Note : Les descriptions sont présentées dans la langue officielle dans laquelle elles ont été soumises.
Back~round of the Invention
This invention relates to heat of fusion materials,
and more particularly, to a nucleating device for use with
such materials.
In recent years much work has been done on materials
for storing heat and cold. This work has been closely related
to the use of solar and other forms of energy for heating
and cooling purposes. Thermal energy storage is particularly
! 30 desirable if one wishes to store daytime excess solar heat
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for later use at night and possibly for use on cloudy days.
Heat storage materials can accumulate or release thermal
energy as specific heat (sensible) or as heat of fusion
(latent) or can utilize both forms of heat storage. In
most cases the heat of fusion of materials is used because -
of its relatively large heat storage capacity per unit weight ~-
of the heat storage material. ~
To be useful for thermal energy storage a material -
preferably should be low cost, available in large quantities,
be easily prepared and relatively non-toxic, non-flammable,
non-combustible and non-corrosive. Among the materials which
meet these general criteria are the large volume chemicals ;~
based on compounds of sodium, potassium, calcium, magnesium,
aluminum and iron. Preferably, these materials are in the
form of salt hydrates to take advantage of the high heat of
fusion of the hydrate groups. Most typically, these low
cost compounds are chlorides, sulphates, nitrates, phosphates,
and carbonates, or mixtures thereof, while additives or
modifiers may include the borates, hydroxides and silicates.
Among the materials which have been proven satis-
factory for this purpose are included the following salt
hydrates:
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Heat of
Fusion
BTU
Chemical Melting per Der.s~ty
Compound Point,F Pound lb/ft
Calcium chloride hexa- CaC12.6H2O 84-102 75 102
hydrate
Sodium carbonate deca- Na2CO3.1OH2O 90-97 106 90
hydrate
Disodium phosphate Na HPO .12H2O 97 114 95
dodecahydrate 2 4
Calcium nitrate tetra- Ca(NO3)2.4H2O 102-108 60 114 -
hydrate
Sodium sulfate deca- Na2SO4.1OH2O 88-90 108 97
Sodium thiosulfate 2 2o3-5H2o 118 120 go 104
pentahydrate
These thermal energy storage materials, as noted,
melt at definite temperatures and store thermal energy as
their heat of fusion or heat of melting. The melting point
may be characterized as the equilibrium crystallization
temperature where the liquid or melt and solid or crystal co-
exist. Unfortunately, all of these materials with well defined -
equilibrium crystallization temperatures or melting points can
and do supercool below their actual melting or freezing point.
In this case, their latent heat of fusion remains latent until ~ -
crystallization starts and is completed, thus leaving only the
specific heat of the melt, which is relatively low, to be used.
While supercooling is generally terminated by spontaneous
crystallization, such crystallizations usually start many
degrees below the actual melting point. For this reason such
supercooling is generally undesirable and is usually avoided
by the use of crystal seeding or nucleating agents as required.
In an article entitled "Nucleation of Supersaturated
Inorganic Salt Solutions" by Dr. Maria Telkes, which was
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` published in Industrial and Engineering Chemistry, Vol.44,
page 1308, June 1952, it is noted that crystallization from
supersaturated solutions may be catalyzed by heterogeneous
materials. This is true if the crystal forms of the two
; materials are similar in atomic arrangement and lattice
spacing. This phenomenon, known as epitaxis, was utilized
in an invention described in U.S. Patent 2,677,243 issued
May 4, 1954 to Dr. Maria Telkes. Therein is described a
~evice that is cooled by artificial means reserving part of
the thermal storage mass in the solid state such that solid
crystals act as nuclei or seeds which automatically induce
crystal formation. While the methods and apparatus in said
patent afford certain advantages, there is in their attain-
ment a need for containers with tubular extensions in which
the nuclei may be held. Since these extensions are difficult
to handle, improvements over this patent or new approaches to
the problem are needed.
Another techni~ue for nucleating thermal storage
materials involves the use of another substance that is partly
soluble or insoluble in the melt and yet has a similar crystal
form, being isomorphous or partly isomorphous or capable of
nucleation by epitaxis. Such a method is described in U.S.
Patent 2,677,664 issued May 4, 1954 to Dr. Maria Telkes. In
this patent there is described the nucleation of sodium
sulphate decahydrate utilizing small amounts of borax of
closely similar crystal form. While the compositions in
this patent are effective, one often finds it difficult in a
given situation to discover a suitable nucleating agent -
based on similar crystal structure or epitaxis.
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Still another example of heterogeneous nucleation
is found in U.S. Patent 2,677,367 issued May 4, 1954 to
Dr. Maria Telkes. In this patent there is described a means
for nucleating disodium phosphate dodecahydrate using glass
' of a definite configuration, which may be a cellular or
sintered mass, the glass being formed to have fractured or
broken or chipped surfaces. This patent also teaches the use
of glass wool as a crystallization promoter wherein the
filaments of the fibers with sharp pointed ends provide the
uneven surface area. Small bundles of this glass wool are
tied to floats such that they are maintained at the top portion
of the melt. The glass container itself may be roughened such
as by etching or by sandblasting to obtain the fractured
surface. While the uneven or fractured glass surfaces do
promote crystallization, it has been found that a glass
crystallization promoter of this type does not work for
~3~ all kinds of salts, a wider range being desired for nucleating
devices and methods. Further, the cells in the material are
too large, making it difficult to prepare crystals therein
and to get satisfactory results.
According to D. Turnbull (Journal of Chemical
Physics, Vol. 18, page 198, February, 1950) nucleation in
melts must be induced by nucleating agents (sometimes called
catalysts of crystal formation). These are influenced by the
thermal history of the melt. Turnbull theorized that these
"Catalysts" are due to microcavities which must be small
and filled with crystals of the material to be nucleated.
It is, therefore, an object of this invention to
create artificial microcavities as an aid to nucleation.
An object of this invention is to obviate
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disadvantages of the prior art nucleating devices.
Another object of this invention is to provide
an improved nucleating device.
A further object of this invention is to provide
an improved method for manufacturing nucleating devices which
have wide ranges of applicability with many materials.
Brief Description of the Invention
The present invention, in one embodiment, resides
in a nucleating device for a melt comprising a solid glass
material which is floatable in said melt while said melt is
in a sealed container and which glass material has a dis~
ordered lattice that has microcavities therein, and solid ~ -
crystals contained in said microcavities which act as nuclei
in the crystallization of said melt when said device is
floating in said melt, said crystals being held in said micro- --~
cavities oriented as to said melt to communicate with said
melt thereby facilitating seeding and recycling of said melt
to solid and solid to melt changes. ~ -
In another embodiment, this invention resides in
a method of preparing a nucleating device for a melt com-
prising: selecting a material having highly disordered
lattices, forming microcavities in said material by removing ~
parts of the lattices, at least partially filling some of -
said microcavities with said melt, and allowing the melt to ~-
solidify so as to deposit solid crystals in said micro-
cavities.
In a particularly preferred embodiment, the
material having the disordered lattice is a soda-silica glass
and is in the form of a float adapted to float on the surface
of the melt. In still other embodiments the glass float is ;
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covered with glass fibers which have been similarly processed
so as to have microcavities containing solid crystals of a
material the same as or of similar crystal structure as the
material to be crystallized.
In accordance with a preferred method of this
invention, a material having a highly disordered lattice
is selected, soda glass being such a starting material.
Microcavities are then formed in the material by removing
parts of the lattice as by leaching. The resulting micro-
cavities are at least partially filled with solid crystals
known to act as nucleating materials for the melt or solution. ~ -
The microcavity/nucleating device is constructed so that it
will float by itself on the melt or is attached to a buoyant
~ body which floats.
- Brief Description of the Drawings
' The invention, both as to its organization and
; method of operation, as~;well as additional objects and
advantages thereof, will be further understood from the
following description when read in connection with the
, 20 accompanying drawings which are not limitative and, in which:
FIGURE 1 iS a two dimensional representation of
the structure of soda-silica glass.
FIGURE 2 is a two dimensional schematic represen-
tation of soda-silica glass after removal of some of the
' surface sodium in the form of compounds, as the silicate,
s thereby forming microcavities therein in accordance with
this invention; and
FIGURE 3 is a cross-sectional elevation view of a
typical container containing a heat of fusion material having
therein a nucleating device constructed in accordance with
this invention.
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Description of the Preferred Embodiment
A nucleating agent or device is described having
microcavities which are made artific~lly according to the
method of this invention. Such nucleating device for a melt
is manufactured starting with a material which has a highly
: disordered lattice and modifying said lattice so that it has
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microcavities therein, particularly near the surface regions.
Among the materials which are suitable for this purpose is
` glass. One such material which is particularly suited is
soda-silica glass which has a lattice structure of the type
illustrated in FIG. 1 in which there is illustrated a schematic
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; representation in two dimensions of soda-silicate glass which
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is one of the simplest forms of glass. As is seen in FIG.l,
the chains of the crystal lattice are composed of silicon and
oxygen atoms which entrap sodium atoms. In other varieties of
t
~, glass, calcium oxide is present in addition to the oxides of
silicon and sodium shown ln FIG. 1. While other forms of
~ glass and other materials having a less disordered lattice may - i
;~ be used, those having a more disordered lattice are preferred
since the surface atoms are more easily removed as compounds
; !~
~ to leave submicroscopic cavities or crevices of a size
; appropriate to house solid crystals of the melt to be nucleated. -
'j These crystals preferably are held such that they are properly
oriented to facilitate seeding.
According to the next step of the method of this
invention, some of the constitutents of the glass are removed
from the surface regions by chemical leaching and/or dissolving
so as to leave numerous submicroscopic crevices or cavities 8
- as is illustrated in the two-dimensional schematic of FIG.2.
. 30 These cavities result, in the case of soda-silicate glass, ~ -~
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, where the sodium atoms have been removed, as compounds from
the outer surfaces. This removal is accomplished in accord-
ance with the method of this invention by chemical leaching,
being basically a neutralization or acid/base reaction resulting
in the dissolving of and removal of glass material. Leaching
is accomplished by heating or boiling the glass in an alkaline
solution. Among the solutions that are satisfactory for this
` purpose are: sodium hydroxide, sodium carbonate, sodium
!, phosphate and others. According to this invention leaching
should be restricted to the surface and is not intended to
etch the glass deeply or remove most of its constitutents.
Preferably, the leaching should be such as to create micro-scopic
sized surface cavities which typically are less than 1000
Angstrom in size (diameter) and preferably are in the range
, of about 5 to about 100 Angstrom or less. Leaching, for
example, can be accomplished by boiling the soda-silicate
glass in a 10% solution of disodium phosphate Na2HPO4 in
water for a period of one-half hour. After this, the glass
1 is washed with water, cooled and drained.
-, 20 As a next step, the previously leached and now
washed glass is immersed in the melt of the subject thermal
storage material so as to force the glass nucleating device
below the liquid surface to fill all the microporous crevices
with melted material. After this treatment, the glass or
nucleating device is allowed to float and in the floating
condition it is covered with powdered solid material of the
same composition as the crystals of the melt. This treatment
assures the presence of solid nuclei in the submicroscopic
crevices. Alternatively, the float may be removed from the
melt and is immediately thoroughly and completely covered with
said powder so that very little or no melt drains from the
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cavities before the powdering. The powdering step may be
omitted, since one can remove the float after its immersion
and allow the melt in the microcavities to crystallize in
time. However, the powdering step is preferred since it
facilitates the crystallization and allows one to use, the
devices of this invention with less time delay and with as-
surance that crystals are in or on the device. No special
mesh size of the powder is needed, since the float is wetted
by the melt which wetting causes the powder to adhere and
. .
causes very small crystals to enter into the microstructure.
~ However, a fine powder is, of course, preferred and is gener-
i ally used. In either procedure the powdered float may be
stored in tightly closed containers for subsequent use or -
may be used immediately.
The above method for preparing nucleating devices
of this invention may be used to create nucleating devices
for use with any of the known thermal storage materials
listed above. The devices may be any configuration and may -
be positioned at any point within the melt. One preferred
nucleating device constructed in accordance with this method
is illustrated in FIG. 3. This nucleating device is in the
form af a float made of a thin walled glass tube 11 sealed
at both ends. The float 11 is covered with glass fibers or
glass cloth 12 of a known type (pre~ferably sodasilicate
glass) sa as to provide the glass with a greatly enlarge~
~ surface area. The glass cloth 12 may be secured to the float
i by any suitable means such as glass strings or other material
13. The actual dimensions of the float 11 may vary greatly
depending upon the size and form of the container 14 which is
to be filled with the heat of fusion material 15 in its
liquid form. The container 14, as depicted herein, is shown
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` in cross-sectional area as being of generally rectangular con-
struction with the float 11 floating in the upper portion
thereof. The top surface 18 of the container 14 is illustrated
as being sloped upwardly such that an air space 20 is located
near one end or corner. This permits the float 11 to stay
within this region and float upon the upper surface of the
~i melt. The container may have other shapes as the exigencies
' of the situation require. Also, the top surface 18 may beshaped to have a protuberant bubble to accommodate the float.
With such a positioning arrangement, one can easily and
quickly locate the float(s) in containers for removal, in-
spection, or the like. One or more nucleating devices can
be used in each container which, as illustrated, is almost
completely filled but for the air space, which typically
- represents 2 to 5% of the container volume. This space is
l left mainly for the convenience in filling and applying a
J' closure or cap 16 which may be cemented or otherwise secured
'~ to close and seal the container completely.
As described above, prior to introduction into the
container, the float 11 with the covering of glass cloth 12
is processed to form the microcavities, in accordance with
this invention, and these microcavities are at least part-
ially filled with crystals of the melt. One such float was
formed of 0.5 inch O.D. glass tubing approximately one and
one-half inches long (whose ends were sealed) and covered
with several layers of glass cloth secured with glass strings
' to the float. Leaching was accomplished by boiling the float
in a 10% solution of disodium phosphate (Na2HPO4) in water
for one-half hour. The float was washed with water, cooled
and drained. Next the float was immersed in melted sodium
thiosulfate pentahydrate such as to saturate all the micro-
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porous cavities. The float is then allowed to rise to the
top and covered with powdered sodium thiosulfate pentahydrate
to assure the presence of solid nuclei in the submicroscopic
crevices. The float was then removed and transferred to its
final container which ha~ been filled with sodium thiosulfate -
pentahydrate as the heat of fusion material. The number of
floats required in a particular container is dependent upon
the size of the container. Similar floats are readily prepared
with sodium sulfate decahydrate or sodium carbonate decahydrate
as the powdered,crystal material used in forming the crystals
in the microcavities.
In nucleating devices made in accordance with this
invention preferably a large portion (typically over half)
of the microcavities should remain above the liquid level of ~-
the melt once the cavities have been filled with the micro-
crystal "seeds". This reduces the possibility of the crystals
going into solution and thereby becoming lost for seeding
purposes. Also, it is desirable as is noted, that the cavities
should be preferably formed only in the surface regions of
the disordered lattice material so that the microcavities
are able to communicate with the melt and/or disseminate
some of the crystals to the melt for seeding purposes as needed.
This dissemination occurs through partial melting of and ~ -resultant flow of melt/crystals from the cavities into the
melted thermal storage materials which melt around 120F. or
below or throughsolvent effects brought about by water vapor
or surface tension effects. In any event the nucleating devices
of this invention are all effective in seeding said melts,
thus affording ready recycling of the melt to solid and solid
to melt changes in said storage materials.
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There has thus been described a relatively simple
nucleating device which is reliable and prevents super-cooling
of the melt and finds particular use in heat of fusion materials
for the storage of thermal energy. There is also described a
unique method of constructing such nucleating devices by leach-
ing the surface of highly disordered lattice material, such
as glass, and filling the resulting microcavities with crystals
of the melt to insure that there are solid nuclei in the sub-
microscopic crevices for seeding purposes.
; 10 Many embodiments may be made of this inventive
concept, and many modifications may be made in the embodiments
hereinbefore described. Therefore, it is to be understood that
all descriptive material herein is to be interpreted merely as
illustrative, exemplary and not in a limited sense. It is in-
tended that various modifications which might readily suggest
themselves to those skilled in the art be covered by the follow-
ing claims, as far as the prior art permits.
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