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27Al pulsed nuclear magnetic resonance study of CeAl2
강기혁,이무희 한국물리학회 2014 Current Applied Physics Vol.14 No.3
The magnetic properties and the electronic structures of a rare-earth aluminum intermetallic compound CeAl2 are investigated by magnetic susceptibility measurements and 27Al pulsed nuclear magnetic resonance (NMR) techniques. The magnetic susceptibility is strongly temperature-dependent, following a CurieeWeiss law down to w12 K, and shows an antiferromagnetic transition at 4 K. The 27Al NMR spectra show a typical powder pattern for a nuclear spin I of 5/2 with the second-order nuclear quadrupole interaction at high temperature and an additional large dipolar broadening between the 4f electron spins of cerium and the 27Al nuclear spins at low temperature. The 27Al NMR Knight shift follows the same temperature dependence as the magnetic susceptibility, suggesting that the 27Al NMR Knight shift originates from the transferred hyperfine field of the Ce 4f electron spins with the hyperfine coupling constant of A ¼ þ5.7 kOe/mB. The spin-lattice relaxation rate 1/T1 is roughly proportional to temperature, as with most non-magnetic metals at high temperature, and then strongly temperaturedependent, increasing rapidly with a peak near the antiferromagnetic transition temperature and decreasing at lower temperature. The temperature dependence of the Korringa ratio K, however, suggests that the antiferromagnetic spin fluctuation signature, which is an enhancement in the Korringa ratio, is washed out owing to the geometrical cancellation of Ce 4f fluctuations at the Al sites.
Orientation of a MgB2 Powder in a Magnetic Field
강기혁,B. J. Mean,김성훈,B. Ndiaye,조병기,J. Y. Kim,이무희 한국물리학회 2010 THE JOURNAL OF THE KOREAN PHYSICAL SOCIETY Vol.57 No.61
We have attempted magnetic orientation of MgB2 powder grains in an epoxy resin under a 7-T horizontal magnetic field. The horizontal field was crucial to get rid of gravitational sedimentation by rotating the mixture of MgB2 powder grains and the epoxy resin along the horizontal axis. Since we did not know the principal axes and the magnitude of the magnetic anisotropy tensor for MgB2, we devised different alignment schemes assuming two cases: case I for the easy axis along the c-axis and case II for the two easy axes perpendicular to the c-axis. For each case, we built different rotation devices: namely, for case I, the rotation axis was parallel to the magnetic field whereas, for case II, it was perpendicular to the magnetic field. For both cases, the powder grains should be aligned along the rotation axis. For assessment of the c-axis alignment, X-ray diffraction (XRD) and 11B nuclear magnetic resonance (NMR) measurements were utilized. However, the XRD and 11B NMR data for both cases were the same as those for the powder sample and showed no evidence of orientation. For possible scenarios, we suggest two possibilities: negligible anisotropy of the magnetic susceptibility tensor and the large size of the ground MgB2 powder particles still in multi-grains.
A review on the Mo-precursors for catalytic hydroconversion of heavy oil
강기혁,김규태,박선영,서필원,서휘민,이철위 한국공업화학회 2019 Journal of Industrial and Engineering Chemistry Vol.76 No.-
MoS2 is known as core material for the hydroconversion of heavy oil in petroleum refinery due to itssuperior coke suppression and desulfurization activities. With the recent interest in applying MoS2 toslurry-bed reactors, it has become important to design appropriate Mo-precursors and to identify factorsfor evaluating them. In this review, various methods for the synthesis of MoS2 from Mo-precursors andtheir physicochemical properties were examined. It was revealed that several properties of Moprecursors(dispersibility and thermal properties) and MoS2 (sulfidation degree, particle size, andstacking) serve as crucial factors in determining catalytic performance. For better catalyst design, thedecomposition pathway of Mo-precursors and the effect of carbonaceous material on MoS2 were alsodiscussed.
신장이식후 이식신에 재발한 IgA 신병증에서 Lovastatin과 Cyclosporin으로 인한 급성 신부전 1예
강기혁,노승현,고행일 인제대학교 1997 仁濟醫學 Vol.18 No.1
Lovastatin은 3-hydroxy-3-methylglutaryl-coenzyme A(HMG-CoA)환원 효소의 경쟁적인 억제제로서 콜레스테롤 합성을 억제하여 총 콜레스테롤과 저밀도 단백질을 감소시키는 효과적인 약제로 알려져 있다. 그러나 lovastatin은 근병증 또는 횡문근 융해와 같은 합병증을 초래 할 수 있으며 특히 cyclosporine과 병행치료를 하는 경우 이러한 합병증의 빈도가 증가한다고 알려져 있다. 저자들은 이식신에 재발한 IgA 신병증에 의하여 생긴 신증후군에서 고지혈증의 치료로 사용하였던 lovastain에 의한 급성 신부전 1예를 경험하였기에 이를 보고하는 바이다. Lovastatin, HMG-CoA reductase inhibitor, is effective in lowering both total cholesterol and low density lipoprotein levels with few side effects. But it has been known that the incidence of rhabdomyolysis is increased in the cases of combination therapy with CsA. The authors experienced a case of 41-year-old male kidney transplant recipient with recurrent IgA nephropathy who developed acute renal failure accompanying massive rhabdomyolysis during the period of Lovastatin-CsA combination therapy. Myalgia, muscle tenderness and weakness, dark colored oliguria were developed, and we diagnosed as rhabdomyolysis with markedly increased levels of serum LDH, CK, AST and positive urine myoglobin. The patient's renal dysfunction and rhabdomyolysis were recovered after conservative management including 9 times of hemodialysis. In transplanted patients receiving cyclosporine A and lovastatin, close observation is important for early detection of rhabdomyolysis.
11B pulsed nuclear magnetic resonance study of HoB4
강기혁,김정훈,이무희,J.Y. Kim,B.K. Cho 한국물리학회 2015 Current Applied Physics Vol.15 No.4
11B pulsed nuclear magnetic resonance (NMR) measurements were performed on a single crystal of HoB4 to investigate disorder-induced effects on the 4f spin structures and dynamics. The 11B NMR spectrum, shift, linewidth, and spin-lattice and spinespin relaxation rates (1/T1 and 1/T2, respectively) were measured down to 3.5 K at a field of 8 T perpendicular to the c-axis. Above the Ne el temperature, TN ¼ 5.7 K, the 11B NMR linewidth is very large and the shift is also large and with negative sign. In addition, both are strongly temperature-dependent and the absolute shift value and linewidth increase at lower temperatures. This fact confirms that the hyperfine field at the boron sites originates mainly from the Ho 4f spins, as this is also the case for the susceptibility. Below TN, the 11B NMR spectrum displays a single, broad and featureless shape with an extremely large linewidth. This behavior is an unexpected result compared with typical NMR spectra in the magnetically ordered state for single crystal specimens, and is significantly different to that of other RB4 substances. Considering frustration and disorder effects on the static and dynamic NMR data, we conclude that this behavior originates from the magnetic frustration and quadrupole moment disorder effects on the NMR static data, since the Ho ions form a Shastry-Sutherland (SS)-type lattice in the ab plane. Above TN, both relaxation rates (1/T1 and 1/T2) are very large as a result of the rapid and random fluctuation of the Ho 4f moments, and they then increase toward the Ne el transition. Below TN, both rates decrease significantly since the 4f spin fluctuations and dynamics are frozen as a result of the antiferromagnetic ordering.