Calcium diphosphates [Ca₅K₄H₂ (P₂O₇) ₄·8H₂O, Ca₃KH (P₂O₇) ₂·4H₂O, Ca₅K₆ (P₂O₇) ₄·12H₂O, and Ca₂P₂O₇-4H₂O] were made by mixing aqueous solutions of tetrapotassium diphosphate and calcium chloride. The first and third diphosphates were new compounds. When the diphosphates were heated up to 1000°C, the thermal reac-tions were observed :
(1) dehydration
M₄P₂O₇·nH₂O→M₄P₂O₇· (n-x) H₂O+xH₂O (0<x≤n)
(2) degradation
M₄P₂O₇+H₂O→2M₂HPO₄
(3) condensation
M₂O₃PO [P (O₂M) O] nPO₃HM+M₂HPO₄
→M₂O₃PO [P (O₂M) O] _n+1PO₃M₂+H₂O (n=0, 1, 2...)
(4) reorganization ordegradation of polyphosphates to diphosphate (for only Ca₂P₂O₇-41-120 above 600°C)
M₂O₃PO [P (O₂M) O] nPO₃M₂ + nM₃PO₄→ (n+1) M₄P₂O₇
where M stands for 1/2Ca, K, and/or H.

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As a series of studies on the dissolution of sparingly soluble inorganic compound with a strongly acidic ion exchange resin (H-R), alkaline earth carbonate (MCO₃) compounds were chosen by considering from a viewpoint of bicarbonate formation as an intermediate. Experiments were carried out by batchwise addition of H-R and by continuous feeding of H-R suspension into an aqueous suspension containing MCO₃ powder such as 4MgCO₃·Mg (OH) ₂·4H₂O (BMC), CaCO₃, SrCO₃, BaCO₃ and those binary mixtures.
The pH, specific conductivity (k : mS/cm) and dissolved amount of M²⁺ ion against H-R (g) added were measured, and the results obtained were analyzed for the dissolution mechanism.
The results obtained were summerized as follows;
1) In an initial stage of H-R addition, increases in M²⁺ concentration and k value were observed. Linear relationship between [M²⁺] (mmol/l) and k, the correlation between the slop and equivalent ionic conductivity of M²⁺ ion, and pH dependence on the stability of HCO₃- ion indicate the formation of bicarbonate compound, M (HCO₃) ₂.
2MCO₃ + 2 (H-R) →M (HCO₃) ₂ + M-R2
2) As more is added, secondary ion exchange reaction between M²⁺ and H-R leads to decrease in [M²⁺] and k with evolution of CO₂ gas.
M (HCO₃) ₂ + 2 (H-R) →M-R2 + 2CO₂ + 2H₂O
3) In general, the reactivity to produce the bicarbonate is proportional to the solubility product, i.e. Mg (HCO₃) ₂>> Ba (HCO₃) ₂ > Ca (HCO₃) ₂> Sr (HCO₃) ₂; the result will be applied to a separation technique.

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The behavior of the β-α transformation in Ca₂P2C₇ (C₂P) was investigated by DTA and isothermal methods. The transformation temperature (Tt) was determined to be 1165±15°C. The β→α transformation began to progress acceleratively above a superheating of 10-20°C over the Tt with an endothermic DTA change. The β→α isotherms were expressed by the first-order rate equation. Apparent activation energies obtained for the β→α by the two methods were extremely high. The rate of the α→β transformation reached a maximum at 1025°C, however the rate was very slow. So, the α→β was confirmed to be easily hindered even by slow cooling. It was suggested that such a high activation energy and slow transformation rate might be attributed to the difficulty of nucleation or movement relating to large P2C₇^4- groups in C₂P.

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The sinterability of hydroxyapatite (HAp) powder, prepared by the spray-pyrolysis technique, was evaluated with increasing temperature from room temperature up to 1200°C (heating rate : 10°C·min^-1) and at fixed temperatures of 1050°C, 1100°C, 1150°C and 1200°C (heating time : 1 to 5 h). The relative density (bulk density/theoretical density) and the state of texture (grain shape, grain-size distribution and average grain size) of the sintered HAp compact were examined. The densification of HAp compact started at around 750°C and was accelerated with increasing temperature up to 1200°C. The densification behavior of HAp compact could be divided into three regions, i.e. (i) 750°C to 950°C, (ii) 950°C to 1100°C, and (iii) 1100°C to 1200°C. In region (i), 750°C to 950°C, HAp particles agglomerated, because of the mass transfer due to the sintering. In region (ii), 950°C to 1100°C, the residual pores in HAp compact were eliminated by the sintering of these agglomerates themselves. In region 1100°C to 1200°C, rapid grain growth (0.34μm (1100°C) -0.75 μm (1200°C)) occurred. The relative densities of HAp compacts sintered at 1100°C, 1150°C and 1200°C for various times (1 to 5 h) were about 95%.

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