Lawrence P. Cook

Liquidus Diagram of the Ba-Y-Cu-O System in the Vicinity of the Ba2YCu3O6+x Phase Field

This paper describes the melting equilibria in the vicinity of the high Tc phase Ba2YCu3O6+x, including evidence for two Ba-Y-Cu-O immiscible liquids. Melting equilibria have been investigated in purified air using a combination of differential thermal analysis (DTA), thermogravimetric analysis (TGA), powder x-ray diffraction (XRD), MgO wick entrapment of liquid for analysis, scanning electron microscopy (SEM) coupled with energy dispersive x-ray analysis (EDS), and hydrogen reduction for determination of copper oxidation state. For relatively barium-rich compositions, it was necessary to prepare the starting materials under controlled atmosphere conditions using BaO. A liquidus diagram was derived from quantitative data for the melts involved in various melting reactions. In general the 1/2(Y2O3) contents of the melts participating in these equilibria were low (mole fraction <4 %). The primary phase field of Ba2YCu3O6+x occurs at a mole fraction of <2.0 % 1/2Y2O3 and lies very close along the BaO-CuOx edge, extending from a mole fraction of ≈43 % CuO to a mole fraction of ≈76 % CuO. It is divided by a liquid miscibility gap and extends on either side about this gap. The topological sequence of melting reactions associated with the liquidus is presented as a function of temperature. Implications for the growth of Ba2YCu3O6+x crystals are discussed.

Cathodoluminescence measurement of strained alumina single crystals

Abstract Alumina single crystals indented with a Vickers hardness indenter were investigated by cathodoluminescence (CL) imaging and spectroscopy in a scanning electron microscope. The spatial resolution of the CL images was approximately 0·4 μm. CL spectra were measured with wavelength resolution of 1·0 nm in the wavelength range of 200 to 900 nm; in the immediate region of the ruby lines (693 nm), spectra were measured with a resolution of 0·15 nm. Stress-induced frequency shifts of the ruby lines were utilized to measure the residual stresses in the region surrounding the indentation. Both hydrostatic and non-hydrostatic stresses were measured. The magnitude of the stresses was observed to vary with distance from the center of the indent and also with crystallographic orientation. After the samples were annealed at 1600°C for 10 h, no measurable stresses remained either in the center of the indent or in the surrounding area.

Flux pinning of Bi2Sr2CaCu2O8+δ/Ag superconductors utilizing (Sr,Ca)14Cu24O41 defects and nanophase Al2O3 and Au particles

Investigations into flux-pinning of Bi 2 Sr 2 CaCu 2 O 8+δ /Ag (Bi-2212/Ag) thick film conductors utilizing micron-sized (Sr,Ca) 14 Cu 24 O 41 and nanophase Al 2 O 3 and Au particles are described. Addition of defect particles delayed and reduced formation of 2212 c-axis texture, while improving film quality by inhibiting Sr:Ca:Cu:O defect formation. Oxide defects were observed to rapidly coarsen in the melt, simultaneously with beneficial increase of 2212 texture. Nanophase Al 2 O 3 reacted rapidly ( < 4 min) with 2212-type melts (Sr:Ca = 1.33-2.07) to form solid-solution (Sr,Ca) 3 Al 2 O 6 with Sr:Ca ratio close to the Sr:Ca composition of the precursor matrix. Addition of nanophase Al 2 O 3 improved the J c (1 T) over the range 20-30 K, e.g. J c (30 K, 1 T)/J c (5 K, 0 T)=(<0.001), 0.005, and 0.065 for 0%, 10% and 46% molar addition, respectively. The addition of nanophase or micron-sized Au completely suppressed 2212 c-axis orientation for the processing temperatures tested.

A review of the crystallography and crystal chemistry of compounds in the BaO-CuOx system

Literature data pertaining to the crystal chemistry, crystallography, and X-ray powder-diffraction properties of phases reported in the system BaO-CuO x are compiled. Reported phases include BaCuO 2−2.12 , BaCuO 2.26−2.39 , BaCuO 2.5 , Ba 2 CuO 3+ x , Ba 2 Cu 3 O 5+ x , BaCu 2 O 2+ x , Ba 3 Cu 5 O 8 , Ba 3 CuO 4 , BaCu 3 O 4 , Ba 3 Cu 2 O 4+ x , Ba 5 Cu 3 O 6+ x , and BaCu 2 O 3 . The existence of some of these phases, however, still has to be verified. Most of these compounds are nonstoichiometric in oxygen. Their oxygen content, and in some cases even their crystal structures, vary with oxygen partial pressure. There are also reported indications that the cations Ba and Cu are not stoichiometric in the cubic BaCuO 2 -type compounds.

Primary phase field of the Pb-doped 2223 high-{Tc} superconductor in the (Bi,Pb)-Sr-Ca-Cu-O system

Both liquidus and subsolidus phase equilibrium data are of central importance for applications of high temperature superconductors in the (Bi, Pb)-Sr-Ca-Cu-O system, including material synthesis, melt processing and single crystal growth. The subsolidus equilibria of the 110 K high-Tc Pb-doped 2223 ([Bi, Pb], Sr, Ca, Cu) phase and the location of the primary phase field (crystallization field) have been determined in this study. For the quantitative determination of liquidus data, a wicking technique was developed to capture the melt for quantitative microchemical analysis. A total of 29 five-phase volumes that include the 2223 phase as a component was obtained. The initial melt compositions of these volumes range from a mole fraction of 7.3 % to 28.0 % for Bi, 11.3 % to 27.8 % for Sr, 1.2 % to 19.4 % for Pb, 9.8 % to 30.8 % for Ca, and 17.1 % to 47.0 % for Cu. Based on these data, the crystallization field for the 2223 phase was constructed using the convex hull technique. A section of this “volume” was obtained by holding two components of the composition at the median value, allowing projection on the other three axes to show the extent of the field.

Lead Zirconate-Titanate Thin Films Prepared by the Laser Ablation Technique

Lead zirconate-titanate (PZT) thin films were prepared by the laser ablation technique. The PZT (Zr/Ti=53/47) target was irradiated using a focused q-switched Nd:YAG laser (15 ns, 100 mJ at 1.064 μ;m). The as-deposited films were amorphous as indicated by X-ray powder patterns, but crystallized readily with brief annealing above 650°C. The dielectric constant and the resistivity of the crystallized films were studied using a parallel-plate type capacitor structure.

Enhanced mass transport in ultrarapidly heated Ni/Si thin-film multilayers

We investigated multilayer and bilayer Ni/Si thin films by nanodifferential scanning calorimetry (nano-DSC) at ultrarapid scan rates, in a temperature-time regime not accessible with conventional apparatus. DSC experiments were completed at slower scan rates as well, where it was possible to conduct parallel rapid thermal annealing experiments for comparison. Postexperimental characterization was accomplished by x-ray diffraction, and by transmission electron microscopy (TEM) and energy-filtered TEM of thin cross sections prepared by focused ion beam milling. We found that rate of heating has a profound effect on the resulting microstructure, as well as on the DSC signal. After heating to 560 °C at 120 °C/s, the general microstructure of the multilayer was preserved, in spite of extensive interdiffusion of Ni and Si. By contrast, after heating to 560 °C at 16 000 °C/s, the multilayer films were completely homogeneous with no evidence of the original multilayer microstructure. For the slower scan rates, we...

Manifestation of anisotropy in melting systematics of RBa2Cu3O7−δ (R=lanthanides)

The conventional isotropic Debye temperature fails to account for the trend of melting temperatures for the high Tc superconductors, RBa2Cu3O7−δ (R-123), as a function of the ionic radius of R3+. We overcame this problem by calculating Debye temperatures using mean sound velocity along the c axis that features an anisotropic layered structure. Using the “improved” Debye temperature, the trend of derived melting temperatures based on the “Lindemann law” matches well with experimental data. This trend is also confirmed by comparing theoretical and experimental Raman active modes corresponding to the Cu–O (plane copper and apical oxygen) and Ba–O (in-plane) bonds in R-123 series.

Melting of Sr14Cu24O41 at oxygen pressures of 0.0075, 0.021 and 0.1 MPa

The eutectic and peritectic melting equilibria of the Sr14Cu24O41 phase have been studied at oxygen pressures of 0.0075, 0.021, and 0.1 MPa. The Sr14Cu24O41 primary phase field was determined using a combination of differential thermal analysis and electron beam microanalysis of quenched liquids. It was found that the incongruent melting of Sr14Cu24O41 takes place at 925, 975, and 1032°C at PO2=0.0075, 0.021, and 0.1 MPa, respectively. The peritectic melting reaction under all conditions was confirmed to be: Sr14Cu24O41→SrCuO2+Liquid+O2. The liquid composition of this melt was found to range from 79.8% (mole fraction, at PO2=0.0075 MPa) to 83.9% CuO (mole fraction, at PO2=0.1 MPa). The copper oxidation state in these liquids, as estimated from thermogravimetric data, ranged from Cu+1.42 (at PO2=0.0075 MPa) to Cu+1.46 (at PO2=0.1 MPa). The eutectic melting in the SrO–CuO system occurs at 918°C (mole fraction of 81.7% CuO), 965°C (mole fraction of 85.0% CuO), and 1005°C (mole fraction of 86.7% CuO) at PO2=0.0075, 0.021, and 0.1 MPa, respectively. The temperature range of the Sr14Cu24O41 primary crystallization field was found to increase from 7°C at PO2=0.0075 MPa to 28°C at PO2=0.1 MPa, but the compositional width remains small (<4%) at all three oxygen pressures. These data provide important points on the Sr–Cu–O phase diagram. The data are discussed with reference to the growth of Sr14Cu24O41 crystals.

A close proximity self-aligned shadow mask for sputter deposition onto a membrane or cavity

In this paper we report on the fabrication of a close proximity shadow mask designed for sputtering into cavities or onto the back surface of freestanding silicon nitride (SiNx) membranes. Sputtering into a well-defined area on a fragile surface is difficult since sputter deposition through a shadow mask separated from the deposition surface typically results in spreading of the deposited material. The area of spreading beyond the desired area of deposition depends on the vertical separation between the shadow mask and the surface of the membrane. In our design, a high degree of accuracy (±5 µm) in the separation (25 µm) between the shadow mask and the deposition surface is achieved. The shadow mask is made from SiNx-coated silicon wafers, using potassium hydroxide (KOH) etching on both sides of the wafer. The design rules chosen to maintain accuracy of the fit between the shadow mask and the deposition surface over various etch conditions, fabrication and methods used for convex-corner compensation and the alignment to the wafer crystal axis, which also contribute to an accurate fit, are discussed. Spreading of deposited material due to sputtering is limited to about 40 µm when this shadow mask is used.