FIELD OF THE INVENTION

This invention is in the field of carbon nanotube arrays or sheets, particularly arrays or sheets which can be single-layered or stacked to form multilayered or multitiered structures and methods of making and using thereof.

BACKGROUND OF THE INVENTION

As the performance and packaging demands on modern electronics have continued to become more and more aggressive, identifying new thermal solutions has become a critical part of the design process. In nearly all thermal management applications, there are numerous heat transfer interfaces where intimate contact is needed to ensure efficient removal of heat from the package. In this space, a wide variety of thermal interface materials (TIMs) have been developed for specific applications (R. Prasher, “Thermal interface materials: historical perspective, status, and future directions,”Proceedings of the IEEE, vol. 94, pp. 1571-1586, 2006).

Carbon nanotube (CNT) arrays are an attractive solution for enhancing thermal transport between surfaces. CNTs can be grown on metal substrates, eliminating concerns associated with pump out or voiding that liquid thermal interface materials (TIMs) and greases may suffer from.

The high in-plane conductivity of individual nanotubes (as high as 3,000 W/m-K) means that even at relatively low CNT densities (typical CNT fill factors are on the order of 1%) the cross plane thermal conductance of a CNT-based TIM can be competitive with that of thermal grease. Furthermore, the favorable deformation mechanics of CNTs allow them to efficiently conform to the asperities of adjoining surfaces, resulting in high contact areas at such interfaces between surfaces.

A key challenge, however, in CNT-based TIMs comes from the difficulty in growing very long CNTs on metal substrates. Unlike CNTs grown on silicon or other inert substrates, the catalyst required for CNT growth suffers from subsurface diffusion when grown on metal substrates, resulting in early termination of tube growth. Furthermore, defects tend to accumulate in CNTs as their height increases, resulting in CNT arrays with conductivities significantly lower than the 3,000 W/m-K limit otherwise achievable with pristine tubes.

Thus, there is a need for overcoming the above-mentioned difficulty in growing long CNT arrays on metal substrates and methods of making materials which have good thermal transport properties.

Therefore, it is an object of the invention to provide CNT arrays or sheets and structures formed thereof and methods of making such structures having good thermal transport properties.

It is also an object of the invention to provide CNT arrays or sheets and structures formed thereof which can provide high levels of compliance at the interface with one or more surfaces.

It is a further object of the invention to provide TIM composites with improved durability needed for burn-in applications.

It is also an object of the invention to provide TIM composites with good compliance and compression properties that allow the size of a device under testing to be changed.

SUMMARY OF THE INVENTION

Single layered or multilayered or multitiered structures, formed by stacking of vertically aligned carbon nanotube (CNT) arrays, and methods of making and using thereof are described herein.

In some embodiments described, two or more CNT arrays are stacked to form multilayered or multitiered structures. Multiple CNT arrays can be stacked, such that the nanostructure elements from opposing arrays form into tiers in the stack and become at least partially interdigitated with one another. Unlike the stacking of a traditional material, stacked arrays of vertically aligned nanostructures do not suffer from a linear (or worse) increase in thermal resistance with increasing thickness. Accordingly, the resulting multilayered structures can mitigate the adverse impact of thickness and boundaries on energy transport as a result of the interdigitation of the nanostructure elements (i.e., CNTs) of the two or more arrays when contacted. In contrast, for a typical material the resistance to heat transfer is directly proportional to the material's thickness, with an additional interfacial resistance at the interfaces of a multilayered structure.

For multilayered or multitiered structures formed by stacking of vertically aligned nanostructure materials of CNT arrays, wherein the CNTs of the arrays at least partially interdigitate within or into one another, effectively increasing the density of the CNTs. Typically, the density of CNTs grown on metal substrates is only about 1% of the total volume. When two adjacent layers of CNT arrays are stacked, for example, the density of heat conducting elements, such as CNTs or structures formed thereof, is effectively doubled. As such the resistance to heat transfer per unit length is reduced in kind.

An advantage of the multilayered or multitiered structures formed by stacking of two or more CNT arrays over traditional bulk materials comes at the interface of the arrays. For example, resistance to heat transfer increases not only due to the increase in thickness of a multilayer stack, but also due to the interfacial resistances between the tiers. Accordingly, between any two adjacent tiers, the boundary between the two tiers is the location of poor heat transfer, relative to the bulk material due to poor contact between the tiers, as well as due to scattering of energy carriers (e.g. electrons or phonons) at the boundary. When interdigitated, the high aspect ratio of the nanostructures, such as CNTs, results in a very high contact area between the tiers minimizing the poor contact area contribution to the inter tier interfacial resistance. Although Kapitza (scattering) resistance cannot be completely eliminated the resistance can be reduced by applying, infiltrating, or backfilling the array or sheet with a polymer, wax, or other secondary material that facilitates thermal/energy transport across the boundary. This transport facilitation may be through the formation of a covalent or weak atomic interaction between the CNT and a secondary material, reduction of acoustic phononic transport mismatch relative to air, or other types of mechanisms.

In some embodiments, nanostructure elements which form the array are vertically aligned carbon nanotubes (CNTs). In some embodiments, the CNT array is grown on a metal substrate which is formed of aluminum, copper or steel or comprises aluminum, copper or steel, or alloys thereof. In another embodiment, the CNT array is formed on a flexible, electrically and thermally conductive substrate, such as graphite. In yet another embodiment, the CNT array is grown on an electrically insulating substrate, such as a flexible ceramic. In one embodiment, the inert support for the CNT array is a piece of metal foil, such as aluminum foil. In some instances only one surface (i.e., side) of the metal foil contains an array of vertically aligned CNTs anchored to the surface or the substrate/support. In other cases, both surfaces (i.e., sides) of the substrate/support, such as a metal foil, contain a coated array of aligned CNTs anchored to the surface. As another example, CNT sheets can be coated on one or both sides and do not require an inert support.

In certain embodiments, a single substrate containing thereon one or more CNT arrays on one or more surfaces, as described herein. In other embodiments, two or more CNT arrays are stacked atop one another and the nanostructure elements of the individual arrays, such as the CNTs or some portion thereof, fully or substantially interdigitate within one another; “substantially,” as used herein, refers to at least 95%, 96%, 97%, 98%, or 99% interdigitation between the nanostructure elements (i.e., CNTs) of the individual arrays. In some embodiments, the extent of interdigitation is in the range of about 0.1% to 99% or at least about 1%, 5%, 10%, 20%, 30%, 40%, 50%, 60%, 70%, 80%, 90%, or 95%. In certain other embodiments, the two CNT arrays may be interdigitated only at the tips of the nanostructure elements (i.e., CNTs) of the individual arrays. By stacking two or more individual CNT arrays, wherein the nanostructure elements of the individual arrays interdigitate when stacked adjacently, it is possible to form multilayered or multitiered structures.

In certain embodiments, one or more individual nanostructure elements, such as CNTs, of the array may navigate through another when adjacent CNT arrays are brought in contact during the stacking process.

In some embodiments, the individual nanostructure elements, such as CNTs, of the array may interdigitate and form into larger structures, such as superstructures, such as, but not limited to, tube bundles, clumps, or rows. Such superstructures may be formed through mechanisms such as capillary clumping or when a polymer coating has been applied to the CNT arrays prior to, during, or following the stacking process.

In certain embodiments, adjacent tiers formed by stacking of CNT arrays are formed via simple dry contact, using entanglement, friction or weak attraction forces between the nanostructures present therein to keep the resulting interdigitated structure together.

In certain other embodiments the CNT arrays on a substrate, alone or as part of a multilayered stack resulting therefrom may optionally be infiltrated or backfilled with a polymer, wax, liquid metal, or other suitable material that solidifies inside the stacked structure in order to hold the interdigitated arrays together.

In some embodiments the polymer, wax, liquid metal, or other suitable material can reduce the transport resistance between the multiple layers or tiers formed, resulting from improved contact area, a reduction in scattering, or other related mechanisms. In yet other embodiment the tiers formed by stacking of arrays may be bonded by use of an adhesive or a phase-change material.

CNT arrays and the multilayered or multitiered structures formed by stacking of such CNT arrays exhibit both high thermal conductance and mechanical durability. The multilayered or multitiered structures formed by stacking of CNT arrays described herein can be used as thermal interface materials (TIMs). Accordingly, such materials are well suited for applications where repeated cycling is required. For example, they can be employed as thermal interface materials during ‘burn-in’ testing of electrical components, such as chips. In some embodiments, the inert support/substrate is a surface of a conventional metal heat sink or spreader. This functionalized heat sink or spreader may then be abutted or adhered to a heat source, such as an integrated circuit package. Such TIM materials can also be placed or affixed in between a heat source and a heat sink or heat spreader, such as between an integrated circuit package and a finned heat exchanger, to improve the transfer of heat from the heat source to the heat sink or spreader.

The CNT arrays and the multilayered or multitiered structures formed by stacking of such CNT arrays described herein can be used as thermal interface materials (TIMs) in personal computers, server computers, memory modules, graphics chips, radar and radio-frequency (RF) devices, disc drives, displays, including light-emitting diode (LED) displays, lighting systems, automotive control units, power-electronics, solar cells, batteries, communications equipment, such as cellular phones, thermoelectric generators, and imaging equipment, including MRIs.

In certain embodiments, the single-layered CNT arrays or multilayered or multitiered structures formed by stacking of CNT arrays are useful as TIMs in low contact pressure and/or low ambient pressure applications, such as in aerospace applications where such TIMs could be used in satellites or space vehicles/systems. In certain embodiments, the multilayered or multitiered structures formed by stacking of CNT arrays are useful at temperatures below ambient, below freezing, or at cryogenic temperatures (such as experienced in space).

The CNT arrays and the multilayered or multitiered structures formed by stacking of such CNT arrays described herein can also be used for applications other than heat transfer. Examples include, but are not limited to, microelectronics, through-wafer vertical interconnect assemblies, and electrodes for batteries and capacitors. Currently, copper and aluminum foil are used as the backing materials for the anode and cathode in lithium ion batteries. The CNT arrays and multilayered or multitiered structures formed by stacking of such CNT arrays could also be used for electromagnetic shielding.

The CNT arrays and multilayered or multitiered structures formed therefrom are useful as TIMs used in thermal management methods, such as in a method including the steps of:

(1) attaching the substrate directly to a thermal or electrical unit head to cover the area of the head completely or matching the size of the device under test;

(2) engaging the thermal or electrical unit head with the attached substrate to the device under test with a pressure of at least 10 psi and engagement temperature less than 150° C.;

(3) holding the engagement under the pressure of at least 10 psi for at least 5 minutes or cycling the engagement at least one to five times, wherein during each engagement the device under test is powering up and heating the thermal or electrical unit head to simulate the device powering up to reach a temperature of at least 50° C.;

(4) disengaging and re-engaging the thermal or electrical head and substrate with the device under test for at least 1,500 cycles of powering up; and

(5) testing the device under test; in order to determine the thermal resistance and/or relative thermal resistance following cycling.

The CNT arrays and multilayered or multitiered structures formed therefrom used as TIMs for burn in or other applications may also be provided in suitable kits.

