5-3-1 Thermal decomposition of coupling agents
The DTA results (Figure 5-1) show that an exothermic peak is detected in all the coated Ag powder samples ； this peak is absent in the uncoated silver powder. To identify what is responsible for the exothermic peak, TGA was used to characterize the pure coupling agents (Zr, Al, Ti), as shown in Figure 5-2. All the coupling agents show significant maximum weight loss in the temperature range 200 to 300℃；however the weight loss percentages of these samples are different. Combining the DTA and TGA results, it is clear that the organic coupling agent is on the silver powder surfaces and the coupling agent layer is decomposed/vaporized from 200 to 300℃. Another approach for the analysis of the thermal decomposition is to identify it in different atmospheres. Figure 5-3 shows that the exothermic peaks shift to a lower temperature (Zr: from 250 to 236℃, Al from 261℃ to 243℃, Ti: from 288℃ to 263℃) when the samples are analyzed in an O2 atmosphere. This suggests that the exothermic peak is related to an oxidation process of the coupling agent. The exothermic peak is absent when the sample is analyzed in N2 atmosphere as shown in Figure 5-4.
5-3-2 Crystallization kinetics of coupling agents
To elucidate what is responsible for the oxidation process, the Ti-based coupling agent was calcined directly at four different isothermal temperatures (350, 400, 500, 600℃) for 2h and the crystallization behavior was determined by XRD. Figure 5-5 shows that the anatase phase is detected until the temperature is higher than 350℃. Since the anatase phase is detected up to temperatures lower than 350℃ and the amount of anatase phase precipitated
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cannot be directly determined by XRD analysis. Therefore, the volume fraction (α) of crystalline anatase phase is estimated from the integrated XRD intensity of the crystalline peak at 2θ=25.30. When the Ti organometallic compound is calcined at 400℃, for a time longer than 12h, the XRD pattern of the anatase phase is reasonably sharp and no anomalous intensity changes for the major peak (101). We therefore consider the intensity of this peak to be the standard for comparing the relative crystallinity. The relative amount of anatase present in the calcined Ti organometallic compounds is determined by comparing the intensity of the major peak of the anatase with the standard specimens. In Figure 5-6, the fraction of TiO2 formed , α, is plotted as a function of reaction time for isothermal reaction at 350, 375, and 400℃. The kinetic data for the isothermal reaction follows a parabolic-like behavior. At 350℃, the fraction of the new phase of TiO2 is almost 27% after reaction for 60 min. When the reaction time is extended to 120min, 50% of TiO2 is obtained. When the temperature is raised to 375℃, and 400℃, 50% of TiO2 was obtained within 15 min and 23 min, respectively. Figure 5-7 shows the temperature dependence of the time at which volume fraction of anatase crystal is 50% in an isothermal crystallization of titanate precursor powders .Combining the results of DTA and XRD, it is found that formation of nano-crystallized titania accompanies the combustion of organometallic compounds.
To obtain the kinetic parameter of Ti precursor powders, the following rate equation is assumed[7]
(
α)
α = kntn−1 1− dt
d (5.1)
Here α is the crystallization fraction at time t, n is the growth morphology
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parameter, and k is related to temperature T by an Arrhenius –type equation
⎟⎠
Where Ea is the activation energy, R is the gas constant and A is a constant.
The intergrated form of Eq. (5.1) can be expressed by the well-known Johnson-Mehl-Avrami (JMA) equation[8, 9]
( )
kt nEquation (5.4) is obtained by taking the logarithms of both sides of Eq.
(5.3) shown in Figure 8, the straight line slope, n, indicates the growth morphology parameter of crystallized phase. The growth morphology parameter n in the crystallization process for Ti precursor powders at varying calcined temperatures is shown in Figure 8. By Eq.(5.4) and Figure 5-8, the growth morphology parameter n is obtained being 1.061, 0.915, 1.016, respectively. Figure 5-8 also provides the kinetic constant data k by the interception of the straight line with the axis of ｛lnln［1/(1-α)］｝.
Equation (5.5) is obtained by taking the logarithms for both sides of Eq.
(5.2).
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RT A E
k = ln −
aln
(5.5)When the left side of ln k in Eq. (5.5) is plotted against the reciprocal of temperature, as shown in Figure 5-9. a straight line is obtained, and then the apparent activation energy can be calculated from the slopes and the activation energy for crystallization of coupling agents is 134.9 kJ mol-1.
The TEM bright field (BF) images and electron diffraction (ED) patterns of the Ti-base coupling agent calcined at 375℃ for 60min are shown in Figures 5-10 and 5-11. It is found that very fine powders have a primary size of about 10nm. Figure 5-10(b) demonstrates the high resolution TEM image of anatase crystal, the (101) spacing of anatase is about 3.61Ao . Figure 5-11 shows the ED pattern of coupling agent calcined at 375℃ for 60min. This result agrees with the crystallites type measured from XRD.
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5-4 Summary
Nanocrystalline titania powders have been synthesized by calcining Ti-base coupling agent directly. The thermal behavior and transformation kinetics of TiO2 nanocrystallites prepared by coupling agents are studied by using isothermal methods. The kinetic data for isothermal reaction follow a parabolic-like behaviour. It is found that the formation of nanocrystallized titania accompanies the combustion of organometallic compounds. The results are summarized as follows:
(1)The isothermal activation energy for for the anatase TiO2 crystallization is 134.9 kJ mol-1.
(2)Both TEM examination and SAED analysis indicate that the anatase TiO2
nanocrystallites with a spherical-like morphology ranging from 10 to 20 nm are formed by calcining Ti-base coupling agent directly.
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References:
1. J. M. Herbert, Ceramic Dielectrics and Capacitors. Electrocomponet Science Monographs,.( Gordon and Breach , New York, 1985) Vol. 6, p.91.
2. W. Noorlander, Electrocomponet Sci. Technol. 5(1978) 33.
3. A. H. Kumar and R. R. Tummala, Int. J. Hybrid Microelectron. 14, (1991) 137.
4. R. K. Bordia and R. Raj, J. Am Ceram. Soc. 68(1985) No.6, 287.
5. C. H. Hsueh and A. G. Evans, J. Am. Ceram. Soc. 68 (1985) No 3, 120.
6. C. J. Shih, S. J. Shih, H. C. Lin, Y. C. Hung, H. H. Yeh, Nanotechnology, 14, ( 2003 )1014.
7. A. Marotta, A. Buri, and G. L. Valenti, J. Mater. Sci. 13(1978) 2483.
8. M. Avami, J. Chem. Phys. 7(1939) 1103.
S. B. Wen, N. C. Wu, S. Yand, and M. C. Wang, J. Mater. Res. 14(1999) 3559.
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100 150 200 250 300 350 400 -10
Figure5-1. DTA analyses for pure Ag and Ag powders coated with coupling agents of (a) Zr (b) Al (c) Ti at a heating rate of 10℃ in air.
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100 200 300 400 500 600 700 0
20 40 60 80 100
Coupling Al
Coupling Ti
W e ig h t ( % )
Temperature
(0C
)Coupling Zr
Figure 5-2. Weight loss of pure coupling agents of (a) Zr (b) Al (c) Ti at a heating rate of 10℃in air.
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100 200 300 400
-10 0 10 20 30 40 50 60
Silver coated Zr
Silver coated Al
(c) (b)
263oC 242oC
236oC
H e at Fl ow ( m W )
Temperature (
0C)
(a)
Silver coated Ti
Figure 5-3. DTA analyses for Ag powder coated with (a) Zr, (b) Al (c) Ti coupling agents at a heating rate of 10℃ in O2
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100 200 300 400 500
-10 0 10 20 30 40 50 60 70
Coupling Ti in N
2
Heat Fl ow ( m W )
Temperature (
0C)
Coupling Ti in Air
Figure 5-4. DTA analyses for Ag powder coated with Ti coupling agents at a heating rate of 20℃ in (a) Air and (b) N2.
97
Figure5-5. XRD patterns of organometallic compounds of Ti calcined at (a) 350℃ (b) 400℃ (c) 500℃ (d) 600℃ for 2 h.
98
99
0 20 40 60 80 100 120
10 20 30 40 50 60 70 80 90
100 4000C
3750C
Crystallization ( % )
Time (min)
3500C
Figure 5- 6. Variation of volume fraction of Anatase crystal wth time. In isothermal crystallization of isopropyl tri(N-ethylenediamino) ethyl titanate
precursor powders.
100
14.8 15.0 15.2 15.4 15.6 15.8 16.0 1.5
2.0 2.5 3.0 3.5 4.0 4.5 5.0 5.5 6.0
ln(time to X c=0.5)
1/T?04
Figure 5-7. Temperature dependence of the time at which volume fraction of anatase crystal is 50% in an isothermal crystallization of titanate precursor
powders.
101
1.0 1.5 2.0 2.5 3.0 3.5 4.0 4.5 5.0 -4
-3 -2 -1 0 1 2
4000C
3750C
n=1.061
lnln(1/1-α)
ln( t, min.)
n=1.016 n=0.915
3500C
Figure 5-8. Determination of the growth morphology parameter (n) in the crystallization process for titanate precursor powders
102
1.48 1.50 1.52 1.54 1.56 1.58 1.60 1.62
-5.0 -4.5 -4.0 -3.5 -3.0 -2.5
lnk
1/T× 103
Ec=134.9kJ/mol
Figure 5-9. Activation energy for crystallization of Ti-base coupling agents
103
103
Figure 5-10. TEM images of the of the Ti-base coupling agent calcined at 375℃ for 60min (a) primary size of about 10nm (b) lattice image showing the lattice spacing of (1 0 1) is 3.61Ao .
d(101)=3.61 A
0Amorphous
104
Figure 5-11. TEM BF images and ED pattern of the Ti-base coupling agent calcined at 375℃ for 60min (a) BF images (b) ED pattern .
101 004 200 211 204
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