Forschungs- & Entwicklungsinformationsdienst der Gemeinschaft - CORDIS

Low firing temperature BST ceramics

In many cases, a further advance in the electronics industry is closely related to a reduction in the processing temperatures of functional materials, e.g., the production of a base-metal electrode capacitor, the reduction of Pb losses in piezo-materials, the integration of passive elements in LTCC modules, etc. Because of the lack of fundamental knowledge about low-temperature sintering mechanisms researchers are forced to apply specific empirical principles for each particular material. Only a few attempts to explain the basic mechanisms of low-temperature sintering have been published so far, and no general principles have been described. Here we explain the fundamental elements of a low-temperature sintering mechanism, called reactive-liquid phase sintering, and show that if a few general conditions are ensured then almost any powder can be sintered at temperatures as much as 400°C lower than its initial sintering temperature.

We performed the sintering experiments involving BaTiO3 on a variety of different BaTiO3 powders, whose original sintering temperature is from 1250 to 1300°C. A small addition of 0.3wt% Li2O was used as a sintering aid in the form of an either polycrystalline Li2O, Li2CO3 or an acetic solution of Li+ ions. Microstructural investigations of the sintered bodies showed the presence of a small concentration of two secondary phases: Li2TiO3 and Ba2TiO4. A high-resolution TEM was used to check the grain boundaries and the triple points for the presence of the liquid-phase residuals; however, no such evidence was found. To understand this unusual sintering behavior a detailed investigation of the reaction mechanism of the sintering was performed.

Based on these experimental results we have developed a general explanation for the low-temperature sintering mechanism, called here reactive liquid-phase sintering. The essential element of reactive liquid-phase sintering is the presence of a low-temperature liquid phase that must be able to directly or indirectly accelerate a reaction with the matrix phase. If we assume the same thermal conditions and the same reaction-limiting process, which is the rate of diffusion through the solid reaction layer then the rate of the reaction depends only on the surface contact areas, which in the case of the reaction between a liquid and a solid is significantly larger. So, the rate of the reaction will be significantly increased when one of the reagents melts.

The next important element of reactive liquid-phase sintering is the nature of the reaction with the matrix phase. The reaction must enhance at least one of three mass-transport processes, which are dominant in such a system during sintering. The most direct way is to increase the volume-diffusion coefficients. The volume-diffusion coefficients are proportional to the vacancy concentration; as a result, the sintering rate will increase when the structural vacancies are generated in the matrix phase. The process called a liquid-phase (-assisted) sintering, where the mass transport goes through the liquid phase by a solution-precipitation method, also promotes the sintering.

This process can be further accelerated by the increase in the solubility of the matrix phase during or after the reaction. Finally, if during the reaction with the matrix phase a temporary or permanent amorphization occurs a viscous flow from the grain surface to the necks between the grains contributes to the sintering.

In the second stage of our investigation we applied the reactive liquid-phase sintering mechanism to successfully sinter a number of powders with very different chemistries. The sintering aids were selected according to the crystal-chemistry of the matrix phase in such a way that they would trigger all the mechanisms required for the reactive liquid-phase sintering.

Certainly, the addition of sintering aids has an influence on the physical properties of ceramics. However, the concentration of the sintering aid is small and in many cases it is entirely incorporated into the crystal lattice of the matrix. With the proper selection of sintering aid we can minimize the influence on the particular physical properties.

An example is the commercial X7R capacitor formulation, which produces its desired dielectric properties as a result of an inhomogeneous core-shell microstructure. By applying the same method as for the SrTiO3 we managed to reduce the sintering temperature from 1090 to 900°C. Due to the low processing temperature the inhomogeneity was well preserved and consequently the obtained dielectric properties are even better than usual. In addition, due to such a low sintering temperature the expensive palladium can be eliminated from the electrodes of the X7R multilayer capacitors, which makes the production significantly cheaper.

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