Akinori Kan

Crystal structure of corundum type Mg4(Nb2-xTax)O9 microwave dielectric ceramics with low dielectric loss

Abstract New microwave dielectric ceramics, i.e. Mg 4 (Nb 2− x Ta x )O 9 (MNT) solid solutions, were synthesized and their microwave dielectric properties and crystal structure were investigated in this study. From the discrete variational Xα (DV-Xα) method, it was found that the Ta–O bonds in the TaO 6 octahedron become more covalent than Nb–O bonds in the NbO 6 octahedron; this result leads to the decrease of the ionicity in the Ta 5+ ion. The dielectric constants of MNT were slightly decreased from 12.4 to 11.5; this result might be due to the covalent interaction of Ta–O bonding. The quality factors of the samples were found to exhibit high value ( Q · f ≒350 000 GHz for x =2) which is comparable to those of Al 2 O 3 .

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

Low-Temperature Sintering and Microstructure of Mg4(Nb2-xVx)O9 Microwave Dielectric Ceramic by V Substitution for Nb

The effects of microstructure on the microwave dielectric properties of Mg4Nb2O9 (MN) were investigated and the possibility of low-temperature sintering of Mg4(Nb2-xVx)O9 (MNV) by V substitution for Nb was also discussed in this study. The quality factors (Qf) of MN ranged from 10338 to 217390 GHz with increasing sintering temperatures up to 1300°C, resulting in the grain growth and densification of the samples. In the case of V substitution for Nb, it was found that the limit of the solid solution is lower than x=0.125; the improvements in the sinterability and apparent density of MNV at x=0.0625 were recognized at the sintering temperature of 1025°C. The dielectric constants (er) and the Qf values of MNV at x=0.0625 sintered at 1025°C were 11.6 and 160256 GHz, respectively. Thus, it was clarified that a small amount of V substitution for Nb is effective in reducing the sintering temperatures and improving the microwave dielectric properties of MNV.

Influence of Zn and Ni substitutions for Mg on dielectric properties of (Mg4-xMx)(Nb2−ySby)O9 (M=Zn and Ni) solid solutions

Abstract The effects of M substitution for Mg and Sb substitution for Nb in the (Mg 4− x M x )(Nb 2− y Sb y )O 9 ( M =Zn and Ni) ( x =0–2, y =0–1.5) solid solutions on the microwave dielectric properties were investigated in this study. The limits of both (Mg 4− x M x )Nb 2 O 9 ( M =Zn and Ni) solid solutions were approximately x =2, whereas that of Mg 4 (Nb 2− y Sb y )O 9 solid solutions was approximately y =1.5. As for the Ni and Zn substitutions for Mg, the quality factor, Q · f values of the (Mg 4− x M x )Nb 2 O 9 ( M =Zn and Ni) solid solutions decreased from 192 268 to 28 400GHz with increasing composition x from 0 to 2. On the other hand, the Q · f values of the Mg 4 (Nb 2− y Sb y )O 9 solid solutions were remarkably improved by the Sb substitution for Nb; the highest Q · f value of 285 423 GHz was obtained at y =1. It was found that an increase in the Q · f values of Mg 4 (Nb 2− y Sb y )O 9 solid solutions may related to the grain size decreased which is opposite to general results and this relation is consistent with that of Al 2 O 3 .

Microwave Dielectric Properties of Perovskite-Like Structured Ba8Ta6(Ni1-xMx)O24 (M=Co, Cu, and Zn) Solid Solutions

Perovskite-like structured Ba8Ta6(Ni1-xMx)O24 (M=Co, Cu, and Zn) solid solutions were prepared by solid-state reaction method and the crystal structure and microwave dielectric properties of the solid solutions were characterized in this study. The structural refinements of all the solid solutions in the composition range of 0–1 fit well with the hexagonal structure with the space group P63cm (No. 185). All the lattice parameters of the solid solutions linearly increased, depending on the composition x and, such variations in lattice parameters were attributed to an increase in the (Ni/M)–O bond. No remarkable differences between the dielectric constants were observed with M substitution for Ni and these dielectric constants ranged from 27 to 29. On the other hand, the Qf value of the solid solutions was significantly influenced by M substitution for Ni. In the case of Ni substitution for Cu, the Qf value decreased from 81000 to 8500 GHz. The temperature coefficient of resonant frequency (τf) varies from 33 to 77 pm/°C with Cu substitution for Ni and such a variation in τf may be related to the increase in the covalency of M–O bonds in an MO6 octahedron caused by an increase in temperature.

Effects of variations in crystal structure on microwave dielectric properties of Y2BaCuO5 system

Abstract The dielectric properties of (Y 2– x R x )BaCuO 5 ( R =Sm, Gd, Dy, Ho, Er and Tm) solid solutions were measured for the application as a new microwave resonator. Moreover, the relationships between the results of the dielectric properties obtained and variation in the crystal structure of (Y 2− x R x )BaCuO 5 solid solutions have been studied in some detail using the results of Reitveld analysis. The result is that e r changes from 12.7 to 19.6 and depend on the ionic radii of the rare-earth which composed the R 2 O 11 polyhedron. Then, τ f was largely improved by substituting R for Y, and in the case of Dy 2 BaCuO 5 compound, the value was −6.4ppm/°C. Using the results of Rietveld analysis obtained at 20 and 80 o C, negative values of τ e were considered to be caused by a decrease in the atomic distances of the R 2 O 11 polyhedron in the direction of the b -axis. As for the Q·f value, it suggests that the difference of Q·f value between x =1 and 2 is reduced by the strain of R 2 O 11 polyhedron which is caused by the difference of ionic radii of Y and R .

Low-Temperature Sintering and Microwave Dielectric Properties of MgO Ceramic with LiF Addition

MgO ceramics with a nominal composition of (1-x)MgO–xLiF (x = 0.02–0.10) were synthesized and the effects of LiF addition and sintering conditions on the microwave dielectric properties of MgO ceramics were investigated in this study. With LiF addition, the MgO ceramics were well-sintered at 950 °C in a covered crucible, and the Qf value of the LiF-added MgO ceramic strongly depended on the amount of LiF and sintering time. As a result, the MgO ceramics with a nominal composition of 0.96MgO–0.04LiF (x=0.04) exhibited the highest Qf value of 751,500 GHz (er = 9.6 and TCf= - 56 ppm/°C) when they were sintered at 950 °C for 100 h in a covered crucible. The endothermic reaction of the MgO ceramics was observed with LiF addition by differential thermal analysis and thermograviometry (DTA–TG), and it was considered that the partial melting of MgO with LiF addition to form the liquid phase enhances the sinterability of the MgO ceramics. The crystal structure analysis of the LiF-added MgO ceramics showed a slight increase in the lattice parameters of the MgO ceramics, suggesting the Li substitution for Mg in the MgO structure.

MICROWAVE DIELECTRIC PROPERTIES OF SM2BA(CU1-XZNX)O5(X = 0 TO 1) SOLID SOLUTIONS

Sm2Ba(Cu1-xZnx)O5 solid solutions with composition x ranging from 0 to 1 were prepared by the solid-state reaction method. The microwave dielectric properties of the solid solutions obtained in this study were as follows: the dielectric constants (er) range from 13 to 20, and the temperature coefficient of the resonant frequency (τf) and the quality factor (Qf) value are -9 to -4 (ppm/°C) and 3000 to 66000 (GHz), respectively. Moreover, it was evaluated that the τf and the Qf values obtained in this work are extremely improved by (Y2-ySmy)BaCuO5 (y=0 to 1) and Y2Ba(Cu1-xZnx)O5 solid solutions, respectively. It was also clarified that the variations in the atomic distances in the Sm2O11 polyhedron and the strain of the polyhedron exerted an influence on the temperature coefficient of the dielectric constant (τe) and the Qf value, respectively.

Cation Distributions and Microwave Dielectric Properties of Spinel-Structured MgGa2O4 Ceramics

The Mg2+ and Ga3+ cation distributions in the MgGa2O4 lattice were characterized by the refinement of the crystal structure and the firing temperature dependence of microwave dielectric properties was described in this study. The crystal structure refinement of MgGa2O4 ceramics fired at different temperatures indicated that the degree of inversion x, which represents the Mg2+ and Ga3+ cation distributions in the 8(a) and 16(d) sites in (Mg1-xGax)[MgxGa2-x]O4, slightly decreases from 0.88 to 0.84 with increasing firing temperature from 1500 to 1600 °C. This implies that the Mg2+ cation preferentially occupies the 8(a) site, i.e., the tetrahedral site, with increasing firing temperature. The dielectric constant (er) of the MgGa2O4 ceramics fired above 1520 °C was almost constant (er = 9.2), whereas their Qf significantly increased from 92,000 to 298,000 GHz, depending on the firing temperature. Such an increase in the Qf may be related to the Mg2+ and Ga3+ cation distributions in the MgGa2O4 lattice.

Microwave Dielectric Properties of xMgO?(1 - x)B2O3 Ceramics

The effects of B2O3 doping on the microwave dielectric properties of a MgO compound were characterized in this study. The MgO and Mg3B2O6 compounds were recognized in the XRPD profiles of xMgO–(1 - x)B2O3 ceramics at a sintering temperature of 1350 °C for 4 h in a covered crucible. From the calculation of weight fraction of the compounds, the slight deviation in the concentrations of B2O3 and MgO from the stoichiometric composition of xMgO–(1 - x)B2O3 ceramics was also observed, and was attributed to the vaporization of B2O3 and the decomposition of Mg3B2O6 compound during the sintering. In the composition range of 0.95–0.99, abnormal grain growth of xMgO–(1 - x)B2O3 ceramics was observed; therefore, the decomposition of Mg3B2O6 compound into liquid phase and MgO compound may play an important role in enhancing the grain growth of the ceramics. The dielectric constant of xMgO–(1 - x)B2O3 ceramics ranged from 6.9 to 9.4 depending on the amount of Mg3B2O6 and MgO compounds, whereas the remarkable variation in the Q f value of the ceramics was observed at x > 0.95. As a result, a maximum Q f value of approximately 773000 GHz was obtained at x = 0.99. Such the variation in the Q f value may be attributed to the sinterability of MgO compound, which is related to microstructural changes in the ceramics.