Welcome to this lecture on the design of materials for recycling. My name is Erik Offerman. I work at the Department of Materials Science and Engineering at the Delft University of Technology in Netherlands. This is the second part of the lecture, and the first part of the lecture was about the complexity of the materials world and how that affects the recycling rates. The second part of this lecture is about moving forward. What are the opportunities? The goal of this lecture is to provide you with insight on how we can design materials for recycling. Now that is an importance and new field to me because as a material scientist, I've always developed new materials and designed new materials to improve the performance of these materials. But now that we have to move to a circular economy, I'm being challenged to think ahead at the moments that materials need to be recycled again. So, therefore, the challenge that we face is how to design materials in such a way that when these materials are recycled, they have the same or better properties whereas at the same time minimize costs, minimize environmental footprint and minimize material losses. I propose four pathways forward for the design of materials for recycling. First path is to design materials with minimal sensitivity for impurities. The second path is to design materials with the desirable mixing of elements during recycling. The third path is to design materials with a reduced number of alloying elements, and a fourth path is to design materials with fewer elements from the periodic table. The first path is to design alloys with minimal sensitivity for impurities. For example, if we take a metallic alloy as you can see here in the figure, it's, for example, an aluminium alloy with a matrix of aluminium which means you have mainly aluminium atoms, and added to that are eight alloying elements, eight chemical elements. So those are indicated by the little spheres with numbers in them from 1 to 8. That is very schematically a metallic alloy. Now if you want to recycle this metallic alloy at a certain moment once you have separated from other waste, you want to smelt the alloy. And in that smelting process, some impurities from the waste that you could not get rid of will be incorporated in this alloy. Now if we can design alloys in such a way that there are less sensitive or at least minimally sensitive to impurities, we can still create high-performance alloys during the smelting process and even increase the recovery rate of these high-performance alloys, because you do not degrade the alloy by the addition of impurities, if you have already taken into account beforehand that some impurities will be added during the smelting process into recycling. The second path forward that I propose is to design material combinations that result in desirable mixtures of elements during recycling. So what do I mean by that? I'll take the example shown in the figure. Suppose we have a metallic alloy again consisting of something like eight alloying elements, and now this alloy is coated with a coating and this coating also has an alloying element or an element of which it is made of. Now these coatings are often added to prevent the metal from corroding or to make a hard outer coating so that you can use it for drilling purposes. Now if you think beforehand about the effects that in end this material has to be recycled, you can think of combinations of elements that are used in the coating and that are used in the metallic alloy that actually, match that you can get a desirable combination of alloying elements once you smelt the combination of the metallic alloy plus the coating. And on the other part of the picture where you see the molten material that has solidified in it nicely has all the alloying elements of the original alloy plus an additional alloying element number nine from the coating. And they fit nicely together. The third path forward is to design alloys with a reduced amount of alloying elements while maintaining or improving performance. What do I mean by that? I suppose you have an alloy with a large number of alloying elements, let's say eight alloying elements, and you want to recycle and smelt it together with another alloy also consisting of eight alloying elements but different alloying elements. Now in this case, you will end up with a new alloy consisting of 16 alloying elements. And there is a higher chance of unfavorable combinations of alloying elements which reduce the properties of your material. Then, when you do it differently, and differently. I mean reducing the complexity of the alloy by reducing the number of alloying elements in it. So suppose you have alloy one consisting of three alloying elements numbered one to three in this case, and you have a second alloy with three different alloying elements numbered four to six in this case. And when you smelt them together, you get a new alloy consisting of six alloying elements. For this alloy, the chance is reduced that you get unfavorable combinations of alloying elements. However, this trick will only work once because now you have an alloy a new alloy with six alloying elements whereas you started with three alloying elements. And the fourth path provides a solution for this. The fourth path forward is to design alloys that use fewer elements from the periodic table to create a wide range of properties. Suppose we have alloy one consisting of just three alloying elements numbered here one to three, and you have a second alloy which also contains the same alloying elements number one to three, but in different concentrations and maybe with a different microstructure and alloy number one. In other words, alloy number one and alloy number two have different properties but they have the same chemical elements. Now if you smelt these two alloys together, you end up with a new alloy also consisting of just three alloying elements. And this is a cycle that can be repeated many times, because if you now want to recycle, re-recycle the alloy again you still have an alloy with only just three chemical elements. And you can combine that again with a second alloy with the same three elements as smelted and to form a new alloy, and this cycle can be repeated over and over. I would like to give you one example of the potential feasibility of the fourth path forward. And there is I would like to illustrate that you can design materials in such a way that they have completely different properties and yet have the same chemical elements. So for steel, it is already known that you can create on the one hand a very high ductile steel with a low strength, and on the other hand, a very strong steel with a very low ductility with just the same chemical alloying elements. So to illustrate that, you see here in the figure two different microstructures and the two different microstructures are at the origin of the two different properties that these alloys have. In one figure you see relatively large grains in green, orange, pink, and greys, and blue, and on the other figure, you see more needle-like type of microstructure. And these two different microstructures can result in a factor of 10 difference in strength between the two different steel rates. So this is for me a new challenge, a new path forward which I think is not an easy path which will require a lot of time but that's I think the case for many aspects when we want to make a transition from a linear economy to a more circular economy. This is a transition which some scientists have calculated to take place over a period of a 100 years and the redesign of all the alloys can be thought of as a task which takes also a similar amount of years. This is the summary of the second part of the lecture. I propose as ways forwards to design materials for recycling along four paths. One, to design materials with minimal sensitivity for impurities. This is a path forward that can be implemented in the near future. The second path forward that I propose is to design materials with desirable mixing of elements during recycling. This is already a bit more complex to do and further into the future. This third part forward that I propose is to design materials with reduced number of alloying elements which will take more time to realize. And the fourth path forward which is also the most promising path forward, is to design materials with fewer elements from the periodic table. And this is also the most radical change that I propose and the most difficult to achieve. However, if we look at the time scales that people estimate to go from a linear economy to a circular economy, it is of the order o100 years. And I think to make new alloys to design alloys with fewer elements from the periodic table, we're talking about similar timescales. So that I also see as a challenge for my own career to move forward. And what we need for that is to control the microstructure for metals.
