Flame Fusion Research Articles

Steady-State Studies in the Conventional Verneuil Process—A Working Model for Burnout

A working model for burnout of additives is presented for the flame fusion growth of crystals (Verneuil process). The case of α—Al2O3:Cr3+ (ruby) is of special interest since the additive (chromium) manifests more than one valence state in the powder, the conventional form of the feed material.

Steady-State Studies in the Conventional Verneuil Process—the Trajection (Melting) Zone

In the conventional Verneuil growth of ruby, it is shown that certain parameters pertinent to the final concentration of the additive in the crystal product (solid solution) are: starting concentration, surface distribution of additive in the powder, trajection distance, and growth rate. The effects of these factors are studied at the trajection (or melting) zone.

Surface and Bulk States of Additives in Alumina Powder

A significant fraction of the losses in the additive, in the crystal growth (conventional Verneuil process) of ruby, is traced to powder technology. Valence states of the additive in the powder are studied and correlated with their escaping tendencies. A working model is proposed and supported by kinetic studies on the powder. From this model is derived certain predictions which are significant to the Verneuil crystal growth of solid solutions.

Dislocations and Stacking Faults in Rutile Crystals Grown by Flame-Fusion Methods

Single crystals of rutile, grown by the Verneuil method, have been chemically thinned and examined by electron transmission microscopy. The density and distribution of dislocations agree with that expected from etch pit data. This density is at least two orders of magnitude below that for rutile films formed by oxidizing titanium carbide. The etching of dislocations and stacking faults is described. Some preliminary results of high-temperature studies in the microscope are reported. These include the discovery of a hitherto unknown slip system, {001} 〈100〉.

Infrared Properties of NiO and CoO and Their Mixed Crystals

The room-temperature reststrahlen spectra of flame fusion grown NiO and CoO, as well as those of three intermediate members of this solid solution series, are presented. A full Kramers-Kronig analysis on all members has been performed and checked with damped oscillator calculations. The resulting refractive indices, extinction coefficients, dielectric constants, effective ionic charges (Szigeti q*), and characteristic frequencies are presented along with the lattice constants and are discussed in the light of their interrelation.

The influence of growth orientation on the texture of synthetic ruby

The frequency of occurrence of mosaic imperfections in ruby single crystals (grown by flame fusion from the vapour phase) has been shown to be markedly dependent on the growth orientation. It is least in crystals having the c axis normal to the growth direction, An explanation is given in terms of the dislocation systems in ruby.

Time dependence of the electrical conductivity in strontium titanate single crystals

Measurements of the apparent electrical conductivity of high resistivity single crystal slices of SrTiO3 are reported. It is shown that the behaviour is strongly dependent on the electrode material employed. In particular gold and silver anodes lead to a substantial increase of current with time followed by a plateau. It is suggested that the effects observed could be accounted for by the formation of an anodic layer at the anode-crystal interface leading to enhanced hole injection. Etching experiments are described which lead to distinct results depending on whether the crystals were grown by the flame fusion method or the fluxed melt method. These results are discussed in relation to the electrical measurements.

Electron microscopy and diffraction of synthetic corundum crystals I. Pure aluminium oxide grown by the verneuil process

Abstract Single crystal aluminium oxide has been chemically thinned with the aid of a jet polishing device and examined in the electron microscope. The behaviour of the dislocations in various plastically deformed crystals has been examined. Tangles of dislocations, commonly found in metals and ionic solids, are absent, and dislocation densities in glide bands are low. Grown-in dislocations apparently participate in the glide process; therefore, they are not strongly pinned by impurities at temperatures permitting plastic deformation. The occurrence of dislocation dipoles is taken as evidence for the existence of cross slipping of basal dislocations at temperatures below those required for slip on {1120} prism planes. No definite evidence for dislocation dissociation has been found and it appears that, normally, such splitting is rare in samples of bulk sapphire. Preliminary studies of dislocation motion in the microscope at temperatures ∼1500°c are reported and discussed.

Ultrasonic Attenuation in Al2O3

Ultrasonic attenuation was measured in single-crystal aluminum oxide (Al2O3) using the pulse technique. Measurements were made at frequencies from 100 to 3000 Mc/sec and at temperatures 4° to 300°K. The observed ultrasonic losses are explained by a scattering from microscopic bubbles in single crystals grown by flame fusion.

Optical Properties and Laser Thresholds of Thirty-nine Ruby Laser Crystals

Measurements were made of optical path uniformity, chromium concentration, fluorescence linewidth, large-angle scatter, and laser threshold on 39 flame fusion rubies. Results show that index of refraction variation is explainable by variation of chrome concentration and wander of the optic axis. Residual stress plays little role in optical defects, but may broaden the 77°K linewidth. Optical quality of lasers cut from regions of boules selected by interferometry remains constant after cutting, but warping destroys their parallelism. Threshold increases with chromium concentration, indicating that the rods are optically thick at the pumping bands. Threshold is not correlated with other optical properties, however.

Concentration and textural inhomogeneities in synthetic ruby

Inhomogeneities of structure in synthetic ruby, grown by flame fusion from the vapour phase, have been studied by x-ray and etching techniques and correlated with chemically estimated variations in chromium concentration. The highest concentrations of chromium were found in those parts of the crystal where there was greatest mosaic misorientation and where the dislocation density was greatest.

STIMULATED EMISSION FROM Y2O3:Nd3+

First Page

A Simple Flame Fusion Apparatus

THE need for large single crystals of refractoryoxides has been ever increasing in recent years. In many instances further technological advances are being hampered by the lack of materials of suitable purity and perfection. One of the major sources of contamination of materials prepared at high temperature is the container in which the material is heated. The flame fusion technique is one high-temperature crystal growth method which overcomes the container problem as the material under investigation acts as its own support. A rather complete review of the advantages, limitations, and operation of flame fusion furnaces is given by Popov1 in a book edited by Shubnikov and Sheftal.

Flame Fusion Growth of C-Type Rare-Earth Oxides

First Page

Behavior of Chromium in the System MgA12O4– A12O3

Single crystals of chromium‐doped magnesium aluminate spinels with varying amounts of excess alumina have been grown by the flame fusion process. Presumably these crystals contain stoichiometrically significant amounts of defects. When these chromium‐doped spinels are annealed at temperatures below their melting point, the spinel structure cannot tolerate such a high concentration of defects and at least some of the excess alumina is exsolved. The chromium ion in magnesium aluminate spinel is pink, whereas in magnesium aluminate spinel containing excess alumina it is green. When exsolution of the alumina occurs during annealing, the aggregate changes to pink. The behavior of the chromium ion is monitored by observing the significant changes in the optical absorption as well as the emission or fluorescence spectra for both single crystals and polycrystalline materials in the system [MgA12O4– A12O3] :Cr.

Synthesis of Calcium Titanate Single Crystals by Flame Fusion Technique

A technique has been developed for growing un‐fractured calcium titanate boules up to 25 mm long and 12 mm in diameter by flame fusion. Large untwinned sin.gle crystals have been prepared from which optical properties, density, hardness, dielectric constant, and melting point have been determined.

Microwave appraisal of maser crystals

The potentialities of microwave spectrometer techniques in assessing the purity and crystalline quality of synthetic maser crystals are illustrated by a comparison of the paramagnetic properties of ruby samples grown from the vapour phase and from powder by flame fusion. Typical spectra presented are discussed in relation to complementary chemical and spectrographic analysis, and it is shown the information obtained is of direct relevance to the accurate control of methods of crystal growth.

Magnetic Anisotropy Measurement of MnO Single Crystal

The magnetic anisotropy of MnO single crystal, grown by the Verneuil method, was measured using a torque balance. The results of the torque measurements at liquid air temperature could be summarized within experimental errors by a formula \begin{aligned} T{=}\text{C}_{1}\text{H}^{2}\sin (2\theta+\varepsilon_{1})+\text{C}_{2}\text{H}^{4}\sin (4\theta+\varepsilon_{2})+\text{C}_{3}\text{H}^{6}, \end{aligned} where H is the applied field intensity, θ the angle of rotation of H relative to the crystal, and e 's and C's are constants which depend in a certain way on the kind of crystallographic axis of suspension. In the case where [111] is the direction of suspension, C 2 =0. The C 3 H 6 term changed its sigh when the rotation was made in the revere sense; thus it represents a rotational hysteresis term. Alternative theoretical interpretations of the new 4θ-term are presented: it arises either from a possible small anisotropy energy within the easy plane of magnetization, or from reversible movements of the ...

Growth of Single Crystals of Incongruently Melting Yttrium Iron Garnet by Flame Fusion Process

Single crystals of yttrium iron garnet (Y3Fe5O12) have been grown using the flame fusion process, even though the compound is reported to melt incongruently. The growth of these single crystals involves a mechanism different from that which has been proposed for the growth of single crystals of incongruently melting mullite. Crystal boules were grown at varying linear growth rates and analyzed with chemical, X‐ray, and metallographic techniques. With high linear growth rates, the samples are uniformly polycrystalline and three‐phase, containing Fe2O3, YFeO3, and Y3Fe5O12. When slow linear growth rates are used, single‐crystal Y3Fe5O12 can be grown. The mechanism is as follows: At the beginning of growth the first phase to precipitate is YFeO3, and during this stage in growth the molten cap becomes enriched in Fe2O3, compared with the Y3Fe5O12 composition. The liquid cap composition thus changes to the limit of the peritectic on the Fe2O3‐rich side, and Y3Fe6O12 then crystallizes from the bottom of the melt as Y3Fe5O12 powder is added to the top of the molten cap. The central sections of these boules are single‐crystal yttrium iron garnet.

ベルヌーイ法によるルチル単結晶合成に及ぼす不純物鉄の影響

ベルヌーイ法によるルチル単結晶合成に及ぼす不純物鉄の影響