Preparation of highly porous ZrB2/ZrC/SiC composite monoliths using liquid precursors via direct drying process

We developed a simple liquid precursor method for the syntheses of porous ZrB₂/ZrC/SiC composite monoliths. Furfuryl alcohol (FA), zirconium n-butoxide, tetraethyl orthosilicate and boric acid are used as the raw materials. By combining the polymerization of FA and gelation of inorganic sols, porous hybrid monoliths are prepared by direct drying the wet gels. The inorganic and organic polymers possibly form interpenetrated network which provides the robustness for the wet gel to withstand the severe changes during dessication. When heat-treated at 1600 °C, hybrid gels are converted into porous ZrB₂/ZrC/SiC monoliths. The microstructure of the ZrB₂/ZrC/SiC monoliths can be easily tailored by controlling the synthesis conditions. The porosities of the ZrB₂/ZrC/SiC monoliths can be tuned around 74.3–81.6%, while the average pore diameters can be tuned ranging from 1.0 to 8.5 μm with pretty narrow distribution. The compressive strengths of such highly porous ceramics are in the range of 1.2–1.9 MPa.     

Effect of ZrC content on the properties of biomorphic C–ZrC–SiC composites prepared using hybrid precursors of novel organometallic zirconium polymer and polycarbosilane

A novel organometallic zirconium polymer was synthesized through the copolycondensation using n-butyllithium, 1,4-diethynylbenzene, phenylacetylene and zirconium tetrachloride as raw materials. Then biomorphic C–ZrC–SiC composites were fabricated from corn stover templates by precursor infiltration and pyrolysis process using hybrid polymeric precursors containing the organometallic zirconium polymer and polycarbosilane. The microstructure, mechanical properties and oxidation resistance of the composites were investigated. With ZrC content increasing, the mechanical properties of the composites were enhanced due to dispersion strengthening and grain fining of the homogeneously dispersed ZrC nanoparticles. The oxidation behavior of C–SiC–ZrC indicated that the oxidation resistance of the composite was reduced at 1000 °C but improved at 1500 °C with the increase of ZrC content. The improved oxidation resistance was mainly attributed to a proper ZrC content, the formation of ZrSiO₄ layer on the surface of the composite, and its matrix microstructure characterized by a nano-sized dispersion of ZrC–SiC phases.     

Mechanical behaviour and microstructure of Cf/ZrC,Cf/SiC and Cf/ZrC – SiC composites

Cf/ZrC, Cf/SiC and Cf/ZrC–SiC composites were successfully prepared by polymer infiltration and pyrolysis (PIP) using polycarbosilane and a liquid ZrC precursor. The densification process, mechanical properties and microstructures were studied in a view of comparison. After the same total 20 PIP cycles, the Cf/ZrC, Cf/SiC and Cf/ZrC–SiC composites had flexural strengths of 50.1±5.3, 285.7±22.6, 141.5±13.1?MPa respectively; elastic moduli of 7.8±0.9, 57.1±3.2 and 45.1±2.6?GPa respectively; and fracture toughness of 2.5±0.2, 10.4±0.9 and 10.9±1.1?MPa m^1/2 respectively. With the introduction of high modulus SiC phase into the ZrC matrix, the densification and modulus of the matrix were improved; as a result, the Cf/ZrC–SiC composite showed higher mechanical properties compared to Cf/ZrC.     

Reaction kinetics and ablation properties of C/C-ZrC composites fabricated by reactive melt infiltration

Carbon/carbon-zirconium carbide (C/C-ZrC) composites were prepared by reactive melt infiltration. Carbon fiber felt was firstly densified by carbon using chemical vapor infiltration to obtain a porous carbon/carbon (C/C) skeleton. The zirconium melt was then infiltrated into the porous C/C at temperatures higher than the melting point of zirconium to obtain C/C-ZrC composites. The infiltration depth as a function of annealing temperature and dwelling time was studied. A model based on these results was built up to describe the kinetic process. The ablation properties of the C/C-ZrC were tested under an oxyacetylene torch and a laser beam. The results indicate that the linear and mass ablation rates of the C/C-ZrC composites are greatly reduced compared with C/SiC-ZrB₂, C/SiC, and C/C composites. The formation of a dense layer of ZrC and ZrO₂ mixture at high temperatures is the reason for high ablation resistance.     

Volatility diagram of ZrB2‐SiC‐ZrC system and experimental validation

A volatility diagram of zirconium carbide (ZrC) at 1600, 1930, and 2200°C was calculated in this work. Combining it with the existing volatility diagrams of ZrB₂ and SiC, the volatility diagram of a ternary ZrB₂‐SiC‐ZrC (ZSZ) system was constructed in order to interpret the oxidation behavior of ZSZ ceramics. Applying this diagram, the formation of ZrC‐corroded and SiC‐depleted layers and the oxidation sequence of each component in ZSZ during oxidation and ablation could be well understood. Most of the predictions from the diagrams are consistent with the experimental observations on the oxidation scale of dense ZrB₂‐SiC‐ZrC ceramics/coatings after oxidation at 1600°C or ablation at 1930 and 2200°C. The reasons for the discrepancy are also briefly discussed.     

A Simple Polymeric Precursor Strategy for the Syntheses of Complex Zirconium and Hafnium-Based Ultra High-Temperature Silicon-Carbide Composite Ceramics

Pyrolyses of chemical precursor systems composed of zirconium or hafnium powders dispersed into blends of the polymethylcarbosilane and poly(norbornenyldecaborane) preceramic polymers provide simple efficient routes to new types of ZrB₂/ZrC/SiC and HfB₂/HfC/SiC composites. Further extensions of this method should now allow the syntheses of a wide variety of complex potential ultra high-temperature zirconium- and hafnium-based composite materials with controllable tuning of their compositions, microstructures, and/or stabilities.     

Monolithic silicon carbide with interconnected and hierarchical pores fabricated by reaction-induced phase separation

Hierarchically porous silicon carbide (SiC) monoliths were fabricated based on polycarbosilane (PCS), divinyl benzene (DVB), and decalin, by a sequence of procedures including catalyst-free hydrosilylation reaction-induced phase separation, ambient-pressure drying, calcination, and HF etching. The influences of ratios of each component on the phase separation were systematically studied. It was found that isotactic polypropylene added as a nonreactive additive could effectively tailor the microstructure and improve the mechanical properties of SiC monoliths. The resultant SiC monoliths mainly consisted of β-SiC nanocrystals, and possessed low bulk density (0.7 g/cm³), high porosity (78%), large specific area (100.6 m²/g), high compressive strength (13.5 ± 1.6 MPa), and hierarchical pores (macropores around 350 nm, mesopores around 4 and 20 nm). These properties make SiC monoliths promising materials for catalyst/catalyst support, gas separator, and the reinforcement of high-temperature composites.     

Synthesis,Characterization, and Microstructure of ZrC/SiC Composite Ceramics via Liquid Precursor Conversion Method

The ceramic precursor for ZrC/SiC was prepared via solution‐based processing using polyzirconoxane, polycarbosilane, and divinylbenzene. The precursor could be transformed into ZrC/SiC ceramic powders at relative low temperature (1500°C). The cross‐linking process of precursor was studied by FT–IR. The conversion from precursor into ceramic was investigated by TGA, XRD. The ceramic compositions and microstructures were identified by element analysis, Raman spectra, SEM, and corresponding EDS. The results indicated that the ceramic samples remained amorphous below 1000°C and t–ZrO₂ initially generated at 1200°C. Further heating to 1400°C led to the formation of ZrC and SiC with the phase transformation of ZrO₂ and almost pure ZrC/SiC could be obtained upon heat‐treatment at 1500°C. During heat treatments, the ceramic sample changed from compact to porous due to carbothermal reduction. The ceramic powders with particle size of 100 nm~400 nm consisted of high crystalline degree ZrC and SiC phases, and Zr, Si, C were well distributed at the different sites in ceramic powders. The free carbon content was lowered to 1.60 wt% in final ZrC/SiC composite ceramics.     

Surface densification of porous ZrC by a laser process

Thin films of 100%-dense zirconium carbide, ZrC, on top of ca. 30%-porous ZrC are obtained by using an ytterbium-doped fibre laser under argon atmosphere. It is shown experimentally and theoretically that oxidation into zirconia or oxycarbide is completely avoided at the working temperature of at least 3420 °C. This type of highly refractory materials – dense at the surface and porous in the bulk – could be used in high temperature applications where both diffusion and thermal barrier properties are needed. This process could also be used to purify from oxygen the surface of carbides obtained by thermal carboreduction of oxides.     

Organic-inorganic composites for novel optical transformation induced by high-energy laser ablation

High-energy continuous-wave (CW) laser has been considered as a significant technology in recent decades. Such laser can destroy conventional materials in an extremely short time, necessitating their protection. In this study, zirconium carbide (ZrC) and silicon carbide (SiC) particle-modified short silicon carbide fiber-reinforced phenolic resin matrix composites (SiC/BPF-ZS) with significant anti-laser performance were designed and prepared. Our results showed that the ceramic particles and SiC fibers rapidly oxidized, leading to the formation of a ceramic coating composed of ZrO₂ and SiO₂. Owing to the formation of the ceramic coating, the reflectivity of the composites improved significantly from 15.8% to 73.2% after ablation at 500 W/cm² for 30 s. Additionally, the SiC fibers played an important role in the formation of a high-reflectivity coating during laser ablation. Contrast experiments indicated that SiC fibers lead to better performance than the carbon fibers. The high reflectivity and low mass ablation rate are demonstrated to be the key factors improving the anti-laser ablation performance of the SiC/BPF-ZS composites.     

Carbothermal conversion of self-supporting organic/inorganic interpenetrating networks to porous metal boride monoliths

In this work, we present a general sol-gel protocol for the synthesis of highly porous monolithic transition metal borides via carbothermal conversion of the organic/inorganic interpenetrating networks (IPNs). The formation of organic/inorganic IPNs is clearly demonstrated by simple oxidation and boiling water treatment. A series of transition metal boride porous monoliths, including CrB₂, ZrB₂, TiB₂, Cr₃C₂/CrB, and ZrB₂/ZrC with porosities ranging from 70% to 85% and pore sizes ranging from 0.5 to 35 μm, have been prepared. In each case, a porous hybrid monolith is obtained by drying the wet gel under ambient pressure. It is believed that the formation of organic/inorganic IPNs strengthens the gel network, so that it can withstand the severe changes during desiccation to give out a monolithic xerogel. Samples are characterized by TG-DSC, XRD, SEM, EDS, TEM, BET, and MIP, and the ceramic monoliths are shown to be well defined and rather homogeneous.     

Preparation and Characterization of Zirconium Carbide Coating on Coated Fuel Particles

Compact and uniform zirconium carbide (ZrC) coatings have been successfully deposited on coated fuel particles using a ZrCl₄+H₂+Ar+C₃H₆ gas mixture. Zirconium tetrachloride (ZrCl₄) powder feeder was especially designed and manufactured to control accurately the flow rate of ZrCl₄ and produce ZrC on an industrial scale. The coating has a large density (6.57 g/cm³), a thickness of 35 μm, a stoichiometry close to Zr/C=1, and a clear interface between the coating and substrate. The coating exhibits an fcc -ZrC phase with a grain size of 11.18 nm and a (111) texture coefficient of 0.57, which corresponds to a polycrystalline microstructure of randomly oriented ZrC grains. The preparation apparatus, processing conditions, properties, microstructures, and morphologies of the ZrC coating are investigated systematically.     

碳化锆陶瓷有机前驱体的热解过程

合成了碳化锆陶瓷有机前驱体,研究了其在热解过程中化学成分和物相组成变化,探讨了从有机高分子向无机陶瓷转化的机理,对碳热还原反应进行了热力学分析。结果表明,前驱体在600℃以下完成了有机结构的断裂、裂解碎片的重排与挥发,600℃以上裂解产物不再具备有机特征;随热解温度升高,无定型碳和单斜相ZrO2逐渐生成,大于1200℃时可检测到立方相ZrC,1400℃时单斜相ZrO2基本消失;1500℃时完成碳热还原反应,在远低于热力学反应温度的条件下生成了高度结晶的纳米尺寸的立方相碳化锆陶瓷。     

High-Temperature Thermal Expansion Behavior of Refractory Materials: I, Selected Monocarbides and Binary Carbides

An apparatus for absolute thermal expansion measurements has been developed and operated to 2843°C. Thermal expansion characteristics have been measured for niobium carbide, zirconium carbide, tantalum carbide, and the carbide solutions 8TaC·ZrC, 4TaC·ZrC, TaC·ZrC, and TaC·4ZrC. Supporting evidence for a high temperature inversion in the monocarbides of zirconium and tantalum has been observed. Evidence of analogous behavior for other carbides is also reported.     

Fabrication and properties of ultra highly porous silicon carbide by the gelation–freezing method

Silicon carbide (SiC) with ultra high porosity and unidirectionally oriented micrometer-sized cylindrical pores was prepared using a novel gelation–freezing (GF) method. Gelatin, water and silicon carbide powder were mixed and cooled at 7 °C. The obtained gels were frozen from ?10 to ?70 °C, dried using a vacuum freeze drier, degreased at 600 °C and then sintered at 1800 °C for 2 h. The gels could be easily formed into various shapes, such as cylinders, large pipes and honeycombs using molds. Scanning electron microscopy (SEM) observations of the sintered bodies showed a microstructure composed of ordered micrometer-sized cylindrical cells with unidirectional orientation. The cell size ranging from 34 to 147 μm could be modulated by changing the freezing temperatures. The numbers of cells for the samples frozen at ?10 and ?70 °C were 47 and 900 cells/mm², respectively, as determined from cross-sections of the sintered bodies. The resulting porous SiC with a total porosity of 86%, exhibited air permeability from 2.3 × 10^?11 to 1.0 × 10^?10 m², which was the same as the calculated ideal permeability, and high compressive strength of 16.6 MPa. The porosity, number of cells, air permeability and strength of the present porous SiC were significantly higher than that reported for other porous SiC ceramics.     

TEM/STEM Observation of ZrC Coating Layer for Advanced High-Temperature Gas-Cooled Reactor Fuel, Part II

The Japan Atomic Energy Agency (JAEA) has started to study and develop zirconium carbide (ZrC)-coated fuel particles for advanced high-temperature gas-cooled reactors. The ZrC coating layer has been fabricated at JAEA by chemical vapor deposition using a pyrolytic reaction of zirconium bromide. The microstructures of the ZrC layers, whose nominal deposition temperatures could be measured and controlled during the deposition process, were characterized by means of TEM and STEM. In the present study, three batches were prepared and compared with each other as well as the previous batches. The crystallographic orientation of ZrC with regard to the growth direction in the ZrC layers deposited at a constant temperature of 1630 K was different from that deposited at varying temperatures in the 1493–1823 K range. A thin layer of turbostratic carbon was observed at the boundary between pyrolytic carbon and ZrC in particles deposited at the highest temperature among those used in this study (the nominal temperature was 1769 K); no such structure was found in a batch deposited at a lower temperature (the nominal temperature was 1632 K). Therefore, precise control of temperature is shown to be critical to the formation of good ZrC coatings.     

Fabrication and properties of Cf/ZrC‐SiC‐based composites by an improved reactive melt infiltration

C_f/ZrC‐SiC composites with a density of 2.52 g/cm³ and a porosity of 1.68% were fabricated via reactive melt infiltration (RMI) of Si into nano‐porous C_f/ZrC‐C preforms. The nano‐porous C_f/ZrC‐C preforms were prepared through a colloid process, with a ZrC “protective coating” formed surrounding the carbon fibers. Consequently, highly dense C_f/ZrC‐SiC composites without evident fiber/interphase degradation were obtained. Moreover, abundant needle‐shaped ZrSi₂ grains were formed in the composites. Benefiting from this unique microstructure, flexural strength, and elastic modulus of the composites are as high as 380 MPa and 61 GPa, respectively, which are much higher than C_f/ZrC‐SiC composites prepared by conventional RMI.     

Fabrication of highly porous biodegradable monoliths strengthened by graphene oxide and their adsorption of metal ions

Ordered porous chitosan–gelatin/graphene oxide (CGGO) monoliths with over 97% porosity were prepared by a unidirectional freeze-drying method and used as adsorbents for metal ions. They were characterized by X-ray diffraction, scanning electron microscopy and thermogravimetric analysis. In addition, their water absorption, wet-state stability and compressive strength were measured. The adsorption behavior of the CGGO monoliths and influencing factors such as pH, graphene oxide (GO) concentration, metal ion concentration as well as the effect of ethylenediaminetetraacetic acid (EDTA) were investigated. The incorporation of GO significantly increased the compressive strength of the CGGO monoliths in both their wet and dry states, and changed their porous structure. They exhibited an extremely high adsorbing ability for metal ions, which decreased at low pH, but increased from 20% to 88% upon the addition of EDTA at low pH. The CGGO monoliths have good stability and can be recycled several times with only a slight loss in adsorption ability. In addition, they are biodegradable, non-toxic, efficient and regenerable.     

Synthesis,Ceramic Conversion and Microstructure Analysis of Zirconium Modified Polycarbosilane

Polymer derived ceramics have been widely being explored as high temperature structural components in aerospace as rocket nozzles, nose tip and leading edges of reusable launch vehicles. Polycarbosilane (PCS) was modified by a condensation reaction with zirconium acetylacetonate [Zr(acac)₄] to form polyzirconocarbosilane (PZrCS). A series of PZrCS were synthesized, which could be transformed into Si–Zr–C ceramic phases on pyrolysis. The ceramic yield of PCS was significantly improved by the introduction of zirconium into the system. The XRD patterns of the PZrCs show the characteristic peaks of ?SiC at 1300 °C and at 1500 °C the characteristic peaks of ZrC and ZrO₂ were observed. The carbothermal reaction in PZrCS was completed at 1650 °C and the resulting ceramic was non-oxide SiC/ZrC phase. The SEM images proved that the increase in concentration of zirconium in the final ceramic decreases the surface uniformity. HRTEM analysis of PZrCS heat treated at 1650 °C shows the evolution of oxide free ZrC/SiC phase with compatible grain boundaries without stacking fault. It could be concluded that the technique of introducing ultra-high temperature ceramic phases into the SiC matrix is an effective approach to improve the high-temperature performance of silicon based ceramics.     

Fabrication of non-oxide ceramic powders by carbothermal-reduction from industrial minerals

The most promising method for obtaining a large variety of non-oxide products having important technical uses is carbothermal-reduction reaction (CRR). By using this procedure, SiC and ZrC/SiC powders are obtained from diatomaceous earth and zircon powder. In this way the synthesized powders are obtained at a relatively low temperature due to good homogenization. Starting C/ZrSiO₄ admixtures having different molar ratios (3:1, 4:1, 5:1 and 7:1) and C/SiO₂ having ratios 1:1, 3:1, 4:1, and 7:1 were heated at temperatures between 1300 and 1600 °C in a controlled Ar flow atmosphere. The phase evolution was a function of the raw materials molar ratios and sintering temperature. The optimal parameters for the synthesis of SiC and ZrC/SiC powders were obtained. The results obtained by EDS analysis are in good agreement with those obtained by XRD analysis for the synthesized carbide powders.