AC Electrical Measurements Research Articles

Multiphased (NaCl)x(KCl)y−x(KBr)1−y single crystals: ac conductivity activation energy versus bulk properties

The ac electrical measurements have been just reported for alkali halide mixed crystals that were melt grown from NaCl, KCl and KBr starting materials. They showed a nonlinear variation of all the electrical parameters with the bulk composition. In this short paper, we show that these mixed systems, depending on their major constituent, are classified into three categories in each of which, the activation energy for the ac conductivity increases linearly with BΩ, where B is the isothermal bulk modulus and Ω the mean volume per atom. In addition, the resulting three straight lines are formed to have almost the same slope.

Pressure-Induced Superconducting State of Europium Metal at Low Temperatures

Divalent Eu (4f;{7}, J=7/2) possesses a strong local magnetic moment which suppresses superconductivity. Under sufficient pressure it is anticipated that Eu will become trivalent (4f;{6}, J=0) and a weak Van Vleck paramagnet, thus opening the door for a possible superconducting state, in analogy with Am metal (5f;{6}, J=0) which superconducts at 0.79 K. We present ac susceptibility and electrical resistivity measurements on Eu metal for temperatures 1.5-297 K to pressures as high as 142 GPa. At approximately 80 GPa Eu becomes superconducting at T_{c} approximately 1.8 K; T_{c} increases linearly with pressure to 2.75 K at 142 GPa. Eu metal thus becomes the 53rd known elemental superconductor in the periodic table.

PP/ABS blends with carbon black: Morphology and electrical properties

AbstractPhase morphology of melt‐mixed polypropylene (PP)/acrylonitrile‐butadiene‐styrene (ABS) blends was found to be blend ratio dependent, viz., matrix‐particle dispersed type of morphology which was observed up to 30 wt % of ABS level beyond which the morphology showed co‐continuous type. The domain size of 80/20 PP/ABS blends was found to decrease significantly at 10 wt % carbon black (CB) level, and in case of 70/30 blends morphology was transformed into co‐continuous type in the presence of CB, which was retained up to 60 wt % ABS. The finer morphological features were associated with the compatibilizing action of CB particles. Continuous network was achieved through aggregated CB particles predominantly in the PP phase wherein one could find CB‐rich PP phase and CB‐less PP phase. Solution experiments further supported the existence of CB particles preferentially in the PP phase. AC electrical conductivity measurements indicated a 3D network‐like structure of CB aggregates in the co‐continuous compositions which showed enhanced electrical conductivity as compared to the matrix‐dispersed type of morphology in 80/20 PP/ABS blends which exhibited insulating behavior. On increasing ABS content in the blends the electrical conductivity decreased progressively due to a difficulty in retaining 3D continuous network of CB aggregates especially at 40/60 composition. Increased processing temperature led to a higher electrical conductivity in the respective blends. Dielectric measurements revealed the existence of metallic type of conduction in the co‐continuous compositions. However, 80/20 blends showed low ε′ value. Overall, structure property relationship studies were conducted in PP/ABS blends with CB. © 2009 Wiley Periodicals, Inc. J Appl Polym Sci, 2009

Studies on multiphased mixed crystals of NaCl, KCl and KI

AbstractMultiphased mixed crystals of NaCl, KCl and KI were grown by the melt method, for the first time. Densities and refractive indices of all the grown crystals were determined and used for the estimation of the composition in the crystal. Atomic absorption spectroscopic measurements were done to estimate the metal atom contents in the crystal. Lattice parameters and thermal parameters (Debye‐Waller factor, mean square amplitude of vibration, Debye temperature and Debye frequency) were determined from the X‐ray powder diffraction data. DC and AC electrical measurements were done at various temperatures ranging from 40 to 150°C. Activation energies were also estimated. The observed lattice parameters showed that the system exhibits three phases each nearly corresponds to NaCl, KCl and KI. The thermal and electrical parameters show a highly nonlinear bulk composition dependence. Results are reported. (© 2009 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

Electrical, rheological and morphological studies in co-continuous blends of polyamide 6 and acrylonitrile–butadiene–styrene with multiwall carbon nanotubes prepared by melt blending

Multiwall carbon nanotubes (MWNT) were incorporated in melt-mixed co-continuous blends of polyamide 6 (PA6) and acrylonitrile–butadiene–styrene (ABS) using a conical twin-screw microcompounder. The state of dispersion of MWNT in the blends was assessed through AC electrical conductivity measurements and melt-rheological investigations. The electrical and rheological percolation threshold in PA6/ABS blends was ∼3–4 and ∼1–2 wt% MWNT, respectively. A unique reactive modifier (sodium salt of 6-amino hexanoic acid, Na–AHA) was employed to facilitate ‘network-like’ structure of MWNT and to confine them in a specific phase. This was achieved by establishing specific interactions with the delocalized ‘π-electron’ clouds of MWNT and melt-interfacial reaction during melt-mixing. The electrical percolation threshold was significantly reduced in the blends (∼0.25 wt%) in the presence of Na–AHA modified MWNT and even coincided with the rheological percolation threshold. Significant refinement in the co-continuous structure was also observed in the presence of Na–AHA modified MWNT.

Studies on NaCl Added ADP Single Crystals

Ammonium dihydrogen orthophosphate (ADP) crystal continues to be an interesting nonlinear optical and dielectric material. In order to understand the effect of NaCl as an impurity on the electrical properties of ADP, we have grown from aqueous solutions pure and NaCl added single crystals and electrically characterized. Density and atomic absorption spectroscopic measurements indicate that the impurity molecules have entered into the crystal matrix of ADP. DC and AC electrical measurements were carried out at various temperatures ranging between 40–150°C along both a- and c-directions. Results indicate an increase of the electrical parameters, viz σdc, ϵ r , tanδ, and σac with the increase of temperature which can be attributed mainly to the increase of thermally generated hydrogen bond vacancies (L-defects). Also, the present study indicates that NaCl addition leads to nonlinear variation of electrical parameters of ADP single crystals with impurity concentration. Activation energies were also estimated.

Electrical measurements on multiphased (NaCl)x(KCl)y−x(KBr)1−y single crystals

Abstract Alkali halide mixed crystals were melt grown from NaCl, KCl and KBr starting materials. DC and AC electrical measurements were carried out on the resulting ternary compositions at temperatures ranging from 308 to 423 K. Activation energies and mean jump frequencies were also estimated. The present study indicates an increase of DC and AC electrical conductivities and dielectric constant with the increase of temperature. Also, it indicates a nonlinear variation of all the electrical parameters (both DC and AC) with the bulk composition, which is explained to be due to the enhanced diffusion of charge carriers along dislocations and grain boundaries.

Rheology, electrical conductivity, and the phase behavior of cocontinuous PA6/ABS blends with MWNT: Correlating the aspect ratio of MWNT with the percolation threshold

AbstractMultiwalled carbon nanotubes (purified, p‐MWNT and ∼ NH2 functionalized, f‐MWNT) were melt‐mixed with 50/50 cocontinuous blends of polyamide 6 (PA6) and acrylonitrile–butadiene–styrene in a conical twin‐screw microcompounder to obtain conductive polymer blends utilizing the conceptual approach of double‐percolation. The state of dispersion of the tubes was assessed using AC electrical conductivity measurements and melt‐rheology. The rheological and the electrical percolation threshold was observed to be ∼ 1–2 wt % and ∼ 3–4 wt %, respectively, for blends with p‐MWNT. In case of blends with f‐MWNT, the rheological percolation threshold was observed to be higher (2–3 wt %) than p‐MWNT but the electrical percolation threshold remained almost same. However, the absolute values were significantly lower than blends with p‐MWNT. In addition, significant refinement in the cocontinuous morphology of the blends with increasing concentration of MWNT was observed in both the cases. Further, an attempt was made to understand the underlying concepts in relation to cocontinuous morphologies that how the geometrical percolation threshold which adversely suffered because of the attrition of tubes under prolonged shear contributed further in retaining the rheological percolation threshold. © 2008 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 46: 1619–1631, 2008

Role of structural modification on the electrical properties of poly(ethylene terephthalate) irradiated with 90‐MeV carbon ion beam

AbstractThin films of poly(ethylene terephthalate) (PET) having a thickness of 100 μm were exposed to different ion fluence of swift heavy ions of carbon in the range of 5 × 1011 – 5 × 1013 ions/cm2. The effect of ion beam on structural and electrical modification has been studied by UV/vis, FTIR, X‐ray diffraction (XRD), Differential Scanning Calorimetery (DSC), and AC electrical measurement techniques. On irradiation, a shift in absorption wavelength toward the red end of spectrum with increase of ion fluence was observed. The intensity of crystalline IR bands and main diffraction peak in XRD pattern were found to decrease with increase in ion fluence. It indicates the loss of crystallinity induced by ion‐beam irradiation. The crystallite size was found to increase on irradiation. The melting temperature (Tm) of PET films increased at a low ion dose (5.0 × 1012 ions/cm2), while it decreased at higher ion fluence (50.0 × 1012 ions/cm2). The dielectric constant (ε′) of PET films was increased with increase of ion fluence. The modifications brought about in the dielectric constant are correlated with chemical and molecular structural changes occurring on irradiation. POLYM. ENG. SCI., 2008. © 2008 Society of Plastics Engineers.

Melt Mixed Composites of Poly(ethylene-co-methacrylic acid) Ionomers and Multiwall Carbon Nanotubes: Influence of Specific Interactions

Multiwall carbon nanotubes (MWNT) were melt-mixed with poly(ethylene-co-methacrylic acid) ionomers (Surlyn) using twin screw microcompounder. The specific interactions existing between the Na+ moieties in Surlyn and the π electron clouds of MWNT were supported by FTIR and Raman spectroscopic analysis. SAXS scattering patterns were found to be progressively broadened in presence of MWNT in Surlyn/MWNT composites. Morphological investigations revealed selective clustering of MWNT in the vicinity of the ionic domains in Surlyn. Further, the domain size of the ionic clusters was found to increase with increasing MWNT content disrupting the ionic pairs apart in the ionic domain. The melt rheological response of Surlyn was significantly affected in presence of MWNT and was profoundly dependent on the ionic clusters. The state of dispersion of MWNT was assessed by AC electrical conductivity measurements. The associated percolation threshold was observed between 1.5–2 wt% of MWNT.

Influence of thermal annealing treatment in oxygen atmosphere on grain boundary chemistry and non‐ohmic properties of SnO2·MnO polycrystalline semiconductors

AbstractThe present work studied the influence of thermal treatment in oxygen rich atmosphere on heterogeneous junctions in Mn‐doped SnO2 polycrystalline system presenting varistor behavior. The samples were prepared by conventional oxide mixture methodology, and were submitted to heat treatment in oxygen rich atmosphere at 900 °C for 2 h. The samples were characterized by X‐ray diffraction, scanning electron microscopy, dc and ac electrical measurements. The results showed that there is an evident relationship between the microstructure heterogeneity and non‐ohmic electrical properties. It was found that for this SnO2·MnO‐based varistor system the heat treatment in oxygen rich atmosphere does not necessarily increase the varistors' properties, which was related to the decrease in the grain boundary resistance. The results are compared with Co‐doped SnO2 varistors and ZnO based varistors. (© 2008 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

Growth and electrical characterization of L‐arginine added KDP and ADP single crystals

AbstractPotassium dihydrogen orthophosphate (KDP) and ammonium dihydrogen orthophosphate (ADP) single crystals added with L‐arginine have been grown by the solution methods. DC and AC electrical measurements were carried out at various temperatures along both a‐ and c‐ directions. Results indicate an increase of the electrical parameters with the increase of temperature which can be attributed mainly to the increase of thermally generated hydrogen bond vacancies (L. defects). Also, the present study indicates that L‐arginine addition leads to reduction of electrical parameters of KDP and ADP single crystals which can be attributed mainly to the decrease of L‐defects due to creation of additional hydrogen bonds by the impurity in random directions. (© 2008 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

Ionic conductivity in new perovskite type oxides: NaAZrMO 6 (A = Ca or Sr; M = Nb or Ta)

New oxides of the type, NaAZrMO 6 (M = Ca or Sr; M = Nb or Ta), have been prepared by the solid-state reaction technique. Phase identification by powder X-ray diffraction (XRD) shows that NaCaZrMO 6 has orthorhombic perovskite type structure ( Pnma) and NaSrZrMO 6 has cubic perovskite type structure ( Pm3 m). The grain morphology observation by scanning electron microscope (SEM) shows well-sintered grains. ac impedance spectra and electrical conductivity measurements in air, oxygen and nitrogen atmospheres indicate that they are probable oxide ion conductors with ionic conductivities of the order of 10 −3 S cm −1 at 750 °C.

Growth and Characterization of Multiphased Mixed Crystals of NaBr and KBr

Mixed crystals of NaBr and KBr were prepared from melt and physically characterized. Bulk composition of the crystals was estimated using the measured density values. X-ray diffraction analysis indicates the existence of two phases in the mixed crystals. Thermal parameters determined from the X-ray powder diffraction data are found to vary nonlinearly with bulk composition. DC and AC electrical measurements were carried out at various temperatures. The results indicate that the electrical parameters increase with the increase in temperature and vary nonlinearly with the bulk composition. Activation energies and mean jump frequencies were also determined.

Studies on multiphased mixed crystals grown from NaBr and KCl

We have grown multiphased binary and ternary mixed crystals by the melt method using the miscible alkali halides, viz. NaBr and KCl and physically characterized. Thermal parameters like Debye–Waller factor, Debye temperature, Debye frequency and mean square amplitude of vibration were determined using the X-ray powder diffraction intensity data. DC and AC electrical measurements were carried out by using the parallel plate capacitor method at various temperatures. Activation energies (DC and AC), mean jump frequency, compressibility and mean sound velocity were also determined. The results obtained are reported here.

Spatial and Intensity Modulation of Nanowire Emission Induced by Mobile Charges

Single-molecule optical experiments carried out in conjunction with externally applied electric fields show deliberate spatial and intensity control over CdSe nanowire (NW) emission. In particular, by applying external fields to electrically isolated (single) NWs, their emission can be localized in areas of the wire closest to the positive electrode. In a few cases, the resulting emission intensity increases over the corresponding zero-field value by nearly an order of magnitude. More often than not, factors of 2-3 are seen. Reversing the field polarity causes the emission to localize in opposite regions of the wire. Emission from individual NWs can therefore be modulated. Complementary ac electric field measurements show that the effect persists up to 500 kHz. To explain the phenomenon, the effective passivation of surface trap states by mobile carriers is speculated. This, in turn, causes local changes in the NW emission quantum yield (QY). To verify the presence of such mobile charges, both ensemble and single NW bundle electrophoresis experiments are conducted. By investigating subsequent NW rotational and translational dynamics, an estimate for the number of mobile carriers is determined. A lower limit (best case) linear charge density of approximately 0.45-1.2 mobile electrons per micrometer of the wire is obtained. Apart from self-consistently explaining the field-induced NW emission modulation, the resulting data and subsequent analysis also suggests that the same mobile carriers may be the root cause of NW emission intermittency. Furthermore, given the ubiquity of stray charges, the resulting hypothesis may have additional applicability toward explaining blinking in other systems, a problem of current interest especially within the context of colloidal QDs.

Simultaneous sudden changes of electrical behavior at the threshold in laser diodes

Precise ac electrical measurements as well as dc I-V plots at forward bias have been used to characterize multi-quantum-well (MQW) laser diodes. Step offsets of apparent conductance, apparent capacitance, junction voltage, series resistance, and ideality factor at lasing threshold were observed. To compare the electrical characteristics of different diodes numerical simulations of the IdV∕dI-I curve of the double-heterostructure (DH) laser were performed. We find that the conventional model used to explain this curve is not very satisfactory. Our simulations demonstrate that the junction voltage, series resistance, and ideality factor for both DH and MQW lasers exhibit the same sudden changes at threshold. Perhaps the electrical behavior near threshold for all laser diodes is very similar.

Ac and dc magnetotransport properties of the phase-separated La0.6Y0.1Ca0.3MnO3 manganite

The physical properties of the La0.6Y0.1Ca0.3MnO3 compound have been investigated, focusing on the magnetoresistance phenomenon studied by both dc and ac electrical transport measurements. X-ray diffraction and scanning electron microscopy analysis of ceramic samples prepared by the sol–gel method revealed that specimens are single phase and have average grain size of ∼0.5 μm. Magnetization and 4-probe dc electrical resistivity ρ(T,H) experiments showed that a ferromagnetic transition at TC ∼ 170 K is closely related to a metal-insulator (MI) transition occurring at essentially the same temperature TMI. The magnetoresistance effect was found to be more pronounced at low applied fields (H ≤ 2.5 T) and temperatures close to the MI transition. The ac electrical transport was investigated by impedance spectroscopy Z(f,T,H) under applied magnetic field H up to 1 T. The Z(f,T,H) data exhibited two well-defined relaxation processes that exhibit different behaviors depending on the temperature and applied magnetic field. Pronounced effects were observed close to TC and were associated with the coexistence of clusters with different electronic and magnetic properties. In addition, the appreciable decrease of the electrical permittivity e′(T,H) is consistent with changes in the concentration of eg mobile holes, a feature much more pronounced close to TC.

Magnetic and structural studies of magnetite at the Verwey transition

Studies of the Verwey transition in zinc doped magnetite (Fe 3− x Zn x O 4, x = 0, 0.0097, 0.011, 0.036) by simultaneous ac magnetic susceptibility ( χ) and electric resistance measurements are presented. The temperature hysteresis of the Verwey transition increases from 0.03 K for stoichiometric magnetite to 0.5 K for Zn doped samples, despite the continuous character of the transition. The results of introductory observations of structural changes are also presented.

Thermopower studies of Pr0.67D0.33MnO3 manganite system

With a view to explaining the structural and electrical transport behaviour of some rare earth manganites, having the general formula Pr0.67D0.33MnO3 (D = Ca, Sr, Pb and Ba), a systematic investigation of the electrical resistivity, thermopower and magnetic properties has been undertaken. These materials were prepared using the citrate–gel route by sintering at 900 °C. All the materials were characterized by x-ray diffraction, scanning electron microscopy, etc measurements. The x-ray data were analysed using the Rietveld method and the variation of various parameters involved in the process as explained. The average crystallite size of the materials has been estimated using the peak broadening method, while TC and TP values were determined from ac magnetic susceptibility and electrical resistivity measurements, respectively. Finally, the magnetoresistance (MR) measurements were also carried out over a magnetic field of 1–7 T and the data clearly exhibit the extrinsic MR in ferromagnetic metallic region.The influence of average A-site cation radius ⟨rA⟩ and cation mismatch parameter σ2 on thermoelectric power (TEP), MR, electrical and magnetic transitions has been analysed. The TEP data at low temperatures (T < TP) have been analysed by considering the magnon concepts, while the high temperature (T > TP) data were explained by the Mott's small polaron conduction mechanism. The effects of ⟨rA⟩ and σ2 on polaron activation and hopping energies were also studied systematically.