Effect of Reduced Al2O3 Mole Ratio on Fabrication of Cordierite Ceramic by Solid-State Sintering Method

Cordierite ceramic was fabricated by reducing Al2O3 mole ratio from 2.0 to 1.4 with kaolin, silicon dioxide and magnesium oxide as the raw materials. The effect of reduced Al2O3 mole ratio on the sintering behaviors, phase transition, main properties, and microstructure were characterized in detail. The results show that cordierite phase becomes the main crystallization phase at 1300 C, and mullite phase can be consumed to produce cordierite phase by reducing Al2O3 mole ratio. But additional quartz phase still exists until 1400 C. Moreover, the open porosity, pore connectivity and pore size increase as Al2O3 mole ratio reduces from 2.0 to 1.4 while the linear shrinkage percent and bulk density decrease with the reduced Al2O3. It is considered that the sintering activity of the raw materials at low temperature decreases due to the increase of chemically pure magnesium oxide and silica with the decrease of Al2O3 mole ratio, while the crystallization process, pore growth and cordierite phase rather than the liquid phase control the densification process at high temperature.


Introduction
Currently, cordierite (2MgO•2Al 2 O 3 •5SiO 2 ) ceramic has been widely studied due to its outstanding properties, and it has many industrial applications, such as catalysts, microelectronics, refractory products, integrated circuit boards, heat exchanger for gas turbines, membranes, thermal shock-resistance tableware and porous ceramics [1][2][3].The stoichiometric composition of cordierite, additives, preparation methods, and sintering process have received extensive concerns for fabricating cordierite ceramic [4,5].Solid-state sintering method [6][7][8] as one kind of the preparation methods, due to its low cost and simple process, is suitable for cordierite industrial production.However, the researchers are willing to investigate the MgO-Al 2 O 3 -SiO 2 system by glass crystallization method [9,10].And the difference between the two fabrication methods causes the distinction of the final cordierite productions, including crystallization process and shrinkage behavior.For example, we have studied the influence of excess MgO on the properties of cordierite ceramic sintered by solidstate method [11], and the results are different from those of Banjuraizah [12] and Hwang [13].The crystallization temperature did not reduce with the increase of MgO while the liquid glassy phase increased and the densification process was improved.Sapphirine phase instead of μ-cordierite phase appeared at low temperature.Moreover, we also have fabricated dense cordierite ceramic by reducing Al 2 O 3 mole ratio from 2.0 to 1.4 using kaolin, attapulgite and basic magnesium carbonate as the raw materials [14] and the distinction of the results from the two different fabrication methods still exists.Besides, the impurities in attapulgite may have a great influence on the final performance of the cordierite ceramics, covering up the real function of reduced Al 2 O 3 , which is not discussed in that paper.Therefore, using the solid-state sintering method, cordierite ceramic was fabricated by chemically pure kaolin, silicon dioxide and magnesium oxide with reduced Al 2 O 3 in this work.The effects of reduced Al 2 O 3 on the properties of cordierite were studied in detail, mainly including sintering behavior, phase compositions, micro-structure properties.

Raw materials and specimen preparation
In this work, kaolin, silicon dioxide and magnesium oxide were used as the raw materials to synthesize cordierite ceramics.All of them came from Sinopharm Chemical Reagent Co., Ltd (Shanghai, China).Based on the non-stoichiometric composition of cordierite {2MgO The raw materials were mixed for 24 h by a high energy planetary ball mill using alumina balls as media at a rotation speed of 180 rpm.After being dried at 105 o C in an oven and sieved through a 48-mesh screen, the powders mixed with certain binder (PVA, 4 ml/30 g) were pressed into cylindrical specimens with Φ25.7 mm ×5 mm under the pressure of 160 MPa.The specimens were dried at 70 o C for 24 h and then sintered at various temperatures between 1200 and 1400 o C at an interval of 50 o C for 2 h.The heating rates were 2 o C/min to 450 o C with a holding time of 1 h and then 5 o C/min up to the final temperatures.

Characterization techniques
Sintering shrinkage behaviors of the samples with reduced Al 2 O 3 were respectively measured between 26 o C and 1400 o C in the horizontal dilatometer (Netzsch, DIL 402C, Germany) with 5 o C/min of heating rate.The crystalline phases were recognized based on the XRD patterns.The measurement was carried out using an X-ray diffractometer (D8 ADVANCE, Bruker Corporation, Germany; Cu Kα radiation) at 40 kV and 30 mA.The diameter shrinkage percent was simply calculated by measuring the diameter of the specimens before and after sintering using a vernier caliper.The open porosity and bulk density were measured based on the Archimedes' principle with distilled water as medium.Scanning electron microscopy (SEM) observation was carried out using a XL-30 ESEM device at 5 keV.The above phase transformation process can be ascribed to the following steps.First, the kaolin loses the crystallization water to produce metakaolin during sintering process, and then further decomposes into mullite and quartz [12].

Crystallization behaviors
Parts of mullite reacts to produce sapphirine at low temperature and then increasing temperature results in the transformation of the sapphirine to the cordierite [11].The left mullite will directly transform to be the cordierite phase at high temperature [12].
As Al 2 O 3 reduces, the added silicon dioxide and magnesium oxide increase (as shown in table I), facilitating the above reaction.Thus, mullite disappears for the samples with 1.8-1.4Al 2 O 3 mole ratio sintered at 1350 o C.However, the excess SiO 2 results in the existence of quartz phase in the samples until 1400 o C.Then, the samples shrunken again in the range of 1190 -1260 o C.And the shrinkage rate had an obvious decline, which can be ascribed to the inhibitory effect of the crystallization process.After 1263 o C, the sintering shrinkage rate decreased again and the shrinkage process entered into the third stage (1263-1351 o C).In this temperature range, cordierite appeared in abundance and became the main crystalline phase, inhibiting the shrinkage process due to its low thermal expansion coefficient.When the temperature exceeded 1353 o C, the shrinkage process intensified as a result of the sintering promotion action of liquid glassy phase [15].By contrast, the samples with 1.4 Al 2 O 3 mole ratio exhibit obvious differences.For the samples with 1.4 Al 2 O 3 mole ratio, there was a small shrinkage from 802 to 928 o C in the dL/L 0 curve before the first sintering shrinkage stage.Because the above temperatures are beyond the main scope of the study, further investigation is required.Then the samples have similar shrinkage process in the first stage except for the lower value of dL/L 0 and the higher initial and end temperatures (974 and 1184 o C) of the shrinkage process.And the result is opposite to the results of Banjuraizah [16] and our previous studies [14].This indicates that reducing Al 2 O 3 mole ratio did not promote the densification process of the cordierite compacts sintered by solid-state sintering method.McMillan [17] proposed that reducing the intermediate oxide of the glass can result in the reduction of the melting temperature of the glass compound.Therefore, in the MgO-Al 2 O 3 -SiO 2 system, the melting temperature should be reduced by reducing Al 2 O 3 mole ratio and the viscosity of the liquid phase should also be decreased at the same melting temperature, resulting in an evident shrinkage process.However, the sintering activity of the samples decreased due to the increase of the added silicon dioxide and magnesium oxide (as shown in Table I) with Al 2 O 3 mole ratio reducing from 2.0 to 1.4.This is because that the chemically pure raw materials are inert and un-reacted at this temperature range [18].And this also becomes the dominant factor for the shrinkage process in the first stage.

Sintering shrinkage behaviors
After 1200 o C, there was no evident shrinkage behavior for the samples with 1.4 Al 2 O 3 mole ratio in the sintering temperature range.And the shrinkage percent was still lower than that of the sample with 2.0 Al 2 O 3 mole ratio.It is concluded that some factors, which may be the crystallization process, pores and the cordierite phase, completely impedes the promotion effect of the liquid phase on the densification process.

Sintering behaviors of cordierite ceramics 3.3.1. Shrinkage percent
Fig. 5 presents the diameter shrinkage percent of the cordierite compacts sintered at 1200-1400 o C for 2 h.For all the samples with 2.0-1.4Al 2 O 3 mole ratio, the shrinkage percent remained unchanged or decreased a little with the temperature increasing from 1200 to 1400 o C, which is in accord with the above dilatometric results in section 3.2.The difference for the samples is attribute to the different thermal-treating schedules between the dilatometric measurement and fabrication process.As the sintering temperature increases, large amount of liquid glass phase appears, resulting in a further densification [18].However, the simultaneous crystallization process can impede the densification process of the samples [19], and the existence of the holes also weakens the densification process [20].Besides, abundant cordierite phase inhibits the shrinkage process as well because of its low coefficient of thermal expansion.Therefore, it can be concluded that the inhibition effects of the crystallization process, pores and cordierite phase reach equilibrium with the densification process caused by the liquid glass phase at high temperature.Moreover, for all the sintered temperatures, the shrinkage percent decreased with the reduced Al 2 O 3 mole ratio.The low sintering activity of the raw materials impedes the densification process at low temperature and the difference of the shrinkage percent between the samples does not reduce due to the influence of the crystallization process, pores and cordierite phase.

SEM
Fig. 7 displays the SEM microphotographs of the cordierite samples with 1.4 Al 2 O 3 mole ratio sintered at 1200-1400 o C and the micro-structures are extremely related to the sintering temperature.As the sintering temperature increased, more liquid phase produced, resulting in a gradual densification.Besides, the pore size increased and the connectivity of the pores gradually strengthened.As sintered at 1200 o C, the particles of the raw materials connected to each other to produce small blocks and then continuous sintering area appeared with strip holes scattering between the blocks.When the temperature increased to 1300 o C, the small blocks disappeared and the continuous sintering area expanded.The small strip pores expanded or merged to form spherical pores.With the sintering temperature increasing to 1400 o C, the substrate further densified and the pore size continued to increase.

Conclusion
The cordierite ceramic was fabricated using kaolin, silicon dioxide and magnesium oxide as the raw materials.It is found that after 1300 o C, cordierite phase becomes the main crystallization phase for all the samples.By reducing Al 2 O 3 mole ratio, the mullite phase is consumed to produce cordierite phase, and the quartz phase still exists until 1400 o C. Besides, the shrinkage percent and bulk density decrease with Al 2 O 3 mole ratio reducing from 2.0 to 1.4 while the open porosity increases.It is considered that the sintering activity of the raw materials at low temperature decreases due to the increase of chemically pure magnesium oxide and silica with the decrease of Al 2 O 3 mole ratio, while the crystallization process and the cordierite phase rather than the liquid phase caused by reducing Al 2 O 3 mole ratio control the densification process at high temperature.Moreover, reducing Al 2 O 3 mole ratio can improve the connectivity of the pores and cause the pores grow larger, which also results in the increase of the open porosity and the decrease of the shrinkage percent and bulk density.

Fig. 1 .
Fig. 1.XRD patterns of cordierite samples with reduced Al 2 O 3 after sintering at 1300 o C.

Fig. 1
Fig.1presents the XRD patterns of cordierite samples with reduced Al 2 O 3 sintered at 1300 o C. For all the samples with different Al 2 O 3 mole ratios from 2.0 to 1.4, it can be observed that cordierite became the main crystalline phase with a few of quartz and mullite phases.As Al 2 O 3 mole ratio decreased from 2.0 to 1.4, the peak intensity of mullite phase was weakened and even disappeared while the quartz phase still existed.It is considered that mullite was consumed with excess MgO and SiO 2 to form cordierite.

Fig. 2 .
Fig. 2. XRD patterns of cordierite samples with reduced Al 2 O 3 after sintering at 1350 o C.When the sintered temperature increased to 1350 o C (as shown in Fig.2), the peak intensity of quartz phase reduced to zero for the samples with 2.0 mole ratio of Al 2 O 3 .In addition, the mullite phase still existed until the mole ratio of Al 2 O 3 was lower than 1.8, which proves again that reducing the mole ratio of Al 2 O 3 can promoted the transformation from mullite phase to cordierite phase.But the quartz phase appeared again and increased a little due to the excess SiO 2 with the mole ratio of Al 2 O 3 reducing from 1.8 to 1.4.The XRD patterns of cordierite samples with reduced Al 2 O 3 sintered at 1400 o C are present in Fig.3.A

Fig. 3 .
Fig. 3. XRD patterns of cordierite samples with reduced Al 2 O 3 after sintering at 1400 o C.

Fig. 4
Fig.4displays the sintering shrinkage behaviors of the mixture samples with 2.0 and 1.4 Al 2 O 3 male ratios.For the sample with 2.0 Al 2 O 3 mole ratio, there are four sintering shrinkage stages from 800 to 1400 o C. The first stage of significant shrinkage process started at 967 o C, and then finished at 1165 o C, which resulted from the viscous sintering process.Then, the samples shrunken again in the range of 1190 -1260 o C.And the shrinkage rate had an obvious decline, which can be ascribed to the inhibitory effect of the crystallization process.After 1263 o C, the sintering shrinkage rate decreased again and the shrinkage process entered into the third stage (1263-1351 o C).In this temperature range, cordierite appeared in abundance and became the main crystalline phase, inhibiting the shrinkage process due to its

Fig. 5 .
Fig. 5. Diametrical shrinkage percent of the cordierite samples with reduced Al 2 O 3 sintered at different temperatures between 1200 and 1400 o C for 2 h.

Fig. 6
Fig. 6 shows the variations of open porosity and bulk density for the samples with reduced Al 2 O 3 sintered at 1200-1400 o C. For all the samples, the open porosity decreased with the temperature increasing from 1200 to 1400 o C. Based on the slopes of the curves, it can be obtained that the open porosity cures can be divided into two parts at 1300 o C. For all the samples, the porosities decreased rapidly before 1300 o C, and then the down trend of the porosity became slowly.It is considered that liquid phase sintering process results in a huge loss of small pores at low temperature, while the abundant cordierite phase and large pores impede the further densification process at high temperature.Besides, the sample with 2.0 Al 2 O 3 mole ratio exhibits the lowest open porosities in the investigated sintering temperature range.And the open porosity increases with reduced Al 2 O 3 mole ratio, which is consistent with the result of shrinkage process.In the low temperature range, the low sintering activity of the raw materials impeding the densification process with the decrease of Al 2 O 3 mole ratio, resulting in the high open porosity.As a result, the increase of liquid phase by reducing Al 2 O 3 at high temperature cannot obviously improve the densification process.Moreover, the increase of liquid phase can also partially evaporate to produce some bubbles, leading to the increase of open pores.The results of bulk density, as shown in Fig. 6 (b), are in agreement with those of sintering shrinkage percent.However, the bulk density is not sensitive to temperature like the open porosity.This is because that the performance of the substrate other than open porosity

Fig. 6 .
Fig. 6.Open porosity and bulk density of the cordierite sintered bodies with reduced Al 2 O 3 as a function of sintering temperature.