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.

Microwave Dielectric Ceramics for Resonators and Filters in Mobile Phone Networks

Since 1970s, many kinds of dielectric resonator materials with improved Q values and improved temperature coefficients have been developed. Representative of them are the systems, (Ba, Sr, Ca)(Zr, Ti)O3, MgTiO3-CaTiO3-La2O3 · TiO2, Ba2Ti9O20, (Zr, Sn)TiO4, (Sr, Ca)[(Li, Nb),Ti]O3, and BaO-Nd2O3-TiO2. In the past 15 years, the Q values of these materials have gradually been improved. Using these new materials, dielectric resonator filters and oscillators are being developed which are more compact and applicable to high power operation. Their main applications are cellular mobile radio systems and SHF-TV systems using direct broadcasting satellite. This paper will present the recent development in Japan of resonator materials and filters.

Recent development of dielectric resonator materials and filters in Japan

Small substitutions of BaZrO 3 into Ba[(Zn,Ni) 1/3 Ta 2/3 ]O 3 are utilized in the commercial preparation of low-loss perovskite microwave dielectrics. The structures of a series of these phases with substitution levels ranging from 1% to 5% BaZrO 3 were examined using high-resolution TEM. For < 2.15% BaZrO 3 the solid solutions retain the ordered 1:2 structure of the Ba[(Zn,Ni) 1/3 Ta 2/3 ]O 3 end-member but are comprised of small ordered domains whose size decreases as the Zr content is raised. The decrease in the size of the domains parallels a decrease in the processing time required to access a low-loss state. Although for pure Ba[(Zn,Ni) 1/3 Ta 2/3 ]O 3 reductions in the degree of cation order produce a large increase in the dielectric loss, the Zr-substituted ceramics retain a very low loss. We believe the low losses of the 1:2 ceramics are derived from the stabilization of the ordering-induced domain boundaries via the partial segregation of the Zr cations. For substitutions between 3% and 5% BaZrO 3 the size of the ordered domains continues to decrease but the system undergoes an abrupt transformation to a cubic 1:1 ordered structure with a doubled perovskite repeat. The structures of these phases have been interpreted using a random layer model in which one site is occupied by Ta and the other by a random distribution of Zn, Zr, and the remaining Ta cations, i.e., Ba{[Zn (2-y)/3 Ta (1-2y)/3 Zr y ] 1/2 [Ta 1/2 ]}O 3 . Although the ordering is confined to nano-sized domains, these ceramics also exhibit low losses, again reflecting the relative stability of the domain boundaries. In this case we believe the low losses reflect the effectiveness of the random layer in stabilizing the anti-phase boundaries.

/pdf/effect-of-ordering-induced-domain-boundaries-on-low-loss-ba-1igfjaki2v.pdf

Effect of Ordering‐Induced Domain Boundaries on Low‐Loss Ba(Zn1/3Ta2/3)O3‐BaZrO3 Perovskite Microwave Dielectrics

Loss of ZnO explains the improved microwave loss quality (Q) for Ba(Zn /SUB 1/3/ Ta /SUB 2/3/ )O3 ceramics which have been sintered at high temperatures or longer times. Ordering of Zn and Ta is complete in 60 h at 1400C, but crystallographic distortion and Q continue to increase up to 120 h. As ZnO volatilizes from the sample, Ba replaces Zn on the B sites and permits additional crystallographic distortion; meanwhile, barium tantalate phases also appear. Crystallographic compositional data are presented to confirm this interpretation.

Microwave Loss Quality of BaZn13Ta2/3O3 Ceramics

Monosized spherical particles of (Zr, Sn)TiO4 with an average diameter of about 0.3 μm were synthesized by the controlled hydrolysis of metal alkoxides. The as-prepared, amorphous, particles were compacted without any sintering aid, crystallized, and then sintered at 1600°C for 3 h into bodies with >96.0% of theoretical density. The dense sintered bodies of (Zr0.80Sn0.20)TiO4 showed good microwave characteristics; ɛr= 40.0, Q= 5000, and τt= 3 ppm/°C at 10 GHz. The dielectric constant was remarkably dependent upon the relative densities of the sintered bodies and the change of the lattice parameters, while Q value was mainly affected by the oxygen deficiency. An increase of the dielectric constant with the substitution of Sn4+ in ZrTiO4 was attributable to the enhancement of ionic polarization with the increase of the c-axis length.

Chemical Processing and Microwave Characteristics of (Zr,Sn)TiO4 Microwave Dielectrics

The dielectric properties at microwave frequencies of Ba(Zn1/3Ta2/3)O3 ceramics prepared by sintering were investigated. These ceramics had lower density but higher loss quality than ceramics hot-pressed at 1400°C. Loss quality was greatly improved by prolonged sintering. The Q of the ceramics measured by the dielectric resonator method was 14 000 at 12 GHz. The ceramics were investigated by X-ray diffraction analysis. It was found that Q improvement corresponds with increased Zn and Ta ordered structures in the ceramics.

Ba(Zn1/3Ta2/3)O3 Ceramics with Low Dielectric Loss at Microwave Frequencies

A classification of p-type superconductors is proposed. According to this classification, PbPr 2.4 Sr 1.6 Cu 2.6 O y ceramic was designed and synthesized as a ceramic having a new crystal structure. This ceramic has a tetragonal symmetry ( a =0.388 nm, c =2.914 nm) and consists of three blocks which are a (Pb/Cu)-O single layer, a Cu-O 5 pyramid and a fluorite-type Pr 2 -O 2 layer. It is expected to be a mother material of superconductors.

A new tupe of crystal structure in the Pb-Pr-Sr-Cu-O system

A review has been made to find the technical trend on the presently developed dielectric ceramics for microwave applications. Some new dielectric ceramics recently developed by authers that is (1) temperature stable and high Q materials of Ba(Zn1/3Ta2/3)O3 (BZT) and Ba(Zn1/3Nb2/3)O3–Ba(Zn1/3Ta2/3)O3 (BZNT) ceramics and (2) temperature stable and high dielectric constant materials of BaO–Sm2O3–TiO2(BST) ceramics and their applications have been introduced.

https://iopscience.iop.org/article/10.7567/JJAPS.24S2.60/pdf

Dielectric Ceramics for Microwave Application

Superconducting properties in a Bi-Sr-Ca-Cu-O system have been studied. Two types of superconductors were observed: one has a transition temperature of 80 K, and the other has two transitions at 80 K and 110 K. XRD measurements suggest that the latter superconductor has at least two phases. The phase which seemed to be responsible for the superconductivity above 80 K was investigated by means of XRD, SEM and XMA.

Superconducting Properties in a Bi-Sr-Ca-Cu-O System

Superconductivity and crystal structure of a Tl-Bi-Sr-Ca-Cu-O system were studied. A Tc-onset temperature of 120 K as determined by AC susceptibility measurement was achieved for the nominal composition of Tl2Bi0.5Sr2Ca3Cu4Ox ceramics sintered at 900°C for 100 h. The superconductive phases synthesized in the Tl-Bi-Sr-Ca-Cu-O system were (Tl, Bi)Sr2CaCu2Ox and (Tl, Bi)Sr2Ca2Cu3Ox ("1212" and "1223" phases). Superconductivity at 120 K may be due to this "1223" phase.

Syunichiro Kawashima

Papers

Ba(Zn1/3Ta2/3)O3 Ceramics with Low Dielectric Loss at Microwave Frequencies

A new tupe of crystal structure in the Pb-Pr-Sr-Cu-O system

Dielectric Ceramics for Microwave Application

Superconducting Properties in a Bi-Sr-Ca-Cu-O System

Superconductive Transition at 120 K in a Tl-Bi-Sr-Ca-Cu-O System