Showing papers on "Texture (crystalline) published in 2007"

Rietveld texture analysis from diffraction images

Abstract: The procedure to perform Rietveld texture analysis from diffraction images collected with CCDs or image plates is shown. In some cases only one transmission image may be sufficient to obtain an Orientation Distribution Function (ODF) if a sufficient number of peaks are included. The images are transformed in spectra and analyzed using the Maud Rietveld program containing some recently developed texture model well suited for this kind of analysis. In this work we will present the results obtained using a custom laboratory image plate camera developed for texture analysis. The instrument may work with a curve image plate detector in reflection condition or with a flat detector in transmission. The reflection condition is used mainly for ceramics and metal-alloys and the transmission mode for polymers and fibres. We will show how with the combination of such camera and the Rietveld Texture Analysis method we were able to analyze the ODF of the martensitic phase of Shape Memory Alloy (monoclinic NiTi SMA) as well as to obtain the quantitative texture of low symmetry polymers in fibre form.

Wicking within forests of micropillars

Abstract: We describe how a wetting liquid brought into contact with a forest of micropillars impregnates this forest. Both the driving and the viscous forces depend on the parameters of the texture (radius b and height h of the pillars, pitch p of the network) and it is found that two different limits characterize the dynamics of wicking. For small posts ( h p ), the speed of impregnation becomes independent of the pillar height, and becomes mainly fixed by the radius of the posts. Copyright c EPLA, 2007

Thin-film silicon solar cells with efficient periodic light trapping texture

Abstract: For solar cells based on thin-film microcrystalline (μc-Si:H) or amorphous silicon (a-Si:H) with absorber layers in the micrometer range, highly effective light trapping and an optimal incoupling of the entire sun spectrum are essential. To investigate and optimize both effects the wave propagation in thin-film silicon solar cells is modeled in three dimensions (3D) solving the Maxwell equations rigorously. A periodic nanostructured texture is investigated as an alternative to the common randomly rough texture. Inverted 3D pyramids with a periodicity of 850nm and structure height of 400nm show promising high quantum efficiencies close to the Tiedje limit.

Optical properties and structure of HfO2 thin films grown by high pressure reactive sputtering

Abstract: Thin films of hafnium oxide (HfO2) have been grown by high pressure reactive sputtering on transparent quartz substrates (UV-grade silica) and silicon wafers. Deposition conditions were adjusted to obtain polycrystalline as well as amorphous films. Optical properties of the films deposited on the silica substrates were investigated by transmittance and reflectance spectroscopy in the ultraviolet, visible and near infrared range. A numerical analysis method that takes into account the different surface roughness of the polycrystalline and amorphous films was applied to calculate the optical constants (refractive index and absorption coefficient). Amorphous films were found to have a higher refractive index and a lower transparency than polycrystalline films. This is attributed to a higher density of the amorphous samples, which was confirmed by atomic density measurements performed by heavy-ion elastic recoil detection analysis. The absorption coefficient gave an excellent fit to the Tauc law (indirect gap), which allowed a band gap value of 5.54 eV to be obtained. The structure of the films (amorphous or polycrystalline) was found to have no significant influence on the nature of the band gap. The Tauc plots also give information about the structure of the films, because the slope of the plot (the Tauc parameter) is related to the degree of order in the bond network. The amorphous samples had a larger value of the Tauc parameter, i.e. more order than the polycrystalline samples. This is indicative of a uniform bond network with percolation of the bond chains, in contrast to the randomly oriented polycrystalline grains separated by grain boundaries.

Highly organized mesoporous TiO2 films with controlled crystallinity : A Li-insertion study

Abstract: A study of electrochemical Li insertion combined with structural and textural analysis enabled the identification and quantification of individual crystalline and amorphous phases in mesoporous TiO2 films prepared by the evaporation-induced self-assembly procedure. It was found that the properties of the amphiphilic block copolymers used as templates, namely those of a novel poly(ethylene-co-butylene)-b-poly(ethylene oxide) polymer (KLE) and commercial Pluronic P123 (HO(CH2CH2O)20(CH2CH(CH3)O)70(CH2CH2O)20H), decisively influence the physicochemical properties of the resulting films. The KLE-templated films possess a 3D cubic mesoporous structure and are practically amorphous when calcined at temperatures below 450 °C, but treatment at 550–700 °C provides a pure-phase (anatase), fully crystalline material with intact mesoporous architecture. The electrochemically determined fraction of crystalline anatase increases from 85 to 100 % for films calcined at 550 and 700 °C, respectively. In contrast, the films prepared using Pluronic P123, which also show a 3D cubic pore arrangement, exhibit almost 50 % crystallinity even at a calcination temperature of 400 °C, and their transformation into a fully crystalline material is accompanied by collapse of the mesoporous texture. Therefore, our study revealed the significance of using suitable block-copolymer templates for the generation of mesoporous metal oxide films. Coupling of both electrochemical and X-ray diffraction methods has shown to be highly advisable for the correct interpretation of structure properties, in particular the crystallinity, of such sol–gel derived films.

Evolution of crystallographic texture during equal channel angular extrusion (ECAE) and its effects on secondary processing of magnesium

Abstract: This work addresses the course of texture evolution during different routes of equal channel angular extrusion (ECAE) of magnesium and the effect of ECAE texture on subsequent deformation. Commercially pure magnesium was deformed at 250 °C through the routes A, B C and C up to four passes. Texture evolution was found to follow a different course for different routes of ECAE. In general, the ECAE textures were asymmetric. An average grain size of 6–8 μm could be achieved after four passes. The improved cold formability in the present investigation has been attributed to the initial non-basal texture and grain refinement introduced by ECAE. The textures introduced by ECAE may be beneficial for cold rolling characteristics of magnesium.

Texture development in Ti-6Al-4V linear friction welds

Abstract: Microstructure and microtexture development in as-welded and post weld heat treated (PWHT) lab-scale (LS) and full-scale (FS) Ti–6Al–4V linear friction welds have been characterized using scanning electron microscopy and electron back-scatter diffraction (EBSD). The full-scale specimens exhibited a plastically affected zone (PAZ) about twice the size compared to lab-scale test welds. At the weld line a region of very fine martensitically formed microstructure was observed in both the LS and FS friction welds. EBSD texture scans across the weld line revealed a sharp texture variation in the PAZ. At the weld line a strong { 1 0 1 ¯ 0 } 〈 1 1 2 ¯ 0 〉 transverse texture for the as-welded and PWHTed condition was observed in both welds. Whereas the lab-scale linear friction welds displayed a dramatic change from almost random to strong transverse texture in the PAZ, the full-scale linear friction welds exhibited transverse texture at the weld line, followed by dispersed bands of transverse and { 1 1 2 ¯ 2 } 〈 1 1 2 ¯ 3 〉 type rolling texture in the PAZ region. The observed difference in LS and FS weld region textures is attributed to a difference in the variant selections during β → α transformation.

Deformation induced martensite in an AISI 301LN stainless steel: characterization and influence on pitting corrosion resistance

Abstract: In austenitic stainless steels, plastic deformation can induce martensite formation. The induced martensite is related to the austenite (g) instability at temperatures close or below room temperature. The metastability of austenite stainless steels increases with the decreasing of stacking fault energy (SFE). In this work, the deformation induced martensite was analyzed by X ray diffraction, electron back scatter diffraction (EBSD), magnetic methods and atomic force microscope (AFM) in samples of a low SFE austenitic stainless steel, AISI 301LN and compared with a medium SFE stainless steel, AISI 316L. Both techniques, X ray diffraction and EBSD, presented similar quantities for the a’-martensite. Texture results indicate that the crystallographic orientation of the formed a’-martensite is {001} and {103} . The morphology of a’-martensite was analyzed by AFM. Corrosion tests showed that deformation reduces pitting corrosion and generalized corrosion resistance in both steels.

Analysis of the γ–ɛ–α′ variant selection induced by 10% plastic deformation in 304 stainless steel at −60 °C

Abstract: The texture changes occurring by phase transformations, induced by 10% plastic deformation of a 304 stainless steel sheet at T = −60 °C have been analysed. The inherited α′ texture is obtained from the austenitic γ texture with variant selection involving the intermediate HCP ɛ phase. The local orientation analysis of the different phases suggested that the applied stress first promotes the ɛ variants in a γ grain. At this stage, a strong variant selection often occurs due to the influence of the external applied stress. Then, the selected ɛ variants transform into α′ variants. The selection of these consecutive α′ variants is more complex because of the superposition of the applied stress and the internal stresses due to the formation of the ɛ variants themselves. We analysed these transformations in austenitic grains at the light of the γ–ɛ and ɛ–α′ transformation mechanisms. The selected variants, calculated according to these mechanisms are in fair agreement with the experimental ones.

Direct observation of amorphous to crystalline phase transitions in nanoparticle arrays of phase change materials

Abstract: We have used time-resolved x-ray diffraction to study the amorphous-crystalline phase transition in 20–80nm particles of the phase change materials Ge2Sb2Te5, nitrogen-doped Ge2Sb2Te5, Ge15Sb85, Sb2Te, and Sb2Te doped with Ag and In. We find that all samples undergo the phase transition with crystallization temperatures close to those of similarly prepared blanket films of the same materials with the exception of Sb2Te that shows the transition at a temperature that is about 40°C higher than that of blanket films. Some of the nanoparticles show a difference in crystallographic texture compared to thick films. Large area arrays of these nanoparticles were fabricated using electron-beam lithography, keeping the sample temperatures well below the crystallization temperatures so as to produce particles that were entirely in the amorphous phase. The observation that particles with diameters as small as 20nm can still undergo this phase transition indicates that phase change solid-state memory technology should scale to these dimensions.

Texture and Anisotropy of Polycrystalline Piezoelectrics

Abstract: Piezoelectricity is manifested in ferroelectric ceramics by inducing a preferred volume fraction of one ferroelectric domain variant orientation at the expense of degenerate orientations. The piezoelectric effect is therefore largely controlled by the effectiveness of the electrical poling in producing a bias in ferroelectric (180°) and ferroelastic (non-180°) domain orientations. Further enhancement of the piezoelectric effect in bulk ceramics can be accomplished by inducing preferred orientation through grain-orientation processes such as hot forging or tape casting that precede the electrical-poling process. Coupled crystal orientation and domain orientation processing yields ceramics with an even greater piezoelectric response. In this paper, preferred orientations of domains and grains in polycrystalline piezoelectric ceramics generated through both domain- and grain-orientation processing are characterized through pole figures and orientation distribution functions obtained using data from a variety of diffraction techniques. The processing methods used to produce these materials and the methods used to evaluate preferred orientation and texture are described and discussed in the context of prior research. Different sample and crystal symmetries are explored across a range of commercial and laboratory-prepared materials. Some of the variables presented in this work include the effects of in situ thermal depoling and the detailed processing parameters used in tape casting of materials with preferred crystallite orientations. Preferred orientation is also correlated with anisotropic properties, demonstrating a clear influence of both grain and domain orientations on piezoelectricity.

Low temperature deposited L10 FePt–C (001) films with high coercivity and small grain size

Abstract: FePt–C films with high coercivity, (001) texture, and small grain size were deposited on MgO∕CrRu/glass substrate by cosputtering FePt and carbon at 350°C. The out-of-plane coercivity measured at room temperature increased from 9.6to15.1kOe when C concentrations increased from 0% to 15%. Further increasing the C contents to 20% and 25% caused the decrease of coercivity to 13.6 and 11.8kOe, respectively. With C doping, a two-layer structure of FePt–C films was formed and fcc-phase FePt particles were found. By optimizing the sputtering process, FePt–C (001) film with coercivity higher than 14.4kOe and columnar FePt grains of 7.5nm in diameter was obtained, which are suitable for ultrahigh density perpendicular recording.

Dramatic morphological change of scallop-type Cu6Sn5 formed on (001) single crystal copper in reaction between molten SnPb solder and Cu

Abstract: Wetting reaction between molten Sn-based solders and Cu produces scallop-type Cu6Sn5. In the present wetting study, a (001) single crystal Cu is used as substrate and a dramatic change in the morphology of Cu6Sn5 is observed: instead of scallop type, the authors observed a rooftop-type Cu6Sn5 grains, elongated along two preferred orientation directions. This was confirmed by electron beam backscattered diffraction and white beam synchrotron x-ray microdiffraction. The results indicate that the nucleation, growth, and ripening behavior of Cu6Sn5 on single crystal substrate can be quite different from the conventional case of wetting on randomly oriented polycrystalline Cu substrates.

Enhanced Photoelectrochemical Activity of Sol−Gel Tungsten Trioxide Films through Textural Control

Abstract: Novel mesoporous tungsten trioxide films with enhanced incident photon-to-current conversion efficiencies have been prepared by a sol−gel route from an aqueous precursor solution containing peroxopolytungstic acid (PPTA). For films heated in air at 500 °C, it was found that film texture depended in a precise and reproducible manner on adjustment of the pH of this precursor solution by addition of a small volume of a selected mineral acid. Mesoporous micrometer-thick transparent films were obtained from PPTA without pH adjustment while mesoporous semi-transparent films resulted when the pH was lowered. The transparent films had specific surface areas of 18 m2/g, average pore diameters of 7.3 nm, and average crystallite sizes of 30 nm. The semi-transparent films possessed specific surface areas of 30 m2/g, average pore diameters of 12.5 nm, and average crystallite diameters of 17 nm. In the case of the semi-transparent films, electron microscopy indicated that the fundamental crystallites formed part of lar...

Determination of the thickness and optical constants of transparent indium-doped ZnO thin films by the envelope method

Abstract: Transparent indium-doped ZnO thin films were deposited by the spray pyrolysis method onto glass substrates The content of indium in the starting solution was 05 at % The crystallographic structure of the film was studied by X-ray diffraction (XRD) XRD measurement shows that the film is crystallized in the wurtzite phase and presents a preferential orientation along the c-axis The texture coefficient (TC), grain size value and lattice constants have been calculated The absorption coefficient and the thickness of the films were calculated from interference of transmittance spectra Optical constants such as the refractive index n and extinction coefficient k have been determined from transmittance spectrum in the ultraviolet–visible–near infrared (UV–VIS–NIR) regions using the envelope method The thickness of the films strongly influences the optical constants