• Journal of Powder Materials
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• v.9 no.6
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• pp.381-393
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• 2002

Recent developments in nanocrystalline and nanocomposite rare earth-transition metal magnets are reviewed and emphasis is placed on research work at IFW Dresden. Principal synthesis methods include high energy ball milling, melt spinning, mold casting and hydrogen assisted methods such as reactive milling and hydrogenation-disproportionation-desorption-recombination. These techniques are applied to NdFeB-, PrFeB- and SmCo-type systems with the aim to produce high remanence magnets with high coercivity. Concepts of maximizing the energy density in nanostructured magnets by either inducing a texture via anisotropic HDDR or hot deformation or enhancing the remanence via magnetic exchange coupling are evaluated. With respect to high temperature applications melt spun $Sm(Co_{0.74}Fe_{0.1}Cu_{0.12}Zr_{0.04})_{7.5}$ ribbons were prepared, which showed coercivities of up to 0.53 T at 50$0^{\circ}C$. Partially amorphous $Nd_{60}Fe_xCo_{30-x}Al_{10}(0{\leq}x{\leq}30)$ alloys were prepared by copper mold casting. The effect of transition metal content on the glass-forming ability and the magnetic properties was investigated. The $Nd_{60}Co_{30}Al_{10}$ alloy exhibits an amorphous structure shown by the corresponding diffraction pattern. A small substitution of Co by 2.5 at.% Fe results In the formation of Fe-rich crystallites embedded in the Nd-rich amorphous matrix. The Fe-rich crystallites show hard magnetic behaviour at room temperature with a coercivity value of about 0.4 T, relatively low saturation magnetization and a Curie temperature of 500 K.

• Proceedings of the Korean Vacuum Society Conference
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• 2011.08a
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• pp.68-69
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• 2011

The process of charge transfer at the interface between two semiconductors or between a metal and a semiconductor plays an important role in many areas of technology. The optimization of such devices requires a good theoretical description of the interfaces involved. This, in turn, has motivated detailed mechanistic studies of interfacial charge-transfer reactions at metal/organic, organic/organic, and organic/inorganic semiconductor heterojunctions. Charge recombination of photo-induced electron with redox species such as oxidized dyes or triiodide or cationic HTM (hole transporting materials) at the heterogeneous interface of $TiO_2$ is one of main loss factors in liquid junction DSSCs or solid-state DSSCs, respectively. Among the attempts to prevent recombination reactions such as insulating thin layer and lithium ions-doped hole transport materials and introduction of co-adsorbents, although co-adsorbents retard the recombination reactions as hydrophobic energy barriers, little attention has been focused on the anchoring processes. Molecular engineering of heterogeneous interfaces by employing several co-adsorbents with different properties altered the surface properties of $TiO_2$ electrodes, resulting to the improved power conversion efficiency and long-term stability of the DSSCs. In this talk, advantages of the coadsorbent-assisted sensitization of N719 in preparation of DSSCs will be discussed.

• Journal of the Korean Magnetics Society
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• v.23 no.1
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• pp.12-17
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• 2013

Nanocrystalline nickel (Ni) tick films were synthesized by direct current electrodeposition at current density from 1 to $30mA/cm^2$ and pH = 4. The basic composition of the bath, which was prepared by dissolving Ni metal particles in HCl, was 0.2M Ni ions. The effects of the current density on the average grain size of Ni deposits were investigated by XRD and SEM techniques. The results showed that the surface roughness was decreased as the saccharin addition was increased up to 2 g/l. The experimental results showed that the increase in the current density had a considerable effect on the average grain size of the Ni deposits. The perpendicular magnetization was raised as the thickness of coating layer was increased.

• Bulletin of the Korean Chemical Society
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• v.34 no.2
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• pp.622-628
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• 2013

Nanocrystalline spinel lithium manganese oxide ($LiMn_2O_4$) powders with narrow-size-distribution, pure-phase particles, and high crystallinity with an average crystallite size of about 70 nm were synthesized at $600^{\circ}C$ for 6 h in air by freeze drying method. Spinel $LiMn_2O_4$ is also prepared by sol-gel using citric acid as a chelating agent. The influence of different parameters such as pH conditions, solvent, molar ratio of citric acid to total metal ions, calcination temperature, starting material on the structure, morphology and purity of this oxide was investigated. The results of sol-gel method show that pure $LiMn_2O_4$ with average crystallite size of about 130 nm can be produced from nitrate salts as starting materials at $800^{\circ}C$ for 6 h in air. The optimum pH and molar ratio of chelating agent to total metal ions are $4{\leq}pH{\leq}6$ and 1.0, respectively. A possible mechanism on the formation of the nanocrystallines synthesized by sol-gel was also discussed. At the end a comparison of the differences between two methods was made on the basis of x-ray diffraction (XRD), scanning electron microscopy (SEM), Fourier transform infrared spectroscopy (FTIR), and thermogravimetric analysis (TGA) tests.

• Journal of the Korean Electrochemical Society
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• v.9 no.4
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• pp.141-150
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• 2006

Nanocrystalline CoW thin/thick film alloys were electodeposited from citrate baths to investigate the influences of metal ion concentration, current density and solution pH on chemical composition, current efficiency, residual stress, surface morphology, and microstructure of the film. Deposit W (tungsten) content in CoW thin/thick film increased with increasing W ion concentration, current density, and solution pH in the plating bath. It was observed that residual stress in CoW thin/thick film decreased with increasing W ion concentration and solution pH. CoW thin film exhibited mixed phases of hop Co [(100) and (002)] and hcp $Co_3W$ [(002) and (201)] at W ion concentration with 0.02 to 0.08 M. The microstructure of CoW thin film at W ion concentration of 0.1 to 0.2 M was close to amorphous phase. The dominant phases were found to be hop Co (002) and hop $Co_3W$ [(200), (002) and (201)] at the current densities of 5, 10, 25, and $100mA{\cdot}cm^{-2}$ CoW thin film at the current densities of 50 and $75mA{\cdot}cm^{-2}$ was close to amorphous phase. At solution pH 8.7, CoW thin film exhibited hcp Co (002) and hop $Co_3W$ [(200), (002) and (201)]. Below solution pH 8.7, CoW thin film exhibited amorphous microstructure. The optimum electrodeposition conditions for CoW thin/thick film were found to be W ion concentration of 0.08 M, current density of $10mA{\cdot}cm^{-2}$, and solution pH 8.7.

• Proceedings of the Korean Vacuum Society Conference
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• 2011.08a
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• pp.75-75
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• 2011

Recently, dye-sensitized solar cell (DSSC) attracts great attention as a promising alternative to conventional silicon solar cells. One of the key components for the DSSC would be the nanocrystalline TiO2 electrode, and the control of interface between TiO2 and TCO is a highly important issue in improving the photovoltaic conversion efficiency. In this work, we applied various interfacial layers, and analyzed their effect in enhancing photovoltaic properties. In overall, introduction of interfacial layers increased both the Voc and Jsc, since the back-reaction of electrons from TCO to electrolyte could be blocked. First, several metal oxides with different band gaps and positions were employed as interfacial layer. SnO2, TiO2, and ZrO2 nanoparticles in the size of 3-5 nm have been synthesized. Among them, the interfacial layer of SnO2, which has lower flat-band potential than that of TiO2, exhibited the best performance in increasing the photovoltaic efficiency of DSSC. Second, long-range ordered cubic mesoporous TiO2 films, prepared by using triblock copolymer-templated sol-gel method via evaporation-induced self-assembly (EISA) process, were utilized as an interfacial layer. Mesoporous TiO2 films seem to be one of the best interfacial layers, due to their additional effect, improving the adhesion to TCO and showing an anti-reflective effect. Third, we handled the issues related to the optimum thickness of interfacial layers. It was also found that in fabricating DSSC at low temperature, the role of interfacial layer turned out to be a lot more important. The self-assembled interfacial layer fabricated at room temperature leads to the efficient transport of photo-injected electrons from TiO2 to TCO, as well as blocking the back-reaction from TCO to I3-. As a result, fill factor (FF) was remarkably increased, as well as increase in Voc and Jsc.

• Journal of the Korean Society for Heat Treatment
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• v.31 no.4
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• pp.171-179
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• 2018

Copper powder dispersed with 4 vol.% of $Cr_2O_3$ was successfully produced by a simple milling at 210 K with a mixture of $Cu_2O$, Cu and Cr elemental powders, followed by Hot Pressing (HP) at 1123 K and 50 MPa for 2h to consolidate the milled powder. The microstructure of the HPed material was characterized by standard metallographic techniques such as XRD (X-ray Diffraction), TEM and STEM-EDS. The results of STEMEDS analysis showed that the HPed materials comprised a mixture of nanocrystalline Cu matrix and $Cr_2O_3$ dispersoid with a homogeneous bimodal size distribution. The mechanical properties of the HPed materials were characterized by micro Vickers hardness test at room temperature. The thermodynamic considerations on the heat of formation, the incubation time to ignite MSR (Mechanically induced Self-sustaining Reaction), and the adiabatic temperature for the heat of displacement reaction between the oxide-metal are made for the delayed formation of $Cr_2O_3$ dispersoid in terms of MSR suppression. The results of TEM observation and hardness test indicated that the relatively large dispersoids in the HPed materials are attributed to the significant coarsening for the high temperature consolidation; this leads to the low Vickers hardness value. Based on the thermodynamic calculation for the operating processes with a limited number of parameters, the formation kinetics and coarsening of the $Cr_2O_3$ dispersoid are discussed.

• Nano
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• v.13 no.12
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• pp.1850141.1-1850141.11
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• 2018

Sodium ion batteries based on the more sodium source reserve than that of lithium have been designed as promising alternatives to lithium ion batteries. However, several problems including unsatisfied specific capacity and serious cyclic stability must be solved before the reality. One of the effective approaches to solve the abovementioned problems is to search for suitable anode materials. In this work, we designed and prepared $FeSe_2$ nanoflakes via a simple hydrothermal method which can be adjusted in composition by Fe precursor. As a potential anode for sodium storage, the optimized $FeSe_2$ electrode was further evaluated in different electrolytes of $NaClO_4$ in propylene carbonate/fluoroethylene carbonate and $NaCF_3SO_3$ in diethylene glycol dimethyl ether. The capacity was about $470mAh\;g^{-1}$ and $535mAh\;g^{-1}$ at $0.5A\;g^{-1}$, respectively, in the voltage between 0.5 V and 2.9 V in the cycle of stabilization phase. Superior performance both in capacity and in stability was obtained in ether-based electrolyte, which affords the property without plugging the intermediates of transition metal dichalcogenides during charge/discharge processes.

• Applied Chemistry for Engineering
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• v.22 no.2
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• pp.185-189
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• 2011

In this work, nano-sized M-ferrites (M=Co, Ni, Cu, Zn) for the decomposition of carbon dioxide were synthesized by the chemical co-precipitation. From the thermogravimetric analysis, it was clear that the maximum weight loss of each sample took place below $350^{\circ}C$. High temperature calcination resulted in more systematic crystallines, smaller specific surface area and larger particle size. An analysis by FTIR in the range of $375{\sim}406cm^{-1}$ revealed the presence of chelates at the octahedral site, which implies the formation of spinel structure in the ferrites. The current work showed that a $500^{\circ}C$ is the optimum heat treatment temperature of metal ferrites for $CO_2$ decomposition reaction.

• Journal of Korean Ophthalmic Optics Society
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• v.10 no.3
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• pp.193-203
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• 2005

Highly c-axis oriented nanocrystalline ZnO thin films on silica glass substrates were prepared by spin coating-pyrolysis process with a zinc naphthenate precursor. Only the XRD intensity peak of (002) phase was observed for all samples. With an increase in heat treatment temperature, the peak intensity of (002) phase increases. No significant aggregation of particle was present. From scanning probe microscopy analyses, three-dimensional grain growth, which was thought to be due to inhomogeneous substrate surface and c-axis oriented grain growth of the ZnO phase, was independent on heal-treatment temperature. Highly homogeneous surface of the highly-oriented ZnO film was observed at $800^{\circ}C$. All the films exhibited a high transmittance (above 80%) in visible region except film heat treated at $1000^{\circ}C$, and showed a sharp fundamental absorption edge at about $0.38{\sim}0.40{\mu}m$. The estimated energy band gap for all the films were within the range previously reported for films and single crystal. ZnO films, consisting of densely packed grains with smooth surface morphology were obtained by heat treatment at $600^{\circ}C{\sim}800^{\circ}C$, expected to be ideal for practical application, such as transparent conductive film and optical device.