Conformal Fabrication of High-Sensitivity Embedded Thick Film Thermopile Heat Flux Sensors: Design, Preparation, and Performance

Aiming to realize the accurate in situ monitoring of heat flux passing through the hot-end components of air vehicle, a embedded thick film thermopile heat flux sensor (ETFTP) was proposed. First, a wished ETFTP structure was designed, which consists of eight 300- $\mu \text{m}$ -thick polyimide (PI) thermal insulators embedded in the substrate, a PI dielectric layer, 80 pairs of platinum–silver (Pt–Ag) thermocouples and an outer PI encapsulation layer. Then, it was conformally prepared on flat and curved aluminum alloy substrates by laser etching combined with laser microcladding (LE-LMC) technologies. Furthermore, the morphology, interfacial characteristic, and performance of the ETFTP were systematically analyzed. The results show that all the heterogeneous interfaces in the ETFTP are tightly bonded, in which the bonding strength between PI film and aluminum alloy substrate is 10.7 ± 3 MPa and that between Ag/Pt electrodes and PI dielectric layer reaches 5 and 4 B level, respectively. Meanwhile, the Ag and Pt thermal electrodes are well sintered by LMC without damaging the PI layer beneath them, and no obvious elemental interdiffusion is detected at the Pt–Ag thermojunctions. The calibration results show that the ETFTP possesses a large output thermoelectric voltage and sensitivity, reaching 1.56 mV and 1.7 $\times \,\,10^{-{2}}$ mV/(kW $\cdot \text{m}^{-{2}}$ ) at an input heat flux of 113.55 kW/ $\text{m}^{{2}}$ , respectively. Besides, it shows good stability and fast dynamic response time (0.30–0.32 s).