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Growth Speed (growth + speed)
Selected AbstractsEquilibrium and growth shapes of crystals: how do they differ and why should we care?CRYSTAL RESEARCH AND TECHNOLOGY, Issue 4-5 2005Robert F. SekerkaArticle first published online: 15 MAR 200 Abstract Since the death of Prof. Dr. Jan Czochralski nearly 50 years ago, crystals grown by the Czochralski method have increased remarkably in size and perfection, resulting today in the industrial production of silicon crystals about 30 cm in diameter and two meters in length. The Czochralski method is of great technological and economic importance for semiconductors and optical crystals. Over this same time period, there have been equally dramatic improvements in our theoretical understanding of crystal growth morphology. Today we can compute complex crystal growth shapes from robust models that reproduce most of the features and phenomena observed experimentally. We should care about this because it is likely to result in the development of powerful and economical design tools to enable future progress. Crystal growth morphology results from an interplay of crystallographic anisotropy and growth kinetics by means of interfacial processes and long-range transport. The equilibrium shape of a crystal results from minimizing its anisotropic surface free energy under the constraint of constant volume; it is given by the classical Wulff construction but can also be represented by an analytical formula based on the ,-vector formalism of Hoffman and Cahn. We now have analytic criteria for missing orientations (sharp corners or edges) on the equilibrium shape, both in two (classical) and three (new) dimensions. Crystals that grow under the control of interfacial kinetic processes tend asymptotically toward a "kinetic Wulff shape", the analogue of the Wulff shape, except it is based on the anisotropic interfacial kinetic coefficient. If it were not for long range transport, crystals would presumably nucleate with their equilibrium shape and then evolve toward their "kinetic Wulff shape". Allowing for long range transport leads to morphological instabilities on the scale of the geometric mean of a transport length (typically a diffusivity divided by the growth speed) and a capillary length (of the order of atomic dimensions). Resulting crystal growth shapes can be cellular or dendritic, but can also exhibit corners and facets related to the underlying crystallographic anisotropy. Within the last decade, powerful phase field models, based on a diffuse interface, have been used to treat simultaneously all of the above phenomena. Computed morphologies can exhibit cells, dendrites and facets, and the geometry of isotherms and isoconcentrates can also be determined. Results of such computations are illustrated in both two and three dimensions. (© 2005 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim) [source] Cover Picture: Acceleration of Calcite Kinetics by Abalone Nacre Proteins (Adv. Mater.ADVANCED MATERIALS, Issue 22 200522/2005) Abstract Abalone utilizes a system of macromolecular matrices and soluble proteins to produce beautiful and mechanically robust shells. The cover shows work by Qiu and co-workers reported on p.,2678, in which AP8 proteins isolated from the shell of red abalone are shown to alter the growth of calcite both by accelerating the rate and modifying the shape from the simple rhombohedra seen in the upper left of the scheme to the more complex form seem in the lower right. The changes are made manifest at an atomic scale through alterations in the growth speed and shape of the atomic steps that form the growth hillocks (background). [source] Natural history of hepatocellular carcinoma including fibrolamellar and hepato-cholangiocarcinoma variantsJOURNAL OF GASTROENTEROLOGY AND HEPATOLOGY, Issue 4 2002Kunio Okuda Abstract The natural history of hepatocellular carcinoma (HCC) varies greatly with the global region, because the carcinogenic factors are not the same among countries. Besides the clinicopathological factors such as tumor characteristics, sex, and age, background liver disease is a major determinant of prognosis. Hepatocellular carcinoma, mainly associated with chemical carcinogens such as aflatoxin, does not have severe background cirrhosis, and grows quickly, whereas HCC developing in association with a virus in a cirrhotic liver generally grows more slowly, and the severity of cirrhosis is the major prognostic factor. The median survival of untreated sub-Saharan African patients is less than 1 month from diagnosis, contrasted by an average survival of 4 months in virus-induced HCC associated with cirrhosis. Tumor characteristics, such as size, number, and growth speed, which vary considerably from case to case, affect the prognosis. Vascular (portal) invasion portends a poor prognosis, and ,-fetoprotein levels also correlate with prognosis. Several distinct clinical types of HCC occur, namely diffuse-type HCC caused by rapid portal spread of cancer cells, febrile-type caused by poorly differentiated sarcomatoid cancer cells, and cholestatic HCC caused by intraductal invasion; all have a short survival. There are several histological variant forms: combined hepato-cholangiocarcinoma behaves like HCC, with a poorer prognosis because of more frequent lymph node metastases; fibromellar carcinoma, which is relatively common in young Caucasian adults, has a good prognosis if diagnosed early, permitting resection; and cholangiolocellular carcinoma, which derives from the canalicular epithelium, is indistinguishable from HCC, with a similar prognosis. © 2002 Blackwell Publishing Asia Pty Ltd [source] Bone vascular supply in monitor lizards (Squamata: Varanidae): Influence of size, growth, and phylogenyJOURNAL OF MORPHOLOGY, Issue 5 2008Vivian de Buffrénil Abstract Bone vascular canals occur irregularly in tetrapods; however, the reason why a species has or lacks bone canals remains poorly understood. Basically, this feature could depend on phylogenetic history, or result from diverse causes, especially cortical accretion rate. The Varanidae, a monophyletic clade that includes species with impressive size differences but similar morphologies, is an excellent model for this question. Cortical vascularization was studied in 20 monitor species, on three bones (femur, fibula, and tibia) that differ in their shaft diameters, and in the absolute growth speed of their diaphyseal cortices. In all species smaller than 398 mm SVL (133,397 mm in sample), bone cortices lack vascular canals, whereas all larger species (460,1,170 mm in sample) display canals. The size 398,460 mm SVL is thus a threshold for the appearance of the canals. The distribution of vascular and avascular bone tissues among species does not precisely reflect phylogenetic relationships. When present, vascular canals always occur in the femur and tibia, but are less frequent, sparser, and thinner in the fibula. Vascular density increases linearly with specific size but decreases exponentially during individual growth. In most species, canal orientation varies between individuals and is diverse in a single section. No clear relationship exists between canal orientation and vascular density. These results suggest that: a) the occurrence and density of bone vascular canals are basically dependant on specific size, not phylogenetic relationships; b) vascular density reflects the absolute growth rates of bone cortices; c) the orientation of vascular canals is a variable feature independent of phylogeny or growth rate. J. Morphol., 2008. © 2007 Wiley-Liss, Inc. [source] Experiment and simulation on interface shapes of an yttrium aluminium garnet miniature molten zone formed using the laser-heated pedestal growth method for single-crystal fibersJOURNAL OF APPLIED CRYSTALLOGRAPHY, Issue 4 2009P. Y. Chen A two-dimensional simulation was employed to study the melt/air and melt/solid interface shapes of the miniature molten zone formed in a laser-heated pedestal growth (LHPG) system. Using a non-orthogonal body-fitting grid system with the control-volume finite-difference method, the interface shape can be determined both efficiently and accurately. During stable growth, the dependence of the molten-zone length and shape on the heating CO2 laser is examined in detail under both the maximum and the minimum allowed powers with various growth speeds. The effect of gravity on the miniature molten zone is also simulated and the possibility of horizontally oriented LHPG is revealed. Such a horizontal system is good for the growth of long crystal fibers. [source] Industrial solidification processes in polybutene-1.POLYMER ENGINEERING & SCIENCE, Issue 1 2003Part I, quiescent melts In the first paper of this series the principles for solidification of crystallizable polymers under heat transfer conditions are discussed for the case of the absence of flow. For industrial polymers, which, in general, are forced to crystallize at large undercoolings, the pertinent kinetic data are extremely rare. In the present paper the growth speeds of spherulites and the number of nuclei per unit volume are presented as functions of temperature for two industrial polybutene-1 grades. [source] | ||||||||||