A model of HPHT color enhancement mechanism in natural gray diamond by nikeborome

VIEWS: 11 PAGES: 10

									  A model of HPHT color
enhancement mechanism in
  natural gray diamonds



  V.G. Vins, O.V. Kononov
  신소재공학부 4학년 오민호
             Abstract (개요)
   HPHT-processing of natural gray
    diamonds is studied and a new model
    of the color enhancement mechanism
    is proposed.
   It is proposed that there occurs a
    polymorphous transformation of
    graphite inclusions into diamond during
    HPHT-processing.
             1. Introduction
   After nitrogen, hydrogen is the most
    abundant impurity in natural diamond.
   Gray diamonds are assumed to grow
    under supersaturation conditions
    created when the temperature falls
    gradually as the cooling kimberlite pipe
    moves towards the surfaces.
      2. Experimental procedure
   The absorption spectra before and
    after HPHT-processing were recorded
    on the entire aperture of the samples.
   T=1800±50℃, P≒6.0 Gpa
   T=1950±50℃, P≒6.5 Gpa
   T=2100±50℃, P≒7.0 Gpa
      3. Results and discussion
   HPHT-processing changed gray
    coloration of diamonds to yellow.
   In order to understand the mechanism
    of HPHT induced color enhancement in
    gray diamonds let us consider the
    origin of gray color and the influence of
    hydrogen on diamond color.
    3.1 The origin of gray color in diamond

   The initial gray color is attributed to uniform
    absorption and reflection of light within the
    entire range of the optical spectrum.
   The uniform absorption and reflection of light
    is due to numerous microscopic graphite
    inclusions.
            3.2 Source of hydrogen

   …, diamond crystallization involves
    hydrocarbons, mainly methane, products of
    its dissociation, hydrogen, hydrogen oxides
    as well as nitrogen hydrides, inert and other
    gases.
   During the formation of carbon crystal
    phases, hydrogen could form CH radicals
    and get trapped at diamond defects and
    diamond crystal surface and at the edge
    faces of graphite crystals.
     3.3 Color enhancement mechanism

   ... transformation of graphite into diamond
    accompanied by transition of hydrogen from
    graphite to diamond.
   Under HPHT pressures and temperatures
    exceeding the diamond-graphite equilibrium
    line (over 1800℃) C-H bonds in graphite get
    spilt.
   As a result of this, hydrogen atoms become
    mobile and diffuse forming a kind of a cloud..
     3.3 Color enhancement mechanism

   These atoms get localized at the nearest
    structural defects...
   Hydrogen having been lost from the surface
    of graphite inclusions, the interatomic
    distance grows larger and weaker in each
    hexagonal structure and in the end, edge
    carbon atoms spilt away from the graphite
    matrix.
     3.3 Color enhancement mechanism

   ..free carbon atoms lose their bonds with
    graphite, diffuse and get trapped at
    vacancies, dislocations, internal surfaces
    and external surfaces of diamond crystals.
   Microscopic graphite inclusions disappear
    completely, thus, removing the gray
    component and making diamond crystals
    more transparent (colorless).

								
To top