A Study on the Percolation Threshold of Polyethylene Matrix Composites Filled Carbon Powder

신순기 대한금속·재료학회 2010 ELECTRONIC MATERIALS LETTERS Vol.6 No.2

This paper investigates the percolation threshold of carbon powder-filled polyethylene matrix composites using the experimental results of the changes in resistivity and relative permittivity for filled carbon powder, the electric field dependence of current, and the critical exponent of conductivity. We found that a formation of infinite clusters is interrupted by a tunneling gap in the volume fraction section of filled carbon powder where the change in resistivity is extremely large. We also found that the critical exponent of conductivity for the universal law of conductivity is satisfied if the percolation threshold is estimated at the volume fraction of carbon powder where a nonohmic current behavior changes into an ohmic one. It is concluded that the percolation threshold should be defined at this volume fraction of carbon powder.

Temperature Dependence of Resistivity of Carbon Black Filled Polymer Matrix Composites Below and Above Percolation Threshold

신순기,권인규 대한금속·재료학회 2011 대한금속·재료학회지 Vol.49 No.10

The effect of the temperature on the resistivity of carbon black filled polymer matrix composites below and above percolation threshold (P_t) was studied based on the electrical conduction mechanism. Temperature coefficient of resistance of the composites below percolation threshold changed from negative to positive at 0.195<P_t<0.210, and the trend decreased with increasing of P_t. The temperature dependence of the resistivity of the composites below P_t can be explained with a tunneling conduction model by incorporating the effect of the thermal expansion of the composites into a tunneling gap. The temperature coefficient of resistance of the composites above P_t has a positive value and its absolute value increased with increasing volume fraction of carbon black. By assuming that the electrical conduction through percolating paths is a thermally activated process and by incorporating the effect of thermal expansion into the volume fraction of the carbon black, the temperature dependence of the resistivity above P_t has been well explained without violating the universal law of conductivity; the apparent activation energy is estimated to be 0.14 eV.

The High Temperature Deformation Behavior of TiC0.98 Grown by the Floating Zone Technique

신순기 대한금속·재료학회 2013 대한금속·재료학회지 Vol.51 No.7

In order to investigate the high temperature deformation behaviors of TiC0.98, crystals were grown by the radio frequency (r.f.) floating zone technique and were deformed by a compression test at temperatures from 1283 to 2133 K and a strain rate from 5.87 × 10‒4 to 6.01 × 10‒3s‒1. TiC0.98 exhibited a marked work-softening phenomena which is characteristic of covalently bonded materials such as Ge and Si, and the phenomena became less clear with an increasing temperature and with a decreasing strain rate. After the softening, work hardening was observed. The rate hardening decreased as the temperature increased. The stress-strain curve can be explained in terms of the increase in the athermal internal stress and the decrease in the thermal effective stress with the increase in dislocation density by the deformation. The upper yield stress was mainly determined by the frictional resistance to dislocation motion, and the athermal internal stress contribution was very small.

탄화물기 소결재료의 액상소결 때의 입성장

신순기,송원수창 (松原秀彰),좌구간건인 (佐久間健人) 대한금속재료학회 1993 대한금속재료학회 학술대회 개요집 Vol.1993 No.1

Effect of Carbon Content on the Electrical Conductivity of Carbon Black-Filled PMCs with Various Matrices

신순기 대한금속·재료학회 2011 ELECTRONIC MATERIALS LETTERS Vol.7 No.3

The electrical conductivity of carbon black-filled polymer matrix composites (PMCs) with various matrices was studied as a function of carbon content to find the break point of the relationship between carbon content and conductivity. Conductivity jumps by as much as ten orders of magnitude at the break point. The critical carbon content corresponding to the break point varies depending on the matrix species and tends to increase with increase in the surface tension of the matrix. To explain the dependency of the critical carbon content on the matrix species, a simple equation was derived under some assumptions, the most important of which is that when the interfacial excess energy introduced by particles of carbon black into the matrix reaches a universal value ( ), the particles of carbon black begin to coagulate so as to avoid any further increase of the energy and to form networks which facilitate electrical conduction. The equation well explains the dependency through surface tension, as long as the difference of the surface tensions between the particles of carbon black and the matrix is not very small.

카본블랙 첨가 PMC(Polyethylene Matrix Composites)의 문턱스며들기(Percolation Threshold)와 절연파괴 강도 임계지수

신순기,Shin, Soon-Gi 한국재료학회 2011 한국재료학회지 Vol.21 No.9

Composites of insulating polyethylene and carbon black are widely used in switching elements, conductive paint, and other applications due to the large gap of resistance value. This research addresses the critical exponent of dielectric breakdown strength of polymer matrix composites (PMC) made with carbon black and polyethylene below the percolation threshold (Pt) for the first time. Here, Pt means the volume fraction of carbon black of which the resistance of the PMC is transferred from its sharp decrease to gradual decrease in accordance with the increase of carbon-black-filled content. First, the Pt is determined based on the critical exponents of resistivity and relative permittivity. Although huge cohesive bodies of carbon black are formed in case of being less than the Pt, a percolation path connecting the conducting phases is not formed. The dielectric breakdown strength (Dbs) of the PMC below Pt is measured by using an impulse voltage in the range from 10 kV to 40 kV to avoid the effect of joule heating. Although the observed Dbs data seems to be well fitted to a straight line with a slope of 0.9 on a double logarithm of (Pt-$V_{CB}$) and Dbs, the least squares method gives a slope of 0.97 for the PMC. It has been found that finite carbon-black clusters play an important role in dielectric breakdown.

WC기 초경합금중 WC/WC界面의 구조와 입계편석

신순기,Sin, Sun-Gi 한국재료학회 2000 한국재료학회지 Vol.10 No.9

WC-Co와 WC-Co 초경합금중 WC/WC 입계의 구조와 입계 편석상태를 알아볼 목적으로 HRTEM과 EDS를 이용하여 연구하였다. 일부의 입계들은 액상에 의하여 분리된 상태로 관찰되었으나, 상당수는 원자적 상태의 연소계면이었다. 또 연속계면 중 WC-Co 합금에서는 Co 상이 편석되어 있었으며, WC-VC-Co 합금에서는 Co와 V이 동시에 편석되어 있음을 알 수 있었다. 그 편석의 폭은 약6nm이었다. 연속 계면 중 V의 편석은 소결 또는 열처리 시에 일어나는 입계 이동을 억제하는 데 효과적인 역할을 할 것으로 여겨졌다. 동시에 이것은 WC-Co 초경합금에서 VC 첨가에 의한 입성상 억제기구를 설명할 수 있는 것으로 사료되었다. The WC/WC grain boundary structure and intergranular segregation in WC-Co and WC-VC-Co cemented carbides were investigated by high-resolution transmission electron microscopy and energy dispersive X-ray spectroscopy in order to elucidate whether contiguous boundaries were present or not at the atomic level. Some grain boundaries were separated by liquid phase, while others were contiguous at the atomic level. Cobalt was found to be segregated to WC/WC grain boundaries in WC-Co. Cobalt and vanadium were co-segregated to grain boundaries in WC-VC-Co. The segregation width in both materials was about 6 nm. These results suggest that the vanadium present in contiguous boundaries acts as an effective barrier to the migration of boundaries during sintering and annealing. This could explain the grain growth inhibiting mechanism of VC added to WC-Co.

전기저항측정에 의한 SiC섬유강화 $Si_3N_4$기 복합재료의 파괴예측

신순기,Sin, Sun-Gi 한국재료학회 2000 한국재료학회지 Vol.10 No.5

섬유강화 세라믹스 복합재료의 파괴예측 가능성을 알아보기 위해서 탄소섬유와 WC분말입자를 전기 전도상으로 이용하여 재료 스스로가 파괴예측 기능을 가지도록 한 SiC섬유강화 $Si_3N_4$세라믹스기 복합재료를 1773K에서 1시간 동안 hot-press하여 제작하였다. 4점 굽힘 시험하는 동안 전기저항 변화를 측정하여 파괴예측 기능을 평가하였다. 그 결과 전기정항은 재료의 파괴거동과 밀접한 관계를 가지면서 변화함을 알았다. 특히 분말형태의 전기전도상의 첨가는 본 복합재료의 파괴과정을 낮은 응력단계로부터 예측하는데 유용하였다. 결과적으로 이러한 재료설RP의 신개 (파괴예측기능)의도입은 $Si_3N_4$기 세라믹스를 구조재료로 이용함에 있어서 큰 문제가 되고 있는 신뢰성 확보에 새로운 기능을 준다고 생각되었다. 생각되었다. SiC fiber reinforced $Si_3N_4$ matrix composites combined with electrical conductive phases of carbon fiber and WC powder fabricated by hot pressing at 1773K. The ability to predict fracture in the ceramic matrix composites was evaluated by measuring simultaneous load-deflection and electrical resistanc difference-deflection curves in four point bending tests. The changes in electrical resistance differences closely corresponded to the fracture behavior of the composites. Different electrical conductive phases are suited to predicting different stages and rates of fracture. These obsevations how that it is possible to perform "in situ" fracture detection in ceramic composites.

TiC-Nb 소결 복합재료의 연성-취성 천이 특성

신순기,Shin, Soon-Gi 한국재료학회 2014 한국재료학회지 Vol.24 No.1

In order to clarify the effect of Nb addition on the ductile-brittle transition property of sintered TiC, TiC-10 mol% Nb composites were researched using a three-point bending test at temperatures from room temperature to 2020 K, and the fracture surface was observed by scanning electron microscopy. It was found that the Nb addition decreases the ductile-brittle transition temperature of sintered TiC by 300 K and increases the ductility. The room temperature bending strength was maintained at up to 1800 K, but drastically dropped at higher temperatures in pure TiC. The strength increased moderately to a value of 320MPa at 1600 K in TiC-10 mol% Nb composites, which is 40% of the room temperature strength. Pores were observed in both the grains and the grain boundaries. It can be seen that, as Nb was added, the size of the grain decreased. The ductile-brittle transition temperature in TiC-10 mol% Nb composites was determined to be 1550 K. Above 1970 K, yieldpoint behavior was observed. When the grain boundary and cleavage strengths exceed the yield strength, plastic deformation is observed at about the same stress level in bending as in compression. The effect of Nb addition is discussed from the viewpoint of ability for plastic deformation.

Computer Simulation for Microstructure Development in Porous Sintered Compacts

신순기,Shin, Soon-Ki,Matsubara, Hideaki The Korean Ceramic Society 2006 한국세라믹학회지 Vol.43 No.4

A Monte Carlo simulation based on Potts model in a three dimensional lattice was studied to analyze and design microstructures in porous sintered compacts such as porosity, pore size, grain (particle) size and contiguity of grains. The effect of surface energy of particles and the content of additional fine particles to coarse particles on microstructure development were examined to obtain fundamentals for material design in porous materials. It has been found that the larger surface energy enhances sintering (necking) of particles and increases contiguity and surface energy does not change pore size and grain size. The addition of fine particles also enhances sintering of particles and increases contiguity, but it has an effect on increment of pore size and grain size. Such a simulation technique can give us important information or wisdom for design of porous materials, e.g., material system with high surface energy and fine particle audition are available for higher strength and larger porosity in porous sintered compacts with applications in an automobile.