• Korean Journal of Materials Research
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• v.13 no.4
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• pp.259-265
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• 2003

For the aim of thin microwave absorbers used in mobile telecommunication frequency band, this study proposed a high permittivity dielectrics(λ/4 spacer) coated with ITO thin films of 377 $\Omega$/sq(impedance transformer). High frequency dielectric properties of ferroelectric ceramics, electrical properties of ITO thin films and microwave absorbing properties of ITO/dielectrics were investigated. Ferroelectric materials including $BaTiO_3$(BT), 0.9Pb($Mg_{1}$3/Nb$_{2}$3/)$O_3$-0.1 $PbTiO_3$(PMN-PT), 0.8 Pb (Mg$_{1}$3/$Nb_{2}$3/)$O_3$-0.2 Pb($Zn_{1}$3$_Nb{2}$3/)$O_3$(PMN-PZN) were prepared by ceramic processing for high permittivity dielectrics,. The ferroelectric materials show high dielectric constant and dielectric loss in the microwave frequency range. The microwave absorbance (at 2 ㎓) of BT, 0.9PMN-0.1PT, and 0.8PMN-0.2PZN were found to be 60%(at a thickness of 3.5 mm), 20% (2.5 mm), and 30% (2.5 mm), respectively. By coating the ITO thin films on the ferroelectric substrates with λ/4 thickness, the microwave absorbance is greatly improved. Particularly, when the surface resistance of ITO films is closed of 377 $\Omega$/sq, the reflection loss is reduced to -20 ㏈(99% absorbance). This is attributed to the wave impedance matching controlled by ITO thin films at a given thickness of high permittivity dielectrics of λ/4 (3.5 mm for BT, 2.5 mm for PMN-PT and PMN-PZN at 2 ㎓). It is, therefore, successfully proposed that the ITO/ferroelectric materials with controlled surface resistance and high dielectric constant can be useful as a thin microwave absorbers in mobile telecommunication frequency band.

• Journal of the Korean Ceramic Society
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• v.32 no.4
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• pp.448-454
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• 1995

The effect of WO3 addition on microwave dielectric properties of BaTiO3-3{(1-x)TiO2-xWO3} system was studied. Addition of WO3 to this system resulted in the formation of BaWO4 and Ba2Ti9O20 phases. Both the dielectric constant (K) and the temperature coefficientof resonant frequency (Tf) were decreased with the amount of WO3 addition. The value of Q$\times$f0 was increased as the amount of WO3 was increased up to x=0.0275, and then decreased when x exceeded 0.03. At x=0.0275, this ceramic showed excellent microwave proprties of K=34-35, Q$\times$f0=50,000-53,000, and near zero ppm/$^{\circ}C$ of Tf.

• Proceedings of the Korean Institute of Electrical and Electronic Material Engineers Conference
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• 2002.07b
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• pp.605-608
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• 2002

In order to improve the microwave dielectric properties of ZnO+$TiO_2$ ceramic systems, we studied the relations among microstructures, phases, and microwave dielectric properties at various mole ratio and sintering temperatures. The optimum composition was found to be 0.2ZnO+0.8$TiO_2$ when sintered at $1100^{\circ}C$, at which we could obtain following results: $Q{\times}f_o$ = 22,500 GHz, ${\varepsilon}_r$ = 73, and $\tau_f=+210ppm/^{\circ}C$.

• The Korean Journal of Ceramics
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• v.2 no.2
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• pp.111-113
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• 1996

Dielectric properties at microwave frequency region of the five-layered compound $Ba_5Nb_4O_{15}$ prepared by the conventional solid state reaction method were investigated. $Ba_5Nb_4O_{15}$ has excellent microwave dielectric characteristics; ${\varepsilon}_r$=38, Q=7500 at 10 GHz, and ${\tau}_l$=+50 ppm/K. Since this compound has a high dielectric constant, high Q and sufficiently stable characteristics, it is useful for the applications at microwave frequencies.

• Proceedings of the Korean Institute of Electrical and Electronic Material Engineers Conference
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• 2003.11a
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• pp.321-324
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• 2003

The effects of various sintering additives such as $Bi_2O_3+V_2O_5$, $BiVO_4$, $B_2O_3$, and $CuO+V_2O_5$ on the low-temperature sintering and microwave dielectric properties of $(ZnMg)TiO_3$ system were studied. Sintering was enhanced by the sintering additives and highly dense samples were obtained for $(Zn_{0.8}Mg_{0.2})TiO_3$ at the sintering temperature of $910^{\circ}C$. $(Zn_{0.8}Mg_{0.2})TiO_3$ with 6.19 mol.%$B_2O_3$ was found to show the best sintering and microwave dielectric properties.

• Korean Journal of Materials Research
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• v.12 no.2
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• pp.160-165
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• 2002

High-frequency dielectric and microwave absorbing properties have been investigated in ferroelectric materials (BaTiO$_3$(BT), (1-x)Pb$Mg_{\frac{1}{3}}Nb_{\frac{2}{3}}$)O$_3$-xPbTiO$_3$(PMN-PT), (1-x)Pb$Mg_{\frac{1}{3}}Nb_{\frac{2}{3}}$O$_3$-xPb(Zn_{\frac{1}{3}}Nb_{\frac{2}{3}}$)O$_3$(PMN-PZN) for the aim of thin microwave absorbers in the frequency range of mobile telecommunication. The specimenns are prepared by conventional ceramic processing and complex permittivity has been measured by transmission/reflection method. The ferroelectric materials show high dielectric constant and dielectric loss in the microwave range and their domiant loss mechanism is considered to be domain wall relaxation. The microwave absorbance of BT 0.9PMN-0.1PT, and 0.8PMN-0.2PZN specimen (determined at 2) are found to be 99.5% (at a thickness of 4.5 mm), 50% (2.5 mm), and 30% (2.5 mm), respectively. It is suggested that PMN-PT or PMN-PZN ferroelectrics are good candidate materials for the spacer of λ/4 absorber. The use of ferroelectric materials is effective in reducing the thickness of absorber with their advantage of high dielectric constant.

• Journal of the Korean Institute of Electrical and Electronic Material Engineers
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• v.11 no.11
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• pp.947-953
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• 1998

The microwave dielectric properties of X BaO-0.15($Nd_{0.87}Bi_{0.13})_2O_3-(0.85-X)TiO_2 (X=0.14~0.17) and 0.16BaO-0.15(Bi_xNd_{1-x})_2O_3-0.69TiO_2$ (X=0.12~0.15) ceramics sintered at 1320~$1380^{\circ}C$ were investigated. The microwave dielectric properties of X BaO-0.15(Nd_{0.87}Bi_{0.13})_2O_3-(0.85-X)TiO_2$ (X=0.14~0.17) can be controlled effectively by adjusting X content : with increasing X from 0.14 to 0.17 both dielectric constant and temperature coefficient of resonant frequency decreased from 94.6 to 86 and from 22 ppm/^${\circ}C to -7 ppm/^{\circ}C$, respectively, while quality factor increased from 1300 to 1920 (at 4GHz). The microwave dielectric properties of 0.16BaO-0.15(Bi_x/Nd_{1-x2}O_3 -0.69TiO_2$ (X=0.12~0.15) can be controlled effectively by adjusting X content : with increasing X from 0.12 to 0.15 both quality factor and temperature coefficient of resonant frequency decreased from 1920 to 1430 and from 9 ppm/^${\circ}C to -10 ppm/^{\circ}C$, respectively, while dielectric constant increased from 87.5 to 92.6.

• Journal of the Korean Institute of Electrical and Electronic Material Engineers
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• v.14 no.3
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• pp.197-201
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• 2001

The microwave dielectric properties of (1-x)Ba(Mg$_{1}$3/Ta$_{2}$3/)O$_3$-xBa(Co$_{1}$3/Nb$_{2}$3)O$_3$(x=0.25~0.5) ceramics depending on the Ba(Co$_{1}$3/Nb$_{2}$3/)O$_3$[BCN] contents and the possibility of application as a microwave dielectric resonator were investigated. The specimens were prepared by he conventional mixed oxide method using there sintering temperature of 1575$^{\circ}C$. It was found that Ba(Mg$_{1}$3/Ta$_{2}$3/)O$_3$[BMT] and BCN formed a solid solution with complex perovskite structure. As the mole fraction of BCN increased, dielectric constant increased while temperature coefficient of resonant frequency decreased. The highest value of quality factor, Qxf$_{0}$=138,205GHz, obtained in the sample of 0.9BMT-0.1BCN ceramics. In the range of x$\geq$0.4, the dielectric constant was about 30. The 0.55BMT-0.45BCN ceramics showed excellent microwave dielectric properties with $\varepsilon$$_{r}$=30.84, Qxf$_{0}$=75,325GHz and $\tau$$_{f}$=2.9015ppm/$^{\circ}C$.EX>.EX>.

• Journal of the Korean Society for Nondestructive Testing
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• v.22 no.4
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• pp.369-376
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• 2002

The dielectric properties of cereal grains such as short-grain rough rice, brown rice and barley with various moisture contents were determined by measuring the attenuation and phase shift of the microwave signal trough the grain samples at 9.5GHz. The microwave free-space transmission measurement system consisted of sample holder, horn antenna and network analyzer. Dielectric constant and loss factor of grain samples increased with moisture content and bulk density and agreed well with previous research results. Moisture density, which is defined as the product of moisture content and bulk density, was proposed as a bulk density and variety compensation factor. The technique for measurement of dielectric properties based on free-space transmission may be useful for other particulate materials.

• Proceedings of the Korean Institute of Electrical and Electronic Material Engineers Conference
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• 2002.07b
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• pp.642-645
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• 2002

Microwave dielectric properties of $ZnWO_4$ ceramic were investigated with calcination and sintering temperatures. The dielectric properties required for such application are high dielectric constant$(\varepsilon_r)$, high $Q{\times}f_o$ value and low temperature coefficient of resonant frequency$(\tau_f)$. These requirement correspond to necessities for size reduction, excellent frequency selectivity, good temperature stability of devices. $ZnWO_4$ ceramics could be sintered at low $1075^{\circ}C$, which was comparatively low temperature for microwave dielectrics. As a result, $ZnWO_4$ showed the dielectric constant of 13, quality factor($Q{\times}f_o$ value) of 22000 and 'temperature coefficient of resonant frequency$(\tau_f)$ of $-65{\pm}5ppm/^{\circ}C$.