Silica Volume Fraction Research Articles

Influence of Ion Size on Short-Range Repulsive Forces between Silica Surfaces

The influence of counterion size on short-range repulsive forces at high salt concentrations was investigated with silica slurries at various pHs. Ions from the lyotropic series of monovalent electrolytes (LiCl, NaCl, KCl, and CsCl) were used to coagulate dispersed slurries. Measurements of viscosity and sedimentation rate were performed. The results at high salt concentrations and volume fractions of silica clearly show that the extent of the short-range repulsive forces correlates with the size of the unhydrated ion. This trend is opposite to the ion-adsorption sequence on silica at low volume fractions of solids or lower ionic strength. Our results are also contrary to the widely accepted hydration force model but concur with the recently developed reference-hypernetted chain statistical mechanics models describing the interaction of ions with solvated surfaces.

Granular modeling approach to electro-packaging materials

The particulate material to be used for electro-packaging of LSI-chip is a compound of polymer matrix and silica particles. In its materials design, a volume fraction of silica must be designed to reduce the difference of thermal expansion coefficients between the compound and the semiconductive device. Since the loading ratio of silica particles in the polymer matrix reaches to 60 vol.% in actual situation, the viscous flow of the compound can be identified as a non-Newtonian fluid; since nonlinear viscosity changes itself even in local with the process time, various defects or porosities have to be left in the final product. In other words, new materials processing design is necessary to rationally determine the geometry and dimensions of a die-set system and to adequately optimize the process parameters. In the present paper, a granular modeling is proposed to describe the rheological behavior of this high-loaded particulate compound. Use of this modeling enables us to make direct process simulations for investigation of the formation of defects and understanding of mechanism to be free from defects. Simulated results indicate a rational possibility for the present granular modeling to be used for the related materials processing design to the particulate electro-packaging materials.

Properties of maxillofacial silicone elastomers reinforced with silica powder.

Compounds of a silicone elastomer suitable for preparing maxillofacial prostheses, reinforced with various amounts of silica powder, have been studied for their mechanical response and wetting properties in terms of contact angle. Tensile strength and elongation at break showed an increase with increasing silica volume fraction up to 35%, whereas the Young modulus displayed small dependence on the silica content and the resistance to tear increased continuously with filler volume fraction (Vf). The wetting properties assessed via the contact angle, seemed to degrade with increasing silica Vf, but a dependence on the elastomer network density has also been recorded.

Order-disorder transition in aqueous dispersions of ionic colloidal silica particles

Order (crystal structure)-disorder (liquid structure) transition of aqueous dispersion of colloidal silica particles (average particle diameter=0.12×10−6 and 0.11×10−6 m) was examined at a silica volume fraction of 3×10−2, and various (analytical) surface charge densities of the particle, σa, and salt concentrations, Cs. The value of σa was changed by varying concentration of the coexisting sodium hydroxide. Phase diagram for the order-disorder transition was determined at σa<1.4 × 10−6 C cm−2. It was found that there existed lower and upper limits of σa for the ordering, at given C s's. The grain size was large near the order-disorder transition points in the σa-C s diagram. The structure of the ordered state was studied using an ultra-small-angle X-ray scattering method. Several orders of Bragg reflection having a 6-fold symmetry was observed for large grains. For small grains, the scattering profile was powder-like. In both cases, the ordered structure had a bodycentered-cubic lattice symmetry. The observed closest interparticle distances were smaller than the average interparticle distance, suggesting the non-space-filling nature of the ordered structure and existence of grain-boundary regions.

Control of crystallization of ionic silica particles in aqueous dispersions by sodium hydroxide.

Crystallization behavior of colloidal silica particles in aqueous dispersion (particle diameter = 0.12\ifmmode\times\else\texttimes\fi{}${10}^{\ensuremath{-}6}$ m) is examined at a silica volume fraction of 3\ifmmode\times\else\texttimes\fi{}${10}^{\ensuremath{-}2}$ and various sodium hydroxide and salt concentrations. We report that the crystallization could be controlled by adding ${10}^{\ensuremath{-}5}$ to ${10}^{\ensuremath{-}4}M$ sodium hydroxide, which increased the surface charge density, $\ensuremath{\sigma}$, of the silica particle. Conditions for crystallization were determined as a function of $\ensuremath{\sigma}$ and salt concentration. The grain size was large near the phase boundary between crystal and liquidlike regions. The structure of the crystals was studied using an ultra-small-angle x-ray scattering method. Several orders of Bragg reflection having a sixfold symmetry was observed for large grains. For small grains, the scattering profile was powderlike. In both cases, the crystals had body-centered-cubic lattice structures. The observed closest interparticle distances were smaller than average interparticle distance, suggesting the non-space-filling nature of the crystal.

Silica-poly(dimethylsiloxane) mixtures: n.m.r. approach to the crystallization of adsorbed chains

This work deals with the crystallization of polymer chains within silica-poly(dimethylsiloxane) mixtures, irrespective of the type of crystalline structure that is actually present in the system. An analysis of the transverse magnetization relaxation function is proposed as a method for melting point determination as well as for determination of the extent of crystallization as a function of time during an isothermal crystallization. We first pay attention to pure poly(dimethylsiloxane) (PDMS), for which we have measured the enthalpy of fusion per mole of structural units and defined the isothermal crystallization kinetics. The results obtained for the mixtures are compared to those obtained for pure PDMS. It turns out that the presence of silica particles does not affect the dimensionality of the crystalline growth. However, it reduces the extent of crystallization and has two antagonistic effects on the overall rate of crystallization. The first is a nucleation effect which is dominant at low silica volume fractions, and the second effect is a topological constraint which becomes dominant and hinders the crystalline growth at high silica volume fractions. As a result of the competition between these two effects, the overall rate of crystallization goes through a maximum with increasing silica volume fraction.

Reaction synthesis of MoSi2 from high purity elemental powders

This paper reports that Molybdenum disilicide (MoSi{sub 2}) is an intermetallic compound that combines a high melting point (2293K) with excellent resistance to high temperature oxidation. With respect to mechanical properties the behavior of polycrystalline MoSi{sub 2} falls into one of three regimes, based on the sue temperature: t {lt} 925{degrees} C: strong + brittle; 925 {lt} T {lt} 1250{degrees} C:strong + ductile and T {gt} 1250{degrees} C:weak + ductile. The majority of recent investigations have focused on the properties of MoSi{sub 2}-based materials processed from commercially available powder. Commercial MoSi{sub 2} powder contains significant amounts of oxygen; for example, a chemical analysis of powder obtained from Cerac Inc. revealed an oxygen content of 0.6 wt%. Consolidation of such powder resulted in a substantial volume fraction of amorphous silica in the final product. MoSi{sub 2} powder produced by the crushing and grinding of arc-melted elemental Mo and Si also exhibited amorphous Si-rich particles after vacuum hot-pressing at 1973K and 30MPa. Grinding of the cast material to powder was done in ethanol but one this was driven off, the powder was air-handled. The high energy mechanical alloying in an argon atmosphere of elemental Mo and Si powder to produce MoSi{sub 2}more » has also been investigated. Following hot pressing, the MoSi{sub 2} made in this fashion still contained a substantial volume fraction of SiO{sub 2}.« less

Thermal expansion of phenolic resin and phenolic-fibre composites

A large number of thermal expansion measurements in the temperature range 20 to 300‡C are presented for the Monsanto phenolic resin SC-1008 as a function of heating rate, position in cured block, curing treatment and repeated heating cycles. Thermal expansion measurements in those directions where the resin dominates are also reported up to 400‡C for composite systems consisting of the phenolic resin reinforced with ∼60% (volume fraction) of either continuous unidirectional silica or bidirectional carbon (rayon precursor) fibres. The large variation in thermal expansion measurements is used to show that current curing treatments for the materials do not yield a resin with predictable expansion response. Annealing at a higher temperature (>200 ‡C) than the maximum used in the curing treatments is shown to stabilize the phenolic resin SC-1008. Then, the mean linear coefficient of thermal expansion for the pure resin up to 100 ‡C is within the range (55±5)×10−6‡C−1.

Effect of pH on the rheological properties of a silica hydrosol

The viscosity of a silica hydrosol is studied as a function of the volume fraction of silica for pHs from 2 to 11 using a capillary flow viscometer. The experimental curves depend on the pH of the sol and exhibit a discrepancy with the behaviour predicted from theoretical expressions. An adsorbed layer must exist on these colloidal particles (equivalent spherical radius of 87–88 Å) which increases the volume fraction of the dispersed phase. The hydrodynamic radius rH including the thickness of the layer is calculated from the viscometric data. The values obtained at pH 2 and 9·7 are 94–98 and 110–115 Å respectively. These analyses are used to explain previously reported results on the variation of gel packing density as a function of pH. The hydrodynamic radius of the particles is also determined from measurements on dilute sols using quasi-elastic light scattering. At pH 9·7, the measured value, 120 Å, is close to that previously calculated. The highest measured rH, 130 Å, is obtained at pH 2. An explanation is proposed to account for the difference from the calculated value. Die Viskosität einer SiO2-kolloidalen Lösung in Wasser wurde mit einem Kapillarviskosimeter (Ubbelohde Röhre) für pH-Werte von 2–11 in Abhängigkeit des SiO2-Volumenanteils untersucht. Die experimentellen Ergebnisse hängen vom pH des Sols ab und weichen vom theoretisch vorhergesagten Verhalten ab. Auf diesen kolloidalen Teilchen mit einem äquivalenten Kugelradius von 87–88 Å muß eine adsorbierte Schicht vorliegen, die den Volumenanteil der dispergierten Phase erhöht. Der hydrodynamische Radius rH, der diese Schicht berücksichtigt, wurde aus den Viskositätsdaten berechnet. Bei pH 2 und 9·7 ergeben sich Werte von 94–98 bzw. 110–115 Å. Diese Überlegungen wurden zur Erklärung früherer Ergebnisse über die Änderung der Gel-Packungsdichte in Abhängigkeit vom pH-Wert herangezogen. Der hydrodynamische Radius der Teilchen wurde auch durch quasielastische Lichtstreuungsmessungen an verdünnten Sols bestimmt. Bei einem pH-Wert von 9·7 liegt der gemessene Wert von 120 Å nahe bei dem obig berechneten Wert. Der höchste Wert für rH wurde bei pH2 gemessen und liegt bei 130 Å. Auf die Abweichung zum berechneten Wert wird eingegangen. La viscosité d'un hydrosol de silice est étudiée en fonction de la fraction volumique de silice pour des valeurs de pH comprises entre 2 et 11, en utilisant un viscosimètre à écoulement. Les courbes expérimentales évoluent avec le pH du sol et montrent un écart par rapport au comportement attendu à partir des expressions théoriques. Une couche adsorbée doit exister à la surface de ces particales colloïdales (rayon de sphère équivalente: 87–88 Å), qui accroît la fraction volumique de phase dispersée. Le rayon hydrodynamique rH qui intègre l'épaisseur de cette couche est calculé à partir des données viscosimètriques. Les valeurs obtenues à pH 2 et 9·7 sont respectivement, de 94–98 et 110–115 Å. Ces analyses sont utilisées pour expliquer des résultats rapportés antérieurement sur la variation de densité du gel en fonction du pH du sol. Le rayon hydrodynamique des particules est également déterminé à partir de mesures sur les sols dilués en utilisant la diffusion quasiélastique de la lumière. A pH 9·7, la valuer mesurée, 120 Å, est proche de celle préalablement calculée. La valuer maximale pour rH mesuré, 130 Å, est obtenue à pH 2. Une explication est proposée pour rendre compte de la différence entre cette valeur et la valuer calculée.

The dispersion of hydrophilic and hydrophobic flame-hydrolysed silicas in short-chained alkanols

The ultrasonic dispersion of Aerosil 200 and some n-alkyl surface-modified derivatives thereof in alkanols have been examined. The sizes of the aggregates formed during this process were investigated from measurements of sol viscosity and quasi-elastic light scattering (QELS). In the former technique, the aggregate volume ratios (AVR) were calculated at very low volume fractions of silica. The AVR values so obtained indicated that the silica monomers were highly aggregated in all systems, as expected. Differences between the various alkanols were found, however. Decreasing continuum polarity from water to hexan-1-ol produced decreasing AVRs. From hexan-1-ol to octan-1-ol, however, the AVRs increased. The n-alkyl surface-modified silicas gave AVRs substantially different to A 200, related most likely to wetting and solvation of the solid surface during dispersion. QELS measurements were subsequently found to be of limited use for these systems, since the aggregate size distributions in these sols were evidently very broad. The results indicated that these size distribution were non-Gaussian. The use of QELS to investigate systems of such high aggregation appears to be limited.

Influence of pH, electrolyte, and poly(vinyl alcohol) addition on the rheological behavior of aqueous silica (Aerosil) dispersions

The rheological behavior of aqueous silica (Aerosil) dispersions was investigated as a function of pH, volume fraction of silica, and electrolyte (NaCl) concentration. At pH = 7 and 9, a gradual transition from Newtonian to pseudoplastic behavior was observed as the silica content was gradually increased (above ∼7%). However, at pH = 3, the apparent Newtonian behavior was maintained up to a silica concentration of ∼10%, above which there was a tendency to slight shear thickening at high shear rates. Plots of η PL versus percentage v v of silica showed a rapid increase in η PL above a critical silica concentration ( ø crit) which was dependent on the pH of the dispersion. For a silica concentration of 9.1% v v , η PL decreased as the pH was increased from 2 to 3, and remained constant up to pH = 5, above which there was a rapid increase in η PL with increase in pH (up to pH = 7). On the other hand, above pH = 5.5 the yield value, τ β , increased rapidly with increase in pH, reached a maximum at pH = ∼7.5 and then decreased rapidly with further increase in pH. These results are explained in terms of the interaction between the silica particles and the size of the units formed. For example, at pH = 7 and 9, double-layer repulsion forces are significant and therefore the rapid increase in η PL above ø crit is due to the considerable repulsion between the particles. However, at pH = 3 (near the iep of silica) these double-layer repulsion forces are reduced and interaction occurs at higher volume fractions. Addition of the electrolyte (NaCl) has the effect of reducing double-layer interactions, thus increasing φ crit although in some cases the increase of surface charge compensates for the reduction due to double-layer compression. Preliminary results on the effect of addition of poly(vinyl alcohol) (PVA) at pH = 3 showed an increase in yield value τ β reaching a maximum at a PVA concentration corresponding to ∼0.5 coverage. Thereafter, the yield value decreased with further increase in PVA coverage. Moreover, the rheological results showed extensive hysteresis in the shear stress-shear rate curves, indicative of thixotropy. The results are explained in terms of bridging flocculation and partial reversibility of the flocculated structure.

Transmission Electron Microscopy of Granular Nickel/SiO2 Cermet Films

Granular nickel films have been prepared by co-sputtering nickel and silica onto glass substrates at room temperature. Seven films of (Ni)1−x(SiO2)x, where x is the volume fraction of silica, were examined by transmission electron microscopy. For all samples, the microstructure consisted of nickel grains embedded in a matrix of silica. The grain size is observed to decrease linearly with x; the distribution of grain sizes varies from 170 to 310 Å for x≃0.1 and from 40 to 55 Å for x≃0.75. The thickness of the intergranular silica is about 22 Å and is independent of composition. The nickel lattice is observed to expand with increasing x; the fractional expansion is inversely proportional to grain size and attains values as large as 10% for large x. These results will prove useful in the interpretation of the magnetic, electrical, and optical properties of granular metal films. In particular, these results have been applied to the interpretation of the depression in magnetic Curie temperature observed in the (Ni)1−x(SiO2)x system.