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15FILTRATION COMBUSTION OF TITANIUM-ALUMINUM-NITROGEN TERNARY SYSTEM IN FLOW-TYPE REACTOR
I. A. Studenikin*", A. V. Linde", A. A. Kondakov", and V. V. Grachev"
aMerzhanov Institute of Structural Macrokinetics and Materials Science, Russian Academy of Sciences, Chernogolovka, Moscow, 142432 Russia *e-mail: [email protected]
DOI: 10.24411/9999-0014A-2019-10168
Combustion in titanium-aluminum-nitrogen system is the interesting object for investigations from the fundamental combustion science point of view as well as from technological point of view because of the opportunity of valuable combustion products obtaining such as binary nitrides of titanium and aluminum, complex ternary nitrides, MAX-phases. The varied regimes of combustion in this system are determined by the competition of parallel reactions, melting of reagents and intermediate phases in the combustion wave, changes of porosity and medium permeability, accordingly, the changes of gas reagent flow and extent of nitridation.
Earlier the combustion of mixtures based on Ti, Al, TiAl, and TiN at nitrogen pressure from
0.1.to 5 MPa with natural gas flow was investigated [1-4]. Depending on the initial conditions (initial mixture composition, gas pressure, thermal-vacuum treatment of initial mixture) different combustion modes (surface, layer-by-layer, spin, multipoint, 2- and 3-stages of front propagation) were observed. As a rule, the phase composition of combustion products consisted of TiN and AlN, intermetallics and ternary compounds (MAX-phases, complex nitrides). Efforts to obtain both monophasic Ti2AlN and TiN-AlN composite have been unsuccessful. On the one hand, it can be explained, that the formation of binary nitrides is preferable from the thermodynamic point of view. On the other hand, generated heat of combustion is sufficient for melting of components. Formed liquid phase decreases medium permeability and blocks supply of nitrogen in reaction zone fixing system in all forms of intermediate states.
Experimental study of filtration combustion modes in nitrogen coflow was carried out in [5-7] by the example of titanium-carbon system. It was revealed that forced flow of gas is a powerful controling factor, which can influence on the mode of combustion wave propagation and phase composition of reaction products.
The aim of this work was investigation of filtration combustion in titanium-aluminum-nitrogen system in flow-type reactor at variation of composition of the initial solid-state reagents mixture, gas mixture composition (nitrogen-argon), value of gas flow and its flow direction (coflow or counter flow) in relation to the direction of combustion front propagation. Three following stages can be separated in this research:
1. Development, fabrication, and setup of experimental stand. Experimental procedure was tested by the combustion of Ti + TiN mixtures in coflow nitrogen. Simultaneously the titanium nitride as one of the initial component for the subsequent experiments was produced.
2. Investigation of coflow and counter flow modes of combustion of Ti + Al + TiN mixtures both in pure nitrogen and in mix with argon to research the opportunity of obtaining MAX-phases.
3. Experimental study of hydrodynamic instability of filtration combustion front with formation of «finger» [8].
The experimental stand consists of:
• vertical steel tube reactor with cooling jacket and five thermal couple ports, reaction camera diameter is 44 mm and its height is 290 mm;
• system of gas feeding into reactor from balloon through gas-pressure reducer;
• gas venting system from reactor through the filter-refrigerant into vacuum pump 2NVR-5DM;
• flowmeter Bronkhorst EL-Flow on entering the reactor and pressure sensors МС2000 on entering and exit from the reactor;
• data acquisition system based on ADC QMLab and IBM PC.
The initial reagents were titanium powder of PTS trademark, aluminum powder of ASD-1 trademark, titanium nitride powder obtained in preliminary experiments and nitrogen (extra pure grade 99.995 wt %). Powder reagents were mixed in cylindrical vessel on the rolls without milling. Gas supply was performed through upper gate and gas extraction through bottom gate. To realize coflow mode the powder mixture was ignited at the top of reaction camera, and for counter flow - at its bottom. At pure nitrogen coflow combustion of Ti + xTiN mixture (where x is from 50 to 70 wt %) plane front of filtration combustion was observed. Combustion velocity was from 1.2 to 2.6 mm/s and increased linearly with increasing of gas flow from 20 to 25.5 l/min and decreasing of dilution ratio x from 70 to 50 wt %. The obtained sintered product can be divided in two parts: a near-wall multiphase layer containing TiN, Ti, and TiN0.3, and a central part containing only TiN. Product quantity of near-wall multiphase layer decreased proportionally from 27 to 5.2 wt % with increasing nitrogen flow and decreasing dilution ratio. Sintered product cleared from external layer was easily milled in the pounder to powder with particles size closed to particles size of initial powder-reagents. Content of nitrogen in monophasic product reached 21.5 wt % (stoichiometric value for TiN is 22 wt %). Obtained titanium nitride powder was used for preparation of Ti + Al + TiN mixtures. Combustion products of Ti + Al + TiN mixtures in coflow of pure nitrogen (Table 1) mainly represent binary nitrides and formation of ternary phases is inconsiderable.
Table 1.
Mixture Initial gas flow, l/min Product type Total Product phase composition ^comb ' , mm/s 7"- max T comb oC Cake height
composition weight gain, % Major phases Secondary phases shrinkage, %
porous cake:
2Ti+Al+ 2TiN 15.15 grey near-wall layer + yellow central part 13.08 TiN AlN 1.99 2256 0.76
1.5Ti+Al+ 0.5TiN 20 melted macroporous 14.32 TiN Ti2AlN, Ti4AlN3, _ _ 7.66
sponge AlN
Changes in gas flow direction on counter mode and gas composition on nitrogen + argon mixture lead to an increase in the ternary phase content in combustion products (Table 2). In this case, combustion velocity increased and temperature decreased in comparison with coflow combustion for the same mixture compositions. There was an essential shrinkage of product by two times compared with the initial height. Sintered product was a grey hard-milled fine porous sponge with a metallic luster on the shear.
At combustion of Ti + Al + TiN mixtures in nitrogen-argon gas coflow a hydrodynamic instability of propagation of plane front was revealed. It was earlier predicted theoretically [7]. The instability of plane front leads to combustion mode with formation of «finger», when inside of porous cylinder a gas channel forms (see Fig. 1). Experimental results and conditions of finger formation are presented in Table 3. Finger propagation velocity after formation was about 1 mm/s. Further on the half-way its velocity increased to ~ 2 mm/s and in the end of process increased to ~ 3 mm/s. Accordingly, gas flow through reactor increased proportionally to finger motion and in the end of process it exceeded start value about in three times.
Table 2.
Mixture Initial gas Gas composition, vol.% Total . weight . gain, % Product phase composition Ucomb™d', j1 , max. T comb. , Cake height
composition flow, l/min N2 Ar Major phases Secondary phases mm/s oC shrinkage, %
2Ti+Al 11.68 50 50 5.39 Ti3AlN, TisAl Ti2AlN 2.36 1536 49.8
2Ti+Al 11.35 63 37 5.33 Ti3AlN, TisAl, Ti2AlN TiN 3.77 1382 57.8
1.5Ti+Al+0.5TiN 11.8 50 50 5.09 Ti2AlN TiAl, TiN, Ti3Al 2.08 1580 41.5
1.5Ti+Al+0.5TiN 11.35 63 37 5.36 Ti2AlN TiAl, TiN, TisAl 2.31 1484 51.1
Fig. 1. Cross section of sample burned with «finger» formation. Table 3.
Mixture composition Gas composition, vol.% Initial gas flow, l/min Product type Total weight gain, % Product phase composition
N2 Ar Major Secondary phases phases
Ti + Al 100 0 10.14 melted cake 5.11 TiAl, TiN Ti2AlN
Ti + Al 98.5 1.5 10.07 melted cake 4.42 TiAl, Ti2AlN
Ti + Al 63 37 11.3 melted cake with formation of finger 4.07 multilayer: TiN, Ti2AlN, TiAl3, AlN, Ti3Al, TiAl3, Ti, Al;
Ti + Al 34 66 12.3 doesn't burn -
1.5Ti+Al+0.5TiN 50 50 11.6 melted cake with formation of finger 4.91 multilayer: TiN, Ti2AlN, TiAl3, AlN, Ti3Al, TiAl3, Ti, Al;
1.5Ti+Al+0.5TiN 50 50 5.8 doesn't burn -
For all cases of finger formation sintered material is essentially inhomogeneous on phase composition in cross section (Fig. 1): melted layer covered finger surface (1) consists of mainly TiN and Ti2AlN, to a less extent TiAh and AlN. The second golden-grey porous layer mainly consists of TiAl, Ti3Al and TiN. The third dark-grey porous layer (3) consists of TiAh, Ti3Al,
unreacted Ti, Al and initial TiN. In the fourth light-grey thin layer near the reactor wall were mainly phases of the initial TiN, Ti and Al and to a less extent TiAb and Ti3Al intermetallics. As a result of carried out research we can draw next conclusions:
1. Experimental stand based on flow-type rector was made and started up. It was equipped with temperature and pressure gages, flowmeter, computer data acquisition system. This experimental stand enables to carry out investigations of filtration combustion modes both in coflow and in counter flow of gas mixtures with different composition.
2. At combustion of Ti + Al + TiN mixtures in coflow of pure nitrogen the combustion products consist mainly of binary nitrides and formation of ternary phases is inconsiderable.
3. Formation of ternary compounds such as MAX-phase (Ti2AlN) and complex nitride (Ti3AlN) was observed at combustion of Ti + Al + TiN mixtures in counter flow of gas mixture of nitrogen with argon.
4. For the first time at filtration combustion of SHS-systems in the forced flow of gas reagent, the theoretically predicted hydrodynamic instability of plane front propagation was experimentally revealed. Evolution of instability leads to initiation of combustion modes with finger formation when a gas channel appears inside of porous solid material. Finger formation was observed only for coflow mode of Ti + Al + TiN mixtures combustion in nitrogen-argon gas mixture.
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