Specific Defect Structures Research Articles

The evolution of γ-Mg17Al12 intermetallic compound during accumulative back extrusion and subsequent ageing treatment

Accumulative back extrusion (ABE) processing, as a novel severe plastic deformation (SPD) method, has been recently justified to be capable of modifying the microstructural characteristics of alloys. In line to its ongoing researches, the present work has been planned to study the evolution of γ-Mg17Al12 intermetallic phase during ABE and subsequent ageing treatment in a high Al-bearing Mg–Al–Zn alloy. The behaviour of γ intermetallic has been systematically examined as following points of view: (i) strain–temperature-dependent morphology changes, (ii) strain-induced dissolution, and (iii) re-ageing behaviour as a function of time and temperature. Aiming to analyse the morphology of eutectic γ compound with respect to the strain and temperature, 2D projections of effective diameter, shape factor and globularity have been made in strain/temperature graphs. The processing conditions (strain and temperature) corresponding to the desired and undesired morphologies are introduced and microstructurally explained through underlying plasticity mechanisms, i.e., ‘necking-thinning-particle separation’ and ‘brittle fragmentation.’ The former mechanism is suggested to be in relation with partial strain-induced dissolution of eutectic γ phase, leading to generation of a supersaturated solid solution. This has resulted to the observation of ‘off-stoichiometry’ phenomena in Mg17Al12 phase and has been justified through dislocation-assisted deformation mechanism at elevated temperature. Surprisingly, a unique re-ageing behaviour has been found for the obtained solid solutions, where a modified kinetics and morphology of γ phase precipitation were characterized. The altered precipitation behaviour is attributed to the specific defect structure achieved by SPD acting as fast diffusion channel for Al solutes.

Research Progress on Brittleness of Iridium

Abstract Due to the high melting point, excellent high temperature strength and anticorrosive property, iridium is the unique material which can be used under extremely hostile environments. However, iridium exhibits an anomalous brittle fracture behavior, a mixed brittle intergranular fracture (BIF) and brittle transgranular fracture (BTF), even though it is of face-centred cubic (fcc) crystal structure. A great deal of efforts have been made to explore the embrittlement mechanisms since the anomalous fracture behavior was recognized in 1960 s, up to now, there has not been a reasonable conclusion yet. This paper emphatically reviewed the possible embrittlement mechanism of iridium, including impurity-induced brittleness, intrinsic brittleness and special defect structure induced embrittlement, discussed the research status quo about the deformation and failure mechanisms of iridium. Finally, the research direction and research method of the embrittlement mechanism of iridium were forecasted.

Ferroic properties of Fe-doped and Cu-doped K0.45Na0.49Li0.06NbO3 ceramics

Ferroelectric and magnetic properties of Fe-doped K0.45Na0.49Li0.06NbO3 (KNLN) and Cu-doped KNLN ceramics were investigated. Comparing with pure KNLN ceramic, Fe-doped KNLN shows ferromagnetic and ferroelectric properties simultaneously while Cu-doped KNLN shows stronger diamagnetic and ferroelectric properties. The magnetic property in KNLN matrix are believed to be induced by point defects. The ferromagnetism of Fe-doped KNLN can be explained by the F-center exchange mechanism, while the antiferroelectric-like loop and diamagnetism of Cu-doped KNLN was explained by the special defect structure and Cu1+ ion at A site.

Nonradiative Recombination via Conical Intersections Arising at Defects on the Oxidized Silicon Surface

Nonradiative recombination of excitations in semiconductors limits the performance of photovoltaics, light-emitting diodes, photocatalysts, and other devices. Herein we investigate the role that two known defects on the oxidized surface of silicon play in nonradiative recombination in silicon nanocrystals. We apply ab initio multiple spawning and multireference electronic structure methods to model the nonradiative processes which follow excitation of two cluster models of silicon epoxide defects that differ in the oxidation state of their respective silicon atoms. We find conical intersections in both clusters, and these intersections are found to be accessible at energies corresponding to visible wavelengths. In both cases, photochemical opening of the epoxide ring precedes nonradiative decay. These results support the hypothesis that conical intersections associated with specific defect structures on the oxidized surface of silicon nanocrystals facilitate nonradiative recombination. Discussion regarding how this hypothesis can be tested experimentally is presented.

Effect of calcination temperature on the photocatalytic activity of CaSb2O6 nanoparticles prepared by co-precipitation method

CaSb2O6 precursors were obtained via co-precipitation technique and subsequently they underwent calcination at different temperatures. The effect of calcination temperature on the crystallite size, specific surface area, crystallinity and defect structure of CaSb2O6 has been studied in detail. It is found that the photocatalytic activity of the as-prepared CaSb2O6 samples for methyl orange (MO) degradation under UV light is strongly dependent on the calcination temperature. The results demonstrated that the CaSb2O6 nanoparticles calcined at 900°C possess the best photocatalytic activity and excellent photocatalytic stability. A possible mechanism of the photocatalysis over CaSb2O6 was also presented.

Photo-degradation of organic dye by zinc oxide nanosystems with special defect structure: Effect of the morphology and annealing temperature

The fabrication of strong photocatalysts applied to the degradation of organic pollutants is necessary in environmental applications. In a single-stage method, acetate precursor and poly vinyl pyrolydine are used to produce ZnO nanostructures with various morphologies in annealing temperatures ranging from 300 to 900°C. The physical properties of the prepared nanostructures were characterized by SEM, TEM, XRD, BET, DRS, CHN analysis and PL spectroscopy. The SEM images exhibit a variety of the as-prepared hexagonal zinc oxides including wires, rods, particles and porous network of welded particles of ZnO nanoparticles. The results of the photocatalytic degradation of methylene blue as an organic dye in aqueous suspension showed that the morphology of ZnO nanostructures influences on the photocatalytic efficiency of ZnO nanostructures, greatly. For the best result, the highest MB degradation occurs by ZnO nanowires within 16min and in others samples, degradation of higher than 95% occurs within 20min. The XRD and PL spectroscopy revealed neither VZn nor Oi are in all of samples but only VO−, VO2− and Zni exist in ZnO nanostructures.

Atomistic insight into the minimum wear depth of Cu(111) surface

In the present work, we investigate the minimum wear depth of single crystalline Cu(111) under single asperity friction by means of molecular dynamics simulations. The atomistic mechanisms governing the incipient plasticity are elucidated by characterizing specific defect structures and are correlated to the observed mechanical and frictional responses of the material. Furthermore, the effect of probe radius on the friction process is studied. Our simulations indicate that the formation of wear impression is closely associated with defect nucleation and the minimum wear depth is equivalent to the critical penetration depth at which plasticity initiates. It is found that the probe radius has a strong influence on the formation of defect structures and the observed mechanical responses.

Photocatalytic studies of Ho–Zr–O nano-composite with controllable composition and defects

With the help of sol–gel method assisted by melting salt, a series of Ho–Zr–O nano-composite with controllable composition and defects have been successfully prepared. Characterization results show that the positions, intensity, and width of the X-ray diffraction peaks of the products have a regular variation with the increase of zirconium element which implies the gradual changes of crystal spacing and product size. At the same time, the molar ratios between holmium and zirconium ions are consistent with the chemical formula and both of them are uniformly distributed in products further showing the perfect formation of targeted materials. Optical properties reveal that diversified defect forms of Ho–Zr–O nano-composite lead to the different absorptions of visible light. Photocatalytic experiments demonstrate Zr0.8Ho0.2O2−δ nano-crystals have excellent visible-light-responsive photocatalytic activities on some familiar dyes (e.g.: methylene blue and Rhodamine B) which results from the special defect structure, better absorption of visible light and larger specific surface area. It follows that Zr0.8Ho0.2O2−δ nano-crystals are a new kind of visible-light-responsive photocatalysts with better prospects in conversion and utilization of solar energy. Also, the present melting salt assisted route might be generalized to synthesize other AxByOz composite oxide nano-crystals with more complicated structures.

RE/ZrO2 (RE = Sm, Eu) composite oxide nano-materials: Synthesis and applications in photocatalysis

Zirconia modified by Samarium/Europium, RE/ZrO2 (RE=Sm, Eu), composite oxide nano-materials have been successfully synthesized by improved sol–gel method. Characterization results show that X-ray diffraction (XRD) peaks of products gradually shift to the lower angle with the increase of rare earth which implies that the lattice distances of RE/ZrO2 nano-materials are gradually enlarged. Moreover, the molar ratios between zirconium and rare earth are consistent with the chemical formula and both of them are uniformly distributed in samples. Optical properties indicate that defect structures and electron configurations of RE/ZrO2 (RE=Sm, Eu) with single phase determine their absorption properties on visible light. Photocatalytic experiments indicate Zr0.8Sm0.2O2−δ nano-crystals have excellent visible-light-responsive photocatalytic activities on Methylene blue and Rhodamine B which results from the special defect structure, suitable electronic configuration, and larger specific surface area. It follows that Zr0.8Sm0.2O2−δ nano-crystals are new visible-light-responsive photocatalysts which can be applied in dye wastewater treatment and environmental protection in the future.

ZrO2/Dy2O3 Solid Solution Nano‐Materials: Tunable Composition, Visible light–Responsive Photocatalytic Activities and Reaction Mechanism

A series of ZrO2/Dy2O3 solid solution nano‐materials with tunable compositions were successfully synthesized by the molten salt–assisted method. Structural characterization results show the positions, intensity, and width of the X‐ray diffraction peaks of products show a regular variation with increasing Dy3+ content which implies the gradually changes of crystal spacing and product size. In addition, all the products are single phase without the coexistence of zirconia and Dy2O3 further demonstrating the perfect formation of targeted solid solutions. Photocatalytic experiments reveal that Zr0.8Dy0.2O2−δ nano‐crystals have pretty photocatalytic degradation activities on Rhodamine B and Methylene blue under visible light irradiations. Reaction mechanism indicates that the excellent photocatalytic activities of Zr0.8Dy0.2O2−δ nano‐crystals result from the special defect structure, smaller size, and larger specific surface area. It follows that Zr0.8Dy0.2O2−δ nano‐crystals are promising visible light–responsive photocatalysts with better prospect in environmental protection and dye wastewater treatment. Moreover, the present molten salt–assisted route might be generalized to synthesize other solid solution nano‐crystals with more complicated structures.

Detection of Fe2+ valence states in Fe doped SrTiO3 epitaxial thin films grown by pulsed laser deposition

We present an X-ray absorption spectroscopy study on Fe-doped SrTiO3 thin films grown by pulsed laser deposition. The Fe L2,3 edge spectra are recorded for doping concentrations from 0-5% after several annealing steps at moderate temperatures. The Fe valence state is determined by comparison with an ilmenite reference sample and calculations according to the charge transfer multiplet model. We found clear evidence of Fe(2+) and Fe(3+) oxidation states independently of the doping concentration. The Fe(2+) signal is enhanced at the surface and increases after annealing. The Fe(2+) configuration is in contrast to the mixed Fe(3+)/Fe(4+) valence state in bulk material and must be explained by the specific defect structure of the thin films due to the kinetically limited growth which induces a high concentration of oxygen vacancies.

Cross-sectional transmission electron microscopy of thin graphite films grown by chemical vapor deposition

Graphene has been the subject of an extraordinary upsurge of interest due to its intriguing properties and potential applications. Recent work has shown that excellent electronic properties are exhibited by large-scale ultrathin graphite films, grown by chemical vapor deposition on a polycrystalline metal and transferred to a device-compatible surface. The properties of such multilayered graphene films could depend strongly on film thickness and uniformity. Unlike the other common methods for analysis in the literature, cross-sectional transmission electron microscopy (TEM) would provide direct, straightforward analysis of film thickness and quality, as well as provide a means to investigate specific defect structures (e.g. wrinkles). However, this approach has not often been pursued due to the sensitivity of graphite to the electron and ion beam damage in such a procedure. Here, an approach to creating cross-sectional TEM samples using a focused ion beam lift-out method is presented, along with the resulting TEM images of thin graphite films and several wrinkle defects. Samples removed from as-grown films on polycrystalline nickel and films removed from measured devices are presented. The benefits and limitations to this approach are discussed.

X-ray characterization of catalytically grown ZnTe and ZnMgTe nanowires

This paper reports on the results of detailed X-ray diffraction studies of ZnTe and ZnMgTe nanowires grown by molecular beam epitaxy. As the aim of proper interpretation of the X-ray measurements of samples consisting of large number of nanowires we defined a virtual unit cell as an averaged one from all really existing cells in the large number of nanowires. The studies revealed that average structure of nanowires is characterized by a minor rhombohedral distortion. The occurrence of this distortion is attributed to the specific defect structure in the majority of nanowires.

Bulk and epitaxial growth of micropipe‐free silicon carbide on basal and rhombohedral plane seeds

AbstractIn this paper the criteria for reasonable choice of alternative to (0001) seed orientation are discussed. Growth conditions necessary to produce bulk 6H‐ and 4H‐SiC crystals on rhombohedral (01‐1n) plane seeds in PVT process are presented. Specific defect structures in such crystals are described in relation to growth conditions. In the second part of the paper, liquid phase epitaxial (LPE) growth using high‐temperature silicon based flux and rhombohedral plane seeds is compared to the LPE on basal plane seeds. In both bulk and epitaxial growth processes the employment of rhombohedral plane seed offers important advantages. (© 2008 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

An Alternative Approach to Analyzing the Interstitial Decay from the End of Range Damage During Millisecond Annealing

ABSTRACTFlash-assist Rapid Thermal Processing (RTP) presents an opportunity to investigate annealing time and temperature regimes which were previously not accessible with conventional annealing techniques such as Rapid Thermal Annealing. This provides a unique opportunity to explore the early stages of the End of Range (EOR) damage evolution and also to examine how the damage evolves during the high temperature portion of the temperature profile. However, the nature of the Flash-assist RTP makes it is extremely difficult to reasonably compare it to alternative annealing techniques, largely because the annealing time at a given temperature is dictated by the FWHM of the radiation pulse. The FWHM for current flash tools vary between 0.85 and 1.38 milliseconds, which is three orders of magnitude smaller to that required for a RTA to achieve similar temperatures. Traditionally, the kinetics of the extended defects has been studied by time dependent studies utilizing isothermal anneals; in which specific defect structures could be isolated. The characteristics of Flash-assist RTP do not allow for such investigations in which the EOR defect evolution could be closely tracked with time. Since the annealing time at the target temperature for the Flash-assist RTP is essentially fixed to very small times on the order of milliseconds, isochronal anneals are a logical experimental approach to temperature dependent studies. This fact presents a challenge in the data analysis and comparison. Another feature of Flash-assist RTP which makes the analysis complex is the ramp time relative to the dwell time spent at the peak fRTP temperature. As the flash anneal temperature is increased the total ramp time can exceed the dwell time at the peak temperature, which may play a significantly larger role in dictating the final material properties. The inherent characteristics of Flash-assist RTP have consequently required the development of another approach to analyzing the attainable experimental data, such that a meaningful comparison could be made to past studies. The adopted analysis entails the selection of a reference anneal, from which the decay in the trapped interstitial density can be tracked with the flash anneal temperature, allowing for the kinetics of the interstitial decay to be extracted.

Predicted pyrochlore to fluorite disorder temperature for A2Zr2O7compositions

In a previous publication the order–disorder pyrochlore to fluorite transformation temperatures for a series of A2Hf2O7pyrochlores were predicted [C.R. Stanek and R.W. Grimes: Prediction of rare-earth A2Hf2O7pyrochlore phases.J. Am. Ceram. Soc.2002,85,p. 2139]. This was facilitated by establishing a relationship between these temperatures and the energy required to introduce a specific defect structure into the perfect pyrochlore lattice. Here an equivalent relationship for A2Zr2O7pyrochlores was generated, and from this the disorder temperatures for a number of compositions including Eu2Zr2O7were predicted.

Fluctuation conductivity and critical currents in YBCO films

A comparative analysis of the results obtained in measurements of the fluctuation conductivity and of the critical current density jc(T) in YBa2Cu3O7−y films containing various numbers of defects is carried out for the first time. It is found that the value and temperature dependence of the fluctuation conductivity are interrelated with the values and temperature dependences of the critical current and resistivity of the samples. It follows from measurements of the fluctuation conductivity that the variation of these temperature dependences is directly related to the variation of the number of defects and, hence, the number of pinning centers in the films studied. It is shown that in films containing practically no defects the jc(T) curve completely follows a model giving jc(T) in high-Tc superconductors with low-angle grain boundaries between crystallization blocks. On the other hand, if the sample has a specific defect structure that is formed under certain conditions in c-oriented epitaxial high-Tc superconducting films, one observes a sharp increase in jc and in the slope of the jc(T) curve.

Creation and annealing of defect structures in silicon-based semiconductors during and after implantations at 77–500 K

Defect structures created in implantations of radioactive 57Mn + ions into silicon-based semiconductors held at temperatures of 77–500 K have been studied by Mössbauer spectroscopy on the 14 keV γ-radiation emitted in the decay of the 57Fe daughter atoms. For implantations at <300 K, the majority of Fe atoms is located in a specific defect structure, likely within an “amorphous pocket”, which anneals partially at 100–200 K and completely at 300–450 K. The latter is proven to occur within minutes by isothermal time delayed measurements and results in a substitutional incorporation of the 57Mn probe atoms. The atomic structure of the annealing defect is compared to that of Fe in amorphous silicon upon 57Mn implantation.

Free‐Standing HVPE‐GaN Quasi‐Substrates: Impurity and Strain Distributions

(a) Linko¨ping University, 581 83 Linko¨ping, Sweden(b) Macquarie University, Sydney 2109 NSW, Australia(c) Chalmers University of Technology, 412 96 Go¨teborg, Sweden(d) Faculty of Physics, Sofia University, Sofia, Bulgaria(Submitted July 22, 2002; accepted October 1, 2002)PACS: 61.10.Eq; 68.55.Jk; 78.55.Cr; 78.60.HkWe report a study of the physical properties of free-standing HVPE-GaN quasi-substrates. A vari-ety of characterization techniques was employed in order to characterise in a comparative way thetwo sides of the films. The as-grown Ga-face was found to have lower density of both, structuraland impurity defects. This leads to a lower concentration of free carriers and residual strain in theGa-face compared to that in the N-face. The optical properties were found to be strongly influ-enced by the specific defect structure in both faces.

Powder diffraction data for the germanides CoGe and Co5Ge7

Precise X-ray powder diffraction data are given for two germanides, CoGe and Co5Ge7. The refined unit cell parameters for CoGe are a=11.630(1) Å, b=3.8014(3) Å, c=4.9347(3) Å and β=100.889(8)° (space group C2/m, Pearson symbol mC16) with volume of the unit cell 214.24(2) Å3; the figures of merit are M20=96, F30=77 (0.0085, 46). The refined unit cell parameters for Co5Ge7 are a=7.6262(4) Å and c=5.8017(5) Å (I4mm, tI24) with volume of the unit cell 337.42(3) Å3; the figures of merit are M20=204 and F22=130(0.0068, 25). The dependence on composition of the unit cell parameters of CoGe is discussed in terms of specific defect structures.