Framboid Mineralogy

Framboids are dominantly made of pyrite. The limiting factors for other minerals forming framboids include the requirements of crystal habit, solubility, and natural abundances of the constituent elements for framboid formation. Detailed examination of reports of non-pyritic framboids reveal microcrystalline material within and associated with framboids (e.g., greigite) and sub-spherical crystalline aggregates (e.g., marcasite, chalcocite-digenite, magnetite). Framboids are sometimes observed replaced by other minerals. Pyrite framboids are often formed during the earliest stages of sedimentation or mineralization and therefore are subject to further reactions with later fluids. Minerals such as copper, cobalt, zinc, and lead sulfides often display framboidal forms that have replaced original pyrite framboids. Likewise, oxidation of pyrite under some conditions can produce iron (oxyhydr)oxide and iron sulfate framboids.

GAS SENSITIVE SEMICONDUCTOR NANOMATERIALS FOR CREATION OF HYDROGEN SENSORS

Co-precipitaion method and sol-gel technique were used to prepare semiconductor microcrystalline and nanosized SnO2/Sb2O5 and Со/SnO2/Sb2O5 (0.15 wt.% Sb) materials aimed to create high sensitive hydrogen sensors. Morphology and phase composition of the obtained samples were studied by SEM, TEM and XRD methods. It was found that microcrystalline SnO2/Sb2O5 material with particle size of 1–30 μm was obtained by a co-precipitation method and nanosized SnO2/Sb2O5 material with particle size of 5–25 μm (an average size – 12 nm) was obtained by a zol-gel method. Only cassiterite phase was detected for both microcrystalline and nanosized materials. Sensitivity measurements of the sensors were carried out with using of air-hydrogen mixtures in the concentration range of 40 – 1145 ppm Н2, and dynamic characteristics (response time and relax time) were evaluated for 40 ppm Н2 at different heater power consumptions – 0.25 and 0.35 W. To increase sensitivities of the sensors, cobalt oxide, a known catalyst for hydrogen oxidation, was added to the resulting SnO2/Sb2O5 materials. It was shown that the sensors obtained by a zol-gel method demonstrate more significant sensitivity to hydrogen concentration in comparison with the sensors obtained by a co-precipitation method. It is probably associated with a higher surface area of the nanomaterial that agrees with its smaller particles as compared with the particles of the microcrystalline material. The Co-containing sensors based on the nanosized SnO2/Sb2O5 material are established to reveal higher sensitivity to Н2 than microcrystalline Co/SnO2/Sb2O5 sensors. The Co-containing sensors based on the nanosized SnO2/Sb2O5 material were found to have better dynamic characteristics than microcrystalline Co/SnO2/Sb2O5 sensors. The sensitivities increase and the response and recovery time decrease were found for both sensor materials at increasing of the sensors heater power consumption. The obtained results can be explained with different degree of energy surface heterogeneity of the used materials. The sensor response time is determined by the time of dynamic equilibrium establishment of the hydrogen oxidation reaction on the sensor surface and the recovery time is determined by the time of desorption of the H2 oxidation reaction products (H2O) from the sensor surface. Because of the processes, the sensor with a gas sensitive layer based on the nanosized material possessing with more homogeneous structure of its surface (according to the obtained TEM data) demonstrates improved gas sensitive properties in comparison with the sensor based on the microcrystalline material. The obtained results concerning the sensitivities to H2 and the dynamic parameters of the created sensors point to possibility of effective usage of the sensors based on the nanomaterial to detect H2 in air in the practice.

Concentration Dependent Fluorescence Lifetime in Nanocrystalline Y2O3:Eu Phosphors

Nanocrystalline (Y1-xEux)2O3 powder was synthesized via a chemical vapour reaction. Xray diffraction revealed the structure of cubic yttria with crystallite sizes of about 5 nm. The Eudopand concentrations x for the samples in the range from 0.003 up to 0.165 were determined by EDX-spectra. The luminescence of the nanopowders was investigated by continuous and timeresolved UV-fluorescence spectroscopy and compared to a microcrystalline Y2O3:Eu phosphor as a reference. The emission spectra show an increasing intensity for higher doping concentrations. However, compared to the microcrystalline material the yield was significantly lower. The lifetime of the 5D0 – 7FJ transition in the nanocrystalline Y2O3:Eu was found to be significantly longer than for the microcrystalline reference sample. For increasing Eu-content the lifetime in the nanocrystalline samples decreased continuously from 3.71 ms to a value of 1.20 ms for the highest doping concentration. The concentration dependent lifetime behaviour was interpreted by energy transfer between Eu ions and from Eu ions to impurities as a competing process to the radiative 5D0 – 7F2 transition.

Specimen Thickness Effects on EBSD Patterns in the Sem

Electron backscatter diffraction (EBSD) in the SEM has become a widely used technique for both automated orientation mapping and the identification of unknown crystalline phases. Despite the rapidly growing use of this technique for the above applications, there has been relatively little fundamental research concerned with determining the thickness of the surface layer that generates the EBSD pattern. EBSD patterns are related through reciprocity to electron channeling patterns. Based on this assumption the thickness of the surface layer that an EBSD pattern is generated in is 2 or 3 extinction distances. Extinction distances at low SEM voltages (10-20 kV) are quite short. Thus, EBSD patterns are generated in very thin surface layers. This paper will discuss the results of a study of the information depth of EBSD in the SEM.Previous research has attempted to determine the information depth of EBSD by obtaining patterns from samples that have had layers of other crystalline or microcrystalline material deposited on the surface.

Properties of silicon films deposited under argon dilution

ABSTRACTFour series of samples, prepared at 250° C by decomposition of a mixture of silane and argon in a radio frequency powered deposition systems (rf-PECVD), have been studied. The dilution rates were 1 %, 1.5 %, 5 % and 10 % of silane in argon and the total pressure was 0.5 Torr for the first series and 0.2 Torr for the others. Structural and transport properties of the materials have been studied as function of power density. Structural studies show the transition from purely amorphous material towards microcrystalline material with increasing rf power density. The transport parameters were measured in the as-deposited, light-soaked and annealed states and compared to those obtained on state of the art material. The best material obtained is clearly device grade material. This study shows that argon dilution allows to tailor the material for a given application.

Electrical/Dielectric Properties of Nanocrystalline Cerium Oxide

ABSTRACTElectrical/dielectric properties of nanocrystalline cerium oxide have been studied using impedance spectroscopy, thermopower, and DC 4-point conductivity. The combined techniques identified the effect of poor electroding on impedance spectra. Incomplete contact between the specimen and the electrode induces an additional arc in the impedance spectra. The additional high resistance feature results from the geometric constriction of current flow at the specimen/electrode interface and can be misinterpreted as a grain boundary response. The defect chemistry, nonstoichiometry, and transport properties were investigated in nanoscale ceria and compared with those of microcrystalline material.

Exchange Anisotropy in Amorphous/Microcrystalline Co-Gd Films

ABSTRACTMagnetization, TEM, and x-ray diffraction studies have been carried out on GdCox films sputtered onto Si or sapphire substrates at ˜90 C, ambient temperature. The composition range studied was x=2−8.5. Over the composition range defined approximately by 5>×>3, the films, which are 1–3 microns thick, exhibit a unidirectionally displaced B-H loop, characteristic of an exchange-biased phase. TEM studies indicated that the samples with the shifted loops indeed consist of a mixture of amorphous and microcrystalline phases. The characteristic size of the microstructure is 10–20 A. Electron diffraction shows a very broad ring characteristic of amorphous phase together with six or seven sharper rings characteristic of crystalline material which index best to the hexagonal GdCo5 structure or to a high temperature hexagonal Gd2Co17 phase. The diffraction pattern remains virtually unchanged over the composition range x=2–8. This leads us to conclude that the microcrystalline material consists of one, or perhaps more than one, metastable phase over the indicated composition range. X-ray diffraction shows only one broad maximum.

Chemistry and Solid State Physics of Microcrystalline Silicon

Various methods for the preparation of microcrystalline (nanocrystalline) silicon are summarized and compared with respect to the possibility of the control of the materials quality and scaling of the deposition process to large area applications. It is shown that the deposition of a pure microcrystalline material is achieved under conditions close to partial chemical equilibrium. The mechanism of the crystallization during the growth will be briefly discussed.The second part of the paper deals with the physical properties of pure microcrystalline silicon which is free of any amorphous phase detectable by X-ray diffraction, i.e. less than about 1 vol%. Several aspects of electric conductivity, optical absorption and Raman scattering which have been frequently misinterpreted in the literature will be reviewed.