Research On The Low Temperature Co-fired NiCuZn Ferrite And Multiplayer Chip Inductor Applications

The main technical revolutions of the electrical information technology are figured of from analog to digital technologies, from fixed frequency to alterative frequency, from plug devices to chip devices. The multilayer chip inductors (MLCI) and the low temperature co-fired ferrites (LTCF) are the key technologies to realize the revolution of the third part. In this dissertation, the investigations were focus on the theories, technics and applications of the LTCF and MLCI. The research mode that combined theoretical analysis, materials preparation and element application was adopted to develop high performance low-fired NiCuZn ferrite, which were very suitable to produce multilayer chip inductors.In the part of theoretical analysis, the key factors to decide magnetic properties of the low-fired NiCuZn ferrite were investigated firstly, which was very helpful to control and improve these magnetic properties during experiments. Then the sintering kinetics of the ferrite was analyzed to give directions to densify the ferrite under low sintering temperature. To design compositions of the NiCuZn ferrite by permeability requirement, the expression to calculate permeability of the ferrite was deduced. Then the genetic algorithm was adopted to optimize compositions of the NiCuZn ferrite. which evidently improved efficiency and speed of new material development.In the part of experiments investigations, the mixed-oxide method was first adopted to produce the NiCuZn ferrite. The influences of compositions on the sintering behaviors, microstructures and magnetic properties of the ferrite were investigated. It was confirmed that 10.2mol% CuO content was appropriate to give attention to both low-fired characteristic and good magnetic properties of the NiCuZn ferrite. Then the effects of calcining temperature, milling time, heating rate and additives on the densification behaviors and magnetic properties of the ferrite were studied. It was confirmed that the optimal calcining temperature was 800℃; the optimal milling time was 24h; the optimal heating rate was≤2.5℃/min. To obtain the NiCuZn ferrite with high permeability, the optimal additives were 1.5wt%Bi₂O₃+0.3wt%WO₃; to give attention to both permeability and Q-factor, the optimal additives were 1.5wt%Bi₂O₃+ 0.3wt%WO₃+0.2wt%Co₂O₃.Then the sol-gel method was adopted to produce the NiCuZn ferrite. X-ray diffraction (XRD) method was used to identify the spinel structure of the dried-gel, sol-gel auto-combustion prepared powders and sintered samples. It was confirmed that the sol-gel auto-combustion prepared powders possessed single phase of the NiCuZn ferrite, and the particle size was in nano-scale. Then the sintering behaviors and magnetic properties of the samples prepared by the sol-gel method and the mixed-oxide method were compared to ensure the advantages and disadvantages of the two methods.Finally, the combining method was adopted to produce the NiCuZn ferrite. It was confirmed that nanocrystalline ferrite particles enhanced densification of the samples obviously. This factor was attributed to that the nanocrystalline particles, which spread around the micron-sized ferrite particles, increased contacting area and inter-diffusion of the particles. Due to the nanocrystalline ferrite particles had the same chemical composition as the NiCuZn ferrite, high performance NiCuZn ferrite with low sintering temperature could be obtained.In the part of MLCI application research, the HFSS software was adopted to design, emulate and optimize the multilayer chip inductor first. It was confirmed that for the 0603-type multilayer chip inductor, when the length of the long side was chosen as 1200μm, length of the short side was chosen as 500μm, width of the line was chosen as 100μm, distance between line to the up and down side of the inductor was chosen as 150μm, a high performance inductor could been obtained. Then the prepared NiCuZn ferrite was chosen to produce the multilayer chip inductors by adopting LTCC technical line. It was confirmed that the produced inductance values were lower than the forecast, which was mainly due to debasement of the permeability. However, the relationship between inductance values and turns of the coils was coincident with the forecast.