Microwave dielectric properties of Ca2P2O7
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 (τf). 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
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: εr=8.4, Q×f=53,500 GHz and τf=−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.
References (13)
- et al.
The effects of Na4P2O7·10H2O addition on the mechanical properties of sintered Ca2P2O7 bioceramics
Mater. Chem. Phys
(1995) - et al.
Vibration spectra of double diphosphates RbLnP2O7 (Ln=Dy, Ho, Y, Er, Tm, Yb)
Vibration Spectroscopy
(2001) - et al.
Effect of porosity and grain size on the microwave dielectric properties of sintered alumina
J. Am. Ceram, Soc.
(1997) - et al.
Microwave dielectric properties and applications of rare-earth aluminates
J. Mater. Res.
(1999) - et al.
The investigation of new phosphor, with particular reference to the pyrophosphates
Brit. J. Appl. Phys.
(1955) The crystal structure of β-Ca2P2O7
Acta. Cryst
(1966)
Cited by (85)
Dense LiF microwave dielectric ceramics with near-zero linear shrinkage during sintering
2022, Ceramics InternationalInterlayer material technology of manganese phosphate toward and beyond electrochemical pseudocapacitance over energy storage application
2021, Journal of Materials Science and TechnologyCitation Excerpt :Therefore, it is imperative to develop a new kind of manganese compound to fill this gap, which can not only retain the benefits but also improve the weaknesses of the traditional manganese compounds [26]. Phosphate material is already well known in the literature by its use in multiple fields, for example, as adsorbent for multivalent metals [27,28], as host medium in laser [29], applied in ceramic industries [30], dielectric [31], electric [32], magnetic [33], catalyst [34], thermo-resistant materials [35], molecular sieves [36], sensors [37], electrochemistry [38] and biocompatible materials [39]. Thus far, the exploration of electrode materials is toward low-cost, high specific energy and power density, and favorable cycling-ability to meet the both economical and high-energy demands [40].
Correlation between crystal structure and microwave dielectric properties of two garnet-type ceramics in rare-earth-free gallates
2021, Journal of the European Ceramic Society