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.     

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.     

Diamond formation by reduction of carbon dioxide at low temperatures

This Communication reports a low-temperature diamond synthesis technique, in which diamonds (10-250 mum) can form at a temperature as low as 440 degrees C by reduction of dense CO2 with metallic Na. The X-ray diffraction pattern of a powder sample shows three reflection peaks, indexed with 111, 220, and 311, corresponding unambiguously to cubic diamond. The Raman spectrum of the product exhibits an intense first-order peak at 1332 cm-1, which is the characteristic signature of the cubic diamond, indicating the formation of well-crystallized diamond. Carbon dioxide is a nontoxic low-energy molecule, abundant on earth. This novel reduction method could allow studies of large-size diamond growth using CO2 as the carbon source.     

INFLUENCE OF PLASMA DENSITY ON NUCLEATED DENSITY OF DIAMOND

The predeposition method for mereasing CH_4 concentration ininitial stage of diamond synthesis by plasma chemical vapor deposition.isused to enhance nucleated density of diamond films.The plasma parametersare diagnosed in situ using the Langmuir double probe.The relation betweenplasma ion density and nucleated densitv of diamond is revealed Increasingplasma ion density results in enhancement of nucleated density of diamondobviously     

Nano-Diamond Synthesis in Strong Magnetic Field

Diamond is one of the five known allotropes of carbon. Diamonds are important due to their esthetic beauty and excellently remarkable properties. Single crystalline diamond has been synthesized by catalytic processes under high temperature and high pressure. Polycrystalline diamond has been prepared by various activated hot filament and plasma chemical vapor deposition methods. Industrial synthesis of diamonds has been limited by the extreme synthetic conditions, purity, crystallinity, size and high cost. Synthetic advancement for future industrial production will require better understanding and exploitation of the mechanism of formation. Current mechanistic uncertainty arises out of limited consideration of atomic scale dynamics. A new comprehensive model by Little addresses atomic scale electronic processes with nonclassical significance of the intermediary states. This new nonclassical, electronic mechanistic perspective identifies the creation, stabilization and condensation of carbon and metal intermediates for pathways from carbon precursors thru solvated carbon and carbides to sp³ carbon condensation as diamond. In order to test these mechanistic ideas and exploit the impact for better diamond synthesis, catalytic carbon condensation has been explored in strong magnetic field in an effort to influence various radicals and high spin metal and carbon intermediates along the pathway for more efficient diamond condensation. In this effort, static magnetic fields in excess of 15 T are observed to direct these carbon and metal intermediary microstates to promote nano-diamond nucleation and growth at atmospheric pressure and temperature of 900°C. The observed nano-diamond formation is consistent with the predicted faster kinetics due to lower potential energies of intermediates along pathways to diamond. The predicted stability of nano-diamond relative to nano-graphite also accounts for the observed nano-diamond in this work. This novel use of static magnetic field for diamond synthesis is compared with advancements since the first synthetics bulk diamond formation by scientists at both ASEA (Sweden) and GE Research Laboratory, Schenectady, NY, 50 years ago. On the basis of this predicted, invented, and intrinsic magnetic influence on the dynamics of carbon condensation and the promise for faster, feasible single crystal diamond formation, this discovery of the use of strong magnetic field for diamond production is asserted a major advancement during this 50-year period since the first successful synthesis.     

Process study of thermal plasma chemical vapor deposition of diamond,part I: Substrate material,temperature, and methane concentration

Effects of process parameters on diamond film synthesis in DC thermal plasma jet reactors are discussed including substrate material, methane concentration and substrate temperature. Diamond has been deposited on silicon, molybdenum, tungsten, tantalum, copper, nickel, titanium, and stainless steel. The adhesion of diamond film to the substrate is greatly affected by the type of substrate used. It has been found that the methane concentration strongly affects the grain size of the diamond films. Increased methane concentrations result in smaller grain sizes due to the increased number of secondary nucleations on the existing facets of diamond crystals. Substrate temperature has a strong effect on the morphology of diamond films. With increasing substrate temperature, the predominant orientation of the crystal growth planes changes from the (111) to the (100) planes. Studies of the variation of the film quality across the substrate due to the nonuniformity of thermal plasma jets indicate that microcrystalline graphite formation starts at the corners and edges of diamond crystals when the conditions become unfavorable for diamond deposition.     

Core–Shell Composites Based on Partially Oxidized Blend of Detonation Synthesis Nanodiamonds

Russian Journal of Applied Chemistry - The results of a study on the production of graphite–diamond nanocompositions by partial oxidation of a detonation synthesis blend in aqueous solutions...     

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.     

A perspective on diamond composites and their electrochemical applications

Diamond composites have gained increasing interests because of their outstanding properties (e.g. robust mechanical properties, high conductivity and activity, good chemical and thermal stability, as well as excellent electrochemical properties) and their promising applications in a wide range of different fields. In this perspective, recent advances on the synthesis of diamond composites are summarized together with state-of-the-art progress in their electrochemical applications. Metal-diamond, alloy–diamond, oxide/carbide/nitride-diamond, sp² carbon/sp³ diamond, and organic-diamond composites are covered in the context of enhancing their performance of electrocatalysis, sensing, water treatment, supercapacitor, and photoelectrochemistry. Ongoing challenges and future perspectives of the synthesis and electrochemical applications of diamond composites are outlined and discussed.     

新金刚石稳定性及晶体结构模型的研究

利用碳黑催化法制备了新金刚石粉末, 并通过粉末X射线衍射(XRD)对不同时效处理后的新金刚石样品进行表征. 结果表明, 新金刚石是一种亚稳态的相, 在室温下, 随着放置时间的推移其晶体结构发生变化. 根据XRD分析和模拟的XRD图谱, 提出了用具有分数占位的“缺陷金刚石”模型来解释新金刚石结构随时间的变化规律. 在该模型中, 原子的占位数χ为0时, 为面心立方结构(FCC), χ为1时, 为金刚石结构. 密度泛函理论计算结果表明, 随χ的增加, 其结构的稳定性也增加. 可见, 新金刚石是由FCC碳向金刚石结构过渡的中间态结构.     

Diamond Nanowires: Fabrication,Structure, Properties,and Applications

C(sp³)? C‐bonded diamond nanowires are wide band gap semiconductors that exhibit a combination of superior properties such as negative electron affinity, chemical inertness, high Young’s modulus, the highest hardness, and room‐temperature thermal conductivity. The creation of 1D diamond nanowires with their giant surface‐to‐volume ratio enhancements makes it possible to control and enhance the fundamental properties of diamond. Although theoretical comparisons with carbon nanotubes have shown that diamond nanowires are energetically and mechanically viable structures, reproducibly synthesizing the crystalline diamond nanowires has remained challenging. We present a comprehensive, up‐to‐date review of diamond nanowires, including a discussion of their synthesis along with their structures, properties, and applications.     

Facile Synthesis of Poly(hydridocarbyne): A Precursor to Diamond and Diamond‐like Ceramics

Polycarbynes have previously been shown to be polymeric precursors to diamond and diamond‐like carbon. Here, we report an incredibly simple method for producing one of these polymers, poly(hydridocarbyne). The method simply requires chloroform, electricity, a solvent and an electrolyte. Since the polymer is soluble, the production of diamond objects of any shape is feasible. It is hoped that the ease of the synthesis will make these types of polymers accessible to scientists from all disciplines and that the potential applications for this material, which range from electrical to biomedical, are finally realized.     

金刚石和类金刚石的常温常压电化学合成

采用线性扫描伏安(LSV)\, X射线粉末衍射和拉曼光谱等方法对电化学还原法从CCl4\|NaCl\|\[BMIM\]BF4体系合成金刚石的可能性进行了研究. LSV研究结果表明, CCl4可在白金研究电极表面直接还原而不需要NaCl作为电子媒介. 采用恒电势电解的方法可在白金研究电极上获得黑色还原产物. 采用X射线粉末衍射和拉曼光谱对研究电极表面形成的黑色产物进行了表征, 在XRD图谱中可观察到金刚石的特征峰, 在拉曼光谱中1 332 cm-1附近可观察到金刚石结构的特征吸收峰, 表明产物中存在金刚石相. 这些结果表明, 采用电化学方法在常温常压下将CCl4转化为金刚石的方法是可行的.     

毫秒脉冲激光合成超细纳米金刚石

通过热力学和动力学的基本理论, 分析了毫秒脉冲激光照射石墨悬浮液合成超细纳米金刚石的机理. 在毫秒脉冲激光与石墨颗粒相互作用形成的碳蒸气羽中, 通过碳蒸气凝聚形成了金刚石核. 与纳秒脉冲激光相比, 毫秒脉冲激光具有较低的功率密度和较长的脉宽, 为金刚石核的生长提供了较小的过冷度, 使得金刚石核的生长速率减小; 而较小的生长速率也为金刚石表面形成sp2杂化结构提供了机会, 它可以有效降低金刚石核的表面能, 促使金刚石核稳定, 但表面的sp2杂化也阻止了金刚石核的外延. 以上两个原因决定了毫秒激光辐照石墨颗粒过程中只能获得超细的纳米金刚石.     

Model of the nucleation and growth of amorphous and crystalline films of diamond-like materials: The (100) plane

The nucleation and growth processes of amorphous and crystalline films of diamond-like materials deposited on the (100) planes of monocrystals with a diamond structure of diamond are modeled. The mechanisms of the formation of monolayers are analyzed from the results of calculating the potential energies of carbon clusters by means of modern semiempirical quantum chemical methods of computer chemistry. The nucleation and growth of an epitaxial film is considered as a replication process, and the deposition of amorphous films is described from the viewpoint of modifying the surface superstructure. Equations establishing the relationship between the structure of amorphous and crystalline films and conditions of the synthesis are obtained as a result of the performed studies.     

Thermal conductivity enhancement of copper–diamond composites by sintering with chromium additive

The thermal diffusivity (TD) and thermal conductivity (TC) of Cu–Cr–diamond composite materials were examined in the temperature range from 50 to 300 °C for diamond volume fractions of 22, 40, 50, 55, and 60 %. The samples were fabricated by the plasma pulse sintering (PPS) method. TC does not increase proportionally with the diamond fraction in the particular composite materials. The highest TD was determined for 50 % diamond volume fraction, and the evaluated TC reached 658 W m^?1 K^?1 at 50 °C. This article complements earlier articles concerning synthesis and characterization of the diamond–copper composites produced by the PPS method.     

Opportunities and challenges of thin-film boron-doped diamond electrochemistry for valuable resources recovery from waste: Organic,inorganic, and volatile product electrosynthesis

Thin-film boron-doped diamond (BDD) electrochemistry has made a tremendous progress in electrochemical synthesis/recovery of high-added value products from aqueous and gaseous waste streams. The distinguished electrochemical characteristic of this electrode has made this material emerging and successfully used in electrosynthetic transformations, besides its destructive and powerful performance in disinfection and detoxification of wastewaters. Organic electrosynthesis is achieved by the oxyl radical oxidation formed at BDD, peroxo compounds electrosynthesis is attained by oxidation of corresponding anions at the BDD surface, whereas electrochemical conversion of SO₂, CO₂, NO₃^?, and NH₃ to value-added products occurs by BDD cathodic reduction process. There are still some challenges needed to address for seamless scale-up and translation into application of this future technology.     

Anodic coupling of guaiacol derivatives on boron-doped diamond electrodes

The anodic treatment of guaiacol derivatives on boron-doped diamond electrodes (BDD) provides a direct access to nonsymmetrical biphenols, which would require a multistep sequence by conventional methods. Despite the destructive nature of BDD anodes they can be exploited for chemical synthesis.     

Highly selective electrosynthesis of biphenols on graphite electrodes in fluorinated media

The direct and selective phenol coupling reaction that provides biphenols still represents a challenge in organic synthesis. The recently developed electrosynthesis on boron-doped diamond anodes with fluorinated additives was developed further to allow the application to less-expensive electrodes and fluorinated media. This advanced protocol allows the highly selective anodic phenol coupling reaction on graphite with a broad scope.     

Adsorption of Dyes in Studying the Surface Chemistry of Ultradispersed Diamond

Russian Journal of Physical Chemistry A - The effect the surface chemistry of ultradispersed diamond (UDD) has on the adsorption of watersoluble dyes is considered. A comparison is made to...