Influence of surface on the nanocrystalline structure formation

In contrast to the common Fe-B-base amorphous alloys, Mössbauer transmission and CEMS studies of the early stages of the nanocrystalline structure formation in the new magnetically soft FeCuNbSiB alloys produced via the amorphous way, revealed that the primary crystallization of the Fe₃Si-like phase is inhibited by the surface. This result supports the role of copper as nucleation agent in the amorphous structure.     

Kinetics of vortex structure formation in magnetic materials

A kinetic scenario for the formation of a vortex phase in magnetic materials is discussed. It is found that such a phase can be generated from fluctuations at the kinetic stage of evolution and can subsequently be fixed as a thermodynamically stable phase. Zh. éksp. Teor. Fiz. 111, 1775–1786 (May 1997)     

Effect of formation of a nanocrystalline structure on the electron work function and ion-electron emission of nickel

The effect of deformational nanostructuring on the electron work function for nickel is investigated. The current-voltage characteristic of the gas discharge with nanocrystalline nickel as the cold cathode is measured. It is found that the electron work function is considerably reduced and the ion-electron emission current in the gas discharge increases upon a decrease in the average size of nickel grains to 100 nm. The observed changes in the electron work function and in the ion-electron emission are analyzed.     

Mechanism of deformation-twin formation in nanocrystalline metals

A theoretical model is proposed to describe the nucleation of deformation twins at grain boundaries in nanocrystalline materials under the action of an applied stress and the stress field of a dipole of junction or grain-boundary wedge disclinations. The model is used to consider pure nanocrystalline aluminum and copper with an average grain size of about 30 nm. The conditions of barrier-free twinning-dislocation nucleation are studied. These conditions are shown to be realistic for the metals under study. As the twin-plate thickness increases, one observes two stages of local hardening and an intermediate stage of local flow of a nanocrystalline metal on the scale of one nanograin. In all stages, the critical stress increases with decreasing disclination-dipole strength. The equilibrium thickness and shape of the twin plate are analyzed and found to agree well with the well-known results of experimental observations.     

X-ray absorption studies of the structure of nanocrystalline oxides

X-ray absorption spectroscopy is a prime tool in probing the local structure in condensed matter. This contribution describes some of the applications of this technique to probe the microstructure of nanocrystalline oxides. By the use of specific examples four areas will be covered, namely the effect of grain size and preparation route on microstructure, the locating of dopant sites and the chemical nature of metal/oxide catalysts. The materials involved are tin oxide, lithium niobate, titanium dioxide and copper/ceria.     

Kinetics of pore formation upon plastic deformation of crystals with a cesium chloride structure

The kinetics of pore formation upon plastic deformation of crystals with a cesium chloride structure was studied as the first stage of a first-order phase transition in a deformable media. The shape and the critical dimensions of microvoids depending on the critical shear stress were found. The number of critical microvoids per unit volume arising per unit time on the surface of a slip plane was determined based on the kinetics of formation of pairs of charged vacancies. The volume fraction of pores at the initial stage of plastic deformation in CsBr and CsI crystals was estimated.     

Aluminium based amorphous and nanocrystalline structure

Aluminium based rapidly quenched alloys of nominal composition Al₉₀Fe₇Nb₃ and Al₉₄Fe₂V₄ were studied by Mössbauer spectroscopy. Samples annealed up to 573 K showed amorphous structure represented by quadrupole doublet. From corrected spectra areas the values of f-factor were calculated. In the case of AlFeNb samples f-factor was estimated as f = 0.26 and for AlFeV f = 0.31. The corresponding Debye temperatures were also calculated. Higher temperature annealing at 773 and 873 K induced deformation of nano- and microcrystalline state. Mössbauer spectra of samples of both compositions (with vanadium as well as with niobium) annealed at 773 K showed superposition of crystalline phases with dominant role of Al₃Fe alloy. During annealing at 873 K, phases with large grains and small amount of FeAl metastable phase were developed (Das et al. Mat. Sci. Eng., A304–A306, 159, 2001; Illeková et al. Mat. Sci. Eng., A375–A377, 946, 2004).     

Kinetics of the formation and structure of oligoperoxide nanolayers and grafted polymer brushes on glass plate surface

Functional oligoperoxide surfactants and coordinating oligoperoxide metal complexes were studied as modifiers of glass flat plates to provide the localization of radical forming sites and other functional fragments in adsorbed polymeric layers of a nanoscale thickness. Both the kinetics of the coating formation and properties of the nanolayers witness the dependence of the packing density of oligoperoxide molecules in the coatings on the oligoperoxide natures, concentrations and conditions of the sorption modification. The availability of definite amount of peroxide groups in formed nanolayer provides the possibility of controlled radical graft polymerization initiated from modified surface leading to reliable surface protection, functionality and targeted surface hydrophilic-hydrophobic properties.      

Magnetic properties and structure of the nanocrystalline Gd-Ti-Ge intermetallic compound

This paper reports an experimental study of the magnetization processes and structure of the Gd-Ti-Ge compound in the initial coarse-grained state and a nanostructured state obtained under torsion at a high quasihydrostatic pressure. It is established that in a nanocrystalline sample, the magnetic ordering temperature is 30 K lower, the coercive force is eight times higher, and the magnetization is 3.7 times lower than their respective values in a coarse-grained sample. The observed changes in the magnetic properties are shown to be related primarily with the conversion of a part of the initial phase with the CeScSi-type lattice to a CeFeSi-type weakly magnetic phase. The effect of structural defects and of partial disorder on the magnetic characteristics of the compound are also discussed.     

Mechanism of the formation of shear microbands under plastic deformation of nanocrystalline materials

The mechanism of deformation localization and formation of shear microbands under plastic deformation of submicrocrystalline and nanocrystalline materials is theoretically discussed in the framework of the dislocation-kinetic approach. An equation of evolution of the density and self-organization of dislocations in these materials is formulated taking into account that the grain boundaries are the main sources, sinks, and barriers to moving dislocations. By solving this equation, it is found that the width of microbands and the distance between them depend on the nanograin size and the degree of plastic deformation. It is also demonstrated that there exists a critical grain size (350 nm in the case of α-Fe) above which no microbands are formed in the nanomaterial. The theoretical results are compared with the data available in the literature.     

Kinetics of muonium formation in liquid helium

The spatial distribution of electron-muon pairs in superfluid helium (He-II) is determined using a new algorithm for reconstructing the muonium (Mu) formation probability. It is shown that because a gap is present in the excitation spectrum of He-II the thermalization time of muons and secondary electrons increases with decreasing temperature. As a result, the average distance in the electron-muon pairs increases and, correspondingly, the muonium formation rate decreases. Pis’ma Zh. éksp. Teor. Fiz. 69, No. 3, 251–256 (10 February 1999)     

Grain boundary structure of nanocrystalline Cu processed by cryomilling

The microstructures of cryogenically ball-milled Cu were investigated by high-resolution electron microscopy. It was found that the grain-size reduction is a dislocation-controlled continuous process which consists of the formation of small-angle grain boundaries (GBs), a gradual increase in misorientations as a result of accumulation of more dislocations and, finally, the formation of large-angle GBs. The GBs were generally curved, wavy or faceted, and heavily strained, which are typical characteristics of nanostructured materials. In addition, extrinsic dislocations were found in many GBs, indicating that most are in a high-energy non-equilibrium configuration, which is consistent with observations in equal-channel angular pressing processed Cu, Ni, and Al-Mg, repetitive corrugation and straightening processed Cu and room-temperature ball-milled Cu. These results support a still-disputed concept that GBs in nanostructured metals processed by severe plastic deformation are mostly in non-equilibrium states.     

X-ray surface radiolysis: Kinetics of the metal-organic interface formation

Grazing incidence x-ray irradiation of a Langmuir monolayer deposited at the interface between an aqueous silver salt solution and the air leads to the formation of a metallic silver layer of thickness about 4.5 nm, in agreement to the x-ray penetration depth at the air – water interface. Moreover using the scattering and fluorescence induced by the x-ray, we show that it is possible to follow the kinetics of formation of the layer. It appear that the silver layer is formed in two steps: first, the formation of small oriented cristallites, and second, the coalescence of the cristallites forming large plates.     

Kinetics of defect formation in thin-film vacuum condensates

Principal factors responsible for ageing of thin-film vacuum condensates on a glass base are considered. A hypothesis concerning the relation between the change in the relative defected area of thin-film coatings and a decrease in internal stresses in these coatings with time is proposed. Assuming that the film material is viscoelastic and obeys the Maxwell-Thompson rheological relation, a mathematical model of the kinetics of defect formation in thin-film vacuum condensates under normal conditions of storage and operation is developed on the basis of this hypothesis. Satisfactory agreement between calculated and experimental data suggests that this model can be recommended for qualitative and quantitative assessment of ageing of thin-film coatings obtained by condensation in vacuum.     

Production, structure, and microhardness of nanocrystalline Ni-Mo-B alloys

Radiography, differential scanning calorimetry, luminescence and high-resolution electron microscopy are used to study the production, nanocrystalline structure, stability, and microhardness of alloys from the Ni-Mo-B system containing from 27 at. % to 31.5 at. % Mo and 10 at. % B. All studies of these alloys indicated that annealing at 600 °C leads to the creation of a granular phase consisting of FCC nanocrystallites with average grain sizes of 15–25 nm, depending on the chemical composition of the alloy. Annealing these nanocrystalline samples isothermally at a temperature of 600 °C has no appreciable effect on the grain size. Structurally, the nanocrystalline phase consists of grains of an FCC solid solution of Mo and B in Ni, dispersed in an amorphous matrix that isolates them from one another. The lattice parameters of the FCC nanocrystallites depend on the alloy composition and the duration of their isothermal anneal. Within this latter time, molybdenum and boron atoms diffuse from the FCC solid-solution lattice into the surrounding amorphous matrix. The stability of the nanocrystalline structure is determined by the thermal stability of the amorphous matrix, whose crystallization temperature increases with the isothermal annealing time due to enrichment by boron and molybdenum. As the structure forms, the alloy becomes harder as the nanocrystalline grains grow in size. This relation between hardness and grain size, which is opposite to the Hall-Petch law, is explained by hardening of the amorphous matrix due to changes in its chemical composition. Fiz. Tverd. Tela (St. Petersburg) 40, 10–16 (January 1998)     

Structural role of polyethylene glycol in the formation of anatase nanocrystalline titania at low temperature

Anatase nanocrystalline TiO₂ thin films were obtained by a sol–gel dip‐coating method, in which the nanocrystallization is effected by a simple hot water treatment of the deposited films at temperatures below 90 °C under atmospheric pressure for 1 h. The dip‐coating sol was prepared by reacting titanium tetra‐n‐butoxide [Ti(OⁿBu)₄] with polyethylene glycol (PEG) in ethanol. Films obtained from a sol that do not contain PEG show no sign of crystallization, demonstrating the importance of PEG in the crystallization process. Raman studies of reaction dynamics show that PEG undergoes a nucleophilic substitution reaction replacing butoxy groups in Ti(OⁿBu)₄. Stoichiometric reactions of Ti(OⁿBu)₄ with PEG in polar and nonpolar solvents were performed, and they yielded different titanium–PEG hybrid polymers, which were isolated and characterized by various spectroscopic techniques such as IR, Raman, solid‐state NMR and MALDI‐TOF‐MS. NMR studies evidenced the location and the way in which PEG is bonded with titanium atoms in the titanium–PEG hybrid polymers. On the basis of these studies, we have proposed structures for these polymers. It is demonstrated that the structure of the obtained polymers plays an important role in the formation of anatase TiO₂ nanoparticles in hot water at temperatures below 90 °C under atmospheric pressure. Copyright © 2009 John Wiley & Sons, Ltd.     

热处理对富硅氧化硅薄膜中硅纳米晶形成的影响

采用磁控溅射法制备了富硅氧化硅薄膜, 然后分别经过一步热处理、两步热处理和快速热处理制备了镶嵌有硅纳米晶的氧化硅薄膜. 实验结果表明, 在硅含量为～ 42.63 at.%的富硅氧化硅薄膜中, 三种热处理均能形成10¹²/cm²量级的硅纳米晶. 其中在两步热处理中, 硅纳米晶的密度最高, 达到2.2× 10¹²/cm², 并且尺寸均匀、结晶完整性好; 一步热处理后的样品中, 硅纳米晶密度较低, 并且部分纳米晶结晶不充分; 快速热处理后的样品中, 硅纳米晶密度最低、尺寸分布不均匀, 并且存在孪晶结构. 分析认为, 热处理初始阶段的形核过程对纳米晶的密度及微观结构有着重要的影响, 两步热处理中的低温段促进了纳米晶的成核, 有助于形成高密度高质量硅纳米晶.