Materials Research Bulletin, Vol. 32, No. 9, pp. 1173-I 179.1997 Copyri#t 0 1997 Elsevia Science Ltd Printed in the USA. All rights reserved 0025-5408/97 S17.00 + .OO
PI1 s0025-5408(97)ooo93-7
EFFECT OF THERMAL OXIDATION TREATMENT ON THE HYDROLYSIS OF AN POWDER
IN AIR
Yuan Qiang Li, Tai Qiu and Jie Xu Department of Materials Science and Engineering, Nanjing University of Chemical Technology, Nanjing 2 10009, P.R. China (Refereed) (Received December 2,1996; Accepted December 12,1996)
ABSTRACT The hydrolysis process was investigated for AlN powder oxidizing treatment at high temperature in air. Samples were characterized by chemical analysis, XRD, and IR analysis. It was found that the hydrolysis of AlN powder in aqueous suspension was strongly dependent on the oxidation temperature. The rate of hydrolysis decreased as the oxidation temperature increased. A significant improvement in water resistance was observed when the powders were treated in air above 800°C. The A&O:, film formed by the oxidation treatment on the surface of AlN particles inhibits the reaction between AlN and water, thereby minimizing hydrolysis. Q 1997EIsevierScknceLtd KEYWORDS: A. nitrides, A. oxides, C. X-ray diffraction, spectroscopy, D. surface properties
C. infrared
INTRODUCTION Aluminum nitride (AIN) has. high thermal conductivity, thermal expansion coefficient close to that of silicon, good mechanical strength, high resistivity, low dielectric constant, and good corrosion resistance. Consequently, it is an attractive material for electronic substrates or packaging, high temperature, structural and refractory materials. Additionally, aluminum nitride powder is proposed to use as a filler for encapsulating material for integrated circuits and a filler for a synthetic resin or rubber admixture (14). However, AlN powder is sensitive to atmospheric moisture due to its high reactivity with water. AlN interacts with water in a hydrolysis reaction to decompose into aluminum hydroxide with the production of ammonia and heat. Therefore, the thermal conductivity of 1173
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AlN ceramics may remarkably decrease because the oxygen content of AlN powder increase by the hydrolysis reaction (5,6). Recently, the hydrolysis of AlN powder against water has been widely investigated (7-10). In addition, many studies have revealed that the chemical stability of AlN powder can be enhanced by modifying its surface chemistry, which forms a hydrophobic surface coating and inhibits the contact between AlN and water (1 I-15). However, little investigate has been reported on the relationship between thermal oxidation treatment and the hydrolysis of AlN powder. The present paper describes the influence of oxidation treatments at high temperature in air on the hydrolysis of AlN powder. The aim is to increase the water resistance of AlN powder. EXPERIMENTAL
PROCEDURES
Raw AlN powder was prepared by the carbothermal reduction-nitridation process of A1203 with carbon at temperature 1600-1700°C for about 5 h in a nitrogen atmosphere. Powder properties are listed in Table 1. The AlN powder was firstly dehydrated in vacuum at 200°C for 5 h, and was subsequently oxidation treatment in air with a small alumina crucible carrying out in a tubular furnace maintained at the desired temperature (f I “c) for 1 h. The net mass gain data were obtained to an accuracy of fl mg using an electrbalance after the oxidation treatment. The experiments were carried out at selected temperature between 600 and 1050°C. The hydrolysis of as-received and oxidation treated powder was evaluated by measuring the variation in pH as a function of time. For this experiment 1.0 g of the powder was dispersed in 200 ml of distilled water having pH of about 5.4 and a temperature 60°C in a flask. For the duration of the hydrolysis process, the solution was continuously stirred with a magnetic agitator. After the hydrolysis, the AlN powder was filtered off and washed with acetone, and then dried at 100°C for 2 h. The nitrogen content of AlN powder before and after hydrolysis was determined by chemical analysis (that is neutralization distillation method). The AlN content was calculated using nitrogen content with the proportion coefftcient AlNiN = 2.926. The phases before and after hydrolysis were characterized by X-ray diffraction (XRD) (Ragaku, Dmax/y B, Cu Ka = 1.542 A) and infrared spectra (IR) (Zeiss 75, v = 4000-400 cm-‘). RESULTS AND DISCUSSION The weight change of the AlN powder exposed to air was strongly dependent on the temperature. The effect of temperature on the weight gain and the AlN content of the AIN powder exposed to air were shown in Figure 1. Two distinct oxidation stages were observed dependence on the exposure temperature. At temperature below 850°C, the weight gain was very small, indicating that the oxidation rate TABLE 1 Main Characteristics of the AlN Powder Chemical composition (wt %) Al
N
0
C
65.3 33.33 0.61 0.10
Spxific surface area Average particle size.
Other impurities
(m*/g)
(Pm)
trace
6.8
3.2
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500 600 700 800 Qoo1ooo11oo Temperature ( “C )
FIG. 1. Oxidation temperature dependence of the weight gain and AlN content of AIN powder. was relatively slow. A slight reduction in AlN content was found with increasing the temperature between 600 and 85O’C. As the temperature was increased above 850°C, the weight gain increased rapidly and the reduction in AlN content became more significant, so that after exposure for 1 h at 1050°C the AIN content was less than 75%. It was seen that the oxidation became dominant and its rate increased suddenly at 850°C. When the AM powder is exposed to an air atmosphere, the following reaction is the dominant oxidation reaction (16):
The X-r,ay diffraction in the AIN powder showed no significant difference before and after oxidation treatment in air. Only AlN peaks were detected, although the AlN content decrease above 85O’C was identified by the chemical analysis. This result suggests that the aluminum oxide film on the surface of AlN may not sufficiently crystalline. Although the A1203 film became denser and thicker by increasing the oxidation temperature, the oxygen content or .AlzOJlayer on the surface of AlN powder also increased drastically. Hence, from a point of llow oxygen or high AlN content, the oxidation temperature should be controlled below 850°C. Figure 2 shows the variation of pH values with hydrolysis time for AlN water suspensions at 6O’C. For all of the AlN powders, it is observed that pH increases with increase of the duration of the hydrolysis time. The ultimate pH values that are remarkable rely on the oxidation temperature while they have initial pH values about 5.4. The higher the oxidation temperature, the lower the ultimate pH value. The as-received AlN powder (namely 65O’C in Fig. 2) exhibits a steep rise after 10 minutes and the pH value increase above 9 after 30 min, which ultimately approaches to a pH > 10. It is indicated that the concentration of ammonia in the suspension increases with the pH increase. As a result, the decomposition AlN appears to be accelerated. Above a pH of 10, the water is mostly saturated with dissolved ammonia producing through the hydrolysis of AlN powder, subsequent ammonia is expelled as a gas with a distinct ammonia odor. The AlN powders oxidized below 8OO’C showed essentially the same pH variation as the as-received powder. While the elapse of time until a steep rise in pH value trend to be extended gradually with the increase of the
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9
-e- 550°C
z1 -A- 750°C + 800% 8 850% -e- s50°c *lo!a"c
8
0
30
80 90 120 l!xl 180 210 Hydrolysis time (min)
FIG. 2 The pH values of aqueous suspensions as a function of hydrolysis of time for the AlN powders oxidized at different temperatures. oxidation temperature for the AlN powder oxidized above 800°C. In contrast, the curves slope gently, and no ammonia odor can be detected during the hydrolysis process. On the other hand, the oxidation temperature has a drastic effect on the AlN content of the hydrolyses. The results are given in Figure 3 which shows that the AlN content in the powder gradually decreases as the hydrolysis times prolong. For the low oxidation temperature-treated powders (<8OO”C),the AIN content abruptly decreases with the progress of the hydrolysis process, indicating that the rates of the hydrolysis are very fast. With the as-received AlN powder, the AlN content decreased to a 100
20 I-F-F-l 0 30 80 90 120150180210 Hydrolysis time (min)
FIG. 3 The AlN content as a function of hydrolysis time for the AlN powders oxidized at different temperatures.
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ALUMINUM
1177
NITRIDE HYDROLYSIS
value of less than 25% after whole hydrolysis. Although the rate of the hydrolysis is slower
than that of the as-received powder, the experiment results are similar to that of the asreceived powder for the powder oxidizing at 75OT, and the residual AlN content is just only 50% after hydrolysis. Within the test span, the AlN content decreases approximately linearly wjith the hydrolysis time. The linearity indicates a direct reaction at the surface of AlN particles, with no or insufficient formation of a protective coating. Evidently low oxidation ‘temperature (<800°C) is not effective to retard the reaction between AlN and water. As expected, large amount of AlN in powder favored the formation of aluminum hydroxide,, which resulted in high ultimate pH values (Fig. 1). These were confumed by X-ray difiaction patterns, as shown in Figure 4. Before hydrolysis, only AIN peaks were detected. When the AlN powders oxidized at low temperature hydrolyzed in suspension, peaks of AlN and AI (bayerite) were detected by XRD as shown in Figure 4, (D) and(E). Those results revealed that most of the AlN was converted into AI( after 180 min immersed in water, so the peaks of AI became stronger. One possible explanation for the remarkable reduction in the AlN content is that the initial oxide formation grown on the AlN particle is a porous Ab0~ structure, which below 8001T may be amorphous, highly defective and thin membrane. Water can easily penetrate into the surface of AlN, therefore the rate of hydrolysis is controlled by the chemical reaction between AlN and water. As the oxidation temperature of 800°C and above, the rate of hydrolysis became slower. As a result, no significant decrease in the AlN content was found in Figure 4. It is worth noting that the AlN content nearly remained unchanged at 850°C. X-ray diffiction patterns recorded showed only peaks related to AlN.
-i-IL A
10
20
30
40 50 60 28(degrees)
70
80
FIG. 4 X-ray diffraction patterns of the AlN powders after hydrolysis at 60°C for 180 min, oxidation treatment at (A) 95O”C, (B) 85O”C, (C) 8OO”C, (D) 75O”C, and (E) 650°C @-receiveid).
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Additional phases such as AI(O A1203were much weaker (Fig. 4(B)). It was suggested that a thicker and denser ahuninum oxide layer was formed on the surface of AlN particles, i.e., a protective coating which inhibited further hydrolysis. Thus, the denser surface film controlled the diffusion of water from the Al203 surface to interior, thereby the rate of hydrolysis became slow above 8OO’C.As the oxidation temperature was increased further (>SSO“C),the AlN content maintained almost constant at about 87.3 wt% (95OoC)and 74.2 wt!%(1050°C)before and after hy~olysis, ~de~ndent of the hydrolysis time. Them was no AI( crystalline phase other than AIN phase in the hydrolyzates as shown in Figure 4, (A). In spite of having initial low AlN content or high oxygen content in powder, the treatment at higher oxidation temperature was successful in suppressing the hydrolysis of the AlN powder. The typical IR spectra for the oxi~on-seated AlN powder after hydrolysis for 180 min at 60°C are shown in Figure 5. For comp~son the spectrum of the as-received powder was also recorded. The IR spectra of the AlN powders oxidized above 800°C(Fig. 5, (B) and (C)) exhibit a broad intense band at 500-900 cm-‘. These bands, which am closely similar to that of the asreceived powder (Fig. 5 (A)), correspond to stretching vibration of the Al-N band (8,17-18). The strongest line centered at 1330 cm-’ may be an overtone of the Al-N stretching fundamental band located at 700 cm-‘(8,17). Besides Al-N, no significant OH and Al-0 bands can be found. However, for the powders oxidized below 8OO’C(Fig. 5 (D)), the
3200
2600
2400
2000
1600
600
Wavenumbers (cm-1)
FIG. 5 IR spectra of the AlN powders (A) before and (B)-(D) after hydrolysis oxidized treatment at (B) 850°C, (C) 950°C, and (D) 65OOC.
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strong, narrow line-width bands at 3650,3540,3460, and 970 cm-’ can be observed in the OH stretching region. All bands exhibited the band characteristics of the bayerite, AI( (8,10). It is also conformed that the oxidation treatment is effective for inhibition of hydrolysis of AlN powder by forming an aluminum oxide coating. CONCLUSION The hydrolysis of AlN powder in aqueous suspension could be suppressed by the oxidation treatment in air at elevated temperature. For treatment below 8OO”c,the rate of hydrolysis is rapid and large amounts of AlN are transformed into a crystalline hydroxide, Al(OH)1 after hydrolysis for 180 min at 6O’C. It was demonstrated that the rate of hydrolysis was controlled by the surface chemical reaction between AlN and water. While, for the oxidation treatment above 8OO”C,the rate of hydrolysis became slower by forming aluminum oxide coating on the surface of AlN particles. No or lesser amount of Al(OH)j could be detected in the hydrolyzates. In this case, the rate of hydrolysis was controlled by the diffusion of water through the compact regions of aluminum oxide film on the surface of AlN. REFERENCES 1. GA. Slack, J. Phys. Chem. Solids. 34,321 (1973). 2. W. Werdecker, F. Aldinger, IEEE Trans. Compon., Hybrids, Manuf: Technol. CHMT-7, 399 (1984). 3. R.R. Tummala,Am. Ceram. Sot. Bull. 68,883 (1989). 4. G.A. Slack, and T.F. McNeil,J. Cyst. Growth. 34,263 (1976). 5. D. Suryanarayana, L.J. Matienzo and D.F. Spencer, IEEE Trans. Compon., Hybrids, Man& Techno’l. 12,330 (1989). 6. A.V. Virkar, T.B. Jackson and R.A. Cutler,J. Am. Ceram. Sot. 72,203 I (1989). 7. A. Abid, R. Bensalemand J. Sealy,J. Mater. Sci. 21, 1301 (1986). 8. P. Bowen, J.G. Highfield, A. Mocellin and T.A. Ring, J. Am. Ceram. Soc.73, 724 (1990). 9. S. Hayashi, K. Hayamizu and 0. Yamamoto, Bull. Chem. Sot. Jpn. 60, 761 (1987). 10. J.G. Highfield and P. Bowen, Anal. Chem. 61,2399 (1989). 11. M. Egashira, Y. Shimizu and S. Takatsuki, J. Mater. Sci. Lett. 10,994 (1991). 12. M. Egashira, Y. Shimizu, Y. Takao, R. Yamaguchi and Y. Ishikawa, J. Am. Ceram. Sot. 77, 1793 (1994). 13. E.A. Glroat and T.J. Mroz, Am. Ceram. Sot. Bull. 73, 75 (1994). 14. J. Takada, J. Sot. Powder Technol. Jpn 29,83 1 (1992). 15. K. Sugiiyama,H. Takahashi,S. Konno, S. Tanaka, M. Uenishi and Y. Hashizume,J. Sue. Powder Technol. Jpn. 29,682 ( 1992). 16. T. Sate, K. Haryu, T. Endo and M. Shimada, J. Mater. Sci. 22,2277(1987). 17. E. Ponthieu, P. Grange, B. Delmon, L. Lonnoy, L. Leclercq, R. Bechara and J. Grimblot, J. Euro. Ceram. Sot. 8,233 (1991). 18. R.A. N;yquist and R.O. Kagel, Infrared Spectra of Inorganic Compounds, p. 495, Academic Press, New Ylork (1971).