In the past decades, the development of embodied robots has transitioned from conceptual prototypes to practical machines capable of performing complex tasks. As an engineer and researcher working on magnetic materials, I have focused on the preparation and performance improvement of sintered NdFeB magnets specifically designed for the actuators and servo motors used in embodied robots. The cost of high-performance sintered NdFeB magnets constitutes a major share of the total cost of embodied robots, since the servo motors consume approximately 20% of the entire robot budget, and the magnets themselves represent around 50% of the motor cost. Therefore, the central goal of my research is to produce lower-cost sintered NdFeB magnets without compromising the required magnetic properties, while also exploring innovative modification routes to enhance their performance. This article summarizes my experimental studies, which include component design, additive optimization, and pilot-scale process control, all aimed at enabling the widespread adoption of embodied robots.
1. Introduction
Embodied robots, as opposed to purely software-based artificial agents, require physical hardware that can interact with the real world. The mechanical structure of an embodied robot depends heavily on high-torque-density motors, especially permanent magnet synchronous motors. Sintered NdFeB magnets are the preferred choice for these motors because they offer high remanence, high coercivity, and excellent magnetic energy product. However, the standard formulations using heavy rare earth elements such as dysprosium and terbium are expensive and may become unsustainable. In this research, I aimed to develop several categories of sintered NdFeB magnets with reduced reliance on expensive elements, using more abundant rare earths such as cerium and gadolinium, while still maintaining adequate magnetic properties for embodied robot applications.
This study comprises three main parts. First, I designed three families of sintered NdFeB magnets with different compositions, varying the concentrations of aluminum, dysprosium, and cerium to identify the optimal balance between cost and magnetic performance. Second, I investigated the effects of six different modifiers—three antioxidants and three lubricants—on the magnetic properties of the three optimized magnets. The optimal modifier combination was then applied to the production of a high-cerium magnet. Third, I scaled up the process to a pilot production level, optimizing key parameters such as jet milling speed, orientation field, sintering temperature, and heat treatment conditions. Throughout this article I emphasize the relevance of these developments to the manufacturing of compact, reliable, and affordable motors for embodied robots.
2. Experimental Materials and Methods
2.1 Design of Alloy Compositions
The principle of using high-abundance elements while reducing the amount of precious elements guided the design of three magnet families. The general compositions are shown in Table 1, where M represents a mixture of additive elements including Al, Co, Zr, Cu, Ga, Ti, etc. I denote the three families as HR-free magnets, low-Dy magnets, and high-Ce magnets.
| Family | Nominal Composition (wt.%) | Variable Element |
|---|---|---|
| I: HR-free | (PrNd)30.8FebalM1.43B1 | Al: 0 – 0.4 |
| II: Low-Dy | (PrNd)22.8Gd1.8Ce7Dy0.3–0.7FebalM1.85B0.92 | Dy: 0.3 – 0.7 |
| III: High-Ce | (PrNd)17Ce12–16FebalM1.55B0.9 | Ce: 12 – 16 |
The HR-free magnet contains no expensive heavy rare earth, relying instead on aluminum and other light elements to enhance coercivity. The low-Dy magnet reduces the dysprosium content substantially by employing gadolinium and cerium as co-additives, which is beneficial for elevated temperature operation in embodied robot joints. The high-Ce magnet maximizes the use of cerium, the most abundant rare earth element, thereby dramatically lowering raw material cost. For each family, I prepared a series of magnets with different contents of the variable element using the conventional powder metallurgy route.
2.2 Sample Preparation
All magnets were fabricated through the standard sintered NdFeB process. The starting raw materials were mixed according to the nominal compositions and melted in a vacuum induction melting furnace with a rapid solidification (strip-casting) wheel. The resulting alloy flakes were then subjected to hydrogen decrepitation, followed by jet milling with a nitrogen or helium atmosphere to obtain fine powders with a median particle size between 2.8 and 3.3 μm. The powder was oriented and pressed in a magnetic field, followed by cold isostatic pressing. The green compacts were sintered in vacuum at temperatures ranging from 1030 °C to 1070 °C, and subsequently underwent two-stage heat treatment. Table 2 lists the major equipment and their roles in the preparation route.
| Equipment | Function |
|---|---|
| Vacuum melting/Strip casting furnace | Melting and rapid solidification to form alloy flakes |
| Hydrogen decrepitation furnace | Crushing alloy flakes by hydrogen absorption |
| Jet mill (AFG) | Fine grinding to single-crystal powder particles |
| Orientation press (BDM-350/G) | Aligning particles in a magnetic field and compacting |
| Vacuum sintering furnace | Sintering and heat treatment of green bodies |
| NIM-2000 magnetic measurement system | Measuring remanence, coercivity, and energy product |
The magnetic properties, namely remanence \(B_r\), intrinsic coercivity \(H_{cj}\), and maximum energy product \((BH)_{\max}\), were evaluated using a closed-circuit permanence meter. The density of sintered samples was determined by the Archimedes method. Microstructural observation was performed using scanning electron microscopy on polished cross sections.
2.3 Modifier Types and Addition Strategies
I selected six commercially available modifiers: three antioxidants (designated A, B, C) and three lubricants (designated D, E, F). The modifiers were introduced at different stages of the jet milling and compacting processes. In the preliminary screening, each modifier was added in amounts of 0.6, 0.8, 1.0, 1.2, and 1.4 mL per kilogram of powder. The powder was first coated with the modifier during jet milling, and the same dose was again added before compaction. In the subsequent combination study, I tested five addition strategies: (A+A), (D+D), (A+D), (D+A), and no modifier. Here, the first letter indicates the modifier added before jet milling, while the second letter indicates the modifier added during the blending step before pressing.
3. Composition Optimization and Magnetic Properties
3.1 HR-free Magnets with Different Al Content
I prepared five HR-free magnets with aluminum contents of 0%, 0.1%, 0.2%, 0.3%, and 0.4%. The measured magnetic properties are summarized in Table 3.
| Al (wt.%) | \(B_r\) (kGs) | \(H_{cj}\) (kOe) | \((BH)_{\max}\) (MGOe) |
|---|---|---|---|
| 0 | 14.64 | 12.20 | 51.20 |
| 0.1 | 14.29 | 12.45 | 48.55 |
| 0.2 | 14.18 | 12.90 | 47.67 |
| 0.3 | 14.16 | 13.42 | 48.07 |
| 0.4 | 14.14 | 13.65 | 48.38 |
As the aluminum content increased, the remanence gradually decreased from 14.64 to 14.14 kGs. This reduction is attributed to the substitution of non-magnetic aluminum within the main phase, which dilutes the saturation magnetization. On the other hand, the coercivity increased from 12.20 to 13.65 kOe because aluminum enriches the grain boundary phase, improves the magnetic isolation between main phase grains, and refines the microstructure. The incremental improvement in coercivity becomes marginal above 0.3% Al, while the reduction in remanence also becomes less steep. Therefore, I selected 0.3% Al as the optimal content for the HR-free magnet, providing a balance between \(B_r = 14.16\) kGs and \(H_{cj} = 13.42\) kOe.
3.2 Low-Dy Magnets with Different Dy Content
The second family was designed to reduce the heavy rare earth content. I prepared five samples with Dy concentrations of 0.3%, 0.4%, 0.5%, 0.6%, and 0.7%. Table 4 reports their magnetic properties.
| Dy (wt.%) | \(B_r\) (kGs) | \(H_{cj}\) (kOe) | \((BH)_{\max}\) (MGOe) |
|---|---|---|---|
| 0.3 | 12.32 | 16.06 | 35.29 |
| 0.4 | 12.19 | 16.43 | 34.43 |
| 0.5 | 12.00 | 16.78 | 34.45 |
| 0.6 | 11.95 | 17.11 | 34.21 |
| 0.7 | 11.94 | 17.21 | 34.24 |
Increasing Dy from 0.3% to 0.7% leads to a slight decrease in remanence, from 12.32 to 11.94 kGs, while the coercivity improves significantly up to 0.6% Dy, after which it tends to saturate. The enhancement in \(H_{cj}\) arises from the formation of a (Nd, Dy)2Fe14B phase with a higher anisotropy field, which suppresses the nucleation of reversed domains at grain surfaces. The loss in remanence is relatively small, making Dy at 0.6% an attractive compromise. Hence, the low-Dy magnet chosen for subsequent modifier studies has the composition (PrNd)22.8Gd1.8Ce7Dy0.6FebalM1.85B0.92.
3.3 High-Ce Magnets with Different Ce Content
The high-Ce family is the most economical because cerium is abundant and much cheaper than neodymium. I prepared magnets with Ce contents of 12%, 13%, 14%, 15%, and 16%. Their magnetic properties are shown in Table 5.
| Ce (wt.%) | \(B_r\) (kGs) | \(H_{cj}\) (kOe) | \((BH)_{\max}\) (MGOe) |
|---|---|---|---|
| 12 | 12.39 | 13.27 | 35.54 |
| 13 | 12.26 | 12.88 | 34.47 |
| 14 | 12.12 | 12.59 | 33.57 |
| 15 | 12.02 | 12.20 | 32.95 |
| 16 | 12.00 | 12.08 | 32.76 |
Unlike aluminum and dysprosium, cerium reduces both remanence and coercivity when it replaces neodymium. The intrinsic magnetic properties of Ce2Fe14B are inherently lower than those of Nd2Fe14B. The decline is relatively linear, but from 15% to 16% the decrease is small. I selected 15% Ce as the optimal content because it provides a good economic benefit without an excessive drop in performance: \(B_r = 12.02\) kGs, \(H_{cj} = 12.20\) kOe, and \((BH)_{\max} = 32.95\) MGOe. This high-Ce magnet is particularly promising for embodied robots where cost is a major constraint and moderate magnetic performance is sufficient for many joints.
The relation between the variable element content \(x\) and the resulting coercivity can be represented by empirical linear fittings. For example, in the low-Dy magnets, \(H_{cj}\) increases with Dy concentration according to a near-linear law:
$$ H_{cj}(x) = H_{cj}(0.3) + \alpha (x – 0.3), $$
where \(\alpha \approx 2.1\) kOe per wt.% Dy in the range from 0.3% to 0.6%, and the slope decreases beyond 0.6%. In the high-Ce magnets, the coercivity decreases linearly with cerium content:
$$ H_{cj}(x) = H_{cj}(12) – \beta (x – 12), $$
with \(\beta \approx 1.2\) kOe per wt.% Ce. Such formulas are useful for predicting magnet grades from a known composition and for designing the optimal balance between cost and performance in real embodied robot applications.
4. Improvement Using Modifiers
4.1 Screening of Individual Modifiers
I applied each of the six modifiers to the three selected magnets. The results consistently showed that antioxidant A and lubricant D outperform the others. Tables 6, 7, and 8 summarize the magnetic properties for the HR-free, low-Dy, and high-Ce magnets, respectively, when the modifier is added at different concentrations. For brevity, I present the complete data for the high-Ce magnet in Table 6, while Tables 7 and 8 list the remanence and coercivity for all three magnets.
| Modifier | Dose (mL/kg) | \(B_r\) (kGs) | \(H_{cj}\) (kOe) |
|---|---|---|---|
| A | 0.6 | 12.27 | 12.18 |
| A | 0.8 | 12.36 | 12.21 |
| A | 1.0 | 12.53 | 12.40 |
| A | 1.2 | 12.62 | 12.65 |
| A | 1.4 | 12.31 | 12.26 |
| B | 1.2 | 12.54 | 12.50 |
| C | 1.2 | 12.42 | 12.40 |
| D | 1.2 | 12.55 | 12.53 |
| E | 1.2 | 12.44 | 12.48 |
| F | 1.2 | 12.32 | 12.39 |
For every magnet family, the optimum dose of both A and D is 1.2 mL/kg. At this dose, antioxidant A yields a remanence of 14.43 kGs and a coercivity of 13.76 kOe for the HR-free magnet; for the low-Dy magnet, \(B_r\) reaches 12.25 kGs and \(H_{cj}\) reaches 17.69 kOe; for the high-Ce magnet, \(B_r\) reaches 12.62 kGs and \(H_{cj}\) reaches 12.65 kOe. Lubricant D also produces substantial gains, but slightly lower than those obtained with A. This suggests that the antioxidant A is more effective in suppressing oxidation at the grain boundaries, while the lubricant D primarily improves particle mobility and green density.
4.2 Combined Addition of Modifiers
I then studied the effect of combining the best antioxidant (A) and the best lubricant (D) in different orders. Table 9 lists the results for the high-Ce magnet as an example, and Table 10 summarizes the outcomes for all three magnets.
| Addition sequence | \(B_r\) (kGs) | \(H_{cj}\) (kOe) | \((BH)_{\max}\) (MGOe) |
|---|---|---|---|
| A + A | 12.42 | 12.34 | 35.72 |
| D + D | 12.29 | 12.21 | 34.64 |
| A + D | 12.54 | 12.50 | 37.24 |
| D + A | 12.61 | 12.69 | 37.58 |
| None | 12.20 | 12.01 | 33.75 |
| Magnet | \(B_r\) (kGs) | \(H_{cj}\) (kOe) | \((BH)_{\max}\) (MGOe) |
|---|---|---|---|
| HR-free | 14.46 | 13.86 | 50.00 |
| Low-Dy | 12.29 | 17.65 | 35.36 |
| High-Ce | 12.61 | 12.69 | 37.58 |
The sequence in which the lubricant D is added before jet milling and the antioxidant A is added after jet milling (D+A) gives the largest enhancement. This order makes physical sense: the lubricant reduces particle agglomeration and friction during milling, producing a finer and more uniform powder; the antioxidant then protects the fresh particle surfaces from oxidation during subsequent handling and compaction. The result is improved grain orientation, denser microstructure, and stronger exchange decoupling. In contrast, adding A before jet milling and D after (A+D) is also beneficial but less effective, because the lubricant cannot fully act on already oxidized surfaces and the antioxidant is consumed during milling. The synergistic effect of D+A adds value to the low-cost high-Ce magnet, making it more attractive for embodied robot motors.
5. Pilot-Scale Production and Process Optimization
Once the optimal modifier combination was established, I scaled up the high-Ce magnet to a pilot production line. The objective was to identify key process parameters that would yield consistent performance in batches large enough for assembling motors for embodied robots. The pilot equipment handled 50 kg batches, and I systematically optimized the jet milling speed, orientation field, sintering temperature, and heat treatment schedule.
5.1 Jet Milling Speed
I tested jet mill speeds from 3800 to 4200 rpm while maintaining a constant lubricant D addition of 1.2 mL/kg. The resulting powder particle size and magnetic properties are summarized in Table 11.
| Speed (rpm) | Particle size \(d_{50}\) (μm) | \(B_r\) (kGs) | \(H_{cj}\) (kOe) | \((BH)_{\max}\) (MGOe) |
|---|---|---|---|---|
| 3800 | 3.2 – 3.3 | 12.30 | 12.25 | 35.88 |
| 3900 | 3.1 – 3.2 | 12.53 | 12.44 | 36.15 |
| 4000 | 3.0 – 3.1 | 12.61 | 12.69 | 37.58 |
| 4100 | 2.9 – 3.0 | 12.57 | 12.36 | 35.94 |
| 4200 | 2.8 – 2.9 | 12.41 | 12.22 | 36.02 |
The optimal speed is 4000 rpm, which produces a median particle size of about 3.0–3.1 μm. Finer particles improve the alignment and sintering activity, but overly fine powders exhibit increased oxidation and surface defects, causing a drop in both \(B_r\) and \(H_{cj}\). The particle size distribution can be described by a log-normal function, and the median size \(d_{50}\) decreases approximately linearly with jet mill speed:
$$ d_{50} = 6.8 – 0.98 \times 10^{-3} \cdot v, $$
where \(v\) is the rotational speed in rpm. This empirical relation is valid in the range of 3800–4200 rpm and helps predict the optimum operating window.
5.2 Orientation Field
I varied the orientation field during die pressing from 1.5 T to 2.2 T, with a constant holding time of 60 seconds and adding antioxidant A at 1.2 mL/kg. Table 12 shows the measured properties.
| Field (T) | Holding time (s) | \(B_r\) (kGs) | \(H_{cj}\) (kOe) | \((BH)_{\max}\) (MGOe) |
|---|---|---|---|---|
| 1.5 | 60 | 12.43 | 12.15 | 36.36 |
| 1.8 | 60 | 12.61 | 12.69 | 37.58 |
| 2.0 | 60 | 12.55 | 12.41 | 36.81 |
| 2.2 | 60 | 12.26 | 12.08 | 35.77 |
At 1.8 T, the orientation degree reaches an optimum. Higher fields do not necessarily improve the alignment because the magnetic powder experiences strong demagnetizing effects during pressing, and excessive field gradients may disturb the particle orientation. There exists an optimal magnetic field strength \(H_{\text{opt}}\) that maximizes the degree of alignment \(f\) according to a Stoner-Wohlfarth type model:
$$ f = \frac{\int_0^{\pi/2} \exp\left(-\frac{E_a \sin^2\theta}{k_B T}\right) \cos\theta \, \sin\theta \, d\theta}{\int_0^{\pi/2} \exp\left(-\frac{E_a \sin^2\theta}{k_B T}\right) \sin\theta \, d\theta}, $$
where \(E_a = \mu_0 M_s H_{\text{app}} V\) is the magnetic alignment energy. In practice, 1.8 T gives the best balance between particle rotation and powder compaction.
5.3 Sintering Temperature
The sintering temperature was scanned from 1030 °C to 1070 °C with a constant holding time of 6 hours. Table 13 lists the density and magnetic properties.
| Sintering temp. (°C) | Density (g/cm³) | \(B_r\) (kGs) | \(H_{cj}\) (kOe) | \((BH)_{\max}\) (MGOe) |
|---|---|---|---|---|
| 1030 | 7.48 | 11.64 | 11.28 | 31.66 |
| 1040 | 7.51 | 11.96 | 11.54 | 33.52 |
| 1050 | 7.49 | 12.36 | 12.33 | 35.74 |
| 1060 | 7.58 | 12.61 | 12.69 | 37.58 |
| 1070 | 7.56 | 12.14 | 12.24 | 35.31 |
As the sintering temperature rises, the density increases and approaches the theoretical value at 1060 °C. Above this temperature, abnormal grain growth occurs and the coercivity decreases because the grain boundary phase becomes too thick or uneven. The optimum sintering temperature for the high-Ce magnet is therefore 1060 °C. The densification process during the isothermal hold can be approximated by the sintering equation:
$$ \frac{\Delta L}{L_0} = K(T) \, t^{1/n}, $$
where \(\Delta L/L_0\) is the linear shrinkage, \(K(T)\) is a temperature-dependent rate constant, \(t\) is the holding time, and \(n\) is an exponent related to the dominant diffusion mechanism. The experimental data agree with \(n \approx 3\), suggesting grain-boundary diffusion controlled densification.
5.4 Two-Stage Heat Treatment
The first-stage annealing temperature was varied from 860 °C to 910 °C while holding for 2 hours, followed by a second-stage anneal at fixed temperatures. Table 14 reports the effect of the first-stage temperature.
| T1 (°C) | \(B_r\) (kGs) | \(H_{cj}\) (kOe) | \((BH)_{\max}\) (MGOe) |
|---|---|---|---|
| 860 | 12.55 | 11.36 | 36.54 |
| 880 | 12.59 | 11.88 | 36.61 |
| 900 | 12.61 | 12.69 | 37.58 |
| 910 | 12.59 | 12.41 | 36.75 |
The best coercivity is obtained at 900 °C. Above this temperature, the rare-earth-rich phase wets the grain boundaries less effectively and may penetrate into the main phase, leading to reduced decoupling and larger grains. Table 15 shows the effect of the second-stage temperature.
| T2 (°C) | \(B_r\) (kGs) | \(H_{cj}\) (kOe) | \((BH)_{\max}\) (MGOe) |
|---|---|---|---|
| 650 | 12.59 | 12.15 | 37.00 |
| 670 | 12.57 | 12.34 | 37.31 |
| 690 | 12.61 | 12.69 | 37.58 |
| 710 | 12.54 | 11.99 | 36.84 |
A second-stage temperature of 690 °C yields the highest coercivity. Thus, the optimized pilot process includes a first-stage anneal at 900 °C for 2 hours and a second-stage anneal at 690 °C for 5 hours. The complete production parameters are listed in Table 16.
| Step | Parameter | Value |
|---|---|---|
| Jet milling | Speed | 4000 r/min |
| Lubricant D | 1.2 mL/kg | |
| Pressing | Orientation field | 1.8 T |
| Holding time | 60 s | |
| Antioxidant A | 1.2 mL/kg | |
| Sintering | Temperature | 1060 °C for 6 h |
| Heat treatment 1 | Temperature | 900 °C for 2 h |
| Heat treatment 2 | Temperature | 690 °C for 5 h |
5.5 Microstructural Characterization
I performed scanning electron microscopy on the sintered high-Ce magnets with and without the optimized modifier treatment. In the unmodified sample, the main phase grains are surrounded by rare-earth-rich phases, but some pores and weak spots are visible. In the modified sample, the grain boundaries are continuous and clean, and the material appears fully dense without observable holes. This dense microstructure is critical for producing motors with high reliability for embodied robots, because it reduces the risk of mechanical failure under cyclic electromagnetic and thermal loads.
The improvement in microstructure can be related to the decrease in porosity \(P\). If the density \(\rho\) of the sintered magnet increases, the porosity decreases according to:
$$ P = 1 – \frac{\rho}{\rho_{\text{theoretical}}}, $$
where \(\rho_{\text{theoretical}} = 7.60\) g/cm³ for the high-Ce magnet. At the optimized condition, \(\rho = 7.58\) g/cm³, giving \(P\) as low as 0.3%. This near-full density contributes to the observed improvement in magnetic performance and is essential for high-efficiency motors used in embodied robot joints.
6. Economic Impact on Embodied Robots
From the perspective of embodied robot manufacturing, the reduction in magnet cost directly helps lower the overall system cost. The high-Ce magnet developed in this work uses about 15% cerium, which is substantially cheaper than neodymium and dysprosium. Table 17 presents a cost comparison of the three magnet families based on the raw material usage.
| Magnet | Rare earth composition | Cost index |
|---|---|---|
| HR-free | 30.8 wt.% (PrNd), no heavy rare earth | 100 |
| Low-Dy | 22.8 wt.% PrNd, 7 wt.% Ce, 0.6 wt.% Dy | 86 |
| High-Ce | 17 wt.% PrNd, 15 wt.% Ce | 72 |
By selecting the high-Ce magnet with the D+A modifier combination and the optimized pilot process, I can produce a magnet with \(B_r = 12.61\) kGs and \(H_{cj} = 12.69\) kOe at a significantly reduced cost. Such a magnet is suitable for many non-critical servo motors in embodied robots, for example those used in wrist joints, grippers, and lower-torque actuators. The low-Dy magnet, on the other hand, provides a higher coercivity of 17.69 kOe, making it appropriate for applications requiring greater thermal stability, such as motors embedded in the torso or hip joints of embodied robots. The HR-free magnet remains the best choice when the highest energy product is required for the main drive joints.
The performance of a permanent magnet motor is often evaluated through its torque density \(\tau\) and power density. The torque of a surface-mounted permanent magnet motor can be expressed as:
$$ \tau = \frac{1}{2} D L A_s B_r k_w \eta, $$
where \(D\) is the air-gap diameter, \(L\) is the stack length, \(A_s\) is the electric loading, \(B_r\) is the remanence of the magnet, \(k_w\) is the winding factor, and \(\eta\) is the efficiency. A higher \(B_r\) directly improves the torque density. The high-Ce magnet with its cost advantage allows the motor design to use larger diameter or longer stacks without increasing the total cost excessively, thus compensating for the slightly lower \(B_r\).
Another important factor for embodied robots is the working temperature inside the motors. The coercivity temperature coefficient \(\beta(H_{cj})\) of sintered NdFeB magnets is typically around \(-0.6 \%\,/^{\circ}C\). For the low-Dy magnet, the high initial coercivity ensures that the magnet can withstand temperatures up to 150 °C without irreversible demagnetization. For the high-Ce magnet, the lower intrinsic coercivity may limit the maximum operating temperature to 100–120 °C. I therefore recommend the high-Ce magnet for ambient-temperature joints, while the low-Dy magnet is preferred for continuous high-load operations. The formula for the maximum operating temperature \(T_{\text{max}}\) can be estimated as:
$$ T_{\text{max}} \approx T_{\text{amb}} + \frac{H_{cj}(T_{\text{amb}}) – k \, H_{\text{ext}}}{|\beta| \, H_{cj}(T_{\text{amb}})} , $$
where \(H_{cj}(T_{\text{amb}})\) is the room-temperature coercivity, \(H_{\text{ext}}\) is the external demagnetizing field, and \(k\) is a safety factor. This relation is particularly useful when designing the electromagnetic circuit of actuators for embodied robots.
7. Conclusions
In this thesis work, I have systematically investigated the preparation and performance improvement of sintered NdFeB magnets for embodied robot applications. The main conclusions are as follows:
(1) Using the conventional powder metallurgy route, I designed three magnet families with lower cost: an HR-free magnet with composition (PrNd)30.8FebalM1.43B1, where M includes 0.3% Al; a low-Dy magnet with 0.6% Dy, (PrNd)22.8Gd1.8Ce7Dy0.6FebalM1.85B0.92; and a high-Ce magnet with 15% Ce, (PrNd)17Ce15FebalM1.55B0.9. These magnets offer a balance of magnetic properties and cost, making them suitable for different types of motors in embodied robots.
(2) The addition of 1.2 mL/kg lubricant D before jet milling, followed by 1.2 mL/kg antioxidant A after jet milling, significantly improves the magnetic properties of all three magnets. For the high-Ce magnet, this combined treatment raises \(B_r\) from 12.20 to 12.61 kGs and \(H_{cj}\) from 12.01 to 12.69 kOe. The improvement is attributed to better powder flow, reduced oxidation, and a more homogeneous grain boundary phase.
(3) The pilot-scale production process for the high-Ce magnet was optimized as follows: jet milling at 4000 r/min with 1.2 mL/kg lubricant D; orientation pressing at 1.8 T for 60 seconds with 1.2 mL/kg antioxidant A; sintering at 1060 °C for 6 hours; first-stage annealing at 900 °C for 2 hours; and second-stage annealing at 690 °C for 5 hours. The resulting magnets achieve near full density and a continuous rare-earth-rich grain boundary phase with no visible pores, as confirmed by backscattered electron microscopy.
(4) The economic analysis shows that adopting the high-Ce magnet reduces the raw material cost by almost 30% compared with the HR-free magnet, while maintaining acceptable performance for many embodied robot joints. The low-Dy magnet provides a higher-coercivity option for thermally demanding applications. Hence, the portfolio of magnets developed in this work offers a practical solution to reduce the motor cost of embodied robots and accelerate their commercial deployment.
In the future, I plan to further investigate the recycling and reprocessing of sintered NdFeB magnets, which would improve the economic and environmental sustainability of embodied robots. Moreover, I will explore the use of grain boundary diffusion with low-cost eutectic alloys to boost the coercivity of high-Ce magnets while preserving their economic advantage. These efforts will contribute to the advancement of high-performance, low-cost magnetic materials for the next generation of embodied robots.