Characterization of cellular ceramics for high-temperature applications

Polymeric sponge replication technique is the most used process to obtain ceramic foams with a cellular structure for filtration applications. This technique is based on an impregnation of a polymeric sponge with ceramic slurry, removal by squeezing, followed by burning out polymer and high temperature sintering. Ceramic filters must present high permeability and strength. However, these parameters are influenced in different ways by the processing method and the consequent cellular structure. In this work the relationship between permeability and strength has been investigated for 10- and 40-ppi (pores per linear inch) Al₂O₃–ZrO₂ filter materials. Characterization included the evaluation of the permeability and strength as well as the microstructural analyses of the fracture surface.     

Preceramic polymer derived cellular ceramics

Ceramic foams were prepared by a self-blowing process of a poly(silsesquioxane) melt at 270 °C. The cell size, the interconnectivity density and the shape of the foam cells were adjusted by a thermal pre-curing procedure of the polymer at 200 °C. Inorganic fillers were used to modify processing behaviour and properties of the pyrolysed ceramic foam. After pyrolysis in inert atmosphere at 1200 °C ceramic composite foams with a total porosity up to 87% were obtained. The open cell ceramic foams had a mean cell diameter of 1.2 mm and a mean strut thickness of 0.2 mm. Interpenetrating phase composites (IPCs) were fabricated by infiltrating the open cellular ceramic preform with Mg alloy melt at 680 °C and a pressure of 86 MPa. The mechanical properties were found to depend on the reactions between the metal and the ceramic forming MgO, Mg₂Si and Al₁₂Mg₁₇ as the major reaction products. The IPCs showed a significantly higher creep resistance at 135 °C, compression strength and elastic modulus compared to the unreinforced magnesium alloys.     

Conventional and microwave-assisted processing of Cu-loaded ICAs for electronic interconnect applications

Isotropically conductive adhesives (ICAs) and inks are widely used for interconnecting components and for printed circuits. Silver (Ag)-filled ICAs and inks are the most popular due to their high conductivity and good reliability. However, the price of Ag is a significant issue for the wider exploitation of these materials in low cost, high volume applications such as printed electronics. In addition, there is a need to develop systems compatible with temperature-sensitive substrates through the use of alternative materials and heating methods. Copper (Cu) is considered as a more cost-effective filler for ICAs and in this work, Cu powders were treated to remove the oxide layer and then protected with a self-assembled monolayer. The treated Cu powder was combined with one of two different adhesive resins to form ICAs that were stencil printed onto glass substrates before curing. The use of conventional and microwave-assisted heating methods under an inert atmosphere for the curing of the Cu-loaded ICAs was investigated in detail. The samples were characterised for electrical performance, microstructure and shrinkage as a function of curing temperature (80–150 °C) and time. Tracks with electrical conductivity comparable to Ag-filled adhesives were obtained for both curing methods and with both resins. It was found that curing could be accelerated and/or carried out at lower temperature with the addition of microwave radiation for one adhesive resin, but the other showed almost no absorption indicating a difference in mechanism for the two formulations.     

Adaptive methods and system for image processing

We present results obtained in the implementation of adaptive methods and a system for image processing. The light-developing nonlinear elements are used in the filtering block of the mirror optical configuration. The processing of gray-scale and of binary images with the help of adaptive optical elements is fulfilled for realization of ground-suppressing, contouring, differentiating, and skeletonizing adaptive filters. A comparison is made between optical and digital methods applied in the pattern processing.     

Sol-gel processing: A versatile concept for special glasses and ceramics

The various parameters related to sol-gel processing are discussed with special reference to those which usually attract less attention but depending on the final product in mind, can play important roles. The versatility of the sol-gel technique in materials preparation is demonstrated by discussing the various products developed at the author’s laboratory by using this processing method.     

Low-temperature processing of porous SiC ceramics

Porous silicon carbide ceramics were fabricated from SiC, polysiloxane, and polymer microbead (as a pore former) at a temperature as low as 800 °C by a simple pressing and heat-treatment process. The effects of polysiloxane and template contents on the porosity and strength of the ceramics were investigated. During heat treatment, the polysiloxane transformed to an amorphous SiOC phase, which acted as the bonding material between SiC particles, and the polymer microbeads decomposed into gases and left pores. The porosity of porous SiC ceramics could be controlled within a range of 26–56 % with the present set of processing variables. The porous SiC ceramics showed a maximal porosity of 56 % when 10 μm SiC particles and 16 % polysiloxane were used with 20 % polymer microbeads. Flexural strength generally increased with increasing polysiloxane content and decreased with increasing polymer microbead content. Typical flexural strength of the porous SiC ceramics was 53 MPa at 42 % porosity.     

电泳沉积及其在新型陶瓷工艺上的应用

介绍了电泳沉积的特点、悬浮液的稳定机制、电泳沉积的机理及动力学原理,并对该技术在制备固体表面陶瓷涂层、孔状结构陶瓷、多层及复合结构、固体氧化物电池、纳米材料及纳米结构陶瓷上的应用进行了总结。     

运用过程方法实施QMS审核

在质量管理体系(QMS)审核中发现,不少组织常常机械地套用ISO 9001标准条款编制质量手册和程序,与组织实际的控制形成两张皮--不能结合组织自身特点识别过程、过程之间的关系以及相互作用未能清楚表述、组织的质量管理体系与整个经营体系脱离、为通过ISO 9001认证而建立体系,基于此种现象,采用过程方法实施审核,引导组织建立更加符合其实际运行过程和产品/服务特点的质量管理体系,是实现审核增值的途径之一.     

Regularization methods for processing fringe-pattern images

A powerful technique for processing fringe-pattern images is based on Bayesian estimation theory with prior Markov random-field models. In this approach the solution of a processing problem is characterized as the minimizer of a cost function with terms that specify that the solution should be compatible with the available observations and terms that impose certain (prior) constraints on the solution. We show that, by the appropriate choice of these terms, one can use this approach in almost every processing step for accurate and robust interferogram demodulation and phase unwrapping.     

Bioactive ceramics: processing, structures and properties

There is an increasing need for bone repair materials for skeletal reconstruction, due to the prevalence of diseases such as osteoporosis and to the growing number of aged and overweight people Worldwide. Although used widely, there are limitations with autograft and allograft, including issues of supply and effectiveness, respectively. This has led to the need for more suitable synthetic biomaterials to replace natural bone, which can be nearly inert or bioactive. This review aims to discuss bioactive implants, coatings and scaffolds made of ceramics, glasses, glass–ceramics and composites. These are able to form a chemical interfacial bond with tissue and can be resorbable or non-resorbable.     

An improvement in processing of hydroxyapatite ceramics

Hydroxyapatite ceramics have been fabricated via two different processing routes, a conventional processing route and an emulsion-refined route. The conventional precipitation processing of powder precursors for hydroxyapatite ceramics results in the formation of hard particle agglomerates, which degrade both the compaction and densification behaviour of the resultant powder compacts. An emulsion-refinement step has been shown to be effective in softening particle agglomerates present in the conventionally processed powder precursor. As a result, the emulsion-refined powder compact exhibits both a higher green density and a higher sintered density than the un-refined powder compact, on sintering at temperatures above 800 °C. The effect of powder agglomeration on densification during both the initial and later stage of sintering is discussed. The attainable sintered density of the conventionally processed material was found to be limited by the presence of hard powder agglomerates, which were not effectively eliminated by the application of a pressing pressure of 200 MPa. These hard powder agglomerates, which form highly densified regions in the sintered ceramic body, commenced densification at around 400 °C which is more than 100 °C lower than the densification onset temperature for the emulsion-refined powder compact, when heated at a rate of 5 °C min^–1. The inter-agglomerate voids, manifested by the differential sintering, resulted in the formation of large, crack-like pores, which act as the strength-limiting microstructural defects in the conventionally processed hydroxyapatite. A fracture strength of 170±12.3 MPa was measured for the emulsion-refined material compared to 70±15.4 MPa for the conventionally processed material, when both were sintered at 1100 °C for 2 h.     

Chemical processing and microstructure of piezoelectric lead titanate ceramics

Lead titanate-based ceramics with different microstructures were prepared from PbO-TiO₂ gels with equimolar and non-equimolar mixtures of the oxides. Deviations from the initial compositions were found in the sintered materials. As a consequence, microstructures with different grain sizes and secondary phases were developed in the resultant ceramics. These microstructures had a strong influence on the piezoelectric behaviour of the materials obtained.     

Preparation of novel calcia-stabilized TZP ceramics

The preparation of tetragonal zirconia polycrystal (TZP) ceramics stabilized by the addition of calcia is presented. These novel ceramics were obtained by means of a fast-firing treatment applied to compacts of nanocrystalline powders with a composition of ZrO₂ — 4 mol% CaO. Powders were synthesized by a nitrate-citrate gel-combustion process and they exhibited the tetragonal phase at room temperature due to their small crystallite size (12–13 nm). These powders were compacted by uniaxial pressing and fired at 1400–1500 °C for 3–5 min, obtaining fine-grained, dense ceramics, which also retained the metastable tetragonal phase. Longer heat treatments increased the average grain size leading to large amounts of the monoclinic phase. It was also found that the critical grain size for the tetragonal-to-monoclinic transformation is about 150 nm for this composition. This small value explains the necessity of a fast-firing treatment of the samples, allowing densification but avoiding a significant grain growth.     

Colloidal processing of lead lanthanum zirconate titanate ceramics

Well-dispersed aqueous slurries of fine ceramic powders with high solids loading are often required for various shape forming techniques such as slip and tape casting in order to fabricate advanced ceramics with a dense and uniform microstructure. Colloidal processing of lead lanthanum zirconate titanate (PLZT) powders was conducted at various pH using ammonium polymethacrylate as a dispersant. Suspensions were characterized by viscosity and zeta-potential measurements. The effect of pH on polymer adsorption and the rheological behavior of the slurries were investigated and stabilization mechanisms discussed. Through optimization of the dispersant concentration and pH, solids loadings of the suspensions up to 50 vol.% with a relatively low viscosity were obtained.     

Thermal processing and properties of highly homogeneous alumina-zirconia composite ceramics

Alumina-zirconia composites were made using a novel processing technique involving nonsolvent precipitation of a dilute polymer solution containing dispersed ceramic powder particles. Seven identical alumina-nominally 15 wt% zirconia composite green bodies were fired at different temperatures. An optimal firing temperature of 1500° C was found, above and below which sample fracture strengths, densities, and tetragonal zirconia contents were lower. Fracture strengths correlated well with porosity. An alumina-14.06 wt% (zirconia-1.35 mol% yttria) composite had excellent resistance to abnormal grain growth upon high-temperature annealing. The tetragonal zirconia content increased with increasing annealing time; this was attributed to the equilibration of zirconia particles with widely varying yttria concentrations.     

Characterization of engineering ceramics by indentation methods

The induction of indentations in brittle materials and their evaluation result in information useful for the characterization of materials. In addition to the fracture-mechanical evaluation of the indentations, i.e. of the incipient cracks resulting from them, the cracks may be used for determining the fracture toughness,information regarding the hardness, critical size of surface defects, characteristic quantities of crack propagation, durability, as well as existing internal stresses.     

Hybrid processing and anisotropic sintering shrinkage in textured ZnO ceramics

Abstract

We have studied the combined effects of the templated grain growth and magnetic alignment processes on sintering, anisotropic sintering shrinkage, microstructure development and texture in ZnO ceramics. Suspensions of 0–10 vol % ZnO template particles were slip cast in a 12 T rotating magnetic field. Sintering and texture characteristics were investigated via thermomechanical analysis and electron backscatter diffraction, respectively. Sintering as well as texture characteristics depend on template concentration. For the studied ZnO system, there is a critical template concentration (2 vol % in this study) above which densification is limited by the templates owing to constrained sintering. Below this limit, the densification is enhanced and the anisotropic shrinkage is reduced, which is attributed to densifying characteristics of the templates.