High-permittivity ceramics

Dielectrics with relative permittivities exceeding 1000 are based on ferroelectric materials and are more sensitive to temperature, field strength and frequency than lower-permittivity dielectrics. Development in the past 50 years has resulted in improvements in stability whilst retaining the desirable high-permittivity 

EIA Code Temperature range/°C EIA Code Capacitance change/% 

Example a capacitor is required for which the capacitance value at 25 °C changes by no more than +7.5% in the temperature range —30 °C to + 85 °C the EIA Code will be Y5F. EIA codes D-R Class II Codes S-V Class III. 

---

Class II/III dielectrics consist of high-permittivity ceramics based on ferro-electrics. They have er values between 2000 and 20 000 and properties that vary more with temperature, field strength and frequency than Class I dielectrics. Their dissipation factors are generally below 0.03 but may exceed this level in some temperature ranges and in many cases become much higher when high a.c. fields are applied. Their main value lies in their high volumetric efficiency (see Table 5.1). 

Class II dielectrics are high-permittivity ceramics based on ferroelectrics (see Chap. 15) and have values of k between 2000 and 20,000. 

Petzelt, J., Rychetsky, L Effective dielectric function in high-permittivity ceramics and films. Ferroelectrics 316, 89-95 (2005)... 

Medium-permittivity ceramics are widely used as Class I dielectrics, and in order to be in this category they need to have low dissipation factors. This precludes the use of most ferroelectric compounds in their composition since ferroelectrics have high losses (tan S >0.003), particularly when subjected to high a.c. fields. 

The following discussion outlines the historical path taken in the development of microwave ceramics combining high permittivity, high Q and zero tf with long term stability and at an economic cost. Key references are given in the overview by R. Freer [10]. 

Technically useful properties of such perovskite ceramics are their high permittivities (relative dielectric constants), the semiconductor properties of certain chemical compositions and their piezoelectric properties. 

Ceramic dielectric exhibit extremely high permittivities... 

Low-permittivity ceramics are widely used for their insulative properties. The major requirements are good mechanical, thermal, and chemical stability good thermal shock resistance low-cost raw materials and low fabrication costs. These include the clay- and talc-based ceramics also known as electrical porcelains. A large-volume use of these materials is as insulators to support high tension cables that distribute electric power. Other applications include lead-feedthroughs and substrates for some types of circuits, terminal connecting blocks, supports for high-power fuse holders, and wire-wound resistors. 

R. C. Kell, A. C. Greenham, and G. C. E. Olds, High-Permittivity Temperature-Stable Ceramic Dielectrics with Low Microwave Loss, J. Am. Ceram. Soc., 56, 352-4 (1973). 

The microwave dielectric properties were measured under TEou mode. The ceramic sintered at 1100 °C shows relatively good microwave dielectric properties of high permittivity (e 58) and high Q f value up to 7500 GHz. Moreover, Figure 3 presents the resonant frequency of the ceramic as a function of the temperature. The Xj value of the ceramic sintered at 1100 °C calculated by Equation (1) is about 24 ppm/°C, which is relatively low. 

Ferroelectric materials, especially polyciystalhne ceramics, are utihzed in various devices such as high-permittivity dielectrics, ferroelectric memories, pyroelectric sensors, piezoelectric transducers, electrooptic devices, and PTC (positive temperature coefficient of resistivity) components. 

Barium titanate has been studied extensively since the end of World War II when it was identified independently in Russia, Japan and USA as a promising material with high permittivity for ceramic capacitors. Its ferroelectric activity is known since 1946 when it was discovered on ceramic samples. Since 1950 s also the single-crystals of desired size and quality are available. BaTiOs undergoes several phase transitions... 

The advantages over natural crystals are the subsequent polarizability, very high permittivity, chemical resistance, free possibility of forming and low-cost manufacturing by the ceramic manufacturing process. 

Several strategies for the design of dielectric microwave ceramics with high permittivities, high Q-factors, and a close to zero temperature coefficient of the resonance frequency, are Usted in Table 8.5. 

---