Microwave-assisted solid-state synthesis of hydroxyapatite nanorods at room temperature

Hydroxyapatite (HAP) is the main inorganic constituent of human hard tissues, such as bones and teeth [1]. HAP is usually used in various physical forms such as granules, rods and coating over metallic implants. For example, Bone itself is a composite with HAP nanorods embedded in the collagen matrix [2]. HAP is commonly employed as a coating for bionert metallic and ceramic substrates, such as titanium and alumina [3, 4]. Dense HAP ceramics can serve as bone substitution, which require their good mechanical properties. Therefore, especially required is the HAP powders which exhibit desired characteristics such as small grain size with narrow size distribution and high specific surface area. According to the previous literatures, a number of methods have been proposed for preparing HAP, such as wet chemical routes based on precipitation at low temperature [5, 6], hydrothermal methods [7, 8] and solid-state reactions [9] etc. The wet chemical methods usually lead to irregular forms of HAP powders and require complex process control. In recent years, the hydrothermal routes towards the synthesis of HAP nanorods have been reported [7, 8]. In spite of its much particular characters in high-temperature and high-pressure conditions, the hydrothermal synthesis route is relatively complex and reaction time is comparatively long. So some attentions have been switched to seek some new methods. The high-temperature solidstate method has been applied to the synthesis of HAP for the past several decades [9]. Due to its too much energy consumption, complex apparatus and techniques, the solid-state reaction becomes gradually unpopular and unsatisfied. At present, there are many compounds which have been synthesized by solid-state reaction at room temperature, such as oxide, sulfide, carbonate, hydroxide and so on [10–13]. However, the synthesis of HAP nanorods at room temperature is seldom reported until now. Recently, it is reported that HAP nanoparticles have been synthesized by microwavemediated metathesis in room temperature solid-state reaction [14]. Unfortunately, the products obtained only took the form of particles with large size and irregular diameter distribution. Microwave irradiation has particular advantages such as rapid volumetric heat-