Afyon Kocatepe University, Afyon Engineering Faculty Department of Ceramic Engineering / Afyon-TURKEY

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1 Key Engineering Materials Online: ISSN: , Vols , pp doi: / Trans Tech Publications, Switzerland Effect of CaF 2 Addition and Type of Alumina on Production of Aluminum Nitride M. Aktürk, C.B. Emrullahoğlu, Ö.F. Emrullahoğlu Afyon Kocatepe University, Afyon Engineering Faculty Department of Ceramic Engineering / Afyon-TURKEY Keywords : AlN, Carbothermal Reduction-Nitride. Abstract. The synthesis of AlN powders by carbothermal reduction-nitridation of various types of Al 2 O 3 powders was investigated. Calcium fluoride (CaF 2 ) was added as a catalyst to promote the reaction. An Al 2 O 3 / C molar ratio of 0.29 and CaF 2 of 3 wt. % were fixed in advance. The reduction-nitridation was performed at the various temperatures from 1400 to 1500 C in N 2 gas flow. The laboratory work essentially consists of characterization of the raw materials, shaping by dry pressing, reaction sintering in tube furnace and characterization of the final products by means of X-ray diffraction. Introduction Aluminum nitride (AlN) has excellent properties as high-performance substrate, owing to the high thermal conductivity, high electrical resistivity, and relatively low dielectric constant. In addition its thermal expansion coefficient is close to that of silicon [1,2]. Recently it has been reported that AlN powder is also a leading candidate as a filler in high thermal conductivity molding compounds for integrated circuit packaging [1]. In the 1950s, the first systematic studies on synthetic processing of AlN powder were developed [3]. At present, three kinds of AlN powders, produced by direct nitridation, carbothermal reduction and vapor phase reaction, are commercially available [4]. The AlN powder is used as a high quality raw powder for achieving high thermal conductivity. However, except of the work of Komeya et.al.[5] few fundamental works on this method have been reported in recent years, in spite of expansion of application fields. The effect of CaF 2 additions and type of alumina on carbothermal reduction-nitridation for preparing AlN powders from the Al 2 O 3 -C-N 2 system was investigated in this work. Consequently, it was confirmed that Ca compounds, such as CaF 2, were excellent nitridation promoters. The characteristics of raw Al 2 O 3 powder are also considered to strongly affect the reaction rate and morphology of the product. All rights reserved. No part of contents of this paper may be reproduced or transmitted in any form or by any means without the written permission of Trans Tech Publications, (ID: , Pennsylvania State University, University Park, USA-03/03/16,23:28:19)

2 966 Euro Ceramics VIII Experimental Raw Materials Two types of Al 2 O 3 powders, grade CT 3000 SG (s.s.a. 7.0 m 2 /g) from Alcoa and Active Alumina (γ-al 2 O 3 ) which was obtained by the heat treatment of Etibank Seydişehir Alumina at 400 C were used. Carbon black (+325 mesh fraction, wt. %, Petkara FEF-N 550 from PETKIM Petro-Chem. Co.-Turkey) and fluorit mineral (from Van Provence of Eastern Turkey) were used as reduction and nitridation promoters. Experimental Procedure An Al 2 O 3 /C molar ratio of 0.29, which is slightly larger than the theoretical value was selected based on the previous study [3]. The powders were mixed by wet ball milling using jet mill with alumina balls for 30 minutes. The slurry was dried in rotary evaporator and screened through 297 µm sieve. About 5 g of powder mixture was uniaxially pressed in a 25 mm diameter steel die at 1 MPa. 10 wt. % PVA was used as binder. The compositions of pressed samples were (1) α-al 2 O 3 C, (2) α-al 2 O 3 -C-CaF 2, (3) γ-al 2 O 3 C and (4) γ-al 2 O 3 -C-CaF 2. They were sintered at temperatures of 1400, 1450 and 1500 C for 90 min in N 2 gas flow of 0.7 l/min. For reference powder mixtures without CaF 2 addition were also examined. The residual C in each product was burned out at 700 C for 2 hours in air. The products were characterized by XRD (Shimadzu) analysis. Results and Discussions Figure 1(a, b, c, d), 2(a, b, c, d) and 3 (a, b, c, d) show the X-ray diffraction patterns of the powder mixtures sintered at temperatures of 1400, 1450 and 1500 C for 90 min. in N 2 gas flow of 0,7 l/min. a, b, c and d show the X-ray diffraction patterns of the powder mixtures sintered of Al 2 O 3, Al 2 O 3 with CaF 2 addition, γ-al 2 O 3 and γ-al 2 O 3 with CaF 2 addition respectively. Materials containing Al 2 O 3 and sintered at temperatures of 1400, 1450 and 1500 C showed only Al 2 O 3 peaks. But materials containing Al 2 O 3 with CaF 2 addition gave Al 2 O 3 and AlN peaks. The highest intensity peak of AlN is at 2θ = 33. Both of the materials containing γ-al 2 O 3 and γ-al 2 O 3 with CaF 2 addition sintered at temperatures of 1400, 1450 and 1500 C gave Al 2 O 3 and AlN peaks. γ-al 2 O 3 containing mixtures gave higher AlN peaks than that of the Al 2 O 3 containing mixtures at all sintering temperatures. The amount of AlN increased with the increasing reaction temperatures.

3 Key Engineering Materials Vols K. Kameya et. al.[4] was confirmed to form CaF 2 5Al 2 O 3 as an intermediate, whereas in this study the intermediate could not be detected in all of the mixtures sintered at temperatures of 1400, 1450 and 1500 C because of the decomposition of CaF 2 5Al 2 O 3 at 1395 C. Figure 1. X-ray diffraction patterns of the powder mixtures sintered at 1400 C Al 2 O 3, Al 2 O 3 with CaF 2 addition, γ-al 2 O 3 and γ-al 2 O 3 with CaF 2 addition Figure 2. X-ray diffraction patterns of the powder mixtures sintered at 1450 C Al 2 O 3, Al 2 O 3 with CaF 2 addition, γ-al 2 O 3 and γ-al 2 O 3 with CaF 2 addition

4 968 Euro Ceramics VIII Figure 3. X-ray diffraction patterns of the powder mixtures sintered at 1500 C Al 2 O 3, Al 2 O 3 with CaF 2 addition, γ-al 2 O 3 and γ-al 2 O 3 with CaF 2 addition Conclusion (1) The nitridation rate tended to increase with increasing reaction temperatures. The nitridation rate were also found to significantly depend on the type of Al 2 O 3. (2) γ-al 2 O 3 containing mixtures gave higher AlN peaks than that of the Al 2 O 3 containing mixtures at all sintering temperatures. This may be a result of larger surface area of γ-al 2 O 3 than α-al 2 O 3. (3) Calcium fluoride added as a catalyst to promote the reaction increased the quantity of AlN. (4) CaF 2 5Al 2 O 3 could not be detected in all of the mixtures sintered at temperatures of 1400, 1450 and 1500 C. References [1] Y. Nagai and G.-C. Lai, "Preparation and Properties of Moisture- Resistant Aluminum Nitride Powder " Key Eng. Mater. Vols (1999) pp [2] Ceramic Industry, Materials Handbook. Special Section, Brick and Clay Record. January 2003 p [3] N. Kuramoto and H. Taniguchi, J. Mater. Sci. Lett., 3[6] (1984) [4] K. Komeya, T. Ide and T. Meguro Synthesis of AlN Powder by Carbothermal Reduction- Nitridation of Various Al 2 O 3 Powders with CaF 2 " Key Engineering Materials Vols (1999) pp [5] K. Komeya, M. Mitsuhashi, T Meguro, J. Ceram Soc. Jpn., 101 [4] (1993)

5 Euro Ceramics VIII / Effect of CaF 2 Addition and Type of Alumina on Production of Aluminum Nitride / DOI References [1] Y. Nagai and G.-C. Lai, "Preparation and Properties of Moisture- Resistant Aluminum Nitride Powder " Key Eng. Mater. Vols (1999) pp /