Manufacturing Bits: June 5

Water insulators; water isn’t alike; why is ice slippery?


Water insulators
North Carolina State University, the Oak Ridge National Laboratory (ORNL) and Texas A&M University have developed what could be considered as water insulators for energy storage applications.

Basically, researchers sandwiched water between two materials, enabling higher power storage devices with more efficiency. More specifically, in the lab, researchers developed a compound called crystalline tungsten oxide dehydrate. The compound consists of crystalline tungsten oxide layers, which are separated by thin layers of water.

This material set is a promising way to store and release energy. “The presence of structural water in tungsten oxides leads to a transition in the energy storage mechanism from battery-type intercalation (limited by solid state diffusion) to pseudocapacitance (limited by surface kinetics). Here, we demonstrate that these electrochemical mechanisms are linked to the mechanical response of the materials during intercalation of protons and present a pathway to utilize the mechanical coupling for local studies of electrochemistry,” according to researchers in the journal ACS Nano.

A metrology technique called atomic force microscopy (AFM) is used to measure the expansion and contraction of the material set at the nanoscale. In the experiment, AFM detected the deformation of the redox-active energy storage materials, according to ACS.

AFM reveals that structural water in tungsten oxide results in smaller deformation rates from ion intercalation, an unexpected finding on the role of structural water that can enable materials with higher power and efficiency energy storage devices. (Source: NC State)

“We tested both crystalline tungsten oxide dihydrate and crystalline tungsten oxide – which lacks the water layers,” said Veronica Augustyn, an assistant professor of materials science and engineering at NC State. “And we found that the water layers appear to play a significant role in how the material responds mechanically to energy storage.

“In practical terms, this means that the material with water layers is more efficient at storing charge, losing less energy,” Augustyn added.

Ruocun “John” Wang, a researcher at NC State, added: “Specifically, we found that the water layers do two things. One, the water layers minimize deformation, meaning that the material expands and contracts less as ions move in and out of the material when there are water layers. Two, the water layers make the deformation more reversible, meaning that the material returns to its original dimensions more easily.”

Water isn’t alike
The University of Basel, the University of Hamburg and the Deutsches Elektronen-Synchrotron (DESY) organization have demonstrated that water isn’t alike.

Water, which is a molecule, consists of a single oxygen atom that is linked to two hydrogen atoms. But water also exists in two different molecular forms. Each form has a different orientation in terms of the spins of the two hydrogen atoms, according to researchers.

The spins can be aligned in the same or opposite direction. One alignment refers to ortho-water, while the other is para-water, according to researchers. The two forms have nearly identical physical properties, however. Each form, though, can exhibit a different chemical reaction, according to researchers from DESY and others.

The two forms must be separated to test the chemical reactions. This was accomplished using an electric prism. In simple terms, researchers send a jet of water molecules through an electric field.

Pre-sorted ortho-water and para-water molecules with differently oriented nuclear spins (blue or red arrows) react with diazenylium ions (center left) at different speeds. (Illustration: University of Basel, Department of Chemistry)

Researchers worked with molecules at temperatures close to absolute zero. “Para- and ortho-water have almost identical physical properties which makes their separation particularly challenging,” said Ardita Kilaj, a researcher from the University of Basel.

“Also, the molecules frequently collide with other molecules, causing nuclear spin orientations to change so that para- and ortho-water transform into one another,” added Jochen Küpper from DESY, who is also a professor from the University of Hamburg. “Para- and ortho-water get deflected differently, allowing us to separate them in space and obtain nearly pure para and ortho samples.”

Stefan Willitsch, a professor from the University of Basel, added: “It was demonstrated that para-water reacts about 25% faster than ortho-water. This effect can be explained in terms of the nuclear spin also influencing the rotation of the water molecules. As a result, different attractive forces act between the reaction partners. Para-water is able to attract its reaction partner more strongly than the ortho-form, which leads to an increased chemical reactivity.”

Why is ice slippery?
The AMOLF, the University of Amsterdam and the Max Planck Institute for Polymer Research (MPI-P) have explained why ice is slippery.

In 1886, a physicist provided the first explanation. When an object touches ice, it creates local contact pressure. Then, the ice melts, thereby creating a liquid water layer that lubricates the sliding.

As it turns out, though, it’s far more complex than that. To understand why ice is slippery, the AMOLF, the University of Amsterdam and MPI-P conducted experiments at temperatures ranging from 0° C to minus 100° C. To explore the slippery nature of ice, researchers performed spectroscopic measurements of the state of water molecules at the surface of the ice. Then, they compared these with molecular dynamics simulations.

Researchers found two types of water molecules exist at the ice surface. One is water molecules that are stuck to the underlying ice. Another is mobile water molecules that are bound by only two hydrogen bonds.

“These mobile water molecules continuously roll over the ice – like tiny spheres – powered by thermal vibrations,” according to researchers. “As the temperature increases, the two species of surface molecules are interconverted: the number of mobile water molecules is increased at the expense of water molecules that are fixed to the ice surface.

“Remarkably, this temperature driven change in the mobility of the topmost water molecules at the ice surface perfectly matches the temperature-dependence of the measured friction force: the larger the mobility at the surface, the lower the friction and vice versa,” according to researchers.

As a result, researchers concluded that the high mobility of the surface water molecules is responsible for the slipperiness of ice.

In the experiments, a steel ball slides over the ice surface which consists of rapidly tumbling mobile water molecules that are only loosely bounded to the underlying ice. (© Nagata / MPI-P)

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