Preparation of titanium nitride ultrafine powders by sol–gel and microwave carbothermal reduction nitridation methods

Titanium nitride ultrafine powders were prepared from tetrabutyl titanate and sucrose by sol–gel and microwave carbothermal reduction methods. The influences of reaction temperature, molar ratio of Ti to C, addition of crystal seeds and amount of NH₄F on the synthesis of titanium nitride were studied. The results show that excess amount of carbon, addition of crystal seeds and NH₄F plays a positive effect on the preparation of TiN at low temperature. The inceptive formation temperature of TiN ultrafine powders is about 800 °C, and pure TiN can be prepared at 1000 °C. Field emission-scanning electron microscopy (FE-SEM) was used to get the micrograph of the TiN powder, it shows that the size of the powders synthesized at 1000 °C is about 0.1–0.5 μm.     

Synthesis and characterisation of self-assembled SiC nanowires and nanoribbons by using sol–gel carbothermal reduction

ABSTRACT

Hexagonal-shaped 3C-SiC nanowires were grafted onto SiC nanoribbons by a sol–gel technique using ferrocene as catalyst. The nanowire diameter (～200?nm) and the nanoribbon width–thickness ratio (20:1) are uniform along their entire length. Their length is about several tens to several hundreds of micrometres. Meanwhile, single SiC nanostructure (nanowire or nanoribbon) was obtained by adjusting temperature field. A novel cooperative growth mechanism of vapour–liquid–solid and vapour–solid was proposed for the self-assembled SiC nanostructure. The self-assembled SiC nanowires and nanoribbons exhibit two strong broad photoluminescence peaks at wavelengths of about 373 and 471?nm, which are significantly shifted to the blue compared with the reported luminescence of SiC nanowires. This study will pave a way for the controllable synthesis of SiC nanowires and nanoribbons, and provide a simple method to connect them together firmly as potential applications for nanodevices in future.     

Synthesis of Ti(C,N) ultrafine powders by carbothermal reduction of TiO2 derived from sol–gel process

Experimental results demonstrate that TiC_1−XN_X ultrafine powders can be synthesized by the sol–gel process. The factors influencing the powder synthesizing process, such as temperature, C/Ti ratio in raw materials, holding time and flow rate of nitrogen gas, are discussed. TiC_0.5N_0.5 powders with particle sizes less than 100 nm were produced at 1550°C. The microhardness of hot-pressed TiC_0.5N_0.5 samples at 1750°C was 19.6 GPa and the relative density was 98.9%.     

Preparation of alumina ultrafine powders through acrylamide,starch and glutaric dialdehyde mediated sol–gel method

Ultrafine alumina powders were synthesized through acrylamide (AM), starch and glutaric dialdehyde mediated aqueous sol–gel process, respectively. Sol and gel formed gradually during drying of the solution due to polymerization reaction between functional groups. The used AM, starch and glutaric dialdehyde could be applied to form perfect matrix for the entrapment of metal ions, giving rise to ultrafine crystalline alumina particles during heating treatment. Al₂O₃ nanoparticles with γ crystalline phase were obtained via heating treatment of the dried precursor in air. Then the γ phase transforms to α phase and pure α-Al₂O₃ powders could be obtained when the heating temperature was 1473 K. Our results provide a new way of aqueous sol–gel process.     

Microwave assisted synthesis of Dy2Ti2O7 ultrafine powders by sol–gel method

Dy₂Ti₂O₇ ultrafine powders ranging from 100 to 300 nm were successfully synthesized by sol–gel method. Particularly, the dried gel precursor was treated at different temperatures (700–1000 °C) via microwave-heating, which contributed to decreasing the grain size and reaction time. The phase composition and structural evolution of the final products were examined by X-ray diffraction (XRD), scanning electron microscope (SEM) and transmission electron microscope (TEM). Furthermore, the resultant powders were selected to fabricate ceramics and rubber based absobers. Their sinterability, mechanical properties and neutron absorption ability were also studied. Results showed that the highest flexural strength of 99.0 MPa were obtained for Dy₂Ti₂O₇ samples when sintered at 1600 °C for 2 h in air atmosphere. Meanwhile, the neutron absorption rate of Dy₂Ti₂O₇ ceramics and rubber based absorbers could reach 97.39% and 80.00% respectively when the thickness of samples was set as 5.0 mm.     

Preparation of ZrB2–SiC–Al2O3 composites by SHS method with aluminothermic reduction

Ternary composites composed of ZrB₂, SiC, and Al₂O₃ with a broad composition range were produced by PTFE-activated combustion synthesis involving aluminothermic reduction of ZrSiO₄, SiO₂, and ZrO₂. Two solid-state combustion systems were investigated: one comprised ZrSiO₄, SiO₂, Al, B₄C, and C and the other consisted of ZrSiO₄, ZrO₂, Al, B₄C, and B powders. It was required for the powder compacts to adopt 3 wt% PTFE as the reaction promoter to initiate and sustain their combustion reaction. The influence of the gas-phase diffusion transport induced by PTFE on the SHS process was confirmed and the activation mechanisms were proposed. With the assistance of PTFE, self-sustaining combustion was established with a distinct combustion wave characterized by a reaction temperature of 1210–1380 °C and a propagation velocity of 2.27–3.16 mm/s. Based on the XRD analysis, the products were ZrB₂–SiC–Al₂O₃ composites with traces of Si, ZrO₂, or ZrC. The synthesized products exhibited a granular morphology. The particle size of the composite powders was in the range of 1–2 μm.     

Synthesis of SiC precursors by a two-step sol–gel process and their conversion to SiC powders

A two-step sol–gel processing was developed to synthesize phenolic resin–SiO₂ hybrid gels as SiC precursors, with tetraethoxysilane (TEOS) and novolac phenolic resin being the starting materials, and oxalic acid (OA) and hexamethylenetetramine (HMTA) being the catalysts. At the first step TEOS was prehydrolyzed under the catalysis of OA. At the second step HMTA was added to facilitate gelation. The influences of the molar ratio of OA/TEOS and prehydrolysis time on the sol–gel reaction were investigated. There existed an optimum OA/TEOS ratio where prehydrolysis time needed to form transparent gels was the shortest. The increase of temperature could accelerate sol–gel reaction. The dried hybrid gels were yellowish transparent glassy solids, with uniform microstructure composed of nanometer-sized particles. The conversion of the gels to silicon carbide powders was complete when heated at 1650°C for 30 min in vacuum. The oxygen and free carbon were 0.43 and 0.50 wt%, respectively, in the powder produced from the gel prepared with starting resin/TEOS being 0.143 g/ml.     

Preparation of nanocrystalline MoSi2 with enhanced lithium storage by sol–gel and carbonthermal reduction method

Herein nanocrystalline MoSi₂ with enhanced lithium storage was successfully synthesized via a sol-gel and carbonthermal reduction method. Reduction of the gel mixture of Mo precursor and Si precursor by carbon at a desired temperature resulted in the formation of MoSi₂ nanoparticles. The gel mixture was obtained through the hydrolysis of TEOS and ammonium molybdate and the polymerization of hydrolysis products of TEOS. The reducing agent carbon was produced via decarburition of sucrose's hydrolysis products, which have been wrapped in the gel during its formation process. Addition of HCl to the mixed solution controlled the hydrolysis and polymerization rate, and enabled the formation of a gel mixture with homogeneously distributed hydrolysis products of ammonium molybdate, TEOS and sucrose. This achievement likely generates a novel route to synthesize non-oxide compounds such as silicide, carbide through the sol–gel and carbonthermal reduction process. In addition, the as-received MoSi₂ nanoparticles showed considerable activities in the reversible lithiation and delithiation process. When using as an anode for Li-ion batteries, MoSi₂ nanoparticles delivered a specific capacity of 325 mAh g^?1 at C/12 and showed an increasing capacity with cycling.     

Preparation and characterization of Li2TiO3 ceramic pebbles by modification of the water-based sol–gel method

Li₂TiO₃ is considered as one of the best candidates for breeding materials. This article adopted a modification water-based sol–gel method to synthesize nano-Li₂TiO₃ powders, which overcomes the poor phase purity, coarse grain, and inferior crushing strength described in the previous literature. In this paper, the thermal effect of the precursor, the crystal phase, and the morphology of the powders were characterized by thermogravimetric analysis/differential thermal analysis (TG/DTA), X-ray diffraction (XRD), and transmission electron microscopy (TEM) techniques. The nano-structured Li₂TiO₃ powders with good dispersion and an average particle size of 20–50 nm were successfully synthesized at 600°C by controlling PH and hydrolysis rate. Moreover, the phase transition temperature for the monoclinic phase β-Li₂TiO₃ was as low as 600°C, which is lower than 750°C using the traditional solid-state method. Meanwhile, the morphology, porosity, crushing load, and thermal conductivity of ceramic pebbles are characterized systematically by using scanning electron microscope (SEM), mercury injection meter, compression strength equipment, and laser scattering method, respectively. Experimental results showed that the Li₂TiO₃ ceramic pebbles with a sphericity of .98, crush load of 48.4 N, and relative density of 90.03 % were successfully prepared at 1050°C for 2 h. This method will provide new guidance for the preparation of tritium breeders.     

In situ synthesis of ZrB2–ZrC–SiC ultra-high-temperature nanocomposites by a sol–gel process

ABSTRACT

ZrB₂–ZrC–SiC is one of the ultra-high-temperature ceramic composites with excellent properties. In this research, high-purity ZrB₂–ZrC–SiC nanopowders were synthesised using a carbothermal reduction reaction at a relatively low temperature (1370°C) from cost-effective zirconium(IV) chloride by a sol–gel method. The effect of heat treatment temperature on the synthesis of ZrB₂–ZrC–SiC composite powder was studied. X-ray diffractometry results showed that the phases ZrB₂, β-SiC and ZrC were synthesised at 1370°C. The mean crystallite sizes for each of the phases were calculated using the Scherrer method. The specific surface area for the sample calcined at 1370°C was 81.479?m²?g^?1. SEM observation revealed that the particles had a size lower than 250?nm. Backscattered electron image and map analysis with scanning electron microscopy showed that a suitable phase homogeneity was achieved, as confirmed by energy-dispersive X-ray spectroscopy.     

In situ synthesis of Al2O3–SiC powders via molten-salt-assisted aluminum/carbothermal reduction method

Al₂O₃–SiC composite powder (ASCP) was successfully synthesized using a novel molten-salt-assisted aluminum/carbothermal reduction (MS-ACTR) method with silica fume, aluminum powder, and carbon black as raw materials; NaCl–KCl was used as the molten salt medium. The effects of the synthesis temperature and salt-reactant ratio on the phase composition and microstructure were investigated. The results showed that the Al₂O₃–SiC content increased with an increase in molten salt temperature, and the salt–reactant ratio in the range of 1.5:1–2.5:1 had an impact on the fabrication of ASCP. The optimum condition for synthesizing ASCP from NaCl–KCl molten salt consisted of maintaining the temperature at 1573 K for 4 h. The chemical reaction thermodynamics and growth mechanism indicate that the molten salt plays an important role in the formation of SiC whiskers by following the vapor-solid growth mode in the MS-ACTR treatment. This study demonstrates that the addition of molten salt as a reaction medium is a promising approach for synthesizing high-melting-point composite powders at low temperatures.     

Synthesis of TiC–TiB2 composite powders via carbothermal reduction and its reaction mechanism

Titanium carbide–titanium diboride (TiC–TiB₂) composite powders were synthesised through a carbothermal reduction method by using titanium dioxide, boric acid, and different carbon sources (namely, carbon black, sucrose, and glucose) as starting materials. The thermal decomposition behaviour of the precursors was studied by thermogravimetry–differential thermal analyser. Phase compositions and morphologies of the synthesised products were characterised by X-ray diffractometer and scanning electron microscope. When n(Ti):n(B):n(C)?=?1.0:2.5:5.0, the blended stock mainly formed TiB₂ with sucrose or glucose as a carbon source, whereas the stock produced TiC when carbon black was the source. At an optimum reaction temperature, the particles of the powders synthesised from carbon black as a carbon source were the smallest at approximately 100?nm. With increasing amount of boric acid in the precursor, the morphologies of the samples changed into less spherical particles, and more flaky grains and small particles with irregular structures were observed.     

Formation of novel mesoporous TiC microspheres through a sol–gel and carbothermal reduction process

Novel mesoporous TiC microspheres with uniform size are synthesized via a sol–gel combined carbothermal reduction process. A microfluidic aerosol nozzle was used to produce droplets which were subsequently dried into gel microspheres under different conditions. The influence of drying temperatures and sol aging time on the diameters of obtained gel microspheres was investigated. The spherical morphology of TiC spheres can be maintained after a two-step heat treatment. Moreover, the TiC microspheres exhibit a high surface area of 267 m²/g and consist of 30–50 nm nano TiC grains and 4.5 nm pores. This unique nanostructure is directly formed from the carbothermal reduction of non-porous and template-free titania/carbon spheres.     

Formation and characterization of p-type semiconductor CuCrO2 thin films prepared by a sol–gel method

This study adopted the sol–gel method to synthesize p-type semiconductor CuCrO₂ films and analyzed the effects of an annealing treatment, under a controlled argon atmosphere by changing the temperature and time, on the phase transformation, micro- and nano-structure, composition, and semiconductor properties of thin films. In the Cu–Cr–O phase transformation system, CuO, Cr₂O₃, and CuCr₂O₄ were the intermediate phases of the reaction for forming CuCrO₂: in the metastable state reaction process, the composite phases changed into a single phase, CuCrO₂; in the stable-state reaction process of CuCrO₂, carbon elements of precursors were released and eliminated; and finally the optoelectronic properties of the CuCrO₂ thin film were adjusted and changed. The CuCrO₂ thin film possessed cell- and polygon-like shaped microstructures. The carbon content in the CuCrO₂ film decreased, so the copper, chromium, and oxygen contents increased accordingly. The optical band gap of CuCrO₂ thin film increased from 2.81 eV to 3.05 eV, while the resistivity decreased. The nanoscale crystal was identified which also of the delafossite CuCrO₂ structure. Using the sol–gel method to prepare the CuCrO₂ thin films, an appropriate annealing temperature and time were helpful in forming the single-phase CuCrO₂; the decrease of precursor elements in the thin film could enhance the band gap and the conductivity of the material.     

Magnetic behavior of nickel–bismuth ferrite synthesized by a combined sol–gel/thermal method

The bulk NiFe_2?xBi_xO₄ ferrites with various Bi³⁺ concentration (x=0, 0.1, 0.15) were synthesized via sol–gel procedure, starting from nickel, bismuth and iron nitrate powders, followed by the conventional thermal treatment. The structural and magnetic properties of the as-prepared ferrites were studied by means of X-ray diffraction, alternating gradient force magnetometry and Faraday balance. The anisotropy constant was determined by the law of approach to saturation (LAS) model. An increasing Bi³⁺ concentration in NiFe_2?xBi_xO₄ leads to a decrease in the saturation magnetization, Néel temperature and the anisotropy constant of the material.     

Synthesis and characterization of boehmitic alumina coated graphite by sol–gel method

Non-wettability property makes graphite a good protecting material against the molten metal and/or slag. Properties like high oxidation potential between 600 and 1200 °C and non-wettability with water at room temperatures limits the usage of graphite in castable refractory applications. In this study, sol–gel method, which is a relatively cheaper process, was used. Boehmitic sol was obtained by hydrolyzing and peptiziting the alkoxide AIP (aluminum isopropoxide) used as alumina source. Then natural flake graphite was mixed with the boehmitic solution for coating of graphite. At 120 °C boehmitic sol coated graphite was dried and gelled. Then heat threaded at 550 °C for γ-Al₂O₃ transformation of boehmite. Products that obtained from the studies were characterized with FTIR and XRD tests. Alumina coated graphite samples were made by repeating the same steps and TG analysis were made to investigate the oxidation behaviour of the samples. Finally, SEM–EDS analyses were carried out to investigate the microscopic properties of the alumina coated graphite powders.     

Synthesis of ultra-fine tantalum carbide powders by a combinational method of sol–gel and spark plasma sintering

Tantalum carbide (TaC) nanopowders were synthesized by a novel method combining the sol–gel and spark plasma sintering (SPS) processes using tantalum pentachloride (TaCl₅) and phenolic resin as the sources of tantalum (Ta) and carbon (C), respectively. Gels of Ta-containing chelate with good uniformity and high stability were prepared by solution-based processing. The products with the structure of carbon-coated tantalum pentoxide (Ta₂O₅) were obtained after pyrolysis at 800?°C. Further heat treatment by SPS resulted in the fast formation of TaC at a relatively low temperature. The effects of the C/Ta molar ratio in the raw materials and the heat treatment temperature on the prepared powders were investigated. With increase in the C/Ta molar ratio from 3.75 to 4.25, the synthesis temperature, oxygen content and average crystallite size of the TaC powders decreased. Furthermore, the oxygen content of the powders prepared at the C/Ta molar ratio of 4.25 could be reduce by increasing the heat treatment temperature from 1400° to 1600°C, which unfortunately also induced a mean crystallite size increase from 30 to 100?nm. The TaC powders obtained at a comparatively low C/Ta molar ratios of 4.25 at 1500?°C had an average particle size of about 50?nm and a low oxygen content of about 0.43?wt%.     

Preparation of Al2O3–SiC composite powder from kyanite tailings via carbothermal reduction process

ABSTRACT

Al₂O₃–SiC composite powders were prepared from kyanite tailings mixed with 20% excess carbon coke via carbothermal reduction (CR) reaction. The optimised synthesis condition for synthesising Al₂O₃–SiC composite powders was at 1600°C for 6?h. The equilibrium relationship curves of the condensed phases were presented and the temperature dependence of the phase composition was also studied. The results show that irregular Al₂O₃ and SiC grains first formed at 1500°C, and the elements C, O, Al, and Si randomly distributed in the each crystal particles. The amount of Al₂O₃–SiC composites increased with the increasing synthesis temperature and reaction time. Finally, Al₂O₃–SiC composite bulk materials were further prepared by pressureless sintering using the synthesised Al₂O₃–SiC composite powders as raw materials, and their mechanical properties were investigated in detail. All these results indicate that the CR method can offer a niche application for the development of kyanite tailings.     

Synthesis of nanosized zirconium carbide powders by a combinational method of sol–gel and pulse current heating

Zirconium carbide nanopowders were synthesized by a novel method combining the advantages of sol–gel method and rapid synthesis using pulse current heating. The core-shelled structure of ZrO₂/C mixture was obtained during the sol–gel process, and further heat treatment in SPS led to the fast formation of ZrC. The particle size of ZrO₂ played an important role in the synthesis of nanosized ZrC powders. In addition, the coalescence and grain growth of ZrC particles could be also limited due to the fast heating rate. As a result, the reactions were thoroughly completed at a relatively low temperature and ZrC nanopowders of 60–100 nm were obtained. The corresponding powders also had low oxygen content (∼0.64 wt%) and residual carbon content (∼0.27 wt%). Additive-free ZrC powders could be sintered to ∼99% relative density with an average grain size of 0.8 μm at low temperature of 1750 °C.