Self-seeding Technique Research Articles

Growth of lead barium niobate (Pb 1− xBa xNb 2O 6) crystals by the vertical Bridgman method : I. Self-seeded growth

Pb 1− x Ba x Nb 2O 6 (PBN) single crystals were grown by the vertical Bridgman method without the loss of PbO during the growth process, which has been a serious problem in the Czochralski growth method. A self-seeded technique using sealed Pt crucibles containing the ceramic charges produced stoichiometric and transparent single-crystal grains of millimeter sizes. The preferred growth direction was found to be [0 0 1] which is the same as in Sr 1− x Ba x Nb 2O 6 (SBN). The grain size increased as the grains propagated, but normal grain selection was not sufficient to produce individual single grains with centimeter size cross sections. The ferroelectric domain structures of tetragonal PBN could be revealed either by etching in hydrofluoric acid or polishing with colloidal silica. The typical domain size on the surface perpendicular to the polar axis was 10–50 μm. The measured spontaneous polarization, P s, was 0.40–0.70 μC/mm 2 depending on the composition. As-grown crystals contained a high concentration of protons (H +) on the order of 10 17/cm 3, which is important for the holographic data storage applications. In addition, the concentration of protons could be increased or decreased by post-growth heat-treatment processing.

Efficient ultraviolet-light (345-nm) generation in a bulk LiIO_3 crystal by frequency doubling of a self-seeded gain-switched AlGaInP Fabry–Perot semiconductor laser

A self-seeded gain-switched semiconductor laser is applied as a fundamental optical source to second-harmonic generation, and improvement of frequency-conversion efficiency is demonstrated. 345-nm ultraviolet light of 0.12-muW average power is generated in a bulk LiIO(3) crystal for 690-nm fundamental light of 10.5-mW average power, which is composed of 500-MHz pulse trains emitted from an AlGaInP diode laser with a 68-ps temporal width and a 0.11-nm spectral bandwidth. It is clearly shown that the narrowing of the spectral bandwidth by the self-seeding technique gives rise to a normalized conversion efficiency of 0.218%/Wcm , which is fourtimes larger than that for pulses without seeding.

Morphology of poly(phenylene sulphide) single crystals grown by a two-stage self-seeding technique

Abstract We have successfully developed a two-stage self-seeding technique to grow poly(phenylene sulphide) (PPS) single crystals from dilute solution, and used these single crystals as model materials to investigate the morphological changes dependent upon the seeding and crystallization conditions. The seeding conditions control the coexistence of different crystal types and determine whether isolated single crystals will be grown. The crystal structure and morphology of PPS single crystals have been investigated by using the wide angle X-ray scattering (WAXS) and transmission electron microscopy techniques including surface decoration by polyethylene. The morphology of the PPS single crystals reveals a highly anisotropic needle-like habit with a very smooth fold surface and very rough growth edges. The aspect ratio of the PPS needle-like single crystals increases as the crystallization temperature and molecular weight increase. Formation of needle-like crystals, lack of sectorization, formation of macroscopically rough growth surfaces, and molecular weight and crystallization temperature dependency of crystal habit suggest a crystal growth model in which the PPS molecules fold parallel to the growth direction during the crystal growth stage from dilute solution.

Three-dimensional model for twinning in polymer single crystals with inclined chains

Abstract A model describing the three-dimensional structure of twinned crystals of poly(paraxylylene) (PPX) in the aL form is proposed. PPX crystals were grown at 218°C in 1-methylnaphthalene at a concentration of 0.005% using the self-seeding technique. A large number of twinned crystals are obtained, corresponding to two basic modes: aLta and aLtb, involving respectively {110} and {130} twinning planes. Lattice images of the boundary in αta crystals occasionally show 110 lines. Since chains are inclined to the surface normal in untwinned crystals, the presence of 110 lattice fringes at the boundary is explained by the collapse onto the carbon substrate of a three-dimensional roof-like structure consisting of two lamellar twin components. This ‘nonplanar twin’ model, in a sense that the two lamellar components do not lie in the same plane, is extended to the crystals with aLtb twins and further demonstrated by electron diffraction tilt experiments. Multiple twins as well as coalesced α–β crystals are als...

Studies of the chemical degradation of polysiloxanes by hydrofluoric acid: (2). Poly(tetramethyl- p-silphenylene siloxane-dimethyl siloxane) block copolymers

Block copolymers of tetramethyl- p-silphenylene siloxane (TMPS) and dimethyl siloxane (DMS) have been selectively degraded with hydrofluoric acid solution. The HF preferentially attacks the SiO bonds, particularly those of the non-crystalline DMS component. The copolymers which were precipitated by the self-seeding technique were etched with 48% HF at 30°C (established as suitable conditions for our work). The crystallinity change on etching these is largest for the sample of lowest TMPS content; it varies from about 20% at zero etching time to 98% after a period of 40 h approximately. The molecular weight, small-angle X-ray period, melting temperature, heat of fusion and degree of crystallinity were all determined as a function of etching time. The correlated results were found to be consistent with a two-phase model in which the DMS component was essentially excluded from the TMPS crystalline core. Some predictions, based on copolymer theories, were found to be consistent with the analysis of the experimental observations.

Morphology and structure of poly(ethylene-cocarbon monoxide) single crystals

Random copolymers from ethylene and carbon monoxide, containing different numbers of carbonyl groups in the main chain, have been crystallized from dilute solutions. The self-seeding technique has been applied. Monolayer single crystals are obtained and some results concerning their morphology and the unit cell dimensions as functions of the carbonyl content are reported. The annealing treatment supports a structural transition previously noted for other polymers.

Controlled crystal-growing procedures in polyethylene involving self-seeding: Some novel twinning habits

Abstract The paper demonstrates the potentiality of the self-seeding technique [3] for the growing of single crystals of polyethylene. It is shown that with appropriate measures, uniform single-layer crystals of strictly controlled habit can be obtained. Concrete recipes are given for the growing technique. The habit features are systematized as functions of crystallization temperature and concentration. Novel twin formations with coincident centers arising from this work are described and analyzed. Their origin is correlated with the result suggested by an associated investigation that the nuclei have a multicomponent structure [4]. The advantages of the new crystal-growing technique for other lines of work are pointed out.