Studies On The Negative Thermal Expansion Materials Of LaFeSi

Due to the special requirements for thermal expansion properties in precisioninstrument and aerospace, La(Fe,Si)13materials with negative thermal expansion (NTE)properties have been explored, and the NTE properties have been optimized by otherelements-doping. Moreover, the mechanism of the broadening of the NTE temperaturerange has been investigated.A series of La(Fe,Si)13and Si, Co, Mn-doping compounds have been prepared in anarc melting furnace. the samples annealed at1100℃for2days formed a singleLa(Fe,Si)13phase with the NaZn13structure. Microstructure and coefficient of thermalexpansion of the compounds were investigated by X-ray diffraction andthermo-dilatometer, respectively. Relation between the NTE properties and dopingelements was discussed. The magnetic phase transition was measured and discussed bySQUID-VSM. Electric conductivity and heat conductivity were measured of one sampleto provide theoretical and experimental foundations for apply of La(Fe,Si)13compounds.The results show that La(Fe,Si)13compounds translate the ferromagnetic toparamagnetic on heating at the Curie temperature. The magnetostrictive effect resultingof the magnetic order-disorder transition offset the normal thermal expansion andcontraction caused by atom anharmonic vibrations, resulting in a NTE phenomenon. Thevalue of NTE (ΔL/L) of LaFe11.5Si1.5is3.6×10-3in the temperature range of170-240K,and the average expansion coefficient is-50.1×10-6.The temperature and the temperature range of the NTE in La(Fe,Si)13compoundscan be changed by some elements doping. Amount of Si-doping increase the NTEtemperature and broaden the temperature range, the value of NTE decrease rapidly andthe temperature of the magnetic order-disorder transition increase. The Si-doping resultin the cure temperature increasing, and the first-order magnetic transition translated tosecond-order. Co-doping has similar effects with Si, but the loss of the value of NTE isless, and the NTE temperature increase by a bigger margin. Typically, the operation-temperature window in the LaFe10.5Co1.0Si1.5compound reaches110K (240-350K), which includes room temperature. Amount of Mn-doping result in the NTE temperaturedecreasing, and the first-order magnetic transition translate to second-order.The measures indicate that electric conductivity and thermal conductivity areSatisfying and stable from liquid nitrogen temperature to room temperature. La(Fe,Si)13compounds are more suitable for practical application than other common NTEmaterials.