Effect of incorporation of hafnium diboride nanofibers on thermomechanical properties of carbon fiber–phenolic composites

Carbon fiber/phenolic (C/Ph) composites were modified with different weight ratios of hafnium diboride (HfB₂) nanofibers to apperceive thermomechanical properties of C/Ph–Hf nanocomposites. Mechanical properties, thermal stability, and ablation resistance of C/Ph–Hf nanocomposites were found to be optimum when the weight percentage of HfB₂ was equal to one. Maximum flexural strength and modulus were obtained with 118 MPa and 1.9 GPa for C/Ph–1%Hf nanocomposite, respectively. Increasing the proportion of HfB₂, by delaying the temperature of thermal degradation of nanocomposites, enhanced the thermal stability and residual of C/Ph–Hf relative to C/Ph in both nitrogen and air environments. In the oxyacetylene flame test at 2500°C for 160 s, the optimum mass ablation rate of C/Ph–1%Hf nanocomposites was found to be 0.0150 g/s compared to 0.068 g/s for blank C/Ph, along with reducing the back surface temperature by 51%. The ablation mechanism of C/Ph–Hf nanocomposites after the oxyacetylene torch test was concluded from the derivations obtained from X-ray diffraction, energy dispersion spectroscopy, and microstructure analyses. These clarified that the formation of high-temperature species, such as HfO₂, HfC, and B₄C owing to oxidation of HfB₂ and subsequent reaction products with char, resulted in an increased ablation resistance of the nanocomposites.     

Sol–Gel Synthesis and Formation Mechanism of Ultrahigh Temperature Ceramic: HfB2

Hafnium diboride (HfB₂) powder has been synthesized via a sol–gel‐based route using phenolic resin, hafnium chloride, and boric acid as the source of carbon, hafnium, and boron, respectively, though a small number of comparative experiments involved amorphous boron as boron source. The effects of heat‐treatment dwell time and hafnium:carbon (Hf:C) and hafnium:boron (Hf:B) molar ratio on the purity and morphology of the final powder have been studied and the mechanism of HfB₂ formation investigated using several techniques. The results showed that while temperatures as low as 1300°C could be used to produce HfB₂ particles, the heat treatment needed to last for about 25 h. This in turn resulted in anisotropic particle growth along the c‐axis of the HfB₂ crystals yielding tube‐like structures of about 10 μm long. Equiaxed particles 1–2 μm in size were obtained when the precursor was heat treated at 1600°C for 2 h. The reaction mechanism involved boro/carbothermal reduction and the indications were that the formation of HfB₂ at 1300°C is through the intermediate formation of an amorphous B or boron suboxides, although at higher temperatures more than one reaction mechanism may be active.     

Oxygen barrier resistance of HfB2-MoSi2-TaB2 coatings in a wide temperature region

HfB₂-MoSi₂-based ultra-high temperature ceramic (UHTC) coatings have shown remarkable antioxidant effects owing to the formation of silicate glass layers with low oxygen permeability in high-temperature environments, which shows great potential in the antioxidation of carbon structural materials. To further enhance the oxidation resistance of the HfB₂-MoSi₂-based coating in a wide temperature region, the influence of volume ratio between HfB₂ and TaB₂ on the antioxidant capacity of the HfB₂-MoSi₂-TaB₂ coatings was investigated. The addition of 15 vol% TaB₂ in the 60HfB₂-40MoSi₂ coating delays the initial oxidation temperature of the 60HfB₂-40MoSi₂ sample from 300 °C to 500 °C, which decreased the oxidation loss by 75.85% during dynamic oxidation. In oxidation process at 900 °C and 1700 °C, the weight gains of the 45HfB₂-40MoSi₂–15TaB₂ coating reduced by 78.56% and 63.14%, respectively. Due to the coexistence of 45 vol%HfB₂ and 15 vol%TaB₂, the suitable Ta⁵⁺ promoted the homogenization and dispersion of Hf/Ta-oxides, which forms the coral-like Hf/Ta oxides skeleton in the glass layer, thus preventing the oxygen penetration and decreasing the inert factor of the HfB₂-MoSi₂ coating at 1700 °C by 51.19%. However, excessive TaB₂ weakened the self-healing ability of the Ta-Hf-Si-O glass layer and inhibited the oxygen barrier effect of the HfB₂-MoSi₂-TaB₂ coating.     

Thermal properties of La2O3-doped ZrB2- and HfB2-based ultra-high temperature ceramics

Thermal properties of La₂O₃-doped ZrB₂- and HfB₂-based ultra high temperature ceramics (UHTCs) have been measured at temperatures from room temperature to 2000 °C and compared with SiC-doped ZrB₂- and HfB₂-based UHTCs and monolithic ZrB₂ and HfB₂. Thermal conductivities of La₂O₃-doped UHTCs remain constant around 55–60 W/mK from 1500 °C to 1900 °C while SiC-doped UHTCs showed a trend to decreasing values over this range.     

Synthesis,Characterization, and Microstructure of Hafnium Boride‐Based Composite Ceramics Via Preceramic Method

The ceramic precursor for HfB₂/HfC/SiC/C was prepared via solution‐based processing of polyhafnoxanesal, linear phenolic resin, boric acid and poly[(methylsilylene)acetylene)]. The obtained precursor could be cured at 250°C and subsequently heat treated at relative lower temperature (1500°C) to form HfB₂/HfC/SiC/C ceramic powders. The ceramic powders were characterized by element analysis, thermal gravimetric analysis, X‐ray diffraction, Raman spectroscopy, and Scanning electron microscopy. The results indicated that the ceramic powders with particle size of 200~500 nm were consisted of pure phase HfB₂, HfC, and SiC along with free carbon as fourth phase with low crystallinity.     

Phase evolution of SiOC-based ceramic nanocomposites derived from a polymethylsiloxane modified by Hf- and Ti-alkoxides

SiOC/HfO₂-based ceramic nanocomposites with in situ formed HfO₂ nanoparticles were prepared via a single-source precursor (SSP) approach starting from a polymethylsilsesquioxane (PMS) modified by Hf- and Ti-alkoxides. By varying the alkyl-group of the employed Hf-alkoxides, SiOC/HfO₂-based ceramic nanocomposites with different HfO₂ polymorphs formed via thermal decomposition of the SSP under the same heat-treatment conditions. Using PMS chemically modified by Hf(OⁿBu)₄, tetragonal HfO₂ phase was formed after the synthesis at 1100°C in Ar, whereas both, tetragonal and monoclinic HfO₂ nanocrystals, were analyzed when replacing Hf(OⁿBu)₄ by Hf(OⁱPr)₄. After oxidation of the synthesized nanocomposites in air at 1500°C, a facile formation of oxidation-resistant HfSiO₄ (hafnon) phase occurred by the reaction of HfO₂ nanocrystals with silica present in the SiOC nanocomposite matrix derived from Hf(OⁱPr)₄-modified SSPs. Moreover the amount of hafnon is dramatically increased by the additional modification of the polysiloxane with Ti-alkoxides. In contrast, ceramic nanocomposites derived from Hf(OⁿBu)₄-modified SSPs, almost no HfSiO₄ is detected after oxidation at 1500°C even though in the case of Ti-alkoxide-modified single-source precursor.     

Synthesis of Ultrafine Hafnium Diboride Powders Using Solution‐Based Processing and Spark Plasma Sintering

Ultrafine hafnium diboride (HfB₂) powders were synthesized by the boro/carborthermal reduction process. Fine‐scale mixing of the reactants was achieved by solution‐based processing using hafnium oxychloride (HfOCl₂·8H₂O) and phenolic resin as the precursor of HfO₂ and carbon respectively. The heat treatment was completed at a temperature range 1300–1500°C for 1h using spark plasma sintering (SPS) apparatus. The crystallite sizes of the synthesized powders were small (<500 nm) and the oxygen content was low (0.85 wt%). The grain growth of HfB₂ could be effectively suppressed using SPS due to the fast heating rate. The effects of temperature and holding time on the synthesis of ultrafine HfB₂ powders were discussed.     

Selection principle of the synthetic route for fabrication of HfB2 and HfB2-SiC ceramics

Densification, microstructure, and mechanical properties of spark plasma sintered HfB₂ and HfB₂-SiC ceramics using HfB₂ powders from borothermal reduction and boro/carbothermal reduction were investigated and compared. It was found that HfB₂ceramics obtained by boro/carbothermal reduction exhibited a significantly higher sinterability compared to that by borothermal reduction. Inversely, HfB₂-SiC ceramics obtained by borothermal reduction exhibited a refined microstructure and better mechanical properties (Vickers hardness: 23.60 ± 2.43 GPa; fracture toughness: 5.89 ± 0.30 MPa.m^1/2) than that by boro/carbothermal reduction. These results indicated that optimal fabrication of HfB₂-based ceramics could be achieved by the selection of synthetic route of HfB₂ powders.     

Anti-oxidation and ablation properties of carbon/carbon composites infiltrated by hafnium boride

To improve the anti-oxidation and ablation properties of carbon/carbon (C/C) composites, they are modified by hafnium boride (HfB₂) using a two-step process of in situ reaction and thermal gradient chemical vapor infiltration. X-ray diffraction is used to monitor the composition of the samples. Scanning electron microscope images show that the carbon fibers are uniformly coated by HfB₂ particles. The oxidation onset temperature of carbon fibers is greatly increased from 300 to 700 °C after HfB₂ coating. After modification with HfB₂, the linear and mass ablation rates of the C/C composites are decreased by 51.80% and 24.32%. During oxidation and ablation, the interface between carbon matrix and fiber is effectively protected by HfB₂ due to the reaction of HfB₂ with the oxygen, and the resultant hafnium oxide may form the liquid film to resist the oxygen at high temperature.     

Effects of SiC or MoSi2 second phase on the oxide layers structure of HfB2-based composites

Monolithic HfB₂, HfB₂-30 vol% SiC and HfB₂-10 vol% MoSi₂ composites were prepared by SPS and oxidized in stagnant air at 1500 °C for 70 min. The microstructure of the oxide layer cross-sections showed that the oxidation extents were as follow: monolithic HfB₂ > HfB₂-30 vol%SiC > HfB₂-10 vol% MoSi₂.According to the EDS Line-scan, only one porous oxide layer containing a minor amount of B₂O₃was found on the HfB₂ oxidized surface whereas a thick silicate glass layer and a porous oxide layer below that existed on the surface of HfB₂-30 vol% SiC. After oxidation, the surface of HfB₂-10 vol% MoSi₂ had a narrow silicate-oxide compact layer covered by a very thin glass layer. X-ray diffraction patterns of the oxidized surfaces showed the monolithic HfB₂,the HfB₂-30 vol% SiC and HfB₂-10 vol% MoSi₂composites contain, upon oxidation, only m-HfO₂ phase, mainly m-HfO₂ with a minor amount of HfSiO₄ and mainly HfSiO₄ with a minor amount of m-HfO₂ phases, respectively. Based on the observations in this study, it is suggested that the elimination of the porous layer and subsequent increase of the HfSiO₄ phase are the main reasons for the better oxidation resistance of HfB₂-10 vol% MoSi₂.     

Spark plasma sintering of HfB2 with low additions of silicides of molybdenum and tantalum

HfB₂-based composites containing 3 vol% silicides of molybdenum or tantalum as sintering additives are densified by spark plasma sintering at 1900–2000 °C. Mechanical properties are measured up to 1500 °C in air. 4-pt Flexural strength values at 1500 °C are 480 MPa (64% of the RT value) for the MoSi₂-doped composite and 290 MPa (49% of the RT value) for the TaSi₂-doped composite. The fracture toughness is insensitive to the temperature change and reaches 5 MPa m^1/2 for the TaSi₂-doped ceramic.     

Screw Dislocation Assisted Spontaneous Growth of HfB2 Tubes and Rods

The mechanism of anisotropic growth of HfB₂ rods has been discussed in this study. HfB₂ powder has been synthesized via a sol–gel‐based route using phenolic resin, hafnium chloride, and boric acid as the source of carbon, hafnium, and boron respectively, though a small number of comparative experiments involved amorphous boron as the boron source. The effects of calcination dwell time and Hf:C and Hf:B molar ratio on the purity and morphology of the final powder have been studied and the mechanism of anisotropic growth of HfB₂ has been investigated. It is hypothesized that imperfect oriented attachment of finer HfB₂ particles results in screw dislocations in the coarser particles. The screw dislocation facilitates dislocation‐driven growth of particles into anisotropic HfB₂ rods.     

High heterogeneous compatibility of HfB2-SiC ceramic and Zr-4 alloy with in-situ assembled interface

HfB₂-based ceramics, such as HfB₂-SiC, are promising materials of neutron control rods. Under nuclear conditions, the interfacial compatibility between the HfB₂-SiC control rod and the guide tube (made of Zr-4 alloy) is critical in the operation. The present study demonstrated the high compatibility of the two heterogeneous materials even at 1400 °C. The formation of an in-situ assembled triple diffusion layer with stable phases and dense structures largely improved the interface compatibility. In addition, the as-produced phases with unique microstructures were organized at the interface. ZrSi formed typical columnar crystals with strong [001] fiber texture, which extended in the direction favorable to the interface. ZrB₂ grains grew as needle shapes and entered the Zr-4 alloy substrate. These unique morphologies clearly revealed an interface with strong stability and high compatibility. The results provide the basis for the application of HfB₂-based ceramics in nuclear infrastructures.     

Low Temperature Carbothermal Reduction Synthesis of ZrC Nanofibers via Cyclized Electrospun PVP/Zr(OPr)4 Hybrid

The fabrication capability of zirconium carbide (ZrC) nanofibers by a novel polymeric solution was examined using electrospinning method. The electrospinnable solution was prepared from the reaction of zirconium n‐propoxide (Zr(OPr)₄) with acetylacetone and acetic acid followed by the addition of polyvinylpyrrolidone (PVP) solution. By utilizing thermal and microstructural analyses such as differential scanning calorimetry–thermogravimetry (DSC–TG), field emission scanning electron microscopy (FE‐SEM), transmission electron microscopy (TEM), X‐ray diffraction (XRD), and Brunauer–Emmett–Teller (BET), the effect of heat treatment type on the morphology and crystallinity of as‐spun PVP/Zr(OPr)₄ hybrid fibers was examined. The results showed that direct carbonization treatment of as‐spun fibers under argon atmosphere led to spherical ZrC aggregates in lack of fibrillar morphology, whereas carbonization coupled with cyclization could be recognized as the unique template to govern the morphology and crystallinity of ZrC nanofibers. Carbonization of the cyclized fibers at 1550°C in flowing argon atmosphere produced the thick, fragmented rosary‐like fibers with a diameter of 357 nm, while through a 100°C decrease in carbonization temperature to 1450°C, the thin, smooth, long, and uniform ZrC nanofibers with 176 nm diameter and a medium surface area of 23 m²/g were obtained.     

Porous ceramics based on high-thermal-stability Al2O3–ZrO2 nanofibers for thermal insulation and sound absorption applications

Al₂O₃ fibers are promising candidates for porous ceramics, but the sudden growth of grains in the fibers above 1200 °C will limit their applications for high temperature. Herein, we reported the successful fabrication of the Al₂O₃–ZrO₂ nanofibers by electrospinning and the nanofiber-based porous ceramics by a combination of gel-casting, freeze-drying and high-temperature sintering. Results show that the addition of Zr could greatly improve the thermal stability (up to 1400 °C) of the Al₂O₃-based nanofibers, owing to the inhibition of the sudden growth of the grains in the fibers at high temperature. The Al₂O₃–ZrO₂ nanofiber-based porous ceramics after sintering at 1100–1400 °C possessed a multi-level pore structure and exhibited high thermal stability, ultra-high porosity (97.79–98.04%), ultra-low density (0.075–0.091 g/cm³) and thermal conductivity (0.0474–0.0554 W/mK), and excellent sound absorption performance with the average sound absorption coefficient of 0.598–0.770. These porous ceramics are expected to be employed in the fields of high-temperature thermal insulation and sound absorption.     

Densification,solid solution formation,and microstructural investigation of reactive pressureless sintered HfB2-TiB2-SiC-MoSi2 quadruplet composite

Dense HfB₂-TiB₂-SiC-MoSi₂ quadruplet composite was produced by a reactive pressureless sintering method at 2050 °C for 5 h. The relative density was improved and reached 98% by in situ formation of SiC and MoSi₂ phases. Microstructural studies proved that SiC and MoSi₂ second phases were mostly formed during the sintering process. Moreover, the Sintering mechanism of the composite was investigated by HSC software. TiB₂ co-matrix was improved the sinterability of the composite by the formation of (Hf,Ti,Mo)–B and (Hf,Ti,Mo)–C solid solutions.Mechanical properties such as Vickers hardness (23.2 GPa), fracture toughness (5.4 MPa m^1/2), and elastic modulus (430 GPa) were effectively enhanced by tailoring the composite.     

HfB2 coating on C/C composites prepared by chemical vapor deposition: Thermodynamics and experimental investigation

A thermodynamic calculation on the HfB₂ coating prepared by chemical vapor deposition (CVD) through HfCl₄-BCl₃-H₂-Ar system was performed, together with the relevant verification experiments. The calculation results indicated that HfB₂ coating could be obtained above 900 °C with the ratios of BCl₃/HfCl₄ and H₂/HfCl₄ higher than 1 and 12, respectively. The experimental results demonstrated that the deposition temperature, H₂ and BCl₃ flow rates had significant effects on the grain size, growth rate and phase composition of HfB₂ coatings. A dense and uniform HfB₂ coating was prepared at 1150 °C with a BCl₃/HfCl₄ ratio of 3 and a H₂/HfCl₄ ratio of 20, whose mass and linear ablation rates were 15.61 mg/s and 15.58 μm/s under oxyacetylene flame.     

Pressureless sintering of HfB2–SiC ceramics doped with WC

Using WC as sintering aid, nearly full dense (～99%) HfB₂–20 vol% SiC ceramics were sintered at 2200 °C for 2 h without external pressure. The densification mechanism, microstructure evolution, mechanical properties and oxidation resistance were investigated. The results indicated that complex chemical reactions of WC in HfB₂–SiC system strongly related to the densification, microstructure and properties. The Young's modulus, fracture toughness and 3-pt bending strength of HfB₂–20 vol% SiC with 10 wt% WC were 511 GPa, 4.85 Mpa m^1/2 and 563 MPa, respectively, which were comparable to some hot pressed HfB₂–SiC ceramics in literature. The oxidation of HfB₂–20 vol% SiC with 10 wt% WC at 1500 °C in air exhibited parabolic kinetics. After oxidation at 1500 °C for 10 h, its weight gain and SiC-depleted layer thickness were 3.7 mg/cm² and 43 μm, respectively, and its residual flexural strength was comparable to or even a little higher than the value before oxidation.     

Structural Influence on the Thermal Conversion of Self‐Catalyzed HfB2/ZrB2 Sol–Gel Precursors by Rapid Ultrasonication of Oxychloride Hydrates

Sol–gel precursors to HfB₂ and ZrB₂ are processed by high‐energy ultrasonication of Hf,Zr oxychloride hydrates, triethyl borate, and phenolic resin to form precipitate‐free sols that turn into stable gels with no catalyst addition. Both precursor concentration and structure (a sol or a gel) are found to influence the synthesis of the diboride phase at high temperature. Decreasing sol concentration increases powder surface area from 3.6 to 6.8 m²/g, whereas heat‐treating a gel leads to residual oxides and carbides. Particles are either fine spherical particles, unique elongated rods, and/or platelets, indicating particle growth with directional coarsening. Investigation of the conversion process to ZrB₂ indicates that a multistep reaction is likely taking place with: (1) ZrC formation, (2) ZrC reacts with B₂O₃ or ZrC reacts with B₂O₃ and C to form ZrB₂. At low temperatures, ZrC formation is limiting, while at higher temperatures the reaction of ZrC to ZrB₂ becomes rate limiting. ZrC is found to be a direct reducing agent for B₂O₃ at low temperature (~1200°C) to form ZrB₂ and ZrO₂, whereas at high temperatures (~1500°C) it reacts with B₂O₃ and C to form pure ZrB₂.     

Development and Characterization of Continuous SiC Fiber‐Reinforced HfB2‐Based UHTC Matrix Composites Using Polymer Impregnation and Slurry Infiltration Techniques

This paper discusses the development of continuous SiC fiber‐reinforced HfB₂‐SiC composite laminates. A range of techniques, based on resin‐based precursors and slurries, for infiltrating porous SiC preforms with HfB₂ powder were developed. While resin‐based precursors proved to be ineffective due to low HfB₂ yield and poor adhesion, the slurry infiltration techniques were effective to varying degrees. The greatest pore filling and composite densities were achieved using pressure and vibration‐assisted pressure infiltration techniques. SiC_f/HfB₂‐SiC laminates were subsequently developed via lamination, cure and pyrolysis of fabrics using a HfB₂‐loaded polymeric SiC precursor, followed by HfB₂ slurry infiltration and preceramic polymer infiltration and pyrolysis (PIP). Repeated PIP processing, for 6–10 cycles, resulted in density increases, from the 3.03–3.22 g/cm³ range after HfB₂ slurry infiltration, to 3.97–4.03 g/cm³ after PIP processing. Correspondingly, there was a decrease in open porosity from approximately 52% to less than 11%. The matrix consisted of discreet, lightly sintered HfB₂ particles dispersed in SiC. The PIP SiC matrix was primarily nanocrystalline after 1300°C pyrolysis, but experienced grain growth with further heat treatment at 1600°C.