Mechanochemical behavior of CaCO3–P2O5–CaF2 system to produce carbonated fluorapatite nanopowder

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Mechanochemical behavior of CaCO3–P2O5–CaF2 system to produce carbonated fluorapatite nanopowder Abbas Fahami, Bahman Nasiri-Tabrizi

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Cite this article as: Abbas Fahami, Bahman Nasiri-Tabrizi, Mechanochemical behavior of CaCO3–P2O5–CaF2 system to produce carbonated fluorapatite nanopowder, Ceramics International, http://dx.doi.org/10.1016/j.ceramint.2014.06.091 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.

Mechanochemical behavior of CaCO3–P2O5–CaF2 system to produce carbonated fluorapatite nanopowder Abbas Fahami, Bahman Nasiri-Tabrizi* Advanced Materials Research Center, Materials Engineering Department, Najafabad Branch, Islamic Azad University, Isfahan, Iran

Abstract The influence of milling time on mechanochemical behavior of CaCO3–P2O5–CaF2 system to produce carbonated fluorapatite nanopowder was studied. Results showed that the formation of n-CFAp was influenced noticeably by the milling time. At the beginning of milling (up to 30 min), CaF2 and CaCO3 were dominant phases, while P2O5 disappeared entirely due to its very high hydrophilic nature. With increasing the milling time to 600 min, the progressive mechanochemical reaction was completed which resulted in the formation of nanosized B–type carbonated fluorapatite. According to the obtained data, crystallite size of the product decreased from 77±4 to 69±3 nm when the milling time increased from 300 to 600 min, respectively. Microscopic observations illustrated that the final product had a cluster-like structure which was composed of both the spheroidal and ellipsoidal particles with an average size of around 45 and 90 nm, respectively. The present findings suggest that the room-temperature solid-state reaction can lead to the formation of nanostructured carbonated fluorapatite which can be a promising candidate for using in biomedical applications. Keywords: Carbonated fluorapatite; Milling time; Nanoparticles; Structural features; Microscopic observation.

1. Introduction The recent advances in bioceramics are predominantly focused on calcium phosphate apatites which have importance in geochemistry, biology, agriculture, and materials science. [13]. Among different forms of calcium phosphates, hydroxyapatite (HAp, Ca10(PO4)6(OH)2), fluorapatite (FAp, Ca10(PO4)6F2) and chlorapatite (CAp, Ca10(PO4)6Cl2] as well as carbonated apatite (CA, Ca10-x/2[(PO4)6-x(CO3)x][(Z)2-2y(CO3)y]), Z = OH, F, Cl) have been considerably applied in biomedical applications as potential bone-substitute materials owing to their biocompatibility and osteoconductivity [4]. More recently, however, attention has been focused on carbonated apatites in which carbonate ions (CO32) substitute in either hydroxyl (OH1), which is known as type A, or phosphate (PO43) sites (as type B) to form a bioactive apatite with enhanced dissolution and osteoclast mediated resorption properties [5]. * Corresponding author. Tel.: +98 3114456551; fax: +98 3312291008 E-mail address: [email protected]; [email protected]

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The inorganic matrix of the bone is based on HAp doped with different quantities of cations as Na+, K+ and Mg2+, and anions like CO32, SO42, Cl1 and F1. Among them, CO32 and F1 play an important role due to their influence on the physical and biological properties of HAp [6]. According to the literature, increasing of F1 concentration decreases the solubility of FAp and fluorhydroxyapatite (FHAp) in chemical and biological media. This deficiency can be compensated by the presence of CO3 2 concentrations simultaneously in apatite structure [6]. Furthermore, the incorporation of carbonate into the apatite structure led to a decrease in crystallinity, a change in crystal morphology, and an enhancement of chemical reactivity due to the weak bonding [7–9]. In fact, the carbonated apatite increases the local concentration of calcium and phosphate ions that are necessary for new bone formation [10]. So, in order to meet the requirements set for the reparation of bones and teeth, the designed carbonated fluorapatite (CFAp) could be easily optimized. The carbonated apatite cements can be utilized in many biomedical applications such as in situ-hardened bone filler [11] or screw fixations [12]. Carbonated hydroxyapatite (CHAp) nanocrystals were successfully applied for the fabrication of CHAp/collagen composite [13]. For all these reasons, the synthesis of CHAp and CFAp is of great value and has been extensively studied, applying different synthesis methods such as precipitation, sol-gel, solid-state reaction, and hydrothermal treatment [14–20]. Depending on the method, powders with different morphologies, stoichiometry and levels of crystallinity can be obtained. Recently, several papers regarding mechanochemical synthesis of HAp and CHAp powders have appeared in the literature [21–23]. The main advantages of the mechanosynthesis of the ceramic powders are simplicity and low cost. A number of bioceramics have been prepared by this technique, such as HAp [24], FAp [25], CAp [26], HAp/Ti [27], FAp/ZrO2 [28], CAp/ZnO [29], and so on. Even though a lot of research has been carried out concerning the preparation of CHAp [8, 30–32], only a limited number of studies have been focused on the production of nanostructured CFAp (n-CFAp). This paper aims at investigating CFAp synthesis through a facile room-temperature mechanochemical reaction. In fact, the effect of milling time has been explored with the purpose of recommending a proper circumstance for the mass production of CFAp nanopowders. Structural and morphological features of the product were also characterized by using XRD, FT-IR, SEM and TEM techniques.

2. Experimental procedures 2.1. CFAp preparation Calcium carbonate (CaCO3), phosphorous pentoxide (P2O5) and calcium fluoride (CaF2) powders were used as reactants for the mechanosynthesis of n-CFAp. All chemicals were purchased from Merck and used without purification. The mole ratio of calcium to phosphorus was in accordance with the stoichiometric Ca/P content in the composition of CFAp

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which was equal to1.67. The designed degree of substitution of PO43– by CO32– was shown by the x value in the general formula of B–type CFAp (Ca10-x/2(PO4)6-x(CO3)xF2), where x value was chosen equal to 1.0. Hence, the general form of the mechanochemical reaction (R1) is as follows: 8.5CaCO3 + 2.5P2O5 + CaF2  Ca9.5(PO4)5(CO3)F2 + 7.5CO2

(R1)

Mechanochemical process was carried out in a high-energy planetary ball mill using hardened chromium steel vials (vol. 125 ml) and balls (20 mm in diameter) under a high-purity argon atmosphere (99.99 % purity) for 30, 60, 180, 300, and 600 min. In all cases, the weight ratio of ball-to-powder (BPR), total powder mass and rotational speed were 15:1, 7 g and 600 rpm, respectively.

2.2. Characterization techniques The phase compositions of the samples were examined by X–ray diffraction (Philips X–ray diffractometer (XRD), Cu– K radiation, 40 kV, 30 mA, 0.02 °S–1 step scan, and 10°  2  90°). The following equation was used to determine the crystallite size and lattice strain of the products [33]: B cos T

0.9O  K sin T D

(I)

where , D,  and  are the wavelength of the X–ray used (0.154056 nm), crystallite size, internal micro-strain and the Bragg angle (°), respectively. If we assume that a crystallite is a sphere of diameter D surrounded by a shell of grain boundary with thickness t, the volume fraction of grain boundary ( f) may be estimated using the following formula [34].

f

ª D º 1 « » ¬ (D  t ) ¼

3

(II)

Values of f were determined by substituting the crystallite size obtained by XRD into D with the assumption that t = 1 nm. The crystallinity degree (Xc) was estimated by taking the sum total of relative intensities of individual characteristic peaks according to the following equation [26]:

XC

Sum I1 : I n FAp

Sum I1 : I n Standard

u 100

(III)

Fourier transform infrared spectroscopy (FT-IR, Perkin Elmer Spectrum 65 FT-IR Spectrometer, USA) was performed to evaluate the functional groups and structural changes of CFAp during milling. All spectra were recorded at ambient temperature in the range 4000–400 cm–1. The morphological features of n-CFAp were evaluated using SEM (LEO 435VP, Cambridge, UK). A more detailed morphological analysis was executed using TEM (Philips CM10, Eindhoven,

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The Netherlands) that operated at the acceleration voltage of 100 kV. Besides, the edge-mode of SEM images was used to estimate the volume fraction of grain boundary of n-CFAp.

3. Results and discussions 3.1. XRD analysis Fig. 1 shows the XRD patterns of the powder mixture after different milling times (30–600 min). The X-ray profile of the 30 min milled sample demonstrates the most intense peaks for CaF2 and CaCO3, while the most intense ones for P2O5 disappeared completely. It should be noted that phosphoric acid was formed immediately upon addition of P2O5 to the reaction mixture due to very high hydrophilic nature of P2O5. Accordingly, the characteristic peaks of P2O5 could not be observed in XRD profiles. A similar trend was observed in the previous studies [26,35]. In the XRD profile of the 60 min milled sample, similar to the previous specimen, CaF2 and CaCO3 were dominant phases. The X-ray patterns of the samples milled for 180–600 min confirmed the progressive nature of the mechanochemical process in the above mentioned system. First, weakly defined peaks for carbonated calcium-deficient fluorhydroxyapatite (CDFHAp; Ca9x/2(HPO4)(PO4)5-x(CO3)x(OH)1-xFx)

appeared for the sample milled for 180 min, while several peaks corresponding to the

raw materials (CaF2 and CaCO3) have been remained relatively stable. The clear resolution of all characteristic diffraction peaks for CFAp can be seen for the 300 min milled sample, and as a result the characteristic peaks of the raw materials disappeared entirely. Further increase in the milling time up to 600 min resulted in further increase in crystalline order of the CFAp phase—further sharpening of the principal diffraction peaks.

3.2. Structural features The crystallite size, lattice strain, volume fraction of grain boundary, crystallinity degree, lattice parameters and unit cell volume of CFAp as a function of milling time are summarized in Table 1. According to this table, with increasing the milling time from 300 to 600 min, the crystallite size declined significantly from 77±4 to 69±3 nm, respectively. In contrast, the lattice strain and crystallinity degree rose notably from 0.0026±0.0001 to 0.0045±0.0002 and from about 42±2 to 52±3%, respectively, when the milling time increased from 300 to 600 min. The volume fraction of grain boundary was about 3.80±0.19 and 4.22±0.21% after 30 and 300 min of milling, respectively. This shows that the average crystallite size declined by increasing the milling time from 30 to 300 min. In accordance with the obtained data, with the increase of the milling time to 600 min, the values of aaxis, caxis, and unit cell volume of CFAp declined obviously and reached about 9.35492 Å, 6.91942 Å and 524.421 Å3, respectively. From the table, it is obvious that the disparities in values of unit cell volume resulted mainly from decreases in the (a) constant, rather than from the (c) parameter and could possibly be related to the structural changes of CFAp during the milling process. These findings showed that the structural features of the mechanosynthesized CFAp was influenced by the milling time.

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3.3. FT-IR spectra Fig. 2 shows the FT-IR spectra of the powder mixture after different milling times (30–600 min). These spectra clearly show the structural changes that occurred during the milling process. In all the samples, two bands belonging to the vibration of the adsorbed water in apatites appeared at 3600–2600 and 1800–1666 cm–1 [6,26]. In the case of 30 min milled sample, the fundamental bands of calcite can be seen at: 715 cm–1 (4 - in-plane bend), 876 cm–1 (2 - out-of-plane bend) and at about 1438 cm–1 (3 - asymmetric stretch) [36]. The band at 1235 cm–1 was assigned to in-plane bending vibration of P–O–H. Besides, the peaks at approximately 1097 and 526 cm–1 belonged to P–O stretching and bending vibrations, respectively [37]. Similar bands were detected in the case of the milled sample for 60 min. However, these peaks fluctuated during milling. With increasing the milling time to 180 min, the appearance of the new bands at about 923 cm1 (1) and 1053–1086 cm1 (3) showed the characteristic bands of phosphate groups in the apatitic lattice [25,28]. It is clear that several bands corresponding to the raw materials (calcite) have been remained relatively stable. This finding is in good agreement with the XRD data. After 300 min of milling, the absence of the bands at 630 and 3568 cm–1 corresponding to OH– liberation mode and the manifestation of a band at 746 cm–1 indicated complete transformation of carbonated fluorhydroxyapatite (CFHAp; Ca10-x/2(PO4)6-x(CO3)x(OH)2-2xF2x) into CFAp [6]. This spectrum demonstrated that the 300 min milled sample had the characteristic bands of the phosphate groups of the apatitic structure at about 573 and 604 cm1 (4), and 1046 and 1098 cm1 (3) [28,29,35]. Moreover, two regions of the spectra were characteristic of carbonate vibrations in apatites: (i) 850–890 cm1 corresponding to 2(CO32) (ii) 1420– 1650 cm1 belonging to 3 vibrations of carbonate groups [38]. The emergence of the carbonated groups revealed that the products contained some CO32 groups in PO43 sites of apatite lattice (B-type substitution) and thus the obtained nanopowders can exhibit higher bioactivity than pure HAp. This feature is very helpful in biomedical applications. A similar trend was observed in the case of the 600 min milled sample. Based on the FT-IR spectra, in the above mentioned system the type of mechanochemical reaction was a gradual transformation so that the reaction extend to a very small volume during each collision and eventually all the characteristic bands for CFAp became apparent after 300 min of milling.

3.4. SEM and TEM observations It is clear that calcium phosphate apatites with appropriate morphological characteristics have better functions in biomedical applications [10], therefore the morphological features as an important aspect of bioceramics were evaluated by using SEM and TEM techniques. Fig. 3 shows the SEM micrographs of the samples after 30 and 300 min milling. As can be seen in Fig.3a and b, no significant changes in size distribution and morphology of the agglomerates/particles occurred after 30 min of milling. In this case, only accumulated fine particles and lamina of polygonal shape on the

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surfaces of larger particles are obvious. From the SEM image, the mean agglomerate size of 30 min milled sample was around 20 m. As evident from Figs. 3d, the mechanical activation has reached a steady state after 300 min of milling where the particles have become homogenized in size and shape. From higher magnification image (Fig. 3e), the CFAp nanopowder consisted of several fine spheroidal and ellipsoidal particles. Moreover, it can be seen that the particles of the products can be attached together at specific surfaces and form elongated cluster-like structure. The edge mode images of the specimens milled for 30 and 300 min is shown in Figs. 3c and f. Based on these images, the volume fraction of grain boundary for the 300 min milled sample is higher than that of the 30 min milled specimen. This confirmed that the average crystallite/particle size decreased by increasing the milling time from 30 to 300 min. Fig. 4 illustrates a detailed insight into the morphology of the powder obtained by milling for 300 min. In accordance with TEM observations, the particles exhibit relatively high tendency to agglomerate. According to literature [39], when two adjacent primary particles collide, the coalescence may take place on the premise that these two particles share a common crystallographic orientation. Hence, two primary particles attach to each other and combine into a secondary one. Since the sizes of the secondary particles are still very small, it is reasonable that they will continue to collide and coalesce which may ultimately lead to the agglomeration. In Fig. 4, it is obvious that the agglomerates consist of smaller particles, from approximately 25 to 100 nm in size which were consisted of mostly ellipsoidal and spheroidal shape nanocrystals with a mean size of around 45 and 90 nm, respectively.

3.5. Reaction mechanism The reaction mechanism steps during the mechanochemical processes are still a much-discussed question. Here, using the gained data by XRD, FT-IR, SEM, and TEM techniques the following reaction mechanism is provided. It should be mentioned that these assessments were performed in accordance with four assumptions: (a) milling vial was sealed and isolated (b) all the reactions occurred in the standard conditions (c) the activity coefficient of raw materials was equal to 1, and (d) the reactions have occurred in the same physical conditions. In the above mentioned system, the formation of n-CFAp was influenced remarkably by the presence of P2O5 in the reaction mixture due to its very high hydrophilic nature. Accordingly, the progressive mechanochemical reaction may occur in several steps as follows. At the beginning of milling: In the first step, H3PO4 was formed according to the following reaction (R2). (3-x/2)P2O5 + (9-3x/2)H2O  (6-x)H3PO4

(R2)

During 30 to 60 min of milling: The formation of CaHPO4 as a result of the reaction of reactants with H3PO4 (R3 and R4). CaF2 + H3PO4  CaHPO4 + 2HF,

G298K = + 68.240 kJ,

(5-x)CaCO3 + (5-x)H3PO4  (5-x)CaHPO4 + (5-x)CO2 + (5-x)H2O

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H298K = + 93.034 kJ

(R3) (R4)

After 180 min of milling: At this stage, stoichiometrically carbonated calcium-deficient fluorhydroxyapatite (CDFHAp) appeared in accordance with the following reaction (R5). (3+x/2)CaCO3 + (6-x)CaHPO4 + xHF  Ca9-x/2(HPO4)(PO4)5-x(CO3)x(OH)1-xFx + (2+x/2)H2O

(R5)

Finally, from 300 to 600 min of mechanical activation: The mechanochemical reaction progressed by the development of CFHAp (R6) and eventually n-CFAp was generated as a consequence of the reaction R7. Ca9-x/2(HPO4)(PO4)5-x(CO3)x(OH)1-xFx + CaCO3 + xHF  Ca10-x/2(PO4)6-x(CO3)x(OH)2-2xF2x + CO2 + xH2O

(R6)

Ca10-x/2(PO4)6-x(CO3)x(OH)2-2xF2x + (2-2x)HF  Ca10-x/2(PO4)6-x(CO3)xF2 + (2-2x)H2O

(R7)

Fig. 5 shows the formation mechanism of n-CFAp as a function of milling duration. Before the mechanical activation, the above mentioned system was composed of one ductile powder (P2O5) and two brittle constituents (CaCO3 and CaF2). At the beginning of milling, phosphoric acid was formed owing to the hygroscopic nature of P2O5. In the next step, a ductile–brittle system was generated as a result of the reaction of phosphoric acid with the calcium phosphate reagents. With continued deformation, both ductile and brittle particles got further refined and the interlamellar spacing decreased. At the final stage, a balance between fracturing brittle particles by trapping and ductile constituents by work hardening caused the formation of n-CFAp. Based on the obtained data, the mechanochemical process is an effective route for producing nanostructured CFAp with appropriate structural features as well as morphological characteristics. The knowledge obtained in this research will contribute to the development of nano-sized carbonated apatite by a facile solidstate process.

4. Conclusions Mechanochemical behavior of CaCO3–P2O5–CaF2 system to produce nanostructured B–type CFAp was examined in terms of milling duration. Results showed that the formation and structural as well as morphological features of n-CFAp were influenced noticeably by the milling time. Based on the XRD and FT-IR results, in the above mentioned system, the type of mechanochemical reaction was a gradual transformation so that the reaction extend to a very small volume during each collision and eventually the clear resolution of all characteristic diffraction peaks for CFAp became apparent after 300 min of milling. From the structural point of view, with increasing the milling time from 300 to 600 min, crystallite size declined significantly from 77±4 to 69±3 nm, respectively. On the contrary, lattice strain and crystallinity degree rose notably from 0.0026±0.0001 to 0.0045±0.0002 and from about 42±2 to 52±3%, respectively, when the milling time increased from 300 to 600 min. In accordance with the TEM observations, n-CFAp consisted of mostly ellipsoidal and spheroidal shape nanocrystals with a mean size of around 45 and 90 nm, respectively. In a nutshell, mechanosynthesis is

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an effective method for producing nanostructured carbonated apatite with appropriate structural and morphological features.

Acknowledgement The authors are grateful to research affairs of Islamic Azad University, Najafabad Branch for supporting this research.

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List of Figures Caption Fig. 1 XRD patterns of the powder mixture after different milling times (30–600 min). Fig. 2 FT-IR spectra of the powder mixture after different milling durations (30–600 min). Fig. 3 SEM micrographs and the edge-mode of SEM images of powder mixture after (a,b,c) 30 and (d,e,f) 300 min of milling. Fig. 4 TEM image of n-CFAp after obtained by milling for 300 min. Fig. 5 The formation mechanism of n-CFAp as a function of milling time.

List of Tables Caption Table 1 Structural features of CFAp as a function of milling time.

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Table 1 Structural features of CFAp as a function of milling time. Series

Phase composition

I II

Ca9.5(PO4)5(CO3)F2 Ca9.5(PO4)5(CO3)F2

MT* (min) 300 600

D (nm) 77±4 69±3

 (Å)

f (%)

0.0026±0.0001 0.0045±0.0002

3.80±0.19 4.22±0.21

MT: Milling time

13

Xc (%) 42±2 52±3

a (Å)

c (Å)

V (Å3)

9.38536 9.35492

6.92748 6.91942

528.46 524.42

Figure1

Figure2

Figure3

Figure4

Figure5