961 research outputs found
Degradation of epoxy coatings under gamma irradiation
Epoxy networks based on Diglycidyl ether of bisphenol A (DGEBA) and cured with Jeffamines (POPA) or polyamidoamine (PAA) were gamma irradiated at 25 1C in air. Dose rates of 50, 200 or 2000 Gy h- 1 for doses up 100 kGy were used. Structural changes were monitored by IR spectrophotometry, DSC and sol–gel analysis. Both networks display some common features: for I Z 200 Gy h- 1, reaction products grow proportionally to time and the rate is a decreasing function of dose rate. The simplest explanation is that peroxy radicals are the main precursors of these products (in the dose rate domain under study), through a unimolecular rearrangement of which an hypothetical mechanism is proposed. DGEBA–POPA are more reactive then DGEBA–PAA networks (according to IR criteria), that can be attributed to the high reactivity of tertiary CH bands in polyoxypropylene segments. The oxidation of these sites leads to methyl ketones. A simple kinetic model in which methyl ketones result from rearrangements of tertiary peroxyls and from tertiary alkoxyls was proposed. It leads to an expression of the radiochemical yield of methyl ketones (G(MK)) of the form GðMKÞ¼ a þ bI-1=2 where a and b are parameters depending of elementary rate constants. Experimental G(MK) values are reasonably well fitted by this equation. In DGEBA–PAA networks, a wide variety of oxidation products, among which amides predominate, can be observed. In these networks, chain scissions predominate over crosslinking, whereas a slight predominance of crosslinking was observed, at least for the lowest dose rate, in DGEBA–POPA
Thermo-oxidative aging of epoxy coating systems
tThe thermo-oxidative behavior of unformulated (unfilled) samples of epoxy coatings has been studiedat five temperatures ranging from 70◦C to 150◦C. Two epoxy networks based on diglycidyl ether ofbisphenol A (DGEBA), respectively, cured by jeffamine (POPA) or polyamidoamine (PAA) were compared.Infrared spectrophotometry (IR), differential scanning (DSC) and sol–gel analysis (SGA) were used tomonitor structural changes.Thermal oxidation leads to carbonyl and amide formation in both systems. POPA systems appear moresensitive to oxidation than PAA ones. Thermal oxidation leads to predominant chain scission as evidencedby the decrease of glass transition temperatures (Tg) and increase of sol fraction
On the functions counting walks with small steps in the quarter plane
Models of spatially homogeneous walks in the quarter plane
with steps taken from a subset of the set of jumps to the eight
nearest neighbors are considered. The generating function of the numbers of such walks starting at the origin and
ending at after steps is studied. For all
non-singular models of walks, the functions and are continued as multi-valued functions on having
infinitely many meromorphic branches, of which the set of poles is identified.
The nature of these functions is derived from this result: namely, for all the
51 walks which admit a certain infinite group of birational transformations of
, the interval of variation of splits into
two dense subsets such that the functions and are shown to be holonomic for any from the one of them and
non-holonomic for any from the other. This entails the non-holonomy of
, and therefore proves a conjecture of
Bousquet-M\'elou and Mishna.Comment: 40 pages, 17 figure
-Martin boundary of killed random walks in the quadrant
We compute the -Martin boundary of two-dimensional small steps random
walks killed at the boundary of the quarter plane. We further provide explicit
expressions for the (generating functions of the) discrete -harmonic
functions. Our approach is uniform in , and shows that there are three
regimes for the Martin boundary.Comment: 18 pages, 2 figures, to appear in S\'eminaire de Probabilit\'e
Phase diagram of a generalized ABC model on the interval
We study the equilibrium phase diagram of a generalized ABC model on an
interval of the one-dimensional lattice: each site is occupied by a
particle of type \a=A,B,C, with the average density of each particle species
N_\a/N=r_\a fixed. These particles interact via a mean field
non-reflection-symmetric pair interaction. The interaction need not be
invariant under cyclic permutation of the particle species as in the standard
ABC model studied earlier. We prove in some cases and conjecture in others that
the scaled infinite system N\rw\infty, i/N\rw x\in[0,1] has a unique
density profile \p_\a(x) except for some special values of the r_\a for
which the system undergoes a second order phase transition from a uniform to a
nonuniform periodic profile at a critical temperature .Comment: 25 pages, 6 figure
Combining astrometry and JUICE -- Europa Clipper radio science to improve the ephemerides of the Galilean moons
The upcoming JUICE and Europa Clipper missions to Jupiter's Galilean
satellites will provide radio science tracking measurements of both spacecraft.
Such data are expected to significantly help estimating the moons' ephemerides
and related dynamical parameters. However, the two missions will yield an
imbalanced dataset, with no flybys planned at Io, condensed over less than six
years. Current ephemerides' solutions for the Galilean moons, on the other
hand, rely on ground-based astrometry collected over more than a century which,
while being less accurate, bring very valuable constraints on the long-term
dynamics of the system. An improved solution for the Galilean satellites'
complex dynamics could however be achieved by exploiting the existing synergies
between these different observation sets. To quantify this, we merged simulated
JUICE and Clipper radio science data with existing ground-based astrometric and
radar observations, and performed the inversion. Our study specifically
focusses on the resulting formal uncertainties in the moons' states, as well as
Io's and Jupiter's tidal dissipation parameters. Adding astrometry stabilises
the moons' state solution, especially beyond the missions' timelines. It
furthermore reduces the uncertainties in (inverse of the tidal quality
factor) by a factor two to four for Jupiter, and about 30-35\% for Io. Among
all data types, classical astrometry data prior to 1960 proved particularly
beneficial. We also show that ground observations of Io add the most to the
solution, confirming that ground observations can fill the lack of radio
science data for this specific moon. We obtained a noticeable solution
improvement when exploiting the complementarity between all different
observation sets. These promising simulation results thus motivate future
efforts to achieve a global solution from actual JUICE and Clipper radio
science data
Entrepreneurial intention studies: A hybrid bibliometric method to identify new directions for theory and research
Fragmentation is the main obstacle to scientific progress on entrepreneurial intention. To address this issue, we systematise the current literature with a hybrid bibliometric method that combines co-citation and bibliographic coupling analysis for the first time in entrepreneurial intention studies to show the field's knowledge base and research fronts and to examine how divergent perspectives have challenged the core knowledge of the field. We highlight three recurring dimensions of entrepreneurial intention studies: (1) personal factors, (2) social factors and (3) investigational settings. In addition to introducing new constructs, divergent perspectives have emphasised the interplay between these components and challenged the mechanisms connecting them. Based on these findings, we extend previous classifications in the literature by providing a framework that integrates divergent perspectives with the field's knowledge base, helping establish future research avenues and improving the theorising process of entrepreneurial intention
Combining astrometry and JUICE-Europa Clipper radio science to improve the ephemerides of the Galilean moons
Context. The upcoming JUICE and Europa Clipper missions targeting Jupiter s Galilean satellites will provide radio science tracking measurements of both spacecraft. Such data are expected to significantly help estimating the moons ephemerides and related dynamical parameters (e.g. tidal dissipation parameters). However, the two missions will yield an imbalanced dataset, with no flybys planned at Io, condensed over less than six years. Current ephemerides solutions for the Galilean moons, on the other hand, rely on ground-based astrometry collected over more than a century which, while being less accurate, bring very valuable constraints on the long-term dynamics of the system. Aims. An improved solution for the Galilean satellites complex dynamics could however be achieved by exploiting the existing synergies between these different observation sets. Methods. To quantify this, we merged simulated radio science data from both JUICE and Europa Clipper spacecraft with existing ground-based astrometric and radar observations, and performed the inversion in different configurations: either adding all available ground observations or individually assessing the contribution of different data subsets. Our discussion specifically focusses on the resulting formal uncertainties in the moons states, as well as Io s and Jupiter s tidal dissipation parameters. Results. Adding astrometry stabilises the moons state solution, especially beyond the missions timelines. It furthermore reduces the uncertainties in 1/Q (inverse of the tidal quality factor) by a factor two to four for Jupiter, and about 30- 35% for Io. Among all data types, classical astrometry data prior to 1960 proved particularly beneficial. Overall, we also show that ground observations of Io add the most to the solution, confirming that ground observations can fill the lack of radio science data for this specific moon. Conclusions. We obtained a noticeable solution improvement when making use of the complementarity between all different observation sets. The promising results obtained with simulations thus motivate future efforts to achieve a global solution from actual JUICE and Clipper radio science measurements
On the dynamical behavior of the ABC model
We consider the ABC dynamics, with equal density of the three species, on the
discrete ring with sites. In this case, the process is reversible with
respect to a Gibbs measure with a mean field interaction that undergoes a
second order phase transition. We analyze the relaxation time of the dynamics
and show that at high temperature it grows at most as while it grows at
least as at low temperature
Indirect ultraviolet photodesorption from CO:N2 binary ices - an efficient grain-gas process
UV ice photodesorption is an important non-thermal desorption pathway in many
interstellar environments that has been invoked to explain observations of cold
molecules in disks, clouds and cloud cores. Systematic laboratory studies of
the photodesorption rates, between 7 and 14 eV, from CO:N2 binary ices, have
been performed at the DESIRS vacuum UV beamline of the synchrotron facility
SOLEIL. The photodesorption spectral analysis demonstrates that the
photodesorption process is indirect, i.e. the desorption is induced by a photon
absorption in sub-surface molecular layers, while only surface molecules are
actually desorbing. The photodesorption spectra of CO and N2 in binary ices
therefore depend on the absorption spectra of the dominant species in the
subsurface ice layer, which implies that the photodesorption efficiency and
energy dependence are dramatically different for mixed and layered ices
compared to pure ices. In particular, a thin (1-2 ML) N2 ice layer on top of CO
will effectively quench CO photodesorption, while enhancing N2 photodesorption
by a factors of a few (compared to the pure ices) when the ice is exposed to a
typical dark cloud UV field, which may help to explain the different
distributions of CO and N2H+ in molecular cloud cores. This indirect
photodesorption mechanism may also explain observations of small amounts of
complex organics in cold interstellar environments.Comment: 21 pages 5 figure
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