Mechanical properties of in situ synthesized mullite‐based composite ceramics with three‐dimensional network structure

The utilization of coal gangue in power plants has become a new anthropogenic discharge source of mercury and attracted much concern in China. It is crucial to obtain the information about the mercury release during thermal treatment of coal gangue. In this study, the mercury release behavior of two coal gangues selected from two power plants were studied under different thermal treatment conditions of heating rate, residence time, and atmosphere. The results of mercury release profile show that the specified release temperature ranges for the different modes of occurrence of Hg are scarcely affected by the heating rate of 10, 20, and 40°C/min. A higher heating rate could promote the Hg release to some extent. The mercury release ratio gradually increases with the extension of residence time for both coal gangues. The oxidizing environment has a positive effect on mercury release <600°C and has a minor effect >600°C. Mercury in coal gangue is more volatile than coal gangue matrix and the mercury in GD coal gangue is more easily released out than that in ED coal gangue.

TiB2-BN composite ceramics combine excellent electrical conductivity, thermal shock resistance, high-temperature resistance, corrosion resistance, and easy processing of TiB2 and BN. However, in practical applications, their high-temperature oxidation resistance is poor and the resistivity distribution is uneven and changes substantially with temperature. A TiB2-BN-SiC composite ceramic with stable and controllable resistivity was prepared by introducing SiC into the TiB2-BN composite ceramics. In this work, spark plasma sintering (SPS) technology was used to prepare TiB2-BN-SiC composite ceramics with various TiB2-SiC ratios and sintering temperatures. The samples were tested by XRD, SEM, and thermal and mechanical analysis. The results show that as the volume ratio of TiB2-SiC was increased from 3:1 to 12:1, the resistivity of the sample decreased from 8053.3 to 4923.3 μΩ·cm, the thermal conductivity increased from 24.89 to 34.15 W/(m k), and the thermal expansion rate increased from 7.49 (10−6/K) to 10.81 (10−6/K). As the sintering temperature was increased from 1650 to 1950 °C, the density of the sample increased, the mechanical properties were slightly improved, and the resistivity, thermal expansion rate, and thermal conductivity changed substantially. The volume ratio and sintering temperature are the key factors that control the resistivity and thermal characteristics of TiB2-SiC-BN composite ceramics, and the in situ from liquid phases of FeB and FeO also promotes the sintering of the TiB2-BN-SiC ceramics.

Novel Bi2O3-added Al2Mo3O12 composite microwave dielectric ceramics for ULTCC applications

We have investigated the effects of sintering temperature on the properties of hot-pressed alumina/hydroxyapatite ceramic composites. The ceramic composites were characterized by field-emission scanning electron microscopy and X-ray diffraction. The mechanical properties of the composites were also assessed. The results showed that the sintering temperature is a critical factor influencing phase composition, sintered microstructure and hence the mechanical properties. The decomposition of Hydroxyapatite to calcium phosphate was accelerated and the compactness and grain size increased with the increasing sintering temperature. Samples sintered at 1250 °C exhibited better mechanical properties as compared to those sintered at other temperatures. Secondary electron micrographs images of fracture surfaces of the composite ceramics sintered at different temperatures: (a) 1100 °C, (b) 1150 °C, (c) 1200 °C, (d) 1250 °C, (e) 1300 °C and (f) the SEM images of the residue extracted by dissolving the composite ceramic sintered at 1300 °C.

Understanding the effect of high-volume fly ash on micro-structure and mechanical properties of cemented coal gangue paste backfill

Enhancing carbonation resistance of low-activity coal gangue-based LC3 cementitious materials with Ca(OH)₂ and gypsum additives

The aim of this study is to determine the effect of changes in the phase composition of Al2O3–Si3N4 ceramics that were obtained using the method of mechanochemical solid-phase grinding on their resistance to the process of long-term thermal exposure, accompanied by the processes of oxidation and softening. The relevance of this research consists of determining the influence of the phase composition of ceramics on the change in their strength and thermophysical parameters, on the basis of which, we can draw a conclusion about the optimal composition of composite ceramics that have great prospects in the field of fire-resistant, heat-resistant, or radiation-resistant structural materials. During this study, the dynamics of the changes in the phase transformations of the xAl2O3–(1−x)Si3N4 ceramics, with variations in the ratio of the components, initiated by the thermal annealing of the samples, was established. According to the assessment of the phase transformations with variations in the ratio of the components, it was found that thermal annealing in an air environment at an Al2O3 concentration in the order of 0.3–0.5 M leads to the formation of an orthorhombic Al2(SiO4)O phase and an elevation in its contribution at concentrations above 0.5 M, which causes a rise in the thermophysical parameters and resistance to high-temperature degradation. During the heat resistance tests, it was found that the formation of the composite ceramics with the Si3N4(SiO2)/Al2(SiO4)O/Al2O3 phase composition results in an increase in the stability of their strength properties when exposed to thermally induced oxidation, which has a negative impact on their resistance to softening and a decrease in hardness. Moreover, the presence of the Al2(SiO4)O phase in the composition of the ceramics causes a slowdown in the processes of thermal oxidation of the Si3N4 phase under prolonged temperature exposure, alongside an increase in the degradation resistance of strength properties by more than 4–7 times, in comparison with the softening data established for single-component ceramics.

Based on three kinds of CuAl alloy powders with Al mass fraction of 0.18, 0.55 and 1.95%, respectively, as raw materials and Cu2O powder as oxidant, three dispersion-strengthened copper alloy powders with Al2O3 content of 0.34, 1.00 and 3.19%, respectively, were prepared by internal oxidation using the optimal internal oxidation parameter. Hot pressing was subsequently adopted to prepare Al2O3 dispersion-strengthened copper (Cu–Al2O3) alloys. The effects of Al2O3 content on microstructures and properties of alloy powders and sintered alloys were studied. The results showed that the microhardness of three alloy powders after internal oxidation was significantly improved and increased with increasing Al2O3 content. But the difference between powders with 1.00 and 3.19% Al2O3 content was small. The relative densities of sintered alloys with lower Al2O3 content, i.e. 0.34 and 1.00%, were both above 99%. However that with higher Al2O3 content, i.e. 3.19%, was only 97.8%. With increasing Al2O3 content, electrical conductivity decreased and hardness increased obviously. The electrical conductivity and hardness (HRB) of three alloys were 86.0, 71.5, 60.0% IACS, and 68.8, 84.3, 91.0, respectively.

Effects of sintering temperatures on phase formation, microstructure and the dielectric properties of near MPB (1-x)BNT-xBT ceramics (x = 0.06 and 0.08) were investigated. The (1-x)BNT-xBT ceramics were prepared using a combustion technique. The phase purity, crystal structure and microstructure of the samples were examined using X-ray diffraction (XRD) and scanning electron microscopy (SEM). Pure perovskite, which consists of rhombohedral and tetragonal phases, was found in the samples sintered below 1200°C. The average grain size tended to increase with increased sintering temperature. The density and the dielectric constant at room temperature increased with increasing sintered temperature up to 1200°C, thereafter they decreased. The ceramic composition with x = 0.08 exhibited the highest density (∼ 5.80 g/cm3) and dielectric constant (∼ 2829).

In this study, spodumene tailings (STs) were investigated as a primary raw material for preparing of foamed ceramics (ST-FCs). The effects of Al2O3 content on the physical properties and pore structure of the foamed ceramics were systematically investigated. Comprehensive analyses of phase composition, crystalline structure, and high-temperature foaming behavior were conducted to elucidate the influence mechanisms of Al2O3 content on the performance of ST-FCs. The results indicate that STs, composed mainly of albite, quartz, and minor muscovite, had a low sintering temperature of 1160 °C, making it an ideal candidate for foamed ceramic production. The Al2O3 content significantly influenced the physical properties and pore structure of ST-FCs, which effect was associated with the appearance of the crystals generated in samples during foaming process. Optimal results were achieved at 24.04 wt % Al2O3, where the ST-FCs prepared at 1140 °C demonstrated a bulk density of 0.405 g/cm3, compressive strength of 7.84 MPa, and favorable pore structure (average pore diameter of 0.173 mm and apparent porosity as low as 0.96 %). These findings strongly indicated that FCs with high performance could be successfully prepared from STs by optimizing the Al2O3 content, which not only mitigated the environmental impact of STs but also effectively reduced the production cost of FCs.

Microstructure and mechanical properties of high strength porous ceramics with high sewage sludge content

The authors of this study systematically investigate the influence of the main components of BaO-containing slag on its viscosity and thermal stability. The results indicate that the viscosity of slag significantly increases with the mass fraction of the crystalline phase. Increasing the slag basicity from 1.00 to 1.10 and the MgO content from 5.0% to 9.5% is an effective way to weaken the influence of crystallization on slag viscosity and improve the stability of BaO-containing slag. Al2O3 content over 11% is necessary to maintain a higher slag temperature, which enhances the ability of BaO-containing slag to resist the influence of crystallization on relative viscosity. When the heat of BaO slag is greatly reduced, increasing the slag basicity from 1.05 to 1.15 and MgO content to over 6.5% can help maintain the thermal stability of slag. The effect of Al2O3 content on the viscosity of slag under conditions of fixed heat is greater than that under constant temperature conditions. As the heat decreases, the Al2O3 content increase from 9% to 13% has a more significant effect on the viscosity and temperature of slag. A lower Al2O3 content in slag is advantageous for reducing the influence of heat fluctuation on slag viscosity.

A phosphate based composite ceramics system, (1 − x)SrZn2(PO4)2–xTiO2 (0 ≤ x ≤ 0.5), was prepared by the solid-state reaction method, and the relations between the phase composition, crystal structure and microwave dielectric properties were investigated. The results show that only two phases of SrZn2(PO4)2 and TiO2 can be obtained in the whole range of x value, and high relative density can be archived for all compositions as suitable sintering temperatures were applied. In the variation process of phase composition ratio, lower increase of dielectric constant value (<14) and wide changing range of temperature coefficient of resonant frequency (from −76 to +110 ppm/°C) can be obtained synchronously. When the phase composition ratio x value are 0.5, composite ceramics above possesses microwave dielectric properties as e = 12.011, Q × f = 24900 GHz and τ f = +91 ppm/°C as the sintering temperature was 1000 °C.

Coal gangue is the waste generated during coal mining and processing. This paper aims to efficiently utilize coal gangue as a resource and reduce environmental load. Using coal gangue as the main raw material and copper tailings as auxiliary materials, magnetic glass-ceramics is prepared by direct sintering method. We examine the effects of different raw material ratios, sintering temperatures, and holding times on the structure and properties of magnetic glass-ceramics. The results show that when the coal gangue content is MGS-60wt%, the sintering temperature is 1180 ℃, and the insulation time is 50 minutes, the water absorption rate of magnetic glass-ceramics is 0.0028%, the volume density is 1.269g/cm3, and the magnetization strength is 0.533Gs. As the sintering temperature gradually increases and the holding time increases, its volume density shows a decreasing trend. The magnetization strength of magnetic glass-ceramics belongs to the weak magnetic range, with the lowest magnetization strength of 0.1Gs.

Coal gangue is used to prepare flake nano-mullite with a molten salt method. In order to increase the utilization value, corundum composite ceramics were prepared from the nanomullite and alumina. The effect of particle size of the raw material size and the content of mullite on mechanical properties of the corundum composite ceramics was studied. It is found that the mullite with appropriate sizes can be filled in between the corundum grains, thus increasing the density of the samples. The mechanical properties were enhanced due to the variation in the crack propagation mode. The optimized sample exhibited flexural strength and fracture toughness of 263.7 MPa and 5.0 MPa·m1/2, respectively. Griffith fracture theory analysis results indicated that the critical crack size reached 173.9 μm and the fracture energy was 127.6 J/m2. Coal gangue has realized deep resource utilization, and the mechanical properties of composite ceramics are higher than alumina ceramics prepared under the same conditions. Flake nanomullite contributes to mechanical properties.