Ladislav Pach

Nucleation of alpha alumina in boehmite gel

Transformation of boehmitc-derived alumina gel to α–Al 2 O 3 in unseeded gels is strongly influenced by the parent phase, θ-Al 2 O 3 . In seeded gels the perfect structure of the epitaxial substrate (α–Al 2 O 3 , 0.3 wt. %) influences the surrounding defect θ-phase resulting in (a) fewer imperfections due to lower transformation enthalpy to α–Al 2 O 3 , and (b) multiple nucleation sites of α–Al 2 O 3 at the α–Al 2 O 3 seed surface. The rate of transformation expressed for the same number of α–Al 2 O 3 nuclei in unseeded and seeded gels indicates that it is faster in the unseeded gel than in the seeded gel.

The influence of water vapor on thermal transformation of boehmite

Boehmite compacts and boehmite gels (seeded and unseeded) were annealed at various temperatures in nitrogen and 1 atm water vapor to determine the influence of water vapor on boehmite transformation to new phases, changes in porosity, and morphology. Water vapor was found to accelerate the phase transformations markedly compared to dry N 2 treatment. The catalyzing effect of water vapor may be due to its interactions on grain surfaces and generation of fast diffusion paths, resulting in nucleation and growth of new phases. This result shows that control of the furnace atmosphere is a useful variable in thermally activated processes.

Morphological control of precipitated calcium carbonates and phosphates by colloidal additives

The precipitation of calcium carbonate and calcium phosphates from solutions with and without additives such as hydroxyethyl cellulose, colloidal silica and potato starch, has been studied by X-ray diffraction and scanning electron microscopy. A constant rate of 10−6 mol s−1 of reactant was added into the mixed solutions with and without additives. Nucleation frequency and morphology were dramatically altered by the addition of colloidal potato starch. It was found to be effective for non-specific nucleation of calcite and hydroxyapatite and specific nucleation of monetite. The nucleation frequency of calcite is increased by adding starch by about a factor of 104. Starch also alters the shape of hydroxyapatite to fibre-like.

Sintering and crystallization of mullite in diphasic gels

Diphasic monolithic mullite gel of stoichiometric composition shows a multiple aggregation of colloidal SiO2 (∼ 12 nm) and AlOOH (∼10 nm) particles. The identity of the colloidal particles is retained up to the crystallization of mullite at a temperature of 1240°C in unpressed or 1220°C in gels cold-isostatically pressed at 1.5 GPa. Intensive sintering is closely followed by crystallization of mullite. Both sintering and crystallization are apparently related with the formation of a silica continuum. Nucleation of mullite appears to occur at points of contact (interface) of SiO2/δ-Al2O3 particles.

Controlled crystallization of vaterite from viscous solutions of organic colloids

Precipitation of calcium carbonate from hydroxyethyl cellulose (HEC) containing aqueous solutions of CaCl{sub 2} and Na{sub 2}CO{sub 3} resulted in an uncommon polymorph, vaterite. In contrast to precipitations without HEC, crystallization in the presence of HEC led to a shell-shaped body consisting of organized vaterite platelets. The artificial shell is a composite of stacks of vaterite and about 2% HEC. In the experimental arrangement used, HEC controlled both nucleation and crystal growth of vaterite. Concentration of HEC also affected the platelets thickness which in turn influenced the shells morphology as well. These results demonstrate the importance of organic-inorganic interface in controlling crystallization.

Sintering of diphasic mullite gel

Abstract A monolithic colloidal gel of mullite composition, prepared from SiO 2 and boehmite sols, was sintered under isothermal conditions. The sintering process was studied by dilatometry, thermogravimetric analysis, bulk density measurements and discontinuous shrinkage measurements. The silica component of stoichiometric colloidal mullite gel prevented crystallization of α-Al 2 O 3 in the system. The initial rapid part of the sintering is controlled by the alumina phase and it is not a viscous sintering process in the sense of Scherers viscous sintering model. The final part of sintering, above approximately 80% of the theoretical density, is controlled by the silica component of the gel and agrees very well with the Scherer model.

Precipitation of hydroxyapatite film under dynamic conditions

Hydroxyapatite was synthesized by reacting Ca(HCO3)2 with (NH)2HPO4 at 25°C. It was demonstrated that, for the crystallization of hydroxyapatite on a substrate as a coating, it is important to suppress the homogeneous nucleation in the bulk volume of the precursor solution. Heterogeneous nucleation on a sol-gel derived SiO2 surface was advantageous for the deposition of film on a substrate under dynamic conditions. This approach led to the formation of a homogeneous, transparent coating (∼1 μm) of hydroxyapatite on the sol-gel derived SiO2 surface on the glass.

Preparation and thermal behavior of CaCO3/SiO2 nanocomposite

Abstract The preparation of CaCO 3 from a supersaturated solution of Ca(HCO 3 ) 2 in a microwave oven was studied at 25°C and pH of ∼7.1 and 7.3, with and without colloidal SiO 2 as an additive. SiO 2 particles controlled the nucleation, size, and shape of crystals and were incorporated into the CaCO 3 /SiO 2 nanocomposite. Incorporation of SiO 2 increased with increased SiO 2 addition into the solution and with increased rate of precipitation. Heating the above nanocomposite precipitate led to crystallization of calcium silicate at a low temperature of 450°C, due to intimate contacts of CaCO 3 and SiO 2 in the composite.

Nanocomposite route to the stabilization of porous ϑ-alumina

Stabilization of ϑ-alumina phase by silica was studied in nanocomposite (diphasic) alumina-silica gels by XRD and BET surface areas measurements. Five wt.% of silica (22 nm particles) increased the crystallization temperature of ϑ to α-alumina by about 100°C from boehmite (10 nm particles) derived alumina. Stabilization of ϑ-alumina was caused by the formation of intimate contact (Al-O-Si) between components by diffusion of silica into the defect alumina structure.

Porous mullite ceramics through diphasic gels of large boehmite and small silica particles

Mullite formation process has been studied in stoichiometric mullite (3Al2O3·2SiO2) diphasic gel containing large boehmite (∼1 μm) and small silica (∼10 nm) particles. It has been found that initial mullitization did not take place inside the silica phase (cristobalite), but took place in the defect θ-alumina phase. θ-alumina was stabilized by silica when the temperature was below 1350°C. At temperatures above 1350°C, mullite crystallized directly. It was suggested that silica diffused into the pores (<1.8 nm) of θ-alumina and prevented the collapse of θ-alumina pore structure. On the other hand, when silica was not present, the pore structure of θ-alumina collapsed and α-alumina crystallized at ∼1100°C. Porous mullite ceramics were obtained by using special diphasic gels containing large boehmite and small silica particles.