Papers by Keyword: Thermal Shock Resistance

Abstract: The isothermal oxidation of the ZrB₂-SiC-ZrC ceramic was carried out in static air at a constant temperature of 1500±15 oC for 20 min. Compared with the original strength of 580 MPa, the strength of the oxidized specimen increased to 655±45 MPa, because the flaws in the surface of the specimen were sealed by the oxide layer. The thermal shock resistance of the specimens before and after the oxidation was measured by the water quenching. The measured ΔT_crit for the oxidized specimen was 641 oC that was obviously greater than 348 °C for the unoxidized specimen. The improvement in the thermal shock resistance was attributed to the formation the oxide layer on the surface of the specimen. The results here pointed to a promising method for improving strength and thermal shock resistance of ZrB₂-based ceramics.

Abstract: The physical properties of some materials used by local foundries were investigated with a view to assessing their suitability for use as low heat duty refractory bricks. The samples were collected from Malamai village, Gezawa Local Government, Kano state; they are Gezawa clay and Burji (Clay). The samples were crushed, ground, sieved and the chemical compositions were determined. The clay samples were treated separately as well as blended with Gezawa clay in different proportions and molded in to bricks. The bricks were dried and fired to 1100. Test for refractoriness, thermal shock resistance, linear shrinkage; bulk density, porosity and compressive strength were carried out on each of the specimen. Burji blended with 50% to 90% Gezawa clay gave improved thermal shock resistance with a refractoriness of 1300 and hence could be used for non ferrous melting cupolas.

Abstract: The investigations centered on how addition of Gezawa clay (GC) affects the refractory properties of the Gezawa stone (GS) (Silica Stone) with a view to finding the optimum percentage of the blend that will give good refractory properties. The samples were crushed, ground, sieved and the chemical compositions were determined. Test samples were produced by blending the Gezawa stone with various proportions of Gezawa clay. The refractoriness of the samples was measured, and the samples were then fired to 1100and standard properties such as thermal shock resistance, cold crushing strength and apparent porosity were determined. The result shows that addition of Gezawa clay to Gezawa stone improves its thermal shock resistance but reduces the refractoriness, while the porosity was found to be within the specified range for fire clay and the cold crushing strength of the samples was high. Compromise could be made in selecting the percentage that would suit certain application.

Abstract: Mullite-aluminium titanate-corundum composite was prepared at 1300°C with refractory clay, aluminium titanate and high alumina grog as raw material, molded at pressure of 50MPa. Effect of raw material ratio on sintering and themal shock resistance of the mullite-aluminium titanate-corundum composite was researched by measurements of apparent porosity, bending strength and residual strength after water-cool, and analyses of XRD and SEM. The results showed that as refractory clay content, apparent porosity of samples decrease, bulk density and bending strength increase. When the weight ratio of refractory clay, aluminium titanate and high alumina grog is 60/10/30, themal shock resistance of sample is excellent, The XRD and SEM analysis results indicated that the mechanical and thermal proprieties are relative to the microstructure and crystal phases of the composite materials.

Abstract: The effects of Si3N4 addition on the room temperature physical properties and thermal shock resistance properties of corundum based refractory castables were investigated using brown corundum, white corundum and alumina micropowder as the starting materials and pure calcium aluminate as a binder. The phase composition, microstructure, mechanical properties of corundum based castables were investigated by means of X-ray diffraction (XRD) and scanning electron microscopy (SEM). The results showed that as the increase in Si3N4 addition, the bulk density decreased and apparent porosity increased, the cold strength deduced. However, the residual strength rate increased. That is, the thermal shock resistance was improved. This is because even though the introduction of Si3N4 inhibited the sintering of material and deduced the compactness, microcracks were produced in the materials due to a difference in thermal expansion coefficient. So the thermal shock resistance of corundum based castable was improved.

Abstract: Silicon nitride nanoceramics were fabricated by hot press sintering two kinds of Si₃N₄ nano-sized powders. The effect of starting powders on microstructure, mechanical properties and thermal shock resistance were investigated. The microstructure of sintered materials consists of spherical grains and the addition of α–Si₃N₄ to starting powders does not affect the grain morphology. The flexural strength, fracture toughness and thermal shock resistance increase with the increase in amount of α–Si₃N₄ starting powders, and the maximum mechanical properties are obtained when the amount of α–Si3N4 powders is 40wt.%. The hardness values decrease with the increase of α–Si₃N₄ amount.

Abstract: Silicon carbide with diffierent granularity and three grain composition was used as raw material. Silicon carbide refractory material was prepared in oxidizing atmosphere at 1400 °C, 1450 °C and 1500 °C for 3 h. Performence of samples were researched by measurements of apparent porosity, bulk density, bending strength at room temperature, thermal shock resistance and thermal expansion rate, and analyzed by SEM. The results showed that samples sintered at 1400 °C have low thermal expansion rate and apparent porosity, high bending strength and bulk density, good thermal shock resistance, compact texture as well. It can be deduced that (1.0-0.5mm) / (0.5-0.1mm) / (45μm) / (5μm) = 50 / 17/ 20/ 13 is the best grain composition to improve integrated performance of silicon carbide refractory material.

Abstract: Low-carbon Al2O3-C refractories were prepared using white fused corundum, α-Al2O3 powders and flake graphite as main raw materials. The critical particle sizes of corundum selected in this experiment were 0.5mm, 1mm and 2mm. The effects of corundum critical particle size on physical, mechanical and thermo-mechanical properties of low-carbon Al2O3-C refractories were investigated. The results show that the increase of critical particle size is conducive to the improvement of thermal shock resistance and fracture energy, but little effect on thermal expansion. The cold modulus of rupture after thermal shock test of samples using 0.5mm critical particle size corundum was 2.09MPa, while using 2mm critical particle size corundum was 2.98MPa. And the fracture energy increased from 265N/mm to 588N/mm when the critical particle size increased from 0.5mm to 2mm. The effects of critical particle size on apparent porosity, bulk density and modulus of rupture were insignificant.

Abstract: Ceramic refractory materials initially contain a multitude of defects such as voids, microcracks, grain boundaries etc. Particularly being exposed to high temperatures above 1000 °C the macroscopic properties such as effective compliance, strength and lifetime are essentially determined by microscopic features of the material. The deformation process and failure mechanisms are going along with the creation of new microdefects as well as the growth and coalescence of cracks. A brittle material damage model for dynamic thermomechanical loading conditions is presented in this paper. Representative volume elements (RVE) include microcrack initiation and growth. The material laws are formulated on the continuum level using appropriate homogenisation methods. To demonstrate the potential of the numerical tools, two examples are presented which are taken from applications. Based on experiments, cyclic thermal shock tests at refractory plates are simulated by FEM. To quantify the thermal shock resistance of ceramics, experiments suggested by Hasselman are simulated numerically supplying a critical temperature slope.

Abstract: The thermal shock resistance of ZrO₂/LaPO₄ composites containing LaPO4 with different particle size was investigated. Microstructure, mechanical properties and RIV para_{Subscript text}meter were evaluated and discussed. On the basis of theoretically calculated RIV values and experimental results, resistance to thermal shock damage would be more strongly favoured with composites containing LaPO₄ calcined at 1100°C for 1 h