Indium, Yttrium ... What is the Use of Rare Metals in Our Smartphones?

Rare earth oxides. From the top and in the clockwise direction: praseodymium (black), cerium (yellow), lanthanum (cream), neodymium (gray), samarium (yellow), gadolinium (white). Credit: Wikimedia Commons.

They are called tantalum, hafnium, praseodymium or terbium: these exotic metals with increasing strategic importance are indispensable in components of our phones such as touch screens, microphones, or transistors. But what are they really for?

Our digital devices contain treasures. Advanced electronics depend on a whole bunch of very different metals from copper, aluminum or steel that we encounter in everyday life. This poses many environmental and geopolitical questions, from cobalt mines in the Democratic Republic of the Congo to Chinese rare earth production, which Beijing threatens to limit exports in its trade war with the United States.

Smartphone manufacturers are not very talkative about the rare metals used in their products. But I still tried to collect information on the use of these exotic elements in everyday electronics.

What are "rare metals"?

"Rare metals" are a catch-all name that covers most of the atoms in the middle of the periodic table of elements. That's several dozens of elementary materials. Some names, such as blue cobalt or very heavy tungsten, are relatively familiar to us. But we also find more surprising things, like gallium, the metal that melts in the hand at the same temperature as chocolate (30°C). Let us mention among other names: tantalum, niobium, indium, hafnium, or palladium.

The best-known rare metals are a group of 17 elements called "rare earth". These were first discovered by the Swedish chemist Carl Axel Arrhenius in 1787 in a rock in the village of Ytterby on an island near Stockholm. They represent a group of metals with properties quite similar to each other and can sometimes be used interchangeably in alloys.

These are scandium, yttrium (named after the village of Ytterby), and 15 atoms of the "lanthanide" family: lanthanum, cerium, praseodymium, neodymium, promethium, samarium, europium, gadolinium, terbium (from "Ytterby" by removing two letters), dysprosium, holmium, erbium ("Ytterby" by removing three letters), thulium, ytterbium (you have it?) and lutetium. Among them, promethium is apart, because it is radioactive and extremely rare in its natural state, with only 500 grams scattered throughout the entire earth's crust; its uses are thus very small and special.

Contrary to what their name indicates, rare earth are not "rare" on Earth. Yttrium is thus 400 times more abundant in the earth's crust than silver. But they are very scattered and do not come in the form of easily exploitable minerals. They can be mined in copper, zinc, or uranium mines, but the process is very expensive and highly polluting. That is why virtually only China assumes to get into it, with almost 100% monopoly over the treatment of rare earth from the ores of other metals.

Where are they in our smartphones?
Screens and LEDs
The first touch screens were resistive, that is to say, it was physically necessary to press it with a finger or a stylus. Our modern smartphones have capacitive screens, with an electrostatic field that recognizes the disturbances caused by the simple touch. For this to work, the screen must be coated with a film that conducts electricity while being transparent. This film is made of indium-tin-oxide (abbreviated "ITO"), the indium being derived from zinc ore.

An LED is schematically made of a semiconductor chip, mounted on a metal frame and encapsulated in a piece of transparent plastic. The semiconductor material of the chip is usually a gallium compound (extracted from the aluminum ore) and at least one other atom, which will determine the "basic" color of the LED. With arsenic combined with phosphorus, an orange-red light is obtained, while with nitrogen or indium, the LED appears blue.

But to make really beautiful colors with an LED, it is necessary to coat the chip with powders of phosphorus. And in there, it's not just phosphorus. The colors are indeed made with different rare-earth cocktails. The most common, yttrium and aluminum garnet ("YAG") doped with cerium, is used to make a yellow color that can, for example, be added to a blue LED (indium) to make it white. Other phosphors with yttrium or europium make red, while europium can also be used for blue and terbium for green. Lanthanum and gadolinium can also be found.

Sound and vibrations
What is the common point between a microphone, a speaker and the vibrations of a phone? All of these elements need magnets to work. It is they who, thanks to their force of attraction, produce the vibrations which can shake the whole apparatus - or produce oscillations finer and translatable in sound.

But beware those are different from the magnets found on our refrigerators. You need tiny, powerful magnets, and above all, that does not weaken over time. The most proven solution in this field is an alloy of neodymium and praseodymium, also used in electric motors and turbines of wind turbines. This mixture may also include some terbium or dysprosium. Tungsten, which is twice as heavy as steel, serves as a weight to amplify vibrations.

Batteries
The ubiquitous batteries in our devices are lithium-ion batteries. Of course, there is lithium, the lightest metal in the world, especially from Australia and the deserts of salt from South America. But cobalt, in the form of cobalt dioxide and lithium (LiCoO2), is generally required to form the cathode of the battery.

Integrated circuit
An integrated circuit is a plate of semiconductor material (generally silicon, sometimes germanium) on the surface of which are connected transistors, and small components also containing semiconductor materials. This is also called a chip. To make silicon conductive in some places only, it must be "doped" by adding impurities: phosphorus, boron, arsenic, antimony, but also indium or gallium. In transistors having to operate at very high frequency, such as those used for Wi-Fi, Bluetooth or 4G, the silicon is replaced by gallium arsenide or silicon-germanium.

With the launch of its 45-nm generation in 2007, Intel began using hafnium to isolate the doors of its transistors. The latter are interconnected on the printed circuits by titanium and tungsten films. The miniaturization of chips would push semiconductor manufacturers to replace cobalt or ruthenium with less and less practical copper at the nanoscale.

Printed circuit board
The printed circuits are the epoxy resin plates, often green in color, on which components are soldered using copper tracks. Gold and silver are not always classified as "rare metals", but I chose to mention them because of their importance in electronics. Excellent electrical conductor, the gold is found in printed circuits, including the connection between the silicon and the pins of the various components. Silver is present in most resistances. Printed circuits also contain very small quantities of palladium, a platinum-group metal.

Finally, one of the common components soldered on printed circuits is the capacitor, a small electric energy tank. Standard capacitors use aluminum, but these are too big to fit into a smartphone. To make them very small, we use tantalum.

All these strange names are therefore vital for our smartphones. But they are also for many advanced applications in chemistry and medicine, as well as for most clean energy. It is a shame when we know that these resources are "dirty" to exploit, ecologically and politically. Hence their stake, which will undoubtedly grow in the future.

This article is courtesy of FrAndroid.