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A modified method to synthesize single-phase forsterite nanoparticles at low temperature
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A modified method to synthesize single-phase forsterite nanoparticles at low temperature S.M. Mirhadi a,n, A. Forghani b, F. Tavangarian c a
Department of Materials Engineering, Shahreza Branch, Islamic Azad University, 86145 311 Shahreza, Isfahan, Iran Department of Mechanical and Industrial Engineering, Louisiana State University, Baton Rouge, LA 70803, United States c Department of Engineering and Technology Management, Morehead State University, Morehead, KY 40351, United States b
art ic l e i nf o
a b s t r a c t
Article history: Received 11 January 2016 Received in revised form 29 January 2016 Accepted 30 January 2016
In this paper forsterite (Mg2SiO4) nanopowder with particle size in the range of 33 and 112 nm was synthesized by a combination of sol–gel and ball milling methods. Magnesium nitrate and silica were used as the sources of magnesium and silicon in the forsterite nanopowder. Thermogravimetry (TG) analysis, X-ray diffraction (XRD), scanning electron microscopy (SEM), transmission electron microscopy (TEM), Fourier transform infrared spectroscopy (FT-IR) and dynamic light scattering (DLS) techniques were utilized to characterize the synthesized powders. Single-phase nanocrystalline forsterite powder with mean crystallite size of about 16 nm was obtained from sol–gel method with subsequent ball milling for 5 h and heat treatment at 750 °C for 1 h. A combination of sol–gel and mechanical activation led to the formation of more homogeneous powder and subsequently lower sintering temperature to produce forsterite powder. In vitro biological studies were performed by immersing the forsterite samples in simulated body fluid (SBF). The results showed that nanostructure forsterite is bioactive and possessed apatite formation ability. & 2016 Elsevier Ltd and Techna Group S.r.l. All rights reserved.
Keywords: A. Milling A. Sol–gel processes E. Biomedical applications
1. Introduction Forsterite with the chemical formula Mg2SiO4 has attracted the attention of many scientists as a bioceramic in the recent decades [1,2]. Si and Mg components of forsterite play a vital role in the growth and mineralization of the bones in human bodies [3,4]. It is proven that Si containing materials such as larnite [5], Wollastonite [6], and tricalcium silicate [7] have excellent bioactivity both in vivo and in vitro. In some studies performed by Carlisle [8,9] it was found that silicon (more than 5 wt%) was uniquely localized in the active growth areas in the bones of young rats (when the Ca/P ratio was 0.7) and involved in the early stage of bone calcification under physiological conditions. Similar studies showed that the growth rate of rats was increased by adding silicon to their diets. Furthermore, it was found that silicon deficiency in rats resulted in skull deformations [4,10]. Magnesium also plays a significant role in the bone fragility, bone-growth, osteoblastic activities and degradation of hydroxyapatite crystals [11]. In clinical experiments, it is proven that Mg-containing materials such as diopside [12], bredigite [13] and merwinite [14] can be promising candidates for bone tissue engineering at load bearing applications. n
Corresponding author. Tel.: þ 98 321 3232706; fax: þ 98 321 3232701. E-mail address: [email protected] (S.M. Mirhadi).
Forsterite has been synthesized by various techniques such as the polymer precursor method [15], heat treatment of the mixed powders prepared by alkoxymethod [16], the sol–gel method [17–19] and ball milling method [20–22]. During the synthesis of forsterite, it is very difficult to avoid the formation of enstatite (MgSiO3) or/and periclase (MgO). Annealing up to 1200–1600 °C is reported to be necessary to obtain single phase forsterite structure [17]. In this paper forsterite was synthesized at much lower temperature, regarding to the conventional synthesis method, by a combination of sol–gel and mechanical activation methods. The forsterite formation mechanism was studied as well. Mechanical activation method partially provides the required activation energy for the forsterite formation and hence decrease the required sintering temperature. Furthermore, the in vitro bioactivity of forsterite was studied by immersing the samples in simulated body fluid (SBF).
2. Experimental procedures 2.1. Samples preparation Forsterite powder was synthesized according to the following steps: sol–gel, ball milling and subsequent annealing at 600–1000 °C
http://dx.doi.org/10.1016/j.ceramint.2016.01.195 0272-8842/& 2016 Elsevier Ltd and Techna Group S.r.l. All rights reserved.
Please cite this article as: S.M. Mirhadi, et al., A modified method to synthesize single-phase forsterite nanoparticles at low temperature, Ceramics International (2016), http://dx.doi.org/10.1016/j.ceramint.2016.01.195i
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for 1 h. The initial materials for the synthesis of forsterite powder are listed in Table 1. Fig. 1 shows the forsterite synthesis procedure. Preparation of the sol–gel is described in [23]. Briefly, proper amount of magnesium nitrate and silica was dissolved in deionized water utilizing magnetic stirrer to result stoichiometric forsterite structure with Mg:Si molar ratio of 2:1. A solution of sucrose with the molar ratio of sucrose to metal equal to 4:1 was prepared. Then this solution was added gently to nitrate solution while it was stirring. Subsequently a solution of polyvinyl alcohol with PVA to metal molar ratio of 0.5:1 was added to the obtained sucrose and nitrate solution and then the pH was adjusted to 1 utilizing nitric acid. Subsequently, the solution was heated at 200 °C for 24 h. The obtained mass was then ball milled for 5 h in a planetary ball mill under ambient conditions. The obtained powder was called RS (stands for raw sample). The milling media consisted of a zirconia vial with five 20 mm zirconia balls. In all milling runs the ball-to-powder weight ratio was 5:1 and the rotational speed of main disc was 500 rpm. Heat treatment of ball milled powders was carried out from 600 to 1000 °C for 1 h in air. Table 1 Initial materials for the synthesis of forsterite powder. Material
Formula
Company
Purity
Magnesium nitrate Silica Polyvinyl alcohol Sucrose Nitric acid
Mg(NO3)2 6H2O SiO2 [–CH2CHOH–]n C12H22O11 HNO3
Sigma Sigma Sigma Sigma Sigma
99.0% 99.5% 99.0% 99.5% 99.9%
Aldrich Aldrich Aldrich Aldrich Aldrich
2.2. Samples characterization To evaluate the thermal behavior of the ball milled powder, thermogravimetry (TG) test was performed up to 1200 °C in air at a heating rate of 10 °C/min. Phase transformation of the obtained powders after heat treatment was investigated by X-ray diffractometry (XRD) using a Philips X'PERT MPD diffractometer with Cu Kα radiation (λ ¼0.154056 nm). The XRD patterns were recorded in the 2θ range of 20–80° (a step size of 0.04° and a time per step of 1 s). The morphology of powder particles was studied by field emission scanning electron microscopy (SEM) (Quanta3D FEG, FEI Company, USA) at acceleration voltages between 20 kV. Forsterite powder particle size was measured by UTHSCSA Image Tool 3 software. The transmission electron microscopy (TEM; JEM1400) technique was utilized to characterize the morphology and the structure of the prepared powders. The particle size distribution of obtained forsterite powder was measured by dynamic light scattering (Zetasizer Nano-ZS). Fourier transform infrared (FT-IR) spectroscopy analysis (Bomem, MB 100) was carried out to identify the functional groups. The spectrum was recorded in the 4000–400 cm 1 region with a resolution of 2 cm 1. The bioactivity of forsterite was evaluated by the procedure described in Kokubo et al. [24]. For this purpose, first the forsterite powders were pressed in the form of cylindrical samples and then immersed in SBF (pH 7.40) at 37 °C for up to 21 days. The ratio of solution volume to sample mass was 200 ml/g. After soaking, the samples were dried at 120 °C for 1 day.
Fig. 1. Schematic representation of the forsterite synthesis procedure.
Please cite this article as: S.M. Mirhadi, et al., A modified method to synthesize single-phase forsterite nanoparticles at low temperature, Ceramics International (2016), http://dx.doi.org/10.1016/j.ceramint.2016.01.195i
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3. Results and discussions 3.1. Thermal analysis Fig. 2 shows the thermal gravimetric (TG) and differential thermal gravimetric (DTG) analyses curves of RS. Three main stages were observed in the TG curve. The first stage (5%) belonged to the region below 200 °C, accompanied with an endothermic peak at around 130 due to the removal of free water. The second stage of weight loss (47%) was observed from 200 to 600 °C which is accompanied with two exothermic peaks at around 320 and 500 °C can be assigned to the decomposition of precursor, burning of carbon and formation of magnesium oxide, respectively [23]. The third stage of weight loss (7%) accompanied with an exothermic peak at around 715 °C is due to the formation of forsterite powder [23,25]. Forsterite forms through diffusion-controlled mechanism. Hence the exothermic peak assigned to the formation of forsterite structure is spread from 650 to about 800 °C. 3.2. XRD phase analysis Based on the results obtained from thermal analysis, RS powders were annealed at various temperatures and the produced powders were characterized by X-ray diffraction method to reveal the phase transformations occur during heat treatment. The XRD patterns of the prepared RS powders before and after sintering at various temperatures for 1 h are shown in Fig. 3. An amorphous structure was observed on the XRD pattern of RS powder. Annealing of RS powders at 600 and 700 °C for 1 h led to the formation of MgO (XRD JCPDS data file no. 43-1022) in the structure. With increasing the annealing temperature to 750 °C, forsterite peaks (XRD JCPDS data file no. 34-0189) were observed in the XRD pattern. With increasing the annealing temperature to 1000 °C, the intensity of XRD peaks increased while their width decreased due to the recovery of internal strain and growth of crystallite size. No other phases were observed in the XRD pattern of annealed samples between 750 and 1000 °C. Forsterite formation mechanism was investigated in our previous study [26]. Initially magnesium oxide forms in the structure and then the produced MgO reacts at the surface of SiO2 to form enstatite. Subsequently, MgO diffuses through the enstatite layer and form forsterite phase. Decreasing the diffusion distances through producing smaller particles and formation of a homogenous powder cause the faster formation of forsterite structure at lower temperatures. Controlling the sol–gel method is difficult due to the sensitivity of chemicals to the environment and subsequently a non-homogenous structure may form. As a result, the chemical composition of the final product may alter and does not match the stoichiometric amount. Saberi et al. [23] reported the synthesis of forsterite structure by the sol–gel method however they observed MgO phase on the XRD patterns of heat treated samples below
Fig. 3. XRD patterns of RS powders after sintering at various temperatures for 1 h.
800 °C. The absence of periclase on the XRD patterns in this study indicates that during mechanical activation, a homogeneous powder mixture was obtained and caused the formation of singlephase nanostructure forsterite. 3.3. SEM evaluation The morphology of the obtained forsterite powders after sintering at 750 and 1000 °C is shown in Fig. 4a and b, respectively. As can be seen in both samples, the obtained powders consist of agglomerates with almost uniform particle size distribution. Fig. 4c and d shows the morphology of obtained forsterite particles at higher magnifications after sintering at 750 and 1000 °C for 1 h, respectively. With increasing the annealing temperature, the particle size of forsterite powder increased due to the grain growth phenomenon. However, both samples showed the formation of forsterite nano-powder. Image tool analysis was used to measure the particle size of forsterite nano-powder. It was found that the mean particle size of obtained forsterite structure after sintering at 750 and 1000 °C was 26 and 32 nm, respectively. 3.4. DLS evaluation
Fig. 2. DTG and TG curves of RS powder.
In order to determine the particle size distribution of forsterite nanopowder, dynamic light scattering (DLS) was performed on forsterite powder obtained after sintering at 750 °C for 1 h. Fig. 5 shows the particle size distribution of the forsterite nonopowder. As can be seen, the forsterite nanopowder particles had a wide range distribution from 33 to 112 nm with the mean particle size of 58 nm.
Please cite this article as: S.M. Mirhadi, et al., A modified method to synthesize single-phase forsterite nanoparticles at low temperature, Ceramics International (2016), http://dx.doi.org/10.1016/j.ceramint.2016.01.195i
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Fig. 4. SEM micrographs of obtained forsterite powders after sintering at (a), (c) 750 and (b), (d) 1000 °C for 1 h.
Fig. 5. The particle size distribution of the forsterite powder obtained after annealing at 750 °C for 1 h.
3.5. TEM evaluation TEM analysis was performed to investigate the morphology and crystallite size of the forsterite nanopowder after sintering at 750 for 1 h. Fig. 6 shows the morphology and size of forsterite nanopowder crystallites. It is obvious that the crystallites of forsterite powder exhibit agglomerative morphologies with irregular shapes. The crystallite size of the obtained forsterite nanopowder was in the range of 7–28 nm with a mean of about 16 nm. 3.6. In vitro bioactivity evaluation
Fig. 6. TEM micrograph of single-phase nanostructure forsterite powder obtained after annealing at 750 °C for 1 h.
samples and soaked in SBF for up to 21 days. Fig. 7 shows the SEM micrograph and the EDX spectra of the immersed sample in SBF for 21 days. After 21-day soaking, small particles (mean size 10 μm) were observed on the surface of forsterite samples. The EDX spectra (Fig. 7b) showed that these particles were composed of calcium and phosphorus which proved the formation of an apatite layer on the surface of forsterite sample after soaking in SBF. 3.7. FTIR evaluation
In order to investigate the bioactivity of the prepared nanostructure forsterite, first the obtained forsterite powder (sintered at 750 °C for 1 h) was uniaxially pressed in the form of cylindrical
The infrared spectra of the prepared forsterite specimens before and after soaking in SBF for 14 and 21 days are shown in Fig. 8.
Please cite this article as: S.M. Mirhadi, et al., A modified method to synthesize single-phase forsterite nanoparticles at low temperature, Ceramics International (2016), http://dx.doi.org/10.1016/j.ceramint.2016.01.195i
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Fig. 7. SEM micrograph and the EDX spectra of the surface of the immersed forsterite in SBF for 21 days.
4. Conclusion This paper showed that a combination of sol–gel and mechanical activation methods forms a more homogenous powder and subsequently affects the easier formation of final product. The results showed that single-phase nanostructure forsterite powder can be obtained after sintering RS powders at 750 °C for 1 h. The absence of other phases such as MgSiO3, MgO and SiO2 in the final product disclosed the beneficial effects of utilizing sol–gel and mechanical activation procedures together. The mean crystallite size and particle size of the obtained nanostructure forsterite were 16 nm and 58 nm, respectively. Furthermore in vitro study showed that nanostructure forstertie, unlike micrometer-sized forsterite, has the apatite formation ability and is bioactive.
Fig. 8. FTIR spectra of forsterite (a) before, and (b), (c) after soaking in SBF for 14 and 21 days.
As can be seen in Fig. 8a, the infrared spectra of forsterite specimens before immersion in SBF corresponded to the characteristic bands of forsterite. The bands related to the characteristic bands of forsterite appeared in the range of 1100–800 cm 1 related to the SiO4 stretching, 650–500 cm 1 related to the SiO4 bending, and at 475 cm 1 due to the MgO6 modes. These results confirmed the formation of forsterite as previously was observed in the XRD pattern. The position of FTIR bands of the prepared forsterite agreed with the results reported in previous studies [27]. With immersing the samples in SBF, new absorption bands related to O– H, C–O, and P–O were observed. The bands at 3500 and 1621 cm 1 are as a result of the formation of hydroxyl groups in the hydroxyapatite. Those bands at 1462 and 1420 cm 1 can be attributed to the formation of carbonate groups of apatite structure [28]. The bands at about 2900 cm 1 may be assigned to H–C–O functional groups [29]. Furthermore, bands related to phosphate groups were located in the range of 1100–1000, and 550–600 cm 1. These three bands are the characteristic bands of apatite crystals which suggest the formation of apatite on the surface layer of forsterite samples after soaking in SBF [30]. With increasing the soaking time the intensity of O–H, C–O, and P–O absorption bands increased as a result of the formation of higher amount of hydroxycarbonate apatite (HCA) on the surface of forsterite samples [28]. These findings showed the formation of apatite on the surface of the specimens after soaking in SBF. The immersion test in SBF and formation of amorphous HCA precipitation on the surface of materials could be regarded as the established test in order to determine the bioactivity of materials [17].
Acknowledgments This investigation was partially supported by the Research and Creative Productions Committee at Morehead State University and by Kimiya Darou Co Ltd. under Contract number 2896. Also the authors thanks members of the Materials Science Laboratory of the Islamic Azad University of Shahreza for their helpful contributions to this work.
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Please cite this article as: S.M. Mirhadi, et al., A modified method to synthesize single-phase forsterite nanoparticles at low temperature, Ceramics International (2016), http://dx.doi.org/10.1016/j.ceramint.2016.01.195i
Contents lists available at ScienceDirect
Ceramics International journal homepage: www.elsevier.com/locate/ceramint
A modified method to synthesize single-phase forsterite nanoparticles at low temperature S.M. Mirhadi a,n, A. Forghani b, F. Tavangarian c a
Department of Materials Engineering, Shahreza Branch, Islamic Azad University, 86145 311 Shahreza, Isfahan, Iran Department of Mechanical and Industrial Engineering, Louisiana State University, Baton Rouge, LA 70803, United States c Department of Engineering and Technology Management, Morehead State University, Morehead, KY 40351, United States b
art ic l e i nf o
a b s t r a c t
Article history: Received 11 January 2016 Received in revised form 29 January 2016 Accepted 30 January 2016
In this paper forsterite (Mg2SiO4) nanopowder with particle size in the range of 33 and 112 nm was synthesized by a combination of sol–gel and ball milling methods. Magnesium nitrate and silica were used as the sources of magnesium and silicon in the forsterite nanopowder. Thermogravimetry (TG) analysis, X-ray diffraction (XRD), scanning electron microscopy (SEM), transmission electron microscopy (TEM), Fourier transform infrared spectroscopy (FT-IR) and dynamic light scattering (DLS) techniques were utilized to characterize the synthesized powders. Single-phase nanocrystalline forsterite powder with mean crystallite size of about 16 nm was obtained from sol–gel method with subsequent ball milling for 5 h and heat treatment at 750 °C for 1 h. A combination of sol–gel and mechanical activation led to the formation of more homogeneous powder and subsequently lower sintering temperature to produce forsterite powder. In vitro biological studies were performed by immersing the forsterite samples in simulated body fluid (SBF). The results showed that nanostructure forsterite is bioactive and possessed apatite formation ability. & 2016 Elsevier Ltd and Techna Group S.r.l. All rights reserved.
Keywords: A. Milling A. Sol–gel processes E. Biomedical applications
1. Introduction Forsterite with the chemical formula Mg2SiO4 has attracted the attention of many scientists as a bioceramic in the recent decades [1,2]. Si and Mg components of forsterite play a vital role in the growth and mineralization of the bones in human bodies [3,4]. It is proven that Si containing materials such as larnite [5], Wollastonite [6], and tricalcium silicate [7] have excellent bioactivity both in vivo and in vitro. In some studies performed by Carlisle [8,9] it was found that silicon (more than 5 wt%) was uniquely localized in the active growth areas in the bones of young rats (when the Ca/P ratio was 0.7) and involved in the early stage of bone calcification under physiological conditions. Similar studies showed that the growth rate of rats was increased by adding silicon to their diets. Furthermore, it was found that silicon deficiency in rats resulted in skull deformations [4,10]. Magnesium also plays a significant role in the bone fragility, bone-growth, osteoblastic activities and degradation of hydroxyapatite crystals [11]. In clinical experiments, it is proven that Mg-containing materials such as diopside [12], bredigite [13] and merwinite [14] can be promising candidates for bone tissue engineering at load bearing applications. n
Corresponding author. Tel.: þ 98 321 3232706; fax: þ 98 321 3232701. E-mail address: [email protected] (S.M. Mirhadi).
Forsterite has been synthesized by various techniques such as the polymer precursor method [15], heat treatment of the mixed powders prepared by alkoxymethod [16], the sol–gel method [17–19] and ball milling method [20–22]. During the synthesis of forsterite, it is very difficult to avoid the formation of enstatite (MgSiO3) or/and periclase (MgO). Annealing up to 1200–1600 °C is reported to be necessary to obtain single phase forsterite structure [17]. In this paper forsterite was synthesized at much lower temperature, regarding to the conventional synthesis method, by a combination of sol–gel and mechanical activation methods. The forsterite formation mechanism was studied as well. Mechanical activation method partially provides the required activation energy for the forsterite formation and hence decrease the required sintering temperature. Furthermore, the in vitro bioactivity of forsterite was studied by immersing the samples in simulated body fluid (SBF).
2. Experimental procedures 2.1. Samples preparation Forsterite powder was synthesized according to the following steps: sol–gel, ball milling and subsequent annealing at 600–1000 °C
http://dx.doi.org/10.1016/j.ceramint.2016.01.195 0272-8842/& 2016 Elsevier Ltd and Techna Group S.r.l. All rights reserved.
Please cite this article as: S.M. Mirhadi, et al., A modified method to synthesize single-phase forsterite nanoparticles at low temperature, Ceramics International (2016), http://dx.doi.org/10.1016/j.ceramint.2016.01.195i
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for 1 h. The initial materials for the synthesis of forsterite powder are listed in Table 1. Fig. 1 shows the forsterite synthesis procedure. Preparation of the sol–gel is described in [23]. Briefly, proper amount of magnesium nitrate and silica was dissolved in deionized water utilizing magnetic stirrer to result stoichiometric forsterite structure with Mg:Si molar ratio of 2:1. A solution of sucrose with the molar ratio of sucrose to metal equal to 4:1 was prepared. Then this solution was added gently to nitrate solution while it was stirring. Subsequently a solution of polyvinyl alcohol with PVA to metal molar ratio of 0.5:1 was added to the obtained sucrose and nitrate solution and then the pH was adjusted to 1 utilizing nitric acid. Subsequently, the solution was heated at 200 °C for 24 h. The obtained mass was then ball milled for 5 h in a planetary ball mill under ambient conditions. The obtained powder was called RS (stands for raw sample). The milling media consisted of a zirconia vial with five 20 mm zirconia balls. In all milling runs the ball-to-powder weight ratio was 5:1 and the rotational speed of main disc was 500 rpm. Heat treatment of ball milled powders was carried out from 600 to 1000 °C for 1 h in air. Table 1 Initial materials for the synthesis of forsterite powder. Material
Formula
Company
Purity
Magnesium nitrate Silica Polyvinyl alcohol Sucrose Nitric acid
Mg(NO3)2 6H2O SiO2 [–CH2CHOH–]n C12H22O11 HNO3
Sigma Sigma Sigma Sigma Sigma
99.0% 99.5% 99.0% 99.5% 99.9%
Aldrich Aldrich Aldrich Aldrich Aldrich
2.2. Samples characterization To evaluate the thermal behavior of the ball milled powder, thermogravimetry (TG) test was performed up to 1200 °C in air at a heating rate of 10 °C/min. Phase transformation of the obtained powders after heat treatment was investigated by X-ray diffractometry (XRD) using a Philips X'PERT MPD diffractometer with Cu Kα radiation (λ ¼0.154056 nm). The XRD patterns were recorded in the 2θ range of 20–80° (a step size of 0.04° and a time per step of 1 s). The morphology of powder particles was studied by field emission scanning electron microscopy (SEM) (Quanta3D FEG, FEI Company, USA) at acceleration voltages between 20 kV. Forsterite powder particle size was measured by UTHSCSA Image Tool 3 software. The transmission electron microscopy (TEM; JEM1400) technique was utilized to characterize the morphology and the structure of the prepared powders. The particle size distribution of obtained forsterite powder was measured by dynamic light scattering (Zetasizer Nano-ZS). Fourier transform infrared (FT-IR) spectroscopy analysis (Bomem, MB 100) was carried out to identify the functional groups. The spectrum was recorded in the 4000–400 cm 1 region with a resolution of 2 cm 1. The bioactivity of forsterite was evaluated by the procedure described in Kokubo et al. [24]. For this purpose, first the forsterite powders were pressed in the form of cylindrical samples and then immersed in SBF (pH 7.40) at 37 °C for up to 21 days. The ratio of solution volume to sample mass was 200 ml/g. After soaking, the samples were dried at 120 °C for 1 day.
Fig. 1. Schematic representation of the forsterite synthesis procedure.
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3. Results and discussions 3.1. Thermal analysis Fig. 2 shows the thermal gravimetric (TG) and differential thermal gravimetric (DTG) analyses curves of RS. Three main stages were observed in the TG curve. The first stage (5%) belonged to the region below 200 °C, accompanied with an endothermic peak at around 130 due to the removal of free water. The second stage of weight loss (47%) was observed from 200 to 600 °C which is accompanied with two exothermic peaks at around 320 and 500 °C can be assigned to the decomposition of precursor, burning of carbon and formation of magnesium oxide, respectively [23]. The third stage of weight loss (7%) accompanied with an exothermic peak at around 715 °C is due to the formation of forsterite powder [23,25]. Forsterite forms through diffusion-controlled mechanism. Hence the exothermic peak assigned to the formation of forsterite structure is spread from 650 to about 800 °C. 3.2. XRD phase analysis Based on the results obtained from thermal analysis, RS powders were annealed at various temperatures and the produced powders were characterized by X-ray diffraction method to reveal the phase transformations occur during heat treatment. The XRD patterns of the prepared RS powders before and after sintering at various temperatures for 1 h are shown in Fig. 3. An amorphous structure was observed on the XRD pattern of RS powder. Annealing of RS powders at 600 and 700 °C for 1 h led to the formation of MgO (XRD JCPDS data file no. 43-1022) in the structure. With increasing the annealing temperature to 750 °C, forsterite peaks (XRD JCPDS data file no. 34-0189) were observed in the XRD pattern. With increasing the annealing temperature to 1000 °C, the intensity of XRD peaks increased while their width decreased due to the recovery of internal strain and growth of crystallite size. No other phases were observed in the XRD pattern of annealed samples between 750 and 1000 °C. Forsterite formation mechanism was investigated in our previous study [26]. Initially magnesium oxide forms in the structure and then the produced MgO reacts at the surface of SiO2 to form enstatite. Subsequently, MgO diffuses through the enstatite layer and form forsterite phase. Decreasing the diffusion distances through producing smaller particles and formation of a homogenous powder cause the faster formation of forsterite structure at lower temperatures. Controlling the sol–gel method is difficult due to the sensitivity of chemicals to the environment and subsequently a non-homogenous structure may form. As a result, the chemical composition of the final product may alter and does not match the stoichiometric amount. Saberi et al. [23] reported the synthesis of forsterite structure by the sol–gel method however they observed MgO phase on the XRD patterns of heat treated samples below
Fig. 3. XRD patterns of RS powders after sintering at various temperatures for 1 h.
800 °C. The absence of periclase on the XRD patterns in this study indicates that during mechanical activation, a homogeneous powder mixture was obtained and caused the formation of singlephase nanostructure forsterite. 3.3. SEM evaluation The morphology of the obtained forsterite powders after sintering at 750 and 1000 °C is shown in Fig. 4a and b, respectively. As can be seen in both samples, the obtained powders consist of agglomerates with almost uniform particle size distribution. Fig. 4c and d shows the morphology of obtained forsterite particles at higher magnifications after sintering at 750 and 1000 °C for 1 h, respectively. With increasing the annealing temperature, the particle size of forsterite powder increased due to the grain growth phenomenon. However, both samples showed the formation of forsterite nano-powder. Image tool analysis was used to measure the particle size of forsterite nano-powder. It was found that the mean particle size of obtained forsterite structure after sintering at 750 and 1000 °C was 26 and 32 nm, respectively. 3.4. DLS evaluation
Fig. 2. DTG and TG curves of RS powder.
In order to determine the particle size distribution of forsterite nanopowder, dynamic light scattering (DLS) was performed on forsterite powder obtained after sintering at 750 °C for 1 h. Fig. 5 shows the particle size distribution of the forsterite nonopowder. As can be seen, the forsterite nanopowder particles had a wide range distribution from 33 to 112 nm with the mean particle size of 58 nm.
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Fig. 4. SEM micrographs of obtained forsterite powders after sintering at (a), (c) 750 and (b), (d) 1000 °C for 1 h.
Fig. 5. The particle size distribution of the forsterite powder obtained after annealing at 750 °C for 1 h.
3.5. TEM evaluation TEM analysis was performed to investigate the morphology and crystallite size of the forsterite nanopowder after sintering at 750 for 1 h. Fig. 6 shows the morphology and size of forsterite nanopowder crystallites. It is obvious that the crystallites of forsterite powder exhibit agglomerative morphologies with irregular shapes. The crystallite size of the obtained forsterite nanopowder was in the range of 7–28 nm with a mean of about 16 nm. 3.6. In vitro bioactivity evaluation
Fig. 6. TEM micrograph of single-phase nanostructure forsterite powder obtained after annealing at 750 °C for 1 h.
samples and soaked in SBF for up to 21 days. Fig. 7 shows the SEM micrograph and the EDX spectra of the immersed sample in SBF for 21 days. After 21-day soaking, small particles (mean size 10 μm) were observed on the surface of forsterite samples. The EDX spectra (Fig. 7b) showed that these particles were composed of calcium and phosphorus which proved the formation of an apatite layer on the surface of forsterite sample after soaking in SBF. 3.7. FTIR evaluation
In order to investigate the bioactivity of the prepared nanostructure forsterite, first the obtained forsterite powder (sintered at 750 °C for 1 h) was uniaxially pressed in the form of cylindrical
The infrared spectra of the prepared forsterite specimens before and after soaking in SBF for 14 and 21 days are shown in Fig. 8.
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Fig. 7. SEM micrograph and the EDX spectra of the surface of the immersed forsterite in SBF for 21 days.
4. Conclusion This paper showed that a combination of sol–gel and mechanical activation methods forms a more homogenous powder and subsequently affects the easier formation of final product. The results showed that single-phase nanostructure forsterite powder can be obtained after sintering RS powders at 750 °C for 1 h. The absence of other phases such as MgSiO3, MgO and SiO2 in the final product disclosed the beneficial effects of utilizing sol–gel and mechanical activation procedures together. The mean crystallite size and particle size of the obtained nanostructure forsterite were 16 nm and 58 nm, respectively. Furthermore in vitro study showed that nanostructure forstertie, unlike micrometer-sized forsterite, has the apatite formation ability and is bioactive.
Fig. 8. FTIR spectra of forsterite (a) before, and (b), (c) after soaking in SBF for 14 and 21 days.
As can be seen in Fig. 8a, the infrared spectra of forsterite specimens before immersion in SBF corresponded to the characteristic bands of forsterite. The bands related to the characteristic bands of forsterite appeared in the range of 1100–800 cm 1 related to the SiO4 stretching, 650–500 cm 1 related to the SiO4 bending, and at 475 cm 1 due to the MgO6 modes. These results confirmed the formation of forsterite as previously was observed in the XRD pattern. The position of FTIR bands of the prepared forsterite agreed with the results reported in previous studies [27]. With immersing the samples in SBF, new absorption bands related to O– H, C–O, and P–O were observed. The bands at 3500 and 1621 cm 1 are as a result of the formation of hydroxyl groups in the hydroxyapatite. Those bands at 1462 and 1420 cm 1 can be attributed to the formation of carbonate groups of apatite structure [28]. The bands at about 2900 cm 1 may be assigned to H–C–O functional groups [29]. Furthermore, bands related to phosphate groups were located in the range of 1100–1000, and 550–600 cm 1. These three bands are the characteristic bands of apatite crystals which suggest the formation of apatite on the surface layer of forsterite samples after soaking in SBF [30]. With increasing the soaking time the intensity of O–H, C–O, and P–O absorption bands increased as a result of the formation of higher amount of hydroxycarbonate apatite (HCA) on the surface of forsterite samples [28]. These findings showed the formation of apatite on the surface of the specimens after soaking in SBF. The immersion test in SBF and formation of amorphous HCA precipitation on the surface of materials could be regarded as the established test in order to determine the bioactivity of materials [17].
Acknowledgments This investigation was partially supported by the Research and Creative Productions Committee at Morehead State University and by Kimiya Darou Co Ltd. under Contract number 2896. Also the authors thanks members of the Materials Science Laboratory of the Islamic Azad University of Shahreza for their helpful contributions to this work.
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