Mechanochemical Activation of Batch During Fabrication of Highly-Porous Cordierite Materials Based on Natural Oxide Compounds

Using X-ray diffraction analysis and infrared spectroscopy techniques, the effect of additives on phase composition variation during mechanochemical activation of a kaolincontaining cordierite batch was investigated. It was shown that introduction of Trilon B permits fixing of the start of cordierite synthesis already during batch activation. It was established that mechanochemical activation of a cordierite batch doped with Trilon B results in an increase of the strength of cordierite samples prepared by polymer matrix duplication.


Introduction
At the moment, one of the most common methods for intensification of solid-state interactions and product quality improvement is mechanochemical activation, which consists in processing of individual solid components or their mixtures in various devices for the purpose of imparting an additional quantity of energy to them [1].Generally [2] this process is divided in two steps: formation of a new surface (material dispersion) and formation and accumulation of crystalline defects (proper activation).Crystalline defects include unit cell defects, dislocations, chemical inhomogeneity, elastic stress, etc. [3,4].Along with such defects, anti-structural defects may occur [4] when an ion or an atom goes over to another sublattice as a result of energy disturbances.In oxides such phenomena are initiated by the presence of non-stoichiometry [5].Most often mechanical activation permits reduction of the initial reaction temperature or ensures the completeness of solid-state synthesis [6][7][8].
This paper investigates the effect of additives on the synthesis of cordierite and the strength of permeable cordierite ceramics obtained by polymer matrix duplication [9,10].materials.Before batch blending the raw stock was previously wet-milled to average grain size of not more than 5 µm and dried.The batch makeup was designed considering maximum approximation to the stoichiometrical cordierite composition and permits obtaining of graphically pure cordierite of a hexagonal modification [10].Powder processing for the purpose of batch blending and activation was carried out in an aqueous medium (powder-towater ratio = 1:1) for 1 h in a "Sand" planetary-type mill at 150 rpm.Ammonium hydroxide (up to pH = 10), glycerin and Trilon B (disodium salt of ethylenediaminetetraacetic acid) were used as additives during aqueous-medium mechanochemical activation.Glycerin and Trilon B, commonly used as surfactants and complexing agents, were added at 1 % of powder weight.
X-ray diffraction phase analysis of powders and sintered materials was carried out on a DRON-3М diffractometer using Co α-radiation.Qualitative phase analysis was performed from diffraction patterns.The presence of specific phases was determined by means of a radiographic mineral identifier and ASTM card register.
IR spectra were obtained using an IFS-66 interference spectrometer (Bruker, Germany).Spectrum recording conditions were: grinding of a sample in an agate mortar, petrolatum oil suspension, KBr glasses, resolution of 1 cm -1 , 100 scans.The spectra obtained were reduced to a zero baseline and complex outlines of absorption bands were separated into individual components determining the wavenumber, halfwidth, peak and integral optical density for each component.Powdered polyphase ceramic materials are structurally nonuniform so outlines of individual bands were approximated by Gaussian functions.The absorbent layer thickness and the amount of petrolatum oil were not controlled when recording the spectra so spectrum intensities varied considerably for various samples.In order to compare the spectra correctly, individual bands were normalized with respect to the total spectrum intensity: A rel. i = D i / ΣD i where A rel. i is the relative intensity, D i is the integral optical density and ΣD i is the total optical density.The spectra obtained were interpreted using literature data [11 -14].
In order to obtain highly-porous samples, a slurry with a disperse-phase-todispersion-medium ratio of 2.0 to 2.2 was subsequently applied onto preactivated polyurethane foam with an average cell diameter of 0.5 to 0.8 mm.After baking polyurethane foam was sintered in air at 1350 to 1390°C.Compressive tests were performed by means of a 2054P-5 testing machine at room temperature and constant strain rate of 2 mm/min.The dependence of the thermal linear expansion factor for cordierite as one of the basic qualitative characteristics of cordierite ceramics on the ratio between intensities of cordierite peaks I 110 (110) d α = 0.490 nm and I 002 (002) d α = 0.468 nm was established in [15].The intensity ratio was calculated from the following: I 110 / (I 110 + I 002 ).The interplanar spacings selected are typical of both hexagonal and orthorhombic cordierite.Cordierite is considered to have satisfactory quality if the peak intensity ratio is from 0.61 to 0.69.

Results and Discussion
Designation and general characterization of cordierite batch activation conditions are given in Tab I.
The batch phase composition achieved after activation is given in Tab.II.Patches of powder diffraction patterns are shown in fig. 1.The presence of enstatite rather than talc in the initial batch appears to result from premilling of raw stock.The greatest changes during activation were observed in quartz and enstatite.The general phase formation characterization was made up using the peak intensity ratio (tab.II).The closely set lines of α-quartz I 100 (101) d α = 0.335 nm and kaolinite I 100 (004) d α = 0.357 nm (I 1 /I 2 ) as well as the lines of enstatite I 100 (420, 221) d α = 0.318 nm and α-quartz I 25 (100) d α = 0.425 nm (I 3 /I 4 ).The activated cordierite batch IR spectra analytical processing data are stated in Tab.III.The isolated absorption peaks may be attributed to two compounds: kaolinite and enstatite -which are present in the batch in the largest amounts.Lines pertaining to quartz, corundum and feldspar could not be isolated individually.

Tab. III Wavenumbers (ν), relative intensities (A) and attribution of spectral lines of cordierite batch powders
Wavenumbers and relative intensities of cordierite batch peaks 0 1 2 3 Attribution of spectral lines ν, cm Fig. 2 shows an attempt of interpreting the experimental findings graphically (the calculated data are given in Tab.IV.Relative intensities for all initial batch peaks are taken for 100 %.The spectral lines are spaced evenly along the abscissa axis as the actual differences are of no significance in this case.Fig. 2 Effect of mechanochemical activation on the intensity of cordierite batch spectral lines: 0 -initial batch (line intensity is taken for 100 %); 1 -batch after activation with Trilon B added; 2 -batch after activation with hydroxide ammonium added; 3 -after activation with glycerin added.Peak intensity in the histogram is summed.
During mechanochemical activation substantial structural changes occur in the batch and they are considerably noticeable in the IR spectra.The most substantial change in peak intensity was observed when Trilon B was introduced in the activation medium.At that, most peaks, which underwent a change in intensity, are characteristic of cordierite IR spectrum (1116, 1081, 1033, 798, 780, 644, 542, 468, 433 cm -1 ) [11].
Thus, the beginning of cordierite synthesis in the kaolin-containing cordierite batch can be established already during mechanochemical activation.Addition of various reagents during activation enables intensification of the changes occurring in the batch composition.Among the alternatives examined, the largest influence is exerted by addition of Trilon B known as a strong complexing agent that appears to facilitate the removal of impurity ions.
Tab. IV Change in the intensity of batch spectral lines during mechanochemical activation (% relative to the initial cordierite batch) Peak intensity, % Wavenumber (ν), cm - General characterization of highly-porous sintered samples prepared by polymer matrix duplication from batch activated samples in various conditions, is given in Tab.V.
Tab. V General characterization of highly-porous cordierite samples Batch T sinter , °С ρ, g/cm 3  Qualitative phase composition Mechanochemical activation of a cordierite batch with ammonium hydroxide added not only leads to the least changes in powder diffraction patterns and IR spectra but also does not permit an increase in the strength of sintered cordierite ceramics.When introducing Trilon B as an additive, it is possible to achieve a significant increase in sample strength.The effect of glycerin appears to result in not only an increase in batch activity but also from its ability to change the flow properties of slurries [16].

Conclusions
During mechanochemical activation of kaolin-containing multicomponent cordierite batch structural changes occur that can be established using X-ray diffraction analysis and IR spectroscopy.
Addition of chemical reagents enables intensification of the batch activation process, where the largest influence was exerted by Trilon B. In the IR spectra of cordierite batch activated with Trilon B added, an increase was observed in the intensity of peaks characteristic of cordierite IR spectrum.
Mechanochemical activation of a cordierite batch with Trilon B added enables a substantial increase of the strength of cordierite samples prepared by polymer matrix duplication.

Tab. I Fig. 1
Fig. 1 Patches of diffraction patterns for cordierite batch and its individual components.Designations: 1 -non-activated initial batch; 2 -batch activated with hydroxide ammonium added; 3 -batch activated with glycerin added;4 -batch activated with Trilon B added; 5, 6talc and kaolin activated with Trilon B added.