Nb, Cr And Re Doping Manganese Nitride Negative Thermal Expansion Materials And Their Physical Properties At Cryogenic Temperatures

The manganese nitride negative thermal expansion (NTE) materials were obtained by elements doping method, and their NTE properties at cryogenic temperatures were also investigated. The mechanism of NTE in these materials is discussed through the analysis on magnetic phase transition.Nb, Cr and Re were selected as doping elements in the Mn₃(Cu_0.6Ge_0.4)N matrix. By investigating their crystal structures and thermal expansion properties, we found that Nb doping can shift the NTE occurring-temperature window to low temperature region, and the NTE operation-temperature (ΔT) are also broadened, theΔT of Mn₃(Cu_0.6 Nb_0.15Ge_0.25)N and Mn₃ (Cu_0.6Nb_0.2Ge_0.2)N can reach 60% larger than Mn₃(Cu_0.6Ge_0.4)N. The coefficient of expansion (CTE) for Mn₃ (Cu_0.6Nb_0.15Ge_0.25)N is of -19.5×10^-6 K^-1 at 165-210K; TheΔT of Mn₃-xCrx(Cu_0.6Ge_0.4)N can shift to low temperature region with the increase of Cr content, Mn_2.55 Cr_0.45(Cu_0.6Ge_0.4)N has a CTE of -19.14×10^-6 K^-1 at 80-150K; The CTE changes with Re doping, Mn₃(Cu_0.35 Re_0.25Ge_0.4)N has a very low linear CTE at 220-315K。By measuring the magnetic property, we found that the NTE transitions of Mn₃(Cu_0.6 Nb x Ge_0.4-x)N are related with two types of magnetic phase transition: firstly, at high temperature region, with low doping concentration, the NTE transition is in a good coincidence to the paramagnetic phase and anti-ferromagnetic phase transition; Secondly, at low temperature region, with high doping concentration, the NTE transition is coincidence to the paramagnetic phase and ferromagnetic phase transition, further studies have shown that such magnetic phase transition may created a spin-glass system, which may be a reason for the broadening of the NTE operation-temperature window. The magnetic studies of low expansion materials show that the"zero-expansion"of Mn₃ (Cu_0.35 Re_0.25Ge_0.4)N may be a result of multi-phase effect.