Processing and characterization of MWCNT reinforced aluminum matrix composites

Metal matrix composites comprising aluminum matrix and multi-wall carbon nanotubes (MWCNTs) as reinforcements are fabricated using cold uniaxial compaction followed by sintering and cold extrusion as secondary processes. The MWCNTs are pretreated with sodium dodecyl sulfate for improved adhesion with aluminum powder. The effect of sintering temperature on the microstructure is explored using differential scanning calorimetric spectrum. The tensile yield and ultimate strength of Al-MWCNTs increased to 90% with 2 wt% addition of MWCNTs. Various theories for the strengthening and stiffening of Al-MWCNTs composites are explored.     

先进陶瓷材料固相烧结理论研究进展

系统介绍了陶瓷材料固相烧结理论的历史和研究进展,综述了用来描述烧结前期、中期和后期的烧结理论和烧结模型.目前烧结理论大多局限于烧结全程的某一阶段,且只研究某一种扩散机制起主导作用,多数理论烧结模型不能完全反映真实烧结参数,烧结单元模型的定量描述不够完善,缺乏描述烧结全程的烧结模型,且大多研究局限于基础研究,如物质的传输机制、致密化过程、气孔和晶粒生长机制.因此,建立多种扩散机制耦合作用的全期烧结模型,进一步研究烧结动力学,用计算机模拟烧结的真实条件,建立能定量描述的烧结模型,是未来烧结理论研究的方向.     

SiC晶须增韧陶瓷基复合材料的研究进展

介绍了SiC晶须增韧陶瓷基复合材料的烧结合成方法，将各动力学因素（晶须含量、混合工艺和烧结温度）对热压烧结法制备SiC晶须增韧陶瓷基复合材料的影响进行了详细阐述，叙述并讨论了SiC晶须增韧的不同机理，并展望了该领域的研究方向。     

分形理论及其在材料科学中的应用

介绍了分形的概念以及分形维数的计算方法 ,重点讨论了分形理论在材料科学 ,如材料磨损表面分析、断裂表面分析、无机材料的烧结与氧化过程分析等方面的应用     

微波加热制备特种陶瓷材料研究进展

特种陶瓷广泛应用于航天航空、电子信息、新能源、机械、化工等新兴工业领域, 其高温制备过程仍以传统燃气窑炉和电加热炉为主; 碳排放高、能耗大, 节能减排形势严峻。当前, 我国面临实现“双碳”目标的巨大压力, 研究推广清洁高效的加热技术迫在眉睫。微波加热是利用材料自身对微波进行吸收, 将电磁能转化为热能, 能量的转移发生在分子水平上, 通过这种方式, 加热在整个材料内外同时产生, 整个材料体系中的温度梯度非常低。除体积加热外, 选择性加热、功率再分配、热剧变以及微波等离子效应等也是微波烧结的显著特征。微波加热具有节能环保、改善制品性能、减少燃烧碳排放等优点, 国内外有许多关于微波合成各种氧化物、碳化物、氮化物陶瓷粉体和微波烧结陶瓷复合材料的报道。本文首先对微波和微波混合烧结的基本理论进行综述, 然后介绍了微波加热制备陶瓷粉体与微波烧结制备陶瓷材料的最新研究进展, 最后总结了微波加热在陶瓷工程制品烧结中的一些研究成果, 体现出微波烧结的优越性, 并提出了微波烧结制备特种陶瓷的关键问题和今后的发展方向。     

碳化硼固相烧结微观结构演化的同步辐射CT观测

利用同步辐射CT (SR-CT) 技术,在碳化硼陶瓷样品烧结过程中对其进行实时投影成像,并应用滤波反投影算法和数字图像处理技术,得到了样品在整个烧结过程中内部微结构演化的二维和三维重建图像,实现了对陶瓷固相烧结过程实时、无损的观测.通过重建图像清晰观测到了陶瓷样品在烧结三个阶段中颗粒接触、烧结颈形成、晶粒和气孔长大、气孔球化并收缩等烧结现象;统计了样品在不同烧结时刻的孔隙率,得到了孔隙率随烧结时间和烧结时间对数的变化曲线,并根据曲线分析了样品在不同烧结时刻致密化速率的变化,得到了烧结中期孔隙率和时间对数的线性关系.实验结果和现有烧结理论相吻合,并为进一步完善烧结理论以及建立扩散和本构模型提供了有效的实验数据.     

Employee Startups in High-Tech Industries

Various theories that can account for employees leaving incumbentfirms to start their own firms in the same industry are discussed.The theories are evaluated using evidence from three new andone old study of such employee startups in high-tech industriesand from studies of high-tech and high-growth startups of allkinds. A new, evolutionary-based theory that builds on and extendsexisting theories is sketched out to provide a novel interpretationof the evidence.     

Nonlocal theories for bending,buckling and vibration of beams

Various available beam theories, including the Euler–Bernoulli, Timoshenko, Reddy, and Levinson beam theories, are reformulated using the nonlocal differential constitutive relations of Eringen. The equations of motion of the nonlocal theories are derived, and variational statements in terms of the generalized displacements are presented. Analytical solutions of bending, vibration and buckling are presented using the nonlocal theories to bring out the effect of the nonlocal behavior on deflections, buckling loads, and natural frequencies. The theoretical development as well as numerical solutions presented herein should serve as references for nonlocal theories of beams, plates, and shells.     

Evolution of fractal features of pores in compacting and sintering process

As an index of molding, the fractal dimension of pores can reflect system varying details as the density. Pores in the powder aggregates with a composition of Fe–2%Cu–0.5%C were researched. The change tendency of dimension of pores as a function of compaction pressure or sintering temperature was investigated. Result indicates that the dimensions of pores in this ternary system decrease both in the process of compacting and sintering. It is convenient to build up some theories by combining compacting and sintering based on the variation of pore dimensions.     

Enhancing densification of zirconia-containing ceramic-matrix composites by microwave processing

Various ceramic-matrix composites containing zirconia were sintered using a 2.45 GHz microwave field. The effects of the addition of zirconia and the processing parameters on the sintering and microstructure development were investigated. The results showed that microwave processing enhanced the densification of these composites considerably. The enhancement in sintered density was up to 46% over conventional sintering, depending on the systems. This revised version was published online in November 2006 with corrections to the Cover Date.     

Acoustic coagulation of cement mixes

Coagulation of cement paste due to ultrasonic vibrations is described. Various mechanisms (theories) of coagulation are compared with the results of the experiments.     

Densification and strength evolution in solid-state sintering Part II Strength model

A compact gains strength in sintering through low-temperature interparticle bonding, followed by further strength contributions from high-temperature densification. On the other hand, thermal softening substantially reduces a compact's strength at high temperatures. Therefore, the in situ strength during sintering is determined by the competition among interparticle neck growth, densification, and thermal softening. Distortion in sintering occurs when the compact is weak. Most strength models for sintered materials are semi-empirical relations based on the sintered fractional density. These models do not include microstructure or sintering cycle parameters; thus, they do not provide guidelines for thermal cycle design to improve compact dimensional control. A strength evolution model is derived which combines sintering theories and microstructure parameters, including interparticle neck size, solid volume fraction, and particle coordination number. The model predicts sintered strength and when combined with thermal softening gives a good prediction of in situ strength. The validity of the model is verified by comparison to experimental data for sintered and in situ strength of bronze and steel powders.     

Ceramic composite components with gradient porosity by powder injection moulding

Various components used in the industries may benefit from having layered structures with gradient porosity in each layer. In this paper, bi-layer composite components with gradient porosity made by Powder Injection Moulding (PIM) have been investigated. The ceramic spinel materials of AR7845 having coarse particle size and AR7820 having fine particle size were used. It shows that AR7820 and AR7845 powders have different sintering behaviour with the fine powder having faster shrinkage as compared to coarse powder. Curling or bending is found in the bi-layer rectangular composite component fabricated from these two powders. This is due to induced stress caused mainly by strain rate mismatch of the two materials during sintering. The degree of curling is also related to thickness ratio of two materials in each layer. Composite components can be designed into cylindrical shape so as to avoid curling as observed in rectangular composite components. No interfacial debonding and part cracking are observed in both rectangular and cylindrical composite components. The microstructure shows that continuously straight joining lines along the interfaces are formed in these composite components.     

Generalized flow potentials in finite elastoplasticity—II. Examples

Various representations of generalized stress potentials are presented that result from a straightforward application of isotropic function theory. A variety of theories are then reduced from the theory as special cases. For comparison, a non-isotropic flow potential is presented.     

混凝土与各向同性岩石强度理论研究进展

强度理论是研究复杂应力状态下材料是否破坏的理论,该文总结了古典强度理论以及混凝土和各向同性岩石等两类现代强度理论,将各项同性现代强度理论分为剪应力强度理论、八面体强度理论与主应力强度理论。通过收集国内外已有相关三轴实验数据对各种主要强度理论进行论述、比较与评价,在此基础上对混凝土和各向同性岩石的主应力空间损伤比强度理论进行展望。     

PROCESSING AND PROPERTIES OF MODIFIED-LEAD TITANATE CERAMICS

Various ways to synthesize modified-lead titanate ceramics such as mixed oxides, coprecipitation and sol-gel are briefly reviewed. It has been demonstrated that the wet-chemical methods allow lower sintering temperatures and a better microstructure control. No strong differences in the K_t/K_p, ratio, were found. Better performances piezoelectric in homogeneous (Ca or lanthanide) modified-lead titanate ceramics have been achieved. From the point of view of an industrial application the coprecipitation processing for the bulk production of these ceramics is suggested.     

The sintering of combustion-synthesized titanium diboride

A comparative study of the sinterability of combustion-synthesized titanium diboride was conducted over the temperature range of 1800 to 2100° C. During the initia! sintering stage, the densification rate was slightly higher in the combustion-synthesized than in the commercially obtained titanium diboride. For sintering times of > 30 min, however, the shrinkage rates for both types of powders were the same. The activation energy for the late sintering stage was 774 ± 46 kJ mol^–1, consistent with 8 volume diffusion mechanism, end was the same for both combustion-synthesized and commercial powders. The microstructures of sintered specimens with initial particle size below 1O µm exhibited a grain size ranging from 5 to over 40 µm after 30 min of sintering. The addition of 5 wt% NbB₂ to the combustion-synthesized resulted in enhanced shrinkage during the initial sintering stage, but did not affect later stage kinetics. Various amounts of additives of CrB₂, NiB and TiC had no effect on early and late stage sintering kinetics, with the exception of 50 wt% TiC which appreciably inhibited densification.     

Production of lightweight aggregates from mining residues, heavy metal sludge, and incinerator fly ash

In this study, artificial lightweight aggregate (LWA) manufactured from recycled resources was investigated. Residues from mining, fly ash from an incinerator and heavy metal sludge from an electronic waste water plant were mixed into raw aggregate pellets and fed into a tunnel kiln to be sintered and finally cooled rapidly. Various feeding and sintering temperatures were employed to examine their impact on the extent of vitrification on the aggregate surface. Microstructural analysis and toxicity characteristic leaching procedure (TCLP) were also performed. The results show that the optimum condition of LWA fabrication is sintering at 1150 degrees C for 15 min with raw aggregate pellets fed at 750 degrees C. The rapidly vitrified surface envelops the gas produced with the increase in internal temperature and cooling by spraying water prevents the aggregates from binding together, thus forming LWA with specific gravity of 0.6. LWA produced by sintering in tunnel kiln shows good vitrified surface, low water absorption rate below 5%, and low cylindrical compressive strength of 4.3 MPa. In addition, only trace amounts of heavy metals were detected, making the LWA non-hazardous for construction use.     

Electric Current-Assisted Sintering of 8YSZ: A Comparative Study of Ultrafast High-Temperature Sintering and Flash Sintering

Electric current-assisted sintering (ECAS) is a promising powder consolidation technique that can achieve short-term sintering with high heating rates. Currently, main methods of performing ECAS are indirect heating of the powder compact in a conductive tool or direct heating with current flowing through the powder compact. Various influencing factors have been identified to explain the rapid densification during ECAS, such as ultrahigh heating rates, extra-high temperatures, and electric field. However, the key consolidation-enhancing factor is still under debate. This study aims at understanding the role of heating rate on the enhanced densification during ECAS of 8 mol% Y₂O₃-stabilized ZrO₂ (8YSZ) by experimental and numerical methods. Two different heating modes, ultrafast high-temperature sintering (UHS, indirect heating) and flash sintering (FS, direct heating), are studied. The novel UHS technique is successfully applied to consolidate the 8YSZ samples. Additionally, finite element methods (FEM) combined with a constitutive model is adopted to predict the densification and grain growth. Furthermore, a comparison of UHS and FS is performed to investigate the thermal effect (heating rate) and athermal effect (electric field) individually. The results indicate that the high heating rate is the key factor of the rapid densification during UHS and FS of 8YSZ.