Microwave dielectric properties and sintering behaviors of scheelite compound CaMoO4

https://doi.org/10.1016/j.jeurceramsoc.2005.09.051Get rights and content

Abstract

The microwave dielectric properties, sintering behaviors of scheelite compound CaMoO4 were investigated using dilatometry, X-ray diffraction, scanning electron microscopy and network analyzer. To improve the sintering property of CaMoO4, samples with different Ca/Mo ratio were prepared. The bulk densities of CaMo(x)O4 (for x = 1.02, 1.05, 1.08) samples were higher than those of pure CaMoO4 over all temperature range. The well-sintered CaMo(x)O4 (for x = 1.02) sample with ∼95.7% of the theoretical density shows Q × f value of 71,000 GHz and dielectric constant (ɛr) = 10.3. The effects of liquid phase formation on sintering process and dielectric properties were discussed. Sample with hot-press sintering was also prepared in order to investigate dielectric properties of fully dense CaMoO4, which exhibited quality factor (Q × f), ∼55,000 GHz and dielectric constant (ɛr) = 11.7.

Introduction

Recently, Molybdenum (Mo) containing mixed oxides have been of practical interest because of their attractive luminescence property1 and possibility of negative electrode (anode) materials.2, 3 The scheelite compound has been investigated as a host crystal for laser applications. Also, the dielectric constant and the anisotropy of the dielectric constant (ɛ′) of single crystals (grown by the Czochralski method) were firstly reported by Brower and Fang.4 The dielectric constant (ɛ′) parallel to a-axis of CaMoO4 single crystal was 24.0 ± 0.2; parallel to c-axis, 20.0 ± 0.2 at 1.59 kHz. The dielectric loss (tan δ) for both orientations of CaMoO4 specimen was 10−3 or less. Unfortunately, not many works have been done on the dielectric properties for microwave application.

Dielectric ceramic substrate with low permittivity and high-Q is required to minimize cross-coupling with conductors and to maintain overall high-Q circuits by lowering power dissipation. Alumina (ɛr = 10, Q × f = 100,000, τf = −50 ppm/°C) is the standard material for the manufacturing of hybrid circuits via thick or thin film technology and for high integrated multilayer circuits produced via co-firing of metallized green tapes. We thought that CaMoO4 ceramic could be selected as a possible candidate for microwave dielectric material because of its low permittivity and high-Q (ɛr = 11.7, Q × f = 55,000, τf = −60 ppm/°C), such alumina.

To investigate possibility of microwave application of CaMoO4 ceramic, the sintering behavior and microwave dielectric properties of dense CaMoO4 ceramic were examined. In our study, as a way of obtaining fully dense ceramic, the specimens were uniaxially pressed during sintering at 1100 °C for 1 h under pressure of 16 MPa (hot-press method). But hot-press method has many problems on the commercial uses, so the samples with different Ca/Mo ratio were prepared to control sintering behavior of CaMoO4.

Section snippets

Experimental procedure

The powders were prepared by conventional mixed oxide method. CaCO3, MoO3 (High Purity Chemical Lab., Japan) powders with 99.99 and 99.9% purity were weighed and mixed for 24 h with stabilized zirconia media and ethanol. The mixed slurry was dried and then powders were calcined at 800–1100 °C for 2 h in air and then the calcined powder was granulated with an organic binder after ball milling for 24 h. The milled powders were uniaxially pressed into disks 8 mm in diameter and 2–4 mm thick under a

Sintering behavior of CaMo(x)O4

Fig. 1 shows bulk densities of pure CaMoO4 and the samples with different Ca/Mo ratio, which are CaMo(x)O4 (Ca/Mo = 1/x for x = 1.0, 1.02, 1.05, 1.08) as a function of sintering temperature. The densities of pure CaMoO4 sample significantly decreased at temperature range from 1200 to 1300 °C and has the maximum value of ∼3.85 g/cm3 (∼90.6% of the theoretical density) at 1000 °C. This result suggests that pure CaMoO4 sample has poor densification characteristic. On the other hand, the densities of CaMo(

Conclusion

In our study, pure CaMoO4 sample shows lower sinterability. So as a way to obtain fully dense ceramics, the sample prepared by hot pressing (uniaxially pressed at 1100 °C for 1 h under pressure of 16 MPa). Hot-pressed sample with ∼99% of the theoretical density exhibited Q × f value of ∼55,000 GHz and dielectric constant (ɛr) = 11.7. Because hot-press method was not suitable for commercializing, the samples with different Ca/Mo ratio were prepared to control sintering behavior of CaMoO4. Different

Acknowledgments

This research was supported by a grant from the Center for Advanced Materials Processing (CAMP) of the 21st Century Frontier R&D Program funded by the Ministry of Science and Technology, Republic of Korea.

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