Note: Descriptions are shown in the official language in which they were submitted.
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THERMAL REGENERATORS AND FABRICATION METHODS
FOR THERMAL REGENERATORS
Field of the Invention
The invention pertains to thermal regenerators and manufacturing methods for
thermal
regenerators) especially monolithic, magnetically active or passive
regenerators.
Background of the Invention
Thermal regenerators are used to transfer heat to and from fluids flowing
through the
regenerator. Typically a thermal regenerator absorbs heat from a fluid flowing
through the
regenerator and then releases heat to the fluid as it flows back through the
regenerator in an
opposite direction. Such a regenerator is a passive regenerator in that it
only provides for the
transfer of heat to and from a fluid while refrigeration is provided by, for
example) the expansion
of a gaseous refrigerant in a regenerative gas cycae.
In addition to passive regenerators, active regenerators have been developed
that provide
refrigeration. For example, an active regenerator can consist of a material
exhibiting the
magneto-caloric effect; application of a magnetic field can then be used
provide heating or cooling
of a fluid flowing through the regenerator.
Both active and passive regenerators preferably have large thermal masses,
large heat
transfer coefficients, and little resistance to fluid flow. In addition,
regenerators should not
conduct heat in directions parallel to the direction of fluid flow through the
regenerator. The
particles of the regenerator material should be solid particles, without voids
and preferably have
low overall porosity in a regenerator. These conditions are generally not
simultaneously satisfied.
Various types of regenerators are known, including wire-screen, foil,
perforated-plate,
and particle regenerators. Wire-screen regenerau~rs have layers of a fine
gauge wire screen that
are stacked inside a container. The wire screens provide openings for fluid
flow and provide
large surface areas for heat transfer. Unfortunavtely, it is difficult to
accurately align the layers
of wire screens so that fluid flow resistance tends to be variable. In
addition, preparation and
cutting of the wire screens is expensive. Because the wire screens are thin, a
very large number
of wire screens must be assembled in the container. This assembly is also
expensive and difficult
to automate.
Foil regenerators use thin foils of a regc;nerator material. In one foil
regenerator, a
metal foil is dimpled and then rolled, forming a series of flow channels.
Metal-foil regenerators
have also been made by photolithographically de:fming and then etching flow
channels in a thin
foil as described in U.S. patent 5,429,177 to Yaron et al. Foil layers with
these etched flow
channels are then stacked to form a regenerator. The foil layers can also be
etched to provide
flow passages between the flow channels.
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Particle regenerators generally are porous, finely divided aggregates of
particles that
permit fluid flow therethrough. The aggregates have large surface areas for
heat transfer between
the particles and the fluid. Conventional particle regenerators use spherical
metallic powders
formed by, e.g., centrifugal or gas atomization. The yield of appropriately
sized particles from
these methods is low and such particles may have internal voids, reducing the
thermal mass per
unit volume of the powder.
Perforated-plate regenerators as described, for example, in U.S. patent
5,101,894 to
Hendricks use a stack of perforated plates. The plates are perforated with
small holes whose
length-to-diameter ratios are such that the holes function as tubes in heating
or cooling a fluid
flowing through the holes.
Because the extent to which a regenerator can heat or cool a fluid depends on
the heat
capacity of the materials of the regenerator, regenerators are preferably made
of materials having
large heat capacities. However, if cooling to very low temperatures is
intended, material
selection is limited because material heat capacities diminish with decreasing
temperature. In
1S addition) many materials with relatively high heat capacities at low
temperatures are brittle, so
that forming the material into a shape suitable for incorporation into a
regenerator is difficult and
expensive. Therefore, regenerators requiring wires, wire screens, plates,
spherical particles, or
foils are difficult and expensive to manufacture for many advantageous
materials. Moreover,
regenerators are usually exposed to mechanical stresses such as differential
pressures and thermal
cycling that cause the particles of a particle regenerator to rub against each
other. Such rubbing
tends to fracture the particles of brittle materials. The fine particles
resulting from the fracture
contaminate the cooling system, clogging the flow passages or migrating out of
the regenerator to
the rest of the cooling system.
Summary of the Invention
The regenerators of the present invention take advantage of the relatively
large low-
temperature heat capacities of the compounds of rare earth metals and
transition metals (RE/TM
compounds)) such as the materials described in U.S. patent 5,186,756 to Arai
et al. Other
compounds with advantageous properties include compounds such as Gds(SiZGez),
Gd5(SixGe,_x)4
(x<_0.5), and alloys of these materials) described in "Effect of alloying on
the giant
magnetocaloric effect of Gd5(Si2Ge2) and other Gd compounds," J. Magnetism and
Magnetic
Materials 176 (1997), L179-L184. Other materials include but are not limited
to the materials
described in U.S. patent 4,082,138 to Miedma et al. and U.S. patent 5,462,610
to Gschneidner et
al.
3S Furthermore, the present invention provides regenerators and fabrication
methods for
regenerators that use irregularly shaped particles of these or other
compounds, including brittle
RE/TM compounds. The regenerators of the present invention are preferably
monolithic
regenerators, i.e. regenerators in which the regenerator particles are bound
together to form a
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self-supporting structure. Because irregularly shaped particles can be used,
the compounds need
not be formed into wires, screens, sheets) perforated plates, or spheres.
Particle fabrication is
inexpensive and can be performed with a grinding apparatus such as a mortar
and pestle. Particle
sizes are readily selected by sieving the particles produced by grinding.
Preferably, the particles
are sieved during grinding to obtain a high yield ~of appropriately sized
particles.
To form a monolithic regenerator, particles of a regenerator material such as
a RE
compound are placed in a mold. An end of the mold is covered by a screen
separator that
contains the particles in the mold. A dilute solution of a binding agent is
then applied to the
particles and flows through the particles and the screen separator. Excess
binding agent is then
removed by forcing a gas flow through the particles in the mold. The gas flow
for removing
excess binding agent can be provided by a source of pressurized gas.
Alternatively, a vacuum
can draw a gas flow through the particles. The application of binding agent
and the removal of
excess binding agent can be repeated until sufficient binding agent remains
among the particles
while still permitting fluid flow therethrough. The number of repetitions is
adjusted according to
the dilution and viscosity of the binding agent.
The binding agent is then cured or hardened and the bound particles form a
monolithic
regenerator that is extracted as a whole from the mold. The screen separator
can remain attached
to the monolithic regenerator or can be removed. The finished monolithic
regenerator can be cut
into smaller regenerators if desired.
Because excess binding agent is removed from the particles, fluid flow through
the
monolithic regenerator is not appreciably impaired. In addition, rates of heat
transfer to and from
the particles are not appreciably changed. The binding agent holds the
particles in fixed
relationship, reducing particle rubbing. Thus, even brittle particles do not
fracture. Low values
of thermal conductivity along the fluid flow direction are maintained because
the contact areas
between adjacent particles are small and provide a poor path for heat
conduction.
To facilitate removal of the monolithic regenerator from the mold, the mold
may be a
tube of a non-stick material such as TEFLON and may be coated with a mold
release agent.
Molds of other shapes and materials are also suitable. A regenerator container
can be substituted
for the mold so that the particles are bound together and to the container.
After the binding agent
hardens, the monolithic regenerator (i. e., the bound particles) and the
regenerator container form
a regenerator assembly that can be inserted in a cooling system. Applying the
binding agent with
the particles molded by the regenerator container instead of a mold reduces
the likelihood of fluid
flow around the monolithic regenerator.
Regenerator assemblies made by these methods comprise one or more layers of
regenerator particles of a RE/TM compound or other compound. The particles may
be brittle and
irregularly shaped. The layers of particles can be separated by screen
separators. In addition,
screen separators can be provided on exterior regenerator surfaces through
which fluid enters or
exits the regenerator during operation. The partiicles of the layers are bound
together by a
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binding agent to form a self-supporting regenerator. The binding agent is
applied so as to control
increases in fluid flow resistance. A 0.5 ~m to 20 ~m coating of a binding
agent on particles
having effective diameters of 100 ~m to 400 ~m is generally satisfactory.
Monolithic regenerators formed in a mold are subsequently attached to a
regenerator
container. This can be done using a mixture of an epoxy and cotton fibers.
Such a mixture does
not penetrate into the monolithic regenerator, maintaining the fluid flow
properties of the
regenerator and effectively sealing the interface between the container and
the regenerator.
During operation, fluid flows through interstices between the particles of the
regenerator and not
between the regenerator and the container. Alternatively, the particles and
the container can be
bound together as the binding agent is applied to the particles) forming a
regenerator assembly.
Alternatively, the regenerator particles can be coated with a metal or polymer
coating
without binding the regenerator particles together. For example, an epoxy
mixture can be applied
that is too dilute to bind the particles but is sufficient to coat or
partially coat the particles. After
such coating) the particles can be confined in a regenerator container by
screen separators or
perforated plates. The screen separators or perforated plates permit fluid
flow through the
particles; the coating on the regenerator particles inhibits particle
fracturing and tends to hold
fractured particles together.
The foregoing and other features and advantages of the invention will become
more
apparent from the following detailed description which proceeds with reference
to the
accompanying drawings.
Brief Description of the Drawings
FIG. 1 is a chart which contains graphs of magnetic entropy and de Gennes
factors for
trivalent lanthanide ions.
FIG. 2 is a schematic diagram of an arc-melting apparatus for preparing
regenerator
materials.
FIG. 3 is a schematic diagram of a grinding apparatus for preparing particles
of
regenerator materials.
FIG. 4 is a graph of volumetric heat capacity as a function of temperature for
exemplary
RE/TM materials, lead, and helium.
FIGS. 5 are schematic diagrams which illustrate a method for fabricating a
single-layer
monolithic regenerator. FIG 5(a) illustrates the application of a binding
agent to a layer of
regenerator particles and the removal of excess binding agent using a gas
flow. FIG. 5(b) shows
extraction of a single-layer monolithic regenerator from a tube.
FIGS. 6 are schematic diagrams which illustrate a method for fabricating a two-
layer
monolithic regenerator. FIG 6(a) illustrates the application of a binding
agent to first and second
layers of regenerator particles and the removal of excess binding agent using
a gas flow. FIG.
6(b) shows extraction of a two-layer monolithic regenerator from a mold. FIG.
6(c) illustrates
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the insertion of the monolithic regenerator of FIG. 6(b) into a regenerator
container. FIG. 6(d)
illustrates the sealing of the monolithic regenerator of FIG. 6(b) into the
container.
FIGS. 7 are schematic diagrams which illustrate a method for fabricating a two-
layer
regenerator directly in a regenerator container. F'IG. 7(a) shows formation of
two layers of
regenerator particles in a container. FIG. 7(b) shows the insertion of a plug
to capture the layers
of particles in the container. FIG. 7(c) illustrates application of a binding
agent to the layers and
the removal of excess binding agent using a gas flow.
Detailed IDescriution
Thermal regenerators using irregularly-shaped particles and methods for
fabricating such
thermal regenerators are provided. The regenerators are particularly suitable
for cooling systems
in which brittle particles of a regenerator material are used. The
regenerators may be used in
conventional regenerative gas-cycle devices such as Stirling, Gifford-McMahon,
and Orifice Pulse
Tube refrigerators. The regenerators may also bc: used in magnetic
regenerative refrigerators
where the application and removal of a magnetic induction causes the heating
and cooling
required for a refrigeration cycle. Aggregates of regenerator particles in
which the particles are
bound together to form a self-supporting structure are referred to herein as
monolithic
regenerators. A regenerator assembly typically comprises an aggregate of
regenerator particles
(such as a monolithic regenerator) and a regeneranor container. The
regenerator container
facilitates mounting the aggregate of regenerator particles in a cooling
system.
A process for fabricating a monolithic regenerator comprises the steps of
selecting a
regenerator material or materials, fabricating the materials, forming
particles of the materials, and
binding the particles into a monolithic regenerator. In addition, before the
monolithic regenerator
can be used in a cooling system, the monolithic ;regenerator must be mounted
in a container for
insertion into the cooling system.
Selection of a regenerator material begins by establishing selection criteria
for
regenerator materials. Materials with large thermal masses within an intended
operational
temperature range are preferred. At low temperatures) materials with large
thermal masses are
selected with reference to the following selection: criteria. First, such
materials have a low Debye
temperature BD. Second, the materials have a magnetic ordering temperature in
an appropriate
range. Third, the materials preferably have a large magnetic entropy. Fourth,
the materials
should have a high number density of magnetic .atoms. Fifth, the materials
should contain
lanthanide elements from either the beginning or the end of the lanthanide
series. Sixth) the
materials should have appropriately broad or narrow heat capacity peaks. These
selection criteria
are explained in more detail below.
Advantageous regenerator materials have low Debye temperatures BD so that the
lattice
contribution to heat capacity is significant at low temperatures. In materials
having high Debye
temperatures B", the lattice contribution to heat capacity is small and the
heat capacity tends to be
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low. Heavy lanthanides, i.e., those having atomic numbers greater than 57,
such as Er and Yb)
have Debye temperatures 6~ that are generally lower than the Debye
temperatures 6D of other
elements, including the 3d transition metals. With reference to FIG. 2, the
volumetric heat
capacities C of representative RE/TM compounds Nd3Ni, Er3Ni, ErNbo.9Coo.~) and
Ero.9Ybo,,Ni as
well as Pb and He are graphed for temperatures below 30 K. Below about 10 K,
the heat
capacities of these representative RE/TM compounds are larger than the heat
capacity of lead.
Thus, the heavy lanthanides are preferred for thermal regenerators for such
low temperatures. In
addition, the magnetic ianthanides have lower magnetic ordering temperatures
than the 3d
transition metals.
A material with a large magnetic entropy S",~,R,~,;~ exhibits a relatively
large magneto-
caloric effect and thus provides superior magnetic cooling. Because S",QB,~r;~
is proportional to the
total angular momentum quantum number J, elements with large values of J
(i.e., with large
magnetic moments) have large magnetic entropy. The heavy lanthanides generally
have relatively
large values of the total angular momentum quantum number J and large magnetic
moments.
Therefore, the heavy lanthanides have relatively high values of magnetic
entropy. While some 3d
transition metals have theoretical magnetic moments comparable to the magnetic
moments of the
lanthanides, the measured magnetic moments are consistently lower than the
theoretical values.
The fourth criterion, that the material preferably has a high number density
of magnetic
atoms, directs that the fraction of the magnetically active component in the
material be large. If
the lanthanide elements are selected, the materials should consist mostly of
lanthanides. A large
fraction of a non-magnetic element or an element with a small magnetic moment
produces a
magnetically dilute material. For non-magnetic regenerators, this
consideration is not important.
In addition, the material should not be porous. It will be apparent that a
porous material has a
reduced number density of the magnetic component as well as a reduced specific
heat per unit
volume.
The fifth criterion arises because the magnetic ordering temperature tends to
follow the
De Gennes factor G given by Equation 1:
G ° J(J+1)(8-1)2 , (1)
wherein g is the gyromagnetic ratio given by Equation 2:
g - 1 + J(J+1)+S(S+1) L(L 1) . (2)
J(J+1)
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With reference to FIG. 1, elements at either end of the lanthanide series have
low
magnetic ordering temperatures (i.e., low G values'i and are thus preferred
for magnetic
regenerators for low temperatures.
The selection of materials based on the shape of the heat capacity peaks as a
function of
temperature generally requires consideration of the specific application of
the thermal regenerator.
For example, layered hybrid regenerators having more than one regenerator
material perform
satisfactorily with materials having relatively narrow heat capacity peaks.
With such layered
regenerators) the heat capacity peaks of the differern materials are matched
with the anticipated
temperature profile of the regenerator. In other applications, a single
material is used over an
extended temperature range. In this case) a broad heat capacity peak is
preferred.
The width and distribution of heat capacity peaks appear to be intrinsic
properties of a
compound with impurities having small effect on the shape of the peaks.
Therefore, regenerator
material selection depends primarily on the heat capacities of the regenerator
compounds with
little consideration of impurities.
For the example embodiments of monolithic magnetic regenerators and methods
for
making such regenerators described herein) the materials Er3Ni, ErNio.9Coo.,,
and Ero,9Ybo,,Ni are
selected. Because the properties of Ero.9Ybo.lNi and ErNio.9Coo., are similar,
magnetic
regenerators were built with Ero,9Ybo.,Ni and Er3Ni;; magnetic regenerators
using ErNio.9Coo., are
expected to be similar. It will be readily apparent that other regenerator
materials can be selected.
The size of particles for a regenerator is chosen to provide large surface
areas for heating
or cooling while maintaining low resistance to fluid flow through the
regenerator. The rate at
which particles transfer heat to and from a fluid moving through a regenerator
depends on the total
particle surface area; smaller particles transfer heat more effectively than
larger particles.
Unfortunately, regenerators of small particles have relatively higher fluid
flow resistances than
larger particles. For the regenerators described herein, particles of
effective diameter of 100 ~m
to 400 ~.m are chosen. (An effective diameter is a dimension proportional to
the ratio of particle
volume to particle surface area.) The selection of particle size can be varied
in consideration of
required regenerator thermal and fluid flow properties.
After regenerator materials are selected, an arc-welter is used to prepare the
regenerator
materials. Because most rare-earth materials are commonly available as ingots
of fixed masses,
the rare-earth ingots must be cut into smaller piece, before alloying with
other materials. Pure
rare earth metals are difficult to cut and small chips are not readily
obtained. Because RE/TM
compounds for thermal regenerators contain a large percentage of a rare earth
metal) it is
convenient to use a mass of a rare earth metal as a~ starting point and then
prepare appropriate
masses of other constituents. Generally, RE/TM ingots of mass of 25-100 g are
conveniently
prepared. Alternatively) a total mass of regenerator material can be specified
and corresponding
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_g_
constituent masses prepared to yield the total tttass of regenerator material
required . A total
regenerator volume can be computed based on porosity and yield values.
With reference to FIG. 2, an arc-melter 200 for preparing regenerator
materials
comprises a stainless steel vessel 202 which defines a chamber 201 with
systems for pumping)
purging, and filling the chamber 201 with an inert gas, typically 99.98 % pure
argon. A water-
cooled copper hearth 215 and a non-consumable tungsten electrode 2I7 are
connected to a high-
current, low-voltage power supply 219. Regenerator components 221 (e.g., Er,
Ni, and Co) are
placed in machined pockets 223 in the water-cooled copper hearth 215. The
chamber 201 is then
purged by repetitively evacuating the chamber 201 with a rough vacuum pump 203
through a
valve 209 and then filling the chamber 201 to pressures of up to 10 psi with
argon gas from a gas
cylinder 205 through a valve 213. Typically five repetitions suffice to
minimize oxygen in the
chamber 201. After purging, the chamber 201 is evacuated and a small piece of
titanium 225 is
melted a few seconds before arc-melting the components 221 to remove any
remaining oxygen.
An arc is then activated, melting the components 221. The components 221 are
alternately melted
and flipped ten times to ensure a homogeneous regenerator material.
After the regenerator material is prepared by arc-melting, particles of the
material are
produced by mechanical crushing. With reference to Fig. 3, a mortar and pestle
apparatus 300 for
crushing the regenerator material comprises a mortar 301 and a pestle 302. The
mortar 301 has a
floor 311 and rotates on a shaft 303 in a direction 310 about an axis 305; the
shaft 303 is held in
place by a bearing plate 309. The pestle 302 rotates on a shaft 304 in a
direction 306 about an
axis 308. The pestle 302 is provided with a flat 313 to facilitate crushing.
The shafts 303, 304
rotate independently and generally have different axes of rotation and
rotation rates.
The axes of rotation 305, 308 of the shafts 303, 304 are generally parallel
and offset by a
distance 320. A grinding end 312 of the pestle 302 is a set a distance 322
from the floor 311 of
the mortar 301. The distances 320, 322 are preferably 2.5 mm and 1.0 mm,
respectively; the
rotation rate of the mortar 301 and pestle 302 are preferably 60 rpm and 150
rpm, respectively.
Pressure 325 is applied to force the pestle 302 toward the mortar 301; the
pressure 325 can be
controlled manually.
A piece of a regenerator material 324 is placed in the mortar 301. The mortar
301 and
the pestle 302 crush the piece of regenerator material 324, producing
irregularly shaped particles
307 of varying sizes. The particles 307 are sieved to separate the particles
307 according to
particle size. The particles 307 are continuously sieved during crushing.
Without continuous
sieving, the particles 307 produced tend to be too small, with effective
diameters of 20-40 ~.m
instead of the 100-400 ~.m diameters desired.
The regenerator material 324 is processed in the mortar 301 continuously;
adding small
amounts in regular intervals allows a higher yield of particles of the
intended size. Without
continuous sieving, the yield of appropriately sized particles from grinding
brittle materials is
typically 5-10 % . With continuous sieving, the yield increases to 25 % or
larger. Particle sizes are
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characterized by effective particle diameters Dp. 'fhe particle size
distribution for ErNio,9Coo,, is
similar to that of Er3Ni and other brittle compounds.
Although particle yields in a given size range are low, mechanical crushing
with the
mortar and pestle apparatus 300 is both reliable and repeatable. In addition)
larger particles can
simply be crushed again to increase the yield of smaller sizes. Smaller
particles can be re-melted
and re-crushed but this is more difficult because fine particles are easily
dispersed by the arc of
the arc-melter 200. Providing deeper machined pockets in the copper hearth 215
better contains
small particles of regenerator material during re-melting. In deeper machined
pockets, turning
samples or extracting finished ingots would be mare difficult. However, with
brittle materials
such as the RE/TM compounds, the samples can he easily fragmented and
extracted from the
machined pockets when arc-melting is complete.
After preparing regenerator particles 307 of ErNio.9Coo., with the selected
range of
effective diameters) the particles 307 are fotmted into a monolithic
regenerator. With reference to
FIG. 5(a), the particles 307 are formed into a cylindrical section by a Teflon
tube 108 having an
outside diameter of 25.4 mm. To ensure ease of extraction, the tube 108 is
internally coated with
an anti-adhesive substance or a mold release agent . An example of such a mold
release agent is
the silicon polymer spray 122-S available from 3M Canada, Inc. It will be
evident that any other
anti-adhesive or mold release agent can be used. It will also be apparent that
molds having cross-
sections and dimensions different from the circular cross-section of the tube
108 can be used. A
screen separator 105 is placed near an end of the tube 108 to seal off the
tube 108 and contain the
particles 307. The screen separator 105 is preferably one or more 400 mesh
stainless steel screens
of wire diameter of about 2.5 pin. The screen separator 105 is supported by a
plug 106 that is
also inserted into the tube 108. Particles 307 of the regenerator material
with sizes ranging from
about 250 ~m to 300 ~m are then placed inside the tube 108.
A binding agent mixture 111 is then applied to the particles 307 in the tube
108. In a
preferred embodiment, the binding agent mixture 111 is a mixture of a binding
agent (6.426 g of
STYCAST 1266 epoxy made by mixing 5.010 ~ 0.002 g of part A and 1.416 ~ 0.002
g of part
B) diluted with 40 mL of toluene. The viscosity of the STYCAST 1266 is about
650 cps before
dilution with the 40 mL of toluene. A valve 113 controls the flow of the
binding agent mixture
111 from a reservoir 114 through a manifold 121 to the panicles 307. The
manifold 121 is
retained on the tube 108 by a plug 122.
The binding agent is diluted to a suitable viscosity. If the binding agent
mixture i l l is
too thin, the binding agent mixture 111 flows through the particles 307
without leaving enough
binding agent among the particles to bind the particles 307. If the binding
agent mixture 111 is
too viscous, the binding agent mixture 111 does not flow between the particles
307 and remains
among the particles 307, clogging the spaces between the particles 307. This
causes unacceptably
large fluid flow resistance in the finished regenerator.
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The proper viscosity for any particular binding agent can be determined by
applying the
binding agent in different viscosities to corresponding test layers of
particles. After the binding
agent is hardened, the structural integrity and flow resistance of the test
layers are assessed)
establishing a suitable range of viscosities.
As shown in FIG. 5(a), the binding agent mixture 111 flows through the
particles 307;
excess binding agent mixture 111 passes through the particles 307 into a
container 115. Because
the binding agent tends to obstruct fluid flow through a regenerator, excess
binding agent mixture
111 is removed. In general, a drop in porosity of 10 % from approximately 50 %
to approximately
40 % is detrimental to particle-type regenerator operation. In addition) if a
thick coating of the
binding agent forms on the regenerator particles 307 ) heat transfer is
impaired. However,
adequate binding agent must remain so that the finished regenerator is
structurally sound. Excess
binding agent also increases longitudinal conduction) i.e. thermal conduction
parallel to fluid flow
through the regenerator.
Excess binding agent mixture 111 is additionally removed by a pressurized gas
flow to the
1 S particles 307 that is provided via a gas line 117. The gas pressure of gas
line 117 is typically
between 20 and 30 psi. A pressure near 20 psi of nitrogen gas is effective
with the diluted
STYCAST 1266 epoxy mixture. A valve 119 controls the flow of the pressurized
gas through the
manifold 12 i . When the valve 119 is open, the gas line 117 applies a
pressure to the particles 307
that forces excess binding agent mixture 111 through the screen separator 105
toward the container
115. Alternatively, excess binding agent mixture 111 can be removed by pulling
a gas flow
through the particles 307 with a suitably-connected vacuum line.
While a single application of the binding agent mixture 111 can be sufficient
to bind the
particles 307 together, the diluted binding agent mixture 111 is preferably
applied and the excess
removed five times. After five applications of the binding agent mixture 111,
the binding agent
mixture is cured for 24 hours at room temperature and then post-cured for 2
hours at 93 ° C,
following the manufacturer's recommended epoxy curing process. After the post-
cure, the
thickness of resulting coating on the particles 307 is estimated to be between
1 ~,m and 10 ~.m on
particles having effective diameters of about 100 Vim.
The number of applications of the binding agent mixture 111 can be altered by
changing
the dilution of the binding agent mixture 111. For the STYCAST epoxy, dilution
ratios of 3:1 to
9:1 by volume of solvent (toluene) to mixed epoxy resin (part A mixed with
part B) are effective.
Mixtures of approximately 5.0 g of STYCAST part A and 1.4 g of STYCAST part B
diluted with
20.0, 30.0, and 50.0 mL of toluene successfully bind 200 pm copper test
particles substituted for
the regenerator particles 307 when excess is removed with pressurized nitrogen
gas at 30, 20, and
20 psi, respectively, measured at the valve 119.
The binding agent is not limited to epoxies or other polymers. For example, a
metal
alloy can be deposited using a vapor which contains a mixture of metals, such
as nickel or more
preferably) a ductile metal such as lead or tin. After deposition) the
deposited metal and the
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particles 307 can be sintered together in a low tem~oerature furnace.
Alternatively, a metal coating
can be applied by immersion in a bath of a low-melting point alloy. Metals
used as binding agents
should have chemical and magnetic properties that do not interfere with the
operation of the
regenerator, and have sufficiently low melting temperatures to permit
sintering without thermally
damaging the particles 307.
It will be understood that solvents other tktan toluene can be used to dilute
the binding
agent mixture 111 so long as the solvent is compatible with the binding agent.
In addition, the
number of applications and the gas flow through the regenerator particles can
be adjusted for
binding agents other than the STYCAST epoxy. It will be apparent that binding
agents that form
thin coatings or otherwise do not clog the pores beaween regenerator particles
permit the use of
smaller particles having superior heat transfer properties in a regenerator.
After the binding agent is cured, the particles 307 form a monolithic
regenerator 129 as
shown in FIG. 5(b). The monolithic regenerator 129 is removed from the tube
108 by applying a
pressure 133 to the monolithic regenerator 129 with an extracting rod 131.
Because the tube 108
is treated with a mold release agent before the parl:icles 307 are introduced
into the tube 108, the
extracting rod 131 applies only a slight pressure to force the monolithic
regenerator 129 from the
tube 108. In FIG. 5(b)) the screen separator is shown as a part of the
monolithic regenerator 129
but the screen separator 105 can be removed. It is useful for the screen
separator 105 to be
retained to contain any regenerator particles 307 that become unbound.
After removal from the tube 108, the monolithic regenerator 129 can be shaped
or cut to
a desired size. For example) a diamond-charged wheel can cut the monolithic
regenerator 129.
Some particles on the surfaces of the monolithic regenerator 129 can be
removed by scraping, but
generally the particles remain bound. The monolithic regenerator 129 can be
readily cut into a
number of smaller monolithic regenerators.
The monolithic regenerator 129 is advantageously glued into place in a cooling
system.
Because the monolithic regenerator 129 is porous, it may be necessary to take
steps to prevent the
glue from invading the pores. An epoxy such as 'undiluted STYCAST 1266 epoxy
could be used
as the glue, but the viscosity of that epoxy is insufficient to prevent
absorption by the monolithic
regenerator 129. Instead of remaining on the surface of the regenerator 129,
the epoxy would
penetrate into the regenerator 129. Accordingly ) before gluing the
regenerator 129, 0.5 g of a
cotton-fiber filler is added to an undiluted STYCAST epoxy mixture to make the
epoxy more
viscous and prevent penetration into the regenerator 129. With reference to
FIG. S(b), an
approximately 0.5 mm layer of the cotton-fiber epoxy mixture is applied to a
perimeter surface
137 of the regenerator 129, without obstructing the ends 135, 136. A second
screen separator can
be provided at the end 136.
With reference to FIGS. 6, a two-layer nnonolithic regenerator is made by a
method
similar to the method of FIGS. 5. In FIG. 6(a), a second layer of regenerator
particles 349 of
Er3Ni is provided in addition to the first layer of particles 307. After
capturing the particles 307
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in the tube 108, a second screen separator 145 is placed to confine the
particles 307 between the
screen separators 105, 145. The second layer of particles 349 is then put into
the tube 108. It
will be apparent that the particles 349 can be either the same or a different
regenerator material
than the particles 307. The total masses of the particles 307, 349 are
approximately 60 g and 66
g, respectively; these masses correspond to approximate layer thicknesses of
about 22 mm and 24
mm, respectively.
Fabrication then proceeds as in the first process. The binding agent mixture
111 is
applied and excess removed five times. The binding agent mixture 111 is cured
and a finished
monolithic regenerator 149 is extracted from the tube 108 as shown in FIG.
6(b).
With reference to FIG. 6(c), a regenerator container 161 is provided that has
a seal
groove 163 for an application such a the second stage of a Gifford-McMahon
refrigerator. A seal
assembly (not shown) is generally inserted into the groove 163 and seals the
regenerator container
161 into a cooling system. The regenerator container 161 defines a central
regenerator chamber
162 and fluid flow channels 165, 167, 169. A layer of lead particles 171 of
diameter 300 ~.m and
1 mm thick is placed at an end of the regenerator chamber 162 and is supported
by a screen
separator 173 that is in turn held in place with a retaining ring 177. The
monolithic regenerator
149 is inserted into the regenerator container 161 such that the particles 349
are nearer the fluid
flow channel 169 than the particles 307.
An end plug 179 is provided for the other end of the regenerator container
161. The end
plug 179 comprises a plug 181, a retaining ring 182, a screen separator 183,
and a perforated
plate 184. The end plug 179 also has fluid flow channels 185. With reference
to FIG. 6(d), the
end plug 179 is inserted into the regenerator container 161 after the
monolithic regenerator 149 is
inserted and a second layer 187 of 300 ~m lead particles is placed in the
regenerator container
161. The end plug is glued into the regenerator container with a cryogenic
epoxy such as
STYCAST 1266. The monolithic regenerator 149 is attached to the regenerator
container 161
with the cotton-fiber/STYCAST 1266 mixture described previously.
The fluid channels 165, 167, 169 permit fluid flow into and out of the
regenerator
chamber 162; the fluid flow channels 185 on the end plug 179 line up with the
fluid flow channels
165, 167. The perimeter surface 137 of the monolithic regenerator 149 must be
sealed to the
inner surface of the regenerator container 161 to prevent fluid flow between
the regenerator
container 161 and the monolithic regenerator 149. Such a bypass fluid flow
would avoid the
monolithic regenerator 149 and would not be effectively cooled by the
monolithic regenerator 149.
In another method, a regenerator assembly is fabricated without the tube 108
by molding
the regenerator particles directly in a regenerator container such as the
regenerator container 161.
With reference to FIG. 7(a), a layer of lead particles 551 of diameter 300 ~cm
is placed at an end
of a regenerator container 561 and is supported by a screen separator 573 and
a perforated plate
574 that are in turn held in place with a retaining ring 577. The regenerator
container 561 defines
a central chamber 562 and fluid channels 565, 567, 569 and a seal groove 563
for sealing the
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regenerator container 561 into a cooling system. Layers 507, 549, 551 of
ErNio,9Coo." Er3Ni, and
lead particles) respectively, are placed in the regenerator chamber 562,
separated by screen
separators 545, 546. The layers 507, 549, 551 have particle masses of about 58
g, 61 g, and 58 g
and extend axially along the regenerator container :561 about 25.0 mm, 24.5
mm, and 15.5 mm,
respectively. When mounted in a cooling system, the layer 551 of lead
particles generally is on
the wanner end.
An end plug 579 is inserted to seal an end of the regenerator container 561.
The end
plug 579 is similar in construction to the end plug I79. With reference to
FIG. 7(b), after
insertion of the end plug 579, the regenerator container 561 is positioned
with the end plug 579
downward. With reference to FIG. 7(c), a binding; agent such as the binding
agent mixture 111 is
then applied as in previous examples. The binding agent mixture 111 is applied
through the fluid
channel 567 and excess binding agent mixture 111 exits the regenerator
container 561 through the
fluid channels 565, 566. Excess binding agent mixture 111 is removed by
directing a gas flow
through a gas line 117. The binding agent mixture 111 is generally applied and
excess removed
five times before curing as discussed previously.
The layer 551 of lead particles provides an additional advantage during
regenerator
fabrication. A mechanical load of approximately :100 N can be applied to the
end plug 579 while
the binding agent is cured. Because lead is ductile, the layer 551 absorbs and
distributes the load,
preventing fracture of the particles in the layers 507, 549. Moreover, if the
particles inadvertently
fragment, then the layer 551 contains the fragmenoed particles) and the
fragments do not
contaminate other portions of a cooling system. It will be apparent that an
additional layer of lead
particles can be provided between the end plug 579 and the layer 507 for
distributing the load
during manufacture and containing particle fragments.
By forming a monolithic regenerator directly in the regenerator container 561,
no further
cutting or mounting is required and extraction from a mold such as the tube
108 of FIGS. 5(a)-
5(b) is not required. In addition, the binding agent seals the layers 507,
549, 551 to the
regenerator container 561 so that fluid flow around the layers 507, 549, 551
is prevented.
It will be apparent that the methods described are suitable for the
preparation of other
regenerators. The number of layers can be readily varied and the layers can
comprise particles of
various materials. Screen separators can be provided within the regenerator
assembly as needed to
separate the layers. Alternatively) the particles can be layered without
screen separators.
Regenerators can be formed in a regenerator container or transferred to a
regenerator container
after the particles of the regenerator are bound. 'JVhile the regenerators and
fabrication methods
are particularly useful for irregularly shaped particles of brittle materials
such as Gds(Si,Gez) and
many RE/TM compounds, other regenerator materials or materials having regular
shapes can be
used.
Successful use of monolithic regenerators 129) 149 requires satisfactory
mounting in a
cooling system. If fluid to be cooled flows arowtd the regenerator and not
through the
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regenerator, the efficiency of the regenerator is diminished. Forming a
monolithic regenerator
within a sleeve that is part of a cooling system is therefore especially
advantageous.
Superior results are obtained when the binding agent mixture 111 is an epoxy
mixture
diluted with a solvent such as toluene, but other polymers can also be used.
In general, a polymer
should be soluble in organic solvents, have a high thermal conductivity, and
have a thermal
expansion coefficient near that of the particles. The polymer should also have
sufficient tensile
strength to bind the particles and should adhere to both the particles and
containers. The tensile
strength of STYCAST epoxy is 20,000 psi. These factors can be traded-off,
particularly if the
binding agent forms thin layers as applied. With thin layers, thermal
expansion and thermal
conductivity are of lesser importance. However, the binding agent should
withstand thermal
cycling.
It will be apparent that the regenerators need not be monolithic and may be
made
structurally sound by similar coating and confinement methods. By applying a
polymer or metal
coating to the regenerator particles and then confining the particles in a
regenerator container, a
regenerator using brittle particles is realized. The coating reduces particle
fracturing and holds
fractured particles together. Because the particle coating does not bind the
particles into monolithic
regenerator, the coating can be thin so that fluid flow through the aggregate
of particles is not
impeded. It will be apparent that the coating need not completely encase the
particles.
Coated regenerator particles can be used to form regenerator assemblies. Such
regenerator assemblies comprise unbound but coated particles held in a
regenerator container. For
such regenerator assemblies, a binding agent is used to coat the particles but
need not bind the
particles into a monolithic regenerator. Screen separators or perforated
plates confine the particles
while permitting fluid flow. It will be apparent that mufti-layer regenerator
assemblies can be
formed of these unbound, coated particles by layering coated regenerator
particles in a regenerator
container.
Having illustrated and demonstrated the principles of the invention, it should
be apparent
to those skilled in the art that the embodiments can be modified in
arrangement and detail without
departing from such principles. For example, regenerator materials other than
RE/TM
compounds, such as Gds(SiZGe2) and Gds(SixGe,_x), (x <_0.5) or other compounds
can be used.
The particles can have various sizes or shapes. The number of layers of
regenerator materials can
be varied and the layers can be in direct contact or separated with screen
separators or perforated
plates. In addition, while the regenerators and methods described use
irregularly-shaped particles,
spherical particles or other regular shapes can be used. Similarly, the
regenerator materials need
not be brittle.
