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1.
《Ceramics International》2020,46(6):7099-7108
ZrB2–ZrC–SiC nanopowders with uniform phase distribution were prepared from cost-effective ZrOCl2·8H2O by a simple sol-gel method. The synthesis route, ceramization mechanism and morphology evolution of the nanopowders were investigated. ZrB2–ZrC–SiC ceramic precursor can be successfully obtained through hydrolysis and condensation reactions between the raw materials. Pyrolysis of the precursor was completed at 650 °C, and it produced ZrO2, SiO2, B2O3 and amorphous carbon with a yield of 39% at 1300 °C. By heat-treated at 1500 °C for 2 h, highly crystallized ZrB2–ZrC–SiC ceramics with narrow size distribution were obtained. With the holding time of 2 h, both the crystal size and the particle size can be refined. Further prolonging the holding time can lead to serious particles coarsening. Studies on the microstructure evolution of the generated carbon during the ceramic conversion demonstrates the negative effect of the ceramic formation on the structure order improvement of the carbon, due to the large amount of defects generated in it by the boro/carbothermal reduction reactions. 相似文献
2.
《Journal of the European Ceramic Society》2019,39(11):3283-3291
In this study, two composition ZrB2–ZrC–WB composites were synthesized by reactive hot-pressing of Zr + B4C + WC powder mixtures at 1900 °C. The microstructure of the resulting composites was characterized by a combination of scanning electron microscopy and X-ray diffraction. It is seen that highly-dense ZrB2–ZrC–WB composites with a homogenous fine-microstructure were obtained after the sintering. The mechanical behavior of the composites was evaluated using by testing under four-point bend testing at room and high temperatures. The results show that the high-temperature strength of the ZrB2–ZrC–WB composites was substantially improved, compared to ZrB2–ZrC-based composites without WB. In addition, the elastic properties, electrical conductivity, hardness and fracture toughness of the composites were measured at room temperature. The results reveal that these properties were comparable to those of ZrB2–ZrC-based composites without WB. 相似文献
3.
《Journal of the European Ceramic Society》2000,20(11):1853-1857
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. 相似文献
4.
P. Sarin P.E. Driemeyer R.P. Haggerty D.-K. Kim J.L. Bell Z.D. Apostolov W.M. Kriven 《Journal of the European Ceramic Society》2010,30(11):2375-2386
High temperature oxidation of ZrB2 and the effect of SiC on controlling the oxidation of ZrB2 in ZrB2–SiC composites were studied in situ, in air, using X-ray diffraction. Oxidation was studied by quantitatively analyzing the crystalline phase changes in the samples, both non-isothermally, as a function of temperature, up to ~1650 °C, as well as isothermally, as a function of time, at ~1300 °C. During the non-isothermal studies, the formation and transformation of intermediate crystalline phases of ZrO2 were also observed. The change in SiC content, during isothermal oxidation studies of ZrB2–SiC composites, was similar in the examined temperature range, regardless of sample microstructure and composition. Higher SiC content, however, markedly retarded the oxidation rate of the ZrB2 phase in the composites. A novel approach to quantify the extent of oxidation by estimating the thickness of the oxidation layer formed during oxidation of ZrB2 and ZrB2–SiC composites, based on fractional conversion of ZrB2 to ZrO2 in situ, is presented. 相似文献
5.
In situ filling of nanomaterials into polymers facilitates the dispersion of the nanofillers and their interface combination with the matrices, and reduces the agglomeration encountered in the nanocomposites prepared by a mechanical mixing method. Polytetrafluoroethylene (PTFE) nanocomposites filled with SiO2 nanospheres (SNS) were fabricated by an in situ sol–gel method in this paper. The SNS in situ filled was highly dispersed in PTFE and showed an excellent combination with the matrix, and the fabricated SNS/PTFE nanocomposites were found a pronounced improvement in stiffness, hardness, glass transition temperature, and hydrophobicity in comparison with the pristine PTFE and the ones prepared by mechanical mixing with the same content. Furthermore, significantly reduced coefficients of friction and volume wear rates were observed on the SNS/PTFE nanocomposites prepared by in situ sol–gel. An operating temperature high up to 200°C and very low volume wear rate were accessible on the optimized SNS/PTFE nanocomposite by in situ filling. The methodology, in situ filling of nanofillers into matrices, might pave a way to prepare nanocomposites with excellent mechanical, thermal, and tribological properties. 相似文献
6.
《Journal of the European Ceramic Society》2014,34(12):2875-2883
A technique for densifying ultra high temperature ceramic composites while minimising grain growth is reported. As-purchased ZrB2 powder was treated with a zirconia-carbon sol–gel coating. Carbothermal reduction at 1450 °C produced 100–200 nm crystalline ZrC particles attached on the surface of ZrB2 powders. The densification behaviour of the sol–gel coated powder was compared with both the as-purchased ZrB2 and a compositionally similar ZrB2–ZrC mixture. All three samples were densified by spark plasma sintering (SPS). The ZrB2 reference sample was slow to densify until 1800 °C and was not fully dense even at 2000 °C, while the sol–gel modified ZrB2 powder completed densification by 1800 °C. The process was studied by ram displacement data, gas evolution, SEM, and XRD. The sol–gel coated nanoparticles on the ZrB2 powder played a number of important roles in sintering, facilitating superior densification by carbothermal reduction, nanoparticle coalescence and solid-state diffusion, and controlling grain growth and pore removal by Zener pinning. The sol–gel surface modification is a promising technique to develop ultra-high temperature ceramic composites with high density and minimum grain growth. 相似文献
7.
Sanjay S. Latthe Hiroaki Imai V. Ganesan A. Venkateswara Rao 《Journal of Porous Materials》2010,17(5):565-571
Wettability of solid surfaces is a crucial concern in our daily life as well as in engineering and science. The present research
work describes the room temperature (27 °C) synthesis of adherent and water repellent silica films on glass substrates using
vinyltrimethoxysilane (VTMS) as a hydrophobic reagent by a single step sol–gel process. The silica sol was prepared by keeping
the molar ratio of tetraethoxysilane (TEOS), methanol (MeOH), water (H2O) constant at 1:14.69:5, respectively, with 0.01 M NH4F throughout the experiments and the VTMS/TEOS molar ratio (M) was varied from 0 to 0.97. The effects of M on the surface structure and hydrophobicity have been researched. The static water contact angle as high as 144° and water
sliding angle as low as 14° was obtained for silica film prepared from M = 0.97. The hydrophobic silica films retained their hydrophobicity up to a temperature of 255 °C and above this temperature
the films became superhydrophilic. The prepared silica films were characterized by Scanning Electron Microscopy (SEM), Atomic
Force Microscopy (AFM), Fourier Transform Infrared (FT-IR) spectroscopy, humidity test and static and dynamic water contact
angle measurements. 相似文献
8.
9.
《Journal of the European Ceramic Society》2014,34(13):3139-3149
ZrB2 powder was coated with 5% ZrOC sol–gel precursor and sintered by SPS. Relative densities >98% were achieved at 1800 °C with minimal grain growth and an intergranular phase of ZrC. Carbon content in the precursor determined the type of reinforcing phase and porosity of the sintered composites. XRD, SEM and EDS studies indicated that carbon deficiency resulted in ZrO2 retention, improving ZrB2 densification with oxide particle reinforcement. Excess carbon resulted in ZrC formation as the reinforcing phase, but could yield porosity and residual carbon at grain boundaries. These two types of ZrB2 composites displayed different densification and microstructural evolution that explain their contrasting properties. In the extreme oxidative environment of oxyacetylene ablation, the composites with ZrC-C maintained superior leading edge geometry; whereas for mechanical strength, a bias towards the residual ZrO2 content was beneficial. This highlighted the sensitivity of processing carbon-precursors in the initial sol–gel process and the carbon content in ZrB2-based composite systems. 相似文献
10.
《Ceramics International》2020,46(1):156-164
Spark plasma sintering (SPS) route was employed for preparation of quadruplet ZrB2–SiC–ZrC–Cf ultrahigh temperature ceramic matrix composites (UHTCMC). Zirconium diboride and silicon carbide powders with a constant ZrB2:SiC volume ratio of 4:1 were selected as the baseline. Mixtures of ZrB2–SiC were co-reinforced with zirconium carbide (ZrC: 0–10 vol%) and carbon fiber (Cf: 0–5 vol%), taking into account a constant ratio of 2:1 for ZrC:Cf components. The sintered composite samples, processed at 1800 °C for 5 min and 30 MPa punch press under vacuumed atmosphere, were characterized by densitometry, field emission scanning electron microscopy, energy dispersive spectroscopy, X-ray diffractometry as well as mechanical tests such as hardness and flexural strength measurements. The results verified that the composite co-reinforced with 5 vol% ZrC and 2.5 vol% Cf had the optimal characteristics, i.e., it reached a relative density of 99.6%, a hardness of 18 GPa and a flexural strength of 565 MPa. 相似文献
11.
Cédric Alexis Clarence Charnay Vincent Lapinte Jean-Jacques Robin 《Progress in Organic Coatings》2013
Hybrid films prepared from TEOS and polyoxazolines (Si–POx–Si) crosslinking agents were coated on different substrates in order to modify their surface properties. The film cohesion and adhesion on substrates were expected through the hydrogen bonding of the polyoxazoline crosslinked network. Low molecular-weight α,ω-unsaturated polyoxazolines (DA-PMOx)s were synthesized by a one step cationic ring-opening polymerization (CROP) of 2-methyl-2-oxazoline (MOx) with a good control over the molecular weight. Based on double thiol-ene coupling (d-TEC) a post-functionalization of DA-PMOx end chains gave in good yield polyoxazoline cross linker (Si–POx–Si). Glass and various polymer substrates (PP, PEI, POM, etc.) were spin coated by the organic–inorganic hybrid films through sol–gel process. AFM, SEM, visible reflectance spectroscopy and contact angle experiments allowed the full characterization of targeted surfaces and demonstrated the efficiency of the polyoxazoline coating. 相似文献
12.
《Ceramics International》2015,41(6):7823-7829
A combined sol–gel and microwave boro/carbothermal reduction technique was investigated and used to synthesize ultrafine ZrB2–SiC composite powders from raw starting materials of zirconium oxychloride, boric acid, tetraethoxysilane and glucose. The effects of reaction temperature, molar ratios of n(B)/n(Zr) and n(C)/n(Zr+Si) on the synthesis of ultrafine ZrB2–SiC composite powders were studied. The results showed that the optimum molar ratios of n(B)/n(Zr) and n(C)/n(Zr+Si) for the preparation of phase pure ultrafine ZrB2–SiC composite powders were 2.5 and 8.0, respectively, and the firing temperature required was 1300 °C. This temperature was 200 °C lower than that require by using the conventional boro/carbothermal reduction method. Microstructures and phase morphologies of as-prepared ultrafine ZrB2–SiC composite powders were examined by field emission-scanning electron microscopy (FE-SEM) and transmission electron microscope (TEM), showing that SiC grains were formed evenly among the ZrB2 grains, and the grain sizes of ZrB2 in the samples prepared at 1300 °C for 3 h were about 1–2 μm. The average crystalline sizes of these two phases in the as-prepared samples were calculated by using the Scherrer equation as about 58 and 27 nm, respectively. 相似文献
13.
《Journal of the European Ceramic Society》2014,34(15):3479-3487
Ceramic foam materials are commonly used for various applications, including catalyst supports or solar receivers. SiC foams are good candidates for the latter application as solar receivers. Its efficiency is directly related to the geometry, which can be evidenced by X-ray microtomography, and optical properties of the receiver. A promising route to add functionalities with homogenous and adhering oxide coatings onto complex SiC foams in a single step process is proposed. This oxide synthetic process is derived from the Pechini method. Foams are fully impregnated by precursor sols with a controlled viscosity making a thin and totally covering coating. 相似文献
14.
The present work reports a study on the effect that a peptiser species has on the crystallisation of alumina gel produced by a sol–gel process to help develop a method for producing α-Al2O3 at low temperature. The white precipitate of aluminium hydroxide, which was prepared with a homogeneous precipitation method using aluminium nitrate and urea in an aqueous solution, was peptised using various peptisers at room temperature to form a transparent alumina sol. The alumina gel obtained from the alumina sol, which was produced using formic acid as the peptiser, was most dominantly crystallised into α-Al2O3 by annealing at 900°C. The optimal [peptiser]/[Al3+] (P/A) molar ratio for the crystallisation into α-Al2O3 was 0.2. The alumina gel began to crystallise into α-Al2O3 with annealing at as low as 500°C when formic acid and a P/A ratio of 0.2 were used. 相似文献
15.
《Journal of the European Ceramic Society》2014,34(2):237-247
In the present work, mullite-bonded porous SiC ceramics were fabricated using reaction bonding techniques. The morphologies, phase composition, open porosity, pore size distribution and mechanical strength of porous ceramics were examined as a function of alumina sources (calcined nano-sized alumina powder and alumina sol prepared from hydrolysis of aluminum isopropoxide) and contents. It was found that the addition of alumina in powder form effectively enhanced the strength and decreased the porosity. In contrast, when alumina was added in sol form, a reverse effect was observed. Moreover, it was revealed that when a portion of calcined alumina was replaced by alumina sol, the mechanical properties improved significantly (more than 30%) as well as porosity compared to the traditional method. Pore size distribution analysis showed that the dispersion of nanosize alumina powder and SiC micro-particles in alumina sol is strongly improved compared to mixing in ethanol. 相似文献
16.
Xin Qi Xudong Luo Ling Zhang Shaoyang Wang Jialiang Zhao 《International Journal of Applied Ceramic Technology》2022,19(5):2723-2733
Adding SiC directly to MgO–C refractories possesses the disadvantages of low dispersion and interfacial bonding strength. Herein, the in situ synthesized SiC was introduced into the MgO–SiC–C refractories to maintain the original excellent performance of MgO–C refractories and reduce the carbon dissolution in molten steel. With the increase of Si and C content in raw materials, the morphology of SiC changed from whisker to network, whose growth mechanism was vapor–solid and vapor–liquid–solid. The network structure and uniform distribution of SiC improved the thermal shock resistance of MgO–SiC–C refractories. According to the analysis of molecular dynamics simulation by Materials Studio software, SiC strengthened the relationship between periclase and graphite to enhance the structure of the compound. 相似文献
17.
Rosa Taurino Elena Fabbri Doris Pospiech Alla Synytska Massimo Messori 《Progress in Organic Coatings》2014
Organic–inorganic hybrid coatings on glass substrates with superhydrophobic properties and with improved scratch resistance were obtained by means of applying a multilayer approach including multiple sol–gel processes. The coatings exhibited a water contact angle (WCA) higher than 150°. Ultraviolet (UV)-curable vinyl ester resins and vinyltriethoxysilane (VTEOS) as coupling agent were employed to increase the adhesion between substrate and the inorganic layers. The surfaces were characterized by means of dynamic contact angle and roughness measurements. Indeed, the occurrence of superhydrophobic behavior was observed. The scratch resistance of the hybrid coatings was tested to evaluate the adhesion of the coatings to the glass substrate. The proposed preparation method for scratch resistant, mechanically stable, superhydrophobic coatings is simple and can be applied on large areas of different kinds of substrates. 相似文献
18.
《Ceramics International》2016,42(8):9488-9495
In this work we successfully obtained ceria–zirconia–alumina samples by the sol–gel technique. These materials were prepared under acidic or basic conditions, using either nitric acid or ammonium hydroxide as the catalyst. A design-of-experiments approach was used in order to optimize the specific surface area, pore structure, and thermal stability of the prepared samples. It was observed that the addition of ceria and zirconia did not affect the formation of γ-Al2O3. The highest surface areas and smallest pore sizes were observed for specimens obtained under acidic conditions and with low to intermediate concentrations of cerium and water. The increase of the heat treatment temperature from 600 °C to 1000 °C led to both a decrease of the surface area and an increase of the mean pore size. This behavior is due to the coalescence of pores upon calcination. Samples with a high concentration of ceria showed an expressive thermal instability at high temperatures. On the other hand, the addition of zirconia increased the thermal stability of these materials. In general, samples with improved thermal stabilities were obtained under basic conditions. 相似文献
19.
Trirutile-structure MgTa2O6 ceramics were prepared by aqueous sol–gel method and microwave dielectric properties were investigated. Highly reactive nanosized MgTa2O6 powders were successfully synthesized at 500 °C in oxygen atmosphere with particle sizes of 20–40 nm. The evolution of phase formation was detected by DTA–TG and XRD. Sintering characteristic and microwave dielectric properties of MgTa2O6 ceramics were studied at different temperatures ranging from 1100 to 1300 °C. With the increase of sintering temperature, density, ?r and Q · f values increased and saturated at 1200 °C with excellent microwave properties of ?r ~ 30.1, Q · f ~ 57,300 GHz and τf ~ 29 ppm/°C. The sintering temperature of MgTa2O6 ceramics was significantly reduced by aqueous sol–gel process compared to conventional solid-state method. 相似文献
20.
In recent years, polyimide (PI) hybrid materials have received considerable attention owing to the dramatic enhancements over their pristine state in thermal stabilities, mechanical properties and other special features by introducing only a small fraction of inorganic additives. In this investigation, hybrid nanocomposite films of titanium dioxide (TiO2) in PI were successfully fabricated by an in situ sol–gel process starting from tetraethyl orthotitanate in the solution of poly(amic acid) in N,N-dimethylacetamide. Neat PI was prepared from the polymerization of 2-(3,5-diaminophenyl)-benzimidazole and pyromellitic dianhydride. The hybrid films were obtained by the hydrolysis–polycondensation of moisture-sensitive titania precursor in poly(amic acid) solution, followed by the elimination of solvents and imidization process. The chelating agent, acetylacetone, was used to reduce the gelation rate of titanium alkoxide. The complete imidization temperature of the poly(amic acid) was delayed; furthermore, the thermal stability of PI was enhanced through the incorporation of the inorganic moieties in the hybrid materials. The chemical and morphological structures of the hybrid materials were characterized by Fourier transform infrared spectroscopy, X-ray diffraction, field emission scanning electron microscopy and transmission electron microscopy. The results show that the TiO2 particles are well dispersed in the PI matrix with particle size between 15 and 30 nm in diameter. 相似文献