Papers by Keyword: Nuclear Microanalysis

Abstract: The results of measurements of the bulk diffusion of tracer oxygen atoms in the oxides LnMnO_3+δ (Ln = La, Nd, Sm) in the temperature range 400 – 750°С are presented. The measurements were carried out on micro-and nanopowders. Nanoscale powders were prepared by mechanical activation. A method based on the study of the kinetics of oxygen isotope exchange between the powder and gaseous oxygen enriched with ¹⁸O isotope was used to obtain data on the diffusion coefficients. The average concentration of ¹⁸O isotope in the powders was measured using NRA technique. The obtained diffusion coefficients lay in the range of 10^-21 - 10^-24 m²/s, the diffusion activation energy for all the oxides have been close to 1 eV. These results suggest that the migration of tracer oxygen in oxides LаMnO_3+δ, NdMnO_3+δ and SmMnO_3+δ at low temperatures is realized via structural defects. As for the oxide LaMnO_3+δ, its oxygen diffusion coefficients at low temperatures have been lower than the values extrapolated from high temperatures. Such behavior of diffusion properties has not been previously observed in other metal oxides. In this regard, the vacancy formation energy in the rare earth manganites has been supposed to increase with decreasing temperature.

Abstract: The mechanically activated oxides MnO₂, Mn₂O₃, Mn₃O₄ and MnO were studied under ambient and non-ambient temperature conditions. Regimes of mechanical treatment were found that allowed one to preserve the single-phase state without an essential change of oxide chemical composition. New data about the temperatures and sequence of phase transitions in mechanically activated manganese oxides as well as about the kinetics of isotope exchange were obtained. Magnetic properties of the treated oxides were measured in the temperature range of 4-300К. It is shown that mechano-activation essentially influenced the temperature and field dependences of magnetization, temperatures of magnetic phase transitions and leads to the appearance of additional magnetic phases. New data on the rates of surface reactions during isotope exchange and oxygen self-diffusion coefficients are obtained.

Abstract: sotope exchange of oxygen ¹⁸О₂ with oxides NdMnO_3+δ was investigated. The oxide was obtained from oxides Nd₂O₃ and Mn₂O₃ using a ceramic technology with annealing in air at 1400°C for 90 hours followed by cooling in a furnace. A planetary mill AGO-2 with a centrifugal factor of g = 60 was used for mechanical treatment of oxides. The study of isotope exchange was carried out by nuclear microanalysis. The concentration of the isotopes ¹⁸O and ¹⁶O in oxides was determined using a Van de Graaff accelerator and ¹⁸O(p, α)¹⁵N and ¹⁶O(d, p)¹⁷O* reactions at the energies of incident beams 762 and 900 keV. Isothermal annealing of powders was carried out in oxygen, enriched to 80% by the isotope ¹⁸O. It was established that the concentration of the isotope ¹⁸O in mechanically activated powders was several times higher than in the initial micropowder under the same conditions of annealing. The effect increased with increasing of mechanical activation time (30 - 300 s). The isotope exchange parameters connected with the processes at the boundaries of the particles of mechano-activated powder and within their volume were analyzed.

Abstract: sotope exchange of oxygen ¹⁸О₂ with oxides Mn₂O₃ and Mn₃O₄ was investigated in the temperature range of 300700°C. It was established that the content of the isotope O¹⁸ in mechanically activated powders was noticeably higher than in the initial ones. Already at relatively low annealing temperatures isotope exchange leads to nearly the maximum possible under the given conditions replacement of atoms ¹⁶O to ¹⁸O. The isotope exchange parameters connected with the processes at the boundaries of the particles of mechanoactivated powder and within their volume were determined.

Abstract: Isotope exchange of oxygen 18О2 with the boundary of nanograins of oxide LaMnO3+ obtained by the method of shock-wave loading was investigated in the temperature range of 400 – 500 °C. It was established that the temperature dependence of the isotope exchange rate is described by the Arrhenius expression, the activation energy and the pre-exponential factor being 1.67 eV and 1.8∙102 cm/s, respectively. Comparison with literature data has shown that for oxide LaMnO3+, a significant difference in activation energies and pre-exponential factors is observed for the isotope exchange rate with a ‘defect-free’ surface and the nanograin boundary. In case of the boundary, these parameters were higher: the activation energy about two times, and the pre-exponential factor, by almost 7 orders of magnitude.

Abstract: It is shown that investigations of the isotope exchange kinetics in nanocrystalline oxides allows one not only to solve problems connected with determination of process parameters and characteristic scenarios, but also to substantially extend experimental opportunities in analysis of diffusion properties of oxides. A specific behavior of the oxygen isotope exchange was studied in nanocrystalline LaMnO3 and ZrO2:Y2O3 oxides. The former oxide is characterized by very small values of oxygen volume diffusion coefficients, while the latter one is, on the contrary, an ionic conductor. The study was carried out using powdered nanomaterials prepared from original ceramic materials by grinding in planetary mills or by laser evaporation. The study revealed future trends in the use of nanomaterials for analysis of the diffusion kinetics in oxides. It is demonstrated that in this case the sensitivity of traditional isotope methods increases considerably and fundamentally new opportunities are provided for analysis of processes on the "gas phase  solid state" interface as well as for the study of volume diffusion in polycrystals. Considering a strong dependence of the isotope exchange rate on the particle size, it is also topical to conduct studies dealing, along with determination of kinetic parameters of the process, with the use of the obtained data for certification of dimensional characteristics of oxide nanopowders.

Abstract: Using nuclear microanalysis (NRA) and electron probe microanalysis (EPMA), concentrations of carbon, oxygen, nitrogen, aluminum, and vanadium were measured on a large group of macrodefects formed in the course of smelting titanium alloys. A remarkable enrichment of the defect material in oxygen and nitrogen atoms was detected; histograms of defect distribution over the concentrations of oxygen, nitrogen, aluminum, and vanadium were obtained. The above results agree with the concepts according to which the defects are formed from the particles that have the melting temperature higher then the temperature of smelting.

Abstract: The kinetics of the isotope exchange between gaseous oxygen enriched with the 18O isotope and two LaMnO3+δ oxide samples – a nanopowder and a bulk nanocrystal – has been studied. The 18O isotope concentration has been measured by the acceleration nuclear microanalysis method. The coefficients of the volume and the nanograin boundary self-diffusion of oxygen have been evaluated at 500 °C. They are equal to 3.5·10 −20 and 1.5·10 −13 cm2/s, respectively.