Environmentally Friendly Growth of Calcium Chlorapatite Whiskers from a Sodium Chloride Flux

Environmentally friendly, high-quality, and transparent-colorless calcium chlorapatite (Ca 5 Cl(PO 4 ) 3 ) whiskers were selectively fabricated by rapid cooling of a sodium chloride flux. The crystal growth was conducted by heating a mixture of solute ((NH 4 ) 2 HPO 4, CaCl 2, and CaCO 3 ) and flux (NaCl) at 1100 °C, holding at this temperature for 10 h, and then cooling it at a rate of 5−200 °C h -1 to 500 °C. The grown crystals were divided into two morphological types, that is, needle and prismatic shapes. The Ca 5 Cl(PO 4 ) 3 whiskers obtained average lengths up to 4.2 mm and widths up to 130 μm. Their form was a hexagonal {101̄0} cylinder with pyramidal {101̄1} end faces. The crystal forms and average sizes obviously depended on the cooling rate of the high-temperature solution. The formation ratio of the whiskers was increased with cooling rate, reaching much higher than 80 wt % at 200 °C h -1 in all growth runs. It is found that Ca 5 Cl(PO 4 ) 3 whiskers were predominantly grown by controlling the cooling rate. Finally, NaCl was found to be a very suitable flux for an environmentally friendly growth of Ca 5 Cl(PO 4 ) 3 whiskers.