International Journal of Heat and Mass Transfer 95 (2016) 264–271 Contents lists available at ScienceDirect International Journal of Heat and Mass Transfer journal homepage: www.elsevier.com/locate/ijhmt Combustion synthesis of porous oxynitride materials under conditions of forced filtration of reacting gas ⇑ Anatoly S. Maznoy a, , Alexander I. Kirdyashkin b, Ramil M. Gabbasov a a Tomsk Scientific Center SB RAS, Akademicheskiy Av., 10/4, Tomsk 634055, Russia b Far Eastern Federal University, Suhanova St., 8, Vladivostok 690950, Russia article info abstract Article history: Combustion synthesis is a promising technique for producing the oxynitride ceramics, which represent a Received 5 June 2015 new class of materials with high resistance to high temperatures and to intensive wear. The different Received in revised form 15 November 2015 experimental ways of control of combustion parameters were considered, such as sample porosity, poros- Accepted 23 November 2015 ity structure of samples, different experimental configurations (coflowing and opposite flow systems), Available online 21 December 2015 and pressure. Kinetic features of nitrogenation of reacting samples (aluminium, silicon oxide and sialon powders) were analyzed at different conditions. Forced nitrogen filtration through porous sample sub- stantially improved the process of combustion synthesis (yield with higher nitrogen content, porous structure). The application of coflowing configuration intensifies the combustion synthesis and provides the re-heating process by convective heat transfer from combustion products to the initial mixture. It was demonstrated that this process leads to super-adiabatic temperature rise in the combustion zone. Coflowing configuration allow one to synthesize porous sialon with the conversion ratio up to 0.995 with combustion rate 0.8 mm/s and maximum combustion temperature about 1900 °C. The parameters of por- ous samples were varied in a wide range: sample porosity: 40–75%, the average sizes of the skeleton ele- ments – from 240 lm to 3.5 mm; average pore-channel size 10–390 mm; percentage of closed porosity 16–30%. Ó 2015 Elsevier Ltd. All rights reserved. 1. Introduction Aluminum oxide [9] or mullite [10] can be also used instead of silicon oxide for combustion synthesis. Synthesis proceeds through Ceramics on the basis of aluminum and silicon oxynitrides, several reaction steps suggested in [11–13]: known as sialons (Si6ÀZAlZOZN8ÀZ, where Z is the degree of substi- tution), are attractive materials for applications in the chemically AlS ! AlL þ AlG aggressive media at high temperatures [1–3]. Due to high temper- þ : ! ;D 0 ¼ ature resistance of porous sialon ceramics, these materials are con- Al 0 5N2 AlN H 318 kJ 0 ð Þ sidered for filters in aggressive media, for porous matrixes in 2Al þ 1:5SiO2 ! 1:5Si þ Al2O3; DH ¼309 kJ 2 radiative gas burners and as a material for block carrier catalysts 3Si þ 2N ! bSi N ; DH0 ¼829 kJ [4,5]. The sialon ceramics can be produced using combustion syn- 2 3 4 b þ þ ! b thesis [6,7] in the presence of nitrogen filtration through the por- Si3N4 AlN Al2O3 SiAlON ous samples. The sialon synthesis takes place in the narrow zone For reaction initiation in the powder samples, a heat impulse is of exothermic chemical reactions of the combustion wave propa- used to melt and evaporate aluminum. The aluminum reacts with gating through the initial sample. The sialon formation proceeds the nitrogen and the silicon oxide with large heat release providing in accord with overall reaction [8]: conditions for self-sustaining combustion synthesis. An exother- mic nitriding reaction of silicon takes place simultaneously with ð6 À 1:5zÞSi þ zAl þ 0:5zSiO2 þð4 À 0:5zÞN2 ! b À Si6ÀzAlzOzN8Àz; the synthesis of the sialon at temperatures above 1350 °C [14].It ð1Þ is known that to obtain porous materials by combustion synthesis it is necessary to sustain the maximum temperature in the com- bustion reaction zone below the melting point of reaction prod- ⇑ Corresponding author at: 10/4 Akademicheskiy Ave., Tomsk 634045, Russia. Tel.: +7 923 412 4765. ucts. To control the amount of heat release in the reaction zone, E-mail addresses: [email protected] (A.S. Maznoy), [email protected] the inert compounds, such as nitrides and oxynitrides [15], are (A.I. Kirdyashkin), [email protected] (R.M. Gabbasov). added to the batch. Parameters of porous structure are typically http://dx.doi.org/10.1016/j.ijheatmasstransfer.2015.11.083 0017-9310/Ó 2015 Elsevier Ltd. All rights reserved. A.S. Maznoy et al. / International Journal of Heat and Mass Transfer 95 (2016) 264–271 265 determined employing granulometric analysis [16]. Since the syn- 4. Heating of cylinder in the furnace with air atmosphere (up to thesis process involves a gas-phase reaction, the specific surface 110 °C, one hour) was used for fixation of porous structure of area should be at the maximum to provide effective reaction pro- powder samples. Excess of aluminum x reacts with H2O with ceeding. Estimates indicate that the size of powder particles should formation of hydrogen making expandable slurry in accord with be less than 100 lm. reaction 2Al + 6H2O = 2Al(OH)3 +3H2. Variation of sample To produce porous sialon with large pore sizes it was suggested porosity was achieved changing ratio of slurry volume to the to perform synthesis in the samples with two-level porous struc- volume of cylinder or the ratio of slurry volume to the volume ture (also known as bi-disperse porous medium, BDPM): macro- of pore-producer. structure carcass with elements having additional micro-porous 5. Dried sample was roasted in incinerator using air atmosphere in structure, formed by highly dispersed powder of reactants. Several order to remove water and pore-producer (450 °C, 45 min). authors consider different experimental configurations for 6. A cylindrical channel was drilled out (6 mm diameter) in the combustion synthesis: preliminary preparation of specially center of sample for reactant flow. structured initial porous samples [17], compressing of granules of different shapes [18], gel casting [19], and freeze casting Porosity of sample was calculated using relationship: process [20]. X a The key element of oxynitride synthesis is providing conditions P ¼ À m i 1 q Á 100%: ð3Þ for effective nitrogen absorption by aluminum and silicon. V i Typically, synthesis is carried out at pressure of 1 Ä 10 MPa in Here m and V are the mass and volume of sample; a is mass of frac- the reactors containing substantially larger amount of nitrogen i tion of i-th component in slurry composition, q is density of i-th than that required for sample nitriding. Absorption of nitrogen i component. through the surface of the sample creates a pressure drop in the reaction zone causing a nitrogen flux to the reaction zone. The temperature, pressure and gas phase composition in the reactor 2.2. Filtration combustion are changing [21], and the process can be only controlled by gas pressure. Fig. 1 presents the experimental setup for combustion synthe- The experimental configuration of synthesis with the use of sis. The experimental setup was equipped by flow-meters Mass- forced nitrogen filtration through the sample and combustion reac- View MV-104 and Mass-View MV-106 (Bronkhorst, Netherlands), tion zone represents an interesting alternative of process organiza- measurement error was 2% in the pressure range 0–1 MPa. For tion. It allows one to intensify the process of synthesis providing pressure measurement a manometer DM5002 (Manotom, Russia) additional heat and mass transfer between reaction zone, combus- was used, accuracy 0.1% in the pressure range 0–1 MPa. For tem- tion products and initial reactants [22]. Combustion synthesis in perature measurements an 8th channel module for thermocouples the presence of nitrogen forced filtration is a poorly studied mode Adam 4018+ (Advantech Co., Taiwan), accuracy 0.1 mV. of combustion process for these systems. The aim of this work was We used the cylindrical flow reactor (volume 0.7 l, inner diam- a detailed experimental study of combustion synthesis of porous eter 80 mm) for synthesis under the forced filtration. Synthesis sialon from preliminary structured bi-disperse porous reaction sys- procedure was as follows. Sample (6) was placed into the reactor tems with the use of forced nitrogen filtration. (4) with the installed thermocouples (9). For temperature mea- surements we used 9.2 thermocouple (tungsten–rhenium, C type, junction diameter 400 lm, maximum measurement temperature 2. Experimental procedure 2500 °C). It was mounted to the outside sample surface in the cylindrical hole (diameter 1 mm, depth 10 mm). Synthesis rate 2.1. Sample preparation Uc was determined from the passage time between thermocouples 9.1 and 9.3 (chromel–alumel, K type). The thermocouples were Synthesis of porous sialons was conducted using powder sam- mounted at 1–2 mm depth in the sample. For synthesis initiation ples. In this work we synthesized beta-sialon with substitution we used ignition tablet (7) with incandescence spiral (8). During þ þ ! degree Z = 4: 4Al 2SiO2 2N2 Si2Al4O4N4. Reason for the experiments thermocouple, pressure and flow meter measure- choice of this compound was a minimum number of components ments were recorded. The gas flow was shut-off when sample in the initial mixture. Initial reactants were aluminum powder (ACD-4, purity 98.8%; <10 lm, RusAl, Russia), natural quarts sand (composition: 98.15% SiO2, 0.67% Al2O3, <10 lm), and inert diluent 1 Si2Al4O4N4, obtained in our laboratory using procedure described 2 3 in [23]. 7 Structuring of reaction samples was performed as follows: 5 1. Initial mixture was prepared in accord with the expected com- 8 x position ratio: 23.7% Al + 26.3% SiO2 + 50% Si2Al4O4N4 + % Al, 9.1 where x =5Ä 10% is the excess of aluminum (here and below N2 9.2 4 – weight percentages are used).