Nozomu Uchida

Origin of shrinkage anisotropy during sintering for uniaxially pressed alumina compacts

Origin of shrinkage anisotropy during sintering is reported on alumina powder compacts made by uniaxial pressing followed by cold isostatic pressing (CIP). Two types of alumina particles were used in this study; one was slightly elongated shape, the other spherical shape. An anisotropic shrinkage occurred during sintering in the compacts made of the elongated shape of particles, even after the CIP. However, no the anisotropic shrinkage happened in the compacts made of the spherical shape of particles, even at the same processing condition to the former. Detailed structure analysis involving the novel immersion liquid technique showed that the particle orientation was found to be an important origin of the shrinkage anisotropy. The particle orientation was developed during uniaxial pressing in the compacts of the elongated shape of particles. Subsequent CIP did not eliminate the shrinkage anisotropy. No particle orientation happened in the compact of the spherical shape of particles.

Sintering deformation caused by particle orientation in uniaxially and isostatically pressed alumina compacts

Effect of particle orientation on deformation during sintering is reported for model systems; one made with industrial grade low soda alumina, which has an elongated particle shape, and the other a special alumina with a spherical particle shape. To ensure the homogeneous packing density of particles, compacts were made by uniaxial pressing followed by cold isostatic pressing. The particle orientation was examined with a polarized light microscope and was found to be an important cause of sintering deformation. In a green body, for elongated shape of particles, the particle orientation occurred during uniaxial pressing, causing the anisotropic sintering shrinkage during sintering and thus the sintering deformation. No particle orientation nor shrinkage anisotropy was noted in the system made with the powder of spherical particle shape.

Binder surface segregation during spray drying of ceramic slurry

Spray-dried granules were observed by a laser microscopy with the immersion liquid technique. The binder distribution in the granules was analyzed from light intensity profiles of the images. The results showed that a surface layer with a large amount of the binder is formed in the spray-dried granule, and the segregation is influenced by initial binder concentration and size of atomized droplet. A computer simulation for soluble binder segregation during spray drying was conducted by considering simultaneously the solvent evaporation, the relative migration between the liquid and the particles, the diffusion, and drying shrinkage. The simulation coincides with the experimental results. To make uniform granules, reducing the amount of binder, liquid content, size of atomized droplet, and drying rate is favorable.

CHARACTERIZATION OF PROCESSING PORES AND THEIR RELEVANCE TO THE STRENGTH IN ALUMINA CERAMICS

Characterization of bulk defects was successfully accomplished in alumina with a transmission optical microscope. The characterization technique used is based on the fact that many ceramics are essentially transparent. Most defects in this particular ceramic were found to be pore. Their size distribution was found to follow a simple power function. With these characteristics of defects, the strength distribution of the ceramics was calculated with Barattas model and compared to the measured strength of the ceramics. A good agreement was found between them when the pore was assumed to be accompanied with cracks 4 times the length of the grain size.

Improvement of the corrosion resistance of alumina–chromia ceramic materials in molten slag

Abstract In the previous study, we found out that the corrosion of ceramic materials in molten slag depended strongly on the viscosity of the slag and the basicity gap between the ceramics and the slag. In this study, we propose experimental equation to predict the corrosion rate of a ceramic material in a molten slag as functions of the viscosity and the basicity gap on the basis of the corrosion tests. Cr2O3–Al2O3 ceramic materials were used for the tests in the SiO2–CaO–B2O3 based model molten slag. The obtained experimental equation is as follows: logΔ D=0.18 ∗ log Δ B−0.55 ∗ log η+C where, ΔD is the depth of corrosion (μm), ΔB the basicity gap between the slag and specimen, η the viscosity of slag (Poise). C is the constant that depends on the conditions of the corrosion. The value is 2.7 for the testing temperature of 1573 K, holding time of 3600 s and rotating speed of specimen of 5.2 cm s−1 within this corrosion test.

Chemical effects of DCCA to the sol-gel reaction process

The effects of drying control chemical additives (DCCA) on the growth of silica particles, gelation time and physical properties of the dry gel were examined in a two-step silica sol-gel process.N,N-dimethylformamide,N,N-dimethylacetamide and ethylene glycol (EG) were applied as DCCAs. The shapes of growing silica particles were distorted spheres on addition of DCCA. EG accelerated the gelation process. Despite the use of DCCA, crack-free, dry gels were obtained only under limited conditions.

Development of anisotropic microstructure in uniaxially pressed alumina compacts

Abstract Alumina compacts are prepared by uniaxial pressing followed by cold isostatic pressing (CIP). Anisotropic microstructures of the compacts and their sintered bodies are reported. Anisotropic grain growth with sintering time was systematically examined through direct measurement of grain dimensions on SEM micrographs in various conditions. Experimental results indicated that average grain size was always larger in the plane parallel to the direction of uniaxial pressing, than in its perpendicular plane, for all sintering conditions. The difference in the average grain size increased in these planes with sintering time. The grain growth followed the cubic law and the rate constants were 4.8×10−21 and 1.9×10−21(m3/s), for the former and the latter, respectively, in specimen uniaxial pressed at 100 MPa and CIPed at 100 MPa. Many intra-granule fractures were observed in the compact in the plane perpendicular the pressing direction comparing with its parallel plane.

Direct observation of non-uniform distribution of PVA binder in alumina green body

To study the effect of binder distribution in a powder granule on the internal structure of the compacts, the internal structure of green bodies with various PVA contents was examined by a liquid immersion technique and SEM. By the liquid immersion technique, a dark three-dimensional network was found in the as-compacted green bodies, but disappeared after binder removal. This result shows that the structure corresponds to binder-rich surface layer of the spray-dried granule. Detailed examination on green bodies after the binder removal showed that the low density region was present at the boundaries of granules and was related to the binder-rich region. The voids between granules increase with increasing the PVA content.

Kinetics of property change associated with atmospheric humidity changes in alumina powder granules with PVA binder

Abstract A microcompression testing machine was used on single Al2O3 powder granules to study their stress–strain behavior as a function of relative humidity. The test granules were prepared by spray-drying slurries containing 3 mass% poly(vinyl alcohol) and Al2O3 powder. The stress–strain curves and granule strength were determined at regular time intervals, after step changes in atmospheric humidity. When stress was applied, deformation increased rapidly with the stress. This stress corresponded to the fracture strength of the granule. In a dry atmosphere, the granules deformed linearly with increasing stress, and a rapid change in strain was noted above a certain critical value of increased stress. In a wet atmosphere, the granules deformed continuously at lower stress. The mechanical properties of the granules changed rapidly with time when the atmospheric relative humidity was changed. Within 3 min of exposure to a new atmosphere, the mechanical properties of the granules reached equilibrium values. The change in properties with humidity was controlled by moisture diffusion in the granules and was reversible.

Direct observation of aggregates and agglomerates in alumina granules

Abstract Aggregates and agglomerates in granules prepared from milled alumina powder were studied with new confocal laser scanning fluorescence microscopy (CLSFM) using the liquid immersion method. Two kinds of ground powders were prepared by agitation bead milling and conventional ball milling and spray drying. The observation by CLSFM was very effective for detecting aggregates and agglomerates of very low amounts in alumina granules. Aggregates and agglomerates were absent in the granule made from powder milled by agitation bead milling, whereas they remained in the granule made from powder milled by ball milling. The volume fractions of these particles were about 2.0 vol.% and under 0.1 vol.% for the granules made from ball-milled and bead-milled powders, respectively.