HPHT Synthesis of Micron Grade Boron-Doped Diamond Single Crystal

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					CHIN.PHYS.LETT.                                                                                Vol. 25, No. 7 (2008) 2667

    HPHT Synthesis of Micron Grade Boron-Doped Diamond Single Crystal in
                            Fe-Ni-C-B Systems ∗
    ZHANG He-Min(                )1 , ZANG Chuan-Yi(          )1 , LI Xiao-Lei(       )1 , MA Hong-An(             )2 ,
    LI Shang-Sheng(             ) , ZHOU Sheng-Guo(           )1 , GUO Wei(        2
                                                                                  ) , JIA Xiao-Peng(           )1,2∗∗
                Institute of Materials Science and Engineering, Henan Polytechnic University, Jiaozuo 454000
                           National Lab of Super Hard Materials, Jilin University, Changchun 130012

                                              (Received 19 January 2008)
   Micron grade boron-doped diamond crystals with octahedral morphology are successfully synthesized in a Fe–Ni–
   C–B system under high pressure and high temperature (HPHT). The effects of the additive boron on synthesis
   conditions, nucleation and growth, crystal morphology of diamond are studied. The synthesized micron grade
   diamond crystals were characterized by optical microscope (OM), scanning electron microscope (SEM), x-ray
   diffraction (XRD) and Raman spectroscopy. The research results show that the V-shaped section of synthetic
   diamond moves downwards to the utmost extent due to 0.3a wt% (a is a constant.) boron added in the synthesis
   system. The crystal colour is black, and the average crystal size is about 25 µm. The crystal faces of synthetic
   diamond are mainly {111} face. The synthesis of this kind of diamond is few reported, and it will have important
   and widely applications.

   PACS: 81. 10. Aj, 61. 72. S−, 81. 10. −h

    Boron-doped diamond is a significant material be-           anvil top face of 44 × 44 mm2 . Using crude graphite
cause of its physical and chemical properties such as          powder (mesh 200) as carbon source and Fe80 Ni20
high hardness superior to cBN, high thermal stabil-            alloy powder (99.9% in purity, mesh 200) as cata-
ity superior to common diamond and chemical inert-             lyst/solvent, two series of experiments with and with-
ness to ferrous materials.[1] In addition, the intensity,      out amorphous boron additive (mesh 200) were per-
impact toughness (TI) and thermal impact toughness             formed. The synthesized diamonds were character-
(TTI) become higher with the crystal size reducing.            ized by optics microscope (OM), scanning electronic
As the development of electronic and optical compo-            microscope (SEM) on a LINK-ISIS JSM5310, x-ray
nents toward miniaturization and high precision, dia-          diffraction (XRD) with Cu Kα radiation (V = 40 kV,
mond grinding tools for finishing and micro-processing          I = 100 mA) and Raman spectroscopy (Renishaw
are needed urgently. For example, processing fibre-             1000) with the excitation wavelength of 514 nm (Ar-
optic connector and the V-shaped groove of WC car-             gon ion laser) after being purified by boiling in strong
bide die. Micron grade boron-doped diamond is ex-              H2 SO4 -HNO3 .
tensively used in the finish, half finish machining and              The relationship between the minimum syn-
precision polishing. Boron is the best candidate for           thesis conditions (pressure and temperature) for
acceptor in diamond, the acceptor level of which is            graphite/diamond conversion and the amount of
estimated to be 0.37 eV above the top of the valence           boron additive is investigated. It is found that the
band.[2] Boron-doped diamond electrodes are of signif-         minimum pressure and temperature for diamond syn-
icant potential as so called nonnative anodes on which         thesis can decline to the utmost with 0.3a wt% boron
oxidation of organics is achieved, without adsorption          additive added. It is well known that in the P–T phase
of substrates or products on the electrode surface.[3]         diagram of carbon, the district for diamond forma-
Experimentally, many studies have been concentrated            tion is a V-shaped region bounded by the diamond-
on the fabrication of boron-doped diamond film.[4−7]            graphite equilibrium line and the metal-diamond eu-
However, there are few reports on the HPHT synthesis           tectic line, as shown in Fig. 1(a). The experimental
of micron grade boron-doped diamond single crystal             results show that the V-shaped region moves down
and it will have important and wide applications. In           to the utmost extent with a certain quantity boron
this Letter, amorphous boron was added to the system           additive added (Fig. 1(b)). Zhang et al.[8] proved that
of powder graphite and Fe-Ni alloy solvent/catalyst to         with an increase of boron, synthesized diamonds in the
synthesize micron grade boron-doped diamond crys-              V-shaped region move ceaselessly, downwards firstly
tals which are almost integrated octahedral at HPHT.           and upwards subsequently. For different catalysts,
    The experiments were performed in a China-type             the metal-diamond eutectic lines are different, corre-
SPD 6 × 1670 cubic high-pressure apparatus with an             sponding to V-shaped regions. When boron is added

   ∗ Supportedby the National Natural Science Foundation of China under Grant No 50572032.
    ∗∗To whom correspondence should be addressed. Email:
    c 2008 Chinese Physical Society and IOP Publishing Ltd
2668                                              ZHANG He-Min et al.                                              Vol. 25

into the reaction system, the property of Fe–Ni alloy        grade boron-doped diamond crystals exhibit many tri-
solvent/catalyst may change at HPHT, which leads             angular or triangular-like pits in the {111} face as
to the moving of the V-shaped regions. At the same           shown in Figs. 3(b) and 3(d). We deduce that the for-
time, the octahedral shape region broadens obviously,        mation of these morphologies of micron grade boron-
so the diamond presents octahedral shape easily. It          doped diamonds is related to dislocations. From tri-
is deduced that boron atoms entered into diamond             angular or triangular-like pits in Fig. 3(d), we deduce
could restrain {111} face growth and lead to {100}           that their formation is resulted from screw disloca-
face growing quickly. According to crystal growth the-       tions. Dislocations, stacking faults, micro-twin lamel-
ory, the face growing rapidly disappears firstly and          lae are the extended structural defect characteristics
leaves the face which grows slowly.                          of real crystals including diamond.[9] Boron atoms are
                                                             slightly bigger than carbon atoms. When boron atoms
                                                             enter into diamond lattices, the lattice disfiguration
                                                             and vacancies appear largely. Finally, dislocation,
                                                             especially screw dislocation are formed more easily
                                                             through the movement of disfiguration and vacancies
                                                             under HPHT.

  Fig. 1. Sketch map of V-shape section for micron grade
  diamonds synthesized with and without boron additive:
  (a) without additive boron, (b) with additive boron.

   Figure 2 shows the typical photographs of the
micron grade diamond crystals synthesized from the
powder catalyst-graphite system with and without
boron additive. The diamond crystals synthesized
from the system of Fe80 Ni20 and graphite with-
out boron additive (Fig. 2(a)) present cub-octahedral
shape and yellow colour. The crystal shape is pre-
dominant with {111} and {100} faces. The average
crystal size is about 30 µm. However, the diamond
crystals synthesized with boron additive (Fig. 2(b))
exhibit black colour, and some of them even opaque.
The crystals exhibit octahedral shape, and the crystal
shape is predominant with {111} face. The average
crystal size is about 25 µm.

                                                               Fig. 3. SEM photographs of micron grade diamond crys-
                                                               tals: (a) and (b) are micron grade diamonds without and
                                                               with boron additive respectively; (c) and (d) are amplifi-
                                                               catory of (a) and (b).

                                                                 In order to research the crystal structure of boron-
                                                             doped diamond, x-ray diffraction was used to analyse
                                                             the impurities in diamond crystals. XRD patterns of
                                                             yellow and black diamond crystals are shown in Fig. 4.
  Fig. 2. Photographs of diamond crystals synthesized (a)    From Fig. 4, we can see that only the characteristic
  without additive and (b) with boron additive.
                                                             peaks of diamond exist in the patterns for both yellow
    In order to analyse the surface characters of the        and black diamond crystals. In addition, we find that
micron grade diamond synthesized with boron addi-            the {220} peak of boron-doped diamond disappears
tive more carefully, we took some SEM photographs            and its {111} and {311} peaks enhance obviously in
of synthetic crystals, which are shown in Fig. 3. It is      Fig. 4(a). This phenomenon is probably related to the
observed that the surfaces of the pure micron grade di-      change of morphology from cub-octahedral shape to
amond crystals exhibit flat and slippery in {111} face        octahedral shape. In other words, boron atoms en-
and few striations in {100} face, as shown in Figs. 3(a)     tered into diamond results in crystal lattice distor-
and 3(c). On the contrary, the surfaces of micron            tion and crystal tropism transforms apparently. The
No. 7                                             ZHANG He-Min et al.                                               2669

above results show that the boron additive promotes
the formation of octahedral diamond, which is consis-
tent with the result of crystal morphology from SEM.

                                                                Fig. 5. Raman spectra of micron grade diamonds (a)
                                                                without boron additive and (b) with boron additive.

                                                                 In a summary, our results show that the V-shape
                                                             section for micron grade boron-doped diamond growth
                                                             is variation, the diamond surface is coarse and the x-
  Fig. 4. XRD spectrum of the micron grade diamond crys-     ray diffraction and Raman peaks are changed to dif-
  tals (a) without boron additive and (b) with boron addi-   ferent extent due to boron added into the reaction
    Figure 5 is the first-order Raman spectra of mi-              The authors are grateful to Y. Tian for his assis-
cron grade diamond single crystals at room tempera-          tance.
ture. Wave number and half peak width of high qual-
ity diamond without any adulteration is 1332.5 cm−1
and 3 cm−1 , respectively.[10] The Raman peak and            References
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