Physico-chemical synthesis of diamond in metastable range

The present paper discusses the synthesis of diamond in its metastable region from carbonaceous gases, in particular methane. The diamond synthesis on seed crystals is made possible owing to the orientation effect of surface forces on the formation of a new phase. Discussed is also the joint crystallization of diamond and graphite. The basic experiments were made on highly dispersed diamond powders. The growth of diamond in kinetic and diffusion regions has been studied. Presented are results of studying the fractionation of stable carbon isotopes when growing a diamond.     

Nucleation of bulk phases in the HCl/H2O system

We report experimental results on the low-temperature uptake of HCl on H(2)O ice (ice). HCl was deposited on the surface at greater than monolayer amounts at 85 K, and the ice substrate was heated. The temperature dependence of the HCl vapor pressure from this phase was measured from 110 to 150 K, with the nucleation of a bulk hydrate phase observed at 150 K. Measurements were conducted in a closed system by simultaneous application of gas phase mass spectrometry and surface spectroscopy to characterize vapor/solid equilibrium and the nucleation of bulk hydrate phases. Combining the nucleation data reported here with data we reported previously (180 to 200 K) and data from two other laboratories (165 and 170 K), the thermodynamic boundaries for the nucleation of both the metastable bulk solution and bulk hydrate phases subsequent to monolayer adsorption of HCl have been determined. The nucleation of the metastable bulk solution phase occurs promptly at monolayer coverage at the ice/liquid coexistence boundary on the binary bulk phase diagram. The nucleation of the bulk hexahydrate occurs from this metastable solution along a locus of points defining a state of constant solution free energy. This measured free energy is -51.2 +/- 0.9 kJ/mol. Finally, the temperature dependence of the HCl vapor pressure from the low-temperature phase is reported here for the first time and is consistent with that of the metastable solution predicted by this thermodynamic model of uptake, extending the range of validity of this model of adsorption followed by bulk solution and hydrate nucleation to a lower bound in temperature of 110 K.     

Phase transitions and nucleation in diamond and graphite

Conclusions It has been shown that the region of diamond thermodynamic stability depends on the dimensions of the carbon particles in small-particle systems. At the high supersaturation realized in vacuum condensation and chemical decomposition, there is an increase in the ratio of the probabilities for diamond and graphite nucleation.Translated from Izvestiya Akademii Nauk SSSR, Seriya Khimicheskaya, No. 6, pp. 1184–1188, June, 1979.     

The fundamentals of the chemical crystallization of diamond from the gas phase

The basic steps of researches for the synthesis of diamond from the gas phase is reviewed. The experimental results obtained on the thermal activation of a medium by using diamond powders as nucleation centers are considered. It has been established that the diamond rate from hydrocarbons exceeds that of graphite. Fractionation of the stable carbon isotopes is observed. The presence of hydrogen influences differently the growth of diamond and that of graphite.

The method of periodical pulse supersaturations is described, enabling one to obtain epitaxial films and isometrical diamond crystals. The method of activation of the gas medium by using an electric discharge allowed to obtain polycrystalline diamond films on the surface of metals and semiconductors. A possibility of the homogeneous formation of diamond in the gas phase has been established. The possibility of realization of this process in the cosmic space is discussed.     

不同压力下碳氢体系低压金刚石的非平衡定态系列相图

系统压力是低压金刚石薄膜生长实验中重要的实验参数，如果采用合理的计算方法定量化地预测出压力对金刚石薄膜生长条件的影响，则可以直接用于指导其实验研究。本文报道根据非平衡热力学耦合理论模型绘制了C-H体系金刚石生长投影相图，经与大量实验结果比较相一致，并系统地计算了压力变化的碳氢体系金刚石生长非平衡定态截面相图，得到了金刚石生长区随压力变化的规律。计算得到的相图与经典平衡相图有本质不同，均有金刚石生长区，因而可以合理解决金刚石低压下连续生长而石墨被腐蚀与经典平衡热力学之间的矛盾。本文的计算结果可以为金刚石生长实验提供定量化的压力条件的选择和优化实验条件。     

Phenomenology of Polymorphism: IV. The Trimorphism of Ferrocene and the Overall Metastability of Its Triclinic Phase

Structural and thermodynamic data reported in the literature for the three crystalline phases of ferrocene are used to build a topological representation of its (p, T) phase diagram. Two phases (orthorhombic and monoclinic) exhibit stable phase regions whose temperature ranges increase with increasing pressure. The triclinic phase is found to be metastable at any temperature and pressure. In fact, in the (p, T) diagram, it is not associated with any state of lowest energy although it transforms into the monoclinic phase according to a transition which is reversible. This transition occurs in the phase region where the orthorhombic phase is the stable one.     

Universal Asymptotics of the Thermodynamic Characteristics of the Nucleation on Small Macroscopic Nuclei of Soluble Surfactants

Asymptotic behavior of thermodynamic characteristics of nucleation on small macroscopic nuclei of soluble surfactants at their complete dissolution in a nucleating droplet is studied. It is taken into account that, in the region of small sizes of nuclei and corresponding small sizes of critical nuclei of liquid phase, the chemical potential of condensate and the work of droplet formation are affected by the presence of dense surfactant adsorption monolayer on the droplet surface. It is shown that, as the limiting surface area per surfactant molecule in adsorption monolayer increases, the behavior of thermodynamic characteristics of nucleation in the region of small nucleus sizes is characterized by the transition from asymptotics at the adsorption of almost all substance comprising nucleus in a monolayer to the asymptotics at constant adsorption. The study performed is not limited by the selection of specific adsorption isotherms; therefore, the obtained asymptotic dependences of thermodynamic characteristics on the nucleus size can be considered as universal for the heterogeneous nucleation on the nuclei of soluble surfactants.     

Phase stability of nanocarbon in one dimension: nanotubes versus diamond nanowires

Since their discovery in 1990, the study of sp2 bonded carbon nanotubes has grown into a field of research in it's own right; however the development of the sp3 analog, diamond nanowires, has been slow. A number of theoretical models have been proposed to compare the relative stability of diamond and graphite at the nanoscale; and more recently, to compare nanodiamonds and fullerenes. Presented here is a study of the phase stability of nanocarbon in one-dimension. The structural energies of carbon nanotubes and diamond nanowires have been calculated using density functional theory within the generalized gradient approximation, and used to determine the atomic heat of formation as a function of size.     

Sensitivity of nucleation phenomena on range of interaction potential

Theoretical and computational investigations of nucleation have been plagued by the sensitivity of the phase diagram to the range of the interaction potential. As the surface tension depends strongly on the range of interaction potential and as the classical nucleation theory (CNT) predicts the free energy barrier to be directly proportional to the cube of the surface tension, one expects a strong sensitivity of nucleation barrier to the range of the potential; however, CNT leaves many aspects unexplored. We find for gas-liquid nucleation in Lennard-Jones system that on increasing the range of interaction the kinetic spinodal (KS) (where the mechanism of nucleation changes from activated to barrierless) shifts deeper into the metastable region. Therefore the system remains metastable for larger value of supersaturation and this allows one to explore the high metastable region without encountering the KS. On increasing the range of interaction, both the critical cluster size and pre-critical minima in the free energy surface of kth largest cluster, at respective kinetic spinodals, shift towards smaller cluster size. In order to separate surface tension contribution to the increase in the barrier from other non-trivial factors, we introduce a new scaling form for surface tension and use it to capture both the temperature and the interaction range dependence of surface tension. Surprisingly, we find only a weak non-trivial contribution from other factors to the free energy barrier of nucleation.     

Nucleation in a simple model for protein solutions with anisotropic interactions

A lattice analog of density functional theory is used to explore the structural and thermodynamic properties of critical nuclei in mixtures of particles with attractive anisotropic interactions. Protein molecules are assumed to occupy the sites on a regular cubic lattice, with effective directional interactions that mimic hydrogen bonding and the solvation forces induced by water. Interaction parameters are chosen to qualitatively reproduce the phase behavior of protein solutions. Our model predicts that critical nuclei of the solidlike phase have nonspherical shapes, and that their specific geometry depends on the nature of the anisotropic interactions. Molecules tend to align in distinctive ways in the core and in the interfacial region of these critical clusters, and the width and structure of the interface are highly affected by the presence of a metastable fluid-fluid critical point. Close to the critical region, the height of the barrier to nucleation is strongly reduced; this effect is enhanced by increasing the anisotropy of the intermolecular interactions. Unlike systems with short-range isotropic interactions, nucleation in our model is initiated by highly ordered clusters in which the order-disorder transition is confined to the interfacial region.     

Phase field theory of interfaces and crystal nucleation in a eutectic system of fcc structure: II. Nucleation in the metastable liquid immiscibility region

In the second part of our paper, we address crystal nucleation in the metastable liquid miscibility region of eutectic systems that is always present, though experimentally often inaccessible. While this situation resembles the one seen in single component crystal nucleation in the presence of a metastable vapor-liquid critical point addressed in previous works, it is more complex because of the fact that here two crystal phases of significantly different compositions may nucleate. Accordingly, at a fixed temperature below the critical point, six different types of nuclei may form: two liquid-liquid nuclei: two solid-liquid nuclei; and two types of composite nuclei, in which the crystalline core has a liquid "skirt," whose composition falls in between the compositions of the solid and the initial liquid phases, in addition to nuclei with concentric alternating composition shells of prohibitively high free energy. We discuss crystalline phase selection via exploring/identifying the possible pathways for crystal nucleation.     

Stresses inside critical nuclei

The usual derivation of classical nucleation theory is inappropriate for crystal nucleation. In particular, it leads to a seriously flawed estimate of the pressure inside a critical nucleus. This has consequences for the prediction of possible metastable phases during the nucleation process. In this paper, we reanalyze the theory for crystal nucleation based on the thermodynamics of small crystals suspended in a liquid, due to Mullins (J. Chem. Phys. 1984, 81, 1436). As an illustration of the difference between the classical picture and the present approach, we consider a numerical study of crystal nucleation in binary mixtures of hard spherical colloids with a size ratio of 1:10. The stable crystal phase of this system can be either dense or expanded. We find that, in the vicinity of the solid-solid critical point where the crystallites are highly compressible, small crystal nuclei are less dense than large nuclei. This phenomenon cannot be accounted for by either classical nucleation theory or by the Gibbsian droplet model.     

Hidden minima of the gibbs free energy revealed in a phase separation in polymer/surfactant/water mixture

We observed a very unusual kinetic pathway in a separating C(12)E(6)/PEG/H(2)O ternary mixture. We let the mixture separate above the spinodal temperature (cloud point temperature) for some time and next cool it into a metastable region of a phase diagram, characterized by two minima of the Gibbs potential, one corresponding to the homogeneous mixture and one to the fully separated PEG-rich and C(12)E(6)-rich phases. Despite the fact that in the metastable region the thermodynamic equilibrium corresponds to the separated phases (global minimum of the Gibbs free energy), we observe perfect mixing of the initially separated phase. The homogeneous state, obtained in this way, does not separate, if left undisturbed. However, many cooling-heating cycles or full separation with visible meniscus above the cloud point temperature induce the phase separation in the metastable region. The metastable region can exist tens of degrees below the cloud point temperature. This effect is not observed in the binary mixture of C(12)E(6)/H(2)O.     

Experimental Gibbs free energy considerations in the nucleation and growth of single-walled carbon nanotubes

Gas feed composition and reaction temperature were varied to identify the thermodynamic threshold conditions for the nucleation and growth of SWNT from methane on supported Fe/Mo catalyst. These reaction conditions closely approximate the pseudoequilibrium conditions that lead to the nucleation and growth of SWNT. These measurements also serve to determine an upper limit of the Gibbs free energy of formation for SWNT. The Gibbs free energy of formation relative to graphite is in good agreement with literature values predicted from simulations for SWNT nuclei containing approximately 80 atoms, while considerably larger than that predicted for bulk (5,5) SWNT. Our estimate over the range 700 to 1000 degrees C of 16.1 to 13.9 kJ mol(-1) falls between the results of these simulations and literature values for diamond.     

Free-energy landscape of nucleation with an intermediate metastable phase studied using capillarity approximation

Capillarity approximation is used to study the free-energy landscape of nucleation when an intermediate metastable phase exists. The critical nucleus that corresponds to the saddle point of the free-energy landscape as well as the whole free-energy landscape can be studied using this capillarity approximation, and various scenarios of nucleation and growth can be elucidated. In this study, we consider a model in which a stable solid phase nucleates within a metastable vapor phase when an intermediate metastable liquid phase exists. We predict that a composite critical nucleus that consists of a solid core and a liquid wetting layer as well as pure liquid and pure solid critical nuclei can exist depending not only on the supersaturation of the liquid phase relative to that of the vapor phase but also on the wetting behavior of the liquid surrounding the solid. The existence of liquid critical nucleus indicates that the phase transformation from metastable vapor to stable solid occurs via the intermediate metastable liquid phase, which is quite similar to the scenario of nucleation observed in proteins and colloidal systems. By studying the minimum-free-energy path on the free-energy landscape, we can study the evolution of the composition of solid and liquid within nuclei which is not limited to the critical nucleus.     

Simultaneous growth of diamond and nanostructured graphite thin films by hot-filament chemical vapor deposition

Diamond and graphite films on silicon wafer were simultaneously synthesized at 850 °C without any additional catalyst. The synthesis was achieved in hot-filament chemical vapor deposition reactor by changing distance among filaments in traditional gas mixture. The inter-wire distance for diamond and graphite deposition was kept 5 and 15 mm, whereas kept constant from the substrate. The Raman spectroscopic analyses show that film deposited at 5 mm is good quality diamond and at 15 mm is nanostructured graphite and respective growths confirm by scanning auger electron microscopy. The scanning electron microscope results exhibit that black soot graphite is composed of needle-like nanostructures, whereas diamond with pyramidal featured structure. Transformation of diamond into graphite mainly attributes lacking in atomic hydrogen. The present study develops new trend in the field of carbon based coatings, where single substrate incorporate dual application can be utilized.     

Thermal vacancy effects on the thermodynamic properties and stability of fullerites

We study the influence of thermal vacancies on equilibrium thermodynamic properties of the high-temperature phase of fullerites taking into account the strong anharmonicity of the lattice vibrations. Treating a crystal with point defects as a quasi-multicomponent system and using the correlative method of the unsymmetrized self-consistent field and the Girifalco interaction potential for the molecular subsystem, we have obtained the vacancy formation parameters for the C(60) fullerite. We also take into account the divacancies. The influence of the lattice defects on the specific heats of fullerites is negligible, since a dominant contribution to them is given by intramolecular degrees of freedom. We have calculated the contributions of vacancies to various thermodynamic properties, the volume thermal expansion coefficient, the isothermal bulk modulus, and the components of the isothermal elastic tensor, depending on the temperature and pressure. Near the estimated triple point, these contributions run to more than 10% and increase still further at a metastable region. We also discuss the influence of defects on the thermodynamic stability of fullerites.     

Nature of phase transitions in crystalline and amorphous GeTe-Sb2Te3 phase change materials

The thermodynamic nature of phase stabilities and transformations are investigated in crystalline and amorphous Ge(1)Sb(2)Te(4) (GST124) phase change materials as a function of pressure and temperature using high-resolution synchrotron x-ray diffraction in a diamond anvil cell. The phase transformation sequences upon compression, for cubic and hexagonal GST124 phases are found to be: cubic → amorphous → orthorhombic → bcc and hexagonal → orthorhombic → bcc. The Clapeyron slopes for melting of the hexagonal and bcc phases are negative and positive, respectively, resulting in a pressure dependent minimum in the liquidus. When taken together, the phase equilibria relations are consistent with the presence of polyamorphism in this system with the as-deposited amorphous GST phase being the low entropy low-density amorphous phase and the laser melt-quenched and high-pressure amorphized GST being the high entropy high-density amorphous phase. The metastable phase boundary between these two polyamorphic phases is expected to have a negative Clapeyron slope.