Interior Of Sample Research Articles

Synthesis of Cu2ZnSnS4nanoparticles and analysis of secondary phases in powder pellets

Sulfur-based kesterite Cu2ZnSnS4 (CZTS) nanoparticles (NPs) have been produced in a wet-chemical synthesis route which provides high yields of CZTS-NPs per synthesis cycle. These NPs can be used as raw material for electronic inks, which can be processed to light-harvesting absorber thin films in photovoltaic devices. In the corresponding phase diagram, the desired kesterite phase is found only in a very narrow, off-stoichiometric composition region. Accordingly, kesterite materials are likely to contain unwanted secondary phases such as ZnS, Cu2S, Cu2SnS3, which may limit the photovoltaic device performance in terms of current-blocking domains and formation of low-energy recombination channels. These phases can form already during the synthesis but may also evolve at high temperatures, for example, during annealing of the thin film. To limit the formation of Cu2S and Cu2SnS3 secondary phases Zn-rich and Cu-poor compositions were chosen corresponding to device-grade CZTS. In this paper we report on the phase and composition analysis of CZTS using X-ray diffraction, Raman spectroscopy with different excitation wavelengths, scanning electron microscopy and energy-dispersive X-ray spectroscopy. The analysis was performed on annealed CZTS pellets prepared from condensed CZTS NP powders. To extract the phase constitution and the composition from the surfaces and from the sample interior the CZTS pellets were horizontally cleaved after the annealing step. The bulk of the pellet consists of the required CZTS phase with very small quantities of Cu2–xS and ZnS. The utmost amount of these secondary phases was found on the top of the sample whereby copper sulfide forms preferentially isolated domains. As these surface-located secondary phases can be removed chemically, CZTS absorber films in good crystalline quality are to be expected.

Bulk effects on topological conduction in three-dimensional topological insulators

The surface states of a topological insulator in a fine-tuned magnetic field are ideal candidates for realizing a topological metal which is protected against disorder. Its signatures are (i) a conductance plateau in long wires in a finely tuned longitudinal magnetic field and (ii) a conductivity which always increases with sample size, and both are independent of disorder strength. We numerically study how these experimental transport signatures are affected by bulk physics in the interior of the topological insulator sample. We show that both signatures of the topological metal are robust against bulk effects. However the bulk does substantially accelerate the metal's decay in a magnetic field and alter its response to surface disorder. When the disorder strength is tuned to resonance with the bulk band the conductivity follows the predictions of scaling theory, indicating that conduction is diffusive. At other disorder strengths the bulk reduces the effects of surface disorder and scaling theory is systematically violated, signaling that conduction is not fully diffusive. These effects will change the magnitude of the surface conductivity and the magnetoconductivity.

Thermal transport in shock wave-compressed solids using pulsed laser heating.

A pulsed laser heating method was developed for determining thermal transport properties of solids under shock-wave compression. While the solid is compressed, a laser deposits a known amount of heat onto the sample surface, which is held in the shocked state by a transparent window. The heat from the laser briefly elevates the surface temperature and then diffuses into the interior via one-dimensional heat conduction. The thermal effusivity is determined from the time history of the resulting surface temperature pulse, which is recorded with optical pyrometry. Thermal effusivity is the square root of the product of thermal conductivity and volumetric heat capacity and is the key thermal transport parameter for relating the surface temperature to the interior temperature of the sample in a dynamic compression experiment. Therefore, this method provides information that is needed to determine the thermodynamic state of the interior of a compressed metal sample from a temperature measurement at the surface. The laser heat method was successfully demonstrated on tin that was shock compressed with explosives to a stress and temperature of ~25 GPa and ~1300 K. In this state, tin was observed to have a thermal effusivity of close to twice its ambient value. The implications on determining the interior shock wave temperature of tin are discussed.

Analysis method of physical processes occurring at the interface between vortex and Meissner regions in high-temperature superconductors

A technique for probing the sample interior using the leading edge of an alternating evanescent magnetic field is suggested. It makes it possible to highlight and study physical processes taking place at the vortex front in superconductors. Successively increasing the field amplitude by equal amounts, one can study the formation and transformation of the magnetic microstructure in response to variation of the sample’s crystal microstructure. For samples with different microstructures, qualitatively new results are obtained. The technique suggested may be used for testing high-temperature superconductor epitaxial films and magnetic materials.

Textural and rheological evolution of basalt flowing down a lava channel

The Muliwai a Pele lava channel was emplaced during the final stage of Mauna Ulu’s 1969–1974 eruption (Kilauea, Hawaii). The event was fountain-fed and lasted for around 50 h, during which time a channelized flow system developed, in which a 6-km channel fed a zone of dispersed flow that extended a further 2.6 km. The channel was surrounded by initial rubble levees of ’a’a, capped by overflow units of limited extent. We sampled the uppermost overflow unit every 250 m down the entire channel length, collecting, and analyzing 27 air-quenched samples. Bulk chemistry, density and textural analyses were carried out on the sample interior, and glass chemistry and microlite crystallization analyses were completed on the quenched crust. Thermal and rheological parameters (cooling, crystallization rate, viscosity, and yield strength) were also calculated. Results show that all parameters experience a change around 4.5 km from the vent. At this point, there is a lava surface transition from pahoehoe to ’a’a. Lava density, microlite content, viscosity, and yield strength all increase down channel, but vesicle content and lava temperature decrease. Cooling rates were 6.7 °C/km, with crystallization rates increasing from 0.03 Фc/km proximally, to 0.14 Фc/km distally. Modeling of the channel was carried out using the FLOWGO thermo-rheological model and allowed fits for temperature, microlite content, and channel width when run using a three-phase viscosity model based on a temperature-dependent viscosity relation derived for this lava. The down flow velocity profile suggests an initial velocity of 27 m/s, declining to 1 m/s at the end of the channel. Down-channel, lava underwent cooling that induced crystallization, causing both the lava viscosity and yield strength to increase. Moreover, lava underwent degassing and a subsequent vesicularity decrease. This aided in increasing viscosity, with the subsequent increase in shearing promoting a transition to ’a’a.

Sintering of zirconia-based nanomaterials studied by variable-energy slow-positron beam

A variable-energy slow-positron beam was applied to the investigations of the tetragonal yttria-stabilised zirconia (YSZ), the YSZ co-doped with small amount of Cr2O3. The initial nanopowders exhibiting the mean particle size of ≈ 20 nm were prepared by co-precipitation technique. Prior the sintering, the nanopowders were calcined and compacted using a pressure of 500 MPa. The ordinary shape parameters of the Doppler-broadened annihilation peak and the relative positronium 3γ-fractions were determined as functions of positron energy. The results are consistent with a remarkable sintering-induced grain growth and disappearance of porosity which is driven out from the sample interior toward a thin subsurface layer.

Defects and Sintering-Induced Diffusion Processes in Yttria-Stabilised Zirconia Nanomaterials Studied by Positron Annihilation Spectroscopy

In the present work, zirconia (ZrO2) nanopowders doped with yttria (Y2O3) and chromia (Cr2O3) were prepared by a co-precipitation technique. The nanopowders were then subjected to a calcination and a successive sintering at elevated temperatures up to 1500 °C. The nanostructures in these nanomaterials were characterized by positron annihilation spectroscopy (positron lifetimes and Doppler broadening measurements) which is a non-destructive technique with a high sensitivity to atomic-scale open-volume defects. It was found that the zirconia-based nanomaterials studied contain vacancy-like defects and nanoscale pores. Diffusion processes induced in these nanomaterials by sintering were investigated also by depth sensitive positron annihilation studies using a variable energy slow positron beam. Sintering was found to cause intensive grain growth and a removal of porosity by a migration of pores from the sample interior toward its surface.

Bright persistent ceramic phosphors of Ce3+-Cr3+-codoped garnet able to store by blue light

We have developed bright persistent phosphors of Ce3+-Cr3+-doped Y3Al5-xGaxO12 (x = 2.5, 3, 3.5) ceramics with green luminescence (Ce3+:5d→4f) via blue-light excitation. The persistent luminance value 5 min after ceasing blue-light excitation for the Ce3+-Cr3+-doped Y3Al2Ga3O12 sample is approximately 3900 times higher than that for a Ce3+-doped Y3Al2Ga3O12 sample and better than that from compact SrAl2O4:Eu2+-Dy3+ powders. The results are consequences of efficient carrier trap formation and effective excitation of the interior of the translucent ceramic samples. A series of Ce3+-Cr3+-doped garnet materials is the best candidate for the persistent phosphor under white-light emitting diode (LED) illumination consisting of blue LEDs and visible phosphors.

Polycrystal Plasticity: Comparison Between Grain - Scale Observations of Deformation and Simulations

The response of polycrystals to plastic deformation is studied at the level of variations within individual grains, and comparisons are made to theoretical calculations using crystal plasticity (CP) . We provide a brief overview of CP and a review of the literature, which is dominated by surface observations. The motivating question asks how well does CP represent the mesoscale behavior of large populations of dislocations (as carriers of plastic strain). The literature shows consistently that only moderate agreement is found between experiment and calculation. We supplement this with a current example of microstructure evolution in the interior of a copper sample subjected to tensile deformation. Nondestructive measurements of orientation fields were performed using the near-field high-energy X-ray diffraction microscopy (nf-HEDM) technique at the Advanced Photon Source (APS). Starting at highly ordered grains, a single two-dimensional slice of microstructure containing ∼150 grains was followed through multiple strain states, where it tracked lattice rotations and defect accumulation of up to 14% elongation. In accord with the literature, at the scale of individual grains, comparison of observations with CP models indicates reasonable qualitative agreement but significant variations between simulation and experiment are apparent. The conclusion is that in order to be able to quantify the effects of microstructure on the distributions of slip, orientation change, and damage accumulation, the empirically derived constitutive relations used in continuum-scale simulations need to be improved. Equally important will be the development of large-scale simulations of polycrystals that directly model dislocations.

Direct Assessment of Phytochemicals Inherent in Plant Tissues Using Extractive Electrospray Ionization Mass Spectrometry

An ambient pressure ionization mass spectrometric strategy called internal extractive electrospray ionization mass spectrometry (iEESI-MS) has been developed and applied for direct profiling of labile phytochemicals inherent in various native plant tissues, including leaves, roots, and fruits. By passing the electrospray solvent through the plant tissue, a variety of phytochemicals, such as amino acids, sugars (e.g., glucose, sucrose, polysaccharides, etc.), and alkaloids, were continuously extracted from the sample interior, driven toward the natural/cut electro-spraying tip, and vaporized into gaseous ions for mass spectrometric interrogation. Phytochemical patterns obtained by iEESI-MS permit a rapid differentiation between various species of ginkgo plant and strawberry maturity stages, as well as characterization of physiological/pathologic conditions of chlorophytum comosum. Our experimental results further demonstrate that the established iEESI-MS approach is potentially useful for direct phytochemomics studies with minimal biodegradation, allowing elucidation of plant metabolism with high speed, specificity, and simplicity of analysis.

Distortion in a 7xxx Aluminum Alloy during Liquid Phase Sintering

The distortion in a sintered 7xxx aluminum alloy, Al-7Zn-2.5Mg-1Cu (wt. pct), has been investigated by sintering three rectangular bars in each batch at 893 K (620 °C) for 0 to 40 minutes in nitrogen, followed by air or furnace cooling. They were placed parallel to each other, equally spaced apart at 2 mm, with their long axes being perpendicular to the incoming nitrogen flow. Pore evolution in each sample during isothermal sintering was examined metallographically. The compositional changes across sample mid-cross section and surface layers were analyzed using energy dispersive X-ray spectroscopy and X-ray photoelectron spectroscopy depth profiling, respectively. The two outer samples bent toward the middle one, while the middle sample was essentially distortion free after sintering. The distortion in the outer samples was a result of differential shrinkage between their outer and inner surfaces during isothermal sintering. The porous outer surface showed an enrichment of oxygen around the large pores as well as lower magnesium and zinc contents than the interior and inner surface of the same sample, while the inner surface was distinguished by the presence of AlN. The differential shrinkage was caused by different oxygen contents in local sintering atmosphere and unbalanced loss of magnesium and zinc between the outer and inner surfaces.

Direct Characterization of Bulk Samples by Internal Extractive Electrospray Ionization Mass Spectrometry

A straight-forward analytical strategy called internal extractive electrospray ionization mass spectrometry (iEESI-MS), which combines solvent extraction of chemicals inside a bulk sample with in situ electrospray ionization mass spectrometry, has been established to directly characterize the interior of a bulk sample with molecular specificity. The method allows both qualitative and quantitative analysis of analytes distributed in a 3-dimensional volume (e.g., 1 ~ 100 mm3) of various synthetic and biological matrices (e.g., chewing gum, leaves, fruits, roots, pork, lung tissues) without either mashing the sample or matrix separation. Using different extraction solvents, online chromatographic separation of chemicals inside the sample volume was observed during iEESI-MS analysis. The presented method is featured by the high speed of analysis, high sensitivity, low sample consumption and minimal sample preparation and/or degradation, offering unique possibilities for advanced applications in plant science, clinical diagnosis, catalyst studies, and materials science.

Preparation and Research of High-Temperature Foam Ceramic

High-temperature foam ceramics were produced from sludge, Zhongxiang porcelain sand, shale and sand, with addition of SiC 0.15%~0.40% as foaming agent, respectively, sintered at 1130°C~1160°C. The phase composition and micro structure of sintered samples were investigated by X-ray diffraction (XRD) and scanning electron microscopy (SEM), and density, compressive strength and thermal conductivity were evaluated. The results showed that the main phase of amorphous phase plagioclase in sintered samples and interior of samples with a large number of uniformly distributed closed pores, rose sintering temperature and increased SiC addition leading to pores size enlarged and density, compressive strength and thermal conductivity diminished.

Directional crystallization of columnar-grained monoclinic Y2O3 under high pressure

A pressure-induced directional crystallization process is described to convert equiaxially-grained cubic Y2O3 into columnar-grained monoclinic Y2O3. The conversion to the columnar-grained structure initiates at the sample surface, propagates into the sample interior and eventually encompasses the entire sample volume. Optimal processing conditions are about 3.0 GPa at 1000 °C. It is proposed that the phase transformation from cubic Y2O3 to the higher density monoclinic Y2O3 provides the driving force for columnar grain growth.

Continuous wave terahertz phase imaging with three-step phase-shifting

We demonstrated a continuous wave terahertz (THz) phase imaging system which can present the depth information of an object's surface interior accurately and non-invasively. With the three-step phase-shifting method, the phase images of samples which can provide the relative optical depth profile of the sample's surface and interior have been achieved. Two samples with different thickness and materials were measured to demonstrate the phase imaging ability of the system. The results show that this method is effective and the measured values agree well with the actual ones. The longitudinal resolution of the system was also measured. To exhibit the advantages of the three-step phase shifting method, the multi-wavelength imaging method was also employed and the results are compared.

Study of the effect of α irradiation on the microstructure and mechanical properties of nanocrystalline Ni

The effect of irradiation damage on the microstructure evolution and mechanical properties in nanocrystalline (nc) Ni with an average grain size of ∼60nm was studied. Samples were irradiated at doses of 1.6×1015, 2.3×1015, 5×1015 and 2.3×1016 He2+ cm−2 by 12MeV He ions at room temperature. Microstructural parameters like domain size and microstrain were studied in detail using the X-ray diffraction (XRD) technique with the simplified breadth method and Williamson–Hall analysis. The average domain size was found to decrease systematically with the increase in irradiation dose. Microscopic observations made with transmission electron microscopy showed dislocation loops and dislocation networks within the grain interior of the irradiated sample. In addition, irradiation at a higher dose showed small amounts of the sand-like black dots inside some grains, which could be due to the accumulation of point defects. Tensile tests performed on the irradiated sample were compared with unirradiated samples. The irradiated sample showed higher ultimate tensile strength and average work-hardening rate as compared to the unirradiated sample. Nanoindentation studies were performed to study the effect of irradiation on deformation parameters like strain rate sensitivity (m) and activation volume (V*). The value of m was found to increase whereas V* was found to decrease with increase in irradiation dose. Fracture surfaces of the tensile sample were investigated by SEM. The fracture morphology of the unirradiated sample showed dimpled rupture with much large dimple diameter and depth. The irradiated sample also showed dimple rupture but with much finer dimple diameter with wide size distribution and shallow depths.

1H-NMR measurements of proton mobility in nano-crystalline YSZ

We report nuclear magnetic resonance (NMR) results on water saturated, dense, nano-crystalline YSZ samples (9.5 mol% yttria doped zirconia) which exhibit proton conductivity at temperatures as low as room temperature. (1)H-NMR spectra recorded under static and magic angle spinning conditions show two distinct signals. Their temperature-dependent behavior and their linewidths suggest that one can be attributed to (free) water adsorbed on the surface of the sample and the other one to mobile protons within the sample. This interpretation is supported by comparison with measurements on a single-crystalline sample. For the nano-crystalline samples motional narrowing is observed for the signal originating from protons in the sample interior. For these protons, the analysis of temperature and field dependent spin-lattice relaxation time T1 points towards diffusion in a confined two-dimensional geometry. We attribute this quasi two-dimensional motion to protons that are mobile along internal interfaces or nanopores of nano-crystalline YSZ.

Effect of Sintering on Defects in Yttria Stabilised Zirconia

A variable energy slow positron beam was utilised to investigate depth dependent effects of sintering on the tetragonal yttria stabilised zirconia nanopowders. Positron implantation was combined with the determination of Doppler broadened profiles of annihilation radiation. The results are consistent with recent positron lifetime data showing that sintering at elevated temperatures leads to a disappearance of pores and a significant grain growth, which is demonstrated by a strong suppression of positronium formation and a substantial decrease in concentration of open volume defects at triple points, respectively, with increasing sintering temperature. An existence of a subsurface layer of a relatively high content of defects was shown in sintered samples and tentatively attributed to arise from a diffusion of open volume defects from the sample interior toward the surface or from a sintering-induced surface modification.

Breakdown of the large-scale circulation inΓ=1/2rotating Rayleigh-Bénard flow

Experiments and simulations of rotating Rayleigh-Bénard convection in cylindrical samples have revealed an increase in heat transport with increasing rotation rate. This heat transport enhancement is intimately related to a transition in the turbulent flow structure from a regime dominated by a large-scale circulation (LSC), consisting of a single convection roll, at no or weak rotation to a regime dominated by vertically aligned vortices at strong rotation. For a sample with an aspect ratio Γ=D/L=1 (D is the sample diameter and L is its height) the transition between the two regimes is indicated by a strong decrease in the LSC strength. In contrast, for Γ=1/2, Weiss and Ahlers [J. Fluid Mech. 688, 461 (2011)] revealed the presence of a LSC-like sidewall temperature signature beyond the critical rotation rate. They suggested that this might be due to the formation of a two-vortex state, in which one vortex extends vertically from the bottom into the sample interior and brings up warm fluid while another vortex brings down cold fluid from the top; this flow field would yield a sidewall temperature signature similar to that of the LSC. Here we show by direct numerical simulations for Γ=1/2 and parameters that allow direct comparison with experiment that the spatial organization of the vertically aligned vortical structures in the convection cell do indeed yield (for the time average) a sinusoidal variation of the temperature near the sidewall, as found in the experiment. This is also the essential and nontrivial difference with the Γ=1 sample, where the vertically aligned vortices are distributed randomly.

A Feasibility Study in Application of a Gamma Scattering Technique for Inspecting Density Variation by Monte Carlo Method

Back-scattering gamma-rays have been extensively used for years as a nondestructive tool for inspecting the materials in different fields of the economy. The intensities of Compton scattering gamma-rays from the scattering medium strongly depend on its electron density and therefore in its mass density. This feature is very useful for using it as a viable tool for inspecting material. This work aims to investigate the feasibility of application of gamma scattering technique for inspecting density variation in some construction objects by Monte-Carlo simulation method. The gamma-ray sources of different energies and strengths have been used to extract the information of density variation for interior of sample by recording the backscattering gamma-rays with a gamma-ray detector. The results of our simulations confirm that the resolution for density variation in the inspected objects is quite good. The results should also prove useful in the optimum design of the nondestructive density gauges.