This study demonstrates the effect of the compaction pressure on the microstructure and properties of pressureless-sintered W bodies. W powders are synthesized by ultrasonic spray pyrolysis and hydrogen reduction using ammonium metatungstate hydrate as a precursor. Microstructural investigation reveals that a spherical powder in the form of agglomerated nanosized W particles is successfully synthesized. The W powder synthesized by ultrasonic spray pyrolysis exhibits a relative density of approximately 94% regardless of the compaction pressure, whereas the commercial powder exhibits a relative density of 64% under the same sintering conditions. This change in the relative density of the sintered compact can be explained by the difference in the sizes of the raw powder and the densities of the compacted green body. The grain size increases as the compaction pressure increases, and the sintered compact uniaxially pressed to 50 MPa and then isostatically pressed to 300 MPa exhibits a size of 0.71 m. The Vickers hardness of the sintered W exhibits a high value of 4.7 GPa, mainly due to grain refinement.

The synthesis of porous W by freeze-casting and vacuum drying is investigated. Ball-milled WO3 powders and tert-butyl alcohol were used as the starting materials. The tert-butyl alcohol slurry is frozen at –25oC and dried under vacuum at –25 and –10oC. The dried bodies are hydrogen-reduced at 800oC and sintered at 1000oC. The XRD analysis shows that WO3 is completely reduced to W without any reaction phases. SEM observations reveal that the struts and pores aligned in the tert-butyl alcohol growth direction, and the change in the powder content and drying temperature affects the pore structure. Furthermore, the struts of the porous body fabricated under vacuum are thinner than those fabricated under atmospheric pressure. This behavior is explained by the growth mechanism of tert-butyl alcohol and rearrangement of the powders during solidification. These results suggest that the pore structure of a porous body can be controlled by the powder content, drying temperature, and pressure.

The effects of drying temperature on the microstructure of porous W fabricated by the freeze-casting process of tert-butyl alcohol slurry with WO3 powder was investigated. Green bodies were hydrogen-reduced at 800oC for 1 h and sintered at 1000oC for 6 h. X-ray diffraction analysis revealed that WO3 powders were completely converted to W without any reaction phases by hydrogen reduction. The sintered body showed pores aligned in the direction of tertbutyl alcohol growth, and the porosity and pore size decreased as the amount of WO3 increased from 5 to 10v ol%. As the drying temperature of the frozen body increased from -25oC to -10oC, the pore size and thickness of the struts increased. The change in microstructural characteristics based on the amount of powder added and the drying temperature was explained by the growth behavior of the freezing agent and the degree of rearrangement of the solid powder during the solidification of the slurry.

The effect of tert-butyl alcohol (TBA) as a freezing solvent on the pore structure of a porous tungsten body prepared by freeze-drying is analyzed. TBA slurries with a WO3 content of 10 vol% are prepared by mixing with a small amount of dispersant and binder at 30oC. The slurries are frozen at -25oC, and pores are formed in the frozen specimens by the sublimation of TBA during drying in air. After hydrogen reduction at 800oC and sintering at 1000oC, the green body of WO3 is completely converted to porous W with various pore structures. Directional pores from the center of the specimen to the outside are observed in the sintered bodies because of the columnar growth of TBA. A decrease in pore directionality and porosity is observed in the specimens prepared by long-duration drying and sintering. The change in pore structure is explained by the growth of the freezing solvent and densification.

The present study demonstrates the effect of magnetic pulse compaction and spark plasma sintering on the microstructure and mechanical property of a sintered W body. The relative density of green specimens prepared by magnetic pulse compaction increases with increase in applied pressure, but when the applied pressure is 3.4 GPa or more, some cracks in the specimen are observed. The pressureless-sintered W shows neck growth between W particles, but there are still many pores. The sintered body fabricated by spark plasma sintering exhibits a relative density of above 90 %, and the specimen sintered at 1,600 oC after magnetic pulse compaction shows the highest density, with a relative density of 93.6 %. Compared to the specimen for which the W powder is directly sintered, the specimen sintered after magnetic pulse compaction shows a smaller crystal grain size, which is explained by the reduced W particle size and microstructure homogenization during the magnetic pulse compaction process. Sintering at 1,600 oC led to the largest Vickers hardness value, but the value is slightly lower than that of the conventional W sintered body, which is attributed mainly to the increased grain size and low sintering density.

목적: 중등도 근시의 초기성인에서 자각적 굴검검사와 개방형, 내부형 자동굴절력계에 의한 측정값을 비교하여 자동굴절력계의 성능과 정확도를 평가하고자 하였다. 방법: 대상자는 만 18세 이상 ~ 20세 이하(평균 18.94±0.54세)로 중등도 근시(–3.00D≤ S.E.<-7.00D) 이상의 초기성인 36안(남자 12명, 여자 6명)을 검사하였다. 자각식 굴절검사는 수동포롭터 BR-7(Shin-nippon)으로 측정하였고, 자동굴절검사는 개방형 자동굴절력계 Nvision K-5001(Shin-nippon)과 내부형 자동굴절력계 KR-800(Topcon)으로 3회 반복 측정하여 평균 값을 사용하였다. 측정값은 구면굴절력과 실린더굴절력, 축으로 표기하고 다시 Power vector 성분으로 SE(=Shp+Cyl/2), J0(=-Cyl/2․cos(2․Ax)), J45(=-Cyl/2․sin(2․Ax))로 나타내어 Paired T-tset와 Pearson 상관계수를 산출하였다. 통계처리는 SPSS 13.0을 사용하였고, p<0.05인 경 우를 통계적으로 유의하다고 판단하였다. 결과: 자각적 굴절검사와 개방형, 내부형 자동굴절력계에 의한 SE는 –4.37±1.04D, -4.50±1.28D, -4.67±1.40D이고 J0는 –0.32±0.51D, 0.01±0.52D, -0.02±0.62D 그리고 J45 는 0.11±0.37D, 0.24±0.62D, 0.09±0.54D로 나타났다. 자각적 굴절검사와 개방형 자동굴절력계에 의한 상관관계는 SE 성분에서 관계식(상관계수) y=-1.01x+0.75(r=0.92, p=0.00), J0 성분은 y=-0.32x+0.13(r=0.14, p=0.43), J45 성분은 y=0.09x+0.07(r=0.12, p=0.47)이고 내부형 자동굴절력계는 SE 성분이 y=-0.95x+0.73(r=0.98, p=0.00), J0 성분은 y=-0.32x+0.17(r=0.21, p=0.22) 그리고 J45 성분은 y=0.10x+0.05(r=0.08, p=0.65)였다. 연속 변수 비교는 자각적 굴절검사값을 기준으로 개방형 자동굴절력계와 차이값에서 SE, J0, J45 성분이 각각 0.13±0.53(p=0.17), -0.33±0.68(p=0.01), -0.13±0.68(p=0.26)로 J0 성분은 유의한 차이를 보였지만 SE, J45 성분은 차이가 없었다. 내부형 자동굴절력계는 SE, J0, J45 성분이 각각 0.30±0.42(p=0.00), -0.30±0.71(p=0.02), 0.02±0.63(p=0.88)로 J45 성분은 차이가 없지만 SE, J0, 성분은 유의한 차이를 나타났다. 결론: 제시된 분석자료는 임상에서 자동굴절력계를 이용하여 굴절이상을 교정하는 안경사에게 기기의 특성을 이해하는데 도움이 될 것으로 사료된다.