Abstract:
A composite adsorbent and heat burst desorption system using the same. The composite adsorbent material includes a backbone and a filler. The backbone comprises a first material having a high thermal conductivity and a plurality of pore. The filler, within the pores of the backbone, comprises a second material having a large specific surface area.

Description:
[0001]    Pursuant to 37 C.F.R. §1.78(a)(4), this application claims the benefit of and priority to prior filed co-pending Provisional Application Ser. No. 61/720,416, filed Oct. 31, 2012, which is expressly incorporated herein by reference. 
     
    
     RIGHTS OF THE GOVERNMENT 
       [0002]    The invention described herein may be manufactured and used by or for the Government of the United States for all governmental purposes without the payment of any royalty. 
     
    
     FIELD OF THE INVENTION 
       [0003]    The present invention relates generally to the rapid absorption of heat pulses and, more particularly, to adsorbents and systems for rapid absorption of heat pulses. 
       BACKGROUND OF THE INVENTION 
       [0004]    Technological advances in electronics, avionics, and automotive industries are often accompanied by a further need for energy storage. However, the energetic needs of these platforms often result in high thermal output. At times, the thermal output may include large bursts of heat (ranging from about 100 kW to about 1 MW) over a relatively short period of time (about 10 seconds to about 100 seconds). Conventional systems of managing the heat produced by these platforms have included micro- and meso-porous materials (e.g., activated carbons, zeolites, silica, and alumina gels) having large specific surface areas (ranging from about 1000 m 2 /g to about 2000 m 2 /g). However, some of these materials (zeolites and silica gels) have poor thermal conductivity due to low intrinsic thermal conductivity or poor consolidation of powders. 
         [0005]    Certain gases and vapors (e.g., NH 3 , CO 2 , MeOH, and H 2 O), known and used as adsorbates, adsorb onto solid adsorbents, such as the micro- and meso-porous materials with large heats of adsorption. Exemplary heats of desorption, on an adsorbate-mass basis, are shown below in Table 1. However, these systems (solid/vapor desorption systems) have conventionally been used as refrigerators or heat pumps that function on a continuous, low instantaneous, cooling power basis. Therefore, these conventional systems have not been suitable for use in adequately adsorbing rapid bursts of heat, particularly over the lifetime of the platform. 
         [0000]    
       
         
               
               
               
               
             
           
               
                   
                 TABLE 1 
               
               
                   
                   
               
               
                   
                   
                   
                 ΔH adsorption  in 
               
               
                   
                 Adsorbent 
                 Adsorbate 
                 [MJ/kg adsorbate ] 
               
               
                   
                   
               
             
             
               
                   
                 Silica gel 
                 CH 3 OH 
                 1.0-1.5 
               
               
                   
                   
                 H 2 O 
                 2.8 
               
               
                   
                 Activated alumina 
                 H 2 O 
                 3.0 
               
               
                   
                 Zeolites 
                 H 2 O 
                 3.3-4.2 
               
               
                   
                   
                 NH 3   
                 4.0-6.0 
               
               
                   
                   
                 CO 2   
                 0.8-1.0 
               
               
                   
                   
                 CH 3 OH 
                 2.3-2.6 
               
               
                   
                 Activated carbon 
                 C 2 H 4   
                 0.4-1.2 
               
               
                   
                   
                 C 4 H 10   
                 0.3 
               
               
                   
                   
                 CH 2 F 2   
                 0.4 
               
               
                   
                   
                 NH 3   
                 2.0-2.7 
               
               
                   
                   
                 H 2 O 
                 2.3-2.6 
               
               
                   
                   
                 CH 3 OH 
                 1.8-2.0 
               
               
                   
                   
                 C 2 H 5 OH 
                 1.2-1.4 
               
               
                   
                   
               
             
          
         
       
     
         [0006]    Other conventional approaches to thermal management have incorporated macroporous foams (e.g., graphitic, aluminum, and copper foams), having high effective thermal conductivities (ranging from about 20 W/m·K to about 200 W/m·K). Yet, macroporous foams are incapable of adsorbing large quantities of adsorbates due to low specific surface areas (ranging from about 10 m 2 /g to about 100 m 2 /g). 
         [0007]    Thus, there remains a need for large high-thermal conductivity adsorbents, particularly for use in systems configured to manage large thermal loads and accommodate short periods of rapid cooling. Furthermore, and in particularly for uses with respect to deployed military platforms, new devices are needed that are light weight, compact, low cost, low maintenance, and safe to operate (with respect to toxicity, flammability, and reactivity). 
       SUMMARY OF THE INVENTION 
       [0008]    The present invention overcomes the foregoing problems and other shortcomings, drawbacks, and challenges of conventional heat adsorption materials and systems. While the invention will be described in connection with certain embodiments, it will be understood that the invention is not limited to these embodiments. To the contrary, this invention includes all alternatives, modifications, and equivalents as may be included within the spirit and scope of the present invention. 
         [0009]    According to one embodiment of the present invention, a composite adsorbent includes a backbone and a filler. The backbone comprises a first material having a high thermal conductivity and a plurality of pore. The filler, within the pores of the backbone, comprises a second material having a large specific surface area. 
         [0010]    Another embodiment of the present invention is directed to a heat burst desorption system for absorbing rapid heat bursts from a heat load. The system includes an adsorbent bed containing an adsorbent and an adsorbate supply containing an adsorbate. The adsorbate is configured to adsorb onto the surface of the adsorbent with a large heat of adsorption. The system operates in an absorption mode and a recharge mode. In the absorption mode, when heat is transferred from the heat load to the adsorbent bed, the adsorbent bed is fluidically coupled to an exhaust such that adsorbate desorbs from surface of the adsorbent and is vented. In the recharge mode, heat is rejected from the system along a second transfer path, the adsorbent bed is fluidically coupled to the adsorbate supply such that adsorbate adsorbs onto the surface of the adsorbent. 
         [0011]    According to still another embodiment of the present invention, a method of absorbing rapid heat bursts from a heat load includes decompressing an adsorbent bed, containing an adsorbent, when heat is generated. When heat generation cases, the adsorbent bed is recharged with an adsorbate, which is configured to adsorb onto a surface of the adsorbent. 
         [0012]    Yet another embodiment of the present invention is directed to a composite adsorbent having a thermally-conductive backbone and a filler. The backbone includes a first material having plurality of pore therein and the filler is within those pores. A second material, comprising the filler, has a large specific surface area. 
         [0013]    In accordance with another embodiment of the present invention, an adsorbent includes a plurality of thermally-conductive elements. Elements of the plurality interface with other elements of the plurality to form a three-dimensional network. The elements may include carbon nanotubes, multi-wall carbon nanotubes, metallic nanowires, semiconducting nanowires, graphene, or a combination thereof. 
         [0014]    An embodiment of the present invention includes a composite adsorbent having a backbone and a filler. A first material, which comprises the backbone, includes a plurality of pores therein and a thermal conductivity greater than about 20 W/m·K. A second material, which comprises the filler, has a specific surface area greater than about 1000 m 2 /g. 
         [0015]    The above and other objects and advantages of the present invention shall be made apparent from the accompanying drawings and the descriptions thereof. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0016]    The accompanying drawings, which are incorporated in and constitute a part of this specification, illustrate embodiments of the present invention and, together with a general description of the invention given above, and the detailed description of the embodiments given below, serve to explain the principles of the present invention. 
           [0017]      FIG. 1  is a flowchart illustrating methods of forming a composite adsorbent in accordance with two embodiments of the present invention. 
           [0018]      FIG. 2  is a schematic representation of one method shown in  FIG. 1 . 
           [0019]      FIG. 3  is a schematic representation of one method shown in  FIG. 1 . 
           [0020]      FIG. 4  is a schematic representation of a three-dimensional composite adsorbent in accordance with another embodiment of the present invention. 
           [0021]      FIG. 5  is a diagrammatic view of a burst desorption system in accordance with one embodiment of the present invention. 
           [0022]      FIGS. 5A and 5B  illustrate operation of the burst desorption system during an absorption mode and a recharging mode, respectively. 
       
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
       [0023]    Turning now to the figures, and in particular to  FIGS. 1-3 , methods for building composite, high-thermal conductivity adsorbents in accordance with embodiments of the present invention are described. A first method, as shown in the flowchart  10  of  FIG. 1 , begins with selection of a material comprising a porous backbone  12  (Block  14 ) for a desired composite adsorbent  16 . The material is generally porous (illustrated as pores  18 ) and may be selected, at least in part, on (1) the thermal conductivity characteristics of the material and (2) the desired degree of heat pulse absorption. Graphitic foams, such as commercially-available POCOFoam and Koppers KFOAM, described in detail in U.S. Pat. No. 6,033,506, the disclosure of which is incorporated herein by reference in its entirety, have a thermal conductivity, low bulk density, and a connected open network of pores with tunable diameters. 
         [0024]    In Block  20  of the flowchart  10 , an outermost layer of the material comprising the backbone  12  is exfoliated and fills pores  18  of the backbone  12 . One method of exfoliation, according to one exemplary embodiment, places the porous backbone  12  into an acidic bath such that ionic solvent may intercalate into the pores  18 . The intercalating species (illustrated as intercalating solvent  22 ) may include, for example, SO 4 , NO 3 , or other like ionic groups. With the intercalating solvent  22  permeated throughout the backbone  12 , the backbone  12  may then be rapidly heated such that outermost layers exfoliate from the backbone  12  and reside within the pores  18  as exfoliate filler  24 . 
         [0025]    If desired, the backbone  12  and/or the exfoliate filler  24  may be surface treated so as to further tune adsorbate-adsorbent interaction within the composite adsorbent  16  (Block  26 ). For instance, polar ionic species (e.g., acidic functional groups, nitrates, and sulfates) may be added to change the polarity of the adsorbent surface of the backbone  12 . Additionally, or alternatively, liquid, vapor, or plasma treatments may be used to further modify the adsorbent surface of the backbone  12  or of the filler  24 . 
         [0026]    A second method of forming a composite adsorbent  28 , also shown in the flowchart of  FIG. 1 , begins with selection of a first material comprising the porous backbone  12 . Again, the backbone  12  is generally porous (illustrated as pores  18 ) (Block  14 ). According to the instant embodiment, the pores  18  are filled with a second material differing from the material comprising the backbone  12 . One such method of incorporating the second material, i.e., a filler  30 , into the pores  18  includes submerging the backbone  12  into a solution  32  containing one or more sol-gel initiators, for example, an aqueous resorcinol-formaldehyde solution (Block  34 ). 
         [0027]    As residual solution is evaporated from the backbone  12 , whether by supercritical or freeze drying, an aerogel or cryogel  30  forms, respectively, within the pores  18  (Block  36 ). If necessary, or desired, a reactive gas species may be directed through the filler  30  to develop a desired distribution of porosity (Block  38 ). And, as described previously with respect to the composite adsorbent  16  of  FIG. 2 , the composite adsorbent  28  and/or aerogel/cryogel filler  30  may be surface treated (Block  26 ), if desired or necessary. 
         [0028]    In some instances, high thermal conductivity may be required to extend in three dimensions. A three-dimensional adsorbent  40  suitable for such uses and according to one embodiment of the present invention is shown and described with respect to  FIG. 4 . The illustrative three-dimensional adsorbent  40  comprises a three-dimensional network of connected, high-thermal conductivity of elements  42  configured to form a dense mesh (on the order of 10 8  interfaces/m to 10 9  interfaces/m). 
         [0029]    Each element  42  may be a carbon nanotubes (CNTs), a multi-wall CNT, a metallic or semiconducting nanowire, graphene, or other 2D structured material. The three-dimensional adsorbent forms a three-dimensional network to transmit heat over a large volume while providing a porous-like structure between adjacent ones of the elements  42 . The illustrative structure permits significant adsorption of adsorbate species without restricting the flow of adsorbate molecules, the necessity of which is described in greater detail below. 
         [0030]    One approach of synthesizing embodiments of the three-dimensional adsorbents  40  may include template-assisted growth from graphene. In this approach, two-dimensional graphene is grown on all outer surfaces of a connected network of metal, e.g., Ni, after which the metal is dissolved to leave the residual, covalently bonded CNT network. 
         [0031]    While the three-dimensional adsorbent  40  may have large inherent thermal conductivity, the high interfacial density may suggest to those of ordinary skill in the art that the effective conductivity of the three-dimensional adsorbent  40  will be dominated by thermal interfacial resistance at nodes  44  formed at the intersection of connected individual elements  42 . Small interfacial thermal resistance is likely to require strong bonding forces at the nodes  44  (e.g., covalent, ionic, or metallic bonds), which minimize the scattering of photons at those interfaces. Mechanisms to promote these nodes  44  may include introduction of metallic nanoparticles (which act as hubs connecting multiple tubes/wires), covalent bonding between individual elements (e.g., branching, covalently bonded CNT structures), or other similar mechanisms. Large specific surface areas of the three-dimensional adsorbent  40  may be derived from the large intrinsic specific surface areas of individual elements  42 . Given that very little area is consumed by the nodes  44 , the three-dimensional adsorbent  40  will, for the large part, maintain the specific surface area of the individual elements  42 , which maximizes the area for gas or vapor molecules to adsorb to the adsorbent. 
         [0032]    Composite materials, as described in the various embodiments of the present invention herein, are light-weight and have a high-thermally conductive. Generally, the high-dielectric porous backbone provides the desired high thermal conductivity while filler materials provide a large surface area, which promotes adsorbate interaction. By minimizing the thermal interfacial resistance between the backbone and the filler, heat transfer through the composite material is highly efficient. 
         [0033]    The composite materials, according to the various embodiments of the present invention, may be utilized in improving systems configured to quickly absorb bursts of heat. That is, these composite materials, when used with suitable gas or vapor adsorbates, rapidly absorb large pulses of heat. In that regard, and with reference now to  FIG. 5 , a burst desorption system  50  is shown and described in accordance with one embodiment of the present invention. The burst desorption system  50  includes a supply, illustrated as a tank  52 , containing pressurized adsorbate  54 , which may be selected, for example, from the listing provided in Table 1 above. The total volume of the tank  52  may be selected so as to compensate for the dissipation of heat required by the particular heat load  56  over the lifetime of the heat load  56  while a maximum pressure within the tank  52  is limited by safety considerations. 
         [0034]    The fraction of the adsorbate to be used during system operation is determined by minimum and operating pressures. Exemplary pressures for the adsorbate tank  52  may range from about 50 bar to 100 bar; however, these pressure ranges are not considered to be limiting. Ideally, the adsorbate may be stored in the liquid phase to minimize the overall volume of the system. 
         [0035]    The adsorbate tank  52  is fluidically coupled to an adsorbent bed  58  containing a composite adsorbent  60  according to embodiments of the present invention. The adsorbent bed  58  is operable over a range of pressures, for example, pressures ranging from about 10 bar to 50 bar, or, as necessary, to atmospheric pressure; however, these pressure ranges are not considered to be limiting. In any event, the adsorbent bed  58  may generally be under a lower pressure after discharge and a higher pressure after recharged, as provided in detail below. In some embodiments, the adsorbent bed  58  may not be pressured over the condensation pressure of the adsorbent  60  as depressurization may lead to rapid expansion within the adsorbent bed  58 , which is likely to irreversibly damage the adsorbent  60 . Furthermore, the maximum pressure of the adsorbent bed  58  may be intrinsically linked to the minimum pressure of the storage tank  52 , as the storage tank  52  is maintained at a higher pressure than the adsorbent bed  58  in order to effectively recharge the adsorbent  60 . 
         [0036]    In use, the system  50  is operable in an absorption mode, during which heat is absorbed from the heat load  56 , and in a recharge mode, during which the system  50  prepares for subsequent heat absorption by transferring absorbed heat to a cooling loop  62  of, for example, a fuel tank (not shown). More specifically, and in the absorption mode illustrated in  FIG. 5A , heat is transferred from the heat load  56  to the adsorbent  60  of the adsorbent bed  58 . Resultantly, the adsorbent bed  58  decompresses, causing some quantity of adsorbate  54  to desorb from the surface of the adsorbent  60 , which dissipates a certain quantity of heat. The quantity of heat is related to the product of the quantity of adsorbate  54  desorbed and the specific heat of desorption. Further dissipation of heat may occur by the expansion of adsorbate while being exhausted via valve  64 . The ordinarily-skilled artisan would readily appreciate that without such an open system, heat may be generated when the adsorbate  54  is recondensed or adsorbed. 
         [0037]    Between heat burst generations, the system  50  is operated in a recharging mode to prepare for a subsequently generated heat burst. In that regard, the exhaust valve  64  is closed so that the adsorbent bed  58  may be re-pressurized from the adsorbate tank  52  via valve  66 . Because adsorption is essentially the inverse process to desorption, some quantity of heat may be generated at the adsorbent bed  58 , which must be rejected for the adsorption process to continue. However, generated heat is offset by the quantity of heat consumed by evaporating and/or decompressing adsorbate  54  from the tank  52 . If the net effect is heat generation, the heat may be dissipated by some other element on the platform, including, for example, the cooling loop  62  of a fuel tank (not shown) or to the ambient environment by way of an air-heat exchanger. However, the rate of available heat rejection may limit the duty cycle of the system  50 . 
         [0038]    The skilled artisan will, with the benefit of the disclosure provided herein, note the particular benefit of a pressure cascade to store adsorbate gas/vapor, to control adsorption/desorption, and to regulate venting of the adsorbate  54 . 
         [0039]    The heat burst desorption system described according to the embodiments of the present invention herein may be configured to manage large thermal loads (about 100 kW to about 1 MW) with a rapid response time (less than about 1 second). Recharging capabilities are possible with a duty cycle of about 5% to about 50%, and, generally, the systems may be operational for extended periods of time, for example, missions ranging from 100 sec to 10,000 sec. 
         [0040]    While the present invention has been illustrated by a description of various embodiments, and while these embodiments have been described in some detail, they are not intended to restrict or in any way limit the scope of the appended claims to such detail. Additional advantages and modifications will readily appear to those skilled in the art. The various features of the invention may be used alone or in any combination depending on the needs and preferences of the user. This has been a description of the present invention, along with methods of practicing the present invention as currently known. However, the invention itself should only be defined by the appended claims.