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Diamond Synthesis HTHP Requirements

  1. Oct 31, 2009 #1
    I have a couple questions regarding the transformation of graphite to diamond.
    As I understand a common method of diamond synthesis involves applying high pressure and high temperature to a graphite sample for a few days and then removing the high temperature and finally the high pressure to insure the carbon takes the diamond form and maintains it at S.T.P.

    According the the carbon phase diagram http://dao.mit.edu/8.231/carbon_phase_diagram.jpg" it is obvious that the introduction of high temperature reduces the necessary high pressure needed to create diamond.

    Questions.
    Could diamond be formed without the introduction of high temperature as long as the applied pressure is above the required pressure according to the phase diagram? And would it stay in the diamond phase once the pressure is released?

    Does high pressure alone have the capacity to heat up the graphite sample, and if so how is this change in temperature calculated?

    Is the time required to make diamond related to the pressure applied, would higher pressures allow for large diamonds to be made more rapidly?
     
    Last edited by a moderator: Apr 24, 2017
  2. jcsd
  3. Oct 31, 2009 #2

    Ygggdrasil

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    In addition to considering the thermodynamics of a chemical process, it is also important to consider the kinetics. Although diamond may be thermodynamically favored over graphite at very high pressures, the conversion of graphite to diamond would be very slow at lower temperatures because the thermal energy available would not be sufficient to break the strong C-C bonds in graphite so that they can be reformed in the diamond lattice (this is why diamonds do not spontaneously convert into graphite at ambient temperature and pressure even though they are less stable than graphite). The high temperature facilitates this bond reformation process and make the reaction proceed at a reasonable rate.
     
  4. Nov 2, 2009 #3

    alxm

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    I've never heard that? As far as I know, you need to dissolve the carbon in something and actually crystallize it. You can't just 'squash' graphite directly (the movie 'Superman' lied to us all); you need crystallization taking place at high temperature and pressure.

    No, it's obviously the opposite. A higher pressure is required at a higher temperature.
    The reason why higher temperatures are used is not because it makes it easier to reach the diamond-stability area. It does the opposite. Higher temperatures (which in turn require higher pressures) are demanded because diamond formation is very slow otherwise.

    Only extremely slowly.

    Yes.

    Pressure does not heat things. At least not things that aren't moving. Where did you get that idea?

    No, the time is related to temperature.
     
    Last edited by a moderator: Apr 24, 2017
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