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An alternative route for preparing silicon-substituted hydroxyapatite powders Ca10(PO4)6−x(SiO4)x(OH)2−x (Si-HAp); using different content of SiO4 (0.588–0.48 M) in the aqueous solution of Si precursor was investigated. Tetramethylammonium silicate (TMAS) was used as a silicon precursor. The experiments were carried out via conventional hydrothermal treatments at 150 °C for 10 h. The Si-HAp nanoparticles obtained were characterized by X-ray diffraction (XRD), Fourier transform IR spectroscopy (FTIR), inductively coupled plasma AES (ICPAES) and scanning electron microscopy (SEM) techniques. A drastically decrease of the diffraction was not observed by increasing the Si content in the structure of the hydrothermally Si-Hap particles produced. A detailed structural analysis conducted by Rietveld refinement indicated that the incorporation of Si occurred in hydroxyapatite (HAp) lattice by partially replacing phosphate (PO43−) groups with silicate (SiO44−) groups. Raman and X-ray photoelectron spectroscopy (XPS) analysis confirmed the substitution of PO43− by SiO44− in the Si-HAp structure. The crystallite size decreases by increasing the Si molar content in the hexagonal HAp structure. TEM observations revealed that the Si-HAp produced have a high crystallinity. A slight decrease of the particle size of the Si-HAp compared to pure HAp was observed (from 60–100 nm to 40–70 nm). The morphological analysis revealed that all the SiHAp powders prepared have the rod-like shape. Additionally, a slight decrease in both length and diameter of nanorods was observed by increasing the silicon content, the minimum size obtained for the powders containing the largest amount of Si (20Si-HAp) 25 and 70 nm. The presence of Si in the structure of the Si-HAp favored densification of the compacts treated at high temperature 1200 °C in comparison with pure HAp. Resumen: Una ruta alternativa de síntesis de polvos de hidroxiapatita (HAp) sustituida con silicio Ca10(PO4)6−x(SiO4)x(OH)2−x, (Si-HAp) utilizando diferentes concentraciones molares de SiO4 (0.588-0.48 M) en la solución precursora de silicio (Si). Se utilizó solución de silicato de tetrametilamonio (TMAS, por sus siglas en inglés). Los experimentos se realizaron mediante tratamiento hidrotérmico convencional a 150 °C y 10 h. Las nano partículas de Si-HAp obtenidas se caracterizaron por difracción de rayos X (XRD), espectroscopia de IR con transformadas de Fourier (FT-IR), análisis por espectrometría de plasma acoplado inductivamente (ICP-AES) y microscopía electrónica de barrido (SEM). El incremento del contenido de Si no cambió drásticamente la intensidad de los picos de difracción. El análisis estructural por refinamiento Rietveld indicó que la incorporación de Si efectivamente ocurrió en la red de HAp reemplazando parcialmente los grupos fosfato (PO43−) con grupos de silicato (SiO44−). Análisis por espectroscopia Raman y (XPS) confirman la sustitución de PO43− por SiO44− en la estructura Si-HAp. El tamaño del cristalito disminuye con el aumento del contenido de Si en la estructura hexagonal de HAp. De las observaciones de TEM, confirman la alta cristalinidad del polvo producido. Se observó una ligera disminución del tamaño de partícula de Si-HAp en comparación con la HAp pura (de 60-100 a 40-70 nm). La morfología de las muestras Si-HAp reveló la obtención nanopartículas de tipo varilla. Se observó una ligera disminución en la longitud y diámetro de los nano varillas con el aumento del contenido de Si, con un diámetro de 25-45 nm y una longitud de 70-90 nm para el 20SiHAp. La presencia de Si en la estructura de la Si-HAp favoreció la densificación de los compactos, particularmente para los obtenidos a altas temperatura de 1200 °C en comparación con la HAp pura. Keywords: Hydrothermal treatment, Silicon hydroxyapatite, Nanopowders, Palabras clave: Tratamiento hidrotérmico, Si-hidroxiapatita, Nanopartículas

The synthesis of partially substituted silicon hydroxyapatite (Si-HAp) nanopowders was systematically investigated via the microwave-assisted hydrothermal process. The experiments were conducted at 150 °C for 1 h using TMAS (C4H13NO5Si2) as a Si4+ precursor. To improve the Si4+ uptake in the hexagonal structure, the Si precursor was supplied above the stoichiometric molar ratio (0.2 M). The concentration of the TMAS aqueous solutions used varied between 0.3 and 1.8 M, corresponding to saturation levels of 1.5–9.0-fold. Rietveld refinement analyses indicated that Si incorporation occurred in the HAp lattice by replacing phosphate groups (PO43−) with the silicate (SiO4−) group. FT-IR and XPS analyses also confirmed the gradual uptake of SiO4− units in the HAp, as the saturation of Si4+ reached 1.8 M. TEM observations confirmed that Si-HAp agglomerates had a high crystallinity and are constituted by tiny rod-shaped particles with single-crystal habit. Furthermore, a reduction in the particle growth process took place by increasing the Si4+ excess content up to 1.8 M, and the excess of Si4+ triggered the fine rod-shaped particles self-assembly to form agglomerates. The agglomerate size that occurred with intermediate (0.99 mol%) and large (12.16 mol%) Si contents varied between 233.1 and 315.1 nm, respectively. The excess of Si in the hydrothermal medium might trigger the formation of the Si-HAp agglomerates prepared under fast kinetic reaction conditions assisted by the microwave heating. Consequently, the use of microwave heating-assisted hydrothermal conditions has delivered high processing efficiency to crystallize Si-HAp with a broad content of Si4+.

Resumen: Compactados de hidroxiapatita estequiométrica (HAp) y solución sólida de magnesio hidroxiapatita (Mg-HAp) fueron preparados mediante la técnica de compactación hidrotérmica en caliente (CHC), a diferentes temperaturas (150, 175 y 200 °C), utilizando una presión de carga uniaxial de 60 MPa durante 6 h, con la adición del 10% en peso de agua desionizada. El material de partida fue obtenido mediante síntesis hidrotérmica a 150 °C durante 10 h. Los productos compactados fueron caracterizados química, física y mecánicamente. De acuerdo con el análisis de resultados, se determinó que un incremento en la temperatura de CHC favoreció la reacción entre las partículas de HAp o Mg-HAp y el agua promoviendo fases con mayor grado de cristalinidad, sin generar subproductos o alteraciones estructurales. Este resultado es particularmente relevante, ya que durante la CHC se produce un empaquetamiento efectivo de las partículas, activado por el mecanismo de disolución-recristalización en presencia del agua, con lo cual se logró una densificación del orden del 70%. Los compactados de HAp y Mg-HAp obtenidos a 200 °C, con porosidad abierta del orden del 30-36%, mostraron valores de máxima densidad aparente de 3,085 ± 0,138 g/cm3 y 2,792 ± 0,021 g/cm3, respectivamente, con una máxima resistencia a la tensión de 7,10 ± 0,20 y 5,89 ± 0,06 MPa. De acuerdo a estos resultados, el material obtenido podría ser utilizado en aplicaciones ortopédicas y/u odontológicas. Abstract: Preparation of hydrothermal hot-pressed (HHP) compacts of the stoichiometric hydroxyapatite and solid-solution of magnesium hydroxyapatite powders (HAp and Mg-HAp) was carry out at different temperatures 150, 175 and 200 °C, using uniaxial pressure load of 60 MPa for 6 h, with the addition of 10% wt. of deionized water as a solvent. The starting powders were obtained by hydrothermal synthesis at 150 °C for 10 h. The HHP compacts were chemical, physical and mechanically characterized. According to the results, it has been determined that an increase of the HHP temperature stimulate the reaction between the added water and the particles of HAp or Mg-HAp, which results on an increase of crystallinity degree without the formation of any addition products or alteration of the HAp structure. This result is particularly relevant, due to during HHP process, the production of effective packing of the particles which is activate by the dissolution-recrystallization mechanism in presence of added water which allow a densification near of 70%. For the HHP compacts of the HAp and Mg-HAp prepared at 200 °C, with porosity near to 30-36% of open pores, achieve the maximum apparent density value of 3.085 ± 0.138 g/cm3 and 2.792 ± 0.021 g/cm3, respectively, with the maximum tensile strength of 7.10 ± 0.20 and 5.89 ± 0.06 MPa. From these results, the obtained HHP compacts might be useful for orthopedics and/or dentist applications. Palabras clave: Hidroxiapatita, Compactación hidrotérmica en caliente, Keywords: Hydroxyapatite, Hydrothermal hot-pressing