Crystal structure and microwave dielectric properties of low temperature sintered MgO ceramic with LiF addition
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The (1x)MgO–xLiF ceramics (x 5 0.02–0.08) were successfully sintered when the ceramics were sintered at 950 °C for 4 h in covered crucible. From the crystal structure analysis, it was found that a small amount of Li1 cation occupied Mg21 site in MgO ceramic; the formation of oxygen vacancy induced by Li substitution for Mg was suggested by the evaluation of the bulk conductivity and the calculation of density of state (DOS) for the (Mg13O43)60 and (Mg11Li2O42)58 cluster models. As for the microwave dielectric properties of the (1x)MgO–xLiF ceramics, the dielectric constant er and the temperature coefficient of resonant frequency values of the ceramic were independent of the lithium fluoride (LiF) content, and these values were approximately 9.5 and 62 ppm/°C. On the other hand, the quality factor ðQ f Þ values strongly depended on the LiF content. As a result, the highest Q f value of 282,230 GHz was obtained at x 5 0.04. From these results, it is determined that the LiF addition is effective in reducing the sintering temperature of MgO without any detrimental effect on the microwave dielectric properties of MgO ceramics.
I. INTRODUCTION
A variety of microwave dielectric ceramics is widely used for the communication systems such as antenna, filter, substrate, etc. For the application to the devices operated at high frequencies such as millimeter wave, the microwave dielectric ceramics with low dielectric constant (er) and high quality factor ðQ f Þ are especially required, though the temperature coefficient of resonant frequency (TCF) of the ceramics must be near zero. As the size reduction of devices is also important as much as the microwave dielectric properties, many attempts have been carried out for the characterization of low temperature cofired ceramics (LTCCs) to miniaturize the multilayer integrated circuit components (MICCs). For the LTCCs, it is necessary to lower the sintering temperature of the dielectric ceramics because the dielectric ceramics and electrode metals such as Ag and Cu should be co-fired in the MICCs. As for the LTCCs with a low dielectric constant, the microwave dielectric properties of Al2O3 based LTCCs have been reported in the literatures.1–4 For example, temperature-stable LTCC which consists of Al2O3, TiO2, and glass has been reported by Dai et al.,4 and the glass as a sintering aid reacts with Al2O3 to form the high-Q crystalline phase to prevent the decrease in the Q f of ceramics by the glass addition. Although the LTCC has
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Address all correspondence to this author. e-mail: [email protected] DOI: 10.1557/jmr.2011.438
a low er and a near-zero TCF, the Q f value of the ceramics (Q 5 1000–1200 at 0.5 GHz)4 is considered to be low for the application to the microwave devices operated at high frequencies. On the other hand, the MgO ceramic is also one of the materials with low dielectric loss. Recently, many attempts have been made to lower the sintering temperature of MgO ceramic for the LTCC applications.5–7 Although the addition of the sintering aids is effectiv
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