Melting and evaporation

Melting and evaporation involve the addition of heat condensation and freezing involve the removal of heat. 

Microwave energy is applied differently from dielectric energy. It is transmitted to an enclosed application chamber by a wave guide, a rectangular pipe. There is microwave interaction with water molecules in frozen books placed in this chamber. Generated heat raises frozen water s temperature to cause melting and evaporation. 

Enthalpies of transformation, melting and evaporation can simply be calculated from these values. 

The determination of quantities of heat is important in a large number of physical and chemical problems. One of the most important of these is the determination of specific heats. The specific heat of a body is closely connected with its chemical nature, and is an exceedingly important quantity. Change of state (melting and evaporation with the reverse processes of sohdification and condensation), all forms of chemical transformation, the creation and the disappearance of mechanical work, the conduction of electricity, the absorption of light and other forms of radiation, are all processes which involve transference of heat. The apparatus with w hich quantities of heat are determined are called calorimeters. 

Micro electrical discharge machining (EDM) is primarily used for hard metals and other electrically conductive materials. When two electrodes separated by a dielectric medium come close, the dielectrical medium breaks down and suddenly becomes conductive. Sparks generated between the electrodes release energy and remove metal by melting and evaporation [30], Murali and Yeo combined micro electro-discharge machining with ultrasound electrode vibration for improved surface quality, and achieved micro features as small as 25 pm on titanium with 20 pm diameter electrodes [30]. 

EDM is one of the most commonly used unconventional micromachining technologies. The process is limited to electrically conductive materials. Electrical arcs are produced in a controlled way between the tool and the workpiece in order to achieve material removal by melting and evaporation. 

Many substances undergo changes in their chemical composition during transformations such as melting and evaporation. Congruent transformations of several compounds at elevated temperatures are only possible if the chemical potentials of constituent elements can be maintained at certain level. Gas molecules such as S2, SO2, SO3, NH3, NH4OH can only exist at elevated temperatures when high partial pressures of the constituent elements (S, S2, N2, H2, CI2) are established and effective in the system. 

Many solids can evaporate directly to the gaseous state without melting to an intermediate liquid state. Examples are solid CO2 (dry ice), and solid I2, which is always present in its container with some purple vapor. The direct transition from solid to gas is called sublimation. The energy required for this transition is the sum of the energies for melting and evaporation ... 

Matter changes state when energy is added or taken away. The addition of energy, usually in the form of heat, increases the speed and kinetic energy of the component molecules. Faster moving molecules more readily overcome the intermolecular attractions that maintain the form of solids and liquids. In conclusion, as the speed of molecules increases, matter changes state from solid to liquid to gas (melting and evaporation). 

By far the most important colored smokes are those in which the dyestuff is gradually evaporated from a mixture of dye and a pyrotechnic heat source. This involves the selection of suitable dyes and of cool-buriing, gas-forming mixtures, which melt and evaporate the dye with a minimum of destruction. However, the recovery may be no more than one-third of the amount of dye in the formulation. 

Common tool electrodes generally incorporate a good electric conductivity and high thermophysical strength in terms of high melting and evaporation points as well as high heat... 

Besides the phenomena of melting and evaporation, also other material removal effects like spalling, leaching, oxidation, or electrolysis take place depending on the workpiece material, electric generator settings, and the dielectric medium (Klocke and Kdnig 2007). 

In processes with relatively high power density, the material only within the spot is heated up to a high temperature, and the material is removed by melting and evaporation. The ratio between them depends on the power density and the relationship of evaporation increases with the power density. Figure 3 shows the material removal mechanisms of different power densities of the electron beam. In the case of a low power density, the temperature at the center of beam on the surface is nearly at the melting point of the workpiece material, and the melt pool enlarges due to the heat conduction. When the power density is increased, vaporization of the... 

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