Molecular simulation of compressive failure in poly(p-phenylene teraphthalamide) crystals

Molecular mechanics simulations are carried out on crystals of poly(p-phenylene teraphthalamide) (PPTA) as a function of an applied axial compressive stress. The vibrational frequencies of the long wavelength acoustic modes which propagate along the chain axis and are polarized perpendicular to the plane of the hydrogen-bonded sheets are found to become imaginary when the imposed stress exceeds the modulus for shear between hydrogen-bonded sheets. The imaginary frequencies denote an elastic buckling instability. This instability occurs at a compressive stress of 0.3 GPa, in good agreement with the experimental result for the stress which causes material failure in PPTA fibres. It is suggested that previous overestimates of compressive strengths based on elastic buckling models occurred due to the use of the torsion modulus as the relevant shear modulus; however, the torsion modulus is not relevant because it represents an average of shear moduli in different directions, while elastic buckling takes place along the direction of easiest shear.     

High-resolution electron microscopy of detonation nanodiamond

The structure of individual nanodiamond grains produced by the detonation of carbon-based explosives has been studied with a high-vacuum aberration-corrected electron microscope. Many grains show a well-resolved cubic diamond lattice with negligible contamination, thereby demonstrating that the non-diamond shell, universally observed on nanodiamond particles, could be intrinsic to the preparation process rather than to the nanosized diamond itself. The strength of the adhesion between the nanodiamond grains, and the possibility of their patterning with sub-nanometer precision, are also demonstrated.     

High-resolution electron microscopy study of belite

The microstructures of belite examined by high-resolution electron microscopy (HREM) show the (1 0 0) and (0 0 1) twin lamellae which are introduced by shear stresses during the transformation of–C₂S to-C₂S.-C₂S formed in the larnite was identified and the orientation relationship between them was also determined by a composite electron diffraction pattern (EDP), which is different from that suggested earlier by Groves. The EDP and the HREM image showed the existence of the unstable phase _L–C₂S.     

Scanning electron microscopy of voided poly(vinylidene fluoride)

Journal of Materials Science Letters -     

Effects of electron radiation on mono- and di-methyl pendant poly(p-phenylene benzobisthiazole) fibers

The effect of electron radiation on mono- and di-methyl pendant poly(p-phenylene benzobisthiazole) polymers (MePBZT and DiMePBZT, respectively) has been investigated. MePBZT is chemically stable (as measured by ¹³C NMR) upon exposure to 1 Grad electron radiation at room temperature. The irradiation of DiMePBZT at 225°C was also carried out, which was found to have no effect up to a dosage of 850 Mrad. Thus, the additional molecular mobility brought about by heating (225°C) did not enhance the reactivity of DiMePBZT. The compressive and tensile properties of DiMePBZT fiber remained unchanged at lower radiation dosages. At a dosage of 850 Mrad, the tensile properties were found to decrease substantially, which may be attributed to defects observed in these fibers. The defects can be attributed to the effects of electron radiation, rather than prolonged heating at 225°C.     

Transmission electron microscopy study of defect-selective etched (010) ScN crystals

The origin of etch pits, formed in (010) scandium nitride single crystals etched in molten potassium hydroxide KOH, is reported. Transmission electron microscopy (TEM) with weak beam condition and large area convergent beam electron diffraction technique were used to identify the nature of dislocations associated with etch pits. The top edge of the etch pits was a square inverted pyramid with the bottom point deviating from the center. TEM investigation demonstrated that a mixed dislocation terminated at the bottom of each etch pit. The Burgers vector of the mixed dislocation is ½ [01¯1¯] and the vector of the dislocation line is [1¯91].     

Electrorheology of undoped poly(p-phenylene) particle-based suspension

Electrorheological (ER) properties of synthesized, pristine poly(p-phenylene) (PPP) particles without any dopant treatment was investigated via flow curves including shear viscosity and yield stress. The ER characteristics were examined as functions of both particle concentration and applied electric field strengths. This undoped, PPP based suspension exhibited normal ER characteristics displayed by conventional semi-conducting polymeric ER materials with doping. The yield stress, which is an important design parameter for ER fluids, satisfied a universal scaling function. We found that the critical electric field strength for the undoped PPP suspension was much higher than the doped PPP suspension. These undoped particles can be used as a model system to test the doped system at low concentration via a universal scaling function.     

Structural studies of poly(p-phenylene benzbisthiazole) films

Poly(p-phenylene benzbisthiazole) (PBT) is one member of a new class of highly-rigid, linear, thermally-stable aromatic heterocyclic polymers. The role of heat-treatment in the improvement of the perfection of crystallinity and mechanical properties of oriented films is discussed. Part of the heat-treatment process seems to be to increase the conjugation length of the polymer chain by increasing the planarity of the molecule, as revealed by visual colour changes and by differential scanning calorimetry. This may in turn account for the improved quality of crystallinity. Considerable detail can be seen in the electron diffraction patterns of heat-treated films. With the exception of the equatorial diffraction peaks this scatter can be accounted for by the detailed molecular transform of the PBT polymer, suitably cylindrically averaged, indicating that the crystal structure is essentially two-dimensional, that is the chains while closely and regularly packed lack longitudinal register. A two-dimensional unit cell with the corresponding molecular packing is proposed which can satisfactorily account for the observed density and for the equatorial diffraction peaks.     

Beam-induced changes in the scanning electron microscopy of poly(oxymethylene)

Samples of injection-moulded Delrin of a particular known morphology were examined in the scanning electron microscope. It was found that the beam affected the specimen in certain characteristic ways, the raster leaving a permanent imprint on the sample surface. By considering the nature of the irradiation process in the scanning electron microscope the amount and type of beam damage could be quantitatively correlated with the operating variables of the instrument. It is hoped that this will provide the groundwork for similar investigations on other systems. The importance of understanding and controlling beam-induced effects in the interpretation of scanning electron microscope images is clearly brought out by the present study. The nature of the beam damage bears a relation to the texture of the sample as inferred by other means. Thus the scanning electron microscope can serve as a device for achieving controlled beam etching in service of structure studies. Delrin is a poly(oxymethylene) manufactured by Du Pont de Nemours and Co. Inc.     

High-resolution transmission electron microscopy of AlAs-GaAs semiconductor superlattices

A new high-resolution transmission electron microscopy (HREM) technique is proposed for observing interfacial structures in semiconductor superlattices in the <1 1 0> cross-section, and is used to characterize AlAs-GaAs interfacial step structures. Analysis of diffracted beam amplitudes shows that the {1 1 1| beam amplitudes of GaAs are minimized at an extinction distance of 14.4 nm, whereas those of AlAs show significantly higher values. This remarkable difference in the {1 1 1| beam amplitudes leads to a marked contrast between HREM images of GaAs and AlAs around this specimen thickness, allowing edge-on observation of the interfacial atomic steps running along the <1 1 0> direction. Artifacts produced by ion milling prevent atomic-scale observations of the interfaces so an artifact-free TEM specimen preparation technique is also presented. The HREM method provides information on the step intervals and straightness of step edges at AlAs-GaAs interfaces fabricated on just-cut and vicinal GaAs (0 0 1) substrates. The effect of the growth interruption method during molecular beam epitaxy on interface smoothing is also demonstrated.     

High-resolution electron microscopy: from imaging toward measuring

High-resolution electron microscopy is discussed as a measuring, rather than an imaging, technique. It is shown that the interpretation or the images could greatly benefit from a quantitative instead of a qualitative approach accompanied by quantitative statistical experimental design.     

High-resolution electron microscopy studies of high-pressure synthesized mercury-based superconductor

Studies of high-pressure synthesized Hg-Ba-Ca-Cu-O superconductor by electron diffraction and high-resolution electron microscopy are reported. The results of electron diffraction indicate that two superconducting phases 1212 and 1223 exist in the sample and their cell parameters area=b=3.8 Å,c=12.7 Å anda=b=3.8 Å,c=15.8 Å, respectively. High-resolution electron micrographs were taken from the two phases Hg-1212 and Hg-1223 and from the region of the intergrowth of the two phases. The relation between the structure and superconducting transitional temperatureT _c is discussed.     

High-resolution electron microscopy studies of nanophases in Nd-Fe-B alloys

High-resolution transmission electron microscopy observations on rapidly solidified Nd-Fe-B alloys are presented. The results show the existence of nanophases of different compositions to the nominal Nd₂Fe₁₄B as-prepared alloy, which was detected prior to the fast-quenching process. In particular, iron-enriched nanophases are commonly found among the wide variety of nanophases in the final alloy. The possible mechanisms, as well as their relevance to the understanding of the properties of these magnetic materials, are also discussed.     

Single fibre deformation studies of poly(p-phenylene benzobisoxazole) fibres

Changes in crystal strain and crystallite orientation of three varieties of PBO fibre (namely PBO AS, HM and HM+) have been investigated during deformation from the analysis of diffraction patterns obtained across single filaments, using a synchrotron X-ray source. Crystal strain was measured from the positions of the meridional reflections and orientation calculated from azimuthal broadening of the equatorial reflections. It has been demonstrated that no difference in crystal strain across the fibre exists, with the calculated strain being equal between fibre skin and core at a given level of stress. Further skin-core crystallite orientation analysis (calculation of the orientation parameter ) proved that the AS fibre was the only PBO variety with a significant difference in orientation across the fibre, with the core region being less oriented due to the processing conditions. The skin and core orientation of all three fibres was found to improve with deformation, with the core of the AS fibre showing a significantly higher rate of improvement. This resulted in a similar level of orientation for both skin and core regions of the PBO AS fibre at high levels of stress. The fibre modulus was found to increase with the increasing initial degree of crystallite orientation. Furthermore, improvement in orientation with external stress was related to _{= 0}, with higher values resulting in greater shear forces on the crystallites and therefore a greater rate of orientation improvement.