| In this thesis, the experimental ideas based on the adjustment of new formulation by co-doped La-Co. La-Zn. Sin-Co into the matrix, making the better crystal degree, the additve concentration of high-coercivity Cr2O3A12O3.the process of appropriate orientation calcination temperature for complete reaction and through two-step fine grind to make partical size close to single-domain were carried out respectively.The strontium ferrite samples were prepared by using the traditional solid phase reaction method. By using the raw material of α-Fe2O3. SrCO3. La2O3. CoO. and through the process steps of mixing, drying. calcination. crushing. grinding. pressing and sintering etc.. the preparation of strontium ferrite samples can be made accordingly. The magnetic coercivity Hcb. remanence magnetic flux density Br. intrinsic coercivity Hcj. and the squareness ratio H?? etc. are measured by the B-H tester. The lattice structures were examined by XRD. The surface morphology of the samples were detected by HR-SEM.The conclusion is as follows:1. La, Co co-doped strontium ferriteBy the study of La-Co co-doped strontium ferrite. we found that the remanent magnetization does not have an obvious improment with the increase value of La/Co ratio in the range of0.5to1.75. and the magnetic flux density of the coercive force H? and intrinsic coercivity of Hcj value raise with the increase of La/Co ratio. illustrating that La/Co doping is fruiteful for the improvement of material magnetic anisotropy. However, excessive doping of lanthanum will lead to the appearence of non-magnetic phase, as well the adverse effects to the magnetic properties. The specific mechanism of La-Co co-doped strontium ferrite can be got a deep understanding after the microstructure morphology, and a series test and analysis.Through the La. Zn co-doped strontium ferrite Sr1-xLaxNZnxFe12-（?）O19（x=0.0.05,0.10,0.15,0.20,0.25）, experimental results demonstrated that:La-Zn replaced strontium ferrite does not change the crystal structure of hexagonal type.The grain size is getting decreased.Br and （BH）max increase with more replacement amount of La/Zn mol ratio. then decrease subsequently. When x=0.15, Br reaches the maximum value, and it increases by2%in value compared with the undoping case. However. H? and Hcj values decrease rapidly with the increase of La-Zn substitution quantity. According to Sm. Co co-doped strontium ferrite, under the case of Co ions quantity invariable, by changing the amount of Sm ions, as we see from the experiment data, the remanence B, decreased with the increase of Sm/Co ratio, the doping effect is not ideal. The possible reason may be that the doping ions had not mixed into the structure of the material.2.The addition of A12O3reveals that with the increasing of additive A12O3. the remanence Br decreases monotonically. and H? rise at beginning then declined, and H? increase monotonically; For La-Co co-doping strontium ferrite in the ratio of1.5. the properties of strontium ferrite magnetic remanence and coercivity of magnetic flux density with different amounts of Cr2O3additive doping cases were studied. With the increase of Cr2C3. the coercivity increased accordingly. As for0.15%additive concentration, the values of Br. magnetic flux density, and residual magnetism coercivity H? were all dropped off. When the additive amount increased to0.3%. Br and magnetic flux density, residual magnetism coercivity H? reached an maximum value of Br=415.45mT. H?=300.8kA/m. H?=374.2kA/m. When the additive amount is more than0.3wt%. the intrinsic coercivity remanent flux density increases in value, and B, and coercive force H? value then drops off. It is due to the overmuch of additive quantity, as well the overbalance of non-magnetic material, thus leads to the change of phase structure of ferrite material.3. Effect of calcination temperature on the magnetic properties of strontium ferriteUnder different calcining temperatures （118O℃.1210℃.1250℃.1270℃.1290℃.1300℃.1310℃） of strontium ferrite Sr0.7La0.3Fe11.75Co0.15O19, the influence to the microstructure and the magneitic properties were studied, and conclusions were as follows:A suitable calcination temperature is important for the better performance strontium ferrite. For Sr0.7La0.3Fe11.75Co0.15O19 system, the most suitable calcination temperature is1290℃, and the optimal magnetic properties can be given by Br=422.85mT. Hcj=343.85kA （?）m,（BH）max=34.99KJ/m3;XRD patterns indicate that, different calcination temperatures don’t change the internal magnet structure and grain morphology of the material. Around the appropriate calcination temperature, the crystalline phase is still the magnetoplumbite structure. At a comparatively low calcination temperature （below1270℃）, the a-Fe2C3phase can be observed due to the under-reaction of the materials. With the reactions going on. the γ-Fe2O3phase can be also observed due to the lack of oxygen or other reasons.SEM figures show that, with the increase of calcination temperature, the grain growth and melting phenomenon appeared, but the hexagonal structure of the grains did not cH4nge.Magnetic properties test showed that, with the increase of calcining temperature. B?.（B11）ma\ and H?are all increased thereupon. When the temperature reaches to1290℃. three parameters are at their maximum value. And then they decrease with the increase of calcinations temperature.H?,is in a particular case.Both at low and high calcination temperatures, it is not conducive to the improvement of magnetic properties.Fine grinding and milling time on the morphology and magnetic properties of strontium ferrites were studied. With the prolonging of milling time, the magnetic particle size becomes smaller, the orientation of magnetic grains becomes better, and the indices of magnetic properties are improved accoringly. When the milling time reaches to9hours, although there are a little improvement in magnetic properties, but it will increase the cost of production. Moreover, the small magnetic particle diameter will raise the difficulty of samples molding, and samples are easy to crack. |
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