Effect of Severe Quasi-Static and Dynamic Deformation on the Lamno3 Structure

STRUCTURE AND PROPERTIES OF COMPACT NANOCRYSTAL OXIDES OBTAINEDBY SEVERE PLASTIC DEFORMATIONS

1T.I.Arbuzova, 1N.M.Chebotaev, 1B.A.Gizhevskii, 2A.V.Fetisov, 2A.Ya. Fishman, 3E.A.Kozlov, 1T.E.Kyrennykh, 2L.I.Leontiev, 2V.L.Lisin, 1S.V.Naumov, 2S.A. Petrova, 1V.P.Pilugin, 1Yu.P.Sukhorukov, 1V.B.Vykhodets, 2R.G.Zakharov, 4M.I.Zinigrad

1Institute of Metal Physics, Ural Division of the Russian Academy of Science, 18 S. Kovalevskaya Str., Ekaterinburg, 620041, Russia

2Institute of Metallurgy, UD RAS, 101 Amundsen Str., Ekaterinburg, 620016, Russia

3Russian Federal Nuclear Center - E.I.Zababakhin Research Institute of Technical Physics, Snezhinsk, Russia

4Natural Science Faculty of College of Judea and Samaria, Science Park, Ariel, 44837 Israel

Abstract

The present work presents the original data concerning using a high pressure torsion method and a shock wave loading technique to produce compact oxide nanomaterials and investigations of the effect of severe plastic deformation on a microstructure, crystal lattice and stability of these compounds. This allowed us to compare two ways of deformation action that can be characterized as quasi-static and dynamic effect, correspondingly.

Particular attention was paid to a stoichiometry and surface composition changes upon severe plastic deformations. A procedure for studying chemistry of the oxide nanomaterials by means of nuclear microanalysis and Rutherford back scattering has been worked through. For surface studies the X-ray photoelectron spectroscopy has been used.

It was shown that both distortion methods permit to produce massive nano-scale oxide materials from the coarse-grained powder during a single technological cycle. Bulk nanocrystalline materials based on LaMnO3 and TiOy were obtained by the quasi-static deformation technique. Nanoscaled ceramics of CuO, Mn3O4 and LaMnO3 were produced by the dynamic deformations. The density of the nanoceramics comes to 99%.

Size effects and specific imperfection of the nanoceramics obtained lead to a set of particularities of physical properties.

Introduction

Production of the solid compact nanomaterials is one of the most actual topics in chemistry, physics of condensed matter, and material science today. The properties of such materials differ substantially from those of the corresponding polycrystalline samples [1]. These materials have grains smaller than 100 nm and often have improved mechanical properties as well as physical and chemical properties interesting for functional applications. They also have a unique crystal structure and high degree defectiveness. Therefore, it is of interest to study methods of the formation and stabilization of the nanocrystalline structure and the related mechanism of structural changes.

The main difficulties in producing compact nanomaterials from nanopowder arises from their pressing and following annealing, which may cause an increase of a grain size and lack of important features for nanomaterials. One of the most promising methods of producing compact nanomaterials is a severe plastic deformation. Apart from providing small mean grain sizes, severe plastic deformation can produce porousless massive samples having densities close to those of the corresponding coarse-grained or single-crystal samples, which cannot be reached by, for example, compacting nanopowders or hot pressing. Deformation results in a high dislocation density, fine grains (crystallites), and high concentration of point defects and stacking faults. These changes cause the formation of a specific nanostructure. The laws of the formation of the structure during plastic deformation are specified by a combination of parameters of the initial structural state of the material, deformation conditions, and the mechanism of deformation.

The methods of severe plastic deformation (such as high pressure torsion, etc) are well designed for metal nanomaterials [2]. An application of this technique for oxides gives rise to some obstacles involving changes of the original chemical composition and reduction of oxides up to metal [3,4]. It could be overcome by particular measures, such as: to take the initial oxides with oxygen overbalance (oxygen positive deviation in stoichiometry), to carry out the experiment under low temperature, etc. These methods need the oxides characterized by a wide homogeneity region, such as lanthanum manganite.

This work presents the original data concerning using a high pressure torsion method [5] and a shock wave loading technique [6,7] to produce compact oxide nanomaterials and investigations of the effect of severe plastic deformation on a microstructure, crystal lattice and stability of these compounds. This allowed us to compare two ways of deformation action that can be characterized as quasi-static and dynamic effect, correspondingly.

Methods for Preparation of Bulk Nanocrystalline Oxides

Bulk nanocrystalline materials based on TiOy, LaMnO3 and some other were obtained by the quasi-static deformation technique [8,9]. To prepare compact nanocrystalline oxides the initial coarse-grain powders were subjected to severe plastic deformation by high pressure torsion in Bridgman anvils [5]. The anvils made of the WC-Co alloy (VK6) were 5 mm in diameter. The coarse-grained powder was placed between the anvils and pressed under pressure as high as 9 GPa. The deformation was effected by the rotation of one of the anvils with respect to the other one. The rate of rotation was 0.3-1 rpm. The experiments were carried out at room temperature in air. The degree of strain was specified by the angle of rotation of the anvil [5]. Crystallite sizes up to 14 nm were archived by this technique. The high pressure torsion caused powder consolidation; as a result, a bulk sample in the form of a biconvex lens, which had a thickness of approx. 0.12 mm at its center and corresponded to the shape of the high pressure cell, were produced. In some cases the samples exfoliated.

For obtaining nanocrystalline ceramics we also used a shock wave loading (the method of dynamic deformations). The spherical explosive systems developed in the Russian Federal Nuclear Center – Research Institute of Technical Physics to produce the materials are the foundation of this method [6]. Shock wave loading is performed by a detonation of an explosive on the surface of the spherical sealed casing with the initial material inside. The initial material is a coarse-grain ceramics with a density of 70-80% from the theoretical one.

In the process of the loading by spherically convergent shock waves the initial ceramics is compacted up to 99% from theoretical density and as a result of compression and shear strains we obtain the nano-scale structure with the crystallite sizes of 10-200 nm. By means of this method we prepared a high density nanoceramics of CuO and Mn3O4 as well as LaMnO3 [7,10].

The important advantage of the methods of quasi-static and dynamic deformations is a combination of producing nanocrystalline structure and the material compacting in one technological process.

Investigations of Structure and Properties

Nuclear microanalysis and Rutherford back scattering.

An accelerator complex situated in the Institute of Metal Physics was used to determine a chemical composition of samples. The complex is based on a 2-MV Van de Graaf accelerator. An oxygen content was established by a nuclear microanalysis technique for the 16O(d,p1)17O reaction, and the Rutherford back scattering was used for cation determination. The energy of the incident beam particles was 900keV.

Concentration distribution

Fig. 1. A dependence of the Cu/O ratio on the distance from the surface of the nanoceramic ball. / Fig. 2. Variation of the crystallite size along the radius of compressed CuO ball

As an example a concentration dependence of the oxygen content on the distance from the ball surface in CuO nanoceramics obtained by a shock wave loading is presented in Fig. 1. The CuO oxide is known to have a narrow region of homogeneity. In the equilibrium state under normal conditions the copper monoxide deviates from the stoichiometric composition CuO to less than 1% [11]. Our investigations show that the ratio of oxygen and copper concentrations CO/CCu varies strongly along the radius of the compressed CuO nanoceramic ball (see Fig. 2).

Isotope exchange

For the same samples an isotope exchange kinetics had been investigated. Nanocrystalline CuO samples were annealed in oxygen atmosphere (0.21 atm), enriched by 80% of the 18О isotope, with the following proton irradiation of 762 keV. It is shown, annealing at 500 о and 700оС brings no changes of the oxygen concentration in the samples and just rises up an isotope exchange (when the isotope composition of the gas phase varied). Reaction spectra with the 18О isotope for the samples annealed at 500оС during 1, 2 and 3 hours are presented in Fig.3. It is evident no difference for different annealing time. In the next figure (Fig. 4) a corresponding 18О concentration cross-section for one-hour annealing is shown. Cross-sections for 2 and 3 hours of treatment are the same. The upper curve belongs to a nanocrystalline state, and the lower one – to a polycrystalline one. From the data obtained, it is clear the 18О oxygenation is a non-diffusion process, as nothing depends on time.

Similar results were revealed for the samples annealed at 700оС. It must only be noted, the amount of the 18О isotope got into the sample at 700оС was twice in comparison with 500оС (the lower curve in Fig.4).

For the results it may be offered the following explanation: nearby the surface it exists sufficiently deep (about a micron) layer enriched with oxygen above the stoichiometry. During annealing an extra oxygen is substituted rapidly (in less than 1 hour) into the 18О isotope, and the following annealing brings no changes as diffusion rates in the basic material at 500 оС are negligible. The same effect takes place both in single and polycrystalline samples, but with less absolute value for concentration.

Fig. 3 18O reaction spectra.

Fig. 4 Concentration cross-section

XRD-studies

X-ray diffraction was applied for phase analysis, monitoring lattice parameters of phases obtained as well as for crystallite size and lattice strain calculations [12]. Typical dependences of crystallite sizes and lattice strains on distortion conditions for nanoscaled LaMnO3 and TiOy oxides are represented in Fig. 5. In the case of LaMnO3 the quasi-static deformation method allows one to obtain nanoscaled grain size even with a small rotation angle (see Fig.5). Further rotating leads as mainly to an increase of the lattice microstrains. In the case of TiOy constant dispersibility of the microstructure is achieved in about 2 turns.

Fig. 5. Crystallite size, D, and lattice strain, ε, of LaMnO3 and TiOy against the rotation angle, φ, upon high pressure torsion deformation.

Fig. 6. Microstructure of CuO nanoceramics. Data of scanning tunnel microscopy

Microstructure

To be brief, let us restrict our consideration with CuO and Mn3O4. According to the data obtained by XRD, SEM and STM the crystallite size of the CuO nanoceramics was 10-100 nm. The results of STM studies of samples obtained by a shock wave loading are shown in Fig.6.

Fig. 7. Crystallite size and shape of the polycrystalline Mn3O4 before (a) and after (b) a shock wave loading.

The shape and size of Mn3O4 crystallites before and after shock wave loading were determined using a scanning electron microscope JEOL-5500. Figs. 7a and 7b show typical microstructures of the initial Mn3O4 polycrystal. It can be seen that the sample is loose with a large number of intergranular voids. The grain size ranges from 1 to 10 µm. The shapes of the grains are also different. Coarse grains have an intricate shape with relatively sharp “broken” edges, while small grains are quasi-spherical in shape. In contrast to the nanoceramic CuO [13], no clearly manifested interrelation between the variation of the grain size and the radius of the sphere is observed in the loaded ceramic Mn3O4. Most of small grains with a size 0.06 µm < d 0.5 µm are located in the central part of the sphere (Figs. 7b-1 and 7b-6). In addition to small grains, large crystallites are also present. It should be noted that fine grains are mainly grouped in the vicinity of boundaries of various cracks. In all probability, the shock waves passing through the material most actively modify the structure in the vicinity of inner boundaries and lead to the formation of nanostructures.

XPS study

Typical X-ray photoelectron spectrum for CuO has two distinctive maxima: a main one, with lesser bond energy corresponding to the d10copper state, and a concerned satellite conforming to the d9configuration.

Preliminary X-ray photoelectron spectroscopy (XPS) investigations of volumetric and nanoscaled (4-6 nm) CuO samples [14] have established an increase of Cu–O-bond ionicity with particle size decreasing. Besides, on the surface of nanosized samples a great amount of adsorbed oxygen has been marked (the Cu/O came to 0.16÷0.23).

In the present work XPS studies of the CuO samples obtained by a shock-wave loading were taken with the help of Multiprob manufactured by Omicron (AlKα with 1486.6eV used as an exciting radiation). The analyzer was standardized and spectra were corrected for "charging effect" using еру C1s low-energy spectrum component.

The spectra obtained for three different samples (a polycrystalline one, and two nanoscaled cut portions cut out of the sphere involved into SWL experiment) are presented in figs. 8,9. Opposite to [14] an increase of Cu–O –bond ionicity was not revealed for nanosized samples. It may be explained by a greater particle size (50-60 nm compared to 4-6 nm in [14]). At the same time as previously in [14] for nanoscaled samples it was observed a significant Cu 2p-line broadening including satellite especially for samples ‘b’ (according to notification in fig.8). It may be caused by a contamination with a surface CuCO3 phase (not revealed by XRD). High intensity of the O 1s-peak at 531.5eV corresponding to an oxygen state in copper carbonateargues for this supposition.

A slight shift of the Cu 2p3/2 peak towards low bond energies and decreasing satellite intensity (fig. 8) can be due to appearance of univalent copper. Bipartition of the Cu 2p3/2 -spectrum into Cu2+ and Cu1+ components reveals approximately 10% of univalent copper.

For nanoscaled material the Cu/O ratio came to 0.10, and to 0.08 for polycrystal, while taking into account only so called structural oxygen both values enlarged to 0.35 and 0.26, correspondingly. The last values were calculated by O 1s- spectrum envelope deconvolution.

To get explanations for the results obtained additional measurements of O 1s and Cu 2p spectra under different angles between sample surface normal and analyzer window ( = 0, 20, 40 и 55) were undertaken. It allowed us to vary the depth, , of a layer under investigation ( = 0cos, where 0 50 Å). It turned out, spectra for  50, 47, 38 and 29 Å differ significantly. Thus, upon changing λ from 50 to 29Å, the Cu1+ fraction went down from 0.24 to 0, and the Cu/O ratio (for structural oxygen only) rose from 0.35 up to 1. Accordingly, in a thin near-surface layer the composition of nanoscaled oxides is close to an average typical for CuO nanopowder obtained by chemical techniques [14].

Taking into account the abovementioned results as well as those obtained anywhere (for example references in [14]), it may be instant the object under investigation has vastly inhomogeneous structure. It resulted from shock wave effect which had lead to the formation of strong strain regions and their following relaxation in oxygen atmosphere.

Fig 8. Copper and oxygen spectra for: (a) polycrystalline object (b) nanocrystalline sample cut out at 1 mm from the surface of the sphere involved into SWL experiment (c) nanocrystalline sample cut out at 4mm

Fig 9. Cu 2p and O 1s-spectra for the nanoscaled sample ‘b’ taken at different angles .

Absorption spectra

Absorption spectra of nanoceramics and nanopowders of strongly correlated oxide CuO and classic semiconductor Cu2O in the range 0.1-3.5 eV were studied to elucidate a peculiarity of electron spectra of strongly correlated systems in nano-scale states (figs.10, 11).

Fig. 10. Absorption spectra of CuO. 1- single crystal; 2,3 –nanocrystalline CuO.

Fig. 11. Absorption spectra of Cu2O at 295K. The grain sizes of nanocrystalline Cu2O are 10-90 nm.

For comparison absorption spectra of CuO and Cu2O single crystals were measured near Eg. The absorption edges of n-CuO and n-Cu2O are strongly smeared. The blue shift of Eg of n-Cu2O is ~0.3-0.4 eV, but n-CuO has red shift of Eg up to 0.15 eV. Anomaly of Eg for n-CuO is due to high defectiveness and electron phase separation [15, 16].

Cupric oxide is perspective material for solar cells, but CuO has large Eg=1.45 eV. The red shift of short wavelength of “transparency window” up to ~0.5 eV (with regard for bands tails) extends opportunity for application of n-CuO as photosensitive material.

Magnetic studies

In the spinel structure of Mn3O4, the Mn2+ ions (S =5/2) occupy tetrahedral sites, while the Mn3+ ions (S = 2) occupy octahedral positions. The unit cell contains six manganese ions, viz., two ions (d5) in tetra positions and four ions (d5) in octa-sites. The Jahn-Teller effect for the Mn3+ ion leads to a distortion of the AB2O4 crystal lattice and results in a complex magnetic order. At T < 33 K, a skewed spin order of the Yafet-Kittel type sets in. The magnetic moments of two A sublattices (Mn2+ ions) are directed along the [010] axis, while the magnetic moments of two B sublattices (Mn3+ ions) are tilted at the angle of 69° to the [010] axis so that the total moments of A and B sites are antiparallel [17]. The intensities of intralattice and interlattice exchange interactions are comparable. The relatively high anisotropy energy and the low exchange energy destabilize the collinearity of spins in the B sublattice. Anisotropy is preserved up to T = 100 K (i.e., in the temperature range much higher than TC). At T =4.2 K, the magnetization does not attain saturation even in fields H = 300 kOe. In the temperature range 33 K <T < 300 K, a spin-spiral structure is observed with a propagation vector directed along the [010] axis. Since the noncollinear magnetic structure cannot pass directly to the paramagnetic state, the Néel spin configuration is stable in the temperature interval 39 K < T < TC.

Magnetic measurements on the initial Mn3O4 polycrystals from which the loaded samples were obtained proved that the temperature dependence of the reciprocal susceptibility at T > TChas a convex hyperbolic shape (Fig. 12). In the molecular field theory for two sublattices, this quantity is described by the relation [18]:

(1)