It has been well documented that nanoparticles could act as effective nucleating agents for various semicrystalline polymers and affect the crystallization kinetics and crystalline morphology. 1À10 For example, only 0.02 wt % multiwalled carbon nanotubes (CNTs) could reduce the half-crystallization time (t 1/2 ) of poly(L-lactide) to 9.6% when isothermally crystallized at 115°C. 11 For nonisothermal crystallization at a cooling rate of 10°C/min, the crystallization temperature of isotactic polypropylene (iPP) containing only 0.005 wt % gold nanoparticles was increased by 8.5°C. 12 The similar nucleation effect was also found in other nanofillers, such as montmorillonite, zinc oxide, monodispersed SiO 2 , etc. 6,9,13 On the other hand, nanoparticles are prone to attract molecular chains and template the growth of polymer crystals. By appropriate lattice matching, a contact between polymer lamellae and substrate surface can be established, leading to the development of new crystalline morphology. 5,7,8,14À17 One such morphology is the "nano-hybrid shish-kebabs" (NHSK) structure proposed by Li et al., where CNTs serve as the shish and disk-shaped polymer single crystals (kebabs) grow epitaxially perpendicular to the surface of CNTs. 5,15 In typical polymer processing operations (e.g., extrusion, injection, and blowing molding), the applications of different flow fields have great influence on the final crystallinity and crystalline morphology, which are intimately associated with their mechanical properties. 18À22 Therefore, the subject of flow-induced crystallization and the relationship with the mechanical performances has attracted a great deal of attention in the community. 23À29 It has been well established that shear flow can significantly enhance the crystallization kinetics of polymers. The accelerating effect can be attributed to the formation of oriented molecular chains, which nucleate and form crystal structure. The lifetime of the shear-induced precursors is closely related to the shear intensity and crystallization conditions. For example, the row nuclei can emerge under the effect of shear fields at low crystallization temperatures (T c ), while they would disintegrate into pointlike precursors at high T c or relax after a long holding time. 24À26 The shish structure can be formed by the ABSTRACT: Combined effects of graphene nanosheets (GNSs) and shear flow on the crystallization behavior of isotactic polypropylene (iPP) were investigated by in-situ synchrotron wide-angle X-ray diffraction (WAXD) and small-angle X-ray scattering (SAXS) techniques. For crystallization under quiescent condition (at 145°C), the halfcrystallization time (t 1/2 ) of nanocomposites containing 0.05 and 0.1 wt % GNSs was reduced to at least 50% compared to that of neat iPP, indicating the high nucleation ability of GNSs. The crystallization rate of iPP was directly proportional to the GNS content. Under a relatively weak shear flow (at a rate of 20 s À1 for 5 s duration) and a low degree of supercooling, the neat iPP exhi...

Solids in nature can be generally classified into crystalline and non-crystalline states [1][2][3][4][5][6][7] , depending on whether long-range lattice periodicity is present in the material. The differentiation of the two states, however, could face fundamental challenges if the degree of long-range order in crystals is significantly reduced. Here we report a unique paracrystalline state of diamond that is distinct from either crystalline or amorphous diamond [8][9][10] . The paracrystalline diamond reported in this work, consisting of sub-nanometersized paracrystals that possess a well-defined crystalline medium-range order up to a few atomic shells 4,5,[11][12][13][14] , was synthesized in high-pressure high-temperature conditions (e.g., 30 GPa, 1600 K) employing fcc-C 60 as a precursor. The structural characteristics of paracrystalline diamond was identified through a combination of X-ray diffraction, highresolution transmission microscopy, and advanced molecular dynamics simulation. The formation of paracrystalline diamond is a result of densely distributed nucleation sites developed in compressed C 60 as well as pronounced second-nearest-neighbor short-range order in amorphous diamond due to strong sp 3 bonding. The discovery of paracrystalline diamond adds a new diamond form to the enriched carbon family 15-17 , which exhibits distinguishing physical properties and can be furthered exploited to develop new materials. Furthermore, this work reveals the missing link in the length-scale between amorphous and crystalline states across the structural landscape, which has profound implications for recognizing complex structures arising from amorphous materials.Amorphous solids refer to materials that do not possess long-range periodicity as exhibited in crystals [1][2][3][4][5][6][7][8][9][10][11] . Consequently, Bragg peaks associated with crystalline arrangements of atoms are absent or obscured in the diffraction signals of amorphous materials, which renders the recognition of their structural organizations notoriously difficult. Due to decades of research effort, it is now understood that structural ordering on the atomic level of amorphous solids is ubiquitous, as manifested by the short-to-medium-range ordering in metallic glasses [5][6][7] and the continuousrandom networks (CRN) of amorphous semiconductors 1-3 . Moving from the short range into the extended length-scale abutting the long-range scale, however, our understanding of the structural arrangements remains much more limited, and it is often complicated by capricious crystalline structural ordering encountered in amorphous materials 14,[18][19][20] . In an attempt to resolve this structural enigma, a paracrystalline structure model was proposed 11,20 , in which nanosized paracrystals, defined as severely distorted crystals, were introduced to the amorphous matrix to account for the crystalline medium range order (MRO). A crucial question to answer is, in the configurational space, are we able to identify a state of matter that is fully packed with...

The effect of shear flow and carbon nanotubes (CNTs), separately and together, on nonisothermal crystallization of poly(lactic acid) (PLA) at a relatively large cooling rate was investigated by time-resolved synchrotron wide-angle X-ray diffraction (WAXD) and polarized optical microscope (POM). Unlike flexible-chain polymers such as polyethylene, and so on, whose crystallization kinetics are significantly accelerated by shear flow, neat PLA only exhibits an increase in onset crystallization temperature after experiencing a shear rate of 30 s(-1), whereas both the nucleation density and ultimate crystallinity are not changed too much because PLA chains are intrinsically semirigid and have relatively short length. The breaking down of shear-induced nuclei into point-like precursors (or random coil) probably becomes increasingly active after shear stops. Very interestingly, a marked synergistic effect of shear flow and CNTs exists in enhancing crystallization of PLA, leading to a remarkable increase of nucleation density in PLA/CNT nanocomposite. This synergistic effect is ascribed to extra nuclei, which are formed by the anchoring effect of CNTs' surfaces on the shear-induced nuclei and suppressing effect of CNTs on the relaxation of the shear-induced nuclei. Further, this interesting finding was deliberately applied to injection molding, aiming to improve the crystallinity of PLA products. As expected, a remarkable high crystallinity in the injection-molded PLA part has been achieved successfully by the combination of shear flow and CNTs, which offers a new method to fabricate PLA products with high crystallinity for specific applications.

The discovery of electrides, in particular, inorganic electrides where electrons substitute anions, has inspired striking interests in the systems that exhibit unusual electronic and catalytic properties. So far, however, the experimental studies of such systems are largely restricted to ambient conditions, unable to understand their interactions between electron localizations and geometrical modifications under external stimuli, e.g., pressure. Here, pressure‐induced structural and electronic evolutions of Ca2N by in situ synchrotron X‐ray diffraction and electrical resistance measurements, and density functional theory calculations with particle swarm optimization algorithms are reported. Experiments and computation are combined to reveal that under compression, Ca2N undergoes structural transforms from R 3true¯ m symmetry to I 4true¯2d phase via an intermediate Fd 3true¯ m phase, and then to Cc phase, accompanied by the reductions of electronic dimensionality from 2D, 1D to 0D. Electrical resistance measurements support a metal‐to‐semiconductor transition in Ca2N because of the reorganizations of confined electrons under pressure, also validated by the calculation. The results demonstrate unexplored experimental evidence for a pressure‐induced metal‐to‐semiconductor switching in Ca2N and offer a possible strategy for producing new electrides under moderate pressure.

Multicomponent alloying has displayed extraordinary potential for producing exceptional structural and functional materials. However, the synthesis of single-phase, multiprincipal covalent compounds remains a challenge. Here we present a diffusioncontrolled alloying strategy for the successful realization of covalent multi-principal transition metal carbides (MPTMCs) with a single face-centered cubic (FCC) phase. The increased interfacial diffusion promoted by the addition of a nonstoichiometric compound leads to rapid formation of the new single phase at much lower sintering temperature.Direct atomic-level observations via scanning transmission electron microscopy demonstrate that MPTMCs are composed of a single phase with a random distribution of all cations, which holds the key to the unique combinations of improved fracture toughness, superior Vickers hardness, and extremely lower thermal diffusivity achieved in MPTMCs. The present discovery provides a promising approach toward the design and synthesis of next-generation high-performance materials.

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Determination of the structures of materials involving more light elements such as boron-rich compounds is challenging and technically important in understanding their varied compositions and superior functionalities. Here we resolve the long-standing uncertainties in structure and composition about the highest boride (termed MoB4, Mo1–x B3, or MoB3) through the rapid formation of large-sized boron-rich molybdenum boride under pressure. Using high-quality single-crystal X-ray diffraction analysis and aberration-corrected scanning transmission electron microscopy, we reveal that boron-rich molybdenum boride with a composition of Mo0.757B3 exhibits P63/mmc symmetry with a partial occupancy of 0.514 in 2b Mo sites (Mo1), and direct observations reveal the short-range ordering of cation vacancies in (010) crystal planes. Large anisotropic Young’s moduli and Vickers hardness are seen for Mo0.757B3, which may be attributed by its two-dimensional boron distributions. Mo0.757B3 is also found to be superconducting with a transition temperature (T c) of ∼2.4 K, which was confirmed by measurements of resistivity and magnetic susceptibility. Theoretical calculations suggest that the partial occupancy of Mo atoms plays a crucial role in the emergence of superconductivity.