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Author Topic: Diamond surface graphitization  (Read 1895 times)

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Corribus

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Re: Diamond surface graphitization
« Reply #15 on: June 01, 2018, 03:45:09 AM »

In theory. But by analogy, a bucket of gasoline left at room temperature in air doesn't just explode, either, even though the oxidation of hydrocarbons is extremely exothermic. The reaction is SLOW.
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pnacze199204

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Re: Diamond surface graphitization
« Reply #16 on: June 01, 2018, 04:00:30 AM »

So slow that we are still thinking about a time greater than the age of the universe? In that case, what about the nano diamond (size less than 1 micrometer)? Would the age of the universe be enough time to get rid of it?
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Corribus

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Re: Diamond surface graphitization
« Reply #17 on: June 01, 2018, 05:17:39 AM »

Well thankfully the original research article in the Wikipedia page you linked to (https://www.sciencedirect.com/science/article/pii/S0925963501006732?via%3Dihub) provides experimental numbers you can use. They report a pre-exponential factor of ~ 2 x 107 nm s-1 Pa-1 and an activation energy of 222 kJ mol-1. The atmospheric pressure of oxygen at room temperature at sea level is ~21000 Pa. So, the pre-exponential factor is ~ 4.3 x 1011 nm s-1. If we assume temperature independence of these factors (not a great assumption, but for rough estimates it's OK), then at room temperature (298 K, say) and regulator atmosphere at sea leavel, I calculated an approximate rate of 5 x 10-28 nm s-1.  Or, 1.5 x 10-20 nm of diamond oxidized per year. To oxidize 1 full nm thickness of diamond (100 face) at room temperature and atmospheric pressure, then, would take on average about 6 x 1019 years. The age of the universe is about 1.4 x 1010 years. So, trillions of times the age of the universe would be an approximate estimate of the time it would take to oxidize 1 nm of diamond at room temperature.

In the paper, the authors do the oxidation reaction in a furnace at, say, 1000 K and concentrated oxygen atmosphere (~80,000 Pa). We can see that raising the temperature and increasing concentration of oxygen so much really increases the rate. In pure oxygen atmosphere of 80,000 Pa, the rate of oxidation becomes 4 nm/s: under these conditions, a 5 mm diamond (with a single 100 face exposed, mind) could evaporate to carbon dioxide in about 2 weeks - but still, if you think about it, that's TWO WEEKS at 1000 K and pure oxygen to oxidize a relatively small diamond (or relatively big, depending on your salary ;)).

Even under regular air the oxidation rate is significant at high temperature (about 4x slower).
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pnacze199204

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Re: Diamond surface graphitization
« Reply #18 on: June 03, 2018, 11:32:58 AM »

Well, I found an article in which it's written that diamond can disappear in sunlight: https://www.nature.com/news/2011/110715/full/news.2011.421.html . 10 billions of years would be sufficient to make 1 microgram of diamond evaporate. And here is the paper: https://www.osapublishing.org/ome/fulltext.cfm?uri=ome-1-4-576&id=220251 . I find that very interesting, but once again, I have a question :D

 It would require removing a few or a dozen atoms from the surface of the diamond per year. In that case, would a diamond have to be in the sun 24 hours a day to make it happen? It is obvious that on Earth we have night and day, so if we cut off the sunlight, would the process have to start from the beginning to excite the atom? I don't know if I have expressed myself well, but I hope you understand me. :D
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Corribus

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Re: Diamond surface graphitization
« Reply #19 on: June 04, 2018, 03:37:22 AM »

I'm not 100% sure what your asking. Quantum events are probabilistic, not deterministic, so turning off the light source does not cause you to "start over" for a single reaction event. Single reaction events are practically instantaneous (not really, but based on a human frame of reference they might as well be). It's the low probability of a single reaction event happening at any given instant that drives the slow macroscopic change from reactant to product.  That said, only having the light on for 50% of the time does effectively double your overall conversion time.
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pnacze199204

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Re: Diamond surface graphitization
« Reply #20 on: June 04, 2018, 07:44:19 AM »

Well, I ask because the speed of the process is highly linear, and as I read, a typicall diamond would lose only a few atoms per year if we put it in strong sunlight. So I wanted to know if cutting off the UV light influences the process. I thought that the atom gets excited becuse of the long exposure to sunlight and finally it pop off, but when the light is off the process has to start over. By the way, to this process happen we need 2 UV photons. Sunlight seems unlikely to do much, am I right?
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Corribus

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Re: Diamond surface graphitization
« Reply #21 on: June 04, 2018, 08:58:14 AM »

Cutting of the light source does influencing the time it would take, but not in the way you are thinking. "Starting over" is not the way photon-driven (or any, really) reactions - which are just ensembles of quantum events - work. 

You can think of it like any other gambling process. Let's say you are in Vegas and you are doing slot machines, where the probability of winning the jackpot is (say) 1/1,000,000 for every pull of the lever. You do this from 10 AM to 12:00 PM and a friend comes by and says, "Hey, do you want to get some lunch?" If you agree to go to lunch, this doesn't cause you to "start over" when you come back, because every pull of the lever has the same chance of being a winner. On the other hand, if you had the time (and money) to use the machine continuously as long as the casino is open, it would be appropriate to say that, on average, it will take you a longer absolute amount of time on average to become a winner if the casino closes 8 hours every night, because you lose 8 hours every 24 hour period of lever-pulling-time. But, if you were to calculate the average number of playing hours it takes to win, it makes no difference if there are casino-closings or not. I.e., it's the number of pulls of the lever per playing time that makes the difference. Needless to say, if you could increase the number of machines you could play at one time, or rig the system to make it more likely that any given pull results in a winner, you can reduce the amount of time it takes to become a millionaire. In the chemistry world, these two things are akin to increasing the energy (temperature, light intensity) or applying a catalyst.
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pnacze199204

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Re: Diamond surface graphitization
« Reply #22 on: June 09, 2018, 05:37:22 AM »

I understand, thank you!

The paper says that to diamond lose its atoms it is requiered the ultraviolet light and even the sunlight or mercury-vapour lamp would be able to damage a diamond structure. They paper says also that there's no threshold, so the process can proceed at very low intenses of light. It is pretty hard to belive that the walvelenght of 380 nm would influence the structure of the stone, even if it is so slow.

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pnacze199204

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Re: Diamond surface graphitization
« Reply #23 on: June 11, 2018, 10:29:04 AM »

By the way, I have read some articles on the Internet and I understood that we can have Two Photon  Absorbtion only with very high intensity light (laser sourse). Would the sullight be enough to excite the state of atom? :) The paper that I linked says yes, but maybe I misunderstand something.
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Corribus

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Re: Diamond surface graphitization
« Reply #24 on: June 13, 2018, 04:02:57 AM »

By the way, I have read some articles on the Internet and I understood that we can have Two Photon  Absorbtion only with very high intensity light (laser sourse). Would the sullight be enough to excite the state of atom? :) The paper that I linked says yes, but maybe I misunderstand something.
TPA is a nonlinear optical phenomenon, meaning the intensity is not linearly correlated to irradiation power. Simultaneous absorption of two photons is proportional to the square of the incident photon intensity (power or fluence, however you want to define it), and the proportionality constant is kind of like a two-photon extinction coefficient. It can be related to molecular electronic structure, but honestly I'd have to think about how it'd be related to the surface properties of a solid. Although it is not true that two photon absorption occurs only with high intensity light, it is true that it will become a more prominent mechanism for light absorption using, e.g., a laser rather than ambient light. Sunlight is a continuum source, meaning it emits light over a broad wavelength range, whereas a laser is (mostly) monochromatic. In sunlight then you would have both one and two photon absorption going on simultaneously. But as before, this is mostly a matter of kinetics rather than "happen/not happen". Two photon absorption will probably be slow - all other things equal - because the photon fluence is low. That doesn't mean it won't happen.

(NB - there are certain selection rules to be concerned with as well. Honestly I don't remember what they are for two photon absorption. So this would impact the likelihood of an absorption event.)
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What men are poets who can speak of Jupiter if he were like a man, but if he is an immense spinning sphere of methane and ammonia must be silent?  - Richard P. Feynman
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