The Study of Purity Improvement on Tl-1223 Thin Films by DC Sputtering and Post-annealing Method

Un-substituted TlBa₂Ca₂Cu₃O _x (Tl-1223)-superconducting thin films have been fabricated on a LaAlO₃ (001) substrate in oxygen by using a two-step method, which includes direct current (DC) magnetron sputtering and post-annealing process. Thallium (Tl) content in amorphous precursor films is found to be important to the crystallization of Tl-superconducting phase. Using the nominal composition of Tl_1+δ Ba₂Ca₂Cu₃ O ₈ (δ = 0.1～0.2) precursor films, the formation is promoted to Tl-1223 and Tl-2223 phase rather than Tl-1212 and Tl-1223 phase with accompanying Tl-rich source pellets. When the annealing process continues, Tl-2223 phase will be converted to Tl-1223 phase at a suitable annealing time and temperature. From the X-ray diffraction pattern, only Tl-1223 (00l) peaks are observed, which shows that the purity of Tl-1223 film is improved significantly by this method. The critical temperature T _c of Tl-1223 film is characterized at 110 K, and the critical current density J _c (77 K, T = 0) is up to 1.5 MA/cm².     

Tl2Ba2CaCu2O8 thin films on 2 in. LaAlO3(0 0 1) substrates

High quality Tl₂Ba₂CaCu₂O₈ (Tl-2212) superconducting thin films are prepared on both sides of 2 in. LaAlO₃(0 0 1) substrates by off-axis magnetron sputtering and post-annealing process. XRD measurements show that these films possess pure Tl-2212 phase with C-axis perpendicular to the substrate surface. The thickness unhomogeneity of the whole film on the 2 in. wafer is less than 5%. The superconducting transition temperatures T_cs of the films are around 105 K. At zero applied magnetic field, the critical current densities J_cs of the films on both sides of the wafer were measured to be above 2 × 10⁶ A/cm² at 77 K. The microwave surface resistance R_s of film was as low as 350 μΩ at 10 GHz and 77 K. In order to test the suitability of Tl-2212 thin films for passive microwave devices, 3-pole bandpass filters have been fabricated from double-sided Tl-2212 films on LaAlO₃ substrates.     

Synthesis of Hg1?XRexBa2Ca2Cu3O8+x Pure Phase at Normal Pressures

High purity ceramics of the superconducting phases Hg_1–xRe_xBa₂Ca₂Cu₃O₈ have been prepared by the vacuum sealed quartz tube techniques. The superconductors were obtained using precursor powders of Ba₂Ca₂Cu₃O_x mixed with stoichiometric additions of ReO₂ and HgO. The reaction was carried out in two steps: Firstly, the Ba₂Ca₂Cu₃O_x was homogenised and annealed with ReO₂ in oxygen atmosphere. Finally, the powders were mixed and pressed with stoichiometric additions of HgO and were encapsulated in an evacuated quartz tube and annealed at different rates and temperatures in order to optimise the purity of the phase.     

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.     

Double electrodeposition and heat treatment process for the fabrication of superconducting oxides on the intensified {1 1 3} <1 2 1> Ag substrate

The Tl-1223 coated conductor (Tl_0.8Pb_0.2Bi_0.2Sr_1.8Ba_0.2Ca_2.2Cu₃O_δ) was synthesized by using electrodeposition on the intensified {1 1 3} <1 2 1> Ag substrate applying the new method of the repeating electrodeposition/heat treatment. The films were fabricated conducting the first electrodeposition and then heat treatment, after that the second electrodeposition on the first electrodeposition/heat treatment coated conductors. The second electrodeposition on the first electrodeposition/heat treatment/the first electrodeposition/heat treatment coated conductor showed the better quality of the Tl-1223 phases than that of the first electrodeposition/heat treatment coated conductor, showing more compact and dense grains on the films. The thin Tl-1223 films caused by the thinning process were discussed by considering the properties of Tl and the epitaxial growth aspects. The purer Tl-1223 grains obtained at the double electrodeposition/heat treatment are due to the growth from Tl-1223 grains already synthesized during the first electrodeposition/heat treatment, facilitating the epitaxial growth easier than that of the Ag substrate. The second electrodeposition process was successfully performed, obtaining 1.9 × 10⁵ A/cm² of J_c at 0 T at 10 K.     

Preparation of Tl2Ba2Ca1 − y CeyCu2O8 − x/2Fx Ce-and F-substituted superconductors

High-T _c superconducting ceramics with the compositions Tl₂Ba₂CaCu₂O_{8 ? x/2} (0 ≤ x ≤ 0.3), Tl₂Ba₂(Ca_{1 ? y} Ce_y)Cu₂O_x (0 ≤ y ≤ 0.2), and Tl₂Ba₂(Ca_0.9Ce_0.1)(Cu_1.98Fe_0.02)F_0.2O_8.01 are synthesized. Partial fluorine substitution for oxygen in Tl₂Ba₂CaCu₂O_{8 ? x/2} (0 ≤ x ≤ 0.1) alters the carrier density in the Cu-O planes of the material, raising its superconducting transition temperature from 106 to 110 K. Partial cerium substitution for calcium in Tl₂Ba₂(Ca_{1 ? y} Ce_y)Cu₂O_x (0 ≤ y ≤ 0.2) reduces T _c. The combined substitution in Tl₂Ba₂(Ca_0.9Ce_0.1)(Cu_1.98Fe_0.02)F_0.2O_8.01 has an insignificant effect on T _c.     

Phase Formation in Tl,Bi-1223 Superconductors by LaAlO3 or LSAT Micro-Particle Addition

Polycrystalline (Tl,Bi)(Sr,Ba)₂Ca₂Cu₃O_x (Tl,Bi-1223) samples were synthesized by mixing the precursor with 5wt% micrometer-size LaAlO₃ or LSAT particles. These particles appeared to provide suitable surface for epitaxial growth of Tl,Bi-1223 phase. The growth zone is typically 5 - 10 m thick and several zones can coalesce to form a larger zone. Consequently, there is a dramatic increase in the magnetization signals, indicating the increased superconducting volume fraction. The enhancement is uniform in the external magnetic field. The T_c of the samples was not affected by the additives and the transition remained sharp. The results suggest that a controlled growth of Tl-1223 grain with improved texture can be achieved by adding the growth centers.     

Synthesis and Processing of Doped Hg1Ba2Ca2Cu3Oy Superconductors

A series of quenching experiments were conducted to understand the sequence of reactions that occur during the synthesis of doped Hg1223, (Hg, A)Ba₂Ca₂Cu₃O _y , A = Re, Bi, and Pb (HgA1223). The formation and decomposition of the intermediate phases during the high-temperature reaction were followed as a function of temperature. HgA1223 phase forms over a wide range of temperatures, 750–950°C, 750–880°C, and 840–880°C for A = Re, Pb, and Bi, respectively. At T<750°C, HgA1212 phase forms for A = Re and Pb. Based on the results of quenching experiments, heat treatment conditions were optimized for the synthesis of pure HgA1223 phase using commercial BaCaCuO precursor powders. A reduced-temperature annealing stage after the high-temperature reaction helps in grain growth and improves the microstructural characteristics of HgA1223 samples. Control of Hg pressure during the reaction is crucial for achieving phase purity, grain growth, and texture in the final products. A novel approach for the control of Hg pressure during the synthesis of HgA1223, which consists of using CaHgO₂ as an external Hg source, is reported. HgA1223 samples synthesized using the new synthesis protocol exhibit improved microstructural and superconducting properties.     

Investigations of Bi substitution in NdBa2Cu3Oy

Attempts to substitute Bi for Nd in orthorhombic NdBa₂Cu₃O _y , prepared in air or oxygen at about 950°C led instead to formation of Ba₂NdBiO₆, a new cubic compound witha=0.8703 nm. The possibility was then explored of preparing superconducting (Nd_1–x Bi _x )Ba₂Cu₃O _y , by first forming the tetragonal phase at 880–950°C in nitrogen or argon followed by reheating in oxygen or air at 250–500°C in order to insert the additional oxygen required to yield the orthorhombic form while avoiding oxidation of Bi³⁺ to Bi⁵⁺. X-ray diffraction studies, electrical conductivity measurements, and thermogravimetric analysis of products indicate that Bi does not enter the NdBa₂Cu₃O _y , lattice in either the tetragonal or the orthorhombic phase. Ba₂NdBiO₆ clearly forms on reheating in oxygen or air even at low temperatures, and evidence is presented that a poorly crystallized oxygen-deficient form of this compound is already present prior to the reheating.     

Doped Tl-1223 Superconductors

Doped Tl-1223 superconductors of the composition Tl_aBi_bPb_cBa_nSr_2?nCa₂Cu₃O_y (a = 0.50 ? 0.76, b = 0, 0.16 and 0.3, c = 0.5, 0.24 and 0, n = 0.15 ? 0.4) were prepared from oxidic Ba-Sr-Ca-Cu precursor material. Upon calcination of the tartrate gels or the nitrates, the respective amounts of Tl₂O₃, PbO and Bi₂O₃ were added to the oxidic powders by milling. The samples were sintered in silver foil. The specimens consisted predominantely of the Tl-1223 phase with small amounts of Tl-1212 and secondary phases. Tc₍₀₎-values ranged between 115 K and 119 K with transition widths around 1 K. Transport critical current densities up to 1.4 kA cm^?2 at 77 K were measured for the assintered samples. Texturing was obtained by repressing and resintering of the superconducting material followed by annealing at 750°C for 50 h in flowing oxygen. The transport critical current densities could be improved up to 7 kA cm^?2 by the thermomechanical treatment. Microstructural studies yielded information on the alignment of the superconducting grains, on secondary phases and the porosities.     

Structural and dielectric properties of ferroelectric Sr1−xBaxBi2(Nb0.5Ta0.5)2O9 and Sr0.5Ba0.5Bi2(Nb1−yTay)2O9 ceramics

Ferroelectric Sr_1−xBa_xBi₂(Nb_0.5Ta_0.5)₂O₉ and Sr_0.5Ba_0.5Bi₂(Nb_1−yTa_y)₂O₉ were synthesized by solid state reaction route. X-ray diffraction studies confirm the formation of single phase layered perovskite solid solutions over a wide range of compositions (x=y=0.0, 0.25, 0.50, 0.75 and 1). The lattice parameters and the Curie temperature (T_c) have been found to have linear dependence on x and y. Transmission electron microscopy (TEM) suggest the lowering of orthorhombic distortion with increasing Ba²⁺ substitution. Variations in microstructural features as a function of x and y were monitored by scanning electron microscopy (SEM). The dielectric constant at room temperature increases with increase in both x and y. Interestingly Ba²⁺ substitution on Sr²⁺ site induces diffused phase transition and diffuseness increases with increasing Ba²⁺ concentration.     

Effects of Mn doping on structural and dielectric properties of sol-gel-derived (Ba0.835Ca0.165)(Zr0.09Ti0.91)O3 thin films

We reported the effects of Mn doping on the structure and dielectric properties of (Ba_0.835Ca_0.165)(Zr_0.09Ti_0.91)O₃ (BCZT) thin films prepared by sol-gel method. The (Ba_0.835Ca_0.165)Mn_x(Zr_0.09Ti_0.91)_1 − xO₃ (x = 0, 0.002, 0.005, and 0.01) thin films exhibited a pure pseudo-cubic perovskite structure with random orientation. Scanning electron microscopy and atomic force microscopy observation showed that increasing Mn-doping amount caused a decrease in particle size and a cluster of the particles, while the film surface remained smooth and crack-free. Compared with the undoped film, Mn doped BCZT thin films exhibited smaller dielectric constant and lower dielectric loss. The figure of merit reached the maximum value of 16.7 with a tunability of 53.6% for the film with 0.5 mol % Mn doping, when a bias electric field of 400 kV/cm was applied at 100 kHz. The results indicated that the Mn doped BCZT thin films are suitable for tunable microwave device applications.     

Microstructure and electrical properties of Ba0.7Sr0.3(Ti1 − xZrx)O3 thin films prepared on copper foils with sol-gel method

Ba_0.7Sr_0.3(Ti_1 − xZr_x)O₃ (x = 0, 0.1, 0.2) (BSZT) thin films have been prepared on copper foils using sol-gel method. The films were annealed in an atmosphere with low oxygen pressure so that the substrate oxidation was avoided and the formation of the perovskite phase was allowed. The X-ray diffraction results show a stable polycrystalline perovskite phase, with the diffraction peaks of the BSZT films shifting toward the smaller 2θ with increasing Zr content. Scanning electron microscopy images show that the grain size of the BSZT thin films decreases with increasing Zr content. High resolution transmission electron microscopy shows the clear lattice and domain structure in the film. The dielectric peaks of the BSZT thin films broaden with increasing Zr content. Leakage current density of Ba_0.7Sr_0.3(Ti_1 − xZr_x)O₃ (x = 0.1) thin film is the lowest over the whole applied voltage.     

Temperature effects on the growth and electrical properties of Er2O3 films on Ge substrates

Er₂O₃ films were grown on Ge (001) substrates at different temperatures by molecular beam epitaxy using metallic Er and molecular oxygen sources with otherwise identical conditions. High-resolution transmission electron microscopy and X-ray photoelectron spectroscopy were used to characterize the microstructures and compositions of the films. The film deposited at room temperature is found to be composed of an Er₂O₃ layer and an ErGe_xO_y interface layer with a thickness of 5.5 nm; the film grown at 300 °C has a mixed structure of Er₂O₃ and ErGe_xO_y and the thickness was found to be reduced to 2.2 nm; the film grown at 450 °C becomes much rougher with voids formed underneath the film, having a mixed structure of three compounds of Er₂O₃, GeO and ErGe_xO_y. The growth mechanisms of the films at different temperatures are suggested. Current images obtained by tunneling atomic force microscopy show that the film grown at 450 °C has much more leaky spots than those grown at RT and 300 °C, which may arise from the formation of volatile GeO in the film.     

Synthesis, crystal structure and thermal decomposition of [La2(CH3CH2COO)6·(H2O)3]·3.5H2O precursor for high-k La2O3 thin films deposition

Lanthanum acetylacetonate La(C₅H₇O₂)₃·xH₂O has been used in the preparation of the precursor solution for the deposition of polycrystalline La₂O₃ thin films on Si(1 1 1) single crystalline substrates. The precursor chemistry of the as-prepared coating solution, precursor powder and precursor single crystal have been investigated by Fourier Transformed Infrared Spectroscopy (FTIR), differential thermal analysis coupled with quadrupole mass spectrometry (TG-DTA-QMS) and X-ray diffraction. The FTIR and X-ray diffraction analyses have revealed the complex nature of the coating solution due to the formation of a lanthanum propionate complex. The La₂O₃ thin films deposited by spin coating on Si(1 1 1) substrate exhibit good morphological and structural properties. The films heat treated at 800 °C crystallize in a hexagonal phase with the lattice parameters a = 3,89 Å and c = 6.33 Å, while at 900 °C the films contain both the hexagonal and cubic La₂O₃ phase.     

Novel Metal-Organic Chemical Vapor Deposition / T1F Annealing Route to Thin Films of Tl1Ba2Ca2Cu3O9+x

Thin films of TI₁Ba₂Ca₂Cu₃O_9+x have been grown on single crystal (110) LaAlO₃ by a metal-organic chemical vapor deposition process employing Ba(hfa)₂.mep, Ca(hfa)2.tet. and Cu(dpm)₂ (hfa = hexafluoroacetylacetonate; dpm = dipivaloylmethanate; tet = tetraglyme; mep =methylethylpentaglyme) as the volatile metal sources. A subsequent phase-selective annealing procedure accomplishes thallination using TIF in a bulk oxide pellet. The resulting films are nearly phase-pure and highly oriented and contain negligible fluoride by windowless energy-dispersive x-ray measurements. The films exhibit transport measured T_c = 103 K and J_c > 10⁴ A/cm² (77 K. 0 T) and at low temperatures retain J_c > 10⁵ A/cm² (5 K, 4.5 T) as measured by magnetic hysteresis.     

Superconductivity at 103 K in CdBa2(Ca0.7Y0.3)Cu2Oy

A cadmium analogue of the mercury system with nominal composition CdBa₂(Ca_1–xY_x)Cu₂O_y has been synthesized. Thex=0 samples contain about 12 vol.% of the 1212 phase but are not superconducting. Thex=0.3 samples are superconducting atT _on = 103 K. The EDX analysis of 18 microcrystals shows a broad cationic distribution of the different components. The observed broad superconducting transition is attributed to the variousT _c of the different microcrystals.     

Preparation and Superconducting Properties of (Tl,Pb)-Based Compounds with 1234-Type Structure

(Tl_1?x Pb _x )(Sr_2?y Ba _y )Ca₃Cu₄O _z samples were synthesized by using a two-step solid state reaction method and examined with respect to the (Tl,Pb)-1234 phase formation and superconducting properties. Samples were prepared from various starting compositions of 0≤x≤0.5 and 0.4≤y≤2.0. Nearly single-phased samples were obtained for (Tl_0.8Pb_0.2)(Sr_2?y Ba _y )Ca₃Cu₄O _z (1.0≤y≤1.25) and (Tl_1?x Pb _x )(SrBa)Ca₃Cu₄O _z (0.15≤y≤0.35). We have found that both the partial substitution of Ba for Sr and the partial substitution of Pb for Tl are effective for the formation of the Tl-1234 phase. Bulk superconductivity with a T _c value of 106 K is observed in the (Tl_0.7Pb_0.3)(SrBa)Ca₃Cu₄O _z sample.     

Effect of M and M-Ca Substitution on the Structure and Superconductivity of GdBa2Cu3O7−δ [M = Mo, Hf ]

The structural and superconducting properties of (Gd_1?x?y Ca _y M _x )Ba₂Cu₃O _z with M = Mo, Hf are investigated using X-ray diffraction, electrical resistivity, and oxygen content measurements. The effect of increasing the Mo concentration in (Gd_1?x Mo _x )Ba₂Cu₃O _z changes the structure from orthorhombic to tetragonal accompanied by a large increase in resistivity and a fast decrease in T _c at the rate of 1.9 K per at.% of Mo, unlike that of Hf substitution in (Gd_1?x Hf _x )Ba₂Cu₃O _z , which maintains the orthorhombic structure and decreases T _c very slowly at the rate of 0.6 K per atm.% of Hf with nearly no change in resistivity. The suppression of T _c by M = Mo, Hf can be counterbalanced by hole doping by Ca which increases T _c with increasing Ca content showing maximum compensation for Mo. A comparative study of M = Mo, Hf doped samples in (Gd_1?x?y Ca _y M _x )Ba₂Cu₃O _z indicates that the valence of the dopant M = Mo^4+,6+, Hf⁴⁺ and its ionic radius play an important role in controlling the structural and superconducting properties of the systems.     

Study on properties of BaxSmyTi7O20 ceramics with CaO-SiO2-B2O3 glass

The effect of CaO-SiO₂-B₂O₃ (CSB) glass addition on the sintering temperature and dielectric properties of Ba_xSm_yTi₇O₂₀ ceramics has been investigated using X-ray diffraction, scanning electron microscopy and differential thermal analysis. The CSB glass starts to melt at about 970 °C, and a small amount of CSB glass addition to Ba_xSm_yTi₇O₂₀ ceramics can greatly decrease the sintering temperature from about 1350 to about 1260 °C, which is attributed to the formation of liquid phase. It is found that the dielectric properties of Ba_xSm_yTi₇O₂₀ ceramics are dependent on the amount of CSB glass and the microstructures of sintered samples. The product with 5 wt% CSB glass sintered at 1260 °C is optimal in these samples based on the microstructure and the properties of sintering product, when the major phases of this material are BaSm₂Ti₄O₁₂ and BaTi₄O₉. The material possesses excellent dielectric properties: ?_r = 61, tan δ = 1.5 × 10⁻⁴ at 10 GHz, temperature coefficient of dielectric constant is −75 × 10⁻⁶ °C⁻¹.