Mechanically activated annealing

Gaffet, E., Malhouroux-Gaffet, N. NanocrystaUine MoSia phase formation induced by mechanically activated annealing. J. AUoy. Compd. 205, 27-34 (1994)... 

Figure 1. XRD patterns for TiH2 upon mechanical activation and annealing under a N2 atmosphere 1 -initial titanium hydride 2 - upon mechanical activation for 1 h 3,4 -upon mechanical activation and annealing at 300°C and 500°C, respectively. ( )-TiH2 (+)-TiN ( .)- Ti20.

Kaolinite is transformed into X-ray amorphous state when activated in air. According to authors [14,15], amorphization involves the destruction of bonds between tetrahedral and octahedral layers inside the package, till the decomposition into amorphous aluminium and silicon oxides. Other researchers [ 16,17] consider that amorphized kaolinite conserves the initial ordering of the positions of silicon atoms while disordering of the structure is due to the rupture of A1 - OH, Si - O - A1 bonds and the formation of molecular water. Endothermic effect of the dehydration of activated kaolinite is shifted to lower temperatures while intensive exo-effect with a maximum at 980°C still conserves. When mechanically activated kaolinite annealed at 1(X)0°C, only mullite (3Al20j-2Si0j) and X-ray amorphous SiOj are observed. In this case, the phase with spinel structure which is formed under thermal treatment of non-activated kaolinite is not observed thus, mechanical activation leads to the formation of other phases. 

The conventional methods of mullite preparation are the following crystallization from melt, high-temperature sintering of oxides, high-temperature annealing of kaolin [32]. Mechanical activation of hydrated oxides brings to decrease of mullite synthesis temperature [32-39]. 

Mechanical activation of Ca(OH)2 is not accompanied by the change in the position of Ca 2p3/2 line. For activated mixture of Ca(OH)2 with HTD, its maximum is shifted, and the bond energy increases by 0.5 eV with respect to the final product, CaTiOs. After annealing of activated mixture at 1000°C and the formation of CaTiOs, a positive shift of Ca 2pin line disappears. 

Figure 6.16. X-ray patterns of zirconium titanates synthesized by mechanical activation and subsequent annealing at 1260°C for 4 h of Zr02nH20+Ti02 H20 mixtures.

Fig. 7.1 shows X-ray diffraction patterns of a mixture of anhydrous magnesium, aluminium and silicon oxides and a mixture of correspondent hydroxides after mechanical activation in a planetary mill followed by annealing at 1260°C for 2 h. In the former case, the reflections of spinel MgAlA (0.245, 0.207,... 

Hexagonal celsian can be transformed into monoclinic modification by mechanical activation of the mixture with modifiers (KF, LiCl at a level of 1-5 mol. %) followed by annealing at 1100°C. 

Figure 7.4. XRD profiles of BaO+Al(OH)j+2Si02aq mixture after mechanical activation and annealing at I200°C for 2 h.

The application of mechanical activation for the synthesis of the mentioned above compounds started with the activation of the mixtures of anhydrous oxides [15]. Then, barium oxide (or carbonate) in the mixture was replaced with barium nitrate [16], while copper oxide was replaced with copper hydroxide or basic carbonate Cu2C03(0H)2 [17]. Later, yttrium and copper nitrates were used instead of oxides, while barium hydroxide Ba(OH)2 or Ba02 were used as barium-containing compound [18] finally, the mixture of oxides was activated in the presence of water added for the purpose of obtaining well-molding pastes [19]. After activation, the mixtures were annealed at increased temperature, but the synthesis... 

It was demonstrated that the use of mechanical activation allows to decrease the temperature of consequent thermal treatment by 200-300°C and to decrease the annealing time, thus, providing the materials of high homogeneity and high dispersion. The quality of materials as prepared is similar or even better than of those prepared by traditional methods of synthesis. The perspectives of the method for chemical industry have been demonstrated using specific examples. 

Mechanical activation of the starting compounds was carried out using high-powered El or AGO planetary ball mills [5]. The ratio of the weights of milling balls and powders was equal to 10 time of activation -1-30 min acceleration (achieved in the drums of the mills) - 40 and 60 g, respectively. After mechanical treatment the powders were air annealed for 2-4 hours at temperatures ranging from 550 to 1000°C. 

In the present study, synthesis of doped ATLS with general formula Laio-x-zAxSi6-yBy027-x/2-y/2-3z/2 (A = Sr B = Al, Fe x = 0- 3 y = 0- 1.5 z = 0 0.67) has been conducted via mechanical activation (MA) of solid precursors and using polymerized ethylene glycol-citric acid polyester precursors (Pechini (Pe)) route. To find optimal parameters of the synthesis, the effect of the dopant nature and its parent compoimds, milling duration, temperature of MA products annealing on the doped ATLS formation has been studied. Phase, structural and microstructural studies have been carried out to clarify the mechanism of Al and Fe-doped lanthanum silicates formation in the mixtures with different parent compounds. 

According to the data of [2004Bok] the particles of Fe and ABN after mechanical activation form aggregates with a size of 300-1000 nm. The Fe particles exhibit a rather broad size distribution (fiom 3-7 to 100 nm) while the minimal size of the BN particles is several tens of nanometers. Annealing of Fe samples with BN after preliminary meehanical activation at 860°C caused the formation of nanosized encapsulated particles of Fc2B. Their surfaee was coated with a shell of ABN with flie fliickness from 5 to 15 nm. The size of the eneapsulated partieles varies within a rather broad range from 30 to 50 nm to several hundred nanometers [2004Bok]. 

Works [40, 91] surveyed y versus temperature for deactivation of 02( Aj ) on quartz at 350- 900 K. The obtained temperature dependencies were in the Arrhenius form with the activation energy of 18.5kJ/mole. A conclusion was drawn up about the chemisorption mechanism of singlet oxygen deactivation on quartz surface. A similar inference was arrived at by the authors of work [92] relative to 02( A ) deactivation (on a surface of oxygen-annealed gold). 

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