Phase transformations in oxides above 2000°C: experimental technique development

Abstract

The oxidation of boride and carbide-based ultra-high-temperature ceramics is the primary limiting factor for their use as aerodynamic surfaces. Understanding the behaviour of the oxides that can result from oxidation of metal borides and carbides at very high temperatures is essential to optimise and tailor the performance of these materials; yet experimental thermodynamic and structural data for refractory oxides above 2000°C are mostly absent. The following techniques that can be applied to fill this gap are discussed: (i) commercial ultra-high-temperature differential thermal analysis for investigation of phase transformations and melting in inert environments to 2500°C, (ii) a combination of laser heating with a splittable nozzle aerodynamic levitator for splat quenching and drop calorimetry from temperatures limited only by sample evaporation, (iii) synchrotron X-ray and neutron diffraction on laser-heated aerodynamically levitated oxide samples for in situ observation of phase transformations in variable atmospheres, refinement of hightemperature structures and thermal expansion. Recent experimental findings include anomalous thermal expansion of the defect fluorite phase of YSZ, thermodynamics of pyrochlore-fluorite transformation from high-temperature structure refinements, and measurement of thermal expansion to the melting temperatures and fusion enthalpies of Zr, Hf, La, Yb and Lu oxides. These methods provide temperatures, enthalpies and volume change for phase transformations above 2000°C, which are required for thermodynamic assessments and calculation of phase diagrams of multicomponent systems.

Thermal Analysis of High Entropy Rare Earth Oxides

“…In a more recent evaluation by Zhang and Jung [ 68 ] evaluation, missing data were estimated from liquidus in RE 2 O 3 -Al 2 O 3 phase diagrams. However, new measurements for fusion enthalpies [ 53 ] are in better agreement with Zinkevich’s assessment. Zhang and Jung’s evaluation varies largely with the values proposed by Konings et al [ 69 ].…”

“…In diffraction experiments on levitated samples, powder-like diffraction patterns are obtained by ensuring the rotation of the solid sample in the gas flow [ 53 , 54 , 58 ]. When sample spheroids are prepared by melt solidification, as in this study, the exact shape of each bead depends on the surface tension of the particular composition, volume change on melting, and stochastic nature of nucleation.…”

“…Diffraction data were collected in 100-°C increments based on the surface temperature of the levitated bead, which was monitored with a single color pyrometer (875–925 nm spectral band, IR-CAS3CS, Chino Co., Tokyo, Japan) with emissivity set to 0.92. Emissivities for rare earth oxides above 2000 °C are unknown [ 51 ], and thermal gradient in laser-heated aerodynamically levitated bead exceeds 100 °C [ 52 , 53 , 54 ]. In this work, the temperatures of diffracted volume were assigned based on phase transformation temperatures obtained from DTA measurements.…”

“…Enthalpies of phase transformation were calculated as the averages of absolute values of endothermic heat effects on heating and exothermic heat effects on cooling. The instrument and methodology were described in detail elsewhere [ 53 , 57 , 58 , 59 ]. Temperature and sensitivity calibrations were performed using melting and phase transition temperatures and enthalpies of Au (1064 °C), Al 2 O 3 (2054 °C), Nd 2 O 3 (A-H, H-X, and X-L at 2077, 2201 and 2308 °C, respectively), and Y 2 O 3 (C-H and H-L at 2348 °C and 2439 °C, respectively).…”

Melting temperature measurement of refractory oxide ceramics as a function of oxygen fugacity using containerless methods

“…All rights reserved inevitable. 11,12 Therefore, an alternative method to study phase equilibria must be sought to avoid sample-crucible reaction at temperatures beyond 1500 °C.…”