Elsevier

Ceramics International

Volume 38, Issue 8, December 2012, Pages 6729-6738
Ceramics International

Synthesis of calcium phosphate-based composite nanopowders by mechanochemical process and subsequent thermal treatment

https://doi.org/10.1016/j.ceramint.2012.05.064Get rights and content

Abstract

One-step mechanochemical process followed by thermal treatment has been used to produce calcium phosphate-based composite nanopowders. Effects of milling and subsequent heat treatment on the phase transition as well as structural features were investigated. The products were characterized by powder X-ray diffraction (XRD), Fourier transform infrared (FT-IR) spectroscopy, scanning electron microscopy (SEM), energy dispersive X-ray spectroscopy (EDX), and transmission electron microscopy (TEM) techniques. The results revealed that the dominant phases after mechanical activation were hydroxyapatite, anatase (TiO2), and periclase (MgO), while after thermal annealing process at 700 °C hydroxyapatite along with geikielite (MgTiO3) and periclase (MgO) were the major phases. In addition, decomposition of hydroxyapatite to tricalcium phosphate (β-TCP) occurred after heat treatment in the range 900–1100 °C which resulted in the formation of tricalcium phosphate-based composite nanopowders. Evaluation of structural features of the samples calculated by X-ray diffraction profiles analysis indicated that the average crystallite size of hydroxyapatite after 10 h of milling and subsequent heat treatment at 700 °C were about 21 and 34 nm, respectively. TEM and SEM studies exhibited that the considerable morphological changes at temperatures ≥900 °C had to be ascribed not only to grain growth, but also for the transformation of hydroxyapatite to β-TCP.

Introduction

The recent trend in bioceramic research is predominantly focused on calcium phosphate-based materials, as they exhibit superior biological and mechanical properties over other materials [1], [2], [3]. Among different calcium phosphates, hydroxyapatite (HAp, Ca10(PO4)6(OH)2) and β-tricalcium phosphate (β-TCP, Ca3(PO4)2) have been considerably employed in the biomedical fields due to their biocompatibility and osteoconductivity [4]. From a thermodynamic standpoint, HAp is the most stable phase in physiological conditions and has the ability for direct chemical bonding to the bone, while β-TCP is found to be resorbable in vivo with new bone growth replacing the implanted β-TCP [5]. According to literature [6], β-TCP is an effectual implant in accelerating the formation of new bone owing to the solubility and biodegradation rate of β-TCP and are much higher than those of HAp.

However, these bioceramics have inherently some weaknesses such as poor toughness and low bending strength, poor corrosion resistance in an acid environment, and poor chemical stability at high-temperatures which have restricted wider applications in the fields of orthopedic and dentistry [7], [8], [9]. Therefore, improvements on structural features as well as mechanical properties of calcium phosphates have been attempted in a number of studies [10], [11], [12]. Theoretical and experimental investigations have shown that such properties of calcium phosphates can be remarkably strengthened by various methods, such as making nanocomposites [13], use of different sintering techniques [14], and adding dopants [11]. In the field of nanocomposites, an ideal reinforcing material for calcium phosphate-based composites has not yet been found. Nevertheless, different approaches have been extensively investigated in order to develop calcium phosphate-based composites, for instance HAp-Al2O3 [15], HAp-ZrO2 [16], HAp-TiO2 [17], poly(lactide-co-glycolide)/β-TCP [18], polyglycolic acid (PGA)/β-TCP [19], and TiO2/TCP [20] composites. These studies exhibited that interfacial reactions occurred during the high temperature processing of composites due to the large interfacial area available for the reactions. Interfacial reactions result in the formation of new phases, influence densification, mechanical properties and even degrade the biological properties of the composite in some cases which often limit their performance [15]. Hence, production and characterization of novel HAp/MgTiO3–MgO and β-TCP/MgTiO3–MgO composite nanopowders provided the main target for current research. It should be noted that despite a large number of studies on the synthesis of HAp and TCP composites [15], [16], [17], [18], [19], [20], no systematic investigations on the preparation of HAp/MgTiO3–MgO and β-TCP/MgTiO3–MgO are performed. Over the past decades, various synthesis methods of such biomaterials have been reported, including wet chemical methods [21], [22], hydrothermal processes [23], solid-state reaction [24], and the sol-gel method [25]. Among them, mechanochemical process has been extended for the production of a wide range of advanced materials [26], [27]. The prominent features of this technique are that melting is not essential and that the products have nanostructural characteristics [24].

In this paper, mechanosynthesis of calcium phosphate-based materials was investigated. In addition, effect of subsequent heat treatment on the phase transformation was evaluated. Since nanostrutured calcium phosphate composite nanopowders with appropriate stoichiometry, high purity and crystallinity enhance densification, osseointegrative, and bioactive properties [5], structural features (crystallite size, lattice strain, and crystallinity) as well as morphological characteristics (particle size and shape, particle distribution and agglomeration) of composite nanopowders were investigated by using XRD, FT-IR, SEM/EDX and TEM techniques.

Section snippets

Composite preparation

The starting reactant materials were anhydrous calcium hydrogen phosphate (CaHPO4, Merck, Germany), calcium oxide (CaO, Merck, Germany), titanium dioxide (TiO2, Merck, Germany), and magnesium (Mg, Merck, Germany). Synthesis of calcium phosphate-based composite nanopowders consists of (i) mechanical activation of powder mixture and (ii) subsequently thermal treatment. For this purpose, anhydrous calcium hydrogen phosphate and calcium oxide powder mixture with Ca/P ratio=1.67 mixed with distinct

Phase determination

Fig. 1 shows the XRD spectra of the samples after mechanical activation and subsequent thermal treatment at different temperatures. The pattern for sample I (Fig. 1a) shows well characterized peaks of pure HAp and the peaks were indexed according to the standard pattern (JCPDS 09–0432). In this sample, diffraction peaks broadened, particularly 2θ=31–35°, indicating that the sample demonstrated poor crystallinity (low fraction of crystalline phase). Therefore, the product of mechanochemical

Conclusions

Novel calcium phosphate-based composite nanopowders with appropriate structural features as well as morphological characteristics were synthesized via one-step mechanochemical process and subsequent thermal treatment. According to XRD data, thermal annealing of the milled powders in the range 700–1100 °C resulted in the formation of HAp- and β-TCP-based nanocomposites with different structural features. The calculated amounts of crystallite size indicated that increasing of the annealing

Acknowledgment

The authors are grateful to research affairs of Islamic Azad University, Najafabad Branch for supporting this research.

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