This review focuses on the materials chemistry and thermodynamics of the REBa2Cu3O7−x (RE = rare earth) series of superconductors. The importance of this basic information for predicting and controlling structure and microstructure of single crystals, thin films, and bulk materials is discussed. Examples of where an understanding of the thermodynamics has led to advances in processing are presented, and future areas of research that are key to the second decade of superconductor processing are highlighted.

Materials chemistry and thermodynamics of REBa2Cu3O7−x

We have undertaken phase-stability studies of Y1Ba2Cu3O7−x(+Ag) and BaCu2O2 (+Ag) over the temperature range 780°C–950°C and oxygen partial pressure range 1.0 × 10 −6 –2.0 × 10 −1 atm. Ag was found to have no effect on the thermodynamic stability of either phase. Partial melting of YBCO was found at temperatures as low as 800°C ± 10°C, with significant peritectic melting at temperatures as low as 900°C ± 10°C. These thermochemical studies have enabled us to determine the optimum conditions for reduced temperature and pressure (RTP) processing of Ag clad Y1Ba2Cu3O7−x pellets and tapes. The classic domain structure with strong texturing over short lengths was produced in samples melt processed by controlling oxygen partial pressure at a fixed temperature. The RTP method has the advantage that the second phase Y2BaCuO5 particle size can be controlled. Moreover, the technique offers the promise of long lengths of flexible Y1Ba2Cu3O7−x tapes with high current carrying capability.

Phase equilibria in the YBCuO system and melt processing of Ag clad Y1Ba2Cu3O7−x tapes at reduced oxygen partial pressures

In order to understand the formation mechanism of the Y2BaCuO5 free region, microstructures concerning incongraent mehing and peritectic reaction were studied in melt-textured Y–Ba–Cu–O oxides. It is found that spherical pores form during incongruent melting of the YBa2Cu3O7−y phase into Y2BaCuO3 and a Ba–Cu–O liquid phase. As the melting goes on, liquid phase flows into the pores and then produces spherical liquid pockets containing a few Y2BaCuO5 particles. During slow cooling of the sample from the peritectic temperature to the temperature where YBa2Cu3O7−y phase is formed, the liquid pockets are converted into YBa2Cu3O7−y phase containing a few Y2BaCuO5 particles. Sometimes, remnant Ba–Cu–O liquid phase is present at the center part of the Y2BaCuO5 free regions due to the incomplete peritectic reaction. It is concluded that formation of spherical pores during incongruent melting of YBa2Cu3O7−y is responsible for the formation of the Y2BaCuO5 free regions.

Origin of the y2ba1cu1o5 free region in melt-textured y-ba-cu-o oxides

This paper describes the melting equilibria in the vicinity of the high T{sub c} phase Ba{sub 2}YCu{sub 3}O{sub 6+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 (Y{sub 2}O{sub 3}) contents of the melts participating in these equilibria were low (mole fraction <4%). The primary phase field of Ba{sub 2}YCu{sub 3}O{sub 6+x} occurs at a mole fraction of <2.0% 1/2 Y{sub 2}O{sub 3} and lies very close along the BaO-CuO{sub x} edge, extending from a mole fraction of {approx}43% CuO to a mole fraction of {approx}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 liquidusmore » is presented as a function of temperature. Implications of the growth of Ba{sub 2}YCu{sub 3}O{sub 6+x} crystals are discussed.« less

/pdf/liquidus-diagram-of-the-ba-y-cu-o-system-in-the-vicinity-of-57og6dmowe.pdf

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

Thermogravimetric measurements of the equilibrium oxygen partial pressure ( p (O 2 )) as a function of temperature and oxygen content have been made on GdBa 2 Cu 3 O y (Gd123) within the range 3×10 −4 atm≤ p (O 2 )≤1 atm and 450°C≤ T ≤800°C. The partial molar entropy Δ S (O 2 ) and enthalpy Δ H (O 2 ) of solution of oxygen in Gd123 are evaluated and discussed. The p (O 2 ) data on Gd123 are compared with measurements performed on NdBa 2 Cu 3 O y (Nd123) at 800°C and literature data for YBa 2 Cu 3 O y (Y123) at 600°C and 800°C. The orthorhombic/tetragonal transition temperature ( T O/T ) for Gd123 and Nd123 as a function of the oxygen partial pressure (1×10 −3 atm≤ p (O 2 )≤1 atm) has been determined by high temperature X-ray diffraction (HTXRD). The oxygen content values for the O/T transition ( y O/T ) at several p (O 2 ) values for Gd123 and Nd123 have been determined by means of thermogravimetric measurements. The obtained data in the y – T diagram supports the applicability of the quasichemical approximation in describing the O/T transition of Gd123 and Nd123 compounds. The effect of the R atom (R=Y, Gd, Nd) ionic size on the oxygen stoichiometry and phase stability of R123 compounds at high temperature is discussed, and a possible explanation is proposed which accounts for the lattice mismatch between Ba–O and CuO 2 layers.

High temperature thermodynamic properties, orthorhombic/tetragonal transition and phase stability of GdBa2Cu3Oy and related R123 compounds

The dependence of the degree of nonstoichiometry of YBa2Cu307–x (123) on temperature and oxygen pressure has been determined by thermogravimetric analysis (TGA) in the temperature range 400° to 950°C and the oxygen pressure range 10–6 to 1 atm (1 atm = 105 Pa). The nature of the decomposition of 123 in the temperature range 750° to 950°C and the oxygen pressure range 10–6 to 10–2 atm has been determined by TGA and X-ray diffractometry (XRD). As the oxygen pressure decreases, the decomposition of 123 follows the sequence 123→ Y2BaCuo5 (211) + BaCuO2° Cu2O→ 211 ° BaCuO2° BaCu2O2→ 211 ° YBa3Cu2Ox (132) ° BaCu2O2→ 211 ° BaCu2O2°BaO. The incongruent melting temperatures have been determined in the oxygen pressure range 10–6 to 1 atm by differential thermal analysis, and the phases formed on solidification have been identified by XRD. The stability diagram for the composition 123 has been constructed.

Stability Diagram for the System Yba2Cu3O7–x

Atomic layer deposition (ALD) has been a promising technique in fabricating membranes and tuning their properties with a precision at the atomic level. Fabrication of zeolitic imidazolate framework (ZIF) membranes using the ligand-induced permselectivation (LIPS) method starts with the formation of an oxide in a mesoporous substrate by ALD and is followed by the transformation of this oxide to ZIF using imidazolate vapor treatment. The objective of the ALD step is to block the mesopores with a thin deposit, that is, one with small penetration depth and small thickness on the top surface of the substrate. Unlike typical ALD on nonporous substrates, where all available sites react per ALD cycle, thin deposit formation in a mesoporous substrate requires that only a small fraction of the available deposition sites (i.e., close to the substrate surface) is subjected to ALD. Consequently, reactant dosing and duration of pulses are important process variables which, together with diffusion and reaction kinetics determine the deposit structure. Quantitative understanding of the interplay of these variables and phenomena can enable the rational design of ALD within mesoporous substrates. Here, we extend our earlier modeling effort considering the coexistence of ALD both inside the pores and on the external surface of the substrate. Finite-volume based models were developed and validated to simulate the two distinct modes of deposition cycle by cycle. The total mass uptake of the substrate with ALD cycles can be predicted using the combined surface deposition and pore reaction–diffusion models as affirmed by in situ quartz crystal microbalance experimental data. The ALD reactor model combined with the deposition model can accurately capture the number of ALD cycles needed to block the pores of the substrate. Based on the model, we designed a modified ALD process and examined the performance of the corresponding LIPS membranes. The present modeling work provides a new understanding of the deposit formation via ALD within mesoporous substrates for a variety of membrane applications. 

Modeling of deposit formation in mesoporous substrates via atomic layer deposition: insights from pore‐scale simulation

Despite recent advances in area-selective deposition (ASD) processes, most studies have focused on single-material ASD. Multi-material ASD processes could provide additional flexibility for fabricating semiconductor devices. In this work, we identify process requirements to sequentially combine two intrinsic ASD processes: (1) poly(3,4-ethylenedioxythiophene) (PEDOT) ASD on SiO2 vs Si–H via oxidative chemical vapor deposition and (2) W ASD on Si–H vs SiO2 via atomic layer deposition. Using ex situ X-ray photoelectron spectroscopy, we show that a preferred orthogonal ASD sequence involves PEDOT ASD on SiO2 vs Si–H, followed by W ASD on Si–H vs PEDOT. We find that the properties of the individual PEDOT and W ASD materials, including resistivity, surface roughness, and growth rate, are affected by the ASD sequence. Furthermore, we successfully demonstrate that orthogonal ASD can be extended to nanoscale starting patterns. The cross-sectional scanning transmission electron microscopy (STEM) with energy-dispersive X-ray spectroscopy analysis shows that the resulting PEDOT thickness on SiO2 depends on feature geometry and dimension. Finally, we demonstrate the feasibility that the PEDOT layer can control the lateral growth of W onto the non-growth surface. 

Self-Aligned Nanopatterning and Controlled Lateral Growth by Dual-Material Orthogonal Area-Selective Deposition of Poly(3,4-ethylenedioxythiophene) and Tungsten

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Stability Diagram for the System Yba2Cu3O7–x

Modeling of deposit formation in mesoporous substrates via atomic layer deposition: insights from pore‐scale simulation

Self-Aligned Nanopatterning and Controlled Lateral Growth by Dual-Material Orthogonal Area-Selective Deposition of Poly(3,4-ethylenedioxythiophene) and Tungsten