Surface microstructural changes of spark plasma sintered zirconia after grinding and annealing

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 a 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. The resulted 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.Peer ReviewedPostprint (author's final draft

Microchannel zeolite 13X adsorbent with high CO2 separation performance

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

Microstructure of Bentonite in Iron Ore Green Pellets

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 compac

Synthesis of ytterbium-doped yttrium oxide nanoparticles and transparent ceramics

In this thesis, the author reports on a ceramic process leading to transparent ytterbium doped yttrium oxide ceramics. The full production route was investigated, namely: powder preparation, compaction and sintering. First, yttria nano-powder was fabricated from a precursor with transient morphology, i.e. yttrium hydroxynitrate platelets that decomposed into spherical yttria nano-particles during calcination. The influence of different dewatering methods on such precursors was investigated. Water removal by freeze-drying was shown to be optimal for the production of non- agglomerated nano-particles compared to other methods that involve solvent removal by evaporation. This was attributed to the ability of freeze-drying to avoid the formation of solid bridges, since water is directly removed by sublimation. In a second step, doping with ytterbium was performed. Two precipitation routes were compared: precipitation of hydroxynitrate platelets and precipitation of amorphous carbonate. The latter route was shown to allow a better distribution of ytterbium in the yttrium oxide matrix after calcination of the corresponding precursor. This was attributed to good cation mixing within the amorphous particles, while formation of the hydroxynitrate platelets resulted in segregation of ytterbium because of the precipitation of an ytterbium-rich secondary phase. Because of the laborious character of the above techniques, an alternative method for synthesizing yttrium oxide powder was developed. This method not only allows for producing weakly-agglomerated nano-particles with equiaxed morphology, but is also a very time-efficient process. Its characteristic feature lies in the ability to skip two processing steps encountered in common precipitation techniques (i.e. filtration and drying). The method is based on combustion synthesis, sulfation, and calcination at high temperature. Through careful tailoring of the process parameters, evolution of the cellular nanostructure towards individual yttrium oxide nano- particles was obtained during firing. A method combining pre-sintering in vacuum followed by hot isostatic pressing (HIP) using glass encapsulation was shown to be successful to produce transparent yttria ceramics from agglomerated powders. Pre- sintering in vacuum of compact agglomerates enables differential sintering, resulting in only intergranular porosity. Subsequent HIP treatment of the pre-sintered samples caused almost complete pore elimination and achievement of transparency. However, macro-defects were present in the final material. From these results it became obvious that the packing homogeneity must be improved. For comparison, green-bodies were produced by slip-casting or pressing followed by cold isostatic pressing (CIP). Rheology studies were performed in order to tailor the milling conditions and the quantity of dispersant to obtain slurries with minimum viscosity and which were suitable for slip-casting. Although slip-cast pellets exhibited similar densities as pressed samples after pre-sintering, they showed more homogeneous optical properties after HIP. This was attributed to density gradients in the pressed compacts. By using the novel powder synthesis and HIP methods developed in this work, yttrium oxide transparent ceramics doped with 30 at% ytterbium were fabricated. The best sample showed a transmittance of 25% at 400 nm for a thickness of 3mm.Godkänd; 2006; 20070314 (evan)</p

Synthesis of Yb:Y2O3 nanoparticles and fabrication of transparent polycrystalline yttria ceramics

More efficient laser materials are needed for space applications in order to save weight and make cost-savings. The fabrication of polycrystalline ceramic materials is a solution to obtain new compositions with better efficiency. In order to produce a Yb doped Y2O3 laser material, we developed a process to obtain yttrium oxide nanosized particles doped with ytterbium and a sintering method that leads to transparent polycrystalline ceramics. First, nano-powder of yttria was fabricated from a precursor with transient morphology, i.d. yttrium hydroxynitrate platelets that decompose into spherical yttria nano-particles during calcination. The influence of different dewatering methods on such precursors was investigated. Water removal by freeze-drying was shown to be optimal for the production of non- agglomerated nano-particles compared with other methods that involve solvent removal by evaporation. This was attributed to the ability of freeze-drying to avoid the formation of solid bridges, since water is directly removed by sublimation. In a second step, doping with ytterbium was performed. Two precipitation routes were compared: precipitation of hydroxynitrate platelets and precipitation of amorphous carbonate. This latter was shown to allow a better distribution of ytterbium in the yttrium oxide matrix after calcination of the corresponding precursor. This was explained in terms of the good cation mixing in the amorphous particles, while formation of the hydroxynitrate platelets resulted in segregation of ytterbium, probably because a second phase with different ytterbium/yttrium composition precipitates in the first stages of the synthesis. Finally, a method combining pre-sintering in vacuum followed by hot isostatic pressing was shown to be successful to produce transparent yttria ceramics from agglomerated powders with high purity. Pre-sintering in vacuum agglomerates of closely-packed particles enables differential sintering, which is responsible for the complete elimination of porosity in the agglomerates and for the formation of intergranular porosity only. Then, hot isostatic pressing treatment of the pre-sintered samples using the glass-canning technique reactivates sintering and coarsening, which causes almost complete pore elimination. Furthermore, a liquid phase formed during sintering because of pollution from the furnace. This liquid phase is thought of as to have helped densification to some extent. The best transparent ceramic showed a transmittance of 43% at 400 nm.Godkänd; 2005; 20061221 (haneit

Growth of thin MFI zeolite films : Characterization by XHR-SEM and HR-TEM

Godkänd; 2011; 20111206 (ysko

Characterization of small defects in MFI membranes by permporometry, separation experiments and XHR-SEM

Godkänd; 2010; 20091214 (linste

Formation of Boundary Film from Ionic Liquids Enhanced by Additives

Room temperature ionic liquids (RTILs) have several properties that make them interesting candidates as base fluids for extreme conditions. However, a lack of compatibility with tribo-improving additives combined with an often overly aggressive nature is limiting their use as base fluids. To overcome these drawbacks, hydrocarbon-imitating RTIL base fluids have recently been developed. In this study, the effects of several common additives in the novel RTIL (P-SiSO) were examined by laboratory tribotesting. A reciprocating steel-steel ball-on-flat setup in an air atmosphere was used, where the lubricant performance was evaluated over a range of loads and temperatures. Surface analyses after testing were carried out using optical profilometry, scanning electron microscopy (SEM), and energy dispersive X-ray spectroscopy (EDS). Neat P-SiSO displayed high performance in the tribotests. At an elevated load and temperature, a shift in lubrication mode was observed with an accompanying increase in friction and wear. Surface analysis revealed a boundary film rich in Si and O in the primary lubrication mode, while P was detected after a shift to the secondary lubrication mode. An amine additive was effective in reducing wear and friction under harsh conditions. The amine was determined to increase formation of the protective Si–O film, presumably by enhancing the anion activity.Validerad; 2017; Nivå 2; 2017-05-10 (rokbeg)</p

An experimental study of micropore defects in MFI membranes

Godkänd; 2013; 20131129 (dankor