First - Principles Study On Microwave Performance Of Ceramic Media

With the rapid developing of modern communication technique, miniaturization and lightweight of microwave device have been increasingly addressed. Recently, much attention has been paid to the development of microwave telecommunication technologies since the increasingly requirement for microwave applications. With the continuing growth of mobile telecommunications, high quality microwave dielectric ceramics are strongly demanded. These applications require microwave substrate materials with high quality factor (Q×f) to achieve high selectivity, moderate dielectric constant (εr) to reduce the delay time of electronic signal, and nearly zero temperature coefficient for resonant frequency (τf) to stabilize the frequency. However, many microwave dielectric ceramics prepared by conventional milling methods with very high sintering temperature, and have excellent microwave dielectric properties. Thus, new manufacturing processes should be proposed to lower the sintering temperature and improve the microwave dielectric properties of the microwave dielectric ceramics.In this paper, the dielectric properties are investigated by combining both the theoretial and experimental methods. The intrinsic and extrinsic factors that affect the dielectric properties of microwave ceramics are discussed. The electronic structure, binding energy, density of states and Mulliken bonding population of the AWO4 (A=Ca, Ba), Mg2TiO4, Mg2SiO4 microwave ceramics are calculated by the ab-initial method, which have been employed to explain the easily crystalling phase of AWO4 (A=Ca, Ba) during the high energy ball milling (HEBM) process. Meanwhile, besed on the theoretical calculation results, we have studied the sintering behavior, microstructure and microwave dielectric properties of ceramics. The results were shown as follows:(1) The numerical calculated binding energy for AWO4 (A=Ca, Ba) clculated are-41.3 eV and -45.1 eV, respectively. Just because of the very low binding energy, the crystal samples of the AWO4 were obtained in the HEBM process for a very short time, i.e.,30 mins. The density of states and Mulliken populations for CaWO4 and BaW04 showed that O2p and W5d states not only contribute to the top of the valence band, but also to the bottom of the conduction band. Thus the W-0 bonds are the covalence bond, which have stronger covalent contribution than the Ca-0 bonds. The Q×fvalues could be attributed to the bond strength of W-0 due to strong covalent interactions. The theoretical dielectric constant of AWO4 (A= Ca, Ba) are 11.2 and 9.3, respectively, coincide with the measured values. The theoretical temperature coefficient of resonance frequency of AWO4 (A= Ca, Ba) are -34 ppm/°C and -42 ppm/°C, respectively. Moreover, the AWO4 (A= Ca, Ba) ceramics were synthesized by HEBM method. All AWO4 ceramics with single phase have dense microstructures and good microwave dielectric properties.(2) The numerical calculated binding energy for Mg2TiO4 clculated is -53.8 eV. Because of this binding energy, the crystal sample of the Mg2TiO4 could not be obtained in the HEBM process. The theoretical dielectric constant is 14.4, coincide with the measured values. The densities of state for Mg2TiO4 showed that O2p and Ti3d states not only contribute to the top of the valence band, but also to the bottom of the conduction band. The Ti-O bonds are the covalence bond, which have stronger covalent contribution than the Mg-O bonds. The Q×fvalues could be attributed to the bond strength of Ti-O due to strong covalent interactions. Moreover, the Mg2TiO4 ceramics were synthesized by HEBM method. All samples showed almost full density and excellent microwave dielectric properties.(3) The numerical calculated binding energy for Mg2SiO4 calculated is -75.3 eV. The densities of state for Mg2SiO4 showed that O2p and Si3d states not only contribute to the top of the valence band, but also to the bottom of the conduction band. The Si-O bonds are the covalence bond, which have stronger covalent contribution than the Mg-O bonds. The Q×fvalues could be attributed to the bond strength of Si-O due to strong covalent interactions. Mg2SiO4 ceramics were synthesized by high energy ball milling method. All ceramics showed almost full density and excellent microwave dielectric properties, especially Q×f=193,800 GHz proved the contribution of covalent bond.