Metal foam
[9] Two patents on sponge-like metal were issued to Benjamin Sosnik in 1948 and 1951 who applied mercury vapor to blow liquid aluminium.
They are light (typically 10–25% of the density of an identical non-porous alloy; commonly those of aluminium) and stiff and are frequently proposed as a lightweight structural material.
This manufacturing method allows for "perfect" foam, so-called because it satisfies Plateau's laws and has conducting pores of the shape of a truncated octahedron Kelvin cell (body-centered cubic structure).
The addition of a thin film can also improve other properties such as corrosion resistance and enable surface functionalization for catalytic flow processes.
[16] Recent studies have demonstrated issues with the uniformity of the thin-film due to the complex geometry of metal foams.
[16] Issues with uniformity have been addressed in more recent studies through the implementation of nanoparticle thin films, leading to improved mechanical and corrosion resistance properties.
Additionally, the interaction or compatibility between different polymers and metals in foam ligands can be explored in order to get an improved understanding of their sensitivity to external forces.
After metal (e.g. aluminium) powders and foaming agent (e.g. TiH2) have been mixed, they are compressed into a compact, solid precursor, which can be available in the form of a billet, a sheet, or a wire.
Production of precursors can be done by a combination of materials forming processes, such as powder pressing,[21] extrusion (direct[22] or conform[23]) and flat rolling.
[25] CMF was developed at North Carolina State University by the inventor Afsaneh Rabiei with four patents in her name, all entitled "Composite Metal Foam and Method of Preparation Thereof" (US Utility Patents 9208912, 8110143, 8105696, 7641984), and CMF is currently proprietary technology owned by the company Advanced Materials Manufacturing.
Other potential applications include nuclear waste (shielding X-rays, gamma rays and neutron radiation) transfer and thermal insulation for space vehicle atmospheric re-entry, with many times the resistance to fire and heat as the plain metals.
The panels were tested against 23 × 152 mm high explosive incendiary rounds (as in anti-aircraft weapons) that release a high-pressure blast wave and metal fragments at speeds up to 1524 m/s.
The thicker sample (16.7 mm thick) was able to completely stop various-sized fragments from three separate incendiary ammunition tests.
[27] In this study, stainless steel CMF blocked blast pressure and fragmentation at 5,000 feet per second from high explosive incendiary (HEI) rounds that detonate at 18 inches away.
[29] Composite metal foam panels, manufactured using 2 mm steel hollow spheres embedded in a stainless steel matrix and processed using a powder metallurgy technique, were used together with boron carbide ceramic and aluminium 7075 or Kevlar back panels to fabricate a new composite armour system.
The highly functional layer-based design allowed the composite metal foam to absorb the ballistic kinetic energy effectively, where the CMF layer accounted for 60–70% of the total energy absorbed by the armour system and allowed the composite armour system to show superior ballistic performance for both Type III and IV threats.
The mild steel cores of the ball rounds penetrated one of the three samples but revealed the benefits of using multiple tiles over a single ceramic faceplate to limit the spread of damage.
The experimental results were compared to commercially available armour materials and offer improved performance with reduced weight.
As the impact velocity increased, so did the effective strength of the CMF layer due to the strain rate sensitivity of the material.
[26] The weight savings afforded by using such novel armour can improve the fuel efficiency of military vehicles without sacrificing the protection of the personnel or the equipment inside.
Puncture tests were conducted on S-S CMF-CSP with different thicknesses of stainless steel face sheets and CMF core.
Various thicknesses of the CMF core and face sheets created a variety of target areal densities from about 6.7 to about 11.7 kg per each tile of 30 x 30 cm.
Upon reaching the 30 minutes' time of exposure, the maximum temperature on the unexposed surface of the steel was 400 °C (752 °F) at the center of the plate directly above the jet burner.
For orthopedic applications, tantalum or titanium foams are common for their tensile strength, corrosion resistance and biocompatibility.
These foams are stiff, fire resistant, nontoxic, recyclable, energy absorbent, less thermally conductive, less magnetically permeable, and more efficiently sound dampening, especially when compared to hollow parts.
[44] Metal foams are popular support for electrocatalysts due to the high surface area and stable structure.
However, the benchmark of electrocatalysts can be difficult due to the undetermined surface area, different foam properties, and capillary effect.
From a manufacturing standpoint, the transition to foam technology requires new production and assembly techniques and heat exchanger design.
Kisitu et al. [51][52] pioneered the experimental investigation of using compressed copper foam for advanced two-phase cooling for high heat flux electronics.
Preliminarly results show that compressed metallic foams can solve several issues faced with microchannels, including clogging, flow instabilities, low CHF, and others.
