Electrical Resistance Evaluation Reliability under High Grade Temperature for Na-AMTEC(Natrium Alkali-Metal Thermal-to-Electric Convertor)

다국어 초록 (Multilingual Abstract)

In general, an electrical circuit, electrical resistance is a measure of the opposition to current flow. Resistance is a measured in ohms, symbolized by the Greek letter omega(Ω). In practical, specialized on Na-AMTEC(Natrium Alkali-Metal Thermal-to-Electric Convertor), the input energy is directly converted to electrical energy. In microscopic view, the energy transforming of one source to another useful forms classified as storage, transportation and conversion is simultaneous taken place in one small circuit. In macroscopic view, thermal energy directly converted into electrical energy, in other words it is energy transforming primarily solar-to-thermal and thermal-to-electric.
In case of metallic substances, free electrons are moving randomly in the crystal structure of it. Due mainly to the electric field across the resistance, free electrons drift from lower potential point to higher potential when voltage is applied. Free electron continually collides with atoms of the substance during drifting motion, this phenomenon prevents the free motion of electrons. The resistance is known to be caused by the collision of free electron with atoms of the substance. Physicist George Simon Ohm suggest the law ohm that electric resistance is equal to voltage per ampere on pure metal. Hence, resistance is defined as the ratio of the applied voltage to the current through the substance. Most of metallic substance with rising temperatures the inter atomic vibrations increase and consequently offer more resistance to the movement of electrons causing the current. Thus, with increasing temperature the resistance of metallic substances increases. Refractory metals are a class of metals that are extraordinarily resistant to heat. The well-known Na-AMTEC is working between 800 K to 1300 K. Most of materials does not meet the criteria of aforementioned working condition in sense of thermo-condition except the five elements two of fifth period (niobium and molybdenum) and three of the sixth period (tantalum, tungsten and rhenium). They all share some properties, including a melting point above 2,200 K and high hardness at room temperature, except ductile transition character of niobium. Pure niobium has a Mohs hardness rating similar to that of pure titanium.
Hence, the selection of candidate materials for Na-AMTEC as electrical electrode, current collector and lead is narrowed down to the minimum requirement of temperature resistance and electrical conductivity. In case of electrical resistance of Na-AMTEC is derived as following that the total resistance, R_T of Na-AMTEC is sum of the following resistances R_B (Resistance of Beta-Alumina Solid Electrolyte, BASE) + R_S (sheet resistance) + R_C (contact resistance) + R_L (lead resistance). Mostly, the sum of the rest of resistances is larger than the resistance of BASE.
● Correlation of Voltage, Ampere and Resistance: V ∝ I , V = RI , R= V/I
● Total electrical resistance of Na-AMTEC: R_T = R_B + R_S + R_C + R_L, R_B < R_S + R_C + R_L
● Correlation of resistances of Na-AMTEC: R_T ∝ 2 R_B
Nonetheless, there is a reliability issue when measuring the electrical resistance which is intricate compared to the concept of electrical conductivity measurement on room temperature by virtue of the continuous temperature deviation during the Na-AMTEC cell test under elevated temperature. This study is to report the effort to develop the methodology of measurement reliability of the electrical resistance under high temperature state.