The Ag/WC electrical contacts were prepared via powder metallurgy using 60 wt% Ag, 40 wt% WC, and small amounts of Co3O4 with varying WC particle sizes. After the fabrication of the contact materials, microstructure observations confirmed that WC-1 had an average grain size (AGS) of 0.27 μm, and WC-2 had an AGS of 0.35 μm. The Ag matrix in WC-1 formed fine grains, whereas a significantly larger and continuous growth of the Ag matrix was observed in WC-2. This indicates the different flow behaviors of liquid Ag during the sintering process owing to the different WC sizes. The electrical conductivities of WC-1 and WC-2 were 47.8% and 60.4%, respectively, and had a significant influence on the Ag matrix. In particular, WC-2 exhibited extremely high electrical conductivity owing to its large and continuous Ag-grain matrix. The yield strengths of WC-1 and WC-2 after compression tests were 349.9 MPa and 280.7 MPa, respectively. The high yield strength of WC-1 can be attributed to the Hall–Petch effect, whereas the low yield strength of WC-2 can be explained by the high fraction of high-angle boundaries (HAB) between the WC grains. Furthermore, the relationships between the microstructure, electrical/mechanical properties, and deformation mechanisms were evaluated.

Although airborne wear particles (AWPs) generated from wheel-rail contacts are the major source of particulate matter (PM) in subway systems, studies on reducing the generation of such particles in order to enhance air quality are extremely rare. Therefore, this study investigated the effect of applying water-lubricant (applying tap water) on improving air quality by reducing the mass concentration (MC) of AWPs from wheel-rail contacts at a train velocity of 73 km/h using a twin-disk rig. An optical particle sizer was used to measure the MC of particles with the diameter range of 0.3 μm~10 μm. The results showed that the generation trends regarding PM1, PM2.5, and PM10 were different for dry and water-lubricated conditions: all three PMs showed an increasing-decreasing trend with slip rate under dry conditions; however, they were almost constant with slip rate under water-lubricated conditions. The particle size distributions were also different for dry and water-lubricated conditions: the peak occurred in multi-modal with the largest peak at approximately 6 μm in diameter under dry conditions; whereas, the peak occurred in bi-modal with the largest peak at approximately 0.9 μm in diameter under water-lubricated conditions. In addition, MCs were mostly smaller under water-lubricated conditions than dry conditions except at approximately 0.9 μm in diameter. Applying water significantly decreased PM1~2.5 and PM2.5~10 by more than 95%. This caused a decrease in PM2.5 and PM10 by 48.1% and 78.5%, respectively. On the other hand, applying water increased PM0.3~1 (i.e., PM1) by 52.8%, possibly owing to the effect of water vapor and mineral crystals from tap water. Overall, these findings indicate that water-lubrication can improve air quality in subway systems by reducing the MC of APWs generated from wheel-rail contacts. This study may provide a reference for future studies seeking to improve air quality in subway systems by reducing AWPs generated from wheel-rail contacts by applying lubricants.

The purpose of this study is to analyze the distribution characteristics of mist spray particle size by devising a rotary mist spraying device to develop the evaporative salt water desalination system. The rotary mist spraying device was consisted of a BLDC sirocco fan, a spinning fan, a fan fixed shaft and a salt water supply device etc. In this study we analyzed the characteristics of spray particle size and distribution according to the variation of sirocco fan surface roughness(Ra, μm), revolutions(rpm) and salt water flow rate(mL/min). When sirocco fan surface roughness( Ra) was in the range of 0.27~7.65 μm, the spray particle size was 0.117~1.360 μm. And then more than 90% of spray particles were found to be less than 0.50 μm. When sirocco fan surface roughness(Ra) was in the range of 12.70~22.84 μm, the spray particle size was 2.51~184.79 μm and more than 98% of spray particles were found to be less than 13.59 μm. To analyze the effect of fan rotation speed on the size and distribution of spray particles, when surface roughness Ra was fixed 0.27 μm and fan rotation speed and salt water flow rate was respectively changed at 3,800~5,600 rpm and 2.77~8.28 mL/min, spray particle size was 0.341~0.541 μm. And when salt water flow rate was 9.74 mL/min and fan rotation speed was 3,800~5,200 rpm, spray particle size was in the range of 29.29~341.46 μm and in case of 5,600 rpm more than 98.23% of spray particles were in the range of 2.51~13.59 μm.

본 논문은 Ni - Al2O3로 구성된 금속-세라믹 이종 입자복합재의 2차원 미세구조(microstructure) 생성과 미세구조 스케일 (scale)에 따라 정의되는 계층적 모델들의 역학적 특성 분석에 관한 내용이다. 이종 입자복합재의 미세구조는 수학적인 RMDF(random morphology description functions) 모델링기법을 복합재의 2차원 RVE(representative volume element) 영 역에 적용하여 생성하였다. 그리고 미세구조 생성에 필요한 가우스 함수들의 개수에 따라 미세구조의 계층적 모델을 정의하였다. 한편 임의 미세구조 내 금속과 세라믹 입자가 차지하는 체적분율(volume fraction)은 RMDF 함수의 레벨을 조정함으로서 설정하였다. RMDF기법에 의한 미세구조들은 가우스 함수들의 개수가 일정할지라도 랜덤하게 생성된다. 이렇게 랜덤 하게 생성되는 미세구조들을 2차원 보(beam) 모델에 적용하여 미세구조의 스케일에 따른 수직응력과 전단응력의 계층적 변 동을 수치 해석적으로 고찰하였다. 또한, 균열해석을 통해 RMDF의 랜덤성과 가우스 함수들의 개수가 균열선단에서의 응력 값에 미치는 영향을 고찰하였다.

입자 복합재료는 입자의 형상, 크기 그리고 분포의 산포특성으로 인해 물성치의 편차가 존재하고, 입자 복합재료를 사용 한 시스템의 거동 또한 산포가 존재한다. 하지만 입자의 산포특성을 고려하기 어려우므로 균질화법을 사용하여 시스템의 거 동을 해석하거나 국부영역에서 미세구조를 적용하여 해석한다. 본 연구에서는 입자의 랜덤적 산포특성을 고려하기 위해 RMDFs(random morphology description functions)를 사용하여 랜덤 미세구조를 생성하였고, 단면 1차 모멘트를 사용하여 가우시안 함수의 수(N)와 입자의 산포특성의 관계를 분석하였다. 그리고 랜덤 미세구조 구조물의 거동을 분석하기 위하여 랜덤 미세구조를 전체에 반영한 외팔보에 multi-scale 해석을 수행하였다. 그 결과 입자의 산포특성과 외팔보의 처짐의 편차 는 N의 증가에 따라 감소하고 N=200에서 수렴하는 것을 확인하였다.

Noncontact direct-printed conductive silver patterns with an enhanced electrical resistivity are fabricated using a silver ink with a mixture of silver nanoparticles and nanoplates. The microstructure and electrical resistivity of the silver pattern are systematically investigated as a function of the mixing ratio of the nanoparticles and nanoplates. The pattern, which is fabricated using a mixture with a mixing ratio of 3(nanoparticles):7(nanoplates) and sintered at 200oC shows a highly dense and well-sintered microstructure and has a resistivity of 7.60 μΩ·cm. This originates a mutual synergistic effect through a combination of the sinterability of the nanoparticles and the packing ability of the nanoplates. This is a conductive material that can be used to fabricate noncontact direct-printed conductive patterns with excellent electrical conductivity for various flexible electronics applications, including solar cells, displays, RFIDs, and sensors.

This study investigates ventilation and processing methods for industrial facilities by analyzing the distribution chart and concentration of fine particles generated. Analyses of fine particles were conducted in crushing rooms, sorting room, Primary manufacturing room and packing room, where the concentration of 0.3~10.0 μm particles were measured for 10 minutes during processing of herb medicines. The result revealed that particles with sizes greater than or equal to PM2.685 took up most of the space, and the concentration of fine particles (PM10) was 1,672.24 μg/m3 in 1th crushing room, 3,144.7 μg/m3 in crushing 2nd room, 262.45 μg/m3 in sorting room, prior to processing (2,302.3 μg/m3 for Aralia continentails and 5,564.9 μg/m3 for Poria cocos), 4,656.5 μg/m3 in Primary manufacturing room, and 20,213.3 μg/m3 in packing room. The concentration of fine particles generated during manufacture of herb medicines was, in comparison to the standard of 150 μg/m3 established in the Indoor Air Quality Control Act, 1.7 to 135 times higher. High concentrations of fine particles were found in all rooms, except for packing, even with partial ventilation. Also, it can be inferred that adequate deodorization system may be required in order to control unpleasant herb odors. Through this study, it is recommended that the use of scrubbers and concealment may facilitate control of fine particles containment.

This study investigates the microstructure and thermal shock properties of polycrystalline diamond compact (PDC) produced by the high-temperature, high-pressure (HPHT) process. The diamond used for the investigation features a 12~22 μm- and 8~16 μm-sized main particles, and 1~2 μm-sized filler particles. The filler particle ratio is adjusted up to 5~31% to produce a mixed particle, and then the tap density is measured. The measurement finds that as the filler particle ratio increases, the tap density value continuously increases, but at 23% or greater, it reduces by a small margin. The mixed particle described above undergoes an HPHT sintering process. Observation of PDC microstructures reveals that the filler particle ratio with high tap density value increases direct bonding among diamond particles, Co distribution becomes even, and the Co and W fraction also decreases. The produced PDC undergoes thermal shock tests with two temperature conditions of 820 and 830, and the results reveals that PDC with smaller filler particle ratio and low tap density value easily produces cracks, while PDC with high tap density value that contributes in increased direct bonding along with the higher diamond content results in improved thermal shock properties.

W-10vol.%ZrC composites reinforced by micrometric and nanosized ZrC particles were prepared by hot-pressing of 25 MPa for 2 h at 1900˚C. The effect of ZrC particle size on microstructure and mechanical properties at room temperature and elevated temperatures was investigated by X-ray diffraction analysis, scanning electron microscope and transmission electron microscope observations and the flexural strength test of the W-ZrC composite. Microstructural analysis of the W-ZrC composite revealed that nanosized ZrC particles were homogeneously dispersed in the W matrix inhibiting W grain growth compared to W specimen with micrometric ZrC particle. As a result, its flexural strength was significantly improved. The flexural strength at room temperature for W-ZrC composite using nanosized ZrC particle being 740 MPa increased by around 2 times than that of specimen using micrometric ZrC particle which was 377 MPa. The maximum strength of 935 MPa was tested at 1200˚C on the W composite specimen containing nanosized ZrC particle.

The mechanical properties and microstructures of aluminum-matrix composites fabricated by the dispersion of fine alumina particles less than 20μm in size into 6061 aluminum alloys are investigated in this study. In the as-quenched state, the yield stress of the composite is 40~85 MPa higher than that of the 6061 alloy. This difference is attributed to the high density of dislocations within the matrix introduced due to the difference in the thermal expansion coefficients between the matrix and the reinforcement. The difference in the yield stress between the composite and the 6061 alloy decreases with the aging time and the age-hardening curves of both materials show a similar trend. At room temperature, the strain-hardening rate of the composite is higher than that of the 6061 alloy, most likely because the distribution of reinforcements enhances the dislocation density during deformation. Both the yield stress and the strain-hardening rate of the T6-treated composite decrease as the testing temperature increases, and the rate of decrease is faster in the composite than in the 6061 alloy. Under creep conditions, the stress exponents of the T6-treated composite vary from 8.3 at 473 K to 4.8 at 623 K. These exponents are larger than those of the 6061 matrix alloy.

Porous Al2O3 dispersed with nano-sized Cu was fabricated by freeze-drying process and solution chemistry method using Cu-nitrate. To prepare porous Al2O3, camphene was used as the sublimable vehicle. Camphene slurries with Al2O3 content of 10 vol% were prepared by milling at 50˚C with a small amount of oligomeric polyester dispersant. Freezing of the slurry was done in a Teflon cylinder attached to a copper bottom plate cooled to -25˚C while unidirectionally controlling the growth direction of the camphene. Pores were subsequently generated by sublimation of the camphene during drying in air for 48 h. The green body was sintered in a furnace at 1400˚C for 1 h. Cu particles were dispersed in porous Al2O3 by calcination and hydrogen reduction of Cu-nitrate. The sintered samples showed large pores with sizes of about 150μm; these pores were aligned parallel to the camphene growth direction. Also, the internal walls of the large pores had relatively small pores due to the traces of camphene left between the concentrated Al2O3 particles on the internal wall. EDS analysis revealed that the Cu particles were mainly dispersed on the surfaces of the large pores. These results strongly suggest that porous Al2O3 with Cu dispersion can be successfully fabricated by freeze-drying and solution chemistry routes.

This paper describes the fabrication of AlN thin films containing iron and iron nitride particles, and the magnetic and electrical properties of such films. Fe-N-Al alloy films were deposited in Ar and N2 mixtures at ambient temperature using Fe/Al composite targets in a two-facing-target DC sputtering system. X-ray diffraction results showed that the Fe-N-Al films were amorphous, and after annealing for 5 h both AlN and bcc-Fe/bct-FeNx phases appeared. Structure changes in the FeNx phases were explained in terms of occupied nitrogen atoms. Electron diffraction and transmission electron microscopy observations revealed that iron and iron nitride particles were randomly dispersed in annealed AlN films. The grain size of magnetic particles ranged from 5 to 20 nm in diameter depending on annealing conditions. The saturation magnetization as a function of the annealing time for the Fe55N20Al25 films when annealed at 573, 773 and 873 K. At these temperatures, the amount of iron/iron nitride particles increased with increasing annealing time. An increase in the saturation magnetization is explained qualitatively in terms of the amount of such magnetic particles in the film. The resistivity increased monotonously with decreasing Fe content, being consistent with randomly dispersed iron/iron nitride particles in the AlN film. The coercive force was evaluated to be larger than 6.4×103Am-1 (80 Oe). This large value is ascribed to a residual stress restrained in the ferromagnetic particles, which is considered to be related to the present preparation process.

In this research, the optimal manufacturing conditions of fine Si powders from Si scrap were investigated as a function of different initial powder size using the high-energy ball milling equipment, which produces the fine powder by means of an ultra high-energy within a short duration. The morphological change of the powders according to the milling time was observed by Scanning electron microscopy (SEM). With the increasing milling time, the size of Si powder was decreased. In addition, more energy and stress for milling were required with the decreasing initial powder size. The refinement of Si scrap was rapidly carried out at 10min ball milling time. However, the refined powder started to agglomerate at 30 min milling time, while the powder size became uniform at 60 min milling time.

습식분쇄의 최적조건을 얻고자 부형제 종류, 농도, 습식분쇄시 rotor speed, bead size, feed rate, 분쇄회수를 달리하여 분쇄한 결과, 부형제로는 gum arabic을 사용하여 5%의 농도로 첨가할 때 가장 좋은 분쇄효과를 나타내었다. Rotor speed를 달리하여 습식분쇄한 결과, rotor speed가 증가할수록 좋은 결과를 나타내어 4,000 rpm에서 가장 우수한 것으로 나타났다. 원료와 직접 부딪혀 분쇄를 하게 되는 bead는 0.4 mm의 크기를 사용하였을 때 가장 작은 사이즈로 분쇄되는 것으로 나타났다. 분쇄가 이루어지는 chamber내에 원료를 공급하는 비율에 따른 결과에서는 40 L/h 의 속도로 공급했을 때 가장 작은 입도분포를 보였다. 분쇄회수에 따른 영향은 횟수에 따라서는 8회 이상으로 분쇄하였을때 0.6 μm이하의 입도분포가 90% 이상으로 나타나 10회를 분쇄하였을때 가장 좋은 효과를 나타내었다. 연속운전과 비연속운전의 비교에서는 연속운전이 더 효율적이었으며 부형제 종류에 따른 영향은 gum arabic을 첨가하여 분쇄하였을때 가장 우수한 결과를 나타내었다. 따라서 적절한 농도와 종류의 부형제 사용과 rotor speed, bead size, feed rate, 용매와의 혼합비의 최적공정을 수립한 습식분쇄기술로 초미세액상칼슘의 제조가 가능하였고, 습식분쇄기술을 식품가공기술로 활용할 수 있는 가능성을 확보하였다.

The pressureless sintering behavior of /Cu powder mixtures, prepared from /CuO and /Cu-nitrate, has been investigated. Microstructural observation revealed that powders with nano-sized Cu particles could be synthesized by hydrogen reduction method. The specimens, pressureless-sintered at for 4 min using infrared heating furnace with the heating rate of /min, showed the relative density of above 90%. Maximum hardness of 16.1 GPa was obtained in /MgO/Cu nanocomposites. The nanocomposites exhibited the enhanced fracture toughness of 4.3-5.7 , compared with monolithic . The mechanical properties were discussed in terms of microstructural characteristics

본 연구에서는 입자강화 복합재료(particle-reinforced composites)의 거동을 예측하기 위하여 Lee and Pyo(2007)에 의해 제안된 계면손상을 고려한 복합재료의 미세역학 탄성모델과 Karihaloo and Fu(1989)의 미세균열 생성모델을 결합하여, 보강입자의 계면손상(imperfect interface)과 기지 내 미세균열을 고려하여 탄성구성모델(constitutive model)의 거동해석을 수행하였다. 제안된 탄성구성모델의 적용성 검증과 주요손상변수가 거동예측에 미치는 영향을 알아보기 위해 일축 하중 하에서의 응력-변형률 관계를 수치적으로 나타내었다. 또한, 기존의 관련 실험결과와 본 해석결과와의 비교를 통하여 제안된 모델의 정확도를 검증하였다.

In T-mixer crystallization, supersaturation is generated by mixing of another solvent or non-solvent in order to reduce the solubility of the compound. Also, T-mixer is a type of continuous crystallization. In order to induce micro-mixing, two solutions were mixed rapidly by T-mixer, which formed high supersaturation. As the results, mean size of HMX crystals decreased with increasing de-supersaturation rate (Rs). Eventually, HMX particles ranging from 0.5 to 5μm can be obtained by T-mixer crystallization. Mixing efficiency in T-mixer increased with increasing Rs values. In T-mixer crystallization without surfactants, homogeneous nucleation was formed when S and Rs was over 54 and 1.6×103/sec. In T-mixer crystallization with surfactants, homogeneous nucleation was formed when S and Rs was over 26 and 7.4/sec.