Growth of lithium borate crystals from the vitreous state
- PDF / 1,289,071 Bytes
- 6 Pages / 576 x 792 pts Page_size
- 0 Downloads / 182 Views
The morphology and the growth mechanism of lithium borate crystals from the vitreous state have been studied for various compositions, X = B/(Li + B), from 0.62 to 0.75. Crystalline phases and morphology of grown crystals varied with the composition. Octahedral and/or spherical Li 2 B 4 O 7 crystals are seen in the specimen with X = 0.62, 0.64, 0.67, and 0.68. The spherical crystal is composed of an octahedral core and fibrous crystals. The size of the core varied with the composition of the starting glass, X. The fibrous crystal growth is supposed to arise from the morphological instability due to the compositional variation caused by the solute pileup at the growth interface.
I. INTRODUCTION
II. EXPERIMENTAL PROCEDURE
The crystallization of glass, which is always thought of as "frozen melt", is particularly suitable for the investigation of the mechanism of diffusion-controlled crystal growth, because mass transports are dominated by the atomic diffusion in glass. Since the crystal growth from the vitreous state is interruptible by quenching,1"6 the growth kinetics can be most directly determined by observation of growing interface in partially crystallized specimen. In order to study the effects of composition on the crystal morphology and the mechanism of diffusioncontrolled crystal growth, crystallization of vitreous lithium borates has been intensively investigated. Lithium borates have the following advantages for studying the crystal growth mechanism using revivification processes, (i) Lithium borates with various compositions can easily be vitrified by rapid cooling.7 (ii) Crystallization of glass can be compared with crystal growth from melt, because lithium borate crystals can be grown from both the melt and the glass with the same composition, (iii) Crystallization process can be observed by an optical microscope, because glass and crystal are colorless and transparent.
A series of lithium borates was used for experiments. The boron mole fraction, X = B/(Li + B), of specimen and corresponding crystalline phases are shown in Table I. The lithium borates with X from 0.62 to 0.75 were prepared using highly pure (4N) Li2B4O7, Li2CO3, and B 2 O 3 powders (Tomiyama Pure Chemicals Ltd.). Powders were weighed and pulverized, and then were poured into a graphite container and heated up to 1000 °C in a quartz tube filled with Ar gas. After homogenizing, the melt was quenched to room temperature. A transparent colorless ingot was obtained by this process.9 Vitreous specimens sliced about 2 mm thick were mirror polished. The as-quenched vitreous specimen was annealed isothermally at 800 °C under the microscope. Crystallization was interrupted by quenching to room temperature. The specimen was changed from colorless and transparent to opaque by crystallization. The crystalline phase was identified by x-ray diffraction analysis (XRD) using a Rigaku x-ray diffractometer (RINT 1000) with Cu Ka radiation and a Ni filter, at a scanning rate (20) of
In addition to these advantages, the crystallization of vitreous lit
Data Loading...
