Decrease In Charge Carrier Concentration Research Articles

Atomistic origin of high grain boundary resistance in solid electrolyte lanthanum lithium titanate

Lanthanum lithium titanate is one of the promising electrolytes for solid-state lithium-ion batteries due to its high bulk ionic conductivity up to ∼10−3 S/cm. However, the practical application of this material has been bottlenecked by high grain boundary (GB) resistance, while the underlying mechanism is still under debate. Here we report a comprehensive study with direct evidence to reveal the origin of high GB resistance in La2/3–xLi3xTiO3 (LLTO). Atomic-scale observations via advanced scanning transmission electron microscopy show that the GBs are uniformly subject to subsurface segregation of La atoms to compensate for the excess surface charges. The La segregation leads to an abrupt decrease of charge carrier concentration neighboring GBs and hence is supposed to have deleterious effect on GB conductivity. The findings suggest a novel mechanism of space-charge-induced cation segregation, which shed lights on the intrinsic origin of low GB ionic conductivity in LLTO.

Influence of Stoichiometry and Aging at Operating Temperature on Thermoelectric Higher Manganese Silicides

Thermoelectric higher manganese silicides, MnSix, were synthesized by magnesioreduction followed by spark plasma sintering with different nominal compositions (x = 1.65, 1.74, and 1.80) and various postsynthesis annealing durations (0, 48, 96, and 336 h). The composite Nowotny chimney-ladder crystal structures of the resulting samples were investigated by synchrotron X-ray powder diffraction. The modulation vector component gamma, generally considered corresponding to the stoichiometry (x) of the material, was accurately determined by Rietveld refinement using a (3 + 1)D superspace approach. Regardless of the initial nominal composition, all the samples have a similar gamma similar to 1.736 after 48 h of annealing at 900 K. This result suggests that MnSix, at a temperature of 900 K, is better described as a defined compound with x close to 1.736, rather than intermediate solid-solution phases with 1.725 < x < 1.75 as predicted by the commonly accepted phase diagram. At the fixed nominal composition MnSi1.74, gamma increases significantly from 1.7313(2) to 1.7411(1) after 336 h of annealing, indicating that the thermal history influences the Si stoichiometry. The evolution of gamma with time is concomitant with a power factor drop (-19%), attributed to a decrease in charge carrier concentration. The drop of the power factor, partially compensated by a decrease in thermal conductivity, results in a -12% reduction of the maximum figure-of-merit ZT, after prolonged annealing under realistic application conditions.

Microstructures and electrical properties of Mn/Co/Ni-doped BaBiO3 perovskite-type NTC ceramic systems

Mn/Co/Ni-modified BaBiO3-based negative temperature coefficient (NTC) ceramics with the formula, x[(3-y-z)Mn/yCo/zNi-ions]+(1−x)BaBiO3 (x = 0.0–0.4, y = 0.96 and z = 0.48), were synthesized using a conventional solid state reaction. The effects of Mn/Co/Ni-substitute content on the phase structures and composition, microstructures and electrical properties were characterized by X-ray diffraction (XRD), scanning electron microscope and resistance–temperature measurements, respectively. XRD pattern analysis results revealed that the compound phases consisted of a main monoclinal BaBiO3 phase and a secondary phase of Ba(Mn, Co, Ni)O3 solid-solution with a hexagonal structure, which were detected at x = 0.05–0.2. In addition, according to the XRD patterns, the main phase changed from BaBiO3 to a rhombohedral Bi8.11Ba0.89O13.05 phase at x = 0.3, and this phase content increased dramatically as the x value increased to 0.4. For the related electrical properties, all of the samples demonstrated the typical characteristics of NTC thermistors across a relatively wide range of temperatures. The values obtained for B constant (B25/85) and the room-temperature resistivity (ρ25) were in the range of 2297–5235 K and 760 Ω cm−1, 620 kΩ cm, respectively, which implied that the electrical properties of the present ceramic systems could be optimized via various substitutions in the Mn/Co/Ni-ions content. The variable values of ρ25 were affected strongly by the changes in crystal structure and the decrease in charge carrier concentration. These composite ceramic materials are suitable candidates for a variety of NTC thermistor applications due to their widely adjustable electrical properties.

Incorporation of indium into the Zn13Sb10 thermoelectric material

The composition of the In-substituted Zn13Sb10 phases follows the Zn13-1.5xInxSb10 charge-balanced formula and the reduction in the Zn content is due to the In3+ oxidation state, compared to the Zn2+ oxidation state. Zn deficiency in the In-doped samples is supported by the X-ray powder diffraction and EDS elemental analyses. Single-crystal X-ray diffraction refinement backs the Zn deficiency by revealing absence of Zn atoms in the interstitial sites and decreased occupancy of the main Zn site.The heat capacity measurements indicate that incorporation of In suppresses the α → α′ (235 K) and α′ → β (254 K) phase transitions, which could be due to the disappearance of some interstitial Zn atoms and/or difficulty in ordering the remaining ones. Both electrical resistivity and Seebeck coefficient data suggest a decrease in charge carrier concentration, stemming from an increased occupancy of the valence band. The thermal conductivity reveals changes in the phonon scattering pathways upon In doping; dominance of point defect scattering in Zn13-1.5xInxSb10 vs. Umklapp scattering in the pristine Zn13Sb10.

Ni-doped ZnO films deposited by RF magnetron sputtering using raw powder target

Pure and nickel-doped zinc oxide thin films were deposited by radio-frequency magnetron sputtering on glass substrates at room temperature using raw powder targets. The dopant concentration was varied from 0 to 4 wt%. Structural characterization of the samples performed with X-ray diffraction confirmed that all films exhibit the hexagonal wurtzite crystal structure with (002) preferred orientation. The average transmittance of all the films is higher than 80% in the visible wavelength region. PL analysis revealed that the ultraviolet emission intensity decreased and exhibited a redshift with increasing Ni concentration. This was consistent with the energy band values (3.35–3.28). A lowest electrical resistivity of 8.42 × 10−4 Ω cm was observed for the pure ZnO films, and by increasing Ni content, the electrical resistivity increases, which was subsequently affected by a decrease in charge carrier concentration. AC and DC conductivities were studied to explore the conduction mechanism. The temperature dependence of both ac conductivity and the parameter s is reasonably well interpreted by the correlated barrier hopping model. Activation energy values deduced from both dc conductivity and relaxation frequency are in the range of 0.49–0.61 eV. The analysis of the parameter s leads to the barrier height Wm values which are in agreement with that proposed by the theory of hopping of charge carriers over the potential barrier between the defect states in the band tail as suggested by Elliott.

Formation of nanodots and enhancement of thermoelectric power induced by ion irradiation in PbTe:Ag composite thin films

Present study demonstrates an enhancement in thermoelectric power of 10% Ag doped PbTe (PbTe:Ag) thin films when irradiated with 200keV Ar ion. X-ray diffraction showed an increase in crystallinity for both PbTe and PbTe:10Ag nano-composite films after Ar ion irradiation due to annealing of defects in the grain boundaries. The preferential sputtering of Pb and Te ions in comparison to Ag ions resulted in the formation of nano-dots. This was further confirmed by X-ray photoelectron spectroscopy (XPS). Such an enhancement in thermoelectric power of irradiated PbTe:10Ag films in comparison to pristine PbTe:10Ag film is attributed to the decrease in charge carrier concentration that takes part in the transport process via restricting the tunneling of carriers through the wider potential barrier formed at the interface of nano-dots.

Depth profiling of strain and carrier concentration by cleaved surface scanning of GaN Gunn-diode: confocal Raman microscopy

Confocal micro-Raman spectroscopy was applied to study the cleaved surface of vertical GaN Gunn-diode structure grown by molecular-beam epitaxy. The analysis of lateral scanning along the cleaved edge reveals the depth profile of elastic strain, quality of the crystal structure, and the concentration of charge carriers. Results are compared with that of axial confocal Raman depth profiling normal to the structure's surface. Decrease of compressive strain near the cleaved edge in the direction from the substrate to the structure's surface and in the growth plane towards the cleaved edge is shown. The decrease in charge carrier concentration in the undoped n0-GaN channel region in comparison with the n+-GaN contact region is identified. Peculiarities of the resulting spatial profiles of free charge carriers and their correlation with the initial doping profile are discussed.

Spectroscopic and electrical sensing mechanism in oxidant-mediated polypyrrole nanofibers/nanoparticles for ammonia gas

Ammonia gas sensing mechanism in oxidant-mediated polypyrrole (PPy) nanofibers/nanoparticles has been studied through spectroscopic and electrical investigations. PPy nanofibers/nanoparticles have been synthesized by chemical oxidation method in the presence of various oxidizing agents such as ammonium persulfate (APS), potassium persulfate (PPS), vanadium pentoxide (V2O5), and iron chloride (FeCl3). Scanning electron microscopy study revealed that PPy nanofibers of about 63, 71 and 79 nm diameters were formed in the presence of APS, PPS, V2O5, respectively, while PPy nanoparticles of about 100–110 nm size were obtained in the presence of FeCl3 as an oxidant. The structural investigations and confirmation of synthesis of PPy were established through Fourier transform infrared and Raman spectroscopy. The gas sensing behavior of the prepared PPy samples is investigated by measuring the electrical resistance in ammonia environment. The observed gas sensing response $$ \left( {{{\Updelta R} \mathord{\left/ {\vphantom {{\Updelta R} R}} \right. \kern-0pt} R} \times 100} \right) $$ at 100 ppm level of ammonia is ~4.5 and 18 % for the samples prepared with oxidizing agents FeCl3 and APS, respectively, and by changing the ammonia level from 50 to 300 ppm, the sensing response varies from ~4.5 to 11 % and ~10 to 39 %, respectively. Out of all four samples, the PPy nanofibers prepared in the presence of APS have shown the best sensing response. The mechanism of gas sensing response of the PPy samples has been investigated through Raman spectroscopy study. The decrease of charge carrier concentration through reduction of polymeric chains has been recognized through Raman spectroscopic measurements recorded in ammonia environment.

Detailed Numerical Modeling of a Hybrid Polymer Electrolyte Fuel Cell

Numerical modeling has been used to investigate the resistive behavior of a novel hybrid polymer electrolyte fuel cell design that utilizes a porous wicking layer to eliminate flooding. The resistance of the porous layer is shown to depend upon current and to increase nonlinearly with layer thickness. The relationship between conductivity and thickness was found to be related to the physical size of the reaction zone in the porous layer, and the scaling of the reaction zone with current was investigated. The porous layer resistance was also found to increase with temperature in most cases due to a decrease in charge carrier concentration at elevated temperatures.

The effect of silver chloride formation on the kinetics of silver dissolution in chloride solution

The precipitation and growth of AgCl on silver in physiological NaCl solution were investigated. AgCl was found to form at bottom of scratches on the surface which may be the less effective sites for diffusion or the favorable sites for heterogeneous nucleation. Patches of silver chloride expanded laterally on the substrate until a continuous film formed. The ionic transport path through this newly formed continuous film was via spaces between AgCl patches. As the film grew, the spaces between AgCl patches closed and ion transport was primarily via micro-channels running through AgCl patches. The decrease of AgCl layer conductivity during film growth were attributed to the clogging of micro-channels or decrease in charge carrier concentration inside the micro-channels. Under thin AgCl layer, i.e. on the order of a micrometer, the dissolution of silver substrate was under mixed activation-Ohmic control. Under thick AgCl layer, i.e. on the order of tens of micrometers, the dissolution of silver substrate was mediated by the Ohmic resistance of AgCl layer.

Thermoelectric Performances of Free-Standing Polythiophene and Poly(3-Methylthiophene) Nanofilms

Thermoelectric performances of free-standing polythiophene (PT) and poly(3-methylthiophene) (PMeT) nanofilms with high tensile strength electrosynthesized from boron trifluoride diethyl etherate (BFEE) are systematically investigated. They display decent electric conductivity (47 and 73 S · cm−1), high Seebeck coefficient (130 and 76 μ V · K−1) and low thermal conductivity (0.17 and 0.15 W · m−1 · K−1) at room temperature. Their figure of merit can reach 3.0 × 10−2 at 250K, higher than that of many other conducting polymers. The decrease of charge carrier concentration resulting from volatile and water-sensitive dopants from BFEE leads to the decrease of electrical conductivity but a substantial increase of the Seebeck coefficient, making their figure-of-merit values maintained at more than 10−2 even after prolonged storage (two months). Moreover, free-standing PT and PMeT exhibit much better thermoelectric performances than those in pressed pellets due to the good arrangement of the polymer chains and preferably oriented structure in films. It therefore provides a way to improve the thermoelectric performances of conducting polymers by controlling regularity of the extended conjugated chain structure and/or the chain packing to achieve high charge mobility.

Doping dependent metal to insulator transition in the (Bi, Pb)-2212 system: The evolution of structural and electronic properties with europium substitution

The present work investigates the effect of europium substitution on the (Bi, Pb)-2212 system in the concentration range 0.5 ≤ x ≤ 1.0. Phase analysis and lattice parameter calculations on the powder diffraction data and the elemental analysis of EDX show that the Eu atoms are successfully substituted into the (Bi, Pb)-2212 system. Resistivity measurements (64–300 K) reveal that the system exhibits superconductivity at x ≤ 0.5 and semiconductivity at x > 0.5. With the complete suppression of superconductivity which is known to be a quasi-two dimensional phenomenon in these materials, a metal to insulator transition takes place at x = 0.6 and the predominant conduction mechanism is found to be variable range hopping between localized states, resulting in macroscopic semiconducting behaviour. The results of electrical and structural properties of the doped (Bi, Pb)-2212 compounds suggest that the decrease of charge carrier concentration and the induced structural disorder are the more effective and dominant mechanisms in the origin of the metal to insulator transition and suppression of superconductivity due to Eu substitution at its Sr site.

Structural and electrical properties of Bi 1.7Pb 0.4Sr 2− xHo xCa 1.1Cu 2.1O 8+ δ system across the metal to insulator transition

The structural and electrical properties of Bi 1.7Pb 0.4Sr 2− x Ho x Ca 1.1Cu 2.1O 8+ δ (0.5 ≤ x ≤ 1.0) system, synthesized by solid state reaction technique are reported across the metal to insulator transition (MIT). The system shows superconducting transition at x = 0.5 and suppression in superconductivity with increasing x. A MIT is found to occur at 0.6 < x ≤ 0.7 and for x ≥ 0.7 the samples show insulating behavior. A detailed resistivity analysis shows that Ho substitution localizes the electronic states of (Bi,Pb)-2212 system at the Fermi level and the transport mechanism in insulating samples is the two-dimensional Mott's variable range hopping. The normal state resistivity increases with x and a considerable drop in localization length takes place at the MIT. The results are interpreted on the basis of decrease in charge carrier concentration and disorder induced carrier localization, due to the substitution of Ho atoms at the Sr site of (Bi,Pb)-2212 system.

Accelerated Degradation Test of Indium Tin Oxide (ITO) Thin Films Deposited by RF Magnetron Sputter

Thermal degradation of indium tin oxide (ITO) thin film has been investigated. ITO thin film was fabricated on glass substrate using RF magnetron sputtering and was characterized. The resistivity of the film which was thermally degraded at high temperatures in air atmosphere was increased highly. Thermally-degraded specimen was analyzed using XPS and Hall measurement to reveal failure mechanism. Result showed that failure mechanism was the decrease in charge carrier concentration and mobility due to oxygen diffusion and chemisorption. Accelerated degradation test (ADT) was performed to predict the lifetime of ITO thin film. The lifetime under normal operating condition could be predicted via statistical analysis and modeling of data acquired from ADT of a short period.

Magnetic and transport properties in the Ti doped cobaltite Ca3Co4−xTixO9 (⩽x⩽0.8) single crystals

Single crystals of Ca3Co4−xTixO9 (x=0, 0.2, 0.4, 0.6, and 0.8) are grown by flux growth technique. The effect of Ti doping at Co site on magnetic and transport properties is studied. All samples show Curie-Weiss behavior at temperatures above 50K except for the x=0.8 sample, and the effective magnetic moment μeff increases with increasing Ti content. The free-doped sample Ca3Co4O9 shows a ferrimagnetic transition at 19K. However, a spin-glass transition at Tf(∼20K) appears for the Ti doped samples and the transition temperature Tf shifts to higher temperature as the Ti content increases. The magnetic moment values M at Tf and 5K under the applied field of 4.5T increase with increasing Ti doping content. The variation of the magnetic properties may be related to the induced extra Co2+ ions in the CoO2 layer by substituting Ti4+ for Co3+. The resistivity ρ of the x=0, 0.2, 0.4, and 0.6 samples shows metalliclike behavior at high temperatures and a broad metal-insulator (M-I) transition appears as the temperature decreases to a certain value Tmin. The value of resistivity and the ratio of ρ5K∕ρ300K increase with increasing Ti content. The results are discussed according to the decrease of charge carrier concentration in the samples, which is confirmed by the Hall coefficient measurement. For the x=0.8 sample, there exists another M-I transition in the vicinity of 269K, which is suggested to originate from the spin-state transition of Co3+ or Co4+ ions.

Application of Field-Enhanced Rapid Thermal Annealing to Activation of Doped Polycrystalline Si Thin Films

AbstractActivation of polycrystalline silicon (poly-Si) thin films doped as n-type using selective ion implantation of phosphorous was performed employing field-enhanced rapid thermal annealing where rapid thermal annealing of halogen lamps is combined with alternating magnetic fields. The ion activation was evaluated using Hall effect measurements incorporating the resistivity, the charge carrier concentration, and the mobility. Statistical design of experiments is attempted in order to clarify the effects and interactions of processes variables on field-enhanced rapid thermal annealing towards ion activation: the three processing variables are furnace temperature, power of halogen lamp, and the alternating magnetic field. Hall effect measurements indicate that the furnace temperature and RTA power are found to be dominant in activating the doped polycrystalline Si in dose. The activation process results from the competition between charge carrier concentration and mobility: the increase in mobility is larger than the decrease in charge carrier concentration.

Transport properties in heavy fermion system PrOs 4Sb 12 probed by radiation-induced disordering

The influence of atomic disordering induced by fast neutron irradiation on resistivity ρ and Hall coefficient R H was studied in single crystal samples of heavy fermion system PrOs 4Sb 12. Both ρ( T) and R H( T) of the initial samples can be described by a two-band model with conductivities in the form of ( σ i ) −1= ρ i0 + A i T 2 (indexes i=1,2 correspond to heavy and light holes), where A 1≫ A 2. Disordering leads to the appearance of electronic-like band as well as Kondo-like term in the resistivity. In both cases the essential decrease in charge carrier concentration under influence of the magnetic field is observed.

Incorporation of boron and vanadium during PVT growth of 6H-SiC crystals

To obtain semi-insulating SiC by vanadium and boron co-doping during PVT growth, a detailed understanding of the dopant (B,V) incorporation is required. Crystal growth of 1.4′′ 6H-SiC on either Si or C face, doped with boron or vanadium, respectively, was performed. For reference purposes, also nominally undoped SiC crystals were grown. It is shown that in nominally undoped crystals nitrogen remains residual impurity acting as a finite source during growth. An exponential decrease of charge carrier concentration in growth direction from 2×10 17 to 8×10 15 cm −3 was observed. In boron doped crystals, charge carrier concentration increases with growth time because of nitrogen decrease and boron loss from the powder charge due to evaporation. However, lateral inhomogeneity throughout a wafer is as low as Δ p/ p=15%. Factors influencing the transfer coefficient of boron are discussed. Semi-insulating behavior was achieved by vanadium doping as shown with electron spin resonance and optical absorption. V content of the source as well as V incorporation in the crystal decrease with growth time. If the solubility limit of V in SiC is exceeded, defects consisting of vanadium-rich precipitates and planar voids occur. The vanadium transfer coefficient depends on the partial pressure of V species and measures about 0.001.

Phase separation of hydrogenated La2-xSrxCu(Fe)O4.

Hydrogenated powdered ${\mathrm{H}}_{\mathit{z}}$${\mathrm{La}}_{2\mathrm{\ensuremath{-}}\mathit{x}}$${\mathrm{Sr}}_{\mathit{x}}$Cu(Fe)${\mathrm{O}}_{4}$ was studied by means of M\"ossbauer spectroscopy, magnetic ac susceptibility, and x-ray powder diffraction. We have shown that hydrogenation of ${\mathrm{La}}_{2\mathrm{\ensuremath{-}}\mathit{x}}$${\mathrm{Sr}}_{\mathit{x}}$${\mathrm{CuO}}_{4}$ leads to a decrease of charge carrier concentration. Low-temperature (T280 K) M\"ossbauer spectra of highly hydrogenated ${\mathrm{H}}_{\mathit{z}}$${\mathrm{La}}_{2\mathrm{\ensuremath{-}}\mathit{x}}$${\mathrm{Sr}}_{\mathit{x}}$Cu(Fe)${\mathrm{O}}_{4}$ samples exhibit the presence of magnetic and nonmagnetic parts. We interpret this as being due to phase separation into magnetic insulating and metallic regions, taking place slightly below room temperature.

The Core-Shell Grain Resistance in the Reduced La-Doped (Ba, Sr)TiO3 Ceramics

Based on the oxygen-vacancy diffusion, a core-shell model of grain resistance is proposed, for the reduced La-doped (Ba, Sr)TiO3 ceramics, to describe the charge-carrier distribution. Complex impedance analysis is adopted to determine the grain resistance and grain boundary resistance. After sintering at 1400°C in reducing atmosphere, the low grain resistance is attributed to the effective electron compensation in the grain core, while the higher grain boundary resistance is thought to be due to the lower charge-carrier concentration in the grain shell. In addition, reoxidation leads to further decrease of charge-carrier concentration on the grain shell by diffusing inward to the grain core.