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Residual Carbon (residual + carbon)
Selected AbstractsPreparation of Nanometer-Sized ,-Alumina Powders by Calcining an Emulsion of Boehmite and Oleic AcidJOURNAL OF THE AMERICAN CERAMIC SOCIETY, Issue 1 2002Chih-Peng Lin This study proposes a method to form ultrafine ,-Al2O3 powders. Oleic acid is mixed with Al(OH)3 gel. The gel is the precursor of the Al2O3. After it is mixed and aged, the mixture is calcined in a depleted oxygen atmosphere between 25° and 1100°C. Oleic acid evaporates and decomposes into carbon during the thermal process. Residual carbon prevents the growth of agglomerates during the formation of ,-Al2O3. The phase transformation in this process is as follows: emulsion ,,-Al2O3,,-Al2O3,,-Al2O3,,-Al2O3. This process has no clear , phase. Aging the mixed sample lowers the formation temperature of ,-Al2O3 from 1100° to 1000°C. The average crystallite diameter is 60 nm, measured using Scherrer's equation, which is consistent with TEM observations. [source] Optical properties of TiO2 thin films prepared by chemical spray pyrolysis from aqueous solutionsPHYSICA STATUS SOLIDI (C) - CURRENT TOPICS IN SOLID STATE PHYSICS, Issue 3-4 2010R. Ayouchi Abstract Titanium dioxide (TiO2) is known to have three different kinds of polymorphous crystalline forms: rutile, anatase, and brookite. The rutile phase is always formed at higher temperatures, while the anatase phase is formed at lower temperatures and transformed into rutile phase above 800 ºC. Various deposition techniques have been developed for depositing TiO2 thin films, including evaporation, sputtering, chemical vapour deposition and thermal oxidation of titanium. Among them, the Chemical Spray Pyrolysis (CSP) technique has many advantages, such as good conformal coverage, the possibility of epitaxial growth and the application to large area deposition. Also, this method is low cost and it is easy to control the deposition growth parameters. In the present work, TiO2 thin films have been deposited on p-Si (001) and fused silica substrates by Chemical Spray Pyrolysis (CSP) method from aqueous solution containing titanium (IV) isopropoxide (Ti[OCH(CH3)2]4. As-deposited thin films show anatase polycrystalline structure, and rutile phase formed for films annealed at 750ºC. SEM images have confirmed a smooth and crack-free surface with low surface roughness. X-ray photoelectron spectroscopy (XPS) combined with 4 keV Ar+ depth profiling has shown that crystallized films correspond to TiO2. Residual carbon coming from the organic precursor solution is only detected at the surface of the film. Thin films deposited on fused silica were highly transparent (more than 85%), with an indirect optical band gap of 3,43 and 3,33 eV for as-deposited and annealed films, respectively, and refractive indexes in the range between 2.01,2.29. Spectroscopic Ellipsometry (SE) also has been used to extract optical parameters. SE data fitted to triple-layer physical model revealed the same tendency to increase refractive index in annealed films. (© 2010 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim) [source] Deposition of HfO2, Gd2O3 and PrOx by Liquid Injection ALD Techniques,CHEMICAL VAPOR DEPOSITION, Issue 3 2005J. Potter Abstract Thin films of hafnium oxide (HfO2), gadolinium oxide (Gd2O3), and praseodymium oxide (PrOx) have been deposited by liquid injection atomic layer deposition (ALD) and for comparison, have also been deposited by "thermal" metal-organic (MO) CVD using the same reactor. The ALD-grown films were deposited on Si(100) over a range of substrate temperatures (150,450,°C) using alternate pulses of [Hf(mmp)4], [Gd(mmp)3], or [Pr(mmp)3] (mmp = OCMe2CH2OMe) and water vapor. X-ray diffraction (XRD) analysis showed that as-grown films of HfO2 were amorphous, but these crystallized into the monoclinic phase after annealing in air at 800,°C. XRD analysis showed that as-grown Gd2O3 and PrOx films had some degree of crystallinity. Residual carbon (0.8,3.3 at.-%) was detected in the HfO2 and PrOx films by Auger electron spectroscopy (AES), but not in the Gd2O3 films. The self-limiting behavior of the precursors was investigated at 225,°C by varying the volume of precursor injected during each ALD cycle and, in each case, oxide growth was not fully self-limiting. We propose a mechanism for this involving ,-hydride elimination of the mmp group, and also propose some general mechanistic principles which may influence the growth of oxides by ALD using other precursors. [source] Processing of Carbon Nanofiber Reinforced ZrB2 Matrix Composites for Aerospace Applications,ADVANCED ENGINEERING MATERIALS, Issue 7 2010Jorge Barcena Ceramic matrix composites (CMCs) based on zirconium diboride (ZrB2) reinforced by vapor grown carbon nanofibers are a potential constituent of reusable thermal protection systems. A manufacturing procedure was devised that involved the fabrication of thin films by tape casting to obtain a layer that could be integrated into a more complex system. Higher thermal conductivities and improved toughness can be expected for nanofiber additions, as compared to the matrix alone. Consolidation by hot-pressing was more effective than pressureless sintering, in terms of the final relative density and flatness of specimens. Examination of microstructures showed that few carbon nanofibers were present in the matrix after consolidation by sintering, which was attributed to a reaction between the nanofibers and zirconium oxide present on the surface of the ZrB2 powder. As a solution, oxygen impurities from the boride powders were removed by reduction with carbon coatings derived from phenolic resin. The deleterious reaction was avoided, but residual carbon remained at the grain boundaries, likely from decomposition of the binder. The use of an alternative binder (PMMA vs. PVB) will be used in future studies to reduce the residual carbon content. Further, consolidation by Spark Plasma Sintering (SPS) will be explored to further reduce the reaction of surface oxides with the nanofibers. Finally, characterization of the microstructure at the nanometric level and further determination of the mechanical and thermal properties will be conducted as part of future studies. [source] In Situ Synthesis and Microstructures of Tungsten Carbide-Nanoparticle-Reinforced Silicon Nitride-Matrix CompositesJOURNAL OF THE AMERICAN CERAMIC SOCIETY, Issue 3 2004Tateoki Iizuka A W2C-nanoparticle-reinforced Si3N4 -matrix composite was fabricated by sintering porous Si3N4 that had been infiltrated with a tungsten solution. During the sintering procedure, nanometer-sized W2C particles grew in situ from the reaction between the tungsten and carbon sources considered to originate mainly from residual binder. The W2C particles resided in the grain-boundary junctions of the Si3N4, had an average diameter of ,60 nm, and were polyhedral in shape. Because the residual carbon, which normally would obstruct sintering, reacted with the tungsten to form W2C particles in the composite, the sinterability of the Si3N4 was improved, and a W2C,Si3N4 composite with almost full density was obtained. The flexural strength of the W2C,Si3N4 composite was 1212 MPa, ,34% higher than that of standard sintered Si3N4. [source] | ||||||||||