Sn-doped BaO-TiO 2 -ZnO microwave ceramics
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oped BaO–TiO2–ZnO (BTZ) microwave ceramic materials were investigated as a function of SnO2 content. Addition of a small amount of SnO2 (0.01–0.06 wt%) lowered the sintering temperature of the system to 1160 °C and also greatly reduced the dielectric loss (tan ␦), which is closely related to the insulation resistivity. The Sn-doped BTZ materials were found to have excellent dielectric properties at 1 GHz with dielectric constant ⑀ ≈ 36, loss tangent tan ␦ 艋 1 × 10−4, temperature coefficient of dielectric constant, ␣⑀ ⳱ 0 ± 30 ppm/°C, and volume resistivity v 艌 1013 ⍀ cm. I. INTRODUCTION
The importance of dielectric ceramics for microwave multilayer ceramic capacitors (MLCC) is increasing because they can be useful in the communications industry for mobile telephones and satellite broadcasting systems. The materials for MLCC are required to have a low dielectric loss, a small temperature coefficient of dielectric constant, and a high volume resistivity. BaO–TiO2 system compounds were first investigated by Rase and Roy.1 They studied the phase equilibria of this system with Ba/Ti ratios of 2:1, 1:1, 1:2, 1:3, and 1:4. Jonker and Kwestroo2 found the presence of the phases with Ba/Ti ratios of 2:5 and 2:9. Negas and Roth3 investigated a BaO–TiO2 crystalline system and found titanium-rich titanate compounds with Ba/Ti ratios of 6:17, 4:13, 1:4, and 2:9, respectively. To lower the sintering temperature and to improve the dielectric properties of BaO–TiO2 system, ZnO was added. The dielectric properties of BaO–TiO2–ZnO (BTZ) system as functions of SnO2 contents were investigated. The system prepared in this work was proved to have superior dielectric properties for microwave applications as follows: First, BTZ system ceramics can be used to make MLCC with high Q (⳱1/tan ␦) and nearzero temperature coefficient of dielectric constant. Second, the sintering temperature of this system was lowered significantly (1160 °C) because of the addition of SnO2 and other dopants, so that the electrode slurry of Pd30/ Ag70 may be used and the cost of monolithic capacitors was greatly reduced. Whereas, the sintering temperature of the system in previous works was as high as 1300– 1400 °C.3–5 II. EXPERIMENTAL
The starting materials of BaCO3, TiO2, ZnO, Nb2O5, and SnO2 powders were weighed according to the composition of 0.64BaTi4O9 + 0.32ZnO + 0.03Nb2O5, and 1550
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J. Mater. Res., Vol. 17, No. 6, Jun 2002 Downloaded: 13 Mar 2015
0.01–0.06 wt% SnO2 was incorporated as sintering agent. The purity of these powders and additives was higher than 99.9%. They were mixed and milled with zirconia balls in distilled water for 1.5 h and then dried and calcined in air at 1100 °C for 2 h. After that, the powders were milled again for 4.5 h and dried. The remilled and dried powders were sieved by No. 80 mesh and pressed isostatically into disks at 4 MPa. The disks were sintered on the Al2O3 plate at 1160 °C for 6 h at a heating rate of 4 °C/min. They were then covered with Ag electrodes for measurement. The capacitance (
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