Article pubs.acs.org/crystal
Controlling the Crystallization of Fluorapatite in Apatite-Mullite Glass-Ceramics Kevin P. O’Flynn and Kenneth T. Stanton* School of Mechanical and Materials Engineering, University College Dublin, Belfield, Dublin 4, Ireland ABSTRACT: Recent studies have shown that by controlling material topography on the nanoscale, it may be possible to elicit a more favorable cell response. It has been reported that apatite-mullite glass-ceramics (AMGCs) can form nanostructures of accurately controlled, self-assembled arrays of apatite nanocrystals, and it is possible that AMGCs may be used to produce bioactive nanoscale topographies. This paper systematically investigates methods for producing these structures. Controlling the crystallization of fluorapatite in apatite-mullite glass-ceramics is possible through the variation in base glass composition and heat treatment regime. Results show that increased amounts of network modifier result in a decreased fluorapatite crystal size. The reduction in crystal size is caused by growth limiting factors such as impingement and the degree of phase separation prior to crystallization. It is also demonstrated that high heating rates produce a more coarsened crystal microstructure. The Scherrer equation is used to verify the increase in the average crystal size from X-ray diffraction data. The heating rate does not affect the unit cell dimensions indicating that the level of substitutions in the unit cell is not increased by higher heating rates. The results can be used to control the resultant crystal morphology to produce microstructures of varying degrees of disorder.
■
INTRODUCTION Bone regeneration is an important aspect of orthopedic clinical treatment. The use of bioactive materials as synthetic bone grafts is attractive as they do not have the same disease transmission potential as allografts, can be synthesized in large quantities unlike autografts, and may have mechanical and biological properties tailorable to specific applications. Bioactive glass-ceramics are such materials suitable for enhancing bone regeneration. They are castable as a glass to accurately produce complex shapes with little or no residual porosity unlike conventional ceramics.1 When devitrified, they are bioactive, proven to be osseoconductive,2 and have high strength and fracture toughness.1,3 There are three main bioactive glass-ceramic systems: apatite-wollastonite developed by Kokubo et al.,4 mica-based developed by Grossman5 and Beall et al.,6 and apatite-mullite.3,7 Apatite-mullite glass-ceramics (AMGCs) are being considered for use in bone grafting,2 osseoconductive coatings,8,9 and dental implants.10 They can be crystallized in monolithic form to form fluorapatite (FAp, Ca 10(PO4) 6F2) and mullite (Al6Si2O13).3 FAp is a chemical analogue of hydroxyapatite (HA, Ca10(PO4)6(OH)2) which is the basis of the natural mineral found in bone, although FAp is more stable in vivo,11 making it more suitable for long-term use as a bone graft. Mullite is a bioinert ceramic phase with excellent mechanical properties.10 It has been shown that the fluorapatite does not form a simple acicular structure but rather crystallizes spherulitcally,12 which can have a significant influence on the fracture properties of the material.13,14 However, there are a number of variables that affect the resultant crystal morphology including the presence of network modifying elements in the base glass, heat treatment ramp rate, time held at the optimum © 2012 American Chemical Society
nucleation temperature, and the crystallization time and hold temperature. This study assesses the effect of these parameters to ascertain how different variables can be optimized to control crystal morphology. A complete understanding of these different heat-treatment parameters on the devitrification of AMGCs is important in developing optimal coatings for orthopedic metal implants. The formation of apatite crystals has been observed in a number of glass-ceramic systems. It has been proposed by Höland et al. that fluorapatite forms in apatite-leucite glassceramics as a result of homogeneous crystallization from a CaO-P2O5 rich glass phase after glass-in-glass phase separation.15 In the study, they show needle-like fluorapatite can be formed but only after holding the material at 700 °C for 8 h and again at 1050 °C for 2 h. A study by Hoche et al. also formed needle-like apatite crystals in the SiO2−Al2O3−CaO− P2O5−K2O−F− system after 15 h at 1200 °C.16 The advantage of apatite-mullite glass-ceramics is they can be crystallized to form apatite needles without the need for such high temperatures and long hold times3,7,10,17,18 and therefore present a significant processing advantage where FAp is the desired bioactive phase. Considering glasses as inorganic polymers allows for the application of a number of concepts when examining glass behavior, in particular, that of network connectivity, which is defined as “the average number of network bonds that link each repeat unit in the network”.19 This idea may then be developed to define the cross-link density of the glass which is the average Received: September 14, 2011 Revised: January 26, 2012 Published: February 21, 2012 1218
dx.doi.org/10.1021/cg201208c | Cryst. Growth Des. 2012, 12, 1218−1226
Crystal Growth & Design
Article
h before the furnace was turned off and the samples were allowed to cool in the furnace. Samples were sectioned using a Buehler Isomet 2000 PrecisionSaw into 2 mm thick disks using a diamond blade. Differential Scanning Calorimetry (DSC). Thermal analysis on the glasses was performed using a Rheometric Scientific STA 1600 (Surrey, UK) with an error of ±1 °C. Samples were placed in matched platinum−rhodium crucibles in a flowing dry nitrogen atmosphere. During heating, silicon tetrafluoride could potentially volatilize from the surface causing anorthite (CaAl2Si2O8) to form instead of FAp. To minimize unwanted surface effects, glass frit was used for all experiments giving a lower surface area per unit volume and the glass composition had previously been optimized to avoid anorthite formation.3 The optimum nucleation temperature (ONT) is defined as the temperature at which the largest number of stable nuclei form per volume element. The relative number of stable nuclei that form during a nucleation hold can be inferred from the crystallization temperature at the peak of the relevant exotherm, for example, Tp1: a greater number of nuclei reduce the barrier to crystallization and lower Tp1. Therefore, at the ONT, Tp1 will be a minimum. In practice, following the method of Marrota et al.,23 the ONT is determined by heating the glass to various temperatures between the start of the glass transition, Tg,onset, and Tp1, then holding at that temperature for 1 h before heating the sample to full crystallization. The hold temperature which produces a minimum Tp1 is the ONT. In all cases for ONT determination, a heating rate of 10 °C·min−1 was used. X-ray Diffraction (XRD). To examine the phases present, powder XRD was performed. Prior to XRD, samples of glass frit were crystallized by heating in the DSC furnace to ensure accurate temperature and heating rate control: this also ensures that XRD measurements closely match the thermal events observed in the DSC traces. Once the sample has reached the desired temperature it was cooled at 40 °C·min−1 to room temperature. The sample was then crushed and sieved to give particle sizes of
