C -H. Chao, H.-Y. Lu/Materials Science and Engineering 4328 (2002)267-276 dendritc partial 000038o a :500nm b m (d) cracked matrix(SEM-SED) (c)early stage of nucleation and (d) interface between glass and the cracked matrix(SEM-BElh ticles dispersed in Fig. 5. General structure of the y= 19.5 sample sintered at 1100C for 48 h,(a) three general features, (b) spherical par Glassy phases amongst the dendritic arms etched away formed to a-cristobalite(Fig. 6(b)), although they are hen preparing for the polished sections have become a found only occasionally. They may have escaped from recess, which makes chemical analysis for their content the grinding stress when located within a residual pore by energy-dispersive X-ray spectroscopy(EDS)imprac- during preparing for the polished sections. a-Grains tical. Although crystallized features are similar to those larger than a l um can clearly be identified from the commonly found in metals, and in the silicate system of characteristic twinning. These grains confined Li,O-SiO2 or BaO-Sio2 [21], preferred crystallo- glassy matrix must have exceeded the critical size for graphic orientations of (101)a reported [13] for devit- phase transformation to a-cristobalite. rified glass surface was not seen in the secondary dendrites. Fig. 6(a) reveals the primary and secondary 3. 2. 2. y= 13.9 samples containing only B-cristobalite dendritic arms where their crystallization by constitu SEM microstructures of the samples sintered at tional supercooling is indicative by the gaps(shown by 1100C for 48 h, containing Na,O and Al, O3, each of arrow) existed between them when viewing along the 6.29 mol%(i. e y= 13.9), are presented in Fig. 7(a)-(c) longitudinal direction. The gap would have contained XRD results(Figs. 3 and 4(a)) have shown that these glass of higher Na2O-content and (so)etching rate than samples contain only B-cristobalite. a-Cristobalite char- the dendrites. A crystallization mechanism involving acterized by lamellar twins [19, 20] is not detected in the expulsion of Nat-ion has indeed been proposed [22] these samples with higher doping levels. The sintered for Na,O-containing borosilicate glass microstructure is represented by five distinctive features Not all ground and polished sample surfaces appear as indicated in Fig. 7(a):(1) residual pores located in to contain the spherical particles. Some of the crystals the glassy matrix beco ome more noticeable; (2)spherical hibiting the cellular growth morphology have trans- particles resemble one of the nucleation types from Fig272 C.-H. Chao, H.-Y. Lu / Materials Science and Engineering A328 (2002) 267–276 Fig. 5. General microstructure of the y=19.5 sample sintered at 1100 °C for 48 h, (a) three general features, (b) spherical particles dispersed in cracked matrix (SEM-SEI), (c) early stage of nucleation and (d) interface between glass and the cracked matrix (SEM-BEI). Glassy phases amongst the dendritic arms etched away when preparing for the polished sections have become a recess, which makes chemical analysis for their content by energy-dispersive X-ray spectroscopy (EDS) imprac￾tical. Although crystallized features are similar to those commonly found in metals, and in the silicate system of Li2O–SiO2 or BaO–SiO2 [21], preferred crystallo￾graphic orientations of (101) reported [13] for devit￾rified glass surface was not seen in the secondary dendrites. Fig. 6(a) reveals the primary and secondary dendritic arms where their crystallization by constitu￾tional supercooling is indicative by the gaps (shown by arrow) existed between them when viewing along the longitudinal direction. The gap would have contained glass of higher Na2O-content and (so) etching rate than the dendrites. A crystallization mechanism involving the expulsion of Na+-ion has indeed been proposed [22] for Na2O-containing borosilicate glass. Not all ground and polished sample surfaces appear to contain the spherical particles. Some of the crystals exhibiting the cellular growth morphology have trans￾formed to -cristobalite (Fig. 6(b)), although they are found only occasionally. They may have escaped from the grinding stress when located within a residual pore during preparing for the polished sections. -Grains larger than 1 m can clearly be identified from the characteristic twinning. These grains confined in a glassy matrix must have exceeded the critical size for phase transformation to -cristobalite. 3.2.2. y=13.9 samples containing only -cristobalite SEM microstructures of the samples sintered at 1100 °C for 48 h, containing Na2O and Al2O3, each of 6.29 mol% (i.e. y=13.9), are presented in Fig. 7(a)–(c). XRD results (Figs. 3 and 4(a)) have shown that these samples contain only
-cristobalite. -Cristobalite char￾acterized by lamellar twins [19,20] is not detected in these samples with higher doping levels. The sintered microstructure is represented by five distinctive features as indicated in Fig. 7(a): (1) residual pores located in the glassy matrix become more noticeable; (2) spherical particles resemble one of the nucleation types from Fig.