UC Riverside Researcher Uses Graphene Quilts To Cool High-Power Electronics

Graphene

Image Source: University of California, Riverside

University of California, Riverside (UCR) Professor of Electrical Engineering and Chair of Materials Science and Engineering Alexander Balandin is leading several projects to explore ways to use the unique capabilities of graphene “quilts” as heat conductors in high-power electronics.

Graphene is a recently discovered single-atom-thick carbon crystal, which reveals many unique properties. In Balandin’s designs, graphene “quilts” (large-area overlapping networks of graphene flakes) will play quite an opposite role of your grandma’s quilts. They will remove heat instead of retaining it.

His work on graphene heat-conducting coats for heat removal from high-power gallium-nitride transistors is being funded by a recently awarded $420,000 grant from U.S. Office of Naval Research (ONR). It aims at an experimental proof-of-concept demonstration to be conducted in Balandin’s Nano-Device Laboratory (NDL).

In addition to the ONR grant, Balandin received a new three-year subcontract with the Interconnect Focus Center (IFC), based at the Georgia Institute of Technology, that deals with graphene interconnects and heat spreaders for three-dimensional (3-D) electronics. According to the International Technology Roadmap for Semiconductors, in the next five years, up to 80 percent of microprocessor power will be consumed by the interconnect wiring—a driver for the search for new interconnect materials and innovative methods of heat removal.

Another recent subcontract awarded to Balandin is with the Functional Engineered Nano Architectonics (FENA) center based at UCLA. In this center, he investigates the problems of energy dissipation in graphene nanostructures and nanodevices. Combined new funding secured by Balandin this month for the three projects exceeds $1 million. The centers’ funding comes from the Semiconductor Research Corporation (SRC) and Defense Advanced Research Project Agency (DARPA).

Most of the current research on graphene has focused on its electronic properties and graphene’s potential for high-speed nano-circuits. Due to its unique structure, electrons travel at extremely high speeds throughout it.

Balandin is focusing on another of graphene’s remarkable properties: it’s extraordinarily high thermal conductivity, which can be used for heat removal in nanoscale and 3-D electronics. The higher speed, higher power densities and increased thermal residence in the state-of-the-art devices result in development of hot spots, performance degradation and thermal breakdown. Balandin’s proposed graphene-based approach for thermal management represents a radical departure from conventional methods and might lead to creation of a new technology for hot-spot spreading.

Because graphene is only one molecule thick, it didn’t lend itself to traditional methods of thermal conductivity measurement. Balandin led a team of researchers that first measured it using an original non-conventional technique in 2008. The procedure involved a non-contact approach on the basis of Raman spectroscopy utilizing the inelastic scattering of photons (light) by phonons (crystal vibrations). The power dissipated in graphene and corresponding temperature rise were detected by extremely small shifts in the wavelength of the light scattered from graphene. That was sufficient to extract the values of the thermal conductivity through an elaborate mathematical procedure.

Balandin’s research group discovered that the thermal conductivity of large suspended graphene sheets varies in the range from about 3000 to 5300 W/mK (watts per meter per degree Kelvin) near room temperature. These are very high values, which exceed those of carbon nanotubes (3,000-3,500 W/mK) and diamond (1,000-2,200 W/mK).

As a result of his findings, Balandin has proposed several innovative graphene-based approaches for thermal management , which might lead to creation of a new technology for local cooling and hot-spot spreading in the high-power-density and ultra-fast chips. A detailed description of Balandin’s graphene and thermal management research can be found in his invited popular science article, “Chill Out,” in the October 2009 issue of IEEE Spectrum, the magazine of the The Institute of Electrical and Electronic Engineers (IEEE)

New Jersey Institute of Technology Reveals Microwave Synthesis of Metal-Carbon Nanotube Composites in Ten Minutes

Figure 1 from U.S. Patent Application 20090304923 shows various types of functionalized multiwall carbon nanotubes manufactured in ten to twenty minutes via microwave radiation by scientists at the New Jersey Institute of Technology.

In U.S. Patent Application 20090304923, New Jersey Institute Of Technology (Newark, NJ) scientists Somenath Mitra (Bridgewater, NJ) and  Yuhong Chen (Frederick, MD) reveal a fast method to make improved soluble carbon nanotube (CNT) composites  in minutes that are at least partially coated with a metal material.

Mitra and Chen developed improved methods for the synthesis, generation or formation of substantially soluble carbon nanotube composites via microwave reactions.  Their methods provide for the rapid, controllable, environmentally-friendly formation of substantially soluble carbon nanotube composites via in-situ microwave-assisted reactions in a process than can be completed in ten minutes to an hour.  The carbon nanotube composites are at least partially coated with nanometal particles (e.g., nanoplatinum particles or other metals), and are substantially soluble in water or in organic solvents (e.g., o-dichlorobenzene (ODCB), chloroform, tetrahydrofuran (THF), ethanol, toluene, hexane and DMF).

The microwave technique produces some carbon nanotubes in 10-20 minutes at temperatures of 120 degrees C to 190 degrees C. The complete product process for other MWNT products can be completed in one hour. The process does not work as well for single wall carbon nanotubes (SWNT).

Figure 4 depicts a scanning electron microscope (SEM) image of original MWNTs (mag=400.00 K X, SEM scale bar is 20 nm) made with microwaves in minutes. 

Current practice provides that conventional approaches to solubilize CNTs are complex, time-consuming, tedious and involve multiple steps. As such, current practice also provides that attempting to incorporate metal materials (e.g., nanometal particles) on and/or in CNTs is very challenging. In addition, the conventional approach to graft ODA on raw CNTs is via thermal treatment. However, not only is this a very time consuming process, which often requires several days to complete, this method also leads to damage to the CNTs in the process.

Mitra and Chen say CNTs are at least partially coated with a metal, and  the effective microwave energy of the process shortens the formation process to about one hour, thereby dramatically improving the performance of the whole formation process  compared to conventional thermal methods in which  just  the solubilizing  process for CNTs can take several days to complete.

Furthermore, in exemplary embodiments, the selected reaction solvent (e.g., ethanol) may help to facilitate the nanometal particle coating portion of the process in just several minutes. Additionally, the advantageous, faster microwave systems and methods of the  disclosure do not alter and/or damage the CNTs during processing, thereby providing a significant commercial and manufacturing advantage as a result. Microwave processing can also reduce the need for solvents, thus it is eco-friendly.

Functionalization allows the chemical structure of the nanotubes to be modified, and other functional groups, polymers, ceramics, biological molecules such as enzymes and other appropriate chemical moieties can be attached. For example, treating with acid generates --COOH groups to which other functionalities can be attached by a variety of chemical reactions. Some functionalization reactions may be, for example, carboxylation, sulfonation, esterification, thiolation, carbine addition, nitration, nucleophylic cyclopropanation, bromination, fluorination, diels alder reaction, amidation, cycloaddition, polymerization, adsorption of polymers, addition of biological molecules and enzymes, etc.

The functionalization may be covalent bonding to the nanotube, or noncovalent adsorption or wrapping. By synthesizing the appropriate functionality, the nanotubes may be rendered soluble in aqueous, organic, polar, nonpolar, hydrogen bonding, ionic liquids, and other solvents so that they can be processed easily.

In general, say the inventors,  during microwave-assisted reactions, the smaller diameter and higher curvature of SWNTs generates more stress in the SWNTs than in the MWNTs. However, it is to be noted that the additional layers in MWNTs may result in more adsorption of microwave radiation by the MWNTs in the disclosed systems and methods. As such, the MWNTs may be easier to handle compared to the SWNTs under the microwave methods  Additionally, the chemical activation parameters may be modified due to further polarization of the dipoles under microwave radiation. 

Arkema Combines Ball Milling with Vapor Deposition to Continuously Produce Highly Pure Carbon Nanotubes

Arkema France (Colombes, FR) reveals in U.S. Patent 7,622,059 a method for synthesis of carbon nanotubes of the highest carbon purity by combining the process of vapor phase chemical deposition. with ball milling.  The nanotubes produced can be used to advantage in all known applications of carbon nanotubes. Inventors Serge Bordere, Patrice Gaillard and Carole Baddour say the process produces 15 grams of carbon nanotubes for each gram of catalyst used.

FIG. 1 is a scanning electron micrograph of the CNT agglomerates obtained according to the prior manufacturing methods.

FIG. 2 is a scanning electron micrograph of the milled CNTs obtained from step b) according to Arkema's invention; and by comparing FIG. 2 with FIG. 1, it may be clearly seen that the process according to the invention results in a very small number of CNT agglomerates with a diameter greater than 200 .mu.m. The final product thus formed is therefore more easily dispersed within a material, in particular a polymer.

FIG. 3 illustrates a milling device according to Arkema's invention which may be installed either within an actual synthesis reactor (6) for synthesizing CNTs by CVD (in situ milling) or in an external loop allowing possible recycling of all or part of the CNTs milled within the reactor (ex situ milling).

The milling device shown in FIG. 3 comprises a system of high-velocity gas jets generated through injectors (2) which entrain the CNT powder onto one or more targets (5) held by a support (4), that has to be subjected to the bombardment of the CNT agglomerates thus reducing the particle size by impact. The fluidization may be carried out by just these injectors (2) and/or in combination with a gas stream diffused by the distributor (3) around these injectors (2). The dimensions of the milling system and the flow rates of incoming gas (1) and (2a) used are suitable for obtaining good fluidization and the desired particle size, depending on the hardness and the density of the catalyst substrate. The distributor (3) is designed to support the catalyst, which is in powder form, at the time T0 of the synthesis.

The form of the milling device will advantageously be adapted according to the materials used and/or the behavior of the fluidized bed. The process may be carried out semi-continuously or in batch mode, but preferably continuously. At least part of the entangled CNT/catalyst network resulting from step a) may be extracted from the synthesis reactor to a milling device operating continuously, semi-continuously or in batch mode, then injected (step c)) either into the same synthesis reactor of step a) or into a second CNT synthesis reactor by fluidized-bed CVD (finishing reactor).

It is also possible to carry out the milling (step b) in the synthesis reactor of step a), provided with milling means as shown by the device in FIG. 3, which avoids having to extract the powder from the reactor and therefore reduces the head losses and the risk of powder fly-off.

Step b) is carried out inside the CNT synthesis reactor (6) by injecting some of the reactive gas or gases and/or an additional gas through injection nozzles (2) distributed over the surface of the distributor (3), the vertical gas jet or jets (1) entraining the particles toward a target (5). The particles consist of CNT agglomerates and/or catalyst. The target (5) is in the form of a cone, made of stainless steel, preventing deposition of particles at the top of the target (5).

This milling makes catalytic CNT growth sites accessible, thereby making it possible, during step c), to grow further CNTs on these now accessible sites, but also on the CNT agglomerates formed during step a), the size and/or the number of which have been reduced thanks to the milling. Growth of the CNTs during step a) and step c) may take place using identical gas sources (which is the case during a process involving in situ milling) or sources that differ both in terms of nature and flow rate (which is especially the case during a process involving ex situ milling). The CNTs synthesized during introduction of synthesis gas and fresh catalyst, during step c), may be subjected to a further milling step d) under the conditions described above. The CNTs thus obtained after step c) or d) are finally recovered.

These CNTs have improved properties, especially their dispersion in a material, in particular a polymer. It is thus possible to introduce a higher quantity of CNTs compared with the prior art, with better distribution and/or homogeneity, thereby improving the final properties of the material containing the CNTs.

These CNTs can be used in all applications in which CNTs are employed, especially in fields in which their electrical properties are desired (depending on the temperature and their structure, they may be conductors, semiconductors or insulators), and/or in fields in which their mechanical properties are desired, for example for the reinforcement of composites (the CNTs are one hundred times stronger and six times lighter than steel) and in electromechanical applications (they can elongate or contract by charge injection). For example, mention may be made of the use of CNTs in macromolecular compositions intended for example for the packaging of electronic components, for the manufacture of fuel lines (gasoline or diesel), antistatic coatings, in thermistors, electrodes, especially in the energy sector, for supercapacitors, etc

Samsung Electro-Mechanics Creates Superior Heat Sinks Using Carbon Nanotubes

In U.S. Patent Application 20090297801,  Samsung Electro-Mechanics Co., LTD researchers Seung Hyun Cho, Byoung Youl Min, Soon Jin Cho and Jin Won Choi  disclose how to manufacture a heat radiation substrate with a metal core using carbon nanotubes for computer chips.  The manufacturing method includes injection-molding mixed powder of carbon nanotubes and metal in a die to fabricate a metal core having through holes., That is followed by molding the entire metal core including the through holes with an insulating resin to fabricate a metal core substrate; then processing the insulating resin in the through holes to form connection holes; and forming a circuit pattern on the metal core substrate in which the connection holes are formed. The amount of the carbon nanotubes in the mixed powder is 20% by weight or less.

When carbon nanotubes are used to manufacture a heat radiation substrate, since the area of the heat radiation substrate is larger than that of a chip, which is a heating element, the temperature of the chip is greatly decreased, so that, in a package system, the size and capacity of a cooling system including a cooling fan and a radiation fin are decreased, or the cooling system is not required, thereby decreasing the manufacturing cost of the heat radiation substrate.

As shown in FIGS. 1A to 1C, carbon nanotubes, which are materials having carbon chains, such as a single-walled nanotube structure (see FIG. 1A), a multi-walled nanotube structure (see FIG. 1B), and a nanotube rope structure, have a thermal conductivity of about 3000.about.6000 W/mK. Carbon nanotubes have very high radiation performance, considering that the thermal conductivities of copper and aluminum, which are used as materials having excellent thermal conductivity, are 350 W/mK and 210 W/mK, respectively. In particular, carbon nanotubes are very useful in the dissipation of heat from a small area such as in a chip because the chain structure thereof determines the direction of heat transfer therethrough.

FIG. 5 is a schematic sectional view showing a multi-layered heat radiation substrate including the metal.  Insulating resin layers 208 and 308 are layered on the substrate, on which the circuit layer is formed, through a general build-up process, and circuit patterns 211 and 311 are formed on the respective insulating resin layers 208 and 308 through a circuit forming process, thus further forming at least one outer circuit layer. Moreover, a solder resist layer 401 may be formed on the outermost circuit layer (see FIG. 5), and then the solder resist layer 401 may be surface-treated using a commonly used solder resist opening process and a nickel/gold plating process.

FIGS. 6A to 6c are views explaining a general radiation method of a substrate.