Thermodynamics of diamond nucleation on the nanoscale

To have a clear insight into the diamond nucleation upon the hydrothermal synthesis and the reduction of carbide (HSRC), we performed the thermodynamic approach on the nanoscale to elucidate the diamond nucleation taking place in HSRC supercritical-fluid systems taking into account the capillary effect of the nanosized curvature of the diamond critical nuclei, based on the carbon thermodynamic equilibrium phase diagram. These theoretical analyses showed that the nanosize-induced interior pressure of diamond nuclei could drive the metastable phase region of the diamond nucleation in HSRC into the new stable phase region of diamond in the carbon phase diagram. Accordingly, the diamond nucleation is preferable to the graphite phase formation in the competing growth between diamond and graphite upon HSRC. Meanwhile, we predicted that 400 MPa should be the threshold pressure for the diamond synthesis by HSRC in the metastable phase region of diamond, based on the proposed thermodynamic nucleation on the nanoscale.     

Chemical nucleation for CVD diamond growth.

A new nucleation method to form diamond by chemically pretreating silicon (111) surfaces is reported. The nucleation consists of binding covalently 2,2-divinyladamantane molecules on the silicon substrate. Then low-pressure diamond growth was performed for 2 h via microwave plasma CVD in a tubular deposition system. The resulting diamond layers presented a good cristallinity and the Raman spectra showed a single very sharp peak at 1331 cm(-1), indicating high-quality diamonds.     

Phase transitions and the nucleation of catalytic nanostructures under the action of chemical, physical, and mechanical factors

Basic concepts of first-order phase transitions are formulated, and the state of the art in this field is considered using nanoparticle nucleation and growth as examples. General equations are presented to describe the nanoparticle size distribution function; the average nanoparticle radius; and the density, morphology, and composition of the new phase. Examples are given to illustrate the catalytic effect of nanoparticles on the kinetics of first-order phase transitions in multicomponent systems. The effect of the acidity (pH) of the medium on phase transitions in solutions and the effect of mechanical stress on the nucleation, evolution, and properties of quantum dots are considered.     

Molecular dynamics studies of homogeneous nucleation in solid-state transitions

Simulations of clusters containing 100 to 250 molecules of TeF₆ successfully reproduce the crystalline packing arrangements observed in electron diffraction investigations of large molecular clusters (～ 10⁴ molecules) of the same material. More remarkably, when the clusters are cooled step by step in MD computations at a rate of ca. 10¹¹ K/s they spontaneously undergo the same bcc to monoclinic phase transition that has been observed experimentally in supersonic flow, despite the million-fold difference in the timescales involved. The existence of such a correspondence over so many orders of magnitude, in itself, imposes severe constraints on what type of molecular mechanism can underlie the transformation. Even more revealing evidence about the molecular behavior associated with the phase change is provided by the simulations. They show the formation of the actual transition complexes along the transition pathway, namely, the critical nuclei of the new phase. These nuclei, which are made up of approximately 20 molecules, can be recognized in the midst of the surrounding matter. Techniques based on molecular orientations, involving Pawley-Fuchs projections and orientational angular distribution functions, make it possible to estimate the size of critical nuclei. One noteworthy result established in the simulations is that the solid-state transition temperature from bcc to monoclinic depends upon particle size in the same manner as does the freezing point.     

Phase transitions in polymer brushes

Liquid crystalline ordering in planar polymer brushes is investigated theoretically by numerical calculations within a self-consistent field approximation. The brushes are formed by macromolecules with mesogenic groups in main chain and immersed in a solvent. Existence of a microphase segregated brush (MSB) regime with a collapsed orientationally ordered intrinsic sublayer and a swollen external sublayer is shown. At small grafting density, the transition from a conventional brush state to the microphase segregated state is a jump-wise first order phase transition for a finite chain length (N). The magnitudes of the jumps in the average characteristics of the brush tend to zero in the limit N → ∞ since this transition occurs only in a vanishingly small part (∝ N^−1/2) of the brush. High compressibility of MSB brush is demonstrated. The origin of phase transition in planar brushes is discussed.     

Phase transitions in potassium nitrate

It is shown that the heat of transition of the phase change II → I at 129° on heating KNO₃ is dependent on the thermal history of the sample, since it involves two steps, viz., II→ III and III→ I at 2° interval. During cooling, the latter step is fast and truly reversible, though with a temperature hysteresis. The former step is sluggish and is dependent both on temperature and time. Our results indicate that KNO₃ can be used for calibration purpose only if the material has not been heated beyond 128° in the immediately preceding three hours.     

Phase transitions in chlorobenzene-cis-decalin mixtures

The dielectric constant ?' and loss tangent tan δ of chlorobenzene-cis-decalin mixtures have been measured in the temperature range 77 K to 330 K and frequency range 0.1 to 100 kHz. On cooling, ?' increases with decreasing temperature upto about 135 K, after which it drops rapidly with decreasing T followed by a slow decrease. This indicates that the liquid mixture goes to an amorphous phase which transforms to a glass phase of restricted dipole rotation below T_g; however, the peak in ?' is due to relaxation in the amorphous phase (α relaxation) and does not give an exact T_g. On heating, the behaviour of the cooling curve is retraced upto 160 K, after which ?' drops suddenly to a value lower than that at 77 K in the glass phase. This indicates the transition to a crystalline phase in which dipole rotational freedom is completely lost. The crystalline phase changes to a eutectic liquid phase of high ?' at a temperature (200 K) lower than the melting point of chlorobenzene and cis-decalin. Dielectric dispersion is observed only in the glass and amorphous phases. The dielectric relaxation time is independent of the concentration of chlorobenzene.     

Phase transitions in mixed-layer Aurivillius phases

The Aurivillius phases Bi₇Ti₄NbO₂₁, CaBi₈Ti₇O₂₇, and SrBi₈Ti₇O₂₇ were synthesized by solid-phase reactions. The results of high-temperature X-ray diffraction and thermal analyses of certain Aurivillius phases are presented. Principal regularities of thermal expansion of the synthesized compounds are established. A correlation between the Curie temperature and the structure of the compounds was found.     

X-ray photoelectron investigation of the surface of diamond and graphite

X-ray photoelectron spectroscopy was used to investigate the surface functional groups of natural diamond and graphite. An analysis of the spectra of the core shells made it possible to establish the presence of hydroxyl (ether), carbonyl, and carboxyl-like groups. It is shown that it is possible to identify acid oxyfunctional groups within the framework of a cluster approach.Translated from Teoreticheskaya i Éksperimental'naya Khimiya, Vol. 25, No. 4, pp. 490–495, July–August, 1989.     

Phase transitions and second-harmonic generation in sodium monothiophosphate

Oxythiophosphate compounds, which contain anions comprised of oxidized phosphorus bound to oxygen and sulfur, are scarce and, in general, poorly characterized. Although alpha-Na3PO3S has been known for over 60 years, little has been revealed about this compound. Here we present thermal analysis and second-harmonic-generation (SHG) efficiency of alpha-Na3PO3S. Details about its transformation to the high-temperature beta phase are discussed. Under vacuum, alpha-Na3PO3S undergoes one endothermic event at 553 degrees C upon heating and two exothermic events at 504 and 467 degrees C upon cooling. beta-Na3PO3S can be trapped upon quenching Na3PO3S from high temperature. We have observed that the beta phase converts back to the alpha phase at room temperature. Interestingly, relative humidity has been determined to catalyze this phase transformation. alpha-Na3PO3S oxidizes at 315 degrees C in air to produce Na4P2O7 and Na2SO4. Upon exposure to 1064-nm incident radiation, noncentrosymmetric alpha-Na3PO3S produces an SHG efficiency 200 times that of alpha-quartz and is nonphase-matchable (type 1).     

Phase transitions in novel disulphide-bridged alkoxycyanobiphenyl dimers

The synthesis, characterization and mesomorphic properties of the first examples of novel disulphide-bridged alkoxycyanobiphenyl dimers are reported. The thermal behaviour of these mesogens was investigated by polarizing optical microscopy, differential scanning calorimetry and X-ray diffractometry. The dimers with a shorter spacer exhibit only the nematic phase while dimers with a longer spacer display nematic as well as smectic phases. X-ray diffraction experiments reveal the intercalated structure of the SmA phase of these dimers and the presence of short range SmA-like order in the N phase (cybotactic nematic) of all the compounds, except the one with the shortest spacer.     

Phase transitions and rheology of aramid solutions

Clearing temperatures of solutions of poly(4-4′-benzanilidylene-terephthalamide) in concentrated sulphuric acid are presented as a function of polymer concentration and average molecular weight. The orientational order parameter <P ₂> is obtained from birefringence measurements. The experimental results are explained by a mean-field type theory similar to the Maier-Saupe model for thermotropic liquid crystals. Molecular flexibility, concentration and molecular weight are taken into account by using simple scaling factors. The birefringence induced by shear flow in an isotropic solution of poly(para-phenylene-terephthalamide) shows a strong pretransitional behaviour. This indicates the occurrence of a flow-induced phase transition.     

Phase transitions in calcium nitrate thin films

Calcium carbonate is a ubiquitous mineral and its reactivity with indoor and outdoor air pollutants will contribute to the deterioration of these materials through the formation of salts that deliquesce at low relative humidity (RH). As shown here for calcium nitrate thin films, deliquescence occurs at even lower relative humidity than expected from bulk thermodynamics and lower than the recommended humidity for the preservation of artifacts and antiques.     

Phase transitions in ternary caesium lead bromide

Phase transitions in ternary caesium lead bromide (CsPbBr₃) were studied by means of DSC, TMA and high temperature X-ray diffraction. The samples were prepared from the solution by water evaporation and from the melt. on the DSC curves as well as on the temperature dependence of the lattice constants of CsPbBr₃ only two effects were found belonging to the earlier published phase transitions at 88 and 130°C and no further effects. Linear thermal expansion coefficient α of individual CsPbBr₃ modifications were calculated from both TMA and high temperature X-ray diffraction. The structural parameters of the room temperature orthorhombic phase were refined and the results are presented. CsPbBr₃ prepared from the solution contained about 10% of CsPb₂Br₅ and so the DSC curve of pure CsPb₂Br₅ was also measured and an effect at a temperature of 68.5°C was found. This revised version was published online in July 2006 with corrections to the Cover Date.