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21SHS OF MACROPOROUS NiAl ALLOYS USED IN ADVANCED RADIANT BURNERS
A. Maznoy*", A. Kirdyashkin", V. Kitler", N. Pichugin", and V. SalamatovA
aTomsk Scientific Center of the Siberian Branch of the Russian Academy of Sciences, Tomsk, 634055 Russia
bMerzhanov Institute of Structural Macrokinetics and Materials Science, Russian Academy of Sciences, Chernogolovka, Moscow, 142432 Russia *e-mail: [email protected]
DOI: 10.24411/9999-0014A-2019-10097
Since the first half of the 20th century, porous radiant burners have been successfully used for heating industrial premises and thermal processing and drying of materials. The key element of the radiant burner is a special gas permeable porous body - an emitter. The emitter takes part in heat exchange with a burning gas-air mixture, becomes red-hot, and emits the IR flux in accordance with the Stefan-Boltzmann law. Fursenko et al. [1] have demonstrated that depending on ignition conditions, the hollow cylindrical Ni-Al burners with the microchannels of less than the critical diameter can function both in the surface stabilized and internal combustion modes when the combustion zone stabilizes in the emitter internal cavity space. They have claimed that the radiation efficiency in the internal combustion mode is twice higher, up to 60% at a firing rate of 160 kW/m2 Maznoy at al. [2] have further found that in the internal combustion mode, the radiation efficiency of a cylindrical Ni-Al burner is close to the level of the highest possible radiant burner efficiency. To create reliable cylindrical burners, it is necessary to use materials that can endure thermo-mechanical stresses during a burner start-up without being destructed. Possible materials to be used are Ni-Al intermetallic alloys. The SHS reaction in the Ni + Al system starts at the melting point of Al and proceeds with a dissolution-precipitation mechanism, which is controlled by nickel diffusion in an aluminum melt. However, when both reagents melt, an abrupt increase in the reaction rate occurs. Rogachev at al. [3] have found that both reagents first melt in a Ni-Al combustion wave, and after that both liquids interact due to a very fast (t ~ 10-3 s) convective-reaction mixing, forming large drops of a melt. The size of the merged drops is 10-100 times larger than the size of the initial metal particles: there are about 105 particles of the initial reagents merged in one globule. This mechanism is called "reaction coalescence". Kirdyashkin et al. [4] have further shown that capillary hydrodynamic phenomena such as the Marangoni convection and thermal-gradient or capillary driven filtration of melts take place in a combustion wave of melting systems. Under the reaction coalescence conditions, the combustion wave front looks like a multitude of randomly generated drops, the size of which determines the porous structure of synthesized intermetallic alloys. The motivation of the present work is to develop methods for controlling the reaction coalescence process along with the production of coarse porous B2 + Lh Ni-Al alloys. The main approach discussed in this paper is the use of a bifunctional additive. The first function of the additive is to form low-temperature eutectics with aluminum oxide, which is inevitably available on the surface of commercial aluminum powders. The second function is to decompose the additive in the combustion wave zone at the formation of gases, which increases the powders moveability and intensifies the reaction coalescence.
The reaction compositions of Ni + 13.5-31.5 wt % Al have been analyzed. Nickel powder PNK UT-1 and aluminum powder ASD-4 with a particle fewer than 10 ^m in size have been used as the initial reagents. The powders have been thoroughly mixed using a Turbula mixer. Chemically pure powders CaO, CaF2, Ca(OH)2, CaCO3 have been used as additives to Ni-Al
mixtures. The self-propagating high-temperature synthesis of porous Ni-Al alloys was carried out according to the following method (Fig. 1a). The pre-mixed blends of powders were laid inside a steel cylindrical shell (inner diameter of 38 mm, the height of 68 mm) and were exposed to the controlled vibroforming to the relative density in the range of p = 0.4-0.5. The shell with the powder mixture was located inside a hermetic reaction chamber equipped with programmable electrical heating. The chamber was vacuumized three times each time followed by filling with argon to the atmospheric pressure. The temperature To of the reaction medium preheating was set. To measure the maximum temperature TM in the combustion wave, a tungsten-rhenium thermocouple of 200 |im thick was installed in the center of the mixture. The combustion wave velocity was defined as U = At//, where At was the passing time of the combustion wave between two K type thermocouples installed on the outer surface of the steel shell at a distance /. To visualize structural transformations in the SHS wave, high-speed imaging was employed using a Motion Pro X-3 camera (up to 10 000 fps). Since the thermocouple method is inertial, spectral pyrometry was additionally used to record instantaneous temperature values in the SHS wave. The dynamic monitoring of emission spectra was performed using an Ocean Optics HR4000 spectrometer according to the scheme shown in the Fig. 1b.
Fig. 1. The experimental scheme of porous materials synthesis (a); the scheme of spectral measurements and high-speed imaging during the SHS (b).
Let us consider the synthesis mechanism for coarse porous materials using an example of the reaction mixture Ni + 20 wt % Al with the relative density of p = 0.46 and preheating value of T0 = 200°C (Table 1, Fig. 2).
The SHS in this mixture is realized with combustion velocities of more than 10 mm/s, and this produces materials with a fine porous and defective structure (Fig. 2a). It is possible to obtain coarse porous materials without defects by using CaO or CaF2 additives (Figs. 4b, 4c). With these additives, a reduction of oxide films occurs on the surface of Al particles. The use of Ca(OH)2 and CaCO3 additives makes it possible to obtain coarse porous materials with an average diameter of Ni-Al elements up to 2 mm (Figs. 2d, 2e). From the high speed video filming data, it has been determined that in a combustion wave zone the decomposition of additives occurs with the formation of gases. The gases increase the mobility of the powder medium and form a fluidized bed. The formation of droplets in the combustion wave is stimulated not only by the reduced melt viscosity but also by the increased mobility of the powder particles of the fluidized bed. The thermocouple measurements have shown that the maximum temperature TM in the combustion wave (about 1430°) is consistent for both the mixture with additives and without additives, and this temperature is 40° lower than the melting point of the alloy with this composition. The spectrometric measurements have shown that in the combustion wave region there are high-temperature scintillations of a short period (t < 50 ms). Their temperature is 100-350°C higher than the solidus temperature, and this fact also increases the liquidus temperature (Fig. 3). It has been found that a planar front of the
combustion wave splits into many foci (Fig. 4), and everything seems to indicate that these superadiabatic scintillations correspond to the moments of melt droplets formation. Heat losses of a melt drop into the ambient, the temperature of which is lower than the solidus one, lead to rapid crystallization of the drop with its shape and size fixation. In such a way, the Ni-Al elements of a porous alloy are formed.
Table 1. The effect of an additive on the SHS parameters and the average size of skeleton elements of synthesized alloys De._
# Additive u, mm/s Tm, °c De, ^m
1 no 15.0 ± 0.5 defects
2 2 % CaO 7.8 ± 0.3 160 ± 20
3 2 % CaF2 3.6 ± 0.2 1430±10 660 ± 30
4 2 % Ca(OH)2 1.1 ± 0.1 1920± 90
5 2 % CaCO3 1.0 ± 0.1 1750±90
Time, s Time, ms
Fig. 3. Thermocouple measurements (part a) and spectrometric measurement data (part b). The reaction mixture of (Ni + 20 wt % Al) + 2 wt % CaCO3.
Fig. 4. Instantaneous time sequent photographs of the SHS process. The reaction mixture of (Ni + 20 wt % Al) + 2 wt % CaCO3.
Let us consider the effect of the initial synthesis conditions given in Table 2 on the average size of skeleton elements De. Without preheating the reaction mixture, the regulation range of the composition is 18-22 wt % Al, and it is 15-20 wt % Al with preheating (Fig. 5, #1, 2). Beyond these ranges, it is not possible to organize self-sustainable combustion, or the alloys are characterized by a substantially non-uniform porous structure. For a mixture with a given
chemical composition, the control of the SHS process is possible by varying the preheating temperature and relative density of the reaction mixture (Fig. 5, #3, 4).
The phase composition of the materials was determined by X-ray analysis using a Shimadzu XRD 600 diffractometer, CuKa radiation, PCPDFWIN and PDF+ databases. The synthesized alloys of (Ni + 13.5-14.5 wt %Al) + 2 wt % CaCO3 composition consist only of the LI2 Ni3Âl phase. For the compositions (Ni + 24-31.5 wt % Al) + 2 wt % CaCO3, the synthesized alloys are characterized only by the B2 NiAl phase. The synthesized alloys of the Ni + 17-22 wt % Al composition mainly consist of a combination of the B2 NiAl and L12 Ni3Al phases. Some minor amounts of the L1o and 7R martensite phases are also available. It has been established that at all ranges of Ni + 13.5-31.5 wt % Al compositions studied, the synthesis products are characterized by an equilibrium composition after an hour's 1100° annealing.
Table 2. Considered synthesis conditions. By the example of 2 wt % CaCO3 additive.
Variable parameters
# Range
Fixed parameters Al, wt.% to, °c
Aluminium concentration, wt %
Sample initial temperature to, °c Sample relative density, p
1
2
3
4
13.5-25 13.5-25 25-350 0.4-0.5
20 20
25 0.46 250 0.46 0.46
25
2500
S
.It 2000 c
S
Jä 1500
3 1000
I 500-1
Cü
^ 0 I.......... ,-,-,-,-,-,-, -,-,-,-,-,-
14 16 18 20 22 0 100 200 300 0.40 0.45 0.50
Al content in the mixture, wt. % Preheating temperature To, "C Sample relative density, p
Fig. 5. Diameter dependence of average Ni-Al elements De in synthesized alloys on SHS conditions. The curves numbers correspond to Table 2.
The experiments have shown that the phase composition of cylindrical emitters has a significant impact on the performance properties of a radiant burner. Emitters of mono-B2 composition and B2 + LI2 alloy can be destroyed when starting the burner using maximum power with a firing rate of 500 kW/m2, and a temperature rise rate in the emitter being more than 10° per second. The destructions are associated with cracks formation or porous emitter separation at the site of the weld joint. For trouble-free operation of B2 + LI2 emitters, it is necessary to perform a smooth start-up of the burner with the firing rate of about 150-200 kW/m2, while the temperature rise rate is no more than 5° per second. Our study has found that emitters made of mono LI2 Ni3Al could not be destroyed under any conditions of the burner start-up.
Thus, macro-porous Ni-Al intermetallic alloys have been successfully fabricated using self-propagating high-temperature synthesis. Testing the cylindrical radiant burners made by the SHS method has shown that the mono-Ll2 alloy is characterized by increased robustness. It is also tolerant to any thermal gradients arising at the burner operation.
1 R. Fursenko, A. Maznoy, E. Odintsov, A. Kirdyashkin, S. Minaev, K. Sudarshan, Temperature and radiative characteristics of cylindrical porous Ni-Al burners, Int. J. Heat Mass Transf., 2016, vol. 98, pp 277-284.
2 A. Maznoy, A. Kirdyashkin, S. Minaev, A. Markov, N. Pichugin, E. Yakovlev, A study on
the effects of porous structure on the environmental and radiative characteristics of cylindrical Ni-Al burners, Energy., 2018, vol. 160, pp. 399-409.
3 A.S. Rogachev, A. Varma, A.G. Merzhanov, The mechanism of self-propagating high-temperature synthesis of nickel aluminides, Part I: Formation of the product microstructure in a combustion wave, Int. J. Self-Propag. High-Temp. Synth., 1993, vol. 2, pp. 25-38.
4 A.I. Kirdyashkin, V.D. Kitler, V.G. Salamatov, R.A. Yusupov, Y.M. Maksimov, Capillary hydrodynamic phenomena in gas-free combustion, Combust. Explos. Shock Waves., 2007, vol. 43, pp. 645-653.
