• Transactions on Electrical and Electronic Materials
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• v.15 no.5
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• pp.284-289
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• 2014

The effects of additives (Nb, Bi and Cr) on the microstructure, varistor properties, and aging behavior of V/Mn/Co/ La/Dy co-doped zinc oxide ceramics were systematically investigated. An analysis of the microstructure showed that all of the ceramics that were modified with various additives were composed of zinc oxide grain as the main phase, and secondary phases such as $Zn_3(VO_4)_2$, $ZnV_2O_4$, and $DyVO_4$. The $Bi_2O_3$-modified samples exhibited the lowest density, the $Nb_2O_5$-modified sample exhibited the largest average grain size, and the $Cr_2O_3$-modified samples exhibited the highest breakdown field. All additives improved the non-ohmic coefficient (${\alpha}$) by either a small or a large margin, and in particular an $Nb_2O_5$ additive noticeably increased the non-ohmic coefficient to be as large as 36. The $Bi_2O_3$-modified samples exhibited the highest stability with variation rates for the breakdown field and for the non-ohmic coefficient (${\alpha}$) of -1.2% and -26.3%, respectively, after application of a DC accelerated aging stress of 0.85 EB/$85^{\circ}C$/24 h.

• The Korean Journal of Ceramics
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• v.6 no.1
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• pp.27-31
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• 2000

A series experiments were performed to evaluate mechanical behavior of non-oxide boride type ceramics formed on the AISI 1040 plain carbon steel. Boronizing was performed in a slurry salt bath consisting of borax, boric acid, and ferro-silicon at $950^{\circ}C$ for 2-6h. The AISI 1040 steel used as substrate material was containing 0.4%C, 0.13%Si, 0.65%Mn, 0.02%P, 0.014%S. The presence of non-oxide boride type ceramics $Fe_2B $ and FeB formed on the surface of steel was confirmed by metallographic technique and X-ray diffraction (XRD) analysis. The hardness of borides measured via Vickers indenter with a load of 2N reached a microhardness of up to 1800 DPN. The hardness of unborided steel was 185 DPN. The fracture toughness of borides measured by means of Vickers indenter with a load of 10N was about 2.30 MPa.$m^{1/2}$. The thickness of boride layers ranged from 72$\mu\textrm{m}$ to 145$\mu\textrm{m}$. Boride layers have a columnar morphology.

• Journal of Powder Materials
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• v.28 no.5
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• pp.381-388
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• 2021

Zirconia has excellent mechanical properties, such as high fracture toughness, wear resistance, and flexural strength, which make it a candidate for application in bead mills as milling media as well as a variety of components. In addition, enhanced mechanical properties can be attained by adding oxide or non-oxide dispersing particles to zirconia ceramics. In this study, the densification and mechanical properties of YSZ-TiC ceramic composites with different TiC contents and sintering temperatures are investigated. YSZ - x vol.% TiC (x=10, 20, 30) system is selected as compositions of interest. The mixed powders are sintered using hot pressing (HP) at different temperatures of 1300, 1400, and 1500℃. The densification behavior and mechanical properties of sintered ceramics, such as hardness and fracture toughness, are examined.

• Journal of the Korean Crystal Growth and Crystal Technology
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• v.8 no.2
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• pp.350-355
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• 1998

Dysprosium monotitanate $(Dy_2TiO_5)$ powder was synthesized by a simple mixed-oxide ceramics process using ethylene glycol (EG). Ethylene glycol, as the organic carrier for the metal cations, was used for polymerization mechanism. The successful used of a non-chelating polymer for the mixed-oxide ceramic process indicated that cation chelation is not the only route for creating stable ceramic precursors. Characterization of the powders by various thermal analysis, microscopy, and diffraction methods has been carried out.

• The Korean Journal of Ceramics
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• v.5 no.4
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• pp.336-340
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• 1999

Glasses in the system $CaO-TiO_2-SiO_2-Al_2O_3-ZrO_2-B_2O_3$ were investigated to find the glass seal compositions suitable for use in the planar solid oxide fuel cell (SOFC). Glass-ceramics prepared from the glasses by one-stage heat treatment at $1,000^{\circ}C$ showed various thermal expansion coefficients (i,e., $8.6\times10^{-6^{\circ}}C^{-1}$ to $42.7\times10^{-6^{\circ}}C^{-1}$ in the range 25-$1,000^{\circ}C$) due to the viscoelastic response of glass phase. The average values of contact angles between the zirconia substrate and the glass particles heated at 1,000-$1,200^{\circ}C$ were in the range of $131^{\circ}\pm4^{\circ}$~$137^{\circ}\pm9^{\circ}$, indicating that the glass-ceramic was in partial non-wetting condition with the zirconia substrate. With increasing heat treatment time of glass samples from 0.5 to 24 h at $1,100^{\circ}C$, the DC electrical conductivity of the resultant glass-ceramics decreased from at $800^{\circ}C$. Isothermal hold of the glass sample at $1100^{\circ}C$ for 48h resulted in diffusion of Ca, Si, and Al ions from glass phase into the zirconia substrate through the glass/zirconia bonding interface. Glass phase and diffusion of the moving ion such as $Ca^{2+}$ in glass phase is responsible for the electrical conduction in the glass-ceramics.

• Proceedings of the Korean Powder Metallurgy Institute Conference
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• 2006.09a
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• pp.321-322
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• 2006

Nano-structured ceramics, which consist of structural elements with nanometer-size crystallites, are expected to show various unusual properties. We developed the novel nano-structured ceramics which consists of $Si_3N_4$ and TiN and a self-lubricant material. The ceramics was fabricated by powder metallurgy process using mechano-chemical grinding process and short-time sintering process. Each grain size of matrix and the self-lubricant particle was under about 50 nm and a few namometer. It showed high wear resistance and low friction coefficient by controlling of microstructure.

• The Korean Journal of Ceramics
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• v.4 no.2
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• pp.156-161
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• 1998

We have measured changes of the non-stoichiometry, $\Delta\delta$, in Fe-doped nicked oxide , by thermogravimetry for four iron fractions, x=0.01, 0.031, 0.057 and 0.10, and three temperatures, T=1273, 1373 and 1473 K. The obtained data can be modelled by a defect structure in which substitutional trivalent iron ions, FeNi, are compensated by cation vacancies, $V_{Ni}$", and (4:1)-clusters. These clusters consist of tetravalent interstitial iron, $Fe_i\;^4$

• Journal of Technologic Dentistry
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• v.33 no.1
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• pp.37-45
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• 2011

Purpose: Heat pressed ceramics, used for all ceramic restorations, have the additional advantage of being technically less change through using of the lost-wax technique. Conceptually, combining the ceramic with the clinically proven reinforcing ability of a metal framework would be advantageous; however, cause of mismatching of fusion between ceramics and metal frameworks which from differences of casting temperature and coefficient of thermal expansion, pressed ceramics could not be used with a metal framework. The purpose of this study was to compare shear bond strength of press-to metal ceramic to porcelain fused non precious metal and feldspatic porcelain fused non precious metal. Methods: The 30 metal specimens were casted in a porcelain fused non precious metal nickel-chromium alloy. They were divided into 3 groups by surface treatment and applied ceramic: $125{\mu}m$ aluminium oxide sandblasting and veneered feldspatic porcelain (group FP), $125{\mu}m$ aluminium oxide sandblasting and had press-to-metal ceramic applied (group PC), porcelain bonder (gold bonder) fused on surface of metal specimens and had press-to-metal ceramic applied (group PCG). In each group 10 metal specimens were used. The press-to-metal ceramic applied 20 specimens had ash-free wax pattern applied, the metal-wax complexes invested, and were pressed with heat press ceramic. All specimens were subjected to shear bond strength test at a crosshead speed of 1.0 mm/min. Results: The results of measured in Mean SD and data were analyzed by one-way AVOVA (p= .05) and Tukey HSD test (p= .05).: group FP $16.090{\pm}1.841$ MPa, group PC $12.620{\pm}1.8256$ MPa, group PCG $10.920{\pm}0.9283$, significant differences between all groups (p < .05). Significant differences were found in each between group FP and group PC, group FP and group PCG (p < .05). Conclusion: The shear bond strength of press-to-metal ceramic to porcelain fused non precious metal was described higher in unused gold bonder group than used gold bonder groups.

• Journal of the Korean Society for Precision Engineering
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• v.29 no.3
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• pp.273-278
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• 2012

$ZrB_2$ has a melting temperature of $3245^{\circ}C$ and a low density of $6.1\;g/cm^3$, which makes this a candidate for application to ultra-high temperature over $2000^{\circ}C$. Beside these properties, $ZrB_2$ has excellent resistance to thermal shock and oxidation compared with other non-oxide engineering ceramics. This paper reviewed briefly 2 research examples, which are related to densification and properties of $ZrB_2$-based ceramics for ultra-high temperature applications. In the first section, the effect of $B_4C$ addition on the densification and properties of $ZrB_2$-based ceramics is shown. $ZrB_2$-20 vol.% SiC system was selected as a basic composition and $B_4C$ or C was added to this system in some extents. With sintered bodies, densification behavior and hightemperature (up to $1400^{\circ}C$) properties such as bending strength and hardness are examined. In the second section, the effect of the SiC size on the microstructures and physical properties is shown. $ZrB_2$-SiC ceramics are fabricated by using various SiC sources in order to investigate the grain-growth inhibition and the mechanical/thermal properties of $ZrB_2$-SiC.

• Journal of the Korea Institute of Military Science and Technology
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• v.25 no.6
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• pp.580-585
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• 2022

In this study, the high-temperature dielectric constant of Si₃N₄ ceramics, a representative non-oxide-based radome material, was evaluated and the cause of the dielectric constant change was analyzed in relation to the oxidation behavior. The dielectric constant of Si₃N₄ ceramics was 7.79 at room temperature, and it linearly increased as the temperature increased, showing 8.42 at 1,000 ℃. As results of analyzing the microstructure and phase for the Si₃N₄ ceramics before and after heat-treatment, it was confirmed that oxidation did not occur at all or occurred only on the surface at a very insignificant level below 1,000 ℃. Based on this, it is concluded that the increase in the dielectric constant according to the temperature increase of Si₃N₄ ceramics is irrelevant to the oxidation behavior and is only due to the activation of charge polarization.