C.D. Brandle

Growth of single crystals of lanthanum aluminate

Abstract Single crystals of lanthanum aluminate have application as a substrate for deposition of thin films of the high temperature superconductors such as YBa 2 Cu 3 O 7 . Single crystals have been grown by the Czochralski technique in the 〈100〉 pseudo-cubic crystallographic orientations in sizes from 16 mm weighing 180 g to 59 mm diameter weighing 2500 g. The design of the crystal growth furnace used for the Czochralski growth of lanthanum aluminate required some modifications from the geometry suitable for the growth of lower melting temperature oxides.

Physical properties of a Y3Al5012 melt

Density, surface tension contact angle (with an Ir wire) and viscosity data were measured for a Y 3 Al 5 O 12 melt from 1970 o C (just above the melting point) to 2070 o C. Density, as determined by the Archimedean two bob technique, was 3.688 g/cm 3 at the melting point, with a volume coefficient of thermal expansion of only 1.8±0.5×10 -5 . The maximum bubble pressure method found the surface tension to be high, 781 dyn/cm at 1970 o C, but also with a low temperature dependence. Data from these two measurements allowed determination of the contact angle of the melt with the iridium suspension wire to be ∼113 o

Growth of single crystals of rare earth gallates

Abstract Single crystals of LaGaO 3 , NdGaO 3 , PrGaO 3 have been grown by the Czochralski technique for evaluation as substrates for thin film growth of superconductors. LaGaO 3 crystals have been grown in several crystallographic orientations in sizes from 16 mm diameter weighing 150 g to 55 mm diameter weighing 1500 g. Crystals of NdGaO 3 and PrGaO 3 have been prepared to test their compatibility with the superconductor films. The influence of reagent purity, growth atmosphere, and the crystal growth furnace design upon the color and quality of the crystals have been examined.

Temperature dependence of photorefractive properties of strontium‐barium niobate (Sr0.6Ba0.4Nb2O6)

Beam coupling gain and response time together with reflectivity and response time of an external ring phase‐conjugate mirror are studied as a function of temperature for a crystal of strontium‐barium niobate. The photorefractive sensitivity is increased by a factor of 5 when the crystal is heated to 65 °C. Self‐pumped reflectivities up to 19% are observed at room temperature. The reflectivity is smaller at elevated temperature.

Growth-induced anisotropy in bismuth - Rare-earth iron garnets

The bismuth‐doped rare‐earth iron garnets, (R3−x−yBixPby)Fe5O12 (Bi:RIG, R=Pr, Nd, Sm, Eu, Gd, Tb, Dy, Ho, Er, Tm, Yb, Lu, and Y), were prepared under constant growth conditions to investigate the influence of ionic species on the bismuth‐based growth‐induced uniaxial anisotropy Kgu. The effect of ionic species on growth‐induced anisotropy in Bi:RIG was not consistent with the ionic size model of site ordering. In particular, Bi:SmIG, Bi:EuIG, and Bi:TbIG displayed high growth‐induced anisotropies, up to 331 000 erg/cm3 at room temperature for x≊0.5. The temperature dependence of these Kgu’s was somewhat higher than that of the well studied Bi:YIG. The site ordering of Bi can be modeled by assuming that small, low‐oxygen‐coordination BiO+3−2ww melt complexes have a strong site selectivity for small, high‐oxygen coordination sites at the growth interface.

New substrates for HTSC microwave devices

A new substrate material LSAT, a 30/70 mole % solid solution between LaA10₃ and Sr₂AITaO₆, has been prepared for the purposes of eliminating twinning, strain and non-isotropic microwave properties found in pure LaA10₃ substrates. At 300 K and 30/70 mole %, LSAT is cubic with lattice parameter of 7.737 Å. The dielectric properties of single crystal LSAT (30/70 mole %) substrates have been measured at 10 GHz and 300 K and determined to be: dielectric constant ε=22.5; and, loss tangent tan(δ) ⩽ 10^-3. High quality c -axis oriented high critical temperature superconducting (HTSC), YBa₂Cu₃0_7-δ (YBCO), thin films have been deposited on (001) single crystal LSAT substrates. The crystal quality of these films is excellent as evidenced by the full width half maximum (FWHM) rocking curve widths of typically 300 arc-seconds. Critical current densities (as measured using magnetization) are about 4 × 10⁶ A/cm² at 10 K. Microwave film properties include an onset transition temperature (T_c) higher than 91 K, transition width (ΔT_c) less than 5 K, surface resistance R_S lower than copper (30 mω) at 85 K and 35 GHz for a film of thickness 2500 Å.

Electrical behavior of a 31‐cm, thin‐film YBaCuO superconducting microstrip

Electrical time domain measurements and transmission response measurements were made using a 31‐cm‐long, YBaCuO superconducting thin‐film microstrip line and a YBaCuO ground plane, each on separate 1‐cm LaGaO3 substrates, with a 125‐μm sapphire substrate serving as the dielectric insulator. Degradation of the performance of the line for currents up to the critical‐current density and for magnetic fields moderately above the lower critical magnetic field HC1 were evaluated in a variety of simple measurements. In addition, an evaluation of an optically switched segment of a superconducting microstrip line on a zirconia substrate is described. Direct evaluations of pulse distortion are not possible for these long serpentine lines on small substrates due to electromagnetic coupling between adjacent line segments. However, this coupling gives rise to distinctive transmission responses which may aid the evaluation of line performance.

Congruent composition for growth of lanthanum aluminate

Single crystals of lanthanum aluminate have application as a substrate for deposition of thin films of the high temperature superconductors. Initial trials at Czochralski growth of this material revealed that only a small fraction of a stoichiometric melt may be crystallized before the onset of inclusions or voids at the growth interface. The composition of lanthanum aluminate Czochralski grown crystals was analyzed using X-ray fluorescence with a special procedure for sample preparation. The crystal composition was found to be non-stoichiometric, with 0.987 gmoles of La per gmole of AI. Crystal growth from melts covering this composition range have confirmed that the greatest yield of good material comes with the above composition.

Nickel containing perovskites

Ceramic samples of compositions from all the categories of possible nickel-containing perovskites were investigated by X-ray diffraction. Promising compositions were melted in a radio frequency (RF) furnace, but the evaporation of nickel oxide makes them unsuitable for Czochralski growth.

Thermophysical properties of a LiCaAlF6 melt

Abstract Density, surface tension and viscosity were measured in a LiCaAlF 6 meltfrom 815 to 915°C. At 815°C (just above the melting temperature) the density, as measured by the Archimedean technique, is 2.505 g⧸cm 3 and the volume coefficient of thermal expansion is 2.90×10 -4 . The surface tension is 157.8 dyn⧸cm under a nitrogen atmosphere and 148.9 dyn⧸cm under argon according to the maximum bubble pressure method. The viscosity was measured with a Brookfield viscometer to be 19 cP at 815°C and had an activation energy of 531 kJ⧸mol. These data will be used to model melt convection and flow for Bridgman and Czochralski growth of single crystals of this material.