Lasisiirtymä

Lasisiirtymä refers to a phenomenon observed in amorphous solids, particularly in glasses. It describes a reversible change in the physical properties of a glass when it is heated or cooled through a specific temperature range. This range is known as the glass transition temperature, often denoted as Tg. Below Tg, the material exists in a rigid, glassy state. Above Tg, the material becomes more viscous and exhibits properties more akin to a supercooled liquid.

During the glass transition, properties such as heat capacity, thermal expansion, and viscosity change abruptly. The glass transition is not a true phase transition like melting, which involves a discontinuous change in enthalpy and entropy at a fixed temperature. Instead, it is a kinetic phenomenon where the structural relaxation time of the material becomes comparable to the experimental timescale. As temperature increases, molecular mobility increases, leading to a decrease in viscosity. Conversely, upon cooling, molecular motion slows down, and the material "freezes" into a rigid structure.

The glass transition temperature is a characteristic property of a given glass and depends on its chemical composition and cooling rate during preparation. For example, pure silica glass has a Tg around 1200 °C, while common window glass (soda-lime silicate) has a Tg around 700 °C. Understanding and controlling lasisiirtymä is crucial in many applications involving glass, from manufacturing to material science, as it dictates the material's behavior under varying thermal conditions.