The function of rare earth elements in NdFeB
Sintered NdFeB, as the name implies, is an alloy material
composed of Nd2Fe14B, a compound composed of three elements: Nd, Fe, and B.
However, sintered NdFeB is not a single phase. It consists of Nd2Fe14B phase
and B-rich phase (also known as Nd1 .1Fe4B4 phase) and Nd-rich phase (also
known as rare-earth-rich phase), of which the Nd2Fe14B phase is 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 are very different. Adding other rare earth elements to sintered NdFeB to replace neodymium can change some properties of the magnet.
The role of heavy rare earth metal Dy instead of Nd
1. Significantly improve the coercivity of the magnet
The anisotropy 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 increase the coercive force Hcj of the magnet. Theoretically, every time 1% (atomic fraction) Dy replaces Nd, the coercive force Hcj of the magnet can be increased by 11.4kA/m, but the increase in coercive force Hcj in practical applications is related to the existence of other components.
2. Reduce the magnetic polarization intensity Js of the magnet
Thereby reducing the remanence Br and the maximum magnetic energy product (BH) m
In theory, every time 1% (atomic fraction) Dy replaces Nd, the magnet's magnetic polarization intensity Js decreases by 90mT
3. Reduce the temperature coefficient of magnet remanence Br and maximum magnetic energy product (BH) m
It should be noted that the addition of heavy rare earth element Dy will significantly increase the raw material cost of sintered NdFeB permanent magnets, so the relationship between cost and magnet performance needs to be comprehensively considered.
The role of heavy rare earth metal Tb instead of Nd
Adding Tb to the sintered NdFeB magnet to partially replace Nd has the same effect as the replacement of Nd by Dy, but the anisotropic field HA of Tb 2Fe14B is higher, so it can more effectively improve the coercivity of the permanent magnet. But Tb has less reserves in rare earth mines than Dy, and the price is higher.
The role of metal Gd and metal Ho replacing Nd
Among the heavy rare earth metals, Gd has the highest reserves, and Gd can also form Gd2Fe14B compounds. The magnetic polarization intensity Js and anisotropic field HA of this compound are obviously lower, but its Curie temperature Tc is the highest. Due to the high reserves of Gd and the low price, some manufacturers add Gd in the form of gadolinium-iron alloy to partially replace Nd to produce low-cost sintered NdFeB. However, its practical use of Gd to replace Nd is a waste. Once it is discovered that Gd has more important uses in the future, it will be found to be an irreversible loss. Replacing Nd with Ho has the same effect and problem.
The role of light rare earth metals La, Ce, Pr instead of Nd
The reserves of light rare earth elements are abundant and the price is relatively cheap. The development of light rare earth metals for the manufacture of sintered NdFeB materials is worth encouraging.
La, Fe, and B metals are difficult to form La2Fe14B, and the temperature is very narrow, but once formed, it is stable below 860°C. Nd accounts for 65%-75% of the cost of sintered NdFeB. At this stage, the cost of La is about one-tenth of Nd. Substituting La for Nd can reduce costs and promote the comprehensive utilization of rare earth resources. With the increase of La content, the magnetic polarization intensity Js, remanence Br, coercive force Hcj and maximum magnetic energy product (BH) m of the alloy will all decrease. La is a non-magnetic atom. Because of magnetic dilution, (BH)m decreases It decreases much faster than Br.
Ce2Fe14B has poor stability and is more difficult to form. With the increase of Ce content, various magnetic properties are reduced. At the same time, the addition of Ce will cause the Curie temperature and temperature stability of the magnet to decrease.
The Pr2Fe14B compound has several basic conditions that can be used as a permanent magnetic material. It can be sintered at about 1060°C to obtain better magnetic properties. Using (PrNd)-Fe metal as a raw material can produce sintered NdFeB permanent magnets with good magnetic properties. It should be noted that Pr is easier to oxidize than Nd, and the amount of Pr must be appropriately controlled for some materials that require high stability.
The role of other metals replacing Fe
The low coercivity and Curie temperature of sintered NdFeB permanent magnet material, poor temperature stability, low working temperature (about 80℃), poor corrosion resistance and other shortcomings limit its application range. For this reason, people the effects of various elements on NdFeB permanent magnet materials have been extensively studied.
1. The effect of cobalt-Co partial substitution of Fe on sintered NdFeB
With the increase of Co content, the Curie temperature of the alloy increases linearly, and the reversible temperature coefficient of magnetic induction decreases significantly. When the Co content is less than 5% (atomic fraction), (BH) m and Br hardly decrease; when the Co content is more than 30%, the magnetic performance parameters are significantly reduced. The added Co content of less than 10% is very beneficial, which not only increases the Curie temperature of the alloy, but also maintains higher magnetic properties, and at the same time the temperature coefficient of magnetic induction is also improved.
2. The role of Al partly replacing Fe
The research results of scholars show that adding a small amount of aluminum Al can significantly increase the coercivity of the ternary Nd-Fe-B material. The research results point out that since the Nd-Fe-Co-B permanent magnet material, the addition of Al can compensate for the decrease in coercivity caused by the addition of Co, so that a Nd-Fe-Co-Al- with higher comprehensive performance can be obtained. B alloy.
3. The role of Cu partially replacing Fe pair
Studies have found that adding a small amount of copper to the (Nd, Dy)-Fe-B and (Nd, Dy)- (Fe, Co)-B systems can significantly increase the coercivity, while Br hardly decreases, so that permanent magnets with high Hcj and high (BH)m can be manufactured.
4. Partial replacement of Fe by other elements
Based on ternary Nd-Fe-B alloy, adding a small amount of niobium Nb or zirconium Zr to replace part of the iron can effectively increase the Hcj and squareness Hk of the alloy, while the Br is reduced very little, and the magnetic flux of the alloy cannot be reduced. loss. The experimental results show that the maximum content of niobium and Nb in Nd-Fe-B alloy is 3% (atomic fraction), adding excessive Nb will make the coercivity of the alloy drop rapidly and make Nd2Fe14B unstable
The addition of gallium Ga can significantly increase the coercivity of the alloy and reduce the irreversible magnetic field. In the Nd-Fe-Co-B series alloy, as the content of Co increases, the Hcj of the alloy decreases, but when Ga is added Bottom, the coercivity increased. It is expected that it is possible to prepare Nd-Fe-B permanent magnet materials with high Curie point and high Hcj in alloys with Ga added
The compound addition of gallium Ga and niobium Nb can significantly improve the temperature stability of the alloy.