Browsing by Author "Chien, R.R."

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A-site strontium doping effects on structure, magnetic, and photovoltaic properties of (Bi1−xSrx)FeO3−δ multiferroic ceramics
(2015-03) Tu, Chi-Shun; Xu, Zhe-Rui; Schmidt, V. Hugo; Chan, Ting-Shan; Chien, R.R.; Son, Hyungbin
Raman spectroscopy, X-ray diffraction (XRD), magnetization hysteresis loop, synchrotron X-ray absorption spectroscopy, and photovoltaic effects have been measured in (Bi1−xSrx)FeO3−δ (BFO100xSr) ceramics for x=0.0, 0.05, 0.10, and 0.15. Raman spectra and XRD reveal a rhombohedral R3c structure in all compounds. A-site Sr2+ doping increases fluctuations in cation-site occupancy and causes broadening in Raman modes. BFO15Sr exhibits a strong ferromagnetic feature due to reduction of Fesingle bondOsingle bondFe bond angle evidenced by the extended synchrotron X-ray absorption fine structure. The heterostructure of indium tin oxide (ITO) film/(Bi1−xSrx)FeO3−δ ceramic/Au film exhibit clear photovoltaic (PV) responses under blue illumination of λ=405 nm. The maximal power-conversion efficiency and external quantum efficiency in ITO/BFO5Sr/Au are about 0.004% and 0.2%, respectively. A model based on optically excited charges in the depletion region between ITO and (Bi1−xSrx)FeO3−δ can well describe open-circuit voltage and short-circuit current as a function of illumination intensity.
Calcium-doping effects on photovoltaic response and structure in multiferroic BiFeO3 ceramics
(2013) Tu, Chi-Shun; Hung, C.-M.; Xu, Zhe-Rui; Schmidt, V. Hugo; Ting, Yi; Chien, R.R.; Peng, Y.-T.; Anthoninappen, J.
Photovoltaic (PV) effects, power-conversion efficiencies, and structures have been systematically measured in (Bi1− x Ca x )FeO3−δ ceramics for x = 0.05, 0.10, and 0.15. The heterostructures of indium tin oxide (ITO) film/(Bi1− x Ca x )FeO3−δ ceramics/Au film exhibit significant PV effects under illumination of λ = 405 nm. The maximum power-conversion efficiency in the ITO/(Bi0.90Ca0.10)FeO2.95 (BFO10C)/Au can reach 0.0072%, which is larger than 0.0025% observed in the graphene/polycrystalline BFO/Pt films [Zang et al., Appl. Phys. Lett. 99, 132904 (2011)]. A theoretical model based on optically excited current in the depletion region between ITO film and Ca-doped BFO ceramics is used to describe the I-V characteristic, open-circuit voltage, and short-circuit current density as a function of illumination intensity. This work suggests that the Ca-substitution can reduce the rhombohedral distortion and stabilize the single-phase structure.
Dielectric and photovoltaic phenomena in tungsten-doped Pb(Mg1/3Nb2/3)1-xTixO3 crystal
(2006) Tu, Chi-Shun; Wang, F.-T.; Chien, R.R.; Schmidt, V. Hugo; Tseng, C.-T.
This work investigates dielectric and photovoltaic behaviors in Pb(Mg1∕3Nb2∕3)0.64Ti0.36O3single crystaldoped with 0.5mol% WO3. Dielectric permittivities measured as functions of temperature and frequency reveal two first-order-type phase transitions upon heating and cooling. The photovoltaic response strongly depends on illumination wavelength, sample thickness, and prior electric-field poling. The relation of photovoltage and light intensity under near-ultraviolet (λ=406nm)illumination for the poled samples can be expressed by an exponential equation. Optical transmission reveals that the cutoff wavelength is near 400nm and indicates a minimum electronic energy gap of ∼3.0eV.
Dielectric/piezoelectric resonance in high-strain Pb(Mg1/3Nb2/3)1-xTixO3 crystals
(2005) Tu, Chi-Shun; Chien, R.R.; Schmidt, V. Hugo; Wang, F.-T.; Hsu, W.-T.; Tseng, C.-T.; Shih, C.C.
This work presents dielectricresonance in Pb(Mg1∕3Nb2∕3)1−xTixO3 crystals after electric (E)-field poling, which is crucial for piezoelectric applications. Dielectricpermittivity has been measured as functions of temperature, frequency, poling E-field strength, and Ti content (x=25% and 34%). Frequency-dependent dielectric spectroscopy after poling exhibits multiple piezoelectricresonances between 0.2 and 1MHz, and can be described by the forced-damped-oscillator model. The resonantspectra show significant changes while phase transitions are taking place.
Effect of diamagnetic barium substitution on magnetic and photovoltaic properties in multiferroic BiFeO3
(2014) Hung, C.-M.; Tu, Chi-Shun; Xu, Zhe-Rui; Chang, L.-Y.; Schmidt, V. Hugo; Chien, R.R.; Chang, W.-C.
Spontaneous magnetization and photovoltaic (PV) effects have been measured in (Bi1- x Ba x )FeO3-δ ceramics for x = 0.05, 0.10, and 0.15. The substitution of Ba2+ ion in the A site of the perovskite unit cell can effectively enhance the ferromagnetic magnetization. The heterostructure of indium tin oxide (ITO) film/(Bi1- x Ba x )FeO3-δ ceramic/Au film exhibits significant PV effects under illumination of λ = 405 nm. The PV responses decrease with increasing Ba concentration. The maximum power-conversion efficiency in the ITO/(Bi0.95Ba0.5)FeO2.95/Au can reach 0.006%. A theoretical model based on optically excited current in the depletion region between ITO film and (Bi1- x Ba x )FeO3-δ ceramics is used to describe the I-V characteristic, open-circuit voltage (V oc), and short-circuit current density (J sc) as a function of light intensity.
Electric field- and temperature-induced phase transitions in high-strain relaxor-based ferroelectric Pb(Mg1/3Nb2/3)1-xTixO3 single crystals
(2007-12-07) Chien, R.R.; Schmidt, V. Hugo; Tu, Chi-Shun; Wang, F.-T.; Shih, I.-C.; Hung, L.-W.; Luo, H.
As a review, we compare results in electric-field- and temperature-induced phase transitions for four Pb(Mg1/3Nb2/3)1 - xTixO3(PMNT-x) single crystals, namely (001)-cut PMNT-x (x = 24%, 33%, and 40%) and (111)-cut PMNT-33%. Dielectric properties and domain states were measured as functions of temperature and electric field. The hysteresis loops of polarization versus electric field at room temperature were also measured. The intermediate monoclinic phases are essential in bridging higher symmetries while the electric-field- or temperature-induced transitions are taking place. Whether or not a monodomain state can be induced by a dc electric field strongly depends on crystallographic orientation, PbTiO3 content, and temperature.
Electric-field effects of dielectric and optical properties in Pb(Mg1/3Nb2/3)0.65Ti0.35O3 crystal
(2005) Tu, Chi-Shun; Wang, F.-T.; Chien, R.R.; Schmidt, V. Hugo; Tuthill, G. F.
Dielectric properties and domain structure have been measured as functions of temperature in a (001)-oriented Pb(Mg1∕3Nb2∕3)0.65Ti0.35O3 (PMNT35%) single crystal with and without a prior dc electric (E)-field poling. Without E-field poling, the dielectric loss exhibits a frequency-dependent maximum in the region of 120–180K, which can be described by a Vogel–Fulcher equation and fractal cluster model which implies structural irregularities within domains. With a prior poling a long-range monoclinic (tetragonal)→tetragonal (monoclinic) transition takes place near 212K upon heating. “Monoclinic (tetragonal)” represents that dominant monoclinic phase domains coexist with a small fraction of tetragonal phase domains. Optical transmission and birefringence were significantly enhanced by a prior E-field poling. The Cauchy equations for ordinary no and extraordinary nerefractive indices were determined between 0.45 and 1.4μm. However, the phase-matching criterion for second-harmonic generation was not found.
Electric-field-induced dielectric anomalies and optical birefringence in Pb(Mg1/3Nb2/3)1-xTixO3 (x=0.10) single crystal
(2006) Tu, Chi-Shun; Wang, F.-T.; Chien, R.R.; Schmidt, V. Hugo; Lim, L.-C.
This work aims to study thermal stability, frequency response, and refractive indices in a (001)-cut Pb(Zn1∕3Nb2∕3)0.90Ti0.10O3 (PZNT10%) crystal before and after an electric (E)-field poling, which are important for piezoelectric and optical uses. Dielectric permittivities have been measured as functions of temperature, frequency, and poling strength. The dielectric absorption reveals a broad dipolar relaxation in the region of 260–310K in the unpoled sample. Piezoelectric resonances were observed in the poled sample and show significant changes while phase transitions are taking place. The unpoled crystal shows almost no birefringence, indicating that the structure averaged over the optically anisotropic ferroelectric domains is optically isotropic. Birefringence appears after an E-field poling and shows a “negative” uniaxial distortion. The Cauchy equations for ordinary no and extraordinary nerefractive indices were determined between 0.45 and 1.4μm. However, the phase-matching criterion for second harmonic generation was not found.
Field-induced intermediate orthorhombic phase in (110)-cut Pb(Mg1/3Nb2/3)0.70Ti0.30O3 single crystal
(2008) Tu, Chi-Shun; Schmidt, V. Hugo; Chien, R.R.; Tsai, S H; Lee, S.-C.; Luo, H.
Field-induced orientational percolation to a ferroelectric phase in relaxor Pb(In1/2Nb1/2)1-xTixO3
(2007-06-05) Tu, Chi-Shun; Chien, R.R.; Hung, C.-M.; Schmidt, V. Hugo; Wang, F.-T.; Tseng, C.-T.
This work presents direct observations of orientational percolation and depolarization in a (001)-cut relaxor Pb(In1∕2Nb1∕2)0.70Ti0.30O3 (PINT30%) crystal by means of dielectric permittivity, depolarization current, domain structure, and hysteresis loop. Electric (E)-field poling induced a 10–300μm size orientational percolation transition in the matrix. The percolation breaks down at depolarization temperature Td≅395K, which is ∼20–40K higher than in rhombohedral Pb(Mg1∕3Nb2∕3)1−xTixO3. Near 420K, the remaining polarization disappears, and the full relaxor-type dielectric dispersion due to polar nanoclusters reappears, and remains evident up to the Burns temperature TB≅520K. Under E=40kV∕cm at room temperature, small regions of [001] tetragonal domains appeared, but most of the crystal exhibited field-induced percolation rhombohedral and monoclinic microdomains embedded randomly in the matrix. This work suggests that this relaxor ferroelectric consists of two components (spherical glassy matrix and polar nanoclusters) as proposed for Pb(Mg1∕3Nb2∕3)O3 by Blinc et al. [Phys. Rev. Lett. 91, 247601 (2003)].
Field-induced polarization rotation in (001)-cut Pb(Mg1/3Nb2/3)0.76Ti0.24O3
(2004-05-11) Chien, R.R.; Schmidt, V. Hugo; Tu, Chi-Shun; Hung, L.-W.; Lou, H.
Polarization rotation of field-induced phase transformations was observed by means of a polarizing microscope on a (001)-cut single crystal Pb(Mg1/3Nb2/3)0.76Ti0.24O3 (PMNT24%) at room temperature with an electric field applied along the [001] direction. A hysteresis loop was also measured for comparison. As the electric field increases, polarizations of rhombohedral R domains rotate toward the [001] tetragonal T001 phase through MA-type monoclinic distortions, i.e., R⃗ MA→T001. The crystal cannot entirely reach the T001 phase as the field approaches 44 kV/cm. In addition, spotlike domains that perhaps correspond to MB-type monoclinic or triclinic phases were also observed. This field-induced phase transformation is strongly irreversible.
Intermediate phases in rhombohedral Pb(Mg1/3Nb2/3)1-xTixO3 crystal
(2008) Tu, Chi-Shun; Chuang, H.-T.; Lee, S.-C.; Chien, R.R.; Schmidt, V. Hugo; Luo, H.
Phase transformations of (111)-cut Pb(Mg1∕3Nb2∕3)1−xTixO3 (x=0.30) single crystals before and after an electric (E)–field poling have been investigated by means of dielectricpermittivity, hysteresis loop, polarization current, and in situx-ray diffraction as a function of temperature. An R-T-C transition sequence was observed in the unpoled sample upon zero-field heating. R, T, and C are rhombohedral, tetragonal, and cubic phases, respectively. After a prior E-field poling, an extra intermediate monoclinic (possible MA) phase was induced and the crystal underwent an R-MA-T-C phase sequence with significant responses in polarization current. The dielectric dispersion was diminished after poling and reappeared in the cubic state. The dielectricpermittivities with and without a prior poling were found to follow the Curie–Weiss equation, ε′=C/(T−T0), above the Burns temperature 505K with the same constants C=1.9×105 K and T0=460K.
Nanotwin and phase transformation in tetragonal Pb(Fe1/2Nb1/2)1-xTixO3 single crystal
(2008) Tu, Chi-Shun; Tseng, C.-T.; Chien, R.R.; Schmidt, V. Hugo; Hsieh, C.-M.
This work is a study of phase transformation in (001)-cut Pb(Fe1/2Nb1/2)1−xTixO3Pb(Fe1/2Nb1/2)1−xTixO3 (x=48%)(x=48%) single crystals by means of dielectric permittivity, domain structure, and in situ x-ray diffraction. A first-order T(TNT)-C(TNT)T(TNT)-C(TNT) phase transition was observed at the Curie temperature TC≅518KTC≅518 K upon zero-field heating. T, TNTTNT, and C are tetragonal, tetragonal nanotwin, and cubic phases, respectively. T(TNT)T(TNT) and C(TNT)C(TNT) indicate that minor TNTTNT domains reside in the T and C matrices. Nanotwins, which can cause broad diffraction peak, remain above TC≅518KTC≅518 K and give an average microscopic cubic symmetry in the polarizing microscopy. Colossal dielectric permittivity (>104)(>104) was observed above room temperature with strong frequency dispersion. This study suggests that nanotwins can play an important role in relaxor ferroelectric crystals while phase transition takes place. The Fe ion is a potential candidate as a BB-site dopant for enhancing dielectric permittivity.
Nanotwins and phases in high-strain Pb(Mg1/3Nb2/3)1-xTixO3 crystal
(2008) Tu, Chi-Shun; Hsieh, C.-M.; Chien, R.R.; Schmidt, V. Hugo; Wang, F.-T.; Chang, W.S.
This work is a study of the thermal stability of (001)-cut Pb(Mg1/3Nb2/3)1−xTixO3 (x=0.30) single crystals before and after an electric (E)-field poling by means of dielectricpermittivity, hysteresis loop, domain structure,polarization current, and x-ray diffraction. An R(RNT)-R(RNT)/T(TNT)-T(TNT)-C transition sequence was observed upon heating in the unpoled sample. R, RNT, T, TNT, and C are the rhombohedral, rhombohedral nanotwin, tetragonal, tetragonal nanotwin, and cubic phases, respectively. R/T indicates coexistence of the R and T phases. R(RNT) and T(TNT) indicate that the RNT and TNTstructures mimic monoclinic phases in the R and T matrices, respectively. After a prior E-field poling, an R(RNT)-T(TNT)-C phase sequence takes place upon heating. The dielectricpermittivity and current density evidenced an additional polarization at 355 K, which is associated with the vanishing of the dielectric dispersion, which reappears near 410 K and remains up to the Burns temperature TB=510 K. This study suggests that nanotwins (RNT and TNT) can play an important role in high-strain piezoelectric crystals while phase transition takes place. Under E=38 kV/cm, [001] T domains randomly appeared in the matrix, suggesting that the matrix consists of a glassy matrix and ferroelectric nanoclusters.
Origins of dielectric response and conductivity in (Bi1-xNdx)FeO3 multiferroic ceramics
(2011) Tu, Chi-Shun; Yang, W.-C.; Schmidt, V. Hugo; Chien, R.R.
The dielectric response and conductivity have been measured in (Bi1−x Nd x )FeO3 (x = 0.0 and 0.05) ceramics as functions of temperature and frequency. A one-dimensional across-barrier model with intrinsic barriers, B (in temperature units), every lattice constant, a, and extrinsic barriers, B + Δ, every distance, d, is introduced to describe the dielectric response and conductivity. The across-barrier hopping is responsible for the high-temperature conductivity and step-like dielectric relaxation in the region of 500–800 K. Good qualitative fits of dielectric dispersion and conductivity are obtained with d = 20–30 nm, B = 8400–8700 K (∼0.72–0.75 eV), and Δ = 2500 K (∼0.215 eV). The resistivity plot of scaled ρ" versus ρ' indicates a contribution of grain boundaries or internal defects to the conductivity.
Phase coexistence and Mn-doping effect in lead-free ferroelectric (Na1/2Bi1/2)TiO3 crystals
(2010) Tu, Chi-Shun; Huang, S.-H.; Ku, C. -S.; Lee, H.-Y.; Chien, R.R.; Schmidt, V. Hugo; Luo, H.
Phase transformations of (001)-cut (Na1/2Bi1/2)TiO3 (NBT) and 1 at. % Mn-doped NBT (Mn-NBT) crystals have been investigated by means of dielectric permittivity, conventional x-ray diffraction (XRD), and high-resolution synchrotron XRD and reciprocal space mapping. An R−R+T−T−C transition sequence was observed in NBT and Mn-NBT upon zero-field heating. R, T, and C are rhombohedral, tetragonal, and cubic phases, respectively. R+T represents that the ferroelastic T phase coexists with the ferroelectric R phase. The Mn dopant can enhance dielectric response and reduce the dielectric loss in the high-temperature region due to the increased degree of ordering.
Phase coexistence of temperature-dependent phase transformation in relaxor ferroelectric Pb(Mg1/3Nb2/3)1-xTixO3 single crystal
(2004) Chien, R.R.; Schmidt, V. Hugo; Tu, Chi-Shun; Hung, L.-W.
Coexistence of monoclinic and tetragonal domains is evidenced in a (001)-cut Pb(Mg1/3Nb2/3)0.61Ti0.39O3 (PMNT39%) single crystal from temperature dependent observation of domain structures and dielectric permittivity. The coexistence persists until the crystal transforms into the cubic phase near 455 K. As temperature increases, the monoclinic domains progressively transform to the tetragonal phase via polarization rotations of monoclinic distortions. This causes increasing volume of the tetragonal domain matrix as temperature increases before the cubic phase takes over.
Phase stability after an electric‑field poling in Pb(Mg1/3Nb2/3)1-xTixO3 crystals
(2004-12-23) Tu, Chi-Shun; Chien, R.R.; Wang, F.-T.; Schmidt, V. Hugo; Han, P.
This work concerns phase thermal stability in (001)-cut Pb(Mg1∕3Nb2∕3)1−xTixO3 crystals after an electric (E)-field poling, which is crucial for piezoelectric applications. Dielectric properties have been measured as functions of Ti content (x=24, 26, 27, 29, 35, and 38%), E-field strength, temperature, and poling process. Domain structures after poling have also been observed by polarizing microscopy. An extra dielectric anomaly, which is mostly associated with the appearance of monoclinic domains, was observed during zero-field heating after poling. A prior poling can reveal a “hidden” phase transition which was not clear in unpoled samples. The monoclinic phase plays an essential role in bridging higher symmetries while phase transitions are taking place.
Phase transformation via a monoclinic phase in relaxor-based ferroelectric crystal (PbMg1/3Nb2/3O3)1-x(PbTiO3)x
(2003-01-24) Tu, Chi-Shun; Schmidt, V. Hugo; Shih, I.-C.; Chien, R.R.
A monoclinic phase is evidenced between rhombohedral and cubic phases in a 〈111〉-cut single crystal (PbMg1/3Nb2/3O3)0.67(PbTiO3)0.33 (PMN-33% PT) from observation of domain structures. Near 360 K the structure begins to distort from the rhombohedral toward the tetragonal phase through monoclinic domains (probably MA type but perhaps MB type). However, the present 〈111〉-cut crystal seems to disfavor the tetragonal phase and persists in the monoclinic phase up to T∼420K, where the cubic phase begins to develop. Temperature-dependent orientations of optical indicatrices of domains indicate polarization rotations within the monoclinic planes. In addition, a previously electric-field-cooled process enhances a long-range transformation.
Piezoelectric response and origin in (001) Pb(Mg1/3Nb2/3)0.70Ti0.30O3 crystal
(2008) Tu, Chi-Shun; Hsieh, C.-M.; Schmidt, V. Hugo; Chien, R.R.; Luo, H.
Converse and direct piezoelectric coefficients (dC33 and dD33) of (001)-cut Pb(Mg1/3Nb2/3)0.70Ti0.30O3 (PMN-30%PT) single crystals have been investigated as a function of poling electric (E) field. E-field-dependent domain structures were observed by using a polarizing microscope. Both dC33 and dD33 exhibit a rapid increase at E=1–2 kV/cm and reach maxima at E=2.5–4 kV/cm. This study suggests that polarization rotation from rhombohedral to monoclinic MA phases plays an important role while the high piezoelectric response builds up. Overpoling phenomenon evidenced by a sudden reduction in piezoelectric coefficient with increasing field is very sensitive to Ti content.