• Bulletin of the Korean Chemical Society
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• 제33권3호
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• pp.839-842
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• 2012

We report for the first time the synthesis of niobium dioxide nanowires on a sapphire substrate by chemical vapor transport method. We identified single crystalline nature of as-synthesized nanowires by scanning electron microscopy and transmission electron microscopy. Niobium dioxide nanowires with their large surface-to-volume ratio and high activities can be employed for electrochemical catalysts and immunosensors. The Raman spectrum of niobium dioxide nanowires also confirmed their identity.

• 한국결정성장학회지
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• 제7권1호
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• pp.76-80
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• 1997

직경 55 mm의 ZnSe 단결정을 수소분위기에서 seeded chemical vapor transport법에 의해 성장하였고, 성장 parameter들이 결정 결함에 미치는 영향을 조사하였다. Chemical etching에 의한 EPD 측정, X-ray rocking curve 측정, photolumlnescence 측정으로 성장된 단결정의 특성을 평가하였다.

• 한국결정성장학회지
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• 제13권4호
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• pp.187-198
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• 2003

Zinc selenide (ZnSe) single crystals hold promise for many electro-optics, acousto-optic and green laser generation applications. This material is prepared in closed ampoules by the physical vapor transport (PVT) growth method based on the dissociative sublimation. We investigate the effects of diffusive-convection on the crystal growth rate of ZnSe with a low vapor pressure system in a horizontal configuration. Our results show that for the ratios of partial pressures, s=0.2 and 2.9, the growth rate increases with the Peclet number and the temperature differences between the source and crystal. As the ratio of partial pressures approaches the stoichiometric value of 2, the rate increases. The mass fluk based on one dimensional (1D model) flow for low vapor pressure system fall within the range of the predictions (2D model) obtained by solving the coupled set of conservation equations, which indicates the flow fields would be advective-diffusive. Therefore, the rate and the flow fields are independent of gravity acceleration levels.

• 한국결정성장학회지
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• 제19권3호
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• pp.116-124
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• 2009

Our computational studies for the physical vapor transport crystal growth of $Hg_2Cl_2-Cl_2$ system evidence suggests that the PVT growth process exhibits the diffusion-dominated behaviors for aspect ratios more than and equal to 10, which would provide purely diffusive transport conditions adequate to microgravity environments less than $10^{-3}g_0$. Also, the regimes of high temperature difference based on the fixed source temperature of $380^{\circ}C$, where ${\Delta}T$ is relatively large enough for the crystal growth of mercurous chloride, the transport rates do not keep increasing with ${\Delta}T$ but tend to some constant value of $2.12\;mole\;cm^{-2}s^{-1}$. For the aspect ratios of 5, 10, and 20, the transport rate is directly proportional to the total pressure of the system under consideration. For Ar = 5, the rate is increased by a factor of 2.3 with increasing the total pressure from 403 Torr to 935 Torr, i.e., by a factor of 2.3. For both Ar = 10 and 20, the rate is increased by a factor of 1.25 with increasing the total pressure from 403 Torr to 935 Torr.

• Korean Chemical Engineering Research
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• 제57권2호
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• pp.289-300
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• 2019

We have conducted a preliminary numerical analysis to understand the effects of double-diffusive convection on the molar flux at the crystal region during the growth of mercurous bromide ($Hg_2Br_2$) crystals in 1 g and microgravity (${\mu}g$) conditions. It was found that the total molar fluxes decay first-order exponentially with the aspect ratio (AR, transport length-to-width), $1{\leq}AR{\leq}10$. With increasing the aspect ratio of the horizontal enclosure from AR = 1 up to Ar = 10, the convection flow field shifts to the advective-diffusion mode and the flow structures become stable. Therefore, altering the aspect ratio of the enclosure allows one to control the effect of the double diffusive natural convection. Moreover, microgravity environments less than $10^{-2}g$ make the effect of double-diffusive natural convection much reduced so that the convection mode could be switched over the advective-diffusion mode.

• 한국정보디스플레이학회:학술대회논문집
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• 한국정보디스플레이학회 2009년도 9th International Meeting on Information Display
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• pp.700-702
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• 2009

We present the epitaxial growth of high-quality ZnO layers by chemical vapor transport (CVT) technique on (01-12) sapphire with a ZnO buffer layer growth by metal-organic chemical vapor deposition (MOCVD). The surface of the grown ZnO epitaxial layers has atomically flats and the RMS is 0.11 nm. PL spectrum of as-grown samples exhibits two emissions originated by interactions between photon and free excitons.

• 한국결정성장학회지
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• 제12권6호
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• pp.272-282
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• 2002

Mercurous bromide ($Hg_{2}0Br_{2}$) crystals hold promise for many acousto-optic and opto-electronic applications. This material is prepared in closed ampoules by the physical vapor transport (PVT) growth method. Due to the temperature gradient between the source and the growing crystal region, the buoyancy-driven convection may occur. The effects of thermal convection on the crystal growth rate was investigated in this study in a horizontal configuration for conditions ranging from typical laboratory conditions to conditions achievable only in a low gravity environment. The results showed that the growth rate increases linearly with Grashof number, and for 0.2 $\leq$ Ar (transport length-to-height, L/H)$\leq$1.0 sharply for Ar=5 and $\Delta$T=30 K. We have also shown that the magnitude of convection decreases with the Ar. For gravity levels of less than $10^{-2}$g the non-uniformity of interfacial distribution is negligible.

• 한국결정성장학회지
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• 제32권1호
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• pp.16-24
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• 2022

A computational study of combined thermal and solutal convection (double diffusive convection) in a sealed crystal growth reactor is presented, based on a two-dimensional numerical analysis of the nonlinear and strongly coupled partial differential equations and their associated boundary conditions. The average Nusselt numbers for the source regions are greater than those at the crystal regions for 9.73 × 10³ ≤ Gr_t ≤ 6.22 × 10⁵. The average Nusselt numbers for the source regions varies linearly and increases directly with the thermal Grashof number form 9.73 × 10³ ≤ Gr_t ≤ 6.22 × 10⁵ for aspect ratio, Ar (transport length-to-width) = 1 and 2. Additionally, the average Nusselt numbers for the crystal regions at Ar = 1 are much greater than those at Ar = 2. Also, the occurrence of one unicellular flow structure is caused by both the thermal and solutal convection, which is inherent during the physical vapor transport of Hg₂Br₂. When the aspect ratio of the enclosure increases, the fluid movement is hindered and results in the decrease of thermal buoyancy force.

• 한국결정성장학회:학술대회논문집
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• 한국결정성장학회 1997년도 Proceedings of the 13th KACG Technical Meeting `97 Industrial Crystallization Symposium(ICS)-Doosan Resort, Chunchon, October 30-31, 1997
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• pp.39-43
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• 1997

Mercurous chloride (Hg$_2$Cl$_2$) has many advantages in its applications to acousto-optic, and opto-electronic devices because it has the unique properties of a broad transmisson range, well into the far infra-red, a low acoustic velocity, a large birefringence, and a high acousto-optic figure of merit[1]. Hg$_2$Cl$_2$ has a high vapor pressure, hence single crystals are usually grown by physical vapor transport(PVT) method in closed silica glass ampoules. We discuss the application of the laser Doppler velocimetry to measure the flow field inside a closed ampoule. The experimental results, are discussed its relationship to computational model and compared to their expectations.

• 한국결정성장학회지
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• 제17권6호
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• pp.231-237
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• 2007

For an aspect ratio (transport length-to-width) of 5, Pr=3.34, Le=0.078, Pe=4.16, Cv=1.01, $P_B=50$ Torr, only thermally buoyancy-driven convection ($Gr=4.83{\times}10^5$) is considered in this study in spite of the disparity in the molecular weights of the component A ($Hg_2Cl_2$) and B which would cause thermally and/or solutally buoyancy-driven convection. The crystal growth rate and the maximum velocity vector magnitude are decreased exponentially for $3{\le}Ar{\le}5$, for (1) adiabatic walls and (2) the linear temperature profile, with a fixed source temperature. This is related to the finding that the effects of side walls tend to stabilize convection in the growth reactor. The rate for the linear temperature profiles walls is slightly greater than for the adiabatic walls far varied temperature differences and aspect ratios. With the imposed thermal profile, a fixed source region, both the rate and the maximum velocity vector magnitude increase linearly with increasing the temperature difference for $10{\le}{\Delta}T{\le}50K$.