Devitrification kinetics and mechanism of Pyrex borosilicate glass
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Cristobalite is formed when initially amorphous Pyrex borosilicate glass (Corning 7740) is fired at temperatures ranging from 700 to 1000 °C. The sigmoidal devitrification kinetics of cristobalite obeys Avrami-like three-dimensional diffusion-controlled kinetics. Activation energy analysis indicates that the diffusion of Na+ in the glass is the rate-limiting step during phase transformation. The above conclusion is further confirmed by calculated and measured results of linear growth rates.
I. INTRODUCTION
Cristobalite was previously reported to be formed when an initially amorphous Pyrex borosilicate glass is fired at elevated temperatures from 700 to 1000 °C.1–3 Since cristobalite has a large thermal expansion coefficient (50 × 10−6 K−1) and a displacive transformation involving a large volume change,4 it can dramatically reduce thermal shock resistance and mechanical strength of glass. It is necessary to prevent cristobalite from forming if any practical application of this glass is needed at elevated temperatures. To do that, the predominant ratecontrolling kinetics during phase transformation have to be identified first. In this study, which is an extension of previous work,1,3,5–7 a more systematic and detailed investigation was conducted to understand devitrification kinetics and mechanism of Pyrex borosilicate glass. XRD analysis was used to determine degree of devitrification and devitrification rate. Devitrification kinetics were analyzed by using Avrami’s equations.8–10 Linear growth rate of cristobalite was determined by measuring the arm’s length of crystals on SEM micrographs as a function of time using an image analyzer and compared with those theoretically calculated.11 II. EXPERIMENTAL
As-received Pyrex borosilicate glass powder (Corning Code 7740, Corning, NY) was used in this study. The glass powder had a median size of 7 m and contained 81 wt% SiO2, 13 wt% B2O3, 4 wt% Na2O and 2 wt% Al2O3. The glass powder had a softening point of 800 °C. No crystallinity was detected by x-ray diffraction (XRD) analysis for the as-received glass powder. The powders were mixed with 5 wt% polyethylene glycol binder in 1-propanol. The suspension was deagglomerated by a high-energy ultrasonic horn, and 1752
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J. Mater. Res., Vol. 16, No. 6, Jun 2001 Downloaded: 26 Mar 2015
mixing was continued using a 3-dimensional mixer (Maschinenfabrik, Basel, Switzerland) for 2 h. The powder mixture was dried, ground, and uniaxially pressed at about 90 MPa to make pellets of 1.3-cm diameter and 0.3-cm height. Samples were sintered isothermally in air at temperatures in the range of 700 –1000 °C for various periods of time. Crystallinity of fired samples was determined by XRD analysis. Microstructure of the sintered samples was examined by SEM and TEM, and elemental distribution was determined by energy dispersive spectroscopy (EDS). Size of crystals was determined by an image analyzer. III. RESULTS AND DISCUSSION A. Identification of crystalline phase
XRD results for the samples fired at differ
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