Steel is an alloy of predominantly iron and carbon and considered as one of the most versatile structural materials in use today. In particular, the use of steel foams has been gaining importance in applications such as electromagnetic shielding and heat exchangers [1]. Such foams are unique in their solid steel makeup but have a larger volume fraction of pores that can be penetrated or infused by other materials, such as metals. Steel foams are advantageous due to their high energy absorption, high strength, ductility, light weight, uniformity and improved thermal properties [3]. Steel foams are currently produced by powder metallurgy by varying the sintering pressure or by using solid or gaseous materials to impart porosity [2]. Here we present an alternative manufacturing technique using heat treatment of a patterned biopolymer-iron oxide composite. In particular, we use carrageenan, a common food thickener, as the biopolymer and carbon source for the derivation of steel. Carrageenan is a biopolymer extracted from seaweed in a sustainable process with positive socioeconomic impact in countries such as the Philippines, Thailand and Indonesia. Furthermore, one gram of iota-carrageenan currently costs $0.62, while one gram of activated carbon black costs $5.23 USD (Sigma Aldrich®). Our goal is to innovate a sustainable process that enables the making of relatively inexpensive steel foams with arbitrary geometry. To this end, we implement the use of a carrageenan-iron oxide composite that we can shape by extrusion. We then heat treat such shapes under an inert atmosphere to carbonize the biopolymer, reduce the iron oxide and finally to react iron and carbon. Here, we study the effect of heat treatment and the mixing ratio between carrageenan and iron oxide on the composition of the steel obtained.

The effect of mixing ratio has been studied by mixing dry powders of iron oxide (Fe₃O₄) nanoparticles with iota-carrageenan in ratios of 0.7, 2.5, 15, and 65 percent carbon. These different mixtures were then each poured into water at 100 °C to induce gellification of the carrageenan and sonicated to disperse the iron oxide particles throughout the polymer matrix. The biopolymer composites were then inserted into a syringe and extruded into different 3-dimensional shapes, such as a cubes and pyramids. Heat treatment of these shapes for 3 hours in a tube furnace under high vacuum to a final temperature of 1350°C following a heating ramp of 5°C/min has so far lead to iron carbide in the form of cementite. The effect of mixing ratio in the composition of the final material is evident by the right shift and magnitude decrease of the peaks in XRD results as the percentage of carbon decreases. After heat treatment we observed a ~60% decrease in the size of the original shapes. Such shrinkage is near isometric as the shapes remain throughout the process. As expected, they are highly porous and brittle.

Ongoing work is on studying mixing ratios at or below 0.7%, where we expect to observe the cementite shifting into steel. Once steel is produced, an alteration of carbon percentages from 0.1% to 1.2% and introduction of other elements will be performed to analyze how different grades of steel can be produced. Besides eliminating the need for carbon from petroleum oil, an expected advantage of the process presented here is the reduction of the final temperature to obtain steel. This is due to the colloidal nature of the precursor and the close contact between the nanoparticles and the surrounding carbon. At a temperature of 1350 °C this process will compare advantageously to the normal temperatures of 2000 °C used for 60% of global steel production. This would translate to less energy required for the process. Another important advantage is the potential to manufacture 3D printed steel shapes by shaping the precursor composite. Both composition of the precursor as well as heat treatment are expected to determine the porosity of the foam. Understanding such relation will allow for tailoring the material porosity for specific applications such as filters for harsh conditions and electromagnetic shielding.

 

References

1. H. Dziemballa, L. Manke, in: I. von Hagen, H.-J. Wieland (Eds.), Steel, Future for the Automotive Industry, Verlag Stahleisen GmbH, D¨usseldorf 2005, pp. 341–348.
2. T. Maki, in: K.A. Taylor, S.W. Thompson, F.B. Fischer (Eds.), Physical Metallurgy of Direct-Quenched Steels, The Minerals, Metals and Materials Society, Warrendale, PA, USA, 1993, pp. 3–16.
3. E. Murad, J.H. Johnston, in: G.J. Long (Ed.), Iron Oxide and Hydroxides in Mössbauer Spectroscopy Applied to Inorganic Chemistry, Plenum, New York, 1987, p. 507.