Ceramics made from calcium hydroxyapatite synthesized from calcium acetate and potassium hydrophosphate

Abstract

The properties of hydroxyapatite powder synthesized from calcium acetate and potassium hydrophosphate and hydroxide were investigated. The associated reaction product (ARP), whose basic component is potassium acetate, modifies the surface of the hydroxyapatite nanocrystals. The components of the ARP can play the role of a salt matrix by preventing coalescence of nanocrystallites and can also serve as a source for a sintering additive which is uniformly distributed due to adsorption during synthesis. A ceramic based on hydroxyapatite with a uniform microstructure and a maximum grain size of 300 nm was obtained by incorporation of sintering additives in the form of ARP at a temperature of 700°C. The ARP is the cause of different solid-phase processes and processes involving the liquid phase. The sites where these processes occur are the boundaries of the grains, which also restrains their growth.Fabrication of ceramics based on hydroxyapatite (HAP) is one of the intensively evolving directions in medical materials science. Powders with high sintering activity must be used to fabricate high-quality ceramic materials. Such powders are usually synthesized by an existing chemical method. Precipitation of the powder from aqueous solutions of the corresponding components is the most common and simplest method of fabricating HAP. There are different methods of fabricating HAP in which phosphoric acid or soluble ammonium, potassium, and sodium phosphates and soluble potassium salts: nitrate, chloride, acetate, are used as the initial compounds (USSR Inventor's Certificate Nos. 710928 and 1450852; RF Patent Nos. 2038293 and 2050317).The powders obtained contain a significant amount of associated reaction products (ARP) which are adsorbed on the surface of the powder during synthesis [1,2]. After drying, the ARP can play the role of a rigid matrix that prevents aggregation of HAP particles less than 100 nm in size. During molding, the ARP present in the powder whose basic component is a soluble salt, for example, NH 4 NO 3 or NaCl [1, 2], can play the role of a plasticizer. In heat treatment, the NH 4 NO 3 salt will most probably not affect the occurrence of such a physicochemical process as sintering, since it decomposes before sintering begins. The compound NaCl has a relatively low melting point and due to the high volatility, can go out of the intermediate product during heat treatment above this temperature. In examining the reaction of synthesis of HAP from calcium acetate and potassium hydrophosphate and hydroxide, we can predict that potassium acetate will be one of the basic components of the associated product: 10Ca(CH 3 COO) 2 + 6K 2 HPO 4 + 8KOH ® Ca 10 (PO 4 ) 6 (OH) 2 + 20KCH 3 COO + 6H 2 O.(1)We also know that the acetate ion, which actively participates in ionic equilibria, decreases the concentration of free calcium ions in the solution due to formation of a CaCH 3 COO + ion pair. Additional effects are introduced by hydrolysis of the acetate ion, and the buffer system formed stabilizes the pH of the reactio...

Calcium Phosphate Powder Synthesized from Calcium Acetate and Ammonium Hydrophosphate for Bioceramics Application

“…Therefore, various stages of NO evolution were accompanied by the formation of H 2 O or CO 2 , and the overall mass loss was a result of several competing multistage processes involving both calcium phosphate species and ammonium acetate. It has been earlier revealed [18,20,23,24,26] that the reaction by-product consisting of carboxylic acids and their ammonium salts undergoes partial and then complete decomposition at temperatures below 800 • C. Therefore, the calcium phosphates powders containing carboxylic acids or their salts (ammonium, sodium [37], or potassium [38]) heat-treated at 400-800 • C were black-colored due to the formation of carbon or products of partial decomposition of organic substances present in the as-prepared samples.…”

Bioceramics Based on β-Calcium Pyrophosphate

“…Sodium phosphates [22,23], sodium carbonate [24], sodium/potassium nitrates [25], and calcium polyphosphate [9,26,27] were used as sintering additives for calcium pyrophosphate ceramic preparation. Potassium carbonate [28,29], potassium chloride [30], and calcium chloride [31] used as sintering additives for ceramic based on hydroxyapatite can also be used as sintering additives for calcium pyrophosphate ceramics. It should be noted, that in investigations [26,27], calcium polyphosphate was introduced in preceramic samples prepared as cement stone via excess of monocalcium phosphate monohydrate.…”

Calcium phosphate powders synthesized from solutions with [Ca2+]/[PO43−]=1 for bioresorbable ceramics

“…[13]). The role of by-products in sintering of ceramics based on these powders has been presented in [14][15][16]. At the same time, there is a lack of such information related to synthesis of the powders of calcium phosphates with Ca/P = 1 for the purpose of bioceramics fabrication.…”