Sung Kang

Synthesis and microstructure of zirconia nanopowders by glycothermal processing

ZrO2 nanoparticles were prepared under high temperature and high pressure conditions by precipitation from metal nitrates with aqueous potassium hydroxide. The effects of synthesis parameters, such as the concentration of starting solution, pH of starting solution, reaction temperature and time, were discussed. The results show that the ZrO2 nanoparticles are obtained at 230−270 ℃. The average size and size distribution of the synthesized particles are below 10 nm and narrow, respectively. The XRD pattern shows that the synthesized particles are composed of crystalline. The synthesis of ZrO2 nanosized crystalline particles is possible under glycothermal conditions in ethylene glycol solution.

Magnetic and magnetoresistive properties related to microstructure in Cu75–Fe5–Ni20 alloys

The electromagnetic properties and microstructures of a Cu75?Fe5?Ni20 alloy have been investigated on isothermal annealing at 1073?K, using a superconducting quantum interference device magnetometer, quantum design, physical property measurement system and transmission electron microscopy. Nanoscale magnetic particles were formed randomly in the Cu-rich matrix after receiving a short annealing due to the phase decomposition in the alloy. With increasing isothermal annealing time, however, rod-type precipitates aligned along the 1?0?0 directions were observed in the matrix, on isothermal annealing at 1073?K. Although the size of the precipitates became larger (from ~10?nm to >300?nm) after further annealing, no significant change (less than 2%) was detected in the MR value. The largest MR value (MR ~16% at H = 7?T and T = 10?K) was attained, in particular, for the as-quenched specimen. This study revealed that several significant influences were introduced into the magnetic and magnetoresistive properties during the phase decomposition process in the Cu?Fe?Ni alloy.

Microstructure Affecting Magnetoresistance of a Cu75?Fe5?Ni20 Alloy

The electromagnetic properties of a Cu75–Fe5–Ni20 alloy have been investigated upon isothermal annealing of the alloy at 873 K, using a superconducting quantum interference device (SQUID) magnetometer and a physical property measurement system (PPMS) instrument. Magnetoresistance (MR) was closely related to the magnetization value in specimens of the alloy. The most striking finding of the present work is that the as-quenched specimen with no visible precipitates attains the largest MR (~16% at H=7 T and T=10 K), although fine precipitates with a proper size were thought to be essential in our previous studies. The present investigation also revealed that several significant effects accompanied the magnetic and magnetoresistive properties, with microstructural evolution occurring during the phase decomposition of a Cu75–Fe5–Ni20 alloy.

Precipitation Behavior and Magnetic Properties of Nano-Scale Particles in a Cu-Fe-Ni Alloy

The precipitation behavior of nano–scale magnetic particles formed in a Cu–Fe–Ni alloy on isothermal annealing at 873K and 1073K have been investigated by means of transmission electron microscopy (TEM). Nano–scale magnetic particles were formed randomly in the Cu–rich matrix after receiving a short annealing due to phase decomposition in the alloy. With increasing the isothermal annealing time, however, the striking features that two or more nano–scale particles with a cubic shape and precipitates with a needle shape were aligned linearly along <100> directions were observed on isothermal annealing at 873K and 1073K, respectively. To investigate the relationship between microstructures and magnetic properties of precipitates in a Cu–Fe–Ni alloy were also carried out the superconducting quantum interference device (SQUID) magnetometer. The present study revealed that several significant influences to magnetic properties were induced during the precipitation process in this alloy.

Microstructural evolutions of a Cu75-Fe5-Ni20 alloy depending on the isothermal annealing temperatures

The microstructures formed in a Cu75-Fe5-Ni20 alloy on isothermal annealing at 1073 K were investigated by means of transmission electron microscopy (TEM) and the EELS element mapping technique, to compare with the precipitation phenomena occurring at 873 K. Cubic and rod precipitates were simultaneously formed in the copper matrix in the initial stage of annealing at 1073 K. This feature shows a sharp contrast to the feature observed at 873 K, since only cubic precipitates were observed in the specimens isothermally aged at lower temperatures. The cubic precipitates tend to stretch their shapes along one of the 〈100〉 directions of the Cu matrix and to have rectangular shapes in the late stage of the annealing at both temperatures. This tendency was, however, more clearly observed in the specimen receiving the isothermal annealing at 1073 K.

TEM study and magnetic measurements of precipitates formed in Cu-Fe-Ni alloy

The precipitation behavior of Cu-Fe alloys with Ni addition on isothermal annealing at 878K was investigated by means of transmission electron microscopy (TEM) and SQUID measurements. Nano-scale coherent particles were randomly formed at the initial stage of the precipitation process. The ordering trend was outstanding that two or more nano-scale particles cubic in shape are linearly aligned at later stages of the precipitation. Finally linear chains of the precipitates were formed along the ≪100≫ orientations of Cu matrices in the ternary Cu-Fe-Ni alloys. The present SQUID measurements revealed several significant influences to magnetic properties were induced during the precipitation in Cu-Fe-Ni alloys.

TEM Study and Magnetic Measurements of Nano–Scale Particles Formed in Cu–Base Alloys

The precipitation behavior of nano–scale particles formed in Cu–base alloys was studied by means of transmission electron microscopy (TEM) and SQUID measurements. Linear arrangements of two or more nano–scale particles cubic in shape were observed in the <100> orientations of matrices in a Cu–Co alloy. Although the trend was less explicit in a Cu–Fe alloy, Fe precipitates accompanying twin–like lattice modulations were found in the decomposition, when no deformation was applied. The present SQUID measurements revealed several significant influences to magnetic properties were induced during the precipitation in Cu–base alloys. Lorentz electron microscopy confirmed that phase transformation from γ → α occurred at the stage that the Fe particles reach to 40~60nm in size.

Formation of Boride Eutectic with Addition of Boron in Mod. 12Cr-1Mo Steels

The behavior of boride formation has been examined with solidification rates and boron contents by observing the solid/liquid interface using directional solidification in Mod. 12Cr-1Mo alloys. The phase transformation temperatures of liquidus, solidus, eutectic formation, and final solidification were analyzed by DTA. In order to analyze the crystallographic structure of the boride, extractive method, extracting the boride from the matrix, was used. The boride was proved as M3B2, and this boride phase expected to be formed by eutectic reaction. It has been also found that the boride formation depends on contents of born as well as the other alloying elements, such as C and W. As increasing boron contents, the amount of boride eutectic increased. Also, the alloying element W was shown to enhance the formation of the boride.