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  • 1
    Electronic Resource
    Electronic Resource
    Electrical conduction in CaF2 and CaF2-Al2O3 nanocomposite films on Al2O3 substrates (1993)
    [S.l.] : American Institute of Physics (AIP)
    Journal of Applied Physics 74 (1993), S. 2658-2664 
    Details
    ISSN: 1089-7550
    Source: AIP Digital Archive
    Topics: Physics
    Notes: Thin films of pure CaF2 and nanocomposite mixtures of Al2O3 with CaF2 were evaporated on Al2O3 substrates. Interdigital electrodes on some of the substrates allowed in situ measurements of the electrical conduction in the films during and subsequent to film growth. Electrical conduction was studied as a function of film thickness, film deposition rate, composition, time, and temperature. The electrical conductivity in pure CaF2 adjacent to an Al2O3 interface sometimes exceeded the bulk CaF2 conductivity (i.e., value at more than 50 nm distance) by as much as a factor of 6700 at 200 °C. The high conductivity is characterized by an activation energy of 0.6±0.1 eV, which is significantly lower than the activation energy of about 1.0 eV for conduction in the bulk. However, this high conductivity is thermally unstable and diminishes in time. A high but stable conductivity was obtained in CaF2 films containing about 10 mol % Al2O3 as a dispersed second phase. At 200 °C, a two-phase film gave a factor of 360 enhancement over the measured bulk CaF2 conductivity and a factor of 7 improvement over the best previously reported conductivity for CaF2-Al2O3 composite materials. The origin of enhanced conduction in CaF2 is attributed to ion transport along dislocations. Dislocations anneal with a characteristic log of time dependence that is recognizable in the annealing behavior of the electrical conductivity. Presumably, the addition of a dispersed second phase of Al2O3 to CaF2 serves both to generate and to pin dislocations; the electrical conductance is thereby enhanced and stabilized.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.354657
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  • 2
    Electronic Resource
    Electronic Resource
    Polarization currents in varistors (1990)
    [S.l.] : American Institute of Physics (AIP)
    Journal of Applied Physics 68 (1990), S. 339-346 
    Details
    ISSN: 1089-7550
    Source: AIP Digital Archive
    Topics: Physics
    Notes: The time, voltage, and temperature dependencies of transient polarization currents are reported for two types of varistors (i.e., ZnO and a SiC composite). The current transients exhibit a power-law time response to a step change in voltage (i.e., I≈I0/tm, where m is slightly less than unity) that persists over a time scale exceeding 10−8–104 s. The polarization current increases linearly with low applied voltage, but at more than a linear rate for higher voltage. The temperature dependence of the polarization current in medium voltage ZnO varistors is described by an Arrhenius plot with a change of slope near 200 K, which suggests thermal activation energies of about 160 and 10 meV. The time dependence of the polarization currents is confirmed and extended to short times by the ac admittance measured as a function of frequency. Transient changes in the ac admittance accompany the transient polarization currents, and exhibit time and temperature dependencies that reveal their close relationship to the polarization currents. By comparing transient admittance data to predictions of the Mott–Schottky theory of a barrier, it is concluded that the theory gives an inadequate account of the ac conductance, even though the voltage dependence of the capacitance is predicted well. Theoretical explanations of the polarization currents that are based upon a distribution of exponential relaxation times are examined. A reasonable account of the polarization current is provided, but the origin of the distribution is uncertain. Possible origins are a distribution of thermal activation energies or electron hopping among randomly distributed donors.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.347138
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  • 3
    Electronic Resource
    Electronic Resource
    Laser fabrication of electrodes on zinc oxide varistors (1985)
    [S.l.] : American Institute of Physics (AIP)
    Journal of Applied Physics 57 (1985), S. 5066-5068 
    Details
    ISSN: 1089-7550
    Source: AIP Digital Archive
    Topics: Physics
    Notes: Near surface properties are reported for ZnO varistors irradiated with high-intensity pulses from both KrF excimer and CO2 lasers. Electrical, optical, and Rutherford backscattering measurements reveal that a thin conducting film is formed by pulsed-laser heating of a varistor surface. The conductive film is evaluated as a varistor electrode.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.335284
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  • 4
    Electronic Resource
    Electronic Resource
    Voronoi network model of ZnO varistors with different types of grain boundaries (1996)
    [S.l.] : American Institute of Physics (AIP)
    Journal of Applied Physics 80 (1996), S. 6516-6522 
    Details
    ISSN: 1089-7550
    Source: AIP Digital Archive
    Topics: Physics
    Notes: Electrical transport in zinc oxide varistors is simulated using two-dimensional Voronoi networks. The networks are assumed to contain randomly distributed grain boundaries of three electrical types: (1) high nonlinearity (i.e., "good'') junctions; (2) poor nonlinearity (i.e., "bad'') junctions; and (3) linear with low-resistivity (i.e., ohmic) junctions. These type classifications are those found in experimental measurements. By varying the type concentrations, the simulated current density versus electric field (J–E) characteristics can be made to conform to the different experimentally observed characteristics of ZnO varistors. These characteristics include the sharpness of switching at the transition between ohmic and nonlinear J–E response (i.e., knee region), as well as the degree of nonlinearity. It is shown that the reduction of the nonlinearity coefficient of bulk varistors, relative to that of isolated grain boundaries, can be explained only by the presence of "bad'' varistor junctions. © 1996 American Institute of Physics.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.363645
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  • 5
    Electronic Resource
    Electronic Resource
    Enhanced ionic conduction mechanisms at LiI/Al2O3 interfaces (1996)
    [S.l.] : American Institute of Physics (AIP)
    Journal of Applied Physics 80 (1996), S. 5150-5157 
    Details
    ISSN: 1089-7550
    Source: AIP Digital Archive
    Topics: Physics
    Notes: Electrical conduction in LiI thin films has been measured in situ during growth as a function of film thickness and temperature. Films grown at 27 °C exhibited an enhancement of the ionic conduction of ∼40× the bulk value in a region extending ∼100 nm above the interface. The enhanced conduction was not stable and decreased with a log(time) dependence during annealing at the growth temperature. The activation energy for conduction was 0.47±0.03 eV, approximately equal to the value for extrinsic conduction in bulk LiI. Films grown at 100 °C, on the other hand, showed no enhancement near the interface, and a nearly linear increase in conductance with thickness after an initial nucleation stage. The conductivity of these films was stable at and above the growth temperature and exhibited an activation energy of 0.75±0.03 eV, which is comparable to the value for intrinsic LiI. X-ray diffraction measurements on the samples showed that all films were cubic LiI. Films grown near room temperature had a strong (002) orientation while those grown at elevated temperatures were (111) with a mosaic spread of 〈0.4°. The data show conclusively that there is no enhancement due to space-charge layer formation at the film/substrate interface. On the other hand, the data are all consistent with a model that assumes the conduction enhancement is caused by the overlapping strain fields of dislocations formed during growth.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.363497
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  • 6
    Electronic Resource
    Electronic Resource
    Influence of ohmic grain boundaries in ZnO varistors (1996)
    [S.l.] : American Institute of Physics (AIP)
    Journal of Applied Physics 79 (1996), S. 273-281 
    Details
    ISSN: 1089-7550
    Source: AIP Digital Archive
    Topics: Physics
    Notes: A realistic model of transport properties of zinc oxide varistors is constructed from two-dimensional Voronoi networks and studied via computer simulations. In agreement with experimental microcontact measurements made on individual junctions, the networks are assumed to contain randomly distributed microjunctions of two types: (1) electrically active with highly nonlinear current-voltage (I-V) characteristics and (2) ohmic, i.e., with linear I-V characteristics. Effects of the ohmic grain boundaries in the network are simulated for various concentrations and resistivities. Shapes of the simulated I-V characteristics and current dependence of the coefficient of nonlinearity of the network are in good agreement with those experimentally observed for thin varistor samples and in the measurements employing various surface electrode patterns. It is found that the breakdown voltage of the networks increases with the number of the ohmic grain boundaries, except when their resistivity is so low that it becomes comparable with that of the ZnO grains. The maximal value of the coefficient of nonlinearity of the network is shown to be insensitive to the presence of the ohmic grain boundaries, regardless of their resistivity and concentration. © 1996 American Institute of Physics.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.362708
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  • 7
    Electronic Resource
    Electronic Resource
    Polarization currents in zinc oxide varistors from 77 to 450 K (1994)
    [S.l.] : American Institute of Physics (AIP)
    Journal of Applied Physics 76 (1994), S. 7367-7371 
    Details
    ISSN: 1089-7550
    Source: AIP Digital Archive
    Topics: Physics
    Notes: The time and temperature dependencies of polarization currents are investigated to higher temperatures and with greater temperature resolution than has been reported previously for zinc oxide varistors. Arrhenius plots of these transient polarization currents yield two thermal activation energies near 140 and 8 meV, which are in reasonable agreement with values reported recently, as well as an additional thermally activated level at 0.66 eV, which becomes evident in the extension of the measurements to T(approximately-greater-than)300 K. Conductance data in this higher temperature range yield the nearly identical energy of 0.68 eV for the intergranular (Schottky) barrier height. The temperature dependence of the exponent m in the power-law expression for the current-time dependence I=I0t−m was carefully examined and found to change twice from 0.5 to 1.0. The nonmonotonic behavior of m(T) can be interpreted as due to resolution of polarization currents from two different traps, each of which evolves from diffusive to dispersive transport as temperature is lowered rather than, for example, a complicated distribution of relaxation times. © 1994 American Institute of Physics.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.357960
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  • 8
    Electronic Resource
    Electronic Resource
    Optical functions of silicon at elevated temperatures (1994)
    [S.l.] : American Institute of Physics (AIP)
    Journal of Applied Physics 76 (1994), S. 3758-3761 
    Details
    ISSN: 1089-7550
    Source: AIP Digital Archive
    Topics: Physics
    Notes: The optical functions of silicon have been measured accurately at elevated temperatures using the two-channel spectroscopic polarization modulation ellipsometer. The wavelength region covered is 240–840 nm (5.16–1.47 eV), and the temperature region covered is room temperature to 490 °C. Using this data, the refractive index n and the extinction coefficient k are both parameterized as functions of temperature T and photon energy E for photon energies below the direct band edge of silicon (∼3.36 eV or 370 nm). In this range, n(E,T) can be fit with five parameters, and k(E,T) can be fit with six parameters.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.357378
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  • 9
    Electronic Resource
    Electronic Resource
    Parameterization of the optical functions of amorphous materials in the interband region (1996)
    Woodbury, NY : American Institute of Physics (AIP)
    Applied Physics Letters 69 (1996), S. 371-373 
    Details
    ISSN: 1077-3118
    Source: AIP Digital Archive
    Topics: Physics
    Notes: A parameterization of the optical functions of amorphous semiconductors and insulators is presented in which the imaginary part of the dielectric function ε2 is determined by multiplying the Tauc joint density of states by the ε2 obtained from the Lorentz oscillator model. The real part of the dielectric function ε1 is calculated from ε2 using Kramers–Kronig integration. The parameters of this model are fit to n and k data for amorphous Si (2 data sets), SiO, As2S3, and Si3N4. Comparative fits are made with a similar parameterization presented earlier by Forouhi and Bloomer [Phys. Rev. B 34, 7018 (1986)]. In all cases, the new parameterization fits the data better. © 1996 American Institute of Physics.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.118064
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  • 10
    Electronic Resource
    Electronic Resource
    New varistor material (1988)
    [S.l.] : American Institute of Physics (AIP)
    Journal of Applied Physics 64 (1988), S. 4229-4232 
    Details
    ISSN: 1089-7550
    Source: AIP Digital Archive
    Topics: Physics
    Notes: A composite varistor material containing silicon carbide, a conductor, and an insulator has been developed. The material has rubberlike flexibility and is easily formed. A higher leakage resistivity (∼1012 Ω cm) and greater nonlinearity (∼10) than for silicon carbide varistors permit the material to be used as a gapless surge suppressor. The breakdown voltage (1–10 kV/cm) and other properties of the material vary with composition. High current capability (〉200 A/cm2) and good energy absorption (〉40 J/cm3) are obtained. Also, the material exhibits a low-temperature coefficient (∼4×10−3/K) and a low dielectric constant (∼10) with no observed loss peak.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.341289
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