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Oxidation kinetics of aluminum nitride at different oxidizing atmosphere
Journal of Alloys and Compounds 465 (2008) 90–96
Oxidation kinetics of aluminum nitride at different oxidizing atmosphere Xinmei Hou a , Kuo-Chih Chou a,∗ , Xiangchong Zhong b , Seshadri Seetharaman c a
Metallurgical and Ecological Engineering School, University of Science and Technology Beijing, Beijing 100083, PR China b High Temperature Ceramics Institute, Zhengzhou University, Henan Province 450052, PR China c Department of Materials Science and Engineering, Royal Institute of Technology, Stockholm, Sweden Received 4 September 2007; received in revised form 15 October 2007; accepted 16 October 2007 Available online 23 October 2007
Abstract In the present work, the oxidation kinetics of AlN powder was investigated by using thermogravimetric analysis, X-ray diffraction (XRD) and scanning electron microscopy (SEM). The experiments were carried out both in isothermal as well as non-isothermal modes under two different oxidizing atmospheres. The results showed that the oxidation reaction started at around 1100 K and the rate increased significantly beyond 1273 K forming porous aluminum oxide as the reaction product. The oxidation rate was affected by temperature and oxygen partial pressure. A distinct change in the oxidation mechanism was noticed in the temperature range 1533–1543 K which is attributed to the phase transformation in oxidation product, viz. alumina. Diffusion is the controlling step during the oxidation process. Based on the experimental data, a new model for predicting the oxidation process of AlN powder had been developed, which offered an analytic form expressing the oxidation weight increment as a function of time, temperature and oxygen partial pressure. The application of this new model to this system demonstrated that this model could be used to describe the oxidation behavior of AlN powder. © 2007 Elsevier B.V. All rights reserved. Keywords: Oxidation kinetics; AlN powder; Oxygen partial pressure; Temperature; Model
1. Introduction Increased attention has been devoted in recent years to AlN materials because of their excellent physical properties such as high thermal conductivity (3.2 W/cm K), low coefficient of thermal expansion ((4.03–6.0) × 10−6 K−1 ) and high electrical resistivity (>4 × 108 cm) [1–3]. AlN materials find large applications as electronic substrates, heat radiation fins and refractory materials [4,5]. However, these materials tend to be oxidized at high temperature under oxidizing atmosphere, which greatly limits their application. For long-term operations, the oxidation behavior of AIN is of major importance. In fact, low levels of oxygen impurities in AlN dramatically affect its final properties [6]. Therefore, the study of AlN oxidation is of great interest for both fundamental science and practical applications. Several articles have reported the oxidation behavior of AlN, both in powder form and as bulk ceramic [5–18]. Among these studies, many techniques including X-ray diffraction, infrared
∗
Corresponding author. Tel.: +86 10 62332646. E-mail address: [email protected] (K.-C. Chou).
0925-8388/$ – see front matter © 2007 Elsevier B.V. All rights reserved. doi:10.1016/j.jallcom.2007.10.066
spectroscopy, thermogravimetric analysis, X-ray photoelectron spectroscopy (XPS), scanning electron microscopy (SEM), and transmission electron microscopy (TEM) have been employed to study the oxidation phenomenon. These studies have determined the crystalline state of the oxide, the composition and chemical bonding states of Al, N, and O atoms, and have developed simple oxidation kinetics rate models. Because the oxidation process is affected by parameters such as: particle size, temperature, the humidity of environment, and characteristics of the sample [5–17], various kinetics models (linear or parabolic) have been proposed. Kim and Moorhead [12] reported that the oxidation kinetics of sintered aluminum nitride followed linear rate law while at higher temperature became parabolic. By contrast, Bellosi et al. [8] investigated the oxidation kinetics of sintered AlN substrates using Y2 O3 as the sintering aid and found that the kinetics depended on the type of sintering aid. It was reported [8] that in the temperature range 1100–1400 ◦ C, the oxidation kinetics followed a linear rate law. Despite the abundance in the experimental data available, the theoretical analysis of the oxidation phenomenon has not made any significant progress. Among the key problems yet to be addressed, a few significant ones are
X. Hou et al. / Journal of Alloys and Compounds 465 (2008) 90–96
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(1) the relationship of the reacted fraction of oxidation ξ with time t at a constant temperature; (2) the relationship between the reacted fraction ξ and temperature T in a non-isothermal process; and (3) The impact of oxygen partial pressure PO2 on the reaction rate. In the present study, the oxidation of AlN powder was carried out both in isothermal as well as non-isothermal modes by thermogravimetric method. XRD and SEM analyses were employed to characterize the oxidation product. The effect of different oxygen partial pressure on the oxidation behavior, which was not looked into by earlier researchers in a systematic way was also investigated. Based on the experimental results, a simple set of equations was developed in order to describe the oxidation kinetics. These equations have also been successfully used in the case of the oxidation of Si–Al–O–N, SiC, and Si3 N4 [18–21] in view of the analogy between the materials studied. 2. Experimental procedure 2.1. Material Commercial AlN powder with an average particle size of 4.34 m and a maximum impurity content of less than 2 wt% was used. XRD analysis showed that the characteristic peaks were all corresponding to that of AlN.
2.2. Oxidation tests The weight change of AlN powder during oxidation was monitored using a Netzsch STA449C thermal analysis system. The TG microbalance is sensitive within ±0.001 mg. The oxidation experiments were carried out in both non-isothermal and isothermal modes. In the non-isothermal mode, the sample (about 26 mg) held in an alumina crucible was placed in the heating furnace. The furnace was then heated from room temperature to the maximum temperature of 1723 K at the heating rate of 10 K/min. In the isothermal experiments, AlN powder (about 26 mg) was also placed in an alumina crucible and the temperature was rapidly raised to the required level in a flowing purified argon gas (<5 ppm O2 ). The argon was purified by passing through silica gel and dehydrite (Mg(ClO3 )2 ) to remove moisture and through tube furnaces containing copper and magnesium at 773 K to remove residual oxygen. After the thermal equilibrium was established, the argon gas was stopped and the oxidizing gas was introduced. The mass increase due to oxidation was then monitored continuously for 2 h at a rate of 1 point/min enabling a quantities kinetic analysis of the results. Two different oxygen partial pressures, viz. 0.35 and 0.95 MPa were employed in order to study the impact of oxygen potential on the oxidation kinetics. In all the experiments, the flow rate was kept constant, i.e., 40 ml/min. The surfaces of the oxidized AlN specimens were analyzed by XRD (Model M21XRHF22, Mac Science, Yokohama, Japan), SEM (Model JSM-5610LV, JEOL, Tokyo, Japan), and X-ray microanalysis (EDAX) (Model INCA 2000, Oxford, UK).
Fig. 1. Oxidation curves under non-isothermal condition with different oxygen partial pressure.
±0.001 mg and the mass of sample for this experiment is about 26 mg. Based on these data, the experimental error was estimated to be within ±0.004%. In view of the relative small experimental uncertainty, the difference of experimental results caused by two different oxygen partial pressures should be able to be identified easily. The trend noticed was that the rate of oxidation was lower at the lower oxygen partial pressure in the gas atmosphere. In both cases, the oxidation was found to start at about 1100 K and became rapid at about 1273 K. The oxidation rate slowed down after 1500 K due to the less amount of residual of materials in the final stage. A second oxidation step was noticed in the temperature range 1533–1543 K. The oxidation was complete when the dimensionless mass change reached a value of 22%. According to the reports in literature, the oxidation product is primarily the thermodynamically stable rhombohedral ␣-Al2 O3 [22]. It has been shown [9] by infrared spectroscopy that the transition from AlN to ␣-Al2 O3 involved formation of two of the so-called transition phases: the monoclinic -alumina and the tetragonal or orthohombic ␦-alumina, which are formed at temperatures ≥1323 K. At temperatures above 1473 K, the transition alumina phases transform to the stable corundum form [22]. In the present investigation, the oxide product is expected to be in the form of ␣-Al2 O3 at temperatures higher than 1543 K, while at lower temperatures, the formation of one of the metastable forms is possible. When the form of product changes, an accompanying volume change can cause the crack formation. This, in turn, can affect the kinetic rate of the reaction. 3.2. Isothermal oxidation of AlN powder
3. Results 3.1. Non-isothermal oxidation of AlN powder Fig. 1 shows the experimental data with different oxygen partial pressure at the heating rate of 10 K/min. In this figure, the percentage of mass change, m/m0 , where m represents the mass change registered and m0 the initial mass, was plotted as a function of time. The sensitive of TG microbalance is within
Since the oxidation rate was found to be significant above 1273 K, the isothermal oxidation experiments were conducted in temperature range from 1423 to 1523 K for 2 h with an interval of 50 K in the oxygen partial pressure of 0.35 MPa. The dimensionless mass change, m/m0 , at the three experimental temperatures is shown in Fig. 2. It is seen in this figure that the oxidation rate increased quickly with increasing temperature during the initial stages. At longer time intervals (beyond
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Fig. 2. A comparison of experimental data with model for isothermal oxidation curves with oxygen partial pressure of 0.35 MPa.
2000 s), the increase in the dimensionless mass change was found to be much slower. This kind of curve indicated that the diffusion step determined the rate of the reaction. Kim et al. [4–7,9–12] suggested that this step could be the diffusion of oxygen through the oxide product layer to the oxide/AlN interface. The completion of the oxidation reaction corresponds to a value of 22% in the dimensionless mass change. Fig. 3 shows the effect of oxygen partial pressure on the oxidation behavior of AlN powder. It can be seen that dimensionless mass change increases more rapidly at the higher oxygen partial pressure, especially at the initial stages. 3.3. Oxidation product analyzed by XRD and SEM In order to understand the oxidation process of AlN, SEM, and XRD analysis were applied to investigate morphological development of the samples before and after oxidation. The results are presented in Figs. 4 and 5, respectively. The XRD analysis shows that the oxidation product is corundum and the intensity of its characteristic peaks increased with oxida-
Fig. 4. XRD patterns of AlN powder before and after oxidation.
tion temperature. Fig. 5 illustrates the SEM micrographs of the raw material as well as those of the oxidized samples at the three experimental temperatures. It can be seen that, compared to the morphology of AlN raw material, the surface of AlN after oxidation became coarser and more tiny crystalline material were formed as the oxidation temperature increased. The tiny crystalline material was identified to be Al2 O3 by EDAX analysis (Fig. 5c). Thus the oxidation reaction can be represented as 3 2 AlN + O2 = Al2 O3 + N2 2
(1)
The present authors have, in an earlier investigation, carried out experiments on the oxidation of Si3 N4 . In this case, the product of oxidation of Si3 N4 , was found to be porous. This was attributed to the escape of N2 from oxide layer [19]. Since the oxidation mechanism of AlN is likely to be somewhat similar, the oxidation product in the present case, viz. Al2 O3 may also be expected to be porous. Fig. 6 shows the microstructure of the AlN specimen oxidized at 1523 K. A comparison with the corresponding structure of the product of oxidation at 0.35 MPa at the same temperature suggested that the product of oxidation at the higher oxygen potential is more porous This could possibly be due to the oxidation reaction being faster at the oxygen partial pressure of 0.95 MPa which is likely to produce more gaseous product in accordance with Eq. (1). 4. Discussion 4.1. Kinetic model
Fig. 3. A comparison of experimental data with model for isothermal oxidation curves at 1523 K with different oxygen partial pressure.
From the XRD result and microstructure analysis, a simplified kinetic model can be proposed to explain the reaction sequence at 1373–1523 K in the oxidizing atmosphere. The oxidation process proceeds first with chemical reaction between AlN and O2 at the outer surface to form a thin layer. This stage proceeds very quickly. If the product layer is dense, then N2 produced will accumulate inside of the sample. At last, the oxide
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Fig. 5. SEM photos before and after oxidation at different temperatures with oxygen partial pressure of 0.35 MPa: (a) raw material; (b) oxidized at 1423 K; (c) oxidized at 1473 K; (d) oxidized at 1523 K.
layer around the AlN grains will break because of the pressure of N2 increase and the porous outer layer is formed and the N2 gas escapes from these porous gaps. To proceed the oxidation reaction, the oxygen has to pass through the oxide product layer by diffusion. Since the diffusion of oxygen through the oxide product is slow and it becomes the rate-controlling step. Therefore, the reaction rate will be determined by the diffusion of gas through the porous outer layer, which is related to the difference of the partial pressures of oxygen across the porous layer. Because the diffusion-controlled stage occupies a large portion of time in the whole process, we can apply the oxidation model of Si3 N4 to treat AlN oxidation problem since both have the similar reaction mechanism. This model has already been successfully employed to treat the oxidation of Si3 N4 and SiAlON material [18,19,21].
From previous analysis, the formula for describing the reacted fraction ξ of oxidation with time t, temperature T, oxygen partial pressure PO2 and other variables have been deduced, respectively as follows [18]: (1) The effect of temperature T on the reacted fraction ξ at constant temperature 3 exp(−E/RT)t ξ =1− 1− (2) BT where BT =
1
oβ oβ (2K0 D0 /vm )(( PO2
−
eq
PO2 )/R20
(3)
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X. Hou et al. / Journal of Alloys and Compounds 465 (2008) 90–96
Thus Eq. (5) will become ⎞3 ⎛
eq PO2 − PO2 exp(−E/RT) ⎟ ⎜ ⎟ (T − T ) ξ = 1−⎜ 1 − 0 ⎠ ⎝ BCP (8)
Fig. 6. SEM photos oxidized at 1523 K with oxygen partial pressure 0.95 MPa.
and E represents the apparent activation energy; BT a funceq eq tion of PO2 , PO2 and R0 , in which PO2 is the oxygen partial pressure in equilibrium with oxide and should be related oβ oβ to temperature T. K0 and D0 are constant independent of temperature but relying on the material; vm a coefficient that depends on substance and reaction; R0 is the radius of the eq whole particle. If the value of PO2 is very small or the temeq perature coefficient of PO2 can be neglected, thus BT will be constant as the oxygen partial pressure and the particle radius are fixed. (2) The effect of oxygen partial pressure PO2 on the reacted fraction of oxidation ξ. Define BP =
1 oβ oβ 2K0 D0 /vm exp(−E/RT)/R20
(4)
where BP is a function of R0 and it will be constant as the particle size is fixed. Thus, Eq. (2) will become
⎞3 eq PO2 − PO2 t ⎟ ⎜ ⎟ ξ =1−⎜ ⎠ ⎝1 − BP ⎛
(5)
(3) The effect of oxygen partial pressure PO2 on the reacted fraction of oxidation ξ in variable temperature process If the system was heated from room temperature T0 to an experimental temperature T at a certain temperature-increasing rate “η” where η = dT/dt, the relationship between temperature T with time t should be
4.2. Application of the new model to oxidation of AlN 4.2.1. The effect of oxygen partial pressure PO2 on the reacted fraction of oxidation ξ in variable temperature At the outset, the present experimental data is employed to check the conformity to Eq. (8). From the results of the non-isothermal experiments, it is seen that the reaction mechanism changes above 1543 K. Hence the present model has been checked only from room temperature up to 1540 K. Please note, the y-axis in the experimental plots, Figs. 1–3, is not the reacted fraction ξ but the dimensionless mass change, m/m0 . In order to use Eq. (8), a transformation of the dimensionless mass change to ξ is required. The relation between ξ and m/m0 can be represented as ξ=
m mmax
(6)
(9)
where mmax is the theoretical maximum increment after a comeq plete oxidation. PO2 represents the oxygen partial pressure in equilibrium with oxide and should be related to temperature eq T. However, the temperature coefficient of PO2 is very small eq and can be neglected. Thus PO2 is a constant and calculated to be 5 × 10−8 . Therefore, one has the following equations, respectively for two corresponding oxygen partial pressures: 3 −22762.81 √ 16.02 1 − 1 − 44838.2 exp T − 298 T (10) 17.14
T = T0 + ηt
3 −22762.81 √ 1 − 1 − 58339.12 exp T − 298 T (11)
define BCP =
The above equations describe the relation of the reacted fraction ξ with time t, temperature T, oxygen partial pressure PO2 and other variables. The parabolic relations for describing AlN oxidation has been reported in literature [12]. The difference between the equations used in the present work and those reported in literature is the expression of the constant. The present equations form is an explicit function that is easy to use. The advantage of this kind of approximate treatment is that one might find an explicit analytic solution. In the following paragraph, the application of the equations to the oxidation of AlN powder will be illustrated.
1 oβ
oβ
(2K0 /vm ) (D0 /R20 η)
(7)
In Fig. 7, the computed percentual dimensionless mass change is compared with the corresponding experimental data under non-isothermal conditions for the two oxygen partial pressures
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5. Conclusion
Fig. 7. A comparison of experimental data with model for non-isothermal oxidation curves with different oxygen partial pressure.
employed in the present work. The excellent agreement between the two confirms the validity of the present theoretical approach to oxidation reactions.
The oxidation of AlN powder in different oxidizing atmosphere has been investigated by using thermogravimetric method. XRD analysis and SEM were also applied to analyze the phase of oxidation product and the surface morphological development. The results of the thermogravimetric studies showed that the oxidation started at about 1100 K and the rate increased evidently after 1273 K. The oxidation rate was affected by temperature and oxygen partial pressure and there was a distinct change of the oxidation mechanism in the temperature range 1533–1543 K. The oxidation product analysed by XRD and SEM is identical to ␣-Al2 O3 , which became obviously porous at elevated temperature and higher oxygen partial pressure. The kinetic analysis of oxidation of AlN powder showed that the diffusion of oxygen through the porous oxide layer was the controlling step. A simple and practical formula has been proposed to describe the oxidation of AlN powder. The effect of oxygen partial pressure on the oxidation behavior is discussed. Good agreement was found between theoretical calculations and the experimental data.
4.2.2. Isothermal oxidation In order to clarify the effect of oxygen partial pressure on the oxidation reaction rate, a series of experiments were carried out regarding the oxidation behavior of AlN in the temperature range from 1373 to 1523 K with an interval of 50 K, the oxygen partial pressure being 0.35 and 0.95 MPa. Based on these data, Eqs. (2) and (5), have been checked, respectively. According to Eq. (2), there are two parameters BT and E that are required to be determined by fitting the experimental data. Based on the present experimental data, these two parameters could easy to be extracted. E and BT were determined to be 323.65 kJ/mol and 3.65 × 10−8 , respectively. Substituting the above E and BT data into Eq. (2), the model describing isothermal oxidation behavior of AlN is as follows: −19464.2 √ 3 (12) 23.94 1 − 1 − 5234.24 exp t T
Acknowledgements
In an analogous way, the equations describing the effect of oxygen pressure on oxidation behavior at 1523 K are as follow:
m0 PO2 eq PO2 R R0 t T vm
PO2 = 0.35 MPa √ 3 23.5 1 − (1 − 0.01381 t)
(13)
PO2 = 0.95 MPa √ 3 23.5 1 − (1 − 0.01773 t)
(14)
For comparison the theoretical predicted lines are also presented in Figs. 2 and 3, respectively. It may be seen that the experimental data and theoretical prediction are in good agreement validating the present theoretical approach.
The authors sincerely thank Quanli Jia, Enxia Xu, and Suping Li, Zhengzhou University, PR China, for their help in the preparation and performance of the thermogravimetry and SEM experiments. The authors would also like to express their thanks to the Science and Technology Committee of Shanghai for their kind support on contract no. 0452NM002. Appendix A oβ
D0
E m mmax
a constant independent of temperature but relying on the material apparent activation energy of oxidation the increment of sample weight theoretical maximum increment after complete oxidation original sample weight partial pressure of oxygen in gas phase oxygen partial pressure in equilibrium with oxide gas constant radius of the original whole particle time in second absolute temperature with K coefficient depending on substance and reaction
Greek symbols η temperature-increasing rate ξ reacted fraction of oxidation References [1] B. Luo, J.W. Johnson, O. Kryliouk, F. Ren, S.J. Pearton, S.N.G. Chu, A.E. Nikolaev, Y.V. Melnik, V.A. Dmitriev, T.J. Anderson, Solid-state Electron 46 (2002) 573–576.
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X. Hou et al. / Journal of Alloys and Compounds 465 (2008) 90–96 G.A. Slack, J. Phys. Chem. Solids 34 (1973) 321–325. S. Strite, H. Morkoc, J. Vac. Sci. Technol. B 10 (1992) 1237–1266. O. Ambacher, J. Phys. D: Appl. Phys. 31 (1998) 2653–2710. J. Chaudhuri, L. Nyakiti, R.G. Lee, Z. Gu, J.H. Edgar, J.G. Wen, Mater. Charact. 58 (2007) 672–679. A.D. Katnani, K.I. Papathomas, J. Vac. Sci. Technol. A 5 (4) (1987) 1335–1340. D. Suryanarayana, J. Am. Ceram. Soc. 73 (4) (1990) 1108–1110. A. Bellosi, E. Landi, A. Tampieri, J. Mater. Res. 8 (3) (1993) 565–572. Y.J. Geng, M.G. Norton, J. Mater. Res. 14 (7) (1999) 2708–2711. E.W. Osborne, M.G. Norton, J. Mater. Sci. 33 (1998) 3859–3865. I. Dutta, S. Mitra, L. Rabenberg, J. Am. Ceram. Soc. 75 (1992) 3149–3153. H.-E. Kim, A.J. Moorhead, J. Am. Ceram. Soc. 77 (4) (1994) 1037–1041. Z. Gu, J.H. Edgar, S.A. Speakman, D. Blom, J. Perrin, J. Chaudhuri, J. Electron. Mater. 34 (2005) 1271–1279.
[14] M. Balat, Calphad 20 (1996) 161–170. [15] D. Sciti, F. Winterhalter, A. Bellosi, J. Mater. Sci. 39 (2004) 6965– 6973. [16] D. Robinson, R. Diekmann, J. Mater. Sci. 29 (1994) 1949–1957. [17] A. Maghsoudipour, F. Moztarzadeh, M. Saremi, J.G. Heinrich, Ceram. Int. 30 (2004) 773–783. [18] K.C. Chou, J. Am. Ceram. Soc. 89 (5) (2006) 1568–1576. [19] X.M. Hou, K.C. Chou, X.J. Hu, H.L. Zhao, J. Alloy Compd., in press. [20] X.M. Hou, K.C. Chou, F.S. Li, Ceram. Int., in press. [21] X.M. Hou, K.C. Chou, F.S. Li, J. Eur. Ceram. Soc., in press. [22] M.W. Chase, C.A. Davies Jr, J.R. Downey, D.J. Frurip Jr, R.A. McDonald, A.N. Syverud (Eds.), JANAF Thermochemical Tables, American Chemistry Society Publication for the National Bureau of Standards, Gaithersburg, MD, 1985.
Oxidation kinetics of aluminum nitride at different oxidizing atmosphere Xinmei Hou a , Kuo-Chih Chou a,∗ , Xiangchong Zhong b , Seshadri Seetharaman c a
Metallurgical and Ecological Engineering School, University of Science and Technology Beijing, Beijing 100083, PR China b High Temperature Ceramics Institute, Zhengzhou University, Henan Province 450052, PR China c Department of Materials Science and Engineering, Royal Institute of Technology, Stockholm, Sweden Received 4 September 2007; received in revised form 15 October 2007; accepted 16 October 2007 Available online 23 October 2007
Abstract In the present work, the oxidation kinetics of AlN powder was investigated by using thermogravimetric analysis, X-ray diffraction (XRD) and scanning electron microscopy (SEM). The experiments were carried out both in isothermal as well as non-isothermal modes under two different oxidizing atmospheres. The results showed that the oxidation reaction started at around 1100 K and the rate increased significantly beyond 1273 K forming porous aluminum oxide as the reaction product. The oxidation rate was affected by temperature and oxygen partial pressure. A distinct change in the oxidation mechanism was noticed in the temperature range 1533–1543 K which is attributed to the phase transformation in oxidation product, viz. alumina. Diffusion is the controlling step during the oxidation process. Based on the experimental data, a new model for predicting the oxidation process of AlN powder had been developed, which offered an analytic form expressing the oxidation weight increment as a function of time, temperature and oxygen partial pressure. The application of this new model to this system demonstrated that this model could be used to describe the oxidation behavior of AlN powder. © 2007 Elsevier B.V. All rights reserved. Keywords: Oxidation kinetics; AlN powder; Oxygen partial pressure; Temperature; Model
1. Introduction Increased attention has been devoted in recent years to AlN materials because of their excellent physical properties such as high thermal conductivity (3.2 W/cm K), low coefficient of thermal expansion ((4.03–6.0) × 10−6 K−1 ) and high electrical resistivity (>4 × 108 cm) [1–3]. AlN materials find large applications as electronic substrates, heat radiation fins and refractory materials [4,5]. However, these materials tend to be oxidized at high temperature under oxidizing atmosphere, which greatly limits their application. For long-term operations, the oxidation behavior of AIN is of major importance. In fact, low levels of oxygen impurities in AlN dramatically affect its final properties [6]. Therefore, the study of AlN oxidation is of great interest for both fundamental science and practical applications. Several articles have reported the oxidation behavior of AlN, both in powder form and as bulk ceramic [5–18]. Among these studies, many techniques including X-ray diffraction, infrared
∗
Corresponding author. Tel.: +86 10 62332646. E-mail address: [email protected] (K.-C. Chou).
0925-8388/$ – see front matter © 2007 Elsevier B.V. All rights reserved. doi:10.1016/j.jallcom.2007.10.066
spectroscopy, thermogravimetric analysis, X-ray photoelectron spectroscopy (XPS), scanning electron microscopy (SEM), and transmission electron microscopy (TEM) have been employed to study the oxidation phenomenon. These studies have determined the crystalline state of the oxide, the composition and chemical bonding states of Al, N, and O atoms, and have developed simple oxidation kinetics rate models. Because the oxidation process is affected by parameters such as: particle size, temperature, the humidity of environment, and characteristics of the sample [5–17], various kinetics models (linear or parabolic) have been proposed. Kim and Moorhead [12] reported that the oxidation kinetics of sintered aluminum nitride followed linear rate law while at higher temperature became parabolic. By contrast, Bellosi et al. [8] investigated the oxidation kinetics of sintered AlN substrates using Y2 O3 as the sintering aid and found that the kinetics depended on the type of sintering aid. It was reported [8] that in the temperature range 1100–1400 ◦ C, the oxidation kinetics followed a linear rate law. Despite the abundance in the experimental data available, the theoretical analysis of the oxidation phenomenon has not made any significant progress. Among the key problems yet to be addressed, a few significant ones are
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(1) the relationship of the reacted fraction of oxidation ξ with time t at a constant temperature; (2) the relationship between the reacted fraction ξ and temperature T in a non-isothermal process; and (3) The impact of oxygen partial pressure PO2 on the reaction rate. In the present study, the oxidation of AlN powder was carried out both in isothermal as well as non-isothermal modes by thermogravimetric method. XRD and SEM analyses were employed to characterize the oxidation product. The effect of different oxygen partial pressure on the oxidation behavior, which was not looked into by earlier researchers in a systematic way was also investigated. Based on the experimental results, a simple set of equations was developed in order to describe the oxidation kinetics. These equations have also been successfully used in the case of the oxidation of Si–Al–O–N, SiC, and Si3 N4 [18–21] in view of the analogy between the materials studied. 2. Experimental procedure 2.1. Material Commercial AlN powder with an average particle size of 4.34 m and a maximum impurity content of less than 2 wt% was used. XRD analysis showed that the characteristic peaks were all corresponding to that of AlN.
2.2. Oxidation tests The weight change of AlN powder during oxidation was monitored using a Netzsch STA449C thermal analysis system. The TG microbalance is sensitive within ±0.001 mg. The oxidation experiments were carried out in both non-isothermal and isothermal modes. In the non-isothermal mode, the sample (about 26 mg) held in an alumina crucible was placed in the heating furnace. The furnace was then heated from room temperature to the maximum temperature of 1723 K at the heating rate of 10 K/min. In the isothermal experiments, AlN powder (about 26 mg) was also placed in an alumina crucible and the temperature was rapidly raised to the required level in a flowing purified argon gas (<5 ppm O2 ). The argon was purified by passing through silica gel and dehydrite (Mg(ClO3 )2 ) to remove moisture and through tube furnaces containing copper and magnesium at 773 K to remove residual oxygen. After the thermal equilibrium was established, the argon gas was stopped and the oxidizing gas was introduced. The mass increase due to oxidation was then monitored continuously for 2 h at a rate of 1 point/min enabling a quantities kinetic analysis of the results. Two different oxygen partial pressures, viz. 0.35 and 0.95 MPa were employed in order to study the impact of oxygen potential on the oxidation kinetics. In all the experiments, the flow rate was kept constant, i.e., 40 ml/min. The surfaces of the oxidized AlN specimens were analyzed by XRD (Model M21XRHF22, Mac Science, Yokohama, Japan), SEM (Model JSM-5610LV, JEOL, Tokyo, Japan), and X-ray microanalysis (EDAX) (Model INCA 2000, Oxford, UK).
Fig. 1. Oxidation curves under non-isothermal condition with different oxygen partial pressure.
±0.001 mg and the mass of sample for this experiment is about 26 mg. Based on these data, the experimental error was estimated to be within ±0.004%. In view of the relative small experimental uncertainty, the difference of experimental results caused by two different oxygen partial pressures should be able to be identified easily. The trend noticed was that the rate of oxidation was lower at the lower oxygen partial pressure in the gas atmosphere. In both cases, the oxidation was found to start at about 1100 K and became rapid at about 1273 K. The oxidation rate slowed down after 1500 K due to the less amount of residual of materials in the final stage. A second oxidation step was noticed in the temperature range 1533–1543 K. The oxidation was complete when the dimensionless mass change reached a value of 22%. According to the reports in literature, the oxidation product is primarily the thermodynamically stable rhombohedral ␣-Al2 O3 [22]. It has been shown [9] by infrared spectroscopy that the transition from AlN to ␣-Al2 O3 involved formation of two of the so-called transition phases: the monoclinic -alumina and the tetragonal or orthohombic ␦-alumina, which are formed at temperatures ≥1323 K. At temperatures above 1473 K, the transition alumina phases transform to the stable corundum form [22]. In the present investigation, the oxide product is expected to be in the form of ␣-Al2 O3 at temperatures higher than 1543 K, while at lower temperatures, the formation of one of the metastable forms is possible. When the form of product changes, an accompanying volume change can cause the crack formation. This, in turn, can affect the kinetic rate of the reaction. 3.2. Isothermal oxidation of AlN powder
3. Results 3.1. Non-isothermal oxidation of AlN powder Fig. 1 shows the experimental data with different oxygen partial pressure at the heating rate of 10 K/min. In this figure, the percentage of mass change, m/m0 , where m represents the mass change registered and m0 the initial mass, was plotted as a function of time. The sensitive of TG microbalance is within
Since the oxidation rate was found to be significant above 1273 K, the isothermal oxidation experiments were conducted in temperature range from 1423 to 1523 K for 2 h with an interval of 50 K in the oxygen partial pressure of 0.35 MPa. The dimensionless mass change, m/m0 , at the three experimental temperatures is shown in Fig. 2. It is seen in this figure that the oxidation rate increased quickly with increasing temperature during the initial stages. At longer time intervals (beyond
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Fig. 2. A comparison of experimental data with model for isothermal oxidation curves with oxygen partial pressure of 0.35 MPa.
2000 s), the increase in the dimensionless mass change was found to be much slower. This kind of curve indicated that the diffusion step determined the rate of the reaction. Kim et al. [4–7,9–12] suggested that this step could be the diffusion of oxygen through the oxide product layer to the oxide/AlN interface. The completion of the oxidation reaction corresponds to a value of 22% in the dimensionless mass change. Fig. 3 shows the effect of oxygen partial pressure on the oxidation behavior of AlN powder. It can be seen that dimensionless mass change increases more rapidly at the higher oxygen partial pressure, especially at the initial stages. 3.3. Oxidation product analyzed by XRD and SEM In order to understand the oxidation process of AlN, SEM, and XRD analysis were applied to investigate morphological development of the samples before and after oxidation. The results are presented in Figs. 4 and 5, respectively. The XRD analysis shows that the oxidation product is corundum and the intensity of its characteristic peaks increased with oxida-
Fig. 4. XRD patterns of AlN powder before and after oxidation.
tion temperature. Fig. 5 illustrates the SEM micrographs of the raw material as well as those of the oxidized samples at the three experimental temperatures. It can be seen that, compared to the morphology of AlN raw material, the surface of AlN after oxidation became coarser and more tiny crystalline material were formed as the oxidation temperature increased. The tiny crystalline material was identified to be Al2 O3 by EDAX analysis (Fig. 5c). Thus the oxidation reaction can be represented as 3 2 AlN + O2 = Al2 O3 + N2 2
(1)
The present authors have, in an earlier investigation, carried out experiments on the oxidation of Si3 N4 . In this case, the product of oxidation of Si3 N4 , was found to be porous. This was attributed to the escape of N2 from oxide layer [19]. Since the oxidation mechanism of AlN is likely to be somewhat similar, the oxidation product in the present case, viz. Al2 O3 may also be expected to be porous. Fig. 6 shows the microstructure of the AlN specimen oxidized at 1523 K. A comparison with the corresponding structure of the product of oxidation at 0.35 MPa at the same temperature suggested that the product of oxidation at the higher oxygen potential is more porous This could possibly be due to the oxidation reaction being faster at the oxygen partial pressure of 0.95 MPa which is likely to produce more gaseous product in accordance with Eq. (1). 4. Discussion 4.1. Kinetic model
Fig. 3. A comparison of experimental data with model for isothermal oxidation curves at 1523 K with different oxygen partial pressure.
From the XRD result and microstructure analysis, a simplified kinetic model can be proposed to explain the reaction sequence at 1373–1523 K in the oxidizing atmosphere. The oxidation process proceeds first with chemical reaction between AlN and O2 at the outer surface to form a thin layer. This stage proceeds very quickly. If the product layer is dense, then N2 produced will accumulate inside of the sample. At last, the oxide
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Fig. 5. SEM photos before and after oxidation at different temperatures with oxygen partial pressure of 0.35 MPa: (a) raw material; (b) oxidized at 1423 K; (c) oxidized at 1473 K; (d) oxidized at 1523 K.
layer around the AlN grains will break because of the pressure of N2 increase and the porous outer layer is formed and the N2 gas escapes from these porous gaps. To proceed the oxidation reaction, the oxygen has to pass through the oxide product layer by diffusion. Since the diffusion of oxygen through the oxide product is slow and it becomes the rate-controlling step. Therefore, the reaction rate will be determined by the diffusion of gas through the porous outer layer, which is related to the difference of the partial pressures of oxygen across the porous layer. Because the diffusion-controlled stage occupies a large portion of time in the whole process, we can apply the oxidation model of Si3 N4 to treat AlN oxidation problem since both have the similar reaction mechanism. This model has already been successfully employed to treat the oxidation of Si3 N4 and SiAlON material [18,19,21].
From previous analysis, the formula for describing the reacted fraction ξ of oxidation with time t, temperature T, oxygen partial pressure PO2 and other variables have been deduced, respectively as follows [18]: (1) The effect of temperature T on the reacted fraction ξ at constant temperature 3 exp(−E/RT)t ξ =1− 1− (2) BT where BT =
1
oβ oβ (2K0 D0 /vm )(( PO2
−
eq
PO2 )/R20
(3)
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Thus Eq. (5) will become ⎞3 ⎛
eq PO2 − PO2 exp(−E/RT) ⎟ ⎜ ⎟ (T − T ) ξ = 1−⎜ 1 − 0 ⎠ ⎝ BCP (8)
Fig. 6. SEM photos oxidized at 1523 K with oxygen partial pressure 0.95 MPa.
and E represents the apparent activation energy; BT a funceq eq tion of PO2 , PO2 and R0 , in which PO2 is the oxygen partial pressure in equilibrium with oxide and should be related oβ oβ to temperature T. K0 and D0 are constant independent of temperature but relying on the material; vm a coefficient that depends on substance and reaction; R0 is the radius of the eq whole particle. If the value of PO2 is very small or the temeq perature coefficient of PO2 can be neglected, thus BT will be constant as the oxygen partial pressure and the particle radius are fixed. (2) The effect of oxygen partial pressure PO2 on the reacted fraction of oxidation ξ. Define BP =
1 oβ oβ 2K0 D0 /vm exp(−E/RT)/R20
(4)
where BP is a function of R0 and it will be constant as the particle size is fixed. Thus, Eq. (2) will become
⎞3 eq PO2 − PO2 t ⎟ ⎜ ⎟ ξ =1−⎜ ⎠ ⎝1 − BP ⎛
(5)
(3) The effect of oxygen partial pressure PO2 on the reacted fraction of oxidation ξ in variable temperature process If the system was heated from room temperature T0 to an experimental temperature T at a certain temperature-increasing rate “η” where η = dT/dt, the relationship between temperature T with time t should be
4.2. Application of the new model to oxidation of AlN 4.2.1. The effect of oxygen partial pressure PO2 on the reacted fraction of oxidation ξ in variable temperature At the outset, the present experimental data is employed to check the conformity to Eq. (8). From the results of the non-isothermal experiments, it is seen that the reaction mechanism changes above 1543 K. Hence the present model has been checked only from room temperature up to 1540 K. Please note, the y-axis in the experimental plots, Figs. 1–3, is not the reacted fraction ξ but the dimensionless mass change, m/m0 . In order to use Eq. (8), a transformation of the dimensionless mass change to ξ is required. The relation between ξ and m/m0 can be represented as ξ=
m mmax
(6)
(9)
where mmax is the theoretical maximum increment after a comeq plete oxidation. PO2 represents the oxygen partial pressure in equilibrium with oxide and should be related to temperature eq T. However, the temperature coefficient of PO2 is very small eq and can be neglected. Thus PO2 is a constant and calculated to be 5 × 10−8 . Therefore, one has the following equations, respectively for two corresponding oxygen partial pressures: 3 −22762.81 √ 16.02 1 − 1 − 44838.2 exp T − 298 T (10) 17.14
T = T0 + ηt
3 −22762.81 √ 1 − 1 − 58339.12 exp T − 298 T (11)
define BCP =
The above equations describe the relation of the reacted fraction ξ with time t, temperature T, oxygen partial pressure PO2 and other variables. The parabolic relations for describing AlN oxidation has been reported in literature [12]. The difference between the equations used in the present work and those reported in literature is the expression of the constant. The present equations form is an explicit function that is easy to use. The advantage of this kind of approximate treatment is that one might find an explicit analytic solution. In the following paragraph, the application of the equations to the oxidation of AlN powder will be illustrated.
1 oβ
oβ
(2K0 /vm ) (D0 /R20 η)
(7)
In Fig. 7, the computed percentual dimensionless mass change is compared with the corresponding experimental data under non-isothermal conditions for the two oxygen partial pressures
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5. Conclusion
Fig. 7. A comparison of experimental data with model for non-isothermal oxidation curves with different oxygen partial pressure.
employed in the present work. The excellent agreement between the two confirms the validity of the present theoretical approach to oxidation reactions.
The oxidation of AlN powder in different oxidizing atmosphere has been investigated by using thermogravimetric method. XRD analysis and SEM were also applied to analyze the phase of oxidation product and the surface morphological development. The results of the thermogravimetric studies showed that the oxidation started at about 1100 K and the rate increased evidently after 1273 K. The oxidation rate was affected by temperature and oxygen partial pressure and there was a distinct change of the oxidation mechanism in the temperature range 1533–1543 K. The oxidation product analysed by XRD and SEM is identical to ␣-Al2 O3 , which became obviously porous at elevated temperature and higher oxygen partial pressure. The kinetic analysis of oxidation of AlN powder showed that the diffusion of oxygen through the porous oxide layer was the controlling step. A simple and practical formula has been proposed to describe the oxidation of AlN powder. The effect of oxygen partial pressure on the oxidation behavior is discussed. Good agreement was found between theoretical calculations and the experimental data.
4.2.2. Isothermal oxidation In order to clarify the effect of oxygen partial pressure on the oxidation reaction rate, a series of experiments were carried out regarding the oxidation behavior of AlN in the temperature range from 1373 to 1523 K with an interval of 50 K, the oxygen partial pressure being 0.35 and 0.95 MPa. Based on these data, Eqs. (2) and (5), have been checked, respectively. According to Eq. (2), there are two parameters BT and E that are required to be determined by fitting the experimental data. Based on the present experimental data, these two parameters could easy to be extracted. E and BT were determined to be 323.65 kJ/mol and 3.65 × 10−8 , respectively. Substituting the above E and BT data into Eq. (2), the model describing isothermal oxidation behavior of AlN is as follows: −19464.2 √ 3 (12) 23.94 1 − 1 − 5234.24 exp t T
Acknowledgements
In an analogous way, the equations describing the effect of oxygen pressure on oxidation behavior at 1523 K are as follow:
m0 PO2 eq PO2 R R0 t T vm
PO2 = 0.35 MPa √ 3 23.5 1 − (1 − 0.01381 t)
(13)
PO2 = 0.95 MPa √ 3 23.5 1 − (1 − 0.01773 t)
(14)
For comparison the theoretical predicted lines are also presented in Figs. 2 and 3, respectively. It may be seen that the experimental data and theoretical prediction are in good agreement validating the present theoretical approach.
The authors sincerely thank Quanli Jia, Enxia Xu, and Suping Li, Zhengzhou University, PR China, for their help in the preparation and performance of the thermogravimetry and SEM experiments. The authors would also like to express their thanks to the Science and Technology Committee of Shanghai for their kind support on contract no. 0452NM002. Appendix A oβ
D0
E m mmax
a constant independent of temperature but relying on the material apparent activation energy of oxidation the increment of sample weight theoretical maximum increment after complete oxidation original sample weight partial pressure of oxygen in gas phase oxygen partial pressure in equilibrium with oxide gas constant radius of the original whole particle time in second absolute temperature with K coefficient depending on substance and reaction
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