| Microwave dielectric ceramic is applied as the key fundamental materials in modern communication technology.With the development of miniaturization,high-frequency and high-performance for communication devices and portable terminals,the low-dielectric constants(?r)microwave dielectric materials with high quality factor(Q×f)value and theirs low temperature co-fired ceramic(LTCC)technology have gradually become more and more important in the 5 generation era.Some garnet-structured ceramics with low-?r and high Q×f have attracted much attentions,but their applications were limited due to their large negative temperature coefficient of resonant frequency(?f).A series A3B2C3O12-type garnet ceramics were prepared to investigate the effects of the Li+and Mg2+ions occupied tetrahedral C-site,octahedral B-site or dodecahedral A-site of garnet structure on the values of ?f Then,the sintering behaviors,crystal structure,vibrational spectrum and microwave dielectric properties of these garnet ceramics were characterized,and the relationships between composition,structure and properties such as ?f and Q×f was investigated by XRD,neutron diffraction,SEM,raman spectrum,far-infrared reflective spectrum,ultraviolet-visible light spectrum,XPS,ac impendence spectrum and magnetic measurement system.Finally,it was found that the Mg2+ with smaller ionic radius occupied A-site with inverse garnet structure could result in "rattling" effect and structural instability,which has a significant effect on the ?f,Q×f and er,thus,a new affect mechanism on the ?f and Q×f values of low-?r microwave dielectric ceramics is proposed.The details are as follow:Firstly,the effect of Li+occupying C-site on ?f in garnet structure and the potential application of Li-rich garnet in LTCC technology was investigated.The Li3Nd3W2O12 ceramic sintered at 940? with a relative density of 93.6%,have a lower ?r(9.8),higher Q×f(36,800 GHz),and a larger negative ?f(-30 ppm/?).A near-zero ?f value(3.6 ppm/?)was achieved in 0.96Li3Nd3W2O12-0.04CaTiO3 ceramics.Besides,Li3Nd3W2O12 ceramics could be co-firing with Ag electrode and exhibited a good chemical compatibility,thus it may be applied in LTCC technology.To explore the effects of Mg2+ and transition metal cations Zn2+,Mn2+,Co2+,Ni2+and Cu2+fully occupied octahedral B-site on ?f in garnet structure.The density Ca3B2GeV2O12 ceramics were obtained with sintering temperature in the range of 1020??1120?,all compounds still have a negative ?f(-61.2?-73.2 ppm/?)with significant divergence in ?r(9.38?11.50)and Q×f(13,207?54,000 GHz),while Ca3Cu2GeV2O12 ceramic have no resonance signals in microwave frequency range.The ?r is correlated to the sum ionic polarization per unit volume;Q×f value can be associated with the part fully filled 3d orbit,un-paired d electron in B-site ions and the magnetic loss.And the ?f is correlated to bond valance of B-site cation,the bond valance of Mg-O(2.11)is higher than Zn-O(1.97),which resulting in the absolute ?f value of Ca3Mg2GeV2O12(-61.2 ppm/?)is lower than Ca3Zn2GeV2O12(-73.2 ppm/?).The fitting analysis of far-infrared reflectance spectra of Ca3Mg2GeV2O12 shown that the active vibration modes in the low wave number range(100?400 cm-1)has a greater contribution to the intrinsic microwave dielectric polarization.And the influence of Mg2+ entry in A-site on ?f and the other properties was investigated,the dense ceramics of A3Y2Ge3O12(A=Ca,Mg)were preparation in the temperature range of 1020??1120?.The Ca3Y2Ge3O12 ceramic has a normal garnet structure,Ca2+ occupied dodecahedral A-site,Y3+ and Ge4+occupied the octahedral B-site and the tetrahedral C-site,respectively;and it exhibits a lower ?r(10.8),higher Q×f(97,126 GHz)and negative ?f(-40.6 ppm/?).The Mg3Y2Ge3O12 has an inverse garnet structure with a mixed Mg2+/2Y3+occupancy in the A-site and 2Mg2+in B-site;it possess a larger ?r(?14.1),lower Q×f(12,600 GHz)and an abnormal positive ?f(+120.5 ppm/?).A near-zero ?f value was obtained in Ca0.5Mg2.5Y2Ge3O12 ceramic by substituted Mg2+for Ca2+.When Mg2+entry into A-site,the bond valence of Mg2+considerably lower and Mg-O bond trends to larger,thus resulting in "rattling"effect and has large polarizability with consequent 30%higher ?r compared to theoretical value.The rattling effect gradually decreased as temperature increased,which eventually lead to the ionic polarization temperature coefficient lower than thermal expansion coefficient,and of changed into a positive value.According to the Raman spectrum,the Mg2+ and Y3+in the A-site which have a larger difference between ionic radii and polarization,leading to a distorted local structure and an enhanced anharmonic vibration,and then resulting in a larger dielectric loss and a smaller Q×f values.The fitting results of the far-infrared reflective spectrum shown that the intrinsic ?r and Q×f values of A3Y2Ge312(A=Ca,Mg)ceramics were in agreement with the measured values.A series of garnet Ca3-xMgxYb2Ge3O12(0?x?3)ceramics were prepared,and the influence of "rattling" effects Mg2+ in the A-site on microwave dielectric properties were confirmed.For ceramics with 0?x?2,the compounds have a normal garnet structure,?r gradually increased from 10.3 to 11.8 whereas the Q×f gradually decreased from 98,000 GHz to 78000 GHz,and ?f in the range of-40 to-56 ppm/?.For ceramics with 2<x?3,the compounds have an inverse garnet structure with Mg2+ occupancy in the A-site,?r quickly increased to 13.5 whereas the Q×f apparently decreased to 19,800 GHz,and the ?f value dramatically changed from the negative value to the positive value(+70.5 ppm/?).A near-zero?f was obtained as x=2.5.The calculations of the ionic polarization temperature coefficient demonstrated that the rattling effect could be controlled by selecting different cations occupancy in the A-site,and ?f also could be controlled.According to the extrapolated values of the far-infrared reflective spectrum in the microwave region,the intrinsic ?r and Q×f values were in agreement with the measured values.Finally,since the olivine-phase Ca2GeO4 often appeared during the synthesis of Ge-containing garnets,and the relationship between sintering characteristics,structure and microwave dielectric properties in Ca2GeO4 ceramics was investigated.The optimum microwave dielectric property with relative density?94.9%,?r?6.76,Q×f?82,400 GHz,and ?f?-67 ppm/? were obtained in the Ca2GeO4 ceramics sintered at 1420?.And a near-zero ?f(+3.6 ppm/?)was obtained for 0.92Ca2GeO4-0.08 CaTiO3 ceramic,which could be a potentional microwave substrate materials. |
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