Conductivity Of Polycrystalline Research Articles

N+-ion implantation induced enhanced conductivity in polycrystalline and single crystal diamond.

With the 200 keV N+-ion implantation technique and a systematic variation of fluence, we report on the formation of highly conducting n-type diamond where insulator-to-metal transition (IMT) is observed above a certain fluence wherein the conductivity no longer obeys the hopping mechanism of transport rather, it obeys quantum corrections to Boltzmann conductivity at concentrations of nN ≥ 2 × 1020 cm−3. The conductivity for ultra-nanocrystalline diamond is found to be high, ∼650 Ω−1 cm−1 with thermal activation energy Ea ∼ 4 meV. Interestingly, with gradual increase in fluence, the conductivity in polycrystalline diamond films has been seen to progress from the hopping mechanism of transport in the case of low fluence implantation to a semiconducting nature with medium fluence and finally a semi-metallic conduction is observed where percolation occurs giving an insulator-to-metal transition. XANES confirms that the long-range order in diamond films remains intact when implanted with low and medium fluences; while implantation at sufficiently high fluences >5 × 1016 cm−2 leads to the formation of a disordered tetrahedral amorphous carbon network leading to metallic conduction resembling a metallic glass behaviour. XPS confirms that the sp2 fraction increases gradually with fluence starting from only 6% in the case of low fluence implantations and saturates at 40–50% for implantation at high fluences. A similar observation can be made for single crystal diamond when implanted at high fluence; it retains long-range order but percolative transport takes place through defects or semi-amorphized regions.

Comparative study of thermal fluctuation induced conductivity in YBa2Cu3O7-d containing Nano-Zn0.95Mn0.05O and Nano-Al2O3 particles

In this report we presented the fluctuation induced excess conductivity in polycrystalline superconducting samples of YBa2Cu3O7-δ (for brevity YBCO or Y123) with nano-Al2O3 (20 nm) and nano- Zn0.95Mn0.05O (30 nm) particles prepared under the same conditions using the solid-state reaction route. The crystal structure and microstructure were analyzed via X-ray diffraction, scanning and transmission electron microscopies. The crystal structure of Y123 was conserved for samples added with both kinds of nanoparticles. The electrical resistivity dependence with temperature (ρ(T)) was measured and the excess conductivity in the mean field region was analyzed using the Aslamazov and Larkin approach. The fluctuation regions and the critical parameters including the coherence lengths, penetration depth and the effective layer thickness were estimated and compared for both series of samples. The critical physical quantities Bc1(0), Bc2(0) and Jc(0) were computed. Improvements in these critical physical quantities were obtained for samples sintered with 0.02 wt% nanoparticles. The nano-Al2O3 particles are more adequate than nano-Zn0.95Mn0.05O particles for enhancement of flux pinning properties of YBa2Cu3O7-δ.

Metal–Insulator Phase Transition in Iron-Doped Vanadium Dioxide Thin Films

The electrical conductivity of polycrystalline V(1 – x)FexO2 films has been investigated in a wide temperature range, which covers both the metal and insulator phase regions. It is shown that with an increase in the iron concentration the metal–insulator phase transition shifts toward lower temperatures, while the temperature range of the transition in doped samples additionally broadens as compared with pure VO2. To explain the temperature dependence of the electrical conductivity of the V(1 – x)FexO2 insulator phase, a hopping conductivity model has been used, which takes into account the effect of thermal vibrations of atoms on the resonance integral. The values of parameter e have been calculated as a function of the degree of VO2 doping.

Large room temperature magnetoresistance of transparent Fe and Ni doped ZnO thin films

The electrical, optical, and magnetic properties of pure, 1%Fe, and 1%Ni doped ZnO thin films grown by spray pyrolysis technique were studied. All samples are transparent (T ≈ 85%) in VIS and near infrared region of wavelength. Ni and Fe doped ZnO layers are paramagnetic. Resistivity versus temperature has semiconducting behavior. Large value of magnetoresistance at 300 K at 1.3 T: MR = 56%/T for 1%Fe doped ZnO and MR = 28%/T for 1%Ni doped samples have been observed. These exceptional values of MR at room temperature originate probably from hopping conductivity in polycrystalline diluted magnetic semiconductor in paramagnetic high dilution limit.

Size effects on the thermal conductivity of polycrystalline platinum nanofilms

The surface and grain-boundary effects on the in-plane thermal conductivity ofpolycrystalline platinum nanofilms have been investigated. The thicknesses ofthe nanofilms range from 15.0 to 63.0 nm and the mean grain sizes measuredby x-ray diffraction vary from 9.5 to 26.4 nm. The thermal conductivities of thenanofilms measured by a direct electrical heating method are greatly reduced fromthe bulk values. The measured results are compared with the values predictedby the Qiu and Tien model and the Kumar and Vradis theory. It is found thatthe reduction in the thermal conductivity is mainly caused by grain-boundaryscattering and the reflection coefficient of electrons striking the grain boundaries isaround 0.35. The relaxation time model is also applied to study the size effectsto check whether the Matthiessen rule is still valid in predicting the in-planethermal conductivity of polycrystalline metallic nanofilms. The results indicate thatby considering only grain-boundary scattering and background scattering theMatthiessen rule is still valid. If surface scattering, however, is included, deviationsof the Matthiessen rule from other theories mentioned above have been found.

Theoretical approaches to superionic conductivity

Recent theoretical approaches to the understanding of superionic conductivity in polycrystalline, glassy and polymeric materials are briefly reviewed. Phase transitions to the superionic conducting state in the AgI family are apparently triggered by cluster formation and strong mobile ion interaction within the clusters. Anomalous conductivity and related physical properties are explained in the cluster induced distortion model. Ionic composites such as AgX : Al2O3 (X = Cl, Br and I) involve conducting and non-conducting phases and the all-important interface between the two whose space charge enhances the conductivity and also trigger phase transitions to exotic polymorphic phases, for which the mechanisms are yet to be explored. Ion hopping dynamics controls the conductivity of superionic glasses. Mode coupling and jump relaxation theories account for the non-Debye relaxation observed in a.c. conductivity of these glasses. The theory of conductivity in polymer electrolytes—still in its infancy—involves their complex structure and glass transition behaviour. Preparative and thermal history, composition and crystallinity control ionic conductivity. New approaches to the synthesis of optimal polymer electrolytes such as rubbery electrolytes, crystalline polymers and nanocomposites must be considered before achieving a comprehensive theoretical understanding.

Magnetic, thermodynamic, NMR, and transport properties of the heavy-fermion semiconductorU2Ru2Sn

We have measured the magnetic susceptibility, specific heat. NMR, electrical resistivity, magnetoresistance, Hall effect, thermoelectric power, and thermal conductivity of polycrystalline U 2 Ru 2 Sn. Some of these properties are compared to those of Th 2 Ru 2 Sn. The experimental data indicate the formation of a narrow energy gap of approximately k B × 160 K in U 2 Ru 2 Sn. Similarities to the behavior of heavy-fermion semiconductors (Kondo insulators) are observed, in particular to CeNiSn. Thus, we believe that U 2 Ru 2 Sn may be classified as a heavy-fermion semiconductor.

Transport Properties And Crystal Chemistry Of Ba-Sr-Bi Oxides

ABSTRACTRhombohedral (R) phases of the binary systems AE-Bi-O (AE = Ca, Sr, Ba) have low temperature (β2) and high temperature (β1) polymorphs and the β1 polymorph is a good oxygen ion conductor. In a previous study, we showed that in the ternary system Ba-Sr-Bi-O, conductivities of polycrystalline R samples are unaffected by Ba:Sr, whereas temperature ranges of the β2↔β1 polymorphic transition are. Recently, we proved that the conductivity of both polycrystalline and single-crystal R samples is sensitive to spatial direction. For single crystals, the conductivity is highest perpendicular to the c-axis. Along the conduction plane, the conductivities are significantly higher than those of polycrystalline R samples; perpendicular to that plane they are significantly lower. The BaBiO3 perovskite (P) phase, predominantly an electronic conductor, has conductivities of 10 to 102 S/cm at 350-800°C in air. Conductivity measurements of the R-P assemblages with different R:P showed that percolation threshold is around 35 volume % P.

Electric properties of polycrystalline PbCd1/3Nb2/3O3

Temperature dependence of dielectric permittivity and electric conductivity of polycrystalline PbCd1/3Nb2/3O3 in the region of phase transition have been presented. Electric properties of polycrystalline PbCd1/3Nb2/3O3 depend on frequency of the measuring field. In the region of para-ferroelectric phase transition a change of electric properties takes place.

Role of Collective and Localized Modes on the Temperature-Dependent Thermal Conductivity in Polycrystalline C60 Fullerite Compacts

Recently observed thermal conductivity of polycrystalline C 60 fullerite compacts has been explained on the basis of a suggested dynamical model of the fullerites which takes into account the collective acoustic phonon modes with frequency dependent relaxation time and localized libronic and orientational diffusive modes with constant relaxation times, in the temperature range 0.7–300 K. Though the bulk of the conduction is via collective modes, the localized modes, too, contribute significantly to the total thermal conductivity.

Thermal conductivity of YBCO(123) and YBCO(211) mixed crystals prepared by MMTG

The a-b and c-axis thermal conductivity of YBa 2Cu 3) 7- x bulk superconducting crystal prepared by modified melt texture growth (MMTG) was studied. The crystals contained finely dispersed Y 2BaCuO 5 particles which were expected to form pinning centres. The conductivity data were analysed based on a proposed model for the mixed crystals using the conductivity of polycrystalline Y 2BaCuO b. The analyses indicate that Y 2BaCuO 5 particles reduce the conductivyt of MMTG crystals in the a-b plane but hardly influence the conductivity along the c-axis.

H2-induced changes in electrical conductance of ?-Ga2O3 thin-film systems

H2-induced changes of electrical conductivity in polycrystalline, undoped β-Ga2O3 thin films in the temperature range of 400–650° C are described. The sheet conductance of these films depends reversibly, according to a power law σ□ ∼ p 1/3, on the partial pressure of hydrogen in the ambient atmosphere of the Ga2O3 film. A bulk vacancy mechanism is excluded by experiments and it is shown that the interaction is based on a surface effect. Changes in conductance are discussed to result from the formation of an accumulation layer due to chemisorption on the grain surfaces. Typical coverages are determined to be approximately 10−4 ML for pH2=0.05 bar and T=600° C. A possible explanation of the σ□ ∼ p 1/3 power law is provided.

Impedance studies of the cell [formula omitted

The present work confirms that the temperature range in which the conductivity of polycrystalline (Na, Sr) β″-aluminas can be obtained by two-probe or four-probe measurements depends upon the composition. A separate determination of the inter- and intragranular resistance is feasible in a limited temperature range for each compositions. The intragranular resisivity of the polycrystalline materials is considerably smaller than the total resistivity. The activation energy of the intragranular conductivity is lower than that of the total conductivity and shows an increase with the Sr content. The activation energy of the total conductivity increases strongly with the Sr content and passes through a maximum.

Microwave conductivity in polycrystalline (BEDT-TTF)2I3 material

Polycrystalline material of the α-phase of (BEDT-TTF)2I3 was compressed to small samples (4 mm × 1 mm, thickness 0.3 mm typically) at a pressure of 10 kbar. Annealing at 70°C yields the superconducting αt-phase.Microwaves (10.2 GHz) enable the measurements of the conductivity for stepwise annealing after every annealing step in always the same sample. For annealing times ⪆ 10 min all conductivity versus temperature curves are intersecting in an isosbestic point at 190 K. This behaviour can be described by a conductivity relation for a two component system, from which was determined the volume fraction of the new grown αt-phase in dependence of the annealing time.Starting annealing (annealing times < 10 min) shows another unexpected phase transformation. After 2 min annealing the conductivity at 200 K increases by more than one order of magnitude, but then decreases of further annealing (5–10 min) down to the value for the unannealed sample.

Defect structure and physical properties of Y 2O 3

Undoped Y 2O 3 is, under given conditions (air, 460°C< T<1100°C) a mixed conductor. At temperatures T>700°C the ionic contribution of conductivity becomes significant. Grain boundaries cause the appearance of an additional relaxation process with substantionally higher activation energy, 1.25 eV, in comparison with the bulk, 0.9 eV. The decrease in conductivity of polycrystalline Y 2O 3 can be attributed to grain boundary effects. Significant softening of Y 2O 3 at temperatures T>700°C corresponds to increased participation of intersitial oxygen ions in the deformation process. Structural data, temperature dependence of electrical conductivity and microhardness, influence of partial oxygen pressure indicate the important role of oxygen intersititials in the defect structure of undoped Y 2O 3 at high temperatures and high oxygen pressure.

Excess electrical conductivity in polycrystalline Y 1Ba 2Cu 3O 7-δ compounds: Beyond the mean-field region

The excess conductivity in various polycrystalline Y 1Ba 2Cu 3O 7-δ single-phase (within 4%) compounds has been measured with a reduced-temperature resolution of 10 -4. The full critical dynamic region was probably penetrated for the first time, and our results admit a superconducting order parameter of dimension two fluctuating in three dimensions.

Electrical and optical properties of In 2Se 3 thin films

In 2Se 3 thin films were grown with good stoichiometry at a substrate temperature around 460 K in the α phase and were shown to remain in the β phase above 480 K. The Hall coefficient and the d.c. conductivity of polycrystalline In 2Se 3 films grown on Pyrex and mica substrates were studied in the temperature range 77–530 K. After thermal treatment above 473 K of the α phase thin film, the electrical resistivity decreased and the sample remained in an irreversible phase. This is explained in terms of structural changes at high temperatures. The mobility behaviour of the β phase annealed thin films is illustrated. We use the Petritz barrier model to explain the activation energy of the mobility as due to the grain boundaries of the polycrystallites. The optical properties (refractive index and absorption coefficient) are also reported. The direct band gaps of In 2Se 3 thin films are 1.43 eV and 1.55 eV for the α phase and β phase respectively. These values are obtained from transmission measurements and are confirmed through photoconductivity measurements.

Bulk and Grain Boundary Ionic Conductivity in Polycrystalline β″- Alumina

en

The relationship between sodium ion diffusion and conductivity in polycrystalline β"-alumina

Using radioactive tracer techniques, the diffusion coefficient of the sodium ions has been determined from room temperature to 600 degrees C. These diffusion results have then been compared with the ionic conductivity of similar beta "-alumina material. From this comparison a determination of the Haven ratio (HR) has also been made and a simple model is proposed that adequately predicts the observed magnitude of HR.

Kinetics of the low-temperature phase transformation of ammonium iodide

The temperature-dependent specific electrical conductivity of polycrystalline, polymorphic NH4I samples was measured in the temperature range between −170°C and +25°C. The conductivity in the ranges of the tetragonal →CsCl-type and CsCl-type→NaCl type transformations can be explained by the formal kinetics of a diffusion-controlled nucleation and growth process. An estimation of the activation energy of the formation of the second-and first-phase nuclei in the respective parent phases by the evaluation of TTT curves yielded energy values of 0.004–0.05 and 0.0001–0.024 eV.