The world is in need of more eco-friendly material, therefore researchers around the globe focus on developing new materials that would improve the environmental quality of products. This need for new green materials has led to the utilization of composites made from raw natural fibers and polymer matrices, and this has become one of the most widely investigated research topics in recent times. Natural fiber composites are an alternative for replacing environmentally harmful synthetic materials and help control pollution problems. In addition, they are low cost, have better mechanical properties and require low production energy consumption. Also, using such materials in construction works, it is possible to improve the sustainability by eliminating construction wastes. Keeping in view all the benefits of natural fiber reinforced polymer composites, this paper first discusses various fabrication techniques employed for the production of these composites and then presents a detailed review of the research devoted to the analysis of their structure and properties by a variety of characterization techniques.

Nanothermite composites containing metallic fuel and inorganic oxidizer are gaining importance due to their outstanding combustion characteristics. In this paper, the combustion behaviors of copper oxide/aluminum nanothermites are discussed. CuO nanorods were synthesized using the surfactant-templating method, then mixed or self-assembled with Al nanoparticles. This nanoscale mixing resulted in a large interfacial contact area between fuel and oxidizer. As a result, the reaction of the low density nanothermite composite leads to a fast propagating combustion, generating shock waves with Mach numbers up to 3. © 2007 American Institute of Physics. ͓DOI: 10.1063/1.2787972͔ Nanothermite materials are comprised of a physical mixture of inorganic fuel and oxidizer nanoparticles. Nonhomogenous distribution of fuel and oxidizer has been observed in the microstructures. 1 This produces random hot spot density distribution and decreases the propagation speed of the combustion wave front. It is, therefore, important to achieve homogenous mixing of the oxidizer and fuel components for faster reaction kinetics. This can be achieved by selfassembly of fuel around the solid oxidizer. Enhancement in the combustion wave speed has already been reported for composites containing porous oxidizers and fuel nanoparticles, 2,3 and also for electrostatically charged selfassembled composites. 4 Recently, we reported that higher combustion wave speeds were achieved for the composites of ordered porous Fe 2 O 3 oxidizer and Al nanoparticles 5 as compared with the one containing porous oxidizer with no ordering of the pores and Al nanoparticles. We have also reported the composite of CuO nanorods and Al nanoparticles exhibiting a combustion wave speed of 1500Ϯ 100 m / s, which enhances to 2200 m / s for the self-assembled composites. 6-8 Interestingly, these higher combustion wave speeds are comparable to the lower end values of the detonation velocities ͑e.g., 2000 m / s for hydrocarbon/alkylene-air mixtures, 9 1500-2700 m / s for metallic azides and fulminates, 10 and about 3000 m / s for ammonium nitrate fuel oil͒ for explosives. 11 In conventional explosives, the gases produced during the chemical reaction develop turbulence due to a combined effect of high pressure and rapid shearing of molecular layers generating a shock wave. In a process called deflagration-todetonation transition ͑DDT͒, the wave propagates in the reactive medium creating localized high pressure at the hot spots and, after a certain run-up distance, rapid deflagration can transition to full detonation. 9 This distance depends on the dimensions of the shock tube and also the level of confinement. 9 In the case of low density superthermites, as the adiabatic reaction temperatures are several thousand degrees, the reaction products can volatilize rapidly 12 resulting in an increased level of turbulence and high localized pressures. Because of the low density and multiphase nature of reaction materials, the corresponding Chapman-Jouguet ͑CJ͒ pressure can be much lower ...

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The world has continued to change rapidly since the last version of this article was written on May 20, 2020. Yet, as this article goes to press, we are aware of two realities; first, that we cannot perennially chase a moving target, but second, that nothing about the fundamental trends that we have identified appear to have changed. India is firmly in the throes of a vicious pandemic that we can only hope will abate with the development of an effective vaccine. Our plea for the widespread provision of adequate health and medical facilities, adequate protection for the elderly, and transfers to those severely affected by the lockdown are absolutely unchanged in the face of the latest data. In contrast, the brutal enforcement of a lockdown with none of these accompanying measures can only worsen outcomes for the poorest and most vulnerable among the population.

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We provide a principled extension of SQL, called SchemaSQL, that offers the capability of uniform manipulation of data and schema in relational multidatabase systems. We develop a precise syntax and semantics of SchemaSQL in a manner that extends traditional SQL syntax and semantics, and demonstrate the following. (1) SchemaSQL retains the flavor of SQL while supporting querying of both data and schema. (2) It can be used to transform data in a database in a structure substantially different from original database, in which data and schema may be interchanged.(3) It also permits the creation of views whose schema is dynamically dependent on the contents of the input instance. (4) While aggregation in SQL is restricted to values occurring in one column at a time, SchemaSQL permits "horizontal" aggregation and even aggregation over more general "blocks" of information. (5) SchemaSQL provides a useful facility for interoperability and data/schema manipulation in relational multidatabase systems. We provide many examples to illustrate our claims. We clearly spell out the formal semantics of SchemaSQL that accounts for all these features. We describe an architecture for the implementation of SchemaSQL and develop implementation algorithms based on available database technology that allows for powerful integration of SQL based relational DBMS. We also discuss the applicability of SchemaSQL for handling semantic heterogeneity arising in a multidatabase system.

We thank Bishnupriya Gupta, Rajeswari Sengupta, Lore Vandewalle and especially James Poterba for helpful comments. Ray acknowledges funding under National Science Foundation grant SES-1851758. The views expressed herein are those of the authors and do not necessarily reflect the views of the National Bureau of Economic Research. NBER working papers are circulated for discussion and comment purposes. They have not been peerreviewed or been subject to the review by the NBER Board of Directors that accompanies official NBER publications.