Yasuharu Miyauchi

Millimeter-wave Dielectric Ceramics of Alumina and Forsterite with High Quality factor and Low Dielectric Constant

Millimeter-wave dielectric ceramics have been used like applications for ultrahigh speed wireless LAN because it reduces the resources of electromagnetic wave, and Intelligent Transport System (ITS) because of straight propagation wave. For millimeter-wave, the dielectric ceramics with high quality factor (Q·f), low dielectric constant(e r ), and nearly zero temperature coefficient of resonant frequency (τ f ) are needed. No microwave dielectric ceramics with these three properties exist except Ba(Mg 1/3 Ta 2/3 )O₃ (BMT), which has a little high τ f . In this paper, alumina (Al₂O₃) and forsterite (Mg₂SiO₄), candidates for millimeter-wave applications, were studied with an objective to get high Q·f and early zero τ f . For alumina ceramics, Q·f more than 680,000 GHz was obtained but it was difficult to obtain nearly zero τ f . On the other hand, for forsterite ceramics, Q·f was achieved from 10,000 GHz of commercial forsterite to 240,000 GHz of highly purified MgO and SiO₂ raw materials, and τ f was reduced a few by adding TiO₂ with high positive τ f .

Piezoelectric Properties of SrBi4Ti4O15-Based Ceramics

Piezoelectric properties of lead-free bismuth layer structure oxide, SrBi4Ti4O15 (SBT) based ceramics, were studied. By means of La substitution and Mn addition, the maximum value of Q (Qmax ) between resonant frequency and anti-resonant frequency was larger than that of SBT. These materials could be utilized in practical applications as substitutes for lead titanate based ceramics (PT) and lead zirconate titanate based ceramics (PZT) which are used mainly in resonators. The reason for the higher Qmax value was identified by microstructual analysis as being due to the absence of a defect structure.

Microwave-Millimeterwave Dielectric Materials

Development of microwave dielectric materials has been expected on the wireless communications in the high speed communication society. There are three important directions for research and development of microwave dielectric materials. We have been studied on these three directions based on the crystallography. First direction: tungstenbronze-type like compounds, Second direction: homologous compounds and third direction: silicate compound such as forsterite. INTRODUCTION Recently, microwave and millimeter-wave telecommunication has been developed for wide applications, such as mobile phone, wireless LAN, and intelligent transport system (ITS). Moreover, in the ubiquitous future system, everyone have computer such as IC-tag, which communicate each other using wireless antenna. Microwave and millimeter-wave dielectric materials are expected to be developed for variety of uses such as miniaturization for mobile phone, transmitter and receiver with high performance for base station, and millimeter applications for ultrahigh speed wireless LAN and ITS. The direction of development of microwave dielectric materials are shown in Fig.1, in which quality factors (Q·f) are shown as a function of dielectric constants (εr). Curve in the figure shows an outline the upper limit of Q·f obtained up to now for a given εr. Here, Q·f and εr are among the three important dielectric properties. Q is inverse of the dielectric loss (tanδ) and the effect of εr in shortening the wavelength is described by the relation λ=λ0/εr . Temperature coefficients of resonant frequency (τf) is also one of the three important properties. τf is expected to be close to zero. There are three directions for research and development of wireless communications. Properties demanded is high εr for the 1st direction, high Q and high εr for the 2nd one and extremely high Q and low εr for the 3rd one. The first direction are mainly on demand for miniaturization of mobile phone parts. The second one is on demand for increasing signal/noise ratio for the main application in mobile phone base station. The third direction is for devise working in millimeter-wave range. We have been studying microwave dielectric materials for all these directions. In this paper, we would like to show some examples in our results on the three directions. EXPERIMENTAL Crystal structure analyses were performed by using the four-circle X-ray diffractometer. The single crystals were grown by self-flux method. Full-matrix least-squares refinement was carried out using RADY program. Accurate lattice parameters were obtained 0 20 60 100 140 10

Controlled Temperature Coefficient of Resonant Frequency of Al2O3-TiO2 Ceramics by Annealing Treatment

The microwave dielectric properties of alumina (Al2O3) ceramics were studied. Our objective was to improve the large negative temperature coefficient of resonant frequency (τf = -60 ppm/°C) of Al2O3 ceramics by the adding rutile (TiO2) with a large positive τf (+450 ppm/°C). A near-zero τf (τf = +1.5 ppm/°C) with excellent microwave dielectric properties (Qf = 117,000 GHz, er = 12.4) was obtained in 0.9Al2O3-0.1TiO2 ceramics sintered at 1350°C for 2 h, followed by annealing at 1000°C for 2 h in air.

Microwave Dielectric Properties of Al2O3-TiO2 Improved by Addition of ZnO

Microwave dielectric properties of rutile (TiO 2 ) and ZnO added alumina (Al 2 O 3 ) ceramics have been studied for millimeter wave applications. Our objective was to achieve high quality factor (Q) and to improve temperature coefficient of resonant frequency (τ f ) of Al 2 O 3 ceramics. A relationship between the crystalline phase and microwave dielectric property was observed. Excellent microwave dielectric properties (Q·f = 187,000 GHz, ϵ r = 12.9) including near zero τ f (τ f = −2.9 ppm/°C) was obtained in 0.89Al 2 O 3 -0.11TiO 2 ceramics with 0.5 wt% of ZnO, when they were sintered at 1300°C for 5 h.

The Improvement of Microwave Dielectric Properties on Al2O3 Ceramics

Al 2 O 3 ceramics are a candidate for millimeter-wave dielectrics. It possesses a high quality factor (Q·f) and a low dielectric constant (ϵ r ), however, a relatively large negative temperature coefficient of resonant frequency (τ f ) is an obstacle. For microwave communicating application, a τ f value near 0 ppm/°C is desirable to keep the frequency stability at various temperatures. On the other hand τ f of TiO 2 is positive value, indicating that the τ f can be improved by doping TiO 2 in Al 2 O 3 . However 0.9Al 2 O 3 -0.1TiO 2 sintered above 1350°C were composed of three phases: Al 2 O 3 , TiO 2 and Al 2 TiO 5 . The τ f value was about -20 –30 ppm/°C. By annealing in 1000–1100 °C, τ f value approached to be 0 ppm/°C with decomposition of AlTi 2 O 5 . A good Q·f was obtained by optimizing the annealing condition.

Microstructures and Microwave Dielectric Properties on Annealed Al2O3-TiO2 Composite Ceramics

The large negative temperature coefficient of resonant frequency ( τ f ) of Al2O3 is a problem for applicable microwave/millimeter wave dielectrics. A Previous study reported that the τ f was improved by sintering with TiO2 and then annealing, where the annealing decompose the secondary phase of Al2TiO5. This study investigated the hold time of annealing. The Quality factor (Qf ) value decreased when the sample was annealed at 1100°C for 2hrs. While Al2TiO5 was not detected by X-ray powder diffraction, it was located around TiO2 by scanning transmission electron microscopy equipped with energy dispersive X-ray spectroscopy. It should be considered that the degradation of the Qf was caused by the existence of noncrystalline Al2TiO5 and new boundaries between noncrystalline Al2TiO5 and TiO2.