Established Materials Technology: Front Page
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MT@TMS Case Study: GraphiMetal Coatings Provide Solderability to Lightweight, High Thermal Conductivity Graphite Foams
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By Ben D. Poquette and Stephen L. Kampe
Posted on:
2/8/2010 12:00:00 AM...
Editors Note: The MT@TMS Case Study is a new, occasional feature of Materials Technology&TMS. Its intent is to highlight how a technology or process developed by TMS members has been used to address a specific problem or need. To suggest a topic for a future case study, contact JOM editor Maureen Byko.
INTRODUCTION
Keystone Materials, LLC utilizes its patent-pending GraphiMetalTM coating process to fully metallize graphite and ceramic foams in a way that does not close the porosity and leaves fluid/air flow though the foams unhindered. Graphite foams exhibit ligament thermal conductivities four times that of copper with one fifth the weight of aluminum alloys, presenting a unique opportunity to radically change the approach to solving many heat transfer problems. GraphiMetal coatings serve to solve factors such as low strength, lack of joinability, and dusting (the release of conductive particles upon handling), all of which have historically prevented the widespread use of graphite foam.
GraphiMetal coatings are based on patent pending enhancements to traditional electroless plating materials and processes, allowing most electroless chemistries available in the literature to be utilized as a starting point for coating with new metals and alloys. GraphiMetal is a readily scalable process and can be applied using traditional electroless plating infrastructure. The technology also does not utilize rare or cost-prohibitive activation materials that could hinder large-scale production. The ability to apply uniform nanostructured metallic coatings to bulk graphitic foam structures creates substantial opportunities to further engineer these materials for a wide range of thermal, electrical, and chemical functionalities.
Keystone Materials was formed by the inventors of the GraphiMetal coating process and retains an exclusive license for the technology from Virginia Tech Intellectual Properties (VTIP). Keystone has developed a pilot line to deposit copper- and nickel-based coatings and continues to work closely with the Oak Ridge National Laboratory (ORNL) and other partners to qualify GraphiMetal coated foams for insertion on commercial and military platforms.
CHALLENGES
Graphite foams present a lightweight replacement for metallic heat exchange materials in thermal management systems. However, the integration of graphite foams has been slowed by their low strength, the dusting of conductive graphite particles, and the lack of an efficient joining method. GraphiMetal coatings provide a solution to these issues.
While at Virginia Polytechnic Institute and State University (Virginia Tech), the founders of Keystone, in cooperation with ORNL, developed a process to produce uniform, nanostructured metallic coatings for graphite foams which allow foam structures to be joined to devices through traditional soldering techniques. From there, the challenge was to transition this patent pending technology from the laboratory to a pilot-scale manufacturing process.
APPROACH
The main goal for Keystone until recently was to scale up the GraphiMetal coating process and take the technology from the “beaker” to the “bathtub” size scale. When Keystone was formed, the GraphiMetal coating process was in its experimental stage. Small foam articles were coated one at a time in a small beaker in a laboratory fume hood.
Upon moving outside of the university umbrella, several new challenges presented themselves and had to be addressed. Once a suitable manufacturing site was selected, issues of waste management and ventilation had to be resolved. The coating process was further analyzed, modified, and optimized to reduce both waste production and the amount of ventilation required to maintain a safe workspace.
During the design of the current pilot-scale coating line, multiple lessons were learned with respect to practical implementation of new materials and processes. Process automation was required to improve consistency and manage the increased quantity of material involved. Simple processes, such as bath stirring and filling of coating tanks as opposed to beakers, now required implementation of chemical-resistant plumbing and mixing tools. When working with large volumes of solution, temperature and chemical uniformity became important factors. Additionally, proper fixturing and implementation were critical. Multiple designs were investigated and evaluated to discover an arrangement that provided uniform deposition across the various parts, maximizing both the cosmetic appearance and the physical performance of the coated items. Because of the increased quantities of materials required for pilot-scale operation, chemical purchasing and storage became issues with a direct financial impact on operations. Additionally, material qualification can consume a significant amount of the precious resources of a young business.
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(Click on images to enlarge.) Uncoated graphite foam, GraphiMetal copper-coated foam and ¼ inch tubing, and copper tubing which has been soldered to the GraphiMetal copper-coated foam. A protective green patina forms on the GraphiMetal copper at soldering temperatures above 200 degrees Celsius. Here, a no-clean SAC 305 solder paste was utilized and reflowed at a temperature of 235 degrees Celsius.
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Cross-section of the solder joint between GraphiMetal copper-coated foam and ¼ inch copper tubing. When stressed to failure, the foam breaks before the solder joint.
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Cross-section of the solder joint between GraphiMetal copper-coated foam and ¼ inch aluminum tubing. Here, the aluminum tubing was pretreated for soldering with a traditional zincate, followed by an electrolytic nickel-copper strike. When stressed to failure, the foam breaks before the solder joint.
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Uncoated and coated graphite foam microprocessor heat sink.
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Being a young company, specific procedures had to be newly developed and qualified in-house for each of the steps involved in the coating process. With a small workforce, this required significant flexibility from everyone involved. Founders and early employees were required to take on multiple roles, including topics outside of their primary expertise. This was only made possible by the high quality of our workforce, and allowed us to maximize the progress that was made, using our initial funding.
This early technical success has been the foundation of our current marketing effort. By overcoming the primary flaws associated with graphite foam, we have provided designers with a new lightweight, high thermal conductivity replacement for traditional metals in thermal management systems. Keystone’s GraphiMetal coatings have sparked significant interest from the private sector, and the company is currently looking for a manufacturing/marketing partner to assist in cost reduction and introduce this technology to new and untapped markets.
RESULTS
The major benefits and successes of GraphiMetal coatings are that they offer a simple, economical way to provide joinability to graphite foams. GraphiMetal copper and nickel coatings facilitate direct joining of graphite foams to other metallic structures using traditional soldering techniques. This solderability solves the major drawback to the incorporation of graphite foam into current platforms. Until now, a robust, economical means to create a structural, high conductivity joint between graphite foam and metallic structures has not existed.
GraphiMetal coatings also make solder joining to aluminum structures possible without the use of highly active, corrosive flux or high vapor pressure, zinc-based solder alloys. In addition, indium-based solder alloys can be utilized to mitigate the effects of coefficient of thermal expansion (CTE) mismatch between the foam and metal substrates.
Providing joinability to graphite foams was the primary goal in the development of GraphiMetal coatings. However, a customer brought to our attention two very important secondary benefits. Dusting has been a major hurdle to the integration of graphite foam into microelectronics. Even with light handling, graphitic foams can produce a fine graphite dust, which due to its conductivity, can short the exposed traces of a circuit board. The uniform metal deposit formed by the GraphiMetal process creates a continuous envelope to contain this dust without negatively affecting foam performance. With this improvement, GraphiMetal foam has become a viable material to replace copper and aluminum finned heat sinks on microprocessors and in power electronics. Additionally, preliminary research performed at Virginia Tech has shown flexural strength increases greater than 30 percent with the application of nanostructured, thin film (< 3 μm) GraphiMetal coatings.
CONCLUSION
In starting a new business, be prepared for a roller coaster and maintain a mindset to take the good with the bad. In our case, the GraphiMetal technology matured as planned, and we were able to create a robust manufacturing process incorporating several improvements over the original discovery. However, this maturation process did consume a significant amount of time. The primary bottleneck was available resources, and the time to market could have been significantly reduced if we had initially pursued a greater level of funding.
Since beginning our major marketing push in the fall of 2009, we have had significant commercial interest in the GraphiMetal technology. In an effort to rapidly increase availability and decrease cost, Keystone is currently looking for a manufacturing/marketing partner for the large-scale production and sales of GraphiMetal coated foams.
Ben Poquette is president of Keystone Materials, LLC, headquartered in Blacksburg, Virginia. Stephen L. Kampe is a professor at the Michigan Technological University, Houghton, Michigan, and co-founder of Keystone Materials.
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