Giorgi Mumladze

The influence of impurities on the physical properties of YBa2Cu3O7−δYBa2Cu3O7−δ superconducting ceramics is investigated. Samples of yttrium ceramics with different Ni, Zn, Co, Fe, and Ga impurity contents were prepared for this study, and the superconducting fraction, the low- frequency susceptibility in an external dc field, and the temperature dependence of the magnetic susceptibility in a weak ac field were measured for each sample. The last of these measurements permits a more precise determination of the critical temperature (the start of the transition) without regard for percolation effects. The experimental results show a stronger suppression of the superconducting state by nonmagnetic impurities, thus confirming the presence of d-state pairs in these superconductors. The degree of suppression of superconductivity by magnetic impurities depends on the preference of the impurities to locate in Cu(1) or Cu(2) sites.  

During the mass formation of aggregates of molecules in a gelatin film dyed with the mixture of chrysophenine and acridine yellow dyes, photo-reorientation, photo-disorientation, and photo-orientation of the molecules are observed. Based on these observations, the photo-induction of granular aniso tropy may be realized.

By using the video microscopy, the picture of the formation of anisotropy photoinduction in the form of grains in time is shown, when anisotropy in the film is induced in individual micrograins and the concentration and sizes of the grains are modulated depending on the light exposition. This phenomenon was observed in the gelatin film dyed with the mixture prepared with the saturated solutions of Chrysophenine and Acridine Yellow mixed with 1 : 1 proportion.

(Bi,Pb)-2223 HTSs (high temperature superconductors) were synthesized from nominally pure (reference) and BN-added Bi1.7Pb0.3Ca2Sr2Cu3Oy (BN)x precursors (x=0,0.10,0.15, and 0.20) by the solid state reaction method using alumina crucibles. The influence of boron nitride addition on the phase formation kinetics and transport properties of (Bi,Pb)-2223 HTSs was studied using X-ray diffraction (XRD), resistivity and critical current density measurements. BN-added compounds reveal a significant enhancement in both the high-T c 2223 phase formation and critical current density compared to the reference specimen

The work is dedicated to a study of anisotropy photoinduction during the light-stimulated molecular aggregations in organic compounds. The case of gelatin or polyvinyl films saturated by azo dyes is considered. By using the video and spectral techniques, the time-formation picture of optical image is studied. It is shown that in this case the image is formed in a similar way as in silver emulsions in the process of photographing — "with the grain distribution".

The anisotropy induction by light in self-induced molecular aggregations ensemble of azodyes has been studied. In the dynamic regime of anisotropy photoinduction the video-microscopy investigations of the film having photoinduced anisotropy in the crossed polarizers have been conducted. It is shown that image formation herein can be carried out in the way of light modulation of the granular anisotropy integral area of the film. According to an active light exposition, both the concentration and sizes of molecular aggregates having photoinduced anisotropy are being modulated

Chemical doping with different elements and compounds at various amounts represents the most suitable approach to improve the superconducting properties of bismuth-based superconductors for technological applications. In this paper, the influence of partial substitution of Sr(BO2)2 for SrO on the phase formation kinetics and transport properties of (Bi,Pb)-2223 HTS has been studied for the first time. Samples with nominal composition Bi1.7Pb0.3Sr2-xCa2Cu3Oy[Sr(BO2)2]x, x=0, 0.0375, 0.075, 0.15, 0.25, were prepared by the standard solid state processing. The appropriate mixtures were calcined at 845 oC for 40 h. The resulting materials were pressed into pellets and annealed at 837 oC for 30 h in air. Superconducting properties of undoped (reference) and Sr(BO2)2-doped (Bi,Pb)-2223 compounds were investigated through X-ray diffraction (XRD), resistivity (ρ) and transport critical current density (Jc) measurements. The surface morphology changes in the prepared samples were examined by scanning electron microscope (SEM). XRD and Jc studies have shown that the low level Sr(BO2)2 doping (x=0.0375-0.075) to the Sr-site promotes the formation of high-Tc phase and leads to the enhancement of current carrying capacity in (Bi,Pb)-2223 HTS. The doped sample with x=0.0375 has the best performance compared to other prepared samples. The estimated volume fraction of (Bi,Pb)-2223 phase increases from ~25 % for reference specimen to ~70 % for x=0.0375. Moreover, strong increase in the self-field Jc value was observed for this dopant amount (Jc=340 A/cm2), compared to an undoped sample (Jc=110 A/cm2). Pronounced enhancement of superconducting properties of (Bi,Pb)-2223 superconductor can be attributed to the acceleration of high-Tc phase formation as well as the improvement of inter-grain connectivity by small amounts of Sr(BO2)2 dopant.

The work is dedicated to anisotropy photoinduction phenomenon in organic compounds, in particular, in the gelatin or polyvinyl films, dyed by azodyes. Here, using the video microscopy is revealed that during the film preparation, when drying, the self-induction of micro particles ensemble having polarizing sensitivity takes place. As a result of acting on that film by polarized light the anisotropy photoinduction in form of "grains" is obtained, where, according to active light exposition the anisotropy integral area is modulated

The present paper reports on the impact of boron carbide (B4C) additive on phase formation and physical properties of Bi(Pb)-2223 high-temperature superconductor (HTS). Refe-rence and B4Cadded Bi(Pb)-2223 samples with nominal composition Bi1.7Pb0.3Sr2Ca2Cu3Oy(B4C)x, x=0, 0.040 and 0.075 were prepared by solid-state reaction method. Precursor powders were manually grinded in agate mortar and sintered at 8500C for 30 hours with intermediate grindings; obtained materials were pressed into pellets and annealed at 8450C for 40 hours, then cooled to room temperature in the furnace. The evolution of Bi(Pb)-2223 phase in prepared samples was investigated by X-ray diffraction (XRD) analysis and volume fraction of Bi(Pb)-2223 phase was estimated from XRD data. The lattice parameters for Bi(Pb)-2223 phase were calculated from XRD patterns. The temperature dependence of electrical resistivity ࣋)ࢀ (was measured by four-probe method. Transmission electron microscope (TEM) and scanning transmission electron Microscope (STEM) were used for microstructural and elemental analysis of synthesized samples. The ࣋)ࢀ (dependences show a twostep superconducting transition with Tc>100 K, indicating the coexistence of high-Tc 2223 and lowTc 2212 phases in prepared materials. XRD analysis confirms that B4C-doping leads to the marked enhancement of Bi(Pb)-2223 phase volume fraction from 56% for un-doped sample up to 88% at x=0.04. Gradual decrease of lattice parameters with increasing doping level can be attributed to the partial substitution of B3+ ions for Cu2+ ions. The areas with completely formed, partly formed and unformed Bi(Pb)-2223 phase have been visually revealed by the TEM/STEM imaging and elemental analysis. 

Doping of a high-temperature Bi(Pb)-2223 superconductor with various additives, which can act as pinning centers and / or accelerate the formation of a superconducting phase, is an effective tool for obtaining Bi(Pb)-2223 samples with improved characteristics. In this paper, we studied the effect of the addition of hexagonal boron nitride (h-BN) on the phase formation and critical transport current density of ceramic samples Bi(Pb)-2223. Samples with a nominal composition of Bi1.7Pb0.3Sr2Ca2Cu3Oy [BN]x, x = 0–0.25 were prepared by the solid-phase reaction method. Two different doping methods were used: the introduction of hand-ground BN powder at the initial stage of synthesis and the addition of BN powder subjected to high-energy grinding in a ball mill before pelletizing. The phase composition of the obtained samples was studied by X-ray diffraction. The microstructure and elemental composition of the samples were studied using a scanning electron microscope with an X-ray microanalyzer. The resistivity and critical current density were measured using the standard four-probe method. The results show that boron nitride is a suitable additive to increase the rate of formation and critical transport current density Bi(Pb)-2223.

Doping of thermoelectric materials with various additives is a promising approach for increasing the functional efficiency of thermoelectric materials of different classes. This paper presents preliminary results of studying the impact of the strontium borate dopant Sr(BO2)2 on the power factor (PF) of the oxide thermoelectric Bi2Sr2Co1.8Oy. Samples with the nominal composition Bi2Sr2–x[Sr(BO2)2]xCo1.8Oy, x = 0–0.15, were prepared by the solid-phase reaction method. The temperature dependences of the resistivity and Seebeck coefficient were measured, and the power factor of the synthesized materials was calculated. The results obtained show that doping with strontium borate enhances the power factor of Bi2Sr2Co1.8Oy thermoelectric.

The present paper reports on the impact of graphene (Gr) addition on the microstructure and thermoelectric properties of Bi2Sr2Co1.8Oy ceramics. Reference and graphene added to Bi2Sr2Co1.8Oy+x wt. % Gr thermoelectric (TE) materials (x=0, 0.15, 0.35, 0.70, and 1.15) were prepared by using the solid-state reaction method. The phase purity and microstructure of the samples were examined by X-ray diffraction (XRD) and scanning electron microscopy (SEM) techniques. XRD analysis revealed that the addition of graphene did not alter the structure of Bi2Sr2Co1.8Oy and this compound remained as a single-phase. SEM analysis showed slight increase of the amount of grains with smaller sizes for 0.35-0.70 wt. % Gr and deterioration of texturing degree for 1.15 wt. % graphene additive. Electrical transport characteristics of prepared materials were studied and values of their power factor (PF) were calculated. It was found that the incorporation of graphene into the Bi2Sr2Co1.8Oy ceramics resulted in a monotonic decrease of electrical resistivity for 0.15-0.70 wt. % Gr while the Seebeck coefficient of all synthesized samples remained practically unaffected. Graphene addition leads to 1.4-fold enhancement of the power factor from 37.0 µW/(m⋅K2 ) at 973 K for the reference sample to 50.7 µW/(m⋅K2 ) for the sample with 0.70 wt. % Gr additive

The processes of heat propagation in five-layer detection pixels of the thermoelectric single-photon detector after absorption of 0.8–1000 eV energy photons are investigated by the method of computer simulation. Design of the detection pixel consisting of successive layers on a sapphire substrate of heat sink Bi2223, thermoelectric sensor CeB₆, absorber Bi2223, and the antireflection layer SiO₂ is proposed. The computer modelling was carried out based on the equation of heat propagation from the limited volume by the use of the three-dimensional matrix method for differential equations. Temporal dependences of the signal intensity for different thicknesses of the layers of the detection pixel are determined. It is shown that the detection pixel SiO₂/Bi2223/CeB₆/Bi2223/Al₂O₃ can register single photons in a wide spectral range from near-IR to X-ray, as well as count the number of simultaneously absorbed photons up to eight. The use of Bi2223 high-temperature superconductor in the design of the detection pixel provides a gigahertz count rate and high system detection efficiency. The simple design of the detection pixel is a prerequisite for the creation of multi-pixel sensors. A detector with such characteristics could be representative of the next generation single-photon detectors in the near future.

The effect of lead borate Pb(BO2)2 dopingon the phase formation and superconducting propertiesof (Bi,Pb)-2223 high-temperature superconductivity (HTS)has been studied. Samples with nominal compositionBi1.7Pb0.3−xSr2Ca2Cu3Oy [Pb(BO2)2]x, x = 0−0.3, wereprepared by the standard solid-state processing. The appropriatemixtures were calcined at 850 ◦C for 30 h. Theresulting materials were pressed into pellets and annealedat 840 ◦C for 30 h in air. X-ray diffraction, resistivity, andcritical current density measurements were performed onthe undoped (reference) and Pb(BO2)2-doped (Bi,Pb)-2223compounds. The dominance of the low-Tc 2212 phase overthe high-Tc 2223 phase was observed in the reference sample.With the increasing of doping level, the 2223 phase issignificantly enhanced and its increase is associated with thedecrease of the 2212 phase. Our results show that the samplewith x = 0.075 is the best in the present study. The calculatedvolume fraction of (Bi,Pb)-2223 phase increases from∼25 % for reference specimen to ∼85 % for x = 0.075 ina short sintering time (60 h). The (Bi,Pb)-2223 sample withx = 0.075 (0.15 wt% boron) reveals significant enhancementof critical current density (from 25 up to 360 A/cm2)compared to the reference sample. Obtained results couldenable us to develop an accelerated and simple methodfor producing a high purity (Bi,Pb)-2223 superconductingmaterials by a proper amount of Pb(BO2)2 doping.

In this paper, the combined effects of B4C-doping and planetary ball milling on the phase evolution, microstructure and transport properties of Bi1.7Pb0.3Sr2Ca2Cu3Oy(B4C)x HTS with x = 0 ÷ 0.125 were studied through X-ray diffraction (XRD), scanning electron microscopy (SEM), resistivity and critical current density measurements. Obtained results have shown that B4C additive leads to the strong acceleration of high-Tc phase formation and substantial enhancement in Jc. High-energy ball milling seems to produce a more homogeneous distribution of refined doped particles in the (Bi,Pb)-2223 HTS which results in an improved intergranular flux pinning and better self-field Jc performance.

The heat evolved into the environment during theoperation of enterprises and autos can be directlyconverted into electrical energy in thermoelectricgenerators (TEGs). To produce TEGs, one needsso-called thermoelectric materials, which must possess low electrical resistivity, thermal conductivity, and high Seebeck coefficient (S).Layered bismuth–strontium cobaltiteBi2Sr2Co1:8Oy crystallizes in monoclinic syngony,and its crystal structure consists of alternating[CoO2] and [Bi2Sr2O4] layers, in which a, c,and lattice constants are identical, but b parametersmismatch each other, so this complex oxidebelongs to so-called misfit-layered phases Bi2Sr2Co1:8Oy is the possible candidate for pbranchesof high-temperature TEGs because itpossesses high Seebeck coefficient S, low resistivity, and thermal conductivity values. Moreover,Bi2Sr2Co1:8Oy cobaltite is stable in air at elevatedtemperatures. Thermoelectric characteristics,such as power factor and thermoelectricfigure-of-merit, where Tis the absolute temperature of layered bismuth–strontium cobaltite, are inferior to the traditionalthermoelectrics based on the layered chalcogenidesof heavy metals but can be improved throughoptimization of its chemical composition orusing advanced preparation methods.Cobaltite thermoelectrics show a resemblance instructure to the high-temperature cuprate superconductors,such as the 2D character of CoO2 andCuO2 conducting layers and the mixed valence(Cu2+/Cu3+, Co3+/Co4+), suggesting that thesetwo systems have similar underlying physics.Our previous reports indicate that the introductionof Pb(BO2)2 into the Bi-2223 superconductor promotesthe formation of the Bi-2223 phase and leadsto the increase in the intergranular critical currentdensity compared to the reference sample.Recently, we found that the power factorof Bi2Sr2Co1:8Oy may be essentially improvedthrough doping by Sr(BO2)2. The aim ofthis work is to check the possibility of improvingthe thermoelectric properties of layeredbismuth–strontium cobaltite through its dopingby Pb(BO2)2.

ProperDoping of a high-temperature superconductor(HTS) Bi(Pb)-2223 is an effective route to prepare Bi(Pb)-2223HTS with enhanced transport characteristics. In this paper,we studied the impact of Ca(BO2)2 doping on high-Tc phaseformation kinetics and transport critical current densityof Bi(Pb)-2223. Samples with a nominal composition ofBi1.7Pb0.3Sr2Ca2-xCu3Oy[Ca(BO2)2]x, x = 0 −0.15 wereprepared by the solid-state reaction method. The phase evolutionof the prepared materials was studied by X-ray diffractionanalysis (XRD). The resistivity and critical current density weremeasured by the standard four-probe method. Scanning electronmicroscopy (SEM) combined with Energy Dispersive X-ray (EDX)microanalysis (SEM-EDX) was used to examine the microstructureand elemental composition of samples. The obtained results showthat Ca(BO2)2 doping leads to the promotion of the high-Tc phaseformation and enhancement of the transport critical currentdensity (Jc) in Bi(Pb)-2223 HTS.Index Terms—Bi(Pb)-2223 phase, critical current density,doping, high-temperature superconductors.