High quality ceramics of Ba((Co0.7Zn0.3)1/3Nb2/3)O3 (BCZN), Ba(Mg1/3Nb2/3)O3 (BMN) and Ba(Mg1/3Ta2/3)O3 (BMT) were prepared by the mixed oxide route using sintering temperatures up to 1620°C. Products with a high degree of cation ordering exhibited dielectric Q×f values from 83,000GHz (BCZN) to 360,000GHz (BMT). High Resolution TEM and aberration-corrected scanning transmission electron microscopy (STEM) revealed ordering domains and type I, II and III boundary structures. High-Angle Angular Dark Field (HAADF) STEM images provided direct evidence of 1:2 ordering and stacking sequences, and the presence of disordered regions within domain boundaries. The exceptionally high Q×f values for BMT are associated with a high degree of B-site ordering and the removal of domain boundaries in large, single domain grains. The catastrophic degradation of Q×f values in BMN after prolonged sintering is associated with formation of a lossy ferroelectric secondary phase (Ba3Nb2O8), and changes to composition and stoichiometry of BMN grains.

AbstractThe title perovskite (0 ≤ x ≤ 3.6) is synthesized from stoichiometric amounts of BaCO3, Ga2O3, and Ta2O5 (1200 °C for 12 h and 1450 °C for 24 h).

Compositions based on REZn1/2Ti1/2O3–CaTiO3(RE=La and Nd), suitable for microwave (MW) applications have been developed by systematically doping the perovskite B site with Ta2O5, Al2O3, and MgO in order to tune the temperature coefficient of the resonant frequency (τf) and improve the MW quality factor (Q ×fr) Optimized compositions, 0.45La(Zn0.395Ti0.385Ta0.01Al0.21)O3–0.55CaTiO3(LZT‐CT) and 0.32Nd(Zn0.45Mg0.05Ti0.5)O3–0.10NdAlO3–0.58CaTiO3(NZT‐CT) were prepared by solid‐state reaction and have been characterized. X‐ray diffraction (XRD) and scanning electron microscopy revealed that both compositions were single phase with a bulk microstructure composed of equiaxed grains (∼10 μm). Electron diffraction and XRD demonstrated that, at room temperature, LZT‐CT and NZT‐CT were orthorhombic with space groupPnma, consistent with ana−a−c+tilt system. Diffraction contrast transmission electron microscopy revealed a complex domain structure consisting of ferroelastic and antiphase domain boundaries. LZT‐CT and NZT‐CT had zero τf, ɛr=49 and 44,Q×fr=29 600 GHz (at 1.957 GHz), and 32 200 GHz (at 1.971 GHz), respectively.

An 8-layer B-site deficient twinned hexagonal perovskite Ba8Ga4-xTa4+0.6xO24 has been synthesized and its structure and microwave dielectric properties characterized. This hexagonal perovskite consists of eight close-packed BaO3 layers stacked by a sequence of (ccch)(2), where c and h refer to cubic and hexagonal BaO3 layers, respectively. The Ba8Ga4-xTa4+0.6xO24 ceramic materials exhibit composition-independent dielectric permittivity e;4 29, improved Qf value with the B-site vacancy content increase, and tunable temperature coefficient of resonant frequency tau(f) from negative to positive. An optimum microwave dielectric performance was achieved for Ba8Ga0.8Ta5.92O24: Qf approximate to 29 000 GHz and tau(f) approximate to 11 ppm/degrees C. The factors controlling the microwave dielectric properties are discussed in comparison with 8-layer twinned analogues and related 10-layer twinned hexagonal perovskites based on their structural and property data.

Commercial glass frits (lead borosilicate glasses) were employed as the sintering aids to reduce the sintering temperatures of BST ceramics. The effects of the glass content and the sintering temperature on the microstructures, dielectric properties and tunabilities of BST ceramics have been investigated. Densification of BST ceramics of 5wt% glass content becomes significant from sintering temperature of 1000°C. The glass content shows a strong influence on the Curie temperature Tc, permittivity and the diffuse transition. X-ray results show all BST ceramics exhibit a perovskite structure and also the formation of a secondary phase, Ba2TiSi2O8. The shift of BST diffraction peaks towards higher angle with increasing the glass content indicates the substitution of Pb2+ in Ba2+ site, which mainly accounts for the diffuse transition observed in these BST ceramics. BST ceramics with 10wt% glass additives possess the highest tunability at all four sintering temperatures. A tunability of 12.2% at a bias field of 1kV/mm was achieved for BST ceramics with 10wt% glass content sintered at 900°C.

This paper presents the processing, modelling and results of a resonant element made from ultra high permittivity dielectric material shaped into a helix. The ceramic helix was made from bulk barium strontium titanate and was produced by viscous plastic processing (VPP). A 1.2 turn and 1.75 turn helix was produced and measured to validate the electromagnetic modelling results. Some of the helices studied in this work are very compact. An optimised device may possess moderate Q factors, due to the low filling factors. The dielectric loss and permittivity of the bulk barium strontium titanate were also measured at microwave frequencies. The quality factors achieved in this paper with dielectric helical resonators, made from barium strontium titanate, are the highest so far attained.

A new six-layer perovskite-related structure Ba 6Na 2Nb 2M 2O 17 (M = P, V), which consists of cubic (c) BaO 3 layers and oxygen-deficient pseudocubic (c') BaO 2 layers stacked in the sequence c'ccccc, is presented. In Ba 6Na 2Nb 2M 2O 17, two-dimensional slabs of the well-known 2:1 octahedral cation-ordered perovskite motif are isolated between layers of tetrahedral units formed by anion vacancy ordering: two consecutive NbO 6 octahedral layers are sandwiched by two single NaO 6 octahedral layers, which, in turn, connect with two isolated MO 4 tetrahedral layers. Both oxides are derived from the 2:1 ordered perovskite structure (e.g., Ba 3ZnTa 2O 9) by ordered removal of O atoms in every sixth BaO 3 layer. Both materials exhibit a relative permittivity of approximately 20-23, Q x f 0 values of approximately 7800-10600 GHz, and negative temperature coefficients of the resonant frequency of approximately -23 to -7 ppm/ degrees C.

Microwave dielectric ceramics have applied extensively in the microwave devices as microwave resonators, filters, dielectric substrates, phase shifters as the key fundamental materials. The advance in microwave dielectric ceramics promotes largely the rapid development of modern communications industry. The recent progresses on the microwave dielectric ceramics have been reviewed in this paper. And the developing trend of microwave dielectric ceramics is discussed.

The dielectric and crystallographic properties of the ceramic Ba3(Co0.6Zn0.4)Nb2O9 have been investigated in relation to the processing schedule. Regimes incorporating slow cooling or a lower-temperature annealing stage are found to enhance the dielectric properties with the unloaded quality factor value increasing up to Qu=24300 at 3.21GHz, as compared with Qu=14425 at 3.21GHz using the standard schedule, while high relative permittivity εr=34 is retained. High-resolution neutron powder diffraction studies reveal the materials to be biphasic assemblages of trigonal perovskites which exhibit varying degrees of cation order. The higher Q samples are found to contain the highest proportion of the fully cation-ordered phase.

Ten weight percent BBZS (Bi2O3, B2O3, ZnO and SiO2) glass was added to x(Ba4Nd9.333Ti18O54)−(1−x)(BaLa4Ti4O15) (BNLT, 0≤x≤1) composite dielectric ceramics to lower their sintering temperature whilst retaining microwave properties useful for low temperature co-fired ceramic and antenna core technology. With the addition of 10wt% BBZS glass, dense BNLT composite ceramics were produced at temperatures between 950 and 1140°C, depending on composition (x), an average reduction of sintering temperature by 350°C. X-ray diffraction, scanning and transmission electron microscopy and Raman spectroscopy studies revealed that there was limited inter-reaction between BLT/BNT and the BBZS glass. Microwave property measurement showed that the addition of BBZS glass to BNLT ceramics had a negligible effect on ɛr and τf, although deterioration in the measured quality factor (Qf) was observed. The optimised composition (xBNT−(1−x)BLT)/0.1BBZS (x=0.75) had ɛr∼61, τf∼38ppm/°C and Qf∼2305GHz.

In situ synchrotron x-ray powder diffraction has been used to study the kinetics of cation ordering and disordering in the microwave dielectric electroceramic Ba3CoNb2O9 with a time resolution of 15s. The method enables the order/disorder temperature (To∕d) in this material of 1430°C to be directly observed. The changes in the rate and degree of cation ordering and in the growth of ordered domains between samples ordered from standard precursor material and then subsequently reordered following an annealing period above To∕d show that small changes in precursor order state and phase assemblage strongly influence the final domain size.

Temperature‐stable, medium‐permittivity dielectric ceramics have been used as resonators in filters for microwave (MW) communications for several decades. The growth of the mobile phone market in the 1990s led to extensive research and development in this area. The main driving forces were the greater utilization of available bandwidth, that necessitates extremely low dielectric loss (high‐quality factor), an increase in permittivity so that smaller components could be fabricated, and, as ever in the commercial world, cost reduction. Over the last decade, a clear picture has emerged of the principal factors, that influence MW properties. This article reviews these basic principles and gives examples of where they have been used to control microwave properties and ultimately develop new materials.

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Three new n = 5 members of the B-cation deficient hexagonal perovskite A(n)B(n-1)O(3n) family, La5Zn0.5Ti3.5O15, La5GaTi3O15 and La5AlTi3O15, have been synthesised and their crystal structures determined. Their microwave dielectric properties and AC conductivities, as well as those of a known material, La5Mg0.5Ti3.5O15, were characterized. These four analogues adopt a 10H structure with an AO(3) stacking sequence of (hhccc)(2) with empty octahedral sites between two hexagonal layers. Zn, Mg, Ga and Al atoms prefer the octahedral sites between two cubic layers to those between cubic and hexagonal layers. The Mg phase exhibits more extensive B-cation order than the Zn, Ga and Al phases. These three new materials exhibit relatively high dielectric permittivity epsilon, 28 - 37, modest Q x f values, 23 000 - 30 000 GHz and a negative temperature coefficient of resonant frequency tau(f) from 237 ppm/degrees C to 255 ppm/degrees C. tau(f) of these four members and other n = 5 La-based and Ba-based members of B-cation hexagonal deficient perovskites are compared and the correlations between their tf and tolerance factors are discussed. Based on the comparison of these four materials with BaLa4Ti4O15, correlations between the defects, conductivity and microwave dielectric loss are also addressed.

Tunable capacitor elements for microwave phase shifter applications have been fabricated by screen printing barium strontium titanate (BST) films on alumina substrates. A Ba0.70Sr0.30TiO3 composition was chosen for the initial devices as it has been shown to exhibit high tunability at room temperature. A vertical capacitor test structure involving a Pt lower electrode, BST film and Ag top electrode has been used throughout the work. The tunability and figure of merit (phase shift/dB of insertion loss) at 2–3GHz were found to be strongly dependent on the sintering temperature of the BST layers, with properties improving as the sintering temperature was increased. However, for sintering temperatures >1280°C, the device properties could not be measured, possibly indicating a problem with the lower Pt electrodes. In order to reduce the sintering temperature required for densification, test structures have also been fabricated using other BST compositions to which sintering aids have been added. Finally, a reflection-type phase shifter (RTPS) based on the capacitor test structure and optimised processing conditions is presented with microwave measurements results.

BaLa4Ti4O15 (BLT) is a hexagonal perovskite-related compound with a temperature coefficient of resonant frequency (τf) of -2 ppm/°C, relative permittivity (εr)∼44 and figure of merit (Q·f)∼44000 GHz. Ba4Nd9.333Ti18O54 (BNT) has a tungsten-bronze-related structure with εr∼78, Q·f≈11000 GHz and τf of +47 ppm/°C. The microstructures and microwave dielectric properties of xBNT–(1-x)BLT (0≤x≤1) composite ceramics have been studied. X-ray diffraction analysis and scanning electron microscopy revealed that there was limited inter-reaction between the two phases and that samples were composed largely of BNT and BLT, although some deterioration in measured εr with respect to calculated values was observed. The optimum compositions were x=0.55 and 0.75 for which εr∼63, τf∼-20 ppm/°C and Q·f>10,000 GHz.

Integration of ferroelectric tunable components into dielectric layers of low temperature co-fired ceramic (LTCC) systems enables the realization of more functional multilayer ceramic modules (MCMs) for telecommunication applications. A composition designed for the low temperature sintering of ferroelectric BST material has been developed based upon previous work. Small amount of manganese is introduced to modify the properties of ferroelectric BaSrTiO3 LTCC compositions. The effects of manganese on sintering behaviour, microstructure, relative permittivity, dissipation factor and tunability are studied. Manganese enhances the sintering of the BST due to an increase in the amount of oxygen vacancies and which yields an improvement in the electrical properties of BST.

The dielectric properties of (1 - x)Ba(Zn1/3Nb2/3)O-3-xBa (Ga1/2Ta1/2)O-3 (BZN-xBGT) microwave (MW) ceramics, with x between 0 and 0.2, and those of 0.9Ba(Zn0.6Co0.4)(1/3)Nb2/3O3-0.1Ba(Ga0.5Ta0.5)O-3 (BZCN-BGT) were studied at MW, terahertz (THz) and infrared (IR) frequencies at temperatures from 10 to 300 K. At room temperature, the temperature coefficient of resonance frequency (tau(f)) near 3 GHz decreases from 28 ppm K-1 in undoped BZN to 2 ppm K-1 in BZN-0.2BGT and reduces to zero in BZCN-BGT. The addition of BGT to BZN depresses the dielectric Q value, but incorporation of Co improves the Q values, yielding Q similar to 30 000 at 3 GHZ in BZCN-BGT. The relative permittivity (epsilon') exhibits only limited variation with composition (epsilon' values in the range 34.4-36.0). IR and THz spectra as well as the low-temperature MW dielectric measurements revealed a weak dielectric relaxation below phonon frequencies, possibly arising from charges caused by inhomogeneous distribution of the B-site ions with differing valences. The IR reflectivity spectrum of BZN-0.2BGT is significantly different (smeared) compared with other compositions, which may be caused by disorder on the B sites and by an amorphous phase at the grain boundaries.