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At least 1,387 records · Page 77
All sourcesNASA NTRSDOE OSTI
DOE OSTI2015

Thermal Microstructural Stability of AZ31 Magnesium after Severe Plastic Deformation

Both equal channel angular pressing and friction stir processing have the ability to refine the grain size of twin roll cast AZ31 magnesium and potentially improve its superplastic properties. This work used isochronal and isothermal heat treatments to investigate the microstructural stability of twin roll cast, equal channel angular pressed and friction stir processed AZ31 magnesium. For both heat treatment conditions, it was found that the twin roll casted and equal channel angular pressed materials were more stable than the friction stir processed material. Calculations of the grain growth kinetics showed that severe plastic deformation processing decreased the activation energy for grain boundary motion with the equal channel angular pressed material having the greatest Q value of the severely plastically deformed materials and that increasing the tool travel speed of the friction stir processed material improved microstructural stability. The Hollomon-Jaffe parameter was found to be an accurate means of identifying the annealing conditions that will result in substantial grain growth and loss of potential superplastic properties in the severely plastically deformed materials. In addition, Humphreys’s model of cellular microstructural stability accurately predicted the relative microstructural stability of the severely plastically deformed materials and with some modification, closely predicted the maximum grain size ratio achieved by the severely plastically deformed materials.

FSW↗
DOE OSTI2016

Microstructure characterization of LAE442 magnesium alloy processed by extrusion and ECAP

The magnesium alloy LAE442 was processed by extrusion and equal channel angular pressing (ECAP) to achieve ultrafine grained microstructure. Detailed characterization of the microstructure was performed by scanning electron microscope, electron back scattered diffraction (EBSD) and transmission electron microscope. The initial, as-cast, microstructure consisted of large grains of ~ 1 mm. The grain refinement due to the processing by severe plastic deformation led to a decrease of the average grain size to ~ 1.7 μm after the final step of ECAP. A detailed characterization of secondary phases showed the precipitation of Al{sub 11}RE{sub 3}, Al{sub 2}Ca and Al{sub 10}RE{sub 2}Mn{sub 7} intermetallic phases. X-ray diffraction measurements proved that Li is dissolved within the magnesium matrix in the as-cast condition. Newly formed Al{sub 3}Li phase was observed after ECAP. The texture formation due to the extrusion and ECAP was different from that in the other magnesium alloys due to the activation of non-basal slip systems as a result of the decrease of the c/a ratio. - Highlights: • Combined extrusion and equal channel angular pressing results in significant grain refinement by factor 1000 approximately. • Al{sub 11}RE{sub 3}, Al{sub 2}Ca and Al{sub 10}RE{sub 2}Mn{sub 7} secondary phases are present in the as-cast material while Li was dissolved in the Mg matrix. • Extrusion and ECAP have no effect on the composition of the secondary phases but they influence strongly their distribution. • Texture evolution is affected by decrease of c/a ratio due to the presence of Li and resulting activation of non-basal slip.

36 MATERIALS SCIENCE↗
DOE OSTI2017

Investigation of nanostructured Al-10 wt.% Zr material prepared by ball milling for high temperature applications

Ground chips of as-cast Al-10 wt.% Zr alloy were subjected to mechanical alloying (MA) with 5 vol.% of nanodiamond addition in a high energy planetary ball-mill. The aim of this work was to investigate the microstructure, phase transformation and mechanical properties of the material both after MA and after subsequent annealing. Optical and transmission electron microscopes were used for morphological and microstructural analysis. The effect of milling time on powder microhardness, Al lattice parameter, lattice microstrain and crystallite size was determined. It was shown that mechanical alloying of as-cast Al-10wt.%Zr alloy during 20 h leads to a complete dissolution of the primary tetragonal Al{sub 3}Zr crystals in aluminum. At the same time, the powder microhardness increases to 370 HV. Metastable cubic Al{sub 3}Zr phase nanoparticles precipitate from the Al solution due to its decomposition after annealing, however, the Al solid solution remains supersaturated and nanocrystalline. Compression tests at room temperature and at 300 °C showed that the strength values of the hot-pressed samples reach 822 MPa and 344 MPa, respectively. - Highlights: •As-cast Al-10 wt.% Zr alloy was mechanically alloyed with 5 vol.% nanodiamond. •The primary tetragonal Al{sub 3}Zr crystals were completely dissolved in Al after 20 h. •Cubic Al{sub 3}Zr phase nanoparticles precipitated from Al solution after aging. •The aged bulk material showed a high strength at room and elevated temperatures.

36 MATERIALS SCIENCE↗
DOE OSTI2018

Precipitation behavior of L12 Al3Zr phase in Al-Mg-Zr alloy

Highlights: • Precipitation behavior of the as-cast Al-3.0Mg-0.25Zr alloy was studied. • Discontinuous and continuous precipitation mechanisms of Al{sub 3}Zr phase are observed. • Precipitation type changes from discontinuous to continuous at two-step annealing. • Developed treatment mode provides high dispersoids dense and maximum aging effect. - Abstract: Increasing the recrystallization resistance and the mechanical properties of aluminum-based alloys is possible due to the formation of a nanoscale L1{sub 2}-structured Al{sub 3}Zr phase. Treatment conditions and alloys composition affect the size and density of dispersoids and their final properties. In this work we analyze the decomposition of the supersaturated solid solution in the as-cast Al-3%Mg-0.25%Zr alloy for different annealing modes to understand the precipitation kinetics of the Al{sub 3}Zr phase in the presence of Mg. We found that both discontinuous and continuous precipitation mechanisms of the Al{sub 3}Zr phase are possible in the studied low-alloyed material. One-step annealing leads to the formation of coarse (17 nm) spherical precipitates of a coherent L1{sub 2}-structured Al{sub 3}Zr phase and discontinuously formed fan-shaped aggregations of the same phase. Two-step annealing provided for the maximum precipitation hardening with the formation of high-density nanoscale (7 nm) dispersoids of the Al{sub 3}Zr phase. This study highlights the importance of the annealing mode of the as-cast material for achieving a high density of the fine L1{sub 2} structured Al{sub 3}Zr phase and the maximum hardening effect.

36 MATERIALS SCIENCE↗
DOE OSTI2018

Microstructure evolution and properties of graphene nanoplatelets reinforced aluminum matrix composites

Highlights: • Lamellar structure of GNPs fibers and Al matrix was obtained in the rolling direction. • Stacking faults were observed in Al grains due to greatly increased GNPs/Al interfaces. • Strength of Al-0.2 wt% GNPs increased 36.8% with negligible electrical conductivity loss. - Abstract: Aluminum matrix composites reinforced with graphene nanoplatelets (GNPs) were prepared by continuous casting and subsequent rolling. Al-GNPs master alloy, prepared by ball milling and cold pressing, was added into Al melts to fabricate the composites. Microstructure evolution of the composites were observed using scanning electron microscopy (SEM), transmission electron microscopy (TEM) and electron back scattered diffraction (EBSD). GNPs distributed uniformly in the as-cast specimen and then transformed into fibers in the rolling direction after deformation and finally a lamellar structure formed in the composites. Raman spectra showed that the structural damage of GNPs mainly came from ball milling. The interface between GNPs and Al was well combined in general while a few microcracks were observed, which reduced the ductility of the composites and no adverse aluminum carbide (Al{sub 4}C{sub 3}) was detected at the interface. Stacking faults were observed interior of the aluminum grain, which may due to the existence of plentiful interface introduced by GNPs. Fracture observation revealed that the load transferred from Al matrix to GNPs. The ultimate tensile strength of Al-0.2 wt% GNPs composites was about 36.8% higher than that of pure Al with the same casting and rolling process, which should result from the lamellar structure and load transfer, while the conductivity of the composites decreased slightly, indicating that interface scattering between Al and GNPs is very limited. The investigation results show that Al-GNPs composite is potential for high strength and high conductivity application.

77 NANOSCIENCE AND NANOTECHNOLOGY↗
DOE OSTI2018

Non-destructive compositional and microstructural characterization of Sardinian Bronze Age swords through Neutron Diffraction

Highlights: • Determination of the manufacturing technique of some “Sa Idda” swords • New data on their provenience • Determination of: alloy composition, dendritic segregation, mineralization phase - Abstract: The Monte Sa Idda type swords belong to a well characterized typological class, whose origin has long been debated and seems to coincide with the Iberian Peninsula. The name comes from the site of the first discovery of a hoard containing this type of swords, among other bronze artefacts, namely Monte Sa Idda, Decimoputzu-Cagliari, Italy. The swords were produced through fusion inside a casting mould. Such kind of casting mould has not been found in Sardinia, and this may suggests that finished objects were either imported or made using clay or sand casting mould. Moreover, the discovery of these types of sword, in hoards and in votive deposits, raises the problem of their functional use. It is unknown if the blades were intended to be used as weapons, and therefore subjected to cold working necessary for thinning and sharpening of the blade, or as ceremonial objects. In order to answer to these questions, a neutron diffraction experiment was performed on three swords belonging to the Monte Sa Idda type. The results obtained are presented and discussed in the paper.

36 MATERIALS SCIENCE↗
DOE OSTI2018

Refinement effect of two rare earth borides in an Al-7Si-4Cu alloy: A comparative study

Highlights: • Refinement performance of Al-La-B and Al-Ce-B refiners in Al-7Si-4Cu alloy was investigated. Due to the orientation relationships between rare earth borides and Al, CeB{sub 6} and LaB{sub 6} can act as the nucleation substrate of α-Al and refine Al-Si-Cu casting alloy effectively. • Al-Ce-B refiner showed better refining efficacy than Al-La-B refiner because of the lower lattice mismatch and less angel difference of the certain orientation relationship between CeB{sub 6} and Al, demonstrated by TEM analysis. • Effects of Al-La-B and Al-Ce-B refiners on alloy defects and tensile properties were investigated. The best refiner addition level in this alloy is considered to be 1.0wt%. Tensile properties of Al-7Si-4Cu alloy were prominently improved owing to the better refined microstructures and much fewer shrinkage defects. - Abstract: In this paper, refinement performance of Al-La-B and Al-Ce-B refiners in Al-7Si-4Cu alloy and their effects on alloy defects and tensile properties were investigated. Due to the orientation relationships between rare earth borides and Al, CeB{sub 6} and LaB{sub 6} can act as the nucleation substrate of α-Al and refine Al-Si-Cu casting alloy effectively. Al-Ce-B refiner showed better refining efficacy than Al-La-B refiner because of the lower lattice mismatch and less angel difference of the certain orientation relationship between CeB{sub 6} and Al, demonstrated by TEM analysis. Tensile properties of Al-7Si-4Cu alloy were prominently improved from 204.4 ± 5.9 MPa to 250.6 ± 6.7 MPa in as-cast state and 335.7 ± 6.5 MPa to 386.0 ± 7.0 MPa in heat treatment state owing to the better refined microstructures and much fewer shrinkage defects. However, when the addition level of the refiners was higher than 1.0 wt%, the amount of gas pores in the alloy significantly increased. Hence, the best refiner addition level in this alloy is considered to be 1.0 wt%.

36 MATERIALS SCIENCE↗
DOE OSTI2024

Ionic liquid mediated one-step perpendicular assembly in block copolymer thin films for ultrafiltration membrane applications

The rapid rise of oil and gas, petrochemical, food processing, and pharmaceutical industries has added a complex mixture of contaminants like oil, particulates, metals, and other organic compounds to wastewater. Multipurpose membranes with precise pore structure can offer a solution to this problem and block copolymers (BCP) can be used to achieve such structures. Achieving vertical assembly of BCP domains provides a perfect template for developing uniform through film channels for separation and transport of material, besides being useful for the rectification of defects in photolithography patterns. Producing such morphologies may require extensive film/substrate processing and is not always feasible for scale-up. With this article, we demonstrate a facile solution casting method to induce vertical domain assembly in as-cast diblock copolymer thin films in the presence of an ionic liquid (IL) additive that preferentially segregates to one block and neutralizes interfacial interactions. We also show the tunability of domain sizes by controlling the additive concentration. These vertically aligned morphologies are important for the development of next-generation lithographic techniques and form excellent templates for ultrafiltration membranes with uniform pore sizes. In conclusion, this article demonstrates how IL additives can be used to obtain stable non-equilibrium morphologies in as-cast BCP films and the effect of BCP molecular mass, block volume fractions, and IL content on self-assembly.

59 BASIC BIOLOGICAL SCIENCES↗
DOE OSTI2020

Effect of temperature on the fatigue cracking mechanisms in A356 Al alloy

This work builds on our previous investigation of the room temperature fatigue cracking mechanisms of an A356 Al alloy. Here, we analyze the elevated temperature fatigue cracking mechanisms in cast and friction stir processed (FSP) A356, and contrast them with the room temperature behavior. Two sets of FSP parameters were used to modify the microstructure of the cast alloy. After heat treatment, both the FSPed microstructures exhibited severe abnormal grain growth (AGG) leading to a very wide grain size distribution (from a few microns to a few millimeters). During room temperature fatigue tests, the FSP conditions exhibited significant improvements in fatigue lives, even up to an order of magnitude. However, with increasing temperature, the difference in the performance of the FSPed and Cast microstructures decreased until, at 200 °C, all three microstructural conditions exhibited similar fatigue response. Detailed electron back scattered diffraction (EBSD) imaging of failed fatigue specimens was used to study crack behavior. At room temperature, cracks initiated at defect sites or along persistent slip bands (PSBs) and propagated transgranularly. The grain boundaries acted as barriers to crack growth. At elevated temperatures, there was a complete role reversal of the grain boundaries. Cracks initiated along grain boundaries and propagated intergranularly in all microstructures. Below, we explore this transition of the cracking mechanism in detail.

Nelaturu, Plalgun↗
DOE OSTI2017

Improved technique for preparation of deuterated-polyethylene targets

An improved method for preparation of self-supporting deuterated polyethylene targets using solvent casting technique is described. The improved method provides recommendations for solvent heating, solvent casting of the thin film, and subsequent heating treatment which is shown to greatly enhance the overall target strength and durability as needed for accelerator experiments. A detailed characterization using both optical and electron microscopy, differential scanning calorimetry, and Fourier transform infrared spectroscopy demonstrates the improvements using the post-casting heating treatment. The effects of heavy ion-induced target degradation and recommendations for use under these conditions is also discussed.

46 INSTRUMENTATION RELATED TO NUCLEAR SCIENCE AND ↗
DOE OSTI2024

Tribological behavior of hybrid Aluminum-TiB 2 metal matrix composites for brake rotor applications

Here, this research investigates the feasibility of hybrid aluminum metal matrix composites (MMC) incorporating TiB 2 particles for brake rotor applications. The composites were produced by incorporating both in-situ, submicron-sized TiB 2 particles and regular micron-sized TiB 2 powders via stir and squeeze casting techniques into A206 aluminum alloy matrix. Systematic adjustments in the fractions of in-situ and ex-situ TiB 2 particles were conducted to evaluate their impact on wear behavior and mechanisms. Combination of both particle types allowed composites with up to 10 vol% of reinforcements. Composites with higher proportions of ex-situ particles demonstrated increased wear resistance compared to those solely composed of in-situ particles, control specimens without TiB 2 , and conventional cast iron counterparts. The lowest wear rate for the hybrid composites sliding against phenolic brake pads was 1.1 × 10 -5 mm 3 /Nm, signifying a 3-fold reduction relative to cast iron sliding against the same pads. Wear analysis elucidated distinctive mechanisms within the hybrid composites, characterized by mild fragmented abrasive wear, adhesive wear, and plastic deformation, accompanied by the formation of an intermixed tribo-oxide layer.

42 ENGINEERING↗
DOE OSTI2012

Electron Backscatter Diffraction (EBSD) Characterization of Uranium and Uranium Alloys

Electron backscatter diffraction (EBSD) was used to examine the microstructures of unalloyed uranium, U-6Nb, U-10Mo, and U-0.75Ti. For unalloyed uranium, we used EBSD to examine the effects of various processes on microstructures including casting, rolling and forming, recrystallization, welding, and quasi-static and shock deformation. For U-6Nb we used EBSD to examine the microstructural evolution during shape memory loading. EBSD was used to study chemical homogenization in U-10Mo, and for U-0.75Ti, we used EBSD to study the microstructure and texture evolution during thermal cycling and deformation. The studied uranium alloys have significant microstructural and chemical differences and each of these alloys presents unique preparation challenges. Each of the alloys is prepared by a sequence of mechanical grinding and polishing followed by electropolishing with subtle differences between the alloys. U-6Nb and U-0.75Ti both have martensitic microstructures and both require special care in order to avoid mechanical polishing artifacts. Unalloyed uranium has a tendency to rapidly oxidize when exposed to air and a two-step electropolish is employed, the first step to remove the damaged surface layer resulting from the mechanical preparation and the second step to passivate the surface. All of the alloying additions provide a level of surface passivation and different one and two step electropolishes are employed to create good EBSD surfaces. Because of its low symmetry crystal structure, uranium exhibits complex deformation behavior including operation of multiple deformation twinning modes. EBSD was used to observe and quantify twinning contributions to deformation and to examine the fracture behavior. Figure 1 shows a cross section of two mating fracture surfaces in cast uranium showing the propensity of deformation twinning and intergranular fracture largely between dissimilarly oriented grains. Deformation of U-6Nb in the shape memory regime occurs by the motion of twin boundaries formed during the martensitic transformation. Deformation actually results in a coarsening of the microstructure making EBSD more practical following a limited amount of strain. Figure 2 shows the microstructure resulting from 6% compression. Casting of U-10Mo results in considerable chemical segregation as is apparent in Figure 2a. The segregation subsists through rolling and heat treatment processes as shown in Figure 2b. EBSD was used to study the effects of homogenization time and temperature on chemical heterogeneity. It was found that times and temperatures that result in a chemically homogeneous microstructure also result in a significant increase in grain size. U-0.75Ti forms an acicular martinsite as shown in Figure 4. This microstructure prevails through cycling into the higher temperature solid uranium phases.

36 MATERIALS SCIENCE↗
DOE OSTI2016

Quantitative Dedoping of Conductive Polymers

Although doping is a cornerstone of the inorganic semiconductor industry, most devices using organic semiconductors (OSCs) make use of intrinsic (undoped) materials. Recent work on OSC doping has focused on the use of dopants to modify a material’s physical properties, such as solubility, in addition to electronic and optical properties. However, if these effects are to be exploited in device manufacturing, a method for dedoping organic semiconductors is required. In this work, we outline two chemical strategies for dedoping OSC films. In the first strategy, we use an electron donor (a tertiary amine) to act as competitive donor. This process is based on a thermodynamic equilibrium between ionization of the donor and OSC and results in only partial dedoping. In the second strategy, we use an electron donor that subsequently reacts with the p-type dopant to create a nondoping product molecule. Primary and secondary amines undergo a rapid addition reaction with the dopant molecule 2,3,5,6-tetrafluoro-7,7,8,8,-tetracyanoquinodimethane (F4TCNQ), with primary amines undergoing a further reaction eliminating HCN. Under optimized conditions, films of semiconducting polymer poly(3-hexylthiophene) (P3HT) dedoped with 1-propylamine (PA) reach as-cast fluorescence intensities within 5 s of exposure to the amine, eventually reaching 140% of the as-cast values. Field-effect mobilities similarly recover after dedoping. Quantitative fluorescence recovery is possible even in highly fluorescent polymers such as PFB, which are expected to be much more sensitive to residual dopants. Interestingly, treatment of undoped films with PA also yields increased fluorescence intensity and a reduction in conductivity of at least 2 orders of magnitude. These results indicate that the process quantitatively removes not only F4TCNQ but also intrinsic p-type impurities present in as-cast films. The dedoping strategies outlined in this article are generally applicable to other p- and n-type molecular dopants in OSC films.

36 MATERIALS SCIENCE↗
DOE OSTI2019

Evolution of Ionomer Morphology from Dispersion to Film: An in Situ X-ray Study

Ion-conducting polymers (ionomers) have been extensively studied in solution, as membranes and substrate-supported thin films for various electrochemical energy-conversion devices, including fuel cells and electrolyzers. Formation of an ionomer film from a solution, however, is not well understood, despite its importance for fabrication of electrodes in energy devices. Here, the evolution of the perfluorinated sulfonic acid morphology upon casting from a solution is observed using in situ grazing-incidence small- and wide-angle X-ray scattering. Aggregate interactions in dispersion directly impact the hydrophilic-domain network of the cast film and the onset of crystallization occurs simultaneously with the solution-to-film transition but continues to evolve on different time scales. In addition, confinement is shown to induce anisotropic morphology at multiple length scales. These results show promise for elucidating the role of casting parameters, drying protocols, and ionomer-solvent interactions in governing film morphology and open new avenues for establishing structure/processing/property relationships for ionomer films and modifying their transport functionality at catalytic interfaces.

30 DIRECT ENERGY CONVERSION↗
DOE OSTI2025

Cold-Sprayed NMC622 Composite as a Cathode for Lithium-Ion Batteries

The growing demand for high-energy, low-cost lithium-ion batteries (LIBs) to power electric vehicles (EVs) necessitates advances in both materials and manufacturing processes. Conventional cathode fabrication methods, such as slurry casting and drying, are energy-intensive and pose challenges for scalability and environmental compliance. In this study, we propose cold-spray (CS) deposition as a solvent-free approach for fabricating LiNi 0.6 Mn 0.2 Co 0.2 O 2 (NMC622) composite cathodes. Powder blends of NMC622, poly(vinylidene fluoride) (PVDF), and carbon black (CB) are directly deposited onto stainless steel and Inconel substrates under varied gas temperatures, pressures (and thus velocities), and standoff distances. The effects of temperature on the deposit morphology, coating density, and volume are systematically investigated. Computational fluid dynamics simulations reveal that increasing the gas temperature enhances the particle velocity, narrows the spray angle, and reduces the mass concentration radially at the nozzle outlet. CS deposition results in a dense cathode microstructure, accompanied by fracture in polycrystalline NMC622 particles. X-ray diffraction analysis further verifies that there are no phase changes during the deposition process. The electrochemical performance of the cold-sprayed cathodes reveals an initial capacity of approximately 96 mAh g –1 for single-crystal NMC622 and 167 mAh g –1 for polycrystalline NMC622. While these values are modest compared to state-of-the-art slurry-cast cathodes, which typically exhibit 180–200 mAh g –1 under optimized conditions. The results demonstrate a competitive performance given the solvent-free nature of the CS process and compare favorably with tape-cast samples made from identical feedstock. In conclusion, he CS process enables the formation of dense, binder-integrated cathode coatings without the need for solvent processing, offering a promising pathway for scalable, energy-efficient dry electrode manufacturing of next-generation LIBs.

Batteries↗
DOE OSTI2023

Auxiliary sequential deposition enables 19%-efficiency organic solar cells processed from halogen-free solvents

High-efficiency organic solar cells are often achieved using toxic halogenated solvents and additives that are constrained in organic solar cells industry. Therefore, it is important to develop materials or processing methods that enabled highly efficient organic solar cells processed by halogen free solvents. In this paper, we report an innovative processing method named auxiliary sequential deposition that enables 19%-efficiency organic solar cells processed by halogen free solvents. Our auxiliary sequential deposition method is different from the conventional blend casting or sequential deposition methods in that it involves an additional casting of dithieno[3,2-b:2’,3’-d]thiophene between the sequential depositions of the donor (D18-Cl) and acceptor (L8-BO) layers. The auxiliary sequential deposition method enables dramatic performance enhancement from 15% to over 18% compared to the blend casting and sequential deposition methods. Furthermore, by incorporating a branched-chain-engineered acceptor called L8-BO-X, device performance can be boosted to over 19% due to increased intermolecular packing, representing top-tier values for green-solvent processed organic solar cells. Comprehensive morphological and time-resolved characterizations reveal that the superior blend morphology achieved through the auxiliary sequential deposition method promotes charge generation while simultaneously suppressing charge recombination. This research underscores the potential of the auxiliary sequential deposition method for fabricating highly efficient organic solar cells using environmentally friendly solvents.

14 SOLAR ENERGY↗
DOE OSTI2015

Correlating highpower conversion efficiency of PTB7:PC 71 BM inverted organic solar cells with nanoscale structures [Unraveling the correlation between the structural aspects and power conversion efficiency in PTB7:PC 71 BM inverted organic solar cells]

Advances in materials design and device engineering led to inverted organic solar cells (i-OSCs) with superior power conversion efficiencies (PCEs) to their conventional counterparts, in addition to the well-known better ambient stability. Despite the significant progress, however, it has so far been unclear how the morphologies of the photoactive layer and its interface with the cathode modifying layer impact device performance. Here, we report an in-depth morphology study of the i-OSC active and cathode modifying layers, employing a model system with the well-established bulk-heterojunction, PTB7:PC 71 BM as the active layer and poly-[(9,9-bis(3 -( N,N -dimethylamino)propyl)-2,7-fluorene)- alt -2,7-(9,9-dioctylfluorene)] (PFN) as the cathode surface modifying layer. We have also identified the role of a processing additive, 1,8-diiodooctane (DIO), used in the spin-casting of the active layer to increase PCE. Using a variety of characterization techniques, we demonstrate that the high PCEs of i-OSCs are due to the smearing (diffusion) of electron-accepting PC 71 BM into the PFN layer, resulting in improved electron transport. The PC 71 BM diffusion occurs after spin-casting the active layer onto the PFN layer, when residual solvent molecules act as a plasticizer. Furthermore, the DIO additive, with a higher boiling point than the host solvent, has a longer residence time in the spin-cast active layer, resulting in more PC 71 BM smearing and therefore more efficient electron transport. This work provides important insight and guidance to further enhancement of i-OSC performance by materials and interface engineering.

14 SOLAR ENERGY↗
DOE OSTI2020

Contribution of cryogenic thermal cycling to the atomic dynamics in a La-based bulk metallic glass with different initial states

Structural rejuvenation in metallic glasses (MGs) induced by cryogenic thermal cycling has been intensively studied. However, the effect of thermal cycling on the atomic dynamics in MGs is still missing. In this work, we present a systematic study on the atomic dynamics in a La-based MG affected by sub-Tg annealing and thermal cycling. We find that the thermal cycling has little effect on the atomic dynamics in both as-cast and annealed MGs although it could significantly increase the free volume and change mechanical properties of the as-cast MG sample. In contrast, both the structure and mechanical properties of the well-relaxed sample are almost unchanged by thermal cycling. Further, the increased open volume, mainly in the form of atomic vacancies, does not greatly promote the atomic dynamics in the as-cast MG sample, indicating that the free volume induced by thermal cycling is not the key factor in stimulating the β-relaxation and the boson peak in the studied MG.

36 MATERIALS SCIENCE↗