Preparation Of Sm₂Co₁₇-based High Temperature Rare-earth Permanent Magnets And Their Microstructure And Magnetic Properties

Sm₂Co₁₇type permanent magnets are the promising candidates for high temperature applications because of their good combination of magnetocrystalline anisotropy and the Curie temperature.For example,they have been used in exactitude machine tool, cymoscope,gyroscope,measure instrument,magnetic beating,permanent magnetic electrical machines and so on.To optimize the microstructure and increase the operating temperature of Sm₂Co₁₇type magnets,efforts had been devoted to investiagate the effects of sintering and annealing treatment on the microstructure and magnetic properties of the magnets.Furthermore,this dissertion had undertaken a systematic study in the effect of component on the magnetic properties and temperature stability.A Sm₂Co₁₇type permanent magnet was provided which retains its magnetic properties and exhibited a linear extrinsic demagnetization curve at elevated temperatures up to 500℃.In addition,the cellular microstructure,domain structure and phase structure had been studied using transmission electron microscopy （TEM）,scaning electron microscopy（SEM）,magnetic force microscopy（MFM）and X-ray diffraction（XRD）.The coercivity mechanism was discussed in detail.It is discovered for the first time that sintering temperature plays an important role in the temperature coefficient of intrinsive coercivity.The results show that the quantities and sizes of Sm and Zr precipitated from the matrix increase with the increasing of sintering temperature,which leading to the increase of intrinsive coercivity coefficient.The magnets with low Fe and high Cu content exhibit low sintering temperature and good temperature stability.Effects of annealing treatment on the phase composition and microstructure of Sm₂Co₁₇-type high temperature permanent magnets had been systematically investigated.The results show that the cellular structure of magnets Sm(Co_balFe_0.197Cu_0.124Zr_0.026)_7.5and Sm(Co_balFe_0.108Cu_0.131Zr_0.042)_7.0firstly increases with the prolonging of annealing time at 830℃.And then with the annealing time extends to 30h the cellular structure becomes half-baked and the coercivity becomes very low.The coarsening velocity of the cellular structure decreases with the increasing of the thickness of 1:5 phase and the content of the matrix,and increases with the increasing of the size and content of 2:17R phase.It is discovered for the first time that,after annealed at 830℃,the magnet with higher Cu and Zr content contains more lamella phase than that with lower.Increasing the annealing time can promote the formation of lamella phase and improve the coercivity.But,when the content of Cu and Zr is too high,non-magnetic phase cotaining Cu and Zr appears in the grain boundary and matrix,resulted in a decrease in the ramanence and energy product.Developed a new-type high temperature rare-earth permanent magnet Sm(Co_balFe_0.11Cu_0.10Zr_0.03)_7.5used at 500℃.It was proved that the main magnetic properties of this magnet have reached a international level.Its magnetic properties at room temperature are B_r=0.982T,H_ci=2400.0kA/m,（BH）_max=200.3kJ/m³,and at 500℃are B_r=0.708T,H_ci=646.7kA/m,（BH）_max=85.4kJ/m³,and it exihibits a linear extrinsic demagnetization curve at elevated temperatures up to 500℃.This magnet can be triumphantly applied in the dynamic systems such as dynamotor,electromotor and so on.The domain structure of Sm₂Co₁₇type magnets is studied.It is discovered that the domain spacing is much smaller than the grain size and larger than the cellular structure.The magnet used at higher temperature shows smaller domain spacing and cell size.Furthurmore,phase transformation from room temperature to 700℃was examined by in-sute XRD techniques firstly,and the coercivity mechanism was also disccused.The results show that,when the temperature is below 300℃,the microstructure of the magnet consists of 2:17R phase,1:5 phase and lamella phase, and the the 1:5 phase acts as a barrier for domain wall displacements.In the intermediate temperature range(300℃＜T＜T_c^1:5),1:5 phase transforms to 2:7 phase, and the coercivity mechanism changes to pinning at the 1:5 phase and nucleation at the 2:7 phase.With increasing the temperature higher than the Curie temperature of the 1:5 cell walls a nucleation mechanism at the 1:5 non-magnetic phase and 2:7 phase may be dominant.This dissertion explored another new-type permanent magnet Sm_0.85Gd_0.15(Co_balFe_0.24Cu_0.07Zr_0.03)_7.5used at 300℃,discussed the detailed forced status of the magnet in radially oriented magnetic field and achieved the perdect framework of the parts.The properties of the magnet at room temperature are B_r=1.069T,H_ci=2225.2kA/m and（BH）_max=221.8kJ/m³,while those at 300℃are B_r=1.0T,H_ci=1313.5kA/m and（BH）_max=183.5kJ/m³,respectively.Theα_20-300℃is -0.023%/℃and theβ_20-300℃is -0.146%/℃.The operation practice by customers of military industry shows that it can satisfy the need of the high-temperature moment motor. Sm(Co_balFe_0.11Cu_0.16Zr_0.04)_7.0magnet had been annealed in microwave field for the first time and a single scale nanocomposite magnet had been attained.Annealing in microwave field provides a new method to preparate high performance nanocomposite magnets.The results show that microwave field can restrain the growth of the grain and the formation of FeCo soft magnetic phase.The grains with the size of 20nm composed of 1:7 main phase and 2:7 phase has been attained and the M_r/M_s reachs 0.86.The transformation from ferromagnetism to paramagnetism is also found with M_s increased from 0.337T to 1.192T and H_cidecreased from 542.6kA/m to zero.