| Microwave dielectric ceramics are a new type of electronic ceramic materials,which have a wide range of applications in wireless communication,radar detection,etc.They can be made into antennas,filters,resonators.RePO4(Re=La,Nd)ceramics have low dielectric constant and dielectric loss and are potential millimeter wave dielectric materials,but the high sintering temperature makes it difficult to be used in practical production.Therefore,in this paper,RePO4(Re=La,Nd)ceramics are investigated to reduce their sintering temperature and improve their dielectric properties by doping with LiF,and the structure-property relationship is studied.Firstly,in this thesis,LaPO4-x wt.%LiF(x=0～5)series ceramics were prepared by the conventional solid-phase method.XRD results showed that the addition of LiF had no effect on the phase composition of the LaPO4 ceramics,and the ceramics of each component were in a single pure phase with a certain degree of shift in the diffraction peaks.Through structural refinement,the unit cell volume and lattice constants showed an upward trend with the increase of x,but the changes were not obvious and within the acceptable range.SEM plots were obtained that part of LiF existed in the amorphous form on the ceramic surface,and the grain size gradually decreased with increasing x?which increased the relative density of the ceramics.A dense structure was obtained at 750℃,at which time the relative greatly improved over the relative density of the autonomously prepared substrate.In terms of dielectric properties,the quality factors of the LaPO4-x wt.%LiF series ceramics are significantly improved compared to the substrate,and the best microwave dielectric properties are obtained at x=3:εr=10.03,Q×f=81,467 GHz,τf=-43.78 ppm/℃.The dielectric response of LaPO4-3 wt.%LiF ceramics was probed by far-infrared reflectance spectroscopy and terahertz timedomain spectroscopy.The main mode of polarization in the microwave and terahertz bands is ion-displacement polarization,while the main sources of dielectric loss are phonon vibrations and increased oxygen vacancy concentration.The best optical and dielectric properties were obtained when x=3 and the sintering temperature was 750℃:n=3.17,εr=10.03,α=1.28 cm-1,tanδ=0.0039.Next,NdPO4 ceramics with the same crystal structure were chosen as the object of study,a series of NdPO4-x wt.%LiF(x=0.0～4.0)ceramics were prepared by the solid-phase reaction method in this experiment.XRD results showed that the ceramics of each component were in a single pure phase.The results of structural refinement show that the lattice constant unit cell volume does not change significantly with the increase of x.SEM images show that part of LiF exists as small amorphous particles on the ceramic surface,and the grains grow with increasing x.When x≥ 4.0,the grain size tends to stabilize,and a denser structure is obtained at 850℃.The relative density is 95%.The relative density is 95%,which greatly improves over the matrix.In terms of dielectric properties,the NdPO4-x wt.%LiF ceramics showed some improvement,and the best microwave dielectric properties were obtained at x＝1.0:εr=10.40,Q×f=55,840 GHz,τf=-43.14 ppm/℃.The dielectric response of NdPO4-1 wt.%LiF ceramics was probed by far-infrared reflectance spectroscopy and terahertz timedomain spectroscopy,where the dominant polarization mode in the microwave and terahertz bands is ion-displacement polarization.The best optical and dielectric properties were obtained when x=1 and the sintering temperature was 850℃:n=3.17,εr=10.04,α=1.29 cm-1,tanδ=0.0039.Finally,the autonomously prepared LaPO4-3 wt.%LiF and NdPO4-1 wt.%LiF ceramics were successfully co-fired with Ag and used as dielectric substrates for parameter optimization using HFSS software to obtain the optimal dimensions of the rectangular microstrip patch antenna,the dimensions of the patch antenna made of LaPO4-3 wt.%LiF ceramics were 18.50 mm × 36.00 mm × 3.23 mm;the resonant center frequency is 2.40 GHz;the bandwidth about 20 MHz;the return loss is-23.33 dB;the maximum gain is 6.74 dB.The dimensions of the antenna designed by NdPO41 wt.%LiF ceramic are 18.00 mm × 36.00 mm × 3.12 mm;the resonant center frequency is 2.43 GHz;the bandwidth is about 20 MHz;the return loss is-18.85 dB;the maximum gain is 6.83 dB. |
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