Temperature compensated niobate microwave ceramics with the columbite structure, M2+Nb2O6
Introduction
With the continuing proliferation of wireless communications technologies operating at microwave frequencies, there is an ever-increasing demand for cheap, but nonetheless high performance, dielectric ceramics. To be effective dielectric resonator materials they should have a sufficiently high relative permittivity to allow miniaturisation of the component (εr > 10), low dielectric losses at microwave frequencies to improve selectivity (Q > 5000, where Q=1/tan δ), and a temperature coefficient of resonant frequency near zero for temperature stability (τf <±20 ppm per °C).1
Microwave ceramics are currently available with low τf and Qf > 100 000 (Qf=Q × fr). However, these are usually made from complex perovskites, such as the mixed metal tantalate perovskites BaZn0.33Ta0.67O3 (BZT) and BaMg0.33Ta0.67O3 (BMT).2, 3, 4 These complex perovskites require fairly high sintering temperatures (> 1400 °C), and the structures and properties of the complex perovskites (often with four or more cations) are proving difficult to predict, and depend strongly upon the degree of ordering.5 Furthermore, tantalum is a relatively expensive metal, the ore tantalite (60% Ta2O5) costing $150 per kg,6 whereas niobium is over 20 times cheaper, with the mineral columbite costing just $8 per kg.6
The binary niobate ceramics, with the formula MNb2O6 where M is a divalent cation, are one of the end members of the perovskite BaM0.33Nb0.67O3 group (the other being BaO), and they are mostly isostructural with the orthorhombic mineral columbite (ZnNb2O6, space group=Pnca (60)).7 The transition metal columbite niobates sinter between 1100 and 1200 °C, much lower than the perovskites,7, 8, 9, 10 and Q of the columbite niobates is higher than that of the M2+Ta2O6 compounds, which do not have the columbite structure.8 As niobium is so much cheaper than tantalum, and because the chemistry of the binary compounds should be easier to investigate than that of the complex perovskites, a study was made of these binary niobates, and these results have been previously reported.11, 12 Amongst the niobates investigated, four in particular exhibited potential for commercial application: these were ZnNb2O6 (ZnNO), MgNb2O6 (MgNO), CaNb2O6 (CaNO) and CoNb2O6 (CoNO). The properties of these materials are shown in Table 1, and it can be seen that these materials have good quality factors, especially considering the low cost and simplicity of the materials compared with the complex perovskites. The τf values, whilst being fairly small compared with materials like titania or calcium titanate, were still too large for many resonator applications. Therefore, the effects of using dopants in an attempt to lower the τf of these niobate compounds were investigated. Because all of the niobates have negative τf values, dopants with large positive τf values and good microwave properties were added. The dopants used were commercially available forms of doped TiO2 (rutile), which has τf=+420 ppm K−1 and Qf=∼48 000 GHz,13, 14 and CaTiO3 which has a τf=+800 ppm K−1 and Qf=∼3600 GHz.15
Section snippets
Sample preparation
All niobate samples were prepared by a standard ceramics mixed-oxide route (oxides at least 99.9% pure). A stoichiometric mixture of the oxides needed to form each columbite compound was ball milled in deionised water with zirconia balls for 2 days, and then dried on a rotary evaporator. The resultant powder was then calcined at a temperature between 1000 and 1200 °C for 12 h in air, and then ball milled again in deionised water with zirconia balls for 2 weeks, and again dried on a rotary
Results and discussion
Initial doping levels were chosen assuming that the total τf of the doped niobate would be simply additive from the proportions of the two individual components. It was not expected that the τf would be so easily predictable, and this indeed proved to be the case. Then, based upon the results of these first experiments, further doping levels were chosen for each niobate. These doping levels are shown in Table 1, as wt.%. In our investigations of the columbite niobate compounds we observed that
Conclusions
It is evident from this study that the behaviour of these four, structurally and chemically very similar, columbite niobate compounds are different from one-another, and difficult to predict, when doped with TiO2 and CaTiO3. It can be generally stated that an increasing amount of dopant increases εr, decreases Q and changes τf from negative to positive, passing through zero at some point. While many of the materials investigated have Qf values too low for practical use as microwave resonator
Acknowledgements
The authors wish to acknowledge the support for this project by the European Community under the Competitive and Sustainable Growth Programme (1998 – 2002), as part of “Functional Oxide Structures for Advanced Microwave Systems”, project No. GRD1-1999-10643.
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