Study On Preparation, Properties And Mechanism Of Low-Temperature Co-fired Glass Ceramic Materials

According to the latest development and shortcoming of lowtemperature co-fired ceramic: substrate materials (LTCCs), the effects ofthe additives and oxide replacement on glass structure, sintering, phasecompositions, dielectric properties, thermal expansion characterizationand mechanical properties of cordierite-based glass-ceramics wereinvestigated by using the differential thermal analysis (DTA), X-raydiffraction (XRD), scanning electron microscope (SEM), energyspectrum analysis, a Hewlett-Packard (HP) AC impedance, a thermalproperty test apparatus and a mechanical property test apparatus. Theinfluences of the ceramic content and sintering or hot-pressingtemperature on firing, phase composition, thermal expansioncharacterization, dielectric property, heat exchange behavior, microstructure and mechanical properties of new glass/ ceramicsystems were studied. The mechanisms of sintering densificationkinetics, crystallization kinetics and phase transition of glass-ceramicsand those of the precipitation of cristobalite and quartz in sinteringprocess for the borosilicate/feldspar composites as well as ofstrengthening and toughening in the AlN/cordierite glass compositeswere put forward respectively in this paper and the results showed as thefollowing:The relationships betwe, en the content of bismuth oxide, sinteringtechnologic parameters and the phase transition, sinteringcharacterization, physical properties and interfacial characteristics ofcordierite-based glass-ceramics were investigated. The addition ofbismuth oxide can lower the phase transformation temperature fromμ-toα- cordierite and enhance the sintering densification. The dielectricconstant and the flexural strength of samples increase with the additionof bismuth oxide in a similar way as that of the density. The dielectricloss manifests "V" curve with bismuth oxide. The thermal expansion coefficients with increasing bismuth oxide exhibit the characteristic oflinear increase. The reasonable addition of bismuth oxide is 3～5wt%.The material fired at 900～950℃in air has a low dielectric constant(～5.3), low dielectric loss (～0.001), low thermal expansion coefficient(～3.5×10^-6K^-1)and high flexural strength (≥130MPa). The polarizationuniversal relation index is in the range of 0.5～0.9. The dielectricproperties of the sample with temperature dependence and frequencydependence possess the excellent stability. The interfacial coalescentbetween the glass-ceramic substrate and Ag electrode is good and thethickness of Ag diffusion layer is about 5μm. The glass-ceramicsubstrate can meet the demand for a long-term reliability in LTCCs.The role of ceria in the cordierite-based glasses was put forward forthe first time. The results show that a small amount of ceria added act asthe network modifier, lower the viscosity of the glass, accelerate phasetransformation of cordierite fromμtoαand enhance sinteringdensification of cordierite-based glass powders. Over ceria added willinhibit the sintering densification. The dielectric properties ofglass-ceramics depend on not only the content of ceria and sinteringtemperature but also the crystalline phases and relative density. Thethermal expansion coefficient of glass-ceramics present the reciprocal "Z" curve with ceria content and it basically decrease according to thelinearity with increasing temperature, which depend upon the amount ofcordierite. The flexural strength of samples firstly increases and thendecreases with the content of ceria, which is similar to the curve of theporosity. The glass-ceramics with 4wt% ceria sintered at 900～950℃has a low dielectric constant(5.3), low dissipation factor (≤0.002), lowthermal expansion coefficient (2.5～2.8×10^-6K^-1) and high flexuralstrength (115MPa), suggesting that it would be a promising material inLTCCs.The effects of ZnO added and the replacement of Al₂O₃ by ZnO onglass structure, crystallization behavior, phase transition, microstructure, sintering dynamics and physical properties were discussed for the firsttime. With increasing ZnO content, the melting temperature, glasstransition temperature and crystallization temperature all decreased.Addition of ZnO can greatly improve the sinterability of samples andalter the type of the crystalline phase. Too much ZnO addition (morethan 3wt%) seems to be needless for deteriorating the physicalproperties of the sample. The reasonable ZnO content is 3wt%. Thecrystallization sequence of crystalline phases in the sample with 3%ZnO corresponds to the position of IR absorption spectra. The dielectricconstants increase with ZnO content and the dielectric loss has thelowest value at 1.5wt%～3wt% ZnO. The thermal expansioncoefficients increase with ZnO content. The flexural strength firstlyincreases and then decreases with ZnO and get to the highest value at1.5wt% ZnO. The glass-ceramics with 1.5～3wt% ZnO sintered at900～950℃has a low dielectric constant(5.0～5.3), low dielectric loss(≤0.002), low thermal expansion coefficient (3.0～4.8×10^-6K^-1) andhigh flexural strength (≥120MPa). The dielectric properties of thesample with temperature dependence and frequency dependence possessthe excellent stability.With increasing replacement amount of Al₂O₃ by ZnO, the meltingtemperature and crystallization temperature of the glass-ceramics lower, and the glass transition temperature firstly decrease and then increase.With the replacement of 5wt% and 8wt%, the predominant crystallinephases in the glass-ceramics were found to beα-cordierite and thesecondary crystalline phase to be gahnite and quartz. When thereplacement was increased to 11wt%, the predominant crystalline phaseswere found to be gahnite and quartz and the secondary phase to beα-cordierite. Only the sample with 8wt% ZnO can be fully sinteredbefore 1000℃. Therefore, the sample fired at 900～925℃has 97.0%theoretical density, a fairly low dielectric constant (5.0～5.2), a lowdielectric loss (≤0.001) at 1MHz, a lower thermal expansion coefficient(4.0～4.2×10^-6K^-1) and a higher flexural strength (≥125MPa), suggesting that it would be a promising material in the electronicpackaging field.The sintering and crystallization mechanisms of the glass-ceramicscontaining ceria were put forward. The sintering mechanism is the liquidsintering process in which Newtonian viscosity flow predominates. Theporosity is determined by time and is given by—lnp=kt+B. Themethod of the sintering kinetic parameter used gives an equation of theform In k=C-E/RT. According to two equations, the sinteringactivation energy calculated of samples with 0wt%, 2wt%, 4wt%and7wt%ceria is 221 kJ/mol, 203.1kJ/mol, 196.0 kJ/mol and 232.0 kJ/molrespectively. However, the mathematic models mentioned above are thesame with the initial and medium-term phases. The results obtained byusing differential thermal analysis and JMA equation illuminate that thecrystallization activation energy of glass-ceramics increases with ceriacontent and adding ceria will inhibit the crystallization of glasses. Thecrystallization mechanism of all the glass is believed to crystallize fromthe surface.Based on the crystal structure and phase separation of glass, thedevitrification mechanisms of cristobalite and quartz in the borosilicateglass/feldspar (anorthite or Sr-celsian) composites in the sinteringprocess were put forward for the first time, which is different from thatof alumina. The unique crystal structure of feldspar makes M²⁺ ion(M=Ca, Sr) exist in the large channels. According to the above analysis, the Al³⁺ion from feldspar is very difficult to diffuse into glass. Incontrast, the M²⁺ ion from feldspar and other ions (Ca²⁺, Zn²⁺)from theglass phase can easily reciprocally diffuse into that increasing thetendency of phase separation in borosilicate glass, which leads to theformation of rich SiO₂ glass phase and rich-R²⁺(R=Ca, Sr, Zn) glassphase. The precipitation ofα-quartz crystal occurs in the compositesduring the firing and some amount ofα-quartz may synchronously transformed into cristobalite. Certainly, elevating temperature andincreasing ceramic content can accelerate the formation ofα-quartz andcristobalite directly from the composite, which leads to having badeffect on the thermal expansion coefficient of the composites.The relationships between the composition and physical propertywere investigated. The results show that the dielectric constant andthermal expansion coefficient as well as hardness increases butdielectric loss of samples decreases with ceramic content. The formationofα-quartz and cristobalite leads to having bad effect on the thermalexpansion coefficient but has little effect on the values of the dielectricconstant of the composites. The formation of aluminum borate ispropitious to the dielectric property and thermal expansion property inthe glass/spinel composites. Adding 10wt% aluminum borate (Al₁₈B₄O₃₃)drastically inhibits the precipitation of quartz and cristobalite in the highSiO₂ glass/aluminum borate composites. The material sintered at lowtemperature between 750℃and 1000℃has good physical propertiesand would be applied to the electronic packaging field.The strengthening and toughening mechanisms of AlN/cordieriteglass composites were put forward. The strengthening mechanism isload transfer and the toughening mechanisms are deflexion, divaricationand transfixion as well as AlN particle pulling out.The relationships between AlN content, hot-pressing technology, sintering characterization and thermo-physical properties of theAlN/cordierite glass composites were systemically investigated. Therelative density of the composites slightly decreases with AlN contentand obviously increases with increasing hot-pressing temperature andholding time. The thermal expansion coefficient and the thermalconductivity increases with increasing AlN. The variety of the thermalconductivity for the samples containing different AlN content hasdifferent characteristic. The dielectric constant and dielectric lossincrease with AlN content and reduce with hot-pressing temperature. The flexural strength and fracture toughness gradually improve withincreasing AlN. The flexural strength and fracture toughness reach amaximum, i.e. from 117MPa and 1.27MPa.m^1/2 of the glass matrix to212MPa and 3.04MPa.m^1/2 of the composites at 40vol% AlN. XRDanalysis indicates that no chemical reaction occurs between cordieriteglass matrix and AlN particles suggesting that the glass matrix haveexcellent chemical compatibility with AlN particles.The composites sintered in vacuum by hot-pressing at lowtemperature (≤1000℃) have a fairly low dielectric constant (5.6～6.6), a low dielectric loss (0.001), a low thermal expansioncoefficient(≤3.8×10^-6K^-1), a high thermal conductivity (6.5Wm^-1K^-1), ahigh mechanical properties and is suitable for high density electronicpackaging.