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Above are some TEM images from the paper In Situ TEM Investigation of Congruent Phase Transition and Structural Evolution of Nanostructured Silicon/Carbon Anode for Lithium Ion Batteries. They show a hollow, conductive fiber with a layer of silicon (Si) around the outside. On the right are some electron diffraction images, taken where the red circles indicate.

What the researchers found is that as more and more lithium (Li) ions enter the Si, the Si layer remains amorphous (order-less) until the composition reaches the level of Li15 Si4. At this atomic composition, the layer attains a crystalline order. A way to think of that is like freezing. Ice is like water with a crystalline order imposed. In this case both the amorphous and crystalline phases are solid, but you sort of freeze to go from one to the other.

The way the researchers could tell this happened is from the electron diffraction images. The dots in the bottom image are caused by crystalline order. The electrons bounce off the regular faces of the crystals and interfere with each other, giving dots.

We mentioned a recent paper that examined silicon/carbon fibers for use in batteries. Shown above is a TEM image from the paper. From it you can tell the fibers are essentially hollow carbon fibers with a layer of silicon around the outside edge.

Transmission electron microscopes (TEMs) work by shooting a beam of electrons through a sample. The sample has to be quite thin to let the electrons through, but since these fibers are around 100 nm thick that is fine. The important thing is that the image is made with electrons. This is different than the sort of images you’re used to, such as those made with cameras, regular microscopes, or even your eyes: all of those images are made by light. Electrons have a smaller wavelength than light, and so electron microscopes work at extremely high resolution: in some cases you can make out individual atoms with them.

Why would anyone care about silicon-coated nano-fibers? Because lithium ions can be stored inside silicon, and that makes these fibers well-suited to storing lithium for lithium ion batteries. Some researchers think we may be able to extend the life of batteries using these.

Let’s talk about some recent science work on lithium-ion batteries, and the journal article reporting the work. Scientists tell each other about their work through articles (or “papers”). This one is called: In Situ TEM Investigation of Congruent Phase Transition and Structural Evolution of Nanostructured Silicon/Carbon Anode for Lithium Ion Batteries.

Sometimes titles are hard to understand unless you break them down into pieces:

• TEM is transmission electron microscopy
• in situ means they’re doing some experiment and watching it happen inside the TEM
• phase transition means atoms are going from one arrangement to another
• structural evolution means the experiment is changing the material somehow
• nanostructured means the size they’re looking at is less than 1000 nanometers
• an anode is the negative electrode in a battery
• lithium ion batteries are like computer or cell phone batteries

This paper is in a journal called Nano Letters. Under the title is a list of names. These are the authors. Basically that is everyone who did the science work, dreamed it up, paid for it, or wrote the paper. (This C&EN article on the paper summarizes and contains a link to it.)

The Clear Science Staff was just reading Energy-critical elements for sustainable development in the current issue of the MRS Bulletin. Energy-critical elements or ECEs are those thought to be important for emerging energy technologies. Notice the elements chosen: the rare earths (pink), the platinum group metals (blue), metalloids used for solar panels (green), and lithium for batteries.

At one time, aluminum was considered a precious metal, much like silver and gold. This was not due to scarcity, since aluminum is 8% of the Earth, making it the most abundant metal, and the third most abundant element. However, the process to separate aluminum metal from its ores and minerals (e.g. bauxite, feldspar) was so difficult that it cost as much as silver.

The capstone of the Washington Monument is made of aluminum, which was at the time a precious metal. P. H. McLaughlin, the master mechanic of the project, is seen above setting it in place.

The Hall-Heroult process, which is the industrial process for producing aluminum metal, was invented in 1886, soon after the Washington Monument was completed. Today we consider aluminum a relatively inexpensive material. A similar discovery will not cause the price of the precious metals to fall, since they are scarce in nature, unlike aluminum.

We’ve discussed how polymers have not only a molecular structure, but also a larger order based on how the long molecular chains orient with respect to each other. Take for example HDPE (high density polyethylene) and LDPE (low density polyethylene). These are familiar household items: for example shampoo bottles are often HDPE and plastic film is often LDPE.

LDPE has a branched chain, and this prevents the polymer from lining up with itself in orderly rows. HDPE is mostly straight, and it does line up. By lining up, HDPE becomes rigid and opaque. LDPE, in contrast, is floppy and transparent. 

Why is some stainless steel magnetic and some isn’t? There are a couple kinds of stainless steel, and their atoms are stacked in different ways. This is exactly like the way cannonballs or marbles are stacked. There is a science for that.

Ferritic stainless is magnetic, but there is another type, called austenitic, which also has nickel in the formulation. In this one, the atoms stack a different way, and it’s no longer magnetic. Magnetism is complicated, but can be affected by the “order” in a material.

BCC is Body-Centered Cubic: It’s like 4 spheres sitting on one, which sits on 4. FCC is Face-Centered Cubic: It’s like a layer of 5 sitting on 4, which sits on 5. Try and stack some apples or cherries and see if you can see the different ways.

Stainless steel is called that because it doesn’t rust or discolor, unlike regular steel or plain iron. This is because of the chromium in the alloy. It’s just enough chromium to cause a chromium oxide layer on the stainless steel.

Chromium oxide forms an adherent, transparent, very thin layer on metal that blocks air and stops corrosion. Iron oxide, a.k.a. rust, flakes off, lets air and water through, is ugly, and doesn’t stop corrosion at all.

The Clear Science research team found this example of a chrome Lamborghini. How pretty! You wouldn’t ever make an iron Lamborghini, because it would soon be a rust Lamborghini. Chromium oxide: keeping stainless steel stainless, and keeping your obnoxious fancy car shiny. (And a DeLorean is stainless steel, BTW.)

PS Aluminum does the same kind of thing. Ever wonder why iron rusts while aluminum and chrome don’t?

To make stainless steel, you add chromium to the basic steel formulation of iron + carbon. So stainless steel is an alloy of iron, carbon, and chromium.

Alloys are different than molecular compounds. The Fe, Cr, and C aren’t bonded to each other. In other words, they are not sharing or exchanging electrons to form molecular bonds. They are just sitting next to each other.