46,705 research outputs found
The Feasibility of a Fully Miniaturized Magneto-Optical Trap for Portable Ultracold Quantum Technology
Experiments using laser cooled atoms and ions show real promise for practical
applications in quantum- enhanced metrology, timing, navigation, and sensing as
well as exotic roles in quantum computing, networking and simulation. The heart
of many of these experiments has been translated to microfabricated platforms
known as atom chips whose construction readily lend themselves to integration
with larger systems and future mass production. To truly make the jump from
laboratory demonstrations to practical, rugged devices, the complex surrounding
infrastructure (including vacuum systems, optics, and lasers) also needs to be
miniatur- ized and integrated. In this paper we explore the feasibility of
applying this approach to the Magneto-Optical Trap; incorporating the vacuum
system, atom source and optical geometry into a permanently sealed micro- litre
system capable of maintaining mbar for more than 1000 days of
operation with passive pumping alone. We demonstrate such an engineering
challenge is achievable using recent advances in semiconductor microfabrication
techniques and materialsComment: 23 pages, 10 figure
Clathration of Volatiles in the Solar Nebula and Implications for the Origin of Titan's atmosphere
We describe a scenario of Titan's formation matching the constraints imposed
by its current atmospheric composition. Assuming that the abundances of all
elements, including oxygen, are solar in the outer nebula, we show that the icy
planetesimals were agglomerated in the feeding zone of Saturn from a mixture of
clathrates with multiple guest species, so-called stochiometric hydrates such
as ammonia hydrate, and pure condensates. We also use a statistical
thermodynamic approach to constrain the composition of multiple guest
clathrates formed in the solar nebula. We then infer that krypton and xenon,
that are expected to condense in the 20-30 K temperature range in the solar
nebula, are trapped in clathrates at higher temperatures than 50 K. Once
formed, these ices either were accreted by Saturn or remained embedded in its
surrounding subnebula until they found their way into the regular satellites
growing around Saturn. In order to explain the carbon monoxide and primordial
argon deficiencies of Titan's atmosphere, we suggest that the satellite was
formed from icy planetesimals initially produced in the solar nebula and that
were partially devolatilized at a temperature not exceeding 50 K during their
migration within Saturn's subnebula. The observed deficiencies of Titan's
atmosphere in krypton and xenon could result from other processes that may have
occurred both prior or after the completion of Titan. Thus, krypton and xenon
may have been sequestrated in the form of XH3+ complexes in the solar nebula
gas phase, causing the formation of noble gas-poor planetesimals ultimately
accreted by Titan. Alternatively, krypton and xenon may have also been trapped
efficiently in clathrates located on the satellite's surface or in its
atmospheric haze.Comment: Accepted for publication in The Astrophysical Journa
Clathrate hydrates as a sink of noble gases in Titan's atmosphere
We use a statistical thermodynamic approach to determine the composition of
clathrate hydrates which may form from a multiple compound gas whose
composition is similar to that of Titan's atmosphere. Assuming that noble gases
are initially present in this gas phase, we calculate the ratios of xenon,
krypton and argon to species trapped in clathrate hydrates. We find that these
ratios calculated for xenon and krypton are several orders of magnitude higher
than in the coexisting gas at temperature and pressure conditions close to
those of Titan's present atmosphere at ground level. Furthermore we show that,
by contrast, argon is poorly trapped in these ices. This trapping mechanism
implies that the gas-phase is progressively depleted in xenon and krypton when
the coexisting clathrate hydrates form whereas the initial abundance of argon
remains almost constant. Our results are thus compatible with the deficiency of
Titan's atmosphere in xenon and krypton measured by the {\it Huygens} probe
during its descent on January 14, 2005. However, in order to interpret the
subsolar abundance of primordial Ar also revealed by {\it Huygens}, other
processes that occurred either during the formation of Titan or during its
evolution must be also invoked.Comment: Astronomy & Astrophysics Letters, in pres
Reduction of phenylacetylenes using Raney Ni-Al alloy, Al powder in the presence of noble metal catalysts in water
Dedicated to Prof. Kenneth Laali on the occasion of his 66th birthday Received mm-dd-yyyy Accepted mm-dd-yyyy Published on line mm-dd-yyyy Dates to be inserted by editorial office Abstract The chemoselective reduction is based on the reaction of Raney Ni–Al alloy with Al powder in water which produces in situ hydrogen to utilize the hydrogenation of the targeted functional groups. Raney Ni–Al alloy with Al powder can reduce phenylacetylenes to the corresponding ethylbenzene (3) in water in excellent yield at 120 °C for 6 h in a sealed tube. In addition, the complete reduction of aromatic ring to ethylcyclohexane (4) required 60 °C for 12 h with Raney Ni–Al alloy, Al powder in the presence of Pt/C. The appropriate selection of the reaction conditions allowed the selective preparation of ethylbenzene as well as ethylcyclohexane from phenylacetylene. 1a : R = H ,1b : R = CH 3 , 1c : R = OCH 3 ,1d : R = C(CH 3)
The catalytic oxidation of organic contaminants in a packed bed reactor
The catalytic oxidation of several hydrocarbons was studied over noble metal and metal oxide catalysts. A fast empirical method was developed to determine the minimum operating temperature required to guarantee complete conversion of the hydrocarbon.\ud
\ud
The influence of the operating parameters such as the inlet concentration and residence time, as well as the chemical character of the component to be oxidized, have been investigated. The results can be described satisfactorily by a simple isothermal, plug flow reactor model and first-order reaction kinetics. In the case of simultaneous oxidation of different components a significant mixture effect was not observed. The presence of water in the feed did significantly inhibit the oxidation of alkanes.\ud
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Of the applied catalysts, Pt was the most effective for the combustion of the alkenes, whereas Pd showed a higher activity for the oxidation of alkanes
Reduction of diphenylacetylene using Al powder in the presence of noble metal catalysts in water
Diphenylacetylenes can be reduced to the corresponding diphenylethanes (2) in water in excellent yield using Al powder and Pd/C at 60 °C for 3 h in a sealed tube. In addition, the complete reduction of both aromatic rings required 80 °C for 15 h with Al powder in the presence of Pt/C. However, the nature of hydrogenated product formed was found to be strongly influenced by the reaction temperature, time, volume of water and the amount of catalyst being employed
Searching for Better Plasmonic Materials
Plasmonics is a research area merging the fields of optics and
nanoelectronics by confining light with relatively large free-space wavelength
to the nanometer scale - thereby enabling a family of novel devices. Current
plasmonic devices at telecommunication and optical frequencies face significant
challenges due to losses encountered in the constituent plasmonic materials.
These large losses seriously limit the practicality of these metals for many
novel applications. This paper provides an overview of alternative plasmonic
materials along with motivation for each material choice and important aspects
of fabrication. A comparative study of various materials including metals,
metal alloys and heavily doped semiconductors is presented. The performance of
each material is evaluated based on quality factors defined for each class of
plasmonic devices. Most importantly, this paper outlines an approach for
realizing optimal plasmonic material properties for specific frequencies and
applications, thereby providing a reference for those searching for better
plasmonic materials.Comment: 27 pages, 6 figures, 2 table
Volatile inventories in clathrate hydrates formed in the primordial nebula
Examination of ambient thermodynamic conditions suggest that clathrate
hydrates could exist in the martian permafrost, on the surface and in the
interior of Titan, as well as in other icy satellites. Clathrate hydrates
probably formed in a significant fraction of planetesimals in the solar system.
Thus, these crystalline solids may have been accreted in comets, in the forming
giant planets and in their surrounding satellite systems. In this work, we use
a statistical thermodynamic model to investigate the composition of clathrate
hydrates that may have formed in the primordial nebula. In our approach, we
consider the formation sequence of the different ices occurring during the
cooling of the nebula, a reasonable idealization of the process by which
volatiles are trapped in planetesimals. We then determine the fractional
occupancies of guests in each clathrate hydrate formed at given temperature.
The major ingredient of our model is the description of the guest-clathrate
hydrate interaction by a spherically averaged Kihara potential with a nominal
set of parameters, most of which being fitted on experimental equilibrium data.
Our model allows us to find that Kr, Ar and N can be efficiently encaged in
clathrate hydrates formed at temperatures higher than 48.5 K in the
primitive nebula, instead of forming pure condensates below 30 K. However, we
find at the same time that the determination of the relative abundances of
guest species incorporated in these clathrate hydrates strongly depends on the
choice of the parameters of the Kihara potential and also on the adopted size
of cages. Indeed, testing different potential parameters, we have noted that
even minor dispersions between the different existing sets can lead to
non-negligible variations in the determination of the volatiles trapped in
clathrate hydrates formed in the primordial nebula.Comment: Accepted for publication in Faraday Discussion
The Success Story of Gold-Based Catalysts for Gas- and Liquid-Phase Reactions: A Brief Perspective and Beyond
Gold has long held the fascination of mankind. For millennia it has found use in art, cosmetic metallurgy and architecture; this element is seen as the ultimate statement of prosperity and beauty. This myriad of uses is made possible by the characteristic inertness of bulk gold; allowing it to appear long lasting and above the tarnishing experienced by other metals, in part providing its status as the most noble meta
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