Nanocomposites and nanomaterials
Ageing phenomena in temperature-sensitive
thick-film nanostructures
Klym H.1, Hadzaman I.2, Kostiv Yu.1
1 Lviv Polytechnic National University, Bandery str., 12, Lviv-79013, Ukraine 
E-mail: halyna.i.klym@lpnu.ua, klymha@yahoo.com
2 Ivan Franko Drohobych State Pedagogical University, I. Franko str., 24, Drohobych-82100, Ukraine
In this investigation of ageing processes in the single as well as two- and three-layered thick-film elements based on NiMn2O4-CuMn2O4-MnCo2O4 ceramics was performed.The ageing test was carried out at long-term isothermal treatment at 170 oC to study the thermal stability of the obtained thick films. General duration of ageing test was 250 h, measurements electrical resistance R was performed at 25 °C after 6, 12, 18, 24, 30, 48, 64, 104, 144, 208 and 250 hours.
It was established that all investigated temperature-sensitive have a characteristic linear dependence of electrical resistance on the temperature region from 298 K to 368 K in semilogarithmic scale. In the single-layered thick films change of electrical resistance at 250 h is ~ -7.5 % and -8.7 %. This behavior of relaxation kinetics occurs due to two independent processes: at first the rapid diffusion process of molecules in the surface layer is dominated and then organic components burn what is represented in diminishing of relative electric resistance over time of degradation test.
Kinetic dependences for studied single-layered thick-film elements show a typical suppressed-exponential dependence on time in accordance with the known relaxation function [1]. Such behavior is caused by burnout of organic compounds. This process is typical for structural-heterogeneous media such as thick-film ceramic structure. Typical increasing form of thermally-induced aging curve with relative saturation during prolongation of test is observed in two-layered thick-film structures in the first 50-150 hours. Maximum drift of electrical resistance is ~ 6 %. Ageing kinetics are described by extended exponential relaxation function. It should be noted that thermally-induced mechanism in these samples was complex including not only the cation redistribution, but also mass transfer processes. 
Unfortunately, the two- and three-layered thick film structures are characterized by relatively high stability, the drift of electrical resistivity is 1.5 %. Such highly stable components are obtained at modification of paste compositions (for achieve the required viscosity) and preparation of thick-film layers based on different compositions of ceramics.