Synthesis and characterization of (Zr1/3Nb1/3Ti1/3)C metal carbide solid-solution ceramic

(Zr_1/3Nb_1/3Ti_1/3)C metal carbide solid-solution ceramic has been successfully fabricated by hot pressing sintering at 2473 K using ZrC, NbC, and TiC powders as raw materials. The results show that the as-prepared solid-solution ceramic possesses a single rock-salt crystal structure of metal carbides and simultaneously exhibits high compositional uniformity from nanoscale to microscale. By taking advantage of these unique features, it shows relatively high hardness of 38.8 ± 4.4 Gpa and elastic modulus of 481.8 ± 31.0 Gpa and relatively low thermal conductivity of 17.1 ± 0.3 W/(m·K) and thermal diffusivity of 6.1 ± 0.1 mm²/s, which may attribute primarily to the presence of solid solution effects.     

One-step synthesis of coral-like high-entropy metal carbide powders

The synthesis of high-entropy metal carbide powders is critical for implementing their extensive applications. However, the one-step synthesis of high-entropy metal carbide powders is rarely studied. Herein, the synthesis possibility of high-entropy metal carbide powders, namely (Zr_0.25Ta_0.25Nb_0.25Ti_0.25)C (ZTNTC), via one-step carbothermal reduction was first investigated theoretically by analyzing chemical thermodynamics and lattice size difference based on the first-principle calculations, and then the ZTNTC powders with particle size of 0.5-2 μm were successfully synthesized experimentally. The as-synthesized powders not only had a single rock-salt crystal structure of metal carbides, but also possessed high-compositional uniformity from nanoscale to microscale. More interestingly, they exhibited the distinguished coral-like morphology with the hexagonal step surface, whose growth was governed by a classical screw dislocation growth mechanism.     

A novel approach to the rapid synthesis of high-entropy carbide nanoparticles

A novel strategy for the rapid synthesis of high-entropy carbide particles is proposed that involves the transformation of multicomponent intermetallic intermediates to multicomponent carbides (high-entropy carbide precursors). (Ti_0.25V_0.25Nb_0.25Ta_0.25)C nanoparticles with a uniform solute distribution were successfully synthesized in an Al matrix by heating Al-Ti-V-Nb-Ta-C powder mixtures at 1500°C for 10 minutes. The multicomponent aluminide intermediates led to the rapid formation of multicomponent carbides during heating to 1100°C, which transformed into a high-entropy solid solution during heating to 1500°C. We developed a new rapid approach for the synthesis of high-entropy ceramic particles.     

Synthesis of high-purity high-entropy metal diboride powders by boro/carbothermal reduction

Synthesis of high-purity high-entropy metal diboride powders is critical to implementing their extensive applications. However, the related studies are rarely reported. Herein we first theoretically studied the synthesis possibility of high-purity high-entropy diboride powders, namely (Hf_0.25Ta_0.25Nb_0.25Ti_0.25)B₂ (HTNTB), via boro/carbothermal reduction by analyzing the thermodynamics of the possible chemical reactions and then successfully synthesized the high-purity and superfine HTNTB powders via boro/carbothermal reduction for the first time. The as-prepared powders exhibited low-oxygen impurity content of 0.49 wt% and small average particle size of 260 nm. Meanwhile, they possessed good single-crystal hexagonal structure of metal diborides and high-compositional uniformity from nanoscale to microscale. This work will open up a new research field on the synthesis of high-purity high-entropy metal diboride powders.     

Synthesis and characterization of the ternary metal diboride solid-solution nanopowders

The synthesis of the multi-component transition-metal diboride (MeB₂) solid-solution powders has been recently attracting considerable attentions. However, the synthesis of the ternary or more component MeB₂ solid-solution powders has rarely been reported until now. To fabricate the ternary MeB₂ solid-solution powders, herein we utilized two kinds of the ternary MeB₂ solid solutions as prototypes, namely (Hf_1/3Zr_1/3Ti_1/3)B₂ (HZTB) and (Ta_1/3Nb_1/3Ti_1/3)B₂ (TNTB). The formation possibility of HZTB and TNTB was first analyzed by the first-principles calculations and then we attempted of fabricated them by a simple molten salt synthesis technique. The first-principles calculations results showed that the mixing Gibbs free energy at room temperature and lattice size difference at 0 K of HZTB and TNTB were (1.666 kJ/mol and 3.146%) and (−3.030 kJ/mol and 1.254%), respectively. This suggested that TNTB solid solution was more prone to being fabricated than HZTB solid solution. The experimental results showed the high purity TNTB solid-solution nanopowders were successfully synthesized by the molten salt synthesis technique at 1373 K with 30% excessive B as precursors while the HZTB solid solution was not able to be synthesized by the molten salt synthesis technique. The as-synthesized TNTB solid-solution nanopowders exhibited the distinguished nanorod morphology with the diameters of 20-40 nm and lengths of 100-200 nm. Meanwhile, they possessed the good single-crystal hexagonal structure and high compositional uniformity from nanoscale to microscale. In addition, their formation mechanism associated to the possible chemical reactions was well interpreted by the thermodynamics analysis.     

Fabrication and characterization of polymer-derived high-entropy carbide ceramic powders

Polymer-derived ceramic (PDC) route has been widely used to fabricate various ceramics or ceramic-matrix composites in recent years. However, the synthesis of high-entropy ceramics via PDC route has rarely been reported until now. Herein, we successfully synthesized a class of high-entropy carbides, namely (Hf_0.25Nb_0.25Zr_0.25Ti_0.25)C (HEC-1), via PDC route. The polymer-derived HEC-1 ceramics consisted of numerous superfine particles with the average particle size ~800 nm. Meanwhile, they possessed a rock-salt structure of metal carbides and high-compositional uniformity from nanoscale to microscale. In addition, the as-obtained HEC-1 ceramics had a low oxygen impurity content of 0.51% and a low free carbon impurity content of 2.56%. This work will open up a new research field on the fabrication of high-entropy ceramics or high-entropy ceramic-matrix composites via PDC route.     

Synthesis of fine zirconium carbide powder by carbothermal reaction of carbon‐coated zirconia particles

In this study, a novel method for the synthesis of fine ZrC powder was presented. It consists of chemical vapor deposition (CVD) of C from CH₄ on ZrO₂ particles followed by carbothermal reaction. Firstly, optimal CVD conditions (1300 K and 30 minutes) yielding the stoichiometric amount of C deposit (23 wt%) were determined. Carbothermal reaction behavior of the carbon‐coated oxide particles was then investigated in Ar flow at 1700‐1800 K for 0‐120 minutes. Mass measurements, XRD and SEM techniques were used to characterize the products at various stages of the process. Lattice constants and mass losses of the samples increased to the levels of ZrC with increasing temperature and time. Almost pure ZrC powder (oxygen content: 0.59 wt%) with a mean particle size of ~170 nm was synthesized at 1800 K within 120 minutes. The present study demonstrates that ZrC powder can be synthesized at lower temperatures and shorter reaction times using C‐coated ZrO₂ powders compared with the conventional method which uses a mixture of ZrO₂ and solid C particles.     

Mechanical properties and grain orientation evolution of zirconium diboride-zirconium carbide ceramics

The effect of ZrC on the mechanical response of ZrB₂ ceramics has been evaluated from room temperature to 2000 °C. Zirconium diboride ceramics containing 10 vol% ZrC had higher strengths at all temperatures compared to previous reports for nominally pure ZrB₂. The addition of ZrC also increased fracture toughness from $～3\text{.5}\text{MPa}\sqrt{\text{m}}$ for nominally pure ZrB₂ to $～4\text{.3}\text{MPa}\sqrt{\text{m}}$ due to residual thermal stresses. The toughness was comparable with ZrB₂ up to 1600 °C, but increased to $4\text{.6}\text{MPa}\sqrt{\text{m}}$ at 1800 °C and 2000 °C. The increased toughness above 1600 °C was attributed to plasticity in the ZrC at elevated temperatures. Electron back-scattered diffraction analysis showed strong orientation of the ZrC grains along the [001] direction in the tensile region of specimens tested at 2000 °C, a phenomenon that has not been observed previously for fast fracture (crosshead displacement rate = 4.0 mm min^?1) in four point bending. It is believed that microstructural changes and plasticity at elevated temperature were the mechanisms behind the ultrafast reorientation of ZrC.     

Reactive hot pressing route for dense ZrB2-SiC and ZrB2-SiC-CNT ultra-high temperature ceramics

The in-situ exothermic reactions between ZrC_0.8, B₄C and Si have assisted densification and allowed to obtain fully dense ZrB₂-31 wt.%SiC ultra-high temperature ceramics within 6 min at 1750 °C. The use of zirconium carbide instead of metallic zirconium in the green body obviated the possibility of in-situ SHS process and allowed to apply the pressure at low temperatures. The latter provided a first densification stage just above 1050 °C. A slight carbon excess was created in the green body to preserve the carbon nanotubes. The developed reactive hot pressing route (1830 °C, 3 min, 30 MPa) has been successfully used to obtain ZrB₂-SiC ceramics containing 8 vol.% of multi-wall carbon nanotubes (MW-CNT). The carbon nanotubes survived the thermal cycle and could be clearly observed in the sintered ceramics. The CNT addition improved the fracture toughness of the composite from 4.3 MPa m^1/2 for ZrB₂-31 wt.%SiC to 6.8 MPa m^1/2 for ZrB₂-29 wt.%SiC-CNT.     

Ultra-high temperature ceramics melting temperature prediction via machine learning

Melting temperature has great influence on the high temperature properties and working temperature limits of ultra-high temperature ceramics (UHTCs) In order to bypass the challenge in the measurement of ultra-high melting points, this paper proposed a novel method to predict UHTCs melting temperature via machine learning. A dataset including more than ten thousand melting temperature data has been established, which covers 8 elements and most of the known non-oxide UHTCs. We built up an element to ceramic system framework by back propagation artificial neural network (BPANN) with the accuracy approaching to 90% and the correlation coefficients approaching to 0.95. Our work provides a probability to get the high accuracy melting temperature of UHTCs, and a more convenient way to develop novel materials with higher working temperature. The given case of melting temperature prediction of Hf-C-N ceramics proves the generality of the artificial neural network (ANN). An inter-validation of melting temperature prediction using our network with materials thermodynamics and density functional theory (DFT) has been demonstrated, indicating that our network is of powerful prediction ability.     

Pursuing enhanced oxidation resistance of ZrB2 ceramics by SiC and WC co-doping

The oxidative degradation of ZrB₂ ceramics is the main challenge for its extensive application under high temperature condition. Here, we report an effective method for co-doping suitable compounds into ZrB₂ in order to significantly improve its anti-oxidation performance. The incorporation of SiC and WC into ZrB₂ matrix is achieved using spark plasma sintering (SPS) at 1800?°C. The oxidation behavior of ZrB₂-based ceramics is investigated in the temperature range of 1000?°C–1600?°C. The oxidation resistance of single SiC-doped ZrB₂ ceramics is improved due to the formation of silica layer on the surface of the ceramics. As for the WC-doped ZrB₂, a dense ZrO₂ layer is formed which enhances the oxidation resistance. Notably, the SiC and WC co-doped ZrB₂ ceramics with relative density of almost 100% exhibit the lowest oxidation weight gain in the process of oxidation treatment. Consequently, the co-doped ZrB₂ ceramics have the highest oxidation resistance among all the samples.     

Synthesis of ternary ceramics (Cr2AlC) by using biochars

In this paper, we report the synthesis of Cr₂AlC by using biochars derived from lignin, Distiller's dried grains with solubles (DDGS), and hemp fibers. Initially, the powders were pyrolyzed at 1350°C for 4 h in Ar atmosphere to form biochars. The ball-milled and sieved biochar powders were then mixed with Cr and Al powders in different stoichiometric ratios according to the C-content of the biochars. The mixed powders were reacted at 1350°C for 4 h in Ar atmosphere. Detailed scanning electron microscopy and X-ray diffraction analysis showed the powders derived from hemp and DDGS biochars were > 90% pure as compared to powders derived from lignin biochar which was 76% pure. It is expected that ternary ceramics derived from biochars can be an addition avenue for carbon-storage.     

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.     

A facile route for the synthesis of high-entropy transition carbides/borides at low temperatures

As the main candidates in the field of ultra-high temperature ceramics, high entropy carbides/borides (HECs/HEBs) have good oxidation resistance properties, high hardness, as well as excellent thermal and electrical conductivities, which are the focused points of research nowadays. In the current study, (Hf,Ta,Zr,Nb,Mo,Ti)C powders were successfully synthesized by a three-step process, including the mixing process of raw oxides and carbon black with spaying Fe(NO₃)₃ solution, carbothermal reduction and subsequent calcium posttreatment. For the preparation of (Hf,Ta,Zr,Nb,Mo,Ti)B₂ powders, during the calcium posttreatment process, equal stoichiometric ratio of B₄C was added for the purpose of boriding reaction. The relevant X-ray diffraction and SEM characterizations indicate the successful preparations of face-centered cubic HECs and hexagonal HEBs. However, slight Mo local segregation was found in the prepared (Hf,Ta,Zr,Nb,Mo,Ti)B₂ powders. The iron generated from Fe(NO₃)₃ promotes the solid solution process between monocarbides during the carbothermal reduction process via the dissolution-diffusion-precipitation mechanism. In the calcium posttreatment process, the liquid calcium ensures the boriding reaction take place at a low temperature. In addition, the residual carbon could be combined with calcium to generate CaC₂ which is easy to be removed by acid leaching, and meanwhile, the added Fe could also be finally eliminated to produce pure HEC/HEB powders. The current method does not require the long-time high energy ball milling of raw materials, but only simple and mild mixing is enough. Therefore, such a facile route has a great potential application prospect for industrially preparing high entropy phase powders in a large scale.     

Solute distributions in tantalum-containing zirconium diboride ceramics

Solute segregation was examined in zirconium diboride and zirconium-tantalum diboride solid solution ceramics that were produced by reactive hot pressing. Microstructural analysis demonstrated that the ZrB₂ and (Zr,Ta)B₂ ceramics reached nearly full relative density and were nominally phase-pure. X-ray diffraction was consistent with full incorporation of Ta into solid solution within the ZrB₂ structure, and energy-dispersive spectroscopy demonstrated that tantalum was well distributed throughout the bulk of the Ta-doped specimens. The weak characteristic X-rays for B led to inaccurate results for total atom concentrations in boride ceramics by energy-dispersive spectroscopy. Atom probe tomography was used to analyze the amount and spatial distribution of Ta species. No obvious Ta segregation was observed in grains or grain boundaries. However, nitrogen strongly segregated to a grain boundary. This study demonstrated that atom probe tomography is an accurate method for characterizing the amount and spatial distribution of metallic and nonmetallic species in ZrB₂ ceramics.     

Synthesis and pyrolysis of non-oxide precursors for ZrC/SiC and HfC/SiC composite ceramics

Multiphase ceramics like ZrC/SiC are promising candidates as ultra-high temperature ceramics for applications in extreme environments. In this work, non-oxide precursors for ZrC/SiC and HfC/SiC composite ceramics were synthesized by a one-pot reaction of three components – metal source, silicon source, and activating reagent. Molecular structures of the precursors were identified by ¹H NMR and FTIR. Transformation process of the precursors to the ZrC/SiC ceramics was investigated via XRD and SEM. After heat-treatment at 1600 °C under argon, the obtained ZrC/SiC and HfC/SiC ceramics features a particle size of 100–200 nm and high metal content without excess carbon. The elemental composition of pyrolyzed ceramics can be tuned by varying the ratio of the reagents in the synthesis of precursors. This strategy also inspires a facile fabrication of composite ceramics with other elemental compositions.     

Processing,microstructure and performance of Al2O3/Er3Al5O12/ZrO2 ternary eutectic ceramics prepared by laser floating zone melting with ultra-high temperature gradient

Directionally solidified Al₂O₃/Er₃Al₅O₁₂(EAG)/ZrO₂ ternary eutectic/off-eutectic composite ceramics with high density, homogeneous microstructures, well-oriented growth have been prepared by laser floating zone melting at different solidification rates from 4 to 400 µm/s. Uniform and stable melting zone is obtained by optimizing temperature field distribution to keep continuous and stable eutectic growth and prevent from cracks and defects. The as-solidified composite ceramic exhibits complexly irregular eutectic structure, in which the eutectic spacing is rapidly refined but dotted ZrO₂ number inside Al₂O₃ phase is decreased as increasing the solidification rate. The formation mechanism of ZrO₂ distributed inside Al₂O₃ matrix is revealed by examining the depression of solid/liquid interface. Furthermore, after heat exposure 1500 °C for 200 h, the eutectic microstructure only shows tiny coarsening, which indicates it has excellent microstructural stability. As increasing the ZrO₂ content, the fracture toughness can be improved up to 3.5 MPa m^1/2 at 20.6 mol% ZrO₂.     

Low-temperature molten salt synthesis of high-entropy carbide nanopowders

Synthesis of the powders is critical for achieving the extensive applications of high-entropy carbides (HECs). Previously reported studies focus mainly on the high-temperature (>2000 K) synthesis of HEC micro/submicropowder, while the low-temperature synthesis of HEC nanopowders is rarely studied. Herein we reported the low-temperature synthesis of HEC nanopowders, namely (Ta_0.25Nb_0.25Ti_0.25V_0.25)C (HEC-1), via molten salt synthesis for the first time. The synthesis possibility of HEC-1 nanopowders was first theoretically demonstrated by analyzing lattice size difference and chemical reaction thermodynamics based on the first-principle calculations, and then the angular HEC-1 nanopowders were successfully synthesized via molten salt synthesis at 1573 K. The as-synthesized nanopowders possessed the single-crystal rock-salt structure of metal carbides and high compositional uniformity from nanoscale to microscale. In addition, their formation mechanism was well interpreted by a classical molten salt-assisted growth.     

Ultra-high temperature ablation behavior of MoAlB ceramics under an oxyacetylene flame

The ultra-high temperature ablation of a polycrystalline, fully dense, predominantly single phase MoAlB ceramic discs under an oxyacetylene flame is examined. The linear ablation rate decreases from 1.3 μm/s during the first 30 s to - 0.7 μm/s after 60 s when the surface temperature reached about 2050 °C (with a flame temperature around 3000 °C). Up to 60 s, the MoAlB is ablation resistant due to the formation of a protective and viscous surface Al₂O₃ layer. As the ablation time is prolonged, the protective Al₂O₃ scale becomes porous and is almost fully destroyed at the central ablation region after 120 s. This accelerates the formation of large amounts of volatile species (mainly B and Mo oxides), resulting in a reduction in the ablation resistance.     

Synthesis and characterization of (HfMoTiWZr)C high entropy carbide ceramics

(HfMoTiWZr)C high entropy carbides (HEC) were prepared from the commercial carbide powders of IVB (Hf, Ti, Zr) and VIB (Mo, W) group metals of the periodic table via high energy ball milling (HEBM) and spark plasma sintering (SPS). Metal carbide powders (HfC, TiC, ZrC, Mo₂C and WC) were HEBM’d for 3 h in a vibratory ball mill, and then SPS’d at different temperatures (1800, 1900, 2000 and 2100 °C). The HEBM’d powders and SPS’d ceramics were characterized in composition, density and microstructure using an X-ray diffractometer (XRD), a scanning electron microscope/energy dispersive spectrometer (SEM/EDS), a particle size analyzer and a pycnometer. Also, microhardness and sliding wear tests were conducted on the SPS’d ceramics. Based on the performed characterization, a single-phase FCC structure was observed at all sintering temperatures indicating a high entropy carbide ceramic, and they all have high hardness and wear resistance values.