• Korean Journal of Materials Research
• /
• v.25 no.5
• /
• pp.258-263
• /
• 2015

To fabricate porous SiC-Si composites for heating element applications, both SiC powders and Si powders were mixed and sintered together. The properties of the sintered SiC-Si body were investigated as a function of SiC particle size and/or Si particle contents from 10 wt% to 40 wt%, respectively. Porous SiC-Si composites were fabricated by Si bonded reaction at a sintering temperature of $1650^{\circ}C$ for 80 min. The microstructure and phase analysis of SiC-Si composites that depend on Si particle contents were characterized using scanning electron microscope and X-ray diffraction. The electrical resistivity of SiC-Si composites was also evaluated using a 4-point probe resistivity method. The electrical resistivity of the sintered SiC-Si body sharply decreased as the amount of Si addition increased. We found that the electrical resistivity of porous SiC-Si composites is closely related to the amount of Si added and at least 20 wt% Si are needed in order to apply the SiCSi composites to the heating element.

• Journal of the Korean Crystal Growth and Crystal Technology
• /
• v.16 no.1
• /
• pp.43-48
• /
• 2006

In this Part 2, the grain growth processes of $Al_2O_3$ ceramics is numerically simulated using Monte Carlo method (MCM) and finite element method (FEM) and the pore sizes are analyzed. As the green ceramics whose thermal conductivities in high temperatures are generally low are sintered by the plasma heat and are rapidly cooled, the grain growth of the sintered body in the center is different from that in the outer. Also, even in the same sintering temperature, the pore size differs according to the pressing pressure. In order to prove the difference, the temperature distribution of the sintered body was analyzed using the finite element method and then the grain growth process associated with pressing pressures and relative densities was simulated using Monte Carlo method.

• Journal of the Korean Ceramic Society
• /
• v.35 no.4
• /
• pp.406-412
• /
• 1998

The reaction bonded alumina ceramics was prepared through the addition of each SiC and ZrO2 powder to the mixture of Al metal powder and Al2O3 The mono sized (3mm) and biodal sized (3mm+5mm) balls were used in attrition milling of Al and starting powders. The milling efficiency of both cases was compared by the analysis of particle size and X-ray diffraction. After the forming and sintering of each powder batchs the weight gains dimensional changes and densities were determined. The specimens were investigated by X-ray diffraction analysis and scanning electron microscope. Bimodal sized balls had better milling effect than single ball size in the milling of Al powder. However in the milling which ceramic powders mono sized the green body during the reaction sintering at 1$600^{\circ}C$ for 5 hour was about 10% The densities attained the values of 92-98% theoretical. The SiC added specimen that was milled with 3mm ball media had 96% theoretical density and dense microstructure.

• Journal of the Korean Ceramic Society
• /
• v.32 no.5
• /
• pp.559-566
• /
• 1995

Alumina powder was coated with stearic acid and then mixed with isotactic polypropylene, atactic polypropylene as binders at 15$0^{\circ}C$ for 2 hours. The mixture was then injection molded at various mold temperatures using injection molding machine to investigate the effect of the molding temperature and debinding parameters on the formation of the defects. The molded specimens were debinded in both air and nitrogen atmospheres. Wicking and solvent methods were also used to enhance debinding efficiency. The specimens were prefired at 120$0^{\circ}C$ and then sintered at 150$0^{\circ}C$ for 3 hours. Various defects were formed at mold temoperature of 3$0^{\circ}C$, 6$0^{\circ}C$ and 10$0^{\circ}C$ and any noticeable defect was not formed at 85$^{\circ}C$. The density of green body increased with mold temperature. Debinding in air atmosphere was more effective than in nitrogen atmosphere. Results also proved that wicking and solvent treatments helped minimize the number of defects.

• Journal of Welding and Joining
• /
• v.27 no.6
• /
• pp.9-16
• /
• 2009

In this paper, the coaxial monitoring system to capture image of weld pool was developed in laser-arc hybrid welding. In order to obtain the reliable image, green laser was used as a illumination system and measuring components such as band pass filter, ND (Neutral Density) filter and shutter speed was designed and optimized. Using this monitoring system, weld pool images were captured according to laser power, welding speed, welding current and interspace between laser and arc through the experiment. ANOVA (Analysis of Variation) was carried out to identify the influence of process variables on bead widths extracted from captured images of monitoring system. Welding speed and current were major factor to affect weld pool.

• Proceedings of the Korean Society for Noise and Vibration Engineering Conference
• /
• 2013.04a
• /
• pp.842-848
• /
• 2013

Developments of Solid-State Gyroscopy during last decades are impressive and were based on thin-walled shell resonators like HRG or CRG made from fused quartz or leuko-sapphire. However, a number of design choices for inertial-grade gyroscopes, which can be used for high-g applications and for mass- or middle-scale production, is still very limited. So, considerations of fundamental physical effects in solids that can be used for development of a miniature, completely solid-state, and lower-cost sensor look urgent. There is a variety of different types of bulk acoustic (elastic) waves (BAW) in anisotropic solids. Shear waves with different variants of their polarization have to be studied especially carefully, because shear sounds in glasses and crystals are sensitive to a turn of the solid as a whole, and, so, they can be used for development of gyroscopic sensors. For an isotropic medium (for a glass or a fine polycrystalline body), classic Lame's theorem (so-called, a general solution of Elasticity Theory or Green-Lame's representation) has been modified for enough general case: an elastic medium rotated about an arbitrary set of axes. Travelling, standing, and mixed shear waves propagating in an infinite isotopic medium (or between a pair of parallel reflecting surfaces) have been considered too. An analogy with classic Foucault's pendulum has been underlined for the effect of a turn of a polarizational plane (i.e., an integration effect for an input angular rate) due to a medium's turn about the axis of the wave propagation. These cases demonstrate a whole-angle regime of gyroscopic operation. Single-crystals are anisotropic media, and, therefore, to reflect influence of the crystal's rotation, classic Christoffel-Green's tensors have been modified. Cases of acoustic axes corresponding to equal velocities for a pair of the pure-transverse (shear) waves have of an evident applied interest. For such a special direction in a crystal, different polarizations of waves are possible, and the gyroscopic effect of "polarizational precession" can be observed like for a glass. Naturally, formation of a wave pattern in a massive elastic body is much more complex due to reflections from its boundaries. Some of these complexities can be eliminated. However, a non-homogeneity has a fundamental nature for any amorphous medium due to its thermodynamically-unstable micro-structure, having fluctuations of the rapidly-frozen liquid. For single-crystalline structures, blockness (walls of dislocations) plays a similar role. Physical nature and kinematic particularities of several typical "drifts" in polarizational BAW gyros (P-BAW) have been considered briefly too. They include irregular precessions ("polarizational beats") due to: non-homogeneity of mass density and elastic moduli, dissymmetry of intrinsic losses, and an angular mismatch between propagation and acoustic axes.

• Korean Journal of Materials Research
• /
• v.28 no.3
• /
• pp.166-173
• /
• 2018

Molybdenum (Mo) is one of the representative refractory metals for its high melting point, superior thermal conductivity, low density and low thermal expansion coefficient. However, due to its high melting point, it is necessary for Mo products to be fabricated at a high sintering temperature of over $1800-2000^{\circ}C$. Because this process is expensive and inefficient, studies to improve sintering property of Mo have been researched actively. In this study, we fabricated Mo nanopowders to lower the sintering temperature of Mo and tried to consolidate the Mo nanopowders through ultra high pressure compaction. We first fabricated Mo nanopowders by a mechano-chemical process to increase the specific surface area of the Mo powders. This process includes a high-energy ball milling step and a reduction step in a hydrogen atmosphere. We compacted the Mo nanopowders with ultra high pressure by magnetic pulsed compaction (MPC) before pressureless sintering. Through this process, we were able to improve the green density of the Mo compacts by more than 20 % and fabricate a high density Mo sintered body with more than a 95 % sintered density at relatively low temperature.

• Proceedings of the Korean Powder Metallurgy Institute Conference
• /
• 1997.10a
• /
• pp.7-7
• /
• 1997

During sintering of very porous green bodies, as obtained by compaction of hard powders - such as tungsten carbide or ceramics - or by injection moulding, important shrinkage occurs. Due to heterogeneous green density field, gravity effects, friction on the support, thermal gradients, etc., this shrinkage is often non-uniform, which' may induce significant shape changes. As the ratio of compact dimension to powder size is very high, the mechanics of continuum is relevant to model such phenomena. Thus numerical techniques, such as the finite element method can be used to simulate the sintering process and predict the final shape of the sintered part. Such type of simulation has much been developed in the last decade firstly for hot isostatic pressing and next for die compaction. Finite element modelling has been recently applied to free sintering. The simulation of sintering should be based on constitutive equations describing the thermo-mechanical behaviour of the material under any state of stress and any temperature which may arise within the sintering body. These equations can be drawn either from experimental data or from micromechanical models. The experiments usually consist in free sintering and sinter-forging tests. Indeed applying more complex loading conditions at high temperature under controlled atmosphere is delicate. Micromechanical models describe the constitutive behaviour of aggregates of spheres from the deformation of two-sphere contact either by viscous flow or grain boundary diffusion. Such models are not able to describe complex microstructure and mechanisms as observed in real materials but they can give some basic information on the formulation of constitutive equations. Practically both experimental and theoretical approaches can be coupled to identify the constitutive equations. Such procedure has been performed for modelling the sintering of compacts obtained by die pressing of a mixture of tungsten carbide and cobalt powders. The constitutive behaviour of this material during sintering has been described by a linear viscous constitutive model, whose functions have been fitted from results of free sintering and sinter-forging experiments. This model has next been introduced in ABAQUS finite element code to simulate the sintering of heterogeneous green compacts of various geometries at constant temperature. Examples of simulations are shown and compared with experiments.

• Journal of Powder Materials
• /
• v.29 no.3
• /
• pp.247-251
• /
• 2022

This study demonstrates the effect of the compaction pressure on the microstructure and properties of pressureless-sintered W bodies. W powders are synthesized by ultrasonic spray pyrolysis and hydrogen reduction using ammonium metatungstate hydrate as a precursor. Microstructural investigation reveals that a spherical powder in the form of agglomerated nanosized W particles is successfully synthesized. The W powder synthesized by ultrasonic spray pyrolysis exhibits a relative density of approximately 94% regardless of the compaction pressure, whereas the commercial powder exhibits a relative density of 64% under the same sintering conditions. This change in the relative density of the sintered compact can be explained by the difference in the sizes of the raw powder and the densities of the compacted green body. The grain size increases as the compaction pressure increases, and the sintered compact uniaxially pressed to 50 MPa and then isostatically pressed to 300 MPa exhibits a size of 0.71 m. The Vickers hardness of the sintered W exhibits a high value of 4.7 GPa, mainly due to grain refinement.

• Journal of the Korean Ceramic Society
• /
• v.49 no.4
• /
• pp.363-368
• /
• 2012

Nitriding and post-sintering behavior of powder mixture compacts were investigated. As mixture compacts are different from simple Si compacts, the fabrication of a sintered body with a mixture composition has engineering implications. In this research, in specimens without a pore former, the extent of nitridation increased with $Si_3N_4$ content, while the highest extent of nitridation was measured in $Si_3N_4$-free composition when a pore former was added. Large pores made from the thermal decomposition of the pore former collapsed, and they were filled with a reaction product, reaction-bonded silicon nitride (RBSN) in the $Si_3N_4$-free specimen. On the other hand, pores from the decomposed pore former were retained in the $Si_3N_4$-added specimen. Introduction of small $Si_3N_4$ particles ($d_{50}=0.3{\mu}m$) into a powder compact consisting of large silicon particles ($d_{50}=7{\mu}m$) promoted close packing in the green body compact, and resulted in a stable strut structure after decomposition of the pore former. The local packing density of the strut structure depends on silicon to $Si_3N_4$ size ratio and affected both nitriding reaction kinetics and microstructure in the post-sintered body.