Clear Science!

The Clear Science staff was going to answer the question “Is there a decay rate in heat at distance from a flame/heat source?” To do that let’s consider one way that heat transports from one place to another: conduction.

Heat is energy. Say we have a flame on the left and no flame on the right. The flame is there because some chemical reaction is happening: chemical bonds are breaking and their energy is being liberated. Because of this the temperature of the flame is high, like 1500 degrees. On the right temperature is only room temperature or 20 degrees.

Heat moves by conduction from high temperatures to low ones. This is a basic property of the universe, and it is described by Fourier’s law. Written above in “math language,” what it says in English is “heat flux is proportional to the negative of the temperature gradient.” Or: heat fluxes from high temp to low.

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.)

Tellurium (Te) is used in a popular kind of solar cell, the CdTe photovoltaic. Only about 150 tonnes (t) of tellurium are produced a year. Looking at the elemental abundance chart we saw before, you see that tellurium is more abundant than gold (Au). But over 2,000 t of gold are produced on Earth per year. Why can’t we produce more tellurium?

It’s because tellurium is a byproduct of copper (Cu) production. The last stage of copper mining is to electrolytically refine copper on flat cathodes in an electrochemical cell. Selenium and tellurium just happen to form at the anodes in these cells, and so they are collected and sold as byproducts.

So using our current technology, you would double tellurium production by doubling copper production. But 16 million tonnes of copper are already produced every year, and that production uses 0.08% of all global energy. Doubling copper production is not realistic. So how would you double tellurium production? You have to start working to figure out another way to get it. The US Department of Energy anticipates a tellurium shortage by 2025 … (and looking back, it was one of the ECEs.)

Reading about Energy Critical Elements (or ECEs) the other day, the Clear Science Staff found ourselves thinking of this plot of elemental abundance in the Earth’s crust. The data comes from this US Geological Survey website. This can give you a good idea of how easy certain elements are to come by. The most common are oxygen (O) and silicon (Si), otherwise known as sand or glass when you put them together (silica SiO2).

Precious metals like platinum (Pt), gold (Au), and osmium (Os) are in the group of lowest occurrence. Rare earths are labeled in blue, and you can easily see they are not the most common but also not the rarest elements. However, this numerical “abundance” is only part of the story. There are other considerations when deciding how easy an element is to come by …  

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.

We mentioned storing electricity in a battery. A battery stores energy electrochemically, meaning that electrons are “stored” in a chemical with high energy. Any chemistry that involves electrons as reactants and products is electrochemistry. A fuel cell is similar to a battery: both have two electrodes (anode and cathode) and an electrolyte.

There is more than one way to distinguish a battery and a fuel cell:

1. Batteries store their chemicals inside the battery, while fuel cells are fed from outside. 
2. Fuel cells often involve catalysis, while batteries may not.
3. Fuel cells operate continuously. Batteries operate in a batch manner, i.e. charge/discharge.

A current (simplified) view of the electrical grid shows that the demand for electricity and the amount of electricity generated must essentially match. This is done by ramping power plants up and down as people change how much electricity they want.

Solar power and wind power are not easy to regulate. But we want to be able to make them a large part of the grid, since they’re green. One way to do this is add electrical storage to the grid, which is like being able to put electricity in a box and save it for later. During periods of low demand you fill up the storage and then use it during high demand. A charged battery is an example of “stored” electricity.

Humanity uses about 15 TW of power. (That’s 15,000,000,000,000 watts. TW means terawatt, which is a trillion watts.) By comparison 120,000 TW of sunlight falls on the Earth. Ideally, this means one hour of sunlight could power us for one year. Practically, the top end of what we could collect would be around 600 TW, which is still a huge number. In other words, solar power could solve a lot of problems.

Solar cells like those you have probably seen work by using sunlight to make electrons move in a circuit, which is electricity. The most common design is made of layers of n- and p-type semiconductors. Light separates an electron and a hole, and the semiconductor layers make them go different directions to recombine. You cleverly make the electron go through a circuit and you get electricity.

The challenges to widespread use of solar cells are cost and intermittency. This is a good problem for scientists and engineers.