Influence Of Crystal Defects Research Articles

Effect of annealing temperature on the photocatalytic activity of nanocrystalline tungsten oxide

Simple acid precipitation method was adopted for the synthesis of nanocrystalline tungsten oxide with sodium tungstate dihydrate as precursor. To study the effect of temperature on the crystal structure and optical properties of tungsten oxide, as-prepared sample is annealed at 300,500 and 700 0C each for 2 hrs. Characterization techniques such as XRD, FTIR, UV–visible and PL are done to analyse the impact of annealing temperatures on the properties of annealed samples. The photocatalytic activity of the annealed samples (W300, W500 and W700) is evaluated by the degradation of MB dye under UV light irradiation. Low temperature annealed samples showed mixed phase of orthorhombic and monoclinic structure while phase pure monoclinic structure was obtained on increasing the annealing temperature to 700 0C. The high crystallinity of the samples is evident from the XRD pattern and the average crystallite size of the samples are obtained in the range 9–25 nm. The influence of crystal defects on the microstrain was carried out by using Williamson Hall analysis and it is found that microstrain decreased with increase of annealing temperature, indicating a lowering of defects arising due to cation or anion vacancies. The optical band gap energies of the samples are measured from diffused reflectance spectrum of the samples using Kubelka-Munk relation and its value was obtained high for the sample W500. Further, PL emission intensity for the excitation wavelength of 250 nm was also high for the sample W500. From the photocatalytic studies, it is evident that the degradation efficiency is high for the sample W500, suggestive of its use as an eminent tool for the treatment of water.

Cold Rolling Deformation Characteristic of a Biomedical Beta Type Ti-25Nb-3Zr-2Sn-3Mo Alloy Plate and Its Influence on α Precipitated Phases and Room Temperature Mechanical Properties During Aging Treatment.

The microstructure characteristics and texture evolution of a biomedical metastable beta Ti–25Nb–3Zr–2Sn–3Mo (TLM; wt%) titanium alloy plate cold rolled at various reductions were studied in this article. <110> texture was easily formed in the TLM alloy plates, and a large number of dislocation tangles were generated in the β matrix in the process of cold rolling deformation. The dislocation lines, dislocation cells, subgrain boundaries, and other crystal defects introduced during cold rolling had a great impact on the morphological characteristics and volume fraction of precipitated phases during aging. These typical crystal defects could be considered as the major triggers of the formation of second phases, and they could also shorten the time of β→α phase transformation. α precipitated phases, with a size range of 150–500 nm, were formed within the β matrix in the cold deformed 34% in conjunction with the aging specimen, resulting in the relatively high tensile strength of 931 MPa and the acceptable elongation of 6.9%. When the TLM alloy plate was cold rolled at a reduction of 60% in conjunction with aging, the maximum value of ultimate strength (1,005 MPa) was achieved, but the elongation value was relatively low owing to the formation of α precipitated phases with a large size around the subgrain boundaries. In this paper, the influence of crystal defects and subgrain boundaries on the morphology characteristics and volume fraction of α precipitated phases and mechanical properties will be discussed in detail.

Effect of High Temperature Annealing on Crystal Structure and Electrical Properties of Multicrystalline Silicon by the Metallurgical Method

The defects of dislocations and grain boundaries (GBs) could significantly reduce the electrical properties of multicrystalline silicon (mc-Si) solar cells. In this manuscript, the influence of crystal defects on the electrical properties of mc-Si was investigated, aims to carry out the research about the influence of crystal defects on the electrical properties of mc-Si. Purified metallurgical grade N type mc-Si ingot was obtained with the method of vacuum directional solidification technique by employing industrial production ingot furnace. The variations in grain boundaries and dislocations were characterized by electron backscatter diffraction (EBSD), and their effects on the electrical properties of mc-Si before and after annealing at different heat insulating time were investigated. Experimental results showed that high temperature annealing process could significantly improve the electrical properties of mc-Si, the minority carrier lifetime was improved from 0.599 μs to 0.770 μs after 10 h insulation, with a significant increase of 28.61%. Experimental results indicated that high temperature annealing process could significantly eliminate the crystal defects of multicrystalline silicon. The crystal defects was eliminated more obviously with prolonged heat preservation time. When treated by high temperature annealing, the proportion of small angle grain boundaries in the same region can be increased by as high as 13.45%. Moreover, the whole grain sizes of partly region of silicon wafer could be increased by 20000μm2, increased by 15.89% compared with the original grain sizes. Based on the experiment phenomena, we attributed the improvement in the electrical properties of mc-Si to the elimination of dislocations, increased grain sizes and migration of GBs.

Lock-in thermography with depth resolution on silicon solar cells

Lock-in thermography (LIT) is the standard method for imaging and evaluating leakage currents in solar cells. For usually applied lock-in frequencies in the order of 10 Hz, silicon solar cells are considered to be thermally thin. Hence, depth-dependent investigations, as they are performed in non-destructive testing and failure analysis of ICs, were not performed until now by LIT. In this contribution two special LIT investigation and evaluation methods are introduced, which have the potential to judge whether some recombination occurs at the top, in the middle, or at the bottom of a Si solar cell. Such investigations can be useful to evaluate e.g. metal-induced recombination or the influence of crystal defects in multicrystalline solar material on the emitter or backside recombination. The methods are tested at a cell containing a diamond scratch in the emitter and backside recombination at the Ag back contact.

Growth, structural, spectral and high-power continuous-wave laser operation of Yb0.11Gd0.89COB crystal

A Yb0.11Gd0.89Ca4O(BO3)3 crystal with new composition was grown by the Czochralski method. The crystal structure was measured and analyzed. The unit-cell parameters of the Yb0.11Gd0.89COB were calculated to be a=0.8089(7) nm, b=1.5987(6) nm, c=0.3545(8) nm, β=101.22°. The absorption and fluorescence spectra were measured. The maximum absorption cross-section of Yb0.11Gd0.89COB crystal was 0.79×10−20 cm2, which occurred at 976 nm with Y polarization. The emission cross-section at 1027 nm was calculated to be 0.33×10−20 cm2. The radiative lifetime τrad was calculated to be 2.74 ms. The Stark energy-level diagram of Yb3+ in the Yb0.11Gd0.89COB crystal field at room temperature was determined. The ground-state energy level 2F7/2 splitting was calculated to be as large as 1004 cm−1 and the zero-line energy was 10246 cm−1. A maximum output power of 9.35 W was achieved in continuous-wave (CW) mode, with the slope efficiency being 42.1%. Chemical etching experiment revealed that the dominating imperfections in the studied Yb0.11Gd0.89COB crystal were dislocations and sub-grain boundaries. The existence of crystal defects could cause light scattering, and degrade laser output efficiency. The influence of crystal defects on laser properties was discussed.

The influence of crystal defects on the elastic properties of tungsten metals

The four stretching process stages of the elastic, plastic, stalemate, and fracture were represented for the metal tungsten by using molecular dynamics method. The young's modulus, yield strain and yield stress were calculated. The microscopic mechanics of the stretching process is analyzed. The energy of FCC and HCP generated was higher than that of BCC, so that the energy of the system increased, and the stress level was lower in the plastic deformation stage. In the late stage of plastic deformation, the growth of the twin belt was of long range ordered periodic structure, which made the system stress change periodically. In the Stalemate Stage of deformation, the other disordered structure, setting in the HCP structure of the twin belt, growed to absorb energy and generate cavity under stress and makes the lattice fracture. The yield stress of metal tungsten decreases monotonically with temperature and vacancy. The effects of temperature and vacancy on the lattice fracture were discussed.

Martensitic transformation in shape memory crystal with defects: Monte Carlo simulations and Landau theory

The efficiency of complex (microscopic and macroscopic) theoretical studies of the influence of crystal defects on the transformational properties of shape memory alloys was demonstrated. To this end, the numerical Monte Carlo simulations were carried out for the microscopic modeling of the influence of vacancies on the martensitic transformation (MT) of a Ni–Mn–Ga shape memory alloy and the dependence of MT temperature on concentration of the vacancies was determined. Applicability of the Monte Carlo method to the description of martensite stabilization was demonstrated. Landau theory of MT in the real (with defects) alloy specimen was then used for the evaluation of mechanical stress and internal pressure, which the vacancies create in the alloy. Time dependencies of the mechanical stress and the internal pressure were found from experimental data on the martensite aging. Concentration of the vacancies that provides the experimentally observed time dependence of MT temperature was estimated.

Influence of Crystal Defects on the Reflectivity of the Aluminum

The effect of ion-assisted deposition of the Al films on their UV reflectance is investigated in this paper. The films' reflectance is measured by a spectrophotometer. The obtained films are examined by using transmission electron microscopy (TEM), X-ray diffraction analysis (XRD), and atomic force microscopy (AFM). The TEM and AFM measurements allow the determination of the size of crystallites in a film and its microstructure. The XRD analysis reveals that the films deposited with argon ion-beam assist are characterized by much higher microstress levels compared to the films deposited without ion assist. The comparison of the Al films’ reflectance measurements indicate that the films with a higher microstress level (hence, higher defect concentration) are characterized by the enhanced reflectance in the UV region. The conducted investigation shows that the defects of the Al films’ crystalline structure affect its optical properties.

Thickness dependence of the piezoresistive effect in p-type single crystalline 3C-SiC nanothin films

The influence of crystal defects on the gauge factor of p-type single crystalline 3C-SiC thin films is reported.

Computational Evaluation of Electrical Conductivity on SiC and the Influence of Crystal Defects

The main electronic characteristics of silicon carbide (SiC) are its wide energy gap, high thermal conductivity, and high break down electric field which make of it of one of the most appropriate materials for power electronic devices. Previously we reported on a new electrical conductivity evaluation method for nano-scale complex systems based on our original tight-binding quantum chemical molecular dynamics method. In this work, we report on the application of our methodology to various SiC polytypes. The electrical conductivity obtained for perfect crystal models of 3C-, 6H- and 4H-SiC, were equal to 10-20-10-25 S/cm. For the defect including model an extremely large electrical conductivity (of the order of 102 S/cm) was obtained. Consequently these results lead to the conclusion that the 3C-, 6H-, and 4H-SiC polytypes with perfect crystals have insulator properties while the electrical conductivity of the crystal with defect, increases significantly. This result infers that crystals containing defects easily undergo electric breakdown.

Defect analysis of hydrogenated nanocrystalline Si thin films

Hydrogenated nanocrystalline silicon (nc-Si:H) is a very promising material for photovoltaic applications. Notwithstanding its wide application as instrinsic layer in solar cells, many issues regarding its electronic and optical properties are not completely understood. The influence of crystal defects and impurities on the physical properties of nc-Si:H is not completely explored yet. The present contribution aims to report on defective states associated with crystal defects and impurities in nc-Si:H films grown by low-energy plasma enhanced chemical vapor deposition (LEPECVD) technique. Electronic levels associated to defect states were investigated by surface photovoltage spectroscopy (SPS) and the role of defects and impurities on the conduction mechanisms at a microscopic level were investigated by conductive atomic force microscopy (C-AFM). The results will be compared with structural analyses

Water vapor nucleation on an infinite substrate with complementary structure: 2. The influence of crystal defects

The simulation of the adsorption of water vapor on the infinite surface of a silver iodide crystal with regularly arranged crystal defects is performed by the Monte Carlo method. Long-range electrostatic and polarization interactions are taken into account by the Ewald method. The work of nucleus formation in the field of crystal defects is calculated by the method of bicanonical statistical ensemble. The interaction with a defect decreases the internal energy, the Gibbs energy, and the entropy of the nucleus. The rough surface of the silver iodide crystal exerts a larger stimulating effect on vapor nucleation than an ideally smooth surface. Saturation vapor pressure above the condensate layer formed on the rough surface can be several times lower than the pressure above the smooth surface. This effect is caused by the cooperative action of surface crystal defects. It may be expected that the surface with a characteristic size of rough elements of 15–20 A is the most efficient for the formation of condensed phase nuclei. Single point crystal defects with extremely small sizes on the substrate surface do not exert a stimulating effect on the formation of the macroscopic condensed phase. Single defects with moderate sizes can lower the barrier of monomolecular film formation; however, such a route of the stimulation of ice-forming activity of the surface is less efficient than the cooperative action of the defects of rough surface.

Theoretical Strength of Solids

The significance of theoretical strength (TS) investigations is explained and a brief overview of the current state of TS calculations is presented on the background of a historical evolution. Influence of crystal defects on TS is outlined by taking into account published theoretical models. The study is completed by a discussion concerning differences between theoretical and experimental results.

Effect of crystallinity on the bulk laser damage and UV absorption of CLBO crystals

The influence of crystal defects on bulk laser damage and UV absorption in CsLiB 6O 10 (CLBO) crystals was investigated. A CLBO crystal grown by the solution stirring method had a 2.5-fold higher laser-induced damage threshold (maximum) than conventional CLBO crystals. This highly damage-resistant CLBO crystal had a lower dislocation density (best value, 6.6×10 3/cm 2) than conventional CLBO crystals (best value, 15.0×10 3/cm 2). The temperature increase during UV generation on the output surface of highly damage-resistant CLBzO crystal was lower than that detected in conventional CLBO crystal.

Influence of crystal defects on the electromigration of the hydrogen in palladium

Electromigration and diffusion of hydrogen in palladium defected through quenching to liquid nitrogen temperature were investigated. It was found that the obtained experimental results cannot be described by means of a single flux of hydrogen which diffuses in a gradient of electric field. In order to effectively explain the obtained results, a theoretical model was elaborated. It assumed the existence of the following two fluxes of hydrogen in the quenched metal: • J i—of migration, according to the interstitial mechanism, and • J i–d—of migration, according to the interstitial-defective mechanism. Dependence of effective valences and diffusion coefficients for both mechanisms of transportation was determined in relation to the initial temperature of the quenching.

Relaxation of band bending on GaAs(001) surface by controlling the crystal defects near the surface

The influence of crystal defects near the surface on horizontal Bridgman-grown GaAs(001) surface is investigated by photoluminescence (PL) and synchrotron radiation photoelectron spectroscopy (SRPES). PL measurements reveal that after heating to 500°C a layer with lower defect concentration exists just under the thermal degraded one. SRPES shows that the surface Fermi level ( E FS) moves to the conduction band minimum when this thermally degraded layer is removed by chemically etching and the etched surface is heated in ultra-high vacuum after rinsing with the deoxygenated and deionized water. These results suggest that the position of E FS for GaAs(001) surface is strongly affected by crystal defects near the surface.

X-ray acoustic resonance in real crystal

The influence of crystal defects and ultrasound absorption on X-ray diffraction under the condition of X-ray acoustic resonance are theoretically investigated. It is shown that the rocking curve and integral intensity of diffraction are very sensitive to defect's type and concentration

Influence of crystal defects on the irreversibility line of YBa2Cu3O7-x single crystals

High quality flux grown and defect enriched peritectically grown YBa2Cu3O7−x single crystals have been investigated by an ac-susceptibility technique. This method allows to determine an irreversibility line from the temperature and field dependence of the peaked imaginary part of the susceptibility, which is due to magnetic losses. For magnetic fieldsHac perpendicular to thec-axis of the crystal, the irreversibility line of the defect enriched crystal shows a shift to higher field values as compared to the perfect crystal, a sign that crystal defects like Y2BaCuO5(211)-precipitates and microcracks act as strong pinning centers. ForHac parallel to thec-axis no clear evidence for a stronger pinning is found. From these results we conclude that different pinning mechanisms are dominating at different field orientations.

Interaction of the dopants Mg and Si in AlxGa1−x As/GaAs heterolayers (MOVPE): application to DQW laser structures

Abstract The interaction of Mg and Si has been studied in GaAs/Al x Ga 1− x As DQW laser structures with a 50 nm Si diffusion barrier. The samples have been exposed to capless heat treatments at 860°C and under Si/SiO 2 and Si 3 N 4 cap layers, and were analysed by SIMS. The Mg diffusion is highly dependent on the surface conditions during heating. A Si barrier is effective for temperature treatments under H 2 /AsH 3 and Si/SiO 2 . It is not effective under a Si 3 N 4 cap where we detected very fast Mg diffusion. The Mg diffusion behaviour is discussed in terms of Si-Mg interaction and the influence of crystal defects.

Techniques of convergent beam electron diffraction.

The techniques required to record standard convergent beam electron diffraction patterns in an analytical electron microscope are discussed in detail, with emphasis on the design of electron optics in commercial instruments. Practical comments are included on specimen preparation, the influence of crystal defects, tilting to major zone axes, and alignment of the instrument. The influence of parameters under experimental control such as probe size, accelerating voltage, temperature, specimen thickness, and convergence angle is discussed in detail. Some comments are included on the alignment and limitations of large angle patterns formed by a defocused probe.