The nanopowder of hydroxyapatite (HAp) synthesized by means of precipitation demonstrates a very high specific area and a good ability to adsorb different ion from solution. The aim of the present work was to investigate properties and thermal behavior of HAp with NH₄NO₃ (by-product of HAp synthesis) and CaCl₂ and densification of nano-sized hydroxyapatite (HAp) powder compacts. Powders of HAp were fabricated via wet-precipitation technique from (NH₄)₂HPO₄ and of Ca(NO₃)₂ solutions at 60^oC with pH=9 and pH=7. Ratio of Ca/P was 1.67, 1.61 and 1.48. CaCl₂ was added to the powder of HAp in quantity of 10 % wt. The powders and ceramics were tested with XRD, FTIR, TGA, TEM, SEM/EDX and dilatometry (up to 1000^oC). DTA experiments showed that increasing of concentration of salt solution leads to higher adsorption of NH₄NO₃ by-product by HAp precipitate from the mother liquor. According to dilatomeric curves NH₄NO₃ can be considered as a densification additive for the HAp compacts since it undergoes melting in the interval of 150-250^oC. Maximum rate of linear shrinkage for the powder of HAp (Ca/P=1.67, pH=9) was located in the interval of 850-950^oC. Reduction of Ca/P value at pH=7 resulted in changing of composition of adsorption layer due to presence of extra phosphate-ion in solution. While heated, extra (PO₄)^3- can react with the main phase, by-product and with an additive affecting the sintering process. When adding CaCl₂, the maximum of the rate linear shrinkage was in interval of 500-700^oC.

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