6,081 research outputs found
Variation of the glass transition temperature with rigidity and chemical composition
The effects of flexibility and chemical composition in the variation of the
glass transition temperature are obtained by using the Lindemann criteria, that
relates melting temperature with atomic vibrations. Using this criteria and
that floppy modes at low frequencies enhance in a considerable way the average
cuadratic displacement, we show that the consequence is a modified glass
transition temperature. This approach allows to obtain in a simple way the
empirically modified Gibbs-DiMarzio law, which has been widely used in
chalcogenide glasses to fit the changes in the glass transition temperature
with the chemical composition . The method predicts that the constant that
appears in the law depends upon the ratio of two characteristic frequencies (or
temperatures). Then, the constant for the Se-Ge-As glass is estimated by using
the experimental density of vibrational states, and the result shows a very
good agreement with the experimental fit from glass transition temperature
variation
Self-organization with equilibration: a model for the intermediate phase in rigidity percolation
Recent experimental results for covalent glasses suggest the existence of an
intermediate phase attributed to the self-organization of the glass network
resulting from the tendency to minimize its internal stress. However, the exact
nature of this experimentally measured phase remains unclear. We modify a
previously proposed model of self-organization by generating a uniform sampling
of stress-free networks. In our model, studied on a diluted triangular lattice,
an unusual intermediate phase appears, in which both rigid and floppy networks
have a chance to occur, a result also observed in a related model on a Bethe
lattice by Barre et al. [Phys. Rev. Lett. 94, 208701 (2005)]. Our results for
the bond-configurational entropy of self-organized networks, which turns out to
be only about 2% lower than that of random networks, suggest that a
self-organized intermediate phase could be common in systems near the rigidity
percolation threshold.Comment: 9 pages, 6 figure
Self-organized criticality in the intermediate phase of rigidity percolation
Experimental results for covalent glasses have highlighted the existence of a
new self-organized phase due to the tendency of glass networks to minimize
internal stress. Recently, we have shown that an equilibrated self-organized
two-dimensional lattice-based model also possesses an intermediate phase in
which a percolating rigid cluster exists with a probability between zero and
one, depending on the average coordination of the network. In this paper, we
study the properties of this intermediate phase in more detail. We find that
microscopic perturbations, such as the addition or removal of a single bond,
can affect the rigidity of macroscopic regions of the network, in particular,
creating or destroying percolation. This, together with a power-law
distribution of rigid cluster sizes, suggests that the system is maintained in
a critical state on the rigid/floppy boundary throughout the intermediate
phase, a behavior similar to self-organized criticality, but, remarkably, in a
thermodynamically equilibrated state. The distinction between percolating and
non-percolating networks appears physically meaningless, even though the
percolating cluster, when it exists, takes up a finite fraction of the network.
We point out both similarities and differences between the intermediate phase
and the critical point of ordinary percolation models without
self-organization. Our results are consistent with an interpretation of recent
experiments on the pressure dependence of Raman frequencies in chalcogenide
glasses in terms of network homogeneity.Comment: 20 pages, 18 figure
How Do State–Business Relations Shape Sustainable Development?
The achievement of the UN Sustainable Development Goals will depend on the ways in which states and businesses engage with one another. While state–business interactions can take many forms, they inherently involve processes of negotiation through which actors in both camps pursue their own interests. Successfully accelerating sustainability, generating inclusion or reducing inequalities will depend on whether such negotiations build on and support interdependencies, create trust, and develop shared ideas about challenges and potential solutions. But the factors that determine the nature and outcomes of state–business relations are not yet well-enough understood, particularly in relation to goals beyond economic growth, where trade-offs are often more apparent.UK Department for International Developmen
New multivariable capabilities of the INCA program
The INteractive Controls Analysis (INCA) program was developed at NASA's Goddard Space Flight Center to provide a user friendly, efficient environment for the design and analysis of control systems, specifically spacecraft control systems. Since its inception, INCA has found extensive use in the design, development, and analysis of control systems for spacecraft, instruments, robotics, and pointing systems. The (INCA) program was initially developed as a comprehensive classical design analysis tool for small and large order control systems. The latest version of INCA, expected to be released in February of 1990, was expanded to include the capability to perform multivariable controls analysis and design
Addressing Market Constraints to Providing Nutrient-Rich Foods: An Exploration of Market Systems Approaches
This Evidence Report asks how a market systems approach could be applied to improve
poor households’ access to nutrient-dense foods. By ‘market systems approach’ we mean methods that identify and address underlying constraints in market transactions, their
supporting functions and the institutional environment in which markets operate, and which are preventing markets from delivering desired outcomes. The report builds on a developing body of research on value chain approaches to nutrition, which has highlighted challenges in developing commercially viable business models that can deliver affordable, available, appropriate and acceptable foods. It explores how market systems approaches could be used to improve the diagnosis of constraints preventing a market from achieving these outcomes, and to develop interventions that change the way the market system works. The report does not seek to make specific recommendations for policy or practice, but rather sets out how a market systems approach could be applied to nutrition in different contexts in order to develop such recommendations.UK Department for International Developmen
Asymmetry of jets, lobe size and spectral index in radio galaxies and quasars
We investigate the correlations between spectral index, jet side and extent
of the radio lobes for a sample of nearby FRII radio galaxies. In
Dennett-Thorpe et al. (1997) we studied a sample of quasars and found that the
high surface brightness regions had flatter spectra on the jet side (explicable
as a result of Doppler beaming) whilst the extended regions had spectral
asymmetries dependent on lobe length. Unified schemes predict that asymmetries
due to beaming will be much smaller in narrow-line radio galaxies than in
quasars: we therefore investigate in a similar manner, a sample of radio
galaxies with detected jets. We find that spectral asymmetries in these objects
are uncorrelated with jet sidedness at all brightness levels, but depend on
relative lobe volume. Our results are not in conflict with unified schemes, but
suggest that the differences between the two samples are due primarily to power
or redshift, rather than to orientation. We also show directly that hotspot
spectra steepen as a function of radio power or redshift. Whilst a shift in
observed frequency due to the redshift may account for some of the steepening,
it cannot account for all of it, and a dependence on radio power is required.Comment: accepted for publication in MNRAS, 10 pages; typos/minor correctio
Time Domain Simulations of Arm Locking in LISA
Arm locking is a technique that has been proposed for reducing laser
frequency fluctuations in the Laser Interferometer Space Antenna (LISA), a
gravitational-wave observatory sensitive in the milliHertz frequency band. Arm
locking takes advantage of the geometric stability of the triangular
constellation of three spacecraft that comprise LISA to provide a frequency
reference with a stability in the LISA measurement band that exceeds that
available from a standard reference such as an optical cavity or molecular
absorption line. We have implemented a time-domain simulation of arm locking
including the expected limiting noise sources (shot noise, clock noise,
spacecraft jitter noise, and residual laser frequency noise). The effect of
imperfect a priori knowledge of the LISA heterodyne frequencies and the
associated 'pulling' of an arm locked laser is included. We find that our
implementation meets requirements both on the noise and dynamic range of the
laser frequency.Comment: Revised to address reviewer comments. Accepted by Phys. Rev.
Simulator test to study hot-flow problems related to a gas cooled reactor
An advance study of materials, fuel injection, and hot flow problems related to the gas core nuclear rocket is reported. The first task was to test a previously constructed induction heated plasma GCNR simulator above 300 kW. A number of tests are reported operating in the range of 300 kW at 10,000 cps. A second simulator was designed but not constructed for cold-hot visualization studies using louvered walls. A third task was a paper investigation of practical uranium feed systems, including a detailed discussion of related problems. The last assignment resulted in two designs for plasma nozzle test devices that could be operated at 200 atm on hydrogen
Reinforced structural plastics
Reinforced polyimide structures are described. Reinforcing materials are impregnated with a suspension of polyimide prepolymer and bonded together by heat and pressure to form a cured, hard-reinforced, polyimide structure
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