Noncrystallographic Branching Research Articles

Preparation of compact spherulite of 2,2′,4,4′,6,6′-hexanitrostilbene (HNS) via specific adsorption of antisolvent combined with polymer

The application value of micro-nano hierarchical structure has been widely recognized in many fields, as it can greatly improve the surface performance of particles. Furthermore, the advantages of micro-nano hierarchical structure also bring new opportunities for the performance regulation of energetic materials, as an important material for both civilian and military applications, whose performance directly affects the application areas. This work developed a novel strategy for controlling the construction of compact spherulite of 2,2′,4,4′,6,6′-hexanitrostilbene(HNS) by utilizing antisolvent specific adsorption to generate noncrystallographic branching combined with the polymer additive to inhibit diffusion. Computational simulations revealed that toluene (TOL) exhibits specific adsorption on the (100) face of HNS through π-π interactions, while the polymer additive (Poly(acrylic acid), PAA) strongly interacts with each crystal face of HNS through hydrogen bonding, effectively inhibiting diffusion. Based on these findings, compact spherulites of HNS were successfully prepared by controlling supersaturation. The HNS spherulites exhibited superior flowability, specific surface area (11.84 m2/g for SP-I, 6.43 m2/g for SP-II), porosity (1.94 % for SP-I and 2.14 % for SP-II), and impact sensitivity (impact energy that can withstand increased from 4 J to 32.5 J for SP-I and to 11 J for SP-II). This work not only validates the effectiveness of antisolvent-induced branching in constructing compact spherulite structures, but also provides valuable insights into controlling crystal morphology. Furthermore, this research offers a promising approach to enhancing the safety and performance of energetic materials.

Structural Peculiarities of Natural Ballas—Spheroidal Variety of Polycrystalline Diamond

Ballas is a rare polycrystalline diamond variety characterized by a radially oriented internal structure and spheroidal outer shape. The origin of natural ballases remains poorly constrained. We present the results of a comprehensive investigation of two classic ballas diamonds from Brazil. External morphology was studied using SEM, high-resolution 3D optical microscopy, and X-ray tomography. Point and extended defects were examined on polished central plates using infra-red, photo- and cathodoluminescence spectroscopies, and electron back-scattering diffraction; information about nanosized inclusions was inferred from Transmission Electron Microscopy. The results suggest that fibrous diamond crystallites comprising ballas are split with pronounced rotation, causing concentric zoning of the samples. Pervasive feather-like luminescing structural features envelop single crystalline domains and most likely represent fibers with non-crystallographic branching. These features are enriched in N3 point defects. Twinning is not common. The nitrogen content of the studied samples reaches 700 at.ppm; its concentration gradually increases from the center to the rim. Annealing of the ballases took place at relatively high temperatures of 1125–1250 °C; the annealing continued even when the samples were fully grown, as suggested by the presence of the H4 nitrogen-related defects in the outer rim. Presumably, the ballas diamond variety was formed at high supersaturation but in conditions favoring a small growth kinetic coefficient. The carbon isotopic composition of the studied ballases (δ13C = −5.42, −7.11‰) belongs to the main mode of mantle-derived diamonds.

Fabrication of micro/nano spherulitic hierarchical energetic materials via noncrystallographic branching induced by polymer additives: A case study of 2,2′,4,4′,6,6′-hexanitrostilbene (HNS)

In this work, the micro/nano spherulitic hierarchical 2,2′,4,4′,6,6′-hexanitrostilbene (HNS) with the excellent specific surface area was prepared for the first time, which was expected to overcome the agglomeration problems of nanoscale HNS. In detail, poly(N-isopropyl acrylamide) (PNIPAM), polysuccinimide (PSI), and polyvinylpyrrolidone (PVP) were selected as additives to induce forming spherulitic HNS. Leaflike crystals were obtained in the presence of PNIPAM, while spherulitic HNS could be fabricated in the presence of PSI or PVP. And the prepared spherulitic HNS owned nearly 6 times the specific surface area of the raw HNS, with decreased impact sensitivity and advanced melting and decomposition temperatures, demonstrating a good desensitization effect, and a favorable energy release of the hierarchical structure. Moreover, morphological changes under the effect of the molecular weight and concentration of PVP were explored. The approximated morphological evolution of spherulitic HNS was consistent with the classical spherulitic growth by noncrystallographic branching. Furthermore, it was verified that preferential adsorption of PVP and PSI on the HNS inhibits the growth steps of the (200) face and introduces stress to generate crystal defects, which triggers noncrystallographic branching and further spherulitic growth. Overall, the strategy of fabricating micro/nano spherulitic hierarchical energetic materials via polymer-additive induced noncrystallographic branching is promising to solve the problems of nanoscale energetic materials in agglomeration and microscale bulk crystals in low activity.

230Th ∕ U isochron dating of cryogenic cave carbonates

Abstract. Cryogenic cave carbonates (CCCs) are a type of speleothem, typically dated with 230Th/U disequilibrium methods, that provide evidence of palaeo-permafrost conditions. In the field, CCCs occur as distinct patches of millimetre- to centimetre-sized loose crystals and crystal aggregates on the floors of cave chambers, so they lack a framework that would allow ages to be validated by stratigraphic order. Correction factors for the initial 230Th (230Th0) are often based on the bulk-earth-derived initial 230Th/232Th activity ratio ((230Th/232Th)0), which is a well-established approach when 230Th0 is moderately low. For samples with elevated levels of 230Th0, however, accuracy can be improved by constraining (230Th/232Th)0 independently. Here, we combine detailed morphological observations from three CCC patches found in Water Icicle Close Cavern in the Peak District (UK) with 230Th/U analyses. We find that individual CCC crystals show a range of morphologies that arise from non-crystallographic branching in response to the chemical evolution of the freezing solution. Results of 230Th/U dating indicate that samples with a large surface area relative to the sample volume are systematically more affected by contamination with 230Th0. By fitting isochrons to these results, we test whether the CCCs in a patch formed during the same freezing event, and demonstrate that (230Th/232Th)0 can deviate substantially from the bulk-earth-derived value and can also vary between the different CCC patches. Where CCCs display elevated 230Th0, isochrons are a useful tool to constrain (230Th/232Th)0 and obtain ages with improved accuracy. Detritus absorbed to the crystal surface is shown to be the most likely source of 230Th0. Our results suggest that some previously published CCC ages may merit re-assessment, and we provide suggestions on how to approach future dating efforts.

Growth mechanism of the spherulitic propylthiouracil–kaempferol cocrystal: new perspectives into surface nucleation

A growth mechanism of spherulitic cocrystals shows three stages of single crystal growth, non-crystallographic branching and surface nucleation. Low supersaturation forms loose spherulites, and high temperature causes “spherulites on spherulites”.

Novel Mode of Noncrystallographic Branching in the Initial Stages of Polymer Fibril Growth.

Spherulites are the most ubiquitous of polycrystalline microstructure of polymers; they develop under a wide range of conditions by the subsequent branching of crystalline lamella that results in an overall spherical shape. Despite significant efforts over decades, the mechanisms behind branching remain unclear. Molecular dynamics simulations in polyethylene reveal the molecular-level origin of noncrystallographic branching and the initial formation of fibrils. We find that the growth of crystalline lamella by reeling in and folding of polymer chains causes surprisingly large local deformation which, in turn, aligns the chains in the neighboring undercooled liquid. Thus, subsidiary grains nucleate with preferred orientations resulting in fibril growth with branching at small angles, consistent with those observed experimentally.

The source of lattice rotation in rotating lattice single (RLS) crystals

Laser heating can be used to produce single crystal architectures in glass with a lattice that rotates at a constant rate. Such metamaterials can offer properties that are disallowed by conventional crystal structures. To establish the mechanism of this lattice rotation, we used transmission electron microscopy (TEM) to directly observe and characterize dislocations in Sb2S3 crystal lines fabricated in Sb-S-I glass as a model system. The lattice rotation calculated from the density and Burgers vectors of edge dislocations agrees with lattice rotation values experimentally determined by electron backscatter diffraction (EBSD) and selected area diffraction patterns (SADP). These results provide the first direct proof of the dislocation mechanism of lattice rotation in rotating lattice single (RLS) crystals, and very likely other forms of growth actuated bending, twisting and noncrystallographic branching as seen in spherulites.

The Source of Lattice Rotation in Rotating Lattice Single (RLS) Crystals

Laser heating can be used to produce single crystal architectures in glass with a lattice that rotates at a constant rate. Such metamaterials can offer properties that are disallowed by conventional crystal structures. To establish the mechanism of this lattice rotation, we used transmission electron microscopy (TEM) to directly observe and characterize dislocations in Sb2S3 crystal lines fabricated in Sb-S-I glass as a model system. The lattice rotation calculated from the density and Burgers vectors of edge dislocations agrees with lattice rotation values experimentally determined by electron backscatter diffraction (EBSD) and selected area diffraction patterns (SADP). These results provide the first direct proof of the dislocation mechanism of lattice rotation in rotating lattice single (RLS) crystals, and very likely other forms of growth actuated bending, twisting and noncrystallographic branching as seen in spherulites.

Decoupled versus coupled growth dynamics of an irregular eutectic alloy

We present an experimental study of irregular growth patterns observed in real time during thin-sample directional solidification of a faceted/nonfaceted eutectic alloy, namely, the transparent 2-amino-2-methyl-1,3-propanediol (AMPD)-succinonitrile (SCN) system. The body-centered cubic SCN crystals are nonfaceted, while monoclinic AMPD crystals grow as faceted needles. At low velocities (<0.3 µms−1), a decoupled growth regime is observed, during which the tip of the AMPD crystals grows ahead of the SCN-liquid interface. At intermediate velocities, an unsteady coupled-growth regime takes place, with intermittent pinning of triple SCN-AMPD-liquid junctions, and frequent noncrystallographic branching. At higher velocities (>1 µms−1), two-phase fingers form.

Crystal nucleation and growth of spherulites demonstrated by coral skeletons and phase-field simulations

Spherulites are radial distributions of acicular crystals, common in biogenic, geologic, and synthetic systems, yet exactly how spherulitic crystals nucleate and grow is still poorly understood. To investigate these processes in more detail, we chose scleractinian corals as a model system, because they are well known to form their skeletons from aragonite (CaCO3) spherulites, and because a comparative study of crystal structures across coral species has not been performed previously. We observed that all 12 diverse coral species analyzed here exhibit plumose spherulites in their skeletons, with well-defined centers of calcification (CoCs), and crystalline fibers radiating from them. In 7 of the 12 species, we observed a skeletal structural motif not observed previously: randomly oriented, equant crystals, which we termed “sprinkles”. In Acropora pharaonis, these sprinkles are localized at the CoCs, while in 6 other species, sprinkles are either layered at the growth front (GF) of the spherulites, or randomly distributed. At the nano- and micro-scale, coral skeletons fill space as much as single crystals of aragonite. Based on these observations, we tentatively propose a spherulite formation mechanism in which growth front nucleation (GFN) of randomly oriented sprinkles, competition for space, and coarsening produce spherulites, rather than the previously assumed slightly misoriented nucleations termed “non-crystallographic branching”. Phase-field simulations support this mechanism, and, using a minimal set of thermodynamic parameters, are able to reproduce all of the microstructural variation observed experimentally in all of the investigated coral skeletons. Beyond coral skeletons, other spherulitic systems, from aspirin to semicrystalline polymers and chocolate, may also form according to the mechanism for spherulite formation proposed here. Statement of SignificanceUnderstanding the fundamental mechanisms of spherulite nucleation and growth has broad ranging applications in the fields of metallurgy, polymers, food science, and pharmaceutical production. Using the skeletons of reef-building corals as a model system for investigating these processes, we propose a new spherulite growth mechanism that can not only explain the micro-structural diversity observed in distantly related coral species, but may point to a universal growth mechanism in a wide range of biologically and technologically relevant spherulitic materials systems.

Spherulitic growth and morphology control of lithium carbonate: the stepwise evolution of core-shell structures

In this paper, we demonstrate morphologies, controlling factors (temperature, supersaturation and additives) and formation processes of lithium carbonate (Li2CO3) spherulites. Li2CO3 crystals were synthesized via a reaction crystallization and the effect of supersaturation was studied by changing temperature, adding antisolvent and altering Li+ ions concentration. Star-like and flower-like polycrystalline particles were produced without additives. By addition of sodium hexametaphosphate (SHMP), spherical particles could be obtained composed of radially arranged crystallites and core-shell structure was evolved at lower temperature. Time-resolved experiments were performed to detect the nucleation time and morphological changes. It was found that SHMP strongly inhibited nucleation and induced noncrystallographic branching. Two formation mechanisms were proposed to illustrate the stepwise evolution of core-shell spheres under different conditions. In addition, sodium tripolyphosphate exhibits similar effects with SHMP, while polymaleic acid modulates a different spherulitic growth mode. These findings provide new insights into spherulitic growth and fabrication of spherical particles.

Spherulitic Growth of Coral Skeletons and Synthetic Aragonite: Nature's Three-Dimensional Printing.

Coral skeletons were long assumed to have a spherulitic structure, that is, a radial distribution of acicular aragonite (CaCO3) crystals with their c-axes radiating from series of points, termed centers of calcification (CoCs). This assumption was based on morphology alone, not on crystallography. Here we measure the orientation of crystals and nanocrystals and confirm that corals grow their skeletons in bundles of aragonite crystals, with their c-axes and long axes oriented radially and at an angle from the CoCs, thus precisely as expected for feather-like or "plumose" spherulites. Furthermore, we find that in both synthetic and coral aragonite spherulites at the nanoscale adjacent crystals have similar but not identical orientations, thus demonstrating by direct observation that even at nanoscale the mechanism of spherulite formation is non-crystallographic branching (NCB), as predicted by theory. Finally, synthetic aragonite spherulites and coral skeletons have similar angle spreads, and angular distances of adjacent crystals, further confirming that coral skeletons are spherulites. This is important because aragonite grows anisotropically, 10 times faster along the c-axis than along the a-axis direction, and spherulites fill space with crystals growing almost exclusively along the c-axis, thus they can fill space faster than any other aragonite growth geometry, and create isotropic materials from anisotropic crystals. Greater space filling rate and isotropic mechanical behavior are key to the skeleton's supporting function and therefore to its evolutionary success. In this sense, spherulitic growth is Nature's 3D printing.

Investigations of spherulitic growth in industrial crystallization

The discrimination between crystal growth and aggregation is of crucial importance for the control of morphology and particle size in crystallization processes, as they are influenced in very different ways by the industrial processing environment. A collection of resembling solution-grown polycrystalline particles that differ widely in chemical nature, like elemental nickel, calcium and sodium carbonate, l-glutamic acid and an aromatic amine have been identified to grow by a spherulitic growth mechanism usually only associated with the crystallization of polymers or melts. The particles are not growing by agglomeration of small individual crystals, as often claimed in the literature. The effect of initial supersaturation, temperature and solvent composition on the spherulitic growth of calcium carbonate (vaterite) has been used to demonstrate how spherulites can grow from solution both by central multidirectional growth (in water) and by unidirectional growth followed by low angle branching (in 90 wt% ethylene glycol). The progression of non-crystallographic branching could be monitored as a function of time at intermediate initial supersaturation values, supplying direct visual evidence for spherulitic growth in this system. A reduction in initial supersaturation and temperature resulted in insufficient branching and dumbbell particles, whereas increased levels of supersaturation rapidly produced fully grown spherulites.

Formation kinetics of fractal nanofiber networks in organogels

The fractal structure of nanofiber networks formed in organogels is identified, using an in situ rheological method, together with a coupled supercritical fluid CO2 extraction/scanning electron microscopy technique. The rheological method allows us to have in-line measurements of the fractal growth of the nanofiber networks. In comparison with conventional light scattering techniques, the results obtained from this technique show much less scatter in data. Unlike conventional fractal aggregates, the growth of this type of fractal pattern is governed by a noncrystallographic branching mechanism, which occurs via self-mismatch nucleation and growth.

Theory of spherulitic crystallization

I show that diffusion-controlled growth can account for the occurrence of spherulites, their small-angle non-crystallographic branching, and the linear form of their radial growth rate. The characteristic dimension of the large-scale morphology is predicted to scale with √δ, where δ is the diffusion length. These results provide quantitative support for the mechanism of spherulitic growth proposed by Keith and Padden.

Observations of Microcrystalline Plagioclase Spherulites With the Transmission Electron Microscope

Two thin sections of macroscopic plagioclase spherulites of approximately 1 cm diameter found in a rhyolitic glass have been studied with the transmission electron microscope (TEM). Orientations of the thin sections were chosen to give views down and perpendicular to the major fiber axis. The crystalline fiber phase is high albite microtwinned on the (010) composition plane, elongated in the major growth direction, [001]. Fiber morphology is non‐polygonal with an average fiber diameter of 2000 Å perpendicular to c*. Fibers are separated by a non‐crystalline residuum layer of approximately constant thickness (300–500 Å). Microtwinning relationships, as well as selected area diffraction (SAD) patterns, reveal both crystallographic and non‐crystallographic branching with the former unexpectedly dominant.

A Phenomenological Theory of Spherulitic Crystallization

To account for spherulitic crystallization from the melt, one must explain the origins (i) of fibrous crystal habits in the absence of appreciable temperature gradients and (ii) of profuse noncrystallographic branching. Attention is drawn to properties held in common by spherulite-forming melts of various types and, in particular, to the facts that (a) they are multicomponent systems, (b) they exhibit small coefficients of self-diffusion, and (c) they crystallize slowly. It is shown that a consequence of these properties is that a plane crystal face cannot grow without suffering an instability of profile. Analagous instabilities lead in metal crystals to a cellular interface but, because of unusual growth kinetics, instability in spherulite-forming melts gives rise to a drastic modification of crystal habit. Bundles of discrete fibers are formed whose widths are determined by δ = D/G, D being the coefficient of self-diffusion and G being the growth rate. δ is generally small in these systems and commensurate with the scale of crystalline disorder in the fibers. It is this circumstance that allows noncrystallographic branching to occur.