The influences of Ni, Cr and Nb as substitution elements for Fe were investigated. The change in microstructure and the magnetic properties have been discussed in detail. Substitution elements Ni, Cr and Nb not only have limited solubility in NaZn13-type (1:13) phase, but also hinder the peritectoid reaction. Ni element mainly enters into La-rich phase while Cr element mainly concentrates in α-Fe phase, which both have detriment effect on the peritectoid reaction, leading to a large residual of impurity phases after annealing and a decrease of magnetic entropy change. Besides, Ni and Cr participated in peritectoid reaction by entering parent phases but slightly entering 1:13 phase, which would cause the disappearance of first order magnetic phase transition. A new phase (Fe,Si)2Nb was found when Nb element substitutes Fe in La(Fe,Si)13, suggesting that Nb does not participate in peritectoid reaction and only exists in (Fe,Si)2Nb phase after annealing. The alloy with Nb substitution maintains the first order magnetic phase transition character.
Magnetic properties and magnetocaloric effects of LaFe11.5−xTxSi1.5 (T = Cr, Ni; x = 0, 0.1, 0.15, and 0.2) have been investigated by magnetic measurements and differential scanning calorimetry and compared with that of Mn, Co substitution. The Curie temperatures (TC) of LaFe11.5−xCrxSi1.5 compounds are decreased from 193 K (x = 0) to 185 K (x = 0.2) with increasing Cr. Although the latent heat and relative cooling power are slightly decreased by Cr doping, the first-order magnetic transition is preserved, which lead to a large entropy change about 21 J/kg·K for all the Cr substituted samples. In contrast, Ni only concentrates in La-rich phase, and due to the decrease of Fe/Si ratio in the main phase by Ni doping, the TCs of LaFe11.5−xNixSi1.5 compounds increase from 193 K (x = 0) to 206 K (x = 0.2) with increasing Ni amount, and the nature of magnetic transition is changed from first order to second order resulting in the decrease of entropy change.
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