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On the basis of the results obtained by Aladashvili et al. (1997) it may be argued that current oscillations in the external circuit (Aladashvili et al., 1988) are associated with “hopping” domain periodic formation and travelling in the bulk of the sample; the domain motion probably occurs along dead end sections of the hopping network, responsible for the drooping region in the current–voltage characteristic (Nguyen and Shklovskii, 1981). In the present work the actual electric field distribution in the hopping domains is measured; it is shown that hopping domain is of triangle shape with equal leading and back edges. Their wall thicknesses are determined. The hopping carrier mobility is measured.

In this specific work we present results of the investigation of the current oscillations in the hopping conductivity region at magnetic field and aniaxial stress, where the dependences of threshold field beginning of oscillations on temperature and on magnetic field and uniaxial pressure are determined. The model to explain the experimental results is presented.

The dependence of the shape of current oscillations observed in the hopping conductivity region in a weakly compensated p-Si on the supply voltage applied to the sample is established. With increasing bias voltage the current pulse shape to form a hopping domain changes significantly. The change in the shape is ascribed to voltage redistribution in corresponding regions in the bulk of the crystal.

The non-ohmic hopping conductivities of a large number of samples of lightly doped by various impurities (P. As, Sb, B and Ga) and weakly compensated n-and p-type Si were investigated. The current-voltage (I-V) characteristics appeared to be different in two temperature regions typical for the studies of Ohmic hopping conductivity: the low-temperature hopping conductivity region with a constant activation energy ε3 (the ε3 region) and the relatively high-temperature hopping conductivity saturation region (the s region). In the ε3 region the conductivity increases with increasing field owing to a Poole-Frenkel-like lowering of the activation energy. In the s region the conductivity decreases with increasing field and shows a negative differential conductance (low-frequency current oscillations have been observed) according to Aladashvili et al. (1988. JETP Lett., 47, 390) as predicted by Nguyen and Shklovskii. It is shown that low-frequency current oscillations are associated with ‘hopping’ domain periodic formation and travel in the bulk of the sample; the hopping domain is of triangular shape parallel to the external field, with equal leading and back edges. Estimates give the velocity c = 0.25 cm s−1, the domain width d = 150μm and the average field E = 300 V cm−1. The mobility of the hopping carriers is measured.

(Bi,Pb)-2223 HTSs (high temperature superconductors) were synthesized from nominally pure (reference) and BN-added Bi1.7Pb0.3Ca2Sr2Cu3O y (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.

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 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 investigation deals with the hopping transport region. The hopping transport can occur in semiconductors at a temperature decrease. In this case a charge transfer takes place in impurity centers located randomly and energy scattered. In the present work experimental data confirming the formation of several electric domains moving simultaneously in the sample bulk with the velocity of c=2.5mm/sec in the region of temperature saturation of hopping conductance are presented. The phenomenon was observed in the samples of weakly doped p - Si with low degree of compensation. The acceptor concentration was 7·1016cm-3 and the compensation was of the order of 10-4. A multidomain was formed within the temperature range of 7-14K. The maximum electric field strength was observed in the temperature region of 10K. Here, supply voltage pulses above the critical value and with the rise time less than Maxwel time of bulk charge formation were applied to the sample. During the whole period of a voltage pulse a drop of the current passing through the sample was observed which is due to the negative differenttial resistance. At this time the current drop was stronger than that during appearance of a single domain. The result can be explained on the basis of the Nguyen and Shklovskii theory predicting the negative differential conduction in the hopping conduction region. A method allowing electric investigations of high - ohmic samples is described.

The influence of addition of lead borate Pb(BO2)2 and boron oxide B2O3 on the phase evolution and superconducting properties of (Bi, Pb)-2223 HTSs synthesized by the solid-state reaction method in alumina crucibles has been studied. X-ray diffraction, resistivity, critical current density, and AC susceptibility measurements were performed on the prepared compounds. Obtained results have shown that boron-containing dopants lead to the drastic enhancement of the (Bi, Pb)-2223 phase formation. Boron-doped samples reveal a significant increase in both the zero resistivity temperature and transport critical current density compared to the undoped specimen. On the other hand, a high content of boron-containing dopants causes the appearance of a very low-T c 2201 phase and leads to a deterioration of coupling between superconducting grain boundaries. Obtained results could enable us to develop a cheap and energy efficient fabrication technology for nearly single (Bi, Pb)-2223 phase superconducting materials via heat treatment of boron-incorporated precursors in an alumina crucibles.

The effect of lead borate Pb(BO2)2 doping on the phase formation and superconducting properties of (Bi,Pb)-2223 high-temperature superconductivity (HTS) has been studied. Samples with nominal composition Bi1.7Pb0.3−xSr2Ca2Cu3O y [Pb(BO2)2] x , x = 0–0.3, were prepared by the standard solid-state processing. The appropriate mixtures were calcined at 850 °C for 30 h. The resulting materials were pressed into pellets and annealed at 840 °C for 30 h in air. X-ray diffraction, resistivity, and critical current density measurements were performed on the undoped (reference) and Pb(BO2)2-doped (Bi,Pb)-2223 compounds. The dominance of the low-T c 2212 phase over the high-Tc 2223 phase was observed in the reference sample. With the increasing of doping level, the 2223 phase is significantly enhanced and its increase is associated with the decrease of the 2212 phase. Our results show that the sample with x = 0.075 is the best in the present study. The calculated volume fraction of (Bi,Pb)-2223 phase increases from ∼25 % for reference specimen to ∼85 % for x = 0.075 in a short sintering time (60 h). The (Bi,Pb)-2223 sample with x = 0.075 (0.15 wt% boron) reveals significant enhancement of critical current density (from 25 up to 360 A/cm2) compared to the reference sample. Obtained results could enable us to develop an accelerated and simple method for producing a high purity (Bi,Pb)-2223 superconducting materials by a proper amount of Pb(BO2)2 doping.