Diamond Phase Transition Research Articles

ТЕРМОДИНАМИЧЕСКИЙ АНАЛИЗ ПРОЦЕССА ФОРМИРОВАНИЯ НАНОСТРУКТУРНОГО ПОЛИКРИСТАЛЛИЧЕСКОГО МАТЕРИАЛА НА ОСНОВЕ НАНОАЛМАЗОВ, МОДИФИЦИРОВАННЫХ НЕАЛМАЗНЫМ УГЛЕРОДОМ (ЧАСТЬ 2)

The paper presents the results of thermodynamic analysis showing that the use of nanodiamonds with a thin graphite-like coating formed by surface graphitization of nanodiamonds makes it possible to increase the thermodynamic stimulus for the formation of a diamond structure under conditions of high pressures and temperatures. For a thin carbon film with a disordered structure (amorphous carbon, soot), the pressure of transition into diamond will exceed the equilibrium pressure due to the lower surface energy of amorphous carbon compared to the surface energy of graphite. In this case, a decrease in the thickness of the non-diamond carbon film on the surface of a diamond particle leads to an increase in the pressure of the graphite–diamond phase transition. The proposed approach provides the possibility of reducing the synthesis parameters of nanostructured polycrystalline diamond materials without additional use of phase transformation catalysts.

Intermediate carbon phase. New experimental data and atomic model

We investigated phase transitions in diamond under cyclic loading near the graphite-diamond equilibrium line at temperatures sufficiently below the Debye temperature (maximum stresses and temperature in the diamond do not exceed 6 GPa and 420 K accordingly). Using Raman and transmission electron microscopy, we reveal the following stages of the transition of diamond to graphite-type phase (onions). The first stage is the plastic deformation by the mechanical twinning along with martensitic phase transition of diamond to intermediate carbon phase (ICP). The next stage is the transition of ICP to onions with a radius below ~5 nm. The proposed theoretical ICP model based on the crosslinking of graphene layers by Frenkel defects displays not only good agreement with the available structural experimental data but also specific properties like enhanced out-of-plane stiffness and metallicity.

Impulse laser cutting of diamond accompanied by phase transitions to fullerene-type onions

We studied a process of diamond cutting by a focused pulsed laser beam in an explosion inside solid matter mode (a 100 ns laser beam impulse focused in diamond generates a shock wave). A cavity size created in diamond by the laser impact is described by a conception of a blast cavity formation after explosion inside solid matter. Diamond cutting is also accompanied by a phase transition of diamond to fullerene-type onions composed of 2 to 5 shells. According to a new phase diagram of carbon (which contains a zone of diamond instability in the 55–115 GPa pressure range), the observed phase transition is possible under ~70 GPa pressure and ~2400 K temperature, which indicates pressure and temperature values during laser cutting. The pressure estimated based on the new phase diagram corresponds to the pressure estimated from the known blast cavity formation model.

Transformation of diamond to fullerene-type onions at pressure 70 GPa and temperature 2400 K

We report the observation of a phase transition of diamond to denser than diamond carbon phase composed from 2 to 3 fullerene-type shells of onions. Raman spectra indicate the fullerene-type of the onions shells. The onions phase is a stable phase in a diamond instability zone of a phase diagram of carbon at pressure 70 GPa and temperature 2400 K. A mixture of diamond and Ni powders was heated by a laser beam under pressure in a diamond anvil cell. Both direct and catalytic diamond to onions transitions were observed during heating. The catalytic transformation includes the following steps. Melting of Ni during the laser heating at pressure 70 GPa, a ‘diamond solution’ (a transfer of carbon atoms from diamond) in liquid Ni and the formation of an equilibrium carbon phase from the supersaturated solution upon cooling. The catalytic process is a reverse one relative to the catalytic synthesis of diamond in a diamond stability zone at pressure around 6 GPa. The main result of our study is the presence of fullerene-type structures in the phase diagram of carbon in the region of diamond instability under high sub-Mbar pressure and wide range of temperatures.

High temperature tribology of heavily boron doped diamond films against steel

Despite the fact that Fe, Co, and Ni catalyze the phase transition of diamond into graphite, the question of the applicability of diamond as a functional coating of a metal-cutting tool is still open. For this reason, our work contains investigation of wear and friction of heavily boron doped diamond films against steel at elevated temperature, as well as influence of boron concentration on diamond film oxidation resistance. The obtained data indicated that minimum CoF value is achieved in the temperature range within 570–670 °C and strongly depends on boron concentration in coating (CoF decreases with increasing of boron concentration). Wear rate has the same dependence as the CoF, whereas oxidation resistance decreases with increasing of boron concentration. Besides, the presented results are first obtained for boron doped diamond films synthesized under high B/C ratio conditions (of up to 333 ppb).

Graphitization wave in diamond induced by uniformly moving laser focus

Laser beam tightly focused inside single-crystal diamond allows conducive microstructures to be fabricated via local phase transition of diamond to graphite. An extended modified region is formed in diamond due to propagation of so called laser-induced graphitization wave, which occurs immediately after the optical breakdown and propagates towards the laser beam even though the position of the laser focus is fixed. This paper is the first to consider the behavior of the graphitization wave when the laser focus uniformly moves towards the laser, which results in the formation of conductive wires of unlimited length in diamond. It has been found that there is an initial transitional period in the wire growth, during which velocities of the laser focus and the wire front are equalized owing to the change in the distance between them. After stabilization of the graphitization front velocity, the axial fluence at the front of the growing wire also reaches a constant value. It has been found that the stable laser fluence at the wire front is practically independent of the laser pulse energy, but it grows with increasing velocity of the laser focus. Such increase finally leads to violation of the physical criterion of the continuous wire growth, since the axial fluence at the wire front becomes higher than the diamond breakdown threshold. It has been shown that the minimal fluence providing propagation of the graphitization wave in diamond can be used to predict the lateral wire dimension.

Simulation of the graphite–diamond transition in an isentropic process

An equation of state for graphite and diamond has been derived in wide density and temperature ranges. A set of equations for the graphite–diamond phase transition has been presented. Hugoniots for graphite and diamond have been calculated. Numerical simulation data for the graphite–diamond transition in the isentropic compression process using a metallic z-pinch with diamond saving have been reported.

Hydrothermal synthesis of well crystallized C8and diamond nanocrystals and pH-controlled C8↔ diamond phase transition

Diamond is a major carbon allotrope, whereas C8 is a rare and mysterious carbon allotrope with a super-cubic crystal structure that is even denser than diamond (by 15%). Little is known about the properties of C8 nanocrystals (NCs) because carbon NCs synthesized under usual conditions have either a graphitic or diamond crystal structure. We develop a hydrothermal method to synthesize well-crystallized C8 (∼6.1 nm) and diamond (∼4.7 nm) NCs. The crystal phase of the synthesized carbon NCs is determined by the pH of the initial precursor solution: acid solution results in diamond NCs while basic solution results in C8 NCs. These two phases of carbon NCs can be reversibly converted to each other by changing the pH from acidic to basic or vice versa under thermal conditions.

Study of a 3D diamond detector with photon and proton micro-beams

To form a 3D diamond detector electrodes were produced in diamond by a femtosecond laser-induced phase transition of diamond to graphite. The process parameters were varied to study the influence on electrode resistivity and induced stress. A technique for a relative measurement of stress induced in 3D diamond detectors is described. The detector was characterised with a 15keV photon micro-beam (Diamond Light Source, Oxford) and a 4MeV proton micro-beam (Ruđer Bošković Institute, Zagreb). The detector shows characteristics consistent with full charge collection. Spatially resolved transient current measurements were obtained with protons for the first time, and the results were compared to simulations of the detector.

Graphite to ultrafine nanocrystalline diamond phase transition model and growth restriction mechanism induced by nanosecond laser processing

To have a clear insight into nanocrystal growth from graphite to diamond upon high energy pulsed laser irradiation of graphite suspension, synthesis of ultrafine nanocrystalline diamonds with laser energy set up from 0.3 J to 12 J, repetition rate of 10 Hz has been studied. The method allows synthesizing ultrafine nanocrystalline particles continuously at the ambient temperature and normal pressure. The particle size is shown independent of laser energy, which is ultrafine and ranges in 2–6 nm. The theoretical grown size of nano-diamonds is found in well agreement with the experiment results. Four kinds of production were found: nano-diamond, spherical carbon nano-particles, flocculent amorphous carbon, and graphene nano-ribbon rolls. A solid-vapor-plasma-liquid coexistence model describing phase transition from graphite to diamond induced by nanosecond laser processing was proposed. Graphene nano-ribbon rolls might be the intermediate phase in the conversion from graphite to diamond.

Graphite–diamond phase transition induced by interlayer charge transfer exciton in visible region

AbstractA new photoinduced phase with interlayer σ bonds was experimentally discovered in a graphite crystal. We theoretically study the mechanism of this nonequilibrium phase transition, in terms of the proliferation of photogenerated interlayer charge transfer (CT) excitations in visible region. At the Franck–Condon state, the resultant electron–hole pair is quite unstable, being easily dissipated into the two‐dimensional (2D) electronic continuum as plus and minus free carriers. However, by a small probability, the electron and the hole are bound as an interlayer CT exciton, due to the Coulomb attraction between them. This exciton self‐localizes, contracting the interlayer distance and buckling the six membered ring of graphite, only around it. Due to plenty of excitations, interlayer σ bonds are cooperatively formed, and thus a tiny sp3 nanoscale domain appears.

Fabrication of polycrystalline diamond microtool using a Q-switched Nd:YAG laser

A Q-switched Nd:YAG laser (1,064 nm, 100 ns) was used to machine 2 × 1.5 × 0.5-mm rhombus-shaped tool inserts from a 60 × 0.5-mm circular disk of polycrystalline diamond. A systematic experimental study was undertaken to examine the effects of pulse repetition rate, feed rate, and number of laser passes on kerf, material removal rate, recast layer, surface morphology, and surface roughness. The optimal laser parameters for generating two-dimensional tool profiles were an average power of 3 W, a pulse repetition rate of 2 kHz, a feed rate of 1 mm/s, and a total of 45 laser passes. The beneficial results were a material removal rate of 0.02 mm3/min, kerf width of 27 μm, cutting edge radius of 6 μm, and surface roughness (Ra) of 0.625 μm. Recast layer formation, undulations, and striations were observed in the laser-cut regions. These features were attributed to the presence of a molten layer of cobalt binder, and amorphous carbon and graphite transitioned from diamond. An intriguing feature is the presence of fine particulate matter ranging in size from nanometers to a few micrometers in the laser-cut regions. It is believed that phase transition of diamond and cobalt during laser machining created thermal expansion mismatch stresses sufficient to fracture the solid into fine fragments.

Formation of diamond-like carbon by fs laser irradiation of organic liquids

The high intensities generated in femtosecond (fs) laser interactions offers the possibility of novel formation routes for diamond and diamond-like carbon materials. Coulomb explosion, a common phenomenon in intensive fs irradiation, has recently been shown to lead to a direct graphite–diamond phase transition on the surface of graphite. In this paper we report the results of fs irradiation of a variety of liquid organic compounds at intensities in the Coulomb explosion regime. The products of laser-induced chemistry under these conditions have been studied using visual/surface enhanced Raman and transmission electron microscopy (TEM). Surface enhanced Raman spectra/TEM show that an intermediate diamond phase, trans-polyacetylene chains and amorphous carbon are present after fs irradiation of liquid alkanes. We also find that the diamond component can be enhanced by irradiation in the presence of certain transition metals; however the origin of this effect is still uncertain. Diamond films deposited in this way are found to exhibit a nano-assembled structure involving individual nanodiamonds extending over an area of about 1 cm 2. This process represents a wet chemical method for room temperature formation of diamond-like carbon films

Size-Dependent Phase Transition of Diamond to Graphite at High Pressures

Size-dependent phase transition of nanosize diamond particles (NDPs) to graphite was found in the course of studies of high pressure high temperature (HPHT) induced transformations of NDPs at 8 GPa and in the temperature range of 873−1623 K. Analysis of the products of HPHT treatment of powder NDPs (with the mean size of 4.5 nm) by X-ray diffraction (XRD), scanning (SEM) and transmission electron (TEM) microscopies, and Raman spectroscopy have shown that at 8 GPa the increase of the treatment temperature results in a gradual enlargement of NDPs. According to XRD data, the temperature increase from 873 to 1473 K at the treatment time of 60 s gives rise to NDP mean size enlargement from initial 4.5 to 8.8 nm. However, the largest NDPs can reach the sizes of 13.5−14.0 nm at 1473 K, as measured directly from TEM images. At 1623 K, size-dependent phase transition of diamond to graphite has been observed. By applying the Arrenius equation to the particle mean size versus treatment temperature plot, activation e...

Z-pinch Acceleration of Massive Cylindrical Liners for Realization of Graphite-to-Diamond Phase Transition

Experimental results on Z-pinch acceleration of massive copper liners for generation of pressure pulses having a magnitude of 20-40 Gpa to investigate the graphite-to diamond phase transition are presented. The phase transition was induced by the cylindrical shock wave generated when the liner struck the steel ampoule filled with graphite or graphite containing composite. The crystal structure of carbon after shock compression was cleaned by the chemical methods and investigated using the electron microscopy analysis. As a result of the investigation it was found out that about 3% of the substance initially presented in the sample was transformed into a diamond phase having an average crystallite size of 1-2 /spl mu/m.

Effect of sample size on simulations and measurements of the phase transition in(GaAs)1−xGe2xand related alloys

Monte Carlo simulations have been made of the growth of (GaAs) 1 - x Ge 2 x in a direction by using the Davis-Holloway model with various sample sizes. Small samples give overestimates of the zinc blende order in thevicinity of the zinc blende to diamond phase transition. This is due to inadequate sampling over the mixture of phase and antiphase domains of GaAs that exists in this region. The effect is relevant to experimental determination of the critical composition at which the phase transition occurs because x-ray measurements on thin films are subject to significant limitations on sample size. Consequently experimental measurements will give overestimates of the Ge concentration at which the phase transition occurs because the zinc blende order will appear to exist beyond this point. These results also apply to other alloys between group III-V and group IV semiconductors and similar effects are to be expected with other growth directions that give a phase transition.

Computer Simulation of Physical and Mechanical Processes Running in the Reaction Cells of High-Pressure Installations in the Course of Synthesis of Diamonds

We study the problem of finite-element simulation of coupled nonlinear nonstationary processes of electric conductivity, heat conduction, and thermoplasticity taking into account the phase transitions in the materials. As a practical application, we develop a procedure of computer simulation of the process of spontaneous crystallization of diamonds. We solve the problem of determination of the fields of temperature, stresses, and concentrations of phases in the reaction volume of a high-pressure installation and in the local diamond–melt–graphite system in the course of crystallization of diamonds. It is discovered that these fields are strongly coupled, the solutions obtained for the reaction mixture and local system are characterized by a pronounced mutual influence, and the effect of self-regulation of pressure in the reaction zone exists and takes the form of oscillations of pressure about the line of the graphite–diamond phase transition.

Possible 3D Carbon Structures as Progressive Intermediates in Graphite to Diamond Phase Transition

Modeling successive contractions of conventional graphite structure in different directions, together with some chemical reactions (consisting of Csp2 to Csp3 hybridization changes, followed by new bonding between closing carbons), simulates a graphite to diamond phase transition. The stable intermediate structures were optimized by a molecular mechanic approach, giving rise to new hypothetical 3D crystal structures which progress along the transition in the order of increasing density from 2.22 to 3.30 gcm−3. There are graphite bilayers with planar and pyramidal Csp2, graphite bilayers with Csp2 and Csp3, diamonds with Csp2 and Csp3, and diamonds with only Csp3. We present eight different models: seven orthorhombic and one cubic Ia-3, the latter close to one structure proposed for supercubane C8. Additionally, some other noncrystalline models, made by optimizing unconstrained clusters from the above crystal models, are proposed. Although these structures are speculative, some of them present synthetic polycrystal X-ray diffraction with their highest peaks among those observed in reported experimental phases between graphite and diamond, which gives more reality to the proposed models.

Regrowth zones in laser annealed, radiation damaged diamond

Focused laser annealing of ion implanted diamond with a 15 μm diameter laser spot produces a variety of effects that depend on the power density of the laser. Channeling contrast muscopy (CCM) provides a relatively straightforward, rapid method to analyse the annealed regions of the diamond to characterize the effects. In order of increasing laser power density, effects that are observed include: regrowth of the end of range damage of the ion implantation, formation of a buried graphitic layer and complete graphitization of the surface of the diamond down to the bottom of the original damage layer. Information provided by CCM leads to an understanding of the causes of these effects and provides insight into the carbon phase diagram in the neighbourhood of the graphite to diamond phase transition. Analysis of the effects of laser annealing by CCM are complicated by the swelling of the diamond lattice caused by the original ion implantation, compaction following regrowth and the effect of the analysis beam irradiation itself.

STUDY OF ELECTRONIC STATES IN THE HYDROGEN-INDUCED STRUCTURAL PHASE TRANSITION FROM DIAMOND C(111)–(2*1) RECONSTRUCTION TO (1*1) STRUCTURE

The electronic structure in the structural phase transition of diamond C(111) surface from (2*1) reconstruction to (1*1) structure induced by adsorbed hydrogen has been studied by DV-X αab initio calculations. The role of the adsorbed hydrogen in the structural phase transition is also discussed.