Papers by Author: Young Do Park

Abstract: The separator is one of the most important parts in PEM fuel cells. Stainless steels are widely used as separator for its good mechanical properties and mass production. However, for a good chemical compatibility, stainless steels need to have high chromium content or surface treatment, which makes separator high cost. Low cost of separator is important for commercial use. In this study, conventional low carbon steel is used as base metal of separator. Low carbon steel is low at cost, but has poor chemical properties for separator. For a good corrosion resistance, low carbon steel needs to be surface treated. To make a uniform surface treated layer on low carbon steel, chromium is conventionally electroplated on the steel and thermally nitrided. Surface treated low carbon steel is investigated using microstructure and element analysis tools. Interfacial contact resistance and polarization test is applied for the properties of fuel cell separator. The results show that chromium nitrided layer uniformly formed on low carbon steel. And the surface treated steel showed a good corrosion resistance as a separator.

Abstract: Utilizing alternating welding process parameters, deposition practices, and welding consumables, particularly during multiple pass welding, it is possible to improve a variety of weld metal properties. There are available a number of phenomena occurring during welding that allow weld metal designers the ability to generate macro- and micro-structural features amenable to implementation of composite theory. These phenomena include solidification microsegregation during dendrite growth, gas-metal reactions between the selected alternating shielding gas composition and weld pool, and solidification microstructural orientation during welding. Additional methods of producing composite welds including specially designed weld compositions, weld metal solidification modification by arc pulsing, and dual wire deposition may be utilized to achieve single pass and multipass composite weld metal deposition. Composite welds are a potential method to solve challenging demands such as high-toughness at low temperature, creep strength at high temperature, and customized design for corrosion, wear, or cracking resistance.

Abstract: Thermoelectric power coefficient measurement techniques have been developed for numerous applications to guarantee material integrity by providing a non-destructive electronic property correlation to material microstructure, phase stability, specific solute additions, and lattice strain. How the electron concentration, the effective mass, and the dominating scattering mechanisms allow for non-destructive evaluation of materials will be described. Because thermoelectric power (TEP) is dependent upon numerous variables, additional non-destructive techniques are necessary to further characterize or classify the material or weldment. The concept of an electronic metallography laboratory is developed using additional collaborative NDE technologies.

Abstract: P-type thermoelectric material Si0.8Ge0.2 was fabricated by mechanical alloying(MA) and Hot-Press Process(HP) The effect of Boron(0.25~2wt%) addition on the thermoelectric properties of p-type Si0.8Ge0.2 alloy was reported. Experiments showed that the electrical conductivity decreased with Boron content at temperature ranging from 500K to 1250K. The carrier concentration measured by the Hall effect measurement also decreased as a function of doping level. With increasing temperature, the Seebeck coefficient and the power factor increased with boron content. Based on measured results, the Figure of Merit (Z) value of 0.5 wt% Si0.8Ge0.2 alloy increased with the small addition of Boron, and reached maximum rapidly; the Z value was 0.9×10-3/K, the highest value among the prepared alloys.

Abstract: The effect of Al content on the microstructural characteristics, tensile and creep properties of Mg-xAl-Zn alloy was investigated. The molten Mg-(3~11)Al-Zn alloys of 700°C were poured into the permanent mould designed to prepare the specimens used in this study. Tensile and creep tests were carried out for each Mg-Al based alloys. Results show that creep resistance decreased while tensile strength improved with increasing Al contents. Microstructural study revealed that morphology and distribution of non equilibrium Mg17Al12 phase is main factor determining the tensile and creep properties of as cast Mg-xAl-Zn alloys. The final purpose of this study is to construct the database of already used Mg-Al based alloy and to develop the heat resistant magnesium alloys to apply transportation systems such as automobile.

Abstract: A copper matrix composite reinforced with in situ TiB2 nanoparticle was successfully fabricated by tubulent in-situ mixing process. The microstructure, mechanical and electrical properties of the in situ composite were investigated. The results showed that the in situ formed TiB2 particles, in which size varying from about 50nm to 200nm, exhibited a homogenous dispersion in the copper matrix. It is shown that the interface between the nanoscale particles and the matrix was clean without a transitional layer. Because of the reinforcement, the hardness and Young’s Modulus of the composite improved with increment of cooling rate. Moreover, the in situ Cu-TiB2 composite exhibited higher electrical conductivity with increasing of cooling rate.

Abstract: The direct extrusion process using the mechanically alloyed powder as raw materials was applied to prepare the thermoelectric materials. The mechanically alloyed powders of Ag added Bi1Sb3Te6 were extruded using pulse discharge sintering method at temperatures ranging from 345°C to 425°C. The fractographs of extruded specimens shows that no grain size changes was observed until 385°C, but became coarser with the increase of the extrusion temperature above 405°C. The X-ray diffraction patterns showed that the intensity of (110) plane increased with extrusion temperature up to 385°C and started to drop down above 405°C. The power factor value (PF) also increased with the extrusion temperature up to 385°C and altered above 405°C. These trends agreed with the variation in intensity of (110) plane of XRD patterns. The figure of merit (Z) value of the extruded specimens at 385°C was 3.1 × 10-3 W/K, which was the highest value among the prepared specimens.

Abstract: A Cu-TiB2 composite was successfully fabricated by in-situ liquid mixing process, and its microstructure, mechanical properties as well as electrical conductivity were evaluated. For Cu-2vol.%TiB2 composite, the hardness was as high as 5GPa and the Young’s modulus was 130GPa. And hardness and Young’s modulus of Cu-6vol.%TiB2 composite was 5.6Gpa and 138GPa, respectively. With the increase of the TiB2 content, hardness and Young’s modulus of Cu-10vol.%TiB2 composite were 20 and 12%, respectively, which was higher than that of Cu-2vol.%TiB2 composite. Young’s modulus of the Cu-TiB2 composite in this paper was in good agreement with the prediction by Hashin-Shtrikman (H-S) model. Furthermore, the electrical conductivity of the Cu-TiB2 composite showed its maximum value of about 78%IACS and decreased with the increase of the TiB2.

Abstract: The n-type Bi2(Te0.94Se0.06)3 thermoelectric compound was prepared by the direct extrusion process using the powder as raw materials. Hot extruded specimens were annealed at 200°C and 350°C for 2hrs. The electrical conductivity, thermoelectric power and thermal conductivity of hot extruded and annealed specimens were measured at room temperature. The fractographs of the specimens showed that the grain size became coarser and a lot of porosities were generated during annealing process. The power factor value (PF) decreased with increasing the annealing temperature. The electric conductivity and thermal conductivity of the specimens also decreased with the increase of annealing temperature. This may be reasoned that the generated porosities affect the thermal conductivity of the specimens prepared in this study. The figure of merit (Z) value of the annealed specimens at 350°C was improved about 10%. The highest Z value of the specimens annealed at 350°C was 2.0 x 10-3/K among the prepared specimens.