改善耐火制品热震稳定性的方法

提出了改进耐火制品热震稳定性的方法：1制品的气孔率适当;2控制原料的颗粒配比;3增加微细裂纹并形成网络结构;4形成界面结合;5添加膨胀性材料。     

提高刚玉质耐火材料热震稳定性的研究

利用微气孔和弥散的ＺｒＯ２能改善耐火材料热震稳定性的特点，在含有一定数量微气孔的刚玉质耐火材料中，加入弥散的ＺｒＯ２，将ＺｒＯ２的增韧作用应用于刚玉质耐火材料热震稳定性的设计中。实验结果表明，当气孔的数量控制在１４％左右，ＺｒＯ２的外加量达６～９ｗｔ％时，能有效地改善刚玉质耐火材料的热震稳定性。在１１００℃，水冷的条件下，经１０次热震试验后，耐压强度保持不变，残余抗折强度为４５％。     

刚玉骨料种类与粒度对刚玉-莫来石材料抗热震性的影响

以莫来石和电熔白刚玉或板状刚玉为骨料,氧化铝粉、氧化硅粉、红柱石粉为基质料,纸浆为结合剂,同时以≤0.15 mm(100目)的刚玉细骨料取代较粗(1～0.15 mm或0.5～0.15 mm)的刚玉骨料,制备了2个系列、不同粒度级配的刚玉-莫来石试样,并分别以抗折强度保持率和弹性模量保持率为评价指标,研究了刚玉骨料种类和粒度对刚玉-莫来石材料性能的影响。结果表明:1)分别以白刚玉和板状刚玉为骨料时,刚玉-莫来石材料热震后的弹性模量保持率与抗折强度保持率并不完全一致;2)以≤0.15 mm骨料代替较粗骨料,虽能提高材料强度,但可能会降低其抗热震性能。分析认为:刚玉-莫来石材料抗临界危险裂纹扩展能力与材料热震强度保持率一致,相对于弹性模量保持率,以强度保持率作为此类材料抗热震性的评价标准更为合适;由于板状刚玉骨料周围残余应力的存在,热震实验后以板状刚玉为骨料的刚玉-莫来石材料具有更高的抗折强度保持率。     

Computer modeling of systematic processing defects on the thermal and elastic properties of open Kelvin-cell metamaterials

Open-cell metamaterials prepared by additive manufacturing or replica techniques are typically prone to processing defects resulting from limited resolution, strut cross-section variations or internal strut porosity. These defects are expected to cause deviations from the ideal (CAD-based or template-based) target microstructures and thus from the envisaged properties. This paper investigates some of these effects in a quantitative manner. Based on computer-generated open Kelvin-cell (tetrakaidecahedral) alumina-based metamaterials, the effective thermal conductivity and elastic constants, mainly Young’s modulus, are calculated in dependence of the voxel size, strut thinning and strut wall thickness. It is shown that the porosity dependence of smooth, straight and full struts agrees closely to the Gibson-Ashby prediction for open-cell foams, while limited resolution and strut thinning leads to property values that tend to be lower and hollow struts lead to higher property values. The Pabst-Gregorová cross-property relation gives an excellent prediction of the conductivity-modulus correlation in all cases.     

Thermal shock resistance of refractory composites with Zirconia and Silicon-Carbide inclusions and alumina binder

With the goal of designing a castable refractory for an aerospace application with optimum resistance to thermal shock, three different particle-reinforced ceramic composites are designed and compared. Different volume fractions of Silicon Carbide (SiC) particles, Zirconia (ZrO₂) bubbles, and Zirconia solid particles dispersed in an alumina (Al₂O₃) matrix are used in the fabrication of these castables. Destructive and nondestructive testing procedures are implemented to evaluate their thermomechanical properties, both before and after a custom designed thermal shock cycle. After demonstrating the applicability of thermal shock indices, the variation of these indices due to thermal shock is measured experimentally and utilized as a design tool. Multiple micro-scale damage mechanisms were observed, all of which are various forms of structural deformation.     

Modeling of elastic properties and conductivity of partially sintered ceramics with duplex microstructure and different grain size ratio

The elastic constants and conductivity of partially sintered single-phase and two-phase ceramics (exemplified by alumina ceramics and alumina-zirconia composites, respectively) with different grain size ratio (from 1:1 to 1:4) are investigated by numerical modeling. The relative elastic moduli of partially sintered two-phase ceramics are shown to be relatively similar to those of single-phase ceramics, whereas the relative conductivity is significantly lower, because of the higher phase contrast. The more the grain size ratio deviates from unity, the higher is the initial packing fraction, and the lower are the relative elastic moduli and conductivity of the partially sintered ceramics. The porosity dependence of the Poisson ratio shows a decreasing trend which is only very weakly affected by the grain size ratio. Correlations between relative Young’s modulus and relative conductivity lie between upper and lower cross-property bounds. For single-phase materials the correlation lies below, for two-phase materials above, the Pabst-Gregorová cross-property relation.     

Fabrication and thermal shock behavior of periclase-forsterite aggregates with micro-nanometer dual-pore-size structures

To improve the service life of periclase-forsterite refractories, it is important to develop aggregates with high thermal shock resistance. In this study, periclase-forsterite aggregates with good resistance to thermal shock and micro-nanopores were prepared using high-silicon magnesite, silica, and silica sol. Microcracks were generated in the multiphase aggregates, which inhibited the continuous propagation of cracks during thermal shock through mismatched thermal expansion coefficients. Based on Hasselman's thermal shock stability factor, the reduction in the average thermal expansion coefficient and improved mechanical characteristics were critical factors in improving the thermal shock resistance of the multiphase aggregates. As a binder, silica sol provided nano-SiO₂ and superplasticity, which facilitated the formation of micro-nanopores and strengthened the combination of the various phases in the aggregates.     

Young's modulus of isotropic porous materials with spheroidal pores

Based on the Eshelby solution for the single-inclusion problem and Wu's specification of this solution to spheroidal pores, we show that the Eshelby–Wu coefficients for Young's modulus, in contrast to their counterparts for the bulk and shear moduli, are quite insensitive to changes of the Poisson ratio. Therefore the Eshelby–Wu coefficients of Young's modulus can be described (to a very good approximation) by a unique function of the aspect ratio, which is calculated in this paper and for which a master curve is obtained by segment-wise fitting. Also the implementation of the Eshelby–Wu coefficients into the well-known effective medium approximations (Maxwell, self-consistent, differential) and our exponential relation is discussed. Irrespective of the model into which the Eshelby–Wu coefficients are implemented, prolate pore shape affects the porosity dependence of Young's modulus only very weakly, whereas oblate pore shape can lead to an arbitrary reduction of Young's modulus.     

大力发展优质合成耐火原料

进入新世纪,应根据我国耐火原料资源丰富的优势,加快发展有自己特色的矾土基和镁砂基的优质合成耐火原料.它们可分为均质料、改性料、转型料三种类型.     

利用有限元法分析耐火砖的热应力和变形

利用有限元法分析计算了G1、G2、C3和C4 4种耐火砖的热应力和变形。本工作为分析高炉内衬整个耐火砖的热应力和变形奠定了基础,对于延长高炉寿命具有重要的意义。     

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.     

Changes in the coefficient of thermal expansion in stressed Gilsocarbon graphite

The thermal expansion coefficient (CTE) of Gilsocarbon graphite samples has been measured using strain gauges whilst the samples were subject to either tensile or compressive static loading. Compressive loading increased the CTE from ∼5.4 × 10⁻⁶ K⁻¹ to ∼8 × 10⁻⁶ K⁻¹ for a compressive load of −50 MPa, whilst tensile loading decreased the CTE to ∼4 × 10⁻⁶ K⁻¹ for a tensile load of 12 MPa. There was also found to be corresponding changes to CTE perpendicular to the loading direction. Independent measurements on other samples using clip gauge extensometers were shown to support the strain gauge results. It is considered that the mechanism for this behaviour is related to closure of micro-cracks under load.     

Fractal characterization of ceramic crack patterns after thermal shocks

This work utilized a combination of experimental evidence and fractal geometric method to assess the effect of crack extension concerning the thermal shock on residual strength of ceramics. Sintered alumina (Al₂O₃) ceramic slabs were bundled and quenched in water under different thermal shock temperatures. The fractal dimension of thermal shock crack patterns on the interior surface and the cooled surface was calculated by the Box-counting method. Fracture energy of a fractal pattern of microcracks in quasi-brittle solids was employed to explain the relationship between crack length and fractal dimensions. The results show that if the crack propagation has the same crack length but a larger fractal dimension, it will absorb more fracture energy. The thermal shock crack patterns of Al₂O₃ ceramics with different grain sizes were analyzed, and the smaller grain size ceramic had a higher fractal dimension of crack patterns than the larger one.     

CaO稳定ZrO2耐火材料的相组成和热膨胀

对CaO稳定ZrO2耐火材料的相组成和热膨胀行为进行了研究.重点讨论了三种具有不同相组成的CaO稳定ZrO2材料的热膨胀特性.研究结果表明,通过调整ZrO2的稳定程度,可以优化材料的相组成和热膨胀行为,从而改善材料的抗热震性能,并有望使耐火材料的综合性能得到提高.     

Cr2O3-ZrO2材料的烧结及抗热震性研究

应用复合覆盖剂方法研究了Cr2O3-ZrO2材料在还原性气氛下的烧结和抗热震性.结果发现Cr2O3-ZrO2材料的烧结完全取决于Cr2O3的烧结;Cr2O3和ZrO2构成复合材料之后,可使其抗热震性明显提高.     

用涂层热线法测量含SiC耐火混合料的热导率

以国标 (GB/T10 2 97- 98)测定非金属固体材料热导率的热线法为基础 ,对热线和热电偶等涂敷一层绝缘涂料 ,测定了含SiC的耐火混合料的热导率。采用高精密仪表及利用多点测试数据线性回归处理法 ,可提高测量结果的可靠性和精度。     

Mechanical characterization at nanometric scale for heterogeneous graphite-salt phase change materials with a statistical approach

A statistical indentation analysis of a series of phase change graphite-salt composite materials for latent heat thermal energy storage applications was investigated using instrumented indentation technique with the aim of isolate the mechanical influence of each phase. This method employs the instrumented indentation technique to extract the in situ hardness and Young's modulus properties of individual components without the need to observe the residual imprints for heterogeneous materials. This approach relies on a large array of imprints (around 1000 indentations performed at 200 nm of indentation depth) and the statistical analysis of the resulting data. A statistical study by a Cumulative Distribution Function fit and Gaussian simulated distributions showed that the mechanical properties for each compound can be isolated when the indentation depth is much lower than the size of the secondary phases.     

Effect of carbon nanofiber additives on thermal behavior of phase change materials

Thermal performance of nanocomposite carbon nanofibers filled paraffin wax was studied experimentally and analytically. The transient temperature response of made nanocomposite was measured during its solidification process and the cooling rate was predicted. It was found that nanocomposite thermal conductivities were enhanced significantly causing the cooling rate to increase. An analytical model was introduced based on one-dimensional heat conduction approach to predict the effective thermal conductivity for the new nanocomposites and its findings showed good agreement with the experimental data. Also, a comparative study was performed to investigate the effect of carbon nanofibers surface characteristics on thermal performance of paraffin wax.     

Effect of glassy bonding materials 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 coefficient of thermal expansion value of a desired porous composite can be easily controlled by choosing the appropriate ratios of the different phases. It was shown that some of LiAlSiO₄ was decomposed to LiAlSi₂O₆ and LiAlO₂, some of LiAlSiO₄ reacted with SiO₂ to form LiAlSi₂O₆ during sintering. With increasing the content of glassy materials, the reaction between LiAlSiO₄ and SiO₂ was accelerated. The Young's modulus increased due to the neck growth between the SiC grains. The 52.5 vol% LiAlSiO₄ (LAS)/SiC ceramics with ∼36% porosity had a combination of near zero coefficient of thermal expansion ∼0.39 × 10⁻⁶ K⁻¹ at room temperature and relatively high Young's modulus ∼59 GPa.     

Poisson's ratio and volume change accompanying deformation of randomly oriented electrospun nanofibrous membranes

ABSTRACT

The present work focuses on the determination of volume change accompanying deformation and Poisson's ratio for electrospun nanofibrous membranes. For this purpose, polyurethane (PU) is considered for the fabrication of electrospun nanofibrous membranes. Three different sample thicknesses are fabricated. Following this, surface morphology analysis and fibre orientation analysis are conducted to investigate the variation of properties between electrospun PU membranes of different thicknesses. Subsequently, PU specimens are subjected to uniaxial extension test where the changes in sample width and thickness are recorded as a function of applied strain. Volume changes are computed while further analysis on the relationship between transverse strains and axial strain provided the values of Poisson's ratio. For all three electrospun PU samples investigated, significant volume changes are observed while the in-plane Poisson's ratio is found to be around 0.55. However, the out-of-plane Poisson's ratio of electrospun PU membranes are not classical and remains undetermined.