H.M. Shao

Synthesis of single-phase HgBa2Ca2Cu3O8+δ superconductor

Abstract The third member (Hg-1223) of the recently discovered homologous series HgBa2Can−1CunO2n+2+δ, having a Tc of 118 K (as synthesized sample) and 133 K (oxygen-annealed sample), was successfully prepared by the solid state reaction and short-time annealing technique using mixtures of the metal oxides HgO, BaO, CaO and CuO with starting composition HgBa2Ca2Cu3O8+δ. We found that the combination of very short annealing times with optimum temperatures 715–725°C gives rise to a very effective sintering of the constituents and avoids an excessive loss of Hg. Samples so obtained display a sharp superconducting transition determined both magnetically and resistively. The X-ray diffraction patterns of both the 118 K phase and the 133 K phase indicate a nearly single phase, corresponding to the tetragonal structure of space group P4/mmm and with lattice parameters a= 3.85 A and c= 15.85 A . The resistivity ϱ of such a sample was very low, ∼ 12x10-3 Ω cm at 300 K, and the ϱ versus T curve was linearly extrapolated to zero resistance at 0 K, being similar to the cases of high-quality high-Tc cuprates. Energy dispersive X-ray analysis (EDX) data of several grains of the Hg-1223 sample are in agreement with the proposed chemical formula.

The synthesis and characterization of HgBa2Ca2Cu3O8+δ superconductors with substitution of Hg by Pb

Abstract We have successfully fabricated ceramic superconductors of Hg-1223 with substitution of Pb for Hg, i.e. Hg1−xPbxBa2Ca2Cu3Oy, by using encapsulation and short time annealing techniques. Samples fabricated in this manner may have nearly single phase structure and some enhancement of Tc can be achieved through this method. The materials of this ceramic with x ranging from 0 to 0.5 exhibit high-temperature superconductivity at temperatures falling in the range from 103 to 135 K. The sample with optimum doping of Pb of x = 0.34 and ° = 0.40 has a highest Tc of 135 K, and its diamagnetic onset temperature Tcdia = 143 K. The composition of this sample has been found experimentally and is specified by a formula of Hg0.66Pb0.34Ba2Ca1.98O8.4. The valence number for the copper constituent is given by V Cu = 2.30 . X-ray diffraction (XRD), transmission electron microscope (TEM) and electron probe microanalyzer (EPMA) analyses have been carried out for detailed investigation. Results indicate that the critical temperature for samples which were post-annealed with either oxygen flow or in high pressure of oxygen gas was increased, while that for those annealed in argon flow was decreased. Furthermore, the as-prepared samples were slightly in the under-doped state with respect to the vacancy. It was shown that Tc parabolically depends on the valence state of copper.

TEM observation of intergrowth structures in the HgBa2Ca2Cu3Oy ceramic superconductor

Abstract Intergrowth structures, which occur in the HgBa 2 Ca 2 Cu 3 O y ceramic superconductor, were observed by means of transmission electron microscopy (TEM). There are at least two types of intergrowth structure: Hg-1212/Hg-1223 and Hg-1223/Hg-1234. The congruence relationships for these intergrowth structures are respectively: 〈1 1 0〉 Hg-1212 |〈1 1 0〉 Hg-1223 , {0 0 1} Hg-1212 |{0 0 1} Hg-1223 ; and 〈1 1 0〉 Hg-1223 |〈1 1 0〉 Hg-1234 , {0 0 1} Hg-1223 |{0 0 1} Hg-1234 . For a stable intergrowth structure, the total strain energy density should have a minimum value. The strain energy density consists of two parts: the tensile strain energy density U t and the shear strain energy density U s . The tensile strain energy density U t stored in the crystal can be expressed in terms of the ratio between the numbers of matching slabs, i.e. f i = N i / N , where N i is the number of slabs corresponding to Hg-12( n i −1) n i , and N is the number of slabs corresponding to the matrix, i.e. Hg-12( n −1) n . The plots of U t vs. f i = 1.2498 and f i = 0.8272 for intergrowth structures of Hg-1212/Hg-1223 and Hg-1234/Hg-1223, respectively. As was observed from the TEM micrograph, two immobile dislocations can be created by an appropriate combination of intergrowth structures of Hg-1212/Hg-1223 and Hg-1223/Hg-1234. A model for the formation of these immobile dislocations is presented.

Characterization of Hg-Ba-Ca-Cu-O 1223 superconductor

Abstract The HgBaCaCuO 1223 superconductors have been characterized in this work. XRD and SEM analyses indicate that Hg-Ba2Ca2Cu3O8+δ is the major superconducting phase in the shape of a bar or plate in the samples. The transition temperature Tc of one sample is 129 K by resitivity, AC and DC susceptibility measurements. Magnetic measurements including the magnetic critical current density and magnetization relaxations indicate that a strong thermally activated flux motion also exists in the Hg-based system of high-temperature superconductors.

Synthesis and X-ray powder diffraction analysis of the high-Tc superconductor HgBa2Ca2Cu3O8+δ

Abstract The third member (Hg-1223) of the recently-discovered homologous series HgBa 2 Ca n −1 Cu n O 2 n +2+δ , which having a T c of 118 K (assynthesized sample) and 133 K (oxygen-annealed sample), was successfully prepared by solid state reaction and short time annealing technique using the mixtures of metal oxides HgO, BaO, CaO and CuO with starting composition HgBa 2 Ca 2 Cu 3 O y . We found that the combination of very short annealing time with optimum temperatures 715–725°C gives rise to a very effective sintering of the constituents and avoids an excessive loss of Hg. This also means that the reaction should be followed by X-ray powder diffraction analysis of the material at each step. Sample so obtained displays a sharp superconducting transition determined both magnetically and resistively. The X-ray diffraction patterns of the 118 K phase and the 133 K phase both indicate a nearly single phase, corresponding to the tetragonal structure of space group of P 4/ mmm and with lattice parameters a = 0.385 nm and c = 1.585 nm. Energy dispersive X-ray spectroscopic analysis (EDX) data of several flat grains of the Hg-1223 sample shows that the cation stoichiometry is an agreement with the proposed chemical formulae.

Texture and chemical composition analyses on the Hg 0.66 Pb 0.33 Ba 2 Ca 2 Cu 3 O y superconductor using the sealed quartz tube technique

Pb-substituted Hg-based superconductor of Hg{sub 0.66}Pb{sub 0.33}Ba{sub 2}Ca{sub 2}Cu{sub 3}O{sub {ital y}} has been fabricated using the sealed quartz tube technique. {ital R}-{ital T} and x-ray diffraction pattern (XDP) measurements show that the specimen has a {ital T}{sub {ital c}} of 135 K and contains mainly the Hg-1223 phase. Scanning electron microscopy/energy dispersive x-ray analysis (SEM/EDX) and transmission electron microscopy/energy dispersive x-ray analysis (TEM/EDX) were employed to study the texture and chemical composition of the specimen. It is found that the specimen contains round-shaped grains with a mixture of Hg-1223, BaCuO{sub 2}, and Ca{sub 0.85}CuO{sub 2} phases, square-shaped grains with a formula of PbBa{sub 2}O{sub 3}, small single crystals with single Hg-1223 phase, and crystal-like layers with a mixture of Hg-1223 and BaCuO{sub 2} phase. We consider that though the doping of Pb can benefit the stabilization of the Hg-1223 phase, it introduces other impurity phases and textures in the specimen at the same time. {copyright} {ital 1996 Materials Research Society.}

Low field irreversibility line in (Hg0.66Pb0.34)Ba2Ca2Cu3O8+δ by third harmonic a.c. susceptibility measurements

Abstract First-harmonic χ1 and third-harmonic χ3 a.c. susceptibility measurements have been carried out on a Hg0.66Pb0.34Ba2Ca2Cu2O8+δ sample. The frequency dependence of the onset temperature of χ3 can be described well by T on (f) = C(2πf) 1 ((z − 1)V) + T g . Extrapolating to f = 0, we derived the vortex glass transition temperature Tg, and established the irreversibility line (IL) for the Pb-doped Hg-1223 sample. In the low-field region (H ⩽ 0.1 T), the power law H irr = 9.0(1 − T irr T c ) 1.95 fits the data rather well; however, deviation from the power law is observed in the higher-field region, indicating the transition in dimensionality from 3D to 2D vortices.

Irreversibility and transport in the high Tc superconductor HgBa2Ca2Cu3O8 + δ

A HgBa2Ca2Cu3O8 + δ superconductor, having a Tc of 133 K (oxygen-annealed sample), was successfully synthesized. A.c. susceptibility, electrical resistivity and critical current density measurements were performed. The irreversibility line (IL) was obtained and a new feature of 1 − t ∝ H1/4 (t = Tirr/Tc is the reduced temperature, where irr is the irreversibility temperature) was found and explained in terms of T. Matsushitas theory. The results of the transport measurements can be looked upon as evidence that there also might be a vortex glass-liquid transition in Hg-based systems. The dimensions of the superconducting CuO2 layer for different compounds was used to explain the observed transition in the IL.

Syntheses and neutron powder diffraction studies of the superconductor HgBa2Ca2Cu3O8+δ by Pb substitution

Substitution of Pb for Hg was performed in the Hg based 1223 phase HgBa2Ca2Cu3O8+delta a (T-c similar to 135 K), resulting in an increase of the superconducting transition temperature to T-c(dia) = 143 K for samples with a nominal composition of Hg0.7Pb0.3Ba2Ca2Cu3O8+delta. The crystal structure of this solid solution has been investigated by neutron powder diffraction (NPD) and synchrotron X-ray diffraction (SXRD) techniques at room temperature (RT). The compound has the same crystal structure as Hg-1223 with the space group P4/mmm and lattice parameters a = 3.8457(1) Angstrom and c = 15.8252(7) Angstrom. Rietveld analysis results indicate that Hg is partially replaced by Pb, and the oxygen content, delta, is 0.450(2). The lattice parameter changes resulting from the Pb substitution are too small to account for the T-c change mimicking the effect of pressure.

The Influence of Mobile Holes in Hg1 − x Pb x Ba2Ca2Cu3O8 + δ Superconductors

By means of X-ray diffraction (XRD), neutron diffraction spectra (NDS), electron probe microanalysis (EMPA), and X-ray photo electron spectroscopy (XPS) etc., we studied the superconductivity of doped-Pb Hg-1223 compound. The crystal parameters a and c and the unit cell volume of the Pb-doped sample do not change considerably, but the intercalated O(4) of the sample increases more than that of the pure-phase Hg-1223. As for the valence number of Hg is still 2 and that of Pb is either 2 or 4, the bond length of Cu(2)–O(3) increases, Raman frequency of the AIg mode of the apical O(3) increases, the bond angle of O(2)–Cu(2)–O(2) is smaller and the hole density is approximately uniformly shared between the three CuO2 planes. The above have been affirmed by our experiment data and calculations based on Bond Valence Sums (BVS). Thus, the enhancement in Tc of the Pb doping effect is due to the variation of the electronic state and its distribution.