The LaFe 11.4 Si 1.6 compounds are prepared by arc-melting and then annealed at different high temperatures from 1323 K(5 h) to 1623 K(2 h) . The powder X-ray diffraction(XRD) and microstructure observations show that large amount of 1:13 phase begins to appear in the LaFe 11.4 Si 1.6 compound annealed at 1423 K(5 h) . In the temperature range from 1423 K to 1523 K,the α-Fe and LaFeSi phases rapidly decrease to form 1:13 phase. The LaFeSi phase is rarely observed by XRD when the as-cast compound is annealed at 1523 K(5 h) . With annealing temperature increasing to 1573 K,LaFeSi phase is detected again in LaFe 11.4 Si 1.6 compound. In LaFe 11.4 Si 1.6 compounds annealed at 1523 K(5 h) ,at 1373 K(2 h) +1523 K(5 h) ,and 1523 K(7 h) +1373 K(2 h) ,the impurity phases including small amount of α-Fe and LaFeSi phase reduce in turn. The magnetic measurement shows that LaFe 11.4 Si 1.6 compounds annealed by above three processes keep the first-order of magnetic transition behavior,and T C are both at about 200 K. But the values of the maximal S M(T,H) of has large difference,they are 9.94,12.66,and 13.96 J/(kg·K) under a magnetic field of 0-2 T,respectively.
The effects of the introduction of Ce to La_(1-x)Ce_xFe_(11.5)Si_(1.5) alloys on 1:13 phase formation mechanism,the first-order magnetic phase transition strengthening characteristics,and magnetocaloric property were studied,respectively.The results show that the formation mechanisms of 1:13 and La Fe Si phases in La_(1-x)Ce_xFe_(11.5)Si_(1.5) alloys are the same as those of Ce_2Fe_(17) and CeFe_2 phases in Ce–Fe binary system,respectively.The substitution of Ce in 1:13 phase which is limited can make the first-order magnetic phase transition characteristics strengthen,which can make thermal and magnetic hysteresis increase,the temperature interval of temperatureinduced phase transition decrease,and the critical magnetic field of field-induced magnetic phase transition(HC)increase,respectively.Owing to the lattice shrink of 1:13phase with the increase in Ce content,the Curie temperatures(TC) show a linear decrease.The maximum change in magnetic entropy gradually increases due to the decrease in temperature interval of temperature-induced phase transition,but the relative cooling capacities are all about80 Jákg-1at magnetic field of 2 T.
The large disc LaFe11.6Si1.4 alloy, which was prepared by medium-frequency induction furnace, was annealed at 1503 K for different time. The main phases were 1:13 phase in the edge parts of the large discs alloy; the impurity phases included α-Fe phase, LaFeSi phase, and even very small amount of La5Si3 phase. The amounts of impurity phases reduced with increasing in annealing time. The magnetic properties in the edge parts of the large discs LaFe11.6Si1.4 alloy were investigated. The magnetic susceptibility had an abrupt change at Curie temperature(TC) as the magnetization in M-T curves. The alloys had almost the same TC(191 K), the magnetocaloric effect(MCE) and relative cooling power(RCP) increased with increasing in annealing time. In addition, for the same alloy, the magnetic hysteresis decreased with the increase in temperature.
The effect of Pr,Nd addition on the magnetic properties and magnetic exchange interaction of gadolinium alloys was systematically studied.Curie temperature T_C and magnetic moment of Gd_(1–x)RE_x(RE=Pr,Nd)systems with x<0.05 were investigated.When x<0.05,Pr and Nd formed respectively with Gd continuous solid solution which has the crystalline structure HCP.Study on the magnetic behavior indicated that at near room temperature,the simple ferromagnetism prevailed in these two systems of alloy.The Curie temperature and magnetic moment of Gd_(1–x)RE_x alloy decreased with RE(RE= Pr,Nd)content x increasing.The de Gennes factor of Gd_(1–x)RE_x alloy which was associated with the exchange interaction between magnetic spin components also decreased with RE content increasing.The above results showed that the magnetic exchange interaction between magnetic atoms in gadolinium could be effectively changed by the Pr,Nd addition.
