Preparation, Dielectric Properties And Mechanism Of Microwave Ceramic Materials With Low Dielectric Constant

Preparation of microwave ceramic materials with low dielectric constant and highquality factor has become a research focus in recent years. In this paper, Sm₂SiO₅ceramics,Sm₄(SiO₄)₃ceramics, Nd₂SiO₅ceramics, Al₂O₃-TiO₂ceramics, MgTiO₃-CaTiO₃ceramicsand (Zn, Mg)TiO₃-TiO₂ceramics were synthesized. The phase composition, density andcrystal microstructure of ceramic materials with low dielectric constant were analyzed byXRD, SEM, EDS, TG-DTA and so on. Microwave dielectric properties were tested byHakki-Coleman opened resonator method. The relationship between microstructure andmicrowave dielectric properties was also investigated. The evolution of dielectric propertieswas discussed.(Zn,Mg)TiO₃-TiO₂composite ceramics were applied to manufacturemultilayer ceramic capacitors (MLCC).1. Sm₂SiO₅ceramics were synthesized by solid phase method. The pure monoclinicSm₂SiO₅phase could be obtained when Sm₂O₃/SiO₂=1:1.05at1350℃. The hexagonalSm₄(SiO₄)₃second phase occurred as temperature increased. The densification of Sm₂SiO₅ceramics increased with increasing temperature. The Sm₂SiO₅ceramics sintered at1500℃exhibited microwave dielectric properties: a dielectric constant (εr) of8.5, a quality factor Q×fof64878.71GHz and a temperature coefficient of resonant frequency (Ï„f) of-37.64ppm/℃.Sm₂SiO₅ceramics had a wide temperature region and small negative Ï„fvalue. They arepromising candidate materials for millimeter-wave devices.2. Sm₄(SiO₄)₃ceramics were synthesized by solid phase method. The pure hexagonalSm₄(SiO₄)₃phase could be obtained when Sm₂O₃/SiO₂=1:1.425-1.6at1350℃-1600℃for4h. when Sm₂O₃/SiO₂=1:1.5at1550℃, Sm₄(SiO₄)₃ceramics exhibited microwave dielectricproperties: a dielectric constant (εr) of9.03, a quality factor Q×f of17470.76GHz (12.40GHz)and a temperature coefficient of resonant frequency (Ï„f) of-24.4ppm/℃. Sm₄(SiO₄)₃ceramics had a wide temperature region and small negative Ï„fvalue.3. Nd₂SiO₅ceramics were synthesized by solid phase method. The hexagonal Nd₄Si₃O₁₂second phase disappeared and the pure monoclinic Nd₂SiO₅phase could be obtained when themolar ratio of Nd2O₃/SiO₂was1:1.05at1450℃. The relative density of Nd₂SiO₅ceramicsincreased with increasing temperature. The Nd₂SiO₅ceramics sintered at1500℃exhibitedmicrowave dielectric properties: a dielectric constant (εr) of7.94, a quality factor (Q×f) of38800GHz and a temperature coefficient of resonant frequency (Ï„f) of-53ppm/℃. Highresonant frequency led to a low dielectric constant and low Q×f value. Nd₂SiO₅ceramics had a wide temperature region. They are promising candidate materials for microwave passivecomponents.4. The0.9Al₂O₃-0.1TiO₂nano-particles were synthesized by novel water-based sol-gelmethod. Compared with the traditional sol-gel method using titanium alcohol salt, this methoddid not need ethanol solvent system using TALH as titanium salt water-based precursor body.The preparation condition of0.9Al₂O₃-0.1TiO₂nano-particles was optimized. The graingrowth exponent (n) values were2.5and4for α-Al₂O₃and rutile, respectively. The activationenergies of grain growth were estimated to be100and107kJ/mol for α-Al₂O₃and rutile.Along suture line emerging after grain boundary diffusion, the nano-sheets of Al₂O₃-TiO₂grew through high temperature self-assembly way. The microwave dielectric behaviors of0.9Al₂O₃-0.1TiO₂ceramics were εr=10.4, Q×f=18000GHz, Ï„f=-10.8ppm/℃(as preparedat1300℃), and εr=13, Q×f=32000GHz, Ï„f=45ppm/℃(post-annealed at1100℃for10h).5. MgTiO₃-CaTiO₃composite ceramics have been prepared via the solid-phase synthesismethod. CaTiO₃was employed to tone negative temperature coefficient of resonant frequency(Ï„f) of MgTiO₃,3ZnO-B2O₃was effective to promote sintering.(a) With the content of CaTiO₃increasing, dielectric constant and temperature coefficientof MgTiO₃-CaTiO₃ceramics increased, and quality factor reduced. The dielectric propertiesdepended on microstructure and grain phase transition closely. Microwave dielectricbehaviors of0.97MgTiO₃-0.03CaTiO₃ceramics were: εr=18.23, Q×f=76529GHz (7.37GHz), Ï„f=-34.68ppm/°C (at1300℃).(b) Right amount of ZB doping0.97MgTiO₃-0.03CaTiO₃, led to the decrease in thesintering temperature, and the dielectric properties had no significant deterioration.Microwave dielectric behaviors of0.97MgTiO₃-0.03CaTiO₃+2wt.%ZB ceramics were: εr=17.96, Q×f=79346GHz (7.47GHz), Ï„f=-34.93ppm/°C (at1225℃).6.(Zn, Mg)TiO₃-xTiO₂composite ceramics were prepared via solid-phase synthesismethod. TiO₂was employed to tone temperature coefficient of resonant frequency (Ï„f) andstabilized hexagonal (Zn, Mg)TiO₃phase.3ZnO-B₂O₃was effective to promote sintering. Themovement of grain boundary was obvious because of the liquid phase sintering. SEM andEDS showed that segregation and precipitation of dissociative (Zn, Mg)TiO₃grains occurredat grain boundary during sintering. SnO₂was used as inhibitor to prevent the grain boundaryfrom moving. The dielectric behaviors of specimen strongly depended on structural transitionand microstructure. We found that1.0wt.%3ZnO-B₂O₃doped (Zn, Mg)TiO₃-0.25TiO₂ceramics with0.1wt.%SnO₂additive displayed excellent dielectric properties (at1000°C): εr =27.7, Q×f=65494GHz (at6.07GHz) and Ï„f=-8.88ppm/°C.7. The above-mentioned material was applied successfully to make multilayer ceramiccapacitors (MLCC), which exhibited excellent electrical properties. The self-resonancefrequency (SRF) and equivalent series resistance (ESR) of capacitor decreased withcapacitance increasing, and the quality factor (Q) of capacitor reduced as frequency orcapacity increased.