291 research outputs found
The efficiency and the demagnetization field of a general Halbach cylinder
The maximum magnetic efficiency of a general multipole Halbach cylinder of
order is found as function of . The efficiency is shown to decrease for
increasing absolute value of . The optimal ratio between the inner and outer
radius, i.e. the ratio resulting in the most efficient design, is also found as
function of and is shown to tend towards smaller and smaller magnet sizes.
Finally, the demagnetizing field in a general -Halbach cylinder is
calculated, and it is shown that demagnetization is largest either at or . For the common case of a Halbach cylinder
the maximum values of the demagnetizing field is either at at
the outer radius, where the field is always equal to the remanence, or at at the inner radius, where it is the magnitude of the field in the
bore. Thus to avoid demagnetization the coercivity of the magnets must be
larger than these values.Comment: 5 pages, 5 figure
Topology optimized permanent magnet systems
Topology optimization of permanent magnet systems consisting of permanent
magnets, high permeability iron and air is presented. An implementation of
topology optimization for magnetostatics is discussed and three examples are
considered. First, the Halbach cylinder is topology optimized with iron and an
increase of 15% in magnetic efficiency is shown, albeit with an increase of 3.8
pp. in field inhomogeneity - a value compared to the inhomogeneity in a 16
segmented Halbach cylinder. Following this a topology optimized structure to
concentrate a homogeneous field is shown to increase the magnitude of the field
by 111% for the chosen dimensions. Finally, a permanent magnet with alternating
high and low field regions is considered. Here a figure
of merit of 0.472 is reached, which is an increase of 100% compared to a
previous optimized design.Comment: 10 pages, 10 figure
A thermoelectric power generating heat exchanger: Part I - Experimental realization
An experimental realization of a heat exchanger with commercial
thermoelectric generators (TEGs) is presented. The power producing capabilities
as a function of flow rate and temperature span are characterized for two
different commercial heat transfer fluids and for three different thermal
interface materials. The device is shown to produce 2 W per TEG or 0.22 W
cm at a fluid temperature difference of 175 C and a flow rate
per fluid channel of 5 L min. One experimentally realized design
produced 200 W in total from 100 TEGs. For the design considered here, the
power production is shown to depend more critically on the fluid temperature
span than on the fluid flow rate. Finally, the temperature span across the TEG
is shown to be 55% to 75% of the temperature span between the hot and cold
fluids.Comment: 9 pages, 11 figure
Determining the minimum mass and cost of a magnetic refrigerator
An expression is determined for the mass of the magnet and magnetocaloric
material needed for a magnetic refrigerator and these are determined using
numerical modeling for both parallel plate and packed sphere bed regenerators
as function of temperature span and cooling power. As magnetocaloric material
Gd or a model material with a constant adiabatic temperature change,
representing a infinitely linearly graded refrigeration device, is used. For
the magnet a maximum figure of merit magnet or a Halbach cylinder is used. For
a cost of \$40 and \$20 per kg for the magnet and magnetocaloric material,
respectively, the cheapest 100 W parallel plate refrigerator with a temperature
span of 20 K using Gd and a Halbach magnet has 0.8 kg of magnet, 0.3 kg of Gd
and a cost of \$35. Using the constant material reduces this cost to \$25. A
packed sphere bed refrigerator with the constant material costs \$7. It is also
shown that increasing the operation frequency reduces the cost. Finally, the
lowest cost is also found as a function of the cost of the magnet and
magnetocaloric material.Comment: 12 pages, 10 figure
Clinical Decision Making of Nurses Working in Hospital Settings
This study analyzed nurses' perceptions of clinical decision making (CDM) in their clinical practice and compared differences in decision making related to nurse demographic and contextual variables. A cross-sectional survey was carried out with 2095 nurses in four hospitals in Norway. A 24-item Nursing Decision Making Instrument based on cognitive continuum theory was used to explore how nurses perceived their CDM when meeting an elective patient for the first time. Data were analyzed with descriptive frequencies, t-tests, Chi-Square test, and linear regression. Nurses' decision making was categorized into analytic-systematic, intuitive-interpretive, and quasi-rational models of CDM. Most nurses reported the use of quasi-rational models during CDM thereby supporting the tenet that cognition most often includes properties of both analysis and intuition. Increased use of intuitive-interpretive models of CDM was associated with years in present job, further education, male gender, higher age, and working in predominantly surgical units
Band gap engineering by Bi intercalation of graphene on Ir(111)
We report on the structural and electronic properties of a single bismuth
layer intercalated underneath a graphene layer grown on an Ir(111) single
crystal. Scanning tunneling microscopy (STM) reveals a hexagonal surface
structure and a dislocation network upon Bi intercalation, which we attribute
to a Bi structure on the underlying Ir(111)
surface. Ab-initio calculations show that this Bi structure is the most
energetically favorable, and also illustrate that STM measurements are most
sensitive to C atoms in close proximity to intercalated Bi atoms. Additionally,
Bi intercalation induces a band gap (eV) at the Dirac point of
graphene and an overall n-doping (eV), as seen in angular-resolved
photoemission spectroscopy. We attribute the emergence of the band gap to the
dislocation network which forms favorably along certain parts of the moir\'e
structure induced by the graphene/Ir(111) interface.Comment: 5 figure
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