Studies On The Thermophysical Properties Of Doped Manganese Nitride Negative Thermal Expansion Materials At Cryogenic Temperature

Due to the special requirements for thermal expansion properties in cryogenic engineering and space technology, doped manganese nitride materials with excellent negative thermal expansion (NTE) properties at cryogenic temperatures have been explored, and the mechanism of the broadening of the NTE operation-temperature window has been investigated. Moreover, application studies on doped manganese nitride materials with excellent negative thermal expansion properties at cryogenic temperatures have been also carried out.Based on the Mn3CuN, a series of doped manganese nitride materials, including Mn3(Cu0.6Ge0.4)N1-xCx, Mn3(Cu0.8-xAgxGe0.2)N, Mn3(Cu0.6-xNixGe0.4)N, Mn3(Cu0.6SixGe0.4-x)N and Mn3(Cu0.5SixGe0.5-x)N, were designed and prepared. Their crystal structures and thermal expansion properties were investigated, and the relationship between thermal expansion properties and doping element was discussed. The results show that (1) The NTE operation-temperature window of Mn3(Cu0.6Ge0.4)N1-xCx shifts toward lower temperature region with increasing C content, but the width of NTE operation-temperature window (?T) and the change value of ?L/L(300K) in the NTE operation-temperature window are independent of C. (2) The NTE operation-temperature window of Mn3(Cu0.8-xAgxGe0.2)N shifts toward higher temperature region and the change value of ?L/L(300K) in the NTE operation-temperature window decreases with increasing Ag content, ?T is independent of Ag. (3) The second phase of Mn-Ni alloy appears in the Ni and Ge co-doped manganese nitride materials. The NTE operation-temperature window shifts toward lower temperature region with increasing Ni content and ?T is independent of Ni. The change value of ?L/L(300K) in the NTE operation-temperature window decreases and results to zero with increasing Ni content. (4) The ?T of Mn3(Cu0.6SixGe0.4-x)N increases with increasing Si content. Especially for Mn3(Cu0.6Si0.15Ge0.25)N, the temperature range of NTE behavior of is 90-190K (?T=100K), coefficient of thermal expansion (CTE) is -16×10-6K-1. (5) The average CTEs of Mn3(Cu0.5SixGe0.5-x)N(x=0.1, 0.15) in the temperature range of room temperature to liquid nitrogen temperature are very small, which are 1.3×10-6K-1 and 1.65×10-6K-1 , respectively. The discoveries of nearly zero and negative thermal expansion materials prepare the ground for further development of cryogenic engineering.The mechanism of the broadening of the NTE operation-temperature window was investigated though situ X-ray diffraction, magnetic susceptibility and special heat experiments. The results show that magnetic phase transition appears in the NTE operation-temperature window, and the type of magnetic phase transition gradually changes with increasing Si content. Mn3(Cu0.6Si0.15Ge0.25)N shows a typical characteristic of spin-glass systems. After theoretical analysis, it is concluded that spin-glass which results from the hyperdispersion of Si at micro scale is the reason for the broadening of the NTE operation-temperature window. This investigation provides theoretical and experimental foundations for exploring new NTE materials with broader NTE operation-temperature window.The composite materials made from nano-Mn3(Cu0.6Si0.15Ge0.25)N modified by a plasma treatment and epoxy resin were prepared. Their thermal expansion properties and thermal conductivities were investigated. The results show that the addition of Mn3(Cu0.6Si0.15Ge0.25)N can significantly decrease CTE and increase thermal conductivity. The average CTE of composite contains 32vol% Mn3(Cu0.6Si0.15Ge0.25)N is 22×10-6K-1 in the temperature range of 190-77 K, which is 42% lower than that of pure epoxy resin. The thermal conductivities are 0.48 W(m·K)-1 at 298K and 0.28 W(m·K)-1 at 77K, respectively, which are 2.8 and 4 times as large as that of pure epoxy resin, respectively. This application study provides a new method for resolving thermal expansion problem and simultaneously improving thermal conductivity of materials in cryogenic engineering.