14 minutes reading time (2815 words)

    Chemical Bonding and the Structure and Properties of Materials

     Introduction:

    The materials used to make things that we use can be grouped into three classes:
    • Metals
    • Ceramics
    • Polymers

    These are related to the classification based on the type of chemical bonding (See figure 1): metals (including alloys) involve metallic bonding: ceramics include materials with both ionic and covalent bonds forming network structures: polymers are examples of molecular solids with varying degrees of crystallinity. Amorphous (or glassy) metals and ceramics have also recently become important. There is obviously a close connection between our chemical classification by bond type and the engineer's classification: it is the structure of a material that determines its properties. This is why a knowledge of structure, at various levels, is important for the engineer as well as the chemist. The establishment of metallurgy, ceramic science, and polymer science on firm theoretical foundations over the last 50 years has led to important new materials and marked improvements in old ones. Materials science is a subject which covers all these areas and has substantial inputs from Chemistry and Physics (See figure 2). It might be argued that the chemistry teacher (and student) doesn't need to get his or her hands (or mind) dirty by venturing into technology. However, it is precisely at this point that our chemistry actually becomes relevant to our everyday lives, which are filled with objects made from a bewildering variety of materials, both old and new. The chemist is in a good position to explain why certain materials are chosen for particular applications, and to predict their properties - physical and chemical - from a knowledge of their structure and bonding. Advances in materials science lie at the heart of the new technologies, especially electronics, and we should know something about the new materials, at least enough to show the relevance of structural chemistry in everyday life. The new Junior Science syllabus to be introduced in 1989 has an Applied Science Section which includes materials, and provides a good introduction to the subject.

    Structure:

    Structure is a word that has different meanings to different scientists and we will distinguish three levels of structure important for understanding materials:

    • local structure - the formula or composition of a material (the ratio/percentage of its constituent elements) and the local or short-range organisation of atoms eg. into molecules, ions, clusters etc., distinguishing between isomers, or alloys.
    • crystal structure - the long-range pattern of organisation of the units (atoms, ilt, molecules) making up the material eg. B.C.C. versus F.C.C. iron, graphite versus diamond.
    • microstructure - the nature, size and shape of crystalline phases or regions present in a material e.g. austenitic versus martensitic steel, HDPE versus LDPE.

    Figure 3 illustrates these three levels of structure.

     The chemist is usually only concerned with levels 1 and 2: the real properties of a material are often critically dependent on its microstrusture. For example, the properties of two steel samples e.g. coathangers, with the same chemical composition and crystal structure, may be markedly different depending on how fast and in what way they were cooled (see Box 1). The rate of cooling affects the microstructure which affects key properties such as strength and hardness. Various types of poly(ethene) are used - they all contain the same ratio of carbon to hydrogen, but differ in molecular structure and degree of crystallinity (see box 2). The microstructure of poly(ethene) thus affects very tangible properties such as strength, flexibility and transparency, which we employ every day in making and using plastic items. Microstructure determines the different properties of glass ceramic (used for fridge-to-oven cookware and cooker tops) and ordinary borosilicate glass. It also influences the spreadability of margarine! These are all familiar examples drawn from everyday life to illustrate the importance of the microstructure of a material.

     What is microstructure?

    First we need to distinguish between a single crystal e.g. a diamond or NaCl crystal, and a polycrystalline solid e.g. diamond dust (for grinding paste) or powdered salt. The difference is one of size - big versus small crystals - which is easy to see if the crystals are separate. But what is the difference between a sugar lump and granulated sugar? A sugar lump is not a single crystal of sugar - it is lots of small crystals stuck together under pressure to give a compact lump What about a piece of metal? When etched with chemical reagent and observed under a microscope most metals are found to be poly-crystalline: lots of small crystals stuck together. Figure 4 illustrates the difference between single crystals and polycrystalline solids. Each tiny piece or grain of the polycrystal is a small, single crystal -the microstructure is often known as the grain structure. The shape (round or elongated) and size (large or small) of the grains can affect the strength and hardness and other properties of a material. Annealing i.e. heating at a temperature below the melting point, increases grain size and makes metals softer and easier to bend. Very small grains are found in the hardest materials. Single crystals of metals are very soft. Most ceramics are also polycrystaltine and firing conditions are carefully controlled to give the optimum grain size. The hardness of glass ceramics is due to the very small grain size.

    What effect does composition have on structure?

    If all the grains in a material have the same chemical composition and the same crystal structure we say the material is single phase i.e. one substance throughout. Often the grains have different chemical composition and/or crystal structure and this is then a multiphase material eg. steel. Figure 5 shows the difference between a single phase and a two phase solid. An alloy is usually a solution of one metal in another and is single phase when molten. On cooling crystals of different composition often separate and we end up with a mixture of two (or more) solid phases.

    Single versus multiphase material

    Single phase: all grains have the same structure and composition - size doesn't matter.

    The phase diagrams used to describe metals (or ceramics or polymers) are extensions of simple phase diagrams, used for example for water (Figure 6a). If only one substance (component) is present the diagrams are simpler - the axes are pressure and temperature, areas represent single phases (eg. solid, liquid, gas), lines represent two phases in equilibrium and points three phases in equilibrium. A solution has at least two components - the solvent (larger component) and the solute (smaller component) - and an extra variable for composition must be introduced. The phase diagram for water and sodium chloride illustrates this nicely (Figure 6). The shaded area is a solution of variable composition: along AB ice separates and this is the freezing point curve of water with added salt; along CB salt separates out and this is the solubility curve of salt in water: at B both salt and ice separate and the mixture sets solid. It is known as a eutectic point - the solid formed at this point is polycrystalline and two-phase, retaining both ice and salt crystals. Alloys can behave in a similar way - some metals are soluble in the molten state but separate out on cooling. (figure 6c). In other alloys the metals remain in solution in the solid also and form a single solid phase e.g. zinc and copper in brasses.

    Composition can also affect the microstructure by determining the number of phases present. Steel is strong and hard because iron carbide precipitates out as a separate phase from BCC iron at low temperatures. Above red-heat iron transforms to an FCC structure and the carbon 'dissolves' filling the octahedral holes and giving a single phase material, still solid, which is easier to work. On cooling the FCC(ẟ) structure reverts to BCC(α) and iron carbide precipitates out again. (See Figure 7).

    FIgure 8 illustrates graphically the strength of a single crystal copper rod, a poly crystalline copper rod and a polycrystalline bronze rod (an alloy of tin and copper). (This point is made excellently in the Unilever film "Discovering Crystals", which describes the importance of crystal structure in a wide range of materials- from margarine to nylon to steel).

     The single crystal material is very soft. The presence of small crystals makes it stronger, but the addition of tin to form an alloy makes it very strong indeed. The chance discovery of this alloy by early man inaugurated the Bronze Age, with its vastly superior tools and weapons to those used in the Stone Age. It was in turn superseded by the iron age, as iron is tougher than bronze.

    Single crystals are often very soft, polycrystalline solids are harder and polycrystalline alloys harder still. The reasons for these differences will be explained further on.

    Thus the atomic or local structure, the crystal structure, and the microstructure of a material all have a role to play in determining its properties.

    Why are metals ductile, ceramics brittle and polymers stretchy?

    Our typical picture of a metal is a material which is strong, but is also ductile and malleable i.e. it can be stretched or squeezed into new shapes without breaking. Strength is measured by how much force (the stress) is required to produce a given extension (the strain) (see Figure 9).

     Ceramics are usually strong, hard and brittle. They do not give easily under stress, but sudden shock usually breaks them. Polymers can have a range of properties but typically they are soft, easily stretched or deformed, but tend to return to their original shape - they neither deform nor break under sudden stress but merely yield and return to shape when the stress is removed. As an example compare soft-drinks containers made from metal, glass (a ceramic) and plastic for softdrinks (Figure 10). The metal can is easily squashed and stays squashed. The glass bottle when dropped or hit sharply breaks - and stays broken. The plastic bottle is easily squashed but returns to its original shape when the force is removed. Figure 9 shows typical stress-strain curves for ceramics, metals and polymers - although within each class of materials there will be a range of behaviour. (Polymers, for example, show glassy, plastic, and rubbery behaviour - all quite different.)

    How can we understand the differences of these three types of material in terms of structure and bonding? For a material to deform under an applied force atoms/molecules/ions must move and chemical bonds must be squeezed, stretched or broken. In a metal (Figure 11a), the atoms can slip over each other without completely breaking or disrupting the metallic bond, and so this occurs easily. All atoms are the same and the bonding forces operate in all directions and the bonding in the final state is indistinguishable from the first, except that the atoms have slipped by one place. The metal stretches and stays stretched.

     If we try to make one layer of an ionic crystal slip over another layer then we have a problem. An ionic crystal consists of alternating positive and negative ions. Moving one layer of ions by one place (Figure 11b) breaks the ionic bond and puts oppositely charged ions next to each other - the result is that the crystal falls apart. An ionic crystal is strong because the bonding resists disruption - but beyond a certain critical point failure occurs. Covalently bonded ceramics behave similarly (Figure 11c). The covalent bonding is strong, directional and resists disruption - but for atoms to move and change position all the bonds between two layers must be broken. Because the bonding is directional, atoms cannot be moved very far without breaking the bond and the material then falls to bits. Diamonds are hard because the covalent bonding between carbon atoms is hard to break - but sudden shock will result in fracture. (Don't try a hammer on a diamond!). Both ionic and covalent network materials are therefore brittle, and the broken parts can be glued back together as they are not distorted. This means we can restore broken china by careful gluing, as the bits fit together to form the final shape.

     In polymers we have a different situation again (Figure 12). Thermoplastic polymers are molecular solids - they consist of large molecules (of various sizes) held together by Van de Waal's bonds, consequenily they are soft and flexible and are fairly easily stretched. When stress is applied the molecules can slide over each other easily (the bonding is weak), but the bonding is never completely broken unless the material is stretched by large amounts.

    Thermosetting polymers have a cross-linked network structure and behave more like covalent network solids. (In a later article the properties of polymers will be discussed in more detail, as the treatment above is necessarily simplified). It can be seen that the nature of the bonding has a profound effect on the properties of the materials we use, and so often taken for granted.

     Why are alloys stronger than pure metals?

    Pure iron or pure aluminium are much too soft to be useful for making things, and are usually employed in the forms of alloys. A metal alloy is a solid-solution of one or more elements in a metal. (Polymers also form alloys, which are solid solutions of one pure polymer of another). The solute atoms in metal alloys are usually other metals but may be small non-metal atoms such as carbon or boron. Usually non-metal atoms such as oxygen, hydrogen or nitrogen weaken and embrittle metals and must be removed in processing. Carbon, however, in small amounts is essential in determining the properties of steel. Small non-metal atoms occupy the holes interstitial alloys (Figure 13a) whereas metal atoms of about the same size as the solvent atoms occupy normal sites forming substitutional alloys (Figure 13b). Brass is an example of a substitutional alloy, with a random distribution of zinc atoms in copper (or copper in zinc). Many brasses are known of different composition (Table 1) because zinc and copper, due to their similarity in size, are soluble over the whole range of composition. But how does zinc make copper stronger? The atoms are not exactly the same size and zinc atoms disrupt the layers and make it harder for the layers to slide over each other - thus the material is stronger. The superior properties of steel over pure iron are due to a different cause. The carbon in steel precipitates out as small hard regions of iron carbide and these disrupt the movement of the crystal planes and make steel stronger than pure iron.

    Composite materials - one plus one is greater than two

    A composite material is one containing at least two phases or materials. A common example is wood which consists of cellulose tubes bonded by a cross-linked polymer, known as lignan. This is similar to modern composite materials where fibres (glass, carbon fibre, polymer) are bonded in a polymer martrix. Horn and bone are other natural examples. Concrete is a composite material where grains of sand are bonded by the crystalline hydration products of cement. Many constructional materials such as particle board, plywood and laminates are also composites. In such materials the properties of two materials are combined to give a new set of properties eg. strength with flexibility. Lightweight modern materials for aircraft or sports equipment are often plastic composites, involving carbon or kevlar fibres to give strength. The whole is greater than the sum of its parts.

    Conclusion

    This article has been a brief look at some aspects of materials science, where chemistry meets the real world. It is interesting that materials science is now an option in the new Junior Certificate course, as well as being part of technology and engineering courses.

    Each branch of materials - metals, ceramics, polymers and composites (Figure 15) is a subject in its own right and we have only touched briefly on their properties. Table 2 compares the typical properties of metals, ceramics and polymers, each of which has its advantages and disadvantages. Many common items, for example, cars (Figure 16) illustrate the uses of all three classes of material. But underlying the uses of each material lies its hidden structure which ultimately determines its characteristic properties. There is certainly more to materials than meets the eye!

     Metals - body, chassis, engine block

    Polymers - tyres, seals, upholstery and trim

    Ceramic - glass (windows, lights), spark plugs

    References:

    1. Gordon, J.E., The Science of strong materials", Penguin 1968
    2. Alexander, W. and Street, A., "Metals in the Service of Man", 8th edition, Penguin 1982
    3. Higgins, R.A., 'The properties of Engineering Materials", Hodder & Stoughton, 1981
    4. Chalmers, A., "The Structure and Properties of Solids", Heyden, 1982
    5. Guinier, A., "The Structure of Matter", Arnold, 1984
    6. Marrison, L.W., "Crystals, Diamonds and Transistors", Penguin, 1966
    7. "Chemistry and Materials", Proceedings Chem Ed-Ireland, 1989, Limerick, 1991
    8. Cotterill, R., "Cambridge Guide to the Material World", CUP, 1985
    9. Radford, D., "The Materials We Use", Batsford, 1983
    10. The World of Plastics British Plastics Federation, 1986.

    DISCOVERY OF THE STRUCTURE OF THE DEOXYRIBONUCLEIC...
    Queensland Branch Schools' Chemistry Lecture 1991 ...
     

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