Browsing by Author "Kumar, Amit"

Now showing 1 - 6 of 6
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    Giant energy density in 001 -textured Pb(Mg1/3Nb2/3)O-3-PbZrO3-PbTiO3 piezoelectric ceramics
    Yan, Yongke; Cho, Kyung-Hoon; Maurya, Deepam; Kumar, Amit; Kalinin, Sergei; Khachaturyan, Armen G.; Priya, Shashank (AIP Publishing, 2013-01-01)
    Pb(Zr,Ti)O-3 (PZT) based compositions have been challenging to texture or grow in a single crystal form due to the incongruent melting point of ZrO2. Here we demonstrate the method for achieving 90% textured PZT-based ceramics and further show that it can provide highest known energy density in piezoelectric materials through enhancement of piezoelectric charge and voltage coefficients (d and g). Our method provides more than similar to 5x increase in the ratio d(textured)/d(random). A giant magnitude of d.g coefficient with value of 59 000 x 10(-15) m(2) N-1 (comparable to that of the single crystal counterpart and 359% higher than that of the best commercial compositions) was obtained. (C) 2013 American Institute of Physics. [http://dx.doi.org/10.1063/1.4789854]
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    Morphology, Crystallization and Melting Behavior of Statistical Copolymers of Propylene with Ethylene, 1-Butene, 1-Hexene and 1-Octene
    Kumar, Amit (Virginia Tech, 2001-06-12)
    In this thesis, the morphology, crystallization and melting behavior of polypropylene copolymers (propylene/ethylene, propylene/1-butene, propylene/1-hexene and propylene/1-octene) has been examined. The multiple melting behavior has been correlated with the presence of alpha and gamma phases and with the occurrence of cross-hatching morphology. The Crystallization and melting behavior of propylene/ethylene and propylne/1-butene are qualitatively similar and compatible with the expectations that the ethylene and butene comonomers are partly included in the propylene lattice. Propylene/1-hexene and propylene/1-octene copolymers exhibit remarkably similar behavior and morphologies consistent with the expectations that the hexene and the octene comonomers are rigorously excluded from the crystal lattice. The origin and the crystallization time dependence of the multiple melting behavior is very different for these two pairs of copolymers. For PE and PB it is consistent with the melting of parent a-phase lamellae and alpha or gamma-phase daughter lamellae. For PH and PO copolymers it is very similar to that observed for ethylene-octene(EO) and ethylene-styrene(ES) copolymers and compatible with the melting of primary lamellae and secondary mosaic or fringed micellar structures.
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    Phase transition and temperature stability of piezoelectric properties in Mn-modified Pb(Mg1/3Nb2/3)O-3-PbZrO3-PbTiO3 ceramics
    Yan, Yongke; Kumar, Amit; Correa, Margarita; Cho, Kyung-Hoon; Katiyar, Ram S.; Priya, Shashank (AIP Publishing, 2012-04-01)
    This study investigates the effect of two different Mn modifiers [MnO2 and Pb(Mn1/3Nb2/3)O-3(PMnN)] on the of phase transitions in Pb(Mg1/3Nb2/3)O-3-PbZrO3-PbTiO3 ceramics. The temperature dependence of polarization derived from measured pyroelectric current indicated change in nature of phase transition with MnO2 doping. This phenomenon was supported by the temperature evolution of the linear softening of low lying hard lattice mode as revealed by Raman analysis. The grain size was found to increase with MnO2 doping (5X) while decrease with PMnN modification (0.5X). Interestingly, the piezoelectric constant of MnO2 modified composition showed negligible degradation (<1%) even after heat treatment very close to the ferroelectric-paraelectric transition temperature. (C) 2012 American Institute of Physics. [http://dx.doi.org/10.1063/1.3703124]
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    Phonon anomalies and phonon-spin coupling in oriented PbFe0.5Nb0.5O3 thin films
    Correa, Margarita; Kumar, Amit; Priya, Shashank; Katiyar, Ram S.; Scott, J. F. (American Physical Society, 2011-01-18)
    We present Raman data on both single-crystal and thin-film samples of the multiferroic PbFe0.5Nb0.5O3 (PFN). We show first that the number and selection rules of Raman lines are compatible with a face-centered-cubic Fm-3m structure, as is known in other ABO(3) relaxors, such as PbSc1/2Ta1/2O3. We then compare Raman data with anomalies in magnetization and the dielectric constant near the magnetic-phase-transition temperature (T-N), the diffuse ferroelectric-phase-transition temperature (T-m), and the pseudostructural-phase transition temperature (Burns temperature similar to T-B). The temperature evolution of the Raman spectra for the PFN film shows measurable changes in phonon positions, intensities, and full width at half maxima near 200, 410, and 650 K-temperatures that match well with experimentally observed T-N, T-m, and T-B, respectively. The increase in frequency with increasing temperature for the lowest-energy F-2g phonon mode is particularly unexpected. These changes suggest the transition of the crystal structure from an ordered phase to a disordered phase near T-B. The Raman study revealed phonon anomalies in the vicinity of T-m and T-N that are attributed to the dynamical behavior of polar nanoregions and spin-phonon coupling owing to its relaxor and multiferroic nature, respectively, which is well supported by dielectric and magnetic properties of the PFN thin film. Softening of the Fe-O mode was observed near the T-N. We correlate the anomalous shift of the Fe-O mode frequency with the normalized square of the magnetization sublattice; agreement with the experimental results suggests strong spin-phonon coupling near T-N owing to phonon modulation of the superexchange integral; however, the shifts in frequency with temperature are small (<3 cm(-1)).
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    Room temperature multiferroic effects in superlattice nanocapacitors
    Dussan, S.; Kumar, Amit; Scott, J. F.; Priya, Shashank; Katiyar, Ram S. (AIP Publishing, 2010-12-01)
    A composite nanocapacitor was fabricated based on ferroelectric PbZr(0.52)Ti(0.48)O(3)( PZT) and half-metallic oxide La(0.67)Sr(0.33)MnO(3)(LSMO) with 5 nm/l nm periodicity, respectively. X-ray theta-2 theta scan and Phi-scan revealed epitaxial growth of nanocapacitors. At room temperature, a microscopic polarization of 11 mu C/cm(2) and magnetization of 50 emu/cm(3) were measured for superlattice nanocapacitors. Local piezo force microscopy measurements revealed switching of polarization under external bias field confirming ferroelectric behavior. Zero field cooling measurements showed the existence of cusp in magnetization at low temperatures indicating spin-glass-like behavior contrary to Pb( Zr(x)Ti(1-x))O(3) /La(x)Sr(1-x)MnO(3) bilayer structure. Frequency dependent dielectric anomaly was observed near room temperature suggesting dynamic magneto (resistance)-dielectric coupling. (C) 2010 American Institute of Physics. [doi:10.1063/1.3528210]
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    Two-phonon coupling to the antiferromagnetic phase transition in multiferroic BiFeO(3)
    Ramirez, Mariola O.; Krishnamurthi, M.; Denev, S.; Kumar, Amit; Yang, Seung-Yeul; Chu, Ying-Hao; Saiz, Eduardo; Seidel, Jan; Pyatakov, Alexander P.; Bush, A.; Viehland, Dwight D.; Orenstein, J.; Ramesh, R.; Gopalan, Venkatraman (AIP Publishing, 2008-01-01)
    A prominent band centered at similar to 1000-1300 cm(-1) and associated with resonant enhancement of two-phonon Raman scattering is reported in multiferroic BiFeO(3) thin films and single crystals. A strong anomaly in this band occurs at the antiferromagnetic Neel temperature, T(N)similar to 375 degrees C. This band is composed of three peaks, assigned to 2A(4), 2E(8), and 2E(9) Raman modes. While all three peaks were found to be sensitive to the antiferromagnetic phase transition, the 2E(8) mode, in particular, nearly disappears at T(N) on heating, indicating a strong spin-two-phonon coupling in BiFeO(3). (c) 2008 American Institute of Physics.