Change In Conduction Mechanism Research Articles

Conduction mechanisms and dielectric relaxations in ternary ZnO/Zn2SiO4/SiO2 composites for energy storage systems

In this paper, ZnO-based composite (ZnO/Zn2SiO4/SiO2) powders were prepared and investigated as a function of SiO2 added amount. Indeed, the X-ray diffraction patterns (XRD) confirm the formation of ternary ZnO/Zn2SiO4/SiO2 composites. The structural defects are shown to increase with the SiO2 amount. In addition, the SEM observations reveal agglomerated and heterogenic grain shape distribution. Furthermore, dielectric properties were investigated as a function of SiO2 added amount. The Nyquist plots are simulated by an electric circuit formed by a parallel distribution of resistance (R), capacitance (C), and a constant phase element (CPE), in all composites. Moreover, using the Jonscher's law, we demonstrate a change in conduction mechanism as a function of SiO2 content. Indeed, for the 10 % SiO2 doping, the conduction process is ensured by defects such as Zn interstitial and the oxygen vacancies, consistent with the Correlated Barrier Hopping (CBH) model. Whereas, for the 20 % SiO2 doping, the conduction is dominated by grain boundaries contribution, according to the Non-Overlapping Small Polaron Tunneling (NSPT) model. Besides, the 10 % SiO2-doped composite exhibits localized relaxation process, while the 20 % SiO2-doped sample reveals simultaneously long-range and localized relaxations. These variations, caused by the increase of SiO2 percent, can be explained by the influence of the growth of Zn2SiO4 phase, and subsequently the development of further interfacial defects. Accordingly, the composites display capacitive behavior, high permittivity, and low dielectric losses, which make them good candidates for energy storage systems.

AC Electric Conductivity of High Pressure and High Temperature Formed NaFePO4 Glassy Nanocomposite.

Olivine-like NaFePO4 glasses and nanocomposites are promising materials for cathodes in sodium batteries. Our previous studies focused on the preparation of NaFePO4 glass, transforming it into a nanocomposite using high-pressure-high-temperature treatment, and comparing both materials' structural, thermal, and DC electric conductivity. This work focuses on specific features of AC electric conductivity, containing messages on the dynamics of translational processes. Conductivity spectra measured at various temperatures are scaled by apparent DC conductivity and plotted against frequency scaled by DC conductivity and temperature in a so-called master curve representation. Both glass and nanocomposite conductivity spectra are used to test the (effective) exponent using Jonscher's scaling law. In both materials, the values of exponent range from 0.3 to 0.9, with different relation to temperature. It corresponds to the electronic conduction mechanism change from low-temperature Mott's variable range hopping (between Fe2+/Fe3+ centers) to phonon-assisted hopping, which was suggested by previous DC measurements. Following the pressure treatment, AC conductivity activation energies were reduced from EAC≈0.40 eV for glass to EAC≈0.18 eV for nanocomposite and are lower than their DC counterpart, following a typical empirical relation with the value of the exponent. While pressure treatment leads to a 2-3-orders-of-magnitude rise in the AC and apparent DC conductivity due to the reduced distance between the hopping centers, a nonmonotonic relation of AC power exponent and temperature is observed. It occurs due to the disturbance of polaron interactions with Na+ mobile ions.

Defect/disorder correlated modification of transport properties from hopping to tunneling processes in BaTiO3–LaFeO3 solid solution

Crystal structure, bandgap, and the changes in the charge conduction mechanisms in ceramics are interrelated, and the underlying physics unifies all these different phenomena. The experimental and theoretical evaluation of the electronic properties of the solid solution of (1 − x)BaTiO3–(x)LaFeO3 (x = 0, 0.015, 0.031, 0.062) is attempted in this work. Bandgap was observed to be tunable with La/Fe doping from 3.2 eV (x = 0) to 2.6 eV (x = 0.06), while the lattice disorder was found to increase. A theoretical assessment confirms a considerable shift of valence band maxima and conduction band minima with an introduction of additional defect states within the bandgap. Electron localization was also confirmed theoretically with doping. Such changes in the electronic properties were experimentally confirmed from dielectric/AC - conductivity/impedance spectroscopy studies. From different transportation models, hopping is a preferred mechanism in the less distorted BaTiO3. However, a large polaron tunneling process can be justified for the doped samples at lower temperatures. Only at higher temperatures, a small polaron tunneling can be justified for the doped samples. The transportation is affected by the grain boundaries as much as the grains themselves. A complete analysis using Nyquist plots reveals the competing contributions of these regions to the transportation mechanism and is correlated to the disorder/distortions in the lattice in terms of the formation of oxygen vacancies.

CaCu3Ti4O12 ceramic via oxalate route: Synthesis, characterization and electrical properties

In this manuscript, a new method for the synthesis of CaCu3Ti4O12 (CCTO) perovskite was adopted via co-precipitation followed by calcination processes. The co-precipitation route was employed to synthesize calcium and copper oxalates precipitated on the TiO2 surfaces and the calcination technique at different temperatures was utilized to follow the single-phase formation. Differential thermal analysis-thermogravimetry (DTA-TG) were used to estimate the proper calcination temperatures and to follow the precursor′s decomposition stages. X-ray diffraction (XRD) revealed the initiation of titanate formation at 700 °C. The complete formation of single-phase was achieved at 1100 °C. In agreement with the XRD measurements, FT-IR spectra confirmed the formation the different phases at different calcination temperatures. The prepared titanate showed giant dielectric properties with values higher than reported in literature. The conductivity measurement indicated semi-conducting properties and revealed a transition agreed well with that obtained in the dielectric properties measurements vs. temperature, attributed to the orientional polarization or change in the entire conduction mechanism from hopping to ionic conduction. The obtained electrical properties enhances the utilization of the entire titanate as capacitors.

Molecular dynamics simulations of the effect of porosity on heat transfer in Li2TiO3

Heat transfer is a key consideration in the development of tritium breeder blankets for future fusion reactors. For solid tritium breeder materials there is a fine balance to be struck between high levels of porosity to encourage tritium release and minimising it to maintain the thermal and mechanical properties. Therefore, in this work we employ molecular dynamics simulations to understand how the introduction of porosity influences the thermal conductivity of lithium metatitanate ceramic breeder material. Our simulations predict that increasing the porosity leads to a decrease in the thermal conductivity which is in good agreement with previous experimental observations. By contrast, we do not observe the increase in the thermal conductivity at high temperatures, that is observed in some experiments. We argue that this increase is a consequence of sintering or some other modification of the experimental sample rather than a fundamental change in the heat conduction mechanism in the crystal matrix.

Improvement of memristive switching of Pb(Zr0.52Ti0.48)O3/Nb:SrTiO3 heterostructures via La doping

The ferroelectric memory with superior stability and high operating speed has broad development prospects in areas such as neural computing and information storage. An effective strategy to improve the performance of the resistive switching (RS) of the ferroelectric memory is to improve the conductivity of ferroelectric thin films while maintaining their ferroelectric properties, because there is a competition between ferroelectric properties and conductivity in the RS behavior. Herein, we prepared La-doped Pb(Zr0.52Ti0.48)O3 (PLZT) thin films on the Nb:SrTiO3 (NSTO) substrate. By controlling La concentration, a compromise between ferroelectricity and conductivity was achieved. The RS on/off ratio reached 2.6 × 104 doped with 2 mol% La. Moreover, the conduction mechanism changes from the Schottky emission controlling PZT/NSTO interface to the coexistence of Schottky emission and space charge-limited mechanism in the PLZT/NSTO heterostructures. The results reveal that the effective matching of the ferroelectricity and conductivity of ferroelectric thin films is necessary for obtaining high RS on/off ratio, which is helpful to the accurate design of high-performance ferroelectric memory devices.

Change in conduction mechanism from Mott variable range to small polaronic hopping in Sr2+ doped Y2 − xSrxNiMnO6.

Change in conduction mechanism from Mott variable range to small polaronic hopping in Sr2+ doped Y2 − xSrxNiMnO6.

Change of Conduction Mechanism in Polymer/Single Wall Carbon Nanotube Composites upon Introduction of Ionic Liquids and Their Investigation by Transient Absorption Spectroscopy: Implication for Thermoelectric Applications.

Polymer composites based on polycarbonate (PC) and polyether ether ketone (PEEK) filled with single-walled carbon nanotubes (SWCNTs, 0.5-2.0 wt %) were melt-mixed to investigate their suitability for thermoelectric applications. Both types of polymer composites exhibited positive Seebeck coefficients (S), indicative for p-type thermoelectric materials. As an additive to improve the thermoelectric performance, three different ionic liquids (ILs), specifically THTDPCl, BMIMPF6, and OMIMCl, were added with the aim to change the thermoelectric conduction type of the composites from p-type to n-type. It was found that in both composite types, among the three ILs employed, only the phosphonium-based IL THTDPCl was able to activate the p- to n-type switching. Moreover, it is revealed that for the thermoelectric parameters and performance, the SWCNT:lL ratio plays a role. In the selected systems, S-values between 61.3 μV/K (PEEK/0.75 wt % SWCNT) and -37.1 μV/K (PEEK/0.75 wt % SWCNT + 3 wt % THTDPCl) were reached. In order to shed light on the physical origins of the thermoelectric properties, the PC-based composites were studied using ultrafast laser time-resolved transient absorption spectroscopy (TAS). The TAS studies revealed that the introduction of ILs in the developed PC/CNT composites leads to the formation of biexcitons when compared to the IL-free composites. Moreover, no direct correlation between S and exciton lifetimes was found for the IL-containing composites. Instead, the exciton lifetime decreases while the conductivity seems to increase due to the availability of more free-charge carriers in the polymer matrix.

Effects of volume and surface conductivity on the surface charge and electric field characteristics of the tri‐post insulator in SF 6 ‐filled ± 500 kV DC‐GIL

Herein, the surface charge characteristics of a tri-post insulator in a ± 500 kV direct current gas-insulated transmission lines (DC-GIL) filled with SF6 are thoroughly studied. The influences of volume and surface conductivity of the insulator on the surface charge and consequent electric field distribution are discussed. An improved method is introduced to obtain the distribution of surface charge with considerable accuracy and efficiency for the calculation. It is found that with increasing the volume conductivity, the dominative conduction mechanism changes from surface conduction to bulk conduction, while the inverse variation appears with increasing the surface conductivity. During the variation of electric conductivity, obvious effect of surface charge accumulation is found on the electric field distribution involving the variation of peak electric field strength and its location. Besides, high charge density usually appears along with low peak field strength during this variation. It can be concluded that the surface charge accumulation should be paid special attention when analysing the insulation characteristics of tri-post insulators. The variation of volume and surface electric conductivity should be thoroughly considered.

Impact of interfacial engineering on MgO-based resistive switching devices for low-power applications

In this work, the resistive switching characteristics of MgO/Al2O3-based resistive random-access memory (ReRAM) devices have been reported. Analysis shows the change in dominant conduction mechanism from space-charge-limited conduction to Schottky emission owing to the incorporation of an Al2O3 insertion layer. The MgO/Al2O3 bilayer ReRAM devices exhibit lower power operation (50.6% reduction) and better switching uniformity as compared to single-layer devices, depending on the stack configuration. This can be attributed to the lower oxygen vacancy accumulation and filament confinement at the MgO/Al2O3 interface, resulting in a more controllable switching operation. Further X-ray photoelectron spectroscopy (XPS) depth profile analysis of the bilayer device reveals that the switching dynamics are correlated directly with the oxygen vacancy concentrations. These findings indicate the importance of interfacial layer engineering in improving the resistive switching properties of MgO-based memory devices, thus allowing for low-power applications.

Tantalum Suboxide Films with Tunable Composition and Electrical Resistivity Deposited by Reactive Magnetron Sputtering

Tantalum-based films with tailored composition, density, and electrical resistivity are of interest for next generation hohlraums for magnetized indirect-drive inertial confinement fusion. Here, we use reactive direct-current magnetron sputtering to deposit tantalum suboxide films with O content in the range of 46–71 at.%. In contrast to a common approach involving varying reactive gas contents, compositional control is achieved kinetically by changing the total chamber pressure and the deposition rate, while keeping the working gas mix of Ar-5%O2 constant. The resultant films are X-ray amorphous with electrical resistivity varying by over seven orders of magnitude. The dominant conduction mechanism changes from metallic to activated tunneling above ∼55 at.% of O, which is characterized by a sharp increase in resistivity and a decrease in the carrier density at low temperatures.

Fabrication of porous Al2O3 ceramics using carbon black as a pore forming agent by spark plasma sintering

Fabrication of ceramic materials containing high amount of porosity with controlled size and structure is a critical factor for various applications. In this study, porous alumina ceramics were prepared by addition of 5–30 vol% carbon black (CB) powders with different specific surface area (SSA) into the alumina matrix as a pore forming agent and application of partial sintering via spark plasma sintering (SPS) technique. After removal of CB particles from the ceramic structures, physical, mechanical, and thermal properties of the porous alumina ceramics were investigated. The open porosity level was detected between 38.48 and 59.81% depending on the type and amount of CB removed. The compressive strength of the sintered samples was measured as ∼150 MPa, which is ∼3 times higher than that of the conventionally sintered samples thanks to the strong neck formation between grains. The large pores formed by removal of CB agglomerates in case of using CB powders with SSA of 107 and 254 m2/g resulted in a change in thermal conduction mechanism from lattice vibration to thermal convection.

Electric conductivity and dielectric relaxation properties of BiFeO3-YMnO3 solid solution

This manuscript presents a systematic study of conductivity and dielectric relaxations in the solid solution of (BiFeO3)1-x -(YMnO3)x for compositional range (0.0 ≤ x ≤ 0.2) as function of frequency, temperature, and magnetic field. The samples have been synthesized by combustion method. The thermal activations of dc-conductivity and relaxation time obey Arrhenius behavior for, x = 0.0, 0.025, 0.05 but follow variable range hopping of small polarons above, x ≥ 0.075 with activation energy 0.45 eV. The compositional time-temperature superposition of dielectric modulus data suggests that conduction mechanism changes with composition. The bulk capacitance decreases with increase in magnetic field.

Electrical Conduction Mechanism of β‐MnTe Thin Film with Wurtzite‐Type Structure Using Radiofrequency Magnetron Sputtering

Manganese telluride (MnTe) compound is known to be a polymorphic chalcogenide. Recently, it has been reported that the MnTe shows nonvolatile memory properties with a significant change in resistance via a polymorphic transition between NiAs‐type (NC) structure (low resistance) and wurtzite‐type (WZ) structure (high resistance). This crystalline polymorphic MnTe is expected to realize a phase‐change memory with fast operation speed and ultralow operation energy. While the NC‐MnTe, generally designated as α‐MnTe, is intensively studied, WZ‐MnTe is still poorly understood. Herein this study, electrical conduction mechanism of a β‐MnTe film with a WZ‐type structure is studied. A resistivity, Hall mobility, and Seebeck coefficient of the WZ‐MnTe film are measured at various temperatures. The temperature dependence of resistivity in the temperature range 120–300 K clearly indicates that the WZ‐MnTe film shows a variable‐range hopping (VRH) conduction. In this temperature region, with decreasing temperature, the conduction mechanism changes from Mott–VRH conduction to Efros–Shklovskii VRH conduction at about 210 K. Furthermore, the low thermally activated Hall mobility, occurrence of Hall‐effect sign anomaly, and relatively low activation energy for thermopower, which are the observed results, suggest that the small polaron hopping conduction is dominant above 310 K.

Combined merits of high dielectric breakdown strength and low sintering temperature acquired in MgO-based dielectric ceramics: from theoretical prediction to experimental validation

Dilemma of dielectric breakdown has perplexed researchers for decades while intrinsic factors accounting for dielectric breakdown strength (DBS) is still unsettled. In this work to address the great deterioration of DBS after LiF acceptor doping in MgO (LMO), high throughout first principles calculation is carried out to screen nearly 30 cations from donor doping perspective. The work function from the theoretical calculation, as a descriptor correlated to electronic property, displays a linear correlation with experimental DBS while Mn is proven to be the best candidate dopant for its greatly improvement on work function of LMO system and therefore a markedly enhanced DBS. Detailed analysis manifests that MnCO3 incorporation further decreases the sintering temperature of LMO to 900 °C and also leads to defect associations between oxygen vacancies and cation substitutional defects, causing distinct change in dielectric behavior and conduction mechanism. Afterwards, beneficial effects of grain refinement and increased grain boundary resistivity on DBS are analyzed using phase field modeling. With all these merits, nearly 85% improvement in DBS is achieved in 1 wt% MnCO3 tailored LMO as well as an ultrahigh Q × f of more than 156000 GHz. This work not only reveals the decisive role of work function in DBS prediction therefore shed light on composition design of dielectrics, but also provides a paradigm combining ions screening, theoretical prediction, experiments and simulation together to accelerate material research.

Structural and Electrical Characterization of Pure and Al-Doped ZnO Nanorods.

Pure and Al-doped (3 at.%) ZnO nanorods were prepared by two-step synthesis. In the first step, ZnO thin films were deposited on silicon wafers by spin coating; then, ZnO nanorods (NR) and Al-doped ZnO NR were grown using a chemical bath method. The structural properties of zincite nanorods were determined by X-ray diffraction (XRD) and corroborated well with the morphologic properties obtained by field-emission gun scanning electron microscopy (FEG SEM) with energy-dispersive X-ray spectroscopy (EDS). Morphology results revealed a minute change in the nanorod geometry upon doping, which was also visible by Kelvin probe force microscopy (KPFM). KPFM also showed preliminary electrical properties. Detailed electrical characterization of pure and Al-doped ZnO NR was conducted by temperature-dependent current–voltage (I–V) measurements on Au/(Al)ZnO NR/n-Si junctions. It was shown that Al doping increases the conductivity of ZnO NR by an order of magnitude. The I–V characteristics of pure and Al-doped ZnO NR followed the ohmic regime for lower voltages, whereas, for the higher voltages, significant changes in electric conduction mechanisms were detected and ascribed to Al-doping. In conclusion, for future applications, one should consider the possible influence of the geometry change of (Al)ZnO NRs on their overall electric transport properties.

Changes in electrical properties and conduction mechanisms of Pd/n-Si diodes due to niobium dopant

A change in electrical properties of silicon (Si) diodes due to niobium (Nb) doping was studied using current–voltage (I-V) and capacitance–voltage (C-V) techniques. The observed ohmic I-V behaviour and a low-voltage capacitance peak on Nb-doped Si diodes indicated that a charge distribution mechanism has been changed and dominated by generation-recombination defect centres in the material. These Nb-induced defect centres are responsible for a reduction in free majority carrier density in the space charge region as a result of charge compensation hence an increase in material resistivity. An increase in material resistivity is confirmed by relatively high series and shunt resistances evaluated on Nb-doped Si diodes. Furthermore, the effects of Nb doping on other diode parameters such as the saturation current, Schottky barrier height, ideality factor and doping density were investigated in the study. Since the observed diode property changes are similar to those induced by promising dopants for silicon radiation-hardness, the results presented here would, therefore, assist in a quest to improve radiation-hardness of silicon by defect-engineering strategy.

Investigation of optical and electrical conduction mechanism of AgYP2O7 prepared by solid state process

Diphosphate materials are promising materials for different applications where their properties need to be widely studied in order to optimize them. The AgYP2O7 compound was prepared following the solid-state reaction method. X-ray powder reveals that the title compound crystallizes in the triclinic system, while differential scanning calorimetry analysis discloses the existence of an endothermic peak around T = 480±10 K indicating the existence of a phase transition. As for optical properties, using reflectance spectroscopy and Kubelka– Munk's procedure, the deduced value of the direct energy gap of Eg =2.29±0.09 eV. The variation in conductivity with temperature confirms the presence of the phase transition evidenced by DSC measurements. The AC conductivity data are well described by Jonsher's law. The thermal behavior of the exponent parameter "s" reveals that the conduction mechanism changes from over large polaron tunneling to non-overlapping small polaron tunneling at the temperature of phase transition.

An overview of oxygen vacancy dynamics in (1 − x)(Bi1/2Na1/2)TiO3–xBaTiO3 solid solution

Across the phase diagram of (Bi1/2Na1/2)TiO3-xBaTiO3 solid solution, the conduction mechanism changes from ionic to electronic through the mixed case at the morphotropic phase boundary, giving rise to distinct DC degradation behaviors.

Combined Merits of High Dielectric Breakdown Strength and Low Sintering Temperature Acquired in Mgo-Baseddielectric Ceramics: From Theoretical Prediction to Experimental Validation

Dilemma of dielectric breakdown has perplexed researchers for decades while intrinsic factors accounting for dielectric breakdown strength (DBS) is still unsettled. In this work to address the great deterioration of DBS after LiF acceptor doping in MgO (LMO), high throughout first principles calculation is carried out to screen nearly 30 cations from donor doping perspective. The work function from the theoretical calculation, as a descriptor correlated to electronic property, displays a linear correlation with experimental DBS while Mn is proven to be the best candidate dopant for its greatly improvement on work function of LMO system and therefore a markedly enhanced DBS. Detailed analysis manifests that MnCO3 incorporation further decreases the sintering temperature of LMO to 900 °C and also leads to defect associations between oxygen vacancies and cation substitutional defects, causing distinct change in dielectric behavior and conduction mechanism. Afterwards, beneficial effects of grain refinement and increased grain boundary resistivity on DBS are analyzed using phase field modeling method. With all these merits, nearly 85% improvement in DBS is achieved in 1 wt% MnCO3 tailored LMO. This work not only reveals the decisive role of work function in DBS prediction therefore shed light on composition design of dielectrics, but also provides a paradigm combining ions screening, theoretical prediction, experiments and simulation together to greatly accelerate the materials research.