Giancarlo Celotti

Densification behaviour and mechanisms of synthetic hydroxyapatites

Starting from Ca(OH)2 and H3PO4 hydroxyapatite powders with three different crystallinity degrees have been prepared and characterized. Densification extent and mechanisms were studied through dilatometric measurements in isothermal regime in the range of temperature 750–1250°C: the influence of different powder features (including the effect of calcination treatment) have been evaluated. Powder characterized by the lowest crystallinity degree has the highest densification extent; overlapping phenomena occurring during the sintering treatments are responsible for unexpectedly low values found for the shrinkage rate, which can easily lead to a misidentification of the rate controlling mechanism. An interpretation of the densification mechanism, consistent with all experimental findings, is proposed.

Carbonated hydroxyapatite as bone substitute

Abstract B-carbonateapatite (CHA) powder was synthesized starting from calcium nitrate tetrahydrate, diammonium hydrogen phosphate and sodium hydrogen carbonate. The powder was fully characterized in terms of phase purity, stoichiometry, morphology, specific surface area and particle size distribution. The thermal stability of the powder in air and CO 2 atmosphere also was evaluated by thermal analysis. Electroacoustic analysis of the water based suspension of the CHA powder was used to determine the stability of the slurry. Porous bodies of CHA were prepared by impregnation of cellulose sponges with a proper slurry of the powder and optimizing the subsequent sintering. The fired samples were characterized in terms of phase purity and carbonate content, microstructure and pore size distribution. The compressive strength also was evaluated, resulting in 6.0±0.5 MPa. First results of in vivo tests on New Zealand White rabbits showed good biocompatibility and osteointegration of the CHA implant, with higher osteoconductive properties and earlier bioresorption, compared to HA samples, used as control.

A novel sol-gel technique for hydroxyapatite preparation

Abstract The concept of biocompatible, osteoconductive material mimicking the structure of natural bone to be used as bone graft has generated a considerable interest in recent decades. This paper reports on a novel and simple sol–gel technique for fabricating pure hydroxyapatite powder. Compositional, microstructural, morphological and mechanical characterisations carried out on the powder as well as on dense and porous sintered bodies, are also reported.

Sintering and characterization of HA and TCP bioceramics with control of their strength and phase purity

HA and β-TCP-based ceramics were prepared using commercial powders. Powder characteristics were defined and the processing parameters studied, aimed at the production of samples with improved microstructural and mechanical properties. The behaviour of HA powder subjected to various thermal treatments was investigated in order to control the formation of secondary phases (α- and β-TCP) during sintering. The optimal thermal treatment required to prepare pure β-TCP powder from the precursors (HA and DCP) was determined and the sintering method required to prepare fully dense β-TCP completely free from α-form, was identified. Translucent hot-pressed β-TCP ceramics with potential applications in aesthetic restorative prostheses were prepared and characterized. The interval of existence of α-TCP and α-TCP as secondary products was also defined. Crystallographic analysis was carried out on the imperfectly known low-temperature α-TCP phase, and a proper monoclinic unit cell determined.

Synthesis of hydroxyapatite-based powders by mechano-chemical method and their sintering

Abstract Ceramic hydroxyapatite (HA) is on the way to gaining credit as one of the most promising and diversified materials for employment in surgery, thanks to its good characteristics of biocompatibility and bioadaptability. The main and probably unique deficiency of these materials obtained by traditional methods consists in its weak mechanical resistance, that does not allow their use when even low loading is involved. To improve the mechanical properties of HA-based ceramic bodies additions are normally necessary. In this paper the authors propose a method to prepare powders and composite ceramic bodies with a matrix comprising HA. The powders are produced by the utilization of a simple and economic mechano-chemical method. The composite ceramic bodies are easily obtained by simple firing of the powders at a suitable temperature (1250 °C). The powders, after sintering, give products that show a flexural strength of more than 100 MPa in standard samples. This value is significantly higher than that usually attainable with present commercially available powders (60 MPa). The transformation in the components (HA and β-tricalcium phosphate) of the composite ceramic arises from the nature of the powders, consisting of highly defective HA. The mechano-chemical process is described together with the employed procedures specifically followed for reactive milling of starting powders. The preparation conditions of defective HA powders and properties acquired through such a method are reported. By this method an intimate mixing of β-tricalcium phosphate (β-TCP) inside the HA matrix is obtained that easily allows the preparation of ceramic bodies with reproducible properties, with no necessity for additions and mixing procedures, that could lead to inhomogeneity.

Development of hydroxyapatite/calcium silicate composites addressed to the design of load-bearing bone scaffolds.

This work deals with the preparation of bioactive ceramic composites to be employed for the development of load-bearing bone substitutes, made of hydroxyapatite (Ca(10)(PO(4))(6)(OH)(2), HA) and bioactive dicalcium silicate (Ca(2)SiO(4), C(2)S) as a reinforcing phase. The composite materials were prepared by Fast Hot-Pressing (FHP), which allowed the rapid sintering of monolithic ceramics at temperatures up to 1500 degrees C, well above the commonly adopted temperatures for the consolidation of hydroxyapatite (1200-1300 degrees C). The purpose was to achieve the grain coalescence of both HA and the strengthening phase, so that to obtain a homogeneous ceramic material characterized by controlled phase composition and improved mechanical strength; the dwell time was reduced as much as possible to prevent HA decomposition and excessive grain growth. The most remarkable result, in terms of phase composition, was the absence of any secondary phases in the final ceramics other than HA and C(2)S, even after sintering at 1500 degrees C. The flexure strength of the composite materials was found to be much higher than that of HA alone. Further mechanical characterization was also carried out on HA and composites, sintered in different conditions, to evaluate the elastic properties and fracture toughness, and properties close to those of mineral bone were found. These preliminary results confirmed that composites of HA and Ca(2)SiO(4) are promising for the development of bioactive load-bearing ceramic bone substitutes with controlled phase composition.

Raman and cathodoluminescence spectroscopies of magnesium-substituted hydroxyapatite powders

Stoichiometric and magnesium-substituted synthetic hydroxyapatite (HA) powders with different Mg contents were characterized by Raman and cathodoluminescence (CL) spectroscopies. The substitution of Ca ions by Mg is presently of great interest because it may improve activity in the first stage of the bone remodeling process. In this paper, we show new evidence that CL spectroscopy has the capability to detect the presence of crystal defects, related to the presence of magnesium substituting calcium in Mg-doped HA powders. The dependence of CL spectra of stoichiometric and magnesium-doped HA powders on their chemical composition was studied, and the results are compared with Raman analysis and data previously collected by other analytical tools. All the investigated powders showed five distinct CL bands; moreover, in magnesium-doped HA, an additional band at higher energy was found. The intensity ratios between selected CL bands showed some relationships with the powder crystallinity and the estimated amount of magnesium both in the HA lattice and in the amorphous surface layer; moreover the band observed only in magnesium-substituted powders could be directly related to the amount of magnesium entered into the HA lattice. Such results can contribute to improve the knowledge of the crystallographic structure of Mg-substituted hydroxyapatite.

Thermal expansion behavior of bulk Bi-based (2223) superconductors

Thermal expansion coefficient of Bi-based 2223 and 2212 ceramics has been measured using dilatometric apparatus. Different typologies of bulk superconducting samples have been tested, in particular (2223) phase fully dense hot-pressed samples both parallel and perpendicular to the direction of applied pressure, pressureless sintered samples, and finally hot pressed (2212) phase specimen, in order to gather information on the contribution of secondary phase to thermal expansion behavior. From the observed {alpha} (thermal expansion coefficient) values, accounting for the sample orientation degree, it was possible to distinguish the contributions along the basic crystallographic directions and to determine the following for (2223) phase: {alpha}{sub {ital a}}=12.2{times}10{sup {minus}6}{degree}C{sup {minus}1}, and {alpha}{sub {ital c}}=20.8{times}10{sup {minus}6}{degree}C{sup {minus}1} for temperatures 200{endash}600{degree}C. Theoretical {alpha} values were then calculated for the different samples and successfully compared with the measured ones. As regards (2212) phase, thermal expansion coefficient was evaluated for oriented sample, and a value for random one extrapolated; the results are very similar to that of (2223), but lower in respect to previously reported data in air atmosphere. {copyright} {ital 1996 Materials Research Society.}

Mechanisms and kinetics of the synthesis of BSCCO superconducting phases via organometallic precursors

Abstract A precursor powder was prepared with the nominal composition Bi1.84Pb35Sr9Ca3Ox by the pyrolysis technique starting from a solution of citrates. Reactions among components were investigated by thermogravimetric and differential thermal analysis in the temperature range 700–850°C. Reaction kinetics and mechanisms involved in the transformation of the organometallic precursors into superconducting phases were examined and mathematically treated by relationships theoretically derived for phenomenological models. The identification of reaction intermediates that play a crucial role in the synthetic process was systematically performed via XRD and a comparison with the solid state reaction route was attempted.