Precipitation and pinning in Ca and Sr-Rich High-Tc superconducting “Bi2Sr2CaCu2O8” ceramics

From a consideration of the phase equilibrium diagram of the system Bi₂O₃-SrO-CaO-CuO, a simple annealing procedure was developed to precipitate Bi_2+xSr_2+xCuO_6+d, Sr₁₄Cu₂₄O_41−x, and Bi₂Sr₃O₆ in high-temperature superconducting Sr-rich “Bi₂Sr₂CaCu₂O₈” ceramics and Ca₂CuO₃ and a liquid in Ca-rich “Bi₂Sr₂CaCu₂O₈” ceramics. The transformation results in an increase of the critical current density of which is believed to express improved pinning properties of the superconducting crystals, in particular an increased pinning energy, which reduces the probability for thermally activated depinning. Possible pinning centers which were introduced during precipitation of the second phases are the surface of the precipitates.     

Thermal Conductivity and Dimensionless Figure of Merit of Thermoelectric Rhodium Oxides Measured by a Modified Harman Method

We have developed a modified Harman method to extract the thermoelectric signal using a squared AC current in the presence of Joule heating, and have measured the thermal conductivity and dimensionless figure of merit of single crystals of the layered rhodium oxide Bi_0.78Sr_0.4RhO_3+d and the pseudo-one-dimensional rhodium oxide Ba_1.2Rh₈O₁₆. We find that these rhodium oxides exhibit a small thermal conductivity of 30 mW/cm K at 200 K and rather large ZT of 0.02 below 200 K. We believe that this method will be a powerful tool for thermal conductivity measurements in sub-millimeter-sized crystals.     

Submillimeter transmission spectra of Sr2Ca2Cu4Oy

Transmission spectra of Sr₂Ca₂Cu₄O_y, which is a part of high T_c superconducting system Bi₂Sr₂Ca_n?1Cu_nO_z, and their temperature evolutions were investigated by monochromatic quasioptical submillimeter spectroscopy. Strong increase of high frequency conductivity in the range 80 to 350K was revealed. The obtained results were analyzed on basis of thermally activated carriers model.     

The syntheses and microstructures of tabular SrTiO3 crystal

In the present paper, the microstructures of SrTiO₃ particles obtained in two different methods were examined by scanning electron microscopy (SEM) and X-ray diffraction pattern (XRD). The two synthesis techniques included a conventional mixed solid method and a molten salt synthesis method (MSS), which proceeded through two steps. In the first, precursor Sr₃Ti₂O₇ particles were synthesized. Tabular SrTiO₃ crystals were synthesized via the superposition of the SrTiO₃ basic cell on the interface of Sr₃Ti₂O₇ particles. The microstructures of SrTiO₃ particles synthesis in those two ways were quite different. The microstructures of SrTiO₃ obtained by MSS method were high purity and obviously tabular in structure. Oriented growth faces included typical (0 0 1), (1 0 0), (1 1 0), etc. The mechanism of the oriented growth of tabular SrTiO₃ could be considered as the superposition of coordination polyhedron Ti–O₆ octahedron basic cell on the interface of Sr₃Ti₂O₇ particles.     

Cost-Efficient Preparation and Enhanced Thermoelectric Performance of Bi0.48Sb1.52Te3 Bulk Materials with Micro- and Nanostructures

A cost-efficient method has been developed based on the combination of hydrothermal exfoliation and spark plasma sintering (SPS) to fabricate Bi_0.48Sb_1.52Te₃ bulk material with multiscale microstructures composed of micro- and nanosized microstructures. The thermoelectric (TE) transport properties of the bulk material with multiscale microstructures were measured along the directions parallel (||) and perpendicular (⊥) to the SPS pressing direction. It is confirmed that the anisotropy of the electrical conductivity (σ) and thermal conductivity (κ) was decreased by the transformation of the microstructure from a single microscale structure to multiscale microstructures. As compared with Bi_0.48Sb_1.52Te₃ bulk material with single microscale microstructures, the κ value of the Bi_0.48Sb_1.52Te₃ bulk material with multiscale microstructures was significantly reduced, the σ value was slightly decreased, while the α value was slightly increased. Thus, a maximum ZT value of 1.1 was achieved at 350 K along the direction perpendicular to the pressing direction, increased by 20%. The enhanced ZT value was mainly attributed to the significant decrease in κ induced by the multiscale microstructures. This work offers a new approach to improve TE performance by multiscale microstructural engineering.     

Epitaxial (La,Sr)TiO3 as a conductive buffer for high temperature superconducting coated conductors

The transport and structural properties of (La,Sr)TiO₃ epitaxial thin films grown by pulsed-laser deposition is presented. In particular, the potential use of (La,Sr)TiO₃ as a conductive buffer layer for subsequent growth of high temperature superconducting films for coated conductors is discussed. Van der Pauw measurements of film resistivity as a function oxidation conditions show that, for undoped LaTiO₃ films, the resistivity increases rapidly as background oxygen pressure is increased, which is consistent with the formation of the LaTiO_3+x phase. Sr doping of LaTiO₃ significantly enhances the conductivity of thin film materials when synthesized under oxidizing conditions. The transport behavior for Sr-doped LaTiO₃ films correlates with structural data showing no significant shift in lattice spacing as oxygen partial pressure is increased during film growth. In addition, the epitaxial growth of (La,Sr)TiO₃ on biaxially textured Ni alloy tapes is demonstrated. These results suggest that (La,Sr)TiO₃ is a viable candidate as a conducting buffer for superconducting film growth on biaxially textured metal tapes.     

Modulation Doping Leads to Optimized Thermoelectric Properties in n-Type Bi6Cu2Se4O6 due to Interface Effects

Heterogeneous composites consisting of Bi₆Cu₂Se_3.6Cl_0.4O₆ and Bi₂O₂Se are prepared according to the concept of modulation doping. With prominently increased carrier mobility and almost unchanged effective mass, the electrical transport properties are considerably optimized resulting in a peak power factor ≈1.8 µW cm⁻¹ K⁻² at 873 K, although the carrier concentration is slightly deteriorated. Meanwhile, the lattice thermal conductivity is lowered to ≈0.62 W m⁻¹ K⁻¹ due to the introduction of the second phase. The modified Self-consistent Effective Medium Theory is utilized to explain the deeper mechanism of modulation doping. The enhancement of apparent carrier mobility is derived from the highly active phase interfaces as fast carrier transport channels, while the reduced apparent thermal conductivity is ascribed to the existence of thermal resistance at the phase interfaces. Ultimately, an optimized ZT ≈0.23 is obtained at 873 K in Bi₆Cu₂Se_3.6Cl_0.4O₆ + 13% Bi₂O₂Se. This research demonstrates the effectiveness of modulation doping for optimizing thermoelectric properties once again, and provides the direct microstructure observation and consistent theoretical model calculation to emphasize the role of interface effects in modulation doping, which should be probably applicable to other thermoelectrics.     

Effect of Nano-ZrW2O8 on the Thermoelectric Properties of Bi85Sb15/ZrW2O8 Composites

In this study, Bi₈₅Sb₁₅/x wt.% ZrW₂O₈ (x?=?0, 0.1, 0.5, 1) thermoelectric nanocomposites were prepared successfully by ball milling and spark plasma sintering. The effect of ZrW₂O₈ nanoparticles on the thermoelectric properties of the Bi₈₅Sb₁₅/ZrW₂O₈ composite was investigated. Thermal conductivity, Seebeck coefficient, and electrical conductivity were measured between 77?K and 300?K. x-Ray diffraction and scanning electron microscopy were adopted for microstructure characterization of the composites. The electrical transport properties are mainly discussed with regard to the microstructures. The results show that nanoinclusions did not grow during sintering. It is found that the thermal conductivity decreases with the addition of a small amount of ZrW₂O₈ nanoparticles, which serve as additional phonon-scattering centers. The obtained thermal conductivity is 0.5?W/m?K for the Bi₈₅Sb₁₅/1?wt.% ZrW₂O₈ composite at 80?K, which is just half of the value for the Bi₈₅Sb₁₅ matrix. However, the electrical transport properties are degraded with increasing content of ZrW₂O₈. The calculated ZT is also degraded due to the poor electrical properties.     

Phase relations and homogeneity region of the hightemperature superconducting phase (Bi,Pb)2Sr2Ca2Cu3O10+d

For the nominal composition of Bi_2.27−xPb_xSr₂Ca₂Cu₃O_10+d, the lead content was varied from x < 0.05 to 0.45. The compositions were examined between 800 and 890‡C which is supposed to be the temperatue range over which the so-called 2223 phase (Bi₂Sr₂Ca₂Cu₃O_10+d) is stable. Only compositions between x < 0.18 to 0.36 could be synthezised in a single phase state. For x <0.36, a lead-containing phase with a stoichiometry of Pb₄(Sr,Ca)₅CuO_d with a small solubiliy of Bi is formed, for x > 0.18 mainly Bi₂Sr₂CaCu₂O_8+d and cuprates are the equilibrium phases. The temperature range for the 2223 phase was found to be 800 to 890‡C but the 2223 phase has extremely varying cation ratios over this temperaure range. Former single phase 2223 samples turn to multiphase samples when annealed at slightly higher or lower temperaures. A decrease in the Pb solubility with increasing as well as decreasing temperature with a maximum at about 850‡C was found for the 2223 phase.     

Spontaneous Outcropping of Self‐Assembled Insulating Nanodots in Solution‐Derived Metallic Ferromagnetic La0.7Sr0.3MnO3 Films

A new mechanism is proposed for the generation of self‐assembled nanodots at the surface of a film based on spontaneous outcropping of the secondary phase of a nanocomposite epitaxial film. Epitaxial self‐assembled Sr–La oxide insulating nanodots are formed through this mechanism at the surface of an epitaxial metallic ferromagnetic La_0.7Sr_0.3MnO₃ (LSMO) film grown on SrTiO₃ from chemical solutions. TEM analysis reveals that, underneath the La–Sr oxide (LSO) nanodots, the film switches from the compressive out‐of‐plane stress component to a tensile one. It is shown that the size and concentration of the nanodots can be tuned by means of growth kinetics and through modification of the La excess in the precursor chemical solution. The driving force for the nanodot formation can be attributed to a cooperative effect involving the minimization of the elastic strain energy and a thermodynamic instability of the LSMO phase against the formation of a Ruddelsden–Popper phase Sr₃Mn₄O₇ embedded in the film, and LSO surface nanodots. The mechanism can be described as a generalization of the classical Stranski–Krastanov growth mode involving phase separation. LSO islands induce an isotropic strain to the LSMO film underneath the island which decreases the magnetoelastic contribution to the magnetic anisotropy.     

High Thermoelectric Performance in PbTe Due to Large Nanoscale Ag2Te Precipitates and La Doping

Thermoelectrics are being rapidly developed for waste heat recovery applications, particularly in automobiles, to reduce carbon emissions. PbTe‐based materials with small (<20 nm) nanoscale features have been previously shown to have high thermoelectric figure‐of‐merit, zT, largely arising from low lattice thermal conductivity particularly at low temperatures. Separating the various phonon scattering mechanisms and the electronic contribution to the thermal conductivity is a serious challenge to understanding, and further optimizing, these nanocomposites. Here we show that relatively large nanometer‐scale (50–200 nm) Ag₂Te precipitates in PbTe can be controlled according to the equilibrium phase diagram and these materials show intrinsic semiconductor behavior with high electrical resistivity, enabling direct measurement of the phonon thermal conductivity. This study provides direct evidence that even large nanometer‐scale microstructures reduce thermal conductivity below that of a macro‐scale composite of saturated alloys with Kapitza‐type interfacial thermal resistance at the same overall composition. Carrier concentration control is achieved with lanthanum doping, enabling independent control of the electronic properties and microstructure. These materials exhibit lattice thermal conductivity which approaches the theoretical minimum above ～650 K, even lower than that found with small nanoparticles. Optimally La‐doped n‐type PbTe‐Ag₂Te nanocomposites exhibit zT > 1.5 at 775 K.     

Solid-State Self-Assembly of Nanostructured Oxide as a Candidate High-Performance Thermoelectric Material

We have focused on the recently reported nanostructured bulk ZnMn_2−x Ga _x O₄ to evaluate whether this type of nanostructured oxide can effectively reduce thermal conductivity. Firstly, powdered samples of ZnMn_2−x Ga _x O₄ (x = 0 to 2) were prepared and the effect of heat treatment on the obtained phases was examined. Secondly, we have picked out the composition of ZnMnGaO₄, in which two distinct types of rectangular nanorods with different compositions spontaneously interlace to form a cross-sectional checkerboard pattern. To confirm the effect of nanostructure on thermal transport properties, the room-temperature thermal conductivity of this nanostructured oxide was evaluated.     

Rare-earth-modified Sr0.5Ba0.5Nb2O6, ferroelectric crystals and their applications as infrared detectors

Rare-earth-modified ferroelectric crystals with the formula (Sr_1−xBa_x)1− 3y/2 R_yNb2₂O₆, where R = La, Nd, Sm, Gd, and Lu, have Been prepared and studied. When R = La, Nd, x ≃ .5 and y = 0.02, the modified material, at room temperature, exhibited twice the pyroelectric coefficient and four times the dielectric constant of the unmodified Sr_1−xBa_xNb₂O₆ (x ≃ .5). Curie temperatures decreased, dielectric constants increased, while loss factor and detector signal-to-noise ratios remained nearly the same with the addition of rare earth doping. The calculated response based on the measured properties agree with the measured response of actual detectors. These properties suggested that the modified SBN are good materials for small element or array pyroelectric infrared detector applications.     

W5O14 Nanowires

We report on the synthesis of quasi‐1D W₅O₁₄ crystals using NiI₂ as a growth promoter. Photoelectron spectroscopy revealed the metallic conductivity of the W₅O₁₄ nanowires, which was also confirmed by direct‐transport measurements on a double‐stranded nanowire. Scanning electron microscopy and transmission electron‐diffraction data are correlated with details of crystal growth revealing the possible mechanism of the formation of this rarely synthesized phase, which was reported as a homogeneous phase only in 1978 by McColm et al., and in the meantime has been declared as a compound that is rare.     

Electrical properties of Zn3P2 single crystals

The electrical properties of Zn₃P₂ single crystals grown by sublimation and by iodine transport, as well as largegrain polycrystalline material grown by sublimation, have been measured by conductivity and Hall effect measurements as a function of temperature. In single crystals scattering is controlled by charged impurity scattering at low temperatures and acoustic lattice scattering at high temperatures, with a typical maximum mobility value of 10 to 20 cm²/V-sec. In polycrystalline samples the mobility is thermally activated with an activation energy of 0.072 eV at higher temperatures and 0.028 eV at lower temperatures. A computer modelling of the Hall effect and conductivity data indicated acceptor levels with ionization energies of ≤_ 0.02 eV, 0.25 eV and 0.47 eV for the sublimed crystals, and of 0.034 eV and 0.14 eV for crystals grown with iodine transport; densities for these levels were also determined. Results of investigation of electrical contacts, vacuum heat treatment, and Auger profiling are also reported.     

Creation of Yb2O3 Nanoprecipitates Through an Oxidation Process in Bulk Yb-Filled Skutterudites

An approach to introduce in situ nanoprecipitates into bulk filled skutterudites is developed through controlling the oxidation process of the fillers. Yb_0.3Co₄Sb₁₂ is selected as the base material, and prolonged oxidation at high temperatures in sealed quartz tubes under a low pressure of oxygen leads to the formation of Yb₂O₃ nanoinclusions. Transmission electron microscopy shows that the Yb₂O₃ nanoprecipitates are created within the skutterudite crystal grains through an internal oxidation mechanism. With increased time of oxidation, the amount of Yb₂O₃ nanoprecipitates is increased and the nanoprecipitates are more uniformly distributed in the matrix. For the samples oxidized for 10 days, the lattice thermal conductivity is reduced by about 19% at 850 K compared with the Yb_0.3Co₄Sb₁₂ matrix. The reduction in the lattice thermal conductivity originates from additional phonon scattering by the Yb₂O₃ nanoprecipitates, leading to a maximum ZT of 1.3.     

The Effects of Constriction Factor and Geometric Tortuosity on Li‐Ion Transport in Porous Solid‐State Li‐Ion Electrolytes

3D focused ion beam tomography is used to analyze the microstructures of Li‐ion conducting Li_6.75La_2.75Ca_0.25Zr_1.5Nb_0.5O₁₂ (LLCZN) garnet porous electrolytes with different levels of porosity and the theoretical effective bulk conductivities of the electrolyte are calculated based on LLCZN volume fraction, constriction factor, geometric tortuosity, and percolation factor. The experimentally measured effective bulk conductivities are consistently lower than the theoretical values when assuming constant bulk conductivity, suggesting the bulk conductivity of the LLCZN decreased with increasing porosity. This work highlights the importance of understanding the full effects of altering the microstructure of solid‐state electrolytes, as this will play a key role in advancing Li‐ion battery technology to higher energy and power densities.     

Enhanced Thermoelectric Properties of Hole-Doped Bi2−x Ba x Sr2Co2O y Ceramics

The influence of Ba doping on the thermoelectric properties of Bi_2?x Ba _x Sr₂ Co₂O _y (x = 0.00, 0.025, 0.05, 0.075, 0.10, 0.125, and 0.15) samples prepared by the solid-state reaction method was investigated from 333 K to 973 K. For the samples with x ≤ 0.075, the electrical resistivity decreased with increase of the Ba doping amount due to p-type doping and they exhibited metallic electrical conductivity behavior, whereas the samples with x ≥ 0.10 exhibited semiconductor-like electrical conductivity behavior. The Seebeck coefficients of all the samples decreased with increase of the Ba doping amount. The thermal conductivity first decreased for x ≤ 0.075, then increased with higher Ba doping amounts. As an overall result, the dimensionless figure of merit (ZT) of Bi_1.925Ba_0.075Sr₂Co₂O _y reached the maximum value of 0.245 at 973 K, being 41% higher than that of the undoped sample.     

Preparation and Enhanced Thermoelectric Properties of p-Type BaFe12O19/CeFe3CoSb12 Magnetic Nanocomposite Materials

A series of p-type xBaFe₁₂O₁₉/CeFe₃CoSb₁₂ (x = 0, 0.05%, 0.10%, 0.20%, 0.40%) magnetic nanocomposite thermoelectric (TE) materials have been prepared by the combination of ultrasonic dispersion and spark plasma sintering (SPS). The effects of BaFe₁₂O₁₉ magnetic nanoparticles on the phase composition, microstructure, and TE properties of the nanocomposite materials were investigated in this work. x-Ray diffraction analysis shows that all the SPSed bulk samples are composed of main phase skutterudite besides a small amount of FeSb₂ and Sb. The TE transport measurements demonstrated that remarkable enhancements in electrical conductivity and Seebeck coefficient can be simultaneously realized by optimizing the doping content of BaFe₁₂O₁₉ magnetic nanoparticles. The lattice thermal conductivity was significantly reduced because of enhanced phonon scattering induced by BaFe₁₂O₁₉ nanoparticles. The highest ZT value reached 0.75 at 800 K for the sample with x = 0.05%, increased by 41.5% as compared with that of p-type CeFe₃CoSb₁₂ bulk material without BaFe₁₂O₁₉ magnetic nanoparticles. This work confirms that doping a small amount of BaFe₁₂O₁₉ magnetic nanoparticles can significantly improve the ZT value of p-type xBaFe₁₂O₁₉/CeFe₃CoSb₁₂ magnetic nanocomposite TE materials.     

Effect of Ca Doping on the Thermoelectric Performance of Yb<Subscript>14</Subscript>MnSb<Subscript>11</Subscript>

Complex Zintl phases possess low thermal conductivity and can be easily doped to modify the transport properties. Therefore, these phases have the potential to be good thermoelectric materials by simply controlling carrier concentration. Yb₁₄MnSb₁₁ is a Zintl phase that has shown promise as a p-type thermoelectric material for high-temperature power generation. A Sn-flux synthetic route was used to make the new phase, Yb₁₃CaMnSb₁₁. The high-temperature thermoelectric properties were measured on polycrystalline hot-pressed pellets and compared with Yb₁₄MnSb₁₁. Substitution of the lighter isovalent Ca for Yb should reduce the lattice thermal conductivity by mass disorder scattering, and a noticeable reduction is seen in thermal diffusivity measurements at high temperature. There may also be a carrier concentration effect by employing the more electropositive Ca.