Pressure Effects on Resistive Transition in (Cu,M)Ba2Ca3Cu4Oy (M=C,Al,Tl,Mg,Zn) Superconductors

Single phase samples with the composition of (Cu, M)Ba ₂ Ca ₃ Cu ₄ O _y (CuM-1234; M=C,Al,Tl,Mg,Zn) have been synthesized using high pressure technique. We have measured the pressure dependence of superconducting transition temperature (T _c ) through insitu resistivity measurements up to 8 GPa for these samples, reproducibly. These samples indicated almost the same T _c -enhancement by applied pressure, in spite of their different ambient T _c values. The enhancement values of 8-10 K at 8 GPa pressure for these samples are found to be comparable to those of Hg-system and (B,C)-system.     

Synthesis and Properties of Willemite,Zn2SiO4, and M2+:Zn2SiO4 (M = Co and Ni)

Fine particles of willemite, -Zn₂SiO₄, were prepared by both solution combustion and sol-gel methods. Both processes yield single-phase, large-surface area (26- and 78-m²/g), sinteractive willemite powders. Thermal evolution of crystalline phases was studied using X-ray powder diffraction patterns. The combustion method favors low-temperature formation of willemite compared to the sol-gel method. The powders, when uniaxially pressed and sintered at 1300°C, achieved 78–80% theoretical density. The microstructures of the sintered body show the presence of equiaxed 0.5- to 4-m grains. Blue pigments of willemite doped with Co²⁺ and Ni²⁺ were also prepared by the combustion process.     

MF2(M=Mg,Ca,Zn,Sr,Ba)对掺Yb3+磷酸盐玻璃性质的影响

熔制了60P₂O₅·6Al₂O₃·(33-x)BaO·xBaF₂·1Yb₂O₃(x=0,3,6,9)和60P₂O₅·6Al₂O₃·27BaO·6MF₂·1Yb₂O₃(M=Mg、Ca、Sr、Zn、Ba)(分子分数)玻璃,测试了其折射率、密度、转变温度、析晶温度、熔点温度、红外光谱和紫外吸收光谱,讨论了二价氟化物MF₂(M=Mg、Ca、Sr、Zn、Ba)对磷酸盐玻璃热稳定性及内部结构的影响,测试了Yb³⁺离子的吸收光谱、荧光光谱、荧光寿命,计算了光谱参数,讨论了MF₂对Yb³⁺磷酸盐玻璃光谱性质的影响,结果表明二价金属氟化物是作为网络外体进入到磷酸盐玻璃结构中,并没有改变磷酸盐玻璃内部的[PO₄]链状结构,二价金属氟化物还有助于提高Yb³⁺离子的受激发射截面,和自发辐射几率,荧光半高宽。     

The electronic and optical properties of Al2O3, MO, and MAl2O4 (M = Zn, Mg)

The electronic properties and the imaginary parts of the dielectric function for nanosized ZnAl2O4 and MgAl2O4 are studied compared with those of B4-ZnO, B1-MgO and alpha-Al2O3 using a first-principles pesudopotential plane-wave method. The results show that both the electronic structures and the optical spectra of ZnAl2O4 and MgAl2O4 are different from those of ZnO, MgO and Al2O3 due to the atomic rearrangement, which agrees with the experimental data. The insight mechanism is also discussed.     

MF2(M=Mg,Ca,Zn,Sr,Ba)对掺Yb3+磷酸盐玻璃性质的影响

熔制了60P2O5.6Al2O3@(33-x)BaO.xBaF2.1Yb2O3(x=0,3,6,9)和60P2O5.6Al2O3.27BaO@6MF2.1Yb2O3(M=Mg、Ca、Sr、Zn、Ba)(分子分数)玻璃,测试了其折射率、密度、转变温度、析晶温度、熔点温度、红外光谱和紫外吸收光谱,讨论了二价氟化物MF2(M=Mg、Ca、Sr、Zn、Ba)对磷酸盐玻璃热稳定性及内部结构的影响,测试了Yb3+离子的吸收光谱、荧光光谱、荧光寿命,计算了光谱参数,讨论了MF2对Yb3+磷酸盐玻璃光谱性质的影响.结果表明二价金属氟化物是作为网络外体进入到磷酸盐玻璃结构中,并没有改变磷酸盐玻璃内部的[PO4]链状结构,二价金属氟化物还有助于提高Yb3+离子的受激发射截面,和自发辐射几率,荧光半高宽.     

M(M= Mg, Ca, Sr, Ba, Ni)Nb2O6的光催化性能的研究

应用固相烧结法制备了半导体光催化材料MgNb2O6,CaNb2O6,SrNb2O6,BaNb2O6,NiNb2O6粉体.利用XRD、UV-Vis漫反射谱、扫描电镜对上述粉体晶体结构、光谱性质和形貌进行了分析.在紫外光下对其降解罗丹明B（RhB）的光催化性能进行了评估.结果表明,碱土金属Sr、Ba对NiNb2O6中Ni的取代能很好地提高铌酸盐的光催化效果,在10h紫外光照射下能分别将RhB的浓度降解到原来的21%和37%.     

Synthesis of (Hg,X)Ba2Ca2Cu3Ox Superconductors

Synthesis and processing parameters for (Hg_1?xBi_x)Ba₂Ca₂Cu_3.1O_y were optimized for growth at temperatures suitable for growth on Ag and Au substrates (<850° C). Use of Bi₂CuO₄ as a Bi source combined with variations of BaCaCuO precursor phase assemblages allowed the formation of some liquid phase during the reaction and resulted in dense (Hg_1?xBi_x)Ba₂Ca₂Cu_3.1O_y samples with aligned colonies of grains. Magnetization measurements in fields up to 30 T indicated good intergrain coupling within the large grain colonies. The results of the Bi doped samples are compared with those of previously reported Re doped samples.     

Preparation and Dielectric Properties of Bi1.5MNb1.5O7 (M = Cu, Mg, Mn, Ni, Zn) Pyrochlore Oxides

Bi_1.5MNb_1.5O₇ (M = Cu, Mg, Mn, Ni, Zn) oxides are prepared by solid-state reactions and are shown, by x-ray diffraction, to have cubic pyrochlore structure with lattice parameters in the range a= 10.495–10.570 Å. The phases with M = Cu, Mg, Mn, and Ni are prepared and characterized for the first time. The 1- and 10-kHz dielectric permittivity and loss tangent of Bi_1.5MNb_1.5O₇ are measured from 80 to 300 K. For M = Mg, Ni, and Zn, exhibits a maximum at 130, 150, and 90 K, respectively.     

Cu0.5Tl0.5Ba2Ca3Cu4−y Zn y O12−δ (y=0, 1.0, 2.0, 3.0, 3.5): Superconductor with Four ZnO2 Planes

A new Cu_0.5Tl_0.5Ba₂Ca₃Cu_4−y Zn _y O_12−δ (y=0, 1.0, 2.0, 3.0, 3.5) superconductor with four ZnO₂ planes is reported. The structure of the material remains tetragonal for all Zn doping concentration. The substitution of Zn at CuO₂ planar site was carried out following Cu_0.5Tl_0.5Ba₂Ca₃Cu_4−y Zn _y O_12−δ (y=0, 1.0, 2.0, 3.0, 3.5) formula. Contrary to all previous studies of Zn doping in all copper oxide high temperature superconductors, the zero resistivity critical temperature T _c(R=0), critical current density and quantity of diamagnetism increase with increased Zn concentration. The onset temperature of superconductivity in these samples was observed at 128 K and T _c(R=0) at 122 K for y=3.5. The volume of the unit cell observed through X-ray diffraction scan is found to decrease with increase Zn doping; promoting an increase in Fermi vector K _F and effective density of states which results in enhanced superconductivity parameters. The synthesis of Cu_0.5Tl_0.5Ba₂Ca₃Cu_4−y Zn _y O_12−δ material by this method is highly reproducible.      

Preparation of Tl2Ba2Ca3Cu4Oy by the capillary synthetic method

A new phase of Tl₂Ba₂Ca₃Cu₄O_y withT _c = 100 K was prepared by the capillary synthetic method using Tl₂O₃, BaCuO₂ and Ca₂CuO₃ as starting materials. The present capillary synthetic method was useful to obtain the single phase of Tl₂Ba₂CaCu₂O_y Tl₂Ba₂Ca₂Cu₃O_y and Tl₂Ba₂Ca₃Cu₄O_y by solid state reactions using Tl₂O₃ BaCuO₂, and Ca₂CuO₃ as starting materials.     

Physical and dielectric properties of Bi4−xRxSr3Ca3Cu2O10 glasses (x = 0.5 and R = Ag, Ni)

The reflectivity spectra of semiconducting Bi₄Sr₃Ca₃Cu₂O₁₀ glasses were measured over the frequency range between 100 and 4000 cm^–1, and were analysed using Kramer-Kronig routines. It is observed that the static and high frequency dielectric constant varies as the bismuth is partially replaced by silver and nickel. The X-ray diffraction and the dc resisitivity measurements shows a semiconducting behaviour for all the samples. The measured values of the activation energy and other physical parameters based on infrared reflectance measurements are consistent with published work on the similar materials, the activation energy W, hopping energy W_H and other physical parameters vary with the nickel and silver substitutions. It is shown that the hopping in these materials occurs in non-adiabatic regime, unlike other transition metal oxide glasses.     

Superconductivity and Fluctuation-Induced Conductivity (FIC) Analysis of (Cu0.5Tl0.5?x M x )Ba2Ca2Cu3O10?δ

Superconductivity has been improved by partial substitution of slightly higher electronegative (M=Bi, Hg) elements at Tl sites in (Cu_0.5Tl_0.5?x M _x )Ba₂O_4??? (x=0,0.25) charge reservoir layer of (Cu_0.5Tl_0.5?x M _x )Ba₂Ca₂Cu₃O_10??? superconductor. These samples were characterized by X-ray diffractometry, dc-resistivity, ac-susceptibility, FTIR absorption spectroscopy, and fluctuation-induced conductivity (FIC) analysis. The FIC analysis has been carried out in the light of Aslamasov?CLarkin (AL) theory on the resistivity versus temperature curves of (Cu_0.5Tl_0.5?x M _x )Ba₂Ca₂Cu₃O_10??? superconductor. The microscopic parameters such as cross-over temperature (T _o ), zero-temperature coherence length ?? _c (0), inter-layer coupling (J), and critical exponents (?? _2D and ?? _3D) have been determined from FIC analysis of these samples and tried to correlate them to the superconductivity order parameters. A?direct correlation between the cross-over temperatures (T _o ) and zero resistivity critical temperature {T _c (R=0)} and carrier concentration in these samples has been observed. The improvement in T _c (R=0) and the shift of 3D AL region to higher temperature values with the doping of Bi and Hg have also been observed.     

Preparation, characterization and dielectric properties of Ba3M0.33Ta4.67O15 (M=Zn, Ni) ceramics

Two novel Ba₃M_0.33Ta_4.67O₁₅ (M=Zn, Ni) ceramics were prepared by conventional high temperature solid-state reactions and characterized by X-ray diffraction (XRD), scanning electron microscopy (SEM), and dielectric measurements methods. Both compounds are paraelectric phases of the filled tetragonal tungsten–bronze (TB) structure at room temperature. At 1 MHz Ba₃Zn_0.33Ta_4.67O₁₅ and Ba₃Ni_0.33Ta_4.67O₁₅ ceramics show high dielectric constants of 108 and 101 combined with low dielectric loss of 0.0052 and 0.0029, respectively.     

Effect of Mg and Be Doping on Superconducting Properties of TlBa2(Ca2?y M y )Cu3O9?δ (y=0 and 1.5 for M=Mg, Be) Superconductors

In order to study the effect of an insulating charge reservoir layer, we have synthesized as-prepared and oxygen annealed TlBa₂(Ca_2?y M _y )Cu₃O_9??? (y=0 and 1.5 for M=Mg, Be) superconductors sample at atmospheric pressure using a solid-state reaction method. It has been found that the sample mainly contains tetragonal-1223 phase, but there exist some nominal derivative phases of TlBa₂CaCu₂O_9??? and TlBa₂Ca₃Cu₄O_9??? superconductors. It is expected that doping of Mg and Be in place of Ca results in an increase in superconducting properties. But contrary to our expectation, both the [T _c (R=0)] and magnitude of diamagnetism are decreased with the doping of Mg and Be in the final compound. This may be due to the scattering of the carriers in CuO₂ planes resulting in the suppression of superconducting properties. The carrier concentration in these planes is optimized by carrying out a post-annealing experiment. Furthermore, FTIR absorption measurements are also incorporated.     

Nb Substituted Cu0.5Tl0.5Ba2(Ca3Nb1)Cu5O_{14-\delta} and Cu0.5Tl0.5Ba2(Ca4Nb1)Cu6O_{16-\delta} Superconductors

The effects of higher electro-negativity Nb substitution on the Ca site in Cu_0.5Tl_0.5Ba₂ (Ca₃Nb₁)Cu₅O $_{14-\delta}$ and Cu_0.5Tl_0.5Ba₂(Ca₄Nb₁)Cu₆O $_{16-\delta}$ superconductors have been investigated. The Nb doping has been found to increase the oxygen contents in Cu_0.5Tl_0.5Ba₂O $_{16-\delta}$ charge reservoir layer of the final compound. The post-annealing in the N₂ atmosphere has been found to improve the superconducting properties; however, O₂ annealing produced the material with inferior superconducting properties. The quantity of diamagnetism is increased with post-annealing in the nitrogen atmosphere; as N₂ annealing helps in the formation of material with optimum carriers concentration in the CuO₂ planes, which is done by the change in the charge state of thallium from Tl³⁺ to Tl¹⁺. The Nb substitution has been found to develop the higher CuO₂ planner phases such as Cu_0.5Tl_0.5Ba₂Can _?2Nb₁Cun ₂ n+4?δ (n=5,6) with enhanced T _c's.     

Thermochemistry and thermal characteristics of Ba2MIIUO6 (MII = Mg, Ca, Sr, Ba)

A series of uranium compounds with the composition Ba₂M^IIUO₆ (M^II = Mg, Ca, Sr, Ba) were synthesized by a solid-phase procedure. The polymorphism for Ba₂SrUO₆ was studied by high-temperature X-ray diffraction, and the coefficients of thermal expansion were determined. The standard enthalpies of formation of crystalline Ba₂M^IIUO₆ at 298.15 K were determined by the reaction calorimetry.     

Synthesis and dielectric properties of a new class of MX6Ti6O19 (M=Ba,Sr and Ca; X=Mg and Zn) ceramics

MX₆Ti₆O₁₉ (M=Ba, Sr and Ca; X=Mg and Zn) ceramics have been synthesized by the conventional solid state ceramic route. The dielectric properties such as dielectric constant (ε_r), loss tangent (tanδ) and temperature coefficient of dielectric constant (τ_εr) of the sintered ceramic compacts are studied using an impedance analyzer up to the 13-MHz region. The zinc compositions have relatively high dielectric constant and low loss tangent compared to the Mg analogue. Out of the samples studied, BaZn₆Ti₆O₁₉ ceramics show excellent dielectric properties. The structure and microstructure of these ceramic samples are also studied using powder X-ray diffraction, FTIR and SEM methods, and the results are presented.     

Superconducting properties of 1212 phase in (Tl, M) Sr2(Ca1-xCrx)Cu2O7 (M=Zr and Ce)

Samples with nominal starting compositions (Tl0.8M0.2)Sr2(Ca1-xCrx)Cu2O7 (1212) where M=Ce and Zr have been prepared and investigated by the d.c. electrical resistance method and powder X-ray diffraction analyses. Most of the samples showed the 1212 as the major phase and 1201 as the minor phase. Both series of samples showed the highest superconducting onset temperature (Tc) at x=0.2. Ce was found to be more favourable for the formation of the 1212 phase compared to Zr. The valence state of Zr, estimated by employing previous results of optimum average Cu valence for superconductivity in the 1212 superconductors, is 4+. The importance of ionic radius of the substituted elements and the effect of starting composition on Tc and phase formation are discussed. © 1998 Chapman & Hall