Crystallization And Phase Evolution Of Cordierite Synthesized By Glass Method Using Talc And Kaolin As A Function Of Excess Mgo Molar
α-cordierite whose composition is 2MgO.2Al2O3.5SiO2 is one of the most interesting phases of the system MgO-Al2O3-SiO2. It can be applied in many fields, and special attention was in its application as insulating materials in high frequency electronics due to its properties such as low thermal expansion coefficient, low dielectric constant, low dielectric loss and high chemical and mechanical durability. It can be prepared by sol-gel synthesis, non-hydrolytic soil-gel route, liquid phase sintering, glass ceramic route and solid state reaction. Various methods with different kind of initial raw materials and additives have been studied to decrease the sintering temperature of α-cordierite to below than 900oC in order to be used as a substrate in electronic packaging. Generally, some flux as B2O3 , Li2O, La2O3 are added to decrease the sintering temperature and modify the properties of α-cordierite ceramic. The additives results in some changes in sintering and ultimately, properties of ceramics. Among those methods, crystallization by glass method are able to produced nearly pure α-cordierite at such lower temperature. However, the researchers are using pure oxides (MgO, Al2O3 and SiO2) with nonstoichiometry of cordierite system. In this work α-cordierite was synthesized using minerals talc and kaolin with excess MgO molar by crystallization glass method. The minerals used in this work already consists of flux element e.g. K2O, Na2O and CaO which could promote sintering of ceramics and lowering the sintering temperature of α-cordierite phase. Therefore no sintering aids was added in the compositions.
Crystallization by glass method is a process of producing samples by melting and fast cooling to form a glass. Glass is a liquid that have been cooled below their freezing points without crystallization. Glass exist in meta-stable state has a free energy higher than that corresponding crystalline phases of the same composition. Nucleation and crystal growth are necessary to transform the glass from the meta-stable to stable state. Although crystallization by glass method required high temperature for melting, but homogeneity distribution of particles was obtained.
The main purpose of the present works are to study the effect of excess MgO in stoichiometry of cordierite and at the same time to obtain pure α-cordierite phase without secondary phase using minerals talc and kaolin and can be sintered below 900oC.
Material And Methods
Standard raw materials, kaolin and talc, were used in synthesis of cordierite ceramics. Powders of technical grade: silicon oxide (purity >99.5%), alumina (99.95%), magnesia (99.90%) were used and added to compensate the stoichiometric composition. To confirm type of powders used in the experiment, all powders were analyzed by XRD. The diffraction pattern of raw materials used is presented in Fig. 1. The elemental composition of kaolin and talc was determined by X-ray Florescence spectroscopy and are detailed in Table 1 whilst Table 2 list the composition of the mixtures.
In this present work, crystallization by glass methods were employed for the preparation of cordierite powders. Homogenized mixture of raw ingredients of a given compositions was melted in alumina crucibles at 1500oC for 4h. The molten glass was quenched in distilled water glass by dropping the crucible into a distilled water to form 'cullets'. The quenching fragmented the glass, which facilitated removal from the crucibles. The cullets were then heat in an oven for 1 h to remove moisture from quenching process and further milled by the following two steps. In the first step, the particles were pulverised in planetory miled (300 rpm speed, 5 minutes using 5 pieces 20mm tungsten ball). The resultant powders were then sieved to 150 µm to remove particles larger than 150µm. Second, the filtered milled powder was dried milled using 10mm dia tungsten ball at 300 rpm speed for 150 minutes. Milling was stop for every 15 mills to avoid heat from generate which would initiate the crystallization and increase the contamination from milling media. The obtained glass powders with particles size in the range 1-3µm was characterized by X-ray diffraction to determine the degree of crystallinity. Round pellet with diameter 23mm were prepared by uniaxial pressing at ~120MPa and were sintered in air under isothermal conditions for 2h at 900oC with 5oC/min heating rate before further characterization by X-Ray diffraction.
In this research 7 samples with increasing MgO molar from the stoichiometry of cordierite was prepared. The analysis of the glass powder was performed on diffractometer with CuKa. The readings were taken on Bragg Brentano geometry between 10 to 90o. Counting time was fixed to 71.5 s for each 0.03 °2θ steps. EVA software was used to determine the present of phases in all samples as a function of MgO molar. While the amorphous phase in semi-crystalline samples was quantified by standardless-methods based on the Rietveld method using TOPAS ver 3 programme. In these methods, the intensity contribution to the amorphous phase and the background are deconvoluted to relate the scattering intensity of the amorphous to its weight fraction. The profile function used was the split-pseudo-Voight function. This method is based on the fact that during heat treatment of the parent glass to form the glass-ceramics, crystalline regions are formed at the expense of the glassy phase. The percentage crystallinity (PCR) in a particular specimen can then be expressed as:
Where AAT is the total area of diffraction pattern, AA is the area of amorphous phase, and AB is the area of background from scattering effect.
The results from pure crystalline, was used to correct for background scattering effects which is come from air and the instrument. It was assumed that the absorption coefficient of the parent glass does not differ significantly from that of the residual glassy phase of the glass-ceramics. Degree of crystallinity in the glass-ceramic specimens are summarized in.
Results and discussion
Characterization Of Initial Raw Materials
Fig. 1 reveals that all diffraction patterns of Al2O3 and SiO2 powders is matched with the reference pattern. MgO powders obtained from the lab has changed to Mg(OH)2. MgO is unstable compound which can react with the moisture to form brucite Mg(OH)2 if it was exposed in air for a lengthy period. Calcination of Mg(OH)2 at 800oC/ 1h was enough to removed the hydrated water and to obtain high crystalline periclase whereas calcination at 600oC show lower crystallinity. Powders taken as kaolin and talc powders were also unpured. Although results from elemental analysis shows that the amount of MgO and SiO2 for talc is 96% and the amount of Al2O3 and SiO2 in kaolin is 94%, however higher percentage of the compounds shouldn't become a conclusion to state the materials are pure, as what have been declared by the supplier. Beside obtaining similar weight percent between chemical analysis and calculated chemical formulation, it should be prove by X-ray diffraction analysis. Samples as kaolin in this work contain kaolinite, muskovite and quartz phase. While raw material as talc is a mixture of talc, dolomite, magnesite and quartz phase. Hence, to obtained accurate molar, the compositions of mixture for a selected stoichiometric of glass ceramic are calculate using elemental analysis results rather than chemical formulation of talc and kaolin. The amount of talc and kaolin in each selected stoichiometric of MgO-Al2O3-SiO2 system was taken in a maximum weight of the total composition whilst, alumina, silica and magnesia are added to compensate the stoichiometry.
Degree Of Crystallinity
example of the diffraction pattern of glass powder before heat treated. Generally all samples gives huge hump which indicate that the samples is amorphous or in a glass form. Some samples show the nucleation of crystalline phase in the middle of huge amorphous hump and the sample is said to be partly crystalline or semi-crystalline. The degree of crystallinity and the amount of amorphous was measured by Rietveld method using TOPAS software and the results is tabulated in Table 3: All samples gives Rwp below than 10 and the convergence was achieved, which mean the quality of the fit is good.
Sample A1 shows higher degree of crystallinity whereas sample A2 indicate highest armorphous content. One of the factors that effect the degree of crystallinity is a queching rate. High quenching rate will prevent the glass from crystallized. Although all samples undergo the same methods of melting and queching, however human error like times taken to take out the samples and dropping it in to the water will also influence the degree of crystallinity. Others factor include, temperature and volume of water, composition of the mixtures, viscossity and melting point of samples. Mechanical milling could also resulted in a completely amorphized powder. This is because the mechanical activation caused the materials loss its crystallinity by distort its crystal structure. Partial crystalline or fully glass can be observed by the appearance of glass whether it is in an opaque or transparent form. Sample with transparent appearance can be classified as glass, where this glasses are formed by cooling from the liquid state at rates fast enough for crystallization to be avoided. However, samples with opaque look may results from slow cooling. For the purposes of attaining useful properties in a glass ceramic, it is necessary to control the process, so that homogeneous glass can be formed and that glass will crystallize such that the microstructure can be controlled by the crystallization conditions. An understanding of nucleation and crystallization of glasses is thus important.
Phase Evolution Of Samples As A Function Of Mgo Molar
After heat treated at 900oC for 2h, all glasses are partial or fully crystallized. Crystallization of a glass involves the transition from random structure of a liquid to the more ordered regular lattice of a crystalline solid. The driving force for crystallization, as with any phase transition, is the lowering of the free energy of the system. In this case, by decreasing the entropy. The ability of a glass to crystallize is determined by kinetic factors and atomic rearrangement are necessary for the transition from the glass to glass-ceramic. As a molten is cooled from its melting temperature with a certain cooling rate, the tendency of the glass to crystallize is thus determined by both kinetic and thermodynamic considerations.
X-ray diffraction studies revealed important information of phase revolution of the samples with increasing MgO molar when the samples were sintered at 900oC /2h, as shown in Fig. 3. Sample A1, shows the presence of three phases: µcordierite, mullite and α-cordierite. However, the intensity of α-cordierite phase is very weak for samples with exacts stoichiometry of cordierite. At this composition, mullite and µ-cordierite present as main phase which was identified, through their characteristic of diffraction peaks. µ is a metastable form that normally crystallize from glass frits betwee 830oC-900oC. Raising the temperature or holding isothermally at longer timer or by the addition of sintering aids will results in the transformation of µ to α phase. Weak and diffusive peaks, corresponding to α-cordierite, are not noticeable in diffraction pattern observed by EVA software, but it can clearly been seen in TOPAS software which have very good resolution. At higher molar ratio MgO, these peaks are getting sharper and intense. µ-cordierite peak is visible in the diffractogram of the powder obtained from stoichiometry 2.8MgO.2Al2O3.5SiO2. The increament of MgO molar from 2 and 2.4 has caused µ-cordierite transformed to α-cordierite and intensity of mullite (3Al2O3.2SiO2) decrease. At 2.6 molar MgO, no µ-cordierite is observed although the samples is sintered at lower sintering temperature (900oC) as what normally obtained by other researchers during cordierite synhesis at lower sintering temperature. In sample A2, nearly half of mullite has transformed to α-cordierite by the present of excess MgO. This is shown by the equation below:
As a result of decomposition of mullite, Al2O3 are left in the sample. Further increase the MgO molar will cause the reaction between MgO and Al2O3 and consequently at 2.8 molar MgO, spinel (MgAl2O4) phase was detected and start to crystallized. Metastable phase spinel (MgAl2O4) forms as a result of the diffusive reaction between MgO and Al2O3. In solid state method, Shi found this reaction occured at 1200oC.
Further increasing of MgO molar ratio up to 2.8 all mullite had decompose and the amount of spinel keep increasing up to 3 molar MgO. Above 3 molar the amount of spinel is decreased in the diffractogram and totally dissapeared in sample A7 with 4 molar MgO. Forsterite phase was detected at sample with 3 molar MgO and keep increasing as the MgO molar increased. This is because the excess MgO will react with residual SiO2 to form Forsterite.
2MgO + SiO2→ Mg2SiO4
The diffraction peaks of α-cordierite increase with the increased in MgO molar up to 3.2 molar. While those of mullite, spinel and µ-cordierite decrease in intensity as a function of MgO molar. The intensity of α-cordierite drops above 3.2 molar MgO when forsterite start to crystallized and increased.
High intensity of α-cordierite with minimum trace element has succesfully being obtained at lower sintering temperature 900oC/2h using minerals talc and kaolin. Excess MgO in stoichiometric composition of cordierite prevent the crystallization of µ-cordierite. β-cordierite phase doesn't exist in the system which indicates that the homogeinity of the amorphous state is high. Through crystallization glass method, α-cordierite from kaolin and talc can be used for electronic application since it gives low trace element and in addition it was fully crystalline at lower sintering temperature.
I would like to thanks Islamic Development Bank (IDB) who has sponsored my PhD study. This work was also supported by Fundamental Research Grant Scheme (9003-00171) from University Malaysia Perlis.
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