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Bioactive glass-ceramics prepared by powder sintering and crystallization of polyphosphate glass containing strontium
Author’s Accepted Manuscript Bioactive glass-ceramics prepared by powder sintering and crystallization of polyphosphate glass containing strontium Vladimir S. Topalović, Snežana R. Grujić, Vladimir D. Živanović, Srdjan D. Matijašević, Jelena D. Nikolić, Jovica N. Stojanović, Sonja V. Smiljanić
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S0272-8842(17)31276-2 http://dx.doi.org/10.1016/j.ceramint.2017.06.061 CERI15583
To appear in: Ceramics International Received date: 14 November 2016 Revised date: 9 June 2017 Accepted date: 9 June 2017 Cite this article as: Vladimir S. Topalović, Snežana R. Grujić, Vladimir D. Živanović, Srdjan D. Matijašević, Jelena D. Nikolić, Jovica N. Stojanović and Sonja V. Smiljanić, Bioactive glass-ceramics prepared by powder sintering and crystallization of polyphosphate glass containing strontium, Ceramics International, http://dx.doi.org/10.1016/j.ceramint.2017.06.061 This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting galley proof before it is published in its final citable form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
Bioactive glass-ceramics prepared by powder sintering and crystallization of polyphosphate glass containing strontium Vladimir S. Topalovića, Snežana R. Grujićb*, Vladimir D. Živanovića, Srdjan D. Matijaševića, Jelena D. Nikolića, Jovica N. Stojanovića, Sonja V. Smiljanićb a
Institute for the Technology of Nuclear and other Mineral Raw Materials, 86 Franchet d’
Esperey St.,11000 Belgrade, Serbia b
University of Belgrade, Faculty of Technology and Metallurgy, 4 Karnegijeva St.,11000
Belgrade, Serbia *
Corresponding author: Tel.: + 381-11-3303-723; fax: + 381-11-3370-387.
[email protected] (S.Grujić)
Abstract Melt-quenching method was employed for obtaining a glass-ceramic with the following
composition
42P2O5·40CaO·5SrO·10Na2O·3TiO2
(mol
%)
glass.
The
crystallization and sintering behavior of glass have been studied by using DTA, HSM, XRD, FTIR and SEM methods. It was determined that the surface and volume crystallization mechanisms act simultaneously in bulk glass samples. The comparison of DTA and HSM data revealed that the sintering and crystallization processes are independent. The sintered calcium phosphate glass-ceramic which contained bioactive βCa3(PO4)2 and β-Ca2P2O7 phases was successfully prepared. It was determined that during crystallization the primary phase in the precipitate was β-Ca(PO3)2. Other phases appearing in the resulting glass-ceramic were: α-Ca2P2O7, γ-Ca2P2O7, Ca4P6O19 and CaHPO4(H2O)2. Crystalline phases containing Sr and Ti were not detected. SEM analysis of the glass-
ceramic microstructure revealed surface crystallization of glass particles and plate-like morphology of crystal growth. The result of the in vitro bioactivity showed that no apatite layer was formed on the surface of the as-prepared glass-ceramic samples after immersion in the simulated body fluid (SBF).
Keywords: A. Glass powder sintering, B. Polyphosphate glass, D. Bioactive glass-ceramics, β-Ca3(PO4)2, β-Ca2P2O7.
Introduction Due to their specific structure, the phosphate glasses have many unique properties that make them good candidates for variety of applications. In general, glasses feature low melting points, low glass transition temperatures, low softening temperatures and high thermal expansion coefficients [1-7]. In recent years, due to their specific dissolution behavior, bioactivity and biocompatibility, an expanding interest has risen for development and application of phosphate-based glasses as biomaterials in medicine [8-11]. In order to better suit the end application, these glasses could be prepared by introducing different metallic oxides in the primary P2O5 - CaO glassy system. Depending on the [O]/[P] ratio, glasses of different structures could be made. The polyphosphate glasses contain less than 50 mol % P2O5 and their structure consists of chains formed by Q2 groups terminated with Q1 groups [4, 12, 13]. As reported earlier, the phosphate – based glasses containing Ca2+ and Na+ ions show bioactivity and have potential applications in both soft and hard tissue engineering [14-18]. Calcium (Ca2+) and strontium (Sr2+) ions behave similarly and the strontium ions could be incorporated into the glass structure. Previous studies showed that
the two elements not only share some chemical and physical characteristics but also exhibit similar involvements in a number of biological processes [19, 20, 21]. It was documented in tissue engineering literature that the release of strontium ions from bioactive glass scaffolds has a therapeutic effect in bone healing [22, 23]. The crystallization behavior as well as the bioactivity of the glasses could be regulated by addition of small amounts of nucleating agents such as TiO2 and ZrO2 [24-28]. By crystallization of these glasses the bioactive calcium phosphate glass–ceramic materials could be prepared. These materials can promote the formation of apatite (HAP) layer on their surface after being exposed to simulated body fluid (SBF) or human plasma [29-31]. For synthesis of the bioactive glass-ceramics, the glass powder sintering route is frequently employed [32, 33]. The phase composition and microstructure of the as-prepared glassceramic have an effect on all their properties including bioactivity and therefore it is essential to determine the sintering and crystallization behavior of the glass powder [34, 35]. In the presented research, glass-ceramic containing bioactive β-Ca3(PO4)2 and βCa2P2O7 crystalline phases has been prepared by using glass powder sintering route. The parent polyphosphate glass 42P2O5·40CaO·5SrO·10Na2O·3TiO2 (mol %) was obtained by a standard melt-quenching procedure. To study in detail the crystallization and sintering behavior of the glass, differential thermal analysis (DTA), Fourier transform infrared spectroscopy (FTIR), hot-stage microscopy (HSM), X-ray diffraction (XRD) and scanning electron microscopy (SEM) methods were used. In vitro bioactivity assessment of the asprepared glass-ceramic was performed by immersion in the simulated body fluid (SBF) at 37 °C for 21 days. For characterization of the samples, SEM and FTIR methods were used.
2. Materials and methods 2.1. Glass preparation The parent polyphosphate glass was prepared by a standard melt quenching procedure. Reagent-grade chemicals, (NH4)2HPO4, Na2CO3, CaCO3, SrCO3 and TiO2 were mixed and homogenized in an agate mortar. To minimize the foaming of the melts, the glass batch was placed in a Pt-crucible and was slowly heated up to T= 190 °C and then was kept at constant temperature for 3 h in order to release the gasses, such as water vapor and NH3. The melting was performed in an electric furnace Carbolite BLF 17/3 at T = 1250 °C for t = 0.5 h and the glass was obtained by quenching of the melt on a steel plate. The obtained glass sample was clear and colorless without visible gas bubbles. The powder X-ray diffraction (XRD) analysis confirmed that the quenched melts were vitreous. The chemical analysis was performed using spectrophotometer AAS PERKIN ELMER Analyst 703. 2.2. Glass crystallization experiments To examine the glass crystallization, non-isothermal and isothermal experiments were performed for powdered and bulk glass samples. The non-isothermal crystallization was studied using a DTA-Netzsch STA 409 EP instrument with Al2O3 powder as a reference material. The powder sample (100mg) was prepared by crushing and grinding the bulk glass in an agate mortar and sieving thus prepared specimen up to the appropriate grain size of less than 0.048 mm. The glass was heated from 20 to 900 C at a heating rate of 10 C min-1. The isothermal crystallization of the bulk glass samples was performed by heating the samples to the crystallization temperature previously determined by DTA at a heating rate
of 10 ºC min-1 in an electric furnace Carbolite CWF 13/13 with an automatic regulation and temperature accuracy of ±1 ºC. The samples were kept at this temperature for 1-3 hours. After the heat treatment the samples were removed from the furnace, cooled in air and then prepared for analysis. 2.3. Preparation of glass-ceramic samples In order to determine the sintering behavior of the glass powder samples a hot-stage microscope (HSM), E. Leitz Wetzlar, equipped with Cannon camera, was used. The samples (<0.048 mm) were pressed into cylinders and the specimens were placed on a platinum plate and then on top of an alumina support, which was in contact with the thermocouple. The temperature was measured with (Pt/Rh/Pt) thermocouple at a heating rate of 10 °C min-1. The sample images were analyzed and the changes of the sample area (A/A0) during the heating were calculated. The glass-ceramics samples were fabricated by sintering of pellets (Ø 10 mm), which were previously prepared from the glass powder (<0.048 mm) by cold pressing in a Manfredi C 95 laboratory hydraulic press at 30 MPa. The temperatures of sintering were selected according to DTA and HSM experiments. The samples were heated at a rate of 10 C min-1 up to the chosen temperature, and then were kept at these temperatures for 3 h. After cooling, the sintered samples were crushed in an agate mortar and the favorable fractions were selected for analysis. 2.4. Characterization The XRD technique was used to identify the phase composition of the crystallized bulk glass samples as well as sintered glass-ceramic samples. The XRD patterns were obtained
using a Philips PW-1710 automated diffractometer with Cu tube operated at 40 kV and 30 mA. The instrument was equipped with a diffracted beam curved graphite monochromator and Xe-filled proportional counter. The diffraction data were collected in the 2θ Bragg angle range of 5 - 70 º, counting for 1 s. The structure of the as-quenched and sintered glass-ceramic samples was studied by Fourier transform infrared spectroscopy (FTIR). All IR absorption spectra were recorded in the wavenumber range of 400–1600 cm-1 (transmission mode) using Thermo Scientific Nicolet iS50 spectrometer. The resolution used was 4 cm-1 and the number of scans was 32. The microstructure of the crystallized bulk glass samples and sintered glass-ceramic samples was examined using scanning electron microscope (MIRA3 XM TESCAN). The samples were gold coated using Leica SCD005 device. 2.5. In vitro bioactivity assessment of the prepared glass-ceramics The bioactivity of the prepared glass-ceramics was evaluated by standard test of immersion in simulated body fluid (SBF) [31]. The glass-ceramic samples in the form of pellets were placed in polyethylene flasks containing 50 ml of SBF solution. The covered flasks were immersed in a water bath and kept at 37 ºC for 21 days. Subsequently, the samples were removed from the SBF solution, washed with distilled water and then airdried at room temperature. The sample surface was examined by SEM method. Additionally, an XRD analysis of the powdered glass-ceramic samples was employed. 3. Results and discussion 3.1 Chemical composition and structure of glass The data for nominal and analyzed chemical composition of the glass samples are given in Table 1.
The structure of phosphate glasses are usually presented using the Q
i
terminology
where i represents the number of bridging oxygens (BO) per tetrahedron [4, 12]. As presented in Table 1, the parent glass belongs to the polyphosphate glasses (x > 0.5), where x is the molar fraction of the network modifier oxide. The structure of these glasses is composed of Q2 chains terminated by Q1 tetrahedral (one bridging and three non-bridging oxygens per tetrahedron). The network connectivity parameter (NC) is defined as the number of bridging oxygens per network forming element and can be calculated using the following equation [36]:
NCTheo
3 P2O5 M 2I O M II O 2 M IV O2
P2O5
(1)
where [P2O5] is the molar fraction of phosphate and [M2IO], [M IIO] and [MIVO2] are the molar fractions of the network modifiers. According to the chemical composition (Table 1), the theoretical network connectivity NC
Theo
= 1.49 was calculated for this glass. For
glasses with molar fraction x of 0.5 < x < 0.67 (metaphosphate and pyrophosphate boundaries), the theoretical average chain length (nav), i.e. the number of phosphate units per phosphate chain could be calculated by the formula given by Bunker [37]:
nav
2 (2) [ M O] [ M O] 2[ M IV O2 ] 1 [ P2O5 ] I 2
II
The contribution of Q1 and Q2 tetrahedra is given as (Q1) = (2x–1)/(1–x) and f(Q2) = (2–3x)/(1–x), where x is the sum of molar fractions for the network modifier oxides.
Accordingly, for this phosphate glass, the values of nav = 4, f(Q1) = 0.42 and f(Q2) = 0.58 were calculated. 3.2 Differential thermal analysis Fig. 1 showed the DTA curve of powder glass sample (< 0.048 mm) recorded at a constant heating rate of 10 ºC min-1.
The glass crystallization was featured on the DTA curve by an exothermic peak with the onset crystallization temperature of Tx = 620 ºC and maximum crystallization peak temperature Tp = 633 ºC. A well-defined endothermic peak, that represented the melting of the sample, was observed after the crystallization peak. The onset melting temperature of Tm= 710 ºC and maximum peak temperature of TM = 740 ºC were marked on the DTA curve (Fig. 1). The glass transition temperature was determined to be Tg = 450 ºC. Based on these characteristic temperatures, the glass stability (GS) to crystallization under heating could be estimated. The Hruby criterion which takes into account Tg, Tx and Tm has been frequently used [38]:
KH
Tx Tg Tm Tx
(3)
According to the Hruby criterion, the higher the value of KH for a certain glass type, the higher is its stability to crystallization. Based on the calculated value of KH = 1.89, it could be concluded that our glass showed a high stability to crystallization.
3.3. Isothermal crystallization of bulk glass sample 3.3.1. XRD analysis The bulk glass samples were isothermally heated at onset crystallization temperature of Tx = 620 °C and crystallization temperature of Tc = 650 °C, previously determined by DTA (Fig. 1), for 1 and 3 h. The phase composition of these samples was determined by XRD analysis and the collected patterns are presented in Fig. 2. The XRD data showed that during the heating of the glass a very complex crystallization process took place, which resulted in the formation of calcium phosphate multiphase glass-ceramics. The results presented in Fig.6 did not show presence of crystalline phases containing strontium and titanium, which suggested that Sr and Ti ions existed in the glass matrix. The sample heated at Tx = 620 ºC for t = 1 h started to crystallize and the primary formed crystalline phases were: β-Ca(PO3)2 (JCPDS 79-0700) and α-Ca2P2O7 (JCPDS 45-1061) which is one of the polymorphic forms of the Ca2P2O7 phase (α, β, γ) [39]. The peak appearing at 2θ ~12º could be attributed to the presence of CaHPO4(H2O)2 (JCPDS 72-1240) phase. Further heating of the sample for t = 3 h induced the crystallization of three new crystalline phases: two polymorphic forms of Ca2P2O7 - β - Ca2P2O7 (JCPDS 09-0346), γ-Ca2P2O7 (JCPDS 230871) and β-Ca3(PO4)2 (JCPDS 70-2076). The results did not show presence of γ-Ca2P2O7 phase in the sample heated at Tc = 650 ºC for t = 1 h. This phase appeared during prolonged heating for t = 3 h. Additionally, a crystallization of three new phases was registered in this sample: NaCaPO4 (JCPDS 76-1456), Na2CaP2O7 (JCPDS 48-0557), Ca4P6O19 (JCPDS 150711). The results showed that this sample did not contain CaHPO4(H2O)2 phase. Fig.2
As reported previously, the crystalline phases β–Ca3(PO4)2 (β–TCP) and β-Ca2P2O7 (β–CPP), found in the resulting glass-ceramics, are bioactive, which means that they have the ability to promote the formation of apatite (HAP) layer after reaction with the surrounding body fluid [40, 41]. In contrast to the β-Ca3(PO4)2 and β-Ca2P2O7 phases, the biocompatibility of the main crystalline phase β-Ca(PO3)2 and the other ones precipitated in the resulting glass-ceramics has not been clearly reported so far in the literature. The glass-ceramics containing bioactive β-Ca3(PO4)2 and β-Ca2P2O7 phases have been prepared also by Zhang and Santos by using two-step heat treatment of bulk calcium phosphate glass samples with molar ratio CaO / P2O5 >1 and high content of TiO2 [42]. 3.3.2. SEM analysis To determine the glass crystallization mechanism and the morphology of crystal growth an SEM analysis of isothermally heated bulk glass samples at Tx= 620 ºC for t = 3 h was performed. Selected micrographs of the fractured surface of the sample are shown in Fig. 3. As could be seen from Fig. 3, both surface and internal (volume) crystallization mechanisms acted simultaneously in the glass. In Fig 3a, the growth of the crystal layer from the glass surface toward the bulk is visible. Under a high magnification the presence of dendrite (Fig. 3b) and lamellar (Fig. 3c) crystals in the surface layer was determined. Also, nano-sized spherulite crystals, which precipitated in the bulk glass, could be observed in Fig. 3d. 3.4. Characterization of glass-ceramic samples 3.4.1. Sintering behavior of glass-powder (HSM analysis) Photomicrographs of the glass-powder compact collected on a hot-stage microscope (HSM) are shown in Fig. 4. The observed temperatures corresponding to the temperatures
of characteristic shapes of the glass determined are: first shrinkage (TFS), maximum shrinkage (TMS), softening (TD), sphere (TS), half ball (THB) and flow (TF) [43]. After the appearance of crystallization it is not possible to determine fixed viscosity points because viscosity will change in a not predictable way.
To determine the sintering behavior of the glass sample, the changes of the relative area A/A0 of the sample as a function of the temperature were determined, where A0 is the initial area and A is the area at the temperature T, and the results are presented in Fig. 5. The physical processes that control the densification kinetics of porous glass bodies by reducing their surface energy are well studied. It was established that the glass particle surface energy was the driving force and the viscous flow was the kinetic path through which the surface area was reduced. However, during sintering of glass powder compacts above the glass transition temperature (Tg), the crystallization of glass particles could occur as a concurrent process. In such a case the viscous flow sintering would be hindered and as a consequence an inappropriate densification of the glass compacts could appear. If the sintering process ends before the crystallization starts, the residual porosity of the glass body could be avoided and dense glass-ceramics materials could be obtained [44]. To determine whether the sintering and crystallization processes proceeded independently, it was useful to compare the results of HSM and DTA analyses obtained in the same range. As could be seen from Fig. 5, the sample started to shrink at TFS = 463 ºC and the maximum shrinkage (A/A0 = 0.75) was observed at TMS = 520 ºC. As shown in Fig. 1, the DTA onset crystallization temperature Tx = 620 ºC was higher than the temperature of maximum shrinkage TMS, (Tx - TMS = 100 ºC), which indicated independence of the sintering
and crystallization processes. Further heating to T = 760 ºC led to expansion of the sample and after this temperature a melting of the sample occurred which was expressed by an abrupt drop of A/A0 values, Fig. 5. Based on the results from the DTA, HSM and XRD analyses previously performed, the glass-ceramics samples were fabricated by sinter-crystallization of the glass pellets at temperature Tx= 620 ºC for t = 3 h. The identification of crystalline phases precipitated in the glass-ceramics samples was performed by X-ray diffraction analysis and the microstructure was analyzed by using scanning electron microscopy (SEM). 3.4.2. XRD analysis of the sintered glass ceramics In Fig. 6, the XRD pattern of the as-prepared glass-ceramics is shown. The XRD data showed that a more intensive crystallization process occurred during the heating of the glass powder compact sample due to the high specific surface of the fine glass powder, which enhanced the surface nucleation. Fig. 6 As in the case of crystallized bulk glass samples (Fig. 2), the main crystalline phase precipitated in the sintered glass-ceramics was β-Ca(PO3)2. Both bioactive phases appeared, but the contribution of the β-Ca2P2O7 phase was smaller than the one of the β-Ca3(PO4)2 phase. The other determined phases were: α-Ca2P2O7, γ-Ca2P2O7 and Ca4P6O19. The XRD patterns revealed also broad amorphous bands that indicated the presence of a residual glassy phase in the glass-ceramics samples. Kasuga et al. studied extensively the bioactive glass-ceramics derived from the glassy system Na2O–CaO–P2O5 with P2O5 < 50 mol % and addition of TiO2. The glass-ceramics with high content of β–TCP and β–CPP were prepared by sinter/crystallization of the glass powder with CaO/P2O5 = 2 and 3 mol % of
TiO2. The formation of apatite (HAP) layer on the surface after reaction with simulated body fluid (SBF) was determined [45]. 3.4.3. FTIR analysis of the sintered glass powder compacts The structural transformations which appeared during the sinter-crystallization of glass powder compact sample at T= 620 ºC for t =3 h were visible in the FTIR spectra (Fig. 7). As seen from Fig. 7, the as-quenched glass revealed the characteristic bands in IR spectra in the wavenumber range of 400 - 1400 cm-1. The assigned peaks were found to be: ~ 500, 750, 885, 990, 1088 and 1243 cm-1. Based on previous FTIR studies of the structure of phosphate glasses, the band in the frequency range of 400 - 600 cm-1 could be assigned to bending vibration of the bridging phosphorous δ (O-P-O) and/or δ (P=O). The peak at 750 cm-1 could be attributed to P-O-P symmetric stretching vibration νs (P-O-P) of the bridging oxygen atoms bonded to phosphorus atoms and the peak at 885 cm-1 was due to the asymmetric vibration of P-O-P bond, νas (P-O-P), (Q2 thetraedra). The peaks at 990 and 1088 cm-1 expressed the symmetric and asymmetric stretching of (PO3)- groups (characteristic for Q1 structural units). The peak at 1243 cm-1 could be assigned to the asymmetric stretching mode of (P=O) bond [4, 46, 47, 48]. The crystallization of the glass during sintering was expressed on the FTIR curve (Fig. 7b) by transformation of the characteristic broad bands into several new peaks. Fig. 7 In the frequency range of 400-600 cm-1, a very weak splitting of the peak could be observed. Two small peaks at ~ 485 and 503 cm-1 as well as an inflection at ~ 564 cm-1 appeared. In the range of 600-800 new peaks at ~ 635, 679 (inflection), 703, 747 and 772 cm-1 are visible. Also, the formation of new peaks at ~ 975, 1020, 1058 and one peak
inflection at ~ 1145 cm-1 was registered on the FTIR curve (Fig. 7b). These transformations of the characteristic bands corresponded to the vibration modes of metaphosphate (PO3)-, pyrophosphate (P2O7)4- and orthophosphate (PO4)3– groups. Usually, the vibration modes of pyrophosphate (P2O7)4- were expressed in terms of (PO3)- and P–O–P group vibrations. These vibrations were distributed with decreasing frequency in the following order: νas PO3 > νs PO3 > νs POP > δ OPO > δ POP, where the frequencies for each type of vibrations in the different pyrophosphate compounds were always within the same frequency range [46, 49, 50]. The symmetric stretching vibration mode at 920-970 cm-1 and the asymmetric one at 980-1100 cm-1 were characteristic for (PO4)3– anion [46, 49, 50]. The presence of phosphate groups determined by the FTIR analysis was in agreement with the results obtained by the XRD analysis (Fig. 6) where the determined crystalline phases precipitated in the glass matrix were: β-Ca(PO3)2, β-Ca3(PO4)2, α-Ca2P2O7, β-Ca2P2O7, γ-Ca2P2O7. 3.4.4. SEM analysis of the sintered glass-ceramics As could be seen from Fig. 8 (a-d), a non-homogenous porous glass-ceramics body was obtained by sintering of the glass powder compact at Tx = 620 ºC for t = 3 h. The microstructure was formed by surface crystallization of glass particles. The SEM micrographs of the fractured surface (Fig. 8 c, d) revealed blocks of crystals formed from the lamellar (plate-like) crystals growing from the surface of the glass particles.
3.5. In vitro bioactivity assessment of the glass-ceramic In Fig. 9 a) and b), the SEM micrographs and XRD pattern of the glass-ceramics after immersion in SBF at 37 ºC for 21 days are shown.
The changes in the surface morphology of the sample indicated that formation of HAP layer on its surface did not occur. Also, the XRD pattern (Fig. 9b) did not confirm the formation of a HAP crystalline phase. The glass–ceramic samples are known to consist of both crystalline phases and residual glassy phase. There is information that some phases are less soluble or completely inert in contact with SBF [51, 52, 53]. Considering these papers the inability of formation of HAp may be explained by low solubility of present bioactive phases in solution.
4. Conclusions For
synthesis
of
the
sintered
glass-ceramic
the
parent
42P2O5·40CaO·5SrO·10Na2O·3TiO2 (mol %) glass was prepared by standard meltquenching method. The crystallization experiments of bulk glass samples revealed that surface and volume crystallization mechanisms occurred simultaneously. The results from the DTA and HSM analyses showed that the sintering and crystallization processes developed independently during heating of the glass powder. The glass-ceramic containing two bioactive crystalline phases, β-Ca3(PO4)2 and β-Ca2P2O7, was prepared by heating of the compacted glass powder at Tx = 620 ºC for t = 3h. The crystalline phases containing Sr and Ti were not detected. The surface crystallization of the glass particles and plate-like morphology of crystal growth were determined. The results of SEM and XRD analyses showed that no hydroxyapatite (HAP) layer was formed on the glass-ceramic surface after immersion for 21 days in SBF solution.
Acknowledgment This research was funded by the Ministry of Education, Science and Technological Development of the Republic of Serbia (Projects 34001 and 172004).
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FIGURE CAPTIONS Fig. 1. DTA curve recorded at a heating rate of 10 ºC min-1 for sample particle size < 0.048 mm. Fig.2. XRD patterns of the bulk glass samples isothermally heated at T = 620 oC and 650 oC for t = 1 and 3 h [(o) Ca(PO3)2 , (β) β-Ca3(PO4)2 , (α) α-Ca2P2O7, (*) β-Ca2P2O7, (γ) γCa2P2O7, (+) NaCaPO4, (Δ) Na2CaP2O7, (#) Ca4P6O19, (?) CaHPO4 (H2O)2].
Fig.3. SEM micrograph of bulk glass samples heated at Tx= 620 ºC, t = 3 h a) crystallized surface layer b) plate–like crystals c) dendrite crystals d) spherulite crystals in the glass bulk. Fig.4. HSM photomicrograph of the glass powder compact. Fig. 5. The changes of A/A0 during HSM measurement. Fig. 6. XRD pattern of the glass powder compact sintered at Tx = 620 ºC for t = 3 h. [(o) βCa(PO3)2 , (β) β-Ca3(PO4)2 , (α) α-Ca2P2O7., (*) β-Ca2P2O7 , (γ) γ-Ca2P2O7, (#) Ca4P6O19]. Fig. 7. FTIR spectra of the glass powder compacts a) parent glass powder compact sample b) sample sintered at Tx= 620 ºC for t = 3 h. Fig.8. SEM micrographs of glass-ceramic obtained by glass powder sintering at Tx = 620 ºC for t = 3h a) and b) free surface; c) and d) fractured surface. Fig. 9. The glass-ceramic sample after immersion in SBF at 37 °C for 21 days a) SEM micrograph of the surface; b) XRD pattern.
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Fig. 4.
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PII: DOI: Reference:
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S0272-8842(17)31276-2 http://dx.doi.org/10.1016/j.ceramint.2017.06.061 CERI15583
To appear in: Ceramics International Received date: 14 November 2016 Revised date: 9 June 2017 Accepted date: 9 June 2017 Cite this article as: Vladimir S. Topalović, Snežana R. Grujić, Vladimir D. Živanović, Srdjan D. Matijašević, Jelena D. Nikolić, Jovica N. Stojanović and Sonja V. Smiljanić, Bioactive glass-ceramics prepared by powder sintering and crystallization of polyphosphate glass containing strontium, Ceramics International, http://dx.doi.org/10.1016/j.ceramint.2017.06.061 This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting galley proof before it is published in its final citable form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
Bioactive glass-ceramics prepared by powder sintering and crystallization of polyphosphate glass containing strontium Vladimir S. Topalovića, Snežana R. Grujićb*, Vladimir D. Živanovića, Srdjan D. Matijaševića, Jelena D. Nikolića, Jovica N. Stojanovića, Sonja V. Smiljanićb a
Institute for the Technology of Nuclear and other Mineral Raw Materials, 86 Franchet d’
Esperey St.,11000 Belgrade, Serbia b
University of Belgrade, Faculty of Technology and Metallurgy, 4 Karnegijeva St.,11000
Belgrade, Serbia *
Corresponding author: Tel.: + 381-11-3303-723; fax: + 381-11-3370-387.
[email protected] (S.Grujić)
Abstract Melt-quenching method was employed for obtaining a glass-ceramic with the following
composition
42P2O5·40CaO·5SrO·10Na2O·3TiO2
(mol
%)
glass.
The
crystallization and sintering behavior of glass have been studied by using DTA, HSM, XRD, FTIR and SEM methods. It was determined that the surface and volume crystallization mechanisms act simultaneously in bulk glass samples. The comparison of DTA and HSM data revealed that the sintering and crystallization processes are independent. The sintered calcium phosphate glass-ceramic which contained bioactive βCa3(PO4)2 and β-Ca2P2O7 phases was successfully prepared. It was determined that during crystallization the primary phase in the precipitate was β-Ca(PO3)2. Other phases appearing in the resulting glass-ceramic were: α-Ca2P2O7, γ-Ca2P2O7, Ca4P6O19 and CaHPO4(H2O)2. Crystalline phases containing Sr and Ti were not detected. SEM analysis of the glass-
ceramic microstructure revealed surface crystallization of glass particles and plate-like morphology of crystal growth. The result of the in vitro bioactivity showed that no apatite layer was formed on the surface of the as-prepared glass-ceramic samples after immersion in the simulated body fluid (SBF).
Keywords: A. Glass powder sintering, B. Polyphosphate glass, D. Bioactive glass-ceramics, β-Ca3(PO4)2, β-Ca2P2O7.
Introduction Due to their specific structure, the phosphate glasses have many unique properties that make them good candidates for variety of applications. In general, glasses feature low melting points, low glass transition temperatures, low softening temperatures and high thermal expansion coefficients [1-7]. In recent years, due to their specific dissolution behavior, bioactivity and biocompatibility, an expanding interest has risen for development and application of phosphate-based glasses as biomaterials in medicine [8-11]. In order to better suit the end application, these glasses could be prepared by introducing different metallic oxides in the primary P2O5 - CaO glassy system. Depending on the [O]/[P] ratio, glasses of different structures could be made. The polyphosphate glasses contain less than 50 mol % P2O5 and their structure consists of chains formed by Q2 groups terminated with Q1 groups [4, 12, 13]. As reported earlier, the phosphate – based glasses containing Ca2+ and Na+ ions show bioactivity and have potential applications in both soft and hard tissue engineering [14-18]. Calcium (Ca2+) and strontium (Sr2+) ions behave similarly and the strontium ions could be incorporated into the glass structure. Previous studies showed that
the two elements not only share some chemical and physical characteristics but also exhibit similar involvements in a number of biological processes [19, 20, 21]. It was documented in tissue engineering literature that the release of strontium ions from bioactive glass scaffolds has a therapeutic effect in bone healing [22, 23]. The crystallization behavior as well as the bioactivity of the glasses could be regulated by addition of small amounts of nucleating agents such as TiO2 and ZrO2 [24-28]. By crystallization of these glasses the bioactive calcium phosphate glass–ceramic materials could be prepared. These materials can promote the formation of apatite (HAP) layer on their surface after being exposed to simulated body fluid (SBF) or human plasma [29-31]. For synthesis of the bioactive glass-ceramics, the glass powder sintering route is frequently employed [32, 33]. The phase composition and microstructure of the as-prepared glassceramic have an effect on all their properties including bioactivity and therefore it is essential to determine the sintering and crystallization behavior of the glass powder [34, 35]. In the presented research, glass-ceramic containing bioactive β-Ca3(PO4)2 and βCa2P2O7 crystalline phases has been prepared by using glass powder sintering route. The parent polyphosphate glass 42P2O5·40CaO·5SrO·10Na2O·3TiO2 (mol %) was obtained by a standard melt-quenching procedure. To study in detail the crystallization and sintering behavior of the glass, differential thermal analysis (DTA), Fourier transform infrared spectroscopy (FTIR), hot-stage microscopy (HSM), X-ray diffraction (XRD) and scanning electron microscopy (SEM) methods were used. In vitro bioactivity assessment of the asprepared glass-ceramic was performed by immersion in the simulated body fluid (SBF) at 37 °C for 21 days. For characterization of the samples, SEM and FTIR methods were used.
2. Materials and methods 2.1. Glass preparation The parent polyphosphate glass was prepared by a standard melt quenching procedure. Reagent-grade chemicals, (NH4)2HPO4, Na2CO3, CaCO3, SrCO3 and TiO2 were mixed and homogenized in an agate mortar. To minimize the foaming of the melts, the glass batch was placed in a Pt-crucible and was slowly heated up to T= 190 °C and then was kept at constant temperature for 3 h in order to release the gasses, such as water vapor and NH3. The melting was performed in an electric furnace Carbolite BLF 17/3 at T = 1250 °C for t = 0.5 h and the glass was obtained by quenching of the melt on a steel plate. The obtained glass sample was clear and colorless without visible gas bubbles. The powder X-ray diffraction (XRD) analysis confirmed that the quenched melts were vitreous. The chemical analysis was performed using spectrophotometer AAS PERKIN ELMER Analyst 703. 2.2. Glass crystallization experiments To examine the glass crystallization, non-isothermal and isothermal experiments were performed for powdered and bulk glass samples. The non-isothermal crystallization was studied using a DTA-Netzsch STA 409 EP instrument with Al2O3 powder as a reference material. The powder sample (100mg) was prepared by crushing and grinding the bulk glass in an agate mortar and sieving thus prepared specimen up to the appropriate grain size of less than 0.048 mm. The glass was heated from 20 to 900 C at a heating rate of 10 C min-1. The isothermal crystallization of the bulk glass samples was performed by heating the samples to the crystallization temperature previously determined by DTA at a heating rate
of 10 ºC min-1 in an electric furnace Carbolite CWF 13/13 with an automatic regulation and temperature accuracy of ±1 ºC. The samples were kept at this temperature for 1-3 hours. After the heat treatment the samples were removed from the furnace, cooled in air and then prepared for analysis. 2.3. Preparation of glass-ceramic samples In order to determine the sintering behavior of the glass powder samples a hot-stage microscope (HSM), E. Leitz Wetzlar, equipped with Cannon camera, was used. The samples (<0.048 mm) were pressed into cylinders and the specimens were placed on a platinum plate and then on top of an alumina support, which was in contact with the thermocouple. The temperature was measured with (Pt/Rh/Pt) thermocouple at a heating rate of 10 °C min-1. The sample images were analyzed and the changes of the sample area (A/A0) during the heating were calculated. The glass-ceramics samples were fabricated by sintering of pellets (Ø 10 mm), which were previously prepared from the glass powder (<0.048 mm) by cold pressing in a Manfredi C 95 laboratory hydraulic press at 30 MPa. The temperatures of sintering were selected according to DTA and HSM experiments. The samples were heated at a rate of 10 C min-1 up to the chosen temperature, and then were kept at these temperatures for 3 h. After cooling, the sintered samples were crushed in an agate mortar and the favorable fractions were selected for analysis. 2.4. Characterization The XRD technique was used to identify the phase composition of the crystallized bulk glass samples as well as sintered glass-ceramic samples. The XRD patterns were obtained
using a Philips PW-1710 automated diffractometer with Cu tube operated at 40 kV and 30 mA. The instrument was equipped with a diffracted beam curved graphite monochromator and Xe-filled proportional counter. The diffraction data were collected in the 2θ Bragg angle range of 5 - 70 º, counting for 1 s. The structure of the as-quenched and sintered glass-ceramic samples was studied by Fourier transform infrared spectroscopy (FTIR). All IR absorption spectra were recorded in the wavenumber range of 400–1600 cm-1 (transmission mode) using Thermo Scientific Nicolet iS50 spectrometer. The resolution used was 4 cm-1 and the number of scans was 32. The microstructure of the crystallized bulk glass samples and sintered glass-ceramic samples was examined using scanning electron microscope (MIRA3 XM TESCAN). The samples were gold coated using Leica SCD005 device. 2.5. In vitro bioactivity assessment of the prepared glass-ceramics The bioactivity of the prepared glass-ceramics was evaluated by standard test of immersion in simulated body fluid (SBF) [31]. The glass-ceramic samples in the form of pellets were placed in polyethylene flasks containing 50 ml of SBF solution. The covered flasks were immersed in a water bath and kept at 37 ºC for 21 days. Subsequently, the samples were removed from the SBF solution, washed with distilled water and then airdried at room temperature. The sample surface was examined by SEM method. Additionally, an XRD analysis of the powdered glass-ceramic samples was employed. 3. Results and discussion 3.1 Chemical composition and structure of glass The data for nominal and analyzed chemical composition of the glass samples are given in Table 1.
The structure of phosphate glasses are usually presented using the Q
i
terminology
where i represents the number of bridging oxygens (BO) per tetrahedron [4, 12]. As presented in Table 1, the parent glass belongs to the polyphosphate glasses (x > 0.5), where x is the molar fraction of the network modifier oxide. The structure of these glasses is composed of Q2 chains terminated by Q1 tetrahedral (one bridging and three non-bridging oxygens per tetrahedron). The network connectivity parameter (NC) is defined as the number of bridging oxygens per network forming element and can be calculated using the following equation [36]:
NCTheo
3 P2O5 M 2I O M II O 2 M IV O2
P2O5
(1)
where [P2O5] is the molar fraction of phosphate and [M2IO], [M IIO] and [MIVO2] are the molar fractions of the network modifiers. According to the chemical composition (Table 1), the theoretical network connectivity NC
Theo
= 1.49 was calculated for this glass. For
glasses with molar fraction x of 0.5 < x < 0.67 (metaphosphate and pyrophosphate boundaries), the theoretical average chain length (nav), i.e. the number of phosphate units per phosphate chain could be calculated by the formula given by Bunker [37]:
nav
2 (2) [ M O] [ M O] 2[ M IV O2 ] 1 [ P2O5 ] I 2
II
The contribution of Q1 and Q2 tetrahedra is given as (Q1) = (2x–1)/(1–x) and f(Q2) = (2–3x)/(1–x), where x is the sum of molar fractions for the network modifier oxides.
Accordingly, for this phosphate glass, the values of nav = 4, f(Q1) = 0.42 and f(Q2) = 0.58 were calculated. 3.2 Differential thermal analysis Fig. 1 showed the DTA curve of powder glass sample (< 0.048 mm) recorded at a constant heating rate of 10 ºC min-1.
The glass crystallization was featured on the DTA curve by an exothermic peak with the onset crystallization temperature of Tx = 620 ºC and maximum crystallization peak temperature Tp = 633 ºC. A well-defined endothermic peak, that represented the melting of the sample, was observed after the crystallization peak. The onset melting temperature of Tm= 710 ºC and maximum peak temperature of TM = 740 ºC were marked on the DTA curve (Fig. 1). The glass transition temperature was determined to be Tg = 450 ºC. Based on these characteristic temperatures, the glass stability (GS) to crystallization under heating could be estimated. The Hruby criterion which takes into account Tg, Tx and Tm has been frequently used [38]:
KH
Tx Tg Tm Tx
(3)
According to the Hruby criterion, the higher the value of KH for a certain glass type, the higher is its stability to crystallization. Based on the calculated value of KH = 1.89, it could be concluded that our glass showed a high stability to crystallization.
3.3. Isothermal crystallization of bulk glass sample 3.3.1. XRD analysis The bulk glass samples were isothermally heated at onset crystallization temperature of Tx = 620 °C and crystallization temperature of Tc = 650 °C, previously determined by DTA (Fig. 1), for 1 and 3 h. The phase composition of these samples was determined by XRD analysis and the collected patterns are presented in Fig. 2. The XRD data showed that during the heating of the glass a very complex crystallization process took place, which resulted in the formation of calcium phosphate multiphase glass-ceramics. The results presented in Fig.6 did not show presence of crystalline phases containing strontium and titanium, which suggested that Sr and Ti ions existed in the glass matrix. The sample heated at Tx = 620 ºC for t = 1 h started to crystallize and the primary formed crystalline phases were: β-Ca(PO3)2 (JCPDS 79-0700) and α-Ca2P2O7 (JCPDS 45-1061) which is one of the polymorphic forms of the Ca2P2O7 phase (α, β, γ) [39]. The peak appearing at 2θ ~12º could be attributed to the presence of CaHPO4(H2O)2 (JCPDS 72-1240) phase. Further heating of the sample for t = 3 h induced the crystallization of three new crystalline phases: two polymorphic forms of Ca2P2O7 - β - Ca2P2O7 (JCPDS 09-0346), γ-Ca2P2O7 (JCPDS 230871) and β-Ca3(PO4)2 (JCPDS 70-2076). The results did not show presence of γ-Ca2P2O7 phase in the sample heated at Tc = 650 ºC for t = 1 h. This phase appeared during prolonged heating for t = 3 h. Additionally, a crystallization of three new phases was registered in this sample: NaCaPO4 (JCPDS 76-1456), Na2CaP2O7 (JCPDS 48-0557), Ca4P6O19 (JCPDS 150711). The results showed that this sample did not contain CaHPO4(H2O)2 phase. Fig.2
As reported previously, the crystalline phases β–Ca3(PO4)2 (β–TCP) and β-Ca2P2O7 (β–CPP), found in the resulting glass-ceramics, are bioactive, which means that they have the ability to promote the formation of apatite (HAP) layer after reaction with the surrounding body fluid [40, 41]. In contrast to the β-Ca3(PO4)2 and β-Ca2P2O7 phases, the biocompatibility of the main crystalline phase β-Ca(PO3)2 and the other ones precipitated in the resulting glass-ceramics has not been clearly reported so far in the literature. The glass-ceramics containing bioactive β-Ca3(PO4)2 and β-Ca2P2O7 phases have been prepared also by Zhang and Santos by using two-step heat treatment of bulk calcium phosphate glass samples with molar ratio CaO / P2O5 >1 and high content of TiO2 [42]. 3.3.2. SEM analysis To determine the glass crystallization mechanism and the morphology of crystal growth an SEM analysis of isothermally heated bulk glass samples at Tx= 620 ºC for t = 3 h was performed. Selected micrographs of the fractured surface of the sample are shown in Fig. 3. As could be seen from Fig. 3, both surface and internal (volume) crystallization mechanisms acted simultaneously in the glass. In Fig 3a, the growth of the crystal layer from the glass surface toward the bulk is visible. Under a high magnification the presence of dendrite (Fig. 3b) and lamellar (Fig. 3c) crystals in the surface layer was determined. Also, nano-sized spherulite crystals, which precipitated in the bulk glass, could be observed in Fig. 3d. 3.4. Characterization of glass-ceramic samples 3.4.1. Sintering behavior of glass-powder (HSM analysis) Photomicrographs of the glass-powder compact collected on a hot-stage microscope (HSM) are shown in Fig. 4. The observed temperatures corresponding to the temperatures
of characteristic shapes of the glass determined are: first shrinkage (TFS), maximum shrinkage (TMS), softening (TD), sphere (TS), half ball (THB) and flow (TF) [43]. After the appearance of crystallization it is not possible to determine fixed viscosity points because viscosity will change in a not predictable way.
To determine the sintering behavior of the glass sample, the changes of the relative area A/A0 of the sample as a function of the temperature were determined, where A0 is the initial area and A is the area at the temperature T, and the results are presented in Fig. 5. The physical processes that control the densification kinetics of porous glass bodies by reducing their surface energy are well studied. It was established that the glass particle surface energy was the driving force and the viscous flow was the kinetic path through which the surface area was reduced. However, during sintering of glass powder compacts above the glass transition temperature (Tg), the crystallization of glass particles could occur as a concurrent process. In such a case the viscous flow sintering would be hindered and as a consequence an inappropriate densification of the glass compacts could appear. If the sintering process ends before the crystallization starts, the residual porosity of the glass body could be avoided and dense glass-ceramics materials could be obtained [44]. To determine whether the sintering and crystallization processes proceeded independently, it was useful to compare the results of HSM and DTA analyses obtained in the same range. As could be seen from Fig. 5, the sample started to shrink at TFS = 463 ºC and the maximum shrinkage (A/A0 = 0.75) was observed at TMS = 520 ºC. As shown in Fig. 1, the DTA onset crystallization temperature Tx = 620 ºC was higher than the temperature of maximum shrinkage TMS, (Tx - TMS = 100 ºC), which indicated independence of the sintering
and crystallization processes. Further heating to T = 760 ºC led to expansion of the sample and after this temperature a melting of the sample occurred which was expressed by an abrupt drop of A/A0 values, Fig. 5. Based on the results from the DTA, HSM and XRD analyses previously performed, the glass-ceramics samples were fabricated by sinter-crystallization of the glass pellets at temperature Tx= 620 ºC for t = 3 h. The identification of crystalline phases precipitated in the glass-ceramics samples was performed by X-ray diffraction analysis and the microstructure was analyzed by using scanning electron microscopy (SEM). 3.4.2. XRD analysis of the sintered glass ceramics In Fig. 6, the XRD pattern of the as-prepared glass-ceramics is shown. The XRD data showed that a more intensive crystallization process occurred during the heating of the glass powder compact sample due to the high specific surface of the fine glass powder, which enhanced the surface nucleation. Fig. 6 As in the case of crystallized bulk glass samples (Fig. 2), the main crystalline phase precipitated in the sintered glass-ceramics was β-Ca(PO3)2. Both bioactive phases appeared, but the contribution of the β-Ca2P2O7 phase was smaller than the one of the β-Ca3(PO4)2 phase. The other determined phases were: α-Ca2P2O7, γ-Ca2P2O7 and Ca4P6O19. The XRD patterns revealed also broad amorphous bands that indicated the presence of a residual glassy phase in the glass-ceramics samples. Kasuga et al. studied extensively the bioactive glass-ceramics derived from the glassy system Na2O–CaO–P2O5 with P2O5 < 50 mol % and addition of TiO2. The glass-ceramics with high content of β–TCP and β–CPP were prepared by sinter/crystallization of the glass powder with CaO/P2O5 = 2 and 3 mol % of
TiO2. The formation of apatite (HAP) layer on the surface after reaction with simulated body fluid (SBF) was determined [45]. 3.4.3. FTIR analysis of the sintered glass powder compacts The structural transformations which appeared during the sinter-crystallization of glass powder compact sample at T= 620 ºC for t =3 h were visible in the FTIR spectra (Fig. 7). As seen from Fig. 7, the as-quenched glass revealed the characteristic bands in IR spectra in the wavenumber range of 400 - 1400 cm-1. The assigned peaks were found to be: ~ 500, 750, 885, 990, 1088 and 1243 cm-1. Based on previous FTIR studies of the structure of phosphate glasses, the band in the frequency range of 400 - 600 cm-1 could be assigned to bending vibration of the bridging phosphorous δ (O-P-O) and/or δ (P=O). The peak at 750 cm-1 could be attributed to P-O-P symmetric stretching vibration νs (P-O-P) of the bridging oxygen atoms bonded to phosphorus atoms and the peak at 885 cm-1 was due to the asymmetric vibration of P-O-P bond, νas (P-O-P), (Q2 thetraedra). The peaks at 990 and 1088 cm-1 expressed the symmetric and asymmetric stretching of (PO3)- groups (characteristic for Q1 structural units). The peak at 1243 cm-1 could be assigned to the asymmetric stretching mode of (P=O) bond [4, 46, 47, 48]. The crystallization of the glass during sintering was expressed on the FTIR curve (Fig. 7b) by transformation of the characteristic broad bands into several new peaks. Fig. 7 In the frequency range of 400-600 cm-1, a very weak splitting of the peak could be observed. Two small peaks at ~ 485 and 503 cm-1 as well as an inflection at ~ 564 cm-1 appeared. In the range of 600-800 new peaks at ~ 635, 679 (inflection), 703, 747 and 772 cm-1 are visible. Also, the formation of new peaks at ~ 975, 1020, 1058 and one peak
inflection at ~ 1145 cm-1 was registered on the FTIR curve (Fig. 7b). These transformations of the characteristic bands corresponded to the vibration modes of metaphosphate (PO3)-, pyrophosphate (P2O7)4- and orthophosphate (PO4)3– groups. Usually, the vibration modes of pyrophosphate (P2O7)4- were expressed in terms of (PO3)- and P–O–P group vibrations. These vibrations were distributed with decreasing frequency in the following order: νas PO3 > νs PO3 > νs POP > δ OPO > δ POP, where the frequencies for each type of vibrations in the different pyrophosphate compounds were always within the same frequency range [46, 49, 50]. The symmetric stretching vibration mode at 920-970 cm-1 and the asymmetric one at 980-1100 cm-1 were characteristic for (PO4)3– anion [46, 49, 50]. The presence of phosphate groups determined by the FTIR analysis was in agreement with the results obtained by the XRD analysis (Fig. 6) where the determined crystalline phases precipitated in the glass matrix were: β-Ca(PO3)2, β-Ca3(PO4)2, α-Ca2P2O7, β-Ca2P2O7, γ-Ca2P2O7. 3.4.4. SEM analysis of the sintered glass-ceramics As could be seen from Fig. 8 (a-d), a non-homogenous porous glass-ceramics body was obtained by sintering of the glass powder compact at Tx = 620 ºC for t = 3 h. The microstructure was formed by surface crystallization of glass particles. The SEM micrographs of the fractured surface (Fig. 8 c, d) revealed blocks of crystals formed from the lamellar (plate-like) crystals growing from the surface of the glass particles.
3.5. In vitro bioactivity assessment of the glass-ceramic In Fig. 9 a) and b), the SEM micrographs and XRD pattern of the glass-ceramics after immersion in SBF at 37 ºC for 21 days are shown.
The changes in the surface morphology of the sample indicated that formation of HAP layer on its surface did not occur. Also, the XRD pattern (Fig. 9b) did not confirm the formation of a HAP crystalline phase. The glass–ceramic samples are known to consist of both crystalline phases and residual glassy phase. There is information that some phases are less soluble or completely inert in contact with SBF [51, 52, 53]. Considering these papers the inability of formation of HAp may be explained by low solubility of present bioactive phases in solution.
4. Conclusions For
synthesis
of
the
sintered
glass-ceramic
the
parent
42P2O5·40CaO·5SrO·10Na2O·3TiO2 (mol %) glass was prepared by standard meltquenching method. The crystallization experiments of bulk glass samples revealed that surface and volume crystallization mechanisms occurred simultaneously. The results from the DTA and HSM analyses showed that the sintering and crystallization processes developed independently during heating of the glass powder. The glass-ceramic containing two bioactive crystalline phases, β-Ca3(PO4)2 and β-Ca2P2O7, was prepared by heating of the compacted glass powder at Tx = 620 ºC for t = 3h. The crystalline phases containing Sr and Ti were not detected. The surface crystallization of the glass particles and plate-like morphology of crystal growth were determined. The results of SEM and XRD analyses showed that no hydroxyapatite (HAP) layer was formed on the glass-ceramic surface after immersion for 21 days in SBF solution.
Acknowledgment This research was funded by the Ministry of Education, Science and Technological Development of the Republic of Serbia (Projects 34001 and 172004).
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FIGURE CAPTIONS Fig. 1. DTA curve recorded at a heating rate of 10 ºC min-1 for sample particle size < 0.048 mm. Fig.2. XRD patterns of the bulk glass samples isothermally heated at T = 620 oC and 650 oC for t = 1 and 3 h [(o) Ca(PO3)2 , (β) β-Ca3(PO4)2 , (α) α-Ca2P2O7, (*) β-Ca2P2O7, (γ) γCa2P2O7, (+) NaCaPO4, (Δ) Na2CaP2O7, (#) Ca4P6O19, (?) CaHPO4 (H2O)2].
Fig.3. SEM micrograph of bulk glass samples heated at Tx= 620 ºC, t = 3 h a) crystallized surface layer b) plate–like crystals c) dendrite crystals d) spherulite crystals in the glass bulk. Fig.4. HSM photomicrograph of the glass powder compact. Fig. 5. The changes of A/A0 during HSM measurement. Fig. 6. XRD pattern of the glass powder compact sintered at Tx = 620 ºC for t = 3 h. [(o) βCa(PO3)2 , (β) β-Ca3(PO4)2 , (α) α-Ca2P2O7., (*) β-Ca2P2O7 , (γ) γ-Ca2P2O7, (#) Ca4P6O19]. Fig. 7. FTIR spectra of the glass powder compacts a) parent glass powder compact sample b) sample sintered at Tx= 620 ºC for t = 3 h. Fig.8. SEM micrographs of glass-ceramic obtained by glass powder sintering at Tx = 620 ºC for t = 3h a) and b) free surface; c) and d) fractured surface. Fig. 9. The glass-ceramic sample after immersion in SBF at 37 °C for 21 days a) SEM micrograph of the surface; b) XRD pattern.
a)
b)
Fig. 3.
Fig. 4.
c)
d)
a)
b)
c)
d)
Fig. 8.
a)
b) Fig. 9.