• 한국세라믹학회지
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• 제52권6호
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• pp.462-466
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• 2015

This paper reports the microstructure of hot-pressed $ZrB_2$-SiC ceramics with added $B_4C$ as characterized by transmission electron microscopy. $ZrB_2$ has a melting point of $3245^{\circ}C$, a relatively low density of $6.1g/cm^3$, and specific mechanical properties at an elevated temperature, making it a candidate for application to environments with ultra-high temperatures which exceed $2000^{\circ}C$. Due to the non-sinterability of $ZrB_2$-based ceramics, research on sintering aids such as $B_4C$ or $MoSi_2$ has become prominent recently. From TEM investigations, an amorphous layer with contaminant oxide is observed in the vicinity of $B_4C$ grains remaining in hot-pressed $ZrB_2$-SiC ceramics with $B_4C$ as an additive. The effect of a $B_4C$ addition on the microstructure of this system is also discussed.

• 한국세라믹학회지
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• 제54권1호
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• pp.43-48
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• 2017

In this study, we investigate the effect of additive size on the densification and thermal conductivity of AlN ceramics with $MgO-CaO-Al_2O_3-SiO_2$ (MCAS) additives. Micro-sized MCAS powder prepared via melting and nano-sized MCAS powder synthesized via the polymeric complex method are used as sintering additives. We analyze the densification behavior of AlN added with 5 wt.% of MCAS by dilatometry as well as by isothermal sintering in the temperature range of $1300{\sim}1700^{\circ}C$. AlN exhibits higher sinterability with nano-MCAS than with micro-MCAS, and both specimens approach their maximum densities when sintered at $1600^{\circ}C$ for 4 h. The thermal conductivities of AlN with 5 wt% of nano- and micro-MCAS additives sintered at $1600^{\circ}C$ are 82.6 and 32.0 W/mK, respectively. We find that nano-MCAS is more effective in sintering of AlN ceramics at lower temperatures, and thus for enhancing their thermal conductivities.

• 한국세라믹학회지
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• 제54권4호
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• pp.285-291
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• 2017

This study investigated the effects of grain size and phase constitution on the mechanical properties of $3Y-ZrO_2$ by varying the sintering conditions. The raw powder prepared by a low-cost wet milling using the coarse solid oxide powders was sintered by both pressureless sintering and hot-pressing, respectively. As increasing holding time at $1450^{\circ}C$ for pressureless sintering, it promoted the microstructural coarsening of matrix grains and the phase transformation to tetragonal phase, whereas the bimodal microstructure embedded with abnormal $cubic-ZrO_2$ grains was observed regardless of sintering time. On the other hand, the specimens hot-pressed at $1300^{\circ}C$ for 2 h reached ~ 97% of relative density with homogeneous fine microstructure and mixed phase constitution. It was found that the proportion of untransformed monoclinic zirconia had the most adverse effect on the biaxial strength compared to the impacts of grain size and density. The pressureless sintering of the low-cost powder for prolonged sintering time to 8 h led to a decent combination of mechanical properties ($H_V=13.2GPa$, $K_{IC}=8.16MPa{\cdot}m^{1/2}$, ${\sigma}=981MPa$).

• The Korean Journal of Ceramics
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• 제5권4호
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• pp.379-385
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• 1999

Minute boehmite crystals with high aspect rations, which were hydrothermally synthesized from gibbsite in $Ba(OH)_2$ solution, occluded Ba with the Ba/Al molar ratio of about 0.03 in their interlayers. Their surface areas were about 14$\m^2$/g. The Ba-intercalated bohemite samples were partly used for producing $BaAl_{12}O){19}$ with low sinterability by externally supplementing $Ba(OH)_2$, and for forming transient aluminas. The surface area of $BaAl_{12}O){19}$ obtained by firing at $1500^{\circ}C$ for 3 h was 5.3$\m^2$/g, which was significantly lower than 12$\m^2$/g of the sol-gel origin. While a mixture ${\gamma}$-alumina and BaO is known to from $BaAl_{12}O){19}$ at $1200^{\circ}C$, solid state reaction between η-alumina transformed from the Ba-intercalated boehmite and BaO formed from $Ba(OH)_2$ deposited on the boehmite started above $1300^{\circ}C$. This suggests that large sized $Ba^{2+}$ ion occluded in η-alumina considerably suppresses the diffusion of $Al^{3+}$ ion. The surface area of the Ba-intercalated boehmite fired at $1400^{\circ}C$ for 3h was as high as 14$\m^2$/g indicative of its potential applicability to combustion catalysts. But it was decreased to 5.0$\m^2$/g after firing at $1500^{\circ}C$ for 3 h, accompanied by abrupt formations of $\alpha$-alumina and $BaAl_{12}O){19}$ as main products. The suppression of $\alpha$-alumina formation up to $1400^{\circ}C$ also suggests the significant blocking effect of $Ba^{2+}$ ion on the diffusion of the component ions.

• Transactions on Electrical and Electronic Materials
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• 제11권3호
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• pp.126-129
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• 2010

(1-X) $(Na_{0.5}K_{0.5})NbO_3-X$ $K_4CuNb_8O_{23}$ (NKN-X KCN) ceramics were produced using the conventional solid state sintering method, and their sinterability and electric properties were investigated. The density, dielectric constant (${\varepsilon}_r$), piezoelectric constant $d_{33}$, electromechanical coupling factor $k_p$ and mechanical quality factor $Q_m$ value of the NKN ceramics depended upon the KCN content and the sintering temperature. In particular, the KCN addition to the NKN greatly improved the mechanical quality factor $Q_m$ value. The ceramic with X = 2.0 mol% sintered at $1,150^{\circ}C$ possesses the optimum properties (${\varepsilon}_r=241$, $d_{33}=78$, $k_p=0.34$ and $Q_m=1,121$). These results indicate that the ceramic is a promising candidate material for applications in lead free piezoelectric transformer and filter materials.

• 센서학회지
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• 제1권1호
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• pp.93-100
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• 1992

이트리아 안정화(安定化) 지르코니아(92 mole% $ZrO_{2}$+8 mole% $Y_{2}O_{3}$)에 0.5 mole% $SiO_{2}$와 $0{\sim}2.0{\;}mole%{\;}Al_{2}O_{3}$가 첨가된 소결체의 소결성, 기계적 및 전기적 성질에 대하여 조사하였다. 소결밀도(燒結密度)는 0.5 mole% $Al_{2}O_{3}$가 첨가되었을 때 가장 높게 나타났으며 1.5 mole%이상 $Al_{2}O_{3}$가 첨가됨에 따라 감소하는 경향을 나타내었다. 비커스 경도(硬度)는 소결밀도(燒結密度)에 비례하여 나타났으며, 0.5 mole% $Al_{2}O_{3}$가 첨가되었을 때 가장 높은 전도성을 나타내었다. 일정한 산소분압(酸素分壓)에 따른 기전력 측정에서도 0.5 mole% $Al_{2}O_{3}$가 첨가되었을 매 가장 높았다. 제작된 산소(酸素)센서들의 응답특성(應答特性) 결과에서 $SiO_{2}$와 $Al_{2}O_{3}$가 각각 0.5 mole% 첨가된 센서의 응답특성이 가장 좋았다.

• 한국세라믹학회지
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• 제22권5호
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• pp.9-16
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• 1985

Agglomeration of Y-PSZ and its related properties were studied. The ultrafine $ZrO_2$ powder containg 3 mol% $Y_2O_3$ was prepared by 1) coprecipitation method b) hot petroleum drying method c) sol-gel method and the characteristics of calcined powders and the microstructures of sintered body were observed. Powder prepared by the coprecipitation method was about 125$\AA$ in crystallite size and 0.1~1.0${\mu}{\textrm}{m}$ of intra-agglomerate pore size when calcined at $600^{\circ}C$ for 1 hour. because of small crystallite size and weak agglo merate strength resultant densification of sintered body was high. But above the temperature of 130$0^{\circ}C$ efflorescent phenomena due to anions attached to powder surface was observed. Powder prepared by hot petroleum drying method was 65$\AA$ in crystallite size and 1~10${\mu}{\textrm}{m}$ of intraagglome-rate pore size and it was observed that the agglemerates were formed during the calcining process. In this case despite of small crystallite size the rate of sintering was slow and the existing lenticular interagglomerate pore was not eliminated to the final stage of sintering. Powder prepared by sol-gel method showed solid agglomertes due to rapi dhydrolysis reaction. In this powder which involves strong solid agglomerates overall sintering rate was determined by the sintering between the agglomerates and therefore sinterability of powders made by sol-gel method was very poor.

• 한국세라믹학회지
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• 제27권4호
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• pp.465-472
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• 1990

The effects of Al2O3 and Cr2O3 addition on the mechanical properties and microstructures of Y-TZP ceramics obtained by co-precipitation method of ZrO2＋3m/o Y2O3, following pressureless sintering at 150$0^{\circ}C$ for 2h were investigated. The addition of Al2O3 and Cr2O3 improved the Y-TZP sinterability and the Al2O3 addition showed the better effect on Y-TZP sintering than that of the Cr2O3 addition. The density and microstructure had the better effect on the bending strength of specimen more than stressinduced phase transformation (SIPT) of ZrO2 from tetragonal to monoclinic phase. The hardness of the specimens was found to be depend on the relative density and the fracture toughness of Y-TZP was found to rely on the amount of SIPT. The grian size of Cr2O3-doped Y-TZP was observed to be relatively smaller and had a narrower distribution than that of Al2O3-doped Y-TZP. If decomposition reaction of Cr2O3 can be controlled at high temperatures, it is anticipated that the mechanical properties of Y-TZP can be much improved by the Cr2O3 addition.

• 한국세라믹학회지
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• 제50권3호
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• pp.218-225
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• 2013

The densification behavior and strength of sintered reaction bonded silicon nitrides (SRBSN) that contain $Lu_2O_3-SiO_2$ additives were improved by the addition of fine Si powder. Dense specimens (relative density: 99.5%) were obtained by gas-pressure sintering (GPS) at $1850^{\circ}C$ through the addition of fine Si. In contrast, the densification of conventional specimens did not complete at $1950^{\circ}C$. The fine Si decreased the onset temperature of shrinkage and increased the shrinkage rate because the additive helped the compaction of green bodies and induced the formation of fine $Si_3N_4$ particles after nitridation and sintering at and above $1600^{\circ}C$. The amount of residual $SiO_2$ within the specimens was not strongly affected by adding fine Si powder because most of the $SiO_2$ layer that had formed on the fine Si particles decomposed during nitridation. The maximum strength and fracture toughness of the specimens were 991 MPa and $8.0MPa{\cdot}m^{1/2}$, respectively.

• 한국분말재료학회지
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• 제17권5호
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• pp.365-372
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• 2010

The evolution of sinterability, microstructure and mechanical properties for the spark plasma sintered(SPS) Ti from commercial pure titanium(CP-Ti) was studied. The densification of titanium with 200 mesh and 400 mesh pass powder was achieved by SPS at $750{\sim}1100^{\circ}C$ under 10 MPa pressure and the flowing $H_2$+Ar mixed gas atmosphere. The microstructure of Ti sintered up to $800^{\circ}C$ consisted of equiaxed grains. In contrast, the growth of large elongated grains was shown in sintered bodies at $900^{\circ}C$ with the 400 mesh pass powder and the lamella grains microstructure had been developed by increasing sintering temperature. The Vickers hardness of 240~270 HV and biaxial strength of 320~340 MPa were found for the specimen prepared at $950^{\circ}C$.