To establish the design of sintered iron yoke used for speaker, various factors of dimension, shape, surface treating, history of iron powder raw materials, and gap between the yoke and permanent magnet core were studied.
The aging properties of B₅, B₂₅, B₁₀₀and the temperature variation of μm, Hc, and B₂₅ were also studied.
NKS-7, NKS-5 type Alnico magnet were used for permanent magnet cores.
All the yokes except for ring specimens were prepared by cutting from sintered iron of 25φ×20mm block.
Specimens used for surface treating test were heated in an atmosphere of steam. Specimens for B₅, B₂₅, B₁₀₀, Bg and φ_T are of ring and pot type.
These specimens were formed by the pressure of 6t/cm² and sintered in the atmosphere of cracked ammonia gas for 3hr at 1100°C.
The yoke of high density contains about 1.0% (wt) of phosphor. Gap flux density (Bg) of pot type yokes are correlated to the dimension of side and of the thickness of bottom ; it sharply decreases at a certain dimension.
The yoke with hole in the bottom has also similar properties.
Yokes heated in the steam has no variation in Bg and φ_T.
Bg or φ_T values were not so much affected by the history of iron powder raw materials and their absolute values were slightly lower than those of carbon-steel (S10C).
Temperature dependence of μm, Hc and B₂₅ were quite similar with those of S10C and exhibits no aging in flux density B₂₅, B₁₀₀