ИСТИНА

In this work the polymer matrix composites with MnAs powder as reinforcing material were obtained. The aim of this paper is to investigate the influence of both polymeric matrix type and processing methods of composite to the temperature behavior of magnetocaloric effect (MCE) and finding the dependence of MCE and unstable magnetic condition to be produced by thermal and magnetic history of sample. The nanosize particles composites were also investigated. The composites were prepared from nano- and microsrystalline metallic powder fixed in polymeric matrix; the volume of polymer is less than 8%. There were obtained both textured by magnetic field and isotropic samples fixed by hydrostatic pressure up to 20 kbar. A bulk MnAs demonstrates a strong temperature and field hysteresis of magnetic properties in magnetic field less than 50 kOe. The maximum value of MCE is ΔT = 0.28 K on heating (at 308 K) and ΔT = 0.88 K on cooling (at T = 306 K) for MnAs in magnetic field 10 kOe. It was shown, that the decreasing of grain size less than 1 μk leads to reduction of the MCE in nanosized composites. It was found, that both the hysteresis and maximum value of MCE are increased in textured composites in compare with bulk MnAs. It is established that the optimum properties, such as giant MCE and low temperature hysteresis, are found in composite with ~1μm particles and polyvinyl acetate matrix hardened at pressure of 10kBar. The maximum of MCE in this composite is ΔT ~ 1,3 K (at 309 K) in 10kOe. These properties are caused by the residual pressure of polymer mold on grains.