The applications for high-temperature, wireless sensor systems are numerous for the energy and manufacturing industries, especially for monitoring the health and process conditions of refractory brick and active components within extreme conditions. Current sensor technologies in these types of applications are primarily based on thermocouples inserted into the reactors via open access ports through the refractory, which typically leads to rapid corrosion by gaseous or molten reactants. There are very few refractory metals and ceramics that are capable of withstanding such extreme and variable conditions, which include high temperature, high pressure, various pO₂ levels and corrosive conditions (molten inorganics or reactive gasses). Therefore, the common materials used in thermocouples, thermistors, pressure sensors, and strain gauges are not appropriate for long-term use in these applications. In this work, the high-temperature stability and electrical properties of conductive ceramics based on silicide/oxide and semiconducting oxides were investigated up to 1400ºC. These composites were patterned onto ceramic substrates and embedded within refractory brick to form embedded thermistors, thermocouples and spallation sensor architectures. In addition, sensor designs that include capacitive and inductor elements for RF passive wireless communication were also evaluated. The presentation will describe sensor design, fabrication processes, and testing protocols up to 1400ºC to assess sensor performance. After sensor testing, the composition and microstructure of the sensors were characterized by XRD, XPS and SEM.

An example of the performance of a thick-film high temperature thermistor sensor produced in this work is shown in Figure 1. The sensor material in the embedded thermistor consisted of a patterned electrically conducting oxide buried within a laminate yttrium-stabilized zirconia (YSZ) substrate. The oxide sensor composition was initially printed on green YSZ substrates, laminated into a preform, and then sintered to 1400°C. The sintered preforms were embedded into chromia-based refractory bricks. The cast bricks were then sintered to 1450°C for 10 hrs in air. After sintering, the bricks were trimmed to expose the sensor material. Platinum wire, platinum mesh, and additional sensor ink was used to make connection to the sensor embedded inside a brick. Prior to testing, the assembled leads were heat treated to 1400°C to pre-sinter the sensor to wire connection. To collect data for Figure 1, the sensor to wire connection was placed at the same level as high temperature refractory for the furnace door with additional insulation to mimic typical high temperature furnace refractory configuration. With this testing arrangement, the leads connection was exposed to only about 500°C maximum temperature over the course of the test. With the leads outside the hot zone, the resistance drift during the temperature hold was recorded from 9.7 to 9.1 ohms over 20 hours.

Acknowledgements:

The authors would like to acknowledge the financial support of DOE NETL under contracts DE-FE0012383 and DE-FE0026171. The authors would also like to acknowledge West Virginia University Shared Research Facilities for support through materials characterization.

Figure 1. Resistance and temperature profile of conductive oxide based thermistor co-sintered inside ceramic substrate and imbedded in a chromia-based brick. The brick was tested to 1350°C.