Amorphous calcium phosphate nano-powders made from radio frequency (RF) plasma spraying
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Amorphous calcium phosphate nano-powders made from radio frequency (RF) plasma spraying Xu Jinling, Rajendra Kumar, K.A. Khor and P. Cheang Advanced Materials Research Centre (AMRC), Nanyang Technological University, 50, Nanyang Avenue, Singapore 639798, Singapore ABSTRACT Predominantly amorphous nano powders of calcium phosphate were produced by radio frequency (RF) plasma and subsequently characterised by SEM, TEM and XRD. Quantitative phase analysis was carried out by the Rietveld method. The starting feedstock was comprised of hydroxyapatite (HA) powders spray dried from a suspension. The input parameters were varied to produce predominantly amorphous calcium phosphate powders. Results showed that as the plate power was increased and the flow rate of feedstock was decreased the as-sprayed powders produced were predominantly amorphous. The average particle size of the powders was about 20-40 nm, depending on process parameters, as confirmed by TEM observation. These powders are thought to have excellent bio-resorbability and are targetted for use in gene and drug delivery technologies. These nano-sized powders are also envisioned to enhance the mechanical properties of bulk products sintered by appropriate techniques. INTRODUCTION Radio frequency (RF) plasma experiments can be traced back to Babat [1] and Reed [2]. A RF plasma is typically formed by the inductive coupling of oscillating electrical energy. It displays unique characteristics, which can be used advantageously in thermal processing of materials. These include powder densification and spheroidisation, induction vacuum spraying (IPVS), plasma flash vapourisation and deposition (PFVD), synthesis of ultra fine powders and recently, suspension plasma spraying (SPS). Other applications include analytical methods such as elemental analysis and treatment of waste materials [3]. In SPS, a water-cooled probe atomises and delivers the suspension axially as fine droplets into a low intensity plasma. The droplets flash dry as they enter the plasma, melt and resolidify before collection by a series of filters. This process essentially combines several processing steps such as oven drying, calcination, spray drying, spheroidisation (flame or plasma) and sieving into a one step operation. The electrodeless RF plasma offers minimum contamination, large plasma volume, low gas velocity and axial injection of feedstock, which allows optimum treatment, and melting of large particles at high throughput [3]. In terms of temperature profile, the RF plasma is more homogenous and possesses less severe thermal gradients resulting in better material treatment, processing and control within the plasma. In addition, the process can be operated under a wide range of pressures in an inert, reducing or oxidising environment [3]. Resorbable bio-ceramics, as the name implies, degrade in the host after implantation. Examples of resorbable bio-ceramics include aluminium calcium phosphate, coralline, plaster
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