On the stoichiometry of zirconium carbide

Abstract

the dependencies of the enhanced thermomechanical properties of zirconium carbide (Zrc x) with sample purity and stoichiometry are still not understood due to discrepancies in the literature. Multiple researchers have recently reported a linear relation between the carbon to zirconium atomic ratio (c/Zr) and the lattice parameter, in contrast with a more established relationship that suggests that the lattice parameter value attains a maximum value at a C/Zr ~ 0.83. In this study, the relationship between C/Zr atomic ratio and the lattice parameter is critically assessed: it is found that recent studies reporting the thermophysical properties of Zrc x have unintentionally produced and characterised samples containing zirconium oxycarbide. to avoid such erroneous characterization of Zrc x thermophysical properties in the future, we propose a method for the accurate measurement of the stoichiometry of ZrC x using three independent experimental techniques, namely: elemental analysis, thermogravimetric analysis and nuclear magnetic resonance spectroscopy. Although a large scatter in the results (Δc/Zr = 0.07) from these different techniques was found when used independently, when combining the techniques together consistent values of x in Zrc x were obtained. Zirconium carbide (ZrC) is a much-promising material, it has received increased interest recently as an alternative material to silicon carbide (SiC) in nuclear fuel applications 1,2 , in next-generation nuclear fusion reactors 3 , and also as an ultra-high-temperature ceramic to be used in ceramic-metal composite heat exchangers in concentrated solar power (CSP) plants 4,5. ZrC (here denoted as ZrC x) is typically non-stoichiometric as it can contain up to 50% of unoccupied carbon sites 6,7 , it has been found that deviations in the stoichiometry of ZrC x can severely affect its thermal and mechanical properties 8,9. Given its potential in high-temperature applications, it is extremely important to define a method that robustly determines its stoichiometry and purity. The purity of ZrC x should always be assessed as the presence of contaminants such as nitrogen or oxygen is detrimental for its performance. For example, if ZrC x is to be used as a nuclear fuel coating in a nuclear reactor, any nitrogen contamination should be avoided due to the production of radioactive 14 C from nitrogen 14 N inside the reactor 10. There are two common methods for defining the stoichiometry of ZrC x. The first one is to evaluate the C/Zr atomic ratio from the lattice parameter measured by X-ray diffraction (XRD) using the correlation published in Jackson & Lee 6. The second one is to quantify through the inert-gas fusion technique the carbon content in ZrC x and calculate the C/Zr atomic ratio assuming that the sample is free from impurities. Both techniques, however, have limitations and when used in standalone approaches can lead to erroneous stoichiometry estimations, as we will proceed to demonstrate later in this paper. The need for an established robust method to measure ...

Kinetic role of C/Zr ratio in the reaction process, combustion behavior, and synthetic product of 70 wt.% (xC–Zr)–30 wt.% Cu

“…According to the fitted XRD patterns in Figure 12 and references, 40–44 the lattice parameter and the x value of the synthesized ZrC x were calculated. The results are shown in Figure 13A,B.…”

The effect of oxygen impurities on the stability and structural properties of vacancy-ordered and -disordered ZrCx

“…There are several possible reasons why the predicted lowtemperature vacancy-ordered phases are not oen observed experimentally. During the various fabrication methods for ZrC x , the sample is heated to around 2000 K, 22 which would likely cause disordering of any vacancies. 5,14 During cooling, this disordered phase may be quenched into the sample.…”

“…5 Furthermore, ultra-high temperature carbides such as ZrC x are notoriously difficult to fabricate and characterise in terms of impurities. 22 ZrC x samples are likely to have Hf, O, and N impurities which may affect the properties and stability. 22 Samples synthesised by Jackson et al 23,24 show that the most signicant impurity is O, at up to 3.3 wt% (in a sample with target C content of 7.3 wt%), or $11 at%.…”

“…22 ZrC x samples are likely to have Hf, O, and N impurities which may affect the properties and stability. 22 Samples synthesised by Jackson et al 23,24 show that the most signicant impurity is O, at up to 3.3 wt% (in a sample with target C content of 7.3 wt%), or $11 at%. 25 Depending on the experimental conditions, oxygen may exist as dissolved oxygen defects, or as ZrO 2 on the surface.…”

“…25 Depending on the experimental conditions, oxygen may exist as dissolved oxygen defects, or as ZrO 2 on the surface. 22 The oxidation pathway from ZrC (cubic Zr with C occupying the octahedral interstitial sites) to ZrO 2 (cubic Zr with O occupying the tetrahedral interstitial sites) is not fully understood, 26 but several studies have attempted to characterise the structure of zirconium oxycarbide at low oxygen concentration. [27][28][29][30] However, the crystallographic locations of the O atoms are challenging to characterise as light atoms such as C and O can oen be difficult to distinguish experimentally using X-ray diffraction, 22,29 which has limited the characterisation of most experimental investigations on the properties of ZrC x .…”