Glass is an amorphous solid characterized by a disordered, random atomic structure, which makes it similar to a liquid on a microscopic level. Unlike crystalline materials, glass lacks long-range atomic order but retains short-range order, much like a liquid. Despite this, it maintains a fixed shape, unlike liquids that flow under gravity.
Glass exhibits four fundamental properties:
(1) **Isotropy**: The atomic arrangement in glass is irregular and lacks directional dependence. In a homogeneous state, its physical and chemical properties—such as refractive index, hardness, elasticity, thermal expansion coefficient, thermal conductivity, and electrical conductivity—are uniform in all directions.
(2) **No Fixed Melting Point**: Unlike crystalline materials, glass does not have a sharp melting point. Instead, it transitions from a solid to a liquid over a temperature range known as the softening range (Tg–T1), where Tg is the glass transition temperature and T1 is the liquidus temperature. At these points, the viscosity of the material corresponds to 10ⴠand 10¹² Pa·s, respectively.
(3) **Metastability**: Glass is typically formed by rapidly cooling a melt. During this process, the viscosity increases sharply, preventing atoms from arranging into a crystalline structure. As a result, glass contains higher internal energy than its crystalline counterpart and exists in a metastable state between the molten and crystalline forms. From a mechanical perspective, it is an unstable high-energy state, with a tendency to transform into a lower-energy crystalline form over time.
(4) **Graduality and Reversibility**: The transformation from a molten state to a solid glass is gradual, with continuous changes in physical and chemical properties. This contrasts with crystallization, which involves abrupt phase changes and property mutations near the crystallization temperature. Glass forms over a wide temperature range, with viscosity increasing steadily until a solid is achieved. When heated, the reverse process also occurs gradually without forming new phases.
Understanding these properties helps explain why glass behaves differently from both solids and liquids, making it a unique and fascinating material in science and technology.
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