93 research outputs found
CSL grain boundary distribution in alumina and zirconia ceramics
The distributions of general and coincidence site lattice (CSL) grain boundaries (GBs) in texture-free alumina and zirconia ceramics sintered at two different temperatures were investigated based on electron backscatter diffraction (EBSD) measurements. Results were compared with the distributions obtained from random 2D spatial models and with calculated random distributions reported in the literature. All alumina samples independent on sintering temperature show the same characteristic deviations of the measured general GB distributions from the random model. No such features can be seen in zirconia. The total fractions of CSL GBs in alumina and zirconia samples are clearly larger, for both sintering temperatures, than those observed in the random simulations. A general GB prominence factor, similar to the twin prominence factor for fcc metals, was defined to simplify the representation of the CSL GB content in zirconia. The observed deviations from the random model show no dependence on sintering temperature nor on lattice geometry. In alumina, however, the change in the CSL GB character distribution with sintering temperature seems to be crystallographically controlled, i.e. directly dependent on the orientation of the CSL misorientation axis
In situ ESEM observation of melting silver and Inconel on an Al2O3 powder bed
A hot stage in an environmental scanning electron microscope (ESEM) was used for in situ infiltration experiments. Pressureless infiltration of a porous Ti-activated Al2O3 preform has been investigated at temperatures up to 1530°C under two atmospheres (He and H2O(g)). A brief description of the operating and the experimental set-up is given. Silver and Inconel (Ni superalloy) infiltration experiments demonstrate the in situ potential of the ESEM at temperatures up to 1500°
Evolution of metal catalyst during CVD synthesis of carbon nanotubes
La dĂ©couverte rĂ©volutionnaire des nanotubes de carbone (CNT) en 1991 a provoquĂ© une intensification des travaux de recherche dans le domaine de la science du carbone. Les propriĂ©tĂ©s fascinantes de ce matĂ©riau offrent une multitude dâapplications potentielles, par exemple comme Ă©metteur de champs, conducteur uni-dimensionnel, condensateur haute capacitĂ© (âsupercapâ), fibres de renforcement ou encore comme rĂ©servoir dâhydrogĂšne. MalgrĂ© dâimmenses progrĂšs techniques, lâamĂ©lioration des mĂ©thodes de synthĂšse en vue dâune application commerciale est encore au centre des recherches. La technique de dĂ©pĂŽt en phase vapeur (CVD) est un candidat prometteur. Dans cette technique, la nuclĂ©ation et la croissance des CNTs sont induites par la dĂ©composition de gaz carburĂ©s (CO, CO2, C2H2, etc.) sur un catalyseur mĂ©tallique Ă des tempĂ©ratures comprises entre 600°C et 1200°C. La CVD est largement utilisĂ©e pour la fabrication Ă grande Ă©chelle de CNTs et beaucoup de progrĂšs ont Ă©tĂ© faits en ce qui concerne la quantitĂ©, les frais de synthĂšse et la puretĂ© des produits. Toutefois, le mĂ©canisme de croissance des nanotubes par CVD reste peu connu. La diffusion du carbone Ă travers le catalyseur mĂ©tallique est souvent considĂ©rĂ©e comme lâĂ©tape dĂ©terminante lors de la croissance des CNTs. Les mĂ©taux les plus rĂ©actifs sont le fer, le cobalt et le nickel, mais leur effet catalytique est dĂ©pendant de plusieurs facteurs tels que la nature du prĂ©curseur, le substrat utilisĂ© et les gaz de rĂ©action. La nature chimique actuelle du catalyseur actif est trĂšs controversĂ©e; on ne sait pas par exemple sâil est prĂ©sent sous forme de mĂ©tal, de carbure ou en phase mĂ©langĂ©e. JusquâĂ prĂ©sent, trĂšs peu dâanalyses insitu de lâĂ©volution chimique et morphologique du catalyseur durant le processus CVD ont Ă©tĂ© faites. Le comportement de catalyseurs Ă base de nickel, cobalt, chrome ou molybdĂšne a Ă©tĂ© analysĂ© sous une atmosphĂšre azote/acĂ©tylĂšne ou azote/acĂ©tylĂšne/ hydrogĂšne Ă des tempĂ©ratures de 600°C et de 750°C. Pour mieux comprendre les propriĂ©tĂ©s des phases mĂ©talliques pendant le processus de synthĂšse, un diffractomĂštre Ă rayons X Ă©quipĂ© avec une table chauffante et un systĂšme de contrĂŽle atmosphĂ©rique a Ă©tĂ© utilisĂ© pour Ă©tudier in-situ lâĂ©volution des revĂȘtements de nitrate mĂ©tallique. Les Ă©chantillons ont Ă©tĂ© ensuite trempĂ©s Ă diffĂ©rents stades de pyrolyse pour ĂȘtre finalement observĂ©s au MEB et MET. Les images au microscope ont montrĂ© que le nickel ainsi que le cobalt et le molybdĂšne peuvent agir comme catalyseurs pour la nuclĂ©ation et la croissance des CNTs, cepandant pas le chrome. La rĂ©duction de la taille des grains rĂ©sultant dâune perte suffisante de volume solide pendant les rĂ©actions rĂ©dox dans le prĂ©curseur catalytique, ainsi que la transformation de ces prĂ©curseurs en une phase mĂ©tallique sont les principales conditions nĂ©cessaires Ă la croissance de CNTs. Les stades de rĂ©action observĂ©s pendant la rĂ©duction du prĂ©curseur ont Ă©tĂ© mis en relation avec la nuclĂ©ation et la croissance des nanotubes. La diffusion de carbone Ă travers les particules mĂ©talliques, marquĂ©e par un agrandissement des paramĂštres cellulaires du mĂ©tal et identifiĂ©e sur les diffractogrammes par un dĂ©placement des pics, est observĂ©e Ă chaque fois que des nanotubes de carbone sont gĂ©nĂ©rĂ©s. Avec le nickel et le cobalt, aucune phase de carbure ne sâest formĂ©e. Avec le fer, la dĂ©composition des phases mĂ©tastables de carbure agit comme une seconde activation de la croissance des nanotubes alors que le molybdĂšne va favoriser la formation de carbures qui vont stopper la croissance des CNTs aprĂšs 20 minutes. Dans tous les cas, il a Ă©tĂ© dĂ©montrĂ© quâun traitement prĂ©liminaire Ă lâhydrogĂšne favorise la croissance des nanotubes.Die revolutionĂ€re Entdeckung von Kohlenstoff- Nanoröhrchen (CNT) im Jahre 1991 liess die Forschungsarbeiten im Bereich der Kohlenstoffwissenschaft intensivieren. Die faszinierenden Eigenschaften dieses einzigartigen Materials ermöglichten eine Vielzahl von potenziellen Anwendungen wie zum Beispiel als Elektronen Feldemissionsquelle, eindimensionale Konduktoren, SuperkapazitĂ€ten, VerstĂ€rkungsfaden oder Wasserstoffspeicher. Trotz der atemberaubenden technischen Fortschritte bemĂŒht man sich immer noch um die Entwicklung einer Synthesemethode fĂŒr die kommerzielle Anwendung. Ein vielversprechender Kandidat ist die Technik der chemischen Gasphasenabscheidung (CVD). Die Keimbildung und das Wachstum von CNTs werden induziert durch die Zersetzung von kohlenstoffhaltigen Gasen (CO, CO2, C2H2, usw.) ĂŒber einem metallischen Katalysator bei Temperaturen zwischen 600°C und 1200°C. CVD ist eine weit verbreitete Technik fĂŒr die Fabrikation von CNT in grossen QuantitĂ€ten und Fortschritte betreffend der Menge, der Synthesekosten und der Reinheit der Produkte, wurden erzielt. Doch das grosse RĂ€tsel der CVD Methode bleibt der Wachstumsmechanismus. Der Hauptreaktionsschritt fĂŒr das Wachstum von Nanoröhrchen scheint die Diffusion von Kohlenstoff durch den Metallkatalysator zu sein. Die reaktivsten Metalle sind Eisen, Kobalt und Nickel, doch deren katalytische Wirkung ist abhĂ€ngig von der Art des Ausgangsmaterials, des benutzten Substrates und der Reaktionsgase. Sehr umstritten ist die aktuelle chemische Beschaffenheit des aktiven Katalysators, zum Beispiel ob er als Metall, Karbid oder als gemischte Phase vorliegt. Bis jetzt wurden nur sehr wenige in-situ Analysen der chemischen und morphologischen Evolution des Katalysators wĂ€hrend des CVD Prozesses durchgefĂŒhrt. Diese Doktorarbeit befasst sich mit der Evolution von nickel-, kobalt-, chrom- und molybdĂ€nbasierenden Katalysatoren unter Stickstoff/Acetylen und Stickstoff/Acetylen/Wasserstoff AtmosphĂ€re bei 600°C und 750°C. Um die Eigenschaften von metallischen Phasen wĂ€hrend des Syntheseablaufs aufzuklĂ€ren, wurde ein Röntgendiffraktometer mit einem Heiztisch und einem AtmosphĂ€renkontrollsystem ausgestattet, welches das in-situ Studium der Evolution von Metallnitrat-Filmen ermöglicht. Die Proben wurden dafĂŒr bei verschiedenen Pyrolysezeiten abgeschreckt und im REM und TEM untersucht. Die Mikroskopiebilder zeigen, dass Nickel sowie Kobalt und MolybdĂ€n unter typischen Nanoröhrchen Synthesebedingungen als Katalysatoren fĂŒr CNTs Keimbildung und Wachstum agieren können, jedoch nicht Chrom. Korngrössenreduktion, resultierend aus dem ausreichenden Festkörpervolumenverlust wĂ€hrend der Redox Reaktion im katalytischen Ausgangsmaterial, und die Transformation des Ausgangsmaterials zu einer metallischen Phase sind die Hauptvoraussetzungen fĂŒr das CNT Wachstum. Die beobachteten Reaktionsabschnitte wĂ€hrend der Reduktion des Ausgangsmaterials werden in Verbindung gebracht mit der Keimbildung und dem Wachstum von Nanoröhrchen. Kohlenstoffdiffusion durch die metallischen Partikel, angezeigt durch eine Vergrösserung der Zellparameter des Metalls und identifiziert in Diffraktogramme als Peak- Verschiebung, wurde beobachtet wann immer CNTs gebildet wurden. Im Nickel- und Kobaltsystem wurden keine Karbidphasen entdeckt. Doch im Vergleich zum Eisensystem, wo die Zerlegung von metastabilem Karbid als zweiter Schub von Nanoröhrchen Bildung dient, wird das CNT Wachstum im MolybdĂ€nsystem nach der Bildung von Karbiden nach 20 Minuten gestoppt. In jedem Fall begĂŒnstigt eine Vorbehandlung mit Wasserstoff die Nanoröhrchen Bildung.The revolutionary discovery of carbon nanotubes (CNT) in 1991 led to intense research activities in the domain of carbon science. The fascinating properties of these unique material has opened a great number of potential applications e.g. as electron field emitters, one-dimensional conductors, supercapacitors, reinforcing fibres or hydrogen storage. Despite these stunning technical progresses there is still much struggle in the development of a synthesis method suitable for commercial applications. A leading candidate is the chemical vapour deposition (CVD) technique. Nucleation and growth of CNTs are induced by the decomposition of carbon-containing gases (CO, CO2, C2H2, etc) over a metallic catalyst at temperatures between 600°C and 1200°. CVD is a widely used technique to generate CNTs in large quantities and much progress has been made from the point of view of the yield, the synthesis costs or the purity of the products. But the great conundrum of CVD process remains the growth mechanism. A key reaction step for nanotube growth seems to be diffusion of carbon through the metal catalyst and the most active metals are iron, cobalt and nickel but their catalytic action depends on the type of precursor, the type of substrate and of the reactive gases used. Highly controversial is the actual chemical nature of the active catalyst e.g. if it is present as metal, carbide or as mixed phase. So far few investigations of the chemical and morphological evolution of the catalyst during CVD process have been performed. This thesis focuses on the evolution of nickel-, cobalt-, chromium- and molybdenum-based catalysts under a nitrogen/acetylene and a nitrogen/acetylene/ hydrogen atmosphere at 600°C and 750°C. In order to elucidate the nature of the catalyst during synthesis runs an X-ray diffractometer equipped with a heating stage and an atmosphere controlling system was used to study in-situ the evolution of metal nitrate films. Samples quenched after different pyrolysis time were investigated by SEM and TEM. The microscopic images showed that nickel, cobalt and molybdenum can act under typical nanotube synthesis conditions as catalyst for CNT nucleation and growth, but not chromium. Grain size reduction resulting from a sufficient solid volume loss during redox reactions in the catalyst precursor and the transformation of these precursors to a metallic phase are the main requirements for nanotube growth. The reaction sequences observed during the reduction of the precursor are put in relation with the nucleation and growth of nanotubes. Diffusion of carbon through the metal particle, indicated by an increase of metal cell parameters identified in diffractograms as peak shifts, was observed whenever carbon nanotubes were generated. In the nickel and cobalt system no carbide phases were detected. In contrast to the iron system, where the break-down of metastable carbides act as a second boost of nanotube formation, the appearance of carbides in the molybdenum system after 20 minutes stops further carbon nanotube growth. In any case hydrogen pre-treatment promotes nanotube growth
Kinetics of the chrysotile and brucite dehydroxylation reaction: a combined non-isothermal/isothermal thermogravimetric analysis and high-temperature X-ray powder diffraction study
The dehydroxylation reactions of chrysotile Mg3Si2O5(OH)4 and brucite Mg(OH)2 were studied under inert nitrogen atmosphere using isothermal and non-isothermal approaches. The brucite decomposition was additionally studied under CO2 in order to check the influence of a competing dehydroxylation/carbonation/decarbonisation reaction on the reaction kinetics. Isothermal experiments were conducted using in situ high-temperature X-ray powder diffraction, whereas non-isothermal experiments were performed by thermogravimetric analyses. All data were treated by model-free, isoconversional approaches (âtime to a given fraction' and Friedman method) to avoid the influence of kinetic misinterpretation caused by model-fitting techniques. All examined reactions are characterised by a dynamic, non-constant reaction-progress-resolved (âα'-resolved) course of the apparent activation energy E a and indicate, therefore, multi-step reaction scenarios in case of the three studied reactions. The dehydroxylation kinetics of chrysotile can be subdivided into three different stages characterised by a steadily increasing E a (αâ€15%, 240-300kJ/mol), before coming down and forming a plateau (15%â€Î±â€60%, 300-260kJ/mol). The reaction ends with an increasing E a (αâ„60%, 260-290kJ/mol). The dehydroxylation of brucite under nitrogen shows a less dynamic, but generally decreasing trend in E a versus α (160-110kJ/mol). In contrast to that, the decomposition of brucite under CO2 delivers a dynamic course with a much higher apparent E a characterised by an initial stage of around 290kJ/mol. Afterwards, the apparent E a comes down to around 250kJ/mol at α~65% before rising up to around 400kJ/mol. The delivered kinetic data have been investigated by the z(α) master plot and generalised time master plot methods in order to discriminate the reaction mechanism. Resulting data verify the multi-step reaction scenarios (reactions governed by more than one rate-determining step) already visible in E a versus α plots
Timing and thermal evolution of fold-and-thrust belt formation in the Ultima Esperanza District, 51°S Chile: Constraints from K-Ar dating and illite characterization
K/Ar dating on llite in Upper Cretaceous low-grade metamorphic pelites in the Torres del Paine area was used to set new time constraints on the development of the Patagonian retroarc fold-and-thrust belt (FTB) caused by the subduction of the Antarctic Plate beneath the South American Plate. The combined use of illite crystallinity (KĂŒbler Index), polytype quantification and K/Ar dating of illite fractions (<0.2, <2 and 2-6 ”m) allowed to distinguish four distinct periods of illite growth based on their K/Ar ages and degree of regional metamorphism: (1) early Cenomanian (98 Ma) illite crystallization, (2) widespread early Campanian (ca. 80 Ma) diagenetic illite growth under anchizonal metamorphic conditions, (3) a significant period of illite formation in the early Paleocene (ca. 60 Ma), and (4) a late stage of illite growth in the early Eocene (55-46 Ma) under epizonal conditions. The earliest indication for the emergent FTB formation in the hinterland is documented in a metapelitic clast (14-9, <2 ”m) within the Upper Cretaceous Cerro Toro conglomerate which yields a K/Ar cooling age of 98.3±1.2 Ma and an epizonal KI value of 0.24 â°2Î. After a certain period of geological quietness an interval of major thrusting and uplift occurred between ca. 60 and 46 Ma. The east dipping Rio Nutria and Rio Rincon thrusts record the onset of thrust and fold activity which can be placed close to 60 Ma. They also mark the frontal thrust towards the less deformed Magallanes foreland basin. In the western part of the internal domain, widespread fault and thrust activity of the frontal wedge and associated thermal overprint continued and is recorded until 46 Ma by K/Ar illite cooling ages. The flexural subsidence that is driven by the thrust sheet loading in the internal domain was responsible for the eastward migration of the foreland depocenter and the rapid increase of sedimentation rate along the monoclinal belt. No Miocene thrusting nor uplift event has been recorded by K/Ar illite dating in the study area
Alburnite, AgâGeTeâSâ, a new mineral species from the RoĆia Montana Au-Ag epithermal deposit, Apuseni Mountains, Romania
Alburnite, ideally AgâGeTeâSâ, was discovered in the CĂąrnicel vein from the RoĆia Montana epithermal Au-Ag ore deposit, Apuseni Mountains, Romania. The new mineral is associated with tetrahedrite, galena, pyrite, sphalerite, chalcopyrite, and tellurides (hessite, altaite, and sylvanite). Associated gangue minerals are rhodochrosite, quartz, calcite, and rhodonite. Alburnite was observed only at the microscopic scale as rounded to sub-rounded grains, veinlets or irregular inclusions hosted mainly by tetrahedrite, hessite, and rhodochrosite. Due to the small size of alburnite grains observed so far it was not possible to determine some macroscopic properties; reported properties are based on microscopic observations. The mineral has a metallic luster and is opaque. It is non-fluorescent and has an estimated Mohs hardness of 4. The mineral shows no cleavage. Density could not be measured because of the small grain size, but calculated density based on the empirical formula is 7.828 g/cmÂł. In plane-polarized light in air, alburnite is gray-blue with a bluish tint. It shows no pleochroism or bireflectance in air. Between crossed polars alburnite is isotropic and internal reflections have not been observed in air. The mineral decomposes in intense light. Reflectance minimum values in air (in percents) are: 470 nm 29.70; 546 nm 28.00; 589 nm 27.35; 650 nm 26.95. The average chemical composition based on 18 electron microprobe analyses from 9 different grains in one polished section is (in wt%): Ag 65.49, Ge 4.82, Te 20.16, S 9.66, total 100.13. The ideal formula of alburnite, AgâGeTeâSâ, based on 15 apfu requires Ag 65.43, Ge 5.50, Te 19.35, S 9.72, total 100.00 wt%. Features of the crystal structure of alburnite were determined based on electron backscattered diffraction and transmission electron microscopy. Alburnite is cubic, space group F43m, with unit- cell parameters a = 10.4(1) Ă
, V = 1125(30) Ă
Âł, Z = 4. The strongest eight calculated XRD lines [d in Ă
(I) (hkl)] are: 6.004(67)(111), 3.136(48)(113), 3.002(100)(222), 2.600(26) (004), 2.123(33)(224), 2.002(61)(115), 1.838(76)(044), and 1.644(12)(026). The name of the new mineral alburnite is derived from the Latin name of the locality. RoĆia Montana Au-Ag deposit was known during the Roman period as Alburnus Maior. The mineral and the mineral name have been approved by the Commission on New Minerals, Nomenclature and Classification, IMA 2012-073
Manufacturing of tubular dead-end membranes by continuous thermoplastic extrusion
Dense and gastight dead-end tubular Ba0.5Sr0.5Co0.8Fe0.2O3-ÎŽ (BSCF) membranes were fabricated by thermoplastic extrusion. Optimal feedstocks for the extrusion of dead-end tubular structures were obtained for a feedstock consisting of 89.83 wt% BSCF powder, 5.63 wt% ethylene vinyl acetate (EVA), 3.56 wt% paraffin wax (PW), and 0.98 wt% stearic acid (SA). Thermogravimetric analysis (TGA) was used to determine the thermal characteristics of the feedstock and to characterize the amount of binder removed during the wicking step. Straight and round dead-end tubular membranes with wall thickness of 0.5Â mm could be achieved after sintering. The gastightness test showed that the membranes were leak-tight up to a pressure of 35Â bar
Quantification of mass transfers and mineralogical transformations in a thrust fault (Monte Perdido thrust unit, southern Pyrenees, Spain)
In fold-and-thrust belts, shortening is mainly accommodated by thrust faults which are preferential zones for recrystallisation and mass transfer. This study focuses on a detachment fault related to the emplacement of the Monte Perdido thrust unit in the southern Pyrenees. The studied fault zone consists of a 10 m thick intensively foliated phyllonite developed within the Millaris marls, of Eocene age. The lithological homogeneity of the hanging wall and footwall allows us to compare the Millaris marls outside the fault zone with the highly deformed marls located in the fault zone and to quantify the chemical, mineralogical and volumetric changes related to deformation processes along the fault. The Millaris marls are composed of detrital quartz, illite, chlorite, minor albite and pyrite, in a micritic calcite matrix. In the fault zone, the cleavage planes are marked by clay minerals and calcite ± chlorite veins attest to fluidâmineral interactions during deformation. The mineral proportions in all samples from both the fault zone and Millaris marls have been quantified by two methods: (1) X-ray diffraction and Rietveld refinement, and (2) bulk chemical analyses as well as microprobe analyses to calculate modal composition. The excellent agreement between the results of these two methods allows us to estimate mineralogical variations using a modification of the Gresens' equation. During fault activation, up to 45 wt% of calcite was lost while the amounts of quartz and chlorite remained unchanged. Illite content remained constant to slightly enriched. The mineralogical variations were coupled with a significant volume loss (up to 45%) mostly due to the dissolution of micritic calcite grains. Deformation was accompanied by pressure solution and phyllosilicates recrystallisation. These processes accommodated slip along the fault. They required fluids as catalyst, but they did not necessitate major chemical transfers
Hybrid magnetic iron oxide nanoparticles with tunable field-directed self-assembly
We describe the synthesis of hybrid magnetic ellipsoidal nanoparticles that consist of a mixture of two different iron oxide phases, hematite (α-Fe2O3) and maghemite (Îł- Fe2O3), and characterize their magnetic field-driven self-assembly. We demonstrate that the relative amount of the two phases can be adjusted in a continuous way by varying the reaction time during the synthesis, leading to strongly varying magnetic properties of the particles. Not only does the saturation magnetization increase dramatically as the composition of the spindles changes from hematite to maghemite, but also the direction of the induced magnetic moment changes from being parallel to the short axis of the spindle to being perpendicular to it. The magnetic dipolar interaction between the particles can be further tuned by adding a screening silica shell. Small-angle X-ray scattering (SAXS) experiments reveal that at high magnetic field, magnetic dipoleâdipole interaction forces the silica coated particles to self- assemble into a distorted hexagonal crystal structure at high maghemite content. However, in the case of uncoated maghemite particles, the crystal structure is not very prominent. We interpret this as a consequence of the strong dipolar interaction between uncoated spindles that then become arrested during field-induced self- assembly into a structure riddled with defects
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