rus
Issue #1/2022
I.F.Khanbekov, V.P.Mikhailov
RESEARCH OF THE ULTRASONIC ACTIVATION PROCESSES OF DIFFUSION AND DESORPTION IN ELECTROVACUUM DEVICES
RESEARCH OF THE ULTRASONIC ACTIVATION PROCESSES OF DIFFUSION AND DESORPTION IN ELECTROVACUUM DEVICES
10.22184/1993-8578.2022.15.1.20.27
INTRODUCTION
Thermo-vacuum processing of some types of electrovacuum devices (EVDs), e.g. of the microwave range, is a long and energy-consuming technological process. The evacuation of microwave EVDs follows the classical scheme with the use of a step-by-step heating of the product body and holding at a temperature of about 550 оС. The duration of such process can reach several tens of hours and is determined by duration of gas diffusion and desorption processes. Obviously, it is impossible to accelerate thermovacuum treatment of EVD, adhering to the classical technology due to the fact that the possibilities of thermodiffusion and thermodesorption in this case are exhausted, and further temperature increase will lead to damage or destruction of the EVD microwave elements. A more effective way to accelerate removal of dissolved gases is to use, in parallel with heating, the non-thermal types of activation of diffusion and desorption, which allows to increase the gas extraction and pumping out [1]. One way to accelerate diffusion and desorption is ultrasonic (US) activation [2]. This method is easy to implement, cost-effective and can significantly improve the quality of degassing and reduce the time of thermo-vacuum treatment of devices.
TEST BENCH FOR ULTRASONIC TREATMENT OF ELECTROVACUUM DEVICES
Equipment for ultrasonic activation of diffusion and desorption processes in the technology of pumping out of microwave EVD is designed to transfer ultrasonic waves generated by piezoelectric transducers to the material of parts of the pumped device. If the piezoelectric plate, which receives the energy of the vibration modulator, is brought into mechanical contact with the body of heated EVD, then in all parts of the structure at the same time will be excited ultrasonic mechanical vibrations with the same frequency, which is experienced by the piezoelectric plate.
Figure 1 shows the mounting scheme for the ultrasonic treatment of the EVD. The pumping-out is carried out as follows. The EVD body 1 is equipped with two concentrators 2, 3 of ultrasonic emitters, two identical piezocrystal plates 5, fixed by pressure washers 4. These plates 5 have different functions in mode of maximum ultrasonic wave transit frequency determination and in mode of pumping out. When determining the frequency of maximum passage of ultrasonic waves through the EVD, one plate generates oscillation under the influence of the signal generator, the amplitude of such oscillations is determined with the aid of an oscillographer, and the other plate receives ultrasonic vibrations passed through the EVD, their amplitude is also recorded on an oscilloscope. In the mode of pumping-out of EVD through the tubulation 6 both plates 5 are connected to the signal generator in a parallel circuit to create a generation at the frequency of maximum passage of ultrasonic waves through the EVD. To ensure stable operation, the elements are made of Langasite family monocrystals, which retain their piezoelectric properties up to a temperature of 1400 °С. Power to the plates 5 is supplied from a signal generator through vacuum-tight current inputs on the flange of the pumping station. Figure 2 shows a rigging for studying diffusion and desorption rates from M0B copper. The toolkit includes tubulation 1 of M0B copper, clamping plates 2, active piezoelectric element 3 and ceramic insulators 4, 5.
STUDY OF DIFFUSION AND DESORPTION PROCESSES
The mechanism of ultrasonic activation of diffusion and desorption processes of gas components from microwave EVD parts is rather complicated and consists of several simultaneously occurring processes, which contribute to acceleration of diffusion, desorption and vacuum evacuation of gas components from the device body. An increase in the intensity of thermodesorption using ultrasonic vibrations occurs primarily by stimulating separation of colloidal particles from the surfaces within the vacuum volume by their mechanical acceleration. Significant quantities of colloidal particles are found on all components of the internal fittings of EVDs. Not only do they slow down the gassing process by covering the surface of the components, but they themselves are also sources of gas flow. Another mechanism for intensification of diffusion and desorption of gas components is compression and stretching of the metal lattice and deformation of intercrystalline boundaries caused by ultrasonic vibrations of polycrystalline metal structure. Let us consider the mechanism of hydrogen gas release from the surface layer of polycrystalline metal structure under the influence of ultrasonic vibrations [3–7]. Crystallite sizes are in a fairly wide range from units to tens of micrometers, the width of intergranular boundaries – from units to tens of angstroms. It is in these intergranular boundaries that the gas sorbed on the crystallite surface and dissolved in the metal volume is mainly found [8–10]. Under the influence of ultrasonic waves, forced elastic oscillations are generated in crystallites. In this case the width of the intercrystalline boundaries also changes with the frequency of the forced oscillations. Under certain conditions a sharp resonance increase in the amplitude of the elastic deformation of the boundaries between the crystallites occurs. In this case, the interaction forces of atoms of dissolved gas with atoms of metal decrease. The gas at intergranular boundaries diffuses to the surface in contact with the vacuum medium at higher speed, then it is desorbed and evacuated by the vacuum system. Another mechanism intensifying the diffusion of gas components is the transfer of additional energy of ultrasonic vibrations to the crystal lattice of the metal and acceleration of gas diffusion within crystallites.
The specific gas flow qi across the metal thickness is limited by the diffusion process, which in this case is described by Fick’s second law:
, (1)
where d – one half of the metal part, Nv – initial molecular concentration of gas in a metal, D – diffusion coefficient of gas in a metal.
The gas diffusion coefficient in a metal is determined as follows:
, (2)
where a – lattice constant of the metal, u – the average thermal speed of the atoms, Eдиф – activation energy of gas diffusion.
The activation energy Eдес of gas diffusion decreases due to the effect of ultrasonic waves on the polycrystalline structure of the metal and the weakening of the interaction forces of the dissolved gas atoms with the metal atoms.
The dwelling time of a gas molecule (atom) on a metal surface is:
, (3)
where τ0 is the minimum dwelling time of a gas molecule (atom) on the surface, determined by the period of thermal oscillation of molecules τ0 ~ 10–13 s, Eдес is the desorption energy.
The desorption rate, i.e. the number of molecules (atoms) of gas desorbing per unit time from a surface unit, can be determined as follows:
, (4)
where is the total number of gas molecules (atoms) adsorbed per surface unit.
The energy Qдес of gas desorption is reduced by the effect of ultrasonic waves on the metal surface and the weakening of the interaction forces between molecules (atoms) of sorbed gas and metal atoms.
The effectiveness of the ultrasonic vibration activation process of diffusion and desorption was experimentally proved. The essence of the experiments was to record the increase in pressure of gas components in the process of pumping out the volume of tubulation of M0B copper. In all experiments the tubulation was heated at a rate of about 2 °C/min in the range of 20÷180 °C. In Fig.3 the temporal dependence of changes in partial pressure and temperature of atomic hydrogen (1 a.u.m.) at different frequencies of ultrasonic vibrations is shown. The graphs show that already at temperatures above 100 °C the partial pressure of atomic hydrogen at simultaneous exposure to ultrasonic vibrations of different frequency and temperature is greater than without the ultrasonic vibrations. At the same time, the diffusion and desorption rates of gas increase due to ultrasonic activation.
Figure 4 shows the temporal dependence of changes in partial pressure and temperature of molecular oxygen (32 a.u.m.) at different frequencies of ultrasonic vibrations. From the graphs it can be seen that at temperatures above 80 °C the partial pressure of molecular oxygen at simultaneous exposure to ultrasonic vibrations of different frequency and temperature is greater than without ultrasonic vibrations. At the same time, gas diffusion and desorption rates increase due to ultrasonic activation.
In Fig.5 the temporal dependence of changes in total pressure and gas temperature at different frequencies of ultrasonic vibrations is shown. The graphs show that at temperatures above 60 °C the total pressure at simultaneous exposure to ultrasonic vibrations of different frequencies and temperatures is greater than without ultrasonic vibrations. When heated to 160 °C with the simultaneous activation of ultrasonic vibrations with a frequency of 55 and 300 kHz, a pronounced maximum of the total pressure (Fig.5) and the release of gas components, in contrast to the use of classical technology with degassing thermal treatment.
CONCLUSIONS
Ultrasonic activation of gas diffusion and desorption processes accompanied with heating allows of increasing the intensity of gas components desorption and diffusion from the internal elements of the processed product, reduces duration of degassing of EVD and, therefore, increases the output of finished products. In the process of evacuation, while heating the device, with simultaneous influence of ultrasonic vibrations, a pronounced maximum of total pressure and gas release is observed as it is connected with decrease of the interaction forces between gas molecules and atoms with the materials of the device vacuum fittings.
PEER REVIEW INFO
Editorial board thanks the anonymous reviewer(s) for their contribution to the peer review of this work. It is also grateful for their consent to publish papers on the journal’s website and SEL eLibrary eLIBRARY.RU.
Declaration of Competing Interest. The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.
Thermo-vacuum processing of some types of electrovacuum devices (EVDs), e.g. of the microwave range, is a long and energy-consuming technological process. The evacuation of microwave EVDs follows the classical scheme with the use of a step-by-step heating of the product body and holding at a temperature of about 550 оС. The duration of such process can reach several tens of hours and is determined by duration of gas diffusion and desorption processes. Obviously, it is impossible to accelerate thermovacuum treatment of EVD, adhering to the classical technology due to the fact that the possibilities of thermodiffusion and thermodesorption in this case are exhausted, and further temperature increase will lead to damage or destruction of the EVD microwave elements. A more effective way to accelerate removal of dissolved gases is to use, in parallel with heating, the non-thermal types of activation of diffusion and desorption, which allows to increase the gas extraction and pumping out [1]. One way to accelerate diffusion and desorption is ultrasonic (US) activation [2]. This method is easy to implement, cost-effective and can significantly improve the quality of degassing and reduce the time of thermo-vacuum treatment of devices.
TEST BENCH FOR ULTRASONIC TREATMENT OF ELECTROVACUUM DEVICES
Equipment for ultrasonic activation of diffusion and desorption processes in the technology of pumping out of microwave EVD is designed to transfer ultrasonic waves generated by piezoelectric transducers to the material of parts of the pumped device. If the piezoelectric plate, which receives the energy of the vibration modulator, is brought into mechanical contact with the body of heated EVD, then in all parts of the structure at the same time will be excited ultrasonic mechanical vibrations with the same frequency, which is experienced by the piezoelectric plate.
Figure 1 shows the mounting scheme for the ultrasonic treatment of the EVD. The pumping-out is carried out as follows. The EVD body 1 is equipped with two concentrators 2, 3 of ultrasonic emitters, two identical piezocrystal plates 5, fixed by pressure washers 4. These plates 5 have different functions in mode of maximum ultrasonic wave transit frequency determination and in mode of pumping out. When determining the frequency of maximum passage of ultrasonic waves through the EVD, one plate generates oscillation under the influence of the signal generator, the amplitude of such oscillations is determined with the aid of an oscillographer, and the other plate receives ultrasonic vibrations passed through the EVD, their amplitude is also recorded on an oscilloscope. In the mode of pumping-out of EVD through the tubulation 6 both plates 5 are connected to the signal generator in a parallel circuit to create a generation at the frequency of maximum passage of ultrasonic waves through the EVD. To ensure stable operation, the elements are made of Langasite family monocrystals, which retain their piezoelectric properties up to a temperature of 1400 °С. Power to the plates 5 is supplied from a signal generator through vacuum-tight current inputs on the flange of the pumping station. Figure 2 shows a rigging for studying diffusion and desorption rates from M0B copper. The toolkit includes tubulation 1 of M0B copper, clamping plates 2, active piezoelectric element 3 and ceramic insulators 4, 5.
STUDY OF DIFFUSION AND DESORPTION PROCESSES
The mechanism of ultrasonic activation of diffusion and desorption processes of gas components from microwave EVD parts is rather complicated and consists of several simultaneously occurring processes, which contribute to acceleration of diffusion, desorption and vacuum evacuation of gas components from the device body. An increase in the intensity of thermodesorption using ultrasonic vibrations occurs primarily by stimulating separation of colloidal particles from the surfaces within the vacuum volume by their mechanical acceleration. Significant quantities of colloidal particles are found on all components of the internal fittings of EVDs. Not only do they slow down the gassing process by covering the surface of the components, but they themselves are also sources of gas flow. Another mechanism for intensification of diffusion and desorption of gas components is compression and stretching of the metal lattice and deformation of intercrystalline boundaries caused by ultrasonic vibrations of polycrystalline metal structure. Let us consider the mechanism of hydrogen gas release from the surface layer of polycrystalline metal structure under the influence of ultrasonic vibrations [3–7]. Crystallite sizes are in a fairly wide range from units to tens of micrometers, the width of intergranular boundaries – from units to tens of angstroms. It is in these intergranular boundaries that the gas sorbed on the crystallite surface and dissolved in the metal volume is mainly found [8–10]. Under the influence of ultrasonic waves, forced elastic oscillations are generated in crystallites. In this case the width of the intercrystalline boundaries also changes with the frequency of the forced oscillations. Under certain conditions a sharp resonance increase in the amplitude of the elastic deformation of the boundaries between the crystallites occurs. In this case, the interaction forces of atoms of dissolved gas with atoms of metal decrease. The gas at intergranular boundaries diffuses to the surface in contact with the vacuum medium at higher speed, then it is desorbed and evacuated by the vacuum system. Another mechanism intensifying the diffusion of gas components is the transfer of additional energy of ultrasonic vibrations to the crystal lattice of the metal and acceleration of gas diffusion within crystallites.
The specific gas flow qi across the metal thickness is limited by the diffusion process, which in this case is described by Fick’s second law:
, (1)
where d – one half of the metal part, Nv – initial molecular concentration of gas in a metal, D – diffusion coefficient of gas in a metal.
The gas diffusion coefficient in a metal is determined as follows:
, (2)
where a – lattice constant of the metal, u – the average thermal speed of the atoms, Eдиф – activation energy of gas diffusion.
The activation energy Eдес of gas diffusion decreases due to the effect of ultrasonic waves on the polycrystalline structure of the metal and the weakening of the interaction forces of the dissolved gas atoms with the metal atoms.
The dwelling time of a gas molecule (atom) on a metal surface is:
, (3)
where τ0 is the minimum dwelling time of a gas molecule (atom) on the surface, determined by the period of thermal oscillation of molecules τ0 ~ 10–13 s, Eдес is the desorption energy.
The desorption rate, i.e. the number of molecules (atoms) of gas desorbing per unit time from a surface unit, can be determined as follows:
, (4)
where is the total number of gas molecules (atoms) adsorbed per surface unit.
The energy Qдес of gas desorption is reduced by the effect of ultrasonic waves on the metal surface and the weakening of the interaction forces between molecules (atoms) of sorbed gas and metal atoms.
The effectiveness of the ultrasonic vibration activation process of diffusion and desorption was experimentally proved. The essence of the experiments was to record the increase in pressure of gas components in the process of pumping out the volume of tubulation of M0B copper. In all experiments the tubulation was heated at a rate of about 2 °C/min in the range of 20÷180 °C. In Fig.3 the temporal dependence of changes in partial pressure and temperature of atomic hydrogen (1 a.u.m.) at different frequencies of ultrasonic vibrations is shown. The graphs show that already at temperatures above 100 °C the partial pressure of atomic hydrogen at simultaneous exposure to ultrasonic vibrations of different frequency and temperature is greater than without the ultrasonic vibrations. At the same time, the diffusion and desorption rates of gas increase due to ultrasonic activation.
Figure 4 shows the temporal dependence of changes in partial pressure and temperature of molecular oxygen (32 a.u.m.) at different frequencies of ultrasonic vibrations. From the graphs it can be seen that at temperatures above 80 °C the partial pressure of molecular oxygen at simultaneous exposure to ultrasonic vibrations of different frequency and temperature is greater than without ultrasonic vibrations. At the same time, gas diffusion and desorption rates increase due to ultrasonic activation.
In Fig.5 the temporal dependence of changes in total pressure and gas temperature at different frequencies of ultrasonic vibrations is shown. The graphs show that at temperatures above 60 °C the total pressure at simultaneous exposure to ultrasonic vibrations of different frequencies and temperatures is greater than without ultrasonic vibrations. When heated to 160 °C with the simultaneous activation of ultrasonic vibrations with a frequency of 55 and 300 kHz, a pronounced maximum of the total pressure (Fig.5) and the release of gas components, in contrast to the use of classical technology with degassing thermal treatment.
CONCLUSIONS
Ultrasonic activation of gas diffusion and desorption processes accompanied with heating allows of increasing the intensity of gas components desorption and diffusion from the internal elements of the processed product, reduces duration of degassing of EVD and, therefore, increases the output of finished products. In the process of evacuation, while heating the device, with simultaneous influence of ultrasonic vibrations, a pronounced maximum of total pressure and gas release is observed as it is connected with decrease of the interaction forces between gas molecules and atoms with the materials of the device vacuum fittings.
PEER REVIEW INFO
Editorial board thanks the anonymous reviewer(s) for their contribution to the peer review of this work. It is also grateful for their consent to publish papers on the journal’s website and SEL eLibrary eLIBRARY.RU.
Declaration of Competing Interest. The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.
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