What is Glass?

The glass is everywhere, and it seems that we all know that shinny, transparent thing.

But what is the glass, chemically?

1. Materials Classification

Materials classification

Generally, the industry materials could be classified as:

• Metals
• Polymers
• Ceramics & glasses
• Composites

The glass is a major type industry material. But what is the difference between glass and the other materials?

2. The characteristics of glass

Difference between crystal and glass structure

• Glass is amorphous

Amorphous means an non-crystalline solid lacks the long-range order, which is the characteristic of crystal solid. As the picture shows, the structure on the left is crystalline, and the structure on the right is an amorphous solid (glass in the picture).
The amorphous solid does NOT have a melting point, does NOT have a regular shape.

• Glass has the glass-transition behavior

The glass transition behavior is a reversible transition in amorphous materials from a hard state to a rubber-like state. It is the characteristic of non-crystalline material. Crystals have melting point, but non-crystal doesn't. This is because the differences between the structures.
Crystals have long-range order structure, when it reaches the melting point, it requires energy to break its long-range order structure, to reach a state with irregular structures. As you can see from the figure on the left, there is a gap at melting point. The gap represents that the energy that was used to break the structure.
For non-crystals, the melting process is shown in the figure on the right. There is no melting point. There is only a so called glass transition temperature. Before the glass transition temperature, the non-crystal behaves the same with crystals. While after the glass transition temperature, the non-crystal converts to a rubber-like state directly, without a gap with heat. Because non-crystal does not have a long range order structure, so there is NOT energy required to break its structure to reach the irregular structure. In other words, the structure has always been irregular, there is not change before and after the glass transition temperature. So this behavior of non-crystals around the glass transition temperature is called glass transition behavior.