1,223 research outputs found
Waste-to-fuel opportunities for British quick service restaurants: A case study
The fast food supply chain is facing increasing operating costs due to volatile food and energy prices. Based on a case study of a major fast food logistics operator, this paper quantifies the potential for fuel generation from the waste generated by quick-service restaurants in Britain. Several fuel pathways and supply chains were mapped to understand the carbon intensity of the various waste-to-fuel opportunities, the number of heavy goods vehicles that might be powered and the key factors that could help companies make better informed decisions related to fuel generation from waste. The research suggested that depending on the scenarios considered, between 13.9 and 17.2 million GJ of energy could be obtained from fuels made from the waste arisings of British quick service restaurants and their distribution centres (DCs), representing between 4.4 and 5.8% of the national energy consumption from heavy goods vehicles (HGVs) and well-to-wheel (WTW) greenhouse gases (GHG) savings of between 652 and 898 thousand tonnes of CO2 equivalent annually. Used cooking oil and burger fat arising from British quick-service restaurants could generate enough energy to power up to 3891 HGVs with FAME diesel (B100), 1622 with HVO diesel (B100) or 1943 with biomethane annually. The paper and card generated by these same establishments could also power an additional 4623 biomethane vehicles, wood pallets could power an additional 73 bioethanol trucks and plastics could also power 341 vehicles running with synthetic diesel. The results showed that collections of separate waste fractions by logistics operators could make a relevant contribution towards the decarbonisation of the supply chain while reducing disposal fees and fuel costs. The carbon emissions resulting from this approach depend greatly on the footprint of the collection and transportation systems used to move waste from the restaurants to the processing plants and return the converted fuel back to the distribution centres where the vehicles are refuelled. Logistics firms are in a privileged position to manage these flows as they can use empty back-haul trips to collect and consolidate waste in distribution centres
Neel order, ring exchange and charge fluctuations in the half-filled Hubbard model
We investigate the ground state properties of the two dimensional half-filled
one band Hubbard model in the strong (large-U) to intermediate coupling limit
({\it i.e.} away from the strict Heisenberg limit) using an effective spin-only
low-energy theory that includes nearest-neighbor exchange, ring exchange, and
all other spin interactions to order t(t/U)^3. We show that the operator for
the staggered magnetization, transformed for use in the effective theory,
differs from that for the order parameter of the spin model by a
renormalization factor accounting for the increased charge fluctuations as t/U
is increased from the t/U -> 0 Heisenberg limit. These charge fluctuations lead
to an increase of the quantum fluctuations over and above those for an S=1/2
antiferromagnet. The renormalization factor ensures that the zero temperature
staggered moment for the Hubbard model is a monotonously decreasing function of
t/U, despite the fact that the moment of the spin Hamiltonien, which depends on
transverse spin fluctuations only, in an increasing function of t/U. We also
comment on quantitative aspects of the t/U and 1/S expansions.Comment: 9 pages - 3 figures - References and details to help the reader adde
Quantum kagome antiferromagnet in a magnetic field: Low-lying non-magnetic excitations versus valence-bond crystal order
We study the ground state properties of a quantum antiferromagnet on the
kagome lattice in the presence of a magnetic field, paying particular attention
to the stability of the plateau at magnetization 1/3 of saturation and the
nature of its ground state. We discuss fluctuations around classical ground
states and argue that quantum and classical calculations at the harmonic level
do not lead to the same result in contrast to the zero-field case. For spin
S=1/2 we find a magnetic gap below which an exponential number of non-magnetic
excitations are present. Moreover, such non-magnetic excitations also have a
(much smaller) gap above the three-fold degenerate ground state. We provide
evidence that the ground state has long-range order of valence-bond crystal
type with nine spins in the unit cell.Comment: 5 pages including 4 figures, uses REVTeX4; final version with some
small extensions; to appear in Phys. Rev.
Tensorial Constitutive Models for Disordered Foams, Dense Emulsions, and other Soft Nonergodic Materials
In recent years, the paradigm of `soft glassy matter' has been used to
describe diverse nonergodic materials exhibiting strong local disorder and slow
mesoscopic rearrangement. As so far formulated, however, the resulting `soft
glassy rheology' (SGR) model treats the shear stress in isolation, effectively
`scalarizing' the stress and strain rate tensors. Here we offer generalizations
of the SGR model that combine its nontrivial aging and yield properties with a
tensorial structure that can be specifically adapted, for example, to the
description of fluid film assemblies or disordered foams.Comment: 18 pages, 4 figure
Ground state and low-lying excitations of the spin-1/2 XXZ model on the kagome lattice at magnetization 1/3
We study the ground state and low-lying excitations of the S=1/2 XXZ
antiferromagnet on the kagome lattice at magnetization one third of the
saturation. An exponential number of non-magnetic states is found below a
magnetic gap. The non-magnetic excitations also have a gap above the ground
state, but it is much smaller than the magnetic gap. This ground state
corresponds to an ordered pattern with resonances in one third of the hexagons.
The spin-spin correlation function is short ranged, but there is long-range
order of valence-bond crystal type.Comment: 2 pages, 1 figure included, to appear in Physica B (proceedings of
SCES'04
In-situ monitoring for CVD processes
Aiming towards process control of industrial high yield/high volume CVD reactors, the potential of optical sensors as a monitoring tool has been explored. The sensors selected are based on both Fourier transform infrared spectroscopy (FTIR) and tunable diode laser spectroscopy (NIR-DLS). The former has the advantage of wide spectral capability, and well established databases. NIR-DLS spectroscopy has potentially high sensitivity, laser spatial resolution, and the benefits of comparatively easier integration capabilities-including optical fibre compatibility. The proposed technical approach for process control is characterised by a 'chemistry based' feedback system with in-situ optical data as input information. The selected optical sensors continuously analyze the gas phase near the surface of the growing layer. The spectroscopic data has been correlated with process performance and layer properties which, in turn establish data basis for process control. The new process control approach is currently being verified on different industrialised CVD coaters. One of the selected applications deals with the deposition of SnO2 layers on glass based on the oxidation of (CH3)2SnCl2, which is used in high volume production for low-E glazing
KIC 4768731: a bright long-period roAp star in theKeplerfield
We report the identification of 61.45 d−1 (711.2 μHz) oscillations, with amplitudes of 62.6 μmag, in KIC 4768731 (HD 225914) using Kepler photometry. This relatively bright (V = 9.17) chemically peculiar star with spectral type A5 Vp SrCr(Eu) has previously been found to exhibit rotational modulation with a period of 5.21 d. Fourier analysis reveals a simple dipole pulsator with an amplitude that has remained stable over a 4-yr time span, but with a frequency that is variable. Analysis of high-resolution spectra yields stellar parameters of Teff = 8100 ± 200 K, log g = 4.0 ± 0.2, [Fe/H] = +0.31 ± 0.24 and v sin i = 14.8 ± 1.6 km s−1.
Line profile variations caused by rotation are also evident. Lines of Sr, Cr, Eu, Mg and Si are strongest when the star is brightest, while Y and Ba vary in antiphase with the other elements. The abundances of rare earth elements are only modestly enhanced compared to other roAp stars of similar Teff and log g. Radial velocities in the literature suggest a significant change over the past 30 yr, but the radial velocities presented here show no significant change over a period of 4 yr
Surfing on a critical line: Rejuvenation without chaos, Memory without a hierarchical phase space
The dynamic behaviour of glassy materials displays strong nonequilibrium
effects, such as ageing in simple protocols, memory, rejuvenation and Kovacs
effects in more elaborated experiments. We show that this phenomenology may be
easily understood in the context of the nonequilibrium critical dynamics of
non-disordered systems, the main ingredient being the existence of an infinite
equilibrium correlation length. As an example, we analytically investigate the
behaviour of the 2D XY model submitted to temperature protocols similar to
experiments. This shows that typical glassy effects may be obtained by `surfing
on a critical line' without invoking the concept of temperature chaos nor the
existence of a hierarchical phase space, as opposed to previous theoretical
approaches. The relevance of this phenomenological approach to glassy dynamics
is finally discussed.Comment: Version to be published in Europhysics Letters. Slight modifs + ref
to "surfing" adde
Low energy theory of the t-t'-t''-U Hubbard Model at half-filling: interaction strengths in cuprate superconductors and an effective spin-only description of La_2CuO_4
Spin-only descriptions of the half-filled one-band Hubbard model are relevant
for a wide range of Mott insulators. In addition to the usual Heisenberg
exchange, many new types of interactions, including ring exchange, appear in
the effective Hamiltonian in the intermediate coupling regime. In order to
improve on the quantitative description of magnetic excitations in the
insulating antiferromagnetic phase of copper-oxide (cuprate) materials, and to
be consistent with band structure calculations and photoemission experiments on
these systems, we include second and third neighbor hopping parameters, t' and
t'', into the Hubbard Hamiltonian. A unitary transformation method is used to
find systematically the effective Hamiltonian and any operator in the spin-only
representation. The results include all closed, four hop electronic pathways in
the canonical transformation. The method generates many ring exchange terms
that play an important role in the comparison with experiments on La2CuO4.
Performing a spin wave analysis, we calculate the magnon dispersion as a
function of U,t,t' and t''. The four parameters are estimated by fitting the
magnon dispersion to the experimental results of Coldea et al. [Phys. Rev.
Lett. 86, 5377, {2001}] for La2CuO4. The ring exchange terms are found
essential, in particular to determine the relative sign of t' and t'', with the
values found in good agreement with independent theoretical and experimental
estimates for other members of the cuprate family. The zero temperature
sublattice magnetization is calculated using these parameters and also found to
be in good agreement with the experimental value estimated by Lee et al. [Phys.
Rev. B 60, 3643 (1999)]. We find a value of the interaction strength U~8t
consistent with Mott insulating behavior.Comment: Submitted to Phys. Rev. B, 24 pages, 8 figures, 78 reference
- …