Electrical Isolation
An insulating thin film layer positioned between a metal substrate and a sensor element prevents electrical shorting at elevated operating temperatures. In sensor systems exposed to extreme environments, a high temperature dielectric buffer acts as a barrier that prevents current leakage into the base metal. This layer maintains its high electrical resistivity even when the operating temperature exceeds eight hundred degrees Celsius.
High resistivity is maintained by ensuring that the material has a wide bandgap and a low concentration of charge carriers at high thermal energy.
Material Selection
Alumina and yttria stabilized zirconia are the primary ceramic materials chosen for this protective layer because of their high thermal and chemical stability. Sputtering these ceramics requires reactive gas mixtures and careful control of deposition rates to achieve the necessary stoichiometry. The density of the resulting film determines its ability to resist electrical breakdown under high voltage conditions.
Diffusion Mitigation
Chemical interactions between the substrate and the sensor are suppressed by the dense structure of the ceramic film. The high temperature dielectric buffer prevents the migration of substrate alloy elements, such as iron and nickel, into the sensitive layer. This isolation protects the electrical stability of the sensor, preventing drift and premature failure of the measurement circuit.
Mechanical Strain
Internal stresses must be managed to prevent cracking of the brittle ceramic layer during thermal expansion of the underlying superalloy. Sputtered films are deposited with compressive residual stresses to counteract the tensile stresses that arise during heating. The thickness of the buffer layer is optimized to balance electrical isolation with thermal shock durability.
