Origin of cristobalite formation during sintering of a binary mixture of borosilicate glass and high silica glass
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Jau-Ho Jeana) Alcoa Electronic Packaging, Inc., San Diego, California 92127 (Received 13 September 1993; accepted 22 November 1993)
It was shown previously1 that cristobalite precipitates out of a mixture of borosilicate glass (BSG) and high silica glass (HSG) when sintered at temperatures ranging from 800 to 1200 °C. In this paper, both direct and indirect evidences are presented to conclude that the formation of cristobalite originates in HSG. It is proposed that the cristobalite is formed as a result of dissolution of HSG in BSG and precipitation at heterogeneously nucleated sites. The process of dissolution and precipitation continues until the whole HSG particle is consumed.
I. INTRODUCTION In a previous paper,1 it was demonstrated that a crystalline phase of cristobalite is continuously precipitated out of an initial amorphous binary glass mixture, containing a low-softening borosilicate glass (BSG) and a high-softening high silica glass (HSG) at temperatures ranging from 800 to 1200 °C. It was suggested that the "formation of cristobalite is more likely to originate in HSG rather than in BSG." In this paper, we re-examine this hypothesis in detail from previously obtained x-ray and thermal expansion data and combine that information with newly obtained SEM, TEM, and electron diffraction data to conclude that it is indeed the HSG where the cristobalite starts to nucleate and grow. II. EXPERIMENTAL Compositions of both BSG and HSG glass powders were analyzed by atomic absorption (AA) and inductively coupled plasma (ICP). The results showed that the BSG contained approximately 75 wt. % SiO 2 , 25 wt. % B 2 O 3 , and a small amount of alkali oxides, such as Na 2 O and K 2 O, and HSG, 95 wt. % SiO 2 and 5 wt. % B 2 O 3 . The BSG and HSG had softening points of 700-750, and >1500 °C, respectively. Particle size distributions of the powders were measured by sedimentation. The results showed that both BSG and HSG had a median size of about 6.5 /u.m. XRD analysis of the glass powders showed that the as-received glass powders were amorphous. No crystallinity was detected for either BSG or HSG powder when sintered separately in air at temperatures ranging from 800 to 1000 °C for 4 h.
^Present address: Department of Materials Science and Engineering, National Tsing Hua University, Hsinchu, Taiwan, Republic of China. J. Mater. Res., Vol. 9, No. 4, Apr 1994 http://journals.cambridge.org
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As in previous experiments,1 a mixture of 40 vol. % BSG and 60 vol. % HSG was mixed with 5 wt. % polyethylene glycol binder in 1-propanol. The suspension was deagglomerated by a high energy ultrasonic horn, and mixing was continued using a Turbula mixer (Glen Mills, Maywood, NJ) for 2 h. The powder mixture was dried, ground, and uniaxially pressed at about 9 X 106 N/m 2 to make pellets 1.9 cm in diameter and 0.3 cm high. Samples were sintered isothermally in air ranging from 800 to 1200 °C for various periods of time. The sintered densities were measured by a water immersion technique, as described previously.1
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