Engineering news
Composite metal foams could possess superior heat insulating qualities than steel, and could be better suited to applications like storing and transporting nuclear or hazardous materials or space exploration, according to new research.
The paper, ‘Experimental and computational studies on the thermal behaviour and fire retardant properties of composite metal foams’, suggests that composite metal foams (CMFs) “offer extremely good thermal insulation, superior thermal stability, and excellent flame retardant performances” compared to base metals and alloys such as steel. CMFs could be a favourable alternative to other metals for heat-sensitive applications and, because of it light weight, for space exploration.
The team from North Carolina State University experimented with same size samples of steel-steel CMF - which consist of steel hollow spheres in a steel matrix - and bulk stainless steel that were 0.75in (19.05mm ) thick and 63.5 by 63.5mm (2.5 by 2.5in). One side of each sample type was exposed to a fire reaching an average temperature of 800°C for 30 minutes. Researchers monitored the length of time it took for the heat to reach the opposite end of the samples and found that it took four minutes for the entire stainless steel piece to reach 800°C while it took double the time for the steel-steel CMF to reach the same temperature.
CMF is made up of metallic hollow spheres embedded in a metallic matrix made of steel, aluminium or metallic alloys. Afsaneh Rabiei, corresponding author of a paper on the research and a professor of mechanical and aerospace engineering at North Carolina State University, said: “The presence of air pockets inside CMF make it so effective at blocking heat, mainly because heat travels more slowly through air than through metal”.
The study also discovered that the composite metal foam made entirely of stainless steel expands 80 percent less than bulk stainless steel at 200°C – and the differential increases at higher temperatures. Another noteworthy advantage is that the composite metal foam expands at a fairly constant rate when exposed to high heat – whereas conventional bulk metals and alloys such as stainless steel expand more rapidly as temperature increases.
“We already knew the CMFs are light-weight materials with outstanding high-velocity impact resistance, and effective radiation shielding, now we know that it can withstand high heat,” Rabiei said. “This difference in thermal conductivity means that the use of composite metal foams could help avoid circumstances where accidents can lead to explosions.”
The team have devised two systems to produce CMFs. The researchers said: “One is based on casting a low melting point matrix material, such as aluminium, around hollow spheres made of a material with a higher melting point, such as steel. This creates aluminium-steel CMFs, for example. The other technique is based on sintering, or baking, the matrix powder around prefabricated hollow spheres. This creates CMFs such as steel-steel, which consist of steel hollow spheres in a steel matrix.”