Thermal Properties of Hf‐Doped ZrB2 Ceramics

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 (Zr_1?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.     

Thermal properties of ZrB2-TiB2 solid solutions

Zirconium diboride ceramics were prepared with additions of up to 50 vol.% TiB₂. The resulting (Zr,Ti)B₂ ceramics formed complete solid solutions based on x-ray diffraction. The addition of TiB₂ resulted in grain size decreasing from 22 μm for nominally pure ZrB₂ to 7 μm for ZrB₂–50 vol.% TiB₂. The thermal conductivity at 25°C ranged from 93 W/m⋅K for nominally pure ZrB₂ to 58 W/m⋅K for ZrB₂–50 vol.% TiB₂. Thermal conductivity was as high as 67 W/m⋅K for nominally pure ZrB₂ at 2000°C, but dropped to 59 W/m K with the addition of 50 vol.% TiB₂. Electrical resistivity measurements were used to calculate the electron contribution to thermal conductivity, which was 76 W/m⋅K for nominally pure ZrB₂ decreasing to 57 W/m⋅K when 50 vol.% TiB₂ was added. The phonon contribution to thermal conductivity did not change significantly for ≤10 vol.% TiB₂. Additions of ≥25 vol.% TiB₂ reduced the phonon contribution to nearly zero for all temperatures.     

Elevated temperature thermal properties of ZrB2-B4C ceramics

The elevated temperature thermal properties of zirconium diboride ceramics containing boron carbide additions of up to 15 vol% were investigated using a combined experimental and modeling approach. The addition of B₄C led to a decrease in the ZrB₂ grain size from 22 µm for nominally pure ZrB₂ to 5.4 µm for ZrB₂ containing 15 vol% B₄C. The measured room temperature thermal conductivity decreased from 93 W/m·K for nominally pure ZrB₂ to 80 W/m·K for ZrB₂ containing 15 vol% B₄C. The thermal conductivity also decreased as temperature increased. For nominally pure ZrB₂, the thermal conductivity was 67 W/m·K at 2000 °C compared to 55 W/m·K for ZrB₂ containing 15 vol% B₄C. A model was developed to describe the effects of grain size and the second phase additions on thermal conductivity from room temperature to 2000 °C. Differences between model predictions and measured values were less than 2 W/m·K at 25 °C for nominally pure ZrB₂ and less than 6 W/m·K when 15 vol% B₄C was added.     

Mechanical,Thermal, and Oxidation Properties of Refractory Hafnium and zirconium Compounds

The thermal conductivity, thermal expansion, Youngs Modulus, flexural strength, and brittle–plastic deformation transition temperature were determined for HfB₂, HfC_0·98, HfC_0·67, and HfN_0·92 ceramics. The oxidation resistance of ceramics in the ZrB₂–ZrC–SiC system was characterized as a function of composition and processing technique. The thermal conductivity of HfB₂ exceeded that of the other materials by a factor of 5 at room temperature and by a factor of 2·5 at 820°C. The transition temperature of HfC exhibited a strong stoichiometry dependence, decreasing from 2200°C for HfC_0·98 to 1100°C for HfC_0·67 ceramics. The transition temperature of HfB₂ was 1100°C. The ZrB₂/ZrC/SiC ceramics were prepared from mixtures of Zr (or ZrC), SiB₄, and C using displacement reactions. The ceramics with ZrB₂ as a predominant phase had high oxidation resistance up to 1500°C compared to pure ZrB₂ and ZrC ceramics. The ceramics with ZrB₂/SiC molar ratio of 2 (25 vol% SiC), containing little or no ZrC, were the most oxidation resistant.     

Phase stability,mechanical and thermodynamic properties of (Hf,Zr, Ta,M)B2 (M= Nb,Ti, Cr,W) quaternary high-entropy diboride ceramics via first-principles calculations

As the high-entropy design concept applied to the diboride ceramic system, high-entropy diboride ceramics with a wide range of composition control, is expected to become a new high-performance material for extreme high-temperature environments. Herein, the effects of four transition metal elements (Nb, Ti, Cr, W) on the phase stability and properties of (Hf, Zr, Ta)B₂-based high-entropy diboride ceramics are systematically investigated via the first-principles calculations. All components were identified as thermodynamically, mechanically and dynamically stable from enthalpy of formation, elastic and phonon spectrum calculations. Among these, compared with the (Hf, Zr, Ta)B₂ ceramics, the addition of Nb and Ti on the metal sublattice is beneficial to improve the mechanical properties of ceramics, including Young's modulus, hardness and fracture toughness, while the introduction of Cr and W weakens the strength of covalently and ionic bonds inside the material, reducing its mechanical properties. The predicted thermophysical properties show that the high-entropy diboride ceramics containing Nb and Ti have better high-temperature comprehensive performance, including higher Debye temperature, thermal conductivity and lower thermal expansion characteristics, which is conducive to the application in extreme high-temperature environments. This research will provide important guidance for the design and development of new high-performance high-entropy diboride ceramics.     

Effect of ZrB2 powders on densification,microstructure, mechanical properties and thermal conductivity of ZrB2-SiC ceramics

With the view to improve the densification behaviour and mechanical properties of ZrB₂-SiC ceramics, three synthesis routes were investigated for the production of ZrB₂, prior to the fabrication of ZrB₂-20 vol. % SiC via spark plasma sintering (SPS). Two borothermal reduction routes, modified with a water-washing stage (BRW) and partial solid solution of Ti (BRS), were utilised, alongside a boro/carbothermal mechanism (BRCR) were utilised to synthesise ZrB₂, as a precursor material for the production of ZrB₂-SiC. It was determined that reduction in the primary ZrB₂ particle size, alongside a diminished oxygen content, was capable of improving densification. ZrB₂-SiC ceramics, with ZrB₂ derived from BRW synthesis, exhibited a favorable combination of high relative density (98.6%), promoting a marked increase in Vickers hardness (21.4 ± 1.7 GPa) and improved thermal conductivity (68.7 W·m^-1K^-1).     

Medium-entropy (Ti,Zr, Hf)2SC MAX phase

Medium- and high-entropy alloys or ceramics for tuning the physicochemical properties of materials by the combination of multiple principal elements have received much interest. Herein, a medium-entropy (Ti, Zr, Hf)₂SC phase was synthesized attributing to the structural and chemical diversity of MAX phases. The crystal structure of (Ti, Zr, Hf)₂SC was determined by the Rietveld refinement of XRD, SEM, and atom-resolved TEM along with EDS elemental analysis. Phase evolution of X-ray diffraction patterns and TG/DSC curves were employed to reveal the synthesis mechanism of (Ti, Zr, Hf)₂SC from 2TiC–Zr–ZrC-2HfH₂-3.2FeS reactant system. The Vicker's hardness and the electrical resistivity of (Ti, Zr, Hf)₂SC were found higher than those of Ti₂SC, but the thermal conductivity of (Ti, Zr, Hf)₂SC was lower.     

Improvement of sinterability and mechanical properties of ZrB2 ceramics by the modified borothermal reduction methods

Starting from ZrO₂ and boron (molar ratio: 1:4), four ZrB₂ powders were synthesized by borothermal reduction method, three of which were designed to introduce minor modifications by combining solid solution with Ti and/or water-washing. The sinterability, microstructures, mechanical properties and thermal conductivity were investigated. In comparison with the conventional borothermal reduction, the modified methods offered significant improvement in terms of densification of ZrB₂ ceramics, particularly the mixture that included water-washing. Owing to the refined particle size and boron residues, ZrB₂ ceramics from the modified borothermal reduction which included water-washing demonstrated nearly full densification, Vickers hardness of 14.0 GPa and thermal conductivity of 82.5 W/mK after spark plasma sintering at 2000 °C for 10 min. It was revealed that the properties of ZrB₂ ceramics could be enhanced utilizing the proposed minor modification, starting from the same raw materials and adopting the same sintering conditions.     

Controllable preparation of porous ZrB2–SiC ceramics via using KCl space holders

In this work, a novel and facile technique based on using KCl as space holders, along with partial sintering (at 1900 °C for 30 min), was explored to prepare porous ZrB₂–SiC ceramics with controllable pore structure, tunable compressive strength and thermal conductivity. The as-prepared porous ZrB₂–SiC samples possess high porosity of 45–67%, low average pore size of 3–7 μm, high compressive strength of 32–106 MPa, and low room temperature thermal conductivity of 13–34 W m⁻¹ K⁻¹. The porosity, pore structure, compressive strength and thermal conductivity of porous ZrB₂–SiC ceramics can be tuned simply by changing KCl content and its particle size. The effect of porosity and pore structure on the thermal conductivity of as-prepared porous ZrB₂–SiC ceramics was examined and found to be consistent with the classical model for porous materials. The poring mechanism of porous ZrB₂–SiC samples via adding pore-forming agent combined with partial sintering was also preliminary illustrated.     

Sintering temperature effect on microstructure,mechanical and electrical properties of multi-layered ZrB2-based ceramics with thin Ti interlayer

Laminated ZrB₂-SiC_w ceramics with a thin Ti interlayer were synthesized via spark plasma sintering at varying temperatures. The effect of sintering temperature on the interlayer morphology, phase composition, and mechanical properties of laminated ZrB₂-SiC_w/Ti ceramics was assessed. With increasing sintering temperature from 1600 ℃ to 1800 ℃, element diffusion between the matrix and the interlayer gradually increased. The green-body ductile Ti gradually transformed into a multiphase mixture with increasing hardness at the interlayer, shortening the crack propagation path. The toughening mechanisms changed from delamination to deflection, leading to a decrease in fracture toughness from 15.30 ± 0.72 to 11.21 ± 0.45 MPa m^1/2. Compared to monolithic ZrB₂-SiC_w ceramics, the introduction of multiple toughening mechanisms significantly improved the toughness of laminated ceramics with a small loss in strength. The electrical conductivity under parallel and perpendicular directions decreased with the decrease in residual Ti, with an important effect on electromagnetic effectiveness, reduced from 61.5 to 45.1 dB.     

High thermal-conductivity rGO/ZrB2-SiC ceramics consolidated from ZrB2-SiC particles decorated GO hybrid foam with enhanced thermal shock resistance

Graphene derivative materials exhibit excellent mechanical and thermal properties, which have been extensively used to toughen ceramics and improve thermal shock resistance. To overcome the thermal agglomeration of graphene oxide (GO) during heating and drying process, ZrB₂-SiC particles decorated GO hybrid foam with uniformly anchored ceramic particles was synthesized by electrostatic self-assembly and liquid nitrogen-assisted freeze-drying process. Densified rGO/ZrB₂-SiC ceramics with varying microstructure, thermal physical and mechanical properties were obtained by adjusting the content of decorated ceramic particles. Although the flexural strength of rGO/ZrB₂-SiC ceramics have an attenuation compared with that of ZrB₂-SiC ceramic, the thermal conductivity, work of fracture and thermal shock resistance are greatly improved. rGO/ZrB₂-SiC ceramics exhibit delayed fracture and increasing R-curve behavior during the crack propagation. The novel preparation technology allows for the well dispersion of rGO in ZrB₂-SiC ceramics and can be easily extended to other ceramic or metal materials systems.     

Enhanced mechanical properties,thermal shock resistance and ablation resistance of Si2BC3N ceramics with nano ZrB2 addition

The mechanical properties, thermal shock resistance, and ablation resistance of nano ZrB₂ modified Si₂BC₃N ceramics were investigated. The results show that ZrB₂ stimulated microstructure evolution obviously. Therefore, the maximum strength and fracture toughness reach 559.6 MPa and 6.77 MPa·m^1/2, which are improved by 61.0% and 29.4%, respectively. Furthermore, the residual strengths of 10 wt% ZrB₂ containing composites tested at 1000 ℃ retain 363.6 MPa, which is much higher than 97.7 MPa of pristine Si₂BC₃N ceramics. Besides, the ablation resistance of ZrB₂ modified Si₂BC₃N ceramics at 3000 ℃ is enhanced remarkably and the linear and mass ablation rates of ZrB₂-10 are only 0.009 mm/s and 1.91 mg/s, respectively. The ablation in the ultra-high temperature zone is totally dominated by the ZrB₂ component, and the thermochemical erosion is determined by the oxidation resistance of ZrB₂ in the thermal affected zone.     

Factorial design to minimize residual oxygen in reaction hot‐pressed zirconium diboride

Processing parameters to minimize the residual oxygen content of ZrB₂ ceramics prepared by reactive hot‐pressing were selected using statistical analysis. Additions of carbon and excess boron were used to react with oxygen present in the starting ZrH₂ and B powders as an impurity. A 3² full‐factorial experimental design was used to determine the carbon and excess boron contents that minimized residual oxygen content in the final ZrB₂ ceramic while also minimizing formation of any impurity phases. Carbon additions were effective at reducing the oxygen content, but resulted in the formation of residual ZrC. Boron additions were also effective at removing oxygen, but to a lesser extent compared to carbon. In addition to being more effective at removing oxygen, carbon additions also improved the densification behavior, whereas boron additions inhibited densification. The stoichiometric reaction resulted in a ZrB₂ ceramic with a relative density of 99%, but that contained 6.9 vol% of (Zr_1?x,Mg_x)O_2?x as a residual phase following reactive hot‐pressing. The composition with a carbon addition to starting oxygen content having a molar ratio of 1, and a boron to zirconium molar ratio of 2.1, resulted in a 95% dense ceramic with only 0.1 vol% of residual (Zr_1?x,Mg_x)O_2?x and trace amounts of ZrC.     

Synthesis of rare earth containing single-phase multicomponent metal carbides via liquid polymer precursor route

Novel high-entropy carbide ceramics (HEC) containing rare earth metals, namely (Ti, Zr, Hf, Ta, La, Y)C, (Ti, Zr, Hf, Ta, Nb, La, Y)C, and (Ti, Zr, Hf, Ta, Nb, Mo, W, La)C were prepared with single-phase structure by polymer precursor method. Controlled co-hydrolysis and polycondensation of equiatomic metal-containing monomers were conducted successively, followed by blending allyl-functional novolac resin as carbon source, and the polymer precursors were obtained as clear viscous liquid solutions. The single-phase formation possibility was theoretically analyzed from the aspects of size-effect parameter δ of the designed compositions. All as-obtained ceramics possessed single face-centered-cubic structure of metal carbides and high-compositional uniformity from nanoscale to microscale. The (Ti, Zr, Hf, Ta, Nb, Mo, W, La)C ceramic powder pyrolyzed at 1800°C exhibited low-oxygen impurity content of 1.2 wt%. Thus, multicomponent high-entropy carbide nanoceramics with over five metal elements containing even rare earth element were firstly synthesized and characterized.     

Effects of hafnium content on microstructures and properties of newly developed (Ti,Hf)(C,N) ceramics

In this study, novel (Ti,Hf)(C,N) ceramics with varying hafnium contents were fabricated via carbothermal reduction–nitridation and subsequent spark plasma sintering. The influence of Hf addition on the mechanical properties, wear properties, and corrosion resistance of the (Ti,Hf)(C,N) ceramics was systematically studied. The introduction of Hf promoted the sintering densification of the ceramics in the sintering process. The prepared (Ti,Hf)(C,N) ceramics exhibited excellent mechanical and wear properties owing to refinement and solution-strengthening mechanisms. The (Ti_0.9,Hf_0.1)(C_0.5,N_0.5) ceramic demonstrated higher Vickers hardness and fracture toughness, measuring 1997 HV5 and 4.28 MPa m^1/2, respectively, compared to the pure Ti(C_0.5,N_0.5) ceramic which exhibited values of 1635 HV5 and 3.94 MPa MPa m^1/2. The wear scar depth of the (Ti_0.9,Hf_0.1)(C_0.5,N_0.5) ceramic sample was 57.36% to that of the Ti(C_0.5,N_0.5) ceramic. Additionally, the addition of Hf improved the corrosion resistance of (Ti,Hf)(C,N) ceramics in a 0.5 M NaOH solution. The potential applications of (Ti,Hf)(C,N) ceramics include machining tools and wear-resistant parts.     

Effect of in situ synthesized and additives on the thermal performance of mullite thermal storage ceramics

High-performance thermal storage ceramics can enable utilization of solar thermal power generation plants. In this work, in situ synthesis was used to prepare mullite thermal storage ceramics. Calcined bauxite, talc, and kaolin were used as raw materials. The effects of additives (e.g., SiC, Si₃N₄, TiC, and ZrB₂) on the density, mechanical durability, phase components, microstructure, and thermal performance of the mullite ceramics were studied. The results showed that the thermal expansion coefficient, thermal conductivity, and heat storage density of the mullite ceramics were affected by their phase components. SiC and Si₃N₄ did not decompose during the in situ syntheses, but TiC and ZrB₂ decomposed. With the addition of 10 wt% SiC, the thermal conductivity improved to 2.72 W (m K)^?1 (298 K). The heat storage density of this material was 688 kJ kg^?1 (273–1073 K). Consequently, the in situ synthesized mullite thermal storage ceramic with added SiC could be a promising candidate material for a compound latent-sensible heat storage system.     

Effect of additives on the thermal conductivity of zirconium diboride based composites – A review

Re-entry space vehicle necessities sharp leading edges for better aerodynamic performance and, hence, require advanced thermal protection materials with improved safety for crew members. Material possessing high thermal conductivity and oxidation resistance are desirable at nose cap and wings leading edge of spacecraft. Consequently, the thermal shock resistance improves due to reduced thermal gradient and stresses. ZrB₂ has drawn strong impetus for futuristic space vehicles as thermal protection materials under extreme thermal environments. This study reviews the effect of the incorporation of non-carbonaceous and carbon additives on the thermal conductivity of ZrB₂ ceramics and based composites. Several factors such as the purity of starting powder, initial particle size, amount of sintering aids, processing route, porosity, the grain size of ZrB₂ matrix, distribution of secondary phases in the matrix and sinter density of the final composite, controls the overall thermal conductivity of ZrB₂ based composites.     

Multiscale toughening of ZrB2-SiC-Graphene@ZrB2-SiC dual composite ceramics

The design of bioinspired architectures is effective for increasing the toughness of ceramic materials. Particularly, a dual composite equiaxial architecture is ideal for fabricating weak interface-toughened ZrB₂-SiC ceramics with isotropic performance. In this paper, ZrB₂-SiC-Graphene@ZrB₂-SiC dual composite ceramics were synthesized via an innovative processing technique of granulating-coating method. ZrB₂-20 vol.% SiC containing 30 vol.% Graphene was selected as weak interface to realize multiscale toughening and improve the thermal shock resistance of ZrB₂-SiC ceramic materials. The incorporation of ZrB₂-SiC-Graphene weak interface into the ZrB₂-SiC matrix improved the damage tolerance and critical thermal shock temperature difference. The design of equiaxial structures moderated the anisotropy of performance in different planes. The graphene sheets incorporated in the ZrB₂-SiC-Graphene interface phase played a key role in multiscale toughening, including macroscopic toughening of crack deflection and microcracks, and microscopic toughening of graphene bridging and pull-out.     

Shock wave-material interaction in ZrB2–SiC based ultra high temperature ceramics for hypersonic applications

We investigate the thermochemical stability of ZrB₂–SiC based multiphase ceramics to hypersonic aerothermodynamic conditions in free piston shock tube with an objective to understand quantitatively the role of thermal shock and pressure. The developed ceramics sustained impulsive thermomechanical shock, under reflected shock pressure of 6.5 MPa and reflected shock temperature of 4160 K in dissociated oxygen, without structural failure. The conjugate heat transfer analysis predicts the surface temperature of ZrB₂–SiC to reach a maximum of 693 and 865 K, for ZrB₂–SiC–Ti. The transient shock-material response is characterized by surface oxidation of the investigated ceramics, when exposed to high enthalpy gaseous environment, as a consequence of the interaction with ultrafast-heated (10⁶ K/s) gas for ~5 ms. Spectroscopic and structural characterization reveals that addition of Ti improves thermomechanical shock resistance, which is attributed to the assemblage of refractory phases. Taken together, ZrB₂–SiC–Ti based multiphase ceramics exhibit favorable shock-material response under impulse loading.     

Densification of ZrB2-based composites and their mechanical and physical properties: A review

This study reviews densification behaviour, mechanical properties, thermal, and electrical conductivities of the ZrB₂ ceramics and ZrB₂-based composites. Hot-pressing is the most commonly used densification method for the ZrB₂-based ceramics in historic studies. Recently, pressureless sintering, reactive hot pressing, and spark plasma sintering are being developed. Compositions with added carbides and disilicides displayed significant improvement of densification and made pressureless sintering possible at ≤2000 °C. Reactive hot-pressing allows in situ synthesizing and densifying of ZrB₂-based composites. Spark plasma sintering displays a potential and attractive way to densify the ZrB₂ ceramics and ZrB₂-based composites without any additive. Young's modulus can be described by a mixture rule and it decreased with porosity. Fracture toughness displayed in the ZrB₂-based composites is in the range of 2–6 MPa m^1/2. Fine-grained ZrB₂ ceramics had strengths of a few hundred MPa, which increased with the additions of SiC and MoSi₂. The small second phase size and uniform distribution led to higher strengths. The addition of nano-sized SiC particles imparts a better oxidation resistance and improves the strength of post-oxidized ZrB₂-based ceramics. In addition, the ZrB₂-based composites showed high thermal and electrical conductivities, which decreased with temperature. These conductivities are sensitive to composition, microstructure and intergranular phase. The unique combinations of mechanical and physical properties make the ZrB₂-based composites attractive candidates for high-temperature thermomechanical structural applications.