Chemical elements are the gifts that keep on giving. This holiday season, it's important to acknowledge how much the chemical elements--and especially the rare-earth elements--keep our photonics industry humming. As I prepare to write my February Photonics Applied feature article on "Photonics Materials", I am reminded of how precious these chemical elements are to the livelihood of our industry and unfortunately, how precarious their supply really is.
For example, take the lanthanide and rare-earth element holmium (Ho), atomic number 67. According to Wikipedia and the Royal Society of chemistry's online periodic table (www.rsc.org/periodic-table), holmium and all the other rare-earth elements including cerium (Ce), neodymium (Nd), erbium (Er), thulium (Tm), and ytterbium (Yb) share something in common: they are at "high risk" in terms of supply. Like many of the rare-earths, holmium rarely occurs in pure form in nature. A soft and malleable silver-white metal (see image), it is extracted from other metallic minerals using caustic and acidic processes. Holmium oxidizes rapidly into several forms of holmium oxide. While benign in its elemental form, many of the isotopes of holmium are radioactive.Although abundant in nature at 1.3 mg/kg of earth (more than 20 times more abundant than silver according to www.lenntech.com/periodic/elements/ho.htm), the environmentally unhealthy separation process means that nearly 97% of the supply comes from China. A recent Forbes article (www.forbes.com/sites/jackperkowski/2012/06/21/behind-chinas-rare-earth-controversy/) highlighted the
controversy, in which China published its first white paper and indicated that it had a smaller percentage (23% of the world's supply) compared to the 36% it was estimated to have. The report raised stock prices for non-Chinese mining companies and recent quotas on exports from China are alarming rare-earth purchasers. Indeed, other nations have simply stopped mining the stuff (see chart).
Because holmium has the highest magnetic moment of any of the rare-earths, it possesses unique properties that make it useful in building ultrastrong magnets, in absorbing nuclear-fission-created neutrons, and as a dopant material in solid-state and fiber lasers. In fact, holmium YAG lasers are used with much efficacy in laser-based surgical applications. For example, a video at http://youtu.be/3TDI8kmU9C0 shows how holmium lasers can remove kidney stones, and another video here shows how a holmium laser is used to treat urethral stricture--an abnormal narrowing of the tube that carries urine out of the body:
So what would the holidays be without our rare-earths? Ask any photonics company and they will tell you, not at all joyful. I'm not a fan of mining and its harmful effects on the environment, but it's clear that the geographic availability of these rare-earth elements needs to be expanded. Stay tuned for my February feature article on photonic materials and the many surprising uses for common and not-so-common chemical elements in our photonics industry. And until then, here's hoping you'll get a big Ho-Ho-Ho from Santa this holiday season!
