Narrow Results

The recovery behavior of 20% plastically deformed AlSi_11.35Mg_0.23 in various stages of isochronal annealing is investigated by positron lifetime (LT). Experimental results show that the positron mean lifetime is a function of annealing temperature. The lifetime of the positron annihilating in a perfect lattice is 187.3 ps. It is 229.8 ps in a 20% deformed one. There are two regions in the isochronal annealing, one of them is related to the point defect and the other to the dislocation. The activation enthalpy for the dislocation is calculated from the isothermal study in the dislocation region from 575–675 K by slow and fast cooling and it is 0.16 ± 0.02 and 0.53 ± 0.06 eV, respectively.

The Fe–C–H interaction near defects in iron structures was studied using qualitative structure calculations in the framework of the atom superposition and electron delocalization molecular orbital. Calculations were performed using three Fe clusters to simulate an edge dislocation, a divacancy; both in bcc iron and a stacking fault in an fcc iron structure. In all cases, the most stable location for C atom inside the clusters was determined. Therefore, H atom was approximated to a minimum energy region where the C atom resides. The total energy of the cluster decreases when the C atom is located near the defects zone. In addition, the presence of C in the defects zone makes no favorable H accumulation. The C acts as an expeller of H in a way that reduces the hydrogen Fe–Fe bonds weakening.

The defects induced by a spike rapid thermal annealing (RTA) process in crystalline silicon (c-Si) solar cells were investigated by the photoluminescence (PL) technique and the transmission electron microscopy (TEM), respectively. Dislocation defects were found to form in the near-surface junction region of the monocrystalline Si solar cell after a spike RTA process was performed at 1100${}^{\circ }$C. Photo J–V characteristics were measured on the Si solar cell before and after the spike RTA treatments to reveal the effects of defects on the Si cell performances. In addition, the Silvaco device simulation program was used to study the effects of defects density on the cell performances by fitting the experimental data of RTA-treated cells. The results demonstrate that there was an obvious degradation in the Si solar cell performances when the defect density after the spike RTA treatment was above $1\times 10^{13}$cm${}^{-3}$.

Owing to the advancement in the field of materials, different range of grades have been developed. The machinability examination of these newer grades must be carried out for future applications. One such newer grade of magnesium with AZ31 is deemed for study during the drilling process. The independent parameters considered are spindle speed (SS), feed rate (FR) and drill bit diameter (DBD). The dependent parameters considered are burr height (BH), burr thickness (BT), drilling time (DT) and surface roughness (SR). Improving hole accuracy is essential for manufacturing superior products, which is discussed in this work. At the same time, the machining time has also to be minimized to increase the production rate. With these objectives, the experimental investigation is made. Further, an analytical model for predicting the responses is developed; later, optimization is carried out to obtain the desired responses through the desirability function approach. The multi-objective optimization suggests the SS of 1100rpm, the FR of 0.198mm/rev., and the DBD of 6mm for reducing the entire dependent is reckoned.

To improve the corrosion resistance of X70 pipeline steel in seawater, nickel coating was prepared on the surface of X70 steel by electroplating method. The corrosion properties of the samples in simulated seawater were studied by macroscopic electrochemical experiment and micro-scanning electrochemical experiment. The systematic characterization of the samples was conducted using a scanning electron microscope (SEM), energy disperse spectroscopy (EDS), and X-ray diffractometer (XRD) techniques. The time-dependent model was established to simulate the localized corrosion by COMSOL Multiphysics. The characterization results show that the nickel coating prepared at 0.045 A is compact and thickest. In the macroscopic electrochemical results, the impedance value of the nickel layer prepared at 0.045 A is 34% and 36% higher than that of the other two coatings, and the current density is 25% and 66% lower than that of the other two coatings. In the micro-scanning electrochemical results, the impedance value of the nickel layer prepared at 0.045 A is 5% and 40% higher than that of the other two coatings, and the current density is 14% and 26% lower than that of the other two coatings. Therefore, 0.045 A is the best electroplating current for preparing nickel coating. The simulation results show that the micropores on the surface of nickel-plated X70 are easy to induce localized corrosion, and the degree of localized corrosion decreases with the increase of micropore diameter.

We summarize recent results on the impact of defects on the electronic properties of single-wall carbon nanotubes. We probe the influence of defects on electron transport in CNFETs by combined scanning gate microscopy (SGM) and scanning impedance microscopy (SIM). Depletion surface potential of individual defects is quantified from the SGM-imaged defect radius as a function of tip bias voltage. This provides a measure of the Fermi level at the defect with zero tip voltage. In the "off" state, transport is first dominated by barriers at depleted defects. It becomes diffusive as the CNFET is turned on, and finally is quasi-ballistic in the regime of "degenerate electrostatic doping". Metallic nanotubes with good contacts show a metal-to-insulator crossover as the gate voltage is varied. In the metallic state we see quantitative agreement with the "twiston" scattering picture.

The capability of graphitic networks to reorganize their structures under irradiation of energetic particles provides the tool for nano-engineering of carbon nanotubes (CNTs). We have studied the effect of 30 keV N⁺ ion irradiation with three different fluences 10¹², 10¹³, and 10¹⁴ ions/cm² on the structural and spectroscopic properties of single-walled carbon nanotubes (SWNTs) and multiwalled carbon nanotubes (MWNTs). Irradiation-induced structural defects and coalescence of the nanotubes are studied using high-resolution transmission electron microscopy (HRTEM). Upon irradiation, some of the radial breathing modes in Raman spectra disappear due to conversion from single-walled to multiwalled structure. We observed a systematic change in intensity of the intermediate frequency mode (IFM) with increasing dose of ion-irradiation and these IFM modes are attributed to structural defects of SWNTs. Dramatic improvement in the intensity ratio G-band to D-band (at ~1335 cm^-1) for ion fluence of 10¹³ ions/cm² indicates improved graphitic structure as a result of reconstruction. Similarly, X-ray Photoelectron spectroscopy studies show improvement in the amount of sp² carbon upon 10¹³ ions/cm²N⁺ ion irradiation dose. At a higher dose (10¹⁴N⁺ions/cm²), vacancy and bent structures having Stone–Wales defects were observed in HRTEM, whereas MWNTs show formation of surface hillock like protrusions leading to formation of fullerene-like structures.

The broad visible photoluminescence (PL) observed in ZnO nanocrystals (NCs) is widely attributed to multiple low lying surface-defects. We have performed steady state and time-resolved PL measurements on size-selected ZnO NCs in the strong confinement regimes. Our results show that radiative relaxation rates and coupling between excitons and surface defect states vary dramatically for sizes between 2 nm and 3 nm. Energy dependent PL lifetimes reveal that relaxation dynamics of these defect states in the blue- and red-edge of the emission are very different from each other.

In this paper we report on the preparation of nanobrushes of ZnO on quartz substrate by a direct atmosphere evaporation method using Zn metal flakes. Activated charcoal was used as a catalyst that facilitated the formation of nanobrushes in which the brush stem was about 15–20 μm in length and the bristles (100–200 nm thick) were made up of nanofibrous ZnO whose tips were 10–15 nm in width and were angled in some cases. These aligned nanobrushes can find potential applications as nanopower generators and high aspect ratio AFM probes by virtue of the piezoelectric property of ZnO. This technique is simple for realizing aligned ZnO nanobrushes with metallic Zn as the source material.

This research demonstrates the capability of controlled, focused ion beam (FIB)–assisted tailoring of morphologies in both multiwall carbon nanotubes (CNTs) and Y junction nonlinear CNT systems through defect engineering. We have shown that a 30 keV FIB Ga⁺ ion beam at low ion milling currents of 1 pA can be used to partially reduce the CNT diameter, to provide electrical conduction bottleneck morphologies for linear CNTs, and to introduce both additive and subractive defects at Y junction locations of Y-CNT samples. Our aim is for this work to provide motivation for additional research to determine the effects of ion-beam-induced changes in modulating the physical and chemical properties of nanotubes.

Nanocrystalline ZnO particles substituted with different concentrations (0–30%) of Mn were synthesized by using a modified ceramic route and characterized by X-ray diffraction, transmission electron microscopy, selected area electron diffraction and energy dispersive X-ray analysis methods. Positron lifetime and coincidence Doppler broadening measurements were used as probes to identify the vacancy-type defects present in them and monitor the changes while doping. The predominant positron trapping center in the undoped ZnO is identified as the trivacancy-type cluster V_Zn+O+Zn, which is negatively charged, and it transformed to the neutral divacancy V_Zn+O on doping with Mn²⁺ ions. The intensity of the defect-specific positron lifetime component got reduced initially indicating partial occupancy of the vacancies by the doped cations but then recovered on further doping due to the additional Zn vacancies created as a result of the increasing strain introduced by the Mn ions of larger radius. The creation of a new phase ZnMn₂O₄ thereafter changed the course of variation of the annihilation parameters, as the positrons got increasingly trapped in the vacancies at the tetrahedral and octahedral sites of the spinel nanomanganite.

We report room temperature ferromagnetism (RTFM) in nanocrystalline Zn_1-xCu_xO(0.03 ≤ x ≤ 0.07) materials synthesized by autocombustion technique. The average particle sizes are in the range of 60 nm. The saturation magnetization and coercivity of 7% Cu-doped ZnO is enhanced significantly in comparison to 3% and 5% Cu-doped ZnO. There is not much variation in the optical band gap due to Cu doping, thus suggesting the uniform distribution of Cu in the ZnO matrix. Micro-Raman and photoluminescence analysis predict the presence of clusters of oxygen vacancies in Cu-doped system which improves with the increase in Cu concentration. This study provides further evidence that oxygen vacancies play an important role in the enhancement of room temperature ferromagnetic property in Cu-doped ZnO.

Spherical Bi₂S₃ nanoparticles (NPs) were prepared by a facile in situ thermal sulfuration method. Different Bi₂S₃ samples were obtained by controlling the sulfuration time. The products were characterized by X-ray diffractometer (XRD), scanning electron microscopy (SEM), Raman and Fourier-transform infrared (FT-IR) methods. The optical properties were examined by UV-visible-near-infrared (UV-Vis–NIR) and photoluminescence (PL) techniques. The results show that the phase of the products after sulfuration is pure and the spherical shape of Bi NPs has been successfully transmitted to Bi₂S₃ samples. The light absorption edges exhibit red shift to 1060 nm while the light emission displays blue shift to 868 nm, compared with the energy bandgap of bulk Bi₂S₃. The reason for the special optical properties of Bi₂S₃ NPs by this in situ sulfuration route is considered to associate with the defects and quantum size effect of NPs.

A molecular structural mechanics method has been implemented to investigate the vibrational characteristics of single-layer graphene (SLG) with defects. By adopting the lumped mass unit to replace carbon atoms, and the beam element with circular cross-section to mimic C–C covalent bonds, SLG is modeled as a space framework. The simulation results show that the chirality almost has no effect on the natural frequency and the vibration mode of SLG, while boundary conditions have great influences. The influences of defects with different number and location on the natural frequencies are also studied. It is concluded that vibration mode is insensitive to the vacancy defect, small hole and short flaw, but large holes and long flaws can affect the vibration characteristics. So the graphene sheet even with small defect effects might be selected as the nanosensor material as well as pristine graphene. The conclusions in this paper may provide some references for the design of nanosensor.