Tungsten Telluride

November 3, 2014

In antiquity, places were often named after a local characteristic or deity. The later case is called a theophorism, from the Greek word for god (Θεος, theos) and bearing (φορος, phoros). Sometimes the connection is not that apparent, as in the name for Woodbridge, a town in Suffolk, England. The name is a variant of its original name, Wodenbrycge (Woden's Bridge). Woden (a.k.a., Odin) is the Norse/Germanic/Anglo-Saxon mythology equivalent of Jupiter and Zeus.

Norse god, Odin, as depicted in an 1825-1827 sculpture by H. E. Freund (1786-1840). Odin, like Zeus, was the father of the gods and the ruler of the abode of the gods. For Odin, the abode of the gods was Asgard. For Zeus, it was Mount Olympus. (Via Wikimedia Commons.)

You would think that the town of Telluride, Colorado was named after the telluride minerals found in Colorado, but that's not the case. Telluride was established near a gold deposit, and the area was later mined for silver. It was subsequently mined for zinc, lead and copper, all of whose ores are associated with silver. However, tellurium and its cousin chalcogenide elements found in the same column of the periodic table, were not mined there.

Tellurium is used in the following applications:

• Tellurium is an important component of iron alloys, such as stainless steel, since it promotes machinability.

• Cadmium telluride (CdTe) is a highly efficient photovoltaic material, so it's used in solar panels. Its chalcogenide sister, Selenium, is used in CIGS (Copper indium gallium selenide) solar cells.

• Mercury cadmium telluride is an important infrared detector at wavelengths as long as 12 μm. It's used as the enabling component in forward looking infrared (FLIR) imaging devices and sensors used in a variety of military and other applications. Lead telluride is used, also, in far-infrared photodetectors.

• In computer applications, tellurium is used in phase-change materials in rewritable optical discs, such as DVD-RW. It's also used as a phase-change memory material.

• Bismuth telluride (Bi₂Te₃) and lead telluride (PbTe) are Thermoelectric materials.

Thermoelectric energy-harvesting is an important niche area for tellurium compounds, and it's enabled electrical power generation for many spacecraft. It's also used on planetary rovers, such as the Curiosity rover, the robotic science mission that's been on Mars since August 6, 2012. The Curiosity rover uses thermoelectrics to convert the heat of the radioactive decay of the plutonium isotope, plutonium-238, (²³⁸Pu) in plutonium dioxide. I wrote about this power source in a previous article (Curiosity Rover Power, November 5, 2012).

The energy output of plutonium-238 by radioactive decay is 560 watts/kg, the half-life of plutonium-238 is 87.7 years, so the power source remains fairly active for many years. Conversion of the heat to electricity is through the Seebeck effect. Lead telluride is very effective as a Seebeck material, since it has a reasonably high electrical conductivity, but a low thermal conductivity. As we know from the second law of thermodynamics, we need a temperature differential to do useful work, so the low thermal conductivity is important.

A basic thermoelectric cell, as shown in the figure, has junctions of n- and p-doped lead telluride, but the energy conversion efficiency is low. The initial power output of the Curiosity rover source was 2,000 watts, but this produced just a little more than a hundred watts of electrical power against the Mars ambient temperature.

A thermoelectric cell. Bismuth and tellurium, common material components in such cells, are mildly toxic. (Image by the author, rendered using Inkscape.)

Telluride compounds still have some surprises, as recent research on tungsten telluride (WTe₂) by scientists at Princeton University (Princeton, New Jersey) and Brookhaven National Laboratory (Upton, New York) has shown. In this case, tungsten telluride was found to exhibit magnetoresistance, which is the change in electrical resistance in response to an applied magnetic field. Magnetoresistance is a useful effect that's presently used in the read heads of computer hard drives, and in various sensors. What's interesting is that the magnetoresistance effect in tungsten telluride, unlike that for other materials, was found not to saturate at any applied magnetic field.[1-2]

Mazhar Ali, a graduate student in the laboratory of Princeton University professor, Robert Cava, first noticed this effect, so follow-up studies were in order.[2] Measurements showed that there was an extremely large positive magnetoresistance of 452,700 percent at 4.5 kelvin for a magnetic field of 14.7 tesla, and 13 million percent at 0.53 kelvin for a magnetic field of 60 tesla.[1] Electron microscopy by Jing Tao of Brookhaven National Laboratory showed the presence of paired tungsten atoms, tungsten dimers, arranged in chains (see figure). It's conjectured that the material's lack of saturation arises from a nearly perfect balance of electrons and holes. Because of this chain structure, tungsten telluride shows this large magnetoresistance only when the magnetic field is applied in a certain direction.[2]

Crystal structure of tungsten telluride (WTe2), a non-magnetic, layered, transition-metal dichalcogenide. (Image: Cava Laboratory/Princeton University, modified for clarity.)

Scientists as playful people, as the example of Richard Feynman demonstrates. Since we already have "giant magnetoresistive" materials (up to about 25,000% change) and "colossal magnetoresistive" materials (up to about 100,000% change), the research team proposed the adjective "ludicrous," instead. The inspiration for this term was the movie, "Spaceballs,"[3] in which the term, "ludicrous speed," was used for a rapid type of travel.[2] The journal editors rejected this term (after all, it was Nature, which is probably the most prestigious scientific journal), so the effect was termed, "large magnetoresistance."[2]

This research was funded by the Army Research Office and the US Department of Energy's Basic Energy Sciences (DOE BES) project "Science at 100 Tesla."[2]

References:

1. Mazhar N. Ali, Jun Xiong, Steven Flynn, Jing Tao, Quinn D. Gibson, Leslie M. Schoop, Tian Liang, Neel Haldolaarachchige, Max Hirschberger, N. P. Ong, and R. J. Cava, "Large, non-saturating magnetoresistance in WTe2," Nature, vol. 514, no. 7521 (October 9, 2014), pp. 205-208.
2. Unstoppable magnetoresistance, Princeton University Press Release, October 9, 2014.
3. Spaceballs (1987, Mel Brooks, Director) on the Internet Movie Database.