Raz Muhammad

Research trends in microwave dielectrics and factors affecting their properties: A review

Abstract Ceramists are constantly looking for materials to be used as dielectric resonators in the telecommunication industry. These applications require materials with ∊r ∼ 4 – 120, Q × f0 > 10 000 GHz and τf ∼ 0 ppm K−1. Additionally, efforts are also underway to lower the sintering temperatures (e. g. ≤ 800 °C) to reduce processing and electrode costs. The simultaneous achievement of all the three properties mentioned above is difficult; nevertheless, some materials have been synthesized fulfilling the criteria for microwave applications. This study is an overview of various studies on materials for possible applications as microwave dielectrics and factors affecting their microwave properties. These factors include crystal structure, defects, fabrication route, and the type and concentration of substituents and additives.

Low loss La 5− x Sr x Ti 4+ x Al 1− x O 17 ceramics for microwave dielectric applications

La5−xSrxTi4+xAl1−xO17 (x = 0, 0.20, 0.25 and 0.30) ceramics were prepared through a mixed oxide solid state sintering route and characterized in terms of phase, crystal structure, microstructure and microwave dielectric properties. La5Ti4AlO17 exhibited reasonable relative permittivity µr ~ 42, a high quality factor Q×fo ~ 18906 GHz but the temperature coefficient of resonance frequency τf ~ −19 ppm/°C was too low for practical applications. In order to tune τf through zero, Sr was substituted for La which increased εr to 47.1, decreased Q × fo to 16678 GHz and decreased negative τf to −4.8 ppm/°C at x = 0.25. The observed decrease in negative τf may be due an increase in tolerance factor and hence change in tilt scheme of BO6 octahedra as a result of substitution of larger cations for smaller cations.

Phase evolution and microwave dielectric properties of A5M5O17-type ceramics

Abstract A number of A5M5O17 (A = Na, Ca, Sr, La, Nd, Sm, Gd, Dy, Yb; B = Ti, Nb, Ta) type compounds were prepared by a solid-state sintering route and characterized in terms of structure, microstructure and microwave dielectric properties. The compatibility of rare earths with mixed niobate/tantalate and titanate phases was investigated. The larger ionic radii mismatch resulted in the formation of pyrochlore and/or mixed phases while in other cases, pure A5M5O17 phase was formed. The samples exhibited relative permittivity in the range of 35 to 82, quality factor (Q × fo) = 897 GHz to 11946 GHz and temperature coefficient of resonance frequency (τf) = -120 ppm/°C to 318 ppm/°C.

Structure-dielectric property relationship in Nb-doped Ca4La2Ti5O17 ceramics

A number of compounds in the Ca4+xLa2−xTi5−xNbxO17 (x = 0–2) series were prepared through a mixed oxide solid-state sintering route. These compounds crystallized into orthorhombic (Pbnm) structure. The relative permittivity (er) and temperature coefficient of resonance frequency (τf) depended on the polarizability of constituent cations while the quality factor (Q × fo) was affected by ordering of cations. These compounds exhibited promising er (74–90) and Q × fo (10208–14116 GHz); however, the observed high τf (157–542 ppm/∘C) requires further investigations to tune it through zero.

Sol–gel synthesis of Na0.4K0.6Ca4Nb5O17 microwave ceramics

The sol–gel method was developed to synthesize A5B5O17-type Na0.4K0.6Ca4Nb5O17 layered perovskite ceramics, using NaNO3, KNO3, CaNO3⋅4H2O and NbCl5 precursors. Samples were calcined at 950°C and sintered at 1200–1350°C. The phase and microstructural analyses of samples were carried out using X-ray diffractometer (XRD) and scanning electron microscope. X-ray diffraction analysis revealed single phase monoclinic symmetry, within the detection limit of in-house XRD facility. Microstructural analysis shows ~ 10 μm elongated rod-like grains. The microwave dielectric properties of the sintered composition at 1300°C were: relative permittivity (er) = 42, quality factor (Q × f0) = 8270 GHz and temperature coefficient of resonant frequency (τf) = -10.1 ppm/°C.