Characteristics of aluminum oxide powders
The subject of investigation in this work is oxygen isotope exchange (OIE) between oxides and oxygen-containing gases 18О2 and С18О2. OIE studies yield information about the rate of processes on the gas – solid interface and oxygen self-diffusion in oxides. In turn, the surface processes can involve some elementary stages, in particular, physical and chemical adsorption. Several types of diffusion processes can be observed also inside oxides, for example, volume and grain boundary diffusion. OIE investigations are of much practical interest, for example, in connection with the problems of catalytic oxidation of metals and development of materials for chemical and electrochemical devices (fuel cells, electrolyzers, sensors, hydrogen storage devices, devices for separation of gas and isotope mixtures, etc.).
Two experimental approaches to OIE examination are known. The pioneer approach was based on measurements of the isotope composition of a gas mixture interacting with oxide. In the alternative approach developed in the last decades, the isotope composition of the oxygen subsystem of oxides was measured, which was studied usually by secondary ion mass spectrometry (SIMS) and nuclear microanalysis (NRA). The present work is devoted to the examination of the isotope composition of solid-state samples. The overwhelming majority of studies by means of this approach were performed on bulk samples. A distinguishing feature of this work is its orientation toward isotope exchange examination in nanoscale oxides. As will be shown below, this results in novel or supplementary data on the surface reaction rates during isotope exchange and the rates of diffusion processes in oxides. Moreover, the investigations into oxygen isotope exchange in oxide nanomaterials are of great practical importance. It is not improbable that the use of nanomaterials may lead to favorable changes in the functional properties of oxides when it is necessary to increase the quantity of absorbed gas or to increase rates of gas absorption or extraction etc.
2. Investigations on bulk samples
Most often, OIE studies on bulk samples involve isothermal annealing of samples in oxygen (or other oxygen-containing gas) enriched with 18О isotope and measurement of concentration profiles (depth distribution) of 18О tracers in the sample. In case of bulk diffusion described by one value of oxygen self-diffusion coefficient
It is seen from expressions (1)-(2) that OIE studies on bulk samples provide measurements both of diffusion coefficients
For strongly anisotropic diffusion systems (HTSC oxides and other oxides with complicated crystal structure and chemical composition), a different route of OIE experiments can be chosen. In some works, step-wise isochronous annealing of samples was carried out in oxygen enriched with 18О isotopes. The isochronous annealing conditions can be selected such that the systems with strong anisotropy of oxygen diffusion coefficients can exhibit a site-plane effect. It consists in successive substitution of 18О atoms for crystallographic oxygen planes or positions occupied by 16О atoms before annealing.
By way of example, Fig. 1 demonstrates the isochronous-annealing curve
The data on the site -plane effect make it possible to obtain information about the fraction of each type of sites in the oxygen subsystem of the oxide, to estimate the activation energy and the frequency factor for oxygen atom transition from one site to another, as well as to examine the effect of phase transformations in oxides on the distribution of oxygen atoms in the lattice sites, etc.
3. Investigations on nanocrystalline powders
The concentration profiles of 18О atoms in a nanoparticle can not be determined experimentally since neither SIMS nor NRA methods ensure the locality of measurements commensurable with the size of nanopowder particles. Therefore, in our OIE studies on nanopowders we measured the average concentration of 18О atoms in a large assembly of powder particles (Gizhevskii et al, 2008); the same assembly was annealed in oxygen enriched with 18О isotopes. In view of the destructive character of SIMS measurements, this technique can scarcely be used to determine the average oxygen isotope concentration in powders. Here, the NRA method, which permits measurement of 18О and 16О atom concentration in the depth to several microns without sample failure, is certainly advantageous.
At first, we shall consider the isotope exchange model, in which only oxygen ions from the external atomic layer of the oxide particle participate. Let only 18О2 molecules be present in the gaseous phase, while the oxide particle contains no 18О atoms at all in the start time. In that case, the differential equation for isotope exchange has the form
A fundamentally different “diffusion” model for the description of OIE (Gizhevskii et al, 2008) is used for processes characterized by the very high surface reaction rate and by comparatively slow volume diffusion of oxygen in the oxide particle. Here, for spherically shaped particles it is easy to obtain
Finally, the OIE model version providing for simultaneous implementation of the relaxation and diffusion mechanisms (Gizhevskii et al, 2008) was considered. In this case, the
Expressions for relaxation-type isotope exchange analogous with (4)-(5) will be valid not only at very slow oxygen diffusion in oxide, when a typical diffusion length is (
From the aforesaid it is seen that the examination of isotope exchange on nanopowders may give data on their dimensional characteristics. Otherwise, the information received from isotope exchange studies on powders and bulk samples is in many respects analogous. This concerns both the diffusion characteristics of oxide materials and the processes on the surface of oxides.
OIE studies on nanopowders attract particular interest since they permit measuring low
Expressions (4),(7),(8) are written for spherically shaped particles with a single value of the particle radius
and for (
The expediency and informativity of experimental studies of isotope exchange on nanopowders is not a trivial problem. First, there may be an unacceptable diversity of annealing conditions for individual particles contained in the powder assembly. Agglomeration of powders aggravates the situation. Second, analysis of expressions (10)-(11) shows that the presence in a nanopowder of large, for example, micron-size particles will result in a very strong decrease in the 18О concentration as compared with that for a powder containing no large particles. Even with a small part of large particles, isotope exchange experiments may turn out senseless since the experimental data on
Taking into consideration the above observations, in this work we devote much attention to the optimization of the technique for experimental studies on nanopowders, in particular, we examine powders obtained by different technologies and analyze the quantitative correspondence between the theoretical and experimental
4. Oxygen isotope exchange with oxide powders
In this work we present OIE data for oxide nanopowders produced by mechanoactivation (grinding), laser sputtering of a ceramic target, and electrical explosion of wire. In some cases, comparative studies on micropowders were also performed. These technologies are characterized by the following features. The powders obtained by laser sputtering of a ceramic target or by electrical explosion of wire are traditional nanopowders with particles ranging within the nanoscale interval. For the nanopowders produced by grinding, the term
4.1. Oxygen isotope exchange with oxide powders LaMnO3+δ
The oxide LaMnO3+δ is a convenient model object for optimization of the OIE technique (Gizhevskii et al, 2008) on nanopowders. In practice, there often occurs uncontrolled doping and diffusion characteristics of oxides are known to be extremely sensitive to the presents of impurities. Naturally, such materials are difficult to be analyzed. At the same time, nonisovalent doping of the cationic sublattice of LaMnO3+δ does not lead to the formation of structural vacancies in the oxygen sublattice and oxygen volume diffusion coefficients change only slightly in this oxide (Fishman et al., 2003). Probably, this effect is connected to variable valence of manganese ions (Mn2+, Mn3+, Mn4+).
The micropowder LaMnO3+δ was synthesized by the standard ceramic technology from oxides La2O3 and Mn3O4. A single-phase manganite powder with the orthorhombic crystal lattice and the particle size of about several microns was obtained. A FRITSCH planetary ball monomill was used to produce a nanostructured material from the initial powder. The grinding was in air in ethyl alcohol. Zirconium dioxide grinding balls and cups were used. The time of grinding was 13 h. According to X-ray diffraction results, the ground powder had an orthorhombic modification with the average coherent scattering domains of about 15 nm.
Isothermal annealings of nano- and micropowders were carried out in oxygen enriched by 80% with 18О isotope. Oxygen pressure was 0.26 atm. The change in the isotope composition of the gaseous atmosphere during annealing was negligible. Previously, stabilization annealings of the powders had been performed in air at the same temperatures as in oxygen containing tracer atoms. Their duration was approximately the same as the maximal time of annealing in 18О2. The annealings were carried out in a quartz tube. The temperature of samples was measured with a chromel-alumel thermocouple with an accuracy of 2ºС.
The concentration of 18О in the samples was determined with NRA using a 2 MV Van de Graaff accelerator (reaction 18О(p, α)15N), the incident beam energy being 762 keV. The sample plane surface was mounted perpendicular to the incident beam axis. The acceptance angle of the nuclear reaction products was 160º. The energy spectra of the reaction products were registered with a silicon surface barrier detector of 10 mm in diameter. The diameter of the primary proton beam was from 1 to 2 mm. The number of incident beam particles reaching the sample was measured with an accuracy of ~1% using a secondary monitor. For control by means of reaction 16О(d, p)17O* when the incident beam particle energy is 900 keV, the content of 16О isotopes in samples was measured. The total oxygen concentration in all the samples was the same to within several percents.
The concentration of 18О and 16О isotopes was measured immediately on powders. For this purpose, the powder particles were pressed into a plate of indium. As a result, a layer containing only particles of oxide with thickness large than 2 µm was formed on the plate of indium. Nondestructive analysis by means of the NRA technique was performed to the depth of about 2 µm. In the test experiments the spectra of reaction products 16О(d, p)17O* and 18О(p, α)15N for a bulk oxide and for powders, which were not annealed in the atmosphere of tracer atoms, did not differ within experimental error. The concentration profiles were calculated using the stopping power data for the examined samples (Vykhodets et al., 1987).
Experimental and calculated
It turned out that the experimental dependences
The results presented in Fig. 2 showed that the technique for OIE examination on oxide nanopowders proposed in work (Gizhevskii et al, 2008) is consistent. It furnishes information about nanopowder dimensional characteristics, reaction rates on the oxide particles surface during isotope exchange, and oxygen volume diffusion coefficients in oxides. The reliability of the technique is supported by the following facts. The best possible fit with the experimental dependences
From Fig. 2 it is seen that the calculated and experimental dependences
From general considerations it was obvious that the value of frequency Г should depend on the oxide type and the kind of oxygen-containing molecules in the gaseous phase. For the theoretical parameter ∆, the presence or absence of such dependence is not trivial. Therefore, we compared OIE kinetics for LaMnO3+δ nanopowders annealed in 18О2 and C18O2. The degree of carbon dioxide enrichment with 18О isotopes was 90%; the pressures of oxygen and carbon dioxide during annealing were similar. The
Figure 3 demonstrates experimental
4.2. Oxygen isotope exchange with α-Mn2O3 oxide powders
Manganese oxides are technically important and scientifically interesting materials. However, for these species, as far as we know, there is no information about OIE, oxygen diffusion coefficients, and surface reaction rates in the interaction with gaseous oxygen. Therefore, in this work we carried out appropriate studies on α-Mn2O3 oxide powders produced by mechanoactivation.
A planetary mill AGO-2 was used to prepare Mn2O3 powders. Nanoscale domains were formed for a much shorter period of time than in the FRITSCH monomill: the grinding time for the examined samples was 60 s. The average particle size during grinding decreased insignificantly: from 1026 to 344 nm. The corresponding data were obtained by method of dynamic light scattering using a laser analyzer. X-ray diffraction studies showed that the phase composition of the ground powder remained unchanged upon heating in air to 950ºС. The difference between LaMnO3+δ and α-Mn2O3 powders produced by mechanoactivation consisted in a pronounced enhancement of the average size of domains with an increase in temperature. In the temperature range from 300º to 700ºС the radius varied from 16 to 35 nm (Petrova et al., 2010). We used these data as reference points in analysis of OIE results. At the same time, it was taken into account that sizes of coherent scattering domains during OIE investigations could differ from those established in work (Petrova et al., 2010) owing to different temperatures and duration of heating.
Experimental and calculated
The data on oxygen diffusion coefficients in manganese oxides were obtained for the first time and can be commented in the following way. The measurement error shown in Fig. 5 was ~50%; it was due mainly to the uncertainty in the powder nanoparticle size in our experiments. We believe that the diffusion coefficient measurement error can be reduced several times if the nanoparticle radius is determined upon each isothermal annealing in gaseous oxygen. Thus, the technique employed can probably provide precision measurements of low values of oxygen volume diffusion in oxides. Of importance is the following finding. The least typical diffusion length (
The oxygen diffusion activation energy in α-Mn2O3 was (1.20 ± 0.08) eV. For oxygen diffusion in oxides, this value is typical of oxygen ion migration energy. When oxygen diffusion was due to structural vacancies in the oxygen sublattice of the oxide, approximately the same value was observed also for the volume oxygen diffusion activation energy. For example, in yttrium-stabilized cubic zirconium oxides, oxygen diffusion activation energies were found (Solmon et al., 1995; Brossmann et al., 2004) to be 1.23 and 1.11 eV, respectively. The pre-exponential factor in the temperature versus diffusion coefficient dependence for α-Mn2O3 turned out to be very low, namely,
In the temperature range 350 – 700ºС in α-Mn2O3 oxide containing no structural vacancies in the oxygen sublattice, the oxygen diffusion coefficients will be evidently much smaller than those established by the authors, since in oxides without structural vacancies the diffusion activation energy is approximately equal to the sum of migration energy and vacancy formation energy. We don’t know any literature data on oxygen vacancy formation energies in manganese oxides.
4.3. Oxygen isotope exchange with cubic zirconium oxide powders
In this section, as distinct from 4.1 and 4.2, we shall consider OIE for gaseous oxygen with an oxide characterized by very fast oxygen volume diffusion coefficients. The condition (
OIE studies (Fishman et al., 2009) were performed on yttrium-stabilized cubic zirconium oxide YSZ containing 9 mol. % of Y2O3. It can be shown that the condition (
YSZ micropowder was produced by coprecipitation of components. For better particle size homogeneity, it was calcined at 1100ºС in air for 3 h. The powder specific surface was 2.15 and 0.64 m2/g before and after calcination, respectively. Figure 6 displays the experimental and calculated with eq. (9)
The temperature versus frequency Г dependence is presented in Fig. 7. The frequency factor Γ0 = 0.84 103 s-1 and the activation energy
Attention is drawn to the very low values of the frequency factor Γ0 in the temperature versus Г dependence, which are about 103 s-1 for LaMnO3+δ and YSZ oxides. Almost the same frequency factor values were obtained also for bulk samples of other oxides: La0.8Sr0.2MnO3+δ (De Souza et al., 1999), La0.8Sr0.2Mn1-yCoyO3±δ (De Souza & Kilner, 1998), La1-xSrxYMnO3-δ (Ruiz-Trejo & Kilner, 1997). These values are by several orders of magnitude smaller than other frequencies characterizing the examined systems. So, the vibration frequency of light atoms in solids is ≥ 1012 s-1, whereas the frequency of gaseous molecule collisions with solid particle atoms during isotope exchange is about 109 s-1. Such a considerable discrepancy in the characteristic frequency values points to a very low concentration of active centers on the surface of oxides participating in isotope exchange. Their identification is of current importance. Moreover, such discrepancies in the characteristic frequency values may be due to the existence of several isotope exchange mechanisms, instead of only one mechanism of dissociation adsorption – desorption (Odzaki, 1979).
The YSZ nanopowder containing 9.5 mol. % Y2O3 was produced by laser sputtering of a ceramic target (Ivanov et al., 2006). It was subjected to sedimentation in isopropyl alcohol to remove particles with diameter of 200 nm or larger from the powder. The specific surface of the nanopowder was (58.6 ± 0.4) m2/g. Upon annealings at 500ºС, it decreased to (53.3 ± 0.8) m2/g, which is likely to be connected with agglomeration of smaller particles.
The experimental and calculated
It was suggested on the basis of these results that the discrepancy in the calculated and experimental
The problems of interpretation of experimental data on specific surface for nanopowders produced by laser sputtering of a ceramic target were also known earlier. They manifested themselves in the following result: the specific surface of sedimentated and non-sedimentated powders were virtually the same according to numerous studies. For example, in work (Kotov et. al., 2004) the specific surface of sedimentated and non-sedimentated Ce0.78Gd0.22O2-δ nanopowder was found to be 57 and 56 m2/g, respectively, though the non-sedimentated powder contained to 8 mass % of large particles. Such results are usually explained in the following way: during sedimentation both large and finest particles are removed. However, this explanation seems doubtful. Some estimations show that small differences in the specific surface for sedimentated and non-sedimentated powders can occur only in the case of multi-layer coating of large particles with nanoparticles. According to the available data, less than one nanoparticle layer on large particles was usually registed on photographs.
It can be supposed as an alternative that large YSZ particles appearing during laser sputtering of a ceramic target are conglomerates of usual nanoparticles. When the BET technique is used, nitrogen molecules penetrate into the space between nanoparticles contained in large particles. That is why the specific surface values for non-sedimentated and sedimentated nanoparticles are almost the same. At the same time, the presence of large particles in nanopowder was reliably established with the OIE technique. These findings show that the OIE technique can yield supplementary (as compared with the BET method) information about the dimensional characteristics of powders and is very sensitive to the presence of small amounts of large particles in nanopowder, which lie beyond the main peak of the size distribution function of powder particles. According to these results, the expected sensitivity of the isotope exchange technique is about 0.01% of the number of particles.
4.4. Oxygen isotope exchange with aluminum oxide nanopowders
In this section we consider OIE in nanopowders produced by electrical explosion of wire. The powders were sedimentated in isopropyl alcohol. The sedimentation regime provided removal of particles with diameter greater than 200 nm from the powder. The main characteristics of the powders are listed in the Table 1.
The values of specific surface
As is seen from the Table 1, the aluminum oxide nanopowders are much more complicated objects than those examined in sections 4.1, 4.2, and 4.3. Since the OIE technique for nanopowders made a good showing in the case of relatively simple oxide systems (see the above sections), it is interesting to apply this method to more complicated species. The major task in this section was to estimate the sensitivity of the OIE technique to the multi-phase state of nanopowders and, in particular, to the presence of an amorphous phase.
Figure 11 demonstrates several
Thus, some OIE regularities for single- and multi-phase oxide nanopowders were found to be of the same type. This is true for linear
No strict theoretical description of
In terms of the above approximations, the expression for
where indexes 1 and 2 relate to the oxide powder amorphous and crystalline phases, respectively;
The results of
It is seen that the “two-phase” model easily provides a satisfactory description of experimental
Thus, the studies of aluminum oxide powders with complex phase compositions showed that the OIE technique is very sensitive not only to the particle size of nanopowders, but probably to the presence of an amorphous phase in them.
5. Oxygen grain boundary diffusion and oxygen isotope exchange in nanocrystalline oxide LaMnO3+δ
Grain boundary diffusion in metal oxides is a poorly studied phenomenon and the data from different authors are controversial. In some of the most reliable works, the presence of enhanced diffusion at the grain boundaries is questioned (Kaur et al., 1995). At the same time, this fact has been reliably determined for metallic systems. The question of the possible common properties of the grain boundary diffusion for oxides and metals is still open. New experimental data on the coefficients of grain boundary diffusion in oxides attract the interest of theoreticians due to the specific properties of the grain boundaries in ionic compounds; in particular, these boundaries are charged. The investigations under consideration are of an applied interest, because metal oxides are widely used as functional materials in applications. The coefficients of the grain boundary diffusion determine the stability of the properties of polycrystalline materials. Presently, most researchers explain this inadequate situation with the diffusion data for oxides citing methodological reasons. It is difficult to obtain correct results primarily because of the strong influence of impurities in the oxides (Kaur et al., 1995). Control of the purity and stoichiometry of a material, especially near grain boundaries, is a complicated problem. In this connection, the study (Vykhodets et al., 2008) dealt with the problem of obtaining correct data on grain boundary diffusion in oxides.
The measurements were carried out for LaMnO3+δ oxide. This is a suitable model object, because it does not have structural vacancies in the oxygen sublattice. Similar systems are characterized by low values of the volume diffusion coefﬁcients of oxygen
The research was carried out for nanocrystalline samples. For them, the conditions to obtain reliable results are more favorable due to the presence of the branching net of the boundaries, which enhances the contribution of the grain boundary diffusion to the diffusion flux. Besides, the nanocrystalline samples were prepared with the method in which the time of the nanostructure formation is very short. This requirement is fulfilled by the shockwave loading method (Kozlov et al., 1997) using explosives. In this case, it is expected that the doping of the nanograin boundaries is weak or absent. The initial material for the shockwave method is a coarse-grained powder of LaMnO3+δ with 15–30-µm grains. According to the results of x-ray diffraction analysis, the material obtained after an explosion was nanocrystalline with an average particle size of 41 nm. These characteristics did not change after further diffraction annealing. Samples 3-mm-thick with an area of 1 cm2 were cut out of this material. Mechanical operations of cutting, polishing, etc., were not performed to avoid the surface contamination of the samples. The scanning electron microscopy investigation of the fractured surface of the samples revealed that the main element of the microstructure of the samples under consideration is a crystallite with an average size of 3µm. The distance between the crystallites was very large (submicron); as a result, 18O2 molecules penetrate freely into the space between the crystallites during annealing. This feature was taken into account in mathematical treatment of
The results presented in Fig. 2 allow one to choose the optimal conditions for the diffusion annealing of compact samples. At 5000C, the C and B kinetic types are expected for
The concentration profiles
Figure 13 shows the results of temperature versus grain boundary diffusion coefficients studies. The grain boundary diffusion activation energy
A value of 0.044 nm was found for boundary width δ. It is an order of magnitude larger than the value δ ≈ 0.5 nm obtained in all of the correct experiments on grain boundary diffusion. Note that using eq. (13) one can only determine an effective thickness of the boundary, δ, disregarding its structure and, correspondingly, diffusion inhomogeneity. For an inhomogeneous boundary with the mixing of tracer atoms between intervals with strong disordering (dislocation cores) and with a weakly distorted lattice, the ln
We know only one work (Brossmann et al., 2004) devoted to oxygen grain boundary diffusion in oxide nanocrystalline. This study was performed on a high-purity oxide ZrO2. The ZrO2 nanocrystalline was produced by sintering of oxide nanoparticles with the average nanograin size 100 nm. Diffusion was examined with the use of the B type kinetics; the grain boundary width value
As was mentioned above, the grain boundary diffusion coefficients
In the studies presented here it was shown that the OIE parameters are sensitive to the dimensional characteristics of oxide powders. Therefore, the approach based on OIE examination shows promise for receiving the information about the size of nanopowder particles. Similar data are obtained with other techniques (electron microscopy, X-ray diffraction, specific surface measurement, dynamic light scattering by means of a laser analyzer etc.); in this respect, the OIE technique has an important specific feature. In addition to the conventional methods, it makes it possible to determine the stoichiometry of oxide powders, is sensitive to chemical activity of powders and, probably, to the presence of amorphous phases and very small amounts of larger particles in nanopowders. These peculiarities of the considered technique are of great interest both for theory and practice, since all existing technologies for nanopowder synthesis lead to the formation of non-equilibrium materials and assemblies consisting of particles with different sizes and shapes. Moreover, when the OIE technique is used on nanopowders, very low oxygen volume diffusion coefficient values can be measured as compared with traditional approaches. The application of the OIE technique for bulk nanocrystalline samples also holds much promise. Already in several works it was shown that considerable progress can be achieved with the use of this technique in the examination of oxygen grain boundary diffusion in oxides.
This work was supported by RFBR (grant 10-03-96016-p_ural_a and grant 09-03-00335_a), the Program of fundamental research of Presidium of Russian Academy of Sciences N 27 “Foundations of fundamental research of nanotechnology and nanomaterials” and the Federal Target Program "Scientific and scientific-pedagogical staff of innovation Russia (contract 02,740. 11.0641).
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