• Bulletin of the Korean Chemical Society
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• 제34권12호
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• pp.3635-3640
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• 2013

We prepared Ni and Pd-modified $TiO_2@SiO_2$ core-shell nanostructures and then analyzed them by scanning electron microscopy, optical microscopy, X-ray diffraction crystallography, FT-IR and UV-Visible absorption spectroscopy. In addition, their CO oxidation performance was tested by temperature-programmed mass spectrometry. The CO oxidation activity showed an order of Ni-$TiO_2@SiO_2$ ($900^{\circ}C$) < Ni-$TiO_2@SiO_2$ ($90^{\circ}C$) < Ni-$TiO_2@SiO_2$ ($450^{\circ}C$) in the first CO oxidation run, and greatly improved activity in the same order in the second run. The $T_{10%}$ (the temperature at 10% CO conversion) corresponds to the CO oxidation rate of $2.8{\times}10^{-5}$ molCO $g{_{cat}}^{-1}s^{-1}$. For Ni-$TiO_2@SiO_2$ ($450^{\circ}C$), the $T_{10%}$ was observed at $365^{\circ}C$ in the first run and at $335^{\circ}C$ in the second run. For the Pd-$TiO_2@SiO_2$ ($450^{\circ}C$), the $T_{10%}$ was observed at a much lower temperature of $263^{\circ}C$ in the first CO oxidation run, and at $247^{\circ}C$ in the second run. The CO oxidation activities of transition metal modified $TiO_2@SiO_2$ core-shell nanostructures presented herein provide new insights that will be useful in developing catalysts for various environments.

• 천문학회보
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• 제35권2호
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• pp.79.2-79.2
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• 2010

We present the results of simultaneous observations of SiO v=1, 2, J=1-0, $^{29}SiO$ v=0, J=1-0, and $H_2O$ $6_{16}-5_{23}$ maser lines performed with the KVN Yonsei 21 m radio telescope from 2009 Nov. to 2010 Jan (ApJ, 719, 126, 2010). We searched for these masers in 47 symbiotic stars and detected maser emission from 21 stars, giving the first time detection from 19 stars. Both SiO and $H_2O$ masers were detected from seven stars of which six stars are D-type symbiotic and one is an S-type star, WRAY 15-1470. In the SiO maser emission, the $^{28}SiO$ v=1 maser was detected from 10 stars, while the v=2 maser detected from 15 stars. In particular, the $^{28}SiO$ v=2 maser emission without the v=1 maser detection was detected from nine stars with its detection rate of 60 %, which is much higher than that of isolated Miras/red giants. The $^{29}SiO$ v=0 maser emission was also detected from two stars, H 2-38 and BF Cyg, together with the $^{28}SiO$ v=2 maser. We conclude that these different observational results between isolated Miras/red giants and symbiotic stars may be related with the presence of hot companions in a symbiotic binary system.

• 한국주조공학회지
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• 제20권1호
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• pp.13-20
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• 2000

In the present study,($10%Al_2O_3+5%Si$)/AZ91 Mg hybrid composite was fabricated using the squeeze casting method. During squeeze casting, Molten Mg was infiltrated into the preform of $10%Al_2O_3+5%Si$ and reaction product of $Mg_2Si$ intermetallic compound was formed by the reaction between molten Mg and Si Powder. Microstructure has been observed and mechanical properties were evaluated for the reaction squeeze cast(RSC) hybrid composite. It was found that Si powder totally reacted with molten Mg to form $Mg_2Si$. Reinforcement($Al_2O_3$) and the reaction product ($Mg_2Si$) are fairly uniformly distributed in Mg Matrix for the squeeze cast hybrid composite. Mechanical Properties were improved with hybridization of reinforcements, namely higher hardness and enhanced wear resistance comparing squeeze cast($15%Al_2O_3$)/AZ91 Mg composite.

• 한국표면공학회지
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• 제33권4호
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• pp.217-221
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• 2000

The gate oxide thickness is becoming thinner and thinner in order to speed up the semiconductor CMOS devices. We have investigated very thin$ SiO_2$ gate oxide layers and found anomaly between the thickness determined with capacitance measurement and these obtained with cross-sectional high resolution transmission electron microscopy. The thicknesses difference of the two becomes important for the thickness of the oxide below 5nm. We propose that the variation of dielectric constant in thin oxide films cause the anomaly. We modeled the behavior as (equation omitted) and determined $\varepsilon_{bulk}$=3.9 and $\varepsilon_{int}$=-4.0. We predict that optimum $SiO_2$ gate oxide thickness may be $20\AA$ due to negative contribution of the interface dielectric constant. These new results have very important implication for designing the CMOS devices.s.

• 공업화학
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• 제8권4호
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• pp.582-588
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• 1997

고체산 촉매인 헤테로폴리산 촉매를 이용하여 대기오염 방지를 위한 옥탄가 향상제인 TAME, ETBE 및 MTBE 합성반응에 대한 실험을 고정층 상압유통식 반응장치에서 수행하였다. 일반적으로 TAME, ETBE 및 MTBE 합성반응에서 헤테로폴리산에 hetero원자와 poly원자를 치환시켜 비교한 결과, 배위된 poly원자가 W, hetero원자가 Si인 $H_4SiW_{12}O_{40}$ 촉매의 활성이 가장 우수하였고, 또한 치환된 금속에 따라 촉매의 활성이 달랐으며 이는 촉매의 산성질과 관련이 있음을 알 수 있었다. 헤테로폴리산 촉매를 이용한 합성반응에서 TAME 경우는 $FeHPW_{12}O_{40}$과 $K_3PM_{o12}O_{40}$이 비교적 활성이 좋았으나, ETBE나 MTBE 경우에 비해서는 다소 활성이 낮았다. 그리하여 본 연구에서는 활성이 좋은 헤테로폴리산 촉매들을 선정하여 각각에 대하여 1:1로 다음과 같이 혼합하여 $H_4SiW_{12}O_{40}$ : $Sr_2SiW_{12}O_{40}$, $H_4SiW_{12}O_{40}$ : $NaH_2PW_{12}O_{40}$, $Fe_{1.5}PW_{12}O_{40}$ : $Mg_2SiW_{12}O_{40}$ 및 $Mg_2SiW_{12}O_{40}$ : $Ba_2SiW_{12}O_{40}$ 실험한 결과, 혼합촉매들이 각각의 단일성분 촉매때보다 TBA의 전화율과 ETBE나 MTBE의 선택율이 더욱 향상되었음을 알 수 있었다.

• 한국세라믹학회지
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• 제25권6호
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• pp.609-614
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• 1988

This study was carried out to investigate the effects of B4C or Y2O3 additives on the tendency of sintering, $\beta$-SiC synthesis and mineral phase changes by reaction bonding of SiC at 145$0^{\circ}C$. At the sintering temperature of 145$0^{\circ}C$, the additives such as B4C or Y2O3 did not improved porosity and bending strength. Added more than 1.5% of Y2O3, 0.5-0.3% of B4C, the formation of $\beta$-SiC was increased. At higher temperature above 145$0^{\circ}C$, it seems that the bodies added B4C, contained 3C form of SiC were denser than that of Y2O3 added. Because the transition of 3Clongrightarrow4Hlongrightarrow6H promoted sintering.

• 한국세라믹학회지
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• 제37권1호
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• pp.77-81
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• 2000

Silica glass films to utilize optical waveguides was fabricated by Aerosol Flame Deposition(AFD) method. As the amount of B2O3 increased in the sol solution of (92-x)SiO2-xB2O3-8P2O5, the thermophoretic deposition rate onto Si substrate was markedly lowered due to vaporizing out of B2O3 and P2O5 during the vaporization and reaction of the aerosol in the flame. GeO2 was added to 62SiO2-30B2O3-8P2O5 in order to control easily the refractive index of glass films. As the amount of GeO2 increased from 2 to 12 wt%, its refractive index increased from 1.4633 up to 1.4716.

• 대한화학회지
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• 제38권1호
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• pp.1-7
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• 1994

알루미노인산염 족($AlPO_4-5$, BAPO-5, 그리고 SAPO-5)분자체의 구조 골격인 가교 $$T_1-OH-T_2$(T_1$ = Al, B 그리고 $T_2=$ P, Si) 안정성 및 가교 OH 그룹 이온성을 연구하기 위하여 반경험적 MNDO 계산 방법으로 연구하였다. 이러한 계산은 Al-OH-P, B-OH-P 그리고 Al-OH-Si덩어리 모델로써 실행하였다. Al-OH-P가교 모델에서 Al대신에 B원자를 치환할 때 B-OH-Si가교에서는 변형이 우선적으로 일어났으나, P 대신에 Si원자를 치환한 Al-OH-Si가교에서는 회전이 우선적으로 일어났다. 이러한 모델 덩어리에서 OH그룹 이온성 및 가교 안정성은 B-OH-P < Al-OH-P < Al-OH-Si 가교 순서로 증가하였다.

• 한국세라믹학회지
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• 제20권1호
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• pp.3-12
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• 1983

In the present study an attempt was made to synthesize the $Al_2O_3$.$2SiO_2$ glass in which atomic ratio is Al:Si=1:1 by sol-gel process. And at such a low temperature as 55$0^{\circ}C$ clear amorphous gel derived glass with Si-O-Al bonding was obtained. $Si(OC_2H_5)_4$ and $Al(NO_3)_3$.$9H_2O$ were used as the precursor and among the mutual solvents only n-butanol gave good results for the synthesis of the gel derived glass. Partial hydrolysis of TEOS with one-fold mol of $H_2O$ prior to the reaction with aluminum nitrate gave the better results., Total oxide content to the total reactants by weight was affective to the results.

• The Korean Journal of Ceramics
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• 제5권3호
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• pp.211-216
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• 1999

Four different $\beta-Si_3N_4$ ceramics with silicon oxynitrides $[Y_10(SiO_4)_6N_2, Yb_4Si_2N_2O_7, Er_2Si_3N_4O_3, \;and La_{10}(SiO_4)_6N_2$, respectivley] as secondary phases have been fabricated by hot-pressing the $Si_3N_4-Re_4Si_2N_2O_7$ (Re=Y, Yb, Er, and La) compositions at $1820^{\circ}C$ for 2h under a pressure of 25 MPa. The high temperature strength and oxidation behavior of the hot-pressed ceramics were characterized and compared with those of the ceramics fabricated from $Si_3N_4-Si_2O_7$ compositions. The $Si_3N_4-Re_4Si_2N_2O_7$composition investigated herein showed comparable high temperature strength to those from $Si_3N_4-Re_2Si_2O_7$ compositions. Si3N4 ceramics from a $Si_3N_4-Y_4Si_2N_2O_7$ composition showed the highest strength of 877 MPa at $1200^{\circ}C$ among the compositions. All $Si_3N_4$ ceramics investigated herein showed a parabolic weight gain with oxidation time at $1400^{\circ}C$ and the oxidation products of the ceramics were $SiO_2$ and $Re_2Si_2O_7$. The $Si_3N_4-Re_4Si_2N_2O_7$ compositions showed inferior oxidation resistance to those from $Si_3n_4-Re_2Si_2O_7$ compositions, owing to the incompatibility of the secondary crystalline phases of those ceramics with $SiO_2$, the oxidation product of Si3N4.Si3N4 ceramics from a $Si_3N_4-Er_4Si_2N_2O_7$ composition showed the best oxidation resistance of 0.375mg/$\textrm{cm}^2$ after oxidation at $1400^{\circ}C$ for 102 h in air among the compositions.