Solid solution synthesis of tantalum carbide‐hafnium carbide by spark plasma sintering

Solid solutions of Tantalum carbide (TaC) and Hafnium carbide (HfC) were synthesized by spark plasma sintering. Five different compositions (pure HfC, HfC‐20 vol% TaC, HfC‐ 50 vol% TaC, HfC‐ 80 vol% TaC, and pure TaC) were sintered at 1850°C, 60 MPa pressure and a holding time of 10 min without any sintering aids. Near‐full density was achieved for all samples, especially in the HfC‐contained samples. The porosity in pure TaC samples was caused by the oxygen contamination (Ta₂O₅) on the starting powder surface. The addition of HfC increased the overall densification by transferring the oxygen contamination from TaC surface and forming ultrafine HfO₂ and Hf‐O‐C grains. With the increasing HfC concentration, the overall grain size was reduced by 50% from HfC‐ 80 vol% TaC to HfC‐20 vol% TaC sample. The solid solution formation required extra energy, which restricted the grain growth. The lattice parameters for the solid solution samples were obtained using X‐ray diffraction which had an excellent match with the theoretical values computed using Vegard's Law. The mechanical properties of the solid solution samples outperformed the pure TaC and HfC carbides samples due to the increased densification and smaller grain size.     

Novel micromesoporous carbon materials synthesized from tantalum hafnium carbide and tungsten titanium carbide

Nanostructured carbide-derived carbons (CDC) were synthesized from Ta₄HfC₅ and WTiC₂ powders via gas phase chlorination within the temperature range from 800 to 1100 °C. The results of X-ray diffraction, high-resolution transmission electron microscopy and Raman spectroscopy showed that the synthesized CDC materials are mainly amorphous, containing relatively small graphitic crystallites. The scanning electron microscopy data revealed the breaking down of the initial carbide particles into smaller ones. The low-temperature N₂ sorption experiments were performed and the specific micropore surface areas up to 1950 m² g⁻¹ and 1880 m² g⁻¹ were obtained for Ta₄HfC₅-CDC and WTiC₂-CDC, respectively. The energy-related properties of the supercapacitors based on 1 M (C₂H₅)₃CH₃NBF₄ solution in acetonitrile and Ta₄HfC₅-CDC or WTiC₂-CDC as an electrode material were investigated using the cyclic voltammetry, electrochemical impedance spectroscopy, galvanostatic charge/discharge and constant power charge/discharge methods. The Ragone plots (using the total material weight or volume of two electrodes) for the supercapacitors based on the Ta₄HfC₅-CDC and WTiC₂-CDC electrodes have been calculated from constant power tests within the potential range from 3.0 to 1.5 V, demonstrating the high gravimetric (28 Wh kg⁻¹) and volumetric (18 Wh dm⁻³) energy densities at high power density (10 kW kg⁻¹ and 15 kW dm⁻³, respectively).     

High-temperature deformation in bulk polycrystalline hafnium carbide consolidated using spark plasma sintering

In this study, we report the three-point flexural strength and fracture toughness of monolithic hafnium carbide up to 2000 °C. HfC with different grain sizes was consolidated using the spark plasma sintering method. Coarse-grained monoliths showed a weak dependence on the strain rate during high-temperature tests at 1600 °C–2000 °C. In contrast, results for the ceramics with a grain size below 20 μm indicated a positive dependence of the yield strength vs strain rate. This allowed us to identify the activation energy for high-temperature deformation in flexure as 370 kJ/mol. This level of activation energy is in satisfactory agreement with reports about the diffusion of C in hafnium carbide.     

Pressureless sintering of single-phase tantalum carbide and niobium carbide

This work presents the results of studies on the preparation of single-phase polycrystalline tantalum carbide and niobium carbide. It has been found that it is possible to obtain polycrystals with high density in the pressureless sintering process at temperatures up to 2000 °C and therefore relatively low temperatures such as for the compounds with one of the highest melting points; TaC – 3985 °C and NbC – 3600 °C. Only carbon as a sintering additive was used. The main role of carbon is to reduce of oxide contamination. It has been shown that the determination of the amount of carbon required to reduce oxide contamination is only possible through the experimental method.     

Oxidation behavior of graphene nanoplatelet reinforced tantalum carbide composites in high temperature plasma flow

Graphene nanoplatelets (GNP) reinforced tantalum carbide (TaC) composites are exposed to a high temperature plasma flow in order to evaluate the effects of GNP on the oxidation behavior of TaC at conditions approaching those of hypersonic flight environments. The addition of GNP is found to suppress the formation of the oxide layer by up to 60%. The high thermal conductivity of GNPs dissipates heat throughout the sample thereby reducing thermal gradients and reducing the intensity of heating at the surface exposed to plasma. In addition, GNPs enhance oxidation resistance by providing toughening which suppresses crack formation and bursting that accelerates oxidation. Scanning electron microscopy (SEM) and high resolution transmission electron microscopy (HR-TEM) reveal that GNPs have the ability to survive the intense high temperature of the plasma. GNPs are believed to seal oxide grain boundaries and hinder the further influx of oxygen. GNPs also provide nano sized carbon needed to induce the localized reduction of Ta₂O₅ to TaC. Micro computed X-ray tomography (MicroCT) validates that the above mechanisms protect the underlying unoxidized material from the structural damage caused by thermal shocks and high shear forces, by reducing thermal gradients and providing toughness.     

Synthesis and characterization of hafnium carbide fine powders

Hafnium carbide powder was synthesized by sol–gel polycondensation of hafnium chloride with citric acid. The starting materials were dissolved in water and mixed homogeneously on a hot plate until precursor gel was formed. Pyrolysis of this gel resulted in monoclinic hafnia which after subsequent heat treatment transformed into hafnium carbide. The obtained materials were analyzed by means of X-ray diffraction and scanning electron microscopy. The results showed that the obtained carbide powders were composed of nearly equiaxed particles with narrow size distribution. Characterization by nitrogen adsorption showed that the obtained powders were micro- and mesoporous materials with high specific surface area.     

Static and dynamic mechanical properties of boron carbide processed by spark plasma sintering

Spark plasma sintering (SPS) has become a popular technique for the densification of covalent ceramics. The present investigation is focused on the static mechanical properties and dynamic compressive behavior of SPS consolidated boron carbide powder without any sintering additives. Fully dense boron carbide bodies were obtained by a short high temperature SPS treatment. The mechanical properties of the SPS-processed material, namely hardness (32 GPa), Young modulus (470 GPa), fracture toughness K_C (3.9–4.9 MPa m^0.5), flexural strength (430 MPa) and Hugoniot elastic limit (17–19 GPa) are close or even better than those of hot-pressed boron carbide.     

The role of microstructure on high-temperature oxidation behavior of hafnium carbide

Microstructure development of the products formed upon oxidation of hafnium carbide (HfC_x, 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 HfO₂C_x 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 HfO₂C_x product.     

High temperature oxidation of two- and three-dimensional hafnium carbide and silicon carbide coatings

The difficulty in using C/C composites as structural components above 2000 °C in an oxidizing atmosphere is their poor lifetime. The solution proposed here consisted in combining two refractory carbides, hafnium and silicon carbides, in coating with a complex architecture, named a three dimensional coating, over a C/C substrate. Such a coating protects the C/C composite at 2000 °C under air. The oxidation of the coating leads to the formation of a Si_xO_yHf_z hafnium-containing silicate liquid, combined with HfO_2(s). This liquid limits oxygen diffusion more than pure SiO₂ does, so it is a better protection against oxidation. Furthermore, HfO_2(s) acts as a frame holding Si_xO_yHf_z in place. From these results, an oxidation mechanism is proposed and discussed.     

Synthesis of ultra-fine hafnium carbide powders combining the methods of liquid precursor conversion and plasma activated sintering

Ultra-fine hafnium carbide (HfC) powders were synthesized using a novel method combining liquid precursor conversion and plasma activated sintering (PAS). Solution-based processing was used to achieve a fine-scale mixing of the reactants, and further treatment by PAS allowed fast formation of HfC. We investigated the effect of the type of acid used during the liquid precursor conversion on the synthesized powders, where mixtures were prepared using salicylic acid, citric acid, or a combination of these. The results show that pure HfC powders (with an average particle sizes of 350 nm) were obtained at a relatively low temperature (1550 °C) using a HfOCl₂·8H₂O precursor with the mixed acids. The oxygen content of the synthesized powders was only 0.97 wt%. The type of acid had a significant effect on the synthesis product. When using only citric acid, the temperature required to produce pure hafnium carbide increased to 1700 °C. In the case of a salicylic acid precursor, pure HfC was not obtained, even at a synthesis temperature of 1700 °C.     

Sintering and densification mechanisms of tantalum carbide ceramics

Dense tantalum carbide (TaC) ceramics were prepared using TaC nanopowder via spark plasma sintering (SPS). The effects of the sintering temperature and applied pressure on the densification and grain growth behaviour of TaC ceramics were investigated. The results showed that high temperature and pressure promoted sintering densification, while their increase caused an increase in the grain size of TaC ceramics. A highly dense TaC ceramic (∼97.19%) with a fine grain size of 2.67 μm was obtained by sintering at 1800 °C for 10 min under 80 MPa. The Vickers hardness, Young's modulus and fracture toughness were 15.60 GPa, 512.66 GPa and 3.59 MPa·m^1/2, respectively. The densification kinetics were investigated using a creep deformation model. Diffusion and grain boundary sliding were proven to be the dominant densification mechanisms based on the stress and grain size exponents combined with the microstructural characteristics. The apparent activation energy of the mechanism controlling densification was 252.94 kJ/mol.     

Oxidation resistance of tantalum carbide-hafnium carbide solid solutions under the extreme conditions of a plasma jet

The oxidation behaviors of tantalum carbide (TaC)- hafnium carbide (HfC) solid solutions with five different compositions, pure HfC, HfC-20 vol% TaC (T20H80), HfC- 50 vol% TaC (T50H50), HfC- 80 vol% TaC (T80H20), and pure TaC have been investigated by exposing to a plasma torch which has a temperature of approximately 2800 °C with a gas flow speed greater than 300 m/s for 60 s, 180 s, and 300 s, respectively. The solid solution samples showed significantly improved oxidation resistance compared to the pure carbide samples, and the T50H50 samples exhibited the best oxidation resistance of all samples. The thickness of the oxide scales in T50H50 was reduced more than 90% compared to the pure TaC samples, and more than 85% compared to the pure HfC samples after 300 s oxidation tests. A new Ta₂Hf₆O₁₇ phase was found to be responsible for the improved oxidation performance exhibited by solid solutions. The oxide scale constitutes of a scaffold-like structure consisting of HfO₂ and Ta₂Hf₆O₁₇ filled with Ta₂O₅ which was beneficial to the oxidation resistance by limiting the availability of oxygen.     

Low-temperature synthesis of tantalum carbide by facile one-pot reaction

Tantalum carbide (TaC) was synthesized by polycondensation and carbothermal reduction reactions from an inorganic hybrid. Tantalum pentachloride (TaCl₅) and phenolic resin were used as the sources of tantalum and carbon, respectively. FTIR of as-synthesized dried complexes revealed formation of Ta-O. Pyrolysis of the complexes at 800 °C/1 h under argon resulted in tantalum oxide which after heat treatment at 1000–1200 °C transformed to tantalum carbide. The mean crystallite size of the precursor-derived TaC ceramics was less than 40 nm and Ta and C elements were homogeneously distributed in the ceramic samples. Mechanism for formation of TaC ceramic was analyzed.     

Electronic structure and thermal conductivity of zirconium carbide with hafnium additions

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.     

Improving electrical conductivity and wear resistance of hafnium nitride films via tantalum incorporation

Transition metal nitrides are being widely applied, as durable sensors, semiconductor and superconductor devices, their electrical conductivity and wear resistance having a significant influence on these applications. However, there are few reports about how to improve above properties. In this paper, tantalum was incorporated into hafnium nitride films through Hf_1-xTa_xN_y [x=Ta/(Hf+Ta), y=N/(Hf+Ta)] solid solution. The electrical conductivity and wear resistance of the films were significantly improved, due to the increase of the electron concentration (tantalum has one more valence electron than hafnium) and the increase in H/E and H³/E² ratios caused by the effect of solid solution hardening, respectively. The highest electrical conductivity of Hf_1-xTa_xN_y films is 8.3×10⁵ S m⁻¹, which is 1.7 times and 5.2 times of that of hafnium nitride and tantalum nitride films, respectively. In addition, the lowest wear rate of films is 1.2×10⁻⁶ mm³/N m, which is only 10% and 48% of that of hafnium nitride and tantalum nitride films, respectively. These results indicate that alloying with another transition metal is an effective method to improve electrical conductivity and wear resistance of transition metal nitrides.     

Mechanical properties and rapid consolidation of binderless nanostructured tantalum carbide

The rapid sintering of nanostructured TaC hard material was investigated with a focus on the manufacturing potential of high-frequency induction heated sintering process. The advantage of this process is that it allows very quick densification to near theoretical density and prohibition of grain growth in nanostructured materials. A dense pure TaC hard material with a relative density of up to 96% was produced with simultaneous application of 80 MPa pressure and induced current within 3 min. The finer the initial TaC powder size, the higher the density and the better mechanical properties. The fracture toughness and hardness values obtained from 10 h milled powder were 5.1 ± 0.3 MPa m^1/2 and 22 GPa, respectively, under 80 MPa pressure and 80% output of total power capacity (15 kW).     

Influence of chemical composition on mechanical properties of spark plasma sintered boron carbide monoliths

Fully dense boron carbide monoliths exhibiting fine microstructure (i.e., submicrometric grain size) are sintered by Spark Plasma Sintering. Two different commercial powder batches, exhibiting different stoichiometries (i.e., B/C ratio and oxygen content) and various amounts of secondary phases (i.e., boric acid and free carbon), are used. Their chemical composition is well‐defined by coupling different methods (Transmission Electron Microscopy associated with XRD analyses, and Instrumental Gas Analysis), and are correlated with their mechanical properties, characterized from meso‐ to macro‐scopic scales by nano‐indentation and ultrasonic pulse echography. The presence of secondary phases (graphite and boric acid) is evidenced in various proportions in each powder batch. If the boric acid disappears during sintering, the graphite remains. However, for the considered amounts of graphite (lower than 1 wt%), the low variations in graphite content have no significant effect on hardness and elasticity values. At the opposite, the presence of oxygen in boron carbide lattice, leading to a boron oxycarbide phase, induces a decrease in both hardness and elasticity properties.     

余留真空喷淋EG的回用

分析了钟纺聚酯工艺停工过程真空喷淋EG质量指标恶化的原因 ,通过控制停工时的各项操作 ,减少停工过程对喷淋EG的污染程度 ,达到对停工余留EG进行回用的目的     

Mechanical and thermophysical properties of carbon/carbon composites with hafnium carbide

Carbon/carbon (C/C) composites with addition of hafnium carbide (HfC) were prepared by immersing the carbon felt in a hafnium oxychloride aqueous solution, followed by densification and graphitization. Mechanical properties, coefficients of thermal expansion (CTE), and thermal conductivity of the composites were investigated. Results show that mechanical properties of the composites decrease dramatically when the HfC content is greater than 6.5 wt%. CTE of the composites increases with the increase of HfC contents. The composites with addition of 6.5 wt% HfC show the highest thermal conductivity. The high thermal conductivity results from the thermal motion of CO in the gaps and pores, which can improve phonon–defect interaction of the C/C composites. Thermal conductivities of the composites decrease when the HfC content is greater than 6.5 wt%, which is due to formation of a large number of cracks in the composites. Cracks increase the phonon scattering and hence restrain heat transport, which results in the decrease of thermal conductivity of the composites.