The phase stability of mesoporous γ-alumina modified with phosphate species was investigated by comparing the effects of phosphoric acid (PA) and ammonium dihydrogen phosphate (ADP). Both treatments delayed the γ–θ–α phase transformation through the formation of aluminum phosphate; however, slight differences in the transformation and formation behaviors were observed. X-ray diffraction and solid-state nuclear magnetic resonance analyses reveal that PA induces more rapid formation and crystallization of aluminum phosphate than ADP, resulting in a more effective delay of the γ–θ–α phase transformation. Infrared spectroscopy suggests that a fraction of NH₄⁺ species in ADP is oriented toward the γ-alumina surface, thereby reducing the effective interaction of POH groups with alumina and suppressing the formation of aluminum phosphate compared with PA. These results demonstrate that not only the presence of phosphorus but also the chemical form and acidity of phosphate species influence surface reactions, diffusion behavior, and phase stability of mesoporous γ-alumina.

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For the synthesis of Fe(II)–substituted hydroxyapatite (HAp), iron needs to be introduced in anionic form. Conventionally, highly soluble iron chlorides or nitrates have been used as iron sources. However, the presence of chloride or nitrate ions during the reaction can result in the incorporation of these anions into the HAp lattice. Therefore, an iron compound that is free from such anions should be selected. In this study, FeO was investigated as an alternative iron source. Although FeO exhibits extremely low solubility and is generally unsuitable as an iron source, its dissolution was enhanced under hydrothermal conditions. Furthermore, the use of an aqueous formic acid solution facilitated the dissolution of FeO. Because formic acid decomposes upon heating to generate carbon monoxide, it provides a reducing environment. This reducing environment stabilizes iron in the Fe²⁺ state. As a result, Fe(II)–substituted HAp was successfully synthesized, and the chemical composition of the product was determined to be Ca_8.8Fe_0.6(PO₄)₆(OH)_0.8.

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The author studied on hydrothermal reactions from 1981 to 2019 at the Research Laboratory of Hydrothermal Chemistry, Kochi University, and one of his research topics was hydroxyapatite. During the research process, a few unique techniques were developed such as high temperature mixing method to prepare hydroxyapatite crystals with high crystallinity, anion exchange of apatite single crystals under alkaline hydrothermal conditions, and hydrothermal hot-pressing technique to prepare hydroxyapatite ceramics. This paper reviews research on hydroxyapatite conducted by the author and his research group.

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Porous β-tricalcium phosphate (β-TCP) ceramics were fabricated by physical foaming, followed by rapid freezing in liquid nitrogen and freeze-drying to preserve the transient bubble architecture prior to sintering. Eight nonionic surfactants with different molecular architectures (DECAGLYN 1-L, BT-7, BT-9, BT-12, TMGCO-7, TRIGLYN 1-KF, TL-10, and GO-440V) were screened under a fixed processing protocol with a constant volumetric surfactant dose (4.0 mL). All specimens retained spherical macro-sized pores after sintering, demonstrating high fidelity of foam-to-pore transfer. The mean pore diameter was tunable from 60 ± 22 μm to 117 ± 45 μm depending on the surfactant, while open porosity remained high (72.6-84.8%) and closed porosity was negligible (0.1-0.85%). Compressive strength followed the expected porosity-strength trade-off; specimens prepared with TMGCO-7, TRIGLYN 1-KF, and GO-440V exceeded 2 MPa. No monotonic correlation was found between pore diameter and HLB, suggesting that HLB alone was not sufficient to predict the resulting pore size in this multicomponent slurry. It was found that, particularly when using linear nonionic surfactants, surface tension influences foamability and foam stability, thereby enabling control of the pore diameter in the resulting porous material.

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Carbonate-containing hydroxyapatite (CHA) is considered a promising material for artificial bones. To obtain CHA porous ceramics with a high carbonate content, two CHA powders (CHA-0.6 and CHA-5.0) containing 0.6 and 5.0 mass% of carbonate, respectively, and having particle sizes of 0.1 and 1.5 µm, were sintered at various temperatures under air, CO₂, and steam atmospheres. Sintering was delayed under CO₂ and steam atmospheres. Both samples were affected by the type of atmosphere in a similar way, but the overall relative density was lower in CHA-5.0. Both samples retained their porous structure with a relatively high carbonate content of 2-3 mass% when sintered at 900-1000 °C under a CO₂ atmosphere. The carbonate content increased to 3-5 mass% when the combination of steam and CO₂ was used as atmospheres at 1000 °C, with the higher carbonate content observed in CHA-5.0. These results demonstrate that porous CHA ceramics with a relatively high carbonate content can be obtained by sintering CHA particles under controlled atmospheric conditions.

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