2,237 research outputs found
Understanding friction mechanisms of Si-DLC/steel interfaces under aqueous lubrication
A key driver in current research on lubricant formulation is the need to move away from older technology that is highly reliant on resources derived from industries associated with high carbon dioxide emissions. In this paper, the adoption of water based (or aqueous) lubrication is explored. This is in direct contrast with most lubricated systems that rely on oil or other petroleum products. In nature, most known biological systems employ aqueous lubrication for tribological contacts, such as those found in cartilage and more widely in mammalian joints including hips and knees giving friction coefficients as low as ÎŒ < 0.002. This is achieved very effectively without the presence of an oil or grease as a base lubricant. In most engineering applications, however, oils and greases are used to achieve desired low friction levels. While effective, this comes with the associated higher costs and carbon footprint of using petroleum derived products. In recent years, certain engineering applications have shifted to aqueous lubrication, a notable example of which is stern tube bearings in maritime applications. These are typically low pressure contacts though. Depending on speed of travel this can range from 100â400 MPa. The research detailed in this paper explores the viability of high pressure contacts lubricated with novel aqueous packages and what effects this shift may have on friction and wear profiles of the system. The work reported herein demonstrates that with some modifications, effective lubrication can be achieved using aqueous lubricant packages
Unitary relation between a harmonic oscillator of time-dependent frequency and a simple harmonic oscillator with and without an inverse-square potential
The unitary operator which transforms a harmonic oscillator system of
time-dependent frequency into that of a simple harmonic oscillator of different
time-scale is found, with and without an inverse-square potential. It is shown
that for both cases, this operator can be used in finding complete sets of wave
functions of a generalized harmonic oscillator system from the well-known sets
of the simple harmonic oscillator. Exact invariants of the time-dependent
systems can also be obtained from the constant Hamiltonians of unit mass and
frequency by making use of this unitary transformation. The geometric phases
for the wave functions of a generalized harmonic oscillator with an
inverse-square potential are given.Comment: Phys. Rev. A (Brief Report), in pres
The Random-bond Potts model in the large-q limit
We study the critical behavior of the q-state Potts model with random
ferromagnetic couplings. Working with the cluster representation the partition
sum of the model in the large-q limit is dominated by a single graph, the
fractal properties of which are related to the critical singularities of the
random Potts model. The optimization problem of finding the dominant graph, is
studied on the square lattice by simulated annealing and by a combinatorial
algorithm. Critical exponents of the magnetization and the correlation length
are estimated and conformal predictions are compared with numerical results.Comment: 7 pages, 6 figure
Legitimating inaction : differing identity constructions of the Scots language.
The Scots language plays a key role in the political and cultural landscape of contemporary Scotland. From a discourse-historical perspective, this article explores how language ideologies about the Scots language are realized linguistically in a so-called âlanguages strategyâ drafted by the Scottish Executive, and in focus groups consisting of Scottish people. This article shows that although the decline of Scots is said to be a âtragedyâ, focus group participants seem to reject the notion of Scots as a viable, contemporary language that can be used across a wide range of registers. The policy document also seems to construct Scots in very positive terms, but is shown to be unhelpful or potentially even damaging in the process of changing public attitudes to Scots
An inverted-sandwich diuranium Ό-η5:η5-cyclo-P5 complex supported by U-P5 Ύ-bonding
Reaction of [U(TrenTIPS)] [1, TrenTIPS=N(CH2CH2NSiiPr3)3] with 0.25â
equivalents of P4 reproducibly affords the unprecedented actinide inverted sandwich cyclo-P5 complex [{U(TrenTIPS)}2(ÎŒ-η5:η5-cyclo-P5)] (2). All prior examples of cyclo-P5 are stabilized by d-block metals, so 2 shows that cyclo-P5 does not require d-block ions to be prepared. Although cyclo-P5 is isolobal to cyclopentadienyl, which usually bonds to metals via Ï- and Ï-interactions with minimal ÎŽ-bonding, theoretical calculations suggest the principal bonding in the U(P5)U unit is polarized ÎŽ-bonding. Surprisingly, the characterization data are overall consistent with charge transfer from uranium to the cyclo-P5 unit to give a cyclo-P5 charge state that approximates to a dianionic formulation. This is ascribed to the larger size and superior acceptor character of cyclo-P5 compared to cyclopentadienyl, the strongly reducing nature of uranium(III), and the availability of uranium ÎŽ-symmetry 5f orbitals
Characterization and wear performance of boride phases over tool steel substrates
This research work was conducted to characterize boride phases, obtained from the powder-pack process, on AISI H13 and D2 steel substrates, and investigate their tribological behavior. The boriding was developed at a temperature of 1273âK with an exposure time of 8âh. X-ray diffraction, scanning electron microscopy, and energy-dispersive X-ray spectroscopy were conducted on the borided material to characterize the presence of the FeB, Fe2B, and CrB phases and the distribution of heavy elements on the surface of the substrates. The adherence of the boride layers was evaluated, in a qualitative form, through the Daimler-Benz Rockwell-C indentation technique. Sliding wear tests were then performed using a reciprocating wear test machine. All tests were conducted in dry conditions at room temperature. A frequency of 10âHz and 15-mm sliding distance were used. The applied Hertzian pressure was 2.01âGPa. Scanning electron microscopy was used to observe and analyze the wear mechanisms. Additionally, the variation of the friction coefficient versus the number of cycles was obtained. Experimental results showed that the characteristic wear mechanism for the borided surface was plastic deformation and mild abrasive wear; for unborided substrates, cracking and spalling were observed
Synthesis and characterization of an fâblock terminal parent imido [U=NH] complex: a masked uranium(IV) nitride
Deprotonation of [U(TrenTIPS)(NH2)] (1) [TrenTIPS = N(CH2CH2NSiPri3)3] with organoalkali metal reagents MR (M = Li, R = But; M = NaâCs, R = CH2C6H5) afforded the imido-bridged dimers [{U-(TrenTIPS)(ÎŒ-N[H]M)}2] [M = L âCs (2aâe)]. Treatmentof 2c (M = K) with 2 equiv of 15 crown-5 ether (15C5) afforded the uranium terminal parent imido complex [U(TrenTIPS)(NH)][K(15C5)2] (3c), which can also be viewed as a masked uranium(IV) nitride. The uraniumâimido linkage was found to be essentially linear, and theoretical calculations suggested Ï2Ï4 polarized UâN multiple bonding. Attempts to oxidize 3c to afford the neutral uranium terminal parent imido complex [U(TrenTIPS)(NH)] (4) resulted in spontaneous disproportionation to give 1 and the uraniumânitride complex [U(TrenTIPS)(N)] (5); this reaction is a new way to prepare the terminal uraniumânitride linkage and was calculated to be exothermic by â3.25 kcal molâ1
CMBfit: Rapid WMAP likelihood calculations with normal parameters
We present a method for ultra-fast confrontation of the WMAP cosmic microwave
background observations with theoretical models, implemented as a publicly
available software package called CMBfit, useful for anyone wishing to measure
cosmological parameters by combining WMAP with other observations. The method
takes advantage of the underlying physics by transforming into a set of
parameters where the WMAP likelihood surface is accurately fit by the
exponential of a quartic or sextic polynomial. Building on previous physics
based approximations by Hu et.al., Kosowsky et.al. and Chu et.al., it combines
their speed with precision cosmology grade accuracy. A Fortran code for
computing the WMAP likelihood for a given set of parameters is provided,
pre-calibrated against CMBfast, accurate to Delta lnL ~ 0.05 over the entire
2sigma region of the parameter space for 6 parameter ``vanilla'' Lambda CDM
models. We also provide 7-parameter fits including spatial curvature,
gravitational waves and a running spectral index.Comment: 14 pages, 8 figures, References added, accepted for publication in
Phys.Rev.D., a Fortran code can be downloaded from
http://space.mit.edu/home/tegmark/cmbfit
Large Deviations in the Superstable Weakly Imperfect Bose Gas
The superstable Weakly Imperfect Bose Gas {(WIBG)} was originally derived to
solve the inconsistency of the Bogoliubov theory of superfluidity. Its
grand-canonical thermodynamics was recently solved but not at {point of} the
{(first order)} phase transition. This paper proposes to close this gap by
using the large deviations formalism and in particular the analysis of the Kac
distribution function. It turns out that, as a function of the chemical
potential, the discontinuity of the Bose condensate density at the phase
transition {point} disappears as a function of the particle density. Indeed,
the Bose condensate continuously starts at the first critical particle density
and progressively grows but the free-energy per particle stays constant until
the second critical density is reached. At higher particle densities, the Bose
condensate density as well as the free-energy per particle both increase
{monotonously}
Anisotropic Bose-Einstein condensates and completely integrable dynamical systems
A Gaussian ansatz for the wave function of two-dimensional harmonically
trapped anisotropic Bose-Einstein condensates is shown to lead, via a
variational procedure, to a coupled system of two second-order, nonlinear
ordinary differential equations. This dynamical system is shown to be in the
general class of Ermakov systems. Complete integrability of the resulting
Ermakov system is proven. Using the exact solution, collapse of the condensate
is analyzed in detail. Time-dependence of the trapping potential is allowed
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