Low Loss Microwave Dielectric Ceramics Research Articles

Defect-related broadband dielectric loss mechanisms of Na1/2Sm1/2Ti1–z(Al1/2Nb1/2)zO3 ceramics

Dielectric ceramics with low dielectric loss in the microwave bands are key materials used in fifth/sixth-generation (5G/6G) telecommunication. Exploring the defect behavior and the broadband dielectric response of ceramics is helpful to understand the loss mechanism and develop novel low-loss microwave dielectric ceramics. In this work, Al/Nb was co-substituted for Ti4+ in Na1/2Sm1/2TiO3 (NST) ceramics with a perovskite structure, and the low frequency, microwave and terahertz (THz) dielectric responses were investigated. In different frequency bands, the variations of dielectric loss with substitution amount are inconsistent. The Al/Nb-substitution increases the dielectric loss in the low frequency and microwave bands, while the dielectric loss in the THz band decreases. The changes in dielectric loss are closely related to the lattice defects in Na1/2Sm1/2Ti1–z(Al1/2Nb1/2)zO3 (NST-zAN, 0 ≤ z ≤ 0.1) ceramics, which are studied by thermally stimulated depolarization current (TSDC) technique. The dielectric relaxation of the defects including oxygen vacancies (VO··) and defect dipoles leads to the dielectric loss in the low-frequency and microwave bands. The dielectric loss in the THz band is mainly affected by the damping of lattice vibration, which is associate with the defect concentration. This work might provide a considerable reference for the development of low-loss dielectric materials.

Lattice dynamics and terahertz response of microwave dielectrics: A case study of Al-doped Ca0.6Sm0.27TiO3 ceramics

Low-loss microwave dielectric ceramics are of great significance for fifth-generation telecommunication (5G) devices used for higher frequencies, while reducing the dielectric loss in the microwave band is still a technical challenge. Lattice dynamics and terahertz response are closely related to the polarization of dielectrics, and can be used as effective methods to explore the loss mechanism of microwave dielectrics. Here, Al3+ was doped into the conventional high-permittivity (εr) Ca1-xSm2x/3TiO3 (x = 0.4) ceramic system, enhancing the Q×f values significantly. Raman spectra, thermally stimulated depolarization currents (TSDC) and terahertz time-domain spectra (THz-TDS) together showed that the stiffness of cation vibration became stronger when z value was larger, indicating a smaller damping of the lattice vibration, and leading to lower extrinsic losses and higher Q×f values. The relationship between lattice dynamics, defect behavior and microwave dielectric loss will provide a significant reference for the development of low-loss microwave dielectric ceramics.

Crystal structure, infrared reflectivity spectra and microwave dielectric properties of CaAl2O4 ceramics with low permittivity

CaAl2O4 ceramics with low permittivity have been prepared via a solid state ceramic route. X-ray diffraction analysis and Rietveld refinement result demonstrated that CaAl2O4 ceramics belonged to the monoclinic structure with P21/n space group. Dense ceramics with homogeneous microstructures were obtained when sintered at around 1450 °C, wherein excellent microwave dielectric properties (εr = 8.9, Qf = 91,350 GHz, τf = −55 ppm/°C) were achieved as well. Moreover, the infrared reflectivity spectra were measured and fitted to understand their intrinsic dielectric properties. Considering the merits of easy preparation, cheap raw materials and most importantly, the outstanding microwave dielectric properties, CaAl2O4 ceramics are expected to be promising candidates for low loss microwave dielectric ceramics.

Structure and microwave dielectric properties of SrLa[Al1−x(Mg0.5Ti0.5)x]O4 (x = 0.2–0.8) ceramics

Dense SrLa[Al1−x(Mg0.5Ti0.5)x]O4 (x = 0.2–0.8) ceramics were prepared by a standard solid state sintering method. The K2NiF4–type layered perovskite structure with I4/mmm space group was revealed by XRD over the whole composition range, indicating that SrLaAlO4 and SrLa(Mg0.5Ti0.5)O4 can form infinite solid solution. The evaluation of the refined structure was investigated by Rietveld analysis of XRD and Raman spectrum. The dielectric constant (εr) and temperature coefficient of resonant frequency (τf) increased with increasing x, while the Qf value first increased and then decreased. The measured εr was significantly lower than the predicted value from Clausius-Mosotti relationship, and their difference approached 0 with increasing x, which is mainly attributed to the increasing normalized bond lengths of Sr/La-O(1) and Sr/La-O(2a) bonds. The optimal microwave dielectric properties with εr = 22.2, Qf = 89,100GHz and τf = −0.1ppm/°C were obtained for x = 0.65, indicating that SrLa[Al1−x(Mg0.5Ti0.5)x]O4 solid solution is a promising candidate as low-loss microwave dielectric ceramics.

Ultrahigh Q values and atmosphere-controlled sintering of Li2(1+x)Mg3ZrO6 microwave dielectric ceramics

Ultrahigh-Q Li2(1+x)Mg3ZrO6 microwave dielectric ceramics were successfully prepared by means of atmosphere-controlled sintering through simultaneously adopting double crucibles and sacrificial powder. This technique played an effective role in suppressing the lithium volatilization and further promoting the formation of the liquid phase, as evidenced by the X-ray diffraction, microstructural observation and the density measurement. Both dense and even microstructure, and the suppression of detrimental secondary phases contributed to low-loss microwave dielectric ceramics with Q×f values of 150,000–300,000GHz. Particularly, desirable microwave dielectric properties of εr=12.8, Q×f=307,319GHz (@9.88GHz), and τf=−35ppm/°C were achieved in the x=0.06 sample as sintered at 1275 °C for 6h.

Characterization of low loss microwave dielectric materials Zn0.92Co0.08ZrNb2O8 based on the complex chemical bond theory

Low loss microwave dielectric ceramics Zn0.92Co0.08ZrNb2O8 with wolframite structure were prepared by the solid state method. Sintering characteristics, phase composition and microwave dielectric properties were investigated systematically. X-ray diffraction results showed that all samples exhibited a wolframite structure. Rietveld refinement was used to analyze the crystal structure. Based on the lattice parameters, the bond ionicity, the lattice energy, the bond energy and the coefficient of thermal expansion were calculated to evaluate the structural characteristics. The e r values could be strongly dependent on the bond ionicity of the Nb–O bonds. The Q·j values were main affected by the lattice energy and the bond energy of the Nb–O bonds. As for the τ f values, it could be mainly affected by the coefficient of thermal expansion of Zn/Co–O bonds. At the sintering temperature of 1200 °C, Zn0.92Co0.08ZrNb2O8 ceramics exhibited the excellent microwave properties of e r = 27.10, Q·f = 61,036 GHz (at 8.54 GHz) and τ f = −9.81 ppm/°C. These results demonstrated that Zn0.92Co0.08ZrNb2O8 ceramics could be promising candidate materials for the application of highly selective microwave ceramic resonators and filters.

A novel low loss microwave dielectric ceramics ZnTi0.6Sn0.4Nb2O8 with wolframite structure

Wolframite-structure ZnTi0.6Sn0.4Nb2O8 was firstly prepared by the conventional solid-state method. X-ray diffraction patterns showed that the samples exhibited wolframite structure instead of ixiolite-structure of ZnTiNb2O8. Microwave dielectric properties of ZnTi0.6Sn0.4Nb2O8 ceramics exhibited a significant dependence on sintering characteristics and crystal structure. The phase transition in ZnTi0.6Sn0.4Nb2O8 ceramics played an important role in reducing dielectric loss. In addition, the bond energy of ceramics was also reported for the first time to analyze intrinsic loss. Typically, the ZnTi0.6Sn0.4Nb2O8 ceramics sintered at 1200°C for 4h exhibited excellent microwave dielectric properties with εr=24.61, Q·f =69,500GHz and τf=−2.67ppm/°C.

Microwave dielectric properties of low loss microwave dielectric ceramics: A0.5Ti0.5NbO4 (A=Zn, Co)

Abstract The microwave dielectric properties of low-loss A0.5Ti0.5NbO4 (A = Zn, Co) ceramics prepared by the solid-state route had been investigated. The influence of various sintering conditions on microwave dielectric properties and the structure for A0.5Ti0.5NbO4 (A = Zn, Co) ceramics were discussed systematically. The Zn0.5Ti0.5NbO4 ceramic (hereafter referred to as ZTN) showed the excellent dielectric properties, with ɛr = 37.4, Q × f = 194,000 (GHz), and τf = −58 ppm/°C and Co0.5Ti0.5NbO4 ceramic (hereafter referred to as CTN) had ɛr = 64, Q × f = 65,300 (GHz), and τf = 223.2 ppm/°C as sintered individually at 1100 and 1120 °C for 6 h. The dielectric constant was dependent on the ionic polarizability. The Q × f and τf are related to the packing fraction and oxygen bond valence of the compounds. Considering the extremely low dielectric loss, A0.5Ti0.5NbO4 (A = Zn and Co) ceramics could be good candidates for microwave or millimeter wave device application.

High εr and low loss microwave dielectric ceramics Ba3LiNb3−xTaxTi5O21

High dielectric constant and low loss microwave dielectric ceramics in the system Ba3LiNb3–xTaxTi5O21 (0 ≤ x ≤ 3) have been prepared through solid state ceramic route. The structure and microstructure of the compositions have been studied using powder X-ray diffraction and Scanning electron microscopic studies. The ceramics could be well densified under 1140 °C and exhibited a single hexagonal phase through the entire range of compositions. They show a linear variation of dielectric properties with the value of x. Their dielectric constant (εr) varies from 78 to 55.5, quality factor Q × f increases from 9780 to 18,480 GHz and temperature variation of resonant frequency reduces from +205 to +70 ppm °C−1 as the content of Ta increases.

Ln 2Mo 3O 12 (Ln = La, Nd): A novel group of low loss microwave dielectric ceramics with low sintering temperature

Ln 2Mo 3O 12 (Ln = La, Nd) ceramics with a defect scheelite related structure were prepared via a solid state reaction method. The La 2Mo 3O 12 ceramics sintered at 930 °C for 2 h exhibited a low dielectric permittivity of 10.1, a high quality factor (Qf value) of 60,000 GHz and a temperature coefficient of −80 ppm/°C at 12.7 GHz. The Nd 2Mo 3O 12 ceramics sintered at 945 °C for 2 h possessed a dielectric permittivity of 8.2, a Qf value of 80,000 GHz, and a temperature coefficient of −60 ppm/°C at 15.8 GHz. This group of ceramics could be good candidates for microwave substrate applications.

New low loss microwave dielectric ceramics in the BaO-TiO2-Nb2O5Ta2O5 system

Microwave ceramic dielectric resonator materials in the BaO-TiO2-Nb2O5Ta2O5 system such as BaTiNb4O13, BaTiTa2Nb2O13, BaTiTa4O13, Ba3Ti4Nb4O21, Ba3Ti4Ta4O21, Ba3Ti5Nb6O28, Ba3Ti5Ta6O28 and Ba3Ti5Nb3Ta3O28 have been prepared by the conventional solid state ceramic route. They have relatively high dielectric constant and high quality factor. The resonator materials are characterized by X-ray diffraction and scanning electron microscopy (SEM) methods. The BaTiNb4O13Ba3Ti5Nb6O28 and Ba3Ti5Nb3Ta3O28 have small temperature variation of the resonant frequency and are possible microwave dielectric resonator materials for practical applications.