Sliding sandwiches
Drs. Wilfred T. Tysoe & Nicholas D. Spencer | TLT Cutting Edge October 2012
Intercalating metal halides reduces interplane interactions and lowers friction.
Graphite can intercalate a wide variety of materials, including alkali metals that store lithium in lithium-ion batteries used in laptop computers.
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LAMELLAR COMPOUNDS CONSIST OF STRONGLY BONDED PLANAR LAYERS that have weak van der Waals forces between them. The presence of the weak interlayer interactions has been suggested to facilitate shear along directions parallel to the layers to lower the friction coefficient.
There are several well-known examples of such lubricious lamellar compounds, the best known of which are graphite and molybdenum disulfide. A product of tribochemical reactions with chlorine-containing additives, ferrous chloride (FeCl
2) is also a layered compound and has low friction.
The weak interlayer forces in lamellar compounds also result in unique properties by allowing chemicals to be “intercalated” between these layers. Graphite, in particular, can intercalate a wide variety of materials, including alkali metals, and this property is exploited to store lithium in lithium-ion batteries that are commonly used in laptop computers, for example.
Since the resulting graphite intercalation compounds also have lamellar structures, they are also expected to display low friction. Because the presence of the intercalated layer separates the graphene (single graphite) sheets, the interlayer interaction energy might be expected to be lower than for graphite, and the intercalation compounds might therefore be expected to have lower friction.
Unfortunately, it is difficult to measure the interaction energy directly. To solve this problem, Drs. Jean-Louis Mansot and Philippe Thomas of the Université des Antilles et de la Guyane in Guadeloupe compared the results of quantum-theory calculations of the structures of intercalation compounds with their frictional behavior.
They examined the behavior of several metal halide intercalation compounds synthesized with ferric chloride (FeCl
3), aluminum chloride (AlCl
3) and antimony pentachloride (SbCl
5). They are all known to form intercalation compounds in which the intercalated molecules occupy all of the spaces between the graphene layers. This results in an expansion of the interlayer spacing of pure graphite (0.335 nm) to ~0.95 nm for the intercalation compounds. All of the metal halide intercalation compounds have a similar repeating graphene-chlorine-metal-chlorine structure.
In order to calculate the interlayer interaction energy, Drs. Mansot and Thomas used density functional theory to calculate the total energy of a unit cell of the intercalation compound, which includes the interaction between the layers, and compared this with the energy of an isolated layer. The difference between the two energies is a measure of the interlayer interaction. The interaction energy between graphene layers in pure graphite (~4.3 kJ/mol) was significantly reduced in the intercalation compounds to ~0.1 kJ/mol. By analyzing the results of the electronic structure calculations, the reduction was traced to both an increase in the distance between the graphene layers and the very weak graphene-intercalant interactions.
This suggests that the intercalation compounds should have lower friction than pure graphite. This was tested by comparing the friction coefficient of graphite and the intercalation compounds under a highpurity argon atmosphere where the friction coefficient of intercalation compounds (~0.09) was indeed significantly lower than that of pure graphite (0.22). In the case of the intercalation compound with ferrous chloride, the friction coefficient remained low for 80 rubbing cycles, while with the other intercalation compounds the friction coefficient slowly rose to that of pure graphite with continued rubbing. This was explored by using Raman spectroscopy to examine the structure of the antimony pentachloride intercalation compound after various numbers of rubbing cycles.
Indeed it was found that the partial deintercalation occurred after only 10 cycles and was complete after 80, rationalizing the observed change in friction. Such a close interplay between theory (in this case, first-principles quantum calculations) and experiment serves not only to test our chemical intuition, but also provides insights into the behavior of the materials.
FOR FURTHER READING:
Delbé, K., Mansot, J.-L., Thomas, Ph., Baranek, Ph., Boucher, F., Vangelisti, R., and Billaud, D. (2012) “Contribution to the Understanding of Tribological Properties of Graphite Intercalation Compounds with Metal Chloride,”
Tribology Letters, 47(3), pp. 367-379.
Eddy Tysoe is a Distinguished Professor of Physical Chemistry at the University of Wisconsin-Milwaukee. You can reach him at wtt@uwm.edu.
Nic Spencer is professor of surface science and technology at the ETH Zurich, Switzerland. Both serve as editors-in-chief of STLE-affiliated Tribology Letters journal. You can reach him at nspencer@ethz.ch.