Hafnium Carbide Research Articles

Oxidation/ablation behaviors of hafnium carbide-silicon carbonitride systems at 1500 and 2500 C

The oxide scales of hafnium carbide (HfC) typically exhibit a porous structure after oxidation/ablation due to the release of gas oxidation products, which allows oxygen penetration to promote the rapid oxidation of the HfC matrices. Here, we report that the oxidation/ablation resistance of HfC was enhanced by the incorporation of amorphous silicon carbonitride (SiCN). HfC-SiCN ceramics with 10 vol % SiCN showed a significant improvement in the oxidation/ablation resistance compared with pure HfC. The HfC-10 vol % SiCN ceramic has a higher density with good mechanical properties. After being oxidized at 1500 °C for 2 h, a dense and homogeneous HfO2-HfSiO4 layer with low oxygen permeability is formed. The ablation resistance of the HfC-10 vol % SiCN ceramic is improved due to the formation of the triple-layer structure oxide with good thermal stability and mechanical scouring resistance. After ablation under an oxyacetylene flame for 60 s, the mass and linear ablation rates of HfC-10 vol % SiCN ceramic are −0.019 mg cm−2 s−1 and -0.156 μm s−1, respectively.

Thermophysical properties for hafnium carbide (HfC) versus temperature from 2000 to 5000 K (experiment)

Hafnium carbide HfC (at%: 42%C; 49%Hf; 7%O) was investigated under rapid heating by a current pulse, in a time of 5–8 μs. The temperature was measured by radiance of a flat surface of a carbide plate (thickness 130 microns) at a given emissivity. This carbide plate was clamped between two K-8 plates, to prevent evaporation of the carbide and to prevent surface discharge at high voltage along the sample. Input specific energy (enthalpy), melting heat, solidus and liquidus temperatures (4000–4200 K), electrical resistance and heat capacity for solid and liquid states were measured up to 5000 K.

Spherical indentations on hafnium carbide- and silicon-carbide-coated carbon–carbon composites after thermal shock test in air

In this study, high-temperature oxidations and indentations on silicon carbide (SiC)- and hafnium carbide (HfC)-coated carbon–carbon (C/C) composites were investigated to prevent the oxidation of C/C composites in air. The SiC and HfC layers were coated to protect the composite from oxidizing in air at a high temperature. High-temperature oxidation tests were performed at 500 °C, 1,000 °C, 1,350 °C, and 1,500 °C for 1 h in air and then cooled to room temperature by thermal shock. This cycle was repeated until the coating layer was damaged. As a result of measuring the weight change according to the thermal cycle and observing the fiber damage in the C/C composites, the oxidation resistance of HfC was evaluated to be superior to that of SiC. The change of the mechanical behavior was investigated using an indentation test with a tungsten carbide ball with a radius of either 3.18 or 7.93 mm before and after the high-temperature oxidation test by thermal shock. The relative elastic modulus was predicted based on the slope of the indentation load–displacement curve during loading or that of the curve during unloading. The relative hardness was also analyzed based on residual displacement after indentation. The hardness and elastic modulus of the HfC and SiC coating were higher than those of C/C composites and the mechanical properties of the HfC-coated C/C composite were relatively good compared with those of the SiC-coated composites prior to the thermal shock test. In particular, in this study, it was found that the mechanical behaviors of HfC-coated C/C composites exposed to temperatures >1,000 °C in air were transferred from elastic to quasi-plastic. The quasi-plasticity of the HfC coating layer was evaluated to be excellent regarding its resistance to mechanical damage as the coating layer was not easily delaminated or damaged even by oxidation.

Growth and Field Emission of Single‐Crystalline Hafnium Carbide Nanowires

Hafnium Carbide Nanowires Single-crystalline hafnium carbide (HfC) nanowires are prepared by chemical vapor deposition (CVD) and their growth morphology as well as electric field-induced emission of electrons is investigated. A high field enhancement factor of 6 × 106 m−1 is obtained from a HfC nanowire due to its unique strucutre, indicating great performance as a potential material for cold field-emission applications. More details can be found in article number 2000007 by Ta-Wei Chiu, Jie Tang, and co-workers.

Studying the properties of carbides in the system ZrC–HfC, TaC–ZrC and TaC–HfC

Considers the chemistry of carbides of zirconium, hafnium and tantalum and their high temperature properties. By hot pressing the obtained dense materials in the systems ZrC– HfC, TaC–ZrC, and TaC–HfC analysed their microstructure, physical and mechanical properties.

Growth and Field Emission of Single‐Crystalline Hafnium Carbide Nanowires

<100>‐oriented hafnium carbide (HfC) nanowires are synthesized on the graphite substrate by chemical vapor deposition. The dependence of temperature, HfCl4content, and flow rate of CH4gas on the morphology of nanowires is investigated herein. The results indicate that the HfC nanowire often kinks to another growth direction at a high HfCl4content or a low temperature due to the destabilization of the catalyst droplet induced by either a faster growth rate or slower thermal activation. Reduced nanowire density and length are observed at the sample synthesized at a lower CH4flow rate. Based on the experimental results, high‐crystallinity nanowires with high aspect ratios and high densities are prepared at a lower HfCl4content, higher temperature, and high CH4flow rate. The field‐emission properties of a single HfC nanowire are also characterized by assembling a single HfC nanowire emitter. An extremely high field enhancement factor of 5.57 × 106 m−1is obtained herein.

Hierarchical core-shell structure of NiCo2O4 nanosheets@HfC nanowires networks for high performance flexible solid-state hybrid supercapacitor

• The hierarchical free-standing core-shell NiCo 2 O 4NSs @HfC NWs /CC was obtained. • HfC NWs network was used as the superhighway for electron transport. • NiCo 2 O 4NSs @HfC NWs /CC exhibits excellent electrochemical properties. • The hybrid supercapacitor shows good performance stability and flexibility. Rationally constructing the electrode with the hierarchical structure, different components, and the flexibility is a promising approach to further improve the electrochemical properties and meet the demand for flexible electronic devices. In this paper, for the first time, hafnium carbide nanowires (HfC NWs ) networks, with the metrics of large specific surface area, excellent conductivity, and chemical/physical stability, are served as nanoscale conductive skeletons on carbon cloth (CC) for supporting the porous NiCo 2 O 4 nanosheets (NSs). Benefiting from the structural advantages, the as-formed free-standing core-shell NiCo 2 O 4NSs @HfC NWs /CC exhibits a high specific capacitance of 2102 F g −1 at a current density of 1 A g −1 , good rate capability (85% capacitance retention at 20 A g −1 ) and outstanding cycling stability (98% capacitance retention after 5000 cycles at 10 A g −1 ). Additionally, assembled with the NiCo 2 O 4NSs @HfC NWs /CC and activated carbon (AC)/CC, the flexible solid-state hybrid supercapacitor displays a high energy density of 53 Wh kg −1 at a power density of 800 W kg −1 and excellent performance stability with good flexibility. More impressively, our work could not only further widen the application of HfC NWs , but also put forward the new thinking of rational construction of high-performance flexible supercapacitors.

Mechanical properties and ablation resistance of HfC nanowire modified carbon/carbon composites

Hafnium carbide nanowires (HfCnws) were in-situ grown in carbon/carbon (C/C) composites, and subsquently the preforms were densified by isothermal chemical vapor infiltration to obtain HfCnws modified carbon/carbon (HfCnws-C/C) composites. Morphology and microstructure of HfCnws were examined, and the effect of HfCnws on the mechanical property and ablation resistance of C/C composites were also investigated. Results show that introducing HfCnws refined the grain size of pyrolytic carbon (PyC). The out-of-plane compression, interlaminar shear and flexual strength of HfCnws-C/C composites increased by 120.80%, 45.60% and 94.65%, respectively compared with pure C/C, and the HfCnws-C/C shows good ablation resistance under oxy-acetylene flame ablation.

Fabrication of ultra-high-temperature nonstoichiometric hafnium carbonitride via combustion synthesis and spark plasma sintering

In this study, nonstoichiometric hafnium carbonitrides (HfCxNy) were fabricated via short-term (5 min) high-energy ball milling of Hf and C powders, followed by combustion of mechanically induced Hf/C composite particles in a nitrogen atmosphere (0.8 MPa). The obtained HfC0.5N0.35 powder exhibited a rock-salt crystal structure with a lattice parameter of 0.4606 nm. The melting point of this synthesized ceramic material was experimentally shown to be higher than that of binary hafnium carbide (HfC). The nonstoichiometric hafnium carbonitride was then consolidated under a constant pressure of 50 MPa at a temperature of 2000 °C and a dwelling time of 10 min, through spark plasma sintering. The obtained bulk ceramic material had a theoretical material density of 98%, Vickers hardness of 21.3 GPa, and fracture toughness of 4.7 MPa m1/2.

Awl‐like HfC nanowires grown on carbon cloth via Fe‐catalyzed in a polymer pyrolysis route

AbstractAwl‐like hafnium carbide (HfC) nanowires were successfully synthesized on carbon cloth in a polymer pyrolysis route via Fe‐catalyzed for the first time. A detailed study of the HfC nanowire morphology was performed. X‐ray diffraction, scanning electron microscopy and transmission electron microscopy were used to investigate the morphology and microstructure of the obtained awl‐like HfC nanowires and the growth mechanism was also discussed in detail. Results show that HfC nanowires wrapped by amorphous HfO2 layer with ~10 nm in thickness display awl‐like structure and the nanowire's surface exhibit prismatic geometry. The diameter of the nanowire decreases from the bottom (~500 nm) to the top (~100 nm). The growth mechanism of the awl‐like HfC nanowires was mainly affected by the combination of bottom‐type vapor‐liquid‐solid (VLS) and solid‐liquid‐solid.

Densification and oxidation behavior of spark plasma sintered Hafnium Diboride-Hafnium Carbide composite

This study reports on the sintering and oxidation behaviors of Hafnium Diboride (HfB2) and Hafnium Carbide (HfC) based ultra-high temperature ceramic (UHTC) composites. Pure Hafnium Diboride, Hafnium Carbide, and HfB2-50 vol% HfC composite were consolidated using spark plasma sintering (SPS) without the use of sintering aids. HfB2-HfC composite displayed a high sintered density of 98% as compared to 87% density of pure HfB2. The increased density of the HfB2-HfC composite is attributed to the bimodal powder distribution, which allowed for the smaller HfC particle to occupy the voids between the larger HfB2 particles resulting in improved packing efficiency. Moreover, the higher planar surface energy, for the (111) and the (200) planes for HfC, contributed to the smaller HfC particle being a significant driving force in the sintering process. Oxidation studies of each material were carried out by exposing them to a high-speed plasma jet in a temperature exceeding 2700 °C. The results of these studies show an improved oxidation resistance of HfB2-HfC composite by 54% and 70% over pure HfC and HfB2, respectively. The enhanced oxidation behavior is attributed to B2O3 filling in the porosity between the HfO2 scale and protecting the underlying material. This study provides a new alternative to improve the sinterability of UHTC diborides by introducing another UHTC (i.e., HfC) to form a fully dense composite without sintering aid and superior oxidation resistance.

Mechanochemical Synthesis of Hafnium Carbide using Amorphous Carbon from Plant Materials

The paper presents the results of studying mechanochemical synthesis of hafnium carbide in the process of vibration treatment of the mixture of powders of hafnium and amorphous carbon obtained by pyrolytic processing of plant materials. Phase compositions, morphology, as well as fractional and chemical compositions of the synthesized products were studied. It has been found that the use of carbon obtained from plant materials is beneficial for the mechanochemical synthesis of highly-dispersed hafnium carbide with low sulfur content.

Reactive Hot Pressing of HfB2–SiC–Ta4HfC5 Ultra-High Temperature Ceramics

Highly disperse and reactive (HfB2–SiC)@(Ta2O5–HfO2–C) composite powders were manufactured by sol–gel technology where Ta2O5–HfO2–C amorphous components were nanostructured and distributed in each other as uniformly as possible. The reactive sintering of the prepared composite powders at a relatively low temperature (1800°С) with an exposure time of 30 min and the pressure 30 MPa yielded (HfB2–30 vol % SiC)–xTa4HfC5 ultra-high temperature ceramics (UHTCs), where x = 5, 10, and 15 vol %, with a relative density of 75–78%. X-ray powder diffraction proved the complete conversion of tantalum and hafnium oxides to complex carbide Ta4HfC5. The average grain size as determined by scanning electron microscopy did not exceed 2–3 µm in HfB2 and 30–60 nm for the Ta4HfC5 phase. Thermal analysis in flowing air showed that, in the temperature range 20–1400°С, the increasing percentage of tantalum–hafnium carbide (the least oxidation resistant phase) leads to a systematic increase in oxidation-induced weight gain; however, a tendency to saturation is observed. The microstructural specifics of the oxidized surface are noticed depending on the composition of HfB2–SiC–Ta4HfC5 ceramics.

A new method to quantitatively control oxygen fugacity in externally heated pressure vessel experiments

Abstract. Oxygen fugacity (fO2) is a fundamental variable affecting phase equilibrium in magmas, and in externally heated pressure vessel experiments it is typically controlled by using redox buffer assemblages. However, these do not allow fine enough resolution; for example, most arc magmas fall between the fO2 imposed by the neighboring Ni–NiO and Re–ReO2 buffers and so does the transition of S2− to S6+ in magmas. Here we propose a new method to quantitatively impose fO2 in hydrous high-P–T experiments in molybdenum hafnium carbide (MHC) pressure vessels by admixing small amounts of hydrogen into the Ar pressure medium. The thermodynamic calculation procedure used to determine the initial amount of hydrogen to be loaded to constrain desired fO2 values was verified by CoPd alloy redox sensor experiments to be accurate within ±0.3 log units for the pressure (P) – temperature (T) range of 940–2060 bar and 800–1100 ∘C. As hydrogen can be slowly lost from the pressure medium due to diffusion through the vessel walls at high T, we also determined the hydrogen permeability of the MHC alloy as a function of T. The such-obtained hydrogen permeability equation for the MHC alloy can be used to determine the rate of fO2 increase for any MHC pressure vessel configuration. As the rate of fO2 increase is slow (e.g., 0.36 log units per day in our setup at T= 1000 ∘C), we propose that H2 addition to the Ar pressure medium is an effective way to accurately impose fO2 in many types of experiments conducted in MHC vessels allowing experimentation up to T= 1200 ∘C and P= 300 MPa.

2.5 D carbon/carbon composites modified by in situ grown hafnium carbide nanowires for enhanced electromagnetic shielding properties and oxidation resistance

Hafnium carbide nanowires (HfCnw) were in-situ grown in 2.5 D needle-punched carbon felts by a catalyst-assisted chemical vapor deposition method. Subsequently, the carbon felts were densified to obtain HfCnw reinforced carbon/carbon (HfCnw-C/C) composites via chemical vapor infiltration. Effects of HfCnw on electrical conductivity, electromagnetic shielding properties and oxidation resistance of C/C composites were investigated for the first time. The results show that the SET of C/C composites with 1.75 wt % HfCnw exceeds 40 dB in the whole frequency range of 8.2–12.4 GHz, implying more than 99.99% of the incident beam was shielded. Moreover, it is found that the relative magnitude relationship of the total shielding effectiveness of composites (SET(S-2h) > SET(S-3h) > SET(S-0h)) is similar with the conductance (σS-2h >σS-3h >σS-0h), which indicates the conductance can effectively predict the relative magnitude of the SET. In addition, the analysis based on oxidation curves demonstrates that HfCnw significantly enhance the oxidation resistance of the composites even with a small amount of HfCnw. The high electromagnetic interference (EMI) shielding effectiveness coupled with relatively low density and good oxidation resistance of HfCnw-C/C composites make itself exhibit remarkable potential as lightweight and high-performance structural-functional integrated materials for future EMI shielding application.

Densification, strengthening and toughening in hafnium carbide with the addition of silicon carbonitride

AbstractHafnium 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.

A controllable and efficient method for the fabrication of a single HfC nanowire field-emission point electron source aided by low keV FIB milling.

A single hafnium carbide (HfC) nanowire field-induced electron emitter with a sharp tip apex is fabricated by Pt deposition and focused ion beam (FIB) milling. The structure of the electron emitter is characterized by scanning transmission electron microscopy (STEM) and atom probe tomography (APT). The HfC nanowire is single-crystalline with a thin oxide layer on its tip surface. The field emission properties are determined by using both in situ transmission electron microscopy (TEM) and a field-emission probe in a high-vacuum chamber. A high current of 173 nA was obtained at a low extraction voltage of 631 V with an emission gap of 5 mm. The emission current is stable at 60 nA for 100 min with a fluctuation of 0.7%. The deduced work function was 3.1 eV. It is suggested that the implanted Ga ions and the oxide layer induce more downward dipoles that are beneficial for lowering the work function and creating a stable surface. When the low keV FIB processing is applied, it takes within 30 minutes to finish a HfC nanowire emitter, establishing an efficient procedure for the preparation of nanowire emitters. These results provide a controllable and fast production method for the fabrication of single nanowire field-emission point electron sources.

Investigation of evaporation of nonstoichiometric hafnium carbide up to 4500 K using laser heating

Investigation of evaporation of nonstoichiometric hafnium carbide up to 4500 K using laser heating

Effect of residual excess carbon on the densification of ultra-fine HfC powder

The residual carbon content of ultra-fine hafnium carbide (HfC) powder was controlled by the optimization of the synthesis process, and the effect of residual carbon on the densification of HfC powder was analyzed. The amount of residual carbon in the HfC powder could be reduced by the de-agglomeration of HfO2 powder before the carbo-thermal reduction (CTR) process. The average particle size of HfO2 powder decreased from 230 to 130 nm after the de-agglomeration treatment. Ultra-fine (d50: 110 nm) and highly pure (metal basis purity: >99.9 % except for Zr) HfC powder was obtained after the CTR at 1600 °C for 1 h using the C/Hf mixing ratio of 3.3. In contrast, the C/Hf ratio increased to 3.6 without the de-agglomeration treatment, indicating that a large amount of excess carbon was required for the complete reduction of the agglomerated HfO2 particles. HfC ceramics with high relative density (>98 %) were obtained after spark plasma sintering at 2000 °C under 80 MPa pressure when using the HfC powder with low excess carbon content. In contrast, the densification did not complete at a higher temperature (2300 °C) and pressure (100 MPa) when the HfC powder contained a large amount of residual carbon. The results clearly indicated that residual carbon suppressed the densification of HfC powder in case the carbide powder had low oxygen content, and the residual carbon content could be controlled by the optimization of the synthesis process. The average grain size and Vickers hardness of the sintered specimen were 6.7(±0.7) μm and 19.6 GPa, respectively.

Microstructure and Mechanical Properties of Vacuum Plasma Sprayed HfC, TiC, and HfC/TiC Ultra-High-Temperature Ceramic Coatings.

To improve the oxidation resistance of carbon composites at high temperatures, hafnium carbide (HfC) and titanium carbide (TiC) ultra-high-temperature ceramic coatings were deposited using vacuum plasma spraying. Single-layer HfC and TiC coatings and multilayer HfC/TiC coatings were fabricated and compared. The microstructure and composition of the fabricated coatings were analyzed using field-emission scanning electron microscopy and energy dispersive X-ray spectroscopy. The coating thicknesses of the HfC and TiC single-layer coatings were 165 µm and 140 µm, respectively, while the thicknesses of the HfC and TiC layers in the HfC/TiC multi-layer coating were 40 µm and 50 µm, respectively. No oxides were observed in any of the coating layers. The porosity was analyzed from cross-sectional images of the coating layers obtained using optical microscopy. Five random areas for each coating layer specimen were analyzed, and average porosity values of approximately 16.8% for the HfC coating and 22.5% for the TiC coating were determined. Furthermore, the mechanical properties of the coating layers were investigated by measuring the hardness of the cross section and surface roughness. The hardness values of the HfC and TiC coatings were 1650.7 HV and 753.6 HV, respectively. The hardness values of the HfC and TiC layers in the multilayer sample were 1563.5 HV and 1059.2 HV, respectively. The roughness values were 5.71 µm for the HfC coating, 4.30 µm for the TiC coating, and 3.32 µm for the HfC/TiC coating.