Refractory high entropy alloys (RHEA) are considered as novel promising high‐temperature materials for structural applications at ultrahigh temperatures primarily due to their attractive mechanical properties. By contrast, the oxidation behavior of RHEA has raised concern owing to pest oxidation, significant weight changes, scale spallation, or even complete oxidation at elevated temperatures. Herein, the currently available literature on high‐temperature oxidation behavior of RHEA is reviewed with respect to alloy composition, mass changes, corrosion products, and scale constitution. While many RHEA indeed suffer from poor oxidation resistance similar to that of pure refractory metals, some RHEA exhibit very good protectiveness, which is attributed to the formation of either well‐known protective scales such as α‐Al₂O₃ or rarely encountered complex oxides such as CrTaO₄. Thermodynamic and kinetic aspects of oxide formation and growth are discussed to understand the oxidation mechanisms typical of RHEA. Further research directions with respect to additional in‐depth studies elucidating the oxidation mechanisms as well as the further consequent improvement of the oxidation resistance of RHEA are developed from the current intermediate stage of research in the field.

Non-contact ratiometric thermometry has applications ranging from in situ physiological measurements to industrial process monitoring. The technique is based on the optical detection of the fluorescence intensity ratio (FIR) of two thermally-coupled levels (TCLs). Here, we report a Yb³⁺/Nd³⁺ co-doped La₂O₃ microcrystal ratiometric thermometer based on upconverted emission in the near-infrared (NIR) after excitation with a 980 nm laser diode, which operates effectively over the wide temperature range from 290–1230 K. The thermometer uses the TCLs of Nd³⁺:⁴F_7/2 (emission peak 762 nm) and Nd³⁺:⁴F_5/2 (emission peak 825 nm). The chosen levels combine desirable characteristics to act as a sensitive temperature sensor over a wide range of elevated temperatures: namely a suitable energy gap (ΔE = 950 cm⁻¹); and weak thermal quenching effects (maximum photoluminescence at 803 and 853 K respectively for two Nd³⁺ emission peaks). This leads to a high relative sensitivity (S_R) of 1334/T², and low temperature uncertainty ΔT_min of 0.1 K (<400 K), 1 K (400–853 K) and 3 K (853–1233 K). In addition to these characteristics, the excellent repeatability of FIR of the two Nd³⁺ emission peaks makes Yb³⁺/Nd³⁺ co-doped La₂O₃ microcrystals a promising non-contact NIR ratiometric thermometer for temperatures up to 1230 K.

The global trend of miniaturization and concomitant increase of functionality in microelectronics, microoptics, and various other fields in microtechnology leads to an emerging demand for temperature control at small scales. In this realm, elastocaloric cooling is an interesting alternative to thermoelectrics due to the large latent heat and good down-scaling behavior. Here, we investigate the elastocaloric effect due to a stress-induced phase transformation in binary TiNi and quaternary TiNiCuCo films of 20 μm thickness produced by DC magnetron sputtering. The mesoscale mechanical and thermal performance, as well as the fatigue behavior are studied by uniaxial tensile tests combined with infrared thermography and digital image correlation measurements. Binary films exhibit strong features of fatigue, involving a transition from Lüders-like to homogeneous transformation behavior within three superelastic cycles. Quaternary films, in contrast, show stable Lüders-like transformation without any signs of degradation. The elastocaloric temperature change under adiabatic conditions is −15 K and −12 K for TiNi and TiNiCuCo films, respectively. First-of-its-kind heat pump demonstrators are developed that make use of out-of-plane deflection of film bridges. Owing to their large surface-to-volume ratio, the demonstrators reveal rapid heat transfer. The TiNiCuCo-based devices, for instance, generate a temperature difference of 3.5 K within 13 s. The coefficients of performance of the demonstrators are about 3.