Laser Melting of Spark Plasma-Sintered Zirconium Carbide: Thermophysical Properties of a Generation IV Very High-Temperature Reactor Material

Melting temperatures of zirconium carbide were investigated in validating a novel thermal analysis technique for refractory materials. Commercial ZrC_0.96 powder was densified by spark plasma sintering to >96% relative density after 6–30 min at 2173–2453 K under 40–100 MPa. Sintered ceramics were heated to >4000 K via pulsed laser heating. Mean values for solidus and liquidus transitions were 3451 and 3608 K, respectively, in fair agreement with the present phase diagram. Postmelting analysis revealed dendritic microstructure and composition consistent with single-phase ZrC. Subsurface gas porosity and ZrC–C eutectic indicate complex processes occurring during melting and freezing.     

High-Temperature Thermophysical Properties of Uranium Monocarbide

Thermal conductivity ( k ), electrical resistivity ( p ), total hemispherical emittance (ε_t), and normal spectral emittance (ε_0.65μ) of dense, arc-cast uranium monocarbide (5.3 wt % total carbon) were measured in the temperature range 1150° to 2050°K. The results were as follows: k , 0.057 cal/sec-cm-deg, 1200° < T < 2050°K, probable error ± 0.002; p, 20.4 × 10⁻⁶+ 114.8 T × 10⁻⁹ ohm-cm, 1175° < T < 2050°K, probable error ± 1.7 × 10⁻⁶; ε_t0.42, 1250° < T < 1980°K, probable error ± 0.02; ε_0.65 0.539 – 0.02 T × 10⁻³ 1150° < T < 1890°K, probable error ± 0.02. Experimental methods are discussed and error sources are analyzed. Uranium monocarbide exhibited typical metallic behavior in its thermophysical properties.     

Thermophysical Properties of α-Tungsten Carbide

Upper and lower bounds for the thermal expansion of polycrystalline tungsten carbide (α-WC) are predicted at ultrahigh temperatures from low-temperature experimental data. The lower bound is obtained from an α _VK_TV model, where α _V is the volume thermal expansion, K_T the isothermal bulk modulus for a randomly oriented polycrystalline sample, and V the molar volume. For many materials, the α _VK_TV product approaches a constant value that is similar to the specific heat at the highest temperatures. The upper bound uses Grüneisen's rule with a constant Grüneisen parameter gamma at temperatures >1.3θ_D (where θ_D is the Debye temperature) and experimental data below that temperature. Literature data for the thermophysical properties of α-WC have been reviewed and used in our α _VK_TV model to calculate a lower bound for the thermal expansion at temperatures >2θ_D and to calculate the temperature dependence of the bulk modulus. The ultrahigh-pressure thermal expansion has been calculated from the lower bound. Model predictions of the thermophysical properties of WC are given for an extended temperature range.     

High-Temperature Zirconium Phosphates

The existence of two zirconium phosphate compounds has been established. The "normal" phosphate, ZrP₂O₇, has a reversible inversion at 300 °C. and dissociates to the zirconyl compound, (ZrO)₂P₂O₇, at 1550°C., with the simultaneous loss of P₂O₅ as vapor. The room-temperature form of ZrP₂O₇ is cubic with refractive index 1.657 ± 0.003. The zirconyl compound has a stable existence to temperatures around 1600°C. and is characterized by a very low linear thermal-expansion coefficient.     

High-Temperature Mechanical Properties of a Uniaxially Reinforced Zircon-Silicon Carbide Composite

Mechanical properties of a monolithic zircon ceramic and zircon-matrix composites uniaxially reinforced with either uncoated or BN-coated silicon carbide monofilaments were measured in flexure between 25° and 1477°C. Monolithic zircon ceramics were weak and exhibited a brittle failure up to about 1300°C. An increasing amount of the plastic deformation was observed before failure above about 1300°C. In contrast, composites reinforced with either uncoated or BN-coated SiC filaments were stronger and tougher than the monolithic zircon at all test temperatures between 25° and 1477°C. The ultimate strength and work-of-fracture of composite samples decreased with increasing temperature. A transgranular matrix fracture was shown by the monolithic and composite samples tested up to about 1200°C, whereas an increasing amount of the intergranular matrix fracture was displayed above 1200°C.     

Research Progress on Preparation and Properties of Zirconium Carbide Matrix Composites

Zirconium carbide(ZrC) exhibits considerable potential for applications as aerospace thermal protection and the Generation-Ⅳ nuclear fuel inert materials due to its high melting point,exceptional hardness,good ablation resistance and low neutron absorption cross-section.Nevertheless,low sinterability of ZrC powders and poor fracture toughness and reliability of bulk ceramics limit their wide applications in extreme environments.This paper reviews the state of the art of preparation and properties of ZrC composites.Optimizing the sintering process and tailoring the chemical constituents of raw powders and sintering aids could improve sinterability to produce dense bulk ceramics.Different additives such as refractory metals,carbides,silicides,oxides,or carbon fibers are introduced into the ZrC matrix in order to improve fracture toughness,oxidation resistance or thermal shock resistance,etc.Further studies are needed to explore the effects of intrinsic defects(vacancies,dislocations,and grain or phase boundaries,etc.) and additives on microstructure and properties at elevated temperatures.     

Ultra‐High Temperature Mechanical Properties of a Zirconium Diboride–Zirconium Carbide Ceramic

The mechanical properties of a ZrB₂‐10 vol% ZrC ceramic were measured up to 2300°C in an argon atmosphere. Dense billets of ZrB₂‐9.5 vol% ZrC‐0.1 vol% C were produced by hot‐pressing at 1900°C. The ZrB₂ grain size was 4.9 μm and ZrC cluster size was 1.8 μm. Flexure strength was 695 MPa at ambient, decreasing to 300 MPa at 1600°C, increasing to 345 MPa at 1800°C and 2000°C, and then decreasing to 290 MPa at 2200°C and 2300°C. Fracture toughness was 4.8 MPa·m^½ at room temperature, decreasing to 3.4 MPa·m^½ at 1400°C, increasing to 4.5 MPa·m^½ at 1800°C, and decreasing to 3.6 MPa·m^½ at 2300°C. Elastic modulus calculated from the crosshead displacement was estimated to be 505 GPa at ambient, relatively unchanging to 1200°C, then decreasing linearly to 385 GPa at 1600°C, more slowly to 345 GPa at 2000°C, and then more rapidly to 260 GPa at 2300°C. Surface flaws resulting from machining damage were the critical flaw up to 1400°C. Above 1400°C, plasticity reduced the stress at the crack tip and the surface flaws experienced subcritical crack growth. Above 2000°C, microvoid coalescence ahead of the crack tip caused failure.     

Mechanical Properties of Hot-Pressed Zirconium Carbide Tested to 2600°C

Tensile, creep, and modulus of elasticity data were determined for hot-pressed zirconium carbide. The material was relatively impure, containing 1 to 2% nitrogen and 1 to 2% free carbon. Tension and creep properties, which were measured to 2600° C, indicated that above 2100°C the strength was 2000 psi or lower and the elongation was 40% or more, thus signifying little structural usefulness. It is proposed that the results were influenced by impurities at the grain boundaries. Room-temperature modulus of elasticity values averaging 51.6 × 10⁶ psi were obtained by dynamic methods.     

Laser Diagnostics of Zirconium Carbide Vaporization

Laser diagnostic methods are developed to study the behavior of laser-ablated zirconium carbide (ZrC). The optical emission spectra from the Zr-C plasma plume is measured from 200 to 500 nm. Emission from zirconium atoms dominated the observed spectra. From the spectra, the average plasma temperature was estimated to be 10500 ± 1500 K. Plume formation is characterized using a CCD camera to capture emission at delay times (with respect to the ablation laser pulse) ranging from 1 μs to 1 ms and using planar laser-induced fluorescence to map Zr atom distributions.     

Physicochemical Transformations During Low-Temperature Synthesis of Zirconium Carbide

Refractories and Industrial Ceramics - Joint reduction of zirconium dioxide and sodium carbonate to form zirconium carbide was studied. Magnesiothermic reduction of ZrO2 was shown to occur at 600...     

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.     

Effect of Zirconium on the Densification of Reactively Hot‐Pressed Zirconium Carbide

Densification mechanisms involved during reactive hot pressing (RHP) of zirconium carbide (ZrC) have been studied. RHP has been carried out using zirconium (Zr) and graphite (C) powders in the molar ratios 1:0.5, 1:0.67, 1:0.8, and 1:1 at 40 MPa, 800°C–1200°C for different durations. The volume fractions of phases formed, including porosity, are determined from the measured density and from Rietveld analysis. Increased densification with an increasing nonstoichiometry in carbon has been observed. Microstructural and X‐ray diffraction observations coupled with the predictions of a model based on the constitutive laws governing plastic flow of zirconium suggest that the better densification of nonstoichiometric compositions arise from the higher amount of starting Zr and also the longer duration of its availability for plastic flow during RHP. Volume shrinkage due to reaction between Zr and C and the gradual elimination of the soft metal phase limit the final density achievable. Based on these observations, a two‐step RHP carried out at 800°C and 1200°C leads to a better densification than a single RHP at 1200°C.     

Oxidation Kinetics of Zirconium Carbide at Relatively Low Temperatures

The isothermal oxidation of ZrC powders was carried out at relatively low temperatures of 380° to 550°C at oxygen pressures of 1.3, 2.6, and 7.9 kPa under a static total pressure of 39.5 kPa, achieved by mixing with argon, using an electromicrobalance. The oxidation kinetics are described by the diffusion-controlled Jander's equation, following rapid oxidation in the early stage. Two activation energies were obtained: 138 kJ · mol⁻¹ below about 470°C and 180 kJ · mol⁻¹ above that temperature. The high- and low-temperature oxidation mechanisms are discussed in connection with the crystallization of cubic ZrO₂, accompanied by the generation of cracks, as well as the formation of carbon in the hexagonal diamond form in the product phase.     

Thermophysical Properties of Stone Fruit

The thermophysical properties of the stone fruits plum, peach, and nectarine were modeled from experimental data as functions of moisture content. Samples were dried to preset moistures in a laboratory cabinet dryer, and the thermal conductivity, specific heat, apparent density, bulk density, and porosity of the fruit were determined. The thermal conductivity and specific heat were found to be linear functions of moisture content, whereas apparent bulk density and porosity followed second-order polynomials. Temperature dependence was not found to be significant.     

碳化锆陶瓷的氧化及其对导电性能的影响

采用高温自蔓延制备的碳化锆粉体作为原料,研究了碳化锆陶瓷在空气中的氧化机制和热压烧结块体的氧化动力学行为。结果显示：在空气中,碳化锆陶瓷在200℃时开始氧化;在200～450℃时,氧化产物为ZrCxO1–x (x=0～0.42);在500～600℃时,生成中间相Zr2O出现;当氧化温度升高到1000℃,氧化产物主要为ZrO2。对烧结体的氧化动力学行为分析发现,在200～450℃的氧化过程中烧结陶瓷表面形成ZrCxO1–x致密氧化层,氧化层会抑制氧原子的扩散。当表层氧化产物为 ZrC0.42O0.58时,氧化反应基本停止,达到一个稳定状态。继续升高温度,由于产物(Zr2O 和 ZrO2)的晶型结构发生较大的变化,烧结块体会开裂破坏。随着固溶氧的增加,ZrCxO1–x (x=0～0.42)的电阻从37μ??cm增加到690μ??cm。     

Plasticity and Creep in Single Crystals of Zirconium Carbide

High-temperature deformation in ZrC single crystals was studied. Seeded crystals were grown by a direct rf-coupling floating-zone process. Yield stresses were measured from 1080° to 2000°C as a function of stress axis orientation. The Burgers vector was shown to be parallel to the 〈110〉 axes by transmission electron microscopy. Slip was observed on {100}, {110}, or {111} planes, depending on the orientation of the stress axis; it always occurred on the most favorably oriented slip system. The dependence of steady-state creep rate on the applied stress indicated that recovery occurred by a dislocation climb mechanism. Examination of the dislocation structure in deformed crystals by transmission electron microscopy supported this conclusion.     

High-Temperature Reaction Between Cemented Tungsten Carbide and Copper

Interdiffusion in the system cemented tungsten carbide-molten copper has been studied in the range ≤1120°C with special emphasis on the effects of WC grain size and Co content. Techniques used for analyzing the diffusion layers obtained are EPMA, optical microscopy, and microhardness measurement. A Cu-bonded WC layer develops with simultaneous diffusion of Co from the cemented carbide into the bulk copper. The Cu-bonded WC layer grows until a Co-rich layer forms at the Cu/WC-Co interface; further heating pushes the Cu-bonded WC layer deep into the bulk cemented carbide without any significant change in layer thickness. When the WC grain size is reduced and the cobalt content increased, the penetration of copper into cemented carbides increases. A tentative mechanism of interdiffusion has been proposed based on the experimental results.     

Pressureless Densification of Zirconium Diboride with Boron Carbide Additions

Zirconium diboride (ZrB₂) was densified (>98% relative density) at temperatures as low as 1850°C by pressureless sintering. Sintering was activated by removing oxide impurities (B₂O₃ and ZrO₂) from particle surfaces. Boron oxide had a high vapor pressure and was removed during heating under a mild vacuum (∼150 mTorr). Zirconia was more persistent and had to be removed by chemical reaction. Both WC and B₄C were evaluated as additives to facilitate the removal of ZrO₂. Reactions were proposed based on thermodynamic analysis and then confirmed by X-ray diffraction analysis of reacted powder mixtures. After the preliminary powder studies, densification was studied using either as-received ZrB₂ (surface area ∼1 m²/g) or attrition-milled ZrB₂ (surface area ∼7.5 m²/g) with WC and/or B₄C as a sintering aid. ZrB₂ containing only WC could be sintered to ∼95% relative density in 4 h at 2050°C under vacuum. In contrast, the addition of B₄C allowed for sintering to >98% relative density in 1 h at 1850°C under vacuum.