Effect of Particle Size on the Pore Characterization and Strength of Porous Cordierite-mullite Ceramics Prepared by a Pore-forming in-situ Technique

The porous cordierite-mullite ceramics were prepared by the pore-forming in-situ technique. The characterizations of porous cordierite-mullite ceramics were determined by an X-ray diffractometer (XRD), a scanning electron microscopy (SEM), and a microscopy measured method, etc., and the effect of particle size on phase composition, pore characterization and strength were investigated. It’s found that particle size affects strongly the formations of cordierite and mullite, and then changes the pore characterization and strength. With the decrease of the particle size, the sintering temperature at which the formations of cordierite and mullite take place extremely fast decreases, the pore size distribution becomes from bi-peak mode to mono-peak mode, the porosity and the median pore size decrease but strength increases. The most opposite mode is the specimen sintered at 1400 oC from the grinded powder with an average particle size of 10.2 μm, which consists of cordierite, mullite and minor spinel, and has a high apparent porosity (40 %), a high compressive strength (58.4 MPa), a small median pore size (6.3 μm) and well-developed necks between particles.


Introduction
With the increasing demand of saving energy and resource, porous ceramics with excellent properties and their environment-friendly fabrication methods have attracted extensive attentions.The properties of porous ceramics greatly depend on the porosity, the pore size distribution and the compositions.Smaller pore size and homogenous pore distribution are thought to be helpful to improve the strength of porous ceramics and decrease the coefficient of thermal conductivity [1][2][3][4][5][6][7].Additionally, the compositions of porous ceramics with lower coefficient of thermal expansion and higher strength are propitious to enhance the high-temperature properties.Usually, a porous structure can be achieved by a conventional powder-processing route with the incorporation of some pore-forming agents such as starch, graphite or organic particulates, or by gel casting, or by injection molding [2].
But the CO 2 emission and the complexity of the fabrication methods limit the development of the porous ceramics.The pore-forming in-situ technique is a good and environment friendly way to prepare porous ceramics with well-distributed pores because it does not produce any more carbon oxide, which utilizing the decomposition of the raw materials to form pores [1][2][3][4][5][6][7].Cordierite-mullite refractory materials are very widely used in ceramic industry as support parts in furnace due to their good thermal-mechanical and thermal shock resistance properties, because cordierite supplies considerable resistance to thermal shock while mullite provides the required strength [8][9][10][11][12][13][14][15][16].Porous ceramics based on cordierite-mullite composite should have excellent properties.
But until now, there are few papers dealing with the porous cordierite-mullite ceramics.In the early work, we have prepared the porous cordierite-mullite ceramics with high porosity and strength through the pore-forming in-situ technique, and investigated the effects of sintering temperature and phase composition on the properties of the cordieritemullite ceramics [15,16].However, the median pore sizes of the porous cordierite-mullite ceramics are > 40 μm, which are harmful to the thermal insulation.In order to decrease the coefficient of thermal conductivity and increase the strength of the cordierite-mullite ceramics, optimizing the pore structure through changing the particle size is an alternative.The particle size has great effect on the reaction sintering, and thus can affect the pore characterization and strength.This will be addressed in the present paper.

Experimental procedure
Talc, kaolinite gangue, Al(OH) 3 , magnesite and silica were used as raw materials.The chemical compositions of raw materials were listed in Tab.I.The powder mixture was consisted of 5.77 wt% talc, 25.28 wt% kaolinite gangue, 42.98 wt% Al(OH) 3 , 8.66 wt% magnesite and 17.31 wt% silica.The above powder was mixed using alumina balls for 3h.After mixed, the powder mixture is named as A, and the average particle size of the mixture A measured by a laser particle size analyzer (Matersizer 2000) was 25.6 μm.Subsequently the mixture A was grinded using tungsten carbide balls for 1h, 4h and 7h, respectively.The average particle sizes of the grinded powders were 17.6 µm, 10.2 and 8.5 µm respectively, and the grinded powders were named as B, C and D corresponding to their average particle sizes.The powders were pressed in cylinders with a height of 36mm and diameter of 36mm at a pressure of about 50MPa and the green compacts after drying at 110°C were heated at different temperature (1370°C-1430°C) for 180 min in an electric furnace, and then furnacecooled.

Tab. I Chemical compositions of raw materials (wt%).
SiO The apparent porosities were detected by Archimedes' Principle with water as medium.The compressive strengths of sintered specimens were measured at room temperature.Phase analysis was carried out by an X-ray diffractometer (Philips Xpert TMP) with a scanning speed of 2° per minute.Pore size distribution and median pore size were obtained by a microscopy measurement method [15][16][17] through an optical microscope (Axioskop 40).Microstructures were observed by a scanning electron microscope (Philips XL30).The liquid phase content in specimen was calculated from the Equilib Mode of SiO 2 -Al 2 O 3 -Fe 2 O 3 -CaO-MgO-K 2 O-Na 2 O-TiO 2 system by the FactSage ® thermochemical software (version 6.1).In this calculation, ELEM, FACT 53 and FToxid databases were used, and the liquid phase came from FToxid-SLAGA.

Phase identification
XRD patterns of specimens sintered at 1370°C and 1400°C are shown in Fig. 1.When the sintering temperature is 1430°C, the phases of specimens A, B, C and D are major cordierite (2MgO•2Al 2 O 3 •5SiO 2 ) and mullite (3Al 2 O 3 •2SiO 2 ), and minor spinel (MgAl 2 O 4 ), so their XRD patterns are not given here.When the average particle size is 25.6 μm, the phase composition of specimen A sintered at 1370°C are cordierite, mullite, spinel, corundum (Al 2 O 3 ) and quartz (SiO 2 ), with the increase of sintering temperature to 1400°C, quartz in specimen A disappears.When the average particle size decreases to 17.6 μm, the phases of specimen B sintered at 1370°C are cordierite, mullite, spinel, corundum, no quartz, with the increase of sintering temperature to 1400°C, the phases of specimen B are cordierite, mullite, spinel.When the average particle size decreased to 10.2 and 8.5 µm, the phases of specimens sintered at different temperature are major cordierite and mullite, minor spinel.It means that the cordierite-mullite ceramics were synthesized successfully and the effect of particle size on phase composition was very obvious.Especially, when the particle size decreases from 25.6 µm to 10.2 µm, the sintering temperature at which the formations of cordierite and mullite take place extremely fast decrease from 1430ºC to 1370ºC.

Pore characterization
Fig. 2. shows the apparent porosities and median pore sizes of specimens sintered at different temperature.When the sintering temperatures are 1370°C and 1400°C, with the decrease of average particle size, viz.from specimen A to D, the apparent porosities and median pore sizes decrease gradually.It is noted that when the average particle sizes are more than 10 µm, the apparent porosities of specimens A, B and C sintered at 1370°C and 1400°C are more than 40% and the median pore sizes are less than 17.3 μm.When the sintering temperature increases to 1430°C, the apparent porosities of specimens B, C and D are less than 40%, and with the decrease of average particle size the apparent porosity decreases sharply; simultaneously, median pore sizes of specimens A and B increase greatly, to 42.4 µm and 27.1 µm, respectively.It confirms that the particle size gives strong effect on the sintering of the porous cordierite-mullite ceramics.The pore size distributions of the specimens sintered at 1370°C and 1430°C are shown in Fig. 3.When the sintering temperature is 1370°C, the pore size distributions of specimens A and B are bimodal.The curve of big pore of specimen A with an average particle size of 25.6 μm is in the range of 100 to 900 μm, when the particle size decreases to 17.6 µm, the curve of big pore of specimen B is in the range of 30 to 300 μm, whereas, when the particle sizes decrease to 10.  Curves of cumulative porous volume (%) versus pore diameter were plotted (Fig. 4.).When the sintering temperature is 1370°C, the cumulative pore-size distribution curves shifted toward lower values with the decrease of the particle size.The median pore sizes of the pores were 11.9, 10.1, 5.9 and 5.8 μm for specimens A, B, C and D, respectively.When the sintering temperature is 1430°C, comparing with the specimens sintered at 1370°C, the curves shift toward right, and the difference between the curves become more obvious.However, the cumulative pore-size distribution curve of specimen C is in the right side of that of specimen D, which may relate to measurement error.
Obviously, the particle size and the sintering temperature give strong effect on the pore characterization.The porosity and pore size distribution of the specimens depend on three factors: 1) The porosity of green compact which depends on the particle size distribution of raw powder and the pressure of compacting.
2) The decomposition of Al(OH) 3 and magnesite and the ignition losses of kaolinite gangue and talc.
3) Reaction sintering.Among the three factors mentioned above, the factor 2 can be ignored because the four mixture powders have the same compositions.The particle size plays three roles.Firstly, smaller particle can decrease the size of pore resulted from particle packing.Secondly, smaller particle also decrease the porosity of green compact.Lastly, smaller particle can enhance the reaction sintering by promoting mass transport [5].As can be seen in the Fig. 5., the microstructures of specimen B and C are rather different.In specimen B with an average particle size of 17.6 µm, there were two types of pores, one are located in the particles, the other are located at particle intersections.Obviously, the latter pore size is larger.When the particle size decreases to 10.2 µm, the size of pore located at particle intersection decreases, the difference between the two types of pores becomes smaller.And thus, with the decrease of the particle size, on the one hand, the difference between the two types of pores become smaller, and then the pore size distribution becomes from bimodal to unimodal and shifts toward lower values (Fig. 3. and Fig. 4.); on the other hand, the porosity decreases and the number of necks between particles increases.Additionally, the sintering temperature affects the pore characteristics changes because of the formation of cordierite and mullite and the sintering.The formation of cordierite and mullite take place only by interdiffusion of Al 3+ , Mg 2+ and SiO 4 4-between the five raw material powders [15].The liquid is a shortcut to diffusion.The liquid phase contents in specimens at 1370ºC, 1400ºC and 1430ºC are 3.51 wt%, 4.88wt and 9.20 wt%, respectively, as shown in Fig. 6.The liquid improves the reaction sintering between particles; simultaneously, the smaller size of particle increases the number of contacting grains.The dual effects promote the formation of cordierite and mullite on the one hand, and result in the decrease of pore volume, the growth of pore and the formation of well-developed necks on the other hand (Fig. 5.).So the sintering temperature at which the formations of cordierite and mullite take place extremely fast decreases, and with the increase of the sintering temperature the curve of pore size distribution of specimen shifts toward higher values (Fig. 3. and Fig.

Compressive strength
The compressive strengths of specimens sintered at different temperature are shown in Fig. 7.The strength increases with the decrease of the particle size and the increase of sintering temperature.The increase of strength may come from three hands.The first is the decreasing porosity (Fig. 2.).The secondary is the small median pore size (Fig. 2.).The last and most important is the formation of well-developed necks between the particles (Fig. 5.).

Conclusions
Particle size affects strongly the formations of cordierite and mullite, and then changes the pore characterizations and strengths of porous cordierite-mullite ceramics.When the particle size decreases from 25.6 µm to 10.2 µm, the sintering temperature at which the formations of cordierite and mullite take place extremely fast decrease from 1430ºC to 1370ºC.With the decrease of the particle size, the pore size distribution becomes from bimodal to unimodal, and the porosity and the median pore size decreases.The decrease of particle size is propitious to the formation of the well-developed necks, which can increase the strength.But when the particle size decreases to 8.5 µm, the degree of sintering increases, resulting in the densification, which is harmful to porous ceramics.
The most apposite mode is the specimen sintered at 1400ºC from the grinded powder with an average particle size of 10.2 µm, which consists of cordierite, mullite and minor spinel, and has a high apparent porosity (40 %), a high compressive strength (58.4 MPa), a small median pore size (6.3

Fig. 2 .
Fig. 2. Apparent porosities and median pore sizes of specimens sintered at different temperature.
2 and 8.5 µm, the pore size distributions of specimens C and D become unimodal, and are in the range of 4 to 100 μm.When the sintering temperature is 1430°C, the pore size distributions of all specimens are almost unimodal.Comparing with the results shown in Fig. 3. (1370°C), the curves in Fig. 3. (1430°C) shift toward right considerably.