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Journal of Non-Crystalline Solids 54 (1983) 163-171 North-Holland PuLIQUID IMMISCIBILITY AND PHASE SEPARATION IN THE QUATERNARY SYSTEMS K20-A! 203-CaO-SiO2 AND Na20-AI203-CaO-SiO 2
I. Introduction
At 1700°C, there is an equilibrium miscibility gap in the CaO-SiO 2 system between liquids containing about 70 mol% and 97 mol% SiO 2 [1]. Metastable extension to lower temperatures occurs which reaches a value of about 60 mol% SiO 2 at a temperature of 1000°C [2,3]. When N a 2 0 or K 2 0 or A1203 are added to the CaO-SiO 2 system, the temperature for equilibrium immiscibility is lower as the region of immiscibility is extended into the ternary field [1,3,4]. Nonequilibrium extension of the surface of the immiscibility region has been mapped to lower temperatures and for a wide compositional region in the ternary system N a 2 0 - C a O - S i O 2 [2], but not for the K 2 0 - C a O - S i O 2 or A1203-CaO-SiO 2 systems, which, on thermodynamic grounds, must behave similarly. Lowering of the immiscibility temperature with third components which lower the liquidus temperature is quite generally to be expected [5]. Such results are found when KzO or m1203 is added to the N a 2 0 - S i O 2 system [6], when m1203 is added to the L i 2 0 - S i O 2 system [7], when m1203 is added to the BaO-SiO 2 system [8], when N a 2 0 is added to the BaO-SiO 2 system [9], and so
on.
In the present study, we have extended observations of liquid-liquid 0022-3093/83/0000-0000/$03.00 © 1983 North-Holland
164
W.D. Kingery et al. / Liquid- liquid immiscibility and phase separation
Received 12 May 1982 Revised manuscript received 7 September 1982
Additions of A1203, Na20 or K20 all lower the immiscibility temperature for liquid-liquid phase separation in the CaO-SiO 2 system as the region of immiscibility is extended into the ternary field. Simultaneous additions of K20 and A1203 or of Na20 and A1203 are found to further extend the compositional region of immiscibility which reaches a maximum with equimolar additions of alkafi and alumina, These results are attributed to the ability of KA10/ or NaA102 units to substitute for SiO2 in the liquid structure. As a consequence of liquid-liquid immiscibility, wollastonite crystallizes from the calcia-rich liquid phase. STEM analysis has been applied to the phase-separated glasses to determine the approximate K20-AI203 ratio and overall composition of each phase to locate tie-lines for the liquid-liquid immiscibility. The tie lines are roughly parallel to the CaO-SiO 2 join.
W.D. K I N G E R Y , P.B. VANDIVER, I.-W. H U A N G and Y.-M. C H I A N G
Ceramics Division, Department of Materials Science and Engineering, Massachusetts Institute of Technology, Cambridge, MA 02139, USA
immiscibility and phase separation to the quaternary A1203-CaO-SiO2 and Na20-A12Oa-CaO-SiO 2.
systems K20-
2. Experimental
All samples were prepared from reagent grade silica gel, aluminum hydroxide, calcium carbonate, potassium carbonate, and sodium carbonate. Samples were wet-mixed in methyl alcohol, dried and melted. Four samples were melted as 150 g batches in fireclay crucibles and furnace cooled. Fibers were drawn and heated in a gradient furnace, which showed that opalescence occurred after heat-treatment in the temperature range 900 to 1400°C. Nineteen additional samples were melted as 10 g batches in platinum containers at 1450°C; after melting, they were withdrawn from the furnace for rapid air cooling. Most were clear and transparent, but a few were opalescent, indicating phase separation at temperature or during cooling. Most compositions studied contained (73.5 + 2.5) mol% SiO2, placing them about in the center of the 1200°C two-liquid region on the CaO-SiO z join. A few had silica contents as low as 70 and as high as 77 mol% SiO 2. Since all compositions are on the CaO-rich side of the consolute composition, the samples with lower SiO 2 concentrations should have lower immiscibility temperatures, but this effect should not be large relative to the scale of our measurements. Samples weighing 0.5 g were treated 4 h on platinum foil at temperatures of 900, 1000, 1100, 1200, 1300 and 1350°C, and the maximum temperature at which opalescence or opacity was observed was recorded. To characterize the nature of the phase separation leading to opalescence or opacity, fractured surfaces were etched with 1% H F 1-4 min at 0-20°C, coated with a thin layer of palladium and examined by scanning electron microscopy and EDAX. To obtain more lightly-etched samples suitable for evaluation of the volume fraction of phases present, samples were etched with a 1 N ammonium bifluoride solution 15 s at 10°C. While the nature of the equilibrium phase diagrams and data for liquid-liquid phase separation in the N a 2 0 - C a O - S i O 2 system suggest that tie lines radiate out from nearly pure SiO 2, this is not necessarily so in the quaternary system where the K20 and AI203 distribution are unknown. Direct determination of the separated phase compositions was attempted for these samples using a dedicated STEM (Vacuum Generators HB5). equipped with a field emission gun, which has a probe diameter of about 25 A at an accelerating voltage of 100 kV. Selected area electron diffraction was done on these same samples. Thin samples were prepared by ion-milling with argon, then coated with a thin layer of carbon to prevent charging, and examined. It was evident from both X-ray analysis of local compositions and imaging of these samples that electron irradiation (100 keV) causes considerable sample degradation. Conventional fixed-probe ( - 25 /~ diameter) analyses showed only Si and O lines in the X-ray spectrum. High magnification imaging of the