Chae Il Cheon

Orientational Coupling Amplification in Ferroelectric Nematic Colloids

We investigated the physical properties of low concentration ferroelectric nematic colloids, using calorimetry, optical methods, infrared spectroscopy, and capacitance studies. The resulting homogeneous colloids possess a significantly amplified nematic orientational coupling. We find that the nematic orientation coupling increases by approximately 10% for particle concentrations of 0.2%. A manifestation of the increased orientational order is that the clearing temperature of a nematic colloid increases by up to 40 degrees C compared to the pure liquid crystal host. A theoretical model is proposed in which the ferroelectric particles induce local dipoles whose effective interaction is proportional to the square of the orientational order parameter.

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...

Preparation of ferroelectric nanoparticles for their use in liquid crystalline colloids

In this paper we summarize our many years of experience in the preparation and optimization of stable colloids of ferroelectric nanoparticles dispersed in an isotropic carrier and in a liquid crystal host. The colloids are of interest for use in electro-optic devices, photorefractive hybrids and nonlinear optical elements. We also outline some of the most interesting features the nanoparticles bring to liquid crystals, along with the potential of these relatively new colloids.

Ferroelectric Particles in Liquid Crystals: Recent Frontiers

In this article we describe electro-optical properties of recently discovered ferroelectric particles/liquid crystal colloids. We show that the presence of ferroelectric particles in a liquid crystal changes its birefringence and dielectric anisotropy. In contrast to the traditional time consuming and expensive chemical synthetic methods, this method to create liquid crystals with enhanced properties is relatively simple and has a great potential. We also demonstrate the performance of these new materials in various devices, including displays, light modulators, and beam steering devices.

Ferroelectric nanoparticle/liquid-crystal colloids for display applications

— A non-synthetic approach to modify liquid crystals (LCs) by dispersing low concentrations of ferroelectric nanoparticles is reported. These dilute colloids are stable and appear similar to a pure LC. However, by changing the concentration and the type of ferroelectric particles, the physical properties of the LC materials can be tailored, including the nematic isotropic transition temperature (TNI), the dielectric constants, the birefringence (Δn), elastic constants, and the threshold voltage. Specifically, doping low concentrations of BaTiO3 nanoparticles (∼1%) into a LC MLC-6609 increases TNI up to ∼40°C. This giant shift of TNI has never been previously reported and indicates strong interactions between the LC and ferroelectric nanoparticles on a molecular level. The doped LC also demonstrates significant enhancement in birefringence (by 10–30%), dielectric anisotropy (by ∼50%), and the elastic constant K33 (by ∼20%). Ferroparticles act as molecular additives and modify the intrinsic properties of LC materials without time consuming and expensive chemical synthesis. The new LC materials are very attractive and suitable for use in displays, switchable lenses, beam steering, as well as other light-controlling devices.

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.

Low temperature hydrothermal epitaxy and Raman study of heteroepitaxial BiFeO3 film

By hydrothermal epitaxy, a heteroepitaxial BiFeO3 (BFO) film of ∼2.5 μm thickness was grown on a (100)-oriented single-crystalline SrTiO3 (STO) substrate at 200 °C, which is at least 250 °C lower than the conventional techniques used thus far. The x-ray diffraction pattern indicated the formation of a single-phase perovskite structure that was highly oriented along the (00l) lattice plane. Pole-figure and Φ-scan results confirmed the cube-on-cube epitaxial relationship of BFO∥STO. The Raman active vibrational modes showed anomalous changes in the vicinity of the Neel temperature (TN), which was attributed to spin-phonon coupling.

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.

New non‐synthetic method to modify properties of liquid crystals using micro‐ and nano‐particles

— The modification of the properties of existing LCs by doping them with ferroelectric micro- and nano-particles will be reported. This approach, in contrast to the conventional time-consuming and expensive chemical synthetic methods, enriches and enhances the electro-optical performance of many liquid-crystal materials. The effect of the ferroelectric particles on the nematic, smectic, and cholesteric phases will be discussed. The performance of these new composite systems in various devices, including displays, light modulators, and beam-steering devices, will be reported.