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Understanding the oxidation behavior of hafnium carbide is crucial to its application in extreme environments. In this work, the transition in high‐temperature oxidation kinetics regimes in hafnium carbide is explained based on phase equilibria considerations supported by observed changes in oxide scale microstructure evolution associated with different transformation pathways. Below, a composition‐dependent critical temperature and oxygen pressure, hafnium carbide first transforms to an amorphous material with nominal composition HfO2C followed by phase separation into carbon and hafnia domains. Subsequently, gaseous transport through a nanometric pore network formed by oxidative removal of phase‐separated carbon becomes rate‐limiting. Above this critical point, the oxidation sequence involves a direct transformation from hafnium carbide to hafnia and gaseous products, leading to dissimilar scale morphologies responsible for the reported transition from gaseous to solid‐state diffusion‐limited oxidation regimes at ultra‐high temperatures. [ABSTRACT FROM AUTHOR]

The synthesis of SiBNC–M (M is metal element) multinary ceramic precursors has mainly been through chemical modification of the SiBNC ceramic precursors with the introduction of the metal element/compound. This misses the simplicity, efficiency, and combined benefits of single reactor synthesis routes. We herein adopt a one‐pot method to synthesize SiBNC–Hf ceramic precursors (PBSHZ) from different Hf sources, and with varying Hf content. The starting materials include hafnium tetrachloride, hafnium dichloride, trichlorosilane, hexamethyldisilazane, and boron trichloride. Fourier transform infrared spectroscopy (FT‐IR), nuclear magnetic resonance (NMR), X‐ray photoelectron spectroscopy (XPS), and thermogravimetric analysis are employed to characterize the structures and properties of the PBSHZ. The results reveal the successful incorporation of Hf into the precursors. Compared to the hafnium tetrachloride source, the precursor synthesized from hafnium dichloride yields more ceramics and shows better solubility, due to fewer Hf–Cl bonds available for reaction. Meanwhile, there is a positive correlation between the rise in the Hf content of PBSHZ and the ceramic yield. However, the solubility and processability of the precursors decline, due to the multiplication of the cross‐linking degree in the molecular structure. Further FT‐IR, NMR, XPS, and thermogravimetry–mass spectrometry (TG–MS) analysis indicate a full polymer‐to‐ceramic transformation at 1000°C pyrolysis temperature. At this point, the SiBNC–Hf ceramics mainly consist of an Si–B–N skeleton, HfN, and free carbon. [ABSTRACT FROM AUTHOR]

Microstructure development of the products formed upon oxidation of hafnium carbide (HfCx, x = 0.65, 0.81, or 0.94) at 1300°C and 0.8 mbar oxygen pressure was investigated using Raman spectroscopy, X‐ray diffraction, electron microscopy, and electron energy‐loss spectroscopy. For all specimens a multilayered oxide scale was observed featuring an outermost porous hafnia layer and an interlayer adjacent to the parent carbide containing hafnia interspersed with carbon. The outermost hafnia features coarse pores presumably formed during initial stages of oxidation to allow rapidly evolving gaseous products to escape from the oxidation front. As the oxidation scale thickens, diffusional resistance results in slower oxidation rates and smaller quantities of gaseous products that are removed via networks of increasingly fine pores until the local oxygen partial pressure is sufficiently low to selectively oxidize the parent carbide. Electron microscopy studies suggest that the oxidation sequence at this stage begins with the transformation of parent carbide to an amorphous material having empirical formula HfO2Cx that subsequently phase separates into hafnia and carbon domains. Hafnia polymorphs in the phase‐separated region vary from cubic to monoclinic as grains coarsen from ca. 2–20 nm, respectively. Immediately adjacent to the phase‐separated region is carbon‐free mesoporous hafnia whose pore morphology is inherited from that of prior carbon domains. The average pore size and pore volume fraction observed in mesoporous hafnia are consistent with predictions from kinetic models that ascribe gaseous diffusion through a pore network as the rate determining step in oxidation behavior of hafnium carbide. These observations imply that high‐temperature oxidation behavior of hafnium carbide under the employed test conditions is linked to microstructure development via phase separation and coarsening behaviors of an initially formed amorphous HfO2Cx product. [ABSTRACT FROM AUTHOR]

The SiHfOC ceramic microspheres were synthesized from the polysilazane and hafnium(IV) acetylacetonate hybrid precursors through a combination of hydrothermal method and polymer‐derived ceramics (PDCs) method. The SiHfOC ceramic microspheres consist of β‐SiC, SiO2, and m‐HfO2 in an amorphous SiOC matrix and are in a spherical morphology with an average size of about 2 μm. Hafnium‐containing nanoparticles with ∼50‐nm size are not only uniformly distributed on the surface of the ceramic microspheres but also embedded in the SiHfOC ceramic microspheres, forming a unique mosaic structure. Pyrolyzing at 1000°C offers a possibility of an in situ formation of nanocrystal nuclei in the amorphous SiOC ceramic matrix, and the subsequent annealing at 1500°C helps the small nanocrystal nuclei grow and separate the amorphous SiHfOC into a multiphase SiOC network containing β‐SiC, SiO2, m‐HfO2 nanocrystalline and free C. This research provides a methodology for preparing multicomponent ceramic microspheres that can efficiently develop high‐performance PDCs modified by transition metal alkoxide. [ABSTRACT FROM AUTHOR]

Carbon fiber/phenolic (C/Ph) composites were modified with different weight ratios of hafnium diboride (HfB2) nanofibers to apperceive thermomechanical properties of C/Ph–Hf nanocomposites. Mechanical properties, thermal stability, and ablation resistance of C/Ph–Hf nanocomposites were found to be optimum when the weight percentage of HfB2 was equal to one. Maximum flexural strength and modulus were obtained with 118 MPa and 1.9 GPa for C/Ph–1%Hf nanocomposite, respectively. Increasing the proportion of HfB2, by delaying the temperature of thermal degradation of nanocomposites, enhanced the thermal stability and residual of C/Ph–Hf relative to C/Ph in both nitrogen and air environments. In the oxyacetylene flame test at 2500°C for 160 s, the optimum mass ablation rate of C/Ph–1%Hf nanocomposites was found to be 0.0150 g/s compared to 0.068 g/s for blank C/Ph, along with reducing the back surface temperature by 51%. The ablation mechanism of C/Ph–Hf nanocomposites after the oxyacetylene torch test was concluded from the derivations obtained from X‐ray diffraction, energy dispersion spectroscopy, and microstructure analyses. These clarified that the formation of high‐temperature species, such as HfO2, HfC, and B4C owing to oxidation of HfB2 and subsequent reaction products with char, resulted in an increased ablation resistance of the nanocomposites. [ABSTRACT FROM AUTHOR]

A variety of the ternary Hf–Ir–B phases formed via the reaction between iridium and hafnium diboride at elevated temperatures was found. The data on the phase and elemental composition, as well as crystal structure, obtained by powder and single‐crystal X‐ray diffraction, scanning electron microscopy/energy‐dispersive X‐ray spectrometer, and time‐of‐flight neutron diffraction analysis unambiguously confirm that HfIr3Bx solid solution, two known ternary borides (HfIr3B4, Hf2Ir5B2), as well as two novel ternary HfIr2.1B1.3 and HfIr5.7B2.7 phases, are formed at elevated temperatures. This result is fundamentally different from that previously obtained by us for the Hf–Ir–C system in which only one binary intermetallic compound, HfIr3, was produced. The measured Vickers microhardness for all the aforementioned ternary borides (13–19 GPa) allows us to consider them hard. The coefficients of thermal expansion of ternary borides were measured by in situ high‐temperature X‐ray analysis. [ABSTRACT FROM AUTHOR]

In this study, we explored the consolidation, solid‐solution formation and high‐temperature properties of tantalum hafnium carbide with the 1 TaC:4 HfC ratio, that is, Ta0.2Hf0.8C. Tantalum hafnium carbide bulks can be consolidated using spark‐plasma sintering only at temperatures exceeding 2200°C. The bulks prepared using a 40‐min dwell at 2200°C had the lattice parameter a = 4.571(9) Å. Based on the three‐point flexural tests, it was observed that the toughness and strength of Ta0.2Hf0.8C remained high at 2000°C (3.4 ± 0.4 MPa m1/2, 500 ± 20 MPa). At 2000°C, majority of carbides show a plastic behavior, but the strain‐stress curves of the SPSed Ta0.2Hf0.8C ceramic were linear. Using the shrinkage rate data during the SPS and the strain‐stress data from the high‐temperature flexure, we estimated the activation energies for the densification and flexure as 820 ± 20 kJ/mol and 1180 ± 120 kJ/mol, respectively. [ABSTRACT FROM AUTHOR]

The lattice thermal conductivity of ZrC with different Hf contents was investigated theoretically. The density of states and electron density differences were calculated for ZrC and (Zr,Hf)C containing 3.125 or 6.25 at% Hf. It was found that the electronic structure did not change significantly with the Hf additions. Lattice thermal conductivities were calculated for all of the compositions by combining first‐principles calculations with the Debye–Callaway model. The theoretical lattice thermal conductivity of ZrC was 68 W·m−1·K−1 at room temperature. When adding 3.125 and 6.25 at% Hf into ZrC, the lattice thermal conductivities decreased to 18 and 15 W·m−1·K−1, respectively. The mechanism for the decreased conductivity is that with the addition of Hf impurities, the frequency of the acoustic phonons decreased, which resulted in decreases in the Debye temperature and lattice thermal conductivity. [ABSTRACT FROM AUTHOR]

The melting temperatures of two different ZrB2 ceramics were studied using laser induced melting. ZrB2 having a low Hf content, produced by reaction hot pressing, had a melting temperature of 3546 K and a commercial grade ZrB2 had a melting temperature of 3553 K. Uncertainty of the temperature measurements was 1% of the absolute temperature, or ~35 K for both materials based upon 2‐sigma and a 95% confidence interval. While these values were consistent with the previously reported ZrB2 melting temperature of 3518 K, this study was able to measure Tm with less uncertainty than previous studies (±45 K). Furthermore, this study assessed the effect of Hf content on melting temperature, finding that melting temperature did not change significantly for hafnium contents of 1.75 to 0.01 at%. This study also measured a normal spectral emissivity of 0.34 for ZrB2 at 3000 K. The emissivity decreased to 0.28 at the melting temperature, then, stabilized at 0.30 in a liquid phase. [ABSTRACT FROM AUTHOR]

The oxidation behavior of SiC fibers coated with (a) undoped polysilazane and (b) precursors containing a mixture of polysilazane and hafnium butoxide in equal weight fractions, is reported. The coatings were prepared by repetitive cycles of nanolayer depositions, as reported in recent publications. The oxidation experiments were carried out at 1400°C in ambient air (Boulder, CO) for up to 100 hours. The extent of degradation of SiC was measured by the recession in the diameter of the fibers as a function of time. The fibers with undoped polymer precursor recessed significantly, whereas the fibers coated with HfSiCNO remained essentially unchanged. These results are in agreement with earlier work from our laboratory where the resilience of hafnon and zircon, as well as hafnia and zirconia, against high‐temperature corrosion in streaming humid environments had been highlighted. [ABSTRACT FROM AUTHOR]

The hafnium‐rich portion of the of the hafnium‐nitrogen phase diagram is dominated by a substoichiometric rocksalt HfN1‐x, the ζ‐Hf4N3−x, the η‐Hf3N2−x, and the elemental Hf phase. The zeta and eta nitride phases have a close packed metal atom stacking sequence but their nitrogen atom ordering has yet to be concretely identified. With respect to the composition of these phases, recent computational studies of their phase stability using density functional theory (DFT) are not in agreement with reported experimental observations. In this work, we re‐examine the phase stability of the zeta and eta phases using DFT combined with enumerated searches using the known metal atom stacking sequences of these phases but with variable carbon concentration and ordering. We have found new structures for the zeta and eta phases that are now in better agreement with experimental findings. Furthermore, we report a new eta phase, η‐Hf12N7, which lies on the convex hull and has a nitrogen atom ordering that is substantially different from the zeta phase. This work also demonstrates the importance of configurational entropy in dictating the finite temperature phase diagrams in this system. [ABSTRACT FROM AUTHOR]

Hafnium silicate (HfSiO4; hafnon) is under consideration as an environmental barrier coating material for high‐temperature applications. However, its rate of formation from mixtures of monoclinic HfO2 and crystalline (β‐cristobalite) SiO2 powders is unknown. Here it has been synthesized and its formation rate measured during their solid‐state reaction at temperatures from 1250°C to 1400°C. Rietveld refinement of X‐ray diffraction patterns indicates that at 1250°C the hafnon phase fraction increases linearly with time, while at the highest reaction temperature, the hafnon phase fraction exhibited a parabolic dependence upon time. Between these two limiting temperatures, a region of linear behavior preceded a transition to parabolic kinetics, with the transition occurring at an earlier time as the reaction temperature increased. Arrhenius relations fitted the kinetics of hafnium silicate formation in both the linear and parabolic regimes. Scanning electron microscopy indicated that the reaction proceeded by diffusion of SiO2 into HfO2, similar to the mechanism by which zirconium silicate has been formed from vitreous SiO2 and tetragonal ZrO2. The initial linear rate of reaction is consistent with the growth of the contact area between the SiO2 and HfO2 particles combined with rapid permeation of the Si4+ and O2− through the initial, incompletely formed hafnon. After a thin hafnon layer had formed between the reactants, the rate of hafnium silicate growth slowed and further growth was governed by the rate of diffusion of Si4+ and O2− through the reaction product consistent with the observed parabolic dependence of the phase fraction upon time. [ABSTRACT FROM AUTHOR]

Hafnium carbide (HfC) possesses low toughness and damage tolerance, which limits its application as high‐temperature structural materials. Here we report a route to fabricate a dense HfC with high toughness by incorporating a two‐component SiCN (silicon carbonitride) sintering aid, which is composed of SiCN and turbostratic carbon in the spark plasma sintering. The addition of the SiCN not only enhances the density (up to ~97%) of the final product, but more importantly results in an increase in toughness from 4.3 to 8.5 MPa m1/2 with only a 10% reduction in the flexural strength. The improvement has been attributed to the unique microstructure of the obtained samples, exhibiting several important characteristics: (a) homogeneously dispersed SiCN and C secondary phases in the HfC matrix to control grain size; (b) HfC grains contain different levels of Si, O, and N to form solid solution; (c) enrichment of free carbon at grain boundaries and triple junctions. [ABSTRACT FROM AUTHOR]

Four different SiOC glass ceramics were synthesized and their fracture toughness (KIc) and fracture surface energy (γ) were assessed by means of the single‐edge precracked beam (SEPB) method. In addition, the elastic moduli were measured and the Vickers indentation behavior (hardness and microcracking) was characterized. In particular, the dependence of KIc on the free carbon content and on the fraction of crystallized nanoparticles (SiC, ZrO2, HfO2) was investigated. An increase in KIc, from about 0.73 to 0.99 MPa √m is observed as the free carbon content is increased from less than 1 to 12 vol%. The addition of Hf and Zr (resulting in 4.5 to 7.8 vol% HfO2 and ZrO2 nanoparticles) was found to increase KIc to an extent similar to the free carbon content. Moreover, predicted KIc values, assuming that the crack travels through all phases accounting for their respective volume fractions, disrupting the weakest links within the structural units, are in agreement with the experimental values. [ABSTRACT FROM AUTHOR]

Bond characteristics, mechanical properties, and high‐temperature thermal conductivity of ultrahigh‐temperature ceramics (UHTCs), hafnium carbide (HfC), tantalum carbide (TaC), and their solid solution composites, were investigated using first‐principles calculations. Mulliken analyses revealed that Ta formed stronger covalent bonds with C than did Hf. Bond overlap analyses indicated that the Hf–C bond possessed mixed covalent and ionic bond characteristics, compared with the more covalent character of the Ta–C bond. Consequently, the overall elastic properties were enhanced with increasing number of Ta–C bonds in the composites. The overall metallicity of the composites also increased with increasing TaC content; thus, the mechanical properties did not improve monotonically. Our results indicate that adding a small amount of TaC to HfC or vice versa to produce a composite would create a new UHTC with greatly improved elastic and mechanical properties as well as high‐temperature thermal conductivity. [ABSTRACT FROM AUTHOR]

Herein, Ta–Hf–C solid solution ceramics were consolidated from nano‐scale Ta–Hf–C solid solution powders for the first time. Four different compositions (4TaC–1HfC, 1TaC–1HfC, 1TaC–3HfC, and 1TaC–4HfC) were prepared by hot‐pressed sintering at 2100°C, 70 MPa pressure and a holding time of 30 minutes. The densification, formation of single‐phase solid solution and mechanical properties of the samples were systemically investigated. Relative density >95% was achieved for all four compositions with some improvement when TaC content was increased. And the formation of single‐phase Ta–Hf–C solid solution was strongly demonstrated by phase analysis and crystal measurement using XRD and TEM. A significant improvement of hardness up to ~30 GPa was achieved, which was much higher than that of pure TaC (18.9 GPa) and HfC (22.1 GPa), due to the high densification and solid solution strengthening mechanism. [ABSTRACT FROM AUTHOR]

Abstract: Ta1−xHfxB2 material is attractive for various aerospace applications. In this study, 2 low‐cost approaches were adopted to synthesize nanocrystalline Ta0.5Hf0.5B2 solid solution and related composite powders. The first was based on carbothermal reduction reaction (CTR) of intimately mixed tantalum‐hafnium‐boron oxide(s) and carbon obtained from aqueous solution processing of TaCl5, HfCl4, B2O3, and sucrose as precursors. It was found that when using this method, due to the low solubility of each other for Ta2O5 and HfO2 and the difference in reactivity of those 2 oxides with carbon (as well as B2O3), individual TaB2 (‐rich) and HfB2 phases always form separately. Those borides tend to remain phase separated due to the slow inter‐diffusion between them. However, it was observed that addition of copper “catalyst” noticeably accelerates the inter‐diffusion and the solid solution formation. The second approach was based on alkali metal reduction reaction, in which TaCl5 and HfCl4 are directly reacted with sodium borohydride (NaBH4). This method yields a single phase Ta0.5Hf0.5B2 solid solution nanopowders in one step at much lower temperatures (e.g., 700°C) by avoiding the oxides formation and the associated phase separation of individual borides as observed in the CTR‐based process. [ABSTRACT FROM AUTHOR]

Ferroelectric (Na1/2Bi1/2)TiO3 and its various solid solutions have been drawing special attention as a new candidate for their lead-based counterparts. In this study, therefore, hafnium- or zirconium-doped grain-oriented (Na1/2Bi1/2)TiO3 ceramics with <001>pc orientation were fabricated by templated grain growth method using anisotropically shaped SrTiO3 template particles. These molten salt synthesized SrTiO3 platelets were tape cast with calcined (Na1/2Bi1/2)TiO3 powders, and then sintered at 1200°C for 6 h. Texture fractions up to 70% have been obtained. Unipolar strains up to >0.25% were measured. Doped ceramics showed a 'relaxor-like' ferroelectric behavior. A broad dielectric peak with a slim hysteresis loop was present for hafnium- or zirconium-doped samples. Electrostrictive coefficient of hafnium-doped (Na1/2Bi1/2)TiO3 ceramics were found to be 0.032 m4/C2, which is much larger than that of PMN-based electrostrictive materials. Electromechanical properties of hafnium- and zirconium-doped (Na1/2Bi1/2)TiO3 ceramics under electric bias were studied as well. [ABSTRACT FROM AUTHOR]

HfC-30 vol% SiC ceramics with a relative density of 99.7% was obtained by pressureless sintering at 2300°C for 0.5 h. The resultant ceramics showed fine microstructure with HfC grain size around 1 μm. The hardness (20.5 ± 0.2 GPa), bending strength (396 ± 56 MPa), and fracture toughness (2.81 ± 0.18 MPa·m1/2) of HfC-30 vol% SiC ceramics were at least 20% higher than those of monolithic HfC ceramics. The influences of SiC particle size, volume fraction, and the oxide impurity on the microstructure evolution of HfC-based ceramics were examined. The results indicate that SiC addition and the oxygen impurity introduced by ball milling play opposite roles in the HfC grain growth during sintering. The oxide impurity introduced by ball milling caused the HfC grain coarsening, whereas SiC particles inhibited the grain growth of HfC significantly. [ABSTRACT FROM AUTHOR]

Sharp leading edge (LE) samples of UHTC (20 vol% SiC- HfB2) and SiC were exposed to simulated hypersonic flight conditions using a direct-connect scramjet rig and their thermal and oxidation responses measured. The measured back-wall temperatures and scale thicknesses were significantly smaller than might be expected from stagnation temperatures at the LE. Furthermore, the scale that formed around the LE was more uniform than expected from the steep drop in cold wall heat flux with distance from the tip. These results were interpreted and rationalized using physics-based models. An aerothermal model in combination with an oxidation model accounted for the observed scale thicknesses at the tip and their slight variation with distance. The scale thicknesses were similar to values reported for exposures in furnaces at temperatures calculated for the tip, but less than those reported in arc jet tests. The formation of hafnon (HfSiO4) and the absence of external glassy layer and of silica in the outer portions of the oxide region are unique to scramjet tested samples, presumably due to the high fluid flow (high shear and evaporation) rates. [ABSTRACT FROM AUTHOR]

Ti3SiC2 shows a unique combination of the properties of both metals and ceramics. However, its stiffness and strength lose rapidly above 1050°C, which is the main obstacle for the high-temperature application of this material. To improve the high-temperature mechanical properties of Ti3SiC2, Zr, Hf, or Nb were used as dopants in Ti3(SiAl)C2. At room temperature, the Zr-, Hf-, or Nb-doped Ti3(SiAl)C2 ceramics have comparable stiffness, hardness, strength, and fracture toughness with those of Ti3(SiAl)C2. At high temperatures, however, a significant improvement in stiffness and strength has been achieved for (Ti1− xTx)3(SiAl)C2 (T=Zr, Hf, or Nb). (Ti1− xT x)3(SiAl)C2 can retain high degrees of stiffness and strength up to 1200°C, which is 150°C higher than those for Ti3(SiAl)C2. [ABSTRACT FROM AUTHOR]

Fine hafnium diboride (HfB2) powders have been prepared by modified carbothermal/borothermal reduction of hafnium dioxide (HfO2) at relatively low temperatures (1500°–1600°C) for 1–2 h. The XRD patterns could be indexed as hexagonal HfB2 and no evidence of HfC, HfO2, or other impurities was observed. Glow discharge mass spectrometer analysis indicates that the synthesized HfB2 powders had high purity. The synthesized HfB2 powders had small average crystallite size (around 1 μm) and low oxygen content (<0.30 wt%). Scanning electron microscopy observation of the as-prepared powders demonstrated quasi-column morphology and laser particle size analysis showed monodispersity (polydispersity 0.005). [ABSTRACT FROM AUTHOR]

Hafnium titanate films are generating increasing interest because of their potential application as high- k dielectrics materials for the semiconductor industry. We have investigated sol–gel processing as an alternative route to obtain hafnium titanate thin films. Hafnia-titania films of different compositions have been synthesized using HfCl4 and TiCl4 as precursors. The HfO2–TiO2 system composition with 50 mol% of TiO2 and 50 mol% of HfO2 has allowed the formation of a hafnium titanate film after annealing at 1000°C. The films exhibited a homogeneous nanocrystalline structure and a monoclinic hafnium titanate phase that has never been obtained before in thin films. The films resulted in the formation of homogeneously distributed nanocrystals with an average size of 50 nm. Different compositions, with higher or lower hafnia contents, produced anatase crystalline films after annealing at 1000°C. [ABSTRACT FROM AUTHOR]

Pyrolyses of chemical precursor systems composed of zirconium or hafnium powders dispersed into blends of the polymethylcarbosilane and poly(norbornenyldecaborane) preceramic polymers provide simple efficient routes to new types of ZrB2/ZrC/SiC and HfB2/HfC/SiC composites. Further extensions of this method should now allow the syntheses of a wide variety of complex potential ultra high-temperature zirconium- and hafnium-based composite materials with controllable tuning of their compositions, microstructures, and/or stabilities. [ABSTRACT FROM AUTHOR]

The homogeneous and heterogeneous reactions of hafnium (Hf) with boron (B) or carbon (C) powders were studied under non-self-propagating high-temperature synthesis (SHS) conditions in searching for chemically aided processes of ultra-high-temperature ceramics at mild and practical conditions. The threshold interactions of the Hf/B and Hf/C powder mixtures consisting of relatively large Hf particles occur at 700° and 800°C, respectively, with no observation of particle melting as previously postulated for igniting SHS reactions. A microscale melting phenomenon at the Hf surface was observed by scanning electron microscopy analysis in heterogeneous reactions between Hf metal strips and B or C powders at the temperature range of 1100°–1200°C, more than 1000°C below the incipient melting point of the metal. The non-SHS conditions allow halting the reactions at intermediate stages for analytical purposes. Kinetic studies of the homogeneous and heterogeneous reactions, heat transfer model analyses, and their association with the investigation of microstructural phenomena were used to postulate the mechanism involved in the observed reactions. [ABSTRACT FROM AUTHOR]

We evaluated the thermal conductivity of HfB2-based ultra-high-temperature ceramics from laser flash diffusivity measurements in the 25°–600°C temperature range. Commercially available powders were used to prepare HfB2 composites containing 20 vol% SiC, some including TaSi2 (5 vol%) and Ir (0.5 or 2 vol%) additions. Samples were consolidated via conventional hot pressing or spark plasma sintering. Processing differences were shown to lead to differences in magnitude and temperature dependence of effective thermal conductivity. We compared results with measured values from heritage materials and analyzed trends using a network model of effective thermal conductivity, incorporating the effects of porosity, grain size, Kapitza resistance, and individual constituent thermal conductivities. [ABSTRACT FROM AUTHOR]

Niobium, tantalum, and hafnium carbonitride nanopowders were obtained by high-energy ball milling of elemental metal and graphite in a nitrogen atmosphere. By adjusting milling parameters, a mechanically induced self-propagating reaction was observed during the formation of carbonitride phases. The stoichiometry of transition metal carbonitrides was controlled by fixing the starting metal-to-carbon molar ratio. Ignition of the reactant mixture was determined by the combined effect of nitrogen and carbon, gas, and solid reagents, respectively. The composition and lattice parameters were determined by X-ray diffraction measurements. Microstructural characterization showed a nanophase structure with a homogeneous chemical composition. [ABSTRACT FROM AUTHOR]

The effect of Hf additions on the thermal properties of ZrB2 ceramics was studied. Reactive hot pressing of ZrH2, B, and HfB2 powders was used to synthesize (Zr1− x,Hf x)B2 ceramics with Hf contents ranging from x = 0.0001 (0.01 at.%) to 0.0033 (0.33 at.%). Room-temperature heat capacity values decreased from 495 J·(kg·K)−1 for a Hf content of 0.01 at.% to 423 J·(kg·K)−1 for a Hf content of 0.28 at.%. Thermal conductivity values decreased from 141 to 100 W·(m·K)−1 as Hf content increased from 0.01 to 0.33 at.%. This study revealed, for the first time, that small Hf contents decreased the thermal conductivity of ZrB2 ceramics. Furthermore, the results indicated that reported thermal properties of ZrB2 ceramics are affected by the presence of impurities and do not represent intrinsic behavior. [ABSTRACT FROM AUTHOR]

Hafnium carbide (HfC)-(5, 10, and 20 vol%) MoSi2 ceramics were pressureless sintered at 1950°C in an argon flux. The materials had nearly full density (96%–98%), with mean grain sizes in the range of 3–4 μm. Depending on the MoSi2 amount (5–20 vol%), the mechanical properties were in the following ranges: hardness 16–15 GPa, Young's modulus 434–385 GPa, fracture toughness 3.6–3.4 MPa·m1/2, and room-temperature 4-point flexural strength 465–383 MPa. [ABSTRACT FROM AUTHOR]