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The Space Between: How Voids in Materials Contribute to 21st Century Society
Materials relationship to society
Throughout history materials have played a crucial role in society. In fact, periods of history are often named after prevalent materials of that time – the Stone Age, Bronze Age, Iron Age, and the Silicon Age now. Our ancestors’ ability to process these materials coincided with changes in society such as differing agricultural practices, weaponry, religious beliefs, and artistic styles, see Figure 1. The relationship of today’s society to materials, materials processes, and the materials science and engineering involved in creating them is no different. We live in a time of rapidly expanding technology and materials form the foundation of all these endeavors; everything is material.
In our The Space Between blog, we take a slightly different viewpoint, namely, there are voids or spacing between atoms, molecules, and materials, which play a very important role in the ultimate properties of materials. We believe this slight shift of viewing materials, through the lens of the voids they all contain, is instructive and can make understanding complex functionality somewhat more accessible. This blog will follow that theme.
What exactly is a void?
In this day and age, one cannot walk out the front door without experiencing the impact of materials of all kinds in our everyday life. What is hidden from our view of the world is the fact that from the seemingly mundane to the latest technological advancements, voids in materials play a critical role. From the shoes on your feet, insulation in your home, sunscreen on your face, filters in your car and kitchen, airplanes, to the lithium ion batteries in your smart phone and electric car; voids in materials provide crucial functions.
Let’s start by first defining what we mean by a void. In a material or solid, a void is a volume that is not occupied by a solid (or liquid). A void has two essential properties, it is:
- a volume measured in length cubed
- occupied by a vacuum or gas (i.e., solid/liquid materials are absent).
In general, there is no size or shape requirement on a void and can be simply thought of as the space where solid material is missing. Furthermore, instead of classifying voids by the materials they are in, we highlight the commonalities of processing, characterization, and functionality of voids independent of material. With this in mind, the most basic way to classify a void is by intent or lack thereof. If the voids are 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 intrinsicvoids.
How voids contribute to 21st Century society
Today, in the 21st Century, the lines between intentional and intrinsic voids are often blurred as the understanding and control of material (and voids) and material processes reach ever-smaller dimensions. Whether intentional or intrinsic, voids must be understood and characterized. The most recognizable examples of void or porosity are single phase foams; i.e. materials that contain approximately spherical, evenly distributed porosity such as a sponge or a cooler made of polystyrene
We have written blogs entries on the role of designed or intentional voids in biomaterials, deep sea buoyancy, and methane hydrate and methane storage in clathrates. But these only scratch the surface.
Voids and deep sea buoyancy
In our deep sea buoyancy blog entry we highlighted the transition from using gasoline as an agent of buoyancy to syntactic foams, which happened in the early 1960s. In 1960 the bathyscaph Trieste descended to the deepest point in the ocean, the Mariana’s Trench using gasoline as a buoyancy agent. However impressive the accomplishment, the enabling material that allowed it to ascend back from this depth was being replaced. The era of gasoline as a source of buoyancy was ending and a new material, a syntactic foam, emerged. Displacing the bathyscaphe (an underwater blimp or dirigible) was a new design of underwater vehicle, the Alvin class submersibles, which were more maneuverable and used this new syntactic foam buoyancy agent.
Syntactic foams have what we call reinforced voids. These reinforced voids are commonly glass microballoons (GMBs) or hollow glass microspheres. One can imagine GMBs as glass “ping-pong balls” only much smaller in size, diameter of a GMB being approximately equal to thickness a human hair, see Fig. 2. When these are embedded into a binder phase, such as epoxy, one gets a syntactic foam. The use of syntactic foam as a buoyancy material that can withstand the crushing pressure experienced at large ocean depths is inextricably linked to the large scale manufacturing of (GMBs). The two men responsible for the industrial scale production of GMBs were Cherry L. Emerson and William R.Cuming. They founded Emerson & Cuming in 1948 near Boston MA. With the launch of Alvin in 1964, the deep sea (and many other) applications of GMBs and syntactic foams quickly expanded. Since the 1960s the applications of syntactic foams have expanded greatly, and today this class of buoyancy material is responsible in large part for offshore oil and gas production, scientific exploration (both deep sea and interplanetary), and many naval applications.
Recently, 52 years after Trieste expedition, man paid another visit to the deepest part of the ocean. This time it was James Cameron in his vehicle Deepsea Challenger. Remarkably, Deepsea Challenger was comprised of 70 percent, by volume, of a low density, high strength material with reinforced voids called syntactic foam, see Fig 3.
Voids make biomaterials bioactive
In our Biomaterial blog entry we introduced the concept of hierarchical porosity, a characteristic needed in today’s bioactive materials. During the 1960s and 1970s, the first generation of biomaterials was chosen mainly for chemical inertness i.e. biocompatibility. In other words, implant materials would not be attacked and rejected by the body. However, no account was taken of the aspects of healing and the growth of new tissue. An important example of this is the use of titanium. Subsequent generations of biomaterials, referred to as bioactive materials, are open to increasing interactions with the body. Today, biomaterials are bioactive and biodegradable, in addition to being biocompatible. We use the term biomimetic to describe them. We define biomimetic as a material or material process that replicates one in nature or biology.
Porosity plays an important role in the tissue healing and regeneration process. To regenerate bone, for example, we need three specific sizes of porosity for optimum functionality. A material with designed porosity on several scales is said to have a hierarchical porosity. A porous scaffold with open cell porosity larger that 100 μm and, in certain cases, as large as 300 μm is needed to allow for permeation and growth of new bone. Pores in the range < 10 μm are important for intensifying adsorption of cell differentiation inducing factors and ion exchange (Holzapfel, Reichert et al. 2013). In addition, an increase in surface area is needed for the proliferation and differentiation of anchorage-dependent cells for tissue regeneration. Nanoscale texture and surface features, such as porosity, facilitate interactions between host cells and the biomaterial. Surface features and properties determine the organization of adsorbed protein layers, which in turn determine specific cellular responses.
Today, biomaterials are not just simply replacement “components” for damaged bone or tissue. Instead, biomaterials and the voids they incorporate are bio-“active” and facilitate regeneration.
Voids trapping and releasing methane
Global warming or climate change has been increasing in the news headlines since the 1980s. Methane in the form of methane hydrate plays a significant role on both sides of the discussion; both as a cause and potential solution. Methane is the major component in natural gas. On one hand methane is one of the most abundant energy sources and the cleanest burning of all fossil fuels. However, sometimes it is not economical to collect it and transport it so it is burned (as a flare) at the source. It is flared off because methane is a greenhouse gas, much worse than carbon dioxide. On the other side of the discussion, methane is being released by melting of a formation called methane hydrate. Methane hydrate is simply methane trapped in a void within an ice crystal cage. These deposits are found across the globe and by some estimates there may be more methane stored as methane hydrate than in tradition natural gas reserves. With climate change and a warming earth, these methane hydrate formations are in danger of melting, thus releasing methane directly into the atmosphere. This blog suggests how we can learn from these natural methane hydrate deposits having methane in the space between ice crystals and even create new more stable material solutions. These new materials can have larger voids giving us the ability to collect methane before it is flared or released into the atmosphere thus conserving this bridge fuel renewable energy.
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 form 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. 4. This dramatic appearance has led to methane hydrate being referred to as burning ice.
There are huge deposits of methane hydrate; located in deep sea formations to arctic permafrost regions of the earth. 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 that of CO2 gas in the atmosphere. With a warming Earth, these deposits can break down releasing the once contained methane gas. As the earth warms up, 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 the hydrate form.
On an atomic or molecular level, methane hydrate is not just simply ice. The crystal structure of methane hydrate is shown in Figure 5a. 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 just 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 form of material is guest-host complex. 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. Thus, methane hydrate can be regarded as (Thallapally, Wirsig et al. 2005) an engineered product. In addition, new organic molecules are being created specifically as guest-host complexes. The molecule is capable of hosting two methane molecules in its central intentional void.
Building the ideal void
Additive manufacturing or solid freeform fabrication (SFF) is a general term used to describe techniques of building three-dimensional parts, layer-by-layer, based on computer models. Interest in this technology as a research tool started in the mid-1990s. A very important feature of SFF is the ability to create nonstochastic foams. A nonstocashtic structure is one in which there is a designed and repeating pattern of material and voids (cells). A stochastic foam structure is a cell structure having a random variation in cell size, shape, and type (open or closed porosity). The term stochastic usually refers to the cell characteristic obtained by traditional foam blowing techniques such as chemical and physical foaming agents, i.e. the type found in seat cushions and Styrofoam insulated coolers.
Traditionally, a component with a complex geometry was manufactured by the removal of material; by drilling, grinding, machining, etc. Additive manufacturing creates a part from “nothing” by adding materials into a finished component. Some additive manufacturing or SFF techniques are:
- Laser sintering/curing
- Three dimensional printing
- Fused deposition
A three-dimensional nonstochastic foam structure consists of repeating units of open cells, see Fig. 6 (Ortona, D’Angelo et al. 2012). These structures and void geometry can be generated based on analysis for individual applications thus minimizing weight while maximizing performance and functionality. Because of the ordered nature of a nonstochastic structure, one can optimize it via computer modeling with far better accuracy than can be done with stochastic foams. So once the structure is designed, one of the SFF methods can be used to make the optimized nonstochastic structure.
How do voids affect us today in the Silicon Age?
The examples above describe important applications and functionalities of voids that occur on many length scales. On the macroscopic length scale, additive manufacturing techniques are making and controlling voids on the millimeter (m‒3) length scale. Two orders of magnitude smaller, hollow glass microballoons, with the average diameter about the diameter of a human hair(7.5 x 10‒5 m), are used extensively to create strong, lightweight materials that provide buoyancy for exploring the deepest depths of the ocean. We now have to decrease 5 orders of magnitude (10‒10 m) to characterize the ice cage that forms around a methane molecule. To get a sense of this length scale one would need to divide the diameter of a human hair into 100,000 equal pieces; one of those pieces would be 10‒10 m. The biomaterial example describes the importance of hierarchical porosity. The voids on 3 separate length scales (10‒4 m, 10‒6 m and 10‒9 m) having very different functionalities are needed for successfully implanting today’s biomaterials into the body.
So, in the present age of silicon, how do voids affect our everyday life? When we discuss silicon we need to think smaller than the methane hydrate cage, down to 10‒11 m; the diameter of a silicon atom is 2.22 x 10‒11 m. This is the diameter of the “void” that provides the functionality needed to make this the Silicon Age. In material science and engineering terms, this void is commonly referred to as an atomic “hole” and even smaller electron hole. In order to make computers and solar panels work, one must control materials on this extremely small length scale.
When we talk about silicon, atomic number 13 in the periodic table (see Figure 7a), we implicitly presume that it is a semiconductor. Very pure silicon is an insulator meaning it doesn’t conduct electricity. Metals are conductors, since they can conduct an electric current. A semiconductor has an electrical conductivity between that of insulating and conducting materials. An insulator can be made semiconducting by either temperature effects (introducing atomic holes or defects) or by the addition of controlled amount of impurities (introducing electronic holes or defects in addition to the atomic holes). Increasing the temperature of silicon can make it semiconducting but it is typically done by the addition of small amounts of other elements or impurities into pure silicon; these other elements are called dopants. Ion implantation is a common method of introducing dopants today. Drabold and Esreicher (2007) sum up semiconductor technology in the following way:
“Semiconductor technology is, to a large extent, the art of defect engineering. Today, defect control is often done at the atomic level.” (Drabold, Estreicher 2007)
Dopants can be added to silicon to leave a net positive charge (p-type) or net negative charge (n-type) leading to an ability of silicon to conduct an electrical current reasonably well. For example when gallium or boron (elements from the column before silicon in the periodic table) are added, electron holes appear. This is a p-type semiconductor. These holes correspond to regions of positive charge in silicon and they give silicon semiconducting functionality. But these p-type semiconductors are only half of the story when discussing computer chips and solar panels. The other, n-type, is typically made using phosphorus or arsenic (elements from the column after silicon in the periodic table) as a dopant. Because they are in the column following silicon on the periodic table they have an extra bonding or valence electron, which becomes a free electron in the silicon. This free electron is responsible for the improved conductivity in the n-type silicon semiconductors. One needs both p and n-types for most of today’s electronic applications. The combination of holes (p-type) and free electron (n-type) and our ability to finely tune the amount and location of them in silicon forms the foundation of the Silicon Age. Applications of this semiconductor technology include computer chips, electronics and solar cells, see Fig. 7. This silicon technology has dramatically changed our society, from the careers we have to the way we work, to the way we write this blog and the electricity and battery that is used to power the computer; atomic and electrical holes play an extremely important role.
But there is so much more
The voids discussed above cover length scales from 10‒11 to 10‒3 m in effective diameter – from a void that can just contain a single silicon atom to voids that allow us to optimize mechanical properties by building nonstochastic porosity via additive manufacturing. All of these voids have important functionality. Our blog only started in September 2014; there so many intentional and intrinsic voids and corresponding functionalities to write about that shape our 21st Century lives and society. Consider these other types and forms of voids:
Hollow fibers and nanotubes, porous and hollow fibers (macro, micro and nanometer scale membranes), hollow particles (macro, micro and nanometer sized spheres), hollow and porous particles, aerogels, Gasar metallic foams (cylindrical voids), nonspherical nano/micro – particle (cuboids, cages, ellipsoids, calabashes, urchins, etc.), nonwoven fibrous media, free volume in polymers, tetrahedral and octahedral holes in crystal lattices, Schottky defects and the list goes on.
The material characteristics that lead to these applications are as diverse as the various void shapes – composite materials for increasing “specific” properties, chemical separations, thermal management, metamaterials, dielectric, drug release, fuel cells, biomaterials, anti-wetting (self cleaning) surfaces.
This blog was prompted by an open invitation from Materials Today for “suggestions for topics and/or presenters that will help demonstrate the role materials science plays in the 21st Century.” Click here to learn more about Materials in Society lecture series: Call for topics.
Read more about ‘The Space Between’ and materials voids by Gary:
- Biomaterials and Much Ado About Nothing(ness)
- To the Bottom of the Sea Using Buoyancy Voids
- A Cause and Solution to Global Warming
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.
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.
DRABOLD, D.A. and ESTREICHER, S.K., 2007. Defect theory: An armchair history. Springer.
HOLZAPFEL, B.M., REICHERT, J.C., SCHANTZ, J., GBURECK, U., RACKWITZ, L., NÖTH, U., JAKOB, F., RUDERT, M., GROLL, J. and HUTMACHER, D.W., 2013. How smart do biomaterials need to be? A translational science and clinical point of view. Advanced Drug Delivery Reviews, 65(4), pp. 581-603.
ORTONA, A., D’ANGELO, C., GIANELLA, S. and GAIA, D., 2012. Cellular ceramics produced by rapid prototyping and replication. Materials Letters, 80(0), pp. 95-98.
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.
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