FIELD OF THE INVENTION

The present disclosure relates to thermally conductive materials for thermal management, including thermal interface materials (TIMs) having boron nitride nanotubes (BNNTs), as well as methods for preparing BNNTs for use in TIMs.

BACKGROUND

As technological advances increase the power densities of modern electronics, heat accumulation becomes a critical performance limitation, reducing efficiency, lifetime, and lifecycle costs. Increasing performance demands for electronics are driving requirements for new thermal management architectures with advanced materials. Thermal management challenges have long been grappled with in systems, within communications systems, controls for more powerful, and quicker-response weapons systems (directed energy laser weapons, railguns), sensors (multi-sensor fusion), and the transport platforms/systems that carry, and often power, them all. Active thermal management, i.e., cooling, only partially solves such challenges, with improved performance offset by increased system weight, volume, power, and energy demands. Improved passive thermal management, on the other hand, dissipates heat and lowers semiconductor junction temperatures (Tj), which improves performance, prolongs component lifetimes, and decreases maintenance and other lifecycle costs. Passive cooling requires advanced materials to conduct heat from deep within circuits to heat sinks and other structures, and in such proximity to circuitry, only electrically-insulating materials may be used. The thermal conductivity of presently available frequently electrically-insulating materials is often inadequate, and these materials are often incompatible with mechanical requirements and fabrication processes. Improved TIMs are important to both improving the performance and lifespan of existing electronic devices, and also enabling the next generation of electronic devices and high-power systems.

Improved TIMs have the potential to benefit all electronics. The highest immediate value and logical first targets are higher-power components, such as power supplies, power converters, AC/DC components, and energy storage and other control systems. High power electronic systems and applications of interest include, for example, sensors such as phased array radar, communications systems (from the most complex high-power RF systems to those as ubiquitous as Wi-Fi), manned or unmanned transport systems, and weapons (especially prototype directed energy systems that cannot advance without solving heat/power conflicts). Improved TIMs are a critical next step towards realizing such devices.

What is needed, then, are improved thermal interface materials suitable for use in electronic devices. These improved TIMs must be both electrically-insulating and thermally conductive, over a broad range of operating temperatures, and have mechanical strength to endure fabrication and repetitive use.

SUMMARY

BNNTs have a number of properties that make them highly desirable for TIMs in electrical components, and may be used in embodiments of the present approach: They are electrically insulating, conduct heat nearly eight times better than copper, survive high temperatures (e.g., oxidation stability 700-900° C. verses carbon 350-550° C.), and molecular in scale, allowing incorporation deep within semiconductor electronics packages. BNNTs, particularly when synthesized from catalyst-free, high-temperature and high-pressure processes, have a high aspect ratio, through which heat may be efficiently conducted. BNNTs can enhance heat transfer within a material when used in conjunction with thermally-conductivity micro-particles and a polymer matrix. As described herein, BNNTs may be used to transform thermally-insulating polymers into thermally-conductive composites (e.g., over an order of magnitude improvement), without losing their vital electrical insulating properties. These BNNT composites may be used in BNNT-modified thermal interface materials, which will improve the performance, efficiency, and reliability of existing designs, and enable the next generation of higher power-density designs.

High quality BNNTs, such as those manufactured by BNNT, LLC of Newport News, Va., generally have few defects, no catalyst impurities, 1- to 10-walls with the peak in the distribution at 3-walls and rapidly decreasing with larger number of walls, and may be preferred as the starting or as-synthesized BNNT material for some embodiments of the present approach. Although dimensions may vary depending on the manufacturing process, BNNT diameters typically range from about 1.5 to about 6 nm, but of course may extend beyond this range. BNNT lengths typically range from a few hundreds of nm to hundreds of microns, but may extend beyond this range depending on, among many factors, the synthesis and collecting processes used to generate the as-synthesized BNNT material.

Previous patents and published applications have suggested the addition of materials including BNNTs into the materials used in electrical components. See, for example, U.S. Patent Publication US2014/0080954 A1 to Raman et al.; and U.S. Pat. No. 6,864,571 B2 to Arik et al. Neither reference describes BNNT materials useful in TIMs and other thermal management systems. Instead, the methods disclosed Raman merely suggest use of BNNTs in bulk, without any regard to synthesis and collection processes, modifications to the as-synthesized material for improving performance, or composite matrices with a BNNT component. Likewise, the methods disclosed by Arik merely suggest “generally aligned nanotubes that extend away from the catalyst layer” i.e. in the out-of-plane and similar out-of-plane heat transfer for limited aspects of the electrical components. Merely dispersing or including the BNNTs into the materials going into an electrical component is insufficient to enhance the thermal management in an electronic system as a general proposition, and wholly inadequate for inclusion in a TIM. The chemical vapor deposition (CVD) growth methods of Arik do not produce high quality BNNTs, i.e. few wall, high aspect ratio, minimal defects and catalyst free, as they take place at temperatures and nitrogen pressures far below what is required for high quality BNNTs.

Under the present approach, a thermal interface material may take the form of a composite matrix, such as a polymer composite matrix. The composite matrix may have a matrix material, a microfiller material, and a BNNT filler material. The TIM may include, but are not limited to, thermal management material form factors such as thermal greases (or “pastes”), adhesives, gap fillers (underfills), gap pads, topside coatings, and printed wiring board.

In some embodiments of the present approach, a BNNT material may be synthesized, deagglomerated, surface-modified, and dispersed within a matrix material. Under the present approach, a BNNT material may be synthesized according to known methods. This description refers to the BNNT material following synthesis as-synthesized BNNT material. Under the present approach, as-synthesized BNNT material may be synthesized according to known methods, and, depending on the synthesis method and conditions, the as-synthesized BNNT material may be low quality or high quality. High quality BNNTs may be preferred for at least some embodiments of the present approach. U.S. Pat. Nos. 9,745,192, 9,776,865, 10,167,195, and International Patent Application PCT/US16/23432, filed Nov. 21, 2017, describe examples of synthesis apparatus and processes for high-quality BNNTs, and each is incorporated by reference in its entirety. Depending on the embodiment, the surface of the synthesized BNNT material may be modified by one or more methods.

Synthesized BNNT material often includes numerous agglomerations, which can interfere with sufficient BNNT dispersion throughout the microfiller material. Thus, in some embodiments, synthesized BNNT material may deagglomerated, such as through one or more of the processes described herein.

Synthesized, deagglomerated BNNT material may also be lyophilized in some embodiments. For example, the BNNTs may be flash frozen in a solvent (water, organic solvent, or combinations thereof), and dried under a low pressure to remove the solvent through sublimation and/or evaporation, resulting in a dried, deagglomerated BNNT material. It should be appreciated that many deagglomeration methods may be used prior to lyophilization, including solvent-based methods and mechanical methods.

In some embodiments, the surface of the synthesized BNNT material may be modified through one or more chemical surface modifications. Depending on the embodiment, BNNT surface modification may occur prior to or after deagglomeration and dispersion of the BNNT. Depending on the embodiment, surface modification of BNNT may be covalent or noncovalent.

Some embodiments of the present approach may take the form of a thermal interface filler, containing refined and deagglomerated boron nitride nanotubes (BNNTs). Refined BNNT materials have reduced residual boron particulate content (not including boron trapped in h-BN nanocages), and in some embodiments the residual boron particle content of less than 20% by weight. In some embodiments, the residual boron particle content is less than 10% by weight, and in some embodiments, the residual boron particle content is less than 3% by weight, and can be less than 1% by weight in some embodiments. Deagglomerated BNNT materials have reduced BNNT agglomeration size and content. For example, in some embodiments, when dispersed in a polymer matrix material, less than 1% of any remaining agglomerations in the BNNTs have two dimensions that exceed 300 μm, as determined using the method described herein. In some embodiments, less than 5% of any remaining agglomerations in the BNNTs have two dimensions that exceed 200 μm. In some embodiments, less than 10% of any remaining agglomerations in the BNNTs have two dimensions that exceed 150 μm. In some embodiments, less than 15% of any remaining agglomerations in the BNNTs have two dimensions that exceed 100 μm. In some embodiments, less than 20% of any remaining agglomerations in the BNNTs have two dimensions that exceed 50 μm. In some embodiments, less than 25% of any remaining agglomerations in the BNNTs have two dimensions that exceed 25 μm.

BNNT material for thermal interface fillers in some embodiments of the present approach may have a residual boron particle content less than 3% by weight, and less than 1% of any remaining agglomerations in the BNNTs have two dimensions that exceed 300 μm. In some embodiments, the residual boron particle content is less than 1% by weight, and less than 10% of any remaining agglomerations in the BNNTs have two dimensions that exceed 150 μm. In some embodiments, the residual boron particle content is less than 1% by weight, less than 10% of any remaining agglomerations in the BNNTs have two dimensions that exceed 150 μm, and less than 5% of any remaining agglomerations in the BNNTs have two dimensions that exceed 200 μm.

In some embodiments, the refined BNNT material has a reduced content of h-BN allotropes, as determined using the method described herein. For example, the residual h-BN allotrope content is less than 30% in some embodiments, less than 10% t in some embodiments, and less than 3% in some embodiments.

Deagglomerated BNNT material may, in some embodiments, be lyophilized, to retain the benefits of deagglomeration when returning to solid form. In some embodiments, BNNTs may be surface modified with a functional group that improves dispersion in a polymer matrix material.

Some embodiments of the present approach may take the form of a thermal interface material, having a polymer matrix material, at least one microfiller; and a refined and deagglomerated BNNT. In some embodiments, the microfiller(s) are about 50% to about 90% by weight. In some embodiments, the refined and deagglomerated BNNT material comprises about 0.1% to about 30% by weight. In some embodiments, the thermal interface material may comprise a dielectric material.

In some embodiments, the present approach may take the form of a method for fabricating a BNNT-enhanced thermal interface material. For example, an as-synthesized BNNT material may be refined to remove (i.e., reduce the amount of) various boron species, such as boron particles, a-BN, and h-BN allotropes. The refined BNNT material may be deagglomerated, to reduce the size of BNNT agglomerations and the mass of BNNTs in agglomerations, to form deagglomerated BNNT material. It should be appreciated that a deagglomerated BNNT material will, when dispersed in the target polymer matrix, have fewer agglomerations and a BNNT material that has not been deagglomerated. The deagglomerated BNNT material may be dispersed into one or more microfillers and a polymer matrix material.

Refining the as-synthesized BNNT material may, in some embodiments, include heating the as-synthesized BNNT material in an inert gas and a hydrogen feedstock at a first temperature, which may be about 500° C. to about 650° C., for a first duration, which may be about 3 minutes to about 12 hours. In some embodiments, refining the as-synthesized BNNT material may also include heating the as-synthesized BNNT material at a second temperature, which may be about 650° C. to about 800° C., for a second duration, which may be about 3 minutes to about 12 hours.

Some embodiments may include lyophilizing the deagglomerated BNNT material prior to dispersing. In some embodiments, the BNNTs may have one or more surface modifications. For example, BNNTs may modified to include a functional group that improves dispersion in the polymer matrix material prior to dispersing.

