A threshold heating rate for single‐stage heat treatments in glass‐ceramics containing seed formers

Abstract

The development of glass-ceramic materials is often achieved using an elementary microstructural strategy that splits the tasks of seed formation and functionality between two types of crystals. This strategy requires customized time-temperature ceramization protocols, which have been so far implemented using empirical parameters. Here, a more fundamental approach is proposed: the extent of overlap O e between seed formation and volume crystallization is evaluated by calorimetric and dilatometric measurements, targeting the computation of a threshold heating rate q t for effective single-stage heat treatments. The applicability of this novel parameter is tested in TiO 2 -doped lithium magnesium aluminosilicate glass-ceramics, whose seed formation stage is thoroughly characterized by Raman spectroscopy and STEM. High-temperature X-ray diffraction demonstrates that insufficient seeding results in potentially weaker performances of the final products, due to large sizes and silica deficiency of the functional quartz solid solution crystals.

Glass formation and devitrification behavior of alkali (Li, Na) aluminosilicate melts containing TiO2

“…The addition of 5 mol% TiO 2 had also a substantial impact on the response of the glasses to a secondary heat treatment, as revealed by DSC upscans at 10 K min -1 (Fig. 3): (i) glass transition and crystallization shifted to lower temperatures in L1T05, L2T05, N1T05 and L1AlT05 as compared to their TiO 2 -free references (similar observations have been reported in other TiO 2 -doped aluminosilicate glasses [19,20,37,38]); (ii) in L1LiT05, the glass transition and crystallization onset appeared instead slightly higher than in L1Li; (iii) the main crystallization event exhibited a remarkable sharpening in L1T05, L2T05 and L1LiT05 as compared to their TiO 2 -free references; (iii) as previously noticed in similar aluminosilicate glasses and ascribed to the formation of TiO 2 -containing seeds [19], the strongest crystallization peak of L1T05 and L2T05 was preceded by a broader and less intense exothermic maximum (T x ). From these results, glass forming ability GFA and glass stability GS c and GS x parameters were calculated as defined in Section 2.3.…”

“…On the other hand, the addition of TiO 2 clearly facilitated devitrification in L1LiT05 with respect to its undoped parent glass, despite the fact that T g was virtually identical in both materials. This latter observation differs from the reduction of low-temperature viscosity usually detected in alkali peraluminous melts upon TiO 2 addition [19,20,37], possibly confirming the re-polymerizing role of this oxide previously suggested by other authors in aluminosilicate melts containing non-bridging oxygens [55]. The glass stability of L1T05 and L2T05, though seemingly unaffected as GS c , was instead heavily reduced by the exothermic event T x assigned to the formation of TiO 2 seeds, as expectable from the known nucleating efficiency of this oxide in LAS glass-ceramics [21].…”

“…As summarized in Table 4, TiO 2 was found to be retained in the amorphous structure of the samples only up to a certain threshold level, above which Ti-bearing crystalline phases could be invariably identified, with crystal sizes and shapes (Fig. 6 and supplementary material) that mostly resembled those classically obtainable during seed formation in glass-ceramics [15,18,19,21]. In other words, we can interpret most of these crystals as the product of low-temperature precipitation due to oversaturation during quenching, which could not be prevented despite the relatively fast cooling rate of our ADL system (roughly between 200 and 400 K s -1 , as shown in Fig.…”

“…Despite being investigated since the early days of glass-ceramic development [5], however, the fundamental laws controlling the nucleation efficiency of TiO 2 in different compositional systems are still not fully understood: while 4 mol% are more than sufficient to induce volume crystallization in lithium aluminosilicate glass-ceramics (LAS) [6], higher additions (6-10 mol%) have been shown to be necessary in magnesium aluminosilicate (MAS) [7], sodium aluminosilicate (NAS) [8] and other multicomponent glasses [9][10][11][12][13]; conversely, TiO 2 was found to detrimentally foster surface nucleation in calcium aluminosilicate matrices [14]. The steady improvement of our analytical capabilities has recently shed new light on the complexity of seed formation, particularly within the LAS and MAS systems: (i) incipient liquid-liquid phase separation [15], (ii) a coupled increase in average coordination number of Ti and Al during crystal nucleation [16][17][18], (iii) the resulting pervasive nanostructural heterogeneity of the residual amorphous matrix [19,20] and (iv) the occurrence of TiO 2 (B) as a metastable precursor to the formation of anatase and rutile [21,22] represent only some of the crucial phenomena involved in TiO 2 nucleation which were disentangled in the past few years.…”

A chemical threshold controls nanocrystallization and degassing behaviour in basalt magmas

“…In contrast, STR proved to be much more stable against crystallization: we observed in this sample a single broad exothermic event starting at 963 °C, i.e., ~285 °C above its glass transition (onset at 679 °C); the subsequent melting process was completed at 1251 °C. The behavior of ETN closely resembled that of synthetic melts in which the early precipitation of nanosized TiO 2 -bearing seeds is exploited to control the subsequent heterogeneous nucleation of aluminosilicate crystals, as for the production of conventional glass-ceramics 147 .…”