Refractories and Industrial Ceramics
Vol. 48, No. 6, 2007
SYNTHESIS OF TITANIUM AND ZIRCONIUM NITRIDES BY BURNING MIXTURES OF THEIR OXIDES WITH ALUMINUM NANOPOWDER IN AIR Yu. A. Amel’kovich,1 A. P. Astankova,1 L. O. Tolbanova,1 and A. P. Il’in1 Translated from Novye Ogneupory, No. 11, pp. 64 – 67, November, 2007.
Original article submitted September 12, 2007. Results of a study of the products of burning mixtures of titania and zirconia with aluminum nanopowder in air are described. It is shown that when these mixtures burn in air the TiN and ZrN phases are stabilized. According to the data of x-ray diffraction phase analysis the materials contain 55% ZrN and 24% TiN. It is most probable that the TiN and ZrN form through formation of element metals.
Synthesis of nitride-containing ceramic powders due to burning of metal powders in air [2] has some advantages over commercial methods and SHS, i.e., lower consumption of energy, no necessity for intricate equipment, and presence of gradually varying layers between different crystalline phases. Combustion of nanopowders yields up to 90% products with submicron sizes [3]. It is known that aluminum, titanium, and zirconium exhibit reducing properties at high temperatures [4]. For example, in combustion of aluminum thermit (Fe2O3 + Al) aluminum reduces iron oxide (III) to metallic iron. The reducing capacity of powdered aluminum in combustion has also been studied for Cr2O3, WO3, MoO3, NiO, and CoO mixtures with aluminum. The use of titanium and zirconium powders at the commercial scale is fraught with a danger of their inflammation [5]. In addition, titanium and zirconium powders are less expensive products than their oxides. We have no published data on the products of combustion of zirconia mixtures with aluminum nanopowders. The aim of the present work consisted in studying the phase composition of the products of synthesis due to burning titania and zirconia mixtures with aluminum nanopowders in air. Methods of experiment. The objects of our study were commercial powders of TiO2 and ZrO2 of grade ch.d.a. and aluminum nanopowder (NPAl) obtained by electric explosion of thin wires in an argon medium. This method is based on sputtering metallic conductors by powerful current pulses (up to 500 kA) due to discharge of a capacitor bank [6]. The energy introduced into the conductor was equal to 1.4 of its sublimation energy. The nanopowders were obtained in a UDP-5G pilot installation of the High-Voltage Research
The recent state of science and technology requires creation of new materials capable of withstanding high temperatures and operating in chemically aggressive media. Among the ceramic materials much attention is attracted today to nitrides of groups III and IV of the Periodic System. It is known that ceramics based on aluminum nitride possess high thermal conductivity [280 W/(m·K)], which is comparable to that of metallic silver. At the same time, AlN is a good dielectric (< 1013 W·m), has a high enough hardness (12 GPa), and is nonwettable by liquid metals (aluminum, gallium, etc.). Zirconium nitride is a material with high hardness and resistance to alkalis. Coatings from titanium nitride possess an elevated wear resistance. Due to their peculiar properties ceramics and coatings based in titanium and zirconium nitrides present interest for engineering and various technologies. The known commercial methods for obtaining titanium and zirconium nitrides, including that of self-propagating high-temperature synthesis (SHS), have some disadvantages, i.e., require intricate equipment and occur under pressure in the presence of pure nitrogen. SHS products are densely sintered materials and are therefore disintegrated in ball mills, which is a high-energy-consuming process. A scientific direction known as “chemical bonding of air nitrogen due to combustion of powder materials and boron” has been developed at the High-Voltage Research Institute of the Tomsk Polytechnic University and is used as a method for fabricating ceramic nitride-containing materials (the combustion products contain over 50 mass % various nitrides [1]). 1
Tomsk Polytechnic University, Russia.
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Yu. A. Amel’kovich et al. m, mg 0
TABLE 1 600°C
5
10
Composition
ti.ox (±5), °C
1 2 3
NPAl NPAl + ZrO2 = 1:1 NPAl + ZrO2 = 1:1
500 280 590
a* (±2%), Vox , % mass %/min
47.5 84.4 49.6
4.8 1.2 1.1
* The growth in the mass of the specimen (Dm ) was calculated in terms of the content of NPAl in it.
DTAAl
15
Number of specimen
TGAl
20
0
150
300
450
600
750
TAl, °C
Fig. 1. Derivatogram of a specimen of aluminum nanopowder: TGAl, change in the mass of the specimen with time; DTAAl, curve of heat emission during heating.
Institute of the Tomsk Polytechnic University [7]. The characteristics of the nanostructure of NPAl were evaluated with the help of a JSM-840 scanning electron microscope (“Geol,” Japan). The specific surface of the nanopowders was measured using the BÉT method with the help of an ASAP2020 device. We determined the effect of oxide additives on the parameters of chemical activity of the initial aluminum nanopowder in advance by evaluating it with the help of data of differential thermal analysis (DTA ) using a Q-150 derivatograph (Hungary). The specimens were heated at a constant rate (10°C/min) in air in a temperature range of 20 – 900°C [8]. The reactivity of the nanopowders was evaluated in terms of the following parameters: the initial temperature of oxidation ti.ox, the degree of oxidation a (the ratio of the mass of the oxidized metal to the mass of the initial metal in the specimen), and the maximum rate of oxidation of the metal (maximum change in the mass of the specimen Vox). A derivatogram for a specimen of aluminum nanopowder is presented in Fig. 1. In accordance with the thermogravimetric curve (TG ) the initial oxidation temperature was understood as the temperature at which the mass started to increase [9]. The degree of oxidation was also determined using the TG dependence, i.e., the growth in the mass of the specimen in the process of oxidation was recala
10 mm
culated for the mass of the oxidized metal divided by the mass of the initial metal in the specimen. The maximum rate of oxidation of the metal was also determined from the TG curve as the most rapid change in the mass of the specimen in a specified temperature range. X-ray diffraction analysis (RDA) of phases in the mixtures was performed using a DRON-3.0 diffractometer by the powder method in Cu Ka-radiation. In order to identify the crystal phases in the nanopowders and in the products of their combustion we used the JCPDS-ICDD database. Results and discussion. In order to study the parameters of chemical activity of mixtures of NPAl with TiO2 and ZrO2 during heating in air we prepared compositions containing equal (1 : 1) numbers of moles of the nanopowder and of the dioxides (dry mixing). Commercial TiO2 and ZrO2 powders were preliminarily screened through a screen with openings 63 mm in diameter. The parameters of the reactivity of the specimens of aluminum nanopowder and of its mixtures with TiO2 and ZrO2 are presented in Table 1. The experimental results show that the temperature at which NPAl without additive starts to be oxidized is 500°C (specimen 1). The presence of ZrO2 in the mixture lowers the initial oxidation temperature to 280°C (specimen 2); addition of TiO2 increases this temperature to 590°C. The degree of oxidation of aluminum increases in the mixture with TiO2 from 47.5 to 49.6%; in the mixture with ZrO2 it increases to 84.4% (specimens 2 and 3). These values of the degree of oxidation were calculated only with allowance for the reaction between the metals and oxygen. The actual values of the degree of oxidation were higher because the products contained nitrides. In addition, the maximum degree of oxidation of aluminum decreased in both cases upon the introduction of dioxides. c
b
10 mm
10 mm
Fig. 2. Microscopic photographs of a cake due to burning a mixture of NPAl and ZrO2 in air: a – c, the upper, intermediate, and lower layers, respectively.
Synthesis of Titanium and Zirconium Nitrides by Burning Mixtures of Their Oxides
a
427
c
b
1 mm
10 mm
10 mm
Fig. 3. Microscopic photographs of a cake due to burning a mixture of NPAl and TiO2 in air: a – c, the upper, intermediate, and lower layers respectively. I, % 100 90 80 70 60 50 40 30 20 10 15
25
35
45
55
2q, deg
65
Fig. 4. Diffractogram of the final products of open burning of a mixture of NPAl and ZrO2: — ZrN; q — ZrO2; — a-Al2O3; ¢ — * g-Al2O3 I, % 100 90 80 70 60 50 40 30 20 10 10
20
30
40
50
60
70
80
2q, deg
Fig. 5. Diffractograms of the final products of open burning of a mixture of NPAl with TiO2: ¨— TiN; q — AlN; ¢ — a-Al2O3; p — g-Al2O3; ¸ — TiO2 .
In order to obtain combustion products a mixture of NPAl with TiO2 and ZrO2 was poured onto a substrate of stainless steel; the weighed portion was shaped as a cone and burned in air. The burning was initiated by passing electric current through a nichrome coil contacting the specimen. The structure of the cake formed was studied layer by layer using electron microscopy. The products of combustion of
NPAl with ZeO2 acquired a violet color due to the presence of compounds with lower degree of oxidation of zirconium. The upper and intermediate (with respect to the substrate) layers were represented by columnar crystals with length ranging from 40 to 200 mm (Figs. 2a, 2b ); the lower layer had a porous structure consisting of whiskers (Fig. 2c ). The conventional diameter of the disaggregated particles did not exceed 8 mm; the content of submicron particles in them was 90%. Combustion of NPAl with TiO2 yielded whiskers in the upper and intermediate layers of the cake in contrast to the mixture of NPAl with ZrO2; faced crystals with irregular shape formed in the lower layer (Fig. 3). The results of a laser analysis of the size of disaggregated particles showed that the products of combustion of NPAl with TiO2 contained 60% particles with submicron conventional diameter; the size of the other particles did not exceed 10 mm. After the desegregation the combustion products were subjected to x-ray diffraction phase analysis in accordance with which the main phase after combustion of the mixture of NPAl with ZrO2 was stoichiometric ZrN and aluminum oxide in the form of a- and g-modifications (Fig. 4). The products of combustion of NPAl with TiO2 contained aluminum and titanium nitrides as well as aluminum oxides and titania (Fig. 5). Discussion of results. In accordance with the data obtained the final products of combustion of TiO2 and ZrO2 mixed with aluminum nanopowder contained the corresponding nitrides; the zirconium nitrides represented the main phase (see Fig. 4). With allowance for the high reducing capacity of aluminum, the process of combustion of the aluminum nanopowder seems to be accompanied by reduction of TiO2 and ZrO2 to the corresponding metals and their subsequent burning that yields nitrides, i.e., 4Al + 3O2 ® 2Al2O3, DG0 = –3164 kJ/mole,
(1)
3TiO2 + 4Al ® 3Ti + 2Al2O3, DG0 = –514.1 kJ/mole, (2) 3ZrO2 + 4Al ® 3Zr + 2Al2O3, DG0 = –1078,4 kJ/mole. (3) It is possible that the combustion of element titanium and zirconium is accompanied by formation of volatile suboxides
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and their interaction with the nitrogen of the air; this is confirmed by the presence of whiskers and acicular crystals in the products of combustion of the dioxides mixed with aluminum nanopowder (see Figs. 2 and 3), i.e., Ti(Zr) + TiO2(ZrO2) TiO(ZrO),
(4)
2TiO(ZrO) + N2 2TiN(ZrN) + O2.
(5)
Thus, we have obtained ceramic powders containing titanium and zirconium nitrides by burning mixtures of respective dioxides with aluminum nanopowder in air. The use of the dioxides allowed us to reduce by several times the cost of the nitride-bearing materials synthesized in the combustion process. CONCLUSIONS We have established experimentally that independent TiN and ZrN phases form due to open burning of titania and zirconia with aluminum nanopowder. The compounds obtained are stoichiometric. Thermodynamic computations show that in all probability the titanium and zirconium nitrides form through formation of elemental substances. In accordance with the data of x-ray diffraction phase analysis the content of zirconium nitride in the final materials amounts to 55% and that of titanium nitride amounts to 24%. The mate-
rials can be used to form batches for fabricating sintered ceramics. REFERENCES 1. A. P. Il’in and A. A. Gromov, Combustion of Aluminum and Boron in Superfine Condition [in Russian], Izd. TGU, Tomsk (2003). 2. A. A. Dits, Oxynitride Ceramic Materials Based on Products of Combustion of Industrial Metal Powders in Air, Author’s Abstract of Candidate’s Thesis [in Russian], Tomsk (2006). 3. L. O. Tolbanova, Synthesis of Ceramic and Nitride-Bearing Materials by Open Burning of Mixtures of Aluminum Nanopowder with W and Mo Nanopowders and Cr Powder, Author’s Abstract of Candidate’s Thesis [in Russian], Tomsk (2007). 4. R. Ripan and I. Ceteanu, Inorganic Chemistry. The Chemistry of Metals, in 2 Vols. [Russian translation], Mir, Moscow (1989). 5. V. Ya. Bulanov and L. I. Kvater, Diagnostics of Metallic Powders [in Russian], Nauka, Moscow (1983). 6. O. B. Nazarenko, Electrically Exploded Nanopowders: Fabrication, Properties, Use [in Russian], Izd. TGU, Tomsk (2005). 7. D. V. Tikhonov, Fabrication of Ultrafine Powders with Complex Composition by Electric Explosion, Author’s Abstract of Candidate’s Thesis [in Russian], Tomsk (2000). 8. A. P. Il’in, A. A. Gromov, and G. V. Yablunovskii, “Activity of aluminum powders,” Fiz. Goren. Vzryv., 37(4), 58 – 62 (2001). 9. W. Wendlandt, Thermal Methods of Analysis, Wiley, New York (1974).
