钕取代对钴铁氧体结构、磁性及电磁波吸收性能的影响

Structure,Magnetic Properties and Electromagnetic Wave Absorption Properties of Neodymium Substituted Cobalt Ferrite

利用共沉淀法合成了钴铁氧体及钕(Nd)取代钴铁氧体,研究了Nd取代对钴铁氧体纳米粒子结构、磁性及电磁波吸收性能的影响。X射线衍射(XRD)和透射电镜(TEM)结果表明,当Nd取代量较低时,可以起到细化晶粒的作用,但是当取代量x≥0.06时,会产生二次相NdFeO_(3),并导致晶粒尺寸变大。磁性能研究结果表明,适量的Nd取代钴铁氧体可以提升材料的矫顽力(H_(c))和磁晶各向异性,这对于提升钴铁氧体的电磁波吸收性能起到了关键作用,磁损耗(C0)和衰减常数有效提升导致电磁波吸收性能增强。相比于钴铁氧体,CoFe_(1.96)Nd_(0.04)O_(4)的反射损耗值提高了61.46%,有效吸收频带拓宽了1.02 GHz。

High-frequency devices with frequencies up to gigahertz have extreme advantages such as long-distance response,fewer wires,and high-speed response,realizing the interconnection of everything.The electromagnetic radiation and interference generated by these high-frequency smart devices,that is,electromagnetic pollution,has become a new type of pollution source after water,atmosphere,and noise.Its characteristic is that it interferes with the normal communication of electromagnetic devices and causes great harm to the human body,and it is not easy to protect.In recent years,cobalt ferrite has been widely used in the field of electromagnetic pollution prevention because of its high coercivity and resistivity,suitable saturation magnetization,large magnetocrystalline anisotropy,good chemical stability,and high Curie temperature.However,cobalt ferrite cannot obtain excellent electromagnetic wave absorption performance and application limitations due to poor impedance matching and high density.Rare earth elements have a large magnetic moment and magnetocrystalline anisotropy due to their special electronic structure,which makes them widely used in a variety of materials.Doping a small amount of trivalent rare-earth ions into spinel ferrite is a promising method to improve its electromagnetic properties.In this paper,cetyltrimethylammonium bromide was used as a surfactant,C_(4)H_(6)CoO_(4)·6H_(2)O,FeCl_(3)·6H_(2)O,and NdCl_(3)·6H_(2)O were used as raw materials to synthesize spinel-type nano-cobalt ferrite with Nd substitution by the co-precipitation method.For cobalt ferrite,the substitution amount of Nd was x(x=0,0.02,0.04,0.06 and 0.08).X-ray diffractometer(XRD),transmission electron microscope(TEM),inductively coupled plasma emission spectrometer(ICP-OES),vibrating sample magnetometer(VSM)and vector network analyzer(VNA)were used to character samples,the research focused on the substitution of Nd ions influence on the microstructure,magnetic properties and electromagnetic wave absorption properties of cobalt ferrite nanoparticles.The results showed that Nd ions substitution would reduce the saturation magnetization of cobalt ferrite,which was because the magnetic behavior of spinel ferromagnetic materials was mainly controlled by the negative Fe^(3+)-Fe^(3+)interaction(spin coupling of three-dimensional electrons).At the same time,there were relatively weak Nd^(3+)-Fe^(3+)interaction(4f-3d coupling)and Nd^(3+)-Nd^(3+)interaction(indirect 4f-5d-4f electronic coupling).When the large-radius Nd ions replaced Fe ions in the octahedral position,the negative exchange interaction between Fe^(3+)-Fe^(3+)in the two sublattices was reduced,thereby reducing the saturation magnetization.When Nd ion substitution rate was high(x≥0.06),the secondary phase NdFeO_(3) produced was antiferromagnetic in nature,which would further reduce the saturation magnetization of the cobalt ferrite.When the substitution amount of Nd ion was low(x≤0.04),it could refine the cobalt ferrite grains and increase the lattice constant.This was because Nd-O bond energy was higher than that of Fe-O,and the introduction of Nd ions into the octahedral position required more energy.The energy required for this process was provided at the expense of crystallization,thus hindering the growth of crystallites.The reduction of grain size played a key role in increasing the coercivity and magnetocrystalline anisotropy of cobalt ferrite,which was because the coercivity was inversely proportional to the grain size.The larger the grain size,the lower the volume fraction of grain boundaries and the less pinning effect of domain walls.However,when x≥0.06,the secondary phase NdFeO_(3) would be produced,and the grain size would become larger,the lattice constant would become smaller,and the morphology of the cobalt ferrite would change from spherical to quadrilateral.This was because there was a solubility limit for the replacement amount of Fe ions by Nd ions,and excess Nd ions tended to accumulate near the grain boundaries in the form of NdFeO_(3).Excessive substitution of Nd ions would reduce the coercivity and magnetocrystalline anisotropy,and this was because the magnetocrystalline anisotropy of cobalt ferrite was caused by Co ions in the octahedral position.Due to the substitution of Nd ions with a larger radius,Co ions migrated from the octahedral position to the tetrahedral position,which showed that the substitution of Nd ions caused the tetrahedral and octahedral cation rearrangement of the cobalt ferrite to have a greater impact on the coercivity and magnetocrystalline anisotropy.By calculating the area of the hysteresis loop,it could be found that Nd ions replacing cobalt ferrite increased the attenuation constant(α)in the form of enhanced hysteresis loss.In this way,the electromagnetic waves entering the material could be dissipated in other forms of energy,thus effectively enhancing the electromagnetic waves'absorption performance of cobalt ferrite.When x=0.04,and the thickness of the absorption layer was 2.5 mm,and the frequency was 13.75 GHz,the minimum reflection loss value was-12.94 dB,and the effective absorbing frequency band was 1.02 GHz(13.07~14.09 GHz).Compared with cobalt ferrite,the reflection loss of CoFe_(1.96)Nd_(0.04)O_(4) was increased by 61.46%,and the effective absorption band was widened by 1.02 GHz.