Browsing by Author "Tu, Chi-Shun"

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A-site strontium doping effects on structure, magnetic, and photovoltaic properties of (Bi 1-x Sr x )FeO 3-δ multiferroic ceramics
(2015) Tu, Chi-Shun; Xu, Zhe-Rui; Schmidt, V. Hugo; Chan, T.-S.; Chien, R. R.; Son, H.
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 FeOFe 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.
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.
Brillouin scattering in a liquid crystal: frequency and temperature dependences
(1994-02-25) Siny, I.G.; Tu, Chi-Shun; Pastalan, Gregory J.; Schmidt, V. Hugo
Brillouin scattering measurements have been carried out in the liquid crystal mixture Merck ZLI-2861 with a low viscosity and a broad nematic phase. Scattering from longitudinal acoustic phonons with different wave vectors was measured by changing the geometries from backscattreing to nearly forward scattering that cover the frequency range from 10 to 1 GHz. In the nematic phase the dispersion in both the hypersonic attenuation and sound velocity is describing by a single relaxation process with the characteristic frequency 3.2 GHz. Evident anomalies in attenuation and hypersonic velocity were found at lower temperatures in the vicinity of the phase transition from a nematic to, probably, a smectic state.
Brillouin scattering in Na1/2Bi1/2TiO3
(1994-02-01) Tu, Chi-Shun; Siny, I.G.; Schmidt, V. Hugo
Measurements of Brillouin scattering in single crystal Na1/2Bi1/2TiO3 are reported from 50 to 900 K which covers a cubic-tetragonal-trigonal succession of phase transitions, ending in a ferroelectric state. An extended dip in the hypersonic sound velocity and an extended maximum in the hypersonic damping were found with the extreme in the internansition region. These anomalies are connected with a response of mixed fluctuations. The Brillouin spectra background confirms the existence of a central peak in light scattering whose intensity increases drastically in the same internansition region.
Brillouin scattering in single crystals of Rb1-x(ND4)xD2AsO4
(1995) Tu, Chi-Shun; Schmidt, V. Hugo
The LA[100] (a-axis) Brillouin backscattering phonon spectra have been measured as a function of temperature (20-370 K) in the mixed deuterated ferroelectric (FE)-antiferroelectric (AFE) system Rb/sub 1-x/(ND/sub 4/)/sub x/D/sub 2/AsO/sub 4/ (DRADA-x) for ammonium concentrations x=0. 0.10 and 0.28. The Brillouin frequency shift with decreasing temperature shows hardening (positive coupling) whose steepness decreases with higher ND/sub 4/ content. For RbD/sub 2/AsO/sub 4/ (DRDA), a Landau-Khalatnikov-like maximum (which persists in weaker form for x=0.10) was observed and the polarization relaxation time is estimated to be /spl tau/-3.8/spl times/10/sup -12//(T/sub c/-T) sec, where T/sub c/ is the ferroelectric transition temperature. For both x=0.10 and 0.28, a broad damping peak anomaly which is stronger in x=0.28 was observed and can be connected with the dynamic order parameter fluctuations. Taking into account earlier NMR and dielectric results, we conclude that the local structure competition between FE ordering and AFE ordering is the origin of these broad damping anomalies.
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.
Conductivity across random barrier distribution as origin of large low-frequency dielectric peak in perovskite crystals and ceramics
(1996-10) Schmidt, V. Hugo; Tuthill, G. F.; Tu, Chi-Shun; Schogoleva, T.V.; Meschia, Steven C. L.
Several perovskite crystals and ceramics show very large dielectric (ϵ′) peaks at high temperature T and low frequency ƒ. In some cases these peaks are in the cubic phase far above any ferroelectric transition. Even at the peaks, the lossy part ϵ″ is larger than the real part ϵ′. The ϵ′ vs T curves for different ƒ follow the same d.c. (low-ƒ) envelope down to some T(ƒ) below which the curve for that ƒfalls below the envelope. Similarly, the conductivity (or ϵ″) data show d.c. and a.c. (high-frequency) envelopes for which data at different ƒ overlap. As a first approximation to a crystal with random barriers impeding conductivity, a model with barriers B (in T units) every lattice constant a = 4 Å and barriers B + Δ every distance d is assumed. The model is fit to permittivity and conductivity data for a strontium titanate single crystal, and a good qualitative fit is obtained.
Critical acoustic behavior of the relaxor ferroelectric Na1/2Bi1/2TiO3 in the intertransition region
(1995-03-01) Siny, I.G.; Tu, Chi-Shun; Schmidt, V. Hugo
A cubic-tetragonal-trigonal sequence of phase transitions in the relaxor ferroelectric Na1/2Bi1/2TiO3 (NBT) has been studied by Brillouin scattering. Pronounced anomalies for both the hypersonic velocity and damping have been found that peak just between two phase transitions in contrast to the ordinary well-known behavior with anomalies in the vicinity of every transition point. The fact that these anomalies are centered in the intertransition region and extend beyond both transitions is attributed to the fluctuations of two coupled order parameters associated with disorder in the Na- and Bi-cation distribution in a manner characteristic of relaxor ferroelectrics. Such a phenomenon seems to be typical of relaxor materials and to be the basis of the diffuse phase-transition dynamics.
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 and relaxation behaviors of (PbMg1/3Nb2/3O3)0.67(PbTiO3)0.33 single crystal
(2006) Tu, Chi-Shun; Schmidt, V. Hugo; Luo, H.; Chao, F.-C.
Dielectric permittivity along the [111] direction has been measured as a function of temperature for a relaxor ferroelectric single crystal (PbMg1/3Nb2/3O3)0.67(PbTiO3)0.33 (PMN-33%PT). A sharp ferroelectric phase transition was observed near 425 K and 429 K for cooling and heating processes, respectively. As temperature decreases, a diffuse phase transition (which begins near 330 K upon cooling) was detected. In addition, the nature of the thermal hysteresis for the dielectric permittivity confirms that these transitions (near 330 and 425 K upon cooling) are diffuse first-order and first-order, respectively. The frequency-dependent dielectric data ∊′111 (ƒ, T) prove the existence of an electric dipolar relaxation process between 350 and 400 K. The activation energy, the Vogel-Fulcher temperature and attempt frequency corresponding to this relaxation process are also calculated.
Dielectric permittivity and magnetoelectric coupling in multiferroic BiFeO3 and (Bi0.95La0.05)FeO3 ceramics
(2011) Tu, Chi-Shun; Ding, Y.; Yang, W.-C.; Wang, T.-H.; Chien, R. R.; Schmidt, V. Hugo; Yao, Y.-D.; Wu, K.-T.
Dielectric permittivity and loss of BiFeO 3 (BFO) and 5 mol% lanthanum-substituted BFO [(Bi 0.95 La 0.05 )FeO 3 or BFO-5% La] ceramics have been carried out as functions of temperature and frequency. A frequency-dependent and broad dielectric shoulder and maximum were observed in BFO and BFO-5% La near 600-700 K. These dielectric responses are likely due to the magnetoelectric coupling while the antiferromagnetic-paramagnetic transition takes place near the Néel temperature. As an approximation, a barrier model with intrinsic barriers B (in temperature unit) every lattice constant a and extrinsic barriers B +Δ every distance d is introduced to describe the low-frequency upturn in dielectric loss in the high-temperature region. Good qualitative fits are obtained for BFO and BFO-5% La. This work suggests that 5 mol% La substitution can enhance dielectric response and considerably reduce electric conductivity.
Dielectric permittivity in paraelectric/ferroelectric coexistence region in several proton glasses
(1995) Howell, Francis L.; Fundaun, I. L.; Stadler, S.; Meschia, Steven C. L.; Tu, Chi-Shun; Schmidt, V. Hugo
We present results for the complex permittivities of several proton glasses of the form M/sub 1-x/(NZ/sub 4/)/sub x/Z/sub 2/AO/sub 4/, where M=Rb, Z=H or D, and A=As or P. All measurements were made perpendicular to the ferroelectric axis, so no effects of domain wall motion occurred. Phase coexistence was apparent in all species. The phosphate glasses exhibited a much narrower coexistence composition range than the arsenate glasses.
Dielectric properties in lead-free piezoelectric (Bi0.5Na0.5)TiO3-BaTiO3 single crystals and ceramics
(2014) Chen, Cheng-Sao; Tu, Chi-Shun; Chen, Pin-Yi; Ting, Yi; Chiu, S.-J.; Hung, C.-M.; Lee, H.-Y.; Wang, S.-F.; Anthoninappen, J.; Schmidt, V. Hugo; Chien, R. R.
The 0.93(Bi0.5Na0.5)TiO3–0.07BaTiO3 (BNB7T) piezoelectric single crystals and ceramics have been grown respectively by using the self-flux and solid-state-reaction methods. The real (ε′) and imaginary (ε″) parts of the dielectric permittivity of BNB7T crystals and ceramics were investigated with and without an electric (E) poling as functions of temperature and frequency. The BNB7T crystal shows a stronger dielectric maximum at Tm~240 °C than the ceramic at Tm~300 °C. The dielectric permittivity of BNB7T ceramic shows an extra peak after poling at an electric field E=40 kV/cm in the region of 80–100 °C designated as the depolarization temperature (Td). A wide-range dielectric thermal hysteresis was observed in BNB7T crystal and ceramic, suggesting a first-order-like phase transition. The dielectric permittivity ε′ obeys the Curie–Weiss equation, ε′=C/(T−To), above 500 °C, which is considered as the Burns temperature (TB), below which polar nanoregions begin to develop and attenuate dielectric responses.
Dielectric relaxation and piezoelectric coupling in the mixed proton-glass crystal K0.61(NH4)0.39H2PO4,
(1993-11-01) Tu, Chi-Shun; Schmidt, V. Hugo; Saleh, Adli A.
Measurements of the dielectric permittivities in the mixed proton-glass crystal K0.61(NH4)0.39H2PO4 are reported from 20 Hz to 200.8 MHz and for fields along both the a and c axes. Between 14 and 50 K, the c-axis permittivity outside the piezoelectric-resonance range shows a temperature-dependent spread in relaxation times but cannot be fit accurately with the Cole-Cole formula. Strong piezoelectric resonance for a field along the c axis was observed in the region between 100 kHz and 10 MHz.
Dielectric, hypersonic, and domain anomalies of (PbMg1/3Nb2/3O3)1-x(PbTiO3)x single crystals
(2001) Tu, Chi-Shun; Tsai, Chih-Long; Schmidt, V. Hugo; Luo, H.; Yin, Z.
Dielectricpermittivities,Brillouinbackscattering spectra, polarization-electric field hysteresis loops, and domain structures have been measured as a function of temperature in relaxor-based ferroelectricsingle crystals(PbMg1/3Nb2/3O3)1−x(PbTiO3)x(PMN-xPT)(PbMg1/3Nb2/3O3)1−x(PbTiO3)x(PMN-xPT) for x=0.24x=0.24 and 0.34. For PMN-24%PT, a diffuse phase transition which is associated with a broad frequency-dependent dielectric maximum was observed near 380 K. As the temperature increases, the PMN-24%PT crystal gradually develops cubic regions and is fully converted into the cubic state near 375 K. An extra dielectric anomaly appears at 370 K, possibly due to the percolating polar cluster induced by an external electric field. PMN-34%PT exhibits a nearly normal ferroelectric phase transition near 445 K from the tetragonal to the cubic phase. In addition, a weak diffuse phase transition observed near 280 K may result from partial conversion of rhombohedral phase to tetragonal phase. The dielectric thermal hysteresis confirms that the transitions near 280 and 445 K are diffuse first order and first order, respectively. The dielectricpermittivities of PMN-24%PT and PMN-34%PT obey the relation, ε′m/ε′(f,T)=1+[T−Tm(f )]γ/2δ2γ,εm′/ε′(f,T)=1+[T−Tm(f )]γ/2δγ2, above the temperature of permittivity maximum Tm.
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.
Direct observation of ferroelectric domains and phases in (001)-cut Pb(Mg1/3Nb2/3)1-xTixO3 single crystals
(2006) Chien, R. R.; Schmidt, V. Hugo; Tu, Chi-Shun; Wang, F.-T.
Real-time direct observation of ferroelectric domains and phases under electric-field poling along [0 0 1] at room temperature in Pb(Mg1/3Nb2/3)0.67Ti0.33O3 (PMNT33%) single crystal has been performed by polarizing microscopy. A hysteresis loop of polarization vs. electric field at room temperature was also measured for comparison. By using relations of crystallographic symmetry and optical extinction, polarizing microscopy reveals orientations of the domain polarizations and their corresponding phases. It also provides direct real-time observation of microcracking phenomena. It was found that the monoclinic phase domains play a crucial role in bridging higher symmetry (tetragonal and rhombohedral) phases while field-induced phase transitions take place.
E-field-induced polarization rotation in Pb(Mg1/2Nb2/3)1-xTixO3 crystal
(2003) Tu, Chi-Shun; Shih, I.-C.; Schmidt, V. Hugo
A sequence of field-induced phase transformations was observed by means of a polarizing microscope on a (111)-cut single crystal Pb(Mg1/3Nb2/3)0.67Ti0.33O3 at room temperature with an electric field applied along the [111] direction. As electric field increases, polarizations of the rhombohedral R domains whose polar directions are not along [111] rotate first toward a tetragonal T phase associated with 90° domain walls. Then the crystal gradually reaches total optical extinction for all polarizer angles at 12 kV/cm, indicating a single [111] rhombohedral R111 domain with polarization aligned with the electric field. These field-induced phase transformations are irreversible and proceed most likely through MA -type monoclinic distortions, i.e., R→MA→T→MA→R111.
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.
Effect of lithium fluoride on thermal stability of proton-conducting Ba(Zr0.8-xCexY0.2)O2.9 ceramics
(2010) Tu, Chi-Shun; Huang, C.-C.; Lee, S.-C.; Chien, R. R.; Schmidt, V. Hugo; Tsai, Chih-Long
In-situ X-ray diffraction (XRD) and micro-Raman scattering have been used to study the thermal stability of lithium fluoride (LiF)-added (7% weight ratio) Ba(Zr0.8−xCexY0.2)O2.9 (BZCY: x = 0.1 and 0.2) proton-conducting ceramic powders as a function of temperature in 1 atm of flowing CO2. This work reveals that LiF-addition can reduce the thermal stability of Ba(Zr0.8−xCexY0.2)O2.9 in CO2 and cause decomposition to BaCO3, and possibly Ba3Ce2(CO3)5F2 (or CeCO3F), and Y2O3-like compound after exposure to CO2 from high temperatures. LiF-related compounds can be removed after calcining (or sintering) in air above 1200 °C, but a minor amount of a Y2O3-like compound could remain after calcining at 1400 °C in air.