Showing papers on "Grain growth published in 2021"

Identifying the origin of the Voc deficit of kesterite solar cells from the two grain growth mechanisms induced by Sn2+ and Sn4+ precursors in DMSO solution

Abstract: Kesterite Cu2ZnSn(S,Se)4 solar cells fabricated from DMSO molecular solutions exhibit very different open circuit voltage (Voc) when the tin precursor has a different oxidation state (Sn2+vs. Sn4+). Here, the grain growth mechanism of the two absorbers was used as a platform to investigate the large voltage deficit issue that limits kesterite solar cell efficiency. The secondary sulfide composed Sn2+ precursor film took a multi-step phase fusion reaction path with secondary SnSe2 existing on the film surface during the whole grain growth, which forms in a very defective surface whereas a uniform kesterite structured Sn4+ precursor film took a direct transformation reaction path along with a top down and bottom up bi-direction grain growth that forms a uniform and less defective surface. Characterizations show that both absorber films exhibit similar bulk electronic properties with comparable band and potential fluctuations, Cu–Zn disorder level and tail states, and the much lower Voc of the Sn2+ device than the Sn4+ device primarily comes from the serious recombination near the junction as revealed by the large ideality factor and reverse saturation current. Our results demonstrate that the large Voc deficit of the kesterite solar cell mainly comes from surface deep defects that originated from the multi-phase fusion grain growth mechanism. The high efficiency (>12%) and low Voc deficit (<300 mV) of Sn4+ processed CZTSSe solar cells highlight that direct phase transformation grain growth is a new strategy to fabricate high quality kesterite absorbers, which can also be applied to other multi-element thin film semiconducting materials.

High strain rate superplasticity via friction stir processing (FSP): A review

Abstract: High strain rate superplasticity by the friction stir processing (FSP), an adaptation of the friction stir welding (FSW), is summarized in this overview article. As a common severe plastic deformation (SPD) processing technique, the microstructures prepared by FSP are characterized by fine grain sizes, being homogeneous with fragmented and dispersed particles, and having a high proportion of high-angle grain boundaries. These attributes are beneficial to the superplastic forming operations at high strain rates and low temperatures. In this monograph, the principles of superplasticity are reviewed, where the importance of grain boundary sliding (GBS), strain rate sensitivity index, grain refinement, and deformation temperature on the remarkable enhancement of ductility is emphasized. Afterwards, FSP is introduced and the effects of the main processing parameters on the heat input and grain size are critically discussed. Finally, the recent progress in the application of FSP for processing of superplastic materials is thoroughly overviewed and the influence of thermal stability against grain growth, addition of alloying elements to form pinning particles, external cooling for obtaining ultrafine grained (UFG) microstructure, FSP process variables such as tool rotation rate and traverse speed, and multi-pass FSP is summarized. Accordingly, this overview presents the opportunities that FSP can offer for controlling the superplastic behavior of materials.

Facile route to bulk ultrafine-grain steels for high strength and ductility

Abstract: Steels with sub-micrometre grain sizes usually possess high toughness and strength, which makes them promising for lightweighting technologies and energy-saving strategies. So far, the industrial fabrication of ultrafine-grained (UFG) alloys, which generally relies on the manipulation of diffusional phase transformation, has been limited to steels with austenite-to-ferrite transformation1–3. Moreover, the limited work hardening and uniform elongation of these UFG steels1,4,5 hinder their widespread application. Here we report the facile mass production of UFG structures in a typical Fe–22Mn–0.6C twinning-induced plasticity steel by minor Cu alloying and manipulation of the recrystallization process through the intragranular nanoprecipitation (within 30 seconds) of a coherent disordered Cu-rich phase. The rapid and copious nanoprecipitation not only prevents the growth of the freshly recrystallized sub-micrometre grains but also enhances the thermal stability of the obtained UFG structure through the Zener pinning mechanism6. Moreover, owing to their full coherency and disordered nature, the precipitates exhibit weak interactions with dislocations under loading. This approach enables the preparation of a fully recrystallized UFG structure with a grain size of 800 ± 400 nanometres without the introduction of detrimental lattice defects such as brittle particles and segregated boundaries. Compared with the steel to which no Cu was added, the yield strength of the UFG structure was doubled to around 710 megapascals, with a uniform ductility of 45 per cent and a tensile strength of around 2,000 megapascals. This grain-refinement concept should be extendable to other alloy systems, and the manufacturing processes can be readily applied to existing industrial production lines. Bulk ultrafine-grained steel is prepared by an approach that involves the rapid production of coherent, disordered nanoprecipitates, which restrict grain growth but do not interfere with twinning or dislocation motion, resulting in high strength and ductility.

Microstructure and mechanical properties of a TiB2-modified Al–Cu alloy processed by laser powder-bed fusion

Abstract: In this work, crack-free samples with a relative density of 99.5 ± 0.1% were produced from a gas-atomized Al–Cu–Ag–Mg–Ti–TiB2 powder via laser powder-bed fusion. The homogeneous equiaxed microstructure without preferred grain orientation shows the α-Al grains’ mean size to be 0.64 μm ± 0.26 μm TiB2 particles with sizes of several tens of nm up to 1.5 μm were observed in the as-built component. Small TiB2 particles of up to approx. 200 nm are located within the α-Al grains, which show a semi-coherent interface to the α-Al phase. Larger TiB2 particles of up to 1.5 μm accumulate in the liquid between the growing α-Al grains during solidification and inhibit grain growth. Al2Cu phase is precipitated at the α-Al grain boundaries. Coarse Al2Cu precipitates, which are slightly enriched with silver and magnesium, are also observed within the grains preferentially precipitated on small pores and TiB2 particles. The novel fine-grained microstructure results in the as-built state in a tensile strength of 401 ± 2 MPa and total elongation at fracture of 17.7 ± 0.8%.

Influence of Sintering Temperature on Microstructure and Mechanical Properties of Ti–Mo–B 4 C Composites

Abstract: Ti–10 wt% Mo–1 wt% B4C composite samples were SPSed under the circumstances of 5 min dwell time, 50 MPa external pressure and sintering temperatures of 1150 °C, 1300 °C and 1450 °C. The role of sintering temperature on the relative density, microstructure and mechanical characteristics of as-sintered specimens were studied. Near fully dense relative density was obtained for the samples sintered at 1450 °C. The best mechanical properties including UTS, elongation, bending strength and micro/macro hardness were achieved for the composites SPSed at the highest temperature. The XRD results and also microscopic photographs disclosed the formation of TiB + TiC in-situ phases. However, there was not any evidence for chemical reaction between Mo and other phases. The role of produced in-situ phases on the grain growth also studied using SEM fractographs.

Rare earth improves strength and creep resistance of additively manufactured Zn implants

Abstract: Poor mechanical strength and creep resistance limit the orthopedic application of biodegradable Zinc (Zn). In present work, cerium (Ce) was alloyed with Zn using laser additive manufacturing technique. As one kind of rare earth element, Ce possessed high surface activity, which effectively interrupted the grain growth and caused the formation of stable intermetallics, thus contributing to grain refinement strengthening and precipitate strengthening. More significantly, Ce alloying activated more pyramidal slip by means of reducing the critical resolved shear stress during plastic deformation, and resultantly formed the sessile dislocations, which caused the accumulated strain hardening and improved the creep resistance. As a result, Zn-Ce alloy exhibited a considerably improved ultimate tensile strength of 247.4 ± 7.2 MPa, and a reduced creep rate of 1.68 × 10−7 s−1. Moreover, it exhibited strong antibacterial activity, as well as favorable cytocompatibility and hemocompatibility. All these results demonstrated the great potential of Zn-Ce alloy as a candidate for bone repair application.

Enhancement of densification and microwave dielectric properties in LiF ceramics via a cold sintering and post-annealing process

Abstract: LiF is a low-firing fluoride with excellent microwave dielectric properties, however densification of LiF ceramics is challenging owing to their low surface free energy. In this study, a cold sintering process (150 °C, 250 MPa, 60 min) was employed to pre-densify LiF ceramics to 78 % relative density. Post-annealing treatments between 650 °C and 800 °C led to significant grain growth which was accompanied by an increase in relative density to 92 %. The microwave quality factor (Qf) increased with increasing annealing temperature to a maximum of 110,800 GHz at 800 °C, 1.5 times higher than the value obtained via conventional sintering (78,800 GHz), with relative permittivity er = 8.2 and temperature coefficient of resonant frequency, τf =–135 ppm/°C. Such high values of Qf and its compatibility with Ag electrode suggest that cold sintered LiF has great potential as a component or additive in low temperature co-fired ceramic formulations.

On the microstructure and solidification behavior of new generation additively manufactured Al-Cu-Mg-Ag-Ti-B alloys

Abstract: Laser powder bed fusion (LPBF) was used to horizontally print A205 ( Al — Cu — Mg — Ag — Ti — B ) metal powder. The samples have been cut in various directions to analyze their phases, grain morphology, size, crystallographic orientation, and distribution of elements via X-ray diffraction (XRD), electron backscatter diffraction (EBSD), and X-ray energy dispersive spectroscopy (XEDS) in a transmission electron microscope (TEM). To monitor deformation behavior and strain distribution, several uniaxial tensile tests accompanied by digital image correlation (DIC) technique were carried out. The results showed that the as-built microstructure fully contains fine equiaxed grains with no strong preferential crystallographic orientation. The XEDS elemental maps confirmed the solute trapping phenomenon taking place during the LPBF. The stress-strain curves obtained by the uniaxial tensile testing revealed a quasi-isotropic behavior in terms of building direction; moreover, the DIC strain contour maps confirmed yield-point phenomenon and Luders bands propagation during uniaxial tensile testing. Finally, a time/temperature interdependence grain growth model was successfully applied based on no diffusion in solid and partial mixing in liquid through the entire melt pool.

Defect Engineering in Solution-Processed Polycrystalline SnSe Leads to High Thermoelectric Performance.

Abstract: SnSe has emerged as one of the most promising materials for thermoelectric energy conversion due to its extraordinary performance in its single-crystal form and its low-cost constituent elements. However, to achieve an economic impact, the polycrystalline counterpart needs to replicate the performance of the single crystal. Herein, we optimize the thermoelectric performance of polycrystalline SnSe produced by consolidating solution-processed and surface-engineered SnSe particles. In particular, the SnSe particles are coated with CdSe molecular complexes that crystallize during the sintering process, forming CdSe nanoparticles. The presence of CdSe nanoparticles inhibits SnSe grain growth during the consolidation step due to Zener pinning, yielding a material with a high density of grain boundaries. Moreover, the resulting SnSe-CdSe nanocomposites present a large number of defects at different length scales, which significantly reduce the thermal conductivity. The produced SnSe-CdSe nanocomposites exhibit thermoelectric figures of merit up to 2.2 at 786 K, which is among the highest reported for solution-processed SnSe.

Spark plasma sintering of B4C and B4C-TiB2 composites: Deformation and failure mechanisms under quasistatic and dynamic loading

Abstract: Spark plasma sintering (SPS) was employed to consolidate powder specimens consisting of B4C and various B4C-TiB2 compositions. SPS allowed for consolidation of pure B4C, B4C-13 vol.%TiB2, and B4C-23 vol.%TiB2 composites achieving ≥99 % theoretical density without sintering additives, residual phases (e.g., graphite), and excessive grain growth due to long sintering times. Electron and x-ray microscopies determined homogeneous microstructures along with excellent distribution of TiB2 phase in both small and larger-scaled composites. An optimized B4C-23 vol.%TiB2 composite with a targeted low density of ∼3.0 g/cm3 exhibited 30–35 % increased hardness, fracture toughness, and flexural bend strength compared to several commercial armor-grade ceramics, with the flexural strength being strain rate insensitive under quasistatic and dynamic loading. Mechanistic studies determined that the improvements are a result of a) no residual graphitic carbon in the composites, b) interfacial microcrack toughening due to thermal expansion coefficient differences placing the B4C matrix in compression and TiB2 phase in tension, and c) TiB2 phase aids in crack deflection thereby increasing the amount of intergranular fracture. Collectively, the addition of TiB2 serves as a toughening and strengthening phase, and scaling of SPS samples show promise for the manufacture of ceramic composites for body armor.

Effect of starting materials and sintering temperature on microstructure and optical properties of Y 2 O 3 :Yb 3+ 5 at% transparent ceramics

Abstract: Y2O3:Yb3+ 5 at% ceramics have been synthesized by the reactive sintering method using different commercial yttria powders (Alfa-Micro, Alfa-Nano, and ITO-V) as raw materials. It has been shown that all Y2O3 starting powders consist from agglomerates up to 5–7 µm in size which are formed from 25–60 nm primary particles. High-energy ball milling allows to significantly decreasing the median particle size D50 below 500 nm regardless of the commercial powders used. Sintering experiments indicate that powder mixtures fabricated from Alfa-Nano yttria powders have the highest sintering activity, while (Y0.86La0.09Yb0.05)2O3 ceramics sintered at 1750 °C for 10 h are characterized by the highest transmittance of about 45%. Y2O3:Yb3+ ceramics have been obtained by the reactive sintering at 1750–1825°C using Alfa-Nano Y2O3 powders and La2O3+ZrO2 as a complex sintering aid. The effects of the sintering temperature on densification processes, microstructure, and optical properties of Y2O3:Yb3+ 5 at% ceramics have been studied. It has been shown that Zr4+ ions decrease the grain growth of Y2O3:Yb3+ ceramics for sintering temperatures 1750–1775 °C. Further increasing the sintering temperature was accompanied by a sharp increase of the average grain size of ceramics referred to changes of structure and chemical composition of grain boundaries, as well as their mobility. It has been determined that the optimal sintering temperature to produce high-dense yttria ceramics with transmittance of 79%–83% and average grain size of 8 µm is 1800 °C. Finally, laser emission at ∼1030.7 nm with a slope efficiency of 10% was obtained with the most transparent Y2O3:Yb3+ 5 at% ceramics sintered.

Ligand-Driven Grain Engineering of High Mobility Two-Dimensional Perovskite Thin-Film Transistors.

Abstract: Controlling grain growth is of great importance in maximizing the charge carrier transport for polycrystalline thin-film electronic devices. The thin-film growth of halide perovskite materials has been manipulated via a number of approaches including solvent engineering, composition engineering, and post-treatment processes. However, none of these methods lead to large-scale atomically flat thin films with extremely large grain size and high charge carrier mobility. Here, we demonstrate a novel π-conjugated ligand design approach for controlling the thin-film nucleation and growth kinetics in two-dimensional (2D) halide perovskites. By extending the π-conjugation and increasing the planarity of the semiconducting ligand, nucleation density can be decreased by more than 5 orders of magnitude. As a result, wafer-scale 2D perovskite thin films with highly ordered crystalline structures and extremely large grain size are readily obtained. We demonstrate high-performance field-effect transistors with hole mobility approaching 10 cm2 V-1 s-1 with ON/OFF current ratios of ∼106 and excellent stability and reproducibility. Our modeling analysis further confirms the origin of enhanced charge transport and field and temperature dependence of the observed mobility, which allows for clear deciphering of the structure-property relationships in these nascent 2D semiconductor systems.

Effect of Nb content on the phase composition, densification, microstructure, and mechanical properties of high-entropy boride ceramics

Abstract: Dense high-entropy (Hf,Zr,Ti,Ta,Nb)B2 ceramics with Nb contents ranging from 0 to 20 at% were produced by a two-step spark plasma sintering process. X-ray diffraction indicated that a single-phase with hexagonal structure was detected in the composition without Nb. In contrast, two phases with the same hexagonal structure, but slightly different lattice parameters were present in compositions containing Nb. The addition of Nb resulted in the presence of a Nb-rich second phase and the amount of the second phase increased as the Nb content increased. The relative densities were all >99.5 %, but decreased from ∼100 % to ∼99.5 % as the Nb content increased from 0 to 20 at%. The average grain size decreased from 13.9 ± 5.5 μm for the composition without Nb additions to 5.2 ± 2.0 μm for the composition containing 20 at% Nb. The reduction of grain size with increasing Nb content was due to the suppression of grain growth by the Nb-rich second phase. The addition of Nb increased Young’s modulus and Vickers hardness, but decreased shear modulus. While some Nb dissolved into the main phase, a Nb-rich second phase was formed in all Nb-containing compositions.

Breakthrough the strength-ductility trade-off in a high-entropy alloy at room temperature via cold rolling and annealing

Abstract: Simultaneous strength–ductility enhancement of a high-entropy alloy via cold rolling and annealing is difficult. Cold rolling and annealing were performed on an as-cast Fe35Co21Ni6Cr18Mn20 high entropy alloy (HEA), the microstructure showed single phase face centered cubic (fcc) solid solution without phase transformation and the grains were fully recrystallized accompanied by additional grain growth. The grain size decreased significantly from 179.4 ± 15.3 μm to 15.2 ± 1.4 μm. A large number of annealing twins and twin boundaries (TBs) (fraction up to 44.2%) were observed. The dislocation slip and twinning were observed as the dominant plastic deformation mechanism. Meanwhile, dislocations plugging was also seen near the twin boundaries. Tensile tests revealed for the first time that the rolling-annealing HEA overcame the strength-ductility trade-off because the yield strength dramatically increased from 516.7 ± 3.4 MPa to 725.4 ± 4.7 MPa and ductility retained excellent (from 0.51 ± 0.01 to 0.53 ± 0.01) as compared with as-cast HEA, this behavior was attributed to the grain refinement and substantial annealing twins including transgranular annealing twins and suspened annealing twins.