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The Space Between: A Cause and Solution to Global Warming
When we discuss voids or porosity in a material, there is an obvious dichotomy of the empty space between solid material – after all one must have the solid material in order to define the void. But this space between doesn’t have to be empty and stagnant; it can be functional.
On one level, this dichotomy of solid/empty space defines the void-as is the case in common foams such as a sponge or a cooler made of polystyrene. Focusing on the empty space and not the solid material can highlight interesting functionality that voids and porosity bring. As it turns out the dichotomy can run deeper. An example is this of methane hydrate.
Methane is the major component in natural gas. But not all natural gas deposits occur in huge gas “pockets” deep underground – sometimes these “pockets” are much smaller. A significant amount of methane is trapped in a solid format as methane hydrate. In this formation methane gas is trapped in a crystal cage made from ice. Visually, on a macroscopic scale, methane hydrate crystals look just like ice. However, when a flame is held near it, the ice cage melts, liberating and burning the methane, see Fig. 1. This dramatic appearance has led to methane hydrate being referred to as burning ice.
On an atomic or molecular level, methane hydrate is not just simply just ice. The crystal structure of methane hydrate is shown in Figure 2 a. From a material science perspective, the ice, represented by two white hydrogen spheres and a red oxygen sphere, forms a cage structure and is similar to a buckyball with each of these “pockets” having a specific size. The space available within the pockets is large enough to host only one methane molecule, represented by a grey carbon sphere and 4 green hydrogen spheres. The chemical formula of methane hydrate is CH4 * n H2O, where n~6 (Circone, Kirby et al. 2005). The general term used to describe this material is guest-host complex. More specifically, methane hydrate is what is called a clathrate. A clathrate is one type of guest-host complex where the guest molecule is not chemically bound in a host’s cage; i.e. it is physically bound in ice. Methane hydrate is also referred to as methane clathrate. Figure 2b is an SEM micrograph showing the facets of the methane hydrate crystal.
Greenhouse gas vs. cleanest burning hydrocarbon
There are huge deposits of methane hydrate; from deep sea formations to arctic permafrost regions of the earth (see Suggested Reading). A current map of these deposits is shown in Figure 3. This material has been given considerable attention both as a source of global warming and as a source for the cleanest burning hydrocarbon fuel.
As a greenhouse gas the effect of methane gas is ~ 21 time more damaging than CO2 gas in the atmosphere. As the average temperature on earth gradually increases, the deposits of methane hydrate are in danger of melting and releasing methane into the atmosphere. Especially at risk are deposits in the shallow arctic shelf regions. With a warming Earth, these deposits can break down releasing the once contained methane gas. As the earth heats, the rate of melting of methane hydrate then increases; the US Department of Energy has identified this as one of the most serious scenarios for accelerating climate change.
On the other hand, methane is considered the bridge fuel to renewable energy. It is the cleanest burning of all fossil fuels. There are significant deposits of methane hydrate that are accessible and researchers are investigating how best to release the methane from hydrate form.
Voids: Intrinsic vs. Intentional
In the broad sense, one can classify a void by “intent.” If the voids or porosity were incorporated by design we call them intentional voids. If they appear because of the nature of the material, natural processes, processing limitations and/or aging during service we call them intrinsic voids.
Today these lines are often blurred as the control and understanding of material (and voids) and material processes reach ever-smaller dimensions. These natural methane hydrate deposits, the pockets within the ice crystals that host methane are intrinsic voids. Within the cage, they host a single methane molecule, having a molecular diameter of ~ 0.4 nanometers (4 x 10-10 m). Engineers can now recreate the earth’s process used to form methane hydrate. As such, the material has also garnered attention as an engineered material; again for the solid storage of natural gas within a cage of water crystals.In 2004 Mitsui in collaboration with Osaka University opened one of the first facilities to produce methane hydrate.Methane hydrate is now and engineered product.
Ice cages are not the only way to contain methane in a sold form. Guest-host complexes have been created from organic solids (Atwood, Barbour et al. 2005), see Fig 4. Many of these are capable of hosting more than one methane molecule. The chemical structure shown in Fig. 4 can host two methane molecules in the central void (Thallapally, Wirsig et al. 2005). With this technology of guest-host complexes one can potentially take methane hydrate production to stranded deposits of natural gas. Because of the difficult location of a significant amount of natural gas, it cannot be economically recovered and transported via pipelines. Instead the natural gas can be processed into methane hydrate (or some other guest-host complex) and shipped in solid form.
As we see, with the present technology, there is as intersection of intrinsic and intentional voids. We can learn from natural processes, such as the conditions that create methane hydrate, in order to design more efficient guest-host complexes.
In my book, Voids in materials: from unavoidable defects to designed cellular materials we provide details on the design, processing, characterization, and functionality derived from atomic scale defects up through macroscale porosity. The basic premise of the book is that,at some level, all solids contain voids, whether intrinsically or intentionally. Sometimes the voids are ignored, at other times they are taken into account, and while at still other times they are the focal point of the research. In this book, we take due notice of all of these occurrences of voids, whether designed or unavoidable defects. We define these voids (or empty spaces in materials), categorize them, characterize them, and describe the effect they have on material properties.
Read more about ‘The Space Between’ and materials voids by Gary:
Gladysz and Chawla’s upcoming book Voids in Materials: From Unavoidable Defects to Designed Cellular Materials is now available for pre-0rder on the Elsevier Store. Use discount code “STC3014” and save up to 30% on your very own copy!
About the Authors
Dr. Gary Gladysz (Twitter: @GMGladysz) is an Associate at Empyreus Solutions, LLC, Seattle WA, USA, where he consults and leads university and government technical interactions. He received his PhD from the New Mexico Institute of Mining & Technology where he participated in the NATO Collaborative Program with the German Aerospace Institute (DLR). Since receiving his PhD, he has led research efforts in university, government and industrial settings. He has extensive research experience designing and characterizing fibrous composite materials, ceramic composites, polymers, composite foams, and thin films.
As a Technical Staff Member at Los Alamos National Laboratory (LANL), he was technical lead for Rigid Composites and Thermoset Materials. In 2005 he was awarded the LANL Distinguished Performance Group Award for his work leading materials development on the Reliable Replacement Warhead Feasibility Project. He has served on funding review boards for LANL, National Science Foundation, ACS, and the Lindbergh Foundation. He has been guest editor on four issues of leading materials science journals, including Journal of Materials Science and Materials Science & Engineering: A. Dr. Gladysz has organized five international conferences/symposia on syntactic foams and composite materials. He started and currently chairs the ECI international conference series on Syntactic & Composites Foams. Dr. Gladysz currently lives near Boston, Massachusetts, USA.
Krishan Chawla is Professor Emeritus of Materials Science and Engineering at the University of Alabama at Birmingham, USA. He received his Ph.D. from the University of Illinois at Urbana-Champaign. His research interests encompass processing, microstructure, and mechanical behavior of materials. He has taught and/or done research at several universities around the world. Professor Chawla has served as a Program Director for metals and ceramics in the Division of Materials Research, National Science Foundation.
He is a fellow of ASM International. Among his other awards are: Distinguished Researcher Award at New Mexico Tech, Distinguished Alumnus Award from Banaras Hindu University, Eshbach Society Distinguished Visiting Scholar award at Northwestern University, Faculty Fellow award at Oak Ridge National Laboratory. Professor Chawla is the author or coauthor of various textbooks in the area of materials and serves on the editorial boards of a number of journals. He is editor of the journal International Materials Reviews.
ATWOOD, J.L., BARBOUR, L.J., THALLAPALLY, P.K. and WIRSIG, T.B., 2005. A crystalline organic substrate absorbs methane under STP conditions. Chemical communications, (1), pp. 51-53.
CIRCONE, S., KIRBY, S.H. and STERN, L.A., 2005. Direct measurement of methane hydrate composition along the hydrate equilibrium boundary. The Journal of Physical Chemistry B, 109(19), pp. 9468-9475.
THALLAPALLY, P.K., WIRSIG, T.B., BARBOUR, L.J. and ATWOOD, J.L., 2005. Crystal engineering of nonporous organic solids for methane sorption. Chemical communications, (35), pp. 4420-4422.
RUPPEL, C. (2011). Methane hydrates and the future of natural gas. MITEI Natural gas Report, Supplementary Paper on Methane Hydrates, 4, 25.
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