Transparent bulk glass-ceramics containing ZnF2, K2SiF6, and KZnF3 nanocrystals are successfully obtained from xKF–xZnF2–(100 – 2x)SiO2 oxyfluoride glasses for the first time to the best of our ...knowledge. The glass transition temperatures of heat-treated samples increase with time and approach values that resemble the temperatures chosen for thermal treatment. During nucleation and crystal growth, the residual glass around the crystals is depleted in fluoride which as glass component usually leads to a decrease in viscosity. The crystallization behavior notably depends on the glass composition and changes within a small range from x = 20 to 22.5 mol %. The occurrence of liquid/liquid phase separation in dependence of the composition is responsible for the physicochemical changes. Two different microstructures of droplet and interpenetrating phase separation and their compositional evolution are observed by replica transmission electron microscopy technique in the multicomponent glassy system. This study suggests that the size and crystal phase of precipitated crystallites can be controlled by the initial phase separation.
The surface crystallization of non-ferroelectric, polar crystals from a glass often leads to piezoelectric surface layers. The surface layer not only shows a crystallographic orientation preference ...(texture) but also prefers one polarity during crystal growth. This cannot be explained by the common periodic bond chain theory or Bravais–Friedel–Donnay–Harker law. For the most frequently studied polar crystals in glass-ceramics, fresnoites of the general composition (Ba/Sr)2Ti(Si/Ge)2O7 or lithium disilicate (Li2Si2O7), the dipoles are, respectively, aligned antiparallel or parallel to the dominant growth direction after crystallization. Hence, the polarity of the crystals cannot be the decisive factor for texture development. Taking the structure of the respective unit cells into account shows that these crystals grow with the corners of coordination polyhedra oriented toward the bulk. This is in agreement with the coordination polyhedra rule, according to which crystal growth along the direction of polyhedral corners should be preferred while the edges show the second-highest growth rate. Furthermore, the planes parallel to the face of the coordination polyhedron undergo the slowest growth.
Depending on the composition of the glass and the annealing procedure supplied, thermal annealing of silicate glasses in the system SiO2/Al2O3/AlF3/Na2O/Gd2O3/SmF3 led to the precipitation of cubic ...or hexagonal NaGdF4 nanocrystals. The glasses were already phase separated after casting and formed a droplet phase supposedly enriched in fluorides and rare earths. The droplets had sizes mainly in the range from 80 to 120 nm. During annealing at temperatures ≥600 °C, multicore particles of cubic or hexagonal NaGdF4 were precipitated inside the amorphous droplet phase. Completely transparent hexagonal NaGdF4 containing nano-glass-ceramics could be derived from this system. Fluorescence spectra and fluorescence decay curves of Sm3+ show crystal phase dependent effects. Sm3+ doped hexagonal NaGdF4 exhibits notably different fluorescence emission spectra and longer fluorescence lifetimes than cubic NaGdF4.
Crystal engineering is a wide field aimed at producing a large range of crystalline phases in a multitude of forms suited for many specific applications. The method of electrochemically induced ...nucleation (EiN) enables to produce oxidic crystalline phases with an orientation alignment in a short period of time. It is based on applying an electric potential between a cathode and an anode contacted by a melt which is locally modified to trigger the nucleation of a desired phase. It is performed at temperatures high enough to prevent nucleation outside of the modified zone but below the liquidus temperature of the melt. Hence, the locally formed nuclei grow throughout the melt, only competing with crystals nucleated in the same area. As the crystals grow directly from the melt, this method also enables the growth of phases difficult to produce using other methods as the crystallization of secondary phases or phase separation during cooling can be prevented. The high temperatures applied lead to comparably fast crystal growth which can cause a high degree of growth selection up to the production of glass-permeated single crystals showing properties comparable to true single crystals, but grown in a matter of minutes.
Electrochemically induced nucleation refers to a method where an applied potential triggers an electrochemical reaction which changes the conditions in the melt so that nucleation can occur where it was previously not significantly possible.
Glasses of the mol composition 2 BaO·TiO2·2 SiO2 + x SiO2 (with x = 0.0, 0.1, 0.2, 0.5, 0.75, 1.5, 2.0, and 3.0 mol) were prepared, polished, and crystallized at 810 °C for 5 h or more. All glasses ...showed the sole crystallization of Ba-fresnoite (Ba2TiSi2O8). The crystal orientations at the immediate surfaces were analyzed by EBSD and showed that all glasses exhibit a nonrandom orientation distribution, i.e., oriented nucleation at the surface. While the glasses with x = 0.0, 0.1, and 0.2 show rather broad orientation preferences in the form of c-axes tilted from the surface by roughly 10–50°, the glasses with x = 1.5 or higher show discrete textures where specific crystal orientations are significantly preferred with orientation spreads of less than ±15°. While the glasses with x = 0 and 0.1 show polygon crystals at the surface and in the bulk, dendritic crystals are observed in the bulk of all other analyzed compositions. Phase separation solely occurred in the glass with x = 1.5 during the applied heat treatments.
In glasses with composition (100 − x)(9.9Na2O·8.8K2O·12.1CaO·6.6Al2O3·62SiO2)·xCaF2 the glass-transition temperature decreases with increasing CaF2 concentration. Samples with x = 12.5 were thermally ...treated at temperatures in the range from 520 to 560 °C for 1−80 h. This leads to crystallization of CaF2. The quantity of crystalline CaF2 increases with increasing crystallization temperature, while the mean crystallite size remains constant. The glass-transformation temperature of partially crystallized samples increases with increasing crystallization temperature and crystallization time and approaches a value equal to the temperature at which the samples were treated. This was explained by formation of a highly viscous layer enriched in SiO2 formed during crystallization, which acts as a diffusion barrier and hinders further crystal growth.
In the past few years, a new phase, Ba
0.5
Sr
0.5
Zn
2
Si
2
O
7
with negative thermal expansion has been described in the literature. Some excess of SiO
2
is necessary to produce glasses from which ...the Ba
0.5
Sr
0.5
Zn
2
Si
2
O
7
phase can be crystallized. Unfortunately, in such glasses usually surface crystallization occurs; however, the addition of nucleating agents such as trace quantities of platinum or relatively high quantities of ZrO
2
is necessary to achieve bulk nucleation. These additional components also affect the crystal growth velocity, which furthermore is different for crystal growth from the surface and in the bulk. In this paper, three different chemical compositions containing different ZrO
2
concentrations, where one composition additionally contains 100 ppm platinum, are studied with respect to their crystallization behaviour. Although the compositions do not differ much, the crystallization behaviour and also the crystal growth velocities are surprisingly different.
The elastic properties of quaternary glass-forming systems within the CaO–MgO–Al
2
O
3
–SiO
2
composition were evaluated. Their compositions differed by not more than 3 mol% for each component from ...each other. After melting, the exact chemical compositions were determined using X-ray fluorescence, and the densities were measured by Archimedes method and pycnometer. This property and its dependency upon small variations in the composition are the main focuses of this paper. Experimental elastic properties such as Poisson’s ratio and Young’s modulus were calculated by measured ultrasound wave velocities (longitudinal and shear) at room temperature. Model calculations of the Poisson’s ratios were not in satisfying agreement with the measured results. The experimentally determined data of the Young’s modulus values ranging between 87 and 91 GPa were compared to different model calculations, which were mostly smaller than the measured ones, ranging—depending on the model—between 70 and 93 GPa. The effect of MgO was also studied by comparing the glasses with similar compositions without any MgO.
Stoichiometric lithium disilicate glass was doped with up to 5 mol% ZrO
2
. The zirconia addition leads to higher viscosities and additionally to a change in the crystallization behavior. The glass ...transition temperatures and the onset of crystallization are shifted to higher temperatures. While in the undoped sample and the glass containing 1 mol% ZrO
2
, lithium disilicate is the sole crystal phase, in the glasses with higher zirconia concentrations, lithium metasilicate, quartz, and ZrO
2
precipitate. Furthermore, with increasing zirconia concentration, the volume nucleation rates are remarkably decreased and the induction times of nucleation are increased. For zirconia concentrations >2 mol%, volume nucleation was not observed even after long heat treatment times; instead, surface crystallization solely occurred. This cannot be explained only by the higher viscosities of the ZrO
2
-containing glass. Hence, zirconia acts as nucleation inhibitor for volume nucleation in stoichiometric lithium disilicate glass.
The thermal properties of the quaternary glass-forming CaO–MgO–Al
2
O
3
–SiO
2
system were evaluated. Different glass compositions with an expected liquidus temperature—the lowest temperature at ...which both the crystal phase and the melt are thermodynamically stable—below 1300 °C were melted from pure raw materials. Each oxide concentration did not deviate by more than ±3 mol% from that of the centre glass composition of 61.2 SiO
2
, 26.3 CaO, 8.6 Al
2
O
3
and 3.8 MgO. Viscosity data and the liquidus temperatures were determined, as well as the exact compositions of the respective glasses by X-ray fluorescence. It was of special interest for this study, whether small compositional changes have a stronger influence on
T
Liq
or
T
3
. The
T
3
-values, which are the temperatures attributed to a viscosity of 10
3
dPa s—showed a much larger variation and are in the range from 1263 to 1363 °C. An exchange of the network formers Al
2
O
3
or SiO
2
by network modifiers CaO and MgO resulted in a decrease in viscosity. The effect of MgO was also shown by a comparison with glasses of similar compositions but without any MgO. The exchange between Al
2
O
3
and SiO
2
did not show a significant difference in viscosity whereas the substitution of CaO by MgO increased the viscosity. The liquidus temperature varied between 1137 and 1192 °C—less than expected for a composition close to an eutectic system.
