Browsing by Author "Chien, R. R."

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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.
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 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.
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.
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.
Electric-field poling effect on thermal stability of monoclinic phase in Pb(Mg1/3Nb2/3)0.74Ti0.26O3 single crystal
(2006) Chien, R. R.; Schmidt, V. Hugo; Tu, Chi-Shun
Phases and domains in a (1 1 0)-cut Pb(Mg1/3Nb2/3)0.74Ti0.26O3 (PMNT26%) single crystal have been investigated as functions of temperature and direct current (DC) electric (E) field by dielectric permittivity, polarizing microscopy, and electric polarization. The unpoled sample has a dominant rhombohedral (R) phase coexisting with monoclinic (M) phase domains, i.e. R/M at room temperature (RT). With 45 kV/cm DC poling applied along [1 1 0] at RT, a single domain of R phase with polar orientation perpendicular to the poling field, i.e. R, was obtained. No microcracking was observed under such high DC field poling. After the poling was removed, the poled sample has R/M microdomains, where the M distortion is close to the R phase. The zero-field-heating domain patterns in the unpoled and poled samples exhibit continuous polarization rotation via an intrinsic M phase in the regions of 355–373 and 365–378 K, respectively. Orthohombic (O) and tetragonal (T) phases were not observed in the temperature-dependent study. The whole crystal becomes cubic (C) phase near 393 and 399 K in the unpoled and poled sample, respectively. In brief, an R/M→M→C transition sequence takes place upon heating for both unpoled and poled samples.
Electric-field-induced and temperature‑induced phase transitions in high-strain ferroelectric Pb(Mg1/3Nb2/3)0.67Ti0.33O3 single crystal
(2006) Chien, R. R.; Tu, Chi-Shun; Schmidt, V. Hugo; Wang, F.-T.
This work is to study electric (E)-ﬁeld-induced and temperature-induced phase transitions in (001)-cut Pb(Mg1/3Nb2/3)0.67Ti0.33O3 (PMNT33%) single crystal, which are critical concerns for piezoelectric applications. Dielectric properties and domain structures (by polarizing microscope) are measured as functions of temperature and E ﬁeld. The hysteresis loop of the polarization versus E ﬁeld at room temperature is also measured. Without any E-ﬁeld application, upon heating a ﬁrst-order-type phase transition sequence rhombohedral (R) → rhombohedral/monoclinic/[001]tetragonal (R/M/T001) → cubic (C) takes place near 350 and 430 K, respectively. Under a dc E-ﬁeld application along [001] at room temperature, [001] tetragonal (T001) phase domains are induced by various phase transition sequences, i.e. R → T001,R → M → T001, R → T → T001,andR→ M → T → T001,asthe E-ﬁeld strength increases. In addition, E-ﬁeld-induced microcracking is observed in this work.
Field-induced phase transitions in relaxor ferroelectrics
(2010) Schmidt, V. Hugo; Chien, R. R.; Tu, Chi-Shun
The Landau free energy expansion as a function of three-dimensional polarization for perovskite relaxor ferroelectrics is examined to find effects of large electric fields in causing transitions from one ferroelectric state to another. Our examination of field-induced effects includes a fourth order expansion to obtain the Landau parameters for the observed temperature- and field-induced rhombohedral (R) tetragonal (T) transitions in an (001)-cut 0.70Pb(Mg1/3Mn2/3)O3-0.30PbTiO3 (PMN-30%PT) crystal. These parameters enabled us to predict the appearance of an orthorhombic (O) phase in (110)-cut PMN-30%PT crystals under an applied field. These results show that also for field-induced effects, the Landau expansion shows qualitative agreement with experiment for relaxor ferroelectrics.
Origin of low-frequency dielectric permittivity in BiFeO3 multiferroic ceramics
(2011) Tu, Chi-Shun; Wang, T.-H.; Schmidt, V. Hugo; Chien, R. R.
A one-dimensional conductivity barrier model is introduced to describe the dielectric response and conductivity of BiFeO 3 (BFO) and (Bi 0.95 Nd 0.05 )FeO 3 (BFO-5%Nd) ceramics as functions of temperature and frequency. Good qualitative fits of conductivity and dielectric permittivity in the intermediate-temperature region (500-800 K) are obtained with intrinsic barriers of B=8700 K (for BFO) and B=8400 K (for BFO-5%Nd), and extrinsic barriers of Δ=2500 K (for BFO and BFO-5%Nd). The phase-shifted conductivity is responsible for a step-like dielectric relaxation in the region of 500-800 K. The experimental conductivity departs from the conductivity-barrier-model fit below 650 K. This deviation is likely caused by the magneto-electric coupling near the antiferromagnetic-paramagnetic transition. This work suggests that the 5% mole Nd-substitution can stabilize the perovskite structure in BFO ceramic.
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 conversion and quantum efficiency in perovskite multiferroic ceramics
(2018-05) Tu, Chi-Shun; Chen, Pin-Yi; Chen, Cheng-Sao; Schmidt, V. Hugo; Chien, R. R.; Lin, Chun-Yen
Junction-driven photovoltaic effects in lead-free perovskite BiFeO3 multiferroic materials have demonstrated promising applications in energy harvesting and optical sensors. This study highlights remarkable photon-to-electron external quantum efficiency (EQE) of ∼9% and light-to-electric power-conversion efficiency (PCE) of ∼0.8% in the heterostructure consisting of A-site neodymium-doped BiFeO3 ceramic and indium-tin-oxide (ITO) thin film under irradiation of wavelength λ = 405 nm. A theoretical p-n-junction model based on the photo-generated carriers was employed to quantitatively describe open-circuit voltage (Voc) and short-circuit current density (Jsc) as functions of irradiation intensity, and to calculate junction widths and carrier densities. The direct band gap and the degree of local disorder (or defect state) were estimated using the photon-energy-dependent optical attenuation coefficient with the Tauc and Urbach relations.
Structural and dielectric properties of (Bi 1-x Ba x )(Fe 1-x Ti x )O 3 multiferroic ceramics
(2014) Wang, T.-H.; Tu, Chi-Shun; Schmidt, V. Hugo; Chien, R. R.; Ding, Y.
Phase transitions, dielectric permittivity, and conductivity of (Bi1-xBax)(Fe1-xTix)O3 (x = 0.05 and 0.1) [BFO-(Ba,Ti)] multiferroic ceramics have been studied as functions of temperature and frequency. In situ synchrotron x-ray diffraction revealed rhombohedral–cubic transitions in the temperature ranges 760–780°C in BFO-5%(Ba,Ti), and 720–750°C in BFO-10%(Ba,Ti). A one-dimensional barrier model with intrinsic barriers B every lattice constant a and extrinsic barriers B + Δ is introduced to describe the dielectric response and conductivity. This work revealed that (Ba,Ti) substitutions can enhance the intrinsic barriers and reduce the hopping rate of charge carriers, thereby giving the desired effect of decreasing the conductivity.
Synthesis and characterization of proton-conducting Ba(Zr0.8-xCexY0.2)O2.9 ceramics
(2010) Chien, R. R.; Tu, S.-S.; Schmidt, V. Hugo; Lee, S.-C.; Huang, C.-C.
X-ray diffraction and micro-Raman scattering have been used to characterize the effects of glycine-to-nitrate (G/N) and zirconium-to-cerium (Zr/Ce) molar ratios on structural properties of proton-conducting Ba(Zr0.8 − xCexY0.2)O2.9 (BZCY) ceramic powders fabricated by using the glycine–nitrate combustion method. Particle sizes of as-synthesized and calcined BZCY powders are estimated by using the Scherrer's formula, and are sensitive to G/N and Zr/Ce ratios. A simple cubic perovskite phase is observed for calcined Ba(Zr0.7Ce0.1Y0.2)O2.9 powders fabricated with G/N ratios of 1/3–3/4. Calcined BZCY (x = 0.0–0.8) powders fabricated with G/N = 1/2 exhibit a single-phase structure and a structural transformation from cubic to possibly rhombohedral for Zr/Ce ≤ 2/6. Particle sizes of as-synthesized and calcined BZCY (x = 0.0–0.8) powders fabricated with G/N = 1/2 vary in the ranges of 5–15 and 34–42 nm, respectively. In-situ temperature-dependent linear shrinkage measurement reveals that smaller-particle BZCY powder can reach densification at a considerable lower temperature.
Temperature-dependent structures of proton-conducting Ba(Zr0.8-xCexY0.2)O2.9 ceramics by Raman scattering and x-ray diffraction
(2012) Tu, Chi-Shun; Chien, R. R.; Schmidt, V. Hugo; Lee, S.-C.; Huang, C.-C.
In situ temperature-dependent micro-Raman scattering and x-ray diffraction have been performed to study atomic vibration, lattice parameter and structural transition of proton-conducting Ba(Zr0.8−xCexY 0.2)O2.9 (BZCY) ceramics (x = 0.0–0.8) synthesized by the glycine–nitrate combustion process. The Raman vibrations have been identified and their frequencies increase with decreasing x as the heavier Ce4+ ions are replaced by Zr4+ ions. The main Raman vibrations of Ba(Ce0.8Y 0.2)O2.9 appear near 305, 332, 352, 440 and 635 cm−1. The X–O ( X=Ce, Zr, Y) stretching modes are sensitive to the variation of Ce/Zr ratio. A rhombohedral–cubic structural transition was observed for x = 0.5–0.8, in which the transition shifts toward higher temperature as cerium increases, except for Ba(Ce0.8Y 0.2)O2.9. A minor monoclinic phase possibly coexists in the rhombohedral matrix for x = 0.5–0.8. The lower-cerium BZCYs (x = 0.0–0.4) ceramics do not exhibit any transition in the region of 20–900 °C, indicating a cubic phase at and above room temperature.