A mechanism for low-temperature sintering

Abstract

We explain the basic mechanism of the low-temperature sintering called reactive liquid-phase sintering. The mechanism involves the presence of a low-temperature liquid phase that must be able to directly or indirectly accelerate a reaction with the matrix phase. The mechanism is explained in details for the case of the low-temperature sintering of BaTiO₃, which was sintered to more than 95% of relative density in 15 min at 820 °C. We have applied reactive liquid-phase sintering to a number of different compounds with very different crystal-chemistry characteristics, and managed to sinter them as much as 400 °C below their original sintering temperatures. A thorough understanding of this sintering mechanism makes it possible to closely control the sintering behavior.

Introduction

Until recently, the rapid developments in semiconductor technology have not been matched by the progress in passive components. This situation was especially critical for the telecommunications industry, where the miniaturization of handset devices plays a key role. An important breakthrough came with the introduction of low-temperature cofired ceramic (LTCC) technology, which has enabled miniaturization, the integration of passive functions and a reduction in costs, and has led to the production, for example, of the well-known Bluetooth module. LTCC modules are produced by co-firing ceramic layers with a three-dimensional Ag-microstrip circuitry. To avoid melting of the Ag-microstrips the firing temperature must be around 900 °C, which is extremely low for a ceramic material and represents the major problem with this technology. For a variety of different reasons lowering the sintering temperature is also important for many other technologies, which means it represents the same challenge for other functional materials, e.g. capacitor materials (the production of a base-metal electrode capacitor), piezo-materials (the reduction of Pb losses), etc.

A number of material-research laboratories have focused their research on reducing the sintering temperatures of functional materials. However, 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,1, 2, 3, 4 and no general principles have been described.

Many researchers have already investigated the low-temperature sintering of BaTiO₃-based ceramics, however, they did not so far clearly explain all the reaction and sintering mechanisms involved in the process.1, 2, 5, 6, 7 As a rule, the investigators used lithium-fluorite salts as a sintering aid and agreed about the mechanism of incorporating the lithium into the titanium sites of BaTiO₃. They assigned an important role in the reduction of the sintering temperature to fluorine ions, either through the incorporation into the oxygen sublattice or the formation of the low-temperature flux. The composition of the flux has not been determined yet, and the investigators have not explained a peculiar correlation between the stoichiometry of the BaTiO₃ and the low-temperature sintering behavior, which was observed during these studies.

Here we explain the fundamental low-temperature sintering mechanism 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 first determined these conditions for the case of BaTiO₃, explained the low-temperature sintering mechanism and showed that no fluorine ions are needed for successful low-temperature sintering. The generalization of the mechanism to other systems with significantly different crystal-chemistry characteristics involved a second research phase.