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  • 1
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    Impact of yttria stabilized zirconia nanoinclusions on the thermal conductivity of n-type Si80Ge20 alloys prepared by spark plasma sintering (2015)
    American Institute of Physics (AIP)
    Details
    Publication Date: 2015-04-11
    Description: Nanocomposites have become a new paradigm for thermoelectric research in recent years and have resulted in the reduction of thermal conductivity via the nano-inclusion and grain boundary scattering. In this work, we report the preparation and thermoelectric study of SiGe-yttria stabilized zirconia (YSZ) nanocomposites prepared by Spark Plasma Sintering (SPS). We experimentally investigated the reduction of lattice thermal conductivity ( κ L ) in the temperature range (30–800 K) of n-type Si 80 Ge 20 P 2 alloys with the incorporation of YSZ nanoparticles (20–40 nm diameter) into the Si-Ge matrix. These samples synthesized by using the SPS technique were found to have densities 〉 95% of the theoretical density. The thermal conductivity, at both low and high temperatures, was measured by steady state and laser flash techniques, respectively. At room temperature, we observed approximately a 50% reduction in the lattice thermal conductivity as result of adding 10% YSZ by volume to the Si 80 Ge 20 P 2 host matrix. A phenomenological model developed by Callaway was used to corroborate both the temperature dependence and reduction of κ L over the measured temperature range (30–800 K) of both Si 80 Ge 20 P 2 and Si 80 Ge 20 P 2  + YSZ samples. The observed κ L is discussed and interpreted in terms of various phonon scattering mechanisms such as alloy disorder, the Umklapp phonon scattering, and boundary scattering. In addition, a contribution from the phonon scattering by YSZ nanoparticles was further included to account for the κ L of Si 80 Ge 20 P 2  + YSZ sample. The theoretical calculations are in reasonably good agreement with the experimental results for both the Si 80 Ge 20 P 2 and Si 80 Ge 20 P 2  + YSZ alloys.
    Print ISSN: 0021-8979
    Electronic ISSN: 1089-7550
    Topics: Physics
    Published by American Institute of Physics (AIP)
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  • 2
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    Near room temperature magnetodielectric consequence in (Li, Ti) doped NiO ceramic (2016)
    American Institute of Physics (AIP)
    Details
    Publication Date: 2016-04-02
    Description: In the quest for high- k dielectrics with decent magnetodielectric (MD) response, ball-milled processed (Li, Ti) doped NiO ceramics have been evaluated by various experimental techniques. Magnetic properties in these ceramics manifest with a pronounced anomaly appearing at ∼260 K, suggesting ferrimagnetic phase (related to cluster magnetism) and co-existence with a glassy-like antiferromagnetic phase at ∼7 K. Room temperature neutron diffraction pattern shows the existence of short-range magnetic correlations. In the magnetically ordered state below 250 K, the magnetic structure is found to be phase coexistence of G -type antiferromagnet and ferrimagnet. Impedance spectroscopy measurements over a wide temperature range can be perfectly described with appropriate microstructural model (internal barrier layer capacitor), based on domain and domain boundary relaxations, justifying the enhancement of the dielectric response. The low-temperature (T 
    Print ISSN: 0021-8979
    Electronic ISSN: 1089-7550
    Topics: Physics
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  • 3
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    Investigation of the cylinder wake under spanwise periodic forcing with a segmented plasma actuator (2015)
    American Institute of Physics (AIP)
    Details
    Publication Date: 2015-01-10
    Description: The wake response to three-dimensional forcing of flow over a circular cylinder was studied. Spanwise-segmented dielectric-barrier discharge plasma actuators were mounted on the cylinder in a square wave pattern for active forcing of the cylinder wake. The buried electrodes were placed periodically to create a spanwise-modulated blowing profile, with the aim of targeting three-dimensional instabilities in the wake. Considerable spanwise variation in the wake was achieved, which was a direct consequence of the difference in the location of shed spanwise vortices from the cylinder, along with the generation of streamwise vorticity. Two distinct power levels were used for forcing the flow, with different flow response observed between the two conditions. With low power, the segmented forcing caused the large-scale spanwise structures in the forcing region to lead those in the no-forcing region, with an accompanying shift away from the centerline and generation of streamwise vorticity. While vortex shedding was not substantially attenuated with low-power forcing, the shedding in the near wake was significantly attenuated with high-power forcing. This attenuation in the shedding strength was accompanied by a decrease in the peak shedding frequency, indicating an increase in the formation length. High-power forcing caused elongation of the Kármán vortices due to the induced strain field and strong differential development of the wake shedding frequency. In both forcing regimes, the wake three-dimensionality increased as shown by the increased width of the spectral peaks.
    Print ISSN: 1070-6631
    Electronic ISSN: 1089-7666
    Topics: Physics
    Published by American Institute of Physics (AIP)
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  • 4
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    Role of phonon scattering by elastic strain field in thermoelectric Sr1−xYxTiO3−δ (2014)
    American Institute of Physics (AIP)
    Details
    Publication Date: 2014-06-13
    Description: Perovskite-type SrTiO 3−δ ceramics are multifunctional materials with significant potential as n-type thermoelectric (TE) materials. The electronic and thermal transport properties of spark plasma sintered polycrystalline Sr 1−x Y x TiO 3−δ (x = 0.05, 0.075, 0.1) ceramics are systematically investigated from (15–800) K. The Sr 0.9 Y 0.1 TiO 3−δ simultaneously exhibits a large Seebeck coefficient, α 〉 −80  μ V/K and moderately high electrical resistivity, ρ ∼ 0.8 mΩ-cm at a carrier concentration of ∼10 21  cm −3 at 300 K resulting in a high TE power factor defined herein as (α 2 σT) ∼ 0.84 W/m-K at 760 K. Despite the similar atomic masses of Sr (87.6 g/mol) and Y (88.9 g/mol), the lattice thermal conductivity ( κ L ) of Sr 1−x Y x TiO 3−δ is significantly reduced with increased Y-doping, owing to the smaller ionic radii of Y 3+ (∼1.23 Å, coordination number 12) compared to Sr 2+ (∼1.44 Å, coordination number 12) ions. In order to understand the thermal conductivity reduction mechanism, the κ L in the Sr 1−x Y x TiO 3−δ series are phenomenologically modeled with a modified Callaway's equation from 30–600 K. Phonon scattering by elastic strain field due to ionic radii mismatch is found to be the prominent scattering mechanism in reducing κ L of these materials. In addition, the effect of Y-doping on the elastic moduli of Sr 1−x Y x TiO 3−δ (x = 0, 0.1) is investigated using resonant ultrasound spectroscopy, which exhibits an anomaly in x = 0.1 in the temperature range 300–600 K. As a result, the phonon mean free path is found to be further reduced in the Sr 0.9 Y 0.1 TiO 3−δ compared to that of SrTiO 3−δ , resulting in a considerably low thermal conductivity κ  ∼ 2.7 W/m-K at 760 K. Finally, we report a thermoelectric figure of merit (ZT) ∼ 0.3 at 760 K in the Sr 0.9 Y 0.1 TiO 3−δ , the highest ZT value reported in the Y-doped SrTiO 3 ceramics thus far.
    Print ISSN: 0021-8979
    Electronic ISSN: 1089-7550
    Topics: Physics
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  • 5
    Electronic Resource
    Electronic Resource
    Structural and transport properties of (Bi,Pb)4Sr3Ca3Cu4−mFemOx (m=0–0.06) glasses: Precursors for high Tc superconductors (1998)
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 108 (1998), S. 2954-2961 
    Details
    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: Microstructure and transport properties (between 80–450 K) of the Fe doped as-quenched Bi3Pb1Sr3Ca3Cu4−mFemOx (m=0.0, 0.02, 0.04, 0.05, and 0.06) type multicomponent glassy precursors for high-temperature superconductors were first reported in this paper. It has been shown from x-ray diffraction, scanning electron microscopy, thermal analysis, density, and oxygen molar volume measurements that single phase homogeneous glasses are formed. Even with small addition of Fe, a large change of glass transition temperature (Tg) indicates appreciable change of glass network structure of the undoped (Bi,Pb)4Sr3Ca3Cu4Ox glass which is reflected in the properties of the corresponding high-temperature annealed glass termed as glass-ceramic (GC) samples. These GC samples are found to be superconductors. Addition of Fe is considered to break up some of the original Cu–O–Cu bonds forming some new Cu–O–Fe and Fe–O–Fe type bonds. A nonlinearity (increase) in the dc conductivity (σdc) of the as-quenched glass sample is observed around m=0.02. This increase of dc conductivity is ascribed to be due to the additional hopping between Fe2+ and Fe3+ which finally destroys superconductivity for m≥0.06. This is also responsible for the nonlinear variation of activation energy of the Fe doped glass and hence change in the superconducting properties of the glass ceramics. The high temperature (above θD/2, θD being the Debye temperature) σdc data of the glasses were explained by considering small polaron hopping conduction mechanism. The dc conductivity of these glasses are found to follow Greaves' variable range hopping model in the intermediate range of temperature. The polaron hopping model of Schnakenberg and Emin can predict the conductivity data in the high-temperature regime. All the Fe-doped glass ceramics showed superconducting transitions with Tc between 100 and 110 K. The corresponding zero resistance temperature, Tco which is maximum (79 K) for m=0, decreases consistently with increase of Fe-concentration and finally superconducting behavior is destroyed for m≥0.06. This behavior is consistent with-pair breaking mechanism. © 1998 American Institute of Physics.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.475682
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  • 6
    Electronic Resource
    Electronic Resource
    Mechanisms of optical response in superconducting YBa2Cu3O7−δ thin films and grain-boundary weak links (1994)
    [S.l.] : American Institute of Physics (AIP)
    Journal of Applied Physics 76 (1994), S. 5829-5838 
    Details
    ISSN: 1089-7550
    Source: AIP Digital Archive
    Topics: Physics
    Notes: The optical response of single grain-boundary weak links in superconducting YBCO thin films has been investigated. At temperatures well below the transition temperature, the grain boundary exhibits an optical response which is qualitatively different from transition edge response. Using a resistively shunted-junction model for the grain-boundary weak link, we find that the optical response below the transition temperature can be described by radiation induced thermal modulation of the critical currents of the weak links. The dependence on the bias current and the temperature distinguishes the weak-link optical response from the transition edge optical response. We discuss novel aspects of a potential radiation detector based on grain-boundary weak-link junctions.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.358396
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  • 7
    Electronic Resource
    Electronic Resource
    Discontinuity of B-diffusion profiles at the interface of polycrystalline Si and single crystal Si (1993)
    [S.l.] : American Institute of Physics (AIP)
    Journal of Applied Physics 73 (1993), S. 3800-3804 
    Details
    ISSN: 1089-7550
    Source: AIP Digital Archive
    Topics: Physics
    Notes: Boron diffusion in polycrystalline Si-on-single crystal Si systems has been studied by secondary ion mass spectrometry. The extrapolated B-diffusion profiles in polycrystalline Si and in the single crystal Si substrate reveal a discontinuity at the polycrystalline Si-single crystal Si interface. The discontinuity in the B profiles is believed to occur due to the blockage of B-defect complexes by the interfacial oxide between polycrystalline Si and the single-crystal Si substrate, as well as the immobility of these defect complexes in single crystal Si. The B in the implant peak region above the B solid solubility limit is found to be immobile in single crystal Si during annealing due to the formation of electrically inactive B-defect complexes. In polycrystalline Si, however, our results show that the B in the peak region spreads out more rapidly than in single crystal Si possibly due to the diffusion of B-defect complexes along grain boundaries. The B-defect complexes are electrically inactive as determined by spreading resistance analysis. If the B concentration is lowered below the solid solubility limit, either by decreasing the dose or by raising the anneal temperature, no discontinuity is observed in the B profile across the polycrystalline Si-single crystal Si interface.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.352886
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  • 8
    Electronic Resource
    Electronic Resource
    Multiphonon hopping conduction in nonconventional chromium-doped Bi3Pb1Sr3Ca3Cu4−nCrnOx (n=0.025–0.2) glasses with nanocrystalline particles and clusters (2000)
    [S.l.] : American Institute of Physics (AIP)
    Journal of Applied Physics 88 (2000), S. 5033-5042 
    Details
    ISSN: 1089-7550
    Source: AIP Digital Archive
    Topics: Physics
    Notes: Transport properties of Cr containing multicomponent oxide glasses Bi3Pb1Sr3Ca3Cu4−nCrnOx (n=0.025, 0.05, 0.1, and 0.2) dispersed with nanocrystalline particles (5–20 nm depending on the values of n) have been reported in the temperature range of 250–450 K. Conductivity of this glass-nanocrystal composite system shows little decrease with increasing Cr content. Above θD/2 (θD is the Debye temperature), conductivity data can be analyzed with small polaron hopping models. Interestingly, unlike undoped Bi4Sr3Ca3Cu4Ox (or Bi-4334) glasses [showing nonadiabatic small polaron hopping (SPH) conduction at T〉θD/2], the Cr doped glasses supports adiabatic SPH conduction mechanism above θD/2 indicating change of glass network structure due to partial substitution of Cu by Cr. But below this temperature Mott's or Greaves' variable range hopping models can be consistently used to fit the experimental conductivity data only with larger (compared to the usual transition metal oxide glasses) values of the density of states at the Fermi level N(EF). The most probable transport mechanism for the entire range of temperature and glass compositions is concluded to be due to multiphonon tunneling of large polarons between the nanoclusters present in the glasses which is also in sharp contrast to the behavior of the undoped (Bi-4334) glass. All the glass samples (except n≥0.2) are found to become superconductors by annealing at higher temperatures. © 2000 American Institute of Physics.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.1317237
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  • 9
    Electronic Resource
    Electronic Resource
    Particle size and magnetic field dependent resistivity and thermoelectric power of La0.5Pb0.5MnO3 above and below metal–insulator transition (2002)
    [S.l.] : American Institute of Physics (AIP)
    Journal of Applied Physics 91 (2002), S. 5125-5134 
    Details
    ISSN: 1089-7550
    Source: AIP Digital Archive
    Topics: Physics
    Notes: The effect of particle size on the transport properties (resistivity and thermopower) of La0.5Pb0.5MnO3 has been investigated both in the presence and in the absence of magnetic field B=0.0–1.5 T (maximum). Grain size, dc conductivity; and the metal–insulator transition temperature Tp of the sample increase with increasing annealing time. Grain size has, however, comparatively little effect on the Seebeck coefficient S. Magnetoresistance is higher for the samples with smaller grain sizes. dc magnetic susceptibility also increases with increasing grain size. High temperature (T〉θD/2) resistivity data well fit the small polaron hopping model. Polaron hopping energy WH decreases but polaron radius rp increases with the increase of grain size. In the metallic regime (for T〈Tp), resistivity data fit well with ρ=ρ0+ρ2.5 T2.5 and the transport mechanism is attributed mainly to the magnon-carrier scattering (∼T2.5). In all the samples with different grain sizes, S changes sign below Tp. In contrast to magnetoresistance, application of magnetic field increases S at low temperature (T〈Tp) for these samples. Thermopower data in the metallic phase (both for B=0.0 and 1.5 T) can be analyzed by considering a spin-wave fluctuation term (∼T4) in addition to the magnon-scattering term similar to the case of resistivity data. Although the variable range hopping mechanism is supported from the resistivity data (for Tp〉T〉θD/2), it is hard to justify this model from the temperature dependent thermopower data. © 2002 American Institute of Physics.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.1459618
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  • 10
    Electronic Resource
    Electronic Resource
    High figure of merit in Eu-filled CoSb3-based skutterudites (2002)
    Woodbury, NY : American Institute of Physics (AIP)
    Applied Physics Letters 80 (2002), S. 598-600 
    Details
    ISSN: 1077-3118
    Source: AIP Digital Archive
    Topics: Physics
    Notes: We report measurements of electrical resistivity, thermopower, thermal conductivity, and Hall coefficient of polycrystalline Eu-doped CoSb3-based skutterudites with compositions Eu0.20Co4Sb12, Eu0.43Co4Sb11.59Ge0.31, and Eu0.42Co4Sb11.37Ge0.50. The relatively high mobility of these compounds, as compared to that of La- and Ce-filled skutterudites, may play a role in the large thermoelectric figure of merit (ZT〉1 at 700 K) of Eu0.42Co4Sb11.37Ge0.50. We discuss the significant potential of these compounds for thermoelectric applications. © 2002 American Institute of Physics.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.1433911
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