Non-uniform Growth Rate Research Articles

Particle sized distribution simulation for an industrial fluidised bed urea granulator by PNB method considering non-uniform growth rate

ABSTRACT The present work is an extension of the PNB method. Contrary to the previous works, it is assumed that the growth rate of particles/granules is not constant along the granulator length and it depends on the granule mass flow rate and molten urea spray mass flowrate. With these assumptions, the governing equations were extracted, and the analytical solution was derived. Since this method is based on the particle number and the measured data are mass-based (defined based on mass fraction distribution), a general procedure was also proposed to transfer between number-based and mass-based quantities. Finally, the results were compared with measured data taken from a large-scale industrial urea granulator and close agreements were observed. This method is introduced as an effective tool for the simulation of other applications, like driers.

Non-uniform Grain Boundary Migration During Static Recrystallization: A Cellular Automaton Study

During static recrystallization, grains often have non-constant and non-uniform growth rates, significantly affecting the recrystallization kinetics and the microstructure after recrystallization. A cellular automaton model was employed in order to evaluate the relative influences of gradients of stored energy, grain boundary curvature, and heterogeneity of grain boundary mobility on the non-uniform migration of grain boundary segments, leading to the formation of protrusions and retrusions. Electron back-scatter diffraction measurements of a cold-rolled copper microstructure served to feed the model. Orientation maps obtained after partial recrystallization were used to assess the model outcome. The model was capable to predict the shapes of recrystallized grains with retrusions and protrusions. Effects of different model assumptions were compared to reveal individual contributions of different factors to grain size distribution, grain shape and boundary roughness. The model predicted a decreasing average grain growth rate as a result of the progressive immobilization of an increasing fraction of grain boundary segments. The model prediction was compared with experimental results, explaining the origin of stationary boundaries and indicating some further improvements necessary to reach quantitative agreement.Graphical

Laser direct metal deposition process of thin-walled parts using variable spot by inside-beam powder feeding

PurposeThe purpose of this paper is to improve the forming efficiency and quality of unequal-width parts fabricated by laser direct metal deposition technology, some experiments were designed.Design/methodology/approachA new method by varying laser spot was adopted to fabricate unequal-width single track using one scanning rather than multi-track overlapping in the way of the inside-beam powder feeding, and the thin-walled parts were fabricated layer by layer. The theoretical model among layer thickness of z-axis, height of single track and the section curve order of single track was established.FindingsThe top surface unevenness of the thin-walled parts could be compensated automatically within the laser defocusing ranges from −2.5 to −5 mm and from 0.5 to 2.5 mm. The growth rate with the large width/height ratio was more than the small ratio, while the set height of the single track was uniform. The problem of non-uniform growth rate could be solved based on a stepped single-track method. The thin-walled parts with the smooth top surface was fabricated layer by layer which had a continuously variable width from 1 to 3 mm by splicing the laser defocusing range.Practical implicationsThe shapes of the to-be-fabricated parts affect variable laser spot process in practical applications. For example, it will be difficult to apply variable laser spot process on the parts with the hole features.Originality/valueThis paper provided a guidance for forming unequal-width parts by laser direct metal deposition based on the inside-beam powder feeding.

Origin of periodic rippling during chemical vapor deposition growth of carbon nanotube forests

Carbon nanotube forests are arrays of roughly vertically aligned nanotubes. Under certain growth conditions, these forests can show a growth instability that gives rise to periodic ripples that are coherent over a forest-sized scale. Previously, we showed that the uniformity and synchronization of the ripples is sufficient for them to behave as diffraction gratings for visible light. Here, we identify the conditions that reproducibly promote the formation of these ripples. We investigate the formation mechanism via ex situ scanning electron microscopy and in situ optical imaging. While the rippling amplitude varies appreciably, the rippling wavelength varies very little and can be estimated from simple mechanical considerations. We provide evidence that the rippling is a consequence of cohesive interactions between nanotubes and the build up of strain, driven by a non-uniform growth rate. The origin of the non-uniform growth rate is explained.

Hydrodynamic Instability During Non-uniform Growth of a Helium Crystal

The Rayleigh-Taylor instability (Taylor in Proc. R. Soc. Lond. Ser. A 201:192, 1950) is an instability of the interface which can be realized when the lighter fluid is propelling the heavier one. We analyze an analog of the hydrodynamic Rayleigh-Taylor instability for the superfluid-solid 4He interface under non-uniform growth rate of the solid phase. The development of the interface instability starts with an accelerated interface growth rate and provided that the magnitude of interface acceleration exceeds some critical value independent of the surface stiffness. The plane and spherical shapes of the liquid-solid interface are considered. For the Richtmyer-Meshkov (Meshkov in Izv. Akad. Nauk SSSR 5:151, 1969) limiting case of an impulsively accelerated interface, the onset of instability does not depend on the sign of the interface acceleration. The investigation of the surface instability can explain the features observed in the experiment (Chavanne et al. in Phys. Rev. Lett. 86:5506–5509, 2001) where the crystal 4He nucleation is triggered by focusing an ultrasonic wave of the large amplitude. The onset and development of the instability are expected at the surface of helium crystals in the course of their abnormal fast growth under large magnitudes of overpressures (Tsymbalenko in J. Low Temp. Phys. 138:795, 2005).

Isotopic investigations of carbonate growth on concrete structures

Stable C and O isotope ratios were measured in carbonate minerals, growing under concrete structures from two locations in the United States. These locations were under a bridge in Michigan and under an overpass in New York. The δ13C of the carbonate samples ranged from −21.6 to −31.4‰ (with respect to V-PDB) and clearly indicated precipitation under non-equilibrium conditions. Indeed, the values in some cases were more negative than could be accounted for by existing models that invoke 4 stages of kinetic fractionation. There have been suggestions that microbial activity involving C from gasoline and other fossil fuel sources might be responsible for the relatively low C isotope ratios measured in these carbonates. To explore this possibility, 14C measurements were made in some of the samples. All samples measured for 14C contained bomb C. The range of 14C concentrations suggested a non-uniform growth rate, although possible fossil fuel-derived carbon in the system needs future investigation. The δ18O values of the carbonates analyzed from Michigan range from 12.5 to 15.7‰ (with respect to V-SMOW), with a mean value of 13.7‰. The δ18O values of the NY samples range from 11.8 to 15.2‰, with a mean value of 13.1‰. The nearly identical mean values at both locations favors incorporation of O from atmospheric CO2 in carbonate precipitation. Additionally, the 210Pb radiometric technique was also attempted to explore the applicability of this technique in dating concrete derived carbonates as well as recent carbonates forming in a wide variety of environments. The results gave ages between 64 and 3.8 a and are consistent when compared with the date the bridge was constructed.

Quasi in situ observation of Si lateral solid phase epitaxy

A quasi in situ observation of Si lateral solid phase epitaxy (L-SPE) has been carried out by an anneal-and-observe technique using a transmission electron microscope (TEM). For this observation, 3 mm Ø Si discs, which were thinned physically and chemically, were cut from a non-heated sample which had been prepared by depositing an amorphous Si (a-Si) film on the patterned amorphous insulator substrate. For the L-SPE growth, the thin specimens were heated in a furnace. The same areas of the same sample were repeatedly observed after an additional heating process at each interval. The direct origin of the (111) facet formation during the L-SPE growth has been precisely revealed by this method. Polygrains due to the random nucleation from the a-Si/a-insulator interface have been found to obstruct further L-SPE growth, while the L-SPE growth continued in the adjacent polygrain-free regions. As a result of this non-uniform growth rate, the (111) facets which nucleated at the polygrains grew into V-shaped valleys and finally caused a zig-zag growth front.

Postpubertal changes in the histomorphometric characteristics of the uterus of the indigenous West African pig.

The epithelial height and myometrial and endometrial characteristics of the fundus, corpus and neck regions of the uteri of 28 virgin West African indigenous gilts in four age groups (6-8, 10-12, 16-18 and 20-22 months) were histometrically evaluated. Though regionally stable, the height of luminal epithelial cells was, respectively, reduced and enhanced in the fundus and neck regions by 10-12 months. The myometrium was thinner in the fundus and was severely reduced in the corpus region in gilts aged 16-18 months. Contrarily, the endometrium was regionally stable in thickness but underwent some thickening in the corpus of 16 to 18-month-old gilts. The respective occurrence of muscular and glandular tissues in the myometrium and endometrium was regionally influenced while older gilts had more circular muscles than the others. Generally, over 70% of the glands and 60% of the muscles were complex glands and longitudinal muscles, respectively. Results depict the nonuniform growth rates of different uterine tissues in the different regions and suggest that the improvement in the litter-carrying capacity of the uterus in these pigs continues well after puberty.

Non-planar solid phase epitaxial growth processes in ion-implanted GaAs

High resolution ion channelling and reflection electron diffraction techniques have been used to examine details of epitaxial regrowth in Ar +-ion-implanted GaAs(100) at furnace anneal temperatures of 400°C or less. In particular, we have investigated the nature and extent of epitaxial regrowth during both isothermal and isochronal annealing for various implant energies and for implant doses above and below the amorphous threshold. Our results indicate the development of a non-planar growth interface during annealing which may lead, ultimately, to complex near-surface crystallization processes. Consistently with our observations and recent results from other laboratories, we propose a model for the epitaxial regrowth of amorphous GaAs layers based upon non-uniform growth rates along the amorphous-crystalline interface which could arise from local stoichiometry imbalance.

Non-Planar Solid Phase Epitaxial Growth Processes in Ion-Implanted GaAs

High resolution ion channelling and reflection electron diffraction techniques have been used to examine details of epitaxial regrowth in Ar+-ion-implanted GaAs(100) at furnace anneal temperatures of 400°C or less. In particular, we have investigated the nature and extent of epitaxial regrowth during both isothermal and isochronal annealing for various implant energies and for implant doses above and below the amorphous threshold. Our results indicate the development of a nonplanar growth interface during annealing which may lead, ultimately, to complex near-surface crystallization processes. Consistently with our observations and recent results from other laboratories, we propose a model for the epitaxial regrowth of amorphous GaAs layers based upon non-uniform growth rates along the amorphous-crystalline interface which could arise from local stoichiometry imbalance.

Hormonal influences on ocular refraction in the rat

WE report here that thyrotrophin (TSH) has a marked influence on ocular refraction in rats. In the clinical experience of one of us (C.C.K.-V.A.), adolescent myopia is not infrequently accompanied by increased height, suggesting a common, possibly hormonal, cause for both phenomena. No data on possible hormonal influences on eye growth or refraction could be found in the literature, but early anatomical studies showed that, during puberty, a growth spurt occurs in the eye after an early post-natal rapid weight increase1–4. In a very detailed study, Weiss2 found relatively less growth of the sagittal eye axis as compared with the vertical and frontal diameters. Non-uniform growth rates along different eye axes might affect refraction.