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The thixoformability and defect formation of AZ91D magnesium alloy are studied with different processing parameters including reheating temperature, time and die temperature. The results indicated that the suitable processing parameters should be reheating temperature between 575-595°C, reheating time more than 75 min, and die temperature over 275°C. Four types of defects, coldshut, liquid segregation, microporosity and cracks, have been observed in the thixoformed products if the processing parameters are not controlled properly. Among these defects, microporosity and cracks are always dominant.

We explain the mechanism of defect screening in GaInN/GaN quantum wells, which are used as active layers in white and blue light emitting diodes (LEDs). Despite the fact that these devices have now been commercially available for some time, the reason for the high luminescence efficiency had not been really understood. The high defect densities in these devices commonly would not allow the use as an optical emitter. We present the mechanism turning an actually poor-quality material into a powerful optical emitter.

Surface conditions of aluminum can influence the final arrangement of nano-pores in fabrication of ordered nanoporous anodic alumina membranes (AAMs). This study is mainly focused on the different applied voltages of aluminum electropolishing by keeping all the other parameters constant. After heat treatment (stress relieving and annealing at 500°C) of pure aluminum sheets, the samples were electropolished at different voltages (10-60V) to obtain desirable surface smoothness, while the temperature of the container was kept constant. The current-time curves were recorded during electropolishing process. The surface roughness obtained in each applied voltage was examined using optical microscope and atomic force microscope (AFM). The process was followed by two-step anodization in order to reach ordered nano-pores. Finally, the influence of surface roughness on regularity of nano-pores was observed using scanning electron microscope (SEM). The SEM images were analyzed to investigate the morphology and the degree of self ordering of pores of the samples by using a new designed analytical method aiming MATLAB and fast Fourier transform (FFT) technique. It was concluded that the electropolishing voltage and the resulted surface roughness and also formed defects can competitively affect the arrangement of membrane's nano-pores. A desired smoothness obtained from electropolishing voltage of 30V. Also 40V provided the best order with respect to the other voltages.

The effects of vacancies on the strength and elastic constants of silicon, such as Young's modulus and Poisson's ratio are investigated using the molecular dynamics simulations with the Stillinger–Weber potential. The defected crystalline cells contain randomly generated defect distributions in the simulation models. The ideal strength is found to be 33.6 GPa at the strain 0.26. The Young's modulus and Poisson's ratio is 148 GPa and 0.252, respectively. It is found that the strength decreases as the point defect fraction increases, and the variation of the strength versus the point defect fraction coincides with a decaying exponential function. In addition, vacancies are shown to reduce the elastic constants. In general, the elastic constants of silicon vary linearly versus the defect fraction.

We have carried out thermally stimulated current (TSC) measurements on as-grown Tl₂Ga₂S₃Se layered single crystals in the temperature range 10–60 K with different heating rates of 0.6–1.5 K s¹. The data were analyzed by curve fitting, initial rise, and peak shape methods. The results were in good agreement with each other. Experimental evidence was obtained for trapping center in Tl₂Ga₂S₃Se crystal with activation energy of 11 meV. The capture cross section and concentration of the traps were found to be 1.5 × 10^-23cm² and 1.44 × 10¹⁰cm^-3, respectively. Analysis of the TSC data at different light excitation temperatures leads to a value of 18meV/decade for the traps distribution.

Structural relaxation by isothermal annealing below the glass transition temperature is conducted on a Zr_64.13Cu_15.75Ni_10.12Al₁₀ bulk metallic glass. The effect of structural relaxation on thermal and mechanical properties was investigated by differential scanning calorimetry and instrumented nanoindentation. The recovery of the enthalpy in the DSC curves indicates that thermally unstable defects were annihilated through structural relaxation. During nanoindentation, the structural relaxation did not have a significant influence on the serrated plastic flow behavior. However, Structural relaxation shows an obvious effect in increasing both the hardness and elastic modulus, which is attributed to the annihilation of thermally unstable defects that resulted from the relaxation.

Positron annihilation lifetime spectroscopy and Doppler broadening annihilation line-shape measurements were carried out in 40 MeV alpha-irradiated undoped InSb. After irradiation the sample was subjected to an isochronal annealing over temperature region of 25°C–400°C with annealing time of 30 min at each set temperature. After each annealing the positron measurements were carried out at room temperature. Formation of radiation induced defects and their recovery with annealing temperature were investigated. A three component positron lifetime analysis was undertaken to observe the trapping of positrons in the sample after irradiation and during annealing. The average positron lifetime value τ_avg = 313 ps at room temperature after irradiation indicated the presence of defects and the high value of τ₂ at room temperature suggested that the probable defects were divacancies. A two stage recovery of defects was observed during post irradiation isochronal annealing over the temperature region 25°C–400°C. The variations in line-shape parameter (S) and defect specific parameter (R) during annealing in the temperature region 25°C–400°C resembled the behavior of τ_avg indicating the migration of vacancies, formation of vacancy clusters and the disappearance of defects between 300°C to 400°C.

A molecular dynamics (MD) based computational intelligence (CI) approach is proposed to investigate the Young modulus of two graphene sheets: Armchair and Zigzag. In this approach, the effect of aspect ratio, the temperature, the number of atomic planes and the vacancy defects on the Young modulus of two graphene sheets are first analyzed using the MD simulation. The data obtained using the MD simulation is then fed into the paradigm of a CI cluster comprising of genetic programming, which was specifically designed to formulate the explicit relationship of Young modulus of two graphene structures. We find that the MD-based-CI model is able to model the Young modulus of two graphene structures very well, which compiles in good agreement with that of experimental results obtained from the literature. Additionally, we also conducted sensitivity and parametric analysis and found that the number of defects has the most dominating influence on the Young modulus of two graphene structures.

Target waves in excitable media such as neuronal network can regulate the spatial distribution and orderliness as a continuous pacemaker. Three different schemes are used to develop stable target wave in the network, and the potential mechanism for emergence of target waves in the excitable media is investigated. For example, a local pacing driven by external periodical forcing can generate stable target wave in the excitable media, furthermore, heterogeneity and local feedback under self-feedback coupling are also effective to generate continuous target wave as well. To discern the difference of these target waves, a statistical synchronization factor is defined by using mean field theory and artificial defects are introduced into the network to block the target wave, thus the robustness of these target waves could be detected. However, these target waves developed from the above mentioned schemes show different robustness to the blocking from artificial defects. A regular network of Hindmarsh–Rose neurons is designed in a two-dimensional square array, target waves are induced by using three different ways, and then some artificial defects, which are associated with anatomical defects, are set in the network to detect the effect of defects blocking on the travelling waves. It confirms that the robustness of target waves to defects blocking depends on the intrinsic properties (ways to generate target wave) of target waves.

The effect of H₂ forming gas annealing on the microwave properties of $\mathrm{\text{Ba}}({\mathrm{\text{Zn}}}_{1/3}{\mathrm{\text{Ta}}}_{2/3}){\mathrm{\text{O}}}_3$ (BZT) dielectric ceramics has been studied. The structural, microwave, DC electrical and optical properties were analyzed by experiment results. With elevated temperature annealing, the microwave loss of BZT was increased. This trend correlated with high DC conductivity of annealed samples, as well as dampened phonons found in Raman spectra. These evidences, together, prove that the enhancement of oxygen vacancy defects induced by oxygen deficient sintering environment is one of the main extrinsic root causes for the high microwave loss in practical ceramic materials.

Positron annihilation technique is applied to study the recovery of radiation-induced defects in 140 MeV oxygen (O${}^{6+}$) irradiated Fe-doped semi-insulating indium phosphide during annealing over a temperature region of 25${}^{\circ }$C–650${}^{\circ }$C. Lifetime spectra of the irradiated sample are fitted with three lifetime components. Trapping model analysis is used to characterize defect states corresponding to the de-convoluted lifetime values. After irradiation, the observed average lifetime of positron ${\tau }_{\mathrm{\text{avg}}}=263$ ps at room temperature is higher than the bulk lifetime by 21 ps which reveals the presence of radiation-induced defects in the material. A decrease in ${\tau }_{\mathrm{\text{avg}}}$ occurs during room temperature 25${}^{\circ }$C to 200${}^{\circ }$C indicating the dissociation of higher order defects, might be due to positron trapping in acceptor-type of defects ($V_{\mathrm{\text{In}}}$). A reverse annealing stage is found at temperature range of 250${}^{\circ }$C–425${}^{\circ }$C for $S$-parameter probably due to the migration of vacancies and the formation of vacancy clusters. Increase in $R$-parameter from 325${}^{\circ }$C to 425${}^{\circ }$C indicates the change in the nature of predominant positron trapping sites. Beyond 425${}^{\circ }$C, ${\tau }_{\mathrm{\text{avg}}}$, $S$-parameter and $R$-parameter starts decreasing and around 650${}^{\circ }$C, ${\tau }_{\mathrm{\text{avg}}}$ and $S$-parameter approached almost the bulk value showing the annealing out of radiation-induced defects.

We have investigated the grain boundary energy of ($11\bar{2}1$) twin boundaries, the formation energies of hydrogen (H) and helium (He) defects in tetrahedral (T) and octahedral (O) interstitial sites at the ($11\bar{2}1$) twin boundary in hcp scandium (Sc) by first-principles calculations based on density functional theory. It is found that the formation energies of the tetrahedral and octahedral interstices H, and tetrahedral interstice He increase significantly towards the ($11\bar{2}1$) twin boundary plane, while the formation energy of the octahedral interstice He atom near the ($11\bar{2}1$) twin boundary plane decreases. To analyze these results, we present the electronic densities of states (DOSs) of H, He and their nearest-neighbor Sc (NN-Sc) atoms in several tetrahedral and octahedral configurations. We have also calculated the formation energies of He-vacancy clusters (He${}_n$V) in the Sc grain boundary, which indicates the stabilities of He${}_n$V clusters depend on the variations of the relaxed vacancy volume near the ($11\bar{2}1$) twin boundary plane.

Vacancy plays a crucial role in mechanical properties of transition metal borides (TMBs). However, the influence of vacancy on hardness of TMBs is unknown. In this paper, the relationship between boron vacancy and mechanical properties of CrB₄ is investigated by first-principle calculations. Two different vacancies including boron monovacancy (MV) and boron bivacancy (BV) are considered. We find that CrB₄ with boron MV is more stable than that of boron BV. The removed atom weakens the deformation resistances, and reduces the elastic stiffness and hardness. The calculated shear modulus, Young’s modulus and theoretical hardness of boron MV are larger than that of boron BV. The reason is that the removed atom weakens the localized hybridization between B and B atoms, and damages the 3D-network B–B covalent bond. However, the bulk modulus of B${}_{\mathrm{\text{-BV4}}}$ is slightly larger than that of perfect CrB₄. This reason is attributed to the formation of triangular pyramid bonding in B${}_{\mathrm{\text{-BV4}}}$ vacancy.

The ferromagnetic (FM) ordering in rutile TiO₂ has been theoretically studied by substituting different $p$-block elements (B, Al, C, Si, N, P and As) doped at oxygen site (B${}_{\mathrm{\text{O}}}$, Al${}_{\mathrm{\text{O}}}$, C${}_{\mathrm{\text{O}}}$, Si${}_{\mathrm{\text{O}}}$, N${}_{\mathrm{\text{O}}}$, P${}_{\mathrm{\text{O}}}$ and As${}_{\mathrm{\text{O}}}$) as well as at titanium site. Ab initio calculations in the frame work of density functional theory indicates that the $p$-block elements (B, C, Al, Si, N, P and As) when substituting the oxygen site give significant amount of magnetic moment, but induce zero magnetic moment in case of substitution at Ti site. Spin–spin interaction for (Ti${}_{12}$O${}_{23}$X)₂ with X = B, Al, C, Si, N, P and As system has also been studied with two different doping distances. Among all the possibilities, carbon substitution at oxygen site (C${}_{\mathrm{\text{O}}}$) results the most stable FM ordering in rutile TiO₂.

The electrical and thermal properties of extruded samples of Bi${}_{85}$Sb${}_{15}$$〈\mathrm{\text{Te}}〉$ modified with ZrO₂ were investigated depending on the dose of gamma radiation in the temperature range $\sim 80÷300$ K and magnetic field strength (H) $\sim 74\times 10^4$ A/m. It was found that an increase in the mobility in the irradiated modified Bi${}_{85}$Sb${}_{15}$$〈\mathrm{\text{Te}}〉$ is associated with the radiation introduction of acceptor (negatively charged) centers, which at low doses are generated mainly in the regions of the efficiency of impurity scattering of charge carriers and partially neutralized centers and, accordingly, to a certain increase in the mobility. In the extruded modified samples of the Bi${}_{85}$Sb${}_{15}$ solid solution, irradiation with gamma quanta results not only in the generation of radiation defects (RDs) (centers), but also accompanied by their rearrangement. This causes a change in the spectrum of localized states and the electron scattering process, which leads to corresponding changes in the presented electrical and thermal parameters.

The appropriate procedure for analyzing experimental data of defects in metals is discussed. The following two diverse procedures have been proposed earlier: either in terms of single vacancy formation with temperature dependent enthalpy and entropy, or by assuming coexistence of vacancies and divacancies with temperature-independent parameters. Using aspects of thermodynamics of the defect formation processes in solids, we show that the former procedure leads to self-consistent parameters.

We investigate the effect of vacancy defect reconstruction on electron transport properties in a (4, 0) zigzag and (5, 5) armchair silicon-carbide nanotubes (SiCNTs) by applying self consistent non-equilibrium Green's function formalism in combination with the density-functional theory to a two probe molecular junction constructed from SiCNTs. The geometry optimization results show that single vacancies and di-vacancies in SiCNTs have different reconstructions. A single vacancy when optimized, reconstructs into a 5-1DB configuration in both zigzag and armchair SiCNTs, and a di-vacancy reconstructs into a 5-8-5 configuration in zigzag and into a 5-2DB configuration in armchair SiCNTs. Analysis of frontier molecular orbitals (FMO) and transmission spectrum show that the vacancy defect increases the band gap of (4, 0) metallic SiCNT and decreases the band gap of (5, 5) semiconducting SiCNT. Bias voltage dependent current characteristic show reduction in overall current in metallic SiCNT and an increase in overall current in semiconducting SiCNT.

The defect centers in TlGaSSe single crystals have been investigated by performing thermoluminescence (TL) measurements with various heating rates between 0.5 K/s and 1.0 K/s in the temperature range of 10–180 K. The TL spectra, with peak maximum temperatures at 39 K and 131 K, revealed the existences of two defect levels. Curve fitting, initial rise and peak shape methods were used to determine the activation energies of two defect centers. The experimental results also showed that the trapping process was dominated by second-order kinetics for the trap related with low temperature peak while the general order (mixed order) kinetics was dominant for the trap donated to high temperature peak. Furthermore, heating rate dependences and traps distributions were studied for two defect centers separately. Thermal quenching effect dominates the behavior of these defects as the heating rate is increased. Also, quasi-continuous distributions were established with the increase of the activation energies from 16 meV to 27 meV and from 97 meV to 146 meV for the traps associated with the peaks observed at low and high temperatures, respectively.

In this paper, α-Mn₂O₃ thin films were fabricated by plasma-assisted molecular beam epitaxy on SrTiO₃ and Nb:SrTiO₃, respectively. The grown samples showed room temperature ferromagnetism (RFM) properties. All the experimental results manifested that the RFM properties in undoped thin films were induced by oxygen vacancies formed during the growth process. Even more, the ferromagnetism of thin films grown on Nb:SrTiO₃ were enhanced, and these results confirmed the fact that oxygen vacancies induced ferromagnetism. That is to say, more oxygen vacancies result the more unpaired electrons induced prominent abnormal spin causing ferromagnetism.

Here, we investigate the following key prediction of a thermodynamical model that interrelates the defect parameters with the bulk elastic and expansivity data: for various defect processes in a given matrix material, a proportionality exists between defect entropies and enthalpies. The investigation is focused on BaF₂ for which ab initio calculations within density functional theory and the generalized-gradient approximation have been recently made as far as the formation and migration of intrinsic defects are concerned, as well as for the elastic constants. Four defect processes have been studied in BaF₂: anion Frenkel formation, fluorine vacancy migration, fluorine interstitial motion and electrical relaxation associated with a single tetravalent uranium. For these processes, the entropies and enthalpies vary by almost two orders of magnitude and reveal a proportionality between them. We find that this proportionality is solely governed by the bulk elasticity and expansivity data, which conforms to the aforementioned thermodynamical model.