CA1077459A - Nucleating device and method of preparing same - Google Patents

Abstract

NUCLEATING DEVICE AND METHOD OF PREPARING SAME

Abstract of the Disclosure A nucleating device is formed of a material, such as glass, having a highly disordered lattice which has been processed to produce microcavities therein each having a size typically in the order of less than 1000 Angstroms. Solid crystals are disposed in the microcavities to aid in the crystal seeding and thereby promote crystallization of a melt or solution of a compound without supercooling. In one embodiment the nucleating device is in the form of a glass float covered with glass fibers processed so as to have microcavities with said crystals disposed in the micro-cavities. Advantageously, the crystals do not have to have the same chemical structure as the compound being crystallized.
According to the method of this invention, micro-cavities are formed in a substance having highly disordered lattices by removing parts of the lattices as by leaching.
At least a portion of the microcavities are then filled with solid crystals. The nucleating device may be in the form of a glass float or it may be in the form of glass fibers which surround the float.

Description

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 1 ~!.

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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 ..~
' ` 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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~ 1077459 : , 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.

-~ . , . ' : ~ '' ' ' , .. , ' ' , 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 ., :
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 ,,~ .. . .
; representation in two dimensions of soda-silicate glass which ,.......................................................................... .
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 ' , _g_ , ~, :

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1(~77459 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 . .

-- , ` 1077459 ` 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-~ ~ .
~ ' ' : . ' , ", ' ' ' ' ' ' ' ' ' ' ' , ' ~ ' ' ' ,, ' . ' ' ~ ' . , 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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Claims (16)

The embodiments of the invention in which an exclusive property or privilege is claimed are defined as follows:

1. 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 disordered 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 microcavities 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.

2. A device according to claim 1 wherein said microcavities are disposed in the surface of said material, thereby to facilitate the seeding of said melt.

3. A device according to claim 1 wherein said glass is a soda-silica glass.

4. A device according to claim 1 wherein said material is in the form of a float adapted to float in said melt.

5. A device according to claim 1 wherein said material is in the form of a glass float covered with glass fibers.

6. A device according to claim 1 wherein said microcavities are about 5 to 1000 Angstroms in diameter.

7. A device according to claim 4 wherein said microcavities are disposed in the surface of said material, thereby to facilitate the seeding of said melt.

8. A device according to claim 1 wherein said cavities are less than about 1000 Angstroms in size.

9. A method of preparing a nucleating device for a melt comprising:
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 microcavities.

10. A method in accordance with claim 9 which includes the step of effecting the placement of crystals for said melt inside said microcavities by at least partially covering said nucleating device with powdered solid material of the same composition as the crystals of said melt.

11. A method according to claim 9 wherein said material is glass.

12. A method according to claim 11 wherein said micro-cavities are formed in the surface of said material.

13. A method according to claim 11 wherein said micro-cavities are formed by leaching said glass with an alkaline solution.

14. A method according to claim 11 wherein said micro-cavities are filled by immersing said material in said melt.

15. A method according to claim 14 which includes the additional step of covering said material with a powdered solid material of the same composition as said melt.

16. A method according to claim 13 wherein said leaching is accomplished by boiling said material in an aqueous solution of disodium phosphate.