ADVANCED MATERIALS FOR DEFENSE APPLICATIONS - PART 2: THERMAL MANAGEMENT
Thermal Materials - Summary of Expansion, Conductivity and Density
By: Martyn Acreman, Defense Market Director; Peter Lewis, Technical Manager of Technology and Innovation; and Nick Farrah, Product Line Director
Materion’s advanced materials are used for thermal management in high-consequence defense equipment across multiple domains, equipment which cannot fail even under the most demanding circumstances. Thermal management solutions are also important for many other markets (including aerospace, automotive, EV and consumer electronics) where reliability, sustainability and life-cycle cost are paramount.
Thermal management in the context of this blog refers to designing systems that allow heat to be transported, stored or expelled in a manner that enables key system elements to operate and continue to operate within a specific temperature range. Controlling the temperature of critical components provides for their optimum performance. Thermal management is most often associated with microelectronics assemblies and systems in which elevated temperatures can significantly reduce system performance and even cause system failure. In general, the demand for increased power density, reduced size, greater reliability, and the ever-present need for lower costs have continued to increase the challenges for thermal management materials.
Thermal management materials include ceramics, metals, alloys, metal matrix composites (MMCs), laminates and even plastics, for some applications. While determining the material properties and characteristics needed for thermal management, note that thermal management applications have widely varying requirements that encompass many aspects of materials science. There are the obvious thermal properties – thermal conductivity, specific heat or heat capacity, CTE (coefficient of thermal expansion) and thermal diffusivity. Then there are additional considerations to take into account such as structural properties, stiffness, strength, fatigue strength and how these properties vary across a temperature range. These certainly can be important considerations for some applications, as can damping properties or frequency response.
Material processing characteristics such as machinability, hermiticity, solderability, suitability for brazing and other available joining techniques, along with whether the material can be nickel plated and/or CVD gold coated should also be considered. Is the material an electrical conductor or insulator? Is it magnetic or non-magnetic? Is it compatible with other materials, gasses or chemicals in the system?
As you can see from the above list of considerations – which is not compete – the selection of materials for thermal management is a complex problem. Not only does it require extensive knowledge of materials, but it is always a multidisciplinary problem set that may need input from physicists, chemists, structural, thermal and corrosion engineers, plus many others.
Below are some considerations and characteristics for a sample of thermal management materials used in defense applications:
- Beryllium Metal (typically S200F or S200FH) – Beryllium metal is a good thermal management solution for very high-end applications, especially when low density is needed. This material is readily available, has excellent thermal properties, features very high specific stiffness, mid-range CTE, can be machined into complex components, is an electrical conductor and can produce gas-tight enclosures. The main disadvantage of beryllium metal for thermal management is cost. For this reason, beryllium-aluminum metal matrix composites (MMCs) are often selected as a good-enough thermal solution at a lower cost than using pure beryllium.
Characteristics of Beryllium Metal Compared to a Beryllium-Aluminum MMC
- Beryllium-Aluminum Metal Matrix Composite – Materion’s example of this is AlBeMet® (AM162H) MMC. This material is produced through powder metallurgy. As you can see in the table above, it has good thermal properties that are close to beryllium, slightly higher CTE and slightly lower thermal conductivity, but at a lower cost. AlBeMet material can be machined to produce complex components, it is E-beam weldable and can be nickel-plated and gold-coated. It is often used for chassis, housings, manifolds, enclosures, frames and cards for thermal management on missile avionics, radar systems and space avionics applications.
- Beryllium Oxide Ceramic – Examples are BW3250 and Thermalox® 995. Several types of beryllium oxide ceramics are available, but BW3250 is an exceptional option for thermal management. It has the highest thermal conductivity of any non-metal, excluding diamond. Note that BeO is an electrical insulator. As with most ceramics, complex geometries can be difficult to achieve and for this reason typical parts are rectangular, rod or tube. The use of ultrasonic machining does allow for higher shape complexity. BeO is frequently used as a heat spreader in defense applications that call for a CTE below 9.0 ppm/oC.
Characteristics of Two Types of Beryllium Oxide Ceramics
- Metal Matrix Composites - A category of materials that offer interesting thermal management solutions, allowing tailored properties to meet specific application needs. There are a lot of options for the metal matrix (aluminum, copper, magnesium, and others) and the reinforcement (diamond, SiC, Si, carbon fibers, graphite and more). These options have led to many varieties of MMC materials on the market that are aimed at thermal management and focus on higher thermal conductivity and low CTE’s. Some have been available for a long time like Mo-Cu and W-Cu which are higher density but have a lower CTE.
There are various grades of metal matrix composites and hypereutectic aluminum-silicon alloys manufactured via a powder metallurgy route. These include aluminum reinforced with SiC and aluminum reinforced with Si. The proprietary blending of high-quality powders produces MMCs with exceptional properties. These materials have a very homogeneous and refined microstructure, are heat treatable, can be machined to complex geometries and have multiple coating and joining options. The table below shows some example materials, such as Materion’s SupremEX® MMCs and AyontEXTM alloys, but many options are available.
*Information from www.samaterials.com; Stanford Advanced Materials
Characteristics of Two Types of Metal Matrix Composites Manufactured via Powder Metallurgy
- Copper Alloys – There are a wide range of copper alloys in various product forms. Many of these alloys are designed with specific applications in mind, such as electrical connectors. The table below shows Materion’s QMet® 300, an excellent material for connectors. Also shown are typical Mo-Cu & W-Cu materials. You can learn more about these different alloys on our website.
Characteristics of Copper Alloys and Copper MMCs
- Laminates or Sandwiched Dissimilar Materials - These materials include Cu/MoCu/Cu, Cu/Mo/Cu and Cu/Invar/Cu and offer interesting combinations of material properties. In this category Materion offers eStainless® Clad materials, which are thermally conductive, fully formable clad laminates of stainless steel and copper, or of stainless steel and aluminum. Think of these as laminates with a combination of the thermal properties of copper or aluminum, supported and protected with the structural properties of stainless steel. Property details are available on our website.
Many of our thermal management materials for defense applications are low density (less than 3 g/cm3), ideally suited to airborne and space applications. We also offer solutions that are not focused on low-density, including copper alloys and metal laminates.
Our team has many years of materials knowledge, and we understand the challenges of selecting suitable materials for thermal management. Our technical support team enjoys discussing options – our specialty is coming up with custom solutions for unique applications. You can contact them with questions at +1.800.375.4205.
** All material property values listed in the above tables are for reference only and should be considered typical values. The materials manufacturers or suppliers’ technical specification/data sheet should be used for property values associated with specific materials.