• Korean Journal of Materials Research
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• v.31 no.12
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• pp.682-689
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• 2021

NbC, HfC, TaC, and their solid solution ceramics have been identified as the best materials for ultrahigh-temperature ceramics. However, their structural stability and elastic properties are mostly unclear. Thus, we investigated structure and elastic properties of (Nb_1-xTa_x)C and (Nb_1-xHf_x)C solid solutions via ab initio calculations. Our calculated results show that the stability of (Nb_1-xTa_x)C and (Nb_1-xHf_x)C increases with the increase of Hf and Ta content, and (Nb_1-xHf_x)C is more stable than (Nb_1-xTa_x)C at the same content of Hf and Ta. The lattice constants decrease with increasing of Hf and Ta content. (Nb_1-xTa_x)C and (Nb_1-xHf_x)C carbides are mechanically stable and brittle. Bulk modulus of (Nb_1-xTa_x)C increases with increasing Ta content. In contrast, bulk modulus of (Nb_1-xHf_x)C decreases with increasing Hf content. Hardness of solid solutions shows the highest values at the (Nb_0.25Ta_0.75)C and (Nb_0.75Hf_0.25)C. In particular, (Nb_0.75Hf_0.25)C shows the highest hardness for the current system. The results indicate that the overall mechanical properties of (Nb_1-xHf_x)C solid solutions are superior to those of (Nb_1-xTa_x)C solid solutions. Therefore, controlling the Hf and Ta element and content of the (Nb_1-xTa_x)C and (Nb_1-xHf_x)C Solid solution is crucial for optimizing the material properties.

• Journal of the Korean Ceramic Society
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• v.47 no.6
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• pp.534-539
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• 2010

$HfB_2$-HfC composites were prepared by reactive hot pressing using Hf and $B_4C$ at temperatures of 1800 and $1900^{\circ}C$ for 60 min under 32 MPa in an Ar atmosphere. The reaction sequences of the $HfB_2$-HfC composite were studied through series of pressureless heat treatments ranging from 800 to $1600^{\circ}C$. The effect of size reduction of the starting powders on densification was investigated by vibration milling. Fully dense $HfB_2$-HfC composites were obtained by size reduction of the starting powders via vibration milling. The oxidation behaviour of the $HfB_2$-HfC composites at $1500^{\circ}C$ in air showed formation of a non-protective $HfO_2$ scale with linear mass gain. Examination of the mechanical properties showed that particle size reduction via vibration milling also led to improved flexural strength, hardness and fracture toughness.

• Proceedings of the Korean Vacuum Society Conference
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• 2016.02a
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• pp.299-299
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• 2016

SiC는 넓은 에너지갭 (Eg=~3.4 eV)을 갖는 반도체로써, 고전압, 고온에서 동작이 가능하여 기존의 Si기반의 파워디바이스를 대체하기 위한 물질로 많은 연구가 이루어지고 있다. 파워 디바이스의 성능 향상을 위해서는 기판과 절연체 사이의 계면에 생성되는 계면 결함을 감소시켜야 한다. 따라서 본 연구에서는 SiC 기판에 high-k 물질인 HfO2를 증착하여 HfO2/SiC 계면에 유도된 결함을 분석하고 이를 감소시킬 수 있는 방법에 대한 연구를 수행하였다. HfO2 박막은 atomic-layer-deposition (ALD) 방법을 이용하여 SiC 기판 위에 $200^{\circ}C$에서 증착하였다. HfO2 박막 증착 후 NH3 분위기에서 rapid thermal annealing 방법을 이용하여 $600^{\circ}C$에서 1분 동안 열처리 진행하였다. Current-voltage (I-V) 측정을 통해 열처리 전 HfO2/SiC의 절연파괴 전압이 약 8.3 V 임을 확인하였다. NH3 열처리 후 HfO2/SiC의 절연파괴 전압이 10 V로 증가하였으며 누설 전류가 크게 감소하는 것을 확인하였다. 또한 capacitance-voltage (C-V) 측정을 통해 열처리 후 flat band voltage가 negative 방향에서 positive 방향으로 이동함을 확인하였고, 이를 통해 NH3 열처리 방법이 HfO2/SiC 계면에 존재하는 결함을 감소시킬 수 있음을 확인하였다. 전자 구조상의 conduction band edge에 존재하는 결함 준위를 분석하기 위해 x-ray absorption spectroscopy (XAS) 분석을 실시하였고, 열처리 전 HfO2/SiC 계면에 많은 결함 준위가 존재함을 확인하였으며, x-ray photoelectron spectroscopy (XPS) 분석을 통해 이 결함 준위가 oxygen deficiency state과 관련됨을 알 수 있었다. NH3 열처리 후 결과와 비교해보면, oxygen deficiency state가 감소함을 확인하였으며 이로 인해 conduction band edge에 존재하는 결함 준위가 감소함을 알 수 있었다. 따라서, NH3 열처리 방법을 이용하여 HfO2/SiC 계면에 존재하는 결함을 감소시킬 수 있으며, HfO2/SiC의 물리적, 전기적 특성을 향상시킬 수 있다는 결과를 도출하였다.

• Journal of the Korean Society of Food Science and Nutrition
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• v.23 no.4
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• pp.540-540
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• 1994

The effect of diets(high fat, high fat high energy, pectin, cellulose) on lipid metabolism were in 70 male rats for 8 weeks. The rats were assigned to either a control(C), high fat (HF) or high fat high energy(HFHE)group for 4 wks. During the next 4wks, reassigned to one of three treatments(high fat, pectin, cellulose) in the HF group and one of three treatment (hign fat high energy, pectin, cellulose) in the HFHE group. Therefore, the total treatment group became 7 (C, HF, HF-P, HF-C, HFHE, HFHE-P, HFHE-C). Parameters evaluated and compared for each diet were body weight, total energy intake, Feed Efficiency Ratio (F.E.R), total lipid, total cholesterol (TC), HDL-cholesterol(HDL-C), HDL-C/TC, triglyceride (TG), phospholipid in serum, and changes in liver and spleen. The results are surmmarized as follows ; 1. There were no signiticant differences in boby weight gain among the groups. 2. Total energy intake was higher in the C group than other groups. Feed Efficiency Ratio (F.E.R) was higher in the HFHE group than other groups. 3. There was no significant difference in the weight gain of liver (100g/B.W.) among the groups. However, the weight gain of spleen (100g/B.W.) was gigher in the C group than other groups (HF and HFHE). 4. Total lipid, TC, TG, and phospholipid increased significantly in all groups for the first 4 wks. However, after 8 wks, pectin and cellulose supplementation groups decreased significantly. Therefore, we can conclude that the HF and the HFHE diet led to increase lipids concentration and the pectin and the cellulose treatments decreased lipids concentration in serum. However, the pectin treatment had greater effect than the cellulose treatment.

• Proceedings of the Korean Vacuum Society Conference
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• 2016.02a
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• pp.237.2-237.2
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• 2016

A pure hafnium-carbide (HfC) coating layer was deposited onto carbon/carbon (C.C) composites using a vacuum plasma spray system. By adopting a SiC buffer layer, we successfully integrated C.C composites with a $100-{\mu}m-thick$ protective coating layer of HfC. Compared to the conventional chemical vapor deposition process, the HfC coating process by VPS showed increased growth rate, thickness, and hardness. The growth behavior and morphology of HfC coatings were investigated by FE-SEM, EDX, and XRD. From these results, it was shown that the addition of a SiC intermediate layer provided optimal surface conditions during the VPS procedure to enhance adhesion between C.C and HfC (without delamination). The thermal ablation test results shows that the HfC coating layer perfectly protected inner C.C layer from thermal ablation and oxidation. Consequently, we expect that this ultra-high temperature ceramic coating method, and the subsequent microstructure that it creates, can be widely applied to improve the thermal shock and oxidation resistance of materials under ultra-high temperature environments.

• Proceedings of the Korean Institute of Electrical and Electronic Material Engineers Conference
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• 2007.11a
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• pp.9-10
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• 2007

In this paper, Thin films of $HfO_2$/Hf were deposited on p-type wafer by Atomic Layer Deposition(ALD). And we studied the electrical characterization of $HfO_2$/Hf/Si MOS capacitor depending on thickness of Hf metal layer. $HfO_2$ films were deposited using TEMAH and $O_3\;at\;350^{\circ}C$. Samples were then annealed using furnace heating to $500^{\circ}C$. The MOS capacitor of round-type was fabricated on Si substrates. Through TEM(Transmission Electron Microscope), XRD(X-ray Diffraction), capacitance-voltage(C-V) and current-voltage(I-V) analysis, the role of thin Hf metal layer for the better $HfO_2$/Si interface property was investigated.

• Journal of the Korean Society of Food Science and Nutrition
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• v.23 no.4
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• pp.541-541
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• 1994

이유기를 지난 웅성흰쥐를 대조식이 (C), 고지방식이 (HF) 및 고지방 고에너지식이 (HFHE) 로 4 주간 사양하고 일부를 희생시킨 후 , HF 군과 HFHE 군 일부에식이섬유 pectin 과 cellulose를 각각 첨가한 HF-P , HF-C, HFHE-P , HFHE-C 식이를 C, HF ,HFHE 식이와 함께 4 주간 더 사양한 후 희생시켜 체중증가량, 총 에너지섭취량, 총식이섭 취량, 식이효율, 간장과 비장의 무게, 혈중 지질의 성상 및 간세포에 미치는 영향을 비교 관 찰한 결과 체중증가는 C 군, HF 군, HFHE 군간의 유의적 차이는 없었으며, 식이섬유 첨가 시 모두 체중이 감소되었다. 총에너지 섭취량은 C 군이 가장 많았고, 식이효율은 HF-HE군 이 가장 높았으며, 식이섬유 첨가에 의해서는 영향을 받지 않았다. 간장의 무게는 C군 , HF 군, HFHE 군 모두 식이의 영향이 없었고, pectin 과 celulose 첨가에 의한 변화도 나타내지 않았다. 비장의 무게는 C군 보다 HF 군과 HFHE 군이 감소하였으나, pectin 과 cellulose 첨가에 의해서는 변화가 없었다. 혈중 총 지질, 총콜레스테롤, 중성지방 및 인지질의 함량은 각 실험군모두 실험기간이 경과함에 따라 증가되었으며, 특히 HFHE 군의 유의성있게 증가 하였고 pectin 과 cellulose를 첨가하였을때는 감소하였으며 pectin이 cellulose 보다 감소효 과가있다.

• Korean Journal of Materials Research
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• v.3 no.2
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• pp.111-120
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• 1993

Abstract In this study, the Mo-Hf-O ingots containing 0.31-1.14at % Hf and 0.08-1.00at % 0 were prepared by plasma arc melting. The change of microstructure depending on the condition of heat treatmen~ was analysed by optical microscophy, auger electron microscophy, and transmission electron microscophy. Molybdenum powder with the oxygen content of 830ppm was compacted, and then melted. The oxygen content of molybdenum ingots was detected to be 40 -130ppm. As the contents of Hf and 0 increased, the grain size of ingots decreased. When molybdenum igot containing l.14at % Hf and 1.00at % C was heat treated, p-molybdenum carbide in grains was transformed into ${\alpha}$-molybdenum carbide at 130$0^{\circ}C$. Between 140$0^{\circ}C$ and 150$0^{\circ}C$, the precipitation of hafnium carbide was due to the reaction of solute Hf and C, and the hafnium carbide was saturated at grain boundaries at 150$0^{\circ}C$. When the sample was heat treated from 150$0^{\circ}C$ to 170$0^{\circ}C$, Hafnium oxide more stable thermodynamically precipitated both at grain boundaries and in grains after hafnium carbide had been dissolved at grain boundaries.

• Asian-Australasian Journal of Animal Sciences
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• v.28 no.11
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• pp.1558-1564
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• 2015

Cutaneous evaporative heat loss in Bos indicus and Bos taurus has been well documented. Nonetheless, how crossbreds with different fractional genetic proportions respond to such circumstances is of interest. A study to examine the physiological responses to cutaneous evaporative heat loss, also lactation period and milk yield, were conducted in Sahiwal (Bos indicus, n = 10, $444{\pm}64.8kg$, $9{\pm}2.9years$), Holstein Friesian (Bos taurus, HF100% (n = 10, $488{\pm}97.9kg$, $6{\pm}2.8years$)) and the following crossbreds: HF50% (n = 10, $355{\pm}40.7kg$, $2{\pm}0years$) and HF87.5% (n = 10, $489{\pm}76.8kg$, $7{\pm}1.8years$). They were allocated so as to determine the physiological responses of sweating rate (SR), respiration rate (RR), rectal temperature (RT), and skin temperature (ST) with and without hair from 06:00 h am to 15:00 h pm. And milk yield during 180 days were collected at days from 30 to 180. The ambient temperature-humidity-index (THI) increased from less than 80 in the early morning to more than 90 in the late afternoon. The interaction of THI and breed were highly affected on SR, RR, RT, and ST (p<0.01). The SR was highest in Sahiwal ($595g/m^2/h$) compared to HF100% ($227g/m^2/h$), and their crossbreds both HF50% ($335g/m^2/h$) and HF87.5% ($299g/m^2/h$). On the other hand, RR was higher in HF87.5% (54 bpm) and both HF100% (48 bpm) and HF50% (42 bpm) than Sahiwal (25 bpm) (p<0.01). The RT showed no significant differences as a result of breed (p>0.05) but did change over time. The ST with and without hair were similar, and was higher in HF100% ($37.4^{\circ}C$; $38.0^{\circ}C$) and their crossbred HF50% ($35.5^{\circ}C$; $35.5^{\circ}C$) and HF87.5% ($37.1^{\circ}C$; $37.9^{\circ}C$) than Sahiwal ($34.8^{\circ}C$; $34.8^{\circ}C$) (p<0.01). Moreover, the early lactation were higher at HF100% (25 kg) and 87.5% (25 kg) than HF50% (23 kg) which were higher than Sahiwal (18 kg) while the peak period of lactation was higher at HF100% (35 kg) than crossbreds both HF87.5% and HF50% (32 kg) which was higher than Sahiwal (26 kg) (p<0.05). In conclusion, sweating and respiration were the main vehicle for dissipating excess body heat for Sahiwal, HF and crossbreds, respectively. The THI at 76 to 80 were the critical points where the physiological responses to elevated temperature displayed change.

• Proceedings of the Korean Vacuum Society Conference
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• 1998.02a
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• pp.39-40
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• 1998

Diffusion barrier characteristics of hf(C,N) thin films for Cu metalliztion was investgated. Hf(C,N) thin films were depposited on Si(100) suvstrates by ppulsed D. C pplasma enhanced metal-organic chemical vappor depposition (ppE-MOCVD) using Tetrakis diethyl amido hafnium (Hf[NC2H5)2]4 : TDEAHf) and N2 as pprecursors. X-ray diffraction analyses sheet resistance measurment and Rutherford backscattering sppectroscoppy analyses revealed that HF(C,N) films pprevent diffusion of Cu fairly well upp to $600^{\circ}C$. At $700^{\circ}C$ however Hf(C,N) films allowed a significant diffusion of Cu into the Si substrate.