Ind. Eng. Chem. Res. 2004, 43, 3141-3150
3141
Formation and Mullitization Mechanism of Nano Premullite Particles Prepared by the Mixed Solvents Method Un-Yeon Hwang,*,† Hyung-Sang Park,† Yong-Ryul Kim,‡ and Kee-Kahb Koo† Department of Chemical Engineering, Sogang University, Seoul 121-742, Korea, and Department of Chemical Engineering, DaeJin University, Kyonggi 487-711, Korea
Fine spherical premullite particles were prepared by employing partial hydrolysis to control the difference in reaction rate of two alkoxides and by controlling the solvent composition, considering the difference in solubility of the double alkoxide. Final particles were formed via aggregation of the growing particles in the droplets and fine particles generated from the dissolved alkoxide. The optimum conditions for the preparation of spherical premullite particles were [Si(OC2H5)4] ) 0.033 mol/L, ([Al(OC4H9sec)3] ) 0.1 mol/L, [H2O] ) 0.5 mol/L, volume fraction of acetonitrile ) 40 vol %, and amount of HPC ) 0.1 g/L. The Al2O3/SiO2 molar ratio of the premullite particles determined by XRF analysis was about 3:2. The premullite particles were found to be amorphous by XRD analysis, and DTA analysis of all premullite powders showed an exothermic peak associated with crystallization to the spinel phase at around 990 °C. The samples that passed through the spinel phase were transformed into the tetragonal mullite at around 1100 °C, and a transformation to orthorhombic mullite took place at about 1250 °C. The ratio of added H2O to Si(OC2H5)4 in the partial hydrolysis reaction had a significant effect on the mullitization mechanism of premullite particles and the composition of the crystal. 1. Introduction Hydrolysis of metal alkoxides has been widely used for the preparation of colloidal metal (hydrous) oxides. Monodisperse colloidal silica spheres can be prepared by a simple procedure from tetraalkoxysilanes in alcoholic solution. The short-nucleation-time model for the growth of monodisperse particles by La Mer and Dinegar,1,2 the aggregation growth model of primary particles by Bogush and Zukoski,3 and the study of Sto¨ber, Fink, and Bohn4 have contributed to knowledge on the chemical formation of spherical oxide particles. However, it is well-known that the preparation of monodisperse spherical oxide particles with titanium or aluminum alkoxide is difficult because the hydrolysis and condensation of these alkoxides are too rapid to control the particle size, size distribution, and morphology. Hydrolysis of highly reactive alkoxides results in the formation of gels or agglomerates. Therefore, it is necessary to retard the hydrolysis reaction and to promote three-dimensional polymerization reactions for the formation of spherical particles. Recently, Lee et al.5 and Ring et al.6 prepared spherical particles of Al2O3 and TiO2 by the hydrolysis of titanium and aluminum alkoxides, respectively, in a mixed solvent of alcohol and acetonitrile. Extensive research has also been done on the synthesis of premullite particles using aluminum and silicon alkoxides as starting materials.7-12 However, no studies on the formation of stoichiometric spherical premullite particles have been reported because the hydrolysis and condensation rates of aluminum and silicon alkoxides are substantially different. In general, * To whom correspondence should be addressed. Fax: 82-2-3272-0331. Tel.: 82-2-705-8475. E-mail: huy1012@ hanmail.net. † Sogang University. ‡ DaeJin University.
two factors are mainly responsible for the homogeneity and mullitization mechanism of premullite particles prepared from alkoxides. First, the choice of the source of the Al2O3 and SiO2 components is of great importance in ensuring the synthesis of premullite particles in which the Al2O3 and SiO2 components are homogeneously mixed and obtaining orthorhombic mullite (Omullite) at low temperature. Second, the hydrolysis reactions of silicon and aluminumalkoxides are very susceptible to processing variables such as acidic and basic catalysts, temperature, and media. In this study, stoichiometric spherical premullite particles were prepared by employing partial hydrolysis to overcome the difference in reaction rate of two alkoxides and a mixed solvent to control the rate of reaction of the double alkoxide; the form of the final particles was controlled by utilizing the difference in solubility of the double alkoxide in the solvents. The main goal of this paper is to elucidate, on the basis of an adaptation of Mie theory, the mechanism of premullite particle formation in alkoxide-alcohol-acetonitrile systems by measuring the turbidity of the solution and the number density and size of the particles during the reaction as functions of the acetonitrile concentration. In addition, the effect of the ratio of water to tetraethyl orthosilicate in the partial hydrolysis reaction of tetraethyl orthosilicate on the mullitization of premullite particles is reported. 2. Experimental Section 2.1. Experimental Apparatus and Materials. Figure 1 shows a schematic of the experimental setup used in this study. Aluminum sec-butoxide [ASB, Al(OC4H9sec)3, assay ≈ 11.0%, Fluka, d420 ) 0.96] and tetraethyl orthosilicate [TEOS, Si(OC2H5)4, Merck, 99.5%] were used as starting materials for alumina and silica, respectively. Acetonitrile (CH3CN, dielectric constant ) 37.5, dipole moment ) 3.92, Yakuri, 99.5%, d ) 0.783)
10.1021/ie0209071 CCC: $27.50 © 2004 American Chemical Society Published on Web 05/14/2004
3142 Ind. Eng. Chem. Res., Vol. 43, No. 12, 2004
prepared by mixing 0.01 g of the sample with 2 g of KBr powder and applying a uniaxial press. Differential thermal analysis (DTA, DuPont 2000) curves were measured in air from room temperature to 1400 °C at a heating rate of 5 °C/min using about 20 mg of sample. 2.4. Determination of Particle Size from Turbidity Data. The turbidity, τ, of a solution or emulsion is given by
τ)
1 ln(I0/I) x
(1)
where I0 and I are the intensities of the incident and transmitted light, respectively, and x is the path length of the transmission cell. From Mie theory14,15
τ ) k*πrs2np
Figure 1. Experimental apparatus.
and n-octanol [CH3(CH2)6CH2OH, Yakuri, 99.5%, d ) 0.826] were used to prepare the mixed solvent. Nitric acid (HNO3, Merck, 65%) was used as a catalyst for the partial hydrolysis of TEOS. To avoid particle aggregation, a sterically stabilizing dispersant, hydroxy propyl cellulose (HPC, Aldrich, average MW ) 100 00), was dissolved in the octanol before mixing, and n-butanol [C4H9OH, Junsei, 99.5%, refractive index (RI) ) 1.3992] was used as a solvent for measurements of the turbidity of the reacting solutions. 2.2. Preparation of Spherical Premullite Particles. TEOS was dissolved in hydrophobic n-octanol at 50 °C, and then a specified amount of HNO3 diluted with water was added to initiate the partial hydrolysis reaction of TEOS. The reaction mixture was allowed to reflux under stirring at 50 °C for 24 h. After completion of the partial hydrolysis reaction of TEOS, the mixture was heated to 80 °C in a water bath. ASB was then added to the partially hydrolyzed TEOS solution, which was then mixed at 80 °C for 24 h. The molar ratio of ASB to TEOS was 3:1. The Al-O-Si mixed alkoxide solutions were cooled to 40 °C. Then, HPC/octanol was added to the solution according to the reaction conditions, and the solution was mixed for 15 min. The water-octanol-acetonitrile solution was added to the mixture and was vigorously stirred for 1 min. The particles formed in the solution were aged at 40 °C for 1 h. The resulting particles were then filtered and dried at 100 °C for 24 h. The amounts of materials used in the experimental runs are listed in Table 1. 2.3. Measurements and Observations. The turbidities of solutions diluted with n-butanol were determined with a UV/vis spectrophotometer (Jasco, V-550). The shape and size distributions of the particles were evaluated with a scanning electron microscope (SEM, Philips 535M). The chemical composition of the premullite particles was assayed with an X-ray fluorescence spectrometer (XRF, Philips/PW 1480). Each sample was heated to 1600 °C, and the crystalline phases in the heat-treated samples were examined with a powder X-ray diffractometer (XRD, Rigaku, scanning speed ) 1°/min, power ) 30 kV, 20 mA, 14° e 2θ e 70°) using Cu KR radiation monochromated by a graphite monochromator. The lattice parameters were computed by a method similar to that reported elsewhere.13 Infrared absorption spectra (Midac Grams-386) were obtained for a wavenumber range between 400 and 1300 cm-1 by the KBr pellet method. Each pellet was
(2)
Here, rs is the radius of the scattering particles, np is the number of particles per cubic meter, and k* is the scattering coefficient of the particles. k* is a function of the two parameters m and R, where m is the ratio of the refractive index of the scattering particles to that of the medium and R ) 2πrs/λm ) πD/πλm, where D is the diameter of the particles and λm is the wavelength of the light. If the concentration of the scattering particles in kilograms per cubic meter and their density are represented by C and F1, respectively
C ) 4/3πrs3npF1
(3)
and eq 2 can be transformed to
3C 3k*C 3C τ ) k*πrs2 ) k* ) 4rsF1 2F1D 4πrs3F1
(4)
If we let c be the total weight of the spheres in 0.1 kg of scattering system, then
(
)( )
200F1λm τ 3πF2 c
)
0
k* R
(5)
where F2 is the density of the entire system. The Mie equation applied to the scattering of a spherical particle can be expressed in the form
J⊥ )
λm2 4π2rs2
J| )
|∑
AnPn′(cos γ)
∞
sin γ
n)1
λm2 4π2rs2
|∑ ∞
n)1
+ Bn
Pn′(cos γ)
dγ
d
An Pn′(cos γ) + dγ
R)
λm2
∞
∑
2π n)1
|
2
d
|
BnPn′(cos γ)
|an|2 + |bn|2 2n + 1
(6)
sin γ
2
(7)
(8)
In the above equations, J⊥ is the total intensity of light scattered by a single sphere in a direction defined by the angle γ when the incident wave is of unit intensity and is plane-polarized with the electric vector perpendicular to the plane of observation. J| refers to the vibration of the electric vector parallel to the plane of observation. The quantity R is the total light energy scattered by a sphere when the incident wave is of unit intensity.
Ind. Eng. Chem. Res., Vol. 43, No. 12, 2004 3143 Table 1. Experimental Conditionsa a. Effect of Acetonitrile Content (units ) mol g/L) name
acetonitrile (vol %)
CA1 CA2 CA3c CA4 CA5
10 30 40 50 70
partial hydrolysis of TEOS TEOS H2O octanol
0.0333
0.0666
0.5
ASB added ASB
0.0999
octanol
H2O
1.063
condensation acetonitrile
octanol
1.9130 5.7370 7.6500 9.5633 13.386
3.656 2.633 1.993 1.357 0.080
0.4334
b. Effect of H2O/TEOS Ratio in the Partial Hydrolysis of TEOSb partial hydrolysis of TEOS name
H/T
HW1 HW2c HW3 HW4
1:1 2:1 3:1 4:1
TEOS
H2O
ASB added
octanol
ASB
condensation octanol
H2O
0.0333 0.0333
acetonitrile
octanol
7.6500
1.993
0.4667 0.5
0.0999
1.063
0.1000 0.1333
0.4000 0.3667
a [HPC] ) 0.1 g/L, [HNO ] ) 0.667 × 10-3 mol/L, [H O] b c 3 2 Tot ) 0.5 mol/L. [Acetonitrile] ) 40 vol %, H/T ) H2O/TEOS mole ratio. CA3 and HW2 represent the same conditions.
The quantities An and Bn refer to the electric and magnetic waves, respectively, and are defined in terms of the quantities an and bn
An ) Bn )
an n(n + 1) bn n(n + 1)
Denoting τ as unit concentration of the scattering material, eq 1 can be transformed to
(τ/φ)0 ) (9)
)
(10) )
( ()
)
npR volume fraction of the spheres
np R φ
[ ]
λm2
1
∞
∑
|an|2 + |bn|2 2n + 1
(4/3)πrs3 2π n)1
where
)
an )
3
(-1)ni(2n + 1)[mSn′(R) Sn(β) - Sn(R) Sn′(β)] mSn′(R) Sn(β) - Sn(R) Sn′(β) + (-1)ni[mGn′(R) Sn(β) - Gn(R) Sn′(β)]
R λm (11)
mSn(R) Sn′(β) - Sn′(R) Sn(β) + (-1)ni[mGn(R) Sn′(β) - Gn′(R) Sn(β)]
() τ c
(12)
(13)
() τ
c
Sn(x) and Gn(x) are related to the Bessel function Jn+1/2(x) by the equations
(πx2 ) πx G (x) ) ( ) 2 Sn(x) )
1/2
1/2
n
2n + 1
)
R V
(16)
( )( )
) 0.01
0
F2 τ F1 φ
(17)
0
where c is the total weight of the spheres in 0.1 kg of the scattering system. If all parameters such as F1, F2, and the wavelength of light are given, the specific turbidity is then simply
where
β ) mR
∑ n)1
|an|2 + |bn|2
where V is the volume of one sphere and φ is the volume fraction of the spheres. The primary concentrations obtained in precise practical work are based on gravimetric determinations. Consequently
bn ) (-1)ni(2n + 1)[mSn(R) Sn′(β) - Sn′(R) Sn(β)]
∞
3π
Jn+1/2(x)
(14)
J-n-1/2(x)
(15)
Sn′(x) and Gn′(x) denote the corresponding derivatives with respect to x.
0
1
∞
∑
) 408.59 R3n)1
|an|2 + |bn|2 2n + 1
(18)
The zero subscript in (τ/c)0 means that this ratio has to be extrapolated to c ) 0 to meet the Mie theory requirement of no mutual interactions among the particles and no secondary scattering. All of the quantities on the left side in eq 5 are experimentally determined, and they allow, therefore, an evaluation of k*/R for the system under investigation. If we now plot (τ/c)0 as a function of R in eq 18, then we can obtain k* from the plot and hence also the particle diameter rs from R ) 2πrs/λm and np from eq 2. R is determined by solving eq 18 using a computer. The variation of the size and number density of the particles
3144 Ind. Eng. Chem. Res., Vol. 43, No. 12, 2004
Figure 2. Specific turbidity, (τ/c)0, as a function of R for various m values.
with time was determined by measuring the yield and turbidity of mixture of n-butanol (100 mL) and the aging sample (5 mL). The change of the yield with aging time was calculated with the following assumption: The density of a mixed solvent (octanol + acetonitrile) was estimated by assuming that the component volumes are additive. We assumed that the density of a particle is the same as the theoretical density (3.17 kg/m3) of mullite. After a certain reaction time, we measured the weights of 5 mL of the reacting solution (WTot1) and 5 mL of the pure mixed solvent (octanol + acetonitrile) (WSol1) with the same composition as the reacting solution. An initial estimate of the weight of the particles in 5 mL of the reacting solution (WPar1) was obtained from the difference WTot1 - WSol1. Therefore, from the relation between the theoretical density and the weight of the particle, we obtained an initial estimate of volume that the particles occupy in solution (VPar1). An initial estimate of the volume of the mixed solvent in 5 mL of the reacting solution (VMSol1) was obtained from the difference 5 mL - VPar1. The second estimate of the weight of the particles in 5 mL of the reacting solution, WPar2, was obtained from the difference WTot - weight of the pure mixed solvent of volume VMSol1. We execuetd additional iterations until the error range was on the order of 10-10. 3. Results and Discussion 3.1. Variation of (τ/c)0 with r. The numerical (τ/c)0 data, valid for the specified R and m values, are given in Figure 2. Such data play a key role in accurate particle size determination in microscopic dispersions of spheres if visible radiation is available. Figure 2 shows that (τ/c)0 increases monotonically until R reaches about 5 and 20 for m ) 1.3 and 1.05, respectively. Turbidity measurements, therefore, give a single-valued particle diameter. We assumed that the refractive index and density of the premullite particles are the same as those of mullite (RI ) 1.654, density ) 3.17 kg/m3). Therefore, the line representing m ) 1.1821 in Figure 2 is applicable to the present work. 3.2. Effect of the Acetonitrile Content on Premullite Particle Formation. Figure 3 shows that the transmittance of mixtures of 100 mL of n-butanol and
Figure 3. Transmittance as a function of time after 5 mL of the CA3 solution was added to 100 mL of butanol.
Figure 4. Dependence of turbidity on solution aging time for different acetonitrile contents.
5 mL of the CA3 solution (acetonitrile ) 40 vol %) sampled after a reaction time of 10 s did not change significantly for at least 30 min. This indicates that there are no mutual interactions among the particles and no secondary scattering by growth of particles or production of new nuclei from unreacted alkoxides and thus satisfies the requirements of Mie theory. The changes in turbidity of the reacting solution and the yield of premullite particles are shown in Figures 4 and 5, respectively. The yield was determined by dividing the weight of particles produced at a given reaction time by the value calculated assuming that all of the alkoxide completely converted. From Figures 4 and 5, it can be seen that the turbidity and yield increase continuously with increasing reaction time. The weights, sizes, and number densities of the particles produced at various acetonitrile contents are summarized in Table 2. As can be seen from Figure 6, the particle size
Ind. Eng. Chem. Res., Vol. 43, No. 12, 2004 3145 Table 2. Characteristicsa of the Reaction Process at Different Acetonitrile Contents
CA2 CA3 CA4 CA5
WPI (g)
PDI (nm)
NDI (1010/mL)
TDI (s)
TND (s)
yield (%)
FPD (nm)
FND (1010/mL)
SEM (nm)
0.5537 0.8731 0.9583 1.1287
334.5 469.3 947.0 1280.0
3.14 1.67 1.47 1.46
120 150 150 180
150 180 180 300
68.5 74.1 81.9 88.5
339.60 467.90 876.59 994.21
7.39 3.76 3.67 3.63
337.5 467.2 877.6 998.2
a WPI ) weight of particle produced after 10-s growth time. PDI ) particle diameter after 10-s growth time. NDI ) number density after 10-s growth time. TDI ) reaction time for the beginning of the increase in particle size. TND ) reaction time for the beginning of the decrease in particle number density. FPD ) final particle diameter. FND ) final particle number density.
Figure 5. Dependence of yield on solution aging time for different acetonitrile contents.
Figure 6. Dependence of particle size on solution aging time for different acetonitrile contents.
decreases at the beginning of reaction and tends to increase after a certain reaction time. Figure 7 shows that the number density of particles increases at the beginning of the reaction and then decreases after a certain reaction time. The synthesis of spherical oxide particles in a mixed solvent is based on the difference in solubility of the alkoxide in the two solvents. In the mixed solvent used here, the alkoxide dissolves in octanol but not in acetonitrile, and water dissolves in
Figure 7. Dependence of particle number density on solution aging time for different acetonitrile contents.
acetonitrile. In such a case, the hydrolysis reaction can be depressed because the water and alkoxide molecules are separated by solvation in different solvents. Therefore, as acetonitrile is added to the alkoxide-octanol solution, the solubility of alkoxide in the mixed solvent decreases. Hence, some of the dissolved alkoxide is separated from the solution in the form of alkoxide emulsion droplets that are stabilized with a stabilizer (HPC), whereas the rest of the alkoxide remains dissolved in octanol. In the next step, the size of the droplets decreases because the liquid droplet transforms into a solid through the hydrolysis and condensation of the alkoxide. At the same time, fine particles are derived from the dissolved alkoxide. Therefore, a decrease of the average particle size and an increase of the number density take place at the beginning of the reaction as shown in Figures 6 and 7. The increase in particle size and the decrease in number density after a certain reaction time can be attributed to a consequence of aggregation of the growing particles in the droplets and fine particles generated from the dissolved alkoxide. From these results, we propose the formation mechanism of premullite spherical particles by the mixed solvent method shown in Figure 8. It is assumed that the solubility of the double alkoxide in the mixed solvent decreases and the amount of alkoxide separated in spherical droplets increases with increasing acetonitrile concentration, leading to an increase in the size of the droplets at fixed stirring speed. This is why the yield and particle size in the initial stage of the reaction increase with increasing acetonitrile concentration, as can be seen in Figures 5 and 6 and Table 2.
3146 Ind. Eng. Chem. Res., Vol. 43, No. 12, 2004 Table 3. Composition of Premullite Particles (HW2) Determined by XRF composition (wt %)
Figure 8. Schematic representation of the formation mechanism of the spherical premullite particles by the mixed solvent method.
From Figures 6 and 7 and Table 2, the particle number density is found to increase rapidly in the early stage of reaction with decreasing acetonitrile concentration; in addition, the average particle size increases, and the number density decreases in a short reaction time. These phenomena are explained as follows: As the hydrophilic acetonitrile concentration decreases, the hydrolysis and condensation rate of the double alkoxide increases because water can react freely with the double alkoxide, and the amount of double alkoxide separated in spherical droplets is reduced so that most of alkoxide exists in the dissolved state in octanol. Hence, more fine particles are produced with decreasing acetonitrile concentration from the solvation state in octanol during the initial reaction period, and then the aggregation of fine particles and growth of large particles take place immediately. Figure 9 shows SEM images of premullite particles derived from solutions with various acetonitrile concentrations in the presence of HPC dispersant. The SEM images show that, at lower acetonitrile concentrations (Figure 9a, CA1 ) 10 vol %), surface agglomeration occurs, and irregularly shaped particles are formed.
components
experimental
theoretical
SiO2 Al2O3 Fe2O3 CaO MgO Na2O P2O5
27.97 71.86 0.06 0.02 0.01 0.07 0.01
28.16 71.83 0.00 0.00 0.00 0.00 0.00
These agglomerated particles can be attributed to the high reaction rate of the alkoxide. Figure 9 indicates that agglomerate-free and large particles are formed when the acetonitrile concentration is increased, and an acetonitrile concentration of at least 40 vol % is required to obtain spherical particles. However, acetonitrile concentrations higher than 50 vol % are shown to result in polydispersed particles. The formation of agglomerate-free particles is from the coverage of particles by HPC, and the formation of polydispersed particles is considered to result from the amount of alkoxide separated from the octanol with increasing acetonitrile concentration, which leads to the emergence of polydispersed droplets during the initial reaction at the same stirring speed. The optimum conditions for the preparation of spherical premullite particles in this work are [TEOS] ) 0.033 mol/L, [ASB] ) 0.1 mol/L, [HPC] ) 0.1 g/L, [acetonitrile] ) 40 vol %, and [H2O] ) 0.5 mol/L. 3.3. Chemical Composition of Premullite Particles. Table 3 shows the chemical composition of premullite particles (HW2) determined by XRF analysis.
Figure 9. SEM images of premullite particles for various acetonitrile contents.
Ind. Eng. Chem. Res., Vol. 43, No. 12, 2004 3147
Figure 10. X-ray diffraction patterns of sample HW1 calcined at various temperatures.
Figure 11. X-ray diffraction patterns of various samples calcined at 1200 °C.
The Al2O3/SiO2 ratio of the premullite particles was about 3:2, which corresponds to the theoretical composition of mullite. Impurities cause the formation of glassy phases in mullite. Because the presence of glassy phases can degrade mechanical properties at high temperature, the powder should contain as small an amount of impurities as possible. The total amount of impurities in our particles is 0.17 wt %, which is much lower than that of commercial mullite. 3.4. XRD Analysis. To understand the sequence of phase development in samples synthesized by partial hydrolysis with [H2O]/[TEOS] ) 1, a series of XRD patterns were recorded and are presented in Figure 10. The dried powders of the premullite are amorphous, and this phase is retained up to 950 °C. Broad XRD peaks of the spinel phase are obtained when the particles are calcined at 992 °C. At 1000 °C, the mullite phase is observed to coexist with the spinel phase. The XRD pattern of the powders calcined at 1100 °C shows that the spinel phase disappears and only the mullite phase exists. At this temperature, the (120) and (210) lines of O-mullite are not distinctly split16,17 but rather form a broad and diffuse maximum. Therefore, the crystalline phase of the powders calcined at 1100 °C is tetragonal mullite (T-mullite). The XRD pattern of the powders calcined at 1200 °C shows that the (120) and (210) lines are distinctly split, a feature that apparently corresponds to well-crystallized O-mullite. The same results are shown in the (250) and (520) lines. The XRD peak intensity is found to be improved when the powders are calcined to 1600 °C. The XRD pattern of the powders calcined at 1600 °C shows only the O-mullite peaks without any significant formation of R-alumina or cristobalite.
XRD patterns for specimens synthesized at various water-to-TEOS ratios and calcined at 1200 °C are shown in Figure 11. Obviously, the larger the H2O/TEOS ratio, the lower the intensity of the XRD peaks. The specimens synthesized at [H2O]/[TEOS] ) 3 and 4 show not only the reflection of T-mullite but also that of a spinel phase. It has been reported that the nature and type of oligomers or polymers depend on the temperature, the concentration of metal alkoxide, the catalysts, and the amount of water. Among these parameters, the amount of water expressed in terms of the molar ratio of water to metal alkoxide is the most important factor affecting the type of oligomers or polymers formed.18,19 The degree of mullitization occurring during calcination is known to be affected by the homogeneity of the SiO2 and Al2O3 components, i.e., the degree of Si-O-Al bond formation in the precursor system.20,21 The mullitization temperature can be brought down considerably by increasing amount of Si-O-Al bonding in the premullite particles. The structures of the silicate species created at various water-to-TEOS ratios were reported by Yoldas19 (Figure 12): partially hydrolyzed monomer, dimmer and/or linear trimer, are created with ratios of 1 or 2, and planar or three-dimensional polymer containing more Si-O-Si oxo bridges are generated for [H2O]/[TEOS] ) 3 or 4. Therefore, it is expected that a homogeneous Al-O-Si double alkoxide, in which silicon and aluminum atoms are mixed evenly on the molecular scale, is formed with H2O/TEOS ratios of 1 or 2 and a nonhomogeneous Al-O-Si double alkoxide, in which two phases of SiO2- and Al2O3-rich areas exist, is generated with the H2O/TEOS ratios of 3 or 4. Also, the homogeneity of the Al-O-Si double alkoxide has an effect on the internal structure of the final premullite particles. The low amount of mullite at higher H2O/TEOS ratios
3148 Ind. Eng. Chem. Res., Vol. 43, No. 12, 2004 Table 4. Lattice Constants, Al2O3 Contents, and Amorphous SiO2 Contents for Various Samples Calcined at Different Temperatures HW1
Figure 12. Types of silicate structures formed at different H2O/ TEOS ratios.
HW3
HW4
lattice constant a (Å) Al2O3 mol % amorphous SiO2a
1600 °C 7.5411 7.5430 60.125 60.402 0.0015 0.004 71
7.5442 60.570 0.0067
7.5458 60.800 0.0093
lattice constant a (Å) Al2O3 mol % amorphous SiO2a
1500 °C 7.5421 7.5443 60.272 60.594 0.0032 0.0069
7.5475 61.049 0.0121
7.5487 61.211 0.0140
lattice constant a (Å) Al2O3 mol % amorphous SiO2a
1400 °C 7.5438 7.5460 60.514 60.832 0.0060 0.0097
7.5484 61.174 0.0135
7.5496 61.349 0.0155
lattice constant a (Å) Al2O3 mol % amorphous SiO2a
1300 °C 7.5484 7.5551 61.173 62.145 0.0135 0.0243
7.5602 62.874 0.0322
7.5648 63.542 0.0393
lattice constant a (Å) Al2O3 mol % amorphous SiO2a
1200 °C 7.5606 7.5676 62.935 63.950 0.0329 0.0435
7.5699 64.270 0.0468
7.5730 64.721 0.0514
Al2O3 mol %
1100 °C 63.376 64.401
64.610
65.016
Al2O3 mol %
Spinal Temperature 63.850 64.853
64.968
65.311
a
HW2
Units of g/1 g of sample.
and the coexistence of T-mullite and the spinel phase at [H2O]/[TEOS] ) 3 and 4 are caused by the inhomogeneous molecular-scale mixing of the two reactants. The lattice constants of mullite were computed by Murthy and Hummel’s method.13 For the orthorhombic system, the relation between the lattice constant is given by the equation
h2 k2 l2 1 ) 2+ 2+ 2 2 d a b c
(19)
In this study, a and b were determined from the average values obtained from the (041) and (401) reflections and from the (250) and (520) reflections, respectively. Constant c was computed using the average values of a and b, the (331) reflection, and eq 19. Ban and Okada22 proposed the relation between the lattice constant a and the chemical composition of mullite (for both pseudo-tetragonal and orthorhombic mullite) as
Al2O3 (mol %) ) 1443a - 1028.06
(20)
The Al2O3 concentrations in mullite crystals calcined at 1200-1600 °C were determined by eq 20. The SiO2 concentration in the crystal was determined from the Al2O3 concentration thus obtained. The concentration of amorphous SiO2 coexisting with mullite crystal was determined from the difference between the bulk and crystalline SiO2 concentrations. Table 4 shows that the Al2O3 content in the mullite crystals, the amount of amorphous SiO2, and the lattice constant a at the same sintering temperature decrease with decreasing H2O/ TEOS ratio used in the partial hydrolysis reaction of TEOS. This indicates that the amount of water is an important factor in the mullitization process, with the highest degree of Al-O-Si bonding being produced at [H2O]/[TEOS] ) 1.
Figure 13. Variation of mullite composition and lattice constants b and c as a function of calcination temperature.
The chemical compositions (mol % Al2O3) of various samples are plotted against the calcination temperature in Figure 13A. Figure 13B and C shows the variation of lattice constants of b and c as a function of calcination temperature for the specimen produced at [H2O]/[TEOS] ) 1. The important feature of Figure 13A is that the chemical composition of the mullite approaches 3 Al2O3/2 SiO2 at low calcination temperature with decreasing H2O/TEOS ratio. Figure 13B and C shows that the
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Figure 14. Infrared spectra of various samples calcined at 1200 °C. Table 5. Assignmentsa of Mullite Bond Wave Numbers (cm-1)23,24 Si-O bond from SiO4 1171 (s) 1120 (s) 960 (ms)
927 (s) 901 (s) 445 (m)
Aliv-O bond from AlO4
Alvi-O bond from AlO6
1165 (s) 832 (s) 740 (m)
613 (m, sh) 567 (s)
a s ) strong, m ) medium, br ) broad, sh ) shoulder, ms ) medium strong.
lattice constants b and c change with calcination temperature, but not significantly. 3.5. FT-IR Analysis. The combined results of the XRD and FT-IR analyses indicate that the conditions of the partial hydrolysis reaction of TEOS have an effect on the change in the chemical composition of mullite and the degree of mullitization. Representative infrared spectra of mullite produced at various H2O/TEOS ratios and calcined at 1200 °C are shown in Figure 14. The assignments of the spectral bands are summarized in Table 5.23,24 Cameron25 and Okada et al.26 reported the relation between the chemical composition of mullite and the absorption bands at 1130 and 1170 cm-1. When mullite is rich in alumina, the intensity of the 1130 cm-1 peak is stronger than that of the 1170 cm-1 peak and vice versa as the composition approaches 60 mol %. Figure 14 shows that the ratio of the intensities I(1130 cm-1)/I(1170 cm-1) increases with increasing H2O/TEOS ratio in the partial hydrolysis reaction of TEOS. This sequence of variation in intensity with the H2O/TEOS ratio reflects the fact that the Al2O3 content in the mullite crystals increases with increasing H2O/TEOS ratio and with increasing concentration of amorphous SiO2 as can be seen in Table 4. 3.6. DTA Analysis. Figure 15 shows DTA curves of premullite particles produced at various H2O/TEOS ratios with HPC in air. The inset in Figure 15 shows the DTA curve of premullite particles produced with
Figure 15. DTA traces of various samples.
[H2O]/[TEOS] ) 1. The first endotherm at 110 °C of the inset can be attributed to the removal of loosely bound or physisorbed water. The large exothermic peak observed from 300 to 400 °C is probably caused by the decomposition of residual organic species such as acetonitrile, octanol, and butoxy groups (-OC4H9) absorbed onto the particle surfaces. The exotherm at 992 °C is attributed to the formation of a spinel phase according to the XRD patterns (Figure 10) of the powders heated to various temperatures. At 1224 °C, the exotherm is attributed to the transformation of T-mullite into O-mullite. From Figure 14, it can be seen that the exothermic peak at about 990 °C is sharpest and highest at the [H2O]/[TEOS] ) 1 and becomes diffused and weakened as the H2O/TEOS ratio is increased. Moreover, the second exothermic peaks are shifted to lower temperatures with decreasing H2O/ TEOS ratio. It is well-known that mullite formation is observed at ∼980 °C through an exothermic reaction when the scale of chemical homogeneity is at the atomic level, and that the formation of mullite is delayed to temperatures higher than 1200 °C when the scale of chemical homogeneity is in the nanometer range (the so-called diphasic precursors).27 From this statement reported in the literature and Figures 11, 13, and 15, premullite particles in which Al2O3 and SiO2 are homogeneously mixed at the atomic level (having higher degree of Al-O-Si bonding) are produced with decreasing H2O/TEOS ratio and transformed into the O-mullite phase at lower temperature, as shown in Figure 15. The specimens synthesized at [H2O]/[TEOS] ) 4 show a double exothermic peak at around 990 °C. Okada and Otsuka20 proposed various compositions for the spinel phase. Hori and Kurita28 attributed the double DTA peaks at about 990 °C to an amorphous premullite and γ-Al2O3, respectively. However, premullite particles prepared at [H2O]/[TEOS] ) 4 were found to be amorphous when calcined up to 1000 °C. Therefore, the double exothermic peaks display the formation of spinel
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phases of two different compositions by the generation of two phases of SiO2- and Al2O3-rich regions caused by a nonhomogeneous reaction. 4. Conclusions In this study, the synthesis of stoichiometric spherical premullite particles and the effect of process variables on the mullitization were investigated, and several conclusions were drawn. 1. The mechanism of formation of monodisperse spherical premullite particles in the present system is summarized as follows: The addition of a mixed solvent to an alkoxide-octanol solution decreases the solubility of the alkoxide, leading to the formation of droplets with the dissolved alkoxide. These droplets are stabilized with a stabilizer. Then, hydrolysis and condensation reactions in the alkoxide droplets decrease the size of the droplets, and fine particles are formed from the alkoxide dissolved in octanol. Subsequently, the final particles are produced by aggregation between the particles produced in the droplets and the fine particles. 2. The optimum conditions for the preparation of spherical premullite particles were found to be [TEOS] ) 0.033 mol/L, [ASB] ) 0.1 mol/L, [HPC] ) 0.1 g/L, [acetonitrile] ) 40 vol %, and [H2O] ) 0.5 mol/L. Low acetonitrile concentrations (CA1 ) 10 vol %) were found to cause the surface aggregation to coarsen and form irregularly shaped particles. Agglomerate-free and large premullite particles were produced when the acetonitrile concentration was increased, whereas acetonitrile concentrations higher than 50 vol % were found to result in polydispersed particles. 3. The Al2O3/SiO2 ratio of the premullite particles was measured to be about 3:2, which corresponds to the theoretical composition of mullite. O-mullite was formed at lower temperatures with decreasing H2O/TEOS ratio. 4. It was found that the amorphous premullite particles were crystallized to a spinel phase at about 990 °C, the spinel phase was transformed into T-mullite at around 1100 °C, and the transformation of T-mullite to O-mullite took place at about 1250 °C. Acknowledgment This work was supported by a Korea Research Foundation grant (KRF-2002-005-D00003). Literature Cited (1) La Mer, V. K.; Dinegar, R. H. Theory, Production and Mechanism of Formation of Monodispersed Hydrosols. J. Am. Chem. Soc. 1950, 72, 4847. (2) La Mer, V. K.; Dinegar, R. H. The Limiting Degrees of Supersaturation of the Sparingly Soluble Sulfates. J. Am. Chem. Soc. 1951, 73, 380. (3) Bogush, G. H.; Zukoski, C. F. Uniform Silica Particle Precipitation: An Aggregative Growth Model. J. Colloid Interface Sci. 1991, 142, 19. (4) Stober, W.; Fink, A.; Bohn, E. Controlled Growth of Monodisperse Silica Spheres in the Micron Size Range. J. Colloid Interface Sci. 1968, 26, 62. (5) Lee, S. K.; Shinozaki, K.; Mizutani, N. Influence of Mixed Solvent on the Formation of Monodispersed Al2O3 Powders by Hydrolysis of Aluminum sec-Butoxide. J. Ceram. Soc. Jpn. 1993, 101, 470.
(6) Ring, T. A.; Jean, J. H. Processing Monosized TiO2 Powders Generated with HPC Dispersant. J. Am. Ceram. Soc. 1986, 65, 1574. (7) Mah, T.; Mazdiyasni, K. S. Mechanical Properties of Mullite. J. Am. Ceram. Soc. 1983, 66, 699. (8) Prochazka, S.; Klug, F. J. Infrared-Transparent Mullite Ceramic. J. Am. Ceram. Soc. 1983, 66, 874. (9) Yoldas, B. E. Microstructure of Monolithic Materials Formed by Heat Treatment of Chemically Polymerized Precursors in the Al2O3-SiO2 Binary. Bull. Am. Ceram. Soc. 1980, 59, 479. (10) Hirata, T.; Minamizono, H.; Shimada, K. Property of SiO2Al2O3 Powders Prepared from Metal Alkoxide. J. Ceram. Soc. Jpn. 1985, 93, 36. (11) Ogihara, T.; Yanagawa, T.; Ogata, N.; Yoshida, K.; Iguchi, M.; Nakata, N.; Ogawa, N. Formation of Monodispersed Oxide Particles by Hydrolysis of Metal Alkoxide in Octanol/Acetonitrile Solution. J. Ceram. Soc. Jpn. 1993, 101, 315. (12) Ogihara, T.; Yanagawa, T.; Ogata, N.; Yoshida, K. Synthesis of Monodispersed, Spherical Fine Mullite Powders by Alkoxide. J. Ceram. Soc. Jpn. 1994, 102, 778. (13) Murthy, M. K.; Hummel, F. A. X-ray Study of the Solid Solution of TiO2, Fe2O3, and Cr2O3 in Mullite (3 Al2O3-iO2). J. Am. Ceram. Soc. 1960, 43, 267. (14) Maron, S. H.; Pierce, P. E.; Ulevitch, I. N. Determination of Latex Particles Size by Light Scattering. J. Colloid. Sci. 1963, 18, 470. (15) Encyclopedia of Emulsion Technology; Becher, P., Ed.; Marcel Dekker: New York, 1983; Vol. 1, p 439. (16) Itatani, K.; Kubozono, T.; Howell, F. S.; Kishioka, A.; Kinoshita, M. Some properties of mullite powders prepared by chemical vapour deposition. J. Mater. Sci. 1995, 30, 1158. (17) Schneider, H.; Merwin, L.; Sebald, A. Mullite formation from noncrystalline percursors. J. Mater. Sci. 1992, 27, 805. (18) Sakka S.; Kamiya, K. Glasses from Metal Alkoholates. J. Non-Cryst. Solids 1980, 42, 403. (19) Yoldas, P. E. Effect of Variation in Polymerized Oxides on Sintering and Crystalline Transformations. J. Am. Ceram. Soc. 1982, 65, 387. (20) Okada, K.; Otsuka, N. Characterization of the Spinel Phase from SiO2-Al2O3 Xerogels and the Formation Process of Mullite. J. Am. Ceram. Soc. 1986, 69, 652. (21) Roy, D. W.; Hoffman, R.; Komarneni, S. Diphasic Xerogels, A New Class of Materials: Phases in the System Al2O3-SiO2 . J. Am. Ceram. Soc. 1984, 67, 468. (22) Ban, T.; Okada, K. Structure Refinement of Mullite by the Rietveld Method and a New Method for Estimation of Chemical Composition. J. Am. Ceram. Soc. 1992, 75, 227. (23) Percival H. J.; Duncan, J. F.; Foster, P. K. Interpretation of the Kaolinite-Mullite Reaction Sequence from Infrared Absorption Spectra. J. Am. Ceram. Soc. 1974, 57, 57. (24) Mackenzie, K. J. D. Infrared Frequency Calculations for Ideal Mullite (3Al2O3-SiO2). J. Am. Ceram. Soc. 1972, 55, 68. (25) Cameron, W. E. Composition and Cell Dimensions of Mullite. Bull. Am. Ceram. Soc. 1977, 56, 1003. (26) Okada, K.; Otsuka, N.; Ossaka, J. Characterization of Spinel Phase Formed in the Kalin-Mullite Thermal Sequence. J. Am. Ceram. Soc. 1986, 69, C251. (27) Taylor A.; Holland, D. The chemical synthesis and crystallization sequence of mullite. J. Non-Cryst. Solids 1993, 152, 1. (28) Hori, S.; Kurita, R. In Mullite and Mullite Composite. Somiya, S., Davis, R. F., Pask, J. A., Eds.; American Ceramic Society: Westerville, OH, 1990; Vol. 6, p 311.
Received for review November 13, 2002 Revised manuscript received March 3, 2004 Accepted March 3, 2004 IE0209071
