UHV Evaportain System

Application

Vaporizing is the change of a solid substance to a gas or a vapor. There is fundamentally no difference between the terms gas and vapor, but gas is used commonly to describe a substance that appears in the gaseous state under standard conditions of pressure and temperature, and vapor to describe the gaseous state of a substance that appears ordinarily as a solid. When heat is added to solid at its melting point, with the pressure kept constant, the molecules of the solid acquire enough energy to overcome the intermolecular forces that bind them together in the solid state, and they escape as individual molecules of vapor until the vaporization is complete.

Evaporating...Vaporization of a solid below its melting point is called Evaporating, which occurs at any temperature when the surface of a solid is exposed in an unconfined space. When, however, the surface is exposed in a confined space and the solid is in excess of that needed to saturate the space with vapor, an equilibrium is quickly reached between the number of molecules of the substance going off from the surface and those returning to it. A change in temperature upsets this equilibrium; a rise in temperature, for example, increases the activity of the molecules at the surface and consequently increases the rate at which they fly off.

The thermal motion of a molecule overcomes the surface tension  of the solid and it evaporates, that is, its kinetic energy exceeds the work function of cohesion at the surface.When the temperature is maintained at the new point for a short time, a new equilibrium is soon established. The pressure exerted by the vapor of a solid in a confined space is called its vapor pressure. It differs for different substances at any given temperature, but each substance has a specific vapor pressure for each given temperature. At its melting point the vapor pressure of a solid is equal to atmospheric pressure