The role of dysprosium, terbium, gadolinium, holmium and other elements in sintering neodymium iron boron

As the name suggests, sintered neodymium iron boron is an alloy material based on the compound Nd2Fe14B composed of three elements: neodymium Nd, iron Fe, and boron B. However, sintered neodymium iron boron is not single-phase, it consists of Nd2Fe14B phase, B-rich phase (also known as Nd1.1Fe4B4 phase), and Nd rich phase (also known as rare earth rich phase), with Nd2Fe14B phase as the main phase or basic term.

Most rare earth elements (RE) form RE 2Fe14B compounds, which are the basic phase of sintered rare earth iron boron permanent magnet materials, accounting for 96% -98% of sintered rare earth iron boron permanent magnets. All RE 2Fe14B compounds have the same crystal structure, but their magnetic properties differ greatly, which means that adding other rare earth elements instead of neodymium in sintered neodymium iron boron can change certain properties of the magnet.

The role of heavy rare earth metal Dy as a substitute for Nd

1. Significantly improve the coercivity of magnets

The anisotropic field HA of Dy 2Fe14B compound is about 2.14 times higher than that of Nd2Fe14B, so replacing Nd with a small amount of Dy can significantly improve the coercivity Hcj of the magnet. In theory, replacing Nd with 1% (atomic fraction) Dy can increase the coercivity Hcj of the magnet by 11.4kA/m. However, in practical applications, the increase in coercivity Hcj is related to the presence of other components.

2. Reduce the magnetic polarization intensity Js of the magnet, thereby reducing the residual magnetism Br and maximum magnetic energy product (BH) m

In theory, for every 1% (atomic fraction) Dy used instead of Nd, the magnetic polarization strength Js of the magnet decreases by 90mT

3. Reduce the temperature coefficient of residual magnetism Br and maximum magnetic energy product (BH) m of the magnet

It should be noted that adding heavy rare earth element Dy will significantly increase the raw material cost of sintered neodymium iron boron permanent magnets, so the relationship between cost and magnet performance needs to be comprehensively considered.

The role of heavy rare earth metal Tb as a substitute for Nd

Adding Tb to partially replace Nd in sintered neodymium iron boron magnets has the same effect as Dy replacing Nd mentioned above, but Tb2Fe14B has a higher anisotropic field HA, which can effectively improve the coercivity of permanent magnets. But Tb has less reserves in rare earth minerals than Dy and is more expensive.

The role of metal Gd and metal Ho as substitutes for Nd

Gd has the highest reserves among heavy rare earth metals and can also form Gd2Fe14B compounds. The magnetic polarization strength Js and anisotropic field HA of this compound are significantly lower, but its Curie temperature Tc is the highest. Due to the high reserves and low price of Gd, some manufacturers add Gd in the form of gadolinium iron alloy to partially replace Nd, in order to manufacture low-cost sintered neodymium iron boron. But its practical use of Gd instead of Nd is a waste, and once Gd is found to have more important uses in the future, it will be found to be an irreparable loss. Replacing Nd with Ho also has the same effect and problem.

The role of light rare earth metals La, Ce, and Pr as substitutes for Nd

The reserves of light rare earth elements are abundant and the prices are relatively cheap. Developing the application of light rare earth metals in the manufacturing of sintered neodymium iron boron materials is worth encouraging.

The formation of La2Fe14B from La, Fe, and B metals is relatively difficult and the temperature is very narrow, but once formed below 860 ℃, it is stable. Nd accounts for 65% -75% of the cost of sintering neodymium iron boron, while the current cost of La is about one tenth of Nd. Replacing Nd with La can reduce costs and promote the comprehensive utilization of rare earth resources. With the increase of La content, the magnetic polarization strength Js, remanence Br, coercivity Hcj, and maximum magnetic energy product (BH) m of the alloy will all decrease. La is a non-magnetic atom, and due to the magnetic dilution effect, the decrease in (BH) m is much faster than that of Br.

The stability of Ce2Fe14B is poor, making it more difficult to form. As the Ce content increases, all magnetic properties decrease, and the addition of Ce can lead to a decrease in the Curie temperature and temperature stability of the magnet.

The Pr2Fe14B compound has several basic conditions for use as a permanent magnet material, and good magnetic properties can be obtained by sintering at around 1060 ℃. Sintered neodymium iron boron permanent magnets with excellent magnetic properties can be manufactured using (PrNd) - Fe metal as raw material. It should be noted that Pr is more prone to oxidation than Nd, and the amount of Pr should be appropriately controlled for certain materials that require high stability.

The role of other metals replacing Fe

The low coercivity and Curie temperature, poor temperature stability, low operating temperature (about 80 ℃), and poor corrosion resistance of sintered neodymium iron boron permanent magnet materials limit their application range. Therefore, people have extensively studied the effects of various elements on neodymium iron boron permanent magnet materials.

1. The effect of cobalt Co partially replacing Fe on sintered neodymium iron boron

With the increase of Co content, the Curie temperature of the alloy linearly increases, and the reversible temperature coefficient α of magnetic induction significantly decreases. When the Co content is less than 5% (atomic fraction), (BH) m and Br hardly decrease; When the Co content is greater than 30%, various magnetic performance parameters significantly decrease. The addition of Co content less than 10% is very beneficial, as it not only increases the Curie temperature of the alloy, but also maintains high magnetic properties, and improves the temperature coefficient of magnetic induction.

2. The role of Al partially replacing Fe

The research results of scholars indicate that adding a small amount of aluminum Al can significantly improve the coercivity of ternary Nd-Fe-B materials. The research results indicate that adding Al to Nd-Fe-Co-B permanent magnet material can compensate for the decrease in coercivity caused by Co addition, thus obtaining Nd-Fe-Co-Al-B alloy with high comprehensive performance.

3. The role of Cu partially replacing Fe

Research has found that adding a small amount of copper (Cu) to the (Nd, Dy) - Fe-B and (Nd, Dy) - (Fe, Co) - B systems can significantly increase their coercivity, while Br hardly decreases, thus enabling the production of high Hcj and high (BH) m permanent magnets.

4. Partial substitution of Fe by other elements

Adding a small amount of niobium Nb or zirconium Zr to partially replace iron on the basis of ternary Nd-Fe-B alloy can effectively improve the Hcj and squareness Hk of the alloy, while the decrease in Br is minimal, and the magnetic flux of the alloy cannot be lost. The experimental results show that the highest content of niobium Nb in Nd-Fe-B alloy is 3% (atomic fraction). Adding excessive Nb will rapidly decrease the coercivity of the alloy and make Nd2Fe14B unstable

The addition of gallium (Ga) can significantly improve the coercivity of the alloy and reduce the irreversible magnetic field hirr. In Nd-Fe-Co-B alloy, as the Co content increases, the Hcj of the alloy decreases. However, with the addition of Ga, the coercivity increases. It is expected that Nd-Fe-B permanent magnet materials with high Curie point and high Hcj may be prepared in alloys with added Ga

The composite addition of gallium Ga and niobium Nb can significantly improve the temperature stability of the alloy

This article mainly refers to the 2014 edition of "Sintered NdFeB Rare Earth Permanent Magnet Materials and Technology" by Zhou Shouzeng et al