Crystalline hydroxyapatite (HAp) powders were prepared at room temperature from heterogeneous reaction between Ca(OH)2powders and (NH4)2HPO4 solutions via the mechanochemical-hydrothermal route. X-ray diffraction, infrared spectroscopy, thermogravimetric characterization, and chemical analysis were performed, and it was determined that the room temperature products were phase-pure, thermally stable HAp with a nearly stoichiometric composition. Dynamic light scattering revealed that the dispersed particle size distribution of the room temperature HAp powders was in the range of 0.15–3.0 μm with a specific surface area of ≈90 m2/g. Both specific surface area measurements and scanning electron microscopy confirmed that the HAp powders consisted of agglomerates containing hundreds of ≈20 nm HAp crystals.

The term “materials cycle” is generally used to describe the synthesis of substances from raw materials (sometimes including the synthesis of manufactured raw materials), the fabrication of shaped materials, their use, and their eventual disposal. It is well known that all materials are extracted from the earth, then are converted to functional products through various fabrication processes, usually involving a high expenditure of energy, which in turn contributes to environmental problems such as global warming.

Stability of fullerenes C60 under hydrothermal conditions (200–800 °C, 100 MPa, 20 min–168 h) has been investigated. The reaction products have been characterized by Raman spectroscopy and x-ray diffraction. The fullerenes were stable up to 500 °C, but they decomposed immediately at 800 ±C into amorphous carbon. In the transition region between 600 and 750 °C, longer times and higher temperatures of the hydrothermal treatment favored decomposition of C60 with the formation of amorphous carbon. Addition of nickel to the C60–H2OO system neither suppressed hydrothermal decomposition of C60 nor induced formation of other phases, except of the amorphous carbon.

BaTiO3, SrTiO3, and BaxSr1−x TiO3 thin films, as well as multilayers in the SrTiO3-BaTiO3 system, have been prepared on Ti substrates in newly constructed flow-system equipment by the hydrothermal-electrochemical method. The synthesis parameters (temperature of 120–200 °C, flow rate of 1–50 cm3/min) allow fabrication of dense, single-phase films with different morphology by controlling nucleation and/or growth rates. The flow system enables also an easy fabrication of SrTiO3/BaTiO3 and BaTiO3/SrTiO3 multilayers with variable chemical composition and microstructure across the film thickness. The multilayers can be prepared in only one experiment by simply changing the kind of flowing solution and/or adjusting the processing conditions.

This paper reviews the past, present, and future of the hydroxyapatite (HAp)-based biomaterials from the point of view of preparation of hard tissue replacement implants. Properties of the hard tissues are also described. The mechanical reliability of the pure HAp ceramics is low, therefore it cannot be used as artificial teeth or bones. For these reasons, various HAp-based composites have been fabricated, but only the HAp-coated titanium alloys have found wide application. Among the others, the microstructurally controlled HAp ceramics such as fibers/whiskers-reinforced HAp, fibrous HAp-reinforced polymers, or biomimetically fabricated HAp/collagen composites seem to be the most suitable ceramic materials for the future hard tissue replacement implants.

Hydroxyapatite whiskers have been prepared by the hydrothermal method. The crystals had diameter, length, and aspect ratio in the range of 1–10 μm, 30–50 μm, and 5–20, respectively. Their Ca/P molar ratio varied from 1.59 to 1.62. The morphology of the crystals can easily be controlled by the concentrations of species in the starting solution, while the Ca/P ratio is almost independent of them. Through the reaction with calcite powder at 600 °C, the Ca/P ratio of the whiskers has been improved even to the stoichiometric value of 1.67. Taking into account morphology and chemical composition of the HAp whiskers, they should not be health hazardous and may find applications as substitutes for asbestos and other fibrous materials which presently have restricted use because of their carcinogenic natures.