Molten Catalyst Research Articles

Crack‐Free Drying of Ceramic Foams by the Use of Viscous Cosolvents

Ceramic foams are being used in numerous industrial applications such as high‐temperature thermal insulation, hot gas particulate filters, molten metal filters, catalyst support, and cores in high‐ temperature structural panels. The growing usage ceramic foams stems from the favorable properties such as low density, low thermal conductivity, and from their thermal and chemical stability. Recently Gauckler and colleagues developed a new method to produce ceramic foams. This method involves introducing gas bubbles into highly concentrated colloidal suspensions. The foam structure is stabilized by the suspended particles which align at the liquid/gas interface. Following the foaming stage, the wet green foams are dried and sintered. We have found that when making large foam bodies (i.e., >100 mm), cracks formation during the drying stage became a major issue. Following the rational of Scherer and Brinker in the drying of gels, we propose a model that explains the cracks formation in the foams and suggest a different procedure for successful drying of this system. Our solution is based on the addition of miscible cosolvents with low volatility to the aqueous phase. In this paper, we present the effectivity of different cosolvents in solving the cracking problems of large objects.

Tracking the catalyzed growth process of nanowires by in situ x-ray diffraction

Quasi-one-dimensional nanostructures of silicon, oxides, and other materials show great promise for a variety of applications. These nanostructures are commonly grown using metal catalyst nanoparticles. This paper investigates the growth mechanism of Au-catalyzed Si nanowires through in situ x-ray diffraction, and the results are compared to previously studied Au-catalyzed ZnO nanorods. The Si nanowires were found to grow from molten catalyst particles, however, the ZnO nanorods were found to grow from solid catalyst particles through a surface diffusion process. From this comparison, the relative bonding types of the catalyst and source material are determined to have a significant effect on the growth mechanism.

Carbon-nanopillar tubulization caused by liquidlike iron catalyst nanoparticles

Amorphous carbon nanopillars that we fabricated with electron-beam induced chemical vapor deposition were tublized by heating at 600°C-650°C in the presence of iron nanoparticles. The tublization process of the amorphous carbon nanopillar was observed in situ by using transmission electron microscopy and scanning transmission electron microscopy. A molten catalyst nanoparticle penetrated the amorphous carbon nanopillar, dissolving it and leaving a graphite track behind. The nanoparticle moved with its shape changing like the movement of an earthworm. Z-contrast images of the molten catalyst nanoparticle revealed that dissolved carbon atoms were diffused through its outer layer. The tubulization mechanism is a solid-quasiliquid-solid mechanism in which the carbon phase transformation is a kind of liquid phase graphitization of amorphous carbon catalyzed by liquefied metal-carbon alloy nanoparticles. [DOI: 10.1380/ejssnt.2006.401]

High pressure–high temperature growth of diamond crystals using split sphere apparatus

An overview of the application of crystal growth fundamentals in the high pressure–high temperature production of diamond by solvent/catalyst technique is presented. The process, also called temperature gradient process, makes use of a molten catalyst to dissolve carbon from a source (graphite or diamond powder) and transport the dissolved carbon to a growth site where they precipitate on a diamond seed. The pressure and temperature requirements for the process are generally around 5.0–6.5 GPa and 1300–1700 °C, depending on the chemistry of the solvent used and the desired crystal geometry. In spite of major progress in the science and technology of diamond growth, large scale commercial production of diamonds single crystals for jewelry or electronic applications has not been feasible until recently. This has been mainly due to the substantial cost associated with the presses needed, and the difficulties in controlling the growth parameters and catalyst chemistry. The recent developments in the commercial production of diamond single crystals utilizing the Split Sphere pressurization apparatus are discussed.

In situ observation of carbon-nanopillar tubulization process

We observed the tubulization process of amorphous carbon nanopillars using an in situ transmission electron microscopy. Amorphous carbon nanopillars were grown by electron-beam-induced chemical vapor deposition on an iron-doped carbon foundation. Amorphous carbon nanopillars were transformed into graphitic tubules by annealing at about 650°C in the presence of iron nanoparticles supplied from the iron-doped carbon foundation. A molten catalyst nanoparticle penetrated an amorphous carbon nanopillar, dissolving it and leaving a graphite track behind. This graphitic nanotube synthesis technique using nanofabrication promises to bring about a great advance in nanotube electronic devices.

Study on the gasification of wastepaper/carbon dioxide catalyzed by molten carbonate salts

This study focuses on waste paper gasification in carbon dioxide atmosphere with molten alkali carbonates including potassium, sodium, lithium carbonate or their intermixtures as catalyst. The molten catalysts is capable of facilitating a desired reaction (C+CO 2→2CO), which was hardly feasible even at a high temperature of 973 K if catalyst absences. It is that the catalysts which provides a gas–liquid interface between carbon and carbon dioxide, replacing original gas–solid, then allows the two reactants contact each other effectively. Further experimental results demonstrate that the intermixture carbonates exhibit strongly enhancement on catalytic activity than any carbonate salts in the form of single. The reaction rate depends on temperature evaluating manners, a rapid heating processes is favorable to the aimed reaction. With respect to recycling of carbon dioxide, the process provides a conversion in 30%.

In situ observation of carbon-nanopillar tubulization caused by liquidlike iron particles.

The tubulization process of amorphous carbon nanopillars was observed in situ by transmis-sion electron microscopy. Amorphous carbon nanopillars were transformed into graphitic tubules by annealing at 650-900 degrees C in the presence of iron nanoparticles. A molten catalyst nanoparticle penetrated an amorphous carbon nanopillar, dissolving it, and leaving a graphite track behind. An iron nanoparticle moved with its shape changing like an earthworm. We concluded that the tubulization mechanism is a solid-(quasiliquid)-solid mechanism where the carbon phase transformation is a kind of liquid phase graphitization of amorphous carbon catalyzed by liquefied metal-carbon alloy nanoparticles.

Repeated use of molten carbonate catalysts for gasification of carbonaceous materials by carbon dioxide

Reactions of carbonaceous materials (activated carbon, graphite, and lignin) with carbon dioxide were carried out at 973 K using alkali metal carbonates (m.p. 891∼1164 K) and eutectic mixtures of these compounds (m.p. 669∼787 K) as catalysts. The reaction occurred when the carbonate catalysts were physically mixed with carbonaceous materials and the melting points of the catalysts were lower than the reaction temperature. However, the reaction did not proceed with physically mixed catalysts when the melting points of the catalysts were higher than the reaction temperature. Repeated use of the molten catalyst was also studied to investigate the possibility of continuous gasification of carbonaceous materials. After the activated carbon (or lignin) was consumed in a batch reactor, the same amount was added in the reactor containing the catalyst and the reaction started again. It was found that the eutectic molten catalyst could be reused without any loss of its catalytic activity by repeating this operation.

Microstructure of high-pressure-synthesized diamonds under rapid quenching and electron irradiation

A type of synthetic diamond single crystal about 0.4–0.5 mm in dimension prepared under high pressure–high temperature (HPHT) in the presence of a FeNi molten catalyst was quenched from HPHT and irradiated with 300 keV electrons at room temperature. Transmission electron microscopy was employed to examine the microstructure of the diamond single crystal before and after electron irradiation. It was found that there exists a large amount of cellular interfaces in the quenched diamond sample, which indicates the growth condition of the diamond under HPHT. Hexagonal dislocation loops about several tens of nanometers in dimension were observed in the high-pressure-synthesized diamond single crystal before electron irradiation, which strongly suggests that a number of vacancies were quenched-in due to rapid quenching from high temperature at the end of diamond synthesis, and were aggregated in the synthetic diamond to form vacancy disks on the (111) plane, the collapse of such vacancy disks forming vacancy-type dislocation loops. After electron irradiation, it was found that defect clusters present as interstitial-character dislocation loops were formed in the electron-irradiated region of the diamond. The interstitial dislocation loops grow with increase of the irradiation time. The present study, in comparison to previous work on ion implantation on diamond, indicates that electron irradiation does not induce a phase transformation but produces interstitial dislocation loops due to the migration of interstitial atoms and vacancies. The result of the study directly indicates that interstitials and vacancies in diamond are mobile at room temperature under electron irradiation. Nitrogen, as the most important kind of impurity contained in the HPHT as-grown diamond, probably acts as nucleation of the interstitial loops.

Some aspects of diamond crystal growth at high temperature and high pressure by TEM and SEM

Diamond crystal growth process at high temperature–high pressure (HPHT) from Fe–Ni–C system was investigated by transmission electron microscopy (TEM) and scanning electron microscopy (SEM). Parallel growth cellular interfaces in the growing diamond crystals were directly observed by transmission electron microscopy (TEM) for the first time. The presence of the cellular growth interface indicates that the diamond crystal grows from solution of graphite in molten catalyst and there exists a narrow constitutional supercooling zone in front of the growth interface during the diamond crystal growth. The formation of the cellular interface may be related to the solubility difference between diamond and graphite in molten catalyst. The successive parallel arrays of layers with cellular interface by TEM suggest that the diamond grows from solution layer by layer, which can be further confirmed by morphologies on the surface of growing diamonds and as-grown diamonds obtained by SEM. Some particle clusters were found on the growing parallel layers and diamond surface by TEM and SEM, which may be the diamond atom clusters transmitted to the growing diamond through diffusion. This study provides direct evidence that the diamond is formed at HPHT through graphite continuous dissolution in the molten catalyst to form a colloidal solution, transition of graphite to diamond under the action of the catalyst, diffusion of the diamond atom clusters to the growing diamond, and collection or unification of the diamond subcritical atom clusters on the growing diamond crystal.

Interface Instability of Diamond Crystals at High Temperatureand High Pressure

Diamond growth instability at high temperature and high pressure (HPHT)has been elucidated by observing the cellular interface in diamondcrystals. The HPHT diamond crystals grow layer by layer from solutionof carbon in the molten catalyst. In the growth of any other crystals fromsolution, the growth interface is not stable and should be of thegreatest significance to understand further the diamond growthmechanism. During the diamond growth, the carbon atoms are delivered tothe growing diamond crystal by diffusion through a diamondcrystal-solution boundary layer. In front of the boundary layer, thereis a narrow constitutional supercooling zone related to thesolubility difference between diamond and graphite in the moltencatalyst. The diamond growth stability is broken, and the flat orplanar growth interface transforms into a cellular interface due to thelight supercooling. The phenomenon of solute trails in the diamonds wasobserved, the formation of solute trails was closely associated withthe cellular interface.

Rate-Limiting Processes in the Formation of Single-Wall Carbon Nanotubes: Pointing the Way to the Nanotube Formation Mechanism

Four rate-limiting processes for the formation of single-wall carbon nanotubes (SWCNT) could be identified by varying furnace temperature, gas type, and pressure in a pulsed-laser evaporation setup. One rate-limiting process accounts essentially for all relevant gas-pressure dependencies and can be quantitatively described using a single gas-specific constant. One thermally activated process is related to fullerene formation, whereas another process, following a T2-law, is discussed in terms of the diffusion of carbon through molten catalyst nanoparticles. The data provide strong support for an “undercooled melt” mechanism of nanotube formation.

Electron microscopy of high-temperature and high-pressure as-grown diamond crystals

Electron microscopy was used to investigate microstructures of diamond single crystals prepared at high temperature and high pressure (HPHT) from the Fe–Ni–C system. Analysis through selected-area electron diffraction pattern suggests that the crystal structure of the HPHT-grown diamond is cubic; polycrystalline diamonds are contained in the diamond. Etch pits on the (111) surface of the diamond by scanning electron microscopy revealed the dislocation motion under the action of applied stress. An array of parallel dislocation lines taken with reflection of [110] was observed directly by transmission electron microscopy (TEM). Fringe distortions and concentric dislocation loops were examined by rotation moiré images, which arose from two overlapping (111) close-packed planes rotated with respect to each other at a 5° angle. The parallel dislocation lines, distorted fringes, and concentric dislocation loops might have derived from the micro-inclusions. The parallel layers with growth cellular interface by TEM provided direct evidence that the diamond grew from solution of carbon in the molten catalyst at HPHT and the growth interface diamond was not stable.

Characterization of a growth-front interface in a HPHT-grown diamond crystal by transmission electron microscopy

The growth-front interface of a diamond single crystal, which was grown from the Fe-Ni-C system under high pressure and high temperature (HPHT), has been directly observed by transmission electron microscopy (TEM) for the first time. The presence of a cellular interface suggests that the diamond is grown from solution and there exists a narrow supercooling zone in front of the solid–liquid interface. Diamond-growth parallel layers were also found, which indicates that the diamond grows from solution layer by layer. It provides direct evidence that the diamond is synthesized through graphite dissolution and transformation to subcritical diamond particles in a molten catalyst, diamond subcritical particle connection to form diamond clusters, diffusion of the diamond clusters to the growing diamond, and unification of the diamond clusters on the growing diamond crystal.

Formation and crystal structure of metallic inclusions in a HPHT as-grown diamond single crystal

One of the most important characteristics associated with crystal growth technology is the entrapment of inclusions by the growing crystal. Diamond single crystals prepared under high temperature-high pressure (HPHT) usually contain metallic inclusions. In the present paper, metallic inclusions in a diamond grown from a Fe-Ni-C system using the HPHT method have been, for the first time, systematically examined by transmission electron microscopy (TEM). Energy dispersive X-ray spectrometry (EDS) , combined with selected area electron diffraction (SAD) patterns, has been used to identify the chemical composition and crystal structure of the metallic inclusions. The metallic inclusions were found to be composed mainly of cubic γ-(FeNi), face-centered cubic (FeNi)23C6, ortho-rhombic Fe3C and hexagonal Ni3C, which may have been formed through the entrapment of molten catalyst by the growth front or through reaction of the trapped melt with contaminants in the diamond.

Periodic instability in growth of chains of crystalline-silicon nanospheres

Chains of crystalline-silicon nanospheres were formed by a self-organized process via an extension of the vapor–liquid–solid mechanism using gold as catalyst. The diameter of nanospheres and the spacing between nanospheres were controlled by changing the growth conditions: the diameter and the spacing were small when the gold deposit was thin and the growth temperature was low. The spacing between the crystalline-silicon nanospheres was kept nearly proportional to the diameter of the chain. In order to reveal the growth mechanism of the chain, we simulated the periodic instability in chain growth, taking account of the supersaturation of silicon in vapor phase and molten catalyst, the kinetic roughening transition at the liquid–solid interface, and the interface tensions. Simulated instability was consistent with the experimental observations.

Growth Mechanism of Chains of Silicon Nanocrystallites

AbstractThe growth mechanism of chains of silicon nanocrystallites was investigated. Energy-filtered transmission electron microscopy observations provided strong evidence that the chains were formed as a result of surface oxidation of silicon nanowires. In addition, the periodic instability in the wetting property of molten catalysts was simulated numerically.

VLS Growth of Si nanowhiskers on a H-terminated Si{111} surface

AbstractWe have grown Si nanowhiskers on a Si{1111} surface via the vapor-liquid-solid (VLS) mechanism. The minimum diameter of the crystalline is 3nm and is close to the critical value for the effect of quantum confinement. We have found that many whiskers grow epitaxially or non-epitaxially on the substrate along the 〈112〉 direction as well as the 〈111〉 direction.In our growth procedure, we first deposited gold on a H-terminated Si{111} surface and prepared the molten catalysts of Au and Si at 500°C. Under the flow of high pressure silane gas, we have succeeded in producing the nanowhiskers without any extended defects. We present the details of the growth condition and discuss the growth mechanism of the nanowhiskers extending along the 〈112〉 direction.

Reactivities of transition metals with carbon: Implications to the mechanism of diamond synthesis under high pressure

The ability for a transition metal to react with carbon increases with its number of electron vacancies in d-orbitals. Elements (e.g. Cu, Zn) with no dvacancies are inert relative to carbon. Elements (e.g. Fe, Co) with few d-vacancies are effective carbon solvents. Elements (e.g. Ti, V) with many d-vacancies are carbide formers. Transition metals with intermediate reactivities can attract carbon atoms in graphite without forming a carbide. Such a moderation of interaction may catalyze the graphite → diamond transition in the diamond stability field and its back conversion in graphite stability field. The catalytic conversion of graphite to diamond, under high pressure in a molten metal, proceeds by nucleation and growth. Graphite will first be disintegrated into flakes by the invasion of liquid metal. These flakes are then puckered by the catalytic action of liquid metal that pulls every other carbon atom away from the basal planes of graphite. The puckering converts a graphite flake into a diamond nucleus that grows by feeding on either carbon atoms dissolved, or graphite flakes suspended in the molten catalyst. The capability of a catalyst to nucleate and grow diamonds under high pressure may be modeled by its atomic size and electronic configuration. This model predicts that the most powerful elemental catalysts are Co, Fe, Mn, Ni and Cr. These transition metals are the most commonly used catalyst components for the commercial production of synthetic diamond under high pressure. The catalytic power, as already determined is based on a microscopic mechanism described in this research, also correlates well with the activation energies calculated from macroscopic kinetics data available in literature

Coal hydroliquefaction using MoCl 3- and NiCl 2-containing salts as catalysts: difference in catalysis between solid and molten catalysts

Both MoCl 3- and NiCl 2-LiCl-NaCl-KCl quaternary solid salts and both MoCl 3- and NiCl 2-LiCl-KCl ternarly molten salts were examined as catalysts for hydroliquefaction of subbituminous Wandoan coal at 400 °C for 1 h in a batch reactor with an initial H 2 pressure of 9.8 M Pa. Conversions (to pyridine solubles) and yields of hexane solubles (HS) ranged from 80 to 92 wt% and from 55 to 65 wt%, respectively. The results suggested that both solid and molten catalysts are highly effective for hydrocracking of coal, with the latter being superior in terms of conversion and HS yield. H 2 consumption of the reaction suggested that the hydrogenating ability of the catalysts decreases in the following order: NiCl 2 (molten) > MoCl 3 (molten) > MoCl 3 (solid) > NiCl 2 (solid). Cracking ability of these catalysts decreases in the order: MoCl 3 (molten) > MoCl 3 (solid) > NiCl 2 (molten) > NiCl 2 (solid). It was tentatively concluded that the molten catalysts have higher hydrogenating and cracking abilities for coal hydroliquefaction than the corresponding solid ones.