Investigations on TlBa2Ca2Cu3Ox superconducting thin films formation on LaAlO3 substrate

Investigations on pure superconducting phase TlBa₂Ca₂Cu₃O_x (Tl-1223) thin films formation, of about 100-125 nm in thickness, on (001) LaAlO₃ single crystal substrate, were made using radio-frequency sputtering deposition of Ba₂Ca₂Cu₃O_x precursor films and ex-situ thallination in sealed quartz tube. The precursor films were thallinated under different conditions of partial oxygen pressure, temperature, time and y thallium source content using unreacted pellets of composition Tl_yBa₂Ca₂Cu₃O_x. In all cases, strongly c-oriented multiphase films were obtained. A correlation between the Tl-1223 phase purity and the precursor film conditions of thallination is established. Temperature and time of thallination as well as the thallium source content and the partial pressure of oxygen play a key role in the quality of the obtained film. The films' onset temperature of the superconducting transition ranges between 90 and 103 K. It is shown that the best samples can be obtained from a dense precursor film and relatively medium thallination time.     

Phase Formation and Grain Growth Kinetics of High-Tc Superconducting Tl-1223 Ceramics

The phase formation and grain growth of (Tl,Pb,Bi)(Sr,Ba)₂Ca₂Cu₃O_9?δ ceramics were investigated in the temperature range between 860°C and 1000°C. It was found that partial melting seems to improve the texture of the samples. The phase purity, however, is limited due to an incomplete re-precipitation process due to the formation of a liquid phase, which is already present above 885°C. The resulting grain growth exponent n, however, appears to differ from the classical exponent of 1/3 for the coexistence of a liquid phase in the temperature range between 885°C and 920°C. For the first time, the activation energy for the high-T_c Tl-1223 phase has been determined showing faster grain growth kinetics than for Y-123 and Bi-2223.     

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.     

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.     

Investigations on Tl-2223 thin films fabricated through ultrasonic spray pyrolysis under oxygen deficient conditions

In this paper we have shown that polycrystalline films corresponding to Tl-2223 phase can be grown by employing high thalliation temperatures and short thalliation times. Ultrasonically deposited precursor films corresponding to Ba₂Ca_2.2Cu_3.3O_x(Ag_y) have been thalliated under high vacuum (∼ 10^-5 torr) at 890°C to obtain single phase Tl-2223 films. An off-stoichiometric and unreacted pellet of composition Tl_2.05Ba₂Ca₂Cu₃O_z has been used as source of Tl. We have shown that oxygen ambient is not necessary for the growth of Tl-2223 phase. The as-thalliated films have T_c’s in the range 123 K ±0.70 K. TheT _c has been found to be independent of the addition of AgNO₃ to the precursor. The zero field transportJ _c has been observed to be > 1.2 X 10⁵ A/cm² at 77 K. NearT _c (110 K-122 K),J _c has been observed to follow the power lawJ _c ∞ (1-T/T _c )_p,p 2. A power law withp tt 1.4 has been observed for the temperature range 70 K-110 K. An optimum doping of Ag has been observed to induce about 25% increase inJ _c and it also leads to uniform and enlarged grain growth. The surface morphology of Ag free samples contains plate like grains having arbitrary shapes. In contrast to this 0.35 Ag doped sample exhibits nearly rectangular plate like grains     

Synthesis of Thick Tl-Based High-T c Oxide Layer Through a Diffusion Process

Tl-based high-T _c oxide layers have been synthesized through the reaction between Tl-free substrate and coating layer containing Tl. The coating layer with the eutectic composition in V₂O₅-Tl₂O₃ system much enhances the formation of Tl-2223 phase without degrading its superconducting properties. The TIF substitution for T1₂O₃ in the coating layer promotes the formation of Tl-1223 phase, which significantly improves the I _c-B performance at higher temperatures. The behavior of F during the reaction has been studied. Thick Tl-1223 layer is formed on Ni tape with good bonding through the diffusion reaction.     

Synthesis of Thick Tl-Based High-Tc Oxide Layer Through a Diffusion Process

Tl-based high-T _c oxide layers have been synthesized through the reaction between Tl-free substrate and coating layer containing Tl. The coating layer with the eutectic composition in V₂O₅-Tl₂O₃ system much enhances the formation of Tl-2223 phase without degrading its superconducting properties. The TIF substitution for T1₂O₃ in the coating layer promotes the formation of Tl-1223 phase, which significantly improves the I _c-B performance at higher temperatures. The behavior of F during the reaction has been studied. Thick Tl-1223 layer is formed on Ni tape with good bonding through the diffusion reaction.     

Effect of cationic size in Hg(Tl/Bi)Ba2Ca2Cu3O8+δ on superconducting and microstructural characteristics

In this paper we have reported investigations on the effect of simultaneous substitution of Bi and Tl at the Hg site in the oxygen deficient HgO_δ layer of HgBa₂Ca₂Cu₃O_8+δ cuprate superconductor. Bulk polycrystalline samples have been prepared by the two-step solid state reaction process (precursor route). It has been observed that the as grown HgBi_0.2?xTI_xBa₂Ca₂Cu₃O_8+δ (withx = 0.00, 0.05, 0.10, 0.15, 0.20) corresponds to the 1223 phase. It has been found that the T_c varies with the average cationic size < R_d > of the dopant cations. The optimumT _c of ～ 131 K has been found for the composition HgBi_0.15TI_0.05Ba₂Ca₂Cu₃O_8+δ This composition leads to the average dopant cation size of ～1.108 å which is very close to the size of Hg²⁺ (～ 111 å). The microstructure for HgBi_0.15TI_0.05Ba₂Ca₂Cu₃O_8+δ has been found to be most dense and this phase exhibits the highest stability. The J_c of the optimum material HgBi_0.15TI_0.05Ba₂Ca₂Cu₃O_8+δ is found to be ～ 1.29 × 10³ A/cm² at 77 K.     

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.     

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 μm 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.     

The Influence of Bi Doping on the In Situ Growth of TlBa2Ca2Cu3O9 High-T c Films

The in situ process—laser ablation in combination with thermal evaporation of Tl₂O—has turned out to be a preparation method for single-phase and epitaxial TlBa₂Ca₂Cu₃O₉ (1223) thin films with T _c values up to 109 K. It was found by several groups that a partial substitution of Tl by Bi simplifies the phase development of the 1223 compound in the usual two-step process. We have investigated the influence of the Bi doping on the in situ growth. X-ray measurements show that the films consisted mainly of the 1223 compound. In 300-nm thin films there was no evidence of a Bi amount in the crystal structure, but thinner films (80 nm) show a small amount of Bi. We concluded that Bi doping supports the phase development of the 1223 compound only in an early stage of the film growth. The Bi-doped films have higher T _c values up to 114 K, higher j _c values up to 6 × 10⁵ A/cm² (77 K, 0 T), and lower surface resistances of 56 mΩ (77 K, 87 GHz) than the undoped films.     

Properties of boron doped Tl-Ba-Ca-Cu-O superconductors

The effects of boron doping on the formation and properties of the Tl-based superconductors have been studied. Up to 10 wt.% boron has been added to the oxides having the nominal composition, Tl_1.8Ba₂Ca_2.2Cu₃O_x, by the usual solid-state reaction method. Boron additions in the range 0.8-1.0% increase the fraction of the Tl-2223 phase and significantly improve the critical temperature of the samples. Higher amounts of boron additions eliminate the Tl-2223 phase, reduce the fraction of Tl-2212 phase and cause separate non-superconducting phases to be formed. The formation of non-superconducting phases does not allow us to obtain pure Tl-2212 phase.     

Preparation of Double Perovskite Ca2NiWO6 as a Buffer Layer for YBa2Cu3O7−δ Coated Conductors

Ca₂NiWO₆ films were deposited on the Y-stabilized ZrO₂ (100) (YSZ) single crystalline substrates by pulsed laser deposition process. X-ray diffraction reveals that the Ca₂NiWO₆ film is coherently strained due to the small lattice mismatch between Ca₂NiWO₆ and YSZ substrate. With this c-axis orientated Ca₂NiWO₆ film as buffer layer, YBa₂Cu₃O_7?δ film has been successfully prepared by pulsed laser deposition. The subsequently deposited YBa₂Cu₃O_7?δ film has a preferred c-axis orientation and a smooth surface. The temperature dependence of resistance indicates a superconducting critical temperature higher than 82 K. Our results demonstrate that the double perovskite Ca₂NiWO₆ is a promising buffer layer candidate for coated conductors.     

Synthesis and characterization of the Indium-doped Tl-1223 phase

Superconductor of type Tl_{1 – x}In_xBa₂(Ca_0.9Y_0.1)₂Cu₃O_{9 –} for x = 0, 0.1, 0.2, 0.3 and 0.5 were synthesized using stoichiometric amount of Ba₂(Ca_0.9Y_0.1)₂Cu₃O_{7 +} precursor, Tl₂O₃ and In₂O₃ in a tube furnace at 870°C. The samples were characterized using the x-ray powder diffraction and the scanning electron microscope (SEM) whereas the composition of these samples were determined by micro-probe analysis. The transition temperature T_c for as-synthesized (Tl, In)-1223 compound, determined from the electrical resistance measurements, varied from 122 K for x = 0 to 100 for x = 0.6 These results indicate that the addition of indium decrease the transition temperature of Tl-1223 phase. This is because the addition of indium changes the Tl-1223 phase to the over-doped region. The argon annealing for some of these samples were studied. The effect of external applied magnetic field on the stage of transition is reported. Our data show that the transition width is increased by the increasing of the external applied magnetic field. The samples are more affected by the magnetic field with increasing of indium content.     

Preparation and Characterization of Nanometer Grained High Temperature Superconducting High-Quality Epitaxial Bi-2223 Thin Films Grown by DC Sputtering

Bi₂Sr₂Ca₂Cu₃O_10+?? HTSC epitaxial thin films with thickness in the order of 6.0?nm were prepared onto (100) aligned SrTiO₃ single-crystal substrates by DC sputtering from stoichiometric targets. As-grown samples were characterized by X-ray diffraction, AC-susceptibility and scanning electron microscopy. X-ray diffraction patterns show that all obtained superconducting thin films were c-axis oriented with a Bi-2223 phase. All reflections (except the substrate ones) can be assigned to the (00l) reflections of the film material (h=k=0, l??0), indicating that the films were grown preferentially with the c-axis normal to the film plane. In order to investigate the crystal quality of these Bi-2223 films, the rocking curves of the (0012) peaks were explored by ??-scans. The rocking curve of the (0012) reflection had a full width at half maximum (FWHM) of 0.30??. This demonstrates that our prepared Bi-2223 thin films have good crystalline quality and high degree of c-axis orientation. The grain size has well known important effects in the magnetic, optical, and electrical properties of metals and alloys. High temperature superconducting thin films, obtained in this work, have nanometer grain size. The mean size of the grains of the samples were determined by X-ray diffraction (XRD) and found to be in the order of 34.8?nm. The superconducting transitions temperature of several Bi-2223 samples is about 103?K. Surface morphology of the films and chemical composition were studied using scanning electron microscopy (SEM) and energy dispersive X-ray microanalysis (EDX).     

Electric,magnetic, and transport critical current density behavior of the 2201, 2212, and 2223 phases of ‘BSCCO’

We have studied the electrical properties of polycrystalline pellets of the high-T _c superconducting phases occurring in the Bi₂O₃-SrO-CaO-CuO (BSCCO) system, having nominal compositions Bi₂Sr₂Cu₁O_6+y, (2201 phase, withT _c = 10 K.) Bi₂Sr₂CaCu₂O_8+y (2212 phase, withT _c = 85 K), and Bi₂Sr₂Ca₂Cu₃O_10+y (2223 phase, withT _c = 110 K). Pellets containing the 2223 phase having zero resistance below 105 K were obtained. For the 2212 and 2223 phases, the transport critical current density was measured as a function of temperature and of the externally applied magnetic field. As previously found for YBa₂Cu₃O_7-x (YBCO), results are consistent with the occurrence of a weak superconducting coupling among the grains. Such coupling was, however, noticeably stronger for the 2223 phase than for the 2212 phase.     

Epitaxial Film Growth of Tl0.78Bi0.22Sr1.6Ba0.4Ca2Cu3O9 on Rolling Assisted Biaxially Textured Nickel Substrates with YSZ And CeO2 Buffer Layers

Epitaxial film growth of Tl_0.78Bi_0.22Sr_1.6Ba_0.4Ca₂Cu₃O₉ ((Tl,Bi)-1223) on rolling assisted biaxially textured substrates with YSZ and CeO₂ buffer layers (RABiTS) has been successfully demonstrated by laser ablation and post-deposition annealing in flowing argon. X-ray diffraction (XRD) θ-2θ spectra showed that the films consisted mainly of c-axis aligned 1223 phase with some intergrown 1212 phase, while XRD Φ-scans of (102) pole figure revealed that the films are also a- and b-axes aligned, with an epitaxy of the «100» of (Tl,Bi)-1223 film on the «110» of the top YSZ buffer layer. Four-terminal electrical transport measurements showed that the zero-resistance transition temperature (T_c) was in the range of 106 - 110 K, and the critical current density (J_c) at 77 K and zero field was about 10⁵ A/cm² for the entire film width (3 mm) of a longer film (14 mm) which was processed differently from the shorter films (7 mm). For a shorter film (7 mm) that showed better ab-in-plane alignment, the magnetization J_c, at 77 K and extrapolated to zero field, calculated from Bean's model using the full film width (3.5 mm) as the appropriate lateral dimension, was 2 × 10⁵ A/cm².     

Novel ceramic substrates for high Tc superconductors

A group of complex perovskite oxides REBa₂NbO₆ (RE=La and Dy) have been synthesized and developed for their use as substrates for both YBa₂Cu₃O_7-δ and Bi(2223) superconductors. These materials have a complex cubic perovskite (A₂BB′O₆) structure with lattice constants,a=8·48?8·60 Å. REBa₂NbO₆ did not show any phase transition in the temperature range 30–1300°C. The thermal expansion coefficient, thermal diffusivity and thermal conductivity values of REBa₂NbO₆ are favourable for their use as substrates for highT _c superconductors. The dielectric constant and loss factor of REBa₂NbO₆ are in a range suitable for their use as substrates for microwave applications. Both YBa₂Cu₃O_7-δ and Bi(2223) superconductors did not show any detectable chemical reaction with REBa₂NbO₆ even under extreme processing conditions. Dip coated YBa₂Cu₃O_7-δ thick films on polycrystalline REBa₂NbO₆ substrate gave aT _c(0) of 92 K and a current density of ～1·1×10⁴ A/cm² and Bi(2223) thick film on polycrystalline REBa₂NbO₆ substrate gave aT _c(0) of 110 K and a current density of ～ 4×10³ A/cm² at 77 K and zero magnetic field. A laser ablated YBa₂Cu₃O_7-δ thin film deposited on polycrystalline REBa₂NbO₆ substrate gave aT _c(0) of 90 K and a current density of ～5×10⁵ A/cm².     

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.     

Superconducting Properties of Spray Deposited TBCCO Films

The present study shows general characteristic features of processes such as spray pyrolysis and heat-treatment, involved in the preparation of Tl-2223/CeO₂/Al₂O₃ superconducting coatings by using aerosol pyrolysis method. The importance of the present study, however, lies in a new process to make a thick Tl-2223 superconducting layer by using spray pyrolysis. In the new method, a relatively thick BaCaCuO precursor film is firstly spray deposited on the buffered alumina substrate and then heat-treated in the presence of thallium source for phase transformation to superconducting phases to assign conductivity to the film. The thickness of the Tl-2223 layer was 2–3 μm. The crystal symmetry was found to be tetragonal with lattice parameters a=3.85, and c=35.7 Å. The T _c was found to be 98 K for the samples heat treated at 870?°C for 60 min. Magnetic characterizations were conducted by using a DC-SQUID magnetometer. From the magnetization measurements, the intragrain critical-current density, J _c, was calculated to be 2×10⁴ A/cm² at 70 K.