1.3 Research Methods for Studying the Magnetic Properties of Rare Earth Permanent Magnets

Permanent - magnet alloys can be made more coercive in two ways. The first one is that deriving the relationship of coercivity with physical or geometrical parameters of the alloy phase according to some physical model, with mathematic treatment afterwards. The other is to observe directly the correlation of coercivity with geometrical parameters of microstructure or physical parameters of phase. Both are supplements and are complementary to each other. The coercivity of rare earth permanent - magnet alloys is very sensitive to the microstructure of the alloys. And observing microstructure and investigating microstructure and magnetic domain relationship and interaction is the foundation of studying coercivity mechanism (Li, Zhu, 2003; Pan, Xiao, 1989).

After it is determined that the magnetic properties of the rare earth permanent - magnet alloys are studied based on the observation of the microstructure and its interaction with domain structure, it is necessary to consider how to achieve the objectives, using TEM, SEM or EPMA? In experiments, some or all of them can be used coordinately. The development of electron, vacuum, and computation technologies has the development and application of SEM, TEM and EPMA reduced. The apparatus with high resolution and sensitivity lay solid foundations for studies of materials.

The author discovered in experiments that the magnetic properties of rare earth permanent - magnet alloys are influenced by many factors. For Co and Al substituted NdFeB alloys, the maximum coercivity occurs at Al concentration of 4%, as Co concentration is 10%, while the maximum coercivity occurs at Al concentration of 2%, as Co concentration is 16%. The substitution of Al can increase the coercivity, while the substitution of Co can elevate the Curie temperature. The combined substitution of Co and Al can increase both the coercivity and the Curie temperature of NdFeB alloy. If an appropriate amount of Nb is added, the coercivity of NdFeB alloy can be greatly increased, while the Curie temperature is also elevated.

Based on the in - situ and dynamic observation to SmCo₅ using JEM - 1000 kV super - high voltage TEM, the crystal nuclei of Sm₂Co₁₇ phase segregated from the parent phase SmCo₅ at 600 - 750 °C grow, collide each other and get together under the phase transition driving force, and thus slowing down the rate of phase transition. At the same time, the crystal nuclei in other regions still grow. This is the “isothermal phase transition”. After above process is finished, the new phase formed at the given temperature cannot continuously grow. It is necessary to increase temperatures to provide more phase transition driving force. The new nucleation will occur and grow in the thermal excitation. This is the “varying - temperature phase transition”. Besides the Sm₂Co₁₇ phase, Sm₂Co₇ phase is segregated from the parent phase SmCo₅ at 600 - 750 °C. There exists the interaction of growth of the Sm₂Co₁₇ and Sm₂Co₇ phases. As temperature is lowered, the phase transition is reversible.

The in - situ observation at 400 - 750 °C to Sm(Co, Cu, Fe, Zr)_7.4 permanent - magnet alloys with large coercivity indicated that the Zr - rich phase with long plate in shape appears, while the cellular microstructure (main phase Sm(Co, Cu, Fe, Zr)_7.4 with the size of 50 nm is surrounded by Sm(Co, Cu, Fe, Zr)₅ with the thickness of 10 nm), which has decisive effect on coercivity of the alloys, is growing. Due to different wall energy between 1:5 and 2:17 phases, the domain wall of the 2:17 phase is pinned by the 1:5 phase in the reversibly magnetized process, thus acquiring large coercivity. The coercivity of the alloys is determined by the pinning field. As the applied magnetic field is larger than pinning field, the alloys are magnetized to saturation state. The coercivity of the alloys is not determined by the crystal size, but is determined by the structure of Sm(Co, Cu, Fe, Zr)_7.4 with the size of 50 nm and Sm(Co, Cu, Fe, Zr)₅ with the size of 10nm (Pan, Xiao, 1989; Pan, Ma, Li, 1993; Pan, Liu, 1990).

Application of the “dynamic crossing” in manufacturing the rare earth permanent magnetic alloys

The good results are obtained from high vacuum in sintering, based on the principle of high activity and easy oxidation of rare earths. For example, in order to obtain high vacuum, a small amount of highly active La and Ce is located in furnace. As La and Ce are easily compounded with oxygen, oxygen in air is decreased.

• Add Zr - Al oxygen absorber into sintering furnace to seize oxygen in air; and then add a chemically active element to reduce oxygen in the furnace so that to increase sintering efficiency and to raise product yield.
• Develop dynamically adjust liquid amount by means of cross - handle of liquid, solid and air phases in hydro - milling to reduce the amount of air phase as much as possible, so that to achieve the target to lower oxygen content in the alloy. That had gotten good result.
• In vacuum melting use mechanic pump to draw to low vacuum, and then add argon into furnace in order to drive out the remained oxygen faster; repeat the above two steps for several times. This was testified as an effective deoxidation process.
• Design a process for “reproducing rejected product because of unqualified performance of rare earth magnetic material due to oxidation” to make use of waste goods and to improve environment. The surface of rare earth ferrous permanent magnetic material is liable to be oxidized in manufacturing process, which may produce Nd₂O₃ and lead to degrade magnetic performance. New process, designed under the principle of “dynamic crossing and supplement beneficials”, is to add element which is inadequate.
• Property of the rare earth metals are active, that is easy to be oxidized to be Nd₂O₃ which make the formation of Nd₂Fe₁₄B become difficult; supplement neodymium can promote the formation of Nd₂Fe₁₄B. This process is a three - dimension melting, one is heat activation, another is draw vacuum and the third is magnetic stirring. Afterwards, by passing through corresponding milling and sintering process the magnetism can be re - produced and the magnetic performance can be recovered. By this way the waste becomes qualified product.

Application of the “dynamic crossing” in developing the rare earth permanent magnetic alloys and raising performance of the alloys

• There are many milling techniques in product process of the rare earth permanent magnetic alloys, among them by hydrogenated crush and then mill to 3 - 4μm is an effective method to reduce oxygen content of the alloys. However, a severe problem occurred in the hydrogenated milling process: crystal size of the powder by hydrogenated milling after sintering was found to be extremely big in test, that the granule size of crystal of permanent magnetic alloy, sintered at 1000°C, is as large as 400nm, which will degrade the coercivity. To resolve this problem a new project was designed - add element dysprosium to make granule of alloy fined. Element Dy is mainly distributed crystal boundary of alloy microtexture so that increase of Dy can make granule become finer. Analysis by X - ray indicates that when Dy content is raised the lattice constant \(a\), \(c\) are decreased. For \(Nd_{15}Fe_{77}B_{8}\) alloy its lattice constant \(a\) was reduced from 0.883nm to 0.870nm, and \(c\) was reduced from 1.226nm to 1.211nm by adding 1% (at.) Dy replaces a few Nd.

  Studies indicated that when add dysprosium in 0.4% (at.) the average size of granule was 7.0 μm; and when add Dy in 2% (at.) the average size of granule was 5.1μm; when add Dy to 3%(at.) the average size of granule reduced to 3.0μm. The result of magnetic measurement showed that the coercivity of the rare earth permanent magnetic alloy was enhanced obviously.

• Analyze functions of elements for the rare earth ferrous permanent magnetic alloys of ternary system or above, and analyze problem existed in material preparation process in order to improve performance of material.

  If the relation between the coercivity and the manufacturing process for the rare earth permanent magnetic alloy is described in functional formulate, or say, coercivity is the function of (element component, melting, milling, forming, sintering, and magnetization). Thus to enhance coercivity it is necessary to optimize every variables in manufacturing process. For example, alloy elements from ternary change to seven, the melting process and heat treatment technique of the alloy should be changed.

  For example, texture solidification status of NdFeB permanent magnetic alloy in melting process, including factors such as columnar crystals, degree of orientation, size of crystal granule, etc., has important influence on the magnetism of the alloy. A desired crystal texture of equiaxial and fine crystal granule is interrelated to melting temperature, molding temperature, cooling speed, design and preparation quality of crystallization apparatus, etc.

  What worth to be discussed is the effect of change in melted alloy composition on above mentioned problems and eliminate \(\alpha\)-Fe crystals. In fact the columnar crystals can be seen directly without using microscope which is mainly quadrilateral crystals of \(Nd_{2}Fe_{14}B\), and there is Nd - rich phase among the columnar crystals. Crystals of casted ingot grow fast along directions of (411) and (410), that can be certified by intensity of diffraction peaks on (410) and (411) planes in X - ray diffraction pattern. There are a lot of equiaxial crystals’ texture in alloy ingot, copper was severely segregation, copper was poor within crystals, and disassociated copper enriched among crystals, which is not good for improving magnetism of the alloy, because of unreasonable melting procedures in melting Sm(Co, Cu, Fe, M)_7.4 (\(M = \text{Zr, Mn}\)).

  Inspired by “dynamic crossing,” the aforementioned alloy ingot was subjected to solid - solution treatment. During this treatment, the elements were fully “dynamically crossed.” Copper accumulated substantially at the crystal boundaries and dissolved the dispersed copper and part of the 1:5 phase of copper on the cell wall, which acted as a pinning function on the domain wall, thereby increasing the alloy's coercivity. The required equipment for this process is simply a crystallizing apparatus with excellent heat - conductive coefficient and a good cooling structure.