Browsing by Author "Hung, C.-M."

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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 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.
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.
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)].
The origin of photovoltaic responses in BiFeO3 multiferroic ceramics
(2012) Tu, Chi-Shun; Hung, C.-M.; Schmidt, V. Hugo; Chien, R. R.; Jiang, M.-D.; Anthoninappen, J.
Multiferroic BiFeO3 (BFO) ceramics with electrodes of indium tin oxide (ITO) and Au thin films exhibit significant photovoltaic effects under near-ultraviolet illumination (λ = 405 nm) and show strong dependences on light wavelength, illumination intensity, and sample thickness. The correlation between photovoltaic responses and illumination intensity can be attributed to photo-excited and thermally generated charge carriers in the interface depletion region between BFO ceramic and ITO thin film. A theoretical model is developed to describe the open-circuit photovoltage and short-circuit photocurrent density as a function of illumination intensity. This model can be applied to the photovoltaic effects in p–n junction type BFO thin films and other systems. The BFO ceramic exhibits stronger photovoltaic responses than the ferroelectric Pb1−xLax(ZryTi1−y)1−x/4O3 (PLZT) ceramics under near-ultraviolet illumination. Comparisons are made with other systems and models for the photovoltaic effect.
Photo-induced electric responses in heterostructure of indium tin oxide/(Bi 1-x Ca x FeO 3-δ /Au,
(2014) Hung, C.-M.; Tu, Chi-Shun; Xu, Zhe-Rui; Schmidt, V. Hugo; Chien, R. R.
Photovoltaic effects in heterostructure of indium tin oxide (ITO)/(Bi 1_x Ca x )FeO 3_δ multiferroic ceramics/Au (x = 0.0 and 0.15) have been measured under illuminations of λ = 405 and 445 nm. Open-circuit voltage (Voc), short-circuit current density (Jsc), and power conversion efficiency (η) show strong dependences on light wavelength and intensity. For λ = 405 nm, V oc and J sc can reach 0.62 V and 0.042 A/m 2 for BiFeO 3 (BFO), and 0.48 V and 0.30 A/m 2 for (Bi 0.85 Ca 0.15 )FeO 2.925 (BFO-15%Ca) at I N 9.1×10 2 W/m 2 . The maximum power conversion efficiency for λ = 405 nm can reach η N 0.002% for BFO and η N 0.0035% for BFO-15%Ca, which are comparable with 0.0025% observed in graphene/polycrystalline BFO/Pt films. A model based on forward p-n junction, reverse p-n junction and photo-excited currents in the interface between ITO film and (Bi 1_x Ca x )FeO 3_δ ceramic, was developed to describe Voc and Jsc as a function of incident light intensity. The theoretical fits agree well with experimental results. The depletion-region widths for λ = 405 nm were calculated as a function of light intensity. The calculated depletion-region widths without illumination are do N 210 nm in BFO and do N 340 nm in BFO-15%Ca.
Photovoltaic phenomena in BiFeO3 multiferroic ceramics
(2012) Hung, C.-M.; Tu, Chi-Shun; Yen, W.D.; Jou, L.S.; Jiang, M.-D.; Schmidt, V. Hugo
Open-circuit photovoltage and short-circuit photocurrent were investigated in BiFeO3 (BFO) ceramics as functions of laser wavelength (λ = 373 and 532 nm), illumination intensity, and sample thickness. BFO ceramics exhibit significant photovoltaic responses under near-ultraviolet illumination of λ = 373 nm. The photovoltaic responses strongly depend on wavelength, light intensity, and sample thickness. The relation between photovoltaic responses and light intensity can be described by exponential equations, E OC = E S[1 – exp(–I/α)] and J SC = J S[1 – exp(−I/β)]. E OC, J SC, E S, and J S are open-circuit photovoltage (V/cm), short-circuit photocurrent density (A/cm2), saturated photovoltage (V/cm), and saturated photocurrent density, respectively. This work suggests that BFO ceramic exhibits stronger photovoltaic responses than the ferroelectric WO3-doped Pb1-x La x (Zr y Ti z )1-x/4O3ceramics and Pb(Mg1/3Nb2/3)1-x Ti x O3 crystals.
Poling effect and piezoelectric response in high-strain ferroelectric 0.70Pb(Mg1/3Nb2/3)O3-0.30PbTiO3 crystal
(2010) Chen, H.-Y.; Tu, Chi-Shun; Hung, C.-M.; Chien, R.R.; Schmidt, V. Hugo; Ku, C. -S.; Lee, H.-Y.
In situ high-resolution synchrotron x-ray diffraction, dielectric permittivity, hysteresis loop, and polarization current, were used to investigate phase transitions of (211)-cut 0.70Pb(Mg1/3Nb2/3)O3–0.30PbTiO30.70Pb(Mg1/3Nb2/3)O3–0.30PbTiO3 single crystal before and after an electric (E)(E) field poling. A rhombohedral (R)–tetragonal (T)–cubic transition sequence was observed upon zero-field heating in both unpoled and poled samples. Before the R–T transition takes place, an extra dielectric and polarization current anomalies near 365 K were observed in the poled sample due to a transition of polarization ordering. The direct piezoelectric coefficient d33d33 exhibits a rapid increase for poling at E=1.0–1.3kV/cmE=1.0–1.3 kV/cm, followed by an overpoling behavior. The increment of polarization ordering plays an important role while the high piezoelectric response builds up.