Where C is the capacitance, e is the dielectric constant of the dielectrics, V and t are the volume of capacitor active body and the active thickness of dielectrics respectively. To achieve greater volume efficiency, reducing the active thickness is much more effective than increasing the dielectric constant alone. Therefore, great efforts have been made to produce a thinner ceramic layer. Unfortunately, serious problems may occur because the dc and ac fields applied to the dielectric layer become stronger with thinner layers. The increase of the dc field results in the decrease of the dielectric constant, while the increase of the ac field results in an increase of the dielectric loss [2]. For barium titanate ceramics, these two problems are more serious, especially the ac field stability for the dielectric loss. As indicated in Fig. 1, the dissipation factor of the MLCC (commercially available one with barium titanate-based dielectrics) shows a strong ac field dependence at room temperature, exceeding the specification of 2.5% for the Electronic Industries Association (EIA) X7R designation (<±15% change in capacitance between −55 ◦C and 125 ◦C) when the ac drive level is above 2 kV/cm. Thus, for MLCC applications, studying the loss mechanism of dielectric materials, to minimize its effect, is of great significance. It is widely accepted that small amounts of manganese are usually incorporated in BaTiO3 as a dielectric loss controller [3, 4]. But the role of Mn has not been fully understood yet. Also, manganese doping is needed for further investigation of relaxor materials, which are characterized by higher dielectric permittivity and lower sintering temperature and become important compositions for MLCC. Moreover, it is well known that Mn doping causes evident aging behavior [5, 6], but its mechanism has not been clarified. Therefore, in this letter, we have examined PMW-PT-PNN relaxor ceramics to discuss the influence of various Mn doping levels on dielectric loss and aging characteristics. A dielectric with the composition 0.27Pb(Mg1/2W1/2)O3-0.33PbTiO3-0.40Pb(Ni1/3Nb2/3)O3 was used throughout this study. The samples were prepared by the two-stage calcination method. Prior to reaction with PbO and TiO2, Columbite NiNb2O6 (NN) and wolframite MgWO4 (MW) were synthesized by calcining analytical reagent (AR) grade Ni(CH3COO)2 and Nb2O5 at 1000 ◦C for 6 h and MgCO3Mg(OH)2 · 6H2O and WO3 also at 1000 ◦C for 2 h. These phases were then batched with Pb3O4, TiO2, excess WO3 and MnO2 to form several compositions modified with 0.05, 0.1, 0.2, 0.3 mol% MnO2. Excess WO3 was added to obtain a core-shell structure [7]. The calcination condition of the batches was 800 ◦C for 2 h. The calcined powders were ball-milled again, ensuring a fine particle size before sintering. Then these dried powders were pressed as disks (10 mm in diameter and 1 mm thick) under the application of 100 MPa pressure. The pressed pellets were buried in the same composition powders to minimize PbO loss and fired in a covered alumina crucible at 1100 ◦C for 1 h with different heating rates. Finally, the silver-coated specimens were heat-treated at 600 ◦C. The dielectric properties of the sintered disk were studied as a function of temperature using an automatic measurement system incorporating an Environmental Chamber (Delta 2300) with an LCR precision meter (HP 4284A). The aging temperature was kept at 25 ◦C, and the aging time varied in the range of 1–100 h.
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Aug 31, 2012
Liuding Wang + 4
