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In order to develop Fe-based nanocrystalline soft magnetic alloys with high saturation magnetic flux density (B s ) and good manufacturability, the effect of the Nb content on the thermal stability, microstructural evolution and soft magnetic properties of Fe 78Àx Si 13 B 8 Nb x Cu 1 (x = 0, 1, 2 and 3) alloys were investigated. It is found that proper Nb addition is effective in widening the optimum annealing temperature range and refining the a-Fe grain in addition to enhancing the soft magnetic properties. For the representative Fe 76 Si 13 B 8 Nb 2 Cu 1 alloy, the effective annealing time can be over 60 min in the optimal temperature range of 500-600°C. FeSiBNbCu nanocrystalline soft magnetic alloys with desirable soft magnetic properties including high B s of 1.39 T, low coercivity (H c ) of 1.5 A/m and high effective permeability (l e ) of 21,500 at 1 kHz have been developed. The enhanced soft magnetic performance and manufacturability of the FeSiBNbCu nanocrystalline alloys are attributed to the high activated energy for the precipitation of a-Fe(Si) and the second phase. These alloys with excellent performance have promising applications in electromagnetic fields like inductors.

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Temperature and humidity are the main factors that cause the decomposition of Nomex paper widely used as turn‐to‐turn insulation material of mining dry‐type transformers. In this study, for understanding the effect of ambient humidity and thermal aging on the properties of Nomex paper, specimens with the initial moisture content of 7% were aged in the oven for 168, 504, 672 h at 180°C, respectively. After thermal aging, corona discharge experiments of unaged and aged specimens with different aging degrees were carried out at different relative humidity (RH) levels from 10 to 70%. The characteristic parameters of corona discharge, such as the average discharge amplitude and the number of discharges, were measured. Then, the surface conductivity and the surface potential decay rate were measured. The morphology and attenuated total internal reflectance Fourier transform infrared spectroscopy were also analysed for explaining the difference of properties between unaged and aged specimens. The experiment results indicate that the average discharge amplitude decreases with the increase of RH or the deepening of the aging degree of specimens, but the number of discharges shows the reversal tendency. The changes in the physical structure and chemical composition, which is induced by the hydrolysis reaction of Nomex paper, accelerate the surface potential decay and increase the surface conductivity.

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