Jung-Hoon Choi

Domain formation in epitaxial Pb(Zr, Ti)O3 thin films

Ferroelectric twin-domain structures in epitaxial Pb(Zr,u200aTi)O3 (PZT) thin films grown on various single-crystal substrates such as MgO(001), KTaO3(001), and SrTiO3(001) were investigated by two-dimensional reciprocal space mapping using synchrotron x-ray diffraction. Each system showed a characteristic domain structure. PbTiO3 thin films grown on MgO(001) showed highly c-axis oriented domain structures consisting of a periodic array of 90° twinlike domains. Perfectly c-axis oriented films were obtained on SrTiO3(001), while the films grown on KTaO3(001) showed a-domain dominant structures with a small amount of c domains embedded in matrix a domains. Contributions of net elastic strain stored in each heteroepitaxial layer and its relaxation to the final domain structures were evaluated considering thermodynamic equilibrium relief of coherency strain by misfit dislocation generation at the film growth temperature. A comparison between theoretical consideration and experimental results clearly demonstrates ...

Enhanced Stability of Laminated Graphene Oxide Membranes for Nanofiltration via Interstitial Amide Bonding

Laminated graphene oxide (GO) has promising use as a membrane because of its high permeance, chemical and mechanical stability, as well as the molecular sieving effect of its interlayers. However, the hydrophilic surface of GO, which is highly decorated with oxygen groups, easily induces delamination of stacked GO films in aqueous media, thereby limiting the practical application. To stabilize GO films in aqueous media, we functionalized a polymer support with branched polyethylene-imine (BPEI). BPEI adsorbed intercalated into the stacked GO sheets via diffusion during filtration. The GO/BPEI membrane obtained exhibits high stability during sonication (>1 h duration, 40 kHz frequency) in water within a broad pH range (2-12). In contrast, the GO film spontaneously delaminated upon sonication. Furthermore, BPEI treatment did not affect the filtration performance of the GO film, as evidenced by the high rejection rates (>90%) for the dye molecules methylene blue, rose bengal, and brilliant blue and by their permeation rates of ca. 124, 34.8, 12.2, and 5.1%, respectively, relative to those of a typical GO membrane.

Enhanced water permeation based on nanoporous multilayer graphene membranes: the role of pore size and density

Bulk scale graphenes containing narrow and dense pores are realized via potassium hydroxide activation of pre-oxidized graphite (size: ca. 3 nm, density: ca. 1015 m−2). A film (20 nm thickness) comprised of this nanoporous graphene displays much enhanced water flux (ca. 37 L m−2 h−1 bar−1) compared to that of conventional graphene oxide membranes (∼6 times), while maintaining the seiving performances of GO laminates with ca. 20% rejection for NaCl and up to 99% rejection for various dyes around 1 nm diameter size. This advantageous property is a result of the fact that in addition to the effect of the interlayer stacking of graphene sheets, nanopores in the graphene generated by using the new method serve as additional channels through which water molecules can diffuse. We believe that the new approach will play a key role in preparing and designing graphene membranes with high flux.

Fabrication and physical properties of lanthanide oxide glass wasteform for the immobilization of lanthanide oxide wastes generated from pyrochemical process

The pyrochemical process, which uses a dry method to recycle used nuclear fuel generates waste LiCl–KCl salt containing radioactive lanthanide elements. To reuse LiCl–KCl salt, the lanthanide elements are separated through a precipitation method promoted by oxygen sparging and the separated fission product of lanthanide oxide should be fabricated into durable wasteforms sustainable for several 1,000xa0years to store in a final geological repository. Herein, we report the fabrication of a borosilicate glass based wasteform with a glass matrix of SiO2–Al2O3–B2O3 having a high waste loading of 50xa0wt% lanthanide oxide. Th physical properties of four kinds of wasteforms having a different lanthanide oxide waste composition were evaluated. To investigate the long-term physical stability of each sample having 50xa0wt% lanthanide oxide waste loading, time–temperature–transformation (TTT) test was conducted at 500 and 700xa0°C for 60 and 180xa0h, and the physical properties were evaluated after each TTT test.

Purification of LiCl–KCl eutectic waste salt containing rare earth chlorides delivered from the pyrochemical process of used nuclear fuel using a reactive distillation process

In the pyrochemical process of used nuclear fuel, the purification of waste salts containing radioactive nuclides can greatly contribute to a radioactive waste reduction. For this reason, the purification of LiCl–KCl eutectic salt containing rare earth chlorides was performed using a series of the phosphorylation process and the distillation process. LiCl–KCl eutectic salt recovered from the purification had a very low concentration (<1xa0ppm) for the rare earth chlorides. The recycling feasibility of the recovered salt was verified through a uranium electro-deposition test using LiCl–KCl eutectic salt as the electrolyte. Based on these results, one body type of reactive distillation equipment with two top covers was designed.

Ultrathin graphene oxide membranes on freestanding carbon nanotube supports for enhanced selective permeation in organic solvents

Among the various factors required for membranes in organic solvent separations, the stability of membrane supports is critical in the preparation of membranes with universal chemical stability, mechanical flexibility, and high flux. In this study, nanoporous freestanding carbon nanotube (CNT) films were fabricated and utilized as supports for enhanced permeation in organic solvents. The excellent chemical stability of the CNT support allowed it to withstand various organic solvents such as toluene, acetone, and dimethylformamide. In addition, the structural stability and high pore density of CNT supports allowed the deposition of an ultrathin selective layer for an enhanced-flux membrane. Membrane performance was demonstrated by depositing a thin graphene oxide (GO) layer on the CNT support; GO was selected because of its high chemical stability. CNT-supported GO membranes effectively blocked molecules with molecular weight larger than ~800u2009gu2009mol−1 while allowing the fast permeation of small molecules such as naphthalene (permeation was 50 times faster than that through thick GO membranes) and maintaining selective permeation in harsh solvents even after 72u2009hours of operation. We believe that the developed CNT support can provide fundamental insights in utilizing selective materials toward organic solvent membranes.

Pore-Size-Tuned Graphene Oxide Frameworks as Ion-Selective and Protective Layers on Hydrocarbon Membranes for Vanadium Redox-Flow Batteries

The laminated structure of graphene oxide (GO) membranes provides exceptional ion-separation properties due to the regular interlayer spacing ( d) between laminate layers. However, a larger effective pore size of the laminate immersed in water (∼11.1 Å) than the hydrated diameter of vanadium ions (>6.0 Å) prevents its use in vanadium redox-flow batteries (VRFB). In this work, we report an ion-selective graphene oxide framework (GOF) with a d tuned by cross-linking the GO nanosheets. Its effective pore size (∼5.9 Å) excludes vanadium ions by size but allows proton conduction. The GOF membrane is employed as a protective layer to address the poor chemical stability of sulfonated poly(arylene ether sulfone) (SPAES) membranes against VO2+ in VRFB. By effectively blocking vanadium ions, the GOF/SPAES membrane exhibits vanadium-ion permeability 4.2 times lower and a durability 5 times longer than that of the pristine SPAES membrane. Moreover, the VRFB with the GOF/SPAES membrane achieves an energy efficiency of 89% at 80 mA cm-2 and a capacity retention of 88% even after 400 cycles, far exceeding results for Nafion 115 and demonstrating its practical applicability for VRFB.

Ultrasmall Grained Pd Nanopattern H2 Sensor

Precise control of the size and interfaces of Pd grains is very important for designing a high-performance H2 sensing channel because the transition of the Pd phase from α to β occurs through units of single grains. However, unfortunately, the grain controllability of previous approaches has been limited to grains exceeding 10 nm in size and simple macroscopic channel structures have only shown monotonic response behavior for a wide concentration range of H2. In this work, for the first time, we found that Pd channels that are precisely grain-controlled show very different H2 sensing behavior. They display dual-switching response behavior with simultaneous variation of the positive and negative response direction within single sensor. The Pd nanopattern channel having smallest grain size/interface among previous works could be fabricated via unique lithographic approaches involving low-energy plasma (Ar+) bombardment. The ultrasmall grain size (5 nm) and narrow interface gap (<2 nm) controlled by Ar+ plasma bombardment enabled both the hydrogen-induced lattice expansion (HILE) (Δ RH2 < 0) and surface electron scattering (Δ RH2 > 0) mechanisms to be simultaneously applied to the single Pd channel, thereby inducing dual-switching response according to the H2 concentration range. In addition, the unique high-aspect-ratio high-resolution morphological characteristics made it possible to achieve highly sensitive H2 detecting performance (limit of detection: 2.5 ppm) without any hysteresis and irreversible performance degradation. These noteworthy new insights are attributed to high-resolution control of the grain size and the interfaces with the Pd nanostructure channel.