• Journal of Korean Society of Environmental Engineers
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• v.27 no.3
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• pp.247-252
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• 2005

The typical treatment method for fluoride polluted water is the flocculation and precipitation method which usually is capable of reducing the fluoride concentration down to the level of about 10 ppm. However, this method is no longer effective for the treatment of contaminated water having less than 10 ppm of fluorides. To remove fluorides in polluted water from the fluoride concentration between 1 to 10 ppm, several adsorbents were prepared mainly based on an activated alumina and the fluoride removal efficiencies of the adsorbents were analyzed. The best fluoride removal efficiency was obtained when the activated alumina treated by sulfuric acid was used as the adsorbent. A proper calcination temperature for the sulfuric acid contained activated alumina was found to be about $500^{\circ}C$. An adsorption isotherm for the adsorbent was also obtained by using Freundlich model. The values of the constants in Freundlich isotherm model were calculated to be K=6.63 and 1/n=0.29 based on the results obtained from the series of batch type adsorption experiments.

• Journal of the Korean Crystal Growth and Crystal Technology
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• v.10 no.4
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• pp.330-336
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• 2000

Effects of microstructure and indentation stress on fracture behavior of glass-infiltrated alumina composite for dental restorative application were investigated by the Hertzian and Vickers indentation method. Indentation stress-strain curve of glass-infiltrated alumina has showed the quasi-plastic behavior - deviation from linearity at high stress and the classical Hertzian cone crack, which could be confirmed the subsurface damage micrographs using bonded-interface specimen technique. The indentation stress-strain curves for the starting preforms are strongly dependent on porosity and microstructure of the preforms. On the other hand, the curves for the infiltrated composites are relatively insensitive to these factors. The failure of composite is originated at quasi-plastic deformation region. Damage and fracture behavior due to Hertzian stress field is theoretically examined, so that the indentation stress field plays a great role in material degradation. After Hertzian indentation annealing processing changes fracture behavior of alumina composite, so that stress field in material is healed through annealing.

• Korean Journal of Materials Research
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• v.9 no.8
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• pp.843-848
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• 1999

The efect of A1 addition and sintering condition on mutization behavior between alumina and silica in alumina-based shell mold was investigated. A1 addition and high sintering temperature increased the degree of mullitization between alumina/silica but decreased the room temperature strentgh of the shell molds. It was identified that the sintered strength of the shell molds was in range of 2.0~2.6kg/$\textrm{mm}^2$. Al addition suppressed deflection of the shell molds at high temperature. Especially, no deflection was observed in the specimen which was sintered at $1000^{\circ}C$ for 1hr with 2.5wt% Al. The specimen sintered at $1500^{\circ}C$ for 4hrs with no Al addition also displayed no deflection.

• Applied Chemistry for Engineering
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• v.8 no.6
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• pp.1048-1053
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• 1997

Modified mordenites by HF treatement, which have different $SiO_2/Al_2O_3$ ratio, and those mixed with $\gamma$-alumina are prepared and used for catalytic cracking of triphenylmethane(TPM) in micro-activity tester(MAT). Dealumination of mordenites decreased the acid content but developed secondary mesopores. The conversion and the selectivity of benzene over modified mordenites with the mesopores were increased. However, for the further dealuminated mordenites, they decreased due to the destruction of pore structure and low acid amount. Accordingly, the maximum cracking activity and the maximum selectivity of benzene were obtained about 17 $SiO_2/Al_2O_3$ ratio of modified mordenites. The modified mordenites mixed with alumina enhanced the cracking activity of TPM compared with the pure mordenites. It can be explained that TPM is cracked at active sites with in large pores of alumina firstly and further cracked into small molecules in zeolite pores as step mechanism of catalytic cracking.

• Journal of the Mineralogical Society of Korea
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• v.26 no.2
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• pp.101-110
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• 2013

Due to the large specific surface area and great reactivity toward environmental contaminants, nanocrystalline mackinawite (FeS) has been widely applied for the remediation of contaminated groundwater and soil. Furthermore, nanocrystalline FeS is rather thermodynamically stable against anoxic corrosion, and its reactivity can be regenerated continuously by the activity of sulfate-reducing bacteria. However, nanocrystalline mackinawite is prone to either spread out along the groundwater flow or cause pore clogging in aquifers by particle aggregation. Accordingly, this mineral should be modified for the application of permeable reactive barriers (PRBs). In this study, coating methods were investigated by which mackinawite nanoparticles were deposited on the surface of alumina or activated alumina. The amount of FeS coating was found to significantly vary with pH, with the highest amount occurring at pH ~6.9 for both minerals. At this pH, the surfaces of mackinawite and alumina (or activated alumina) were oppositely charged, with the resultant electrostatic attraction making the coating highly effective. At this pH, the coating amounts by alumina and activated alumina were 0.038 and 0.114 $mmol{\cdot}FeS/g$, respectively. Under anoxic conditions, arsenite sorption experiments were conducted with uncoated alumina, uncoated activated alumina, and both minerals coated with FeS at the optimal pH for comparison of their reactivity. Uncoated activated alumina showed the higher arsenite removal compared to uncoated alumina. Notably, the arsenite sorption capacity of activated alumina was little changed by the coating with FeS. This might be attributed to the abundance of highly reactive hydroxyl functional groups (${\equiv}$AlOH) on the surface of activated alumina, making the arsenite sorption by the coated FeS unnoticeable. In contrast, the arsenite sorption capacity of alumina was found to increase substantially by the FeS coating. This was due to the consumption of the surface hydroxyl functional groups on the alumina surface and the subsequent occurrence of As(III) sorption by the coated FeS. Alumina, on the surface area basis, has about 8 times higher FeS coating amount and higher As(III) sorption capacity than silica. This study indicates that alumina is a better candidate than silica for the coating of nanocrystalline mackinawite.

• Composites Research
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• v.12 no.1
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• pp.68-75
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• 1999

This study has observed that the dynamic behavior of Metal Matrix Composites (MMCs) in low velocity impact varies with impact velocity. MMCs with 15 fiber volume percent were fabricated by using the squeeze casting method. The AC8A was used as the matrix, and the alumina and the carbon were used as reinforcements. The tensile and vibration tests conducted yielded the yielded the tensile stress and elastic modulus of MMCs The low pass filter and instrumented impact test machine was adopted to study dynamic behaviors of MMCs corresponding to impact velocity. Stable impact signals were obtained by using the low pass filter. Impact corresponding to impact velocity. Stable impact signals were obtained by using the low pass filter. Impact energy of unreinforced alloy and MM s increased as the impact velocity increased. The increase of crack propagation energy was especially prominent, but the dynamic toughness of each material did not change much. To show the relation between crack initiation energy and dynamic fracture toughness, a simple model was proposed by using the strain energy and stress distribution at notch. The model revealed that crack initiation energy is proportional to the square of dynamic fracture toughness and inversely proportional to elastic modulus.

• Journal of the Korean Ceramic Society
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• v.41 no.7
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• pp.513-517
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• 2004

Porous alumina ceramics with aligned plate-shaped pores were fabricated by using thermoplastic microsphere in order to show the anisotropy in thermal conductivity. The mixed powder of alumina and microsphere was pressed under 15 MPa till 20$0^{\circ}C$ to deform polymer into platelet-shape and sintered at 1,00$0^{\circ}C$ for 1 h. The sintered specimen with 10 wt% microsphere has 45.3% porosity and the bending strength of 44 MPa. The microstructural investigation confirmed the pore structure of platelet-shape, the thermal conductivities for vertical and parallel directions are 3.803 W/mK and 7.818 W/mK, respectively, the ratio between two directions exceeds 2.

• Journal of the Korean Ceramic Society
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• v.39 no.9
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• pp.871-877
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• 2002

Alumina slurriers, having various amount of alumina and ratio of organic additives, were prepared for tape casting. The relative viscosities were compared to investigate influence of composition on stability of the slurry and plotted as a function of powder fraction. They raised with increasing powder fraction of slurries, revealing a exponential function curve, which means that stability of slurry was not affected by amount and composition of organic additives. Cast green tapes were tested under tensile condition at room temperature. The increase in alumina ratio and binder ratio was found to decrease strain to failure of green tapes from 363% to 45% and from 68% to 25%, respectively. Tensile strength of green tapes increased abruptly with increasing alumina ratio, which showed its maximum at 1 MPa. On other hand, Tensile strength increased continuously from 0.5 MPa to 4 MPa with increasing binder ratio. Mechanical properties of them were affected seriously and lost their properties by elevating temperature from 20$^{\circ}C$ to 80$^{\circ}C$.

• Korean Journal of Materials Research
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• v.4 no.5
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• pp.590-599
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• 1994

A work on the brittle to ductile transition (BDT) in single crystal alumina has been performed to understand and assess the dynamics of dislocation mobility around crack tip of brittle material. The critical stress intensity factor and yield strengths were obtained from bending test using precracked specimens at elevated temperatures. It was found that the BDT temperature was dependent on strain rate and orientation of specimen : for (1120) fracture surface, $1034^{\circ}C$, $1150^{\circ}C$ for $4.2 \times 10^{-6}$, $4.2 \times 10^{-7}s^{-1}$ respectively. Under a 4 point bending test, the moving distance of dislocation generated near crack front in ductile range is determined by an etch pits method. The velocity of dislocation in sapphire obtained from the double etching method was applied to modelling study.

• Proceedings of the Korean Powder Metallurgy Institute Conference
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• 2003.04a
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• pp.59-59
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• 2003

경사기능 재료는 위치에 따른 특성 변화를 갖고 있어 변화하는 주변 환경에서 사용되는 재료의 응용성을 높일 수 있는 장점이 있다. 적합한 경사기능 재료의 제조를 위해서는 탄성률, 강도, 인성 등의 위치에 따른 변화를 예측하는 것이 필요하다. 그러나 재료 내에서의 탄성률 변화 등으로 인해 경사기능 재료에서 인성을 정량적으로 표현하는데는 어려움이 있으므로 이를 정량적으로 측정하고, R-curve를 결정하는 것은 재료의 응용성에 중요한 인자가 된다. 본 연구에서는 폴리우레탄 스폰지를 이용하여 밀도의 분포가 연속적으로 다른 폴리 우레탄 스폰지를 제조하고 이에 알루미나 분말 슬러리를 이용하여 slip casting을 행하였다. 그 후 폴리 우레탄 스폰지를 탈지한 후 알루미나 성형체를 소결하여 연속적인 기공률 분포가 다른 다공성 알루미나를 제조하였으며, 이에 Al을 용침하여 $Al_2O_3-Al$ 경사기능 재료를 제조하였다. 이러한 $Al_2O_3-Al$ 경사기능 재료에 대해 파괴인성 및 R-curve 특성을 CT(conpact tension)시편으로 측정하였으며, 이를 균일한 복합체의 파괴인성과 비교하였다. 또한 잔류 응력 특성을 파악하기 위해 실험적이 응력 데이터를 Moire interferometry를 이용하여 결정하였다. 또한 이를 유한요서 해석법(FEM)에 의한 계산치와 비교하였다. 서로 다른 조성 분포를 갖는 $Al_2O_3-Al$ 경사기능 재료와 균일한 복합체의 파괴인성을 비교한 결과 동일한 Al조성에서도 서로 다른 파괴 인성치가 나타났다. $Al_2O_3-Al$ 경사기능 재료에서 파괴인성에 영향을 줄 수 있는 인자로는 술수한 $Al_2O_3$의 파괴인성에 Al금속의 소성변형에 의한 인성증진 효과, 그리고 경사기능 재료에서 상호 조성차이에 따른 잔류응력을 고려할 수 있을 것이다. 이중 $Al_2O_3-Al$ 경사기능 재료의 파괴인성에 미치는 잔류응력의 영향을 고려하기 위해 이의 잔류응력에 대해 실험에 의한 유추된 잔류응력과 FEM계산에 의해 유추된 잔류 응력을 비교, 분석하였다.