Manufacturing Bits: May 18

Mystery of MXenes
Aalto University has studied the surface composition and provided some new insights into MXenes, a promising set of materials used for energy storage and related applications.

A class of two-dimensional inorganic compounds, MXenes consist of thin atomic layers. The materials are based on transition metal carbides, nitrides or carbonitrides.

These materials have extraordinary electronic, optical, mechanical, thermal and catalytic properties. “These materials have the general composition M_(n+1)X_n, where M is a transition metal and X a carbon or nitrogen atom, with n corresponding to atomic thickness,” according to Aalto University.

MXenes are durable and conductive materials. They could one day improve energy storage systems. Other applications include computing, communications and health care.

To date, though, MXenes remain a bit of a mystery. The structure and composition of the surfaces are somewhat unknown for many MXenes, according to researchers in The Journal of Physical Chemistry Letters.

Researchers hope to gain some new insights into the structures and properties of MXenes, thereby paving the way for these materials into systems.

In the lab, researchers analyzed the surface functionalization for titanium (Ti) and niobium (Nb) carbides and nitrides of varying thickness. “Using a multiscale computational scheme, we study the trends in distribution and composition of the surface functional groups −O, −OH, and −F on two-dimensional (2D) transition metal carbides and nitrides (MXenes),” said Rina Ibragimova, a researcher at Aalto, in The Journal of Physical Chemistry Letters. Others contributed to the work.

“Using a combination of cluster expansion, Monte Carlo, and density functional theory methods, we study the distribution and composition of functional groups at experimentally relevant conditions,” Ibragimova said. “We show that mixtures of functional groups are favorable on all studied MXene surfaces. The distribution of functional groups appears to be largely independent of the type of metal, carbon, or nitrogen species and/or number of atomic layers in the MXene. We further show that some properties (e.g., the work function) strongly depend on the surface composition, while others, for example, the electric conductivity, exhibit only a weak dependence.”

Solid lubricants
TU Wein, Purdue University, Universidad de Chile and others have demonstrated MXenes’ ability to work as a solid lubricant under extreme conditions.

MXenes is a novel nanomaterial that belongs to a 2D material class. MXenes is made of multiple ultra-thin, single atomic layers of titanium and carbon. Each layer bonds weakly with the layer above or below it, giving MXenes its lubricating properties.

“MXenes-coated specimens demonstrate a 6-fold friction reduction and an ultralow wear rate over 100,000 sliding cycles, outperforming state-of-the-art 2D nanomaterials by at least 200% regarding their wear life,” writes Phillip Grutzmacher, in a paper published in ACS Nano.

Unlike other thin-film materials, MXenes does not react sensitively to water molecules in its surroundings. It also has high heat resistance. Due to its stable and long-wear nature, MXenes can be used in many extreme environments, such as in space. While lubricating oil would evaporate in the vacuum of space, MXenes can be used in the form of a dry powder. MXenes can also be used in the steel industry, where equipment can easily reach temperatures of several hundred degrees Celsius.

Artificial kidneys
Nephria Bio, a U.S.-based spin-off of the South Korean medical device company EOFlow, has signed a licensing agreement with Drexel University to use MXene materials as a filter in wearable artificial kidney devices.

MXene has gained traction since its discovery in Drexel’s College of Engineering in 2011. This includes the ability to filter tiny particles from liquid. This makes it a prime candidate for use in dialysis.

The technology could allow many of the millions of people suffering from end-stage kidney disease worldwide to move more freely, without spending hours each week anchored to large dialysis machines.