Rotating-crystal Method Research Articles

Numerical simulation of flow and mass transfer during growth of the long seed KDP crystals under 2D translation method

A novel motion method, namely, the two-dimensional translation method (2D TM) had been proposed by our team to effectively utilize convection to enhance morphological stability and mass transfer. To evaluate this new method and apply it to the long seed KDP crystals growth, numerical simulations on the hydrodynamics and mass transfer involved in the growth of the long seed KDP crystals through this new method have been performed. Comparisons with the rotating crystal method (RCM) indicates that 2D TM improves the value and distribution homogeneity of the surface supersaturation. As the translational velocity increases, the value of the surface supersaturation on the prismatic faces increases. However, as the translational distance becomes long, there is a decrease in the value of the prismatic surface supersaturation, but the homogeneity of the surface supersaturation is improved. Moreover, the increase of the crystal size not only reduces the value of the surface supersaturation on the prismatic faces, but also degrades the homogeneity of the surface supersaturation. A further improvement in the magnitude and distribution homogeneity of the prismatic face supersaturation can be observed through modifying the crystal mounting method.

Growth study and characterization of KDP crystals grown by 2D translation method

Transparent potassium dihydrogen phosphate (KDP) crystals that are grown from different translational velocities and orientation configurations were obtained during growth through a novel motion mode called 2D translation method. The grown crystals were investigated by microhardness, dielectric, chemical etching, thermal analysis and laser damage threshold studies. A higher mechanical hardness is observed in crystals grown by the 2D translation method than that grown by the rotating-crystal method. Low dielectric loss and less dislocation density show that the 2D translation method grown crystals contain minimum defects. The thermal stability and damage threshold of the 2D translation method grown crystals are higher than those of the crystal grown using the rotating-crystal method. Moreover, the properties of various crystals grown by the 2D translation method under different conditions are compared, and results show that the grown crystals' quality is best when the translation velocity is 0.04 m/s.

Numerical simulation of flow and mass transfer for large KDP crystal growth via solution-jet method

A novel technique of growing large crystals of potassium dihydrogen phosphate (KDP) named solution-jet method is proposed. The aim is to increase supersaturation on the pyramidal face, especially for crystal surface regions close to the rotation axis. The fluid flow and surface supersaturation distribution of crystals grown under different conditions were computed using the finite-volume method. Results indicate that the time-averaged supersaturation of the pyramidal face in the proposed method significantly increases and the supersaturation difference from the crystal center to edge clearly decreases compared with the rotating-crystal method. With increased jet velocity, supersaturation on the pyramidal face steadily increases. Rotation rate considerably affects the magnitude and distribution of the prismatic surface supersaturation. With increased crystal size, the mean value of surface supersaturation averaged over the pyramid gradually decreases; conversely, standard deviation increases, which is detrimental to crystal growth. Moreover, the significant roles played by natural and forced convection in the process of mass transport are discussed. Results show that further increased jet velocity to 0.6 m/s renders negligible the effects of natural convection around the pyramid. The simulation for step propagation indicates that solution-jet method can promote a steady step migration and enhance surface morphology stability, which can improve the crystal quality.

Numerical simulations of flow and mass transfer during potassium dihydrogen phosphate single crystal growth via the three-dimensional motion growth method

A novel solution-based crystal growth method, namely, the three-dimensional motion growth method (3D MGM) is proposed to effectively utilize convection for simultaneous enhancement of morphological stability and mass transfer. To evaluate this new growth method, numerical simulations of flow and mass transfer are carried out on growth of potassium dihydrogen phosphate (KDP) crystals subjected to 3D MGM. The supersaturation field of the crystal surface is presented as functions of translational velocity and distance since it is critically involved in the processes of morphological instability and inclusion formation; the dependences of surface supersaturation on the solution flow surrounding the crystal surface are analyzed in detail. The correlations between thickness of the solute boundary layer and translational velocity and distance are described. The roles of natural and forced convection in mass transfer under different conditions are discussed, with results showing that the effects of natural convection are significant only at low translational velocities. The Damkohler number is introduced to measure the significance of mass transport limitations in the solution system of 3D MGM crystal growth, where it implies that with the increase of translational velocity, the limitations in mass transport decrease. Comparisons with the traditional rotating-crystal method indicate that 3D MGM has advantages in terms of the distribution homogeneity of surface supersaturation. The model of steps moving on the prismatic face demonstrates that the step-train propagates more stably on the crystal surface when the 3D MGM is applied.

Femtosecond x-ray diffraction using the rotating crystal method

We demonstrate the rotating-crystal method in femtosecond x-ray diffraction. Structural dynamics of a photoexcited bismuth crystal is mapped in a pump-probe scheme by measuring intensity changes of many Bragg reflections simultaneously.

The rotating-crystal method in femtosecond X-ray diffraction

We report the first implementation of the rotating-crystal method in femtosecond X-ray diffraction. Applying a pump-probe scheme with 100 fs hard X-ray probe pulses from a laser-driven plasma source, the novel technique is demonstrated by mapping structural dynamics of a photoexcited bismuth crystal via changes of the diffracted intensity on a multitude of Bragg reflections. The method is compared to femtosecond powder diffraction and to Bragg diffraction from a crystal with stationary orientation.

Kinematical intensity profiles obtained for single and multiple diffraction in perfect spherical crystals

The `intensity' (power) profiles obtained with the rotating-crystal method are derived in the framework of the kinematical theory for a spherically shaped crystal bathed in a parallel monochromatic incident beam. It is shown that in the case of the ψ scanning technique, the `particle size effect' can cause appreciable broadening of the Bragg intensity profiles.

The measurement of integrated intensities

Expressions for the integrated intensities of the Laue and of the rotating-crystal methods are given, which take the spectral distribution of the incident radiation into account. The correlation between the integrated intensities of both methods is elucidated. A procedure is described for the measurement of integrated intensities on an absolute scale using radiation with a continuous spectral distribution superimposed by sharp (spectral) lines. It is shown that crystal-monochromatized radiation may also exhibit, besides sharp lines, continuous tails. Neglecting these tails causes errors up to 2% for observed integrated intensities.

Some New Techniques in Reflection High Energy Electron Diffraction (RHEED) Application to Surface Structure Studies

A new apparatus for reflection high energy electron diffraction (RHEED) provided with an electron gun with a sharply focused electron beam and operated under ultrahigh vacuum condition has been constructed. It has a specially devised fluorescent screen which enables us to take new type RHEED patterns corresponding to sections of the reciprocal lattice rods and similar to LEED patterns. The oscillating- and the rotating-crystal methods have been applied to observe the intensity distributions along the reciprocal lattice rods. On the basis of experimental results obtained in the studies of the Si(111)7×7, Si(111)√19×√19 Ni and Si(111)5×1 Au structures, it is concluded that the RHEED method including the new technique is superior in principle for the surface structure research to LEED almost in all respects. It is expected that RHEED will find wide applications in the field of surface research taking the place of LEED.

Energy-dispersive spectroscopic methods applied to X-ray diffraction in single crystals

Two single-crystal energy-dispersive crystallographic methods (the fixed-crystal method and the rotating-crystal method) are described and investigated. Formulae for integrated intensities are derived for mosaic and perfect single crystals. Experimental results and a comparison between measured and calculated integrated intensities for a perfect germanium crystal are given. Special features and possible applications of the methods are discussed.

Die Intensität der an gebogenen Glimmerspaltflächen in höheren Ordnungen reflektierten Röntgenstrahlung

Abstract Reflection properties of the mica-cleavage plane (001) for x-rays were investigated up to the 20th order with the aid of an x-ray spectrometer and by the rotating-crystal method. They were compared with calculated structure amplitudes. The results of the different procedures were in good agreement. Intense spectra can be obtained especially in the 3rd and 5th, and still in the 4th, 8th, and 11th orders of reflection.

Die Intensität der an gebogenen Glimmerspaltflächen in höheren Ordnungen reflektierten Röntgenstrahlung

Reflection properties of the mica-cleavage plane (001) for x-rays were investigated up to the 20th order with the aid of an x-ray spectrometer and by the rotating-crystal method. They were compared with calculated structure amplitudes. The results of the different procedures were in good agreement. Intense spectra can be obtained especially in the 3rd and 5th, and still in the 4th, 8th, and 11th orders of reflection.

The crystal structure of orthorhombic hydrazonium sulphate

Crystalline hydrazonium sulphate exists in two modifications, one orthorhombic and the other monoclinic. The crystal structure of the orthorhombic modification has been determined by the Laue and the rotating-crystal methods. The unit cell, containing four units of the chemical formula, has a-8-251, b-9.159, c-5-532 A. The space group is D~-P2t2121. The atomic co-ordinates were determined by means of Patterson-Harker diagrams and two-dhnensional Fourier syntheses. The sulphate group has the configuration of a nearly regular tetrahedron with the S-O distances 1.48-1.49 A. The O-O distances between different sulphate groups are greater than 3 A. The hydrazonium ion may be considered to be of the trans type, the N-N distance being 1.40 A. One of its nitrogen atoms forms three hydrogen bonds with the N-H. . .O distances 2-73 (two) and 2.77 A., and the other may be regarded as forming three bifurcated hydrogen bonds with the distances 2.79-2.99 A. Besides the oxygen atoms linked by the hydrogen bonds, each N~H6 ++ group has three more oxygen neighbours, the total number of oxygen neighbours surrounding an N~He ++ group being eleven.

The Crystal Structure of Scorodite

The crystal structure of scorodite was determined by means of the Laue and the rotating-crystal methods. Xyiay data obtained from oscillation photographs taken with MoKoc radiation showed the structure to be built upon a simple orthorhombic unit cell with dimensions: a=10.40kX, b=8.94kX, c=10.13kX, containing eight units of the chemical formula, FeAsO4⋅2H2O. All the atoms are in general positions of the space group D162h Pbca. The values of atomic parameters were &termined by the method of trial and error. The arsenate group has the configuration of a slightly distorted tetrahedron. Each iron atom is surrounded by four oxygen atoms, each hem four different arsenate groups, and two water molecules at the corners of a distorted octahedron, the latter occupying its adjacent corners.

The absorption factor for the powder and rotating-crystal methods of X-ray crystal analysis

A method has been devised for calculating the absorption factor for the powder method. It may be used for all values of μr, where μ is the linear absorption coefficient and r the radius of the specimen. It is, however, especially suitable when μr exceeds 2. For smaller values of μr, for which the calculations are tedious and become less accurate, it is better to use Claassen's graphical method. By such a combination of the two methods a set of data was calculated for which the absorption factor may easily be obtained for any value of μr and for any angle of reflection, with an error not exceeding 1 per cent. The data may also be applied for the calculation of the absorption factor in the lines belonging to the layer of zero order in rotation photographs.