B位掺杂钙钛矿结构化合物LaSr2Mn2-xFexO7的电磁性能

研究了以Fe离子(Fe3 )掺杂取代部分锰(Mn)离子的层状钙钛矿结构化合物LaSr2Mn2-xFexO7(O≤x≤O.3)的电性和磁性.结果发现:Fe3 的掺杂抑制了LaSr2Mn2-xFexO7低温的反铁磁性交换作用,使Neel温度TN由x=0的138K降至x=0.3的l0lK.同时,LaSr2Mn2-xFex07的自旋玻璃态、电荷有序和金属一绝缘体转变温度都由于Fe3 取代Mn离子而受到抑制.产生上述现象的原因是因为Mn位被取代导致产生无序状态.由于Fe3 取代Mn粒子,致使双交换作用通道被打破,化合物的电阻率随x的增加而迅速增加.所有化合物的电阻率在低温阶段都可以用Mott的变程跳跃理论来拟合;在高温阶段可以用最近小声子极化理论来拟合.     

Manual turbostratic stacked graphene transistor: A study on electrical properties and device potential

This study investigated the potential of turbostratic stacking of graphene few-layers produced using the consecutive electrochemical delamination method for electronic applications. The temperature dependence of electrical conductivity was measured to identify the transport mechanisms and band overlap. By lowering the temperature from 298 to 20 K, it was shown that these highly disordered structures follow nearest neighbor hopping through the variable range hopping mechanism. Variations in band overlap for samples versus carrier concentration were extracted and show that trilayer graphene has the best electrical properties at a mobility of about 1000 cm²/V s and the lowest band overlap at 28.5 meV which is promising structure for optoelectronic applications.     

Band gap engineering and low temperature transport phenomenon in highly conducting antimony doped tin oxide thin films

A huge band gap tuning and low temperature transport phenomenon in highly transparent antimony doped tin oxide thin film (Sb:SnO₂) under the influence of swift heavy ions irradiation (SHII) is reported. Structural analysis shows an enhancement in crystallinity at initial fluence of irradiation followed by amorphization at higher fluences. Films were also well studied for their surface morphology by atomic force microscopy and scanning electron microscopy. Band gap analysis reveals a drastic band gap narrowing around 1.1 eV upon SHI irradiation. Transport measurements show that the high conductivity and the carrier concentration decrease upon increase in the fluence of irradiation. The mechanism of charge carrier transport investigated at low temperature is attributed to nearest neighbor hopping (NNH) and variable range hopping (VRH) in different temperature regimes. Origin of the band gap tuning is understood in framework of Burstein–Moss (BM) shift, Quantum Confinement (QC) effect and band-tailing states in amorphous semiconductors.     

A-site Ca2+ substitution induced polyvalent Cu cations and correlated polaron relaxations in colossal permittivity Li1-xCaxCuNb3O9

LiCuNb₃O₉ has been reported newly a colossal permittivity (CP) perovskite, in which the B-site NbO₆ octahedra play a bridging role in the polaron hopping. However, how the A-site modification affects the origin of the polarons and further the CP behaviours remains unexplored. To this end, A-site Ca²⁺ was incorporated to form Li_1-xCa_xCuNb₃O₉, and the local states, dielectric relaxations and conduction behaviours were comprehensively studied. The substitution induces the polyvalent Cu cations, i.e. Cu⁺/Cu²⁺/Cu³⁺. Bond valence sum calculations imply that Cu²⁺ and Cu³⁺ are underbonded, and Cu⁺ is overbonded, while B-site Nb⁵⁺ shows slightly different with theoretical pentavalence. All the compositions exhibit a similarly room-temperature CP response, but present two dielectric relaxations, i.e. T_R1:170–300 K and T_R2:260–400 K. Comprehensive investigations on universal dielectric response and bulk dc conductivity indicate that the T_R1 follows the variable-range-hopping where the electron hopping between the mixed Cu⁺/Cu²⁺, while T_R2 contributes from the Cu³⁺ nearest neighbor hopping.     

Dielectric studies of proton migration and relaxation in wet cellulose and its derivatives

Dielectric relaxation data are reported over a frequency range 400 Hz to 12 kHz and over a temperature range 193 to 323K on cellulose, cellulose acetate and ethyl cellulose with various water contents. A relaxation peak shifted to the low temperature side of the beta process was observed for cellulose acetate with less than 4% water. The effect of water addition in the case of cellulose and ethyl cellulose was initially to increase the amplitude of the loss associated with side group motion. Above 4% water content, the low temperature relaxation in cellulose acetate moves to lower temperatures and an increase in the loss is observed at high temperatures. Similar behaviour was observed in cellulose and ethyl cellulose. A transformation of the frequency dependent conductivity data allowed identification of a hopping conduction process at high temperature. The lowest temperature processes are analyzed in terms of dipole relaxation and the higher temperature features in terms of proton migration.     

High‐Temperature Dielectric Relaxation in Pb(Mg1/3Nb2/3)O3–PbTiO3 Single Crystals

We reported the dielectric properties of Pb(Mg_1/3Nb_2/3)O₃–PbTiO₃ single crystal in the temperature range of 300–1073 K and the frequency range of 100 Hz–10 MHz. Our results showed the coexistence of both true‐ and pseudo‐relaxor behaviors in the crystal. The true relaxor behavior related to the paraelectric‐ferroelectric phase transition occurs at~423 K. The pseudo‐relaxor behavior appearing at~773 K was found to be related to oxygen vacancies. Further investigation reveals that the pseudo‐relaxor behavior has fine structure: it contains two oxygen‐vacancy‐related relaxation processes. The low‐temperature relaxation process is a dipolar relaxation created by the hopping motions of the oxygen vacancies, and the high‐temperature relaxation process is a Maxwell‐Wagner relaxation caused by the sample/electrode contacts.     

Electrical Properties of Ceria-Doped Yttria

The electrical conductivity and ionic domain of several ceriadoped yttria compositions, with up to 10 cation% dopant, were studied as a function of temperature (800° to 1100°C) and oxygen partial pressure (10⁻¹⁵ to 10⁵ Pa). The ionic conduction in ceria-doped yttria involves the transport of interstitial oxygen ion defects. The increase of electronic conductivity under reducing conditions with increasing dopant concentration suggests hopping of small polarons between cerium ions with different valences as the dominant conduction mechanism.     

Impedance spectroscopic behaviour of spark plasma sintered nanocrystalline scandia stabilized zirconia (SSZ)

Nanocrystalline scandia (Sc₂O₃) doped zirconia was prepared by dissolution and co-precipitation method. The formation of the nanostructured nearly cubic phase has been observed from transmission electron microscope and X-ray diffraction studies. Spark plasma sintering retains the nano/submicrograin structure. The frequency dependent conductivities were measured in the frequency range from 100 Hz to 1 MHz and at temperatures 300–800 K. The impedance diagram shows the grain interior and grain boundary resistance contributions. The frequency dependent conductance spectra show the DC plateau (at low frequency region) and dispersive region suggesting the correlated hopping motion of ions. The dielectric-loss spectra suggest the presence dielectric relaxation, which is related to enthalpy of ion hopping. Characteristic hopping frequencies were calculated and appear to be related to the thermally activated ion hoping process.     

Electrochemical capacitance spectroscopy and capacitive relaxation of the changeover process in iron hexacyanoferrate molecular compound

In this work it was proposed the use electrochemical capacitance spectroscopy (ECS) to evaluate the storage process during changeover in FeHCF compound. The approach is equivalent to electrochemical impedance spectroscopy (EIS) albeit from such analysis it was possible to focus attention on the capacitive and dielectric relaxation instead of the dispersive relaxation related to charge transfer. From such approach it was possible to obtain complementary information on the role played by [Fe²⁺(CN)₆]⁴⁻ vacancies during the changeover process. It was observed that Fe³⁺(NC)₅OH⁻ clusters located in these vacancies mediate an electronic and ionic coupled trapping/detrapping process giving rise to what was considered an electrochemical dipolar relaxation whose K⁺ ionic specie with Fe³⁺(NC)₅OH⁻ clusters are the key entities in controlling the general process. The capacitive response reaches its maximum value at the changeover due to saturation of the electronic occupation at Fe³⁺(NC)₅OH⁻ clusters. The nature and the dynamics of the capacitive relaxation process were either discussed.     

Low-frequency dielectric dispersion in polymer-derived amorphous silicon carbonitride ceramics

The dielectric response of polymer-derived amorphous silicon carbonitride (PDC-SiCN) material was studied over a wide frequency range from 1 mHz to 10 MHz. Aside from presenting a relaxation process over 10⁵ to 0.03 Hz frequency range, the material showed a strong low-frequency dielectric dispersion (LFDD), which resulted in extremely high permittivities (up to 10⁵) at frequencies lower than 0.03 Hz. It is identified that this LFDD resulted from relaxation process was not induced by conductivity. Therefore, PDC-SiCN material showed two relaxation processes. Based on its microstructure, two polarization mechanisms were proposed for PDC-SiCN: a mechanism based on polarons generated by unpaired electrons present in the material and an interfacial polarization mechanism as a result of different properties of SiC_xN_4−x matrix and free-carbon clusters. Interfacial polarization process was simulated based on a parallel plate model involving two different materials. The result indicated that the relaxation time of an interfacial polarization can be distributed over a broad time range in the material.     

Mechanisms of the relaxations in (In + Nb) co-doped TiO2 ceramics

(In_0·5Nb_0.5)_0.1Ti_0·9O₂ ceramic sample was prepared by sol-gel method. The dielectric properties of this sample were investigated in a temperature range of 5–320 K. Two thermally activated relaxations were found. The low-temperature relaxation (P0-relaxation) appearing around 50 K follows the Vogel-Fulcher law and is ascribed to be a low-temperature Maxwell-Wagner relaxation caused by frozen electrons. The intermediate-temperature relaxation (P1-relaxation) occurring around 150 K obeys the Arrhenius law. Thermally activated depolarization current (TSDC) investigations reveal that it contains two relaxation processes (P1’ and P1 peaks). This relaxation was argued to be a polaronic relaxation caused by electrons hopping between Ti³⁺ and Ti⁴⁺ ions. TSDC also reveals a high-temperature relaxation (P2 peak) near room temperature, which is related to humidity sensing property.     

Thermally stimulated depolarization current properties of Co4Nb2O9 ceramics

Thermally stimulated depolarization current in magnetoelectric antiferromagnet Co₄Nb₂O₉ were investigated above Néel temperature, and five current peaks (denoted as P0 ~ P4 in the order of ascending temperature) were observed. These peaks are related to holes which can be trapped by cobalt vacancies, their surrounding medium (self-trapped), internal, and surface barrier layers. In low-temperature range, the holes are bound to cobalt vacancies. The polarization caused by bound holes yields P0 peak at 46 K. In middle-temperature range, the holes are localized by the surrounding medium forming polarons. The hopping motions of the self-trapped holes create P2 peak at ~120 K. The microdisplacements around their locating positions for the self-trapped holes lead to P1 peak, whose peak temperature strongly depends on the poling temperature but lower than that of P2 peak. In high-temperature range, the holes are trapped by internal and surface barrier layers giving rise to P3 peak at ~150 K and P4 peak at ~230 K, respectively.     

Dielectric and electric relaxations induced by the complex defect clusters in (Yb+Nb) co‐doped rutile TiO2 ceramics

The effect of the Yb+Nb substitution for Ti on the microstructure, crystal structures, and dielectric properties of (Yb_1/2Nb_1/2)_xTi_1?xO₂ (0.01≤x≤0.1) ceramics is investigated in this study. The results reveal that the solid solubility limit of the (Yb_1/2Nb_1/2)_xTi_1?xO₂ ceramics is x=0.07, and the average grain sizes considerably decrease from 12 μm to 6 μm with x increasing from 0.01 to 0.1. Three types of dielectric relaxations are observed at temperature ranges of 10‐30 K, 80‐180 K, and 260‐300 K, caused by the electron‐pinned defect dipoles, polaron hopping, and interfacial polarizations, respectively. The conduction mechanism changes from nearest‐neighbor‐hopping to polaron hopping mechanism, which is confirmed by ac conductivity measurements. The present work indentifies the correlation between the colossal permittivity and polaron hopping process in the titled compound.     

基于模糊粗糙集和事例推理的凝汽器真空故障诊断

针对凝汽器真空故障诊断的不确定性和复杂性,提出一种基于模糊粗糙集和事例推理的凝汽器故障诊断新方法。首先,运用模糊粗糙集属性约简方法对故障特征进行约简和权重分配,不仅提取了反映故障的主要特征量,降低了特征变量之间的非线性相关性,而且避免了人的主观性对权重分配的影响。然后,在分析凝汽器真空故障特征的基础上,建立凝汽器真空故障树,以约简特征作为条件对故障树根节点进行归纳检索,有效地减少了候选事例的数量,再通过最近邻法检索故障树叶节点,对凝汽器真空故障进行智能定位。通过对汽轮机凝汽器历史故障特征数据集仿真,验证了该方法的有效性。     

Sublinear dispersive conductivity in polymethyl methacrylate at temperatures above the glass transition

Dynamic electrical analysis shows that conductivity conditions the electrical properties of a polymer at high temperatures and low frequencies. In this paper we show the possibilities of the electric modulus formalism to study the properties of carrier transport and space charge relaxation processes in polymethyl methacrylate. Asymmetric Argand's plots are observed in the temperature range between 150 and 210 °C. This asymmetry is related to power-law dependencies in the real part of the conductivity, of the form ωⁿ with n<1, as a result of correlated ion hopping. The complex part of the electric modulus exhibits a peak in the low frequency range that can be associated with these conductive processes. In the case of the relaxation time related to conductive processes, it has been observed that Maxwell time is higher than the characteristic time associated with the crossover frequency which determines the transition from dc to ac regime. This fact is explained by the presence of deep traps. Finally, a maximum in the value of n is observed between 180 and 190 °C which may be related to coupling between charge transport and chain segment motions.     

Giant dielectric constant in Nd2NiO4+δ ceramics obtained by spark plasma sintering

The crystal structure and dielectric properties of Nd₂NiO_4+δ ceramics prepared by spark plasma sintering (SPS) process were characterized. The crystal structure belonged to orthorhombic system with a space group of F mmm, and the excess oxygen content, δ, was about 0.192. Temperature-stable giant dielectric constants (?′ ∼ 10⁵) up to high frequency (5 MHz) over a wide temperature range (200–500 K) were observed in the present ceramics. After comparing the activation energies of dielectric relaxation and electrical conduction, we found the giant dielectric response should be mainly attributed to the adiabatic small polaronic hopping process, while the polaronic hopping process was closely related to the charge ordering in the present ceramics.     

Impurity effects on polaron dynamics in graphene nanoribbons

The impurity effects on the dynamics of polarons in armchair graphene nanoribbons are numerically investigated in the scope of a two-dimensional tight-binding approach with lattice relaxation. The results show that the presence of an impurity changes significantly the net charge distribution associated to the polaron structure. Moreover, the interplay between external electric field and the local impurities plays the role of drastically modifying the polaron dynamics. Interestingly, nanoribbons containing mobile polarons are noted to take place even when considering high impurity levels, which is associated with the highly conductive character of the graphene nanoribbons. This investigation may enlighten the understanding of the charge transport mechanism in carbon-based nanomaterials.     

DC Conduction and Electric Modulus Formulation of Lithium‐Doped Bismuth Zinc Vanadate Semiconducting Glassy System

The conduction parameters related to dc conductivity and electric modulus formulation in the temperature range 313–533 K for xLi₂O (100 ? x) (50V₂O_5.20 Bi₂O₃. 30 ZnO) glass system have been studied. The temperature‐dependant dc conductivity is analyzed using the Mott's model for transition metals and modified Mott's VRH model. It is observed that Mott's model is in good agreement with the experimental data in high‐ and intermediate‐temperature region as a strong electron–phonon interaction seems to be prevalent. The conduction in these glasses has been attributed to small polaron hopping at the vanadium sites in nonadiabatic regime. The ion‐polaron effect (coupling between ions and polarons) leads to drop of the effective mobility and it can manifest itself as decrease in the dc conductivity with the addition of lithium content. The composition and temperature dependence of imaginary part of electric modulus () collapsed onto a single master curve, confirms the presence of same type of relaxation phenomenon in all the studied glass compositions.     

Dielectric Behavior of Lead-Silicate Glasses Containing Iron

The ac conductivity of lead-silicate glasses containing Fe was measured in the range 4.5° to 500° K to investigate the electronic hopping mechanism in these amorphous materials. Classes containing 0.0, 2.0, and 10.0 wt% Fe₂O₂ were prepared under normal atmospheric conditions. The ac conductivity, which increased with increasing Fe concentration, followed an Arrhenius expression at the higher temperatures. The activation energy for ac conduction increased with increasing Fe concentration, in contrast to the usual decrease in activation energy observed by other workers. The frequency of the conductivity σ (ω) was approximately of the form σ (ω)αω^-n where the exponent increases with increasing Fe concentration and temperature. This behavior is explained by assuming that a distribution of hopping distances and activation energies is involved in the ac electron transport mechanism. At low temperatures, the curves for tanδ vs T for all samples showed a broad relaxation peak at ∼ 70°K. Although this peak is frequency-dependent, it was independent of Fe concentration. The results did not indicate electron hopping between localized states in the temperature range studied.     

Evaluation of Polaron Transport in Solids from First-principles

Polarons are formed in polar or ionic solids, either molecular or crystalline, due to local distortions of the lattice induced by charge carriers. Polaron hopping is the primary mechanism of charge transport in these materials, such as functional ceramic compounds, with applications in photovoltaics, thermoelectrics, two-dimensional electron gas transistors, magnetic sensors, spin valve devices, and memories. Understanding the fundamental physics of polaron hopping is, therefore, of prime technological importance. This article provides a brief physical background of polarons and their hopping mechanism, focusing on first-principles calculations of polaron properties. Herein, we review recent selected studies applying the density functional theory (DFT), and describe the merits and challenges in applying DFT for such calculations, highlighting the need to address both electronic and vibrational aspects. The vibrational component of the polaron is evaluated based on structural and total energy calculations, whereas the electronic component is derived from both total energy and electron density calculations. To address the most compelling challenge of calculating polaron properties using DFT, which is the issue of electron localization, we propose to employ calculations of selected vibrational properties, such as the sound velocity, shear modulus, and Grüneisen parameter, to represent the polaron hopping energy; all of which originate from the stiffness of inter-atomic bonds. Such methodology is expected to be more straightforward than the existing ones, however demands standardization.