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Topic Review
Ring Expansion and Ring Contraction
Ring expansion and ring contraction reactions in the course of organic synthesis refer to a set of reactions which can lead to the expansion or contraction of an existing ring. This often makes it possible to access structures that would be difficult if not impossible to synthesise with single cyclization reactions. Ring expansions are valuable because they allow access to larger systems that are difficult to synthesize through a single cyclization due to the slow rate of formation. Ring contractions are useful for making smaller, more strained rings from larger rings. Expansions are classified by the mechanism of expansion and the atom(s) added; contractions are characterized simply by the reactive intermediate which performs the contraction.
  • 2.7K
  • 01 Dec 2022
Topic Review
Rice Husk Reinforced Polymer Composites
Rice (Oryza sativa L. genus) is the primary source of daily food intake and has become the world’s second most important cereal crop sector due to the demand of billions of human beings. Rice husk (RH) should never be burned, due to various reasons, such as the ashes, harmful gases, and fumes that contribute to air pollution. Typically, the RH can be used as biochar, extracted silica, or husk itself. In general, RH is a hull to protect seeds or grains. It is formed from rigid materials, is water-insoluble, and is abrasive, with a high level of cellulose–silica structures. The exterior of the hulls consists of silica covered with a cuticle, with a small amount of silica content at the innermost epidermis. The exploitation of RH residues in biocomposites offers multiple advantages, for example, reducing the relative amount of constituents derived from synthetic polymers, such as resin polymers and some additives. The tensile strength is mainly used to evaluate the strength behavior of a composite material. 
  • 1.3K
  • 04 Aug 2021
Topic Review
Rice Husk
The development of engineered silica particles by using low-cost renewable or waste resources is a key example of sustainability. Rice husks have emerged as a renewable resource for the production of engineered silica particles as well as bioenergy.
  • 3.4K
  • 17 Jan 2022
Topic Review
Rice Hulls
Rice hulls (or rice husks) are the hard protecting coverings of grains of rice. In addition to protecting rice during the growing season, rice hulls can be put to use as building material, fertilizer, insulation material, or fuel. Rice hulls are part of the chaff of the rice.
  • 6.1K
  • 10 Oct 2022
Topic Review
Riboflavin-Induced Photochemistry
Riboflavin (RF), which is also known as vitamin B2, is a water-soluble vitamin. RF is a nontoxic and biocompatible natural substance. It absorbs light (at wavelengths of 380 and 450 nm) in the presence of oxygen to form reactive singlet oxygen (1O2). The generated singlet oxygen acts as a photoinitiator to induce the oxidation of biomolecules, such as amino acids, proteins, and nucleotides, or to initiate chemical reactions, such as the thiol-ene reaction and crosslinking of tyramine and furfuryl groups. 
  • 815
  • 23 Feb 2023
Topic Review
Rheology of Gels and Yielding Liquids
Gels are understood as soft viscoelastic multicomponent solids that are in the incomplete phase separation state, which, under the action of external mechanical forces, do not transit into a fluid state but rupture like any solid material. Gels can “melt” (again, like any solids) due to a change in temperature or variation in the environment. In contrast to this type of rheology, yielding liquids (sometimes not rigorously referred to as “gels”, especially in relation to colloids) can exist in a solid-like (gel-like) state and become fluid above some defined stress and time conditions (yield stress). At low stresses, their behavior is quite similar to that of permanent solid gels, including the frequency-independent storage modulus. The gel-to-sol transition considered in colloid chemistry is treated as a case of yielding. However, in many cases, the yield stress cannot be assumed to be a physical parameter since the solid-to-liquid transition happens in time and is associated with thixotropic effects. 
  • 570
  • 28 Sep 2023
Topic Review
Rheological Behavior of Cement Paste
The rheological behavior of cement paste displays several complexities such as non-linearity, yielding, time-dependence and chemical changes. The most common testing procedures allowing for the characterization of this behavior have been reported. For flow behavior, the most relevant protocol consists of the application of a ramp sweep (shear rate or shear stress) so that the flow curve (shear stress as a function of the shear rate) is at steady state flow. The structural build-up can be characterized by static yield stress measurements or the SAOS protocol for various resting times. The non-linearity of flow behavior corresponds to a shear-thinning/shear-thickening phenomenon that can be represented by the Herschel–Bulkley model. If the shear-thinning phenomenon can be related to the flocculation state, the origins of shear thickening are not clearly elucidated. Theories based on order–disorder transition, particle clustering, or viscous/inertial regime transition have been developed to explain the appearance of shear thickening. 
  • 505
  • 17 Aug 2023
Topic Review
Rhenium(I) Tricarbonyl Complexes As Photosensitisers
Photodynamic therapy (PDT) is emerging as a significant complementary or alternative approach for cancer treatment. Re(I) tricarbonyl complexes is a new generation of metal-based photosensitisers for PDT that are of great interest in multidisciplinary research.
  • 1.3K
  • 27 Oct 2020
Topic Review
Rhenium in Biological Imaging
Rhenium (Re) is widely used in the diagnosis and treatment of cancer due to its unique physical and chemical properties. Re has more valence electrons in its outer shell, allowing it to exist in a variety of oxidation states and to form different geometric configurations with many different ligands. The luminescence properties, lipophilicity, and cytotoxicity of complexes can be adjusted by changing the ligand of Re.
  • 385
  • 30 Mar 2023
Topic Review
Rhamnus (Genus)
Rhamnus is a genus of about 110 accepted species of shrubs or small trees, commonly known as buckthorns in the family Rhamnaceae. Its species range from 1 to 10 meters tall (rarely to 15 m) and are native mainly in east Asia and North America, but found throughout the temperate and subtropical Northern Hemisphere, and also more locally in the subtropical Southern Hemisphere in parts of Africa and South America. Both deciduous and evergreen species occur. The leaves are simple, 3 to 15 centimeters long, and arranged alternately or in opposite pairs, or almost paired (subopposite). One distinctive character of many buckthorns is the way the veination curves upward towards the tip of the leaf. The plant bears fruits which are black or red berry-like drupe. The name is due to the woody spine on the end of each twig in many species. In the United States , Buckthorn is considered an invasive species by many local jurisdictions and state governments, such as Minnesota or Wisconsin.
  • 1.3K
  • 31 Oct 2022
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