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하윤철,정대영 한국물리학회 2010 THE JOURNAL OF THE KOREAN PHYSICAL SOCIETY Vol.57 No.61
A series of processes for the fast fabrication of nanoporous anodic alumina membranes with highaspect-ratio, self-ordered pore arrays was developed based on a high-field 2-step anodization in a 0.3 M oxalic electrolyte. The dielectric breakdown commonly driven by the high electric field was circumvented by using a linear sweep of the initial voltage from 0 to 140 V, followed by a constant voltage of 140 V for the first step and by using a controlled growth rate that was adjusted by varying the electrolyte concentration while applying an instantaneous constant voltage of 140 V for the second step. A thick nanoporous film of about 120 um was grown within 2 hours with an average interpore distance of 310 nm and an average pore size of 50 nm, where the aspect ratio of the pores was over 2000. In order to overcome the problems associated with a thick barrier layer formed during the high-field anodization, we applied a pulsed electrochemical detachment technique to remove the base Al metal. A through-hole membrane with a pore size of about 210 nm was fabricated after widening the pores through a chemical etching of the pore walls. These novel processes ensure reliable fabrication of a high-field nanoporous anodic alumina membrane and provide a new template for nano-scale research.
하윤철 한국전기화학회 2022 한국전기화학회지 Vol.25 No.3
The development of non-flammable all-solid-state batteries (ASSLBs) has become a hot topic due to the known drawbacks of commercial lithium-ion batteries. As the possibility of applying sulfide solid electrolytes (SSEs) for electric vehicle batteries increases, efforts for the low-cost mass-production are actively underway. Until now, most studies have used highenergy mechanical milling, which is easy to control composition and impurities and can reduce the process time. Through this, various SSEs that exceed the Li+ conductivity of liquid electrolytes have been reported, and expectations for the realization of ASSLBs are growing. However, the high-energy mechanical milling method has disadvantages in obtaining the same physical properties when mass-produced, and in controlling the particle size or shape, so that physical properties deteriorate during the full process. On the other hand, wet chemical synthesis technology, which has advantages in mass production and low price, is still in the initial exploration stage. In this technology, SSEs are mainly manufactured through producing a particle-type, solution-type, or mixed-type precursor, but a clear understanding of the reaction mechanism hasn’t been made yet. In this review, wet chemical synthesis technologies for SSEs are summarized regarding the reaction mechanism between the raw materials in the solvent.
하윤철,조주현 한국물리학회 2010 THE JOURNAL OF THE KOREAN PHYSICAL SOCIETY Vol.57 No.61
A novel process for the synthesis of nano-sized metallic Sn powders in liquid media was developed based on a high-energy electrical explosion technique. The liquid, i.e., ethanol in this work, prevented the agglomeration of the powders produced by electrical explosions of conductive wires, protected the metal surface from oxidation, and enabled a size-based separation of the nanopowders. By repeatedly charging a capacitor and transferring the stored electrical energy to the Sn wire, we produced several tens of grams of Sn powders within an hour. Large-sized powders were easily removed by gravitational sedimentation or centrifugation. Scanning electron microscopy and X-ray diffraction analyses showed that the resulting Sn nanopowders had a narrow size distribution with an average size of ∼50 nm in a highly-crystalline metallic form.
Wire Explosion Synthesis of a Sn/C Nanocomposite as an Anode Material for Li Secondary Batteries
하윤철,Chungil Kang,Chuhyun Cho,Young-Ugk Kim,박철민,손헌준 한국물리학회 2011 THE JOURNAL OF THE KOREAN PHYSICAL SOCIETY Vol.59 No.61
As a top-down approach for nanomaterial synthesis, a "wire explosion in liquid media" process was applied to the preparation of a Sn/C nanocomposite used as an alternative anode material for lithium secondary batteries. Using a highly-dispersed Sn nanoparticle suspension produced by electrical explosion of Sn wires in ethanol followed by gravimetric sedimentation for size separation, addition of a carbon precursor and pyrolysis, we prepared nano-sized Sn particles embedded in a carbon matrix. The physical properties of the nano-sized Sn and its carbon composite were characterized by scanning electron microscopy, transmission electron microscopy, X-ray diffraction and differential scanning calorimetry. The electrochemical property was analyzed by cycled charge/discharge tests.?The resulting Li storage performance of the Sn/C nanocomposite showed an initial discharge capacity of over 400 mAhg^(-1) and improved capacity retention of 340 mAhg^(-10 after 40 cycles. The simplicity and scalability of nanocomposite synthesis provided by the "wiire explosion in a liquid medium" process make this method a promising tool not only for a new nanomaterial investigation but also for a commercial production of nanomaterials.
직류전기철도 전식대책 실증실험(1) : 누설전류 배류시스템
하윤철(Yoon-Cheol Ha),배정효(Jeong-Hyo Bae),하태현(Tae-Hyun Ha),이현구(Hyun-Goo Lee),김대경(Dae-Kyeong Kim),최정희(Jeong-Hee Choi) 대한전기학회 2007 대한전기학회 학술대회 논문집 Vol.2007 No.10
With the wide spread of direct current (DC) electric railroads in Korea, the stray currents or leakage currents from negative return rails become a pending problem to the safety of nearby underground infrastructures. The most widely used mitigation method for this interference is the stray current drainage method, which connects the underground metallic structures to the rails with diodes (polarized drainage) or thyristor (forced drainage). Although this method inherently possesses some drawbacks, its cost effectiveness and efficiency to protect the interfered structures has been the main reason for the wide adoption. In this paper, we show the field test results for the application of stray current drainage system to a city gas pipeline paralleling a depot area of a metropolitan rapid transit system. The process for optimal positioning is briefly illustrated. The effectiveness of constant voltage, constant current, and constant potential drainage schemes was also described.