Study On Thermal Stability Of Nanocomposite Nd₂Fe₁₄B/α-Fe Permanent Magnetic Materials

The low cost nanocomposite Nd₂Fe₁₄B/α-Fe magnet is a new kind of magnetic material developed since the end of 1980s. This magnet consists of two phases, magnetically hard Nd₂Fe₁₄B and soft a-Fe. The rotation of magnetic moments in the a-Fe phase is controlled by Nd₂Fe₁₄B phase through exchange coupling effect, which gives this magnet high remanence and high-energy product. However the low curie temperature of Nd₂Fe₁₄B phase limits its application in the field requring high temperatures. In recent years, a lot of researches have been done on the thermal stability of sintered NdFeB permanent magnetic materials and conventional bonded NdFeB permanent magnetic materials. But there is little research on the thermal stability of nanocomposite Nd₂Fe₁₄B/α-Fe magnetic material. In the current investigation, the influence of alloy composition on the thermal stability and magnetic property of the nanocomposite Nd₂Fe₁₄B/α-Fe magnetic material are investigated. The effect of additional elements on inoxidizability of magnetic material is also discussed.The main methods to improve working temperature of NdFeB include to improve room temperature coercivity and to improve Curie temperature. Zr dopants can refine the grain and significantly improves the room temperature coercivity. So most of the alloys in this study contain Zr. Firstly, the effect of Nd on magnetic property and thermal stability in nanocomposite Nd_xFe_93.xB₆Zr₁ (x=9-11at %) was investigated. The results showed that Nd₁₀Fe₈₃B₆Zr₁ exhibited an acceptable comprehensive magnetic property (a B_r value of 0.932T and a H_Cj value of 780kA/m) and an outstanding thermal stability: -0.154%/℃ for the temperature coefficient of remanence a and -0.39%/℃ for the temperature coefficient of coercivity β at 180℃, respectively. After the Nd content was fixed at 10at%, the effect of B on magnetic property and thermal stability in nanocomposite Nd₁₀Fe_93-xZr₁B_x(x=5, 6, 7, 8) was investigated. The results showed that remanence and the thermal stability of remanence decreased with the increase in the B content. For the coercivity and thermal stability of coercivity, morderate B content was found. The coercivity andthermal stability of coercivity increased, and then decreased with the increase in the B content. NdioFes2B7Zri exhibited an optimal coercivity and thermal stability of coercivity, a //cj value of 795kA/m at 25 °C and a -0.37%/°C for the p at 180°C, respectively.Based on the above results, the effect of Zr, Co, Dy, and Ga was studied. The results showed that NdioFe83ZriB6 exhibited a comprehensive property. It possessed a better remanence and coercivity and improved thermal stability. The Co dopants can significantly improve romanence and reduce a, however the coercivity decreased and P increased with the increase in the Co content. The Dy addition and the combined Dy and Co addition can significantly reduce a and p. The combined Dy and Co had a better effect on thermal stability of magnetic material than Dy addition only. NdioFe82.5B6ZriGao.5 exhibited a higher remanence and coercivity and can significantly reduce a and p.The effect of Zr, Nb, Co, and Dy adition on inoxidizability in the nanocomposite Nd2Fei4B/a-Fe magnetic material was studied by thermo gravimetric analysis (TGA). The results showed that Nb and Zr addition can improve inoxidizability of magnetic material. The combined Zr and Co additions can effectively improve anti-oxidation of samples. The Dy addition can slightly improve inoxidizability of magnetic material.