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Consolidation of GeO2 soot body prepared by flame hydrolysis reaction
] O U R N A L OF
ELSEVIER
Journal of Non-Crystalline Solids 171 (1994) 228-235
Consolidation of G e O 2 s o o t body prepared by flame hydrolysis reaction Shigeki Sakaguchi * NTT Opto-Electronics Laboratories Tokai, Ibaraki 319-11, Japan
Received 27 October 1992; revised manuscript received 10 February 1994
Abstract
The consolidation behavior of GeO 2 soot bodies prepared with a flame hydrolysis reaction from vapor-phase GeCI 4 was examined based on differential thermal analysis (DTA). GeO 2 soot deposited on silicon substrates, which are useful to collect deposits due to their cleanliness, has both amorphous and crystalline phases, depending on the deposition conditions. The DTA curve for amorphous soot has a broad endothermic peak which corresponds to the consolidation process of the soot body. By contrast, the curve for the crystalline phase does not have such a peak except for one at the melting temperature. The amorphous soot is possibly consolidated into transparent solid glass in a narrow range between the DTA peak temperature and the crystallization temperature. Based on the above results, transparent GeO 2 glass layers were obtained by the consolidation of soot layers deposited on alumina substrates used to avoid thermal expansion mismatch.
I. Introduction
The synthesis of glass through consolidation of a soot (sub-micrometer glass particles) body prepared by using a flame hydrolysis reaction is useful for fabricating high optical quality glass products such as silica glass optical fibers [1]. At the same time, synthesis of pure materials such as silica glass is processed at relatively low temperatures compared with melting methods. In this method, where the soot body is formed and subsequently consolidated into transparent glass, the consolidation process is one of the critical techniques to obtain high optical quality glass [2].
* Corresponding author. Telefax: + 81-492 87 7878. E-mail: [email protected].
The consolidation of a silica soot body is well described by Scherer as viscous sintering [3]. The sintering kinetics are demonstrated for isothermal condition based on knowing several material parameters such as the soot configuration and the viscosity [4]. However, these parameters are not always available for the various types of soot. So, Sakaguchi and Sun have recently developed an experimental method for determining the consolidation conditions by differential thermal analysis (DTA) in which these material parameters are not needed [5,6]. This method is based on the idea that a substantial change in thermal properties occurs as densification progresses [7]. On the other hand, it is well known that germania is a typical material which can form stable glass with single component other than silica. In practice, transparent solid glass has been synthe-
0022-3093/94/$07.00 © 1994 Elsevier Science B.V. All rights reserved SSDI 0022-3093(94)00236-G
s. Sakaguchi / Journal of Non-Crystalline Solids 171 (1994) 228-235
sized by the vapor-phase axial deposition (VAD) technique for drawing optical fibers [8,9]. In addition, it is reported that GeO 2 powder prepared by thermally activated hydrolysis reaction of halide raw materials indicates both amorphous and crystalline phases depending on the reaction conditions [10]. GeO 2 also has some features that differ from silica, such as an obvious tendency toward crystallization and a process temperature lower than that of silica. Especially, the crystallization characteristics are important because the crystallization causes excess optical loss due to scattering. Since these features seem to be common to many glass-forming materials, some attention must be paid when an attempt is made to synthesize glass other than silica using hydrolysis reaction. Therefore, it is useful to investigate the glass synthesis process for GeO 2, which is a typical glass-forming material, through the consolidation of a soot body. This report deals with GeO 2 glass synthesis through consolidation of a soot body prepared by flame hydrolysis reaction. The consolidation process is examined by means of DTA and phase analysis. Optical properties of GeO 2 glass layers obtained by consolidating soot layers are also demonstrated.
2. Experimental procedure 2.1. Soot preparation
Gaseous GeC14 vaporized by a bubbling technique [1] was introduced into an oxy-hydrogen flame and hydrolyzed to form GeO 2 soot. The soot samples for measurements were collected by deposition on silicon substrates which were kept at about 150°C on a heat plate. The silicon substrates are useful for collecting soot samples because of their cleanliness. The deposition procedure is described in the literature in detail [11]. In the deposition process, the oxy-hydrogen flame was adjusted so that it did not melt the soot directly. The raw material gas was supplied into the flame for about 15 rain. A soot layer with a maximum thickness of about 1.5 mm was typically obtained.
229
Soot deposition on alumina substrates was also performed to obtain glass layers. To this purpose, alumina substrates were used to avoid fracture of the glass layers due to thermal expansion mismatch. 2.2. Measurements on soot body
The soot bodies on silicon substrates were analyzed by using conventional DTA, thermogravimetric analysis (TGA), X-ray diffractometry (XRD) and scanning electron microscopy (SEM). In the DTA and TGA analyses, soot samples with a weight of 15-20 mg were charged in platinum crucibles and heated from room temperature to 1500°C at a heating rate of 20°C/min in air. XRD (Cu K s ) measurements were made on powdered samples. Cross-sections of the soot layers for SEM observation were prepared by cleavage of the substrates and carbon coating. Some soot samples were heat-treated in order to examine their temperature-dependent phase transition and morphological changes. The samples were heated to specific temperatures at a heating rate of 10°C/min, kept for 30 min at the specified temperature and cooled with the poweroff in a flowing argon-oxygen atmosphere (ratio, 3:1) using a muffle furnace. Optical loss measurements were performed on glass layers. A He-Ne laser light was launched in the glass layer using a prism coupling technique [12] and the intensity of the light scattered from the surface was measured along the light trace [13].
3. Results 3.1. G e O 2 soot
The GeO 2 soot body deposited on the silicon substrates shows two typical appearances: one is relatively densely packed and glossy white at the deposition center area where the flame tip touched, and the other is smoky, unpacked and soft at deposition edge area. The appearance of the deposited soot depends on the distance from the point where the flame tip touched. Soot sam-
230
S. Sakaguchi/ Journal of Non-Crystalline Solids 171 (1994) 228-235
pies collected from these two regions were used for measurements. Fig. 1 shows an example of an SEM observation of the cross-section of a dense soot layer. The soot body is formed by uniform spherical particles with a diameter of approximately 0.1 rzm. This appearance is very similar to that of silica soot [14]. As for the smoky soot, it was very soft, so it was not possible to observe the crosssection by SEM. Fig. 2 shows the results of X R D measurements for the two contrasting samples, dense and smoky soot. It is clear that the dense soot is amorphous, whereas the smoky soot is crystalline (hexagonal) [10,15]. The phase of the sample seems to be related to its appearance. The smoky sample includes a certain small quantity of amorphous particles, because a slight background is observed. From three deposition samples, similar results were obtained, i.e., the dense soot is amorphous and the smoky soot is crystalline.
C
< >IZ LLI I"Z
10
40
20 ANGLE,
60
2 O
Fig. 2. X R D patterns of dense and smoky soot.
Consequently, both amorphous and crystalline phases may be formed by a flame hydrolysis reaction similar to the case of thermally activated hydrolysis [10] and their phases are correlated to their appearance. Such deposition characteristics showing two phases differs from the case of SiO 2 soot whose state is completely amorphous [16].
3.2. Thermal analysis
Fig. 1. A typical example of SEM photograph of the cross-section of a G e O 2 dense soot layer.
Differential thermal analysis and T G A measurements were made on the two types of soot to investigate the consolidation behavior. Fig. 3 shows D T A and T G A curves for dense soot. In the D T A curve, a broad endothermic peak is seen in the temperature range of 200-800°C. The peak temperature is observed around 730°C. This peak is a quasi-endothermic phenomenon explained as a result of a substantial change in heat conduction as the densification progresses [7]. Rapid densification occurs when the temperature exceeds the peak temperature and it is almost completed at the point where the D T A
S. Sakaguchi / Journal of Non-Crystalline Sofids 171 (1994) 228-235
curve recovers its base line (--- 800°C). The peak temperature indicates the onset of densification [6]. This result shows that it is possible to consolidate the present G e O 2 soot body at temperatures higher than 800°C. On the other hand, a clear sharp endothermic peak is observed around 1110°C. This peak corresponds to the melting of the crystalline phase. Since no clear crystallization peak is recognized in the D T A curve, the dense soot seems to have formed a small amount of the crystalline phase during deposition, even though X R D does not indicate a crystalline state (Fig. 2). In the T G A curve, weight losses are observed in the temperature ranges < 200°C and from 400 to 600°C. The loss at < 200°C corresponds to a small peak in the D T A curve. However, the loss from 400 to 600°C is not necessarily related to the broad peak. I suggest that this fact supports the hypothesis that the broad D T A peak is caused not by a reaction causing weight loss but by consolidation. Weight loss is also seen when the temperature exceeds 1300°C. This loss is attributed to evaporation of G e O 2. At temperatures > 600°C, the T G A curve seems to increase. This increase is not explained in the present experiment.
100
~
~
TGA
231
115
200
400
600
800
1000
"~
1200
1400
1600
TEMPERATURE, "(2
Fig. 4. D T A curve for reheating the same sample as shown in Fig. 3, which was heated to 1300°C and cooled rapidly to room temperature.
A D T A curve obtained by reheating the same sample as shown in Fig. 3 is shown in Fig. 4. This curve was taken for the sample, which has already solidified by heating to 1300°C for D T A and subsequent rapid cooling to room temperature. Although a small melting peak is recognized around 1 l l0°C, no resolved crystallization peak is seen in the course of reheating. This absence means that the reheated sample is mostly amorphous and is stable enough to maintain its glass state against such reheating. From this curve, the glass transition temperature, Tg, is determined to
92.5
0 x w
85
t 730.9
1116.1
i
1 12 2oo
40o
6oo
80o
1 0 0 0 1200 1400 1600
TEMPERATURE,
Fig. 3. Thermal analysis for dense soot. Curves represent D T A ( l o w e r ) and T G A ( u p p e r ) .
L
200
i 400
i 600
i 800
1(300
TEMPERATURE,
i
i
1200
1400
°C
Fig. 5. D T A curve for smoky soot.
1600
S. Sakaguchi /Journal of Non-Crystalline Solids 171 (1994) 228-235
232
Fig. 5 shows a D T A curve for smoky soot sample. A clear melting peak is seen at 1112°C. Although there is no broad endothermic peak, as seen in Fig. 3, the curve descends towards the endothermic side until the melting peak appears. At temperatures higher than the melting point, the curve recovers the base line, 3.3. X R D measurement ,4 >.I-'-
z
W
z
10
20
40 ANGLE,
60
28
Fig. 6. X R D patterns for dense soot heated to various temperatures: as-deposited and 875, 900 and 950°C.
be 544°C (average of three samples with a scattering range of 16°C), which agrees with the published data [17].
As described in Section 3.2., the dense soot, which is amorphous, can be consolidated at temperatures > 800°C. In addition, a melting peak is observed despite the fact that there is no resolved crystallization peak in the D T A curve. To clarify the crystallographic change in the densification process of the G e O 2 soot, X R D measurements were made for the soot heated to various temperatures. Fig. 6 shows a series of X R D measurements. The X R D measurements of the soot samples heated to 875 and 900°C indicate that the resultant solids are all in an amorphous state. By contrast, the measurement for the sample heated to 950°C shows that the solid is crystalline (hexagonal). This fact means that crystallization temperature of G e O : is in the vicinity of 950°C, although crystallization is not resolved in the DTA
Fig. 7. A n example of micrograph for the cross-section of G e O 2 glass layers formed on an alumina substrate.
S. Sakaguchi /Journal of Non-Crystalline Solids 171 (1994) 228-235
curve (see Fig. 3). The temperature obtained is close to a previously reported value [18]. Finally, when the soot body is amorphous, transparent G e O 2 glass can be obtained at temperatures ranging from the D T A peak ( = 800°C) to the crystallization temperature ( ~ 950°C), because the amorphous phase can cause viscous sintering. This range is more than 160°C less than melting temperature, and supports the previous result that transparent preforms are obtained by consolidation of soot prepared by the VAD technique at around 800°C [8]. 3.4. GeO 2 glass layer This section describes an attempt to obtain G e O 2 glass layers through the consolidation of soot layers based on the above result. Fig. 7 is a micrograph of the cross-section of transparent GeO2 glass layers formed on an alumina substrate. To obtain the glass layers, soot layers having uniform thickness are first deposited on the substrate by a traverse of the oxy-hydrogen flame as described in the literature [11]. By removing reaction gas downstream of the burner, dense soot layers are obtained. The refractive index is varied to form a light waveguide structure by introducing SiO 2 which decreases the index. Then the deposited soot layers are consolidated into transparent glass layers at 850°C in an argon-oxygen flowing atmosphere. In this experiment, alumina substrates are used, because the thermal expansion of alumina is close to that of G e O z glass [19]. Although it is possible to consolidate the soot layers deposited on silicon substrates into transparent glass, the glass layers are fractured due to thermal expansion mismatch. As shown in Fig. 7, two layers are formed on the substrate, and light is guided in the upper layer (core). This layer is G e O 2 glass and has a relatively high refractive index compared with the lower layer (clad). The lower layer is SiO2-doped GeO 2 glass and its index is less. In this experiment, the SiC14 ratio is controlled at 10% of the whole raw material gas, and a relative refractive index difference of about 0.8% is expected [1]. As
233
a result, a clear planar waveguide of GeO 2 glass is obtained for the first time. An optical transmission loss of 6 (_+ 0.8) d B / cm at a wavelength of 0.633 ~m was measured in the GeO~ glass layer shown in Fig. 7.
4. Discussion
To summarize the aspects obtained from the present experiments for the synthesis of G e O 2 glass via a soot body prepared by flame hydrolysis, (1) GeO 2 particles can form both amorphous and crystalline phases depending on the deposition. conditions, (2) the D T A curve for amorphous soot has a broad endothermic peak which is attributed to the consolidation behavior and (3) it is possible to obtain a transparent glass from soot (amorphous) in a narrow temperature range, which is less than the melting temperature, between the endothermic peak and the crystallization temperatures. In the present experiment, amorphous and crystalline G e O 2 soot are obtained. In the VAD process, in which GeO 2 is used as a dopant for controlling the refractive index and has an amorphous and a crystalline phase depending on the deposition condition, amorphous G e O 2 is formed when the deposition temperature is > 400°C [16]. The dense soot, which is obtained from the center area of the deposition and is amorphous, corresponds to the fact that the center area has temperatures needed for the deposition of amorphous particles. On the other hand, it is demonstrated that hydrolysis generally produces a crystalline (hexagonal) form [10]. This is the form of smoky soot, taking the deposition temperature effect into account. The temperature at which smoky soot is formed is less than that for the formation of amorphous particles. This discussion is related to Ueda's work in which he has investigated formation of G e O 2 particles in an oxy-hydrogen flame by the light scattering technique [20]. It is known that (1) GeO 2 particles are formed immediately at the burner outlet and are then vaporized as the flame temperature increases and (2) when the flame
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S. Sakaguchi /Journal of Non-Crystalline Solids 171 (1994) 228-235
temperature begins to decrease at some distance from the burner outlet, the particles re-appear [20]. The particles, which reappear at the downstream flame, are directly formed from the vapor as amorphous soot similar to the formation of SiO 2 particles. It is thought that these particles form dense soot when they deposit on the substrate. Calculation of the equilibrium constant, log Kp, for the hydrolysis reaction of GeC14 shows that it is < 3 [7]. For such a small equilibrium constant, it is known that the reaction product directly grows on the substrate surface from the vapor phase as particles, whiskers or thin films [21]. Such particle formation on the surface by condensation from the vaporized state appears to form crystallites. Thus, it is thought that hexagonal G e O z, which is stable at low temperatures, appears as smoky soot in the present experiment. As described above, the dependence of soot phase on the deposition condition is attributed to the temperatures at which particles form the soot body, i.e., the flame temperature in addition to deposition surface temperature. Thus, by keeping the deposition from condensing from vaporized GeO 2 in low temperature regions, it is possible to obtain totally amorphous soot. In the D T A curves for the dense soot (Fig. 3) and the reheated (Fig. 4), there is no evidence for crystallization during heating. This fact means that GeO2 maintains a glass state in a normal heat process. However, a melting peak is resolved in the DTA curve for dense soot (Fig. 3). This fact indicates that the crystalline form is not induced during the heating but exists in as-deposited soot similar to VAD soot. It is plausible that a small amount of the vaporized phase is condensed to form crystalline Particles even in the dense body. The weight loss at temperatures < 200°C seem to result from the dissociation of gaseous substances such as H 2 0 and HCI, which are reaction products in the flame hydrolysis. It represents the release of water molecules adsorbed on the soot surface. The loss in the range from 400 to 600°C seems to correspond to the dehydration of surface hydroxyl groups similar to the dehydration behavior of SiO 2 glass [22]. G e O 2 is strongly
moisture-sensitive, so that it shows a noticeable weight loss compared with SiO 2 which exhibits no weight loss [7]. In the D T A data for the smoky soot (Fig. 5), the curve descends towards the endothermic side until melting occurs. This descent occurs because of low density which causes poor thermal conductivity [23]. Morphological changes do not occur until the particles melt, because they are crystalline and therefore no obvious viscous flow occurs. Once an amorphous soot body is formed, it can be consolidated into transparent solid glass. It is preferable to heat the soot body to temperatures at which the soot does not deform because of viscous flow and as high as possible in order to obtain bubble-flee glass. However, since X R D measurements on heat-treated samples indicate that crystallization occurs when the sample is held at a high temperature, crystallization gives the upper limit for consolidation. Finally, the temperature for consolidation is limited to a narrow range: the lower limit, at which densification begins, is easily detected by D T A as the peak temperature and the upper limit is the crystallization temperature. Such a limitation in obtaining transparent G e O 2 glass is an important characteristic different from SiO 2 soot which shows no limitation due to crystallization. Transparent G e O 2 glass layers are obtained by consolidation of soot layers formed on alumina substrate. The measured optical loss of 6 d B / c m is very high compared with that of SiO 2 glass layer which is typically < 0.1 d B / c m [24]. Such a high loss seems to be caused by scattering due to micro-crystallites, because the melting peak is observed in the D T A curve (Fig. 3). When the loss is improved, it seems to be possible to use G e O 2 glass layers for a non-linear optical device.
5. Conclusion
G e O 2 soot prepared by flame hydrolysis reaction has amorphous and crystalline forms depending on the deposition conditions. It is possible to consolidate amorphous GeO2 soot fully
S. Sakaguchi /Journal of Non-Crystalline Solids 171 (1994) 228-235
into transparent glass in a temperature range less than the melting temperature. The lower limit of the temperature at which the consolidation begins is detected by the DTA data as a broad endothermic peak. The upper limit is given by the crystallization temperature. It is demonstrated that DTA data can be used for investigating the consolidation behavior for synthesis of GeO 2 glass. In addition, transparent GeO 2 glass layers are obtained by consolidating soot layers deposited on alumina substrates and an optical loss of 6 d B / c m is measured. The author thanks Dr Kiyomasa Sugii for encouragement, Dr Shiro Takahashi for useful discussions, Takeshi Kitagawa for deposition experiments, Yoshiki Nishida for XRD measurements and Takashi Inukai for optical loss measurements.
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. McChesney, 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.
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[5] S. Sakaguchi and C.K. Sun, Am. Ceram. Soc. 91st Ann. Meet. Abs. (1989) 48-G-89. [6] S. Sakaguchi and C.K. Sun, Am. Ceram. Soc. 91st Ann. Meet. Abs. (1989) 49-G-89. [7] S. Sakaguchi, to be published in J. Nucl. Sci. [8] H. Takahashi and I. Sugimoto, IEEE J. Lightwave Tech. LT-2 (1984) 613. [9] T. Hosaka, S. Sudo and K. Okamoto, Electron. Lett. 23 (1987) 24. [10] S. Tomaru, M. Kawachi and T. Edahiro, Jpn. J. Appl. Phys. 19 (1980) 1197. [11] M. Yasu, M. Kawachi and M. Kobayashi, Trans. IECE. J68-C (1985) 454 (in Japanese). [12] P.K. Tien, Appl. Opt. 10 (1971) 2395. [13] A. Miki, Y. Okamura and S. Yamamoto, Trans. EIC, J71-C (1988) 453 (in Japanese). [14] E. Potkay, H.R. Clark, I.P. Smyth, T.Y. Kometani and D.L. Wood, IEEE J. Lightwave Tech. 6 (1988) 1338. [15] ASTM card 36-1463. [16] /(,1. Kawachi, S. Sudo, N. Shibata and T. Edahiro, Jpn. J. Appl. Phys. 19 (1980) L69. [17] K. Takahashi, N. Mochida, H. Matsui, S. Takeuchi and Y. Gohshi, Yogyo-Kyokai-Shi 84 (1976) 482. [18] E.H. Fontana and W.A. Plummer, Phys. Chem. Glasses 7 (1966) 139. [19] Kyocera, Kyoto, Catalog. [20] T. Ueda, Nensho Kenkyu 80 (1989) 18 (in Japanese). [21] Yo-Gyo-Kyokai, ed., Ceramic Processing: Powder Preparation and Forming (Yo-Gyo-Kyokai, Tokyo, 1984) (in Japanese). [22] M.L. Hair, J. Non-Cryst. Solids 19 (1975) 299. [23] W. Woodside and J.H. Messmer, J. Appl. Phys. 32 (1961) 1688. [24] Y. Ohmori, T. Kominato, H. Okazaki and M. Yasu, in: Tech. Digest OFC'90, San Francisco, Jan. 1990, Vol. 1, WE2.
ELSEVIER
Journal of Non-Crystalline Solids 171 (1994) 228-235
Consolidation of G e O 2 s o o t body prepared by flame hydrolysis reaction Shigeki Sakaguchi * NTT Opto-Electronics Laboratories Tokai, Ibaraki 319-11, Japan
Received 27 October 1992; revised manuscript received 10 February 1994
Abstract
The consolidation behavior of GeO 2 soot bodies prepared with a flame hydrolysis reaction from vapor-phase GeCI 4 was examined based on differential thermal analysis (DTA). GeO 2 soot deposited on silicon substrates, which are useful to collect deposits due to their cleanliness, has both amorphous and crystalline phases, depending on the deposition conditions. The DTA curve for amorphous soot has a broad endothermic peak which corresponds to the consolidation process of the soot body. By contrast, the curve for the crystalline phase does not have such a peak except for one at the melting temperature. The amorphous soot is possibly consolidated into transparent solid glass in a narrow range between the DTA peak temperature and the crystallization temperature. Based on the above results, transparent GeO 2 glass layers were obtained by the consolidation of soot layers deposited on alumina substrates used to avoid thermal expansion mismatch.
I. Introduction
The synthesis of glass through consolidation of a soot (sub-micrometer glass particles) body prepared by using a flame hydrolysis reaction is useful for fabricating high optical quality glass products such as silica glass optical fibers [1]. At the same time, synthesis of pure materials such as silica glass is processed at relatively low temperatures compared with melting methods. In this method, where the soot body is formed and subsequently consolidated into transparent glass, the consolidation process is one of the critical techniques to obtain high optical quality glass [2].
* Corresponding author. Telefax: + 81-492 87 7878. E-mail: [email protected].
The consolidation of a silica soot body is well described by Scherer as viscous sintering [3]. The sintering kinetics are demonstrated for isothermal condition based on knowing several material parameters such as the soot configuration and the viscosity [4]. However, these parameters are not always available for the various types of soot. So, Sakaguchi and Sun have recently developed an experimental method for determining the consolidation conditions by differential thermal analysis (DTA) in which these material parameters are not needed [5,6]. This method is based on the idea that a substantial change in thermal properties occurs as densification progresses [7]. On the other hand, it is well known that germania is a typical material which can form stable glass with single component other than silica. In practice, transparent solid glass has been synthe-
0022-3093/94/$07.00 © 1994 Elsevier Science B.V. All rights reserved SSDI 0022-3093(94)00236-G
s. Sakaguchi / Journal of Non-Crystalline Solids 171 (1994) 228-235
sized by the vapor-phase axial deposition (VAD) technique for drawing optical fibers [8,9]. In addition, it is reported that GeO 2 powder prepared by thermally activated hydrolysis reaction of halide raw materials indicates both amorphous and crystalline phases depending on the reaction conditions [10]. GeO 2 also has some features that differ from silica, such as an obvious tendency toward crystallization and a process temperature lower than that of silica. Especially, the crystallization characteristics are important because the crystallization causes excess optical loss due to scattering. Since these features seem to be common to many glass-forming materials, some attention must be paid when an attempt is made to synthesize glass other than silica using hydrolysis reaction. Therefore, it is useful to investigate the glass synthesis process for GeO 2, which is a typical glass-forming material, through the consolidation of a soot body. This report deals with GeO 2 glass synthesis through consolidation of a soot body prepared by flame hydrolysis reaction. The consolidation process is examined by means of DTA and phase analysis. Optical properties of GeO 2 glass layers obtained by consolidating soot layers are also demonstrated.
2. Experimental procedure 2.1. Soot preparation
Gaseous GeC14 vaporized by a bubbling technique [1] was introduced into an oxy-hydrogen flame and hydrolyzed to form GeO 2 soot. The soot samples for measurements were collected by deposition on silicon substrates which were kept at about 150°C on a heat plate. The silicon substrates are useful for collecting soot samples because of their cleanliness. The deposition procedure is described in the literature in detail [11]. In the deposition process, the oxy-hydrogen flame was adjusted so that it did not melt the soot directly. The raw material gas was supplied into the flame for about 15 rain. A soot layer with a maximum thickness of about 1.5 mm was typically obtained.
229
Soot deposition on alumina substrates was also performed to obtain glass layers. To this purpose, alumina substrates were used to avoid fracture of the glass layers due to thermal expansion mismatch. 2.2. Measurements on soot body
The soot bodies on silicon substrates were analyzed by using conventional DTA, thermogravimetric analysis (TGA), X-ray diffractometry (XRD) and scanning electron microscopy (SEM). In the DTA and TGA analyses, soot samples with a weight of 15-20 mg were charged in platinum crucibles and heated from room temperature to 1500°C at a heating rate of 20°C/min in air. XRD (Cu K s ) measurements were made on powdered samples. Cross-sections of the soot layers for SEM observation were prepared by cleavage of the substrates and carbon coating. Some soot samples were heat-treated in order to examine their temperature-dependent phase transition and morphological changes. The samples were heated to specific temperatures at a heating rate of 10°C/min, kept for 30 min at the specified temperature and cooled with the poweroff in a flowing argon-oxygen atmosphere (ratio, 3:1) using a muffle furnace. Optical loss measurements were performed on glass layers. A He-Ne laser light was launched in the glass layer using a prism coupling technique [12] and the intensity of the light scattered from the surface was measured along the light trace [13].
3. Results 3.1. G e O 2 soot
The GeO 2 soot body deposited on the silicon substrates shows two typical appearances: one is relatively densely packed and glossy white at the deposition center area where the flame tip touched, and the other is smoky, unpacked and soft at deposition edge area. The appearance of the deposited soot depends on the distance from the point where the flame tip touched. Soot sam-
230
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pies collected from these two regions were used for measurements. Fig. 1 shows an example of an SEM observation of the cross-section of a dense soot layer. The soot body is formed by uniform spherical particles with a diameter of approximately 0.1 rzm. This appearance is very similar to that of silica soot [14]. As for the smoky soot, it was very soft, so it was not possible to observe the crosssection by SEM. Fig. 2 shows the results of X R D measurements for the two contrasting samples, dense and smoky soot. It is clear that the dense soot is amorphous, whereas the smoky soot is crystalline (hexagonal) [10,15]. The phase of the sample seems to be related to its appearance. The smoky sample includes a certain small quantity of amorphous particles, because a slight background is observed. From three deposition samples, similar results were obtained, i.e., the dense soot is amorphous and the smoky soot is crystalline.
C
< >IZ LLI I"Z
10
40
20 ANGLE,
60
2 O
Fig. 2. X R D patterns of dense and smoky soot.
Consequently, both amorphous and crystalline phases may be formed by a flame hydrolysis reaction similar to the case of thermally activated hydrolysis [10] and their phases are correlated to their appearance. Such deposition characteristics showing two phases differs from the case of SiO 2 soot whose state is completely amorphous [16].
3.2. Thermal analysis
Fig. 1. A typical example of SEM photograph of the cross-section of a G e O 2 dense soot layer.
Differential thermal analysis and T G A measurements were made on the two types of soot to investigate the consolidation behavior. Fig. 3 shows D T A and T G A curves for dense soot. In the D T A curve, a broad endothermic peak is seen in the temperature range of 200-800°C. The peak temperature is observed around 730°C. This peak is a quasi-endothermic phenomenon explained as a result of a substantial change in heat conduction as the densification progresses [7]. Rapid densification occurs when the temperature exceeds the peak temperature and it is almost completed at the point where the D T A
S. Sakaguchi / Journal of Non-Crystalline Sofids 171 (1994) 228-235
curve recovers its base line (--- 800°C). The peak temperature indicates the onset of densification [6]. This result shows that it is possible to consolidate the present G e O 2 soot body at temperatures higher than 800°C. On the other hand, a clear sharp endothermic peak is observed around 1110°C. This peak corresponds to the melting of the crystalline phase. Since no clear crystallization peak is recognized in the D T A curve, the dense soot seems to have formed a small amount of the crystalline phase during deposition, even though X R D does not indicate a crystalline state (Fig. 2). In the T G A curve, weight losses are observed in the temperature ranges < 200°C and from 400 to 600°C. The loss at < 200°C corresponds to a small peak in the D T A curve. However, the loss from 400 to 600°C is not necessarily related to the broad peak. I suggest that this fact supports the hypothesis that the broad D T A peak is caused not by a reaction causing weight loss but by consolidation. Weight loss is also seen when the temperature exceeds 1300°C. This loss is attributed to evaporation of G e O 2. At temperatures > 600°C, the T G A curve seems to increase. This increase is not explained in the present experiment.
100
~
~
TGA
231
115
200
400
600
800
1000
"~
1200
1400
1600
TEMPERATURE, "(2
Fig. 4. D T A curve for reheating the same sample as shown in Fig. 3, which was heated to 1300°C and cooled rapidly to room temperature.
A D T A curve obtained by reheating the same sample as shown in Fig. 3 is shown in Fig. 4. This curve was taken for the sample, which has already solidified by heating to 1300°C for D T A and subsequent rapid cooling to room temperature. Although a small melting peak is recognized around 1 l l0°C, no resolved crystallization peak is seen in the course of reheating. This absence means that the reheated sample is mostly amorphous and is stable enough to maintain its glass state against such reheating. From this curve, the glass transition temperature, Tg, is determined to
92.5
0 x w
85
t 730.9
1116.1
i
1 12 2oo
40o
6oo
80o
1 0 0 0 1200 1400 1600
TEMPERATURE,
Fig. 3. Thermal analysis for dense soot. Curves represent D T A ( l o w e r ) and T G A ( u p p e r ) .
L
200
i 400
i 600
i 800
1(300
TEMPERATURE,
i
i
1200
1400
°C
Fig. 5. D T A curve for smoky soot.
1600
S. Sakaguchi /Journal of Non-Crystalline Solids 171 (1994) 228-235
232
Fig. 5 shows a D T A curve for smoky soot sample. A clear melting peak is seen at 1112°C. Although there is no broad endothermic peak, as seen in Fig. 3, the curve descends towards the endothermic side until the melting peak appears. At temperatures higher than the melting point, the curve recovers the base line, 3.3. X R D measurement ,4 >.I-'-
z
W
z
10
20
40 ANGLE,
60
28
Fig. 6. X R D patterns for dense soot heated to various temperatures: as-deposited and 875, 900 and 950°C.
be 544°C (average of three samples with a scattering range of 16°C), which agrees with the published data [17].
As described in Section 3.2., the dense soot, which is amorphous, can be consolidated at temperatures > 800°C. In addition, a melting peak is observed despite the fact that there is no resolved crystallization peak in the D T A curve. To clarify the crystallographic change in the densification process of the G e O 2 soot, X R D measurements were made for the soot heated to various temperatures. Fig. 6 shows a series of X R D measurements. The X R D measurements of the soot samples heated to 875 and 900°C indicate that the resultant solids are all in an amorphous state. By contrast, the measurement for the sample heated to 950°C shows that the solid is crystalline (hexagonal). This fact means that crystallization temperature of G e O : is in the vicinity of 950°C, although crystallization is not resolved in the DTA
Fig. 7. A n example of micrograph for the cross-section of G e O 2 glass layers formed on an alumina substrate.
S. Sakaguchi /Journal of Non-Crystalline Solids 171 (1994) 228-235
curve (see Fig. 3). The temperature obtained is close to a previously reported value [18]. Finally, when the soot body is amorphous, transparent G e O 2 glass can be obtained at temperatures ranging from the D T A peak ( = 800°C) to the crystallization temperature ( ~ 950°C), because the amorphous phase can cause viscous sintering. This range is more than 160°C less than melting temperature, and supports the previous result that transparent preforms are obtained by consolidation of soot prepared by the VAD technique at around 800°C [8]. 3.4. GeO 2 glass layer This section describes an attempt to obtain G e O 2 glass layers through the consolidation of soot layers based on the above result. Fig. 7 is a micrograph of the cross-section of transparent GeO2 glass layers formed on an alumina substrate. To obtain the glass layers, soot layers having uniform thickness are first deposited on the substrate by a traverse of the oxy-hydrogen flame as described in the literature [11]. By removing reaction gas downstream of the burner, dense soot layers are obtained. The refractive index is varied to form a light waveguide structure by introducing SiO 2 which decreases the index. Then the deposited soot layers are consolidated into transparent glass layers at 850°C in an argon-oxygen flowing atmosphere. In this experiment, alumina substrates are used, because the thermal expansion of alumina is close to that of G e O z glass [19]. Although it is possible to consolidate the soot layers deposited on silicon substrates into transparent glass, the glass layers are fractured due to thermal expansion mismatch. As shown in Fig. 7, two layers are formed on the substrate, and light is guided in the upper layer (core). This layer is G e O 2 glass and has a relatively high refractive index compared with the lower layer (clad). The lower layer is SiO2-doped GeO 2 glass and its index is less. In this experiment, the SiC14 ratio is controlled at 10% of the whole raw material gas, and a relative refractive index difference of about 0.8% is expected [1]. As
233
a result, a clear planar waveguide of GeO 2 glass is obtained for the first time. An optical transmission loss of 6 (_+ 0.8) d B / cm at a wavelength of 0.633 ~m was measured in the GeO~ glass layer shown in Fig. 7.
4. Discussion
To summarize the aspects obtained from the present experiments for the synthesis of G e O 2 glass via a soot body prepared by flame hydrolysis, (1) GeO 2 particles can form both amorphous and crystalline phases depending on the deposition. conditions, (2) the D T A curve for amorphous soot has a broad endothermic peak which is attributed to the consolidation behavior and (3) it is possible to obtain a transparent glass from soot (amorphous) in a narrow temperature range, which is less than the melting temperature, between the endothermic peak and the crystallization temperatures. In the present experiment, amorphous and crystalline G e O 2 soot are obtained. In the VAD process, in which GeO 2 is used as a dopant for controlling the refractive index and has an amorphous and a crystalline phase depending on the deposition condition, amorphous G e O 2 is formed when the deposition temperature is > 400°C [16]. The dense soot, which is obtained from the center area of the deposition and is amorphous, corresponds to the fact that the center area has temperatures needed for the deposition of amorphous particles. On the other hand, it is demonstrated that hydrolysis generally produces a crystalline (hexagonal) form [10]. This is the form of smoky soot, taking the deposition temperature effect into account. The temperature at which smoky soot is formed is less than that for the formation of amorphous particles. This discussion is related to Ueda's work in which he has investigated formation of G e O 2 particles in an oxy-hydrogen flame by the light scattering technique [20]. It is known that (1) GeO 2 particles are formed immediately at the burner outlet and are then vaporized as the flame temperature increases and (2) when the flame
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temperature begins to decrease at some distance from the burner outlet, the particles re-appear [20]. The particles, which reappear at the downstream flame, are directly formed from the vapor as amorphous soot similar to the formation of SiO 2 particles. It is thought that these particles form dense soot when they deposit on the substrate. Calculation of the equilibrium constant, log Kp, for the hydrolysis reaction of GeC14 shows that it is < 3 [7]. For such a small equilibrium constant, it is known that the reaction product directly grows on the substrate surface from the vapor phase as particles, whiskers or thin films [21]. Such particle formation on the surface by condensation from the vaporized state appears to form crystallites. Thus, it is thought that hexagonal G e O z, which is stable at low temperatures, appears as smoky soot in the present experiment. As described above, the dependence of soot phase on the deposition condition is attributed to the temperatures at which particles form the soot body, i.e., the flame temperature in addition to deposition surface temperature. Thus, by keeping the deposition from condensing from vaporized GeO 2 in low temperature regions, it is possible to obtain totally amorphous soot. In the D T A curves for the dense soot (Fig. 3) and the reheated (Fig. 4), there is no evidence for crystallization during heating. This fact means that GeO2 maintains a glass state in a normal heat process. However, a melting peak is resolved in the DTA curve for dense soot (Fig. 3). This fact indicates that the crystalline form is not induced during the heating but exists in as-deposited soot similar to VAD soot. It is plausible that a small amount of the vaporized phase is condensed to form crystalline Particles even in the dense body. The weight loss at temperatures < 200°C seem to result from the dissociation of gaseous substances such as H 2 0 and HCI, which are reaction products in the flame hydrolysis. It represents the release of water molecules adsorbed on the soot surface. The loss in the range from 400 to 600°C seems to correspond to the dehydration of surface hydroxyl groups similar to the dehydration behavior of SiO 2 glass [22]. G e O 2 is strongly
moisture-sensitive, so that it shows a noticeable weight loss compared with SiO 2 which exhibits no weight loss [7]. In the D T A data for the smoky soot (Fig. 5), the curve descends towards the endothermic side until melting occurs. This descent occurs because of low density which causes poor thermal conductivity [23]. Morphological changes do not occur until the particles melt, because they are crystalline and therefore no obvious viscous flow occurs. Once an amorphous soot body is formed, it can be consolidated into transparent solid glass. It is preferable to heat the soot body to temperatures at which the soot does not deform because of viscous flow and as high as possible in order to obtain bubble-flee glass. However, since X R D measurements on heat-treated samples indicate that crystallization occurs when the sample is held at a high temperature, crystallization gives the upper limit for consolidation. Finally, the temperature for consolidation is limited to a narrow range: the lower limit, at which densification begins, is easily detected by D T A as the peak temperature and the upper limit is the crystallization temperature. Such a limitation in obtaining transparent G e O 2 glass is an important characteristic different from SiO 2 soot which shows no limitation due to crystallization. Transparent G e O 2 glass layers are obtained by consolidation of soot layers formed on alumina substrate. The measured optical loss of 6 d B / c m is very high compared with that of SiO 2 glass layer which is typically < 0.1 d B / c m [24]. Such a high loss seems to be caused by scattering due to micro-crystallites, because the melting peak is observed in the D T A curve (Fig. 3). When the loss is improved, it seems to be possible to use G e O 2 glass layers for a non-linear optical device.
5. Conclusion
G e O 2 soot prepared by flame hydrolysis reaction has amorphous and crystalline forms depending on the deposition conditions. It is possible to consolidate amorphous GeO2 soot fully
S. Sakaguchi /Journal of Non-Crystalline Solids 171 (1994) 228-235
into transparent glass in a temperature range less than the melting temperature. The lower limit of the temperature at which the consolidation begins is detected by the DTA data as a broad endothermic peak. The upper limit is given by the crystallization temperature. It is demonstrated that DTA data can be used for investigating the consolidation behavior for synthesis of GeO 2 glass. In addition, transparent GeO 2 glass layers are obtained by consolidating soot layers deposited on alumina substrates and an optical loss of 6 d B / c m is measured. The author thanks Dr Kiyomasa Sugii for encouragement, Dr Shiro Takahashi for useful discussions, Takeshi Kitagawa for deposition experiments, Yoshiki Nishida for XRD measurements and Takashi Inukai for optical loss measurements.
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[5] S. Sakaguchi and C.K. Sun, Am. Ceram. Soc. 91st Ann. Meet. Abs. (1989) 48-G-89. [6] S. Sakaguchi and C.K. Sun, Am. Ceram. Soc. 91st Ann. Meet. Abs. (1989) 49-G-89. [7] S. Sakaguchi, to be published in J. Nucl. Sci. [8] H. Takahashi and I. Sugimoto, IEEE J. Lightwave Tech. LT-2 (1984) 613. [9] T. Hosaka, S. Sudo and K. Okamoto, Electron. Lett. 23 (1987) 24. [10] S. Tomaru, M. Kawachi and T. Edahiro, Jpn. J. Appl. Phys. 19 (1980) 1197. [11] M. Yasu, M. Kawachi and M. Kobayashi, Trans. IECE. J68-C (1985) 454 (in Japanese). [12] P.K. Tien, Appl. Opt. 10 (1971) 2395. [13] A. Miki, Y. Okamura and S. Yamamoto, Trans. EIC, J71-C (1988) 453 (in Japanese). [14] E. Potkay, H.R. Clark, I.P. Smyth, T.Y. Kometani and D.L. Wood, IEEE J. Lightwave Tech. 6 (1988) 1338. [15] ASTM card 36-1463. [16] /(,1. Kawachi, S. Sudo, N. Shibata and T. Edahiro, Jpn. J. Appl. Phys. 19 (1980) L69. [17] K. Takahashi, N. Mochida, H. Matsui, S. Takeuchi and Y. Gohshi, Yogyo-Kyokai-Shi 84 (1976) 482. [18] E.H. Fontana and W.A. Plummer, Phys. Chem. Glasses 7 (1966) 139. [19] Kyocera, Kyoto, Catalog. [20] T. Ueda, Nensho Kenkyu 80 (1989) 18 (in Japanese). [21] Yo-Gyo-Kyokai, ed., Ceramic Processing: Powder Preparation and Forming (Yo-Gyo-Kyokai, Tokyo, 1984) (in Japanese). [22] M.L. Hair, J. Non-Cryst. Solids 19 (1975) 299. [23] W. Woodside and J.H. Messmer, J. Appl. Phys. 32 (1961) 1688. [24] Y. Ohmori, T. Kominato, H. Okazaki and M. Yasu, in: Tech. Digest OFC'90, San Francisco, Jan. 1990, Vol. 1, WE2.