Effect of pH and impurities on the surface charge of zinc oxide in aqueous solution

The surface charge on some commercially available zinc oxide powders in aqueous solution has been studied. Aqueous suspensions of zinc oxide could not be stabilized electrostatically because of the decreasing pH of suspensions in the region between 7.2 and 12 as a result the transformation of colloidal Zn(OH)_2(S) particles to Zn(OH)_2(aq) ions. The major cationic impurity in the zinc oxide powders is lead, whereas in the aged suspensions of the same powders, zinc and sulphate ions dominate. These ions have the the most significant effect in creating an electrical double layer shell around the zinc oxide particles. The effect of these ions on the surface potential of zinc oxide was studied and the point of zero charge and charge reversal were found to be a direct function of the zinc and sulphate ions' concentration in the supernatant.     

Triggering the aqueous interparticle association of γ‒Al2O3 hierarchical assemblies using divalent cations and cellulose nanofibers

Understanding aqueous dispersion, rheological properties and colloidal stabilisation mechanisms of hierarchically assembled ceramic powders is important for progress in the fields of catalysis, separation and/or adsorption. The present study was designed to evaluate the rheological and sedimentation behaviour of highly loaded aqueous suspensions (up to φ_A = 0.126) containing AlN-powder-hydrolysis-derived, micron-sized, mesoporous, gamma alumina (MA) particulates with a high surface area (～180 m²/g) dispersed with sodium polyacrylate (NaPAA). The as-prepared suspensions were prone to sedimentation and segregation. However, when divalent cations (Mg²⁺, Ca²⁺) or cellulose nanofibers were added, the formation of interparticle association networks in the aqueous suspensions containing MA particles was triggered, facilitating their long-term resistance to sedimentation lasting more than 12 weeks.     

Colloidal dispersion of nano-scale ZnO powders using amphibious and anionic polyelectrolytes

This article describes the dispersion of aqueous suspensions containing nano-scale ZnO powder by utilizing a hybrid of chemical dispersant and mechanical mixing/grinding process. The chemical dispersants included anionic or amphibious polyelectrolytes, i.e., sodium salt of polymethylacrylic acid (PMAA-Na) or polyacrylamide/(α-N, N-dimethyl-N-acryloyloxyethyl) ammonium ethanate (PDAAE). The optimum critical concentrations for each dispersants to achieve the lowest viscosity, smallest final sediment volume and particle size (d₅₀) for the nano-ZnO suspensions, 3 wt.% for PMAA-Na and 5 wt.% for PDAAE, were identified. The finely dispersed nano-ZnO powders were transferred to prepare sputtering target. The root-mean-square roughness (R_RMS) of thin films deposited by utilizing such a target was found to be 2.05 nm, which was lower than the R_RMS of the film (≈27.57 nm) deposited by using a commercial ZnO target comprised of micro-scale granules.     

Surface Modification of Si3N4 Powders by Coprecipitation of Sintering Aids

Two commercial Si₃N₄ powders were coated with sintering aids by coprecipitation. Lanthanum and yttrium nitrates were used as sintering aid precursors. Electrokinetic sonic amplitude measurements and X-ray photoemission spectroscopy analysis were used to investigate electrokinetic behavior and surface properties, respectively. Coprecipitation produced different effects on the composition of the coating layer depending on the actual features of the starting Si₃N₄ powders. The electrokinetic behavior of aqueous suspensions with coated powders depended strongly on the additives, their solubility, and the rate of oxidation of the coated layer. The coprecipitation conditions had to be carefully controlled to obtain reproducible composition and morphology of the coating layers. Treatments of the starting powder, pH, and washing volumes were optimized to tailor the coating layer and improve the coprecipitation yield.     

Phase and microstructural evolution of yttrium-doped nanocrystalline alumina: A contribution of advanced microscopy techniques

Well-dispersed nano-crystalline transition alumina suspensions were mixed with yttrium chloride aqueous solutions, with the aim of producing Al₂O₃-Y₃Al₅O₁₂ (YAG) composite powders. DTA analysis allowed to highlight the role of yttrium on the α-phase crystallization path. Systematic XRD and HRTEM analyses were carried out in parallel on powders calcined in a wide temperature range (600-1300 °C) in order to follow phase and microstructural evolution. A thin, homogeneous yttrium-rich layer was yielded on the alumina particles surface; yttrium diffusion into the alumina matrix was negligible up to 1150 °C whereas, starting from 1200 °C, aggregates of partially sintered alumina particles appeared, stuck together by yttrium-rich thin films. Moreover, in the yttrium-richer zones, such as alumina grain boundaries and triple joints, yttrium-aluminates precipitated at alumina particles surface. Finally, at 1300 °C, alumina-YAG composite powders were produced, in which YAG was homogenously distributed among the alumina grains.     

Surface modification of SiC powders by hydrolysed aluminium coating

SiC powders are surface modified to behave like alumina in aqueous suspensions by coating the powders with in situ generated hydrolyzed aluminium from dilute aqueous aluminum nitrate solutions in a pH range of 3–4.5 using hexa-methylene-tetramine as the base generator at ambient temperature. By examining the zeta potential and rheological properties of aqueous suspensions of the powder with different Al-coverage, it was observed that the coated powder begins to show alumina-like surface properties at an Al-coverage of 0.1 mg/m², in contrast to 0.5 mg/m² that was the minimum value reported earlier for observing the effect. This is explained by proposing a coating mechanism which proceeds through adsorption of a layer of cationic hydrolysed aluminum molecules, such as [Al(OH)(H₂O)₅]²⁺ and [Al₂(OH)₂(H₂O)₈]⁴⁺, during which the particles attain alumina-like surface charge properties. The modified powder retained alumina-like surface characteristics when stored under moist conditions and in acidic suspensions. The low value of Al-coverage and the resulting low increase (<10%) of powder surface area are advantageous for preparation of concentrated SiC and SiC–Al₂O₃ composite suspensions in acidic aqueous medium with rheological properties similar to that of their counterparts containing alumina only.     

The effects of carboxylic acids on the aqueous dispersion and electrophoretic deposition of ZrO2

The agglomeration, electrokinetic properties and electrophoretic deposition behaviour of aqueous suspensions of ZrO₂ with carboxylic acid additives were studied in comparison with conventional pH adjustment. It was found that citric acid imparted negative zeta-potential values and electrosteric stabilisation to particles in suspensions at all pH levels. The examination of additions of carboxylic acids to ZrO₂ suspensions revealed that these reagents cause a sharp drop in zeta-potential at distinct addition levels, which correspond to surface saturation of the particles with negatively charged carboxylate groups. Adsorption cross sections of citric acid, EDTA and oxalic acid were evaluated from these results, showing that both citric acid and EDTA coordinate to ZrO₂ surfaces by two carboxylate groups while oxalic acid is coordinated by one group. The use of carboxylic acids was shown to facilitate superior electrophoretic deposition in comparison with zeta-potential modification by conventional pH adjustment through improved suspension stability.     

Abatement of emissions in small-scale combustors through utilisation of blended pellet fuels

This paper presents the results of a study carried out on a small-scale 10 kW fixed-bed combustor fired by blended pellet fuels burning on a thin depth fuel-bed. The use of the thin fuel-bed may contribute to major advances in the introduction of low-cost, fully automated small-scale combustors by eliminating de-ashing mechanisms, and improving operating conditions. The pellets were made by pressing the mixture of finely ground lignite or its by-product (xylite) with wood chips. The xylite (woody lignite) and the wood chips are wastes produced during the production of lignite fuel and timber, respectively. Therefore, using these materials to produce the high quality pellet fuel contributes to environmental conservation. Water and calcium hydroxide suspensions were used as pelletising agents playing the role of binders. The major objective was to study the improvement of the quality of the blended pellet fuels in regard to their combustion and emission behaviours when burned on the thin fuel-bed. The ignition and combustion behaviour of the pellets were compared to those of conventional lignite briquettes. The study proved that the pellets on the thin fuel-bed ignited faster and had steady burnout. The emissions of NO_x, SO₂, CO and hydrocarbon compounds were comparatively lower than those from burning of lignite briquettes in a domestic combustor. The addition of calcium hydroxide suspensions played not only the role of binding together the raw material blends, but also provided favourable Ca/S molar ratio optimum for considerable reduction of the SO₂ emissions. The need to reduce further the specific emissions of SO₂ leads to exceeding the proportional value of Ca/S molar ratio, which lowers the calorific value of the pellets. Thermogravimetric studies helped in establishing the comparative pyrolysis behaviours of the lignite and wood chips.     

Aqueous processing of carbothermally prepared Ca-α-SiAlON and β-SiAlON Powders: powder and suspension characterisation

Properties of carbothermally prepared Ca-α-SiAlON and β-SiAlON powders and aqueous suspensions thereof were determined. The isoelectric points of Ca-α-SiAlON and β-SiAlON were 3·4 and 4·6. After addition of deflocculant, Dolapix CE64, the behaviour of both suspensions is nearly identical. The isoelectric points become 5·5 and 5·3, respectively. Despite differences in bulk composition, grain size distribution, grain size and shape, both SiAlON suspensions show a similar dependence of a zeta potential on pH. Optimum slip casting properties, i.e. lowest viscosity values (below 10 mPa s), the highest absolute zeta potential values, the smallest floc size and sediment volume were found between pH 10–11 for both powders. The potentials of the different suspension characterisation techniques were compared and zeta potential and viscosity measurements were found the most convenient.     

The fluorescence of suspensions of powdered coals

The fluorescence spectra of aqueous suspensions of powders of polystyrene, two phenol–formaldehyde resites and a humic acid have been shown to be similar to the fluorescence spectra of their solutions in polar solvents. Fluorescence occurs both from individual aromatic structures and from excimers caged in the solid powders. Accordingly, the synchronous fluorescence (Δ=20 nm) of aqueous suspensions of a range of powdered coals showed the majority of the aromatic structures present in the coals to consist of substituted mono- and bi-aromatic rings. Excimers and exciplexes were also present. Excitation spectra of the aqueous suspensions confirm this interpretation. The fluorescence of powdered coals suspended in chloroform arises from the material extracted into the solvent. Accordingly the fluorescence spectra are dominated by the excimers and exciplexes formed in the solution; this fluorescence is enhanced by energy transfer from the smaller aromatic systems.     

Inter-particle spacing in aqueous suspensions of nanopowders and its effects on particle packing,green body formation and fabrication of alumina

Despite the flexibility offered by a slurry-based processing of ceramics, fabrication of high-density products from nanopowders via conventional techniques is a challenge. The rheological behavior of nanopowder suspensions and the sintering processes of nanopowders have been thoroughly studied in the literature; however, the link between has commonly been overlooked. In this study, the packing behavior of alumina nanopowders and its relation to the rheological behavior of suspensions and the final sintered densities were investigated. For this reason, the availability of inter-particle spacing in aqueous nanopowder suspensions (IPS_aq) was discussed. Funk-Dinger's relation was modified by means of electric double layer and hydration layer formed around oxide powders in aqueous media. The effects of IPS_aq on the green body formation and alumina fabrication were explored in terms of densities and packing behavior. It was concluded that low viscosities or high green body densities do not necessarily lead to high-density end products. Yet, the availability of interparticle spacing in liquid media is vital to obtain efficient packing and high density in sintered products.     

Dispersion of TiO2 nanopowders to obtain homogeneous nanostructured granules by spray-drying

This work deals with the dispersion and stabilisation of nanosized TiO₂ particles in an aqueous medium as a first step in the preparation of spray-dried nanostructured powders.A colloidal route leading to the production of titania nanostructured feedstocks to obtain nanostructured powders was developed. The process was based on the production of homogeneous and concentrated TiO₂ nanosuspensions dispersed in deionised water with a suitable control of pH and the use of an appropriate anionic dispersant. Concentrated suspensions could be obtained by mixing with an ultrasounds probe at different times depending on the dispersing conditions.Homogeneous suspensions prepared were then reconstituted by spray drying into free-flowing powders with an adequate granule size distribution for diverse purposes, such as atmospheric plasma spraying coatings.     

Electrophoretic deposition of multiferroic BiFeO3 sub-micrometric particles from stabilized suspensions

The electrophoretic deposition (EPD) was applied to BiFeO₃ (BFO) powders, one of the most interesting multiferroic compounds characterized by simultaneous magnetic and ferroelectric activity, to form homogeneous films. The preparation and characterization of stable BFO colloidal suspensions in aqueous, organic and mixed solvents were investigated by zeta potential measurements at room temperature in the presence of surfactants. BFO thin films were then deposited on steel substrates from stabilized BFO suspensions, by adjusting the preparative parameters to optimize the film quality. The compositional, morphological and electrical characteristics of the obtained BFO films, together with thickness measurements, were studied using SEM, XRD, AFM, EIS and optical surface profilometer. EPD method applied to BFO stable suspensions produced homogeneous thickness BFO films, free from pinholes and cracks, that were successively sintered and characterized also in terms of photocatalytic response.     

Atmospheric plasma spraying coatings from alumina–titania feedstock comprising bimodal particle size distributions

In this work, Al₂O₃–13 wt% TiO₂ submicron-nanostructured powders were deposited using atmospheric plasma spraying. The feedstocks were obtained by spray drying two starting suspensions of different solids content, prepared by adding nanosized TiO₂ and submicron-sized Al₂O₃ powders to water. The spray-dried granules were heat-treated to reduce their porosity and the powders were fully characterised in both untreated and thermally treated state. Comparison with two commercial feedstocks was carried out. Characterisation allowed a temperature for the thermal treatment to be chosen on the basis of the sprayability of the feedstock and the preservation as much as possible of the submicron-sized structure of the unfired agglomerates.Optimisation of the deposition conditions enabled the reconstituted powders to be successfully deposited, yielding coatings that were well bonded to the substrate. The coating microstructure, characterised by SEM, was mostly formed by a matrix of fully molten particles where the presence of semi-molten feedstock agglomerates was also observed.Moreover, microhardness, toughness, adhesion and tribological behaviours were determined, and the impact of the granule characteristics on these properties was studied. It was found that changing the feedstock characteristics allows controlling the coating quality and properties. In general, good mechanical properties were obtained using a feedstock comprising a binary mixture of submicrometric Al₂O₃ and nanometric TiO₂ particles in the spray-dried powder.     

Aqueous slip casting and hydrolysis assisted solidification of MgAl2O4 spinel ceramics

Abstract

Abstract

This paper reports on synthesis of MgAl₂O₄ spinel (MAS) powders with six different chemical compositions and the consolidation of the synthesised MAS powders following an aqueous slip casting and hydrolysis assisted solidification (HAS) routes. The synthesised MAS powders were surface passivated against hydrolysis before being dispersed in distilled water to obtain suspensions with 41–45?vol.‐% solid loading using suitable dispersing agents. In the case of the HAS process, the consolidation of suspensions was achieved in non‐porous moulds under ambient conditions by the incorporation of AlN equivalent to 1–5?wt‐%Al₂O₃ into the suspension. The stoichiometric MAS powder consolidated by slip casting and dry pressing routes was sintered along with those consolidated by HAS route at 1550–1650°C for 1?h. Various characterisation techniques were utilised to evaluate the effect of composition and consolidation technique on slurry characteristics and sintered properties of MAS ceramics.     

Aqueous and Nonaqueous Colloidal Processing of Difficult‐to‐Densify Ceramics: Suspension Rheology and Particle Packing

Aqueous and nonaqueous colloidal processing of zirconium diboride (ZrB₂) and boron carbide (B₄C) has been investigated. The aqueous and nonaqueous ZrB₂ and B₄C suspension formulations have been optimized. The suspensions were cast into green bodies using slip casting. The correlation between the state of dispersion with the rheological properties of the suspensions and the resulting packing density was observed in both aqueous and nonaqueous processing. The attractive interactions between powder particles in water were difficult to overcome with electrical double layer or electrosteric repulsion. Reasonably low viscosity aqueous ZrB₂ suspensions up to 45 vol% solids could be prepared. It was not possible to produce low viscosity (viscosity below 1 Pa·s at shear rate of 100 s^?1) aqueous B₄C suspensions with solid content above 30 vol%. Slip casting of the weakly aggregated ZrB₂ suspensions resulted in low packing densities (~55% relative density) of the green bodies. On the other hand, dispersion of powder particles in nonaqueous media (cyclohexane and dodecane) enabled suspensions with lower viscosities and a higher maximum solid concentration (up to 50 vol%) to be prepared. The well‐dispersed nonaqueous suspensions promoted an efficient particle packing, resulting in higher green densities (64% and 62% relative density for ZrB₂ and B₄C, respectively) compared to aqueous processing. The significantly high green densities are promising to allow densification of the materials at lower sintering temperature.     

Effect of blend ratio on rheological properties of aqueous SiC suspensions

The relationship between blend ratio and rheological properties of concentrated suspensions is of great importance since it is the key to get high solid suspensions. The rheological properties of bidisperse aqueous suspensions made of two SiC powders with different particle size [d_(0.5)=1.63 and 18.43 μm, respectively] has been studied as a function of blend ratio ξ (the volume fraction of larger particle size). The results showed that the value of critical blend ratio ξ, at which the viscosity is minimized, is in close relation to the shear rate applied. At shear rates below 10 S⁻¹, the critical ξ was greater than 70%. But at shear rates from 10 to 500 S⁻¹, ξ turned to be 50%. The change of shear region from shear-thinning behavior to shear-thickening behavior may be used to account for the variation of critical ξ. Dynamic oscillatory tests showed that the moduli and the linear viscoelastic region of suspensions with higher ξ are smaller than those with low ξ and the increment of blend ratio ξ leads to the change of suspension from nearly an elastic response to a liquid like response.     

Ba0.6Sr0.4TiO3–MgO ceramics from ceramic powders prepared by improved aqueous gelcasting-assisted solid-state method

Based on aqueous gelcasting-assisted solid-state method (AGASSM), improved aqueous gelcasting-assisted solid-state method (IAGASSM) was proposed to prepare the 45 wt% Ba_0.6Sr_0.4TiO₃–55 wt% MgO (BSTM) ceramic powders. It is found that the BSTM ceramic powders prepared by IAGASSM are the most uniform with the smallest particles (D_av = 0.83 μm) than those prepared by solid-state method (SSM) and AGASSM. The phase compositions of the BSTM ceramic powders and ceramics from the prepared ceramic powders are the same whatever ceramic powder preparation method is adopted. Compared with SSM and AGASSM, the BSTM green samples and ceramics from ceramic powders prepared by IAGASSM are the most uniform. Furthermore, it is found that adopting IAGASSM to prepare ceramic powders could not only improve the dielectric properties of the BSTM ceramics considerably, but also decrease their sintering temperature.     

Enhancing Effects of Ultrasound Treatment on the Preparation of TiO2 Photocatalysts

TiO₂ nanoparticles were synthesized by the hydrolysis and condensation of TiCl₄ in a mixed solvent of iso-propyl alcohol and water with or without ultrasound treatment. As-prepared powders were characterized by X-ray diffraction (XRD), field emission scanning electron microscopy (FE-SEM), energy filtering transmission electron microscopy (EF-TEM), particle size analysis and BET surface area analysis. The specific surface area, thermal stability and crystallization of the as-prepared samples treated with ultrasound were higher than those of samples treated without ultrasound. To examine the photocatalytic activity of the as-prepared TiO₂, the photodegradation of MB which is a typical dye resistant to biodegradation has been investigated on TiO₂ powders in aqueous heterogeneous suspensions. The photocatalytic degradation of a aqueous solution of methylene blue shows a remarkable increase when it is carried out with ultrasound in all cases.     

Flotation of metal hydroxide precipitates. II. Flotation of zinc and cobalt hydroxides

Flotation of zinc and cobalt hydroxide precipitates was investigated at total initial metal concentrations of 1 × 10^?2 M with sodium dodecylbenzenesulphonate (DBSNa) and dodecyldimethylbenzylammonium bromide (DDMBABr), both at initial concentrations of 1 × 10^?4 M. In particular, the granulometric analysis of precipitates and measurements of their electrokinetic potential were carried out over a wide range of acidity of aqueous precipitate suspensions. Moreover, adsorption of the flotation collectors by the precipitates as well as the rate of their sedimentation were measured. It was found that the extent of collector sorption, changes in the electrokinetic potential of precipitate grains and their aggregation affected the flotability and course of flotation of the examined precipitates. These dependencies were irregular and could be discussed in terms of chemisorption of collectors on the surface of the precipitate grains.