Comparison of Mechanical Properties of Silicon Nitrides with Controlled Porosities Produced by Different Fabrication Routes

Three different series of porous silicon nitride ceramics with volume fraction porosities in the range 0–0.5 were fabricated using different preparation routes: (i) partial sintering, (ii) the addition of fugitive inclusions, and (iii) partial hot pressing. The use of different sintering additives and firing conditions, depending on the preparation route, gives rise to different materials within a certain porosity range with variations in terms of microstructure and grain boundary phase. Mechanical properties, elastic moduli, and strength have been evaluated separately for each series of materials. Porosity dependences of Young's modulus, shear modulus, Poisson's ratio, and fracture strength have been assessed and a comparison of the different materials is made and discussed in relation to their microstructural features.     

不同铝基原料对MgO-Al-C材料性能和显微结构的影响

以烧结镁砂骨料、电熔镁砂细粉、Al粉、N220炭黑为原料,酚醛树脂为结合剂,制备MgO-Al-C材料,研究了三种铝基原料(造粒氧化铝微粉、氧化铝空心微珠、六铝酸钙)对其常温抗折强度、高温抗折强度、抗热震性及抗氧化性的影响,并借助XRD和SEM对其物相组成及显微结构进行分析。结果表明,造粒氧化铝微粉是多孔结构,可吸收热应力,加入量为1%(质量分数,下同)时,可提高材料的常温抗折强度和抗热震性,明显改善材料的抗氧化性。氧化铝空心微珠是中空结构,可缓冲热应力,加入量为3%时,可明显提高材料的常温抗折强度,并具有较高的抗热震性和抗氧化性。六铝酸钙的热膨胀系数较低,可赋予材料较好的韧性,加入量不超过5%时,样品具有较好的抗热震性。     

The prediction of elastic modulus of the mullite fiber network based on the actual structure architecture

Accurately establishing the relationship between the network architecture characteristics and performance of fibrous porous ceramics is instructive for structural design and performance control. In the present work, fibrous, high porous (82.87–90.02%), low density (0.247–0.512 g/cm³) and low elastic modulus (50.62–188.56 MPa) mullite ceramics were fabricated by freeze casting. The three dimensional network architectures were characterized by X-ray tomography technique and quantitatively analyzed by 3D image analysis software (imorph, www.imorph.fr). The radius (5.04 µm), types, lengths (64.72–96.49 µm) and orientations (0.87–1.45, anisotropy parameter) of fiber segments in the network architecture were investigated. The extracted results were employed to predict the Young's modulus of the mullite fibrous porous ceramics according to a model based on the bending and axial compression of single fiber segment. The predicted Young's modulus agreed well with the experimental results. The differences of Young's modulus and Poisson ratio between the prediction and the model of Markaki and Clyne were compared. The comparison showed that the difference became larger when the aspect ratio of the fiber segment was less than 6 due to the effect of axial compression. The predicted Poisson ratio had a certain dependence on fiber segment aspect ratio and got close to the constant (1/π) reported by Markaki and Clyne with the increase of fiber segment aspect ratio.     

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.     

Thermal schock acoustic emission and microstructure of refractories II. Basic refractories

Acoustic emission generated on thermal shocks makes it possible to investigate the processes of cracking that take place in magnesia and magnesia-chromite refractories separately at heating and cooling stages, as well as on successive thermal shocks. It is dependent on the raw material type, technological process and the microstructure of products (i.e. number, size and shape of pores and cracks). The damage present after 20 thermal shocks in the material tested was illustrated by the microstructure examinations.     

Effect of LiAlSiO4 particle size on the properties of LAS/SiC porous ceramics with near zero thermal expansion

Near zero thermal expansion porous ceramics were fabricated by using SiC and LiAlSiO₄ as positive and negative thermal expansion materials, respectively, bonded by glassy material. The microstructure, mechanical properties, and thermal expansion behavior of LAS/SiC porous ceramics with different particle sizes of LiAlSiO₄ were investigated. The results indicated that the coefficient of thermal expansion of the LAS/SiC porous ceramics decreased from 0.5206×10⁻⁶ to −1.1053×10⁻⁶ K⁻¹ with increasing the LiAlSiO₄ particle size from ~45 µm to ~125 μm. It was attributed to the reduction in the reaction between LiAlSiO₄ and SiO₂ as the particle size of LiAlSiO₄ increased. Young’s modulus increased from 36 MPa to 54 MPa as the sintering temperature increased from 850 °C to 950 °C because of the good bonding between the SiC grains and the glass materials.     

Elastic properties of porous oxide ceramics prepared using starch as a pore-forming agent

The elastic properties, in particular the tensile modulus (Young's modulus) and Poisson ratio, of porous alumina, zirconia, and alumina–zirconia composite ceramics are studied using the resonance frequency method and the results compared with theoretical predictions. Starch is used as a pore-forming agent, so that the resulting microstructure is essentially of the matrix-inclusion type (with large bulk pores, connected by small throats when a percolation threshold is exceeded). It is found that for this type of microstructure the porosity dependence of the Young's modulus is significantly below the upper Hashin–Shtrikman bound and the power-law prediction; it corresponds well, however, to a recently proposed exponential relation and to an empirical volume-weighted average of the upper and lower Hashin–Shtrikman bounds. Results for all three types of ceramics indicate that – in the porosity range considered, i.e. up to approximately 50% – the Poisson ratio depends only slightly on porosity.     

SiO_2对环氧胶层弹性模量和泊松比的影响

用应变片电测技术研究了SiO2填料对环氧树脂胶层弹性模量的影响,测定了变化SiO2填料添加量时环氧胶层的变形情况。结果表明:在所采取的试验条件下,填料SiO2的添加量小于45％(树脂重)时,增加填料导致其弹性模量明显提高而泊松比减小,添加量为45％时所测得的弹性模量比无填料试样提高了大约22．5％而泊松比降低了约19．4％,分析了弹性模量和线膨胀率对环氧胶层内应力的综合影响。     

Hot elastic modulus of Al2O3–SiC–SiO2–C castables

This work discusses an investigation of the hot elastic modulus and crack generation of two Al₂O₃–SiC–SiO₂–C castable compositions throughout two thermal cycles in an oxidizing atmosphere. A high temperature ultrasonic technique carried out using a long bar mode, thermogravimetric, X-ray diffraction, apparent porosity analyses and thermodynamic calculations were evaluated in order to understand the results. Significant changes in the castables’ elastic modulus values were observed with temperature, which were related to the decomposition of hydrated phases, antioxidant reactions, changes in the liquid phase viscosity, and formation and closure of microcracks in the castable microstructure. The results attained are fundamental for providing data for thermo-mechanical computing simulations by finite element analyses and for the design of large refractory structures, such as blast furnace runners for the steel industry.     

The Poisson ratio of polymer blends,effects of adhesion and correlation with the Kerner packed grains model

The moduli and Poisson ratio of polystyrene-polyethylene blends with different compositions have been determined. In some cases a polystyrene-polyethylene copolymer was added to the blends to provide for adhesion between the components. It is shown that the Kerner packed grains model can be used to predice accurately both modulus and Poisson ratio of these blends. Deviations of both modulus and Poisson ratio from the predicted values occur in some blends containing block copolymer as a consequence of the copolymer-induced formation of a continuous low modulus phase at a relatively low concentration of the low modulus material. In the case of non-adhesion between the components only the Poisson ratio was found to deviate significantly from the predicted value. This is explained by assuming hole-like behaviour of the polyethylene particles due to non-adhesion and the misfit of the coefficients of thermal expansion of polystyrene and low density polyethylene.     

Micromechanics Analysis of Porous and Filled Ceramic Composites

A finite element analysis was used to calculate the internal stresses and deformations everywhere within spherically filled or porous ceramic materials. This analysis accounts for interactions between inclusions and demonstrates the importance of these interactions on stress fields around the inclusions. Modulus of elasticity and Poisson's ratio were calculated for porous and filled ceramics. The predicted modulus is compared with experimental data for two glass composites; the agreement is excellent.     

Elastic Energy at Fracture and Surface Energy as Design Criteria for Thermal Shock

The physical properties which affect the propagation of cracks in specimens fractured by thermal shock are discussed. The driving force for crack propagation is provided by the elastic energy stored at fracture. The mechanism of energy dissipation which will tend to arrest the propagating cracks is provided by the "effective surface energy" required to produce the newly formed crack surfaces. An expression is derived applicable to a body of spherical shape for the mean area traversed by cracks nucleated by thermal shock. Three numerical examples are given for materials with widely different physical properties, and their fracture behavior is predicted. Good agreement with experiment was obtained. Thermal shock damage resistance parameters suitable for the relative comparison of the "degree of damage" to be expected in materials fractured by thermal shock are proposed. The criteria for a low degree of damage are high values of Young's modulus of elasticity, Poisson's ratio, and effective surface energy and low values of strength. Recommendations are made for the selection of materials for severe thermal shock, where the best materials available are known to fail.     

Combination of Brazilian test and digital image correlation for mechanical characterization of refractory materials

Flexibility of refractories is an interesting property to improve thermal shock resistance of refractories. It can often be obtained by promoting a nonlinear mechanical behaviour which reduces brittleness. Among industrial refractory materials developed in this aim, magnesia hercynite is of a particular interest for cement industry. As linings for rotary kilns, magnesia hercynite can be submitted to tensile and compressive stresses. Since tensile strength is usually much lower than compressive one for brittle materials, the mechanical characterization is, in such case, more significant in tension than in compression. To overcome difficulties involved by direct tensile test, an indirect tensile test (Brazilian test) has been applied here and combined to digital image correlation in order to measure kinematic fields on the surface of the sample during loading. This combination has allowed to accurately measure initial elastic properties (Young’s modulus and Poisson ratio), to detect crack initiation and to analyze fracture process.     

Young’s modulus measurements of magnesia–spinel composites using load–deflection curves, sonic modulus, strain gauges and Rayleigh waves

The extent of interlinking of the microcracking and a decrease in strength and modulus values were determined to be a function of both spinel particle size and volume fraction to allow calculation of thermal shock parameter, R. Measurements of Young’s modulus were carried out both at room temperature and after thermal shock testing by using the load–deflection curves (defined as “mechanical” modulus) and by the sonic modulus technique. The values obtained from these methods were significantly different for quenched/unquenched samples. To understand the basis for these differences, strain gauge and Rayleigh wave methods were also used to determine Young’s modulus of the composites. Modulus values obtained from these methods confirmed the differences measured, and provided a guide to the values to be used in calculating thermal shock parameters. The mechanical modulus technique was considered the most meaningful indicator of Young’s modulus for a situation in which large mechanical strains were to be applied to the materials during thermal shock.     

Gamma-ray irradiation effect on microstructure and physical performances of porous silica

In this paper, the gamma irradiation effect on the microstructure and physical performances of porous silica, including mechanical, thermal, and optical performances, are systematically investigated by using molecular dynamics and density-functional theory-based methods. The study of bond angle distribution, pair distribution function, coordination number distribution, and average ring size distribution show that, after gamma-ray irradiation, the microstructure of porous silica is obviously modified. The tight packing of SiO₂ tetrahedrons in the porous silica network is broken by gamma-ray irradiation. Defects of three-coordinated Si and non-bridging oxygen are induced by gamma-ray irradiation. Moreover, we find that the defects concentrations rapidly grow as gamma-ray dose increases. The mechanical, thermal, and optical performances of porous silica are all seriously degenerated by gamma-ray irradiation. Our results show that, for mechanical performance, Young's modulus, Bulk modulus, and Shear modulus first decrease and then keep stable as gamma-ray dose increases, but the change of Poisson's ratio is slight. For thermal performance, the thermal conductivity decreases exponentially as gamma-ray dose increases. For optical performance, light absorption coefficients increase exponentially and light transmittance drops as gamma-ray dose increases in the working range (photon energy range around 3.5 eV) of inertial confinement fusion. Present work is expected to be valuable for studying the degradation mechanism of silicate materials under gamma radiation and developing gamma-ray irradiation protection technology.     

Elastic Properties of Model Porous Ceramics

The finite-element method (FEM) is used to study the influence of porosity and pore shape on the elastic properties of model porous ceramics. Young's modulus of each model is practically independent of the solid Poisson's ratio. At a sufficiently high porosity, Poisson's ratio of the porous models converges to a fixed value independent of the solid Poisson's ratio. Young's modulus of the models is in good agreement with experimental data. We provide simple formulas that can be used to predict the elastic properties of ceramics and allow the accurate interpretation of empirical property–porosity relations in terms of pore shape and structure.     

Mechanical properties and microstructure evolution of MgO–Al–C slide plate refractories in presence of Al powder-modified magnesia aggregates

MgO–Al–C slide plate refractories were fabricated using sintered magnesia and modified sintered magnesia as aggregates, fused magnesia aggregates and fines, Al powder and carbon black (N220) as fines, and thermosetting phenolic resin as the binder. Al powder-modified magnesia aggregates were prepared and characterized and were introduced into the MgO–Al–C slide plate refractories. The effects of the modified aggregates on the properties, phase composition, and microstructure were investigated. 1) The Al powder-modified magnesia aggregates exhibited considerably high bonding strengths and low Al powder shedding ratios, thus meeting the preparation requirements of MgO–Al–C slide plate refractories. 2) At high temperatures, more needle-like and fibrous Al₄C₃, AlN and octahedral MgAl₂O₄ were generated on the surface of the modified magnesia aggregates, which enhanced the bond between the matrix and the aggregates and increased the hot modulus of rupture of the material. 3) Non-oxide Al₄C₃ and AlN phases were formed in situ and had high thermal conductivity and low coefficient of expansion; this could relieve the internal thermal stress of the material and create a toughening effect, improving the thermal shock resistance of the material.     

Structure and properties of lightweight magnesia refractory castables with porous matrix

To improve the energy-saving capacity of magnesia refractory castables for working lining of high-temperature kilns, this study presents the researches on microstructure and properties of lightweight magnesia refractory castables with porous matrix fabricated by direct foaming method. The results show that formation of closed-pores in the matrix significantly enhanced high-temperature thermal insulation performance of castables with minor changes of slag corrosion resistance. The thermal conductivity of the lightweight magnesia castables at 1000 °C was below 1.2 W/m·K, which is 47.8% lower than that of the referenced magnesia castable. The increasing content of SDS (foaming agent, over 0.02 wt%) led to increments of size and number of large-sized pores, resulting in the significantly decreased density and mechanical performances. The slag resistance mechanism reveals that, in addition to intergranular penetration, the accumulation of slag and penetration between adjacent pores were the major ways of slag mass transfer in lightweight magnesia castables. In conclusion, controlling the size (below 53.2 μm), number and distribution of closed-pores in the matrix is effective to realize the coupling of high thermal insulation, mechanical properties and slag resistance for lightweight magnesia castables used in the metallurgical field.     

Effect of composition and high-temperature annealing on the local deformation behavior of silicon oxycarbides

Silicon oxycarbides with varying compositions were investigated concerning their elastic and plastic properties. Additionally, the impact of thermal annealing on their elastic properties was assessed. Phase separation of SiOC seems to have no significant impact on Young’s modulus (high values of β-SiC compensate the low values of the vitreous silica matrix) and hardness. However, it leads to an increase in Poisson’s ratio, indicating an increase in the atomic packing density. The phase composition of SiOC significantly influences Young’s modulus, hardness, brittleness and strain-rate sensitivity: the amount of both β-SiC and segregated carbon governs Young’s modulus and hardness, whereas the fraction of free carbon determines brittleness and strain-rate sensitivity. Thermal annealing of SiOC glass-ceramics leads to an increase in Young’s modulus. However, the temperature sensitivity of Young’s modulus and Poisson’s ratio is not affected, indicating the glassy matrix being stable during thermal annealing. A slightly improved ordering of the segregated carbon and the β-SiC nanoparticles upon thermal annealing was observed. It is suggested that this is responsible for the increase in Young’s modulus.     

Some Physical Properties of High-Density Thorium Dioxide

The values for a number of physical properties are reported for a very high density form of thorium dioxide. When specimens of a mixture of 99½% ThO² and ½% CaO, by weight, were hydrostatically pressed at 30,000 lb. per sq. in. and heat-treated for 1 hour at 1800°C., they attained 99.0% of theoretical density. All the test specimens were extremely brittle. Physical-property values determined at room tempera- ture were the following: lattice constant; bulk and theoretical (X-ray) densities; compressive and impact strengths; Knoop hardness; modulus of rupture and Young's modulus, determined by a static method; Young's modulus and the shear modulus, determined by a dynamic method; Poisson's ratio and the bulk modulus, calculated from the dynamic-test data; and the velocity of sound through the material. The properties determined at elevated temperatures were the following : linear thermal expansion modulus of rupture and Young's modulus, determined by a static method; Young's modulus and the shear modulus, determined by a dynamic method; and Poisson's ratio, calculated from the elevated-temperature dynamic-test data. "Martin's diameter" grain counts were taken for the material both before and after heat-treatment.