Studies on the Carbothermal Preparation of Titanium Carbide from Different Gel Precursors

Different mixed organic–inorganic gels as precursors for the synthesis of titanium carbide and oxycarbides have been prepared in aqueous and organic solutions starting either from rutile or titanium alkoxide. The sol–gel processes have been controlled by complexing additives such as H₂O₂, acetic acid or ethyl aceto-acetate. Upon pyrolysis up to ∼600°C, composites of finely divided particles of amorphous titania and carbon are formed. Monitoring of the high-temperature treatment by TG, XRD and nitrogen adsorption up to 1600°C revealed a three-step carbothermal reduction mechanism through lower titanium oxides and oxycarbides. The intimate mixture of titania and carbon leads to a considerable lowering of the onset of the reaction temperature in comparison with rutile/carbon black and rutile/gel mixtures. Delayed reactions in the final steps, however, may be due to the interruption of the intimate contact of the reactants by pore generation. During the high temperature processes microporous and mesoporous intermediate materials have been prepared; the final products at 1600°C are finely divided particles of oxycarbides with oxygen contents of 1·2–3·4 wt% and grain sizes lower than 1 μm.     

Synthesis of rod-like ZrB2 powders

Rod-like ZrB₂ powders were synthesised at 1500°C in vacuum by boro/carbothermal reduction using ZrO₂, B₄C and graphite as the starting materials. During the heating process, the ZrB₂ grains primarily grow along the c axis to form a rod-like morphology without any heterogeneous catalyst. The final products are pure rod-like ZrB₂ particles, which are thought to be promising starting powders to prepare high performance ultrahigh temperature ceramics with unique microstructures such as textured one through tape casting process.     

Gas‐Phase and Solid‐State Simultaneous Mechanism for Two‐Step Carbothermal AlON Formation

AlON powders were synthesized by two‐step carbothermal reduction nitridation method, which includes thermal treatment of Al₂O₃/C mixtures at 1200°C–1600°C for 2 h, followed by subsequent heating at 1750°C for 1.5 h in N₂ flow. The effects of soaking temperatures of the first step on phase compositions and morphologies of the final products were investigated. It is found that the variation in precalcination does not have impact on phase compositions of the final products, which are all single‐phase AlON. However, it impacts the AlON morphology significantly. Lower precalcining temperature results in severer agglomeration of AlON powder. Obvious terrace surface morphology was also observed on AlON particles with lower precalcination. Both the agglomeration and terrace‐like morphology are attributed to the gas‐phase reaction induced by the residual carbon in the AlON formation process. An AlON formation mechanism including simultaneous solid‐state reaction between Al₂O₃ and AlN, and gas‐phase reaction among Al (g), O₂ (g), and N₂ (g) with the presence of residual carbon is proposed based on the experiment, kinetics, and thermodynamics. The mechanism was further examined by carefully designed control experiments, which was confirmed to be both experimentally and theoretically valid.     

Synthesis,Characterization, and Consolidation of Cr2(C,N) Solid Solution Powders

Single‐phase Cr₂(C,N) powders, which are mainly composed of large worm‐like particles with the diameter of 1–3 μm, have been successfully synthesized by carbothermal reduction nitridation (CRN) method at 1200°C for 1 h in N₂ atmosphere for the first time. The phase evolution and morphology were investigated using XRD, TG‐DSC, SEM, and TEM techniques. The reactions during the synthesis process were proposed and analyzed. The surface of Cr₂(C,N) determined by XPS is mainly composed of four elements, Cr, N, C, and O. Furthermore, the attempts to consolidate Cr₂(C,N) powders were carried out using spark plasma sintering (SPS), and the fully densified bulk ceramics were obtained at 1200°C. The effect of sintering temperature on phase composition and mechanical properties were also discussed.     

Microstructure and ablation behavior of carbon/carbon composites infiltrated with Zr–Ti

Carbon/carbon(C/C) composites infiltrated with Zr–Ti were prepared by chemical vapor infiltration and reactive melt infiltration. Their microstructure and ablation behavior at different temperatures and time were investigated. The results show that C/C composites infiltrated with Zr–Ti have good interface cohesion between carbon fibers, pyrocarbon and carbide. Compared with C/C composites and C/C–ZrC composites, the synthesized sample with Zr_0.83Ti_0.17C_0.92 and Ti_0.82Zr_0.18C_0.92 exhibits better ablation resistance at 2500 °C due to the newly formed protective layer composed of ZrTiO₂ pinned by ZrO₂ grains after ablation. The ablation resistance of the sample with Zr_0.57Ti_0.43C_1.01 increased gradually with the decrease of temperature from 3000 °C to 2000 °C, whereas the ablation resistance of the sample with Zr_0.83Ti_0.17C_0.92 and Ti_0.82Zr_0.18C_0.92 first increased obviously and then decreased slightly. In addition, the work indicates that surplus particles or liquid phases of oxides cannot protect the matrix, and that the liquid oxides may even cause severe ablation. Furthermore, a protective layer of oxides tends to be formed with the increase of ablation time.     

Preparation and Characterization of UltraHigh‐Temperature Ternary Ceramics Ta4HfC5

Tantalum hafnium carbide (Ta₄HfC₅) powders were synthesized by solvothermal treatment and carbothermal reduction reactions from an inorganic hybrid. Tantalum pentachloride, hafnium chloride, and phenolic resin were used as the sources of tantalum, hafnium, and carbon, respectively. Pyrolysis of the complexes at 1000°C/1 h initiated the carbothermal reduction to result in multiplex phases including tantalum carbide and hafnium oxide which after heat treatment at 1400°C–1600°C transformed to single‐phase solid solution Ta₄HfC₅ by solid solution reaction. The mean crystallite size of Ta₄HfC₅ particles was less than 80 nm, and the composition of Ta, Hf, and C elements was near stoichiometric and homogeneously distributed in the powder samples. XRD pattern for Ta₄HfC₅ powders was analyzed.     

Fabrication and optimization of 3D-Cf/HfC-SiC-based composites via sol-gel processing and reactive melt infiltration

In this work, 3D-C_f/HfC-SiC-based composites were fabricated and optimized via reactive melt infiltration (RMI) of Si into porous C_f/HfC-C preforms prepared by a sol-gel processing. The physical and chemical processes involved during the fabrication were identified and analyzed in details. It is revealed that fibers and interphase of the composites can be eroded during carbothermal reduction process, which can be further aggravated during RMI, with the formation of Hf-containing substance on the fibers surface. The fibers and interphase degradation is mainly induced by the reactions between HfO₂ and C/SiC interphase layers at elevated temperatures. Accordingly, a two-step carbothermal reduction treatment was proposed for the optimization of the fabrication procedure. As a result, less fiber/interphase erosion and improved mechanical properties are achieved in the composites, with the bending strength increased by ～49 % (from 214.1 ± 15.7 MPa to 319.0 ± 26.0 MPa).     

Effect of SiC nanoparticles on in-situ synthesis of SiC whiskers in corundum–mullite–SiC composites obtained by carbothermal reduction

Corundum–mullite–SiC composites were synthesised using a carbothermal reduction method. The effects of SiC nanoparticles and sintering temperatures on the phase transformation of the composites and the synthesis of SiC whiskers were studied by X-ray diffraction, scanning electron microscopy, and transmission electron microscopy. Results indicated that corundum, mullite, and SiC whiskers were produced as final products at 1600–1650 °C. SiC whiskers were formed through the vapor–solid mechanism. The added SiC nanoparticles worked as nucleating agents to facilitate the carbothermal reduction of aluminosilicates and formation of SiC whiskers. The sample with the added SiC nanoparticles exhibited a high yield of β-SiC of 17.1%. Furthermore, the SiC nanoparticles decreased the formation temperature of SiC whiskers from the original 1600 °C to 1500 °C, and the porosity of the composites was increased from 56.7% to 64.7% by increasing the partial pressure of SiO gas. This study provides an insight into the more efficient synthesis of composites with SiC whiskers through the carbothermal reduction of aluminosilicates.     

Fabrication and microstructure of 3D Cf /ZrC-SiC composites: through RMI method with ZrO2 powders as pore-making agent

3D C_f/ZrC–SiC composites were prepared by a combination process of slurry infiltration and reactive melt infiltration. ZrO₂ powders and ZrSi₂ alloy, both of which reacted with amorphous carbon, were used as pore-making agent and infiltrator, respectively. After carbothermal reduction at 1650 °C, X-ray diffraction analysis revealed that ZrO₂ powders were completely converted into ZrC by reacting with amorphous carbon, and an in-situ formed submicron porous configuration was observed at the areas containing ZrO₂. Results showed that the matrix in composites mainly consisted of SiC, ZrC and a small quantity of residual metal. SEM and TEM images revealed the formation of ZrC or SiC intergranular particles in the matrix and the characteristic around the residual resin carbon. The composites had a bending strength of 94.89±16.7 MPa, fracture toughness of 11.0±0.98 MPa m^1/2, bulk density of 3.36±0.01 g/cm³, and open porosity of 4.64±0.40%. The formation mechanisms of ZrC–SiC dual matrix and intrabundles׳ structure were discussed in the article.     

Thermal decomposition of trichloroethylene under a reducing atmosphere of hydrogen

The thermal reaction of trichloroethylene (TCE: C₂HCl₃) has been conducted in an isothermal tubular flow reactor at 1 atm total pressure in order to investigate characteristics of chlorinated hydrocarbons decomposition and pyrolytic reaction pathways for formation of product under excess hydrogen reaction environment. The reactions were studied over the temperature range 650 to 900 °C with reaction times of 0.3–2.0 s. A constant feed molar ratio C₂HCl₃: H₂ of 4: 96 was maintained through the whole experiments. Complete decay (99%) of the parent reagent, C₂HCl₃ was observed at temperature near 800 °C with 1 s reaction time. The maximum concentration (28%) of C₂H₂Cl₂ as the primary intermediate product was found at temperature 700 °C where up to 68% decay of C₂HCl₃ occurred. The C₂H₃Cl as highest concentration (19%) of secondary products was detected at 750 °C. The one less chlorinated methane than parent increased with temperature rise subsequently. The number of qualitative and qualitative chlorinated products decreased with increasing temperature. HCl and dechlorinated hydrocarbons such as C₂H₄, C₂H₆, CH₄ and C₂H₂ were the final products at above 800 °C. The almost 95% carbon material balance was given over a wide range of temperatures, and trace amounts of C₆H₆, C₄H₆ and C₂HCl were observed above 800 °C. The decay of reactant, C₂HCl₃ and the hydrodechlorination of intermediate products, resulted from H atom cyclic chain reaction via abstraction and addition replacement reactions. The important pyrolytic reaction pathways to describe the important features of reagent decay, intermediate product distributions and carbon mass balances, based upon thermochemical and kinetic principles, were suggested. The main reaction pathways for formation of major products along with preliminary activation energies and rate constants were given.     

Two-step synthesis process for high-entropy diboride powders

High-entropy diboride powders were produced by a two-step synthesis process consisting of boro/carbothermal reduction followed by solid solution formation. Nominally phase-pure (Hf,Zr,Ti,Ta,Nb)B₂ in a single-phase hexagonal structure had an average particle size of just over 400 nm and contained 0.3 wt% carbon and 0.3 wt% oxygen. The fine particle size was due to the use of high-energy ball milling prior to boro/carbothermal reduction, which led to a relatively low synthesis temperature of 1650°C. Oxygen and carbon contents were minimized by completion of the boro/carbothermal reduction reactions under vacuum. This is the first report of synthesis of a nominally phase pure high-entropy diboride powder from oxides using a two-step process.     

Microwave Absorbing Performances of Silica Matrix Composites Reinforced by Carbon Nanotubes and Carbon Fiber

Multilayered composites consisting of silica, carbon nanotubes (CNTs), and continuous carbon fibers (C_f) were prepared by hot‐pressing technique. Microstructures of different layer presented few pores of the composites. The thermal stability of the composites was analyzed by TG/DTA measurement. After being heat treated at 400°C for 10 h, the composites retained the equivalent shielding property compared to room temperature, and the impedance matching property at material/wave interface was improved slightly. The multilayer CNTs/C_f/silica composites have not only the excellent absorbing properties but also the outstanding thermal stability, and it can be a promising candidate for high‐temperature electromagnetic interference shielding applications.     

Sol-Gel synthesis and characterization of SiC–B4C nano powder

In the present research, SiC–B₄C nano powders were synthesized through sol–gel process in water–solvent–catalyst–dispersant system. In order to evaluate the formation mechanism of the product during sol-gel process, TEM, SEM, DTA/TG, BET, XRD, FTIR and DLS analysis methods were employed. The nanometric size of precursor was controlled by dispersing agents and controlling pH inside the sol. DLS analysis revealed that the particles of the precursor inside the sol were below 10 nm. FTIR results indicated that the (Si–O–B) bonds were formed in the dried gel powder, due to hydrolysis and condensation reactions. DTA analysis confirmed that the synthesis temperature was lower than 1400 °C. XRD results implied the presence of cubic β-SiC and the rhombohedral B₄C phases, which were formed simultaneously in the SiC–B₄C nanopowder. BET analysis indicated a high surface area for the particles of about 171.42 m²/g, and that the surfaces of these particles were meso porous. SEM analysis exhibited that SiC– B₄C particle size was in the range of 20–40 nm with homogenous morphology. Ultimately, the TEM/EDS microstructural analysis showed that B₄C and SiC particles were formed simultaneously and uniformly in the final product.     

Composite W–Cu powders by joint reduction of oxides in combustion mode

Composite powders W–Cu were prepared via joint reduction of WO₃ and CuO oxides with Mg?C combined reducer in a combustion mode by using the method of coupled reactions. Combustion phenomenology and the processes of phase and microstructure formation were investigated by thermocouple and copper- wedge techniques combined with XRD, SEM, and EDS analyses. Thermal conditions of combustion and phase composition and microstructure of products were found to depend on a Mg/C ratio in green mixtures. It was established that the magnesiothermic reaction was preceded by the stage of low-caloric carbothermal reduction. Slow propagation of combustion wave was found to favor the complete reduction of oxides and formation of target W–Cu composite nanopowder.     

Processing of dense high-entropy boride ceramics

Dense (Hf_0.2,Zr_0.2,Ti_0.2,Ta_0.2,Nb_0.2)B₂ high-entropy ceramics with high phase purity were produced by two-step spark plasma sintering of precursor powders synthesized by boro/carbothermal reduction of oxides. The reacted powders had low oxygen (0.404 wt%) and carbon (0.034 wt%) contents and a sub-micron average particle size (∼0.3 μm). Powders were synthesized by optimizing the excess B₄C content of the reaction mixture and densified by a two-step spark plasma sintering process. The relative density increased from 98.9% to 99.9% as the final sintering temperature increased from 2000 °C to 2200 °C. The resulting ceramics were nominally single-phase (Hf,Zr,Ti,Ta,Nb)B₂ with oxygen contents as low as 0.004 wt% and carbon as low as 0.018 wt%. The average grain size increased from 2.3 ± 1.2 μm after densification at 2000 °C to 4.7 ± 1.8 μm after densification at 2100 °C, while significant grain growth occurred during sintering at 2200 °C. The high relative densities, low oxygen and carbon contents, and fine grain sizes achieved in the present study were attributed to the use of synthesized precursor powders with high purity and fine particle size, and the two-step synthesis-densification process. These are the first reported results for dense high-entropy boride ceramics with high purity and fine grain size.     

Phase evolution and microstructure characteristics during the carbothermal reduction of Hf(C,N,O) ceramics derived from a precursor

In this study, nanosized Hf(C,N,O) ceramics were successfully prepared from a novel precursor synthesised by combining HfCl₄ with ethylenediamine and dimethylformamide. Subsequently, the carbothermal reduction of these Hf(C,N,O) ceramics into hafnium carbide was investigated. The Hf(C,N,O) ceramics comprised Hf₂ON₂ and HfO₂ nanocrystals and amorphous carbon. Upon carbothermal reduction, conversion began at 1300 °C, when HfC first appeared, and continued to completion at 1500 °C, resulting in irregularly shaped crystallites measuring 50–150 nm. Upon increasing the dwelling time, the oxides were completely converted into carbides at 1400 °C. Furthermore, nitrogen was introduced into the reaction to catalyse the conversion of oxides into carbides considering the beneficial gas–solid reaction between CO and Hf₂ON₂. We expect that the ceramics prepared in this study will be suitable for the fabrication of high-performance composite ceramics, with properties superior to those of current materials.     

Comparative Study of the Physico-Chemical Properties of Nanocrystalline CuO–ZnO–Al2O3 Prepared from Different Precursors: Hydrogen Production by Vaporeforming of Bioethanol

The physico-chemical and catalytic properties of CuO–ZnO–Al₂O₃, synthesised by sol–gel process (SG), impregnation method (IMP) and a combination of both preparative procedures (ISG), were comparatively studied. Samples were characterised with thermogravimetric-differential thermal analysis (TG–DTA), X-ray powder diffraction (XRD), X-ray photoelectron spectroscopy (XPS) techniques and oxygen chemisorption. XPS study was not consistent with the bulk findings and revealed the presence of Cu²⁺, Cu⁺ and/or Cu⁰ species at the catalysts surface. Surface analysis revealed also that copper enrichment occurred mainly at the surface of SG and IMP solids. The reducibility of the mixed oxides catalysts was always modified with respect to that of pure copper oxides phases and the reduction of CuO was markedly affected by the presence of ZnO–Al₂O₃. Temperature programmed reduction (H₂-TPR) analysis showed that the temperature corresponding to maximum reduction rate of copper oxide was ca. 256 °C for IMP sample and ca. 296 °C for both SG and ISG solids. These latter showing a high resistance to reduction suggest a strong interaction of copper species with ZnO–Al₂O₃, limiting thus copper particles sintering. CuO particle size was found to be ca. 20 nm for both SG and ISG solids and ca. 40 nm for IMP catalysts. Besides, at 300 °C SG and ISG samples showed superior amount of reversible O₂ uptake with respect to IMP solids. Catalytic activity of CuO–ZnO–Al₂O₃ was measured with bio-ethanol steam reforming reaction. SG catalysts exhibited both high selectivity to hydrogen and high stability with time on stream than IMP and ISG catalysts. This was attributed both to the particles size of copper species, their amount on the catalytic surface and to their strong interaction with ZnO–Al₂O₃.     

Synthesis of Nanosized Cesium Ferrite by Precursor and Combustion Method: A Comparative Study

Thermolysis of cesium hexa(carboxylato)ferrate(III) precursors, Cs₃[Fe(L)₆].xH₂O (L = formate, acetate, propionate, butyrate), has been carried out in flowing air atmosphere from ambient temperature to 1000°C. Various physico‐chemical techniques, that is, simultaneous TG‐DTG‐DTA, XRD, TEM, IR, Mössbauer spectroscopy, have been employed to characterize the intermediates and end products. After dehydration, the anhydrous precursors undergo exothermic decomposition to yield various intermediates, that is, cesium carbonate/propionate/butyrate and α‐Fe₂O₃. A subsequent decomposition of these intermediates leads to the formation of cesium ferrite above 800°C. Similar ferrite has been prepared by the combustion method at comparatively lower temperature (600°C) and in less time than that of conventional ceramic method.     

Synthesis of nano-titanium diboride powders by carbothermal reduction

The nano-sized titanium diboride particles were synthesized by carbothermal reduction process. In this study, carbothermal reduction process was used by controlling reaction rate and duration time. TiO₂, B₂O₃ and carbon resin were used as starting materials with a molar composition; TiO₂:B₂O₃:C = 1:2:5. The mixture was placed in a graphite crucible and pushed into a heating zone maintained at 1500 °C and Ar was flown for a period of 20 min. After reaction, the crucible was pulled out from the heating zone to cooling zone of the furnace for the rapid cooling. The average particle size of the agglomerated product was found to be ∼500 nm, which was composed with small primary particles of <100 nm in size. After milling, the large agglomerate was reduced to primary particles.     

Synthesis of SiC whiskers by VLS and VS process

This study investigates the mechanisms of SiC whisker formation in the carbothermal reduction of quartz to SiC in different gas atmospheres. Reduction of quartz by graphite was studied in Ar, H₂, and CH₄–H₂–Ar gas mixture in a laboratory fixed bed reactor. The reduction products were characterised by XRD, SEM and TEM. Whiskers were not formed in the carbothermal reduction of quartz in argon. Two types of SiC whiskers were observed in the carbothermal reduction of quartz in H₂ and CH₄–H₂–Ar gas mixture. In the process of reduction at 1400–1600 °C in H₂ and at 1200–1600 °C in CH₄–H₂–Ar gas mixture, whiskers with hexagonal shape with diameter 100–800 nm and length up to tens of microns were formed by the VLS mechanism under catalytic effect of iron. The whiskers with the characteristics of cylindrical shape and high aspect ratio were synthesized in CH₄–H₂–Ar gas mixture at 1400–1600 °C by VS mechanism.