Synthesis of nano-sized hydroxyapatite powders through solution combustion route under different reaction conditions

Abstract

Calcium hydroxyapatite, Ca₁₀(PO₄)₆(OH)₂ (HAp) was synthesized by combustion in the aqueous system containing calcium nitrate–diammonium hydrogen orthophosphate with urea and glycine as fuels. These ceramics are important materials for biomedical applications. Thermo-gravimetric and differential thermal analysis were employed to understand the nature of synthesis process during combustion. Effects of different process parameters namely, nature of fuel (urea and glycine), fuel to oxidizer ratio (0.6–4.0) and initial furnace temperature (300–700 °C) on the combustion behavior as well as physical properties of as-formed powders were investigated. A series of combustion reactions were carried out to optimize the reaction parameters for synthesis of nano-sized HAp powders. The combustion temperature (T_f) for the oxidant and fuels were calculated to be 896 °C and 1035 °C for the stoichiometric system of urea and glycine respectively. The stoichiometric glycine–calcium nitrate produced higher flame temperature (both calculated and measured) and powder with lower specific surface area (8.75 m²/g) compared to the stoichiometric urea–calcium nitrate system (10.50 m²/g). Fuel excess combustion in both glycine and urea produced powders with higher surface area. Nanocrystalline HAp powder could be synthesized in situ with a large span of fuel to oxidizer ratio (φ) in case of urea system (0.8 < φ < 4) and (0.6 < φ < 1.5) for the glycine system. Calcium hydroxyapatite particles having diameters ranging between 20 nm and 120 nm could be successfully synthesized through optimized process variable.

Introduction

Calcium phosphate based bioceramics have proven to be attractive materials for biomedical applications [1]. Among these bioceramics, particular attention has been given to hydroxyapatite (HAp) with an ideal chemical formula being Ca₁₀(PO₄)₆(OH)₂ for its excellent biocompatibility, bone bonding ability, structural and compositional similarity to that of the mineralized matrix of human bone, and beta-tri-calcium phosphate (β-TCP) for its bioresorbability [2], [3]. These materials are extensively used in the form of porous granules, sintered and porous blocks, and powders for different surgical applications in the field of dentistry and orthopaedics. Though HAp is an excellent biomaterial, its use under load bearing applications such as artificial joints have been restricted for its inherent low toughness (0.8–1.2 MPa-m^1/2) and flexural strength (<140 MPa) [4]. Several researchers have shown that the mechanical properties of fabricated hydroxyapatite could be significantly enhanced by controlling the important parameters like particle size, shape, its distribution and agglomeration [5].

Various routes to synthesize HAp powders have been developed, most common of which is the wet chemical precipitation [6]. Others include hydrothermal reaction [7], mechanochemical–hydrothermal synthesis [8] and sol–gel synthesis [9], [10]. However, these methods have several disadvantages including difficulties to maintain the pH value above 9 during the initial solution, formation of calcium deficient HAp that on further heat treatment easily decomposes to β-TCP.

Solution combustion method is widely used to synthesize various nanocrystalline oxide ceramic powders and the process is thoroughly reviewed by various authors [11], [12], [13], [14], [15]. The solution combustion is a versatile method that involves a very rapid exothermic and self-sustaining chemical reaction between an oxidant (preferably nitrates of the desired metal cations) and a suitable organic fuel, (such as urea, glycine, carbohydrazide, etc.) in an aqueous solution [16], [17]. Exothermicity of the reaction supplies the heat required to sustain the combustion and once initiated, no external heat source is needed. The reaction is initiated at a fairly low temperature (viz., ∼300 °C) and rapid temperature rise during progress of combustion and subsequent fast cooling ensure nucleation of a very large number of crystallites and prevent particle growth. The resulting product is soft agglomerates of very fine particulates that can easily be dispersed to fine powders. Also, the high combustion temperature helps elimination of volatile impurities. The key features of this method are its ability to quickly produce desired phases with high purity, better homogeneity and ultra fine powders in a single step.

The basis of combustion synthesis technique comes from the thermo-chemical concepts used in the field of propellants and explosives chemistry. In this field, calculation of effective constituents of a fuel to oxidizer mixture is of paramount importance. According to this concept, the gaseous products of combustion reaction are CO₂, H₂O and N₂. Jain et al. [18] developed a simple method of calculating the oxidizing and reducing character of the mixture. The maximum heat is released when the fuel to oxidizer ratio is stoichiometric. Stoichiometric composition of fuel to oxidizer is calculated using the total oxidizing and reducing valences of the reactants, which serve as numerical coefficient for stoichiometric balance. It is expressed as follows:

$$\phi =\frac{\text{sum of all oxidizing and reducing valences in fuel}}{\text{sum of all oxidizing and reducing valences in oxidizers}}$$

The highest temperature generated in a combustion reaction is called the ‘flame temperature (T_f) that confers the most significant effect on the powder characteristics. Besides, other process parameters including nature of fuel, fuel to oxidizer ratio (φ) and the initial furnace temperature that may control flame temperature directly or indirectly influence and hence the resulting powder properties. Another important feature in this system is the evolution of large volumes of gases in combustion effectively cool the product and consequently prevents particles overgrowth.

Varma et al. [19] utilized a polymeric combustion method via a solution of calcium nitrate and ethyl phosphate to synthesize calcium phosphate powders. A modified self-propagating combustion synthesis in the calcium nitrate–diammonium hydrogen orthophosphate (DAP)–organic fuel was utilized by Tas [20] and Ghosh et al. [21], [22], [23] who successfully synthesized single phase HAp as well as HAp/β-TCP composite powder through aqueous solution combustion method. In the above method, the main combustion reaction was provided by calcium nitrate–fuel that subsequently consumes DAP to form HAp. Han et al. [24] and Yuan et al. [25] have successfully synthesized nanocrystalline HAp powder and nano tubes through sol–gel auto combustion technique.

In the present investigation, synthesis of hydroxyapatite ceramic powders by the modified aqueous solution combustion process is presented. A few important combustion parameters like nature of fuel, fuel to oxidizer ratio and initial furnace temperature have been considered to study their influence on the combustion behavior and consequently on powder characteristics such as crystallite size, specific surface area, morphology and particle size distribution. Maximum flame temperatures were calculated theoretically for the combustion reactions involving the fuels, urea and glycine, and also for different φ ratios. The progress of combustion reactions of calcium nitrate–diammonium hydrogen orthophosphate (DAP)–urea and glycine system have been followed through differential thermal analysis (DTA) and thermo-gravimetric analysis (TGA). Possible mechanisms of the reactions in these systems are indicated.