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The Secret of Glass: Why Do Liquids Turn into Glass?
Glass is a common material in our daily lives and can be found in windows, cups and electronic products. But how is glass formed? What physical mysteries are hidden in this process? This article takes a closer look at the process by which liquid turns into glass and the science behind it.
Concept of glass transition
When we talk about the glass transition (also called the glass-liquid transition), we are referring to the process of gradually changing from a hard and relatively brittle "glassy" state to a viscous or rubbery state. This change occurs when the temperature of the material is increased and is reversible. Glass is defined as an amorphous solid that exhibits this transition, while the reverse process is called vitrification and is usually achieved by supercooling a liquid.
The glass transition process represents a smooth and delicate physical phenomenon, which, apart from the change in temperature, is not accompanied by obvious changes in the material structure.
Transition temperature and material properties
For every material, there is a specific glass transition temperature (Tg), which is the key parameter defining the glass state. Generally speaking, the value of Tg is always lower than the melting temperature (Tm) of the corresponding crystalline state of the material, because the energy state of glass is higher than that of crystal. For example, polystyrene and polymethyl methacrylate have a Tg of approximately 100°C (212°F).
Glass-forming ability of materials
The ability of some materials to transform into glass rather than into a crystalline state when cooled rapidly is known as glass-forming ability. It depends on the composition of the material and can be predicted by stiffness theory. For example, some polymers can easily form a glassy structure when cooled or compressed very slowly.
Non-equilibrium states and dynamics
The glassy state is a kinetically locked nonequilibrium state whose properties, such as entropy and density, depend on the thermal history. At this time, the internal degrees of freedom of the liquid gradually deviate from equilibrium. Many scientists believe that the true equilibrium state should be a crystal, and glass is a locked state.
During the glass transition, temperature and time can be considered interchangeable quantities, a key concept in glass science.
Kauzman paradox
As the liquid is supercooled, the entropy difference between the liquid and solid phases gradually decreases. By extrapolating the heat capacity of a supercold liquid to below the glass transition temperature, the temperature at which the entropy difference is zero can be calculated, called the Kauzman temperature. This paradox has not yet been clearly answered and has sparked widespread discussion in the scientific community.
Example of a specific material: Silicon dioxide
Silicon dioxide (SiO2) not only exists in many different crystalline forms, but also in the form of glass. These forms are closely related to their chemical structures and provide a rich reference as we explore the properties of glass.
ConclusionThe formation of glass is a new realm full of mystery and science. It not only involves the interaction of thermodynamics and kinetics, but also triggers our deep thinking about the nature of matter. As research deepens, will we discover more mysteries about glass in the future?
