High Concentration Of Grain Boundaries Research Articles

Nanotwinned Copper Foil for “Zero Excess” Lithium–Metal Batteries

The “zero excess” lithium–metal battery cell concept, in which the pristine negative electrode consists only of the current collector, while all lithium is present only in the positive electrode active material, promises substantial improvements in energy density. However, the achievement of stable cycling for more than just a few cycles requires a careful design and adjustment of all cell components─especially the current collector at the negative electrode. Herein, we provide a comparative investigation of commercial, polycrystalline copper foil, characterized by a rather random [111], [200], and [220] orientation and a high concentration of grain boundaries, and nanotwinned copper foil, which has a solely [111]-oriented grain structure. Remarkably, the plated lithium on the foil with [111] orientation reveals a much more homogeneous morphology independent of the current density applied, while several dendritic lithium deposits are observed on the polycrystalline copper foil. This also translates into higher Coulombic efficiency and stabilized cycling performance of Cu[111]||NMC811 cells, highlighting the important impact of the crystal orientation of the current collector.

Cross-scale microstructure design of precursor-derived SiC-AlN nanoceramic composites hybrid with ex-situ ZrB2

ZrB2/SiC–AlN nanoceramic composites were densified at 1950 ​°C by hot pressing using an organic-precursor-derived SiC and commercially available AlN and ZrB2. A cross-scale microstructure was constructed by distributing the ZrB2 secondary phase (∼421 ​nm) within the SiC–AlN solid solution matrix. The substructure of the SiC–AlN matrix was agglomerated by nanograins with an average size of only 62 ​nm. ZrB2 connected around the majority of pores within the SiC–AlN matrix and contributed to the formation of numerous weak interfacial bonding, resulting in improved strength and toughness. The highest flexural strength and fracture toughness of 579 ​± ​52 ​MPa and 6.7 ​± ​0.1 ​MPa ​m1/2 were obtained from a 10 ​wt%-ZrB2/SiC–AlN sample, respectively. The high concentration of grain boundaries of the ZrB2/SiC–AlN nanoceramic composites resulted in heat insulation characteristic. The thermal diffusivity and conductivity were 3.6 ​mm2⋅s−1 and 14.3 ​W·(m·K)−1 at 1400 ​°C, respectively, while the electrical resistivity was 3.9×103 ​Ω·cm for the 10 ​wt%-ZrB2/SiC–AlN sample.

In situ ion irradiation of amorphous TiO2 nanotubes

Understanding of structural and morphological evolution in nanomaterials is critical in tailoring their functionality for applications such as energy conversion and storage. Here, we examine irradiation effects on the morphology and structure of amorphous TiO2 nanotubes in comparison with their crystalline counterpart, anatase TiO2 nanotubes, using high-resolution transmission electron microscopy (TEM), in situ ion irradiation TEM, and molecular dynamics (MD) simulations. Anatase TiO2 nanotubes exhibit morphological and structural stability under irradiation due to their high concentration of grain boundaries and surfaces as defect sinks. On the other hand, amorphous TiO2 nanotubes undergo irradiation-induced crystallization, with some tubes remaining only partially crystallized. The partially crystalline tubes bend due to internal stresses associated with densification during crystallization as suggested by MD calculations. These results present a novel irradiation-based pathway for potentially tuning structure and morphology of energy storage materials.Graphical abstract

Femtosecond laser rejuvenation of nanocrystalline metals

Nanocrystalline metals are distinct from traditional engineering materials due to their high concentration of grain boundaries and corresponding structural disorder at grain boundaries. The effect of local disorder in nanocrystalline materials manifests in ways reminiscent of fully amorphous materials, such as mesoscale shear localization and pressure-dependent yielding, owing to the high concentration of grain boundaries and their predominance in governing plasticity. Relaxation processes in nanocrystalline materials that facilitate reconfigurations of grain boundaries and lower their energy, such as low temperature annealing, have been shown to enhance mechanical strength. However, processes that raise the energy of a nanocrystalline metal have not been observed, limiting the tunability of properties and the prospect for suppressing shear localization. Here, we use femtosecond laser processing as a unique non-equilibrium process that can generate complex stress states due to ultrafast electronic excitation and subsequent relaxation events. Experiments on nanocrystalline Al-O and Cu-Zr alloys indicate that sub-ablation femtosecond laser pulses cause up to an 87% reduction in hardness with no change in grain size, which can be ascribed to grain boundary-mediated processes. Parallels between our results and rejuvenation processes in glassy systems will be discussed in the context of controlling metastable structural configurations through novel processing routes.

Nanomaterials Applications in Modern Metallurgical Processes

In recent years, improvement of metals mechanical properties becomes one of the main challenges in materials and particularly in metallurgical industry. Mostly, an alloying process is typically applied to reach metals enhanced performance. This work, however, describes a different methodology, where WC and TiC nanoparticles used as a modifiers and then gas-dynamic treatment (GDT) are applied. These processes were investigated on a hypoeutectic casting aluminum A356 alloy. Microstructural evaluation illustrated that a coarse Al grains were refined as well as eutectic Si particles were formed. Subsequent mechanical properties tests revealed that aluminum elongation enhanced while strength remained unchanged. Addition of WC and TiC enhanced the elongation by 20-60%, depends on the mold area. A combined treatment, using GDT with addition of TiCN nanoparticles showed even improvement in both, elongation and strength by 18 and 19%, respectively. Moreover, based on the electron microscopy studies, this behavior was attributed to a grain-size strengthening mechanism, where a high concentration of grain boundaries serves as dislocation movement blockers

Modification Performance of WC Nanoparticles in Aluminum and an Al-Si Casting Alloy

The influence of a modifier based on tungsten carbide (WC) nanoparticles is investigated first using 1 kg of bulk aluminum and then in a real industrial process using a commercial Al-Si casting alloy. The modifier is prepared by two different approaches, and its influence is investigated in pure aluminum and in commercial aluminum alloy A356. Microstructural studies show that the mean grain size in pure aluminum is reduced by 11.5 pct. Such a change usually causes an improvement in the mechanical properties of metals. Accordingly, the mechanical properties of the A356 alloy modified with WC nanoparticles are determined after T6 heat treatment and compared with unmodified specimens of the same alloy. The results obtained in the modified A356 alloy reveal unusual behavior of the mechanical properties, where the elongation of the alloys improved by 32 to 64 pct, while the tensile strength and yield strength remained unchanged. This behavior is attributable to a grain-size strengthening mechanism, where strengthening occurs due to the high concentration of grain boundaries, which act as obstacles to the motion of dislocations in the lattice.

Effect of the Al-doping on the electrical and thermoelectric response of TbMnO3 polycrystalline samples: Evidence of polaronic transport

High-quality, polycrystalline Tb1−xAlxMnO3 (x = 0–0.2) samples were prepared by means of standard solid state reaction. The incorporation of the Al ions into the TbMnO3 lattice is verified by X-ray diffraction measurements. The zero-field electrical conductivity, thermopower, and thermal conductivity of these samples are carefully measured and analyzed. The pristine compound shows low electrical conductivity, high and positive thermopower values (∼300 μV/K at 300 K) in the whole temperature range measured, and rather low thermal conductivity (∼5 W m−1K−1 at 300 K). Al substitution leads to a dramatic reduction in resistivity and stabilization of the ferromagnetic phase at low temperatures. The low value of the electrical conductivity constitutes a major obstacle for thermoelectric applications of this material. Nevertheless, the incorporation of Al ions at the Tb site provokes a slight reduction in the value of the thermopower and a slight increase in the thermal conductivity. For the tested samples dκ/dT > 0 (the behavior of an amorphous solid) is verified in the whole temperature range (2–390 K). Nevertheless, dκ/dT > 0 here should be related not to quenched structural disorder but rather to unusually large dynamic lattice distortions accompanying charge transport. The lower values of κ at low-temperatures (as compared to those reported for single crystals) can be linked to the high concentration of grain boundaries. The variation in both the resistivity and thermopower with temperature is well described by the polaronic model, which, nevertheless, is not linked to the stabilization of a ferromagnetic phase in the compound. Although the values of the figure of merit ended up being too small to thermoelectric applications, it is noted that the addition of Al ions improve the values of this parameters by more than one order of magnitude. The improvement of the low electrical conductivity upon Al addition is behind this technical achievement.

Monodisperse ceria nanospheres: Synthesis, characterization, optical properties, and applications in wastewater treatment

Monodisperse ceria nanospheres have been synthesized by a facile solvothermal method, and their morphology and microstructures have been revealed by a combination of X-ray diffraction (XRD), scanning electron microscopy, high-resolution transmission electron microscopy (HRTEM), Raman spectroscopy and N 2 adsorption. It is demonstrated that the as-synthesized powders are highly uniform CeO 2 in spherical shape with cubic fluorite structure. HRTEM and XRD studies show that each ceria nanosphere is composed of dozens of nanocrystals with the average size of 8.5 nm. The direct optical band gap of the ceria nanospheres estimated from the ultraviolet–visible absorption spectrum is 2.7 eV, which is evidently red-shifted with respect to the bulk material ( E g = 3.19 eV); the reduced band gap could be resulted from the high concentration of grain boundaries and defects present in the ceria nanospheres. In addition, the ceria nanospheres exhibit a strong blue luminescence at 504 nm and a broad orange luminescence centered at 645 nm. As a result of the large specific surface area, ceria nanospheres are revealed to be an excellent sorbent for the removal of poisonous pollutants present in water, such as chromium ions and rhodamine B. The removal efficiency of chromium ions is as high as ∼94%.

Novel Solution Route Synthesis of Low Thermal Conductivity Nanocrystalline Bismuth Telluride

A novel synthesis approach based on a solution route has been developed for the fabrication of nanocrystalline bismuth telluride. The method consists of dissolving both bismuth and tellurium into the same organic solvent with the assistance of complexing agents and one-step coprecipitation of bismuth telluride. The synthesized nanocrystalline bismuth telluride powders possess rhombohedral crystal structure and are nanosheet/nanorod-like with an average size of between 30 and 40 nm. The thermal conductivity of the hot-pressed compact consolidated from the as-synthesized nanopowders is 0.39-0.45 Wm(-1)K(-1) in the temperature range of 323 to 523 K, which is at most one third of that of bulk bismuth telluride-based materials reported in the literature. Such low thermal conductivity of the investigated bismuth telluride is mainly attributed to substantially high concentration of grain boundaries provided by nanostructuring to scatter phonons intensively.

Raman and conductivity studies of boron-doped microcrystalline diamond, facetted nanocrystalline diamond and cauliflower diamond films

We present a large amount of data showing how the electrical conductivity and Raman spectra of boron-doped CVD diamond films vary as a function of both B content and film type — in particular, diamond crystallite size. Three types of film have been investigated: microcrystalline diamond (MCD), faceted nanocrystalline diamond (f-NCD) and ‘cauliflower’ diamond (c-NCD). For the same B content (measured by SIMS), the conductance of MCD films was much higher than those for the two types of smaller grained films. Multi-wavelength laser Raman spectroscopy showed that Fano interference effects were much reduced for the smaller grain-sized material. The position of the Lorentzian contribution to the 500 cm − 1 Raman feature was used to estimate the B content in each type of film, and compared to the value measured using SIMS. We found that the Raman method overestimated the concentration of B by a factor of ~ 5 for the f-NCD and c-NCD films, although it remains reasonably accurate for MCD films. The shortfall may be explained if only a small fraction of the B found in the small-grained films is being incorporated into substitutional sites. We conclude that in diamond films with a high concentration of grain boundaries, the majority of the B (80% in some cases) must be present at sites that do not contribute to the continuum of electronic states that give rise to metallic conductivity and the Fano effects. Such sites may include (a) interstitials, (b) the surface of the crystallites, or (c) bonded within the non-diamond carbon impurities present at the grain boundaries. This suggests that heavy doping of nanograined diamond films will give rise to a material with many different conducting regions, and possibly different conducting pathways and mechanisms.

Boron Doping of Microcrystalline and Nanocrystalline Diamond Films: Where is the Boron Going?

AbstractWe present data showing how the electrical conductivity and Raman spectra of boron doped ‘cauliflower’-type nanocrystalline (c-NCD) CVD diamond films vary as a function of B content. The conductivity is roughly linear as a function of B content between an onset threshold of ∼5×1020 cm−3 up to ∼6×1021 cm−3, with the higher concentrations giving near metallic conductivity values. The onset threshold may be due to compensating donors due to the large number of impurities and defects in these films. The position of the Lorentzian contribution to the 500 cm−1 Raman feature was used to estimate the B content and compared to the value measured using SIMS. We found that the Raman method overestimated the concentration of B by a factor of ∼5 for these c-NCD films. The shortfall may be explained if only a small fraction of the B found in the small-grained films is being incorporated into substitutional sites. We conclude that in diamond films with a high concentration of grain boundaries, the majority of the B (80% in some cases) must be present at or in the grain boundaries.

Nanostructured Co1−xNix(Sb1−yTey)3 skutterudites: Theoretical modeling, synthesis and thermoelectric properties

The properties of Te-doped Co(Sb1−yTey)3 and Te-Ni double-doped Co1−xNix(Sb1−yTey)3 nanostructured skutterudites were evaluated by means of x-ray powder diffraction, and transport properties measured on the synthesized samples have been compared with ab initio theoretical modeling. Theoretical optimal dopant contents have been evaluated according to the maximum value of the power factor, calculating the electronic transport properties from the ab initio material band structure using semiclassical Boltzmann transport theory. The samples have been synthesized by chemical alloying with Te substitution for Sb up to 2.5at.% and Ni substitution for Co up to 2.0at.%. X-ray powder diffraction has been performed on all samples to reveal information about phase purity and Rietveld refinement was performed for the phase composition and cell parameter. The thermoelectric properties of the resulting consolidates were investigated in a temperature range from 300to723K using various measurement facilities. A standardization and round robin program was started among the participating evaluation laboratories in order to ensure reliability of the data obtained. The significant reduction in thermal conductivity, when compared to highly annealed CoSb3, could be proved which is caused by the nanostructuring, resulting in a high concentration of grain boundaries. A combination of substitution levels for Ni and Te has been found resulting in the largest ZT value of 0.65 at 680K among unfilled skutterudite materials.

The Impact of Nanostructuring on the Thermal Conductivity of Thermoelectric CoSb3

AbstractThe high concentration of grain boundaries provided by nanostructuring is expected to lower the thermal conductivity of thermoelectric materials, which favors an increase in their thermoelectric figure‐of‐merit, ZT. A novel chemical alloying method has been used for the synthesis of nanoengineered‐skutterudite CoSb3. The CoSb3 powders were annealed for different durations to obtain a set of samples with different particle sizes. The samples were then compacted into pellets by uniaxial pressing under various conditions and used for the thermoelectric characterization. The transport properties were investigated by measuring the Seebeck coefficient and the electrical and thermal conductivities in the temperature range 300 K to 650 K. A substantial reduction in the thermal conductivity of CoSb3 was observed with decreasing grain size in the nanometer region. For an average grain size of 140 nm, the thermal conductivity was reduced by almost an order of magnitude compared to that of a single crystalline or highly annealed polycrystalline material. The highest ZT value obtained was 0.17 at 611 K for a sample with an average grain size of 220 nm. The observed decrease in the thermal conductivity with decreasing grain size is quantified using a model that combines the macroscopic effective medium approaches with the concept of the Kapitza resistance. The compacted samples exhibit Kapitza resistances typical of semiconductors and comparable to those of Si–Ge alloys.

Chemical alloying and characterization of nanocrystalline bismuth telluride

A novel chemical alloying method has been developed for the fabrication of nanocrystalline thermoelectric alloy Bismuth telluride, Bi2Te3. The method consists of a combination of solution chemical method and thermal processing under controlled heating conditions. The components have been coprecipitated from a solution and the precursor consists of a solid solution of the different intermediate compounds and exhibits high reactivity. Calcination and hydrogen reduction of the precursor at moderate temperature, 350 °C, for 2 h resulted in the alloying of these elements to obtain the pure phase of the thermoelectric material in fine powder. The method is simpler than conventional melt processing and produced a 95–98% yield in laboratory scale. High concentration of grain boundaries provided by nanostructuring is expected to lower the thermal conductivity of thermoelectric material and further increase the performance.

Nanostructured Co1−xNixSb3 skutterudites: Synthesis, thermoelectric properties, and theoretical modeling

Nanostructured skutterudite Co1−xNixSb3 has been synthesized by chemical alloying with Ni substitution for Co up to 27.5 at. %. High concentration of grain boundaries provided by nanostructuring is expected to lower the thermal conductivity of thermoelectric materials and could thus increase their thermoelectric dimensionless figure-of-merit ZT. Material preparation comprises two key stages, coprecipitation of the precursor, and thermal processing to produce the skutterudite. By modeling the chemistry of the metal ions in aqueous solution, optimum coprecipitation conditions were achieved. The precipitated precursor consists of a solid solution of the different intermediate compounds, and exhibits high reactivity. Calcination and reduction of the precursor resulted in the alloying of these elements and in the formation of skutterudite at a temperature as low as 723–773 K. Unfilled CoSb3 skutterudites were prepared by chemical precipitation from aqueous solutions to achieve powders with a very small grain size (∼40 nm). Compacted samples were produced from this powder by uniaxial pressing under various conditions. Thermal conductivity, electrical resistivity and Seebeck coefficient of the resulting compacts were measured in a temperature range from 150 to 575 K. Measurement procedures were standardized for absolute accuracy and reproducibility between the DLR, Cologne and NEDO, Cardiff Laboratories. It was found that the thermal conductivity can be decreased by almost an order of magnitude at the highest concentration of grain boundaries compared to highly annealed CoSb3. Scanning Seebeck microthermoprobe examination, facilitated spatially resolved measurement of Seebeck coefficient S, providing information on samples’ homogeneity and on its effect on local S. Indications on the formation of an additional Ni containing phase were found. The electronic structure of Ni-doped skutterudites has been investigated by means of fully periodic density functional theory calculations and a topological analysis of the resulting electron densities. Ni substitution for Co doubles the electronic charge transfer from the pnicogen ring to the metal frame and fills the region of the CoSb3 band gap with occupied states, thus explaining the increase of electrical conductivity observed experimentally. The effect of the Ni substitution on the thermal conductivity is analyzed. The computed changes of the cell parameter with rising Ni content differ with those found by x-ray powder diffraction, thereby suggesting that the structural hypothesis of Ni replacing Co in the cubic frame of the skutterudite is only approximate or possibly incorrect.

ChemInform Abstract: POLYCRYSTALLINE SILICON ON COATED STEEL SUBSTRATES

AbstractDie Arbeit faßt die experimentellen Verfahren für die Abscheidung von Si aus SiH4 auf Stahlsubstraten und die Eigenschaften der erhaltenen Si‐Schichten zusammen.

Polycrystalline Silicon on Coated Steel Substrates

Steel is the most economical substrate for the deposition of silicon. At temperatures used for the chemical vapor deposition of silicon, however, a barrier layer must be used to prevent the diffusion of iron from the substrate into the silicon layer. Tungsten was found to be ineffective as a diffusion barrier when silicon was deposited by the thermal decomposition of silane at 900°C and above. Borosilicate deposited by the oxidation of a silane‐diborane mixture was found to be an effective barrier at temperatures up to 1150°C. Silicon deposited on borosilicate/steel substrates is polycrystalline. The microstructure and crystallographic properties of silicon depend strongly on the substrate temperature, deposition rate, and extent of doping. Silicon layers deposited at low temperatures and high rates consist of small crystallites with a strong preferred {110} orientation, while those deposited at high temperatures and low rates consist of larger crystallites with more random orientation. Silicon p‐n junctions deposited on borosilicate/steel substrates show poor electrical characteristics because of the high concentration of grain boundaries, and solar cells have low conversion efficiencies.