Importance of Shear in the bcc-to-hcp Transformation in Iron

Iron shows a pressure-induced martensitic phase transformation from the ground state ferromagnetic bcc phase to a nonmagnetic hcp phase at ≈13 GPa. The exact transformation pressure (TP) and pathway are not known. Here we present a multiscale model containing a quantum-mechanics-based multiwell energy function accounting for the bcc and hcp phases of Fe and a construction of kinematically compatible and equilibrated mixed phases. This model suggests that shear stresses have a significant influence on the bcc↔hcp transformation. In particular, the presence of modest shear accounts for the scatter in measured TPs. The formation of mixed phases also provides an explanation for the observed hysteresis in TP

Analytic binary alloy volume-concentration relations and the deviation from Zen`s law

Alloys expand or contract as concentrations change, and the resulting relationship between atomic volume and alloy content is an important property of the solid. While a well-known approximation posits that the atomic volume varies linearly with concentration (Zen`s law), the actual variation is more complicated. Here we use an apparent size of the solute (solvent) atom and the elasticity to derive explicit analytical expressions for the atomic volume of binary solid alloys. Two approximations, continuum and terminal, are proposed. Deviations from Zen`s law are studied for 22 binary alloy systems

Exploring the Contribution of Parental Perceptions to Childhood Anxiety

Parental rearing practices such as over-involvement are associated with childhood anxiety; however, little is known about the contribution of parental perceptions to child anxiety. This study explores the relationship between maternal and paternal perceptions of parenting and childhood anxiety. The perceived rearing behaviors and parental sense of competence (i.e., satisfaction and efficacy) of the parents of anxious children (n = 59) were compared with those of a non-clinical control sample (n = 44). In line with the findings from the literature that addresses externalizing disorders, parental sense of competence was significantly associated with childhood outcomes. Logistical regression suggested that paternal efficacy beliefs, acceptance, and maternal satisfaction were associated with an absence of clinical anxiety and lower levels of anxiety symptoms in children. Parental perceptions may thus provide an important area for understanding childhood anxiety

Usability and Usage of Interactive Features in an Online Ebook for CS Teachers

There are too few secondary school computing teachers to meet international needs for growing secondary school computing education. Our group has created an ebook to help prepare secondary teachers to teach the programming and big data concepts in the new AP Computer Science Principles course. The ebook was designed using principles from educational psychology, specifically worked examples and cognitive load. The ebook interleaves worked examples and interactive practice activities, which we believe will lead to more efficient and effective learning than more typical approaches to learning programming. This paper reports the results from initial studies of our ebook. First, we conducted a usability study comparing three different ebook platforms. Next, we conducted a study of teacher use of the ebook. Ten teachers worked through the first eight chapters of the ebook at their own pace. Five of the ten teachers completed the first eight chapters which is a 50% completion rate. Significantly, teachers who used more of the interactive features in the ebook did better on the post-tests and reported higher confidence in their ability to teach the material than teachers who used few of the interactive features

An atmospheric perspective on North American carbon dioxide exchange: CarbonTracker

We present an estimate of net CO2 exchange between the terrestrial biosphere and the atmosphere across North America for every week in the period 2000 through 2005. This estimate is derived from a set of 28,000 CO2 mole fraction observations in the global atmosphere that are fed into a state-of-the-art data assimilation system for CO2 called CarbonTracker. By design, the surface fluxes produced in CarbonTracker are consistent with the recent history of CO2 in the atmosphere and provide constraints on the net carbon flux independent from national inventories derived from accounting efforts. We find the North American terrestrial biosphere to have absorbed –0.65 PgC/yr (1 petagram = 10^15 g; negative signs are used for carbon sinks) averaged over the period studied, partly offsetting the estimated 1.85 PgC/yr release by fossil fuel burning and cement manufacturing. Uncertainty on this estimate is derived from a set of sensitivity experiments and places the sink within a range of –0.4 to –1.0 PgC/yr. The estimated sink is located mainly in the deciduous forests along the East Coast (32%) and the boreal coniferous forests (22%). Terrestrial uptake fell to –0.32 PgC/yr during the large-scale drought of 2002, suggesting sensitivity of the contemporary carbon sinks to climate extremes. CarbonTracker results are in excellent agreement with a wide collection of carbon inventories that form the basis of the first North American State of the Carbon Cycle Report (SOCCR), to be released in 2007. All CarbonTracker results are freely available at http://carbontracker.noaa.gov

Atoms in the Surf: Molecular Dynamics Simulation of the Kelvin-Helmholtz Instability using 9 Billion Atoms

We present a fluid dynamics video showing the results of a 9-billion atom molecular dynamics simulation of complex fluid flow in molten copper and aluminum. Starting with an atomically flat interface, a shear is imposed along the copper-aluminum interface and random atomic fluctuations seed the formation of vortices. These vortices grow due to the Kelvin-Helmholtz instability. The resulting vortical structures are beautifully intricate, decorated with secondary instabilities and complex mixing phenomena. This work performed under the auspices of the U.S. Department of Energy by Lawrence Livermore National Laboratory under Contract DE-AC52-07NA27344.Comment: Description of video submitted to APS DFD Gallery of Fluid Motion 200