Beijing National Laboratory for Condensed Matter Physics and State Key Laboratory of Magnetism, Institute of Physics, Chinese Academy of Sciences , Beijing 100190, P. R. China.
J Am Chem Soc. 2015 Feb 11;137(5):1746-9. doi: 10.1021/ja510693a. Epub 2015 Feb 2.
MnCoGe-based compounds undergo a giant negative thermal expansion (NTE) during the martensitic structural transition from Ni2In-type hexagonal to TiNiSi-type orthorhombic structure. High-resolution neutron diffraction experiments revealed that the expansion of unit cell volume can be as large as ΔV/V ∼ 3.9%. The optimized compositions with concurrent magnetic and structural transitions have been studied for magnetocaloric effect. However, these materials have not been considered as NTE materials partially due to the limited temperature window of phase transition. The as-prepared MnCoGe-based compounds are quite brittle and naturally collapse into powders. By using a few percents (3-4%) of epoxy to bond the powders, we introduced residual stress in the bonded samples and thus realized the broadening of structural transition by utilizing the specific characteristics of lattice softening enforced by the stress. As a result, giant NTE (not only the linear NTE coefficient α but also the operation-temperature window) has been achieved. For example, the average α̅ as much as -51.5 × 10(-6)/K with an operating temperature window as wide as 210 K from 122 to 332 K has been observed in a bonded MnCo0.98Cr0.02Ge compound. Moreover, in the region between 250 and 305 K near room temperature, the α value (-119 × 10(-6)/K) remains nearly independent of temperature. Such an excellent performance exceeds that of most other materials reported previously, suggesting it can potentially be used as a NTE material, particularly for compensating the materials with large positive thermal expansions.
基于 MnCoGe 的化合物在马氏体结构相变过程中经历巨大的负热膨胀（NTE），从 Ni2In 型六方结构转变为 TiNiSi 型正交结构。高分辨率中子衍射实验表明，单元体积的膨胀可以达到 ΔV/V ∼ 3.9%。已经研究了具有协同磁结构转变的优化成分的磁热效应。然而，这些材料尚未被认为是 NTE 材料，部分原因是相变的温度窗口有限。所制备的基于 MnCoGe 的化合物相当脆，自然会坍塌成粉末。通过使用少量（3-4%）的环氧树脂将粉末粘结，可以在粘结样品中引入残余应力，从而利用晶格软化的特定特性通过应力强制来拓宽结构转变。结果，实现了巨大的 NTE（不仅是线性 NTE 系数 α，还有操作温度窗口）。例如，在 MnCo0.98Cr0.02Ge 化合物中，观察到平均 α̅ 高达-51.5 × 10(-6)/K，操作温度窗口从 122 到 332 K 宽达 210 K。此外，在室温附近 250 到 305 K 的区域，α 值（-119 × 10(-6)/K）几乎与温度无关。这种优异的性能超过了以前报道的大多数其他材料，表明它可以作为 NTE 材料使用，特别是用于补偿具有较大正热膨胀的材料。
