Liliana B. Garrido

Zircon based ceramics by colloidal processing

Abstract Zircon is an excellent raw material to elaborate ceramics with good thermo-chemical properties. Also, mullite–zirconia materials can be prepared by reaction sintering of zircon–alumina mixtures. Concentrated aqueous alumina–zircon suspensions were prepared to study the effect of the slip properties on the density and microstructure of the compacts obtained by slip casting and pressure filtration at 8 MPa. The effect of the degree of dispersion and solid volume concentration on the viscosity of the suspension was studied. Optimization of the amount of polyacrylate dispersant added and pH of the suspension improved the solid loading and consequently the green and sintered properties of the cast samples.

Dispersion of aluminum hydroxide coated Si3N4 powders with ammonium polyacrylate dispersant

Abstract Si3N4 powders coated with aluminum hydroxide were prepared and dispersed in water with ammonium polyacrylate (NH4PA) to produce 11.8 vol.% slips. The influence of the Al(OH)3 coating on the rheological behavior of 11.8 vol.% aqueous Si3N4 slips with NH4PA was investigated. The coating of Al(OH)3 on Si3N4 particles resulted in a bayerite-like surface with a small amount of gibbsite-like structure which improved the adsorption of a NH4PA dispersant and effectively reduced the viscosity of the 11.8 vol.% slurries. The viscosity values of slips with 0.11 wt.% NH4PA at pH 9.5 decreased from 33 mPa s for the uncoated powder to about 2 mPa s for the coated powders. Lower viscosity values and higher maximum packing fraction of the coated Si3N4 slips with respect to the uncoated one were found in the pH range studied (9.1–10.3) due to the greater NH4PA adsorption of the coated powders.

Influence of starch type on characteristics of porous 3Y-ZrO2 prepared from a direct consolidation casting method

Starch consolidation casting has been successfully used to produce porous ceramics with complex shapes at a relatively low producing cost. In this work, porous 3Y-ZrO2 ceramics were produced using two types of starch (corn and potato). Concentrated (50-52 vol. (%)) aqueous suspensions of the different 3Y-ZrO2-starch mixtures (i.e. starch to zirconia weight ratios between 0.20 and 0.52) were thermally consolidated and the effect of the type and amount of added starch on properties of resultant dried product was examined. After starch granules burnout, the effect of sintering temperature varying between 1000 and 1500 oC on the porosity evolution, interconnection and morphology of pores and microstructure of final ceramic was followed by water absorption measurements, Hg porosimetry and scanning electron microscopy SEM. Typical microstructure of the ceramic sintered at 1500 oC consisted of large nearly spherical pores (created by starch) uniformly distributed in a dense ZrO2 matrix. As the content of starch in the dried product (expressed as volume fraction) Xst is increased from 0.23 to 0.65, porosity of the ceramic sintered at 1500 oC increased from 38 to 52 vol. (%) for corn starch, while it achieved 50 to 54 vol. (%) for potato starch; thus, lower Xst of potato starch developed a porous zirconia with higher porosity. In this case, the pore structure contained pores having sizes above 50 µm and resulted completely interconnected while for those prepared using corn starch the pore interconnection occurred for Xst above 0.50.

Rheological properties of Si3N4, pseudoboehmite and bayerite coated Si3N4 suspensions with ammonium polyacrylate dispersant

Abstract Si3N4 powders coated with pseudoboehmite and bayerite were prepared by precipitation using AlCl3 solution and Al13 polymer solution, respectively. The resulting powders were dispersed in water with ammonium polyacrylate (NH4PA) at pH 9.4 to produce 18 vol% slips. The influence of the pseudoboehmite and bayerite coatings on the rheological properties of 18 vol% aqueous Si3N4 slips with NH4PA were studied. In addition, the effect of the amount of NH4PA solution added on the stabilization of the uncoated and coated Si3N4 slips were determined. Lower viscosity values and greater stability of the coated Si3N4 slips with respect to the uncoated one were found due to the greater NH4PA adsorption of the coated powders. However, the best stabilization was obtained with the bayerite coated Si3N4 powder. This was attributed to the greater NH4PA adsorption and lower specific surface area of the bayerite coated powder. The Si3N4 slips had a pseudoplastic behavior. The addition of NH4PA resulted in slips with high apparent yield stress and viscosity values due to the appreciable amount of NH4PA remaining in solution. The viscosity increased from 98 mPa s without NH4PA to about 155 mPa s for NH4PA concentration ≥0.28 wt.%. The flow curves of the suspensions followed the Herschel–Bulkley model. The variation of τ0, k and n constants of the model were used to represent the rheological properties of the slips. The minimum viscosity of the bayerite coated slips (3 mPa s) was obtained with the addition of 0.14 wt.% NH4PA. In this condition, the slips exhibited a Newtonian behavior (τ0=0, n=1). The pseudoboehmite coated slips were stabilized with 0.42 wt.% NH4PA, having a viscosity value of 6.6 mPa s. The τ0, k and n constants of the pseudoboehmite coated slips had an intermediate value between the Si3N4 and the bayerite coated one.

Comparison of different zirconia powders for slip casting of alumina–zirconia ceramics

Abstract Abstract Two commercial 3 mol.-% yttria partially stabilised zirconia powders, 0·3 wt-%Al2O3 doped (Al doped Y-PSZ) and without Al2O3 (Y-PSZ), were used to produce alumina (Al2O3)–zirconia (ZrO2) slip cast composites. The effect of the substitution of Al2O3 and/or Y-PSZ by Al doped Y-PSZ on the rheological properties of concentrated aqueous Al2O3–ZrO2 slips was studied. In addition, the density of green cast samples was studied and related to the degree of slip dispersion. The viscosity and yield stress values of Al2O3 Al doped Y-PSZ suspensions were higher than those of Al2O3 Y-PSZ ones for all the compositions and solid loading studied. This behaviour could be explained by a larger effective size of the Al doped Y-PSZ particles, which resulted in a higher effective solid volume fraction and a lower amount of free liquid available for flow. The substitution of Y-PSZ by Al doped Y-PSZ in the Al2O3–ZrO2 mixtures resulted in a less dense packing of cast samples.

Influence of yttria-alumina surface coating on the colloidal processing of silicon nitride slips

Abstract Silicon nitride (Si3N4) powders coated with 6 wt.% Y2O3 and 4 wt.% Al2O3 were prepared by coprecipitation. For comparison, the Si3N4 powder containing the same amount of additives was also prepared by mechanical mixing. The resulting powders were dispersed in water with ammonium polyacrylate (NH4PA) to produce 32 vol.% slips. The influence of pH on the dispersion properties of 32 vol.% coated Si3N4 slips with NH4PA were investigated and compared with those of the mixed Si3N4 slips. The microstructure and density of green cast samples were determined and related to the degree of slip dispersion. In addition, the influence of the surface coating of Si3N4 on the sintering behavior was investigated. Lower viscosity values of the coated Si3N4 slips were found in the pH range studied. The better slip dispersion of the coated Si3N4 slips resulted in higher green density values. A linear correlation between the sintered density and the green density was found. The densification and the α- to -β phase transformation of Si3N4 were not affected and enhanced, respectively, with the surface coating of Si3N4 particles with sintering aids.

Processing of Yttria-Alumina Coated Silicon Nitride Slips by Slip Casting

Si3N4powders coated with 6 wt% Y2O3and 4 wt% Al2O3were prepared by coprecipitation. The resulting powders were dispersed in water at different pH values and with addition of various amounts of ammonium polyacrylate (NH4PA) to produce 32 vol% slips. The influence of the amount of NH4PA solution added and pH on the rheological properties of 32 vol% coated Si3N4slips were studied. In addition, the sintered density of cast samples was determined and related to the degree of slip dispersion. The adsorption of the NH4PA on the coated particle surface was rather high and the surface became saturated near 0.86 mg/m2at pH 9.2. High NH4PA concentrations (1.7–3 wt%) were necessary to obtain well dispersed 32 vol% coated Si3N4slips at pH 9.2. The best stabilization was obtained with the addition of 2.3 wt% NH4PA; in this condition, the viscosity reached a minimum value of 35 mPa.s at 100 s−1. The slip viscosity increased with increasing pH from 9.2 to 10.2. Slips with low viscosities gave a more dense packing of cast samples and consequently higher sintered density values.

Ammonium polyacrylate adsorption on “aluminium hydroxides and oxyhydroxide” coated silicon nitride powders

Si3N4 powders coated with pseudoboehmite, Al hydroxide with a OH/Al mol ratio (R) lower than 3 and bayerite, were prepared from different coating solutions and dispersed in water with an ammonium polyacrylate (NH4PA) to produce 11.8 vol% slips. The coated powders showed isoelectric points at pH 7, 7.5 and 7.5 close to that of the precipitate coatings of pseudoboehmite, Al hydroxide (R<3) and bayerite, respectively, indicating that the surface coverages were almost complete. The bayerite coated powders had the lowest specific surface area values. The coatings on Si3N4 particles were shown to improve the adsorption of NH4PA dispersant on Si3N4. The maximum adsorption of the polyelectrolyte at pH 9.5 increased from 0.01 for the uncoated powder to 0.03, 0.055 and 0.08 mg/m2 for the coated powders with pseudoboehmite, Al hydroxide (R<3) and bayerite, respectively.

Improvement of ammonium polyacrylate adsorption on Si3N4 powders by an aluminium hydroxide coating

Abstract A precipitate coating Al(OH) 3 on Si 3 N 4 particles was shown to improve the adsorption of ammonium polyacrylate (NH 4 PA) dispersant on Si 3 N 4 . The coating of Al(OH) 3 on Si 3 N 4 particles resulted in a bayerite-like surface with a small amount of gibbsite-like structure which improved the adsorption of a NH 4 PA dispersant. The maximum adsorption at pH 9.5 increased from 0.01 to 0.075 mg/m 2 for the as-received and coated Si 3 N 4 powders, respectively.

Densification and mechanical properties of ZrO2–CaAl4O7 composites obtained by reaction sintering

Abstract Composites of the ZrO2–CaO–Al2O3 system exhibit high corrosion resistance and high chemical stability, these being suitable properties for enhanced performance as refractories. In this work composites based on ZrO2–CaAl4O7 (CA2) were developed by reaction sintering of two pure ZrO2 powders with different particle size (d50 = 0.44 μm and d50 = 8 μm) mixed with high-alumina cement. The effects of CaO to ZrO2 molar ratio, particle size of ZrO2 and sintering temperature on densification, c-ZrO2 formation, microstructure and mechanical properties (Vickers hardness, dynamic Youngs modulus and compressive strength) were determined. Porosity and mean pore size of composites reduced with decreasing both particle size of ZrO2 and CaO content. The highest compression strength of ∼350 MPa and hardness of 6 GPa corresponded to dense ZrO2-based composite with 15 vol.% CA2 sintered at 1400°C.