Ultra-high Temperature Ceramics Research Articles

New insight into the structural, mechanical and thermodynamic properties of Mo2C MXene as the ultrahigh temperature ceramics

Molybdenum-based carbide (Mo2C) is a promising ultrahigh temperature ceramics due to the high melting point, high hardness, better corrosion resistance and excellent thermal stability. However, the crystal structures of some reported Mo2C remain controversy until now. In particular, how to adjust the balance between the strength and ductility is still a big challenge for the applications of these ultrahigh temperature materials. Based of the above problems, we apply the first-principles method to study the structural stability, mechanical and thermodynamic properties of six Mo2C carbides. The calculated results show that two novel Mo2C: the orthorhombic (Pna21) and orthorhombic (Pnma) phases are firstly predicted. However, the published hexagonal (P63/mmc) and hexagonal (P-6m2) phases are dynamical instability. Furthermore, it is found that the calculated hardness and elastic modulus of the predicted orthorhombic (Pna21) Mo2C are close to the orthorhombic (Pbcn) and hexagonal (P-3m1) Mo2C. Importantly, four Mo2C exhibit better ductility. Essentially, the Mo–C bond in layered structure plays an important role in strength and ductility. In addition, the calculated melting point follows the order of the orthorhombic (Pnma) > orthorhombic (Pna21) > hexagonal (P-3m1) > orthorhombic (Pbcn). The excellent thermodynamic properties of all Mo2C are determined by the lattice vibration of C and Mo–C bond. Therefore, we believe that four Mo2C not only adjust the balance between the strength and ductility, but also show good thermodynamic properties, which are potentially used in various ultrahigh temperature industries.

Pressureless synthesis and consolidation of the entropy-stabilized (Hf0.25Zr0.25Ti0.25V0.25)B2-B4C composite by ultra-fast high-temperature sintering (UHS)

Entropy-stabilized Ultra High-Temperature Ceramics (UHTC) offer a groundbreaking solution to the challenges of extreme environments, showcasing enhanced mechanical properties, thermal stability, and resistance to oxidation at high temperatures. The consolidation of UHTC by ultra-fast high-temperature sintering (UHS) significantly reduces processing times and temperature and can produce dense high-performance ceramics with superior mechanical properties. This study reports the pressureless synthesis and consolidation of the entropy-stabilized (Hf0.25Zr0.25Ti0.25V0.25)B2-B4C composite through UHS within 1minute, starting from transition metal diboride powders. B4C acts as an effective sintering aid, promoting the densification of the system and the formation of a nearly single-phase hexagonal diboride with a diboride-eutectic phase. Furthermore, a secondary minor hexagonal phase rich in V and Zr is formed close to the eutectic regions. Sintering currents of 40A were necessary to reach densities higher than 90% under pressureless conditions, achieving nano hardness higher than 27.3GPa, comparable with high-entropy diborides produced by Spark Plasma Sintering. The study highlights the entropy-stabilized phase formation, diffusion, densification, and grain mechanisms involved during UHS. The work contributes to understanding entropy-stabilized ceramics produced by UHS as a faster and less energy-consuming process than more conventional sintering methods.

Temperature‐dependent heat transfer deterioration in ordered graphene/ceramic composites

AbstractBoosting heat transfer of ultrahigh temperature ceramics (UHTCs) with dimensional‐crossover graphene has become an increasing challenge for improving the thermal protective performance of new‐generation spacecraft. Yet its characteristics of anisotropic and high interfacial thermal resistance inevitably cause heat transfer deterioration at high temperatures. Here we develop a few ZrB2/graphene/ZrB2 interface‐dominated diffuse models that are driven by bonding configurations and thermal converger. It has been found that forming stronger bonding configurations by Zr‐C interfacial bonding and designing a preferred thermal converger by ordered graphene are both unarguable ways of suppressing high‐temperature heat transfer deterioration. Each Zr‐C interfacial bonding serves as an independent sluice gate to accelerate heat flow. More high‐frequency phonons are excited at high temperature, which augments the interfacial thermal conductance from 38.5 to 335.4 MW·m−2·K−1 at 700 K. Impressively, a concept of thermal converger inspires us to adopt ordered graphene for changing the diverging behavior in isotropic ZrB2 composites. Its convergent process combines with anisotropic characteristics to transform the isotropic heat flow into the highly ordered graphene. Their thermal conductance can be regulated from minimum (2.5) to maximum (80.8 W·m−1·K−1) by rotating the ordered graphene from 90° to 7.5°. This work paves an optimal way for manipulating the heat transfer of UHTCs.

From molecular precursors to ultra-high temperature ceramics: A novel synthesis of hafnium carbonitride nanoceramics

Hafnium carbonitride (HfCxN1–x) ceramics have drawn considerable interest due to their exceptional mechanical and thermophysical properties. Herein, we report a novel single-source precursor with Hf–N bonds as the main chain and fabricate HfCxN1–x ceramics after pyrolysis of the precursor. The synthesis, ceramic conversion, and microstructural evolution of the single-source precursor as well as the derived HfCxN1–x ceramics treated under various atmospheres were investigated. The results indicate that in an argon atmosphere, the nitrogen content within HfCxN1–x decreases with rising temperature. While under a nitrogen atmosphere, the high concentration of N2 facilitates the rapid conversion of HfO2 to Hf7O8N4, which subsequently promotes the transformation of the HfCxN1–x solid solution ceramics. During this process, there is also an inhibitory effect of N2 on the tendency of HfN into HfC. Moreover, the desired chemical composition of HfCxN1–x can be regulated by adjusting the N2 concentration in the heat treatment atmosphere. The present work proposes a novel strategy for the single-source precursor-derived carbonitride ceramics and provides a deep understanding of the preparation and property modulation of HfCxN1–x ceramics.

Utilization of handheld LIBS and XRF analyzers for impurity detection in 3D-printed ultra-high-temperature ceramics

In this study, we implement handheld laser-induced breakdown spectroscopy (LIBS) and X-ray fluorescence (XRF) analyzers to detect elemental impurities in additively manufactured ultra-high-temperature ceramics (UHTCs). Spectral data were collected from digital light processing (DLP) 3D-printed alumina (Al2O3) samples at various processing stages. These stages included high-temperature debinding and sintering phases used to bake out organic impurities and improve grain cohesion of the ceramic. Spectral analysis revealed the presence of organic impurities such as H and C together, with inorganic impurities such as Na, Si, Ca, and Fe. A reduction in elemental impurities in the spectra was observed as the ceramic samples were processed, validating the effectiveness of handheld analyzers for in situ rapid impurity detection and quality control in the manufacturing of 3D-printed UHTCs.

An Ultra-Broadband Electromagnetic Wave Absorbing ZrB2-Based Ceramic Composite Inspired by Barbule of Peacock.

Broadband electromagnetic wave (EMW) absorbing ceramic materials are highly required for the thermal parts of aerocraft. As members of ultrahigh temperature ceramics, ZrB2-based ceramics have great potential for applications in more extreme environments relative to the currently used silicon-based and oxide-based ceramics. However, ZrB2 is not among the traditional EMW absorbing material candidates due to its high conductivity, which induces the strong reflection of EMW due to the impedance mismatch with free space. Herein, ZrB2-based ceramic with a bionic microstructure inspired by peacock barbules is proposed. Boron nitride nanotubes acting as polarization centers inside the ZrB2-based material cause massive EMW dissipation. The ceramic shows an ultra-broadband absorption of 9.6GHz (<-10dB from 8.4 to 18GHz), almost covering the entire X and Ku bands, superior to the reported ceramics. The polarization centers successfully turn the ZrB2-based ceramic from EMW reflecting material to an excellent EMW absorbing material by the bionic barbule interspersed microstructure. The simulated metamaterial of the ceramic achieves an ultra-broad absorption (lower than -15dB) in the range of 2-40GHz. This work provides valuable insights for the development of broadband absorption material for high-temperature environments.

Thermal properties of MB2-WC (M = Ti, Zr, Hf) and tungsten and their stability after deuterium plasma exposure

The thermal properties of ultra-high temperature ceramics (UHTCs) in the MB2-WC (M = Ti, Zr, Hf) system and tungsten were studied for potential application as plasma-facing materials in fusion power plants. The sintered UHTC and tungsten samples were subjected to deuterium plasma or protons irradiation. Thermal diffusivity was measured using the laser flash method, and superficial thermal conductivity was analyzed through atomic force microscopy. Results showed that the thermal properties did not degrade when exposed to relevant environments and remained stable over a range of temperatures, unlike the reference tungsten material. Thermal conductivity ranged from 61 to 68 W m−1 K−1 for TiB2-2(WC-6Co), from 53 to 63 W m−1 K−1 for ZrB2-6WC, from 67 to 75 W m−1 K−1 for HfB2-6WC, and from 180 to 119 W m−1 K−1 for tungsten across the temperature range from room temperature to 1200 °C. The increasing trend of thermal effusivity, over 19000 J s−0.5 m−2 K−1 at 1200 °C, justifies further testing and of UHTC materials for fusion applications.

Oxidation and ablation resistance of (TiZrHf)C medium-entropy ceramic coating on C/C composite constructed in-situ at low temperature down to 900°C

The low-cost construction of ultra-high temperature high/medium-entropy ceramic coatings is pivotal for advancing their engineering applications. The study successfully prepared (TiZrHf)C medium-entropy ceramic coatings on C/C composite surfaces using an in-situ molten salt disproportionation reaction at temperatures as low as 900°C. The (TiZrHf)C coatings, approximately 20 μm thick, showed uniform distribution of Ti, Zr, and Hf, typical of medium-entropy ceramics. The process entails Hf reducing Zr⁴⁺ and Ti⁴⁺ to divalent states, which then disproportionate and react with carbon to form the coatings. High-temperature oxidation tests revealed larger oxide grain sizes and dense boundaries in coatings with higher Ti content, indicating superior oxidation resistance. Additionally, ablation tests demonstrated that a suitable amount of liquid-phase TiO₂ formed on the composite surface improves ablation resistance by stabilizing the oxide layer. This cost-effective, highly designable method promotes medium/high-entropy ultra-high temperature ceramics' engineering application.

Production of complex microchanneled parts of ZrB2-MoSi2 ultra-high temperature ceramics by freeze casting and pressureless spark plasma sintering

A novel approach for manufacturing complex parts of ultra-high-temperature ceramics (UHTCs) is proposed, combining freeze casting with pressureless spark plasma sintering (SPS). With the aid of MoSi2 sintering additive (15 vol% or more), this combination enables the production of fully dense ZrB2-based UHTC parts at 1900 °C. Producing complex-shaped samples including an intricate network of internal microchannels seems feasible, by using additive manufacturing techniques in the production of templates for the external silicone mould and the internal microchannel network to be used during the freeze casting process. Although defects are prone to occur either during freeze drying, due to thermal stresses arising from the expansion mismatch between the internal resin template and the ceramic preform, or during the resin burn-out or SPS cycles, the resulting samples exhibited a fully dense microstructure and similar hardness (17 ± 1 GPa) as the bulk material. Thus, the proposed approach shows promise in the production of arbitrarily complex-shaped UHTC parts that may find application in numerous industrial sectors including aerospace, aviation, and energy generation and storage.

Deuterium plasma induced preferential erosion in ultra-high temperature ceramics TiB2 and ZrB2*

Abstract Steady-state deuterium plasma exposures were performed on ultra-high temperature ceramics titanium diboride (TiB2) and zirconium diboride (ZrB2) using the PISCES-RF linear plasma device (LPD) as early screening for first wall, plasma-facing applications. Deuterium plasma exposures were performed using 40 eV ion energies at 240, 525, and 800 °C sample temperatures and 90 eV ion energies at 240 °C sample temperatures to analyze TiB2 and ZrB2 surface morphology and chemistry evolution behavior. Post-plasma exposure chemistry characterization of the near surface ( &lt; 50 nm) region of the samples all show transition metal enrichment, indicating boron preferential erosion. Transition metal to boron fractions vary with plasma exposure temperature under the 40 eV ion energy; metal enrichment is maximized at 800 °C and then minimized at 525 °C. SEM micrographs of all plasma exposed sample surfaces show no significant or noticeable plasma induced damage from cracking or blistering.

Characterization of (Zr,Ti)B2-SiC composites obtained by hot press sintering of ZrB2-SiC-TiO2 powder mixtures

The ability to enhance mechanical and oxidation properties for severe environmental applications has led to substantial academic interest in multiphase ultra-high temperature ceramics (UHTC). The purpose of this work is to study the in-situ solid solution formation of (Zr,Ti)B2 from ZrB2 and TiO2 in a ZrB2-SiC composite using hot pressing reaction sintering. For this, a mixture of 10, 20, and 30 % vol% SiC with ZrB2 was mixed with 2.0 wt% TiO2. Hot pressing sintering was performed with a load of 20 MPa at a final temperature of 1850 °C/30 min in an argon atmosphere. The microstructures, crystalline phases, densities, mechanical properties, and oxidation resistance of the composites were examined and compared with ZrB2-SiC samples lacking TiO2. In samples where TiO2 was added, the matrix grain size slightly decreased, the fracture mode was mainly intergranular, and the SiC grain morphology changed the aspect ratio to be more equiaxed. The solid solution (Zr,Ti)B2 was produced, and it was demonstrated by EDS elemental map images and the XRD analysis that Ti atoms incorporate into the ZrB2 crystalline structure. The development of solid solutions showed no impact on relative densities or Vickers hardness. However, the solid solution formation favored an improvement in fracture toughness, probably owing to the smaller matrix grain size and intergranular fracture mode. Samples exhibiting (Zr,Ti)B2 formation presented lower oxidation resistance than undoped samples in the same oxidizing condition.

A perspective to efficient synthesis of zirconium carbide via novel pyro-vacuum method: lower temperatures and enhanced purity

The use of ultra-high temperature ceramics (UHTCs) as a novel additive in the refractory industry is becoming increasingly popular. However, the synthesis of such materials is associated with some commercial obstacles, mainly high-temperature synthesis methods. In the present study, the pyro-vacuum method is presented as a new method to decrease the final product's synthesis temperature and oxygen content. Some thermodynamic aspects and phase evolution of the materials during the synthesis procedure are described for the synthesis of non-oxide material. Conclusively, it seems that by applying vacuum conditions, the final UHTC phases can be synthesized at significantly lower temperatures (&gt;400 °C lower, for ZrC), if adequate powder mixtures are considered. Also due to phase analysis, it was found that the oxygen content of the final phase is lower than the conventional routes and other references. The process provides promising prospects for the economic synthesis of UHTCs.

Superior ablation resistance of plasma sprayed SiC based coating by structural optimized powders

Pristine SiC coating prepared by plasma spraying is difficult to achieve good ablation resistance at high temperature due to its weak SiO2 scale. Besides incorporation by ultra-high temperature ceramics, the modification of the SiC coating from its feedstock powder is necessary. In the current work, the mixed powders of SiC, Si and C, where the SiC was encapsulated and then carbonized by polydopamine, denoted as SiC@C–Si, were employed as the feedstocks, compared with the mixed case, i.e. SiC–Si–C. The SiC@C–Si coating exhibited superior anti-ablation properties, with a linear ablation rate of 0.62 μm/s, 57 % lower than the SiC–Si–C coating. This is attributed to the fact that nano-carbon formed by carbonization of polydopamine is more prone to reacting with Si than micron-sized C powder, thereby forming a dense microstructure after ablation. Besides, in-situ formed SiC by the depletion reaction increases the flow resistance and decelerates the failure of SiO2 film, thus improving the ablation resistance.

Microstructure evolution of zirconium boride whiskers prepared by electrospinning and their toughening effect

AbstractZirconium boride (ZrB2) whiskers hold significant potential for toughening ultra‐high temperature ceramics (UHTCs). Yet, their fabrication remains challenging due to limited knowledge of the microstructure evolution. Herein, high‐purity ZrB2 whiskers were successfully prepared by electrospinning combined with carbothermal reduction. Specifically, a precursor solution containing Zr–O–C–B networks stabilized through hydrogen bonds with polyvinyl pyrrolidone was synthesized as a spinnable ZrB2 precursor. High‐purity ZrB2 whiskers with a diameter of ca. 200 nm and a length of 5–10 µm were obtained by precise heat treatment of ZrB2 precursor fibers. In this process, ZrO2 grains formed initially and transformed into ZrB2 grains through carbothermal reduction, which further converted into ZrB2 whiskers through a solid‐liquid‐solid mechanism. To testify to the toughening effect for UHTCs, the ZrB2 whiskers were added into ZrB2 powders and followed by sintering. This strategic addition of 5 wt% ZrB2 whiskers led to a 19% enhancement in the fracture toughness of ZrB2 bulk. This study illuminated the preparation strategy and formation mechanism for ZrB2 whiskers and their application for toughening UHTCs.

Invited Article: The oxidation kinetics and mechanisms observed during ultra-high temperature oxidation of (HfZrTiTaNb)C and (HfZrTiTaNb)B2

Ultra-high temperature ceramics (UHTCs), most notably transition metal carbides and borides, exhibit melting temperatures exceeding 3000 °C, making them appropriate candidates to withstand the extreme temperatures (∼2000 °C) expected to occur at the leading edges of hypersonic vehicles. However, their propensity to react rapidly with oxygen limits their sustained application. The high entropy paradigm enables the exploration of novel UHTC compositions that may improve on the oxidation resistance of conventional refractory mono-carbides and -diborides. The oxidation kinetics of candidate high entropy group IV + V (HfZrTiTaNb)C and (HfZrTiTaNb)B2 materials were evaluated at 1500–1800 °C using Joule heating in one atmosphere 0.1%–1% oxygen/argon gas mixtures for times up to 15 min. Possible mechanisms based on the resulting complex time, temperature, and oxygen partial pressure dependencies are discussed. The carbides formed porous and intergranular oxides. Oxidation resistance was improved upon a continuous external scale formation. The diborides formed dense external scales and exhibited better oxidation resistance compared to the carbides. This improvement was attributed to the formation of liquid boria. Both compositions showed an unexpected reduction in material consumption at 1800 °C for all times tested, compared to results at lower temperatures. An in-depth analysis of the composition and morphology of the oxide scale and sub-surface regions for specimens tested at 1800 °C revealed that the formation of denser group IV-rich (Hf, Zr, Ti) oxides mitigated the formation of the otherwise detrimental liquid-forming group V (Ta, Nb) oxides, leading to the improved oxidation resistance.

Ablation behaviour and mechanical performance of ZrB2-ZrC-SiC modified carbon/carbon composites prepared by vacuum infiltration combined with reactive melt infiltration

The development of advanced aircraft relies on high performance thermal-structural materials, and carbon/carbon composites (C/C) composited with ultrahigh-temperature ceramics are ideal candidates. However, the traditional routes of compositing are either inefficient and expensive or lead to a non-uniform distribution of ceramics in the matrix. Compared with the traditional C/C-ZrC-SiC composites prepared by the reactive melt infiltration of ZrSi2, C/C-ZrB2-ZrC-SiC composites prepared by the vacuum infiltration of ZrB2 combined with reactive melt infiltration have the higher content and more uniform distribution of the introduced ceramic phases. The mass and linear ablation rates of the C/C-ZrB2-ZrC-SiC composites were respectively 68.9% and 29.7% lower than those of C/C-ZrC-SiC composites prepared by reactive melt infiltration. The ablation performance was improved because the volatilization of B2O3, removes some of the heat, and the more uniformly distributed ZrO2, that helps produce a ZrO2-SiO2 continuous protective layer, hinders oxygen infiltration and decreases ablation.

Transformation of the Surface of HfB2–SiC–C(graphene) Ultrahigh-Temperature Ceramics in a High-Velocity Flow of Dissociated Nitrogen

Transformation of the Surface of HfB2–SiC–C(graphene) Ultrahigh-Temperature Ceramics in a High-Velocity Flow of Dissociated Nitrogen

High-temperature oxidation and ablation behavior of (Zr1/3Hf1/3Ti1/3)C ceramic

With the increasing requirements on thermal protection systems of aerospace aircraft, multi-component ultra-high temperature ceramics (MC-UHTCs) are emerging and considered to be candidate materials. In this work, (Zr1/3Hf1/3Ti1/3)C ceramics were prepared by hot pressed sintering, and the behaviors from oxidation to ablation of the ceramics over a wide temperature range was systematically investigated for the first time. The oxidation rate is controlled by oxygen diffusion at initial stage with high activation energy and is slower at lower oxygen partial pressure. The intermediate layer (Zr,Hf,Ti)CxOy, along with the dense oxide layer formed through preferential oxidation, plays a crucial role in hindering oxygen diffusion. Diffusion of Zr, Hf, and Ti elements during oxidation results in Ti enrichment on the ablation surface, enhancing ablation protection. However, high temperatures and prolonged ablation times cause Ti-rich oxides dissipation and significant evaporation of TiO, resulting a porous surface, compromising ablation protection efficacy.

Novel high-entropy ultra-high temperature ceramics with enhanced ablation resistance

Novel high-entropy ultra-high temperature ceramics with enhanced ablation resistance

Enhancing Ablation Resistance of TaB2-Based Ultra-High Temperature Ceramics by Mixing Fine TaC Particles and Dispersed Multi-Walled Carbon Nanotubes.

Ultra-high temperature ceramics (UHTCs) have been widely applied in many fields. In order to enhance the comprehensive properties of TaB2-based UHTCs, the first collaborative use of fine TaC particles and dispersed multi-walled carbon nanotubes (MWCNTs) was employed via spark plasma sintering (SPS) at 1700 °C. The derived UHTCs exhibited an average grain size of 1.3 μm, a relative density of 98.6%, an elastic modulus of 386.3 GPa, and a nano hardness of 21.7 GPa, leading to a greatly improved oxidation resistance with a lower linear ablation rate at -3.3 × 10-2 μm/s, and a markedly reinforced ablation resistance with mass ablation rate of -1.3 × 10-3 mg/(s·cm2). The enhanced ablation resistance was attributable to the physical pinning effect, sealing effect and self-healing effect. Thus, this study provides a potential strategy for preparation of UHTCs with bettered ablation resistance and physical properties.