Browsing by Author "Cao, Hu"

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    Conformal sensor skin approach to the safety-monitoring of H-2 fuel tanks
    Dong, Shuxiang; Cao, Hu; Bai, Feiming; Yan, Li; Li, Jiefang; Viehland, Dwight D.; Gao, Y. K. (AIP Publishing, 2004-05-01)
    A conformal sensor skin approach has been developed for safety monitoring of H-2 fuel tanks. Small piezoelectrically driven sound resonance cavities were embedded in a porous polymer. When placed on a structural composite plate, it was found feasible to detect the leakage of small concentrations of H-2 in real time. (C) 2004 American Institute of Physics.
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    Correlation between non-180 degrees domain structures in (1-x)PbA(1/3)Nb(2/3)O(3)-xPbTiO(3) single crystals (A=Mg or Zn) under an applied 001 electric field
    Topolov, V. Yu; Cao, Hu; Viehland, Dwight D. (American Institute of Physics, 2007-07-15)
    This work is devoted to the study of heterophase states in (1-x)PbA(1/3)Nb(2/3)O(3)-xPbTiO(3) or PAN-xPT (A=Mg or Zn) single crystals under an electric field E parallel to[001]. The possibility of elastically matched phases in the transformational sequences of Pm3m -> P4mm -> Cm, Pm3m -> Cm, and P4mm -> Cm are considered with regards to non-180 degrees domains and changes in lattice parameters. Systems of diagrams linking domain types with interfaces between coexisting polydomain phases in PMN-0.28PT and PZN-0.045PT crystals are given. An important role of an intermediate P4mm phase in achieving stress accommodation has been identified by analysis of lattice parameters of PMN-0.28PT, and the elastic compatibility conditions between non-180 degrees domain structures in the P4mm and Cm phases have been determined for various heterophase states. Two scenarios of changes in the lattice parameter a(T) in the P4mm phase of PMN-0.28PT have first been discussed to reveal possibilities of stress accommodation in heterophase states. It has been shown that the correlation between domain structures in different heterophase states of studied crystals stems from coordinated unit-cell behavior under E parallel to[001].
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    Direct high-resolution transmission electron microscopy observation of tetragonal nanotwins within the monoclinic M(C) phase of Pb(Mg(1/3)Nb(2/3))O(3)-0.35PbTiO(3) crystals
    Bhattacharyya, Somnath; Jinschek, J. R.; Cao, Hu; Wang, Yu. U.; Li, Jiefang; Viehland, Dwight D. (AIP Publishing, 2008-04-01)
    We report on the direct observation of tetragonal nanodomains within an average monoclinic M(C) phase of Pb(Mg(1/3)Nb(2/3))O(3)-0.35PbTiO(3) single crystals by high-resolution transmission electron microscopy. These nanodomains are geometrically arranged in alternating layers of twins. The findings are consistent with the fundamental underlying assumptions of the ferroelectric adaptive phase theory.
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    Dynamic origin of the morphotropic phase boundary: Soft modes and phase instability in 0.68Pb(Mg(1/3)Nb(2/3)O(3))-0.32PbTiO(3)
    Cao, Hu; Stock, Chris; Xu, Guangyong; Gehring, Peter M.; Li, Jiefang; Viehland, Dwight D. (American Physical Society, 2008-09-04)
    We report neutron inelastic scattering and high-resolution x-ray diffraction measurements on single crystal 0.68Pb(Mg(1/3)Nb(2/3)O(3))-0.32PbTiO(3)) (PMN-0.32PT), a relaxor ferroelectric material that lies within the compositional range of the morphotropic phase boundary (MPB) where the piezoelectric properties of PMN-xPT compounds are close to maximal. Data were obtained between 100 and 600 K under zero and nonzero electric field applied along the cubic [001] direction. The lowest-energy, zone-center, transverse optic phonon is strongly damped and softens slowly at high temperature; however, the square of the soft-mode energy begins to increase linearly with temperature as in a conventional ferroelectric, which we term the soft-mode "recovery," upon cooling into the tetragonal phase at T(C). Our data show that the soft mode in PMN-0.32PT behaves almost identically to that in pure PMN, exhibiting the same temperature dependence and recovery temperature even though PMN exhibits no well-defined structural transition (no T(C)). The temperature dependence of the soft mode in PMN-0.32PT is also similar to that in PMN-0.60PT; however, in PMN-0.60PT the recovery temperature equals T(C). These results suggest that the temperature dependence and the energy scale of the soft-mode dynamics in PMN-xPT are independent of concentration on the Ti-poor side of the MPB, but scale with T(C) for Ti-rich compositions. Thus the MPB may be defined in lattice dynamical terms as the concentration where T(C) first matches the recovery temperature of the soft mode. High-resolution x-ray studies show that the cubic-to-ferroelectric phase boundary shifts to higher temperatures by an abnormal amount within the MPB region in the presence of an electric field. This suggests that an unusual instability exists within the apparently cubic phase at the MPB.
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    Electric-field-induced orthorhombic to monoclinic M-B phase transition in 111 electric field cooled Pb(Mg1/3Nb2/3O3)-30%PbTiO3 crystals
    Cao, Hu; Li, Jiefang; Viehland, Dwight D. (American Institute of Physics, 2006-10-15)
    Structural phase transformations of [111] electric field cooled (FC) Pb(Mg1/3Nb2/3)O-3-0.30PbTiO(3) crystals have been performed by x-ray diffraction. A phase sequence of cubic (C)-> tetragonal (T)-> orthorhombic (O)-> monoclinic (M) was found in the FC condition. The finding of a=b>c demonstrates that the M phase is of the M-B type. (c) 2006 American Institute of Physics.
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    Epitaxially induced high temperature (> 900 K) cubic-tetragonal structural phase transition in BaTiO3 thin films
    Bai, Feiming; Zheng, H. M.; Cao, Hu; Cross, L. E.; Ramesh, R.; Li, Jiefang; Viehland, Dwight D. (AIP Publishing, 2004-11-01)
    For (001)(c) oriented BaTiO3 thin films, it has been found that epitaxial constraint can result in a dramatic increase in the temperature of a tetragonal (T) structural phase transition. For 2000-Angstrom-thick films grown directly on SrTiO3 substrates, a T-->cubic (C) phase transition was found on heating at >950 K, where the lattice constant changed smoothly with temperature. It was also found for films of the same thickness that the T-->C transition is nearly restored to that of bulk crystals by the use of a buffer layer, which relaxes the epitaxial constraint. These results provide evidence of an epitaxially induced high temperature structural phase transition in BaTiO3 thin films, where the ferroelectric (internal) and structural (external) aspects of the phase transition are decoupled. (C) 2004 American Institute of Physics.
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    Fragile phase stability in (1-x)Pb(Mg1/3Nb2/3O3)-xPbTiO(3) crystals: A comparison of 001 and 110 field-cooled phase diagrams
    Cao, Hu; Li, Jiefang; Viehland, Dwight D.; Xu, Guangyong (American Physical Society, 2006-05-09)
    Phase diagrams of [001] and [110] field-cooled (FC) (1-x)Pb(Mg1/3Nb2/3O3)-xPbTiO(3) or PMN-xPT (0.15 <= x <= 0.38) crystals have been constructed, based on high-resolution x-ray diffraction data. Comparisons reveal several interesting findings. First, a region of abnormal thermal expansion (c not equal a) above the dielectric maximum was found, whose stability range extended to higher temperatures on application of electric field (E). Second, the rhombohedral (R) phase of the zero-field-cooled (ZFC) state was replaced by monoclinic M-A in the [001] FC diagram, but with monoclinic M-B in the [110] FC. Third, the monoclinic M-C phase in the ZFC and [001] FC diagram was replaced by an orthorhombic (O) phase in the [110] FC diagram. Finally, in the [001] FC diagram, the phase boundary between tetragonal (T) and M-A phases was extended to lower PT content (x=0.25); whereas in the [110] FC diagram, this extended region was entirely replaced by the O phase. These results clearly demonstrate that the phase stability of PMN-xPT crystals is quite fragile-depending not only on modest changes in E, but also on the direction along which E is applied.
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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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    Intermediate ferroelectric orthorhombic and monoclinic M-B phases in [110] electric-field-cooled Pb(Mg1/3Nb2/3)O-3-30%PbTiO3 crystals
    Cao, Hu; Bai, Feiming; Wang, Naigang; Li, Jiefang; Viehland, Dwight D.; Xu, Guangyong; Shirane, Gen (American Physical Society, 2005-08-01)
    Structural phase transformations of [110] electric-field-cooled Pb(Mg1/3Nb2/3)O-3-30%PbTiO3 (PMN-30%PT) crystals have been performed by x-ray diffraction. A phase sequence of cubic (C)-> tetragonal (T)-> orthorhombic (O)-> monoclinic (M-B) was found on field cooling (FC); and a rhombohedral (R)-> M-B-> O sequence was observed with increasing field, beginning from the zero-field-cooled condition at room temperature. The application of the [110] electric field induced a dramatic change in the phase sequence in the FC condition, compared to the corresponding data for PMN-30%PT crystals in a [001] field, which shows that the phase sequence in the FC condition is altered by the crystallographic direction along which a modest electric field (E) is applied. Only when E is applied along [110] are intermediate O and M-B phases observed.
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    Low symmetry phase in Pb(Zr0.52Ti0.48)O-3 epitaxial thin films with enhanced ferroelectric properties
    Yan, Li; Li, Jiefang; Cao, Hu; Viehland, Dwight D. (AIP Publishing, 2006-12-01)
    The authors report the structural and ferroelectric properties of Pb(Zr0.52Ti0.48)O-3 (PZT) epitaxial thin films grown on (001), (110), and (111) SrRuO3/SrTiO3 substrates by pulsed laser deposition. A monoclinic C (M-c) phase has been found for (101) films, whereas (001) and (111) ones were tetragonal (T) and rhombohedral (R), respectively. The authors find that the ferroelectric polarization of the M-c phase is higher than that in either the T or R ones. These results are consistent with predictions (i) of epitaxial phase diagrams and (ii) that the enhanced ferroelectric properties of morphotropic phase boundary PZT are related to a low symmetry monoclinic phase. (c) 2006 American Institute of Physics.
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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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    Monoclinic M-C vs orthorhombic in 001 and 110 electric-field-cooled Pb(Mg1/3Nb2/3O3)-35%PbTiO3 crystals
    Cao, Hu; Li, Jiefang; Viehland, Dwight D.; Xu, Guangyong; Shirane, Gen (AIP Publishing, 2006-02-01)
    Structural phase transformations in Pb(Mg1/3Nb2/3O3)-35%PbTiO3 crystals with an electric field (E) applied along [110] and [001] directions have been performed by x-ray diffraction. In the field-cooling process, a phase transformation sequence of Cubic(C)-> Tetragonal(T)-> Orthorhombic(O) was found for E//[110]; whereas a sequence of C -> T -> monoclinic(M-C) was found for E//[001]. Our results establish that the stability of M-C relative to O (or limiting M-C) is altered by the direction along which E is applied.
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    Nanodispersed DO(3)-phase nanostructures observed in magnetostrictive Fe-19% Ga Galfenol alloys
    Bhattacharyya, Somnath; Jinschek, J. R.; Khachaturyan, Armen G.; Cao, Hu; Li, Jiefang; Viehland, Dwight D. (American Physical Society, 2008-03-10)
    Few nanometer large (<2 nm) inclusions of DO(3)-phase structure have been identified in the A2 matrix of highly magnetostrictive Fe-19% Ga alloys by high-resolution transmission electron microscopy (HRTEM). In addition, we have found that these nanostructures include a high density of {100} line defects with a Burgers vector of a(bcc)/2 < 100 >. This dispersion of DO(3)-phase nanostructures formed within the main A2 matrix and the lifting of their coherency by defects are consistent with a recent theory for the structure and properties of magnetostrictive Fe-Ga and Fe-Al alloys.
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    Neutron and x-ray diffraction study of cubic [111] field-cooled Pb(Mg1/3Nb2/3)O-3
    Stock, Chris; Xu, Guangyong; Gehring, Peter M.; Luo, Hongjie; Zhao, X.; Cao, Hu; Li, Jiefang; Viehland, Dwight D.; Shirane, Gen (American Physical Society, 2007-08-01)
    Neutron and x-ray diffraction techniques have been used to study the competing long- and short-range polar order in the relaxor ferroelectric Pb(Mg1/3Nb2/3)O-3 (PMN) under a [111] applied electric field. Despite reports of a structural transition from a cubic phase to a rhombohedral phase for fields E>1.7 kV/cm, we find that the bulk unit cell remains cubic (within a sensitivity of 90 degrees-alpha=0.03 degrees) for fields up to 8 kV/cm. Furthermore, we observe a structural transition confined to the near surface volume or "skin" of the crystal where the cubic cell is transformed to a rhombohedral unit cell at T-c=210 K for E>4 kV/cm, for which 90 degrees-alpha=0.08 +/- 0.03 degrees below 50 K. While the bulk unit cell remains cubic, a suppression of the diffuse scattering and concomitant enhancement of the Bragg peak intensity is observed below T-c=210 K, indicating a more ordered structure with increasing electric field yet an absence of a long-range ferroelectric ground state in the bulk. The electric field strength has little effect on the diffuse scattering above T-c, however, below T-c the diffuse scattering is reduced in intensity and adopts an asymmetric line shape in reciprocal space. The absence of hysteresis in our neutron measurements (on the bulk) and the presence of two distinct temperature scales suggests that the ground state of PMN is not a frozen glassy phase as suggested by some theories but is better understood in terms of random fields introduced through the presence of structural disorder. Based on these results, we also suggest that PMN represents an extreme example of the two-length scale problem, and that the presence of a distinct skin may be necessary for a relaxor ground state.
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    Phase transformations in highly electrostrictive and magnetostrictive crystals: structural heterogeneity and history dependent phase stability
    Cao, Hu (Virginia Tech, 2008-05-08)
    Ferroelectric and ferromagnetic materials have been extensively studied for potential applications in sensors, actuators and transducers. Highly electrostrictive (1-x)Pb(Mg1/3Nb2/3)-xPbTiO₃ (PMN-xPT) and highly magnetostrictive Fe-xat.%Ga are two such novel materials. Both materials systems have chemical disorders and structural inhomogeneity on a microscale, giving rise to an interesting diversity of crystal structures and novel macroscopic physical properties, which are dependent on thermal and electrical histories of the crystals. In this thesis, I have to investigated phase transformations in these two systems under thermal and field (electric/magnetic) histories, using x-ray and neutron scattering techniques. In PMN-xPT crystals, x-ray and neutron diffractions were performed along the different crystallographic orientations and for different thermal and electrical histories. Various intermediate monoclinic (M) phases that structurally “bridge” the rhombohedral (R) and tetragonal (T) ones across a morphtropic phase boundary (MPB) have been observed. Systematic investigations of (001) and (110) electric (E) field-temperature phase diagrams of PMN-xPT crystals have demonstrated that the phase stability of PMN-xPT crystals is quite fragile: depending not only on modest changes in E (≤ 0.5kV/cm), but also on the direction along which E is applied. Structurally bridging monoclinic Mc or orthorhombic (O) phases were found to be associated with the T phase, whereas the monoclinic Ma or Mb phases bridged the Cubic (C) and R ones. In addition, neutron inelastic scattering was performed on PMN-0.32PT to study the dynamic origin of the MPB. Data were obtained between 100 and 600 K under various E applied along the cubic [001] direction. The lowest frequency zone-center, transverse optic phonon was strongly damped and softened over a wide temperature range, but started to recover on cooling into the T phase at the Curie temperature (TC). Comparisons of my findings with prior ones for PMN and PMN-0.60PT suggest that the temperature dependence and energy scales of the soft mode dynamics in PMN-xPT are independent of PT concentration below the MPB, and that the MPB may be defined in composition space x when TC matches the temperature at which the soft mode frequency begins to recover. High-resolution x-ray studies then showed that the C–T phase boundary shifted to higher temperatures under E by an expected amount within the MPB region: suggesting an unusual instability within the apparently cubic phase at the MPB. In Fe-xat.%Ga alloys, the addition of Ga atoms into the b.c.c. α-Fe phase also results in diversity of crystal structures and structural inhomogeneity, which are likely the source of its unusual magneto-elastic properties. I have carefully investigated decomposition of Fe-xat.%Ga alloys subjected to different thermal treatments by x-ray and neutron diffraction for 12 < x < 25. Quenching was found to suppress the formation of a DO₃ structure in favor of a high-temperature disordered bcc (A2) one. By contrast, annealing produced a two-phase mixture of A2 + DO₃ for 14 < x < 20 and a fully DO₃ phase for x = 25. A splitting of the (2 0 0) and (0 0 2) Bragg peaks observed along the respective transverse directions indicated that Fe-xat.%Ga –crystals' are composed of several crystal grain orientations (or texture structures), which are slightly tilted with respect to each other. In order to investigate the local structural distortions and heterogeneities, neutron diffuse scattering was performed on Fe-x%Ga alloys for different thermal conditions. Diffuse scattering around a (100) superlattice reflection was found for 14 < x < 22 in the furnace-cooled condition, indicative of short-range ordered DO₃ nanoprecipitates in an A2 matrix. This diffuse intensity had an asymmetric radial contour and an off-centering. Analysis (x=19) revealed two broad peaks with c/a–1.2: indicating that the DO₃-like nanoprecipitates are not cubic, but rather of lower symmetry with a large elastic strain. The strongest diffuse scattering was observed for x=19, which correspondingly had maximum magnetostriction: indicating a structural origin for enhanced magnetostriction.
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    Role of Nanoscale Precipitates on the Enhanced Magnetostriction of Heat-Treated Galfenol (Fe1-xGax) Alloys
    Cao, Hu; Gehring, Peter M.; Devreugd, Christopher P.; Rodriguez-Rivera, J. A.; Li, J.; Viehland, Dwight D. (American Physical Society, 2009-03-27)
    We report neutron diffuse scattering measurements on highly magnetostrictive Fe1-xGax alloys (0.14 < x < 0.20) with different thermal treatments. This diffuse scattering scales with magnetostriction and exhibits asymmetric peaks at the (100) and (300) reciprocal lattice positions that are consistent with the coexistence of short-range ordered, coherent nanometer-scale precipitates embedded in a long-range ordered, body-centered cubic matrix. A large peak splitting is observed at (300) for x=0.19, which indicates that the nanoprecipitates are not cubic and have a large elastic strain. This implies a structural origin for the enhanced magnetostriction.
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    Structural origin of the relaxor-to-normal ferroelectric transition in Pb(Mg1/3Nb2/3O3)-xPbTiO(3)
    Cao, Hu; Li, Jiefang; Viehland, Dwight D. (American Institute of Physics, 2006-08-01)
    A relaxor-to-normal, or micro-to-macrodomain, transition in Pb(Mg1/3Nb2/3O3)-xPbTiO(3), has been found under electric fields applied along [001] for x >= 0.24. It is shown that an intermediate tetragonal ferroelectric phase is important to the sharpness of this transition, as observed in its dielectric response. (c) 2006 American Institute of Physics.
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    Structural phase transformation and phase boundary/stability studies of field-cooled Pb(Mg1/3Nb2/3O3)-32%PbTiO3 crystals
    Cao, Hu; Bai, Feiming; Li, Jiefang; Viehland, Dwight D.; Xu, Guangyong; Hiraka, Haruhiro; Shirane, Gen (American Institute of Physics, 2005-05-01)
    Structural phase transformations in (001)-oriented (1-x)Pb(Mg1/3Nb2/3O3)-32%PbTiO3 crystals have been investigated by x-ray diffraction. A C -> T -> M-C sequence was observed in both the field-cooled and zero-field-cooled conditions. Most interestingly, an anomalous increase in the C -> T phase boundary with increasing field has been observed, which is seemingly a common characteristic of crystals whose compositions are in the vicinity of the morphotropic phase boundary, irrespective of the width of the T and M-C phase regions. (C) 2005 American Institute of Physics.
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    Triclinic phase in tilted (001) oriented BiFeO3 epitaxial thin films
    Yan, Li; Cao, Hu; Li, Jiefang; Viehland, Dwight D. (AIP Publishing, 2009-03-01)
    We report a triclinic phase in perovskite BiFeO3 (BFO) epitaxial thin films grown on (130) and (120) oriented SrTiO3 substrates. The lattice constants of the BFO thin films changed with tilt of the substrates from (001) toward (110). These lattice parameters result from the epitaxially engineering of structurally bridging phases of the lowest possible symmetry.