Thin-film thermocouples for in situ measurement: Design, preparation, testing and applications

In the process of aero-engine development, the temperature parameter of the engine is a main factor affecting the performance and safety of the aircraft. In recent years, in order to achieve the higher thrust weight ratio of the engine, the hot components' working temperature of the new aeroengine continues to increase. For example, the temperature in front of the turbine of the F119 is as high as 1950K. Although the traditional test methods can be used in testing the high temperature, it is still not suitable to the harsh environment in aeroengine for the requirement of in-situ, on-line and real-time test [1]. Optical fiber cannot tolerate a high temperature up to 1100K. Sapphire fiber temperature sensor, which is complicated in conversion and susceptible to cross interference, is hard to apply to online monitoring. The unprotected thermocouples fail in the chemical reaction with high temperature up to 2000K. For the armored thermocouples, the insulation layer of it will reduce because of the ultra-high temperature so that it is easy to lead to electric leakage, resulting in shunting error and drift control. In addition, the large size limits the application of armored thermocouples. The device of infrared radiation is complex and the precision is not great enough for the online measurement of hot components in aeroengines. With the development of micro electro mechanical system (MEMS) technology, the ceramic based high temperature thin-film thermocouples (TFTCs) are mentioned. Compared with the traditional thermocouple, TFTCs have the typical characteristics of the two-dimensional, which leads to the advantages of small thermal capacity and rapid response to measure the transient temperature change accurately. The thickness of the hot junction is micro or nano scale, which makes it more suitable for the temperature measurement of hot components in aeroengines. According to the problems in improving technical index of reliable working temperature, working time, and response time, research of TFTCs are focused on extending material system, improving overall structure, explaining the mechanism of electrical characteristics of the sensitive coating at high temperature. Therefore, the variety of materials, the stability of the structure and the instantaneous of response are discussed systematically. It is no doubt that TFTCs will continue to play an important role in temperature measurement continually. In this chapter, we focus on the design, preparation, testing and applications of TFTCs, especially tungsten-rhenium TFTCs proposed by Xi'an Jiaotong University. Tungsten-rhenium TFTCs for Silicon carbide based ceramic matrix composites are designed by finite element numerical analysis and fabricated based on radio frequency (RF) magnetron sputtering technology. Range analysis of thermal stress and optimal design are introduced. Measurement study of residual stress by nano-indentation technology are done to ensure the mechanical properties to make a protected sensor. Effect of magnetron sputtering parameters on adhesion properties, thermoelectric properties and heat treatment on thermoelectric properties are analyzed. It exhibits a high thermoelectric voltage of 35.51 mV at 1420°C, which provide application value for high temperature in-situ sensing in air.