High dielectric constant and small temperature coefficient bismuth-based dielectric compositions
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M. F. Yan and W.W. Rhodes AT&T Bell Laboratories, Murray Hill, New Jersey 07974 (Received 23 August 1989; accepted 7 May 1990)
We have studied the crystal structure and the dielectric properties of a series of Bi-based ceramic compositions as a function of compositional variation and sintering temperature. These dielectrics have dielectric constants between 70 and 165 and their temperature coefficients are within ±500 x 10"6/°C. The precise temperature coefficient can be controlled via compositional changes such that dielectrics with temperature coefficients within ±50 x 10~6/°C are easily obtainable. The room temperature dissipation factor is smaller than 0.001 or equivalently, the Q value is greater than 1000. The electrical resistivity is greater than 1014 ohm-cm. Furthermore, these dielectrics are sinterable below 960 °C, rendering it possible to use silver or high silver metallization as the internal electrode in making the multilayer ceramic capacitors. I. INTRODUCTION
Ceramic capacitors are used in a variety of functions in electrical circuits. For bypassing, coupling, and filtering, the important properties are large capacitance value, low equivalent series resistance (ESR), and low equivalent series inductance (ESL). Thus, ferroelectric ceramics with high dielectric constant (K > 1500) are used as the dielectric material. For timing and tuning applications, the properties of primary concern are high Q (low loss) and the stability of the capacitance values. Furthermore, in a tuned circuit the stability of the circuit's resonant frequency may require proper compensation for temperature, humidity, voltage, and current. For these purposes one uses temperature-stable and temperature-compensating ceramic dielectrics, which are the subject of this paper. Temperature-stable and temperature-compensating dielectrics are nonferroelectrics. Commercial formulations consist primarily of TiC
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