Thermoelectric properties of p-type Bi0.5Sb1.5Te3 compounds fabricated by spark plasma sintering

P-type thermoelectric Bi_0.5Sb_1.5Te₃ compounds were prepared by the spark plasma sintering method with temperature ranges of 300–420^∘C and powder sizes of ∼75 μm, 76–150 μm, 151–250 μm. As the sintering temperature increased, the electrical resistivity and thermal conductivity of the compound were greatly changed due to an increase in the relative density. The Seebeck coefficient and electrical resistivity were varied largely with decreasing the powder size. Subsequently, the compound sintered at 380^∘C with the powders of ∼75 μm showed the maximum figure-of-merit of 2.65 × 10⁻³K⁻¹ and the bending strength of 73 MPa.     

Solid state rapid quenching method to synthesize micron size Li4Ti5O12

Li₄Ti₅O₁₂ spinel has been synthesized by a new solid-state rapid quenching method adopting solid-state synthesis followed by quenching leading to stoichiometric phase pure micron size material. By simple freezing of the solid-state reaction occurring at 800 °C using ice cold medium, micron sized particulates are achieved. The powder has been characterized by X-ray diffraction, SEM, etc. By this synthesis uniform morphology has been attained and particle size of fine particulates of Li₄Ti₅O₁₂ of an average size of 1–2 μm has been achieved. High rate characteristic and excellent cyclability are the highlights of newly synthesized material. Diffusion coefficient of Lithium has been evaluated and it is found to be 1.4 × 10⁻¹⁵ cm² s⁻¹. A maximum reversible lithium insertion capacity of 154 mAhg⁻¹ at 2 C rate has been achieved. Results are discussed in this paper.     

Thermoelectric properties of Bi and Cu co-doped Ca3Co2O6single crystals

Single crystals of Bi and Cu-doped Ca₃Co₂O₆were synthesized in a molten K₂CO₃flux. Using an obtained single crystal of (Ca_0.985(5)Bi_0.015(5))₃(Co_0.990(3)Cu_0.010(3))₂O₆elongated to the c-axis direction of the crystal structure, the electric resistivity (ρ) and Seebeck coefficient (S) were measured from room temperature to over 1000 K in air. The single crystal showed p-type semiconducting behavior with ρ values of 1.8 Ω cm at 303 K and 0.017 Ω cm at 1000 K. The S values were +254 μ VK^{− 1} at 325 K, +360 μ VK^{− 1} at 420 K, and +214 μ VK^{− 1} at 1000 K. The power factor (S ² ρ ^{− 1}) increased with an increase of temperature and attained 2.70 × 10^{− 4} Wm^{− 1}K^{− 2} at 1000 K.     

Effects of sputtering pressure on the characteristics of lithium ion conductive lithium phosphorous oxynitride thin film

Lithium phosphorous oxynitride(Lipon) thin films as a lithium ion conductive electrolyte were prepared by radio frequency reactive sputtering in N₂ plasma. The properties of the amorphous Lipon solid electrolyte were investigated as a function of N₂ pressure during reactive sputtering. The ionic conductivity and the electrochemical stability of Lipon thin films improved drastically as the N₂ pressure decreased. The ionic conductivity closed to 10⁻⁶ S cm⁻¹ and obtained a stability window of 1.0–5.0 V with an N₂ pressure of 5 mTorr, where the number of nitrogen bonds between the phosphate groups were more than those formed at higher pressure. It was possible to fabricate the Li//LiCoO₂ complete thin film battery using this Lipon solid electrolyte, which exhibited excellent discharge characteristics close to the theoretical capacity (ca. 69 uAhcm⁻²−um⁻¹) and showed a considerably high rate capability.     

Characteristics of thick-film CO2 sensors based on NASICON with Na2CO3-CaCO3 auxiliary phases

Potentiometric CO₂ sensors were fabricated using a NASICON (Na_1+x Zr₂Si_xP_3−x O₁₂) thick film and auxiliary layers. The powder of a precursor of NASICON with high purity was synthesized using the sol-gel method. Using the NASICON paste, an electrolyte was prepared on the alumina substrate through screen printing and then sintered at 1000^∘C for 4 h. In the present study, as new auxiliary phases, a series of Na₂CO₃-CaCO₃ system was deposited on the Pt sensing electrode. The electromotive force (EMF) values were found to be linearly dependent on the logarithm of the CO₂ concentration in the range of 1000–10000 ppm. The device to which Na₂CO₃-CaCO₃ (1:2) was attached showed good sensing properties at low temperatures.     

Micro-structure and ferroelectric properties of BiFeO3 thin films formed on Pt-coated r-plane sapphire substrates

Magnetoelectric BiFeO₃ (BFO) materials exhibit ferroelectric and ferromagnetic properties simultaneously, therefore they have a potential to be applied in magnetic as well as ferroelectric devices. BFO thin films were formed by depositing sol-gel solutions on Pt-coated r-plane sapphire dielectric substrates. We did not observe any secondary phase such as Bi₂Fe₄O₉ on the r-plane sapphire substrates, which is generally observed on Si substrates. We observed small ferroelectric grains of about 0.1 μm on Pt/sapphire structures. The leakage current density in BFO films was found to be decreased dramatically after optimizing process conditions of stoichiometric BFO chemical solution. The leakage current densities were in the range of 10^{− 7} A/cm² at room temperature and 10^{− 9} A/cm² at 80 K under 0.4 MV/cm applied electric field. The main reason for low leakage current is considered to be reduction of oxygen vacancies due to the presence of exclusive Fe^{3 +} valance state in the films. An applied electric field higher than 0.5 MV/cm was required to pole the BFO films, which made it difficult to obtain the saturated polarization at room temperature. We could measure the saturated remanent polarization in the BFO films at 80 K and the obtained remanent polarization was 100 μC/cm².     

Photoresist patterned thick-film piezoelectric elements on silicon

A fundamental limitation of screen printing is the achievable alignment accuracy and resolution. This paper presents details of a thick-resist process that improves both of these factors. The technique involves exposing/developing a thick resist to form the desired pattern and then filling the features with thick film material using a doctor blading process. Registration accuracy comparable with standard photolithographic processes has been achieved resulting in minimum feature sizes of <50 μm and a film thickness of 100 μm. Piezoelectric elements have been successfully poled on a platinised silicon wafer with a measured d ₃₃ value of 60 pCN⁻¹.     

Effect of Concentration of SH U 555A Labeled Human Melanoma Cells on MR Spin Echo and Gradient Echo Signal Decay at 0.2, 1.5, and 3T

In the current study the effect of increasing concentrations of superparamagnetic iron oxide labeled cells on the MRI signal decay at magnetic field strengths of 0.2, 1.5, and 3 T was evaluated. The spin echo and gradient echo cellular transverse relaxivity was systematically studied for various concentrations (N = 1, 5, 10, 20, 40, and 80 cells/μl_gel) of homogeneously suspended SH U 555A labeled SK-Mel28 human melanoma cells. For all field strengths investigated a linear relationship between cellular transverse relaxation enhancement and cell concentration was found. In the spin echo case, the cellular relaxivities [i.e., d(ΔR ₂)/dN] were determined to 0.12 s⁻¹ (cell/μl)⁻¹ at 0.2 T, 0.16 s⁻¹ (cell/μl)⁻¹ at 1.5 T, and 0.17 s⁻¹ (cell/μl) at 3 T. In the gradient echo case, the calculated cellular relaxivities (i.e., d(ΔR ₂ ^* )/dN) were 0.51 s⁻¹ (cell/μl)⁻¹ at 0.2 T, 0.69 s⁻¹ (cell/μl)⁻¹ at 1.5 T, and 0.71 s⁻¹ (cell/μl)⁻¹ at 3 T. The proposed preparation technique has proven to be a simple and reliable approach to quantify effects of magnetically labeled cells in vitro. On the basis of this quantification well suited tissue specific models can be derived.     

Structural and electrical properties of Sr(Ti, Fe)O3-δ materials for SOFC cathodes

Doped strontium titanates are very versatile materials. Iron doped SrTiO₃ can be used, for example, as a material for resistive gas sensors and fuel cell electrodes. In this paper, two compositions based on Fe doped SrTiO₃ were studied as possible candidates for cathode application in SOFCs. Namely, SrTi_0.65Fe_0.35O₃ and SrTi_0.50Fe_0.50O₃ were examined. A chemical reactivity between electrode and YSZ electrolyte material was investigated, since Sr containing cathode materials in contact with YSZ electrolyte are prone to form insulating phases. Electrical conductivity of bulk samples showed relatively low total conductivities of 0.4 S cm⁻¹ and ~2 S cm⁻¹ for STF35 and STF50 respectively. Suitability for cathode application was studied by Electrochemical Impedance Spectroscopy in a symmetrical electrode configuration. Area specific resistance (ASR) was determined in the temperature range from 600°C to 800°C. At 790°C samples show polarization ASR of approximately 0.1 Ω cm². It can be expected that further reduction of electrode ASR can be obtained by introduction of ceria barrier layer and tailoring of the electrode microstructure.     

Titanium Dioxide Wrapped MOFs-Derived Cobalt-Doped Porous Carbon Polyhedrons as Sulfur Hosts for Advanced Lithium–Sulfur Batteries

Abstract

To overcome the shortcomings (low-activity materials and the poor cycle stability) of lithium-sulfur (Li-S) batteries, rational design of cathode host materials is critical. Herein, we developed core-shell Co@NPC@TiO₂ nanostructures, coating titanium dioxide (TiO₂) on cobalt-doped nanoporous carbon (Co@NPC) derived from the thermal decomposition of zeolitic imidazolate framework-67 (ZIF-67) for sulfur electrodes. The synthesized Co@NPC@TiO₂ has excellent electrical conductivity and strong restrictions on polysulfides, the sulfur cathodes with core-shell structure demonstrate a high initial discharge capacity of 1400 mAh g^?1 at 0.1 C, compared with porous carbon, the electrical conductivity and cyclic stability are improved obviously.     

Improving the rate performance of LiCoO2 by Zr doping

Zr-doped LiCoO₂ cathode materials for lithium ion batteries were synthesized by an ultrasonic spray pyrolysis method. The synthesized powders with less than 1 mol% Zr had a single phase layered structure while those with 5 mol% Zr had a little secondary phase, Li₂ZrO₃. The cycle stabilities of Zr-doped and undoped LiCoO₂ were compared at various charge–discharge rates. The Zr-doped LiCoO₂ showed much improved cycle stability compared to the undoped, especially at a high C-rate of 3C (4.2 mA/cm²). To investigate the reasons of the improvement, changes of the lattice parameters and the interatomic distances of Co–Co and Co–O of the doped and the undoped powders were analyzed using XRD and EXAFS. The lattice parameters, a and c, increased in the powders with less than 1 mol% Zr, but decreased in the powder with 5 mol% Zr. On the other hand, the interatomic distances of Co–Co and Co–O did not change with Zr doping. From these results, the improved cycle stability is thought to be due to the expanded inter-slab distance, which enhances Li-ion mobility during charge/discharge processes.     

Preparation and characterization of LiNi0.80Co0.20– x Al x O2 as cathode materials for lithium ion batteries

LiNi_0.80Co_{0.20− x} Al _x O₂ samples (x = 0.025, 0.050 and 0.100) were prepared by a solid-state reaction at 725^∘C for 24 h from LiOH⋅H₂O, Ni₂O₃, Co₂O₃ and Al(OH)₃ under oxygen flow. LiNiO₂ simultaneously doped by Co-Al has been tried to improve the cathode performance. The results showed that substitution of optimum amount Al and Co at the Ni-site in LiNiO₂ improved cycling performance. As a consequence, LiNi_0.80Co_0.15Al_0.05O₂ has 178.2 mAh/g of the first discharge capacity and 174.0 mAh/g after 10 cycles. Differential capacity vs. voltage curves indicated that the Co-Al co-doped LiNiO₂ showed suppression of the phase transitions compared with pure LiNiO₂.     

Ferroelectric properties of (Bi3.15 Nd 0.85 )Ti 3 O 12 with decreasing film thickness

Film texture and ferroelectric behaviors of (Bi_3.15Nd_0.85)Ti₃O₁₂ (BNdT) of layered-perovskite structure deposited on Pt/TiO₂/Si substrate are dependent on the film thickness. When the film thickness is reduced from ∼240 to ∼120 nm, BNdT grains evolve from a rod-like morphology to a spherical morphology, accompanied by a decrease in average grain size. At the same time, P-E hysteresis transforms from a square-shaped hysteresis loop (2Pr ∼24.1 μC/cm² at 240 nm) to a relative slimmer hysteresis loop (with a lower 2Pr = 19.8 μC/cm² at 120 nm). The nonvolatile polarization (Δ P) shows a maximum at the film thickness of 160 nm, where Δ P was measured to be 14.7 μC/cm² and 6.8 μC/cm² at 5 V and 3 V, respectively. A small amount of excess bismuth in the film thickness of 130 nm, introduced by co-sputtering, can lead to a much enhanced remanent polarization (2Pr of 21.3 μC/cm² at 5 V and 15.2 μC/cm²at 3 V), which is promising for low voltage FRAM applications.     

Thermal expansion and electrochemical properties of La0.7AE0.3CuO3−δ (AE=Ca, Sr, Ba) cathode materials for IT-SOFCs

Alkaline earth metals (Ca, Sr, Ba) substituted lanthanum copper oxides were investigated to evaluate their potentials as cathode materials for intermediate temperature solid oxide fuel cell (IT-SOFC). The crystal structure, thermal expansion and electrochemical performance of La_1−x AE _x CuO_3−δ (x = 0.3, AE=Ca, Sr, Ba) were studied by X-ray diffraction, thermal dilatometer and impedance spectra, respectively. By lowering the size of the A-site cation in ABO₃ perovskite, a lower thermal expansion and polarization resistance were obtained forCa-doped LaCuO_3−δ cathode which showed an area specific resistance of 0.19 Ω cm⁻² under open circuit potential conditions at 800°C, and a polarization overpotential of 52 mV at a current density of 0.1 A/cm² at 700°C, being a potential candidate of cathode material for IT-SOFCs.     

Control of 0.2CaTiO3-0.8Li0.5Nd0.5TiO3 microwave dielectric ceramics by additions of Bi2Ti2O7

Ceramics of 0.2CaTiO₃-0.8Li_0.5Nd_0.5TiO₃) have been prepared by the mixed oxide route using additions of Bi₂O₃-2TiO₂ (up to 15 wt%). Powders were calcined 1100^∘C; cylindrical specimens were fired at temperatures in the range 1250–1325^∘C. Sintered products were typically 95% dense. The microstructures were dominated by angular grains 1–2 μm in size. With increasing levels of Bi₂O₃-2TiO₂ additions, needle and lath shaped second phases developed. For Bi₂Ti₂O₇ additions up to 5 wt%, the relative permittivity increased from 95 to 131, the product of dielectric Q value and measurement frequency increased from 2150 to 2450 GHz and the temperature coefficient of resonant frequency (τ _f ) increased from −28pp/^∘C to +22pp/^∘C. A product with temperature stable τ _f could be obtained at ∼2 wt% Bi₂Ti₂O₇ additions. For high levels of additives, there is minimal change in relative permittivity, the Qxf values degrade and τ _f becomes increasingly negative.     

Development of 2-2 piezoelectric ceramic/polymer composites by direct-write technique

The Micropen™ direct-write technique was used to fabricate ceramic skeletal structures to develop piezoelectric ceramic/polymer composites with 2–2 connectivity for medical imaging applications. A lead zirconate titanate PZT paste with ∼35 vol.% solids loading was prepared as a writing material and the paste’s rheological properties were characterized to evaluate its feasibly for Micropen deposition. A serpentine pattern was designed and deposited in AutoCAD and with a 100 μm pen tip, respectively. After debinding and sintering, the microstructural analysis showed that the ceramic structures were fully densified, with good bonding among layers. Typical single-layer thickness was ∼50 μm, and sintered line width was ∼120 μm. The composites containing 30–45 vol.% PZT were fabricated within 1 cm² area, with thicknesses ranging from 350 to 380 μm. Their electromechanical and dielectric properties were measured and found similar to that of composites fabricated by other techniques. The k _t was ∼0.61, d ₃₃ was 210–320, with Q _m of ∼6 and dielectric constant of 650–940.     

Structure and dielectric properties of BaTi4O9 thin films for RF-MIM capacitor applications

BaTi₄O₉ thin films were grown on a Pt/Ti/SiO₂/Si substrate using RF magnetron sputtering. A homogeneous BaTi₄O₉ crystalline phase developed in the films deposited at 550^∘C and annealed above 850^∘C. When the thickness of the film was reduced, the capacitance density and leakage current density increased. Furthermore, the dielectric constant was observed to decrease with decreasing film thickness. The BaTi₄O₉ film with a thickness of 62 nm exhibited excellent dielectric and electrical properties, with a capacitance density of 4.612 fF/μm² and a dissipation factor of 0.26% at 100 kHz. Similar results were also obtained in the RF frequency range (1–6 GHz). A low leakage current density of 1.0 × 10⁻⁹ A/cm² was achieved at ± 2 V, as well as small voltage and temperature coefficients of capacitance of 40.05 ppm/V² and –92.157 ppm/^∘C, respectively, at 100 kHz.     

Co-planar type single chamber solid oxide fuel cell with micro-patterned electrodes

A co-planar type single chamber solid oxide fuel cell (SC-SOFC) with linearly patterned electrode structures on the same surface as the electrolyte has been fabricated by robo-dispensing method. Paste materials of NiO-SDC-Pd cermet and (La_0.7Sr_0.3)_0.95MnO₃ (LSM) were selectively deposited onto a substrate of yttria stablized zirconia (YSZ) by extrusion through a syringe nozzle. The dispensed pastes were solidified upon solvent evaporation, and the anode and the cathode were sintered at 1350^∘C for 2 h and 1200^∘C for 1h, respectively. We have fabricated SC-SOFCs that have a single electrode pair with varying anode-to-cathode distances and interdigitated patterned electrodes with 2, 4, and 8 multiple pairs. The electrode microstructures of the resulting cells were examined by SEM. The electrochemical performance of the SC-SOFCs was also analyzed using impedance spectroscopy and a DC source meter.     

Praseodymium-cerium oxide thin film cathodes: Study of oxygen reduction reaction kinetics

Praseodymium-Cerium Oxide (Pr_xCe_1-xO_2−δ; PCO), a potential three way catalyst oxygen storage material and solid oxide fuel cell (SOFC) cathode, exhibits surprisingly high levels of oxygen nonstoichiometry, even under oxidizing (e.g. air) conditions, resulting in mixed ionic electronic conductivity (MIEC). In this study we examine the redox kinetics of dense PCO thin films using impedance spectroscopy, for x = 0.01, 0.10 and 0.20, over the temperature range of 550 to 670°C, and the oxygen partial pressure range of 10⁻⁴ to 1 atm O₂. The electrode impedance was observed to be independent of electrode thickness and inversely proportional to electrode area, pointing to surface exchange rather than bulk diffusion limited kinetics. The large electrode capacitance (10⁻²F) was found to be consistent with an expected large electrochemically induced change in stoichiometry for x = 0.1 and x = 0.2 PCO. The PCO films showed surprisingly rapid oxygen exchange kinetics, comparable to other high performance SOFC cathode materials, from which values for the surface exchange coefficient, k ^q , were calculated. This study confirms the suitability of PCO as a model MIEC cathode material compatible with both zirconia and ceria based solid oxide electrolytes.     

Electrochemical characteristics of LiFePO4/LiCoO2 mixed electrode for Li secondary battery

LiFePO₄ cathode active material was synthesized using a solid state method and a mixed cathode was prepared by adding LiCoO₂. A coin cell was prepared using the mixed cathode and its electrochemical performances were evaluated. The LiFePO₄ material improved both in thermal stability and rate capability. The discharge capacity of the 5 wt.% LiCoO₂-added LiFePO₄ material was 139.4 mAh/g at 0.2 C rate, and it showed a capacity retention of 64.2% even at 5 C rate in comparison to 0.2 C rate. The cycle performance of the bare LiFePO₄ material was excellent, but the capacity fading of 20 wt.% LiCoO₂-added LiFePO₄ material became significant during charge/discharge cycle. However, 5～10 wt.% LiCoO₂-added LiFePO₄ material showed good thermal stability, high rate capability and good cycle performances.