Welcome back.

Now as it turns out,

we know that there are a variety of different type of silicate systems.

And we're going to be looking at an example of one in particular that's

referred to as soda-lime-silicate glass, or sometimes abbreviated as SLS.

When we look at the structure, and I'm describing it here in a relatively simple

way by showing the four oxygens that surround the silica.

And what we see then is those four covalent

bonds between the Si4+ and the O2-.

And what we see is the structure spread out in two dimensions, but

obviously it would be in three dimensions.

Now as it turns out, if you take a look at the middle of the structure,

what's happened is an oxide of sodium,

Na2O, has been added to the silica.

And as a result, because it is not the same charge as the Si4+,

in order for that material to be incorporated in the structure,

what happens is bridging oxygens are broken.

And so now what we have is an oxygen which is not bridging,

and so you can actually see two of those,

as a result of the presence of the Na+.

Let's look at a more concentrated solution in which the concentration

of sodium has been increased in the silica glass.

So here is our structure that we're going to be considering.

When we look at the atoms, or the ions in this case, they're color-coded and

what we see are the red circles

represent the O2-, the blue circles represent the Si4+,

and the yellow circles represent the presence of the Na+.

Now when we look through the structure, we're going to actually see some single

lines, and those single lines represent the presence of bridging oxygens.

The dotted line represent where, as a result of the addition of the sodium,

we have non-bridging oxygens.

And as a consequence of having bridging and non-bridging,

we wind up developing sections in this glassy structure where we can

highlight them nicely by putting in the background and looking at the background.

In the case of this beige background,

what we're highlighting is regions of the structure which are actually

well-developed silica tetrahedra that have bridging oxygens.

And the open pathways in there, in the white areas, indicate regions where,

because of the presence of the sodium, what we have is the non-bridging.

And so our structure now is divided up into two parts,

the bridging area and the non-bridging area.

Now when the material is transforming from above the equilibrium melting temperature,

as the temperature drops in the silica systems,

because of the interconnected nature of these structures, what we find is

there is a large resistance to the organization of the structure.

And consequently the viscosity of the material

increases dramatically as the temperature is dropped.

Now as we break up these patches,

as indicated in these beige sections of the structure, we're having less and

less units that are part of this three-dimensional

network of bridging oxygen tetrahedra.

And so as a result of that, because there's not as large of a volume

that needs to be organized, it turns out that at a given temperature,

as we increase the concentration of these sodium ions,

what we find is a corresponding reduction in the glass transition temperature.

Now, when we look at the potential candidates to do this,

it turns out that if we look at sodium, potassium, lithium,

those are the elements that are in the first column.

So we have Li2O, K2O, and Na2O, and

these are materials that are typically added to the silicates.

Now it turns out that the word soda comes from the sodium oxide and

now when we look at the lime which corresponds to the calcium ions,

now what we're looking at is the materials that wind up

breaking up the bridges between the silica tetrahedra.

Now it turns out that you can organize the elements in that boron,

sodium, and potassium or calcium.

Each of those, in terms of inserting into the silicate structure,

have a valance which is less than 4, and that assists in breaking up

the network and thereby reducing the glass transition temperature.

Now when you start looking at other elements like phosphorus, for example,

what you wind up doing is to increase and

that tends to reinforce the structure of these bridging ions.

And what we find is that there is a corresponding

increase in the glass transition temperature.

It also turns out that when we're looking at the glass transition temperature,

it turns out that it is proportional to the bond strength

that existed between the cations and the anions.

Now let's think about what would happen if we took a piece of glass and

we put it into a circuit.

And the circuit that we're looking at here is a furnace, and the glass rod is inside

and it's an SLS, that is, a soda-lime-silicate.

It's in a furnace that's connected to a light bulb and

there's a power supply to complete the circuit.

Now if we have the power supply going and we look at the light bulb at room

temperature, what we would find is because of the large resistance of the glass rod,

no current is passing through the rod and consequently the bulb doesn't come on.

Now if we actually turn around and activate the furnace, increase

the temperature, thereby increasing the temperature of the rod, what we find is

we're beginning to see the development of the current and the light bulb comes on.

This is a consequence of the fact that we have ions that are being transported

through the structure.

Now the electrical conduction in ionic solids.

We've talked about electrical conduction in metallic materials, but

we can also have electrical conduction in ionic materials and in some polymers.

And the process of ionic conductivity is going to depend on

the behavior of the ions, and that's going to be a diffusion process.

So what we actually wind up doing is using the behavior for

the conduction equation.

And the first term we're going to consider in that equation is the mobility term.

And what you see here is the fact that the mobility

is going to be related to the temperature but also to the diffusivity of the cation.

And we know from module four when we discussed the diffusion process,

we know that that diffusivity is an exponentially dependent

behavior with respect to temperature.

So we're going to have a diffusivity that's going to be a function

of temperature as well as the concentration of ions that are diffusing.

And we can then put this back into the conductivity equation, and

what we see is that the number, which is going to be dependent upon

the concentration that we have added, the particular charge of the material that

we are dealing with and the associated mobility.

And that's going to be given by a relationship which is exponential.

Now if we look at a number of different compositions, and

what can be done with these different compositions is that the transport

of the Na+ can be determined in a couple of different ways.

In the first case what can be done is we can look at

the way we normally consider the diffusion process and

look at the diffusion of the cation through the structure.

Alternatively, what we can do is, because we're looking at the activation energy,

we can describe the conductivity as a function of temperature

using this exponential behavior.

And thereby determining what the activation energy Q is for this process,

by plotting it the traditional way,

of the log of the conductivity versus the reciprocal of the temperature.

And when you extract the conductivity activation energy that way,

and you compare it to experiments in using diffusion,

what you see is the activation energies are very, very similar.

What this ultimately tells us is that the transport in

this particular material is controlled by the ions,

either the Na+ or the Ca+ or the Al3+,

depending upon which of the cations that we have in solution in the silicate glass.

Thank you.

6.9 Soda Lime Silicate

Material Behavior

Meet the Instructors