SG and SSR approach in the preparation of precursors and inuence on superconducting properties of Tl-1223 superconductors

The paper presents the comparative analysis of the Sol-gel (SG) and solid-state reaction (SSR) route for the synthesis of precursors for thallium-based superconductors. Samples were prepared a two-step method and by sealed quartz tube technique in ambient pressure. Heat treatments of precursors showed that to obtain high-purity precursors, without any carbonate contamination for SG methods is sucient to heat treatments at 915 o C temperatures and for SSR methods 945 o C. The diamagnetic onset temperature of the superconducting transition for the TlBa 2 Ca 2 Cu 3 O 8+δ samples prepared by precursors SG at 915 o C and SSR at 945 o C is about T = 120 K and full screening of applied ac magnetic elds observed at T ≈ 102 K and T ≈ 94 K, respectively. The value of J c transport critical current densities for SSR obtained 128 A/cm 2 , whereas for the SG sample and exactly 174 A/cm 2 . We could conclude that the, using the wet chemistry offers some advantages in comparison with the classical solid-state ceramics processing, especially, better chemical homogeneity and higher reactivity of the precursor powder.


Introduction
Thallium-and mercury-based superconductors are generating considerable interest, as these systems set records for the transition temperature to the superconducting state. The Tl-1223 and Hg-1223 had the transition temperatures T c ≈115K and T c ≈133K when they prepared at ambient conditions. And when they synthesized under high pressures, the critical temperature can reach 133.5 K and 164 K, respectively [1][2][3][4]. The formula unit and crystal structure of the TlBa 2 Ca n − 1 Cu n O 2n+2+δ system similar to that of the HgBa 2 Ca n − 1 Cu n O 2n+2+δ system, where (n) is the number of adjacent Cu-O layers. Mercury-and thallium are very toxic, volatile at high temperature, and what is more important for this family, to achieve high purity superconductivity phase, critically depends on the used precursor and synthesis conditions. Preparation of high purity and reaction ability precursor, in the various articles, was solved by diverse methods [5][6][7][8][9][10][11].
S.L. Loureiro et al. investigated importance [5] of average copper valence in the precursor for synthesized homologous series of Hg-1223 superconductors under high pressure. Have to note that, so that avoid problems elimination residual carbonate in the precursor, therefore in work, calcium oxide and barium nitrates were used as starting materials. A precursor was prepared by solid-state reaction methods. They show that average copper valence is a crucial dependence on the oxygen intake of non-stoichiometric phases during the precursor synthesis at a speci c temperature, time, and cooling process treatment. By S. Lee and coauthors [6] proposed a method of freeze-dried for synthesized highly homogeneous precursors. For starting, materials have used nitrates. As a result, for Hg-1223 samples prepared from freeze-dried precursor were obtained 75% of Hg-1223 superconducting fazes and T c ~133 K. Metalorganic chemical vapor deposition methods are presented in articles [7,8]. Precursor lms have derived by employ Ba(hfa) 2 mep, Ca(hfa) 2 tet, Cu(dpm) 2 as metal sources. In this case, for the nal synthesis necessarily need Thallium Fluorine for nucleating the Tl-1223 phase. In results were obtained high transport properties samples, with very short transition temperature. Sin et al. [9] have studied simultaneously the in uence of the Rhenium addition and in-situ gelation process using acrylamide monomers on the synthesis of precursors. The authors conclude that the rhenium based precursor is much more stable against moisture and carbonation. The urea combustion method for synthesizing precursors is present by T.M. Mendonca and coauthors [10]. The results showed that the in urea samples observed the single phase 99% vol., of the Hg-1223 phase. Brylewski et al. [11] have been reporting about the sol-gel method for synthesized Ba 2 Ca 2 Cu 3 O x precursors. The starting materials were used nitrates and nitrates tetrahydrate, and as complexing agent ethylenediaminetetraacetic acid. Subsequently, was observed 89.1% of the high volume fraction of the (Hg,Pb)-1223 phase, The authors think, that EDTA chelating agent may be a reason for this.
The subject of this work is to synthesize the precursors separately by a sol-gel and solid-state reaction methods and based on that the Tl-1223 superconductors. It is mast note that for both ways, for starting materials was used oxide and carbonate-containing materials. For sol-gel route was used poly(vinyl alcohol)/poly(vinyl acetate) as the complexing agent [12,13]. We present a comparative analysis with the data of samples TlBa 2 Ca 2 Cu 3 O 8+δ obtained by the SG and SSR approach. The characterization of the superconducting parameters has investigated by the x-ray diffraction, the FTIR analysis. The phase method was used to study the real parts − 4πχ' of the linear susceptibility. Errors in determination of χ' at higher frequencies than 1 kHz do not exceed 1% when − 4πχ > 0.1. For 4πχ < 0.1 the errors are increased in proportional to diminishing the magnitude of susceptibility and frequency. For the measurements of intergranular critical current densities, we used the method of high harmonics. The error of measurement of high harmonics was approximately 2% when the measured signal was less than 0.2 µV and no more than 0.5% when the signal was higher. The measurements mainly performed at h = 1 Oe, f = 20 kHz, and H = 0. The Earth's magnetic elded was shielded to less than 10 − 3 Oe by use of Permalloy screens [14][15].

Experimental
For synthesized TlBa 2 Ca 2 Cu 3 O 8+δ samples, we used the two-step method. In the rst stage, a Tl-free precursor has prepared before proceeding to the second stage, where Tl 2 O 3 has added to the precursor before nal sintering. We note that for both methods, starting materials have utilized powders materials gel was dried slowly, the temperature rate 1 o C per minute up to 300 o C. The result was a black powder that was then grounded in an agate mortar. Then, the powdered was calcined at 900 o C in the air with a heating rate of 2 o C /min, for 12 h, with two intermediate grindings.
The resulting powders were ground, separated into six parts, and pressed in the form of a disc. For the eliminates the CO 2 from precursors, each pellet was individually annealed tube-type furnace at different In the second step both Ba 2 Ca 2 Cu 3 O x precursors prepared by SG and SSR methods separately was mixed with Tl 2 O 3 according to the composition TlBa 2 Ca 2 Cu 3 O 8+δ and After nal grinding the powder was pressed into a disc-shaped pellet 6 mm in diameter, and 3 mm thick, by using a hydraulic press under a pressure of 400 MPa. The samples have wrapped in a platinum foil then individually put into quartz tubes and from quartz tubes were evacuated up to 10 − 3 Torr and sealed. Thereafter, a quartz tube has inserted into a programmed mu e furnace. The temperature of the furnace was raised at a rate of 25 o C/min up to 900 o C and held at this temperature for 8 h. After the synthesis completed the furnace was quickly cooled to room temperature. X-ray powder diffraction (XRD) patterns were obtained on a Dron-3 + PC diffractometer with CuKα radiation. The Fourier transformed IR of the samples was taken in the region 400-4000 cm − 1 on a Cary 600 series FTIR Spectrometer using the KBr disc technique. Scanning resolution 0.5 sm − 1 . The samples were pulverized into a ne powder and then mixed with potassium bromide powder using a weight ratio of 1:100. The IR absorption spectra were measured immediately after preparing the discs.

Results And Discussion
As described above, to achieve the higher purity superconductivity phase critically depends on the used precursor. To get precursors with optimal properties best way is thermal annealing at the oxygen partial pressure, especially if precursors will be prepared from oxide containing carbonates. The synthesis of precursors in oxygen pressure provides the elimination of the carbonates in samples and the cation homogeneity and the oxygen content. The Absorption spectra of the precursors, with annealed in various temperatures, are shown in Fig. 1(a,b) for the range 400-4000 cm − 1 . The signature of the ν 3 -triply degenerated stretching mode of carbonate at ∼1460 − 1360 cm − 1 is observed as SSR and for SGprecursors annealed at 700-900 o C temperatures. The ν 2 -doubly degenerated stretching modes of carbonate (870 − 860 cm − 1 ) are observed only in precursors that have synthesized at 700 o C and 800 o C temperature [16][17][18]. By contrast, for SG-precursor annealed at 915 o C remain CO 3 2− impurity species are not observed and also for SSR-precursor annealed at 945 o C.
The X-ray diffraction pattern of Ba 2 Ca 2 Cu 3 O y precursor powder, prepared by the SG method at 800, 900°С and 915°С temperatures are plotted in Fig. 2 (a,b,c) [19][20][21]. From XRD measurement results, we can conclude that the sensitivity of X-ray diffraction to xing carbonate is not su cient. The temperature dependencies of the susceptibility versus temperature for the Tl-1223 samples prepared on the based precursors that are synthesized by SSR and SG methods are present in Fig. 4 (a,b) Figure 1 IR analysis of Ba2Ca2Cu3Oy after annealing in owing oxygen partial pressure of 0.5 bar: for SSRprecursor at 700oC-945oC (b) and for SG-precursor at 700oC -915oC.
Page 10/12  Temperature dependences of the real -4πχ' part of ac susceptibility for the SG and SSR samples. Dependencies of the Jc transport critical current densities versus precursor synthesis temperature, for the SSR and SG samples.