As-grown Rod Research Articles

Evolutions of rod diameter, molten zone and temperature gradient of oxide eutectic ceramics during laser floating zone melting

Laser floating zone melting (LFZM) is an important directional solidification (DS) technique in oxide ceramic fabrication owing to its very high temperature gradient and rapid cooling rate. In this work, variations and evolutions of rod diameter, molten zone, and temperature gradient are explored in fabrication of Al2O3-based eutectic ceramics. At low solidification rate of 4 μm/s, periodical oscillations with almost the same period (~275 s) are produced in melting zone shape/size, as-grown rod diameter, temperature distribution, and temperature gradient. The temperature gradient fluctuates between 3.6 × 103 K/cm and 6.9 × 103 K/cm with an average value of about 5.3 × 103 K/cm. As increasing the solidification rate, the temperature gradient decreases and the periodical oscillation weakens. Based on the evolutions of the temperature field, interface location and molten zone shape, the formation mechanism of periodical oscillation during LFZM processing is evaluated.

Low-Temperature Properties of Whispering-Gallery Modes in Isotopically Pure Silicon-28

Whispering Gallery (WG) mode resonators have been machined from a boule of single-crystal isotopically pure silicon-28. Before machining, the as-grown rod was measured in a cavity, with the best Bragg confined modes exhibiting microwave $Q$-factors on the order of a million for frequencies between 10 and 15 GHz. After machining the rod into smaller cylindrical WG mode resonators, the frequencies of the fundamental mode families were used to determine the relative permittivity of the material to be $11.488\pm0.024$ near 4 K, with the precision limited only by the dimensional accuracy of the resonator. However, the Q-factors were degraded by the machining to below $4\times10^4$. Raman spectroscopy was used to optimize post-machining surface treatments to restore high $Q$-factors. This is an enabling step for the use of such resonators for hybrid quantum systems and frequency conversion applications, as silicon-28 also has very low phonon losses, can host very narrow linewidth spin ensembles and is a material commonly used in optical applications.

Directional solidification and growth characteristics of Al2O3/Er3Al5O12/ZrO2 ternary eutectic ceramic by laser floating zone melting

Directionally solidified Al2O3/Er3Al5O12/ZrO2 ternary eutectic ceramic in situ composite rods with length of 110 mm have been fabricated by laser floating zone melting. The microstructural characteristics of steady growth zone, initial growth zone and solid/liquid interface are investigated under high temperature gradient. In the steady growth zone, the eutectic spacing (λ) is rapidly decreased as increasing the growth rate (V), and the corresponding relationship between growth rate and eutectic spacing is determined to be λ = 11.14 × V −1/2. The temperature gradient has been measured to be about 5.3 × 103 K/cm. In the initial growth zone, the melting process and temperature distribution are recorded by infrared thermal imager, and several unstable complex microstructures are observed. In the quenched zone, the regular eutectics with minimum eutectic spacing of 200 nm are obtained. Moreover, the solid/liquid interface during solidification shows convex interface morphology and the interface height is gradually decreased as increasing the growth rate. The eutectic growth behaviors at the center and edge of the as-grown rod are compared and discussed.

Enhanced electrical and thermoelectric properties from textured Bi1.6Pb0.4Ba2Co2Oy/Ag composites

Bi1.6Pb0.4Ba2Co2Oy/3 wt% Ag composites have been textured using the laser floating zone technique. Microstructural observations have shown the formation of relatively high amounts of secondary phases and cracks along the plate-like grains in the as-grown materials. In spite of these microstructural features, electrical properties have not been drastically decreased. Annealing of these as-grown rods has enhanced the grains connectivity, decreased the contents of the secondary phases, and healed the cracks, thereby increasing their electrical performances. It has been deduced that oxygen content in the thermoelectric phase has not been significantly modified through annealing, as demonstrated by the very similar Seebeck values at room temperature in the as-grown and annealed samples. As a consequence of the low electrical resistivity values in the annealed materials, they reached higher power factor values than the as-grown ones. The highest power factor at room temperature is in the range of those reported for single crystals, while at 650 °C, it is the highest reported in Bi–Ba–Co–O polycrystalline family so far.

Templated Electrosynthesis of Zinc Oxide Nanorods

Electrosynthesis of zinc oxide has been performed in track etched polymer membranes to yield nanorods of defined diameter and controlled length. The electrodeposition process involves two steps: (i) electroreduction of either hydrogen peroxide or nitrate ions to alter the local pH within the pores and (ii) precipitation of the metal oxide within the pores. Synthesis at 22 °C via the reduction of hydrogen peroxide yielded polycrystalline zinc oxide nanorods. When deposition was performed at 90 °C, using the reduction of nitrate to control the local pH, zinc oxide nanorods which displayed the same growth direction along their entire length were obtained. The length of all as-grown rods increased with the integrated charge passed. The growth direction of the ZnO rods obtained at the higher temperature was unusual, being perpendicular to the (101̄1) plane.

Single-crystal growth and superconductivity of (La 1−xSr x) 2CaCu 2O 6+δ

The effects of growth conditions and composition on the crystal growth of La-2126-phase superconducting materials have been studied by the floating-zone method. The composition of La-2126-phase single crystals was (La 1− x Sr x ) 2CaCu 2O 6+ δ ( x = 0.0 7 5 – 0.2 5 ). The crystals were grown under oxygen pressures from 1 bar to 11 bar. A smooth planar solid–liquid growth interface of (La 1− x Sr x ) 2CaCu 2O 6+ δ ( x = 0.0 7 5 ) tended to break down into a cellular interface when the growth velocity was higher than 0.5 mm/h, or the Sr concentration x > 0.0 7 5 . When the planar solid–liquid growth interface broke down into a cellular interface, the single-crystal size in an as-grown rod decreased abruptly and the as-grown rod was not of single phase. Various small sizes of CaO second phases were found inside the as-grown single crystals of (La 1− x Sr x ) 2CaCu 2O 6+ δ ( x ⩾0.125). Based on the occurrence of these CaO second phases inside the as-grown crystals, the estimated solubility of as-grown single crystals is x = 0.0 7 5 for (La 1− x Sr x ) 2CaCu 2O 6+ δ . The melting point of the La-2126 materials increases with increasing oxygen pressure. Cubic-centimeter-size single crystals were successfully grown for (La 1− x Sr x ) 2CaCu 2O 6+ δ ( x = 0.0 7 5 ). The superconducting transition temperature of the as-grown (La 1− x Sr x ) 2CaCu 2O 6+ δ single crystals increases from T c = 2 8 K to 46 K with increasing the Sr concentration x from 0.075 to 0.25 at 11 bar oxygen pressure.

Crystal growth and superconductivity of (La1−xCax)2CaCu2O6+δ

Abstract The effects of growth conditions and composition on the crystal growth of (La1−xCax)2CaCu2O6+δ (x=0.05–0.15) superconducting materials have been studied by using the floating zone method. A smooth solid-liquid growth interface of (La1−xCax)2CaCu2O6+δ (x=0.05–0.06) tended to break down into a cellular interface when the growth velocity was higher than 0.5 mm/h. When the solid–liquid interface became cellular, the single crystal size in an as-grown rod decreased abruptly and the as-grown rod was not single phase. Various sized CaO inclusions were found inside the as-grown single crystals of (La1−xCax)2CaCu2O6+δ (x≥0.075). Cubic centimeter size single crystals were successfully grown for both (La0.95Ca0.05)2CaCu2O6+δ and (La0.94Ca0.06)2CaCu2O6+δ. The as-grown single crystals of (La1−xCax)2CaCu2O6+y under 1 bar pressure of oxygen were not superconducting. The superconducting temperature of the (La1−xCax)2CaCu2O6+y crystals grown at 11 bar oxygen is 28 K. The application of high pressure oxygen during crystal growth is an effective method to get bulk superconductivity in the crystals.

Macrosegregation in Bridgman growth of Terfenol-D and effects of annealing

Giant magnetostrictive material Terfenol-D (Tb x Dy 1− x Fe 2− y , 0.27⩽ x⩽0.30, 0.05⩽ y⩽0.1) was directionally solidified by vertical Bridgman method within a sealed quartz crucible. Macrosegregation of RE elements along the growth axis occurred in an abnormal way. RE elements concentration decreased significantly with increasing axial distance, which is different from the typical macrosegregation with a distribution coefficient, k, less than unity. This was due to the compositional inhomogeneity of the initial melt. During the melting of the prealloyed ingot in the quartz crucible, eutectic RE-rich phase melted first and flowed out into the gap between the crucible and the ingot. This RE-rich phase which flowed to the bottom did not mix well with the melt by convection because the temperature distribution in the melt is quite stable and the density of the RE-rich phase is higher than that of the REFe 2 phase. Macrosegregation was reduced very much in the rod grown after the homogenization of the initial melt. Even though the magnetostrictive strain was very different from part to part in the as-grown rod due to the macrosegregation, each specimen came to have a similar magnetostriction after the heat treatment above the eutectic temperature (890° C). During the high-temperature annealing, excess RE-rich phase was observed to be melted and flowed out from the specimen leaving pores between REFe 2 matrices. Therefore, composition of the crystal approached to the stoichiometic value regardless of the initial composition. That makes magnetostriction of the samples similar to one another. Melt extraction during the annealing is thought to be the result of poor wetting between the two phases.

Effect of growth conditions and compositions on the morphology and stability of the solid-liquid interface of Bi [sbnd]Sr [sbnd]Ca [sbnd]Cu [sbnd]O grown by a floating zone method

The effects of growth conditions and a variety of nominal compositions of a feed rod on the crystal growth and the stability of the solid-liquid interface of Bi Sr Ca Cu O oxide was studied by using a floating zone method. A variety of growth morphologies at the solid-liquid interface of an as-grown rod have been observed by a polarised optical microscope. The results showed that a planer solid-liquid interface tended to break down into a cellular interface while increasing the velocity from 0.2 mm/h or deviating from the nominal optimal composition of the feed rod Bi 2.1Sr 1.9Ca 1Cu 2O x. Fe-substitution in Bi-2212 depresses the stability of the solid-liquid interface of Bi Sr Ca Cu O oxide. The size of the single crystals of Bi-2212 in an as-grown rod decreases with the increase of the growth velocity and Fe-substitution. An indispensable condition for growth of large size Bi-2212 single crystals with a thickness more than 0.2 mm along the c-axis is that the solid-liquid interface maintains at a stable planar growth fron in an as-grown rod.

Crystallography and morphology of as-grown and coarsened Al-Al3Ni directionally solidified eutectic

The structure of the rodlike Al-Al3Ni eutectic directionally solidified vertically at rates ranging from 0.8 to 36.5 cm/h was studied in as-grown and coarsened conditions. The observed crystallographic relations are growth direction ∥ [010]A13Ni ∥ [011]A1or [21l]A1 and (111)A1 ∥ (102)A13Ni in the zone of the growth direction. The facet structure of as-grown rods is most pronounced at low growth rates. Elevated temperature exposure of the composite produces fiber coarsening and improved definition of the facets, both effects becoming more rapid with increasing growth rate. The facets themselves are, in many cases, of such crystallographic orientations that nearly equal atomic densities occur on both sides of the boundary. We conclude that, although faceting occurs to a marked degree, the related shape anisotropy is probably of second-order importance in calculations of coarsening rates. However, the presence of facets does preclude fiber foreshortening during coarsening.