Low Temperature Synthesis of Porous Silicate Ceramics

Impregnation of a polyurethane sponge with kaolin, feldspar, silica, fusible glass slurry followed by temperature treatment in air in the temperature range 800-1000 C leads to the formation of aluminosilicate ceramics with a set pore size. The low-temperature synthesis of porous ceramics is based on the stage-by-stage formation of low-temperature eutectics and thermodestruction of polyurethane sponge.


Introduction
Porous ceramics is extensively used in different fields of engineering [1][2][3][4].For porous ceramics used as a filtering material, its porous structure and properties connected with it, namely, pore size, permeability, specific surface area, etc., are of determining significance.At present, the most extensively used manufacturing methods of porous permeable ceramics are the following: processing of monofractional starting materials; foaming method; method of burning-out of additives; chemical method of pore formation [5][6].Each manufacturing method of porous permeable ceramics has both advantages and essential disadvantages.Specifically, ceramics prepared by the foaming method and chemical method of pore formation is characterized by low permeability due to predominantly closed porosity, whereas ceramics obtained by the method of burning-out of additives has an inhomogeneous porous structure, which is connected with the fact that a homogeneous distribution of components in the volume of the mixture is difficult to obtain.This is why the search for new manufacturing technologies for porous ceramics is being continuously carried out.
The aim of the work is to perform low-temperature synthesis of kaolin-based porous ceramics with a certain pore size by the method of burning-out of additives.А decrease in the sintering temperature of ceramics is expected using fusible silicate additives that serve as a binder of metakaolin or sillimanite grains.Polyurethane sponge was chosen as a porous matrix.

Preparation of samples and experimental procedure
In the work, a mixture of kaolin (35 wt.%) with feldspar (30 wt.%) and quartz (35 wt.%) additives was used.This mixture is referred to as the "paste".Such a mixture is usually used in manufacturing of art ceramics by the method of molding in gypsum molds.Feldspar serves as a diluent.Ground glass was introduced in the composition, and its content was varied from 4 to 80 %.A slurry was prepared by adding water to the paste.
To record structural-phase transformations proceeding during sintering and to determine sintering conditions providing the synthesis of a strong product, the prepared slurry was poured in gypsum moulds and held in them until separation from the moulds.The obtained intermediate product in the form of disks 40 mm in diameter and 10 mm in thickness was dried under natural conditions.Then the disks were placed in a furnace.The sintering temperature was varied from 800 to 1000 0 C. The sintering time ranged from 60 to 120 min.In the preparation of porous ceramics, polyurethane sponges of different pore size were used: (7÷0.1)mm (I type), (0.5÷2) mm (II type), and (0.1÷1) mm (III type).The sponge was impregnated by the slurry with continuous stirring.The sponge size was 100 x 50 x 50 mm.After drying under natural conditions, the semiproduct was heat treated in a furnace in the temperature range 800-1000 0 C. The heat treatment time was varied from 60 to 120 min.
The structural-phase transformations were investigated by X-ray diffraction in Cu Kά radiation (diffractometer Siemens D-500), IR spectroscopy (Specord M 80 spectrometer), electron microscopy (Jeol 6400), and the adsorption-structural static volume method (ASAP-2000M).Particle size was measured on a laser particle sizer in wet dispersing regimen with the using of continuous stirring and ultrasound application (Analysette 22 COMPACT unit).

Powder characteristics
In Tab.I, kaolin, feldspar, and glass compositions are presented.In Tab.II and Fig. 1, characteristics of paste and glass powders are given.From these data it is evident that the content of particles with sizes 1-5 µm in crushed glass is smaller than in the paste.Moreover, large size particles are present.In the paste, large particles (which are likely to be aggregates of kaolin or feldspar) are smaller than the large particles in glass.

X-ray data
In Fig. 1, changes of crystallograms of the paste-glass mixture with increasing sintering temperature are illustrated.Along with the disappearance of the lines of kaolin, a decrease in the feldspar content and the formation of aluminosilicate (sillimanite) is noted (Fig. 2 and 3).The presence of a halo in the crystallograms, indicates the presence of an amorphous phase in the samples.As a rule, the halo intensity appears to be maximal in the 850-950 0 C range of sintering temperatures.
In the high-temperature region, lines assigned to silicates of different compositions, specifically, CaAl 2 SiO 6 , CaSiO 3 , Na 2 Si 2 O 5 , Na 2 Ca 6 Si 4 O 15 , inorganic aluminosilicate (K, Na)AlSi 3 O 8 , etc., are registered.Both the structure and content of inorganic silicates change significantly differ for mixtures of different compositions and change with increasing sintering temperature.In view of the presence of a large number of silicates of different composition, their insignificant contents, and the superposition of their X-ray diffraction lines, identification of the whole set of formed silicates is complicated.
At T s = 800 0 C, changes in the feldspar and quartz contents are connected with the change in the content of the paste in the mixtures (Fig. 4a).At T s = 900 0 C, the larger the nce of Sintering, 39 (2007) 39-49   The change of halo intensity and lines of quartz specifies active participation of an amorphous phase (glass) and SiO 2 in the formation of silicates.

IR-data
In Tab. 3 and Fig. 5, IR-spectra of the investigated samples are presented.The IRspectrum of the initial paste (slurry) (Fig. 5a) is the superposition of the spectra of kaolin and feldspar [7,8].

Porous ceramic
Since polypropylene sponge was used as a porous ceramic matrix the character of its thermodestruction called for investigation.During heat treatment in air, the largest weight loss occurs in the temperature range 160-400 0 C (Fig. 6).The internal surface of pores in ceramics of type I, II, and III is different (Fig. 8).In ceramics of type I it has a "glazed" view in greater pores (Fig. 8 a').In ceramics of type III, the surface view surface corresponds to a higher degree to a ceramic surface (Fig. 8 b').It should be noted that, in all types of porous ceramics, depending on the pore size, both extreme cases, and their combination can be observed.According to the X-ray microanalysis data, as the pore size decreases, the number of elements which enter the composition of aluminum silicate (Al, Fe, Ti) increases, and the number of elements which enter the composition of glass (Na, Ca, Mg) decreases.Taking into account the disperse composition of glass and paste particles, it can be assumed that this is caused by the difference between the particle size and the pore size.Large glass particles cannot penetrate into small pores, whereas small paste particles can easily enter them.
The structural-phase reorganization of the components of the mixtures does not differ from that in pore-free ceramics.

Discussion
Synthesis of porous silicate ceramics using porous polyurethane sponge is well known (foam rubber) [9,10].Porous glass is most often obtained from ground broken domestic and industrial glass by the foaming method [11].However, in the preparation of porous ceramics on the base of clay minerals, ground glass is not introduced into the slurry.The sintering temperature ranges from 1300 to 1500 0 C. At the same time, a polyurethane sponge is not used in the manufacturing of porous glass, since the formation of a porous structure homogeneous over the volume with a homogeneous pore size fails, though the sintering temperature is not higher than 1000 0 C.In principle, this is caused by the size of glass particles.In the preparation of ceramics one should avoid slurry lamination and precipitation of glass particles on the bath bottom, whereas in the synthesis of foam glass, the problems of saturation of the whole volume of the sponge with glass particles and extremely critical conditions of sintering together of particles rather than their melting takes arise.
The fraction of glass used in the work and continuous stirring of the slurry enabled lamination prevention and provided its penetration into the bulk of the sponge.Moreover, the distribution of kaolin particles (aggregates) among glass particles enabled us to prevent transformation of the porous body in a monolithic body at temperatures higher than the glass melting point.
Taking into account that the manufacturing conditions of this ceramics are close to those of fine ceramics [12], it should be noted that, in the synthesis of porcelain, a large number of fusing agents is used.This is why, during heating, a large amount of liquid phase forms in tiles.In the manufacture of faience, the liquid phase is practically absent.Depending on the requirements of a ceramic product, its sintering can be performed in a wide temperature range 1100-1420 0 C.
The obtained data on the change in the kaolin content in the synthesis of mixtures with a large paste content in the temperature range 800-1000 0 C are described in the framework of decomposition of kaolin and its transformation into metakaolinite-sillimanite [13].
Softening of feldspars begins at 1000-1100 0 C, whereras for the mixture of feldspar and quartz, which is present in kaolinite, an eutectic with a melting point 990 0 C is characteristic [14].This is why, in the feldspar-silica eutectic melt, dissolution of kaolinite anhydride must begin.Moreover, in the considered case, ground sodium silicate glass, the softening of which begins at 650-700 0 C, was introduced into the kaoline-feldspar-silica mixture.At T ≥ 800 0 C, in the glass melt, kaolinite anhydride, feldspar, and quartz grains are present.The disappearance of feldspar, a decrease in the quartz content, a decrease in the intensity of the halo in the X-ray diffraction pattern, identification of newly formed silicates and mullite indicate that transformations are realized not only in the Al 2 O 3 -SiO 2 binary system, but also phase formation in the Na   9).In this case, the glass melt and eutectic melt play the role of nepheline syenite, which is added to speed up the sintering process [12].This is why, not only strong tiles, but also a porous matrix can be obtained at rather low temperatures.The decrease in the intensity of the halo (the decrease in the amount of the glass phase) and the presence of newly formed silicates indicates that aluminosilicate particles are bound by glass ceramics (partially crystallized glass).In Fig. 10, a simplified scheme of formation of porous silicate ceramics is presented.

Conclusions
1. On the base of a kaoline-quartz-feldspar-ground fusible glass composition, porous ceramics was obtained in the temperature range 800-1000 0 C. 2. The low-temperature synthesis of ceramics is based on the stage-by-stage formation of lowtemperature eutectics between feldspar, silica, and newly formed products of the interaction between the glass melt and aluminosilicate.3. The use of the fine fraction of ground glass and continuous stirring conditions enabled prevention of lamination of the slurry and impregnation of the polyurethane sponge over the whole volume.4. The use of polyurethane sponges with a certain pore size enabled us to prepare porous ceramics with a controlled pore-size range.

Fig. 1
Fig. 1 Particle-size distribution in paste (a) and ground glass waste (b).

Fig. 6
Fig. 6 Weight loss of sponges during treatment in air.t tr = 120 min.

Fig. 7
Fig. 7 The view of porous ceramics obtained on the base of sponges with different pore size.(a) from sponge of type I; (b) type II; (c) type III.The composition of the slurry 80 wt.% paste + 20 wt.% glass.T s = 850 0 C; t s = 120 min.

Fig. 8
Fig. 8 Micrographs of the internal surface of pores for ceramics of type I (a, a') and type III (b, b').

Fig. 10
Fig. 10 Simplified scheme of the formation of porous ceramics.