Lecture 17: Metallic Bonds and Properties of Metals

Lecture 17: Metallic Bonds and Properties of Metals Summary Metals are formed from atoms that have partially-filled electronic bands: the valence electrons are loosely bound, forming a sea of electrons throughout the solid. The liberated valence electrons are free to move around the fixed cations: they are shared by all of the atoms in the solid, rather than fixed to one specific atom. The bonds that form between metal atoms are called metallic bonds. Unlike ionic or covalent bonds, where the electrons involved in the bond are closely bound up with the ions, metallic bonds don’t belong to one ion or another. Metals are characterized by their high electrical conductivity, high thermal conductivity, high heat capacity, ductility, and luster. Each of these properties can be correlated to the electronic structure and the sea of electrons picture. Because the electronic bands in metals overlap—there is no band gap—electrons can move freely through the closely-spaced energy states. If an electron at the surface is perturbed, it is easy for the energy to transfer across the solid: the Drude model for conduction relates electron drift to applied field, yielding conductivity. The free electrons can also vibrate, so solid metals can efficiently transfer thermal energy, or heat. The thermal conductivity of metals is directly proportional to the electrical conductivity through the Wiedemann-Franz law. The properties of metals can also be controlled via alloying. For example, bronze is a metal alloy with Cu and Sn in a 4:1 ratio. Though the presence of Sn lowers the overall conductivity to 10% of pure copper, bronze is much more resistant to corrosion. Brass is a metal alloy with Cu and Zn in a 2:1 ratio. Though the conductivity is 28% of pure copper, the brass is much more malleable than pure copper. Two mechanical properties are shared by most metals: malleability and ductility. Both are a measure of how much a material can deform without breaking. Malleable materials can be compressed a lot without breaking, while ductile metals can be stretched a lot without breaking. High malleability and ductility allow metallic solids to be drawn into wires, or hammered into complex shapes. For example, a platinum rod 1cm in diameter and 10cm long can be drawn into ≈ 2777 km of wire just by pulling. Surprisingly, these mechanical properties can be directly correlated to the electronic structure. As the solid is deformed and ions move around, the sea of electrons can easily move around, allowing atoms to rearrange without breaking enough bonds to lead to fracture. Periodic trends in metallic bond strength are directly related to electronic structure. Metallic bonds are typically weakest for elements with nearly empty (Cs) or nearly full (Hg) valence subshells, and strongest for elements with half-filled valence shells (W). When the valence band is half-filled with electrons, the ratio of bonding orbitals:antibonding orbitals is maximized. In MO theory, atomic orbitals combined to form bonding and antibonding states. In the band picture, the bottom of the valence band represents 100% bonding character and the top of the band represents 100% antibonding character. Therefore, half-filled valence bands yield the strongest metallic bonds. For transition metals, the atomic orbitals are so close in energy that they often overlap, creating wide energy bands with shared characteristics. Metals in groups 6-9 are most likely to have ≈ half-filled valence bands, so the metallic bonds they form are strongest. 43 Why this matters Corrosion of metals is the worst. The global cost of corrosion is estimated to be $2.5 trillion. Yes, trillion! This is equivalent to 3.4% of the global GDP in 2013. We don’t think much about corrosion in our day-to-day lives. We think about it as the rusting of the spokes of a bike wheel or some old-looking gears. But if a pipe corrodes enough, it can crack. If a boat corrodes enough, it can sink. These are extremely serious problems that involve not only a dramatic loss of money, but also a serious safety risk and potential loss of life. If you dig into the topic of corrosion, you’ll find that many industries have adopted what they call, “corrosion management systems.” What this really means is that they are aware it’s a big problem, and they’re going to try to do something about it. But what can they do? The answer to that question is, of course, chemistry. And because of chemistry, $500 billion annually is saved by preventing, or at least dramatically slowing, corrosion. Much more work needs to be done. The automotive industry is an excellent example of an industry that has moved from minimal corrosion control in the 1970’s when the life of a car was typically set by the corrosion of the body and frame, to state-of-the-art corrosion control through advanced painting/coating technology and the use of corrosion-resistant materials. In this chapter I mentioned the use of alloying as a way to make metals more resilient, with the example of bronze which is copper with a dash of tin. This is exactly how anti-corrosion coatings are developed and they can come in the form of paints or additional metal alloy layers or treatment of existing metal surfaces. Large structural metals like those used in bridges or wind turbines are often treated with zinc and aluminum-based coatings to provide long-term corrosion prevention, while many steel and iron fasteners are coated with a thin layer of cadmium to block hydrogen absorption, which can lead to stress cracking. Nickel-chromium and cobalt-chromium alloys are often used as corrosion prevention coatings because they have very low levels of porosity which makes them extremely resistant to moisture, which in turn inhibits rusting. Oxide ceramics like alumina (which remember we’ve already discussed when we talked about ionic bonds!) can make excellent and strong coatings. The chemistry of corrosion is a highly active and extremely important area of research, and deeper understanding of the chemistry of how it happens, as well as how chemistry can be used to prevent it, represents a continued massive challenge and enormous opportunity. Why this employs How can metal make a job? Let’s go with an example that’s a big part of our future, and is actually really cool: 3D printing. Of metals. In a few short years, 3D printing of metals has gone from concept to fringe to near-mainstream. Many companies can now 3D print metal parts and many 44 Images of corroded gears are in the public domain. Images of corroded pipeline and ship © sources unknown. This content is excluded from our Creative Commons license. For information, see https://ocw.mit.edu/fairuse. innovations are still happening at these companies. What kind of people do these companies hire? Those who know about the electron sea, of course! There are many companies in this space, including one of our very own, an MIT-based spin-out called Desktop Metal that formed in 2017. One of the exciting aspects of 3D metal printing is that there’s no one standard, at least not yet. So many different companies are working on pushing the frontiers of different technologies, from Desktop Metal which uses something they call Bound Metal Deposition, to Concept Laser which sinters metals with a laser and is now owned by GE, to Arcam (also now owned by GE) which uses an electron beam to melt the metal, to Xact Metal which uses something called Powder Bed Fusion, to ExOne which uses something called binder jetting, to Vader Systems which utilizes magnetic fields, to Cytosurge which claims nanometer resolution, to many more companies and many more approaches to the printing process. All of these approaches require continued development of new chemistries (“inks”) and new ways to manipulate those into tailored, complex, and high-resolution shapes. Example Problems 1. Metals vs. semiconductors a) Why are metals better electrical sonductors than semiconductors? b) How often do doped semiconductors reach the electrical conductivity levels of metals? 2. Why is silver (Ag) more electrically conductive than platinum (Pt)? 3. Rank the following metals in order of increasing metallic bond strength, and explain your reasoning: A. Co B. Cd C. W D. Ni 45 Solutions 1. a) Metals do not have a band gap, so their electrons can freely conduct, giving rise to the “sea of electrons” effect b) Doping in semiconductors is never enough to reach the conductivity levels of metals. This is because the concentration of dopants (the impurities that contribute electrons or holes to the bulk material) is extremely small and is not enough to alter the material’s band structure. 2. Ag is more electrically conductive than Pt because of atomic radius–increased atom size leads to increased atomic spacing, which increases electron movement. 3. Metallic bonds tend to be weakest for elements that have nearly empty or nearly full valence shells. They are strongest for those with half-filled valence shells. Therefore, weakest: Cd, Ni, Co , strongest: W 46