Environmentally Friendly Growth of Calcium Chlorapatite Whiskers

Oct 17, 2006 - The cationic valency (1+) of the flux is different from that (2+) of the solute. ... At 1100 °C, Ca5Cl(PO4)3 had a solubility of appro...
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CRYSTAL GROWTH & DESIGN

Environmentally Friendly Growth of Calcium Chlorapatite Whiskers from a Sodium Chloride Flux Teshima,*,†

Katsuya Shuji Oishi*,†

Kunio

Yubuta,‡

Satoru

Ooi,†

Takaomi

Suzuki,†

Toetsu

Shishido,‡

and

2006 VOL. 6, NO. 11 2538-2542

Department of EnVironmental Science and Technology, Faculty of Engineering, Shinshu UniVersity, 4-17-1 Wakasato, Nagano 380-8553, Japan, and Institute for Materials Research, Tohoku UniVersity, Katahira-cho, Aoba-ku, Sendai 980-8577, Japan ReceiVed June 17, 2006; ReVised Manuscript ReceiVed August 23, 2006

ABSTRACT: Environmentally friendly, high-quality, and transparent-colorless calcium chlorapatite (Ca5Cl(PO4)3) whiskers were selectively fabricated by rapid cooling of a sodium chloride flux. The crystal growth was conducted by heating a mixture of solute ((NH4)2HPO4, CaCl2, and CaCO3) and flux (NaCl) at 1100 °C, holding at this temperature for 10 h, and then cooling it at a rate of 5-200 °C h-1 to 500 °C. The grown crystals were divided into two morphological types, that is, needle and prismatic shapes. The Ca5Cl(PO4)3 whiskers obtained average lengths up to 4.2 mm and widths up to 130 µm. Their form was a hexagonal {101h0} cylinder with pyramidal {101h1} end faces. The crystal forms and average sizes obviously depended on the cooling rate of the high-temperature solution. The formation ratio of the whiskers was increased with cooling rate, reaching much higher than 80 wt % at 200 °C h-1 in all growth runs. It is found that Ca5Cl(PO4)3 whiskers were predominantly grown by controlling the cooling rate. Finally, NaCl was found to be a very suitable flux for an environmentally friendly growth of Ca5Cl(PO4)3 whiskers. Introduction Apatites are the principal phosphate ore mineral and are a major source of phosphorus used as fertilizer.1 They are also well-known as the main constituent of bones and teeth in the human body. In addition, a variety of apatites have been widely used for industrial applications, such as fluorescent lamp phosphors, laser hosts, and biocompatible materials.1 Natural apatites (Ca5(Cl,F,OH)(PO4)3) include three mineral species, that is, chlorapatite, fluorapatite, and hydroxyapatite, and exhibit large variations in Cl, F, and OH content.1 They sometimes contain strontium (up to 15 wt % SrO), cerium (up to 12 wt % Ce2O3), and other elements.1a In carbonate apatites, the PO4 group is partially replaced by a CO3(OH) group.1b Among these apatites, calcium chlorapatite has the formula Ca5Cl(PO4)3 (pentacalcium chloride tris(phosphate)). The crystals of Ca5Cl(PO4)3 belong to the hexagonal system (P63/m)1d,1e,2 or the pseudohexagonal system with the monoclinic space group P21/b at room temperature.3 A crystalline powder of Ca5Cl(PO4)3 has been synthesized by a solid-state reaction method.2b,4 Chlorapatite crystals have been pulled from the melt by use of the Kyropoulos method; however, vaporization of CaCl2 at the melting point severely limited the size of the crystals.5 Crystals of Ca5Cl(PO4)3 have been grown by the CaCl2 and NaCl flux methods.3b,6,7 Additionally, barium chlorapatite (Ba5Cl(PO4)3) and strontium chlorapatite (Sr5Cl(PO4)3) have also been grown by the NaCl flux method.8,9 In the CaO-CaCl2-P2O5 system, there is one ternary compound, chlorspodiosite (Ca2ClPO4), that melts incongruently at approximately 1040 °C into another solid (Ca5Cl(PO4)3) and liquid.3b,6,10 In growing Ca5Cl(PO4)3 crystals from a CaCl2 flux, therefore, the growth must take place above 1040 °C.6 In the case of NaCl flux, crystal growth of Ca5Cl(PO4)3 has to be performed at above 800 °C (eutectic temperature ) 795 ( 5 °C, eutectic composition ) 0.03 mol % Ca5Cl(PO4)3/ * Corresponding author. E-mail: [email protected] (K.T.); [email protected] (S.O.). † Shinshu University. ‡ Tohoku University.

99.97 mol % NaCl).7 The forms of the grown crystals were a hexagonal prism and needle with pyramidal end faces.6-9 In our previous study,7 NaCl was found to be a suitable flux for growing the chlorapatite crystals. Sodium chloride has some advantages (the following sentences) for the growth of Ca5Cl(PO4)3 whiskers. It has a common anion (Cl-) with the solute. Another similarity is established by there being no large difference in the cationic radii between the flux (Na+) and the solute (Ca2+). The cationic valency (1+) of the flux is different from that (2+) of the solute. In this way, there is a similarity and a difference between NaCl and Ca5Cl(PO4)3. Furthermore, NaCl is abundant in nature, and is also harmless to human beings and environment. In this report, we describe the growth of Ca5Cl(PO4)3 whiskers (synonym: needle crystals) by rapid cooling of a NaCl flux. In particular, we focus on the selective and environmentally friendly growth of Ca5Cl(PO4)3 whiskers. Furthermore, the effect of the cooling rates of the high-temperature solution on the crystal forms was studied. The crystallinity, imperfections, morphology, and density of the resulting crystals were examined. In recent years, one-dimensional materials such as whiskers, rods, and tubes have been of great importance for various applications in electronic, mechanical, and chemical engineering because they exhibit improved unique properties.11 In general, whiskers are needle-shaped single crystals with mostly theoretical strength because of their perfect geometry. Experimental Section Calcium chlorapatite (Ca5Cl(PO4)3) whiskers were grown by cooling NaCl flux. A stoichiometric mixture of reagent-grade (NH4)2HPO4 (0.084-0.251 g, Wako Pure Chemical Industries, Ltd.), CaCO3 (0.0960.286 g, Wako Pure Chemical Industries, Ltd.), and CaCl2 (0.0120.035 g, Wako Pure Chemical Industries, Ltd.) powders was used as a solute, and reagent-grade NaCl (24.670-24.809 g, Wako Pure Chemical Industries, Ltd.) was chosen as the flux. Mixtures containing solutes of 0.05, 0.10, and 0.15 mol % were prepared. The solubility curve measured in our previous study7 was applied to decide the solute concentration. At 1100 °C, Ca5Cl(PO4)3 had a solubility of approximately 0.15 mol %. The solubility of Ca5Cl(PO4)3 in NaCl was relatively low, although the solubility curve had an appreciable

10.1021/cg0603671 CCC: $33.50 © 2006 American Chemical Society Published on Web 10/17/2006

Environmentally Friendly Growth of Ca5Cl(PO4)3 Whiskers

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Figure 1. Optical micrograph and SEM image showing typical Ca5Cl(PO4)3 whiskers grown from the NaCl flux. temperature coefficient of solubility.7 The mixtures were about 25 g in weight and were put into platinum crucibles capacities of 30 cm3. The lids were loosely fitted and the crucibles were then placed in an electric furnace with silicon carbide heating elements. The crucibles were heated to 1100 °C at about 45 °C h-1 and held at this temperature for 10 h. Subsequently, they were cooled to 500 °C at 5-200 °C h-1 (14 different cooling rates). In the case where the cooling rate is 5 °C h-1, the total growth period is 154 h. When the rate is 200 °C h-1, however, it is only 37 h and shortened to 1/5-1/4. After the cooling program was completed, the crucibles were allowed to cool to room temperature. The crystalline products were then separated by dissolving the flux in warm water. The obtained whiskers were observed by use of an optical microscope (NIKON, SMZ800) and a scanning electron microscope (SEM, HITACHI, S-4100). Phases and elongated directions of the crystals were studied by X-ray diffraction (XRD, SHIMADZU, XRD-6000). An energy-dispersive X-ray spectrometer (EDS, HORIBA, EMAX5770Q) was used to study any variations in the concentration of the major constituents in the grown crystals. The high-resolution transmission electron microscopy (HRTEM) and electron diffraction observations were carried out on JEM-2010 (JEOL) and JEM-2000EXII (JEOL) instruments operated at 200 kV to analyze the crystallinity and elongated direction of the grown crystals. The presence of impurities from the NaCl flux and Pt crucible was also observed. The length, L (parallel to the 〈0001〉 directions), and width, W (perpendicular to the 〈0001〉 directions), of the Ca5Cl(PO4)3 crystals were measured. On the basis of the aspect ratio (L/W) of the crystals, they were divided into two morphological types: whiskers (needles) that had L/W > 10 and prisms with L/W e 10. After each growth run, the average length (Lav) and width (Wav) of the first 30 largest crystals with needle and prismatic forms were calculated. Variation in the weight ratio of whiskers and prism crystals with cooling was examined.

Figure 2. X-ray diffraction patterns (CuKR) of Ca5Cl(PO4)3 crystals. (a) Whiskers of which a well-developed face was laid in parallel with the holder plate; (b) pulverized crystallites; (c) Ca5Cl(PO4)3 JCPDS data.2b

Results and Discussion Large and idiomorphic needle- and prism-shaped Ca5Cl(PO4)3 crystals with average lengths up to 4.2 mm and widths of 130 µm (solute concentration, 0.10 mol %; cooling rate, 60 °C h-1) were grown from the NaCl flux. The typical Ca5Cl(PO4)3 whiskers are shown in Figure 1. Relatively long whiskers were much greater than 4.5 mm in length. The grown whiskers were colorless and transparent. Both whiskers and prismatic crystals were grown from all growth runs. Figure 2 shows XRD profiles of data for the transparent-colorless whiskers (Figure 2a), pulverized crystallites (Figure 2b), and Ca5Cl(PO4)3 (chlorapatite) JCPDS (Figure 2c).2b They were identified as Ca5Cl(PO4)3 by their powder XRD patterns (Figure 2b), using data given on the literatures (Figure 2c).2b The Ca5Cl(PO4)3 crystals including needle and prism shapes were mainly obtained as aggregates. The aggregates of grown crystals look just like cotton and have a silky luster (Figure 1a). In all growth runs, the obtained crystals were greater than 80 wt % of the values calculated from the lever rule between starting and eutectic compositions. The resulting whiskers could be readily separated from the flux in warm water because NaCl was easily soluble. Figure 3 shows variations in the weight ratio of whiskers with cooling rates. The solute concentrations are, respectively, (a)

Figure 3. Variation in the formation ratio of Ca5Cl(PO4)3 whiskers with cooling rates (solute concentration: (a) 0.05, (b) 0.10, and (c) 0.15 mol %).

0.05, (b) 0.10, and (c) 0.15 mol %. At all solute concentrations (Figure 3a-c), the formation ratios of whiskers increased clearly as the cooling rate increased. These figures implied that the ratios of prismatic crystals decreased with an increase in cooling rates. In the case of 0.05 mol % solute concentration (Figure 3a), as the cooling rate increased from 5 to 200 °C h-1, the whisker ratios increased from 54 to 92 wt %. In the case where the solute concentration was 0.15 mol % (Figure 3c), the formation ratio of whiskers was only about 3 wt % (cooling rate: 5 °C h-1). However, it drastically increased with an

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Teshima et al.

Figure 4. Relationship between cooling rate and grown whisker sizes (solute concentration: (a) 0.05, (b) 0.10, and (c) 0.15 mol %).

Figure 5. Relationship between cooling rate and grown prismatic crystal sizes (solute concentration: 0.10 mol %).

Figure 6. (a) Typical Ca5Cl(PO4)3 whisker and (b) drawing of whisker habit.

increase in cooling rate, reaching about 89 wt % at 200 °C h-1. The variation of the 0.10 mol % solute concentration (Figure 3b) is relatively similar to that of 0.15 mol %. The Lav and Wav of the whiskers are plotted against the cooling rate in Figure 4. In the case of the 0.05 mol % solute concentration (Figure 4a), large whiskers of Lav ) 0.56 mm and Wav ) 13 µm (aspect ratio ≈ 43) were grown from a cooling rate of 10 °C h-1. The Lav and Wav values decreased with increasing cooling rate. The whiskers grown from a mixture containing 0.05 mol % solute were much smaller than those grown from mixtures containing 0.10 or 0.15 mol % solute. When the solute concentrations were 0.10 and 0.15 mol % (Figure 4b,c), the Lav and Wav values decreased with an increase in the cooling rate, except for some temperature gradient regions in which prismatic crystals were preferentially grown. A mixture

containing a solute of 0.10 mol % produced whiskers with Lav values of 1.8-4.2 mm and Wav values of 50-204 µm (cooling rates ) 5-200 °C h-1). When the solute concentration ) 0.15 mol %, Lav and Wav values were, respectively, 1.3-3.6 mm and 42-204 µm. The sizes of the whiskers grown from a mixture containing 0.15 mol % solute were slightly smaller than those of 0.10 mol %, because the number of crystal nuclei (0.15 mol %) was relatively higher than that of 0.10 mol %. These whisker sizes were obviously dependent on the number of nuclei. Figure 5 shows the variation in the Lav of the prismatic Ca5Cl(PO4)3 crystals with the cooling rate (solute concentration ) 0.10 mol %). Large crystals with Lav ) 1.3 mm were grown at a cooling rate of 5 °C h-1. The Lav values gradually decreased with increasing cooling rate, reaching about Lav ) 100 µm at 200

Environmentally Friendly Growth of Ca5Cl(PO4)3 Whiskers

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Figure 7. (a) Lattice image, (b) TEM micrograph, and (c) diffraction pattern of a typical Ca5Cl(PO4)3 whisker.

°C h-1. As a result, the selective growth of Ca5Cl(PO4)3 whiskers was successfully conducted by controlling the cooling rates. The basic form of Ca5Cl(PO4)3 whiskers and prismatic crystals was a hexagonal cylinder with pyramidal end faces. The surfaces of these crystals were very flat. A typical example is shown in Figure 6. The orientated whiskers were investigated by the XRD method to determine the Miller indices of the crystal faces (Figure 2a). In the diffraction pattern (Figure 2a), the diffraction intensity of the (303h0) plane (2θ ) 32.3°) with a hexagonal setting was predominant. The indices of side faces were {101h0} and were the same result as our previous study.6-9 The interfacial angles between side and pyramidal faces were 51 ( 2°. This value was in good agreement with the calculated interfacial angle (51.0°) between the {101h0} and {101h1} faces. Figure 7a shows the lattice image of a typical whisker grown at cooling rate of 200 °C h-1. The whisker was of very good crystallinity because no defects were observed in this image. The TEM micrograph and the corresponding selected area diffraction (SAD) pattern of a typical whisker grown at a cooling rate of 200 °C h-1 are shown in images b and c of Figure 7, respectively. The crystal planes in the SAD pattern were confirmed to be in accordance with an apatite structure. As seen in Figure 7, the elongated direction clearly corresponded to the 〈0001〉 directions. In the case of Ba5Cl(PO4)3 crystals, the surface tension of the (101h0) face was smaller than that of the (101h1) face.12 This result indicates that the (101h0) face is more stable than the (101h1) face and the crystal elongates predominantly in the 〈0001〉 directions.12 From these results of SEM, XRD, and TEM, we conclude that the Ca5Cl(PO4)3 whiskers and prismatic crystals were bounded by well-developed six-sided {101h0} faces with pyramidal end {101h1} faces. This morphology was similar to those of the Ca5Cl(PO4)3 crystals grown from a CaCl2 or NaCl flux by a slow-cooling6,7 and Ca5F(PO4)3 crystals from a KF flux.13 The density of the Ca5Cl(PO4)3 whiskers was pycnometrically determined to be 3.16 ( 0.02 g cm-3. This was in good agreement with the literature value (3.17 g cm-3).2b

Variations in the concentration of the major constituents in the Ca5Cl(PO4)3 whiskers were investigated by the EDS and EPMA analyses. Calcium, chlorine, phosphorus, and oxygen atoms were homogeneously distributed in the whiskers. Sodium from the flux was not detected in the whiskers. Conclusions Highly crystalline calcium chlorapatite (Ca5Cl(PO4)3) whiskers were readily and selectively grown by the rapid cooling of a sodium chloride used as the flux. Furthermore, this rapid cooling technique is more environmentally friendly than the previous slow cooling7 one. The obtained crystals were divided into two morphological types: needle and prism shapes. The Ca5Cl(PO4)3 whiskers exhibited were transparent and colorless. Many Ca5Cl(PO4)3 whiskers with average lengths of up to 4.2 mm and widths of 130 µm were obtained from high-temperature solutions containing a solute of 0.05-0.15 mol %. The crystal forms and sizes were dependent on the cooling rate. As the cooling rate was increased, the formation ratio of whiskers was gradually increased, reaching much higher than 80 wt % at 200 °C h-1 in all growth runs. In the case of 200 °C h-1, the total growth period was only 37 h and shortened to 1/5-1/4. Environmental damages (e.g., by exhausting greenhouse gases) and product costs are thought to be reduced by this rapid cooling method. The whiskers and prismatic crystals, which were hexagonal cylinders ({101h0} faces) with pyramidal end {101h1} faces, elongated in the 〈0001〉 directions. The obtained whiskers had no defects (TEM observation), and the major components were homogeneously distributed in the whiskers (EDS and EPMA analyses). Acknowledgment. This research was partially supported by The Salt Science Research Foundation, 0609, and Iketani Science and Technology Foundation, 0181023-A. A part of this work was performed under the interuniversity cooperative research program of Advanced Research Center Metallic Glasses, Institute for Materials Research, Tohoku University.

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References (1) See for example: (a) Wenk, H.-R.; Bulakh, A. Minerals: Their Constitution and Origin; Cambridge University Press: Cambridge, U.K., 2004; pp 376-387. (b) Johnsen, O. Minerals of the World; Princeton University Press: Princeton, NJ, 1994; pp 236-238. (c) Sinkankas, J. Mineralogy; Van Nostrand Reinhold Company: New York, 1964; pp 416-423. (d) Roy, D. M.; Drafall, L. E.; Roy, R. Phase Diagrams: Materials Science and Technology; Alper, A. M., Ed.; Academic Press: New York, 1978; pp 185-239. (e) Kingery, W. D.; Bowen, H. K.; Uhlmann, D. R. Introduction to Ceramics, 2nd ed.; John Wiley & Sons: New York, 1976; pp 646-703. (2) See for example: (a) Hughes, J. M.; Cameron, M.; Crowley, K. D. Am. Mineral. 1989, 74, 870. (b) JCPDS card 33-271. (3) See for example: (a) Mackie, P. E.; Elliott, J. C.; Young, R. A. Acta Crystallogr., Sect. B 1972, 28, 1840. (b) Prener, J. S. J. Electrochem. Soc. 1967, 114, 77. (4) Kanazawa, T.; Monma, H. Kogyo Kagaku Zasshi 1970, 73, 687. (5) Johnson, P. D. J. Electrochem. Soc. 1961, 108, 159.

Teshima et al. (6) Oishi, S.; Kamiya, T. Nippon Kagaku Kaishi 1993, 1129. (7) Oishi, S.; Sugiura, I. Bull. Chem. Soc. Jpn. 1997, 70, 2483. (8) Oishi, S.; Michiba, N.; Ishizawa, N.; Rendon-Angeles, J. C.; Yanagisawa, K. Bull. Chem. Soc. Jpn. 1999, 72, 2097. (9) Oishi, S.; Mitsuya, M.; Suzuki, T.; Ishizawa, N.; Rendon-Angeles, J. C.; Yanagisawa, K. Bull. Chem. Soc. Jpn. 2001, 74, 1635. (10) Levin, E. M.; Robbins, C. R.; McMurdie, H. F. Phase Diagrams for Ceramists; The American Ceramic Society: Westerville, OH, 1969; p 474 (Figure 3800). (11) See for example: (a) Iijima, S. Nature 1991, 354, 56. (b) Endo, M.; Muramatsu, H.; Hayashi, T.; Kim, Y. A.; Terrones, M.; Dresselhaus, M. S. Nature 2005, 433, 476. (c) Oishi, S.; Iida, D.; Suzuki, T.; Shishido, T. Bull. Soc. Sea Water Sci. Jpn. 2002, 56, 26. (12) Suzuki, T.; Nakayama, K.; Oishi, S. Bull. Chem. Soc. Jpn. 2004, 77, 109. (13) Oishi, S.; Kamiya, T. Nippon Kagaku Kaishi 1994, 800.

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