Minoru Hashiba

Dispersion of ZrO2 particles in aqueous suspensions by ammonium polyacrylate

The effects of poly anionic-electrolyte (ammonium polyacrylate, PAA) as a dispersant on two kinds of ZrO2 (monoclinic and yttria-doped tetragonal zirconia) aqueous suspensions were examined by the measurements ofζ-potential and viscosity, the sedimentation test and the determination of the wet point and flow point of the powders. Additions above 2.5 wt% PAA to zirconia gave a negative highζ-potential above −30 mV, and then −45 and −30 mV were obtained for monoclinic and tetragonal zirconia above 5 wt% PAA, respectively. A high negativeζ-potential above −30 mV was retained with 5 wt% PAA for a change in pH over a wider range (pH 6 to 10 for monoclinic ZrO2, 7 to 9 for tetragonal ZrO2) in comparison to that of ZrO2 without dispersant. The increase of theζ-potential resulted in a decrease in the viscosity. The evaluation of dispersion by the sedimentation test was correlated well with the value ofζ-potential and the viscosity of the suspensions. The presence of native positive charge of monoclinic and tetragonal zirconia powders required an excess amount of PAA to attain dispersion of the suspension. There was a small difference in the least amount of PAA required to attain good dispersion between monoclinic and tetragonal ZrO2. The difference was also indicated by changes of the flow point on PAA addition. Addition of 0.1% PAA to monoclinic ZrO2 and 0.25 wt% to tetragonal ZrO2 gave a maximum value of the flow point, whereas the positiveζ-potential fell to zero. Measurement of the flow point was a simple and useful technique for rapid evaluation of a required amount of dispersant for ZrO2 suspensions.

The zeta-potential measurement for concentrated aqueous suspension by improved electrophoretic mass transport apparatus: application to Al2O3, ZrO3 and SiC suspensions

The zeta-potentials for Al2O3, ZrO2 and SiC particles in concentrated suspensions were determined by an improved mass transport apparatus in order to obtain reliable and reproducible data on particle dispersion in the slip casting process. The values of isoelectric point for each particle were as follows: pH ≃ 8 for Al2O3, ≃ 5 for monoclinic ZrO2, ≃ 6.5 for tetragonal ZrO2, ≃ 4 for α-SiC and ≃ 5.5 for β-SiC. It was suggested that the values of zetapotentials for those particles were affected by the chemical and physical state of the particle surfaces, i.e. crystal structure, comtamination and hydration, etc. The apparatus used in this experiment will be also available for use with suspensions containing surfactants in practical slip casting.

FLUIDITY AND DISPERSION OF ALUMINA SUSPENSION AT THE LIMIT OF THICKENING BY AMMONIUM POLYACRYLATES

The limit of thickening of an alumina suspension by ammonium polyacrylates (PAA) and its molecular weight dependence of the limit were determined from the lowering of the flow point to be a measure of simultaneous promotion of the thickening and the dispersion. PAA of a smaller molecular weight gave a lower flow point minimum and a thicker alumina suspension retaining fluidity up to 85 wt % alumina for PAA of molecular weight 2500. The suspension thickened to the limit has the smallest gap between the flow point and the wet point, supporting Daniels statement on good dispersion. The average water layer thickness, calculated by dividing the amount of water of suspension at the limit of thickening by the particle numbers, indicated no linearity with the chain length of the PAA. The thickening for PAA with molecular weights smaller than 21 000 resulted in a limit in the average water layer thickness of ∼30 nm being accompanied by dilatant flow. The suspension at high solid loadings showed various extension features on the glass plate with changes in the PAA concentration around the flow point minimum. The flow behaviour of the alumina suspension around the limit of thickening was characterized by the Bingham model with two parameters of the yield stress, σ0, and the Bingham viscosity, η. Increase in the fluidity on PAA addition was strongly attributed to a greater lowering of σ0 than of η. A balanced ratio between the two parameters in the apparent viscosity under a suitable shear rate was suggested to be necessary for the flow of the castable thick suspension.

Bubble-free electrophoretic deposition of aqueous zirconia suspensions with hydroquinone

Electrophoretic deposition (EPD) is a colloidal processing technique for ceramics in which charged particles move toward, and are deposited upon an electrode with the opposite charge [1–3]. The EPD technique has many advantages, such as the fast, easy, and uniform formation of complex shapes, good control of deposition thickness, high green density. This method has been used to fabricate thin films, multilayered composites, functional materials, etc. Non-aqueous suspensions are usually preferred for the EPD process to prevent electrolysis of the solvent and to obtain bubble-free deposition. On the other hand, an aqueous shaping system is advantageous from the viewpoints of ecology, safety, and cost. Water is decomposed into hydrogen and oxygen when a DC current is passed through an aqueous medium. There have been some reports of the fabrication of a bubble-free deposit from aqueous suspension [4, 5]. Recently, Uchikoshi et al., investigated the EPD characteristics of positively charged particles onto various cathodic substrates using aqueous alumina suspensions [5]. They found that no macro pores were formed in the EPD deposit on a palladium cathode, and that the green density and sintered properties of the deposits were the same as those produced by the slip casting process. It is well known that hydroquinone (HQ) is readily oxidized to quinone (Q) at high pH in alkaline solution [6]:

Simultaneous fabrication of a composite with low thermal expansion and high strength in the eucryptite — yttria-stabilized PSZ system

Low thermal expansion coefficient and high mechanical strength have been attained simultaneously in a composite with microstructures in which the low thermal expansion grains are surrounded by a continuous texture of the grains with high strength. Alloying eucryptite with yttria-stabilized PSZ (partially stabilized zirconia) simultaneously achieved the low thermal expansion coefficient, 1.45 × 10−6, and the high bending strength, 220 MPa, by adjusting the composition and controlling the microstructures by changing the starting powders and the milling durations. Grain size and reactions between the coupled materials determined the critical processing conditions.

Aluminum titanate-tetragonal zirconia composite with low thermal expansion and high strength simultaneously

Abstract Aluminium titanate was strengthened by alloying with 3 mol% yttria stabilized zirconia to attain a small expansion coefficient of 2 × 10 −6 and a high strength of 100 MPa, simultaneously. Long milling and optimization of the composition of the alloy to an equal weight of the component oxides and also of the firing temperature at 1400 °C were important for the purpose. In the optimal conditions, aluminum titanate grains of 3 μm was surrounded by zirconia matrix of ca. 0.3 μm grains containing zirconium titanate where would be a balance between the tension in the matrix and the expansion force of aluminum titanate grains. Formation of zirconium titanate was important for the strengthening to decrease the lowering of the strength brought about by the unstabilization of PSZ.

Internal friction, crack length of fracture origin and fracture surface energy in alumina-zirconia composites

Two series of alumina-zirconia composites, i.e. alumina-unstabilized zirconia and alumina-partially stabilized zirconia with 3 mol % Y2O3, with different zirconia content were slip casted and fired at 1550°C for 3 h. Elastic constant, bending strength and fracture toughness were measured. Internal friction was determined to follow the formation of cracks, nondestructively, which could be one of the fracture origins. The crack length of the fracture origin and the fracture surface energy were calculated by applying Griffiths fracture theory. Microstructures of the fracture surfaces were observed using a scanning electron microscope. For the unstabilized zirconia system, the increase in the internal friction of the order from 10−4 to 10−3 was a guide to find the formation of cracks which lead to the fracture. The increase in the cracks becoming a fracture origin lead to the increase inKlc and also to the apparent increase in the fracture surface energy. For the partially stabilized zirconia system, the increase in the fracture surface energy with an increase in zirconia content, keeping low internal frictions of the order of 10−4, indicates the intrinsic strengthening of the grain boundaries in comparison to the unstabilized zirconia system. Internal friction is the most suitable nondestructive physical quantity to find the microcracks which leads to the fracture.

Composition Dependence of Internal Friction in Al2O3–3Y–ZrO2 Composites

An apparatus was developed for measuring internal friction automatically with high accuracy at high temperatures. The composition dependence of the internal friction in an Al2O3–3Y–ZrO2 system was investigated at room temperature using it. The results showed that no cracks were produced in the 3Y–ZrO2 contents less than 40%, and that an energy absorption peak existed at about 20% 3Y–ZrO2 content.

Extensions of polyacrylic acid ammonium salts in the adsorption layer to fluidize alumina slurries

An average shell volume occupied in the adsorption layer on alumina by a polyacrylic acid ammonium salts molecule (PAA) defined as the average area occupied by an adsorbed PAA on alumina multiplied by the average thickness of the water layer at the limit of thickening, has been calculated from the adsorbed amount of PAA and from the flow points of alumina in the presence of PAA of different molecular weights. A steric effect of the PAA dispersant on the dispersion of alumina resulted in a flow with no yield stress. This was due to the change of the extension of PAA in the adsorption shell from a sphere to an ellipsoid with a long axis which exceeded the effective distance that the van der Waals attraction force reaches at a molecular weight for the PAA of between 10 000–20 000.

Effectiveness of a dispersant for the thickening of alumina slurries whilst retaining the fluidity

Concentrated alumina slurries were fluidized in an optimum amount of polyacrylates (PAA) in the content region where a good dispersion was obtained. Dispersion was necessary, but not sufficient for fluidization. The expected role of PAA in the thickening was retention of a water-reducing ability to retain the fluidity. Thickening of the alumina slurry to the limit of retaining the fluidity was achieved by the amount of PAA at which the flow point showed a minimum. The flow of alumina slurries around the limit was approximated by the Bingham model which was characterized by the yield stress and the Bingham viscosity. The flow curves of the slurries containing insufficient PAA had a yield stress which decreased with increasing amount of PAA and disappeared at an optimum amount of PAA to give a minimum flow point. The yield stress again increased retaining the low Bingham viscosity with increasing amount of PAA, to exceed the optimum amount for molecular weights smaller than 10 000. On the other hand, the Bingham viscosity increased without increasing the yield stress with increasing amount of PAA, to exceed the optimum amount for molecular weights larger than 20 000. The effect of PAA on the dispersion and flow behaviour could be explained by the electrostatic stabilization based on the Derjaguin-Landau, Verwey-Overbeek (DLVO) theory for the PAA with a molecular weight smaller than 10 000, and by the steric stabilization for the PAA with a molecular weight larger than 20 000.