Evaporation rate of metal under vacuum

1 Vacuum state and its characteristics To study the evaporation of a substance under vacuum, it is necessary to first understand the characteristics of the vacuum state. The so-called "vacuum" problem is essentially the problem of gas molecular density. Through some calculations, the characteristics of “vacuum” can be more profoundly understood from the quantitative height. According to the relationship between the number of molecules per unit volume and vacuum degree at T=273.16K, Table 1 shows.

Table 1 relationship between vacuum degree and gas molecular density / (number of molecules. cm3) can be seen from Table 1: In the standard state, per cm3 volume of air, contains 2.687X1019 molecules, at a low vacuum of 1.333Pa, The number of molecules per cubic centimeter of volume is reduced to 3.535×1014; at a high degree of vacuum of 1.333×104 Pa, there are still 3.535×1010 molecules. It can be seen that the degree of vacuum is different and the degree of gas leanness is also different.

Table 2 shows the relationship between the distance between gas molecules and the degree of vacuum when T = 273.16K, as shown in Table 2 below.

From Table 2 it can be seen that under different degrees of vacuum, although the number of collisions has not been so frequent, the movement of gas molecules is reduced by the deadline of receiving the documents from 2001-09 to 30, and The degree of reduction depends on the degree of gas thinness, and the molecules are not clear, but because the distance between molecules is much larger than the diameter of the molecules (about 108 cm), there is bound to be a "broad" space for movement between molecules. Under standard conditions, air molecules move at an average speed of 4.45X104cm/s. The diameter of the air molecules is about 108cm. From the average free path, the average distance through 106cm will hit other molecules. 1010 times. If the vacuum conditions, the situation is very different, for example, in the high degree of vacuum 1. 104Pa, although the number of molecules is still up to 3.535X1010, but a molecule through 50m on average, will meet with other molecules Once, only about 10 times per second. Visible in the "vacuum"

Under conditions, due to the thin gas, the gas molecules or between gas molecules and other particles (such as electrons, ions or evaporated molecules) is small, which constitutes an important feature of the movement of gas molecules under vacuum conditions.

At any temperature, the substance has an "instinctive evaporation rate."

However, at atmospheric pressure, a large number of molecules that have evaporated have almost completely been impacted by air molecules and forced back to the surface of the original melt, so that the net rate of evaporation is practically negligible. However, under a vacuum state, since the gas is thin, the vaporized substance gas molecules are not collided so frequently, and the number of collisions of the molecules on the surface (for example, the wall) for a certain period of time is relatively reduced. It can be seen that the evaporation rate of a substance depends not only on the instinct evaporation rate of the substance, but also on the ability of the evaporated molecules to leave the evaporation surface, which is limited by collisions between gas molecules and between gas molecules and the wall of the device.

The rate of movement of gas molecules obeys the rules of Maxwell's distribution.

However, in the process of vacuum evaporation, gas molecules often deviate from the Maxwell distribution law due to the presence of vapor sources and the dragging phenomenon of adding momentum. Under the premise of condensation, the evaporation of the substance in the vacuum makes the pressure, density and temperature of the gas distribute unevenly in space, there is the transport of momentum, mass and energy, and the vapor flow acts as directional motion, which is a typical non-equilibrium process. However, when the vacuum and evaporation temperature are constant, it is a steady-state process.

2 The space process and the rate of evaporation of matter in a vacuum state The space process of material evaporation is essentially the movement process of vaporized gas molecules. The constant movement and continuous collision of vaporized gas molecules is the most fundamental feature of the evaporation process. Although the microscopic mechanism of evaporation of various substances is not the same, the spatial macroscopic processes shown by the evaporation process are the same, as exemplified by the evaporation of metals.

Temperature; is the depth of the melt, 1 is the distance between the evaporation surface and the condensation surface. Known as the heat transfer interface layer below the melt surface, it is the concentration interface layer on the melt surface.

What is depicted is only a simplification, and it is assumed that the diffusion of the evaporated metal particles is performed by the residual gas. If the mean free path of the metal vapor particles is longer, the diffusion process in the orienting molecular flow can be changed by reducing the influence of the residual gas pressure or shortening the distance between the evaporator and the condenser, resulting in an isotropic condition not Complex existence.

In the steady state process, the evaporation rate of the metal at the evaporation surface Fv, the rate of migration of the metal vapor particles in the gas space and the condensation rate at the condensation surface are equal, and it can be seen that both under normal pressure and under vacuum conditions The material evaporation process includes the following four processes.

Heat transfer: Heat is supplied from a heat source to the evaporation source, and latent heat of vaporization required for vaporization is transferred to the evaporation surface to maintain the temperature of the melt itself.

Evaporation: Part of the surface metal molecules get rid of the evaporation surface into the gas space.

Migration: The diffusion of metal vapors in the gas phase shifts.

Condensation: Condensation of molecules of metal vapor that reach the condensing surface occurs, and if the temperature of the condenser is higher than the melting point of the metal, it condenses into a liquid. On the contrary, it condenses into a solid.

The metal evaporates at atmospheric pressure and evaporates under vacuum conditions. The difference is only in the third process, that is, the gas phase migration process. If evaporating at atmospheric pressure, this turbulence is carried out in the space under atmospheric pressure; if it is carried out in vacuum, this turbulent motion is carried out in a dilute gas. Under the same conditions, the rate of the total process is closely related to the characteristics of the migration process. According to the principle of thermodynamics, the degree of vacuum has little effect on the metal vapor pressure, but it has a significant effect on the kinetics of the evaporation process, because if the evaporated metal gas molecules can not smoothly leave the evaporation space, the evaporation process will slow or even stop. .

If "evaporation-condensation" reaches equilibrium, the rate can be established: Consider an evaporation surface, and let Vx be the evaporation rate of evaporated molecules perpendicular to the evaporation surface. dnx represents the number of molecules per unit volume between Vx and Vx+dVx. The number, because only the rate is greater than a certain rate Vg molecules can break away from the surface gravity evaporation, therefore, the number of molecules evaporated per unit time per unit surface: the gas diffusion layer of the surface Fk is reduced to Pk; T melt is melted The dn1 of the body is based on Maxwell's distribution law. The number of molecules whose velocity is between Vx and Vx + dVx is: a few times. Substituting (4) into (3), the integral is: Boltzmann's constant, m is a The mass of the molecule, q, is the heat of vaporization of each molecule.

Similarly, a similar rate can be obtained for the condensation process. The only difference is that the lower integration limit should be zero. If evaporation and condensation reach a dynamic equilibrium, then N evaporation=N condensation, and the state equation of the ideal gas P=nkT is substituted. After, you can get: Degree is closely related.

3 The law of evaporation rate of substances in vacuum conditions N evaporation = 1 In summary, the inter-temporal relocation of the material evaporation and the vacuum centigrade heat to overcome the static pressure above the bubble and the curvature of the microbubbles in the evaporation, Some of the molecules returning to the liquid phase in the gas phase are elastically reflected at the phase interface and cannot be completely condensed. According to the principle of dynamic equilibrium, the number of vaporized molecules should also be reduced accordingly. Therefore, the evaporation rate should be multiplied by a condensation factor, and the unit time Expressed from the mass of evaporation per unit area, that is, an important formula for the evaporation rate under conditions, it determines the relationship between the theoretical maximum evaporation rate and the vapor pressure and temperature. However, some corrections are needed in the following cases. 121. When the condensing surface temperature Tk is high, the metal condensed on the condensing surface also has a certain vapor pressure Pk at this temperature. In this case, the vaporized metal is actually reduced considerably. Reduced evaporation rate. Therefore, the formula (8) should be amended to: In fact, formula (8) only adapts to the situation where there is little residual gas in the system, that is, the degree of vacuum is high. When the amount of residual gas can not be ignored, the formula (8) should be modified as follows: If both of the above conditions need to be considered, it should be: surface evaporation, or “general evaporation.” This evaporation is performed under rough vacuum. At this time, the corresponding mean free path A is far less than the linear dimension d0 of the evaporation space. That is, when the metal melt enters the zone, bubbles may appear in the metal melt and “boiling evaporation” occurs. This stage is quite in vacuum, and the corresponding mean free path is smaller than Or equal to the linear dimension of the evaporation space, ie, A is quite high, and the corresponding mean free path is much larger than the linear dimension of the evaporation space, ie, A>>d. At this time, there are few residual gas molecules in the system, and collisions between gas molecules occur. It can be neglected that there is only collision between vaporized gas molecules and the wall of the device. Therefore, the internal friction between the gas molecules is not present, and the diffusion resistance of the evaporated metal molecules is negligible, thus creating the best migration conditions for the evaporation process. It ensures that the metal vapor particles can reach the condensation surface at the maximum rate.

Obviously, the order of the rate and the corresponding mean free path of the above three evaporation modes are: boiling evaporation (general evaporation (Xdo)) In summary, and according to available points are very important for vacuum engineering design and practical operation control The conclusion.

At a certain degree of vacuum, the evaporation rate of the metal increases with increasing temperature.

At a certain temperature, the evaporation rate of metal appears as three different stages with the change of vacuum degree and corresponds to “general evaporation”. When the degree of vacuum is high to a certain extent, the evaporation rate of metal is independent of the change of vacuum degree. to reach maximum.

The "instinct evaporation rate" of a substance cannot be considered as "the evaporation rate under vacuum." Because at a certain temperature, the evaporation rate of the material will change with the degree of vacuum. However, it can be said that the instinct evaporation rate of a substance is an evaporation rate that corresponds to a certain temperature, and the requirement for the “vacuum degree” varies depending on the metal, and cannot be generalized. It is worth pointing out that although the “instinct evaporation rate” and the “maximum evaporation rate” are equal, they are obviously different concepts: the instinct evaporation rate “is an inherent property of matter, and it depends on the structure of the material and the temperature The maximum evaporation rate is a description of the kinetics of the evaporation process.At a certain temperature, whether the evaporation rate reaches a maximum depends only on the environmental factors, that is, the degree of vacuum. Under the same conditions, the greater the evaporation rate of instinct The maximum evaporation rate of the material is also greater.

The same metal, at each temperature, has a "maximum evaporation rate" as the degree of vacuum changes. However, it must be pointed out that the word “maximum” here only makes sense at this temperature. At different temperatures, the metal evaporation has a corresponding maximum evaporation rate as the degree of vacuum changes, and the corresponding high temperature “max. Evaporation rate "is greater than the corresponding maximum evaporation rate", ie Wl273> Wll23> W973 (see).

In the vacuum degree that does not reach the "molecular distillation", the evaporation resistance of the substance still exists, so it cannot be said that the evaporation of the substance under vacuum is "without any hindrance". However, compared with atmospheric evaporation, vaporization of the material under vacuum can reduce the chance of condensation of the evaporating particles, thereby creating better vapor transport conditions for the evaporation process and ensuring that the particles can reach the condensing surface with a large evaporation rate. Only when the vacuum conditions reach "molecular distillation" can the best vapor transport conditions be achieved and the vaporous particles will reach the condensing surface at the maximum evaporation rate without any hindrance.

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