Refractories containing fused and sintered alumina aggregates: Investigations on processing,particle size distribution and particle morphology

The present study investigated pressed high-purity alumina refractories containing either white fused or tabular (sintered) alumina aggregates under comparable conditions. Using factorial experiments especially the effects of the pressing pressure, the particle size distribution model and the particle morphology were evaluated. White fused alumina exhibited a higher refractoriness under load as well as a lower total compression and creep rate in creep in compression experiments. However, tabular alumina had a higher cold crushing strength and Young's modulus before and after thermal shock. Yet, no significant effect regarding the relative loss of the Young's modulus due to thermal shock was determined. Generally, a higher pressing pressure reduced the apparent porosity and increased the cold crushing strength, the Young's modulus and the refractoriness under load. The batches according to a recently suggested modified Andreasen particle size distribution model contained a considerably higher amount of the coarsest particle fraction, while the medium particle size fractions were reduced. Surprisingly, for both alumina raw materials the modified Andreasen model resulted in a virtually identical apparent density and a slightly lower apparent porosity compared to the conventional Andreasen model. Furthermore, the thermomechanical properties were essentially unaffected, while the cold crushing strength and the Young's modulus were somewhat lower. For both raw materials the addition of blocky coarse grain fractions yielded a lower apparent porosity and higher apparent density compared to angular grains due to improved particle packing. Remarkably, the creep in compression and the creep rate were reduced as well. Consequently, the modified Andreasen model together with a designed particle morphology might allow the fabrication of shaped alumina products with a much higher content of coarse grained particles resulting in at least similar or even improved physical, mechanical and thermomechanical properties irrespective of the used alumina raw material.     

Influence of in situ phase formation on properties of calcium zirconate refractories

This study investigates chemical, physical, mechanical, and thermomechanical properties of calcium zirconate (CaZrO₃) refractories. The applied fused raw material contained some residual cubic zirconia, which impairs the corrosion resistance of CaZrO₃ refractories. To adjust the stoichiometry, CaCO₃ was added. Furthermore, stoichiometric amounts of CaCO₃/monoclinic ZrO₂ were added to promote an in situ pore formation without altering the phase composition. After firing cubic zirconia was only found in the coarse grained aggregates. CaCO₃ additions increased the apparent porosity and reduced the thermal expansion coefficient due to the stoichiometric adjustment. Surprisingly, a higher porosity did not result in an improved thermal shock resistance, which can be attributed to a shift to larger pore sizes. This might cause a lower crack density, which is usually associated with a reduced thermal shock resistance. Future work will address the formation of smaller pores and the evaluation of the thermal shock resistance under practical conditions.     

Advanced refractories for titanium metallurgy based on calcium zirconate with improved thermomechanical properties

Calcium zirconate refractories exhibit a promisingly high corrosion resistance in contact with titanium alloy melts. In the present article, we improved the thermomechanical properties of calcium zirconate based refractories by altering their microstructure and chemical composition. In a full factorial experimental design we investigated the effects of an insitu phase formation, of the addition of coarse grained MgO aggregates as well as the addition of SrCO₃. Based on these factors, the chemical, physical, mechanical and thermomechanical properties as well as the resulting microstructure were thoroughly characterized. Strikingly, the addition of SrCO₃ resulted in significantly improved mechanical properties before and after thermal shock. The improved thermal shock resistance can be attributed to a lower determined thermal expansion coefficient, a homogeneous pore size distribution with a reduced pore size and a better bonding between the matrix and the coarse grained aggregates.     

Porous cordierite-based ceramics processed by starch consolidation casting – Microstructure and high-temperature mechanical behavior

Porous cordierite-based ceramics with different microstructural features and mechanical behavior were formed by starch consolidation casting (SCC) using native potato and corn starches and sintered at 1275, 1300 and 1330 °C. The composition and microstructure of the ceramic materials were investigated via quantitative phase analysis using X-ray diffraction (with Rietveld refinement), the Archimedes method, mercury porosimetry, scanning electron microscopy and optical microscopy with stereology-based image analysis. The mechanical behavior of samples was evaluated by diametral compression tests at room temperature, 1000 and 1100 °C. The type of starch used and the sintering temperatures were the main factors determining the characteristics of the developed porous microstructures. Materials prepared with corn starch achieved the lowest porosity and the lowest values of mean chord length, mean pore distance and pore throat size. Because of these features, these materials thus presented, in general, higher values of apparent Young's modulus, elastic limit and mechanical strength than those prepared with potato starch. Despite the presence of a silicate glassy phase, both porous materials, mainly those prepared with corn starch, still enhanced the basic mechanical properties at high temperature, in particular, the mechanical strength and the apparent Young's modulus due to the special combination of the porous microstructure features.     

The effect of compressive stress on the Young's modulus of unirradiated and irradiated nuclear graphites

The Young's moduli of unirradiated and high temperature (800–1000°C) irradiated graphites for HTGR were measured by the ultrasonic method in the direction of applied compressive stress during and after stressing. The Young's moduli of all the tested graphites decreased with increasing compressive stress both during and after stressing. In order to investigate the reason for the decrease in Young's modulus by applying compressive stress, the mercury pore diameter distributions of a part of the unirradiated and irradiated specimens were measured. The change in pore distribution is believed to be associated with structural changes produced by irradiation and compressive stressing. The residual strain, after removing the compressive stress, showed a good correlation with the decrease in Young's modulus caused by the compressive stress. The decrease in Young's modulus by applying compressive stress was considered to be due to the increase in the mobile dislocation density and the growth or formation of cracks. The results suggest, however, that the mechanism giving the larger contribution depends on the brand of graphite, and in anisotropic graphite it depends on the direction of applied stress and the irradiation conditions.     

The Dependence of Pore Size Distribution on Porosity in Hot Isostatically Pressed Porous Alumina

Pore size distributions in porous alumina bodies produced by the capsule-free hot isostatic pressing technique have been determined experimentally. The distribution of pore diameter has been found to be dependent on the size of the pre-sintered powders and the amount of open porosity in the sintered body. An empirical model has been developed to predict the modal pore size as a function of median particle size and open porosity. The pore size distributions were found to widen with reduced porosity. They were also shown to be positively skewed. The skew reduced with decreasing porosity. The pore size variation with porosity for specimens produced with a sintering aid could not be described by the same mathematical functions developed for specimens produced by solid-state sintering.     

Pressure slip casting of coarse grain oxide ceramics

This contribution investigates the pressure slip casting of large coarse grain oxide ceramic bodies with a water soluble organic additive system. This organic additive system allows the preparation of a stable and pumpable slip containing alumina rich magnesia aluminate spinel of a size of up to 3 mm and an easy demolding of crack free, dimensionally stable bodies with negligible gradients due to sedimentation. Cut out samples of fired bodies are examined on apparent porosity, dynamic elastic modulus, modulus of rupture, and pore size distribution. Computer tomography showed very homogenous and dense bodies. The effects of different maximum grain sizes as well as possible sedimentation and segregation of the slip on the mechanical properties and microstructure are evaluated by using the Student's t-test. The most promising results of this study indicate that it is possible to reproducible fabricate coarse grain ceramics for refractory and other high temperature applications by pressure slip casting.     

The microstructure evolution and mechanical properties of MgO-C refractories with recycling Si/SiC solid waste from photovoltaic industry

In the present work, the recycling of Si/SiC solid waste from photovoltaic industry for MgO-C refractories preparation has been introduced. The influence of solid waste powders as antioxidant additive on microstructure evolution, mechanical properties and thermal shock resistance of MgO-C refractories has been investigated systematically. With 4?wt% Si/SiC rich solid waste addition, the MgO-C refractories exhibited the highest strength (4.39?MPa) and residual Young's modulus (7.86?GPa) after firing at 1400?°C, compared to only Si or SiC-addition. The presence of iron in the solid waste also promoted the formation of MgO and Mg₂SiO₄ whiskers via catalyst-assisted method. Moreover, a dissolution-saturation-precipitation growth mechanism was used to explain the formation process of the whiskers. The improvements in strength as well as thermal shock resistance can be attributed to the microstructural evolution.     

Influence of random pore defects on failure mode and mechanical properties of SiC ceramics under uniaxial compression using discrete element method

To investigate the relationship between micro-defects in ceramic materials and macro mechanical properties and behaviours, a computational model of SiC ceramics with randomly oriented elliptical pores was established using the discrete element method (DEM). The effects of pore defect content and its aspect ratio on the failure mode, stress-strain curve and mechanical properties of specimen were investigated under uniaxial compression. The effective Young's modulus which was obtained from DEM simulations was compared with the predictions of Mori-Tanaka scheme (MTS) and Self-Consistent scheme (SCS) at various pore defect densities. The results showed that the compressive strength and crack initiation stress decrease nonlinearly as the pore defect content increases. Furthermore, the smaller the aspect ratio of the elliptical pore defects was, the more obvious the weakening trend was. As the pore defect content increases, the failure mode of the specimen changed from brittle fracture to tensile-shear mixing and then to axial splitting. The stress-strain curves showed a certain “softening” period during the loading process. The effective Young's modulus obtained from the DEM simulations coincides with the approximations of MTS and SCS at low pore densities. However, when the pore defect density became larger, the DEM simulation results were slightly lower than the theoretical results of the Mori-Tanaka scheme, which only considers the weak interaction between defects.     

Studies of composites made by impregnation of porous bodies. 1. Sulphur impregnant in portland cement systems

Porous bodies formed by autoclaving portland cement-silica mixtures and by normally curing portland cement were characterized by measuring Young's modulus, microhardness and porosity. These bodies were impregnated almost completely with molten sulphur. The bodies were characterized again. It was found that the mechanical properties of the composites could be described by a form of Reuss' mixing law. Equations relating the improvement of the mechanical properties of the composite to the properties of the porous body were derived for both Young's modulus and microhardness.     

Pore evolution of microporous magnesia aggregates with the introduction of nano-sized MgO

Microporous refractories applied in the working-lining of metallurgical furnaces have been rapidly developed in recent years owing to the outstanding mechanical properties, thermal insulation performance and slag resistance, the pore structure of which plays a critical role in the variation of service performance. Meanwhile, the microporous magnesia aggregates were prepared in our previous research with the introduction of nano-sized particles to overcome the shortcomings of high thermal conductivity, poor thermal shock resistance and slag penetration resistance, however, the pore evolution during sintering still remains to be investigated. Hence, in this study, the pore evolution of microporous magnesia aggregates is explored specifically and the effect of nano-sized MgO on pore structure and sintering is simultaneously discussed. The sintering model of microporous magnesia was built for analyzing the pore structure evolution process. The results revealed that a micro-nano double-scale sintering model developed by the introduction of nano-sized MgO dramatically promoted the sintering kinetic force and boundary migration velocity. The sintering pressure discrepancy and free energy change per unit mole of specimens were respectively increased by ～43 times and ～48 times, which effectively improved the closed porosity and pore distribution homogeneity, while reduced the pore size. Meanwhile, the high sintering diving force lead to the significant improvement of direct bonding degree and grain size of microporous magnesia. With the addition of 3 wt% nano-sized MgO, the optimal sintering properties with closed porosity of 6.4%, bulk density of 3.23 g/cm³ and median equivalent pores diameters of 4.07 μm were achieved. The exploration of pore evolution in microporous magnesia aggregates contributed to the fabrication and industrialization development of microporous refractories.     

Studies of composites made by impregnation of porous bodies. 2. Polymethyl methacrylate in portland cement systems

Porous bodies formed by autoclaving portland cement-silica mixtures and by normally curing portland cement were characterized by measuring Young's modulus, microhardness and porosity. These bodies were impregnated with methyl methacrylate and irradiated, the procedure being carried out twice. The bodies were almost completely impregnated. Increases in mechanical properties were greater for microhardness but less for Young's modulus when compared to sulphur impregnation. It was concluded that polymethyl methacrylate forms a stronger bond with the matrices studied than does sulphur.     

Thermal shock damage evaluation of refractory castables via hot elastic modulus measurements

When refractory castables are submitted to continuous thermal changes, crack nucleation and/or propagation can take place resulting in a loss of mechanical strength and overall degradation of such materials. This work evaluates the thermal shock damage cycling of high-alumina and mullite refractory castables designed for petrochemical application. Hot elastic modulus analyses were carried out after 0, 2, 4, 6, 8 and 10 thermal cycles (ΔT=800 °C) in order to investigate the microcracking evolution due to the temperature changes. Additionally, apparent porosity, hot modulus of rupture, erosion and work of fracture measurements were also performed. According to the attained results, it was detected at which temperature range the stiffening or embrittlement took place in the mullite-based refractory (M-SA) microstructure. Nevertheless, the damage induced by the thermal shock tests was not permanent, as further increase of the elastic modulus results was observed for all evaluated samples after annealing. On the other hand, the alumina-based composition containing a sintering additive (TA-SA) presented enhanced mechanical strength, high thermal stability and E values. Simulations indicated that refractories with high E values (∼140 GPa, such as those attained for alumina-based castable) showed a reduced amount of stored elastic strain energy even under severe thermal stresses, which seems to be a key aspect for the engineered design of thermal shock resistance materials.     

Failure Variability in Porous Glasses: Stress Interactions,Crack Orientation,and Crack Size Distributions

Fracture behavior of porous glass is investigated through a combined finite element–fracture mechanics approach. In contrast to earlier studies, here, simulations embody flaw size distributions in addition to pore–pore stress interactions and crack orientation along pore surfaces. Fracture strength of porous glass shows a steep decrease up to 20% porosity and then levels off due to interacting pores. Weibull modulus varies because of the decreased probability of interactions in microstructures containing less than 2% porosity or the smallest pore diameter =48  μm. Weibull modulus strongly depends on crack size distributions for porosity less than 2% and pore–pore stress interactions for porosity greater than 5%.     

Porosity and mechanical properties of cement mortar

The effect of volume fraction porosity on the mechanical properties of cement mortar is studied. It is shown that both the Young's modulus and fracture toughness decrease with porosity. Although the flexural strength also decreases with porosity the linear relationship is largely fortuitous. Maximum size pores do not act as critical flaws in controlling flexural strength. The critical crack size is several times larger than the maximum pore size due to stable crack growth according to the crack growth resistance curve concept applied to cement mortar.     

Correlations between pore structure parameters and gas permeability of corundum porous materials

Corundum porous materials with different contents of calcium hexaluminate formed in situ were prepared using pure calcium aluminate cement as the calcium source. The surface fractal dimensions of the porous materials were calculated based on the experimental data of mercury intrusion. Correlations between pore structural parameters and the permeability coefficients k₁ and k₂ of the porous materials were then studied based on the grey system theory. The results showed that pores in the corundum porous materials have great fractal characteristics. The surface fractal dimension was a significant pore structural parameter that reflected the complexity of pore shape, pore surface, and pore-size distribution, which had the maximum correlation coefficient with the permeability of this type of porous materials. The apparent porosity and pore-size distribution had relatively high correlation coefficients to the permeability as well. Increasing the apparent porosity and the volume percentage of larger pores, and decreasing the volume percentage of smaller pores all benefited the permeability of the porous materials. In addition, the mean pore size and median pore size showed lower correlation coefficients to the permeability—especially for porous materials with a wide pore-size distribution.     

氧化镁铁铝尖晶石耐火材料的制备

以铁铝尖晶石和镁砂为原料,采用烧结法制备了氧化镁铁铝尖晶石耐火材料.检测了各烧后试样的体积密度、显气孔率和常温耐压强度,利用应力应变法检测了烧后试样的弹性模量,利用X射线衍射(XRD)检测了烧后试样的物相组成,采用扫描电子显微镜(SEM)观察和分析了烧后试样的显微结构.研究结果表明: 1600 ℃时各试样体积密度最大,显气孔率最小,试样达到了烧结;镁砖中加入铁铝尖晶石会引起材料常温强度降低,铁铝尖晶石加入量在3%～4%为宜;铁铝尖晶石以颗粒形式加入的试样的弹性模量比以细粉形式加入的试样要大,所以铁铝尖晶石以颗粒形式加入的试样的抗热震性相对较好;热力学计算表明:当加热温度高于182 ℃时, MgO与FeAl_2O_4开始反应生成MgAl_2O_4;从显微结构照片也可以看出, MgO与FeAl_2O_4中的FeO发生互扩散,FeO扩散进镁砂颗粒中,MgO扩散进铁铝尖晶石内部,与Al_2O_3反应生成MgAl_2O_4,在镁砂颗粒周围形成MgAl_2O_4环,并伴有微裂纹产生.     

Pressure slip casting of coarse-grained alumina-carbon materials

This study presents investigations on the application of pressure slip casting to produce shaped coarse-grained alumina-carbon refractories. Slurries containing alumina particle size fractions ≤3 mm, a modified coal-tar pitch, graphite, and carbon black were prepared and examined by rheological measurements and pressure filtration tests on a laboratory scale. A suitable combination of organic additives was chosen. The graphite content was found to have a significant effect on the flow behavior and the apparent porosity of the samples produced from the slurries. Scale-up experiments were performed in a modified commercial pressure casting machine. It was possible to cast dimensionally stable samples with a size of approximately 220 mm × 220 mm × 40 mm. As shown by means of computed tomography, a homogeneous distribution of the coarse grains over the whole sample was achieved. Quenching tests demonstrated the thermal shock resistance of the carbon-bonded alumina material obtained after the pyrolysis of the modified coal-tar pitch.     

Spark plasma sintering and mechanical properties of compounds in TiB2-SiC pseudo-diagram

The influence of spark plasma sintering (SPS) parameters (temperature, time, pressure) and the role of particle size on densification, microstructure and mechanical properties of commercial additive-free TiB₂, SiC and composites thereof were studied by X-ray diffraction, scanning electron microscopy, the ultrasonic method and indentation. Three particle sizes of SiC and 2 of TiB₂ were processed. An optimal cycle was found for TiB₂ and SiC: 2000?°C, 3?min dwell time, and 100?MPa applied at 600?°C. The relative density of pure SiC increases linearly from 70% to 90% when the initial particle size decreases from 1.75?µm to 0.5?µm. Pure TiB₂ was densified up to 87%. Using 2.5?wt% SiC in TiB₂, the relative density increases to 97%. Young's modulus and the hardness of all samples were measured, with results discussed. The higher properties were obtained for additive-free TiB₂–5%SiC with a relative density of 97% and with the Young's modulus and Vickers hardness values being close to 378?GPa and 23?GPa, respectively.     

Internal friction of capsule-free HIPed porous alumina

Porous alumina were sintered by conventional sintering and capsule-free Hot Isostatic Pressing (HIPing), at temperatures between 800 and 1500 °C under pressures 0.1 MPa or 200 MPa for 1 h or 50 h. Young’s modulus and internal friction of samples were measured by resonance method. The results show that Young’s modulus is mainly dominated by porosity of material. Capsule-free HIPed porous materials have slightly higher Young’s modulus than conventionally sintered ones at the same porosity. Internal friction is governed by both porosity and specific surface area. Capsule-free HIPed porous alumina has lower internal friction coefficient than conventionally sintered ones at the similar porosity or at the similar specific surface area. Enhanced surface-self diffusion under high gas pressure reduces internal friction coefficient, and affects internal friction more than Young’s modulus.