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.     

Plasma-Sprayed-Yttria Layers for Corrosion Resistance

Crucible corrosion in the growth of YBa₂Cu₃O_{7- x} (YBCO) superconducting single crystals is one of the major obstacles in obtaining high-quality crystals. Both metallic and ceramic crucibles have shown significant corrosion that affects the size and properties of YBCO crystals. Plasma spraying has been successfully used to produce yttria coatings on alumina substrates. Yttria coatings with thicknesses between 30 and 100 μm have shown corrosion rates of 3 to 4 μm/h; these rates are 5 to 10 times lower than that of the alumina substrate.     

Acicular Mullite Crystals in Vitrified Kaolin

The microstructure of vitrified kaolin ceramic tapes has been studied via scanning and transmission electron microscopy (SEM and TEM). The sintered samples contained crystalline phase of predominantly stoichiometric mullite (3Al₂O₃·2SiO₂), which consisted of high aspect ratio, acicular crystals that are often referred to as secondary mullite. These crystals were interlocked and embedded in an aluminosilicate glass matrix of inhomogeneous composition. The glass matrix contained an average of ∼3.63 wt% K as determined by energy-dispersive X-ray analysis (EDS), whose composition could be approximated to 5Al₂O₃·16SiO₂·0.1MgO·0.3K₂O·0.15TiO₂·0.12Fe₂O₃. The acicular crystals have approximately the stoichiometric composition of Al₂O₃:SiO₂= 3:2. They have grown along a specific crystallographic orientation along the [001] axis. The crystal growth front exhibited facetting on the {110) planes with microfacetting on both the {100) and {010) planes.     

Fabrication of Transparent Tellurite Glasses Containing Potassium Niobate Crystals by an Incorporation Method

To fabricate transparent oxide glasses containing ferroelectric KNbO₃ crystals, a new method in which KNbO₃ particles are directly incorporated into TeO₂─K₂O─Nb₂O₅ glasses has been developed. Transparent TeO₂-based glasses containing KNbO₃ crystals with a diameter of ∼ 10 μm have been first successfully fabricated by adjusting temperature and time for incorporation. A small difference in the refractive indexes, n , between TeO₂-based matrix glasses ( n = 2.0) and incorporated KNbO₃ crystals ( n = 2.21) is a significant reason for the transparency. This new method is applicable for the fabrication of new transparent glasses containing other functional materials with high refractive indexes.     

Growth of Barium Ferrite Single Crystals

A study has been made of the growth of barium ferrite single crystals from a sodium carbonate flux. Crystals weighing up to 5.6 gm. have been grown by cooling melts slowly from about 1250°C. The phase diagram of the system BaO-Na₂O-Fe₂O₃ has been studied in the region of the BaFe₁₂O₁₉ composition. The effects of variations in batch composition, heating cycle, and seeding have been studied from the point of view of crystal yield and quality.     

Mechanical Properties of Single-Crystal α-Si3N4

Single crystals of α-Si₃N₄ several millimeters in diameter and several millimeters long have been grown by chemical vapor deposition. Some of the microstructural and mechanical properties have been evaluated using X-ray diffracometry, optical and transmission electron microscopy, and high-temperature microhardness testing. The crystallographic growth direction determines the quality of the crystals, including the density of internal microcracks and the nature and quantity of special boundaries. The measurement of crack lengths associated with microindentations has shown that cleavage in α-Si₃N₄ is relatively isotropic. Finally, indentation fracture toughness values agree well with theoretical predictions based on recent bond-energy calculations.     

High-Strength Zirconia Fibers

Fine-grained polycrystalline zirconia fibers have been formed from an acetate precursor. The fibers contained a Y₂O₃ additive, which inhibited grain growth (grain size ≤0.5 μm) and allowed the tetragonal phase to be retained at room temperature. Fibers with diameters in the range 2 to 5 μm had strengths in the range 1.5 to 2.6 GPa.     

Formation of β-Silicon Nitride Crystals from (Si,Al,Mg,Y)(O,N) Liquid: I, Phase, Composition, and Shape Evolutions

Precipitation, growth, and coarsening of Si₃N₄ crystals in (Si,Al,Mg,Y)(O,N) liquids at 1680°C has been studied. The initial nucleation of β-Si₃N₄ occurs mostly on α-Si₃N₄ because of the very high supersaturation of the liquid. After a brief period of growth, the crystals then undergo accelerated coarsening, decreasing the crystal concentration by almost 100 times with little change in the total crystal volume. Meanwhile, the crystals gradually transform from β-Si₃N₄, by substituting Si-N with Al-O, to β'-SiAlON of various compositions. The evolution of aspect ratio strongly depends on the Si/(Al,Mg,Y) ratio, which is rationalized by cation segregation to the interface driven by the acidity-basicity differential between the liquid and the crystal.     

Grain Growth Control and Solid-State Crystal Growth by Li2O/PbO Addition and Dislocation Introduction in the PMN–35PT System

Grain growth behavior and solid-state single crystal growth (SSCG) in the Pb(Mg_1/3Nb_2/3)O₃–35 mol% PbTiO₃ (PMN–35PT) system have been investigated with varying Li₂O/PbO ratios. The effect of dislocation density on crystal growth has also been studied. For SSCG, a BaTiO₃ single-crystal seed was embedded in a polycrystalline PMN–PT matrix. During annealing, a PMN–PT single crystal grew from the seed at the cost of the small matrix grains. Addition of Li₂O dopant first enhanced and then reduced abnormal grain growth in the matrix. In the 2 mol% Li₂O and 6 mol% PbO excess PMN–PT samples annealed at 1200°C, considerable single-crystal growth occurred without formation of abnormally large grains in the matrix. Increasing the dislocation density in the BaTiO₃ seed crystal resulted in enhanced growth of single crystals. These results were explained in terms of interface reaction-controlled nucleation and growth, based on crystal growth theories.     

Formation of β-Silicon Nitride Crystals from (Si,Al,Mg,Y)(O,N) Liquid: II, Population Dynamics and Coarsening Kinetics

Precipitation, growth, and coarsening of Si₃N₄ crystals in (Si,Al,Mg,Y)(O,N) liquids at 1680°C has been studied. Contrary to the common observation in kinetics, coarsening rates of crystals in length and width are found to accelerate when the total volume of crystals remains little changed. This is attributed to the concomitant β-Si₃N₄ to β'-SiAlON transformation, which introduces an additional driving force for crystal dissolution and reprecipitation. As a result of the additional driving force, which has a nonmonotonic size dependence, the normalized size distribution is expected to evolve with time, initially broadening, then shifting skewing as the transformation passes the midpoint, and finally converging to a sharp distribution as the transformation completes. These evolutions have been observed in all the compositions studied.     

Study of Devitrification of Lithium Glass

An etching technique has been found which allows the devitrification phenomena in glass to be followed. Using this technique a study was made on 30 mole % Li₂O-SiO₂ glass at temperatures of 520°, 560°, and 600°C. By studying the average crystal size versus time at a given temperature, the growth rate was determined. It was found to be 7.5 × 10⁻⁴, 8 × 10⁻³, and 3 × 10⁻² cm. per hour for the three temperatures. The nucleation of crystals appeared to be linear with time at each temperature. An activation energy of 49 kcal. per mole was found for the growth process. Crystals were identified as Li₂Si₂O₅.     

Growth of Large Optical-Quality Yttrium and Rare-Earth Aluminum Garnets

An improved flux consisting of PbO-PbF₂ and B₂O₃ was developed for the growth of garnets. Rare-earth aluminum garnet crystals weighing in excess of 100 g and modified yttrium aluminum garnet crystals weighing up to 60 g were obtained using this flux. Lead contamination was reduced to noncritical levels in these crystals by working with large excesses of A1₂O₃ in the melt. Excellent optical quality has been produced as indicated both by visual examination and by outstanding laser performance.     

Thermal Expansion of Y2SiO5 Single Crystals

The thermal expansion of Y₂SiO₅ crystals has been measured for the principal crystallographic directions and two orthogonal directions in the (010) plane in the temperature range 25° to 200°C. This monoclinic crystal has strongly anisotropic expansions with coefficients which range from 0.6 × 10⁻⁶/°C for [100] to 11.4 × 10⁻⁶/°C for [001]. Third-order polynomials have been calculated from the expansion curves. Data for the β angle and cell volume as a function of temperature are also given. The thermal expansion of Y₂SiO₅ crystals is not affected by doping with 5% Tb.     

Observation and Control of Deviations in Molar Composition in Single-Crystal Growth of Mixed Oxides

A technique is described for the preparation of optically transparent crystals of CaO.2Al₂O₃, 12CaO.7Al₂O₃, and 3Y₂O₃.5Al₂O₃. The main difficulties in obtaining optically transparent crystals of these materials by direct Czochralski growth from the component oxides arise from deviations in molar composition.     

Patterning of Non-Linear Optical Crystals in Glass by Laser-Induced Crystallization

This paper reports recent progress in the patterning of non-linear optical crystals on the glass surface by laser irradiation. Two techniques for the writing of crystal lines have been developed, i.e., rare-earth (samarium) atom heat processing and transition metal atom heat processing, in which a continuous wave Nd:YAG laser (wavelength: λ=1064 nm) is irradiated to the glasses containing rare-earth (RE: Sm³⁺, Dy³⁺) ions or transition metal (TM: Ni²⁺, Fe²⁺, V⁴⁺) ions. The writing of crystal lines such as β-BaB₂O₄, Sm _x Bi_{1− x} BO₃, and Ba₂TiGe₂O₈ showing second harmonic generations has been successful. It is clarified from the azimuthal dependence of second harmonic intensity and polarized micro-Raman scattering spectra that crystal lines consist of highly oriented crystals along the crystal line growth direction. It is also possible to write two-dimensional crystal bending or curved lines by just changing the laser scanning direction. The mechanism of the laser-induced crystallization has been proposed.     

Floating Zone Crystal Growth and Phase Equilibria: A Review

The thermal-imaging floating zone technique can be used to grow crystals of yttrium iron garnet (Y₃Fe₅O₁₂), aluminum-doped yttrium orthoferrite (YFe_0.88Al_0.12O₁₂), and magnetite (Fe₃O₄), which represent peritectic compounds, solid-solution crystals, and atmosphere-sensitive materials, respectively. The reactions involved in floating zone crystal growth are explained on the basis of phase diagrams. A review of crystal growth reports, including unpublished findings by the present authors, demonstrates how the crystallization processes, the reaction with the ambient atmosphere, and the composition variation in the obtained crystals can be explained or controlled on the basis of phase equilibrium. The floating zone technique is applicable to a variety of materials and remains a handy tool for materials research; however, its industrial application may be limited.     

Surface and Bulk Crystallization of Glass-Ceramic in the Na2O-CaO-ZnO-PbO-Fe2O3-Al2O3-SiO2 System Derived from a Goethite Waste

A goethite waste from zinc hydrometallurgical processes has been used to produce a glass-ceramic in the Na₂O-CaO-ZnO-PbO-Fe₂O₃-Al₂O₃-SiO₂system. The surface and bulk microstructure of this glass-ceramic have been studied by using scanning electron microscopy (SEM) and transmission electron microscopy (TEM). The surface was comprised of crystalline and glassy areas. Two different types of crystalline growth and two morphologies were observed in the crystallized and glassy zones, respectively. The bulk microstructure was composed of a homogeneously distributed dendritic network comprised of small crystallites of magnetite. A glassy matrix was observed surrounding the magnetite network. Further heat treatment produced the precipitation of a nonstoichiometric zinc ferrite with magnetite crystals, being the nucleating agents of the secondary phase.     

Congruent Composition and Li-Rich Phase Boundary of LiNbO3

The congruent composition of LiNbO₃ was determined precisely by correlating crystal growth compositions with the Curie temperatures of samples quenched from a melt before and after crystal growth and of various sections of crystals grown from that melt. The initial melt composition was determined by control of Li₂CO₃ and Nb₂O₅ contents. The melt compositions were varied from 47 to 49 mol% Li₂O. The variation of T_e with melt composition was found to follow T_e = 9095.2 − 369.05C + 4.228C², where C is mol% Li₂O. High-temperature DTA was used to determine T_e with a precision of ±2°C. Above 1000°C the Li-rich phase boundary was found to be a function of temperature by determining the weight gain after complete lithiation of congruently grown crystals. This boundary curves slightly toward lower Li₂O content as temperature increases. The congruent composition of LiNbO₃ contains 48.45 mol% Li₂O and has a measured Curie temperature of 1138°± 2°C.     

Phase Diagram for a Portion of the System Li2O-Nd2O3-P2O5

In the ternary system Li₂O-Nd₂O₃-P₂0₅, part of the phase diagram relevant to the growth of single LindP₄O₁₂ (LNP) crystals was examined. LNP melts incongruently and decomposes into NdP₃O₉ and liquid at the peritectic temperature of 970°C. For the crystal growth, an Li₂O-P₂O₅ mixture should be used as a flux. The melt compositions from which LNP nucleates were clarified.     

Liquid-Phase Growth of Small Crystals for Seeding α-SiAlON Ceramics

Single-phase small crystals of Li-, Mg-, Ca-, Y-, Nd-, and Yb-α-SiAlONs have been obtained by liquid-phase sintering for various compositions and processing conditions. These crystals are suitable for seeding grain growth in α-SiAlON ceramics. The influence of chemical and processing parameters (starting composition and powders, green density, liquid content, heating schedule, nitrogen pressure, and temperature) on the size and morphology of seed crystals has been investigated. The results are compared with those for β-Si₃N₄ crystal formation, and the differences are discussed in terms of nucleation and growth kinetics during liquid-phase sintering.