Colloidal Boehmite Research Articles

Monazite Coatings on Fibers: I, Effect of Temperature and Alumina Doping on Coated‐Fiber Tensile Strength

Monazite was continuously coated onto Nextel 720 fibers, using an aqueous precursor and in‐line heat treatment at 900°–1300°C. Some experiments were repeated with alumina‐doped precursors. Coated fibers were heat‐treated for 100 h at 1200°C. Coatings were characterized by optical microscopy, scanning electron microscopy, and analytical transmission electron microscopy. Coated‐fiber tensile strengths were measured by single‐filament tensile tests. The precursors were characterized by X‐ray diffractometry, differential thermal analysis/thermogravimetric analysis, and mass spectrometry. Coated‐fiber tensile strength was lower for fibers coated at higher deposition temperatures. Heat treatment for 100 h at 1200°C decreased tensile strength further. The coatings were slightly phosphate‐rich and enhanced alumina grain growth at the fiber surface, but phosphorus was not detected along the alumina grain boundaries. Fibers with alumina‐doped coatings had higher tensile strengths than those with undoped coatings after heat treatment for 100 h at 1200°C. Alumina added as α‐alumina particles gave higher strengths than alumina added as colloidal boehmite. Alumina doping slowed monazite grain growth and formed rough fiber–coating interfaces after 100 h of heat treatment at 1200°C. Possible relationships among precursor characteristics, coating and fiber microstructure development, and strength‐degradation mechanisms are discussed in this paper.

Anomalous stability of aqueous boehmite dispersions induced by hydrolyzed aluminium poly-cations

Aqueous dispersions of colloidal boehmite rods turn into strong gels when the concentration of (1–1) electrolyte concentrations becomes exceeds 50 mM. However, after addition of aluminium chlorohydrate (ACH) the rods remain stable up to salt concentrations as high as 2 M. Moreover the boehmite-ACH dispersions with an aspect ratio of 19 quickly separate into an isotropic and a liquid crystal nematic phase above a typical threshold concentration of 2 v/v%. It is known that ACH forms polynuclear cations at mild acidic conditions. The anomalous stability as encountered in these dispersions is explained by assuming that these hydrolyzed poly-cations cause a shift of the charge carrying surface.

Preparation and Properties of Colloidal Core−Shell Rods with Adjustable Aspect Ratios

Colloidal boehmite (AlOOH) rods were used as cores for the preparation of rods with a silica shell. Via a three-step coating procedure silica rods with adjustable aspect ratios up to 20 were obtained by varying the amount of the added silica precursor tetraethoxysilane (TES). Silica rods are stable in various solvents such as water, ethanol, propanol, and dimethylformamide (DMF). Moreover, polymer-grafted silica rods can be dispersed in ethanol/toluene mixtures and cyclohexane. Incorporation of fluorescein isothiocyanate (FITC) in the silica shell enables the study of long-time self-diffusion processes in dispersions of rodlike particles with fluorescence recovery after photobleaching (FRAP). Fluorescent silica rods also may be used in direct imaging studies with confocal scanning laser fluorescence microscopy (CSLM).

Aqueous Dispersions of Colloidal Boehmite: Structure, Dynamics, and Yield Stress of Rod Gels

The structure, dynamics, and strength of aqueous boehmite gels were studied by yield stress measurements and light scattering as a function of the particle aspect ratio and ionic strength. The yield stress was found to be a strongly varying function of the salt concentration. At low ionic strength, yield stress results from extensive double layers surrounding the rods, leading to a “colloidal glass”. Addition of LiCl decreases the yield stress and enhances the particle dynamics in the gel when [LiCl] < 10mM. For 10 mM < [LiCl] < 75 mM, particle aggregation induces the formation of a space-filling heterogeneous network with fractal dimension d = 2.35, causing a rise in the yield stress and renewed freezing of the particle dynamics. When the LiCl concentration exceeds 75 mM, the yield stress decreases again and the now very turbid gels show syneresis. For the attractive rods, the peculiar salt dependence of the gel strength can be explained by polydispersity in interactions, caused by inhomogeneities in particle surface chemistry and shape.

Decay of Flow Birefringence in Dispersions of Charged Colloidal Rods: Effect of Double-Layer Interactions

The rotational diffusion of charged colloidal boehmite needles in aqueous dispersions was assessed from the decay rate of optical birefringence after cessation of a shear flow. The strong dependence of the characteristic relaxation time τ r on the ionic strength and the boehmite rod concentration can be described by a rescaling of the particle concentrations in terms of an effective rotational volume which takes the electric double layers into account. This dynamic rescaling is obeyed over a wide range of initial shear rates. Rotational relaxation times diverge when the effective particle concentration approaches the threshold density for the formation of a space-filling gel.

Continuous fibre reinforced mullite matrix composites by sol–gel processing: Part I Fabrication and microstructures

The fabrication of ceramic composites based on a mullite matrix uniaxially reinforced with either high strength carbon or Nicalon-SiC continuous fibres is described. Sols of high solids contents of up to 40 vol% were prepared using colloidal silica sol with either α-alumina powder or colloidal boehmite as reactants for the preparation of the mullite matrix, 3Al2O3·2SiO2. Single-stage infiltration was used for the preparation of prepregs for laminating into composites. The composites were consolidated by uniaxially hot-pressing at 1300 or 1550°C, depending on the matrix precursors and the final phases in the matrix. Highly densified composites with fibre volume fractions of 32–60% were obtained and the microstructures characterised by mercury porosimetry, X-ray diffraction, scanning electron microscopy and transmission electron microscopy.

Orientational Order in Sediments of Colloidal Rods

Sediments of colloidal boehmite rods have been studied with polarization microscopy to investigate the presence (and origin) of orientational order. We find, among other structures, a “frozen” nematic texture as well as a structure which resembles the “oriented glass” found in recent simulations with non-Brownian ellipsoids.

Shrinkage Reduction in Silica‐based Refractory Cores Infiltrated with Boehmite

Infiltration of silica‐based refractory cores with colloidal boehmite slurry has the effect of greatly reducing sintering shrinkage, as required for precision casting. Only 300 ppm of equivalent alumina is sufficient to decrease the shrinkage from RT to 1600°C more than three‐fold. Microstructural observations and XPS measures show that the silica grains are homogeneously coated with a reaction film which hinders viscous flow.

Phase Separation of Mixtures of Colloidal Boehmite Rods and Flexible Polymer

Phase separation experiments have been performed on mixtures of colloidal polyisobutene grafted boehmite rods and flexible polymers. Two systems of rods have been studied, one with an average length and diameter of 71 nm and 11.1 nm, and the other with an average length and diameter of 253 nm and 9.4 nm. The polymers used have molecular weights varying from 35 to 186 kg mol-1, leading to radii of gyration of 5.9 to 20 nm. Bi- and triphasic equilibria involving a dilute isotropic phase, a concentrated isotropic phase, and a nematic phase are observed. The phase behavior is compared to a recent theory, which is an extension of the treatment for sphere/polymer mixtures (H. N. W. Lekkerkerker et al., Europhys. Lett. 20, 559 (1992)). The main features of the phase behavior can be understood on the basis of this simple theory. There remain significant discrepancies between theory and experiment which are attributed to the limitations of the theory on the one hand and to the polydispersity of the colloidal rods and the formation of metastable and nonequilibrium states such as colloidal gels and glasses on the other hand.

Phase Behavior of Aqueous Dispersions of Colloidal Boehmite Rods

ADVERTISEMENT RETURN TO ISSUEPREVArticleNEXTPhase Behavior of Aqueous Dispersions of Colloidal Boehmite RodsP. A. Buining, A. P. Philipse, and H. N. W. LekkerkerkerCite this: Langmuir 1994, 10, 7, 2106–2114Publication Date (Print):July 1, 1994Publication History Published online1 May 2002Published inissue 1 July 1994https://doi.org/10.1021/la00019a016RIGHTS & PERMISSIONSArticle Views894Altmetric-Citations128LEARN ABOUT THESE METRICSArticle Views are the COUNTER-compliant sum of full text article downloads since November 2008 (both PDF and HTML) across all institutions and individuals. These metrics are regularly updated to reflect usage leading up to the last few days.Citations are the number of other articles citing this article, calculated by Crossref and updated daily. Find more information about Crossref citation counts.The Altmetric Attention Score is a quantitative measure of the attention that a research article has received online. Clicking on the donut icon will load a page at altmetric.com with additional details about the score and the social media presence for the given article. Find more information on the Altmetric Attention Score and how the score is calculated. Share Add toView InAdd Full Text with ReferenceAdd Description ExportRISCitationCitation and abstractCitation and referencesMore Options Share onFacebookTwitterWechatLinked InReddit PDF (12 MB) Get e-Alerts Get e-Alerts

Preparation of (non-)aqueous dispersions of colloidal boehmite needles

A novel hydrothermal alkoxide method is presented for the preparation of stable, aqueous dispersions of fairly monodisperse, charged colloidal boehmite needles. A polymer coating procedure for the needles is described which leads to sterically stabilized dispersions in organic solvents.

Extrusion of α‐Al2O3–Boehmite Mixtures

A water‐based binder system consisting of colloidal boehmite (AlO(OH)) and 0.3 wt% poly(vinyl alcohol) was developed for low‐pressure piston‐type extrusion of alumina. Batches using this binder system and two different types of commerical alumina powder were characterized with regard to extrusion pressure, densification, and physical properties in the green and fired states.

Preparation of Colloidal Boehmite Needles by Hydrothermal Treatment of Aluminum Alkoxide Precursors

Fairly monodisperse colloidal boehmite fibrils with a high aspect ratio were synthesized by hydrothermal treatment at 150°C of an acidified aqueous alkoxide solution, prepared by adding an aqueous HCl solution to an aluminum alkoxide precursor. The average particle length could be controlled between about 100 and 500 nm by varying the initial amounts of alkoxide and acid. Using two different alkoxides in a 1:1 molar ratio yielded the most needlelike product, having a particle length standard deviation of 40%. The boehmite particles were polycrystalline and contained 0.14 mol of excess H2O per mol of AlOOH, bound to the particle surface.

Reaction Injection Molding of Silica-Alumina Mixtures Using Heterocoagulation

ABSTRACTA colloidal processing technique based on the heterocoagulation of selected ceramic oxide systems, and analogous to reaction injection molding of polymers, is described. Commercial colloidal silica and boehmite sols, which have oppositely charged particles in the pH range 2.0-9.0, and mixtures of these suspensions with micron sized oxide powders, have been used to determine whether a liquid to solid transformation could be induced when the two materials are simultaneously injected at high velocities into a mold cavity. Rheological changes have been measured as a function of relative flow rates of the two suspensions, solids loading and oscillatory frequency. Viscosity is observed to increase by as much as six orders of magnitude under optimized conditions.

Effect of Hydrothermal Conditions on the Morphology of Colloidal Boehmite Particles: Implications for Fibril Formation and Monodispersity

The synthesis of colloidal boehmite (AlOOH) is studied by heating basic aluminum chloride solutions under constant stirring. The temperature and Al2O3: Cl molar ratio influence the product morphology. Synthesis at 140°C generates highly fibrous polycrystalline particles that are on average 360 nm long, 30 nm broad, and 8 nm thick. They contain 0.11 mol of excess H2O per 1 mol of AlOOH. Synthesis at temperatures between 140° and 190°C produces broader fibrils and less excess H2O. Preparation at 220°C eventually produces fully crystalline platelike boehmite particles about 260 nm long, 95 nm broad, and 14 nm thick, without excess H2O. Fibril synthesis requires an Al2O3:Cl molar ratio exceeding 1.0 to yield noncoagulated particles. The fibrils are fairly monodisperse with 20% standard deviation in their length for an Al2O3: Cl molar ratio about 1.0.

Hybrid Gels for Homoepitactic Nucleation of Mullite

Hybrid gels, defined as gels from mixtures of polymerically and colloidally derived sols, offer many opportunities for crystalline microstructure development upon heating. In this study, hybrid mullite gels are formed by mixing a colloidal boehmite—silica sol with a polymeric aluminum nitrate—tetraethoxysilane‐derived sol. The polymeric gel crystallizes in situ to form mullite that acts as seed crystals for homoepitactic nucleation during the subsequent transformation of the colloidal component of the hybrid gel. Compared with the entirely colloidal gel, the introduction of a 30 wt% polymeric gel fraction results in an increase in apparent nucleation frequency from ∼5x1011 to ∼1x1014 nuclei / cm3 at 1375°C, a reduction in high‐temperature grain size from 1.4 to 0.4 μm at 1550°C, and an increase in the degree of microstructural homogeneity, as evidenced by intragranular pore removal.

Flow behavior of hydroxides during decomposition

A compressive creep study of Georgia kaolin, magnesium hydroxide and colloidal boehmite during the dehydroxylation reaction has shown that the materials undergo creep deformation during decomposition. The flow behavior during the dehydroxylation reaction also indicated that there is a strain rate at zero applied stress. This is explained on the basis of coalescence of very fine particles formed during decomposition. The total creep rate for boehmite can be represented by e ̇ T = e ̇ γ + Aσ , where only ≐ γ has been found to be temperature dependent.

Effect of Silica on Transformations of Fibrillar Colloidal Boehmite and Gamma Alumina

The addition of silicic acid to fibrillar colloidal boehmite retarded the conversion at high temperature of gamma alumina to theta and alpha and permitted the formation of an intermediate kappa. Unusually sharp X–ray diffraction patterns for gamma, theta, and kappa transition aluminas are presented.

Fibrillar Colloidal Boehmite; Progressive Conversion to Gamma, Theta, and Alpha Aluminas

The conversion of boehmite to transition aluminas and to alpha (corundum) was studied with (a)fibrillar colloidal boehmite, about 50 a.u. in diameter, (b)finely divided boehmite in the form of thin elongated platelets, and (c)macroscopic boehmite crystals. In the range 500o to 700o C., (a)was converted to fibrillar gamma, essentially unchanged in size or shape, (b)was dehydrated to gamma, but the platelets appeared to be partly cracked longitudinally about every 70 a.u., and (c)was similarly cracked into much smaller crystallites of gamma. At about 1000oC., (a)yielded only rodlike theta alumina, whereas (b)and (c)appeared to give a mixture of theta and delta aluminas, characterized by round holes or pores developed from the cracks. In the range 1000o to 1100oC., all the aluminas were converted to alpha, at a rate which was strongly dependent on temperature. The conversion of (a)and (b) to alpha could be followed by subsequent examination with the electron microscope. The alpha crystals, once nucleated, apparently grew rapidly to 0.5 or 1.0 μ in size, since there was no evidence of smaller alpha crystals. The smaller alpha crystals appeared to grow while embedded in a glassy matrix which must have been amorphous alumina. This phase acted as a bridge between the diminishing theta particles and the growing alpha crystals and may have consisted of the alumina which was in the process of being transported.