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Eingeladener Vortrag
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Silver in LTCC - Interfacial reactions, transport processes and influence on properties of ceramics
(2009)
Due to addition of minor amounts of suitable additive combinations to BaTiO3 powder the sintering temperature was reduced to 900 °C. Excellent densification (more than 98% of theoretical density) and good dielectric properties were achieved after sintering at 900 °C for 2 hours. Dielectric properties can be improved by modifying the composition of sintering additives and additon of further dopants. The low sintering temperature of newly developed capacitor material allows a co-sintering with pure silver electrodes. No interfacial reaction between silver electrodes and ceramics was observed by microprobe analysis. The dielectric constants of about 1200 to 3000 and the dielectric losses of about 0.008 to 0.02 were measured (f = 1 kHz, T = 25 °C). The temperature dependency of dielectric constant for new capacitor materials is low, almost X7R characteristics. Furthermore, the densification and the dielectric properties of embedded capacitors in LTCC were investigated.
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.
Ceramic components with complex shape cannot be produced frequently by usual ceramic forming and sintering processes. Therefore, numerous joining methods were developed and introduced in industrial scale. Nowadays, multi-stage Mo-Mn-process and active brazing are preferentially used, if temperature-stable and gastight joints are required. Unfortunately, both processes involve cost-intensive thermal processes: hydrogenous atmosphere is essential for metallization in Mo-Mn-process and active brazing takes place under vacuum. Thermal processes can be drastically simplified by using Reactive Air Brazing (RAB). Joining under air atmosphere is an interesting alternative, especially to join oxide ceramic components among themselves. So far, main disadvantage of RAB is low strength of join connections.
Aim of this investigation was the development of high-strength, thermal shock resistant and gastight ceramic-ceramic joints by RAB. Therefore,
- commercial, silver and copper oxide containing RAB soldering composition was modified by addition of ceramic particles with low thermal expansion coefficients (TEC). Hence, thermal misfit between TEC of solder and ceramic components was significantly reduced.
- RAB soldering paste was replaced with newly developed RAB composite tapes, produced by ceramic “doctor blade” technology. Thereby, improved potential exist to tailor the brazing layer relating to composition, thickness and thickness uniformity.
Gastight alumina-alumina, alumina-zirconia and zirconia-zirconia joints with strongly improved strength were produced by novel composite tapes. No strength degradation of joints was observed after thermal cycling up to 700°C.