A new method for the investigation of orientation relationships in meteoritic plessite

The orientation relationship (OR) between the bcc and fcc phase in the plessite microstructure of the iron meteorites Watson, Agpalilik and Gibeon has been analysed in a scanning electron microscope using electron back‐scattered diffraction (EBSD). A very strong OR exists, independently on the analysed plessite type and the observed spreading of single orientation data. The agreement between the experimental orientation distribution and existing models varies for each meteorite. The black plessite in the Agpalilik corresponds to the Nishiyama‐Wassermann model whereas the duplex plessite of the Gibeon meteorite shows an OR close to the Kurdjumov‐Sachs model. The Watson meteorite is strongly deformed so that a general OR is difficult to determine due to the blurred experimental orientation distribution. (© 2004 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

Electron and optical microscopy studies of extraterrestrial minerals in NWA4047 meteorite

Analytical electron microscopy and optical microscopy were used to determine the elemental and mineral composition of NWA4047 meteorite. The meteorite was found in 2005 in Morocco, and in 2006 was preliminary classified as H4‐5 ordinary chondrite. The main crystalline meteorite minerals: olivines, pyroxenes, kamacite and taenite, feldspars, as well as troilite identified in our sample represent extraterrestrial minerals typical of the ordinary chondrite of high iron content. Low‐Ca orthopyroxene called bronzite has been revealed. Single enstatite and clinopyroxene crystals have also been identified. Olivines in the meteorite contain 18 mol% of fayalite. The meteorite is a monomict breccia, in which fragments of petrographic types 4, 5, and 6 are present, with the groundmass being type 4. Some of clasts contain large grains of feldspars and high content of feldspars, which is typical of petrologic type 6. Chemical and mineral composition, petrologic types of chondrules, their abundance and sizes, the presence of troilite‐kamacite veins, and nonoxidised iron phases prove that the chondrite belong to H group, i.e. to the olivine‐bronzite group. Our results show that the NWA4047 meteorite is H4‐6 ordinary chondrite. The weathering grade of the meteorite W0/1 is the same as previously established, but the shock grade S3 seems to be higher in comparison with the previous classification. (© 2008 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

Thermodynamic and lamella models relationship for the eutectic system benzoic acid – cinnamic acid

The present investigation reveals the relationship between excess thermodynamic functions and the growth habits of the eutectic phases from the melt by continuous melt‐growth technique. Excess thermodynamic functions computed for different compositions of the benzoic acid – cinnamic acid eutectic system have been found consistent with the criteria of spontaneity and Planck formulation, and their reliability has been ascertained by the application of Guggenheim lattice theory. The results on the kinetics of anisotropic growth of the eutectic phases from the melt, evidentially evince the dislocation mechanism. Evidences have been obtained for a parabolic variation of mechanical strength with growth velocity of the eutectic material grown anisotropically from the melt at different intervals, which offer supporting complement to the dislocation mechanism governing the dependence of growth velocity on supercooling ΔT in the solidus – liquidus interface in a form : V = k(ΔT)². A moderate anisotropic growth region has been explored by unique results of strength properties and microscopic results as well, to growing a layer of lamellae in a unidirectional lamina. An anisotropic eutectic composite lamellae lamina developed by moderate growth velocity (7.3 × 10^‐8m³s^‐1), is of greater interest offering optimum hardness, approximately varying between three‐and eight fold average increase in different modes of the mechanical strength in comparison to its isotropic growth carried out in an ice‐bath (∼273K), and manifold superior to its constituent phases irrespective of the growth mode. The directional lamina of uniform microstructural parameter lamellae, indicates that there is a perfect lamella‐ matrix equilibrium for which excess thermodynamic functions do vanish. (© 2004 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

Structural and Density Changes of Rhenium Single Crystals Upon Rolling and Annealing

The rhenium crystals were rolled in the {1000} 〈112 0〉 orientation. Rolling produces cracks in crystals at low deformation (ϵ = 30%). It has been shown that microcracks in the volume of the crystal arise at a more earlier deformation stage than visible surface cracks. Recrystallization during subsequent annealing is strongly intensified with the start of crack formation. Stepwise facets of recrystallizated grains are often observed, this fact being connected with twinning lamellae recrystallization. Microcracks in the crystal body are completely eliminated during high temperature annealing. This process caused by diffusion over microcracks surfaces.     

Improved determination of fcc/bcc orientation relationships by use of high‐indexed pole figures

The use of variants to analyze fcc/bcc orientation relationships is demonstrated by EBSD data. Because of multiply occupied poles in the stereographic projections low indexed pole figures are not always suitable. This is mostly caused by the convolution of all scattered individual orientation data as the single poles cannot resolved in the pole figure. Pole figures of higher indexed lattice planes more reliably reflect the character of the orientation relationship since no overlapping of poles occurs. (© 2005 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

Theoretical Study of Intragranular Heterogeneity of Deformation and Crystal Rotation

The paper deals with the problem of the intragranular heterogeneous deformation. The grains of a deformed polycrystal are divided in general into subgrains (or deformation zones). In each subgrain a different set of slip systems and consequently a different crystal rotation may occur. We have examined theoretically both the homogeneous and the heterogeneous deformation in tension of a grain using the model of the plastic inclusion(s) in an elastic matrix. The crystal structure and orientation as well as the interactions between slip systems have been taken into account. For both situations (homogeneous and heterogeneous) we have compared resulting deformation energies, active slip systems and crystal rotations. We conclude that for some configurations of heterogeneity it is possible to minimize the total deformation energy.     

Numerical simulation of heat conduction for the growth of anisotropic layered GaSe crystals

In this report, we present the usage of a second rank cylindrical conductivity tensor which we derived to simulate the crystal growth processes of a layered compound GaSe in a cylindrical enclosure by directional solidification. Use of such a tensor is inevitable in the simulations of the growth of highly anisotropic crystals having layered structure, since the crystallographic orientation of the grown material is not necessarily aligned with the ampoule symmetry. Using the finite difference control volume approach in 3D, we solved transient heat conduction equation for a highly anisotropic solid in a cylindrical enclosure. We obtained sloped thermal fields and isothermal surfaces and the magnitudes of the slopes are strong functions of both azimuthal angle and growth orientation. The results showed that the orientation of the crystallographic axes of GaSe in the enclosure is quite effective in the steady and the transient fields, isotherms, and axial and radial temperature gradient within the material. Increase of Bi number decreases the magnitude of the slope of isothermal surface. Anisotropy of the conductivity seems to be effective in the orientation of the growth direction of the resulting crystal within the cylindrical ampoule. (© 2004 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

Structural properties of bulk GaN substrates: Impact of structural anisotropy on non‐polar and semi‐polar crystals

In this paper we show the structural parameters and structural anisotropy of the bulk GaN substrates of various crystallographic orientations (00.1), (10.0), (11.0) and (20.1) obtained by ammonothermal method. The structural parameters were investigated using high resolution X‐Ray Diffraction. Perfect crystalline structure manifests in very narrow peaks in X‐ray rocking curves. The full width at half maximum (FWHM) values of 16 and 18 arcsec for the symmetrical and asymmetrical peaks, respectively, have been observed. In addition, we observed structural anisotropy in the non‐polar and semi‐polar crystals, depending on the orientation of the sample relative to the X‐ray beam. It is conducted that this anisotropy is a intrinsic property of non‐polar and semi‐polar GaN substrates.     

The influence of substrate temperature on the growth of sexiphenyl on mica(001)

Sexiphenyl thin films were grown by Hot Wall Epitaxy on air‐cleaved mica (001) surfaces at substrate temperatures between 293 K and 440 K. For the entire temperature range, organic thin films show nano‐needle like morphology. The nano‐needles grown at low substrate temperature (293 K) are shortest, and their growth is accompanied by a simultaneous formation of flat islands which disturbs the growth of nano‐needles. On the contrary, unusually long nano‐needles with typical lengths up to the mm range evolve during the growth at a substrate temperature close to the material's thermal desorption temperature at about 440 K. X‐ray diffraction reveals two different crystalline orientations for nano‐needles in the entire temperature range. At low substrate temperatures dominantly the (11 ) plane of the β‐phase is formed parallel to the mica (001) surface. At elevated temperatures another strong texture becomes dominant which is close to the (11 ) crystal orientation. In contrast to this, crystallites with the preferred orientation (001) parallel to the surface of the substrate are formed at low substrate temperature (293 K). This crystal orientation can be associated with flat islands observed in the early growth stage. (© 2007 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

On orientational fluctuations in nematic liquid crystals

The factors determining the spectral density of intrinsic optical noise of a nematic liquid crystal (NLC), caused by fluctuations of director orientation in a frequency range of 0.01–1 Hz, have been experimentally established for the first time for (10–100)-μm layers, characterized by rigid anchoring at the boundaries and a quasi-homeotropic macrostructure. A model developed for estimating the spectral density of this noise for an NLC layer with a previously deformed macrostructure and finite molecular anchoring energy has been approved.     

Three‐dimensional thermoelastic stresses in off‐axis oriented single crystals with hexagonal symmetry

A three‐dimensional (3D) thermoelastic stress analysis is carried out on a single crystal with axisymmetric geometry but with a hexagonal crystallographic symmetry. The crystallographic orientation is off‐axis with respect to the cylindrical coordinate system. By applying a Fourier series expansion with respect to the rotational angle ϕ of the cylindrical coordinates, the 3D boundary value problem is reduced to a sequence of 2D ones on the meridian plane, which are solved by the finite‐element method. In our example, the off‐axis orientation is towards a direction of high symmetry, and therefore only four of the six stress tensor components are non‐zero. In the end, the stress tensor is projected onto the slip system of the crystal. (© 2007 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

A Direct Method for the Determination of Orientation Relationships Using TEM

A new method for the determination of orientation relationships has been developed. The method is based on a recently proposed direct method for orientation determination using a transmission electron microscope (TEM). The determination of the orientation relationship can be done by two different but generally applicable approaches. The first approach is especially applicable for crystals with orthogonal base vectors, the second approach is especially applicable for crystals with non-orthogonal base vectors and the general bicrystal.     

Oriented orthorhombic Lead Oxide film grown by vapour phase deposition for X‐ray detector applications

Several materials are under investigations for flat panel x‐ray detector applications. Among them, PbO shows interesting properties, i.e. high sensitivity, large stopping power and high resistivity at room temperature. However, the exploitation of PbO is limited by the difficulty to obtain good quality films constituted by a single phase. In this paper, we describe a new approach for the vapour phase growth of orthorhombic PbO films. The grown layers show a single phase, good crystallinity, and preferential orientation along the c axis. Optical characterization evidenced the presence of a broad defect band. Gold contacted films showed very high electrical resistivity and appreciable response to X‐ray radiation.     

Neutron Interferometer with very thin Lamellae

We describe the manufacturing and testing of a monolithic perfect crystal interferometer with very thin lamellae, optimized for neutron phase contrast imaging where high spatial phase resolution is required. The two main characteristics of our new interferometer design are thin lamellae (t=0.56 mm) for beam splitter S and analyzer A, and mirrors M1, M2 of twice the thickness (t_M=1.12 mm) of Sand A. The different stages of crystal preparation and the successful test with X‐rays are reported.     

Growth of boron nitride films on w‐AlN (0001), 4° off‐cut 4H‐SiC (0001), W (110) and Cr (110) substrates by Chemical Vapor Deposition

Boron Nitride is a promising group 13–group 15 compound material that exhibits various interesting properties like wide band gap, chemical stability, attractive mechanical properties and other. The growth behavior of this material has not been investigated in sufficient details to tailor properties of the resulting films. In this work we present the results on the growth of turbostratic boron nitride (t‐BN) thin films at a relatively high growth rate of 3 μm/h with the aim to investigate the potential use of boron trichloride in combination with ammonia as precursors for growth. Deposition experiments were conducted in a vertical cold wall high temperature chemical vapor deposition reactor in the temperature range 1000°C–1700°C depending on the substrate used. Templates of w‐AlN (0001), 4° off‐cut 4H‐SiC (0001), Cr (110) and W (110) were employed as substrates for the BN growth. As‐grown BN layers were characterized by Scanning Electron Microscopy, X‐Ray Diffraction, Electron Diffraction and Raman Spectroscopy. The results indicate that temperature and N/B ratio have a great influence on the crystallinity of the deposited films. For AlN and SiC substrates, a temperature of 1600°C and N/B ratio in range between 3 and 7.5 were identified as the best parameters for the growth of a 2 μm thick t‐BN layer with a spacing between basal planes of about 3.36 Å compare to the 3.33 Å spacing between basal planes of hexagonal or rhombohedral BN (h‐BN or r‐BN). For Cr and W substrates which have a lower mismatch with h‐BN (1 and 8.8 %), layers of t‐BN were deposited at much lower temperature (1000°C–1150°C) with a spacing between basal planes of 3.5 Å and morphology similar to that observed on SiC substrates. We obtained t‐BN layers with in plane strong disorder but out of plane orientation (c‐axis normal to the surface).     

Irrational orientation relationship derived from rational orientation relationships using EBSD data

The determination of the orientation relationship (OR) between α ‐Fe matrix (bcc) and γ ‐Fe precipitates (fcc) is discussed using orientation data collected by electron backscatter diffraction (EBSD). The comparatively low accuracy of EBSD is compensated by the high number of measurements what allows a general statement regarding to the mean OR existing in a sample. The representation and discussion is realized on a part of the Bain zone in a {001} pole figure. A discussion of some selected rational OR which are commonly used for the phase boundary characterization between α and γ shows that the pole figures describing a transformation from γ → α are different to those for α → γ. A technique is proposed based on at least three misorientation angles between the experimental OR to the rational OR's as reference. For the misorientation angle distribution a refinement is applied to extract the mean values. They are used to detect the mean OR, what is also possible for only a few or even a single precipitate if the number of measurements describing the phase boundary is sufficiently high. (© 2007 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

Modelling of the Transport Processes in Bridgman grown Gallium Antimonide

In this work the thermal, velocity and species fields in the melt during the crystal growth by the vertical Bridgman method, has been studied. The simulations were focused on the special case of GaSb, which is a semiconductor of high technological importance. The simulations have been carried out both in 2 and 3‐D. In both cases the momentum (Navier‐Stockes), energy and mass transport equations were solved. The wall‐to‐wall radiation has also been included. In the two‐dimensional case an axisymmetric global model was developed taking into account the different elements present inside the real Bridgman growth system. In order to study the transport processes in the whole system during a complete growth process, the time dependence has also been considered. In the three‐dimensional case, the mathematical domain is restricted to the melt. These simulations were developed in order to study the influence of the ampoule tilting on the dopant distribution in the melt.     

Three dimensional numerical study of the effect of large Grashof number on HEM crystal growth

Heat transfer and fluid flow in HEM crystal growth of silicon in cylindrical cavity is studied numerically. The walls of the crucible are heated to a fixed temperature. The exchanger that causes and induces natural convection is seated at the middle‐bottom of the crucible. The finite‐volume method is employed to solve the governing equations with proper boundary conditions. The effects of transport mechanism on the temperature distribution, melt flow, pressure and stream function are presented. We focus our work on the pressure field which has not yet been studied in HEM crucible. Also, we extend our work on a wide range Grashof number and for large numbers until 10¹² not yet studied in HEM furnace. It is found that the onset of flow fluctuations appears at Gr = 10¹⁰. Uniform temperature is observed in the entire melt at high Grashof number with development of a thermal boundary layer close to the exchanger. The thermal boundary layer thickness is calculated for strong buoyancy regime. Besides, for very high Gr number, buoyancy has less effect on temperature and then on melt‐crystal interface shape. During enlarging Gr, pressure evolution is related to temperature variation more than flow pattern.     

High resolution neutron and X‐ray diffraction studies as a function of temperature and electric field of the ferroelectric phase transition of RDP

Neutron and high resolution X‐ray diffraction investigations on perfect single crystals of RbH₂PO₄ (RDP), a hydrogen bonded ferroelectric of KDP type are reported. The results of crystal structure analysis from diffraction data, below and above the paraelectric ‐ ferroelectric phase transition, support a disorder ‐ order character of [PO₄H₂]^‐‐groups. The tetragonal symmetry of the paraelectric phase with the double well potential of the hydrogen atoms obtained by diffraction, results simply from a time‐space average of orthorhombic symmetry. According to the group ‐ subgroup relation between the tetragonal space group I42d and the orthorhombic Fdd2 a short range order of ferroelectric clusters in the tetragonal phase is observed. With decreasing temperature the ferroelectric clusters increase and the long range interaction between their local polarisation vectors leads to the formation of lamellar ferroelectric domains with alternating polarisation directions at T_C = 147 K. From the high resolution X‐ray data it is concluded that below T_C the ferroelastic strain in the (a,b)‐plane leads to micro‐angle grain boundaries at the domain walls. The tilt angle is enhanced by an applied electric field parallel to the ferroelectric axis. The resulting dislocations at the domain walls persist in the paraelectric phase leading to a memory effect for the arrangement of twin lamellae. With increased electric field the phase transition temperature T_C is decreased. (© 2004 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

Amorphous tin–iron oxide thin films with 3D reticular porous morphology for lithium‐ion batteries

Porous reticular tin–iron oxide composite have been prepared by electrostatic spray deposition method for lithium‐ion batteries. The structure of depsitied electrodes are investigated by X‐ray Diffraction (XRD), which shows that the film is amorphous. Electrochemical characterization by galvanostatic charge‐discharge tests demonstrate that the conversion reactions in these electrodes enable initial discharge capacities of about 1551 mAh/g for tin–iron oxide thin film electrode, and the reversible capacity stayed in the range of 769–601 mAh/g during the successive 30 cycles. The high porosity of the 3D reticular structure can provide more reaction sites on the electrode surface and accommodate volume variation of Sn particles during Li alloying–dealloying. Such a composite film are expected to be applied as attractive anodes for lithium‐ion batteries.