Stock material or miscellaneous articles – Sheet including cover or casing – Filled with gas other than air; or under vacuum
Reexamination Certificate
1999-05-06
2003-04-08
Ahmad, Nasser (Department: 1772)
Stock material or miscellaneous articles
Sheet including cover or casing
Filled with gas other than air; or under vacuum
C428S036910, C428S036900, C428S068000, C428S074000, C428S075000, C428S138000, C428S213000, C428S215000, C428S457000, C428S469000
Reexamination Certificate
active
06544618
ABSTRACT:
TECHNICAL FIELD OF THE INVENTION
The present invention relates to an insulation composite and, more particularly, a multilayer insulation composite.
BACKGROUND OF THE INVENTION
A common type of insulation is multilayer insulation, which is especially useful for cryogenic applications. Multilayer insulation typically consists of alternating layers of highly reflecting material, such as aluminum foil or aluminized polyester (e.g., Mylar) film, and a low-conductivity spacer material or insulator, such as fiberglass mat or paper, glass fabric, or nylon net. Between twenty and forty such layers are commonly used for cryogenic applications including, for example, laboratory dewars, piping, on-site storage vessels, and transportation vessels (e.g., as part of tank trucks). In addition, multilayer insulation is advantageously kept under a high vacuum, thereby further enhancing the insulating properties of the multilayer insulation. Multilayer insulation has a very low heat transfer due to the fact that all modes of heat transfer—conductive, convective, and radiative—are minimized. The multiple layers of reflecting material have a low emissivity and, thereby, inhibit radiative heat transfer. Convective heat transfer is inhibited by lowering the pressure (i.e., creating a vacuum) between the insulation layers. Finally, the presence of spacer material inhibits conductive heat transfer through thermal short-circuits (physical contact) that might otherwise exist between the layers of reflecting material.
Despite the satisfactory performance of conventional multilayer insulation composites in many applications, there remains a need for an improved multilayer insulation composite. The present invention seeks to provide such a multilayer insulation composite, particularly a multilayer insulation composite that provides satisfactory, if not superior, thermal performance, preferably with a reduced overall mass and/or thickness. These and other objects and advantages of the present invention, as well as additional inventive features, will be apparent from the description of the invention provided herein.
BRIEF SUMMARY OF THE INVENTION
The present invention is an insulating composite comprising (a) a first thermally reflective layer having a reflective surface and an opposite surface, (b) a second thermally reflective layer having a reflective surface and an opposite surface, and (c) a porous metal oxide film having a thickness of 20 &mgr;m or less that is positioned between the first and second thermally reflective layers such that there is substantially no direct physical contact (thermal bridging) between the first and second thermally reflective layers. The present invention further includes an insulating element in which the insulating composite is disposed within an air-impermeable container.
DESCRIPTION OF THE PREFERRED EMBODIMENTS
The present invention provides an insulating composite having at least two thermally reflective layers with a porous metal oxide film positioned therebetween.
Any material that is effective in inhibiting radiative heat transfer can be used as the thermally reflective layer. Typically, such materials will have a reflective (e.g., polished) surface. The thermally reflective material preferably is characterized by a low emissivity. Also, the thermally reflective material typically will be in the form of a sheet or strip. Thus, the thermally reflective layer generally will have a reflective surface and an opposite surface. Suitable thermally reflective layers include, for example, aluminum foil. Other suitable thermally reflective layers include polymeric (e.g., polyester, polyamide, polyimide, or polyolefin) substrates having aluminum deposited on one or both surfaces thereof. Such a thermally reflective layer is commercially available as aluminized polyester (e.g., Mylar) film. Other thermally reflective materials having a low emissivity, such as gold and silver, can be deposited instead of aluminum on the aforementioned substrates in certain applications. The thermally reflective layer can have any suitable thickness, preferably about 10-100 &mgr;m.
The thermally reflective layers can be the same or different. In particular, each thermally reflective layer can be constructed of the same or different material, and in the same or different manner, as other thermally reflective layers. In a preferred embodiment, all of the thermally reflective layers (e.g., the first and second thermally reflective layers) are aluminum foil, which is polished at least on one side.
The insulating composite of the present invention can further include additional thermally reflective layers, e.g., third, fourth, fifth, etc. thermally reflective layers. The discussion herein of the “first” and “second” thermally reflective layers is equally applicable to these additional (e.g., “third,” “fourth,” “fifth,” etc.) thermally reflective layers. Thus, the present inventive insulating composite can comprise successive layers of a thermally reflective material having a reflective surface and an opposite surface, such that the porous metal oxide film separates the layers of thermally reflective material. Because radiant-heat transfer is inversely proportional to the number of thermally reflective layers and directly proportional to the emissivity of these layers, radiant-heat transfer is minimized by using multiple layers of a low-emissivity thermally reflective material.
Any suitable porous metal oxide film (i.e., a porous continuous sheet or expanse of metal oxide) can be used in the insulating composite of the present invention consistent with ensuring that there is substantially no direct physical contact (thermal bridging) between the first and second thermally reflective layers, preferably between any of the thermally reflective layers, and that optimally there is no direct physical contact (thermal bridging) at all between the first and second thermally reflective layers, ideally between any of the thermally reflective layers. The porous metal oxide film most preferably is substantially coextensive or entirely coextensive with at least one of the first and second thermally reflective layers, and preferably, in some embodiments, both of the first and second (or even all of the) thermally reflective layers.
The porous metal oxide film can comprise any suitable type of metal oxide, such as, for example, silica, alumina, titania, zirconia, ceria, and magnesia. The metal oxide preferably is silica, such as, for example, fumed (or pyrogenic) silica, precipitated silica, silica aerogel, and silica xerogel, with fumed silica being particularly preferred. The metal oxide can be in the form of discrete individual particles, which can be in aggregated or non-aggregated form.
The porous metal oxide film can have any suitable density, typically about 2 g/cm
3
or less (e.g., about 0.1-1.5 g/cm
3
), preferably about 1 g/cm
3
or less (e.g., about 0.1-0.8 g/cm
3
), and most preferably about 0.7 g/cm
3
or less (e.g., about 0.1-0.5 g/cm
3
). It is preferred that the porous metal oxide film have as low a density as possible inasmuch as lower densities generally provide improved thermal performance of the present inventive insulating composite.
The porous metal oxide film has a thickness of about 20 &mgr;m or less, preferably about 10 &mgr;m or less. The porous metal oxide film more preferably has a thickness of about 5 &mgr;m or less, most preferably about 1 &mgr;m or less, although typically at least about 200 nm (e.g., about 200 nm to about 1 &mgr;m). It is preferred that the porous metal oxide film be as thin as possible inasmuch as thinner layers generally provide improved thermal performance of the present inventive insulating composite. The minimum spacing between the thermally reflective layers of the present inventive insulating composite is a function of the thickness of the porous metal oxide film (although, due to the nature of many thermally reflective layers which are not amenable to perfectly parallel spacing, the spacing between the layers typically will vary from the thickness of the poro
Ahmad Nasser
Cabot Corporation
LandOfFree
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