According to most crystal growth theories, the "as grown" crystal morphology is dominated by the slow-growing faces and the fast-growing faces may "grow out" and not be represented in the final crystal habit. In this paper a brief survey is given of the recently developed correlation between the evolution of both fast- and slow-growing surfaces, their relative growth rates, and the crystallographic structure of crystal. It is shown that even the fast-growing faces may increase their sizes. On the other hand, the slow-growing faces may decrease and, in consequence, disappear from crystal morphology. Such a behaviour of slow- and fast-growing surfaces influences the growth and evolution of both low- and high-index faces.
Czochralski growth of cm size decagonal AlCoNi single crystals from Al-rich high-temperature solutions is described using native seeds oriented parallel to all symmetrically different crystal directions. Morphological observations allow first hints with respect to anisotropic growth rates. But only classical detachment experiments according to the original idea of Jan Czochralski reveal quantitative results of kinetically limited growth rates. Geometric conditions of wetted plane interfaces as well as aspects of constitutional supercooling affect the detachment experiments. Thus, in only one specific orientation of the decagonal quasicrystal quantitative data for the maximum growth rate could be obtained.
Bibliometric data on the growth dynamics of papers devoted to the Czochralski method of crystal growth and citations to these papers published in the scientific literature are analyzed using mathematical functions based on progressive nucleation mechanism and power-law approaches. It is shown that the basic concepts of progressive nucleation mechanism originally developed for crystal growth can satisfactorily be applied to explain the citation behavior of papers published on the Czochralski method of crystal growth. It is also found that: (1) cumulative number N(t) of papers and cumulative number L(t) of citations to them at time t are mutually related, and (2) as determined by the plot of the parameter L(t)/t^2 against the number Δ N(t) of papers published per year, the citation behavior of papers published on the Czochralski method follows three distinct periods: 1954-1988, 1988-2001 and 2001-2012, which are related to the dependence of the number Δ N(t) of papers published per year on publication time t.
On the occasion of the centennial of the invention of the Czochralski crystal growth process by the Polish scientist Jan Czochralski, a review of selected strategies for the automatic control of this process is given. This review provides a sketch of the fundamental challenges of controlling the Czochralski process and the basic concepts of feedback control. Both early and modern approaches to the control of the Czochralski process are described. The discussion focuses on questions related to feed-forward control, feedback control, and state estimation. The presented methods rely on simple mathematical process models in contrast to the finite element model-based approaches typically used in crystal growth process design and analysis. Such mathematical models motivate both the structure and parameters of the chosen controller. A comprehensive list of references to background literature on this topic completes this survey.
In the past decade there has been an explosive growth in the consumption of sapphire driven by the demands of the next generation of energy efficient general lighting based on GaN LEDs. This application requires orienting these rhombohedral corundum crystals such that the substrate surface is the c-plane; a basal plane defined using hexagonal axes. Sapphire crystals form a strong facet on the c-plane, and growth in that direction generally results in crystals with high defect densities, particularly dislocations, and low angle grain boundaries. To overcome this drawback, the usual methodology is to grow the crystal in the a-direction and then core drill rods perpendicularly which are then sliced into c-plane substrates. For all crystal growth techniques commonly employed for sapphire, this approach suffers from poor material utilization. Although this has generally been viewed as an acceptable trade-off in the manufacturing process as long as 2" substrates were the dominant market, as substrate diameters have increased towards 150 mm and larger, this compromise is no longer seen as a viable alternative because of the low material utilization and the high energy consumption of the growth process. This has led to a renewed look at the Czochralski process for more efficient c-axis substrate production.
This study analyses the phenomenon of constitutional supercooling, which is one of the major problems in industrial growth of heavily doped (>10^{20} atoms/cm^3) silicon crystals by the Czochralski technique. The systematic study is based on theoretical models and experimental data considering the effect of three important dopants (B, P, and As) in dependence of the relevant growth parameters for the Czochralski process. Based on these results, conclusions will be drawn for the stability limits of the Czochralski growth of dislocation-free heavily doped silicon crystals in dependence of the doping species and their concentration.
This study examines the effect of slow crystal dummy rotation on three-dimensional oscillatory instability and time-dependent supercritical flow states in a Czochralski melt flow experimental model. To enable further comparison with numerical modelling, the experiments are carried out using a 20 cSt silicone oil as an experimental liquid and in a large diameter crucible, which allows one to work in a narrow temperature interval, so that temperature dependence of all the thermophysical properties of the experimental liquid can be neglected. The measurements confirm, partially qualitatively and partially quantitatively, earlier numerical predictions on destabilization ofthe Czochralski convective flow by a slow rotation. A simple power dependence of flow oscillations frequency on the Grashof and rotational Reynolds numbers that fits all the experimental runs was found.
The Hartman-Perdok theory explains the relation between crystal structure and morphology and provides the atomic surface topology of the crystal-melt interface. Hartman-Perdok theory has been applied to CaYAlO_{4} as model for all other ABCO_{4} compounds with a K_{2}NiF_{4} crystal structure. F forms are {002}, {101}, {103}, {110}, {112}, {200}, {211} and {213}. The strongly anisotropic shape caused by the perovskite-like AlO_{6} layers || {001} is very distinct in all theoretical growth forms. The form with formal charges is planar following {001} with {101} and {110} as lateral forms. Disordering of the boundary ions results in the disappearance of {110}. At lower effective charge on oxygen ions, q_{O}, the ordered forms are still tabular, while {110} and {112} are the only lateral faces. At still lesser negative q_{O} {112} appears as well. On the disordered models {112} replaces {110}. Crystals show often variations in colour parallel to the {110} interface due to the surface topology of {110}. YBa_{2}Cu_{3}O_{7-x} has, for x=1, the following F forms: {001}, {101}, {103}, {112} and {114}. The theoretical growth form of this tetragonal phase is tabular following {001} with {101} as lateral form. For x=0 the growth form shows important {101} and minor {103} and {001}. When the boundary ions on (001) are ordered, the outermost layer of {001} contains half of the Cu^{+} (x=1) or Cu^{3+} and O^{2-} (x=0) ions in a c(2×2) quadratic lattice which reduces the {001} growth rate significantly. An (1×2) reconstructed {010} surface can be traced for the orthorhombic polymorph which results into the appearance of {010} on the ordered growth form. Otherwise the presence of {010} on as-grown crystals must be due to external factors.
Multipod flower-like zinc oxide (ZnO) nanowires have been successfully synthesized on Si(111) substrates using a pulsed laser deposition prepared Zn film as "self-catalyst" by the simple thermal evaporation oxidation of the metallic zinc powder at 850°C without any other catalysts or additives. The pre-deposited Zn films by pulsed laser deposition on the substrates can promote the formation of the ZnO nuclei effectively. Also it can further advance the growth of the flower-like ZnO nanowires accordingly. X-ray diffraction, scanning electron microscope, high-resolution transmission electron microscopy, Fourier transform infrared spectrum, and photoluminescence were used to analyze the structure, morphology, composition and optical properties of the as-synthesized products. The results demonstrate that the nanowires were single crystalline with hexagonal wurzite structure, grown along the [0001] in the c-axis direction. Room temperature photoluminescence spectrum of the ZnO nanowires shows a nearband-edge ultraviolet emission (peak at ≈ 384 nm) and a deep-level green emission (peak at ≈ 513 nm). In addition, the growth mechanism of the flower-like ZnO nanowires is discussed in detail.
We study the heteroepitaxial growth of thin layers by means of the modified phase-field model with the incorporated anisotropy. The influence of elastic and surface energies on the layer growth is considered. For numerical solution of the model, an explicit numerical scheme based on the finite element method is employed. The obtained computational results with various anisotropy settings demonstrate the anisotropic thin-layer pattern growth.
The root of single-crystalline turbine blade made of CMSX-4 superalloy were studied. The studied blade was produced by the Bridgman technique in industrial ALD furnace at withdrawal rate of 3 mm/min. The samples for investigations were cut from the blade root parallel to the withdrawal direction. Metallographic sections of longitudinal samples planes were prepared for further investigations. The samples were analysed using scanning electron microscopy and the Laue diffraction studies. The crystal orientations in macro-scale were determined by analysis of the Laue pattern and local crystal orientations were studied by electron backscattered diffraction technique. Morphology of dendrites were examined by analysis of scanning electron microscopy macro-images. Study of subgrain structure was performed by X-ray diffraction topography. The sharp parallel contrast bands, visible on the X-ray topograms, were related with dendrite cores, arranged with the same direction. Additionally, the low angle boundaries were formed in certain samples, visible on the topograms as contrast shifts. Step changes of local crystal orientation in certain areas were observed on the electron backscattered diffraction maps. The electron backscattered diffraction crystal orientation maps were related to the misorientation visualized in topograms.
The creep-rupture tests were performed on a single crystal rods made of CMSX-4 superalloy obtained at withdrawal rates of 3 and 5 mm/min. After the rupture the microstructure and fracture surface were examined and correlated with X-ray crystal rotation measurements by the Ω-scan method. The conclusions about the crystal lattice rotation during creep test were provided.
Large-scale molecular dynamics simulations of the high-pressure transformations of the xenon/water system were performed involving special purpose molecular dynamics machines. We investigated several systems of different sizes and geometry at the suitable simulational conditions (density, temperature, etc.), which are similar to the experiments conducted on the xenon hydrates. A binary mixture (ice water + Xe) undergoes at high pressure a long evolution and the Xe-guest atoms, enclosed inside the water molecules, were observed and analyzed. Even for the thin slabs, starting with the capture of the guest atoms by the water molecules, the water clusters around the xenon atoms are formed. The results show that such a hydrate-like formation preserves its structural stability over a long period of the simulation time of order of nanosecond. The molecular dynamics simulations were performed on a basis of the MDGRAPE-2 modifications of the DL_POLY general purposes package, with the efficient treatment of the Ewald real and reciprocal-space components of the Coulombic and Van der Waals forces. The MDGRAPE-2 accelerates the calculations of the Coulomb and Van der Waals forces, without applying a spherical cut of a fixed distance. 31.15.Qg, 61.20.-p, 61.50.Ah, 81.10.Aj, 91.60.Gf
We report the importance of interface engineering in heteroepitaxy with examples of plasma-assisted molecular beam epitaxial ZnO growths on (0001) sapphire substrates and on (0001) GaN/sapphire templates, whose interfaces are engineered to improve and to control properties of ZnO films. The growth of rocksalt structure MgO buffer on Al_2O_3 (0001) is developed for ZnO epitaxy. By employing the MgO buffer layer, the formation of 30^o rotated mixed domains is prohibited and two-dimensional layer-by-layer growth of ZnO on sapphire substrate is achieved. High-resolution X-ray diffraction reveals the superior improvement in a crystal quality of ZnO films with an MgO buffer. Polarity of wurtzite structure ZnO films on Ga-polar GaN/sapphire templates is controlled by changing interface structures. By forming a single crystalline, monoclinic Ga_2O_3 interfacial layer between GaN and ZnO through O-plasma pre-exposure on the Ga-polar GaN surface, O-polar ZnO films are grown. By forming the ZnO/GaN heterointerface without an interfacial layer through the Zn pre-exposure on the Ga-polar GaN surface, Zn-polar ZnO films are grown.
Due to fact that Cu-based alloys are very sensitive to the thermal treatments, in scanning electron microscopy and X-ray diffraction observations, according to the different quenching techniques Cu-26.04%Zn-4.01%Al alloys can display different product phases such as martensite and precipitation. Rapidly cooling sample was formed as M18R structure from the ordered DO_3 phase and in the monoclinic case, β=89.1° determined for sample. It was seen that a α-precipitation (fcc) phase in the matrix is slowly cooling.
Magnesium single crystals of three different orientations were deformed by tension at room temperature to investigate the geometrical criterion resulting from the Schmid law for activation of basal and non-basal slip systems. Changes of crystallographic orientations of investigated single crystals were systematically measured during deformation and the geometrical criterion was examined in respect to the changes of crystallographic orientations during deformation. The geometrical criterion itself failed to explain lack of activity of non-basal slip systems in the deformed magnesium single crystals. Therefore the ratio of activation volumes of soft to hard slip systems is considered to be introduced for a better understanding of the deformation in hcp metals.
Pure barium titanate BaTiO_3 (BT) and BT doped with two different transition elements Fe^{3+} and Ni^{2+} at 5 mol.% (BT5Fe and BT5Ni, respectively) as constant concentration in powder form have been prepared by sol-gel method using barium acetate, titanium(IV) n-butoxide, iron and nickel nitrates as precursor materials. The microstructure of BT and the influence of Fe and Ni dopants on it were investigated by X-ray diffraction and Fourier transform infrared spectroscopy. X-ray diffraction shows that tetragonal phase is dominant for pure BT sintered for 4 h at 800°C. Scanning electron microscopy and transmission electron microscopy were used to study surface morphology and particle size distribution for BT5Fe and BT5Ni, respectively. The presence of hydroxyl defects were verified by Fourier transform infrared spectroscopy for (BaTi_{1-x}Fe_{x}O_{3-x/2}) (BT_xFe), where x=0.005, 0.01, 0.05, and 0.07 and (BaTi_{1-x}Ni_{x}O_{3-x/2}) (BT_xNi), where x=0.005, 0.01, 0.03, 0.05, and 0.07.
JavaScript is turned off in your web browser. Turn it on to take full advantage of this site, then refresh the page.