In addition, glass is further divided into: toughened glass, porous glass (ie foam glass, with a pore size of about 40nm, used for seawater desalination, virus filtration, etc.), conductive glass (used as electrodes and aircraft windshield glass), glass-ceramics , Opal glass (used for lighting devices and decorative items, etc.) and hollow glass (used as door and window glass), etc.
Isotropic
The molecular arrangement of glass is irregular, and its molecules are statistically uniform in space. In an ideal state, the physical and chemical properties of homogeneous glass (such as refractive index, hardness, elastic modulus, thermal expansion coefficient, thermal conductivity, electrical conductivity, etc.) are the same in all directions.
No Fixed Melting Point
Because glass is a mixture and non-crystalline, it has no fixed boiling point. The transformation of glass from solid to liquid is carried out within a certain temperature range (that is, the softening temperature range). Unlike crystalline substances, it has no fixed melting point. The softening temperature range is Tg~T1, Tg is the transition temperature, T1 is the liquidus temperature, and the corresponding viscosities are 1013.4dPa·s and 104~6dPa·s, respectively.
Metastability
The glassy substance is generally obtained by the rapid cooling of the melt. When the molten state changes to the glassy state, the viscosity increases sharply during the cooling process, and the particles are too late to be arranged regularly to form crystals without releasing the latent heat of crystallization. Therefore, the glassy state Substances have higher internal energy than crystalline substances, and their energy is between the molten state and the crystalline state, which is a metastable state. From a mechanical point of view, glass is an unstable high-energy state. For example, there is a tendency to transform from a low-energy state, that is, a tendency to crystallize. Therefore, glass is a metastable solid material.
Gradual Reversibility
The process of glassy substance from molten state to solid state is gradual, and the change of its physical and chemical properties is also continuous and gradual. This is obviously different from the crystallization process of the melt. The crystallization process will inevitably appear a new phase. Near the crystallization temperature point, many properties will undergo mutations. The glassy substance is completed in a wide temperature range from the molten state to the solid state. As the temperature gradually decreases, the viscosity of the glass melt gradually increases, and finally solid glass is formed, but no new phase is formed in the process. On the contrary, the process of heating the glass to melt is also gradual.