Specific features of the morphology and distribution of gas inclusions in single-crystal sapphire ribbons grown by the Stepanov method

The distribution of gas inclusions in single-crystal sapphire ribbons with different crystallographic orientations, grown by the Stepanov method, is analyzed. In all ribbons gas inclusions are observed only in thin surface layers located at a depth of about 100 μm. One characteristic feature is the presence of several zones with different distributions and morphologies of inclusions. We observed inclusions of the two types: cylindrical pores and microbubbles. The differences in the morphology and zonality of the gas-inclusion distribution are consistent with the α-Al₂O₃ crystallography.     

Investigation of the crystallization features,atomic structure,and microstructure of chromium-doped monticellite

A series of Cr⁴⁺:CaMgSiO₄ single crystals is grown using floating zone melting, and their microstructure, composition, and crystal structure are investigated. It is shown that regions with inclusions of second phases, such as forsterite, akermanite, MgO, and Ca₄Mg₂Si₃O₁₂, can form over the length of the sample. The composition of the single-phase regions of the single crystals varies from the stoichiometric monticellite CaMgSiO₄ to the solid solution Ca_{(1 ? x)}Mg_{(1 + x)}SiO₄(x = 0.22). The Cr:(Ca_0.88Mg_0.12)MgSiO₄ crystal is studied using X-ray diffraction. It is revealed that, in this case, the olivine-like orthorhombic crystal lattice is distorted to the monoclinic lattice with the parameters a = 6.3574(5) Å, b = 4.8164(4) Å, c = 11.0387(8) Å, β = 90.30(1)^o, Z = 4, V = 337.98 ų, and space group P2₁/c. In the monoclinic lattice, the M(1) position of the initial olivine structure is split into two nonequivalent positions with the center of symmetry, which are occupied only by Mg²⁺ cations with the average length of the Mg-O bond R _av = 2.128 Å. The overstoichiometric Mg²⁺ cations partially replace Ca²⁺ cations (in the M(2) position of the orthorhombic prastructure) with the average bond length of 2.347 Å in the [(Ca,Mg)-O₆] octahedron. The average distance in SiO₄ distorted tetrahedra is 1.541 Å.     

Physicochemical investigation into the specific features of the phase transformations in electrodeposited nanocrystalline oxotungstate films

The structure of oxotungstate films (as-deposited and subjected to heat treatment at temperatures of up to 600°C) prepared through electrodeposition on platinum and gold polycrystalline substrates is investigated using different physicochemical methods. It is shown that the oxotungstate films consist of X-ray amorphous hydrated mixtures of isopoly compounds, predominantly in the form of paratungstates with [H₂W₁₂O₄₂]¹⁰⁻ anions. The structural transformation with an increase in the temperature in air is accompanied by the loss of water, the transformation of paratungstate anions into more oxidized forms, their destruction, and the crystallization of nonstoichiometric hydroxylated oxide phases.

     

Fabrication and characterization of metallic glasses with a specific microstructure for micro-electro-mechanical system applications

Metallic glass microstructures with high aspect ratios for micro-electro-mechanical system applications have been fabricated by micro-electro-discharge machining and selective electrochemical dissolution methods. Micro-holes and three-dimensional microstructures machined on the La₆₂Al₁₄Ni₁₂Cu₁₂, Zr₅₅Al₁₀Ni₅Cu₃₀ and Cu₄₆Zr₄₄Al₇Y₃ bulk metallic glasses by micro-electro-discharge machining are evaluated by using X-ray diffraction, scanning electron microscopy, and nanoindentation. The experimental results demonstrate that the machined samples kept their amorphous structure without devitrification, and their machining characteristics are related to the thermo-physical properties of the alloys and the electrode diameters. Porous, single-pore and thin-walled Zr-based metallic glass tubes with micro-pore structures can be prepared by selective electrochemical dissolution method. The high aspect ratio microstructures fabricated by the two methods have the potential applications as micro-nozzles, polymer micro-injection molding tools, micro-channels or micro-flow meters in micro-electro-mechanical system devices.     

Investigation into the specific features of the changes in the crystal structure of stabilized zirconia upon thermochemical reduction

The atomic structure of Zr_0.87Y_0.13O₂ single crystals unannealed and annealed at temperatures of 400 (1 h) and 600°C (1 h) is investigated using X-ray diffraction. It is revealed that these crystals have a superstructure: the cubic unit cell parameters a of the unannealed sample and samples annealed at 400 and 600°C are equal to 10.3130(16), 10.3083(13), and 10.3092(14) Å, respectively. The site occupancies of the oxygen atoms are found to decrease under annealing.     

The mechanism of the rotation of single-crystal spheres during sintering

A dislocation mechanism is presented which explains the fast rotation of particles as observed during the sintering of rows and loose packings of monocrystalline copper spheres. The experimentally determined data agree well with the data determined by calculation. They are important for the production of sintered filters.     

Crystal structure and specific features of formation of vibrational spectra of Pb2GeS4

The structure of Pb₂GeS₄ single crystals is confirmed and the polarized Raman spectra of these crystals are recorded. The vibrational spectra was considered in terms of analysis of intramolecular vibrations of [GeS₄] tetrahedral formations, the main structural elements of this compound.     

X-ray reflectometry of the specific features of structural distortions of He+-implanted Si(001) surface layers

The structural changes in the surface layers of silicon substrates, implanted by helium ions with energies from 2 to 5 keV and doses to 6 × 10¹⁵–5 × 10¹⁷ cm^?2, has been studied by high-resolution X-ray reflectometry. The damaged layer is found to have a total thickness comparable with the total ion path length (estimated from the SRIM model) and a multilayer structure: a strongly amorphized layer with reduced density, a porous (incapsulated) layer, and a deformed layer. The thickness of sublayers, their density ρ(z), and the mean strain (～5 × 10^?3) have been determined. The characteristic pore size is estimated to be 5–20 nm. It is shown that the presence of a nanoporous layer facilitates the formation of diffuse scattering, which can be used to diagnose layers by high-resolution X-ray reflectometry.     

Melt menisci during single-crystal tube growth by the Stepanov method

The stability of the liquid menisci formed during the growth of single-crystal tubes by the Stepanov method and, in particular, by the floating zone method used to purify crystals, has been investigated. The results obtained can be used in practice to grow crystals by various capillary shaping methods.     

Effect of minor alloying additions on the solidification of single-crystal Ni-base superalloys

Grain boundary elements, such as carbon and boron, were initially removed from single-crystal alloys, but more recently they have been added back into single-crystal alloys for increased castability and defect tolerance. The mechanism(s) for the increased castability is not completely understood. In this study, carbon was added to the second generation, single crystal, Ni-base superalloy and CMSX-4, to form MC-type carbides. Then either nitrogen or boron was added to the carbon-containing alloy to alter the carbide morphology and the castability of the alloy. The segregation coefficients of the alloying elements in CMSX-4 were measured with varying techniques, but no changes in partitioning were observed. The addition of carbon to CMSX-4 did reduce the number of solidification defects observed in the samples. The addition of carbon and boron to CMSX-4 did not significantly change the number of solidification defects compared to the carbon-only alloy. However, the addition of nitrogen and carbon resulted in an increase in the number of solidification defects in the CMSX-4 samples compared to the baseline CMSX-4 and carbon-containing CMSX-4 samples. The effects of these alloy additions on the carbide morphology and solidification defects are discussed.     

MOVPE growth of single-crystal hexagonal AlN on cubic diamond

We have obtained single-crystal aluminum nitride (AlN) layers on diamond (1 1 1) substrates by metalorganic vapor-phase epitaxy (MOVPE). When the thermal cleaning temperature of the substrate and growth temperature of the AlN layer were below 1100 °C, the AlN layer had multi-domain structures mainly consisting of rotated domains. An interface layer, consisting of amorphous carbon and poly-crystal AlN, was formed between the AlN layer and the diamond substrate. On the other hand, when the thermal cleaning temperature and growth temperature were above 1200 °C, a single-crystal AlN layer was grown and no interface layer was formed. Therefore, we attribute the multi-domain structures to the interface layer. Even at the growth temperature of 1100 °C, by performing the thermal cleaning at 1200 °C, the single-crystal AlN layer was obtained, indicating that the thermal cleaning temperature of the substrate is a critical factor for the formation of the interface layer. The epitaxial relationship between the single-crystal AlN layer and the diamond (1 1 1) substrate was determined to be [0 0 0 1]_AlN∥[1 1 1]_diamond and [1 0 1¯ 0]_AlN∥[1 1¯ 0]_diamond. The AlN surface had Al polarity and no inversion domains were observed in the AlN layer.     

Edge misfit dislocations in the GeSi/Si(001) pair: Conditions and specific features of high-quantity generation

Dislocation structure of Ge_xSi_1?x films (x=0.4?0.8) grown by molecular-beam epitaxy on Si(001) substrates was studied by means of transmission electron microscopy. It was found that the density of edge MDs formed at the early stage of plastic strain relaxation in the films could exceed the density of 60° MDs. In our previous publications, a predominant mechanism underlying the early formation of edge misfit dislocations (MD) in Ge_xSi_1?x/Si films with x>0.4 was identified; this mechanism involves the following processes. A 60° glissile MD provokes nucleation of a complementary 60° MD gliding on a mirror-like tilted plane (111). A new edge MD forms as a result of interaction of the two complementary 60° MDs, and the length of the newly formed edge MD can then be increased following the motion of the “arms” of the complementary 60° MDs. Based on this scenario of the edge MD generation process, we have calculated the critical thickness of insertion of an edge MD into GeSi layers of different compositions using the force balance model. The obtained values were found to be more than twice lower than the similar values for 60° MDs. This result suggests that a promising strategy towards obtaining dislocation arrays dominated by 90° dislocations in MBE-grown Ge_xSi_1?x/Si films can be implemented through preliminary growth on the substrate of a thin, slightly relaxed buffer layer with 60° MDs present in this layer. The dislocated buffer layer, acting as a source of threading dislocations, promotes the strain relaxation in the main growing film through nucleation of edge MDs in the film/buffer interface. It was shown that in the presence of threading dislocations penetrating from the relaxed buffer into the film nucleation of edge MDs in the stressed film can be initiated even if the film thickness remains small in comparison with the critical thickness for insertion of 60° MDs. Examples of such unusual MD generation processes are found in the literature.     

Crystal and molecular structure of the complex boron triferrocenyl-pyridine

The title compound is C₃₅H₃₂NBFe₃, monoclinic,P2₁/n,a=14.204 (4),b=11.582 (4),c=17.547 (6) Å,=101.46 (2)°. The structure was solved by the heavy-atom method and refined by least-squares techniques to anR factor of 0.037 for 3148 observed reflections. The X-ray study confirms that in the solid state the structure of the title compound is similar to that inferred from chemical and spectroscopic evidence. The B atom is coordinated by three ferrocenyl groups and a pyridine ring in a distorted tetrahedral array. The molecule has a nearly three-fold axis normal to the plane defined by the ferrocenyl groups. The B-N distance of 1.656 (5) Å is larger than that obtained for other compounds studied. Bond lengths and angles for the whole structure are all in accord with similar compounds. The pyridine and cyclopentadienyl rings are planar. The H atoms of the cyclopentadienyl rings are displaced significantly toward their corresponding Fe atom. The molecules in the crystal are packed at normal van der Waals distances. No unusually short intermolecular contacts are noted.     

Volatilization of boron from E-glass melts

Boron is known to volatilize from borate and borosilicate melts at high temperatures. The rate of boron loss from E-glass melts, as measured using the B–O overtone band in the infrared spectrum of this glass, has been measured at several temperatures and partial pressures of water vapor. The rate of volatilization increases dramatically with increasing partial pressure of water vapor in the atmosphere surrounding the melt at constant temperature and with increasing melting temperature at constant partial pressure of water vapor. The boron concentration decrease is accompanied with an increase in hydroxyl concentration. The net effect of changes in boron and hydroxyl concentration is an increase in glass density, refractive index, and glass transformation temperature.     

Hydrothermal growth of boron nitride microcrystals

Boron nitride (BN) crystals with size of several micrometers have been successfully synthesized by hydrothermal method. The reactants used in our experiments were boric acid (H₃BO₃), sodium azide (NaN₃) and white phosphor (P). The samples were characterized by X‐ray diffraction (XRD), Fourier transformation infrared spectroscopy (FTIR), selective area electron diffraction (SAED). It is found that the existence of Cl^‐ in the reaction mixture has much effect on the synthesis of BN. (© 2005 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

On the deposition mechanism of boron in silicon CVD epitaxy

The deposition mechanism of boron doping in CVD silicon epitaxy has been investigated by exposing silicon substrates to B₂H₆ H₂ doping gas mixtures at epitaxy temperatures and examining the effect by dopant profile measuring in an afterwards intrinsically in-situ deposited epitaxial silicon layer. It has been shown that boron is deposited increasing its concentration on the surface linearly with prolonged exposition time and desorbed by purging the surface in pure hydrogen. In the latter case its content decreases linearly proportional to the predeposited concentration. The desorbed boron builds up a secondary doping source which maintains a parasitic boron flow for reincorporation during following layer growth.     

On the nature of unique boron sites in borate glasses

The ¹¹B NMR spectra of borate glasses can be accurately computer-simulated using a simple model for the electric field gradient at a boron site in the BO₃ and BO₄ structural units, assuming these structural units are relatively undistorted. Deviations from the average OBO bond angle in these two sites are determined from the simulations to be less than ± 2° in borate glasses. There is no evidence for increasing distortion in the fundamental structural units with increasing metal oxide content.     

Controlled growth of filamentary crystals and fabrication of single-crystal whisker probes

The growth of filamentary crystals (whiskers) on a single-crystal substrate through the vapour-liquid-solid mechanism is described. The possibility of fabricating oriented systems of whiskers on the basis of this mechanism of crystal growth is noted. A phenomenon that is important for nanotechnology is noted: the existence of a critical diameter of whiskers, below which they are not formed. The phenomenon of radial periodic instability, which is characteristic of nanowhiskers, is described and the ways of its elimination are shown. The possibility of transforming whiskers into single-crystal tips and the growth of crystalline diamond particles at their apices are noted as important for practice. Possible applications of systems of whiskers and tips are described briefly. Particular attention is paid to the latest direction in whisker technology—fabrication of single-crystal whisker probes for atomic force microscopy.     

Classification of hydrated oxygen compounds of boron

The known structural classifications of borates are based on the analysis of boron-oxygen radicals consisting of B-triangles and B-tetrahedra sharing O atoms to form more complicated structures. Such classifications ignore the possible presence and the role of hydrogen bonds, which may be the only bonds or additional bonds linking the [BO₃]-and [BO₄]-groups. A new detailed classification of hydrated boron compounds taking into account hydrogen bonds between the B-polyhedra is proposed.