J.M. Porto López

Thermal Evolution of Alumina Prepared by the Sol-Gel Technique

The thermal evolution of an alumina gel synthesized by hydrolysis of aluminium alkoxide (sol-gel technique) was studied by thermal analysis (DTA and TGA), X-ray diffraction, FTIR and NMR spectroscopies, and specific surface area measurements. Between 400 and 900°C, γ- and δ-aluminas were formed showing aluminium vacancies preferentially located in tetrahedral sites. The atomic rearrangements produced during α-alumina formation are oriented to the progressive elimination of tetrahedral aluminium in the ultimate phase. The evolution of the specific surface area during heating is explained by changes in structure and microstructure.

Influence of temperature and additives on the microstructure and sintering behaviour of hydroxyapatites with different Ca/P ratios.

Sintering of two hydroxyapatite (HA) samples with different Ca/P ratios was studied as a function of thermal pretreatments, sintering temperature and additives (0–0.6 ion % Li+ or 0–5 ion % Mg2+). The samples were sintered in air and characterized by density measurements, scanning electron microscopy, differential thermal analysis, X-ray diffraction and dilatometry. Upon sintering, samples with Ca/P ratio of 1.51 (HA C) transformed to β-Ca3(PO4)2 and Ca10(PO4)6(OH)2, resulting in materials with low densities and containing agglomerated β-Ca3(PO4)2 when sintered above 1200°C. Samples with a Ca/P ratio of 1.77 (HA S), without β-Ca3(PO4)2, showed better sinterability and homogeneous microstructures. Li+ additions favoured liquied-phase sintering and reduced the β-Ca3(PO4)2 content in sintered materials. Mg2+ additions did not result in higher densities, but inhibited the hydroxyapatite grain growth rate. A significant percentage of the added Mg2+ was incorporated into the β-Ca3(PO4)2 structure.

Synthesis of Fe-FeAl2O4-Al2O3 by high-energy ball milling of Al-Fe3O4 mixtures

Physicochemical and structural changes induced by mechanical activation of Al–Fe3O4 mixtures are studied. After 37 min, the system undergoes a self-sustained reaction with formation of α-Fe, FeAl2O4 and α-Al2O3. The evolution of the composition follows a three-step reaction. The solid solution spinel Fe[Al2−xFex]O4, (0.13<x<0.29), is obtained through a mechanochemical-thermal route.

Analysis of the microstructural evolution in hydroxyapatite ceramics by electrical characterisation

Abstract The aim of this work is to determine the effect of lithium or magnesium addition on the microstructure of hydroxyapatite ceramics sintered at 1200°C. The samples were characterised by density measurements, scanning electron microscopy (SEM), X-ray diffraction (XRD) and Vickers microhardness. Dielectric properties, dc-conductivity and microhardness were related to the microstructures developed by the sintered materials. Furthermore, significant effects due to the presence of Li+ or Mg+2 in these ceramics were established.

Preparation and in vitro evaluation of porous silica gels

Porous silica gels with high surface areas were prepared from tetraethylothosilicate and polyacrylic acid (PAA) of high molecular weight in acidic media by a sol-gel method. PAA content and ageing temperatures were varied in order to obtain different microstructures. Samples were sintered at temperatures up to 400 degrees C, and subjected to in vitro evaluation by soaking them in acellular inorganic solutions at 37 degrees C and pH 7.3. Surface precipitation of carbonate-apatite on some of the gels was observed by FTIR spectroscopy, scanning electron microscopy and EPMA. Silica dissolution and re-precipitation phenomena were also observed. The relationship between both phenomena during the in vitro test is discussed mainly in terms of structural and microstructural features of the gel.

Analysis by nuclear magnetic resonance and raman spectroscopies of the structure of bioactive alkaline-earth silicophosphate glasses

Abstract Structural knowledge of bioactive glass systems is necessary to understand their biological properties and improve their mechanical and clinical performance. In this work, the local structure of P and Si in bioactive glasses of the system CaO-SiO 2 -P 2 O 5 -CaF 2 with different Mg or Sr contents was determined, by means of 31 P and 29 Si magic-angle sample-spinning nuclear magnetic resonance (MASS NMR) and Raman spectroscopies. The results show that phosphorus is located in calcium-orthophosphate domains, surrounded by silicate species containing 1, 2 and 3 bridging-oxygens per SiO 4 tetrahedra. The type and relative distribution of the silicate units do not depend on the nature of the alkaline earth cations present in the system. However, the increase of RO content decreases the degree of polymerization of silicon tetrahedra, while PO 4 monomeric units remain unchanged. An interpretation of the medium-range structure of the glass is suggested.

Effect of induced structural modifications on the physicochemical behavior of bentonite

Abstract The effect of impact and friction milling on the structure and texture of a bentonite, in its natural and in several monoionic forms (H + , Na + , Ca 2+ , Al 3+ ), is studied. An increase of treatment time shows progressive damage to the crystalline structure. The presence of different exchangeable ions does not exert a great influence on the behavior of the mineral. The physicochemical and structural alterations are studied by XRD, DTA, TGA, IR, CEC and swelling index.

Magnetic and structural study of mechanochemical reactions in the Al-Fe3O4 system

The solid state reaction between Al and Fe3O4 (magnetite) using mechanochemical activation of powder mixtures under Ar atmosphere is studied. The phase evolution during the reaction is analyzed by X-ray diffraction (XRD), vibrating sample magnetometry (VSM), differential thermal analysis (DTA) and scanning electron microscopy (SEM). At 37 minutes of high-energy ball-milling the disappearance of reactive phases and the production of α-Fe, FeAl2O4 and α-Al2O3 is observed, together with significant changes in the magnetic behavior of the system. The composition and properties of samples heated up to 1200°C are also investigated. The behavior of the saturation magnetization Ms is interpreted on the basis of the formation of a variable composition spinel phase Fe [Alx Fe2−x] O4 with 0 ≤ x ≤ 2 and a canting effect due to the presence of Al3+ ions in the spinel structure.

Improvement of the microstructure and microhardness of hydroxyapatite ceramics by addition of lithium

Abstract The aim of this work is to improve sintered density, microhardness and microstructure of hydroxyapatite (HA) ceramics, sintered in air at 1200 °C with the addition of lithium ions. Samples were characterized by density measurements, X-ray diffraction (XRD), Vickers microhardness, and scanning electron microscopy (SEM). Also, conductivity vs. time curves and dissipation factor measurements between 100 Hz-1 MHz were made in order to relate composition and microstructure characteristics with electrical behavior.

Thermal behaviour of bioactive alkaline-earth silicophosphate glasses

The thermal behaviour (sinterability and first crystallization) of a series of alkaline-earth silicophosphate glasses has been studied by differential thermal analysis (DTA), X-ray diffraction (XRD) and scanning electron microscopy (SEM). The samples were prepared from a base bioactive glass of the system CaO−P2O5−SiO2−CaF2, by (a) slightly changing the F/O ratio; (b) replacing part of the CaO by SrO or MgO; and (c) increasing the total alkaline earth concentration by MgO additions. The results show that the addition of MgO is the most effective way of improving sinterability. In these samples, a decrease of the glass transition temperature, together with an increase in the temperature of the first crystallization, is observed. The difference between both temperatures is proposed to be an adequate indicator of the sinterability. The initial stages of the first crystallization (which produces an oxo-fluorapatite), and its composition dependence, are discussed in terms of the results of sinterability, and the classical theory of nucleation.