EVALUATION OF SOME THICK FILM MATERIALS FOR TEMPERATURE, FORCE, AND HUMIDITY SENSORS M. HROVAT, D. BELAVIC*, J. HOLe loief Stefan Institute, lamova 39, 61000 Ljubljana, Slovenia * Iskra HIPOT, Trubarjeva 7, 68310 Sentjemej, Slovenia 1. Introduction Thick film technology is a well known, mature and versatile technology for producing hybrid circuits. Also, for some applications it provides a relatively inexpensive and convenient way to produce sensors which can be integrated in to hybrid circuits. The aim of this paper is to present some results obtained with commercial thick film pastes for temperature and strain sensors (thick film conductors and resistors, respectively), and results on thick film humidity sensors based on Nb205 doped Ti02. 2. Temperature sensors Thermocouples were made with different combinations of thick film conductors based on Ag, Au, AulPd, AuIPt, AglPd, AgIPt, Ni and Cr, printed and fired on alumina substrates. Noble metal based pastes (Du Pont) were frred at 850°C, while air-frring Ni and Cr pastes (Electro Science Labs.) were frred at 750°C. Absolute EMFs (at 25°C) and nonlinearity (dependence of thermocouple EMF on temperature) are presented in Table 1[1,2]. Table I: Absolute EMF and nonlinearity of thick film thermocouples Conductor EMF (25 "C) Nonlinearity Conductor EMF (25 0c) Nonlinearity combination b.tVIK) [o/crIK] combination [J.1VIK) [o/crIK] AulPd-Au 16.85 0.24 AulPd-AglPd 8.29 0.35 AglPd-Au 6.62 0.22 AuJPt-AglPd 2.38 0.18 AulPd-Ag 14.41 0.22 AuJPt-Au 10.75 0.13 AuJPt-Ag 9.06 0.24 AuJPt-AulPd 6.61 0.08 Au-Ag 0.04 ? AgIPt-Ag 1.49 -0.Q7 Ni-Cr 13.65 - 0.22 AglPd-AglPt 5.48 0.13 AglPd-Ag 6.15 0.21 The prototype of a thick film heat flux sensor with 110 thermocouples, connected in series, was designed [1]. Heat flux sensors, realized in thick film technology have some good characteristics, for example low thermal resistivity and relatively high operating temperatures [3,4]. The thermal resistance layer is made of thick film multilayer dielectric pastes, printed and frred on Ah03 substrates. Multilayer dielectrics are glass-like materials with higher heat resistivity than alumina substrates. The heat flow is perpendicular to the ceramic substrate. The temperatures on the top (cold junction) and underneath (hot 267 W. K. Jones et al. (eds.). MCM ClMixed Technologies and Thick Film Sensors. 267-272. © 1995 Kluwer Academic Publishers.