Pretreatments of natural fibers and their application as reinforcing material in polymer composites—A review

In recent years, natural fibers reinforced composites have received much attention because of their lightweight, nonabrasive, combustible, nontoxic, low cost and biodegradable properties. Among the various natural fibers; flax, bamboo, sisal, hemp, ramie, jute, and wood fibers are of particular interest. A lot of research work has been performed all over the world on the use of natural fibers as a reinforcing material for the preparation of various types of composites. However, lack of good interfacial adhesion, low melting point, and poor resistance towards moisture make the use of natural fiber reinforced composites less attractive. Pretreatments of the natural fiber can clean the fiber surface, chemically modify the surface, stop the moisture absorption process, and increase the surface roughness. Among the various pretreatment techniques, graft copolymerization and plasma treatment are the best methods for surface modification of natural fibers. Graft copolymers of natural fibers with vinyl monomers provide better adhesion between matrix and fiber. In the present article, the use of pretreated natural fibers in polymer matrix‐based composites has been reviewed. Effect of surface modification of natural fibers on the properties of fibers and fiber reinforced polymer composites has also been discussed. POLYM. ENG. SCI., 2009. © 2009 Society of Plastics Engineers     

Dark fermentative hydrogen production by defined mixed microbial cultures immobilized on ligno-cellulosic waste materials

Mixed microbial cultures (MMCs) based on 11 isolates belonging to Bacillus spp. (Firmicutes), Bordetella avium, Enterobacter aerogenes and Proteus mirabilis (Proteobacteria) were employed to produce hydrogen (H₂) under dark fermentative conditions. Under daily fed culture conditions (hydraulic retention time of 2 days), MMC6 and MMC4, immobilized on ligno-cellulosic wastes – banana leaves and coconut coir evolved 300–330 mL H₂/day. Here, H₂ constituted 58–62% of the total biogas evolved. It amounted to a H₂ yield of 1.54–1.65 mol/mol glucose utilized over a period of 60 days of fermentation. The involvement of various Bacillus spp. – Bacillus sp., Bacillus cereus, Bacillus megaterium, Bacillus pumilus and Bacillus thuringiensis as components of the defined MMCs for H₂ production has been reported here for the first time.     

Some aspects of fabrication of micro devices by electrochemical micromachining (ECMM) and its finishing by magnetorheological fluid

Electrochemical micromachining (ECMM) is an advanced machining process for machining of electrically conducting materials. In the present work, a micro nozzle and a fluidic mixer having complex structures are made using masking technique by ECMM process. Mask is made of 50-μm transparent sheet and the micro nozzle and micro mixer are fabricated on an 800-μm thick copper sheet. The resulting rough inner walls and bottom surface of micro nozzle are finished using magnetorheological fluid-based finishing process. Surface finish of the nozzle is significantly improved after finishing. A comparison of width, depth, and surface roughness of the micro nozzle is also carried out before and after finishing. The mixing behavior of two fluids is visualized by microscope in micro mixer. The rough inner walls of the mixer’s channels act as obstructions and result in zigzag path of flowing fluid. Hence, mixing occurs at microscopic level because of rapid molecular diffusion.     

Million atom molecular dynamics simulations of materials on parallel computers

Recent advances in computing tecnology — parallel computer architectures, portable software and development of robust O(N) algorithms — have revolutionized the field of computer simulation. Using the space-time multiresolution molecular dynamics algorithms it is possible to carry out multimillion atom simulations of materials in different ranges of density, temperature and uniaxial strain.     

Large-scale atomistic modeling of nanoelectronic structures

Large-scale molecular-dynamics simulations are performed on parallel computers to study critical issues on ultrathin dielectric films and device reliability in next-decade semiconductor devices. New interatomic-potential models based on many-body, reactive, and quantum-mechanical schemes are used to study various atomic-scale effects: growth of oxide layers; dielectric properties of high-permittivity oxides; dislocation activities at semiconductor/dielectric interfaces; effects of amorphous layers and pixellation on atomic-level stresses in lattice-mismatched nanopixels; and nanoindentation testing of thin films. Enabling technologies for 10 to 100 million-atom simulations of nanoelectronic structures are discussed, which include multiresolution algorithms for molecular dynamics, load balancing, and data management. In ten years, this scalable software infrastructure will enable trillion-atom simulations of realistic device structures with sizes well beyond μm on petaflop computers     

Multiresolution atomistic simulations of dynamic fracture in nanostructured ceramics and glasses

Multimillion atom molecular-dynamics (MD) simulations are performed to investigate dynamic fracture in glasses and nanostructured ceramics. Using multiresolution algorithms, simulations are carried out for up to 70 ps on massively parallel computers. MD results in amorphous silica (a-SiO₂) reveal the formation of nanoscale cavities ahead of the crack tip. With an increase in applied strain, these cavities grow and coalesce and their coalescence with the advancing crack causes fracture in the system. Recent AFM studies of glasses confirm this behavior. The MD value for the critical stress intensity factor of a-SiO₂ is in good agreement with experiments. Molecular dynamics simulations are also performed for nanostructured silicon nitride (n-Si₃N₄). Structural correlations in n-Si₃N₄ reveal that interfacial regions between nanoparticles are amorphous. Under an external strain, nanoscale cavities nucleate and grow in interfacial regions while the crack meanders through these regions. The fracture toughness of n-Si₃N₄ is found to be six times larger than that of crystalline -Si₃N₄. We also investigate the morphology of fracture surfaces. MD results reveal that fracture surfaces of n-Si₃N₄ are characterized by roughness exponents 0.58 below and 0.84 above a certain crossover length, which is of the order of the size of Si₃N₄ nanoparticles. Experiments on a variety of materials reveal this behavior. The final set of simulations deals with the interaction of water with a crack in strained silicon. These simulations couple MD with a quantum-mechanical (QM) method based on the density functional theory (DFT) so that chemical processes are included. For stress intensity factor K=0.4 MPa m^1/2, we find that a decomposed water molecule becomes attached to dangling bonds at the crack or forms a Si-O-Si structure. At K=0.5 MPa m^1/2, water molecules decompose to oxidize Si or break Si-Si bonds.     

A divide-and-conquer/cellular-decomposition framework for million-to-billion atom simulations of chemical reactions

To enable large-scale atomistic simulations of material processes involving chemical reactions, we have designed linear-scaling molecular dynamics (MD) algorithms based on an embedded divide-and-conquer (EDC) framework: first principles-based fast reactive force-field (F-ReaxFF) MD; and quantum-mechanical MD in the framework of the density functional theory (DFT) on adaptive multigrids. To map these O(N) algorithms onto parallel computers with deep memory hierarchies, we have developed a tunable hierarchical cellular-decomposition (THCD) framework, which achieves performance tunability through a hierarchy of parameterized cell data/computation structures and adaptive load balancing through wavelet-based computational-space decomposition. Benchmark tests on 1920 Itanium2 processors of the NASA Columbia supercomputer have achieved unprecedented scales of quantum-mechanically accurate and well validated, chemically reactive atomistic simulations—0.56 billion-atom F-ReaxFF MD and 1.4 million-atom (0.12 trillion grid points) EDC–DFT MD—in addition to 18.9 billion-atom non reactive space–time multiresolution MD. The EDC and THCD frameworks expose maximal data localities, and consequently the isogranular parallel efficiency on 1920 processors is as high as 0.953. Chemically reactive MD simulations have been applied to shock-initiated detonation of energetic materials and stress-induced bond breaking in ceramics in corrosive environments.