High Br permanent ferrite material production research

High Br permanent magnetic ferrite materials, whose magnetic properties reaching Br=0.411mT, HCB=276.2kA/m, HCJ=279kA/m, (BH)max=32.1kJ/m3 are mainly used for producing permanent magnetic DC micro-motor and high-grade speakers.

In order to produce High Br permanent magnetic ferrite materials with YF 28 presintered ones, it needs to add additive, which amount is as less as possible, and better to increase Br and HCJ simultancitily. Now, mixed rare-earth oxid can meet this requirement. In this thesis, it will introduce a new way, which is to mix SiO2, CaO, mixed rare-earth oxid and dispersant into YF 28 persintered material, and then to produce permanent magnetic ferrite material with the magnetic properties of Br=0.411mT, HCB=276.2kA/m, HCJ=279.8kA/m, (BH)max=32.1kJ/m3. And it also discuss and analyze the influence on permanent magnetic materials by mixed rare-earth oxid and sintering temperature influence on HCB.

Experiment Method

Grind roughly the YF28 presintered granular material to 5~7 µm, and then add nano-oxid (SiO2<0.6wt%、CaO<0.75wt%), submicron mixed rare-earth oxid (<0.6wt%), and some appropriate dispersant. After that, the mixture should be wet-grinded in a sand (or ball) mill to 0.90~0.95µm, the water content of mixture should be within 35~40wt%, magnetic field >400kA/m, blank density >3.1g/cm3. Next, sinter in electric or coal kiln with temperature 1220~1230℃, and keep the temperature for 1~1.5 hours. At last, test sample size of Ф20*10mm, and test particle size with WLP-208 fisher sub sieve sizer, and test magnetic performance with TYU-2000 magnetic material auto measuring machine.

Results and Discussion

  1. Experiment Result
  2. If we use YF28 presintered material to produce high Br permanent magnetic ferrite materials, usually we should select presintered material with radial contractility 14~16%. The purpose to add SiO2—CaO is mainly to increase remanence Br and to increase coercivity HCB and HCJ at the same time. If adding SiO2—CaO only, it is the same technic to produce Y28 permanent magnetic ferrite materials. If mixing Al2O3 with SiO2—CaO, the remanence Br will be decreased a lot, and can not meet the expected result. If mixing the SiO2—CaO with appropriate dispersant and mixed rare-earth oxid, the remanence Br will increase a little, while coercivity HCB and HCJ will be increased a lot. Please find properties in the table below.

    Magnetic Performance of property after-changed YF 28 presinterd powder

    No.
    Model
    Br/mT
    HCB/kAm-1
    HCJ/kAm-1
    (BH)MAX/kJM-3
    Remark
    01
    YF28
    402
    157.9
    159.7
    28.97
    blank
    1
    E8—3
    408
    278.3
    284.4
    31.38
    lab
    2
    U94
    411
    276.2
    279.8
    32.1
    coal kiln
    3
    U95
    412
    274.9
    278.9
    32.2
    electric kiln
  3. Add mixed rare-earth oxid to increase magnetic property
  4. In order to produce high Br permanent magnetic ferrite material, we select a kind of rare-earth oxid which mixed with La, Ce, Nd, Pr etc as additive. In YF28 presintered material, if we do not change the quantity of SiO2—CaO, but change that of mixed rare-earth oxid, we can find the magnetic propertity changed as shown in the chart below. It’s shown that when the quantity of mixed rare-earth oxid is between 0.2~0.4wt%, the remanence Br and coercivity HCB and HCJ of Sr permanent magnetic ferrite material will be increased, and it will reach the max value when the quantity is 0.4wt%. When it >0.4wt%, Br, HCB and HCJ are all decreased with HCB decreased fastest.

    The principle of action for La3+、Ce3+、Pr3+、Nd3+ in Sr permanent magnetic ferrite is unkonwn now. Maybe the reason is that Ce3+ (2.4µB)、Pr3+(3.5µB)、Nd3+(3.5µB) are all paramagnetic ion. So, Pr3+ contributes to increase HCJ (Pr3+ is uaually used to increase HCJ in producing NdFeb process); Ce3+ and Nd3+ contribute to increase Br; La3+ is mainly used to replace some Sr2+ and stabilize magnet and plumbum structure. During our expriment, we also find that the mixed rare-earth oxid can lower sintering temperature, but its own melting point is very high. Maybe the reason of lowering sintered temperature is that La3+ replaces some Sr2+, and this causes the rest Sr2+ and SiO2—CaO become into liquid state during high-temperature sintering period.

  5. The influence of sintering temperature on coercivity HCB
  6. The HCB and HCJ can be increased under the condition that the quantity of Br in high Br permanent magnetic ferrite material to be decreased to the least, but adjusting HCB is more difficult than Br and HCJ. So, we need to pay more attention to the factors which can affect HCB.  µoHCB≤Br, so to increase Br is the precondition to increase HCB; Considering technics, we should reduce posture and increase density and tropism, and only single-domain particle can realize tropism;HCB< HCJ∝KJ/MS, so HCB can be restricted by HCJ (this mainly lies on magnetic crystal anisotropism).

    According to the theory of coercivity, in the ideal condition, the sintered particle is as small as single-domain size, without cover. So, the coercivity will be the max in the magnetization process with magnetization vector removed only. In our experiment, when sintering temperature is higher, the HCB and Br will be increased simultaneously, and HCB reaches the max value on a certain temperature; if the temperature is rising continuously, Br will also increase, but HCB opposite. And it decreases faster than HCJ. The reason is that the particle size is larger than single-domain size, and it becomes into cover. So the best sintering temperature is the one when HCB reaching the max value.

    Sr ferrite magnetic property changing chart

    The chart shows how the mangetic property of Sr ferrite change with the changing of the content of mixed rare earth when the content of CaO—SiO2 in YF28 presintered material is fixed.

Conclusion

Adding SiO2—CaO mixed rare-earth oxid and dispersant into YF28 persintered material, we can produce High Br permanent magnetic ferrite material with the magnetic properties of Br=0.41mT, HCB=276kA/m, HCJ=279kA/m, (BH)max=32kJ/m3. The best additive quantity of mixed rare-earth oxid is 0.4wt%. We should control the sintered temperature to make coercivity HCB to reach the max value.