Hyunik Yang

Strained gallium nitride nanowires

Gallium nitride nanowires were synthesized on silicon substrates by chemical vapor deposition using the reaction of gallium and gallium nitride mixture with ammonia. Iron nanoparticles were used as catalysts. The diameter of nanowires is uniform as 25 nm and the lengths are 20–40 μm. The nanowires have single crystalline wurtzite structure with a few stacking faults. A careful examination into x-ray diffraction and Raman scattering data revealed that the separations of the neighboring lattice planes along the growth direction are shorter than those of bulk gallium nitride. The nanowires would experience biaxial compressive stresses in the inward radial direction and the induced tensile uniaxial stresses in the growth direction. The shifts of the band gap due to the stresses have been estimated using the experimental data, showing that the reduction of the band gap due to the tensile stresses can occur more significantly than the increase due to the compressive stresses. The temperature-dependent photolumi...

Single-crystalline gallium nitride nanobelts

Single-crystalline wurtzite gallium nitride nanobelts were synthesized by thermal reaction of gallium, gallium nitride, and ammonia using iron and boron oxide as catalysts. The structure of nanobelts was investigated by high-resolution transmission electron microscopy with electron energy-loss spectroscopy. They have a distinctive triangle tip and thick side edges. The widths are 200–300 nm, the thickness of belt plane is about 1/10 of the width, and the lengths are up to a few tens μm. The growth direction is uniformly perpendicular to the [010] direction.

Synthesis of silicon nitride nanowires directly from the silicon substrates

We present a novel method to synthesize high-density silicon nitride nanowires directly from the silicon substrates via a catalytic reaction under ammonia or hydrogen flow at 1200 °C. Gallium, gallium nitride, and iron nanoparticles deposited on the silicon substrate were used as catalysts. Gallium nitride can act as a nitrogen source under hydrogen flow. The average diameter of the nanowires is 40 nm and their length is about 300 μm. The silicon nitride nanowires consist of a defect-free single-crystalline α-phase crystal grown with various growth directions.

Triangular gallium nitride nanorods

Gallium nitride nanorods were synthesized by a chemical vapor deposition using the reaction of gallium/gallium nitride with ammonia. All nanorods have, exclusively, a triangle cross section with an average diameter of 50 nm. They consist of single-crystalline wurtzite structure crystal grown with the [010] direction. X-ray diffraction and Raman spectroscopy data suggest no shift of the lattice constants from those of the bulk. Temperature-dependent photoluminescence exhibits the I2 and free-to-bound emission peaks. The present triangular gallium nitride nanorods would be free from the stress, having the band-gap energy of the bulk.

Optimal Design of a Hybrid Composite Flywheel with a Permanent Magnet Rotor

A design method to optimize the dimensions of a hybrid composite flywheel rotor and a permanent magnet is presented. The flywheel rotor consists of multiple rims of advanced composite materials with a permanent magnetic rotor attached inside the flywheel. The pressure distribution at the inner surface from the centrifugal forces of the magnet is considered together with the centrifugal body forces in the flywheel rotor. The size of the magnet also maintains the required induced voltage. An analytical solution of the displacements and stresses for each ring has been obtained and expressed in terms of a symmetric stiffness matrix. Using the stiffness matrix, the continuity conditions between the rings and the pressure due to the magnet can be easily considered. An optimum design is thus performed maximizing the total stored energy (TSE) with the size of the magnet and the thickness of each composite ring as design variables. For that purpose, the sensitivities of TSE and the Tsai-Wu failure index with respect to the design variables are derived by using the global stiffness matrix and the relationship of the magnetic size and the induced voltage. As a result, the optimal design using the hybrid composite rims has significantly reduced the stresses in a rotor and attained about 2 times the total energy of cases of using each material alone. The present optimization approach can be used to develop an efficient flywheel rotor for various applications.

Direct synthesis of aligned silicon carbide nanowires from the silicon substrates

Aligned silicon carbide nanowires were synthesized directly from the silicon substrates via a novel catalytic reaction with a methane-hydrogen mixture at 1,100 degrees C, with a mean diameter of 40 nm and length of 500 microm; they consist of a single-crystalline zinc blende structure crystal in the [111] growth direction; X-ray diffraction, Raman, and infrared spectroscopy confirm the synthesis of high-purity silicon carbide nanowires.

Design of a Pedestrian Protection Airbag System Using Experiments

The safety of vehicles has been studied in the past on the basis of the safety of passengers. Recently, pedestrian safety has also attracted attention. The pedestrian protection performance of vehicles has gradually improved but is still not sufficiently good. Devices such as the pop-up hood and the pedestrian protection airbag system are generally used to protect pedestrians. Since these devices are relatively newer than other parts of an automobile, a standard design process has not yet been rigorously established. There have been studies that demonstrated how to determine the design parameters of the devices using computer simulations. In this research, a novel design process for a pedestrian protection airbag is proposed, which utilizes the results of the experiments. Some parameters of the pedestrian protection airbag system, which cannot be determined by simulations, are selected and obtained using experiments. They are the folding method of the airbag, the height type of the airbag and the inflator type. The design results using simulations are incorporated in the initial design of the proposed process. The experimental facilities are prepared on the basis of a mid-sized passenger vehicle, and the headform tests are performed according to the protocol of the European New Car Assessment Programme test. The proposed design process selects the rolling and folding method, the small height type and the bottom spurt inflator type. The resultant design is evaluated and analysed using experiments, and it is found to be satisfactory.

An improved dynamic model of friction draft gear with a transitional characteristic accounting for housing deformation

ABSTRACT A white-box friction draft gear model has been developed. All components of the draft gear are considered. The distinctive feature of the model, as compared to its predecessor, is the transitional characteristic, which accounts for the effect of elastic deformations of the draft gear housing on the position of the friction wedge system components under loading. The adjustment of the model parameters for improved agreement with experimental data is discussed. The new model can be used in the simulation of shunting impacts for single cars and car groups represented by detailed finite-element models. An example of the simulation is presented and compared with experimental data obtained using a shunting hump test stand.

Wear simulation for the centre plate arrangement of a freight car

The bodies of many railway freight cars in many countries of the world are coupled to the running gear by means of a body centre plate that makes a friction pair with a centre bowl. During motion, the bogie is rotated and moved with respect to the car body. This leads to wear on the contact surfaces. Lubrication is inexpedient in this case because the friction forces damp the vibrations (so-called bogie hunting) during motion. Usually, centre plates exhibit noticeable wear after two years of operation. Reducing wear requires knowing details of the wear process which, in turn, requires computer simulation of freight car motion for an operation period of 10–15 years. The purpose of this paper is to develop a universal method for wear simulation of friction pairs that could be used, in particular, for the centre plate of a freight car.

Collisional quenching of vibrationally excited methyl-substituted pyrazine and pyridine series by CO2

Abstract Collisional quenching of vibrationally excited methyl-substituted pyrazine and pyridine molecules by CO 2 has been investigated using the infrared emission of C–H stretching vibrations in gas phase. The methyl substitution increases the collisional quenching efficiency, but its enhancement effect decreases as the number of methyl groups increases. The pyrazine series are on average more efficiently quenched compared to the pyridine series. The quenching efficiency of dimethylpyrazine and dimethylpyridine isomers depends on the position of two methyl groups. We suggest that the collision pathways involving the ring nitrogen atom may be the efficient quenching channels.