• Journal of the Korean Crystal Growth and Crystal Technology
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• v.30 no.5
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• pp.168-173
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• 2020

To analyze and describe the behavior and mechanisms occurring in the thermal oxidation process of AlN, bulk AlN single crystals were thermally treated with different temperatures. As a result, it was confirmed that full-scale oxidation of bulk AlN and growth of Al-oxide occurred from the temperature of 800℃, which confirmed that the weight% of O elements tended to increase while the N elements decreased with increasing the temperature. In the case of thermal treatment at 900℃, the grown Al-oxides were merged with neighboring Al-oxides and began to form α-Al₂O₃ poly-crystals. During thermal treatment at the temperature of 1000℃, hexagonal pyramidal shaped poly-crystalline α-Al₂O₃ was clearly observed. Through the X-ray diffraction pattern analysis, the changes of surface crystal structure according to the temperature of bulk AlN were investigated in detail.

• Journal of the Korean Crystal Growth and Crystal Technology
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• v.19 no.4
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• pp.190-195
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• 2009

Na-tetrasilicic fluorine mica powders were synthesized by skull melting method. The staring materials having chemical composition of $Mg_3(OH)_2Si_4O_{10}:Na_2SiF_6:SiO_2=8.3:24.8:66.9$ mol% were charged into a cold crucible of 13 cm in diameter and 14cm in height and heated by R.F. generator at working frequency of 2.84 MHz. The materials were maintained for 1hr as a molten state and cooled down in the container. In this study, the specific electric resistance of mica was estimated and the columnar and plate shaped mica were synthesized.

• Korean Journal of Materials Research
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• v.29 no.7
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• pp.432-436
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• 2019

Zinc oxide(ZnO) micro/nanocrystals are grown via thermal evaporation of ZnO powder mixed with Mn powder, which is used as a reducing agent. The ZnO/Mn powder mixture produces ZnO micro/nanocrystals with diverse morphologies such as rods, wires, belts, and spherical shapes. Rod-shaped ZnO micro/nanocrystals, which have an average diameter of 360 nm and an average length of about $12{\mu}m$, are fabricated at a temperature as low as $800^{\circ}C$ due to the reducibility of Mn. Wire-and belt-like ZnO micro/nanocrystals with length of $3{\mu}m$ are formed at $900^{\circ}C$ and $1,000^{\circ}C$. When the growth temperature is $1,100^{\circ}C$, spherical shaped ZnO crystals having a diameter of 150 nm are synthesized. X-ray diffraction patterns reveal that ZnO had hexagonal wurtzite crystal structure. A strong ultraviolet emission peak and a weak visible emission band are observed in the cathodoluminescence spectra of the rod- and wire-shaped ZnO crystals, while visible emission is detected for the spherical shaped ZnO crystals.

• Journal of the Korean Crystal Growth and Crystal Technology
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• v.4 no.2
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• pp.148-156
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• 1994

Three composition in the system of $YO_{1.5}-BaO-CuO$ were grown using a cold crucible (skull) melting technique with a 50 kW R.F. induction generator operating at 4 MHz as the power source. The starting materials were prepared by conventional ceramic powder processing methods, loaded into the skull, and melted at about $1200^{\circ}C$. For this study, compositions near the $YBa_2Cu_3O_X$ region were selected. The growth rates used ranged from 4 cm/hr to 0.25 cm/hr. The relation between the microstructures and the starting composition of each ingot was determined using metallograph, X-ray diffraction, and energy dispersive X-ray analysis. Both $YBa_2Cu_3O_X$ and $Y_2BaCuO_5$ needle-shaped crystals, aligned with the growth direction, were formed in the $CuO-BaCuO_2$ eutectic matrix of the $YBa_2Cu_7O_x and YBa_5Cu_{11}O_x$ ingot.

• Macromolecular Research
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• v.11 no.6
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• pp.511-513
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• 2003

Hydroxyl groups were introduced onto the surface of a silicon wafer by O$_2$ plasma treatment. Poly(l-lactide) (1-PLA) was attached onto the surface-modified silicon wafer by the ring-opening polymerization of l-lactide using the hydroxyl group as an initiator. Lamellar single crystals of 1-PLA were grown directly on the 1-PLA-attached silicon wafer from a 0.025% solution in acetonitrile at 5$^{\circ}C$. A well-separated, lozenge-shaped, monolayered lamellar single crystal was prepared because the 1-PLA-attached silicon wafer acts as an initial nucleus.

• Journal of the Korean Crystal Growth and Crystal Technology
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• v.24 no.4
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• pp.145-150
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• 2014

GaN nanorods were grown on the apex of GaN stripes by three dimensional selective growth method. $SiO_2$ mask was partially removed only on the apex area of the GaN stripes by an optimized photolithography for the selective growth. Metallic Au was deposited only on the apex of the GaN stripes and a selective growth of GaN nanorods was followed by a metal organic vapor phase epitaxy (MOVPE). We confirmed that the shape and size of the GaN nanorods depend on growth temperature and flow rates of group III precursor. GaN nanorods were grown having a taper shape which have sharp tip and triangle-shaped cross section. From the TEM result, we confirmed that threading dislocations were rarely observed in GaN nanorods because of the very small contact area for the selective growth. Stacking faults which might be originated from a difference of the crystal facet directions between the GaN stripe and the GaN nanorods were observed in the center area of the GaN nanorods.

• Applied Science and Convergence Technology
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• v.27 no.5
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• pp.95-99
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• 2018

We discuss the structural and optical characteristics of GaN nanowires (NWs) grown on Si(111) substrates by a plasma-assisted molecular-beam epitaxy. The GaN NWs with high crystal quality were formed by adopting a new growth approach, so called Ga pre-deposition (GaPD) method. In the GaPD, only Ga was supplied without nitrogen flux on a SiN/Si surface, resulting in the formation of Ga droplets. The Ga droplets were used as initial nucleation sites for the growth of GaN NWs. The GaN NWs with the average heights of 60.10 to 214.62 nm obtained by increasing growth time. The hexagonal-shaped top surfaces and facets were observed from the field-emission electron microscope images of GaN NWs, indicating that the NWs have the wurtzite (WZ) crystal structure. Strong peaks of GaN (0002) corresponding to WZ structures were also observed from double crystal x-ray diffraction rocking curves of the NW samples. At room temperature, free-exciton emissions were observed from GaN NWs with narrow linewidth broadenings, indicating to the formation of high-quality NWs.

• Journal of the Korean Crystal Growth and Crystal Technology
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• v.18 no.3
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• pp.115-121
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• 2008

Microstructure of glass-ceramics obtained by heat treating silicate glass containing 50 wt% electric arc furnace dust (EAF dust) and nucleation agents were observed. The crystallization temperature, $T_c$ of glassy specimen measured around $850^{\circ}C$ from different thermal analysis, so the heat treatment condition to obtain glass-ceramic specimen was selected as $950^{\circ}C/15 hr$. The nucleation agent, $TiO_2$ showed the superior effect on enhancing franklinite crystal growth which has stronger mechanical properties and more durable chemical resistance than willemite phase. Some specimens containing $TiO_2$ had the augite crystal phase and increasing $TiO_2$ amount decreased a fraction of willemite and increased a franklinite. Especially, the specimen with 5 wt% $TiO_2$ showed no willemite crystal phase and $1{\sim}2\;{\mu}m$ franklinite crystals dispersed uniformly in glassy matrix. Also, the specimens containing 5 wt% $TiO_2$ mixed with $Fe_2O_3$ showed a dendrite-shaped franklinite crystals caused by coalescence of small crystallites.

• Journal of the Mineralogical Society of Korea
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• v.1 no.2
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• pp.117-130
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• 1988

The morphology of the Ca-montmorillonites from the Gampo area was investigated by the use of scanning and transmission electron microscope. These bulk samples show an irregular or rugged surface with well developed cavities and relict outline of original ash material at low magnification by SEM, indicating that montmorillonites are derived from volcanogenic material. Two different types of morphology of montmorillonites by SEM were observed which appear to depend very much on the space available for crystal growth. One is honeycomb structure montmorillonite growing in an open space. The other is the closely packed aggregates of intergrown montmorillonite showing platy particles where available space for the growth is limited. Several different habits of montmorillonite within a single sample can be observed by TEM. The dominant habit observed is the irregularly shaped and foliated aggregates and the platy shaped particles. In general, platy shaped particles are relatively abundant in the samples from Yongdongri compared with those from Jugjeonri area.

• Journal of the Korean Crystal Growth and Crystal Technology
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• v.28 no.5
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• pp.217-221
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• 2018

Selective laser melting (SLM) technique is one of the additive manufacturing processes, in which functional, complex parts can be directly manufactured by selective melting layers of powder. SLM technique has received great attention due to offering a facile part-manufacturing route and utilizing a hard-to-manufacturing material (e.g. Ti6Al4V). The SLM process allows the accurate fabrication of near-net shaped parts and the significant reduction in the consumption of raw materials when compared to the traditional manufacturing processes such as casting and/or forging. In this study, we focus the high-speed additive manufacturing of Ti6Al4V parts in the aspect of manufacturing time, controlling various process parameters.