On the kinetics of crystal growth from a supercooled melt

We present a theoretical analysis of the dynamics of crystal growth from a supercooled melt. A molecular theory of crystal growth that pays proper attention to the structure at the liquid-solid interface is discussed.

Crystal growth from undercooling melts under strong gradients of temperatures generating in short-duration microgravity

Crystal growth of bulk CdTe in short-duration microgravity is performed by the unidirectional cooling method. The largest growth grains in microgravity samples are 4X2mm. The cooling profiles indicate undercooling melts in microgravity. Cooling melt samples in microgravity generate strong gradient of temperature due to stop thermal convections. Temperature distribution in the melt is calculated by the one-dimensional equation of heat conduction, and about 100 K-undercooling is considered to occur at the cooling surface.

Crystal growth of melt-textured Nd-123 under 1% oxygen partial pressure atmosphere

Crystal growth of melt-textured Nd-123 pseudo-crystals was investigated via an isothermal solidification with top-seeding technique under a 1%O2 in N2 atmosphere. Non-steady state solidification was observed at low undercooling, in contrast to an almost linear growth at higher undercooling. Similar to processing in air, the substitution of Nd/Ba was found to decrease from the seed position to the edge of the crystal. In addition, the volume fraction of Nd-422 particles decreased in the solid as solidification proceeded. As a result of these microstructural inhomogeneities, the critical temperature and the critical current density varied within the crystal even for samples processed isothermally, despite the narrow solid solution range of the Nd-123 phase under a reduced pO2 atmosphere.

Cents and Sustainability : Securing Our Common Future by Decoupling Economic Growth from Environmental Pressures

The focus of Cents and Sustainability is to respond to the call by Dr Gro Brundtland in the seminal book Our Common Future to achieve, 'a new era of economic growth - growth that is forceful and at the same time socially and environmentally sustainable'. With the 20th anniversary of Our Common Future in 2007, it is clearly time to re-examine this important work in a modern global context. Using the framework of ‘Decoupling Economic Growth from Environmental Pressures’, Cents and Sustainability investigates a range of new evidence and research in order to develop a deeper understanding of how, and under what conditions, this 'forceful sustainable growth' is possible. With an introduction by Dr Jim MacNeill (former Secretary General to the Brundtland Commission, and former Director, OECD Environment Directorate 1978 -1984), the book will carry forewords from Dr Gro Brundtland (former Chair of the World Commission on Environment and Development), Dr Rajendra Pachauri (Chief, Intergovernmental Panel on Climate Change (IPCC), and joint recipient of the 2007 Nobel Peace Prize on behalf of the IPCC), and Dr Kenneth Ruffing (former Deputy Director and Chief Economist of the OECD Environment Directorate 2000 - 2005). Beginning with a detailed explanation of decoupling theory, along with investigation into a range of issues and barriers to its achievement, the book then focuses on informing national strategies for decoupling. Then putting this into action the book focuses on five key areas of decoupling, namely greenhouse gas emissions, biodiversity, freshwater extraction, waste production, and air pollution, and in each case showing compelling evidence for significant cost effective reductions in environmental pressures. The book concludes with a detailed case study of the groundbreaking application of public interest litigation to combat air pollution in Delhi, India.

Study of Pinna nobilis growth from inner record: How biased are posterior adductor muscle scars estimates?

Previous studies have shown that the external growth records of the posterior adductor muscle scar (PAMS) of the bivalve Pinna nobilis are incomplete and do not produce accurate age estimations. We have developed a new methodology to study age and growth using the inner record of the PAMS, which avoids the necessity of costly in situ shell measurements or isotopic studies. Using the inner record we identified the positions of PAMS previously obscured by nacre and estimated the number of missing records in adult specimens with strong abrasion of the calcite layer in the anterior portion of the shell. The study of the PAMS and inner record of two shells that were 6 years old when collected showed that only 2 and 3 PAMS were observed, while 6 inner records could be counted, thus confirming our working methodology. Growth parameters of a P. nobilis population located in Moraira, Spain (western Mediterranean) were estimated with the new methodology and compared to those obtained using PAMS data and in situ measurements. For the comparisons, we applied different models considering the data alternatively as length-at-age (LA) and tag-recapture (TR). Among every method we tested to fit the Von Bertalanffy growth model, we observed that LA data from inner record fitted to the model using non-linear mixed effects and the estimation of missing records using the calcite width was the most appropriate. The equation obtained with this method, L = 573*(1 - e(-0.16(t-0.02))), is very similar to that calculated previously from in situ measurements for the same population.

A maximum likelihood approach for estimating growth from tag–recapture data

The Fabens method is commonly used to estimate growth parameters k and l infinity in the von Bertalanffy model from tag-recapture data. However, the Fabens method of estimation has an inherent bias when individual growth is variable. This paper presents an asymptotically unbiassed method using a maximum likelihood approach that takes account of individual variability in both maximum length and age-at-tagging. It is assumed that each individual's growth follows a von Bertalanffy curve with its own maximum length and age-at-tagging. The parameter k is assumed to be a constant to ensure that the mean growth follows a von Bertalanffy curve and to avoid overparameterization. Our method also makes more efficient use nf thp measurements at tno and recapture and includes diagnostic techniques for checking distributional assumptions. The method is reasonably robust and performs better than the Fabens method when individual growth differs from the von Bertalanffy relationship. When measurement error is negligible, the estimation involves maximizing the profile likelihood of one parameter only. The method is applied to tag-recapture data for the grooved tiger prawn (Penaeus semisulcatus) from the Gulf of Carpentaria, Australia.

Directional grain growth from anisotropic kinetic roughening of grain boundaries in sheared colloidal crystals

The fabrication of functional materials via grain growth engineering implicitly relies on altering the mobilities of grain boundaries (GBs) by applying external fields. Although computer simulations have alluded to kinetic roughening as a potential mechanism for modifying GB mobilities, its implications for grain growth have remained largely unexplored owing to difficulties in bridging the widely separated length and time scales. Here, by imaging GB particle dynamics as well as grain network evolution under shear, we present direct evidence for kinetic roughening of GBs and unravel its connection to grain growth in driven colloidal polycrystals. The capillary fluctuation method allows us to quantitatively extract shear-dependent effective mobilities. Remarkably, our experiments reveal that for sufficiently large strains, GBs with normals parallel to shear undergo preferential kinetic roughening, resulting in anisotropic enhancement of effective mobilities and hence directional grain growth. Single-particle level analysis shows that the mobility anisotropy emerges from strain-induced directional enhancement of activated particle hops normal to the GB plane. We expect our results to influence materials fabrication strategies for atomic and block copolymeric polycrystals as well.

Nucleation of a Stable Solid from Melt in the Presence of Multiple Metastable Intermediate Phases: Wetting, Ostwald's Step Rule, and Vanishing Polymorphs

In many systems, nucleation of a stable solid may occur in the presence of other (often more than one) metastable phases. These may be polymorphic solids or even liquid phases. Sometimes, the metastable phase might have a lower free energy minimum than the liquid but higher than the stable-solid-phase minimum and have characteristics in between the parent liquid and the globally stable solid phase. In such cases, nucleation of the solid phase from the melt may be facilitated by the metastable phase because the latter can ``wet'' the interface between the parent and the daughter phases, even though there may be no signature of the existence of metastable phase in the thermodynamic properties of the parent liquid and the stable solid phase. Straightforward application of classical nucleation theory (CNT) is flawed here as it overestimates the nucleation barrier because surface tension is overestimated (by neglecting the metastable phases of intermediate order) while the thermodynamic free energy gap between daughter and parent phases remains unchanged. In this work, we discuss a density functional theory (DFT)-based statistical mechanical approach to explore and quantify such facilitation. We construct a simple order-parameter-dependent free energy surface that we then use in DFT to calculate (i) the order parameter profile, (ii) the overall nucleation free energy barrier, and (iii) the surface tension between the parent liquid and the metastable solid and also parent liquid and stable solid phases. The theory indeed finds that the nucleation free energy barrier can decrease significantly in the presence of wetting. This approach can provide a microscopic explanation of the Ostwald step rule and the well-known phenomenon of ``disappearing polymorphs'' that depends on temperature and other thermodynamic conditions. Theory reveals a diverse scenario for phase transformation kinetics, some of which may be explored via modem nanoscopic synthetic methods.

Carbon nanoflake growth from carbon nanotubes by hot filament chemical vapor deposition

We report the growth of carbon nanoflakes (CNFs) on Si substrate by the hot filament chemical vapor deposition without the substrate bias or the catalyst. CNFs were grown using the single wall carbon nanotubes and the multiwall carbon nanotubes as the nucleation center, in the Ar-rich CH4-H-2-Ar precursor gas mixture with 1% CH4, at the chamber pressure and the substrate temperature of 7.5 Ton and 840 degrees C, respectively. In the H-2-rich condition, CNF synthesis failed due to severe etch-removal of carbon nanotubes (CNTs) while it was successful at the optimized Ar-rich condition. Other forms of carbon such as nano-diamond or mesoporous carbon failed to serve as the nucleation centers for the CNF growth. We proposed a mechanism of the CNF synthesis from the CNTs, which involved the initial unzipping of CNTs by atomic hydrogen and subsequent nucleation and growth of CNFs from the unzipped portion of the graphene layers. (C) 2013 Elsevier Ltd. All rights reserved.

Fatigue crack-growth from a circular notch under high-levels of biaxial stress

A series of experiments have been conducted on cruciform specimens to investigate fatigue crack growth from circular notches under high levels of biaxial stress. Two stress levels (Δσ₁= 380 and 560 MPa) and five stress biaxialities (λ=+1.0, +0.5, 0, −0.5 and −1.0; where λ=σ₂/σ₁ were adopted in the fatigue tests in type 316 stainless steel having a monotonic yield strength of 243 MPa. The results reveal that fatigue crack growth rates are markedly influenced by both the stress amplitude and the stress biaxiality. A modified model has been developed to describe fatigue crack growth under high levels of biaxial stress.

Growth and spectral properties of Er:Gd2SiO5 crystal

Er3+ -doped Gd2SiO5 (Er:GSO) single crystal with dimensions of circle divide 35 x 40 mm(3) has been grown by the Czochralski method. The absorption and fluorescence spectra of the Er:GSO crystal were measured at room temperature. The spectral parameters were calculated based on Judd-Ofelt theory, and the intensity parameters Omega(2), Omega(4) and Omega 6 are obtained to be 6.168 x 10(-20), 1.878 x 10(-20), and 1.255 x 10(-20) cm(2), respectively. The emission cross-section has been calculated by Fuechtbauer-Ladenbury formula. (c) 2007 Elsevier B.V. All rights reserved.