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Tham khảo tài liệu 'introdution to thermal analysis techniques and applications part 2', kỹ thuật - công nghệ, cơ khí - chế tạo máy phục vụ nhu cầu học tập, nghiên cứu và làm việc hiệu quả | This page intentionally left blank THERMAL EVENTS 2.1 Intnxiuction The utf i le referred 10 in the definition of thermal analysis in Chapter I. is very often in the xrlùl state at least al the Stan of the investigation. The thermal behaviour of liquid CM also be studied using special techniques sec below but gases arc not normally the principal reactants in thermal analysis experiments. 2.2 The Solid State The main characteristic feature of the solid state is the relatively ordered arrangement of the constituent atoms molecules or tons. Just as he concept of an ideal gas is useful in describing the behaviour of real gases the concept of a perfect solid or perfect crystal is useful as the reference point for real solids. A perfect crystalline solid has a completely ordered arrangement of its constituents while real solids have imperfections of many kinds When the order present is marginally greater than for liquids but considerably less than in a perfect crystal. the substance is sometimes referred to as a non-crystallinc solid . When liquids composed of complex molecules or ions c.g. sucrose or silicates and a vast number of organic polymers are cooled rapidly a glass may be formed. A glass resembles a solid in many of its physical properties c.g. rigidity but differs in that the constituents do not show the regular lattice arrangement of a crystalline solid. Glasses are thus examples of noncrystalline solids They do not melt at a sharply-defined temperature but soften over a temperature interval. This transition from the rigid glassy Mate to a more flexible form is known as the glass transition and the temperature interval over which this change occurs. known as Ik g av.v transition Irmi troture. Tv 6 . is of tremendous importance in iIk practical use of polymers. Crystalline solids may be classified according to the dominant bonding forces between the constituents in the crystal. i.e. as molecular covalent ionic or metallic crystals Ị2.7Ị. In molecular crystals .