The wireless communications have been tremendously developed in the recent ubiquitous age. The utilizable frequency region has been expanded to millimeterwaves because of shortage of conventional frequency regions. For the ultra high frequencies of millimeterwaves, dielectrics with ultra high quality factor Q and low dielectric constant e
 r
 are desired. Low e
 r
 is necessary to short the delay time of signals and to make devices small in size. Silicates for candidate of millimeterwave materials have low dielectric constant, because of silica-oxygen tetrahedra composed of half covalent bonds. Forsterite (Mg2SiO4) as such kind of silicates has high Q ⋅ f of 270000 GHz and low e
 r
 of 6.8. The temperature coefficient of resonant frequency τ
 f
 was improved by adding rutile TiO2 with high τ
 f
 of plus 450 ppm/∘C. In this paper, these results for forsterite are summarized.

Forsterite ceramics for millimeterwave dielectrics

Microwave dielectric ceramics are vital for filters, dielectric resonators, and dielectric antennas in the 5G era. It was found that the (Cu1/3Nb2/3)4+ substitution can effectively adjust the TCF (...

Design of a High-Efficiency and -Gain Antenna Using Novel Low-Loss, Temperature-Stable Li2Ti1–x(Cu1/3Nb2/3)xO3 Microwave Dielectric Ceramics

Nonstoichiometric Mg2SiO4 forsterite dielectric ceramics with various Mg/Si ratios (R=2.025, 2.05, and 2.2) were prepared and characterized. The appearance of a MgSiO3 secondary phase could be effectively suppressed by adjusting the nonstoichiometry, and a single-phase forsterite structure was obtained in the present ceramics with Mg/Si ratios of 2.025 and 2.05. The microwave dielectric characteristics were significantly improved in the nonstoichiometric Mg2SiO4 ceramics on eliminating the MgSiO3 secondary phase, where an enhanced Q×f value and a suppressed temperature coefficient of resonant frequency λf were obtained. The best microwave characteristics were achieved in the nonstoichiometric forsterite ceramics with an Mg/Si ratio of 2.05: ɛr=7.5, Q×f=114 730 GHz, and λf=−59 ppm/°C.

Effects of Mg/Si Ratio on Microwave Dielectric Characteristics of Forsterite Ceramics

Zn 2 SiO 4 ceramics synthesized by the conventional solid-state method exhibited a low Q × f value, possibly due to the formation of a ZnO second phase. However, with a small ZnO reduction from the Zn 2 SiO 4 ceramics, the ZnO second phase disappeared and grain growth occurred due to the formation of a Si-rich liquid phase. Specimens with a large grain size exhibited an improved Q × f value. In particular, the ceramics with nominal composition Zn 1.8 SiO 3.8 sintered at 1300°C exhibited improved microwave dielectric properties of e r = 6.6, Q × f = 147000 GHz, and τ f = -22 ppm/°C.

Effect of Zn/Si Ratio on the Microstructural and Microwave Dielectric Properties of Zn2SiO4 Ceramics

Re 3 Ga 5 O 12 (Re: Nd, Sm, Eu, Dy, Yb, and Y) garnet ceramics were synthesized and their microwave dielectric properties were investigated for advanced substrate materials in microwave integrated circuits. The Re 3 Ga 5 O 2 ceramics sintered at 1350°-1500°C had a high-quality factor (Q xf) ranging from 40 000 to 192173 GHz and a low-dielectric constant (e r ) of between 11.5 and 12.5. They also exhibited a relatively stable temperature coefficient of resonant frequency (if) in the range of -33.7 to - 12.4 ppm/°C. In particular, the Sm 3 Ga 5 O 12 ceramics sintered at 1450°C exhibited good microwave dielectric properties of e r = 12.4, Qxf= 192173 GHz, and τ f = -19.2 ppm/°C.

Synthesis and Microwave Dielectric Properties of Re3Ga5O12 (Re: Nd, Sm, Eu, Dy, Yb, and Y) Ceramics

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 .

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

The influence of M (M = Zn and Mg) substitution for Ni on the microwave dielectric properties and the crystal structure of Ba 8 Ta 6 (Ni 1-x M x )O 24 ceramics was investigated in this study. The Ba 8 Ta 6 (Ni 1-x Zn x )O 24 (BTNZ) solid solutions showed a single phase in the composition range of 0-1, whereas the limit of Ba 8 Ta 6 (Ni 1-x Mg x )O 24 (BTNM) solid solutions was approximately x=0.75; the lattice parameters of both solid solutions increased linearly, depending on the composition x. Although the dielectric constants (e r ) of BTNZ were almost constant over the whole composition range, those of BTNM slightly decreased from 27.8 to 24.3; the decrease in the dielectric constant of BTNM is due to the change in relative density of the sample. The quality factors (Q xf) of both solid solutions were improved by the M substitution for Ni; the maximum Q × f values of BTNZ and BTNM were 91729 and 93127 GHz, respectively. Moreover, the temperature coefficients of resonant frequency (τ f ) of BTNZ and BTNM varied from 33 to 40 ppm/°C and from 33 to 26 ppm/°C, respectively.

Microwave dielectric properties of Ba8Ta6(Ni1-xMx)O24 (M = Zn and Mg) ceramics

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 .

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

Effects of the number of fins, fin pitch and wind velocity on air-cooling were investigated using experimental cylinders for an air-cooled engine of a motorcycle. Experimental cylinders that had a various number of fins and fin pitches were tested in a wind tunnel. Then the temperature inside of the cylinder, on the surface of the fins and in the space between the fins was measured. Results indicated that the heat release from the cylinder did not improve when the cylinder had more fins and too narrow a fin pitch at lower wind velocities, because it was difficult for the air to flow into the narrower space between the fins, so the temperature between them increased. We also obtained the expression of average fin surface heat transfer coefficient derived from the fin pitch and the wind velocity. This expression is useful for the fin design of an air-cooled cylinder.

http://www.jstage.jst.go.jp/article/jsmeb/49/3/49_3_869/_pdf

Air-Cooling Effects of Fins on a Motorcycle Engine

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 .

Soichi Ishihara

Papers

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

Microwave dielectric properties of Ba8Ta6(Ni1-xMx)O24 (M = Zn and Mg) ceramics

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

Air-Cooling Effects of Fins on a Motorcycle Engine

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