First Electrochemical Growth of δ-Al2O3 Single Crystal Nobuhito Imanaka,* Toshiyuki Masui, and Young Woon Kim Department of Applied Chemistry, Faculty of Engineering and Handai Frontier Research Center, Osaka University, 2-1 Yamadaoka, Suita, Osaka 565-0871, Japan Received November 14, 2003;
CRYSTAL GROWTH & DESIGN 2004 VOL. 4, NO. 4 663-665
Revised Manuscript Received May 7, 2004
ABSTRACT: Single crystals of δ-Al2O3 were successfully grown for the first time by dc electrolysis of the Al3+ ion conducting Al2(MoO4)3 solid electrolyte at 11 V, 900 °C. δ-Alumina is the intermediate phase in the transformation from γ- to θ-alumina, and is stable only in the narrow calcination temperature range between 650 and 950 °C and has not been ever grown as a single-crystal form. Because it is necessary to heat the starting material above the melting point of alumina to grow a single crystal by the conventional methods via melt, single-crystal growth of the most stable R-Al2O3 has precedence over the intermediate phase such as δ-Al2O3. On the contrary, the presently developed electrochemical method can be simply applicable at moderate temperatures around 900 °C for the single-crystal growth, and, therefore, it becomes possible to grow artificially such an intermediate phase as δ-Al2O3 in a single-crystal form. The compound δ-Al2O3 has been reported as a member of the alumina family. It was recognized as an intermediate stage from γ- to θ-Al2O3 phase transition before the final transformation into R-Al2O3 above 1100 °C.1 Although many references on single-crystal growth of R-Al2O3 have been reported until now,2-10 there are no reports on the growth of δ-Al2O3 so far. This is probably because δ-Al2O3 can be produced only by the calcination of γ-Al2O3 between 650 and 950 °C.1 To grow an alumina single crystal by the conventional procedures such as the Czochralski,3-5 Verneuil,6 and the floating zone methods,7-9 it is necessary to melt the starting material above the melting point of alumina (2054 °C). Even in the chemical vapor deposition method, crystal growth occurred between 1550 and 1800 °C.10 Therefore, it is reasonable that the most stable R-Al2O3 is grown by these methods, and single-crystal growth of the intermediate phase such as δ-Al2O3 is significantly difficult. In our previous communication,11 in contrast, we have elucidated that scandium oxide single crystal was successfully grown by a simple dc electrolysis method at temperatures below 1000 °C despite its high melting point (2550 °C).12 The selective growth of c-type cubic Sc2O3 crystals has been achieved by electrolyzing the Sc3+ ion conducting solid electrolyte of Sc2(MoO4)3.13,14 In this process, successive dc electrolysis supplies Sc metal from inside the solid electrolyte by reducing Sc3+ ions migrated torward the cathodic surface and finally the stable c-type cubic Sc2O3 single crystals are grown in air atmosphere at moderate temperatures. In this communication, δ-Al2O3 was electrochemically grown in a single-crystal form at moderate temperatures as low as 900 °C by using Al2(MoO4)3 as the Al3+-conduction solid electrolyte, and it has been evidenced for the first time that this method is effective to grow intermediate phase forms such as δ-Al2O3, which is extremely difficult to prepare in a single-crystal form by the conventional methods via a melt process. Because the Al2(MoO4)3 solid electrolyte can only conduct Al3+ cation and the electrolysis can be carried out at 900 °C, it becomes possible to grow the δ-Al2O3 single crystal without difficulty. A stoichiometric amount of Al(OH)3 (purity 99.9%) and MoO3 (purity 99.9%) was ground and mixed in a mortar. The mixture was calcined at 750 °C for 12 h in air. The * Corresponding author. E-mail:
[email protected]. Department of Applied Chemistry, Faculty of Engineering, Osaka University, 2-1 Yamadaoka, Suita, Osaka 565-0871, Japan. Tel.: +81-6-6879-7352. Fax: +81-6-6879-7354. E-mail:
[email protected].
Figure 1. Schematic illustration of the δ-Al2O3 single-crystal growth method by dc electrolysis.
resulting powder was made into pellets (10 mm in diameter and 0.8 mm in thickness) and sintered at 750 °C for 12 h in air. The sample pellet sintered was placed between two ion-blocking Pt electrodes for the crystal growth electrolysis as shown in Figure 1. The electrolysis was carried out by dc voltage at 11 V at 900 °C for 3 days in air. The particles grown were characterized with a scanning electron microscope (SEM, Hitachi S-4300SD). The sample was sputter-coated with a gold layer before SEM observation to minimize any possible surface charging effects. The particle size distribution and the average particle size were determined by measuring the mean length of the shortest and the longest diameters in a particle for more than 300 particles on the SEM photograph. Selected area electron diffraction (SAED) measurement was performed with a transmission electron microscope (Hitachi, H-800) equipped with a tilting device and operating at 200 kV. The prepared particles were rinsed out from the matrix with ethanol and then supported on an amorphous carbon film mounted on a copper grid. Images were recorded under axial illumination at an approximate Scherzer focus, with a point resolution better than 0.194 nm. The diameter of the objective aperture was 20 µm, which was large enough to include some low-indexed diffraction spots from the sample. The current change during the electrolysis is plotted in Figure 2. The current rapidly decreased right after the electrolysis due to the formation of insulator alumina particles and went on to gradually decrease with particle growth. After 2 days of electrolysis (1.728 × 105 s), the current became almost constant because the entire surface
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Figure 5. Electron diffraction pattern of the δ-Al2O3 particles. Figure 2. Time dependence of the current for the dc electrolysis (11 V) of the Al2(MoO4)3 pellet at 900 °C.
(0.5 V). A considerably higher dc voltage as compared with the decomposition voltage of Al2(MoO4)3 facilitates the Al2(MoO4)3 decomposition to Al3+ ions and the MoO3 formation at the anode according to the following half-cell reaction:
2[MoO4]2- f 2MoO3 + O2 + 4e-
Figure 3. SEM photograph of the cathodic surface of the Al2(MoO4)3 pellet after electrolyzing at 900 °C for 3 days at 11 V.
Because MoO3 produced at the anode readily vaporizes during the electrolysis because the electrolysis was conducted at 900 °C, which is above the MoO3 sublimation temperature of around 750-800 °C, Al3+ ions are successively produced at the anodic surface and conduct through the Al2(MoO4)3 bulk by ionic conduction from the anode to the Pt cathode direction during electrolysis. The Al3+ ions reaching the cathode reduce to Al metal state, as illustrated in Figure 1. The Al metal is immediately oxidized to Al2O3 from the surface of the Al deposits due to electrolyzing in air.
2Al3+ + 3/2O2 + 6e- f Al2O3
Figure 4. Particle size distribution histogram of the δ-Al2O3 particles.
was covered with alumina particles by the successive electrolysis. Figure 3 shows an SEM image of the prepared particles obtained by the electrolysis of Al2(MoO4)3 at 900 °C and 11 V for 3 days. The cathodic surface was covered with welldefined polyhedral particles, while no deposits were recognized on the anodic surface. The particle formation was not observed on another pellet of Al2(MoO4)3 polycrystal subjected to the same experimental conditions without the electrolysis. Well-defined polyhedral tabular particles ranging from 1.0 to 5.0 µm were obtained by electrolysis. The particle size distribution histogram of the particles shown in Figure 4 was obtained from the calculation of the diameter for more than 300 particles on the SEM photograph. The average particle size was 3.1 µm, and the standard deviation was estimated to be 0.74 µm. The dc voltage applied (11 V) in the electrolysis was much higher than the decomposition voltage of Al2(MoO4)3
Because the Al3+ ions are steadily and gradually supplied from inside the Al2(MoO4)3 electrolyte by the electrolysis, Al metal is successively supplied on the cathodic surface. As a result, size similarly controlled aluminum oxide single crystals are grown on the cathode surface of the Al 2(MoO4)3 solid electrolyte. The electrolysis was carried out at 900 °C where alumina in a δ-form should be stable. To identify whether each particle is exactly a δ-Al2O3 single crystal or not, selectedarea electron diffraction (SAED) pattern measurements were carried out. The sharp diffraction spots in Figure 5 clearly indicate the superior crystallinity of the particle. The net pattern was consistently identical to that of [20 -14 2] zone axis pattern of a tetragonal structure. The electron diffraction spots can be consistently indexed as those of tetragonal δ-Al2O3, which has the space group P4 h m2 with a lattice constant of a0 ) 0.5599 nm and c0 ) 2.3657 nm, respectively.15 The diffraction spots obtained above explicitly elucidates that the δ-Al2O3 particle exist as a single-crystal form on the surface of Al2(MoO4)3 polycrystalline matrixes. In conclusion, single crystals of intermediate δ-Al2O3 form were successfully grown for the first time by a simple dc electrolysis method at a temperature below 1000 °C. This method is applicable at moderate temperatures around 900 °C where δ-Al2O3 can be formed and it can be successfully realized to grow such an intermediate phase of δ-Al2O3 in a single-crystal form. Acknowledgment. The present work was partially supported by a Grant-in-Aid for Scientific Research No. 15550172 from The Ministry of Education, Science, Sports and Culture. This work was also partially supported by the
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Industrial Technology Research Grant Program in ‘02 (project ID: 02A27004c) from the New Energy and Industrial Technology Development Organization (NEDO) based on funds provided by the Ministry of Economy, Trade and Industry, Japan (METI).
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(6) Alford, W. J.; Bauer, W. H.; Matolka, R. W. U. S. Govt. Res. Dev. Rep. 1967, 67, 151. (7) Revcolecshi, A.; Collongues, R. C. R. Acad. Sc. Paris, Ser. C. 1968, t.266, 1767-1769. (8) Gasson, D. B.; Cockayne, B. J. Mater. Sci. 1970, 5, 100104. (9) Dhalenne, G.; Revcolevschi, A.; Collongues, R. Mater. Res. Bull. 1972, 7, 933-942. (10) Schaffer, P. S. J. Am. Ceram. Soc. 1965, 48, 508-511. (11) Imanaka, N.; Kim, Y. W.; Masui, T.; Adachi, G. Cryst. Growth Des. 2003, 3, 289-290. (12) Berard, M. F.; Wirkus, C. D.; Wilder, D. R. J. Am. Ceram. Soc. 1968, 51, 643-647. (13) Imanaka, N.; Ueda, T.; Okazaki, Y.; Tamura, S.; Adachi, G. Chem. Mater. 2000, 12, 1910-1913. (14) Imanaka, N.; Kobayashi, Y.; Tamura, S.; Adachi, G. Solid State Ionics 2000, 136-137, 319-324. (15) Repelin, Y.; Husson, E. Mater. Res. Bull. 1990, 25, 611-621.
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