A statistical approach to strength evaluation incorporating porosity effect for porous ceramics

ABSTRACT It has not been clear whether the conventional effective volume proposed for dense brittle materials can be applied satisfactorily to the strength evaluation of porous ceramics. In the present study, a modified effective volume was proposed by incorporating the porosity effect in the statistical evaluation of strength properties of porous ceramics. The modified effective volume was derived as the conventional effective volume multiplied by a function of porosity p. In this work, a power function of (1 + p)^a was adopted as the simplest porosity function. To clarify the applicability of the modified effective volume, bending tests were conducted using smooth and notched specimens of 3 wt% MgO partially stabilised zirconia with six different porosities. The porosity dependence appeared in the relation between the conventional effective volume and the mean strength of various zirconia ceramics with different porosities. The exponent a of the porosity function was determined from experimental data obtained by using identically shaped specimens with distinct porosities, and the modified effective volume was calculated for several types of specimens used in the experiments. It was revealed that the mean strength was almost uniquely correlated with the modified effective volume independent of porosity. The experimental correlation verified the applicability of the modified effective volume to strength evaluation of porous ceramics.     

Effects of porosity and pore size distribution on mechanical strength reliability of industrial-scale catalyst during preparation and catalytic test steps

This paper investigates the effects of porosity and pore size distribution on crushing strength reliability of industrial-scale catalysts during their preparation and after reactor tests. Six supports, fabricated from two types of γ-alumina (type A, 119?m²/g; and type B, 343?m²/g), were used as starting materials. The supports were impregnated using a copper/zinc nitrate solution. The supports and catalysts, before and after the reactor tests, were characterized using XRD, ICP, SEM, and BET methods and were subjected to single particle crushing strength tests. The results revealed that the strength and Weibull modulus of all samples were decreased after the impregnation process due to the emergence of new flaws. In contrast, the partial sintering/necking of the interparticles during the reaction led to an increase of the crushing strength of the used catalysts. Furthermore, the porosity of the supports and catalysts from type A to type B γ-alumina were increased, leading to the strength decrease. Nevertheless, their Weibull modulus and strength reliability were increased due to the narrowing of pore size distribution in the range of mesopores.     

孔隙形状及孔隙率对多孔材料弹性性能的影响

利用通用单胞法(GMC)计算了不同孔隙形状及孔隙率对多孔材料等效弹性参数的影响,计算中分别采用二维方形、圆形孔隙模型和三维立方体、球形孔隙模型模拟多孔材料。不同孔隙率下等效弹性参数的计算结果表明: 不同孔隙形状下,多孔材料等效弹性参数随孔隙率增大的退化程度不同;通过对比二维简化模型与三维模型的差异,发现二维简化模型对多孔材料等效弹性参数的估算值偏低。进一步将GMC计算结果和已有文献实验结果进行比较,发现两者具有较高的吻合度。最后将GMC模型与有限元、经验模型进行对比,得出GMC模型的局限性。综合计算结果,GMC具有一定的计算精度,可应用于工程实际分析中。     

Characterization of strength properties of thin polycrystalline silicon films for MEMS applications

The aim of this work is to characterize the strength properties of polycrystalline silicon (polysilicon) with the use of tensile and bending test specimens. The strength of thin polysilicon films with different geometry, size and stress concentrations has been measured and correlated with the effective size of the specimen and its stress distribution. The test results are evaluated using a probabilistic strength approach based on the weakest link theory with the use of STAU software. The use of statistic methods of strength prediction of polysilicon test structures with a complex geometry and loading based on test values for standard material tests specimen has been evaluated.     

以冰为模板制备超轻多孔氧化锆块材

以冰为模板,经真空冷冻干燥过程制备轻质ZrO2多孔材料和水玻璃为粘结剂的超轻多孔ZrO2块材,研究了其微观结构、形成机理和隔热耐火性能.结果表明,冰是一种理想的制作多孔材料的模板,可同时获得多种尺度的孔结构,还可获得多孔和层状两种结构复合的微观结构.ZrO2多孔材料的微观结构为奇特的周期性层状结构.添加水玻璃作为粘结剂,制备出多孔和层状两种微结构复合的ZrO2多孔块材,在400℃煅烧6h后仍良好地保持原有的微观结构,孔隙率达87%,表观密度仅为0.50 g·cm-3,超轻且机械强度有较大提高,在1300℃丁烷气火焰灼烧下表现出较好的隔热耐火性能.     

BP模型在预测多孔NiTi合金孔隙度和屈服强度上的应用

基于BP神经网络的原理,建立了热爆法合成多孔NiTi形状记忆合金孔隙度和压缩屈服强度的神经网络模型.该模型拟合了温度、Ti粉颗粒尺寸和生坯密度对热爆产物孔隙度和压缩屈服强度的影响.当温度、Ti粉颗粒尺寸和生坯密度发生变化时,对反应产物的性能进行了预测,其预测值和实验值非常吻合.因此该模型可用于多孔NiTi形状记忆合金的性能控制,通过对性能的合理调控,使其与所替换的生物组织更加匹配,同时提高研究效率,降低成本.     

新型塑性加工制备球形孔多孔金属及其性能

将铝板塑性加工成半球孔层金属结构薄层,以此为基本单元按一定方式连接形成球形孔多孔金属,研究了孔结构对球形孔多孔金属性能的影响.结果表明,平板层相连的球形孔结构,使其强度高于同样条件下堆积的空心金属球结构.这种新型球形孔多孔金属的强度较高,能够有效地吸收能量,铝板的厚度和孔隙率对其压缩和能量吸能性能有明显的影响.     

聚乳酸涂层改善网状羟基磷灰石陶瓷支架力学性能的研究

采用有机泡沫浸渍法制备出高孔隙率的三维网状羟基磷灰石(HA)生物陶瓷支架,并采用不同浓度的聚乳酸(PDLLA)溶液涂覆网状HA支架,以达到增强补韧网化陶瓷支架的目的.通过扫描电镜观察PDLLA涂覆网状HA陶瓷支架的显微结构和测试力学强度,研究了不同浓度PDLLA溶液涂覆对网状HA支架的涂层厚度、显微结构和力学性能的影响.实验结果表明:涂覆一定量的PDLLA涂层可以显著提高网状HA生物陶瓷支架的抗压强度,同时保持网状陶瓷支架的贯通性和孔隙率.     

Weibull stress distribution for static mechanical stress and its stress/strength analysis

The method's steps to estimate the Weibull shape (β) and scale (η) parameters, based only on the ratio of the maximal and minimal principal stresses (σ₁/σ₂) and on the designed reliability (R(t)) are given in Section 4.1 . The method's efficiency is based on the following facts: (1) The square root of σ₁/σ₂ represents the base life on which the Weibull lifetimes are estimated (see Equation  61 ). (2) The mean of the logarithms of the expected lifetimes (g(x)) is completely determined by the determinant of the analyzed stress matrix (see Equation  13 ). (3) The Weibull distribution is a circle centered on the arithmetic mean (μ), and it covers the whole principal stresses' span (see Figure  5 ). (4) σ₁/σ₂ and g(x) completely determine the σ_1i and σ_2i values, which correspond to any lifetime in the Weibull analysis (see Equation  54 ). And (5) σ₁/σ₂ and η completely determine the minimal and maximal lifetime, which corresponds to any σ_1i and σ_2i values (see Equation  57 ). Additionally, by using the addressed stress β and η parameters, when the stress is either constant or variable, the formulation to estimate the designed R(t) index is given. The steps to determine both the material's strength average (μ_M) for a desired R(t) index and the R(t) index, which corresponds to a used μ_M value, are given.     

凝胶注模法制备多孔BiScO3-PbTiO3高温压电陶瓷及性能研究

为了拓展BiScO_3-PbTiO_3体系陶瓷的应用领域,采用凝胶注模工艺,对初始固相体积分数10%的准同型相界附近的BiScO_3-0.642PbTiO_3(记作BS-PT642)/混合物经球磨、注模、脱模、烘干、排胶,在1 100℃烧结,制备出多孔高温压电陶瓷.利用X射线衍射仪对多孔陶瓷进行物相分析,通过扫描电子显微镜观察样品的新鲜断面和抛光热腐蚀后表面的显微结构,并对其压电、介电和水声性能进行研究.研究表明,多孔BS-PT高温压电陶瓷为立方钙钛矿相结构,具有高静水压优值(HFOM)和高孔率结构,孔隙通道密实化清晰可见,没有显著的缺陷,且气孔之间相互连通,其中一个活跃的压电陶瓷相和一个被动空气相呈三维互连.样品的气孔率为31.8%,静水压优值为1 020×10~(-15)Pa~(-1),介电常数和压电常数分别为εr=620,d_(33)=213 pC/N.     

碳化硅多孔陶瓷气孔率和强度影响因素

研究了陶瓷粘结剂含量、碳化硅颗粒粒径以及烧结温度对高温气体过滤用碳化硅多孔陶瓷抗弯强度和气孔率的影响. 利用X射线衍射测试了多孔陶瓷烧结后的物相组成. 陶瓷粘结剂含量的增加使碳化硅多孔陶瓷的气孔率快速下降, 在陶瓷粘结剂含量15wt%时, 碳化硅多孔陶瓷可具有较高的气孔率(37.5%)和抗弯强度(27.63MPa). 随着碳化硅颗粒粒径从300?m减少到87um, 碳化硅多孔陶瓷的气孔率和抗弯强度可同时提高, 气孔率从35.5%增加到了42.4%, 而抗弯强度从19.92MPa增加到了25.18MPa. 碳化硅多孔陶瓷的烧结温度从1300℃增加到1400℃过程中, 其气孔率从38.7%迅速下降到35.4%, 而其抗弯强度一直在27MPa左右, 没有大幅变化, 所以该多孔陶瓷的烧结温度应该选在陶瓷粘结剂熔点(1300℃)附近, 不宜过高.     

Fracture strength distribution of porous ceramics under quasi-static load

The purpose of this paper is to estimate the fracture strength distribution of porous ceramics under quasi-static load. Four-point bending test was performed for SiC-porous ceramics at room temperature under quasi-static load. Fracture strength distributions obtained in the above test were estimated with the aid of a conventional probabilistic time-dependent fracture model on the basis of the slow crack growth concept in conjunction with two-parameter Weibull distribution. The results showed that the estimated fracture strength distribution curves were not in good agreement with the experimental data at stress rates. Porous ceramics have damage-tolerable property due to failure of a lot of grain boundaries. Therefore, this is because the dispersion of applied stress was not considered in the conventional model. A new probabilistic time-dependent fracture model considered the dispersion of applied stress was proposed based on Markov process in conjunction with local load sharing rule. The fracture strength distribution curves estimated the aid of the new model were in reasonably good agreement.     

ZrO2陶瓷的强韧化技术

介绍了ZrO2 陶瓷的增韧机理及各种增韧技术。     

球形孔通孔和闭孔泡沫铝合金的超声衰减性能

研究了球形孔通孔和闭孔泡沫铝合金在1 MHz～10 MHz的超声衰减性能.结果表明:泡沫铝合金的超声衰减性能决定于其孔结构;通孔泡沫铝合金的超声衰减系数α随着孔径d的减小、孔隙率Ps减小和比表面积Sv的增加而增大;闭孔泡沫铝合金的超声衰减系数α随孔径d的减小、孔隙率Ps的增加和比表面积Sv的增加而增大;当孔径d、孔隙率Ps相近时,闭孔泡沫铝合金的超声衰减性能优于通孔泡沫铝合金;在1 MHz～10 MHz二者是具有良好阻尼性能的轻质材料.其衰减机制为在弹性范围内超声应力波在具有大量孔隙界面的泡沫铝合金中的衰减.     

A novel method to measure the porosity of porous materials

ABSTRACT

Porosity is the single most important feature of porous materials. On the basis of the research work, a new method to measure the porosity of porous materials is proposed in this paper. This method is a system of equations composed of six formulas, so it is called model equation method. It is not only applicable to space holder technique but also applicable to metal prepared by pure powder metallurgy. Compared with mass volume method and immersion medium method, our new method is more simple and efficient. More importantly, it can be used as a standard measure. This study provides a fundamental basis for the measurement of porosity in porous materials.     

原位反应结合碳化硅多孔陶瓷的制备与性能

以碳化硅(SiC)和氧化铝(Al₂O₃)为起始原料、石墨为造孔剂, 通过原位反应结合工艺制备SiC多孔陶瓷. XRD分析表明多孔陶瓷的主相是SiC, 结合相是莫来石与方石英; SEM观察到多孔陶瓷具有相互连通的开孔结构. 坯体在烧结前后具有很小的尺寸变化, 线收缩率约在±1.5%内. 多孔陶瓷的开口孔隙率随烧结温度和成型压力的增大而减小, 随石墨加入量的增加而增大; 而体密度具有相反的变化趋势. 随着石墨粒径的增大, 多孔陶瓷的孔径分布呈现双峰分布. 抗弯强度随烧结温度和成型压力的增大而增大, 随石墨加入量的增大而减小. 于1450℃保温4h烧成的样品在0～800℃的平均热膨胀系数为6.4×10^-6/K. 多孔陶瓷还表现出良好的透气性、抗高温氧化和耐酸腐蚀性, 但耐碱腐蚀性相对较差.     

Effect of pore structure on mechanical properties of porous TiAl

Based on two sets of TiAl powder, two kinds of porous TiAl were separately fabricated by powder metallurgical route including four stages. The porous TiAl with single pore structure (SPS) was prepared using pre-alloyed TiAl powder prior mechanical ball milling. Another porous TiAl with composite pore structure (CPS) was manufactured depending on composite mixture of Ti/Al elemental powders. The sintering was achieved at much lower temperature for the pre-alloyed power than for the elemental composite mixture. Compressive mechanical tests indicate that much higher mechanical strength can be obtained for SPS than for CPS at the same porosity. It was suggested that the difference of mechanical properties is ascribed to the variety of the compressive deformation process.     

造孔剂含量对多孔BiScO3-PbTiO3高温压电陶瓷性能的影响

为研究造孔剂含量对多孔BiScO_3-PbTiO_3(记作BS-PT)高温压电陶瓷性能的影响,采用炭黑作为造孔剂,通过传统固相烧结法对不同初始固相体积分数的准同型相界附近的BS-PT/炭黑混合物制备出多孔高温压电陶瓷.利用X射线衍射仪对多孔陶瓷进行物相分析,通过扫描电子显微镜观察样品的新鲜断面和抛光热腐蚀后的表面显微结构,并对其压电、介电和水声性能进行研究.结果表明:制备的不同初始BS-PT体积分数多孔压电陶瓷均为立方钙钛矿相结构,孔隙通道密实化清晰可见,没有显著的缺陷,气孔的形状是长条形的,长度从几微米到几十甚至上百微米不一,气孔之间是孤立的;随着造孔剂炭黑的体积分数从20%增加至50%,气孔率线性地从8.3%增加至22.1%,εr、d33及d31逐渐下降,静水压优值(HFOM)逐渐增大.多孔压电陶瓷的性能强烈依赖于孔隙空间结构,如气孔率、孔径、孔隙形态及连接方式.     

孔隙分布不均匀性对陶瓷材料强度的影响

由于工艺特殊性,一些陶瓷材料中不可避免地存在孔隙、夹杂等缺陷,且孔隙、夹杂大多分布不均匀,而这种不均匀性对陶瓷材料力学性能有较大影响.根据分布不均匀性特点,以陶瓷材料中孔隙分布为例,定义了均匀度对孔隙分布的不均匀性进行量化,提出了利用图像处理确定均匀度的方法.根据图像处理结果,将材料分为富含孔隙的等效基体相和不含孔隙的等效颗粒相,利用相互作用直推估计(IDD)法研究了材料强度随均匀度的变化关系.结果表明:孔隙分布均匀度对材料强度有较大影响,孔隙分布的均匀度越小,材料强度越低,并且在均匀度低于0.2时造成强度的剧烈降低,当均匀度大于0.5时,对材料强度的影响很小.