Jose Maria Calderon Moreno

Carbon nanocells and nanotubes grown in hydrothermal fluids

Abstract For the first time multiwall carbon nanocells ( ∅ nm ) and multiwall carbon nanotubes have been artificially grown in hydrothermal fluids from amorphous carbon, at temperatures below 800°C, in the absence of metal catalysts. Carbon nanocells were the result of bubble growth of graphitic multiwalls at 600°C. Pristine multiwall nanotubes grow at higher temperatures. At 800°C, the condensed solids consist of defect-free sp 2 -bonded multiwall nanocrystals. Hot hydrothermal fluids seem essential for the growth of curled patches of sp 2 -bonded carbon at low temperatures. Hydrothermal synthesis of finite sp 2 -bonded nanotubes and carbon nanocrystals is possible in moderate conditions and easy to reproduce.

Decomposition of silicon carbide in the presence of organic compounds under hydrothermal conditions

The synthesis of different carbon polymorphs such as under hydrothermal conditions through decomposition of graphite, diamond, amorphous carbon or diamond-like silicon carbide in the presence of organic compounds carbon, fullerenes, carbon nanotubes, etc., has attracted instead of pure water. The organic compounds decompose considerable interest for a long time because of their into various C–O–H fluids; the main components are CO, importance in science and technology. There are great OH, CO , and C H radicals. It is very well known that 2 1 x uncertainties about the phase stabilities of these polythese fluids play a significant role in creating a highly morphs as some of them do not find a place in the carbon reducing environment in the system and also assist in the pressure–temperature (P–T ) diagram and are also known dissociation of silicon carbide and precipitation of the for their contrasting physical properties. The exact carbon phase. physico–chemical phenomena responsible for their formaHydrothermal experimental runs were carried out in the tion are yet to be understood. Attempts to synthesize these pressure–temperature range of 100–200 MPa and 600– forms with varied conditions and techniques, sometimes 8508C employing conventional Roy–Tuttle reactor vessels even violating the thermodynamic principles, have met and gold tubes. The starting materials such as silicon with a fair amount of success. The stabilities of graphite carbide and organic compounds in 1:1 mol were encapsuand diamond in nature were mainly controlled by P–T– lated in gold tubes and placed them in the reactor vessels fO in the C–O–H system [1–6]. The role of C–O–H after ensuring for any leakage. The silicon carbide used in 2 fluids [7,8], as well as the hydrothermal and organic origin the present study is fine grained b-SiC powder with a 2 21 [9,10] of these polymorphs, especially with reference to specific area of less than 8 m g , and the organic diamond genesis, prompted the material scientists to compounds used in the present investigation are malonic explore the possibility of synthesizing them at fairly low acid, glycolic acid, citric acid, sucrose, formic acid, acetic pressure and temperature conditions. The hydrothermal acid, ascorbic acid, succinic acid and malic acid. All these technique is highly promising for reactions involving compounds are of pure reagent grade chemicals from volatiles as they attain the supercritical fluid state and Wako and Kanto, Japan. The experiments were quenched supercritical fluids are known for their greater ability to after 40–80 h. The capsules were cut open and the run dissolve non-volatile solids [11]. Silicon carbide powder products were dried and examined under an optical microhas been used for the synthesis of carbon polymorphs scope. It is interesting to note that sufficient gas evolved [12–14], and Gogotsi et al. [15] have reported decomposiwith a pungent smell while opening the capsule, which tion of silicon carbide in supercritical water and discussed indicates the capsule was intact during the run and possibly the formation of various carbon polymorphs. Here we the presence of CH (methane) as one of the gas phases 4 explore the possibility of producing carbon polymorphs inside the capsule. The run products were further characterized by Cu K X-ray diffraction (XRD), scanning a electron microscopy (SEM) and micro Raman spectros*Corresponding author. 1 E-mail address: bbmysore@yahoo.com (B. Basavalingu). copy (Ar laser).

Rapidly solidified eutectic composites in the system Al2O3–Y2O3–ZrO2: ternary regions in the subsolidus diagram

Abstract The fabrication of a ternary eutectic ceramic composite by rapid quenching in the system Al 2 O 3 –Y 2 O 3 –ZrO 2 prompted us to study the solidification behavior and phase formation from melts of three-phase composites in the ternary oxide phase diagram. Solidified phases were determined by X-ray diffraction (XRD) and Raman spectroscopy. The occurrence of cation substitution in the stoichiometric phases, promoted by the rapid phase separation during quenching and the composition of solid solutions in the quenched composites were investigated. The results were used to determine the compositional limits of solid-phase regions, in order to construct a tentative subsolidus diagram for arc-image melted composites. The microstructural study indicates that it is feasible to fabricate ternary eutectic composites containing zirconia in the system by rapid quenching, even at cooling rates above 10 3 K s −1 . Eutectic composites with the zirconia-based ionic conductor phase have excellent prospects for a wide range of high temperature applications in technological devices.

Autogenic synthesis of green- and red-emitting single-phase Pr(2)O(2)CO(3) and PrO(1.833) luminescent nanopowders.

This Article reveals a rare synthesis of pure Pr(2)O(2)CO(3) (POC) nanopowder by thermolysis (700 °C) of a single chemical precursor in an autogenic reaction. The autogenic thermolysis of praseodymium acetate is a solvent-free, efficient, and straightforward approach yielding luminescent POC nanoparticles. The as-prepared POC nanopowder converted to PrO(1.833) (PO) powder via combustion. Methodical morphological, structural, and compositional characterizations of POC and PO powders are carried out, supported by mechanistic elucidation and the photoluminescent properties.

Preparation and characterization of alumina supported silicalite membranes by sol–gel hydrothermal method

Two types of unsupported zeolites (silicalite-1 and silicalite-2) and porous alumina discs supports were prepared by the hydrothermal sol–gel synthesis method. The influence of the raw materials used as SiO 2 source, the temperature of the thermal treatment and the presence of the ceramic support on the crystallization of zeolites were studied. The reaction products were characterized by X-ray diffraction (XRD), IR spectroscopy (IR) and scanning electron microscopy (SEM) studies. The SiO2 source had a significant effect on the final zeolite obtained: the use of colloidal silica sol (ZCS) as SiO 2 source in the synthesis led to ZSM-11 (silicalite-2) crystals, while the sodium silicate solution (ZSS) produced the ZSM-5 (silicalite-1) type. The presence of the alumina support influences the crystallization process of ZSM-5, as it improves nucleation and the ordering of the crystals.