Relevancy between thermochromic and magnetic property of La_(1-x)Sr_xMnO_3 (x=0.2 and 0.33) smart radiation thin film materials prepared by magnetron sputtering

摘要

La_(1-x)Sr_xMnO_3 thin films with x=0.2 and 0.33 were prepared by magnetron sputtering method for its potential application on thermal control of spacecraft. The materials show a phase transition from ferromagnetic metal phase with a low infrared emittance to paramagnetic insulator phase with a high emittance. Because of the thermochromic property, they can automatically change their infrared emittance greatly in response to environment temperature and thermal load and keep the spacecraft electronic components working normally. A superconduction quantum interference device magnetometer was used to study magnetization over the temperature range 100-315 K. Temperature dependence of total hemispherical emittance was carried out in a liquid nitrogen cooled vacuum chamber by a steady state calorimetric method. Thermal emittance results indicate an obvious tuneability of both La_(0.8)Sr_(0.2)MnO_3 and La_(0.67)Sr_(0.33)MnO_3 thin films. Compared to La_(0.8)Sr_(0.2)MnO_3 thin film, La_(0.67)Sr_(0.33)MnO_3 thin film has a higher transition temperature and bigger emittance tuneability. Based on phase separation concept, the thermal emittance was fitted by two-energy-level Boltzmann distribution of metal phase volume fraction f_H. The measured magnetization curves were fitted in terms of mean-field approximation theory. The volume fraction of ferromagnetic phase f_M was not coinciding with the f_H very well. This result showed the f_H and the f_M plays a different role on thermal emittance property.