Chang Won Ahn - Academia.edu (original) (raw)

Papers by Chang Won Ahn

Research paper thumbnail of Low Temperature Sintering and Piezoelectric Properties of CuO-Doped (K0.5Na0.5)NbO3Ceramics

Ferroelectrics Letters Section, 2008

(Na0.5K0.5)NbO3 ceramics require sintering below 1,000C to avoid the problem of Na-volatilization... more (Na0.5K0.5)NbO3 ceramics require sintering below 1,000C to avoid the problem of Na-volatilization which results in lower resistivity. In this study we report the effect of CuO on the sintering behavior of (Na0.5K0.5)NbO3 ceramics. It was found that (Na0.5K0.5)NbO3+ 1.5 mol% CuO ceramics sintered at 950 °C for 2 hr exhibit excellent piezoelectric properties of d33 = 100 pC/N, kp =

Research paper thumbnail of Enhanced ferroelectricity in perovskite oxysulfides

Physical Review Materials, 2019

A sulfur element is a promising anion dopant for synthesizing new multifunctional materials and f... more A sulfur element is a promising anion dopant for synthesizing new multifunctional materials and for exploring unusual physical phenomena. However, owing to its volatility, sulfur substitution to oxide materials is challenging, and thus the sulfurization effects on the associated properties have been limitedly studied. Here, a facile method for sulfurization to a perovskite oxide Pb(Zr,Ti)O 3 is developed and demonstrated. A thiourea (CH 4 N 2 S) solution is used as a precursor for the sulfurization and its doping-level control. By manipulating the sulfur concentration (x), we systematically examine the physical properties of sulfur-doped Pb(Zr,Ti)O 3−x S x films. An enhancement in the tetragonality and ferroelectricity by sulfurization is observed with the band-gap reduction, which is consistent with our theoretical predictions. In the sulfurized films, the ferroelectric phonon modes become softened progressively, probably due to the substitution of apical oxygens with sulfur atoms. Our work is of practical interest for designing ferroelectric photovoltaic devices with high performances.

Research paper thumbnail of Phase transition behavior and mechanical properties of (1- x )(Bi 1/2 Na 1/2 )TiO 3 - x SrTiO 3 lead-free piezoelectric ceramics

Sensors and Actuators A: Physical, 2017

Highlights  A ferroelectric to ergodic relaxor phase transition occurred with increasing ST conc... more Highlights  A ferroelectric to ergodic relaxor phase transition occurred with increasing ST concentration of the BNST ceramics.  The BNST with 25 mol% ST showed the highest strain due to the electric-field-induced phase transition from ergodic relaxor to ferroelectric phase.  The elastic compliance values of the BNST ceramics gradually decreased with increasing ST concentration.  All tensors of elastic compliance values of BNST ceramics are almost half of those of commercial PZT-based ceramics.

Research paper thumbnail of Robust nanogenerators based on graft copolymers via control of dielectrics for remarkable output power enhancement

Science advances, 2017

A robust nanogenerator based on poly(tert-butyl acrylate) (PtBA)-grafted polyvinylidene difluorid... more A robust nanogenerator based on poly(tert-butyl acrylate) (PtBA)-grafted polyvinylidene difluoride (PVDF) copolymers via dielectric constant control through an atom-transfer radical polymerization technique, which can markedly increase the output power, is demonstrated. The copolymer is mainly composed of α phases with enhanced dipole moments due to the π-bonding and polar characteristics of the ester functional groups in the PtBA, resulting in the increase of dielectric constant values by approximately twice, supported by Kelvin probe force microscopy measurements. This increase in the dielectric constant significantly increased the density of the charges that can be accumulated on the copolymer during physical contact. The nanogenerator generates output signals of 105 V and 25 μA/cm(2), a 20-fold enhancement in output power, compared to pristine PVDF-based nanogenerator after tuning the surface potential using a poling method. The markedly enhanced output performance is quite stab...

Research paper thumbnail of Research on Lead-free Piezoelectric Films for MEMS Applications

Physics and High Technology, 2013

Research paper thumbnail of Lead-free piezoceramics – Where to move on?

Journal of Materiomics, 2016

Lead-free piezoceramics aiming at replacing the market-dominant lead-based ones have been extensi... more Lead-free piezoceramics aiming at replacing the market-dominant lead-based ones have been extensively searched for more than a decade worldwide. Some noteworthy outcomes such as the advent of commercial products for certain applications have been reported, but the goal, i.e., the invention of a lead-free piezocermic, the performance of which is equivalent or even superior to that of PZT-based piezoceramics, does not seem to be fulfilled yet. Nevertheless, the academic effort already seems to be culminated, waiting for a guideline to a future research direction. We believe that a driving force for a restoration of this research field needs to be found elsewhere, for example, intimate collaborations with related industries. For this to be effectively realized, it would be helpful for academic side to understand the interests and demands of the industry side as well as to provide the industry with new scientific insights that would eventually lead to new applications. Therefore, this review covers some of the issues that are to be studied further and deeper, so-to-speak, lessons from the history of piezoceramics, and some technical issues that could be useful in better understanding the industry demands. As well, the efforts made in the industry side will be briefly introduced for the academic people to catch up with the recent trends and to be guided for setting up their future research direction effectively.

Research paper thumbnail of Preparation of LiFe PO4 Using Chitosan and its Cathodic Properties for Rechargeable Li-ion Batteries

Bulletin- Korean Chemical Society

The LiFePO4 powder was synthesized by using the solid state reaction method with Fe(C2O4)․2H2O, (... more The LiFePO4 powder was synthesized by using the solid state reaction method with Fe(C2O4)․2H2O, (NH4)2HPO4, Li2CO3, and chitosan as a carbon precursor material for a cathode of a lithium-ion battery. The chitosan added LiFePO4 powder was calcined at 350 o C for 5 hours and then 800 o C for 12 hours for the calcination. Then we calcined again at 800 o C for 12 hours. We characterized the synthesized compounds via the crystallinity, the valence states of iron ions, and their shapes using TGA, XRD, SEM, TEM, and XPS. We found that the synthesized powders were carbon-coated using TEM images and the iron ion is substituted from 3+ to 2+ through XPS measurements. We observed voltage characteristics and initial charge-discharge characteristics according to the C rate in LiFePO4 batteries. The obtained initial specific capacity of the chitosan added LiFePO4 powder is 110 mAh/g, which is much larger than that of LiFePO4 only powder.

Research paper thumbnail of Silk fibroin-based biodegradable piezoelectric composite nanogenerators using lead-free ferroelectric nanoparticles

Nano Energy, 2015

Silk fibroin-based biodegradable composite-type nanogenerators are demonstrated with controllable... more Silk fibroin-based biodegradable composite-type nanogenerators are demonstrated with controllable lifetime for powering to the implantable devices. The 2D thin filmand 1D wire-type composites consist of the well-dispersed lead-free ferroelectric (BaTiO 3 , ZnSnO 3 , Bi 0.5 (Na 0.82 K 0.18) 0.5 TiO 3 , and K 0.5 Na 0.5 Nb 0.995 Mn 0.005 O 3) nanoparticles. Ag nanowires are used to enhance the dispersion of the nanoparticles and polyvinylpyrrolidone prevents Ag nanowires from connecting with each other. A maximum output voltages and current densities of 2.2 V and 0.12 mA/cm 2 in the thin film, and 1.8 V and 0.1 mA/cm 2 in the wire are obtained under the motion of a foot step for when 30 wt% KNN:Mn nanoparticles are well-dispersed in the solution because of the largest piezoelectric coupling figure of merit. The properties of water-soluble composite films are also controlled with the glycerol content up to two days.

Research paper thumbnail of Dielectric and ferroelectric properties of La, Nd and V substituted Bi 4 Ti 3 O 12 ceramics

, Dielectric and ferroelectric properties of LaFeO 3 particles derived from metal organic framewo... more , Dielectric and ferroelectric properties of LaFeO 3 particles derived from metal organic frameworks precursor, Ceramics International,

Research paper thumbnail of Photocatalytic Hydrogen Generation from Water-methanol MixturesUsing Nanocrystalline ZnFe$_2$O$_4$ under Visible Light Irradiation

Journal of the Korean Physical Society, 2009

A spinel crystal structure of ferrite containing zinc (ZnFe2O4) was synthesized by using the poly... more A spinel crystal structure of ferrite containing zinc (ZnFe2O4) was synthesized by using the polymer complex method. A pure phase ZnFe2O4 was obtained at a relatively lower temperature of 900 • C. The as-synthesized nanoparticles are agglomerates of crystals with an average size of ∼50 nm, as estimated from the X-ray diffraction analysis. The band gap of the n-type semiconducting metal oxide, as determined by using an ultraviolet-visible diffuse reflectance spectrometer was found to be 1.90 eV (653 nm). The photocatalytic activity of ZnFe2O4 was investigated by using the photoreduction of water under visible light (≥420 nm), which was found to be much higher than that of the well-known TiO2−xNx photocatalyst. This superior performance is attributed to the larger surface area, the uniform and well-dispersed deposition of the Pt co-catalyst over the surface, and the small band gap of the chemically-prepared ferrite photocatalyst.

Research paper thumbnail of Raman- and Brillouin-scattering studies on lead-free piezoelectric Bi0.5(Na0.78K0.22)0.5−x TiO3 ceramics with A-site vacancies

Journal of the Korean Physical Society, 2013

The Raman and Brillouin spectra of lead-free piezoelectric Bi0.5(Na0.78K0.22)0.5−xTiO3 ceramics w... more The Raman and Brillouin spectra of lead-free piezoelectric Bi0.5(Na0.78K0.22)0.5−xTiO3 ceramics with A-site vacancies were investigated for x = 0 ∼ 0.05. The Raman spectra measured at room temperature showed seven broad bands over the range of 50-1000 cm −1 , reflecting strong anharmonicity, inherent A-site disorder and the resulting breakdown of the Raman selection rule. The splitting of the A1 mode near 270 cm −1 indicated that the tetragonal symmetry was maintained over the whole x range, consistent with the X-ray diffraction results. The Brillouin spectra exhibited distributed longitudinal and transverse acoustic modes, in addition to central peaks. The distributed acoustic modes were attributed to the scattering events at all scattering angles from 0 to 180 • due to multiple reflections and refractions at the grain boundaries. The relaxation time obtained from fitting the broad central peak by using a Debye function exhibited a slowing-down behavior in the paraelectric phase. The formation of the central peak was attributed to the existence of nano-sized polar regions due to the anharmonic vibrations and the hopping motions of the A-site cations.

Research paper thumbnail of The effect of annealing temperature on the piezoelectric and dielectric properties of lead-free Bi0.5(Na0.85K0.15)0.5TiO3 thin films

Journal of the Korean Physical Society, 2012

We fabricated Bi0.5(Na0.85K0.15)0.5TiO3 (BNKT) lead-free thin films on Pt(111)/TiO2/SiO2/ Si(100)... more We fabricated Bi0.5(Na0.85K0.15)0.5TiO3 (BNKT) lead-free thin films on Pt(111)/TiO2/SiO2/ Si(100) substrates by using a chemical solution deposition method. The BNKT thin films were annealed at 650, 700, 750, or 800 • C in an O2 atmosphere. X-ray diffraction patterns revealed a pure perovskite structure at annealing temperatures from 650 to 800 • C. A field emission electron microscope study revealed an increase in grain size with increasing annealing temperature. BNKT thin films annealed at 700 • C had a high remnant effective piezoelectric coefficient of 63.6 pm/V and a low leakage current density of 5.15 × 10 −6 A/cm 2 .

Research paper thumbnail of Piezoelectric and Ferroelectric Properties of Lead-Free Bi4−xNdxTi2.97V0.03O12Ceramics

Ferroelectrics, 2006

We fabricated lead-free Bi4−xNdxTi2.97V0.03O12 (x = 0.25∼1.00; BNdTV-x) ceramics using the conven... more We fabricated lead-free Bi4−xNdxTi2.97V0.03O12 (x = 0.25∼1.00; BNdTV-x) ceramics using the conventional solid-state reaction method and investigated their ferroelectric and piezoelectric properties. BNdTV ceramics exhibited the phase transition from orthorhombic to pseudo-tetragonal structure with increasing Nd concentration. At the morphotropic phase boundary of x = 0.85, BNdTV ceramics showed the highest values of Pr (13 μC/cm), kt (17%) and Qm

Research paper thumbnail of Phase Transition and Electrical Properties of BiAlO3-Modified (Bi0.5Na0.5)TiO3Piezoelectric Ceramics

Ferroelectrics, 2010

Lead-free piezoelectric (1-x)(Bi0.5Na0.5)TiO3–xBiAlO3 (BNT-BA, x = 0.00, 0.025, 0.050, 0.075, 0.1... more Lead-free piezoelectric (1-x)(Bi0.5Na0.5)TiO3–xBiAlO3 (BNT-BA, x = 0.00, 0.025, 0.050, 0.075, 0.100) ceramics were synthesized by a conventional solid- state reaction method. The BNT-BA ceramics exhibited enhanced ferroelectric and piezoelectric properties at x = 0.050. The x-ray diffraction analysis revealed a pure perovskite phase for x ≤ 0.075. A rhombohedral to pseudocubic phase transition was observed at x = 0.075 and maximum values for the electric field-induced strain (S = 0.13%) and dynamic piezoelectric coefficient (d*33 = 163 pm/V) were obtained at this composition. The enhancement in the electric field-induced strain is attributed to the rhombohedral to pseudocubic phase transition.

Research paper thumbnail of Dielectric Relaxation and Electrical Properties of Lead-Free LiNbO3-Substituted Na0.5K0.5NbO3Ferroelectric Ceramics

Ferroelectrics, 2010

ABSTRACT Na0.5K0.5NbO3 ceramics and LiNbO3-substituted Na0.5K0.5NbO3 ceramics were prepared by th... more ABSTRACT Na0.5K0.5NbO3 ceramics and LiNbO3-substituted Na0.5K0.5NbO3 ceramics were prepared by the solid-state reaction method. The crystallization and grain morphology of the two types of ceramics were confirmed by X-ray diffraction and atomic force microscopy studies, respectively. The 5 mol% LiNbO3 substituted Na0.5K0.5NbO3 ceramics shows well-saturated ferroelectric P-E hysteresis loops. The dielectric and electrical properties of the ceramics were investigated in the frequency range from 10 Hz to 1 MHz and the temperature range from 25°C to 600°C. Analysis of the complex impedance relaxation by the Cole-Cole plot showed one impedance relaxation for LiNbO3 substituted Na0.5K0.5NbO3. The contributions of electrical conduction in the above mentioned temperature and frequency ranges are discussed.

Research paper thumbnail of Dielectric and Piezoelectric Properties of Lead-Free Na 0.5 Bi 4.5−x La x Ti 4 O 15 and Na 0.5 Bi 4.5−x Nd x Ti 4 O 15 Ceramics

Ferroelectrics, 2002

The dielectric and piezoelectric properties of lead-free bismuth layered structure Na 0.5 Bi 4.5 ... more The dielectric and piezoelectric properties of lead-free bismuth layered structure Na 0.5 Bi 4.5 m x La x Ti 4 O 15 (NBLT) and Na 0.5 Bi 4.5 m x Nd x Ti 4 O 15 (NBNT) ceramics are studied. The T c and the l r of the NBLT and NBNT ceramics has been decreased and the shape of

Research paper thumbnail of Low-frequency dielectric relaxation and ac conduction of SrBi2Ta2O9 thin film grown by pulsed laser deposition

Applied Physics Letters, 2002

Bi-excess SrBi 2 Ta 2 O 9 ͑SBT͒ thin films on Pt/Ti/SiO 2 /Si substrate were prepared by pulsed l... more Bi-excess SrBi 2 Ta 2 O 9 ͑SBT͒ thin films on Pt/Ti/SiO 2 /Si substrate were prepared by pulsed laser deposition technique. The SBT structure was characterized by x-ray diffraction studies. The ferroelectric properties were confirmed by P-E hysteresis loops at different applied electric fields. The dielectric constant and the ac conductivity of the Pt/SBT/Pt capacitor were investigated in the frequency range from 0.01 Hz to 100 kHz and in the temperature range from 25 to 400°C. The thermal activation energy of 0.90 eV is observed in the frequency dependent dielectric constant. The activation energy for conduction process is calculated as 0.91 eV from the slope of ac conductivity at the lowest frequency. The low-frequency dielectric relaxation and the ac conductivity of Bi-excess SBT thin film are discussed in relation to the electrical conduction of SBT/Pt junction.

Research paper thumbnail of Ferroelectric and piezoelectric properties of Na0.52K0.48NbO3 thin films prepared by radio frequency magnetron sputtering

Applied Physics Letters, 2009

ABSTRACT

Research paper thumbnail of Controlling polymer shape through the self-assembly of dendritic side-groups

Nature, 1998

V. Percec*, C.-H. Ahn*, G. Ungar†, DJP Yeardley†, M. Mo¨ller‡ & SS Sheiko‡ * The WM Keck ... more V. Percec*, C.-H. Ahn*, G. Ungar†, DJP Yeardley†, M. Mo¨ller‡ & SS Sheiko‡ * The WM Keck Laboratories for Organic Synthesis, Department of Macromolecular Science, Case Western Reserve University, Cleveland, Ohio 44106-7202, USA † Department of Engineering Materials ...

Research paper thumbnail of Low-Temperature Sintering of Bi 0.5 (Na,K) 0.5 TiO 3 for Multilayer Ceramic Actuators

Journal of the American Ceramic Society, 2015

We investigated the influence of CuO amount (0.5-3.0 mol%), sintering temperature (900°C-1000°C),... more We investigated the influence of CuO amount (0.5-3.0 mol%), sintering temperature (900°C-1000°C), and sintering time (2-6 h) on the low-temperature sintering behavior of CuOadded Bi 0.5 (Na 0.78 K 0.22) 0.5 TiO 3 (BNKT22) ceramics. Normalized strain (S max /E max), piezoelectric coefficient (d 33), and remanent polarization (P r) of 1.0 mol% CuO-added BNKT22 ceramics sintered at 950°C for 4 h was 280 pm/V, 180 pC/N, and 28 lC/cm 2 , respectively. These values are similar to those of pure BNKT22 ceramics sintered at 1150°C. In addition, we investigated the performance of multilayer ceramic actuators made from CuO-added BNKT22 in acoustic sound speaker devices. A prototype sound speaker device showed similar output sound pressure levels as a Pb(Zr,Ti)O 3-based device in the frequency range 0.66-20 kHz. This result highlights the feasibility of using low-cost multilayer ceramic devices made of lead-free BNKT-based piezoelectric materials in sound speaker devices.

Research paper thumbnail of Low Temperature Sintering and Piezoelectric Properties of CuO-Doped (K0.5Na0.5)NbO3Ceramics

Ferroelectrics Letters Section, 2008

(Na0.5K0.5)NbO3 ceramics require sintering below 1,000C to avoid the problem of Na-volatilization... more (Na0.5K0.5)NbO3 ceramics require sintering below 1,000C to avoid the problem of Na-volatilization which results in lower resistivity. In this study we report the effect of CuO on the sintering behavior of (Na0.5K0.5)NbO3 ceramics. It was found that (Na0.5K0.5)NbO3+ 1.5 mol% CuO ceramics sintered at 950 °C for 2 hr exhibit excellent piezoelectric properties of d33 = 100 pC/N, kp =

Research paper thumbnail of Enhanced ferroelectricity in perovskite oxysulfides

Physical Review Materials, 2019

A sulfur element is a promising anion dopant for synthesizing new multifunctional materials and f... more A sulfur element is a promising anion dopant for synthesizing new multifunctional materials and for exploring unusual physical phenomena. However, owing to its volatility, sulfur substitution to oxide materials is challenging, and thus the sulfurization effects on the associated properties have been limitedly studied. Here, a facile method for sulfurization to a perovskite oxide Pb(Zr,Ti)O 3 is developed and demonstrated. A thiourea (CH 4 N 2 S) solution is used as a precursor for the sulfurization and its doping-level control. By manipulating the sulfur concentration (x), we systematically examine the physical properties of sulfur-doped Pb(Zr,Ti)O 3−x S x films. An enhancement in the tetragonality and ferroelectricity by sulfurization is observed with the band-gap reduction, which is consistent with our theoretical predictions. In the sulfurized films, the ferroelectric phonon modes become softened progressively, probably due to the substitution of apical oxygens with sulfur atoms. Our work is of practical interest for designing ferroelectric photovoltaic devices with high performances.

Research paper thumbnail of Phase transition behavior and mechanical properties of (1- x )(Bi 1/2 Na 1/2 )TiO 3 - x SrTiO 3 lead-free piezoelectric ceramics

Sensors and Actuators A: Physical, 2017

Highlights  A ferroelectric to ergodic relaxor phase transition occurred with increasing ST conc... more Highlights  A ferroelectric to ergodic relaxor phase transition occurred with increasing ST concentration of the BNST ceramics.  The BNST with 25 mol% ST showed the highest strain due to the electric-field-induced phase transition from ergodic relaxor to ferroelectric phase.  The elastic compliance values of the BNST ceramics gradually decreased with increasing ST concentration.  All tensors of elastic compliance values of BNST ceramics are almost half of those of commercial PZT-based ceramics.

Research paper thumbnail of Robust nanogenerators based on graft copolymers via control of dielectrics for remarkable output power enhancement

Science advances, 2017

A robust nanogenerator based on poly(tert-butyl acrylate) (PtBA)-grafted polyvinylidene difluorid... more A robust nanogenerator based on poly(tert-butyl acrylate) (PtBA)-grafted polyvinylidene difluoride (PVDF) copolymers via dielectric constant control through an atom-transfer radical polymerization technique, which can markedly increase the output power, is demonstrated. The copolymer is mainly composed of α phases with enhanced dipole moments due to the π-bonding and polar characteristics of the ester functional groups in the PtBA, resulting in the increase of dielectric constant values by approximately twice, supported by Kelvin probe force microscopy measurements. This increase in the dielectric constant significantly increased the density of the charges that can be accumulated on the copolymer during physical contact. The nanogenerator generates output signals of 105 V and 25 μA/cm(2), a 20-fold enhancement in output power, compared to pristine PVDF-based nanogenerator after tuning the surface potential using a poling method. The markedly enhanced output performance is quite stab...

Research paper thumbnail of Research on Lead-free Piezoelectric Films for MEMS Applications

Physics and High Technology, 2013

Research paper thumbnail of Lead-free piezoceramics – Where to move on?

Journal of Materiomics, 2016

Lead-free piezoceramics aiming at replacing the market-dominant lead-based ones have been extensi... more Lead-free piezoceramics aiming at replacing the market-dominant lead-based ones have been extensively searched for more than a decade worldwide. Some noteworthy outcomes such as the advent of commercial products for certain applications have been reported, but the goal, i.e., the invention of a lead-free piezocermic, the performance of which is equivalent or even superior to that of PZT-based piezoceramics, does not seem to be fulfilled yet. Nevertheless, the academic effort already seems to be culminated, waiting for a guideline to a future research direction. We believe that a driving force for a restoration of this research field needs to be found elsewhere, for example, intimate collaborations with related industries. For this to be effectively realized, it would be helpful for academic side to understand the interests and demands of the industry side as well as to provide the industry with new scientific insights that would eventually lead to new applications. Therefore, this review covers some of the issues that are to be studied further and deeper, so-to-speak, lessons from the history of piezoceramics, and some technical issues that could be useful in better understanding the industry demands. As well, the efforts made in the industry side will be briefly introduced for the academic people to catch up with the recent trends and to be guided for setting up their future research direction effectively.

Research paper thumbnail of Preparation of LiFe PO4 Using Chitosan and its Cathodic Properties for Rechargeable Li-ion Batteries

Bulletin- Korean Chemical Society

The LiFePO4 powder was synthesized by using the solid state reaction method with Fe(C2O4)․2H2O, (... more The LiFePO4 powder was synthesized by using the solid state reaction method with Fe(C2O4)․2H2O, (NH4)2HPO4, Li2CO3, and chitosan as a carbon precursor material for a cathode of a lithium-ion battery. The chitosan added LiFePO4 powder was calcined at 350 o C for 5 hours and then 800 o C for 12 hours for the calcination. Then we calcined again at 800 o C for 12 hours. We characterized the synthesized compounds via the crystallinity, the valence states of iron ions, and their shapes using TGA, XRD, SEM, TEM, and XPS. We found that the synthesized powders were carbon-coated using TEM images and the iron ion is substituted from 3+ to 2+ through XPS measurements. We observed voltage characteristics and initial charge-discharge characteristics according to the C rate in LiFePO4 batteries. The obtained initial specific capacity of the chitosan added LiFePO4 powder is 110 mAh/g, which is much larger than that of LiFePO4 only powder.

Research paper thumbnail of Silk fibroin-based biodegradable piezoelectric composite nanogenerators using lead-free ferroelectric nanoparticles

Nano Energy, 2015

Silk fibroin-based biodegradable composite-type nanogenerators are demonstrated with controllable... more Silk fibroin-based biodegradable composite-type nanogenerators are demonstrated with controllable lifetime for powering to the implantable devices. The 2D thin filmand 1D wire-type composites consist of the well-dispersed lead-free ferroelectric (BaTiO 3 , ZnSnO 3 , Bi 0.5 (Na 0.82 K 0.18) 0.5 TiO 3 , and K 0.5 Na 0.5 Nb 0.995 Mn 0.005 O 3) nanoparticles. Ag nanowires are used to enhance the dispersion of the nanoparticles and polyvinylpyrrolidone prevents Ag nanowires from connecting with each other. A maximum output voltages and current densities of 2.2 V and 0.12 mA/cm 2 in the thin film, and 1.8 V and 0.1 mA/cm 2 in the wire are obtained under the motion of a foot step for when 30 wt% KNN:Mn nanoparticles are well-dispersed in the solution because of the largest piezoelectric coupling figure of merit. The properties of water-soluble composite films are also controlled with the glycerol content up to two days.

Research paper thumbnail of Dielectric and ferroelectric properties of La, Nd and V substituted Bi 4 Ti 3 O 12 ceramics

, Dielectric and ferroelectric properties of LaFeO 3 particles derived from metal organic framewo... more , Dielectric and ferroelectric properties of LaFeO 3 particles derived from metal organic frameworks precursor, Ceramics International,

Research paper thumbnail of Photocatalytic Hydrogen Generation from Water-methanol MixturesUsing Nanocrystalline ZnFe$_2$O$_4$ under Visible Light Irradiation

Journal of the Korean Physical Society, 2009

A spinel crystal structure of ferrite containing zinc (ZnFe2O4) was synthesized by using the poly... more A spinel crystal structure of ferrite containing zinc (ZnFe2O4) was synthesized by using the polymer complex method. A pure phase ZnFe2O4 was obtained at a relatively lower temperature of 900 • C. The as-synthesized nanoparticles are agglomerates of crystals with an average size of ∼50 nm, as estimated from the X-ray diffraction analysis. The band gap of the n-type semiconducting metal oxide, as determined by using an ultraviolet-visible diffuse reflectance spectrometer was found to be 1.90 eV (653 nm). The photocatalytic activity of ZnFe2O4 was investigated by using the photoreduction of water under visible light (≥420 nm), which was found to be much higher than that of the well-known TiO2−xNx photocatalyst. This superior performance is attributed to the larger surface area, the uniform and well-dispersed deposition of the Pt co-catalyst over the surface, and the small band gap of the chemically-prepared ferrite photocatalyst.

Research paper thumbnail of Raman- and Brillouin-scattering studies on lead-free piezoelectric Bi0.5(Na0.78K0.22)0.5−x TiO3 ceramics with A-site vacancies

Journal of the Korean Physical Society, 2013

The Raman and Brillouin spectra of lead-free piezoelectric Bi0.5(Na0.78K0.22)0.5−xTiO3 ceramics w... more The Raman and Brillouin spectra of lead-free piezoelectric Bi0.5(Na0.78K0.22)0.5−xTiO3 ceramics with A-site vacancies were investigated for x = 0 ∼ 0.05. The Raman spectra measured at room temperature showed seven broad bands over the range of 50-1000 cm −1 , reflecting strong anharmonicity, inherent A-site disorder and the resulting breakdown of the Raman selection rule. The splitting of the A1 mode near 270 cm −1 indicated that the tetragonal symmetry was maintained over the whole x range, consistent with the X-ray diffraction results. The Brillouin spectra exhibited distributed longitudinal and transverse acoustic modes, in addition to central peaks. The distributed acoustic modes were attributed to the scattering events at all scattering angles from 0 to 180 • due to multiple reflections and refractions at the grain boundaries. The relaxation time obtained from fitting the broad central peak by using a Debye function exhibited a slowing-down behavior in the paraelectric phase. The formation of the central peak was attributed to the existence of nano-sized polar regions due to the anharmonic vibrations and the hopping motions of the A-site cations.

Research paper thumbnail of The effect of annealing temperature on the piezoelectric and dielectric properties of lead-free Bi0.5(Na0.85K0.15)0.5TiO3 thin films

Journal of the Korean Physical Society, 2012

We fabricated Bi0.5(Na0.85K0.15)0.5TiO3 (BNKT) lead-free thin films on Pt(111)/TiO2/SiO2/ Si(100)... more We fabricated Bi0.5(Na0.85K0.15)0.5TiO3 (BNKT) lead-free thin films on Pt(111)/TiO2/SiO2/ Si(100) substrates by using a chemical solution deposition method. The BNKT thin films were annealed at 650, 700, 750, or 800 • C in an O2 atmosphere. X-ray diffraction patterns revealed a pure perovskite structure at annealing temperatures from 650 to 800 • C. A field emission electron microscope study revealed an increase in grain size with increasing annealing temperature. BNKT thin films annealed at 700 • C had a high remnant effective piezoelectric coefficient of 63.6 pm/V and a low leakage current density of 5.15 × 10 −6 A/cm 2 .

Research paper thumbnail of Piezoelectric and Ferroelectric Properties of Lead-Free Bi4−xNdxTi2.97V0.03O12Ceramics

Ferroelectrics, 2006

We fabricated lead-free Bi4−xNdxTi2.97V0.03O12 (x = 0.25∼1.00; BNdTV-x) ceramics using the conven... more We fabricated lead-free Bi4−xNdxTi2.97V0.03O12 (x = 0.25∼1.00; BNdTV-x) ceramics using the conventional solid-state reaction method and investigated their ferroelectric and piezoelectric properties. BNdTV ceramics exhibited the phase transition from orthorhombic to pseudo-tetragonal structure with increasing Nd concentration. At the morphotropic phase boundary of x = 0.85, BNdTV ceramics showed the highest values of Pr (13 μC/cm), kt (17%) and Qm

Research paper thumbnail of Phase Transition and Electrical Properties of BiAlO3-Modified (Bi0.5Na0.5)TiO3Piezoelectric Ceramics

Ferroelectrics, 2010

Lead-free piezoelectric (1-x)(Bi0.5Na0.5)TiO3–xBiAlO3 (BNT-BA, x = 0.00, 0.025, 0.050, 0.075, 0.1... more Lead-free piezoelectric (1-x)(Bi0.5Na0.5)TiO3–xBiAlO3 (BNT-BA, x = 0.00, 0.025, 0.050, 0.075, 0.100) ceramics were synthesized by a conventional solid- state reaction method. The BNT-BA ceramics exhibited enhanced ferroelectric and piezoelectric properties at x = 0.050. The x-ray diffraction analysis revealed a pure perovskite phase for x ≤ 0.075. A rhombohedral to pseudocubic phase transition was observed at x = 0.075 and maximum values for the electric field-induced strain (S = 0.13%) and dynamic piezoelectric coefficient (d*33 = 163 pm/V) were obtained at this composition. The enhancement in the electric field-induced strain is attributed to the rhombohedral to pseudocubic phase transition.

Research paper thumbnail of Dielectric Relaxation and Electrical Properties of Lead-Free LiNbO3-Substituted Na0.5K0.5NbO3Ferroelectric Ceramics

Ferroelectrics, 2010

ABSTRACT Na0.5K0.5NbO3 ceramics and LiNbO3-substituted Na0.5K0.5NbO3 ceramics were prepared by th... more ABSTRACT Na0.5K0.5NbO3 ceramics and LiNbO3-substituted Na0.5K0.5NbO3 ceramics were prepared by the solid-state reaction method. The crystallization and grain morphology of the two types of ceramics were confirmed by X-ray diffraction and atomic force microscopy studies, respectively. The 5 mol% LiNbO3 substituted Na0.5K0.5NbO3 ceramics shows well-saturated ferroelectric P-E hysteresis loops. The dielectric and electrical properties of the ceramics were investigated in the frequency range from 10 Hz to 1 MHz and the temperature range from 25°C to 600°C. Analysis of the complex impedance relaxation by the Cole-Cole plot showed one impedance relaxation for LiNbO3 substituted Na0.5K0.5NbO3. The contributions of electrical conduction in the above mentioned temperature and frequency ranges are discussed.

Research paper thumbnail of Dielectric and Piezoelectric Properties of Lead-Free Na 0.5 Bi 4.5−x La x Ti 4 O 15 and Na 0.5 Bi 4.5−x Nd x Ti 4 O 15 Ceramics

Ferroelectrics, 2002

The dielectric and piezoelectric properties of lead-free bismuth layered structure Na 0.5 Bi 4.5 ... more The dielectric and piezoelectric properties of lead-free bismuth layered structure Na 0.5 Bi 4.5 m x La x Ti 4 O 15 (NBLT) and Na 0.5 Bi 4.5 m x Nd x Ti 4 O 15 (NBNT) ceramics are studied. The T c and the l r of the NBLT and NBNT ceramics has been decreased and the shape of

Research paper thumbnail of Low-frequency dielectric relaxation and ac conduction of SrBi2Ta2O9 thin film grown by pulsed laser deposition

Applied Physics Letters, 2002

Bi-excess SrBi 2 Ta 2 O 9 ͑SBT͒ thin films on Pt/Ti/SiO 2 /Si substrate were prepared by pulsed l... more Bi-excess SrBi 2 Ta 2 O 9 ͑SBT͒ thin films on Pt/Ti/SiO 2 /Si substrate were prepared by pulsed laser deposition technique. The SBT structure was characterized by x-ray diffraction studies. The ferroelectric properties were confirmed by P-E hysteresis loops at different applied electric fields. The dielectric constant and the ac conductivity of the Pt/SBT/Pt capacitor were investigated in the frequency range from 0.01 Hz to 100 kHz and in the temperature range from 25 to 400°C. The thermal activation energy of 0.90 eV is observed in the frequency dependent dielectric constant. The activation energy for conduction process is calculated as 0.91 eV from the slope of ac conductivity at the lowest frequency. The low-frequency dielectric relaxation and the ac conductivity of Bi-excess SBT thin film are discussed in relation to the electrical conduction of SBT/Pt junction.

Research paper thumbnail of Ferroelectric and piezoelectric properties of Na0.52K0.48NbO3 thin films prepared by radio frequency magnetron sputtering

Applied Physics Letters, 2009

ABSTRACT

Research paper thumbnail of Controlling polymer shape through the self-assembly of dendritic side-groups

Nature, 1998

V. Percec*, C.-H. Ahn*, G. Ungar†, DJP Yeardley†, M. Mo¨ller‡ & SS Sheiko‡ * The WM Keck ... more V. Percec*, C.-H. Ahn*, G. Ungar†, DJP Yeardley†, M. Mo¨ller‡ & SS Sheiko‡ * The WM Keck Laboratories for Organic Synthesis, Department of Macromolecular Science, Case Western Reserve University, Cleveland, Ohio 44106-7202, USA † Department of Engineering Materials ...

Research paper thumbnail of Low-Temperature Sintering of Bi 0.5 (Na,K) 0.5 TiO 3 for Multilayer Ceramic Actuators

Journal of the American Ceramic Society, 2015

We investigated the influence of CuO amount (0.5-3.0 mol%), sintering temperature (900°C-1000°C),... more We investigated the influence of CuO amount (0.5-3.0 mol%), sintering temperature (900°C-1000°C), and sintering time (2-6 h) on the low-temperature sintering behavior of CuOadded Bi 0.5 (Na 0.78 K 0.22) 0.5 TiO 3 (BNKT22) ceramics. Normalized strain (S max /E max), piezoelectric coefficient (d 33), and remanent polarization (P r) of 1.0 mol% CuO-added BNKT22 ceramics sintered at 950°C for 4 h was 280 pm/V, 180 pC/N, and 28 lC/cm 2 , respectively. These values are similar to those of pure BNKT22 ceramics sintered at 1150°C. In addition, we investigated the performance of multilayer ceramic actuators made from CuO-added BNKT22 in acoustic sound speaker devices. A prototype sound speaker device showed similar output sound pressure levels as a Pb(Zr,Ti)O 3-based device in the frequency range 0.66-20 kHz. This result highlights the feasibility of using low-cost multilayer ceramic devices made of lead-free BNKT-based piezoelectric materials in sound speaker devices.