Pressure Sensors For High Temperature Applications Research Articles

A MEMS-Based High-Fineness Fiber-Optic Fabry-Perot Pressure Sensor for High-Temperature Application.

In this paper, a high-fineness fiber-optic Fabry–Perot high-temperature pressure sensor, based on MEMS technology, is proposed and experimentally verified. The Faber–Perot cavity of the pressure sensor is formed by the anodic bonding of a sensitive silicon diaphragm and a Pyrex glass; a high-fineness interference signal is obtained by coating the interface surface with a high-reflection film, so as to simplify the signal demodulation system. The experimental results show that the pressure sensitivity of this sensor is 55.468 nm/MPa, and the temperature coefficient is 0.01859 nm/°C at 25~300 °C. The fiber-optic pressure sensor has the following advantages: high fineness, high temperature tolerance, high consistency and simple demodulation, resulting in a wide application prospect in the field of high-temperature pressure testing.

Design of SiC-Doped Piezoresistive Pressure Sensor for High-Temperature Applications.

Within these studies the piezoresistive effect was analyzed for 6H-SiC and 4H-SiC material doped with various elements: N, B, and Sc. Bulk SiC crystals with a specific concentration of dopants were fabricated by the Physical Vapor Transport (PVT) technique. For such materials, the structures and properties were analyzed using X-ray diffraction, SEM, and Hall measurements. The samples in the form of a beam were also prepared and strained (bent) to measure the resistance change (Gauge Factor). Based on the results obtained for bulk materials, piezoresistive thin films on 6H-SiC and 4H-SiC substrate were fabricated by Chemical Vapor Deposition (CVD). Such materials were shaped by Focus Ion Beam (FIB) into pressure sensors with a specific geometry. The characteristics of the sensors made from different materials under a range of pressures and temperatures were obtained and are presented herewith.

All-sapphire-based fiber-optic pressure sensor for high-temperature applications based on wet etching.

In this paper, we proposed an all-sapphire-based extrinsic Fabry-Perot interferometer (EFPI) pressure sensor based on an optimized wet etching process, aiming to improve the quality of the interference signal. The sapphire pressure sensitive diaphragm (SPSD) was fabricated by wet etching solutions with different mixture ratios of H3PO4 and H2SO4 at 280°C. The differences of mixture ratios affect the surface roughness of SPSD. SPSDs with surface roughness of 3.91nm and 0.39nm are obtained when the mixture ratios of H3PO4 and H2SO4 is 1:1 and 1:3, respectively. We constructed pressure sensing test system adopting these two kinds of SPSD and performed comparative test. The experiment results show that the demodulation jump can be solved and cavity length fluctuation is decreased to ±5nm when the surface roughness of SPSD is 0.39nm.

Capacitive Pressure Sensor With Integrated Signal-Conversion Circuit for High-Temperature Applications

In this paper, a capacitive pressure sensor for high-temperature application is proposed. This sensor includes three parts: a capacitive pressure-sensitive chip fabricated from green tapes using lamination technology, a C–V conversion circuit fabricated using high-temperature-resistant electronic components and a PCB board, and a thermal-insulation shell fabricated from a high-temperature-resistant alloy and aerogel. Due to the aerogel, which is a thermal-insulation material, being filled between the circuit board and the rear-end shell, when the air intake temperature reaches 350°C, the temperature transmitted to the C–V conversion circuit board can be kept below 60°C; this ensures normal operation of the C–V converter circuit. The developed sensor can record pressure values from room temperature to 350 °C. According to the pressure–voltage curves at different temperatures obtained from tests, the sensor’s sensitivity is 38 mV/bar, and its repeatability error is less than 1.948%. The results show that the proposed sensor has a high operating temperature and is more reliable and linear than existing sensors.

Batch-producible MEMS fiber-optic Fabry-Perot pressure sensor for high-temperature application.

A fiber-optic Fabry-Perot pressure sensor based on a micro-electro-mechanical system (MEMS) and CO2 laser fusion technology is developed and experimentally demonstrated for high-temperature application. The sensing heads are batch-fabricated by anodically bonding the micromachined Pyrex glass wafer and local gold-plated silicon wafer. The separated sensing head and the single-mode fiber are fused together to form the Fabry-Perot cavity using the CO2 laser. In order to improve the measurement accuracy in a high-temperature environment, a fiber Bragg grating is used as a temperature sensor for temperature decoupling. The experimental results show that the fiber-optic Fabry-Perot pressure sensor has a maximum nonlinearity of 0.4%. The maximal error of the pressure after temperature decoupling is less than 1.05% over a pressure range of 0-0.5MPa and a temperature range of 20°C-350°C. The batch fabrication technology makes the sensors low cost and high uniformity.

Microbubble-based fiber-optic Fabry-Perot pressure sensor for high-temperature application.

Using arc discharge technology, we fabricated a fiber-optic Fabry-Perot (FP) pressure sensor with a very low temperature coefficient based on a microbubble that can be applied in a high-temperature environment. The thin-walled microbubble can be fabricated by heating the gas-pressurized hollow silica tube (HST) using a commercial fusion splicer. Then, the well-cut single-mode fiber (SMF) was inserted into the microbubble, and they were fused together. Thus, the FP cavity can be formed between the end of the SMF and the inner surface of the microbubble. The diameter of the microbubble can be up to 360μm with the thickness of the wall being approximately 0.5μm. Experimental results show that such a sensor has a linear sensitivity of approximately -6.382 nm/MPa, -5.912 nm/MPa at 20°C, and 600°C within the pressure range of 1MPa. Due to the thermal expansion coefficient of the SMF being slightly larger than that of silica, we can fuse the SMF and the HST with different lengths; thus, the sensor has a very low temperature coefficient of approximately 0.17pm/°C.

Evanescent-mode-resonator-based and antenna-integrated wireless passive pressure sensors for harsh-environment applications

A wireless pressure sensor for high-temperature applications is demonstrated based on a microwave evanescent-mode cavity resonator. Cavity deformation resulting from applied external pressure can be detected by measuring the resonant frequency change of the sensor. Compact sensor size is achieved by loading a cylindrical post inside the cavity resonator. In addition, a patch antenna is seamlessly integrated with the pressure sensor, without additional volume. This pressure sensor is able to survive high temperatures by adopting passive structures and robust ceramic/metallic materials. Fully-dense silicoaluminum carbonitride (SiAlCN) ceramic is used herein owing to its excellent thermal–mechanical properties and manufacturability as a Polymer-Derived Ceramic (PDC). A PDC soft-lithography technique is developed to fabricate the ceramic pressure sensor. In order to wirelessly interrogate the pressure sensor at high temperatures, a robust interrogation antenna is designed and fabricated with a wide fractional bandwidth. Finally, the cavity deformation of pressure sensor versus external pressure is measured at high temperatures up to 800°C. The resonant frequency decreases from 11.75 to 11.56GHz, when the applied external force on the sensor increases from 0 to 5N at 800°C.

An optical absolute pressure sensor for high-temperature applications, fabricated directly on a fiber

Fiber optical pressure sensors show great promise in monitoring harsh environments as sensitive readout electronics can be located remotely. In this paper, a process based on thin film techniques and focused ion beam (FIB) machining is demonstrated that allows the fabrication of a Fabry–Pérot interferometer-based sensor directly on top of an optical fiber. Besides its extremely small size and rugged monolithic construction, the silica-based sensor has the advantage that thermal mismatch is minimal and a vacuum reference pressure cavity is present, enabling application in high-temperature environments. Operation in an environment up to 600 °C is demonstrated. Furthermore, a possible route toward mass fabrication of such sensors is presented, avoiding the need for a FIB operation.

Wireless micromachined ceramic pressure sensor for high-temperature applications

In high-temperature applications, such as pressure sensing in turbine engines and compressors, high-temperature materials and data retrieval methods are required. The microelectronics packaging infrastructure provides high-temperature ceramic materials, fabrication tools, and well-developed processing techniques that have the potential for applicability in high-temperature sensing. Based on this infrastructure, a completely passive ceramic pressure sensor that uses a wireless telemetry scheme has been developed. The passive nature of the telemetry removes the need for electronics, power supplies, or contacts to withstand the high-temperature environment. The sensor contains a passive LC resonator comprised of a movable diaphragm capacitor and a fixed inductor, thereby causing the sensor resonant frequency to be pressure-dependent. Data is retrieved with an external loop antenna. The sensor has been fabricated and characterized and was compared with an electromechanical model. It was operated up to 400/spl deg/C in a pressure range from 0 to 7 Bar. The average sensitivity and accuracy of three typical sensors are: -141 kHz Bar/sup -1/ and 24 mbar, respectively.

Characterization of highly doped n- and p-type 6H-SiC piezoresistors

Highly doped (/spl sim/2/spl times/10/sup 19/ cm/sup -3/) n- and p-type 6H-SiC strain sensing mesa resistors configured in Wheatstone bridge integrated beam transducers were investigated to characterize the piezoresistive and electrical properties. Longitudinal and transverse gauge factors, temperature dependence of resistance, gauge factor (GF), and bridge output voltage were evaluated. For the n-type net doping level of 2/spl times/10/sup 19/ cm/sup -3/ the bridge gauge factor was found to be 15 at room temperature and 8 at 250/spl deg/C. For this doping level, a TCR of -0.24%//spl deg/C and -0.74%//spl deg/C at 100/spl deg/C was obtained for the n- and p-type, respectively. At 250/spl deg/C, the TCR was -0.14%//spl deg/C and -0.34%//spl deg/C, respectively. In both types, for the given doping level, impurity scattering is implied to be the dominant scattering mechanism. The results from this investigation further strengthen the viability of 6H-SiC as a piezoresistive pressure sensor for high-temperature applications.

SOI pressure sensor

Abstract A single-crystal silicon piezoresistor has a high sensitivity to strain but has the problem of leakage at the pn junction at elevated temperatures. This usually limits the applications of these devices to less than ≈ 150 °C. This paper presents a technique for fabricating a single-crystal silicon-on-oxide (SOI) pressure sensor for high-temperature applications. The SOI structure is formed from the substrate by a combination of trench etching and anisotropic etching techniques. The desired SOI island is under-etched using an anisotropic etch and then separated from the substrate by an oxidation step. The problems associated with etching convex corners have been avoided by using two trench etchings. The remaining cavities are refilled by polysilicon deposition. By using this technique, a good-quality single crystal can be achieved, without requiring lengthy or expensive processing.