A method for suppressing thermionic electron emission based on TTM-MD simulation

Abstract

The application discloses a method for inhibiting thermionic emission based on TTM-MD simulation, which adjusts the energy distribution and delay time of double pulses to regulate the electron temperature, and obtains the maximum delay time introduced under the condition of meeting the processing requirements according to the relationship between the temperature rising time and the delay. The method can obtain the maximum delay introduced under the condition of meeting the processing requirements, reduces the electron temperature, and inhibits the thermionic emission.

Description

TECHNICAL FIELD

[0001] The application belongs to the technical field of laser, and particularly relates to a method for inhibiting thermionic emission based on TTM-MD simulation. BACKGROUND

[0002] With the rapid development of manufacturing industry, the requirement for machining precision is increasing, and the traditional mechanical machining method cannot meet the machining requirement of complex shape in micrometer and nanometer scale. The mature precision manufacturing method has extremely high machining resolution, but is limited by the substrate and machining environment, and has a long machining step, and is not suitable for general machining scene, and has limitations in economy and environmental protection.

[0003] The popularization of femtosecond laser provides a direction for precision machining in micro-nano scale, which has the characteristics of short pulse width and high power, can effectively reduce the area affected by heat and reduce the thermal damage to the substrate material, and realizes cold machining. In the ablation machining, it is hoped to limit the area where material removal occurs; in the sintering machining, it is hoped that the irradiated parts are connected to each other to avoid material removal.

[0004] The removal of material is mainly caused by the thermionic emission effect. Under the irradiation of femtosecond laser, the energy of photons is first absorbed by electrons, the energy of electrons is increased to form hot electrons, and then the energy is transmitted to the lattice through the energy coupling between electrons and lattices to realize the heating of the material. However, when the energy absorbed by the electrons in a short time is too much, that is, the "temperature" of the electrons is too high, the electrons may be excited to free electrons, which is called thermionic emission. At this time, the material particles are positively charged and repel each other, and then are blown away to cause material removal.

[0005] The reduction of electron temperature to inhibit thermionic emission can be realized by controlling the energy, but pure reduction of laser energy may lead to the failure to realize the machining requirement; in order to compensate for the reduction of energy, the exposure time is increased, which may cause the heat to be deposited around the substrate and the machining area, and then the heat affected zone is expanded, and the advantages of femtosecond laser cannot be fully played. SUMMARY

[0006] The application provides a method for inhibiting thermionic emission based on TTM-MD simulation, which aims to obtain the maximum delay that can be introduced under the premise of meeting the machining requirement, and reduces the electron temperature to inhibit thermionic emission.

[0007] In order to achieve the above purpose, the application provides a method for inhibiting thermionic emission based on TTM-MD simulation, which adjusts the energy distribution and delay time of double pulses to control the electron temperature, and obtains the maximum delay time that can be introduced under the premise of meeting the machining requirement according to the relationship between the heating time and the delay.

[0008] The further technical scheme of the present application is that a beam splitter and a displacement platform are used to split and introduce the laser into a delay.

[0009] The further technical scheme of the present application is that the temperature of the material under femtosecond laser irradiation is expressed as:

[0010]

[0011] m i 、v i is the mass and speed of each atom, and U is the potential energy of the system, is the Langevin thermostat describing the electron-lattice energy transfer; C e 、k e 、g p , S respectively represent the heat capacity, thermal conductivity, electroacoustic coupling coefficient and external energy of the electron; T e 、T l respectively represent the temperature of the electron and the lattice.

[0012] The further technical scheme of the present application is that the TTM-MD simulation model is two adjacent copper nanoparticles, and the melting point of the copper nanoparticles at the nanoscale is determined to select the simulation laser energy.

[0013] The further technical scheme of the present application is that the melting point of the copper nanoparticles at the nanoscale is determined, and the specific method comprises:

[0014] The relationship between the system potential energy and the temperature is obtained by simulation under the conditions of constant rate slow heating and heating with a high-energy femtosecond laser.

[0015] The further technical scheme of the present application is that the melting point is determined to be 1100K-1200K, and the simulation laser energy flux density is 60J / m 2 -80J / m 2 .

[0016] The further technical scheme of the present application is that the MD simulation based on the microcanonical ensemble is used to obtain the regulation effect of the introduced delay on the electron temperature.

[0017] The further technical scheme of the present application is that the maximum delay time is 16ps-20ps.

[0018] The method for inhibiting thermionic emission based on TTM-MD simulation provided by the embodiment can study the regulation effect of double-pulse laser with delay on electron temperature on the atomic scale. The electron temperature is effectively regulated by adjusting the energy distribution and delay time of the double pulse. Under certain constraints, the relationship between the temperature rise time and the delay is considered, and the maximum delay that can be introduced under the premise of meeting the processing requirements is obtained as a reference, thereby reducing the electron temperature to inhibit thermionic emission.

[0019] It should be understood that the foregoing general description and the following detailed description are only exemplary and explanatory, and cannot limit the present disclosure. BRIEF DESCRIPTION OF DRAWINGS

[0020] The accompanying drawings, which are incorporated into and form part of the specification, illustrate embodiments consistent with the present application and, together with the description, serve to explain the principles of the application

[0021] Figure 1 is a potential-temperature curve in the embodiment of the present application;

[0022] Figure 2 is an electron / lattice temperature-time curve in the embodiment of the present application;

[0023] Figure 3 is a maximum electron temperature-delay curve in the embodiment of the present application;

[0024] Figure 4 (a) is a temperature rise time-delay curve in the embodiment of the present application, Figure 4 (b) is Figure 4 (a) is a curve after differentiation of the curve. DETAILED DESCRIPTION

[0025] The present application will be described in further detail below with reference to the drawings and embodiments. It should be understood that the specific embodiments described herein are only for the purpose of explaining the present application, and not limiting the present application. In addition, it should be noted that only the parts related to the present application are shown in the drawings, not all the structures.

[0026] Before the example embodiments are discussed in more detail, it should be mentioned that some example embodiments are described as processes or methods depicted as flowcharts. Although the flowcharts depict the steps in a sequential order, many of the steps can be performed in parallel, concurrently or simultaneously. In addition, the order of the steps can be rearranged. The process can be terminated when its operations are completed, but can also have additional steps not included in the flowcharts. The process can correspond to a method, function, routine, subroutine, etc.

[0027] The reduction of electron temperature to suppress thermionic emission can be achieved by controlling the energy, and a feasible solution is to split the laser pulse, introduce a picosecond delay, and make the first beam of laser enter the material first, and the second beam of laser reaches before the electron-lattice energy coupling is completely finished, thereby effectively suppressing the additional material removal caused by thermionic emission under the premise of reaching the target temperature.

[0028] A method for suppressing thermionic emission based on two-temperature equation-molecular dynamics (TTM-MD) simulation, which regulates the electron temperature by adjusting the energy distribution and delay time of double pulses; and according to the relationship between the heating time and the delay, the maximum delay time introduced under the premise of meeting the processing requirements is obtained.

[0029] Specifically, the laser can be split and a small delay can be introduced by using a beam splitter and a displacement platform. For the microscopic behavior of the material under femtosecond laser, it is necessary to simulate it by coupling two-temperature equation (TTM) and molecular dynamics (MD). In MD, the substance is modeled as a cluster of atoms with a certain speed (kinetic energy) and a certain force (potential energy) to combine with each other, and the temperature of the lattice is in one-to-one correspondence with the kinetic energy of the atoms. Therefore, the heating of the material under femtosecond laser irradiation can be described by the following two equations:

[0030]

[0031] Where m i , v i is the mass and velocity of each atom, U is the potential energy of the system, is the Langevin thermostat describing the electron-lattice energy transfer; C e , k e , g p , S represent the heat capacity, thermal conductivity, electro- acoustic coupling coefficient and external energy of the electron; T e , T l represent the temperature of the electron and the lattice.

[0032] The model used for simulation is two adjacent copper nanoparticles, and the coordinates and velocities of each atom are calculated. First, the melting point at the nanoscale is determined, and the laser energy used in the subsequent simulation is selected based on this. The relationship between the system potential energy and the temperature is investigated under the conditions of slow heating at a constant rate and heating with a high-energy femtosecond laser, respectively. For example Figure 1The curve shows that under the condition of constant rate heating, the potential energy of the system changes sharply at the temperature of 1100K-1250K, and its melting point should be in the range of 1175K±75K; while under the action of femtosecond laser, the sharp change of potential energy occurs at 1200K-1350K, and its melting point is in the range of 1275K±75K. Under the two conditions, the difference of the melting point range can be explained as follows: under the condition of slow heating, the temperature (kinetic energy) and potential energy of the system have enough time to reach equilibrium, under the action of femtosecond laser, the system is rapidly heated, and the kinetic energy has not reached equilibrium with the potential energy at the same temperature, and the system has already risen to a higher temperature. Therefore, the melting point range under the two conditions is acceptable, and here 1150K slightly higher than the critical temperature is taken as the melting point, and the femtosecond laser with an energy flux density of 60J / m 2 ~80J / m 2 , preferably 70J / m 2 is selected for simulation.

[0033] Under the conditions of single pulse, double pulses (35 / 35) with the same energy and delay of 2ps-12ps, the change of electron / lattice temperature with time is simulated with a total energy flux density of 70J / m 2 , as shown in Figure 2 . In the MD simulation, the microcanonical ensemble (NVE) is selected, that is, the volume of the simulation box of the system is constant, and the total energy change of the system except the heating source / cold source is not considered. Since the energy coupling between electrons and lattices occurs within tens of ps, this time window can be ignored compared with the heat dissipation time between copper thin film and room temperature environment in macroscopic, so the external heat dissipation of the system can be ignored in the simulation. Under the condition of introducing delay, the copper nanoparticles under the action of femtosecond laser can finally reach the same temperature after a slightly prolonged coupling time, that is, the double pulses with delay can achieve the same processing effect as the single pulse; at the same time, it can be seen from the graph that the highest temperature of the electron presents a downward trend with the introduction of delay, and the introduction of delay has a certain regulating effect on the electron temperature, which can effectively inhibit the damage of hot electron emission to the processing surface.

[0034] Further study the regulating effect of pulse delay on the highest temperature of the electron, and consider adjusting the energy distribution of the double pulses so that the energy of the double pulses is not the same (20 / 50, 50 / 20), as shown in Figure 3The maximum electron temperature increases with the delay time in a form similar to exponential decay in the short delay time, and decreases by hundreds of K as the delay time increases to 10 ps. The energy distribution of the pulse also affects the regulation of the electron temperature. When using the combination of large pulse / small pulse, the regulation of the maximum temperature by the delay time is more obvious in the short delay time (about 10 ps); but when using the combination of small pulse / large pulse, the rate of decrease of the electron temperature is slow, and is always higher than the other two cases. This phenomenon is easy to explain: the energy coupling between the electron-lattice is large at high electron temperature, and the electron is first excited to a high temperature after using a large pulse, and the energy is transmitted to the lattice at a faster rate, so compared with the combination of small pulse / large pulse, the energy remaining in the electron system after the second pulse is introduced is less, that is, the regulation of the maximum electron temperature is better.

[0035] It should be noted that, as mentioned before, the simulation here is carried out without considering the external heat dissipation, so after introducing the delay, it must be ensured that the entire coupling time is still much smaller than the actual external heat dissipation time in the processing, so that the simulation is still applicable to the premise of ignoring the external heat dissipation, that is, the second pulse must be introduced when the coupling under the action of the first pulse is not completed, otherwise it cannot be regarded as a double pulse, but "two single pulses that cannot complete the processing", which limits the delay that can be introduced.

[0036] From Figure 2 It can be seen that the temperature curve of the lattice with time is a convex function, and the slope finally tends to 0, so when the two temperature rising processes are superimposed, if the previous process has been close to completion, increasing the delay will only increase the temperature rising time equally. Taking the melting point 1150K as an example, the melting time curve with the change of delay is shown in Figure 4 (a). The slope has approached 1 after 30 ps, which is consistent with the case mentioned above that the previous process is close to completion, that is, the two pulses have no relationship after 30 ps, at which time it is very likely that the processing purpose cannot be achieved, and it is no longer meaningful to study the regulation of the maximum electron temperature by the delay. It can also be seen that the slope changes significantly before 20 ps, so the curve is fitted and differentiated, as shown in Figure 4 (b). The first derivative of the fitted curve is a convex function, and its slope is almost only half at about 16 ps, at which time the relationship between the two pulses is very weak, so the maximum delay time that can be introduced can be obtained near this time, and the double pulse at this delay can achieve the processing purpose while minimizing the electron temperature.

[0037] In summary, a method for suppressing thermionic emission based on TTM-MD simulation is provided, which is used to study the regulation of electron temperature by double-pulse laser with delay at atomic scale. The energy distribution and delay time of double-pulse laser are adjusted to effectively regulate the electron temperature. Under certain constraints, the relationship between the temperature rise time and the delay is considered, and the maximum delay that can be introduced under the premise of meeting the processing requirements is obtained.

[0038] In this document, the terms "comprise", "comprising", or any other variation thereof, are intended to cover a non-exclusive inclusion, such that a process, method, article, or apparatus that comprises a list of elements does not include only those elements but can also include other elements not expressly listed or inherent to such process, method, article, or apparatus.

[0039] The above is a further detailed description of the present application in combination with specific preferred embodiments, and cannot be regarded as limiting the specific implementation of the present application to these descriptions. For ordinary skilled persons in the art to which the present application belongs, a number of simple deductions or substitutions can be made without departing from the concept of the present application, and all of them should be regarded as falling within the protection scope of the present application.

Claims

1. A method of suppressing thermionic electron emission based on TTM-MD simulation, characterized by, The method regulates electron temperature by adjusting energy distribution and delay time of double pulse, and obtains the maximum delay time introduced under the condition of satisfying processing requirement according to the relationship between temperature rising time and delay time. The temperature rising of material under femtosecond laser irradiation is expressed as: m i 、v i is the mass and velocity of each atom, U is the potential energy of the system, is the Langevin thermostat describing the electron-lattice energy transfer; C e 、k e 、g p , S represent the heat capacity, thermal conductivity, electron-phonon coupling coefficient and the applied energy of the electron, respectively; T e 、T l represent the temperature of the electron and lattice, respectively.

2. The method of claim 1, wherein the TTM-MD simulation is based on a model of a field emission device. The laser is split and introduced delay by using beam splitter and displacement platform.

3. The method of claim 1, wherein the TTM-MD simulation is based on a model of a field emission display (FED) device. The TTM-MD simulation model is two adjacent copper nanoparticles, which is used to select simulation laser energy by determining the melting point of copper nanoparticles at nanometer scale.

4. The method of claim 3, wherein the TTM-MD simulation is based on a model of the electron emission device. The melting point of copper nanoparticles at nanometer scale is determined, and the specific method includes: The relationship between potential energy and temperature is obtained by simulating under the conditions of constant rate slow heating and heating with a high-energy femtosecond laser respectively.

5. The method of claim 3, wherein the TTM-MD simulation is performed using a software package. The melting point is determined to be 1100K-1200K, and the simulated laser energy flux density is 60J / m 2 ~80J / m 2 .

6. The method of claim 5, wherein the TTM-MD simulation is based on a model of the electron emission device. It is obtained by MD simulation based on microcanonical ensemble that the delay has a regulating effect on electron temperature.

7. The method of claim 5, wherein the TTM-MD simulation is based on a model of the electron emission device. The maximum delay time is 16ps-20ps.

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