Science of entropy-stabilized ultra-high temperature materials: synthesis, validation and properties
September 17-20, 2017
A multidisciplinary effort to identify new ultra-high temperature materials combines both computational and experimental efforts exploring the potential of multi-principle element transition metal carbides, nitrides, and borides for improved properties. The elements of focus for this project include the group IV, V, and VI transition metals (Ti, Zr, Hf, V, Bn, Ta, Cr, Mo, W) combined as carbides, nitrides, or borides. This presentation will describe synthesis, validation, and evaluation of these Entropy Stabilized Ultra-High Temperature Materials (ES-UHTM).
Both thin film physical vapor deposition and field-assisted bulk sintering techniques are used with the goal to synthesize single phase ES-UHTM. A five-cathode deposition tool has been successfully used to prepare thin film carbides such as (Nb,W,Ti,Zr,Ta)C as a single phase with the rocksalt structure. Films are prepared with thicknesses up to several microns to facilitate fundamental property measurements. Multiple compositions of equimolar, five-component, metal diborides and as well as several metal carbides have also been successfully fabricated as bulk specimens via high-energy ball milling and spark plasma sintering. X-ray diffraction, scanning electron microscopy, energy dispersive spectroscopy, and aberration corrected scanning transmission electron microscopy with high-angle annular dark-field and annular bright-field (HAADF and ABF) imaging and nanoscale compositional mapping have all been used to evaluate the success in achieving single phase states with random and homogeneous distributions of cations. Experimental successes in achieving single phase compositions are compared to theoretical predictions described in a companion presentation.
Material properties such as thermal conductivity, diffusivity and oxidation resistance are evaluated to assess performance relative to theoretical prediction and conventional UHTCs. Thermal conductivity is determined using the time domain thermal reflectance technique on thin film specimens for comparison to theoretical predictions for these multiple principle element compositions. Thin film diffusion couples (two-layer films), in which four of the principle transition metals are found in both films and one element of each thin film differs, are prepared to study diffusivity in these ES-UHTMs. Oxidation resistance is been characterized using Joule heating of bulk specimens to ultra-high temperatures (T>1500C) in controlled argon-oxygen atmospheres to determine oxide phases formed and their distribution, comparing carbides and borides of the same metal composition. Properties of these unique ES-UHTM are evaluated with the intent to enable tailoring of material performance via exploration of the large compositional space available.
Elizabeth Opila, Patrick Hopkins, Donald Brenner, Jon-Paul Maria, Stefano Curtarolo, Kenneth Vecchio, and Jian Luo, "Science of entropy-stabilized ultra-high temperature materials: synthesis, validation and properties" in "Ultra-High Temperature Ceramics: Materials for Extreme Environment Applications IV", Jon Binner, The University of Birmingham, Edgbaston, United Kingdom Bill Lee, Imperial College, London, United Kingdom Eds, ECI Symposium Series, (2017). http://dc.engconfintl.org/uhtc_iv/34