Experimental Study Of Thin Film Sensors For Measuring Temperature And Heat Flux Inside Fuel Cells

Fuel cells（FCs） are the devices which continually convert the chemical energy ofthe fuel and oxidant into electrical energy through the electrochemical reactiondirectly. With the rapid development of modern communication technology, fuel cellsare gradually applied to the mobile electronic products, and developed to a morecompact and miniature structure.Because of their high energy density, good reliability, portability, simplestructure and environmental compatibility, passive micro direct methanol fuelcells（μDMFCs） are widely used as main power source of portable devices.Temperature, heat flux and other factors affect performance and lifetime of passiveμDMFCs.With the micromation of the fuel cell, conventional techniques for measuringtemperature and heat flux have no longer been suitable for passive μDMFCs. Due tothe volume problem of conventional thermocouples, it is difficult to lay them in themicro fuel cells（μFCs）. Even if conventional thermocouples are arranged in the flowchannel of fuel cell reluctantly, the heat that the wires released shall interfere the heatflux distribution within the local fuel cell. Furthermore, the volume is so large that itincreases the assembly difficulty of fuel cell, even leads to fuel leakage during fuelcell working.With micro electro mechanical systems（MEMS） technology widely used in theproduction of micro sensors and μFCs, micro sensor has been gradually introducedinto temperature measurement inside fuel cell.On the basis of previous work, the vacuum evaporation technique was applied toproduce thin film thermocouples and transient thin film heat flux gauges, andcalibrations were performed to determine the sensor’s sensitivity and temporalresponse.In order to measure the temperature and heat flux inside the fuel cells, silicondioxide （SiO2） was selected as insulating substrate, the overall dimension of whichwas8mm long,8mm wide, and0.1mm thick. Two different metals cobalt and stibiumwere chosen as thermode materials. Thin film thermocouples consisted of13Co-Sbjunctions, which were connected in series. The overall dimension of thin filmthermocouples was4.6mm long,4.6mm wide, and0.17⁰.18μm thick. Transient thinfilm heat flux gauges were designed and fabricated using Co-Sb thermopilesconsisting of11Co-Sb junctions deposited on both sides of the0.15μm thick thermalresistance layer. The overall dimension of transient thin film heat flux gauge was6mm long,6mm wide, and0.47⁰.48μm thick.By comparing dynamic characteristics of thin film sensors with different thicknesses of SiO2protective layers, we obtained that within a certain range,thickness of SiO2protective layers impacted thin film sensors measurement precision.The greater the thickness of the protective layer, the lower the sensors respond rate,but the higher the thermal sensitivity.Finally, to do further test and validation for sensors performance, we measuredtemperature variety at the moment of a match being lighting up with thin filmthermocouple, measured heat flux variety at the moment of a boiling tube beingstopping heating and a match being lighting up with transient thin film heat fluxgauge. The sensitivities of thin film thermocouples and transient thin film heat fluxgauge were0.83843μV/oC and2.20575×10^-8V/（W/m²）. The steady time constants ofthin film thermocouples and transient thin film heat flux gauge were72ms and36msrespectively. The experimental results proved that the thin film sensors had someadvantages in response time, application range and dimensions, which could satisfythe requirement of temperature and heat flux measurement inside the micro space ofFCs, with great development prospects.