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Ring rolling process is usually used to fabricate large‐sized ring, such as, tower flange for wind power electric generator. Many kinds of seamless ring are used in wind power electric generator and manufactured by ring rolling process. In general, final part is machined after forming with shape of plain square section. Since interests for near net shaping of seamless ring have been increased gradually because of green energy, it is necessary to develop the technology for shape ring rolling with respect to the market demands and cost. Therefore, we studied the process and die design for shape ring rolling of large sized ring over 3,500 mm out diameter by experiment and FEM simulation. Ring rolling process is very difficult to solve by FEM method because of equilibrium state and size effect, etc. Moreover, shape ring rolling is more difficult to solve the problem that two plastic deformation zones are different each other, that is main roll and conical roll. Also since conical roll has a shape, deformation velocity field is very much complex and the deformed section passed axial roll is different section and velocity field. The FE simulations are performed to analyze process variables affected in forming of profiled ring. Therefore, the main features of used FE model are: (1) it adopts a transient or unsteady state full ring mesh to model the deformation processes and shape development; (2) the mandrel and conical rolls are modeled using coupled heat‐transfer elements; (3) the model involves the full process from blank through perform to final profiled ring. From these calculated results, we have proposed the mechanisms of various tools, such as mandrel and conical roll. The calculated results are compared experimental results. Calculated results can predict the tilting of profiled ring and then process variables to form large sized ring.

We report on the study of the low‐frequency excess noise of a GaN nanowire (NW) device from room temperature to 135 K. A Lorentzian noise is found to be embedded in the 1/f noise background, and becomes more significant as the bias current increases or the temperature decreases. The temperature dependance of the associated characteristic time, extracted from the spectrum, shows two thermally activated regions with different activation energies, which originate from different carrier‐trapping levels in the NW device. The temperature dependence of the 1/f noise is difficult to unveil due to the presence of the strong Lorentzian noise.

We report the temperature (T) dependance (77K to 300K) of the low‐frequency electric noise of GaN nanowires. Our results show that these GaN nanowires exhibit the 1/f‐like excess noise. A Lorentzian‐like feature is observed embedded in the 1/f noise when the applied bias current is large enough. Our four‐wire measurement results reveal that the characteristic time τL of the Lorentzian‐like noise of the nanowire decreases with T.

We have designed and fabricated a photocathode test-stand in order to develop the polarized electron source in Korea. The test-stand consists of a photocathode gun, a light source, a cylindrical condenser and a mini-Mott chamber. We report the status of the test-stand and the future plan for the development of a polarized electron source.