12,974 research outputs found
Maximum performance of piezoelectric energy harvesters when coupled to interface circuits
This paper presents a complete optimization of a piezoelectric vibration energy harvesting system, including a piezoelectric transducer, a power conditioning circuit with full semiconductor device models, a battery and passive components. To the authors awareness, this is the first time and all of these elements have been integrated into one optimization. The optimization is done within a framework, which models the combined mechanical and electrical elements of a complete piezoelectric vibration energy harvesting system. To realize the optimization, an optimal electrical damping is achieved using a single-supply pre-biasing circuit with a buck converter to charge the battery. The model is implemented in MATLAB and verified in SPICE. The results of the full system model are used to find the mechanical and electrical system parameters required to maximize the power output. The model, therefore, yields the upper bound of the output power and the system effectiveness of complete piezoelectric energy harvesting systems and, hence, provides both a benchmark for assessing the effectiveness of existing harvesters and a framework to design the optimized harvesters. It is also shown that the increased acceleration does not always result in increased power generation as a larger damping force is required, forcing a geometry change of the harvester to avoid exceeding the piezoelectric breakdown voltage. Similarly, increasing available volume may not result in the increased power generation because of the difficulty of resonating the beam at certain frequencies whilst utilizing the entire volume. A maximum system effectiveness of 48% is shown to be achievable at 100 Hz for a 3.38-cm3 generator
Drink wise, age well; reducing alcohol related harm among people over 50: a study protocol
Background: Evidence suggests that the use of alcohol among older adults (defined as those aged 50+) has increased in recent years, with people aged 55-64 now more likely to exceed the recommended weekly guidelines than any other age group. Methods/ design: This is a quasi-experimental study with a before-after design. A postal questionnaire will be sent to 76,000 people aged 50 and over registered with a general practice in five different 'demonstration' (intervention) and control areas in the UK. Multiple interventions will then be delivered in demonstration areas across the UK. At the end of the programme, a postal questionnaire will be sent to the same individuals who completed it pre-programme to establish if there has been a reduction in alcohol use, at-risk drinking and alcohol related problems. Qualitative interviews with clients and staff will explore how the interventions were experienced; how they may work to bring about change and to identify areas for practice improvements. Discussion: This study protocol describes a multi-level, multi-intervention prevention-to-treatment programme which aims to reduce alcohol-related harm in people aged 50 and over
Polarization of interacting bosons with spin
We demonstrate rigorously that in the absence of explicit spin-dependent
forces one of the ground states of interacting bosons with spin is always fully
polarized -- however complicated the many-body interaction potential might be.
Depending on the particle spin, the polarized ground state will generally be
degenerate with other states, but one can specify the exact degeneracy. For T>0
the magnetization and susceptibility necessarily exceed that of a pure
paramagnet. These results are relevant to recent experiments exploring the
relation between triplet superconductivity and ferromagnetism, and the
Bose-Einstein condensation of atoms with spin. They eliminate the possibility,
raised in some theoretical speculations, that the ground state or positive
temperature state might be antiferromagnetic.Comment: v4: as published in PR
Surface Partition of Large Fragments
The surface partition of large fragments is derived analytically within a
simple statistical model by the Laplace-Fourier transformation method. In the
limit of small amplitude deformations, a suggested Hills and Dales Model
reproduces the leading term of the famous Fisher result for the surface entropy
with an accuracy of a few percent. The surface partition of finite fragments is
discussed as well.Comment: 4 pages, 1 figur
Bi-Polaron and N-Polaron Binding Energies
The binding of polarons, or its absence, is an old and subtle topic. Here we
prove two things rigorously. First, the transition from many-body collapse to
the existence of a thermodynamic limit for N polarons occurs precisely at
U=2\alpha, where U is the electronic Coulomb repulsion and \alpha is the
polaron coupling constant. Second, if U is large enough, there is no
multi-polaron binding of any kind. Considering the known fact that there is
binding for some U>2\alpha, these conclusions are not obvious and their proof
has been an open problem for some time.Comment: 4 page
The MAJORANA 76Ge neutrino less double-beta decay project: A brief update
At present, MAJORANA is a research and development (R&D) project to
investigate the feasibility and cost of constructing and operating a one ton
decay experiment with ~1000 kg of Ge detectors fabricated from germanium
enriched to 86% in . The study will include three separate cryostats with
various types of detectors: un-segmented, un-segmented point-contact, minimally
segmented, and highly segmented. One cryostat will contain at least 30 kg of
enriched (preferably point-contact) detectors. The performance of the cryostats
and detectors as well as background levels will be investigated. The goal of
the demonstrator project is to reach a discovery sensitivity of ~ 1026 y.Comment: 3 pages, no figure
Atomic levels in superstrong magnetic fields and D=2 QED of massive electrons: screening
The photon polarization operator in superstrong magnetic fields induces the
dynamical photon "mass" which leads to screening of Coulomb potential at small
distances , is the mass of an electron. We demonstrate that this
behaviour is qualitatively different from the case of D=2 QED, where the same
formula for a polarization operator leads to screening at large distances as
well. Because of screening the ground state energy of the hydrogen atom at the
magnetic fields has the finite value .Comment: 12 pages, 2 figure
Correlation equalities and upper bounds for the transverse Ising model
Starting from an exact formal identity for the two-state transverse Ising
model and using correlation inequalities rigorous upper bounds for the critical
temperature and the critical transverse field are obtained which improve
effective results.Comment: 8 pages, 1 figur
Stability of Matter in Magnetic Fields
In the presence of arbitrarily large magnetic fields, matter composed of
electrons and nuclei was known to be unstable if or is too large.
Here we prove that matter {\it is stable\/} if and
.Comment: 10 pages, LaTe
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