ISRN Ceramics

Volume 2013 (2013), Article ID 317830, 5 pages

http://dx.doi.org/10.1155/2013/317830

Research Article

Low Temperature Synthesis of α-Alumina with a Seeding Technique

Kazuhiro Inoue,¹ Masachika Hama,¹ Yoshio Kobayashi,¹ Yusuke Yasuda,² and Toshiaki Morita²

¹Department of Biomolecular Functional Engineering, College of Engineering, Ibaraki University, 4-12-1 Naka-narusawa-cho, Hitachi, Ibaraki 316-8511, Japan

²Hitachi Research Laboratory, Hitachi Ltd., 7-1-1 Omika-cho, Hitachi, Ibaraki 319-1292, Japan

Received 26 August 2013; Accepted 29 September 2013

Academic Editors: K. L. Bing, D. Jia, W.-C. Oh, and W.-H. Tuan

Copyright © 2013 Kazuhiro Inoue et al. This is an open access article distributed under the Creative Commons Attribution License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.

Abstract.

This paper reports a method for producing α-Al₂O₃ at low temperature, which uses a seeding technique. White precipitate of aluminum hydroxide, which was prepared with a homogeneous precipitation method using aluminum nitrate and urea in aqueous solution, was peptized by using acetic acid at room temperature and then transformed to a transparent alumina sol. To the alumina sol α-Al₂O₃ particles were added as a seed, and then the sol containing α-Al₂O₃ particles was transformed to an α-Al₂O₃-seeded alumina gel by drying the sol at room temperature. The nonseeded alumina gel was amorphous or fine crystallites even after being annealed at 600°C and was crystallized to γ-Al₂O₃ at 700°C. The α-Al₂O₃ seeding promoted crystallization of alumina gel to α-Al₂O₃. The promotion of crystallization was made remarkable with a decrease in α-Al₂O₃ particle size and an increase in α-Al₂O₃ particle content in weight for the final seeded alumina gel. With an α-Al₂O₃ particle size of 150 nm and an α-Al₂O₃ particle content of 5%, the seeded alumina gel was partially crystallized to α-Al₂O₃ by annealing at a temperature as low as 700°C and mostly at 900°C.

1. Introduction

Not only electroconductive materials such as gold, silver, and copper but also electric-insulating materials are quite essential for producing electronic devices such as integrated circuits [1–3]. Alumina is one of representative electric-insulating materials. Since it has high thermal conductivity and high chemical stability at high temperature compared to other electro-insulating materials such as glass, plastic, and paper [4–6], it is promising as the electric-insulating material used in the electronic devices.

There are various crystal structures in the alumina. Among the various crystal structures, alumina that reveals electric insulation is of α type. The α-alumina has been conventionally produced by annealing aluminum hydroxide derived from aluminum salts or minerals at temperatures higher than 1000°C [7–10]. The α-alumina can be also produced by fabricating amorphous alumina with methods using liquid phase such as precipitation and sol-gel process and then annealing it at high temperature [11–13]. These methods need the high temperatures, which brings about much consumption of energy. Accordingly, low temperature processes for producing α-alumina are desired for saving energy.

Our research group has studied effects of nanocrystallites seeding on various titanates such as PZT and BST fabricated by the sol-gel method for the last decade [14–17]. As a result, their crystallization temperatures were lowered with the seeding. The titanates were epitaxially crystallized on surface of the crystallites, which provided the lowering of crystallization temperatures, though its mechanism is still unclear. Accordingly, the crystallization temperature of alumina, that is, the temperature for transformation of amorphous alumina to α-alumina, is likely to be also lowered with the seeding technique.

Our previous work proposed methods for preparing alumina sol by utilizing a homogeneous precipitation method and for fabricating transparent alumina films [18]. Since the method used aluminum salt and did not need heating for peptization of aluminum hydroxide precipitate, cost and energy for the fabrication of alumina could be saved. The obtained alumina film was amorphous or fine crystallites even after being annealed at a temperature as high as 500°C and was crystallized to γ-Al₂O₃ at 900°C, which implied that the alumina was required to be annealed at further higher temperatures for transformation to α-Al₂O₃.

The aim of the present work is to study the effect of α-alumina seeding on crystallization of alumina fabricated with our previous alumina and to lower annealing temperature for production of α-Al₂O₃.

2. Materials and Methods

2.1. Materials

Aluminum nitrate enneahydrate (Al(NO₃)₃·9H₂O) (98.0%) and urea (99.0%) were used as starting material for aluminum hydroxide and a precipitation inducer for preparation of aluminum hydroxide, respectively. Acetic acid (99.7%) was used as a peptizer for aluminum hydroxide. Two kinds of α-Al₂O₃ particles were used as seed crystallines, namely, α-Al₂O₃ (nanoalumina) (99.85%, Ionic Liquids Technologies) and α-Al₂O₃ (microalumina) (99.0%). According to their data in catalogue, they have particle sizes of 150 nm and 35–50 μm, respectively. Except for the nanoalumina, all chemicals were purchased from Kanto Chemical Co., Inc., and were used as received. Water that was ion-exchanged and distilled with Yamato WG-250 was used in all the preparations.

2.2. Methods

2.2.1. Preparation

Precipitate of aluminum hydroxide was prepared by the homogeneous precipitation method or by the following procedure. The Al(NO₃)₃ and the urea were dissolved in water in a hermetically sealed glass bottle at initial concentrations of 0.2 M Al(NO₃)₃ and 5 M urea. The mixture was stirred at 80°C for 8 h, which produced white precipitate. The obtained precipitate was aged at room temperature for 12 h after the preparation, washed by repeating centrifugation, removal of supernatant, addition of the water, and sonication over three times, and then peptized at 25°C (room temperature) with the addition of acetic acid at [acid]/[Al³⁺] molar ratios of 0.15. The precipitate was transformed to a transparent sol (alumina sol) in ca. 12 h after the peptization. The α-Al₂O₃ particles were dispersed in the sol at 1–5% in weight for final alumina gel seeded with the α-Al₂O₃ particles. Then, the sol containing the particles was casted onto a petri dish. Drying in air at room temperature converted the sol into a solid alumina gel. The solid gel was pulverized with a mortar into powder and then annealed in air at various temperatures.

2.2.2. Characterization

Actual Al concentrations in particle colloid solutions were measured by inductively coupled plasma (ICP) emission spectroscopy. ICP measurement was performed with a Shimadzu ICPS-7510 atom emission spectrometer. Emission was detected at a wavelength of 396.153 nm. Samples for ICP were prepared by dissolving the particles with aqua regia completely and then diluting the obtained solution with water to adjust its concentrations to concentrations suitable for the measurements.

The gels were characterized by X-ray diffractometry (XRD) and thermal analysis (TG-DTA). Powder, which was obtained by pulverizing alumina gel with a mortar, was used as samples for XRD measurements. The XRD measurements were carried out with a Rigaku Ultima IV X-ray diffractometer at 40 kV and 30 mA with CuKα ₁ radiation. There are mainly two types in crystal structures of Al₂O₃: α and γ. Their XRD intensities were compared to study on crystallinity of alumina gel. α-Al₂O₃ and γ-Al₂O₃ reveal XRD peaks at 43.4 and 45.8 degrees that are not overlapped, respectively. XRD intensity ratio of α-Al₂O₃/γ-Al₂O₃ was defined as the peak intensity (counts per second (cps)) of α-Al₂O₃ at 43.4 degree divided by that of γ-Al₂O₃ at 45.8 degree. When neither the XRD peak of α-Al₂O₃ nor that of γ-Al₂O₃ was detected, the ratio was considered as zero. TG-DTA was performed in air at a heating rate of 10°C/min with a Rigaku Thermo plus EVO TG8120 thermal analyzer. Samples for TG-DTA measurements were obtained in the same manner as that for the XRD samples.

3. Results and Discussion

3.1. Alumina Gels

The ICP measurement showed that an Al concentration in the sol was 0.717 M, which indicated that 71.0% of the Al(NO₃)₃ was transformed to the alumina sol. According to our previous work, the as-prepared gel obtained from the sol was transparent. Figure 1 shows XRD patterns of alumina gels annealed at various temperatures. A few peaks were detected around 20, 30, and 40 degrees in the as-prepared alumina gel. They were roughly assigned to those of pseudoboehmite, according to the reference [19]. Since these peaks were broad and diffuse, the as-prepared gel was amorphous or fine crystallites. No dominant peaks appeared even for annealing at a temperature as high as 600°C; the alumina was still amorphous or fine crystallites For 700°C, peaks were detected at 45.8 and 66.8 degrees. They were probably attributed to γ-Al₂O₃ (JCPDS card number 29-0063), which indicated that transformation from poorly crystallized pseudoboehmite to γ-Al₂O₃ began at 700°C. Peaks due to γ-Al₂O₃ other than the two peaks also appeared over 700°C, and they became clearly detected with an increase in annealing temperature. This result indicated that the annealing promoted the crystallization of alumina gel.

Figure 1: XRD patterns of nonseeded alumina gels. Sample (a) is as-prepared alumina. Samples (b), (c), (d), (e), and (f) were alumina gels annealed at 500, 600, 700, 800, and 900°C, respectively. ○: γ-Al₂O₃.

3.2. Alumina Gels Seeded with α-Al₂O₃ Particles

3.2.1. Effect of Seeding of α-Al₂O₃ Particles

Figure 2 shows XRD patterns of alumina gels seeded with microalumina particles. For the annealing temperatures of 25 (as prepared) to 800°C, there was no large difference in the XRD patterns between the nonseeded (Figure 1) and the seeded. This indicated that the seeding did not affect crystal structure of alumina gel in the range of annealing temperatures from 25 to 800°C. For 900°C, peaks were detected at 25.6, 35.2, 43.4, 52.6, and 57.5 degrees besides the peaks of γ-Al₂O₃. They were attributed to α-Al₂O₃ (JCPDS card number 42-1468). The nonseeded alumina gel annealed at 900°C revealed no α-Al₂O₃ peaks, as shown in Figure 1(f). Accordingly, the crystallization of alumina gel to α-Al₂O₃ was effected by the seeding. The alumina gel was probably crystallized epitaxially on α-Al₂O₃ particle surface.

Figure 2: XRD patterns of alumina gels seeded with 1% microalumina. Sample (a) is as-prepared alumina. Samples (b), (c), (d), (e), and (f) were alumina gels annealed at 500, 600, 700, 800, and 900°C, respectively. ⚫: α-Al₂O₃, ○: γ-Al₂O₃.

3.2.2. Effect of α-Al₂O₃ Particle Size

Figure 3 shows XRD patterns of alumina gels seeded with nanoalumina particles. For the annealing temperatures of 25 (as prepared) to 700°C, there were no large differences in the XRD patterns among the nonseeded alumina gel (patterns (a)–(d) in Figure 1), the alumina gel seeded with microalumina particles (patterns (a)–(d) in Figure 2), and the alumina seeded with nanoalumina particles (patterns (a)–(d) in Figure 3). For 800°C, peaks due to α-Al₂O₃ were detected at 25.6, 35.2, 43.4, 52.6, and 57.5 degrees besides the peaks of γ-Al₂O₃. No α-Al₂O₃ peaks appeared in the microalumina-seeded alumina annealed at 800°C (Figure 2(f)). This indicated that a seeding effect of the small α-Al₂O₃ particles on crystallization was dominant compared to the large α-Al₂O₃ particles. A raise of annealing temperature to 900°C provided that most outstanding peaks were attributed to α-Al₂O₃.

Figure 3: XRD patterns of alumina gels seeded with 1% nanoalumina. Sample (a) is as-prepared alumina. Samples (b), (c), (d), (e), and (f) were alumina gels annealed at 500, 600, 700, 800, and 900°C, respectively. ⚫: α-Al₂O₃, ○: γ-Al₂O₃.

Figure 4 shows XRD intensity ratio of α-Al₂O₃/γ-Al₂O₃ plotted as a function of annealing temperature. The plot of the nonseeded alumina was also shown. Since the nonseeded alumina was not crystallized to α-Al₂O₃, the intensity ratio was 0 in the whole range of annealing temperature examined. For the micro-alumina particles, the ratio was 0 at 25–800°C, and increased faintly to 0.39 at 900°C. In contrast, the ratio started to increase at 800°C and reached 3.19 at 900°C.

Figure 4: XRD intensity ratios of α-Al₂O₃/γ-Al₂O₃ as a function of annealing temperature. △: nonseeded alumina; ▲: 1% microalumina-seeded alumina; ■: 1% nanoalumina-seeded alumina; ○: 3% nanoalumina-seeded alumina; ⚫: 5% nanoalumina-seeded alumina.

A total apparent surface area of α-Al₂O₃ particles with a small size is larger than that with a large size at the same amount of α-Al₂O₃ particles. The large surface area of nanoalumina particles provided effective epitaxial crystallization, compared to the small area. Consequently, the crystallization to α-Al₂O₃ was remarkably promoted with the seeding of nanoalumina particles.

3.2.3. Effect of α-Al₂O₃ Particle Content

Whereas the content of α-Al₂O₃ particles was 1% in Figure 3, the α-Al₂O₃ content was raised to study an effect of α-Al₂O₃ particle content in this section. Figure 5 shows XRD patterns of alumina gels seeded with nanoalumina particles at 3%. For the annealing temperatures of 25 and 500°C, there were no large differences in the XRD patterns between the 1%-nanoalumina-seeded alumina (Figure 3) and the 3%. At 600°C, peaks due to α-Al₂O₃ were faintly observed at 35.2, 43.4, and 57.5 degrees, though not observed for 1%. This indicated that the crystallization to α-Al₂O₃ was more promoted with the increase in α-Al₂O₃ content. These peaks became strong with an increase in annealing temperature. Figure 6 shows XRD patterns of alumina gels seeded with nanoalumina particles at 5%. The promoted crystallization to α-Al₂O₃, which was observed at 3%, became more dominant with a further increase in α-Al₂O₃ particle to 5%.

Figure 5: XRD patterns of alumina gels seeded with 3% nanoalumina. Sample (a) is as-prepared alumina. Samples (b), (c), (d), (e), and (f) were alumina gels annealed at 500, 600, 700, 800, and 900°C, respectively. ⚫: α-Al₂O₃, ○: γ-Al₂O₃.

Figure 6: XRD patterns of alumina gels seeded with 5% nanoalumina. Sample (a) is as-prepared alumina. Samples (b), (c), (d), (e), and (f) were alumina gels annealed at 500, 600, 700, 800, and 900°C, respectively. ⚫: α-Al₂O₃, ○: γ-Al₂O₃.

XRD intensity ratio of α-Al₂O₃/γ-Al₂O₃ versus annealing temperature was also plotted as shown in Figure 4. The ratios for 3 and 5% started to increase at 700°C, and reached 3.54 and 4.28 at 900°C, respectively.

The large α-Al₂O₃ particle content corresponded to a large apparent surface area of α-Al₂O₃ particles. Accordingly, a mechanism similar to that for the α-Al₂O₃ particle size, which was based on surface area of α-Al₂O₃ particles, was also considered for the effect of α-Al₂O₃ particle content; the large apparent surface area of α-Al₂O₃ particles at the large particle content promoted remarkably the crystallization to α-Al₂O₃.

4. Conclusions

The homogeneous precipitation using aluminum nitrate and urea in aqueous solution was performed for producing the white precipitates of aluminum hydroxide. The transparent alumina sol was successfully produced by the peptization of the aluminum hydroxide with the addition of acetic acid at room temperature. The alumina seeded with α-Al₂O₃ particles was fabricated by drying the sol containing the α-Al₂O₃ particles at room temperature. The nonseeded alumina gel was amorphous or fine crystallites even after being annealed at 600°C and was crystallized to γ-Al₂O₃ by annealing at 700°C. For the alumina seeded with α-Al₂O₃ particles, the crystallization of alumina gel to α-Al₂O₃ was promoted remarkably with a decrease in α-Al₂O₃ particle size and an increase in α-Al₂O₃ particle content in film. The alumina seeded with the nanoalumina particle size of 150 nm at the α-Al₂O₃ particle content of 5% started to be crystallized to α-Al₂O₃ by annealing at a temperature as low as 700°C.

Conflict of Interests

The nanoalumina and the other chemicals were supplied by Ionic Liquids Technologies and Kanto Chemical, respectively, with no financial support from the two companies.

Acknowledgment

This work was partially supported by Hitachi Ltd.

References

M. V. Jacob, K. Bazaka, M. Weis, D. Taguchi, T. Manaka, and M. Iwamoto, “Fabrication and characterization of polyterpenol as an insulating layer and incorporated organic field effect transistor,” Thin Solid Films, vol. 518, no. 21, pp. 6123–6129, 2010.

J. U. Jansen and L. D. B. MacHado, “A new resin for photocurable electrical insulating varnishes,” Nuclear Instruments and Methods in Physics Research B, vol. 236, no. 1–4, pp. 546–551, 2005.

A. Masuda, H. Umemoto, and H. Matsumura, “Various applications of silicon nitride by catalytic chemical vapor deposition for coating, passivation and insulating films,” Thin Solid Films, vol. 501, no. 1-2, pp. 149–153, 2006.

X. Shen, X. Nie, H. Hu, and J. Tjong, “Effects of coating thickness on thermal conductivities of alumina coatings and alumina/aluminum hybrid materials prepared using plasma electrolytic oxidation,” Surface and Coatings Technology, vol. 207, pp. 96–101, 2012.

J. Musil, J. Blažek, P. Zeman, Š. Prokšová, M. Šašek, and R. Čerstvý, “Thermal stability of alumina thin films containing γ-Al₂O₃ phase prepared by reactive magnetron sputtering,” Applied Surface Science, vol. 257, pp. 1058–1062, 2010.

K. Vanbesien, P. De Visschere, P. F. Smet, and D. Poelman, “Electrical properties of Al₂O₃ films for TFEL-devices made with sol-gel technology,” Thin Solid Films, vol. 514, no. 1-2, pp. 323–328, 2006.

V. Isupov, L. Chupakhina, G. Kryukova, and S. Tsybulya, “Fine α-alumina with low alkali: new approach for preparation,” Solid State Ionics, vol. 141-142, pp. 471–478, 2001.

I. N. Bhattacharya, P. K. Gochhayat, P. S. Mukherjee, S. Paul, and P. K. Mitra, “Thermal decomposition of precipitated low bulk density basic aluminium sulfate,” Materials Chemistry and Physics, vol. 88, no. 1, pp. 32–40, 2004.

E. Ryshkewitch, Oxide Ceramics, Academic Press, New York, NY, USA, 1960.

J. S. Reed, Principles of Ceramics Processing, Wiley-Interscience, New York, NY, USA, 2nd edition, 1995.

M. Shojaie-Bahaabad and E. Taheri-Nassaj, “Economical synthesis of nano alumina powder using an aqueous sol-gel method,” Materials Letters, vol. 62, no. 19, pp. 3364–3366, 2008.

F. Mirjalili, M. Hasmaliza, and L. C. Abdullah, “Size-controlled synthesis of nano α-alumina particles through the sol-gel method,” Ceramics International, vol. 36, no. 4, pp. 1253–1257, 2010.

Q. Fu, C.-B. Cao, and H.-S. Zhu, “Preparation of alumina films from a new sol-gel route,” Thin Solid Films, vol. 348, no. 1, pp. 99–102, 1999.

T. Tanase, A. Nishikata, Y. Iizuka, Y. Kobayashi, M. Konno, and T. Miwa, “Low-temperature synthesis of single-phase lead zirconate titanate thin film with a nm-seeding technique,” Journal of the Ceramic Society of Japan, vol. 110, no. 1286, pp. 911–915, 2002.

T. Tanase, Y. Kobayashi, T. Miwa, and M. Konno, “Low-temperature synthesis and dielectric properties of single-phase lead zirconate titanate thin film with a nano particle seeding technique,” in Proceedings of the Materials Research Society Symposium, vol. 784, pp. 151–156, 2004.

T. Tanase, Y. Kobayashi, T. Nabatame, T. Miwa, and M. Konno, “Dielectric properties of lead zirconate titanate thin films seeded with barium strontium titanate nanoparticles,” Thin Solid Films, vol. 471, no. 1-2, pp. 71–75, 2005.

Y. Kobayashi, Y. Iizuka, T. Tanase, and M. Konno, “Low-temperature synthesis of single-phase barium strontium titanate thin film with a nm-seeding technique and its dielectric properties,” Journal of Sol-Gel Science and Technology, vol. 33, no. 3, pp. 315–321, 2005.

K. Inoue, H. Kurebayashi, M. Hama, Y. Kobayashi, Y. Yasuda, and T. Morita, “Fabrication of transparent self-supporting alumina films by homogeneous precipitation process,” The Ceramic Society of Japan, vol. 121, pp. 494–497, 2013.

W. Cai, H. Li, and G. Zhang, “An innovative approach for pseudoboehmite precipitation from seeded sodium aluminate solutions,” Journal of Physics and Chemistry of Solids, vol. 71, no. 4, pp. 515–518, 2010.