Electrochemical corrosion of silicon carbide ceramics in H2SO4

Sintered silicon carbide materials have found widespread use due to their high corrosion stability. This corrosion stability can be affected by electrochemical processes. Electrochemical corrosion experiments conducted on a SSiC material in H₂SO₄ at different voltages and subsequent detailed investigation of the formed surfaces was carried out. The first time a systematic local measurement of the thickness of the oxide layers was carried out. The measurements revealed the formation of SiO₂ surface layers with thickness up to 125 μm. The measured values also showed a strong deviation from grain to grain. The thickness of the layers does not correlate with the crystallographic orientation of the grains or the SiC-polytypes. The data indicate that the behaviour is caused by the variation of the resistivity of the grain boundaries. The measured thicknesses as a function of the electrical charge transferred indicate that the electrochemical oxidation results in the SiO₂ and carbon dioxide.     

In situ growth of TiC whiskers in Al2O3 matrix for ceramic machine tools

TiC whiskers have been synthesized via a carbothermal reduction technique in an α-Al₂O₃ matrix within a temperature range of 1380–1580 °C in an argon atmosphere. The raw materials consist of TiO₂, carbon, nickel and NaCl. Various mixing procedures and reaction temperatures were used. The yield of the whiskers is mainly dependent on the mixing procedures and the morphology of the synthesized composite powders is mainly dependent on the synthesis temperatures. The majority of the synthesized whiskers display an ideal aspect ratio of 10–30 with a diameter of 1–3 μm. No significant influence on the growth of the TiC whiskers by the present of the Al₂O₃ matrix powder can be noted.     

The effect of Al2O3-MgO additives on the microstructure of spark plasma sintered silicon nitride

It was shown that spark plasma sintered silicon nitride with a high content of Al₂O₃ and MgO consists of α and β silicon nitride, the main phase being α silicon nitride. The increase in the sintering temperature did not lead to significant changes in the phase composition as occurs in silicon nitride added with Al₂O₃-Y₂O₃. It was found that increasing in SPS temperature above 1650 °C leads to an insignificant increase in the density. A complex shaped equiaxed grain microstructure was shown in both cases. However, doping with aluminum and yttrium oxides allows obtaining an elongated grain microstructure. The Hall-Petch effect was observed for the microhardness of the investigated SPSed silicon nitride. The microhardness of the described ceramics was rather high and more than 1900 HV compared to the pressureless sintered at 1800 °C silicon nitride with the microhardness equal to 1511 HV.     

Improved thermal shock performance of Al2O3-C refractories due to nanoscaled additives

Carbon bonded alumina refractories with approximately 30 wt.-% residual carbon after coking are widely used as functional components such as submerged entry nozzles, monobloc stoppers and ladle shrouds in steel casting operations. Compositions with less residual carbon after coking based on nanoscaled magnesium aluminate spinel (MgAl₂O₄), alumina nanosheets (α-Al₂O₃) and carbon nanotubes (CNTs) either as single additives or combinations have been investigated according to their physical, mechanical and thermo-mechanical properties. The combination of nanoscaled powders based on carbon nanotubes and alumina nanosheets lead to superior thermal shock performance with approximately 30% less residual carbon in comparison to commercial available material compositions.     

Microstructure evolutions of SiC whiskers at high temperature and its effects on silicon carbide ceramics

SiC whisker with a single-crystal structure is promising in enhancing the strength and toughness of advanced structural ceramics, owing to its excellent properties. However, studies on its microstructure evolution at high temperature (>2000 °C) are scarce. Herein, SiC whiskers were calcined at 2100 °C, and XRD, SEM, and TEM were employed to analyze microstructure evolutions. Compared with raw whiskers, XRD results indicated serious annihilation of stacking faults after calcination. The annihilation led to the fracture of whiskers and the formation of β-SiC grains, and then partial grains underwent the phase transformation to form hexagonal prism and triangular prism α-SiC grains with diameters of about 10 μm, according to SEM and TEM results. Furthermore, SiC ceramics containing different whisker contents were innovatively fabricated by pressureless solid-state sintering. The flexural strength and fracture toughness of SiC ceramic containing 10 vol% whiskers were 540 MPa and 5.1 MPa m^0.5, resulting in 38% and 11% higher values than those without whiskers, respectively.     

Electrochemical corrosion of silicon carbide ceramics in sodium hydroxide

Sintered silicon carbide ceramics have found widespread use due to their high corrosion stability. This corrosion stability can be affected by electrochemical processes. Electrochemical corrosion experiments conducted on an SSiC material in NaOH at different voltages and subsequent detailed investigation of the formed surfaces were carried out. Systematic local measurement of the corrosion rate was carried out using the AFM technique. The results revealed the recession of the SiC grain surfaces under anodic electrochemical loading, with the extents differing strongly from grain to grain. The recession rates were not found to correlate with the SiC grain orientations or polytypes. Rather, the data and the observed microstructure indicated that the behaviour was caused by variations in the resistivities of the grain boundaries.     

Al2O3/Cu-O composites fabricated by pressureless infiltration of paper-derived Al2O3 porous preforms

Al₂O₃/Cu-O composites were fabricated from the paper-derived alumina matrix infiltrated with a Cu-3.2?wt% O alloy. Paper-derived alumina preforms with an open porosity ranging from ～ 14 to ～ 25?vol% were prepared by sintering of alumina-loaded preceramic papers at 1600?°C for 4?h. Pressureless infiltration at 1320?°C for 4?h of the preforms with Cu–O alloy resulted in the nearly dense materials with good mechanical and electrical properties, e.g. fracture toughness up to 6?MPa?m^0.5, four-point-bending strength up to 342?MPa, Young's modulus up to 281?GPa and electrical conductivity up to 2?MS/m depending on the volume fraction of copper alloy in the composites. The technological capability of this approach was demonstrated using prototypes in various engineering fields fabricated by lamination, corrugating and Laminated Object Manufacturing (LOM) methods.     

Microstructure and microwave dielectric properties of Al2O3 added Li2ZnTi3O8 ceramics

Recently, the rapid development of advanced communication systems increasingly strongly demands high-performance microwave dielectric ceramics in microwave circuits. Among them, Li₂ZnTi₃O₈ ceramics have been one of the most widely investigated species, due to its high quality factor, moderate firing conditions and low cost. However, the dielectric constants of the already reported Li₂ZnTi₃O₈ ceramics are fixed in a narrow range, limiting their wider applications. To adjust the dielectric constant of the Li₂ZnTi₃O₈ based ceramics, in this work Li₂ZnTi₃O₈ ceramics added with different amounts of Al₂O₃ (0–8?wt%) were prepared by conventional solid-state reaction. The microstructure and microwave dielectric properties of the samples were investigated. Due to the addition of Al₂O₃, the sintering temperature of the ceramics would be increased somewhat. Some Al³⁺ ions could substitute for Ti⁴⁺ ions in Li₂ZnTi₃O₈, and the added Al₂O₃ would react with ZnO to produce a ZnAl₂O₄ phase accompanying with the formation of TiO₂ phase, which would inhibit the growth of Li₂ZnTi₃O₈ grains. The dielectric constant of the finally obtained ceramics would be reduced from 26.2 to 17.9, although the quality factors of the obtained ceramics would decrease somewhat and the temperature coefficient of resonant frequency would deviate further from zero.     

以水玻璃为硅源喷雾干燥制备SiC前驱体

以廉价的水玻璃和炭黑为原料,通过制备均匀混合的前驱体,利用碳热还原反应合成出超细SiC粉体。比较研究了前驱体喷雾干燥与搅拌干燥两种制备方法对体系碳热还原反应的影响,重点考察了喷雾干燥制备过程中主要工艺参数对前驱粉体密度及收率的影响。结果表明:喷雾制备的前驱体具有更高的反应活性,在1550℃下反应2h就可使SiO2转化率达到89.4%,搅拌干燥制备的前驱体转化率只有65.2%。喷雾干燥过程中,适当提高进口温度、喷雾头转速并降低料液的固体含量,可获得高收率、高产率的前驱粉体,利于整个制备工艺生产效率及产率的提高。     

Recent progress in the pore size control of silicon carbide ceramic membranes

The demand for separation and purification applications under harsh conditions has grown strongly in recent years. Silicon carbide (SiC) ceramic membranes have broad prospects in this aspect due to their unique characteristics, but its pore size control is a crucial problem. Therefore, it is of great significance to develop simple and feasible methods for precise control of the pore size of SiC membranes to improve membrane selectivity and expand their application range. This review describes the pore formation process in the preparation of SiC membranes, focusing on the selection of SiC particles, sintering temperature, sacrificial template, sintering aids, oxidation process and other factors affecting the pore size and analysis. Finally, the control of SiC membrane pore size is summarized and the outlook is proposed.     

Microstructure design and preparation of Al2O3/TiC/TiN micro-nano-composite ceramic tool materials based on properties prediction with finite element method

The objective of this paper was to improve the accuracy of semi-empirical method used to design ceramic cutting tool materials. The mechanical properties were predicted by employing finite element model of material microstructure, so as to design microstructure and prepare new ceramic materials. Based on the Voronoi and randomness method, the microstructure model representing the complexity and randomness of micro-nano-composite ceramic material microstructure was established. Combining the representative volume element (RVE) of ceramic material microstructure with mechanical tests, the simulations of mechanical tests were conducted to acquire the flexure strength, fracture toughness and hardness of materials. The microstructure models with various parameters were designed and the material properties were predicted to determine the optimal microstructure parameters. Then, The ceramic cutting tool materials possessing the optimal microstructure parameters were developed for machining ultra-high strength steels. The results showed that the mechanical properties of ceramic materials first improved and then declined as the nano-scale TiC volume fraction increased. To obtain the best comprehensive mechanical properties, the contents of micro-scale TiN, TiC and nano-scale TiC were set as 20%, 10% and 10%, respectively. The prepared ceramic materials possessed the flexure strength of 881.4 MPa, the fracture toughness of 7.8 MPa m^1/2, and the Vickers hardness of 20.8 GPa. This research is beneficial to the development of cutting tool design theory and the improvement of the tool life.     

Seamless joining of silicon carbide ceramics through an sacrificial interlayer of Dy3Si2C2

A ternary carbide Dy₃Si₂C₂ coating was fabricated on the surface of SiC through a molten salt technique. Using the Dy₃Si₂C₂ coating as the joining interlayer, seamless joining of SiC ceramic was achieved at temperature as low as 1500 °C. Phase diagram calculation indicates that seamless joining was achieved by the formation of liquid phase at the interface between Dy₃Si₂C₂ and SiC, which was squeezed out under pressure and continuously consumed by the joining interlayer. This work implies the great potential of the family of ternary rare-earth metal carbide Re₃Si₂C₂ (Re = Y, La-Nd) as the sacrificial interlayer for high-quality SiC joining.     

Fabrication of carbon-coated boron carbide particle and its role in the reaction bonding of boron carbide by silicon infiltration

To tackle the dissolution problem of boron carbide particles in silicon infiltration process, carbon-coated boron carbide particles were fabricated for the preparation of the reaction-bonded boron carbide composites. The carbon coating can effectively protect the boron carbide from reacting with liquid Si and their dissolution, thus maintaining the irregular shape of boron carbide particles and preventing the growth of boron carbide particles and reaction formed SiC regions. Furthermore, the nano-SiC particles, originated from the reaction of the carbon coating and the infiltrated Si, uniformly coated on the surfaces of boron carbide particles, thus forming a ceramic skeleton of the nano-SiC particles-coated and -bonded boron carbide particles. The Vickers hardness, flexural strength and fracture toughness of the composites can be increased by 26 %, 45 %, and 37 % respectively, by using carbon-coated boron carbide particles as raw materials.     

Preparation of Al2O3-coated expanded graphite with enhanced hydrophilicity and oxidation resistance

Expanded graphite (EG), as a new kind of functional carbon-based material, is a vital supporting material and heat transfer enhancer for preparing highly conductive form-stable composite phase change materials (PCMs). However, the hydrophobic nature of EG makes it difficult to incorporate with inorganic PCMs. In this work, we intended to solve this drawback and a modified EG named Al₂O₃-coated EG which was characterized by enhanced hydrophilicity was developed via a heterogeneous nucleation technique and subsequent heat treatment. Experiments found that the Al₂O₃ layer on the surface of EG was uniform and essentially amorphous, and was well-bonded to EG via chemical interactions between oxygen atoms from Al₂O₃ and carbon atoms from EG. The hydrophilicity and oxidation resistance of Al₂O₃-coated EG could be enhanced by increasing the amount of Al₂O₃. Most importantly, compared with EG, the water contact angle of Al₂O₃-coated EG dropped from 90.7° to 33.9° when only 4.4?wt% Al₂O₃ was used, indicating that the hydrophilicity of EG could be greatly enhanced by low cost. Moreover, molecular dynamics (MD) simulation of the hydrophilicity of EG and Al₂O₃-coated EG proved that the preparation of Al₂O₃-coated EG was an efficient and feasible method to improve the hydrophilicity of EG.     

Mechanical properties of SiCp/Al2O3 ceramic matrix composites prepared by directed oxidation of an aluminum alloy

Silicon carbide particulate reinforced alumina matrix composites were fabricated using DIrected Metal OXidation (DIMOX) process. Continuous oxidation of an Al-Si-Mg-Zn alloy with appropriate dopants along with a preform of silicon carbide has led to the formation of alumina matrix surrounding silicon carbide particulates. SiC_p/Al₂O₃ ceramic matrix composites fabricated by the DIMOX process, possess enhanced mechanical properties such as flexural strength, fracture toughness and wear resistance, all at an affordable cost of fabrication. SiC_p/Al₂O₃ matrix composites were investigated for mechanical properties such as flexural strength, fracture toughness and hardness; the composite specimens were evaluated using standard procedures recommended by the ASTM. The SiC_p/Al₂O₃ ceramic matrix composites with SiC volume fractions from 0.35 to 0.43 were found to possess average bend strength in range 158-230 MPa and fracture toughness was found to be in range of 5.61-4.01 MPa√m. The specimen fractured under three-point loading as observed under scanning electron microscope was found to fail in brittle manner being the dominant mode. Further the composites were found to possess lower levels of porosity, among those prepared by DIMOX process.     

High temperature mechanical behavior of Al2O3-MgO-C refractories for steelmaking use

The advantages of Al₂O₃-MgO-C (AMC) refractories are achieved mainly by the incorporation of graphite and the formation of spinel by solid reaction between alumina and magnesia. Regarding other members of oxide-C refractories (such as MgO-C bricks) and others properties (such as the slag corrosion resistance or the PLC), the information about the mechanical behavior of this type of refractories is scarce. In this work, the mechanical behavior of commercial AMC brick used in steelmaking ladles was studied by stress-strain curves in compression at RT and 1000 °C (nitrogen atmosphere). Before the mechanical testing, a comprehensive characterization of AMC materials was performed by several techniques: XRD, DTA/TGA, SEM/EDS, aggregate size distribution analysis and densities, porosities and thermal expansion measurements. Mechanical parameters such as fracture strength and strain, yield stress and Young modulus were determined together with the main characteristics of the fracture. In order to study the transformations occurred during the stay at high temperature, the specimens tested at 1000 °C were analyzed by the same techniques used for the as-received bricks characterization (with the exception of the thermal expansion analysis). The AMC refractories displayed differences in the mechanical behavior and its dependence on the testing temperature. These results were explained considering the differences in the composition and microstructure of both refractories and in their thermal transformations.     

High-temperature mechanical behavior of Al2O3/graphite composites

Uniaxial compressive creep behaviour of spark-plasma-sintered Al₂O₃/graphite particulate composites has been studied at temperature between 1250 and 1350 °C. Values of stress exponent, n, ranging from 1 to 1.4 and, activation energy, Q, of 600 ± 40 kJ/mol have been determined. With 10 vol% graphite in the composite, the creep deformation of the composite is controlled by the fine-grained Al₂O₃ matrix, where Coble creep has been identified as the dominant creep mechanism.     

Kinetics of geopolymerization: Role of Al2O3 and SiO2

The early-stage reaction kinetics of metakaolin/sodium silicate/sodium hydroxide geopolymer system have been investigated. The setting and early strength development characteristics, and associated mineral and microstructural phase development of mixtures containing varying SiO₂/Al₂O₃ ratios, cured at 40 °C for up to 72 h, were carefully studied. It was observed that setting time of the geopolymer systems was mainly controlled by the alumina content. Essentially, the setting time increased with increasing SiO₂/Al₂O₃ ratio of the initial mixture. Up to a certain limit, the SiO₂/Al₂O₃ ratio was also found to be responsible for observed high-strength gains at later stages. An increase in the Al₂O₃ content, i.e. for low SiO₂/Al₂O₃ ratio, led to products of low strength, accompanied by microstructures with increased amounts of Na-Al-Si-containing “massive” phases (grains). EDAX analyses showed that the SiO₂/Al₂O₃ ratios of geopolymer gel phases were quite similar to those of the starting mixtures, but with an overall lower Na content. Most importantly, this study clearly demonstrates that the properties of resulting geopolymer systems can be drastically affected by minor changes in the available Si and Al concentrations during synthesis.     

Effect of phosphorus addition on the hydrotreating activity of NiMo/Al2O3 carbide catalyst

A series of phosphorus promoted γ-Al₂O₃ supported NiMo carbide catalysts with 0–4.5 wt.% P, 13 wt.% Mo and 2.5 wt.% Ni were synthesized and characterized by elemental analysis, pulsed CO chemisorption, BET surface area measurement, X-ray diffraction, near-edge X-ray absorption fine structure, DRIFT spectroscopy of CO adsorption and H₂ temperature programmed reduction. X-ray diffraction patterns and CO uptake showed the P addition to NiMo/γ-Al₂O₃ carbide, increased the dispersion of β-Mo₂C particles. DRIFT spectra of adsorbed CO revealed that P addition to NiMo/γ-Al₂O₃ carbide catalyst not only increases the dispersion of Ni-Mo carbide phase, but also changes the nature of surface active sites. The hydrodenitrogenation (HDN) and hydrodesulfurization (HDS) activities of these P promoted NiMo/γ-Al₂O₃ carbide catalysts were performed in trickle bed reactor using light gas oil (LGO) derived from Athabasca bitumen and model feed containing quinoline and dibenzothiophene at industrial conditions. The P added NiMo/γ-Al₂O₃ carbide catalysts showed enhanced HDN activity compared to the NiMo/γ-Al₂O₃ catalysts with both the feed stocks. The P had almost no influence on the HDS activity of NiMo/γ-Al₂O₃ carbide with LGO and dibenzothiophene. P addition to NiMo/γ-Al₂O₃ carbide accelerated CN bond breaking and thus increased the HDN activity.     

Microstructure refinement approaches of melt-grown Al2O3/YAG/ZrO2 eutectic bulk

Microstructure developments of melt-grown Al₂O₃/YAG/ZrO₂ ceramic bulks were investigated by controlling composition, cooling rate, heterogeneous nucleation sites and melt superheating treatment. The solidification microstructure of sample with hypereutectic composition (ZrO₂ 20 mol%) is finer than that with hypoeutectic or eutectic ones. With increasing the cooling rate, microstructure of melt-grown samples develops from colony to dendrite and finally to cell. The microscopy and the components of samples vary with the melt superheating temperature and the type of heterogeneous nucleation sites. The microstructure evolutions of melt-grown Al₂O₃/YAG/ZrO₂ eutectic relate to the melt undercooling level and the solid–liquid interfaces stability.