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The influence of ZrO2 addition on the microstructure and the magnetic properties of Nd-Dy-Fe-B magnets
Abstract
The microstructure of Nd-Dy-Fe-B magnets can be refined and, consequently, the coercivity of sintered magnets can be improved with the addition of small amounts of ZrO2. By such an addition the temperature coefficients of the magnetic properties were found to be smaller. The microstructure has been examined by means of optical microscopy and high-resolution transmission electron microscopy. The interface changes explain the magnetic properties improvements.
... Wecker and Schultz (1990) found zirconium additions to enhance the coercivity significantly in rapidly quenched ribbons of neodymium-poor (stoichiometric or near stoichiometric) alloys. Numerous increases in coercivity in different materials were also reported by (Bao et al., 2009;Besenicar et al., 1992;Cheng et al., 1998;Jurczyk, 2000;McGuiness et al., 1998McGuiness et al., , 2001Pollard et al., 1988;Wu et al., 2002;S. Y. Zhang et al., 2007;Z. ...
... According to Matzinger et al. (1996), zirconium additions in the form of ZrB2 inhibit grain growth in solid-HDDR processed magnets due to the direct contact with the high-melting ZrB2 platelets. Grain growth reduction has been observed by other authors (Bao et al., 2009;Besenicar et al., 1992;Betancourt & Davies, 2003;Bollero et al., 2001;Wu et al., 2002;Yi et al., 2000;Yu et al., 2011;Z. Zhang et al., 2019). ...
... Therefore, high-melting sulfides such as MoS 2 [34] and WS 2 [ 38 , 39 ] have been selected as the precipitates since they are formed only at GB for pinning sites but do not react with the 2:14:1 phase or form defects in the interior of grain. In addition to the sulfides, the oxides such as SiO 2 [40] and ZrO 2 [41] have also been demonstrated effective for grain refinement. These refractory precipitates can be simultaneously used with the low-melting additives. ...
The magnetic performance of Nd-Fe-B magnets depends on their grain boundary structure. Intergranular addition and grain boundary diffusion (GBD) process are effective approaches for enhancing coercivity with low material cost. This review summarizes the development of grain boundary modification techniques with emphasis on our recent work using cost-effective non-rare earth (non-RE) sources for GBD. Up to now, heavy rare earth (HRE) based compounds, metals and light rare earth (LRE) based alloys have been successfully employed as the diffusion sources for coercivity enhancement. Inspired from the previous investigations on the intergranular addition of non-RE compounds and alloys for Nd-Fe-B magnets, in 2015, we firstly proposed a novel GBD process based on diffusion source of MgO. After that, various non-RE diffusion sources have been developed. The fundamentals of non-RE additives and non-RE diffusion sources for hard magnetic properties enhancement of Nd-Fe-B magnets are summarized here based on both the experimental and computational results. In particular, the properties-microstructure relationships of non-RE GBD modified magnets are discussed. The non-RE alloys or compounds modify the composition and structure of the grain boundary by diffusing into the intergranular regions, resulting in enhanced coercivity and corrosion resistance. Recently, we used Al-Cr coatings for both coercivity enhancement and surface protection, which shortens the production process and makes non-RE diffusion sources more competitive. The opportunity and future directions for non-RE GBD are also discussed in this review.
... Therefore, optimization of the microstructure by modifying intergranular phase is of great importance for obtaining satisfactory magnetic properties. It has been reported that the permanent magnetic properties of Nd-Fe-B magnets can be greatly influenced with addition of micro-sized refractory oxides powders, including Dy 2 O 3 , Tb 4 O 7 , ZrO 2 and MgO [2][3][4]. The addition of these oxides is found to effectively increase the coercivity, which is attributed to hardening the border region of the Nd 2 Fe 14 B grains, refining the grain size and modifying the composition and morphology of the Nd-rich phase. ...
... Using less or more zirconium resulted in materials with lower coercivities. Zirconium is known to substitute for neodymium in the Nd Fe B structure [7], and to result in the formation of nanoscale ZrB platelets [8]. However, it seems clear that the large increase in coercivity for a narrow range of zirconium additions must also be related to the role of zirconium in controlling the rate of the HDDR process: too little and there is a tendency for the material to over-disproportionate and form a coarse disproportionated structure that is difficult to recombine into an ideal microstructure ; too much and the disproportionation reaction is prevented from going to completion. ...
A range of ten Pr- and Zr-containing Nd-Fe-B-based alloy samples were disproportionated in a flow of hydrogen gas using a specially modified vibrating-sample magnetometer. The magnetization of the samples was measured as a function of temperature in order to monitor the changes taking place in the material during the hydriding and disproportionating reactions. The disproportionation reaction, which results in the formation of iron, was observed to vary significantly, depending on the zirconium content of the alloy. Substituting praseodymium for neodymium also had an effect on the progress of the disproportionation reaction, with two-stage behavior being replaced by a single-stage reaction as more praseodymium is added at the expense of neodymium. The intrinsic coercivities of bonded magnets produced from HDDR powders demonstrated that small substitutions of zirconium can substantially increase the coercivity providing that not too much zirconium is added, and that almost completely substituting the neodymium in the alloy with praseodymium leads to the best coercivities.
HDDR (hydrogenation-disproportionation-desorption-recombination) process is an effective means by which the anisotropy Nd2Fe14B-base magnetic powder can be made. In this text, the influence rules of d-HDDR process and the additive element Ti on the magnetic performance of Nd13Fe80.1-xTixB6.5 Zr0.1Cu0.3 (x = 0, 1.0, 2.0) alloy are studied. The results indicate that: In the d-HDDR process, the hydrogen press and disproportionation time is the key to cause materials to produce anisotropy; Element Ti enhances the coercive force of NdFeB alloy notably, it has two functions: On the one hand, forming lower temperature fusant on the grain boundaries, restraining grain growth, improving the structure of rich in Nd phase and making the rich in Nd phase distribute equably along the boundaries; In addition, element Ti makes the NdFeB grain boundaries smooth and clear, enhances the main phase grain surface anisotropy, makes back magnetic domain nucleate difficultly, thereby, the coercive force is enhanced, the best adding dosage of element Ti is 1.0at%. The best performances of the 2# alloy Nd13Fe79.1Ti1.0B6.5 Zr0.1Cu0.3 treated by d-HDDR process are as follows: Br = 1.39T; iHc = 1006 kA/m; (BH)max = 169.66 kJ/m3; DOA = 0.797.
HDDR (hydrogenation-disproportionation-desorption-recombination) process is an effective means by which the anisotropic Nd2Fe14B-base magnetic powder can be prepared. vacuum HDDR process, i.e. the alloy is heated to the temperature about 820°C in vacuum and then HD reactions occur under the certain hydrogen press are with the addition of the alloying element Ga affects the magnetic properties of Nd13Fe64.4-yCo16 B6.5Zr0.1Gay (y=0.1, 0.3, 0.5, 1.0) alloy was studied. The results indicated that in the vacuum HDDR process, the HD steps are the key to spur the alloy on to anisotropy, and applying the low-vacuum high-temperature schedule to the desorption step will make sure that the alloy is anisotropic. The alloying element Ga contributes to refining the grains of main phase and improving the comprehensive magnetic properties of the alloy, of which the best atom fraction is 0.5%. Treated by the vacuum HDDR process, the Nd13Fe63.9Co16 B6.5Zr0.1Ga0.5 alloy provides its best magnetic properties, i.e. iHc=1018 kA·m-1, Br=1.32 T, (BH)max=118.6 kJ·m-3 and DOA=0.787.
Chemical surface modification can be used as a method for corrosion protection of sensitive powders based on intermetallic alloys between rare earth and transition metals /1/. Surface coating is used for preventing fine powders, based on Nd-Fe-B, Sm2Fe17-xTax and Sm2Fe17-xTaxN3-δ prepared by HDDR processing and mechanically alloying, from hydrolysis. Powders coated by chemisorbed organic substance, after exposing to a humid atmosphere, do not show any chemical or physical change. Different coating agents were used and the sufficient amount of various materials was optimised with the emphasis on minimising their quantity. Simple experiment shows that the surfactant is successfully chemisorbed on the powder surface and that the coated powders are hydrophobic indefinitely. Magnetic properties were measured on samples after they were exposed to the same corrosion tests. Measurements on coated and bonded samples were compared with the measurements of non-coated samples. By using Auger electron spectroscopy the thickness of the coating was controlled. In order to distinguish the nature of the bonding between the powder surface and the surface-active substance FT-IR spectroscopy in absorbance and diffuse reflection modes was used. The protection of the fine particles is based on the formation of a covalent bond between the hydroxyl groups at the particle surface and the surface-active substance. The monomolecular layer of organic substance does not damage the magnetic properties of the powder, but successfully protects the powder against humidity.
The HDDR (hydrogenation-disproportionation-desorption-recombination) process was a special method to produce anisotropic Nd 2Fe 14B-based magnet powder. The magnetic properties of Nd 11.2Fe 66.5-x Co 15.4B 6.8Zr 0.1Ga x (x=0, 0.1, 0.3, 0.5, 1.0) alloys after the modified HDDR (i.e., low vacuum dehydrogen treatment was appended between HD treatment and high vacuum dehydrogen treatment) and the appending of microelement Ga have been investigated. It was found that the modified HDDR process was effective to enhance magnetic properties of the powder and fine main phase grain size. The appending of microelement Ga could change the disproportionation character of the alloy and improve the magnetic properties of the magnet powder. The main phase grain size of the magnet powder produced by modified HDDR process was smaller than those produced by conventional HDDR process.
The effects of temperature and time of disproportionated reaction and dehydrogenation on the magnetic properties during the preparation of isotropic HDDR magnetic powder were studied. The changes of reaction temperature and grain size with different hydrogen pressure of dehydrogenation were researched by X-ray diffraction analysis (XRD) and differential thermal analysis (DTA). The results showed that the best performance could be acquired with the disproportionation temperature of 820°C for 1.5 h, the recombination temperature of 820°C for 1 h. The influence of different hydrogen pressure of dehydrogenation was studied by DTA simulating HDDR. With the hydrogen pressure of dehydrogenation increasing, the rate of reaction slowed, peak of α-Fe of alloy was enhanced by XRD, grain size calculated by Scherrer formula found that with the hydrogen pressure of dehydrogenation increasing, the size became smaller.
Chemical surface modification can be used as a method for corrosion protection of sensitive powders based on intermetallic alloys between rare earth and transition metals. Surface coating is used for preventing fine powders, based on Nd-Fe-B, or Sm2Fe17−xTaxN3−δ prepared by HDDR processing and mechanically alloying, from hydrolysis. Powders coated by chemisorbed organic substance, after exposing to a humid atmosphere, do not show any chemical or physical change.
Different coating agents were used and the sufficient amount of various materials was optimised with the emphasis on minimising their quantity. Simple experiment shows that the surfactant is successfully chemisorbed on the powder surface and that the coated powders are hydrophobic indefinitely. Magnetic properties were measured on samples after they were exposed to the same corrosion tests. Measurements on coated and bonded samples were compared with the measurements of non-coated and bonded samples. By using Auger electron spectroscopy the thickness of the coating was controlled. In order to distinguish the nature of the bonding between the powder surface and the surface-active substance FT-IR spectroscopy in absorbency and diffuse reflection modes was used. The protection of the fine particles is based on the formation of a covalent bond between the hydroxyl groups at the particle surface and the surface-active substance. The monomolecular layer of organic substance does not damage the magnetic properties of the powder, but successfully protect the powder against humidity.
The HDDR (Hydrogenation-Disproportionation-Desorption-Recombination) process is a special method to produce anisotropic NdFeB powders for bonded magnet. The effect of the modified HDDR process on magnetic properties of -based magnet with several composition and that of microelement Ga, disproportional temperature and annealing temperature on , grain size were investigated in order to produce anisotropic powder with high magnetic properties. It was found that modified HDDR process is very effective to enhance magnetic properties and to fine grain size. The addition of Ga could change disproportionation character remarkably of the alloy and could improve magnetic properties of magnet powder. Increasing annealing temperature induces significant grain growth. And grain size produced by modified HDDR process is significantly smaller than those produced by conventional HDDR process.
In this paper, we have looked at the influence of praseodymium and zirconium on the disproportion stage of the HDDR process in order to explain the observed variations in magnetic properties.
In this work we have studied in detail the effect of MgO on magnetic properties and intergranular microstructure. We have found that both coercivity and thermal stability can be remarkably enhanced by the intergranular addition of MgO. For Nd22Fe71B7 magnets with 2 wt % MgO addition, the coercivity at room temperature and 180 °C are enhanced from 17.0 and 3.2 kOe to 22.1 and 5.2 kOe, respectively, and the reversible and irreversible flux loss from room temperature to 180 °C is reduced from 25.4% and 5.2% to 20.5% and 0.5%, respectively. Microstructural studies reveal that a new intergranular Nd–O–Fe–Mg phase with a composition close to Nd70O23Fe3Mg2 appears in the magnets with MgO addition. The improvement of magnetic properties by the MgO addition is believed to be due to the appearance of a Nd–O–Fe–Mg intergranular phase, which probably hinders the propagation of the domain walls between Nd2Fe14B grains. It is further found that the addition of Mg or O alone into the intergranular regions of the magnets does not lead to the formation of this Nd–O–Fe–Mg intergranular phase, and thus, cannot substantially improve the coercivity and the thermal stability of the magnets. © 1997 American Institute of Physics.
A review is given on the basic properties of 2:14:1 hard magnetic phases as well as technological procedures used for manufacturing R-Fe-B and R-Fe-C permanent magnets. First, we analyse the phase diagrams, crystal structure of hard magnetic phases and composition ranges in which these phases form solid solutions, as a result of various types of substitution. Then, the magnetic properties of and -based compounds are described. The routes used for manufacturing R-Fe-B and R-Fe-C permanent magnets are detailed. The magnets' microstructure as well as their coercivities are described in correlation with composition and manufacturing routes. The properties of nanostructure magnets are then presented. Finally, the stability of Nd-Fe-B and Nd-Fe-C magnets in various working conditions is analysed.
The positive influence of ZrO2 addition on the magnetic
properties, temperature coefficients and corrosion resistance of
Nd-Dy-Fe-B magnets were established. The improvements in these
parameters were attributed to microstructural changes. The phase
composition was analyzed using HREM, and besides the known phases, an
additional Zr boride phase was found, which was identified using crystal
structure analysis as a ZrB2 plate-like phase. Due to the
formation of the new phase and oxidation of Nd from the Nd-rich grain
boundary phase, the sensitivity of the grain boundaries is diminished to
a great extent, and as a consequence the corrosion process does not
proceed in the sample. It remains limited to the exposed surface, which
is a worthwhile contribution to the improvement of the resistance
against corrosive conditions
The effect of MgO addition on the coercivity, thermal stability and microstructure of Nd22Fe71B7 magnets has been investigated. Results show that both the coercivity and thermal stability of NdFeB magnets can be improved by the MgO additive. Microstructure studies reveal that a new intergranular NdOFeMg phase with a composition close to Nd70O22Fe5Mg1−3 appears in the magnets with MgO addition. The improvement of the properties of the magnets may be correlated with the appearance of this new phase. It was further found that the introduction of an appropriate amount of oxygen and Mg together is beneficial for the magnetic properties and thermal stability of NdFeB magnets.
A comparative study of the processing of highly coercive, isotropic NdFeB-type magnet powders was carried out using two different techniques. Nd15Fe77B8 alloys with additions of Dy (1 at.%) and Zr (0.1, 0.3 and 0.7 at.%) were processed by: (a) a combination of intensive milling-and-annealing and (b) the hydrogenation-disproportination-desorption-recombination (HDDR) process. A marked retarding effect on grain growth due to the Zr addition was observed. Intensively milled and annealed samples containing both Zr and Dy additions showed a large improvement in coercivity in comparison with the additive-free alloys, reaching an optimum value of 2.5 T for 0.1 at.% Zr and 1.0 at.% Dy. The improved microstructure resulted in a wide processing window in which high-coercivity material could be produced. A ZrB2 lamellar phase was observed using transmission electron microscopy; this phase appears to have a grain-growth-inhibiting effect during HDDR processing of NdDyFeBZr materials. A maximum coercivity of 1.9 T was obtained for the HDDR-processed material. At recombination temperatures above 800°C explosive grain growth was observed using Kerr microscopy.
Cast alloys with a range of compositions based on Nd15Fe77B8 with additions of Dy and Zr were produced by a conventional casting method. Zr was added prior to casting in the form of either zirconium or zirconia. The materials were processed using the hydrogen decrepitation, disproportionation, recombination (HDDR) technique in order to develop the fine microstructures necessary for high coercivities. Magnetic measurements on bonded samples produced from the HDDR powders indicated that additions of Zr tended not to affect the recombination temperatures at which the material began to develop high coercivity, but was clearly observed to extend the temperature range over which high coercivity material could be produced. Microstructural analysis revealed that the Zr-doped samples were less prone to the explosive grain growth exhibited by the Zr-free samples at these temperatures.
During the eight year history of NdFeB sintered magnets a large evolution in both the composition of materials and the process technology has been observed. The purpose of this paper is to evaluate these changes. Particular emphasis is set on those processes that have reached the production stage. Some interactions between the evolutions in composition and those in the processes are described. Finally, some trends for possible major future changes are evaluated.
The microstructure of a sintered magnet made from Zr-containing NdFeB material was investigated using optical microscopy and scanning and transmission electron microscopy. The authors observed three phases in addition to those found in the ternary alloy. Two of these phases were Zr-rich, one having a lamellar structure and containing some iron and neodymium, and a second Zr-rich phase containing a small amount of iron. A substantial amount of Zr was found in the NdFe4B4 phase with Zr occupying some of the iron sites. A third phase took the form of small coherent precipitates within grains of the hard magnetic phase and was shown to be richer in Zr than the surrounding matrix. Magnetic measurements show an enhanced coercivity over the ternary alloy, which is attributed to the presence of coherent precipitates within the hard magnetic phase.
The characteristics of the high energy product NdFeB magnets
obtained by the powder metallurgy route have been improved by
appropriate vanadium additions. Specific combinations of V and Co lead
to an intrinsic coercive force of about 1350 kA/m (17 kOe) in Dy-free
magnets, without any large decrease of remanence. If 1.5-at.% Dy is
added, load line down to - B /μ H =0.5 can be achieved
at 180°C without irreversible losses. Corrosion resistance is
improved by blocking the selective oxidation of the Nd-rich phase. As a
consequence, organic-coated magnets are able to withstand 15 days at
115°C-1.75 bars in a H2O saturated atmosphere without any
degradation. It is concluded that this set of new properties will
enlarge the application fields of Nd-F-B permanent magnets