Nae-Lih Wu

Phase identification of the new 126 K Ba–Ca–Cu–O superconductor

Abstract New superconducting oxides that have a composition of Ba2Ca2Cu3(CaxCuy)O8+δ with 0.4≥x≥0.9 and 0.7≥y≥0, and show an optimum Tc∼126 K were identified. Structural analysis, based on X-ray diffraction, indicated a common tetragonal unit cell among these oxides, which has lattice constants a=3.85 A and c=28.2 A with an I/4mmm symmetry. The cell consists of a rock-salt (BaO)2 block sandwiched between two perovskite Ca2Cu3O6 blocks with interstitial Cax+2 and Cuy+2 located, e.g. at the 4d site between the two BaO planes. In other words, a so-called 0223 cell with interstitial CaxCuy. The oxides differ from one another by the amounts of the inserted cations, i.e. the values of x and y. The presence of these interstitial cations offers a new way to adjust the carrier concentration. These oxides are unstable against the incorporation of H2O and CO2 in open air. The decayed products (Ca,Cu,H,CO2)2Ba2Ca2Cu3O8+δ have an expanded lattice constant c∼33.8 A and a lower Tc∼90 K. A structure model for the decayed phases was also proposed.

Low-temperature melt growth of YBa2Cu3O7−x/silver composites in partial vacuum

Abstract A new version of the YBa 2 Cu 3 O 7− x (the 123 oxide) melt-growth (MG) process, which is carried out under greatly reduced oxygen pressures (in partial vacuum) at temperatures not higher than 950°C, is described. Some salient aspects of this process are demonstrated with examples of processing 123/Ag composites in bulk and 123-on-Ag tape forms under the conditions of 0.01⩽ P (O 2 )⩽10 Torr and 920⩽ T ⩽950°C. In addition to the 123 domain structure, Y 2 BaCuO 5 inclusions and strong pinning typical of MG-123, the bulk composites thus synthesized contain uniformly dispersed Ag inclusions, which are effective in suppressing cracking within the 123 domains, while the 123-on-Ag tapes show c -axis preferential orientation of the 123 film normal to the Ag substrate, good oxide-substrate adhesion, and particularly negligible Ag melting. On the basis of microstructural observations, the mechanisms for the engulfment of Ag inclusions during solidification and for the enhancement in resistance to crack growth by the Ag inclusions are revealed.

Synthesis of Tl2CanBa2Cun+1O6+2n (n=1, 2) from stoichiometric reactant mixtures

Abstract New powder synthesis methods have been developed for the preparation of single-phase Tl 2 CaBa 2 Cu 2 O 8 (the 2122 compound) and Tl 2 Ca 2 Ba 2 Cu 3 O 10 (the 2223 compound) powders from stoichiometric reactant mixtures. The 2122 compound was prepared from a stoichiometric mixture of Tl 2 O 3 , CuO and CaBa 2 CuO 4 , while the 2223 compound was prepared from the same mixture but with additional CaO and CuO to match the correct (Tl:Ca:Ba:Cu=2:2:2:3) stoichiometry. For the 2122 compound, XRD single-phase samples with zero-resistivity temperatures ( T c, zero ) above 100 K were obtained by using a one-step calcination at 830° C for a period between 6 and 23 h. A maximum of T c, zero of 116 K was obtained for 14 h of calcination. For the 2223 compound, XRD single-phase samples with T c, zero between 110 and 118 K were obtained by employing a two-step calcination procedure consisting of a first calcination at 830° C for 5 h and a second calcination at 870° C for a period between 2 and 6 h. Both the 2122 and 2223 samples thus prepared exhibit sharp superconducting transitions and typically have a saturation Meissner effect of about 20% of (-1/4π) under a field of 20 G. Powder melting is completely eliminated during the preparation of the 2122 compound and much reduced, by employing the pre-calcination step, during the preparation of the 2223 compound. Compared with the conventional method, the new methods require less stringent calcination conditions, and hence allow for a better process control of the synthesis of Tl 2 Ca n Ba 2 Cu n +1 O 6+ n superconducting powders.

Preparation of tin oxide gels with versatile pore structures

Hydrous tin oxide gels were subjected to consecutive solvent-evaporation (SE) and CO2 supercritical (CS) drying steps, followed by re-hydration at low (30%) humidity, and the effects on pore structure of the SE drying duration and particle surface potential of the gel were investigated. The difference in surface potential has been found to impose significant variations in initial pore size distribution, while the SE drying step tends to narrow such differences and simultaneously reduce the mean pore size. It is demonstrated that varieties of pore structures have thus been obtained by simply varying these two parameters.

Silver-accelerated phase transformation from YBa2Cu3O7−x to YBa2Cu4O8

Abstract The effects of silver on the phase transformation from YBa 2 Cu 3 O 7- x (the 123 compound) to YBa 2 Cu 4 O 8 (124) via either 123+CuO→ 124 (the synthesis reaction) or 123 → 124 + Y-Ba-Cu oxides (the decomposition reaction) were investigated, by using SEM, EDX and quantitative XRD analyses and T c measurements, under the conditions (temperature: 810–880°C, oxygen pressure: 1–50 atm) where 124 is the thermodynamically stable phase. It was found that both reactions were greatly accelerated by Ag. In particular, the transformation rates increase tremendously at high oxygen pressures where Ag melts. These results suggest that the liquid Ag is much more effective in accelerating the reactions than the solid. The Ag-accelerated synthesis reaction produces 124 as the sole product phase, while the decomposition reaction, which is slower than the synthesis one results, in addition to 124, in Y 2 BaCuO 5 and an amorphous Ba-Cu-O phase. EDX indicated no Ag within the 124 grains resulting from both reactions. The 124-Ag composites thus prepared have a dense structure with Ag filling the voids existing between the neighboring oxide grains and exhibit bulk superconductivity with T c s typically at 78–79 K and 75–80 K for those prepared by the synthesis and decomposition reactions, respectively.

Sub-solidus relations in CaOBaOCuO with and without carboxyl contamination

Abstract The sub-solidus relations in the pseudo-ternary CaOBaOCuO system have been investigated at temperatures between 900 and 1000°C in both oxygen and (natural) air, and a tremendous effect has been found to result from the minute amount (∼ 450 ppm) of CO 2 present in the latter atmosphere. In flowing oxygen, CaBa 4 Cu 3 O 8+ x , CaBa 7 Cu 3 O 11 and Ba 1− y Ca y CuO 2 (0≦ y ≦ 0.08) solid solutions have been identified. On the other hand, in stagnant natural air, which contains ∼450 ppm CO 2 , there have been found oxycarbonates including CaBa 4 Cu 2 O 6+δ (CO 3 ), CaBa 8 Cu 3 O 8 (CO 3 4 , and Ca 1− x Ba 8 Cu 4 O 10−2 x (CO 3 ) 3+ x ( x =0 ∼ 0.35) solid solutions, which account for several controversial ternary “oxides” reported in the literature. The ternary phase diagrams for both atmospheres at 900°C are shown, while those at higher temperatures, up to 1000°C, are elucidated from the thermal stability studies of the detected oxides and oxycarbonates. Structural models are proposed for CaBa 8 Cu 3 O 8 (CO 3 ) 4 and Ca 1− x Ba 8 Cu 4 O 10−2 x (CO 3 ) 3+ x .

Oxygenation and superconductivity of Tl2CaBa2Cu2O8−x

Abstract Oxygenation of Tl 2 CaBa 2 Cu 2 O 8− x between 250°C and 800°C was studied. The curves which plotted the amount of oxygen adsorbed as a function of oxygenation temperature indicated the presence of two oxygen adsorption processes, including one which prevailed at temperature below 400°C and the other between 500°C and 700°C. When the compound was oxygenated consecutively at increasing temperatures, the T c of the oxide first increased and then decreased, exhibiting a maximum after oxygenation at 400°C. The T c increased almost linearly with the amounts of oxygen adsorbed below 400°C but showed no unique (one-to-one) correlation with the overall oxygen content of the oxide.

Observation and identification of a 107 K superconducting family in the CaBa2Ca2Cu3O9−δ system

Abstract A new family of C-free high-temperature superconducting oxides in the Ba–Ca–Cu–O system showing a T c up to 107 K has been synthesized under high pressure. Structural analyses, based on X-ray and electron diffractions and high resolution electron microscopy, have revealed that these oxides can be represented by CaBa 2 Ca 2 Cu 3 O 9− δ [Ca-1223-Ba] with a tetragonal subcell that has lattice constants a ∼3.85 A and c ∼16.5 A and is isostructural to Tl-1223-Ba with the Tl sites occupied mainly by Ca. These oxides have the same active block of [(CuO 2 )(Ca)(CuO 2 )(Ca)(CuO 2 )] but with cation mixing and substitution of Ca by Cu within the charge reservoir block [(BaO)(CaO)(BaO)]. This results in the observed variations in composition and superstructure but has only minor influence on T c .

New reaction routes for preparing single-phase Tl2CaBa2Cu2O8 and Tl2Ca2Ba2Cu3O10 powders from stoichiometric mixtures

Single-phase Tl2CaBa2Cu2O8 and Tl2Ca2Ba2Cu3O10 powders were prepared from stoichiometric mixtures by the following two reactions: (1) CaBa2CuO4+Tl2O3+CuO→Tl2CaBa2Cu2O8 and (2) Tl2CaBa2Cu2O8+CaO+CuO→Tl2Ca2Ba2Cu3O10. Single-phase powders were obtained at 830°C for the first reaction, and at 870°C for the second reaction. At these temperatures, the extent of powder melting and the rate of Tl loss were small. Powders thus prepared showed superconducting temperatures (both the zero-resistivity and diamagnetic onset temperatures) of above 110 K and a saturation Meissner effect of 20% of (-1/4π) at a field of 20 G.

The structures of the 126 Ba2Ca2Cu3(CaxCuy)O8+δ the 116 K Ba2Ca3Cu4(CaxCuy)O10+δ and their transformed products

Abstract Two new superconducting Ba Ca Cu oxides were identified as Ba 2 Ca 2 Cu 3 (Ca x Cu y )O 8+δ and Ba 2 Ca 3 Cu 4 (Ca x Cu y )O 10+δ with T c of 126 K and 116 K, respectively. The cations (Ca x Cu y ), 0.4≤x≤0.9and0≤v≤0.7, are inserted between two adjacent BaO layers to stabilize the structures and to adjust the Cu-valences. Both compounds show the body-centered symmetry, but with different lattice constants of ( a,c ) = (3.8, 28.2) and (3.8, 34.8) A, respectively. These compounds are unstable against the incorporation of CO 2 and H 2 O in open air. The transformed products (Ca u/2 Cu v/2 C x H y ) 2 Ba 2 Ca 2 Cu 3 O 8+δ and (Ca u/2 Cu v/2 C x H y ) 2 Ba 2 Ca 3 Cu 4 O 10+δ have an expanded c of 33.8 and 40.2A, and a lower T c of 90 and 114 K, respectively. The structure models for these decay products are proposed.