#clathrates

Quasi-liquid layer controls growth mechanisms of ice-like materials

Clathrate hydrates are crystalline structures formed at the bottom of seafloors, created by water molecules trapping methane, carbon dioxide or other molecules. While these materials are underutilized in technology, a University of Oklahoma researcher is helping scientists better understand them through a trailblazing study. Alberto Striolo, a professor in OU's Gallogly College of Engineering, co-authored an article published in the Proceedings of the National Academy of Sciences that addresses a key challenge toward utilizing hydrates: their slow growth rates. He and his fellow researchers have discovered an unusual interfacial layer on the hydrate that impacts its growth rate. Striolo is the college's Asahi Glass Chair in Chemical Engineering and Lloyd and Jane Austin Presidential Professor. He is also the director of the college's Online Master of Science in Sustainability and the Materials Science and Engineering doctoral program.

Ocean temperatures are also breaking records in the North Atlantic and Mediterranean this week.

The ocean has warmed unabated since 1970, having absorbed more than 90% of excess heat from the climate system, and is now seven degrees hotter than normal.

Why is this bad?

Rising global temperatures decrease oxygen solubility in water, increase the rate of oxygen consumption via respiration, and reduce the introduction of oxygen from the atmosphere and surface waters into the ocean interior by increasing stratification and weakening ocean overturning circulation.

Low-oxygen zones increase production of N2O (a potent greenhouse gas), reduce biodiversity, alter food webs, and negatively affect food security and livelihoods. Both acidification and rising temperature are linked with deoxygenation and combine with low-oxygen conditions to affect biogeochemical, physiological, and ecological processes.

Global warming is the primary cause of ongoing deoxygenation. Models project further oxygen declines during the 21st century, even with ambitious emission reductions.

But wait, there's more.

How could ocean warming be catastrophic?

Clathrates are deposits of methane trapped within ice on the bottom of the ocean, usually off the continental shelf where decaying biological material has flowed from the land into the seas for millennia.

Methane is a potent greenhouse gas, possessing global-warming potential 72× greater than carbon dioxide.

Sudden release of large amounts of natural gas from long-frozen methane clathrate deposits likely were responsible for Earth's sudden runaway warming 630 million years ago, the Permian-Triassic extinction event, and the Paleocene-Eocene Thermal Maximum.

Our greatest hope right now is that most clathrates lie deep under the ocean where temperatures are less likely to rise rapidly enough to melt their ice caps.

However, some methane clathrate deposits are much shallower, making them far more vulnerable to warming. A deposit off Canada in the Beaufort Sea and another in the East Siberian Arctic Shelf are as shallow as 45 meters, so even slight ocean warming could quickly release gas from the currently frozen methane.

Not less than 1,400 gigatons of methane currently lurk under the Arctic submarine permafrost, with up to 50 gigatons of methane hydrate highly likely to be abruptly released at any time.

A release on this scale would increase the methane content of the planet's atmosphere by a factor of twelve, equivalent in greenhouse effect to doubling the level of CO2.

Recognizing this threat led to the "Clathrate Gun" hypothesis. A 2012 study concluded that melting these Arctic methane clathrates would mean a 1000-fold free methane increase in a single pulse, increasing atmospheric temperatures by more than 6°C in 80 years.

(map of methane clathrate deposits worldwide)

The worst part of all this horror is that such a sudden warming is also likely to set off other methane clathrate deposits across the world, sending Earth into an ever-increasing temperature spiral akin to those ancient extinction events.

Excerpt from this story from Inside Climate News:

The slowdown of a key ocean current could release methane that is frozen in layers of organic seabed sediments along some of the world’s coastlines, a new study shows.

Cold temperatures and high pressure on sea floors currently sequester about one-sixth of the world’s methane, a potent but short-lived greenhouse gas, in an ice-like form called methane hydrate, or clathrates. Sudden thawing of those clathrates could result in a surge of methane emissions that would spike the planet’s fever. The new research, published today in the Proceedings of the National Academy of Sciences, shows that some of the shallower layers in the Atlantic Ocean could be more vulnerable than previously thought to warming that could release that methane, and that such events have happened in the distant past.

The trigger for such warming and thawing, according to the study, is a large inflow of fresh, frigid water from melting Arctic ice, which can disrupt the Atlantic Meridional Overturning Current, a slow ocean heat pump, pushing cold water in the Arctic deep down and southward, and warm water to the surface and northward.

Temperature, density and salinity contrasts drive the pump. But in recent decades, the influx of water from rapidly melting Arctic ice, especially the Greenland Ice Sheet, appears to be weakening the current, which could warm the ocean at depths of 300 to 1,300 meters to destabilize methane hydrates buried 20 to 30 feet deep in the seabed.

It’s happened before, said University of California, Santa Barbara climate scientist Syee Weldeab, who led the detailed analysis of ocean and coastal conditions in the Gulf of Guinea. The study covers a 500-year slice of time during the Eemian Age, about 125,000 years ago.

The global average temperature was about 1 to 2 degrees Celsius warmer than now, due to changes in the amount of sunlight reaching Earth. The new study’s analysis of trace elements in the shells of ancient ocean microorganisms shows that was enough to free some methane from its icy state, he said.

Elusive predicted water structure created in the laboratory

Clathrate hydrates are complex water structures that contain foreign guest molecules inside a host water-molecule shell. A predicted clathrate hydrate phase structure has been stably synthesized in the lab and may play an important role in future material science research. Water molecules are made up of just three atoms: two hydrogen atoms bound to a single oxygen atom. Individual water molecules can weakly bind to one another and other molecules, changing their collective physicochemical properties. Clathrate hydrates, in particular, are lattices of water molecules that self-assemble around guest substances to create hydrogen-bonded frameworks. These frameworks belong to Frank-Kasper (FK) phases because of their geometric arrangement as close-packed tetrahedra. Remarkably, the framework is made entirely of weak bonds between water and guest molecules, making some predicted clathrate hydrate structures very difficult to synthesize.

Making the structure of 'fire ice' with nanoparticles

Cage structures made with nanoparticles could be a route toward making organized nanostructures with mixed materials, and researchers at the University of Michigan have shown how to achieve this through computer simulations.

The finding could open new avenues for photonic materials that manipulate light in ways that natural crystals can't. It also showcased an unusual effect that the team is calling entropy compartmentalization.

"We are developing new ways to structure matter across scales, discovering the possibilities and what forces we can use," said Sharon Glotzer, the Anthony C. Lembke Department Chair of Chemical Engineering, who led the study published today in Nature Chemistry. "Entropic forces can stabilize even more complex crystals than we thought."

While entropy is often explained as disorder in a system, it more accurately reflects the system's tendency to maximize its possible states. Often, this ends up as disorder in the colloquial sense. Oxygen molecules don't huddle together in a corner—they spread out to fill a room. But if you put them in the right size box, they will naturally order themselves into a recognizable structure.

A long-sought-after class of "superdiamond" carbon-based materials with tunable mechanical and electronic properties was predicted and synthesized by Carnegie's Li Zhu and Timothy Strobel. Their work is published by Science Advances.

Carbon is the fourth-most-abundant element in the universe and is fundamental to life as we know it. It is unrivaled in its ability to form stable structures, both alone and with other elements.

A material's properties are determined by how its atoms are bonded and the structural arrangements that these bonds create. For carbon-based materials, the type of bonding makes the difference between the hardness of diamond, which has three-dimensional "sp3" bonds, and the softness of graphite, which has two-dimensional "sp2" bonds, for example.

Despite the enormous diversity of carbon compounds, only a handful of three-dimensionally, sp3-bonded carbon-based materials are known, including diamond. The three-dimensional bonding structure makes these materials very attractive for many practical applications due to a range of properties including strength, hardness, and thermal conductivity.

"Aside from diamond and some of its analogs that incorporate additional elements, almost no other extended sp3 carbon materials have been created, despite numerous predictions of potentially synthesizable structures with this kind of bonding," Strobel explained. "Following a chemical principle that indicates adding boron into the structure will enhance its stability, we examined another 3D-bonded class of carbon materials called clathrates, which have a lattice structure of cages that trap other types of atoms or molecules."

Molecular prison forces diatomic inmates to cell floor

Organic prison too crowded for molecular inmates to move about freely

A team of scientists including Carnegie's Tim Strobel and Venkata Bhadram now report unexpected quantum behavior of hydrogen molecules, H2, trapped within tiny cages made of organic molecules, demonstrating that the structure of the cage influences the behavior of the molecule imprisoned inside it.

A detailed understanding of the physics of individual atoms interacting with each other at the microscopic level can lead to the discovery of novel emergent phenomena, help guide the synthesis of new materials, and even aid future drug development.

But at the atomic scale, the classical, so-called Newtonian, rules of physics you learned in school don't apply. In the arena of the ultra-small, different rules, governed by quantum mechanics, are needed to understand interactions between atoms where energy is discrete, or non-continuous, and where position is inherently uncertain.

The research team -- including Anibal Ramirez-Cuesta, Luke Daemen, and Yongqiang Cheng of Oak Ridge National Laboratory, as well as Timothy Jenkins and Craig Brown of the National Institute of Standards and Technology-used spectroscopic tools, including the state-of-the-art inelastic neutron spectrometer called VISION at the Spallation Neutron Source, to examine the atomic-level dynamics of a special kind of molecular structure called a clathrate.

Neutrons provide a novel picture of thermal conductivity in complex materials

The engineering of thermal conductivity in semiconducting materials is a central issue in the development of modern nano- and microtechnologies. Low thermal conductivity is important in materials used in technology products, as it provides thermal insulation and thus reduction of heat transfer, ensuring the products do not overheat.

Some representative of the clathrates family, complex chemical substances containing cages that trap atoms, are particularly important to study in this context as they have a range of important applications. Their thermoelectric properties make them very effective for harvesting wasted heat and converting it into electricity. To date, little is known about the exact mechanism underpinning the low thermal conductivity observed in complex structures such as clathrates.

Thermal energy is mainly carried by atomic vibrations called phonons, which are quasiparticles travelling with the sound velocity. Heat propagation and conductivity is directly related to the time a phonon travels in a material before it collides with defects or other phonons. This characteristic time is called the phonon lifetime. As such, understanding individual phonon properties is also fundamental for applications such as waste heat recovery through thermoelectric conversion. Shortening phonon lifetimes achieves low thermal conductivity, and this is a strategy which has led to extensive research around the 'phonon engineering' of thermoelectric materials.