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The utilization of open cell metal foams in thermal applications

Roll Type Ultra-Thin Foam Copper (7)

The thermal aspects of many emerging technologies are increasingly recognized as a critical factor that limits their performance, size, and cost. Reticulated Metal Foams (RMF), originally developed for structural applications over two decades ago, are now being recognized as effective solutions to many thermal management problems. RMF offers a cost-effective and high-performance thermal management technology that can be integrated with advanced electronic and photonic devices, as well as other challenging applications. The metal foam-based thermal technology is generic, flexible, and scalable. It is generic in terms of its compatibility with various cooling media, ranging from DI water, inert fluorocarbons, and jet fuel to air He or Ar. It is flexible in terms of its compatibility with various semiconductor devices and substrates, including Si, GaAs, SiC, SiN, and many other ceramic, metallic, or composite materials. The metal foam-based thermal technology is scalable in both size and performance, making it applicable not only to discrete devices but also to Hybrid Multi Chip Modules (HMCM) integrating photonic and electronic devices, as well as double-sided Printed Wiring Boards (PWB) with constraining cores. The performance of the metal foam-based advanced heat exchangers is also scalable, along with its cost, to address a wide range of military and commercial applications. In a broader sense, this technology takes advantage of the materials and technologies developed under DoD contracts, creating synergies that increase the value of R&D spending to the US economy.

In electro-optic systems, thermal displacements can result in alignment-related optical losses, and it is often necessary for detectors and lasers to operate within a limited range of wavelengths or temperatures. To address these challenges, self-regulating thermal management systems with built-in temperature control capabilities may offer significant advantages. One such technology is the metal foam-based thermal management system, which can regulate temperatures within a narrow range (ΔT~10℃ to 20℃) over a wide interval of approximately 30℃ to 100℃.

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The advanced Integral RMF-based Heat Exchanger(HX) technology provides several advantages over state-of-the-art approaches, including significant cost, volume, weight, and performance benefits. For instance, current thermal management technology for high-performance power modules relies on heat spreading through a relatively thick (~0.32 cm) copper heat spreader thermal base, which is thermally coupled to a liquid or air-cooled heat sink through a thermally conducting organic compound. This approach increases both the volume and weight of the power modules, limits the heat flux, and reduces the capabilities of advanced power devices. As a result, the current approach necessitates the use of more devices, increases material costs, and limits the reliability of thermal and electrical interfaces between silicon and copper due to the large Coefficient of Thermal Expansion (CTE) mismatch.

Another state-of-the-art approach involves using heat pipes to remove heat from modules to an external heat exchanger. However, this approach also relies on soft interfaces to thermally couple the heat pipes to modules, which significantly increases thermal resistance. Additionally, heat pipes add cost, volume, and weight to the system.

In brief, the utilization of a RMF-based heat exchanger (HX) can effectively minimize the number of thermal interfaces, enhance the effective film coefficients, and increase the effective surface area for heat transfer. This approach can be further optimized by incorporating phase change materials encapsulated by thin microspheres to control and/or reduce coolant temperatures. Additionally, this technique has the potential to significantly mitigate the instabilities that are inherent in nucleate boiling heat transfer. Furthermore, the structural compatibility of this approach with semiconductor devices and substrates enables direct attachment.

The utilization of an integral compact heat exchanger obviates the need for highly resistive external thermal interfaces, such as thermal pads, thermal pastes, or thermal epoxies, which are typically employed to couple electronic and photonic HMCM with external air or liquid cooled heat sinks. Furthermore, the integral compact heat exchanger offers a large surface area and improved film coefficients, which significantly enhance thermal performance. The heat exchanger is composed of a reticulated foam, fabricated from a high thermal conductivity material, such as Al, Cu, Ag, SiC, or graphite. This reticulated metal foam (RMF) can be metallurgically bonded to high-performance electronic or opto-electronic devices, or to the insulating thermal bases of MCM/HMCM, for optimal performance, using soldering or brazing techniques. Commercially available metal foams are configured with pore densities of 5, 10, 20, and 30 pores per inch (ppi), and are approximately at 8% theoretical density, fabricated from 1100 Al, C102 Cu, or Ag. The initial macro-structure of the foam is considered isotropic, with randomly oriented ligaments. In the micro-scale, the foam structure consists of cells shaped as 14-sided polygons (dodecahedrons). The key parameters of the foam are its thermal conductivity, heat transfer surface area, and mechanical compliance.

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The effective or apparent thermal conductivity is a crucial physical property that is closely linked to the thermal performance of heat exchangers based on reticulated metal foams (RMFs). RMFs utilized for advanced heat exchanger applications with high performance are constructed from materials with the highest thermal conductivity, such as silver (Ag), copper (Cu), and aluminum (Al). However, the extended length and intricate shape of the thermal path that travels through the porous media significantly reduces the effective thermal conductivity of the RMF. This observation is particularly relevant to low-density foams (~5% to 10%) as manufactured. Nevertheless, the low density and open structure of the RMF offer a unique capability to scale and customize the thermal conductivity of RMF for any high-performance application. This feature enables the optimization of both the cost and thermal performance of heat exchangers for a given application. The high ductility of pure metals allows the RMF to undergo significant inelastic deformation without ligament failure. Additionally, intermediate annealing steps can be applied to further increase the density of the foam structure up to 50%. Since thermal conductivity is a vector quantity, its value is a function not only of the amount of compression (as it is for the effective surface area) but also of the direction of compression. An experimental and theoretical study was conducted to enhance our comprehension of the thermal conductivity of RMFs and its relationship with the amount of directional compression.

Through electrical resistance measurements and thermal conductivity calculations, indicate that the thermal and electrical conductivity of foam are either directly or inversely proportional to the increase in foam density, as expressed by the equation 1/(1+εp), depending on the direction of compression applied. Specifically, these properties are reduced in the direction of compression, while they are isotropically increased in the plane perpendicular to the direction of compression. This phenomenon can be attributed to the fact that compression leads to an increase in the thermal or electrical resistance by increasing the number of resistive elements that are in series in the direction of compression. Conversely, it reduces the thermal or electrical resistance by increasing the number of resistive elements that are in parallel in the perpendicular directions to densification. It is noteworthy that, regardless of the direction of compression, the actual density of the foam always increases.

The local convective energy transfer rates, or the local film coefficients, play a significant role in the overall thermal transfer performance of RMF-based heat exchangers (HXs). RMF provides flow passages with a small hydraulic diameter, which is inversely proportional to the convective conductance h. The hydraulic diameter is a measure related to the pore density, ligament size, and the relative density of the foam. Although the as-fabricated value of the relative density is typically less than 10%, it can be increased up to 50% through compression. From the perspective of designing and fabricating prototype HXs, the compressibility of the bulk form scales the thermal performance of the foam, allowing us to tailor the material for any given application. As a result, the development cycles are shortened, and the cost and time to market indices are significantly improved.

One of the key features of the RMF-based HX technology is the ability to increase the as-fabricated specific density of the foam through successive compression and annealing steps. This process results in an increase in specific surface area, local convective heat transfer, effective thermal conductivity perpendicular to the plane of compression, and flow resistance of the RMF matrix. By selecting the amount and direction of compression, as well as the initial pore size and relative density, the properties of RMF can be tailored for a given application.

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Increasing the as fabricated specific density of the foam by successive compression and annealing steps is one of the key features of the RMF based HX technology. The compression increases the specific surface area, the local convective heat transfer, the effective thermal conductivity in the direction perpendicular to the plane of compression, and the flow resistance of RMF matrix. Selecting the amount and the direction of compression as well as the initial pore size and relative density allow us tailor the properties of RMF for a given application .

In summary, the current study illustrated that the RMF based heat exchangers offer significant advantages over the alternative current approaches to compact heat exchangers. Among those advantages the most note worthy ones are:

  1. a) Large specific surface area
  2. b) Superior and scalable thermal performance
  3. c) Low weight and volume
  4. d) Compatibility with a large range of coolants
  5. e) High structural compliance and specific stiffness.
  6. f) Hard bondability to low expansion substrates.
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Lu

Our materials research team from Tsinghua University postdoctoral researcher lin and Harbin Institute of Technology researcher Mu, Nanjing University of Technology researcher Wei, they share their expertise in foam metal materials article.

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WE were established in 2003, located in the Gaoxin Zone of Guangdong-Guangxi Cooperation Special Experimental Zone, covering an area of 70 mu, with a plant of about 30,000 square meters, with more than 170 employees, is an advanced new material technology enterprise integrating research and development, production and sales.

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