Effect of Al(OH) 3 Content on the Microstructure and Strength of Porous Cordierite-Mullite Ceramics Prepared by an In-Situ Pore Forming Technique

: Five porous cordierite-mullite ceramics with similar porosity and different neck characteristics were prepared from Al(OH) 3 , magnesite, silica and clay using an in-situ pore-forming technique. The phase composition, pore and neck characteristics and strength of the porous ceramics were investigated by an X-ray diffractometer (XRD), a scanning electron microscopy (SEM) and a microscopy measured method, etc. The experimental results showed that Al(OH) 3 content had a significant effect on the pore size distribution and neck characteristics (neck size distribution, total value of neck size and phase composition) and then affecting the strength. With an increase in Al(OH) 3 content, the median pore size decreased, the total length of necks and the uniformity of neck size increased, also the mullite content of necks increased, resulting in the increase of strength of the porous cordierite-mullite ceramics. When the Al(OH) 3 content was 64.9 wt%, the porous cordierite-mullite ceramics had the best performance of high apparent porosity of 45.1 % and high compressive strength of 55.9 MPa.


Introduction
Porous cordierite ceramic has been widely used in insulating refractories due to its low thermal conductivity coefficient and high thermal-shock resistances at elevated temperatures [1][2][3]. However, some disadvantages of cordierite, such as a narrow range of firing temperature, a low refractoriness, and poor mechanical properties, limited its application in high-temperature industry. Mullite has a high strength, a high refractoriness, as well as a good thermal-shock resistance [4,5], therefore, the properties of porous cordierite can be improved by adding mullite as a reinforcing phase [6][7][8].
In recent years, considerable attention had been devoted towards the preparation methods of porous ceramics. Usually, a porous structure could be achieved by adding some pore-forming agents [9,10], by gel-casting [11,12] or by foaming method [13], etc. Benhammou [10] fabricated porous cordierite ceramics using oil shale as a natural poreforming agent. Zhang [14] prepared the cordierite-mullite-alumina ceramic foams by impregnation of polymer performs with an optimized stock suspension. Suzuki [15] fabricated porous mullite ceramics by gel-casting. Obviously, the disadvantages of the above methods including the CO 2 emission, the complex process and the high cost limited their application in the refractory industry. The in-situ pore-forming technique [3,7,[16][17][18][19], which utilizes the release of gases (such as H 2 O) caused from the decomposition of solid materials (such as Al(OH) 3 ) to form pores in-situ, is an environmentally friendly way to produce porous ceramics with well-distributed pores.
As we know, porous ceramic consists of pore and solid phase. Yan [1,7] prepared the pore porous ceramics by the in-situ pore forming technique and found that the pore structure was interconnected [1,7] and the solid phase was isolated by pores. Therefore, the strength of porous ceramics mainly depended on the strength of necks, accordant with the sintering theory of M. Randall [20]. In our previous work [3,7,16], the porous cordierite-mullite ceramics had been prepared by the in-situ pore-forming technique and the influences of pore characteristics and phase composition on the strength of the porous ceramics were fully discussed. Unfortunately, until now, the reaction sintering process of the porous cordieritemullite ceramics prepared by the in-situ pore-forming technique and the effects of the neck characteristics on the strength has not been understood. In present work, the porous cordieritemullite ceramics with similar apparent porosity and different neck characteristics were prepared by changing Al(OH) 3 content and the effects of neck characteristics on the strength of the porous ceramics were investigated.

Experimental procedure
Al(OH) 3  The apparent porosity was detected by Archimedes' Principle with water as a medium. The compressive strength of sintered samples was obtained according to Chinese standard GB/B 3997. . Phase analysis was carried out by X-ray diffractometry (XRD, X'Pert Pro, Philips, Netherlands) with a scanning speed of 2°/minute. In addition, the relative contents of the identified phases were calculated by the semi-quantitative analysis in HighScore 3.0 works on the basis of the RIR (=Reference Intensity Ratio) values. The microstructure was observed by a scanning electron microscope (SEM, JSM-6610, JEOL Company, Japan) equipped with energy dispersive X-ray spectroscopy (EDS, QUANTAX200-30, BRUKER Company, Germany). Pore and neck characterizations were obtained by a microscopy measurement method based on Micrographs using image Analysis and Process System (MIAPS) software. And the neck parameters in an area of 1.19 mm 2 of each sample were counted using ten Micrographs. The content and chemical composition of the liquid phase at 1410 ºC were calculated from the SiO 2 -Al 2 O 3 -Fe 2 O 3 -CaO-MgO-K 2 O-Na 2 O-TiO 2 system using the Equilibrium Mode of the Factsage 6.2 thermochemical software. In this calculation, FACT 53 and FToxid databases were chosen, and the liquid phase came from FToxid-SLAG. The viscosity of liquid phase was calculated by the Viscosity Mode of Factsage 6.2 using the glass database.    Apparent porosities and median pore sizes of the samples are shown in Fig. 2. With an increase in Al(OH) 3 content, viz. from sample A to E, the apparent porosity changes little (the error range is ±1% according to Chinese standard GB/T 2998-2001), but the median pore sizes of the samples are rather different. From sample A to C, the median pore size of samples decreases slightly from 134.6μm to 120.0 μm, but from sample C to E, it decreases sharply from 120.0μm to 54.1μm. It confirms that the Al(OH) 3 content of samples gives little effect on the apparent porosity but gives significant effect on the median pore size of the porous ceramics.  In order to investigate the change of the pore size distribution, the micrographs of samples A and E are given in Fig. 4. It can be seen that the distribution of solid phase and pores of sample E is more homogeneous than sample A. And the pore size of sample E is much smaller than sample A. Correspondingly, the neck size and solid particle size of sample E are much smaller than sample A, but the total number of necks of sample A are much larger than sample E.

Discussion
The Al(OH) 3 content strongly affected the phase compositions, microstructures and strengths of the porous ceramics through changing the microstructure evolution during sintering process. The early work [17] proved that Al(OH) 3 and magnesite powders decomposed into pseudomorphs with micropores at 300 ºC and 670 ºC, respectively. When the sample in present work was heated at the temperature above 670 ºC, a skeletal structure which formed from the Al(OH) 3 pseudomorphs (AlPs) would filled with the small powders including magnesite pseudomorphs (MaPs), silica and clay, such as the microstructure of sample E sintered at 900 ºC (Fig. 6). When the temperature continued to rise, the reaction sintering process taking place in the samples could be divided into four stages, as illustrated in Fig. 7.
(1) Formation of local liquid phase. According to our early works [21], MgO decomposed from MgCO 3 firstly reacted with the production of the kaolinite gangue to form local liquid phase with high viscosity at 1200 ºC in the Al(OH) 3 -kaolinite gangue-MgCO 3 system; so, similarly, the magnesite pseudomorphs also firstly reacted with the production of the clay to form local liquid phase in present work before the sintering temperature of 1410 ºC. When the liquid phase contacted with the AlPs and silica, both Al 2 O 3 in the AlPs and the SiO 2 in silica began to dissolve in this liquid (Fig. 7a). The higher the magnesite content, the higher the liquid phase content, so the liquid phase content of sample A was higher than that of sample E (Fig. 8).
(2) Spread and penetration of liquid phase. Once the local liquid phases were formed, they would begin to spread into the AlPs and silica particles and then penetrate into the micropores of the AlPs due to the capillarity (Fig. 7b). The solubility of Al 2 O 3 and SiO 2 into the liquid phase aided the spread and penetration. As a result, the small pores in the AlPs were filled with liquid phase, and the number of small pores decreased greatly (Fig. 3). When the contents of Mg 2+ , Al 3+ and SiO 4 4-in liquid phase reached the equilibrium concentration, cordierite precipitated out (Fig. 7b).
(3) Reaction sintering and particle rearrangement. With the penetration of liquid phase in the AlPs, the dissolution rate of Al 2 O 3 into liquid phase increased because of the increasing contact area between the liquid phase and the AlPs, and then the Al 3+ content in the penetrated liquid phase increased. This would generate the chemical potential gradient of Al 3+ , SiO 4 4-and Mg 2+ between the exterior and interior of the Al(OH) 3 pseudomorph. As a consequence, mullite mainly precipitated in the interior of the Al(OH) 3 pseudomorph (Fig. 7c) and the cordierite mainly precipitated in the exterior. Additionally, the penetration of liquid phase also led to the segregation of the AlPs. When the Al(OH) 3 content was low, the liquid phase content was high and the viscosity of liquid phase was low (Fig. 8, sample A), the fragments of the AlPs were connected into a large particle through rearrangement, and then the micropores content in pseudomorphs decreased and the pores between the pseudomorph and silica particles became larger, as shown in Fig. 7c; but, when the Al(OH) 3 content was high, the liquid phase content was low and the viscosity of liquid phase was high (Fig. 8, sample E), and then the rearrangements of the fragments were inhibited, leaving a lot of pores between the fragments (Fig. 7e); this was the reason why the pore size decreased with an increase in Al(OH) 3 content. (4) Neck formation and particles densification. With the development of the reaction sintering process, the particles became dense and they were connected by necks. When the Al(OH) 3 content was low, the necks between the particles consisted of major interconnected cordierite and minor isolated mullite, whereas, when the Al(OH) 3 content was high, the necks consisted of major interconnected mullite and minor isolated cordierite; which could be verified by the results in Fig. 9.
The compressive strength of the porous ceramics depended on the characteristics of the necks, including neck size distribution and neck phase composition.  On one hand, the neck size distributions of the samples are shown in Fig. 10, and the total numbers, average sizes, total lengths and standard deviation of size of the necks are listed in Tab. IV. It can be seen that with an increase in Al(OH) 3 content, the total number of necks increased but the average neck size decreased because the decrease of liquid phase content in samples (Fig. 8) inhibited the rearrangement of particles and the growth of necks (Fig. 7e). However, the increases of total length of necks and the uniformity of neck size were conducive to enhancing the strength of necks. On the other hand, the necks mainly consisted of mullite and cordierite, and the mullite content of necks increased with an increase in Al(OH) 3 content, as illustrated in Fig. 7, Fig. 11 and Tab. V. As reported in Ref. [22], the strengths of mullite and cordierite were 200 MPa and 120 MPa respectively, so the strength of necks increased with an increase in Al(OH) 3 content without considering the neck size characteristics.
When taking the above two hands into consideration, the increasing neck strength increased the strength of the porous ceramics with an increase in Al(OH) 3 content (Fig. 5). Fig. 11

Conclusions
Five porous cordierite-mullite ceramics with different pore characteristics and neck characteristics were prepared by changing Al(OH) 3 content through an in-situ pore-forming technique. The Al(OH) 3 content had little effect on the apparent porosity but had a significant effect on the phase composition, pore size distribution, neck characteristics and strength.
With an increase in Al(OH) 3 content, the median pore size decreased, the total length of necks and the uniformity of neck size increased, also the mullite content of necks increased, resulting in the increase of strength of the porous cordierite-mullite ceramics. When the Al(OH) 3 content was 64.9 wt%, the porous cordierite-mullite ceramics consisting of 62 wt% mullite and 24 wt% cordierite had the best performance of high apparent porosity of 45.1 % and high compressive strength of 55.9 MPa.