No description available.

Zirconia is a metal oxide with high band gap. It is commonly used as catalysts in many industrial practices. In recent years, its high-energy-gap value and redox properties also render it as an excellent photocatalyst, which can eliminate or reduce a variety of pollutants. The purpose of this work is to prepare and characterize the chromium-doped ordered porous zirconia. The main purpose of doping chromium into the zirconia is to avoid the Martensitic transformation of zirconia under high temperatures by volume change and pore structure change, thus reducing the cracking and inferior mechanical properties. With emulsion-free polymerization for the synthesis of polystyrene (PS) particles, controlling the particle diameter less than 200 nm is possible. A polystyrene template is thus produced by gravity sedimentation of these PS particles. Final Cr-doped zirconia is obtained by infiltration of a precursor solution, a mixture consisting of zirconium n-propoxide, n-propanol, acetylacetone, and chromium (III) nitrate nonahydrate, into the PS template, followed by drying and calcination at elevated temperatures. A systematic study on the pore structure and physical properties by XRD and Raman is conducted by varying the precursor concentration, the calcination temperature, and the dopant concentration. The results show that, unlike the pure zirconia, the pore structure of Cr-doped zirconia remains stable under higher calcination temperatures. Without any phase transformation, the doped Cr, evidenced from the EDS mapping, tends to help stabilize the zirconia at tetragonal phase. The average surface area and pore diameter of Cr-doped zirconia from BET measurement are 19 ~ 21 m2/g and 25 -45 nm, far better than the bulk zirconia. The improved surface properties are also confirmed by SEM observations.

The present work was produced as part of the MUZIC consortium, a collaboration between a multi-university team from the UK and industrial partners working on the field of nuclear energy, fabrication of alloys and nuclear research. The aim of the project is to establish a multidiscipline mechanistic understanding of the corrosion and breakaway processes of zirconium alloys used as fuel cladding materials in the nuclear industry. A better understanding of the corrosion mechanism of zirconium alloys will not only aid the development of better performing alloys, but will also allow more accurate models to be developed to reliably predict the service life of existing alloys. This could lead to higher burn-up, increase of energy production and reduction of nuclear waste produced.This work seeks to provide a better understanding of the role of residual stresses in the oxide, which are produced during oxidation due to high Pilling-Bedworth ratio and their impact on oxide phase transformation and oxidation kinetics by employing high energy synchrotron X-ray diffraction techniques. This is achieved by observing how stresses change as oxide growth approaches and passes through transition of the corrosion kinetics, their evolution across the oxide thickness, in situ characterising stresses and phase growth early in oxidation process and how stress changes can affect corrosion properties.It was found that relatively high compressive stresses in the two oxide crystal structures are present. The stresses relax with time up to moments before transition where a possible threshold stress magnitude is reached to aid an extensive tetragonal to monoclinic phase transformation. This generalised tetragonal to monoclinic transformation is believed to produce highly stressed monoclinic crystal structure grains and cause defects in the oxide. The above observation is further supported by a decrease of the tetragonal zirconia content. This is the moment that the oxide looses its protective character and a transition of the corrosion kinetics occurs. By comparing different materials it was observed that the minimum magnitude of the tetragonal phase is lower in better performing alloys while the tetragonal content is some cases was relatively low. It is suggested that the amount of the tetragonal phase, in the oxide layer, is not as important as the rate of it transforming into monoclinic. The extent of tetragonal to monoclinic transformation, that introduces defects in the oxide, defines how protective an oxide layer is. The present work provides a contribution to the available knowledge of the importance of residual stresses in the oxide layer and metal substrate of zirconium alloys and how they can affect corrosion rates or act as a precursor to the corrosion transition.

<p> An investigation is reported of oxygen diffusion in monoclinic zirconia at elevated temperatures. </p> <p> A method was developed for production of solid zirconia spheres of a quality suitable for diffusion measurements. The spheres, 60μ and 90μ diameter were subsequently used for the determination of the coefficient of self-diffusion of oxygen in stoichiometric zirconia at 800°C, 850°C, 900°C, 950°C and 1000°C. Oxygen O18 isotope exchange using mass spectrometry for the gas analysis was employed for this investigation. </p> <p> The oxygen self-diffusion coefficent was found to conform to the Arrhenius equation (see online text for equation) Theoretical considerations indicate that this diffusivity represents virtually the lattice diffusion of oxygen in zirconia. </p> <p> Diffusivities of oxygen in zirconia scale calculated from zirconium oxidation studies are 104 times higher and are believed to be due to short-circuit diffusion through line defects. To substantiate this hypothesis, "slabs" of stoichiometric zirconia scale and irregular, but equiaxed particles of the same material were used for oxygen diffusion experiments employing the same method. The diffusivities for the slabs were 10^3 times higher than those for spheres, supporting the validity of the short-circuit diffusion theory for zirconia scale. </p> <p> Oxygen concentration drop across zirconia scale on metal, during its formation by parabolic kinetics was calculated for 600°C and 850°C, and was found to be 0.04 g/cm3 approximately, while the concentration drop across the interface between the oxide and the oxygen-saturated metal was about 1.07 g/cm3. </p> / Thesis / Doctor of Philosophy (PhD)

Thermal behavior and molecular packing of a series of £\-helical poly(L-glutamates), with n-alkyl side chain of various lengths (m(number of carbons in the alkyl group) = 1, 2, 6, 12,18), were studied by means of differential scanning calorimetry, polarizing light microscopy and X-ray diffraction. For polymers of m = 1 and 2, There is a pseudohexagonal structure below ca. 130 oC and above this temperature the stable phase is the hexagonal columnar phase. There exists a layered structure in the polymer of m = 6, as well as a solvent induced hexagonal columnar structure which formed during solution casting process. In the polymer of m = 12, a layered structure was formed in the temperature range between 20 to 255 oC. However, for longer side chain, m = 18, tendency of crystallization of alkyl long side chain forced the backbone to pack into layer structure. There are two distinct melting temperature at ca. 60 oC, and the enthalpy are ca. 53 and 19 J/g, which corresponding to the melting of hexagonal and monoclinic side chain crystallines. The polymers with longer side chain (m = 6, 12 and 18) tend to be lyotropic liquid crystalline phase within lamellar inter-rod distance of 1.25 nm in solution state, and the structure will remain after drying. However, the inter-rod distance will collapse at the temperature above ca. 200 ¢J and will not recover after cooling.

Neutral diorganotin compounds of the 1,3-dithiole-2-thione-4,5-dithiolate ligand have been prepared. The syntheses of Me₂Sndmit, Et₂Sndmit, Bu₂Sndmit, (C₈H₁₇)₂Sndmit, (C₁₀H₂₁)₂Sndmit and (C₁₄H₂₉)₂Sndmit are described. For the purposes of indicating the formation of different structural phases, D.S.C. powder patterns are reported for initial and recrystallised samples of Me₂Sndmit, Et₂Sndmit and Bu₂Sndmit. Since Et₂Sndmitshowed different tin environments by solid phase state N.M.R. and clearly different powder patterns after recrystallisation, the compound was recrystallised from various solvents in an attempt to determine how many crystalline forms exist. To date, two forms have been identified: orthorhombic and monoclinic. The single crystal X-ray structure analyses of these are described. It has also been shown that upon heating a transition from the orthorhombic to the monoclinic form occurs at 140°C. The crystal strict of Me₂Sndmit has also been determined and is reported, along with Mossbauer parameters for Me₂Sndmit and Me₂Sndmio (dmio = 1,3-dithiole-2-one-4,5-dithiolate). Anionic organotin bis-dmit complexes of the form [RSn(dmit)₂][Q]⁺ have also been prepared, with a view to investigating their electrochemical properties. The syntheses of complexes with R chain length ranging from 4 to 18 carbons are described. The problem of formation of [Sn(dmit)₃][Q]₂ is also discussed. D.S.C. curves and cyclic voltammograms for [C₁₄H₂₉Sn(dmit)₂][C₁₄H₂₉NMe₃] and [NEt₄][(dmit)₂SnC₁₀H₂₀Sn(dmit)₂][NEt₄] are also described. Chiral macrocycles have potential uses as catalysts for asymmetric bond-forming reactions and as selective to metal ions. A chiral macrocyclic derivative of dmit was synthesised from methyl-4,6-<I>O</I>-benzylidene-2,3-bis-<I>O</I>-[(2-iodoethoxyethyl)-ethyl]-α-D-glucopyranoside. Since reaction of this with dmit proved to be unexpectedly problematic, attempted reactions of the sugar derivative with Na₂dmit<I> in situ</I>, isolated Na₂dmit, [Zn(dmit)₂][NEt₄]₂, [Ph₂Sn(dmit)I][NEt₄] and Cs₂dmit are described.

<p> The magnetic susceptibility of samarium orthophosphate (SmPO4) has been measured over the temperature range from 0.4 to 270°K. A theoretical expression for the susceptibility has been developed assuming a crystal field of cubic symmetry and fitted to the experimental data. </p> / Thesis / Master of Science (MSc)

Diploma thesis deals with laboratory preparation of the main clinker mineral - alit. The theoretical part summarizes the existing knowledge and the practical part deals with the methodology of preparation of the alite. The main goal was to prepare and study the monoclinic phases of alite, another goal was to optimize the triclinic alite.

No / This paper reports on the growth of benzamide crystals in the presence of 2'-aminoacetophenone. The resulting self-replicating intergrowth of benzamide crystals gives rise to ordered crystal aggregates in which individuals share a common c*. This behavior is interpreted using the concept of a motif capper additive which is able to halt the extension of structural motifs at the surface of a growing crystal. In this case the additive was selected to terminate the hydrogen-bonding ribbons, which extend along the b axis of the benzamide structure.

The effect of dental technical tools on the phase composition and roughness of 3/4/5 yttria-stabilized tetragonal zirconia polycrystalline (3y-/4y-/5y-TZP) for application in prosthetic dentistry was investigated. Additionally, the X-ray diffraction methods of Garvie-Nicholson and Rietveld were compared in a dental restoration context. Seven plates from two manufacturers, each fabricated from commercially available zirconia (3/4/5 mol%) for application as dental restorative material, were stressed by different dental technical tools used for grinding and polishing, as well as by chewing simulation and thermocycling. All specimens were examined via laser microscopy (surface roughness) and X-ray diffraction (DIN EN ISO 13356 and the Rietveld method). As a result, the monoclinic phase fraction was halved by grinding for the 3y-TZP and transformed entirely into one of the tetragonal phases by polishing/chewing for all specimens. The tetragonal phase t is preferred for an yttria content of 3 mol% and phase t″ for 5 mol%. Mechanical stress, such as polishing or grinding, does not trigger low-temperature degradation (LTD), but it fosters a phase transformation from monoclinic to tetragonal under certain conditions. This may increase the translucency and deteriorate the mechanical properties to some extent.