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Consolidation of silica glass soot body prepared by flame hydrolysis reaction
]OURNAL OF
ELSEVIER
Journal of Non-Crystalline Solids 171 (1994) 249-258
Consolidation of silica glass soot body prepared by flame hydrolysis reaction Shigeki Sakaguchi
*
NTT Opto-Electronics Laboratories, Tokai, Ibaraki 319-11, Japan
Received 31 July 1992; revised manuscript received 10 February 1994
Abstract
The consolidation process of a silica glass soot body prepared by a flame hydrolysis reaction is investigated by differential thermal analysis (DTA). The DTA data for the silica soot show a broad endothermic peak ranging from 300 to 1450°C, without any weight loss. This peak is a quasi-endothermic phenomenon indicating consolidation process, and is supported by temperature calculation based on the assumption that a substantial change occurs in the thermal properties of the soot as densification progresses. A similar endothermic peak is observed in samples of doped silica soot. The effects of dopants such as GeO 2, P205, B203 and TiO 2 on the consolidation temperature are successfully detected by DTA, and are quantitatively determined as a function of dopant concentration. DTA measurements are useful for determining the soot consolidation behavior.
I. Introduction
It is well known that high optical quality silica glass intended for optical waveguides is fabricated by the consolidation of a soot (fine glass particles) body prepared by flame hydrolysis reaction [1]. In this method, the consolidation process is one of the key techniques for obtaining highly transparent glass which is free from scattering defects such as bubbles caused by closed pores and by vaporization [2]. Thus, it is very important to understand the consolidation behavior in order to fabricate optical quality silica glass. The consolidation of a soot body has been well described by Scherer from the point of view of
* Corresponding author. Telefax: + 81-242 877 878. E-mail: [email protected].
viscous flow sintering [3]. Sintering kinetics are described under isothermal conditions by knowing parameters such as soot density and viscosity [4]. Since these parameters are not always available, an experimental technique, which can directly examine the consolidation behavior, would seem to be useful. Sakaguchi and Sun have recently reported attempts to determine the consolidation process of a silica soot body by differential thermal analysis ( D T A ) [5,6]. They showed [5,6] that soot densification tends to progress abruptly when the t e m p e r a t u r e exceeds a certain critical value and that this critical t e m p e r a t u r e is successfully detected in a D T A measurement. The detection is based on the idea that, when thermal properties such as thermal conductivity change as densification progresses, this phenomenon can be detected as an apparent heat exchange by DTA.
0022-3093/94/$07.00 © 1994 Elsevier Science B.V. All rights reserved SSDI 0022-3093(94)00233-D
250
S. Sakaguchi/Journal of Non-Crystalline Solids 171 (1994) 249-258
On the other hand, dopants such as GeO2,
P205, B203 and TiO 2 are added to silica glass to obtain the desired optical properties such as refractive index. It is known that the addition of dopants decreases the consolidation temperature of the doped silica soot. Since the consolidation of a soot body is recognized as viscous sintering [3], it is understood that the densification is accelerated due to the decreased viscosity of the solid solution resulting from doping. Namely, the densification of doped silica soot is linked to the formation of a solid solution. In previous analyses dealing with sintering [2,4] or deposition [7,8] for doped silica soot, consolidation and related dopant behavior are not discussed. Therefore, it still remains an important problem to clarify the consolidation behavior of doped silica soot in order to obtain high optical quality doped silica. This report deals with determining the consolidation process of a silica soot body by DTA along with a temperature calculation which is aimed at clarifying the DTA data. Further, the consolidation of doped silica soot is examined quantitatively and the dopant behavior is also discussed with respect to solid solution formation.
2. Experimental procedure 2.1. Soot preparation
2.2. D T A and TGA measurements
The soot bodies were analyzed by DTA and thermogravimetric analysis (TGA) using commercial instruments in a temperature range from room temperature to 1500°C, in a dry argon/ oxygen (1:1) flow, and at a heating rate of 20oCmin - 1 Accuracy of the temperature is less than + 1.5°C. The soot samples with a weight of 15-20 mg were charged in platinum crucibles for measurement. 2.3. Concentration measurements
For doped soot samples, the dopant concentration was analyzed by inductively coupled plasma (ICP) measurements for sample solutions prepared by (a) HF digestion and (b) methanol extraction [10]. In method (a), all the glass components, whether or not they form solid solution, are thoroughly dissolved in the solution [11], while only the constituents extracted by methanol are dissolved in the solution in method (b) [12]. Measurement error was < 3%. To examine the compositional change of the doped soot in the course of consolidation, doped soot samples were heat-treated to specific temperatures in a dry argon/oxygen (1 : 1) flow at a heating rate of 3°C min- 1, a dwell time of 2 h and a cooling rate of 5°C min-~.
3. Results 3.1. Silica soot
Silica glass soot bodies were deposited on silicon substrates, which are kept ~ 150°C on a heat plate, by a flame hydrolysis reaction [9]. Raw gaseous chloride materials such as SiCI 4, GeCI4, PCI3, BCI 3 and TiCI 4 vaporized by the bubbling technique [1] were hydrolyzed into oxides in an oxy-hydrogen flame. The relative densities of the soot bodies were typically one-tenth that of solid glass [5]. The dopant concentration was varied by changing the flow ratio of the starting materials. The cross-sections of soot bodies were observed by scanning electron microscopy (SEM). The soot samples for SEM observation were prepared by cleavage of substrates and subsequent carbon coating.
Fig. 1 shows a typical example of an SEM observation of a cross-section of deposited soot, The soot particle diameter is typically measured as around 0.1 Ixm by SEM observation. A typical example of DTA and TGA curves for silica soot are shown in Fig. 2. A broad endothermic peak ranging from 300 to 1450°C can be seen in the DTA curve. The curve begins to descend slowly in the endothermic direction, above 300°C, and continues to descend until it observes the peak temperature at 1298°C. Then the curve turns abruptly upward and finally recovers its baseline around 1450°C. This peak configuration shows high reproducibility for several measurements. The average peak value for five
251
S. Sakaguchi /Journal of Non-Crystalline Solids 171 (1994) 249-258
100
o 80 TGA
117 0
= 200
90
"1" uJ
913 i 400
i 600
i 800
i 1000
TEMPERATURE,
i 1200
14(X)
1600
*C
Fig. 3. DTA and TGA curves obtained from silica soot bodies doped with P205, B203 and TiO2.
Fig. 1. A typical example of SEM observation for the crosssection of a soot body.
110
TGA lO0 ~" £9
samples is 1298°C, with a scattering range of 3.8°C. Such a peak is also observed when the heating rate is changed to 10 and 40°Cmin -1. The peak height slightly varies with the rate. The peak seems to be enhanced when the rate is high. On the other hand, the T G A curve shows no weight loss, indicating that no chemical reactions such as dehydration occur during heating. This fact means that the broad endothermic peak is not a real heat exchange caused by a certain chemical reaction but a quasi-heat exchange reflecting consolidation behavior [5] as discussed below. The peak temperature corresponds to the onset of densification. 3.2. Doped silica soot
1298
0
2(]0
400
600
800
1000
TEMPERATURE,
1200
1400
1600
"C
Fig. 2. A typical example of DTA and TGA curves obtained from silica glass soot prepared by flame hydrolysis.
3.2.1. D T A measurements Fig. 3 shows D T A and T G A curves obtained from a doped silica soot body with dopants of P205, B 2 0 3 and TiO 2. Such doped soot is typical material for optical waveguide [9]. In the D T A curve, a broad endothermic peak is observed similar to that of silica soot. The peak temperature for the doped soot (913°C) is noticeably low compared with that for pure silica soot (1298°C).
S. Sakaguchi/Journal of Non-Crystalline Solids 171 (1994) 249-258
252
in Table 1. The lines in Fig. 4 are drawn using these values. This expression is valid, because densification is dominated by viscous flow which is proportional to the inverse of temperature. Slopes, a, for P205 and B203 are ~ 0.02 (K-1 tool% -~) and are approximately three times higher than the value of 0.006 for GeO 2. The slope for TiO 2 is even lower at 0.004. Constants, /3, for these systems fall in the range from 0.66 to 0.7 (K-I). They are very close to each other and converge to that of SiO 2 (0.636).
Table 1 Experimental constants a and /3 for various systems System
a ( K - 1 mol% - 1)
/3 ( K - l)
GeO 2-SiO 2 P2Os-SiO 2 B203-SiO 2 TiO 2 - S i O 2 SiO 2
0.00636 _+0.00092 0.0170 _-/-0.003 0.0234 _+0.0024 0.00432 + 0.00153
0.676 -I-0.016 0.703 _+0.022 0.680+_0.037 0.662 -+ 0.006 0.636 + 0.004
Another peak is observed at 117°C in the DTA curve. This peak seems to be water molecule release. In practice, the TGA curve exhibits a weight loss around 120°C. This fact suggests that the moisture-sensitive dopants such as P205 and B203 in the as-deposited soot absorb water. As shown in Fig. 3, the effect of dopants is seen as the decrease in the peak temperature. Since the peak temperature corresponds to the onset of densification [5], it is clear that DTA measurement is useful for determining the consolidation process even for doped silica soot. In addition, it is possible to examine the consolidation behavior without knowing the parameters such as viscosity, density and dopant concentration of the doped soot. This absence is the great advantage of this method. It should be noted that the peak temperature is defined as the threshold consolidation temperature, Tcth.
3.2.3. Compositional change To examine compositional change in doped soot during heating, four component soot bodies, the DTA curve of which is shown in Fig. 3, were heated to various temperatures. Fig. 5 shows plots of compositional change with heating temperature for HF digestion (Fig. 5(a)) and methanol extraction (Fig. 5(b)). In the analysis by HF digestion, no significant compositional change is observed. The lines represent the average values. These data mean that
1.1
.." /.
3(
\
3.2.2. Dopant effect To measure the effect of dopant on consolidation of doped soot, DTA measurements were performed for various binary systems. Fig. 4 shows plots of the inverse of Tcth versus dopant concentration for GeO2-SiO 2, P205-SIO2, B203-SIO2 and TiO2-SiO 2 systems. The dopant concentration was determined by ICP for HF solutions of soot samples. For each binary system, the inverse of TCth increases as the dopant concentration increases. A linear relation between them is obtained, as 1 / r c t h = aM+/3,
(1)
where M is the mole fraction of the dopant, and a and/3 are experimental constants whose values are determined by least square method and listed
B2Oa
0.9
/
I
//o
•
QI S
0 0 0,p-
o i,~ /
PaOs
,,
0.8 c-
,
o "~
---" " " ~
D
O
. ..-..-" """"
"
GeO2
"
I0.7
,I,,-
~,..~(;~,"
~,o.p
/IU2
0.6
0.5
I 2
I 4
I 6
I 8
I 10
I 12
CONCENTRATION,
I 14
I 16
I 18
20
mol %
Fig. 4. Plots of the inverse of Tctla with dopant concentration for various types of binary system: G e O 2 - S i O 2, P 2 0 5 - 5 i O 2 , B 2 0 3 - S i O 2 and TiO 2 - S i O 2. Lines are drawn using the values listed in Table 1.
S. Sakaguchi/Journal of Non-Crystalline Solids 171 (1994) 249-258
the glass composition hardly changes during the entire consolidation process, indicating that no significant dopant volatilization occurs. By contrast, the composition of the extracted solution clearly changes with heating temperature. Lines
253
are smoothly drawn to describe the data. At low temperatures, a substantial amount of dopant is detected, while no constituents are detected at temperatures > 900°C, which corresponds to the onset of densification.
42
-
40
_._s!o~._ a,..
A
,,,
/X
38
8 3~ E
m
P
o U
/
P205
~-~°-~..~....~-..--.-~..-. ---.~.-.-. --.-~--.-..-.i~-----i~:
~... 0
I
,
200
I
I
I
400
,
I
600
800
I
I
I
1000
I 1200
Temperature, °C
12
I
b
10 m B
, SiO2 c o o Q.
E o
_- . . . . . . . . . x.
~-/ ' , []- -.:'.:T---ox ~2JT...T.-' .-~-?-.L ....a
I
I 200
I
I 400
,
I 600
I
I 800
I
1000
,
I
1200
Temperature,°C
Fig. 5. Compositional change in doped silica soot heated to various temperatures analyzed through (a) HF digestion and (b) methanol extraction. Analytical values are weight percents for elements in the solutions and notations are given by the oxide forms for respective elements. Lines in (a) represent average values and lines in (b) are smoothly drawn to represent the data.
254
S. Sakaguchi/Journal of Non-CrystaUine Solids 171 (1994) 249-258
4. Discussion
4.1. Temperature calculation As shown in Fig. 2, a broad endothermic peak is seen in the DTA curve for silica soot. This peak is primarily understood as a quasi-endothermic phenomenon which is caused by a substantial change in thermal properties such as thermal conductivity as densification progresses [10]. In addition, the DTA peak is closely related to the morphological observation of the soot densification process, i.e., densification progresses abruptly when the temperature exceeds the DTA peak [5]. It is understood that the mass transportation energy for sintering is balanced with the surface energy release during sintering [13]. Thus, no heat exchange should be expected in DTA measurement and this is seen in a report that deals with silica sphere sintering [14]. Nevertheless, it is shown experimentally that the DTA measurement clearly detects an endothermic peak. It is well known that porous materials having very low densities such as those composed of thin solids and large air gaps show very low thermal conductivities. The soot density is generally very low, typically one-tenth that of solid glass [5], so that the thermal conductivity of a soot sample is expected to be very low. Therefore, a large temperature distribution is formed in the soot when it is heated externally. In DTA measurement, a temperature difference is induced between the sample surface and the ambience depending on the temperature distribution in the sample. Thus, the sample temperature is expected to be lower relative to that of the ambient. Such a temperature difference is detected as heat absorption in the DTA curve. As densification progresses, the thermal conductivity increases to reach that of solid glass, decreasing the temperature gradient in the sample. This process appears as the recovery of the DTA curve to the base line. Finally, evaluation for the temperature profile in the sample seems to be an interpretation for DTA behavior. A previous calculation supports the basic idea described above by indicating that the temperature difference is induced when the sample is
heated, although it is not close enough to describe the DTA curve [10]. In the present report, this concept is discussed further by introducing the apparent heat release in a low temperature region to describe the real curve. Suppose that the soot body has an infinite cylindrical shape. Then, the temperature distribution in the body is given in a cylindrical coordination, when internal heat generation is expected, as follows:
aT
la2r C~l ~r2+-I 0t
1or H r -~r ) +--'Cp
(2)
where T is the temperature, t is the time, r is the radial distance, k is the thermal conductivity, C is the heat capacity (1050 Jkg - 1 K -1 [15]), p is the density and H is the heat generated by unit volume per unit time. The heat generation is assumed to be described by a Gaussian profile versus temperature as
H = H 0 exp( - ( T - To)2)/g,
(3)
where H 0 is a constant, To is the median temperature and g is the configuration parameter which is determined by g = - ( ( T h - To)2)/(ln(1/2)),
(4)
when Th, the temperature at which the generation shows half its height, is given. When the temperature exceeds the peak value, densification progresses, i.e., the soot density begins to increase. For a composite material consisting of particles and air gaps, the thermal conductivity is given by [16]
kg where kg is the thermal conductivity for a solid body (2.69 WInK -1) and k a is that for air (0.025 W I n K - l ) , c is a constant and 4~ is the parameter dependent on the density given by
4, = 1 - p/pg.
(6)
The change in soot density, which occurs in the temperature range higher than the peak temperature around 1300°C, is experimentally deter-
S. Sakaguchi /Journal of Non-Crystalline Solids 171 (1994) 249-258
255
0.5
0
-0.5
-1
-1.5
-2
-2.5 0
~ 200
, 400
' 600
' 800
TEMPERATURE,
= 1000
' 1200
' 1400
1600
"C
Fig. 6. Calculated temperature difference between the surface and the center of a cylindrical soot body (solid curve). The dashed curve shows the measured DTA curve. mined as p / p g = - 1.33 × 1 0 - 8 ( T - 1250) 3 + 2.4 x 1 0 - 5 ( T - 1250) 2 + 2.43 × 1 0 - 3 ( T - 1250) + 0.12 [10]. The t e m p e r a t u r e difference between the surface and the center of the soot is numerically calculated using a finite difference approximation for Eq. (2) under the boundary conditions OT/Or = 0
(7)
at r = 0 and aT~Or = - h( T - T,)
(8)
at r = a, where T 1 is the ambient temperature, h is the heat transfer coefficient ( = 1 W m -2 K [17]) and a is the soot radius (2 ram). For simplicity, T 1 is changed stepwise for a short dwell time. Fig. 6 shows an example of the calculated t e m p e r a t u r e difference (solid line) and the measured D T A curve (dashed line). The numerical parameters assumed are 380 J s - ~ m -3 for H 0 and 150 and - 2 5 0 ° C for T o and T h, respectively. In this calculation, a value of 0.084 W inK-1 is assumed for k; then e = 0.7773. The calculated temperature difference between the center and the surface of the soot (solid line) increases as the cylinder is heated. Once densification progresses, the difference decreases to a small value. This tendency agrees well with the behavior of the measured D T A curve shown by the dashed line. When small thermal conductivity is assumed for calculation,
the difference becomes small. Similarly, the calculation gives a smaller difference for a lower heating rate. Therefore, the present assumption that the substantial change in thermal property, which occurs as the densification progresses, causes such endothermic peak is valid. The physical meaning of the assumed heat release unfortunately cannot be clarified in the present study. It is plausible that the fine soot particles are rearranged during heating, maintaining a macroscopic configuration of the soot body. As shown in Fig. 1, the soot particles are loosely bound to each other. In other words, the soot body is formed by a weak binding force such as Van der Waals force. Thus, it seems valid to conclude that a certain heat release occurs as a result of particle rearrangement during the heating process. As described above, the present results offer a fundamental understanding of the D T A behavior based on the concept that a substantial change in thermal properties such as thermal conductivity causes a quasi-endothermic phenomenon. This idea is very useful for characterizing composite materials, which are composed of fine particles and large air gaps. These materials have small thermal conductivity, because of low density, compared with that of a sintered body. 4.2. Effect o f dopant Differential thermal analysis measurements quantitatively give the effect of dopant on consolidation of doped soot by decreasing the consolidation temperature, as shown in Fig. 4. This effect seems to reflect the melting temperature of the dopant. The melting temperatures of crystalline P205 and B 2 0 3 are 450 and 580°C, respectively, and those for G e O 2 and TiO 2 are 1110 and 1840°C, respectively [18]. Dopants with lower melting temperatures have a greater effect on decreasing the consolidation temperature. By contrast, dopants with high melting temperatures have a slight effect. The dopant effect described by Eq. (1) is extended to evaluate the consolidation temperature of multicomponent systems. Generally, a certain physical property of a multicomponent glass such
S. Sakaguchi /Journal of Non-Crystalline Solids 171 (1994) 249-258
256
as refractive index is approximately represented by the linear additivity law, when the dopant concentration is small. This fact can similarly be applied to the consolidation temperature as 1/Tct h = E K i M i,
(9)
where K i (K-1 m o l % - l ) and M i (mol%) are the contribution coefficient and the mole fraction, respectively, of the ith component. By comparing Eqs. (9) and (1) for i = 2 (binary system), K for the dopant is given by a and /3. When i = 2, Eq. (9) becomes 1/Tct h = KI(100 - M2) + K 2 M 2 = ( Kz - K 1) M2 + 100K 1,
where subscript 1 and 2 represent matrix and dopant, respectively, so that a = K 2 - K 1 and /3 = 100K a. Thus, K ( = K 2) for the dopant is given by K = ,~ + / 3 / 1 0 0 .
(a0)
Finally, K is given when a and/3 are obtained in the corresponding binary system. For example, Tcth is calculated for the fourcomponent soot, the DTA curve of which is shown in Fig. 4. The values for M and K are 90.44, 1.72, 6.32 and 1.52 mol% and 0.00636, 0.0240, 0.0302 and 0.0109 (K -1 mole% -1) for SiOz, P205, B203 and TiO 2, respectively. The value Tct h is calculated as 941°C. This agrees fairly well with the measured value of 913°C. Thus, it seems to be valid to evaluate the consolidation temperature of multicomponent systems based on Eq. (9). 4.3. Formation o f solid solution
Dopant addition leads to a decrease in the consolidation temperature. To lower the consolidation temperature, the dopants must have formed a solid solution prior to the onset of densification, otherwise the viscosity does not decrease. In Fig. 3, dehydration reaction is observed ~ 120°C. Since dopants such as P20~ and B203 are strongly moisture-sensitive, they easily absorb water. Thus, the release of water suggests that the dopants are independently present in the as-deposited soot. This discussion is also sup-
ported by the previous results that a dopant such as GeO 2 is possibly present as its own soot separating from the silica matrix [7,8]. In addition, compositional analysis for heattreated doped soot samples shown in Fig. 5 seems to demonstrate the formation of solid solution. To compare the results obtained by Hf digestion and methanol extraction, the concentrations of P205 and B203 in as-deposited soot agree with each other. Since oxides such as P205 and B203 are easily extracted because of their solubility [12], this result means that they are present in the as-deposited soot independent of the silica matrix without forming a solid solution. This is consistent with the DTA data (Fig. 3). As for TiO: and SIO2, which exhibit smaller values with extraction than with HF digestion, some amount is suspended in the extracted solution, because they do not dissolve in alcohol. It is known that there are two distribution peaks in the soot particle size, = 0.1 Ixm and = 0.01 Ixm [3,8]. Thus, if a certain amount of the smaller particles is composed of SiO 2 or TiO2, they are possibly suspended in the methanol solution. When the soot is heated to temperatures > 900°C, no constituent is detected in the extracted solutions. This fact means that the dopants form a solid solution. Since no weight loss except for dehydration is observed (Fig. 3), the dopants diffuse without volatilization into the silica matrix as a result of heating for consolidation. The formation of solid solution is completed prior to the onset of densification, because the temperature at which no constituent is detected (Fig. 5(b)) agrees with the DTA peak temperature (Fig. 3). Then, densification occurs because of its lowered viscosity of formed solid solution. When the deposition surface temperature is high, diffusion seems to occur simultaneously with deposition, resulting in the formation of a solid solution. This discussion corresponds to the case that dopant concentration increases with increased deposition temperature [7]. Compositional analysis of the doped soot indicates that glass components are deposited independently described above. This result may be further discussed from the point of view of reactivity of the starting materials. Thus, the equilib-
S. Sakaguchi/ Journal of Non-Crystalline Solids 171 (1994) 249-258
Temperature, °C 1000 600 400 200 15@ 100 I I I I I I ~ ( s )
5o
-
100
141
~
o~ _
-so
0.5
13)
I
1
I
I
I
1.5 2 1000/T, K-1
2.5
Fig. 7. Calculated curves for the equilibrium constant, log kp for various hydrolysis reactions: (1) SiCI4 +2H20 ~ SiO2 + 4HCI, (2) TiCI4+2H20-+TiO2+4HC1, (3) GeC14+2H20 --+GeO 2 + 4HCI, (4) BCI3 + 3/2H20 ~ 1/2B203 +3HCI and (5) PCI3 +3/2H20+ 1/202 ~ 1/2P20~ + 3HCI.
rium constant, k o, was calculated using a thermodynamics database [19] for the following reactions: SiC14 + 2 H 2 0
, SiO 2 +
4HCI,
PC13 + 3 / 2 H 2 0 + 1 / 2 0 2 BCI 3 + 3 / 2 H 2 0 TiCI 4 + 2 H 2 0 GeCI 4 + 2 H 2 0
Fig. 7 shows calculated curves for log kp. All the chlorides except GeC14 have high reactivity. It is known that particles are easily formed at a log k o > 3 and that particles, whiskers or thin films are grown on a substrate at a log k o < 3 [20]. With the exception of GeC14, the log k p values for these reactions are high enough to complete the reaction so that they easily form their own clusters in the flame [21]. Once clusters are formed, they are independently deposited on the substrate, because collision seems to dominate the deposition [7]. G e O 2 may be formed on the surface of the deposited particles because of low reactivity. From the above discussion, the following process is suggested regarding the consolidation of doped silica soot as schematically shown in Fig. 8: (1) The dopants are independently deposited in as-deposited soot. They do not form solid solutions at this stage. (2) When the soot is heated for consolidation, the dopants diffuse into the silica matrix to form a solid solution. In other words, a solid solution is formed by heating for consolidation. (3) Once a solid solution is formed, it accelerates the densification due to its lowered viscosity. (4) Consequently, consolidation is completed at a lower t e m p e r a t u r e and doped silica is finally formed.
5. Conclusion
, 1 / 2 P 2 0 5 + 3HC1, Differential thermal analysis m e a s u r e m e n t performed on a silica glass soot body prepared by flame hydrolysis reaction detects a broad endothermic peak. A theoretical temperature calcu-
, 1 / 2 B 2 0 3 + 3HCI, , TiO 2 + 4HCI, , G e O 2 + 4HC1.
Si02 Rich
sio2
~OzJ~
257
0"llJm
---~
g
O
~
t~
~
Dopant = 0.01 IJm • As Deposited
~/D°pant
×O~).~ Rich zz~
• Solid-SolidReaction • Diffusion
• Sintering • Necking
• Consolidation
Fig. 8. Schematic of the consolidation process of a doped silica glass soot body.
258
S. Sakaguchi /Journal of Non-Crystalline Solids 171 (1994) 249-258
lation in the soot supports the idea that the endothermic phenomenon is caused by thermal property change during heating for consolidation. The effect of dopants on decreasing the consolidation temperature of doped silica soot bodies is successfully detected by DTA measurements without knowing the dopant concentration, density and viscosity. A linear relation between the inverse of the consolidation temperature and the dopant concentration was obtained quantitatively. Compositional change in doped soot with heating temperature suggests that the dopants, which are independently present in as-deposited soot, complete the formation of solid solution due to heating prior to the onset of densification. It is consequently demonstrated that DTA measurement clearly reflects consolidation behavior. The author would like to thank Drs Shiro Takahashi, C. Jacob Sun and Kazuo Fujiura for useful discussions. References [1] See, for example, Tyngye Li, ed., Fiber Fabrication, Optical Communications, Vol. 1 (Academic Press, New York, 1985). [2] M.F. Yan, J.B. MacChesney, S.R. Nagel and W.W. Rhodes, J. Mater. Sci. 15 (1980) 1371. [3] G.W. Scherer, J. Am. Ceram. Soc. 60 (1977) 236.
[4] G.W. Scherer and D.L. Bachman, J. Am. Ceram. Soc. 60 (1977) 239. [5] S. Sakaguchi and C.K. Sun, Am. Ceram. Soc. 91st Ann. Meet. Abs. (1989) 49-G-89. [6] S. Sakaguchi and C.K. Sun, Am. Ceram. Soc. 91st Ann. Meet. Abs. (1989) 48-G-89. [7] M. Kawachi, S. Sudo, N. Shibata and T. Edahiro, Jpn. J. Appl. Phys. 19 (1980) L69. [8] E. Potkay, H.R. Clark, I.P. Smyth, T.Y. Kometani and D.L. Wood, J. Lightwave Tech. 6 (1988) 1338. [9] M. Yasu, M. Kawachi and M. Kobayashi, Trans. IECE Jpn. J68-C (1985) 454. [10] S. Sakaguchi and C.K. Sun, unpublished report. [11] R.A. Levy and T.Y. Kometani, J. Electrochem. Soc. 134 (1987) 1565. [12] K. Kumamaru, H. Matsuo, A. Okamoto, M. Yamamoto and Y. Yamamoto, Anal. Chem. Acta 186 (1986) 267. [13] W.D. Kingery, H.K. Bowen and D.R. Uhlmann, Introduction to Ceramics (Wiley-Interscience, New York, 1976). [14] M.D. Sacks and T. Tseng, J. Am. Ceram. Soc. 67 (1984) 526. [15] N.P. Bansal and R.H. Doremus, eds., Handbook of Glass Properties (Academic Press, New York, 1986). [16] W. Woodside and J.H. Messmer, J. Appl. Phys. 32 (1961) 1988. [17] R.H. Perry, ed., Perry's Chemical Engineer's Handbook, 6th Ed. (McGraw-Hill, New York, 1979). [18] R.C. Weast, ed., CRC Handbook of Chemistry and Physics (CRC, Boca Raton, FL, 1986). [19] Nippon Netsu Sokutei Gakkai, ed., Malt (Kagaku Gijutsu Sha, Tokyo, 1985). [20] Yo-Gyo-Kyokai, ed., Ceramic Processing: Powder Preparation and Forming (Yo-Gyo-Kyokai, Tokyo, 1984). [21] G.D. Ulrich and J.W. Richl, J. Colloid Sci. 87 (1982) 257.
ELSEVIER
Journal of Non-Crystalline Solids 171 (1994) 249-258
Consolidation of silica glass soot body prepared by flame hydrolysis reaction Shigeki Sakaguchi
*
NTT Opto-Electronics Laboratories, Tokai, Ibaraki 319-11, Japan
Received 31 July 1992; revised manuscript received 10 February 1994
Abstract
The consolidation process of a silica glass soot body prepared by a flame hydrolysis reaction is investigated by differential thermal analysis (DTA). The DTA data for the silica soot show a broad endothermic peak ranging from 300 to 1450°C, without any weight loss. This peak is a quasi-endothermic phenomenon indicating consolidation process, and is supported by temperature calculation based on the assumption that a substantial change occurs in the thermal properties of the soot as densification progresses. A similar endothermic peak is observed in samples of doped silica soot. The effects of dopants such as GeO 2, P205, B203 and TiO 2 on the consolidation temperature are successfully detected by DTA, and are quantitatively determined as a function of dopant concentration. DTA measurements are useful for determining the soot consolidation behavior.
I. Introduction
It is well known that high optical quality silica glass intended for optical waveguides is fabricated by the consolidation of a soot (fine glass particles) body prepared by flame hydrolysis reaction [1]. In this method, the consolidation process is one of the key techniques for obtaining highly transparent glass which is free from scattering defects such as bubbles caused by closed pores and by vaporization [2]. Thus, it is very important to understand the consolidation behavior in order to fabricate optical quality silica glass. The consolidation of a soot body has been well described by Scherer from the point of view of
* Corresponding author. Telefax: + 81-242 877 878. E-mail: [email protected].
viscous flow sintering [3]. Sintering kinetics are described under isothermal conditions by knowing parameters such as soot density and viscosity [4]. Since these parameters are not always available, an experimental technique, which can directly examine the consolidation behavior, would seem to be useful. Sakaguchi and Sun have recently reported attempts to determine the consolidation process of a silica soot body by differential thermal analysis ( D T A ) [5,6]. They showed [5,6] that soot densification tends to progress abruptly when the t e m p e r a t u r e exceeds a certain critical value and that this critical t e m p e r a t u r e is successfully detected in a D T A measurement. The detection is based on the idea that, when thermal properties such as thermal conductivity change as densification progresses, this phenomenon can be detected as an apparent heat exchange by DTA.
0022-3093/94/$07.00 © 1994 Elsevier Science B.V. All rights reserved SSDI 0022-3093(94)00233-D
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S. Sakaguchi/Journal of Non-Crystalline Solids 171 (1994) 249-258
On the other hand, dopants such as GeO2,
P205, B203 and TiO 2 are added to silica glass to obtain the desired optical properties such as refractive index. It is known that the addition of dopants decreases the consolidation temperature of the doped silica soot. Since the consolidation of a soot body is recognized as viscous sintering [3], it is understood that the densification is accelerated due to the decreased viscosity of the solid solution resulting from doping. Namely, the densification of doped silica soot is linked to the formation of a solid solution. In previous analyses dealing with sintering [2,4] or deposition [7,8] for doped silica soot, consolidation and related dopant behavior are not discussed. Therefore, it still remains an important problem to clarify the consolidation behavior of doped silica soot in order to obtain high optical quality doped silica. This report deals with determining the consolidation process of a silica soot body by DTA along with a temperature calculation which is aimed at clarifying the DTA data. Further, the consolidation of doped silica soot is examined quantitatively and the dopant behavior is also discussed with respect to solid solution formation.
2. Experimental procedure 2.1. Soot preparation
2.2. D T A and TGA measurements
The soot bodies were analyzed by DTA and thermogravimetric analysis (TGA) using commercial instruments in a temperature range from room temperature to 1500°C, in a dry argon/ oxygen (1:1) flow, and at a heating rate of 20oCmin - 1 Accuracy of the temperature is less than + 1.5°C. The soot samples with a weight of 15-20 mg were charged in platinum crucibles for measurement. 2.3. Concentration measurements
For doped soot samples, the dopant concentration was analyzed by inductively coupled plasma (ICP) measurements for sample solutions prepared by (a) HF digestion and (b) methanol extraction [10]. In method (a), all the glass components, whether or not they form solid solution, are thoroughly dissolved in the solution [11], while only the constituents extracted by methanol are dissolved in the solution in method (b) [12]. Measurement error was < 3%. To examine the compositional change of the doped soot in the course of consolidation, doped soot samples were heat-treated to specific temperatures in a dry argon/oxygen (1 : 1) flow at a heating rate of 3°C min- 1, a dwell time of 2 h and a cooling rate of 5°C min-~.
3. Results 3.1. Silica soot
Silica glass soot bodies were deposited on silicon substrates, which are kept ~ 150°C on a heat plate, by a flame hydrolysis reaction [9]. Raw gaseous chloride materials such as SiCI 4, GeCI4, PCI3, BCI 3 and TiCI 4 vaporized by the bubbling technique [1] were hydrolyzed into oxides in an oxy-hydrogen flame. The relative densities of the soot bodies were typically one-tenth that of solid glass [5]. The dopant concentration was varied by changing the flow ratio of the starting materials. The cross-sections of soot bodies were observed by scanning electron microscopy (SEM). The soot samples for SEM observation were prepared by cleavage of substrates and subsequent carbon coating.
Fig. 1 shows a typical example of an SEM observation of a cross-section of deposited soot, The soot particle diameter is typically measured as around 0.1 Ixm by SEM observation. A typical example of DTA and TGA curves for silica soot are shown in Fig. 2. A broad endothermic peak ranging from 300 to 1450°C can be seen in the DTA curve. The curve begins to descend slowly in the endothermic direction, above 300°C, and continues to descend until it observes the peak temperature at 1298°C. Then the curve turns abruptly upward and finally recovers its baseline around 1450°C. This peak configuration shows high reproducibility for several measurements. The average peak value for five
251
S. Sakaguchi /Journal of Non-Crystalline Solids 171 (1994) 249-258
100
o 80 TGA
117 0
= 200
90
"1" uJ
913 i 400
i 600
i 800
i 1000
TEMPERATURE,
i 1200
14(X)
1600
*C
Fig. 3. DTA and TGA curves obtained from silica soot bodies doped with P205, B203 and TiO2.
Fig. 1. A typical example of SEM observation for the crosssection of a soot body.
110
TGA lO0 ~" £9
samples is 1298°C, with a scattering range of 3.8°C. Such a peak is also observed when the heating rate is changed to 10 and 40°Cmin -1. The peak height slightly varies with the rate. The peak seems to be enhanced when the rate is high. On the other hand, the T G A curve shows no weight loss, indicating that no chemical reactions such as dehydration occur during heating. This fact means that the broad endothermic peak is not a real heat exchange caused by a certain chemical reaction but a quasi-heat exchange reflecting consolidation behavior [5] as discussed below. The peak temperature corresponds to the onset of densification. 3.2. Doped silica soot
1298
0
2(]0
400
600
800
1000
TEMPERATURE,
1200
1400
1600
"C
Fig. 2. A typical example of DTA and TGA curves obtained from silica glass soot prepared by flame hydrolysis.
3.2.1. D T A measurements Fig. 3 shows D T A and T G A curves obtained from a doped silica soot body with dopants of P205, B 2 0 3 and TiO 2. Such doped soot is typical material for optical waveguide [9]. In the D T A curve, a broad endothermic peak is observed similar to that of silica soot. The peak temperature for the doped soot (913°C) is noticeably low compared with that for pure silica soot (1298°C).
S. Sakaguchi/Journal of Non-Crystalline Solids 171 (1994) 249-258
252
in Table 1. The lines in Fig. 4 are drawn using these values. This expression is valid, because densification is dominated by viscous flow which is proportional to the inverse of temperature. Slopes, a, for P205 and B203 are ~ 0.02 (K-1 tool% -~) and are approximately three times higher than the value of 0.006 for GeO 2. The slope for TiO 2 is even lower at 0.004. Constants, /3, for these systems fall in the range from 0.66 to 0.7 (K-I). They are very close to each other and converge to that of SiO 2 (0.636).
Table 1 Experimental constants a and /3 for various systems System
a ( K - 1 mol% - 1)
/3 ( K - l)
GeO 2-SiO 2 P2Os-SiO 2 B203-SiO 2 TiO 2 - S i O 2 SiO 2
0.00636 _+0.00092 0.0170 _-/-0.003 0.0234 _+0.0024 0.00432 + 0.00153
0.676 -I-0.016 0.703 _+0.022 0.680+_0.037 0.662 -+ 0.006 0.636 + 0.004
Another peak is observed at 117°C in the DTA curve. This peak seems to be water molecule release. In practice, the TGA curve exhibits a weight loss around 120°C. This fact suggests that the moisture-sensitive dopants such as P205 and B203 in the as-deposited soot absorb water. As shown in Fig. 3, the effect of dopants is seen as the decrease in the peak temperature. Since the peak temperature corresponds to the onset of densification [5], it is clear that DTA measurement is useful for determining the consolidation process even for doped silica soot. In addition, it is possible to examine the consolidation behavior without knowing the parameters such as viscosity, density and dopant concentration of the doped soot. This absence is the great advantage of this method. It should be noted that the peak temperature is defined as the threshold consolidation temperature, Tcth.
3.2.3. Compositional change To examine compositional change in doped soot during heating, four component soot bodies, the DTA curve of which is shown in Fig. 3, were heated to various temperatures. Fig. 5 shows plots of compositional change with heating temperature for HF digestion (Fig. 5(a)) and methanol extraction (Fig. 5(b)). In the analysis by HF digestion, no significant compositional change is observed. The lines represent the average values. These data mean that
1.1
.." /.
3(
\
3.2.2. Dopant effect To measure the effect of dopant on consolidation of doped soot, DTA measurements were performed for various binary systems. Fig. 4 shows plots of the inverse of Tcth versus dopant concentration for GeO2-SiO 2, P205-SIO2, B203-SIO2 and TiO2-SiO 2 systems. The dopant concentration was determined by ICP for HF solutions of soot samples. For each binary system, the inverse of TCth increases as the dopant concentration increases. A linear relation between them is obtained, as 1 / r c t h = aM+/3,
(1)
where M is the mole fraction of the dopant, and a and/3 are experimental constants whose values are determined by least square method and listed
B2Oa
0.9
/
I
//o
•
QI S
0 0 0,p-
o i,~ /
PaOs
,,
0.8 c-
,
o "~
---" " " ~
D
O
. ..-..-" """"
"
GeO2
"
I0.7
,I,,-
~,..~(;~,"
~,o.p
/IU2
0.6
0.5
I 2
I 4
I 6
I 8
I 10
I 12
CONCENTRATION,
I 14
I 16
I 18
20
mol %
Fig. 4. Plots of the inverse of Tctla with dopant concentration for various types of binary system: G e O 2 - S i O 2, P 2 0 5 - 5 i O 2 , B 2 0 3 - S i O 2 and TiO 2 - S i O 2. Lines are drawn using the values listed in Table 1.
S. Sakaguchi/Journal of Non-Crystalline Solids 171 (1994) 249-258
the glass composition hardly changes during the entire consolidation process, indicating that no significant dopant volatilization occurs. By contrast, the composition of the extracted solution clearly changes with heating temperature. Lines
253
are smoothly drawn to describe the data. At low temperatures, a substantial amount of dopant is detected, while no constituents are detected at temperatures > 900°C, which corresponds to the onset of densification.
42
-
40
_._s!o~._ a,..
A
,,,
/X
38
8 3~ E
m
P
o U
/
P205
~-~°-~..~....~-..--.-~..-. ---.~.-.-. --.-~--.-..-.i~-----i~:
~... 0
I
,
200
I
I
I
400
,
I
600
800
I
I
I
1000
I 1200
Temperature, °C
12
I
b
10 m B
, SiO2 c o o Q.
E o
_- . . . . . . . . . x.
~-/ ' , []- -.:'.:T---ox ~2JT...T.-' .-~-?-.L ....a
I
I 200
I
I 400
,
I 600
I
I 800
I
1000
,
I
1200
Temperature,°C
Fig. 5. Compositional change in doped silica soot heated to various temperatures analyzed through (a) HF digestion and (b) methanol extraction. Analytical values are weight percents for elements in the solutions and notations are given by the oxide forms for respective elements. Lines in (a) represent average values and lines in (b) are smoothly drawn to represent the data.
254
S. Sakaguchi/Journal of Non-CrystaUine Solids 171 (1994) 249-258
4. Discussion
4.1. Temperature calculation As shown in Fig. 2, a broad endothermic peak is seen in the DTA curve for silica soot. This peak is primarily understood as a quasi-endothermic phenomenon which is caused by a substantial change in thermal properties such as thermal conductivity as densification progresses [10]. In addition, the DTA peak is closely related to the morphological observation of the soot densification process, i.e., densification progresses abruptly when the temperature exceeds the DTA peak [5]. It is understood that the mass transportation energy for sintering is balanced with the surface energy release during sintering [13]. Thus, no heat exchange should be expected in DTA measurement and this is seen in a report that deals with silica sphere sintering [14]. Nevertheless, it is shown experimentally that the DTA measurement clearly detects an endothermic peak. It is well known that porous materials having very low densities such as those composed of thin solids and large air gaps show very low thermal conductivities. The soot density is generally very low, typically one-tenth that of solid glass [5], so that the thermal conductivity of a soot sample is expected to be very low. Therefore, a large temperature distribution is formed in the soot when it is heated externally. In DTA measurement, a temperature difference is induced between the sample surface and the ambience depending on the temperature distribution in the sample. Thus, the sample temperature is expected to be lower relative to that of the ambient. Such a temperature difference is detected as heat absorption in the DTA curve. As densification progresses, the thermal conductivity increases to reach that of solid glass, decreasing the temperature gradient in the sample. This process appears as the recovery of the DTA curve to the base line. Finally, evaluation for the temperature profile in the sample seems to be an interpretation for DTA behavior. A previous calculation supports the basic idea described above by indicating that the temperature difference is induced when the sample is
heated, although it is not close enough to describe the DTA curve [10]. In the present report, this concept is discussed further by introducing the apparent heat release in a low temperature region to describe the real curve. Suppose that the soot body has an infinite cylindrical shape. Then, the temperature distribution in the body is given in a cylindrical coordination, when internal heat generation is expected, as follows:
aT
la2r C~l ~r2+-I 0t
1or H r -~r ) +--'Cp
(2)
where T is the temperature, t is the time, r is the radial distance, k is the thermal conductivity, C is the heat capacity (1050 Jkg - 1 K -1 [15]), p is the density and H is the heat generated by unit volume per unit time. The heat generation is assumed to be described by a Gaussian profile versus temperature as
H = H 0 exp( - ( T - To)2)/g,
(3)
where H 0 is a constant, To is the median temperature and g is the configuration parameter which is determined by g = - ( ( T h - To)2)/(ln(1/2)),
(4)
when Th, the temperature at which the generation shows half its height, is given. When the temperature exceeds the peak value, densification progresses, i.e., the soot density begins to increase. For a composite material consisting of particles and air gaps, the thermal conductivity is given by [16]
kg where kg is the thermal conductivity for a solid body (2.69 WInK -1) and k a is that for air (0.025 W I n K - l ) , c is a constant and 4~ is the parameter dependent on the density given by
4, = 1 - p/pg.
(6)
The change in soot density, which occurs in the temperature range higher than the peak temperature around 1300°C, is experimentally deter-
S. Sakaguchi /Journal of Non-Crystalline Solids 171 (1994) 249-258
255
0.5
0
-0.5
-1
-1.5
-2
-2.5 0
~ 200
, 400
' 600
' 800
TEMPERATURE,
= 1000
' 1200
' 1400
1600
"C
Fig. 6. Calculated temperature difference between the surface and the center of a cylindrical soot body (solid curve). The dashed curve shows the measured DTA curve. mined as p / p g = - 1.33 × 1 0 - 8 ( T - 1250) 3 + 2.4 x 1 0 - 5 ( T - 1250) 2 + 2.43 × 1 0 - 3 ( T - 1250) + 0.12 [10]. The t e m p e r a t u r e difference between the surface and the center of the soot is numerically calculated using a finite difference approximation for Eq. (2) under the boundary conditions OT/Or = 0
(7)
at r = 0 and aT~Or = - h( T - T,)
(8)
at r = a, where T 1 is the ambient temperature, h is the heat transfer coefficient ( = 1 W m -2 K [17]) and a is the soot radius (2 ram). For simplicity, T 1 is changed stepwise for a short dwell time. Fig. 6 shows an example of the calculated t e m p e r a t u r e difference (solid line) and the measured D T A curve (dashed line). The numerical parameters assumed are 380 J s - ~ m -3 for H 0 and 150 and - 2 5 0 ° C for T o and T h, respectively. In this calculation, a value of 0.084 W inK-1 is assumed for k; then e = 0.7773. The calculated temperature difference between the center and the surface of the soot (solid line) increases as the cylinder is heated. Once densification progresses, the difference decreases to a small value. This tendency agrees well with the behavior of the measured D T A curve shown by the dashed line. When small thermal conductivity is assumed for calculation,
the difference becomes small. Similarly, the calculation gives a smaller difference for a lower heating rate. Therefore, the present assumption that the substantial change in thermal property, which occurs as the densification progresses, causes such endothermic peak is valid. The physical meaning of the assumed heat release unfortunately cannot be clarified in the present study. It is plausible that the fine soot particles are rearranged during heating, maintaining a macroscopic configuration of the soot body. As shown in Fig. 1, the soot particles are loosely bound to each other. In other words, the soot body is formed by a weak binding force such as Van der Waals force. Thus, it seems valid to conclude that a certain heat release occurs as a result of particle rearrangement during the heating process. As described above, the present results offer a fundamental understanding of the D T A behavior based on the concept that a substantial change in thermal properties such as thermal conductivity causes a quasi-endothermic phenomenon. This idea is very useful for characterizing composite materials, which are composed of fine particles and large air gaps. These materials have small thermal conductivity, because of low density, compared with that of a sintered body. 4.2. Effect o f dopant Differential thermal analysis measurements quantitatively give the effect of dopant on consolidation of doped soot by decreasing the consolidation temperature, as shown in Fig. 4. This effect seems to reflect the melting temperature of the dopant. The melting temperatures of crystalline P205 and B 2 0 3 are 450 and 580°C, respectively, and those for G e O 2 and TiO 2 are 1110 and 1840°C, respectively [18]. Dopants with lower melting temperatures have a greater effect on decreasing the consolidation temperature. By contrast, dopants with high melting temperatures have a slight effect. The dopant effect described by Eq. (1) is extended to evaluate the consolidation temperature of multicomponent systems. Generally, a certain physical property of a multicomponent glass such
S. Sakaguchi /Journal of Non-Crystalline Solids 171 (1994) 249-258
256
as refractive index is approximately represented by the linear additivity law, when the dopant concentration is small. This fact can similarly be applied to the consolidation temperature as 1/Tct h = E K i M i,
(9)
where K i (K-1 m o l % - l ) and M i (mol%) are the contribution coefficient and the mole fraction, respectively, of the ith component. By comparing Eqs. (9) and (1) for i = 2 (binary system), K for the dopant is given by a and /3. When i = 2, Eq. (9) becomes 1/Tct h = KI(100 - M2) + K 2 M 2 = ( Kz - K 1) M2 + 100K 1,
where subscript 1 and 2 represent matrix and dopant, respectively, so that a = K 2 - K 1 and /3 = 100K a. Thus, K ( = K 2) for the dopant is given by K = ,~ + / 3 / 1 0 0 .
(a0)
Finally, K is given when a and/3 are obtained in the corresponding binary system. For example, Tcth is calculated for the fourcomponent soot, the DTA curve of which is shown in Fig. 4. The values for M and K are 90.44, 1.72, 6.32 and 1.52 mol% and 0.00636, 0.0240, 0.0302 and 0.0109 (K -1 mole% -1) for SiOz, P205, B203 and TiO 2, respectively. The value Tct h is calculated as 941°C. This agrees fairly well with the measured value of 913°C. Thus, it seems to be valid to evaluate the consolidation temperature of multicomponent systems based on Eq. (9). 4.3. Formation o f solid solution
Dopant addition leads to a decrease in the consolidation temperature. To lower the consolidation temperature, the dopants must have formed a solid solution prior to the onset of densification, otherwise the viscosity does not decrease. In Fig. 3, dehydration reaction is observed ~ 120°C. Since dopants such as P20~ and B203 are strongly moisture-sensitive, they easily absorb water. Thus, the release of water suggests that the dopants are independently present in the as-deposited soot. This discussion is also sup-
ported by the previous results that a dopant such as GeO 2 is possibly present as its own soot separating from the silica matrix [7,8]. In addition, compositional analysis for heattreated doped soot samples shown in Fig. 5 seems to demonstrate the formation of solid solution. To compare the results obtained by Hf digestion and methanol extraction, the concentrations of P205 and B203 in as-deposited soot agree with each other. Since oxides such as P205 and B203 are easily extracted because of their solubility [12], this result means that they are present in the as-deposited soot independent of the silica matrix without forming a solid solution. This is consistent with the DTA data (Fig. 3). As for TiO: and SIO2, which exhibit smaller values with extraction than with HF digestion, some amount is suspended in the extracted solution, because they do not dissolve in alcohol. It is known that there are two distribution peaks in the soot particle size, = 0.1 Ixm and = 0.01 Ixm [3,8]. Thus, if a certain amount of the smaller particles is composed of SiO 2 or TiO2, they are possibly suspended in the methanol solution. When the soot is heated to temperatures > 900°C, no constituent is detected in the extracted solutions. This fact means that the dopants form a solid solution. Since no weight loss except for dehydration is observed (Fig. 3), the dopants diffuse without volatilization into the silica matrix as a result of heating for consolidation. The formation of solid solution is completed prior to the onset of densification, because the temperature at which no constituent is detected (Fig. 5(b)) agrees with the DTA peak temperature (Fig. 3). Then, densification occurs because of its lowered viscosity of formed solid solution. When the deposition surface temperature is high, diffusion seems to occur simultaneously with deposition, resulting in the formation of a solid solution. This discussion corresponds to the case that dopant concentration increases with increased deposition temperature [7]. Compositional analysis of the doped soot indicates that glass components are deposited independently described above. This result may be further discussed from the point of view of reactivity of the starting materials. Thus, the equilib-
S. Sakaguchi/ Journal of Non-Crystalline Solids 171 (1994) 249-258
Temperature, °C 1000 600 400 200 15@ 100 I I I I I I ~ ( s )
5o
-
100
141
~
o~ _
-so
0.5
13)
I
1
I
I
I
1.5 2 1000/T, K-1
2.5
Fig. 7. Calculated curves for the equilibrium constant, log kp for various hydrolysis reactions: (1) SiCI4 +2H20 ~ SiO2 + 4HCI, (2) TiCI4+2H20-+TiO2+4HC1, (3) GeC14+2H20 --+GeO 2 + 4HCI, (4) BCI3 + 3/2H20 ~ 1/2B203 +3HCI and (5) PCI3 +3/2H20+ 1/202 ~ 1/2P20~ + 3HCI.
rium constant, k o, was calculated using a thermodynamics database [19] for the following reactions: SiC14 + 2 H 2 0
, SiO 2 +
4HCI,
PC13 + 3 / 2 H 2 0 + 1 / 2 0 2 BCI 3 + 3 / 2 H 2 0 TiCI 4 + 2 H 2 0 GeCI 4 + 2 H 2 0
Fig. 7 shows calculated curves for log kp. All the chlorides except GeC14 have high reactivity. It is known that particles are easily formed at a log k o > 3 and that particles, whiskers or thin films are grown on a substrate at a log k o < 3 [20]. With the exception of GeC14, the log k p values for these reactions are high enough to complete the reaction so that they easily form their own clusters in the flame [21]. Once clusters are formed, they are independently deposited on the substrate, because collision seems to dominate the deposition [7]. G e O 2 may be formed on the surface of the deposited particles because of low reactivity. From the above discussion, the following process is suggested regarding the consolidation of doped silica soot as schematically shown in Fig. 8: (1) The dopants are independently deposited in as-deposited soot. They do not form solid solutions at this stage. (2) When the soot is heated for consolidation, the dopants diffuse into the silica matrix to form a solid solution. In other words, a solid solution is formed by heating for consolidation. (3) Once a solid solution is formed, it accelerates the densification due to its lowered viscosity. (4) Consequently, consolidation is completed at a lower t e m p e r a t u r e and doped silica is finally formed.
5. Conclusion
, 1 / 2 P 2 0 5 + 3HC1, Differential thermal analysis m e a s u r e m e n t performed on a silica glass soot body prepared by flame hydrolysis reaction detects a broad endothermic peak. A theoretical temperature calcu-
, 1 / 2 B 2 0 3 + 3HCI, , TiO 2 + 4HCI, , G e O 2 + 4HC1.
Si02 Rich
sio2
~OzJ~
257
0"llJm
---~
g
O
~
t~
~
Dopant = 0.01 IJm • As Deposited
~/D°pant
×O~).~ Rich zz~
• Solid-SolidReaction • Diffusion
• Sintering • Necking
• Consolidation
Fig. 8. Schematic of the consolidation process of a doped silica glass soot body.
258
S. Sakaguchi /Journal of Non-Crystalline Solids 171 (1994) 249-258
lation in the soot supports the idea that the endothermic phenomenon is caused by thermal property change during heating for consolidation. The effect of dopants on decreasing the consolidation temperature of doped silica soot bodies is successfully detected by DTA measurements without knowing the dopant concentration, density and viscosity. A linear relation between the inverse of the consolidation temperature and the dopant concentration was obtained quantitatively. Compositional change in doped soot with heating temperature suggests that the dopants, which are independently present in as-deposited soot, complete the formation of solid solution due to heating prior to the onset of densification. It is consequently demonstrated that DTA measurement clearly reflects consolidation behavior. The author would like to thank Drs Shiro Takahashi, C. Jacob Sun and Kazuo Fujiura for useful discussions. References [1] See, for example, Tyngye Li, ed., Fiber Fabrication, Optical Communications, Vol. 1 (Academic Press, New York, 1985). [2] M.F. Yan, J.B. MacChesney, S.R. Nagel and W.W. Rhodes, J. Mater. Sci. 15 (1980) 1371. [3] G.W. Scherer, J. Am. Ceram. Soc. 60 (1977) 236.
[4] G.W. Scherer and D.L. Bachman, J. Am. Ceram. Soc. 60 (1977) 239. [5] S. Sakaguchi and C.K. Sun, Am. Ceram. Soc. 91st Ann. Meet. Abs. (1989) 49-G-89. [6] S. Sakaguchi and C.K. Sun, Am. Ceram. Soc. 91st Ann. Meet. Abs. (1989) 48-G-89. [7] M. Kawachi, S. Sudo, N. Shibata and T. Edahiro, Jpn. J. Appl. Phys. 19 (1980) L69. [8] E. Potkay, H.R. Clark, I.P. Smyth, T.Y. Kometani and D.L. Wood, J. Lightwave Tech. 6 (1988) 1338. [9] M. Yasu, M. Kawachi and M. Kobayashi, Trans. IECE Jpn. J68-C (1985) 454. [10] S. Sakaguchi and C.K. Sun, unpublished report. [11] R.A. Levy and T.Y. Kometani, J. Electrochem. Soc. 134 (1987) 1565. [12] K. Kumamaru, H. Matsuo, A. Okamoto, M. Yamamoto and Y. Yamamoto, Anal. Chem. Acta 186 (1986) 267. [13] W.D. Kingery, H.K. Bowen and D.R. Uhlmann, Introduction to Ceramics (Wiley-Interscience, New York, 1976). [14] M.D. Sacks and T. Tseng, J. Am. Ceram. Soc. 67 (1984) 526. [15] N.P. Bansal and R.H. Doremus, eds., Handbook of Glass Properties (Academic Press, New York, 1986). [16] W. Woodside and J.H. Messmer, J. Appl. Phys. 32 (1961) 1988. [17] R.H. Perry, ed., Perry's Chemical Engineer's Handbook, 6th Ed. (McGraw-Hill, New York, 1979). [18] R.C. Weast, ed., CRC Handbook of Chemistry and Physics (CRC, Boca Raton, FL, 1986). [19] Nippon Netsu Sokutei Gakkai, ed., Malt (Kagaku Gijutsu Sha, Tokyo, 1985). [20] Yo-Gyo-Kyokai, ed., Ceramic Processing: Powder Preparation and Forming (Yo-Gyo-Kyokai, Tokyo, 1984). [21] G.D. Ulrich and J.W. Richl, J. Colloid Sci. 87 (1982) 257.