• 한국세라믹학회지
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• 제46권5호
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• pp.452-455
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• 2009

A cost-effective route to synthesize $\beta$-SiAlONs from Si mixtures by reaction bonding followed by post-sintering was investigated. Three different z values, 0.45, 0.92 and 1.87, in $Si_{6-z}Al_zO_zN_{8-z}$ without excess liquid phase were selected to elucidate the mechanism of SiAlON formation and densification. For RBSN (reaction-bonded silicon nitride) specimens prior to post-sintering, nitridation rates of more than 90% were achieved by multistep heating to $1400^{\circ}C$ in flowing 5%$H_2$/95%$N_2$; residual Si was not detected by XRD analysis. An increase in density was acquired with increasing z values in post-sintered specimens, and this tendency was explained by the presence of higher amounts of transient liquid phase at larger z values. Measured z values from the synthesized $\beta$-SiAlONs were similar to the values calculated for the starting compositions. Slight deviations in z values between measurements and calculations were rationalized by a reasonable application of the characteristics of the nitriding and post-sintering processes.

• 한국세라믹학회지
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• 제50권4호
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• pp.280-287
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• 2013

Plate-type phosphor is a promising substitute in overcoming the issues related to the powder phosphor paste mixed with resin. In this research, $Ca-{\alpha}-SiAlON:Eu^{2+}$ plate phosphor ($Ca_xSi_{12-(m+n)}Al_{m+n}O_nN_{16-n}:Eu_y$) was investigated for the varied compositions (m,n) of the host crystal with the fixed Eu content (y). Densification was promoted for the compositions with increasing 'm' values for the m=2n relationship. Dictated by the Eu concentration inside the phosphor crystal, photoluminescence intensity was stronger in ${\alpha}2$ specimen (m = 3.0, n = 1.5) containing the second phases when compared to ${\alpha}1$ specimen (m = 1.5, n = 0.75) comprising a single-phase ${\alpha}$-SiAlON. The concentration of Eu in the non-emitting amorphous interfacial glass phase was 2~4 times of the designed Eu concentration inside the ${\alpha}$-SiAlON crystal.

• Journal of Welding and Joining
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• 제32권4호
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• pp.55-62
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• 2014

The use of light-weight Al alloys in the automotive industry is increasing to meet requirements for fuel efficiency and emission reduction. Joining Al alloy to the conventional steel sheet is also very important issue with the increased use of Al alloy, and several joining processes have been introduced to enhance joining strength between dissimilar metals. This paper deals with a galvanic corrosion in the dissimilar metal joining. Salt spray tests up to 2000 hours were conducted on a self-piercing rivet, spot welded, adhesive bonded and weld-bonded joints, and cross-sections and tensile shear strength according the salt spray duration were analyzed at every 500-hour. Self-piercing rivet joint had relative low initial strength but the joint strength did not change regardless of the salt spray duration. The strength of other joints (spot welded, adhesive bonded and weld-bonded joints) decreased with the increase of salt spray duration and the corrosion behaviour of each joint was discussed.

• KSII Transactions on Internet and Information Systems (TIIS)
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• 제6권12호
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• pp.3352-3365
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• 2012

Group key agreement (GKA) is a cryptographic primitive allowing two or more users to negotiate a shared session key over public networks. Wu et al. recently introduced the concept of asymmetric GKA that allows a group of users to negotiate a common public key, while each user only needs to hold his/her respective private key. However, Wu et al.'s protocol can not resist active attacks, such as fabrication. To solve this problem, Zhang et al. proposed an authenticated asymmetric GKA protocol, where each user is authenticated during the negotiation process, so it can resist active attacks. Whereas, Zhang et al.'s protocol needs a partially trusted certificate authority to issue certificates, which brings a heavy certificate management burden. To eliminate such cost, Zhang et al. constructed another protocol in identity-based setting. Unfortunately, it suffers from the so-called key escrow problem. In this paper, we propose the certificateless authenticated asymmetric group key agreement protocol which does not have certificate management burden and key escrow problem. Besides, our protocol achieves known-key security, unknown key-share security, key-compromise impersonation security, and key control security. Our simulation based on the pairing-based cryptography (PBC) library shows that this protocol is efficient and practical.

• 한국재료학회지
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• 제32권10호
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• pp.403-407
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• 2022

This research investigated the effect of Si addition on the microstructure, mechanical properties, electric and thermal conductivity of as-extruded Al 6013 alloys. As the content of Si increased, the area fraction of the second phase increased. As the Si content increased, the average grain size decreased remarkably, from 182 (no Si addition) to 142 (1.5Si), 78 (3.0Si) and 77 ㎛ (4.5Si) due to dynamic recrystallization by the dispersed second particles in the aluminum matrix during the hot extrusion. As the Si content increased, the yield strength and ultimate tensile strength increased. The maximum values of yield strength and ultimate tensile strength were 224 MPa and 103 MPa for the 6013-4.5Si alloy. As the amount of Si added increased, the electrical and thermal conductivity decreased. The electrical and thermal conductivity of the Al6013-4.5Si alloy were 44.0 % IACS and 165.0 W/mK, respectively. The addition of Si to Al 6013 alloy had a significant effect on its thermal conductivity and mechanical properties.

• 한국산학기술학회논문지
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• 제17권10호
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• pp.432-438
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• 2016

본 연구는 야관문 추출물이 알코올 투여된 생쥐의 간장에 미치는 영향을 알아보고자 시행하였다. 대조군(Con), 알코올투여군(AL) 알코올-야관문 200 mg/kg 투여군 (AL-LC 200군), 알코올-야관문 400 mg/kg투여군 (AL-LC 400군)으로 나누어 구분하였다. 대조군과 알코올투여군은 1차 증류수를 알코올-야관문 투여군은 야관문을 각각 200 mg/kg과 400 mg/kg 농도로 10 일간 경구투여 하였다. 모든 실험군은 경구투여 7일후 12시간 절식시킨 다음 대조군은 1차 증류수, 알코올투여군과 알코올-야관문투여군은 50% ethanol (MERCK, USA)을 10 g/kg농도로 12시간 간격으로 5회 경구투여 하였다. 혈액의 AST, ALT 효소활성과 간장의 조직학적 실험을 하여 다음과 같은 결론을 얻었다. AST, ALT은 AL군 보다 알코올-야관문 복합투여군에서 수치가 감소하였다. 특히 AST 활성은 AL군에 대하여 AL-LC 200군과 AL-LC 400군 모두 활성이 유의적으로 감소하였다. 조직학적 결과는 간장에서 AL군은 대부분에서 괴사와 변형이 일어났고 중심정맥 주변 간세포 내에 지방방울이 축적되었다. AL-LC 200군에서는 중심정맥의 일부가 부풀어진 모습을 보였고 간세포의 팽창과 작은 지방 방울이 축적되었다. AL-LC 400군에서는 중심정맥은 정상적인 모습을 보이며 일부 간세포내의 작은 지방 방울이 축적 되었고 대부분은 정상적인 모습을 보였다. 이상의 연구결과로 보아 야관문 추출물이 알코올투여한 생쥐의 간장손상에 대한 보호 효과가 있어 기능성 건강 소재로서의 활성 용도가 높을 것으로 사료된다.

• 한국결정성장학회지
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• 제28권5호
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• pp.217-221
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• 2018

Selective laser melting(SLM)은 적층 제조 방법 중 하나로서, 분말을 선택적으로 용융하여 기능적이고, 복잡한 형상을 즉각적으로 제작 가능하다는 장점이 있다. Ti-6Al-4V 합금의 경우 높은 융점과 산화문제로 인하여 SLM 공정도입에 어려움을 겪고 있다. 본 연구에서는 SLM 장비를 사용하여 Ti-6Al-4V 파트를 성공적으로 적층하였으며 스캔속도, 레이저 파워의 변수를 조절하여 적층 가능 조건을 도출하였다. 또한 적층 된 Ti-6Al-4V 파트의 형상 정밀도, 면밀도 및 기계적특성 평가를 통하여 일반적으로 사용되는 공정조건(스캔 속도 200~700 mm/s)으로 제작된 제작품 물성치와 비교를 통해 Ti-6Al-4V 합금의 고속 적층 영역에서의 최적 공정을 확립하였다.

• 대한전자공학회논문지SD
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• 제38권8호
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• pp.13-19
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• 2001

분자선 결정 성장법을 이용하여 성장된 서로 다른 장벽층의 두께를 갖는 $Al_{0.3}Ga_{0.7}N$ heterojunction field effect transistors (HFETs) 를 제작하여 그 특성을 비교, 관찰하였다. $Al_{0.3}Ga_{0.7}N$층의 두께에 따른 특성의 비교로부터 최적의 2 차원 전자개스 (2 dimensional electron gas) 를 가질 수 있는 $Al_{0.3}Ga_{0.7}N$/GaN HFET 소자 구조를 얻을 수 있었다. $L_g=0.6$ ${\mu}m$ 와 $W_g=34\;{\mu}m$ $Al_{0.3}Ga_{0.7}N$/GaN (20 nm/2 mm) HFET에서 Imax ($V_{gs}=1\;V$) 와 $G_{m,maX}$는 각각 1.155 A/mm 및 250ms/${\mu}m$ 이었으며 $F_t=13$ GHz 와 $F_{max}=48$ GHz의 우수한 고주파 특성을 얻을 수 있었고 2 inch 기판상에서 제작된 소자들은 5% 이하의 매우 균일한 DC 특성을 나타내었다. 이와 더불어 게이트-드레인 간의 간격에 따른 소자의 특성을 관찰함으로서 소자의 항복전압과 고주파 특성과의 상관관계를 고찰하였다.

• 한국세라믹학회지
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• 제47권6호
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• pp.503-508
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• 2010

$Eu^{2+}$-doped $\beta$-SiAlONs ($Si_{6-z}Al_zO_zN_{8-z}:Eu_y$) are recognized as promising phosphor materials to build an white LED for lighting application due to its excellent absorption/emission efficiency in the long wave length region. In this research, the fabrication of $\beta$-SiAlON:Eu plate phosphor by sintering was investigated with fixed Eu content(y) and varied composition of the host lattice(z). The addition of the activator $Eu_2O_3$ lead to enhanced densification by forming the transient liquid phase. The refinement of a composition by the calculated lattice parameter indicated that the measured composition of the fabricated specimens is nearly same to that of designed one. The single phase $\beta$-SiAlON:Eu plate with relative density of 96.4% was achieved by addition of 2 wt% CaO, which implies the possibility of full densification by adjusting the processing variables.

• 한국분말재료학회지
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• 제24권3호
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• pp.235-241
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• 2017

Titanium alloys have high specific strength, excellent corrosion and wear resistance, as well as high heat-resistant strength compared to conventional steel materials. As intermetallic compounds based on Ti, TiAl alloys are becoming increasingly popular in the aerospace field because these alloys have low density and high creep properties. In spite of those advantages, the low ductility at room temperature and difficult machining performance of TiAl and $Ti_3Al$ materials has limited their potential applications. Titanium powder can be used in such cases for weight and cost reduction. Herein, pre-forms of Ti-Al-xMn powder alloys are fabricated by compression forming. In this process, Ti powder is added to Al and Mn powders and compressed, and the resulting mixture is subjected to various sintering temperature and holding times. The density of the powder-sintered specimens is measured and evaluated by correlation with phase formation, Mn addition, Kirkendall void, etc. Strong Al-Mn reactions can restrain Kirkendall void formation in Ti-Al-xMn powder alloys and result in increased density of the powder alloys. The effect of Al-Mn reactions and microstructural changes as well as Mn addition on the high-temperature compression properties are also analyzed for the Ti-Al-xMn powder alloys.