Crystal Fragments Research Articles

Recognition and characterisation of high-grade ignimbrites from the Neoproterozoic rhyolitic volcanism in southernmost Brazil

Neoproterozoic magmatism in southern Brazil is associated with translithospheric shear belts and strike-slip basins in a post-collisional setting related to the last stages of the Brasilian-Pan African Orogenic Cycle. It evolved from an association of high-K calc-alkaline, leucocratic-peraluminous and continental tholeiitic magmas, to an association with shoshonitic magmas and, eventually, to an association with magmas of the sodic mildly alkaline series. This magmatism varies from metaluminous to peralkaline and exhibits alkaline sodic affinity. A large volcanism is related to this alkaline sodic magmatism and is named the Acampamento Velho Formation. This unit was coeval with subaerial siliciclastic sedimentation in post-collisional basins preserved in the region. The Acampamento Velho Formation consists of pyroclastic and effusive volcanic deposits, which are mainly silicic, emplaced under subaerial conditions. The best exposures of this volcanism occur on the Ramada and Taquarembó plateaus, located southwest of Rio Grande do Sul in southernmost Brazil. The pyroclastic flow deposits are composed mainly of juvenile fragments such as pumices, shards and crystal fragments. Welding is very effective in these units. High-grade ignimbrites occur at the base and intermediate portions of the deposits and rheoignimbrites are observed at the top. The pre-eruptive temperature calculations, which were obtained at the saturation of zircon, revealed values between 870 °C and 978 °C for Taquarembó Plateau and 850 °C–946 °C for Ramada Plateau. The calculated viscosity values vary from 6.946 to 8.453 log η (Pas) for the rheoignimbrites and 7.818 to 10.588 log η (Pas) for the ignimbrites. Zr contents increase toward the top of the pyroclastic sequence, which indicates an increase in peralkalinity and determines the reduction in viscosity for clasts at the upper portions of the flows. The patterns of the structures of the ignimbrites and rheoignimbrites in the Taquarembó and Ramada plateaus accords well with successive pyroclastic flows that halts en masse. In this model the entire pyroclastic flow halts en masse, so complex vertical changes in grain size and composition are interpreted as recording deposition from successive discrete pyroclastic flows. The stratification observed in intermediate units in Taquarembó Plateau might reflect in this case variation in eruptive dynamics and short pauses.

Ar diffusion and solubility measurements in plagioclases using the ultra-violet laser depth-profiling technique

Abstract We describe the first direct measurements of Ar diffusion and solubility in plagioclases using ultra-violet (UV) laser ablation depth-profiling and noble gas mass spectrometer analyses of experimentally treated (599–1000 °C, 50–200 MPa of Ar) crystal fragments of labradorite and oligoclase. Labradorite 40 Ar gain diffusion profiles were measured, yielding an activation energy of 26.72±4.58 kcal mol −1 (118.0±19.16 kJ mol −1 ) and a frequency factor of 9.77×10 −9 (+8.79×10 −8 , −8.79×10 −9 ) cm 2 s −1 (95% confidence). The Ar solubility in labradorite was measured yielding a value of <0.2 ppb bar −1 , which is similar to or lower than many rock forming minerals. The labradorite diffusion parameters indicate Ar closure temperatures of 211 °C for a spherical diffusion geometry, and 243 °C for a planar diffusion geometry (for 100 µm-diameter grains, with cooling rates of 10 °C Ma −1 ). The data indicate that labradorite is less Ar retentive than K-feldspar at low temperatures, but more Ar retentive than K-feldspar at high temperatures, corroborating previous work on plagioclase. The relatively slow Ar diffusion rates in labradorite at magmatic temperatures may explain the common observation of older ages in large plagioclase grains in acidic volcanic systems. Supplementary material: Details of the UV laser depth profiles obtained from the labradorite and oligoclase samples used in this study are available at www.geolsoc.org.uk/SUP18608 .

On the surface energy of crystals of inert gases

The dependences of the specific surface energy (σ) and its isochoric temperature derivative (∂σ/∂T) V on the degree of compression (V/V 0) of the crystal are calculated on the basis of the Mie-Lennard-Jones pair potential of interatomic interaction. The calculations are performed for all face-centered cubic crystals of inert gases (from Ne to Rn) to the degree of compression V/V 0 = 0.016 along three isotherms: 1K, T m and 300 K, where T m is the melting temperature at zero pressure (V/V 0 = 1). The activation processes such as the creation of vacancies and self-diffusion are taken into account in the calculations. It is shown that the isotherm σ(V/V 0) reaching its maximum at (V/V 0)max sharply decreases upon further compression. The surface energy becomes negative (σ(V/V 0) fr =0) at V/V 0 ≤ (V/V 0) fr < (V/V 0)max which should stimulate the process of crystal fragmentation, i.e., an increase in the specific (per atom) intercrystallite surface. It is shown that at high temperatures the condition of fragmentation holds in the crystal in the case of uniform tension, but it is already in the region of the liquid phase. The values of σ, (∂σ/∂T) V , the vacancy concentration and the fraction of the diffusion atoms are estimated at the points: V/V 0 = 1, (V/V 0)max and (V/V 0) fr at 1 K, Tm and 300 K. The size evolution of the surface and activation parameters is studied using neon as an example.

Natural age dispersion arising from the analysis of broken crystals: Part II. Practical application to apatite (U–Th)/He thermochronometry

We describe a new numerical inversion approach to deriving thermal history information from a range of naturally dispersed single grain apatite (U–Th)/He ages. The approach explicitly exploits the information about the shape of the 4He diffusion profile within individual grains that is inherent in the pattern of dispersion that arises from the common and routine practice of analysing broken crystals. Additional dispersion arising from differences in grain size and in U and Th concentration of grains, and the resultant changes to helium diffusivity caused by differential accumulation and annealing of radiation damage, is explicitly included. In this approach we calculate the ingrowth and loss, due to both thermal diffusion and the effects of α-ejection, of helium over time using a finite cylinder geometry. Broken grains are treated explicitly as fragments of an initially larger crystal. The initial grain lengths, L0, can be treated as unknown parameters to be estimated, although this is computationally demanding. A practical solution to the problem of solving for the unknown initial grain lengths is to simply apply a constant and sufficiently long L0 value to each fragment. We found that a good value for L0 was given by the maximum fragment length plus two times the maximum radius of a given set of fragments. Currently whole crystals and fragments with one termination are taken into account. A set of numerical experiments using synthetic fragment ages generated for increasingly complex thermal histories, and including realistic amounts of random noise (5–15%), are presented and show that useful thermal history information can be extracted from datasets showing very large dispersion. These include experiments where dispersion arises only from fragmentation of a single grain (length 400μm and radius 75μm, c. 6–50% dispersion), including the effects of grain size variation (for spherical equivalent grain radii between 74 and 122μm, c. 10–70% dispersion) and the combined effects of fragmentation, grain size and radiation damage (for eU between 5 and 150ppm, c. 10–107% dispersion). Additionally we show that if the spherical equivalent radius of a broken grain is used as a measure of the effective diffusion domain for thermal history inversions then this will likely lead to erroneous thermal histories being obtained in many cases. The viability of the new technique is demonstrated for a real data set of 25 single grain (U–Th)/He apatite ages obtained for a gabbro sample from the BK-1 (Bierkraal) borehole drilled through the Bushveld Complex in South Africa. The inversion produces a well constrained thermal history consistent with both the (U–Th)/He data and available fission track analysis data. The advantage of the new approach is that it can explicitly accommodate all the details of conventional schemes, such as the effects of temporally variable diffusivity, zonation of U and Th and arbitrary grain size variations, and it works equally effectively for whole or broken crystals, and for the most common situation where a mixture of both are analysed. For the routine application of the apatite (U–Th)/He thermochronometry technique with samples where whole apatite grains are rare our experiments indicate that 15–20 single grain analyses are typically required to characterise the age dispersion pattern of a sample. The experiments also suggest that picking very short crystal fragments as well as long fragments, or even deliberately breaking long crystals to maximise the age dispersion in some cases, would ensure the best constraints on the thermal history models. The inversion strategy described in this paper is likely also directly applicable to other thermochronometers, such as the apatite, rutile and titanite U–Pb systems, where the diffusion domain is approximated by the physical grain size.

Evolution of Poly(propylene) Morphology in the Rubbery State under Uniaxial Strain

The evolution of the crystalline morphology of two poly(propylene) films deformed in the rubbery state using wide‐ and small‐angle X‐ray scattering and extensional rheology experiments is presented. The mechanical behavior can be either isotropic or anisotropic depending on the manufacturing process. The microstructure change of the polymer is linked to the thermomechanical behavior of PP films below and close to the melting temperature. The reorientation of the crystalline phase during the deformation is shown to occur quickly when the deformation temperature is largely lower than the melting temperature and for low strain rates. The formation of a fibrillar structure oriented along the stretching direction following the fragmentation of initial crystals is highlighted.

Natural age dispersion arising from the analysis of broken crystals. Part I: Theoretical basis and implications for the apatite (U–Th)/He thermochronometer

Over the last decade major progress has been made in developing both the theoretical and practical aspects of apatite (U–Th)/He thermochronometry and it is now standard practice, and generally seen as best practice, to analyse single grain aliquots. These individual prismatic crystals are often broken and are fragments of larger crystals that have broken during mineral separation along the weak basal cleavage in apatite. This is clearly indicated by the common occurrence of only 1 or no clear crystal terminations present on separated apatite grains, and evidence of freshly broken ends when grains are viewed using a scanning electron microscope. This matters because if the 4He distribution within the whole grain is not homogeneous, because of partial loss due to thermal diffusion for example, then the fragments will all yield ages different from each other and from the whole grain age. Here we use a numerical model with a finite cylinder geometry to approximate 4He ingrowth and thermal diffusion within hexagonal prismatic apatite crystals. This is used to quantify the amount and patterns of inherent, natural age dispersion that arises from analysing broken crystals. A series of systematic numerical experiments were conducted to explore and quantify the pattern and behaviour of this source of dispersion using a set of 5 simple thermal histories that represent a range of plausible geological scenarios. In addition some more complex numerical experiments were run to investigate the pattern and behaviour of grain dispersion seen in several real data sets. The results indicate that natural dispersion of a set of single fragment ages (defined as the range divided by the mean) arising from fragmentation alone varies from c. 7% even for rapid (c. 10°C/Ma), monotonic cooling to over 50% for protracted, complex histories that cause significant diffusional loss of 4He. The magnitude of dispersion arising from fragmentation scales with the grain cylindrical radius, and is of a similar magnitude to dispersion expected from differences in absolute grain size alone (spherical equivalent radii of 40–150μm). This source of dispersion is significant compared with typical analytical uncertainties on individual grain analyses (c. 6%) and standard deviations on multiple grain analyses from a single sample (c. 10–20%). Where there is a significant difference in the U and Th concentration of individual grains (eU), the effect of radiation damage accumulation on 4He diffusivity (assessed using the RDAAM model of Flowers et al. (2009)) is the primary cause of dispersion for samples that have experienced a protracted thermal history, and can cause dispersion in excess of 100% for realistic ranges of eU concentration (i.e. 5–100ppm). Expected natural dispersion arising from the combined effects of reasonable variations in grain size (radii 40–125μm), eU concentration (5–150ppm) and fragmentation would typically exceed 100% for complex thermal histories. In addition to adding a significant component of natural dispersion to analyses, the effect of fragmentation also acts to decouple and corrupt expected correlations between grain ages and absolute grain size and to a lesser extent between grain age and effective uranium concentration (eU). Considering fragmentation explicitly as a source of dispersion and analysing how the different sources of natural dispersion all interact with each other provides a quantitative framework for understanding patterns of dispersion that otherwise appear chaotic. An important outcome of these numerical experiments is that they demonstrate that the pattern of age dispersion arising from fragmentation mimics the pattern of 4He distribution within the whole grains, thus providing an important source of information about the thermal history of the sample. We suggest that if the primary focus of a study is to extract the thermal history information from (U–Th)/He analyses then sampling and analytical strategies should aim to maximise the natural dispersion of grain ages, not minimise it, and should aim to analyse circa 20–30 grains from each sample. The key observations and conclusions drawn here are directly applicable to other thermochronometers, such as the apatite, rutile and titanite U–Pb systems, where the diffusion domain is approximated by the physical grain size.

Temporal variation in volcanic ash texture during a vulcanian eruption at the Sakurajima volcano, Japan

We discuss the process of vulcanian eruptions on the basis of temporal variation of the texture of andesitic volcanic ash ejected from the Sakurajima volcano, Japan. Falling ash samples were collected every 2–50min for petrological analysis. These ash samples contained juvenile particles, altered rock fragments, and fragments of crystals; here, we subdivide the juvenile particles into blocky, vesicular, and fluidal particles. Because the crystallinities of the vesicular and fluidal particles are lower than those of the blocky particles, we refer to the vesicular and fluidal particles as low-crystallinity particles (LCPs) and the blocky particles as high-crystallinity particles (HCPs). The temporal variation observed in ash texture reveals that the ratio of LCPs to HCPs (LCP/HCP) was small at the initiation of the eruption and increased as the eruption progressed. The higher microlite crystallinity in the HCPs indicates crystallization under lower pressures for longer durations relative to the LCP magma. The increase of the LCP/HCP ratio during a single vulcanian eruption suggests that the emission of less viscous gas-rich magma under a cap rock increases as the eruption progresses. Such temporal variation in ash texture reflects the transition from a sudden explosion to continuous ash emission during a vulcanian eruption.

Molecular Insights into Diphenylalanine Nanotube Assembly: All-Atom Simulations of Oligomerization

Self-assembling peptides represent a growing class of inexpensive, environmentally benign, nanostructured materials. In particular, diphenylalanine (FF) self-assembles into nanotubes with remarkable strength and thermal stability that have found use in a wide variety of applications, including as sacrificial templates and scaffolds for structuring inorganic materials and as interfacial "nanoforests" for superhydrophobic surfaces and high-performance supercapacitors and biosensors. However, little is known about the assembly mechanisms of FF nanotubes or the forces underlying their stability. Here, we perform a variety of molecular dynamics simulations on both zwitterionic and capped (uncharged) versions of the FF peptide to understand the early stages of self-assembly. We compare these results to simulations of the proposed nanotube X-ray crystal structure. When comparing the zwitterionic and uncharged FF peptides, we find that, while electrostatic interactions steer the former into more ordered dimers and trimers, the hydrophobic side chain interactions play a strong role in determining the structures of larger oligomers. Simulations of the crystal structure fragment also suggest that the strongest interactions occur between side chains, not between the charged termini that form salt bridges. We conclude that the amphiphilic nature of FF is key to understanding its self-assembly, and that the early precursors to nanotube structures are likely to involve substantial hydrophobic clustering, rather than hexamer ring motifs as has been previously suggested.

On self-diffusion and surface energy upon compression or tension of an iron crystal

The dependences of the activation parameters (formation of vacancies and self-diffusion) and specific surface energy on the volume fraction (V/V0) are calculated in terms of the Mie-Lenard-Jones pair potential of interatomic interaction for bcc-Fe along the 300-K and 3000-K isotherms. It is shown that under strong compressions (V/V0 1), the surface energy has a negative value, which must lead to the crystal structure fragmentation.

Deformation-induced structure evolution of oriented β-polypropylene during uniaxial stretching

β phase isotactic polypropylene (β-iPP) with the β phase lamellae oriented parallel to the melt extrusion direction, was employed to investigate the deformation-induced structure evolution at various temperatures (25, 80, 110, 130 and 140 °C). The orientation change of β phase during deformation greatly influences the β to α phase transformation and its temperature dependence. At temperatures lower than 110 °C, the orientation of β phase is almost unchanged during deformation, and void or crack forms before fragmentation and reorientation of β phase. The crystal size change of β phase is small, and the defolding of the β lamellae triggers the β to α phase transformation. As the deformation temperature rises to 130 and 140 °C, the reorientation of β phase occurs gradually upon stretching, and the size of micro-voids decreases due to the fact that less β crystal fragmentation takes place at high draw temperature than that at low temperature. The β to α phase transformation is mainly induced by intra-lamella slip, and the trend of crystal size change of β phase is larger. The chains orientation changes from perpendicular to the stretching direction in β phase to parallel to the stretching direction in α phase is achieved by the chains defolding of β phase along the stretching direction at temperatures lower than 110 °C, and it is through the chains reorientation of β phase along the stretching direction at temperatures of 130 and 140 °C. The crystal size of α phase of the deformed β-iPP during deformation depends on the dynamic balance of the breakage of existing α crystal and the formation of new crystal through phase transformation. Specially, at draw temperature of 25 °C, the slippage of β phase relieves the breakage of α phase crystal, which indicates that the high content of β phase crystal really accounts for the toughening effect on iPP.

Diamondiferous Archean rocks of the Olondo greenstone belt (western Aldan–Stanovoy shield)

Diamond from metaultramafic rocks of the Mesoarchean (2.96–3.0 Ga) Olondo greenstone belt, located in the western Aldan–Stanovoy shield, has been studied. Diamonds occur in lenses of olivine–serpentine–talc rocks within metaultramafic rocks of intrusive habit, whose composition corresponds to peridotite komatiites. All diamonds from the metaultramafic rocks are crystal fragments 0.3 to 0.5 mm in size. Morphological examination has revealed laminar octahedra, their transitional forms to dodecahedroids, crystals with polycentric faces, and spinel twins. The crystals vary in photoluminescence color: dark blue, green, yellow, red, or albescent. Characteristic absorption bands in crystals point to nitrogen impurities in the form of A and B1 defects and tabular B2 defects. The crystals studied belong to the IaA/B type, common among natural diamonds. The overall nitrogen content varies from <100 to 3800 ppm. The relative content of nitrogen in B1 centers varies from 0 to 94%, pointing to long stay in the mantle. The carbon isotope ratio in the diamonds, 13C = −26‰, is indicative of involvement of subducted crust matter in diamond formation in the Archean.

Recent analog of gypsified microbial laminites and stromatolites in solar salt works and the Miocene gypsum deposits of Saudi Arabia and Egypt

Accumulation of microbial mats and stromatolites dominate in the crystallization ponds of solar salt works west of Alexandria, Egypt. These microbial mats are laminar in the permanent submerged part of the ponds. The microbial mats commonly form sites for growth of gypsum crystals during periods having higher salinity. In the dominant submerged part of the pond, domal stromatolites are common around groundwater seepage holes. In the shallow, intermittent margin of the ponds, the laminated microbial structure forms laterally close-linked hemispheroidal stromatolite type, with unidirectional and multidirectional ripple mark-like morphology on their surface. The microbial laminite and stromatolite types in the modern solar salt works are similar to the organic-rich Miocene gypsum beds of El-Barqan (west Alexandria, Egypt) and Rabigh (north Jeddah, Saudi Arabia). The Miocene organic-rich beds consist of interlayered dark-colored microbial laminae and light-colored gypsum laminae. These beds may have three different variations: regular even lamination, laterally closed-linked hemispheroidal stromatolites, and/or discrete hemispheroidal stromatolites. Petrographic examination of the microbial laminites and stromatolites in the solar salt works and the Miocene gypsum beds indicate that the dark-colored, organic-rich laminae are composed of micritized microbial laminae and/or brown organic filaments. In El-Barqan area, the light-colored gypsum-rich laminae are composed of either gypsum crystal fragments, or lenticular and prismatic gypsum. These gypsum crystals are either entrapped within the microbial filaments or are nucleated at the surface of the microbial laminae to form a radial pattern, whereas in Rabigh area, the light-colored gypsum-rich laminae are composed of secondary porphyrotopic, poikilotopic, or granular gypsum crystals. By comparison of the microbial structure in the Miocene gypsum beds with the recent occurrence of the microbial laminites and stromatolites in the solar salt works, it is demonstrated that the organic-rich Miocene gypsum beds were formed in a very shallow salina with slightly fluctuating brine levels.

In situ small-angle X-ray and neutron scattering measurements on a blend of deuterated and hydrogenated polyethylenes during uniaxial drawing

In situ small-angle X-ray and neutron scattering (SAXS and SANS) measurements were conducted on an isotropic blend of deuterated polyethylene and hydrogenated polyethylene (HPE; 3 wt%) during uniaxial drawing at 125 °C to clarify the formation mechanism of the shish-kebab from the isotropic film. In the early stage of drawing, two-spot scattering patterns were observed along the drawing direction in both the SAXS and SANS patterns, and the patterns gradually shifted to lower angles, which indicated that the isotropic lamellar crystals rotate and orient to align and that the long period increases because of the elongation of the amorphous region. In the late stage of drawing, the long-period peak gradually shifted to higher angles and the peak width broadened and finally disappeared at the local drawing ratio RD∼6.0, suggesting fragmentation of lamellar crystals (kebabs) and pulling of the polymer chains out of the kebab. However, only the 2D SANS patterns exhibited streak-like scattering in the direction normal to drawing above RD∼2.4, suggesting the preferential orientation and stretching of the HPE chains to form the extended chain crystal (shish). By analyzing the SANS data using the multicore-shell cylinder model, the diameter of the shish that consisted of HPE decreased from 9.0 to 4.5 nm when the drawing was increased from RD=3.8 to 8.0. In situ small-angle X-ray and neutron scattering (SAXS and SANS) measurements were conducted on an isotropic blend of deuterated polyethylene (97 wt%) and hydrogenated polyethylene (3 wt%) during uniaxial drawing at 125 °C to clarify the formation mechanism of shish-kebab from isotropic film. Figure shows that 2D SANS and SAXS patterns in the various drawing ratio, RD, conditions. The drawing direction is longitudinal. In RD=8, the streak-like scattering in the normal direction of drawing became stronger only in the SANS, showing that more hydrogenated polyethylene chains were merged into the shish.

Structural features and mechanical properties of austenitic Hadfield steel after high-pressure torsion and subsequent high-temperature annealing

Mechanisms of structure fragmentation and strengthening of single crystals of a Hadfield steel after warm torsion under high-pressure torsion (HPT) and subsequent annealing in a temperature range of 400–800°C have been studied. Multiple twinning and formation of ultrafine carbides upon HPT at 400°C (P = 5 GPa) promote rapid fragmentation of the microstructure. They are responsible for the high mechanical properties of the steel after HPT and the thermal stability of the microstructure up to an annealing temperature of 500°C.

On the surface properties and baric fragmentation of iron

The dependence of the specific surface energy (σ) on the normalized volume (V/V 0) and temperature (T) for the body-centered cubic (BCC) lattice of iron has been studied on the basis of the Mie-Lennard-Jones potential of interatomic interaction. It is shown that below the definite value of the normalized volume (V/V 0)fr the specific surface energy of the BCC-Fe lattice passes to the negative range of values: σ(V/V 0)fr = 0, and the (V/V 0)fr value increases almost linearly with temperature. In the case of compression, when (V/V 0) < (V/V 0)fr, the exothermal process of crystal fragmentation into dendritic domains with the maximum possible specific intercrystalline surface takes place. In the case of nanofragmentation, surface pressure (P sf) emerges, leading to self-packing of the formed nanocrystals. The dependence of the σ(V/V 0) and P sf (V/V 0) functions on the size and shape of the BCC-Fe nanocrystals has been studied at different temperature values T = 1500–3500 K. It has been shown that the P sf function increases with a decrease in the size; this occurs more strongly, the more the nanocrystal deviates from the most thermodynamically stable cubic shape. The dimensional compression of the lattice parameter of BCC-Fe nanocrystals has been studied. The specific (per volume unit) amount of heat released during fragmentation of the BCC-Fe lattice at high pressures and temperatures has been estimated.

Genesis and hydrocarbon significance of vesicular welded tuffs: A case study from the Fengcheng Formation, Wu-Xia area, Junggar Basin, NW China

Abstract On the basis of a large number of core and thin-section observations, the petrologic and pore evolution characteristics of welded tuffs in the Fengcheng Formation in the Wu-Xia area, Junggar Basin, are evaluated, and the genesis of vesicular welded tuffs and the formation mechanism of vesicles are analyzed. Volcanic activity in the Fengcheng Formation is characterized by welded pyroclastic rocks. There are predominant plastic shards with rounded edges and obvious devitrification in the rocks. Plastic debris (fiamme) is commonly deformed and wrinkled, and crystal fragments are usually corroded. A lot of lithophysae occur in the welded tuffs, and the welded texture is obscured due to the occurrence of these lithophysae features. Vesicles are the cavities of lithophysae, which are formed by solidification contraction. They have little or no late filling cement because the volatile constituent and water vapour in the bubbles have escaped with the help of devitrification. Lithophysae distribution within a cooling unit are characterized by vertical zonation: increasing upwards but decreasing gradually later. The cavities of lithophysae have greatly improved the reservoir quality of welded tuffs. Influenced by the intensive welding process, lithophysae are not subject to collapse or damage. In the study area, deeply buried vesicular welded tuffs are important hydrocarbon reservoir rocks, and the vesicular welded tuff has good petroliferous properties.

Numerical implementation of a damage‐coupled material law for semicrystalline polyethylene

PurposeThe purpose of this paper is to provide a computational procedure for a novel damage‐coupled material law for semicrystalline polyethylene. Using a damage mechanics approach, the model seeks to gain insight into the mechanical behaviour of polyethylene considering the microstructure and degradation processes occurring under uniaxial tension.Design/methodology/approachThe material morphology is modelled as a collection of inclusions. Each inclusion consists of crystalline material lying in a thin lamella attached to an amorphous layer. The interface region interconnecting the two phases is the plane through which loads are carried and transferred by the tie molecules. It is assumed that the constitutive model contains complete information about the mechanical behaviour and degradation processes of each constituent. After modelling the two phases independently, the inclusion behaviour is found by applying some compatibility and equilibrium restrictions along the interface plane.FindingsThe model provides a rational representation of the damage process of the intermolecular bonds holding crystals and of the tie‐molecules connecting neighbouring crystallites. The model is also used to analyze the degree of relationship between some of the material properties and the mechanical responses.Practical implicationsIn practice, the numerical model clearly helps to understand the influence of the different microstructure properties on the tensile mechanical behaviour of semicrystalline polyethylene – an issue of particular interest in improving material processability and product performance.Originality/valueTo the authors’ knowledge, a phenomenon such as microstructural degradation of polyethylene has not received much attention in the literature. The proposed model successfully captures aspects of the material behaviour considering crystal fragmentation and tie‐molecule rupture.

Growing Fragmented Potentials for Gas–Surface Reactions: The Reaction between Hydrogen Atoms and Hydrogen-Terminated Silicon (111)

Systematic molecular fragmentation of nonconducting crystals has been combined with modified Shepard interpolation to develop an automated procedure to study the dynamics of gas–surface reactions. Any level of molecular electronic structure theory can be used to construct the gas–surface potential energy surface. The new methodology has been applied to the interaction of hydrogen atoms with the hydrogen-terminated silicon (111) surface at normal incidence. Direct hydrogen abstraction, hydrogen atom exchange, hydrogen atom sorption, and inelastic scattering processes were all found to be significant.

Merging boron solid state and molecular chemistry: Energy landscapes in the exo/endo closo-borane complex Sc[B24H24

The crystal structure of ScB12 suggests the possible existence of the closo B24H242− borane and derived exo and endohedral complexes. The extraction of the B24 ‘perfect’ truncated octahedron from the ScB12 crystal structure and the minimization of the energy by means of quantum-chemical computations leads to a snub cube structure. The two found stable exohedral structures are energetically more favorable than the endohedral complex by only ∼1 and ∼9kcal/mol. The optimized geometry of the B24H242− molecular structure can be derived from the crystal fragment through a shrinkage plus pseudo-rotation of the B24 cage. Analogous solid structures embedding truncated octahedra are also compared with the title structure.

Simple evolution of complex crystal species

Cairns-Smith has proposed that life began as structural patterns in clays that self-replicated during cycles of crystal growth and fragmentation. Complex, evolved crystal forms could then have catalyzed the formation of a more advanced genetic material. A crucial weakness of this theory is that it is unclear how complex crystals might arise through Darwinian evolution and selection. Here we investigate whether complex crystal patterns could evolve using a model system for crystal growth, DNA tile crystals, that is amenable to both theoretical and experimental inquiry. It was previously shown that in principle, the evolution of crystals assembled from a set of thousands of DNA tile types under very specific environmental conditions could produce arbitrarily complex patterns. Here we show that evolution driven only by the dearth of one monomer type could produce complex crystals from just 12 monomer types. When a monomer type is rare, crystals that use few of this monomer type are selected for. We use explicit enumeration to show that there are situations in which crystal species that use a particular monomer type less frequently will grow faster, yet to do so requires that the information contained in the crystal become more complex. We show that this feature of crystal organization could allow more complex crystal morphologies to be selected for in the right environment, using both analysis in a simple model of self-assembly and stochastic kinetic simulations of crystal growth. The proposed mechanism of evolution is simple enough to test experimentally and is sufficiently general that it may apply to other DNA tile crystals or even to natural crystals, suggesting that complex crystals could evolve from simple starting materials because of relative differences in concentrations of the materials needed for growth.