Ferrite permanent magnets are mainly produced from iron oxide combined with strontium or barium compounds. Compared with NdFeB and SmCo magnets, aimants en ferrite have lower remanence and maximum energy product.
Cependant, ferrite offers several important advantages, including abundant raw materials, relatively low production cost, forte résistance à la corrosion, electrical insulation, and good resistance to demagnetization.
Ferrite magnets are widely used in household appliance motors, automotive auxiliary motors, haut-parleurs, capteurs, et équipements industriels. Improving their performance is not a matter of simply increasing one raw material. It requires systematic optimization of formulation, préparation de poudre, particle orientation, formation, frittage, usinage, et magnétisation.
1. Define the Performance Target
The principal parameters used to evaluate ferrite permanent magnets include:
- Rémanence, Br;
- Coercitivité, Hcb;
- Coercitivité intrinsèque, Hcj;
- Produit énergétique maximal, (Bh)maximum;
- Coefficient de température;
- Mechanical strength;
- Dimensional stability.
Higher remanence helps increase the usable magnetic flux produced by the magnet. Higher intrinsic coercivity improves resistance to demagnetization. The maximum energy product indicates the magnetic energy density that the material can provide under defined conditions.
These parameters are not completely independent. Some methods that increase coercivity may reduce remanence. Excessive emphasis on sintered density may cause grain growth and reduce coercivity.
Manufacturers should therefore define performance targets according to the actual requirements of the motor, loudspeaker, sensor, or other application.
2. Control Raw Materials and Formulation
Strontium ferrite usually uses SrFe₁₂O₁₉ as its principal magnetic phase. The ratio of strontium to iron, raw material purity, impurity content, and additives all influence phase composition and magnetic properties.
If the ratio is inappropriate, residual α-Fe₂O₃ or other non-target phases may remain after sintering, reducing the proportion of effective magnetic material.
Silicon, aluminum, and other impurities can also influence grain growth and grain-boundary structure.
Manufacturers should inspect iron oxide, strontium carbonate, and other raw materials for:
- Chemical composition;
- Particle size;
- Moisture;
- Impurity content;
- Batch consistency.
Accurate weighing and uniform mixing are essential for reducing variations between production batches.
3. Optimize Calcination and Magnetic Powder Preparation
Calcination promotes the chemical reaction between the raw materials and forms the hexagonal ferrite magnetic phase.
If the calcination temperature is too low, the reaction may remain incomplete. If it is too high, excessive particle growth may occur, making subsequent fine grinding more difficult.
After calcination, the material normally undergoes crushing and milling. Powder particle size affects coercivity, compact density, magnetic orientation, and sintering behavior.
If the particles are too coarse, domain structure and particle orientation may be unsuitable. If they are excessively fine, they may agglomerate, become difficult to disperse, and reduce forming efficiency.
The goal is therefore not simply to achieve the smallest possible average particle size. Manufacturers must control:
- Mean particle size;
- Particle-size distribution;
- Particle morphology;
- Degree of agglomeration;
- Slurry dispersion;
- Powder surface condition.
4. Improve Magnetic Particle Orientation
Anisotropic ferrite magnets are pressed under an external magnetic field. This allows the easy magnetization axes of the particles to align in the required direction.
A higher degree of orientation generally produces higher remanence in the intended magnetization direction.
Wet pressing allows magnetic particles to rotate more freely in a slurry and can help produce higher density and improved orientation. Dry pressing can offer higher productivity and convenient dimensional control.
The choice between wet and dry pressing therefore depends on magnetic performance, shape, tolerance, productivity, and cost requirements.
Improving orientation requires coordinated control of:
- Orientation field strength;
- Slurry viscosity;
- Solid content;
- Particle dispersion;
- Pressing rate;
- Dewatering behavior;
- Mold design.
If the powder is strongly agglomerated, even a powerful magnetic field may not orient the individual particles effectively.
5. Optimize the Sintering Process
Sintering densifies the pressed compact and develops the final grain and grain-boundary structure.
Sintering temperature, holding time, heating rate, cooling rate, and furnace atmosphere all affect density and magnetic properties.
If the temperature is too low, densification may be insufficient, resulting in low remanence and reduced mechanical strength. If the temperature is too high or the holding time is too long, excessive grain growth may reduce coercivity.
| Sintering condition | Microstructural characteristics | Possible performance effect |
| Insufficient sintering | High porosity and weak particle bonding | Low density, remanence, and mechanical strength |
| Appropriate sintering | Balanced density, grain size, and grain boundaries | Good overall magnetic performance |
| Excessive sintering | Coarse or abnormally grown grains | Reduced coercivity and increased deformation |
| Uneven temperature | Different densification levels within the magnet | Inconsistent dimensions and magnetic properties |
Manufacturers can use sintering trials, density measurements, and microstructural analysis to determine an appropriate processing window.
Two-step sintering and other advanced heat-treatment methods are also being investigated to control densification and grain growth while reducing energy use.
6. Use Additives and Element Substitution Carefully
Small additions of CaCO₃, SiO₂, and other substances can modify sintering behavior, grain boundaries, and grain growth.
Some high-performance strontium ferrites use lanthanum and cobalt substitution to improve magnetocrystalline anisotropy and magnetic performance.
Cependant, a larger additive quantity does not automatically produce better properties. Excessive non-magnetic phases may dilute the magnetic phase and reduce remanence. Different additives may also interact with one another.
Formulation development should therefore use controlled experiments, microstructural analysis, and magnetic testing. Raw material cost, process stability, and industrial scalability should be considered together.
7. Control Machining and Magnetization Quality
Sintered ferrite is a hard and brittle ceramic material. Grinding may cause edge chipping, fissuration, or dimensional variation.
Machining damage can reduce mechanical strength and alter assembly gaps or the magnetic circuit. Grinding depth, feed speed, abrasive selection, coolant condition, and fixture pressure should therefore be controlled.
During magnetization, the fixture and coil should be designed according to the magnet’s orientation direction, géométrie, and pole configuration.
If the magnetizing field is insufficient, the ferrite magnet may not reach magnetic saturation. If the magnetization direction is incorrect, the usable flux and field distribution may not meet the application requirements.
For motor arc segments and multipole magnetic rings, manufacturers should evaluate surface flux, variation among poles, pole position, and magnetic field waveform rather than relying only on a single measurement point.
8. Establish Full-Process Quality Control
Ferrite magnet performance cannot be improved reliably through final inspection alone. Raw materials, calcined powder, milled powder, slurry, green compacts, sintered bodies, machined components, and magnetized products should all be monitored.
Recommended inspection items include:
- Raw material composition;
- Powder particle size and distribution;
- Phase composition;
- Degree of particle orientation;
- Green compact density;
- Sintered density;
- Dimensions and appearance;
- Br, Hcb, Hcj, et (Bh)maximum;
- Surface magnetic flux and field distribution;
- Mechanical strength;
- Environmental reliability.
Process data should be linked to production batches so that any variation can be traced to raw materials, milling, pressing, frittage, usinage, or magnetization.
JLMAG Innovation supplies ferrite, aimants en caoutchouc, aimants moulés par injection, and different types of magnetic assemblies. Its ferrite portfolio includes isotropic and anisotropic materials for motors, haut-parleurs, toys, and industrial applications.
Overall, improving ferrite magnet performance requires a balance among magnetic phase content, particle size, orientation, sintered density, and grain-boundary structure. Only by integrating formulation control, powder engineering, magnetic-field orientation, sintering optimization, usinage, magnetization, and quality inspection can manufacturers achieve stable and repeatable performance in large-scale production.
