High Temperature Electrical Conductivity Research Articles

Oxidation behavior and electrical properties of metal interconnects with Ce-doped Ni–Mn spinel coatings

To improve the high-temperature oxidation resistance and electrical conductivity of ferritic stainless steels, protective Ce-doped NiMn2O4 spinel coatings were fabricated on the surface of SUS430 steel by electrophoretic deposition (EPD). The phase structure and microstructure of Ce-doped NiMn2O4 in both powder and coating forms were characterized using X-ray diffraction (XRD) and scanning electron microscopy (SEM). The high-temperature oxidation of the NiMn2O4 spinel coating before and after Ce doping in the air at 800 °C for 168 h was studied by weight gain experiments. The area-specific resistance (ASR) of coatings was measured by a standard four-probe method. It was found that the Ce-doped NiMn2O4 spinel powder displayed a stable structure, high crystallinity, fine grain size, and decreased agglomeration when the Ce content was fixed at 0.05 mol∙L−1. The oxidation kinetics of NiMn2O4-coated SUS430 steel before and after Ce doping obeyed a parabolic law with parabolic rate constants of 4.58 × 10−15 g2 cm−4 s−1 and 1.83 × 10−15 g2 cm−4 s−1, respectively. When oxidized at 800 °C for 50 h, the ASR value of the coated samples before and after Ce doping stabilized at about 15.2 mΩ∙cm2 and 14.5 mΩ∙cm2, respectively. This work demonstrated that the Ce-doped NiMn2O4 spinel coating improved the high-temperature oxidation resistance and the electrical conductivity of metal interconnects.

Synthesis of Composite Materials based on TiB2–TiC–Al2O3 and CrB2-Al2O3 in the Combustion Conditions

Borides and carbides attract the attention of developers of heat-resistant and super hard structural materials due to a unique combination of their resistance to high-temperature oxidation, high hardness, wear resistance, electrical and thermal conductivity and etc. The article presents experimental results on obtaining composites based on TiB2-TiC-Al2O3, CrB2-Al2O3 by a method combining self-propagating high-temperature synthesis (SHS) and mechanical activation (MA). The influence of the composition of the initial components, the conditions of SHS and preliminary MA on the formation of the microstructure and phase composition of the SHS-composite based on titanium carbide and titanium, chromium borides has been studied. The SHS products were examined by X-ray diffraction analysis and a scanning electron microscope. High-temperature phases of borides of chromium, titanium, aluminum oxide and their spinel are found in SHS products.

Influence of Surface Modification Using a Rare Earth Element on Properties of Ferritic Stainless Steel Metallic Interconnects

Iron and chromium based ferritic stainless steels have been considered to be a promising material for the metallic interconnect of solid oxide fuel cells (SOFCs) and solid oxide electrolysis cells (SOECs). However they have a drawback of chromium evaporation led by a scale growth at the steel surface when exposed at a high temperature oxidation atmosphere, which results in chromium poisoning of the air electrode and subsequent performance degradation of SOFCs and SOECs. Application of a surface modification or protective coating onto the metallic interconnects has been introduced to prevent the degradation by mitigating the chromium evaporation. In this study, ferritic stainless steels modified with a rare earth element of neodymium was investigated on the properties of high temperature oxidation and electrical conductivity. Detailed analyses on phase formation, microstructure evolution, and area specific resistance were carried out to figure out effects of the surface modification on the steels. Accordingly, the surface modified steels revealed decrease in the area specific resistance which was led by the reduction of scale growth since the rare earth element altered the chromium oxidation behavior.

Pressure-induced structural phase transitions in natural kaolinite investigated by Raman spectroscopy and electrical conductivity

Abstract We investigated the structural, vibrational, and electrical transport properties of natural kaolinite and its high-pressure polymorphs by Raman scattering and electrical conductivity measurements at 293–673 K and up to 10.0 GPa using diamond-anvil cell. Upon compression, kaolinite underwent two structural transitions from kaolinite I to kaolinite II to kaolinite III phases at pressures of 2.9 and 6.5 GPa, respectively, which was disclosed by the inflection point in the pressure-dependent Raman shifts and electrical conductivity. Upon decompression, kaolinite III directly transformed to kaolinite I at 0.8 GPa without the appearance of kaolinite II. Additionally, the influence of temperature on the structural transformation of natural kaolinite was explored by high-temperature and high-pressure electrical conductivity measurements and negative temperature-dependent transition pressure correlations were obtained. A phase diagram of natural kaolinite was established for the first time and the kaolinite I-kaolinite II and kaolinite II-kaolinite III phase transition boundaries were determined: P (GPa) = 4.298–0.00462 T (K) and P (GPa) = 8.895–0.00799 T (K), respectively. Furthermore, our acquired phase diagram can be applied to understand the stability field of high-pressure polymorphs of kaolinite in the Earth’s interior and may provide a phase transition model for other kaolin-group minerals.

Superionic iron oxide–hydroxide in Earth’s deep mantle

Water ice becomes a superionic phase under the high pressure and temperature conditions of deep planetary interiors of ice planets such as Neptune and Uranus, which affects interior structures and generates magnetic fields. The solid Earth, however, contains only hydrous minerals with a negligible amount of ice. Here we combine high pressure and temperature electrical conductivity experiments, Raman spectroscopy and first-principles simulations to investigate the state of hydrogen in the pyrite-type FeO2Hx (x ≤ 1), which is a potential H-bearing phase near the core–mantle boundary. We find that when the pressure increases beyond 73 GPa at room temperature, symmetric hydroxyl bonds are softened and the H+ (or proton) becomes diffusive within the vicinity of its crystallographic site. Increasing temperature under pressure, the diffusivity of hydrogen is extended beyond the individual unit cell to cover the entire solid, and the electrical conductivity soars, indicating a transition to the superionic state, which is characterized by freely moving protons and a solid FeO2 lattice. The highly diffusive hydrogen provides fresh transport mechanisms for charge and mass, which dictate the geophysical behaviours of electrical conductivity and magnetism, as well as geochemical processes of redox, hydrogen circulation and hydrogen isotopic mixing in Earth’s deep mantle.

Linking the Electrical Conductivity and Non-Stoichiometry of Thin Film Ce1-xZrxO2-δ by a Resonant Nanobalance Approach.

Bulk ceria-zirconia solid solutions (Ce1−xZrxO2−δ, CZO) are highly suited for application as oxygen storage materials in automotive three-way catalytic converters (TWC) due to the high levels of achievable oxygen non-stoichiometry δ. In thin film CZO, the oxygen storage properties are expected to be further enhanced. The present study addresses this aspect. CZO thin films with 0 ≤ x ≤ 1 were investigated. A unique nano-thermogravimetric method for thin films that is based on the resonant nanobalance approach for high-temperature characterization of oxygen non-stoichiometry in CZO was implemented. The high-temperature electrical conductivity and the non-stoichiometry δ of CZO were measured under oxygen partial pressures pO2 in the range of 10−24–0.2 bar. Markedly enhanced reducibility and electronic conductivity of CeO2-ZrO2 as compared to CeO2−δ and ZrO2 were observed. A comparison of temperature- and pO2-dependences of the non-stoichiometry of thin films with literature data for bulk Ce1−xZrxO2−δ shows enhanced reducibility in the former. The maximum conductivity was found for Ce0.8Zr0.2O2−δ, whereas Ce0.5Zr0.5O2-δ showed the highest non-stoichiometry, yielding δ = 0.16 at 900 °C and pO2 of 10−14 bar. The defect interactions in Ce1−xZrxO2−δ are analyzed in the framework of defect models for ceria and zirconia.

High-temperature electrical and thermal transport properties of polycrystalline PdCoO2

The layered delafossite ${\mathrm{PdCoO}}_{2}$ has been predicted to be one of very few materials with a thermopower that is highly anisotropic and switches sign between different crystallographic directions. These properties are of interest for various applications, but have been difficult to verify because sufficiently large crystals have not been available. We report measurements of the high-temperature electrical resistivity, thermal conductivity, and thermopower of phase-pure ${\mathrm{PdCoO}}_{2}$ powder compacts prepared by a highly Pd-efficient synthesis route. While the electronic transport of the polycrystalline samples is dominated by that of the Pd planes, the thermopower exhibits a well-defined deviation from the in-plane character at temperatures above $600\phantom{\rule{4pt}{0ex}}\mathrm{K}$, which is indicative of opposing trends in the Seebeck coefficients within and perpendicular to the delafossite layers. The experimental data are consistently described by a combination of effective-medium models based on the main axes transport quantities. The results support the predicted ambipolar thermopower anisotropy in ${\mathrm{PdCoO}}_{2}$.

Two-phase interfacial area characteristics and transport under the transition from subcooled boiling to saturated boiling flows

This study focuses on the analysis of the transition from the subcooled boiling to saturated boiling flow. During this process, vapor generates drastically in a short developing length, which makes it difficult to predict the interfacial characteristics of two-phase flow. To investigate the characteristics of this process, experiments were performed in a vertical annulus with 19.1 mm inner diameter and 38.1 mm outer diameter. The two-phase interfacial parameters including time-averaged local vapor fraction, interfacial area concentration, and bubble interfacial velocity are obtained using high-temperature miniature four-sensor electrical conductivity probes. In this study, the conductivity probe signal is processed using the newly-developed signal processing algorithm developed for small spherical bubbles by Shen and Nakamura (2014) , instead of the algorithm used in the previous subcooled boiling experimental studies Ozar (2009) and Kumar et al. (2019), [3] for large bubbles developed by Kataoka et al. (1986). The flow conditions cover a wide range of inlet subcooling temperature at 12.9–21.2 °C, heat flux at 57.9–197.3 kW/m2, and inlet flow rate at 0.265–0.536 m/s. The vapor fractions are cross-validated with impedance void meter measurements. The state-of-art two-group interfacial area transport equations (IATE) [6], (Ozar, 2009) and (Brooks and Hibiki, 2016) is evaluated with the experimental data. The result shows that the two-group IATE gives fairly good predictions on the transition phenomena. Besides, a defect was identified in the condensation model (Park et al., 2007) used in IATE, which is the bubble boundary diameter classifying the two condensation mechanisms: heat-transfer-controlled and inertia-controlled condensation. A rigorous approach to estimate this boundary diameter is established in this study.

Site preference of Y and Mn in nonstoichiometric BaTiO3 from first principles

$\mathrm{Ba}\mathrm{Ti}{\mathrm{O}}_{3}$ is often doped with Y and/or Mn to realize a range of desired properties. Yet, existing canonical models of their defect chemistry cannot explain various observed phenomena outside of the high temperature electrical conductivity measurements to which they were fit. Existing models assume Y substitutes exclusively for Ba or Ti despite experiments showing Y is amphoteric, substituting predominantly for Ba or Ti depending on the cation nonstoichiometry. Existing models assume Mn forms isolated ${\mathrm{Mn}}_{\text{Ti}}$ exclusively, but experiments have shown complexes of ${\mathrm{Mn}}_{\mathrm{Ti}}$ with native oxygen vacancies (${\mathrm{Mn}}_{\mathrm{Ti}}\text{\ensuremath{-}}{\mathrm{v}}_{\text{O}}$) form in significant concentrations. Additionally, recent computational works in ${\mathrm{SrTiO}}_{3}$ suggest A-site substitutional defects may form in greater concentrations in a configuration with reduced symmetry relative to the on-site geometry. To address these inconsistencies, we developed a hybrid functional density functional theory informed grand canonical defect model with the ability to simulate specific nonstoichiometries without ad hoc assumptions about the bulk chemical potentials. Using this model, the site preference of Y and Mn in $\mathrm{Ba}\mathrm{Ti}{\mathrm{O}}_{3}$ as a function of the Ba/Ti ratio was evaluated in the context of a more complete set of defects including: native cation vacancies, on-site and reduced symmetry A-site defects, isolated B-site defects, and ${\mathrm{X}}_{\text{B}}\text{\ensuremath{-}}{\mathrm{v}}_{\text{O}}$ defects. The results reproduce experimental observations of yttrium's amphotericity and significant concentrations of ${\mathrm{Mn}}_{\mathrm{Ti}}\text{\ensuremath{-}}{\mathrm{v}}_{\text{O}}$. Both and Y and Mn are found to substitute predominantly for Ba at Ba/Ti = 0.99 and Ti at Ba/Ti = 1.01, but neither are found to substitute exclusively for Ba or Ti within the range of experimentally accessible Ba/Ti ratios at 1400 ${}^{\ensuremath{\circ}}\mathrm{C}$.

Effect of Yb2O3 stabilizing impurity on the structure and properties of (ZrO2)0.9-x(Sc2O3)0.1(Yb2O3)x crystals

The effect of introduction of 0.5 – 2 mol.% Yb2O3 additional doping oxide into ZrO2 – 10 mol.% Sc2O3 solid solutions on the phase composition, structure and electrophysical properties has been studied. ZrO2 stabilization due to the joint effect of 10 mol.% Sc2O3 and 2 mol.% Yb2O3 provides for the formation of transparent homogeneous crystals with a cubic structure having high phase stability. Mechanical grinding of the crystals has not changed their phase composition, the powders retaining the initial fluorite crystalline structure. We show that the high-temperature electrical conductivity of the crystals decreases with an increase in the Yb2O3 concentration.

Electromechanical losses in carbon- and oxygen-containing bulk AlN single crystals

Bulk single-crystalline aluminum nitride (AlN) is potentially a key component for low-loss high-temperature piezoelectric devices. However, the incorporation of electrically active impurities and defects during growth of AlN may adversely affect the performance of piezoelectric resonators especially at high temperatures. The electrical conductivity and electromechanical losses in bulk AlN single crystals are analyzed in the temperature range of 300–1200 K with respect to various contents of growth-related impurities in them. For AlN with [O]/[C] ≤ 1, an increase of electrical conductivity due to thermal activation of charge carriers in the temperature range of 850–1200 K has been observed and was determined to be a major contribution to electromechanical losses Q−1 rising up to maximum values of about 10−3 at 1200 K. As the oxygen content in AlN increased, the magnitude and the activation energy of high-temperature electrical conductivity increased. In oxygen-dominated AlN, two major thermally activated contributions to electromechanical losses were observed, namely, the anelastic relaxations of point defects at temperatures of 400–800 K and electrical conductivity at T > 800 K.

Influence of vanadium doping on the structural and electric properties of Sr0.5Ca0.5Mn1–xVxO3–δ

The solid solution Sr0.5Ca0.5Mn1–xVxO3–δ (0 ≤ x ≤ 0.125) has been obtained via a citrate-nitrate synthesis route. Powder X-ray diffraction (XRD) data for x = 0, 0.025 and 0.05 reveal the formation of perovskite-type orthorhombic structure with Pbnm space group (SG). Further increase of the vanadium content results in the appearance of the tetragonal crystal structure (SG I4/mcm) so that the composition of x = 0.075 is formed by the orthorhombic and tetragonal phase constituents in the 1:3 vol ratio. The single-phase sample at x = 0.1 is characterized by tetragonal symmetry of its unit cell. The comparison between the calculated (XRD) and pycnometric density values is indicative of 6-fold oxygen coordination of vanadium dopants. High-temperature electrical conductivity (σ) and Seebeck coefficient (S) measurements give evidence to the presence of electronic charge carriers that can be associated with Mn3+ cations appearing in response to vanadium doping. The largest thermoelectric power factor S2σ = 1.5 µW·K−2·cm−1 is attained in the two-phase composition x = 0.075 at 450 °C.

Synthesis and characterization of vacancy-doped neodymium telluride for thermoelectric applications.

Thermoelectric materials exhibit a voltage under an applied thermal gradient and are the heart of radioisotope thermoelectric generators (RTGs), which are the main power system for space missions such as Voyager I, Voyager II, and the Mars Curiosity rover. However, materials currently in use enable only modest thermal-to-electrical conversion efficiencies near 6.5% at the system level, warranting the development of material systems with improved thermoelectric performance. Previous work has demonstrated large thermoelectric figures of merit for lanthanum telluride (La3-x Te4), a high-temperature n-type material, achieving a peak zT value of 1.1 at 1275 K at an optimum cation vacancy concentration. Here we present an investigation of the thermoelectric properties of neodymium telluride (Nd3-x Te4), another rare-earth telluride with a similar structure to La3-x Te4. Density functional theory (DFT) calculations predicted a significant increase in the Seebeck coefficient over La3-x Te4 at equivalent vacancy concentrations due to an increased density of states (DOS) near the Fermi level from the 4f electrons of Nd. The high temperature electrical resistivity, Seebeck coefficient, and thermal conductivity were measured for Nd3-x Te4 at various carrier concentrations. These measurements were compared to La3-x Te4 in order to elucidate the impact of the four 4f electrons of Nd on the transport properties of Nd3-x Te4. A zT of 1.2 was achieved at 1273 K for Nd2.78Te4, which is a 10% improvement over that of La2.74Te4.

Structural Study and Electrical Properties of Bi1.5–xCaxSb1.5CuO7 Pyrochlore-Type Solid Solution Series

Abstract Pyrochlore solid solution Bi1.5−xCaxSb1.5CuO7 has been prepared by conventional ceramic solid state reaction with 0 ≤ x ≤ 0.4. The samples were characterized by X-ray powder diffraction (XRPD), scanning electron microscopy, and electrical conductivity measurements. The pyrochlore single phase formation was confirmed for all of the x-fractions. The variation of the electrical resistivity versus x of all samples measured by means of a four-probe device at room temperature indicates that these still remain insulators. High-temperature electrical conductivity measurements at various frequencies using a device with two electrodes allow us to note the maximum electrical conductivity of 4.7 × 10−3 S.cm−1, which has been reached at x = 0.2, generating an activation energy of 0.14 eV. For this fraction, Rietveld refinement of the corresponding XRPD pattern confirmed the typical cubic Fd-3m space group with a cell parameter of a = 10.4089(1) Å.

High-temperature electrical conductivity of the xNBT–(1-x)LMT ceramics: verification of Meyer-Neldel rule

Electrical conductivity behaviour of x(Na0.5Bi0.5)TiO3-(1-x)La(Mg0.5Ti0.5)O3 ceramics with x ranging from 0.95 to 0.8 was analysed in the temperature range from 300 K to 800 K at microwaves (1 MHz – 1GHz frequency region). Analysis of calculated DC conductivity parameters has shown that the activation energy changes at about 690 K which could be associated with the phase change in these materials. The logarithm of preexponential factor of DC conductivity changes linearly according to the activation energy, i.e. it follows Meyer-Neldel rule both below and above the phase transition point.

High Temperature Electrical Conductivity and Modulus Study of a Lead-Free Na0.5Bi0.5TiO3-SrTiO3 Ceramic

Abstract Electrical conductivity study of a lead-free 0.95Na0.5Bi0.5TiO3-0.05SrTiO3 ceramic was investigated by impedance spectroscopy in the high temperature range from 400 °C to 650 °C. A substantial decrease in conductivity was observed as compared to the previously reported NBT based samples. Frequency and temperature dependent ac conductivity analysis was done employing some empirical models. In this temperature range, the temperature dependence of frequency exponent ‘S’ suggests the dominance of co-related barrier hopping (CBH) for ac conduction. The sample shows non-Debye type of relaxation which is explained adequately by using Gaussian and modified KWW functions.

Enhancement of grain growth and electrical conductivity of La0.8Sr0.2MnO3 ceramics by microwave irradiation

We studied crystallization, grain growth and electric properties of La0.8Sr0.2MnO3 (LSM) ceramics which were produced using the microwave-treatment. While co-precipitated nanoparticles remain mainly amorphous, the microwave irradiated particles are crystallized into LSM and La2Mn2O7 at 550 °C, due to higher dielectric polarizability of La. This, in turn, decreases the amount of the second phase La2O3 in calcined powder and promotes the growth of perovskite grains during sintering at 1400 °C. Larger grains of LSM ceramics lower the activation energy of small polaron hopping from 0.35 eV to 0.24 eV and increases high-temperature electric conductivity. In addition, high crystallinity of LSM ceramics from the microwave-treatment suppresses a chemical reaction with ZrO2 and NiO in a temperature range of 900 – 1100 °C under oxidizing and reducing ambiances. These results show that LSM ceramics from the microwave-assisted reaction meet requirements for an interconnect layer for solid oxide electrolysis cells.

Enhancement of thermoelectric properties by lattice softening and energy band gap control in Te-deficient InTe1−δ

The InTe has intrinsically low lattice thermal conductivity κL originating from the anharmonic bonding of In1+ ion in the lattice, which scatters the phonons. Here we report the enhancement of thermoelectric properties in Te-deficient InTe1−δ (δ = 0, 0.01, 0.1, and 0.2) polycrystalline compounds by lattice softening and energy band gap opening. Te-deficiency gives rise to more weak chemical bonding between In1+ atoms and In3+Te2− clusters than those of pristine InTe, resulting in the reduction of κL near the room temperature. The weak ionic bonding is confirmed by the increase of lattice volume from the X-ray diffraction and lattice softening by the decrease of Debye temperature with increasing Te-deficiency. We observed the low lattice thermal conductivity κL of 0.53 W m−1 K−1 at 300 K for InTe0.99, which is about 25 % decreased value than those of InTe. The Te-deficiency also induces energy band gap so that the electrical resistivity and Seebeck coefficient are increased due to the decrease of carrier concentration. Temperature-dependent thermoelectric properties shows the high Seebeck coefficient at high temperature and high electrical conductivity near room temperature, resulting in the temperature-insensitive high power factor S2σ over a wide temperature range. Owing to the temperature-insensitive high power factor and intrinsic low lattice thermal conductivity by Te-deficiency, the thermoelectric performances of figure-of-merit ZT and engineering ZTeng are enhanced at mild temperature range (≤550 K).

High temperature electrical conductivity and electrochemical investigation of La2-xSrxCoO4 nanoparticles for IT-SOFC cathode

Single-phase Ruddlesden popper of La2-xSrxCoO4 nanopowders with x = 0.7, 0.9, 1.1 and 1.3, were successfully synthesized by a modified sol-gel method. Structural stability and morphology of the prepared samples were examined using HT-XRD analysis, FE-SEM and SEM techniques. HT-XRD analysis of the samples, in the range of room temperature to 850 °C, revealed that the structure of all samples was tetragonal. The electrical conductivity measurements, in the range of room temperature to 850 °C, indicated that by increasing the temperature the electrical conductivity mechanism inverts from variable range hopping to the nearest-neighbor hopping of small polarons. In addition, it was found that by increasing Sr concentration the structure of the sintered samples becomes more stable. The electrochemical characterization was carried out using the impedance spectroscopy (EIS) measurements on symmetrical cells at three different temperatures, 650 °C, 750 °C and 850 °C. The area specific polarization resistance (ASR) of La2-xSrxCoO4-CGO-La2-xSrxCoO4 symmetrical cell, in oxygen flow, was obtained about 1.07, 0.35, 0.33 and 0.43 Ωcm2 at 850 °C for the samples with x = 0.7, 0.9, 1.1 and x = 1.3, respectively. According to our EIS results, the main rate-limiting step for La2-xSrxCoO4 cathode performance is the dissociation process of oxygen at the surface of cathode at 650 °C and the charge transfer limiting in the cathode/electrolyte at 750 °C and 850 °C. Our results showed that the samples with Sr contents of x = 0.9 and x = 1.1 can be the promising cathodes for IT-SOFC applications.

Effect of Fillers on Viscosity and Electrical Conductivity of Glass Composite Sealants

Development of a reliable sealant or sealing system remains one of the top priorities in planar solid oxide fuel cells (SOFC) technology. Glass matrix composites as sealing materials for SOFCs are known to enable design flexibility, e.g., by control of such properties as viscosity, seal operating temperatures, and visco-elastic properties. The objective of this study was to find suitable fillers for structural strengthening as well as for improving long-term cyclic sealability of the alkali borosilicate glass composite sealants. Alkali ion containing compliant sealing glass was developed by present authors. Two kinds of fillers, Al2O3 and 8 mol % Y2O3-stabilized zirconia (8YSZ), were used up to 15% for the preparation of glass composite sealants. The effects of these fillers and their content on the high temperature viscosities and electrical conductivities of the sealants were investigated. Furthermore, glass composite to STS444 seal assemblies were subjected to thermocycling between 450°C and 750°C. The composition of a glass composite sealant to survive over 12,000 h of consecutive cyclic test was identified. In this presentation, effect of fillers on properties and cyclic sealing behavior of the glass composites will be also discussed.