X-ray diffraction and calorimetric studies of powder nanocrystalline systems based on ZrO<Subscript>2</Subscript>(Y) and Al<Subscript>2</Subscript>O<Subscript>3</Subscript> with second insoluble component

Four nanocrystalline ZrO₂(Y)- and Al₂O₃-based powders synthesized by plasma spray pyrolysis have been studied. It has been shown that the ZrO₂(3Y)-based system with second component Al₂O₃ forms a nonequilibrium solid solution ZrO₂(3Y, Al) with the tetragonal structure. It has been found that the existence of an component (ZrO₂(Y)) insoluble in the coarse-grained state in Al₂O₃-based systems causes the delay of the γ → α transformation and decreases the size of the coherently scattering domains of formed nanosized modifications of Al₂O₃.     

Magnetic properties of BiFe<Subscript>0.93</Subscript>Mn<Subscript>0.07</Subscript>O<Subscript>3</Subscript> powders obtained by ultrasonic spray pyrolysis

Samples of BiFe_0.93Mn_0.07O₃ with different specific surface area were synthesized for the first time by ultrasonic spray pyrolysis. The resulting powders consist of porous particles of a spherical shape of medium size ~0.5 μm and have record values of residual magnetization and coercive force. It is found that the magnetic properties of the porous powder particles are determined by the distortion of the crystal lattice and the presence of uncompensated magnetic moments of iron ions on the surface.     

Spherical shape BaO-ZnO-B<Subscript>2</Subscript>O<Subscript>3</Subscript>-SiO<Subscript>2</Subscript> glass powders prepared by spray pyrolysis

Fine-sized BaO-ZnO-B₂O₃-SiO₂ (BZBS) glass powders were directly prepared by high temperature spray pyrolysis. The hollow glass powders prepared at low preparation temperature of 1000 °C had a low density of 2.65 g/cm³. However, the densities of the BZBS powders obtained at preparation temperatures of 1200 and 1400 °C were each 3.92 and 4.13 g/cm³. The mean size of the BZBS glass powders prepared by spray pyrolysis at preparation temperature of 1400 °C was 0.98 μm. The glass transition temperature (T_g) of the prepared BZBS glass powders was 518.9 °C. The dielectric layers formed from the prepared BZBS glass powders with a dense structure had a clean surface and a dense inner structure without voids at the firing temperature of 580 °C. The transparencies of the dielectric layers formed from the prepared BZBS glass powders were higher than 90% within the visible range. PACS 42.70.Ce; 85.60.Pg; 71.55.Jv     

X-ray emission study of the electronic structure of nanocrystalline Al<Subscript>2</Subscript>O<Subscript>3</Subscript>

Valence states of metal ions and the phase composition of nanocrystalline Al₂O₃ (of the original oxide and the oxide irradiated by high-energy Fe⁺ ions) are studied by using x-ray emission Al L_{2, 3} and O Kα spectra. It is established that the shape of the Al L_{2, 3} spectra strongly changes as one goes from the original (bulk) Al₂O₃ to nanocrystalline oxide, while the O Kα spectra remain practically unchanged. Moreover, irradiation by high-energy Fe⁺ ions results in slight additional changes in the x-ray spectral characteristics of the aluminum oxides under study. The obtained experimental data are compared with the results of theoretical calculations of the electronic structure of α and γ phases of Al₂O₃ performed using the LDA formalism. Using the results of x-ray spectral studies, electronic structure calculations, and x-ray diffraction analysis, it is shown that the revealed spectral differences between the nanocrystalline state of aluminum oxide and the bulk material can be interpreted as a phase transition from the α phase to the γ phase of Al₂O₃ with an addition of bayerite.     

A study of the structure and properties of nanostructured ZrO<Subscript>2</Subscript>-Y<Subscript>2</Subscript>O<Subscript>3</Subscript>-Al<Subscript>2</Subscript>O<Subscript>3</Subscript> composite powders

The correlation between temperature treatment conditions and the ratio of components in nanostructured fibrous powders with a composition of ZrO₂-Y₂O₃-Al₂O₃ and their porous crystal structure and physicochemical properties is studied. The dependences of the ratio between zirconia tetragonal and monoclynic phases on the treatment temperature and the alumina content are found to have a nonmonotonic character. The growth of zirconia crystallite size is suppressed by introduced nanocrystalline alumina in a temperature range of 600–1200°C, which is caused by the processes of ternary solid solution formation. The bulk and picnometric density values of materials are proportional to the temperature of heat treatment. The temperature dependence of the specific surface and the size of oxide grain particles has an inversely proportional character. With increasing alumina content in the powders, the specific surface increases, while the picnometric and bulk densities decrease.     

Al<Subscript>2</Subscript>O<Subscript>3</Subscript> plasma production during pulsed laser deposition

A Nd:YAG laser operating in second harmonic (532 nm), 3 ns pulse duration, 150 mJ pulse energy, and 10 Hz repetition rate, is employed to irradiate Al₂O₃ target placed in high vacuum. The produced plasma is investigated by an ion collector used in time-of-flight configuration and by a mass quadrupole spectrometer, in order to determine the equivalent plasma temperature and the atomic and molecular composition. Pulsed laser deposition technique has been used to produce thin films on different substrates placed close to the target. Different surface analyses, such as energy dispersive X-ray fluorescence (EDXRF), X-ray photoelectron spectroscopy (XPS) and surface profilometry are employed to characterize the produced films. Measurements of ablation yield, plasma equivalent temperature, acceleration voltage and characterization of grown thin films are presented and discussed.     

The transport and thermal properties of glassy LiPO<Subscript>3</Subscript>/crystalline Al<Subscript>2</Subscript>O<Subscript>3</Subscript> (ZrO<Subscript>2</Subscript>) composite electrolytes

Glassy LiPO₃/crystalline Al₂O₃ and glassy LiPO₃/crystalline ZrO₂ (0–12.5 vol.% of oxide fillers) composite solid electrolytes have been prepared by glass matrix softening. Their thermal and transport properties have been investigated by differential scanning calorimetry (DSC) and impedance spectroscopy methods. The addition of ZrO₂ leads to a decrease in the crystallization temperature of LiPO₃ glass. It was found that the conductivity behavior depends on the nature of the dispersed addition. In the case of the Al₂O₃ addition, the increase in the electrical conductivity is observed. The ionic conductivity of the LiPO₃/10% Al₂O₃ composite reaches 5.8 × 10^?8 S/cm at room temperature. In contrast, the conductivity in the LiPO₃/ZrO₂ composite system decreases.     

Laser performance of holmium-doped Lu<Subscript>3</Subscript>Al<Subscript>5</Subscript>O<Subscript>12</Subscript> and (Lu,Y)<Subscript>3</Subscript>Al<Subscript>5</Subscript>O<Subscript>12</Subscript> crystals pumped by Tm:fiber laser

Optical properties of Ho³⁺-doped Lu₃Al₅O₁₂ and (Lu,Y)₃Al₅O₁₂ crystals were investigated and compared. Substitution of Y for Lu in the host garnet Lu₃Al₅O₁₂ results in broad absorption and emission spectra, and improvements in the laser behavior of Ho³⁺. Pumped by Tm:fiber laser, a maximum output power of 5.02 and 5.73 W of Ho-doped Lu₃Al₅O₁₂ and (Lu,Y)₃Al₅O₁₂ have been obtained, respectively. The center lasing wavelength are 2124.5 and 2123.0 nm for Lu₃Al₅O₁₂ and (Lu,Y)₃Al₅O₁₂, respectively.     

Electronic excitations and defects in nanostructural Al<Subscript>2</Subscript>O<Subscript>3</Subscript>

A time-resolved cathodo-and photoluminescence study of nanostructural modifications of Al₂O₃ (powders and ceramics) excited by heavy-current electron beams, as well as by pulsed synchrotron radiation, is reported. It was found that Al₂O₃ nanopowders probed before and after Fe⁺ ion irradiation have the same phase composition (the γ-phase/δ-phase ratio is equal to 1), an average grain size equal to ～17 nm, and practically the same set of broad cathodoluminescence (CL) bands peaking at 2.4, 3.2, and 3.8 eV. It was established that Al₂O₃ nanopowders exhibit fast photoluminescence (PL) (a band at 3.2 eV), whose decay kinetics is described by two exponential stages (τ₁ = 0.5 ns, τ₂ = 5.5 ns). Three bands, at 5.24, 6.13, and 7.44 eV, were isolated in the excitation spectrum of the fast PL. Two alternate models of PL centers were considered, according to which the 3.2-eV luminescence either originates from radiative relaxation of the P^? centers (anion-cation vacancy pairs) or is due to the formation of surface analogs of the F⁺ center (F _S ⁺ -type centers). In addition to the fast luminescence, nano-Al₂O₃ was found to produce slow luminescence in the form of a broad band peaking at 3.5 eV. The excitation spectrum of the 3.5-eV luminescence obtained at T = 13 K exhibits two doublet bands with maxima at 7.8 and 8.3 eV. An analysis of the luminescent properties of nanostructural and single-crystal Al₂O₃ suggests that the slow luminescence of nanopowders at 3.5 eV is due to radiative annihilation of excitons localized near structural defects.     

Silver conductor fabrication by laser direct writing on Al<Subscript>2</Subscript>O<Subscript>3</Subscript> substrate

A novel technology for manufacturing circuitry, differing from conventional methods, called laser micro-cladding electronic pastes is presented. In this article, high-quality silver conductors were made on an Al₂O₃ substrate by this method. The process and corresponding mechanisms are described in detail. The experimental results show that the conductive lines produced have a minimum line width of 20 m, which is much smaller than the current limitations of conventional processes. The resistivity can reach the order of 10^-6 cm, which is the same level for pure bulk silver. Optical micrographs and SEM morphology observations were also carried out. PACS 81.15.Fg; 42.70.Hj; 42.62.-b; 42.62.Cf; 61.80.Ba     

Properties of an Al<Subscript>2</Subscript>O<Subscript>3</Subscript>/Si interface

The phase chemical composition of an Al₂O₃/Si interface formed upon molecular deposition of a 100-nm-thick Al₂O₃ layer on the Si(100) (c-Si) surface is investigated by depth-resolved ultrasoft x-ray emission spectroscopy. Analysis is performed using Al and Si L_{2, 3} emission bands. It is found that the thickness of the interface separating the c-Si substrate and the Al₂O₃ layer is approximately equal to 60 nm and the interface has a complex structure. The upper layer of the interface contains Al₂O₃ molecules and Al atoms, whose coordination is characteristic of metallic aluminum (most likely, these atoms form sufficiently large-sized Al clusters). The shape of the Si bands indicates that the interface layer (no more than 10-nm thick) adjacent to the substrate involves Si atoms in an unusual chemical state. This state is not typical of amorphous Si, c-Si, SiO₂, or SiO_x (it is assumed that these Si atoms form small-sized Si clusters). It is revealed that SiO₂ is contained in the vicinity of the substrate. The properties of thicker coatings are similar to those of the 100-nm-thick Al₂O₃ layer and differ significantly from the properties of the interfaces of Al₂O₃ thin layers.     

Comparison between Si/SiO<Subscript>2</Subscript> and InP/Al<Subscript>2</Subscript>O<Subscript>3</Subscript> based MOSFETs

The discrete breathers in graphane in thermodynamic equilibrium in the temperature range 50–600 K are studied by molecular dynamics simulation. A discrete breather is a hydrogen atom vibrating along the normal to a sheet of graphane at a high amplitude. As was found earlier, the lifetime of a discrete breather at zero temperature corresponds to several tens of thousands of vibrations. The effect of temperature on the decay time of discrete breathers and the probability of their detachment from a sheet of graphane are studied in this work. It is shown that closely spaced breathers can exchange energy with each other at zero temperature. The data obtained suggest that thermally activated discrete breathers can be involved in the dehydrogenation of graphane, which is important for hydrogen energetics.     

Fabrication of Al<Subscript>2</Subscript>O<Subscript>3</Subscript>/Al structure by nitric acid oxidation at room temperature

A thick Al₂O₃/aluminum (Al) structure has been fabricated by oxidation of Al with 68wt% and 98wt% nitric acid (HNO₃) aqueous solutions at room temperature. Measurements of the Al₂O₃ thickness vs. the oxidation time show that reaction and diffusion are the rate-determining steps for oxidation with 68wt% and 98wt% HNO₃ solutions, respectively. Observation of transmission electron micrographs shows that the Al₂O₃ layer formed with 68wt% HNO₃ has a structure with cylindrically shaped pores vertically aligned from the Al₂O₃ surface to the Al₂O₃/Al interface. Due to the porous structure, diffusion of HNO₃ proceeds easily, resulting in the reaction-limited oxidation mechanism. In this case, the Al₂O₃/Al structure is considerably rough. The Al₂O₃ layer formed with 98wt% HNO₃ solutions, on the other hand, possesses a denser structure without pores, and the Al₂O₃/Al interface is much smoother, but the thickness of the Al₂O₃ layer formed on crystalline Al regions is much smaller than that on amorphous Al regions. Due to the relatively uniform Al₂O₃ thickness, the leakage current density flowing through the Al₂O₃ layer formed with 68wt% HNO₃ is lower than that formed with 98wt% HNO₃.     

Modification of the Al<Subscript>2</Subscript>O<Subscript>3</Subscript> surface by high-energy bismuth ions

This paper reports on an atomic-force microscopy study of the surface of α-Al₂O₃ single crystals irradiated by Bi ions with energies of 710, 557, 269, and 151 MeV. The shape of the radiation defects produced by single ions was established to depend on the ionization energy loss. The threshold ionization density above which the surface topography is observed to change lies in the 27–35 keV/nm interval. Possible mechanisms of defect formation in the thermal-spike model, namely, a phase transition and the creation of thermoelastic stresses in the high-energy ion track, are considered.     

EPR study of ordered Al<Subscript>2</Subscript>O<Subscript>3</Subscript>-based aerogel

Samples of oriented aerogel based on aluminum oxide are studied by the electron paramagnetic resonance (EPR) technique under steady-state and pulse conditions. At least two types of paramagnetic centers interacting with Al nuclei on the surface of aerogel are revealed. Their spectra are characterized by the presence or absence of superhyperfine structure in the EPR spectra, respectively. The X-ray irradiation at room temperature gives rise to the formation of additional long-lived paramagnetic centers of the second kind. Their characteristic decay times for the “fast” and “slow” processes are determined. The interaction of induced paramagnetic centers with protons located on the surface of aerogel is revealed.     

Synthesis and magnetic properties of FeNi<Subscript>3</Subscript>/Al<Subscript>2</Subscript>O<Subscript>3</Subscript> core-shell nanocomposites

In this study, FeNi₃/Al₂O₃ core-shell nanocomposites, where individual FeNi₃ nanoparticles were coated with a thin layer of alumina, were fabricated by a modified sol-gel method. Several physical characterizations were performed on the samples of FeNi₃/Al₂O₃ nanocomposites with different thickness of Al₂O₃ shell. The encapsulation of FeNi₃ nanoparticles with alumina stops FeNi₃ agglomeration during heat treatment, and prevents interaction among the closely spaced magnetic FeNi₃ nanoparticles. The Al₂O₃ insulating shell improves the soft magnetic properties of FeNi₃. The study of the complex permeability of the samples shows that the real part μ’ of the permeability of the sample with Al molar content of 20% (Al/(Fe+Ni)) is as high as 12, and independent of frequency up to at least 1 GHz. The tunneling magnetoresistance arising from the presence of the Al₂O₃ shell have also been studied.     

Formation and mechanical properties of alumina ceramics based on Al<Subscript>2</Subscript>O<Subscript>3</Subscript> micro- and nanoparticles

Alumina micro- and nanopowders with the particle size from 200 μm to 40 nm synthesized by the sol-gel method are studied. The particle size dependence of γ-Al₂O₃→α-Al₂O₃ phase transformation is studied by differential thermal analysis, X-ray diffraction method, and transmission electron microscopy. X-ray diffraction data show that for alumina nanoparticles γ-Al₂O₃→θ-Al₂O₃ phase transformation occurs at 900°C, and for micro-particles it occurs in the temperature range 1150–1200°C. The alumina ceramics produced of alumina nanoparticles is shown to have higher flexural strength under three-point bending than the ceramics produced of micro-particles. The obtained results demonstrate that alumina particle size reduction stabilizes the formation of α-Al₂O₃ at lower temperatures, due to which the grain growth rate decreases and the flexural strength of monolithic oxide ceramics increases.     

Thermal hysteresis of a simulated Al<Subscript>2</Subscript>O<Subscript>3</Subscript> system

Thermal hysteresis in a simulated Al₂O₃ system has been investigated using a Molecular Dynamics (MD) method. Simulations were done in the basic cube under periodic boundary conditions containing 3000 ions with Born-Mayer type pair potentials. The system was cooled down from 7000 K to 0 K and heated up from 0 K to 7000 K by the same cooling/heating rate of 1.7178×10¹⁴ K/s. The temperature dependence of the system density upon cooling and heating shows thermal hysteresis. The differences between structure and dynamics of the models obtained by cooling (MOBC) and heating (MOBH) at three different temperatures of 2100 K, 3500 K and 5600 K have been detected. Calculations show that the differences in the dynamics of the systems are more pronounced than those in the structure. Furthermore, dynamical heterogeneities in MOBC and MOBH at the temperature of 2100 K have been studied through a non-Gaussian parameter and comparison of partial radial distribution functions (PRDFs) for the 10% most mobile or immobile particles with their corresponding mean ones. Cluster size distributions of the 10% most mobile or immobile particles in MOBC and MOBH at the temperature of 2100 K have been obtained. Calculations show that differences in dynamical heterogeneities are pronounced.