• Journal of the Korean Ceramic Society
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• v.54 no.2
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• pp.121-127
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• 2017

Gadolinium tetraboride ($GdB_4$) was synthesized by reduction of $Gd_2O_3$ using boron carbide in presence of carbon. Effect of temperature on product quality was investigated. Pure $GdB_4$ powder was obtained in vacuum at $1500^{\circ}C$. Pressureless sintering experiments revealed that sintering takes place only above $1600^{\circ}C$. A maximum density of 77.1% of the theoretical value was obtained at $1800^{\circ}C$ by pressureless sintering. Hot pressing resulted in 95.5% of theoretical density at the lower temperature of $1700^{\circ}C$ under 35 MPa pressure. Hardness and fracture toughness of dense $GdB_4$ were measured and found to be 21.4 GPa and $2.3MPa{\cdot}m^{1/2}$, respectively. After exposure to air at $900^{\circ}C$, the formation of a porous and non-protective oxide layer was observed.

• Journal of Powder Materials
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• v.20 no.3
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• pp.221-227
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• 2013

$TiB_2$-reinforced iron matrix composite (Fe-$TiB_2$) powder was in-situ fabricated from titanium hydride ($TiH_2$) and iron boride (FeB) powders by the mechanical activation and a subsequent reaction. Phase formation of the composite powder was identified by X-ray diffraction (XRD). The morphology and phase composition were observed and measured by field emission-scanning electron microscopy (FE-SEM) and energy-dispersive X-ray spectroscopy (EDS), respectively. The results showed that $TiB_2$ particles formed in nanoscale were uniformly distributed in Fe matrix. $Fe_2B$ phase existed due to an incomplete reaction of Ti and FeB. Effect of milling process and synthesis temperature on the formation of composite were discussed.

• Journal of the Korean institute of surface engineering
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• v.44 no.2
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• pp.55-59
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• 2011

In this study, the mechanical properties of boronized 316L austenitic stainless steel have been investigated. Boronizing was carried out in solid medium consisting of Ekabor powder at $900^{\circ}C$ and $1000^{\circ}C$ for 2, 4 and 8 hours, respectively. The properties of sample were analyzed by field emission scanning electron microscope, X-ray diffractometer, Glow discharge spectrometer, micro-hardness tester and ball-on-disk wear tester. Increasing the boronizing time and temperature, the hardness of boronized samples were shown over Hv 2000 and the thickness of boride layers were also increased linearly. XRD patterns of samples were revealed the presence of borides such as FeB, $Fe_2B$, CrB, $Cr_2B$ and $Ni_3B$. Friction coefficient of boronized STS 316L was shown the low value at $900^{\circ}C$ for 8 hours and $1000^{\circ}C$ for 4 hours, respectively.

• Korean Journal of Materials Research
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• v.20 no.4
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• pp.217-222
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• 2010

Zirconium diboride (ZrB2) and mixed diboride of (Zr0.7Ta0.3)B2 containing 30 vol.% silicon carbide (SiC) composites were prepared by hot-pressing at $1800^{\circ}C$. XRD analysis identified the high crystalline metal diboride-SiC composites at $1800^{\circ}C$. The TaB2 addition to ZrB2-SiC showed a slight peak shift to a higher angle of 2-theta of ZrB2, which confirmed the presence of a homogeneous solid solution. Elastic modulus, hardness and fracture toughness were slightly increased by addition of TaB2. A volatility diagram was calculated to understand the oxidation behavior. Oxidation behavior was investigated at $1500^{\circ}C$ under ambient and low oxygen partial pressure (pO2~10-8 Pa). In an ambient environment, the TaB2 addition to the ZrB2-SiC improved the oxidation resistance over entire range of evaluated temperatures by formation of a less porous oxide layer beneath the surface SiO2. Exposure of metal boride-SiC at low pO2 resulted in active oxidation of SiC due to the high vapor pressure of SiO (g), and, as a result, it produced a porous surface layer. The depth variations of the oxidized layer were measured by SEM. In the ZrB2-SiC composite, the thickness of the reaction layer linearly increased as a function of time and showed active oxidation kinetics. The TaB2 addition to the ZrB2-SiC composite showed improved oxidation resistance with slight deviation from the linearity in depth variation.

• Journal of the Korean Chemical Society
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• v.27 no.3
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• pp.178-182
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• 1983

The conductances of sodium, potassium, ammonium, tetramethylammonium, and tetraethylammonium iodides, picrates(Pic) of sodium and potassium, and tetrabutylammonium tetraphenyl-boride have been measured in ethylene carbonate-acetone mixtures at $25{\circ}C$. The limiting equivalent conductances of the electolytes were computed by Fuoss-Kraus equation and the order was $(C_4H_9)_4NB(ph)_4 at any composition of the mixtures. The dissociation constants of the salts showed that the mixtures are good ionizing solvents for the salts. The order of limiting ionic equivalent conductance, $Na^+, is consistent with exactly the reverse order found for solvation number. Effective solvated radii calibrated by the Nightingale method showed that picrate ion seems to be unsolvated and that iodide ion seems to be solvated to some extent in the mixtures.

• Korean Journal of Materials Research
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• v.24 no.5
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• pp.255-258
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• 2014

Zirconium boride is an artificial or which is rarely found in the nature. $ZrB_2$ is popular in the hard material industry because it has a high melting point, excellent mechanical properties and chemical stability. There are two known methods to synthesize $ZrB_2$. The first involves direct reaction between Zr and B, and the second is by reduction of the metal halogen. However, these two methods are known to be unsuitable for mass production. SHS(Self-propagating High-temperature Synthesis) is an efficient and economic method for synthesizing hard materials because it uses exothermic reactions. In this study, $ZrB_2$ was successfully synthesized by subjecting $ZrO_2$, Mg and $B_2O_3$ to SHS. Because of the high combustion temperature and rapid combustion, in conjunction with the stoichiometric ratio of $ZrO_2$, Mg and $B_2O_3$; single phase $ZrB_2$ was not synthesized. In order to solve the temperature problem, Mg and NaCl additives were investigated as diluents. From the experiments it was found that both diluents effectively stabilized the reaction and combustion regime. The final product, made under optimum conditions, was single-phase $ZrB_2$ of $0.1-0.9{\mu}m$ particle size.

• Composites Research
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• v.31 no.4
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• pp.117-121
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• 2018

Various ceramic particulate such as TiC, $TiB_2$, $Al_2O_3$ reinforced SUS431 matrix composites were successfully fabricated by a novel liquid pressing infiltration process. Microstructures of the SUS431 composite were analyzed to determine manufacturability of composites. $Al_2O_3$-SUS431 composite had lots of defects due to poor wettability between the $Al_2O_3$ and steel matrix. On the other hand, TiC was uniformly dispersed in the SUS431 matrix than $TiB_2$ and $Al_2O_3$ due to good wettability and interfacial properties.

• Analytical Science and Technology
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• v.12 no.3
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• pp.235-242
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• 1999

It is impossible to measure the fraction of the precipitates which are neither plenty nor distiguishable on micrographs, using point counting method or image analyzer. In this study, phase fraction of sigma, carbide and boride which are important to mechanical properties of Ni base superalloy Udimet 720 has been measured using a quantitative X-ray diffraction technique combined with electrochemical extraction. The alloys had been exposed at $800^{\circ}C$ for various times up to 3000 hours to have a variation of the amount of the minor precipitates. The amount of sigma had increased exponentially with increasing exposure time up to 3000 hours before saturation. It can be argued that the finishing point of precipitation is around 5000 hours and maximum amount of sigma to be produced is about 5% in weight. The amounts of $M_{23}C_6$ and $M_3B_2$ were maintained constant at the level of 0.1~0.5% in weight, regardless of exposure time.