Structural And Process Optimization Of MnZn Ferrite Magnetic Stripes For Induction Heating

Induction cooker is a new type of kitchen cooker. Different from traditional gas flame heating and resistance heating, induction cooker has shown many advantages, such as high efficiency, fast heating rate, environment friendliness, high safety and reliability. Since the 1960 s, induction cooker has made a great progress in appearance, control system and heating energy efficiency. At present, the energy efficiency of the induction cooker in our country is generally between 80% and 90%. MnZn ferrite stripes are one of the most important parts in the induction cooker. As a common form of soft magnetic materials, it gathers and guide magnetic field lines, and makes more magnetic field lines into the cooker, which produces more eddy currents. It is found by the indduction cookers assembled with different MnZn ferrite stripes have important effects on the energy efficiency. However, there is currently litter research about the MnZn ferrite in induction cooker, and no specific standard in selecting magnetic stripes. There are also many magnetic stripes suppliers who produce different kinds of MnZn ferrite stripes. In this thesis work, a variety of MnZn ferrite stripes have been investigated. Based on the energy efficiency information of the induction cookers assembled with different Mn Zn ferrite stripes, the relationship between MnZn ferrite stripes and energy efficiency was inveatigated. Using ANSYS finite element simulation, the changes of magnetic flux in the pan with the various magnetic stripes were analyzed. Finally, the MnZn ferrite were prepared by traditional ceramic route for induction heating application.Firstly, the chemical composition, microstructure and magnetic properties of different types of Mn-Zn ferrite magnetic strip materials were analyzed and compared by scanning electron microscopy, X- ray diffraction, physical property measurement, and soft magnetic measurments. Large differences were found in chemical composition for various stripes. MnZn ferrite strips prepared by primary materials have better magnetic properties than those prepared by recycled materials. Combined with the energy efficiency data, the relationship between the energy efficiency of induction cooker and magnetic properties of magnetic strips were studied. It is found that the most important magnetic properties to the energy efficiency are permeability, loss and coercivity. The energy efficiency has little relationship with the Curie temperature. Based on above snalysis, the properties standard of MnZn ferrite stripes was developed for induction cooker application as permeability μa>3000, loss PS<10W/kg, and coercivity HC<5A/m under 25 kHz and 100 mT.In ANSYS simulation, different models were designed to investigate the effects ofthe permeability, thickness, and number of Mn Zn ferrite stripes and the permeability, thickness of pan, and distance between the stripes and the pan on the magnetic flux at the bottom of the cooker. The results show that when the permeability of magnetic stripes is less than the permeability of pan, magnetic stripes have litter influence on the magnetic flux in pan. When the permeability of pan is less than the permeability of magnetic stripes, more magnetic stripes, higher permeability, larger thickness, less distance between stripes and pan are beneficial for larger magnetic flux in pan.Finally, the preparation of the MnZn ferrite was studied. A MnZn ferrite formula without doping for high permeability and low loss was selected. The presintering process, molding process and sintering process were investigated systematically. Experimental results show that using the procedure including 2 hours ball milling, 2 hours presintering under 1000℃,forming under 100 MPa after granulation, then 3 hours sintering under 1350℃, final product with uniform structure can be obtained. Under the condition of 25 kHz and 100 mT, the properties of the prepared magnets are permeability μa>4000, loss PS<5W/kg, and coercivity HC<3A/m, which meets the requirement of the properties standard for MnZn ferrite stripes obtained previously in this thesis work.