Papers by Keyword: Hydrothermal Processing

Abstract: The growing demand for functionality of implants is necessary to cater for the growing elderly population in need of repair for damaged or diseased tissues and organs. Radiofrequency magnetron sputtered crystalline hydroxyapatite thin films with preferred crystal orientation on polished titanium substrates were produced without additional substrate heating. No hydroxyl absorption bands were detected in hydroxyapatite thin films even after the addition of water vapour during the sputtering process or hydrothermal post-treatment of the films.

Abstract: Synthetic hydroxyapatite (HAp) has weak antibacterial and mechanical properties. The antibacterial activity of HAp can be enhanced by strontium cation substitution and incorporation of peroxide ion via hydrothermal processing at 100 °C and 150 °C temperature using 50% H₂O₂ solution. The starting reagents and products of HAp processing were analyzed by thermal conversion elemental analyser – isotopic ratio mass spectrometer in order to determine the δ¹⁸O values. Using different host materials it could be possible to determine differences of δ¹⁸O values between untreated and hydrothermally treated HAp samples. The enhanced antibacterial properties of processed HAp samples were investigated using Pseudomonas aeruginosa and Staphylococcus aureus. Hydrothermal processing by H₂O₂ solutions provides additional antibacterial activity and peroxide content of hydrothermally treated samples affect δ¹⁸O values.

Abstract: Hydroxyapatite (Ca₁₀(PO₄)₆(OH)₂) can be obtained from calcium carbonates through dissolution-precipitation reaction in a phosphate solution under a hydrothermal condition, with keeping its external shape. In this study, we assumed preparation of hydroxyapatite honeycombs from a calcite (CaCO₃) honeycomb. Calcite honeycomb was hydrothermally treated in a phosphate solution. After hydrothermal processing for 24 h, calcite transformed partially to hydroxyapatite phase and its external shape was kept. Moreover, specific surface areas of the specimens were increased after the hydrothermal processing. Consequently, this processing is useful to prepare honeycomb structure of hydroxyapatite from calcium carbonates.

Abstract: Peroxide ions in apatite provides an additional resource for imparting an antibacterial capability in apatite. A hydrothermal process has been developed for including peroxide ions into the apatite lattice. Three oxygen generation compounds, hydrogen peroxide, ammonium persulphate and paracetic acid were investigated for peroxyapatite generation. Hydrogen peroxide provides the highest peroxide containing apatite. Both the oxygen generation and the apatite lattice formation represented the two critical factors for producing peroxyapatite. Unlike with high temperature processing, the cooling rate did not influence the retained peroxide content. This new process provides a building block for investigating antibacterial properties of peroxyapatite in a low temperature process.

Abstract: This paper represents the study of hydrothermal processing of mica-titanium pearlescent pigment precursor. Mica-titanium pearlescent pigment precursor was prepared by boiling sulfuric acid hydrolysis method. Then the precursor was put into an autoclave and heated at 180°C～240°C for 4～12 hours. The results showed that the crystallinity of TiO₂ increased with longer hydrothermal time and higher hydrothermal temperature. The morphology of mica-titanium pearlescent pigments which was prepared by high-temperature calcination and hydrothermal method was also studied.

Abstract: In this work, hexagonal solid solutions of (Gd_0.95Eu_0.05)(OH)₃ with two distinctive morphologies of nanorods and nanotubes were successfully synthesized via hydrothermal treatment of mixed nitrate solutions in the presence of ammonium hydroxide. The hydroxide samples exhibited characteristic Eu³⁺ photoluminescence through the energy transfer from Gd³⁺ to Eu³⁺ and the self-excitation of Eu³⁺. The hydroxide precursors transformed into cubic (Gd_0.95Eu_0.05)₂O₃ at ~500 °C via an intermediate monoclinic (Gd_0.95Eu_0.05)OOH phase, and the Eu³⁺ coordination accordingly experienced symmetry changes from D_3h to C_2v, and then to C₂/S₆. The cubic (Gd_0.95Eu_0.05)₂O₃ well retained the original morphologies of their polycrystalline precursors and exhibited a single-crystalline character at 1000 °C. Greatly enhanced photoluminescence (~5000 times that of the hydroxides) was observed for the phase conversion.

Abstract: In this study, well-aligned single-crystalline zinc oxide (ZnO) nanorods arrays were synthesized on Si substrate by simple hydrothermal process in dilute aqueous solution which consisting of zinc nitrate tetrahydrate (Zn(NO3)2•4H2O) and hexamethyltetramine (C6H12N4, HMT) at 90 °C, in the autoclave. ZnO thin film was pre-deposited on Si substrate by the sputter deposition to serve as the seed layer. Aluminum nitrite (Al(NO3)3) also has been mixed into reaction solution. Al doped ZnO nanorod arrays were fabricated successfully. ZnO nanorods were characterized by high resolution X-ray diffraction, field emission scanning electron microscopy, transmission electron microscopy, energy dispersive x-ray spectroscopy, and photoluminescence measurement.

Abstract: Ba0.6Sr0.4TiO3 nanopowders (~ 80 nm in size) were prepared by low temperature hydrothermal process. The process and synthesis mechanism were discussed in detail. Furthermore, the powders were pressed into disc-shaped pellets and sintered at different temperatures. The frequency dependence of the dielectric constant and loss angle tangent of the sintered BST ceramics are measured from room temperature to 500°C and a diffuse dielectric anomaly by the dielectric relaxation was found.

Abstract: Many researchers have assumed that a combination of hydroxyl apatite (HAP) and collagen (COL) may be the best solution for bone replacement and have prepared their composites by several techniques [1]. However, such HAP/COL composite had no nanostructure similar to bone, and consequently indicated no bone-like mechanical properties. These results demonstrate that the chemical composition similar to bone only is insufficient for bone metabolism and mechanical properties. Mechanical and biological performance of this type of materials could be improved by adding TiO2 within the initial mixture of nanostructured composites [2]. Ternary nanostructured systems consisting of hydroxyl apatite, TiO2 aerogel and collagen were prepared for the first time by hydrothermal procedure in high pressure conditions. Among many advantages, the synthesis method proposed in this paper could lead to formation of chemically bonded compounds as a consequence of high pressure conditions. The resulted material could find applications in bone tissue regenerative medicine, either in powder form for bone defects treatment, or in matrix form as osteoconductive coating for metal implants. Further studies are necessary to evaluate the osteoconductive properties.

Abstract: Two-step sintering was performed to prepare hydroxyapatite (HAp) with a high density and a fine microstructure utilizing hydrothermally synthesized nano powders. Its effectiveness was dependent on starting chemicals for precursors of HAp, powder processing condition, initial temperature in the two-step sintering, heating rate to the initial temperature, and holding time at final temperature. HAp powder, hydrothermally treated after aging of precipitate of Ca(OH)2 and H3PO4 for 48 h, led to HAp having a high phase stability at elevated temperatures, a high density, and a fine microstructure after two-step sintering with heating rate of 10°C/min to 1275°C.