The Low Temperature Sintering Of Mg₄Nb₂O₉ Microwave Dielectric Ceramics

Recently, a newly developed microwave dielectric material Mg₄Nb₂O₉ (MN ) with excellent microwave dielectric properties (Q ·f = 197 000 GHz, ε_r =12.6) was reported. The microwave dielectric properties are comparable to those of Al₂O₃.Thus, MN is a suitable material for microwave applications, such as substrates and resonators at high frequency. However, the higher sintering- temperature(T=1400 ℃) and the poor temperature coefficient of resonant frequency (Ï„_f=-72 ppm/℃) of MN ceramics make it virtually useless for commercial applications. To match the low-temperature co-fired ceramic (LTCC) technical and the request of high frequency components, microwave dielectric ceramics are developing in higher frequency and low-sintering temperature. Thus, it is worth to lower the sintering temperature and improve the microwave dielectric properties of MN ceramics.In this thesis, effects of composition and sintering temperature upon the sintering characteristics, phase components, microstructure and microwave dielectric properties of MN ceramics were investigated. Modification of microstructure and microwave dielectric properties were conducted by CaTiO₃ addition. Li substitution for Mg, Ta , Sb and V substitution for Nb and Li+2CO₃-V₂O₅ co-doped were conducted to lower the sintering temperature of MN ceramics. The principal experiment results were shown as follows:(1) The sintering temperature of MN ceramics can be reduced to 925 ℃ with V₂O₅ and Li₂CO₃ co-doped. This can attribute to the low-melt point liquid phases of LiVO₃. XRPD showed well chemical compatibility of MN ceramics with Ag. With increasing of Li₂CO₃t the dielectric constant (ε_r) of MN ceramics have somewhat increased but the quality factor (Q·f) have been suppressed. The decrease of Q·f values can attribute to the lattice defects in MN due to the substitution of Li* for Mg²⁺,as well as the small grain size and second phase. The ε_r of 13.7, Q·f value of 78 000 GHz were obtained for the specimens with 1.5%V₂O₅ and 1.5% Li₂CO₃ sintered at 925 ℃. The excellent microwave dielectric properties and chemical compatibility (Ag) of MN ceramics permit the potential application as a novel LTCC substrate.(2) With Li⁺ substitution for Mg²⁺, the (Mg_4-xLi_x)(Nb_2-0.08V_0.08)O_9-δ solid solution can be well sintered at 950 ℃. XRD showed that, in the range of x≤0.3, acomplete solid solution is formed. As x increased, the a-axis parameters of (Mg 4_xLix)(Nb2₀mV00S)O9_s linearly increased and c-axis parameter decreased, these phenomena obey the general known Vegard law and can be explained by the difference in the ionic radii of Li* and Mg2+. The er of 12.7,(2' f value of 14 000 GHz are obtained for the (Afg38LiO2)(A'!&2_OO8FOO8)09₎, specimens sintered at 950 °C.The MgA(Jal_xVx)2O9 solid solutions were prepared by a conventional solid-state reaction method. In the range of composition x* 0.2, the XRPD patterns can be identified as a corundum type structure and no secondary phase appeared. With increasing of composition x, the XRPD patterns can be identified as a major phase of Mg4(Tal_xVx)2O9 and Mg3(VO4)2 as a minor phase. Lots of porosities can be observed in the grains, this is considered to be due to a volatility of V2O5 at the elevated temperatures. Mg4(Jax_xVx)2O9 ceramics have the poor microwave dielectric properties, this is ascribed to the poor sinterability and the secondary phase.The Mg4(SbNb1_xVx)O9 solid solutions were prepared by a conventional solid-state reaction method. In the range of composition xz 0.3, the XRPD patterns can be identified as a corundum type structure and no secondary phase is detected. The temperature coefficient of resonant frequency is improved by Sb and V substitution for Nb, the dielectric constant and quality factor decreased with increasing the content of V. The er =9.71, Q ? / =35 000 GHz and rf - -22.6 ppm/*C were obtained for the Mg4(SbNb09V01)O9 ceramics sintered at 1300 *C.(3) The multiphase of Mg4Nb2O9 ICaTiO3 was first synthesized with a low sintering temperature by 5mol%V2O5 addition. For the composition with *=0.2 0.4, XRPD patterns can be indexed as mixtures of major phase Mg4Nb209 associated with secondary phases of CaNb2O6and Mg5Nb40l5, and CaTiO3 as a minor phase. The Raman spectra, SEM, EDS and XRD profiles of (l-x)Mg4Nb209 + xCaTiO3 (x*0.5) are characteristic of coexistence of Mg4Nb209 and CaTiO3. The form multiphase of Mg4Nb209 /CaTiO3 can attribute to the lower sintering temperature, which impede the chemical reaction between Mg4Nb209 and CaTiO3. The relative dielectric constants (er) and the temperature coefficient of resonant frequency (rf) of samples increase from 9 to 32 and -44 to 134 ppnVC respectively with composition x increasing from 0.2 to 0.7. In contrast, the Q ? f values of samples drastically decrease from 80 000 to 19 500 GHz with increasing the content x. As a result, a er value of 20 and a Q ? / value of 48 000 GHz with a rf value of -12 ppnVC were obtained for the new0.5Mg4Nb2O9 + 0.5CaTiO3 multiphase ceramics sintered at 1150 "C for 5h.