Evidence for High-Temperature Forms of Zirconium and Tantalum Monocarbides

Evidence has been found which suggests the existence of high-temperature crystalline modifications of both zirconium carbide and tantalum carbide. The high-temperature inversion in zirconium carbide, indicated by a sharp decrease in spectral emisivity, in electrical resistivity, and in density, was reversible. No permanent changes in microhardness or in X-ray diffraction patterns were found after cooling to room temperature. Emissivity measurements showed a hysteresis effect of approximately 400° C for zirconium carbide. A similar, although less sharply defined, effect was noted for tantalum carbide. No such effect was found in solid solutions of these two monocarbides.     

Preparation and characterization of three-dimensional carbon fiber reinforced zirconium carbide composite by precursor infiltration and pyrolysis process

Three-dimensional carbon fiber reinforced zirconium carbide composite (3D C/ZrC) was fabricated for ultra high temperature applications by precursor infiltration and pyrolysis (PIP) process using the mixture of zirconium butoxide (Zr(OC₄H₉)₄) and divinylbenzene (DVB) as precursor of zirconium carbide. The micro-structural, mechanical and ablative properties of the 3D C/ZrC composite were studied. The flexural strength of the composite was 107.6 MPa, the elastic modulus was 28.8 GPa, and the fracture toughness was 7.03 MPa m^1/2. The mass lose rate and linear recession rate of the 3D C/ZrC composite during oxyacetylene torch test was 0.012 g/s and -0.002 mm/s, respectively. The formation of ZrO₂ melt on the surface of the composite contributed mainly the excellent ablative property.     

Mechanical and Thermophysical Properties of Zr–Al–Si–C Ceramics

The mechanical and thermophysical properties of quaternary-layered carbides, Zr₂[Al(Si)]₄C₅ and Zr₃[Al(Si)]₄C₆ have been investigated and compared with those of Zr₂Al₃C₄ and Zr₃Al₃C₅. These four carbides are generally alike in mechanical and thermophysical properties due to their similar crystal structures that consisting of alternatively stacked ZrC layers and Al₃C₂/[Al(Si)]₄C₃ slabs. The layer thickness of zirconium carbide and aluminum carbide has effects on their properties. Thicker layer of zirconium carbide and/or thinner layer of aluminum carbides are in favor of stiffness, hardness, thermal, and electrical conductivities, but go against density, specific stiffness, Debye temperature, and coefficient of thermal expansion.     

Up-scalable preparation of nano zirconium carbide powder in liquid polymeric precursor and its pyrolysis mechanism

Nano size ZrC powder was prepared by liquid polymeric precursor method. Zirconium n-butoxide (Zr(OⁿBu)₄) and benzoylacetone (BA) were mixed directly with different molar ratios to synthesize transparent liquid zirconium carbide single-source precursors. The carbon content in the precursor could be changed by adding different amount of BA. X-ray pure ZrC was obtained when the molar ratio of BA/Zr(OⁿBu)₄ was 4.6:1. The viscosity of the precursor was very low (<8 mPa s) without the addition of solvents. Zirconium carbide powders were fabricated by the pyrolysis at 800 °C in argon and subsequent heating at various temperatures in vacuum for carbothermal reduction reaction. The pyrolysis behavior, phase composition and transformation, and microstructure of the as-fabricated ZrC powders were analyzed. The gases of CH₄, CO and CO₂ released due to decomposition and evaporation of the organic component and transformation from ZrO₂ to ZrC during pyrolysis resulted in total 60–70% mass loss. The average grain size of the synthesized X-ray pure ZrC powders was less than 30 nm. Meanwhile, the pyrolysis mechanism of nano zirconium carbide powder was deduced.     

TEM/STEM Observation of ZrC Coating Layer for Advanced High-Temperature Gas-Cooled Reactor Fuel, Part II

The Japan Atomic Energy Agency (JAEA) has started to study and develop zirconium carbide (ZrC)-coated fuel particles for advanced high-temperature gas-cooled reactors. The ZrC coating layer has been fabricated at JAEA by chemical vapor deposition using a pyrolytic reaction of zirconium bromide. The microstructures of the ZrC layers, whose nominal deposition temperatures could be measured and controlled during the deposition process, were characterized by means of TEM and STEM. In the present study, three batches were prepared and compared with each other as well as the previous batches. The crystallographic orientation of ZrC with regard to the growth direction in the ZrC layers deposited at a constant temperature of 1630 K was different from that deposited at varying temperatures in the 1493–1823 K range. A thin layer of turbostratic carbon was observed at the boundary between pyrolytic carbon and ZrC in particles deposited at the highest temperature among those used in this study (the nominal temperature was 1769 K); no such structure was found in a batch deposited at a lower temperature (the nominal temperature was 1632 K). Therefore, precise control of temperature is shown to be critical to the formation of good ZrC coatings.     

Spark Plasma Sintering of Superhard B4C–ZrB2 Ceramics by Carbide Boronizing

A carbide boronizing method was first developed to produce dense boron carbide‐ zirconium diboride (“B₄C”–ZrB₂) composites from zirconium carbide (ZrC) and amorphous boron powders (B) by Spark Plasma Sintering at 1800°C–2000°C. The stoichiometry of “B₄C” could be tailored by changing initial boron content, which also has an influence on the processing. The self‐propagating high‐temperature synthesis could be ignited by 1 mol ZrC and 6 mol B at around 1240°C, whereas it was suppressed at a level of 10 mol B. B₈C–ZrB₂ ceramics sintered at 1800°C with 1 mole ZrC and 10 mole B exhibited super high hardness (40.36 GPa at 2.94 N and 33.4 GPa at 9.8 N). The primary reason for the unusual high hardness of B₈C–ZrB₂ ceramics was considered to be the formation of nano‐sized ZrB₂ grains.     

Preparation and Characterization of Zirconium Carbide Coating on Coated Fuel Particles

Compact and uniform zirconium carbide (ZrC) coatings have been successfully deposited on coated fuel particles using a ZrCl₄+H₂+Ar+C₃H₆ gas mixture. Zirconium tetrachloride (ZrCl₄) powder feeder was especially designed and manufactured to control accurately the flow rate of ZrCl₄ and produce ZrC on an industrial scale. The coating has a large density (6.57 g/cm³), a thickness of 35 μm, a stoichiometry close to Zr/C=1, and a clear interface between the coating and substrate. The coating exhibits an fcc -ZrC phase with a grain size of 11.18 nm and a (111) texture coefficient of 0.57, which corresponds to a polycrystalline microstructure of randomly oriented ZrC grains. The preparation apparatus, processing conditions, properties, microstructures, and morphologies of the ZrC coating are investigated systematically.     

Deposition Mechanism for Chemical Vapor Deposition of Zirconium Carbide Coatings

Zirconium carbide (ZrC) coatings were fabricated by chemical vapor deposition (CVD) using ZrCl₄, CH₄/C₃H₆, and H₂ as precursors. Both thermodynamic calculation results and the film compositions at different temperatures indicated that zirconium and carbon deposited separately during the CVD process. The ZrC deposition rates were measured for CH₄ or C₃H₆ as carbon sources at different temperatures based on coating thickness. The activation energies for ZrC deposition demonstrated that the CVD ZrC process is controlled by the carbon deposition. This is also proven by the morphologies of ZrC coatings.     

Synthesis of transition metal carbides and high-entropy carbide TiZrNbHfTaC5 in self-shielding DC arc discharge plasma

The paper shows the feasibility of synthesizing micro- and nano-sized particles of binary metal carbides (Me–C) and high-entropy carbide (HEC) TiZrNbHfTaC₅ by vacuum-free electric arc method. The method is based on the effect of self-shielding of the reaction volume from atmospheric oxygen by carbon monoxide CO, which is generated during arcing in air. The synthesis results in a solid solution with a NaCl-type carbide with a cubic lattice, which simultaneously contains atoms of titanium, zirconium, niobium, hafnium, tantalum, and carbon. The lattice parameter of the HEC TiZrNbHfTaC₅ phase is ～4.532 Å that is in line with the known data on this compound. The synthesis product contains micro-sized particle agglomerates of transition metal carbides. The synthesis products also contain nano-sized particles with a shell-core structure, in which the core can consist of metal carbide (TiC, ZrC, NbC, HfC, TaC) or HEC TiZrNbHfTaC₅, and the shell is a graphite phase.     

Comparative Studies of Pressure Sensitization of Zirconium Carbide and Zirconium Silicate on Burning Rates in a RDX‐AP based Composite Propellant

In a systematic study to compare the effects of the values of burning rate and pressure exponent in RDX‐AP based composite propellant, various compositions with varying percentages of zirconium carbide (ZrC) and zirconium silicate (ZrSiO₄) were formulated to select a suitable candidate. Various rocket parameters of each formulation were theoretically predicted by the NASA CEC‐71 program and the burning rate was evaluated in pressure range of 3–11 MPa. In addition, density, sensitivity, and thermal properties of compositions having maximum effects on pressure exponent’s values were also evaluated. It was concluded that ZrSiO₄ enhances the pressure exponent “n” value substantially, whereas ZrC doesn’t have significant effects on it as compared to base composition and also provides higher density values of composite propellant formulated.     

A convenient, general synthesis of carbide nanofibres via templated reactions on carbon nanotubes in molten salt media

Carbide nanofibres were synthesized by reaction of various transition metals with carbon nanotubes using a molten LiCl-KCl-KF salt system as a reaction medium. Metal sources included titanium, zirconium, hafnium, vanadium, niobium and tantalum powders. Multi-walled carbon nanotubes were used both as a carbon source and also as a template for the preparation of titanium carbide, zirconium carbide, hafnium carbide, vanadium carbide, niobium carbide and tantalum carbide nanofibres. Generally, the carbide products were characterized by scanning electron microscopy, transmission electron microscopy, selected-area electron diffraction and electron energy loss spectroscopy. The polycrystalline carbide nanofibres produced in these reactions have a similar morphology to that of their multi-walled carbon nanotube precursors. However, when using titanium mixed with titanium dioxide as a titanium source, both polycrystalline and straight, single crystal titanium carbide nanofibres are formed.     

Synthesis and pyrolysis behavior of a soluble polymer precursor for ultra-fine zirconium carbide powders

A soluble polymer precursor for ultra-fine zirconium carbide (ZrC) was successfully synthesized using phenol and zirconium tetrachloride as carbon and zirconium sources, respectively. The pyrolysis behavior and structural evolution of the precursor were studied by Fourier transform infrared spectra (FTIR), differential scanning calorimetry, and thermal gravimetric analysis (DSC–TG). The microstructure and composition of the pyrolysis products were characterized by X-ray diffraction (XRD), laser Raman spectroscopy, scanning electron microscope (SEM) and element analysis. The results indicate that the obtained precursor for the ultra-fine ZrC could be a Zr–O–C chain polymer with phenol and acetylacetone as ligands. The pyrolysis products of the precursor mainly consist of intimately mixed amorphous carbon and tetragonal ZrO₂ (t-ZrO₂) in the temperature range of 300–1200 °C. When the pyrolysis temperature rises up to 1300 °C, the precursor starts to transform gradually into ZrC, accompanied by the formation of monoclinic ZrO₂ (m-ZrO₂). The carbothermal reduction reaction between ZrO₂ and carbon has been substantially completed at a relatively low temperature (1500 °C). The obtained ultra-fine ZrC powders exhibit as well-distributed near-spherical grains with sizes ranging from 50 to 100 nm. The amount of oxygen in the ZrC powders could be further reduced by increasing the pyrolysis temperature from 1500 to 1600 °C but unfortunately the obvious agglomeration of the ZrC grains will be induced.     

Interaction of certain oxides,carbides and high-melting metals at high temperatures

Conclusions The greatest stability in contact with BeO is shown by tungsten: with MgO, by molybdenum and tungsten; and with stabilized zirconium oxide, by molybdenumThe most resistant material in contact with the molybdenum up to 2100° is tantalum carbide, and the interaction between molybdenum and zirconium, hafnium and niobium carbide begins at 1800–2200°.     

Preparation of ZrC/SiC porous self-supporting monoliths via sol-gel process using polyethylene glycol as phase separation inducer

We present a straightforward method via sol-gel process using polyethylene glycol (PEG) as phase separation inducer to prepare zirconium carbide/silicon carbide (ZrC/SiC) porous monoliths. Organic/inorganic hybrid gels are prepared using zirconium oxychloride, furfuryl alcohol, and tetraethyl orthosilicate as major starting materials. In the presence of PEG, crack-free hybrid monoliths are obtained by drying the wet gels under ambient pressure, whereas in the absence of PEG, the wet gels break into pieces as expected. PEG plays a key role in maintaining the macroscopic shape of the monoliths. After ceramization at 1300–1500?°C, ZrC/SiC porous monoliths are obtained. SEM and mercury intrusion porosimetry data show that PEG also has strong influence on the microstructures of the monoliths. The compressive strengths of the ceramic monoliths are in the range of 0.3 to 0.7?MPa. And their compressive behavior starts to differ due to the changes in their microstructures, especially the pore structure.     

Synthesis and Evolution of Zirconium Carbide via Sol–Gel Route: Features of Nanoparticle Oxide–Carbon Reactions

A sol–gel route was used to synthesize nanocrystalline zirconium carbide (ZrC). The starting materials were zirconium propoxide, with carbon introduced by furfuryl alcohol (FA). A block copolymer surfactant was used to homogenize the oxide and carbon components. ZrC was produced at a low temperature of 1250°C and complete conversion achieved at 1450°C. The powder was nanocrystalline size less than 100 nm. Phase changes were studied using X‐ray diffraction and Raman spectroscopy. Rietveld analysis followed the changes in crystallite size and lattice parameters of the phases during carbothermal reduction. Morphology changes were observed using nitrogen gas sorption. High‐resolution TEM and EDS were used to image the carbide lattice, surface oxides, and graphene‐like carbons. The results indicate that nanoparticle carbothermal synthesis involves agglomeration and necking as the most viable mode of mass transport to complete the carbothermal reduction.     

Spark plasma sintering of quadruplet ZrB2–SiC–ZrC–Cf composites

Spark plasma sintering (SPS) route was employed for preparation of quadruplet ZrB₂–SiC–ZrC–C_f ultrahigh temperature ceramic matrix composites (UHTCMC). Zirconium diboride and silicon carbide powders with a constant ZrB₂:SiC volume ratio of 4:1 were selected as the baseline. Mixtures of ZrB₂–SiC were co-reinforced with zirconium carbide (ZrC: 0–10 vol%) and carbon fiber (C_f: 0–5 vol%), taking into account a constant ratio of 2:1 for ZrC:C_f components. The sintered composite samples, processed at 1800 °C for 5 min and 30 MPa punch press under vacuumed atmosphere, were characterized by densitometry, field emission scanning electron microscopy, energy dispersive spectroscopy, X-ray diffractometry as well as mechanical tests such as hardness and flexural strength measurements. The results verified that the composite co-reinforced with 5 vol% ZrC and 2.5 vol% C_f had the optimal characteristics, i.e., it reached a relative density of 99.6%, a hardness of 18 GPa and a flexural strength of 565 MPa.     

Grain boundary driven mechanical properties of ZrB2 and ZrC‐ZrB2 nanocomposite: A molecular simulation study

In this study, we report the grain boundary driven mechanical behavior of 2 polycrystalline ultra‐high‐temperature ceramics (UHTCs), zirconium diboride (ZrB₂) and zirconium carbide (ZrC) with zirconium diboride (ZrC‐ZrB₂). These nanocomposites were investigated using large‐scale molecular dynamics simulations. First, the atomistic models of the polycrystalline ZrB₂ and ZrC‐ZrB₂ nanocomposites were subjected to tensile loading to determine their elastic constants and tensile strengths. It was found that the presence of nanoparticles imparts an insignificant effect on the mechanical properties of ZrB₂. It has also been observed that the failure mechanisms of both the ZrB₂ and ZrC‐ZrB₂ nanocomposite are driven by grain boundary deformation. At any instant during the applied load transfer, local tensile stress distribution data indicate that atomic stress becomes much higher near the grain boundaries compared to other locations. The authors performed additional sets of simulations to obtain tensile and shear properties of grain boundary material. When these properties were compared with the adjacent single crystal and overall polycrystalline material properties, it was found that the shear strength and stiffness of the grain boundary materials are significantly lower than the single crystal or polycrystal ZrB₂. It is believed that the overall deformation and failure properties of ZrB₂ and its composite are controlled by the properties of grain boundary. Hence, the addition of nanoparticles played an insignificant role on the mechanical properties of ZrB₂.     

The ZrC–C eutectic structure and melting behaviour: A high-temperature radiance spectroscopy study

A fast spectro-pyrometer has been employed for radiance measurements of zirconium carbide samples laser-heated to very high temperature, for compositions 0.7 ≤ C/Zr ≤ 2.61 and in a spectral range 0.550 μm ≤ λ ≤ 0.900 μm. The ZrC–C eutectic temperature has been taken as the radiance reference. The measured normal spectral emissivity (NSE) ?_λ of solid zirconium carbide is close to 0.6 at 0.650 μm, in agreement with previous literature. Its high-temperature behaviour, value in the liquid, carbon-content and wavelength dependences in the visible-near infrared range have been determined here for the first time. Liquid zirconium carbide seems to interact with electromagnetic radiation in a more metallic way than the solid. A considerable NSE increase has been observed at increasing carbon content, which can be interpreted on the basis of preferential growth along the “c” plane of the carbon lamellae in the eutectic structure.     

Low Temperature Carbothermal Reduction Synthesis of ZrC Nanofibers via Cyclized Electrospun PVP/Zr(OPr)4 Hybrid

The fabrication capability of zirconium carbide (ZrC) nanofibers by a novel polymeric solution was examined using electrospinning method. The electrospinnable solution was prepared from the reaction of zirconium n‐propoxide (Zr(OPr)₄) with acetylacetone and acetic acid followed by the addition of polyvinylpyrrolidone (PVP) solution. By utilizing thermal and microstructural analyses such as differential scanning calorimetry–thermogravimetry (DSC–TG), field emission scanning electron microscopy (FE‐SEM), transmission electron microscopy (TEM), X‐ray diffraction (XRD), and Brunauer–Emmett–Teller (BET), the effect of heat treatment type on the morphology and crystallinity of as‐spun PVP/Zr(OPr)₄ hybrid fibers was examined. The results showed that direct carbonization treatment of as‐spun fibers under argon atmosphere led to spherical ZrC aggregates in lack of fibrillar morphology, whereas carbonization coupled with cyclization could be recognized as the unique template to govern the morphology and crystallinity of ZrC nanofibers. Carbonization of the cyclized fibers at 1550°C in flowing argon atmosphere produced the thick, fragmented rosary‐like fibers with a diameter of 357 nm, while through a 100°C decrease in carbonization temperature to 1450°C, the thin, smooth, long, and uniform ZrC nanofibers with 176 nm diameter and a medium surface area of 23 m²/g were obtained.     

Volatility diagram of ZrB2‐SiC‐ZrC system and experimental validation

A volatility diagram of zirconium carbide (ZrC) at 1600, 1930, and 2200°C was calculated in this work. Combining it with the existing volatility diagrams of ZrB₂ and SiC, the volatility diagram of a ternary ZrB₂‐SiC‐ZrC (ZSZ) system was constructed in order to interpret the oxidation behavior of ZSZ ceramics. Applying this diagram, the formation of ZrC‐corroded and SiC‐depleted layers and the oxidation sequence of each component in ZSZ during oxidation and ablation could be well understood. Most of the predictions from the diagrams are consistent with the experimental observations on the oxidation scale of dense ZrB₂‐SiC‐ZrC ceramics/coatings after oxidation at 1600°C or ablation at 1930 and 2200°C. The reasons for the discrepancy are also briefly discussed.