Polycrystalline Bi Research Articles

Analysis of Thermoelectric Properties of Hot-deformed Bismuth Telluride Sintered Materials

Bismuth telluride (Bi2Te3)-based alloys are widely used for thermoelectric cooling and power generation at low temperatures, and they are the only commercially available materials for thermoelectric applications below 500 K. The rhombohedral unit cell with a large c/a lattice constant ratio consisting of - Te-Bi-Te-Bi-Te- stacked layers inevitably brings about a large anisotropy in transport properties, which is why texturing is very important in polycrystalline Bi2Te3</sub alloys for maximum performance. In this report, p and n-type polycrystalline Bi2Te3</sub alloys were synthesized and hot-deformed to investigate the effects of texturing on thermoelectric properties. Hot deformation (HD) induces the strong alignment of (001) orientations along the compression direction, and a remarkable increase in the orientation factor F of (001) orientations is observed after HD in both p and n-type materials. All of the hot-deformed polycrystalline samples showed increased electrical conductivity (σ), power factor (PF), and thermal conductivity (k) in the in-plane (IP) direction, and vice versa in the out-of-plane (OOP) direction, which makes the IP direction of the hot-deformed bodies more favorable for module fabrication in terms of power factor, for both p and n-type Bi2Te3</sub-based alloys. However, due to the different degree of anisotropy of k and σ, the figures of merit (zT) were maximized in the OOP direction in the p-type materials after HD, whereas the zTs of the n-type materials were higher in the IP direction. This occurs because the anisotropy factor of electron conduction is higher than that of hole conduction, which more than offsets the advantage of the smaller k in the c-direction of n-type Bi2Te3</sub. The maximum zT of 1.33 (OOP) and 0.90 (IP) were obtained after HD from the p and n-type Bi2Te3 alloys, respectively, which were 9.3% and 18.4% higher than those of as-sintered materials.

Improvement in organization of Cu–O coordination and super-electrons in Bi-2212 ceramic matrix with Ag/Sr partial substitution

This study rationalizes the reason for an increase in the electrical conductivity, crystal quality, surface morphology, and superconductive transition temperatures for Bi-2212 superconductors via scientific facts and discussions depending on replacing the optimum Ag/Sr sites in the system for the first time. Every experimental result and related calculations indicate the successful replacement of monovalent Ag+ ions for the divalent Sr2+ ions in the polycrystalline Bi-2212 structure. Moreover, it is observed that the bulk Bi-2212 ceramic prepared with a partial substitution level of x = 0.01 is observed to possess the highest electrical conductivity and superconductive transition temperatures of 84.68 K and 86.26 K for the Tcoffset\\documentclass[12pt]{minimal} \\usepackage{amsmath} \\usepackage{wasysym} \\usepackage{amsfonts} \\usepackage{amssymb} \\usepackage{amsbsy} \\usepackage{mathrsfs} \\usepackage{upgreek} \\setlength{\\oddsidemargin}{-69pt} \\begin{document}$$T_{c}^{offset}$$\\end{document} and Tconset\\documentclass[12pt]{minimal} \\usepackage{amsmath} \\usepackage{wasysym} \\usepackage{amsfonts} \\usepackage{amssymb} \\usepackage{amsbsy} \\usepackage{mathrsfs} \\usepackage{upgreek} \\setlength{\\oddsidemargin}{-69pt} \\begin{document}$$T_{c}^{onset}$$\\end{document}, respectively. Besides, in the case of the optimum replacement level, the Bi-2212 ceramic exhibits the most uniform and smoothest surface appearance, highest interactions between the grains, best microcrystal distributions/orientations, and largest average crystalline distribution along the crystal structure. In this regard, this study becomes a leader in broadening the application spectra including the heavy-industrial technologies, innovative, advanced engineering-related sectors, and large-scale application fields for Bi-2212 superconducting ceramics.

Predicting the thermoelectric figure of merit in p-type BiSbTe-based alloys using artificial neural network modeling

The polycrystalline Bi0.5Sb1.5Te3 compound has drawn promising thermoelectric material over the decades with a figure of merit (zT) close to unity and numerous experiments have been conducted through a trial-and-error approach to further improve their zT by tuning the chemical composition and processing approaches. Despite a lot of studies available on BiSbTe alloys, choosing the right combination of alloy system with high zT is still a challenging task. Herein, we produced a p-type Bi0.5Sb1.5Te3 alloy through casting followed by high-energy milling to enhance the zT via reducing thermal conductivity. The experimental results revealed that a peak zT of 0.95 was achieved at 300 K due to a substantial reduction in thermal conductivity via enhanced carrier/phonon scattering at refined grain boundaries. In addition to the current scenario, the machine learning (ML) assisted artificial neural network (ANN) modeling was performed to predict the zT value by applying the composition, electrical conductivity, Seebeck coefficient, thermal conductivity, and temperature as the input parameters. The predicted zT by ANN modeling shows excellent accuracy with experimental findings about an average absolute error of 9.34 %. Therefore, the developed ANN modeling shows a protocol for composition-based prediction of zT with reduced experimental costs, time consumption, and excellent accuracy in terms of the theoretical and experimental databases.

Dielectric and Optical Properties of δ‐Bi2O3 Quaternary Semiconducting Solid Solutions

Quaternary compositions of polycrystalline Bi1.74Dy0.14W0.12−xScxO3 (x = 0.02, 0.03, 0.04, 0.05, 0.06) solid solutions are synthesized by the solid‐state reaction method. The 4a site symmetry of the space group occupied by Sc3+ ion retains the cubic fluorite‐type over a wide temperature range (450–700 °C) for low Sc3+ content without losing the δ‐phase. Dielectric and ionic conductivity by complex impedance in the frequency range from 0.1 to 100 kHz suggests a temperature‐ and Sc3+‐dependent relaxation process. The ionic conductivity increases with the Sc3+ content over the whole tested temperature range. An oxygen ion conductivity of 0.102 Scm−1 at 700 °C and an activation energy of 0.32 eV are achieved for x = 0.06. Optical properties and Rietveld refinement indicate a bandgap reduction due to a bond length reduction (Bi—O). These materials have potential in photocatalysis and water‐splitting technology due to their UV and visible region absorption capabilities.

Grain size effect on the transport properties of polycrystalline Bi(000l) films grown in MgO(001) and glass

The crystal growth and transport properties of the polycrystalline Bi(0001) films grown on MgO(001) and glass substrates were compared. The morphology and electron transport properties with different thickness have been analyzed. The stress-induced bandgap leads to the shift of transition temperature from semimetal to semiconductor transition. Compared with MgO(001) substrates, Bi(0001) thin films on glass substrates have the higher semimetal-semiconductor transition temperature. Furthermore, the electron transport performance is improved, which is attributed to the strain-free van-der-Waals epitaxial preferred orientation grown on the glass substrate. The strain effect on the transport performance is greater than that on in-plane grain boundaries.

Bandgap modulation and phase boundary region of multiferroic Gd, Co co-doped BiFeO3 thin film

Ferroelectric polarization is a crucial factor to induce photovoltaic effect in ferroelectric materials. Here, a novel modulation of bandgap by Gd and Co co-doped BFO is found for a polycrystalline Bi0.9Gd0.1Fe0.85Co0.15O3 thin film prepared by the sol–gel process. The ferroelectric properties, magnetic properties, and bandgap of the BFO films were altered by doping Gd and Co. This work has led to a greater understanding of bismuth ferrate, and it proposes the Bi0.9Gd0.1Fe0.85Co0.15O3 thin film for the possibility of better preparation of high conversion efficiency ferroelectric photovoltaic devices.

Octahedra tilt influencing physical properties of polycrystalline Bi0.9Co0.1FeO3 ceramics

ABSTRACT Recently, multiferroic materials such as BiFeO3 have fascinated researchers for their potential application in memory storage devices. However, high leakage, weak magnetism and magnetoelectric coupling in this system restrict its practical applications. To solve this, the octahedra tilt influencing electrical, magnetic and thermal properties of polycrystalline Bi0.9Co0.1FeO3 ceramics via solid state route is studied here. Cobalt doped BiFeO3 affect the structural arrangement and tunes the crystal engineering of BiFeO3. In particular, the temperature-dependent dielectric behaviour of Bi0.9Co0.1FeO3 shows a significant dielectric anomaly corresponding to the magnetoelectric coupling in this sample. Further, the maximum magnetization of Bi0.9Co0.1FeO3 is 8.70 emu/ g which is two orders higher in magnitude compared to pristine BiFeO3.

Exchange bias effect in polycrystalline Bi0.5Sr0.5Fe0.5Cr0.5O3 bulk

Bulk Bi0.5Sr0.5Fe0.5Cr0.5O3 (BSFCO) is a new compound comprising the R3c structure. The structural, magnetic property and exchange bias (EB) details are investigated. The material was in the super-paramagnetic (SP) state at room temperature. Exchange bias usually occurs at the boundary between different magnetic states after field cooling (HFC) acts on the sample. Here the result shows that changing HFC from 1 to 6 T reduces the HEB value by 16% at 2 K at the same time. Meanwhile, HEB diminishes as the ferromagnetic layer thickness increases. The variation of (the thickness of ferromagnetic layer) tFM with the change of HFC leads to the tuning of HEB by HFC in BSFCO bulk. These effects are obviously different from the phenomenon seen in other oxide types.

Optimization of large magnetoresistance of polycrystalline Bi film

The oriented growth and morphology of polycrystalline bismuth (Bi) films deposited on insulating substrates is investigated based on the molecular beam epitaxy method. The film thickness effect on the resistance-temperature characteristics and magnetoresistance properties is analyzed. The Bi film resistivity with thickness range [25, 65] nm is investigated using the four-probe method at 2–300 K. The magnetic field contributes to opening the energy gap of Bi thin films and a more obvious opening of the energy gap is observed in the thinner films. The Bi film has the weak antilocalization (WAL) effect and the thickness increase affects the WAL effect. The large magnetic field is beneficial to the observation of the WAL effect. The temperature-dependent resistivity of Bi thin films is described based on the Ziman-Faber theory. The competition between the metal surface state and the bulk state explains the semimetal-semiconductor transition and the magnetoresistance effect in the thickness range [25, 65] nm. In addition, the thickness effect on the magnetoresistance property is researched, which provides a new approach to specify the Bi thin film magnetoresistance.

Electric Field‐Induced Microstructural Evolution in Polycrystalline Bi2O3‐Doped ZnO in Presence of a Secondary Liquid Phase

Herein, a spectrum of electric field–induced microstructural evolution phenomena are observed in polycrystalline Bi2O3‐doped ZnO, and the underlying mechanisms are revealed. An applied electric field can drive the migration of bismuth (Bi) toward the negative electrode (including the motion of the charge‐neutral Bi‐rich liquid phase via electrochemical coupling), which produces a Bi‐free zone near the anode. The field‐induced migration of Bi creates a junction between electron‐conducting ZnO and ion‐conducting Bi2O3‐doped ZnO (via Bi2O3‐enriched liquid‐like intergranular films [IGFs]), where an oxygen ion current is converted to an electron current while generating O2 gas, generating a porosity belt. Aberration‐corrected electron microscopy further reveals the formation of three distinct types of grain boundaries (GBs). “Clean” ZnO GBs are observed near the anode, where Bi is depleted by the electric field. Bi2O3‐enriched liquid‐like IGFs (disordered GBs) are observed in the middle section. Near the cathode, electrochemical reduction induces an GB disorder–order transition to form fast‐moving Bi2O3‐enriched ordered GBs, which causes abnormal grain growth. Herein, several new field–matter interaction phenomena are discovered and an exemplar of field effects on microstructural evolution are established at multiple length scales in the presence of GB complexion (phase‐like) transitions.

Polycrystalline bismuth nanowire networks for flexible longitudinal and transverse thermoelectrics.

This paper reports on the preparation and the characterization of structural, electrical and thermoelectric properties of nanocomposite films formed from three-dimensional networks of polycrystalline bismuth (Bi) nanowires (NWs). The samples were fabricated by electrodeposition within polycarbonate (PC) templates with crossed cylindrical nanopores, yielding self-supported networks of Bi crossed nanowires (CNWs) with mean diameter values ranging from 23 nm to 230 nm. Temperature changes in electrical resistance and thermopower were studied by considering electric and thermal currents flowing in the plane of the films. While the values of the Seebeck coefficient are close to those of polycrystalline Bi for diameters greater than 100 nm, a progressive decrease in thermopower appears at smaller diameters, due to an increasing contribution of surface charge carriers as the diameter decreases. Transverse thermoelectricity based on the Nernst effect was also demonstrated on a network of Bi CNWs 230 nm in diameter. Such hierarchical architectures based on Bi CNWs are extremely robust, offering a reliable solution for the next generation of flexible thermoelectric devices.

Anisotropy and Crystallite Misalignment in Textured Superconductors

A misalignment of anisotropic crystallites causes small values of anisotropy and decreases the critical current density of textured polycrystalline superconductors. To relate the crystallite misalignment and out-plane anisotropy, the magnetic properties of the textured Bi2223 polycrystalline superconductor were investigated. A distribution of orientation angles of crystallites was determined using different data: scanning electron microscopy images and hysteresis magnetization loops when an external magnetic field was applied at different angles with respect to the texturing plane of the sample. It was demonstrated that the standard deviation of the distribution and the magnetic disorder angle of crystallites in textured samples can be determined from the magnetization data in perpendicular directions. These data may be either the irreversible magnetization measured for two different orientations of the sample or the simultaneously measured magnetization projections parallel and perpendicular to the magnetic field.

Energy storage performance of polycrystalline Bi0.85Ho0.05Sm0.1FeO3 ceramics

This article analyzes the energy storage performance of polycrystalline Bi0.85Ho0.05Sm0.1FeO3 ceramics. The sample is synthesized by a solid-state reaction and, it crystallizes in the R3c phase without any traces of secondary phases. The electron density distribution and type of bonding are analyzed using the maximum entropy method. The energy storage capability of polycrystalline Bi0.85Ho0.05Sm0.1FeO3 ceramics was determined to be 0.212 J/cm3 and 0.332 J/cm3 at 2 Hz and 100 Hz frequencies, respectively.

Electron density distribution influencing the electrical and magnetic properties of polycrystalline Bi0.9Sm0.1FeO3 ceramics

Abstract This article reports the structural properties that influence the electrical and magnetic behaviours of polycrystalline Bi1−x Sm x FeO3 (x = 0, 0.1) ceramics. The samples are synthesized by a two-step solid state reaction. X-ray diffraction patterns expose two characteristic peaks corresponding to (104) and (110) planes around 31°. Bi1−x Sm x FeO3 (x = 0, 0.1) samples crystalize primarily in the R3c phase along with the traces of secondary phases. The Rietveld refinement analysis reveals that the tilt angle of Bi0.9Sm0.1FeO3 reduces due to the twisting of FeO6 octahedra compared to pristine BiFeO3. The electron density distribution and type of bonding are analyzed using the maximum entropy method. The microstructural analysis reveals that the Bi0.9Sm0.1FeO3 sample has a reduced average particle size compared to pristine BiFeO3. The influence of samarium ions in the bismuth site deviates the canting angle of the modulated spiral spin arrangement and the charge density distribution of the Bi0.9Sm0.1FeO3 sample as a result, the electrical and magnetic behaviours are improved compared with bare BiFeO3.

The structural and dielectric properties of Bi(Fe0.95Sm0.05)O3 ceramic

This article exhibits that how a polycrystalline Bi(Fe0.95Sm0.05)O3 ceramic was synthesized using Solid-State-Reaction route technique. The structural and dielectric characteristic properties of the ceramic have also been studied. X-Ray Diffraction technique has been used for the identification of single-phase orthorhombic symmetry. It has been observed that the minor substitution of Samarium (i.e. Sm; a rare-earth element) at B-site in Bismuth ferrite (BiFeO3) suppresses the impurity section formation which is a basic disadvantage in BiFeO3 based ceramics. The grains were found homogeneously spread on the ceramic surface. The detailed investigations of grain and grain boundaries have delivered numerous interesting and useful outcome for the further device applications.

Disorder effect and thermoelectric properties of Bi1−xCaxCu1−ySeO with Cu vacancy

BiCuSeO exhibits remarkable low thermal conductivity among oxides. The lone pair of the Bi3+ and the point defects brought about by elemental doping are thought to be the origin of the low thermal conductivity. In a recent study, however, the Cu atoms in BiCuSeO play a more significant role in reducing the lattice thermal conductivity than the above two factors. In this work, two series of polycrystalline Bi1−xCaxCu0.975SeO (x = 0.025, 0.050, 0.075, 0.100, 0.125, 0.150) and Bi0.95Ca0.05Cu1−ySeO (y = 0.015, 0.025, 0.035) are synthesized by solid-state reaction. All the materials are of single phase. Point defects introduced by partial elemental substitution and atom vacancy are expected to reduce the lattice thermal conductivity, however, which is not followed by the prediction for Bi1−xCaxCu0.975SeO with 0.050 ≤ x ≤ 0.125 and Bi0.95Ca0.05Cu1−ySeO with y = 0.015, 0.025, and 0.035 in the temperature range where the resistivity decreases while the thermopower increases with temperature. This is in sharp contrast with those BiCuSeO with partial elemental substitution or Cu deficiency exhibiting either both the electrical resistivity and thermopower increase or decrease with increasing temperature. Our results support that the Cu atoms in BiCuSeO play a more significant role in reducing the lattice thermal conductivity than the lone pair of the Bi3+ and the point defects brought about by elemental doping.

Synergistic Texturing and Bi/Sb‐Te Antisite Doping Secure High Thermoelectric Performance in Bi0.5Sb1.5Te3‐Based Thin Films

AbstractBi2Te3‐based thin films are attracting increasing attention due to their considerable wearability and flexibility feature. However, the relatively low performance compared to their bulk counterparts limits their development and wider application. In this work, synergistic texturing and Bi/Sb‐Te antisite doping are used to achieve a high room‐temperature ZT of ≈1.5 in p‐type Bi0.5Sb1.5Te3 thin films by a magnetron sputtering method. Structural characterization confirms that carefully tuning the deposition temperature can strengthen the texture of as‐prepared polycrystalline Bi0.5Sb1.5Te3 thin films, leading to significantly enhanced carrier mobility and electrical conductivity. Simultaneously, rational engineering of the deposition temperature can induce antisite doping between Bi/Sb and Te, which can reduce the carrier concentration and make it closer to the optimized level. In turn, a high power factor of 45.3 µW cm−1 K−2 and a maximized ZT of ≈1.5 at room temperature are obtained. This high power factor and ZT are highly competitive to other state‐of‐the‐art p‐type thin‐film‐based thermoelectric materials, showing great potentials for practical applications.

Plausible multiferroic and magneto-electric behaviour of polycrystalline Bi0.85Gd0.05La0.1FeO3 at room temperature

The structural, magnetic and magneto-electric (ME) properties of polycrystalline BiFeO3 and Bi0.9–xGdxLa0.1FeO3 (x = 0.00 and 0.05) samples have been investigated. X-ray diffraction patterns confirm the formation of single phases in all the samples with ‘R3c’ space group. Magnetic anisotropy and ferroelectric hysteresis behaviour are observed in Bi0.85Gd0.05La0.1FeO3 through magnetization and polarization measurements respectively. In addition, the same compound shows weak ferromagnetism at room temperature with a significantly enhanced remnant magnetization of ~ 23.6emu.mol−1, concomitant with an exchange bias. ME measurements on the compound show a non-linear dependence of magnetic and electric vectors.

The impact of the addition of Bi2Te3 nanoparticles on the structural and the magnetic properties of the Bi-2223 high-Tc superconductor

The polycrystalline granular BSCCO high Tc superconductors (HTSC) have limitations in various applications. These limitations appear due to the flux pinning's weakness and the weak links between the grains comparatively in high temperatures and high applied magnetic fields. Bi2Te3 nanoparticles are artificially introduced into the Bi-2223 HTCS matrix to be employed as effective flux pinning centers to enhance the flux pinning capability and the critical current density. The effect of the additive of Bi2Te3 nanoparticles on the structural and physical properties of Bi-2223 were investigated for the polycrystalline (Bi1.6Pb0.4Sr2Ca2Cu3O10+δ)1-x/(Bi2Te3)x where (x = 0.00,0.01,0.02& 0.03). The phase structure/formation, volume fraction, the lattice constants were described by X-ray powder diffraction (XRD) measurements. Diamagnetic signal has been investigated with two onset temperatures (Tc1&Tc2) for the common BSCCO phases (Bi-2223 and Bi-2212) which confirm the XRD obtained data without any indication for unwanted impurities. The magnetic interactions between Bi2Te3 nanoparticles addition and the superconductor matrix are discussed at 5 and 50 K. The relation between the microstructure, BSCCO phase's contents, the hysteresis loops, the calculated critical current densities, and the flux densities were also reported for the samples. Nano-Bi2Te3 shows a great impact on the BSCCO superconducting properties and influences its flux densities and the flux pinning mechanisms as reported experimentally and theoretically. Consequently, the additive of Bi2Te3 nanoparticles must be carefully controlled to balance the microstructure and superconducting parameters of the BSCCO HTSC.

Boosting the thermoelectric performance of n-type Bi2S3 by hierarchical structure manipulation and carrier density optimization

Te-free Bi 2 S 3 -based thermoelectric materials show great potential for eco-friendly and industrial scale-up applications because of their high-abundance, low-cost, low-toxicity, and low-thermal-conductivity features. However, their low figure of merit, ZT limits their further applications. In this work, we report a high ZT of ~0.8 at ~760 K in n-type polycrystalline Bi 2 S 3 by a combination of hierarchical structure manipulation and carrier density optimization. A step-by-step fabrication by using mechanical alloying, high-pressure and high-temperature treatment, spark plasma sintering, and annealing leads to unique micro/nanostructures in polycrystalline Bi 2 S 3 including refined grains, high-density Bi-rich nanoprecipitates, significant lattice distortions, and nanopores that confirmed by comprehensive characterizations, which contribute to significantly suppressed lattice thermal conductivity of 0.41 W m −1 K −1 at ~760 K. A further 0.5 mol% CuCl 2 -doping triggers impurity band in the electronic structure of Bi 2 S 3 and narrows the bandgap for optimizing the carrier concentration at ~1 × 10 20 cm −3 , confirmed by both experimental results and first-principles density functional theory calculations. The optimized carrier concentration and maintained low lattice thermal conductivity give rise to a high power factor of ~5.3 μW cm −1 K −2 and high ZT that ranks as a top value. This work provides a new route to achieve high thermoelectric performance in n-type polycrystalline Bi 2 S 3 . • A record-high ZT of ~0.8 at 760 K is achieved in n-type polycrystalline Bi 2 S 3 . • CuCl 2 -doping triggers impurity band of Bi 2 S 3 and narrows its bandgap. • An optimized n e of ~1 × 10 20 cm −3 leads to a high S 2 σ of ~5.3 μW cm −1 K −2 at 760 K. • Unique micro/nanostructures significantly suppress κ l to 0.41 W m −1 K −1 at 760 K.