Temperature Coefficients Of Remanence Research Articles

Exploring Ho substituted Y-Fe-B nanocrystalline alloys and hot worked magnets

Aiming to balance the utilization of rare earth (RE) resources and develop Y-Fe-B based permanent magnets, Ho is employed as strategic substitution for enhancing the magnetic properties and thermal stability of nanocrystalline Y-Fe-B alloys. Ho substituting Y can enhance the coercivity of Y-Fe-B alloys while maintaining their excellent thermal stability. 30 at.% Ho substitution leads to an abnormal increase of remanence J r and (Y0.7Ho0.3)2Fe14B alloy exhibits good magnetic properties with remanence J r = 0.73 T, intrinsic coercivity H cj = 303 kA m−1, and maximum energy product (BH)max = 66 kJ m−3. High thermal stability with temperature coefficient of remanence α = −0.124%/K and temperature coefficient of coercivity β = −0.245%/K were obtained between 300–400 K. The results for RE-rich (Y1−xHox)2.5Fe14B alloys also show that the magnetic properties change with Ho content are similar to those of (Y1−xHox)2Fe14B alloys, but the coercivity is higher. In addition, nanocrystalline (Y0.5Ho0.5)2.5Fe14B magnets were prepared by hot-pressing and hot deformation process. Due to the lack of low melting point RE-rich phase, this alloy is difficult to be densified and deformed. The formation of high temperature RE2O3 and RE6Fe23 phases and the lack of continuously distributed RE-rich grain boundary phase are responsible for the poor texture of hot deformed magnet. The hot deformed magnet has the magnetic properties of J r = 0.50 T, H cj = 739 kA m−1, and (BH)max = 40 kJ m−3 together with high thermal stability. The micro-analysis demonstrated the chemical segregation of Y and Ho elements. Higher proportion of Ho than Y existed in main phase and grain boundary phase indicate excess Y were precipitated as Y-rich oxides.

Enhancement of coercivity and thermal stability for Nd-Fe-B sintered magnets by diffusing Dy-Co-M (M=Cu, Al) alloys

Commercial N52 sintered NdFeB magnets were processed by grain boundary diffusion (GBD) with Dy-Co-M (M=Cu, Al) alloys. The coercivity of magnets greatly increase to 17.62 and 18.83 kOe respectively when diffusing Dy58Co25Cu17 and Dy58Co25Al17 alloys, which are obviously higher than that of Dy58Co42 GBD-treated magnet with 16.64 kOe. Further thermal stability studies indicate that the thermal stability of Dy58Co25Cu17 and Dy58Co25Al17 GBD-treated magnets is further improved compared to the Dy58Co42 GBD-treated magnet. The results show that the temperature coefficients of remanence (20–120 °C) are reduced from –0.148%/°C to –0.134%/°C and –0.132%/°C by Dy58Co25Cu17 and Dy58Co25Al17 GBD-treatment, respectively. Besides, the irreversible magnetic flux losses (120 °C) for Dy58Co25Cu17 and Dy58Co25Al17 diffusion magnets are 4.76% and 2.79%, respectively. Microstructural analyses demonstrate that the presence of Cu and Al elements reduces the excessive accumulation of Dy and Co on the surface in the diffusion magnets and improves the diffusion depth and utilization of Dy and Co. Furthermore, the flow of Co from the triple junction phase to the thin grain boundary phase is promoted, which contributes to the uniform distribution of Co. In addition, the dynamic evolution of the magnetic domain structure during the temperature rise process was studied. This work provides insight into the preparation of high-performance and high-thermal stability magnets.

Improvement of thermal stability by co-introducing Dy and Co with dual-alloy method in NdFeB magnets

A dual-main-phase magnet with excellent thermal stability was prepared by co-mixing two types of magnetic powders, which was the conventional N55-grade powder without Dy elements, while the Co and Dy contents reached 50 wt% and 6 wt% respectively in another Co-rich powder. Studies showed that the magnetic properties along with the thermal stabilities of the magnets exhibited significant differences with the changes in the mixing ratio of the two powders. Especially, when mixing the two powders in equal proportions, a great remanence temperature coefficient of −0.0725 %/°C at 20–100 °C was obtained with a rather high Curie temperature of 581 °C, possessing its magnetic properties with Br = 12.58 kGs, Hcj = 15.9 kOe, and (BH)max = 36.88 MGOe. Note that as the proportion of Co-rich powder increased to 2/3, although the Curie temperature of the prepared magnets further increased to 622 °C, both the remanence temperature coefficient and the magnetic properties deteriorated significantly. The phase composition and microstructure analyses revealed that when the proportion of added Co-rich component exceeded 1/2, the RE(Fe,Co)2B phase transformed into the RE(Fe,Co)4B phase, and a significant amount of RE(Fe,Co)1.8B0.2 phase precipitated at the grain boundaries, leading to a deterioration in magnetic performance. In addition, dynamic domain evolution analyses under variable temperature conditions demonstrated the positive effect of the appropriate introduction of Co/Dy on the anti-demagnetization in high temperatures. This work provides insights and an experimental foundation for the development of high-performance magnets with high thermal stability, used in high-temperature environments.

Significantly improved magnetic properties and thermal stability for sintered Nd-Fe-B magnets via grain boundary diffusion of DyCo alloy

Improving the coercivity in high temperature applications is one of the critical issues of NdFeB-based magnets. In this work, DyCo alloy is selected as the diffusion source for grain boundary infiltration to prepare high coercivity NdFeB magnets with excellent thermal stability. Increasing the diffusion temperature until 950 °C is beneficial in elevating the coercivity. Magnetic hardening enhancement of interfacial region contributed by the formation of (Nd, Dy)2Fe14B shell and the elevated magnetic isolation effect between adjacent matrix phase grains are responsible to the coercivity improvement. The optimum magnetic properties with the coercivity of 1573 kA/m, the remanence of 1.33 T, and the maximum magnetic energy product of 338 kJ/m3 can be achieved at 900 °C. The significant increase of Curie temperature to 51 K is attributed to the higher exchange energy between Co-Co and Fe-Co. Moreover, the thermal stability including the temperature coefficient of remanence α and coercivity β are all improved considerably.

Efficient co-diffusion of Tb and Co in a sintered Nd-Fe-B magnet by low-melting point alloys

Co-diffusion of heavy rare-earth elements and Co has been reported to improve the remanence and coercivity of Nd-Fe-B magnet at high temperatures without trade-off. Two low-melting point alloys Pr50Tb20Cu15Co15 and Y15Pr40Tb15Cu15Co15 were designed in this work to simultaneously enhance the remanence and coercivity at 150°C in sintered magnet by grain boundary diffusion. The remanence increased from 1.24 T to 1.28 and 1.31 T at 150°C together with the increase of coercivity from 0.3 T to 0.89 and 0.77 T after diffusion. The temperature coefficients of remanence (20–150°C) were reduced from −0.11 %/°C to −0.07 and −0.06 %/°C by Pr50Tb20Cu15Co15 and Y15Pr40Tb15Cu15Co15 respectively. The improvement of temperature coefficients is attributed to the microstructure effect without changing the Curie temperature. The low melting point of the diffusion alloys and Y are attributed to breaking the remanence–coercivity dilemma with reduced Tb and Co usage.

Ultrahigh coercivity and excellent thermal stability of hot-deformed Nd-Fe-B magnets by the diffusion of (Pr,Nd)-Dy-Cu-Al

The effects of the addition of (Pr0.25Nd0.75)10Dy80Cu5Al5 alloy powders on the magnetic properties and microstructure of the hot-deformed Nd-Fe-B magnets were investigated in this work. Through the addition of (Pr0.25Nd0.75)10Dy80Cu5Al5, an ultrahigh coercivity of 3.14 T was obtained in a 6-mm-thick hot-deformed magnet, and the temperature coefficient of coercivity and remanence were improved from −0.475%/℃ and −0.145%/℃ to −0.383%/℃ and −0.110%/℃ at 20 −180 ℃, respectively, showing the excellent thermal stability. Microstructure observations indicated that the formation of thick nonferromagnetic intergranular phase, the reduction of grain size and aspect ratio and the formation of the (Nd,Pr,Dy)2Fe14B main phase were achieved after the addition of (Pr0.25Nd0.75)10Dy80Cu5Al5, which were the main reasons for the enhancement of coercivity and thermal stability.

Study of the anisotropic Sm2Fe17N3 powders with high performance

The strip casting technique was first used in this work to prepare the Sm2Fe17 parent alloys. Next, the Sm2Fe17N3 powders were synthesized using a gas–solid reaction of Sm2Fe17 powders with N2. Then, the anisotropic Sm2Fe17N3 powders with high performance were prepared by a surfactant-assisted grinding method in gasoline solvent. It was found that the coercivity (iHc) of ground powders increases with decreasing the average particle size (Dm), while the saturation magnetization (Ms) decreases. The maximum value of iHc of 17.5 kOe was obtained at the Dm = 1.3 µm. The ground powders with Dm of 3.1 µm show a remanence ratio (Mr/Ms) over 95% and a maximum energy product [(BH)max] of 35 MGOe. The Sm2Fe17N3 powder exhibited a very high iHc of 36.1 kOe at 10 K. Above 300 K, Sm2Fe17N3 powders mixed with epoxy resin showed a low remanence temperature coefficient of α(RT-100°C) = −0.077% °C−1.

Study on temperature stability and mechanical properties of sintered Nd<sub>25.5</sub>Dy<sub>6.5</sub>Co<sub>13</sub>(Fe, M)<sub>bal</sub>B<sub>0.98</sub> magnet

The sintered Nd<sub>25.5</sub>Dy<sub>6.5</sub>Co<sub>13</sub>Fe<sub>bal</sub>M<sub>1.05</sub>B<sub>0.98</sub> magnet (Co13 magnet) and Nd<sub>30</sub>Dy<sub>1.5</sub>Co<sub>0.5</sub>Fe<sub>bal</sub>M<sub>1.05</sub>B<sub>0.98</sub> magnet (35SH magnet) are prepared by strip casting, hydrogen decrepitation, jet milling, orienting compression, sintering and annealling. The maximum energy product (<i>BH</i>)<sub>max</sub> and coercivity <i>H</i><sub>cj</sub> of Co13 magnet at room temperature are 30.88 MGOe and 19.01 kOe, which are lower than those of 35SH magnet. By adding Co and Dy, the remanence temperature coefficient α , curie temperature <i>T</i><sub>C,</sub> and max operating temperature <i>T</i><sub>W</sub> significantly increase form –0.136%/℃ to –0.065%/℃ (25–180 ℃), 310 ℃ to 454 ℃, and 160 ℃ to 200 ℃ respectively. Mechanical property test and fracture analysis show that owing to the high content of Co in the magnet, the proportion of cleavage fracture in the main phase grains increases, and the bending strength Rbb decreases compared with the Rbb of 35SH magnet, which is nearly twice that of 2∶17 type Sm-Co magnet. The reason for Rbb decreasing might be that Co element preferentially replaces Fe in the 2∶14∶1 main phase, which leads the lattice to be distorted and the grain strength of the main phase to decrease. The microstructure analysis shows that there exists a high Co region in the grain boundary phase of Co13 magnet, and its composition is close to (Nd,Dy)(Fe,Co)<sub>3</sub>, which might be one of the reasons for coercivity <i>H</i><sub>cj</sub> decreasing.

Effect of Pr<sub>80</sub>Cu<sub>20</sub> grain boundary addition on microstructure and magnetic properties of (Pr, Nd, Dy)<sub>32.2</sub>Co<sub>13</sub>Cu<sub>0.4</sub>Fe<sub>bal</sub>B<sub>0.98</sub><i>M</i><sub>1.05</sub> magnet

With the aim of increasing Cu concentration to regulate the distribution of Co elements in RE-rich phase, the low-melting-point Pr<sub>80</sub>Cu<sub>20</sub> intergranular alloy is introduced into the (Pr, Nd, Dy)<sub>32.2</sub>Co<sub>13</sub>Cu<sub>0.4</sub>Fe<sub>bal</sub>B<sub>0.98</sub><i>M</i><sub>1.05</sub> (<i>M</i> = Al, Ga, Zr) magnet. Comparing with the original magnet, the remanence of PrCu-doped magnet is basically unchanged, and the coercivity is increased by approximately 1.3 kOe. Simultaneously, the Curie temperature, remanence temperature coefficient and irreversible flux loss are slightly improved. Microstructural study reveals that the Co-lean phase and the Co-rich phase coexist in the grain boundary in the 2<sup>nd</sup>-annealed original magnet. However, for the PrCu-doped magnet, the uniform distribution of Cu and Co elements in the intergranular phase are evidently improved, resulting in the elimination of the Co-rich phase. Since the <i>R</i><sub>2</sub>(Fe, Co)<sub>17</sub> (<i>R</i> = Pr, Nd, Dy) soft magnetic phase easily coexist with the Co-rich phase and are detrimental to the coercivity, the elimination of Co-rich intergranular phase may be an important reason for the higher coercivity of the 2<sup>nd</sup>-annealed PrCu-doped magnet than that of the original magnet.

Optimization of the Temperature Coefficients of Remanence and Coercivity in Sm2co17 Magnets by Dy88cu12 Doping

Optimization of the Temperature Coefficients of Remanence and Coercivity in Sm2co17 Magnets by Dy88cu12 Doping

Optimization of the Temperature Coefficients of Remanence and Coercivity in Sm2co17 Magnets by Dy88cu12 Doping

Optimization of the Temperature Coefficients of Remanence and Coercivity in Sm2co17 Magnets by Dy88cu12 Doping

Effect of La–Y co-substitution on magnetic properties and microstructure of NdFeB alloy ribbons

Committed to obtaining cost-effective NdFeB based permanent magnets, Nd27–xLa3YxFebalAl0.1Cu0.1B1 (x = 0–3) alloys were fabricated to detect the magnetic properties and microstructure. When x = 1.8, coercivity of 1004 kA/m and the magnetic remanence of 0.75 T are obtained, which are close to those of the original Nd30FebalAl0.1Cu0.1B1 alloy ribbons. The temperature coefficient of coercivity (β) and the temperature coefficient of remanence (α) of the LaY-substituted alloys are better than those of the original alloys. The research results manifest that La is mainly distributed in the grain boundary phases and plays the role of refining the main grains, optimizing the grain boundary phases and improving the wettability between the main phases and the grain boundary phases, while Y tends to enrich in the main phases and enhances the short-range exchange coupling.

Improvement of coercivity and thermal stability of Nd-Fe-B sintered magnets by intergranular addition of Tb80Fe20 alloy

To improve the coercivity and temperature stability of Nd-Fe-B sintered magnets for high-temperature applications, the eutectic Tb80Fe20 (wt%) alloy powders were added into the Nd-Fe-B sintered magnets by intergranular method to enhance the coercivity (Hcj) and thermal stability. The microstructure, magnetic properties and thermal stability of the Nd-Fe-B magnets with different Tb80Fe20 contents were studied. The experimental results demonstrate that the coercivity (Hcj) of the sintered Nd-Fe-B magnet is significantly enhanced from 14.12 to 27.78 kOe, and the remanence (Br) decreases not obviously by introducing 4 wt% Tb80Fe20 alloy. Meanwhile, the reversible temperature coefficients of coercivity (β) and remanence (α) of the Nd-Fe-B magnets are increased from −0.5634%/℃ to −0.4506%/℃ and −0.1276%/℃ to −0.1199%/℃ at 20–170 ℃, respectively. The Curie temperature (TC) of the Nd-Fe-B magnet is slightly enhanced with the increase of Tb80Fe20 content. Moreover, the irreversible flux magnetic loss (hirr) is obviously reduced as Tb80Fe20 addition increases. Further analysis of the microstructure reveals that a modified microstructure, i.e. clear and continuous RE-rich grain boundary layer, is acquired in the sintered magnets by introducing Tb80Fe20 alloy. The associated mechanisms on improved coercivity and thermal stability were comprehensively researched.

Development of cost-effective nanocrystalline multi-component (Ce,La,Y)-Fe-B permanent magnetic alloys containing no critical rare earth elements of Dy, Tb, Pr and Nd

Here we first report the fully abundant rare earth (RE)-based nanocrystalline multi-component (Ce,La,Y)-Fe-B alloys containing no critical RE elements of Nd, Pr, Dy, and Tb by melt-spinning technique. The roles of La and Y substitutions for Ce have been fully understood. La plays a positive role on both thermal stability and room-temperature (RT) magnetic properties. The enhanced coercivity Hcj by partial substitution of La is attributed to the increases of anisotropy field HA and the formation of continuously distributed grain boundaries resulting from the suppression of CeFe2 phase. Although Y substitution is not benefit for Hcj, both remanent polarization Jr and thermal stability have been effectively improved since Y2Fe14B shows relatively high saturation magnetization Ms and a positive temperature coefficient of HA over a certain temperature range. In addition, RE element segregation has been confirmed, La prefers to enter into the grain boundaries than Ce and Y prefers to remain in the 2:14:1 phase. Based on these understanding, a series of melt-spun (Ce,La,Y)-Fe-B alloys have been designed. A relatively good combination of magnetic properties with maximum energy product (BH)max=7.4 MGOe, Hcj=400 kA/m, and Jr=0.63 T has been obtained in [(Ce0.8La0.2)0.7Y0.3]17Fe78B6 alloy, together with high Curie temperature (Tc=488 K) and low temperature coefficients of remanence (α=-0.255 %/K) and coercivity (β=-0.246 %/K).

Structures and magnetic properties of the Co7Hf melt-spun ribbons

Co7Hf ribbons have been prepared by melt spinning technique. The effects of wheel speeds on the Structures and magnetic properties were investigated. At wheel speed of 35 m/s, the Co7Hf ribbons show the best intrinsic coercivity, remanence and maximum energy density, 2.0 kOe, 53 emu/g and 4.7 MGOe, respectively. XRD and TEM results show that Co7Hf ribbons are mainly composed of Co7Hf phase and Co phase. DSC and TEM results show that amorphous phase appeared at high wheel speed. The grain size refinement and the formation of amorphous phase were the main reasons for the changes of magnetic properties. Meanwhile, the excellent high temperature stability of Co7Hf ribbons was observed. In range of 27–500 °C, the temperature coefficients of remanence (α) and coercivity (β) of Co7Hf ribbons are −0.11%/°C, −0.19%/°C, respectively.

Investigation on the grain boundary diffusion of Dy2O3 film prepared by electrophoretic deposition for sintered Nd-Fe-B magnets

Based on the diffusion source of Dy2O3 film prepared by electrophoretic deposition (EPD), grain boundary diffusion (GBD) is performed to improve properties of sintered Nd-Fe-B magnets. Effects of EPD and GBD processes on microstructure and magnetic properties are investigated in details. The studied results show that 30 s of EPD time is the most suitable for the preparation of diffusion source, and the optimal magnetic properties with Hci of 934 kA/m, Br of 1.41 T and (BH)max of 349 kJ/m3 can be achieved under GBD condition of 800 °C/6 h followed by 500 °C/1 h, enhancing by 27.6%, 0.7% and 13.7%, respectively, comparing to those of as-prepared magnet. The formation of (Nd, Dy)2Fe14B shell surrounding Nd2Fe14B core and improved distribution and structure of rare earth-rich (RE-rich) phase jointly contribute to the improvement of coercivity. Moreover, Thermal stability (temperature coefficient of remanence and coercivity) is improved considerably. Dense Dy2O3 film and formed filamentous substance covering the surface together promote the improvement of corrosion resistance.

Enhanced magnetic properties in chemically inhomogeneous Nd-Dy-Fe-B sintered magnets by multi-main-phase process

Homogeneous substitution of Dy for Nd in the hard magnetic 2:14:1 phase can effectively enhance coercivity to ensure the high temperature operation, however, inevitably deteriorate remanence at expense. In this work, we performed a comparative investigation of the two magnets prepared by multi-main-phase (co-sintering Nd2Fe14B and (Nd, Dy)2Fe14B powders) and single-main-phase (sintering (Nd, Dy)2Fe14B powders) approaches. The comparative investigation reveals that at the same Dy substitution level (2.16 wt%), such chemically inhomogeneous multi-main-phase magnet possesses better room-temperature magnetic properties as well as thermal stability than those of the single-main-phase one with homogenous Dy distribution in the matrix grains. Room-temperature magnetic properties Hcj = 1664 kA/m, Br = 1.33 T and (BH)max = 350.4 kJ/m3 for the multi-main-phase magnet are all better than those for the single-main-phase magnet with Hcj = 1536 kA/m, Br = 1.29 T and (BH)max = 318.4 kJ/m3. In addition, over the temperature range from 295 to 423 K, both the temperature coefficients of coercivity and remanence for the multi-main-phase magnet are also lower than that for the single-main-phase magnet. Such superior magnetic performance is attributed to the short-range magnetic interactions inside individual 2:14:1 phase grains and the long-range magnetostatic interactions between adjacent grains with inhomogeneous Dy distribution. Our work provides a feasible approach of enhancing coercivity and retaining energy product simultaneously in the Nd-Dy-Fe-B permanent magnets.

Performance improvement and element segregation behavior in Y substituted nanocrystalline (La,Ce)–Fe–B permanent magnetic alloys without critical RE elements

Abstract Developing Ce, La, or Y based rare earth (RE) permanent magnets is a practical way to balance the utilization of RE resources. In this work, Y substitution for Ce and La is found to not only enhance the hard magnetic properties, but also significantly increase the Curie temperature Tc and thermal stability of the melt-spun (Ce0.7La0.3)2Fe14B alloys. To further optimize the magnetic performance, the RE-rich alloy with [(Ce0.7La0.3)0.8Y0.2]17Fe78B6 composition has been prepared. The good combination of magnetic properties with maximum energy product (BH)max = 7.1 MGOe, intrinsic coercivity Hcj = 357 kA/m, and remanent polarization Jr = 0.70 T has been obtained, together with low temperature coefficients of remanence (α = −0.255%/K) and coercivity (β = −0.246%/K). Importantly, RE element segregation has been confirmed in RE-rich (Ce,La,Y)–Fe–B alloys by microstructure and composition analysis. La prefers to enter into the intergranular phase than Ce and Y prefers to remain in the hard magnetic main phase, which results in increased Tc, improved magnetic properties, and enhanced thermal stability.

Bulk SmCo3 nanocrystalline magnets with magnetic anisotropy

In bulk SmCo3 nanocrystalline materials with small grain sizes of 10–20 nm, it is a challenge to realize a strong crystal texture that is crucial for them as high-performance permanent magnets. Here we fabricated an anisotropic bulk SmCo3 nanocrystalline magnet with a strong (00l) texture combined with a small grain size of 10–20 nm using a high-pressure thermal compression (HPTC) approach from amorphous precursors. This magnet had an obvious magnetic anisotropy with Mr///Mr⊥ = 1.53 and a maximum energy product of 4.2 MGOe (33.8 kJ/m3) which were obviously higher than those Mr///Mr⊥ = 1.01 and 1.9 MGOe (15.1 kJ/m3) of the isotropic magnets prepared using the same precursors. These anisotropic magnets showed a coercivity temperature coefficient of β = –0.31 %/°C and a remanence temperature coefficient of α = –0.14 %/°C. The deformation and annealing temperatures played important roles on the microstructures and magnetic properties of these anisotropic SmCo3 nanocrystalline magnets. These findings make an important step toward the fabrication of oriented bulk SmCo3 magnets for practical applications.

Evolution of Microstructure, Magnetic Properties, and Thermal Stabilities of Isotropic Alnico Ribbons

The magnetic properties of alnico alloys are highly dependent on the spinodally decomposed nanostructure. In this article, we report the fabrication of alnico nanoribbons and the effects of various heat treatments on the magnetic properties, spinodal decomposition, microstructure, and temperature stability. It is found that the spinodal decomposition occurs in a wider range of temperatures from 800 °C to 860 °C. The scanning electron microscopy (SEM) and transmission electron microscopy (TEM) analyses of the microstructure confirmed that the nanostructure phases develop in a wide temperature range. Aspect ratio of the order of ~20 is obtained by optimizing the heat treatment conditions. Magnetic properties of H cj = 815 Oe, B r = 6.70 kG, and (BH) max = 1.88 MGOe are obtained after simplified heat treatment. The magnetic properties of the alloys are measured at low (10 K) and at high (800 K) temperature, and it is shown by calculating temperature coefficient of remanence (α) and temperature coefficient of coercivity (β) that alnico ribbons possess extraordinary temperature stability at low and high temperatures.