by Thomas J Thompson on October 30, 2010

Indium Ingots

I will grant you that Indium finger isn’€™t a good title for a Bond movie, but Indium may certainly be worth hoarding.

Let’s start with the basics. Indium is a chemical element with chemical symbol In and atomic number 49. It is rare, very soft, malleable and is easily fusible. It is a post-transitional metal that is chemically similar to aluminum or gallium. Zinc ores are the primary source of indium and is named for the indigo blue line in its spectrum that was the first indication of its existence in ores, as a new and unknown element.

Here€™s why it’€™s important€“ today’€™s mobile touchscreen gadgets, along with all liquid crystal displays, rely on it, and it could be gone within the decade.

Indium is the principal component in indium tin oxide (ITO). ITO has unique qualities that make it unique. It is a rare example of a material that is both electrically conducting and optically transparent, which means it does not absorb photons of light. Absorption occurs when a photon’€™s energy matches that needed to knock an electron into an excited state. In a metallic conductor, where there is a free-flowing “€œsea”€ of electrons with many different energy states, his almost always happens. Accordingly, almost all metals are highly absorbing and entirely opaque. Not so ITO. It is transparent like glass, but also conducts.

ITO changed the way touchscreen works. The common methods, prior to ITO, were to use infrared LEDs ranged around the screen to fire beams that are blocked by a touch, but those were bulky and required a lot of power to run; or to use a stylus and two layers of ITO separated by a slight gap. Tapping this resistive screen with the stylus brought the two layers together, allowing a current to pass. New touchscreen devices utilize the fact that your finger is conductive to do away with the stylus. Touching the screen changes its capacitance at that location, a change picked up by a single layer of ITO.

The problem is that no one is sure how much indium there is left. The US Geological Survey estimates that known reserves of indium worldwide amount to 16,000 tons (63% in China). At the current rate of consumption, those reserves will be exhausted by 2020. Those numbers don’t take into account recycling or any new sources of indium. According to Indium Corporation, the largest processor of indium, claims that, on the basis of increasing recovery yields during extraction, recovery from a wider range of base metals (including tin, copper and other polymetallic deposits) and new mining investments, the long-term supply of indium is sustainable, reliable and sufficient to meet increasing future demands.

According to James Mitchell Crow writing in New Scientist magazine, the increasing demands for ITO promise to make ITO rare and, therefore, more expensive. The touchscreen market is currently projected at $1.47 billion and will balloon to $2.5 billion by 2017. This means that the race to find a replacement for ITO are on! Some of the replacements under consideration are zinc oxide, but it’€™s not as conductive, transparent or physically resilient as ITO. Another consideration is to stretch the current reserves of indium by mixing it with cadmium oxide. Doing so may reduce the amount of indium necessary per screen by 80%. Unfortunately, cadmium is highly toxic and prone to cracking. More futuristic thoughts include the development of conducting polymers, but these are often prone to ultraviolet light and oxygen.

So is it the end of the touchscreen era? Probably not €“ thanks to nanotechnology.

One solution may be carbon nanotubes. Carbon is a chemical chameleon. In some guises, it is the most light-absorbing material known. Pare it down to nanoscale structures, however, and it becomes transparent. Carbon nanotubes are essentially graphene sheets rolled up into tiny cylinders. Graphene, the wonder material behind the award of this year’€™s Nobel prize in physics, consists of sheets of graphite just a single atom thick. The problem is that individual nanotubes are highly conductive, but the electrons racing across their surface stop dead when they get to the end of a nanotube and have to jump to the next.

Another idea may be metal nanowires. Experiments with silver nanowires have shown transparency of 85 percent and a conductivity only a fraction behind that of ITO. Unfortunately, silver nanowires are 10 times as expensive to produce as top-grade ITO. Other concepts include a mechanical switch behind every pixel, registering the force as the screen is touched, but using pressure-sensing technology means doing away with the protective glass cover, making it more susceptible to damage. Another possibility is an optical technology that incorporates a light-detecting element into each pixel. These light sensors turn the screen into a scanner that can detect and follow a finger. However, it needs significant processing power to continually analyze the screen surface and works only a quarter as fast as a traditional laptop touchpad.

In any case, such innovations do not address the more fundamental problem that, touch or no touch, the electrodes that supply power to the pixels of LCD displays themselves depend on ITO. That will be solved only by the development of new materials that mimic ITO’€™s intensely desirable combination of transparency and conductivity.