Preparation, Characterization Of Magnetic Particles And Their Induction Heating Application In The Thermoplastic Polymers

In this dissertation, a series of magnetic particles suiting for induction heating application of thermoplastic polymers were prepared and characterized. The preparation process, crystal structure control, magnetic properties, heating and heat transfer behavior in thermoplastic polymer were studied. The main research results are as follows:A series of ultrafine spinel ferrites (Ni_1-xZn_xFe₂O₄) were synthesized at relatively low temperature by two soft chemical methods (EDTA complexing method and stearic acid method) using nitrates as raw materials. The effects of Zn content on the crystal structure and magnetic properties were studied. The reaction mechanism of different methods was preliminary discussed. The results indicated the pure phase products could be obtained at 900℃by stearic acid method and severe aggregations existed. The products obtained by EDTA conplexing method had high dispersible and loose structure. The study on the magnetic properties found the process conditions such as calcination temperature, time and the content of Zn ion had a large impact on the magnetization, coercivity, remanence and Curie temperature. The particles could ensure the induction heating of thermoplastic polymer by properly adjusting particles size, composition and magnetic properties. The magnetic spectra showed the capacity of magnetic energy storage and loss were related to the densification and grain growth of Ni_1-xZn_xFe₂O₄. The increasing energy loss and storage suited for the induction heating at moderate frequency could be achieved by increasing the calcination temperature and the content of Zn ion.A series of Y type hexaferrite (Ba2Co_2-xZn_xFe₁2O₂2) powders were prepared by EDTA complexing method and stearic acid method. The effects of calcination temperature, initial pH, ion concentration on the microstructure and magnetic properties were systemically investigated. The reaction process was discussed in detail. The results showed the products via EDTA method had uniform hexagonal shape and narrow particle size distribution, which were obtained by multi-step reactions. The structure and composition of products had a large dependence on the conditional stability constant of complexes precursor. The substitution of Co²⁺by Zn²⁺could not change the crystal structure except for expanding the lattice. The Curie temperature of products has a linear relation:Τ_c=352.66-146.28x. With increasing substitution, the real and imaginary parts of complex permeability increase and the resonance frequency decrease. The increasing calcination temperature and substitution could improve the loss in 1～10MHz frequency. The products obtained by stearic acid method were small in particle size and broad in distribution. The existence of BaCO₃ had good stabilization on the transition phase y-Fe2CO₃. Compared with EDTA method, products with the same composition had smaller magnetization and larger coercivity, which was not suitable for the induction heating under low alternating magnetic fields.Ultrafine nickel powders with controllable morphology and size were prepared by reducing nickel salts solution. The effects of the solvent ratio, pH value, Ni²⁺ concentration, temperature, different surfactants and concentration on the structure and magnetic properties are systemically studied. The results show the surfactants with chain structure can improve the thermal stability of products and control particles growth and aggregation. The obtained 50-900nm nickel powders showed evident ferromagnetism. The saturation magnetization was below the corresponding bulk counterpart and decreased with smaller size due to the different surface area. With increasing particle size, the real and imaginary parts of complex permeability increase and the loss peak shifts to lower frequency.Based on the magnetic field interaction theories, the parameters including hysteresis, eddy current loss, skin effect and heat transfer were preliminarily calculated to understand the uniform, Curie temperature control and quick induction heating process in thermoplastic polymer. The results indicated the dependence of hysteresis energy density of Ni-Zn ferrites, Y type hexaferrites and nickel powders on the composition and particles size in saturation fields. The impacts of eddy current loss, skin effect on the induction heating may neglect in 1～10MHz when Ni-Zn ferrites, Y type hexaferrites and< 1.1μm nickel powders are as susceptors. The heating rate is associated with the volume fraction, frequency, hysteresis energy density and the thermal properties. Under 1.5MHz frequency,150Oe alternating field, the heating rates of 10%volume fraction of Ni_0.7Zn(0.3)Fe₂O₄, Ba₂Co_1.5Zn_0.5Fe₁₂O₂₂and 150nm nickel in PEEK polymer are theoretically 69.3,17.0and 8.6℃/s.Three kinds of magnetic particles/PPESK composites layers were produced by solution impregnation process and their hysteresis heating behaviors were investigated. The results showed the field strength had a large impact on the finial temperature and heating rate of composites. Enough field strength is necessary to Curie temperature control and quick heating rate in the practical application. The experiment heating rates of 10% volume fraction of Ni_0.7Zn_0.3Fe₂O₄, Ba₂Co_1.5Zn_0.5Fe₁2 and 150nm nickel in PPESK matrix are 9.7,6.2 and 5.5℃/s, which shows good induction heating characteristics.