| 1. |
- Agthe, Michael, et al.
(författare)
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Dynamic growth modes of ordered arrays and mesocrystals during drop-casting of iron oxide nanocubes
- 2014
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Ingår i: CrystEngComm. - : Royal Society of Chemistry (RSC). - 1466-8033. ; 16:8, s. 1443-1450
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Tidskriftsartikel (refereegranskat)abstract
- The growth modes of self-assembled mesocrystals and ordered arrays from dispersions of iron oxide nanocubes with a mean edge length of 9.6 nm during controlled solvent removal have been investigated with a combination of visible light video microscopy, atomic force microscopy and scanning electron microscopy. Mesocrystals with translational and orientational order of sizes up to 10 mu m are formed spontaneously during the final, diffusion-controlled, drop-casting stage when the liquid film is very thin and the particle concentration is high. Convection-driven deposition of ordered nanocube arrays at the edge of the drying droplet is a manifestation of the so called coffee-ring effect. Dendritic growth or fingering of rapidly growing arrays of ordered nanocubes could also be observed in a transition regime as the growth front moves from the initial three-phase contact line towards the centre of the original droplet.
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| 2. |
- Aguilar-Mamani, Wilson, 1981-
(författare)
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Crystallization of NBA-ZSM-5 from kaolin
- 2017
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Doktorsavhandling (övrigt vetenskapligt/konstnärligt)abstract
- ZSM-5 is an aluminosilicate zeolite with high Si/Al ratio with suitable properties for catalysis, ion exchange, adsorption and membrane applications. The main goal of this thesis was to study the growth of ZSM-5 zeolite crystals from inexpensive natural sources of silica and alumina, as well as n-butylamine (NBA) as a low-cost structure directing agent.The first objective of this work was to develop pathways to synthesize ZSM-5 crystals from kaolin clay or diatomaceous earth, two inexpensive natural sources of silica and alumina (Paper I). In the case of kaolin, a heat treatment was used in order to form amorphous metakaolinite. Subsequently, dealumination of the raw materials by acid leaching made it possible to reach appropriate Si/Al ratios and to reduce the amount of impurities. Finally, leached metakaolinite or diatomaceous earth was reacted with sodium hydroxide and NBA. After synthesis optimization, both sources of aluminosilicates were found to behave differently during the course of synthesis and to lead to slightly different reaction products. The final products exhibited Si/Al ratios in the range 10-20. The use of leached diatomaceous earth allowed to reach higher yield of ZSM-5 crystals within comparable synthesis times. However, low amounts of mordenite were inevitably formed as a by-product, which was related to the high calcium content of diatomaceous earth. Therefore, the rest of the thesis focused on the kaolin system.In order to study the growth mechanism of ZSM-5 from leached metakaolinite, a proper methodology to gain local compositional data by energy dispersive spectroscopy (EDS) on aluminosilicates was developed (Paper II). Zeolite A was used as a model system that could be ion-exchanged with various elements. In order to evaluate the reliability of the measurements, inductively coupled plasma-sector field mass spectrometry (ICP-SFMS) and EDS were compared. The EDS method developed in this work resulted in molar ratios very close to theoretical values and was therefore found more reliable than ICP-SFMS. Therefore, the method developed for zeolite A was applied in the rest of the thesis work to study the formation and growth of ZSM-5 crystals.The second part of this work focused on the kaolin system in order to understand the nucleation and growth processes of the ZSM-5 crystals. This system was heterogeneous, due to the formation of a gel upon heating of the synthesis mixture. First, the internal structure of the gel was investigated (Paper III). Second, a kinetic study was performed and compared with microstructural observations (Paper IV). Finally, the mechanisms leading to Al-zoning and dendritical growth of the zeolite crystals were investigated (Paper V). The characterization of the intermediate phases during the different stages of the hydrothermal synthesis were analyzed by different analytical techniques, such as inductively coupled plasma-sector field mass spectrometry (ICP-SFMS), dynamic light scattering (DLS), extreme high resolution-scanning electron microscopy (XHR-SEM), energy dispersive spectroscopy (EDS), high resolution-transmission electron microscopy (HR-TEM), X-ray diffraction (XRD) and nitrogen gas adsorption.These investigations led to several important conclusions: 1) The walls of the gel were shown for the first time to be inhomogeneous and to possess a biphasic internal structure consisting of a mesoporous skeleton of aluminosilicate nanoparticles embedded in a silicate-rich soluble matrix of soft matter. 2) The kinetic study and microstructural evidences indicated that the early crystals were fully embedded inside the gel phase and that crystal growth was retarded, as the formation of the gel occurred simultaneously with the early growth of the crystals. Hence, nucleation and growth appeared to be solution mediated. 3) Finally, the Al zoning of the crystals was related to the biphasic internal structure of the gel, since the silicate-rich matrix was preferentially consumed first. 4) The dendrites present at the surface of the crystals during most of the growth process were shown to be caused by the presence of a web of nanoparticles, most likely originating from the mesoporous skeleton inside the gel.In the future, these findings are expected to lead to optimized synthesis pathways of catalysts with homogeneous properties and to contribute to the development of poor regions in Bolivia.
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| 3. |
- Aguilar-Mamani, Wilson, et al.
(författare)
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Internal structure of a gel leading to NBA-ZSM-5 single crystals
- 2018
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Ingår i: Journal of porous materials. - : Springer. - 1380-2224 .- 1573-4854. ; 25:6, s. 1551-1559
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Tidskriftsartikel (refereegranskat)abstract
- Porous gel structures are formed during the synthesis of the zeolite ZSM-5 due to the reaction between a source of aluminosilicate, sodium hydroxide, water and a structure directing agent, such as e.g. tetrapropylammonium (TPA) or n-butylamine (NBA). In the present work, the formation of the gel in a heterogeneous system leading to the crystallization of NBA-ZSM-5 zeolite from leached metakaolin was studied extensively. The solid and liquid phases obtained after separation were analyzed by inductively coupled plasma sector field mass spectrometry, dynamic light scattering, extreme high resolution-scanning electron microscopy, energy dispersive spectroscopy, high resolution-transmission electron microscopy, X-ray diffraction and nitrogen gas adsorption. The main gel phase formed after hydrothermal treatment exhibited a sponge-like structure resembling those forming in (Na, TPA)-ZSM-5-based systems. For the first time, the walls of the main gel were shown to be inhomogenous and to possess a biphasic internal structure consisting of a mesoporous skeleton of aluminosilicate nanoparticles embedded in a silicate-rich soluble matrix of soft matter. The data presented in this paper is of primary importance to understand the mechanism by which the gel is consumed and contributes to the growth process of the zeolite crystals.
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| 4. |
- Aguilar-Mamani, Wilson, et al.
(författare)
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Solution-mediated growth of NBA-ZSM-5 crystals retarded by gel entrapment
- 2018
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Ingår i: Journal of Crystal Growth. - : Elsevier. - 0022-0248 .- 1873-5002. ; 487, s. 57-64
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Tidskriftsartikel (refereegranskat)abstract
- The synthesis of flat tablet-shaped ZSM-5 crystals from a gel using metakaolin as aluminosilicate source and n-butyl amine as structure directing agent was investigated. The evolution inside the solid phase was characterized by X-ray diffraction, scanning electron microscopy, energy dispersive spectroscopy, thermogravimetry and mass spectrometry. A kinetic study indicated that the nucleation of the majority crystals occurred concurrently with the formation of the gel upon heating the starting liquid suspension. Microstructural evidences undeniably showed that the gel precipitated on ZSM-5 crystals and mineral impurities originating from kaolin. As a result, crystal growth was retarded by gel entrapment, as indicated by the configuration and morphology of the embedded crystals. The results presented herein are harmonized with a solution-mediated nucleation and growth mechanism. Our observations differ from the autocatalytic model that suggests that the nuclei rest inside the gel until released when the gel is consumed. Our results show instead that it is crystals that formed in an early stage before entrapment inside the gel that rest inside the gel until exposed at the gel surface. These results illustrate the limitation of the classical method used in the field to determine nucleation profiles when the crystals become trapped inside the gel.
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| 5. |
- Aguilar, Wilson, et al.
(författare)
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Comparison between leached metakaolin and leached diatomaceous earth as raw materials for the synthesis of ZSM-5
- 2014
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Ingår i: SpringerPlus. - : Springer. - 2193-1801. ; 3:1
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Tidskriftsartikel (refereegranskat)abstract
- Inexpensive raw materials have been used to prepare ZSM-5 zeolites with SiO2/Al2O3 molar ratios in the range 20 - 40. Kaolin or Bolivian diatomaceous earth was used as aluminosilicate raw materials and sodium hydroxide and n-butylamine were used as mineralizing agents and template. Dealumination of the raw materials by acid leaching made it possible to reach appropriate SiO2/Al2O3 ratios and to reduce the amount of iron and other impurities. After mixing the components and aging, hydrothermal treatment was carried out and the products were recovered The results clearly show for the first time that well-crystallized ZSM-5 can be directly prepared from leached metakaolin or leached diatomaceous earth using sodium hydroxide and n-butylamine as mineralizing agents and template under appropriate synthesis conditions. A longer induction time prior to crystallization was observed for reaction mixtures prepared from leached diatomaceous earth, probably due to slower digestion of the fossilized diatom skeletons as compared with that for microporous leached metakaolin. The use of leached diatomaceous earth allowed higher yield of ZSM-5 crystals within comparable synthesis times. However, low amounts of Mordenite formed, which was related to the high calcium content of diatomaceous earth. Another considerable advantage of diatomaceous earth over kaolin is that diatomaceous earth does not require heat treatment at high temperature for metakaolinization.
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| 6. |
- Aguilar, Wilson, et al.
(författare)
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Dendritic growth of NBA-ZSM-5
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Ingår i: Microporous and Mesoporous Materials. - 1387-1811 .- 1873-3093.
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Tidskriftsartikel (övrigt vetenskapligt/konstnärligt)abstract
- Crystallization of ZSM-5 zeolite from a gel using n-butylamine as structure-directing agent was studied. Extreme high-resolution transmission and scanning electron microscopy showed the presence of dendritic features that are present at the crystal surface during most of the reaction time that become smoother towards completion of the crystallization. In addition, a web that likely stems from the gel, comprised of alumina-rich nanoparticles between the dendrites at the surface of the crystals was also identified. When the gel is not in direct contact with the crystal surface, dendrites and the web are not observed, and the crystals grow faster. Thus, the alumina-rich web retards the crystal growth and cause the formation of dendritic features.
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| 7. |
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| 8. |
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| 9. |
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| 10. |
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| 11. |
- Bhuiyan, Iftekhar Uddin, et al.
(författare)
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Consideration of X-ray microtomography to quantitatively determine the size distribution of bubble cavities in iron ore pellets
- 2013
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Ingår i: Powder Technology. - : Elsevier BV. - 0032-5910 .- 1873-328X. ; 233, s. 312-318
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Tidskriftsartikel (refereegranskat)abstract
- X-ray microtomography data of iron ore green pellets of approx. 12 mm in diameter were recorded using a commercial instrument. The reconstructed volume after thresholding represented a unique dataset consisting of a three-dimensional distribution of equiaxed objects corresponding to bubble cavities. This dataset was used to successfully validate a stereological method to determine the size distribution of spherical objects dispersed in a volume. This was achieved by investigating only a few cross-sectional images of this volume and measuring the profiles left by these objects in the cross-sectional images. Excellent agreement was observed between the size distribution of the bubble cavities obtained by directly classifying their size in the reconstructed volume and that estimated by applying the aforementioned stereological method to eight cross-sectional images of the reconstructed volume. Subsequently, we discuss the possibility of calibrating X-ray tomography data quantitatively using the size distribution of the bubble cavities as a figure of merit and the results obtained by applying the stereological method to SEM images as reference data. This was justified by considering the validity of the stereological method demonstrated by tomography, the accurate thresholding made possible by back-scattered electron imaging and the solid reproducibility of the results obtained by SEM. Using different threshold values for binarization of the X-ray microtomography data and comparing the results to those obtained by SEM, we found that X-ray microtomography can be used after proper calibration against SEM data to measure the total porosity of the bubble cavities but can only provide a rough estimate of the median diameter because of the limited resolution achieved in this study.
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| 12. |
- Bhuiyan, Iftekhar Uddin, et al.
(författare)
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Microstructure of Bentonite in Iron Ore Green Pellets
- 2014
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Ingår i: Microscopy and Microanalysis. - 1431-9276 .- 1435-8115. ; 20:1, s. 33-41
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Tidskriftsartikel (refereegranskat)abstract
- Sodium-activated calcium bentonite is used as a binder in iron ore pellets and is known to increase strength of both wet and dry iron ore green pellets. In this article, the microstructure of bentonite in magnetite pellets is revealed for the first time using scanning electron microscopy. The microstructure of bentonite in wet and dry iron ore pellets, as well as in distilled water, was imaged by various imaging techniques (e.g., imaging at low voltage with monochromatic and decelerated beam or low loss backscattered electrons) and cryogenic methods (i.e., high pressure freezing and plunge freezing in liquid ethane). In wet iron ore green pellets, clay tactoids (stacks of parallel primary clay platelets) were very well dispersed and formed a voluminous network occupying the space available between mineral particles. When the pellet was dried, bentonite was drawn to the contact points between the particles and formed solid bridges, which impart strength to the solid compact.
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| 13. |
- Bhuiyan, Iftekhar Uddin, et al.
(författare)
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Quantitative image analysis of bubble cavities in iron ore green pellets
- 2011
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Ingår i: Powder Technology. - : Elsevier BV. - 0032-5910 .- 1873-328X. ; 214:3, s. 306-312
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Tidskriftsartikel (refereegranskat)abstract
- Scanning electron microscopy and image analysis was used for quantitative analysis of bubble cavities in iron ore green pellets. Two types of pellets prepared with and without addition of flotation reagent prior to balling were studied. The bubble cavity porosity amounted to 2.8% in the pellets prepared without addition of flotation reagent prior to balling. When flotation reagent was added prior to balling, the bubble cavity porosity increased by a factor of 2.4 and the median bubble diameter was decreased slightly. It was also shown that mercury intrusion porosimetry is not suitable for determination of the distribution of bubble cavities. Finally, our data suggested that the difference in total porosity determined by mercury intrusion porosimetry and pycnometry between the two types of pellets was due to the bubble cavities.
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| 14. |
- Bleken, Bjørn Tore L., et al.
(författare)
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Unit cell thick nanosheets of zeolite H-ZSM-5 : Structure and activity
- 2013
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Ingår i: Topics in catalysis. - : Springer Science and Business Media LLC. - 1022-5528 .- 1572-9028. ; 56:9-10, s. 558-566
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Tidskriftsartikel (refereegranskat)abstract
- nosheets of zeolite H-ZSM-5 were synthesized and characterized by X-ray diffraction, transmission electron microscopy (TEM), N2- physisorption, FT-IR spectroscopy, 27Al and 29Si MAS NMR spectroscopy in addition to catalytic testing in conversion of methanol to hydrocarbons (MTH). It was found that Rietveld analysis, involving anisotropic broadening parameters, gave average crystallite dimensions in good agreement with TEM images. The selectivities in MTH is intact in the mesoporous nanosheet H-ZSM-5 with the largest difference being a higher C3/C2 ratio compared to regular H-ZSM-5.
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| 15. |
- Carabante, Ivan, et al.
(författare)
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Reutilization of porous sintered hematite bodies as effective adsorbents for arsenic(V) removal from water
- 2014
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Ingår i: Industrial & Engineering Chemistry Research. - : American Chemical Society (ACS). - 0888-5885 .- 1520-5045. ; 53:32, s. 12689-12696
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Tidskriftsartikel (refereegranskat)abstract
- A method was developed to enhance the arsenic adsorption capacity of porous bodies of sintered hematite. The method comprised the formation of a coating of 1 wt % iron oxide nanoparticles on the raw material. The nanoparticles showed two distinct habits: spherical habit, likely ferrihydrite, and acicular habit, likely goethite and/or akaganéite. The specific surface area of the hematite raw material increased from 0.5 to 3.75 m2/g, and the adsorption capacity increased from negligible to 0.65 mg of [As]/g as calculated from equilibrium and breakthrough adsorption data. Equilibrium adsorption data of arsenate on the adsorbent from a solution at pH 5 followed the Langmuir model, while breakthrough adsorption data for a 500 μg/L arsenate solution at pH 5 followed the Thomas model. The adsorbed arsenic could be desorbed using distilled water at pH 12. These results show the potential for the reutilization of waste products comprising coarse hematite bodies as adsorbents.
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| 16. |
- Cardenas, Edgar, et al.
(författare)
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Influence of the internal structure of the gel on Al-zoning in NBA-ZSM-5 crystals
-
Ingår i: Microporous and Mesoporous Materials. - 1387-1811 .- 1873-3093.
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Tidskriftsartikel (övrigt vetenskapligt/konstnärligt)abstract
- The crystallization of ZSM-5 from a gel comprising n-butylamine as structure directing agent was investigated. The samples were characterized by X-ray diffraction, nitrogen gas adsorption, extreme high-resolution transmission and scanning electron microscopy, and energy dispersive spectroscopy. The gel was found to be composed by a silica-rich matrix embedded in a skeleton of alumina-rich nanoparticles. During growth of the crystals, the silica-rich matrix is consumed first, and an increasing fraction of the alumina-rich nanoparticles are utilized later in the growth process. This leads to a non-uniform consumption of the gel walls during crystal growth. Consequently, the Si/Al ratio of the gel is steadily decreasing, which is accompanied by a corresponding decrease in the Si/Al ratio from the center to the outer surface of the crystals, i.e. Al-zoning of the ZSM-5 crystals.
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| 17. |
- Cardenas, Edgar, et al.
(författare)
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Microstructural evolution of condensed aggregates during the crystallization of ZSM-5 from a heterogeneous system
- 2021
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Ingår i: Journal of Crystal Growth. - : Elsevier. - 0022-0248 .- 1873-5002. ; 568–569
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Tidskriftsartikel (refereegranskat)abstract
- The microstructural evolution of precursors of ZSM-5 zeolite crystallized from a heterogeneous system using fumed silica, sodium aluminate and tetrapropylammonium ions as reagents is investigated. Entities previously described by Ren et al. (Chem. Mater. 2012, 24, 10, 1726–1737) as condensed aggregates, were extensively studied using scanning electron microscopy, and energy dispersive spectroscopy. It was observed that the condensed aggregates first comprise a core of nanocrystals that is enveloped by a shell of amorphous gel phase. During crystallization, the amorphous shell surrounding the core is converted into ZSM-5 crystals that grow to a film surrounding the core. The crystals in the film grow competitively with nutrients provided by the liquid phase from the surroundings, while the nanocrystals in the core show little or no signs of growth.
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| 18. |
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| 19. |
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| 20. |
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| 21. |
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| 22. |
- Ciurans Oset, Marina, 1993-
(författare)
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Microstructure and Mechanical Properties of Plasma Atomized Refractory Alloys
- 2023
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Licentiatavhandling (övrigt vetenskapligt/konstnärligt)abstract
- Plasma centrifugal atomization is a method widely used in the production of spherical powders of metals and alloys with relatively low melting points. A novel plasma centrifugal atomization process suitable for high melting point materials (i.e. 3500 ᵒC and above) was developed by Metasphere Technology AB, currently Höganäs Sweden AB. In this process, feedstock material in the form of crushed powder with particle sizes in the range 400-1000 µm is fed into a rotating crucible and subsequently melted by the glow discharge of a plasmatron. Due to high rotational speeds, a melt film forms at the edge of the crucible and breaks into fine droplets that are ejected into the reactor chamber and solidified in a whirl of cold inert gases. Capability of the plasmatron to reach very high temperatures, combined with extremely rapid cooling of the ejected droplets, allow for the fabrication of fine powders of refractory alloys exhibiting metastable phases that cannot be obtained otherwise. Oil drilling, ore processing and metal shaping applications, among other, require tool materials capable of withstanding harsh working conditions under heavy loads. Owing to their physical, chemical and mechanical properties, tungsten-carbon alloys are among the most suited materials for such applications. Melting followed by rapid solidification of tungsten-carbon mixtures with 3.9 wt.% C results in a biphasic structure composed of WC lamellae inserted in a W2C matrix, known as cast tungsten carbide (CTC). Due to the metastable nature of both phases present, CTC exhibits exceptional mechanical properties. CTC is mainly used as reinforcing dispersed phase in metal matrix composite hardfacing overlays, which are deposited by plasma transferred arc (PTA) welding or laser cladding onto steel tools.High-entropy alloys (HEAs) are defined as multi-component solid solutions with equimolar or near-equimolar concentration of all principal elements. Owing to their outstanding mechanical, corrosion, erosion, oxidation and radiation resistance properties compared to conventional alloys, HEAs are among the most suited materials for aerospace and nuclear applications. Several processing routes have allowed for laboratory-scale production of HEAs. Nevertheless, size and shape of bulk components that can be thus produced are largely limited. In a quest for up-scaling the processing of high-end bulk HEA components, plasma centrifugal atomization of pre-alloyed refractory HEA spherical powders suitable for additive manufacturing was envisaged.In this work, capabilities of the novel plasma centrifugal atomization for processing of refractory alloys into fine spherical powders have been evaluated based on two different material systems, namely CTC and a refractory HEA containing Ti, V, Zr, Nb, Mo, Hf, Ta, W. Challenges of local mechanical characterization of micron-sized powders have been addressed and a robust method for testing of individual particles has been developed. Mechanical properties such as hardness and fracture toughness of plasma atomized CTC powders have been extensively investigated and related to the corresponding thermal stories. Experimental results suggest significant straining of the crystal lattice in the case of as-atomized CTC, possibly due to extremely high cooling rates experienced by the solidifying particles. This has been ruled out the main reason for the outstanding mechanical properties of plasma atomized CTC compared to both spheroidized CTC and conventional cast & crushed CTC. Effective stress relieve was possible upon heat treatment. Plasma atomization of the refractory HEA yielded similar results, where an extremely fine microstructure with no noticeable chemical segregation was obtained. Indentation hardness of this novel microstructure was found to be approximately 25% higher than that of similar alloys reported in literature. HEA powder thus produced was then consolidated into bulk HEAs with very simple geometries, proving that this powder can be further processed into components of more or less complexity for pre-defined applications.
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| 23. |
- Ciurans Oset, Marina, 1993-
(författare)
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Plasma-Assisted Centrifugal Atomization of Refractory Alloys and Compounds
- 2024
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Doktorsavhandling (övrigt vetenskapligt/konstnärligt)abstract
- Near-net-shaping through powder metallurgy results not only in reduced material waste, but also reduced energy consumption, and strict control over the structure and properties of the final materials. The development of fabrication technologies suitable for the production of high-quality, fine metallic powders of conventional and novel alloys, with optimized mechanical, physical, and functional properties, is crucial to this manufacturing approach. A plasma-assisted centrifugal atomization unit optimized for the production of spherical cast tungsten carbide (CTC) powder was developed by Metasphere Technology AB, and subsequently acquired by Höganäs AB. In the standard implementation of this process, feedstock material in the form of crushed powder with particle sizes in the range of 400-1000 µm is fed into a rotating crucible, melted by the glow discharge of a plasma torch, and atomized into a dispersion of fine droplets that are ejected into the reactor chamber, and solidified in a whirl of cold gases. The capability of the plasma torch to melt materials with melting temperatures above 3 000 ᵒC, combined with the extremely rapid solidification of the ejected droplets, allows for the fabrication of spherical powders of refractory alloys exhibiting metastable phases that cannot be obtained otherwise.The main objectives of this work were to better understand the role of the centrifugal atomization mechanism on the microstructure and the mechanical properties of spherical CTC powders thus produced, particularly compared to other conventional powder fabrication routes, and to explore the capabilities of the pilot-scale plasma centrifugal atomization unit at Höganäs Sweden AB (Luleå, Sweden) for the design and development of novel refractory alloys.The challenges of local mechanical characterization of micron-sized hard carbide powders have been addressed. A robust method for testing individual particles has been developed, based on Vickers microindentation of polished powder specimens and atomic force microscopy (AFM) topography imaging of the indented surfaces. This method enabled a reliable comparison among CTC powders fabricated by different methods, evidencing the mechanical superiority of the centrifugally-atomized spherical powder. Subsequently, the microindentation hardness, the micro-pillar compressive strength, and the resistance to cyclic compressive loading of entire particles were extensively investigated in centrifugally-atomized CTC powders subjected to different heat treatments. The extremely high cooling rates experienced by the solidifying particles were concluded to result in the refinement of the CTC lamellar structure and significant straining of the crystal lattice. Moreover, rather complex stress relaxation phenomena through the entire particles were observed as a result of the different heat treatments, and attributed to local WC-to-W2C phase transformations at the surface of the particles.In order to showcase the capabilities of the plasma-assisted centrifugal atomization unit for alloy development, a suitable processing route for the fabrication of spherical powders of refractory multi-principal element alloys has been developed. In particular, a near-equiatomic refractory high-entropy alloy containing Ti, V, Zr, Nb, Mo, Hf, Ta, and W has been used as model alloy. Starting from a blend of the corresponding elemental powders, the preparation of suitable granulated feedstock material by partial sintering followed by cryogenic milling was considered. Subsequently, in-situ alloying of the powder blend in the melt and simultaneous atomization was envisaged in order to avoid the very time-consuming step of cryogenic milling. Size and microstructure refinement, chemical homogenization, and degradation of the fabricated spherical powders were investigated upon successive re-atomization runs. The indentation hardness, phase stability, prospects of consolidation into bulk alloys by spark plasma sintering, and the hydrogen storage and permeability properties of the alloys thus produced have been extensively investigated.
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| 24. |
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| 25. |
- Ciurans Oset, Marina, 1993-, et al.
(författare)
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Role of the microstructure and the residual strains on the mechanical properties of cast tungsten carbide produced by different methods
- 2024
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Ingår i: Journal of Materials Research and Technology. - : Elsevier. - 2238-7854 .- 2214-0697. ; 30, s. 3640-3649
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Tidskriftsartikel (refereegranskat)abstract
- Cast tungsten carbide (CTC) is a biphasic, pearlitic-like structure composed of WC lamellae in a matrix of W2C. Besides excellent flowability, spherical CTC powders exhibit superior hardness and wear resistance. Nevertheless, the available literature generally fails to explain the physical mechanisms behind such a phenomenon. In the present work, the microstructure and the mechanical properties of the novel centrifugally-atomized spherical CTC have been extensively investigated. This material exhibited an extremely fine microstructure, with WC lamellae of 27-29 nm in thickness and bulk lattice strains of 1.0-1.4 %, resulting in a microindentation hardness of 31.4 ± 1.6 GPa. The results of this study clearly show that centrifugally-atomized CTC is mechanically superior to both spheroidized CTC and conventional cast-and-crushed CTC. In addition, the effect of a series of heat treatments on the bulk fracture toughness and the fatigue life of entire CTC particles was also investigated. The reduction of residual stresses in the bulk of particles upon annealing dramatically increased the indentation fracture toughness, whereas the bulk microindentation hardness remained essentially unaffected. Regarding the fatigue life of entire particles under uniaxial cyclic compressive loading, local phase transformation phenomena at the surface of the particles upon heat treatment were concluded to play the most critical role. Indeed, the cumulative fatigue damage was minimized in surface-carburized CTC powders, where compressive stresses were induced at the outermost surface.
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| 26. |
- Ciurans Oset, Marina, 1993-, et al.
(författare)
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Use of AFM topography images to determine microindentation hardness of cast tungsten carbide powders
- 2022
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Ingår i: International journal of refractory metals & hard materials. - : Elsevier. - 0263-4368. ; 107
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Tidskriftsartikel (refereegranskat)abstract
- Hardness is defined as the resistance of a material to localized plastic deformation. Owing to their non-destructive nature, static indentation hardness tests are widely used in industry. Hardness testing is particularly useful for the mechanical characterization of materials that cannot be tested otherwise, e.g. powdered materials. In this study, challenges related to Vickers microindentation hardness testing of hard brittle cast tungsten carbide (CTC) powders were extensively investigated. Test load was optimized to obtain sufficiently large crack-free indentations allowing for precise measurement of the diagonal lengths. The influence of the operator and imaging technique on the measured hardness value was evaluated. Topography of residual imprints was investigated using atomic force microscopy (AFM) and a systematic and operator bias-free method to locate the indentation vertexes was developed. Results suggested that measurement variability introduced by AFM scanning and post-processing was as low as 3.1% and 1.3% with respect to the mean hardness value, respectively. Since the variability due to the measuring system can be isolated, the homogeneity of powders can be reliably evaluated from the hardness measurements thus obtained.
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| 27. |
- Engström, Andreas, et al.
(författare)
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Synthesis of a TiCN – SiC polyhedron and elongated crystals nanopowder at low nitrogen concentration
- 2012
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Ingår i: Materials letters (General ed.). - : Elsevier BV. - 0167-577X .- 1873-4979. ; 81, s. 148-150
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Tidskriftsartikel (refereegranskat)abstract
- At room temperature diluted TiCl4 and CCl4 were reduced by sodium particles and mixed with a polycarbomethylsilane (PCS) solution to yield a precursor. It was dried and subsequently annealed at 1300 °C, 1400 °C and 1450 °C in a tube furnace using argon with 10 ppm N2. After the 1450 °C annealing a nanocrystalline powder of TiC0.5 N0.5–SiC polyhedron and elongated crystals was obtained. At the low nitrogen concentration during annealing a gradual nitration is proposed. It is promoted by carbon gaseous species, precursor oxidation, a sufficient temperature and a summarised nitrogen surplus compared to the titanium and carbon amount.
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| 28. |
- Garcia, Gustavo, et al.
(författare)
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Preparation of zeolite A with excellent optical properties from clay
- 2015
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Ingår i: Journal of Alloys and Compounds. - : Elsevier BV. - 0925-8388 .- 1873-4669. ; 619, s. 771-777
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Tidskriftsartikel (refereegranskat)abstract
- Discoloration of zeolite A powder is a common problem when natural raw materials such as kaolin clay are used because of the formation of colored iron compounds. In this study, we report on a novel method to produce zeolite A with excellent optical properties, from clays. The brightness is as high as 94.5 and the yellowness is as low as 3.0. The product is comprised of intergrown zeolite A crystals with cubic habit and a length ranging between 0.5 and 2 μm. Good optical properties are obtained when the raw material contains magnesium, as some natural raw materials do, or alternatively, when a magnesium compound is added to the raw material. Magnesium probably forces iron inside colorless extraneous magnesium aluminosilicate compounds. This simple process appears very promising for the preparation of zeolite A with good optical properties from inexpensive natural raw materials.
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| 29. |
- Garcia, Gustavo, et al.
(författare)
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Selective synthesis of FAU-type zeolites
- 2018
-
Ingår i: Journal of Crystal Growth. - : Elsevier. - 0022-0248 .- 1873-5002. ; 489, s. 36-41
-
Tidskriftsartikel (refereegranskat)abstract
- In the present work, parameters influencing the selectivity of the synthesis of FAU-zeolites from diatomite were studied. The final products after varying synthesis time were characterized by scanning electron microscopy, energy dispersive spectroscopy, X-ray diffraction and gas adsorption. It was found that high concentrations of NaCl could completely inhibit the formation of zeolite P, which otherwise usually forms as soon as maximum FAU crystallinity is reached. In the presence of NaCl, the FAU crystals were stable for extended time after completed crystallization of FAU before formation of sodalite. It was also found that addition of NaCl barely changed the crystallization kinetics of FAU zeolite and only reduced the final FAU particle size and SiO2/Al2O3 ratio slightly. Other salts containing either Na or Cl were also investigated. Our results suggest that there is a synergistic effect between Na+ and Cl-. This is attributed to the formation of (Na4Cl)3+ clusters that stabilize the sodalite cages. This new finding may be used to increase the selectivity of syntheses leading to FAU-zeolites and avoid the formation of undesirable by-products, especially if impure natural sources of aluminosilica are used.
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| 30. |
- Garcia, Gustavo, et al.
(författare)
-
Synthesis of zeolite Y from diatomite as silica source
- 2016
-
Ingår i: Microporous and Mesoporous Materials. - : Elsevier BV. - 1387-1811 .- 1873-3093. ; 219, s. 29-37
-
Tidskriftsartikel (refereegranskat)abstract
- Bolivian diatomite was successfully used as a silica source for the synthesis of zeolite Y. Prior to synthesis, the diatomite was leached with sulfuric acid to remove impurities and aluminum sulfate was used as an aluminum source. The raw materials were reacted hydrothermally at 100 °C in water with sodium hydroxide and different Na2O/SiO2 ratios were investigated. The final products were characterized by scanning electron microscopy, X-ray diffraction, gas adsorption and inductively coupled plasma-atomic emission spectroscopy. Diatomites originating from different locations and therefore containing different types and amounts of minerals and clays as impurities were investigated. After optimization of synthesis time, zeolite Y with low SiO2/Al2O3 ratio (3.0–3.9) was obtained at a high yield for high alkalinity conditions (Na2O/SiO2 = 0.85–2.0). Lower Na2O/SiO2 ratios resulted in incomplete dissolution of diatomite and lower yield. Nevertheless, decreasing alkalinity resulted in a steady increase of the SiO2/Al2O3 ratio in zeolite Y. Consequently, it was possible to synthesize almost pure zeolite Y with a SiO2/Al2O3 ratio of 5.3 for a Na2O/SiO2 ratio of 0.6, albeit at a low yield. In this respect, diatomite enables the synthesis of high silica zeolite Y and behaves similarly to colloidal silica in traditional syntheses, with both sources of silica having in common a high degree of polymerization. Interestingly, the presence of minerals and clays in the starting diatomite had marginal effects on the outcome of the synthesis. However, their dissolution resulted in presence of calcium and magnesium in the zeolite Y crystals. Finally, overrun of all investigated compositions resulted in the formation of zeolite P nucleating and growing onto dissolving zeolite Y crystals, which was shown to be triggered when aluminum was completely depleted at high alkalinity
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| 31. |
- Garcia Mendoza, Javier Gustavo, 1980-
(författare)
-
Synthesis and applications of low silica zeolites from Bolivian clay and diatomaceous earth
- 2017
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Doktorsavhandling (övrigt vetenskapligt/konstnärligt)abstract
- The aim of the present work was to develop synthesis routes to produce synthetic zeolites with industrial attractiveness from non-expensive Bolivian raw materials, such as clays and diatomite. In particular, the work was focused on the synthesis of low-silica zeolites with the LTA and FAU structures. The raw materials as well as intermediate and final zeolite products were structurally characterized by different techniques, namely scanning electron microscopy (SEM), X-ray diffraction (XRD), nitrogen gas adsorption, inductively coupled plasma mass spectrometry (ICP-MS). Besides, the final properties were evaluated for intended applications i.e. brightness by UV-VIS spectroscopy for LTA-type zeolite A in detergents and the CO2 breakthrough adsorption of FAU-type zeolite X for gas separation. The first part of this study deals with the synthesis of low silica zeolite A from Bolivian montmorillonite clay (Paper I). The clay was fused at high temperature with NaOH to render the material more reactive. The raw montmorillonite had a SiO2/Al2O3 ratio of 4 and sodium aluminate was added to the mixture to decrease this ratio to 2. An optimization of the synthesis time was performed. For the sake of comparison, the same treatment was applied to commercial kaolin. The final zeolite product from Bolivian montmorillonite exhibited high brightness despite the presence of iron in appreciable amount in the starting material and the final product. It was concluded that magnesium, present in the raw material, exerted a masking effect on iron. The latter was incorporated into extraneous magnesium aluminosilica compounds, thereby increasing brightness and strongly decreasing the overall yellowness. This simple method appears as a promising alternative to the complex and costly techniques suggested to reduce the iron content in natural raw materials, especially kaolin. The second part of the study addresses the synthesis, characterization and evaluation of FAU-type zeolites i.e. Y and X from diatomite and chemical grade reagents, respectively. With regard to the synthesis of zeolite Y, the diatomite was leached in sulfuric acid to remove impurities prior to synthesis, this step also resulted in dealumination, which rendered possible the synthesis of ZSM-5 zeolite (Paper II). However, extra aluminum had to be added in the form of aluminum sulfate for the synthesis of zeolite Y (Paper III). In this case, the raw materials were reacted hydrothermally at 100°C in aqueous medium with sodium hydroxide. Variations in parameters such as the Na2O/SiO2 ratio and synthesis time were investigated. As a result, micro-sized crystals of zeolite Y were obtained at a Na2O/SiO2 ratio of 0.9, which produced zeolite Y with a SiO2/Al2O3 ratio of 3.9. Also, zeolite Y with a SiO2/Al2O3 ratio of 5.3 in low yield at a Na2O/SiO2 ratio of 0.6 was achieved. In this way, diatomite behaved similarly to colloidal silica in traditional syntheses having a high degree of polymerization. Zeolite Y with a SiO2/Al2O3 ratio of 5.3 might be useful for the production of ultra-stable zeolite Y for use as FCC catalyst. Addition of NaCl to this system was also found to completely inhibit the formation of zeolite P and to retard the effects of overrunning by a synergic effect of Na and Cl ions (Paper IV). As an innovate application, FAU-type zeolite X films were growth on steel monoliths as structured adsorbent for CO2 removal (Paper V). Thicknesses 3 and 10 µm on steel monoliths of 1600 cells per square inches (c.p.s.i.) were investigated. The produced structured adsorbents showed reasonable CO2 adsorption capacity but with a very low pressure drop compared with traditional packed beds of zeolite beads. The CO2 breakthrough fronts were very sharp and the mass transfer resistance was very low compensating the presence of less adsorbing material by reducing cycle time and increased cycle frequency to increase throughput. To summarize, zeolites with promising characteristics were successfully synthesized from Bolivian raw materials in this thesis work. However, further characterization is required to qualify these products for industrial applications. Moreover, this study might help the development of poor regions of the Bolivian Altiplano and open up for large scale production, since the methods developed in this work are simple and non-expensive.
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| 32. |
- Hedlund, Jonas, et al.
(författare)
-
Microchannel Zeolite 13X Adsorbent with High CO2 Separation Performance
- 2021
-
Ingår i: Separation and Purification Technology. - : Elsevier. - 1383-5866 .- 1873-3794. ; 277
-
Tidskriftsartikel (refereegranskat)abstract
- Uniform 13X films with thicknesses of 3 and 11 µm were grown on supports in the form of steel monoliths with a cell density of 1600 cpsi and microchannels width of 0.5 mm. Sharp breakthrough fronts and a dynamic uptake of 3.4 mmol CO2 g-1 zeolite were observed in the forwarding step of breakthrough experiments for a feed of 10 % CO2 in N2 with a high flow rate at 293 K and 1 bar. Numerical modeling showed that the adsorption process was very fast and that the transport of CO2 in the thin zeolite layer was the rate-limiting step, the mass transfer resistance for the 11 µm film is 2.2 times lower than zeolite 13X pellets and 100 times lower than zeolite 4A beads. Axial dispersion, pressure drop, and gas film resistance were shown to be negligible. The steel monolith support provides good mechanical strength and excellent thermal conductivity for the 13X films. The combination of properties makes this adsorbent a good performer when compared with other types of structured zeolite adsorbents in reported literatures. This microchannel adsorbent is a promising alternative to traditional adsorbents in processes of fast CO2 separation with short cycle times.
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| 33. |
- Karim, Zoheb, et al.
(författare)
-
Nanoporous membranes with cellulose nanocrystals as functional entity in chitosan : removal of dyes from water
- 2014
-
Ingår i: Carbohydrate Polymers. - : Elsevier BV. - 0144-8617 .- 1879-1344. ; 112, s. 668-676
-
Tidskriftsartikel (refereegranskat)abstract
- Fully biobased composite membranes for water purification were fabricated with cellulose nanocrystals (CNCs) as functional entities in chitosan matrix via freeze-drying process followed by compacting. The chitosan (10 wt%) bound the CNCs in a stable and nanoporous membrane structure with thickness of 250-270 μm, which was further stabilized by cross-linking with gluteraldehyde vapors. Scanning electron microscopy (SEM) studies revealed well-individualized CNCs embedded in a matrix of chitosan. Brunauer, Emmett and Teller (BET) measurements showed that the membranes were nanoporous with pores in the range of 13-10 nm. In spite of the low water flux (64 L m-2 h-1), the membranes successfully removed 98%, 84% and 70% respectively of positively charged dyes like Victoria Blue 2B, Methyl Violet 2B and Rhodamine 6G, after a contact time of 24 h. The removal of dyes was expected to be driven by the electrostatic attraction between negatively charged CNCs and the positively charged dyes.
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| 34. |
- Karimi, Somayeh, et al.
(författare)
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A simple method for blocking defects in zeolite membranes
- 2015
-
Ingår i: Journal of Membrane Science. - : Elsevier BV. - 0376-7388 .- 1873-3123. ; 489, s. 270-274
-
Tidskriftsartikel (refereegranskat)abstract
- The abatement of defects in zeolite membranes is essential for achieving high selectivity. In the present work, a simple and effective method for blocking defects in ultra-thin (ca. 0.5 μm) MFI zeolite membranes has been developed. The method is based on deposition of an ultra-thin (∼15 nm) layer of amorphous silica on the top surface of the membrane. Permporometry data indicated that the amount of defects, especially defects larger than 4 nm, in the membranes was significantly reduced after the modification. In mixture separation experiments, the CO2/H2 separation factor increased dramatically after blocking the defects in a defective membrane that was selected for the experiments. For instance, at 263 K and 9 bar feed pressure, the CO2/H2 separation factor increased from 8.5 to 36 after modification of the membrane, whereas the CO2 flux only decreased by ca. 40%.
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| 35. |
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| 36. |
- Korelskiy, Danil, et al.
(författare)
-
An experimental study of micropore defects in MFI membranes
- 2014
-
Ingår i: Microporous and Mesoporous Materials. - : Elsevier BV. - 1387-1811 .- 1873-3093. ; 186, s. 194-200
-
Tidskriftsartikel (refereegranskat)abstract
- In the present work, two ultra-thin MFI membranes, prepared using hydroxide and fluoride ions as mineralizing agents, respectively, were carefully examined by permporometry. The amount of micropore defects, as determined by permporometry, differed significantly between the two different membranes. For the first time, it was demonstrated that the micropore defects determined by permporometry were most likely open grain boundaries. The results were verified by direct observation of the open grain boundaries by a state-of-the-art XHR-scanning electron microscopy instrument. In addition, the permporometry data were also consistent with permeation data using 1,3,5-trimethylbenzene (TMB) as a probe molecule, separation data using an equimolar mixture of n-hexane and TMB, and nitrogen adsorption data.
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| 37. |
- Korelskiy, Danil, et al.
(författare)
-
Characterization of flow-through micropores in MFI membranes by permporometry
- 2012
-
Ingår i: Journal of Membrane Science. - : Elsevier BV. - 0376-7388 .- 1873-3123. ; 417-418, s. 183-192
-
Tidskriftsartikel (refereegranskat)abstract
- Permporometry was used for the first time to characterize flow-through micropore defects down to 0.7 nm in size in MFI zeolite membranes. Helium was used as the non-adsorbing gas and n-hexane or benzene was used as the adsorbate. The helium flow through zeolite pores was estimated using percolation theory and the remaining flow was assigned to flow-through defects. The area distribution of flow-through defects was estimated from the data using a simple model and similar results were obtained using both adsorbates. The total area of defects determined using n-hexane as the adsorbate was as low as about 0.7% of the membrane area and defects with a width below 1 nm constituted 97% of the total defect area for the best membrane. The permporometry results were supported by n-hexane/1,3,5-trimethylbenzene separation experiments. The permporometry data were also consistent with HR-SEM observations indicating the presence of narrow open grain boundaries, and absence of large cracks and pinholes
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| 38. |
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| 39. |
- Korelskiy, Danil, et al.
(författare)
-
Reprint of: An experimental study of micropore defects in MFI membranes
- 2014
-
Ingår i: Microporous and Mesoporous Materials. - : Elsevier BV. - 1387-1811 .- 1873-3093. ; 192, s. 69-75
-
Tidskriftsartikel (refereegranskat)abstract
- In the present work, two ultra-thin MFI membranes, prepared using hydroxide and fluoride ions as mineralizing agents, respectively, were carefully examined by permporometry. The amount of micropore defects, as determined by permporometry, differed significantly between the two different membranes. For the first time, it was demonstrated that the micropore defects determined by permporometry were most likely open grain boundaries. The results were verified by direct observation of the open grain boundaries by a state-of-the-art XHR-scanning electron microscopy instrument. In addition, the permporometry data were also consistent with permeation data using 1,3,5-trimethylbenzene (TMB) as a probe molecule, separation data using an equimolar mixture of n-hexane and TMB, and nitrogen adsorption data
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| 40. |
- Leduc, Jennifer, et al.
(författare)
-
Electronically-Coupled Phase Boundaries in α‑Fe2O3/Fe3O4 Nanocomposite Photoanodes for Enhanced Water Oxidation
- 2019
-
Ingår i: ACS APPLIED NANO MATERIALS. - : American Chemical Society (ACS). - 2574-0970. ; 2:1, s. 334-342
-
Tidskriftsartikel (refereegranskat)abstract
- Photoelectrochemical (PEC) water splittingreactions are promising for sustainable hydrogen productionfrom renewable sources. We report here, the preparation of α-Fe2O3/Fe3O4 composite films via a single-step chemical vapordeposition of [Fe(OtBu)3]2 and their use as efficient photoanode materials in PEC setups. Film thickness and phase segregation was controlled by varying the deposition time and corroborated through cross-section Raman spectroscopy and scanning electron microscopy. The highest water oxidationactivity (0.48 mA/cm2 at 1.23 V vs RHE) using intermittent AM 1.5 G (100 mW/cm2) standard illumination was found forhybrid films with a thickness of 11 μm. This phenomenon is attributed to an improved electron transport resulting from ahigher magnetite content toward the substrate interface and an increased light absorption due to the hematite layer mainly located at the top surface of the film. The observed high efficiency of α-Fe2O3/Fe3O4 nanocomposite photoanodes is attributed to the close proximity and establishment of 3D interfaces between the weakly ferro- (Fe2O3) and ferrimagnetic (Fe3O4) oxides, which in view of their differential chemical constitution andvalence states of Fe ions (Fe2+/Fe3+) can enhance the charge separation and thus the overall electrical conductivity of the layer.
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| 41. |
- Mangalaraja, R.V., et al.
(författare)
-
Combustion synthesis of Y2O3 and Yb-Y2O3 : Part 1: Nanopowders and their characterization
- 2008
-
Ingår i: Journal of Materials Processing Technology. - : Elsevier BV. - 0924-0136 .- 1873-4774. ; 208:1-3, s. 415-422
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Tidskriftsartikel (refereegranskat)abstract
- Nanosized yttrium oxide and ytterbium doped yttrium oxide powders were prepared by ceramic combustion techniques such as flash combustion, citrate gel decomposition and glycine combustion using urea, citric acid and glycine respectively as fuels. As synthesized precursors and calcined powders were characterized for their structural, particle size and morphology, and the optimization of calcination process by differential scanning calorimetry and thermal gravimetry. The thermal analyses together with XRD results demonstrate the effectiveness of the combustion process for the synthesis of pure phase nanocrystalline powders. Nanocrystalline pure yttria powders were obtained by the calcination of as-prepared precursors at 1100 °C for 4 h.
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| 42. |
- Mangalaraja, R.V., et al.
(författare)
-
Microwave assisted combustion synthesis of nanocrystalline yttria and its powder characteristics
- 2009
-
Ingår i: Powder Technology. - : Elsevier BV. - 0032-5910 .- 1873-328X. ; 191:3, s. 309-314
-
Tidskriftsartikel (refereegranskat)abstract
- Microwave assisted combustion synthesis is used for fast and controlled processing of advanced ceramics. Single phase and sinter active nanocrystalline cubic yttria powders were successfully synthesized by microwave assisted combustion using the organic fuels urea, citric acid and glycine as reducing agents. The precursor powders were investigated by thermogravimetry (TG) and differential scanning colorimetry (DSC) analyses. The as-prepared precursors and the resulting oxide powders calcined at 1100 °C in oxygen atmosphere were characterized for their structure, particle size and morphology. The thermal analyses (TG/DSC), X-ray diffraction (XRD) and Fourier transform infra red (FT-IR) results demonstrate the effectiveness of the microwave assisted combustion synthesis. The scanning electron microscopy (SEM) observations show the different morphologies of as-prepared powders and transmission electron microscopy (TEM) shows the particle sizes in the range of 30-100 nm for calcined powders for different fuels. The results confirm that the homogeneous, nano scale yttria powders derived by microwave assisted combustion have high crystalline quality and the morphology of the as-prepared precursor powders depends on the nature of organic fuel used.
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| 43. |
- Mangalaraja, Ramalinga Viswanathan, et al.
(författare)
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Sintering, microstructural and mechanical characterization of combustion synthesized Y2O3 and Yb3+-Y2O3
- 2009
-
Ingår i: Journal of the Ceramic Society of Japan. - : Ceramic Soc of Japan. - 1882-0743 .- 1348-6535. ; 117:1371, s. 1258-1262
-
Tidskriftsartikel (refereegranskat)abstract
- The present work highlights the microstructural features and mechanical properties of Y2O3 prepared with and without Yb3+ doping that processed through combustion synthesis involving various organic fuels such as urea, citric acid and glycine. Properties such as powder-now, particle packing, green density, % of shrinkage, sintered density, grain size, Vicker's microhardness (H-v) and fracture toughness (K-IC) were analyzed and compared with respect to the fuel sources. The as combusted precursors were calcined at 1100 degrees C for 4 h under oxygen atmosphere to obtain fully crystalline Y2O3 powders. Cylindrical pellets were fabricated as test specimens and sintered at 1600 degrees C for 3 h. The SEM images of the sintered yttria samples show an average grain size of < 3 mu m irrespective of the fuels. However, the mechanical properties show significant dependence on the fuels used. A maximum hardness of 6.8 +/- 0.1 and 7.0 +/- 0.1 GPa was obtained for Y2O3 and Yb3+ doped Y2O3 derived from glycine fuel. Whereas the maximum fracture toughness of 2.6 +/- 0.3 MPa m(1/2) was obtained for the samples derived from urea. The Yb3+ doping found to increase the bulk hardness of yttria from 0.2 to 0.6 GPa. The study contributes to appropriately select the fuels for obtaining high dense, mechanically stable yttria ceramics through combustion process.
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| 44. |
- Mangalaraja, Ramalinga Viswanathan, et al.
(författare)
-
Synthesis of nanocrystalline yttria through in-situ sulphated-combustion technique
- 2009
-
Ingår i: Ceramic Society of Japan. Journal. - : Ceramic Society of Japan. - 1882-0743 .- 1348-6535. ; 117:1370, s. 1065-1068
-
Tidskriftsartikel (refereegranskat)abstract
- An in-situ sulphated-combustion reaction was conducted on the precursor mixture consisting of yttrium nitrate, organic fuels (urea, citric acid and glycine) and 10 mol% ammonium sulphate [(NH4)(2)SO4] at 500 degrees C. Effect of sulphate addition on yttria particles morphology has been analyzed with respect to the types of fuels. In un-sulphated combustion, the calcined yttria powders showed rectangular particle morphology and low specific surface area. Whereas in sulphated-combustion spherical shaped yttria particles were achieved for glycine fuel. In the case of citric acid fuel, yttria powders with high specific surface area [26 m(2)/g] were obtained. For all the fuels, the sulphated-combustion reaction produced nanocrystalline yttria powders and they also had primary crystallite size below 4 nm in the as prepared conditions. Upon calcination at 1100 degrees C, these powders attained mean particle size of 50 nm which was confirmed by TEM.
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| 45. |
- Melk, Latifa, et al.
(författare)
-
Surface microstructural changes of Spark Plasma Sintered Zirconia after grinding and annealing
- 2016
-
Ingår i: Ceramics International. - : Elsevier BV. - 0272-8842 .- 1873-3956. ; 42:14, s. 15610-15617
-
Tidskriftsartikel (refereegranskat)abstract
- Spark plasma sintered zirconia (3Y-TZP) specimens have been produced of 140 nm 372 nm and 753 nm grain sizes by sintering at 1250 C, 1450 C and 1600 C, respectively. The sintered zirconia specimens were grinded using a diamond grinding disc with an average diamond particle size of about 60 µm, under a pressure of 0.9 MPa. The influence of grinding and annealing on the grain size has been analysed. It was shown that thermal etching after of ruff grinding of specimens at 1100 C for one hour induced an irregular surface layer of about a few hundred nanometres in thickness of recrystallized nano-grains, independently of the initial grain size. However, if the ground specimens were exposed to higher temperature, e.g. annealing at 1575 °C for one hour, the nano-grain layer was not observed and the final grain size was similar to that achieved by the same heat treatments on carefully polished specimens. Therefore, by appropriate grinding and thermal etching treatments, nanograined surface layer can be obtained which increases the resistance to low temperature degradation.
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| 46. |
- Mouzon, Johanne, et al.
(författare)
-
Alternative method to precipitation techniques for synthesizing yttrium oxide nanopowder
- 2007
-
Ingår i: Powder Technology. - : Elsevier BV. - 0032-5910 .- 1873-328X. ; 177:2, s. 77-82
-
Tidskriftsartikel (refereegranskat)abstract
- Yttrium oxide nano-powder has been successfully synthesized by a novel approach. In the first step, a foamy structure was produced by combustion synthesis using yttrium nitrate and glycine. This was followed by the addition of sulfate ions and calcination at 1100 °C for 4 h. The sulfated and unsulfated powders were characterized by X-ray powder diffraction (XRD), scanning electron microscope (SEM), transmission electron microscopy (TEM) and the Brunauer-Emmett-Teller method (BET). The sinterability of the resulting powders was also studied by dilatometry. The studies indicated that this method not only allows for producing weakly-agglomerated nano-particles, but is also a very time-efficient process in comparison to precipitation techniques. Moreover, the possibility of performing all processing steps in a fully automated batch reactor was also considered.
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| 47. |
- Mouzon, Johanne, et al.
(författare)
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Comparison between slip-casting and uniaxial pressing for the fabrication of translucent yttria ceramics
- 2008
-
Ingår i: Journal of Materials Science. - : Springer Science and Business Media LLC. - 0022-2461 .- 1573-4803. ; 43:8, s. 2849-2856
-
Tidskriftsartikel (refereegranskat)abstract
- Slip casting and uniaxial pressing were compared as first consolidation stages prior to cold isostatic pressing (CIP) to produce translucent yttria ceramics. In the first step, yttria slurries suitable for slip casting were prepared. The viscosity was optimized with respect to the starting agglomeration state, amount of dispersant, milling time, and number of milling balls. Secondly, pellets were prepared either by slip casting or uniaxial pressing and then cold-isostatically pressed. Finally, the pellets were made translucent by a combination of pre-sintering and hot isostatic pressing (HIP). Although slip-cast and pressed samples exhibited similar green-body densities after CIP and pre-sintering, the samples prepared by slip casting were more homogeneous in terms of translucency and microstructure throughout their bodies. This was attributed to the ability of slip casting to minimize density gradients during packing, and to the beneficial effect of ball-milling to remove larger agglomerates before casting. Therefore, slip casting as a first consolidation stage prior to CIP appears to be more suitable than uniaxial pressing in order to prepare homogeneous optical ceramics.
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| 48. |
- Mouzon, Johanne, et al.
(författare)
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Comparison of two different precipitation routes leading to Yb doped Y2O3 nano-particles
- 2007
-
Ingår i: Journal of the European Ceramic Society. - : Elsevier BV. - 0955-2219 .- 1873-619X. ; 27:4, s. 1991-1998
-
Tidskriftsartikel (refereegranskat)abstract
- Two different precipitation routes leading to (YbxY1-x)2O3 nano-particles (with x = 0; 0.027; and 0.31) were compared, namely, precipitation of hydroxynitrate platelets and amorphous carbonate spherical particles. For both methods, the particle morphology was observed by scanning electron microscopy. X-ray diffraction studies of the unit cell, energy dispersive X-ray analysis and inductive coupled plasma spectroscopy were used to check the ytterbium distribution. The precipitation of amorphous carbonate was found to produce particles with uniform morphology and homogeneous distribution of ytterbium, while hydroxide precipitation favours the formation of hard and dense ytterbium-rich agglomerates. These differences are discussed in terms of precipitation, growth and agglomeration behaviour. The sinterability of both resulting powders is also discussed.
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| 49. |
- Mouzon, Johanne, et al.
(författare)
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Cryo-SEM method for the observation of entrapped bubbles and degree of water filling in large wet powder compacts
- 2011
-
Ingår i: Journal of Microscopy. - : Wiley. - 0022-2720 .- 1365-2818. ; 242:2, s. 189-196
-
Tidskriftsartikel (refereegranskat)abstract
- There are generally two problems associated with cryogenic scanning electron microscopy (cryo-SEM) observations of large wet powder compacts. First, because water cannot be vitrified in such samples, formation of artefacts is unavoidable. Second, large frozen samples are difficult to fracture but also to machine into regular pieces which fit in standard holders, especially if made of hard materials like ceramics. In this article, we first describe a simple method for planning hard cryo-samples and a low-cost technique for cryo-fracture and transfer of large specimens. Subsequently, after applying the entire procedure to green pellets of iron ore produced by balling, we compare the influence of plunge- and unidirectional freezing on large entrapped bubbles throughout the samples as well as the degree of water filling at the outer surface of the pellets. By carefully investigating the presence of artefacts in large areas of the samples and by controlling the orientation of the sample during freezing and preparation, we demonstrate that unidirectional freezing enables the observation of large entrapped bubbles with minimum formation of artefacts, whereas plunge freezing is preferable for the characterization of the degree of water filling at the outer surface of wet powder compacts. The minimum formation of artefacts was due to the high packing density of the iron ore particles in the matrix
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| 50. |
- Mouzon, Johanne, et al.
(författare)
-
Effect of drying and dewatering on yttria precursors with transient morphology
- 2006
-
Ingår i: Journal of The American Ceramic Society. - : Wiley. - 0002-7820 .- 1551-2916. ; 89:10, s. 3094-3100
-
Tidskriftsartikel (refereegranskat)abstract
- The influence of drying and dewatering of a yttrium hydroxynitrate precursor with transient morphology was investigated. The ability of this precursor to form soft agglomerated nanoparticles after calcination is dependent on the dewatering method. Freeze drying leads to finer particles than other dewatering methods that involve removal of the solvent from its liquid state. As water is directly removed by sublimation during freeze drying, this method inhibits the formation of solid bridges between hydroxynitrate platelets. These bridges, which form with the other dewatering methods, destabilize the spheroidization process of the platelets during subsequent firing at high temperatures.
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