Preparation Of Different Sizes Of Porous Spherical Calcium Phosphate Ceramic Granules And Control Of Pore Structure

Calcium phosphate ceramic is a kind of materials which are mainly composed of phosphate. They are similar with human bone and teeth in inorganic composition elements and structure, which results in bone conductivity and excellent biological activity. This kind of materials is now widely used in hard tissue repair and as biological replacement materials. Calcium phosphate ceramic can be made into mass, powders and granules, in order to meet different requirements in application. Spherical granules, because of their high density when accumulated, good flow property and impossibility of reunion, have attracted more and more attention in recent years. In this dissertation, a method called droplet-freezing process is introduced to prepare spherical calcium phosphate ceramic granules. Also there is a further exploration about the preparation process, and the factors which affect the sizes and pore structure of granules.pH of SBF which immersing granules is in the range of 7～7.6, and granules can be basement for carbonated hydroxyapatite (CHA) to deposit. Addition of inorganic additives, especially sulfate can promote deposition of CHA.In order to reduce the sintering temperature of calcium phosphate ceramics, inorganic additives were added into the ceramic slurry. Soluble phosphate glass phase was formed below the sintering temperature, which promotes sintering. But inorganic additives can also reduce thermal stability of the materials, which makes load at fracture of granules low. Addition of starch as the organic additives can improve the suspension stability of ceramic slurry, which makes the ceramic powder suspended, and improves the uniformity of the granules’ composition and the stability of the preparation. Silicone, as the condensate with appropriate density and viscosity can make the slurry droplets solidified adequately. However phenyl ether and heptane have certain requirement of height, when used as condensate. Sintering process mainly depends on the thermal analysis curves. Preserve heat at certain temperature to ensure complete removal of adsorbed water and organic additives. Factors which influence sizes of spherical granules includes internal cause (viscosity) and external causes (diameter of dropping devices’ outlet). Diameter of dropping devices’ outlet plays a decisive role in the size of granules. Corresponding to a certain diameter of the outlet, there is a suitable viscosity of organic additive (adhesive), which makes preparation stable. Process will become infeasible when the viscosity of organic additive is too low or too high. Starch adhesive prepared by alkali paste method has the proper viscosity for preparation process. Viscosity of adhesive can be changed slightly by changing the temperature when preparing the adhesive in order to prepare spherical granules with smaller size (2～3mm). However, viscosity of adhesive can be increased largely by increasing the content of starch in order to prepare spherical granules with larger size (3～5mm).Pores of spherical granules which were prepared by droplet-freezing method mainly come from the space of solvent which was sublimated after crystallizing in the process of freeze drying. The pore (whose diameter is larger than 21.43μm) structure presents large equiaxed pores in the center region of granules, with small and irregular pores in the edge area. Through controlling the condensing temperature, pore structure with different porosity and mean pore size can be obtained. Granules with proper mean pore size and large porosity can be made by freezing at -25℃. Mean pore size of granules freezing at -15℃ and -20℃ can be reduced by sintered, while mean pore size of granules freezing at -25℃ and -30℃ can be increased. Adding carbon powder as pore forming agent will hinder the crystallization of solvent, which results reduction of pore density made by sublimation of solvent. However, burning of the pore forming agent will change the construct of pores of granules. Large mean pore size of granules can be made by adding 5% pore forming agent. Large porosity of granules can be made by adding 10% pore forming agent. However, mostly pores of granules are small pores (smaller than 21.43μm), and pore forming agent affects only structure, amount and porosity of big pores.