination efficiency of silicon dioxide is found to be from 0.0026 to 0.035 at surface roughness factors of 1.01 to 1.10. Compared to the oxygen recombination activity, the nitrogen recombination activity showed a more than five times higher sensitivity to the roughness factor of the SiO2-coated surface.

Development, Characterisation and High-Temperature Suitability of Thin-Film Strain Gauges Directly Deposited with a New Sputter Coating System.

New sensor and sensor manufacturing technologies are identified as a key factor for a successful digitalisation and are therefore economically important for manufacturers and industry. To address various requirements, a new sputter coating system has been invented at the Institute of Micro Production Technology. It enables the deposition of sensor systems directly onto technical surfaces. Compared to commercially available systems, it has no spatial limitations concerning the maximum coatable component size. Moreover, it enables a simultaneous structuring of deposited layers. Within this paper, characterisation techniques, results and challenges concerning directly deposited thin film strain gauges with the new sputter coating system are presented. Constantan (CuNiMn 54/45/1) and NiCr 80/20 are used as sensor materials. The initial resistance, temperature coefficient of resistance and gauge factor/k-factor of quarter-bridge strain gauges are characterised. The influence of a protective layer on sensor behaviour and layer adhesion is investigated as well. Moreover, the temperature compensation quality of directly deposited half-bridge strain gauges is evaluated, optimised with an external trimming technology and benchmarked against commercial strain gauges. Finally, the suitability for high-temperature strain measurement is investigated. Results show a maximum operation temperature of at least 400 °C, which is above the current state-of-the-art of commercial foil-based metal strain gauges.