NSRRC Activity Report 2022

Neutron Science 079 Z ero thermal expansion (ZTE) materials are deployed in many applications to relieve or avoid thermal stress and are thus essential in engineering applications—for instance, they are used in spacecraft, optical instruments, and containers on liquid natural gas ships. However, most ZTE materials are unstable under harsh conditions, limiting their lifetime and applications in uncontrolled environments. Even commercial invar alloys (Fe–36Ni) rapidly corrode in seawater, moist atmospheres, or phosphate/sulphate solutions. Xianran Xing (University of Science and Technology Beijing, China) and coworkers have researched novel negative thermal expansion and ZTE materials for many years. So far, they have discovered many potential candidates in the binary R x M y (using x:y = 1:2, 1:13, 2:17, etc. ) compound family. 1–3 Xing, Kun Lin and Xin Chen (both also from University of Science and Technology Beijing, China) reported a new pseudobinary (Zr,Ta)(Fe,Co) 2 alloy (ZTFC) in the cubic Laves phase that Novel ZTE Material with High Seawater Corrosion Resistance Zr/Ta forms a passive protection layer in air and renders pseudobinary intermetallic (Zr,Ta)(Fe,Co) 2 durable ZTE materials. exhibits high environmental, cyclic thermal, and phase stabilities. 4 Thermal expansion and the associated magnetic properties of ZTFC were characterized using X-ray powder diffraction, neutron powder diffraction (NPD) at WOMBAT , thermodilatometry, and magnetometry. In addition, the chemical stability of ZTFC was verified through electrochemical testing, atomic force microscopy, transmission electron microscopy, Auger electron spectroscopy, and X-ray photoelectron spectroscopy. The coefficient of thermal expansion (CTE), ∆ l / l 0 , of several Zr x Ta 1−x Fe 1.7 Co 0.3 compounds is plotted in Fig. 1(a) with those of iron and invar for comparison. The thermal expansion of this compound was effectively tuned using chemical modification and optimized to a value of almost zero (α l = 0.21[2] × 10 −6 K −1 at x = 0.2 [ZTFC], 100–360 K). The CTE of ZTFC was approximately one order of magnitude smaller than that of invar, despite its slightly Fig. 1 : Thermal expansion of ZTFC and related alloys. (a) ∆ l / l 0 of Zr x Ta 1−x Fe 1.7 Co 0.3 (x = 0–0.4), Fe, and invar alloys measured using a dilatometer; inset shows the lattice constant a of ZTFC measured using neutron diffraction. (b) Comparison of coefficients of thermal expansion and the corresponding temperature windows with conventional ZTE alloys. (c) ∆ l / l 0 of ZTFC along three perpendicular directions exhibits isotropic thermal expansion. (d) ∆ l / l 0 of ZTFC in the 1 st and the 101 st cycles. Each cycle involves a thermal shock from 77 to 400 K. (e) ∆ l / l 0 of ZTFC before and after immersion in a 3.5 wt% NaCl solution for 14 days. [Reproduced from Ref. 4]