DENSIFICATION OF ALUMINUM NITRIDE-BASED CERAMIC MATERIALS SYNTHESIZED BY COMBUSTION OF ALUMINUM IN AIR

Aluminum nitride based materials were synthesized by combustion of aluminum powder in air. When aluminum nitride powder is used as a diluent, mostly aluminum nitride product with small amount of alumina was obtained. For reactant mixtures consisting of 33 wt% of aluminum and 67 w% of alumina, materials with a high content of aluminum oxynitride (AION) were formed. Synthesized aluminum nitride based powders with and without in-situ added sintering aids were characterized and tested for their sintering ability. Densification characteristics of combustion synthesized powders by pressureless sintering in nitrogen atmosphere are also reported.     

Preparation of tabular corundum aggregate from different sources alumina raw powder: Sintering kinetics and establishment of kinetics model

In this paper, we report the use of four types of commercial alumina raw powders as raw materials for the preparation of tabular corundum aggregates under the same conditions. The influence of the transition phases of alumina raw powder on the sintering kinetics of tabular corundum is discussed, the sintering model of materials with pseudomorphic structure is established, and the mechanisms underlying the different performances of various commercial tabular corundum samples are evaluated. The following conclusions were drawn based on the results of the study. (1) A double tetrakaidecahedron model was established and was shown to satisfactorily describes the sintering mechanism of alumina raw powder with pseudomorphic structure, which accords with the porosity change trend of sintered body and provides a basis for perfecting the sintering theory. (2) Compared with the other transition phases, γ-Al₂O₃ shows the largest phase transformation volume contraction, which provides the driving force for the sintering process via an increase in surface energy and mainly acts in the densification and grain growth stages. Thus, high-quality refractory raw materials are prepared with optimized physical properties and Intracrystalline pores or pore clusters in the crystal structure. The preparation of these high-quality refractory products is of importance for prolonging the life of these materials and also meeting rising energy demands.     

微晶氧化铝高档耐磨材料的研制

在确定了微晶氧化铝高档耐磨材料晶相组成的基础上,研究确定了微晶氧化铝高档耐磨材料的生产工艺。普通氧化铝粉在应用前必先煅烧,使γ-Al2O3转变为α-Al2O3,为了降低晶型转化温度研制了晶型转化助剂。研制的复合烧结助剂,降低了微晶氧化铝高档耐磨材料坯体的烧结温度。添加稀土化合物,使微晶氧化铝高档耐磨材料的性能得到了显著提高。     

Near Zero Shrinkage of an Low-Temperature Co-Fired Ceramic Package by Constrained Sintering Using Screen Printed Alumina Paste

Conventional free sintering of low-temperature co-firing ceramic (LTCC) technology has several merits such as sintering temperature below 1000°C that enables co-firing with electrode materials of silver or copper metal and multilayer structure formation. But due to the free sintering process, large shrinkage occurs. To fabricate electronic devices and components with near zero shrinkage within x, y directions constrained sintering (CS) technology is required. In this study a constrained sintering paste (CSP) utilizing alumina powder, which has a higher sintering temperature than LTCC powders, was fabricated for CS technology. The effect of CSP formulated using alumina powder on shrinkage was studied according to variation in paste composition. As a result ceramic package structure with a cavity was fabricated with shrinkage control of 0.028%, which is far smaller than the current CS technology shrinkage of approximately 0.1%.     

Impact of spinel forming systems (Fe-/Mg-/Mn-Al-O) as functional coating materials for carbon-bonded alumina filters on steel melt filtration

Since solid, non-metallic inclusions influence considerably the quality of casted steel products, carbon-bonded alumina foam filters are used in secondary metallurgical treatments to remove these particles from steel melts. In order to attain a significant improvement of the filtration process, five different carbonaceous spinel compounds from the Fe-/Mg-/Mn-Al-O systems are applied on carbon-bonded alumina filters in this study and investigated with regard of their filtration efficiency. However, these spinel compounds decompose partially during sintering at 1400?°C under reducing atmosphere, wherefore the resulting coatings contain not only spinel compounds, but also oxidic and metallic components. The subsequent interaction with molten steel leads to the development of multicrystal structures on the filter surface, which stem from interfacial reactions between coating materials, molten steel, and inclusions. As a result of this procedure, a reduction of almost 60% alumina inclusions is measured with the aid of an automatic SEM, whereby spinel compounds from the Fe-Mn-Al-O system achieve highest filtration efficiencies.     

Constrained Densification Kinetics of Alumina/Borosilicate Glass + Alumina/Alumina Sandwich Structure

The densification kinetics and mechanism of a low-temperature cofirable borosilicate glass (BSG) + alumina during the constrained sintering of a sandwich structure of alumina/(BSG + alumina)/alumina has been studied. The densification kinetics becomes slower when the BSG + alumina tape is constrained during firing. However, a viscous flow-controlling mechanism of the BSG also is still operative during free and constrained sintering. The densification behavior of constrained sintering can be mathematically described by free sintering, using the viscous analogy for the constitutive equations of a porous sintering glass.     

In situ Spinel Expansion Design by Colloidal Alumina Suspension Addition

The incorporation of nano powders into refractory castables it is not a straightforward procedure, due to their agglomeration and sintering drawbacks. Considering the alumina grain size effect on the in situ spinel formation and the associated overall volumetric expansion, alumina–magnesia refractory castables containing different alumina sources were evaluated by the assisted sintering technique. Reducing the size of fine tabular alumina (<200 μm) led to lower expansion levels, indicating the main role of alumina grain size in this property. Regarding the reactive aluminas (nano alumina powder, hydratable alumina, and colloidal alumina), the composition containing colloidal alumina performed remarkably better, leading to the lowest in situ spinel expansion level as a consequence of its high sinterability. The use of nano scaled alumina suspensions was the most suitable alternative to inhibit the shortcoming of nano powder agglomeration. Conversely, the castable containing the nano alumina powder did not behave as expected. The present work pointed out that the use of nano powders in refractory castables is only feasible if the compound is fully dispersed. Otherwise, cheaper raw materials could provide even better results than those of nano agglomerated powders.     

Effect of diopside addition on sintering and mechanical properties of alumina

In this paper, diopside was introduced in alumina as a sintering aid and fine structural alumina matrix ceramic materials were fabricated by pressureless sintering. The relative density, hardness, fracture toughness and bending strength of the new fabricated composites were measured. Tribological tests were carried out at a given rotation speed of 160 rpm and in a normal load ranged from 50 to 200 N. The experiment results show that the introduction of diopside can enhance densification rate, which may contribute to the improvement in mechanical properties and result in enhanced wear resistances. The effects of diopside on mechanical properties and microstructures of fine structural alumina matrix ceramic materials were analyzed and discussed.     

Pressureless sintering of alumina matrix ceramic materials improved by Al–Ti–B master alloys and diopside

Al–Ti–B master alloys and diopside are simultaneously introduced in alumina matrix ceramic materials as sintering aids. Fine structural alumina matrix ceramic materials are fabricated by pressureless sintering during which liquid phase, leading to interface reactions between alumina matrix and additives, is formed. Hardness, fracture toughness and bending strength of the composites are measured. The effects of diopside on mechanical properties and fracture mechanism of fine structural alumina matrix ceramic materials are analyzed together with the microstructure observations on fracture surfaces, the polished surfaces and the indentation cracks.     

Creep study on alumina and alumina/SWCNT nanocomposites

Alumina and alumina/SWCNT nanocomposites have been sintered by spark plasma sintering, obtaining relative densities higher than 99%. Microstuctural characterization revealed a grain microstructure in the submicron range, where alumina/SWCNT nanocomposites exhibited a good CNTs dispersion thought the ceramic matrix. Creep experiments performed in both materials showed a similar mechanical behavior, where the addition of CNTs seems to have a negligible effect on the strain rate, in contrast to results reported by other authors. Grain boundary sliding accommodated by lattice diffusion has been identified as the high temperature deformation mechanisms in both samples, alumina and alumina/SWCNT nanocomposites. We have discussed about the role of CNTs influence on the plasticity of these composites.     

Effect of TiO2 on Initial Sintering of Al2O3

Alpha alumina with additions of TiO₂ sintered more rapidly than "pure" alumina. The rate of initial sintering increased approximately exponentially with titania concentration up to a percentage beyond which the rate of sintering remained approximately constant or decreased slightly with additional titania. The concentration which produces the maximum rate of sintering is thought to be the solubility limit of TiO₂ in Al₂O₃. For alumina particles larger than about 2 μm, the kinetic process was mainly grain-boundary diffusion. With smaller particles, volume diffusion increased. The "solubility limit" increased with decreasing particle size, indicating an excess surface concentration of TiO₂. The data may be interpreted in terms of a region of enhanced diffusion at the grain boundary that increases with TiO₂ concentration. With small alumina particles, this region is large enough to become a significant portion of the volume of the particle, and the small particles appear to sinter by volume diffusion kinetics, but the diffusion coefficient corresponds to an enhanced diffusion coefficient.     

Microstructural investigation and performance evaluation of slip-cast alumina supports

In this work we successfully obtained slip-cast alumina supports with tubular shape. It was investigated the influence of both the starting powder particle size and heat treatment program on the pore structure and water permeability of the prepared materials. This study is supported by a series of experimental tests, including Archimedes method, mercury intrusion porosimetry, scanning electron microscopy, and cold crushing tests. We observed that the heat treatment temperature exhibited a more significant effect on the porosity than the sintering time. It was noticed that, in a general way, the higher the sintering temperature, the smaller the porosity and the larger the apparent density of the prepared materials. In addition, the raise of the sintering temperature from 1100 to 1500 °C increased the cold crushing strength of the tested samples. This behavior was not observed for F2, which seems to be related to the poor sinterability of the starting alumina powder used in its preparation. Among the samples prepared in this work, F2 showed the highest water permeability, followed by F3 and F1. The water permeation behavior of these materials is discussed on the basis of their mean pore size, porosity, genus per unit volume, and pore network tortuosity.     

Creep behaviour of alumina/YAG composites prepared by different sintering routes

Al₂O₃-5 vol.% Y₃Al₅O₁₂ (YAG) composite powders have been prepared by surface doping of α-alumina powders by an yttrium chloride aqueous solution. Two commercial, one submicron-sized, the other ultra-fine, alumina powders were compared as matrix materials. YAG phase was yielded by an in situ reaction promoted by the subsequent thermal treatment of the doped powders. In particular, a flash soaking into a tubular furnace kept at a fixed temperature in the range 1050-1150 °C was employed, for inducing the crystallization of yttrium-aluminates on the alumina particles surface, but avoiding a relevant crystallites growth. After that, aqueous suspensions of the calcined powders were dispersed by ball-milling and cast into porous moulds or simply dried in a oven. Slip cast green bodies were densified by pressure-less sintering, while powdered samples were consolidated by hot pressing or spark plasma sintering. The low- and high-temperature mechanical performances of the sintered materials were investigated and related to monolithic aluminas behaviour as well as to the composites microstructures. It is shown that the hot-pressed and spark plasma sintered composites present a significantly lower creep rate as compared to reference, monolithic alumina samples.     

New Ceramic Materials Based on Aluminum Oxide

A classification of the liquefying additives used for fabrication of sintered alumina ceramics is given. The basic principles guiding the chooice of additives for fabrication of high-performance aluminum-oxide materials with a low sintering temperature are formulated. Based on these principles, a range of eutectic systems is proposed which, used as additives to alumina, enables one to fabricate dense ceramic materials at calcination temperatures of 1300 – 1550°C. Specific sintering features of these ceramics are discussed.     

Large-scale fine structural alumina matrix ceramic guideway materials improved by diopside and Fe2O3

Diopside and Fe₂O₃ were introduced in alumina matrix ceramic materials. Large-scale fine structural alumina matrix ceramic guideway materials were fabricated by the technology of pressureless sintering, during which liquid phase sintering took place and new phases such as 3Al₂O₃·2SiO₂, CaO·Al₂O₃·2SiO₂ and CaO·6Al₂O₃ were produced by the chemical reactions taking place among alumina and the additives. The hardness, the fracture toughness and the bending strength of the guideway products were tested. The influences of diopside and Fe₂O₃ additions were studied by microstructural observations and mechanical properties evaluations. Meanwhile, the expected improvement of mechanical properties compared with pure alumina was indeed observed. The fracture mechanism and porosity of large-scale fine structural alumina matrix ceramic guideway materials were analyzed.     

Effect of freeze-drying treatment on the optical properties of SPS-sintered alumina

This study looks at the influence of alumina powder processing on the preparation of transparent alumina by Spark Plasma Sintering (SPS). Zeta potential measurements were carried out on alumina suspensions in order to determine the best dispersion conditions. Stable slurries were submitted to a spray freeze drying process and their sintering behavior was compared with the corresponding non spray freeze dried powders. Transparent alumina samples were successfully prepared from alumina powders by Spark Plasma Sintering. An optical model considering pore and grain size distributions has been developed to obtain information about porosity in dense materials. It was found that the final density and, accordingly, the optical properties were improved when spray freeze dried starting powder was used.     

Microstructure stabilization of a novel glass/YSZ composite coating material by adding alumina particles

Glass-based composite coating materials incorporating particles of alumina or YSZ were prepared by reaction sintering. It was revealed that phase evolution played a key role on thermal expansion behavior of the composite coating materials. Both precipitating of t-ZrO₂ crystals and adding YSZ inclusions could raise CTEs of the glass-based matrix, while the formation of zircon produced the reverse effect. Especially, alumina additives retarded the crystallization of the base glass and reduced reaction rates between YSZ and the glass matrix remarkably. Thus, the Al₂O₃/YSZ/glass tri-composites could serve as an environmental barrier coating for intermetallics and superalloys because of the stabilized microstructure.     

Activation Energy for the Sintering of Two-Phase Alumina/Zirconia Ceramics

In an earlier paper we reported measurements of the activation energy for sintering from constant-heating-rate experiments with alumina/5% zirconia. Here, results from a full range of compositions in this two-phase system are described. They show that the activation energy remains in the range 700 ± 100 kJ/mol when the composition changes from 5 to 95 vol% zirconia. In comparison, pure zirconia sinters with an activation energy of 615 ± 80 kJ/mol and pure alumina with the energy of 440 ± 45 kJ/mol. The addition of 2.8 mol% yttria to zirconia does not have a measurable effect on the activation energy. The grain size dependence of the sintering rate suggests boundary-diffusion-controlled sintering. These activation energies are phenomenologically correlated with the interfacial energies in alumina, zirconia, and two-phase alumina/zirconia, suggesting that the bonding at the interface influences diffusional transport.     

Transient liquid phase sintering of high-purity mullite for high-temperature structural ceramics

High-purity mullite ceramics, promising engineering ceramics for high-temperature applications, were fabricated using transient liquid phase sintering to improve their high-temperature mechanical properties. Small amounts of ultrafine alumina or silica powders were uniformly mixed with the mullite precursor depending on the silica-alumina ratio of the resulting ceramics to allow for the formation of a transient liquid phase during sintering, thus, enhancing densification at the early stage of sintering and mullite formation by the reaction between additional alumina and the residual glassy phase (mullitization) at the final stage of sintering. The addition of alumina powder to the silica-rich mullite precursor resulted in a reaction between the glassy silica and alumina phases during sintering, thereby forming a mullite phase without inhibiting densification. The addition of fine silica powder to the mullite single-phase precursor led to densification with an abnormal grain growth of mullite, whereas some of the added silica remained as a glassy phase after sintering. The resulting mullite ceramics prepared using different powder compositions showed different sintering behaviors, depending on the amount of alumina added. Upon selecting an optimum process and the amount of alumina to be added, the pure mullite ceramics obtained via transient liquid phase sintering exhibited high density (approximately 99%) and excellent high-temperature flexural strength (approximately 320 MPa) at 1500 °C in air. These results clearly demonstrate that pure mullite ceramics fabricated via transient liquid phase sintering with compositions close to those of stoichiometric mullite could be a promising process for the fabrication of high-temperature structural ceramics used in an ambient atmosphere. The transient liquid phase sintering process proposed in this study could be a powerful processing tool that allows for the preparation of superior high-temperature structural ceramics used in the ambient processing atmosphere.     

Dilatometric study of anisotropic sintering of alumina/zirconia laminates with controlled fracture behaviour

Al₂O₃ and ZrO₂ monoliths as well as layered Al₂O₃/ZrO₂ composites with a varying layer thickness ratio were prepared by electrophoretic deposition. The sintering shrinkage of these materials in the transversal (perpendicular to the layers, i.e. in the direction of deposition) as well as in the longitudinal (parallel with layers interfaces) direction were monitored using high-temperature dilatometry. The sintering of layered composites exhibited anisotropic behaviour. The detailed study revealed that sintering shrinkage in the longitudinal direction was governed by alumina (material with a higher sintering temperature), whilst in the transversal direction it was accelerated by the directional sintering of zirconia layers. For interpretation of such anisotropic sintering kinetics, the Master Shrinkage Curve model was developed and applied. Crack propagation through laminates with a different alumina/zirconia thickness ratio was described with the help of scanning electron microscopy and confocal laser microscopy.