Novel hydrogen decrepitation behaviors of (La, Ce)-Fe-B strips

Since its discovery in 1980s, Nd₂Fe₁₄B has been the strongest permanent magnets, which can provide a constant magnetic field once magnetized.^1–3 Despite the close-to-ideal magnetic performance, Nd-Fe-B magnet is currently in a dilemma due to the increasing demand and tightening supply of closely relied rare earth (RE) Nd/Pr/Dy/Tb.⁴ In this regard, potential substitutes La/Ce with high crustal abundance and low cost have stimulated intense research.^5–9 Though La₂Fe₁₄B and Ce₂Fe₁₄B compounds exhibit inferior intrinsic properties to Nd₂Fe₁₄B,^10,11 recent investigations have revealed that preferable magnetic performance can be obtained in the (Nd, La, Ce)-Fe-B sintered magnets.^12–15 For instance, remanence B_r = 13.7 kGs, coercivity H_cj = 12.0 kOe and maximum energy product (BH)_max = 45.0 MGOe have been achieved at 20 wt. % Ce substitution.¹² Consequently, the low-cost (Nd, La, Ce)-Fe-B sintered magnet is appealing for mass production.

Hydrogen decrepitation (HD) has been widely applied for large-scale fabrication of the Nd-Fe-B magnetic powders.^16–19 Upon hydrogenation, the initial Nd-Fe-B alloys are crushed into small particles ascribed to the lattice expansion, subsequent surface stresses, and final intergranular fracture. The coarse powders produced by HD process are brittle and friable, being more suitable for the fine re-crushing (e.g. jet milling) than the conventional powders.¹⁶ The HD process has been demonstrated to exert a profound effect on the particle size and shape, and microstructure of the final magnets.^16–19 La and Ce substitution in both the RE₂Fe₁₄B matrix and RE-rich intergranular phases may result in distinct HD behaviors compared to the conventional Nd-Fe-B strips, influencing the microstructure and magnetic properties to a certain extent. However, in spite of extensive studies^20–24 on the processing of RE-Fe-B magnets, including strip casting, sintering and annealing, little is known on how La and Ce substitution affects the hydrogen absorption. It is necessary to study the HD behaviors of (La, Ce)-Fe-B strips to further modify the magnetic properties and to promote La/Ce for mass production.

Alloys with nominal compositions of Nd_31.5Fe_balB_1.0 and (La, Ce)_31.5Fe_balB_1.0 were prepared by induction melting and subsequent strip-casting technique. The raw materials are high-purity (above 99.5%) La-Ce alloy (35 wt. % La-65 wt. % Ce), Fe-B alloy (81.5 wt. % Fe-18.5 wt. % B), Nd and Fe metal. Detailed compositions of Nd-Fe-B and (La, Ce)-Fe-B strips (listed in Table I) were identified by ion coupled plasma (ICP) analysis. Room-temperature magnetization curves of Nd-Fe-B and (La, Ce)-Fe-B were measured by Physical Property Measurement System (PPMS). Thermomagnetic curves were measured by vibration sample magnetometer (VSM) at 2 K/min with an external field of 200 Oe.

The hydrogenation of Nd-Fe-B and (La, Ce)-Fe-B strips were performed in a homemade Sieverts-type apparatus. The sample accurately weighing 100 g was loaded into a stainless steel holder connected to a thermocouple to detect the reactor temperature. The pressure inside the sample holder was monitored by pressure sensor. The mass of absorbed hydrogen was determined by the gravimetric method. The Nd-Fe-B strip is hydrogenated at the starting hydrogen pressure of 2.5 MPa. For the (La, Ce)-Fe-B strip, the hydrogen pressures are varied to be 2.5, 4.0 and 5.5 MPa. X-ray diffraction (XRD-6000) was used to identify the phase constituents. The microstructure was observed under a field-emission scanning electron microscopy (FE-SEM SIRION-100).

Fig. 1 compares the magnetic properties of Nd-Fe-B and (La, Ce)-Fe-B strips. The magnetization saturates at 90 kOe, the value at which is then regarded as saturation magnetization M_s. Curie temperature T_C of 2:14:1 phase is determined via the thermomagnetic curve (380∼650 K). Obviously, Nd-Fe-B exhibits higher M_s and T_C (158 emu/g and 581.4 K) than (La, Ce)-Fe-B (121 emu/g and 458.6 K), which accords well with previous publications.¹⁰ The hysteresis loops for Nd-Fe-B and (La, Ce)-Fe-B are not shown here, since further modification on the processing procedures (e.g. sintering and annealing) is required to fabricate (La, Ce)-Fe-B bulk magnets. However, the available investigations^25–27 have revealed that the (La, Ce)-Fe-B system exhibits acceptable magnetic performance, e.g. B_r = 7.39 kGs, H_cj = 3.55 kOe and (BH)_max = 8.29 MGOe. Consequently, in spite of the deteriorated magnetic properties upon La-Ce substitution for Nd, (La, Ce)-Fe-B is still a promising candidate with moderate performance to plug the gap between Nd-Fe-B and ferrites. Therefore, the following research on the HD behavior of (La, Ce)-Fe-B strip is necessary for further commercial application.

Fig. 2 compares the HD behaviors of Nd-Fe-B and (La, Ce)-Fe-B strips. The hydrogen pressure P_H2 vs. time and temperature vs. time plots are presented by exposing the strips to pure hydrogen environment with changing pressure from 2.5 to 5.5 MPa. For the Nd-Fe-B system, the hydrogen absorption process leads to decreased hydrogen pressure from the initial 2.5 MPa to 0.7 MPa (Fig. 2(a)). The Nd-rich phase in the Nd-Fe-B strips begins to absorb hydrogen at room temperature, which is an exothermic reaction. Along with the increased reactor temperature, the hydrogen absorption into the Nd₂Fe₁₄B matrix phase^28–30 is then activated. After completing the HD process, the corresponding weight increase of the Nd-Fe-B system is 1.14 g (Table II). Comparably, the hydrogenation effect in (La, Ce)-Fe-B strips upon the same initial hydrogen pressure P_H2 of 2.5 MPa is remarkably weakened, as demonstrated by the higher end pressure of 1.6 MPa and lower exothermic effects in Fig. 2(b). It is in good accordance with the low weight increase of 0.31 g, which is merely one third of that for the Nd-Fe-B strip. Increasing the initial P_H2 to 4.0 MPa can improve the hydrogen absorption capability of (La, Ce)-Fe-B system, as the end P_H2 is reduced to 2.8 MPa. It indicates that 1.2 MPa hydrogen has been consumed during the HD process, which is higher than 0.9 MPa in Fig. 2(b). However, further increasing the hydrogen pressure to 5.5 MPa, the hydrogen absorption behavior cannot be continually improved, as demonstrated by the similar total loss of hydrogen (1.2 MPa) and resemble exothermal valleys (Fig. 2(d)). This is further confirmed by the same weight increase of the reactor for (La, Ce)-Fe-B system hydrogenated under 4.0 and 5.5 MPa (0.48 g). It appears that the (La, Ce)-Fe-B strip exhibits a similar hydrogen absorption pattern to the conventional Nd-Fe-B one except that the amount of absorbed hydrogen is smaller.

Fig. 3 shows the XRD profiles of Nd-Fe-B and (La, Ce)-Fe-B strips after decrepitation at different hydrogen pressures. The 2:14:1 tetragonal phase is formed for all samples with the characteristic reflections corresponding to those of RE₂Fe₁₄B. It is concluded that similar to the Nd-Fe-B strip hydrogenated at 2.5 MPa, increasing P_H2 from 2.5 to 5.5 MPa do not change the tetragonal structure of 2:14:1 phase in the (La, Ce)-Fe-B system. Besides, compared to the standard JSPDS data of Nd₂Fe₁₄B tetragonal phase (space group P4₂/mnm, PDF#70-1403, in the bottom of Fig. 3), typical reflections for 2:14:1 phase shift to lower Bragg angle, suggesting the slightly increased lattice parameters after HD process. For the (La, Ce)-Fe-B system, increasing the hydrogen pressures from 2.5 to 4.0 and 5.5 MPa can shift the characteristic diffraction peaks of 2:14:1 phase to the lower side, indicating the lattice expansion and enhanced hydrogen absorption capability, which is in good agreement with the former results (Fig. 2 and Table II).

From the SEM images in Fig. 4(a), both the intergranular and transgranular fractures can be observed in the conventional Nd-Fe-B system, suggesting that the Nd-rich intergranular phase and Nd₂Fe₁₄B matrix phase absorb hydrogen upon decrepitation at the pressure of 2.5 MPa. For the (La, Ce)-Fe-B strip hydrogenated at the same P_H2 of 2.5 MPa, less cracks exist and the final particle size is much larger, which indicates that the HD process is insufficient. Increasing the P_H2 to 4.0 and 5.5 MPa, the fracture morphology is much similar to that of the Nd-Fe-B ones. It proves that higher P_H2 can enhance the HD process, being consistent with the HD behaviors in Fig. 2.

The above results demonstrate that the Nd-Fe-B strip possesses the most obvious exothermic effect upon HD at 2.5 MPa. Comparably, increasing P_H2 from 2.5 to 5.5 MPa can gradually enhance the hydrogen absorption behaviors of (La, Ce)-Fe-B strip, which though are still inferior to that of Nd-Fe-B system. Given that Nd-Fe-B and (La, Ce)-Fe-B strips have the similar total rare earth (TRE) contents (Table I), their excessive RE contents relative to the stoichiometric RE₂Fe₁₄B are basically identical. Early work has shown that the hydrogen absorption capability is directly proportional to the content of RE-rich phase.¹⁶ It is therefore inferred that the volume fraction of RE-rich phase differs a lot between Nd-Fe-B and (La, Ce)-Fe-B strips. Previous research also revealed that the oxidation degree of strips highly affects the hydrogen absorption behaviors.³¹ Results show that the oxygen content for the (La, Ce)-Fe-B strip is 560 ppm, much higher than that of the Nd-Fe-B one (116 ppm, see Table I). It should be attributed to the higher oxygen affinity of La and Ce than Nd.^32,33 Therefore more RE-rich intergranular phase of the (La, Ce)-Fe-B strip appears in the form of oxide, lowering the amount of active RE-rich metallic phase and decreasing the channel for hydrogen diffusion. In the following, the hydrogen absorption into the 2:14:1 matrix phase is significantly weakened. It explains the lower hydrogen consumption, less exothermic effect, and smaller weight increase after the completed HD process for (La, Ce)-Fe-B system.

In this work, different HD behaviors between (La, Ce)-Fe-B and conventional Nd-Fe-B strips have been systematically investigated. XRD characterization reveals that increasing P_H2 are not deteriorating the 2:14:1 tetragonal structure of (La, Ce)-Fe-B system even hydrogenated upon a high pressure of 5.5 MPa, similar to Nd-Fe-B decrepitated at 2.5 MPa. Since the Nd₂Fe₁₄B-type tetragonal crystal structure plays an indispensable role in affording large H_A and M_s,¹⁰ the present work demonstrates that La and Ce substitution can guarantee the stability of 2:14:1 tetragonal structure, being promising to yield moderate magnetic performance. Besides, increasing hydrogen pressure can gradually enhance the hydrogen absorption capability of (La, Ce)-Fe-B system, which is still inferior to that of Nd-Fe-B. It should be ascribed to the higher oxygen affinity of La and Ce than Nd, leading to the oxidation of RE-rich phase in (La, Ce)-Fe-B strips and deteriorated hydrogenation behaviors. Consequently, in order to modify the microstructure and enhance the magnetic properties of RE-Fe-B magnets with high La-Ce substitution, the precise oxygen control becomes more important. To sum up, the knowledge of HD behaviors is necessary for developing low-cost and high-performance (Nd, La, Ce)-Fe-B sintered magnets, which acts as a part of endeavor to balance the usage of rare earth sources.

This work was supported by the National Natural Science Foundation of China (Nos. 51590881, 51571176 and 51622104), the National Key Research and Development Program of China (No. 2016YFB0700902), the Key Research and Development Program of Zhejiang Province (No. 2017C01031), and the China Postdoctoral Science Foundation (No. 2017M611984).