• Bulletin of the Korean Chemical Society
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• 제29권2호
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• pp.328-334
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• 2008

The effects of catalyst preparation on the reaction route and the mechanism of biphenol (BP) hydrogenation, which consists of a long series-reaction, were studied. Pd/C catalysts were prepared by incipient wetness method and precipitation and deposition method. The reaction behaviors of the prepared catalysts and a commercial catalyst along with the final product distributions were very different. The choice of the catalyst preparation conditions during precipitation and deposition including the temperature, pH, precursor addition rate, and reducing agent also had significant effects. The reaction behaviors of the catalysts were interpreted in terms of catalyst particle size, metal distribution, and support acidities.

• 한국세라믹학회지
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• 제47권2호
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• pp.183-188
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• 2010

$Ce_{0.8}Gd_{0.2}O_{1.9}$(GDC20) powder was synthesized by milling of $CeO_2$ slurry and Gd oxalate precipitation. The mixture of $CeO_2$ powder and Gd precipitates calcined at $600^{\circ}C$ for 2 h showed the particle size distribution similar to that of $CeO_2$ powder, which had been milled during the synthesis process. Attrition milling of the calcined powder with an average particle size of $0.36\;{\mu}m$ for 2 h resulted in a decrease in the particle size to $0.24\;{\mu}m$. Although the milled powder consisted of small particles(<$1\;{\mu}m$), a small amount of fine platy $Gd_2O_3$ particles, which had been survived in the milling process, was observed. Sintering of the powder compacts for 4 h showed relative densities of 80.7% at $1300^{\circ}C$ and 97% at $1400^{\circ}C$, respectively. Densification was found to almost complete at $1500^{\circ}C$, resulting in a dense and homogeneous microstructure with a relative density of 99.5%.

• 한국세라믹학회지
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• 제48권6호
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• pp.630-635
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• 2011

Bismuth vanadate($BiVO_4$) with monoclinic phase as photocatalyst under visible light is synthesized by precipitation reaction in hot water. Properties such as crystal phase, particle morphology and visual light absorbance as well as the effects of thermal treatment for $BiVO_4$ powders are investigated. $BiVO_4$ powders with both single monoclinic phase and 0.2 ${\mu}m$ in particle size are synthesized when precipitate is stirred in water for 5 h at 95$^{\circ}C$. Well-developed monoclinic phase and light absorption property under 535 nm are observed as a result of thermal treatment for 1 h at 300$^{\circ}C$ after precipitation reaction in water for 5 h at 95$^{\circ}C$. Degradation of monoclinic crystal is found in firing above 350$^{\circ}C$, and particle growth is occurred in firing above 550$^{\circ}C$.

• 대한화장품학회지
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• 제22권2호
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• pp.52-69
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• 1996

Nano-Sized TiO$_2$ particles with diameter between 2 and 5 nm are synthesized in Water/Triton X-100/n-Hexane microemulsion. Particles show the amorphous structure and partially hydroxide form. The optical absorbance of particles appears at 250nm and band edge at 340nm. Gold metal is deposited on the surface of TiO$_2$ particles by reduction reaction of Au(III) ion with sodium hypophosphite. The size of gold-deposited particles is 20nm, and the optical absorbance appears at 270nm and at 550nm. So particles show the red color. The dense precipitation is formed by aggregation in the TiO$_2$ nano-sized particles of about 5nm size. But the bulky precipitation is formed by agglomeration phenomena in the gold-deposited particles of 20nm size. And also gold-deposited particles is easily dispersed by being re-dispersed in PEG/Water solution. This study has compared those things measuring the SPF characteristics of the cosmetics made of the synthesized particles. If the particle size is controlled appropriately, then the SPF value will be higher, or more colorless cosmetics will be made.

• 한국재료학회지
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• 제24권4호
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• pp.214-220
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• 2014

In this research, a precipitation method was used to synthesize ${\beta}-Ga_2O_3$ powders with various particle morphologies and sizes under varying precipitation conditions, such as gallium nitrate concentration, pH, and aging temperature, using ammonium hydroxide and ammonium carbonate as precipitants. The obtained powders were characterized in detail by XRD, SEM, FT-IR, and TG-DSC. From the TG-DSC result, GaOOH phase was transformed to ${\beta}-Ga_2O_3$ at around $742^{\circ}C$, and weight loss percent was about 14 % when $NH_4OH$ was used as a precipitant. Also, ${\beta}-Ga_2O_3$ formed at $749^{\circ}C$ and weight loss percent was about 15 % when $(NH)_2CO_3$ was used as a precipitant. XRD results showed that the obtained $Ga_2O_3$ had pure monoclinic phase in both cases. When $(NH)_2CO_3$ was used as a precipitant, the particle shape changed and became irregular. The range of particle size was about $500nm-4{\mu}m$ based on various concentrations of gallium nitrate solution with $NH_4OH$. The particle size was increased from $1-2{\mu}m$ to $3-4{\mu}m$ and particle shape was changed from spherical to bar type by increasing aging temperature over $80^{\circ}C$.

• 공업화학
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• 제20권2호
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• pp.234-240
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• 2009

마그네슘염과 알칼리를 출발물질로 하는 수산화마그네슘 제조에서 공정 변수가 생성되는 입자의 크기, 형상 및 응집도에 미치는 영향에 대해 완전 요인배치(full factorial) 실험계획법(Design of experiment)으로 연구하였다. 수열합성에서는 알칼리/$MgCl_2$ 몰 비가 낮을수록, $MgCl_2$ 농도와 수열처리 온도가 높을수록 평균입경은 커지고 1차 입자 크기와 2차 입자 크기가 유사하지만, 침전법으로 제조한 수산화마그네슘은 1차 입자의 응집현상으로 2차 입경은 커지며, 응집성은 알칼리원의 종류와 $MgCl_2$의 농도에 따라 달라졌다. 알칼리원의 종류에 따른 차이는 $NH_4OH$를 사용한 경우 NaOH를 사용한 것보다 입자의 크기가 커지고 분산성이 우수하였다. 크기 및 표면 상태가 다른 샘플을 선정하여 LDPE와 EVA 혼합수지에 혼합하여 난연 특성을 비교하였을 때, 50, 55% 함량에서는 일반적으로 2차 입경 크기가 감소함에 따라 LOI값은 상승하지만, UL-94에서는 1차 입자가 작은 경우에 더 우수한 등급을 받는 경우가 있었다. 60%의 높은 함량에서 입자의 제조경로와 무관하게 UL-94는 V0 등급이지만, 미립이더라도 응집성있는 경우 한계산소지수가 상대적으로 낮다는 것을 알 수 있었다.

• Macromolecular Research
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• 제12권2호
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• pp.233-239
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• 2004

We prepared thermally robust fully crosslinked poly(methyl methacrylate-co-divinyl benzene) ［poly(MMA-co-DVB)］microspheres successfully by precipitation polymerization in the absence of a stabilizing agent. The DVB concentration plays a pivotal role not only in the formation of the individually stable microspheres but also in the polymerization characteristics, including the particle size, the uniformity of size, the polymerization yield, and the thermal properties. The number-average diameter of the microspheres increased linearly, from 0.72 to 2.15 $\mu\textrm{m}$, and the particle size distribution became narrower, by elevating the uniformity from 1.35 to 1.12, as the DVB concentration increased from 20 to 75 mol%. In addition, the yield of the polymerization increased, from 73.4 to 98.6%, as the DVB concentration increased. Since the prepared particles possess fully crosslinked microstructures, no glass transition temperatures were observed, but all the samples prepared with DVB concentrations ranging from 20 to 75 mol% possess enhanced thermal properties. Based on the DSC and TGA data, the thermal stability of the mesospheres prepared by the precipitation polymerization is significantly improved as a result of crosslinking with DVB.

• 한국세라믹학회지
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• 제35권5호
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• pp.451-457
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• 1998

$Li_2$$ZrO_3$ powder which is one of the candidates of breeding materials for the fusion reactor was syn-thesized by a precipitation-combustion process. Although precipitates from the reaction between zirconium nitrate and citric acid were existed in a precursor solution. $Li_2$$ZrO_3$ could easily be obtained by using the mixed fuel of urea and citric acid in stoichiometric composition. The phases of as-synthesized powder con-sisted of $Li_2$$ZrO_3$ and small amounts of $Li_6$$Zr_2O_3$ and $Li_2$$ZrO_3$ The latter phases disappeared after the cal-cination at $1100^{\circ}C$ for 2 h. The primary particle size and the specific surface area of as-synthesized powders were smaller than 20nm and 10-14 $M^2$/g, respectively. The primary particle size of the precipitation-combustion synthesized powders was affected by the size of precipitates present in a precursor solution.

• Journal of Astronomy and Space Sciences
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• 제36권2호
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• pp.45-60
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• 2019

The Earth's outer radiation belt has long received considerable attention mainly because the MeV electron flux in the belt varies often dramatically and at various time scales. It is now widely accepted that the wave-particle interaction is one of the major mechanisms responsible for such flux variations. The wave-particle interaction can accelerate electrons to MeV energies, explaining the observed flux increase events, and can also scatter the electrons' motion into the loss cone, resulting in atmospheric precipitation and thus contributing to flux dropouts. In this paper, we provide a review of the current state of research on relativistic electron scattering and precipitation due to the interaction with electromagnetic ion cyclotron (EMIC) waves in the inner magnetosphere. The review is intended to cover progress made over the last ~15 years in the theory and simulations of various issues, including quasilinear resonance diffusion, nonlinear interactions, nonresonant interactions, effects of finite normal angle on pitch angle scattering, effects due to rising tone emission, and ways to scatter near-equatorial pitch angle electrons. The review concludes with suggestions of a few promising topics for future research.

• 한국세라믹학회지
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• 제33권5호
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• pp.495-506
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• 1996

Monodispersed colloidaly yttria(Y2O3) can be used in a variety of applications such as phosphors. IR transparent materials and fine ceramics. For preparing monodispersed yttria homogeneous precipitation has been regarded as a fovorable method that is monodispersed yttria can be obtained through calcining monodispersed colloidal compound of yttrium (eg:Y(OH)CO3.nH2O)which can be prepared by homogeneous precipitation with urea. It is however still required to find out the quantitative effects of important variables of precipitation such as concentration of yttrium and urea reaction temperature and initial pH of reactant even though homogeneous precipitation of Y3+ with urea has been studied extensively. Among the effects of these variables we investiga-ted 1) the effect of yttrium concentration on the shape and size of precipitate and the reation rate 2) range of yttrium concentration required to make monodispersed colloidal particles 3) the reason for limited concentra-tion range of yttrium and 4) the effect of ultrasonic radiation on the limited concentration.