Complex oxides for dielectric resonator applications

Abstract

The rapid progress in microwave telecommunication generated a huge demand for dielectric resonators as a result the production of dielectric resonators emerged as one of the fastest growing area today in the electronic ceramic manufacturing. More new materials are being synthesized to achieve superior dielectric properties and also cost effective. Currently, ceramic oxides with perovskite structure, in particular A(B’1/3B”2/3)O3, exhibit better microwave properties. For a material to be used as an dielectric resonator it should have a high dielectric constant (ε) low dielectric loss, tan δ, and the temperature coefficient of resonant frequency (τf) should be close to + 0 ppm/0C. Chapter I discusses the background of the thesis with an introduction to dielectric properties of the materials, perovskite structure, possible types of ordering in the perovskite structure, the importance of 1:2 ordering in the improvement of low loss (high quality factor) materials, different methodologies to synthesize these materials,which have been studied in the subsequent chapters, sintering methods have all been discussed. Literature review of perovskite oxides and the factors influencing the structure and the properties of the dielectric resonator materials, particularly in the A(B’1/3B”2/3)O3, have been discussed. Characterizations of ordering in the perovskite structure have been done using powder X-ray diffraction. In chapter II we discuss the, inconsistency in the microwave dielectric properties of Ba(Zn1/3Ta2/3)O3 (Fig. 1), the quality factor for the ceramic pucks sintered at same temperature show a huge variation. These huge variation may arise from the initial processing condition.Ba(Zn1/3Ta2/3)O3 was synthesized using four different ways and within which we also varied the calcination time. (i) The initial raw materials were hand ground and calcined at 10000C for 12 to 84 hours, the powder X-ray diffraction pattern on the calcined powder showed disordered cubic phase. The particle size increased marginally with increase in the calcinations time. The densitiy of the pellets sintered at 14750C was close to 70% of the theoretical density. The low frequency upto (20MHz) The ielectric constant (ε) was different for pellets with different initial calcination time, the dielectric loss were less than zero. (ii) The initial raw materials were ball-milled with ethanol and calcined at 10000C for 12 to 84 hours, the powder X-ray diffraction pattern on the calcined powder showed disordered cubic phase, until 48 hours of calcinations, above that even at 10000C the powder pattern shows the appearance of 100 super structure reflection corresponding to 1:2 ordering. The particle size of the calcined powders are in the same range. The density of the pellets sintered at 14750C was around 65 % of the theoretical density. The low frequency (20MHz) dielectric constant (ε) was different for pellets with different initial calcination time and their dielectric loss were less than zero. To study the effect of calcination time and ball milling time, using ethonal as the solvent, on the dielectric properties, six different samples were synthesized while varying the ball milling time shows the 1:2 ordering peak starts appearing, ~17.5 2θ, after 24 hours of calcinations at 10000C, similar ordering peaks also appears for 2 hours ball milling time. However, increasing the ball-milling time (4 hours and 6 hours) shows the appearance of (100) peak even at 12 hours of calcination. When the ball-milling time is further increased (8 hours and 10 hours) the ordering peak starts appearing only after 24 hours similar to 1 and 2 hours ball milling time. Initial calcination time and their dielectric loss were less than zero. The density of the pellets sintered at 14750C was around 70 % of the theoretical density. The low frequency (20MHz) dielectric constant (ε) was different for pellets with different calcination time and the dielectric loss were less than zero. (iv) To study the effect of solvent, calcination time and the ball milling hours on the dielectric properties of Ba(Zn1/3Ta2/3)O3 the raw materials were ball milled using acetone as a solvent. Above 36 hours of calcinations at 10000C, all the powder pattern show 100 super structure reflection corresponding to 1:2 ordering. The density of the pellets sintered at 14750C was around 70 % of the theoretical density. The low frequency (20MHz) dielectric constant (ε) was different for pellets with different calcinations time and the dielectric loss for all the pellets were less than zero. In chapter III NaCl –KCl flux method was used to synthesis BaZn1/3Nb2/3O3 and BaMg1/3Nb2/3O3 at various temperatures and at various soaking times. The powders synthesized using flux method show the disordered cubic perovskite structure at low temperatures compared to that of the powder synthesized using solid state method. However, irrespective of temperature or soaking time the powder X-ray pattern doesn’t show any evidence for 1:2 ordering without impurities, flux method may not be appropriate method to achieve 1:2 ordering in the BaZn1/3Nb2/3O3 and BaMg1/3Nb2/3O3 system. In Chapter IV three different composites and solid solution involving eight layered perovskites were synthesized (1-x)[Ba(Co1/3Ta2/3)O3] – x[(Ba(Co1/8Ta3/4)O3] it either forms a solid solution or forms composites, no other impurities were found in the powder X-ray pattern The dielectric constant change linearly for all the “x values and the dielectric loss are below zero. These composites are highly promising and can be tuned to zero τf values. In (1-x)[Sr(Zn1/3Ta2/3)O3] – x[(Sr(Zn1/8Ta3/4)O3] all the ceramics synthesized either forms a solid solution or a stable composites of the end members. The low frequency dielectric constant increases from ε=22 for x= 0.0 to a maximum value of ε=32 for x=0.2 and their dielectric loss are below zero for all the “x” values. We attempted to synthesize Sr8ZnTa6O24 by two different methods and both the methods gave different powder x-ray pattern. To understand the structure clearly, we synthesized (1-x) (Ba8ZnTa6O24) – x (Sr8ZnTa6O24) from the powder pattern it is clear that the transition occurs between the x=0.1 to x=0.2, below x=0.1 powder pattern is similar to Ba8ZnTa6O24 and above x=0.2 the powder pattern are all similar with slight increase in the 2θ value as the “Sr” concentration is increased.