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Soft X-ray XAFS: local structure of mullite gels prepared from modified aluminium alkoxides
J O U R N A L OF
~ I , I , I t ~ ~l~ ELSEVIER
Journal of Non-Crystalline Solids 177 (1994) 187-192
Soft X-ray XAFS: local structure of mullite gels prepared from modified aluminium alkoxides Nagao Kamijo a,, Norimasa Umesaki a Katsunori Fukui a Carlo Guy Kiyoharu Tadanaga b Masahiro Tatsumisago b, Tsutomu Minami b
a
a Osaka National Research Institute, AIST, 1-8-31 Midorigaoka, lkeda, Osaka 563, Japan b Department of Applied Materials Science, Osaka Prefecture University, 1-1 Gakuen-cho, Sakai, Osaka 593, Japan
Abstract
Using a laboratory X-ray absorption fine structure facility, the local structure of mullite gels prepared from chemically modified aluminium alkoxides has been determined. The gels were dried and heat treated at several temperatures between 60 and 1200°C and the bond lengths and coordination numbers around the aluminium and silicon in each gel were obtained. It is found that the coordination number of oxygen around aluminium is reduced from 6 (in gels dried at 60°C) to ~ 4 (in gels heat-treated at 200, 400 and 800°C). Above 800°C, the coordination number then returns to approximately 5. Double shell models are found to provide better fits for both A1-O and Si-O bonds in all measured gels during curve-fitting analyses. The results indicate that more than two types of coordination coexist for both the A1-O and Si-O sites. Local structure models of the gels are discussed. 1. Introduction
Mullite in the composition 3A1203.2SIO 2 is commonly used as a refractory material and is generally synthesized by heating amorphous aluminosilicate gels of the same composition [1]. One way to make ultra-fine and homogeneous mullite powders is by the controlled hydrolysis and condensation of an alcoholic solution of chelated aluminium alkoxides, which are called chemically modified alkoxides and silicon alkoxides. The formation process of mullite depends on the preparation method of the gel [2]. To study the formation process Ikeda et al. [3] have re-
* Corresponding author. Tel: +81-727 51 9536. Telefax: +81-727 51 9631.
ported X-ray absorption fine structure (XAFS) results on mullite gels prepared by the adsorption of silicic acid on aluminium hydroxide. In this paper, changes in the local structure of aluminium and silicon during the heating process of the various mullite gels are analyzed and attempts are made to interpret the formation process of chemically modified aluminium alkoxide. X-ray absorption fine structure spectroscopy has been used extensively as an aid in determining the local coordination chracteristics of amorphous materials. However, XAFS experiments in the soft X-ray region 700-3000 eV appear less developed than those in the hard X-ray region; the reason may lie in experimental difficulties, such as the use of specialized monochromators, vacuum path, sample preparation and even theoretical background [4]. Here, we compare the extended X-ray absorption fine structure (EX-
0022-3093/94/$07.00 © 1994 Elsevier Science B.V. All rights reserved
SSDI 0 0 2 2 - 3 0 9 3 ( 9 4 ) 0 0 2 9 8 - 2
N. Kamijo et al. /Journal of Non-Crystalline Solids 177 (1994) 187-192
188
AFS) and X-ray absorption near edge structure (XANES) of AI-K and Si-K edges for various mullite gels with reference samples to demonstrate the use of soft X-ray XAFS in ceramics.
x(k) =
A(k) k E -U,. ~ [ fJ('n') [ exp( •
× e x p ( - 2R~ ) sin(2kRj + 2a + 0j). (2)
2. E x p e r i m e n t a l
The samples used in these studies were prepared from aluminium sec-butoxide, ethylacetoacetate, tetraethyl-orthosilicate (TEOS), 2-propanol and water. The molar ratio of aluminium sec-butoxide : ethylacetoacetate : TEOS was fixed at 3 : 3 : 1 to give the mullite composition 3A120 3 • 2SiO 2. The gels obtained were dried at 60°C for 1 week and heated to 200, 400, 800 and 1200°C, respectively. The detailed preparation method of these samples is described elswhere [5]. Aluminium and silicon K-edge XAFS data for all samples were collected in transmission mode using a laboratory XAFS facility (Technos EXAC 800), which consists of a rotating anode X-ray generator operated with a thin Be window (25 Ixm). Energy selection was made using a (002) plane PET monochromator (2d--8.742 .A), with the transmission signal recorded using a solid state detector (SSD). Details of the X-ray spectrometer used here have already been described in a previous paper [6]. The A1 K- and Si K-edge absorption spectra of the mullite gels were collected up to 400 eV above the absorption edge for EXAFS studies and up to about 60 eV for XANES studies. Data analysis was made according to standard procedures [7]. The threshold energy, E0, was taken as the photon energy at the first inflection point of the edge. After the subtraction of the Victoreentype baseline from the pre-edge reg!on, the EXAFS spectrum k3x(k) versus k(in A -1) was extracted from the background by the moving average method or automatic background removal technique [8]. Fourier transformation of the EXAFS spectra k3x(k) yields the radial structure function in R-space:
× e x p ( - i 2 R ) dk,
2°)2k 2)
Ri
(1)
Here, W(k) is a Hanning window function [9] which restricts the Fourier transform to within a finite range, k 3 is used as a weighting term to compensate for the diminishing amplitudes at high k-values. R~ is the interatomic distance between the absorber and the jth neighbouring atom, Nj is the coordination number and o) is the mean square relative displacement resulting from thermal vibration and static disorder (Debye-Waller factor). Non-linear least-squares fitting in the filtered k-space of the Fourier transform was carried out utilizing the theoretical phase and amplitude functions of Makale et al. [101.
3. R e s u l t s
In Fig. l(a), the XANES spectra of the aluminium K-edge for mullite gels dried at 60°C and those heated at 200, 400, 800 and 1200°C are shown. In Fig. l(b), the XANES spectra of the reference samples are shown: a-AI20 3, ~-A120 3, ct-Al(OH) 3 and amorphous Ai20 3 respectively [11]. These XANES spectra are normalized to the maximum of the absorption and are of good enough quality for comparison to be useful in detecting structural differences in the local environment surrounding the aluminium. Unfortunately, most of the features of these spectra are due to multiple scattering between aluminium and neighboring oxygens and are not well understood. Therefore, XANES spectra can only be used at best as fingerprints for various samples. As shown in the figures, the spectral features of the gels dried at 60°C and those heat-treated at 200 and 400°C are quite similar to those of the reference samples (~/-A1203, o~-AI(OH)3 and amorphous A1203). In Figs. 2(a) and (b), the XANES spectra for
iV. Kamijo et al. /Journal of Non-Crystalline Solids 177 (1994) 187-192
e-
e'~
<
E0 1.54
1.60
1.57
Photon Energy (KeV)
b amorph_A1203
F~ < o
<
S E2
__J 1.54
c~-A1203 El
1.57 1.60 Photon Energy (KeV)
Fig. 1. (a) AI K-XANES of the mullite gels dried at 60°C and those h eated at 200, 400, 800 and 1200°C. E o indicates the edge energy of the spectra ( E o = 1.567 keV). (b) A1 K-XANES of the reference samples. E l , E 2, E 3 and E 4 indicate the edge energy of each spectrum. ( E i = 1.568, E 2 = 1.565, E 3 = 1.566, E 4 = 1.565 keV).
189
the silicon K-edge of the gels mentioned above are shown with the reference samples S i O 2 glass, quartz and nepheline [12]. These XANES spectra are normalized in a manner similar to the aluminium K-edge. The spectra of the gel dried at 60°C and those heat-treated at 200 and 400°C show similar features to those of SiO 2 glass and nepheline, which suggests that the local structure around silicon of these gels is similar to the glass-like structure. However, those heat-treated at 800 and 1200°C show mullite-like features. The EXAFS spectra on the A1 K-edge of the gels are of poor quality because of inhomogeneity in the sample thickness during soft X-ray transmission studies. Additionally a small backscattering amplitude and a large disorder factor within the oxygen atoms further complicate analysis. EXAFS oscillations out to a maximum k of only 10.0 A-1 can be used, while the usable EXAFS starts at 2.3 ,~-1. For such a narrow range in k-space, only two or three clearly defined oscillations, and at times a noisy fourth oscillation, can be included in the Fourier transform and curvefitting. Analysis of such a narrow range results in uncertainties in the amplitude information retrieved from the EXAFS. This uncertainty may also influence calculation of the coordination numbers and Debye-Waller factors generated from least-squares fitting of k3x(k) data. In spite of these difficulties, however, we believe reasonable bond distances and coordination numbers around aluminium have been obtained. In Table 1, data are presented using restricted DebyeWaller factors in the curve-fitting analysis and the coordination numbers obtained normalized with that of the reference sample (et-A120 3) [11]. The results are summarized as follows: (1) A double shell model allows better fitting during curve-fitting, indicating that more than two types of bonding between aluminium and neighbouring oxygens exist in gels dried at 60°C and heat-treated at 1200°C, respectively. (2) The coordination number of 6 obtained in the gel treated at 60°C decreases to 4-5 in gels heat-treated at 200, 400 and 800°C and increases to ~ 6 in the gel heat-treated at 1200°C. (3) Fairly large AI-O distances, 2.1-2.5 A, were obtained in all gels, with the normal AI-O o
N. Kamijo et al. /Journal of Non-Crystalline Solids 177 (1994) 187-192
190
Table 1 Parameters obtained by curve-fitting anlysis, where N is the coordination number, R (A) the bond distance and tr 2 the Debye-Waller factor, for 3A1203-2SIO 2 gel (AI-O) T (°C) 60 200 400 800
< O
°~
1200
R (,~,)
N
0.2 (.~2)
1.81 2.22
4 2
0.004 0.004
4-5 4 4 1-2 4-5
0.005 0.006 0.004 0.005 0.005
2.08-2.16 2.16-2.37 2.22-2.37 1.86 2.50
O ..O
<
1.88
1.84
1.92
Photon Energy (KeV)
b
°~
SiO2_Glass
4<
Si02-Qua-rtz
#
o
< !
!
I
I
f
I
I
I
I
I
1.84 1.88 Photon Energy(KeV')
I
I
I
1.92
Fig. 2. (a) Si K-XANES of the mullite gels dried at 60°C and those heated at 200, 400, 800 and 1200°C. E 0 indicates the edge energy of the spectra ( E 0 = 1.853 keV). (b) Si K-XANES of the reference samples, their edge energy being 1.852 keV.
distances (1.81-1.86 .~) additionally found in gels dried at 60°C and heat-treated at 1200°C. In the case of the gel heat-treated at 1200°C, the small AI-O distance of 1.86 .~ may suggest the coexis-
tence of crystalline mullite in the gel and is consistent with evidence already obtained through X-ray powder diffraction studies [5]. To aid analysis, we have also obtained the EXAFS spectra for the Si K-edge in a similar manner to those of AI. The spectra were of good quality with the bond distances, coordination numbers and Debye-Waller factors around silicon listed in Table 2. The results are summarized as follows. (1) The use of a double shell model is required in the curve-fitting procedure for all gels, again indicating more than one type of bonding exists between the silicon and oxygen atoms. (2) Si-O bond distances of 1.60-1.64 ,~ and 1.92-2.07 .~ were found in all the gels. (3) The total coordination number around silicon is found to be 4 for all the gel. Table 2 Parameters obtained by curve-fitting analysis, where N is the coordination number, R (.~) the bond distance and cr 2 the Debye-Waller factor, for 3A1203-2SIO 2 gel (Si-O) T (°C)
N
o.2 (~2)
60
[ 1.60 ~, 2.04
R (A)
2 2
0.001 0.003
200
[ 1.64 ~ 2.09
2 2
0.003 0.003
400
[ 1.60 2.03
2 2
0.002 0.002
800
[ 1.64 2.07
2 2
0.001 0.001
1200
{ 1.64 1.92
2 2
0.003 0.003
N. Kamijo et al. /Journal of Non-Crystalline Solids 177 (1994) 187-192
4. Discussion From the above, one can then suggest the following structural models for aluminium during the various stages of sol-gel formation: aluminium is still coordinated with OH or chelated to ethylacetoacetate at 60-200°C as species (I) or (II); a part of these groups is removed at 400800°C to give (III) or (IV). The sample is then transformed into the gel including crystalline
mullite at 1200°C. Here, an AI-OH type bonding is supposed to be slightly longer than AI-O: this is shown in scheme 1. Accordingly, for the silicon neighbour, on the other hand, three types of bonding, Si-OH, SiOR and Si-O-Si (AI), are proposed. The shorter bond length is consistent with bridging-type oxygen and the longer with Si-OH or Si-OR groups, respectively. Thus, we are able to construct a following model for the silicon (scheme 2).
/
/ \ OH
A1
O
OH / [ O--C
OH
191
~C--O
/
C
or
/ O--C
p
\ /
O--C
/
OH
\
Ar
C
o-d
dH
60-200°C (I)
\
(II) O
\ /0..
, O--/A1
O
/
A1cO
O
or
/
O
0
\
0
(III)
O~ / 0 . .
HO O OH O~AI / \ AI/__ O
400-800°C
0 (IV)
?
i-o + Mullite 0 1200°C S c h e m e 1.
(HO)
RO (RO) H O - - - - ~ S i / 0 " AI
/
HO
or
RO k
.Si
RO----)Si/
(Si)
/
O ""Si
/
HO 60-200°C
(AI)
O"
(AI)
HO--.Si~ / O.... .O A1 Si I (Si) (AI) 800-1200°C
S c h e m e 2.
+ Mullite
192
N. Kamijo et al. /Journal of Non-Crystalline Solids 177 (1994) 187-192
5. Conclusion
References
Using AI and Si K-edge XAFS the local structure of mullite gels prepared from chemically modified aluminium alkoxides has been investigated. The results are as follows. (1) The coordination number of oxygen around AI decreased from 6 in gel dried at 60°C to 4 in the gels heat-treated at 200, 400 and 800°C. This then recovers to 5-6 in the gel heat-treated at 1200°C. (2) Double shell models allow a better fit in both AI-O and Si-O bonds in all gels during curve-fitting, indicating that there exist more than two types of bonding around A1 and Si. Local structure models around the AI and Si atoms have been proposed. (3) XANES spectra of the AI and Si K-edge for each gel have been compared both with each other and with their reference samples. The results indicate that these gels exhibit similar structures with amorphous AI203 or disordered AI203 around aluminium, and with S i O 2 glass around silicon.
[1] S. Somiya, ed., Ceramic Transaction: Mullite and Muilite Matrix Composites, Vol. 6 (American Ceramic Society, Westerville, City, OH, 1990) section III, p. 167. [2] K. Okada and N. Otsuka, J. Am. Ceram. Soc. 69 (1986) 652. [3] Y. Ikeda, T. Yokoyama, S. Yamashita and H. Wakita, Jpn. J. Appl. Phys. 32, Suppl. 32-2 (1993) 670. [4] P. Lagarde, in: X-ray Absorption Fine Structure, ed. S.S. Hasnain (Ellis Horwood, New York, London, 1991) p. 594. [5] K. Tadanaga, N. Tohge and T. Minami, to be published. [6] H. Wakita, S. Yamaguchi and K. Taniguchi, in: X-ray Absorption Fine Structure, ed. S.S. Hasnain (Ellis Horwood, New York, 1991) p. 685. [7] H. Maeda, J. Phys. Soc. Jpn. 56 (1987) 2777. [8] J.W. Cook Jr. and D.E. Sayers, J. Appl. Phys. 52 (1981) 5024. [9] G.D. Bergland, IEEE (July 1969) 41. [10] G.A. Makale, B.W. Veal, A.P. Paulikas, S.K. Chan and G.S. Knapp, J. Am. Chem. Soc. 110 (1988) 3763. [11] N. Kamijo, H. Kageyama, K. Kawabata, K. Nishihagi, Y. Uehara and K. Taniguchi, in: X-ray Absorption Fine Structure, ed. S.S. Hasnain (Ellis Horwood, New York, 1991) p. 613. [12] N. Kamijo and N. Umesaki, Jpn. J. Appl. Phys. 32, Suppl. 32-2 (1993) 658.
~ I , I , I t ~ ~l~ ELSEVIER
Journal of Non-Crystalline Solids 177 (1994) 187-192
Soft X-ray XAFS: local structure of mullite gels prepared from modified aluminium alkoxides Nagao Kamijo a,, Norimasa Umesaki a Katsunori Fukui a Carlo Guy Kiyoharu Tadanaga b Masahiro Tatsumisago b, Tsutomu Minami b
a
a Osaka National Research Institute, AIST, 1-8-31 Midorigaoka, lkeda, Osaka 563, Japan b Department of Applied Materials Science, Osaka Prefecture University, 1-1 Gakuen-cho, Sakai, Osaka 593, Japan
Abstract
Using a laboratory X-ray absorption fine structure facility, the local structure of mullite gels prepared from chemically modified aluminium alkoxides has been determined. The gels were dried and heat treated at several temperatures between 60 and 1200°C and the bond lengths and coordination numbers around the aluminium and silicon in each gel were obtained. It is found that the coordination number of oxygen around aluminium is reduced from 6 (in gels dried at 60°C) to ~ 4 (in gels heat-treated at 200, 400 and 800°C). Above 800°C, the coordination number then returns to approximately 5. Double shell models are found to provide better fits for both A1-O and Si-O bonds in all measured gels during curve-fitting analyses. The results indicate that more than two types of coordination coexist for both the A1-O and Si-O sites. Local structure models of the gels are discussed. 1. Introduction
Mullite in the composition 3A1203.2SIO 2 is commonly used as a refractory material and is generally synthesized by heating amorphous aluminosilicate gels of the same composition [1]. One way to make ultra-fine and homogeneous mullite powders is by the controlled hydrolysis and condensation of an alcoholic solution of chelated aluminium alkoxides, which are called chemically modified alkoxides and silicon alkoxides. The formation process of mullite depends on the preparation method of the gel [2]. To study the formation process Ikeda et al. [3] have re-
* Corresponding author. Tel: +81-727 51 9536. Telefax: +81-727 51 9631.
ported X-ray absorption fine structure (XAFS) results on mullite gels prepared by the adsorption of silicic acid on aluminium hydroxide. In this paper, changes in the local structure of aluminium and silicon during the heating process of the various mullite gels are analyzed and attempts are made to interpret the formation process of chemically modified aluminium alkoxide. X-ray absorption fine structure spectroscopy has been used extensively as an aid in determining the local coordination chracteristics of amorphous materials. However, XAFS experiments in the soft X-ray region 700-3000 eV appear less developed than those in the hard X-ray region; the reason may lie in experimental difficulties, such as the use of specialized monochromators, vacuum path, sample preparation and even theoretical background [4]. Here, we compare the extended X-ray absorption fine structure (EX-
0022-3093/94/$07.00 © 1994 Elsevier Science B.V. All rights reserved
SSDI 0 0 2 2 - 3 0 9 3 ( 9 4 ) 0 0 2 9 8 - 2
N. Kamijo et al. /Journal of Non-Crystalline Solids 177 (1994) 187-192
188
AFS) and X-ray absorption near edge structure (XANES) of AI-K and Si-K edges for various mullite gels with reference samples to demonstrate the use of soft X-ray XAFS in ceramics.
x(k) =
A(k) k E -U,. ~ [ fJ('n') [ exp( •
× e x p ( - 2R~ ) sin(2kRj + 2a + 0j). (2)
2. E x p e r i m e n t a l
The samples used in these studies were prepared from aluminium sec-butoxide, ethylacetoacetate, tetraethyl-orthosilicate (TEOS), 2-propanol and water. The molar ratio of aluminium sec-butoxide : ethylacetoacetate : TEOS was fixed at 3 : 3 : 1 to give the mullite composition 3A120 3 • 2SiO 2. The gels obtained were dried at 60°C for 1 week and heated to 200, 400, 800 and 1200°C, respectively. The detailed preparation method of these samples is described elswhere [5]. Aluminium and silicon K-edge XAFS data for all samples were collected in transmission mode using a laboratory XAFS facility (Technos EXAC 800), which consists of a rotating anode X-ray generator operated with a thin Be window (25 Ixm). Energy selection was made using a (002) plane PET monochromator (2d--8.742 .A), with the transmission signal recorded using a solid state detector (SSD). Details of the X-ray spectrometer used here have already been described in a previous paper [6]. The A1 K- and Si K-edge absorption spectra of the mullite gels were collected up to 400 eV above the absorption edge for EXAFS studies and up to about 60 eV for XANES studies. Data analysis was made according to standard procedures [7]. The threshold energy, E0, was taken as the photon energy at the first inflection point of the edge. After the subtraction of the Victoreentype baseline from the pre-edge reg!on, the EXAFS spectrum k3x(k) versus k(in A -1) was extracted from the background by the moving average method or automatic background removal technique [8]. Fourier transformation of the EXAFS spectra k3x(k) yields the radial structure function in R-space:
× e x p ( - i 2 R ) dk,
2°)2k 2)
Ri
(1)
Here, W(k) is a Hanning window function [9] which restricts the Fourier transform to within a finite range, k 3 is used as a weighting term to compensate for the diminishing amplitudes at high k-values. R~ is the interatomic distance between the absorber and the jth neighbouring atom, Nj is the coordination number and o) is the mean square relative displacement resulting from thermal vibration and static disorder (Debye-Waller factor). Non-linear least-squares fitting in the filtered k-space of the Fourier transform was carried out utilizing the theoretical phase and amplitude functions of Makale et al. [101.
3. R e s u l t s
In Fig. l(a), the XANES spectra of the aluminium K-edge for mullite gels dried at 60°C and those heated at 200, 400, 800 and 1200°C are shown. In Fig. l(b), the XANES spectra of the reference samples are shown: a-AI20 3, ~-A120 3, ct-Al(OH) 3 and amorphous Ai20 3 respectively [11]. These XANES spectra are normalized to the maximum of the absorption and are of good enough quality for comparison to be useful in detecting structural differences in the local environment surrounding the aluminium. Unfortunately, most of the features of these spectra are due to multiple scattering between aluminium and neighboring oxygens and are not well understood. Therefore, XANES spectra can only be used at best as fingerprints for various samples. As shown in the figures, the spectral features of the gels dried at 60°C and those heat-treated at 200 and 400°C are quite similar to those of the reference samples (~/-A1203, o~-AI(OH)3 and amorphous A1203). In Figs. 2(a) and (b), the XANES spectra for
iV. Kamijo et al. /Journal of Non-Crystalline Solids 177 (1994) 187-192
e-
e'~
<
E0 1.54
1.60
1.57
Photon Energy (KeV)
b amorph_A1203
F~ < o
<
S E2
__J 1.54
c~-A1203 El
1.57 1.60 Photon Energy (KeV)
Fig. 1. (a) AI K-XANES of the mullite gels dried at 60°C and those h eated at 200, 400, 800 and 1200°C. E o indicates the edge energy of the spectra ( E o = 1.567 keV). (b) A1 K-XANES of the reference samples. E l , E 2, E 3 and E 4 indicate the edge energy of each spectrum. ( E i = 1.568, E 2 = 1.565, E 3 = 1.566, E 4 = 1.565 keV).
189
the silicon K-edge of the gels mentioned above are shown with the reference samples S i O 2 glass, quartz and nepheline [12]. These XANES spectra are normalized in a manner similar to the aluminium K-edge. The spectra of the gel dried at 60°C and those heat-treated at 200 and 400°C show similar features to those of SiO 2 glass and nepheline, which suggests that the local structure around silicon of these gels is similar to the glass-like structure. However, those heat-treated at 800 and 1200°C show mullite-like features. The EXAFS spectra on the A1 K-edge of the gels are of poor quality because of inhomogeneity in the sample thickness during soft X-ray transmission studies. Additionally a small backscattering amplitude and a large disorder factor within the oxygen atoms further complicate analysis. EXAFS oscillations out to a maximum k of only 10.0 A-1 can be used, while the usable EXAFS starts at 2.3 ,~-1. For such a narrow range in k-space, only two or three clearly defined oscillations, and at times a noisy fourth oscillation, can be included in the Fourier transform and curvefitting. Analysis of such a narrow range results in uncertainties in the amplitude information retrieved from the EXAFS. This uncertainty may also influence calculation of the coordination numbers and Debye-Waller factors generated from least-squares fitting of k3x(k) data. In spite of these difficulties, however, we believe reasonable bond distances and coordination numbers around aluminium have been obtained. In Table 1, data are presented using restricted DebyeWaller factors in the curve-fitting analysis and the coordination numbers obtained normalized with that of the reference sample (et-A120 3) [11]. The results are summarized as follows: (1) A double shell model allows better fitting during curve-fitting, indicating that more than two types of bonding between aluminium and neighbouring oxygens exist in gels dried at 60°C and heat-treated at 1200°C, respectively. (2) The coordination number of 6 obtained in the gel treated at 60°C decreases to 4-5 in gels heat-treated at 200, 400 and 800°C and increases to ~ 6 in the gel heat-treated at 1200°C. (3) Fairly large AI-O distances, 2.1-2.5 A, were obtained in all gels, with the normal AI-O o
N. Kamijo et al. /Journal of Non-Crystalline Solids 177 (1994) 187-192
190
Table 1 Parameters obtained by curve-fitting anlysis, where N is the coordination number, R (A) the bond distance and tr 2 the Debye-Waller factor, for 3A1203-2SIO 2 gel (AI-O) T (°C) 60 200 400 800
< O
°~
1200
R (,~,)
N
0.2 (.~2)
1.81 2.22
4 2
0.004 0.004
4-5 4 4 1-2 4-5
0.005 0.006 0.004 0.005 0.005
2.08-2.16 2.16-2.37 2.22-2.37 1.86 2.50
O ..O
<
1.88
1.84
1.92
Photon Energy (KeV)
b
°~
SiO2_Glass
4<
Si02-Qua-rtz
#
o
< !
!
I
I
f
I
I
I
I
I
1.84 1.88 Photon Energy(KeV')
I
I
I
1.92
Fig. 2. (a) Si K-XANES of the mullite gels dried at 60°C and those heated at 200, 400, 800 and 1200°C. E 0 indicates the edge energy of the spectra ( E 0 = 1.853 keV). (b) Si K-XANES of the reference samples, their edge energy being 1.852 keV.
distances (1.81-1.86 .~) additionally found in gels dried at 60°C and heat-treated at 1200°C. In the case of the gel heat-treated at 1200°C, the small AI-O distance of 1.86 .~ may suggest the coexis-
tence of crystalline mullite in the gel and is consistent with evidence already obtained through X-ray powder diffraction studies [5]. To aid analysis, we have also obtained the EXAFS spectra for the Si K-edge in a similar manner to those of AI. The spectra were of good quality with the bond distances, coordination numbers and Debye-Waller factors around silicon listed in Table 2. The results are summarized as follows. (1) The use of a double shell model is required in the curve-fitting procedure for all gels, again indicating more than one type of bonding exists between the silicon and oxygen atoms. (2) Si-O bond distances of 1.60-1.64 ,~ and 1.92-2.07 .~ were found in all the gels. (3) The total coordination number around silicon is found to be 4 for all the gel. Table 2 Parameters obtained by curve-fitting analysis, where N is the coordination number, R (.~) the bond distance and cr 2 the Debye-Waller factor, for 3A1203-2SIO 2 gel (Si-O) T (°C)
N
o.2 (~2)
60
[ 1.60 ~, 2.04
R (A)
2 2
0.001 0.003
200
[ 1.64 ~ 2.09
2 2
0.003 0.003
400
[ 1.60 2.03
2 2
0.002 0.002
800
[ 1.64 2.07
2 2
0.001 0.001
1200
{ 1.64 1.92
2 2
0.003 0.003
N. Kamijo et al. /Journal of Non-Crystalline Solids 177 (1994) 187-192
4. Discussion From the above, one can then suggest the following structural models for aluminium during the various stages of sol-gel formation: aluminium is still coordinated with OH or chelated to ethylacetoacetate at 60-200°C as species (I) or (II); a part of these groups is removed at 400800°C to give (III) or (IV). The sample is then transformed into the gel including crystalline
mullite at 1200°C. Here, an AI-OH type bonding is supposed to be slightly longer than AI-O: this is shown in scheme 1. Accordingly, for the silicon neighbour, on the other hand, three types of bonding, Si-OH, SiOR and Si-O-Si (AI), are proposed. The shorter bond length is consistent with bridging-type oxygen and the longer with Si-OH or Si-OR groups, respectively. Thus, we are able to construct a following model for the silicon (scheme 2).
/
/ \ OH
A1
O
OH / [ O--C
OH
191
~C--O
/
C
or
/ O--C
p
\ /
O--C
/
OH
\
Ar
C
o-d
dH
60-200°C (I)
\
(II) O
\ /0..
, O--/A1
O
/
A1cO
O
or
/
O
0
\
0
(III)
O~ / 0 . .
HO O OH O~AI / \ AI/__ O
400-800°C
0 (IV)
?
i-o + Mullite 0 1200°C S c h e m e 1.
(HO)
RO (RO) H O - - - - ~ S i / 0 " AI
/
HO
or
RO k
.Si
RO----)Si/
(Si)
/
O ""Si
/
HO 60-200°C
(AI)
O"
(AI)
HO--.Si~ / O.... .O A1 Si I (Si) (AI) 800-1200°C
S c h e m e 2.
+ Mullite
192
N. Kamijo et al. /Journal of Non-Crystalline Solids 177 (1994) 187-192
5. Conclusion
References
Using AI and Si K-edge XAFS the local structure of mullite gels prepared from chemically modified aluminium alkoxides has been investigated. The results are as follows. (1) The coordination number of oxygen around AI decreased from 6 in gel dried at 60°C to 4 in the gels heat-treated at 200, 400 and 800°C. This then recovers to 5-6 in the gel heat-treated at 1200°C. (2) Double shell models allow a better fit in both AI-O and Si-O bonds in all gels during curve-fitting, indicating that there exist more than two types of bonding around A1 and Si. Local structure models around the AI and Si atoms have been proposed. (3) XANES spectra of the AI and Si K-edge for each gel have been compared both with each other and with their reference samples. The results indicate that these gels exhibit similar structures with amorphous AI203 or disordered AI203 around aluminium, and with S i O 2 glass around silicon.
[1] S. Somiya, ed., Ceramic Transaction: Mullite and Muilite Matrix Composites, Vol. 6 (American Ceramic Society, Westerville, City, OH, 1990) section III, p. 167. [2] K. Okada and N. Otsuka, J. Am. Ceram. Soc. 69 (1986) 652. [3] Y. Ikeda, T. Yokoyama, S. Yamashita and H. Wakita, Jpn. J. Appl. Phys. 32, Suppl. 32-2 (1993) 670. [4] P. Lagarde, in: X-ray Absorption Fine Structure, ed. S.S. Hasnain (Ellis Horwood, New York, London, 1991) p. 594. [5] K. Tadanaga, N. Tohge and T. Minami, to be published. [6] H. Wakita, S. Yamaguchi and K. Taniguchi, in: X-ray Absorption Fine Structure, ed. S.S. Hasnain (Ellis Horwood, New York, 1991) p. 685. [7] H. Maeda, J. Phys. Soc. Jpn. 56 (1987) 2777. [8] J.W. Cook Jr. and D.E. Sayers, J. Appl. Phys. 52 (1981) 5024. [9] G.D. Bergland, IEEE (July 1969) 41. [10] G.A. Makale, B.W. Veal, A.P. Paulikas, S.K. Chan and G.S. Knapp, J. Am. Chem. Soc. 110 (1988) 3763. [11] N. Kamijo, H. Kageyama, K. Kawabata, K. Nishihagi, Y. Uehara and K. Taniguchi, in: X-ray Absorption Fine Structure, ed. S.S. Hasnain (Ellis Horwood, New York, 1991) p. 613. [12] N. Kamijo and N. Umesaki, Jpn. J. Appl. Phys. 32, Suppl. 32-2 (1993) 658.