Materials Engineering

Ihe presence of atomic carbon in MgO and the experimentally well documented fact that an internal equilibrium exists between defect-bound OH-and H + O-lead to a very complex pattern of defect equilibria and defect reactions in the bulk and at the surface of the MgO crystals. The most important extrinsic bulk defects are the bare and the OH-com-[~"]" ] OH'V~I' , and [OH V~ H~ ~ pensated cation vacancies, ~ , of which the latter yields CO" (Hz)~e 0~ interstitial hydrogen molecules (H&) ~, interstitial C atoms C~, which may enter the available cation vacancies, and the positive hole, ~0~'. Reactions between C atoms entering the 0 V" 0 and [O'V~{' at the surface lead to the formation of COz and CO molecules and to the formation of double F and single F centers respectively. The free electrons in the F centers are held responsible for the formation of Mg-organic commpounds. Reaction between the C atoms and interstitial Hz molecules lead to hydrocarbon formation. The strain fields associated with the interstitial C atoms and the positive holes provide the driving force for segregation of the carbon into the elastically relaxed subsurface zone and for fluctuating reaction kinetics.

The multitude of OH stretching bands in the near IR in arc-fused MgO can be divided into two main groups, a triangular shaped band at 5548 cm "I (50OK) which strongly shifts and splits to a triplet at 3571, 3566 and 3550 cm ~ upon cooling to 82 K, a quadruplet at 529?, 5512, 5527 and 5342 cm "~ (82 K), plus a broad background absorption extending from 3600 -3300 em "I . The H-H band at 4150 cm ~ points at molecular H formed by charge transfer from [OH'V" HO~---~'(H~)~ O~ ~. The broad background absorption i~s " asslgned to Interstitial protons, H 4 . The 5500 and 1 5550 cm" groups are assigned to the diamagnetic de-• II | • II • fects [OH V~]and ~. V~ .O~ respectively. Their multiplet structure is explained by interaction of the OH oscillator with near-by C~ atoms on interstitial sites surrounded by an elastic strain halo which causes local lattice expansion.

Using i0 nsec, 265 nm laser flashes and a time-of-flight mass spectrometer freshly cleaved, arc-fusion grown HgO and CaO single crystals were analyzed for their carbon content. The positive ion mass spectra from NgO show that the Ng isotopes 24, 25, and 26 form ions even at low laser densities. With the appearance of the 0 + ions a signal was invariably observed at m/e=12 which could + be unambiguously assigned to C + ions. No trace of C,~ cluster ions, typical for graphite or any other form of polymerized carbon, could be detected. At the same time the absence of carbonate and carbide was ascertained by infrared spectroscopy. The results support the view that the carbon contained in these alkaline earth oxides, grown by arc-fusion, is in the atomic state.

Using the uC(d,p)13C method and an ultrahigh vacuum chamber for in situ heat treatments up to i170 K and down to liquid nitrogen temperature, the carbon content was measured of a variety of arc-fusion grown MgO single crystals of rated purity grades 98.5-99.99%. The crystals were free of occluded COz , CO~-, and carbide, but contained H~ dissolved in the lattice or as OH-, sometimes also as gas inclusions in microcavities. Carbon was found to be present in the bulk in concentrations of the order of 2000-3000 at.ppm. The carbon was strongly enriched in a <i ~m thick subsurface zone. Heat treatments in vacuum up to i170 K caused characteristic variations of the subsurface carbon concentrations. Heat treatments in air removed the carbon from the subsurface zone. At room temperature the carbon came back fast. At 470 K it reappeared with time-dependent oscillations. If cooled to 77 K and reheated with 1 K/min the carbon became mobile at about 200 K. The results indicate that the mobile species is atomic carbon interstitially dissolved in the MgO.

The CO 2 evolution from coarse MgO powder (70-120 um 0), obtained by crushing arc-fused single crystals, was measured isothermally between 470-1270 K in strictly O2-free Ar and in 0 2 . Even under Ar carbon dioxide was found to evolve in substantial quantities, up to 1.8 mg/g MgO, following a complicated temperatureand time-dependent pattern. Under 0 2 a similar behavior was found, but the quantity of CO 2 evolved was nearly the same. At certain temperatures a fluctuating kinetics was observed indicating a 45 mln periodicity. Traces of CO were also evolved. Freshly crushed MgO crystals yielded more CO. Thermogravimetry showed that the total weight loss of the MgO samples is larger than can be accounted for by CO 2 and CO evolution alone.

High purity and industrial purity grade MgO single crystals from arc-fusion melts show temperature-and time-dependent anomalies in the thermal expansion behavior at about 520 K and 720 K, systematic changes in the linear thermal expansion coefficient during successive heating and cocling cycles, and a linear shrinkage during room temperature annealing. This anomalous behavior can be related to the presence of carbon which is atomically dissolved in the MgO lattice, not only in the subsurface zone but also in the bulk.

Hydrocarbons, both saturated and unsaturated, including aromatics, are found to evolve from crushed arc-fused MgO single crystals under conditions which exclude external sources of contamination. The hydrocarbon evolution was studied between 700-1200 K. Two maxima are observed at about 850 K and at 950 K followed by a minor peak at about 1150 K due to methane. Alkanes occur in small concentrations only in the lower temperature range. Olefines are dominant andrmainly responsible for the 850 and 950 K maxima. The concentration of benzene rises above 870 K and stays fairly constant up to the highes/£ temperatures studied. The total amount of hydrocarbon evolved from 70-120 um ~ MgO is as high as 25~g/g MgO.

In the range 300-1270 K the d.c. conductivity of highpurity arc-fused Mg0 single crystals was found to be very different during heating and cooling and after room temperature annealing. Measurements with and without guard ring and by the 4-point method gave essentially the same results. Two distinctly different activation energies were found: the 2.5 eV mechanism, already reported in the literature, which is characteristic for high temperatures, > i000 K, and a i.i eV mechanism which progressively builds up in the 700-870 K interval during heating. Between 700-870 K the conductivity rises sharply and cooling yields I.i eV-branches which can be cycled many times. Room temperature annealing, however, destroys the i.I eV mechanism and leads again to sharply rising conductivity curves in the 700-870 K interval. Remarkable instabilities and occasional short circuiting phenomena were observed during cooling from temperatures slightly above 870 K. In this temperature region the conductivity reacted very fast to a change from Ar to 0 I. Together with supporting evidence provided by other methods the conductivity behavior appears to be governed by the presence of atomic carbon in the Mg0.

mobility was found to be as low as 130 K. From these Franceschetti, Department of Physics, Memphis experiments it is concluded that carbon is atomically State University, Memphis, TN 38152, U.S.A. dissolved in the MgO. The C atoms probably occupy Numerical simulation results are presented for the extrinsic cation vacancies introduced in the MgO by time evolution of the interstitial concentration profile co-dissolved "water", but at the same time they must also be on interstitial sites where they acquire an in a metal subject to an applied potential difference. extremely high mobility. Simulation results are presented for five simple theoretical models of interstitial electrotransport : a

Using 2-, 3-and 4-electrodes configurations the direct current conductivity of MgO single crystals of nominally highest purity (with respect to cation impurities), grown by arc-fusion, was studied in argon or oxygen between 500-1200K with special reference to both hysteresis effects during heating and cooling cycles and conductivity phenomena which occur underneath the surface in a thin subsurface zone. The samples contained 250-2500 at.-ppm carbon and typically 800 at.-ppm hydrogen. Below 1000 K the low temperature (LT) conductivity mechanism is characterized by an activation energy of 1.12 0.2 eV, distinctly lower than that of the high temperature region (HT) approximately 2.4 eV. By annealing at 300K the LT mechanism progressively builds up and causes a very pronounced conductivity increase between 700-9OOK, unaffected or even enhanced by 02. Above 900 K, O2 decreases the conductivity. The LT mechanism is proposed to be due to defect electrons on anion sites corresponding to O-in the O*-structure which are a consequence of the presence of carbon and hydrogen-dissolied in the MgO and formally derived from the dissolution of traces of CO, and H,O IJ. Gem. Phvs. Solids 43. 129 (1982)].

MgO with a high concentration of defects was produced by the dehydration of high-purity Mg(OH), and the conductivity was studied in the temperature range 20-250°C. An atmosphere of hydrogen increased the conductivity by up to a factor of 10' and produced a characteristic conductivity maximum when the sample was heated. The hydrogen interacts with the sample at V$-centers which are located at the surface of the dehydrated material. These centers are supposed to have a catalytic activity, i.e. they decompose hydrogen molecules into atoms and further into electrons and protons which contribute to the current.

CdSe/CdS core/shell nanocrystals have been synthesized through a low cost and simple twophase thermal route. The optical spectroscopy and structural characterization evidenced the core/shell structure of the CdSe/CdS nanoparticles. The X-ray diffraction patterns of CdSe and CdSe/CdS nanoparticles exhibited peak positions corresponding to those of their bulk cubic crystal structures. The X-ray photoelectron spectroscopy data confirmed the elemental composition of the CdSe/CdS nanoparticles. The absorption spectra of core/shell nanoparticles showed red shift with respect to the core CdSe nanoparticles. The photoluminescence study indicates that the intensity of the emission maximum is considerably increased in the core/shell structure as compared with the parent material, and the capping of CdS nanoparticles with CdSe material exhibit a near band-edge emission, indicating a successful passivation by removing surface defects. The highresolution transmission microscope images of the bare and core/shell nanoparticles ascertained the monodispersed and well-defined spherical particles. The average particle sizes for CdSe and CdSe/CdS nanoparticles are 2.5 and 5 nm, respectively, thus confirming, the larger diameter of CdSe/CdS core/ shell nanostructure than the core CdSe nanoparticles.

The synthesis of TiO 2 nanorods with anatase structure has been achieved from the necking of truncated nanoparticles by oriented attachment using titanium ethylenediaminetetraacetic acid (EDTA) chelated complex as a molecular precursor. The preparation was carried out under mild conditions using a simple solvothermal process. The influence of EDTA over the growth of nanocrystallites and the various other factors which contribute to the development of 1D TiO 2 nanostructure are investigated. At a relatively lower temperature, titania nanopowders are obtained, and the anatase phase crystallization is verified by wide angle X-ray diffraction. The evolution of rod-shaped TiO 2 with tapered edges has been confirmed by transmission electron micrographs. The well aligned lattice fringes of TiO 2 nanorod towards [001] direction is investigated by HRTEM. The SEM images show the surface configuration of overall aggregates of titania crystallites consisted of primary particles which are densely packed in an orderly texture. The moderate shift in the absorption band towards higher energy region of the absorption spectrum confirms the weak carrier confinement effect in the sample.

h i g h l i g h t s g r a p h i c a l a b s t r a c t < CdS/ZnS core/shell nanorods were synthesized using two-step solvothermal approach. < The nanoparticles were characterized by XRD, EDX, SEM, UVevis and TEM. < SEM images revealed the surface roughness after ZnS shell growth. < TEM microscopy offers evidence for the formation of core/shell nanostructures.

The atomistic structure and phonon transport in aluminosilicate glasses made via an interfacial mixing model of the Molten Core process were studied using molecular dynamics simulations. In the simulations, silica glass was brought in contact with different size alumina crystals (to afford core glasses with 4, 18, 24, 29, and 41 mole% alumina concentrations), followed by a melt-quench process to enable mixing of the phases. The atomistic structure of the resulting glasses and radius of gyration calculations of resultant Al-O-Al connected clusters were evaluated. Variation in the 1-dimensional thermal transport in each glass was also determined and showed that increased alumina concentration in the glasses resulted in increased transport of thermal energy. Results of the structural analyses showed a double

The molecular dynamics (MD) computer simulation technique has been used to study the structure of lithium aluminosilicate (LAS) glasses and the diffusion of lithium ions. Five kinds of lithium aluminosilicate glasses with different R (ratio of the concentration of Al to Li) values are simulated. The structural features of the simulated glasses are analyzed using Radial Distribution Functions (RDFs) and Pair Distribution Functions (PDFs). With the increase of R, the environments of the Li ions alter from bonding to non-bridging oxygen to bonding to the bridging oxygen associated with tetrahedral Al. The diffusion coefficients and activation energy of lithium ion diffusion in simulated lithium aluminosilicate glasses were calculated and the values are consistent with those in experimental glasses. When R equals 1.00, lithium ions have the lowest activation energy for diffusion. The relationship between the activation energy for lithium diffusion and the composition of these glasses is similar to that previously observed for sodium in sodium aluminosilicate glasses.

Molecular dynamics computer simulations were used to study the atomistic structure of intergranular films (IGFs) between two basal oriented Si 3 N 4 crystals or between combined basal and prism oriented crystals. Ordering of the ions into the IGF induced by the crystal surfaces was observed using density profiles of the ions, although that ordering is effected by the roughness of the crystal surface. Density profiles of the sum of all ions misleadingly shows a rapid decay in the density oscillations and apparent ordering into the IGF. However, this is an artifact of the coincidence of the maximum in the peaks of one species with the minimum of another species and the actual oscillations of individual species extend into the IGF farther than the sum profile indicates. This result would have important implications regarding the density oscillations observed in physical experiments with regard to the actual extent of ordering into the IGF induced by the crystal surface.

Molecular dynamics simulations were performed to study the effect of the composition of the intergranular film (IGF) on anisotropic and isotropic grain growth in R-Al 2 O 3 . In the simulations, the IGF is formed while in contact with two differently oriented alumina crystals, with the alumina (0001) basal plane on one side and the (112 h0) prism plane on the other. Five different compositions in the IGFs were studied. Results show preferential growth along the [112 h0] of the (112 h0) surface in comparison to growth along the [0001] direction on the (0001) surface for compositions near a Ca/Al ratio of 0.5. Such preferential growth is consistent with anisotropic grain growth in alumina, where platelets form because of faster growth of the prism orientations than the basal orientation. The simulations also show the mechanism by which Ca ions in the IGF inhibit growth on the basal surface. At compositions with high or low Ca/Al ratios, growth along each surface normal is equivalent, indicating isotropic grain growth, although the attachment rates are quite different, which may indicate differences between normal grain growth and abnormal, but isotropic, grain growth. The simulations provide an atomistic view of attachment onto crystal surfaces, affecting grain growth in alumina.

Previous molecular dynamics simulations have shown that compression of silica glass surfaces occurs upon formation of an interface with a model crystal and that a structural change caused by this process is retained after glass and crystal are separated. The remnant structural modification caused by this stress was an increase in the concentration of siloxane bond angles less than 1508 in the near surface region of the glass. It was expected that the structural modification associated with interface formation and separation could represent an increase in the concentration of less stable siloxane bonds, particularly in the presence of water molecules. It was also recognized that a decreased stability could indicate greater reactivity with water molecules. Thus, water reaction on silica surfaces was simulated before and after stress modification and the subsequent structural relaxations in the glass surface were observed. Decreased stability, represented by a greater number of bond ruptures, existed after interface formation and removal. These bond ruptures were Si-O bonds breaking and reforming siloxane bonds with an angle nearer the average and also Si-O bonds breaking to react with water forming silanols. A greater number of silanols formed after interface formation and removal than before, demonstrating a greater reactivity with water after interface formation and separation.

The structure of soda‐lime alumino‐borosilicate glass was studied using molecular dynamics simulations of samples of varying compositions containing ~20 000 atoms each. Pair distribution functions (PDFs) of cations to oxygen were used for comparison to available experimental data to evaluate consistency between simulations and experiment. Additional PDFs and coordination of the network forming cations (Al/B/Si) to network modifiers (Ca/Na) were examined, which is difficult to measure experimentally. The results are consistent with available experimental data regarding cation‐oxygen bond lengths and network former to oxygen coordination numbers. Si and Al are predominantly 4‐coordinated, with a small concentration of overcoordinated species similar to experimental data. B varied as 3‐coordinated, BO3, and 4‐coordinated, BO4, as a function of the amount of Ca2+ and Na+ present, the ratio of Al2O3 to B2O3, and the fictive temperature of the sample, similar to experimental data. The sim...

The transport properties of the perovskites KTaO3 are calculated using first-principles methods. Our study is based on Boltzmann transport theory and the relaxation time approximation, where the scattering rate is calculated using an analytical model describing the interactions of electrons and longitudinal optical phonons. We compute the room-temperature electron mobility and Seebeck coefficients of KTaO3, and SrTiO3 for comparison, for a range of electron concentrations. The comparison between the two materials provides insight into the mechanisms that determine roomtemperature electron mobility, such as the effect of band-width and spin-orbit splitting. The results, combined with the efficiency of the computational scheme developed in this study, provide a path to investigate and discover materials with targeted transport properties.

Using first-principles electronic structure calculations, we show that ferromagnetic Heusler compounds Co2MnX (X= Si, Ge, Sn) present non-trivial topological characteristics and belong to the category of Weyl semimetals. These materials exhibit two topologically interesting band crossings near the Fermi level. These band crossings have complex 3D geometries in the Brillouin zone and are characterized by non-trivial topology as Hopf links and chain-like nodal lines, that are protected by the perpendicular mirror planes. The spin-orbit interaction split these nodal lines into several zero-dimensional Weyl band crossings. Unlike previously known topologically non-trivial Heusler materials, these majority-spin band crossings lie in the band gap of minority spin bands, potentially facilitating its experimental realization.

This lecture describes recent numerical and experimental studies of Computational Fluid Dynamics (CFD) and combustion in practical applications. The studies have been performed in a range of laboratory platforms where controlled flow and combustion take place. The lecture will cover a wide range of practical industrial applications including burners' swirl flames, premixed and non-premixed combustion, internal combustion engines, fuel cells, hydrogen utilization and applications, dust and hydrogen safety studies. The numerical approaches used in the above studies range from simple ones that are numerically feasible for practical applications such as those based on Reynolds Averaged Navier-Stokes (RANS) equations to complex and numerically very expensive such as Large Eddy Simulation (LES) methods. Experimentally, high speed images of flow and combustion processes as well pressure-time traces are widely available for a range of experimental conditions but flow field measurements are less common due to the seeding difficulty associated with such transient processes. This lecture will also review recent experiments where the flame front was imaged using Laser Induced Fluorescence (LIF) from the hydroxyl radical of the combustion process.

sche Mizellenkonzentration nur yon nichtionogenen ober-fl~ichenaktiven Substanzen. Anionische Mizellen zeigen keine Aufnahme fiir das Sulfonat, w~ihrend kationische MizeHen ein fluoreszierendes Aggregat mit dem Sulfonat bei Konzentrationen unterhalb der kritischen Mizellenkonzentration der oberfl~ichenaktiven Substanzen bilden. Die Verwendung yon fluoreszierenden Substanzen erm/Sglicht es, no& kritische Mizellenkonzentrationen unter 0,0025 g/100 ml zu bestimmen.

Therma~ly stimulated de polarisation current (TSDC) and time-dependent conduction current of unirradiated and y-irradiated pure and CoCl2-doped PYA films have been measured. Doses in the range 21-75 Mrad were used. The TSDC of pure and doped PV A before and after irradiation revealed a T relaxat!on peak. The induc~~ changes in the peak current (/,...) of the glas: relaxatIon peak are composition and dose dl!pendent. A marked decrease in 1.'SDC. corresponding ~o th~ r;tain. glass rela~ation peak is observed with ageing tIme. fhe role of y-madlatton In perturbing the orientation of the dipoles and/or chain segments was inferred from the disappearance of some sub-T relaxation peaks. The thermally activated mobility of charge carriers i~ confirmed from calculations of drift mobility at different y doses and temperatures.

For 3D reconstructions of whole immune cells from zebrafish, isolated from adult animals by FAC-sorting we employed array tomography on hundreds of serial sections deposited on silicon wafers. Image stacks were either recorded manually or automatically with the newly released ZEISS Atlas 5 Array Tomography platform on a Zeiss FEGSEM. To characterize different populations of immune cells, organelle inventories were created by segmenting individual cells. In addition, arrays were used for quantification of cell populations with respect to the various cell types they contained. The detection of immunological synapses in cocultures of cell populations from thymus or WKM with cancer cells helped to identify the cytotoxic nature of these cells. Our results demonstrate the practicality and benefit of AT for high-throughput ultrastructural imaging of substantial volumes.

Sodium alginate (TiO2) sand composites were prepared by solution casting. Purified sand was added in the composite films to increase electrical conductivity. Electrical properties such as conductivity, capacitance, dielectric constant, and loss tangent of the composites were investigated. The current voltage characteristics for all the composites showed ohmic behavior. All the electrical properties have been found to improve with the incorporation of sand (SiO2) but 6% sand containing composite exhibits the best electrical properties. The mechanical properties tensile strength (TS), elongation at break (Eb) and Young modulus for 6% sand containing composite film are found to be 4.445 MPa, 9.76%, and 72.8 MPa respectively. The experimental results reveal that the blended films exhibit higher stability and improved mechanical properties of both tensile strength and elongation at break in dry state. Water absorption properties of the composites are found to decrease with the increase o...

Structural, thermal profiles and kinetic behavior of two different sol-gel samples of MgAl layered double hydroxides (LDHs) were studied. In situ FT-IR study was used to follow the chemical composition changes, and thermal evolution of specific chemical species in each stage was monitored with a dynamic sampling mass spectrometer (TPD-MS analysis). The identified gas products were mainly OH, H 2 O, CO 2 , and organic species, mainly C 2 H 5 , originated from the alkoxy groups. The thermal evolution of intercalated compounds occurred in at least four steps related to dehydration, dehydroxylation of layers, interlayer anion decomposition, shrinkage and collapse of the layered structure, and finally crystallization of spinel MgAl 2 O 4 . Kinetic analysis in terms of activation energy (Ea), suggest that decomposition mechanism is a complex process that involves decomposition of several species and phase transformations.

h i g h l i g h t s g r a p h i c a l a b s t r a c t Semiconducting Mg(Zn)Al(Ga)eLDH/ ITO thin films prepared by solegel. LDH thin films show a turbostratic morphology made up of porous flakes. Electrochemical treatments change the flaky structure into a nanoparticle array.

The intrinsic defects of (Na 0.5 K 0.5 )NbO 3 (NKN) ceramics fabricated by high-energy ball-milling have been studied. In order to obtain the different amounts of Na/K evaporation, NKN ceramics were sintered in air (NKN exposed samples) and in calcined NKN powder (NKN buried samples), respectively. Compared with the dielectric and piezoelectric properties of the buried samples, the exposed samples exhibited a donor doped behavior at room temperature. A dielectric relaxation, induced by intrinsic oxygen vacancies, was detected in paraelectric phase region. The activity energies of intrinsic oxygen vacancies are 1.04 eV and 1.47 eV for the exposed samples and buried samples, respectively. The difference of activity energies resulted from singly ionized oxygen vacancies translating into doubly ionized oxygen vacancies with the increase of Na/K evaporation.

The rheological behavior and tape‐casting performance of ethanol‐based Pb(Mg1/3Nb2/3)O3–PbTiO3 (PMN–PT) slurries is studied. A nanocrystalline powder obtained by mechanochemical activation was used. Suspensions were prepared up to solid contents of 32 vol%, significantly higher than those usually reported for nonaqueous systems. The feasibility of replacing toxic solvents commonly used for tape casting of functional ferroelectric materials, often toluene, with ethanol is demonstrated. The density, phases and microstructure of the green tapes and sintered ceramics are presented. Cube‐shaped templates were successfully aligned during the tape casting for the processing of 〈001〉‐textured PMN–PT ceramics by templated grain growth. An effect of the lamination of tape stacks on the template orientation is described.

Variable feed machining has recently been proposed as a significant method to improve cutting tool life particularly for hard and diffucult to machine materials. This method, which is easy to apply in industry, has been shown to improve tool life in the order of 40% in certain cases. This paper presents a reliability model for the quantitative study of the effect of feed variation on tool wear and tool life. To better compare processes with two different wear modes, a reliability model taking simultaneously into account both flank and face wear has been developed. With this model, which is based on experiments, the tool life for the constant and variable feed cases was calculated from the reliability function. The mean time to failure, obtained from the reliability function, provides an accurate evaluation for any probabilistic distribution. The proposed method is therefore a general approach that can be used for analyzing cutting tool life under any conditions and for any equipment and material.

We report the results of experiments leading to the production of an image of a polarized 129Xe sample prepared by spin exchange with Cs, optically pumped with a spectrally narrowed 894.3 nm diode laser. Representative images of the average electron spin polarization are shown. Appreciable cesium electron polarization values were achieved, and a nuclear polarization of about 2.5% was measured for 129Xe. The absolute nuclear polarization was measured by water-calibrated free induction decay of the nuclear magnetic resonance signal, and the polarized xenon imaged using a 2 T magnetic resonance imaging system.

We study the diffusion of 57 Fe probe atoms in Fe/FeSi/Si/FeSi/Fe multilayers on Si(111) prepared by molecular beam epitaxy by means of 57 Fe conversion electron Mössbauer spectroscopy (CEMS). We demonstrate that the application of FeSi boundary layers successfully inhibits the diffusion of 57 Fe into the Si layer. The critical thickness for the complete prevention of Fe diffusion takes place at a nominal FeSi thickness of t FeSi = 10-12 Å, which was confirmed by the evolution of the isomer shift δ of the crucial CEM subspectrum. The formation of the slightly defective c-FeSi phase for thicker FeSi boundary layers (∼20 Å) was confirmed by CEMS and reflection high-energy electron diffraction (RHEED). Ferromagnetic resonance (FMR) shows that, for t FeSi = 0-14 Å, the Fe layers in all samples are antiferromagnetically coupled and we observe an oscillatory antiferromagnetic coupling strength with FMR and superconducting quantum interference device (SQUID) magnetometry for varying FeSi thickness with a period of ∼6 Å.

Epitaxial Si-based ferromagnet/semiconductor structures demonstrate strong antiferromagnetic coupling (AFC) as well as resonant-type tunneling magnetoresistance, which vanishes at temperatures above T $\ sim $50 K [1]. Magnetoresistance effects in Fe/ ...

Silica layers fabricated by the sol-gel method are usually amorphous and exhibit a relatively low refractive index (n~1.46 at about 0.6 mm), high optical transmittances at UV-VIS and near-IR ranges, as well as high thermal stability. Such layers can be used as high reflection or antireflection coatings, dielectric mirrors, bandpass filters, etc. However, crack-free layers with a maximum thickness of about 400 nm can be prepared in one coating cycle from sols of simple silicon alkoxides. This paper presents an approach for the preparation of crack-free silica layers that allows us to increase this maximum thickness. The approach is based on the dip-coating technique used for applying stable silica sols containing fumed silica. These sols were obtained from input silica sols based on tetraethyl orthosilicate (TEOS) with concentrations 1 and 2 mol/l, ethanol, HCl and water (RW 1.75). Fumed silica was dispersed in water/ TritonX-100 solution and added into some of the input sols. Zeta potentials, average dimensions and diffusion coefficients of silica agglomerates in sols containing fumed silica are presented. Both types of sols were deposited onto silica slides and silicon wafers by the dip-coating technique using different withdrawing velocities 100,200 and 300 mm/min. Prepared silica gel layers were thermally treated at 450 or 900 C. One of silica sols with fumed silica has also been used for the preparation of a silica-titania sol with a molar ratio Ti/Si ¼ 3/7. Both these sols have been employed for the preparation of a Bragg mirror consisting of three pairs of high-index silica-titania and low index silica layers on a silica slide. The appearance and thickness of prepared single silica and silica-titania layers have been characterized by optical microscopy and scanning electron microscopy. Optical properties of layers have been determined by spectral ellipsometry and UV-VIS-NIR transmission spectrometry. The measurements have shown that characteristics of layers fabricated from both types of the sols differ and that one can obtain stable transparent silica layers with thicknesses up to 650 nm and refractive indices in a range from 1.41 to 1.44 at 600 nm using TEOS sols containing fumed silica. It has been found that larger layer thicknesses can be achieved by using silica sols with fumed silica, by increasing the alkoxide concentration, by using increased withdrawing velocities, and by decreasing heat-treatment temperatures.

The formation mechanism of ZnO nanopowders was investigated by appropriate experimental methods. The studied samples have been obtained using a wet chemical route based on Zn(NO3)2·nH2O (ZAH) - (CH2)6N4 (HMTA) system in which the molar ratio of the components was 4:1 and 2:1. From the same solution, depending on the experimental conditions used, gels, precipitates or hydrothermally grown powders were obtained. Thermal decomposition of the as-prepared powders was analysed by simultaneous thermogravimetric and differentialthermal analysis (TG/DTA) and differential scanning calorimetry (DSC) methods. X-ray diffraction method (XRD), FT-IR Spectroscopy and Scanning Electron Microscopy (SEM) were used to study the structure and morphology of the obtained samples and their evolution with thermal treatment. The correlation between the preparation parameters and the properties of the resulted ZnO powders was established.

The influence of the additives (10 weight% rutile, anatase and zirconia) on the atomic scale structure and microstructure of a base glaze (60 wt% feldspar, 15 wt% quartz, 8 wt% kaolin, 12 wt% dolomite and 5 wt% zinc oxide) are investigated. The morphology and structure of the glazes were determined by Scanning Electron Microscopy, X-ray diffraction, infra-red spectroscopy, as well as 27 Al and 29 Si magic angle spinning (MAS) NMR. Microscopy and X-ray diffraction revealed that the opacity was caused by crystals formed during thermal treatment. Although there are clearly crystalline phases present NMR shows that the heat treated glazes are dominated by the glassy component meaning the opacity is largely conveyed by the added oxide. The relationship between composition, structure and thermal expansion coefficient of the glazes is examined.

A technique to deposit Nb-doped films of TiO 2 by radio-frequency magnetron sputtering was investigated. Nb wires are put onto the sputter track of a metallic titanium target and the oxidation state of the target is controlled by the intensity of a Ti line from the plasma emission. The sputtered films are analyzed with X-ray diffraction, RBS (Rutherford back scattering), energy dispersive X-ray spectroscopy, X-ray photoelectron spectroscopy and SEM (scanning electron microscopy). After post-heating at 400 °C, all films exhibit the anatase structure. The films are polycrystalline with a Nb/[Nb + Ti] content from 2 to 17 at.%. RBS and SEM indicate that the films consist of two sub-layers with the surface layer containing more Nb and exhibiting a clearer columnar structure. There is an optimum oxidation state of the target in the transition region between metallic and oxidic modes where the lowest resistivity of 7 × 10 -4 Ωcm is achieved. Nb is incorporated as Nb 5+ into the anatase lattice, but in suboptimal films, Ti vacancies (acceptors) may compensate the donor effect of Nb. The oxygen content in the films is higher than for stoichiometric TiO 2 . O interstitials may increase the mass density of the films. The lattice parameter a and the unit cell volume increase with the Nb content in a similar manner as observed for single crystals of TiO 2 :Nb.

In the present study, La-doped MgB 2 superconductors with different doping level (Mg 1-x La x B 2 ; x=0.00, 0.01, 0.03 & 0.05) have been synthesized by solid-state reaction route at ambient pressure. Effect of La doping have been investigated in relation to microstructural characteristics and superconducting properties, particularly intragrain critical current density (J c ). The microstructural characteristics of the as synthesized Mg(La)B 2 compounds were studied employing transmission electron microscopic (TEM) technique. The TEM investigations reveal inclusion of LaB 6 nanoparticles within the MgB 2 grains which provide effective flux pinning centres. The evaluation of intragrain J c through magnetic measurements on the fine powdered version of the as synthesized samples reveal that J c of the samples change significantly with the doping level. The optimum result on J c is obtained for Mg 0.97 La 0.03 B 2 at 5K, the J c reaches ~1.4x10 7 A/cm 2 in self field, ~2.1 x 10 6 A/cm 2 at 1T, ~2.5 x 10 5 A/cm 2 at 2.5T and ~1.8 x 10 4 A/cm 2 at 4.5T. The highest value of intragrain J c in Mg 0.97 La 0.03 B 2 superconductor has been attributed to the inclusion of LaB 6 nanoparticles which are capable of providing effective flux pinning centres.

We report on the synthesis and structural/ microstructural studies of iron based fluorine doped LaOFeAs superconductors. We have successfully synthesized fluorine doped superconducting LaO 1-x F x FeAs materials by choosing lower temperature (∼1150 0 C) and longer synthesis duration (∼60 hours) as compared to the standard values of these employed in the pioneering first contribution [Kamihara et al 2008 J Am Chem Soc 130 3296]. Decrease of lattice parameters as determined by x-ray diffraction confirm the substitution of fluorine. The superconducting transition temperature is 27.5 K which is observed at doping level of x=0.2. This superconducting material LaO 1-x F x FeAs exhibits interesting microstructural characteristic. This relates to the existence of another structural phase, besides the standard phase, having c parameters of ∼ 12.67Å. This suggests existence of modulated structure, similar to the cuprates, in these new oxypnictides. This phase may have new impacts on this new high-T C family.

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 Oxidized species from high temperature corrosion during refining were identified.  Phase identification by micro-XRD retained spatial information and phase layout.  Troilite content and continuous magnetite layers are key for proper passivation.  Mössbauer complimented XRD data by identifying additional nanocrystalline phases.