Lamellar Direction Research Articles

Anisotropic Lattice Thermal Conductivity in Highly Ordered PEDOT Fibers

AbstractWhen it comes to sustainable and efficient energy solutions, organic semiconductors can play an important role in thermoelectric applications, since they are non‐toxic, cheap, made of abundant chemical species, and show intrinsically low thermal conductivities. Their electrical conductivity can be optimized via doping. Yet, thermal conduction should be as low as possible and, to this end, the atomic scale mechanisms behind heat transport –e.g. the correlation between morphology and thermal conductivity or the role of doping– should be understood in detail. Fully atomistic molecular dynamics calculations of the lattice thermal conductivity of doped poly(3,4‐ethylenedioxythiophene) (PEDOT) highly ordered, quasi‐crystalline nanofibers are presented here. It is found that the conductivity along the backbone direction is not necessarily the highest, but it depends on the length of the PEDOT chains, thus the degree of anisotropy depends on the the aspect ratio of the nanofiber. Indeed, transport along the lamellar direction can be of the same order or higher than that of the backbone if their lengths are comparable. These results challenge the usual expectation that thermal conduction along the backbone largely exceeds those along the lamellar and π − π direction and have the important consequence that the anisotropy could be leveraged in thermal management applications.

Steam oxidation properties of graphene reinforced bioinspired laminated CoCrFeNiMn high-entropy alloy matrix composites at 1000 ℃

Laminated CoCrFeNiMn high-entropy alloy (HEA) matrix composites reinforced with 1.0 wt% graphene nanoplatelets (GNPs) were fabricated by mechanical ball milling and flake powder metallurgy, and were isothermal oxidized at 1000 ℃ for 100 h. Vacuum hot-pressure sintering (VHPS) shows a distinct nacre structure with the microstructure composed of FCC matrix phase, Cr23C6, CrMn1.5O4 precipitated phases, numerous dislocations and twins. High-temperature oxidation tests showed that the composites had mass gains at 12 h intervals from 12 to 100 h respectively. The oxidation kinetic curve changes from a linear pattern in the early stages to an exponential pattern in the later stages, which demonstrates better long-term oxidation resistance. The anisotropy of the laminated structure results in excellent resistance to high-temperature steam oxidation in the vertical lamellar direction. Observation of the cross-section reveals that although Cr2O3 (inner layer) is present, (Mn, Cr)3O4 and Mn3O4 are the dominant oxides (outer layer). Elemental depletion zones for Mn and Cr exist in the region of the matrix near the oxide scales. The results show that the oxidation resistance of the laminated GNPs/CoCrFeNiMn composites is mainly influenced by the diffusion of Mn and Cr elements, the microstructure of the laminations and the internal oxidation.

Construction of a permeable metal-support interface for glycerol oxidation by the topological transformation of 2D precursor

Aiming at achieving a trade-off of electronic-interacted and permeable metal-support interfaces, we developed a feasible approach for constructing semi-continuous TiOx/Pt structure through the topological transformation of ultrathin PtCl62−/TiMgAl-layered double hydroxides (LDHs) to limit the migration degree of metal ions perpendicular to the lamellar direction. Structural characterization revealed that most Pt sites are permeable to the reactants in the ultrathin TiOx/Pt/TiMgAlOx-U catalyst. Despite that, there is still electron transfer to form abundant Pt0-oxygen vacancy(Vo)-Ti3+sites in the interfacial structure. In the glycerol oxidation reaction, the TiOx/Pt/TiMgAlOx-U exhibited a much higher reaction rate(289 molglycrol·molPt−1·h−1), which is 1.7 times of the bulk TiOx/Pt/TiMgAlOx-B and 5.2 times of the TiOx/Pt/TiO2 reference, and simultaneously maintained a decent glyceric acid selectivity. Mechanism study revealed that the Pt0-Vo-Ti3+interfacial structure could promote the activation and fluxion of active oxygen species, and was favorable for the activation of primary hydroxyl groups of glycerol and the aldehyde intermediate.

Equilibrium Melting Temperature of the Hexagonal Crystals of Polybutene-1 and Its Copolymer

TheLamellar structures of form I in polymorphic isotactic polybutene-1 (iPB-1) and forms I and I′ in butene-1/ethylene random copolymer (iPB-1/C2) were investigated by studying their melting behavior using the combination of small-angle X-ray scattering (SAXS) and fast scanning chip calorimetry (FSC) techniques. Hexagonal form I was transformed from tetragonal form II obtained by crystallization of a homogeneous melt of iPB-1 and iPB-1/C2. Hexagonal form I′ was crystallized directly from a heterogeneous melt of iPB-1/C2. Despite the same hexagonal crystallographic structure, forms I and I′ have obviously distinct melting behaviors due to different ways of crystal formation. Well-defined linear dependences of the melting temperature against the reciprocal lamellar thickness were observed for forms I and I′ with frozen chain mobility in crystals. The slopes of the thus obtained melting lines for forms I and I′ are different. Their extrapolations to imaginary infinitely large lamellar thickness provide the equilibrium melting temperature Tm∞ for forms I and I′, which is associated with the stability of the lamellar crystals. In spite of crystal defects in form I′, a higher Tm∞ is observed for this form in iPB-1/C2 than Tm∞ for form I in both iPB-1 and iPB-1/C2. The difference in Tm∞ indicates a less perfect structure of form I crystal lamellae that is caused by the fragmentation of form II crystal lamellae during the polymorphic phase transition from form II to I. The lamellae broke into crystal blocks along the lamellar direction. The smaller lateral sizes of the fragmented lamellae clearly induce a decrease in the stability of crystals, as expressed in a lower Tm∞ for form I.

Design of a novel (CoCrNi)84.5Al15.5 eutectic medium-entropy alloy with hierarchically nano-lamellar microstructure through additively manufactured compositionally graded materials

Eutectic medium/high-entropy alloys (E-M/HEAs) exhibit excellent castability and mechanical properties. This present work developed a novel CoCrNiAl based E-MEAs through a high-throughput method based on compositionally graded materials by laser directed energy deposition. The microstructure in the CoCrNiAlx (x ≈ 0−0.8) compositionally graded alloy changed from columnar to equiaxed with increasing Al content, and the fraction of body-centered cubic (BCC) phase increased simultaneously. Especially, a eutectic lamellar microstructure with face-centered cubic (FCC) and ordered BCC (B2) was obtained when the Al content reached 15.5 at%, and it showed a better strength-ductility combination than other reported cast eutectic alloys. The mechanical-property improvement of this novel E-MEA is related to its hierarchically nano-lamellar microstructure, in which the orientation relationship (OR) between FCC and B2 lamellae conforms to the Kurdjumov-Sachs OR, and spherical BCC and ordered FCC phases precipitate from B2 and FCC lamellae, respectively. Furthermore, the fracture mechanisms of this novel E-MEA were also investigated by in-situ tensile test and transmission electron microscope, indicating that the crack modes were closely related to the eutectic lamellar direction relative to the tensile axis, and the considerable ductility stemmed from the cooperative deformation between FCC and B2 phases caused by various deformation mechanisms.

Preparation and characterization of paraffin/expanded graphite composite phase change materials with high thermal conductivity

Paraffin/expanded graphite (PCM/EG) composite phase change material was prepared. The microstructure, thermal properties and cycle stability of PCM/EG were characterized. The results show that the thermal conductivity of PCM/EG increases with EG content and block density increasing. When EG content is 20%, paraffin can be adsorbed completely and there is no obvious leakage during 60 melting and solidification cycles. EG lamella structure is formed during compression process of PCM/EG, which is perpendicular to pressure direction and beneficial to enhance the directional thermal conductivity. The thermal conductivity of PCM/EG20%-800(b) along EG lamellar structure direction is 7.82 W m−1 k−1, which is 47.8% higher than that perpendicular to EG lamellar direction. In addition, experimental studies were carried out on PCM/EG melting process to reveal the effects of EG content, block density and EG lamellar direction on temperature control performance. The results show that the average temperature of heating surface in melting process of PCM/EG20%-800 is 46.2 °C, which is 4.0% lower than PCM/EG16%-800. In addition, when heat flow direction is parallel to EG lamellar structure direction, the average surface temperature is 44.8 °C, which means a better temperature control effect. The present work provides an efficient method to enhance PCM/EG heat storage and temperature control performance under fixed latent heat.

Morphology Modulation in Self-Assembly of Chiral 2-Hydroxy-2-Phenylacetic Acids in Polymeric Diluents

This study focused on the chirality effects that control the lamellar bending sense in self-assembled crystals of chiral 2-hydroxy-2-phenylacetic acids. 2-Hydroxy-2-phenylacetic acid or mandelic acid (MA) was crystallized in the presence of poly(4-vinyl phenol) (PVPh), and its crystalline structures and morphologies were assessed using polarized optical microscopy (POM) and scanning electron microscopy (SEM). MA of two opposite chiral forms (S- and R-) was crystallized with PVPh as the morphology modulator; with adjustment of the PVPh content, the morphology of MA crystals transforms from ring-banded spherulites to highly dendritic spherulites. For MA/PVPh (50/50 wt./wt.) blend and neat MA at same Tc, the dendritic spherulites are packed with single crystals where the lamellae bend at a specific direction varying with Tc and chirality. Contrary to conventional thought, the bending senses of the MA lamellae in the dendritic spherulites are not solely governed by the MA molecular chirality (S or R), but also by Tc. Only at high Tc (>65 °C) is the lamellar bending direction in dendritic spherulites of (S)-MA or (R)-MA blended with PVPh dictated by the chirality, i.e., displaying counterclockwise and clockwise bending direction for (S)-MA/PVPh and (R)-MA/PVPh, respectively. Nevertheless, at low Tc (45 °C), the bending sense of dendritic spherulites displays an opposite direction from those at the higher Tc, which is to say that the chirality alone does not control the lamellar bending direction.

Crevice corrosion of U75V high-speed rail steel with varying crevice gap size by in-situ monitoring

Crevice corrosion of high-speed rail steel U75V was studied with varying gap size by using self-designed device. Increasing gap size inhibited exterior cathodic reduction reaction of oxygen, inducing decrease of potential difference between interior and exterior. Gap size decrease after increase induced increase of potential difference after decrease, not completely reversible because rust kept expanding, acting as weaker depolarizer than oxygen. Anodic dissolution of interior was under ohmic-control, fastest near crevice mouth and slowest near but not at deepest site. Corrosion initiation was observed, closely related to pearlite nodules with larger interlamellar spacing and different lamellar directions, inducing deep attack.

Effect of V on the microstructure and brittle-to-ductile transition of directionally solidified high-Nb TiAl alloy

The influence of V addition on the microstructure and high-temperature tensile performance of Ti46Al7Nb0.4W0.6Cr alloy, fabricated by using the cold crucible directional solidification method, is comprehensively investigated. The results show that V addition can refine the columnar grain width and promote the brittle-to-ductile transition (BDT) of the TiAl alloy. After the addition of 2 at.% V, the ultimate tensile strength and elongation increased at 973–1173 K. The fracture mode change from interlamellar to translamellar; this is attributed to the decrease in the angle between the lamellar alignment and growth direction. The BDT behaviour of V-containing TiAl alloy at 1073 K results from the transition from dislocation-dominated to twin-dominated deformation as indicated by transmission electron microscopic investigations.

The smooth and notched three-point bending fatigue behavior of directionally solidified high-Nb TiAl alloy

To understand the fatigue behavior of directionally solidified (DS) TiAl alloys, the Ti46Al7Nb alloy fabricated by cold crucible DS method was investigated by microstructure characterization and three-point bending fatigue tests. Results show that the microstructure of the alloy is composed of α 2 /γ lamellae with β segregation, and the angle between lamellar arrangement and DS growth direction is 45°. Smooth and notched specimens were prepared to evaluate the fatigue performance of the TiAl alloy under pre-damage conditions. The finite element results indicate that the notched have more obvious stress concentration. The three-point bending fatigue limits of the smooth and notched specimens are 468 MPa and 213 MPa, respectively. The crack propagation path indicates that there are cleavage microcracks with different shapes around the main crack in the notched specimen, while the main crack in the smooth one is surrounded by many corrugated cracks perpendicular to the lamellar direction. The whole process of fatigue crack initiation and propagation under three-point loading is established. Fracture morphology shows that notched specimens are mostly brittle cleavage fracture along the lamellar facets, while the smooth are trans-lamellar fracture with plastic characteristics. The degree of fatigue deformation of the smooth specimen is greater than that of the notched one, which mainly reflects on high density dislocation and dislocation intersection, as well as more sub-grain boundaries and twins. • The high-Nb TiAl alloy with well-aligned lamellae was fabricated by CCDS. • The three-point bending fatigue limits of the smooth and notched specimens are 468 MPa and 213 MPa, respectively. • The corrugated secondary cracks formed around the main crack are conducive to the release of stress concentration. • The smooth specimen exhibits high-density dislocation intersection, as well as more sub-grain boundaries and twins.

Revealing the Side‐Chain‐Dependent Ordering Transition of Highly Crystalline Double‐Cable Conjugated Polymers

AbstractWe developed a series of highly crystalline double‐cable conjugated polymers for application in single‐component organic solar cells (SCOSCs). These polymers contain conjugated backbones as electron donor and pendant perylene bisimide units (PBIs) as electron acceptor. PBIs are connected to the backbone via alkyl units varying from hexyl (C6H12) to eicosyl (C20H40) as flexible linkers. For double‐cable polymers with short linkers, the PBIs tend to stack in a head‐to‐head fashion, resulting in large d‐spacings (e.g. 64 Å for the polymer P12 with C12H24 linker) along the lamellar stacking direction. When the length of the linker groups is longer than a certain length, the PBIs instead adopt a more ordered packing likely via H‐aggregation, resulting in short d‐spacings (e.g. 50 Å for the polymer P16 with C16H32 linker). This work highlights the importance of linker length on the molecular packing of the acceptor units and the influences on the photovoltaic performance of SCOSCs.

Revealing the Side-Chain-Dependent Ordering Transition of Highly Crystalline Double-Cable Conjugated Polymers.

We developed a series of highly crystalline double-cable conjugated polymers for application in single-component organic solar cells (SCOSCs). These polymers contain conjugated backbones as electron donor and pendant perylene bisimide units (PBIs) as electron acceptor. PBIs are connected to the backbone via alkyl units varying from hexyl (C6 H12 ) to eicosyl (C20 H40 ) as flexible linkers. For double-cable polymers with short linkers, the PBIs tend to stack in a head-to-head fashion, resulting in large d-spacings (e.g. 64 Å for the polymer P12 with C12 H24 linker) along the lamellar stacking direction. When the length of the linker groups is longer than a certain length, the PBIs instead adopt a more ordered packing likely via H-aggregation, resulting in short d-spacings (e.g. 50 Å for the polymer P16 with C16 H32 linker). This work highlights the importance of linker length on the molecular packing of the acceptor units and the influences on the photovoltaic performance of SCOSCs.

Connectivity and Neuronal Synchrony during Seizures.

There is uncertainty regarding when and which groups of neurons fire synchronously during seizures. While several studies found heterogeneous firing during seizures, others suggested synchronous neuronal firing in the seizure core. We tested whether neuronal activity during seizures is orderly in the direction of the excitatory neuronal connections in the circuit. There are strong excitatory connections laterally within the septotemporally organized lamella and inhibitory trans-lamellar connections in the hippocampus, which allow testing of the connectivity hypothesis. We further tested whether epileptogenesis enhances synchrony and antiseizure drug administration disrupts it. We recorded local field potentials from CA1 pyramidal neurons using a small microelectrode array and kindled rats by a rapid, recurrent hippocampal stimulation protocol. We compared cross-correlation, theta phase synchronization, entropy, and event synchronization. These analyses revealed that the firing pattern was correlated along the lamellar, but not the septotemporal, axis during evoked seizures. During kindling, neuronal synchrony increased along the lamellar axis, while synchrony along the septotemporal axis remained relatively low. Additionally, the theta phase distribution demonstrated that CA1 pyramidal cell firing became preferential for theta oscillation negative peak as kindling progressed in the lamellar direction but not in the trans-lamellar direction. Last, event synchronization demonstrated that neuronal firings along the lamellar axis were more synchronized than those along the septotemporal axis. There was a marked decrease in synchronization and phase preference after treatment with phenytoin and levetiracetam. The synchrony structure of CA1 pyramidal neurons during seizures and epileptogenesis depends on anatomic connectivity and plasticity.SIGNIFICANCE STATEMENT We could improve the efficacy of brain stimulation to treat seizures by understanding the structure of synchrony. Electrical stimulation may disrupt seizures by desynchronizing neurons, but there is an uncertainty on which groups of neurons fire synchronously or chaotically during seizures. Here, we demonstrate that neurons linked by excitatory connections fire synchronously during seizures, and this synchrony is modulated by epileptogenesis and antiseizure drugs. Closed-loop brain stimulation carefully targeted to disrupt synchrony may improve the treatment of seizures.

High-strength and multi-functional gypsum with unidirectionally porous architecture mimicking wood

Porous gypsum materials are of notable significance and demonstrate a broad application in construction and biomedical engineering; nevertheless, it is challenging to simultaneously enhance their mechanical and functional properties. Here by replicating the designs of natural wood, new gypsum materials containing unidirectional micro-pores were fabricated by employing a modified ice-templating technique. The retention of ice-templated architectures was realized based on the hydration-induced self-hardening characteristic of gypsum without using the traditionally long-time freeze drying treatment. The porosities of the wood-like gypsum can be readily adjusted in wide ranges from lower than 60 vol.% to over 80 vol.% by controlling the solid loads in initial slurries. The wood-like gypsum exhibits a notable improvement in compressive strength and energy absorption efficiency along the lamellar direction by more than twofold compared to the unfrozen gypsum materials containing random pores of similar porosity. Additionally, it displays the lowest level of thermal conductivity among gypsum materials along the normal direction of lamellae, which ranges from above 0.21 to ~0.1 W (m K)−1 depending on porosity, thereby demonstrating an outstanding effectiveness for thermal insulation. It can also be easily modified from hydrophilic to hydrophobic towards enhanced water resistance. The high strength and multi-functionalities along with the feasible fabrication approach make the wood-like gypsum appealing for applications, e.g., as construction and biomedical materials. The unidirectionally open pores may also provide new possibilities for regulating the cell behavior in gypsum-based bone implants.

Alignment of linear polymeric grains for highly stable N-type thin-film transistors

Summary Temperature-insensitive properties are attractive for most electronics, including polymeric semiconducting devices. Especially, polymeric field-effect transistors (FETs) with high mobility have been important research targets due to their broad applications. However, polymeric FETs with stable charge transport operating at extremely cold or hot zones are faced with enormous challenges. In this study, the polyacrylonitrile was found to significantly tune the sizes of pre-aggregates of polymers in solutions and the crystallinity of the polymeric films. The orientation of 5–25 μm linear grains in the films were prepared through the bar-coating process with polyacrylonitrile as an additive, which stabilized the electron mobility over a wide range of temperatures. The linear-grain morphology of the film contributed to reducing the holes and grain boundaries in the transport paths of carriers. Typically, the top-gate FETs based on P(NDI2OD-T2) displayed a stable electron transporting behavior from 200 to 460 K, with mobility greater than 3.5 cm2V−1s−1.

β0 precipitation in α2 lamellae of the β-solidifying multiple-phase γ-TiAl alloy

The precipitation behavior of β0 particles in the α2 lamellae of a β-solidifying multiple-phase γ-TiAl alloy, i.e., Ti-42.05Al-3.96Nb-1.01Mo-0.21B (at.%) alloys, was investigated using conventional transmission electron microscopy (TEM) and high-resolution transmission electron microscopy (HRTEM). It has been found that β0 phase can precipitate in α2 lamellae when aging-treated at 950–1150 °C for 6 h. This is a process controlled by diffusion of Mo element. The β0 precipitates nucleate initially at the α2/γ lamellar interfaces and grow firstly along the α2 lamellar width direction then along the length direction. The lamellar spacing of α2 lamella and growth stage of β0 precipitates codetermined the morphologies of β0 precipitates. All β0 phases precipitated from α2 matrix follow the Burgers OR: 101β0//0001α2,1¯11β0//112¯0α2, and the interface dislocations between α2/β0 interfaces are commanded to accommodate the lattice misfit. Besides, the decomposition of α2 → γ also occurs during the aging treatments. The α2/γ interface ledges promote the nucleation of β0 phase, but the precipitated β0 particles at the α2/γ ledges impede the α2 → γ transformation. The coordination of α2 → β0 and α2 → γ transformations is influenced by the aging temperature. When the aging temperature is 1150 °C, the amount of β0 precipitation in α2 lamellae up to the maximum.

Atomic-scale insights into interface thermal resistance between epoxy and boron nitride in nanocomposites

The out-plane thermal conductivity of hexagonal boron nitride nanosheet (hBNNS)/epoxy resin (EP) composites in the perpendicular to the lamellar layers direction as well as the interface thermal resistance (ITR) between EP and hBNNS in nanocomposites are investigated via molecular dynamics (MD) simulations, and compared with those of the graphene nanosheet (GNS)/epoxy nanocomposites and experimental results. Results show that the out-plane thermal conductivity of the hBNNS/EP is higher than that of GNS/EP at the same volume fraction of the fillers. The obtained ITR between EP and GNS is (4.48-8.43)×10−9 m2KW−1, consistent with the previous reported experimental and simulation results. Interestingly, the calculated ITR between EP and hBNNS is (2.42–3.62)×10−9 m2KW−1, smaller than that between EP and GNS. The ITRs exponentially decay with the number of layers of hBNNS (or GNS). This work provides insights into predicting and understanding the thermal conductivity of hBN and graphene-based nanocomposites using computational approaches.

Lamellar Bilayer to Fibril Structure Transformation of Tough Photonic Hydrogel under Elongation

Synthetic hydrogels possessing both macroscopic anisotropic structure and toughness, which are analogous to the load-bearing bio-tissues such as muscles and tendons, are rarely available. Studying the molecular mechanism of tough and anisotropic hydrogel under deformation is beneficial to understand the load-deformation functions of soft bio-tissues. In this work, the deformation-induced structure transformation of a macroscopically anisotropic and tough hydrogel has been investigated to understand the role of structure evolution for enhanced toughness. At rest, the hydrogel possesses a well-defined hierarchical structure in which self-assembled nanometer thick lamellar bilayers are alternatively stacked in hundred nanometer thick hydrogel matrixes. Stretching along the lamellar direction induces structure transformation from lamellar bilayers to hierarchical fibrous structures aligned along the deformation axis. The generated hierarchical structures consist of micrometer thick fiber bundles made from nanometer thick fibrils analogous to tropo-collagen bundles or microfibrils of the tendon. The fibrous structure formed at large elongation is associated with damage and rupture of the bilayers, which underpins the molecular mechanism of the unique mechanical behaviors of the tough lamellar hydrogel.

In-situ tensile deformation behavior of powder metallurgy Ti6Al4V alloys

In this study, in-situ tensile deformation behavior of powder metallurgy (PM) Ti6Al4V alloys was investigated to analyze the crack initiation and propagation. Accordingly, the fracture mechanisms of the as-sintered and forged PM alloys were summarized. At the initial stage of plastic deformation, cracks appeared in the stress concentration area of pores in the as-sintered Ti6Al4V alloy, and the crack propagation direction was along the phase boundary. Due to the existence of pores, early fracture was obtained, resulting in low elongation of 6.3%. After forging, the crack initiation occurred between α lamellar structure, and the propagation direction was along the lamellar direction. The fine lamellar structure in different directions in the forged PM Ti6Al4V alloy can hinder the crack propagation, thus improving the plasticity. As a result, better comprehensive mechanical performance was obtained in the forged sample, with UTS of 960 MPa, YS of 850 MPa, and EL of 16%.

Spacing homogenization in lamellar eutectic arrays with anisotropic interphase boundaries

We analyze the effect of interphase boundary anisotropy on the dynamics of lamellar eutectic solidification fronts, in the limit that the lamellar spacing varies slowly along the envelope of the front. In the isotropic case, it is known that the spacing obeys a diffusion equation, which can be obtained theoretically by making two assumptions: (i) the lamellae always grow normal to the large-scale envelope of the front, and (ii) the Jackson-Hunt law that links lamellar spacing and front temperature remains locally valid. For anisotropic boundaries, we replace hypothesis (i) by the symmetric pattern approximation, which has recently been found to yield good predictions for lamellar growth direction in presence of interphase anisotropy. We obtain a generalized Jackson-Hunt law for tilted lamellae, and an evolution equation for the envelope of the front. The latter contains a propagative term if the initial lamellar array is tilted with respect to the direction of the temperature gradient. However, the propagation velocity of the propagative wave modes are found to be small, so that the dynamics of the front can be reasonably described by a diffusion equation with a diffusion coefficient that is modified with respect to the isotropic case.