표면코팅을 이용한 알루미나/실리카 복합분말의 합성

沈秀萬 홍익대학교 산업기술연구소 1995 産業技術 Vol.5 No.-

In order to densify mullite at low temperatures, fine composite powders of silica coated-alumina were prepared with TEOS. The step hydrolysis of TEOS prevented the formation of free silica particles and resulted in a homegeneous coating of silica on alumina particles. High green density of 62% could be obtained from slip-casting under the same despersion condition as for silica particles. since the characteristics of the composite powder surface are identical to those of silica. Sintering up to 1300℃ shows that the silica coating suppresses the contact of the alumina particles and causes viscous sintering suggesting a full densification of mullite at low sintering temperatures.

알루미나-YAG 복합재료의 제조 및 미세구조

沈秀萬 홍익대학교 산업기술연구소 2002 産業技術 Vol.12 No.-

A mixture of Al2O3 and Y2O3 powders was obtained from a slurry of fine Al2O3 powder containing precipitates of Y hydroxide. Al2O3-30 vol% YAG composites were fabricated by sintering the powder compacts up to 1600℃. The formation of crystalline phases in the samples, sintering characteristics, and microstructures were investigated. In the powder calcined at 600℃, fine Y2O3 particles were found to agglomerate with α-Al2O3 particles. At 1600℃, well-crystallized YAG phase was present along with α-Al2O3, and no other phases such as YAM and YAP were observed. A considerable densification occurred at temperatures 1300~1500℃. Sintering of the powder compact at 1600℃ for 6 hrs resulted in the relative density of 97.6%. YAG particles did not affect densification and grain growth of the composite despite the presence of 30 vol% YAG phase. The YAG particles in 2~5 ㎛ sizes were found to be uniformly distributed in Al2O3 grains of 5~10 ㎛ size.

분쇄침전을 이용한 Gd이 도핑된 CeO₂ 분말의 합성

沈秀萬,朴漢壽 홍익대학교 산업기술연구소 2006 産業技術 Vol.16 No.-

GDC(Ce_(0.8)Gd_(0.2)O_(1.9)) powder was synthesized by milling and precipitation and its thermal decomposition, phase formation, and milling characteristics were investigated. Decomposition of Gd precipitates present in the as-prepared powder were found to occur at ~400℃ and complete above 700℃. As calcination temperature increased, crystalline CeO_2 phase was well developed. After calcination at 800℃ for 2 h, the powder having average particle size of 0.36 ㎛ was obtained. Due to good milling characteristic, attrition milling of the calcined powder for 2 h resulted in a decrease in the particle size to 0.24 ㎛. Large particles(>1 ㎛), contained in ~10% of the calcined powder, were removed and the resulting powder consisted of nano-size primary particles.

구연산젤법에 의한 YAG분말의 합성

沈秀萬,鄭炳周 홍익대학교 산업기술연구소 1998 産業技術 Vol.8 No.-

Citrate gel was prepared by drying a solution that Y and Al nitrates had been dissolved in a molar ratio of Y:Al=3:5 in citric acid solution. The gel was found to completely decompose at ∼500℃. Since the citrate function group made the chemical composition of the particles relatively homogeneous, crystallization occurred at 800℃ resulting in YAG phase and a small amount of metastable YAH. However, after calcination at 900℃ for 4 hr, well-crystallized single phase YAH powder could be obtained.

세라믹 미분체의 기계화학적 혼합

沈秀萬 弘益大學校 産業技術硏究所 2005 産業技術 Vol.15 No.-

Milling of YAG precipitates resulted in decomposition of the preipitates at room temperature with an aid of high energy mixing. YAG phase, which appeared to form by mechanochemical reaction between the decomposed products Y_(2)O_(3) and AlOOH, was found in the powder. other phases along with YAG were reacted to form single phase YAG at 1100℃. However, no mechanochemical rection between Ni hydroxide and YSZ was observed during milling because of no reactivity between them. Instead Ni hydroxides existed as agregates between YSZ particles. Some of the hydroxides were found to be coated on YSZ particle surface.

알루미나-이트리아 미분혼합체에서의 YAG 상형성 거동

沈秀萬,金宰泳 弘益大學校 産業技術硏究所 2004 産業技術 Vol.14 No.-

YAG phase formation was studied during calcination of Y₂O₃ or Y hydroxide and nanosized Al₂O₃ powders with various crystal structure (AlOOH, γ-Al₂O₃, and δ-Al₂O₃). The results were compared with those of a mixture of Y₂O₃ and Al₂O₃ nanopowders. Phase development in the powders which had been calcined at temperatures between 600℃∼1200℃ was investigated with XRD. A mixture of nano Al₂O₃ and Y hydroxide powders showed YAG phase in addition to Y₂O₃ and δ-Al₂O₃ at 1200℃. The formation of YAG phase attributes to ease of the reaction between δ-Al₂O₃ nanoparticles and fine Y₂O₃ crystallites which had been decomposed from Y(OH)₃. Whereas, a mixture of nano Al₂O₃ and Y₂O₃ powders did not contain YAG because of slow solid-state reaction between powders.

산화물 혼합법으로 합성한 다결정 YAG의 소결특성

沈秀萬 홍익대학교 산업기술연구소 2001 産業技術 Vol.11 No.-

A mixture of Al_2O_3 powder and Y hydroxide was prepared and its compact was sintered. The formation of crystalline phases and sintering characteristics were investigated. In contrast to the results from other researchers, in the compact sintered at 1400℃, YAM was found to exist in a considerable amount along with Y_2O_3, and to converted to YAG at 1500℃.YAP,which was observed in a very small amount at 1500℃, completely disappeared at 1600℃ and resulted in a single YAG phase. The specimen containing 131 ppm SiO_2, after pressureless sintering at 1600℃ for 4 hrs, showed bulk density of 96.3%, which was much higher than that(77.4%) of the specimen without SiO_2, suggesting significant effect of sintering aid on densification. The sintered specimen consisted of well developed YAG grains in ∼5㎛ size, as a result of enhanced grain growth.

정전기적으로 분산된 콜로이드 실리카 입자의 코팅

沈秀萬,崔鎔成 홍익대학교 산업기술연구소 1996 産業技術 Vol.6 No.-

Spherical colloidal particles of silica in 0.1㎛ diameter were synthesized dispersed in water electrostatically by adjusting pH. The particles in suspensions were deposited using dip-coating on a soda-lime glass substrate. Total potential energy between the particles and the substrate was calculated and the result was discussed with the adhesion and arrangement of the deposited particles. Particles deposited from a suspension of pH=4 were removed after washing with deionlzed water due to the flocculation of the particles arising from an attraction force between the particles. Deposition of a suspension of pH=8 resulted in a dense coating that the particles were uniformly adhered. Whereas, the deposition with a suspension of pH=6 resulted in a realtively porous coating that the particles were less uniformly adhered. The dense and uniform packing of the particles was attributed to a repulsion force existing between the particles.

시멘트가 혼합된 다공성 세라믹스의 압축강도

沈秀萬,金宰榮 홍익대학교 산업기술연구소 2003 産業技術 Vol.13 No.-

Porous ceramics mixed with cement were fabricated to develop lightweight insulation bricks and panels with low thermal conductivities. A fine mixture of potter's clay, kaolin, and clay was mixed with portland cement up to 10~50 vol%. Slurries were prepared by adding water and a foaming agent into the powder and ball milling. The foamed slurry was poured into a mold and cured at room temperature for 48 hrs. Samples were fired between 800℃~1000℃ and then compressive strengths were measured. Various sizes of pores were formed in the fired samples and the compressive strength was increased with firing temperature due to porcelain formed as a result of the reaction between cement and clay ingredients. The results of microstructure and strength showed that it is possible to fabricate porous ceramics from cement containing additives.

MgO의 첨가가 ZrO₂-Y₂O₃의 전기전도도에 미치는 영향

沈秀萬 홍익대학교 산업기술연구소 1994 産業技術 Vol.4 No.-

Small amounts of MgO were substituted for Y_2O_3 in Y_2O_3-stabilized ZrO_2 in order to have equivalent oxygen vacancy concentrations. The electrical conductivities of the samples were measured using a complex impedance method. The sample of (ZrO_2)_0.882(Y_2O_3)0.10(MgO)0.018 had a similar conductivity to (ZrO_2)0.88(Y_2O_3)0.12 while (ZrO_2)_0.902(Y_2O_3)0.08(MgO)0.018 exhibited a lower conductivity than that of (ZrO_2)0.90(Y_2O_3)0.10. The effect of the addition of MgO on the conductivity of Y_2O_3-stabilized ZrO_2 could not be clearly evaluated due to the presence of trace amounts of second phases. monoclinic ZrO_2 and MgO However as compared with the activation energies for (ZrO_2)0.90(Y_2O_3)0.10 and (ZrO_2)0.88(Y_2O_3)0.12 samples a decrease in the activation energy was found from the sample containing MgO.