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| 2. |
- Laarz, E, et al.
(författare)
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Aqueous silicon nitride suspensions: Effect of surface treatment on the rheological and electrokinetic properties
- 1997
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Ingår i: Key Engineering Materials. - 1013-9826 .- 1662-9795. ; 132-136, s. 285-288
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Tidskriftsartikel (refereegranskat)abstract
- The effect of surface treatment on the colloidal stability and rheological properties of concentrated aqueous silicon nitride suspensions has been studied. Oxidized, leached and as-received, non-treated powders were studied using electroacoustic and rheological methods. At pH=10, the silicon nitride powder is well dispersed irrespective of surface treatment. However, the viscosity was affected by the surface treatment; oxidation caused a slightly higher and leaching a significantly lower viscosity. Several possible causes of the leaching effect on viscosity was discussed, e.g. leaching causing a smooth particle surface or facilitating break-up of strong, liquid immobilizing agglomerates. Additionally, the effect of added dispersants – anionic small molecules and polyelectrolytes – were also briefly mentioned.
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| 3. |
- Laarz, E, et al.
(författare)
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Dispersing multi-component and unstable powders in aqueous media using comb-type anionic polymers
- 2006
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Ingår i: Journal of The American Ceramic Society. - 0002-7820 .- 1551-2916. ; 89, s. 1847-1852
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Tidskriftsartikel (refereegranskat)abstract
- We have investigated the effect of polymeric dispersants on the rheological properties and consolidation behavior of concentrated cemented carbide (WC-Co) and magnesia (MgO) suspensions. The relatively novel types of comb-type anionic polymers with grafted non-ionic side chains are effective dispersants also in multi-component powder mixtures with a complex solution and surface chemistry and result in more robust suspensions at significantly higher solids loading compared with e.g., a traditional cationic polyelectrolyte. Direct force measurements on comb-type dispersants with different lengths of the grafted ethylene oxide side chains showed that the dispersants adsorb onto a MgO surface and infer a repulsion where the range scales with the length of the poly ethylene oxide side chains. The compressibility and the consolidation behavior of MgO particle networks in response to a centrifugal force field could be related to the estimated thickness of the adsorbed comb-type dispersants.
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| 4. |
- Laarz, E, et al.
(författare)
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Dispersing WC-Co powders in aqueous media with polyethylenimine
- 2000
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Ingår i: International journal of refractory metals & hard materials. - 0263-4368. ; 18, s. 281-286
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Tidskriftsartikel (refereegranskat)abstract
- The use of aqueous WC¯Co suspensions for powder metallurgical processing of hard metal powders is expected to have environmental as well as economic advantages over the currently used systems with organic solvents. Without a suitable dispersing agent, however, poor mixing and aggregation of the different particulate constituents will occur in water and requirements on homogeneity and colloidal stability of the hard metal slurry cannot be met. We evaluated the effect of polyethylenimine (PEI) on the flow properties of concentrated aqueous WC¯Co suspensions. The investigated parameters were polymer concentration, polymer molecular weight, pH, powder solubility, and ageing time. We found that simple mixing of powder, water, and PEI (at concentrations above 0.3 wt%) is sufficient to obtain well-dispersed, low-viscous suspensions without the need for pH adjustment or further additives. Almost identical flow behavior was observed over the investigated pH range 7¯10.8. Effects of ageing and powder solubility on the colloidal stability were negligible over the investigated time span of 36 h. Adsorption of PEI with Mw=10,000 results in thinner adsorbed layers as compared to PEI with Mw=25,000, which is reflected by a lower suspension viscosity at high solids loading (40 vol%).
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| 5. |
- Laarz, E, et al.
(författare)
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Dissolution and deagglomeration of silicon nitride in aqueous medium
- 2000
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Ingår i: Journal of The American Ceramic Society. - 0002-7820 .- 1551-2916. ; 83, s. 2394-2400
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Tidskriftsartikel (refereegranskat)abstract
- Silicon nitride undergoes hydrolysis and dissolution when subjected to an aqueous environment. Molecular dynamics simulations suggest that hydrolysis proceeds through nucleophilic attack of water with the formation of an intermediate molecular complex involving a penta-coordinated silicon. We found that the dissolution of an oxidized silicon nitride powder resembles that of silica: the dissolution rate could be described using a simple kinetic equation with a dissolution activation energy of 52 kJ/mol. The deagglomeration of a fine silicon nitride powder under mild agitation was evaluated; we show that the peptization kinetics at room temperature is dominated by the break-up of particle-particle bonds due to hydrodynamic friction and cluster attrition. For break-up of hard agglomerates of small particles the dissolution of interparticle necks will play an important role
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| 6. |
- Laarz, E, et al.
(författare)
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Silicon nitride colloidal probe measurements: Interparticle forces and the role of surface-segment interactions in poly(acrylic acid) adsorption from aqueous solution
- 2001
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Ingår i: Journal of The American Ceramic Society. - 0002-7820 .- 1551-2916. ; 84, s. 1675-1682
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Tidskriftsartikel (refereegranskat)abstract
- Direct measurements of forces between silicon nitride surfaces in the presence or poly(acrylic acid) (PAA) are presented. The force-distance curves were obtained at pH > pH(iep) with an atomic force microscopy (AFM) colloidal-probe technique using a novel spherical silicon nitride probe attached to the AFM cantilever. We found that PAA adsorbs onto the negatively charged silicon nitride surface, which results in an increased repulsive surface potential. The steric contribution to the interparticle repulsion is small and the layer conformation remains flat even at high surface potentials or high ionic strength. The general features of the stabilization of ceramic powders with PAA are discussed; we suggest that PAA adsorbs onto silicon nitride by sequential adsorption of neighboring segments ("zipping"), which results in a flat conformation. In contrast, the long-range steric force found in the ZrO2/PAA system at pH > pH(iep) arises because the stretched equilibrium bulk conformation of the highly charged polymer is preserved via the formation of strong, irreversible surface-segment bonds on adsorption.
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| 7. |
- Laarz, E, et al.
(författare)
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The effect of anionic polyelectrolytes on the properties of aqueous silicon nitride suspensions
- 2000
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Ingår i: Journal of the European Ceramic Society. - 0955-2219 .- 1873-619X. ; 20, s. 431-440
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Tidskriftsartikel (refereegranskat)abstract
- The effect of anionic polyelectrolytes on the electrokinetic and rheological properties of concentrated Si3N4 suspensions was investigated experimentally. We have compared novel anionic methacrylic acid comb copolymer modifications with grafted poly(ethylene oxide) chains, PMMA-PEO, with poly(acrylic acid) (PAA); the rheological response as a function of pH was very similar, thus illustrating that the grafted PEO chains have a minor influence on the colloidal stability. The polyelectrolyte adsorption and, thus, colloidal stability at pH> is mainly governed by the surface charge of the solid phase. We found that the colloidal stability and the steady shear rheology of the Si3N4 suspensions with addition of anionic polyelectrolytes could be generalized using a division into four different pH regimes, following a previous study by Hackley. The viscosity minimum around pH 7 was attributed to electrosteric stabilization. The effect of excess addition of polyelectrolyte was also discussed; we attribute the significant increase in viscosity to the ionic strength increase caused by the release of the associated counterions.
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| 8. |
- Yanez, JA, et al.
(författare)
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Viscoelastic properties of particle gels
- 1999
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Ingår i: Journal of Colloid and Interface Science. - 0021-9797 .- 1095-7103. ; 209, s. 162-172
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Tidskriftsartikel (refereegranskat)abstract
- The effect of strength of attraction and volume fraction on the mechanical properties of alumina particle networks were investigated. Alumina particle gels were formed reversibly and in situ in the rheometer by cooling alumina particle suspensions with adsorbed poly(12-hydroxy stearic acid) suspended in a marginal solvent, hexanol. The collapse of the polymer layer with decreasing solvency (temperature) induces flocculation when the long-range van der Waals force overcomes the remaining steric repulsion. The gelation temperature depends on volume fraction. At the gel temperature, Tgel, the gel becomes predominantly elastic; at temperatures below Tgel, the elasticity increases with decreasing temperature. We find that the elastic modulus data, measured over a wide range of volume fraction (0.2 < φ < 0.425) and temperature (10-14°C), follows: G = G0(φ - φg)s. This scaling suggests the prefactor and exponent to be independent of temperature. We present some arguments for why subjecting a particle gel to a preshear procedure might result in an temperature-dependent prefactor. By invoking such an effect, we are able to rescale and collapse previously published moduli data on presheared suspensions according to the (φ - φg) expression.
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