Chapter 878: Using Light as a Knife
Looking at the title on the curtain, everyone present highlighted that some were happy and some were sad.
Whether it is a pulse laser or a continuous laser, it can be said that Chang Haonan has put it all together.
Especially Academician Hou, it felt like he punched the air and then dodged his waist.
Fortunately, although the process was a bit tortuous, the result is still relatively optimistic.
It doesn’t matter if the mental preparation was in vain.
The physical preparation is not in vain, it is always a good thing.
Hou Xun took a few deep breaths, trying to calm down.
The face that had just flushed gradually returned to normal.
Of course, there are also some people whose faces are not so good-looking.
They were in the opposite situation to Hou Xun. They originally came to the capital with great expectations, but in the end they were disappointed.
But I couldn’t say anything——
This project is obviously closely related to the military industry, and it is normal to keep it secret before it officially starts.
Moreover, they actually never said what specific technical route they would pursue.
It’s just that the Nature article that Chang Haonan published before was indeed very influential, so the person who was sent here followed academic conventions and guessed that he would continue to work on continuous lasers along the lines of the paper.
Unauthorized expectation and unauthorized breach of defense are considered appropriate.
However, breaking the defense is still breaking the defense.
To say how violent the reaction was to this...
That's not the case.
First of all, Chang Haonan, who is in charge of the project, and Academician Hou Xun, who is the most influential person on the scene, have reached an agreement. No matter what the reasons are, others do not have the confidence to oppose these two people at the same time.
Furthermore, as Chang Haonan said before, China’s accumulation in laser processing is quite weak.
Except for the Haojing Institute of Optics and Mechanics, the research on this by other units has basically stayed at a superficial level, and the time, energy and money invested are limited.
The sunk costs are not high.
So at most it feels a bit pity.
There will be no frustration like Hou Xun had before when "the work that I have been working on for half my life has been denied in principle".
In short, after a small interlude, the meeting quickly got back on track.
After hesitating for a moment, a researcher from the Institute of Applied Optics raised his hand:
“Professor Chang, I don’t mean anything else...but the article you posted in Nature The data of the paper..."
The other party hesitated a little.
Obviously, this issue is somewhat sensitive and it is difficult to ask straightforwardly.
If you are not careful, you may be considered to be provoking trouble.
However, it is not difficult for Chang Haonan to understand the meaning.
“All data, calculation processes, and conclusions are of course true.”
He answered bluntly:
“It involves academic ethics. Our research team is quite strict about this issue.”
While speaking, he switched to another PPT page.
Shown above are several key charts in the paper.
Here, Chang Haonan continued:
“In fact, according to our calculations, using continuous laser to perform multi-degree-of-freedom scanning drilling with variable path, beam quality and power, It is indeed possible to avoid the problems of oxidation, stress damage and poor contour roughness caused by traditional thermal processing.”
“But the hardware requirements for the laser are too high, let’s not talk about it. It is impossible for us, even in developed countries, to come up with equipment that meets the requirements in a short time. Ultra-short pulse lasers, especially picosecond or even femtosecond lasers whose duration is below the lattice relaxation time, are in principle The thermal effect cannot be activated, so it is completely cold processing, which is more suitable as a key research direction in a developing country.”
When hearing this, most people in the venue looked at Chang Haonan and gradually changed their eyes -
You guys who are engaged in military industry have really dirty hearts...
In the wrong Leave a glimmer of hope for others in the direction of your work.
But this glimmer of hope is still real.
It seems that you can get it if you work hard.
Just tell me whether you want to follow or not! If there is even a little bit of instability in one’s heart, one will definitely become jealous...
Following that, Chang Haonan showed several pictures about the calculation process:
< br>"The mechanism of thermal effect machining is relatively clear and is not the focus of our research, so the calculation at the theoretical level is not difficult. I have also demonstrated it in the paper, so I will only briefly explain it today, and the focus is to show it to you comrades. An idea of using mathematical methods to guide materials science research... I call it, computational materials science! "
The paper published by Chang Haonan is not long, but he judged that most fields of materials science There is a high probability that researchers cannot understand at least one-third of the content.
That is the part involving mathematical principles.
And this happens to be the core technology that he will use to guide the next project.
So, Chang Haonan first spent about twenty minutes talking about his results like a group meeting.
The result...
Actually, I still received a blank look.
But that doesn't matter.
The calculation process itself will be completed by Chang Haonan himself.
The purpose of his introduction this time is to help others build confidence in computational materials science.
Otherwise, if you work with a skeptical attitude, problems will easily arise.
……
Under the continuous bombardment of theory and reality, the thoughts at the symposium were gradually unified.
After Chang Haonan finished the first part of the introduction, Academician Hou first spoke:
“Professor Chang, can you tell us specifically what aspects of work we need to be responsible for next? "
"Of course."
Chang Haonan picked up the cup, took a sip of water, and then continued:
"Comrades who have been studying ultrashort pulse lasers should be aware that so far, regarding the interaction mechanism between laser and materials, the industry generally still uses the two-temperature equation proposed by Soviet scientists in 1974, that is, The temperatures of the electronic system and the lattice system are expressed by the Fourier heat conduction equation, and the two equations are related by the energy transfer term. "
"This equation is simple enough in form, but it can only describe electrons. The energy conversion and temperature changes between the crystal lattice cannot explain the statistical laws of microscopic particle movement, nor can it simulate the state of the material after laser processing."
"Even if there are many scholars in the future. Parameter corrections have been made on the basis of this model, but the shortcomings of the dual-temperature model are theoretical and cannot be overcome by simply adding fixed terms. "
"If you want to use the dual-temperature model to guide industry. Production requires different corrections for each processing link, and the correction method in turn requires a lot of experiments to obtain, so there is no guidance..."
Not everyone Everyone could understand this part of Chang Haonan's mechanism introduction, but Academician Hou, who was sitting in the first place on his left, nodded frequently.
In his past research, he was really troubled by the description error of the two-temperature model.
"So, we need a new model..."
Chang Haonan did not switch the PPT this time, but got up and came to a blackboard placed in the corner of the conference room. There are several big words written on it -
Ablation threshold model.
“This is an idea I put forward based on the principles of molecular dynamics. Of course, it is still in a relatively preliminary state.”
Chang Haonan wrote another character below.
φ.
“The ablation threshold, I define, is the laser energy flux sufficient to destabilize the crystal lattice, cause irreversible damage to the system, and remove at least one layer of material.”
"This threshold does not distinguish between action mechanisms and can be used to describe both cold ablation and thermal ablation."
"In the process of molecular dynamics calculations, I found that for picoseconds or shorter For pulse waves, the deviation of the ablation threshold is very small, causing the ablation to transform from a statistical attribute to a relatively deterministic behavior. In this way, we eliminate the uncertainty of the processing parameters and only need to consider the material to be processed. itself, and of course the characteristics of the laser source.”
Even people who don't understand the previous mechanism part will see their eyes light up at this time.
“If we only need to consider materials and laser sources, then the complexity of this system... wouldn’t it be similar to traditional mechanical processing?”
Perhaps due to excitement, Academician Hou’s voice was filled with excitement. A little out of shape.
“Exactly.”
Chang Haonan turned around and returned to the conference table, but did not sit back in his seat:
"Traditional machining requires knowledge of the hardness, stiffness, thermal expansion coefficient, shear strength... and so on of materials and tools in advance, which we collectively refer to as mechanical properties."
"But ultra-short laser processing is another Based on a set of logic, what we need to do is to find these properties that correspond to mechanical properties and can determine the optical processing effect, and then combine them with the ablation model, and finally achieve the same mechanistic method for any known material. Control processing! ”
(End of this chapter)