The Heat Effect Of Magnetic Phase Transition Materials Investigated By Differential Scanning Calorimetry Method

With the discovery of giant manetocaloric effect in first-ordertransition materials, great attention has been paid to the development ofroom-temperature magnetic refrigeration and its applications in recentyears. Within the known room-temperature magnetic refrigeration materials,MnFe（P,Si） compound is potential for applications with its advancedmagnetocaloric properties and its low cost. Cam Thanh et al studied thepreparation and magnetocaloric properties of the compound. The resultsshow that MnFeP_1-xSi_xcompounds form in the Fe2P-type hexagonalstructure in the component range of0.44≤x≤0.60, and undergo first-orderphase transition from ferromagnetic to paramagnetic. Due to the latent heatof first-order phase transition, it is very difficult to measure the specificheat of first-order phase transition materials with conventional method.However, differential scanning calorimetry method has the advantage ofeasy implementation, shorter measure time, less sample and high accuracy,which leads to its prevalent use in specific-heat measurement. This thesismainly reported on the heat effect and phase transition in MnFe（P,Si） seriescompounds obtained by analyzing the specific heat.The results show that the specific heat of Mn_2-xFe_xP_0.51Si_0.49（x=0.8,0.85,0.9,0.95,1.0） series compounds is abnormal when they are near tophase transition. With the increase of Fe content, the abnormal peak movestowards the high temperature region. When x=0.8, the compound has theminimum phase transition temperature，the value is280.8K. When x=1.0,the compound has the maximum phase transition temperature，the value is345.7K. This result is consistent with magnetization measurement. Theentropy of this kind of compound shows stepwise change near the phasetransition. In other words, the entropy at phase transition does not changecontinuously, indicating the phase transition is of first order. Throughentropy—temperature curve, entropy change and temperature span of thiscompounds have been determined. The entropy change Sand temperature span Tgrow with respect to the growth of x content. When the Fecontent is1.0, the entropy change and temperature span reach theirmaximum,10J/mol-K and16.2K.（2）The abnormal peak temperature of specific heat （phase transitiontemperature） of Mn_1.3Fe_0.7-xP_0.45Si_0.55（x=0.0,0.02,0.04,0.06,0.08,0.1）series compounds experience growth and drop with the growth of x content.Peak temperature changes in the range of239.2～303.8K. When x=0.1,the phase transition temperature of the compound is the lowest, while whenx=0.04, the phase transition temperature of the compound is the highest.When x is changing between0.0and0.04, entropy—temperature curve ofcompounds displays obvious staged change. When x content continue togrow, staged change becomes less and less obvious. The entropy changeand temperature span of this kind of compound do not have any pattern.When the Fe component is0.7, the entropy change of the compoundreaches maximum,5.3J/mol-K. In contrast, when the Fe component is0.62,the temperature span of the compound reaches maximum,8.9K.（3）Within the phase transition, the specific heat of theMn_1.2Fe_0.8-xCoxP_0.48Si_0.52（x=0.0,0.01,0.03,0.05） series compoundsdisplays endothermic peak, the peak temperature decreases with theincrease of Co content, and it could be adapted with respect to roomtemperature. With the increase of x content, the entropy change andtemperature span of this series compound become smaller and when x=0.0,the entropy change of the compound reaches maximum,7.8J/mol-K.The present research also investigated the influence of DSC differentheating and cooling scan rate on the phase transition of the abovementioned compounds. We find that peak width and peak temperature ofphase transition are very sensitive to the scan rate. With the growth ofheating/cooling rate, peak temperature increase/decrease, peak width growsand phase transition thermal hysteresis also grows. Based on this change ofthermal hysteresis led by different scan rate, extrapolation method is usedto determine the actual size of phase transition and thermal hysteresis of MnFe（P,Si） compound.