• Korean Journal of Materials Research
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• v.2 no.3
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• pp.172-179
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• 1992

$Al_2O_3/ZrO_2$ composites were fabricated by pressureless sintering from commercial powders and/or nano composite powder of $Al_2O_3/ZrO_2$. The Properties of the composites such as density, strength, hardness and fracture toughness were evaluated. Microstructures and fracture surfaces ware also examined. The flexural strength remains unchanged(~640 MPa) as long as the content of commercial powders is not extreamly high, and depends on microstructures of the composites. Fracture toughness(4.3-5.3 $Mpa{\cdot}m^{1/2}$) increases with increasing content of commercial powders. Fractography shows that failure-initiating sources are 1)surface flaws resulting from machining damage, 2)crack-shaped voids formed due to $ZrO_2$ agglomeration, and 3)surface separation caused by inhomogeneous blending and by sinterability difference between nato composite powder and commercial powders of $Al_2O_3/ZrO_2$. Failure mode of the composites was mainly transgranular.

• Journal of the Korean Ceramic Society
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• v.54 no.4
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• pp.292-297
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• 2017

${\beta}-SiAlON$, based on its high fracture toughness, good strength and low abrasion resistance, has been adopted in several industrial fields such as bearings, turbine blades and non-ferrous metal refractories. In general, ${\beta}-SiAlON$ is fabricated by reactive sintering using expensive $Si_3N_4$ and AlN as starting materials. On the other hand, in this study, a cheaper ${\beta}-SiAlON$ starting powder synthesized by SHS was employed to improve price competitiveness compared to that of the reactive sintering process. ${\beta}-SiAlON$ ceramics with various content of the sintering additive $Y_2O_3$ up to 7 wt% were fabricated by conventional pressureless sintering at $1800^{\circ}C$ for 2 to 8 h under $N_2$ pressure of 0.1 MPa. The specimen with 3 wt% $Y_2O_3$ exhibited the best mechanical properties: hardness of 14 GPa, biaxial strength of 830 MPa, fracture toughness of $5MPa{\cdot}m^{1/2}$ and wear rate of about $3{\times}10^{-6}mm^3/N{\cdot}m$.

• Journal of Korean Society of Occupational and Environmental Hygiene
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• v.7 no.2
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• pp.289-297
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• 1997

Work-related low back pain(below LBP) is one of the major cause of morbidity, disability, limitation of activity and economic loss. Therefore the work-related LBP is one of the major issue in the field of industrial safety and health. This study was performed for detecting the risk factors and proposing the effective control programs of work-related LBP. The subjects were male workers employed at the welding and metal factory. The data was collected by self-reported questionnaire, interview and checking abdomen muscular and grasping power for two days on October, 1993. The contents of questionnaire were as follow: the experience of LBP, general characteristics, physical characteristics, employment status, type of work and working environment. The number of cases was 104 with a history of work-related LBP, so the prevalence of work-related LBP was 35.0%, and the number of controls was 140 without any history of LBP. As a result, marital status, educational level, abdomen muscular power, tenure, category of job, satisfaction of job, working posture, satisfaction for table and chair and lifting materials showed a statistical significance between the case and control groups. 284 Lifting jobs were quantified by NIOSH lifting equation method and ergonomic computer modelling methods. There were no significant differences in the action limit and disc compression force between group with LBP and without LBP. But in the lifting frequency and cumulative disc compression force there were significant differences. Therefore work-related LBP should be prevented by the ergonomic and environmental control.

• Journal of the Korean Crystal Growth and Crystal Technology
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• v.10 no.3
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• pp.226-232
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• 2000

Corrosion mechanisms by molten steel and slag were investigated in the zirconia/graphite composite as a material of submerged entry nozzle (SEN) using for producing high quality steel. Most of corrosions were started by the dissolution of zirconia particles into molten steel and oxidation of graphite, but subsequently three modes of corrosion were observed. Firstly, the penetration of slag into zirconia matrix was induced to the diffusion of stabilizing agent outward cubic zirconia grains, and the destabilization of cubic to fine monoclinic zirconia particles, which is enhanced to the decomposition and dissolution of them into slag. Secondly, molten slag penetrates into large cubic zirconia particles along grain boundary and decomposed them to fine cubic grains, which is also enhanced to the dissolution of zirconia grains into slag. Lastly, reaction between carbon and cubic zirconia was formed porous ZrC and enhanced the dissolution of it into slag.

• Journal of the Korean Geotechnical Society
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• v.39 no.11
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• pp.33-40
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• 2023

This study explores the performance of pile foundations in highly expansive soil, incorporating magnesium oxide-based refractory materials. A controlled model chamber, housing a fixed pile, was utilized to induce ground expansion through fused magnesia (FM). The investigation focused on measuring the vertical displacement of FM-sand mixtures and the axial load on the pile in relation to depth and time. The study varied the amount of FM content (FM_c) at 30%, 50%, and 70%. The upward movement exhibited an augmentation with increasing FM_c, tapering off with depth as accumulation progressed toward the mixture surface. Compression and tensile forces were both evident along the pile for FM_c at 30% and 50%, while only a tensile force was observed at an FM_c of 70%. These results offer valuable insights for the analysis of pile behavior within FM-sand mixtures.

• Journal of the Korean Applied Science and Technology
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• v.34 no.2
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• pp.260-270
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• 2017

Zirconia has white color and physical, chemical stability, also using in high temperature materials and various industrial structural ceramics such as heat insulating materials and refractories due to their low thermal conductivity, excellent strength, toughness, and corrosion resistance. If hydrophobically modified zirconia is introduced into a hydrophobic acrylate coating solution, the hardness, chemical, electrical, and optical properties will be improved due to the better dispersibility of inorganic particle in organic coating media. Thus, we introduced $-CH_3$ group through silylation reaction using either trimethylchlorosilane(TMCS) or hexamethyldisilazane(HMDZ) on zirconia surface. The $Si-CH_3$ peaks derived from TMCS and HMDZ on hydrophobically modified zirconia surface was confirmed by FT-IR ATR spectroscopy, and introduction of silicon was confirmed by FE-SEM/EDS and ICP-AES. In addition, the sedimentation rate result in acrylate monomer of the modified zirconia showed the improved dispersibility. Comparison of the sizes of a pristine and the modified zirconia particles, which were clearly measured not by the normal microscope but by particle size analysis, provided a pulverizing was occurred by physical force during the silylation process. From the BET analysis data, the specific surface area of zirconia was approximately $18m^2/g$ and did not significantly change during modification process.

• Applied Chemistry for Engineering
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• v.20 no.6
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• pp.638-645
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• 2009

High alumina refractory used in a coal gasifier was analyzed and the degradation mechanism by molten slag was investigated. The depth of refractory severely damaged by slag varied between 12~40 mm, including the adhered slag layer. The sample also showed the cracks formed in parallel to the slag/refractory interface. The degree of degradation varied with the micro-structures in the refractory. Fused alumina grains showed the uneven boundary and pore formation just along the edges, while the tablet alumina showed the slag penetrated between sintered alumina around which the formation of Al-Fe phase was observed. Calcium aluminate cements were not observed at the high temperature zone near the slag/refractory interface, probably due to dissolution into molten slag. Around large grains of alumina, rod shape alumina, which appeared to be recrystallized during cooling, were observed, and large pores were also formed around those grains. Therefore, in high alumina refractories, hot molten slag dissolves the bonding phase and rod-shape alumina phase is recrystallized upon cooling. During this process, cracks are developed due to structural change, and the degradation occurs by physical causes such as structural spalling.

• Journal of Energy Engineering
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• v.13 no.4
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• pp.281-290
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• 2004

Spent refractories from a coal gasifier after 1000 hours of operation were analyzed for crystalline phases, chemical composition and microstructures as a function of slag penetration depth, and the slag corrosion mechanism was determined. The chemical corrosion of chromia refractory occurred via reaction between Cr$_2$O$_3$ of the refractory and FeO and A1$_2$O$_3$ in the slag. The FeO reacted with Cr$_2$O$_3$ at the slare/refractory interface and formed FeCr$_2$O$_4$. After all FeO were consumed, Al in the penetrating slag substituted Cr in Cr$_2$O$_3$, forming (Al, Cr)$_2$O$_3$, at the edges of the particle, which were broken to form fragments rich in Al. The corrosion resistance of Cr$_2$O$_3$ varied with the particle size and the extent of sintering, and the higher resistance was observed in the larger and more sintered particles. There was no chemical change in ZrO$_2$, but showed the effects of physical corrosion: the grain boundaries became more wavy, and ZrO$_2$ grains were split in the corroded area. The slag penetration depth increased in the refractory samples farther down from the feed nozzles.

• Resources Recycling
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• v.29 no.6
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• pp.65-72
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• 2020

Black dross is a dark gray dross generated during the aluminum recycling process that uses flux, and contains NaCl, KCl, Al2O3, MgO, etc. Black dross is separated into soluble substances (NaCl, KCl) and insoluble substances (Al2O4, MgO) through the dissolution process. Soluble materials can be reused as salt flux, and Al2O3 and MgO can be upcycled to various ceramic materials through the synthesis process. In this study, Mayenite was synthesized using Al2O3 and MgO recovered from black dross, and the synthesis was performed according to the mixing ratio and reaction temperature. It was confirmed that when Mayenite was synthesized using black dross (spinel) and CaCO3, precursors were changed to Mg0.4Al2.4O4 and CaO at 700 ℃, and to Ca12Al14O33 (Mayenite) after 800 ℃. In the mixing conditions experiment, it was confirmed that the Mayenite XRD peak increased with increase of the CaCO3 content, and the Mg0.4Al2.4O4 XRD peak decreased. As a result of the BET analysis of the synthesized powder, the surface area decreased as the fine particles were grown and agglomerated in the process of generating mayenite.