H. M. O'bryan

Oxygen stoichiometry in Ba2YCu3Ox

Abstract Thermogravimetry (TG), evolved gas analysis (EGA), x-ray powder diffraction and dilatometry have been used to characterize the oxygen stoichiometry in the 93K superconductor, Ba2YCu3Ox (7.0>x>6.0). Above 400°C equilibration with ambients containing oxygen occurs rapidly. A reversible structural transformation occurs with changing oxygen stoichiometry going from orthorhombic at x=7.0 to tetragonal at x=6.0. The upper limit of x, which is achieved by annealing in oxygen at 400–500°C, shows the sharpest superconducting Tcs. The tetragonal phase is not superconducting. Structural models are proposed for the tetragonal structure.

Thermal analysis of rare earth gallates and aluminates

Dilatometry, high-temperature x-ray diffraction, differential thermal analysis, and differential scanning calorimetry have been performed on LaGaO{sub 3}, NdGaO{sub 3}, PrGaO{sub 3}, SmAlO{sub 3}, and LaAlO{sub 3} single crystals grown by the Czochralski technique. First order phase transitions have been located at 145 {degree}C for LaGaO{sub 3} and 785 {degree}C for SmAlO{sub 3}, and {Delta}{ital H} has been measured for the LaGaO{sub 3} transition. Second order transitions have been identified for LaGaO{sub 3}, PrGaO{sub 3}, NdGaO{sub 3}, and LaAlO{sub 3}. The usefulness of these compounds as substrates for high temperature superconducting films is discussed in terms of thermal expansion matching.

Thermal expansion — An empirical correlation

Abstract The average thermal expansion coefficient (αL) of a large variety of cubic and close-packed materials is shown to depend upon the coincidence of preferred orbital extension and site symmetry as well as melting point (Mp in °C). The product α L M p ⋍ 0.016 tends to hold for close-packed structures and α L M p ⋍ 0.027 for rectilinear arrays. Deviations occur when orbital extension tends to sp3 symmetry for an ion in an octahedral site or the site symmetry is reduced to tetrahedral or planar.

Some effects of CO2, CO and H2O upon the properties of Ba2YCu3O7

Abstract Thermogravimetric (TG) curves are presented for heating Ba 2 YCu 3 O 7 up to 1000°C in CO 2 , CO, several mixtures of CO 2 in O 2 , and in wet O 2 and wet CO 2 . In pure CO 2 there is a small weight loss up to about 400°C, presumably due to O 2 evolution caused by the low P O 2 . This decrease in weight is followed by a large weight gain up to about 950°C at which point the carbonate formed just begins to decompose. The behavior in CO is similar except the loss of O 2 is much greater. In wet O 2 there is a very slight weight gain beginning around 300°C followed by the normal loss of weight associated with O 2 evolution decomposition of any hydroxide formed earlier. Sintered toroids and bars of Ba 2 YCu 3 O 7 were given isothermal treatments in pure CO 2 , 1% CO 2 in O 2 , and wet O 2 at selected temperatures. Treatments in CO 2 for 1000 minutes at 100°C intervals led to a progressive decrease in conductivity and J c until at 500°C the material was completely semiconducting. Behavior in 1% CO 2 in O 2 was similar. In wet O 2 the conductivity and J c went through a minimum at about 200°C. Samples degraded by CO 2 or H 2 O could be nearly regenerated by firing above 900°C in O 2 and either annealed or slow cooled in O 2 .

Process-related problems of YBa2Cu3Ox superconductors

Abstract Ceramic processing of YBa 2 Cu 3 O x -based superconductors and process-related problems are described. Conditions for calcination, sintering and for microstructural development are important considerations for obtaining the superconducting properties. We examine different forming techniques for preparing superconducting components with different sizes, shapes and configurations. We also identify and discuss six problem areas related to (i) the incomplete decomposition of BaCO 3 precursor, (ii) the slow oxidation kinetics of YBa 2 Cu 3 O x ceramics, (iii) the sensitivity to water of the YBa 2 Cu 3 O x phase, (iv) the reactivity of the YBa 2 Cu 3 O x phase with substrates and co-firing materials, (v) the low critical-current density in YBa 2 Cu 3 O x bulk ceramics, and (vi) the low mechanical strength of YBa 2 Cu 3 O x ceramics.

Thermal expansion and transitions of single crystal lithium niobates from -60 to 250°C

The thermal expansion of single crystals of lithium niobate was investigated over the temperature range from -60 to 250°C along both the a- and c-crystallographic axes. Samples of as-grown crystals having the congruent composition and samples after the in-diffusion of Li2O to give the stoichiometric composition were measured. The data were successfully fitted to a third order polynomial equation.An anomalous decrease in the thermal expansion coefficient along the c-axis was observed near 120°C for the congruent crystal. Differential thermal analysis, electrical capacitance and acoustic velocity measurements all show weak but repeatable effects in this same temperature range for congruent material. The nature of the transition which gives rise to these changes has not yet been explained but is speculated to arise from some defect order/disorder process.

Correlation of the thermal expansion coefficients of rare earth and transition metal oxides and fluorides

The product of the average linear thermal expansion coefficient (αl) times melting point in °C (Mp) tends to be αlMp ⋍ 0.020 for the oxides of the 4f-rare earths and the comparable fluorides that have the tysonite structure. Correlation of rutile-like structures are based on deviations from αlMp = 0.027 caused by the cations moving the anions into sites with planar symmetry. The deviation tends to be proportional to the average cation to nearest neighbor anion distance. Additional effects are also discussed.

Reduction of Ba2YCu3O7 and Y2Cu2O5 by H2

Abstract The reduction of Ba 2 YCu 3 O 7 and Y 2 Cu 2 O 5 was studied in 15% H 2 -85% N 2 . Although considerable structure is evident in the thermogravimetric (TG) curves. X-ray diffraction studies of partially reduced samples did not reveal any obvious intermediates. At 980 ° C Ba 2 YCu 3 O 7 is reduced to BaO, Ba 2 Y 2 O 5 and Cu. Near 800 °C there is a plateau in the TG curve corresponding to Cu + and 2Cu. This suggests the formation of YCuO 2 ; however, there is no X-ray evidence in the powder pattern. Similarly, YCuO 2 does not appear as a stoichiometric intermediate in the reduction of Y 2 Cu 2 O 5 by H 2 . X-ray diffraction shows the presence of Cu at an early stage in the reduction of Ba 2 YCu 3 O 7 suggesting a surface reaction which is nearly completed before substantial reduction occurs in the interior of the particle.

Sensitivity of Tc to quenching in YBa2Cu4O8-δ

The superconducting transition temperature Tc of sintered YBa2Cu4O8−δ (124) heat-treated in one atmosphere of oxygen and quenched at temperature TQ (400°C<TQ<800°C) has been studied by electrical, magnetic, X-ray, TEM, and thermogravimetric measurements. Two transitions are observed in susceptibility measurements - a weak one at 81 K which is independent of TQ and a strong one near 70 K which moves to lower temperatures as TQ exceeds ∼400°C, reaching 30 K at TQ=800°C. This behavior is similar to that observed in YBa2Cu3O7−δ (123), even though the amount of oxygen lost at high TQ is much less than in 123. Structural evidence indicates that the dominant phase is 124. A straightforward interpretation of the data would associate the dominant phase (124) with the stronger susceptibility transition (at Tc≤69 K), but this is in conflict with previous studies by other which suggest Tc (124) ≈ 80 K. Several solutions to this discrepancy are proposed.

Application of thermal analysis to study the crystal growth, defect structure and thermal expansion of LiNbO3

Abstract Single crystals of LiNbO 3 are used in a variety of electro-optical devices. For switches, the lightguides are generally fabricated by diffusing TiO 2 into prescribed paths on single crystal wafers. A valuable property of LiNbO 3 is its high ferroelectric transition, T c ≈1140°C, which allows the processing to be achieved without depoling the crystal. Highly perfect homogeneous crystals are necessary for such devices. Since LiNbO 3 melts incongruently, the crystals must be grown at exactly the proper non-stoichiometric melting point. The T c is highly dependent upon Li concentration and hence DTA can be used as an analytical tool to determine the exact congruent composition, i.e., 48.45 M% Li 2 Q, and as a means of subsequent quality control. In addition T c is a function of the general defect structure of the material and is used to study the effects of additives such as TiO 2 upon the cation vacancy content and phase equilibria. For precise alignment of optical fibers with the lightguides it is necessary to know the thermal expansion values for these highly anisotropic crystals. Such data are obtained through careful dilatometric measurements. These measurements also reveal a poorly defined transition in congruent LiNbO 3 crystals at about 120 °C.