Ceramic membranes made from mixed oxygen-ionic and electronic conducting perovskite oxides can selectively separate oxygen from air at elevated temperatures. These membranes have several potential applications that require a continuous supply of oxygen. For example, they may be an alternative for cryogenic production of oxygen or alternative electrode materials in solid oxide fuel cells. Of particular significance is the partial oxidation of methane to syngas (CO + H2). By combining air separation and partial oxidation of natural gas into a single step, the need for expensive oxygen production by cryogenic means may be eliminated. Combined with existing processes for gas-to-liquid production such as Fisher-Tropsch and methanol synthesis, the MIEC membrane technology represents a very attractive route for conversion of natural gas to liquid fuels. The research in this field was initially concerned with the search for materials with the optimum oxygen flux. Today, the long term stability of the membranes is probably the main issue. The membranes have to be stable under operating conditions, which include mechanical stability and chemically compatibility with other materials like sealing and support materials. However, the current understanding of the long term chemical and mechanical reliability is poor and this is one of the major challenges for solid state ionic research. The aim of this work has been to investigate the mechanical properties and the chemical stability of La0.5Sr0.5Fe1-xCoxO3-δ (x = 0, 0.5, 1) materials when they are exposed to thermal and chemical gradients. The chemically induced stresses due to reduction of the valence state of the transition metals are of particular importance with respect to the mechanical stability. In paper I, the oxygen non-stoichiometry, investigated by thermogravimetrical analysis, and thermal end chemical expansion, studied by dilatometry and high temperature X-ray diffraction, of La0.5Sr0.5Fe1-xCoxO3-δ materials are reported. The oxygen deficiency was observed to increase with decreasing partial pressure of oxygen and increasing temperature corresponding to expectations and previous reports. At ambient temperature the thermal expansion coefficient of the materials were in the range 15- 18·10-6 K-1. Above a certain temperature thermal reduction of the material take place, and the thermal expansion coefficient due to chemical expansion raise to 16-36·10-6 K-1. The chemical expansion εc, defined as the linear expansion due to a change in partial pressure of oxygen at constant temperature, reached a maximum in the range 0.036-0.039 for the materials studied at 800ºC. The change in ionic radii of the transition metals is the main contribution to the chemical expansion. The crystal structure of the perovskite materials were shown to be slightly rhombohedral at ambient temperatures and a transition to cubic phase were observed above 300ºC. This non-linear thermal expansion behavior is a major challenge for the applications of the mixed conductor materials. La0.5Sr0.5Fe1-xCoxO3-δ membranes in an oxygen partial pressure gradient will have different oxygen deficiency on either side of the membrane. The increasing oxygen deficiency is accompanied by a volume expansion as shown in paper I, and this will lead to chemically induced stresses. These stresses and the failure that might follow can be prevented by creep of the materials. Creep is also important due to dimensional stability. In paper II, the steady-state creep performance under compression of La0.5Sr0.5Fe1-xCoxO3-δ (x = 0.5, 1) as a function of temperature, atmosphere, load and two different grain sizes is reported. The stress exponent found for the materials was close to unity and an unusual low inverse grain size exponent close to one was found for one of the materials. The activation energy of the two materials was not equal and the influence of secondary phases on the creep was discussed. The obtained creep behavior and microstructural investigation after measurements point to a diffusion related mechanism for the creep. Higher creep rates are found under reducing conditions and this suggest that creep relaxation of mechanical or chemical induced stresses may enhance the mechanical stability of oxygen permeable membranes. In Paper III, the mechanical properties of La0.5Sr0.5Fe1-xCoxO3-δ (x = 0.5, 0.75, 1) were investigated by several methods. Fracture strength was measured by four-point bending, fracture toughness was measured by SENB and SEVNB methods and finally Young’s modulus were investigated by four-point bending and resonant ultrasound spectroscopy. Four-point bending showed a non-linear ferroelastic behavior at ambient temperature due to rhombohedral crystal structure. Above the ferroelastic to paraelastic transition temperature the materials showed elastic behavior, however, at temperatures from about 800ºC a non-elastic respond was observed due to creep. The measured fracture strength and fracture toughness were observed to increase with increasing temperature, which was attributed to frozen-in stress gradients in the materials during cooling due to different oxygen stoichiometry. These stress gradients caused the low fracture strength and fracture toughness at ambient temperature. At higher temperatures, the stresses are assumed to relax resulting in a higher strength and fracture toughness. At high temperature, the non-linear respond made systematic errors in the calculated strength and fracture toughness. The Young’s modulus was measured from four-point bending and by resonant ultrasound spectroscopy for two of the materials. These data obtained by these two different methods were not in good agreement, which demonstrate the difficulty to obtain reliable data for the Young’s modulus of such materials by four-point bending. The presented findings have demonstrated the importance of understanding ferroelasticity and chemically induced stresses in order to comprehend the mechanical properties of such mixed valence state perovskite materials. A high oxygen flux is required in order to realize the oxygen permeable membrane technology. At the same the chemical stability of the materials in a pO2 gradient must be good for a sufficient long period of time. The oxygen flux performance and the long term stability of La0.5Sr0.5Fe1-xCoxO3-δ (x = 0, 0.5, 1) are the topics of Paper IV and V. Oxygen fluxes through the membranes are found as a function of oxygengradient and temperature in a oxygen permeation cell using air and inert gas on each side. The oxygen flux was observed to increase with decreasing pO2 on the secondary side until the surface exchange became rate limiting and the fluxes reach a constant value. By further increase of the pO2 gradient, the flux seemed to decrease and this was attributed to the pO2 dependence of the surface exchange coefficient. The apparent activation energy of the oxygen permeation was in good accordance with previous investigation of similar materials. After about 5 week of exposure in an oxygen gradient at about 1150°C, the membranes were carefully examined by electron microscopy for evidence for kinetic demixing and decomposition. Dependent of the overall composition of the membrane, different secondary phases were formed at the primary surface of the membrane. For the cobalt containing materials, isolated grains or clusters of grains of cobalt oxide were formed. In case of the La0.5Sr0.5FeO3-δ membrane, a dense and about 20 µm thick layer of the secondary phase SrFe12O19 was formed at the primary side. The overall (La+Sr)/(Fe+Co) ratio was also seen to influence on the phase formed at the primary side. Kinetic demixing was also demonstrated in all the membranes although the metal concentration profiles were not drastically changed from the initial concentrations. The formation of secondary phases was reflected in the (La+Sr)/(Fe+Co) ratio across the membrane. The largest deviation from the nominal stoichiometry was seen close to the surfaces indicating steeper chemical gradients close to the surfaces. These phenomena may strongly limit the long term stability of thinner membranes e. g. films on a porous substrate.

When remelting aluminium scrap, metal losses due to dross generation is a common problem. Reduction of these losses will give substantial economic and environmental benefits. Dross is generated when aluminium metal oxidizes and films of oxide envelope molten metal. When a cold metal object is immersed in a melt, the heat of the melt around this is transferred so rapidly into the object that a shell of melt often solidifies to the surface of the object. When scrap with low bulk density is charged to a melt, solidification of melt on the cold scrap prevents melt from entering the cavities in the bulk of the scrap, and the bulk density remains low. Thus the scrap tends to float on the melt surface. Submersion of this scrap is important to avoid oxidation and subsequent dross generation. One solution to this is to roll scrap to a strip and feed it into the melt. This system has been examined by studying feeding of a continuous, thin aluminium plate into molten aluminium. Also, the effect of lacquer was considered, as well as feeding the plate into a launder with melt flowing along the surface of the plate. An analytical, one-dimensional, steady-state model has been developed to describe the melting and the melting mechanisms. It is based on a shell solidifying on the plate surface and a gap introducing a thermal resistance 1/hg between the shell and the plate. The thermal resistance 1/hl of the boundary layer of the melt is included. Depending on these resistances, the initial temperature of the plate and the melt temperature, a shell will form, and the plate will penetrate a distance P into the melt before it melts away. An experimental apparatus was designed and constructed to feed aluminium plate from a coil into a melt bath at a specified velocity. The plate could be withdrawn rapidly to “freeze” the situation like it was below the melt surface. The penetration depth P of the plate could be measured and shell formation observed. More than 200 experiments were performed, and by comparing the penetration depth at different feeding velocities and melt temperatures to model predictions, the two heat-transfer coefficients hl and hg could be determined by curve fitting. They agree reasonably well with values found in the literature and calculated from boundary-layer theory. In a few experiments, the plate feeding was recorded on video tape, and the cross section of some plates was studied in a microscope. Feeding of somewhat thicker plates was also tried. This gave valuable background information for comparing the experiments to the model. We believe that snap-off of the plate due to low mechanical strength around the melting temperature may affect the measurement of the penetration depth of the plate. Attempts were also made to measure the temperature in the plate by attaching thermocouples to its surface. The obtained temperature profiles in the plate were compared to the model predictions, but the method needs improvement. A criterion for formation of a shell is formulated and tested against experimental observations. Qualitative agreement is achieved. Even if there is no shell formation, it seems that there will be an air film with thermal resistance 1/hg. This indicates that the melting rate will be independent of whether a shell is formed or not. Two additional models with only one heat-transfer coefficient are also developed in order to challenge the main model. From this analysis it is found that the use of two heat-transfer coefficients is necessary to describe the system. The model should be of direct interest when feeding rolled scrap into molten aluminium. Improvement of the model can be attained by reconsidering the assumptions made, but then numerical methods must undoubtedly be applied. These new models should include the snap-off mechanism.

The microsystem processing of today is based on an assortment of diverse and contrasting fabrication techniques, sprung from the microelectronic industry. Microsystem technology has primarily been developed to meet the demands for scaled down low-cost sensor and actuator systems. These devices are based on a number of contrasting principles, e.g. capacitance of mechanical membranes, microfluidic or piezoelectric actuation, electrostatic or heat deflection, etc. Clearly the disparate designs of these devices require microfabrication processes that are flexible and unrestricted, allowing deep and vertical structures to be obtained. The scope of this thesis is microstructuring and microsystem fabrication of primarily polymer materials by means of ion track lithography. Ion track lithography in combination with microstructuring unwraps unique and untouched grounds. The available building blocks in these microsystems include high aspect ratio (depth-to-width ratio) capillaries and metal wires of nanometric width, deep vertical cavities as well as conventional double-sided thin film lithographic patterns. The blocks have been combined to materialise a few technology demonstrators integrated in flexible polyimide foils and laminates, commercially attractive materials with excellent mechanical, temperature, and high frequency electronic properties. A magnetic field sensing structure based on the magnetoresistance as well as inductor coils for microwave circuitry have been fabricated and characterised. The various fabrication processes have been evaluated with respect to resolution, aspect ratio, and verticality. We can present a deep vertical micro? and nanofabrication technique, overriding the natural anisotropy or isotropy of many of the diverse materials employed in the microsystem field. We have obtained pores, capillaries with aspect ratios of up to 400, clusters of electrodeposited magnetic and non-magnetic nanowires with similar aspect ratio, solid complex structures with close to vertical walls as well as through hole microvias in flexible printed circuitboards.

In the modern process of continuous Direct Chill (DC) hot top casting of aluminium extrusion ingot with gas slip, poor surface quality of the cast ingot can still be a problem. In the worst cases pronounced surface wrinkling may occur coupled with periodic zones of reduced grain size, macrosegregation and exudation at the surface. The observed surface irregularities are believed to be linked to periodic oscillations or folding of the free molten aluminium surface in the mould, the meniscus, resulting in varying solidification conditions. The focus of this work is to gain a better understanding of the dynamics of the meniscus and the effect it has on ingot surface formation.

Thermal spraying is the name of a large group of coating deposition techniques used to deposit thick layers for a variety of applications. The principle is to melt the material, and rapidly propel the droplets towards a substrate where they flatten and solidify. When properly used, the substrate is not significantly heated. Spraying enables deposition of practically any material with a stable molten phase, on any solid material. Sprayed ceramics are used to reduce wear by sliding and by hard particles. However, due to the defect-filled microstructure resulting from spraying, the coatings typically have poor mechanical and tribological properties compared to dense, sintered materials. By varying the spraying parameters, the microstructure of the coatings was influenced, and also the wear rate and cohesion, which is difficult to quantify. Improved tribological properties resulted from spraying with axial particle injection equipment and using narrowly size distributed, spherically shaped powder particles, compared to conventional equipment and powder particles. A new method to seal the open pores of sprayed ceramic coatings by electrolysis is proposed and evaluated. It was found that almost complete sealing could be obtained, in some cases giving a drastically improved wear behaviour. The studied electrolytically deposited sealants were Pb, Sn, Cu and Ni. The latter was found to give the best performance. It also proved possible to apply PVD coatings to the sprayed ceramics, and it was shown that sprayed and Ni-sealed alumina can be superior to ball bearing steel to support thin, low friction PVD coatings.

The lateral double-diffused MOS (LDMOS) transistor has traditionally been a high-voltage device used in switching applications. The use as a high-frequency device has become more important lately since the LDMOS offers an low cost solution for telecommunication applications. An important property of the LDMOS concept is that it can be manufactured in virtually the same process used in standard CMOS production. It only requires one extra process step, which is easily implemented. The other important aspect that gives the LDMOS the good high-frequency performance is that the channel length is a process parameter and not a lithography parameter. This thesis investigates the LDMOS transistor primarily from two aspects. The first is the high-voltage performance. For a high-voltage device the most important parameter is the breakdown voltage. The second most important parameter is the on-resistance that has the property of being in contradiction of the breakdown voltage and usually trade-offs are made to achieve acceptable performance. In the thesis several methods to improve the breakdown voltage/on-resistance relation are presented. The other part covers the high-frequency behavior of the LDMOS transistor. High-frequency characterization has been made to gain valuable information for the fundamental understanding of the physical mechanisms inside the transistor. A large part of the thesis covers modeling and parameter extraction of the devices. A new general method for parameter extraction of small-signal equivalent circuit models is presented, which has the appealing properties of not needing any approximation during the extraction which is common with other techniques.

QC 20101020

No description available.

<p>The main objective of this industrially important work was to gain an increasing understanding of the properties of materials such as CMSX-4 nickel base super alloy, mould powder used in continuous casting of steel and coke used in blast furnace, with special reference to the thermal diffusivities. The measurements were carried out in a wide temperature range, solid, liquid, glassy and crystalline states.</p><p>For CMSX-4 alloy, the thermal conductivities were calculated from the experimental thermal diffusivities. Both the diffusivities and conductivities were found to increase with increasing temperature. Microscopic analysis showed the presence of intermetallic phases such as NiTi and NiTi2 below 1253 K. In this region, the mean free path of the phonons is likely to be limited by scattering against lattice defects. Between 1253 K and solidus temperature, these phases dissolved in the alloy adding to the impurities in the matrix, which, in turn, caused a decrease in the thermal diffusivity. This effect was confirmed by annealing the samples at 1573 K. The thermal diffusivities of the annealed samples measured at 1277, 1403 and 1531 K were found to be lower than the thermal diffusivities of non-annealed samples and the values did not show any noticeable change with time. It could be related to the attainment of equilibrium with the completion of the dissolution of γ and γ´ phases during the annealing process.</p><p>Liquid CMSX-4 does not show any change of thermal diffusivity with temperature. It may be attributed to the decreasing the mean free path being shorter than characteristic distance between two neighbouring atoms.</p><p>On the other hand thermal diffusivities of mould powder having glassy and crystalline states decrease with increasing temperature at lower temperature and are constant at higher temperature except for one glassy sample.</p><p>Analogously, the thermal diffusivity measurements of mould powder did not show any significant change with temperature in liquid state. It is likely to be due to the silicate network being largely broken down.</p><p>The thermal diffusivity is increased with increasing crystallisation degree of mould powder, which is expected from theoretical considerations.</p><p>The coke sample, taken from deeper level of the blast furnace, is found to have larger thermal diffusivity. This could be correlated to the average crystallite size along the structural <i>c</i>-axis, L_c, which is indicative of the higher degree of graphitisation. This was also confirmed by XRD measurements of the different coke samples. The degree of graphitisation was found to increase with increasing temperature. Further, XRD measurements of coke samples taken from different levels in the shaft of the blast furnace show that the graphitisation of coke was instantaneous between room temperature and 1473 K.</p>

No description available.