Search

[EN] Synthetic gibbsite (Al(OH)3) was mechanically activated by attrition milling for 24 h, with various grinding ball-to-powder weight ratios (0, 5, 10, and 20), and characterized by thermal analysis (TG-DSC). Further, we determined the corresponding kinetic parameters using the model-free and model-fitting methods from the Thermogravimetric Analysis (TG) data set. We found that the activation energies provided by both models agree very well. At temperatures higher than 350 °C, the milled samples (GB5, GB10, and GB20) lose mass very slowly, while the unmilled sample (GB0) decomposes faster. In addition, we demonstrated that the decomposition mechanism of each sample engages multi-step reactions, and the corresponding activation energies change with the increasing milling conditions., [ES] Se ha activado mecánicamente gibbsita sintética, Al(OH)3, mediante molienda por atrición durante 24 h. Se han empleado diversas relaciones de bolas de molienda a polvo (0, 5, 10 y 20, en peso) y se caracterizó mediante análisis térmico (TG-DSC). Con estos resultados se determinaron los correspondientes parámetros cinéticos a través de los análisis sin modelo y con modelo de ajuste del conjunto de datos de análisis termogravimétrico (TG). Se ha encontrado que las energías de activación proporcionadas por ambos modelos concuerdan muy bien. A temperaturas superiores a 350 °C las muestras molidas (GB5, GB10 y GB20) pierden masa muy lentamente, mientras que la muestra sin moler (GB0) se descompone más rápido. Además, se demuestra que el mecanismo de descomposición de cada muestra involucra reacciones con varias etapas, y las correspondientes energías de activación cambian con el incremento de la energía de molienda.

In this article a review of the use of Calcium phosphates in Biomedical Engineering has been done. A brief revision of the first applications in the field of medicine of these phosphates is carried out, to later comment on the current situation. The different types of existing calcium phosphates are summarized and where they are applied in the field of biomedicine. Likewise, the latest advances in its processing and its applications are discussed. The properties that these materials have to show in order to predict their behavior when used as biomaterials are also reviewed. Finally, the future of these materials and their applications is described.

[EN] The main concepts of CALPHAD method (CALculation of PHase Diagrams; Computer Coupling of Phase Diagrams and Thermochemistry), focused on calculation of phase equilibrium diagram of ceramic systems are presented. An overview of thermodynamic assessment of ceramic systems is addressed. Generalist thermodynamic models used for description of multicomponent ceramic phases are summarized. Application of CALPHAD method is depicted with calculations of specific phase diagrams for ceramic materials development.

Robust mechanical properties of refractory ceramics are critical to their performance in industrial applications. To investigate the effects of glass on the sinterability and mechanical properties of refractory ceramics, a controlled amount of glassy phase (10 wt%) was added to mullite (Al6Si2O13)-alumina (Al2O3) and hibonite (CaAl12O19)-alumina composites. Two liquid compositions within the Al2O3-SiO2-CaO system were chosen to produce a glass at 1600 °C that exists in equilibrium with a mullite-alumina composite and a hibonite-alumina composite. We performed a comparative study of the mechanical properties (hardness, flexural strength, and creep resistance) of mullite-alumina and hibonite-alumina composites with and without glassy phases. The observed microstructures clearly affected the mechanical properties. The strong interface created between the three phases in each subsystem and the reduced amount of residual porosity in the composites with an added glassy phase may explain the increase in strength, hardness, and creep resistance of the composites. However, better mechanical properties and finer grain sizes of mullite make the mullite-alumina (-glass) system a better candidate for refractory applications that require improved mechanical properties.

[EN] The history of calcium phosphates in the medicine field starts in 1769 when the first evidence of its existence in the bone tissue is discovered. Since then, the interest for calcium phosphates has increased among the scientific community. Their study has been developed in parallel with new advances in materials sciences, medicine or tissue engineering areas. Bone tissue engineering is the field where calcium phosphates have had a great importance. While the first bioceramics are selected according to bioinert, biocompatibility and mechanical properties with the aim to replace bone tissue damaged, calcium phosphates open the way to the bone tissue regeneration challenge. Nowadays, they are present in the majority of commercial products directed to repair or regenerate damaged bone tissue. Finally, in the last few decades, they have been suggested and studied as drug delivering devices and as vehicles of DNA and RNA for the future generation therapies., [ES] La historia de los fosfatos de calcio en el campo de la medicina comienza en 1769, cuando se descubre la primera evidencia de su existencia en el tejido óseo. Desde este momento los fosfatos de calcio despiertan un gran interés entre la comunidad científica. Su estudio se ha desarrollado en paralelo a nuevos avances en la ciencia de materiales, la medicina o la ingeniería de tejidos. Uno de los campos en que más repercusión han tenido es en la reparación del tejido óseo. Si bien las primeras biocerámicas fueron seleccionadas en base a su bioinercia, biocompatibilidad y propiedades mecánicas, con el objetivo de remplaza rel hueso da ̃nado, los fosfatos de calcio abrieron las puertas al desafío de la regeneración ósea. Nuevos conceptos como biorreabsorción, bioactividad y osteo inducción aparecieron con ellos, convirtiéndoles en excelentes candidatos a resolver este reto. En la actualidad forman parte de la composición de la mayoría de productos comerciales cuyo objetivo es la reparación o la regeneración del tejido óseo. Además, en las últimas décadas se han postulado como soportes en la liberación controlada de fármacos, así como vehículos de material genético para las terapias génicas del futuro.

Four compositions in the BaO-AlO-SiO system modified with BO were investigated with regard to their use as glassy seals in anode supported SOFC or iT-SOFC. The glassy system studied contains varying percentages of SiO, AlO, BO, and high content of BaO as modifier (67-74%-wt). Their glass transition (Tg) and maximum densification point lies between 630 and 680°C, and 734°C to 828°C, respectively. The viscosities of the four glasses are situated between 10 to 10 Pa.s, in the sealing range (730-830°C). Heat treatment at 850°C, revealed crystalline phases identified as BaAlSiO (hexacelsian) and BaSiO. The thermal expansion coefficient (TEC) of the obtained glasses (8.8-10.5ppm/K) was comparable to zirconia (YSZ) electrolyte, and shows chemical compatibility and high characteristic bond strength (up to 33±7MPa). The compositions with higher BaO content in the system studied seem to be good candidates for iT-SOFC sealant application.

In this work the fabrication of buried optical waveguides by femtosecond laser inscription in the 0.8CaSiO3–0.2Ca3(PO4)2 eutectic glass doped with Nd3+ ions is reported. The glass samples were prepared by melting the eutectic powder mixture in a Pt–10 wt.% Rh crucible at 1600 °C and pouring it in a preheated brass mould. Afterwards, the glass was annealed to release the inner stresses. Buried waveguides were fabricated by focusing beneath the surface a pulsed Ti:sapphire laser with a pulsewidth of 120 fs working at 1 kHz. Two adjacent parallel tracks were written to define a region where an increase in the refractive index occurs. The effects produced by the variation of the laser pulse energy as well as the lateral separation between tracks, scanning speed and focusing distance were studied. After the laser processing, the near-field intensity distribution at 633 nm of the waveguide's modes was studied demonstrating the confinement of both, the TE as the TM polarizations. In order to diminish the losses induced by colour centres absorption, heat treatments were carried out in the samples. The waveguide's modes were compared with respect to the samples without heat treatments. The spectroscopic properties of the neodymium ions have been characterized to evaluate in what extent their optical properties could be modified by the waveguide fabrication process and to elucidate the potential application of such waveguides as integrated laser sources

In this work the formation of buried optical waveguides by femtosecond laser inscription in the 0.8CaSiO3-0.2Ca3(PO4)2 eutectic glass doped with Nd3+ ions is reported. The glass samples were prepared by melting the eutectic powder mixture in a Pt-Rh crucible at 1600 ºC and pouring it in a preheated brass mould. Afterwards, the glass was annealed to relieve the inner stresses. The buried waveguides were fabricated by focusing under the surface a pulsed Ti:sapphire laser with a pulsewidth of 120 fs working at 1 kHz. Two adjacent filaments were written to define a region where an increase in the refractive index occurs. The effects produced by the variation of the laser pulse energy as well as the distance between tracks, scanning speed and focusing distance were studied. After the laser processing the near-field intensity distribution of the waveguide's modes at 635 nm was studied. In order to diminish the losses by colour centres absorption a heat treatment was carry out in the samples. The waveguide's modes were compared with those of the samples without heat treatment. The spectroscopic properties of the neodymium ions have been characterized to evaluate if their optical properties have been modified by the waveguide fabrication. Laser experiments are in progress to evaluate the ability of the waveguides for 1064 nm laser light generation under 800 nm optical pumping.

comportamiento en servicio, ha sido, a lo largo del tiempo, un tema de controversia en el campo de la cerámica y de los refractarios. Sin embargo, en la tecnología refractaria moderna es posible obtener mezclas homogéneas a través de rutas sofisticadas de procesamiento. Como resultado, las velocidades de reacción son generalmente lo suficientemente altas para alcanzar el equilibrio o cuasi-equilibrio durante la cocción convencional del material así como en su comportamiento en servicio. Por lo tanto, hoy en día, los diagramas de equilibrio son válidos no sólo para determinar la tendencia termodinámica de equilibrio durante la cocción del material refractario, sino también para predecir el estado final de equilibrio y, con una buena comprensión de las relaciones de equilibrio, también es posible conocer el camino a través del cual el sistema refractario evoluciona hacia el estado final durante su aplicación. En consecuencia, los diagramas de equilibrio son una potente herramienta para el diseño de nuevos materiales refractarios, así como para una mejor comprensión de su comportamiento en servicio en condiciones de alta temperatura y agentes corrosivos. En el presente trabajo se pretende mostrar, a través de algunos ejemplos pertinentes, cómo el proceso de fabricación de algunos materiales refractarios tradicionales puede ser explicado y también cómo es posible mejorar y / o diseñar nuevos materiales refractarios mediante el uso de los diagramas de equilibrio de fases apropiados. El presente trabajo recopila y revisa la última conferencia plenaria impartida por el Profesor Salvador De Aza sobre el tema

The isoplethal sections CaAl2O4–MgO and CaAl4O7–MgO of the Al2O3–MgO–CaO ternary system have been experimentally established at 1 bar total pressure and air of normal humidity. The sections obtained provide new data and information that are in disagreement with thermodynamic evaluations and optimizations of the Al2O3–MgO–CaO ternary system published to date. These differences arise mainly from the inclusion, or exclusion, of the binary compound Ca12Al14O33, mayenite, as a stable phase in the reported studies of the system. The presence or absence of this compound within the system has an important impact on the solid state and melting relationships of the whole ternary system. The present study confirms the solid-state compatibility CaAl2O4–MgO and CaAl2O4–MgO–MgAl2O4 up to 1372°± 2°C, the peritectic melting point of the later mentioned subsystem.

Partially deuterated Ca3Al2(SiO4)y(OH)12−4y–Al(OH)3 mixtures, prepared by hydration of Ca3Al2O6 (C3A), Ca12Al14O33 (C12A7) and CaAl2O4 (CA) phases in the presence of silica fume, have been characterized by 29Si and 27Al magic-angle spinning–nuclear magnetic resonance (MAS–NMR) spectroscopies. NMR spectroscopy was used to characterize anhydrous and fully hydrated samples. In hydrated compounds, Ca3Al2(OH)12 and Al(OH)3 phases were detected. From the quantitative analysis of 27Al NMR signals, the Al(OH)3/Ca3Al2(OH)12 ratio was deduced. The incorporation of Si into the katoite structure, Ca3Al2(SiO4)3−x(OH)4x, was followed by 27Al and 29Si NMR spectroscopies. Si/OH ratios were determined from the quantitative analysis of 27Al MAS–NMR components associated with Al(OH)6 and Al(OSi)(OH)5 environments. The 29Si NMR spectroscopy was also used to quantify the unreacted silica and amorphous calcium aluminosilicate hydrates formed, C–S–H and C–A–S–H for short. From 29Si NMR spectra, the amount of Si incorporated into different phases was estimated. Si and Al concentrations, deduced by NMR, transmission electron microscopy, energy dispersive spectrometry, and Rietveld analysis of both X-ray and neutron data, indicate that only a part of available Si is incorporated in katoite structures.

Hydrogarnet (Ca12Al2(OH)12) and katoite of composition Ca3Al2(SiO4)0.176(OH)11.3 were obtained by hydration of tricalcium aluminium oxide, and for katoite synthesis, by addition of amorphous silica. The thermal dehydration was monitored in situ, by neutron thermodiffractometry, from room temperature to 700 °C at atmospheric pressure and at a heating rate of 2 °C/min. On heating, powder neutron diffraction patterns were collected every 300 s. Cell parameters were fitted by the Pawley method. Hydrogarnet decomposed to yield Ca12Al14O32(OH)·mH2O and Ca(OH)2 that eventually transformed to CaO. For katoite, phases of formula Ca12Al14−ySiyO32(O,OH)1+y/2 related to mayenite were formed. Complementary annealing experiments, for 10 h, at higher temperatures and subsequent quenching lead to the formation of mayenite and traces of Ca3SiO5. Thermogravimetric curves were in agreement with the thermodiffractometry experiments. Also, neutron diffraction data allowed to measure thermal expansion coefficients, at atmospheric pressure, between 25 and 250 °C for hydrogarnet and katoite: 1.89 × 10−5 ± 0.09 and 1.63 × 10−5 ± 0.07 °C−1, respectively.

In the present investigation, the sliding wear behavior is described for Al2O3/ZrO2 micro/nanocomposites and monolithic alumina of similar grain size under defined conditions of a constant sliding speed and different loads (20–150 N). Nano ZrO2 particles (1.7 vol%) were observed uniformly distributing throughout the composites, and most of them were located within the matrix alumina grains. The wear rate of the alumina and the micro/nanocomposites increased as the contact load increased and a clear transition in friction and wear behavior was observed in both materials. However, the nanocomposite wear resistance at low contact loads was one order of magnitude higher than that of the alumina. In the severe regime, no difference was observed among the materials. The low wear rate (10−7 mm3·(N·m)−1) along with low pullout indicates higher wear resistance of micro/nanocomposites in the mild regime compared with monolithic alumina. Based on the morphological observation of worn surfaces by scanning electron microscope and on residual stress analysis performed by neutron diffraction, some wear mechanisms of Al2O3–ZrO2 micro/nanocomposites are proposed. The high wear resistance of the nanocomposites is discussed in terms of fracture resistance properties and residual stress. Improvements in mechanical and tribological properties of these composites make them promising candidates for biomedical applications.

High alumina refractory castables based on the Al2O3-MgO-CaO diagram ternary system were prepared using tabular alumina, white electrofused corundum, calcined alumina, synthetic spinel, dead-burned magnesia, dolomite, and calcium aluminate cement as starting raw materials. Two kinds of slags with 9.02 and 4.14 CaO/SiO2 ratios were studied for slag resistance by means of crucible tests. The corrosion thickness increases with increase in magnesia content in all the designed compositions. The slag penetration decreases with increases in spinel content. Taking into account these results a refractory castable composition for its positioning into a steelmaking ladle was chosen. Microstructural studies by SEM of samples taken from the slag line and wall in a steelmaking ladle were carried out. The correct amount of spinel required for practical applications was determined by the Al2O3-MgO-CaO-SiO2 diagram quaternary system. A detailed model of the attack mechanisms is proposed.

[ES] Se ha estudiado la influencia del humo de sílice sobre el mecanismo de hidratación del monoaluminato de calcio (CaAl2O4) a temperatura ambiente y con una relación agua/sólidos de 0,5. La evolución de las fases hidratadas, que se forman en los primeros instantes de la reacción, se ha estudiado por difracción de rayos X de laboratorio con una resolución temporal de 36 min. La integración de picos de difracción seleccionados de las fases: Ca2Al2O5·8H2O (Ca2Al2(OH)10·3H2O), CaAl2O4 y Ca4Al2(OH)12(CO3)·5H2O, ha permitido establecer la cinética de la reacción. Así, se ha puesto de manifiesto que el humo de sílice provoca una drástica disminución en el periodo de inducción de la hidratación y acelera la formación de los hidratos cristalinos metaestables Ca2Al2(OH)10·3H2O y Ca4Al2(OH)12(CO3)·5H2O. Estos resultados se han confirmado por Termogravimetría, Análisis Térmico Diferencial y Microscopía Electrónica de Barrido. El mecanismo de hidratación propuesto se apoya en datos previos de Resonancia Magnética Nuclear de Sólidos (RMN) de 29Si de los autores de este articulo., [EN] The influence of the amorphous silica on the mechanism of hydration of the calcium monoaluminate (CaAl2O4) has been studied at ambient temperature with a water/solids ratio of 0.5. The evolution of the hydrated phases, formed in the first instants of the reaction, has been studied by laboratory X ray diffraction with a time resolution of 36 min. The integration of selected diffraction peaks of the phases: Ca2Al2O5·8H2O (Ca2Al2(OH)10·3H2O), CaAl2O4 and Ca4Al2(OH)12(CO3)·5H2O, has allowed to establish the kinetics of the reaction. This way, it has been shown that the amorphous silica causes a drastic decrease in the period of induction of the hydration and accelerates the formation of the metastable crystalline hydrates Ca2Al2(OH)10·3H2O and Ca4Al2(OH)12(CO3)·5H2O. These results have been confirmed by Thermogravimetry, Differential Thermal Analysis and Scanning Electric Microscopy. The hydration mechanism proposed confirms previous data of Nuclear Magnetic Resonance of Solids (RMN) of 29Si of the authors of this paper.

[EN] Since the late 1960´s, a great interest in the use of bioceramic materials for biomedical applications has been developed. In a previous paper, the authors reviewed crystalline bioceramic materials "sensus stricto", it is to say, those ceramic materials, constituted for non-metallic inorganic compounds, crystallines and consolidates by thermal treatment of powders at high temperature. In the present review, the authors deal with those called bioactive glasses and glassceramics. Although all of them are also obtained by thermal treatment at high temperature, the first are amorphous and the second are obtained by devitrification of a glass, although the vitreous phase normally prevails on the crystalline phases. After an introduction to the concept of bioactive materials, a short historical review of the bioactive glasses development is made. Its preparation, reactivity in physiological media, mechanism of bonding to living tissues and mechanical strength of the bone-implant interface is also reported. Next, the concept of glass-ceramic and the way of its preparation are exposed. The composition, physicochemical properties and biological behaviour of the principal types of bioactive glasses and glass-ceramic materials: Bioglass®, Ceravital®, Cerabone®, Ilmaplant® and Bioverit® are also reviewed. Finally, a short review on the bioactive-glass coatings and bioactive-composites and most common uses of bioactive-glasses and glass-ceramics are carried out too., [ES] Desde finales de los años sesenta, se ha despertado un gran interés por el uso de los materiales biocerámicos para aplicaciones biomédicas. En un trabajo previo, los autores hicieron una revisión de los denominados materiales biocerámicos cristalinos en sentido estricto, es decir, de aquellos materiales, constituidos por compuestos inorgánicos no metálicos, cristalinos y consolidados mediante tratamientos térmicos a altas temperaturas. En el presente trabajo, los autores revisan el desarrollo de los vidrios bioactivos (biovidrios) y de las vitrocerámicas bioactivas. Si bien todos ellos son obtenidos también por tratamiento térmico a altas temperaturas, los primeros son amorfos y los segundos son obtenidos por desvitrificación de un vidrio, si bien la fase vítrea normalmente predomina sobre las fases cristalinas. Después de una introducción al concepto de material bioactivo, se expone una breve revisión histórica del desarrollo de los vidrios bioactivos. A continuación se describe su obtención, reactividad en suero fisiológico artificial, mecanismo de unión al tejido vivo y resistencia mecánica de la interfaz hueso-implante. Posteriormente, se expone el concepto de material vitrocerámico y el proceso de su obtención así como también se describen los principales tipos de vidrios y vitrocerámicos bioactivos (Bioglass®, Ceravital®, Cerabone®, Ilmaplant® and Bioverit®), sus composiciones, sus propiedades físico-químicas y sus comportamientos biológicos. Finalmente, se lleva a cabo también una corta revisión de los recubrimientos con vidrios bioactivos y de los materiales compuestos (composites) bioactivos así como de los usos más comunes de los vidrios y vitrocerámicos bioactivos.

Magnesia-graphite refractory materials are used in large quantities in the steelmaking process. The chemical characterization of this type of refractories is an arduous task that requires a rigorous set of laboratory tests and analyses. In the present paper, proper characterization of magnesia-graphite refractories has been approached by X-ray powder diffraction combined with Rietveld methodology. The quantitative phase analysis of a MgO-graphite refractory has been 68.3 wt% of MgO, 8.1 wt% of graphite, 13.5 wt% of Al2O3, 4.4 wt% of SiC, 0.6 wt% of Si, 1.2 wt% of Al, 1.5 wt% of AlPO4 and 2.4 wt% of silicone. These results have been checked and validated with those obtained by other analysis procedures used to determine the crystalline and non-crystalline phases present in these materials.

The thermal decomposition mechanism of synthetic Al(OH)3 (gibbsite) was studied in situ by neutron thermodiffractometry in an ambient atmosphere from room temperature to 6001C with 501C steps. Gibbsite decomposed to yield AlO. (OH) (boehmite) and then poorly crystallized v-Al2O3. Rietveld analysis was used to refine the cell parameters’ variation of gibbsite and its thermal expansion coefficients were obtained: for the a-axis

The purpose of this study was to investigate the effect of four sterilization methods (Steam autoclave, Hydrogen peroxide plasma, Ethylene oxide, and Gamma sterilization) on the surface chemistry and in vitro bioactivity of polycrystalline pseudowollastonite (psW). psW samples obtained by solid-state reaction sintering were sterilized and soaked in Kokubo et al.’s proposed simulated body fluid (SBF) up to 30 days. The sterilization procedure was found to result in no significant chemical changes in the surface of the samples. On the other hand, a Ca/P layer, of different thickness, identified as hydroxyapatite (HA) like, was developed on all the samples after soaking, although the Ethylene oxide-sterilized samples present a non-homogeneous and B68% thinner HA layer. The psW samples before soaking were analyzed by X-ray diffraction, raman spectroscopy, and scanning electron microscopy (SEM). The interfacial reaction product was examined by SEM fitted with an energy-dispersive X-ray analyzer. Additionally, changes in ionic concentrations at the psW/SBF interface were measured

Magnesia–graphite refractories are currently essential for the iron and steel industry. As a result, their physico-chemical characterization is crucial with a view to predicting their service life. In this work, a methodology for the complete analytical characterization of such materials has been developed. Magnesia– graphite refractories generally contain elemental phases (C, Si, and Al), non-oxide and oxide inorganic phases (SiC, MgO, and Al2O3), organic phase (binders), and various additives and impurities. Consequently, their chemical characterization is especially difficult and requires the joint use of the following techniques: X-ray diffraction, differential thermal and thermogravimetric analyses, inductively coupled plasma–atomic emission spectroscopy, flame emission, Fourier transform infrared spectrometry, gravimetry, and C/S elemental analyzer (LECO). The data obtained by the different techniques match satisfactorily and provide validity to the methodology developed. The results obtained for each element analyzed, and the precision of each determination method used, are quite acceptable.

[EN] A strong interest in the use of ceramics for biomedical engineering applications developed in the late 1960´s. Used initially as alternatives to metallic materials in order to increase the biocompatibility of implants, bioceramics have become a diverse class of biomaterials, presently including three basic types: relatively bioinert ceramics; bioactive or surface reactive bioceramics and bioresorbable ceramics. This review will only refer to bioceramics “sensus stricto”, it is to say, those ceramic materials constituted for nonmetallic inorganic compounds, crystallines and consolidated by thermal treatments of powders to high temperatures. Leaving bioglasses, glass-ceramics and biocements apart, since, although all of them are obtained by thermal treatments to high temperatures, the first are amorphous, the second are obtained by desvitrification of a glass and in them vitreous phase normally prevails on the crystalline phases and the third are consolidated by means of a hydraulic or chemical reaction to room temperature. A review of the composition, physiochemical properties and biological behaviour of the principal types of crystalline bioceramics is given, based on the literature data and on the own experience of the authors., [ES] A finales de los años sesenta se despertó un gran interés por el uso de los materiales cerámicos para aplicaciones biomédicas. Inicialmente utilizados como una alternativa a los materiales metálicos, con el propósito de incrementar la biocompatibilidad de los implantes, las biocerámicas se han convertido en una clase diversa de biomateriales, incluyendo actualmente tres tipos: cerámicas cuasi inertes; cerámicas bioactivas o reactivas superficialmente y cerámicas reabsorbibles o biodegradables. En la presente revisión se hace referencia a las biocerámicas en sentido estricto, es decir, a aquellos materiales constitutitos por compuestos inorgánicos no metálicos, cristalinos y consolidados mediante tratamientos térmicos a altas temperaturas. Dejando aparte los biovidrios, los vitrocerámicos y los biocementos, puesto que, si bien todos ellos son obtenidos por tratamiento térmicos a altas temperaturas, los primeros son amorfos, los segundos son obtenidos por desvitrificación de un vidrio, prevaleciendo normalmente la fase vítrea sobre las fases cristalinas, y los terceros son consolidados mediante una reacción química o hidráulica a temperatura ambiente. Así pues, teniendo en cuenta la abundante bibliografía sobre el tema y la experiencia propia de los autores, se presenta una revisión de la composición, propiedades fisicoquímicas, aplicaciones y comportamiento biológico de los principales tipos de biocerámicas cristalinas.

A study of slow-crack propagation in zirconia-toughened alumina (ZTA) ceramics is presented. Different ZTA composites, using either unstabilized or yttria-stabilized zirconia, have been tested. The materials were prepared in the range 0-15 vol% ZrO2, using two processing methods: powder mixing and a modified colloidal route. Crack propagation tests have been conducted for crack velocities from 10-12 to 10-2 m/s by means of a double-torsion method. The influence of the processing conditions and compositions on the microstructures and the effect of these microstructures on the slow-crack-growth behavior have been studied. The option that these composites may offer in terms of the lifetime of ceramic joint prostheses is discussed.

Use of spinel-containing high alumina refractory concretes has increased significantly over the last few years. Currently the high cost of sintered or electrofused spinels limits their applications. This paper describes new and inexpensive new spinel-containing refractory cements obtained by a reaction-sintering process between dolomite and alumina. The reaction-sintering process occurs in two well de.ned steps: (A) reaction and (b) sintering. The reaction also happened in two successive stages: (a) dolomite decomposition in the range 700-900 °C, and (b) reaction of lime and magnesia with alumina, in the range from 900 to 1250 °C. A selected composition containing: 43±t25 wt.% CaAl2O4, 15±3 wt.% CaAl4O7 and 42±22 wt.% MgAl2O4, showed a hydration and dehydration behaviour similar to that of commercial calcium aluminate cements. The evolution of the hydrated pure CaAl2O4 cement up to 1500 °C has also been studied.

[ES] El presente trabajo es una revisión sobre el proceso de hidratación de los cementos de aluminatos de calcio, de alto contenido en alúmina, con adiciones de microsílice y alumina. En el se describen los principales productos de hidratación de estos materiales. Así mismo se exponen los mecanismos de reacción que han sido establecidos por distintos autores utilizando técnicas diversas. Finalmente se describe el proceso de deshidratación térmica de estos materiales., [EN] The present work is a literature review on the hydration process of calcium aluminate cements with high alumina contents with alumina and microsilica additions. The main hydrations products of these materials are described as function of temperature. Their reaction mechanisms, studied through different methods and techniques by several authors are discussed. The products of dehydration at high temperatures are also described.

[ES] El presente trabajo es una revisión sobre el proceso de hidratación de los cementos de aluminatos de calcio de alto contenido en alúmina. En el se describen los principales productos de hidratación de estos materiales en función de las relaciones de agua/sólidos y temperatura. Se exponen los mecanismos de reacción que han sido establecidos por distintos autores en la literatura y se describe la microestructura típica de estos materiales., [EN] The present work is a literature review on the hydration process of calcium aluminates cements with high alumina contents. The main hydration products of these materials are described as function of the water/solid ratio and temperature. Their reaction mechanisms, studied by several authors and also their typical microstructure are discussed.

Mono-phase bio-ceramics (alumina and zirconia) are widely used as femoral heads in total hip replacements (THR) as an alternative to metal devices. Unfortunately, the orthopaedic community reports significant in-vivo failures. Material scientists are already familiar with composites like alumina–zirconia. Since both are biocompatible, this could prove to be a new approach to implants. This paper deals with a new generation of alumina–zirconia nano-composites having a high resistance to crack propagation, and as a consequence may offer the option to improve lifetime and reliability of ceramic joint prostheses. The reliability of the above mentioned three bio-ceramics (alumina, zirconia and zirconia toughened alumina) for THR components is analysed based on the study of their slow crack-growth behaviour. The influence of the processing conditions on the microstructure development, of the zirconia toughened alumina composites and the effect of these microstructures, on its mechanical properties, are discussed.

A comparative study of slow crack propagation in alumina, zirconia and zirconia toughened alumina (ZTA) ceramics is presented keeping in mind the option that ZTA materials offer to improve the lifetime and reliability of ceramic joint prostheses. Different ZTA composites, using either unstabilized or yttria stabilized zirconia, have been tested. The materials were prepared in the range 0 - 15 ZrO2 vol. %, by two different processing methods: powder-mixing and a modified colloidal route. Crack propagation tests have been conducted for crack velocities from 10-12 to 10-2 m/s by means of double torsion method. The influence of the processing conditions on the microstructure development, and the effect of these microstructures on the slow crack-growth behaviour, have been studied.

[ES] La técnica de doble torsión tiene numerosas ventajas a la hora de evaluar el crecimiento subcritico de grieta en un material cerámico. Estas ventajas, entre otras, son: independencia del factor de intensidad de tensiones, KI, con respecto a la longitud de grieta, configuración de carga en cuatro puntos y facilidad de prefisuracion. Sin embargo, aun hoy en día, algunas preguntas sobre esta técnica quedan sin respuesta. En el presente trabajo se hace referencia a estas cuestiones, se revisan las expresiones analíticas de la flexibilidad y el factor de intensidad de tensiones, KI, propuestas en la literatura científica para la configuración particular de la doble torsión, y se examina el método de trabajo experimental, ensayo de relajación de carga, tratando de definir las condiciones en las cuales los datos obtenidos son validos., [EN] The double torsion test configuration has many advantages for the evaluation of subcritical crack growth parameters and fracture toughness. These advantages are: crack-length independence, four-point loading, simple specimen geometry and ease of precraking. However , a number of unresolved questions need to be answered about the double torsion specimen and experimental techniques involved in its use before this testing can be adopted as a standard. In this paper these questions are addressed, the analyses of the compliance and KI for the double torsion configuration are revised, and the load relaxation test in double torsion is examined in order to define the conditions under which de data are valid.