Microwave dielectric properties of Ca2P2O7

doi:10.1016/S0955-2219(03)00138-9

Journal of the European Ceramic Society

Volume 23, Issue 14, 2003, Pages 2589-2592

https://doi.org/10.1016/S0955-2219(03)00138-9 Get rights and content

Abstract

Microwave dielectric properties of α- and β-Ca₂P₂O₇ ceramic materials were investigated, by a network analyzer at the frequency of 10 GHz. It was found that β-Ca₂P₂O₇ could be sintered at 1150 °C for 2 h with high bulk density (98% TD) and α-Ca₂P₂O₇ could be sintered at 1290 °C/4h with bulk density of 95.8% TD. The dielectric constant (ε_r) of β-Ca₂P₂O₇ sintered at 1150 °C/2 h is 8.4, the temperature coefficient of resonant frequency (τ_f) is −53 ppm/°C, and the Q×f value is 53,500 GHz. α-Ca₂P₂O₇ sintered at 1290 °C/4 h has ε_r of 7.8, τ_f of −97 ppm/°C and Q×f value of 14,115 GHz.

Introduction

With the rapid development of modern microwave communication system, such as mobile telephones, high quality microwave dielectric ceramics have attracted much scientific and commercial attention. In the case of microwave substrate application, low dielectric constant less than 10 are ideal values for integrated circuits. In addition, substrate materials should have low dielectric loss (tanδ) and low temperature coefficient of resonant frequency (τ_f). Materials that have been investigated which could be possible substrate include Al₂O₃ and rare-earth aluminates (such as LaAlO₃).1, 2 Although these compounds exhibit good microwave dielectric properties, the search for new materials with much better sintering and microwave dielectric properties continues.

Pyrophosphate Ca₂P₂O₇ is a compound with dichromate structure. Its current applications lie mainly in the fields of luminescence and biomaterials.3, 4 The crystal structure of Ca₂P₂O₇ has been extensively studied.5, 6 X-ray diffraction studies of Ca₂P₂O₇ show that it exists in three different forms depending on the temperature of firing:⁷ $γ- Ca_{2} P_{2} O_{7} 750 °C → β- Ca_{2} P_{2} O_{7} 1171 °C → α- Ca_{2} P_{2} O_{7}$

Lattice parameters of the two crystal phases are listed in Table 1. The crystal structures of α and β-Ca₂P₂O₇ generated using the ATOMS software developed by Eric Dowty,⁸ are shown in Fig. 1. β-Ca₂P₂O₇ is tetragonal and the high temperature form of α-Ca₂P₂O₇ crystallizes in the monoclinic space group. Each of the two independent pyrophosphate groups is an essentially eclipsed configuration with a P–O–P angle of 131°, 138° for β-Ca₂P₂O₇ and angle of 130° for α-Ca₂P₂O₇.

Apart from the investigation of crystal structure and luminescence characteristics, to the best of our knowledge, no dielectric properties have been reported for Ca₂P₂O₇. The intention of this study is to investigate the microwave dielectric properties of α- and β-Ca₂P₂O₇. The microwave dielectric properties are discussed from the point view of bond valence and infrared (IR) reflectivity analysis.

Section snippets

Experimental procedure

Synthesis of Ca₂P₂O₇ was conducted by using traditional solid-state reaction techniques. High purity CaCO₃ (99.9%) and (NH₄)₂HPO₄ (99%) were used as raw materials. Stoichiometric mixtures of starting materials were homogenized by ball milling with ZrO₂ media in ethanol for 24 h and calcined at 1000 °C and 1250 °C for 2 h in order to obtain β and α form Ca₂P₂O₇, respectively. The calcined powders were then milled again, uniaxially pressed into pellets at the pressure of 1000 kg/cm² and sintered

Results and discussion

The XRD patterns of calcined and sintered specimens are shown in Fig. 2. It shows that the powder calcined at 1000 °C/2 h exhibits a single phase of β-Ca₂P₂O₇, and a pure α form could be obtained when it was fired at 1250 °C/2 h. β-Ca₂P₂O₇ sintered at 1150 °C/2 h and α-Ca₂P₂O₇ sintered at 1290 °C/4 h exhibit the same phase as their calcined powders, respectively. A bulk density as high as 98% theoretical density (TD) is obtained when β-Ca₂P₂O₇ is sintered at 1150 °C/2 h. For α-Ca₂P₂O₇, bulk

Conclusions

Ca₂P₂O₇ has low dielectric constant less than 10 because of the covalent character of the P–O bond. Its intrinsic losses are mainly from the contribution of ionic Ca–O bond. The Q value of α-Ca₂P₂O₇ is much lower than that of β-Ca₂P₂O₇ owning to its weaker Ca–O bond and inhomogeneous microstructure such as micro-cracks introduced by the phase transformation. The β-Ca₂P₂O₇ sintered at 1150 °C/2 h has excellent microwave dielectric properties: ε_r=8.4, Q×f=53,500 GHz and τ_f=−53 ppm/°C.

Acknowledgements

This research was supported by a grant from the Centor 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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Microwave dielectric properties of Ca2P2O7

Abstract

Introduction

Section snippets

Experimental procedure

Results and discussion

Conclusions

Acknowledgements

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