Carbide and nitride fuels possess the desired thermophysical, neutronic and mech- anical properties desired for use in advanced NPP. However, fabrication processes to non-oxide ceramics via carbothermic reduction-nitridation are predominantly from decades ago with renewed efforts in the last decade in their infancy. The mechanism of carbothermic reduction has been studied, however there is much confusion as to the exact mechanism with fewer studies available for the nitridation reaction. It’s prudent that these mechanisms be fully understood in order to best exploit the re- action for the fabrication of non-oxide fuel from the large stockpiles of mixed oxide fuel left from reprocessing of SF.
ZrC and ZrN are both promising candidates for use in next generation fuels due to their high melting temperatures, superior thermal conductivity, good mechanical properties and increasing positive irradiation results with both materials being stud- ied by academic and industry research for use in these applications. However, this review has highlighted some areas where data are largely scattered or limited, with impurities, vacancies and microstructure all affecting reported values. Fabrication experience of powders is limited with the majority of thermal, mechanical and irra- diation properties being carried out on commercially available ZrC and ZrN powders with quantitative analysis of ceramics rarely reported. It is clear that production of dense, pure and stoichiometric ceramics is not yet fully achieved, hindering the reporting of accurate thermophysical data.
Irradiation properties of ZrC are limited with initial studies of defect formation, swelling, mechanical and thermal properties being reported. ZrN has received fewer studies than the carbide, characterising only defect formation, swelling and mech- anical properties of irradiated samples. Firstly a consistent best practice method to the fabrication and processing of ZrN needs to be developed to reduce scatter of thermophysical properties of ZrN across the literature, this will then allow for irradiation effects on thermal properties of ZrN to be studied, to build a robust knowledge base of the effects of radiation on the thermophysical properties.
3 Materials and Methods
The aims of experiments described in this chapter were to fabricate ZrN and ZrCxNy
powders through a two-step carbothermic reduction-nitridation processing route and to produce dense ceramics. The ceramics were then characterised to examine their microstructure and thermophysical properties, such as thermal conductivity, heat capacity and electrical properties as a function of stoichiometry and impurities (such as carbon and oxygen). The experimental techniques used throughout this work are described in detail here.
3.1 Starting Powder Fabrication
3.1.1 Carbothermal Reduction-Nitridation
3.1.1.1 Development of Carbothermic Reduction
Powders of monoclinic ZrO2 (99%, 1-5µm, Sigma Aldrich, Gillingham, UK) and
carbon black (∼40 nm, ABCR, Karlsruhe, Germany) were mixed in a ratio as to achieve a ZrC powder with a C/Zr mole ratio of 1 and 0.7 (sample names 1Zr-3C and 1Zr-2.7C respectively, Table 3.1) according to the Equation 3.1; (where x is equal to 0 or 0.3 for the samples 1Zr-3C and 1Zr-2.7C respectively).
ZrO2+ (3 − x)C −→ ZrC1−x+ 2CO (3.1)
Sample Mass of ZrO2 / g Mass of C / g Theoretical stoichiometry
1Zr-3C 20.000±0.001 5.854±0.001 ZrC1.00
1Zr-2.7C 20.000±0.001 5.073±0.001 ZrC0.70
The powders were homogenised by forming a non-aqueous slurry using acetone for the ease of removal due to its low boiling point and then ball milled for 12 h using ZrO2 milling media and dried to completeness at 373 K.
Thermograviometric analysis and differential thermal analysis (TGA/DTA) were performed to determine the reaction onset temperature to be used for the carbo- thermal reduction process. TGA/DTA measures the change in weight of a sample using a microbalance during a thermal cycle. As well as the weight change the temperature difference between the sample and reference crucibles under identical heat flow is measured, providing detail on any exothermic or endothermic processes occurring during the thermal cycle. In this work a Netzsch STA 449F1 was used under flowing argon (100 ml/min). The mixed ZrO2 and carbon powder (60 mg)
was placed in the alumina crucible in TGA/DTA and then heated to 1773 and 1873 K. Resulting powders from the TGA/DTA experiments were analysed by XRD and a temperature of 1873 K was chosen. Powders were then subjected to a 4 h dwell at 1873 K (+10 K/min) under Ar in a graphite furnace (FCT systeme GmbH, Franken- blick, Germany) to produce the corresponding carbides by carbothermal reduction.
3.1.1.2 Development of Nitridation
The resulting powders from the carbothermal reduction were further wet ball milled for 12 h in acetone using ZrO2 milling media and dried as before at 373 K. Nitrid-
ations of dried powders (≤ 1 µm, agglomerates 1-5 µm) were carried out for 4-24h at a temperature between 1800-1873 K (+35 K/min) under a flowing atmosphere of 10% hydrogen-90% nitrogen in a tube furnace (Lenton, Derbyshire, UK). The reaction parameters are given in Table 3.2. Powders from all reactions were stored in a vacuum desiccator.
3.1 Starting Powder Fabrication
Itera- tion
Starting powder Starting C/Zr
ratio Dwell Temperature / K Dwell time / h 1 1Zr-3C 3 1873 4 2 1Zr-3C 3 1800 8 2 1Zr-2.7C 2.7 1800 8 3 1Zr-3C 3 1873 8 3 1Zr-2.7C 2.7 1873 8 4 1Zr-3C 3 1800 24 4 1Zr-2.7C 2.7 1800 24 5 1Zr-3C 3 1873 24 5 1Zr-2.7C 2.7 1873 24
Table 3.2: Nitridation reaction parameters
The as-fabricated ZrC (3 g) powders were placed in Al2O3 crucibles in the tube
furnace, the working tube had an internal diameter of 50 mm and a length of 1200 mm. Flowing argon (0.5 L/min) was used to purge the tube for 1h (argon was chosen as it is heavier than air) before reaction, the gas was then changed to the hydrogen doped nitrogen gas (10% H2-90% N2, 0.5 L/min) and the heating programme started.
After nitridation the powders were wet ball milled for 12 h in acetone using ZrO2
milling media and dried as before at 373 K.
3.1.1.3 Powder Chemical Analysis
Quantitative chemical analyses of the powders were carried out via combustion and oxidation analysis techniques.
Combustion analysis of carbon content was performed using combustion and infrared (IR) detection of the resulting CO/CO2 gas (Horiba, EMIA-V2 series, Palaiseau,
France) at the University of Limoges and Horiba. Standards of non-ferrous steel with varying carbon contents, from 0.044 to 4.44 wt% were used to generate a calibration curve. Samples of 0.5-1 g were placed in an alumina crucible with tungsten and tin as a combustion accelerator (due to the carbide and nitrides having very high melting and combustion temperatures) and heated to 3273 K in O2.
Oxygen and nitrogen content were measured using a separate instrument (Horiba, EMGA-930) at Horiba Europe research centre (Horiba, Jobin Yvon SAS, Palaiseau, France), steel standards of 0.0425 and 0.0023 wt% O and 0.0009 and 0.0018 wt% N
were used for calibration. Powders (∼30 mg ) were weighed into a nickel crucible and then crushed before heating to 3273 K under He. Nitrogen and oxygen gases were separated by gas chromatography. Oxygen was reduced to CO/CO2 and de-
tected by two non-dispersive IR detectors and nitrogen detected as N2 by a thermal
conductivity cell.
For the oxidation analysis technique the samples were oxidised at 1173 K in oxygen and the resultant gases detected by quantitative IR spectroscopy and thermal con- ductivity (FlashEA 1112, Thermofisher, Loughborough, UK) at Queen Mary Uni- versity, however oxygen was not detected. Samples of around 3 mg were weighed into a tin crucible and carbon and nitrogen content is determined by conversion into N2 and CO2 which are then separated in a gas chromatography column. An
aspartic acid standard was used to generate a calibration curve using 5 standard measurements of increasing weight (1-5 mg).
For both techniques each sample was repeated 3 times for reproducibility and the results are given as an average of the 3 measurements and the error as the standard deviation of those 3 measurements, error in the measurements can also arise from air trapped between the powders.
3.1.1.4 FactSage
FactSage software (Ver. 6.2) was used to calculate gaseous species under the ni- tridation reaction conditions. The software uses thermodynamic databases of pure compounds to calculate changes in enthalpy, entropy, heat capacity and Gibbs en- ergy of the compounds input to the calculation under definable parameters such as pressure and temperature. The calculation uses a Gibbs energy minimisation algorithm and so the results correspond to the lowest possible Gibbs energy for the particular chemical reaction or equilibrium and the specified conditions.[132]