Pitch-based carbon foam and composites and use thereof
A thermally conductive carbon foam is provided, normally having a thermal conductivity of at least 40 W/m·K. The carbon foam usually has a specific thermal conductivity, defined as the thermal conductivity divided by the density, of at least about 75 W·cm 3 /m·° K·gm. The foam also has a high specific surface area, typically at least about 6,000 m 2 /m 3 . The foam is characterized by an x-ray diffraction pattern having “doublet” 100 and 101 peaks characterized by a relative peak split factor no greater than about 0.470. The foam is graphitic and exhibits substantially isotropic thermal conductivity. The foam comprises substantially ellipsoidal pores and the mean pore diameter of such pores is preferably no greater than about 340 microns. Other materials, such as phase change materials, can be impregnated in the pores in order to impart beneficial thermal properties to the foam. Heat exchange devices and evaporatively cooled heat sinks utilizing the foams are also disclosed.
1. A process for producing a thermally conductive carbon foam, comprising:
(1) liquefying a synthetic or mesophase pitch;
(2) heating the liquefied pitch under non-oxidizing, superatmospheric conditions to produce a carbon foam;
(3) coking said carbon foam by heating said carbon foam under non-oxidizing, superatmospheric conditions;
(4) carbonizing the carbon foam by heating in a non-oxidizing environment at a temperature higher than that for coking in step (3), and
(5) further heating at a yet higher temperature to yield a graphitic carbon foam, wherein after coking step (3), said heatings are performed without oxidatively stabilizing the carbon foam.
2. The process of claim 1 wherein the pitch is liquified in step (1) by heating under non-oxidizing conditions including a pressure lower than the superatmospheric pressure producing the carbon foam in step (2).
3. The process of claim 1 wherein the pressure of said superatmospheric conditions producing said carbon foam in step (2) is less than about 69 atm (1000 psi).
4. The process of claim 1 wherein the heating in step (2) is carried out under a static superatmospheric pressure.
5. The process of claim 1 wherein said heating in step (2) is carried out at a temperature no greater than about 480° C.
6. The process of claim 1 wherein, said heating of the liquified pitch under non-oxidizing, superatmospheric conditions in step (2) comprises heating at a temperature between about 420° C. and about 480° C.
7. The process of claim 6 wherein the heating in step (2) is carried out such that at least some foaming occurs at said temperature between about 420° C. and about 480° C.
8. The process of claim 1 wherein the heating in step (2) is carried out with no release of pressure until the formation of the carbon foam is substantially complete.
9. The process of claim 1 wherein said heating in step (3) is carried out at a temperature above 500° C.
10. The process of claim 1 wherein the temperature in step (3) is held constant for at least 15 minutes.
11. The process of claim 1 wherein the pressure is held substantially constant during each of steps (2) and (3).
12. The process of claim 1 wherein the pressure in steps (2) and (3) is identical.
13. The process of 1 wherein the temperature during coking in step (3) is no more than 1000° C.
14. The process of claim 1 wherein the coked foam is cooled prior to being carbonized in step (4).
15. The process of claim 1 wherein the pressure in steps (2) and (3) is substantially identical.
16. The process of claim 1 wherein said thermal conductivity is at least 40 W/m·K.
17. The process of claim 1 , wherein said thermal conductivity is between 40 W/m·K and 187 W/m·K.
18. The process of claim 1 , wherein said thermal conductivity is between 75 W/m·K and 187 W/m·K.
19. The process of claim 1 , wherein said thermal conductivity is between 100 W/m·K and 187 W/m·K.