J. Torrent-Burgués

Nucleation of calcium carbonate at different initial pH conditions

The precipitation of calcium carbonate from mixing calcium chloride and potassium bicarbonate/carbonate solutions at 25°C has been studied doing unseeded batch pH-drift experiments. The record of pH has been used to determine induction times. The pH and pCa measurements were used for calculations of species concentration and supersaturation. SEM indicated that vaterite was the predominant phase formed. The dependence of the induction time on supersaturation was analyzed with different models. The results indicate that a good fit is obtained from the plot of ln ti versus l/ln2β but the small size of the critical nucleus indicates that an atomistic model is more realistic. The nucleation parameters have been determined using an empirical expression of the growth rate of vaterite taken from the literature, together with the experimentally measured induction times.

Precipitation of stoichiometric hydroxyapatite by a continuous method

In this paper we present the precipitation of hydroxyapatite (HA), Ca 5 (OH)(PO 4 ) 3 , from highly concentrated CaCl 2 and K 2 HPO 4 solutions, carried out by a continuous method in a MSMPR reactor. The procedure consists of adding the reagents in a ratio Ca to P equal to 1.67, maintaining a temperature of 85°C, inert N 2 atmosphere inside the reactor, and monitoring and adjusting automatically the pH by means of a pH-stat system (pH =9.0 ± 0.1). Under these conditions HA with a Ca to P ratio equal or close to the stoichiometric composition (Ca/P = 1.667), with a high yield (up to 99%) and a high production rate (up to 1.17 g/l.min) is obtained at steady state. The CSD, morphology, crystallinity of the precipitates and impurities present fit the requirement for its biomedical applications.

Precipitation of calcium carbonate from solutions with varying Ca2+carbonate ratios

Abstract The precipitation of calcium carbonate from solutions with the same initial supersaturation and different initial [ Ca ] T ([ HCO − 3 ] + [ CO 2− 3 ]) ratio, has been studied making unseeded and seeded experiments at T = 25°C. In unseeded experiments induction times ( t i ) varied in the order: t i ( [ Ca ] T ΣC = 4) > t i ( [ Ca ] T ΣC = 1 4 ) > t i ( [ Ca ] T ΣC = 1) . In seeded experiments the higher growth rates correspond to runs where [ Ca ] T ΣC = 4 , whereas the lower ones are observed in runs where [ Ca ] T ΣC = 1 4 ) . The lower rate of nucleation (high t i ) and higher rate of growth observed when [ Ca ] T ΣC = 4 can be explained, respectively, by the formation of a lower concentration of ion pairs in the bulk solution and a higher concentration of pairs CaCO 0 3 on the crystal surface.

Continuous Precipitation of Hydroxyapatite from Ca/Citrate/Phosphate Solutions using Microwave Heating

The precipitation of hydroxyapatite (HAp) in a continuous process using microwave heating and Ca/citrate/phosphate solutions is presented. Calcium deficient HAp particles in the nanometric size range (<100 nm), with needle-like morphology, were obtained. These chemical and morphological characteristics do not change with the residence time, and are similar to those of HAp obtained in a batch process using also microwave heating and Ca/citrate/phosphate solutions. On the other hand, the steady state is reached faster than in the precipitation of HAp in a MSMPR reactor using non-complexing solutions and conventional heating. In the latter process submicronic (<1 mikrom) needle-like particles are obtained.

Growth habit and surface morphology of L-arginine phosphate monohydrate single crystals

The results of a study of the growth habit and the surface topography of L-arginine phosphate monohydrate (LAP) single crystals as a function of supersaturation are described and discussed. Apart from a change in the growth habit with supersaturation, it was observed that most of the as-grown faces of LAP exhibit isolated growth hillocks and macrohillocks and parallel bunched layers and that the formation of bunched layers is pronounced on faces showing macrohillocks. Observations of bunching of growth layers emitted by macrohillocks on the {100} faces revealed that, for the onset of bunching close to a macrospiral, there is a characteristic threshold distance whose value depends on the interstep distance and supersaturation, but is independent of step height. The theoretical habit of LAP deduced from PBC analysis showed that all faces exhibiting growth hillocks and macrohillocks are F faces. Analysis of the results on bunch formation revealed that growth of LAP takes place by the direct integration of growth entities at the growth steps, that the bunching is facilitated by an increasing value of the activation energy for their integration, and that the observed dependencies of threshold distance on interstep distance, supersaturation and step height are qualitatively in agreement with van der Eerden and Muller-Krumbhaars theory of bunch formation.

Crystal Size Distribution of Hydroxyapatite Precipitated in a MSMPR Reactor

This paper is aimed to analyse the crystal size distribution (CSD) of hydroxyapatite (HA) Ca 5 (OH)(PO 4 ) 3 obtained in a continuous flow MSMPR reactor at 85 °C and pH constant = 9.0, at steady state. The kinetic of precipitation is calculated and the mechanism of particle formation is discussed. The method of moments in crystal volume coordinates, supposing a nucleation-growth-agglomeration mechanism for particle formation, has been used to solve the population balance equation and to calculate the nucleation rate (B o ), volume growth rate (G) and the aggregation kernel (β). The crystal size distribution graphs of precipitates and the low values of the nucleation rate show the inconsistency of this classical model to explain HA precipitation. However, they are in agreement with a mechanism of nucleation-aggregation-agglomeration-growth followed by a large-scale aggregation.

Atomic force microscopic study of step bunching and macrostep formation during the growth of L-arginine phosphate monohydrate single crystals

Abstract The experimental results of the formation of step bunches and macrosteps on the {100} face of L-arginine phosphate monohydrate crystals grown from aqueous solutions at different supersaturations studied by using atomic force microscopy are described and discussed. It was observed that (1) the step height does not remain constant with increasing time but fluctuates within a particular range of heights, which depends on the region of step bunches, (2) the maximum height and the slope of bunched steps increases with growth time as well as supersaturation used for growth, and that (3) the slope of steps of relatively small heights is usually low with a value of about 8° and does not depend on the region of formation of step bunches, but the slope of steps of large heights is up to 21°. Analysis of the experimental results showed that (1) at a particular value of supersaturation the ratio of the average step height to the average step spacing is a constant, suggesting that growth of the {100} face of L-arginine phosphate monohydrate crystals occurs by direct integration of growth entities to growth steps, and that (2) the formation of step bunches and macrosteps follows the dynamic theory of faceting, advanced by Vlachos et al.

Observations of cleavage steps, slip traces and dislocation hollow cores on cleaved {100} faces of l-arginine phosphate monohydrate single crystals by atomic force microscopy

Abstract The results of an atomic force microscopy study of the nature of cleavage steps, observation of slip traces and formation of hollow cores at the centres of dislocations on the {100} faces of l -arginine phosphate monohydrate (LAP) single crystals grown from aqueous solutions are described and discussed. It was observed that: (1) most of the cleavage steps and all the slip traces are of elementary height, a = 1.085 nm; (2) the origin of a cleavage step may or may not have a hollow core; and (3) close to its origin, the curvature of a cleavage step may be positive or negative or may change from positive to negative. The results suggest that slip traces observed on the cleaved surfaces of LAP are formed during the cleavage process while the rounding and the rearrangement of elementary cleavage steps take place immediately after the occurrence of cleavage. Analysis of the results also shows that the dislocations responsible for the origin of hollow cores always represent a stress field state corresponding to a trapped solution of different local interface supersaturations.

AFM Study of the Behaviour of Growth Steps on the (100) Faces of KDP Crystals and the Tapering Phenomenon

The experimental results of an investigation of the surface morphology of the as-grown (100) faces of KDP single crystals grown on seeds from aqueous solutions containing different concentrations of Fe 3+ and Cr 3+ impurities are described and discussed. It was found that the behaviour of steps in terms of their mutual spacing and height does not depend on the presence of an impurity in the growth medium. and that the association of elementary steps to form bunched steps on the growing surface is a statistical process. The bunching of steps and the formation of tapered faces are related phenomena. Step bunching on a face and its tapering are not related with the presence of an impurity in the medium but impurities favour these processes. Our analysis of the evolution of the spacing of elementary steps indicates that the Schwoebel effect is responsible for step bunching and face tapering.

On the formation of dislocation etch pits on L-arginine phosphate monohydrate single crystals

The results of the etching behaviour of different faces of L-arginine phosphate monohydrate (LAP) crystals in distilled water, methanol and ethanol are described. It was found that all the three solvents produce dislocation etch pits on different faces of LAP crystals, and that the capability of revelation of etch pits by a solvent on different faces depends on their crystallographic orientation. The observations are interpreted from the standpoint of Cabreras thermodynamic theory of dislocation etch-pit formation.