Bi-rich Composition Research Articles

Topological Electronic Transition Contributing to Improved Thermoelectric Performance in p-Type Mg3Sb2- xBix Solid Solutions.

Topological electronic transition is the very promising strategy for achieving high band degeneracy (NV) and for optimizing thermoelectric performance. Herein, this work verifies in p-type Mg3Sb2- xBix that topological electronic transition could be the key mechanism responsible for elevating the NV of valence band edge from 1 to 6, leading to much improved thermoelectric performance. Through comprehensive spectroscopy characterizations and theoretical calculations of electronic structures, the topological electronic transition from trivial semiconductor is unambiguously demonstrated to topological semimetal of Mg3Sb2- xBix with increasing the Bi content, due to the strong spin-orbit coupling of Bi and the band inversion. The distinct evolution of Fermi surface configuration and the multivalley valence band edge with NV of 6 are discovered in the Bi-rich compositions, while a peculiar two-step band inversion is revealed for the first time in the end compound Mg3Bi2. As a result, the optimal p-type Mg3Sb0.5Bi1.5 simultaneously obtains a positive bandgap and high NV of 6, and thus acquires the largest thermoelectric power factor of 3.54 and 6.93 µW cm-1 K-2 at 300 and 575 K, respectively, outperforming the values in other compositions. This work provides important guidance on improving thermoelectric performance of p-type Mg3Sb2- xBix utilizing the topological electronic transition.

Visible-range and self-powered bilayer p-Si/n-Bi2S3 heterojunction photodetector: The effect of Au buffer layer on the optoelectronics performance

The present research is conducted to investigate the effect of Au coating on the physical properties of the nanostructured Bi2S3 for optoelectronics applications. The Bi2S3 films are deposited using the chemical vapor deposition (CVD) at three different temperatures. The structural analysis shows the formation of polycrystalline orthorhombic Bi2S3 films and Bi-rich composition. In addition, the morphological studies indicate dense and compact films with nanorod-like morphology on the surface. Optical evolution of Bi2S3 films demonstrates an increase in the optical energy band gap (Eg) once the deposition temperature increases. Moreover, the addition of Au coating increases the absorption intensity and Eg of the samples in comparison with the Free-coated samples. The Mott-Schottky analysis indicates the highest carrier concentration amount (7.67 ×10+18 cm−3) for the Au-coated samples which are deposited at 420 °C. The photodetector characteristics of the fabricated p-Si/n-Bi2S3 heterojunction devices show meaningful improvement in the quality parameters (QPs), including gain (G) and sensitivity (S). The charge transportation mechanism of the devices is discussed in detail calculating ideality factor (n), carrier mobility (µ), and trap-filled limited potential (VTFL).

Important role of Cu in suppressing bipolar conduction in Bi-rich (Bi,Sb)2Te3

Cu doping has been known to enhance thermoelectric performance of p-type (Bi,Sb)2Te3 alloys by suppressing bipolar conduction at higher temperatures. However, detailed mechanisms are not provided yet. Herein, we investigate the unchallenged role of Cu doping in (Bi,Sb)2Te3 alloys by means of Bi-rich compositions of Bi0.6Sb1.4Te3 and Bi0.7Sb1.3Te3, which were chosen since they exhibit much inferior thermoelectric performance owing to severe bipolar conduction. It was found that Cu doping increases non-degenerate mobility and density-of-states effective mass of valence band as well as hole concentration. Small amount of Cu doping enhanced maximum thermoelectric figure-of-merit of Bi0.6Sb1.4Te3 composition from 0.25 to 0.98.

The spin Hall effect of Bi-Sb alloys driven by thermally excited Dirac-like electrons.

We have studied the charge to spin conversion in Bi1-x Sb x /CoFeB heterostructures. The spin Hall conductivity (SHC) of the sputter-deposited heterostructures exhibits a high plateau at Bi-rich compositions, corresponding to the topological insulator phase, followed by a decrease of SHC for Sb-richer alloys, in agreement with the calculated intrinsic spin Hall effect of Bi1-x Sb x . The SHC increases with increasing Bi1-x Sb x thickness before it saturates, indicating that it is the bulk of the alloy that predominantly contributes to the generation of spin current; the topological surface states, if present, play little role. Unexpectedly, the SHC is found to increase with increasing temperature, following the trend of carrier density. These results suggest that the large SHC at room temperature, with a spin Hall efficiency exceeding 1 and an extremely large spin current mobility, is due to increased number of thermally excited Dirac-like electrons in the L valley of the narrow gap Bi1-x Sb x alloy.

Ion dynamics of non-stoichiometric Na0.5+xBi0.5-xTiO3-δ: A degradation study

Sodium bismuth titanate, a well-known piezoelectric material is reported as the new family of fast oxide ion conductors. In the current study, we investigated the influence of A-site non-stoichiometry on the electrical conductivity and ion dynamics of sodium bismuth titanate ceramics. For this, we synthesized the system Na0.5+xBi0.5-xTiO3–δ with x = −0.02, −0.01, 0.00, 0.01 and 0.02. The samples synthesized were almost single phase with rhombohedral (R3c) symmetry. The scanning electron micrographs showed the formation of dense grains with bulk density > 96%. The microstructure appeared to be very sensitive to non-stoichiometry. The conductivity data isotherms with frequency showed non-linear variation and were well fitted with Jonscher power law. We have observed super linear frequency dependent behaviour in Bi rich composition and sub-linear behaviour with the increase in Na content. To study the degradation mechanism, Na0.5+xBi0.5-xTiO3-δ (x = −0.01, 0.00, 0.01) samples were kept in reducing environment for 48 h. Further, for the analysis of conduction mechanism and the qualitative/quantitative estimation of charge carriers before and after reducing conditions, various models were used.

Microstructure and Critical Current Properties of Bi-2212 Round Wires Fabricated With Different Nominal Compositions

The microstructure and superconducting properties of Bi-2212 wires seem to be strongly influenced by the nominal composition of the starting powder. We fabricated Bi-2212 round wires with different starting compositions. The highest <i xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">J</i> <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">c</sub> values of 175 kA/cm <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sup> at 4.2 K and 10 T were obtained in the Bi-rich sample with a Bi-rich composition. We carried out quenching experiments from the melting state of Bi-2212. We observed obviously smaller grains of impurity phases in the wire with high <i xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">J</i> <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">c</sub> than in the low- <i xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">J</i> <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">c</sub> wires in the melting state, which suggested that this is one of the reasons for the enhancement of the <i xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">J</i> <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">c</sub> values in these wires.

異なる初期組成粉末を用いた Bi2212 超伝導丸線材の組織観察

Bi2212 round wires are one of candidates for application in high magnetic fields and low temperatures because of its high performance of Jc. Furthermore, round wires are suitable to make complicated coils and Rutherford cables. Bi2212 round wires are fabricated by simple heat treatment process of partial melting and slow cooling. However, the Jc values are sensitive to reaction temperatures, especially maximum reaction temperature (Tmax). In this research, we fabricated Bi2212 round wires with different compositions. We prepared four powders with the nominal compositions of between Bi=2.00 and Bi=2.25. Highest Ic value of 260 A (Jc=200 A/cm2) at 4.2 K and 10 T were obtained in Bi2.25 sample with Bi rich composition. From the observations of microstructures, we observed small amount of bridging of filaments in high Ic sample. To understand the growth mechanism of bridging, we report the results of quench experiments during heat processing.

Electrical resistivity of liquid Bi–Sb alloys

The electrical resistivities of liquid Bi–Sb alloys have been measured by DC four-probe method within Bi-rich composition through a wide temperature range. The distinct anomaly of a hump shape was observed on resistivity–temperature (ρndash;T) curves for liquid Bi–Sb alloys on heating at the relatively high temperatures. These anomalies have revealed the temperature-induced liquid–liquid phase transition in Bi–Sb melts. The DSC results for BiSb20wt% alloy further prove the existence of liquid–liquid transition. Measuring the ρ–T curves first on heating and then on cooling we have found that on cooling the ρ–T curve remained linear. It means that the postulated liquid–liquid transition may be irreversible.

Thermal analysis of the compositional shift in a transient liquid phase during sintering of a ternary Cu-Sn-Bi powder mixture

In this work, differential scanning calorimetry (DSC) and microstructural analysis were used to study the transient-liquid-phase sintering (TLPS) of a Cu-Sn-Bi powder mixture. During sintering, the liquid phase shifts from a Sn-rich (i.e., ∼90 wt pct Sn) to a Bi-rich (i.e., >78 wt pct Bi) composition. In addition, the presence of Bi creates two melting events: a Sn:Bi eutectic reaction at 139 °C and a reaction involving the melting of (Bi) at 191 °C. The Sn:Bi eutectic melting event was fully transient. The melting event at 191 °C was consistent with the formation of a terminal Bi-rich liquid phase. The rate of compositional shift toward this terminal liquid phase at 260 °C was dependent on the rate of the reaction of the Sn with the Cu powder to form intermetallic phases. For mixtures made with medium and fine Cu powder, the terminal Bi-rich composition was reached after isothermal hold times of 20 and 15 minutes, respectively. This resulted in a new melting point for the mixture of 191 °C. For coarse Cu powders, the rate of the compositional shift toward a Bi-rich composition was much slower. The liquid phase remained at a hypoeutectic Sn-Bi composition estimated at 80 wt pct Sn, while the mixture maintained a melting point of 139 °C.

Growth of Phase-Controlled Bi2Sr2Can-1CunOy Thin Films Prepared by Off-Axis Sputtering using He Gas

We prepared Bi2Sr2Can-1CunOy (BSCCO) (n=2, 3) thin films by rf magnetron sputtering with a single target using a mixture gas of He+O2 and off-axis geometry. Since He gas can effectively produce active O2+ and O+ particles by the Penning ionization effect, we expected the film crystallinity to improve as a result. We investigated the changes in the crystallinity and the superconductivity of the films using the various compositions of the targets. The phase of the films depended on the composition of the targets. From the results, we found that the optimum target compositions for good crystallinity were the stoichiometric composition of Bi:Sr:Ca:Cu=2:2:1:2 for Bi-2212 films and the Bi-rich composition of Bi:Sr:Ca:Cu=3:2:2:3 for Bi-2223 films. Therefore, the sputtering gas of He+O2 and off-axis geometry are effective in obtaining the phase-controlled BSCCO thin films. In addition, we confirmed the effects of in situ and ex situ annealing for the crystallinity and the superconductivity of the films.

The dependence of Bi 9Sr 11Ca 5 oxide on the stoichiometry of Bi-2212 and its reversible phase transition in oxidising and non-oxidising and its reversible phase transition in oxidising and non-oxidising annealing conditions

A molar ratio of 2:2:1:2 for Bi, Sr, Ca and Cu, respectively, showed impurity phases after the completion of the synthesis process described here. The sample was decomposed by high-temperature annealing under nitrogen and then the Bi2212 phase was reformed by reoxygenation but the original impurities remained. The X-ray diffraction results show the predominant impurity phase and the phase after decomposition is a Cu-free compound. The composition is identified as Bi 2Sr 11Ca 5 oxide which goes through a phase transition as a result of oxygen uptake and loss. The phase equilibria between the Cu-free composition and Bi2212 shows the latter can be reduced by lowering the molar ratio of Sr and Ca followed by post annealing in N 2 and then O 2 and can be removed entirely by using a Bi-rich composition.

Effects of an Applied Magnetic Field on Directional Solidification of Off-Eutectic Bi-Mn Alloys

AbstractOff-eutectic compositions of Bi-Mn were directionally solidified in applied transverse magnetic fields up to 3 kG to determine the effects odn thermal and solutal convection. For Bi-rich compositions, the magnetic field appeared to increase mixing as determined from thermal, morphological, chemical and magnetic analyses. For Mn-rich compositions morphological and chemical analyses suggest some reduction in mixing due to application of the magnetic field. Conductivity gradients in the melt are suggested as a possible mechanism for the observed results.