• Proceedings of the Materials Research Society of Korea Conference
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• 2009.05a
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• pp.33.2-33.2
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• 2009

Dielectric and piezoelectric properties of Lead-free $(1-x)(Bi_{0.5}K_{0.5})TiO_3-xBiFeO_3$ceramics prepared by a conventional solid state reaction method were investigated in the range of x = 0~10 mol%. Piezoelectric coefficient was increased from 31 pC/N at x = 0 mol% to 64 pC/N at x = 6 mol% then decreased with increasing x. Electromechanical coupling factor ($K_p$) was increased up to 0.18 at x = 10 mol%. On the other hand, mechanical quality factor ($Q_m$) was decreased. Grain size was not much changed with various x and a single perovskite with tetragonal symmetry was maintained at all compositions forming a solid solution between $(Bi_{0.5}K_{0.5})TiO_3$ and $BiFeO_3$. Depolarization temperature ($T_d$) was gradually decreased with increasing x from $302^{\circ}C$ at x = 0 to $245^{\circ}C$ at x = 10 mol%.

• Transactions on Electrical and Electronic Materials
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• v.14 no.1
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• pp.32-35
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• 2013

In this paper, a manufacturing process was developed for fabricating high-quality AlGaN/GaN high electron mobility transistors (HEMTs) on silicon carbide (SiC) substrates. Various conditions and processing methods regarding the ohmic contact and pre-metal-deposition $BCl_3$ etching processes were evaluated in terms of the device performance. In order to obtain a good ohmic contact performance, we tested a Ti/Al/Ta/Au ohmic contact metallization scheme under different rapid thermal annealing (RTA) temperature and time. A $BCl_3$-based reactive-ion etching (RIE) method was performed before the ohmic metallization, since this approach was shown to produce a better ohmic contact compared to the as-fabricated HEMTs. A HEMT with a 0.5 ${\mu}m$ gate length was fabricated using this novel manufacturing process, which exhibits a maximum drain current density of 720 mA/mm and a peak transconductance of 235 mS/mm. The X-band output power density was 6.4 W/mm with a 53% power added efficiency (PAE).

• Proceedings of the KIEE Conference
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• 1999.07d
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• pp.1910-1912
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• 1999

This paper was described electrical characteristics of Metal/GaN contact for application of GaN thin films. The lowest contact resistivity was $1.7\times10^{-7}[\Omega-cm^2]$ at Ti/Al Structure. Mean while, GaN MESFETs have been fabricated with a 250 nm thick channel on a high resistivity GaN layer grown by GAIVBE system. For a gate-source diode reverse bias of 35 V, the gate leakage current was $120{\mu}A$. From the data, we estimate the transconductance for our GaN MESFET to be 25 mS/mm.

• Korean Journal of Materials Research
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• v.14 no.9
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• pp.634-641
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• 2004

The photocatalytic performance of $TiO_2$ thin films coated on porous alumina balls using various aqueous $TiOCl_2$ solutions as starting precursors, to which 1.0 $mol\%$ transition metal ($Ni^{2+},\;Cr^{3+},\;Fe^{3+},\;Nb^{3+},\;and\;V^{5+}$) chlorides had been already added, has been investigated, together with characterizations for $TiO_2$ sols synthesized simultaneously in the same autoclave through hydrothermal method. The synthesized $TiO_2$ sols were all formed with an anatase phase, and their particle size was between several nm and 30 nm showing ${\zeta}-potential$ of $-25{\sim}-35$ mV, being maintained stable for over 6 months. However, the $TiO_2$ sol added with Cr had a much lower value of -potential and larger particle sizes. The coated $TiO_2$ thin films had almost the same shape and size as those of the sol. The pure $TiO_2$ sol showed the highest optical absorption in the ultraviolet light region, and other $TiO_2$ sols containing $Cr^{3+},\;Fe^{3+}\;and\;Ni^{2+}$ showed higher optical absorption than pure sol in the visible light region. According to the experiments for removal of a gas-phase benzene, the pure $TiO_2$ film showed the highest photo dissociation rate in the ultraviolet light region, but in artificial sunlight the photo dissociation rate of $TiO_2$ coated films containing $Cr^{3+},\;Fe^{3+}\;and\;Ni^{2+}$ was measured higher together with the increase of optical absorption by doping.

• Proceedings of the Korean Society for Technology of Plasticity Conference
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• 2009.10a
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• pp.72-75
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• 2009

Design and Manufacturing processes of Ti-6Al-4V profiled ring-products were investigated with three-dimensional FEM simulation and experimental analyses. FEM simulation for the ring-rolling process was used to calculate the state variables such as strain, strain rate and temperature. In the simulation results of strain and temperature distributions for a plane ring rolling process, the strain level at the surface area is higher than that at the mid-plane, but the temperature level at the surface area is lower than that at mid-plane due to heat transfer between the workpiece and the work roll. These distributions showed a great influence on the evolution of microstructure in different positions. In order to induce the uniform deformation of the profile ring and reduce the applied load, the final blank was prepared by two-step processes. The mechanical properties of Ti-6Al-4V alloy ring products made in this work were investigated with tensile and impact tests and analyzed with the evolution of microstructures during the ring rolling process.

• Proceedings of the Korean Society for Technology of Plasticity Conference
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• 2004.05a
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• pp.114-117
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• 2004

The isothermal forging design of a Ti-6Al-4V wing shape was performed by 3D FE simulation. The design focuses on near-net shape forming by the single stage. The process variables such as the die design, pre-form shape and size, ram speed and forging temperature were investigated. The minimization of forging load and uniform strain distribution in a given forging condition were considered as main design factors. The FE simulation results fur the final process design were compared with the isothermal forging tests. Finally, the modified process design for producing the uniform Ti-6Al-4V wing product without forming defects was suggested.

• Proceedings of the Korean Institute of Electrical and Electronic Material Engineers Conference
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• 2006.12a
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• pp.123-125
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• 2006

The $TiO_2$ sol was prepared hydrothermally in an autoclave from aqueous $TiOCl_2$ solutions as a starting precursor. Hollow fibers were obtained when the sol-gel-derived $TiO_2$ sol was treated chemically with a NaOH solution and subsequently heated in the autoclave under various conditions. A systematic analysis of the influence of different NaOH concentrations on the formation of nanotubes was carried out. The details of the nanotubular structure were investigated by using transmission electron microscopy (TEM), scanning electron microscopy (SEM) and X-ray diffraction (XRD). From the TEM images, the outer and the inner diameters of the tubes were measured to be about 8 and 4 nm, respectively, the lengths were measured to be several hundreds of nanometers.

• Electrical & Electronic Materials
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• v.6 no.5
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• pp.436-445
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• 1993

2단계하소법에 의해 낮은 하소온도에서 미세하고도 균일한 PZT분말을 합성하였다. 우선 Zr $O_{2}$와 Ti $O_{2}$ 혼합분말을 1차하소하여 (Z $r_{0.53}$ $Ti_{0.47}$) $O_{3}$(ZTO) 분말을 합성하고 이 ZTO 고용분말에 PbO와 N $b_{2}$ $O_{5}$을 혼합한 후 650-800.deg.C에서 2시간 하소하여 PZT 분말을 합성하였는데 얻어진 분말은 고상반응법에 비해 미세할 뿐만 아니라 XRD 분석결과 710.deg.C의 낮은 하소온도에서도 PZT 단일상을 나타내었다. 2단계하소법에 의해 하소온도를 낮출 수 있는 주된 이유로는 고상반응법에서는 중간생성물인 PbTi $O_{3}$상의 생성이 수반됨으로 850.deg.C 이상 되어야만 안전한 PZT가 생성될 수 있는 점을 들 수 있다. 또 2단계하소법에 의하면 950.deg.C이하의 낮은 소결온도에서도 치밀화가 미루어지는 소결이 가능함을 알 수 있었는데 이와같이 소결온도를 낮출 수 있는 것은 고상반응법에 비해 미세한 PZT 분말을 사용하였기 때문이라 풀이된다.이된다.

• Electrical & Electronic Materials
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• v.4 no.4
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• pp.304-311
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• 1991

본 연구에서는 p형(100) 실리콘 기판 위에 LPCVD법으로 산화막과 다결정 실리콘을 증착하고 그 위에 Magnetron Sputtering법으로 티타늄을 500.angs.을 증착한 후, 열처리 온도 500-900.deg.C 사이에서 열처리 시간을 변화시키면서 N$_{2}$ 분위기 속에서 급속 열처리하여 티타늄 실리사이드를 형성하고 그 특성을 조사하였다. 500-600.deg.C 온도 범위에서 10초간 열처리한 시료에서는 실리사이드상은 나타나지 않고, 산소등의 불순물이 티타늄 박막 내로 확산되어 600.deg.C에서 면 저항이 최대값을 보였으며 열처리 온도는 675-750.deg.C로 높이자 TiSi상이 나타나면서 면저항이 감소되고 결정립의 크기가 크게 증가하였다. 또한 열처리온도 800.deg.C에서 나타나기 시작한 TiSi$_{2}$상은 열처리 온도 850.deg.C까지 TiSi상과 공존하면서 면저항과 reflectance는 계속 감소했다. 900.deg.C에서 10초간 열처리한 시료에서는 orthorhombic구조의 완전한 실리사이드 상만 나타났다. 최종적인 티타늄 실리사이드 박막의 두께는 1200.angs.이며 비저항은 18.mu..OMEGA.cm였다.

• Electrical & Electronic Materials
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• v.5 no.2
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• pp.207-215
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• 1992

본 연구에서 xPb(A $l_{0.5}$N $b_{0.5}$) $O_{3}$-(1-x)Pb(Z $r_{0.52}$ $Ti_{o.48}$) $O_{3}$계의 조성변화에 따른 유전 및 압전특성에 관해 실험한 결과 다음과 같은 결론을 얻었다. PAN의 첨가량이 증가함에 따라 c축은 수축되고 a축은 팽창하여 tetragonality는 감소하였고 grain의 크기와 Curie온도 또한 PAN의 첨가량에 따라 감소하였으나 밀도와 유전상수는 PAN의 양이 5mol%까지 증가하다가 그 이상에서는 감소하는 경향을 보였다. PAN의 첨가량이 증가함에 따라 시편의 비저항은 증가하였고 Kp는 PAN의 양이 5mol%첨가시 60%로 최대치를 보였으나 Qm은 최소치를 나타내었다.다.