Effect of Ionic Radius and Resultant Two-Dimensionality of Phonons on Thermal Conductivity in M<Subscript><Emphasis Type="Italic">x</Emphasis></Subscript>CoO<Subscript>2</Subscript> (M = Li,Na, K) by Perturbed Molecular Dynamics

Phonon thermal conductivity calculations for Li _x CoO₂, Na _x CoO₂, and K _x CoO₂ (x = 1, 0.5) have been carried out by perturbed molecular dynamics to clarify the dependence of thermal conductivity on alkali-metal vacancy concentration in these materials. While thermal conductivity decreased for all compounds upon introduction of alkali-metal vacancies, the magnitude of the decrease is strongly dependent on the size of the alkali-metal ion. Further numerical analyses using fictitious physical parameters reveal that, with increasing ionic radius, the two-dimensionality of the phonons in the CoO₂ layers, which are responsible for overall thermal conductivity, is enhanced, resulting in lower thermal conductivity in vacancy-free compounds as well as ineffectiveness of alkali-metal vacancies in lowering thermal conductivity. In contrast, for systems with smaller alkali-metal ionic radius, even though higher thermal conductivity is predicted when no vacancies are present, vacancies are quite effective in significantly lowering thermal conductivity by modifying phonon states in the CoO₂ layers, more so than in systems with larger alkali-metal vacancies.     

Relation between Seebeck Coefficient and Lattice Parameters of (Ca2?y Sr y CoO3) x CoO2

The effect of dopant content on the Seebeck coefficient of the Sr²⁺-doped (Ca_2?y Sr _y CoO₃) _x CoO₂ (y?=?0 to 0.4) system was investigated. This system can be described as a misfit layered structure of (CoO₂) and (Ca_2?y Sr _y CoO₃) layers, which have different b lattice parameters denoted by b ₁ and b ₂, respectively. Due to the solubility limit of Sr in (Ca₂CoO₃) _x CoO₂, systematic investigations were available only up to y?=?0.2. Nevertheless, the structural uniqueness enabled partially quantitative analysis of the correlation between the crystal structure and Seebeck coefficient of the system. Substitution of Sr for Ca leads to lattice expansion which accompanies an anisotropic change of the lattice parameters. Among the lattice parameters considered, the increase in the lattice parameter b ₂ of the insulating layer was larger than the change of any other lattice parameter, which induced in-plane stress in the conducting layer. As a result, as the b ₁/b ₂ misfit ratio of (Ca_2?y Sr _y CoO₃) _x CoO₂ is decreased, the Seebeck coefficient also decreases. Practical guidance for selecting dopants to enhance thermoelectric performance is proposed.     

High Proton Conductivity in β-Ba2ScAlO5 Enabled by Octahedral and Intrinsically Oxygen-Deficient Layers

Proton conductors are promising materials for clean energy, but most available materials exhibit sufficient conductivity only when chemically substituted to create oxygen vacancies, which often leads to difficulty in sample preparation and chemical instability. Recently, proton conductors based on hexagonal perovskite-related oxides have been attracting attention as they exhibit high proton conductivity even without the chemical substitutions. However, their conduction mechanism has been elusive so far. Herein, taking three types of oxides with different stacking patterns of oxygen-deficient layers (β-Ba₂ScAlO₅, α-Ba₂Sc_0.83Al_1.17O₅, and BaAl₂O₄) as examples, the roles of close-packed double-octahedral layers and oxygen-deficient layers in proton conduction are shown. It is found that “undoped” β-Ba₂ScAlO₅, which adopts a structure having alternating double-octahedral layer and double-tetrahedral layer with intrinsically oxygen-deficient hexagonal BaO (h') layer, shows high proton conductivity (≈10⁻³ S cm⁻¹ above 300 °C), comparable to representative proton conductors. In contrast, the structurally related oxides α-Ba₂Sc_0.83Al_1.17O₅ and BaAl₂O₄ exhibit lower conductivity. Ab initio molecular dynamics simulations revealed that protons in β-Ba₂ScAlO₅ migrate through the double-octahedral layer, while the h′ layer plays the role of a “proton reservoir” that supplies proton carriers to the proton-conducting double-octahedral layers. The distinct roles of the two layers in proton conduction provide a strategy for developing high-performance proton conductors.     

Influence of the metallic absorption layer on the quality of thermal conductivity measurements by the transient thermo-reflectance method

This work investigates numerically the influence of a metallic absorption layer on the laser-based measurements of the thermal conductivity of dielectric (SiO₂) and semiconductor (Si) electronic materials. The validity of the approach and the obtained results are assessed by comparison with experimental measurements obtained for gold-covered silicon dioxide samples. The results reveal the presence of behaviors associated with thermally thin and thermally thick absorption layers, depending on the ratio between the thickness of the absorption layer and the heat penetration depth. Optimal performance of the transient thermo-reflectance method was found to exist for thicknesses of metal layers falling between the identified thermally thin and thermally thick layers.     

Features of electron transport in relaxed Si/Si1 − x Ge x transistor heterostructures with a high doping level

The low-temperature electrical and magnetotransport characteristics of partially relaxed Si/Si_{1 ? x} Ge _x heterostructures with an electron conduction channel in an elastically strained nanoscale silicon layer are investigated. It is demonstrated that the electron gas in the system exhibits 2D properties. A dependence of the conductivity along layers in the system on the degree of elastic-stress relaxation in it is observed. To understand the observed regularities, the potential and the electron distribution over the structure layers are calculated in detail for samples with different layer strains and doping levels. For the structure with x = 0.25, the parameters of the potential barrier and characteristics of the quantum well formed in the Si layer are estimated. It is established that the characteristics of the potential formed near interfaces strongly depend on the initial parameters of the system, in particular, on the degree of the plastic relaxation of elastic stresses and on the doping level. The formation of a thin tunneling-transparent barrier near the upper interface can lead to the redistribution of electrons between the 2D and 3D conduction channels in the structure, which ensures the spread of the measured transport characteristics of the samples during the measurements. The interlayer tunneling transitions of carriers from the 2D state in the Si transport channel to the 3D state of the Si_{1 ? x} Ge _x crystal matrix, which are separated by a tunneling-transparent potential barrier near the heterointerface, were observed for the first time during transport in the direction transverse to the layer plane.     

Scaling Behavior of Resistive Switching in Epitaxial Bismuth Ferrite Heterostructures

Resistive switching (RS) of (001) epitaxial multiferroic BiFeO₃/La_0.67Sr_0.33MnO₃/SrTiO₃ heterostructures is investigated for varying lengths scales in both the thickness and lateral directions. Macroscale current–voltage analyses in conjunction with local conduction atomic force microscopy (CAFM) reveal that whilst both the local and global resistive states are strongly driven by polarization direction, the type of conduction mechanism is different for each distinct thickness regime. Electrode‐area dependent studies confirm the RS is dominated by an interface mechanism and not by filamentary formation. Furthermore, CAFM maps allow deconvolution of the roles played by domains and domain walls during the RS process. It is shown that the net polarization direction, and not domain walls, controls the conduction process. An interface mechanism based on barrier height and width alteration due to polarization reversal is proposed, and the role of electronic reconstruction at the interface is further investigated.     

A First-Principles Study of the Role of Na Vacancies in the Thermoelectricity of Na<Subscript><Emphasis Type="Italic">x</Emphasis></Subscript>CoO<Subscript>2</Subscript>

First principles calculations using the generalized gradient approximation to density functional theory have been carried out to evaluate formation energies of defects and the resultant changes in electronic structure of NaCoO₂ and Na_0.5CoO₂. The calculated formation energies confirm that Na vacancies form readily in this material, particularly through volatilization at elevated temperatures, consistent with experimentally observed behavior. Numerical analysis of the change in charge distribution upon Na vacancy formation shows that the vacancy plays a crucial role in modifying the electronic properties of the material. In these p-type thermoelectric materials, Na vacancies act as a reservoir for the minority carrier (electrons), removing them from the CoO₂ layer while simultaneously increasing the concentration of majority carriers (holes) available for conduction in the CoO₂ layer.     

Origin of Phonon Glass–Electron Crystal Behavior in Thermoelectric Layered Cobaltate

Measurement of local disorder and lattice vibrations is of great importance for understanding the mechanisms whereby thermoelectric materials efficiently convert heat to electricity. Attaining high thermoelectric power requires minimizing thermal conductivity while keeping electric conductivity high. This situation is achievable by enhancing phonon scattering through specific structural disorder (phonon glass) that also retains sufficient electron mobility (electron crystal). It is demonstrated that the quantitative acquisition of multiple annular‐dark‐field images via scanning transmission electron microscopy at different scattering‐angles simultaneously allows not only the separation but also the accurate determination of static and thermal atomic displacements in crystals. Applying the unique method to the layered thermoelectric material (Ca₂CoO₃)_0.62CoO₂ discloses the presence of large incommensurate displacive modulation and enhanced local vibration of atoms, largely confined within its Ca₂CoO₃ sublayers. Relating the refined disorder to ab initio calculations of scattering rates is a tremendeous challenge. Based on an approximate calculation of scattering rates, it is suggested that this well‐defined deterministic disorder engenders static displacement‐induced scattering and vibrational‐induced resonance scattering of phonons as the origin of the phonon glass. Concurrently, the crystalline CoO₂ sublayers provide pathways for highly conducting electrons and large thermal voltages.     

Thermoelectric Figure of Merit Enhancement in Bi2Te3-Coated Bi Composites

This study examines the thermoelectric behavior of composites containing hydrothermally processed tellurium-coated bismuth particles of various sizes. Since only a very thin layer of Bi₂Te₃ forms on the particle surface, the high-pressure compacted composite is still dominated by bismuth as the main ingredient (??96% Bi). Thermoelectric figure of merit ZT values are derived from measurements of thermal conductivity, electrical resistivity, and Seebeck coefficient. As expected, a ZT value almost three times higher than that of bismuth is found. This enhancement appears to be caused mainly by lowered thermal conductivity due to the significant number of grain boundaries, short phonon mean free path in the coating layers, and lattice mismatch.     

Solid-State Synthesis and Thermoelectric Properties of Al-Doped Mg2Si

The electronic transport and thermoelectric properties of Al-doped Mg₂Si (Mg₂Si:Al _m , m?=?0, 0.005, 0.01, 0.02, 0.03) compounds prepared by solid-state synthesis were examined. Mg₂Si was synthesized by solid-state reaction (SSR) at 773?K for 6?h, and Al-doped Mg₂Si powders were obtained by mechanical alloying (MA) for 24?h. Mg₂Si:Al _m were fully consolidated by hot pressing (HP) at 1073?K for 1?h, and all samples showed n-type conduction, indicating that the electrical conduction is due mainly to electrons. The electrical conductivity increased significantly with increasing Al doping content, and the absolute value of the Seebeck coefficient decreased due to the significant increase in electron concentration from 10¹⁶ cm^?3 to 10¹⁹ cm^?3 by Al doping. The thermal conductivity was increased slightly by Al doping, but was not changed significantly by the Al doping content due to the much larger contribution of lattice thermal conductivity over electronic thermal conductivity. Mg₂Si:Al_0.02 showed a maximum thermoelectric figure of merit of 0.47 at 823?K.     

Improved performance of ITO/TiO2/HfO2/Pt random resistive accessory memory by nitrogen annealing treatment

Resistive switching (RS) characteristics of TiO₂/HfO₂ bilayer memory devices annealed under N₂ and O₂ ambient were investigated in this work. It was found that the improved RS properties were obtained in N₂ annealing atmosphere, which exhibited good endurance of more than 100 times in direct current measurement mode and data retention properties of 10⁴ s at 85 °C. To clarify the effect of annealing treatment on the devices in various atmospheres, conduction mechanism, which is related to the RS properties was analyzed. The results showed that the space charge limited current (SCLC) was the dominant conduction mechanism in HRS for the as prepared device; for the device annealed in N₂, the conduction mechanism was dominated by Pool–Frenkel emission. It can be induced that the conduction mechanism variation in N₂ was attributed to the increase of oxygen vacancies in the switching layer, which was the main reason for the improvement of RS characteristics. Lastly, we switched the operation voltage from the Pt to the ITO electrode of the double layer RRAMs, and found that better endurance and smaller operation voltages were obtained when it was applied on the Pt electrode.     

Effects of tail states on the conduction mechanisms in silicon carbide thin films with high carbon content

Hydrogenated amorphous silicon carbide (a-SiC_x:H) films of different carbon content (x) were deposited by radio frequency plasma enhanced chemical vapor deposition (PECVD) system. Apart from the X-ray photoelectron spectroscopy (XPS) and UV-Visible transmission analyses, the resistivity measurements between 293 K and 450 K were emphasized to assess the eventual transport mechanisms. The film resistivities are unexpectedly found relatively low, especially for high carbon content. In the frame of exclusive band conduction, the apparent thermal activation energies, evaluated from Arrhenius type plot remain too low compared to half values of the optical gaps.Numerical analyses were undertaken by extending conduction from the band conduction about the mobility edge inside the band gap by including the nearest neighbor hopping (NNH) conduction across the localized tail states. By successfully fitting the formulated conductivity expression to the experimental results, parameters such as tail states distributions, true activation energies to the mobility edge have been retrieved.     

Electrical and Thermal Conductivity and Conduction Mechanism of Ge<Subscript>2</Subscript>Sb<Subscript>2</Subscript>Te<Subscript>5</Subscript> Alloy

Ge₂Sb₂Te₅ alloy has drawn much attention due to its application in phase-change random-access memory and potential as a thermoelectric material. Electrical and thermal conductivity are important material properties in both applications. The aim of this work is to investigate the temperature dependence of the electrical and thermal conductivity of Ge₂Sb₂Te₅ alloy and discuss the thermal conduction mechanism. The electrical resistivity and thermal conductivity of Ge₂Sb₂Te₅ alloy were measured from room temperature to 823 K by four-terminal and hot-strip method, respectively. With increasing temperature, the electrical resistivity increased while the thermal conductivity first decreased up to about 600 K then increased. The electronic component of the thermal conductivity was calculated from the Wiedemann–Franz law using the resistivity results. At room temperature, Ge₂Sb₂Te₅ alloy has large electronic thermal conductivity and low lattice thermal conductivity. Bipolar diffusion contributes more to the thermal conductivity with increasing temperature. The special crystallographic structure of Ge₂Sb₂Te₅ alloy accounts for the thermal conduction mechanism.     

Carrier transport mechanism in aluminum nanoparticle embedded AlQ3 structures for organic bistable memory devices

Electrical bistability is demonstrated in organic memory devices based on tris-(8-hydroxyquinoline)aluminum (AlQ₃) and aluminum nanoparticles. The role of the thickness of middle aluminum layer and the size of the nanoparticles in device performance is investigated. Above a threshold voltage, the device suddenly switches from a low conductivity OFF state to a high conductivity ON state with a conductivity difference of several orders of magnitude. The OFF state of the device could be recovered by applying a relatively high voltage pulse. The electronic transition is attributed to an electric field induced transfer of charge between aluminum nanoparticles and AlQ_3. The type of charge carriers responsible for conductance switching is investigated. The charge carrier conduction mechanism through the device in ON and OFF states is studied by temperature dependent current–voltage characteristics and analyzed in the framework of existing theoretical models. The conduction mechanism in the OFF state is dominated by field-enhanced thermal excitation of charge carriers from localized centers, whereas it changes to Fowler–Nordheim tunneling of charge carriers in the ON state. The device exhibited excellent stability in either conductivity states. The results indicate the strong potential of the device towards its application as a nonvolatile electronic memory.     

Thermoelectric Properties and Conduction Mechanism of CaCu3Ti4O12 Ceramics at High Temperatures

The Seebeck coefficient and electrical conductivity of CaCu₃Ti₄O₁₂ (CCTO) ceramics were measured and analyzed in the high temperature range of 300°C to 800°C, and then the electrical conduction mechanism was investigated by using a combination of experimental data fitting and first-principles calculations. The Seebeck coefficient of the CCTO ceramic sintered at 1050°C is negative with largest absolute value of ～650 μV/K at 300°C, and the electrical conductivity is 2–3 orders greater than the value reported previously by other researchers. With increasing sintering temperature, the Seebeck coefficient decreases while the electrical conductivity increases. The temperature dependence of the electrical conductivity follows the rule of adiabatic hopping conduction of small polarons. The calculated density of states of CCTO indicates that the conduction band is mainly contributed by the antibonding states of Cu 3d electrons, therefore small-polaron hopping between CuO₄ square planar clusters was proposed. Possible ways to further improve the thermoelectric properties of CCTO are also discussed.     

Magnetic-field dependences of the conductivity and hall factor in MBE-grown Cd<Subscript>X</Subscript>Hg<Subscript>1−<Emphasis Type="Italic">X</Emphasis></Subscript>Te layers

Dependences of conductivity and the Hall factor on magnetic field in Cd_XHg_1?XTe (mercury cadmium telluride, MCT) have been studied at 77 K. Films of n-type conduction with X=0.21–0.23 were grown on (013) GaAs substrates by MBE. As the magnetic field increases from 0 to 1 T, conductivity and the Hall factor decrease by a factor of 3–5. These dependences are well described in terms of a two-layer model with high and low mobility of electrons in the layers. The analysis of data obtained with layer-by-layer etching has shown that an MCT film can be described as a two-layer structure. In this case, a thin layer with high density and low mobility of electrons is located near the interface with a CdTe buffer. A high density and low mobility of electrons can be attributed to the high defectiveness of this layer. Studies by transmission electron microscopy have demonstrated the presence of a network of dislocations in MCT film near the interface with the buffer layer.     

Sputtered p-Type Sb2Te3/(Bi,Sb)2Te3 Soft Superlattices Created by Nanoalloying

In this work, p-type nanoscale ??soft superlattices?? consisting of multilayer stacks of 25?nm Sb₂Te₃ on 25?nm (Bi_0.2Sb_0.8)₂Te₃ were fabricated by nanoalloying. With this technique, nanoscale layers of the elements Bi, Sb, and Te are deposited by sputtering onto a Si/SiO₂ substrate and subsequently annealed to induce interdiffusion and a solid-state reaction to form the final superlattices. Different combinations of annealing temperatures were used in the annealing process. The in-plane electronic properties (Seebeck coefficient, electrical conductivity, charge carrier concentration, and carrier mobility) of these soft superlattices were examined. The cross-plane thermal conductivity was determined using time-domain thermal reflectance (TDTR). Secondary-ion mass spectrometry (SIMS) depth profiles reveal that the nanostructured thin films exhibit high stability against thermal interdiffusion during the annealing process. X-ray patterns of the samples display very strong texture with preferred c-orientation of the crystallites after the heat treatment. Scanning electron microscopy (SEM) cross-section images of the films show distinctly polycrystalline structure with increasing grain size for higher annealing temperatures, as confirmed by x-ray diffraction (XRD) analysis. Very high power factors exceeding 40???W/cm?K², similar to values for bulk single crystals with comparable compositions, are observed for the soft superlattices. The nanostructure appears to be stable up to 300°C. For a sample annealed at 150°C, a thermal conductivity as low as 0.45?W/mK was determined. Based on different assumptions concerning the degree of anisotropy of the transport properties, a cross-plane figure of merit ZT of 0.6 to 1.9 can be estimated for the thin films annealed at 300°C.     

Hopping conductivity in polycrystalline photoconductive Pb3O4 layers

Ac conductivity has been studied in polycrystalline Pb₃O₄ layers in the frequency range f = 10²–10⁵ Hz at temperatures of 293–370 K; the conductivity has been measured in the dark and under illumination. Specific features of the charge-transport process are discussed. Analysis of the experimental data in terms of the Mott theory shows that the conduction is by the hopping mechanism and the charge transport occurs in the energy gap near the Fermi level. Microscopic parameters governing the conduction process, such as density of localized states and the average carrier’s hopping distance, are determined for different temperatures.     

Effect of Ti Substitution on Thermoelectric Properties of W-Doped Heusler Fe2VAl Alloy

Effects of element substitutions on thermoelectric properties of Heusler Fe₂VAl alloys were evaluated. By W substitution at the V site, the thermal conductivity is reduced effectively because of the enhancement of phonon scattering resulting from the introduction of W atoms, which have much greater atomic mass and volume than the constituent elements of Fe₂VAl alloy. W substitution is also effective to obtain a large negative Seebeck coefficient and high electrical conductivity through an electron injection effect. To change the conduction type from n-type to p-type, additional Ti substitution at the V site, which reduces the valence electron density, was examined. A positive Seebeck coefficient as high as that of conventional p-type Fe₂VAl alloy was obtained using a sufficient amount of Ti substitution. Electrical resistivity was reduced by the hole doping effect of the Ti substitution while maintaining low thermal conductivity. Compared with the conventional solo-Ti-substituted p-type Fe₂VAl alloy, the ZT value was improved, reaching 0.13 at 450 K.     

Scattering of phonons by a spatially correlated system of Fe atoms and the low-temperature anomaly of thermal conductivity in HgSe:Fe crystals

The thermal conductivity κ of HgSe:Fe samples with various content N _Fe of Fe impurity was studied in the temperature range of 8–60 K. It was found that the dependence of the thermal conductivity κ on N _Fe is unconventional at low temperatures. For T<12 K, the value of κ first decreases with an increase in the Fe concentration up to N _Fe=5×10¹⁸ cm^?3 and then increases and attains a maximum for N _Fe=(1–2)×10¹⁹ cm^?3. A further increase in Fe concentration brings about a steady decrease in thermal conductivity. The electron-and phonon-related thermal conductivity of HgSe:Fe crystals with consideration of the effects caused by the ordering of trivalent Fe ions was analyzed. It is shown that both the electron-and phonon-related contributions to thermal conductivity at low temperatures are increasing functions of Fe concentration in the range of 5×10¹⁸<N _Fe<(1–2)×10¹⁹ cm^?3. However, the electronic contribution is too small to account for the experimental increase in thermal conductivity. An analysis of the lattice contribution to thermal conductivity showed that an anomalous increase in thermal conductivity is caused by a reduction in the Rayleigh scattering of phonons by a system of Fe ions with mixed valence and is related to the spatial ordering of Fe³⁺ ions.