Influence of Sintering Temperature on Crystallization Behavior of Cordierite synthesized from Non-Stoichiometric Formulation

Cordierite body with formulation of non-stoichiometric composition (2.5 MgO. 1.8 Al 2 O 3 . 5 SiO 2 ) was synthesized using conventional techniques with standard raw materials. The sintering and crystallization behavior of the compositions was observed by Differential Thermal Analysis and Thermogravimetric Analysis (DTA/TG). The sequence of reaction and phase transformation was analyzed using X-ray Diffraction (XRD) technique and Rietveld structural refinement after sintering the samples at different temperature regarding the information from the DTA. The Scanning electron microscopy (SEM) was employed for morphology analysis. The DTA curve shows the crystallization temperature, T c occur at 1259°C.Rietveld quantitative phase analysis results reveal that α phase Cordierite constitutes up to 96.4 wt% when the samples was sintered for 2 hours at the optimal temperature of 1375°C. The SEM micrograph revealed that the sample was heat treated at 1375°C obtained densified body with well alignment of crystal structure.


Introduction
Cordierite (nominal composition 2MgO. 2Al 2 O 3 .5SiO 2 ) know as of the useful crystalline phases in MgO-Al 2 O 3 -SiO 2 ternary system. Recently, Cordierite has been focuses in numerous important applications due to its excellent properties, such as low thermal expansion coefficient (3.0 x10 -6 / o C), excellent thermal shock resistance, low dielectric constant (4.0-5.0), good chemical durability, excellent refractoriness, and mechanical properties [1][2][3]. Moreover, cordierite has lower sintering temperature and lower density than the alumina, it also much cheaper and can be synthesized using mineral talc and kaolin [4]. Among the three cordierite polymorph, α-phase is more preference due to it has unique properties, which are very low coefficient of thermal expansion, dielectric constant and dielectric loss, density and stable at elevated temperature [5]. Although D-cordierite is a promising material for many applications, but it is difficult to crystallize and sinter high purity D-cordierite phase at low temperature because of its sintering temperature is near to the incongruent melting point of the cordierite. It has the incongruent melting point because the solid compound of cordierite does not melt to form the liquid of its composition, but instead dissociates to form a new solid phase and the liquid.
Logvinkov et al [6] reported that the change in the crystal lattice of cordierite should be considered as changes arising from composition solid solution rather than phase transformation due to thermodynamic instability of cordierite. Additionally, Torres and Alarcon [7] mentioned that the initial crystalline phase form was cordierite within μ-cobalt and after annealing it transformed to α-cordierite. The μ-cordierite growth along the particle, surface and nucleation and form μ-cordierite dendritic, while the α-cordierite growth within the μ-cordierite dendritic and some Al and Si took place in the tetrahedral sites after long period of annealing at 1100 o C , which introduced some transformation to βcordierite. Therefore, numerous of studies had been carried out with the purpose of accelerate phase transformation of MgO-Al 2 O 3 -SiO 2 with cooperation of additive [8][9][10][11][12]. Nevertheless, the TiO 2 was recommended as most effective crystallization catalyst in numerous papers [13][14][15][16]. The work of synthesis cordierite through solid state reaction with stiochiometric composition has been carried out by C.Liu et al [17] and founded that, 87 wt% of α-Cordierite formed at optimal sintering temperature of 1350 o C for 3 hours. However, the quantity of α-cordierite observed trending down as elevated the sintering temperature as the result of crystallization of secondary phase such as mullite.
Recently, non-stoichiometric cordierite composition (2.8MgO.1.5Al 2 O 3 .5SiO2) has been discovered to give high purity of D-cordierite phase at lower temperature using control crystallization of glass method [4]. The present of high concentration of secondary phase such as forsterite, spinel and mullite (which have different coefficient of thermal expansion and different volume) in cordierite synthesis using stoichiometric composition might affect the performance and its application especially in evaluated temperature atmosphere. The Cordierite body synthesized through glass method by using non-stoichiometric cordierite formulation 2.8MgO.1.5Al 2 O 3 . 5SiO 2 obtaining single phase α-phase Cordierite (100 wt%). However, in previous work, the cordierite with same formulation has been synthesized by solid state reaction and more than 10 wt% of secondary phase such as enstatite was observed. The formation of secondary phase was mainly contributed by excess of MgO in the system. Hence, in this present study, the Cordierite body has been synthesized through solid state reaction by alternated the mole ratio of MgO and Al 2 O 3 to 2.5MgO. 1.8Al 2 O 3 . 5SiO 2 in order to obtain high purity (>95%) stable α-cordierite.

Materials and method
α-Cordierite with non-stoichiometric formulation was synthesized using reagent grade oxides (MgO, Al 2 O 3, and SiO 2 ) with 5 wt% of TiO 2 as nucleating catalyst. The magnesia was obtained for R&M Marketing, Essex, U.K, Alumina and silica were obtained from MajuSaintifikSdn.Bhd. Table 1 showed the elemental analysis of the raw materials, while Table 2 gives the details on weight precent of the non-stoichimetric cordierite formulation composition in this study. The particle size of the reagent oxide was sieved and determined by using particle size analyzer to ensure the median of the particle size d 50 less than 30 μm. The components were weighted and mixed thoroughly by using jar mill together with 10 mm diameter alumina balls as media at rotation speed of 136 RPM for 36 hours. The batchers were compacted at 7 tones into pellet form of 20 mm in diameter by using hydraulic powder press (CARVER AUTO 3890). The green body compact were sintered at different temperature, which were 90 o range at a scanning rate of 1 o /min. Crystal structure analysis was carried out with whole diffraction powder pattern data by utilized Rietveld Refinement method using XpertHighScore Plus software. Differential Thermal Analysis and Thermogravimetric Analysis (DTA/TG) (Rigaku TG 8120) technique was employing to identify the thermal behavior of the batch. The thermal analysis was made in range 40-1500 o C by using corundum crucible as an inert reference material at heating rate of 10 o C /min. The Microstructure of the sintered samples was observed by scanning electron microscopy, SEM (Hitachi TM3000). 3 Results and discussion

Crystallization behavior
Differential Thermal Analysis and Thermogravimetric Analysis (DTA/TG) curves recorded on the thermal behaviour of the non-stoichimetric cordierite composition of 2.5Mg.1.8Al 2 O 3 .5SiO 2 from 40 to 1500 o C are shown in Figure 1. As can be seen in Figure  1, the thermogravimetric (TG) curve of the sample has been segregated into 7 regions. The regions A to F was corresponding to the mass losses, while there was mass gain occur at region G which is related to the crystallization and phase transformation. At the first region, the initial weight losses about 0.73% occurs in the range between 40 and 190 o C which is corresponding to the elimination of water physically adsorbed when wet method mixing route was carried out. Followed by the major weight change about 3.61% losses in the temperature range of 190 to 388 o C, which can attributed to the dehydration of the MgO. The existence of endothermic peak in DTA plot at 588 o C further supports the observation.
The mass losses about 1.84% and 1.17% was observed in temperature range 388 to 447 o C (region C) and temperature range 447 to 569 o C (region D) respectively. The mass losses occur in region C is contributed by the decomposition or organic burn out of the sample. While in region D, the small endothermic peak was observed at 564 o C, which is correlated to the phase transformation of silica. There is no significantly weight change in region E. However, the transition temperature, Tg endothermic peak of the sample was presence at 772 o C in the DTA curve at this particular peak. The exothermic peak of was detected at 979 o C and reaching at endothermic at 1163 o C in the DTA curve and about 0.14% weight losses in region F . In this region the nucleation of glass might take place due to the nucleation temperature (T n ), is usually taken in the nucleation range, which is between T g and softening point (T d ) [18] and it can be calculated by using formulation: Considering the endothermic peak at 1163 o C is softening point of the samples, thus the nucleation temperature, T n of sample was located at 1132.67 o C. A shape exothermic crystallization peak was presence at 1259 o C, which is corresponding to the phase transformation of μ-Cordierite to α-Cordierite [19]. Moreover, thermogravimetric curve in region G shown mass gain causes by solid state reaction of decomposition and leading formation of α-Cordierite and leading the variation of samples density of the sample. The sequence of reaction and phase transformation occurs in region G was detailed by the X-ray Diffraction (XRD) pattern.

Phase evolution
To investigate the sequence of reactions and phase transformation, the samples were heattreated at different temperatures. According to crystallization temperature, T c , and information of Figure 1 Figure 4 illustrated the Scanning electron microscopy (SEM) images of the samples heat treated at different temperature for 2 hours. It was found that, the sample has heat treated at 1375 o C (Figure 4f) obtained optimal morphology and fined microstructure as well as α-Cordierite hexagonal crystal structure was formed over the surface of the sample. Conversely, the formation of secondary phase induce the rough morphology as can been seen in Figure 4a. With corroborating the results obtained through XRD, in can be noticed that, the morphology of samples show better appearance more denser body as diminish of secondary phase Figure 5, showed the comparison between the samples sintered at 1250 o C and 1375 o C. As can be seen in Figure 5a, the formation of dissimilar and complex crystal structure of secondary phase has inferior alignment with other phase and induce pore between each other. As a result, the performance in terms of mechanical strength, thermal expansion coefficient, and dielectric constant has impacted. On the other hand, the approximately single phase of α-Cordierite present well alignment of the crystal structure.

Conclusions
The high purity of α phase Cordierite has been successfully synthesized from nonstoichimetric formulation 2.5MgO.1.8Al 2 O 3 . 5 SiO 2 using a conventional technique with standard raw materials. The X-ray diffraction technique and the Rietveld quantitative phase analysis results reveal that α-Cordierite constitutes up to 96.4 wt% of the samples sintered for 2 hours at the optimal temperature of 1375 o C. With corroborating of XRD patterns and DTA curve, the μ-Cordierite has been confirm completely transformed to α-Cordierite at sintering 1275 o C. SEM micrograph revealed that fully dense with well crystal structure alignment α-Cordierite was successfully crystallized.