Prepration and Properties of α-Cordierite Glass-Ceramics from Perlite Tailings

Author:Yu Yong Sheng

Supervisor:li zhen

Database:Doctor

Degree Year:2019

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Pages:158

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Nowadays,cordierite glass-ceramic has been regarded as one of the most promising optimal alternative to low temperature co-fired ceramic substrate for its amazing properties such as favorable mechanical properties,low dielectric constant and loss and excellent compatibility with semiconductors(such as Si and GaAs)in thermal expansion.For decades,cordierite glass-ceramic has been enjoying great developments in doping modification and heat processing.Yet it unignorably remains a problem that most cordierite glass-ceramic are mainly dependent on high-purity chemical reagents which causes high cost and sintering temperature.In this case,it would be of much significant to realize the substitution of minerals/mine tailings for pure chemical reagents in the fabrication of cordierite glass-ceramic and efficient utilization of nature minerals considering the low cost and high abundance advantages of natural minerals.Furthermore,as far as concerned the substitution of natural minerals has little been explored.Based on perlite tailings as main raw material,the preparation ofα-cordierite glass-ceramic was systematically studied in this work.At first,the effects of the impurities from perlite tailings on the cordierite glass-ceramic have been studied and the optimal content of perlite tailings has been evaluated.And the structure,crystalline phase,microstructure,dielectric properties,mechanical properties and thermal expansions of the parent-glasses and glass-ceramic have been systematical studied by fourier transform infrared spectroscope(FT-IR),X-ray powder diffraction(XRD),differential scanning calorimetry(DSC),scanning electron microscope(SEM),transmission electron microscope(TEM)and so on.The effect of excess Mg/Al/Si on the physical chemistry properties and crystallization of the finalα-cordierite glass-ceramic has been investigated.And the relevance between the properties and the crystallization kinetics has been explained.The main results and innovations in this paper are as follows:1.Based on the idea formula of cordierite glass-ceramic(Mg2Al4Si5O18,MgO:Al2O3:SiO2=2:2:5)and perlite tailings as the main raw material,α-cordierite glass-ceramic was successfully fabricated.And the effects of the impurities from perlite tailings on the cordierite glass-ceramic has been studied compared with that made only from pure chemical reagents with the same formula.It turns out that the impurities,mainly composed of K2O,Na2O,Fe2O3 and CaO,serve as the glass network modifier and sintering aids,and can cause damage to glass network,reduce the viscosity of molten glass and the transition temperature(Tg)of parent-glass,rise the temperature gap between Tg and the first crystallization temperature(Tp1),thus promote the sintering with high density.At the same time,the impurities could also boost the formations ofμ/α-cordierite and the transformation ofμ-cordierite intoα-cordierite,lower the sintering temperature of singleα-cordierite from 1000 to 875 oC,lower the porosity together with more harmonious thermal expansion to the Si semiconductor and satisfying dielectric constant,implying that the as-obtained cordierite glass-ceramic can be an optimal alternative for commercial LTCC substrate.2.With no mineralizing agents other than the impurities from perlite tailings,the content of perlite tailings has been explored to evaluated the optimal content and the effect of increasing impurities on the product in the fabrication ofα-cordierite glass-ceramic.The increasing impurities can damage the integrality network of the glass-ceramic,rise the temperature gap between Tp1 and Tg and increase the density,hinder the formation ofμ-cordierite,yet promote the formation ofα-cordierite and the transformation ofμ-cordierite intoα-cordierite.In detail,When the content perlite tailings is>40 wt.%(the content of impurities is>6.43 wt.%),the singleα-cordierite glass-ceramic could be obtained at 900 oC,and when the content comes to 6.43 wt.%,the singleα-cordierite glass-ceramic could be obtained at temperature as low as 875 oC with a bending strength of(112 MPa),dielectric constant(10 MHz)of 6.62 and dielectric loss of 0.024(10 MHz),and thermal expansion of 2.68×10-6 K-1.3.The impurities from perlite tailings serve as glass network modifier and could promote the sintering of cordierite glass-ceramic.Taking M01 as contract,the excess content of MgO could further weaken the glass network,lower the Tg and Tp,lower the crystallization temperature ofμ-andα-cordierite,transformμ-cordierite intoα-cordierite.And the excesses of Al2O3 and SiO2 cause little damage to the glass network,rise the Tg.And it is interesting to find that the excess of Al2O3 could transformμ-cordierite intoα-cordierite,otherwise the excess of SiO2 hinders the formation of bothμ-andα-cordierite.In the sintering and crystallization processes,μ-cordierite is always first formed,and the transform ofμ-cordierite intoα-cordierite can be promoted by high sintering temperature and long sintering time.Due to the intension of the influence,the order of the effect of the content of Mg/Al/Si on singleα-cordierite glass-ceramic could be list as follow:MgO(850℃)<SiO2(925℃)<Al2O3(950℃).As the content of Mg/Al/Si increase,the bending strength of the glass-ceramic increases;the thermal expansion rises as the content of Al increases,while the content of Mg/Si has irregular effect.And the thermal expansion of all the samples decreases as the sintering temperature rises.The residual MgO and Al2O3 in the glass can increase the dielectric constant.The dielectric constant decreases with the increase content of SiO2 which can improve the glass network.The dielectric loss changes irregularly with the change of excess MgO,but increases as the sintering temperature rises and decrease as the content of Al2O3 and SiO2 increases.As a whole,the excess MgO,Al2O3 and SiO2 can affect the dielectric properties of the singleα-cordierites glass-ceramic,and the affect degree can be displayed as MgO>Al2O3>SiO2.And singleα-cordierites glass-ceramic fabricated with excess Al2O3shows more matchable thermal expansion with GaAs,while the ones with excess MgO and SiO2 shows more matchable thermal expansion with Si.With improved formulas,singleα-cordierites glass-ceramic prepared with the lowest temperatures(850 oC,950oC and 925 oC for MgO,Al2O3 and SiO2,respectively)can meet the requirements of LTCC substrate showing excellent properties with high potention.4.The crystallization activation energies ofμ-andα-cordierite were calculated by both Kissinger equation and Ozawa equation respectively.And the effects of excess Mg/Al/Si on the crystallization of the cordierite were evaluated in viewing of the kinetics analysis.The result indicates that the crystallization activation energies ofμ-andα-cordierite are 375 kJ/mol,409 kJ/mol(Kissinger equation)and 351 kJ/mol,413kJ/mol(Ozawa equation)respectively,according to the stoichiometric formula M10.For the parent-glass with excess MgO,the crystallization activation energies ofμ-cordierite andα-cordierite come to 269359 kJ/mol,388421 kJ/mol(K)and 204382 kJ/mol,382419 kJ/mol(O),respectively.For the parent-glass with excess Al2O3and SiO2,the crystallization activation energies ofμ-cordierite andα-cordierite are 379536 kJ/mol,286345 kJ/mol(K)and 247401 kJ/mol,297383 kJ/mol(O),respectively.Compared with the formula M01,the energy barriers ofμ-cordierite decrease as the amount of excess MgO increase(O),and the energy barriers ofα-cordierite goes up first and then down as the content of Mg/Al/Si increases,indicating that the increase of MgO can reduce the energy barriers ofμ-cordierite and promote its crystallization,while the increase of Mg/Al/Si could cut down the energy barriers ofα-cordierite and promote its crystallization.5.The Avrami index of crystalline growth,n,ofα-cordierite has been measured by Augis-Bennett equation to evaluate the effect of the amount of Mg/Al/Si on theα-cordierite,and the results are also verified through the SEM technology.It shows that the Avrami index ofα-cordierite in stoichiometric formula M01 is about 3,indicating that the crystal is bulk nucleation and grew in two-dimensional.In case of excess MgO(the mole ratio is 2.4),the Avrami index ofα-cordierite is about 2(in detail,n=2.39),indicating the crystal is bulk nucleation with a main one-dimensional growth and minor two-dimensional grew to some degree.In the other cases,the Avrami index remains about 3,meaning that the crystal is bulk nucleation and grew in two-dimensional.For the formula with excess Al2O3(mole ratio is 2.2),the n is about 1.59(more close to 2),indicating a main bulk nucleation with minor surface nucleation and one-dimensional growth;with more amount of excess Al2O3(>2.2 in mole ratio),the n is close to 1,indicating the surface nucleation and one-dimensional growth.For the formula with excess SiO2,theα-cordierite is bulk nucleation and one-dimensional growth with the n close to 2 as the mole ratio of SiO2 is≤5.4;and theα-cordierite shows the surface nucleation and one-dimensional growth with the n close to 1 as the mole ratio of SiO2 is≥5.4.Compared with the n=3 in formula M01,excess MgO,Al2O3 and SiO2 have considerable effects on the growth model of crystal.Excess MgO in formula favors bulk nucleation and two-dimensional growth.As the content of Al2O3 and SiO2 increase,the nucleation model turns from bulk(M31,M36M37)to surface(M32M35,M38M40),with one-dimensional growth.And the above is also supported by the SEM images.6.The relevance between the properties and the crystallization kinetics has been explained.The effect of the crystallization mechanism ofα-cordierite on the dielectric properties for singleα-cordierite glass-ceramics has been evaluated.According to the degree of the effect,the order is as follow:bulk nucleation and two-dimensional growth<bulk nucleation and one-dimensional growth<surface nucleation and one-dimensional growth.And singleα-cordierite glass-ceramics which follows the last model shows the closest dielectric index(5)toα-cordierite with low dielectric loss(0.0044-0.016).Among the factors which mainly influence the functional performance of LTCC substrate,the dielectric properties and thermal expansion count much more.Glass-ceramics with excess Al2O3 match GaAs and glass-ceramics with excess Al2O3/SiO2 match Si in the view of thermal expansion.Thus singleα-cordierite glass-ceramics with the surface nucleation and one-dimensional growth model would be optimal alternatives for the low temperature co-fired ceramic substrate.