Study on Carburization of Th-W Filament (2nd Report) Cold Electric Resistance and Operating Temperature by Carburization

Abstract

From the result in the 1st report, we found that it was suitable to adopt the range of 2.45∼3.16% as the carbon content in the carbide layer. Theoretically, we led the following formula representing the relation between the degree of carburization (S) and the ratio of increase in cold electric resistance (λ), λ=\cfrac11-S \left(1-\cfracρ_Wρ_W_2C\ ight)\labeleq1
where ρ_W and ρ_W2C are the cold electric specific resistances in the uncarburized core and the carburized sheath.
If the carbide layer is all of W₂C composition, the value of ρ_W⁄ρ_W2C is 0.07, but ρ_W⁄ρ_W2C is experimentally about 0.3 . The operating temperature of the carburized filament is not directly determined , because the total emissivity of W₂C has not been correctly found heretofore, so that using the actual data in the transmitting tube working, the following formula was theoretically determined. R_θ=fo \cfrac1-S \left(1-\cfracρ_Wρ_W_2C\ ight)1-S \left(1-\cfracρ_W(T)ρ_W_2C(T)\ ight)\labeleq2
where R_θ f_θ is either the ratio of the hot-to-cold electric resistance in a Th-W filament carburized, or not respectively, and ρ_W(T), ρ_W2C(T) is ρ_W,ρ_W2C at operating temperature T respectively. At any time, the degree of carburization is obtained from the formula (1)by measuring the cold electric resistance and the operating temperature of the carburized Th-W filament is known from the formula (2). The expansion of filament by carburization is increasing as ThO₂ content in W is decreasing, but the amount is negligibly small.