Research On The Preparation And Properties Of LiMgPO₄-Based Microwave Dielectric Ceramics With Low Dielectric Constant

LiMgPO4-based ceramics are promising for low temperature co-fired ceramic (LTCC)applications due to their low intrinsic sintering temperature, high quality factor (Q×f), low dielectricconstant (εr) and bulk density, as well as good chemical compatibility with silver. In this paper, theLiMgPO4-based ceramics were prepared by the conventional solid-state ceramic method. The effectsof raw material compositions and preparation processes on the phase compositions, microstructuresand microwave dielectric properties of the ceramics were systematically investigated by DSC-TG,XRD, SEM, EDS, network analyzer etc.At first, the preparation process of LiMgPO4ceramics was investigated. The LiMgPO4ceramicswere prepared by the conventional solid-state ceramic method. The calcination temperature range ofLiMgPO4ceramics was determined initially by DSC-TG. Then the effects of main preparation processparameters on the phase compositions, microstructures and microwave dielectric properties ofLiMgPO4ceramics were investigated. The results revealed that it was not beneficial to the obtainmentof single-phase LiMgPO4ceramic whenever at too low or high calcination temperature. The Q×fvalue was improved by the increase in relative density when sintering at a suitable temperature andheating rate. However, the influences of these parameters on the temperature coefficient of resonantfrequency (τf) of LiMgPO4ceramics were little.The single-phase LiMgPO4ceramic with a densemicrostructure and high relative density of95.4%was obtained when it was calcined at750C andsintered at900C with a heating rate of3C/min, and showed good microwave dielectric properties ofεr=6.72, Q×f=72232GHz, and τf=-58.2ppm/°C.The effects of Ni2+substitution on the phase compositions, microstructures and microwavedielectric properties of LiMgPO4ceramics were investigated. The results revealed that LiMg1-xNixPO4(x=0.1,0.2,0.3,0.4,0.5,1.0) solid solutions with orthorhombic crystal structure were formed by thesubstitution of Ni2+for Mg2+. The cell volume of the solid solutions decreased with the increase of x.The sinterability of LiMg1-xNixPO4ceramics was improved due to the substitution of Ni2+for Mg2+,and the suitable sintering temperature could be decreased to800C for the specimens with x≥0.3. Arelative density higher than96%could be achieved for the LiMg1-xNixPO4ceramics. With x increasingfrom0to1.0, the εrvalue varied from6.83to6.99, the εmvalue (porosity corrected) graduallyincreased, whereas the Q×f value reduced to12322GHz. The variation of the τfvalue was found to benegligible. Finally, the Ba3(VO4)2ceramic with a large positive τfvalue was chosen in order to tailor thelarge nagative τfvalue of LiMgPO4ceramic,(1-x)Ba3(VO4)2-xLiMgPO4(x=0.40~0.60) compositeceramics could be prepared by sintering mixtures of these two phases. The results revealed thatBa3(VO4)2and LiMgPO4coexisted in the sintered ceramics without forming new phases. A relativedensity higher than97%could be achieved for the composite ceramics sintered at850-875C for2h.The grain size of the composite ceramics increased with increasing LiMgPO4content. With xincreasing from0.40to0.60, the Q×f value gradually increased, whereas the εrand τfvalues decreased.A near-zero τfvalue could be achieved by changing the relative content of the two phases. Typically,the composite ceramic with x=0.45sintered at850C for2h showed excellent microwave dielectricproperties of εr=9.67, Q×f=50700GHz, and τf=1.1ppm/C.