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20XV International Symposium on Self-Propagating High-Temperature Synthesis
THE EXCLUSIVE ROLE OF THE INITIATION TEMPERATURE IN THE START OF NANOSCALE SOLID-STATE REACTIONS
V. G. Myagkov
Kirensky Institute of Physics, Federal Research Center KSC SB RAS, Krasnoyarsk, 660036 Russia e-mail: miagkov@iph.krasn.ru
DOI: 10.24411/9999-0014A-2019-10105
Control and predictability of the synthesis of new materials is one of the most fundamental challenges in material science. The prediction of the phases in binary systems that will be formed during the thin-film solid-state reaction has been a subject of numerous studies. and different empirical rules have been developed for predicting first phase formation. Studies of solid-state reactions in nanolayers showed three fundamental features that strongly distinguish them from bulk powders:
(i) Formation of only the first phase at the film reagent interface at a certain temperature called the initiation (formation) temperature Tin. As the annealing temperature is increased. other phases can occur and form the phase sequence.
(ii) The threshold of the reaction. characterized by intense intermixing at the interface and formation of compounds. arises at temperatures above initiation (formation) temperature Tin. The values of Tin in the first phase can be about room temperature or even at cryogenic temperatures.
(iii) Migration of the dominant diffusing species through the interface during first phase formation.
The formation of only the first phase among equilibrium phases. low initiation temperatures. and migration of the dominant diffusing species are unique. unexplained features of solid-state reactions in nanofilms. From the above results it follows that the first phase and its initiation temperature Tin are control characteristics of the thin-film solid-state reactions. The initiation temperatures Tin of the first phase for most bilayers (multilayers) lie below 400°C. However, many thin-film reactions are initiated near room temperature and even at cryogenic temperatures. At such low temperatures, diffusion is extremely small and cannot provide the transfer of atoms in the solid state. This suggests an alternative view, in which chemical interactions rather than diffusion ones take place, plays a crucial role in the initiation and kinetics of interphase reactions in the solid state [1-3].
Previously, it was shown that the initiation temperatures Tin are close or coincide with the solid-state transformation temperatures Tk of the reagent-based binary system, such as orderdisorder transitions, the superionic transition, the spinodal decomposition and others [1-3]. In particular, initiation temperatures Tin(Cu/Au), Tin(Pd/Fe), Tin(Co/Pt), Tin(Ni/Fe), and Tin(Cu/Fe) of reactions in the Cu/Au, Pd/Fe, Co/Pt, Ni/Fe, and Cu/Fe bilayers coincide with the minimum temperature of the order-disorder phase transition in Cu-Au, Pd-Fe, Co-Pt and the eutectoid decomposition temperature in the Fe-Ni, Cu-Fe binary systems, respectively. Martensitic transitions are not diffusion transformations; however, they also satisfy (Tin = Tk) equality. It is well established that the ordered B2 alloys, such as NiTi, AuCd, NiAl have reversible low-temperature martensitic transformations. in which the high-temperature austenite B2-phase develops into a low-temperature martensitic phase through a complex process of the formation of intermediate phases. We have shown earlier that the initiation temperatures Tn(Ti/Ni) < 150°C, Tin(Ni/Al) ~ 180°C, Tin(Cd/Au) = 67°C in Ti/Ni, Ni/Al, Cd/Au bilayers, respectively [1-3]. These temperatures are close or coincide with reverse martensitic
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V. G. Myagkov
ISHS 2019 Moscow, Russia
transformation starting temperatures As(B2-TiNi) ~ 100°C, As(B2-NiAl) ~ 180°C, As(B2-CdAu) = 67°C. As mentioned above, with increasing of the temperature of the bilayer above Tin leads to the beginning of intermixing of the reagents and first phase synthesis on the interface and consequently physical characteristics of the film samples, such as electrical resistance, magnetization, transparence, heat release begins radically change. Obviously, the start temperature of these changes is the reaction initiation temperature Tin. In most cases the energetic properties of thermite nanocomposites were investigated by differential thermal analysis (DTA), thermogravimetric analysis (TGA), and differential scanning calorimetry (DSC). In this case the initiation temperature Tin is the temperature at which heat release starts.
The one important characteristic of reactive multilayer films is the ignition temperature Tig which can be defined as a minimum temperature of onset of a self-sustaining reaction for given experiment. As known, the self-sustaining regime of reaction arises then the rate of heat generation Qreaction overcomes the rate of heat losses Ql oss (Qreaction > Q oss ). Unlike Tig the initiation temperature Tin is start temperature of reaction at which the rate of heat generation Qreaction less than the rate of heat losses Qloss (Qreaction < Qloss) and so always the initiation temperature Tin is less than the ignition temperature Tig (Tin < Tig). As discussed above the initiation temperature Tin is threshold temperature: no reaction below Tin and reaction initiate just the temperature of sample overcomes Tin. Thus, the initiation temperature Tin is fundamental temperature for given reaction couple. In contrast to Tin, the ignition temperature Tig is a kinetic quantity that depends on the heating rate and the rate of heat loss.
In contrast to the universally accepted diffusion mechanism, the above clearly demonstrates that the same chemical interactions underlie and control both the thin-film solid-state reactions and corresponding solid-state transformations [1-3]. Under the impact of chemical interactions above the initiation temperature T > Tin the chemical bonds in the reactants are broken. the reacting atoms are transfer through of the reaction product layer and the synthesis of new compounds are occurring. This proves that there is no reaction below Tin. The equality Tin = Tk indicates that low-temperature solid-state thin-film reactions in A/B bilayers occur only in AB binary systems, which have corresponding low-temperature solid-state transformations. Therefore, the study of reactions in A/B bilayers with different layer ratios is a study of the A-B phase diagram.
1. V.G. Myagkov, L.E. Bykova, A.A. Matsynin, M.N. Volochaev, V.S. Zhigalov, I.A. Tambasov, Yu.L. Mikhlin, D.A. Velikanov, G.N. Bondarenko, Solid state synthesis of Mn5Ge3 in Ge/Ag/Mn trilayers: Structural and magnetic studies, J. Solid State Chem., 2017, vol. 246, pp. 379-387.
2. V. Myagkov, O. Bayukov, Yu. Mikhlin, V. Zhigalov, L. Bykova, G. Bondarenko, Longrange chemical interactions in solid-state reactions: effect of an inert ag interlayer on the formation of L10-FePd in epitaxial Pd(001)/Ag(001)/Fe(001) and Fe(001)/Ag(001)/Pd(001) trilayers, Phil. Mag., 2014, vol. 94, no 23, pp. 2595-2622.
3. V.G. Myagkov, V.C. Zhigalov, L.E. Bykova, G.N. Bondarenko, Long-range chemical interaction in solid-state synthesis:chemical interaction between Ni and Fe in epitaxial Ni(001)/Ag(001)/Fe(001) trilayers, Int. J. Self-Propag. High-Temp. Synth., 2009, vol. 18, no. 2, pp. 117-124.
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