Change In Conduction Mechanism Research Articles

Tunnel conduction regimes, white-light emission and band diagram of porous silicon–zinc oxide nanocomposites

Porous-Silicon/Zinc Oxide (PS-ZnO) nanocomposites can be processed by a combination of electrochemical etching of PS and sol-gel infiltration of ZnO, which leads to complex interfaces of ZnO permeated in the PS matrix. In this work, an approach consisting on the determination of transport mechanisms and identification of electroluminescence emission bands from the heterojunction, has been followed to derive the energy band diagram. Charge transport at room temperature has been determined in ITO/PS-ZnO/p+-Si/Al structures through current-voltage characteristics, showing that direct and Fowler-Nordheim (FN) tunneling are the main conduction mechanisms at low and high bias voltages, respectively. White electroluminescence composed of three main bands (~490, ~550 and ~620nm) is observed at forward bias. Emission starts at a threshold voltage of 3.9±0.2V, in coincidence with the change in conduction mechanism from direct to FN tunneling. A band diagram of the device is derived from the emission and transport properties, which is coherent with previous electronic and microstructural properties of the starting n-ZnO and p+-Si.

Conduction Mechanisms in BaTiO3–Bi(Zn1/2Ti1/2)O3 Ceramics

Polycrystalline BaTiO3–Bi(Zn1/2Ti1/2)O3 (BT–BZT) ceramics have superior dielectric properties for high‐temperature and high‐energy density applications as compared to the existing materials. While it has been shown that the addition of BZT to BT leads to an improvement in resistivity by two orders of magnitude, in this study impedance spectroscopy is used to demonstrate a novel change in conduction mechanism. While nominally undoped BT exhibits extrinsic‐like p‐type conduction, it is reported that BT–BZT ceramics exhibit intrinsic n‐type conduction using atmosphere‐dependent conductivity measurements. Annealing studies and Seebeck measurements were performed and confirmed this result. For BT, resistivity values were higher for samples annealed in nitrogen as compared to oxygen, whereas the opposite responses were observed for BZT‐containing solid solutions. This suggests a fundamental change in the defect equilibrium conditions upon the addition of BZT to the solid solution that lowered the carrier concentration and changed the sign of the majority charge carrier. This is then also linked to the observed improvement in resistivity in BT–BZT ceramics as compared to undoped BT.

Low temperature performance of sodium–nickel chloride batteries with NaSICON solid electrolyte

Abstract A new type of sodium–nickel chloride batteries using NaSICON solid electrolytes (Na 1 + x Zr 2 Si x P 3 − x O 12 ) was successfully investigated at much lower operation temperature of 195 °C compared with that of conventional Zebra batteries adapting same electrochemistry. The gradual phase transition in NaSICON from monoclinic to rhombohedral was perceived at 100–180 °C by high temperature XRD investigation. From symmetrical Na/NaSICON/Na cell test, the abrupt increase of cell resistance was observed at 177–180 °C, which reveals the change of ionic conduction mechanism in NaSICON solid electrolytes due to phase transformation. The sintered NaSICON solid electrolytes exhibit full densified morphology but somewhat lower average flexural strength of ~ 98 MPa compared with that reported for the β″-Alumina solid electrolytes commercially available. The low temperature electrochemical performances of sodium–nickel chloride batteries were compared by using NaSICON and β″-Alumina solid electrolytes at 195 °C. The lower internal resistances of the cell using NaSICON were confirmed by impedance spectroscopy and cyclic voltammetry tests. Also proto-type cell tests revealed the clear advantages of NaSICON cell (N-Cell) over β″-Alumina cell (β-Cell) at low temperature performances below 200 °C due to high Na + ionic conductivity.

Dc Voltage Insulating Properties of Various Inorganic Materials in Hydrogen Atmosphere at High Temperatures

SUMMARYIn this study, the dc voltage insulating properties in a hydrogen atmosphere at high temperatures (600 to 850 ) were evaluated for alumina (AlO), magnesia (MgO), silicon nitride (SiN), and mica (KMg(SiAl)O(OH)) to comprehend the difference in the insulating properties of oxide, nitride, and minerals. The activation energies of the electrical conductivity of alumina and magnesia in hydrogen were larger than those in air. On the other hand, the electrical conduction values for silicon nitride and mica in hydrogen were the same as those in air. Therefore a low oxygen partial pressure would have some influence on the electrical conduction of oxides. Increasing the temperature did not result in a large change in the electrical conduction mechanism in any of the materials in either atmosphere. The maximum partial discharge (PD) in hydrogen tended to increase compared to that in air at high voltage. The applied voltage at which the maximum amount of PD started to increase rapidly became lower with increasing temperature in all materials and in both atmospheres. The total amount of PD tended to decrease with increasing temperature in all materials and in both atmospheres. However, above a certain temperature, the total amount of PD either increased or showed a slight decrease.

Super-capacitive behavior of carbon nano tube doped 11-(4-cyanobiphenyl-4-oxy) undecan-1-ol

In this study, the influence of carbon nano tube (CNT) doping on ac conductivity mechanism and electronic properties of the 11-(4-cyanobiphenyl-4-oxy) undecan-1-ol (LC11) has been analyzed by means of dielectric and impedance spectroscopy measurements. It has been revealed that CNT doping enhances the capacitive behavior that results a super-capacitor ability for the low frequency region up to 100Hz. From this point of view, CNT doped LC11 may be proposed as a new composite material for super-capacitor applications. In addition, it has been determined that the conductivity mechanism changes from Nearly Constant Loss model to Super Linear Power Law behavior by adding CNT for the low frequency region. For the high frequency region, the conductivity mechanism approaches to Correlated Barrier Hopping mechanism by doping CNT to LC system.

Measuring Ligand-Dependent Transport in Nanopatterned PbS Colloidal Quantum Dot Arrays Using Charge Sensing.

Colloidal quantum dot arrays with long organic ligands have better packing order than those with short ligands but are highly resistive, making low-bias conductance measurements impossible with conventional two-probe techniques. We use an integrated charge sensor to study transport in weakly coupled arrays in the low-bias regime, and we nanopattern the arrays to minimize packing disorder. We present the temperature and field dependence of the resistance for nanopatterned oleic-acid and n-butylamine-capped PbS arrays, measuring resistances as high as 10(18) Ω. We find that the conduction mechanism changes from nearest neighbor hopping in oleic-acid-capped PbS dots to Mott's variable range hopping in n-butylamine capped PbS dots. Our results can be understood in terms of a change in the interdot coupling strength or a change in density of trap states and highlight the importance of the capping ligand on charge transport through colloidal quantum dot arrays.

A probabilistic model for estimation of ionically permeable inhomogeneities in polymer coatings

The mechanism of ionic conduction through polymer coatings has been investigated. The presence of highly permeable areas in the coating (designated as D type areas), impose an electrochemical inhomogeneity which is identified by change in the ionic resistance as measured by DC method. The effect of coating thickness was studied on D type behaviour of coatings. A structural model is proposed to explain the changes in conduction mechanism by variation of coating thickness. Statistical models based on numerical (simulation-based) and analytical (probabilistic model) analysis of D type areas distribution have been advanced to estimate the number of D areas in any given coating based on the percentage of D type. Using these models the number of permeable (D type) areas can be estimated based on the D type percentage of the coating.

Comparative study of Mn3+ and Mn2+ doping effects on structure and electrical properties of BiFeO3 thin films

BiFeO3 (BFO) thin films doped differently with 5mol% Mn3+ and 5mol% Mn2+ were prepared on (111)Pt/Ti/SiO2/Si substrates by a sol–gel method with rapid thermal annealing. Systematic analyses about the two different doping effects and their underlying mechanisms were performed. Pure and doped BFO films were well crystallized in perovskite structure, whilst doping of Mn3+ and Mn2+ resulted in slight distortion in the lattice structure. After doping, smaller grain size, diminished structural defects and denser microstructure were confirmed. Both Mn3+ and Mn2+ dopants could reduce the concentration of oxygen vacancies in BFO films, whereas Mn3+doping exhibited better ability. As a result, the Mn3+ doped film showed the highest dielectric permittivity among those three samples, εr=105 at 100kHz; meanwhile, the Mn3+ doped film showed a superior 2Pr value of 79.5µC/cm2, compared to 63.2µC/cm2 for the Mn2+ doped film and unachievable intrinsic ferroelectric polarizations in the undoped BFO film because of high leakage contribution. The leakage current density in medium/high electric field region was dramatically decreased by Mn doping, accompanied with a change of conduction mechanism from the space charge limited conduction to the Schottky mechanism.

Transport properties of Ar+ irradiated resistive switching BiFeO3 thin films

BiFeO3 thin films were irradiated by Ar+ ions with different fluences. The rectifying and resistive switching behaviour were retained on the Au/BiFeO3/Pt stack, and the ON/OFF ratio clearly depends on the Ar+ fluence. It was observed that the transport in high resistance state changes from Poole–Frenkel conduction to space-charge-limited conduction after irradiation. While the conduction of the low resistance state is dominated by both the interface and the bulk thin film in the pristine devices, however, it is only dominated by the interface in the irradiated devices. The observed change of conduction mechanism was explained by additionally created oxygen vacancies (OVs) during irradiation, which also improves the stability of resistive switching.

Pb dopant induced changes in structural, optical and electrical properties of CdSe thin films

Thin films of Pb doped CdSe at 1% and 5% dopant concentrations have been prepared by thermal evaporation technique using inert gas condensation method on glass substrates. The effect of Pb doping on structural, optical and electrical properties of CdSe thin films has been studied. Elemental composition of the thin films has been analyzed using Energy Dispersive X-ray analysis (EDX) spectra. Transmission electron microscope (TEM) images show the spherical nature of the nanoparticles. X-ray diffraction spectra indicate the presence of hexagonal phase of CdSe in undoped and Pb doped CdSe thin films, and formation of cubic phase of PbSe with the increase in amount of Pb dopant. A decrease in the band gap with increase in Pb doping in CdSe lattice has been observed due to the formation of band tails in the band gap and increase in crystallite size after doping. The photoluminescence (PL) spectra of thin films have been studied and enhancement in the PL intensity is observed after Pb doping. The dark conductivity of the prepared thin films has also been studied and two types of conduction mechanisms have been observed. Hall measurements indicate change in conduction mechanism from n-type to p-type after Pb doping in CdSe.

Comparative study of multiwalled carbon nanotube/polyethylene composites produced via different techniques

A comparative study of multiwalled carbon nanotube/polyethylene (MWCNT/PE) composites produced by different methods (thermal mechanical mixing, coagulation precipitation technique and in situ ethylene polymerization with a Ziegler–Natta catalyst supported on MWCNTs) has been performed. The morphology and structure of the produced composites were investigated by optical, scanning and transmission electron microscopies. MWCNT incorporation into PE matrix results in the change of conductivity mechanism from conductivity typical of a system with local disorder and effects of weak interaction of electrons (quantum correction to conductivity) to conductivity that can be described within the theory of Coulomb blockade or quasi‐one‐dimensional variable‐range hopping conductivity with variable length of hops (Mott's law) between localized states. The electromagnetic (EM) response of composites was investigated in a broadband region (118–510 GHz). It was found that the dielectric properties of MWCNT/PE composites correlate with their electrophysical properties determined by the MWCNT distribution within the polymer matrix. Typical TEM image of MWCNT in polyethylene matrix. Distance between nanotubes filled with PE varies within a wide range

Conduction mechanism switch for SnO2 based sensors during operation in application relevant conditions; implications for modeling of sensing

In this paper we present experimental evidence for a conduction mechanism switch for gas sensors based on SnO2 porous, thick film sensing layers under exposure to CO in the presence of humidity. In dry air and CO concentration below 250ppm the conduction is controlled by the surface depletion layers formed by the trapping of electrons from conduction band on adsorbed oxygen species. In the presence of humidity and at CO concentration exceeding 3ppm the surface depletion layers are replaced by surface accumulation layers and the conduction mechanism changes accordingly. We also present a model that takes into account the influence of humidity at low CO concentrations.

Impedance spectroscopic investigation of electro active regions, conduction mechanism and origin of colossal dielectric constant in Nd1 − xSrxFeO3 (0.1 ≤ x ≤ 0.5)

Impedance spectroscopic studies of single phase Nd1−xSrxFeO3 (0.1≤x≤0.5), synthesized by solid state reaction method, have been carried out to investigate the electro active regions, conduction mechanism and origin of colossal dielectric constant in the wide temperature (77–358K) and frequency (1–1.5×107Hz) range. These results show that three electro active regions, i.e., grains, grain boundaries and electrode semiconductor contacts are present in these materials. The conduction mechanism changes from Mott variable range hopping to adiabatic small polaronic hopping with the increase in temperature. Moreover, by increasing the Sr concentration the activation energy of conduction and the temperature at which conduction mechanism changes decrease, while the localization length increases. The permittivity of grains is from 8 to 10 while the high permittivity observed at higher temperatures is extrinsic, due to the formation of different Schottky barriers.

Electronic transport and conduction mechanism transition in La1∕3Sr2∕3FeO3 thin films

We report on the electronic transport properties of epitaxial La1∕3Sr2∕3FeO3 films using temperature dependent resistivity, Hall effect, and magnetoresistance measurements. We show that the electronic phase transition, which occurs near 190 K, results in a change in conduction mechanism from nonadiabatic polaron transport at high temperatures to resistivity behavior following a power law temperature dependence at low temperatures. The phase transition is also accompanied by an abrupt increase in apparent mobility and Hall coefficient below the critical temperature (T*). We argue that the exotic low temperature transport properties are a consequence of the unusually long-range periodicity of the antiferromagnetic ordering, which also couples to the electronic transport in the form of a negative magnetoresistance below T* and a sign reversal of the Hall coefficient at T*. By comparing films of differing thicknesses, stoichiometry, and strain states, we demonstrate that the observed conduction behavior is a robust feature of La1∕3Sr2∕3FeO3.

Reversible changes in temperature dependence of electric conductivity of hydrogenated amorphous silicon caused by proton irradiation

We investigate temperature dependence of electric conductivity and photoconductivity of hydrogenated amorphous silicon (a-Si:H) thin films due to energetic proton irradiation and clarify the change in dominant electric conduction mechanism. We also investigate variations of the temperature dependence after proton irradiation at different temperatures and the recovery from radiation damage due to thermal annealing. The results show that the high fluence proton irradiation changes the dominant electric conduction mechanism from the band transport of thermally excited carriers through the extended states to the hopping transport based on the carrier transport via localized states near the Fermi level. The hopping conduction has the weak temperature dependence which disappears at elevated temperature and the strong temperature dependence based on the band conduction appears again. Also, the proton irradiation at high temperature never provoke the hopping conduction. However, the band conduction after irradiation is not completely restored even after thermal annealing, indicating that thermally stable radiation defects remain. We conclude that these dominant electric conduction mechanism changes originated from proton irradiation and thermal annealing, and are simply caused by the increase or decrease in localized density of states.

Influence of Conduction Mechanism Changes on the Sensor Performance of SMOX Based Gas Sensors

Abstract DC electrical resistance measurements on undoped polycrystalline SnO2 thick film sensing layers are performed to check if, under CO exposure in realistic application conditions (dry or humid air), a change in conduction mechanism is possible. The results show for CO concentrations below 250 ppm in dry air, that the conduction through the sensing layer is controlled by the surface depletion layer. Increasing the CO concentration in humid air, a change in conduction mechanism can be observed at CO concentrations below 15 ppm. In the present studies, the possibility and consequences of changes in the conduction mechanism for sensing with SnO2, as the most studied and used SMOX, are evaluated.

Rapid synthesis and electrical transition in p-type delafossite CuAlO2

Single phase CuAlO2 has been synthesized within 1.5 hours. AC and DC electrical measurements revealed change in conduction mechanism at 375 °C.

DC Voltage Insulating Properties of Various Inorganic Materials in Hydrogen Atmosphere at High Temperatures

SUMMARY In this study, the dc voltage insulating properties in a hydrogen atmosphere at high temperatures (600 to 850 ) were evaluated for alumina (AlO), magnesia (MgO), silicon nitride (SiN), and mica (KMg(SiAl)O(OH)) to comprehend the difference in the insulating properties of oxide, nitride, and minerals. The activation energies of the electrical conductivity of alumina and magnesia in hydrogen were larger than those in air. On the other hand, the electrical conduction values for silicon nitride and mica in hydrogen were the same as those in air. Therefore a low oxygen partial pressure would have some influence on the electrical conduction of oxides. Increasing the temperature did not result in a large change in the electrical conduction mechanism in any of the materials in either atmosphere. The maximum partial discharge (PD) in hydrogen tended to increase compared to that in air at high voltage. The applied voltage at which the maximum amount of PD started to increase rapidly became lower with increasing temperature in all materials and in both atmospheres. The total amount of PD tended to decrease with increasing temperature in all materials and in both atmospheres. However, above a certain temperature, the total amount of PD either increased or showed a slight decrease.

Temperature dependence of the conduction mechanisms through a Pb(Zr,Ti)O3 thin film

The conduction mechanisms through a lead zirconate titanate (PZT) thin film grown by pulsed laser deposition with a La0.67Sr0.33MnO3 (LSMO) buffer layer on epitaxial Pt (111) were assessed in the 230–330K temperature range. X-Ray diffraction and transmission electron microscopy evidenced a columnar growth of (001)- and (011)-oriented PZT grains. The leakage current through the Pt/PZT/LSMO/Pt structure was then systematically measured. From current vs. time curves, a threshold voltage was found below which stable and reproducible current values are obtained, thus avoiding resistance degradation. The conduction mechanism changes from interface controlled at low temperatures to bulk controlled around room temperature. The hopping-type conductivity evidenced above 270K is consistent with the extended defects and columnar microstructure of the PZT film.

Conduction mechanism in undoped and antimony doped SnO2 based FSP gas sensors

The conduction mechanism in highly porous thick film SnO2 sensing layers, obtained by a direct deposition technique using FSP (flame spray pyrolysis) and the role of antimony (Sb) additives is investigated by simultaneous DC electrical resistance and work function changes measurements in various ambient atmospheres. The results have shown that the conduction mechanism changes according to the ambient atmosphere from a depletion layer controlled one to flat band situation and to an accumulation layer controlled one. In comparison to former studies performed on polycrystalline thick film layers, obtained from pre-processed powders, the measurements indicate that the grain size of the sensing material has a big influence on the conduction mechanism. In the case of undoped FSP-SnO2 sensors the studies showed that the grains are fully depleted in oxygen containing atmospheres. By increasing the number of free charge carriers in the bulk of the semiconductor, it was found that the addition of Sb changes the conduction mechanism dramatically. In contrast to the undoped material, there is a direct switch from a conduction mechanism controlled by the depletion layer to the degenerated semiconductor surface situation. Additionally, an initial upwards band bending is observed, even in the absence of atmospheric oxygen, which indicates a surface effect of Sb.