On the mechanism of zirconium nitride formation by zirconium, zirconia and yttria burning in air

Journal of Solid State Chemistry 230 (2015) 199–208

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On the mechanism of zirconium nitride formation by zirconium, zirconia and yttria burning in air Ekaterina Malikova a, Julia Pautova a, Alexander Gromov a,b,c,d,n, Konstantin Monogarov a,d, Kirill Larionov a, Ulrich Teipel b,c a

Tomsk Polytechnic University, 30, Lenin Prospekt, Tomsk 634050, Russia Fraunhofer Institute of Chemical Technology, Joseph-von-Fraunhofer Str. 7, 76327 Pfinztal, Germany c Nürnberg University of Technology Georg Simon Ohm, 12, Keßlerplatz, 90489 Nürnberg, Germany d Semenov Institute of Chemical Physics, 4, Kosygina str., 119991 Moscow, Russia b

art ic l e i nf o

a b s t r a c t

Article history: Received 16 March 2015 Received in revised form 23 June 2015 Accepted 7 July 2015 Available online 10 July 2015

The combustion of Zr and (Zr þZrO2) powdery mixtures in air was accompanied by major ZrN stabilization. The synthesis of cheap ZrN with the high yield in air was facile and utile. The influence of Y2O3 additive on the content of ZrN the solid combustion products (SCP) was investigated. The reagents and SCP were analyzed by BET, DTA–TGA, XRD, SEM and EDS. Burning temperature was measured by thermal imager. The yield of ZrN in the SCP has been varied by the time regulation of the combustion process. The burning samples were quenched at a certain time to avoid the re-oxidation of the obtained ZrN by oxygen. The quenching of the burned (Zr þ ZrO2) samples with the Y2O3 additive was allowed increasing the ZrN yield in SCP up to 66 wt%. The chemical mechanism of ZrN formation in air was discussed and the probable source of ZrN massive formation is suggested. & 2015 Elsevier Inc. All rights reserved.

Keywords: Zirconium Zirconia Yttria Zirconium nitride Combustion

1. Introduction ZrN has a combination of the useful physical properties high melting temperature 2990 °C, high hardness 2800 HV, high thermal conductivity 20 Wm  1 K  1, good wear and corrosion resistance, Seebeck coefficient is from  6 to  7 μV K  1, electrical resistance 13.6 μΩ cm (at 300 K) [1,2]. ZrN powder is used in a wide variety of ceramic and composite applications [2–5]. Existing methods of ZrN production (carbothermal synthesis, direct nitriding of Zr powder, decomposition of the organic Zr-contained precursor) are rather power-consumed and provide low-yield of ZrN (up to 30% on the nitridation stage) [6,7]. Combustion of powdery reagents is a technique for advanced materials production by exothermic combustion reactions [6,8], when the material burning in gaseous oxidizer or in the composition with solid oxidizer. The most important problem of the combustion synthesis of ZrN in N2 is that it normally demands high pressure (up to 300 MPa) [9,10]. The reduction of ZrO2 by graphite in N2 results in the contamination of solid combustion products (SCP) by residual graphite and metal carbides [11,12]. n Corresponding author at: Fraunhofer Institute of Chemical Technology, Energetic Systems Department, Joseph-von-Fraunhofer Str. 7, 76327 Pfinztal, Germany. Fax: þ 49 721 4640 450. E-mail address: [email protected] (A. Gromov).

http://dx.doi.org/10.1016/j.jssc.2015.07.007 0022-4596/& 2015 Elsevier Inc. All rights reserved.

Direct diffusive nitriding of Zr powder requires the use of highpurity nitrogen or hazard reagents (azides, hydrides etc.) [13,14]. The reaction products have to be added to the initial powder as well [15]. However, micron-sized Zr powder can be easily ignited and burned in air (78 vol% of N2) at pressure 1 atm with the yield of ZrN up to 38 wt% [16]. ZrN formation in air was not confirmed by some earlier studies [17]. The efficiency of the combustion synthesis for the obtaining of nitrides in air was proven for the powdery metals Al, Ti, W, Mo, Cr, La and B [18–25]. Combustion synthesis route of ZrN production is the advanced method compared to the approach on the ZrN synthesis from ZrCl4 and Li3N described by Gillan et al. [26]. While the ZrCl4 is the intermediate product for Zr production, its price on the market is nearly twice higher than for the Zr metal. The price of Li3N is close (or even a bit higher) to the price of gold. ZrO2 instead is the hightonnage product, easily available [27]. It is one side of the problematic application of ZrCl4 and Li3N usage as reagents. The second is assuming environmentally dirty Cl-contained products formation (including gaseous Cl2). Li3N is strongly hydrophilic and flammable in air as well. Combustion synthesis in the contrary is the absolutely environmentally friendly method. Air as the source of nitrogen is free. ZrO2 powder is an inert diluent by micron-sized Zr combustion in air added for the increasing the ZrN content in SCP. Al2O3 and TiO2 additives were proven as the nitrides formation promoters by

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Fig. 1. Heat transfer mechanism for (a) micron-sized metal particles and (b) micron-sized metal particles, isolated by nano- or sub-micron oxide particles: gray – metal particles, white – oxide particles, the heat transfer is shown by arrows.

the combustion of the powdery mixtures (Al þAl2O3) and (Ti þTiO2) in air [18,19]. The aim of nano- or sub-micron oxide powders additive to micron-sized metal powder by combustion is to isolate large metal particles from each other and to localize the heating inside one particle (or particle aggregate) and to avoid coalescence of the particles and unburned large droplets formation in SCP (Fig. 1) [18]. The mechanism of oxide diluent effect on the combustion needs to be studied. In the case of Al and Ti the additives of their oxides increased the yield of nitrides in the SCP by metals' combustion in air. Three factors were used to increase the yield of ZrN in SCP: – The quenching of the combustion process (to regulate the burning time and avoid ZrN after-oxidation); – The additive of the ZrO2 to Zr (to provide their interaction and further suboxides ZrO or Zr2O reaction with nitrogen as for Al [19]); – The additive of the Y2O3 to (Zr þZrO2) mixtures (to check the catalytic effect of Y2O3). The chemical chain of ZrO2 polymorph transformation during high-temperature treatment is normally represented by (Eq. (1)) when temperature increases [28–31]

ZrO2

T = 1172 ° C (monoclinic)

⟷

ΔH = 8.4 kJ/mol

ZrO2

T = 2347 ° C (tetragonal)

⟷

ΔH = 13.0 kJ/mol

ZrO2

(cubic)

(1)

Y2O3 is one of the known ceramic stabilizers of ZrO2 (tetragonal) [32–35]. The hypothesis of this work was that yttria could stabilize ZrO2 (tetragonal) during the combustion. The Y2O3 was added to initial (Zr þZrO2) mixture with the aim to reduce the content of ZrO2 (monoclinic) in the SCP [28–30]. This work is devoted to the study of the Zr and (Zr þZrO2) powders burning in air with or without Y2O3 additive.

2. Experimental 2.1. Powders characterization and Zr non-isothermal oxidation The initial Zr, ZrO2, and Y2O3 powders were purchased from RUSAL, Russia (Table 1, Fig. 2). They were micron-sized powders with irregular particles' shape. The apparent densities of the powders are given in Table 1. The XRD patterns (made by Rigaku Table 1 Characteristics of the reagents for combustion. Zr

ZrO2

Y2O3

Mean-surface particle size (by BET) (μm) Particle shape (SEM)

24.3

0.5

8.2

Bulk density (standard) (kgm Melting temperature (°C) Apparent density (kgm  3)

)

2.2. Combustion of (Zr/air) Powdery cone-like Zr samples of equal mass (5 g) were burned in air in a self-propagating mode ignited from the top by tungsten wire [6]. The experimental setup was previously proposed in [16,18,19]. The combustion were carried out in static air with the initial temperature T¼ 25 °C, relative humidity 60% and atmospheric pressure P ¼1 atm. Photo/video camera “Canon 115” was employed for the visualization of the combustion. The ignition temperature was selected as 500 °C (the temperature of igniting tungsten wire) to provide the sustainable ignition [9]. However, according to DTA–TGA curves (see Fig. 3), 290 °C was enough for the ignition of Zr in air. The burning process in airfollowed by the local ignition (Fig. 4). The burning spots propagation for Zr powder was much faster compared to Al (“fingering” combustion regime [37]). The bright stage of the combustion (this stage corresponds to the first exo-peak on DTA, see Fig. 3) was followed by the sample cooling, when the main part of the metal have already been reacted. The thermal images and the temperature histories for the Zr combustion were obtained with the thermal imager Jade J 530 SB (Fig. 4). The fast high-temperature stage of the combustion was only present during the process of the Zr combustion in air (Fig. 4a). The maximum burning temperature was 1590 °C and it was lower than the melting point of the reagents (see Table 1). 2.3. Solid combustion products

Characteristic (method)

3

Max B, Japan) of the powders are shown on Fig. 2. According to the results of the XRD and DTA–TGA (made by STA 449, Netzsch, Germany) metal content in Zr powder was 92.1 wt% and 6.2 wt% was represented by zirconium hydrides (Fig. 2a). The residual 1.7 wt% was amorphous films on particles. The oxide content in ZrO2 and Y2O3 was 99.8 wt% defined by the XRD and DTA–TGA methods. ZrO2 of 90% tetragonal modification with 10% of monoclinic one were used. The ZrO2 powder was produced by plasma condensation method (Fig. 2b). Fragmented spherical hollow semi-spheres (Fig. 2b) were the main morphological features for such powder. For the Zr and Y2O3 powders the spheroidal and irregular shape of particles are specific according to SEM and EDS (made by JEOL JSM 700 and HITACHI S-3400N, Japan). DTA–TGA characterization of Zr powder (Fig. 3) was applied to analyze a thermal behavior of the powder. The reaction of the Zr powder with air occurred intensively at a linear heating at 10 K/ min. The reactivity parameters [36] were used for the Zr powder thermal characterization. The temperature of the oxidation onset was 290 °C. Zr powder had relatively high rate of intensive oxidation (0.11 mg/s) in the temperature range of 460–470 °C (see Fig. 3). The transformation (oxidation) degree was 88.9% up to 800 °C assuming the Zr-ZrO2 reaction by slow heating (theoretical mass increasing 135.2 wt%) . The (Zr þZrO2) and (Zr þZrO2 þY2O3) powdery mixtures were prepared by mechanical mixing in pure ethanol for 15 min with following drying for 12 h in vacuum (residual pressure 0.08 MPa). The apparent density of the powdery mixtures was 1400 kgm-3 for the (50 wt% Zrþ50 wt% ZrO2) and 1600 kgm-3 for (80 wt% Zrþ 20 wt% ZrO2).

Spheroidal Hollow spheres 1860 1100 2125 2700 1780 1220

Irregular 1150 2415 1150

The SCP of the burned Zr powders were collected and analyzed after samples cooling. Phase composition of the SCP was determined by XRD analysis. The SCP contained ZrN, Zr, and ZrO2 in two crystal modifications: tetragonal and monoclinic according to the XRD patterns (Fig. 5). The SCP of Zr powder combustion contained large quantity of the unburned metal. The SCP of Zr in air was represented by highly-porous sponge (see Fig. 5).

E. Malikova et al. / Journal of Solid State Chemistry 230 (2015) 199–208

Fig. 2. XRD patterns (CuKα monochromated) and SEM images of the powders for combustion synthesis: (a) Zr; (b) ZrO2; (c) Y2O3.

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2.4. Quenching in combustion of (Zr/air) The burned Zr samples were quenched by the fast cooling to avoid the formed ZrN re-oxidation by oxygen. The quenching was made using the samples fast fragmentation (during  0.5 s) by the impact pressing of the sample with the heavy steel plate with the room temperature. The steel plate (200  200  5 mm3) was placed over the top of the sample (diameter 20 mm) upon the combustion occurred. The steel plate was cut the sample to small fragments (1–2 mm) by the fixed time of the combustion (20, 60, and 90 s after ignition) to find the maximum yield of the ZrN in the SCP. The combustion was stopped due to the temperature drop down in the sample. The quenched sample was mechanically broken (Fig. 6). The first quenching was made at 20 s after ignition when the burning temperature reduced to 700 °C (see Fig. 4a). The second quenching was made at 60 s when the temperature reduced significantly and stabilized at  300 °C until the very end of combustion (660 s). The third quenching was made for reference at 90 s after ignition. The SCP composition was nearly not changed and contained ZrN (23–27 wt%), Zr (45–53 wt%), and ZrO2: tetragonal (10–13 wt%) and monoclinic (12–19 wt%). The content of residual Zr was slightly decreased with the combustion time increasing, but the other phases content in SCP was in the same level. 2.5. Combustion of ((Zrþ ZrO2)/air)

Fig. 3. DTA–TGA curves for Zr powder under non-isothermal heating in air (10 °C/ min).

The Zr powder with 10–90 wt% of ZrO2 was preliminary mixed with the step 10 wt%. The mixtures were poured onto a steel plate to shape a cone-like sample. The combustion of (Zr þ ZrO2)

Fig. 4. Temperature history and view of the sample (5 g) burning in air: (a) Zr; (b) (Zr þ ZrO2).

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203

Fig. 5. XRD pattern and SEM image of the SCP of Zr (5 g) in air.

Fig. 6. Quenching scheme of the burned powdery sample (2) with massive metal plate (1).

mixtures and Zr without additive visually had no substantial difference: the combustion wave was covered the whole sample surface after the local ignition. The mixtures ((10–20 wt%) Zr þ (80–90 wt%) ZrO2) were not ignitable due to high content of ZrO2. The maximal burning temperature for the mixtures (Zr þZrO2) was 97 °C higher than for the Zr (see Fig. 4b). 2.6. Combustion of ((Zr þZrO2 þY2O3)/air) Y2O3 was added to the (Zr þZrO2) powdery mixtures in amount of 1, 2 and 3 wt%. The burning scenario for the (Zr þ ZrO2 þY2O3) samples was the same as previously described. The quenching time was 30, 60 and 90 s for the samples with and without Y2O3 additive. The samples with each quantity of the additive were quenched during three period of time, i.e. the combustion process was broken at 30, 60 and 90 s. The burning time of more than 90 s was out of interest because ZrN re-oxidation took place intensively after 90 s. Fragility and sponginess of the SCP were the results of intensive mass-transfer due to the gas evolving by combustion (Fig. 7). According to XRD study the SCP of all samples contained ZrN, Zr, and ZrO2 in two modifications (tetragonal and monoclinic, see Fig. 7a). The Y-contained phases were not found by XRD (Fig. 7b). The content of ZrN was higher in the case of Y2O3 usage.

3. Discussion 3.1. Zr non-isothermal oxidation and comparison with the combustion The burning scenario is affected by the powdery sample

density (Table 1) in the case of pressed samples. For the freely poured powders the only two factors affected the self-propagated combustion regime: ignition temperature and the critical minimal mass of the sample. In this work we selected the mass of 5 g that is significantly more than the critical for Zr (0.5 g) [6]. For the nonpressed samples, especially for gas phase combustion, the more important parameter is thermal conductivity of the powder (see Fig. 1). According to Fig. 3 the non 100 % value of the oxidation degree (88.9% exactly) for Zr powder was probably due to the formation of volatile intermediate products released from the crucible by heating [16]. Two stage oxidation of Zr powder in air (1st stage – 290–525 °C, 2nd stage 525–800 °C, see Fig. 3) was presented by three parallel reactions (Table 2). Fast reactions of Zr with N2 and O2 mostly took place during stage 1 of heating, while slower ZrN oxidation occurred under the higher temperature at the stage 2. However, if micron-sized ZrN powder was slowly heated in air, the oxidation was started (by DTA–TG) at the T 41200 °C [38]. Thus, ZrN formation by fast non-isothermal oxidation of the Zr powder in air occurs by the T o525 °C in parallel with ZrO2. Combustion of Zr was accompanied by the flame torch of the hydrogen [16]. Hydrogen evolved from zirconium hydrides by combustion [39]. ZrH2 was formed upon Zr powder storage in water (Fig. 2a). The ZrH2 decomposition occurred at T 4800 °C (Eqs. (2) and (3) [29, 40–42]) (s)-Zr (s) þH2 (g)  130

kJ

(2)

(s)-Zr (s) þ½H2 (g)  62

kJ

(3)

ZrH2 ZrH

Possibly that evolved H2 and its immediate ignition is the main reason of the very low temperature of Zr powders ignition in air. Since the maximum burning temperature of Zr in air was lower

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Fig. 7. XRD pattern and SEM image of the SCP of: (a) (80 wt% Zr þ 20 wt% ZrO2) at the quenching time 60 s; (b) (80 wt% Zrþ 20 wt% ZrO2 þ3 wt% Y2O3) at the quenching time 30 s.

Table 2 Probable reactions for the Zr non-isothermal heating and combustion in air. Stage of slow heating(DTA)/T(°C)

Reaction

Stage of combustion/T(°C)

Reaction

1, Fast /290–525

ZrH(s) þ ½O2(g)-ZrO2(s) þ ½H2 (g)  62kJ Zr (s) þ ½N2(g)-ZrN (s) þ365 kJ Zr (s) þ O2(g)-ZrO2(s) þ 1082 kJ Zr (s) þ O2(g)-ZrO2(s) þ 1082 kJ ZrN (s) þ2O2(g)-ZrO2(s) þ ½N2(g) þ 717 kJ

Ignition/ 4290

ZrH2(s)-Zr (s) þ H2(g)  130 kJ ZrH (s)-Zr (s) þ½H2 (g)  62 kJ H2 (g) þ ½O2 (g)-H2O (l) þ286 kJ Zr (s) þ ½N2(g)-ZrN (s) þ 365 kJ Zr (s) þ O2(g)-ZrO2(s) þ 1082 kJ Zr (s) þ ZrO2 (s)-ZrO (Zr2O) (g) ZrO(Zr2O) (g) þ ½N2(g)-ZrN (s) þO2(g) Zr (s) þ O2(g)-ZrO2(s) þ 1082 kJ ZrN (s) þ2O2(g)-ZrO2(s) þ½N2(g) þ 717 kJ

2, Slow/525–800

1, Fast/290–1590

2, Slow/1590–300

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Fig. 8. SEM/EDS of the SCP with the highest ZrN content: (a) (80 wt% Zr þ20 wt% ZrO2) at the quenching time 60 s; (b) (80 wt% Zr þ20 wt% ZrO2 þ 3 wt% Y2O3) at the quenching time 30 s.

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than the melting point of the reagents one could assume that the burning heat dissipated in another form of energy: irradiation and ionization. The very bright glow of Zr by burning was used since the first chemical flashes for photoimages making. The complete combustion process (with ZrN re-oxidation stage) was very long compared to other metals [16] and took 660 s. The temperature during re-oxidation stage (60 s from ignition and further) was low 300 °C (see Fig. 4a). However, the SCP of Zr powder combustion contained large quantity of the unburned metal what indicates the low diffusion degree of gaseous oxidizer to the solid cores of Zr particles. The SCP structure nevertheless was evidently accompanied by the gas–solid mass transfer according to SEM images (see Fig. 5). Thus, the probability of gaseous products appearance (H2, ZrO, Zr2O) in the burning system is higher compared to slow DTA–TGA non-isothermal heating (see Table 2). 3.2. Quenching of combustion and additives effect on Zr combustion Zr/air. The content of ZrN was increased from 23 to 27 wt% after the samples quenching at 60 s. This value was inside the error bar of quantitative XRD and could be an indicator of ZrN once formed in the first few seconds of burning was not oxidized then (see

Fig. 4). It was a lot of unburned Zr metal in the SCP (45–53 wt%). The content of ZrO2 for the both crystal modifications (tetragonal and monoclinic) presented in the quenched samples was nearly the same at the different burning times and, thus, we conclude that the processes of ZrO2 and ZrN formation were mainly stopped after the first few seconds of burning and only diffusive lowtemperature Zr oxidation gave the input into the ZrO2 content growth in SCP. Not high enough burning temperature (T 41590 °C), the short period of the SCP stabilization (approximately 5 s) and sharp drop down in temperature from 1200 °C to 700 °C after 15 s of combustion (see Fig. 4a) resulted in high amount of the unburned Zr in the SCP (see Fig. 5). (Zr þZrO2)/air. A little slump was observed after the first temperature maximum on the burning curve (Fig. 4b) and then the second temperature raise was fixed. The main difference of the combustion process for the (Zr þZrO2) mixtures from Zr was higher combustion temperature. The highest temperature (1790 °C) was found for the sample (80 wt% Zrþ 20 wt% ZrO2) (Fig. 4b). The oxide additive led to the temperature increase because of the heat isolation of separate metal particle by smaller ZrO2 particles [18]. So, the heat dissipation by burning was lower in the case of ZrO2 additive and thus, maximal combustion

Fig. 9. Phase content (quantitative XRD) for the SCP of (Zr þZrO2) at the different burning times (30, 60 and 90 s).

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temperature was higher in that case. The burned (Zr þZrO2) samples were quenched by the same way as for Zr powder. The only difference was that the first quenching was made at 30 s after ignition when the burning temperature noticeably reduced from 1790 °C to  1000 °C (see Fig. 4b), the second quenching at 60 s was applied when the temperature reduced significantly and stabilized at  700 °C, and the third one was applied at 90 s for reference. Both elemental nitrogen and oxygen were uniformly distributed inside the SCP (Fig. 8a). This is the indicator of simultaneous ZrN and ZrO2 formation. The phase composition for the SCP of (Zr þZrO2) is presented on Fig. 9. The content of unburned Zr in the SCP for the samples ((90–80 wt%) Zrþ (10–20 wt%) ZrO2) was high (25–38 wt%) with nitride yield 35–36 wt% and oxides content 15–23 wt% (Fig. 8a). The other samples ((60–30 wt%) Zr þ(40–70 wt%) ZrO2) contained 56–81 wt% of oxides, and, thus, they were not interesting for the future investigation: the target product was ZrN. The optimal time for the achievement of the maximal conversion of reagents was 60 s (Fig. 8). The highest yield (42 wt%) of ZrN in SCP was obtained by burning of the mixture (80 wt% Zr þ20 wt% ZrO2) (see Fig. 8). So, this mixture with the highest ZrN content in the SCP was selected for the further investigation of Y2O3 additive influence. (Zr þ ZrO2 þ Y2O3)/air. The SCP compositions of the mixture (80 wt% Zr þ20 wt% ZrO2) at quenching time 30, 60 and 90 s was compared with the SCP of the same mixture with the 1, 2, and 3 wt% Y2O3 additive (Fig. 10). The content of ZrN became higher with the burning time raising (1 wt% Y2O3). But the ZrN content became lower and the content of oxides increased with the burning time if 2–3 wt% Y2O3 was added (see Fig. 10). Thus, the Y2O3 additive has reduced ZrN formation time and the re-oxidation process was started before 60 s in the contrast with the samples without additives. The nitride content became higher on  45% average by 2–3 wt% Y2O3 additives. The optimal burning time was 30 s for those samples (see Fig. 10). The total amount of ZrO2 was lower for all samples with Y2O3 additive. The content of unreacted Zr came down to 12–17 wt%. The higher was the percentage of the Y2O3 additive, the lower was the residual metal content in SCP was found. The elemental nitrogen and oxygen were uniformly distributed inside the SCP (Fig. 8b). So, the Y2O3 effect on the SCP formation could be estimated just indirectly. The Y2O3 acted as a catalyst that promoted the formation of the ZrN by burning. It can be two possible reasons of the ZrN content growth in SCP: 1. The alloying of the metal powders with rare-earth metals leads to the surface oxide destruction [43]. Thus, the metal became chemically more reactive and the temperature of its reaction with nitrogen was reduced. 2. The Y2O3 is used for some reactive metals [31] to decrease their melting temperature. It promotes ZrN formation by the combustion for the mixtures (Zr þZrO2) because of the chemical binding of nitrogen was started at the lower temperature in this case [18–21, 44]. As a result, the total reaction time in the burning wave increased, and the yield of ZrN was also increased. However, the final opinion on the Y2O3 action on the Zr combustion process is a subject for the future study and discussion. In this work we stop, thus, on the two possible suggestions given. The chemical reactions by Zr with ZrO2 combustion (see Table 2) are mostly known, but the nitrogen diffusion and leading nitridation reactions is characteristic for Zr. Moreover, the combination of ZrO2 and Y2O3 additives to Zr and burning time varying

207

Fig. 10. Phase content (quantitative XRD) for the SCP of (80 wt% Zrþ 20 wt% ZrO2) mixture with Y2O3 additive (1,2 and 3 wt%) and without (0 wt%) additive.

give the possibility of ZrN yield increasing in SCP in a factor of 2.9 (66 wt% vs. 23 wt%).

4. Conclusion and the future work The combustion of Zr with ZrO2 and Y2O3 additives was studied in this work. The non-pressed powders were used as reagents and air was applied as the source of N2. The new combustion phenomenon has been found: metal (Zr) chemically reacts with its oxide (ZrO2) in the burning wave. The probable mechanism of this interaction would be the formation of the volatile zirconium suboxides ZrO and Zr2O [45], but this phenomenon needs to be investigated by spectroscopic methods. The principal possibility of up to 66 wt% of ZrN obtaining in air (1 atm) by the powdery mixtures (Zr þ ZrO2 þY2O3) combustion in air was experimentally proved in this study. The yield of ZrN produced by the combustion of the (Zr þZrO2) mixtures was

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higher than that for the pure Zr powder. The quenching of the burned samples allowed increasing the ZrN yield in SCP due to avoiding the re-oxidation of the formed ZrN. The obtained SCP contained both monoclinic and tetragonal oxides (ZrO2 (monocl.) and ZrO2 (tetrag.)) in the approximately equal proportion. The best ZrN content was obtained for the samples with 3 wt% Y2O3 additive and quenched at 30 s after ignition. The ZrO2 (monocl.) content was succeeded in reducing on 56% (19 wt% vs. 34 wt%) in this case. The Y2O3 additive can be applied as the catalyst of the ZrN formation, while the mechanism of its action yet to be studied. The total combustion time was decreased by Y2O3 additive as well. The content of ZrN phase was higher on 45 wt% for the (80 wt% Zr þ20 wt% ZrO2 þ3 wt% Y2O3) mixture under the quenching time 30 s and achieved 66 wt% in comparison with pure Zr.

Acknowledgments This work was supported by Tomsk Polytechnic University (Russia) via RF Federal Target Program contract RFMEFI58114X0001 and Alexander-von-Humboldt Foundation (Germany (Grant no. 1141703)).

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