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Growth Sequence (growth + sequence)
Selected AbstractsColonization by barnacles on fossil Clypeaster: an exceptional example of larval settlementLETHAIA, Issue 4 2008ANA G. SANTOS The presence of c. 1450 individuals of the balanid barnacle Balanus crenatus Bruguière encrusting the test of a clypeasteroid sea urchin from the Late Miocene of the Guadalquivir Basin (southwestern Spain) allows proposing a settlement pattern linked to the growth of the encrusting organism. The possible influence of dip angle was controlled by dividing the test into four concentric zones ranging from lowest margin to apex (0,15°, 15,30°, 30,50° and 0°). Contour diagrams were prepared to identify areas of highest barnacle density as well as size categories distribution in relationship to the pitch of the sea urchin test. The orientation of balanid tests was recorded and plotted on rose diagrams from 0° to 180°. Four size categories of barnacles were distinguished: (1) < 1 mm, (2) 1,2 mm, (3) 2,3 mm and (4) 3,4 mm; these correspond to a growth sequence ranging from post-larval forms to juveniles. Two areas of maximum settlement density are situated on the posterior margin of the test, on aboral as well as oral surfaces. The aboral surface shows the maximum number of barnacles. Two groups of individuals are defined on the basis of their location, that is, those encrusting the posterior medium part of the urchin, and those located on the anterior half. The results suggest that larval settlement was initially controlled by the availability of free space and afterwards by an intensification effect. Orientation and dip of the test may have played a secondary role in the settlement of the larvae. Substrate colonization seems to have been closely related to the biostratinomic history of the sea urchin test and although several scenarios are possible, our data are congruent with a synchronous settlement of both surfaces (aboral and oral) by one spat or several. [source] Zirconocene-catalysed propene polymerisation: kinetics, mechanism, and the role of the anionMACROMOLECULAR SYMPOSIA, Issue 1 2004Fuquan Song Abstract The olefin polymerisation activity of metallocene catalysts strongly depends on the counteranion provided by the activator system. The relative activities of a number of new diborate anions [Z(BAr3)2], have been quantified (Z = CN, NH2, N(CN)2; Ar = C6F5 or o -C6F4C6F5). The kinetic parameters for the initiation, propagation and termination steps of propene polymerisations catalysed by (SBI)ZrCl2 have been determined using quenched-flow kinetic and batch techniques [SBI = rac -Me2Si(1-Ind)2]. Comparison of two activator systems, (i) CPh3[B(C6F5)4] / triisobutylaluminium (TIBA) and (ii) methylaluminoxane (MAO) shows, surprisingly, that the concentration of species actively involved in chain growth at any one time is comparable for both systems, although the MAO-activated catalyst is about 20 times less active than the borate system. It is concluded that the counteranion remains sufficiently strongly bound to the metal centre throughout the chain growth sequence to modulate the energetics of monomer insertion. A model suggesting that the monomer binding follows an associative interchange (Ia) mechanism is proposed. [source] Breath-hold water and fat imaging using a dual-echo two-point dixon technique with an efficient and robust phase-correction algorithmMAGNETIC RESONANCE IN MEDICINE, Issue 2 2004Jingfei Ma Abstract A two-point Dixon technique using a novel phase-correction algorithm and commercially available dual-echo fast gradient-echo pulse sequence is presented. The phase-correction algorithm determines the directional rather than phase distribution of signals due to field inhomogeneities. Specifically, a region-growing scheme uses precalculated spatial gradients of the signal phase to guide the growth sequence, so there is no need to manually select the seeds or use an empirical angular threshold. Further, the determination of the signal direction of a given pixel is based on both the amplitude and phase of the surrounding pixels, the direction of which has already been determined. The advantages of this algorithm include its easy implementation, computational efficiency, and robustness in the presence of pixels with large phase uncertainty. The feasibility and usefulness of the technique are demonstrated in vivo with artifact-free water and fat images of an entire abdomen in a single breath-hold. Magn Reson Med 52:415,419, 2004. © 2004 Wiley-Liss, Inc. [source] Multi-generational carbonate assemblages in martian meteorite Allan Hills 84001: Implications for nucleation, growth, and alterationMETEORITICS & PLANETARY SCIENCE, Issue 1 2004Catherine M. CORRIGAN These regions contain familiar forms of carbonate, as well as textural forms previously unreported including carbonate rosettes, planiform "slab" carbonates, distinct "post-slab" magnesites, and carbonates interstitial to feldspathic glass and orthopyroxene. Slab carbonates reveal portions of the carbonate growth sequence not seen in the rosettes and suggest that initial nucleating compositions were calcite-rich. The kinetically controlled growth of rosettes and slab carbonates was followed by an alteration event that formed the magnesite-siderite layers on the exterior surfaces of the carbonate. Post-slab magnesite, intimately associated with silica glass, is compositionally similar to the magnesite in these exterior layers but represents a later generation of carbonate growth. Feldspathic glasses had little or no thermal effect on carbonates, as indicated by the lack of thermal decomposition or any compositional changes associated with glass/carbonate contacts. [source] Gate-lag and drain-lag effects in (GaN)/InAlN/GaN and InAlN/AlN/GaN HEMTsPHYSICA STATUS SOLIDI (A) APPLICATIONS AND MATERIALS SCIENCE, Issue 6 2007J. Kuzmik Abstract Gate and drain-lag effects are studied in (GaN)/InAlN/GaN and InAlN/AlN/GaN HEMTs grown on sapphire. Electron trapping on the surface states between the gate and the drain forming the net negative charge up-to ,2 × 1013 cm,2 is found to be responsible for the gate-lag effect in the (GaN)/InAlN/GaN HEMTs. If the polarization charge at the device surface is decreased by GaN capping, then density of the trapped charge is not changed, however the electron de-trapping process becomes faster. The drain-lag effect is caused by electron injection and trapping in the source-gate area reaching ,1 × 1013 cm,2 of the trapped charge in the steady state. In the studied voltage range the InAlN/AlN/GaN HEMT is shown to be gate-lag-free suggesting that this parasitic transient can be avoided if thin AlN is used in the epi-layer growth sequence. It is assumed that this breakthrough quality relates to the decreased or even reverted electric field in the InAlN layer if AlN is inserted. Surface states need not to be generated in this case. (© 2007 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim) [source] Improvements in crystalline quality of thick GaN layers on GaAs (111)A by periodic insertion of low-temperature GaN buffer layersPHYSICA STATUS SOLIDI (C) - CURRENT TOPICS IN SOLID STATE PHYSICS, Issue 7 2003H. Murakami Abstract Thick and high quality GaN layer growth using periodic insertion of low-temperature (LT)-grown GaN buffer layers was investigated by metalorganic hydrogen chloride vapor phase epitaxy (MOHVPE). Both morphological and optical properties of GaN epitaxial layers were drastically improved by inserting a second LT-GaN buffer layer. Also, the thickness of the second buffer layer was found to affect the quality of subsequently grown epitaxial layers. The full-width at half maximum (FWHM) value of X-ray diffraction for () plane (, scan) of the GaN layer with double buffer layer structure decreased to 608 arcsec whereas that with single buffer structure was 3600 arcsec. These results indicate that the free-standing GaN substrate with low dislocation density can be possible by reiterating the growth sequence of buffer layer and epitaxial layer. (© 2003 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim) [source] Microstructural, chemical and textural records during growth of snowball garnetJOURNAL OF METAMORPHIC GEOLOGY, Issue 6 2009M. ROBYR Abstract The growth history of two populations of snowball garnet from the Lukmanier Pass area (central Swiss Alps) was examined through a detailed analysis of three-dimensional geometry, chemical zoning and crystallographic orientation. The first population, collected in the hinge of a chevron-type fold, shows an apparent rotation of 360°. The first 270° are characterized by spiral-shaped inclusion trails, gradual and concentric Mn zoning and a single crystallographic orientation, whereas in the last 90°, crenulated inclusion trails and secondary Mn maxima centred on distinct crystallographic garnet domains are observed. Microstructural, geochemical and textural data indicate a radical change in growth regime between the two growth sequences. In the first 270°, growth occurred under rotational non-coaxial flow, whereas in the last 90°, garnet grew under a non-rotational shortening regime. The second population, collected in the limb of the same chevron-type fold structure, is characterized by a spiral geometry that does not exceed 270° of apparent rotation. These garnet microstructures do not record any evidence for a modification of the stress field during garnet growth. Concentric Mn zoning as well as a single crystallographic orientation are observed for the entire spiral. Electron backscatter diffraction data indicate that nearly all central domains in the snowball garnet are characterized by one [001] axis oriented (sub-)parallel to the symmetry axis and by another [001] axis oriented (sub-)parallel to the orientation of the internal foliation. These features suggest that the crystallographic orientation across the garnet spiral is not random and that a relation exists among the symmetry axis, the internal foliation and the crystallographic orientation. [source] | |||||||||||||