Nanometer-scale Grain Research Articles

Recrystallization of bulk nanostructured magnesium alloy AZ31 after severe plastic deformation: an in situ diffraction study

The magnesium alloy AZ31, which has undergone high-pressure torsion processing, was subjected to in situ annealing microbeam synchrotron high-energy X-ray diffraction and compared to the as-received rolled sheet material that was investigated through in situ neutron diffraction. While the latter only exhibits thermal expansion and minor recovery, the nanostructured specimen displays a complex evolution, including recovery, strong recrystallization, phase transformations, and various regimes of grain growth. Nanometer-scale grain sizes, determined using Williamson–Hall analysis, exhibit seamless growth, aligning with the transition to larger grains, as assessed through the occupancy of single-grain reflections on the diffraction rings. The study uncovers strain anomalies resulting from thermal expansion, segregation of Al atoms, and the kinetics of vacancy creation and annihilation. Notably, a substantial number of excess vacancies were generated through high-pressure torsion and maintained for driving the recrystallization and forming highly activated volumes for diffusion and phase precipitation during heating. The unsystematic scatter observed in the Williamson–Hall plot indicates high dislocation densities following severe plastic deformation, which significantly decrease during recrystallization. Subsequently, dislocations reappear during grain growth, likely in response to torque gradients in larger grains. It is worth noting that the characteristics of unsystematic scatter differ for dislocations created at high and low temperatures, underscoring the strong temperature dependence of slip system activation.Graphical

Grain Refinement and Strengthening of an Aluminum Alloy Subjected to Severe Plastic Deformation through Equal-Channel Angular Pressing

In the present study, the microstructure, mechanical properties, and stored energy of an aluminum alloy containing iron-rich fine precipitates, subjected to severe plastic deformation through equal-channel angular pressing (ECAP), were investigated using X-ray diffraction, scanning electron microscopy, transmission electron microscopy, and atomic force microscopy. Up to four passes through ECAP resulted in significant nanometer-scale grain refinement, as well as the accumulation of lattice defects, such as dislocations and mesoscopic shear planes. This resulted in a noticeable enhancement in the Vickers microhardness and the flow stress after ECAP. Differential scanning calorimetry results showed that the ECAP’ed material exhibited two exothermal peaks at 222 ± 2 °C and 362 ± 2 °C, with total thermal effects of ΔH = 4.35 and 6.5 J/g, respectively. Slight increases in the ECAP’ed material microhardness and flow stress were observed at 200 °C. The heat release, at a relatively low temperature, and the slight improvement in the mechanical properties were attributed to the evolution of low- and high-angle misorientation, with the strain and the pinning of tangled dislocation caused by the existing fine particles. The second peak was attributed to grain growth, resulting in a significant softening of the material.

Efficient semi-transparent perovskite quantum dot photovoltaics enabled by asymmetric dielectric/metal/dielectric transparent electrodes

CsPbI3 perovskite quantum dots (CsPbI3-PQDs) have a high potential as semi-transparent photovoltaic absorbers because of the facile control of film thicknesses, size-tunable optical band gaps, and nanometer-scale grain sizes suppressing light scattering. Conventional semi-transparent CsPbI3-PQD solar cells showed low photovoltaic performances due to the low electrical conductivity of the graphene electrodes. Here we report that dielectric/ultra-thin metal/dielectric (DMD) electrodes with excellent optical transmittance and electrical conductivity deliver superior photovoltaic performances in the semi-transparent CsPbI3-PQD solar cells. Particularly, the asymmetric DMD electrodes composed of MoOx 15 nm/Au 10 nm/MoOx 35 nm (asym-MAM) stacks exhibit higher optical transmittance than that of the symmetric MoOx 15 nm/Au 10 nm/MoOx 15 nm (sym-MAM) stacks. The optical simulation confirms that asym-MAM stacks reduce the parasitic absorption loss in metal interlayers. Therefore, the asym-MAM stacks with a high electrical conductivity show a higher average visible transmittance (AVT) than that of the sym-MAM. Consequently, the semi-transparent CsPbI3-PQD solar cells fabricated using the asym-MAM transparent top electrodes show a power conversion efficiency of 11.3% with a considerable AVT values of 23.4% (400–800 nm) and 20.0% (380–780 nm), respectively, which were calculated from their transmittance spectra. This is the highest-efficiency semi-transparent PQD solar cells that can be used for solar window applications requiring an AVT value of over 20%.

Alloying reactions in nanostructured multilayers during intense deformation

Abstract Under intense deformation of metallic multilayer samples, a nanometer-scale layer thickness and grain size develops during repeated cold-rolling. Along with the evolution of the highly refined microstructure, a nanoscale interfacial alloying occurs that can result in an amorphization reaction. The deformation of multilayers exhibits driven system behavior during alloying. As the length scale of the layer thickness converges to the length scale of the mixing zone during rolling, amorphization develops in appreciable volumes. The results from selected experiments demonstrate that the relative specific interfacial area is the key microstructural metric to describe the deformation-driven alloying.

Gd0.2Ce0.8O2 Diffusion Barrier Layer between (La0.58Sr0.4)(Co0.2Fe0.8)O3−δ Cathode and Y0.16Zr0.84O2 Electrolyte for Solid Oxide Fuel Cells: Effect of Barrier Layer Sintering Temperature on Microstructure

Combining high-performance (La0.58Sr0.4)(Co0.2Fe0.8)O3-δ (LSCF) cathodes with Y-doped ZrO2(YDZ) electrolytes in solid oxide fuel cells leads to the formation of SrZrO3 (SZO) as secondary phase with exceedingly low oxygen ion conductivity and poor catalytic capability. A promising prevention strategy is the insertion of Gd-doped CeO2 (GDC) as a reaction barrier. In this work, screen-printed GDC layers were sintered on YDZ substrates at temperatures varying from 1100 to 1400 °C. Subsequently, screen-printed LSCF was sintered on top at 1080 °C. The goal of this work was to understand microstructure formation during the two subsequent sintering processes by the analysis of nanometer-scale elemental distributions, crystal structures, and grain sizes of this extended cathode/electrolyte interface as a function of the GDC sintering temperature. Various representative regions on all samples were analyzed by transmission electron microscopy combined with energy dispersive X-ray spectroscopy with high spatial resol...

Anisotropic grain growth kinetics in nanocrystalline nickel

ABSTRACTIntriguing properties exhibited by nanocrystalline metals, including a high level of mechanical strength, arise from their nanometer-scale grain sizes. It is critical to determine the evolution of grain size of nanocrystalline materials at elevated temperature, as this process can drastically change the mechanical properties. In this work, a nanocrystalline Ni foil with grain size ∼ 25 nm was annealed in situ in an X-ray diffractometer. X-ray diffraction peaks were analysed to determine the grain growth kinetics. The grain growth exponents obtained were ∼ 2–4 depending upon the crystallographic direction, indicating the anisotropic nature of the grain growth kinetics.

Nanographene synthesis employing in-liquid plasmas with alcohols or hydrocarbons

Graphenes of nanometer-scale grain size (nanographenes) were synthesized using in-liquid plasmas with alcohols or hydrocarbons. This method of nanographene synthesis showed a trade-off relationship between crystallinity and synthesis rate. The high crystallinity of nanographenes synthesized with alcohols was evaluated from the small full width at half maxima (FWHM) of the G band in Raman scattering spectra. On the other hand, in the case of using hydrocarbons such as n-hexane and benzene, a significantly high synthesis rate was obtained but the crystallinity of nanographenes was low. It was found that hydroxyl groups and oxygen atoms of liquid sources play important roles in determining the crystallinity of synthesized nanographenes.

Rate-Equation-Based Grain Model for the Carbonation of CaO with CO2

A rate-equation-based grain model was developed to describe and predict the complex behaviors of the carbonation reaction of CaO with CO2 in calcium looping. In the model, the assumption of a uniform CaCO3 film at the grain scale was replaced with the product island morphology, and the rate equation theory was used to calculate the growth of product islands; this modified grain model was integrated into a particle scale model in which gas diffusion inside a CaO particle and the pore plugging effect were considered. The macroscopic kinetics of the carbonation reaction—including the initial fast stage and the later product layer diffusion stage—could be predicted successfully using the developed theory and was validated by comparison with experimental data. The effects of structural parameters on the carbonation kinetics were discussed. Furthermore, a nanometer-scale grain design criterion for CaO sorbent was proposed to optimize particle structure and achieve maximum CaO conversion in the initial fast stag...

Generalized type III internal stress from interfaces, triple junctions and other microstructural components in nanocrystalline materials

The microstructure in polycrystalline materials consists of four types of geometric objects: grain cells, grain boundaries, triple junctions and vertex points. Each of them contributes to internal stress differently. Due to experimental limitations, the internal stresses associated with the microstructural components are difficult to acquire directly, particularly for polycrystalline materials with nanometer-scale grain sizes. Using newly developed computational methods, we obtained the type III internal stress associated with each of these microstructural objects in a stress-free nanocrystalline Cu. We found significant variation of the internal stresses from grain to grain, and their magnitudes descended in the order of vertex point, triple junction, grain boundary and grain cell. We also examined the effect of grain size and temperature. The change in the internal stresses inside the grains is found to follow a scaling relation of Ad−x, using the mean grain diameter d from our results. For pressure, we found x=1 and the effective interface stress A∼1Nm−1, and for shear stress x=0.75 and A∼14.12Nm−1. On the other hand, the directly calculated interface stress is about 0.32–0.35GPa for hydrostatic pressure and 12.45–12.60GPa for von Mises shear stress. We discuss issues in treating the two-dimensional interface stress and one-dimensional triple junction line tension in nanocrystalline materials, as well as the potential impact of the type III internal stress on mechanical behavior of poly- and nano-crytalline materials.

Proton conductivity of columnar ceria thin-films grown by chemical vapor deposition

Columnar thin films of undoped ceria were grown by metal-organic chemical vapor deposition. The films, deposited on Pt-coated MgO(100) substrates, display a columnar microstructure with nanometer scale grain size and ~30% overall porosity. Through-plane (thickness mode) electrical conductivity was measured by AC impedance spectroscopy. Proton conduction is observed below 350-400 °C, with a magnitude that depends on gas-phase water vapor pressure. The overall behavior suggests proton transport that occurs along exposed grain surfaces and parallel grain boundaries. No impedance due to grain boundaries normal to the direction of transport is observed. The proton conductivity in the temperature range of 200-400 °C is approximately four times greater than that of nanograined bulk ceria, consistent with enhanced transport along aligned grain surfaces in the CVD films.

Preparation of Ce-Doped (Ba0.87Sr0.04Ca0.09)(Ti0.90Zr0.04Sn0.06)O3 Nano-Crystalline Powders and Ceramics Using the Sol-Gel Method, and their Dielectric Properties

We prepared Ce-doped (Ba0.87Sr0.04Ca0.09)(Ti0.90Zr0.04Sn0.06)O3 (BSCTZS) powders and ceramics using the solgel process. The samples were characterized by means of thermogravimetric, Fourier-transform infrared, X-ray diffraction, and transmission and scanning electron microscope analysis. We also determined the dielectric properties of the ceramics. The results indicated that the SrZrO3 cubic perovskite phase first appeared at a calcining temperature of 600 °C, and facilitated formation of the BaTiO3-based solid solution. The as-synthesized powders calcined at 900 °C for 2 h had a nanometer-scale grain size, with the grains mainly composed of the cubic BaTiO3 phase. After sintering, the ceramics consisted entirely of the cubic BaTiO3 phase. The average grain size and the relative density decreased as the Ce concentration increased. Moreover, the maximum dielectric constant (max) decreased and the Curie temperature (Tm) shifted to a lower temperature. Furthermore, the Ce-doped BSCTZS ceramics had a relatively high dielectric constant over a wide temperature range. When the Ce concentration was 0.15 mol%, the BSCTZS ceramics met the Electronic Industries Alliance (EIA) Y5V specifications and the r (25 °C) was higher than 7500. These merits of the BSCTZS nanopowders and ceramics prepared using the sol-gel method indicate that they will permit a significant reduction in the thickness of multilayer ceramic capacitors and will help meet the current demand for device miniaturization in the electronics industry.

Preparation of Ce-Doped (Ba0.87Sr0.04Ca0.09)(Ti0.90Zr0.04Sn0.06)O3 Nano-Crystalline Powders and Ceramics Using the Sol-Gel Method, and their Dielectric Properties

We prepared Ce-doped (Ba0.87Sr0.04Ca0.09)(Ti0.90Zr0.04Sn0.06)O3 (BSCTZS) powders and ceramics using the solgel process. The samples were characterized by means of thermogravimetric, Fourier-transform infrared, X-ray diffraction, and transmission and scanning electron microscope analysis. We also determined the dielectric properties of the ceramics. The results indicated that the SrZrO3 cubic perovskite phase first appeared at a calcining temperature of 600 °C, and facilitated formation of the BaTiO3-based solid solution. The as-synthesized powders calcined at 900 °C for 2 h had a nanometer-scale grain size, with the grains mainly composed of the cubic BaTiO3 phase. After sintering, the ceramics consisted entirely of the cubic BaTiO3 phase. The average grain size and the relative density decreased as the Ce concentration increased. Moreover, the maximum dielectric constant (max) decreased and the Curie temperature (Tm) shifted to a lower temperature. Furthermore, the Ce-doped BSCTZS ceramics had a relatively high dielectric constant over a wide temperature range. When the Ce concentration was 0.15 mol%, the BSCTZS ceramics met the Electronic Industries Alliance (EIA) Y5V specifications and the r (25 °C) was higher than 7500. These merits of the BSCTZS nanopowders and ceramics prepared using the sol-gel method indicate that they will permit a significant reduction in the thickness of multilayer ceramic capacitors and will help meet the current demand for device miniaturization in the electronics industry. Keywords: Barium titanate, Ce-doped ceramics, Dielectric properties, Sol-gel process, Y5V, Thermogravimetric Analysis, BSCTZS, BSCTZS Powders, Ce Concentration, BSCTZS Xerogels

Microstructure and properties of sol–gel derived PbTiO 3/NiFe 2O 4 multiferroic composite thin film with the two nano-crystalline phases dispersed homogeneously

Ferroelectric/ferrimagnetic PbTiO 3/NiFe 2O 4 composite thin films with nano-scale PbTiO 3 and NiFe 2O 4 phases homogeneously dispersed in each other were synthesized by sol–gel in situ method. The phase structure and morphology of the composite thin films and their dielectric, ferroelectric and ferromagnetic properties were measured respectively in detail. Results show that the ferroelectric/ferrimagnetic composite thin film with PbTiO 3 and NiFe 2O 4 phases is successfully synthesized at heat-treatment temperature of 850 °C. It is significantly isotropic due to the nanometer scale grain size of the constituent PbTiO 3 and NiFe 2O 4 phases. A capacitance of 60–100 pF with a good frequency independence over the conventional applied frequency range, ferroelectric hysteresis loops with the applied electric field of 500 kV/cm, a saturation magnetization higher than 4.8 MT, and a coercive field of 5280 A/m with well suitable ferrimagnetic properties are obtained from the thin film with the composition ratio PbTiO 3/NiFe 2O 4 of 50/50 post heat-treated at 850 °C. The ferroelectric/ferrimagnetic composite thin film is desirable for multifunctional device application.

Novel Grain Boundary Solute Architecture in a Nanostructured Ultra-High Strength 7075 Aluminium Alloy

In recent years, the pursuit of higher strength metals and alloys has led researchers to nanometer scale grain refinement. New nanocrystalline engineering techniques have successfully increased properties for a wide range of materials. Here we report a nanocrystalline 7075 alloy processed by high-pressure torsion that exhibits ultra-high strength and features a hierarchical solute architecture. The new hierarchy of solute architecture was discovered through high-resolution characterisation using novel techniques we have developed in atom probe tomography. These new techniques – nanotexture and fine scale solute cluster measurements, are the focus of this paper. Our results indicate that nanometer-scale engineering of solid solutions could offer a pathway towards a new generation of super-strong alloys that hold promise for creating entirely new regimes of property-performance space.

Structure and thermomechanical behavior of NiTiPt shape memory alloy wires

The objective of this work is to understand the structure–property relationships in polycrystalline NiTiPt (Ti 42.7 at.% Ni 7.5 at %Pt) with a composition showing pseudoelasticity at ambient temperatures. Structural characterization of the alloy includes grain size determination and texture analysis while the thermomechanical properties are explored using tensile testing. Variation in heat treatment is used as a vehicle to modify microstructure. The results are compared to experiments on Ni-rich NiTi alloy wires (Ti–51.0 at.% Ni), which are in commercial use in various biomedical applications. With regards to microstructure, both alloys exhibit a <1 1 1> fiber texture along the wire drawing axis; however, the NiTiPt alloy grain size is smaller than that of the Ni-rich NiTi wires, while the latter materials contain second-phase precipitates. Given the nanometer-scale grain size in NiTiPt and the dispersed, nanometer-scale precipitate size in NiTi, the overall strength and ductility of the alloys are essentially identical when given appropriate heat treatments. Property differences include a much smaller stress hysteresis and smaller temperature dependence of the transformation stress for NiTiPt alloys compared to NiTi alloys. Potential benefits and implications for use in vascular stent applications are discussed.

Deformation-induced alloying and reactions in nanostructured multilayers

Deformation-induced alloying reactions in multilayers depend on the refinement of the layer structure and the atomic-scale mixing at the interfaces. Under intense deformation of multilayer samples, a nanometer-scale layer thickness and grain size develops during repeated cold rolling. As the length scale of the layer thickness converges to that of the interfacial mixing zone during rolling, alloying and amorphization can develop in appreciable volumes. The relative specific interfacial area is then a key metric to describe deformation-driven alloying. The examination of amorphous Al alloys suggests that the deformation-induced crystallization reactions could be linked to quenched-in atomic arrangements that are also evident from structural studies. Deformation-induced devitrification reactions depend on the formation of shear bands and therefore implicitly on the process conditions.

MEMS-based formaldehyde gas sensor integrated with a micro-hotplate

This paper presents a novel micro-fabricated formaldehyde gas sensor with enhanced sensitivity and detection resolution capabilities. The device comprises a quartz substrate with Pt heaters as a micro-hotplate and deposited formaldehyde-sensing layer on it. A sputtered NiO thin film is used as the formaldehyde-sensing layer. A specific orientation of NiO becomes more apparent as the substrate temperature increases in the sputtering process, which helps the formation of NiO material with a correct stoichiometric ratio. The gas sensor incorporates Pt heating resistors integrated with a micro-hotplate to provide a heating function and utilizes Au inter-digitated electrodes. When formaldehyde is present in the atmosphere, oxydation happens near the sensing layer with a high temperature caused by the micro-hotplate and causes a change in the electrical conductivity of the NiO film. Therefore, the measured resistance between the inter-digitated electrodes changes correspondingly. The application of a voltage to the Pt heaters causes the temperature of the micro-hotplate to increase, which in turn enhances the sensitivity of the sensor. The nanometer scale grain size of the sputtered oxide thin film is conducive to improving the sensitivity of the gas sensor. The experimental results indicate that the developed device has a high stability (0.23%), a low hysteresis value (0.18%), a quick response time (13.0 s), a high degree of sensitivity (0.14 Ω ppm−1), and a detection capability of less than 1.2 ppm.

Shear wave attenuation and dispersion in melt‐bearing olivine polycrystals: 2. Microstructural interpretation and seismological implications

The torsional forced oscillation tests of melt‐bearing olivine aggregates reported by Jackson et al. [2004] consistently show a peak in attenuation that is absent from melt‐free aggregates tested under similar conditions and grain sizes. Characterization by SEM shows that the melt resides in triple junction tubules and larger pockets as previously described. TEM imaging and EDS analysis reveals that olivine‐olivine grain boundaries are characterized by a region ≤1 nm wide which is structurally and chemically distinct from olivine grain interiors. From the possible mechanisms that can produce an anelastic attenuation peak, melt squirt can be eliminated for our samples and experimental conditions. We attribute the observed attenuation peak to elastically accommodated grain boundary sliding, requiring that the grain boundaries are weak relative to olivine grain interiors but have a significantly higher viscosity than bulk melt. While the nanometer scale grain boundary structure in the melt‐bearing aggregates is essentially the same as for melt‐free aggregates studied previously, elastically accommodated sliding in the latter is apparently inhibited by tight three‐grain edge intersections. The exponentially increasing high temperature background attenuation in both types of aggregate is attributed to diffusionally accommodated grain boundary sliding. Extrapolation to mantle grain sizes shows that the broad peak may be responsible for nearly frequency independent attenuation in partially molten regions of the upper mantle.

Alloying reactions in nanostructured multilayers during intense deformation

Abstract Under intense deformation of metallic multilayer samples, a nanometer-scale layer thickness and grain size develops during repeated cold-rolling. Along with the evolution of the highly refined microstructure, a nanoscale interfacial alloying occurs that can result in an amorphization reaction. The deformation of multilayers exhibits driven system behavior during alloying. As the length scale of the layer thickness converges to the length scale of the mixing zone during rolling, amorphization develops in appreciable volumes. The results from selected experiments demonstrate that the relative specific interfacial area is the key microstructural metric to describe the deformation-driven alloying.

Grain boundary effects on the magneto-transport properties of Sr 2FeMoO 6 induced by variation of the ambient H 2–Ar mixture ratio during annealing

Using the sol–gel method, polycrystalline double perovskites Sr 2FeMoO 6 with nanometer-scale grain size have been synthesized in the ambience of a controlled stream of different gaseous H 2–Ar mixtures during annealing. X-ray diffraction (XRD) results indicated that with an increasing proportion of H 2, the component of the SrMoO 4 impurity present in the crystallites first decreased, but then increased again with further increase of the H 2 proportion. The XRD results were further supported by independent sample analysis using Raman spectroscopy. Systematic investigation of the transport properties of the samples indicated that the SrMoO 4 impurity at the grain boundary played a key role in determining the measured resistance and magnetoresistance of Sr 2FeMoO 6. This impurity effect was found to be correlated to the variation of the mixture ratio of the ambient H 2–Ar gas stream surrounding the sample during annealing.