Tsutomu Tsunooka

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 .

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

Development of Forsterite with High Q and Zero Temperature Coefficient τf for Millimeterwave Dielectric Ceramics

High-Q pure forsterite (Mg2SiO4) ceramics with Qf = 240,000 GHz were developed by the usual solid sintering process, using raw materials of highly purified MgO and SiO2.The τf (temperature coefficient of the resonant frequency) was a large and negative: -60 to –70 ppm/oC. To adjust the τf value to zero, rutile (TiO2) with high τf (450 ppm/oC) was added. When sintering temperature was selected as low as possible, TiO2 remained in the forsterite specimen, which raised τf from a negative to positive values. For instance, the 30 wt% TiO2 added forsterite ceramics sintered at 1200oC yield a high density (3.3 g/cm ), which same as that of the same forsterite ceramics sintered at 1350oC. A part of added TiO2 works for sintering aid and residual ones do for arising the τf value from negative to positive. Improved forsterite ceramics have τf = 0 ppm/ ̊C, εr = 11 and Qf = 82,000 GHz, when they contains 24 wt% of TiO2 and sintered at 1200oC for 2h.

Quality Factor of Forsterite for Ultrahigh Frequency Dielectrics Depending on Synthesis Process

Forsterite is a candidate ultrahigh frequency dielectric for wireless communications. The quality factor Q of forsterite (Mg2SiO4) for microwave dielectrics is affected by calcination conditions. The shape of calcined powder particles remains the same as that of fine grains of raw silica materials. This is one of the key points for high Q, because of the realization of a high green density. From the observation of the section of calcined grains by definition (EPMA), it was clarified that Si ions remain in the core part of the grains. The remaining silica affected Q by forming a glassy phase and enstatite (MgSiO3).

Synthesis of High-Quality Forsterite

To establish a process that produces high-quality forsterite stably, calcining and sintering conditions were investigated chiefly and two kinds of silica with different forms, and grain sizes were used as starting raw materials. On the basis of the quality factor (Qf) for forsterite, the sintered samples prepared using powders calcined for 10–24 h, were found to be more stable than those in the case of 2–4 h, and in the case of 24 h of calcination, the samples showed a single phase of forsterite with fine grains. Silica with an amorphous form and a small grain size of 0.25 µm brought a higher Qf value and a wider permissible temperature range of sintering than silica with a crystalline form and a coarse grain size of 0.82 µm. Concerning the sintering temperature, the sample sintered above 1400 °C showed a high Qf value. The Qf value of the sample calcined at 1175 °C for 24 h and sintered at 1450 °C for 2 h using fine-grain amorphous silica of 0.25 µm size, was improved to 219,200 GHz.

Dielectric Properties of Sr(Ni1/3Nb2/3)O3–Ba(Ni1/3Nb2/3)O3 Ceramics at Microwave Frequencies

Complex perovskite ceramics, the system Sr(Ni1/3Nb2/3)O3(SNN)–Ba(Ni1/3Nb2/3)O3(BNN) system, have been studied in view of finding materials for use as dielectric resonators at microwave frequencies. A nearly zero temperature coefficient of resonant frequency is obtained for solid solution ceramics with 0.73SNN–0.27BNN composition which corresponds to the order-disorder phase boundary. The relative densities of those ceramics sintered in O2 atmosphere are more than 99.5%, while those sintered in air are 97~98.5%. The dielectric constant and the unloaded Q of the ceramics sintered in O2 atmosphere are 34 and 10,000 at 10 GHz respectively.

Synthesis of Forsterite with High Q and Near Zero TC f for Microwave/Millimeterwave Dielectrics

With the advent of ubiquitous age, the high quality dielectric materials have been required for the wireless communications available to the millimeter-wave as well as microwave frequencies. The utilizable region for the frequency has been expanding to the millimeter-wave region because of the shortage of radio frequency (RF) resources. These high frequencies would be expected for ultra high speed LAN, ETS and car anti-collision system on the intelligent transport system (ITS) and so on. Silicates are good candidates for microwave/millimeterwave dielectrics, because of their low dielectric constant e r and high quality factor (High Q). Forsterite (Mg₂SiO₄) is one of the silicates with low e r of 6.8 and Qㆍf of 240000 ㎓. In this paper, we reviewed following three categories for synthesis of forsterite: (1) Synthesis of high Qㆍforsterite (2) Adjust the temperature coefficient of resonant frequency TC f (3) Diffusion of SiO₄- and Mg-ions on the formation of forsterite

Research & Developments for Millimeter-Wave Dielectric Forsterite with Low Dielectric Constant, High Q, and Zero Temperature Coefficient of Resonant Frequency

Forsterite Mg2SiO4 is a candidate for millimeter-wave dielectrics because of its high Q and low dielectric constant er. Commercial forsterite has been improved with a high Q of 240,000 GHz using high-purity and fine raw materials, and the temperature coefficient of resonant frequency (TCf) can also be adjusted to near-zero ppm/°C by adding 24 wt % rutile compared with that in a previous study. In this study, the TCf, TCe, and er of forsterite ceramics with rutile added are studied for the tuning conditions. Zero ppm/°C TCf of the forsterite with 30 and 25 wt % rutile added was achieved at 1200 °C for 2.5 and 2.25 h, respectively. The er values of the near-zero TCf forsterite with 30 and 25 wt % rutile added are 11.3 and 10.2, respectively.

Ag-Doped Bi-Sr-Ca-Cu-O Superconductor Prepared by Floating Zone Method

Ag-doped Bi2Sr2CaCu2Oy bulk samples were prepared by floating zone (FZ) method aiming at enhancement of critical current density (Jc) and lowering of contact resistance at current terminal. It was found that Ag particles tend to be elongated along the growth direction, and be trapped in the oxide grains. Jc value was not significantly affected by Ag-doping up to 10%, but the contact resistance at current terminal was lowered to about 1/1000 that of undoped sample.

Microwave Dielectric Properties of (Sr, Ba)[(Ni1/3Nb2/3), Zr]O3 Ceramics

In the system (Sr, Ba)[(Ni1/3Nb2/3), Zr]O3, compositions in the Sr(Ni1/3Nb2/3)O3-rich region have an hexagonal superstructure and a negative temperature coefficient of resonant frequency (τf). Nearly zero τf is obtained in compositions at the order-disorder phase boundary. The unloaded Q value of the compositions in the superstructural region is higher than those in the simple cubic one. It increases with the increase of hexagonally ordered structure for the compositions in the superstructural region, but does not depend on porosity or grain size of the ceramics.