Sintering relaxors

G.A. Smolenskii [20] was a pioneering researcher and at the time relaxors were being intensively studied in Russia. Since then they have attracted very considerable interest because of the following attributes. Firstly the low sintering temperatures and high peak permittivity values which are attractive for the... 

Fig. 7.9 shows the temperature dependence of the dielectric constant and dielectric loss at 1 kHz for the PMN-PT ceramics obtained by sintering the calcined powders from a soft-mechanochemical route at 1200°C for 2 h. A diffuse phase transition, being typical for a relaxor, is observed for each ceramics. As x increases from 0 to 0.2, the maximum dielectric constant, K, , increases from 13000 to 27000. The temperature correspondent to K ,... 

The use of excess PbO to achieve densifrcation is quite common in the sintering of PZT. Accurate control of the atmosphere in binder burnout and sintering is important to assure reproducible electrical, dielectric, and piezoelectric properties. The same understanding of processing and control is needed in the manufacture of relaxor dielectric materials and relaxor MLC using the excess PbO route. Successful implementation can result in MLC reliability equal to that of BaTi03 MLC [41]. 

In addition to the multilayered ceramic capacitors just described, many of the barium titanate-based compounds that exhibit high dielectric constants are used in single-layer tape-cast capacitor devices. Relaxor materials such as lead magnesium niobate (PMN), which are characterized by high dielectric constants, broad dielectric maxima, and low sintering temperatures, have been manufactured in thin sheets by tape casting. 

In addition to the high k of many relaxor compositions they also have a broad peak in the permittivity versus temperature range, even in the absence of additives and even in the form of single crystals. This behavior is attributed to nanoscale (-lOnm)-ordered regions, which are too small to yield the sharp phase transition of normal ferroelectrics. As a result, spontaneous polarization and associated ferroelectric properties are retained over a very broad temperature range. Another attractive feature of relaxors is that dense polycrystalline ceramics are achievable at relatively low sintering temperatures (<900°C), which allows a significant reduction in the amount of Pd used in Ag-Pd metallizations for electrodes in multilayer capacitors (see Section 31.7). 

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