Values Of Dislocation Density Research Articles

Carrier concentration effect and other structure-related parameters on lattice thermal conductivity of Si nanowires

Lattice thermal conductivity (LTC) of Si bulk and nanowires (NWs) with diameter 22, 37, 50, 56, 98 and 115 nm was investigated in the temperature range 3–300 K using a modified Callaway model that contains both longitudinal and transverse modes. Using proper equations, mean bond length, lattice parameter, unit cell volume, mass density, melting temperature, longitudinal and transverse Debye temperature and group velocity for all transverse and longitudinal modes were calculated for each NW diameter mentioned. Surface roughness, Gruneisen parameter and impurity were used as adjustable parameters to fit theoretical results with experimental curves. In addition, values of electron concentration and dislocation density were determined. There are some phonon scattering mechanisms assumed, which are Umklapp and normal processes, imperfections, phonon confinement, NW boundaries, electrons scattering and dislocation. Dislocation density less than 10 $$^{{14}}$$ m $$^{{-2}}$$ for NWs and 10 $$^{{12}}$$ m $$^{{-2}}$$ for bulk has no effect on LTC. Also, electron concentration less than 10 $$^{{22}}$$ m $$^{{-3}}$$ for NWs and 10 $$^{{16}}$$ m $$^{{-3}}$$ for the bulk has no effect. On increasing dislocation density and electron concentration, LTC comparably decreases.

An investigation of effect of annealing at different temperatures on microstructures and bulk textures development in deformed Zircaloy-4

In the present work, the effect of various annealing temperatures on the microstructural and textural evolution in deformed Zircaloy-4 has been investigated. Zircaloy-4 was subjected to a rolling reduction of 90% corresponding to a true strain of 2.3 along the ND - ED and ND - TD planes of the as-received Zircaloy-4bar at room (298K) and cryogenic (77K) temperatures. Following this, the various deformed conditions of the samples were subjected to annealing treatment at various temperatures such as 400°C, 450°C and 500°C. The deformed samples were characterized by a high dislocation density and nano scale subgrains, which after annealing at various temperatures were marked with annealing twins of nano - size along with some other dislocation configuration. A higher dislocation density and hardness (258.54±2.91HV) values were observed after annealing the cryorolled alloy along the ND - TD plane at 450°C. This high cold work energy provided by cryorolling along the ND - TD plane, after annealing at 450°C produced an optimum twin growth, which impeded the dislocation motion and slightly increased the hardness along this plane. Finally, the textures evolved after annealing indicated the role of annealing twins combined with <a> prismatic, <c+a> pyramidal & basal glide activity.

Determination of Dislocation Density in Lath Martensite by Means of Electron Backscatter Diffraction

As a novel procedure for determining dislocation density, a software was improved with which data obtained by Scanning Electron Microscope (SEM) measurements can be collected and the value of superficial dislocation density can be calculated. Applying this method we investigated cold rolled lath martensitic steel samples. Besides dislocation density values, microstructure mapped by Electron Backscatter Diffraction (EBSD) will be discussed.

Analyses of determination conditions of n-GaN dislocation density by triple-axis X-ray diffraction

Dislocation densities of two hydride vapor phase epitaxy-grown hexagonal GaN samples, which are Si doped and unintentionally doped respectively, are determined by triple-axis X-ray diffractometry and van der Pauw variable temperature Hall-effect measurement. The dislocation densities of these two samples should be at the same level from the X-ray testing, the -FWHM (full width at half maximum) values of all corresponding reflections for these two samples are almost the same. But from the Hall-effect measurements, the dislocation density values should be different from each other remarkably, because the unintentionally doped sample belongs to Mott transition material, while the Si-doped one does not. This fact indicates that the X-ray testing is perhaps inaccurate under some conditions, although the triple-axis X-ray diffractometry is a highly suitable technique for discriminating different kinds of structural defects such as edge and screw dislocations that lead to characteristic broadening of symmetric and asymmetric Bragg reflection. The experimental result obtained so far (say, for hot-electron bolometer) shows that the dislocation density value from mobility fitting model is in good accordance with that from -FWHM fitting using Srikant method. The anomaly that the dislocation density from -FWHM fitting is much lower than that from mobility fitting for the same sample (sample 59#), indicates that dislocations located in grain boundary may not be tested by triple-axis X-ray diffractometry. According to mosaic model, the layer is assumed to consist of single crystallites, called mosaic blocks, which are assumed to be slightly misoriented with respect to each other. The out-of-plane rotation of the block perpendicular to the surface normal is of the mosaic tilt, and the in-plane rotation around the surface normal is of the mosaic twist. The average absolute values of tilt and twist angles are directly related to the FWHM values of the corresponding distributions of crystallographic orientations. So, the X-ray testing can determine the average orientation of the grains with the same interplanar distance, excluding the information about the grain boundary at which X-ray cannot interfere because of disdortion of lattice. The experimental results and calculation analyses indicate that the dislocation density value from Srikant model is accurate when the ratio of twist angle to tilt angle exceeds 2.0, or the magnitude of the lateral coherence length is larger than 1.5 m.

Determination of dislocation density of a class of n-GaN based on the variable temperature Hall-effect method

An analytical model for electron mobility in a class of wurtzite n-GaN, whose carrier concentration is over 1018 cm-3 (Mott's critical limit), is developed. With the dislocation density and two donor levels serving as the important parameters, the proposed model can accurately predict the electron mobility as a function of temperature. The edge and screw dislocation densities in two samples, which are respectively grown on sapphire (001) by metal organic chemical vapor deposition and hydride vapor phase epitaxy, are determined by using this model which is discussed in detail. It is shown that the data-fitting of H-T characteristic curve is a highly suitable technique for accurately determining the edge and screw dislocation densities in n-GaN films. Quantitative analyses of donor concentration and donor activation energy indicate that the impurity band occurs when the carrier concentration is under 1017 cm-3, much lower than the critical carrier concentration of Mott transition (1018 cm-3). Such a behavior can also be confirmed by the results from solving the Boltzmann transport equation by using the Rode iterative method. Another anomaly is that the dislocation density in Mott transition material perhaps is lower than that of material with carrier concentration under 1018 cm-3. This fact indicates that the cause of Mott transition should not be the shallow donor impurities around dislocation lines, but perhaps the deeper donor impurities or other defects. In the theoretical model calculation, two transition characteristics together with the donor distribution and its energy equilibrium are taken into account. Based both on the Mott transition and the H-like electron state model, the relaxation energies for the shallow-donor defects along the screw and edge dislocation lines are calculated by using an electrical ensemble average method. Besides, an assumption that should be made is that there are 6 shallow-donor defect lines around one dislocation line. The research results show that the Hall mobility should be taken as the live degree of the ionizing energy for the shallow-donor defects along the dislocation line. The experimental results indicate that our calculation function can be best fit by the experimental curve, with the values of dislocation density being between our model and others determined by X-ray diffraction or by chemical etching method, which are all in good agreement with each other. The method reported can be applied to the wurtzite n-GaN films grown by various preparation technologies under any condition, with the peak-mobility temperature about or over 300 K, whose Hall mobility near 0 K perhaps is over 10 cm2/(Vs) and even 100 cm2/(Vs).

STRUCTURE-PHASE STATES, MECHANICAL AND TRIBOLOGICAL PROPERTIES OF THERMOMECHANICALLY STRENGTHENED BEAM

Using the methods of modern materials science the researches and the comparative analysis are performed for the structural and phase states, dislocation substructure, mechanical and tribological properties of the surface of the thermo-mechanically strengthened and non-strengthened I-beam DP155 from low carbon steel used for mine monorails. It was found that accelerated cooling of the beam section in line 450 of the mill at “EVRAZ – Consolidated West-Siberian Metallurgical Plant” (rolling speed is 6 m/s, the water pressure on the sections of shelf cooling is 0.22 – 0.28 MPa, the temperature before the refrigerator is approximately 800 °С) forms a high defect surface layer structure characterized by higher (relatively to the unhardened state) values of hardness, wear resistance and scalar dislocation density. In the non-heat-resistant state, the microhardness of the samples is 2.70 ± 0.33 GPa, and Young’s modulus is 269.6 ± 27.1 GPa. Thermo-mechanical hardening of the material leads to a decrease in its microhardness up to 3.30 ± 0.29 GPa and to an increase in the Young’s modulus up to 228.2 ± 25.7 GPa, respectively. In addition, an increase in the range of microhardness values from 2.20 – 3.80 GPa to 2.64 – 4.60 GPa and a decrease in the Young’s modulus range from 208.0 to 403.0 GPa to 184.1 to 278.2 GPa is established during thermomechanical hardening of steel. It is shown that the thermomechanical strengthening of steel leads to an increase in the wear resistance of the surface layer in approximately 1.36 times (the wear rate varies from 5.3·10 –5 mm 3 /N·m to 2.9·10 –5 mm 3 /N·m) and an increase in the friction coefficient by 1.36 times (from 0.36 to 0.49). In the non-strengthened state the dislocation chaos structure is observed (the dislocation scalar density is (0.9 . 1.0)·10 10 cm –2 ). High-temperature rolling and subsequent accelerated cooling of the samples lead to the formation of a strip dislocation substructure in the grains of ferrite and a reticular dislocation substructure in martensite grains (the average scalar dislocation density in the surface layer is 4.5·10 10 cm –2 ). The possible reasons for the observed regularities are discussed.

Evolution of dislocation structure in neutron irradiated Zircaloy-2 studied by synchrotron x-ray diffraction peak profile analysis

Dislocation structures in neutron irradiated Zircaloy-2 fuel cladding and channel material have been characterized by means of high-resolution synchrotron x-ray diffraction combined with whole peak profile analysis and by transmission electron microscopy (TEM). The samples available for this characterization were taken from high burnup fuel assemblies and offer insight into the evolution of the dislocation structure after the formation of dislocation loops containing a c component. Absolute dislocation density values are about 4–15 times higher for the whole peak profile compared to TEM analysis. Most interestingly, the diffraction analysis suggests that the total dislocation density, as well as the a loop density, increases with fluence for the cladding material type. This trend is also inferred from a Williamson-Hall representation but contradicts the TEM observations. The c loop density evolution is more complicated and doesn't display any particular trend. In addition, the diffraction analysis highlights the presence of well-developed shoulders adjacent to the basal reflections and noticeable peak asymmetry particularly for the channel samples that experienced slightly lower operation temperatures than the clad. The findings are discussed in respect of the perceived irradiation induced growth mechanisms in Zr alloys.

Electrochemical sensing of glucose and photocatalytic performance of porous Co3O4 films by nebulizer spray technique

In this present work an attempt has been made to deposit the porous structured Co3O4 films using nebulizer spray pyrolysis technique. The structural, optical and vibrational properties of the as deposited films were characterized through various analytical tools like XRD, SEM, photoluminescence and Raman. The parameters such as crystallite size, stacking fault probability, dislocation density and lattice parameter values were deduced from XRD patterns. The film with better porosity observed for Co3O4 film was obtained at higher deposition temperature (Tsub = 673 K). Cyclic voltammograms of obtained FTO/porous Co3O4 films show high current response during the addition of glucose indicating more suitability for biosensor applications. The photocatalytic measurements of Co3O4 films over glass were evaluated for different organic dyes such as Rhodamine B, Methlyene blue and Methyl orange under visible light.

Fabrication and characterization of Sr-doped PbS thin films grown by CBD

In this work, undoped and Strontium-doped lead sulfide (PbS) thin films were deposited on glass substrates by chemical bath deposition (CBD) technique. The effect of Sr doping concentration on the structural, morphological and optical properties of the PbS thin films was studied. Changing in crystallite size, microstrain, dislocation density and band gap energy values of the PbS thin films as a function of Sr concentration were investigated by scanning electron microscopy (SEM), X-Ray diffraction (XRD) and UV–visible spectroscopy measurements. Energy dispersive X-Ray analysis (EDS) results indicated that nanostructured PbS films is doped with Sr without causing much lattice strain. According to XRD experiments crystallite size of the films is decreasing from 17.90 to 9.14nm with increasing Sr-doping. UV–visible analysis showed that the band gap value of PbS thin films increased from 1.73ev to 2.19eV by changing Sr doping concentration from 0% to 5%.

An evaluation of diffraction peak profile analysis (DPPA) methods to study plastically deformed metals

A range of diffraction peak profile analysis (DPPA) techniques were used to determine details of the microstructure of plastically deformed alloys. Four different alloys were deformed by uniaxial tension and compression to a range of strains. The methods we have considered include, the full-width, Williamson-Hall methods, Warren-Averbach methods, and van Berkum's alternative method. Different metals were chosen to understand the effect of the deformation microstructure and crystal structure, a nickel alloy, two stainless steel alloys and two titanium alloys.The dislocation density values found by Williamson-Hall and Warren-Averbach methods were found to be close to those expected from TEM results of similar metals. When using the Warren-Averbach methods the results suggest that systematic errors in the dislocation density are introduced by three main factors: (1) separation of instrumental broadening, (2) separation of size and strain broadening, and (3) separation of dislocation density and arrangement. Which suggests in many cases the simpler Williamson-Hall method may be preferable.The other main parameters that can be determined by DPPA are the crystal size and the dislocation arrangement. The work suggests that further investigation is needed to understand what use if any these parameters have for quantifying the deformed microstructure of plastically deformed metals.

Aging effect of the precursor solution on the structural, morphological, optical and electrical properties of ternary CdZnO thin films suited for optoelectronic applications

Precursor solution aging effect on the physical properties of ternary CdZnO thin films is reported in this paper. Nanostructured CdZnO thin films were fabricated on glass substrates by spray technique using perfume atomizer from fresh precursor solution and from solutions with different aging periods (1, 2, 3 and 4days). XRD studies confirm the polycrystalline nature of the films. All the films exhibit cubic crystal structure with a strong (1 1 1) preferential orientation irrespective of aging period of the starting solution. The CdZnO film prepared from 3days aged solution possess a high crystallite size value of 38.72nm and low strain and dislocation density values of 0.89×10−3 and 0.667×1015lines/m2 respectively which supports for its enhanced crystallinity. Cauliflower shaped nanostrucutres are observed for all the films from the SEM images and the CdZnO film prepared from 3days aged precursor solution exhibit better surface morphology. Optical band gap decreased with increase in aging period of the starting solution which was attributed to quantum confinement effect. Electrical resistivity of the CdZnO films decreased with aging period and the film coated from 3days aged solution exhibited a minimum resistivity of 0.056×10−2Ωcm.

GaAsP solar cells on GaP/Si with low threading dislocation density

GaAsP on Si tandem cells represent a promising path towards achieving high efficiency while leveraging the Si solar knowledge base and low-cost infrastructure. However, dislocation densities exceeding 108 cm−2 in GaAsP cells on Si have historically hampered the efficiency of such approaches. Here, we report the achievement of low threading dislocation density values of 4.0–4.6 × 106 cm−2 in GaAsP solar cells on GaP/Si, comparable with more established metamorphic solar cells on GaAs. Our GaAsP solar cells on GaP/Si exhibit high open-circuit voltage and quantum efficiency, allowing them to significantly surpass the power conversion efficiency of previous devices. The results in this work show a realistic path towards dual-junction GaAsP on Si cells with efficiencies exceeding 30%.

Developing a method for the evaluation of dislocation parameters from the Rietveld refinement procedure

In this investigation, the ability of Rietveld refinement method was used to simultaneously refine the structure and microstructure and evaluate the linear defects of cubic crystals. To do this, the basis of Stephans theory, about the anisotropic strain broadening, was developed and the values of dislocation density as well as the fraction of dislocation types were estimated. The reliability of this procedure was checked by selecting four different nanocrystaslline samples and evaluating the microstructure of these materials. Finally, the results were compared with those extracted from the whole powder pattern modeling method. Good agreement was achieved between the results of two methods.

Effect of Cu-doping on optical, electrical and magnetic properties of chemically synthesized MnS nanocrystals

Manganese sulfide and Cu-doped manganese sulfide nanocrystals have been synthesized by wet chemical technique. The structural, optical, electrical and magnetic properties of as-synthesized nanocrystals have been investigated. The average crystallite size and lattice strain of the samples have been calculated from powder X-ray diffraction patterns using the Williamson–Hall analysis. The results show that the average crystallite size decreased while both the lattice strain and the dislocation density values increased in the Cu-doped MnS nanocrystals. The surface morphology of Cu-doped MnS nanocrystals has lesser particle size than undoped sample and it shows spherical like structures with little agglomeration. The chemical composition of the prepared samples has been obtained from EDAX. It clearly indicates the presence of Cu ions in the MnS lattice. UV–visible spectroscopy shows a blue shift in the optical band gap with doping. The photoluminescence spectra on the doped sample show a quenching of the PL intensity due to strain induced by doping. The electrical conduction, dielectric and impedance properties of as-synthesized nanocrystals have been investigated in the frequency range 50 Hz–5 MHz and temperature range 323–473 K which are greatly affected by doping with Cu. The vibrating sample magnetometer measurement revealed that the undoped MnS has paramagnetic behavior while the Cu-doped MnS has superparamagnetic behavior. On Cu-doping, the saturation magnetization and remanence increases while the coercivity decreases.

Improvement of the optoelectronic properties of tin oxide transparent conductive thin films through lanthanum doping

This work highlights some physical investigations on tin oxide thin films doped with different lanthanum content (ratio La–to-Sn = 0–3%). Such doped thin films have been successfully grown by spray pyrolysis onto glass substrates at 450 °C. X-ray diffraction (XRD) patterns showed that SnO2:La thin films were polycrystalline with tetragonal crystal structure. The preferred orientation of crystallites for undoped SnO2 thin film was along (110) plane, whereas La-doped ones have rather preferential orientations along (200) direction. Although the grain size values exhibited a decreasing tendency with increasing doping content confirming the role of La as a grain growth inhibitor, dislocation density and microstrain values showed an increasing tendency. Also, Raman spectroscopy shows the bands corresponding to the tetragonal structure for the entire range of La doping. The same technique confirms the presence of La2O3 as secondary phase. Moreover, SEM images showed a porous architecture with presence of big clusters with different sizes and shapes resulting from the agglomeration of small grains round shaped. Photoluminescence spectra of SnO2:La thin films exhibit a decrease in the emission intensity with La concentration due to the decrease in grain size. Optical transmittance spectra of the films showed high transparency (∼80%) in the visible region. The dispersion of the refractive index is discussed using both Cauchy model and Wemple–Di-Domenico method. The optical band gap values vary slightly with La doping and were found to be around 3.8 eV. It has been found that La doping causes a pronounced decrease in the sheet resistance by up to two orders of magnitude and allows improving the Haacke's figure of merit (Φ) of the sprayed thin films. Moreover, we have introduced for a first time a new figure of merit for qualifying photo-thermal conversion applications. The obtained high conducting and transparent SnO2:La thin films are promising to be useful in various optoelectronic applications.

A new method to determine the dislocation density in wurtzite n-GaN

We develop a new method to determine the edge and screw dislocation density in wurtzite n-GaN film. The method is to fit the van der Pauw variable temperature Hall-effect measurements with a analytic expression of low-field electron mobility in n-GaN. Our calculations take the comprehensive effect between the dislocation line and the shallow-donor defects as the main cause to depress the carrier mobility. Because of the crystal distortion near the dislocation line, the energy is so high that shallow-donor defects in the GaN crystal can be captured near the dislocation line. In other words, the shallow-donor defects distribute in lines along the dislocation line, but the shallow-donor defects along the screw and edge dislocation line have different energy levels. The shallow-donor defects take energy from lattice and the carrier, which is in relaxation process, then deliver the energy through ionizing. So, it is found that the following assumptions need to be made in order to obtain the model function for the mobility over a wide temperature range: i) there are 6 shallow-donor defect lines around one dislocation line; ii) two donor energy levels belonging to the screw and edge dislocation respectively must be taken into account; iii) the exchange energy between the carrier and the shallow-donor defect is ħωLO, the energy value of polar optical phonon. Under these assumptions, experiments indicate that our calculation function can fit the experimental curve best. The values of dislocation density from our model and others determined by x-ray diffraction or by chemical etching method are in good agreement, and the values of donor energy levels from our model and Rode iterative method to solve the Boltzmann equation are also in good accordance with each other. This method is applicable for the wurtzite n-GaN films grown by various preparation technologies under any condition, which is for the sample with the peak-mobility temperature about or under 200 K, not for the sample with the peak-mobility temperature about or above 300 K, which room-temperature mobility usually is about or less than 100 cm2/(V·s).

Structure investigation of large single crystals of gallium antimonide grown by the Czochralski method in the [100] direction

Investigation of the properties of undoped single crystals of gallium antimonide with a diameter greater than 60 mm, grown by the Czochralski method in the crystallographic direction [100] has been carried out. We have found that a decrease in the dislocation density in bulk undoped single crystals of gallium antimonide can be achieved using conventional processing methods during the process of growing and by doping with isovalent indium impurities. It has been shown that the introduction of ageing for one hour in the process of manufacturing at a temperature close to the temperature of crystallization at the stage of the exit of the crystal to a given diameter, and further heat treatment in the postcrystallization state at a temperature of 650°C for three hours, allows one to reach the dislocation density value of (3–5) × 103 cm–2 in undoped single crystals of gallium antimonide with a diameter ~60 mm. It is established that the formation of a dislocation in large-sized single crystals of gallium antimonide is caused by two temperature ranges, as illustrated by the traces of dislocations with different morphology. The values of the critical stresses of the formation of a dislocation in the temperature range of 420–690°C are determined. It is shown that doping by isovalent impurities (In concentrations up to (2–4) × 1018 at/cm3) leads to a significant increase in the critical stress of the formation of a dislocation and, consequently, the reduction in their average density of up to (4–5) × 102 cm–2. This opens up the prospect of obtaining large-sized single crystals of gallium antimonide with a small number of dislocations.

Changes in Estimated Dislocation Density during Creep in Martensitic Heat-Resistant Steel

An equation for dislocation density during creep was introduced for martensitic heat-resistant steel using Orowan’s equation and a thermal activation equation of Kauzmann type based on several assumptions. The dislocation density discussed here corresponds to the number of dislocations swept out of an imaginary deformable domain in an activated state and is not the so-called mobile dislocation density. The one adjustable parameter, the actual deformable domain size, was adopted in the equation as a size intermediate of the lath martensite and packet. The activation energy and volume, and , were calculated using creep curves for 9Cr-1W tempered martensitic steel as a function of creep strain. The changes in the dislocation density during creep were estimated nondestructively to be roughly to using the values of and . The obtained densities were approximately comparable to the values of total dislocation density reported for directly observed ferritic/martensitic steels. It was found that the dislocation density of 9Cr-1W steel initially increased and then decreased in a transition creep range, which relate to hardening and recovery, respectively. This point of inversion from hardening to recovery can be confirmed easily and nondestructively far before the time to minimum creep rate and as a matter of course the time to rupture. It is suggested that monitoring of the dislocation density can predict the unexpected decrease in long-term rupture strength in advance.

Determination of Intrinsic Strain in Poly(Vinylpyrrolidone)-Capped Silver Nano-Hexapod Using X-Ray Diffraction Technique

Poly(vinylpyrrolidone)-capped silver nano-hexapods (AgNHs) have been prepared by chemical reduction method using ethylene glycol as a reducing agent, for the study of their structural and elastic properties. HRTEM study reveals that the prepared nanocrystals are hexapod in shape with an average size of approximately 50 nm. From the analysis of the X-ray diffraction pattern, intrinsic strain produced in the nano-hexapod due to dislocation of silver atoms, has been determined from the W-H plot. The lattice constant of the AgNHs has also been determined from Nelson-Riley plot. Higher value of dislocation density confirms that there may exist intrinsic strain in the nanocrystals.

STRUCTURAL STUDIES OF FERROELECTRIC BATIO3 NANO PARTICLES AND VACUUM EVAPORATED BATIO3 NANO SCALE THIN FILMS

Barium titanate (BaTiO3) nanoparticles were prepared by wet chemical method using commercially available materials barium chloride, titanium dioxide and oxalic acid. Nano scale thin films of different thickness were coated on pre - cleaned glass substrate by using vacuum evaporation technique under a vacuum of 2x10-5 Torr. The X - ray analysis showed that the particles have tetragonal structure. The deposited films of a lower thickness were found to be amorphous in nature, whereas the crystallinity increases with increase of thickness. The estimated value of grain size (D), strain and dislocation density (δ) were also reported in this paper.