EFFECT OF SEVERE DEFORMATION ON
MICROSTRUCTURE AND STOICHIOMETRY OF SOME
OXIDES OF TRANSITION METALS
1
N.M.Chebotaev, 2A. Gedanken, 1B.A.Gizhevskii,
3
A.V.Fetisov, 3A.Ya. Fishman, 4E.A.Kozlov, 1T.E.Kurennykh,
3
L.I.Leontiev, 1S.V.Naumov, 1A.M.Patselov, 3S.A. Petrova,
1
V.P.Pilugin, 1V.B.Vykhodets, 3R.G.Zakharov, 5M.I.Zinigrad
1
Institute of Metal Physics, Ural Division of the Russian Academy of
Science, 18 S. Kovalevskaya Str., Ekaterinburg, 620041, Russia
2
Department of Chemistry, Bar-Ilan University, Ramat-Gan,
52900, Israel
3
Institute of Metallurgy, UD RAS, 101 Amundsen Str., Ekaterinburg,
620016, Russia
4
Russian Federal Nuclear Center - E.I.Zababakhin Research Institute of
Technical Physics, Snezhinsk, Russia
5
Natural Science Faculty and the Materials Research Center of the
Academic College of Judea and Samaria, Science Park, Ariel,
44837 Israel
Inroduction
Production of the solid compact nanomaterials (bulk NM) 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 interesting physical and
chemical properties for functional applications. They also have a unique
crystal structure and a high degree defectiveness. Therefore, it is of
interest to study the methods of the formation and stabilization of the
nanocrystalline structure and the related mechanism of structural
transformations.
The main difficulties in producing bulk NM from nanopowder arises
from their pressing and following annealing, which may cause an
increase of a grain size and lack of important features for NM. One of
the most promising methods of producing bulk NM is a severe plastic
deformation. Apart from providing small mean grain sizes, severe plastic
deformation can produce poreless massive samples having densities
close to the densities of the corresponding coarse-grained or singlecrystal samples, these densities 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 NM [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
stoichiomtry), to carry out the experiment under low temperature, etc.
These methods need the oxides characterized by a wide homogeneity
region, such as lanthanum manganites.
Methods for preparation of bulk nanocrystalline oxides
To prepare compact nanocrystalline oxide the initial coarse-grain
powder of LaMnO3 was subjected to severe plastic deformation by high
pressure torsion in Bridgman anvils [5]. This method we name the
method of quasi-static deformations. The anvils (fig.1) made of the VK6
WC-Co alloy were 5 mm in diameter. The 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]. The experimental conditions and the
properties of the samples produced are given in the Table 1. The shear
strain under pressure caused powder consolidation; as a result, a bulk
sample in the form of a biconvex lens, which had a thickness of ~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 LaMnO3 nanocrystalline ceramics we also used a
shock wave loading (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 (fig.2) performs 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 other
oxides [7,8]. The important advantage of the methods of quasi-static and
dynamic deformations is a combination of producing nanocrystalline
structure and the material compacting one technological process.
1
2
3
1.
2.
3.
Starting material
Hermetically sealed capsule
External jacket
Fig.1 A high-pressure plant with a Fig2. A scheme of loading by
torsion attachment with (a) and spherical shock waves
without (b) restrictive rings:
1-sample, 2-anvils;3-thermocouple
Experimental
XRD-experiment
X-ray diffraction was applied for phase analysis, determination of lattice
parameters of phases obtained as well as for crystallite size and lattice
strain calculations using Hall-Williamson method [9,10]. XRD studies
of the initial and distorted samples were carried out with the automatic
diffractometer DRON-UM1 (CuKβ-radiation, the Bragg-Brentano
geometry, a pyrographite monochromator on the reflected beam).
Patterns were taken in a continuous and scan mode with a step
Δ(2θ)=0.02; 0.04 depending on the reflection half-width. For the scan
mode exposition time varied from 50 to 200s per step according to the
reflection intensity. Silicon with a=0,54309(1)nm was used as an
external standard.
High pressure torsion deformation was applied to an
orthorhombic modification of LaMnO3 (sp.gr.Pnma) with crystal lattice
parameters a=0,5655(1) nm, b=0,7722(1) nm, c=0,5528(1) nm.
XRD analysis of the initial sample for a shock-wave loading revealed a
two-phase mixture of rhombohedral (R 3 c) and cubic (Pm 3 m) phases
with lattice parameters aR=0,5522(1) nm, cR=1,3324(2) nm and
aC=0,3886(1) nm, correspondingly.
XPS study of nanocrystalline LaMnO3
For surface studies of the LaMnO3 samples x-ray photoelectron
spectra (Multiprob manufactured by Omicron, MgKα with 1253.6 eV
used as an exciting radiation) were taken. Spectra were reduced for
charging correction. Measurements were undertaken in a vacuum of 10 -8
mm Hg without preliminary surface purification, as Ar treatment and
heating caused changes in the surface conditions.
Disturbance of a stoichiometry of CuO nanoceramics
Nanocrystalline oxides prepared by the method of dynamic
deformations have a high degree nonstoichiometry. Using CuO
nanoceramics as an example we investigated this phenomenon by means
of nuclear microanalysis and Rutherford back scattering (RBS).
A nuclear 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 (NM) for
the 16O(d,p1)17O reaction, and the Rutherford back scattering (RBS) was
used for copper determination. The energy of the initial beam particles
was 900keV. A plain surface of a sample was installed perpendicular to
the initial beam. Energy spectra from nuclear reaction products were
registered by skin-deep silicon detectors at the angle of 160° for both
oxygen and copper spectra. For counting protons of the 16O(d,p1)17O
reaction a back-scattering deuteron absorber (a lavsan film of 16μm)
was mounted. The initial deuteron beam was of 0.5mm in diameter. It
determined the size for the analysis zone, over which measured oxygen
and copper concentrations were averaged. The number of the initial
beam particles caught the sample (radiation dose) was counted by a
second monitor of 1% accuracy. It determined a minimum error for the
oxygen and copper concentrations measured. Preliminary tests showed
an insufficiency of such accuracy for the purposes of the present
investigation. Due to this it was decided to refuse separate use of the
NM and RBS methods and the reaction products were registered
simultaneously on both oxygen and copper nuclei, i.e. under fully equal
radiation dose. It allowed us to exclude a radiation dose error from the
common precision of composition measurements for the sample surface
irradiated. As a result, a root-mean-square error for the oxygen content
came to 0.4%. It also must be noted, upon using the experimental
scheme described above the analyzed zones for both NM and RBS
methods coincided. The oxygen and copper concentrations were
calculated from comparison between spectra of a sample under interest
and a standard one with an oxygen content constant in depth. The CuO
sample with an oxygen content assumed to be 50 % was used as a
standard.
A dependence of the oxygen content on the distance from the ball
center to the external surface was studied. For this purpose a plain
sample was cut out from the ceramic ball involved into a shock wave
loading. Using the NM and RBS methods the concentration profiles of
the elements could be obtained up to a 1μm depth nondestructively. In
this connection, all precautions against contaminating surface layers
with carbon atoms upon sample cutting were taken. An appropriate
monitoring was carried out with the help of the NM method by the
12
C(d,p)13C reaction. No uncontrolled carbon alloyage of the surface
layers was established. Sample movement under the beam was made
distantly by a step motor with a step 0.7mm. A radiation dose at each
measuring point on the sample was 2x1017 deuteron/cm2 and no effects
on the chemical composition of the sample were fixed for this dose. A
concentration dependence of the oxygen content on the distance from
the ball centre is presented in fig.19. Upon treating spectra of nuclear
reactions and RBS a total content of the oxygen and copper was
assumed as 100%.
Results and discussion
XRD-studies of the nanocrystalline LaMnO3
Treatment conditions and features of the LaMnO3 samples are collected
in the tables 1, 2.
Table 1. Treatment conditions and characteristics of the LaMnO3
samples involved into a quasi-static deformation by high pressure torsion
Sample
Treatment conditions
No.
1
2
3
4
5
initial
9ГПа, ω=0,3 φ=60°
9ГПа, ω=0,3 φ=180°
9ГПа, ω=0,3 n=1
9ГПа, ω=0,3 n=3
Crystal lattice parameters, nm
a
b
c
0,56551
0,56482
0,56433
0,56403
0,5641
0,77221
0,77232
0,77223
0,77253
0,7772
0,55281
0,55262
0,55283
0,55293
0,5531
CrystalLattice
Volume, lite size strain ε, %
3
<D>, nm
nm
0,24141
0,24102
0,24093
0,24093
0,2422
~μm
22
21
23
23
0,46
0,96
1,20
1,40
Table 2. Treatment conditions and characteristics of the LaMnO3
samples involved into a dynamic deformation by shock wave loading
Samp
le No.
Treatment
conditions
6
initial
dynamic deformations, centre
dynamic deformations, 1114mm from
ball centre
dynamic deformations, 2023mm from
ball centre
7
8
9
X-ray phase
analysis
Crystal lattice
parameters, nm
Crystallite size Lattice strain ε,
<D>, nm
%
cR
ac
R 3 c Pm 3 m aR
R 3 c Pm 3 m R 3 c Pm 3 m
75%
25% 0,55221 1,33242 0,38871 ~μm
~μm
-
100%
-
-
0,39011
38
0,21
40%
60%
0,55265 1,33707 0,38895
31
48
0,20
0,38
60%
40%
0,55255 1,34017 0,38865
33
62
0,42
0,68
Figs.3, 4 show parts of the XRD patterns for the initial and distorted
samples of LaMnO3. XRD profiles were described by a pseudo-Voigt
function. Examples of modeling are presented in figs.5, 6.
initial (No.6)
exploded, centre (No.7)
exploded, periferic (No.9)
2500
1200
(004)
initial (No.1)
strained (No.4)
Intensity, rel.units
Intensity, rel.units
2000
800
(220)R
(208)R
1500
(220)C
1000
400
500
(410)
60.0
60.5
0
58
59
60
61
62
61.0
61.5
2
63
2
Fig.3. Experimental XRD pattern
of the lanthanum manganite
samples before and after high
pressure torsion for the (410) and
(004) reflections
700
strained
calculated
(410)
(004)
600
500
400
300
200
exploded
calculated
(220)R
1500
(220)C
Intensity, rel.units
Intensity, rel.units
Fig.4. Experimental XRD pattern
of the lanthanum manganite
samples before and after shockwave loading for the (220)R,
(208)R and (220)C reflections.
Indexes R and C indicate
rhombohedral and cubic phase,
correspondingly
(208)R
1000
500
100
0
-100
58
59
60
61
62
63
2
Fig.5.
Experimental
and
calculated patterns of the (410) и
(004) reflections for quasistatically
distorted
LaMnO3
(No.4)
0
60.0
60.5
61.0
61.5
2
Fig.6.
Experimental
and
calculated patterns of the (220)R,
(208)R and (220)C reflections for
dynamically distorted LaMnO3
A value and character of a line broadening at the XRD patterns
of the distorted samples as well as an 'express-evaluating' comparison of
the ratio between pure physical broadening of the two lines β 2/β1 and
tgθ2/tgθ1 or ctgθ2/ctgθ1 proof to be caused by both crystal lattice strain
and decreasing a coherent scattering domain size. So, experimental
broadening β(2θ) is a sum of size-induced, βs, and strain-induced, βd,
broadenings.
0.05
0.09
040
321
211
0.08
123
004
0.07
-1
020
101
200
220
-1
042
022
220
200
*, nm
*, nm
0.04
202
002
0.06
111
100
0.05
110
0.03
* =0.04228+0.00512 s
0.04
* =0.02631+0.00212 s
0.03
0
1
2
3
4
5
s,nm
6
7
8
0.02
-1
0
1
2
3
4
s,nm
Fig.7 A dependence of the
adjusted β*(2θ)=β(2θ)cosθ/λ line
broadening on the scattering
vector s=2sinθ/λ for the quasistatically
distorted
LaMnO3
sample (No.2)
7
8
0.07
220
208
024
0.06
0.06
200
220
0.05
211
111
0.05
300
104
012
0.04
110
*, nm
-1
-1
006
*, nm
6
Fig.8 A dependence of the
adjusted line broadening on the
scattering
vector
for
the
dynamically distorted LaMnO3
sample (No.7)
018
0.07
5
-1
0.04
100
110
214
202
0.03
0.03
0.02
* =0.02842+0.00431 s
* =0.0179+0.00628 s
0.01
0.02
0
1
2
3
4
s,nm
5
6
7
8
-1
Fig.9 A dependence of the
adjusted line broadening on the
scattering
vector
for
the
rhombohedral phase of the
dynamically distorted LaMnO3
sample (No.9)
0
1
2
3
4
s,nm
5
6
7
8
-1
Fig.10 A dependence of the
adjusted line broadening on the
scattering vector for the cubic
phase of the dynamically
distorted LaMnO3 sample (No.9)
To separate size and strain parts of the reflection width the HallWilliamson method was used [9,10]. Figs.7-10 show dependences of an
adjusted line broadening β*(2θ) on a scattering vector s=2sinθ/λ for the
LaMnO3 samples deformed by both methods. The adjusted broadening
was calculated as β*(2θ)=β(2θ)cosθ/λ. The fact, the broadening is
increased with an increase of the scattering vector confirms the
supposition of two types widening sources. Extrapolating the β*(s)
dependence on the β*(s=0) value allows to determine the size-induced
part of the broadening. And the strain-induced part can be found directly
from the slope of the graph.
In the case of the sample involved into a shock-wave loading a
phase transition took place in the centre of the ball. As a result a single
cubic phase with a=0.3901(2) nm has been formed. For the ‘peripheral’
samples taken far from the ball centre all calculations of the size and
strain terms were made separately for each (rhombohedral and cubic)
phase.
5.56
c, Å
5.54
5.52
1.35
5.50
1.30
1.25
5.48
7.78
aR,cR,aC,nm
1.20
b,Å
7.76
7.74
0.55
0.50
7.72
0.45
5.66
0.40
0.35
0
5.65
5
10
15
20
25
a, Å
r,mm
5.64
0
200
400
600
800
1000
1200
rotation angle , grad
Fig.11 A dependence of the crystal
lattice parameters on the rotation
angle, φ, for the quasi-static
deformation by high pressure
torsion
Fig.12 A dependence of the crystal
lattice parameters of the coexisting
phases on the distant of the ball
centre for the dynamic deformation
by shock-wave loading
Figs. 11-14 represent dependences of the crystal lattice
parameters, crystallite sizes and lattice strains on the distortion
conditions.
Crystal lattice changes for the samples involved into a shockwave loading are of a certainly anisotropic character. In the samples
subjected to a deformation up to n=1 changes in the cell parameters
leave crystal lattice volume practically unaltered. On a higher extent of
deformation (n=3) a unit cell volume enlarges significantly. It may be
probably caused by changes in the deformation character and/or by a
decrease of an oxidation number [3,4].
The dynamic deformation of the two-phase sample led to the
ratio changes between the two phases without any changes of the lattice
parameters (within measurement errors).
65
1050
1000
0.7
<D>C
60
55
50
, %
<D>, nm
1.0
900
40
0.5
0.5
45
40
0.4
35
20
0.3
30
25
0
0.2
0.0
0
200
400
600
800 1000 1200
rotation angle , grad
<>C
0.6
,%
950
<D>, nm
<>R
<D>R
1.5
0
200
400
600
800 1000 1200
rotation angle , grad
20
0
5
10
15
r, mm
Fig.13 A dependence of the
crystallite size and lattice strain on
the rotation angle, φ, during the
quasi-static deformation by the
high-pressure torsion
20
25
0
5
10
15
20
25
r, mm
Fig.14 A dependence of the
crystallite size and lattice strain on
the distant of the ball centre during
the dynamic deformation by the
shock-wave loading
The quasi-static deformation method brings to a considerably
smaller grain size even with a small rotation angle. Further rotating leads
as mainly to an increase of the lattice microstrains. As can be seen from
the results above, it is interesting to investigate a behavior of quasistatic deformed samples upon slight deformation extent (φ<60°), as well
as to study an effect of heavy deformation (with φ>1100°). It is of the
particular interest to establish an effect of the rotation speed on a grain
size, lattice strains and crystal lattice transformations.
For nanocrystal oxides the method of dynamic deformations results
in larger crystallite size and less microstrains. It can be connected with
high residual temperatures affected a sample after a shock wave loading.
XPS study of a nanocrystalline LaMnO3
X-ray photoelectron studies of LaMnO3 surface conditions of the
samples involved into high pressure torsion were carried out for the
initial orthorhombic modification of LaMnO3 (No.1) and the sample
No.2 distorted with a small rotation angle (φ<60°).
A mean crystallite size for the nanoscale LaMnO3 was 22 nm and
the lattice strain archived 0.46%. The b and c crystal lattice parameters
remained nearly unchanged comparatively to the initial sample whereas
the a parameters increased slightly (table 1). Spectra of the following
elements had been obtained: O1s, Ar2p, N1s, La3d, Mn2p, C и W
(figs.15-17). The results are summarized in table3,4.
Table 3. Characteristic reflection for the LaMnO3 involved into high
pressure torsion.
La 3d3/2
ref.
3d
sat
Sample No.1
851.8 eV
855.4 eV
3+
4+
640.7 eV
642.5 eV
1
2
3
4
529.2 eV
531.0 eV
532.3 eV
533.6 eV
Sample No.2
853.3 eV
857.4 eV
Mn 2p3/2
640.6 eV
643.5 eV
O 1s
–
531.5 eV
532.8 eV
534.1 eV
One can see a remarkable difference between surface conditions
of the material before and after distortion both in the basic element
content and in adsorbed and impure elements. Compared to the initial
state where La and Mn were determined in the equal ratio the distorted
sample contained less quantity of the lanthanum. At the same time the
amount of the surface carbon increased significantly and argon was
detected. It was also established a quantity of the nitrogen (in three
different states).
Table 4 Chemical content of the samples involved into high pressure
torsion calculated with and without oxygen and carbon
Sample No.1
frac., at.%
Eb,eV
35
0.00
835
2.77
642
3.00
400
3.16
530
65.52
285
25.55
Sample No.1
frac., at.%
Eb,eV
35
0.00
835
30.99
642
33.61
400
35.40
-
Ref.
W4f
La3d5/2+3/2
Mn2p3/2
N1s
O1s
C1s
Ar2p1/2+3/2
Ref.
W4f
La3d5/2+3/2
Mn2p3/2
N1s
Ar2p1/2+3/2
526
528
530
532
534
536
538
Энергия
связи, эВ
Bond
energy,
eV
Fig.15. O 1s - spectrum for the
LaMnO3 involved into high
pressure torsion: 1-initial No.1,
2-distorted sample No.2
Sample No.2
frac., at.%
Eb,eV
35
0.85
835
1.06
642
3.40
400
1.76
530
57.61
285
33.87
248
1.45
Sample No.2
frac., at.%
Eb,eV
35
9.99
835
12.47
642
39.89
400
20.70
248
16.96
636
638
640
642
644
646
Энергия
связи, эВ eV
Bond energy,
Fig.16. Mn 2p3/2-spectrum for
the LaMnO3 involved into high
pressure torsion: 1- initial No.1,
2-distorted sample No.2
The great part of the surface is made up of the oxygen atoms,
belonging both to the manganite and to adsorbed compounds. Four
oxygen reflections determined at the initial sample (fig.15) are common
for the majority of perovskite-like compounds. Three former peaks
relate to the basic structure, and in accordance to ample studies the
second one contains also carbonate component localized on the surface.
O1s peak at 533.6-534.1 eV corresponds to the adsorbed water.
By XPS analysis in the distorted samples the tungsten undetected
by XRD was also revealed randomly, due perhaps to the anvils made
from WC alloy.
X-ray photoelectron studies of LaMnO3 surface conditions of the
samples involved into shock wave loading were carried out for the initial
LaMnO3 sample (No.6) and the sample No.9 from the edge of the
ceramics ball. It also revealed changes of the surface content between
the initial and distorted samples (tables 5,6)
Table 5. Characteristic reflection for the LaMnO3 involved into shock
wave loading
La 3d3/2
ref.
Sample No.6
3d
сат
851.8 eV
855.5 eV
3+
4+
–
642.0 eV
Sample No.9
852.1 eV
855.9 eV
Mn 2p3/2
848.1 eV (2nd state)
852.2 eV (2nd state)
–
642.2 eV
O 1s
1
2
3
4
529.4 eV
530.8 eV
532.2 eV
533.7 eV
530.1 eV
531.5 eV
532.6 eV
533.7 eV
As with the samples obtained by the high pressure torsion, a
decrease of a general oxygen content and noticeable increase of the
surface located carbon is distinguished. Surface changes in the element
content of the nanoscaled LaMnO3 are connected with a prehistory of
the samples as well as with defect peculiarities and adsorption power of
the nanocrystalline materials.
Table 6 Chemical content of the samples involved into shock wave
loading calculated with and without oxygen and carbon
W4f
La3d5/2+3/
2
Mn2p3/2
N1s
O1s
C1s
Ar
W4f
La3d5/2+3/
2
Mn2p3/2
N1s
Ar
830
35
0.00
35
0.00
835
642
400
530
285
243
1.72
8.20
9.23
79.01
0.72
1.12
835
642
400
530
285
244
1.05
1.12
7.58
54.59
35.47
0.19
Sample No.9
frac., at.%
Sample No.6
frac., at.%
ref.
825
Sample No.9
frac., at.%
Eb,eV
Sample No.6
frac., at.%
Eb,eV
ref.
835
840
Eb,eV
35
835
642
400
243
845
850
855
0.00
Eb,eV
35
0.00
8.49
40.44
45.54
5.52
835
642
400
244
10.58
11.23
76.25
1.94
860
865
Энергия связи, эВ
Bond
energy, eV
Fig. 17. La 3d -spectrum for
the LaMnO3 involved into high
825
830
835
840
845
850
855
860
Энергия
связи, eV
эВ
Bond
energy,
Fig. 18. La 3d -spectrum for
the LaMnO3 involved into
865
pressure torsion: 1–initial No.1,
shock wave loading: 1–initial
2-distorted No.2
No.6, 2-distorted No.9
The absence of the low-energy structural oxygen peak at the
spectrum of the nanoscaled sample No.9 points probably to a huge
carbonate group allocating on the sample surface or indicates (as
supposed by authors) some changes in the basic oxygen structural
positions.
In the sample obtained by dynamic deformations with shock
wave loading a light shift of the lanthanum peaks up to the high energy
side is observed. For the oxygen peaks this movement is even more
obvious. The lanthanum second state (fig.18) discovered in the
nanocrystalline sample No.9 is probably relative to the metal lanthanum.
(probably caused by a two-phase state of the oxide.)
Disturbance of a stoichiometry of CuO nanoceramics
It is known that CuO is compound having narrow region of
homogeneity. In equilibrium under normal conditions copper monoxide
has departure from stoichiometric composition CuO less as 1% [11]. Our
investigations show that the ratio of concentrations oxygen and copper
CO/CCu varies strongly along the radius of the compressed CuO
nanoceramic ball (Fig. a). In our case the radius of CuO ball is about 23
mm. In the outer layers of the ball oxygen concentration CO is bigger
than copper concentration CCu up to several percentages. Variation of CO
and CCu is slightly different along varies radiuses. Maximum difference
of CO and CCu reaches 6-8%. These compositional data are confirmed by
chemical analysis produced by methods of titration and H2-reduction. It
must be emphasized that such kind disturbance of composition is
observed in monohpase CuO nanoceramics. Precision XRD
measurements and X-ray emission spectroscopy study do not reveal any
second phases in our nanoceramics. We note that special
thermotreatments and quenches can’t change CuO composition so
strongly [a].
Possible reasons of such kind disturbance may be synthesis of
peroxide ions (O-O)2- or/and formation of solid solution of CuO and O.
We have not reliable information about existence these compounds but
the possibility formation of these compounds under high pressure of
oxygen was considered in [12]. In our experiments loading of CuO
blank by spherically convergent shock waves gives rise to high pressure
and temperature. In central part of the blank at the radius R<1 mm
pressure reaches some megabars. Under extremal conditions
decomposition of CuO occurs partially on Cu2O and O. Free oxygen is
under high pressure in the germetically sealed casing and initiates
formation of peroxide or solid solution.
Fig.19 A dependence of the
oxygen/copper ratio on the
distance from the surface of the
nanoceramic ball.
CuO
1.06
1.05
Ratio CO/CCu
1.04
1.03
1.02
1.01
1.00
0.99
0.98
0
1
2
3
4
5
6
7
8
Distance from sample edge,mm
Conclusion
Analysis of the data obtained confirms both distortion methods
permit to produce volumetric nano-scale materials, including lanthanum
manganite, from the coarse-grained powder during a single
technological cycle. Whereas thin plates obtained by the quasi-static
deformation are not always strong enough this method brings to a
considerably smaller grain size even with a small rotation angle. Further
rotating leads as mainly to an increase of the lattice microstrains. As a
merit of the dynamical deformation method a high density (up to 99%)
and good mechanical properties can be considered. Moreover, this
technique permits to get comparatively great amount of the material
during one processing cycle.
Besides LaMnO3, bulk nanocrystalline materials based on TiO and
ZrO2:Y2O3 (YSZ) were obtained by the quasi-static deformation
technique [13,14]. In these oxydes XRD analysis revealed a decrease of
crystallite size (up to ~100 nm) and an increase of lattice microstrains
started at the rotation angles of 15-20о. At the same time, a constant
dispersibility of the microstructure is achieved in 2 turns for the TiO and
in one turn for the YSZ case. And the TiO ceramics is characterized by a
non-monotone dependence of the microstrain on the rotation angle. At
the high extent of deformations and small crystallite size the strain value
decreases, probably due to more efficient stress relaxation through a
grain-boundary slipping when decreasing crystallite size. For the TiO
oxide an increase of a deformation rate led to an increase of a cubic cell
parameter. This indicates significant changes of the crystal lattice point
defect upon severe plastic deformation of the titanium monoxide. Such a
behavior could be caused by a shutting down (annihilating) oxygen and
titanium vacancies during high pressure torsion and/or by changing the
oxygen content.
Nanoscaled ceramics of CuO and Mn3O4 were produced by the
dynamic deformations [7,8]. According to the data obtained by XRD,
SEM and STM the crystallite size of the CuO nanoceramics was 10-100
nm and 50-200 nm for the Mn3O4. Certain regularity of the crystallite
size could be observed along the radius of the nanoceramic ball. The
density of the nanoceramics comes to 99%. Size effects and specific
imperfection of the ceramics obtained lead to a set of particularities of
physical properties [16,17,18,19].
Acknowledgment
The financial support from the Russian Foundation for Basic
Research is gratefully acknowledged (project No.04-03-34971)
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