Infulence of Mechanochemical Activation of a Charge on Properties of Mullite-Tialite Materials

The influence of mechanochemical activation (MCA) of a kaolin-containing charge on the strength of mullite-tialite materials (obtained using methods of semidry molding and polymeric matrix duplication) was studied. It is shown that spectral and X-ray indexes of crystallinity of kaolin activated under similar conditions could be used as criteria of MCA efficiency. Parameters of mullite-tialite charge (containing kaolin, alumina and anatase) activation were optimized.


Introduction
Under modern conditions, when ecological problems in industrialized regions acquire still more acuity, a new approach to the use of raw materials is required.Depletion of traditional first-grade raw material deposits and rise of transportation costs necessitate introduction of second-grade raw materials with a lower content of required components and large amount of impurities into industrial production.Thus, one of the most important tasks posed for ceramic material science is complex use of mineral raw materials (including second-grade ones) without profound processing [1].
Many natural ceramic materials when sintered produce (as one of the crystal phases) mullite (3Al 2 O 3 •2SiO 2 ), which is remarkable for its high refractory properties, strength and chemical resistance.To obtain roentgenographically pure mullite (where impurity phases do not exceed 3-5 %), stoichiometric compositions of kaolins with alumina are used.Introduction of additives that improve thermal stability of mullite ceramics enable obtaining of products resistant to thermal shock.Aluminium titanate (tialite) is one of these additives.Mullite-tialite materials consisting of mullite, tialite and a small amount of a glassy phase, can be obtained by sintering of a charge that contains kaolin, alumina and titanium dioxide.
This paper studies the influence of mechanochemical activation (MCA) conditions on the strength of mullite-tialite materials obtained by methods of semidry molding [2] and polymeric matrix duplication [3,4].powders for charge mixing and activation was performed (dry and in an aqueous medium with the powder / water ratio of 1:1) during 1 hour using a planetary grinding mill "Sand" with a rotation speed of 150 rpm.Hydrochloric acid (рН up to 1), ammonia (рН up to 10), glycerol, Trilon B (disodium salt of ethylenediaminetetraacetic acid), carbamide and ethanol were used as additives during mechanochemical activation in aqueous medium.Glycerol and other agents, generally used as complexing agents and surface-active agents, were added in the amount of 1 % of the powder weight.
Cylindrical specimens were made from activated powders by means of compaction under pressure of 50 MPa.These specimens were heat treated in electric furnaces with lanthanum-chromite heaters during 1.5 hours at the temperature of 1580 0 С.In order to obtain highly porous specimens, we prepared a slip (with a dispersed phase and dispersion medium ratio of 2.0-2.2),which later on was applied on preactivated foamed polyurethane with a cell diameter of 0.5-0.8mm.After foamed polyurethane annealing, highly porous specimens were sintered similar to molded materials.Tests of tensile strength at compression were made using the 2054Р-5 machine at room temperature with a constant deformation rate of 2 mm/min.
In order to characterize changes in the crystalline structure of kaolins, the X-ray Hinckley index [5,6] and spectral index [6,7] were used.X-ray structure phase analysis was performed using a DRON-4 diffractometer with Со-α-radiation.The X-ray crystallinity index С was determined from relations of difrractogram peaks of kaolinite [2,5].Infrared spectrums were obtained using the conventional technique on a «Specord M-80» spectrometer.The spectral crystallinity index А was determined using the following formula: where: A -is the spectral crystallinity index; I total -total intensity; I bckg -background intensity; I 1100 and I 1035 -intensities of absorption peaks.
In order to obtain general characteristics of the phase composition, relations between intensities of peaks of crystalline substances were used.We chose closely set lines of mullite I 1 (210) d α = 0.339 nm, I = 100 %, mullite I 2 (120) d α = 0.343 nm, I = 95 % and tialite I 3 (110) d α = 0.336 nm, I = 100 % as the base lines.The I 1 /I 2 ratio enabled mullite characterization, whereas I 1 /I 3 and I 2 /I 3 showed the relative content of tialite in the materials.

Results and Discussion
The process of charge mix preparation requiring mechanical mixing of components (either dry or in aquatic medium), could be considered as a process of mechanochemical activation (MCA) of the charge.Usually, MCA is associated with distortions of the crystal lattice and development of various defects in its structure [8].Table 1 shows the crystallinity index of Kyshtym deposit kaolin when activated, dry and in aqueous medium.
It is known [5] that the X-ray Hinckley index is sensitive to displacement of kaolin layers and to the 120-degree turn of separate layers, and the more ordered is the structure, the higher is the factor.The spectral crystallinity index characterizes the status of silicon-oxygen and alumina-oxygen covalent bonds, and the more ordered is the structure, the lower is the index [5][6][7].
Kyshtym deposit kaolin compared to first-grade kaolins [2,5] is characterized by irregularity of both kaolin layers and covalent bonds status.Dry activation virtually does not affect silicon-oxygen and alumina-oxygen bonds, but it results in significant displacement and turning of separate layers.Activation in neutral water medium has very little influence (if any) on the structure of kaolin.Activation in acid medium improves the spectral crystallinity index significantly without changing the arrangement of layers.This improvement apparently occurs due to the removal of isomorphically-substituted ions.Activation in alkaline medium results in an increase of the kaolin structure irregularity.
A study of diffractograms of the activated charge, which contain kaolin, alumina and titanium dioxide, showed that during the activation process phase-formation reactions occurred in the charge.The composition of products generated is unidentifiable.However, the influence of activation conditions on this composition is unambiguously verified by roentgenography.Table 2 contains general characteristics of mullite-tialite materials, obtained using semidry molding of the activated charge.In all the cases studied, the material composition (along with tialite) included two types of mullite: stochiometric and the so-called calcined, which is generated during kaolinite calcination.The most remarkable thing about calcined mullite is that there is no peak d α = 0.343 nm on the material's difractogram in the interval of Bragg's angles concerned.Change in the ratio of peaks I 1 /I 2 to 1.70 instead of 1.05 verifies the presence of both types of mullite.
The largest amount of calcined mullite could be found in specimens after charge activation in an alkaline medium.Addition of surface-active and complexing agents in the water medium reduces the relative content of tialite.A relationship between material strength and phase composition was not found.
Fig. 1 represents bar charts, which illustrate the strength of materials obtained using semidry molding (а) and polymeric matrix duplication (b) from activated charge.
The strongest molded samples were obtained from the charge activated in acid medium.Among all the additives used, the most positive influence on strength was exerted by trilon B. In both cases, activation apparently resulted in removal of part of isomorphically substituted ions.
The highest strength of high-porous samples was noted after activation of the charge in acid and neutral media, and also in water medium with addition of glycerol.General characteristics of highly porous mullite-tialite materials obtained by the method of polymeric matrix duplication are given in Table 3.

Material designation
Activation medium ρ, g/cm 3  σ compr., MPa The difference in density of highly porous materials is determined by different covering power of slip, since conditions of slip preparation and application on a polymeric matrix were identical.
The phase composition of mullite-tialite materials, obtained using semidry molding and polymeric matrix duplication (like in the case of some other polyphase materials [4,10]) is different.Highly porous materials have a higher abundance ratio of mullite, and they contain more calcined mullite.There is no immediate dependence of strength on the phase ratio neither for highly porous nor for molded materials; however, one may note that the most strong samples are characterized by a higher abundance ratio of calcined mullite and lower abundance ratio of tialite.
Figure 2 represents diagrams of the dependence of the strength of molded (Table 2) and highly porous (Table 3) samples on the crystallinity index of activated kaolin (Table 1).
The strength of both molded and highly porous mullite-tialite samples depends on the value of the spectral crystallinity index, i.e. on the status of silicon-oxygen and alumina- oxygen bonds.Spectral crystallinity decrease and increase of structure orderliness result in an increase of strength.It should be noted that among raw materials included in the composition of the charge kaolin as a representative of laminar aluminosilicates is the most susceptible to activation components.Fig. 2 Influence of crystallinity index of kaolin on strength at compression of molded (firm line) and highly porous (dotted line) mullite-tialite samples.Thus, mechanochemical activation of mullite-tialite charge should be aimed, first of all, at reduction of spectral index kaolin crystallinity.Such an effect could be achieved using activation in water medium with additives, which facilitate removal of isomorphically bonded ions from the crystal lattice.
The influence of the X-ray crystallinity index on the strength of molded mullite-tialite materials was not discovered.With highly porous materials, the higher the X-ray crystallinity index (and, therefore, the more orderly kaolin layers are positioned), the higher is the strength.

Conclusion
• The crystallinity index of kaolin activated under similar conditions may be used as criteria of mechanochemical activation of mullite-tialite charge containing kaolin, alumina and anatase.
• Activation should be aimed first of all at reduction of the spectral crystallinity index, i.e. at removal of isomorphically substituted ions in the structure of kaolin.
• To obtain highly porous materials, it is necessary to control the X-ray index (which during the MCA process should not fall considerably).
• Of all variants of charge activation for manufacturing articles using the method of semidry molding or the method of polymeric matrix duplication, the highest strength may be obtained for activation in a water medium with рН=1.

Fig. 1 -
Fig. 1-Strength at compression of molded (а) and highly porous (b) materials made from activated charge.

Table 1 -
Influence of activation on the kaolin crystallinity index

Table 2 -
General characteristics of materials, obtained using semidry molding of the activated charge