Investigation into the physical properties of glass.

Investigation into the physical

properties of glass.

Introduction 2

I have been set the task of investigating the physical properties of glass. Glass is very widely used (20 billion sq ft of glass is manufactured every year) but the methods used to manufacture it and the properties it possesses aren’t as widely known. I aim to not just cover the properties of normal window glass but also to cover the different types of glass that exist, including bullet proof, safety, tempered, one-way bullet proof, optical lenses and radiation proof glass.

The different properties I aim to investigate include young’s modulus, impact resistance, transmittance, reflection and refraction.

How glass is made 7, 12

In this section the sources agreed about most things, where they didn’t agree it was due to simplification aimed at a younger audience.

This is a very brief explanation of how glass is made because the task was to investigate the physical properties of the chosen material. The only manufacturing processes I have covered are the processes that explain some of the properties of glass.

Most transparent solids are made by melting and cooling a different solid, when the particles in the molten solid cool and solidify their structure changes and the particles don’t have a regular crystalline structure anymore, they now have an amorphous structure with no covalent bonds which absorb light.

Heating silica until it melts and then cooling it makes glass. Chemicals added to the silica prevent the glass from becoming brittle and decrease the melting point of the substance. This saves heat energy that is important to save on production costs. When the silica and added materials cool they aren’t in any sort of order, the particles are positioned randomly, which gives glass one property of liquids, the random position of their particles. Added to this is the fact that the electrons in the newly formed glass don’t absorb energy from the visible area of the spectrum. This is a very basic explanation about what gives glass its most important property.

Different types of glass

Annealed glass 11

When I refer to annealed glass I will be referring to soda lime annealed glass. There are many other types of annealed glass which have different chemicals added to during the manufacturing process, soda lime annealed glass has soda and lime added to it during manufacture and it is the ordinary glass you see in windows. It is made by the procedure described above.

Tempered glass 8, 9, 10, 11

Tempered glass is annealed glass that had already been heated to a very high temperature, but not high enough for the glass to reach its softening point, and then cooled rapidly. The reason for heating and cooling the annealed glass is that glass breaks when under tension. To prevent the glass from breaking you need to make it harder for the surfaces or edges to be under tension. To do this you must compress the glass along its surface. Heating the glass forces it to expand. When the glass is removed out of the 1200 F tempering oven, air is applied to the surface of the glass. This cools the surface but leaves the middle hot. When the middle cools it contracts, the surface contracts with it, meaning that the surface has contracted twice and is now under compression. To break the glass this compression must first be overcome. It takes a lot of force to overcome the compression because in glass cracks can only form in areas of tensile stress where the atoms are already being pulled apart from each other. This process makes tempered glass much stronger than the annealed glass it derived from.

Tempered glass has a wide range of applications because of its many properties. Tempered glass is used where it can be helpful or even necessary to have glass that is impact resistant. In public buildings it would be dangerous to have glass that breaks into large, sharp pieces. New bus stops are going to have tempered glass installed for obvious safety reasons. In some areas of manufacturing it isn’t law that tempered glass is installed but implementing it is cost effective because there aren’t regular breakages. Examples of this are refrigerators, furniture tops, oven doors and fireplace guards (where the heat resistant properties are useful.) For security applications tempered glass isn’t useful alone because a bullet can go through most kinds of tempered glass and if someone put their mind to it they could probably smash a window with a little effort. Tempered glass is slightly denser than annealed glass because of the compression involved. This means that if tempered glass is in place in a skylight or high window by law a safety screen may have to be implemented.

Physical properties of tempered glass. 11, 17, 18, 19, 10

In my research I found that some sources differed I the values given. I concluded that this was due to the differences in grades of glass. Where possible I used the most common values.

Physical properties of annealed glass. 11, 17, 18, 19, 10

This is a preview of the whole essay

Physical properties of tempered glass. 11, 17, 18, 19, 10

In my research I found that some sources differed I the values given. I concluded that this was due to the differences in grades of glass. Where possible I used the most common values.

Physical properties of annealed glass. 11, 17, 18, 19, 10

In the table you can see that with a lower young’s modulus tempered glass actually elongates to a greater degree than annealed glass. I couldn’t find any explicit explanation for this is my research but I think that the reason for this surprising result is that the centre of tempered glass is in tension, which would have the effect of aiding its elongation because the molecules are repelling each other before any force is applied. I think that the difference is so small because glass only elongates a tiny amount before it fractures. This would explain the small difference between annealed glass and tempered glass’ young’s modulus but it is based on theory and I haven’t had the time or resources to investigate any further.

The tensile strength of annealed glass is so much larger than the tensile strength of tempered glass because of reasons I have explained above. The reason that there is a big difference between the tensile strengths of the two types of glass is that the young’s modulus is a measure of the how much force it takes the material to elongate and tensile strength is a measure of how much force it takes to fracture the material.

The compressive strength of tempered glass is a lot larger than the compressive strength of annealed glass because the edges of tempered glass are under compression, the particles are being pushed into each other, it takes a lot more force to prise apart the particles whilst they are under compression than when they aren’t. The only disadvantage of tempered glass compared to annealed glass is that when there is a crack in the glass the tension force in the centre of the glass creates the common spider web pattern that you see at car accidents. Every window in the car apart from the front windscreen is made out of tempered glass.

Heat-treated glass

This is just another term for tempered glass described above.

Bullet-proof glass 7

Ordinary annealed glass will shatter if a bullet hits it. In some situations it is desirable to see through glass but be safe from an assassin or enemy soldier with a gun. The cars the president of the USA travels around in are bulletproof, as are the cars of high-ranking members of the FBI and CIA.

It isn’t the glass that is bulletproof; a layer of tough transparent polycarbonate is embedded in the middle of two pieces of annealed or tempered glass. The manufacturing process of inserting the polycarbonate is called lamination. After lamination the glass is obviously quite a bit thicker than the ordinary glass we are used to seeing but it is so strong it can stop one or more bullets depending on the force the bullet hits the glass with.

One layer of bulletproof glass isn’t usually enough to stop a bullet from a reasonably powerful gun. Most bulletproof glass is at least 4 layers thick; the extra layers mean that the glass can absorb more energy because it is the layers that absorb the bullets energy.

One-way bullet-proof glass 7

One way bullet-proof glass works by having one side of the material made out of glass, and one side made out of a flexible material. When a bullet hits the brittle side of the material the glass shatters, absorbing some of the energy over a large area. This leaves only a little energy that needs to be absorbed, which is easy for the laminated material because the glass broke over a large area. If a bullet hits the flexible material first the glass is forced away from the flexible material, the bullet can then breach the flexible material. This gives the person being shot at a chance to reply, which could be useful if you were being shot at.

Smart windows 7, 1

Smart windows are a relatively new idea that aims to get rid of the need for blinds. The idea is that just by turning a knob your window will suddenly go from transparent, to translucent and then to opaque. One objective of this project is that the windows would save the company money in heating, lighting and air conditioning costs. I could see why this might work if blinds hadn’t been invented but since blinds have been invented the only application for smart windows is a system where the windows automatically darken as the light levels outside increase.

Even if I think the idea is a waste of time smart windows are apparently the future and the physics behind them is interesting.

Suspended particle devices (sometimes called light valves or SPDs) are another type of smart window. SPDs work by particles aligning themselves when electricity passes through them. When the particles are aligned they allow light to pass through. These particles are suspended in a liquid film that allows the particles to move freely when the electricity isn’t turned on. The liquid is trapped between two layers of glass. The electricity is conducted to the liquid film via a conductive material that is used to coat the panes of glass.

A rival to SPD is polymer dispersed liquid crystals. They work in a similar way to liquid crystals. Liquid crystals are hardly cutting edge; they are used in calculators, mobile phones and laptops. In these applications when electricity is applied to the crystals they change shape and allow light to pass through. The non-scientific term for polymer dispersed liquid crystals is switchable windows. This particular strain of smart window is called switchable because it works on a binary basis; there are only two states on and off. When electricity is applied to the crystal light can pass through because the crystals align themselves parallel to each other.

The last type of smart window is the electrochromic window. Unlike the liquid crystal windows electrochromic windows darken when voltage is added and let light pass through when voltage is taken away. Electrochromic windows don’t operate on a binary basis like liquid crystal windows. You can change the transparency of the glass by turning a dial that adjusts the amount of electricity allowed in.

Electrochromic windows work because the electrochromic layer, usually Tungsten Oxide, doesn’t let as much light through when ions are present. When a voltage is applied between the two layers of conducting oxide hydrogen or lithium ions are attracted to the negative cathode past the electrochromic layer. Whilst the ions are in the electrochromic layer the optical properties of the layer change and the glass changes from being transparent to being opaque.

Lead crystal glass 12

Glass that is made from potassium silicate formulas and contains lead in the form of lead oxide is called lead embedded glass, lead glass or crystal glass. Crystal glass is used for making expensive tableware and cheap jewellery. Crystal has a higher refractive index than soda lime glass. Some lead crystal glass types have a refractive index of 1.8, which is much higher than ordinary window glass.

In my research I haven’t managed to find the reason for lead glass having a large refractive index but I can put forward a theory using other information I found in my research. The refractive index of a material is the ratio of the speed of light in a vacuum to the speed of light in the chosen material. The reason for the change in the speed of the light is electronic polarisation. Light is electromagnetic radiation and its electric field distorts the electron cloud of the adjacent atoms. This is polarisation. As a result of the polarisation a proportion of the energy may be absorbed, transmitted or absorbed and immediately re-emitted at a different speed. I know that when you give electrons energy they go up an orbit, that is the absorption of the light, I think that the light is re-emitted when the electron goes down a level. Adding lead to the glass obviously changes the speed of light, making it slow down. I think that the reason for this is that lead has a higher atomic number than silicon or any other chemical commonly added to glass. The greater the atomic number, the greater the energy difference between electron levels. The appearance of metals supports this, they reflect light well because metal atoms are joined with a sea of free electrons. I know that when you give electrons energy they go up an orbit, that is the absorption of the light, we can assume that the light is re-emitted when the electron goes down a level, and this is what causes refraction and reflection. Refraction happens because the light wave is at an angle to the glass. The part of the wave that hits the material first will slow down first which has the effect of angling the wave. The greater the difference between the refractive indexes, the greater the effect will be. This is because refractive index=speed of wave in vacuum/speed of wave in material.

By using Snell’s law we can work out the refractive index of a material using the angles of incidence and refraction or the angles of incidence or refraction from the other angle and the refractive index. Snell’s law is n1*sin i=n2*sin r. Calculating the angles or refractive index of light passing from air to another medium is easy because the refractive index of air is so close to 1. To work out the refractive index you can also use the law speed of light in vacuum/speed of light in material. The speed of light slows down when it enters materials with electrons that absorb the part of the electromagnetic spectrum that we see as visible light. It is only certain materials that absorb visible light because the electrons only absorb photons that have a close energy to their band gap energy. The band-gap energy is the energy it takes for an electron to move up a layer. Different types of light have different levels of energy, which is why you are able to buy coloured sunglasses, in coloured sunglasses the electrons have a band-gap energy that isn’t close to the colour that you see.

What makes glass transparent 7, 3

We see glass so regularly that we never think about the properties it has. We don’t think about what makes glass transparent.

Glass is quite unusual because it is a solid and transparent. This is so unusual that the only solids in nature that are transparent are ice and volcanic glass called obsidian. There are lots of naturally occurring liquids and gases which are transparent, water being the obvious example.

The reason that liquids and gases are more likely to be transparent is that in solids the ordered arrangement of the atoms means that the substance has a greater density, there aren’t as many gaps for the light to pass through. If you think about it that isn’t very surprising. The more particles there are, the more chance the light has to be absorbed. Deep down in the ocean there is so many particles of water above you that no light can pass through, fish there have had to evolve their own phosphorescence to survive.

The reason that any material absorbs light is because of what makes up an atom. Light particles are called photons, when these photons collide with an electron the electron can absorb the energy or the photon and either transform it into heat, store it (luminescence), reflect it back towards its source. In some cases the electron can’t absorb the energy of the photon and it carries on.

We can apply this to understand why glass blocks out UV light. The electrons in the atoms that make up the glass absorb the energy of the photons in the UV end of the spectrum whilst leaving the weaker energy photons in the visible light range. If the glass absorbs the energy from part of the visible spectrum the light that passes through the glass will be seen as the colour that passed through.

Is glass a liquid or a solid 13, 14, 15, 3

Whilst conducting my research I noticed that the debate about whether glass was a solid or liquid was a lot wider than I first imagined it would be. In this section of my report I will be explaining the various viewpoints and putting my own viewpoint.

The “glass is a liquid” argument in some cases said that antique windows were thicker at the bottom which proves that glass flowed downwards. From reading various sources I concluded that it was the way that glass used to be made which meant that the windows were thicker at the bottom. One source that I read said that for glass windows to be 5 percent thicker at the bottom would take 10 million years. This wasn’t the end of the argument though. There is the debate that glass is actually a super-cooled liquid. The theory is that if you get a liquid and cool it very quickly before the molecules have time to align themselves into regular patterns then you will get a glass. The most suitable liquids for turning into a glass are liquids that are very viscous at their freezing point. The more viscous the liquid, the greater the difficulty the particles have forming a crystalline structure.

Glass is constantly (but very slowly) trying to crystallise, people don’t actually know what would happen if a window were left for millions of years, there are theories that it might become opaque, unchanged or a rigid puddle on the floor. No one can know for certain which theory is correct; you just have to weigh up the differences between the theories and see which one seems to be based mostly on fact. You could say that glass is a super-cooled liquid or an amorphous solid or even a state that is somehow in-between solid and liquid.

Terminology specific to glass 17

There are many words that I have used which are specific to glass. In this section I will explain what they mean.

Anneal: To remove stresses in glass by controlled cooling.
Annealing point: The temperature at which the internal strains in glass are reduced to an acceptable level.
Blister: An imperfection in the glass, usually a bubble.
Check: A crack on the surface of glass.
Cullet: Waste glass, usually recycled.
Dice: Tempered glass’s fracture.
Softening point: The temperature at which a uniform fibre 0.5 or 1 mm in diameter and length elongates under its own weight at a rate of 1mm per minute when the upper 10cm of its length is heated in a furnace which temperature increases by 5 degrees every minute.
Tempered glass: Glass that has been cooled quickly from its softening point to increase certain properties.
Transition point: The temperature at which molten glass turns into glass.

Bibliography

In the bibliography I will list the web sites I used for research purposes. In the report I put a number next to a statement, the number corresponds to the sources listed below. Some sections of my report were compiled from a number of sources, where this is the case there will be more than one number next to the section.