Rodration EJJktr & Defkflrt.5 in Solidr, Vol. 141, pp. 35-41 Reprints available directly from the publisher Photocopying permitled by license only C) 1998 OPA (Overseas Publishers Association) N.V. Published by license under the Gordon and Breach Science Publishers imprint. Printed in India DEPENDENCE OF FLAME CHARACTERISTICS ON THE BUBBLE GENERATION AND HYDROXYL CONTENT IN SILICA GLASS E.H. SEKIYA*, D. TORIKAI and C.K. SUZUKI UNICA MP-University of Campinas, Faculty of Mechanical Engineering, Department of Materials Engineering, C.P. 6122, 13081-970 Campinas-SP, Brazil (Received I2 December 1997) High quality silica glass was prepared by flame fusion Verneuil technique using two types of silica powders: purified natural quartz and synthetic sol-gel silica. Two different types of flames, GC2/02 and LPG/02, with various conditions of mixture ratios were used. Bubble generation and hydroxyl incorporation were analyzed in these as-fused materials. Silica glass prepared with purified natural quartz powder showed large concentration of bubbles in the most recently fused region. The bubbles were generated from the fluid inclusions present in the grain. Most of the bubbles collapsed during the fusion process, and depending on the fusion condition it was possible to obtain bubble-free silica glass. A strong dependence of hydroxyl concentration on the flame mixture ratio was also observed. Keywords. Silica glass; Flame fusion; Bubbles in vitreous silica; Hydroxyl in vitreous silica 1. INTRODUCTION Silica glass can be obtained by using various manufacture processes, which define the final quality and characteristics of the material [I]. The flame fusion by Verneuil technique is one of the most useful methods to * Corresponding author. E-mail: sekiya(nifem.unicamp.br [391]/3S
36/[392] E.H. SEKIYA et al. get high purity and transparent silica glass [2]. By this process, silica glass with different properties can be obtained just by changing the raw material (silica powder) or fusion conditions. For optical applications, especially for optical fiber, the silica glass must be bubble-free with very low OH content, as they affect mechanical resistance and optical attenuation [ 1,3]. In spite of the mass production of bubble-free silica glass, the mechanisms of bubble collapse are not completely well understood. In this work, the flame fusion technique was used to correlate fusion parameters, such as flame type and mixture ratios, powder allotropy, grain size distribution with bubble formation and OH incorporation. 11. INSTRUMENTATION The laboratory-scale Verneuil furnace with two pre-mix metallic burners with V arrangement was used for powder fusion at the high temperature zone of the flame. The fused grains were then added to the uppermost part of a rotating rod-like substrate growing by deposition [2,4]. Two kinds of gases were used as a flame source: commercial LPG (liquefied petroleum gas, composed of 50% propane and 50% butane), and the commercial Star Flame GC2 gas (hereafter referred to only as GC2), composed of propene, propane, methyl-acetylene and hydrogen, with about 94% of propene. Due to the burner and flame characteristics, the mixture ratios were limited to some values, such as, 0.23-0.33 for LPG/02 and 0.22-0.40 for GC2/02. 111. RAW MATERIAL As raw materials, two kinds of silica powders were tried: (i) purified quartz powder, produced by Kyushu Ceramics Co. (Kyucera) by grinding and purifying natural quartz with a total metallic impurity content lower than IOppm; and (ii) high purity synthetic amorphous powder (total of metallic impurities lower than looppb), produced by Mitsubishi Kasei Corp. (MKC) from tetramethoxy silane sol-gel process. The particle-size distribution averages for Kyucera and MKC powders were respectively 100 and 160 Fin.
FLAME CHARACTERISTICS [393]/37 IV. CHARACTERIZATION TECHNIQUES Optical Inspectoscopy Optical inspectoscopy was used to observe and estimate the bubble concentration in fused silica. In this technique the samples are immersed in a liquid with refractive index as close as possible to the vitreous silica refraction index, in a dark room, and observed with the aid of an intense and collimated light beam. The light, passing through the samples, is scattered by the bubbles. The image is recorded by a high resolution film [4]. Determination of Hydroxyl Concentration The concentration of OH in the fused silica was determined by the infrared absorption technique in the region of 2.7 pm using the relation proposed by Hetherington and Jack [3]. For each fused ingot, a sample plate of about 2mm thickness was cut parallel to the ingot axis containing the central region. These sample plates were prepared to get a high degree of transparency and parallel faces (* 50 pm). The infrared spectra were measured by an IR Perkin Elmer Spectrometer, model 1600, FTIR series. V. RESULTS AND DISCUSSION The ingots of silica glass were obtained with different fusion conditions described in Table I. The fusion efficiency was observed to be dependent on the powder allotropy, showing an average efficiency of about 27% for crystalline powder and 40% for amorphous powder. The higher aggregation of the amorphous powder in comparison with the crystalline one can be explained by the high reactive surface of the amorphous grain at temperatures below the melting point of the crystalline phase. The fused silica from crystalline powder shows a large concentration of bubbles in the most recently fused region ( head of the ingot) when compared with its base, as can be observed in the Fig. l(a>. The fusion test in quartz powder [5] shows a good correlation between the number of grains containing fluid inclusions and the
38/[394] E.H. SEKIYA el al. TABLE I Fusion conditions and samples characteristics Sample* Ratio Growth Fusion EJficiencvt Bubbleq con- OH con- LPG/O2 ("C) ratio time (min) (%) centration centration or GC'2/02 Wmin) (bubbles/cm3) (ppm) L-K I 0.26-1650 0.15 180 27 75 172 L-M 1 0.28-1710 0.37 60 39 70 219 G-K I 0.22-1740 0.15 204 22 922 189 G-K2 0.33-1730 0.34 163 34 476 191 G-K3 0.40-1720 0.16 158 25 3 100 G-M 1 0.22 N I750 0.16 170 27 878 167 G-M2 0.33-1730 0.29 171 43 395 204 G-M3 0.38-1800 0.33 120 48 25 127 G-M4 0.40-1760 0.34 182 41 4 80 * L = LPGI02 flame; G = GC2/02 flame; K =crystalline powder; M =amorphous powder. 'The temperature is the direct measurement by optical pyrometer (without correction from emissivity factor). $The efficiency represents the percentage of the total amount of powder used for fusion that was converted into silica ingot. nthe "head" region of ingot has not been considered for the estimation of bubble concentration. LPG/O>=O. 26 (3C2/02=0.22 H 10 mm FIGURE 1 Observation of bubbles by optical inspectoscopy: samples obtained with (a) LPG/02 flame and (b) GC2/02 flame. number of grains with bubbles after the test. The microscopic analysis of the powders showed that about 10% of the grains presented some type of fluid inclusions. Therefore, considering a case that each grain (- 100 pm in average) with fluid inclusion generates one bubble, the number of bubbles would be on the order of 2 x 105cm-3. But, the
FLAME CHARACTERISTICS [395]/39 quantitative analysis of the bubble concentration is lower than 1000 cmp3, and depending on the fusion condition, the bubble concentration sharply decreases almost to zero. In fused samples with GC2/02 flame, a decrease in the bubble concentration was observed with increasing ratio of GC2/02 mixture, and for GC2/02 = 0.40, a bubble-free silica glass was obtained, as shown in Fig. l(b). Figure 2 illustrates the effect of bubble collapse as a function of the gas mixture ratio of the GC2/02 flame. This result seems to be similar to the phenomenon described in the consolidation process of porous silica preform obtained by Vapor-phase Axial Deposition (VAD) process [6]. Although the critical diameter of the bubbles is dependent on many factors such as temperature and type of gas inside the bubble and the surrounding atmosphere, we could find a suitable condition to produce bubble-free vitreous silica by using a GC2/02 reductive flame. The hydroxyl concentration in fused silica was also observed to be dependent on the flame mixture ratio of GC2/02, as illustrated in Fig. 3. Lower hydroxyl contents were observed in samples fused with higher Mixture Ratio of GC2/02 FIGURE 2 Concentration of bubbles as a function of the mixture ratios of GC2/02 (vertical bars limit the maximum and minimum values observed in the same sample).
40/[396] E.H. SEKIYA et ul 220-120- 0.20 0.24 0.28 0.32 0.36 0.40 Mixture Ratio of GC2/02 FIGURE 3 Hydroxyl content in flame-fused silica as a function of the mixture ratios of GC2/02 (vertical bars limit the maximum and minimum values observed in the same sample). mixture ratios of GC2/02, and the concentration of OH as low as 80 ppm was obtained for GC2/02 = 0.40. Such results may be explained by the different fusion environment. Increasing the mixture ratio of gases increases the concentration of radicals and molecules such as CO, CO2, H2, C2 [7], with carbon acting as a dehydroxylating agent [8]. VI. CONCLUSIONS The fluid inclusions present in the purified quartz powder, generate a large number of bubbles. However, most of such bubbles suffer collapse under the flame treatment during the fusion process. Fusion of silica with GC2/02 flame indicates that the bubble concentration decreases with increasing ratio, and for GC2/02 = 0.40, bubble-free silica glass was obtained. The smallest hydroxyl concentration was observed in samples obtained with higher mixture ratio of GC2/02 flame.
Acknowledgments FLAME CHARACTERISTICS [397]/41 The authors are grateful to FAPESP and CNPq for financial support of this research. They also thank Prof. Celso Davanzo and his group for the infrared spectroscopy measurement, Kyushu Ceramics Co. and Mitsubishi Kasei Corp. for supplying the silica powder, and the White Martins Company for supplying the Star Flame GC2 gas. References [l] Bruckner, R., J. Non-Cryst. Solids, 5 (1970), 123-175. [2] Torikai, D., Suzuki, C.K., Shimizu, H., Ishizuka, T., Yagi, J., Orii, K. andmiyakawa, T., J. Non-Cryst. Solids, 179 (1994), 328-334. [3] Hetherington, G. and Jack, K.H., Phys. Chem. Glasses, 3(4) (1962), 129-133. [4] Sekiya, E.H., Shinohara, A.H., Shimizu, H., Torikai, D., Nagai, Y.E. and Suzuki, C.K., Proceeding of 12th Brazilian Congress of Engineering and Material Science, Aguas de Lindoia (1996), pp. 40-43. [5] Torikai, D., Hummel, D.C.A. and Suzuki, C.K., Proceeding of 8th Brazilian Congress of Engineering and Material Srience, UNICAMP, Campinas (1988), pp. 428-43 1. [6] Izawa, T. and Sudo, S., Optical Fibers: Materials and Fabrication, KTK Scientific Publishers, Tokyo (1987), Chapter 4, pp. 77-135. [7] Mavrodineanu, R. and Boieax, R., Flame Spectroscopy, John Wiley and Sons, New York (1965), Chapter 1, pp. 12-44. [S] Elmer, T.H. and Meissner, H.E., J. Am. Ceram. Soc., 59(5-6) (1976), 206-209.