Selective Growth of Calcium Molybdate Whiskers by Rapid Cooling of a Sodium Chloride Flux Teshima,*,†
Yubuta,‡
Sugiura,†
Katsuya Kunio Shiori Yoko Morinobu Endo,#,§ Toetsu Shishido,‡ and Shuji Oishi*,†,§
Fujita,†
Takaomi
Suzuki,†,§
CRYSTAL GROWTH & DESIGN 2006 VOL. 6, NO. 7 1598-1601
Department of EnVironmental Science and Technology, Department of Electrical and Electronic Engineering, Faculty of Engineering, and Institute of Carbon Science and Technology, 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 December 24, 2005; ReVised Manuscript ReceiVed May 9, 2006
ABSTRACT: Long whiskers of calcium molybdate (CaMoO4) were selectively grown by rapid cooling of a NaCl flux. The crystal growth of CaMoO4 was conducted by heating a mixture of solute and flux at 900 °C for 10 h and then cooling to 500 °C at a rate of 270 °C‚h-1 or quenching in a furnace with the electric power turned off. The obtained whiskers were colorless and transparent. Transmission electron microscopy images showed that the grown whiskers were of a very good crystallinity. The major constituents were homogeneously distributed throughout the whiskers. Sodium and chlorine atoms from the flux were not detected. Whiskers grown from the solutions were up to 5.2 mm long and 7 µm in diameter. The whiskers were cylindrical in shape with average aspect ratios ranging from 150 to 750, and they elongated dominantly in the 〈110〉 directions. Introduction
Experimental Section
Nanostructural materials have attracted much interest because of their novel properties that differ from those of bulk materials. In particular, one-dimensional (1D) materials, such as whiskers, rods, and tubes, are of importance for various applications in electronic, mechanical, and chemical engineering because they exhibit improved unique properties.1-4 For instance, it is wellknown that mechanical properties, such as strength and toughness, of monolithic ceramics can be improved by the dispersion of a whisker.5 Generally, whiskers are needle-shaped single crystals with mostly theoretical strength due to their perfect geometry. There have been many studies on the fabrication of 1D materials by various techniques, including vapor growth and solution growth.1-7 Calcium molybdate (CaMoO4) is a major luminescence material and has been frequently studied over the last few decades.8 The crystals of CaMoO4 belong to the tetragonal system with space group I41/a.9 Calcium molybdate has a melting point of 1468 °C.10 The single crystals of CaMoO4 have been grown by Czochralski10,11 and flux methods.4,12-16 In the flux growth, the following fluxes have been used successfully: Na2MoO4,12-14 LiCl,15,16 NaCl,4 KCl,13 and CaCl2.15 Both wellformed octahedral crystals and whiskers were grown from the NaCl flux.4 The whiskers were elongated in the 〈001〉 directions.4 No report on the selective growth of CaMoO4 whiskers from a NaCl flux has been published. In this paper, we report on the selective growth of CaMoO4 whiskers from a NaCl flux. Additionally, the effect of the cooling conditions on crystal growth was studied. The morphology, density, lattice parameters, and imperfections of the resulting crystals were examined.
Calcium molybdate whiskers were grown by the flux method using reagent-grade CaCO3, MoO3, and NaCl. A equimolar mixture of CaCO3 and MoO3 powders was used as a solute. Sodium chloride powders were used as the flux. A mixture containing solute of 3 mol % was employed. The mass of the mixture was approximately 25.5 g (25.0 g as the binary system CaMoO4-NaCl). The mixture was put into a 30 cm3 platinum crucible. After the lid was loosely fitted, the crucible was placed in an electric furnace with silicon carbide heating elements. The temperature conditions are as follows [in this study, the solution temperatures are considered to be the same as the furnace (crucible) temperatures]: (Run no. 1) The crucible was heated to 900 °C at a rate of about 45 °C‚h-1, held at this temperature for 10 h, and then cooled to 500 °C at a rate of 270 °C‚h-1. When the cooling program was completed, the crucible was allowed to cool to room temperature. (Run no. 2) The crucible was heated to 900 °C at about 45 °C‚h-1 and held at this temperature for 10 h. Subsequently, the electric power was turned off, and the crucible was cooled quickly to room temperature in the furnace. The crystalline products were then separated by dissolving the flux in warm water. The obtained crystals were examined using an optical microscope and a scanning electron microscope (SEM, Hitachi, S-4100). The crystal phases were identified by X-ray diffraction (XRD, Shimadzu, XRD6000). The lattice parameters were obtained on the basis of the powder XRD data (step size ) 0.02°, 25 e 2θ/°e 30). To derive accurate lattice parameters, silicon (purity ) 99.9%) was added as an internal standard. A SEM equipped with an energy-dispersive X-ray spectrometer (EDS, Horiba, EMAX-5770Q) 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 L (length) and D (diameter) of the CaMoO4 whiskers grown were measured, and the average sizes (Lav and Dav) were calculated.
Results and Discussion * Corresponding authors: (K.T.) E-mail:
[email protected]; (S.O.) E-mail:
[email protected]. † Department of Environmental Science and Technology, Shinshu University. ‡ Tohoku University. # Department of Electrical and Electronic Engineering, Shinshu University. § Institute of Carbon Science and Technology, Shinshu University.
Long whiskers of CaMoO4 having sizes of up to 5.2 mm in length and 7 µm in diameter were selectively grown from the NaCl flux, without growing octahedral CaMoO4 crystals. Typical whiskers are shown in Figure 1. Only CaMoO4 whiskers were grown at both growth runs (Run no. 1 and no. 2). The obtained whiskers were identified as CaMoO4 by their XRD
10.1021/cg050673z CCC: $33.50 © 2006 American Chemical Society Published on Web 06/09/2006
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Figure 3. The dependence of the solution temperature on the passing time. (Run no. 1: the cooling rate ) 270 °C‚h-1 and Run no. 2: quenching). Figure 1. Optical micrograph showing CaMoO4 whiskers grown from a NaCl flux.
Figure 4. SEM photograph showing cylindrical CaMoO4 whiskers.
Figure 2. X-ray diffraction patterns (Cu KR) of CaMoO4 whiskers (Run no. 2). (a) Pulverized crystallites; (b) CaMoO4 ICDD PDF.9
patterns, using data given on the ICDD PDF (Figure 2).9 The whiskers were mainly obtained as aggregates from the crucible wall. The aggregates of grown whiskers look just like cotton and have a silky luster. The obtained respective whiskers were colorless and transparent. For the cooling rate of 270 °C‚h-1 (Run no. 1), Lav of the whiskers was approximately 3.5 mm long. Most whiskers had diameters in the region of 5-7 µm (Dav ) 6 µm). The aspect ratio, Lav/Dav, was about 583. On the other hand, in the case of quenching (Run no. 2), Lav and Dav were about 382 and 2.5 µm, respectively (aspect ratio ) 159). The whisker sizes were clearly dependent on the cooling rates. Long whiskers reaching millimeter size were able to be synthesized by controlling the cooling rates. In addition, nanowhiskers may also be grown through cooling technique changes (for example, a much higher cooling rate). Figure 3 shows the dependence of the solution temperature on the passing time. The whisker growth is deeply related to the solubility4 of CaMoO4 in NaCl and the cooling rates of the high-temperature solutions. In our previous study,4 the solubility curve was determined at temperatures of between 800 and 1100 °C. At 800 and 850 °C, CaMoO4 was dissolved in NaCl at a concentration of about 0.50 and 1.75 mol %, respectively. The solubility gradually increased with increasing temperature. At 900 °C, CaMoO4 had a solubility of about 3.25 mol %. Judging from the solubility curve, at 890 °C, CaMoO4 was dissolved in NaCl at a concentration of about 3 mol %. As a result, the decrease in the solution temperature lower than 890 °C led to nucleus occurrence and subsequent crystal growth, since the solute concentration was fixed at 3 mol % in this study. To control the solution temperature near the saturation point, therefore, is very important for crystal growth. When the cooling rate is 270 °C‚h-1, that is, the temperature gradient per minute
Figure 5. Lattice image of a typical whisker.
is 4.5 °C, nucleus generation and crystal growth proceed relatively slowly. The time needed to decrease the temperature from 890 to 850 °C is 9 min. Crystals also grew at 850 °C or less and continued growing until their eutectic temperature (790 °C) for 25 min. In our previous research,4 octahedral CaMoO4 crystals were grown by slow-cooling at 5 °C‚h-1 of NaCl flux. The temperature gradient used in this study is much larger than that of the flux slow-cooling method. Consequently, without growing bulk crystals, very long whiskers reaching 5.2 mm were obtained in this study. On the other hand, in the case of Run no. 2, about 5 min was required for the solution temperature to decrease from 890 to 850 °C (around the nucleus generation region). The time needed to decrease to 790 °C was about 17 min. At 790 °C or less, the gradient difference in temperature between the two experiments (Run no. 1 and no. 2) was very small. For the quenching experiment (Run no. 2), since the temperature gradient at the first stage of crystal growth was relatively large, a large number of small whiskers were grown, and their surface became rough. The difference between Run no. 1 and no. 2 was not relatively large, as is 4 min from 890 to 850 °C, however, the temperature gradients had a significant effect on the whisker growth such as size and structure. A mixture containing 3 mol % solute produced 2.10 g of whiskers. About 88 mass % of the solute (2.39 g) employed
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Figure 6. TEM micrograph of CaMoO4 whisker grown at Run no. 1 and its diffraction pattern.
was recovered in the form of whiskers. In a calculation using the starting composition and eutectic composition (0.3 mol % CaMoO4-99.7 mol % NaCl), the masses of CaMoO4 whiskers grown and powders contained in the eutectic mixture were, respectively, 2.16 and 0.23 g. The mass of the obtained whiskers was about 97% of the calculated value. The experimental mass agreed well with the calculated one. During these growth runs, evaporation of the NaCl flux was less than 1 mass %. In the growth of CaMoO4 whiskers, the influence of the flux evaporation is negligible. The platinum crucibles were found to be undamaged after use. The NaCl flux did not attack the crucibles. The resulting whiskers could be readily separated from the flux in warm water because NaCl was easily soluble. The SEM photograph of typical CaMoO4 whiskers is shown in Figure 4. The whiskers had a cylindrical form, and their surfaces were very smooth. Figure 5 shows the lattice image of a typical whisker. The whisker was of a very good crystallinity because no defects were observed in this image. The TEM micrograph (Figure 6a) and the corresponding diffraction pattern (Figure 6b) of a typical whisker grown at Run no. 1 are shown in Figure 6. We demonstrated indexing for reflection spots (Figure 6c, upper: index; lower: interplanar spacing). As seen in Figure 6, the elongated direction clearly corresponded to the 〈110〉 direction. In addition, the small number of whiskers having 〈001〉 elongation direction was also detected in our TEM observations (not shown in this paper). Finally, the predominant elongation was found to be in the 〈110〉 direction. On the other hand, the whiskers grown at Run no. 2 elongated in the 〈110〉
directions only. Furthermore, we have previously reported that the elongation direction of the CaMoO4 whiskers grown at 5 °C‚h-1 of cooling rate was mainly 〈001〉.4 These results indicated that elongation directions of flux-grown whiskers were dependent on the cooling rates of the high-temperature solution. Variations in the concentration of the major constituents in the CaMoO4 whiskers were investigated by the EDS method. Calcium and molybdenum atoms were distributed almost homogeneously in the whiskers. According to the results, oxygen atoms are also considered to be distributed almost homogeneously. Sodium and chlorine from the flux were not detected in the whiskers. In addition, flux inclusions were rarely observed in the whiskers. On the basis of the powder XRD data, the a and c axes of the CaMoO4 whiskers were 5.555(3) and 11.033(3) Å, respectively. These values agree well with those (a ) 5.556 Å and c ) 11.032 Å)9 from the literature. Conclusions Long CaMoO4 whiskers, which were of a very good crystallinity, were easily and selectively grown from a NaCl flux. They were colorless and transparent and were up to 5.2 mm in length and 7 µm in diameter. The average aspect ratios of the whiskers were in the region of about 150 to 750. The whiskers were ordinarily obtained as aggregates that look like cotton and had a silky luster. The whiskers, which were cylindrical in shape, elongated in the 〈110〉 directions predominantly. The obtained whiskers had a very good crystallinity, and the major constituents were homogeneously distributed in the whiskers. The
Selective Growth of Calcium Molybdate Whiskers
whisker sizes, that is, length and diameter, and the elongation directions were dependent on the cooling rates. To synthesize short and narrow whiskers, cooling rates have to be increased. Acknowledgment. This research was supported by the CLUSTER of the Ministry of Education, Culture, Sports, Science and Technology. A part of this work was supported by The Salt Science Research Foundation, No. 0609. A part of this work is performed under the inter-university cooperative research program of Advanced Research Center Metallic Glasses, Institute for Materials Research, Tohoku University. References (1) Iijima, S. Nature 1991, 354, 56. (2) Endo, M.; Muramatsu, H.; Hayashi, T.; Kim, Y. A.; Terrones, M.; Dresselhaus, M. S. Nature 2005, 433, 476. (3) Oishi, S.; Sugiura, I. Bull. Chem. Soc. Jpn. 1997, 70, 2483.
Crystal Growth & Design, Vol. 6, No. 7, 2006 1601 (4) Oishi, S.; Iida, D.; Suzuki, T.; Shishido, T. Bull. Soc. Seawater Sci. Jpn. 2002, 56, 26. (5) Chen, I. W.; Rosenflanz, A. Nature 1997, 389, 701. (6) Koyama, T.; Endo, M. Jpn. J. Appl. Phys. 1974, 3, 1175. (7) Tibbetts, G. G. Appl. Phys. Lett. 1983, 42, 666. (8) Koepke, Cz.; Lempicki, A. J. Lumin. 1990, 47, 189. (9) ICDD PDF 29-351. (10) Cockayne, B.; Ridley, J. D. Nature 1964, 47, 1054. (11) Ivleva, I. I.; Galagan, B. I.; Aleynik, A. P.; Denker, B. I.; Osiko, V. V. Cryst. Prop. Prep. 1991, 36-38, 169. (12) Oishi, S.; Takao, K.; Hirao, M. Chem. Express 1993, 8, 81. (13) Oishi, S.; Sugiura, I.; Yokote, Y.; Kobayashi, T.; Wakabayashi, S. Nippon Kagaku Kaishi 1997, 406. (14) Oishi, S.; Morikawa, H.; Hoshikawa, K.; Kitamura, K.; Kobayashi, T.; Wakabayashi, S. Nippon Kagaku Kaishi 1999, 93. (15) Packter, A.; Roy, B. N. Krist. Tech. 1971, 6, 39. (16) Arora, S. K.; Batra, N. M.; Rao, G. S. T. J. Am. Ceram. Soc. 1985, 68, C-240.
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