Structure and conditions for the formation of fullerite crystallites in Sn-C<Subscript>60</Subscript> films

The structure of tin-fullerite films with different thicknesses of the Sn coating layer (50, 100, 200, 300, 450, and 700 nm) and the conditions for the formation and growth of fullerite crystallites on the tin surface during sample exposure in air have been investigated. The methods of X-ray diffraction; scanning electron, transmission electron, and atomic force microscopy; and X-ray microanalysis were used to reveal changes in the structure and phase composition of the tin-fullerite films. Fullerite crystallites in the form of plates and bolts grown under internal stress have been found on the surface of tin films with thicknesses of 50, 100, and 200 nm. The incubation period of crystallite formation is established to be 12–22 months, depending on the thickness of the tin layer.     

Preparation of nickel oxide– and nickel–silica nanocomposites by spray pyrolysis

Nickel oxide–silica and nickel–silica nanocomposites were prepared by spray pyrolysis of aqueous sols of silica nanoparticles containing nickel nitrate hexahydrate, without and with ethanol, respectively. During pyrolysis, the silica nanoparticles were restructured, losing their identities, while the nickel oxide or nickel particles in the composites grew by coalescence and sintering. Compactness of the composites, higher in nickel–silica composites than their counterpart nickel oxide, increased with the temperature of preparation and the concentration of nickel nitrate. Nickel always formed larger crystallites than its counterpart nickel oxide, due to its higher sintering rate. At 500 °C, the crystallite growths of nickel oxide and nickel were inhibited by the low sintering rate and the formation of nickel intermediates, respectively, while at 1000 °C their phase-pure crystallites continuously grew with the increase in the nitrate concentration.     

Some Aspects of Interactions in the Mo–W–Al2O3–H2 System: Formation of Polyoxides

The possibility of formation of molybdenum and tungsten polyoxides in the Mo–W–Al₂O₃–H₂ system at T = 2400 K and P = 1 bar in a controlled Ar + H₂ atmosphere has been investigated by the method of thermodynamic analysis. The formation of polyoxides is found to occur both due to the processes involving Al₂O₃ melt and in the absence of the latter. It is established that metals (Mo and W) and their mono-, di-, and even trioxides (in the latter case, mediated polymerization occurs) can be used as initial components to form polyoxides. It is shown that polyoxides themselves may interact with one of their main sources: Al₂O₃ melt.

     

Structure and mechanical properties of foils made of nanocrystalline beryllium

The phase composition and structural features of (45–90)-μm-thick foils obtained from nanocrystalline beryllium during multistep thermomechanical treatment have been established using electron microscopy, electron diffraction, electron backscattering diffraction, and energy-dispersive analysis. This treatment is shown to lead to the formation of a structure with micrometer- and submicrometer-sized grains. The minimum average size of beryllium grains is 352 nm. The inclusions of beryllium oxide (ВеО) of different modifications with tetragonal (sp. gr. P4₂/mnm) and hexagonal (sp. gr. P6₃/mmc) lattices are partly ground during deformation to a size smaller than 100 nm and are located along beryllium grain boundaries in their volume, significantly hindering migration during treatment. The revealed structural features of foils with submicrometer-sized crystallites provide the thermal stability of their structural state. Beryllium with this structure is a promising material for X-ray instrument engineering and for the production of ultrathin (less than 10 μm) vacuum-dense foils with very high physicomechanical characteristics.     

Electron microscopy of bismuth oxychloride crystallites

Investigations on the crystal structure and morphology of bismuth oxychloride crystallites as a function of the growth conditions are described. A decrease in the concentration of the reactant or an increase in the temperatur during the reaction improves the crystallinity of the bismuth oxychloride precipitate. The crystallites are usually platelets with the c-axis normal to the crystallite plane.     

Mechanisms of Interaction of Molybdenum and Tungsten Polyoxide with Aluminum Oxide Melt under Reducing Conditions

The main chemical reactions between Mo and W polyoxides and Al₂O₃ melt in a controlled Ar + H₂ atmosphere (T = 2400 K, P = 1 bar) during sapphire growth by horizontal directional solidification have been investigated. Under these thermodynamic conditions, the melt and products of its dissociative evaporation may actively react with the tungsten heater and molybdenum thermal screens of the crystallization system. It is shown that the polyoxides formed during evaporation do not directly interact with the melt; this interaction occurs only with participation of reagents exhibiting pronounced reducing properties (Al, H₂, H, WO, Al₂O, AlH, AlH₂, AlH₃). It is established that most of processes occur with participation of aluminum hydrides. A particular role of Mo(W) dioxide–W(Mo) polyoxide functional pairs in the interaction with the melt is determined.

     

Transformation of the Structure of ZnO–C Powders during Mechanical Activation and Spark Plasma Sintering

The processes of grinding and transformation of the structure of the ZnO–C powder mixture at the mechanical activation by ball milling in an inert atmosphere have been investigated. The mechanical activation of the ZnO–C powder mixture is shown to lead to the formation of graphene oxide shells on the surface of crushed zinc oxide particles. The influence of the duration of activation of powder mixtures on the dynamics of their compaction during spark plasma sintering and structure of the ceramics obtained has been investigated. It is established that an increase in the activation duration to 100 h leads to a monotonic decrease in the density of sintered ceramics. The carbon shell formed during sintering at ZnO grain boundaries is shown to have the structure of reduced graphene oxide.

     

Study on the Morphological Change of Growing Ag Crystallites by in-situ SEM Observation

The morphology of the initial growth stage of Ag thin films on Mo(110) substrate was investigated in-situ by the scanning electron microscope (SEM). A great majority of the Ag crystallites showed a truncated Wulff's equilibrium shape of the face-centered-cubic (fcc) structure with epitaxial orientation [1 0]Ag//[11 ]Mo, (111)Ag//(110)Mo with the substrate temperature of 400 °C. In-situ SEM observation during the growth revealed two kinds of particular morphological change of Ag crystallites, namely (1) rotation of an Ag crystallite and (2) reformation of the crystal faces of the Ag crystallite. However, a small number of Ag crystal particles showed Wulff-polyhedron truncated by the (100) plane parallel to the substrate surface (oriented to the 〈100〉 axis normal to the substrate surface).     

Mössbauer study in the glass system PbO · 2B2O3 · Fe2O3

The Mössbauer technique has been employed to study the structure and crystallite formation in the glass system PbO · 2B₂O₃ containing upto 30 wt% Fe₂O₃. Like alkali borate glasses, this glass system also exhibits a broadened quadrupole doublet and iron ions are present in Fe³⁺ state. Above about 20 wt%, the crystallites of magnetically ordered states have been identified. Susceptibility variation with concentration suggests the formation of a superparamagnetic state.     

The Effect of Solid and Liquid Inclusions on Morphology of Crystallization Front

The interaction between crystallization front and solid particles was studied for substances characterized by the normal growth mechanism (continuous growth). It has been measured the critical velocity below which the particle of radius R is repulsed by moving interface and above it is captured. It has been shown that critical velocity is proportional to R⁻(1.4–1.8) what satisfactorily agrees with Chernov-Temkin's theory. Data have been received about change of interface morphology at capture of solid particles for growth from pure and impurity-contained melt. The dendrites have been found to split at interaction with particles and concentration inhomogeneities. Taking for example Al-Cu alloy, possibility has been shown to disperse the dendritic structure by formation concentration inhomogeneities in the melt.     

In situ observations in the melt growth of Pb5Ge3O11 crystals

The interface motion during the growth process of Pb₅Ge₃O₁₁ from the melt is visualized in an in situ observation system. The formation of the striation aggregation is observed, and the rates of the interface motion R_im are measured. It turns out that there are sharp changes in the growth rate during the formation of the striation aggregation. It has been found that the typical fluctuation in the rates can be associated with the kinetics and transport mixed control during the crystal growth. The non-uniform composition normal to the interface gives rise to the striation aggregation. Of particular importance in the investigation are the typical interfacial melt flows from the corners to the center of the interface observed in the experiments. The relation between the rates of this interfacial melt flow R_if and the time t is also provided showing that the change of supersaturation adjacent to the interface gives rise to the interfacial melt flow.     

Dynamics of low temperature PECVD growth of microcrystalline silicon thin films: Impact of substrate surface treatments

Microcrystalline silicon (μc-Si) films have been deposited on polyimide, Corning glass and c-Si(0 0 1) by rf plasma-enhanced chemical vapour deposition (PECVD) using both SiF₄–H₂ and SiH₄–H₂ plasmas. The effect of substrate pre-treatment using SiF₄–He and H₂ plasmas on the nucleation of crystallites is investigated. Real-time laser reflectance interferometry monitoring (LRI) revealed the existence of a ‘crystalline seeding time’ that strongly impacts on the crystallite nucleation, on the structural quality of the substrate/μc-Si interface and on film microstructure. It is found that SiF₄–He pre-treatment of substrates is effective in suppressing porous and amorphous interface layer at the early nucleation stage of crystallites, resulting in direct deposition of μc-Si films also on polyimide at the temperature of 120 °C.     

Self organized nano crystallinity of magnetite precipitated from a 4.9Na2O · 33.3CaO · 17.1Fe2O3 · 44.7B2O3 glass

Glasses with the mol% composition 4.9Na₂O · 33.3CaO · 17.1Fe₂O₃ · 44.7B₂O₃ were melted, rapidly quenched using a twin roller technique, and subsequently tempered in the range from 550 to 620 °C. This led to the crystallization of magnetite with mean crystallite sizes in the 10-20 nm range. Using higher temperatures resulted in a larger quantity of formed crystallites and slightly larger mean crystallite sizes. Larger tempering times did not lead to substantial crystal growth. The time law of Ostwald ripening was not followed. This is explained by an increase in viscosity of the residual glassy phase during nucleation and crystal growth. Here, the smaller iron concentration near the crystals leads to higher viscosities and to the formation of a diffusional barrier around the crystals, which reduces further crystal growth. The crystallization stops, if T_g of the residual glassy phase is equal to the tempering temperature. Magnetite nano crystals with sizes in the 10-20 nm range offer a wide range of applications, such as the preparation of ferrofluids or of materials for medical diagnostics and therapy.     

Metastable states in high carbon C–(Fe,Ni, Co) films obtained by three-electrode ion-plasma sputtering

Abstract

The effect of ion-plasma deposition on the structure of high-carbon films (at. %) Fe–(20–84) % С, Co–(5–52) % С, Ni–(7–61) % С was investigated. The lattice periods and crystallite sizes of nonequilibrium phases in the as-deposited state and after heating are determined. The temperatures of the beginning and end of the decay of metastable phases during heating at a constant speed are established. The transition from an amorphous to an equilibrium crystalline state in Fe–C films passes through the stage of formation and subsequent decomposition of an intermediate, metastable hcp phase of variable composition. The electrical and hysteretic magnetic properties of the films were measured in the as-deposited state and after heat treatment. The compositions and conditions for producing films with low values of the temperature coefficient of electrical resistance and high coercive force are established. Thus, high-carbon films of Ni–61% C in the as-depoteted state and Fe–69% C films after heating to 900?K are characterized by small TCR values (± 10^?6 К^?1) over a wide temperature range.     

Effects of the Li-evaporation on the Czochralski growth of γ-LiAlO2

Two-inch-diameter γ-LiAlO₂ single crystals were grown from the melt by Czochralski method. The crystals were examined by optical methods, high-resolution X-ray diffraction and transmission electron microscopy (TEM). Inductively coupled plasma optical emission spectrometry (ICP-OES) was used to determine the Li/Al ratio in the residual melts. The Li-evaporation from both melt and grown crystal is the main problem in the γ-LiAlO₂ growth and has to be controlled by acting on the vertical temperature gradient. Shallow gradients increase the Li-evaporation from the crystal surface resulting in boules with a milky rim. On the other hand, steep gradients may induce cracks in the boule and enhance the Li₂O escape from melt with consequent variation of the composition. ICP-OES investigations reveal that melt compositions can vary in the range from 46.5 to 50 mol% Li₂O to obtain transparent LiAlO₂ crystals. Beyond this value, the formation of inclusions inside the crystals is probable. We have established an optimized growth assembly, which allows remaining the melt composition stoichiometric. The as-grown crystals exhibit defects like subgrains, twins and a core of voids and fine-grained inclusions. The latter could be characterized by TEM as submicron LiAl₅O₈ crystallites.     

Ordered structure and preferred orientation of boehmite films prepared by the sol-gel method

Several boehmite films were produced using the sol-gel route. The drying process, the film structure evolution and the final texture of the samples were investigated by means of combined X-ray diffraction and weight variation. A remarkable (0 1 0) preferred orientation (about 97% of the crystallites) was found for xerogel films with thickness ranging from 1.5 to 24 μm. The ordered structure is already present in the sol state and is appeared and enhanced during drying, before gelling. This phenomenon may be attributed to the particular boehmite structure that induces special crystallite organization in the aquagel.     

Optical properties of the chopped and non-chopped vacuum evaporated polyaniline thin film

Non-chopped and chopped polyaniline (PANI) thin films were deposited onto glass substrate by vacuum evaporating polyaniline base powder. The effect of chopping on the structural and optical properties of the film is reported in this paper. To the authors’ knowledge there are no reports on the chopping of PANI thin films. The effect of annealing on the chopped and non-chopped thin film was also studied. The FTIR studies indicate absence of benzene ring deformation at 607 cm^–1 in the chopped films. There is a marked decrease in refractive index in the unannealed chopped films. The SEM data of chopped films show smaller crystallites and more uniform films. Chopping produces growth flux interruption preventing large crystallite growth and film with lower defects and voids.     

Growth and Characterisation of Silicon Crystallites on Laser Structured Glass

Selective growth of silicon crystallites on glass, seeded from silicon saturated metallic solution droplets is demonstrated. These droplets are deposited in micropore arrays which are generated in the glass substrate by ultrashort laser pulses. In this way, an equidistant distribution of crystallites can be achieved by preferred nucleation in the micropores. Additionally, a crystallite selection occurs corresponding to the tip geometry. The material transport is governed by the vapour-liquid-solid (VLS) mechanism. The morphological characterization of micropores and grown crystallites is performed by Scanning Electron Microscopy (SEM), Atomic Force Microscopy (AFM) and Focused Ion Beam (FIB) method.     

Metallurgically grown Schottky junction in directionally solidified eutectic Ag–Si alloys

Metallurgically grown M(metal)–S(semiconductor) phase boundaries in directionally solidified Ag Si alloys were investigated relative to their behaviour as Schottky junctions. By the aid of point contacts I/V characteristics have been measured which correspond to these ones of Schottky diodes. The Si crystallites were proved to be n-doped. Reverse breakdown voltages of 8 … 24 V are not incompatible with the purity of the used materials. In forward direction the value of diode factor n = 1.1 … 5 shows the existence of several current flow mechanisms. In order to be able to interpret the characteristics it is necessary to have exact information on the specific resistivity of the Si crystallites and on the content of grain boundaries and dislocations in silicon. Beyond that the complicated shape of Si crystallites prevents the necessary determination of the interface size.     

Two‐ and three‐dimensional growth modes of nitride layers

Heteroepitaxial three dimensional (3D) and two dimensional (2D) growth modes of nitride layers on sapphire substrates are discussed. It is shown that the 3D or 2D growth mode of AlGaN layers depends predominantly on the growth conditions of the underneath low temperature (LT) nucleation layer. Commonly described in literature 3D growth mode is achieved on LT GaN or AlN nucleation layer grown relatively fast. Successive growth of secondary layer at high temperature begins from separated sites, where individual 3D crystallites are formed. Threading dislocations present in crystallites bend on their facets, which reduces the quantity of dislocations. However, slight crystallographic misorientations between crystallites lead to the creation of new dislocations during coalescence of the crystallites. As a result, edge and mix dislocations appear at similar densities of about 10⁹ cm^‐2. Modification of growth conditions of LT AlN nucleation layer, especially reduction of their growth rate, leads to drastic changes in properties of the layer. Successive growth of secondary AlGaN layer at high temperature starts evenly on whole surface retaining atomic flatness. Thus growth at high temperature occurs only by 2D mode. Therefore, it is possible to grow a very thin AlGaN layers directly on top of LT nucleation layer. Such layers contain large number (10¹⁰ cm^‐2) of edge dislocations, and relatively small number (less then 10⁸ cm^‐2) of mix dislocations. It is also shown that the decisive factor determining the growth mode of AlN nucleation layer is a growth of the first few atomic layers on substrate surface. The slow growth of these few first atomic layers decide about the 2D growth mode, and the fast one about the 3D one. The model explaining this difference is presented as well. (© 2007 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)