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- BaTiO3 (1)
- BaTiO3 and titanates (1)
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Integration of magnetic functional components in LTCC circuit boards calls for co-firing of dielectric and ferrite tapes. Ni-Cu-Zn ferrites with permeability of µ = 900 were fully densified at the standard LTCC sintering temperature of 900°C. Co-firing of this ferrite with dielectric tapes requires the adaptation of the shrinkage behavior of the materials as well as the thermal contraction during the cooling period – especially in the temperature range below the transformation point of the glass phase of the dielectric tape. To match these preconditions, a new dielectric LTCC material with steep sintering curve and high thermal expansion coefficient was designed. Sintered multilayer laminates composed of Ni-Cu-Zn ferrite and tailored dielectric tapes are free of cracks and possess no open porosity. No significant interdiffusion between the ferrite and dielectric tapes was found using microprobe analysis. Pure ferrite laminates show homogeneous microstructure with large grains up to 50 µm in diameter. However, growth of ferrite grains does not take place near the interface between ferrite and release or dielectric tapes. There is a strong correlation between permeability and volume fraction of large ferrite grains. Compared to pure ferrite laminates the permeability of co-sintered Ni-Cu-Zn ferrite layers is drastically reduced to 400, that is a decrease of more than 50%.
The sintering temperature of BaTiO3 powder was reduced to 900 °C due to the ZnO-B2O3-Li2O-Nb2O5-Co2O3 addition. Excellent densification was achieved after sintering at 900 °C for 2 h. The low sintering temperature of newly developed capacitor materials allows a co-firing with pure silver electrodes. The dielectric constant and the temperature stability of the dielectric constant are strongly correlated with the composition of the ZnO-B2O3-Li2O additives. A high dielectric constant up to 3000 and a dielectric loss less than 0.024 were measured on multilayer capacitors sintered at 900 °C with silver inner electrodes.
By addition of LiF-containing sintering additives to commercial BaTiO3 powder, more than 98% of the theoretical density was reached at a sintering temperature of 900 °C both on powder compacts and laminates. Dielectric measurements were performed on ceramic samples in the temperature and frequency ranges from -20 °C to 125 °C and from 103 to 106 Hz, respectively. High relative permittivity (εr ~ 3160) and low dielectric loss (tan δ ~ 0.014) were measured for tapes of the favoured material. The breakdown strength for tapes with a thickness of about 80 µm is 30 kV/mm. The microprobe analysis showed, that no interfacial reaction between the dielectric layer and the Ag-electrode had occurred.
The newly developed barium titanate ceramics completely densifying at 900 °C turned out to be promising for integration of buried capacitors in LTCC multilayers.