Tape Cast Alumina Research Articles

Evolution of microstructure and defects in sintering of tape-cast alumina laminates observed by synchrotron X-ray multiscale tomography

Processing-induced defects affect the performance and reliability of multilayered ceramics. They are introduced from the raw powder, or during tape casting, drying, thermo-compression, binder burnout, and sintering. Using synchrotron X-ray multiscale tomography, we investigated how the microstructural heterogeneities and various defects evolve in the sintering of tape-cast alumina laminate. The strength-limiting defects were identified as large defects at the layer interface and peculiar crack-like defects formed around large inclusions contained in the raw powder. Although a slurry might be uniform in colloidal processing, the particle packing in the tape was heterogeneous. This heterogeneity determined the spatial distribution of pores and formed complex pores in later stages. Moreover, we found the formation of flake-like sheets in slurry due to the self-assembly of polyhedral alumina crystals. This phenomenon has the potential to be applied in the manufacture of 2D micro-sheets controlled to the thickness of a single particle.

Thermal shock behaviour of laminated multilayer refractories for steel casting applications reinforced by residual stresses

Ceramic multilayer structures based on tape cast alumina and zirconia substrates have been manufactured for use as carbon-free refractory materials. The laminates were reinforced via residual stresses due to shrinkage mismatch or differences in thermal expansion in order to achieve an improved thermal cycling capability. Thermal shock tests have been carried out using a plasma test stand. The impact of layer sequence and residual stresses has been demonstrated via measurement of Young's modulus and microstructure images of the layered structures. Hasselman parameters as well as the crack propagation behaviour at interfaces have been analysed via wedge splitting test.

Correlation Between Anisotropic Green Microstructure of Spherical‐Shaped Alumina Particles and Their Shrinkage Behavior

Dimensional control is one of the basic problems in ceramic processing, especially for tape cast ceramic sheets which are used to build up multilayer structures for high‐integrated components. Uncontrolled anisotropic shrinkage can cause geometrical distortion and circuit failure of multilayer ceramics during sintering. The understanding of the relationship between green tape microstructure and shrinkage anisotropy is of great importance for further miniaturization of multilayer devices. In this study, alumina powders with spherical particle shape were used to cast green tapes. The microstructure as well as the pore orientation and the shrinkage behavior were analyzed. According to the sintering theory, grain growth and pore elimination are the two most important mechanisms to describe sintering shrinkage. In this work, three‐dimensional shrinkage behavior of tape cast alumina powders with spherical particle shape was investigated and correlated with pore orientation in the microstructure. Specifically, the reason for the different shrinkage in z‐direction compared to the lateral shrinkage is in focus. The study is based on experiments as well as on mathematical visualization.

Properties and cyclic fatigue of glass infiltrated tape cast alumina cores produced using a water-based solvent

Objective The purpose of this study was to investigate the properties of tape cast alumina composite produced using a water-based solvent and its possible clinical use as an all ceramic crown system in a fixed partial denture. Durability of the system will be measured by fatigue test to simulate the masticating conditions of the oral cavity. Methods The optimal weight ratio of water-based alumina tape was determined by tensile strength, shrinkage ratio and durability. The coefficient of thermal expansion, fracture toughness, biaxial flexural strength and flexural strength after fatigue test of a composite produced from alumina tape at optimal weight ratios were determined and compared to In-Ceram alumina core (control). Results The weight ratio of alumina/(alumina + binder + plasticizer) of 0.84 and binder/(binder + plasticizer) of 0.5 was observed to be the optimal composition for achieving excellent composite properties. Coefficient of thermal expansion of the sintered alumina tape was observed to be 7.3 × 10 −6/°C, and this value was increased to 7.5 ×10 −6/°C after infiltrating the sintered tape with glass. The fracture toughness and biaxial flexural strength of glass infiltrated alumina tape was observed to be 4.6 MPa m 1/2 and 498 MPa, respectively. After cyclic loading for 10 2–10 6 cycles, no significant change in the biaxial flexural strength was observed between the glass infiltrated alumina core and the In-Ceram alumina core ( p > 0.05). Significance The observed properties provide evidence that the water-based tape cast alumina–glass composite is suitable for clinical use as an all ceramic crown system in a fixed partial denture.

Characterization of protective coatings for planar automotive gas sensors

Alumina support material suitable for use as a planar automotive gas sensor support was coated in thin films of yttria-stabilised zirconia (YSZ) and titania. The morphology, composition, thickness and homogeneity of the coating was measured. The coating was applied to the ‘green’ form of a tape cast alumina substrate which was subsequently fired at 1500 °C to produce the final form of the coated alumina. The YSZ coating gave a continuous ∼5 μm thick coating with no evidence of mixed oxide formation between the YSZ and the alumina substrate. XRD indicated a face centred cubic Y doped ZrO 2 or primitive tetragonal Zr 0.9Y 0.1O 1.95 phase. The titania coatings were much thinner (<1 μm) with signs of trace amounts of aluminium titanium oxide (Al 2TiO 5) as well as rutile titania in XRD. Spot analysis using X-ray photoelectron spectroscopy showed a fairly regular titania coverage. Atomic force microscopy analysis showed a particle size of 1–3 μm for the YSZ coating and ∼0.5 μm for titania.

Influence of the slip composition on the properties of tape-cast alumina substrates

Tape-casting process was used to produce alumina (Al 2O 3) substrates in an aqueous system with poly(vinylalcohol) (PVA) and glycerine as binder and plasticizer, respectively. Various compositions of Al 2O 3 slips were prepared by dispersing the powder in water with ammonium polyacrylate (NH 4PA), with subsequently additions of different amounts of PVA and glycerine. The influence of the slip composition on the rheological properties of the slips and the properties of the resultant green tapes were studied. In addition, the properties of the sintered tapes were investigated and related to those of the green tapes. PVA and glycerine did not affect the dispersion properties of the Al 2O 3 powder which were governed by the adsorbed NH 4PA. Glycerine additions enhanced the flexibility of the green tapes but also produced a decrease in the tensile strength and in the drying shrinkage. The increase in the PVA content increased the tensile strength but resulted in a markedly decrease in the green density of the tapes. The slip composition 5 wt.% PVA–5 wt.% glycerine produced green tapes with sufficient tensile strength and flexibility which had higher green and sintered density and lower sintering shrinkage. Thus, this composition was adequate for the production of Al 2O 3 substrates by the tape-casting process.

Application of Water-Based Alumina Tapes Prepared Using PVP Binder for All-Ceramic Dental Crowns

Aqueous alumina tapes were prepared by a doctor blade using polyvinyl pyrrolidone (PVP) as a binder and gelatin and polyvinyl alcohol (PVA) as additional binders to improve drying rate and mechanical properties of the tapes containing PVP. As binder was prepared by mixing 89.5 wt% of PVP, 10 wt% of PVA, and 0.5 wt% of gelatin, tape properties were optimized showing complete drying in 3h, tensile strength of 1 MPa, and high flexibility. Marginal gap of all-ceramic dental crowns prepared using the tapes was about 56 µm indicating the tape cast alumina dental crowns are clinically applicable.

Polarized Light Study of Tape Cast Alumina Green Body

Polarized Light Study of Tape Cast Alumina Green Body

偏光顕微鏡を用いたアルミナテープ成形体の粒子配向評価

偏光顕微鏡を用いたアルミナテープ成形体の粒子配向評価

Anisotropic shrinkage during sintering of particle-oriented systems—numerical simulation and experimental studies

High shear rates involved in processes such as tape casting and injection molding result in anisotropic shrinkage during subsequent densification of the green bodies. Anisotropic shrinkage with more than 14% difference in the shrinkages between the casting and transverse directions was observed in tape cast alumina. To explain the measured shrinkage anisotropy in tape cast alumina and its variation as densification progresses, sintering of oriented ellipses is simulated. The simulated shrinkage anisotropy decreases as sintering progresses because of the increasing neck length in the particle-oriented direction. Similar trends were seen for experimental results on tape cast alumina. The decrease is more significant when surface diffusion dominates over grain boundary diffusion. Similar atomistic models were also used to simulate the sintering shrinkages in liquid-phase sintered alumina. Again, anisotropy decreases as sintering progresses. The predictions from solid state sintering models were then validated by sintering alumina with 500–1000 ppm of yttria. It is proposed that the addition of yttria lowers the grain boundary diffusion rate with respect to the surface diffusion and hence lowers the shrinkage anisotropy in accordance with the simulation model.

Effect of Size Distribution of the Starting Powder on the Pore Size and its Distribution of Tape Cast Alumina Microporous Membranes

Thin disc type pure alumina membranes have been prepared by tape casting technique. Pore size distribution and pore volume have been determined by mercury porosimetry. Initial particle size of the alumina powder and the size distribution are found to have a strong influence on the ultimate median pore size and pore size distribution of the fired membranes. The spread of the particle size distribution of the powders is expressed by ‘quartile ratio’ which represents the size ratio corresponding to the cummulative finer percentages of 75 and 25 in the particle size distribution curve. With higher quartile ratio ( wider particle size distribution ) not only the median pore size increases but also the distribution tends to be bimodal. This is explained on the basis of certain basic sintering behaviors of the fine powders in general. ©

Formatation of pore structure in tape-cast alumina membranes – effects of binder content and firing temperature

Disc type ceramic aluminium oxide membrane has been prepared by tape casting technique. Thickness of this single layer membrane is in the range 200–300 μm. Porosity and pore size distribution have been determined by mercury porosimetry. Polymeric binder content of the green tape and the firing temperature are found to have strong influence on the average pore size, pore size distribution and overall porosity. Higher binder content promotes agglomeration of the ceramic particles, which on firing leads to wider pore size distribution and formation of closed pores. Pore coarsening is observed with increasing firing temperature.

Axisymmetric Textures in Alumina

The occurrence of axisymmetric textures prevails over a wide range of processing approaches for alumina-based materials. Mechanisms ranging from dislocation slip, recrystallization, anisometric grain rotation and grain growth all can contribute to the development of textures with similar characteristics. This paper documents neutron texture measurements of axisymmetric textures in hot-pressed, hot-forged and tape-cast alumina. Strong basal textures were introduced in the tape-cast samples during long term annealing in finer, non-tabular aluminas if glass and dopant additions are introduced. The implications of these results versus prior reports of textures in alumina and alumina matrix composites are discussed.