2016 年 57 巻 10 号 p. 1771-1775
To improve the coercivity of Dy free Nd2Fe14B magnet, it is necessary to prevent oxidization of Nd-rich phase formed in grain boundaries on the basis of the information of oxygen behavior in this material which has been not well revealed so far. The present study aims to clarify the thermodynamic property of oxygen in the Fe-Nd-O system. The solubility of oxygen in molten Fe-Nd alloy coexisting with solid Nd2O3 has been measured at 1473 and 1673 K. On the basis of this results, the standard Gibbs energies for the dissolution of oxygen into molten Fe-Nd alloy at 1473 and 1673 K have been determined to be −397.3～−411.6 and −314.6～−396.8 kJ/mol, respectively. The dissolution behavior of oxygen into molten Fe-Nd alloy is thermodynamically discussed and found to be dominated by chemical property of neodymium. Addition of calcium or neodymium fluoride is found promising for effective deoxidation of the present alloy.
Among the magnets currently used, sintered Nd-Fe-B magnet has the highest magnetic properties which meet the requirements in many commercial products such as voice coil motors for hard disk drives, industrial motors and generators, devices for magnetic resonance imaging, speakers and vibrators for mobile phones, as well as variety of automotive applications. Above all, the demand for motors of hybrid cars is eminently growing nowadays. For this application, the commercial Nd-Fe-B magnets are promising due to its highest magnetic energy product among all kinds of magnets, but its coercivity is relatively low even at room temperature and critically deteriorated at temperatures beyond 200℃ where motors are usually operated. As the countermeasure to this problem, Dy is conventionally added to improve the coercivity of Nd-Fe-B magnets at such higher temperatures. Dysprosium being rare metal as well as strategic material, which supply are totally dependent on the import from foreign countries and may not be stable depending on the international situation, it is significantly important to develop Dy free Nd-Fe-B magnets having high coercivity.
Hono et al.1) have found that the grain refinement of Nd-Fe-B alloy improves coercivity of Nd-Fe-B magnet. However, too small grain size causes a decrease in coercivity. This critical size depends on oxygen concentration2). Presence of Nd-rich phase in the boundaries of Nd2Fe14B grains increases coercivity; however, oxygen segregates at grain boundaries of Nd-Fe-B alloy and thereby almost Nd-rich phase is oxidized in its manufacturing process3). Therefore, to improve coercivity of Nd2Fe14B magnet, prevention of oxidization of Nd-rich phase would be contributing. For the sake of this purpose, it is quite important to know the thermodynamic property of Nd-Fe-B-O system. Up to now, even the solubility of oxygen in the Nd-Fe-B system has not been clarified except for that in Nd4). Against these backgrounds, the present study aims to determine the oxygen solubility in molten Nd-Fe alloy and to consequently clarify thermodynamic properties of this system as well as to propose a possible deoxidation process in practical operations.
Iron-neodymium alloy were prepared by arc-melting of Fe plates (99.9% in purity) and Nd rods (99.9% in purity) weighed to have intended alloy compositions listed in Table 1. Before melting, the Nd rods were well polished to remove oxidized layer on the surface. Figure 1 shows the compositions of alloys and experimental temperatures plotted on the Fe-Nd binary phase diagram5). Pre-melted Fe-Nd alloy was put into an Nd2O3 crucible (29-mm in outer diameter, 23-mm in inner diameter, and 55-mm in height). Then, the crucible was placed in an Fe holder (40-mm in outer diameter, 30-mm in inner diameter, and 90-mm in height), which was sealed with an Fe lid by welding after purging argon gas into the holder. This Fe holder was placed and heated in an electric resistance furnace up to 1473 or 1673 K and held for the intended time from 12 to 240 h in a flowing argon gas atmosphere.
| No. | Temperature, T/K | Fe (at%) | Nd (at%) |
|---|---|---|---|
| 1 | 1673 | 91.1 | 8.9 |
| 2 | 76.8 | 23.2 | |
| 3 | 44.2 | 55.8 | |
| 4 | 29.2 | 70.8 | |
| 5 | 0 | 100 | |
| 6 | 1473 | 39.6 | 60.4 |
| 7 | 22.2 | 77.8 |
Experimental compositions of Nd-Fe alloy plotted in the Nd-Fe phase diagram.
After intended holding time, the Fe holder was quickly withdrawn from the furnace and water quenched. The Fe holder was cut and the Nd2O3 crucible was taken out from the holder, the cross section of which in the radial direction was submit to the following analyses. The microstructure and composition of Fe-Nd alloy were observed and measured by SEM-EDS (Scanning Electron Microscope with Energy Dispersive Spectroscopy). The Nd2O3 crucible was analyzed by XRD (X-ray Diffraction Analysis, whose radiation source was Co) to identify the equilibrium oxide phase with Fe-Nd alloy. The oxygen concentration of the Fe-Nd alloy was measured by the inert gas extraction infrared absorption analysis.
Figure 2 shows the change in concentration of oxygen with holding time for the experiment using Fe-23.2at%Nd alloy at 1673 K. The errors have been evaluated by taking the standard deviation for the analyses on each condition or point and reflected in error bars. The oxygen concentration decreases with time on the contrary to the expectation for normal cases where only dissolution of oxide into the metal occurs. It can be explained as follows. At the beginning of the experiment, the oxygen concentration of the portion firstly melted which was enriched with Nd was higher than oxygen solubility of completely melted Fe-23.2at%Nd alloy at 1673 K. Therefore, Nd2O3 gradually formed as inclusions in the molten metal until complete melting of Fe-23.2at%Nd due to the significant decrease in oxygen solubility. Then, Nd2O3 inclusions migrated and sticked to the crucible wall and were then removed from the metal phase. That is the reason why the total oxygen concentration involving possibly formed Nd2O3 inclusions decreases with a lapse of time. Consequently, the present alloys with higher Fe content whose initial oxygen concentration are higher than oxygen solubility need a long time to achieve equilibration. According to results of these preliminary experiments, the holding time of Fe-rich alloys has been determined to be 240 h.
Change in oxygen concentration with time in molten Fe-23.2at%Nd alloy at 1673 K.
On the other hand, equilibration time for the Fe-Nd alloys richer in Nd beyond the composition of Fe-23.2at%Nd has been determined as follows. Sano et al.4) have investigated oxygen solubility of neodymium at 1473 K, and holding time for equilibration has been determined to be 12 h. Nagata et al.6) have evaluated the diffusivity of oxygen in molten iron, the order of which was 10−4～10−5/cm2・s−1. Andrew et al.7) have derived the relationship between diffusion time and diffusivity in the cylindrical coordinate system. On the basis of their investigations, the holding time for equilibration of oxygen in iron at 1473 K has been estimated to be 12 h. Hence, the holding time for the molten Fe-Nd alloys except for Fe-rich region has been determined to be 24 h in the present study.
3.2 Identification of oxide phase equilibrated with Fe-Nd alloysThe samples after the equilibration were observed by SEM-EDS. Figure 3 shows the mapping images for the elements in the Fe-23.2at%Nd held at 1673 K. Warm or bright colors show high concentration region, while cold or dark and black colors show low concentration region. EDS analysis has shown that neither precipitation nor newly formed phase was observed near the interface between the Fe-Nd alloy and the Nd2O3 crucible. The Nd2O3 crucibles after the equilibration were analyzed by XRD. Figure 4 shows an XRD analysis result for the Fe-23.2at%Nd alloy held at 1673 K. No compounds other than Nd2O3 were detected in the present alloys of the Fe-Nd-O system. Therefore, the oxide phase equilibrated with molten Fe-Nd alloy has been identified to be Nd2O3.
Element mapping analysis of Fe-23.2at%Nd alloy held at 1673 K.
XRD analysis result for the crucible of Fe-23.2at%Nd held at 1673 K.
The oxygen solubility of molten Fe-Nd alloys and molten Nd equilibrated with solid Nd2O3 are shown in Table 2 and Fig. 5. The errors have been evaluated by taking the standard deviation for the analyses on each condition or point and reflected in error bars. The oxygen solubility increases with an increase in neodymium contents of alloys and temperature.
| No. | Temperature, T/K | Composition (XNd) | O Concentration (mass%) |
|---|---|---|---|
| 1 | 1673 | 0.089 | 0.0108 ± 0.00323 |
| 2 | 0.232 | 0.0288 ± 0.00224 | |
| 3 | 0.558 | 0.252 ± 0.0644 | |
| 4 | 0.708 | 0.519 ± 0.0370 | |
| 5 | 1 | 0.997 ± 0.00591 | |
| 6 | 1473 | 0.604 | 0.112 ± 0.0212 |
| 7 | 0.778 | 0.253 ± 0.0773 | |
| 8 | 1 | 0.3254) |
Dependence of oxygen solubility of molten Fe-Nd alloys on neodymium concentration.
Equations (1) through (4) show the reaction between molten neodymium in Fe-Nd alloy and solid neodymium oxide, standard Gibbs energy for this reaction8) and its equilibrium constant respectively.
| \[{\rm 2Nd} (\text{l in Fe-Nd alloy}) + 3/2{\rm O}_2({\rm g}) = {\rm Nd}_2 {\rm O}_3({\rm s})\] | (1) |
| \[\Delta G^\circ = -1813 + 0.285{\rm T}\ ({\rm kJ/mol})\] | (2) |
| \[K = a_{{\rm Nd}_2{\rm O}_3} / \left( \left( \gamma_{\rm Nd} X_{\rm Nd} \right)^2 \cdot P_{{\rm O}_2}^{3/2} \right)\] | (3) |
| \[K = \exp (-\Delta G^\circ / RT)\] | (4) |
| \[P_{{\rm O}_2} = \left( a_{{\rm Nd}_2 {\rm O}_3({\rm s})} / \left( X_{\rm Nd}^2 \times \exp (-\Delta G^\circ / RT) \right) \right)^{2/3}\] | (5) |
| No. | Temperature, T/K | Composition (XNd) | Equilibrium $P_{{\rm O}_2}$, $P_{{\rm O}_2}$/atm |
|---|---|---|---|
| 1 | 1673 | 0.089 | 3.88 × 10−27 |
| 2 | 0.232 | 1.08 × 10−27 | |
| 3 | 0.558 | 3.36 × 10−28 | |
| 4 | 0.708 | 2.44 × 10−28 | |
| 5 | 1 | 1.54 × 10−28 | |
| 6 | 1473 | 0.604 | 3.59 × 10−33 |
| 7 | 0.778 | 2.19 × 10−33 | |
| 8 | 1 | 1.16 × 10−33 |
The reaction and standard Gibbs energies for the dissolution of oxygen into molten Fe-Nd alloy are expressed by eqs. (6) and (7);
| \[1/2{\rm O}_2({\rm g}) = {\rm O}(X_{\rm O}, \text{in Fe-Nd alloy})\] | (6) |
| \[\Delta G^\circ = -RT \ln \left( \gamma_{\rm O} X_{\rm O}/P_{{\rm O}_2}^{1/2} \right)\] | (7) |
Dependence of standard Gibbs energies for dissolution of oxygen into molten Fe-Nd alloys on neodymium concentration.
Dependence of the standard Gibbs energies for the reaction of eq. (6) on neodymium content at 1673 K are examined on the basis of the data on Table 4. The standard Gibbs energies for the reaction of eq. (6) at 1673 K are obtained as eq. (8) as a quadratic function of neodymium molar fraction by the least square method.
| \[\Delta G^\circ (X_{\rm O}, \text{in Fe-Nd}) = -298{.}09 - 201X_{\rm Nd} - 88X_{\rm Nd}^2\ ({\rm kJ}/{\rm mol})\] | (8) |
Fe-60.4at%Nd:
| \[\Delta G^\circ = -470{.}1 + 0{.}0492T\ ({\rm kJ}/{\rm mol})\ (1473\ {\rm to}\ 1673\,{\rm K})\] | (9) |
Fe-77.8at%Nd:
| \[\Delta G^\circ = -489{.}1 + 0{.}0522T\ ({\rm kJ}/{\rm mol})\ (1473\ {\rm to}\ 1673\,{\rm K})\] | (10) |
Temperature dependence of standard Gibbs energies for dissolution of oxygen into molten metals.
One of the present authors has investigated the standard Gibbs energies for the dissolution of oxygen into molten Ti-Al alloys and calculated those into hypothetical molten Ti and Al using regular solution model13). However, the mixing enthalpy for the Fe-Nd alloy is too small to compensate the energy gap between regular solution model and real solution of the present study. As shown in Fig. 7, the standard Gibbs energies for the dissolution of oxygen into molten Ti-Al alloys are situated between the lines for dissolution energies for molten titanium and aluminum in proportion to molten Ti-Al composition. It means that oxygen atoms in molten Ti-Al alloy are uniformly dispersed in the solution. However, the standard Gibbs energies for the dissolution of oxygen into molten Fe-Nd alloy are quite closer to that into molten Nd and very far from than that into molten Fe. It indicates that oxygen atoms in molten Fe-Nd alloy preferentially exist near neodymium atoms due to very strong affinity between oxygen and neodymium. Namely, the dissolution behavior of oxygen into molten Fe-Nd alloy is thermodynamically dominated by chemical property of neodymium.
4.2 Estimation on deoxidation proposed in practical processesThe standard Gibbs energies for the dissolution of oxygen into Fe-Nd alloy experimentally obtained can predict deoxidation from molten Nd-Fe-B alloy which is raw material of Nd-Fe-B magnet. Among the candidates for deoxidizing elements, silicon and aluminum may not be promising due to less stability as oxides than neodymium oxide14). Hence, we take up deoxidation by adding calcium or neodymium fluoride (NdF3) for the proposal in practical deoxidation processes. The composition of Nd-Fe-B magnet is basically Nd2Fe14B; therefore, molten Nd-Fe-B alloy is simulated to be Fe-12.5at%Nd alloy. The standard Gibbs energy for dissolution of oxygen into Fe-12.5at%Nd at 1673 K is derived from eqs. (8), (11) and (12) as eq. (13).
| \[{\rm O}({\rm X}_{\rm O}, \text{in Fe-Nd alloy}) = {\rm O}({\rm mass\%O}, \text{in Fe-Nd alloy})\] | (11) |
| \[\Delta G^\circ = -RT \ln (X_{\rm O}/{\rm mass\%O})\] | (12) |
| \[\Delta G^\circ ({\rm mass\%O}, \text{in Fe-12.5at}\%\text{Nd}) = -366{.}05\,{\rm kJ/mol}\] | (13) |
| \[{\rm Ca}({\rm l}) + 1/2{\rm O}_2({\rm g}) = {\rm CaO}({\rm s})\] | (14) |
| \[\Delta G^\circ = -658 + 0{.}113T\ ({\rm kJ}/{\rm mol})\] | (15) |
| \[2/3{\rm Nd}({\rm l}) + 1/2{\rm O}_2({\rm g}) + 1/3{\rm NdF}_3({\rm l}) = {\rm NdOF}({\rm s})\] | (16) |
| \[\Delta G^\circ = -661 + 0{.}101T\ ({\rm kJ}/{\rm mol})\] | (17) |
| \[{\rm Ca}({\rm l}) + {\rm O}({\rm mass\%O}, \text{in Fe-12.5at}\%\text{Nd}) = {\rm CaO}({\rm s})\] | (18) |
| \[\Delta G^\circ = -RT \ln (a_{{\rm CaO}({\rm s})}/a_{{\rm Ca}({\rm l})} f_{\rm O}[{\rm mass\%O}]) = -102{.}9\,{\rm kJ/mol}\] | (19) |
| \begin{align*} & 2/3{\rm Nd}({\rm l}) + {\rm O}({\rm mass\%O}, \text{in Fe-12.5at}\%\text{Nd}) + 1/3{\rm NdF}_3({\rm l}) \\ &\quad = {\rm NdOF}({\rm s}) \end{align*} | (20) |
| \begin{align*} \Delta G^\circ &= -RT \ln \left( a_{{\rm NdOF}({\rm s})}/a_{{\rm Nd}({\rm l})}^{2/3} f_{\rm O}[{\rm mass\%O}] a_{{\rm NdF}_3({\rm l})}^{1/3} \right) \\ &= -126{.}2\,{\rm kJ/mol} \end{align*} | (21) |
Dependence of oxygen concentrations of molten Fe-12.5at%Nd deoxidized by additions of calcium and neodymium fluoride at 1673 K on activity of Ca and NdF3.
The authors are grateful to the Elements Strategy Initiative Center for Magnetic Materials for their financial support to this research in the Elements Strategy Project, launched by Ministry of Education, Culture, Sports, Science and Technology (MEXT).
