Patent Description:
Nanoporous materials generally include a framework supporting a random structure of voids, or pores. They have many uses, for example, thermal insulation, liquid storage media and wick materials for capillary pump loop and heat pipe thermal management systems.

Capillary pump loop (CPL) and heat pipe (HP) thermal management systems use capillary action to remove heat from a source. For particular aerospace applications, CPLs and HPs need to operate over a long distance and against gravity. They have many applications, for example, as reliable cooling systems for high power density and high heat load systems such as compact directed energy weapons (DEW), air- and spacecraft avionics, radar, communications, and flight control electronics operating in high g-acceleration environments or at remote locations from the primary cooling system. In general, a CPL or HP includes a wicking structure that uses liquid coolant to move heat between an evaporator and a condenser. The ability to move coolant over long distances and against gravitational forces depends critically on the permeability and pore size properties of the wick. The highest performing wick materials are theorized to have interconnected open porosities of <NUM>% or more and pore cell sizes less than <NUM> nanometers.

There is currently no satisfactory way of making high performance wick materials with both nanometer dimension pores and high porosity/permeability. Current state-of-art CPL/HP wicks have micrometer and larger size pores which significantly limit the capillary liquid pumping pressure and thus condenser-evaporator separation and the g-acceleration factor they can effectively operate under. Current wick manufacturing technology melts or fuses polymer and/or metal powders to produce monolithic tube or slab wicks. These melt/sintered wicks have pore diameters in the range of ~<NUM>,<NUM> to <NUM>,<NUM> (-<NUM> to <NUM>). In addition, the pores are not regularly or uniformly distributed along the wick length. Further, the melted/sintered wicks have low porosities of the order of only <NUM>-<NUM>%. An alternative wick structure used in CPL/HP systems is tubes with axial grooves/channels. While such wicks have higher and more uniform porosity, they have much larger capillary pores and are less able to develop high working capillary pressures.

<CIT> discloses a vinyl chloride-based copolymer porous body contains a vinyl chloride-based copolymer as the main component. The vinyl chloride-based copolymer porous body has continuous pores having a pore size of <NUM> to <NUM>, the pores have a skeletal diameter of <NUM> to <NUM>, and the vinyl chloride-based copolymer has a thickness of <NUM> or more. Such a vinyl chloride-based copolymer porous body can be produced by a production method including the steps of heating and dissolving the vinyl chloride-based copolymer in a solvent to obtain a vinyl chloride-based copolymer solution; cooling the vinyl chloride-based copolymer solution to obtain a precipitated product; and separating and drying the product to obtain a porous body containing the vinyl chloride-based copolymer as the main component.

<CIT> discloses organic polymer aerogels, articles of manufacture, and uses thereof. The aerogels include an organic polymer matrix and microstructures dispersed within the aerogels, which provides for superior thermal conductivity and mechanical properties.

<CIT> discloses fiber-reinforced organic polymer aerogels, articles of manufacture and uses thereof. The reinforced aerogels include a fiber-reinforced organic polymer matrix having an at least bimodal pore size distribution with a first mode of pores having an average pore size of less than or equal to <NUM> nanometers (nm) and a second mode of pores having an average pore size of greater than <NUM> and a thermal conductivity of less than or equal to <NUM> mW/m- K at a temperature of <NUM>° C.

<CIT> relates to foams made by polymerizing high internal phase emulsions (HIPE) containing polyelectrolytes. The emulsions comprise a continuous oil phase and a co- or discontinuous aqueous phase. The resulting foams are useful as a separations medium.

The present description encompasses a nanoporous wick structure derived from, for example, thermoplastic or thermoset polymer gels in which a gelation solvent is carefully removed so as to preserve an expanded monolithic gel structure consisting of intertwined and/or crosslinked polymer molecular fibrils.

Features of example implementations of the invention will become apparent from the description, the claims, and the accompanying drawings in which:.

A representative nanoporous gel structure is shown in <FIG>. In an embodiment, nanoporous foam or cellular materials are derived from, for example, thermoplastic or thermoset polymer gels in which the gelation solvent is carefully removed so as to preserve the expanded monolithic gel structure consisting of intertwined and crosslinked polymer molecular fibrils indicated, for example, at <NUM> and <NUM>. The nanoporous gel structure of <FIG> is magnified, the scale of <NUM> is shown in the lower right corner.

A method of making a nanoporous open-cell structure as shown in <FIG> is discussed with reference to <FIG>. In <FIG> the aerogel structures are derived from polymer gels in which the gelation solvent is exchanged/removed through lyophilization or critical fluid processing (zero-surface tension drying methods) to produce a low density, free-standing, open pore body of <NUM> - <NUM>% porosity with sub-<NUM> pores. In a first step <NUM>, a solvent is heated. The solvent is selected from toluene, xylene, benzene or mixtures thereof; trichloroethane, trichlorobenzene or mixtures thereof; polybutenes, decalin or mixtures thereof. Next, in step <NUM>, a polymer powder is dissolved in the solvent. In an embodiment, the polymer is polyethylene however, other polymers such as polypropylene, polybutylene or mixtures thereof may be used.

The heated solvent-polymer solution is cooled in step <NUM> so that the polymer forms fibrils or thread-like particles which intertwine and form a soft coherent body. The coherent body has a low yield stress compared with normal solids and resists movement and flow as a free liquid. The yield stress of the gel depends on the particular polymer-solvent combination, the polymer molecular weight and concentration and can vary from <NUM> to over <NUM>,<NUM> Pa. The polymer fibrils may exist in a physically intertwined aggregate structure or as chemically covalently crosslinked molecules. In step <NUM>, the solvent is exchanged for one with a critcal point less than approximately <NUM> or a solvent with a freezing point less than approximately <NUM> and sublimation vapor pressure greater than <NUM> Pa. Finally, in step <NUM>, the replaced solvent is carefully removed by critical point drying or lyophilization leaving the nanoporous polymer gel structure intact in a dry state. Solvents used in step <NUM> include liquid carbon dioxide, methane, ethane, propane, ethylene or mixtures thereof when the solvent is removed by critical point drying. Solvents used in step <NUM> also include menthol, camphene, tert-butanol or mixtures thereof when the solvent is removed through lyophilization.

In <FIG>, showing an example not according to the invention, the first three steps <NUM>, <NUM> and <NUM> for producing the aerogel structure are the same as the process shown in <FIG>, only steps <NUM> and <NUM> are altered in this production method to permit normal evaporative drying of the gel structure. In fourth step <NUM>, the solvent used to dissolve the polymer and form a gel is exchanged with a solvent having a surface tension less than approximately <NUM> mN/m and more preferably less than <NUM> mN/m. Examples of low surface tension solvents include but are not limited to: fluroalkanes and alkyl compounds such as perfluoro-hexane, - heptane, -octane, hexafluorobenzene or mixtures thereof. Further examples of low surface tension solvents include certain dialkyl ethers such as di- methyl, ethyl, propyl, isopropyl, butyl ethers or mixtures thereof. These exchange solvents also exhibit appreciable vapor pressures below <NUM> aiding their evaporation and removal from polymer gels. In fifth step <NUM> these low surface tension exchange solvents are evaporated to produce intact nanoporous monolthic polymer gel structures in a dry state.

A nanoporous open-cell foam as described above has a variety of applications. In an example, a nanoporous foam is used as a wick structure in a capillary pumped loop (CPL) or heat pipe (HP). A representative CPL/HP <NUM> is shown in <FIG>. It is a two-phase heat transfer device that moves heat from heat source <NUM> to heat sink <NUM>. Conduits <NUM> and <NUM> form a loop between evaporators <NUM>, <NUM> and <NUM>, adjacent to heat source <NUM>, and condenser <NUM>, adjacent to heat sink <NUM>. HP/CPL <NUM> is filled with a coolant liquid such as ammonia or Freon®, for example. Heat source <NUM> causes the liquid in evaporators <NUM>, <NUM> and <NUM> to vaporize and travel to condenser <NUM> through conduit <NUM>. In condenser <NUM>, heat sink <NUM> causes the vapor to cool and condense back into liquid which is transferred back to evaporators <NUM>, <NUM> and <NUM> through conduit <NUM>. HP/CPL <NUM> uses capillary action to move vapor and liquid through the system. Capillary action relies on intermolecular forces to cause liquid to flow in narrow spaces without assistance.

To take advantage of capillary action, HP/CPL <NUM> uses a nanoporous wick in evaporators <NUM>, <NUM> and <NUM> as shown in <FIG>. Performance of CPLs and HPs are critically dependent on the capillary wick used for internal transport of liquid phase coolant between condenser and evaporator. The capillary wick controls coolant flow rate and thus governs system heat load capability and its maximum operating g-force and separation distance between evaporator and condenser. Nanoporous materials derived from polymer gels with pore diameters less than <NUM> nanometers greatly increase CPL/HP capillary pressures and thereby increase limits of condenser-evaporator separation and/or height as well as their g-acceleration environmental limit.

In an embodiment, nanometer-dimension, intertwined fibrillar or tendrillar gel structures and particular chemical production methods are used to make aerogel wicks that enable extremely high performance CPL and HP systems. These aerogel structures may be fabricated in a variety forms and shapes as illustrated in <FIG> and used in different HP/CPL designs. For example, a wick structure may be formed as an annular tube <NUM> shown in <FIG>, a solid cylinder <NUM> as shown in <FIG> or a slab <NUM> as shown in <FIG> having a variety of dimensions. In addition, the nanoporous open-cell foam may be combined with micropore wicks as shown at <NUM> in <FIG> and discussed in more detail below.

Another advantage of the invention is that the nanoporous gel structures may be incorporated or deposited in microporous bodies to reduce or convert their effective pore cell size from the micrometer scale down in to the nanoscale region. Examples of pore cell size reduction or conversion are shown in more detail in version of wick structures illustrated in <FIG>, an example of which is shown in <FIG>. Chemically crosslinked and/or physically intertwined aerogel structures may be used to convert current microporous technology open pore foams and wicks into bi-pore structures. In this example, the coarser micropore foam or wick material serves as a scaffolding or host medium in which aerogel is grown or deposited. This effectively converts the micropore foam or wick into a nanoporous structure. A micropore polyethelene wick <NUM> of <FIG> has a plurality of pores <NUM> resulting in approximately <NUM>% porosity. Although pores <NUM> are shown as tubes extending along the length of wick <NUM>, they make take a variety of forms, including grooves around the circumference as shown at <NUM> in <FIG> or a solid cylinder of microporous foam. A graph showing a distribution of pore sizes relative to <NUM> microns is shown at <NUM> in <FIG>. A nanoporous open-cell foam <NUM> of <FIG> may be added to micropore wick <NUM> of <FIG>, resulting in a graph <NUM> in <FIG> showing a distribution of pore sizes relative to. <NUM> microns (<NUM>). Although a specific micropore wick is shown in <FIG>, it should be understood that this method may be applied to any micropore wick or foam.

The range of capillary pressures that can be developed with nanoporous wicks is shown in <FIG> and compared with pressures generated by wicks with micrometer (µm) sized pores. Pore diameter in nanometers (nm) is plotted vs. capillary pressure in megapascals (MPa). Nanoporous wicks, those having pore sizes of <NUM> or less, exhibit capillary pressures for water, ammonia and Freon-<NUM> that are much higher than the capillary pressures available for wicks with micrometer - sized pores greater than <NUM>,<NUM> (<NUM>).

Nanoporous wicks can be made from a variety of thermally and chemically stable polymer materials enabling them to be used with a wide selection of polar and non-polar liquid coolants ranging from Freon to water. A wide variety of polymers from low temperature capability polyethylene (PE) to much higher service temperature polymers like polyimides that are compatible with wide choice of coolant fluids may be used.

In a further example, nanoporous open-cell foam as described above is used as thermal insulation. <FIG> is a graph showing a plot of gas phase thermal conductivity in mW/mK (milliWatts per milliKelvin) vs. pore diameter in nanometers. Microporous materials generally have pore diameters of <NUM>,<NUM> and higher, in other words, materials to the right of line <NUM> in the graph. Nanoporous materials such as those described above generally have pore diameters in the range of <NUM> or less, shown to the left of line <NUM> in the graph. The performance advantage of nanoporous insulating foams over microporous foams is also achieved over a wide operating temperature range as illustrated by the -<NUM>, <NUM> and <NUM> temperature curves in <FIG>.

Claim 1:
A method of forming a nanoporous open-cell foam comprising the steps of:
heating a first solvent selected from the group consisting of toluene, xylene, benzene and mixtures thereof, trichloroethane, trichlorobenzene and mixtures thereof, and polybutene, decalin and mixtures thereof;
dissolving a polymer powder selected from the group consisting of polyethylene powder, polypropylene powder, polybutylene powder or a polypropylene and polybutylene mixture in the heated solvent;
cooling the solvent polymer mix to form a polymer fibril structure;
exchanging the first solvent for a second solvent selected from the group consisting of liquid carbon dioxide, methane, ethane, propane, ethylene and mixtures thereof or menthol, camphene, tert-butanol and mixtures thereof; and
removing the second solvent from the polymer fibril structure, wherein the removing step comprises lyophilization or critical point drying.