An elucidating study on physical and structural properties of 45S5 glass at different sintering temperatures

Journal of Non-Crystalline Solids 412 (2015) 24–29

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An elucidating study on physical and structural properties of 45S5 glass at different sintering temperatures N.A. Zarifah a, W.F. Lim b, K.A. Matori a,b,⁎, H.A.A. Sidek a, Z.A. Wahab a, N. Zainuddin c, M.A. Salleh b, B.N. Fadilah a,b, A.N. Fauzana b a b c

Department of Physics, Faculty of Science, Universiti Putra Malaysia, UPM Serdang, Selangor 43400, Malaysia Materials Synthesis and Characterization Laboratory, Institute of Advanced Technology, Universiti Putra Malaysia, UPM Serdang, Selangor 43400, Malaysia Department of Chemistry, Faculty of Science, Universiti Putra Malaysia, 43400 UPM Serdang, Selangor, Malaysia

a r t i c l e

i n f o

Article history: Received 10 September 2014 Received in revised form 30 December 2014 Accepted 4 January 2015 Available online xxxx Keywords: 45S5; Sintering; Porosity

a b s t r a c t This paper presents a study of physical and structural properties of 45S5 bioactive glass (BG) at different sintering temperatures. The 45S5 BG, one of the most important formulations, was composed of 45% SiO2, 24.5% Na2CO3, 24.5% CaCO3 and 6% P2O5. The aim of this research was to gain a better understanding on the effect of heat treatment towards phase transformation of the 45S5 BG. Each composition of the 45S5 BG was sintered at three different temperatures, which were 800 °C, 1000 °C and 1200 °C. The formation of crystallization or phase changes before and after the heat treatment was being identified using X-ray diffraction analysis. Bonding of the BG before and after sintering was being investigated using Fourier transform infrared spectroscopy while morphology of the 45S5 BG was determined by field emission scanning electron microscopy (FESEM). The X-ray pattern indicated the presence of silicate and phosphate in the glass system. Amorphous glassy phases with sodium calcium silicate (Na2Ca3Si6O16) and sodium calcium phosphate (NaCaPO4) were observed in the sintered glass. A decrement in density was due to formation of additional porosity which occurred due to decomposition of oxide materials in the glass. This was confirmed by FESEM results, which proved an increase in the porosity at high temperature. © 2015 Elsevier B.V. All rights reserved.

1. Introduction Bioactive glasses (BGs) are synthetic amorphous materials with high biocompatibility [1]. Due to this specific property, they can be used as implant materials in the human body in order to replace and/or repair diseased or damaged bone in orthopedic [2], cranio-maxilla [3] and periodontal surgeries as well as a filling material for human teeth. Various biomaterials have been used in biomedical applications, which include glass–ceramic, apatite wollastonite A–W and β-wollastonite [CaO·SiO2] in a MgO–CaO–SiO2 glassy matrix hydroxyapatite (HA) [Ca10(PO4)6(OH)2] and β-tricalcium phosphate (TCP) [Ca3(PO4)] [4–6]. These bioactive glasses are suitable candidates for tissue engineering due to their excellent osteoconductivity and bioactivity, ability to deliver cells, and controllable biodegradability [7]. These features make bioactive glasses the promising scaffold materials for tissue engineering. Bioactive glasses (BGs) can accelerate bone growth three times than that of hydroxyapatite when being implanted in bone. In comparison ⁎ Corresponding author at: Department of Physics, Faculty of Science, Universiti Putra Malaysia, UPM Serdang, Selangor 43400, Malaysia. E-mail addresses: [email protected] (N.A. Zarifah), [email protected] (W.F. Lim), [email protected] (K.A. Matori), [email protected] (H.A.A. Sidek), [email protected] (Z.A. Wahab), [email protected] (N. Zainuddin), [email protected] (M.A. Salleh), [email protected] (B.N. Fadilah), [email protected] (A.N. Fauzana).

http://dx.doi.org/10.1016/j.jnoncrysol.2015.01.005 0022-3093/© 2015 Elsevier B.V. All rights reserved.

with other bioceramic materials, BGs are more advantageous in the aspect of the ability to bond faster to the bone [8], and to dissolve in the body and are osteogenic [9]. Hench and co-workers were the first to develop BGs comprising of silicate composition which were able to bond to tissue in 1969 [4]. The BG with a formulation of 45S5, having a composition of 45% SiO2, 24.5% Na2O, 24.5% CaO, and 6% P2O5 by weight has been widely used in biomedical applications due to its ability to integrate with bone. Srivastava and Pyare had made a comparative study between CuO substituted 45S5 BGs and undoped/pure glass ceramic. The research disclosed a decrease in glass nucleation and crystallization temperature as the Cu content was increased from 1% to 4%. However, this effect was not observed for doping of 1% CuO. In addition, it was found that crystallization would decrease bioactivity proses but increasing chemical durability, density, microhardness and flexural strength of the BGs [10]. On the other hand, Lefebvre et al. [11] had reported regarding structural transformation of the 45S5 BGs during thermal treatment. Findings deduced the occurrence of phase separation accompanied with crystallization of Na2CaSi2O6 phase, which led to the appearance of silicate and phosphate phases at a temperature of 580 °C [11]. In addition, a new crystallize phosphate phase ascribed to Na2Ca4(PO4)2SiO4 was also detected [11]. In one study, synthesis of 45S5 BG through an aqueous solution based sol–gel method was successfully accomplished by Cacciotti et al., who discovered that a calcination process would influence

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crystalline phase evolution of the material [12]. Results disclosed that the amorphous phase of the synthesized powder was retained until 800 °C. Beyond 800 °C, sodium calcium silicate phase was observed in the BG treated at 900 °C and 1000 °C. When the temperature reached 1200 °C, the wollastonite phase in the presence of small traces of cristobalite and calcium phosphate silicates was detected [12]. In the present work, porous BG ceramic materials are fabricated. The aim of this study is to investigate the optimization, structural, physical and mechanical properties of the porous BG ceramic via a careful understanding of the process–microstructure–biomechanical property relations by varying sintering temperatures of the 45S5 BGs. 2. Methodology A series of 45S5 glasses was produced via water quenching using reagent grades of silicon (IV) oxide (SiO2) (99.99%, metal basis, Alfa Aesar, Ward Hill), calcium carbonate (CaCO3) (99.5%, metal basis, Alfa Aesar, Ward Hill), Na2CO3 (99.99%, metal basis, Alfa Aesar, Ward Hill), and phosphorus (V) oxide (P2O5) (99.99%, Alfa Aesar, Ward Hill) as starting materials. The chemicals were weighed accurately using an electronic balance and thoroughly mixed by ball milling process at 500 rpm for 20 h in order to improve homogeneity of the powders. Then, the batch was melted at 1380 °C for about 2 h. Then, the molten samples were rapidly quenched in water. The obtained frits were dried overnight at room temperature. After the drying process, the glass frits were crushed and ground for 1 day and sieved at 63 μm. The powdered glass was pressed into cylindrical pellets using a stainless steel mold with a diameter of 13.0 mm and pressure of ~5 tonne. Each pellet was heated at 800, 1000 and 1200 °C. Some of the samples were crushed into fine powders using agate mortar and pestle. X-ray diffraction (XRD) analysis was run using PANalytical X'Pert Pro PW3050/60 diffractometer at a diffraction angle (2θ), ranging from 10° to 70°. Infrared (IR) spectra were recorded using Fourier transform infrared spectroscopy (FTIR) in a frequency range of 280–4000 cm−1 at room temperature. Densities of the glasses were determined using a densimeter that utilized Archimedes' method with ethanol as the immersion liquid. Finally, field emission scanning electron microscopy (FESEM) was carried out on the 45S5 glasses coated with gold (Au) to observe microstructure of the glass powder surface using Nova NanoSEM 30 Series under high vacuum. Energy-dispersive X-ray (EDX) was carried out to detect the presence of elements in the samples. According to Ulery and Drees [13] and Hong et. al. [14], the Au might interfere with elements at energy 2.2 keV and P is among the potential elements that would overlap to some degrees at energies of 2.1–2.2 keV. In order to prevent the overlapping issue, the EDX analysis was done before coating the samples with Au. After the EDX, the samples were coated with Au for the FESEM inspection to prevent charging. 3. Results and discussion 3.1. Density and molar volume The dependence of sintering temperature on density, d and molar volume, Vm of the BG samples is shown in Fig. 1. The d of pure 45S5 BG decreased with an increase in the sintering temperature. The highest d value (2.255 g cm−3) was attained in the un-sintered sample while the lowest d value (0.44 g cm− 3) was obtained at 1200 °C. It was found that Vm of the BG samples demonstrated an opposite trend with respect to that of density. This agreed well with Saddeek, who mentioned the relationship between the d and the Vm of materials [15]. The development of non-bridging oxygen may flayer-up the glass system and thus increase the Vm. The increase in Vm may cause a decrease in oxygen packing density and mass density [16]. This behavior seems to oppose the normal expected phenomenon in which commonly the density would increase with an increase in the sintering temperature. This sintering process is complicated because of the occurrence of

Fig. 1. Density and molar volume of 45S5 BG at different sintering temperatures. Lines are drawn to guide the eye.

phase changes. There will be reactions that may possibly occur during glass sintering at different temperatures, which involve oxidation–reduction, phase transition, and/or solid solution formation. In these reactions, decomposition of oxide materials in the glass may induce some reactive sintering processes, which usually generate additional pores, and thus leading to a reduction in the density. The decrease in density of the glass might be due to low densification exhibited by Na2CO3 particles. It has been reported that decomposition of Na2CO3 occurred in two consequences, as conveniently described in Eq. (1) and Eq. (2) with overall rate, which was extremely low and the reaction was incomplete at temperature up to 1200 °C [17]. Na2 CO3 ¼ Na2 O þ CO2

ð1Þ

1 Na2 O ¼ 2Na þ O2 2

ð2Þ

According to Jong and Lee, the thermal decomposition of pure Na2CO3, started from its melting point (850 °C) and continued as the temperature was increased at a very slow rate [18]. The decomposition of Na2CO3 during the sintering process would produce air trap in the glass and therefore affect the density. 3.2. XRD XRD results of un-sintered and 45S5 powdered glass that was sintered at 800, 1000 and 1200 °C suggested the consecutive transformation of the parent glass into crystalline phases. Fig. 2 shows the XRD patterns of 45S5 before and after sintering at various temperatures (800–1200 °C). The absence of sharp XRD peaks in the un-sintered sample (Fig. 2(a)) indicated the presence of a short range order in the 45S5 samples, which might be attributed to the amorphous phases. The detection of relatively broad peaks at diffraction angles between 15° and 40° reflected the presences of amorphous phases in the sample. In addition, XRD patterns of samples sintered at 800 °C (Fig. 2(b)) deduced the detection of amorphous glassy phase and crystalline phases of anorthic-Na2Ca3Si6O16 (sodium calcium silicate) and orthorhombic-NaCaPO4 (sodium calcium phosphate) with International Centre for Diffraction Data (ICDD) reference codes of 00-023-0671 and 01-076-1456, respectively. The presence of these silicate and phosphate in the glass system would easily cause the occurrence of crystallization in the bioglass [19]. It has been reported earlier that 45S5 BG would be partially crystallized at a starting temperature of 650 °C [20]. Two different crystalline phases associated with one consisting of sodium calcium silicate and the other containing also phosphorus were

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Fig. 2. XRD spectrum of 45S5 at different sintering temperatures [(a) before sintering, (b) 800 °C, (c) 1000 °C, and (d) 1200 °C].

being detected [20]. Salman et al. also found the presence of sodium calcium silicate but with different chemical formulas, which is combeite (Na2Ca2Si3O9) and whitlockite (Ca(PO4)2) [21]. Elbatal and Elkheshen as well as Majhi et. al. had also reported on the presence of sodium calcium silicate but with different chemical compound formulas [(Na2CaSi3O8) and (Na2CaSi3O9)] [19,22]. As the sintering temperature was increased to 1000 °C (Fig. 2(c)), the amorphous glassy phase was still present with a decrease in the intensities of the NaCaPO4 peaks. Besides, additional XRD peaks were detected, which were ascribed to hexagonal-quartz (SiO2) with ICDD reference code of 01-083-0540. The low intensities of SiO2 peaks indicated poorer crystallization due to the presence of a high content of amorphous SiO2 in the sample. The gradual decrease in the intensities of NaCaPO4 with the emergence of SiO2 might be due to an Oswald ripening effect [23], which has caused a differential grain growth to occur after nucleation of principal crystalline phases present in the sample and large crystallites would grow at the expense of smaller ones. After sintering at 1200 °C (Fig. 2(d)), it was seen that XRD peaks associated with NaCaPO4 became more dominant when compared with

the peaks of SiO2 with amorphous glassy phase still present. Besides, the intensities of the XRD peaks of NaCaPO4 were increased after sintering at 1200 °C, indicating maturation of crystallization. 3.3. FTIR Fig. 3 shows the infrared absorption spectra of un-sintered and sintered 45S5 BG sample. The absorption spectra of un-sintered and sintered 45S5 glass showed the bands centered at ~ 450, ~ 560, ~ 760, and ~1010 cm−1 and weak bands at 1430, and 1500 cm−1 also shoulder at ~950 cm−1. The first band at 450 cm−1 is indicated a vibration of angular deformation of Si–O–Si band rocking vibrational modes between SiO4 tetrahedra [24]. The band at 560 cm−1 can assign to Si–O–Ca asymmetric bending mode that exists in the glass. This absorption band agreed the expectation of increasing the bridging oxygen as the silica network is depolymerized by calcium [25]. The peaks at 660–770 cm−1 with three small kinks at 660,710,770 cm−1 can be attributed to P–O–P stretching vibration, denoting the presence of crystalline phosphate in the glasses [26]. This finding is similar with Higazy

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120

27

CO3

100

d

80 Si-O-Ca

60

P-O-P

40 20

PO4

Si-O-Si

0

c

100 80 60 40 20 0

b

100 80

%T

60 40 20 0 100

a

-OH

80 60 40 Si-O (NBO)

20 0

0

500

1000

1500

2000

2500

3000

3500

4000

-1

Wavenumber (cm ) Fig. 3. FTIR spectra of 45S5 at different sintering temperatures [(a) before sintering, (b) 800 °C, (c) 1000 °C, and (d) 1200 °C]. Lines are drawn to guide the eye.

and Bride who studied the CO3O4–P2O5 system [27]. They observed a stretching frequency of P–O–P unit at the 730–780 cm−1 range. The presence of bands at 950 cm−1 at low temperature is assigned for the existence of silicon oxides with non bridging oxygen (Si–O, NBO). The peak at 1010 cm−1 corresponds to PO4 vibration; although the asymmetric vibration Si–O–Si is very strong at 1000–1050 cm− 1 region, causing in the overlapping with the stretching PO4 vibrational peak. The double peak at 1430 and 1500 cm−1 assigned to the symmetric vibrational mode of the CO3 group [28]. As the temperature increased, this peak seems to diminish slowly with decomposition of CaCO3 and Na2CO3 in the glass system. A broad absorption band coming from OH groups appeared at around 3450 cm−1 which due to the absorption of water. 3.4. FESEM The microstructure and morphology of the glass at different sintering temperatures were observed by FESEM as shown in Fig. 4. When the heat treatment was sintered at low temperature (800 °C), the microstructure of the expanded glass mainly was constituted for small irregular porous around 240 μm, while at 1000 °C the porosity

produced was more homogeneous and the pore was well delimited by the wall of tiny pores with an average pore size of ~310 μm. When the heat treatment was carried out at high temperature, the porosity produced was greater when compared to the one at low temperature and the average size of the pore was about 810 μm. The micrograph of the sintered 45S5 was the same as that of ceramic foams by Marques and Bernadin which was obtained from plain glass cullets that expanded via bubble apparent density and the minimum value of density was corresponded to 0.2171 g cm−3. However, the pores were not interconnected to each other. The conclusion could be further confirmed via a decrease in density, which might be related to the formation of more induced crystallization porosity at a higher temperature. There is a tendency for grain growth to occur since there is an interfacial energy associated with a grain boundary, and small grains have a high surface to volume ratio. Grain growth retards densification by increasing the length of the diffusion path, but it is most damaging when the grain grows around pores, leaving a large pore volume that is not intercrossed by grain boundaries [29]. EDX spectra of all the samples have been presented in Fig. 4 for the detection weight percentage of O, Si, P, Ca, and Na by quantitative analysis. A quantitative analysis was performed using INCA microanalysis

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Fig. 4. FESEM micrographs and EDX spectra with calculated Ca/P ratio of 45S5 at different sintering temperatures [(a) 800 °C, (b) 1000 °C, (c) 1200 °C].

system on a standardless analysis. A standardless analysis would quantify the elements by calculating the area under the peak of each identified element and after taking into account for the accelerating voltage of the beam to produce the spectrum. The calculation was done to create a sensitivity factor that would convert the area under the peak into the weight percentage. The calculated Ca/P ratios (mean ± standard deviation) shown in the figure were calculated by taking into consideration the elemental weight percentage of the elements associated with Ca and P. It has been also included in Fig. 4 for the Ca/P ratio of all samples. Indeed, in the 45S5 sample before undergoing sintering, the Ca/P ratio was 4.27 ± 0.53 which slightly differed from the theoretical value (5.2). This theoretical ratio was determined by the 45S5 composition of 45% SiO2, 24.5% Na2O, 24.5% CaO and P2O5 6%, where S denoted a

network former SiO2 in 45% by weight, followed by a Ca/P molar ratio of 5.2 [29]. The calculated Ca/P ratio decreased from 4.26 to 1.85 as the glass underwent a sintering process and with the increase in temperature. As the samples were sintered, there was a possibility for the calcium to decompose. Furthermore, the slight changes in relative intensity ratios as well as the changes in elemental concentration could have also contributed to the decrease in the Ca/P ratio with the increase of sintering temperatures. However the Ca/P value was higher than 1, making it useful for implantation in the body while the compounds with Ca/P ratio less than 1 were not suitable for biological implantation due to the high solubility. The acquisition of Ca/P ratio that was higher than 1.67 might be due to the existence of CaO in the samples [30].

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4. Conclusions Crystallization of 45S5 at different temperatures (800–1200 °C) was studied by using densities, XRD, FTIR, and FESEM. It was suggested that the density decreased due to the presence of additional porosity that occurred as a result of decomposition of oxide materials in the glass. This finding was confirmed by FESEM results, which showed the increase in porosity at high temperature. Sodium calcium silicate (Na2Ca3Si6O16) and sodium calcium phosphate (NaCaPO4) were identified with certainty. Nevertheless, the formation of phosphorus rich secondary crystals at higher temperatures was considered to be highly possible. The intensity of the signal characteristic of NaCaPO4 was increased after treatment at 1200 °C, indicating maturation of the crystallization of this phase. Acknowledgment The authors gratefully acknowledge the financial support from the Malaysian Ministry of Higher Education (MOHE) through the Exploratory Research Grant Scheme (5527189). One of the authors (W.F. Lim) would like to thank the financial support from the Universiti Putra Malaysia Post-Doctoral Fellowship. References [1] M. Surajit, K. Debabrata, r D. Someswa, B. Debabrata, S. Chidambaram, Indigenous hydroxyapatite coated and bioactive glass coated titanium dental implant system — fabrication and application in humans, J. Indian Soc. Periodontol. 15 (2011). [2] S. Mistry, D. Kundu, S. Datta, D. Basu, C. Soundrapandian, Indigenous hydroxyapatite coated and bioactive glass coated titanium dental implant system — fabrication and application in humans, J. Indian Soc. Periodontol. 15 (2011) 215–220. [3] G. Sàndor, T. Lindholm, C. Clokie, Bone regeneration of the cranio-maxillofacial and dento-alveolar skeletons in the framework of tissue engineering, in: N.A.P. Ferretti (Ed.), Topics in Tissue Engineering, University of Oulu, 2003, p. 19. [4] L.L. Hench, The story of bioglass, J. Mater. Sci. Mater. Med. 17 (2006) 967–978. [5] H. Fujita, H. Iida, K. Ido, Y. Matsuda, M. Oka, T. Nakamura, Porous apatite–wollastonite glass–ceramic as an intramedullary plug, J. Bone Joint Surg. (Br.) 82 (2000) 614–618. [6] N. Madan, N. Madan, V. Sharma, M. Gulati, D. Pardal, Tooth remineralization using bio-active glass a novel approach, Baba Farid Univ. Dent. J. 2 (2011) 64–67. [7] Q.Z. Chen, I.D. Thompson, A.R. Boccaccini, 45S5 Bioglass®-derived glass–ceramic scaffolds for bone tissue engineering, Biomaterials 27 (2006) 2414–2425. [8] M.N. Rahaman, D.E. Day, B.S. Bal, Q. Fu, S.B. Jung, L.F. Bonewald, A.P. Tomsia, Bioactive glass in tissue engineering, Acta Biomater. 7 (2011) 2355–2373. [9] S. Yue, P.D. Lee, G. Poologasundarampillai, J.R. Jones, Evaluation of 3-D bioactive glass scaffolds dissolution in a perfusion flow system with X-ray microtomography, Acta Biomater. 7 (2011) 2637–2643.

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