• Proceedings of the Korean Society of Propulsion Engineers Conference
• /
• 2007.11a
• /
• pp.354-358
• /
• 2007

An experimental study was performed to understand spray characteristics of the slinger injector. system for the micro turbojet engine. In this fuel injection system, fuel is sprayed and atomized in the combustor by centrifugal forces of engine shaft. This experimental apparatus consist of a high speed rotating Spindle, slinger injector, pressure tank and acrylic case. The droplet size and velocity were measured by PDPA(Phase Doppler Particle Analyzer) and spray was visualized by using Nd-Yag laser-based flash photography. From the test results, the droplet size(SMD) is largely affected to rotational speed, mass flow rate and the number of injection orifice. From the this experimental study, we could understand the spray characteristics of the slinger injection system and obtain the optimum shape of the slinger injector nozzle which is suitable for the micro turbojet engine.

• Journal of the Korean Society of Propulsion Engineers
• /
• v.14 no.5
• /
• pp.51-56
• /
• 2010

An experimental study was performed to understand spray characteristics of the V type rotating fuel nozzle with orifice diameters by using high speed rotational system. The experimental apparatus consist of a high speed rotational system, fuel injection system and acrylic case. The droplet size and velocity were measured by PDPA(Phase Doppler Particle Analyzer) and spray was visualized by using Nd-Yag laser-based flash photography. From the test results, droplet size is reduced with increasing orifice diameter up to the critical value. When increasing orifice diameter over than this critical value, droplet size is not decreased with increasing the orifice diameter. This is due to the irregular distribution of the liquid sheet around the inner surface of injection orifice.

• Journal of the Korean Ceramic Society
• /
• v.45 no.12
• /
• pp.827-831
• /
• 2008

Spark plasma sintering (SPS) of AlN ceramics were carried out with ${Y_2}{O_3}$ as sintering additive at a sintering temperature $1,550{\sim}1,700^{\circ}C$. The effect of ${Y_2}{O_3}$ addition on sintering behavior and thermal conductivity of AlN ceramics was studied. ${Y_2}{O_3}$ added AlN showed higher densification rate than pure AlN noticeably, but the formation of yttrium aluminates phases by the solid-state reaction of ${Y_2}{O_3}$ and ${Al_2}{O_3}$ existed on AlN surface could delay the densification during the sintering process. The thermal conductivity of AlN specimens was promoted by the addition of ${Y_2}{O_3}$ up to 3 wt% in spite of the formation of YAG secondary phase in AlN grain boundaries because ${Y_2}{O_3}$ addition could reduced the oxygen contents in AlN lattice which is primary factor of thermal conductivity. However, the thermal conductivity rather decreased over 3 wt% addition because an immoderate formation of YAG phases in grain boundary could decrease thermal conductivity by a phonon scattering surpassing the contribution of ${Y_2}{O_3}$ addition.

• Journal of Oral Medicine and Pain
• /
• v.26 no.4
• /
• pp.395-406
• /
• 2001

지난 수십년동안, 다양한 레이저기기를 이용하여 치아 경조직에 대한 응용 실험을 수행되었다. 그러나, 발생한 주요한 문제는 열발생과 플라즈마 형성과 같은 부작용이 지속된다는 점이었다. 실망스러운 실험결과에 따라 치아 삭제에 대한 레이저의 응용에 대한 연구가 거의 진행되지 못하였다. 본 연구의 목적은 국내에서 처음으로 개발된 얼비움레이저기기를 사용하여 다양한 에너지 수준과 다양한 펄스빈도에서 법랑질의 삭제율을 비교하고자 하였다. 본 연구에 사용된 레이저기기는 국내에서 최초로 개발된 $2.94{\mu}m$ 펄스를 $250{\mu}s$ 방출하는 얼비움야그레이저 기기이다. 기기의 에너지는 20 mJ에서 350 mJ까지 다양하게 방출하며, 최대 출력은 3.5 W이다. 22개의 치아를 에폭시 레진에 포매하고 경화하였다. 치아의 법랑질을 회전 다이아몬드 칼을 흐르는 물과 함께 사용하여 협측 또는 설측으로 평탄하게 삭제하였다. 제작된 법랑질표면은 6개의 구획으로 구분하여 에너지 수준과 펄스빈도에 따라 조사하였다. 본 연구에서 사용된 표본 수는 132개이며, 삭제율을 조사하기 위한 모든 표본은 22개의 실험 군으로 분류하였다. 실험결과는 다음과 같다. 1. 에너지 수준간에는 유의한 삭제율의 차이가 있었다. 2. 40 및 80 mJ로 조사한 경우에는 펄스빈도 간에 차이가 없었지만, 120 mJ의 출력으로 동일한 에너지를 조사한 경우에는 20 Hz가 40 또는 80 mJ로 조사한 경우 보다 가장 삭제효과가 높았다. 3. 연속조사 한 경우 조사시간 경과에 따라 법랑질의 삭제율은 증가되지 않았다. 결론적으로 효과적으로 법랑질을 삭제하기 위해서는 적절한 에너지와 펄스의 레이저를 사용해야 하며, 또한 항상 정확한 초점거리를 유지해야 할 것으로 사료된다.

• Korean Journal of Materials Research
• /
• v.5 no.7
• /
• pp.781-785
• /
• 1995

from the $K_{6}$P$_4$ $O_{13}$ flux using a temperature cooling method. According to SHG outpower measurement, phase matching angle is $\theta$=90$^{\circ}$, $\Phi$=23.3$^{\circ}$and angular acceptance of $\Phi$ direction is about 2 degree, Deviation of phase matching angle due to index inhomogeneity in KTP crystal is 0.17 degree. A 20mW green laser was obtained with the combination of a intracavity Nd ; YAG and KTP crystal.

• International Journal of Korean Welding Society
• /
• v.2 no.2
• /
• pp.26-31
• /
• 2002

Considering the fuel consumption of car, a light structure of aluminum alloys is desired fer car body nowadays. However, fusion welding of aluminum alloys has some problems of reduction of joint efficiency, porosity formation and hot cracking. In the present work, investigation to improve the joint performance of laser welded joint has been carried out by addition of Cu, Ni, and Zr to A6NO 1 alloy welds. Aluminum alloy plate of 2.Omm in thickness with filler metal bar was welded by twin beam Nd: YAG laser facility (total power: 5kW). The filler metals were prepared by changing the chemical compositions for adding the elements into the weld metal. Thirteen filler metal bars were prepared and pre-placed into the base metal before welding. Ar gas shielding with a flow rate of 10 1/min was used. The defocusing distance is kept at 0 mm. At travel speeds off 3 to 9 and at laser power of 5kW (front beam 2kW rear beam 3kW), full penetration welds were obtained, whereas at travel speeds of 12 to 18 m/min and same power, partial penetration was observed. The joint efficiency of laser-welded joint was improved by the addition of Cu, Ni, and Zr due to the solid solution hardening, grain refining and precipitation hardening. The type of hardening has been further considered by metallurgical examination.

• The Journal of Advanced Prosthodontics
• /
• v.6 no.6
• /
• pp.434-443
• /
• 2014

PURPOSE. The purpose of this study was to evaluate and compare the effects of different surface pretreatment techniques on the surface roughness and shear bond strength of a new self-adhering flowable composite resin for use with lithium disilicate-reinforced CAD/CAM ceramic material. MATERIALS AND METHODS. A total of one hundred thirty lithium disilicate CAD/CAM ceramic plates with dimensions of $6mm{\times}4mm$ and 3 mm thick were prepared. Specimens were then assigned into five groups (n=26) as follows: untreated control, coating with $30{\mu}m$ silica oxide particles ($Cojet^{TM}$ Sand), 9.6% hydrofluoric acid etching, Er:YAG laser irradiation, and grinding with a high-speed fine diamond bur. A self-adhering flowable composite resin (Vertise Flow) was applied onto the pre-treated ceramic plates using the Ultradent shear bond Teflon mold system. Surface roughness was measured by atomic force microscopy. Shear bond strength test were performed using a universal testing machine at a crosshead speed of 1 mm/min. Surface roughness data were analyzed by one-way ANOVA and the Tukey HSD tests. Shear bond strength test values were analyzed by Kruskal-Wallis and Mann-Whitney U tests at ${\alpha}=.05$. RESULTS. Hydrofluoric acid etching and grinding with high-speed fine diamond bur produced significantly higher surface roughness than the other pretreatment groups (P<.05). Hydrofluoric acid etching and silica coating yielded the highest shear bond strength values (P<.001). CONCLUSION. Self-adhering flowable composite resin used as repair composite resin exhibited very low bond strength irrespective of the surface pretreatments used.

• Journal of the Korean Ceramic Society
• /
• v.41 no.7
• /
• pp.541-547
• /
• 2004

Addition of large particles restrains densification by small particles in mixed particle systems. In the present study, large SiC whiskers or particles were introduced into small particles for restraining densification and the mixtures were sintered using yttrium aluminum garnet (Y$_3$A1$\sub$5/O$\sub$12/, YAG) as a sintering additive. By controlling the content of large SiC whiskers or particles and the applied pressure during sintering, porous SiC ceramics, with a porosity ranging from 0.3% to 39%, were fabricated. Porosity increased with increasing the content of restraining materials. SiC whiskers were more effective than large SiC partcles for restraining densification. Permeability of the porous SiC ceramics increased with increasing the porosity. Flexural strength decreased with increasing porosity. A noticeable increase in strain to failure was observed in the porous ceramics with a porosity ranging from 18% to 39%.

• The Korean Journal of Ceramics
• /
• v.4 no.4
• /
• pp.323-330
• /
• 1998

Mixed oxide compounds of potential usefulness for fibre coatings (hexagonal celsian, $BaAl_2Si_2O_8$ and lanthanum hexaluminate, $LaAl_{11}O_{18}$) or for matrix materials (yttrium aluminium garnet, $Y_3Al_5O_{12}$) were prepared by hybrid sol-gel synthesis and their thermal crystallisation was monitored by thermal analysis, X-ray diffraction and multinuclear solid state MAS NMR. All the gels convert to the crystalline phase below about $12200^{\circ}C$, via amorphous intermediates in which the Al shows and NMR resonance at 36-38 ppm sometimes ascribed to Al in 5-fold coordination. Additional information about the structural changes during thermal treatment was provided by $^{29}Si$, $^{137}Ba$ and $^{89}Y$ MAS NMR spectroscopy, showing that the feldspar framework of celsian begins to be established by about $500^{\circ}C$ but the Ba is still moving into its polyhedral lattice sites about $400^{\circ}C$ after the sluggish onset of crystallization. Lanthanum hexaluminate and YAG crystallise sharply at 1230 and $930^{\circ}C$ respectively, the former via $\gamma-Al_2O_3$, the latter via $YAlO_3$. Yttrium moves into the garnet lattice sites less than $100^{\circ}C$ after crystallisation.

• The Transactions of the Korean Institute of Electrical Engineers C
• /
• v.52 no.8
• /
• pp.335-341
• /
• 2003

The composites were fabricated respectively 61vo1.% $\beta$ -SiC and 39vo1.% Ti $B_2$ spray-dried powders with the liquid forming additives of l2wt% $Al_2$ $O_3$＋ $Y_2$ $O_3$ by pressureless annealing at 1$700^{\circ}C$, 175$0^{\circ}C$, 180$0^{\circ}C$ for 4 hours. The result of phase analysis of composites by XRD revealed $\alpha$ -SiC(6H), Ti $B_2$, and YAG(A $l_{5}$ $Y_3$ $O_{12}$ ) crystal phase. The relative density, the Young's modulus and fracture toughness showed respectively the highest value of 92.97%, 92.88Gpa and 4.4Mpaㆍ $m^{\frac{1}{2}}$ for composites by pressureless annealing temperature 1$700^{\circ}C$ at room temperature. The electrical resistivity showed the lowest value of 8.09${\times}$10$^{-3}$ ㆍcm for composite by pressureless annealing temperature 1$700^{\circ}C$ at $25^{\circ}C$. The electrical resistivity of the SiC-Ti $B_2$ composites was all positive temperature cofficient resistance(PTCR) in the temperature ranges from $25^{\circ}C$ to $700^{\circ}C$.