Browsing by Author "Ge, Wenwei"

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    Alternating and direct current field effects on the structure-property relationships in Na0.5Bi0.5TiO3-x% BaTiO3 textured ceramics
    Ge, Wenwei; Maurya, Deepam; Li, Jiefang; Priya, Shashank; Viehland, Dwight D. (AIP Publishing, 2013-06-01)
    The influence of alternating (ac) and direct current (dc) fields on the structural and dielectric properties of [001](PC) textured Na0.5Bi0.5TiO3-7%BaTiO3 (NBT-7%BT) ceramics has been investigated. X-ray diffraction measurements revealed that the depolarization at temperature T-d in poled samples resulted from a tetragonal -> pseudo-cubic transition on heating. Moderate ac drive and dc bias had opposite influences on T-d: ac drive decreased the T-d, whereas dc bias increased it. These investigations suggested an effective method to expand the working temperature range of NBT-x%BT textured ceramics to a high temperature. (C) 2013 AIP Publishing LLC.
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    Crystallographic direction dependence of direct current field induced strain and phase transitions in Na0.5Bi0.5TiO3-x%BaTiO3 single crystals near the morphotropic phase boundary
    Luo, Chengtao; Ge, Wenwei; Zhang, Qinhui; Li, Jiefang; Luo, Haosu; Viehland, Dwight D. (AIP Publishing, 2012-10-01)
    The crystallographic dependence of the DC electric (E) field induced strain and phase transition in Na0.5Bi0.5TiO3-5.6%BaTiO3 (NBT-5.6%BT) single crystals has been investigated. An induced transition between pseudocubic and tetragonal structures was observed under E = 10 kV/cm for fields applied along the < 001 > direction, whereas an induced transition between pseudocubic and rhombohedral structures was observed when the E-field was applied along < 111 >. Our results show near the morphotropic phase boundary that the phase stability of NBT-x%BT is dependent on the crystallographic direction along which E was applied. (C) 2012 American Institute of Physics. [http://dx.doi.org/10.1063/1.4757877]
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    Effect of Mn substituents on the domain and local structures of Na1/2Bi1/2TiO3-BaTiO3 single crystals near a morphotropic phase boundary
    Yao, Jianjun; Yang, Yaodong; Monsegue, Niven; Li, Yanxi; Li, Jiefang; Zhang, Qinhui; Ge, Wenwei; Luo, Haosu; Viehland, Dwight D. (AIP Publishing, 2011-03-01)
    The ferroelectric domain and local structures of Na1/2Bi1/2TiO3-x% BaTiO3 (NBT-BT) and 0.14 at. % Mn substituted Na1/2Bi1/2TiO3-x% BaTiO3 (Mn:NBT-BT) single crystals with x=5.5 near a morphotropic phase boundary have been investigated by transmission electron microscopy. Increased ferroelectric ordering and enhanced in-plane octahedral tilting were observed for Mn: NBT-BT compared with NBT-BT. Bragg-filtered lattice images revealed that the size of the in-phase tilt domains of Mn:NBT-BT were on the order of 2 to 8 nm, with a tendency of alignment along {110}. (C) 2011 American Institute of Physics. [doi:10.1063/1.3573801]
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    Electric-field-dependent phase volume fractions and enhanced piezoelectricity near the polymorphic phase boundary of (K0.5Na0.5)(1-x)LixNbO3 textured ceramics
    Ge, Wenwei; Li, Jiefang; Viehland, Dwight D.; Chang, Y. F.; Messing, G. L. (American Physical Society, 2011-06-01)
    The structure, ferroelectric and piezoelectric properties of < 001 > textured (K0.5Na0.5)(0.98)Li0.02NbO3 ceramics were investigated as a function of temperature and dc bias E. X-ray diffraction revealed an orthorhombic (O) -> tetragonal (T) polymorphic phase boundary (PPB). Phase coexistence was found near the PPB over a 30 degrees C temperature range, where the relative phase volume fractions changed with temperature. Furthermore, increasing E applied along the < 001 > texture direction resulted in a notable increase in the volume fraction of the T phase at the expense of the O phase, effectively shifting the O -> T boundary to lower temperature. An enhancement in the piezoelectric properties was found to accompany this increase in the T volume fraction.
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    Enhanced piezoelectric and ferroelectric properties in Mn-doped Na0.5Bi0.5TiO3-BaTiO3 single crystals
    Zhang, Qinhui; Zhang, Yaoyao; Wang, Feifei; Wang, Yaojin; Lin, Di; Zhao, Xiangyong; Luo, Haosu; Ge, Wenwei; Viehland, Dwight D. (AIP Publishing, 2009-09-01)
    High piezoelectric and ferroelectric properties have been found in Mn-doped Na0.5Bi0.5TiO3-BaTiO3 single crystals, which were grown by a top-seeded solution method. The electrical resistivity, dielectric constant, and ferroelectric and piezoelectric properties were all found to be notably enhanced by Mn. The piezoelectric constant d(33) and electromechanical coupling coefficients k(t) and k(31) were found to be as high as 483 pC/N, 0.56, and 0.40, respectively. These values are much higher than those previously reported for Pb-free piezoelectric crystals, demonstrating the real potential for alternative lead-free systems for sensor and piezoelectric applications.
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    Evolution of domain structures in Na1/2Bi1/2TiO3 single crystals with BaTiO3
    Yao, Jianjun; Yan, Li; Ge, Wenwei; Luo, L. A.; Li, Jiefang; Viehland, Dwight D.; Zhang, Qinhui; Luo, Haosu (American Physical Society, 2011-02-16)
    The domain structures of Na1/2Bi1/2TiO3-BaxTiO3 (NBT-x%BT) crystals for x = 0, 4.5, and 5.5 have been investigated by polarized light and piezoresponse force microscopies. The results show that BaTiO3 (i) refines the size of polar nanoregions and enhances their self-organization, and (ii) suppresses the formation of proper ferroelastic domains at high temperatures in the paraelectric state, and it rather favors the formation of improper ones that form below the ferroelectric Curie temperature and that elastically accommodate the ferroelectric ones.
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    Hierarchical domains in Na1/2Bi1/2TiO3 single crystals: Ferroelectric phase transformations within the geometrical restrictions of a ferroelastic inheritance
    Yao, Jianjun; Ge, Wenwei; Luo, L.; Li, Jiefang; Viehland, Dwight D.; Luo, Haosu (AIP Publishing, 2010-05-01)
    We have found unique hierarchical domains in Na1/2Bi1/2TiO3 single crystals. A tetragonal ferroelastic domain structure is unchanged on cooling from 520 to 25 degrees C. Polar microdomains then nucleated on cooling in the rhombohedral ferroelectric phase within the geometrical restrictions imposed by the inherited ferroelastic domains. (C) 2010 American Institute of Physics. [doi:10.1063/1.3443717]
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    Influence of dc-bias on phase stability in Mn-doped Na0.5Bi0.5TiO3-5.6 at. %BaTiO3 single crystals
    Ge, Wenwei; Cao, Hu; Li, Jiefang; Viehland, Dwight D.; Zhang, Qinhui; Luo, Haosu (AIP Publishing, 2009-10-01)
    Mn-doped Na0.5Bi0.5TiO3-5.6 at. %BaTiO3 (Mn:NBT-5.6% BT) single crystals were grown by a top-seeded solution growth method. Tetragonal and rhombohedral phases were found to coexist in the as-grown condition. The dielectric and structural properties were studied as a function of temperature under dc electrical bias. An induced phase stability change from rhombohedral to tetragonal phases occurred under an electric-field applied along the < 001 > direction. (C) 2009 American Institute of Physics. [doi:10.1063/1.3253412]
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    The influence of Mn substitution on the local structure of Na0.5Bi0.5TiO3 crystals: Increased ferroelectric ordering and coexisting octahedral tilts
    Yao, Jianjun; Ge, Wenwei; Yan, Li; Reynolds, William T. Jr.; Li, Jiefang; Viehland, Dwight D.; Kiselev, Dmitry A.; Kholkin, Andrei L.; Zhang, Qinhui; Luo, Haosu (American Institute of Physics, 2012-03-15)
    The ferroelectric domain structure of pure Na1/2Bi1/2TiO3 (NBT) and 1 at.% Mn-doped NBT (Mn-NBT) crystals was investigated by piezoresponse force microscopy. The correlation length of the polar regions was found to increase upon Mn substitution. High resolution transmission electron microscopy revealed that the coherency of the lattice across the domain boundaries between polar regions was also enhanced. Selected area electron diffraction showed that Mn favored coexisting 1/2 (ooo) and 1/2 (ooe) oxygen octahedral tiltings, over only 1/2 (ooo) for pure NBT. (C) 2012 American Institute of Physics. [http://dx.doi.org/10.1063/1.3699010]
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    Lead-free and lead-based ABO(3) perovskite relaxors with mixed-valence A-site and B-site disorder: comparative neutron scattering structural study of (Na1/2Bi1/2) TiO3 and Pb(Mg1/3Nb2/3)O-3
    Ge, Wenwei; Devreugd, Christopher P.; Phelan, Daniel; Zhang, Qinhui; Ahart, Muhtar; Li, Jiefang; Luo, Haosu; Boatner, Lynn A.; Viehland, Dwight D.; Gehring, Peter M. (American Physical Society, 2013-11-01)
    We report the results of neutron elastic-scattering measurements made between -250 degrees C and 620 degrees C on the lead-free relaxor (Na1/2Bi1/2)TiO3 (NBT). Strong, anisotropic, elastic diffuse scattering intensity decorates the (100), (110), (111), (200), (210), and (220) Bragg peaks at room temperature. The wave-vector dependence of this diffuse scattering is compared to that in the lead-based relaxor Pb(Mg1/3Nb2/3)O-3 (PMN) to determine if any features might be common to relaxors. Prominent ridges in the elastic diffuse scattering intensity contours that extend along < 110 > are seen that exhibit the same zone dependence as those observed in PMN and other lead-based relaxors. These ridges disappear gradually on heating above the cubic-to-tetragonal phase transition temperature T-CT = 523 degrees C, which is also near the temperature at which the dielectric permittivity begins to deviate from Curie-Weiss behavior. We thus identify the < 110 >-oriented ridges as a relaxor-specific property. The diffuse scattering contours also display narrower ridges oriented along < 100 > that are consistent with the x-ray results of Kreisel et al. [Phys. Rev. B 68, 014113 (2003)]; these vanish near 320 degrees C, indicating that they have a different physical origin. The < 100 >-oriented ridges are not observed in PMN. We observe no equivalent relaxor-specific elastic diffuse scattering from the homovalent relaxor analogues K0.95Li0.05TiO3 (A-site disordered) and KTa0.95Nb0.05O3 (B-site disordered). This suggests that the < 110 >-oriented diffuse scattering ridges are correlated with the presence of strong random electric fields and invites a reassessment of what defines the relaxor phase. We find that doping NBT with 5.6% BaTiO3, a composition close to the morphotropic phase boundary with enhanced piezoelectric properties, increases the room-temperature correlation length along [1 (1) over bar0] from 40 to 60 angstrom while doubling the associated integrated diffuse scattering. Similar behavior was reported by Matsuura et al. [Phys. Rev. B 74, 144107 (2006)] for morphotropic compositions of PMN doped with PbTiO3. Finally, we comment on the recent observation of monoclinicity in NBT at room temperature by placing a strict bound on the strength of the (1/2 1/2 1/2) superlattice reflection associated with the Cc space group based on the atomic coordinates published in the x-ray study by Aksel et al. [Appl. Phys. Lett. 98, 152901 (2011)] for NBT. We show that a skin effect, analogous to that reported in the relaxors PZN-xPT and PMN-xPT, can reconcile our neutron single-crystal data with the x-ray powder data of Aksel et al. [Appl. Phys. Lett. 98, 152901 (2011)]. Our finding of a skin effect in a lead-free, A-site disordered, heterovalent relaxor supports the idea that it arises in the presence of strong random electric fields.
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    Monoclinic M-B phase and phase instability in 110 field cooled Pb(Zn1/3Nb2/3)O-3-4.5%PbTiO3 single crystals
    Yao, Jianjun; Cao, Hu; Ge, Wenwei; Li, Jiefang; Viehland, Dwight D. (AIP Publishing, 2009-08-01)
    We report the finding of a monoclinic M-B phase in Pb(Zn1/3Nb2/3)O-3-4.5%PbTiO3 single crystals. High precision x-ray diffraction investigations of [110] field cooled crystals have shown a transformation sequence of cubic(C)-> tetragonal(T)-> orthorhombic(O)-> monoclinic(M-B), which is different from that previously reported [A.-E. Renault , J. Appl. Phys. 97, 044105 (2005)]. Beginning in the zero-field-cooled condition at 383 K, a rhombohedral (R)-> M-B -> O sequence was observed with increasing field. Coexisting M-B and O phases were then found upon removal of field, which fully transformed to M-B on cooling to room temperature.
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    Monoclinic M-C phase in (001) field cooled BaTiO3 single crystals
    Cao, Hu; Devreugd, Christopher P.; Ge, Wenwei; Li, Jiefang; Viehland, Dwight D.; Luo, Haosu; Zhao, X. Y. (AIP Publishing, 2009-01-01)
    We report here the finding of a new phase in BaTiO3 single crystals. High precision x-ray diffraction investigations of (001) field cooled crystals have shown the presence of a monoclinic M-C phase for temperatures below 300 K. This M-C phase is shown to be stable upon removal of the electric field. The results demonstrate the presence of structurally bridging low symmetry phases that lie between tetragonal and rhombohedral ones in this classic perovskite system: indicating a means to achieve high piezoelectricity in Pb-free systems via domain engineering.
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    A monoclinic-tetragonal ferroelectric phase transition in lead-free (K0.5Na0.5)NbO3-xLiNbO3 solid solution
    Ge, Wenwei; Ren, Yang; Zhang, Jialiang; Devreugd, Christopher P.; Li, Jiefang; Viehland, Dwight D. (American Institute of Physics, 2012-05-15)
    A monoclinic ferroelectric phase with space group Pm has been discovered in lead-free (K0.5Na0.5)NbO3-5%LiNbO3 solid solution ceramics by high energy synchrotron x-ray powder diffraction measurements. At ambient temperature, the lattice parameters of this monoclinic structure were (a(m), b(m), c(m); beta) = (4.015 angstrom, 3.944 angstrom, 3.987 angstrom; 90.34 degrees). This monoclinic phase transformed to a tetragonal (P4mm) one on heating between 340 K and 360K. The results demonstrate the presence of structurally bridging low symmetry monoclinic phase in (K0.5Na0.5)NbO3-x%LiNbO3 solid solution system: indicating a means to achieve high piezoelectricity in Pb-free systems via domain engineering. (C) 2012 American Institute of Physics. [http://dx.doi.org/10.1063/1.4716027]
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    Observation of partially incoherent <110> boundaries between polar nanodomains in Na1/2Bi1/2TiO3 single crystals
    Yao, Jianjun; Ge, Wenwei; Yang, Yaodong; Luo, Liang; Li, Jiefang; Viehland, Dwight D.; Bhattacharyya, Somnath; Zhang, Qinhui; Luo, Haosu (American Institute of Physics, 2010-09-15)
    The ferroelectric domain structure of Na1/2Bi1/2TiO3 single crystals was investigated by transmission electron microscopy (TEM) and piezoforce microscopy (PFM). Bright-field TEM and PFM images revealed the presence of polar nanodomains, whose boundaries had a tendency to align along the < 110 >. High resolution TEM images revealed planar defects along these < 110 > boundaries, which partially relaxed the coherency of the lattice. (C) 2010 American Institute of Physics. [doi:10.1063/1.3488879]
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    Raman spectroscopic study of Na1/2Bi1/2TiO3-x%BaTiO3 single crystals as a function of temperature and composition
    Luo, Liang; Ge, Wenwei; Li, Jiefang; Viehland, Dwight D.; Farley, Charles; Bodnar, Robert J.; Zhang, Qinhui; Luo, Haosu (American Institute of Physics, 2011-06-01)
    A Raman spectroscopic study of Na1/2Bi1/2TiO3-x%BaTiO3 (NBT-x%BT) single crystals with x=0 and 5.6 has been performed as a function of temperature from 25 to 600 degrees C. The general features of the Raman spectra for the various compositions were similar over the region of the phase diagram investigated, with only subtle changes between rhombohedral (R), tetragonal (T) and cubic phases. The peaks were broad, with no significant narrowing on cooling through a phase transition. We find evidence of an oxygen octahedral rotational mode in the paraelectric state. On cooling near and below the ferroelectric Curie temperature, a gradual change in intensity of the A-O and B-O peaks occurred with decreasing temperature. Evidence of a ferroelectric -> antiferroelectric transition was found near 200-250 degrees C, consistent with the onset of dispersion in the dielectric constant. The phase transition mechanism was discussed. The findings indicate the presence of a broad distribution of quasistatic local structural distortions, which only have subtle differences in the various average structures. (C) 2011 American Institute of Physics. [doi: 10.1063/1.3587236]
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    Role of coexisting tetragonal regions in the rhombohedral phase of Na0.5Bi0.5TiO3-xat.%BaTiO3 crystals on enhanced piezoelectric properties on approaching the morphotropic phase boundary
    Yao, Jianjun; Monsegue, Niven; Murayama, Mitsuhiro; Leng, W. N.; Reynolds, William T. Jr.; Zhang, Qinhui; Luo, Haosu; Li, Jiefang; Ge, Wenwei; Viehland, Dwight D. (AIP Publishing, 2012-01-01)
    The ferroelectric domain and local structures of Na0.5Bi0.5TiO3-xat.%BaTiO3 (NBT-x%BT) crystals for x = 0, 4.5, and 5.5 have been investigated by transmission electron microscopy. The results show that the size of polar nano-regions was refined with increasing xat. %BT. The tetragonal phase volume fraction, as identified by in-phase octahedral tilting, was found to be increased with BT. The findings indicate that the large electric field induced strains in morphotropic phase boundary compositions of NBT-x%BT originate not only from polarization rotation but also polarization extension. (C) 2012 American Institute of Physics. [doi: 10.1063/1.3673832]
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    Ultrahigh electromechanical response in (1-x)(Na0.5Bi0.5)TiO3-xBaTiO(3) single-crystals via polarization extension
    Ge, Wenwei; Luo, Chengtao; Zhang, Qinhui; Devreugd, Christopher P.; Ren, Yang; Li, Jiefang; Luo, Haosu; Viehland, Dwight D. (American Institute of Physics, 2012-05-01)
    The dielectric, ferroelectric, and electric field-induced strain response of [001]-and [101]-oriented 0.944Na(0.5)Bi(0.5)TiO(3)-0.056BaTiO(3) (0.944NBT-0.056BT) single crystals were investigated as a function of temperature and dc bias (E). An ultrahigh electromechanical response with large amplitude longitudinal piezoelectric coefficients as high as d(33)=2500 pm/V was found in [001](PC) oriented 0.944NBT-0.056BT single crystals near a depolarization temperature of T-d = 130 degrees C. In-situ XRD revealed that the enhanced piezoelectric properties resulted from a polarization extension between a polar pseudocubic phase with a slight tetragonal (P4bm) distortion and a polar tetragonal one with a large tetragonal distortion of c/a = 1.02. Our findings indicate a potential approach to high performance lead-free piezoelectrics, via polarization extension. (C) 2012 American Institute of Physics. [http://dx.doi.org/10.1063/1.4709619]
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    Volatile and nonvolatile magnetic easy-axis rotation in epitaxial ferromagnetic thin films on ferroelectric single crystal substrates
    Wang, Z.; Wang, Y.; Ge, Wenwei; Li, J.; Viehland, Dwight D. (AIP Publishing, 2013-09-01)
    We explored the relationship between phase transformation and magnetoelectric effect by depositing epitaxial CoFe2O4 films on < 110 > oriented Pb(Mg,Nb)O-3-PbTiO3 (PMN-PT) with three different PbTiO3 contents (PMN-28PT, PMN-29.5PT, and PMN-30PT). Electric-field controlled rhombohedral to orthorhombic phase transformation was confirmed by both piezoelectric and dielectric constant measurements. A giant in-plane (IP) uniaxial strain in CoFe2O4 film was induced due to dramatic lattice parameter change trigged by phase transition. Magnetic easy axis can be rotated from IP < 110 > to IP < 001 >. More importantly, the phase transformation could be either reversible or irreversible, resulting in either volatile or nonvolatile magnetic easy axis rotations. (C) 2013 AIP Publishing LLC.