(1. 曲靖师范学院 磁性材料及器件研究中心，曲靖 655011；
2曲靖师范学院 物理与电子工程学院，曲靖 655011)
摘 要: 为深入解磁制冷材料Ni-Mn-Ga合金的热膨胀特性，利用SEM、XRD、DSC及PPMS系统分别对合金Ni54+xMn19-xGa27 (x=0.2，0.6，1.0)的组分、结构、相变及热膨胀特性进行实验测试。结果表明：随着Ni含量的增加，合金的马氏体相变温度逐渐增加。当x从0.2增至1.0时，合金正反马氏体相变峰温分别从274、282 K增至300、309 K，且存在7~10 K的热滞后。在升降温过程中，x为0.2和0.6的合金出现两个连续的马氏体相变、x=1.0的合金发生磁-结构耦合转变，相变温区分别为33.5 K、35.1 K、27.5 K。零场热应变曲线表明，合金具有各向同性的热膨胀特性。马氏体相与奥氏体相的热膨胀系数分别为5.02×10-6~10.31×10-6 K-1和3.74×10-6~7.72×10-6 K-1之间。马氏体相变过程中合金出现正热膨胀行为和奇异的负热膨胀行为，最大的负热膨胀系数约为-139.84×10-6 K-1。结合实验数据，从微观的角度对Ni-Mn-Ga的负热膨胀效应进行初略讨论。
关键字: 磁制冷；Ni-Mn-Ga合金；马氏体相变；热膨胀特性；负热膨胀行为
（1. Center for Magnetic Materials and Devices, Qujing Normal University, Qujing 655011, China;
2. College of Physics and Electronic Engineering, Qujing Normal University, Qujing 655011, China）
Abstract:The magnetic refrigeration materials Ni54+xMn19-xGa27 (x=0.2, 0.6, 1.0) with nominal composition were prepared to reveal the thermal expansion properties of Ni-Mn-Ga alloys. The actual compositions, crystal structures, martensitic transformation temperatures, magnetic transition and zero field thermal strain of the alloys were experimentally investigated by scanning electron microscope equipped with energy-dispersive spectrometer (SEM-EDS), X-ray diffractometer (XRD), differential scanning calorimetry (DSC), vibrating sample magnetometer (VSM) and standard strain-gauge connected with physical properties measurement system (PPMS), respectively. The results show that the nominal composition of these alloys agree with the actual composition. The results from DSC and magnetic transition measurements show that the martensitic transformation temperatures gradually increase with Ni content increasing. Moreover, the peak temperatures of direct and reverse martensitic transformations increase from 274 K, 282 K (x=0.2) to 300 K, 309 K (x=1.0), respectively, and they have a distinct thermal hysteresis of about 7 ~ 10 K. Two successive martensitic transformations and magnetic-structural coupling transition are observed in alloys x=0.2, 0.6, and x=1.0, respectively, during cooling and heating cycles. The temperature window of phase transformations is about 33.5 K, 35.1 K and 27.5 K for x=0.2, 0.6 and 1.0. Zero field thermal strain curves illustrate that the alloys studied exhibit an isotropic thermal expansion property. The thermal expansion coefficients range from 5.02 to 10.31×10-6 K-1 and 3.74 to 7.72×10-6 K-1 at martensite and austenite phases, respectively, for Ni54+xMn19-xGa27 alloys. More importantly, the alloys exhibit an obvious and abnormal negative thermal expansion behavior in the vicinity of the martensitic transformation. The maximal negative thermal expansion coefficient occurs in alloy with x of 1.0 is about -139.84×10-6 K-1. This abnormal negative thermal expansion behavior may well be related to the nucleation and orientation of martensitic variants during the martensitic transformation. Based on the experimental data, the negative thermal expansion effect of Ni-Mn-Ga alloys was discussed briefly from microcosmic point of view.
Key words: magnetic refrigeration; Ni-Mn-Ga alloy; martensitic transformation; thermal expansion property; negative thermal expansion behavior
