Jeong Seog Kim

Weak ferromagnetism in the ferroelectric BiFeO3–ReFeO3–BaTiO3 solid solutions (Re=Dy,La)

The binary and ternary solid solutions, BiFeO3–BaTiO3, BiFeO3–ReFeO3–BaTiO3 (Re=Dy,Pr,La), and BiFeO3–BaFeO2.5–BaTiO3 have been explored for attaining ferromagnetic ferroelectrics in bulk ceramics and understanding the effect of rare earth orthoferrites ReFeO3 on the spontaneous magnetization. The coexistence of ferromagnetism and ferroelectricity has been observed over the composition range of 0.2⩽x⩽0.4 in the (1−x)BiFeO3–xBaTiO3 at room temperature. The introduction of DyFeO3 and LaFeO3 expands the composition range of the coexistence. The most superior ferromagnetic ferroelectrics obtained in this study are the 0.65BiFeO3–0.025DyFeO3–0.325BaTiO3 (Pr=5 μC/cm2,Mr=0.1 emu/g), 0.4875BiFeO3–0.025DyFeO3–0.4875BaTiO3 (Pr=7 μC/cm2,Mr=0.06 emu/g), and 0.475BiFeO3–0.05LaFeO3–0.475BaTiO3 (Pr=3.2 μC/cm2,Mr=0.2 emu/g). The spontaneous magnetization strongly depends on both the type and amount of the substitution components, DyFeO3, LaFeO3, PrFeO3, and BaFeO2.5 rather than the degree of G-type antiferromagnetic orde...

Ferroelectric and ferromagnetic properties of BiFeO3–PrFeO3–PbTiO3 solid solutions

Solid solutions of xBiFeO3–yPrFeO3–zPbTiO3 (x+y+z=1) and (1−w)BiFeO3–wPbTiO3 have been explored for finding ferroelectromagnetic bulk material, in which ferroelectricity and ferromagnetism coexist simultaneously. The coexistence has been observed only in some ternary composition samples, that is, 0.2BiFeO3–0.2PrFeO3–0.6PbTiO3 and 0.4BiFeO3–0.2PrFeO3–0.4PbTiO3. In the ternary solid solutions, spontaneous magnetic moments disappear with the decrease of PrFeO3 content to y<0.2 independently of BiFeO3 content. When PrFeO3 content remains constant at y=0.2, the ternary solid solutions become paraelectric with the decrease of PbTiO3 content to z⩽0.2. The ferroelectromagnetic solid solutions have the noncentrosymmetric and doubled perovskite unit cell with a space group I4cm (a=b≈5.4 A, c≈7.9 A). Addition of Ta2O5 dopant substantially changes the polarization–electric-field and magnetization–magnetic field curves of the 0.2BiFeO3–0.2PrFeO3–0.6PbTiO3. The binary solid solutions of (1−w)BiFeO3–wPbTiO3 do not exhib...

Low-temperature solution growth of ZnO nanotube arrays

Summary Single crystal ZnO nanotube arrays were synthesized at low temperature in an aqueous solution containing zinc nitrate and hexamethylenetetramine. It was found that the pH value of the reaction solution played an important role in mediating the growth of ZnO nanostructures. A change in the growth temperature might change the pH value of the solution and bring about the structure conversion of ZnO from nanorods to nanotubes. It was proposed that the ZnO nanorods were initially formed while the reaction solution was at a relatively high temperature (~90 °C) and therefore enriched with colloidal Zn(OH)2, which allowed a fast growth of ZnO nanocrystals along the [001] orientation to form nanorods. A decrease in the reaction temperature yielded a supersaturated solution, resulting in an increase in the concentration of OH− ions as well as the pH value of the solution. Colloidal Zn(OH)2 in the supersaturated solution trended to precipitate. However, because of a slow diffusion process in view of the low temperature and low concentration of the colloidal Zn(OH)2, the growth of the (001) plane of ZnO nanorods was limited and only occurred at the edge of the nanorods, eventually leading to the formation of a nanotube shape. In addition, it was demonstrated that the pH might impact the surface energy difference between the polar and non-polar faces of the ZnO crystal. Such a surface energy difference became small at high pH and hereby the prioritized growth of ZnO crystal along the [001] orientation was suppressed, facilitating the formation of nanotubes. This paper demonstrates a new strategy for the fabrication of ZnO nanotubes on a large scale and presents a more comprehensive understanding of the growth of tube-shaped ZnO in aqueous solution at low temperature.

Crystal structure and phase transitions of Sr1±xBi2±yTa2O9 ceramics

Abstract Crystal structural parameters, such as space group, lattice parameters, apparent valences (AV), and site occupancies were analyzed in Sr 1± x Bi 2± y Ta 2 O 9 ceramics with varying ceramic compositions and diffraction temperature. The crystal structure was refined by Rietveld method using neutron and X-ray powder diffraction data. The phase transition temperature T c was measured on these ceramics. The spontaneous polarization P s was calculated from the refined parameters. The measured T c decreased with increasing Sr-content. The crystal structure of Sr 1 Bi 2 Ta 2 O 9 changed from A21am at room temperature to B2cb at temperature above 400°C and to a higher symmetry one eventually. The cation place exchanges were observed between Sr 2+ and Bi 3+ and the calculated AVs were in accordance with the cation exchanges.

Dielectric properties and crystal structure of Ba6-3x(Nd, M)8+2xTi18O54 (M=La, Bi, Y) microwave ceramics

Crystal structure and dielectric property of tungsten-bronze type microwave dielectric ceramics, i.e., BaOLa2O34TiO2 and Ba6-3x (Nd, M)d8+2x Ti18O54 (M = Y, Bi and x = 0.5, 0.7), are analysed. The optimum properties obtained in Ba(Nd1-xBix)2O34TiO2 were εr = 89–92, Qf = 5855–6091 GHz, and τf = −7–+7 ppm/°C x = 0.04–0.08. The Y-substitution in BaO(Nd1-xYx)2O34TiO2 reduces the dielectric constant εr. Both the Y and Bi substitutions make τεr positive. The relative dielectric constant εr and temperature coefficient τεr are 109.5 and −180 ppm/°C in BaOLa2O34TiO2, 76 and +40 ppm/°C in BaO(Nd0.77Y0.23)2O34TiO2, respectively. The crystal structures were refined by Rietveld method using x-ray and neutron diffraction data. The most reliable results were obtained by refining the cation positions using the x-ray data and the oxygens from the neutron with a superlattice structure model Pnam(c-axis ≈ 7.6 Å). The refined structures show that the a/c ratios are related to the apical oxygen displacements of the Ti–O octahedra. The substitution of the small radius atom, Y, produced a structure of severely tilted and distorted Ti–O octahedra and large a/c ratio, while the large radius atom, La, small a/c ratio. Differential scanning calorimetry analysis showed heat anomaly indicating suspected phase transition in these materials. The relation between τεr and octahedron tilting in tungsten-bronze type material is discussed in relation with complex perovskite structure.

Low temperature firing of PZT thick films prepared by screen printing method

Abstract PZT thick films were prepared on alumina substrate by screen printing method. The paste was made from PbTiO 3 –PbZrO 3 –Pb(Mn 1/3 Nb 2/3 )O 3 ternary system with 1 wt.% excess PbO and organic vehicles. Screen-printed thick films were fired at 750–950 °C for 1 h either in air or PbO atmosphere (PbZrO 3 +10 mol%ZrO 2 ). Pyrochlore phase was observed in PZT thick films fired in air at 950 °C in addition to a perovskite phase, while only a perovskite phase was formed in PbO atmosphere. PZT thick films fired in PbO atmosphere showed denser microstructure and better ferroelectric properties than those fired in air. The PZT thick film fired at 800 °C in PbO atmosphere showed very well developed P–E hysteresis, remanent polarization of 28.0 μC/cm 2 and coercive field of 52.7 kV/cm. The PZT thick film fired at 800 °C in PbO atmosphere can be applied to fabrications of piezoelectric microactuators or microsensors using Si micromachining technology.

Crystal Structures and Low Temperature Cofiring Ceramic Property of (1-x)(Li, RE)W2O8–xBaWO4 Ceramics (RE=Y, Yb)

The crystal structures and low temperature cofiring ceramic (LTCC) microwave dielectric properties of ABW2O8 (A=Li, Na, Zn, B=Y, Yb, Mn, Fe) and the (1-x)LiYW2O8–xBaWO4 binary system have been characterized. LiYW2O8 has a new type of crystal structure in-between the wolframite (MgWO4) and scheelite (BaWO4) structures: the space group P2 and the lattice parameters of a=9.9815(5), b=5.7913(3), c=5.0058(2) A, and β=94.2158(2)°. The new materials LiREW2O8 (RE=Y, Yb) have highly positive τf values (+63.8 ppm/°C for RE=Y) in contrast to the negative τf values of BaWO4 and MgWO4. τf can be tailored to near zero by combining LiYW2O8 and BaWO4. The effects of sinter-additives on τf, Qf0 (GHz), and sintering temperature have been also characterized. The crystal structure has been analyzed by the Rietveld refinement method using neutron diffraction data.

Ferroelectric and ferromagnetic properties of 0.2BiFeO3–0.2RFeO3–0.6ATiO3 (R=Pr, Nd and A=Ba, Pb) and 0.8BiFeO3–0.2BaTiO3

Crystal structure, ferromagnetic and electric polarizations have been characterized for attaining the ferroelectromagnetic materials in the ternary and binary solid solutions consisting of ferroelectric and antiferromagnetic end members: BiFeO 3 (BF), PrFeO 3 (PF), NdFeO 3 (NF), BaTiO 3 (BT),and PbTiO 3 (PT). The XRD and neutron diffraction data collected in the temperature range 10-300 K have been analyzed by the Rietveld refinement method. Every prepared sample shows weak ferromagnetization with a maximum remanent magnetization M r =0.5 emu/g in the 0.8BiFeO 3 0.2BaTiO 3 . The 0.2BF-0.2NF 0.6PT(0.0075 mol Ta 2 O 5 ), 0.8BF-0.2BT, and 0.2BF-0.2PF-0.6PT(0.0075 mol Ta 2 O 5 ) having the noncentrosymmetric structure 14cm (a=b0.54 nm, c0.78 nm) show a ferroelectricity with remanent polarization Pr = 1.2-3 μC/cm 2 .

Neutron, Electrical, and Magnetic Investigations of the PrFeO3–PbTiO3 System

The solid solutions of the (1-x)PrFeO3–xPbTiO3 system were explored for the purpose of finding ferroelectromagnetism. Neutron diffraction data were collected at room temperature to 700°C and refined by the Rietveld method for magnetic and crystal structure analyses. Dielectric permittivity (er) and loss tangent were measured by a low-frequency impedance analyzer for varying frequency. The hysteresis curves of the polarization-electric field and magnetization-magnetic field were measured at room temperature. At room temperature, the crystal structure symmetry of the solid solutions change from orthorhombic to tetragonal i.e., Pbnm, I4 cm, and P4 mm with increasing PbTiO3 content. The solid solution at x=0.8 undergoes structural transition from tetragonal to cubic phase with increasing temperature in a series of I4 cm, I4/mcm (P4/mmm), and Pm3m. The solid solution composition of 0.4≤x≤0.6 exhibited ferromagnetism and the compositions of x≥0.4 were ferroelectric.

Role of (Bi1/2K1/2)TiO3 in the dielectric relaxations of BiFeO3-(Bi1/2K1/2)TiO3 ceramics

Temperature-dependent dielectric relaxations of (1 − x)BiFeO3-x(Bi1/2K1/2)TiO3 (BF-BKT) lead-free piezoceramics (0.4 ≤ x ≤ 0.8) were investigated via impedance spectroscopic techniques. Regardless of the compositions, the dielectric maximum temperatures exhibit a frequency-dependent dispersion, originating from a Debye relaxation due to the presence of oxygen vacancies. It was also observed that there exist local dielectric maxima due to the relaxation of polar nanoregions as a shoulder on the lower temperature side. The onset temperature for the Debye-type relaxation decreased with decreasing BKT content, gradually overlapping with the low-temperature dielectric dispersion from the relaxation of polar nanoregions. It is proposed that the role of BKT in the BF-BKT system is to enhance the random fields that favor a relaxor state and to suppress the Debye-type relaxation of oxygen vacancy related dipoles.