Effect of Severe Deformations on Microstructure and Properties of Some Oxides of Transition Metals
1T.I. Arbuzova, 1B.A.Gizhevskii, 2A.V. Fetisov, 2T.I.Filinkova, 2A.Ya.Fishman, 3E.A.Kozlov, 4I.B. Krynetsky, 1T.E.Kurennykh, 2L.I.Leontiev, 1S.V.Naumov, 2S.A.Petrova, 1V.B.Vykhodets, 2R.G.Zakharov, 5M.I.Zinigrad
1Institute of Metal Physics, UD RAS, 18S.Kovalevskaya St., Ekaterinburg, 620041, Russia
2Institute of Metallurgy, UD RAS, 101 Amundsen St., Ekaterinburg, 620016, Russia
3RFNC - E.I.Zababakhin Research Institute of Technical Physics, Snezhinsk, Russia
4Moscow State University, Leninskie gori, Moscow, Russia
5Natural Science Faculty of College of Judea and Samaria, Science Park, Ariel, 44837 Israel
Abstract
Physicochemical properties of the nanocrystalline oxides obtained by the high-pressure torsion, shock-wave loading and mechanochemical methods have been investigated. It is shown the properties of the nanostructural oxides differ greatly from those of the poly- and monocrystalline materials. Relationships among structural characteristics of the nanoscaled oxides and their prehistory, way and conditions of producing have been detected. Spectral, magnetic, electrical and diffusion features have been established.
Introduction
Producing of bulk (massive) materials with submicron and nanocrystalline structure is one of the main tasks of modern material science. Mechanical and physicochemical properties of these materials are perspective both in constructional and functional use. In this connection different ways to form and stabilize nanocrystalline structure in volumetric material are interesting. At present numerous methods for producing nanocrystalline powders are known: steam condensation in the inert gas, deformation in ball mills, various chemical techniques [1].
The main aim of the present study carried out in the frame of Israeli-Russian scientific cooperation was producing oxide nanomaterials by severe plastic deformations and investigating their physicochemical properties.
At the previous stages [2, 3] there were solved problems concerning choosing and basing ways of synthesize and study of the oxide nanocrystalline materials, together with readjusting the existing investigating techniques to a specificity of the oxide systems. The nanocrystalline oxides were produced by the high-pressure torsion, shock-wave loading and mechanochemical methods. The present paper presents a whole set of the results obtained:
- features of the chemical and phase composition, structure, and electron (valence) state of the surface and bulk atoms in the nanocrystalline oxides produced by different methods;
- magnetic and electrical properties of the nanocrystalline oxides;
- an oxygen non-stoichiometry and oxygen isotope exchange kinetics in the nanooxides;
- nanostate stability upon interaction between an oxide and gas phase redox atmosphere.
The objects under investigation were simple 3d-metal oxides (CuO, TiO etc.) as well as more complicate oxides AMnO3 with perovskite structure and spinels Mn3O4. These oxides are characterized by a wide homogeneous range and a set of physicochemical properties holding promise for both fundamental research and practical applications.
1. PHYSICOCHEMICAL PROPERTIES OF THE NANOOXIDES
1.1. The CuO oxide: Production and physicochemical properties
Nanostructural CuO samples were prepared by a shock-wave loding method (dynamic deformations). The structure and properties of those objects depend on the layer of the pressed out ball they were cut from (table 1).
Table 1. Treatment conditions and characteristics of the copper oxides
Name / Chemical composition / Distance from the ball surface, mm / Crystallite size D, nmSample 1 / CuO0.99 / 6 / 30
Sample 2 / CuO0.99 / 8 / 40
Sample 3 / CuO1.04 / 0.5 / 80
For reference single crystals and coarse-grained (15μm) polycrystalline CuO were used.
1.1.1. Surface studies of the nanocrystalline CuO
Our X-ray photoelectron spectroscopy and optical studies [4, 3 with refs.] revealed distinctly inhomogeneous surface structure of the material involved. Shock wave loading caused the formation therein of strongly deformed regions (first of all on the grain boundaries), whose surface layer relaxed with time (up to ~30 Å deep).
1.1.2. Oxygen content studies of the nanoscaled CuO oxides
Oxygen content was determined by a nuclear microanalysis technique and Rutherford back scattering. Earlier these results were introduced in [2, 3]. For the abovementioned samples it was shown an inhomogeneous oxygen distribution and noticeable fluctuations in comparison with the coarse-grain composition
1.1.3. Microstructure of the CuO oxides
The results of SEM and TEM studies [2, 3] testify the nanocrystalline status of producing oxide materials. They coincides with XRD data given grain size of 10-100 nm and give an opportunity to choose certain parts of the bulk sample according to their specific structural characteristics and properties.
1.1.4. Isotope exchange studies of the CuO oxide
To analyze isotope exchange kinetics nanocristalline and polycrystalline CuO samples of micron grain size were annealed in the oxygen atmosphere enriched up to 80% with 18О isotope and irradiated with protons at 762 keV to get spectrum of the 18О(p, α)15N reaction at the 18О isotope. The oxygen pressure upon annealing was 0.21 atm correlated to the partial oxygen pressure in air.
Nuclear microanalysis data presented together with the XPS results in [2] verified the conclusion concerning distinctively inhomogeneous structure of the nanocrystalline CuO oxide obtained by a shock-wave loading. This inhomogeneity mould the character of the isotope exchange kinetics, non-diffusion nature of which in the temperature–time interval involved became apparent as an abnormal time dependence of the 18О content in the samples. The following explanation was supposed: close to the sample surface there exists rather a deep layer (up to a micron) enriched with an oxygen above stoichiometric amount. During annealing in the 18О atmosphere this excess oxygen is rapidly (quicker then in 1 hour) exchanged with the 18О isotope and the following annealing affects nothing, as a diffusion rate in the bulk at 500оС is quiet low. The analogous but smoother effects were noted in mono- and polycrystalline samples.
1.1.5. Absorption spectra of the copper oxides
The copper monooxide exemplifies the materials with strong electron correlations. We established [5, 3] conversion of bulk CuO samples to a nanostate brought out a distinctive red shift of an absorption edge. It is caused by intergap levels with high density of states taking place for the 3d-nanooxides. This phenomenon gives an opportunity to manage spectral characteristics of the nanomaterials, in particular, to vary a band-gap effective width, in other words to vary a spectral window. It opens up new possibilities for developing selective covers for nanooxide based solar heaters.
1.1.6. Thermal expansion peculiarities of the compact CuO nanoceramics
Here the results of thermal expansion studies of the compact CuO nanoceramics with different crystallite sizes throughout the earlier work [6] are presented.
It is shown at low temperatures (below 80К) all the samples under investigation have regions with anomalous thermal expansion that is with a < 0. Earlier the negative thermal-expansion coefficients have been detected for a set of oxide compounds [7,8], but these results related to coarse-grain materials or single crystals. As against the nanostructural materials represent non-equilibrium metastable systems with high concentration of defects, chemical composition abnormalities and particularities in electronic spectrum. In this connection a particular attention was paid for reasons of abnormal thermal properties appeared for the nanostructural oxides.
There were chosen 3 samples with crystallite sizes of 20, 70, and 90 nm. Thermal expansion has been measured with the help of strain-gauge dilatometer with the unit strain sensitivity equal to 5.10-7 in the temperature range of 4.7- 250K upon heating with the rate of 1K/min. As evident from the data obtained (Fig.1) a temperature dependence of thermal expansion for all sample can be divided into two parts: low- and high-temperature one, below and above 80 К correspondingly. High-temperature range is characterized by monotonous relative lengthening of the sample with increasing temperature, whereas in the low-temperature range regions with zero and even negative coefficients of thermal expansion occur. In the high-temperature part the temperature dependence of thermal expansion approximates to a line, the smaller crystallites size the higher temperature at which the dependence becomes a linear one.
Average thermal expansion coefficients calculated for the high-temperature ranges are presented in table 2. In the bottom line the thermal expansion data for the CuO single crystal along c axis according to [9] is given.
Table 2 Thermal expansion coefficients of the CuO nanoceramics
Sample / Crystallite size, nm / Temperature range / a, 10-6 К-11 / 20 / 80 - 180 / 6.3
2 / 70 / 80 - 190 / 5.4
3 / 90 / 80 - 150 / 4.2
4 / single crystal / 80 - 150 / 1.8
Evidently, the nanoscaled oxides represent higher thermal expansion coefficients in comparison with single crystals. For comparison, a nanocrystalline copper sample with an average crystallite size equal to 8nm has thermal expansion coefficient equal to a = 31.10-6 К-1, twice higher than a = 16.10-6 К-1 for a coarse-grained sample [10]. The authors explained this effect through great difference between thermal expansion coefficients of grain boundaries (interfaces) and grain bodies.
Fig. 1 Thermal expansion of the CuO nanoceramics with different crystallite size: 120 nm, 2 - 70 nm, 3 - 90 nm.
The dependence of the thermal expansion coefficient on the crystallite size established in our study is confirmed by literature data on a heat capacity [11].
Let us dwell on thermal expansion of the nanostructural CuO oxides in the low-temperature region. Below 80К thermal expansion of all samples under investigation has abnormal non-monotone temperature dependence, at that samples 1 and 2 in the temperature range of 6-20 К have negative a values and sample 3 has nearly zero coefficient a in a rather wide interval of 10-40 K. Thus, there is no any clear dependence on a crystallite size.
Low-temperature anomalies of thermal expansion of the CuO nanoceramics can be conditioned by a specific character of nanocrystalline state of transition metal oxides. Results obtained by XPS and Rutherford back scattering unambiguously revealed reallocation of cations and anions in the nanosized grains, that is valence redistribution in cation and anion sublattices. As a result charge carrier spectrum tuning takes place, mixed valence (MV) centers with orientation degeneracy appear, and a tendency towards phase segregation may arise. These effects generally are responsible for low-temperature anomalous properties of the crystal systems. We considered the influence of phase segregation and tunnel MV centers on thermal expansion.
Four cations and anions nearest to the crystal lattice defect with an electron and a hole tunneling among them were taken as a model of a center with orientation degeneracy. Taking into account the tunneling, energy levels of such a quadruple orientation-degenerated MV center with tetragonal symmetry are split into the states corresponding to the (A1 + B1 + E) irreducible representation of the C4V group. Failing random crystal fields quadruple orientation degeneracy of dipole centers is removed only due to tunneling and Hamiltonian spectrum (1) consists of a neighboring doublet and two singlet levels:
(1)
where Dt – tunneling split parameter. The structure of the states specified for the above centers is similar to that of the Jahn-Teller Tb3+ ions in TbVO4, TbAsO4 crystals [12].
The corresponding contribution from MV centers to the heat capacity DС, and thermal expansion coefficient (TEC) - Da(T) is described by the expression
: (2)
where x and N0 the concentration and amount of the MV centers in the crystal, correspondingly (x = N0/N), Ω0 – the crystal volume per a formula unit (Ω0 = Ω/N), K – isothermal compression modulus, - full-symmetric deformation.
Typical temperature dependences for the characteristics mentioned are presented in Figs.2,3. It is clear low-temperature properties of the systems under investigation can be varied due to the contribution of MV centers.
Similar expression has been got for TEC of a solid methane in [13]. It has been shown upon homogeneous expansion of a crystal lattice a potential barrier height for a tunneling particle most likely decreased. As a result the value occurs to be positive and abnormal – negative – contribution from tunneling centers into the thermal expansion coefficient a(T) can take place
Fig.2. Temperature dependence of the dipole centers contribution Da(T) to TEC,.Darel is related to Da(T) as: Da(T) = f Darel, where / Fig.3. Temperature dependence of Da:
G/Dt=0.5(1), 1.5(2), 5(3); ¶Dt/¶eA/¶G/¶eA =-0.6, where G - is a random fields dispersion.
Let us dwell on the TEC value for the CuO system without random crystal fields. Easy to estimate, for the typical values of x~10-2 experimental magnitudes of 4x10-6 observed at the minimum of the TEC temperature dependence can be archived only under sufficiently strong influence of deformation on the tunneling parameter, that is when At the same time a tunneling split parameter must be about 50К.
Thus, one may assume orientation-degenerated MV centers essentially affect low-temperature properties of the oxides and may cause an abnormal TEC behavior due to the negative contribution from the MV centers into the total effect. In this case, distinctive heterogeneities of random strain distribution and defectiveness depending on the part of the pressed out ball where the samples were taken from should respond for fluctuations in their properties.
Another possible cause for the anomalies observed may be low-temperature phase segregation taking place in the nanoscaled oxide system. It is clear due to kinetic restrictions the matter may touch only effects concerned with separation of the charge carriers sufficiently labile at experimental temperatures. In particular, it may be cooperative effects in the MV centers system (orientation degeneracy like any other in the system of interacting centers gives spinodal instability). Manifestation of the corresponding mechanism in low-temperature thermal expansion anomalies was demonstrated by a qualitative simple model of phase segregation in quasi-binary peeling systems with nearest neighbors’ interaction. It was supposed each phase had its own cell parameter aA(x,t), aB(x,t). In the two-phase region changes of the sample linear dimensions calculated under a ‘lever rule’:
(4),