Structure of Materials : Correlation between bond and physical properties

 Correlation between bond and physical properties

In the last lecture, we learned about materials tetrahedron. This was about the structure, properties, processing, and applications of materials.

     Now, the processing is very important here because the processing of structure, especially the microstructure, and structure affects the properties. The phase fractions may alter, and the microstructure may alter, which will give rise to change in the properties, and that will affect the applicability of the material. The processing is also related to applications because processing is related to cost and ease of processing and manufacturability. So, materials tetrahedron gives the relationship between four important aspects of materials engineering, and then we discussed the classification of materials.

So, the classification of materials was basically in 4 categories, and they are metals and alloys, ceramics and glasses, polymers and elastomers, and hybrids or composites. The above-mentioned classification, based on the type of materials, for example, metals are strong and ductile, and they have high electrical and thermal connectivity, but they are poor corrosion resistance, but ceramics, on the other hand, have very high strength, but they have very poor ductility they are very brittle, and they have low cost in thermal expansion and poor electrical and thermal conductivity. Plastics are ductile; they can be stretched too long lengths; they also have good toughness; they have corrosion resistance, but poor high-temperature properties and the hybrids are something which  you intentionally make. The classification of materials was explained based on the nature of atomic bonding. For example, metals are ductile, strong, and tough because of metallic bonding. Ceramics are ionically and covalently bonded; that is why ceramics are hard and brittle and strong. Plastics are a mix of covalent bonding and secondary bonding of van der Waals bonding, and that is why they are softer, and hybrids are a mixture of both of them.

     Then we looked at the scale of the structure. So, when we talk about structure has various scales. So, first, a scale is the macro, second is micro, third is nano fourth is atomic. Now it is so in the context of materials engineering we are interested in microstructure, which is the structure and distribution of phases grains grain boundary, impurities, inclusions at the micron level, or some micron level scale. Nanostructure could be again related to the same as microstructure. So, micro and nanostructures are something that can be observed by microscopy, and they have a very profound impact on the properties of materials. The fourth one is the atomic structure, and this atomic structure is innate to materials, materials tend to adopt atomic structure based on the bonding and based on energy calculations. So, based on the energetic, they tend to adopt specific configurations which determine their properties.

         So, what we will do next is we will go backward, we will start looking from the atomic structure and then go to nano, then go to micro and macrostructures. So, but before we go to atomic structures, let us get into bonding, we were looking at it last time, and we defined a term called as bond energy.

From the bond energy concept, we discussed earlier, the separation distance, r, and E is the energy corresponding to the separation distance at which energy is minimum. This is the bond energy of the materials, and this bond energy has a profound correlation with the properties of materials. So, let me now get into the bonding, there are three kinds of bonding that we primarily come across or study, first is ionic bonding, second is covalent bonding, and third is metallic bonding. These are all primary bonding methods.

Moreover, there is another class of bonding, which is called secondary bonding, and the reason for this classification is because primary bonding is typically characterized by high to moderate bond energy, and secondary bonding is characterized by low bond energy.
        So, I will show you the bond energy magnitudes later on as we move on. However, there is a substantial difference between bond energy of 2 things, and that is why materials which are purely covalently or ionically or mechanically bonded are stronger than secondary bonding.

So, ionic bonding happens when two elements have large differences in their electronegativities. For example, Sodium and Chlorine, Sodium has one extra electron in the outer shell, and Chlorine has 7, so to have stable configuration, it borrows 1 electron from this side. So, that Sodium becomes stable, and Chlorine becomes stable, and a bond formed between 2 of them is called an ionic bond.
        So, the bond energy of Sodium has a value of about 0.9, and the bond energy of Chlorine has a value of roughly 3. So, this difference is a substantial difference between the two, and this is related to the bond energy, so and that is why ionic bonds are typically very strong. So, other examples of these materials could be any. So, Magnesium oxide, which is ionically bonded solid, similarly, Calcium fluoride, Cesium chloride, Calcium oxide, etc. are ionically bonded results in high strength and high melting point.

So, high bond energy implies a high melting point, high elastic modulus, low coefficient of thermal expansion. As a result, most ionic solids have these characteristics. Then you will go into details of ionic bonding a little later when we study the ionically bonded material structures.

Then second is covalent bonding, for example, Silicon has four unpaired electrons, and Silicon tends to share its electrons with neighboring silicon atoms. So, this is not giving or taking, that is sharing. So, neighboring Silicon will have one electron, and it will pair. Similarly, this Silicon will have one electron here they will pair, and likewise, this Silicon on the outside will also be pairing with four neighbors. So, as a result, Silicon has fourfold coordination. So, it pairs up every silicon atom in the silicon lattice is in the silicon structure is paired up with four silicon atoms; it has four neighbors. So, Silicon is none carbon; basically, diamond has this kind of structure, and Silicon carbide, zinc oxide, which is partially ionic. However, it also has a very strong covalent character.
            So, all these materials they are covalently bonded gases such as Cl2, Br2, F2 tend to be covalently bonded group IV elements. So, these materials, like silicon oxide, zinc oxide, and silicon carbide, have quite a strong covalent character, have similar characteristics, have high bond energy. A covalent bond typically is characterized by high bond energy, and as a result, covalent materials also have similar characteristics.

Covalently bonded solids have a high melting point, high elastic modulus, and low coefficient thermal expansion. So, the third primary bonding is metallic bonding, and most metals have this kind of bonding present all matter. All metals are bonded with metallic bonding.

 From the figure, the volume in which atoms are there, and the positive atoms are surrounded by the sea of free electrons, which are unpaired electrons. There is no such sea of free electrons in ionic or covalent bonding. The bond energy, Eb, of metals is lower than Eb of ionic or covalent, it is not always true, but in most cases, it is true. Metals have lower bond energy as compared to ceramics, which are ionically, covalently bonded. However, the metallic bond is very strong as compared to the secondary bond, and this is typically bonding in metals and their alloys.

So, now the other kind of bonding that we discussed was called a secondary bonding, this is weaker than the primary bonds, and arises from the interaction between charged dipoles on the surface. So, normally, what happens to that center of negative and center positive charges will coincide. However, let us say you have a center of positive charge and center of a negative charge; similarly, you have a neighbor who also has a similar kind of configuration. For example, center a positive charge here, the center of negative charges here these 2 attract each other they form a secondary bond. So, this is because of asymmetric, let us say asymmetric charge distribution, and that is when you have asymmetric charge distribution, you will have the formation of electric charge dipoles. So, this happens in things like hydrogen. Most of the gases have this kind of van der Waals kind of a structure because they have fragile water molecules that are bonded. Then the second type could be you have present, for example, in polymers, you have these polymer chains.

So, let us say these are all at polyethylene chains. So, basically, (C2H4)n, these chains are all covalently bonded. So, within the chain, you have covalent bonding, but between the chains, the interaction is Vander Waals. So, since in polymer chains are randomly oriented, that is why polymers a little malleable because the interaction between chains is very weak. For example, these chains will have to branch the groups that will be present.
        Similarly, you have groups present here, and there is some interaction between these groups on both sides, and again that is secondary in nature, this happens in polymers things like HCl. So, many organic compounds have this kind of behavior, and this could be because of permanent dipole moment, so polymers like PTFE, PVDF, PVC are having a material permanent dipole moment, and still it is the polymer.
        So, you can also have examples are PVDF, or you can have p PVC; some of them may have permanent moments; some of them may not have permanent moments, but still, you will have this secondary bonding. So, now, let me come to some energies. So, for example, let me compare some compounds like lithium fluoride.

four, lithium is from the top and left of the periodic table, and fluoride is the right side, then you have sodium chloride. Then let me take an example of magnesium oxide, calcium fluorite, Al2O3, now you will see how this bond energy is related to enthalpy. So, if I take the values the value for NaCl is 640 kJ/mol, magnesium oxide is a value which is 1000 kJ/mol, calcium fluorite has a value of 1548 kJ/mol, and Al2O3 has a value of 3060 kJ/mol, these values can be found in literature and standard textbooks.
        Now, which of these you expect to have the highest melting point and which of these you expect to have the lowest melting point. So, as you suggested earlier, Al2O3 will have the highest melting point. So, the melting point is 20500C. So, let us say this is Tm, and this is delta h, enthalpy of atomization, which is nothing but related to bond energy. And NaCl should have the lowest melting point, i.e., 8010C, lithium fluoride has 8500C, MgO has 28500C, and calcium fluoride has 14200C, there are some exceptions in the middle, but by and large this rule follows.

        There are a lot of other reasons you see the melting point is related to the behavior of the character of atoms. So, bond energy is one important factor. However, by and large, this is true except for calcium fluoride, as you increase the bond energy, the melting point increases, and this is true about melting point and boiling point, and similarly, you will have a decrease in the coefficient of thermal expansion.

So, if I now compare the fourth group elements, let us say four elements in group IV, we have Carbon, Silicon, Germanium and if I look at the bond energy diamond has 347 kJ/mol, Silicon has 176kJ/mol, germanium has 149kJ/mol and tin has 146kJ/mol. I compare this with another material, which is silicon carbide, which has 308kJ/mol, and if you look at the values of the melting point diamond is about 35000C. So, you can again see the compounds which have lower bond energy; they have a lower melting point, and this is true if you compare the values to these as well. So, of course, you can see that the bond energy. So, this is enthalpy of atomization, not exactly the bond energy related to that, but the trend is fairly similar. What you get there is also right here, if I look at certain metals, for example, do not compare the values of two compounds directly because that may not be very comparable.
However, let us compare some metals, such as Copper and Gold.

So, Copper has a bond energy of 56.4 kJ/mol, Gold has a value of 60 kJ/mol, Aluminum has the value of 54 kJ/mol, Nickel has a value of 71.6 kJ/mol, Zinc has a value of 21.9 kJ/mol, Tungsten has a value of 212.3 kJ/mol and Iron has a value of 104 kJ/mol and by our experience we know which has highest melting point of these. So, Tungsten has highest melting point, 34100C, which is very high, of course, you know that it is 15350C, Aluminum we know it is 6670C, Copper is 10830C, and Gold as 21630C, Nickel is again high bond energy, it is melting point is 14530C, and Zinc, of course, is very low it is 4200C. Bond energies for different classes of materials are not exactly comparable because they have a lot of other factors to go behind. However, by and large, for the same category of materials, there is a trend that as the bond energy increases, the melting point increases, and this is also to about the boiling point ah. So, this is how it will so to summarize this part.

I would write three kinds of bonding ionic covalent metallic and secondary and let us first talk about the bond energy Eb. So, Eb for the ionic bond is typically large. Eb for covalent bonding is variable because of something like Bismuth or Tin. So, for bismuth, it is low, but for a diamond, it is high for the ionic bond; it is typically large, but there are some compounds like NaCl for which it is not very high, metallic bonding it is variable again low to moderate-high. The low will be things like zinc-lead moderate will be for things like Copper-Tungsten. So, of course, defining the boundary is not very easy, and secondary bonding is very small; it will have bond energy, which will be less than 10 kJ/mol. So, typically, it is less than 5 to 10 kJ/mol. It is even lower than that. So, ionic we are looking at typically values greater than 100 kJ/mol covalent; we are looking at values that are between 50.

Let us repeat 350 kJ/mol metallics will be anywhere from 20 to 350 kJ/mol, and these will be very low less than 10 kJ/mol. Of course, less than 10, but typically on the lower side, they would be less than 1kJ/mol, and another thing that you want to know is the nature. Now ionic bond typically is nondirectional; it does not have particular the externality; on the other hand, a covalent bond is very directional. So, if you look at Silicon, for example, Silicon, this is the central silicon atom; it is bonded by four silicon atoms, four silicon atoms also bound these silicon atoms.
        So, as a result of maintaining these four bonds, it has to follow a certain geometrical framework, and that is why covalent bonds tend to be very directional in nature. So, this is Silicon or carbon, for example, Silicon or carbon. So, this is directional typically this you will see in ceramics, or this could be semiconductors also as well as ceramics metals, of course, is nondirectional and because of this moderate bond energy, that is why metals are ductile also. If bond energy was very high, they would not be deforming very easily. So, most metals are moderate or low bond energy that is very different; they deform easily, and this is secondary bonding tends to be in polymers. For example, it is directional because it is
between the chains. So, that is why it is directional.

So, some properties, of course, you know bond energy, I have told you that bond energy has it correlates with Tm it has a correlation with the modulus of elasticity, E, and it has no correlation with α. So, we will finish this part here, and we will move on to the next part.

 

For Next Lecture Click below

 Structure of Materials : Bonding in Materials

Structure of Materials : Correlation between bond and physical properties

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