• Journal of Magnetics
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• v.15 no.4
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• pp.159-164
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• 2010

The Zn, Co and Ni substituted manganese ferrite powders, $Mn_{1-x}$(Zn, Co, Ni)$_xFe_2O_4$, were fabricated by the solgel method, and their crystallographic and magnetic properties were studied. The Zn substituted manganese ferrite, $Zn_{0.2}Mn_{0.8}Fe_2O_4$, had a single spinel structure above $400^{\circ}C$, and the size of the particles of the ferrite powder increased when the annealing temperature was increased. Above $500^{\circ}C$, all the $Mn_{1-x}$(Zn, Co, Ni)$_xFe_2O_4$ ferrite had a single spinel structure and the lattice constants decreased with an increasing substitution of Zn, Co, and Ni in $Mn_{1-x}$(Zn, Co, Ni)$_xFe_2O_4$. The Mossbauer spectra of $Mn_{1-x}Zn_xFe_2O_4$ (0.0$\leq$x$\leq$0.4) could be fitted as the superposition of two Zeeman sextets due to the tetrahedral and octahedral sites of the $Fe^{3+}$ ions. For x = 0.6 and 0.8 they showed two Zeeman sextets and a single quadrupole doublet, which indicated they were ferrimagnetic and paramagnetic. And for x = 1.0 spectrum showed a doublet due to a paramagnetic phase. For the Co and Ni substituted manganese ferrite powders, all the Mossbauer spectra could be fitted as the superposition of two Zeeman sextets due to the tetrahedral and octahedral sites of the $Fe^{3+}$ ions. The variation of the Mossbauer parameters are also discussed with substituted Zn, Co and Ni ions. The increment of the saturation magnetization up to x = 0.6 in $Mn_{1-x}Co_xFe_2O_4$ could be qualitatively explained using the site distribution and the spin magnetic moment of substituted ions. The saturation magnetization and coercivity of the $Mn_{1-x}$(Zn, Co, Ni)$_xFe_2O_4$ (x = 0.4) ferrite powders were also compared with pure $MnFe_2O_4$.

• Journal of the Korean Magnetics Society
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• v.19 no.3
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• pp.113-119
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• 2009

$^{57}Fe_xCu_{1-x}O$(x = 0.0, 0.02) powders were prepared by sol-gel method and their crystallographic and magnetic hyperfine properties have been studied using X-ray diffraction and $M{\ddot{o}}ssbauer$ spectroscopy (MS). The crystal structure of the samples is found to be monoclinic without any secondary phases and their lattice parameters increase with increasing annealing temperature ($T_A$), which is attributed to an increase in oxygen-vacancy content. MS measurements at room temperature indicate that $Fe^{3+}$ ions substitute $Cu^{2+}$ sites and ferromagnetic phase grow with increasing $T_A$. Magnetic hyperfine and quadrupole interactions of $^{57}Fe_{0.02}Cu_{0.98}O$ ($T_A=500^{\circ}C$) in the antiferromagnetic state at 17 K have been studied, yielding the following results: $H_{hf}=426.94\;kOe$, ${\Delta}E_Q=-3.67\;mm/s$, I.S.=0.32 mm/s, ${\theta}=65^{\circ}$, ${\phi}=0^{\circ}$, and ${\eta}=0.6$.

• Journal of Positioning, Navigation, and Timing
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• v.6 no.3
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• pp.125-133
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• 2017

Time comparison is necessary for the verification and synchronization of the clock. Two-way satellite time and frequency (TWSTFT) is a method for time comparison over long distances. This method includes errors such as atmospheric effects, satellite motion, and environmental conditions. Ionospheric delay is one of the significant time comparison error in case of the carrier-phase TWSTFT (TWCP). Global Ionosphere Map (GIM) from Center for Orbit Determination in Europe (CODE) is used to compare with Bernese. Thin shell model of the ionosphere is used for the calculation of the Ionosphere Pierce Point (IPP) between stations and a GEO satellite. Korea Research Institute of Standards and Science (KRISS) and Koganei (KGNI) stations are used, and the analysis is conducted at 29 January 2017. Vertical Total Electron Content (VTEC) which is generated by Bernese at the latitude and longitude of the receiver by processing a Receiver Independent Exchange (RINEX) observation file that is generated from the receiver has demonstrated adequacy by showing similar variation trends with the CODE GIM. Bernese also has showed the capability to produce high resolution IONosphere map EXchange (IONEX) data compared to the CODE GIM. At each station IPP, VTEC difference in two stations showed absolute maximum 3.3 and 2.3 Total Electron Content Unit (TECU) in Bernese and GIM, respectively. The ionospheric delay of the TWCP has showed maximum 5.69 and 2.54 ps from Bernese and CODE GIM, respectively. Bernese could correct up to 6.29 ps in ionospheric delay rather than using CODE GIM. The peak-to-peak value of the ionospheric delay for TWCP in Bernese is about 10 ps, and this has to be eliminated to get high precision TWCP results. The $10^{-16}$ level uncertainty of atomic clock corresponds to 10 ps for 1 day averaging time, so time synchronization performance needs less than 10 ps. Current time synchronization of a satellite and ground station is about 2 ns level, but the smaller required performance, like less than 1 ns, the better. In this perspective, since the ionospheric delay could exceed over 100 ps in a long baseline different from this short baseline case, the elimination of the ionospheric delay is thought to be important for more high precision time synchronization of a satellite and ground station. This paper showed detailed method how to eliminate ionospheric delay for TWCP, and a specific case is applied by using this technique. Anyone could apply this method to establish high precision TWCP capability, and it is possible to use other software such as GIPSYOASIS and GPSTk. This TWCP could be applied in the high precision atomic clocks and used in the ground stations of the future domestic satellite navigation system.

• Journal of Korean Society of Coastal and Ocean Engineers
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• v.23 no.4
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• pp.313-325
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• 2011

Finite element grid system using h-refinement on the Yellow Sea was constructed based on previous study (Suh, 1999b) from 14 K to 210 K and special attention was concentrated on refining the coastal zone. In grid generation, depth change between adjacent points and non-dimensional tidal wave length ratio were considered. As a result approximately a quarter of the total nodes are located nearby 5 m of shallow area. Accurate bathymetry data using 30's and ETOPO1 with open boundary conditions of 8 major tidal constituents extracted automatically from FES2004 have been applied. In tidal simulation a 3-dimensional nonlinear harmonic model was setup and tidal amplification due to changes in vertical turbulent and bottom friction were simulated. In this study not only 8 major tidal constituents but also nonlinear shallow tides $M_4,$, $MS_4$ and long period $M_f,$, $M_{sf}$ were reproduced. It is found that implication of spatial variation of friction coefficient plays a very important role in reproduction of astronomical and shallow tides which are computed by iterative computation of nonlinear terms. Also it should be considered differently with respect to tidal periods. To understand the distribution of tidal asymmetry, amplitude ratio of $M_4/M_2$ and phase differences $2g(M_2)-g(M_4)$ were calculated. Tidal distortion ratio marks up to 0.2 on the west coast showing shallow coastal characteristics and somewhat wide range of ebb-dominances in front of Mokpo area are reproduced.

• Journal of the Korean Society of Marine Environment & Safety
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• v.16 no.1
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• pp.31-41
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• 2010

Harmful dinoflagellate, Cochlodinium polykrikoides, has always occurred in southern coastal waters every year during summer, but it did not occur in 2009. A comparative study on outbreak in 2007/2008 and non-outbreak of C. polykrikoides in 2009 was performed based on environmental parameters and phytoplankton appearances. Samples were obtained at 13 stations from Yeosu to Wando waters in 2007-2009. The heavy rainfall was associated with the decrease of surface water temperature below $20^{\circ}C$ in July, the decrease of salinity below 31psu in August and increase of pH to 8.4 in Augus, 2009. concentration of DIN in 2009 was approximately too times as high as those of 2007 and 2008, and the ratioes of N:P and Si:N in 2009 showed Redfield ratio to be above the value of 16. This indicates that Redfield ratioes were significantly different between 2007/2008 and 2009. Dominant species during the field survey were shown to be Chaetoceros spp., Skeletonema costatum and Thalassiosir spp. in diatoms and to be Ceratium spp. in dinoflagellate. In partiuclar, Gonyaulax polygramma occurred in August, 2009 instead of C. polykrikoides. Consequently, the massive rainfall and shortage of sunshine contributed to considerable variation in environmental parameters which were associated with delay in the timing of rapid growth phase of C. polykrikoides.

• Journal of the Korean Society of Marine Environment & Safety
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• v.23 no.2
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• pp.187-193
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• 2017

This study evaluated tidal stream energy resources according to tidal flow properties around Uido off the west coast of, Jeollanam-do, South Korea. A feasibility study was first carried out through the collection of bathymetry data and tidal phase information. For this simulation, a depth-averaged 2D ADCIRC (Advanced Circulation) model for real sea situations was applied to a Finite Element Method (FEM) approach for tides given the variation of tidal current speed. Hydrodynamics were simulated with 4 major tidal constituents (M2, S2, K1, and O1) after setting up 4 observation points. From the real depth-averaged model simulation results, it was found that the spring tide Higher High Water (HHW) and tidal current speed values at the 4 observation points were about 2.2 m and 1.33 m/s, respectively. The ADCIRC model results were analyzed with reference to the Korea Hydrographic and Oceanographic Agency's (KHOA) observed data for verification. Furthermore, using topographical characteristics via the Tidal Flux Method (TFM), tidal energy density distribution was calculated, indicating a maximum tidal energy density of about $1.75kW/m^2$ for the 5 assessment areas around Uido. The tidal energy density was evaluated with consideration given to topographical characteristics as well as tidal elevation and tidal current speed to determine an optimum tidal farm candidate.

• YAKHAK HOEJI
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• v.51 no.2
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• pp.133-139
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• 2007

A simple and sensitive HPLC-MS method for quantitation of 10${\alpha}$-methoxy-9,10-dihydrolysergol (MDL), the main metabolite of nicergoline, in human plasma was developed and the bioavailability parameters of MDL was assessed in Korean healthy male volunteers. Clomipramine was used as an internal standard. MDL and internal standard in plasma sample were extracted using ethyl acetate. A centrifuged upper layer was then evaporated and reconstituted with mobile phase of 10 mM ammonium acetate-acetonitrile (10 : 90, v/v). The reconstituted samples were injected into a Zorbax SB-C8 column (2.1${\times}$150 mm,5 ${\mu}$m) at a flow-rate of 0.3 ml/min. Using MS with selected ion monitoring (SIM) mode, MDL and clomipramine were detected without severe interference from human plasma matrix. MDL produced a protonated molecular ion ([M+H]$^+$) at m/z 287. Internal standard produced a protonated molecular ion ([M+H]$^+$) at m/z 315. A linear relationship for MDL was found in the range of 2.5${\sim}$100 ng/ml. The lower limit of quantitation (LLOQ) was 2.5 ng/ml with acceptable precision and accuracy. The intra- and inter-day validation for all coefficients of variation (R.S.D.%) were found less than 15%. Main pharmacokinetic parameters of 30 mg of nicergoline were revealed as follows: AUC$_t$ 321.1${\pm}$64.5 ng${\cdot}$hr/ml, C$_{max}$, 51.2${\pm}$25.3 ng/ml, T$_{max}$ 3.6${\pm}$1.5 hr, K$_{el}$ 0.12${\pm}$0.07 hr$^{-1}$ and t$_{1/2}$ 7.6${\pm}$3.4 hr. Inter subject variations and race differences were shown in comparison with the published data in the literature.

• Korean Journal of Materials Research
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• v.3 no.2
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• pp.175-184
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• 1993

Abstract Co-and/or AI-added Nd-Fe-B-based magnetic alloys were fabricated by using vacuum induction melting frunace, and melt-spun ribbons were made of the magnetic alloys with single roll rapid quenching method. The variation of magnetic properties of the melt-spun ribbons as a function of Cuwheel velocity (Vs) were investigated. Bonded magnets were made of the optimally quenched ribbon fragments, and the magnetic properties of the melt-spun ribbons and the bonded magnets were studied, relating to the microstructure and crystalline structure. Cu-wheel surface velocity had a strong effect on the magnetic properties of the melt-spun ribbons, and the maximum properties were obtained around Vs =20m/sec. The optimally quenched ribbon had a cellura-type microstructure, in which fine N$d_2$F$e_14$B grains were surrounded by thin Nd-rich phase. In case of a 2.1at% AI-added melt-spun ribbon, the magnetic properties were as follows: iHc, Br, and (BH)max were 15.5KOe, 7.8KG and 8.5MGOe respectively. And resin bonded magnets were fabricated by mixing optimally quenched ribbon fragments with 2.5wt % polyamide resin, compacting and binding at room temperature. The iHc, Br and (BH)max of bonded magnet were lO.2KOe, 4.4KG and 3.3MGOe respectively. And hot-pressed magnets were made by pressing the overquenched ribbons at high temperature. The magnetic properties of hot-pressed magnets were better than those of bonded magnets, and when the holding time was 8 minutes, the iHc, Br, and (BH)max of the hot-pressed magnet were 1O.8KOe, 7.3KG and 8.0MGOe respectively. Domain structure was mainly maze pattern, which means that the easy magnetization axis could be aligned, and the domain width of the hot-pressed magnets was smaller than that of bonded magnets.

• The Journal of Korean Institute of Electromagnetic Engineering and Science
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• v.16 no.4 s.95
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• pp.380-388
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• 2005

This paper presents analysis results of the effects of RF filter characteristics on the system performance of impulse radio. The impulse radio system transmits modulated pulses having very short time duration and information can be extracted in receiver side based on cross-correlation between received and transmitted pulses. Accordingly, the pulse distortion due to in-band group delay variation can cause serious system performance degradation. In general, RF bandpass filters inevitably cause group delay difference to the signal passing through the filter which is proportional to its skirt characteristic due to its resonance phenomenon. For time as well as frequency domain analysis, small signal scattering parameter $S_{21}$ and its Fourier transform are used to characterize output pulse waveform under the condition that the input and output ports are matched. The output pulse waveform of the filter is predicted based on convolution integral between input pulse and filter transfer function, and resulting BER performances in the BPM and PPM based impulse radio system are calculated.

• Journal of Conservation Science
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• v.25 no.1
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• pp.17-24
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• 2009

The feasibility of the geopolymer as a cultural asset restoration material was studied by investigating compressive strength and chromaticity change. Metakaolin that was synthesized by calcination of the kaolin at $750^{\circ}C$ for 6 hours was used as a geopolymeric starting material. Kaolin lost its crystallinity and changed into non-crystalline phase during calcination. NaOH solution and water glass were used as an initiator for the geopolymeric reaction. As the concentration of NaOH solution and water glass increased the compressive strength increased. When alumina was substituted with metakaolin, the compressive strength decreased at a small amount of alumina, but increased at a large substitution. For the most composition of geopolymers, the change of chroma values remained within the limit of slight variation after exposure to sunlight for 8 and 100 days. However, even small amount of organic pigment addition increased chroma values of metakaoline. It was shown that geopolymer had excellent chroma value change over epoxy resins.