Professor Hanington's Speaking of Science: Rare element cerium

Apr 27, 2023

White LED made with cerium

As promised a few weeks ago we are going through the rare earth elements this month for two reasons. One, sadly is: as of late there is not much interesting physical science news making headlines within the United States nor the world for that matter and as most readers know, I don't cover anything related to biology. I leave that for Hannah who is working hard at MIT these days.

The second reason is: the subject of rare earth elements will soon become a very hot topic because China has begun to limit the exportation of these important elements, especially within the last few months. In fact, some of the customers of AHV have forwarded questionnaires to inquire if AHV uses such rare earths in any of their products and will we be impacted. While motors, magnets and special exotic electronics may, we are not really touched by this trade embargo. Therefore, based on the above, we continue our study with the rare earth element cerium.

Cerium, with an atomic number of 58 is a soft, ductile, and silvery-white metal that will, much like silver, develop an oxide tarnish of when exposed to air. You may have first heard about cerium when shopping for a self-cleaning electric oven. For decades, inside the stove, Cerium(III) oxide is used in the walls to help prevent the build-up of cooking grease by acting as a catalyst for the oxidation of hydrocarbons. Or, if you are a do-it-yourselfer, and are making a telescope, you will find that the mirror grinding process uses cerium oxide. This compound lowers the surface energy of the glass by forming a cerium oxygen silicone material that is softer than the underlying glass and allows the surface to take on a smooth polish, it is usually the last step completed before aluminizing the mirror.

Cerium is also the main metal in the alloy called: mischmetal. This metallic solid has the interesting property that it easily sparks when struck or scraped with iron and is often used as lighter flints. Mischmetal, German for "mixed metal" also contains several other rare-earth elements: 55% cerium, 25% lanthanum, and up to 18% neodymium. You can thank the German chemist Carl Auer von Welsbach for discovering this strange alloy in his quest for trying to perfect the street gas-mantle lighting systems in his hometown. Think of him the next time you use a spark-striker for a torch.

The stability of pigments when exposed to bright light can be enhanced by the addition of cerium. Cerium sulfide powder is usually used where dark-red color is required because it has good thermal stability and is non-toxic. It offers a hue that is resistant to darkening caused by sunlight. You may consider it a good bet that many road signs incorporate cerium during their manufacture.

Another use for the element is as an additive (16% Ce by weight) with aluminum to create a castable eutectic alloy that has excellent high temperature strength. This alloy is used notably in the casting of high performance engine cylinder heads. By allowing the engine to run hotter, power output can be increased dramatically. Other automotive applications for cerium are in catalytic converters for the oxidation of CO and NOx emissions in the exhaust gases.

Cerium is the most abundant of all the rare-earth elements making up 66 ppm of the Earth's crust, a value slightly behind that of copper (68 ppm), making its listing as a rare-earth really not so rare. Cerium occurs in various minerals with the important commercial sources being monazite and bastnäsite and is considered one of the easiest rare earths to extract owing to the stable +4 oxidation state in aqueous solutions. For those who want to refine cerium from bastnäsite, just wash with hydrochloric acid to remove calcium carbonate, roast in air to convert to cerium dioxide that can be now leached out with muriatic acid.

One of the most recent uses of cerium is in the last decade has been in the construction of white LEDs. Most "white" LEDs in production today use a 460nm blue gallium nitride LED covered by a yellowish phosphor coating usually made of cerium doped yttrium aluminum garnet crystals which have been powdered and bound in a type of clear viscous adhesive. The LED chip emits blue light, part of which is converted to yellow by the YAG:Ce. Since yellow light stimulates the red and green receptors of the eye, the resulting mix of blue and yellow light gives the appearance of white.

You’ve probably seen closeup images of the Sun before, revealing sunspots, solar storms or something else entirely. But you’ve never seen the Sun quite like this. These images were recently captured by the Daniel K. Inouye Solar Telescope, the most powerful solar telescope on the planet.

Gary Hanington is Professor Emeritus of physical science at Great Basin College and Chief Scientist at AHV. He can be reached at [email protected].

Here is a really old one that follows on the puzzle from two weeks ago where I O A was the spelling of a state using only three letters (for Iowa). Now, even better, name a state that can be spelled with just two letters.

Answer: Ohio (oO)

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