共查询到20条相似文献,搜索用时 46 毫秒
1.
V. P. Ershov D. A. Kas’yanov V. I. Rodchenkov D. A. Sergeev 《Crystallography Reports》2008,53(2):344-350
The growth and dissolution of the isolated (100) face of a KDP crystal at exposure of the phase boundary to focused acoustic fields have been investigated. Visualization tools have been developed and the scale and dynamics of the arising acoustic flows have been established. A quadratic dependence of the dissolution rate on the sound pressure has been obtained; such a dependence is in agreement with the theoretical concepts about the dependence of the mass exchange rate in the acoustic boundary layer on the field parameters. A significant growth response of the face under study, normally exposed to ultrasound along the acoustic axis in the prefocal and postfocal planes of a spherical concentrator (f = 1.4 MHz), has been revealed. It is shown that the mechanisms of mass exchange enhancement in these acoustic modes are radically different. The results obtained show a possibility of controlling growth and dissolution of crystals by varying the parameters of an inhomogeneous acoustic field. 相似文献
2.
Mohammad Hossein Tavakoli Hossein KarbaschiFeridoun Samavat Ebrahim Mohammadi-Manesh 《Journal of Crystal Growth》2010,312(21):3198-3203
A set of 2D finite element numerical simulation of induction heating process for an oxide Czochralski crystal growth system has been made for a range of f=1–100 kHz applied frequency of driving current. It was shown that the frequency selection has a marked effect in all basic induction phenomena, including electromagnetic field distribution, skin depth, coil efficiency, and intensity and structure of heating in the growth setup. 相似文献
3.
Oxygen distribution in a Si crystal (100 mm in diameter) has been studied by the absorption method in the range of the absorption band of interstitial oxygen, λ = 5.81 μm. Large-scale fluctuations (~1 cm) of the oxygen concentration (N 0) along the growth axis were determined. Depending on the melt height, the regions of the chaotic and quasiperiodic changes were established, as well as the region of the constant N 0 value, and their relation to turbulent, quasiperiodic, and stationary modes of melt convection in crystallization. The values of the critical Rayleigh number for the melt transition from stationary to quasiperiodic (3 × 103) and from quasiperiodic to turbulent (1.7 × 104) convection modes are determined for growth of silicon crystals by the Czochralski method. The dominating modes of N 0 concentration oscillations at two incommensurable frequencies, f 1 = 1.3 × 10?3 and f 2 = 6 × 10?4 Hz, are assumed to be related to the oscillatory transfer of oxygen from the walls of the quartz crucible to the crystallization front and restructurization of the convective flow pattern of the melt in the course of crystal growth. 相似文献
4.
At given conditions, especially at higher supersaturation, the growth rate of a close packed, perfect crystal face depends on the formation rate of two-dimensional nuclei and on the propagation rate of the monoatomic layers. This multinuclear multilayer growth as well as the advancement rate of growth steps have been studied experimentally on electrocrystallization of silver. The advancement rate of mono- and polyatomic growth steps has been measured on screw dislocation-free (100) crystal faces. For low overvoltages a linear dependence of the rate on overvoltage has been found. A strong influence of the surface condition of the crystal face — “fresh” or “aged” on the step advancement rate has been established. It was also found that on a “fresh” surface mono- and polyatomic steps advance with the same rate. The average monoatomic step spacing of the polyatomic step has been determined. The kinetic constants of the step growth rate are established and a conclusion regarding the mechanism of electrolytic deposition of silver is drawn. The initial current—time curves were recorded on applying potentiostatic pulses on a perfect crystal face. The shape of these curves coincides very well with those theoretically calculated for the cases of multinuclear growth. On the basis of the theoretical dependences, one can determine from these curves the formation rate J of two-dimensional nuclei at a given overvoltage η since the rate of step advancement is known. A linear dependence of log J on 1/η has been established. The values of the pre-exponential term in Volmer's equation and the specific edge energy of the two-dimensional nucleus have been determined. The surface condition of the crystal face influences strongly also the process of two-dimensional nucleation. 相似文献
5.
Tiden N. N. Clementev E. S. Alekseev P. A. Gvasaliya S. N. Nefedova E. V. Lazukov V. N. Sadikov I. P. 《Crystallography Reports》2006,51(1):S85-S87
The recent studies of the PrNi system have shown that, at a temperature slightly higher than the ferromagnetic ordering temperature (T C ~ 21 K), softening of some part of magnetic excitations occurs near the Γ point of the Brillouin zone. The magnetic modes observed have been qualitatively described by a model taking into account the crystal field and the exchang interaction within the mean-field random-phase approximation. To refine the model parameters and obtain a complete set of excitation modes, inelastic neutron scattering measurements on PrNi single crystals and polycrystals have been performed at T = 23 K. An acoustic branch related to the level with high excitation energy has been observed for the first time. The character of its dispersion suggests that the low-energy acoustic mode is responsible for the magnetic phase transition. 相似文献
6.
A systematic investigation of the influence of B content on the magnetoimpedance (MI) effect in melt-spun Fe91−xZr5BxNb4 (FZBN, 0 ? x ? 30) ribbons has been performed within a frequency range, f ∼ 310-1110 kHz and under a varying dc magnetic field (Hdc) up to 70 Oe. The MI effect is not observed in the sample with x ? 5 but within the range 8 ? x ? 30. A distinct MI effect has been observed with a maximum change of 180% at around 1.1 MHz in the sample with x = 20, coincident with a saturation magnetic field of 66 Oe and a field sensitivity of about 7%/Oe. Magnetic measurements reveal that the MI effect and B content dependence of the effect are closely related to coercivity of the FZBN alloy series, except for the sample with 20 at.%. The drastic MI ratio observed in the sample with x = 20 is ascribed to its special microstructure. The mechanism of the MI effect in FZBN alloys and of the significant MI value appearing at a B content of x = 20 is discussed in this paper. 相似文献
7.
We report a computational model to simulate vapour growth morphology of urea crystal by considering molecular anisotropy and surface relaxation of different crystal faces. It has been argued that the faces' growth occurs through the adsorption of molecular layers rather than a slice of thickness dhkl. The molecular layer is a 2-D periodic arrangement of molecules in which each molecule has same the orientation. The molecular orientations in a slice of thickness dhkl may differ from each other and depend on crystallographic orientation of the slice. The discussed approach has been employed to simulate vapour growth shape of crystalline urea by calculating attachment energy of molecular layers using Hartee–Fock and density functional theories. The calculated growth morphology is in good agreement with the vapour grown shape of urea crystal. The role of thermal and growth kinetics affecting the vapour growth morphology has been discussed. The observed polar growth morphology of urea crystal has also been discussed particularly in the context of different atomic environments of (111) and (?1?1?1) faces. 相似文献
8.
E. A. Sulyanova D. N. Karimov S. N. Sulyanov B. P. Sobolev 《Crystallography Reports》2014,59(1):14-21
A Sr0.7Ce0.3F2.3 crystal (CaF2 type, sp. gr. $Fm\bar 3m$ ), obtained by quenching from melt, has been studied for the first time by X-ray diffraction. Fluorine vacancies and interstitial anions are found in the 8c and 32f sites, respectively. The defect ratio in the Sr0.7Ce0.3F2.3 structure corresponds to the tetrahedral cluster configuration of defects {Sr4 ? n Ce n F26}. The defect structure of quenched (at a rate of ~25 K/min) crystal differs from that of a crystal grown from melt (cooling at a rate of ~3 K/min) by the displacement of some cations (presumably Ce3+) along the threefold axis to the 32f site and the anisotropy of thermal vibrations of ions in the cluster core (F int(32f)3). The concentration dependence of the lattice parameters of quenched Sr1 ? x Ce x F2 + x phases (x = 0–0.5) is described by a third-order polynomial: a = 5.80009 + 1.166518 × 10?3 x ? 1.124969 × 10?5 x 2 + 8.258155 × 10?8 x 3. The compositional dependence of microdistortions is also nonlinear; maximum microdistortions are observed in the SrF2 crystal. They decrease with an increase in the cerium concentration x to ~ 0.35. The minimum in the range x = 0.30–0.35 correlates with a composition corresponding to the peak (at x ~ 0.29) in the melting curves of the fluorite phase estimated from the phase diagram of the SrF2-CeF3 system (the method of thermal analysis). 相似文献
9.
Specific features of the Laue diffraction of X-rays are considered for quasiforbidden reflections within the wavelength range between the K-absorption edges of Ga and As in a GaAs crystal. It is established that scattering for the 200 reflection is of the dynamical nature in the wavelength range where the real part of the structure factor F r200 is zero and diffraction is determined by the imaginary part of the atomic form factor f″ (ω) alone. It is shown that the position of the minimum of the function R i = f(λ) is sensitive to the crystal non-stoichiometry predicted by the numerical calculations within the theory taking into account both the real and the imaginary parts of the structure factor. 相似文献
10.
The growth conditions of pure and Cr3+-doped Al2−xInx(WO4)3 single crystals, using top-seeded solution growth (TSSG) technique, have been studied. A series of experiments have been performed at different In concentrations, x=0.0, 0.3, 0.6 and 1.0, as well as at different concentrations of Cr3+ (0.0, 0.1, 0.2, 0.5 and 1.0) in at% with respect to the initial total concentration of Al and In in the starting solutions. The basic parameters of the crystal growth are varied over a wide range: seed orientation, speed of rotation, axial and radial temperature differences in the solution and the solution cooling rate. The investigated relations between the basic defects in the crystals and these parameters result in determination of the optimal conditions for growth of defect-free crystals. Distribution coefficients of Al, In and Cr have been determined, so the growth of crystals with given compositions is possible. Values of Dq/B (crystal field strength) for the various crystal compositions are calculated from the optical absorption spectra. The calculated values show that the discussed solid solutions have weak crystal field and are suitable for media with broadband emission spectra. 相似文献
11.
A. U. Sheleg E. M. Zub A. Ya. Yachkovskii L. F. Kirpichnikova 《Crystallography Reports》2002,47(4):577-580
The a, b, c lattice parameters of a [(CH3)2NH2]2 · CuCl4 crystal have been measured by the X-ray diffraction method within the temperature range of 100–300 K. The temperature dependences of thermal expansion coefficients αa = f(T), αb = f(T), and αc = f(T) along the principal crystallographic axes and thermal expansion coefficient of the unit-cell volume αV = f(T) are determined. It is found that all the three parameters, a, b, and c, vary with temperature in a complicated way and show jumplike anomalies in the a = f(T), b = f(T), and c = f(T) curves at phase-transition temperatures T c1 = 255 K and T c2 = 279 K. An incommensurate phase with the modulation wave vector q i = (1/2 + δ)(a* + c*) is revealed in the temperature range 279–296 K. It is shown that the incommensurability parameter δ increases with an increase in temperature. 相似文献
12.
R. A. Khasanov N. M. Nizamutdinov N. M. Khasanova V. M. Vinokurov G. S. Morozov A. O. Krivtsov 《Crystallography Reports》2012,57(5):751-757
The conditions for equivalent positions on the (hkl) face of growing crystal are derived using symmetry elements of the space group. It is shown by the example of the sp. gr. D 2h 16 that the conditions of equivalent position formation coincide with conditions of the reflection of diffracted beams by crystal. It is established that electron spin resonance (ESR) centers in barite, SO 4 ? (I) and SO 4 ? (II), with only two conjugate spectra with equal intensity out of four, and SO 4 ? (III), with a different intensity of conjugate spectra K ??M = 2, are localized into the growth pyramid of the (001) face with a [010] step. SO 2 ? , SO 3 ? ,, and SO 4 ? (IV) centers, having an identical intensity of the conjugate ESR spectra with K ??M = 2, are localized into the growth pyramid of the (210) face with a growth step [001]. 相似文献
13.
The Laplace-Young capillary equation for the shape of an axisymmetric liquid-vapor interface has been solved numerically for boundary conditions relevant to a model of the floating zone process. The stability of these solutions with respect to axisymmetric and asymmetric perturbations which conserve volume has been determined via the conjugate point criterion of the calculus of variations. The liquid zone shape is governed by five dimensionless parameters: , , , , and , where Rm and Rf are the radii of the melting and freezing solids, respectively, L is zone length , V is the zone volume, ρ is the density difference between liquid vapor, g is the gravitational acceleration, γ is the liquid-vapor surface tension, and Ω is the constant angular velocity of the uniformly rotating zone. For growth of constant diameter crystals, the angle øf, measured between the meniscus and the growth axis at the freezing interface, is constant. For Rm = Rf, ?R = 0, and øf = 0, the maxi mum value of ? for which a stable liquid zone exists has been calculated for various values of L/Rf. For some values of ?, two different stable liquid zones with different volumes (but all other parameters identical) give the same value of øf. 相似文献
14.
The accurate X-ray diffraction study of a Ca3Ga2Ge4O14 crystal (sp. gr. P321, Z = 1) has been performed using repeated X-ray diffraction data sets collected on a diffractometer equipped with a CCD area detector at 295 and 100 K. The asymmetric disorder in the atomic positions in Ca3Ga2Ge4O14 is described in two alternative ways: with the use of anharmonic atomic displacements (at 295 K R/wR = 0.68/0.60%, 3754 reflections; at 100 K R/wR = 0.90/0.70%, 3632 reflections) and using a split model (SM) (at 295 K R/wR = 0.74/0.67%; at 100 K R/wR = 0.95/0.74%). An analysis of the probability density function that defines the probability of finding an atom at a particular point in space shows that, at 295 K, five of the seven independent atoms in the unit cell are asymmetrically disordered in the vicinity of their sites, whereas only three atoms are disordered at 100 K. At both temperatures the largest disorder is observed at the 3f site on a twofold axis, which is a prerequisite for the formation of helicoidal chains of atoms along the c axis of the crystal and can serve as a structural basis for multiferroic properties of this family of crystals with magnetic ions. 相似文献
15.
L. D. Iskhakova V. Yu. Ivanov A. A. Mukhin A. M. Balbashov S. V. Lavrishchev A. S. Orekhov 《Crystallography Reports》2012,57(4):549-554
Multiferroic crystals Ln 1 ? x MnO3 (Ln = Eu or Tb; Ln?? = Y or Ho) have been grown by floating-zone melting; the compositions of these crystals are close to the point of concentration transition from the orthorhombic structure to the hexagonal phase. Scanning microscopy has revealed that the crystal boules have a regular lamellar structure in the form of alternating layers of the orthorhombic and hexagonal phases; this structure is typical of the materials obtained by the directional solidification of eutectic composites. The local chemical and phase compositions and the unit-cell parameters of the phases have been determined. The microstructure and morphology of the lamellar structures are described for different growth conditions. The mutual orientation of the crystallographic axes of the orthorhombic and hexagonal phases in the neighboring layers has been determined by electron backscatter diffraction. The orientational features are considered from the point of view of the crystal structure of both phases. 相似文献
16.
Zhiyu He Beijun Zhao Shifu Zhu Jiawei Li Yi Zhang Wenjuan Du Wei Huang Baojun Chen 《Journal of Crystal Growth》2011,314(1):349-352
A large, crack-free CdGeAs2 single crystal measuring 15 mm in diameter and 45 mm in length was grown in a vertical three-zone tubular furnace by a modified vertical Bridgman method, i.e. quasi-seed technique with small temperature gradient and descending quartz ampoule. High-purity, single phase CdGeAs2 polycrystallite for crystal growth was synthesized using a rocking furnace with temperature oscillation techniques. Various measuring means, including X-ray diffractometer(XRD), Fourier transform infrared spectroscopy(FTIR), and Field emission scanning electron microscope(FE-SEM) were adopted to characterize the as-grown crystal. It is found that the cleavage plane of the as-grown crystal is {1 0 1} face; the crystal is integrated in structure and crystallized well; etch pits in the shape of pentagon on (1 1 2) face have been observed for the first time using the new preferential etchant we prepared. All these results encouragingly indicate that the modified vertical Bridgman method is a convenient and effective way for high quality CdGeAs2 crystal growth. 相似文献
17.
S. D. Milovidova A. S. Sidorkin O. V. Rogazinskaya A. A. Sidorkin 《Crystallography Reports》2002,47(5):812-814
The unipolar state of a chromium-and L, α-alanine-doped ferroelectric triglycine sulfate (TGS) crystal has been studied. The experimental data on the distribution of internal bias fields with respect to a seed are considered. The possible mechanisms of the formation of an internal bias field during the growth of TGS crystals with a low impurity concentration are considered. 相似文献
18.
Rajnikant V. K. Gupta E. H. Khan S. Shafi S. Hashmi Shafiullah B. Varghese Dinesh 《Crystallography Reports》2001,46(6):963-966
The crystal structure of cholest-4-ene-3,6-dione (C27H44O2) has been determined by X-ray diffraction methods. The compound crystallizes in the monoclinic crystal system (space group P21) with the unit cell parameters a = 10.503(4) Å, b = 8.059(1) Å, c = 14.649(1) Å, β = 105.4(2)°, and Z = 2. The structure has been refined to an R value of 0.035 for 2252 observed reflections. Ring A of the steroid nucleus exists in a sofa conformation, while rings B and C adopt a chair conformation. The five-membered ring D exhibits a half-chair conformation. The molecules in the unit cell are linked together by the C-H?O hydrogen bonds. 相似文献
19.
The molecular and crystal structure of 4-ethoxycarbonyloxy-1-oxo-1H-phthalazine 2-carboxylic acid ethyl ester has been elucidated by X-ray diffraction methods. The compound crystallizes in the orthorhombic crystal system (space group Pc) with the unit cell parameters a = 10.758(9), b = 4.631(3), c = 14.957(7) Å, β = 107.30(6)° and Z = 2. The structure has been solved by direct methods and refined to a final R value of 0.060 for 870 observed reflections [F o > 4σ(F o)]. The presence of a large number of intra-and intermolecular interactions makes the molecule look like a four-ring structure. The structure is stabilized by C-H···O hydrogen bonds. 相似文献
20.
The paper deals with conductivity, thermoelectric power and field effect measurements on amorphous Ge specimens prepared by the decomposition of germane gas in a rf glow discharge. Substrate temperatures Td of 300, 400 and 500 K were used during deposition. The sign of the thermoelectric power S is negative throughout the temperature range investigated (200–500 K). Above 300 K, the conductivity activation energy in specimens prepared at Td = 500 K lies between 0.40 and 0.43 eV; it is equal to the gradient of the S versus 1/T curves, suggesting transport in the extended electron states. Below room temperature there is an increasing contribution in all specimens from electron hopping transport in localized states lying about 0.25 eV below ?C. Both conductivity and thermoelectric power results can be interpreted satisfactorily in terms of these two current paths. Hopping at the Fermi level has not been observed. The preliminary field effect measurements indicate that, as in amorphous Si, ?f lies near a density of state minimum. The density of states at ?f is appreciably higher than that in similarly prepared Si specimens. 相似文献