Solution coated hydrothermal BaTiO3 for low-temperature firing

Details Solution coated hydrothermal BaTiO3 for low-temperature firing Solution coated hydrothermal BaTiO3 for low-temperature firing

1997-04-28

2001-01-02

Jones, Deborah (Department: 1755)

Compositions: ceramic

Ceramic compositions

Titanate, zirconate, stannate, niobate, or tantalate or...

C501S139000

Utility Patent

active

06169049

ABSTRACT:

FIELD OF THE INVENTION
This invention relates to an improved barium titanate based dielectric composition and, more particularly, to a barium titanate composition wherein barium titanate crystallites are coated with a Bismuth metal-organic to achieve both a low-firing temperature and improved dielectric properties.
BACKGROUND OF THE INVENTION
Barium titanate (BaTiO
3
) based compositions are commercially used as capacitor dielectrics, particularly for multilayer ceramic (MLC) capacitors. Present manufacturing techniques produce MLCs with thickness layers of 5-10 &mgr;m. Yet, with decreasing operating voltages of MLCs, thickness layers are expected to be less than 5 &mgr;m. In order to achieve these thickness layers and produce reliable MLCs, average grain size of the sintered BaTiO
3
ceramics need to be one-tenth the size of the layer thickness (i.e., ~0.5 &mgr;m).
In addition to producing thinner layers for MLCs, manufacturers are actively pursuing methods for lowering the cost of MLCs. In particular, electrode materials make up a significant portion of the production costs for MLCs. Palladium-silver electrodes (e.g., 70/30 Pd-Ag) are substantially lower in price compared to platinum and pure palladium electrodes. Yet, in order to utilize Pd/Ag electrode materials, BaTiO
3
-based dielectric materials must be sintered at or below 1100° C.
In order to prepare BaTiO
3
-based ceramics that can be sintered below 1100° C. and have high densities with average grain sizes of 0.5 &mgr;m or less, a combination of hydrothermally-prepared powders with primary crystallites in the nanometer-sized range and a fluxing agent or a low-melting glass powder to promote densification of BaTiO
3
by liquid phase sintering below 1100° C. is needed. Fluxing agents and low-melting glass powders have been used to densify BaTiO
3
at lower sintering temperatures. Fluxing agents that have been used for flux-sintering BaTiO
3
include lithium fluoride, boron oxide, copper oxide, lead germanate, cadmium silicate, and mixtures of cadmium oxide and Bismuth oxide. Also, low-melting glass compounds, such as borosilicate glass powders that contain significant amounts of PbO, BaO, Bi
2
O
3
, CdO, and ZnO modifiers, have been used to promote densification of BaTiO
3
at lower sintering temperatures.
Kumar et al. describe a low-firing BaTiO
3
that is sintered from hydrothermal BaTiO
3
mixed with 3-5 wt % Bi
2
O
3
. (See: Kumar et al., “Preparation of Dense Ultra-Fine Grain Barium Titanate-Based Ceramics”, International Society of Applied Ferroelectrics (ISAF) Conference Proceedings, pp 70-73, 1992). Kumar et al. report that BaTiO
3
ceramics can be sintered to high densities as low as 850° C. using this process. On the other hand, Burn (“Flux-Sintered BaTiO
3
Dielectrics”,
Journal of Materials Science,
17, 1398-1408, 1982) indicates that commercially available BaTiO
3
can be sintered to high densities only around 1100° C. using the fluxing agents mentioned above. Average grain size of the low-firing BaTiO
3
ceramics prepared by Kumar et al. were in the range of 0.15-0.20 &mgr;m.
Kumar et al. also did preliminary work on using a Bismuth-based solution to coat hydrothermal BaTiO
3
crystallites so as to form a low-firing BaTiO
3
material. (See: Kumar et al., “Densification and Dielectric Properties of Hydrothermal BaTiO
3
with Different Bi
2
O
3
Sources”,
Ferroelectrics,
Vol. 154, 283-288, 1994). Bismuth oxide powder was also mixed with the hydrothermal BaTiO
3
crystallites, and the sintering results from the conventionally mixed batch was compared with the two solution-coated batches. BaTiO
3
ceramics with 3.0 wt % equivalent Bi
2
O
3
addition could be sintered to >90% theoretical density as low as 800° C. for the two solution-coated batches. Yet, even though sintering results for solution-coated batches looked promising, the two solution-coated BaTiO
3
powders did not show consistent densification results with each other. No details were given regarding the composition of the Bismuth-based solutions
Accordingly, it is an object of this invention to provide an improved BaTiO
3
ceramic that can be fired at a relatively low firing temperature.
It is a further object of this invention to provide an improved BaTiO
3
ceramic that exhibits consistent densification results.
It is another object of this invention to provide an improved BaTiO
3
ceramic that exhibits improved dielectric properties, even when fired at a low temperature.
SUMMARY OF THE INVENTION
Hydrothermal BaTiO
3
crystallites were coated with Bismuth solutions prepared from Bismuth metal-organics and anhydrous solvents. The Bismuth metal-organics were Bi 2-ethylhexanoate and Bi-neodecanoate. Bismuth oxide was also used as a comparison to the Bismuth solutions. BaTiO
3
ceramics with either 3.0 wt % equivalent Bismuth oxide or 5.0 wt % equivalent Bismuth oxide were made by sintering the compacts between 700° C. and 1000° C. BaTiO
3
ceramics that were coated by Bi-neodecanoate densified >90% theoretical as low as 800° C. for 3.0 wt % equivalent Bi
2
O
3
. Average grain sizes of 0.2-0.4 &mgr;m were observed for Bi-coated BaTiO
3
ceramics. Dielectric K versus temperature measurements of Bismuth-coated BaTiO
3
ceramics, sintered in the lower temperature ranges, showed consistently superior dielectric characteristics.

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