Sol-gel fabrication of compact, crack-free alumina film

Compact, crack-free alumina film was fabricated using an alumina sol with a high Al₂O₃ content. With the addition of ethylacetoacetate (CH₃COCH₂COOC₂H₅, EAcAc), the stable sol could be prepared with a molar ratio of aluminum sec-butoxide (Al(O-sec-Bu)₃, ASB) to water up to 1:25. It was found that EAcAc could notably decrease the surface tension of the liquid in the gel pores. The EAcAc modification layer on the colloidal particle retarded greatly the densification of the Al₂O₃ gel film and provided a long-lasting structural relaxation during heating. Therefore, the formation of cracks was effectively prevented in this alumina material. The alumina gel film contained a high Al₂O₃ content and there was a rather small mass loss during sintering. The critical thickness of Al₂O₃ sol-gel film was eight times higher than that could be achieved via the general sol-gel route and a film thicker than 0.8 μm was prepared by a single-step dipping operation.     

The preparation of AI2O3/Ni composites by a powder coating technique

In the present study, nickel particles are coated onto the surface of alumina powder by an impregnation technique. The densification behaviour and the microstructural evolution of the nickel coated alumina powder during sintering are investigated. The strength and the toughness of the resulting Al₂O₃/Ni composites are determined. As the nickel content is less than 13 vol%, fully dense composites can be prepared by pressureless sintering. The matrix grain size decreases as nickel inclusions are added. The strength and the toughness of alumina can be increased by 23 and 42% by adding 5 and 8vol% nickel, respectively. The toughening effect is attributed to plastic deformation of ductile inclusions and crack deflection by the inclusions. The strengthening effect is attributed to microstructural refinement.     

Microstructure and properties of alumina-SiC nanocomposites prepared from ultrafine powders

Alumina-Silicon Carbide nanocomposites were produced and studied under different aspects: characteristics of the starting materials, processing, microstructure and mechanical properties. The raw materials were two kinds of fine SiC powders (30 and 45 nm) and two Al₂O₃ powders (60 and 140 nm). Different compositions (amounts of SiC in the range 0.5–5 vol%) were performed and the characteristics of the resulting materials compared. The oxygen enrichment in SiC nanopowder due to specific powder treatments was controlled, in order to optimize powder processing routes. Densification tests of Al₂O₃-SiC powder mixtures were performed both by pressureless sintering and hot pressing route. The addition of SiC reduced the densification rate and favoured a refinement of the matrix. Improvement of mechanical properties over monolithic alumina was obtained in composites with the 45 nm SiC. The study pointed out that the critical factor for the success of these materials is the choice of the raw SiC powders in terms of grain size and state of agglomeration. The addition of this ultrafine SiC strongly affected the microstructural evolution, even at low volumetric fractions. The results do not substantiate any remarkable effect by dispersoids in the tested nanosize range.     

Effect of small amounts of additives on the sintering of high-purity Y-TZP

Superfine Y-TZP powders of high purity were prepared by using clean-room facilities. The effects of several kinds of minute or small amounts of additives on the densification and microstructural development of the Y-TZP powder were investigated. It was found that ≤1 wt% ferric or calcium oxides did not affect the densification of the Y-TZP powder compacts, while the addition of sodium oxides retarded the densification and that of copper oxide accelerated it, and these effects are most obvious at an additive level of 1 wt%. The retardation of the densification by sodium oxide was found to result from the agglomeration effect of the powder, and the formation of the eutectic liquid phase between zirconia and copper oxides promoted the densification and grain growth during sintering of copper oxide-doped powder. In addition, sodium and copper oxides both destabilize the tetragonal Y-TZP and lead to the formation of monoclinic phase. This revised version was published online in November 2006 with corrections to the Cover Date.     

Sintering Aids in the Consolidation of Boron Carbide (B4C) by the Plasma Pressure Compaction (P2C) Method

Boron carbide (B₄C) powder has been densified by a novel method of powder consolidation known as Plasma Pressure Compaction (P²C). The P²C technique allows for rapid consolidation of powder by Joule heating of the powder bed. Powder is placed in graphite dies, and uniaxial pressure and low-voltage, high-amperage (10 V, 5000 amps maximum) direct current are applied to achieve densification. Pure B₄C powder was consolidated at lower temperature and hold time to densities equal to those achieved by conventional hot pressing. With the addition of a small amount of alumina (Al₂O₃) as a sintering aid, densities as high as 97% theoretical were attained.     

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.     

Bonding of alumina ceramics with nanoscaled alumina powders

Nanoscaled Al₂O₃-powders can be employed for diffusion bonding of alumina ceramics. In order to accomplish bonding of the ceramics, Al₂O₃-nanopowder with a median particle size of 14 nm in diameter is sandwiched between two commercial microcrystalline corund discs, followed by uniaxially hot compressing of the assembly in vacuum at 80 MPa and 1100 °C for 2 h. Scanning electron microscopic investigations reveal a nanocrystalline structure of the joint with a mean grain size of about 50 nm in diameter and extensive consolidation of the powder without substantial shrinkage void formation. Microhardness measurements across the interface yield a value of 200 HV. In order to achieve complete densification and strength enhancement of the joint material, the sample is subsequently sintered at 1500 °C and 1600 °C for several hours in air. It was found that the hardness of the joint depends strongly on the porosity content and/or grain size and that a hardness of 1700 HV is obtained when both a mean particle size of about 1 μm and complete densification of the joint is achieved. The results show for the first time that Al₂O₃-nanopowders are suitable for diffusion bonding of alumina ceramics. Possible mechanisms are discussed.     

Effects of milling and particle size distribution on the sintering behavior and the evolution of the microstructure in sintering powder compacts

In this investigation, it was observed that the contamination arising from the milling medium has a great influence on the sintering behavior. The densification rate of the powder milled with ZrO₂ balls is largely inhibited, and the activation energy of ZrO₂-doped alumina is significantly enhanced. The possible reasons for the enhancement of the activation energy were discussed. The effects of particle size distribution of powder compacts on the microstructural evolution of alumina during sintering were proposed and discussed.     

Electrical properties of ZnO/alumina nano composites for high voltage transmission line insulator

The main aim of the present study is to evaluate the impact of the nano-ZnO on the dielectric properties of alumina bodies used in high voltage insulators. In this work, Zinc oxide/alumina nanocomposites were prepared by sol–gel method. The ratios of ZnO were 5, 6 and 7 mass %. The effect of ZnO on the densification, microstructure, mechanical and electrical properties of the prepared bodies were evaluated after sintering at a temperature ranging from 1550 to 1750 °C. Results revealed that incorporation of 7 mass % ZnO enhanced the breakdown voltage, electric resistivity and the dielectric loss, which are the most important factors to evaluate high-voltage insulators. In addition, incorporation of 7 mass % ZnO enhanced the densification and mechanical properties of the alumina nano composites .     

SCC modification by use of amorphous nano-silica

In this study two different types of nano-silica (nS) were applied in self-compacting concrete (SCC), both having similar particle size distributions (PSD), but produced through two different processes: fumed powder silica and precipitated silica in colloidal suspension. The influence of nano-silica on SCC was investigated with respect to the properties of concrete in fresh (workability) and hardened state (mechanical properties and durability). Additionally, the densification of the microstructure of the hardened concrete was verified by SEM and EDS analyses. The obtained results demonstrate that nano-silica efficiently used in SCC can improve its mechanical properties and durability. Considering the reactivity of the two applied nano-silicas, the colloidal type showed a higher reactivity at early age, which influenced the final SCC properties.     

Processing of spodumene-modified mullite ceramics

The use of β-spodumene (Li₂O·Al₂O₃·4SiO₂) has been investigated as a liquid-phase sintering aid for the densification of mullite processed from fumed silica and ESP (alumina) dust. XRD, DTA, SEM and Vickers indentation were used to characterize the effect of spodumene on the phase relations, sintering behaviour, microstructure and mechanical properties of mullite. The results show that the presence of spodumene significantly reduces the porosity, improves the sintering behaviour and enhances the formation of mullite at 1550 °C. Spodumene-modified mullite ceramics also have better physical and mechanical properties. This revised version was published online in November 2006 with corrections to the Cover Date.     

Effect of CeO2 on reaction-sintered mullite-ZrO2 ceramics

The effect of ceria on mullite formation and the sintering of zircon and alumina powders was investigated. Quantitative X-ray powder analysis was used to determine the formation of mullite and zirconia of both monoclinic and tetragonal forms. Scanning electron microscopy and electron-probe microanalysis were used for microstructural analysis. It was found that the addition of CeO₂ enhanced the formation of mullite and increased the fraction of tetragonal zirconia. The addition of CeO₂ caused the formation of mullite directly from reaction of zircon with alumina without decomposition of zircon into zirconia and silica. In addition to forming a liquid phase, the ceria essentially formed a solid solution with zirconia. The fracture toughness of the mullite-zirconia composites was about 5.5–6.0 MPa m^1/2.     

Enhanced thermal stability in K<Subscript>2</Subscript>O-metakaolin-based geopolymer concretes by Al<Subscript>2</Subscript>O<Subscript>3</Subscript> and SiO<Subscript>2</Subscript> fillers addition

Based on the principle of stability of geopolymer gel as refractory binder, a geopolymeric paste in the K₂O–Al₂O₃–SiO₂ system was developed and used to produce refractory concretes by adding various amount of α-quartz sand (grain size in the range 0.1 μm to 1 mm) and fine powder alumina (grain size in the range 0.1–100 μm). The consolidated samples were characterized before and after sintering using optical dilatometer, DSC, XRD and SEM. The total shrinkage in the range of 25–900 °C was less than 3%, reduced with respect to the most diffused potassium or sodium based geopolymer systems, which generally records a >5% shrinkage. The maximum shrinkage of the basic geopolymer composition was recorded at 1000 °C with a 17% shrinkage which is reduced to 12% by alumina addition. The temperature of maximum densification was shifted from 1000 °C to 1150 or 1200 °C by adding 75 wt% α-quartz sand or fine powder alumina respectively. The sequences of sintering of geopolymer concretes could be resumed as dehydration, dehydroxylation, densification and finally plastic deformation due to the importance of liquid phase. The geopolymer formulations developed in this study appeared as promising candidates for high-temperature applications: refractory, fire resistant or insulating materials.     

Effect of powder reactivity on microwave sintering of alumina

Effect of the reactivity of starting alumina powder of varying crystallinity on the sintering behavior in microwave process was studied. From X-ray amorphous to highly crystalline alumina, powders were obtained by conventional heating of compacts made of the precursor amorphous powder by heating it at different temperatures from 800 to 1500 °C. These samples were then sintered in a multimode microwave field of 2.45 GHz for 10 min at 1500 °C. The microwave effect on densification of the various alumina powders was evaluated by comparing the microwave and conventional sintering data. The results show significant microwave enhancement in the densification of the samples without any pretreatment. This enhancement became less significant as the temperature of the pretreatment increased and finally diminished. Since the pretreatment at elevated temperatures made the powder more stable thermodynamically, this study indicates that the sintering enhancement of a ceramic material in microwave is a metastability-related phenomenon.     

CO sensing devices based on indium oxide nanoparticles prepared by laser ablation in water

A simple and effective solution route for synthesizing colloidal indium oxide (In₂O₃) nanocrystallites, i.e. laser ablation in liquid (LAL), is reported. The morphology and chemical structure of the as-prepared samples were analyzed by scanning electron microscopy (SEM), transmission electron microscopy (TEM), X-ray diffraction (XRD), and X-ray photoelectron spectra (XPS). The results showed the formation of In₂O₃ nanoparticles with a bimodal distribution, consisting mainly of particles of small diameters (2-5 nm). Sensor devices prepared by spraying directly the LAL colloidal solutions on interdigitated alumina substrates exhibited good sensing properties for the detection of CO at very low concentrations.     

Relationships between processing,microstructure and properties of dense and reaction-bonded silicon nitride

Some aspects of processing, microstructure and properties of the various types of silicon nitride are discussed. Special emphasis is placed on the relationships between powder properties, process conditions, densification and microstructure, as well as the interdependence between microstructure and properties. After summarizing the areas of crystal structure and thermodynamic properties, and processing of the different types of Si₃N₄, the state-of-the-art of dense and reaction-bonded silicon nitride is given. For both types the formation mechanisms and microstructure, relationships between powder properties, additives (in the case of dense Si₃N₄), process conditions, and densification and microstructure, as well as data and microstructural effects of various mechanical, thermal and thermo-mechanical properties, are outlined. Advanced processing techniques, such as sintering, gas-pressure sintering, post-sintering, and the different routes of hot-isostatic pressing (starting with powder compacts, reaction-bonded Si₃N₄ or pre-sintered Si₃N₄ and the resulting properties, are discussed.     

An investigation on microwave sintering of Fe, Fe–Cu and Fe–Cu–C alloys

The powder characteristics of metallic powders play a key role during sintering. Densification and mechanical properties were also influenced by it. The current study examines the effect of heating mode on densification, microstructure, phase compositions and properties of Fe, Fe–2Cu and Fe–2Cu–0·8C systems. The compacts were heated in 2·45 GHz microwave sintering furnaces under forming gas (95%N₂–5%H₂) at 1120 °C for 60 min. Results of densification, mechanical properties and microstructural development of the microwave-sintered samples were reported and critically analysed in terms of various powder processing steps.     

Intrinsic Effect of Nanoparticles on the Mechanical Rupture of Doubled‐Shell Colloidal Capsule via In Situ TEM Mechanical Testing and STEM Interfacial Analysis

The discovery of Pickering emulsion templated assembly enables the design of a hybrid colloidal capsule with engineered properties. However, the underlying mechanisms by which nanoparticles affect the mechanical properties of the shell are poorly understood. Herein, in situ mechanical compression on the transmission electron microscope and aberration‐corrected scanning transmission microscope are unprecedentedly implemented to study the intrinsic effect of nanoparticles on the mechanical properties of the calcium carbonate (CaCO₃)‐decorated silica (SiO₂) colloidal capsule. The stiff and brittle nature of the colloidal capsule is due to the interfacial chemical bonding between the CaCO₃ nanoparticles and SiO₂ inner shell. Such bonding strengthens the mechanical strength of the SiO₂ shell (166 ± 14 nm) from the colloidal capsule compared to the thicker single SiO₂ shell (310 ± 70 nm) from the silica hollow sphere. At elevated temperature, this interfacial bonding accelerates the formation of the single calcium silicate shell, causing shell morphology transformation and yielding significantly enhanced mechanical strength by 30.9% and ductility by 94.7%. The superior thermal durability of the heat‐treated colloidal capsule holds great potential for the fabrication of the functional additives that can be applied in the wide range of applications at elevated temperatures.     

Effect of bauxite addition on densification and mullitization behaviour of West Bengal clay

The effect of bauxite addition on the densification and mullitization of reaction sintered bauxite-clay mixture had been studied in the temperature range 1400–1500°C. The maximum bulk density (2·89 g/cc) and minimum apparent porosity (0·58%) was achieved by addition of 50 wt% bauxite. The impurities present in bauxite and clay formed liquid phase which helped in particle diffusion to aid densification. The X-ray diffraction of sample fired at 1500°C showed cristoballite phase gradually disappearing and at the same time mullite and α-Al₂O₃ phase appearing at a higher level of bauxite addition. The in situ nascent alumina formed was reactive that facilitated the formation of secondary mullite by solution precipitation mechanism. The presence of bauxite also changed the morphology of the mullite particles. Two types of mullite were distinctly observed in the SEM photographs: elongated primary mullite and equiaxed secondary mullite.     

Preparation and sintering behaviour of submicronic Bi4Ti3O12 powders

Submicronic powders of Bi₄Ti₃O₁₂ with different morphologies were prepared by both the oxalate coprecipitation and the conventional mixing oxides methods. Compacts of the two calcined powders were sintered at 850–1100 °C in air, and the densification process was studied by non-isothermal and dilatometric experiments. A rapid densification (> 97% theoretical density) below 875 °C took place in the Bi₄Ti₃O₁₂ oxalate powder which was attributed to an extremely uniform pore-size distribution in the green compact. The possible formation of a transient liquid which promotes densification also was taken into account. The development of plate-like morphology in the conventional Bi₄Ti₃O₁₂ powder, broad pore-size distribution, and the plate-like colony formation, hindered rapid densification of the green compacts at low temperature. Microstructural development was studied; preliminary dielectric and electrical results are also reported.