Subhas G. Malghan

Effect of polymer molecular weight on the absorption of polyacrylic acid at the alumina-water interface

The absorption and electrokinetic characteristics of alumina suspensions in the presence of polyacrylic acid as a dispersant have been studied. The adsorption isotherms exhibit high-affinity Langmuirian behaviour. The adsorption density decreases with increasing in pH, while it increases with increasing molecular weight of the polymer. Electrokinetic studies indicate specific adsorption at and above the isoelectric point of the alumina sample. Possible mechanisms of interaction between alumina and polyacrylic acid are discussed.

The surface chemistry of silicon nitride powder in the presence of dissolved ions

Colloidal processing of silicon nitride (Si3N4) powders depends largely on the control of reactions at the solid-solution interface. The role of dissolved ions in the surface chemistry of Si3N4 powders has been investigated, and the implications of these results for the effects of impurities, contaminants and additives in processing are discussed. The interaction of ions at the solid-solution interface was characterized by particle electrokinetic behaviour determined from electroacoustic measurements in moderately concentrated suspensions. Ions were classified according to chemical similarity and surface specificity. Specific adsorption was inferred from the movement of the isoelectric point relative to the endemic “native” value. Most simple univalent electrolytes behaved indifferently towards the Si3N4 surface, with the exception of fluoride which specifically adsorbed and may have formed a strong complex with surface silicon sites. The alkaline-earth cations exhibited a similar weak specificity. In the presence of hydrolysable transition metal cations, powder surface chemistry was controlled by the adsorption of hydroxy metal complexes and by the solubility of a surface-precipitated metal hydroxide phase. Oxo anions, such as sulphate and carbonate, adsorbed specifically on the Si3N4 surface, but the interactions were weaker than previously observed on metal oxide surfaces.

Surface chemical studies on alumina suspensions using ammonium poly(methacrylate)

The interaction of \alpha-alumina with ammonium poly(methacrylate) (APMA) has been investigated through adsorption and electrokinetic measurements. The adsorption isotherms exhibit high affinity, Langmuirian behaviour at highly acidic pH and the adsorption density is found to decrease with increasing pH. On the contrary, the percentage desorption increases with increasing pH. Electrokinetic studies indicate specific adsorption between alumina and APMA resulting in a shift of the isoelectric point to acidic pH values. Electrophoretic mobility values indicate that the alumina suspension could be well dispersed in the pH range 5-11, consequent to polymer adsorption. These results were corroborated by the dispersion tests. Dissolution tests reveal the release of aluminium species from alumina while co-precipitation experiments confirm the chemical interaction of aluminium species with APMA in the bulk solution. Hydrogen bonding, electrostatic and chemical interactive forces are postulated to govern the adsorption process. FTIR spectroscopic data lend further support to the interaction mechanisms proposed.

Role of polyacrylate in modifying interfacial properties and stability of silicon nitride particles in aqueous suspensions

The role of ammonium polyacrylate in modifying the interfacial properties of silicon nitride powder was examined systematically in order to understand the basic mechanism of dispersion. The adsorption of the polyacrylate on silicon nitride powder was found to be dependent on both the pH and its residual (equilibrium) concentration. The interfacial charge properties of the silicon nitride powder can be modified to a large extent by the adsorbed polyacrylate and equilibrium pH. Well-dispersed silicon nitride suspensions can be obtained in both the neutral and alkaline pH range by the application of ammonium polyacrylate. The enhanced electrostatic repulsion between the dispersant-modified powder surfaces was found to be the primary mechanism governing the dispersion. Also, the steric interactions of the adsorbed polyacrylate with aqueous media were demonstrated in this work to have a positive contribution to the dispersion. The latter is attributed to the hydrophilic nature of the polyacrylate. The dispersing functions of the adsorbed polyacrylate were understood to arise from the two oxygen atoms in the carboxylic group of the polyacrylate. The charged oxygen atom in the carboxylic group was responsible for the measured interfacial charge properties of the dispersant-modified powder. The uncharged oxygen atom had an additional contribution to the hydrophilic nature of the adsorbed dispersant layer.

Mechanistic understanding of silicon nitride dispersion using cationic and anionic polyelectrolytes

Abstract The influence of interface properties of silicon nitride powder on the dispersion and particle size distribution in aqueous environments was investigated in the presence of cationic and anionic polyelectrolytes. The isoelectric point of as-received silicon nitride powder at pH 6.6 shifted to 4.0 in the presence of 1500 ppm ammonium polymethacrylate, an anionic polyelectrolyte (APE). However, no isoelectric point could be observed up to pH 10.8 when 1500 ppm quaternized polyamine epoxychlorohydrin, a cationic polyelectrolyte (CPE), was present in the silicon nitride suspensions. The dispersion characteristics of silicon nitride powder in the presence of various concentrations of the two polyelectrolytes and by pH variation were examined. The slightly better dispersion of silicon nitride powder in the presence of CPE than that in the presence of APE is explained in terms of differences in the adsorption characteristics and surface charge concentration in aqueous environments. A mechanistic understanding of the role of interface chemistry and, in particular, polyelectrolyte adsorption on the dispersion of silicon nitride powder is presented.

Effects of soxhlet extraction on the surface oxide layer of silicon nitride powders

Abstract An aqueous soxhlet extraction procedure was used to surface-clean five commercial silicon nitride powders. The solid-solution interface properties were characterized before and after extraction by electrokinetic sonic amplitude measurements. The isoelectric point (pHiep) was found to increase significantly for some powders after treatment. The powder surface was analyzed using X-ray photoelectron spectroscopy and X-ray induced Auger electron spectroscopy before and after extraction. The surface oxygen content and oxide layer thickness decrease after treatment. A linear correlation was found between oxide thickness and (pHiep), which yields a pristine (pHiep) of about 9.7 for the unoxidized Si3N4 particle.

The oxidation of an aluminum nitride powder studied by bremsstrahlung-excited Auger electron spectroscopy and x-ray photoelectron spectroscopy

Bremsstrahlung-excited Auger electron spectroscopy (AES) was used to study the oxidation kinetics of an aluminum nitride powder oxidized in air at 750, 800, 850, and 900 ‐ C. An equation was derived to calculate the average surface oxide film thickness from the aluminum AES spectra. The oxidation of this powder was found to follow a parabolic rate law within this temperature range. The measured activation energy was 230 6 17 kJymol (55 6 4 kcalymol). Analysis with x-ray photoelectron spectroscopy (XPS) showed that in addition to the nitride N 1s peak, there was a second N 1s peak. This peak has been observed in previous studies and can be attributed to N‐ O bonding either within the growing oxide film or at the Al 2O3yAlN interface.

Population balance model for solid state sintering II. Grain growth

A novel population balance model based on size interval-by-size interval marching algorithm has been developed for the transient grain growth kinetics during solid state sintering. This model incorporates the coupled phenomena of densification and grain growth and exhibits reasonable capability of predicting the temporal evolution of grain size distribution for a given initial grain size distribution and time-temperature sintering cycle. In this, part II of our work, we present the coupled governing equation for sintering kinetics. We employ a size interval-by-size interval marching algorithm for the solution of the resulting population balance equation and validation of the model against experimental data for zirconia compacts sintered at 1400, 1500 and 1600°C for different times. In addition, the model has been successfully tested against published experimental data on sintering of alumina compacts.

Standard reference materials for particle size analysis of ceramic powders by gravity sedimentation

Abstract Two standard reference materials (SRM), one a nitride powder and the other of an oxide powder, of mean size in the range of 1.0 μm, were developed to assist in the measurement and analysis of fine size powders. SRM 659, a silicon nitride powder and SRM 1978, a zirconium oxide powder, are intended for calibration and evaluation of instruments used to measure particle size distribution using Sedigraph, a gravitational sedimentation method. In the development of these SRMs, the parameters of the powder deagglomeration procedures were studied to determine their effects on the measured particle size distribution of powders. The parameters studied included power output and time of sonication, initial pH of the suspension, and dispersant concentration. The morphology, density, surface area, and surface chemistry of these powders were also measured.

Binder distribution in Si3N4 ceramic green bodies studied by stray-field NMR imaging

Nuclear magnetic resonance (NMR) imaging is an emerging technology which provides a unique material-diagnostic technique by in situ internal mapping. It can provide information not only on material distribution, but also on the chemical and physical characteristics of materials. However, due to the nuclear dipole dipole interaction in solid state materials, NMR spectroscopic signals are normally very broad. NMR imaging based on these unresolved broad lines is extremely difficult, and resolution is poor. The binder distribution was studied in ceramic green bodies with a stray-field NMR imaging facility at a proton frequency of 163 MHz near the edge of a 9.394 T superconducting magnet. The 1H nuclear spin echo signal from silicon nitride green bodies containing 10 wt% of either polyethylene glycol or polyvinyl alcohol as a binder was detected at 163 MHz. NMR images show a good homogeneity of the binder distribution in the cross-sections of the samples. Overall results show that the distribution of polyethylene glycol in Si3N4 green bodies is more homogeneous than that of polyvinyl alcohol under similar processing parameters. NMR spectroscopic results also indicate a higher moisture content in the green bodies containing a polyvinyl alcohol binder.