Influence of magnesia on sintering stress of alumina

The sintering stress and the densification of MgO-doped Al₂O₃ were measured with a self-loading apparatus and a thermomechanical analyzer, respectively. The densification started at 950 °C and finished at about 1450 °C. The measured surface tensions were 0.7–0.8 N m⁻¹ in the intermediate sintering stage but drastically decreased to 0.2 N m⁻¹ in the final stage of sintering.     

Design of double-layer ceramic absorbers for microwave heating

We propose a guide for designing double-layer ceramic absorbers in microwave heating by optimizing the thickness based on the analysis of reflection loss (RL) of a double-layer absorber consisting of a high-loss SiC layer and a low-loss Al₂O₃ layer. The calculated reflection losses for individual layers of SiC and Al₂O₃ show that the former with a thickness of 0.0054 m has the maximum microwave absorption while the latter in the thickness range up to 0.1 m is identified as a poor microwave absorbing material with RL larger than −0.4 dB. By using a 0.0054-m-thick SiC layer as the susceptor, the absorption in the Al₂O₃ layer and of the entire double-layer absorber increases significantly. The results demonstrate that high microwave absorption throughout the heating process can only be achieved in a sample with a small thickness in which a slight absorption peak shift during heating (less than one eighth-wavelength in the medium) occurs.     

OH and COOH functionalized single walled carbon nanotubes-reinforced alumina ceramic nanocomposites

Alumina ceramics reinforced with 1 wt.% single-walled carbon nanotube (SWCNT) were fabricated via spark plasma sintering (SPS) of composite powders containing carboxyl (COOH) or hydroxyl (OH) group functionalized single-walled carbon nanotubes. The samples were SPS’ed at 1600 °C under 50 MPa pressure for holding time of 5 min and at a heating rate of 4 °C/s. The effects of CNT addition having different surface functional groups on microstructure, conductivity, density and hardness were reported. It was shown that nanotube addition decreased the grain sizeof alumina from 3.17 μm to 2.11 μm for COOH-SWCNT reinforcement and to 2.28 μm for COOH-SWCNT reinforcement. The hardness values of the composites are similar for all samples but there is 4.5 and 7.5 times increase in electrical conductivity with respect to monolithic alumina for COOH-SWCNT and OH-SWCNT, respectively. It was also shown by TEM and FEG SEM observations that transgranular fracture behaviour of alumina was changed to mostly intergranular fracture mode by the addition of both types of CNTs which may be due to location of CNTs along the grain boundaries. A significant grain size reduction in alumina is considered toresult fromthe suppressing effect of CNTs during sintering.     

Fabrication and characterization of composite sol-gel coatings on porous ceramic substrate

Highly porous ceramic materials were coated using a composite sol-gel method. Alumina powder is dispersed in a silica sol-gel solution and then dip-coated on the substrate. The resulting coatings present a composite microstructure in which crystalline alumina grains are linked to each other by an amorphous silica phase. In this work, we show that, by accurately controlling the sol-gel parameters (water, solvent and silica precursor (TEOS) ratio, pH and ageing time of the sol) and also the powder grain size distribution it is possible to obtain crack-free thick films (more than 20 μm in one step). These coatings present good adherence to the substrate, decrease the roughness and also close the surface porosity of the substrate. Coating mechanical properties have been evaluated thanks to micro-indentation measurements and linked to coating structural evolution with the thermal treatment temperature.     

Optimization of binder removal for ceramic microfabrication via polymer co-extrusion

The objective of this study is to assess the feasibility of solvent extraction (SE) for partial binder removal in the context of polymer co-extrusion with a thermoplastic binder component. Polymer co-extrusion is able to produce multilayered, functionally graded and/or textured structures in an efficient manufacturing process, but requires a polymer binder system with suitable flow characteristics. Traditionally, the binder is removed by thermal debinding (TD), which, however, is prone to form cracks or blisters, both of which are attributed to a lack of initial pore space that allows pyrolysis products to escape. The primary focus of this work is to demonstrate that a binder system with a high soluble binder content is suitable for conventional polymer co-extrusion and to document that a two-step binder removal process involving both SE and TD eliminates debinding defects. The overall fabrication process is documented for the extrusion of solid ceramic rods and co-extrusion of tubes, where alumina powder was batched with polyethylene butyl acrylate (PEBA) as backbone polymer and polyethylene glycol (PEG) as water soluble binder. SE for specimen with varying PEBA:PEG ratios was tested in water at three different temperatures for various times. The 1:1 mixture showed a PEG removal up to 80 wt.% of the original PEG content after 6 h extraction; after subsequent thermal debinding, rods and tubes sintered successfully without defects, demonstrating the viability of the process.     

Sintering behavior of nano alumina powder shaped by pressure filtration

In the present study, the sintering behavior of a commercial nano alumina powder with an initial particle size of 100 nm was investigated. The shrinkage response of the powder formed by pressure filtration (PF) during non-isothermal sintering was measured in a laser assisted dilatometer at three different heating rates of 2, 10 and 25 °C min⁻¹ up to 1400 °C. In order to calculate the activation energy of sintering, constant rate of heating (CRH) was employed and the activation energy was found to be 608 ± 20 kJ mol⁻¹ for iso-density method. The heating rate was demonstrated to have a vital role on densification behavior and final grain size. The mean grain size of the full dense specimens decreased from 875 to 443 nm when the heating rate increased from 2 to 25 °C min⁻¹.     

Synthesis of alumina nanofibers by a mercury-mediated method

Alumina nanofibers were successfully synthesized in mercury media at room temperature. Structure and morphology of the nanofibers were characterized by TEM, EDX, FESEM, XRD, TG, DTA and N₂ adsorption–desorption. The results show that the as-grown alumina nanofibers are amorphous, and have diameters of 5–15 nm and lengths up to several micrometers. After calcinated at 850 °C for 2 h, the amorphous alumina nanofibers convert to γ-Al₂O₃ nanofibers. The mechanism for the growth of alumina nanofibers was discussed and a model representing the growth process was presented. During the process, mercury will be produced by metathesis reaction of HgCl₂ and Al, Al atoms continuously dissolve into mercury and diffuse to amalgam/air interface, and then Al atoms react with oxygen and water in air, finally alumina nanofibers can be formed.     

Reaction sintering of colloidal processed mixtures of sub-micrometric alumina and nano-titania

The fabrication of composites formed by alumina grains (95 vol%) in the micrometer size range and aluminium titanate nanoparticles (5 vol%) by reaction sintering of alumina (Al₂O₃) and titania (TiO₂) is investigated. The green bodies were constituted by mixtures of sub-micrometric alumina and nano-titania obtained from freeze-drying homogeneous water based suspensions, and pressing the powders. The optimization of the colloidal processing variables was performed using the viscosity of the suspensions as control parameter. Different one step and two step sintering schedules using as maximum dwell temperatures 1300 and 1400 °C were established from dynamic sintering experiments. Specimens cooled at 5 °C/min as well as quenched specimens were prepared and characterized in terms of crystalline phases, by X-ray diffraction, and microstructure by scanning electron microscopy of fracture surfaces.Even though homogeneous final materials were obtained in all cases, full reaction was obtained only in materials treated at 1400 °C. The microstructure of the composites obtained by quenching was formed by an alumina matrix with bimodal grain size distribution and submicrometric aluminium titanate grains located inside the largest alumina grains and at triple points. However a cooling rate of 5 °C/min led to significant decomposition of aluminium titanate. This fact is attributed to the small size of the particles and the effect of the alumina surrounding matrix.     

A new direct coagulation casting process for alumina slurries prepared using poly(acrylate) dispersant

Direct coagulation casting (DCC) of concentrated aqueous alumina slurries prepared using ammonium poly(acrylate) dispersant has been studied using MgO as coagulating agent. Addition of small amounts of MgO increased the viscosity of the concentrated alumina slurries with time and finally transformed it in to a stiff gel. Sufficient working time for degassing and casting could be achieved by cooling the slurries to a temperature of ∼5 °C after proper homogenization after the addition of MgO. The DCC slip with alumina loading in the range of 50–55 vol% showed relatively low viscosity (0.12–0.36 Pa s at shear rate of 93 s⁻¹) and yield stress (1.96–10.56 Pa) values. The wet coagulated bodies prepared from slurries of alumina loading in the range of 50–55 vol% had enough compressive strength (45–211 kPa) for handling during mould removal and further drying. The coagulated bodies prepared from slurries of alumina loading in the range of 50–55 vol% showed linear shrinkage in the range of 4.8–2.3 during drying and 17.1–16.2 during sintering respectively. Near-net-shape alumina components with density >98% TD could be prepared by the DCC process.     

Joining of alumina with a porous alumina interlayer

In the joining of structural ceramics, a porous interlayer is generally believed to deteriorate the mechanical properties of the joint. This paper, however, shows that a porous interlayer can sustain high adhesion strength when cavities or interfacial cracks are eliminated. The characteristic of the new slurry approach, described in this work, is that a pure alumina slurry interlayer is dried between two adjoining dense alumina plates and sintered with a negligible external pressure to form the porous interlayer. The effect of slurry concentration was studied to optimize the microstructure of interlayer. By controlling the interlayer microstructure and nature of the flaws, it was possible to fabricate high-strength bonds. The new slurry approach opens up the possibility of pure diffusion bonding which requires neither high pressure during heat treatments nor flat surfaces.     

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.     

Preparation of photocatalytic TiO2 coatings by gel-dipping with polysaccharides

Commercial tiles are being produced in vast quantities. The main properties of tiles are well established but there is an increasing interest in producing ceramics with tailored-properties and advanced functionalities. One way of adding value to commercial tiles is to deposit a photocatalytic coating to obtain ‘smart’ tiles for environmental reasons, e.g. for the (photo) degradation of organic pollutants in air or in a liquid. Here, we show the manufacture of ‘smart’ tiles by formation of TiO₂ coatings onto commercial tiles by a colloidal processing route based on the immersion of the substrate into a homogeneous aqueous ceramic suspension and its consolidation by agar thermogelation. The effect of the processing parameters (withdrawal rate, solid loading and gelling agent content) and the grain size on the photocatalytic activity of the final coated tiles is reported and discussed. Final coatings properties depend on the viscosity of the suspension, particle size, withdrawal rate, solid loading and gelling agent content, and hence, this dependence affects the photocatalytic activity of the coatings.     

Effects of different production techniques on glass–alumina functionally graded materials

Glass–alumina functionally graded materials were obtained using two different methods: percolation, which was representative of natural transport based processes, and plasma spraying, which was representative of constructive processes. The specimens produced in this way were investigated to evaluate the effect of production techniques on the final microstructure and gradient, which, in turn, govern the properties and performances of the graded systems. Moreover, post-production heat treatments were performed in order to improve the reliability of the materials examined.     

Fabrication of alumina parts by electrophoretic deposition from ethanol and aqueous suspensions

This paper discusses the effect of the properties of alumina suspensions in ethanol and in water, on green and sintered ceramic parts formed by electrophoretic deposition. The results of the study demonstrate that a small amount of water present in ethanol suspensions as a hidden additive due to the hygroscopicity of alumina powder and ethanol can detrimentally affect the behaviour of the suspension, thus lowering the reliability of the process. Electrophoretic deposition from aqueous suspensions appears to be advantageous over ethanol, from a reliability standpoint, and due to higher achievable green and sintered densities of the deposits and higher deposition rates. Dolapix CE64 appears to be superior surfactant in water as it results in deposits with the lowest green and sintered porosities.     

Influence of corrosion and mechanical loads on advanced ceramic components

Advanced oxide ceramics are prospective materials for severe application conditions, including corrosion, particularly, in oxygen-rich environments, combined with the action of mechanical loads. The corrosion behavior and mechanical strength decrease of oxide ceramics, such as high alumina, alumina–mullite and zirconia-based ceramics, were studied in water steam supercritical conditions (elevated temperatures and pressures). The strength decrease under the action of the studied aggressive environment is mostly dealt with the glassy phase dissolution and intergranular corrosion for alumina–mullite and high alumina ceramics, while degradation of zirconia-based ceramics is also dealt with the phase transformation. The influence of structure defects related to processing of the ceramics on corrosion is considered.     

Calcination and associated structural modifications in boehmite and their influence on high temperature densification of alumina

A systematic study is reported on the calcination of boehmite and its associated structural changes, and their effect on densification features. Boehmite precursor gels have been calcined in the temperature range 250-1200 °C. The associated structural changes are identified by FTIR and XRD. The specific surface area measurements indicated a relatively high value of 169 m²/g for boehmite calcined at 400 °C; this value reduced to 4 m²/g on calcination at 1200 °C. In the temperature range 400-1000 °C, the coordination of aluminium changes from a quasioctahedral to a tetrahedral nature, which reverts to octahedral at 1200 °C. The precursor containing γ-alumina gives a 92.1% theoretical density, on sintering at 1500 °C due to the highly unstable quasioctahedral coordination. Boehmite precursors calcined at 400 °C and 1000 °C produced a density of 88.2% and 96.9%, respectively, in the sintered compact at 1500 °C. Boehmite calcined to α-alumina (1200 °C) possesses an octahedral structure having a density of 97.6% at 1500 °C.     

High strength alumina joints via transient liquid phase bonding

Low melting boron oxide, instead of metallic materials in other methods of transient liquid phase bonding, was taken as braze in joining alumina in this paper. Pure boron oxide melts at low temperature and reacts with alumina matrix to form a stable high melting compound. This transient liquid phase bonding has the advantage of producing a ceramic joint for high temperature applications at low processing temperature. In this study, alumina pieces coated with boron oxide layers in various thicknesses were bonded at 800 °C for various times in air under minor loading. The average flexural strength of joints were measured by means of four point bending, while the microstructure of the cross-section and fractured surface was observed by means of scanning electron microscopy. Phases at joints were identified by low angle X-ray diffraction. The maximum flexural strength reaches a value of 155 MPa after joining at 800 °C for 15 h with a 21 μm interlayer. Three compounds, 3Al₂O₃–B₃O₃, 2Al₂O₃–B₃O₃ and 9Al₂O₃–2B₃O₃ have been found at the joint. It is also found that 2Al₂O₃–B₃O₃ whiskers dominate at the joint with the maximum strength.     

Effects of particulate metallic phase on microstructure and mechanical properties of carbide reinforced alumina ceramic tool materials

Alumina-based composite ceramic tool materials reinforced with carbide particles were fabricated by the hot-pressing technology. Choice of metallic phase added into the present composite ceramic was based on the distribution of residual stress in the composite. The effects of metallic phase on microstructure and mechanical properties of composites were investigated. The metallic phase could dramatically improve room temperature mechanical properties by refining microstructure, filling pores and enhancing interfacial bonding strength. However, it also led to sharp strength degradation at high temperature because the metallic phase was easier to be oxidized and get soft at high temperature in air. The effects of metallic phase on strengthening and toughening were discussed. The improved fracture toughness of composite with metallic phase was attributed to the lower residual tensile stress in the matrix and the interaction of more effective energy consuming mechanisms, such as crack bridged by particle, crack deflection and intragranular grain failure.     

Fabrication and characterization of ceramic coatings with alumina–silica sol-incorporated α-alumina powder coated on woven quartz fiber fabrics

The formation and properties of dense silica/α-alumina coatings derived from alumina–silica sol-incorporated coarse alumina powder (median particle size d₅₀=0.509 μm) using Al(NO₃)₃·9H₂O and tetraethyl orthosilicate as precursors have been investigated. Phase compositions and thermal evolution analysis of the coatings as well as interface microstructure between the coatings and the matrix were characterized by means of XRD, XRF, DSC-TG, TEM, SEM supplemented with EDS, and tensile strength tester. Silica/α-alumina coatings combined with quartz fiber matrix are homogeneously integrated with distinct inter-diffusion near the interface. The heat conduction mechanism of the quartz fiber matrix was influenced by the dense silica/α-alumina ceramic coatings, and the prepared coatings provide practical thermodynamic stability and desired mechanical strength for the woven quartz fiber fabrics.