Crystal Bundles Research Articles

Evolution of the microstructure of Sn–Ag–Cu solder joints exposed to ultrasonic waves during solidification

Cu/SAC405/Cu solder joints were fabricated using a modified reflow-soldering procedure. The samples were first maintained at 260°C for 320s, following the conventional reflow-soldering methodology. The reflow process was then interrupted and the samples were exposed to ultrasonic waves (USW) while they were cooling in air. Compared with a sample reflow-soldered conventionally, the solidification of the Sn–Ag0.04–Cu0.005 (SAC405) solder filler metal alloy under the influence of USW resulted in significant changes to the microstructure of the solder joints. The thickness of the Cu6Sn5 intermetallic layer at the Cu/SAC interface of the USW-solidified soldered joint decreased by as much as 76%. The β-Sn dendrite width was also reduced by as much as 67%, and the SAC matrix was filled with bundles of acicular Cu6Sn5 crystals. The formation of Ag3Sn plates was also prevented, and the size of the rod-like SAC ternary eutectic matrix was reduced from 700nm to 50nm. This behaviour is attributed to the effect of cavitation and liquid metal streaming induced by the USW on nucleation and the whole solidification process. The presence of Cu6Sn5 bundles and the refined eutectic and β-Sn dendrites in the matrix led to an average improvement in the hardness of the solder bulk by 45%.

Amelogenin-Collagen Interactions Regulate Calcium Phosphate Mineralization in Vitro

Collagen and amelogenin are two major extracellular organic matrix proteins of dentin and enamel, the mineralized tissues comprising a tooth crown. They both are present at the dentin-enamel boundary (DEB), a remarkably robust interface holding dentin and enamel together. It is believed that interactions of dentin and enamel protein assemblies regulate growth and structural organization of mineral crystals at the DEB, leading to a continuum at the molecular level between dentin and enamel organic and mineral phases. To gain insight into the mechanisms of the DEB formation and structural basis of its mechanical resiliency we have studied the interactions between collagen fibrils, amelogenin assemblies, and forming mineral in vitro, using electron microscopy. Our data indicate that collagen fibrils guide assembly of amelogenin into elongated chain or filament-like structures oriented along the long axes of the fibrils. We also show that the interactions between collagen fibrils and amelogenin-calcium phosphate mineral complexes lead to oriented deposition of elongated amorphous mineral particles along the fibril axes, triggering mineralization of the bulk of collagen fibril. The resulting structure was similar to the mineralized collagen fibrils found at the DEB, with arrays of smaller well organized crystals inside the collagen fibrils and bundles of larger crystals on the outside of the fibrils. These data suggest that interactions between collagen and amelogenin might play an important role in the formation of the DEB providing structural continuity between dentin and enamel.

Bundles of fluid fibers formed by bent-core molecules

The internal structures, stability, and elastic properties of free-standing filament bundles of bent-core smectic liquid crystals are studied using polarizing microscopy, scanning electron microscopy, and a customized cantilever containing, temperature controlled heat stage for force measurements. We find that in bundles, the individual filaments fuse together to form semiseparable filaments with radii approximately two and three times larger than the individual fibers. We also find that the effective surface tension of wide bundles is about 10% smaller than a single filament. Finally, we describe the metastable coexistences of single fibers within the bundles that lead to bending of the bundles. All of these observations were explained with simple macroscopic models.

A New Approach to Efficiently Disperse Aggregated Palygorskite Into Single Crystals via Adding Freeze Process Into Traditional Extrusion Treatment

This paper reports a processing technology of natural nanorod materials. Palygorskite is a type of natural nanorod hydrous clay mineral, and its basic unit is about 20-40 nm in diameter and 1-5 ?m in length. Low-cost single crystals of palygorskite can be produced in mass by simply adding freeze process into traditional extrusion treatment. The dispersion state of palygorskite was investigated by X-ray diffraction (XRD), particle size distribution, SEM, transmission electron microscopy (TEM), and viscosity techniques. The effects of the freeze process and treatment time on the dispersive quality of palygorskite were also studied. The results indicated that the new approach was efficient to disperse palygorskite ore into nanometer-scale particles and the crystal bundles of palygorskite into single crystals. Moreover, the new approach had much lower breakage of the long fine fiber of palygorskite compared with the dry process. The freeze process had remarkable influence on the improvement of the dispersion of palygorskite ore. The viscosity of the sample treated with the freeze process was enhanced twice from 472 to 3894 mpa·s, which was about 2.6 times that of the sample treated without the freeze process. The treated samples showed very uniform fibers about 20-40 nm in width and 1-5 ?m in length, as observed from TEM, which is in good agreement with the perfect rod-shaped crystals of palygorskite. In addition, the treated samples showed XRD patterns similar to that of the untreated palygorskite ore, which suggests that the treated samples retained their original structure.

Role of 20-kDa Amelogenin (P148) Phosphorylation in Calcium Phosphate Formation in Vitro

The potential role of amelogenin phosphorylation in enamel formation is elucidated through in vitro mineralization studies. Studies focused on the native 20-kDa porcine amelogenin proteolytic cleavage product P148 that is prominent in developing enamel. Experimental conditions supported spontaneous calcium phosphate precipitation with the initial formation of amorphous calcium phosphate (ACP). In the absence of protein, ACP was found to undergo relatively rapid transformation to randomly oriented plate-like apatitic crystals. In the presence of non-phosphorylated recombinant full-length amelogenin, rP172, a longer induction period was observed during which relatively small ACP nanoparticles were transiently stabilized. In the presence of rP172, these nanoparticles were found to align to form linear needle-like particles that subsequently transformed and organized into parallel arrays of apatitic needle-like crystals. In sharp contrast to these findings, P148, with a single phosphate group on serine 16, was found to inhibit calcium phosphate precipitation and stabilize ACP formation for more than 1 day. Additional studies using non-phosphorylated recombinant (rP147) and partially dephosphorylated forms of P148 (dephoso-P148) showed that the single phosphate group in P148 was responsible for the profound effect on mineral formation in vitro. The present study has provided, for the first time, evidence suggesting that the native proteolytic cleavage product P148 may have an important functional role in regulating mineralization during enamel formation by preventing unwanted mineral formation within the enamel matrix during the secretory stage of amelogenesis. Results obtained have also provided new insights into the functional role of the highly conserved hydrophilic C terminus found in full-length amelogenin.

Thermal decomposition of calcite: Mechanisms of formation and textural evolution of CaO nanocrystals

Field emission scanning electron microscopy (FESEM), two-dimensional X-ray diffraction (2DXRD), and transmission electron microscopy coupled with selected area electron diffraction (TEMSAED) analyses of the reactant/product textural relationship show that the thermal decomposition of Iceland spar single crystals according to the reaction CaCO3(s) → CaO(s) + CO2(g) is pseudomorphic and topotactic. This reaction begins with the formation of a mesoporous structure made up of up to four sets of oriented rod-shaped CaO nanocrystals on each rhombohedral cleavage face of the calcite pseudomorph. The four sets formed on (101̅4)calcite display the following topotactic relationships: (1) (12̅10)calcite//(110)CaO; (2) (1̅104)calcite┴ (110)CaO; (3) (1̅018)calcite//(110)CaO; and (4) (01̅14)calcite┴(110)CaO; with [841]calcite//[11̅0]CaO in all four cases. At this stage, the reaction mechanism is independent of PCO2 (i.e., air or high vacuum). Strain accumulation leads to the collapse of the mesoporous structure, resulting in the oriented aggregation of metastable CaO nanocrystals (~5 nm in thickness) that form crystal bundles up to ~1 μm in cross-section. Finally, sintering progresses up to the maximum T reached (1150 °C). Oriented aggregation and sintering (plus associated shrinking) reduce surface area and porosity (from 79.2 to 0.6 m2/g and from 53 to 47%, respectively) by loss of mesopores and growth of micrometer-sized pores. An isoconversional kinetic analysis of non-isothermal thermogravimetric data of the decomposition of calcite in air yields an overall effective activation energy Eα = 176 ± 9 kJ/ mol (for α > 0.2), a value which approaches the equilibrium enthalpy for calcite thermal decomposition (177.8 kJ/mol). The overall good kinetic fit with the F1 model (chemical reaction, first order) is in agreement with a homogeneous transformation. These analytical and kinetic results enable us to propose a novel model for the thermal decomposition of calcite that explains how decarbonation occurs at the atomic scale via a topotactic mechanism, which is independent of the experimental conditions. This new mechanistic model may help reinterpret previous results on the calcite/CaO transformation, having important geological and technological implications.

Nannobacteria, Organic Matter, and Precipitation in Hot Springs, Viterbo, Italy: Distinctions and Relevance

Abstract Tiny (50-200 nm) spheroids were first discovered by Folk through SEM work on the hot springs of Viterbo Italy. He termed these small, spherical structures “nannobacteria,” and proposed that they may be important agents in precipitation of CaCO3, as needle-like crystals of the mineral aragonite, and as bundles of such needle-like crystals (termed “fuzzy dumbbells”), or as elongated crystals of the mineral calcite. During the past 15 years, nanometer-scale spheroids have been discovered in the geological, medical, and astronomical worlds. There can be no doubt as to their existence, but their significance and origin remain a subject of continuing controversy. Even the spelling (“nanno-“), which has been the standard in biology, geology, and paleontology going back to the 19th century, has been questioned. Whether or not they are truly bacteria or any form of life has been a subject of heated debate.

Experimental determination of growth rate effect on U 6+ and Mg 2+ partitioning between aragonite and fluid at elevated U 6+ concentration

Results are reported from an experimental study in which the partitioning of U and Mg between aragonite and an aqueous solution were determined as a function of crystal growth rate. Crystals, identified as aragonite by X-ray diffractometry and micro-Raman spectroscopy, were grown by diffusion of CO 2 from an ammonium carbonate source into a calcium-bearing solution at temperatures of 22 and 53 °C. Hemispherical bundles (spherulites) of aragonite crystals were produced, the growth rates of which decreased monotonically from the spherulite interiors to the edges and thus provide the opportunity to examine the influence of growth rate on crystal composition. Element concentration ratios were measured using electron microprobe (EMP) and fluid composition was determined by inductively coupled plasma-mass spectrometry (ICP-MS) and atomic absorption (AA). Growth rates were determined directly by addition of a Dy spike to the fluid during the experiment that was subsequently located in an experimentally precipitated spherulite using secondary ion mass spectrometry (SIMS). At 22 °C both U/Ca and Mg/Ca partition coefficients exhibited a strong growth rate dependence when crystal growth rates were low, and became independent of growth rate when crystal growth rates were high. The U/Ca ratios in aragonite increase between 22 and 53 °C; in contrast Mg/Ca ratios show inverse dependence on temperature.

Biomimetic synthesis of oriented hydroxyapatite mediated by nonionic surfactants

Highly oriented organization of hydroxyapatite (HAP) nanorods was achieved through asimple reflux method using mixtures of triblock copolymer pluronic P123 and tween-60 asthe mediated agents. Raft-like organized complexes were prepared when the nanorods wereonly directed by the mixed surfactants. Bundles of nanorod-like HAP crystals were obtainedwhen urea was used as the cosurfactant. These HAP nanorods with a large amount ofuniform 4 nm worm-like mesopores were arranged in parallel to each other along thec axis of HAP. The raft-like complexes might be mediated by the reverse lamellar micelles.And the added urea transformed the reverse lamellar micelles into hexagonalones due to its association with the surfactant molecules by hydrogen bonds,resulting in the formation of bundles of nanorod-like HAP crystals. The regulation ofthe oriented HAP complexes in morphology extends the understanding ofbiomineralization and permits controllable design of biomimetic materials. In addition, thec-axis oriented raft-like HAP complex has great potential in selective bio-absorption andseparation.

A new overgrowth of mazzite on faujasite zeolite crystal investigated by X-ray diffraction and electron microscopy

A new zeolite overgrowth was synthesized by adding an amount of as-synthesised EMT crystals to a synthesis gel of mazzite (MAZ). After crystallisation, the overgrowth sample was made up of two major phases namely MAZ and faujasite (FAU). EMT crystals were transformed into FAU during the crystallisation process of the overgrowth sample. Bundles of needle-like crystals of mazzite overgrew faujasite particles. Weight fractions of phases in the overgrowth, the crystal structure of the phases and the distribution of cations in the overgrowth are determined. The growth of mazzite crystals onto the surface of faujasite crystals is not well oriented and a reliable epitaxial relationship between the FAU support and the overgrown material was therefore not determined.

Superficial performance and pore structure of palygorskite treated by hydrochloric acid

In order to amend the superficial performance of palygorskite and improve its application, the natural palygorskite(NP) was treated in the dipping and ionic exchanging experiments using 6 mol/L hydrochloric acid treatment. The performance and pore structure of the treated palygorskite(TP) were investigated by means of microscope analyses, FT-IR, XRF, BET-SSA and full hole distribution analytical techniques. The results show that the hydrochloric acid treatment can make the gracile and aggregating compact crystal bundles inside palygorskite clay broken and dispersed, the roughness of microcrystalline surface increases, which not only can dissolve or remove dolomite but vary the superficial performance of palygorskite to some degree. The specific surface area and pore volume increase a lot, while the mean pore size decreases. The pore structure of TP changes remarkably compared with that of NP after 6 mol/L hydrochloric acid treatment, and the relevant physicochemical performance can be improved.

Microstructure of pharyngeal tooth enameloid in the parrotfish Scarus rivulatus (Pisces: Scaridae)

The microstructure of parrotfish pharyngeal teeth was examined using scanning electron microscopy to infer possible mechanical properties of the dentition with respect to their function. Parrotfish tooth enameloid is formed from fluorapatite crystals grouped into bundles. In the upper and lower pharyngeal jaw, the majority of the crystal bundles are orientated either perpendicularly or vertically to the enameloid surface. The only exception is in the trailing apical enameloid in which the majority of bundles are orientated perpendicularly or horizontally to the trailing surface. A distinct transition occurs through the middle of the apex between the leading and trailing enameloid in teeth of the lower pharyngeal jaw. This transition appears less distinct in the teeth of the upper pharyngeal jaw. Enameloid microstructure indicates that shear forces predominate at the apex of the teeth. In the remainder of the enameloid, the microstructure indicates that wear is predominant, and the shear forces are of less importance.

Histology of the galeaspid dermoskeleton and endoskeleton, and the origin and early evolution of the vertebrate cranial endoskeleton

ABSTRACT The histological composition of the galeaspid cephalothoracic skeleton has been much debated: here we attempt to resolve this through the analysis of well-preserved remains of galeaspids from Yunnan Province, and Tarim Basin. Xinjiang Uygur Autonomous Region, China. Our results indicate that the galeaspid dermoskeleton is dominantly composed from an acellular laminar bone in which the mineral is organised into cylindrical crystal bundles that are arranged into three orthogonal sets with associated extrinsic fiber spaces, a unique histology for which the term galeaspedin is coined. This is permeated by a coarse vascular plexus that divides the dermoskeleton into upper and lower zones, and the upper zone into distinct tesserae which, like the bounding vascular network, arc polygonal in outline. The outer surface of the dermoskeleton is ornamented by a series of tubercles centered on tesserae, the latter composed partly from galeaspedin, and partly from a capping layer of microspherulitic, acellular bone, similar to the limiting layer of bone of elasmoid scales. Neither dentine nor enameloid is present, nor do the tissue compositions or their arrangement indicate an odontogenic origin. The endoskeleton is composed of an outer zone of globular calcified cartilage in contact with the dermoskeleton through a poorly mineralized intermediate zone. The inner zone is finely laminated, resulting from progressive zones of calcification embracing the calcospherites in a direction away from the dermoskeleton. There is no persuasive histological evidence for the presence of appositional perichondral bone. As in osteostracans, the galeaspid endoskeleton is interpreted as an expanded neurocranium. However, the presence of a calcified cartilaginous neurocranium in galeaspids in the absence of a perichondral bone layer indicates that these two histogenic components have distinct evolutionary origins. The presence of perichondral bone is a synapomorphy of osteostracans and jawed vertebrates, while the presence of a mineralized neurocranium unites galeaspids to this clade (possibly also including pituriaspids).

Origin of pedogenic needle-fiber calcite revealed by micromorphology and stable isotope composition—a case study of a Quaternary paleosol from Hungary

Pedogenic needle-fiber calcite was studied regarding its morphology, texture and stable isotope composition from the paleosol of the Quaternary Várhegy travertine (Budapest, Hungary). The needle-fiber calcite is composed of 40–200 μm long monocrystals. Smooth rods as well as serrated-edged crystals with calcite overgrowths were identified by SEM. Needles have several textural varieties: randomly distributed crystals in vugs and pores with calcite hypocoatings, bundles of subparallel crystals forming coatings around grains and alveolar structure with bridging needles in vugs. The morphological study of needle-fiber calcite suggests that needles are calcified fungal sheaths and produced by fungal biomineralization, a common process in recent and fossil soils and calcretes. The stable isotope composition of needle-fiber calcite (average: δ 18 O = - 7.1 ‰ and δ 13 C = - 7.3 ‰ vs. V-PDB) indicates significant incorporation of organically derived CO 2 and probably biological influence on needle genesis. Dissolved host rock travertine and/or atmospheric CO 2 could also contribute some carbon to the acicular calcite.

Relationship between enamel erosion and liquid flow rate

The association of severe erosion with certain habits of consuming soft drinks prompted a study of erosion in relation to fluid flow. In the main experiments, citric acid solution (pH 3.2) was directed at polished enamel surfaces at flow rates of 26, 67 or 126 ml min(-1) through outlets of diameters 1.0, 2.6 or 5.0 mm for up to 20 min. Erosion depth increased with time of exposure and total volume of solution, with increasing flow rate and with decreasing outlet diameter. Scanning electron microscopy of acid-treated surfaces and of the subsurface enamel indicated that the clearance of dissolution products, rather than the mechanical dislodgement of partly dissolved crystal bundles, was the main process involved. For an exposure time of 20 min, erosion increased with (liquid velocity)(1/4). In a secondary experiment, it was found that the erosive potential of a low-erosive drink did not increase with increasing liquid velocity. The rapid increase in the rate of erosion caused by citric acid at low liquid velocities shows that reproducible stirring is essential for precision and intercomparison of in vitro studies. It also accounts for the rapid erosion seen when erosive drinks are consumed from a straw placed labial to the anterior teeth, or are "swished" between the teeth.

Pelagosite and coniatolite crusts associated with zones of sea spray on coastal terraces, Mohammedia (Northwest Morocco)

Clusters of finely laminated, pelagosite and micro-pelagosite occur on exposed terraces on the Atlantic coastline of Morocco. These structures occupy a substrate of potholed, calcareous Quaternary sandstone, which is receding rapidly due to biogenic and mechanical erosion processes. Sandstone overhangs that are weathered into craggy prominences in the tidal zone are preferential sites of carbonate precipitation. These restricted environments at times become supersaturated with carbonate. Crusts are strongly indurated and exhibit a variety of structures, the most common of which are laterally linked spheroidal aragonite crusts reminiscent of biogenic hemispheroids (LLH), and stacked hemispheroids (SH). They encrust a variety of vertical substrate irregularities occasionally forming dripstone coniatolite. Vertical relief of the spheroids varies from 10–200 μm and all forms are characterized by branching bundles of crystals with concentrical layers of radial fibrous aragonite. The vertical cliff faces provide a modern example of palagonite formed by cement precipitation facilitated by sea spray on variable microtopography. Although many of these aragonitic crusts resemble lithified algal stromatolites, their dripstone morphologies, nanostructures and lack of desiccation cracks, strongly suggest that they are formed by physico-chemical precipitation from seawater. The vertical distribution of aragonite is controlled by CO2 entrapment and release, sea-spray Ca, and the pH of the surrounding areas.

Comparative floral structure and systematics in Crossosomatales (Crossosomataceae, Stachyuraceae, Staphyleaceae, Aphloiaceae, Geissolomataceae, Ixerbaceae, Strasburgeriaceae)

Floral structure of all putative families of Crossosomatales as suggested by molecular studies was comparatively studied. The seven comprise Crossosomataceae, Stachyuraceae, Staphyleaceae, Aphloiaceae, Geissolomataceae, Ixerbaceae, and Strasburgeriaceae. The entire clade (1) is highly supported by floral structure, also the clades (in sequence of diminishing structural support): Ixerbaceae/Strasburgeriaceae (2), Geissolomataceae/Ixerbaceae/Strasburgeriaceae (3), Aphloiaceae/Geissolomataceae/Ixerbaceae/Strasburgeriaceae (4), and Crossosomataceae/Stachyuraceae/Staphyleaceae (5). Among the prominent floral features of Crossosomatales (1) are solitary flowers, presence of a floral cup, imbricate sepals with outermost smaller than inner, pollen grains with horizontally extended endoapertures, shortly stalked gynoecium, postgenitally united carpel tips forming a compitum, stigmatic papillae two- or more-cellular, ovary locules tapering upwards, long integuments forming zigzag micropyles, cell clusters with bundles of long yellow crystals, mucilage cells, seeds with smooth, sclerified testa and without a differentiated tegmen. Clade (2) is characterized by large flowers, petals forming a tight, pointed cone in bud, stamens with long, stout filaments and sagittate anthers, streamlined, conical gynoecium, antitropous ovules, rudimentary aril, lignified, unicellular, T-shaped hairs and idioblasts with striate mucilaginous cell walls. Clade (3) is characterized by alternisepalous carpels, punctiform stigma formed by postgenitally united and twisted carpel tips, synascidiate ovary, only one or two pendant ovules per carpel, nectary recesses between androecium and gynoecium. Clade (4) is characterized by pronounced ‘pollen buds’. Clade (5) is characterized by polygamous or functionally unisexual flowers, x-shaped anthers, free and follicular carpels (not in Stachyuraceae). Crossosomataceae and Aphloiaceae, although not retrieved as a clade in molecular studies, share several special floral features: polystemonous androecium; basifixed anthers without a connective protrusion; stigma with two more or less decurrent crests; camplyotropous ovules and reniform seeds; simple, disc-shaped nectaries and absence of hairs. © 2005 The Linnean Society of London, Botanical Journal of the Linnean Society, 2005, 147, 1–46.

Scanning Electron Microscopy of Softened Enamel

After exposing enamel specimens to 0.3% citric acid at pH 3.2 for various times, the acid was titrated to pH 7 before rinsing the specimens in water. After freeze-drying the specimens were examined by scanning electron microscopy. This procedure eliminates artefacts due to drying and mineral precipitation. The results showed that the outer region of softened enamel is much more delicate than previously thought, even after short (5- to 20-min) etching times. Mineral was lost from both prism boundaries and the prism bodies, resulting in a surface presenting thin, separate crystal bundles. In further studies, replicas of subsurface pores, created by resin impregnation, showed the softening depth to be several times greater than is suggested by techniques based on removing the softened enamel by physical forces. The results point to a need for improved methods of measuring softening depth. More importantly, it appears that the outer region of the softened layer remaining after an erosive challenge might be too fragile to resist frictional forces in vivo.

Intershell conductance in multiwall carbon nanotubes

In the past decade the interest in molecular electronic devices has escalated. The synthesis of molecular crystals has improved, providing single crystals or thin films with mobility comparable with or even higher than amorphous silicon. Their mechanical flexibility admits new types of applications and usage of electronic devices. Some of these organic crystals also display magnetic effects. Furthermore, the fullerene and carbon nanotube allotropes of carbon are prominent candidates for various types of applications. The carbon nanotubes, in particular, are suitable for molecular wire applications with their robust, hollow and almost one-dimensional structure and diverse band structure. In this thesis, we have theoretically investigated carbon based materials, such as carbon nanotubes, pentacene and spiro-biphenalenyl neutral radical molecular crystals. The work mainly deals with the electron structure and the transport properties thereof. The first studies concerns effects and defects in devices of finite carbon nanotubes. The transport properties, that is, conductance, are calculated with the Landauer approach. The device setup contains two metallic leads attached to the carbon nanotubes. Structural defects as vacancies and bending are considered for single-walled carbon nanotubes. For the multi-walled carbon nanotubes the focus is on inter-shell interaction and telescopic junctions. The current voltage characteristics of these systems show clear marks of quantum dot behaviour. The influence of defects as vacancies and geometrical deformations are significant for infinite systems, but in these devices they play a minor role. The rest of the studies concern molecular crystals, treated with density-functional theory (DFT). Inspired by the enhance of the electrical conductivity obtained experimentally by doping similar materials with alkali metals, calculations were performed on bundles of single-walled carbon nanotubes and pentacene crystals doped with potassium. The most prominent effect of the potassium intercalation is the shift of Fermi level in the nanotube bands. A sign of charge transfer of the valence electrons of the potassium atoms. Semi-conducting bundles become metallic and metallic bundles gain density of states at the Fermi level. In the semi-conducting pristine pentacene crystals structural transitions occur upon doping. The herringbone arrangement of the pristine pentacene molecules relaxes to a more π-stacked structure causing more dispersive bands. The charge transfer shifts the Fermi level into the lowest unoccupied molecular orbital band and turns the crystal metallic. Finally, we have studied molecular crystals of spiro-biphenalenyl neutral radicals. According to experimental studies, some of these materials show simultaneous electrical, optical and magnetical bistability. The electronic properties of these crystals are investigated by means of DFT with a focus on the possible intermolecular interactions of radical spins.

Differential scanning calorimetry, small‐angle X‐ray scattering, and wide‐angle X‐ray scattering on homogeneous and heterogeneous ethylene‐α‐copolymers

AbstractThe objective of this work was to use both X‐ray and differential scanning calorimetry techniques in a comparative study of the lamellar and crystalline structures of heterogeneous and homogeneous ethylene‐α‐copolymers. The samples differed in the comonomer type (1‐butene, 1‐hexene, 1‐octene, and hexadecene), comonomer content, and catalyst used in the polymerizations. Step crystallizations were performed with differential scanning calorimetry, and the crystallinity and lamellar thicknesses of the different crystal populations were determined. Wide‐angle X‐ray scattering was used to determine crystallinities, average sizes of the crystallites, and dimensions of the orthorhombic unit cell. The average thickness, separation of the lamellae, and volume fractions of the crystalline phase were determined by small‐angle X‐ray scattering (SAXS). The results revealed that at densities below 900 kg/m3, polymers were organized as poorly organized crystal bundles. The lamellar distances were smaller and the lamellar thickness distributions were narrower for the homogeneous ethylene copolymers than for the heterogeneous ones. Step‐crystallization experiments by SAXS demonstrated that the long period increased after annealing. © 2001 John Wiley & Sons, Inc. J Polym Sci Part B: Polym Phys 39: 1860–1875, 2001