Influence of the Ca/P ratio and cooling rate on the allotropic α↔β-tricalcium phosphate phase transformations

Abstract Alpha-tricalcium phosphate (α-TCP) is a component of particular interest in the formulation of apatitic calcium phosphate based bone cements. During the setting reaction of this type of cements, a calcium deficient hydroxyapatite with composition similar to bone hydroxyapatite is formed. But the purity levels of the commercially available products based on α-TCP are generally low, and their prices are prohibitive. Such stringent conditions are pushing most of the researchers in the area of bone cements towards preparing α-TCP in their own laboratories. The low degree of purity of α-TCP based products is generally due to an incomplete β-TCP → α-TCP phase transformation and/or to a partial α-TCP → β-TCP reversal during cooling. Despite extensive research efforts, contradictory reports exist about the effects of composition of the starting material, cooling rates and the importance of quenching for maintaining α-TCP purity. The present work aims at shedding further light on this issue by controlling the relevant parameters of wet synthesis of the starting β-TCP powders and their heat treatment schedule in order to transform them in high purity α-TCP products. The key experimental variables investigated included the composition of the starting β-TCP powders, namely the Ca/P ratio, the maximum heat treatment temperatures, and the cooling rates.