Carbon nanotube induced polymer crystallization: The formation of nanohybrid shish–kebabs

AbstractCarbon nanotubes (CNTs) have attracted tremendous attention in recent years because of their superb optical, electronic and mechanical properties. In this article, we aim to discuss CNT-induced polymer crystallization with the focus on the newly discovered nanohybrid shish–kebab (NHSK) structure, wherein the CNT serves as the shish and polymer crystals are the kebabs. Polyethylene (PE) and Nylon 6,6 were successfully decorated on single-walled carbon nanotubes (SWNTs), multi-walled carbon nanotubes (MWNTs), and vapor grown carbon nanofibers (CNFs). The formation mechanism was attributed to “size-dependent soft epitaxy”. Polymer CNT nanocomposites (PCNs) containing PE, Nylon 6,6 were prepared using a solution blending technique. Both pristine CNTs and NHSKs were used as the precursors for the PCN preparation. The impact of CNTs on the polymer crystallization behavior will be discussed. Furthermore, four different polymers were decorated on CNTs using the physical vapor deposition method, forming a two-dimensional NHSK structure. These NHSKs represent a new type of nanoscale architecture. A variety of possible applications will be discussed

Progressive Failure Simulation of Security Cable Barriers

Perimeter security cable barriers are widely used by various agencies all over the world to defeat threat vehicle penetration. New barrier designs require crash test validation prior to implementation. Full-scale vehicular crash tests are costly, whereas designs via finite element simulations are time consuming and require specialized skills. Based on full-scale crash tests, an innovative and simple algorithm has been developed to model the progressive failure of security cable barriers. A multi-body approach based on the first principles of physics was developed to substantially reduce computer runtime. The solution algorithm uses a large number of small time steps. Nonlinear vehicle and cable forces and deformations are calculated based on compatibility conditions. This methodology has been validated against three full-scale crash tests. This cable barrier model, displaying simulation results graphically in a time series, provides realistic response parameters of a security cable barrier design in less than 10 minutes of runtime with reasonable accuracy

Quantitative microstructure characterization of Ag nanoparticle sintered joints for power die attachment

The samples of sintered Ag joints for power die attachments were prepared using paste of Ag nanoparticles at 240 °C and 5 MPa for 3 to 17 minutes. Their microstructural features were quantitatively characterized with scanning elec-tronic microscopy, transmission electron microscopy, X-ray diffraction and image analysis method. The resulting nor-malized thickness, pore size and porosity decreased, and grain size increased with increasing the sintering time. A time dependence of the form t1/n with n close to 2 or 3 can be further derived for the kinetics of the thinning, densification and grain growth within the sintered Ag joints

Effect of fiber inclination on crack bridging stress in brittle fiber reinforced brittle matrix composites

behavior of brittle matrix composites is strongly affected by the bridging of cracks by fibers. In random fiber composites, fibers can lie at an angle to the crack plane. Under such conditions, the bridging stress for a certain crack opening is governed by various micromechanisms including fiber debonding, fiber bending and rupture as well as matrix spalling. While fiber debonding has been widely investigated, the coupled fiber bending/matrix spalling mechanism has received little attention. In this paper, the fiber bending/matrix spalling mechanism is analyzed by treating the fiber as a beam bent on an elastic foundation with variable stiffness and the possibility of spalling. The foundation stiffness and spalling criterion are derived from a finite element analysis. The bridging stress due to bending alone as well as the total bridging stress are then obtained for the case with brittle fibers. Through this analysis, the effect of various microstructural parameters (such as fiber and matrix moduli, matrix spalling strain and fiber/matrix interfacial friction) on the behavior of random fiber composites can be studied. Prediction of maximum bridging stress for inclined fibers based on the present model is shown to be in good agreement with experimental results.Peer Reviewedhttp://deepblue.lib.umich.edu/bitstream/2027.42/29939/1/0000296.pd

SR Proteins Collaborate with 7SK and Promoter-Associated Nascent RNA to Release Paused Polymerase

RNAP II is frequently paused near gene promoters in mammals, and its transition to productive elongation requires active recruitment of P-TEFb, a cyclin-dependent kinase for RNAP II and other key transcription elongation factors. A fraction of P-TEFb is sequestered in an inhibitory complex containing the 7SK noncoding RNA, but it has been unclear how P-TEFb is switched from the 7SK complex to RNAP II during transcription activation. We report that SRSF2 (also known as SC35, an SR-splicing factor) is part of the 7SK complex assembled at gene promoters and plays a direct role in transcription pause release. We demonstrate RNA-dependent, coordinated release of SRSF2 and P-TEFb from the 7SK complex and transcription activation via SRSF2 binding to promoter-associated nascent RNA. These findings reveal an unanticipated SR protein function, a role for promoter-proximal nascent RNA in gene activation, and an analogous mechanism to HIV Tat/TAR for activating cellular genes