• 한국세라믹학회지
• /
• 제48권2호
• /
• pp.152-159
• /
• 2011

The green and hard machining characteristics of dental ceramics are of great interest to dental industry. The green bodies of TZP/$Al_2O_3$ composites were prepared by the cold isostatic pressing, and machined on the CNC lathe using PCD (polycrystalline diamond) insert under various machining conditions. With increasing nose radius of PCD insert, surface roughness initially increased due to increased cutting resistance, but decreased by the onset of sliding fracture. The lowest surface roughness was obtained at spindle speed of 1,300 rpm and lowest feed rate. Hard bodies were prepared by pressureless sintering the machined green bodies at several temperatures. The grinding test for sintered hard body was conducted using electroplated diamond bur with different grit sizes. During grinding, grain pull out in the composite was occurred due to thermal expansion mismatch between the alumina and zirconia. The strength of the composite decreased with alumina contents, due to increased surface roughness and high monoclinic phase transformed during grinding process. The final polished samples represented high strength by the elimination of a phase transformation layer.

• 한국자동차공학회논문집
• /
• 제18권2호
• /
• pp.61-66
• /
• 2010

A virtual manufacturing system that is composed of JMatPro, a material modeler and $DEFORM^{TM}$-HT, a finite element package is applied to the hot press forming process: high temperature material properties for each phase such as flow stress, elastic modulus, Poisson's ratio, thermal expansion coefficient, in addition to TTT curve are predicted by JMatPro and taken into $DEFORM^{TM}$-HT to predict the material behavior considering phase transformation and heat transfer simultaneously. In order to verify the accuracy of computation, the residual stress and the springback were compared with the experimental measurements. Both the predicted and measured principal residual stresses and amount of springback were in good agreement. It was also found that the residual stresses generated from hot press forming are not negligible as it has been generally assumed, although the springback deformation is quite small.

• Progress in Superconductivity
• /
• 제3권1호
• /
• pp.5-12
• /
• 2001

Our experimental research focuses on manipulating pinning deflects to alter the phase diagram of vortex matter, creating new vortex phases. Vortex matter offers a unique opportunity for creating and studying these novel phase transitions through precise control of thermal, pinning and elastic energies. The vortex melting transition in untwinned YB $a_2$C $u_3$ $O_{7-}$ $\delta$/ crystals is investigated in the presence of disorder induced by particle irradiation. We focus on the low disorder regime, where a glassy state and a lattice state can be realized in the same phase diagram. We follow the evolution of the first order vortex melting transition line into a continuous transition line as disorder is increased by irradiation. The transformation is marked by an upward shift in the lower critical point on the melting line. With columnar deflects induced by heavy ion irradiation, we find a second order Bose glass transition line separating the vortex liquid from a Bose glass below the lower critical point. Furthermore, we find an upper threshold of columnar defect concentration beyond which the lower critical point and the first order melting line disappear together. With point deflect clusters induced by proton irradiation, we find evidence for a continuous thermodynamic transition below the lower critical point..

• 대한전기학회논문지:전력기술부문A
• /
• 제55권12호
• /
• pp.561-569
• /
• 2006

Because of the trend of deregulation, power industry is going through an unprecedented transformation in North America and Europe, and there are a host of acquisitions and mergers by the private sector to position themselves to take advantage of new business opportunities. Deregulation has accelerated the development of smaller generators and fuel cells will gradually become more attractive to mainstream electricity users as they improve in capability and decrease in cost. Fuel Cell technology is surveyed and the potential of using fuel cell as a distributed generation source is presented. This paper recommends the fuel cell power plants as alternative energy sources for distributed generation in Jeju Island, Korea. This will help in increasing fuel efficiency, at least double the current thermal plants', increasing the reliability of power supply, reducing the dependency on the HVDC link, providing quality power to the growing infrastructure, and maintaining clean air in meeting the free-trade international island.

• 한국세라믹학회지
• /
• 제32권12호
• /
• pp.1337-1348
• /
• 1995

TZP/SUS- and ZT/SUS-functionally gradient materials (FGM) were fabricated by pressureless sintering in Ar-atmosphere. The sintering defects such as warping, frustrum formation, splitting and cracking which originated from shrinkage and sintering behaviors of metal and ceramics different from each other could be controlled by the adjustment with respect to the particle size and phase type of zirconia. The residual stresses generated on the metal and ceramic regions in FGM were characterized with X-ray diffraction method, and relaxed as the thickness and number of compositional gradient layer were increased. The residual stress states in TZP/SUS-FGM have irregular patterns by means of the different sintering behavior and cracking at ceramic-monolith. While in ZT/SUS-FGM, compressive stress is induced on ceramic-monolith by the volume expansion of monoclinic ZrO2 at phase transformation. Also, compressive stress is induced on metal-monolith by the constraint of warping which may be created to the metal direction by the difference of coefficient of thermal expansions. As a consequence, it has been verified that the residual stress generated on FGM is dominantly influenced by the thickness and number of compositional gradient layer, and the sintering defects and residual stress can be controlled by the constraint of the difference of shrinkage and sintering behaviors of each component.

• 반도체디스플레이기술학회지
• /
• 제6권3호
• /
• pp.13-17
• /
• 2007

Effect of Co substitution on crystal structures of two nickel silicides, NiSi and $NiSi_2$, is investigated by using an ab initio calculation. Relaxed NiSi and $NiSi_2$ structures are calculated and the calculated lattice parameters are in good agreement with experimentally determined lattice parameters within about 2%. A Co atom substitutes a Ni and Si site, respectively, to evaluate the preferable site between them. Co prefers Ni site to Si site in both NiSi and $NiSi_2$. The calculated total energy also indicates that the Co substitution to Ni site stabilizes both the NiSi and $NiSi_2$ structures. Co also prefers Ni site in $NiSi_2$ to that in NiSi, indicating that $NiSi_2$ becomes more stable than NiSi with Co substitution. As Co addition to NiSi improves its thermal stability experimentally, this indicates that the energy barrier between the two phases is high enough to prevent the phase transformation from NiSi to $NiSi_2$ up to high temperature.

• 한국재료학회지
• /
• 제13권4호
• /
• pp.213-218
• /
• 2003

Fibrous monolithic control of$ Al_2$$O_3$ -$ZrO_2$composite was investigated by multi-pass extrusion process. To obtain sound $Al_2$$O_3$-X $O_2$sintered bodies, burning out and sintering process were carefully carried out. The sintered bodies showed continuous, fibrous monolithic microstructure without any swelling. Many microcracks were observed at the $Al_2$$O_3$-$ZrO_2$interfaces due to the mismatching of thermal expansion coefficient between $Al_2$$O_3$ and $ZrO_2$phase. Most of m- $ZrO_2$grains included twin defects such as (001), (010) and (011) type to accommodate the phase transformation induced stress.

• 대한기계학회논문집A
• /
• 제30권10호
• /
• pp.1284-1292
• /
• 2006

Heat treatment is a controlled heating and cooling process to improve the physical and/or mechanical properties of metal products without changing their shapes. Today finite element method is widely used to simulate lots of manufacturing processes including heat treatment and surface hardening processes, which aims to reduce the number of time- and cost-consuming experimental tryouts. In this study we tried, using this method, to simulate the full carburizing process that consists of carburizing, diffusing and quenching, and to predict the distribution of carbon contents, phase fraction and hardness, thermal deformation and other mechanical characteristics as the results. In the finite element analysis deformation, heat transfer, phase transformation and diffusion effects are taken into consideration. The carburizing process of a lock gear, a part of the car seat recliner, that is manufactured by the fine blanking process is adopted as the analysis model. The numerical results are discussed and partly compared with experimental data. And a combination of process parameters that is expected to give the highest surface hardness is proposed on the basis of this discussion.

• 한국재료학회지
• /
• 제8권5호
• /
• pp.464-469
• /
• 1998

HF 세정후 자연 산화막의 존재가 급속 열처리 장비를 이용, 아르곤 분위기에서 열처리할 때 티타늄 실리사이드 형성을 관찰하였다. 고분해능 단면 투과 전자 현미경 관찰 결과 기판 온도가 상온일 때 자연산화막(native oxide)이 존재함을 확인하였으며 기판 온도가 40$0^{\circ}C$일 때는 실리콘 기판과 티타늄 박막의 계면 부위에서 자연산화막, 티타늄 및 실리콘이 혼합된 비정질층이 존재함을 확인하였다. 티타늄을 증착하는 동안 기판 온도를 40$0^{\circ}C$로 유지했을 때는 C54~$TiSi_2$상이 형성되는데 요구되는 급속 열처리(Rapid Thermal Annealing : RTA)온도가 기판 온도를 상오느로 유지 했을 때보다 $100^{\circ}C$정도 감소함을 확인하였다. 이 같은 결과는 산소불순물을 함유한 비정질 층이 핵생성 자리를 제공하여 이 상의 형성이 촉진된다는 사실을 말한다. 기판온도 $400^{\circ}C$에서 형성된 티타늄 실리사이드막의 경우 비저항 $\mu$$\Omega$cm임을 확인하였다.

• 한국결정성장학회지
• /
• 제9권5호
• /
• pp.475-479
• /
• 1999

Langasite ($La_{3}Ga_{5}SiO_{14}$) was found to have wide application as a promising piezoelectric material. It has high thermal stability of the frequency and large electromechanical coupling factor. For the further development of new compounds with langasite type structure, powders in the La-Al-Si-O system were synthesized by a modified Pechini process. The evolution of the crystalline phase during calcination was studied using TG-DTA, XRD and TEM for the precursor powders. Decomposition proceeded via dehydration and removal of excess solvents at low temperatures ($T<500^{\circ}C$), followed by the crystallization of lanthanum aluminum silicate ($T>800^{\circ}C$) and phase transformation to $LaAlO_{3}$ phase ($T>1200^{\circ}C$). Transmission electron microscopy (TEM) of the calcined powders showed diffuse hollow rings corresponding to an amorphous phase at $800^{\circ}C$ and clear diffraction patterns corresponding to a crystalline phase from the P321 space group ($T<1200^{\circ}C$) and the R3m ($T<1200^{\circ}C$).