The synthesis and structural and retrostructural analyses of a library containing 10 triphenylenes functionalized with self-assembling benzyl ether and phenyl propyl ether dendrons are reported. ...These dendronized triphenylenes adopt a crown rather than discotic conformation. Their crown conformation mediates the self-assembly of the discotic triphenylene unit in helical pyramidal columns and in chiral spheres. The chiral spheres are generated from short segments of helical pyramidal columns that are spherically distorted. Therefore, the chirality of the sphere is determined by a short helical pyramidal column that represents the inner part of the supramolecular sphere. Both the helical pyramidal columns and the chiral spheres represent supramolecular architectures that were self-assembled for the first time from discotic molecules. The helical pyramidal columns self-organize in various hexagonal and rectangular lattices, while the chiral spheres self-organize into cubic and tetragonal periodic arrays and into a quasiperiodic 12-fold liquid quasicrystal. The helical sense of the helical pyramidal columns and of helical spheres is selected by a stereocenter that can be incorporated either in the alkyl groups of the dendron or in the triphenylene part of the dendritic crown via donor−acceptor interactions. The self-assembly process of the dendronized triphenylene donor can be programmed by a new supramolecular “polymer effect” generated by donor−acceptor interactions.
High cis content (81−99%) cis−transoidal polyphenylacetylene (PPA) jacketed with amphiphilic self-assembling dendrons, poly(3,4 − 3,5)mG2−4EBn with m = 8, 10, 12, 14, 16, and (S)-3,7-dimethyloctyl, ...were synthesized by Rh(C⋮CPh)(nbd)(PPh3)2 (nbd = 2,5-norbornadiene)/N,N-(dimethylamino)pyridine (DMAP) catalyzed polymerization of macromonomers. The resulting cylindrical PPAs self-organize into hexagonal columnar lattices with intracolumnar order (Φh io) and without (Φh). The polymers with m = 12, 14, and 16 exhibit also a hexagonal columnar crystal phase (Φh,k). The reversible Φh,k-to-Φh io-to- Φh phase transition in these dendronized PPAs was analyzed by a combination of differential scanning calorimetry and small and wide-angle X-ray diffraction experiments performed on powder and oriented fibers. In the Φh,k and Φh io phases, the dendronized PPAs form helical porous columns. The helical pore disappears in the Φh phase. This change is accompanied by a decrease of the external column diameter that is induced by stretching of the polymer backbone along the axis of the cylinder. The helix sense of the porous PPA is selected by homochiral alkyl dendritic tails. This transition is generated by an unprecedented conversion of the PPA backbone from the cis−cisoidal conformation in the Φh,k and Φh io phases to the cis−transoidal conformation in the Φh phase. Under the same conditions, the pristine cis-PPA undergoes cis−trans isomerization and irreversible intramolecular 6π electrocyclization of 1,3-cis,5-hexatriene sequences followed by chain cleavage. These processes are eliminated in the dendronized cis-PPA below its decomposition temperature.
The present experimental study focuses on the energy storage performance of Therminol 55-TiO2 nanofluids for the absorption of solar energy. Photothermal conversion efficiency is enhanced using ...Fresnel lens and secondary reflectors with a glass-type evacuated absorber tube. The focal length of the Fresnel lens is 150 mm, and that of the secondary reflector is 70 mm. The optical absorbance, extinction coefficient, and thermal conductivity of nanofluids at 100, 250, 350, and 500 ppm are reported. The optical path length of the energy storage medium is 1 cm. The optical performance of the nanofluids is analyzed in the range of 400 to 800 nm. Compared to base fluid, the prepared concentrations show higher absorbance in the measured range of wavelength. The optimum concentration is found to be 250 ppm, and its specific heat is measured in the temperature range of 27 to 117°C and is found to vary from 1.85 to 2.19 J/g °C. The thermal conductivity of the maximum concentration of nanofluid is 0.134 W/mK. The optical absorbance test confirms the stability of nanofluids. Maximum temperature and photothermal conversion efficiency are obtained.
In the present study, reduced graphene oxide (rGO) is synthesized from graphite using modified Hummer and chemical reduction methods. Various characterizations techniques are carried out to study the ...in-plane crystallite size, number of layers, presence of functional groups and surface morphology. Different concentrations of 0.01, 0.1, and 0.3 g/l of rGO/water nanofluids are prepared by dispersing the flakes in DI water. The colloidal stability of 0.3 g/l concentration is measured after 5 days using Zetasizer and found to be stable. The rGO/water nanofluids are then used to study the effect on the enhancement of critical heat flux (CHF) in pool boiling heat transfer. Results indicate an enhancement in CHF ranging from 145 to 245 % for the tested concentrations. The mechanisms of CHF enhancement are analyzed based on surface wettability, surface roughness, and porous layer thickness. The macrolayer dryout model sufficiently supports the mechanism of CHF enhancement of thin wire with rGO deposits, which is not reported yet.
The synthesis, structural, and retrostructural analysis of two libraries containing 16 first and second generation C 3-symmetric self-assembling dendrimers based on dendrons connected at their apex ...via trisesters and trisamides of 1,3,5-benzenetricarboxylic acid is reported. A combination of X-ray diffraction and CD/UV analysis methods demonstrated that their C 3-symmetry modulates different degrees of packing on the periphery of supramolecular structures that are responsible for the formation of chiral helical supramolecular columns and spheres self-organizable in a diversity of three-dimensional (3D) columnar, tetragonal, and cubic lattices. Two of these periodic arrays, a 3D columnar hexagonal superlattice and a 3D columnar simple orthorhombic chiral lattice with P2221 symmetry, are unprecedented for supramolecular dendrimers. A thermal-reversible inversion of chirality was discovered in helical supramolecular columns. This inversion is induced, on heating, by the change in symmetry from a 3D columnar simple orthorhombic chiral lattice to a 3D columnar hexagonal array and, on cooling, by the change in symmetry from a 2D hexagonal to a 2D centered rectangular lattice, both exhibiting intracolumnar order. A first-order transition from coupled columns with long helical pitch, to weakly or uncorrelated columns with short helical pitch that generates a molecular rotator, was also discovered. The torsion angles of the molecular rotator are proportional to the change in temperature, and this effect is amplified in the case of the C 3-symmetric trisamide supramolecular dendrimers forming H-bonds along their column. The structural changes reported here can be used to design complex functions based on helical supramolecular dendrimers with different degree of packing on their periphery.
The objective of this work is to evaluate the corrosion behaviour of nanographene oxide reinforced aluminium (Al/GO) metal matrix composites with different immersion time periods using the immersion ...corrosion technique. The Al/GO composites were fabricated by the ultrasonic gravitational stir casting process. The corrosions of Al/GO were evaluated using a scanning electron microscope. The experimental results revealed that the corrosion rate decreased and weight losses increased with increasing immersion time periods. The nonimmersed Al/GO composites exhibited higher microhardness values compared to the immersed Al/GO composites.
Thermal performance evaluation of TiO
2
-coated copper mesh wick in a cylindrical heat pipe with graphite nanofluid is experimentally analyzed at various inclinations, nanoparticle concentrations and ...power levels. Boiling heat transfer from the evaporator of a heat pipe depends on both thermal conductivity of the nanoparticle and suspension stability of the nanofluid. The lower the density of the nanoparticle, the better the suspension stability. Spherical graphite nanoparticles having lower density and good thermal conductivity quicken the heat transfer rate and hence the vaporization of base fluid. A hydrophilic coating of TiO
2
is done on the copper wick structure to reduce the contact angle of graphite nanofluid. Results showed that the heat pipe worked well at 60° inclination compared to the other tested orientations. For this optimum condition, a reduction in 168.75% of thermal resistance is obtained compared with DI water with uncoated wick at horizontal position and also an improvement in thermal efficiency of 94.07% for 1.0 mass% particle loading. The enhancement in equivalent thermal conductivity is 90.87% for 1.0 mass% compared with DI water. Results from the repeatability test also confirm that the hydrophilic coating over the wick is stable, and results are repeatable.
The unburned carbon (UC) in the ash in Indian power plants is presently measured offline by manually collecting the samples and analyzing it by loss on ignition or thermogravimetric analysis methods. ...The manual collection and offline analysis of samples take 6-8 h. By this time, the existing operating conditions of the boiler may be different from the conditions when the samples were collected. The combustion optimization and corrective maintenance activities can be performed immediately by plant operation and maintenance team if the UC values are available online along with furnace flame intensity and the percentage of CO in flue gases at economizer outlet. The presence of high amounts of the UC in ash decreases the boiler efficiency and reduces the utilization of ash in cement industries. In this paper, 20 ash samples were collected from five Indian thermal power plants of range 150-500 MW. An experimental setup has been developed for the measurement of the UC using the percentages of CO 2 and CO obtained by thermal oxidation of ash. The experimental results were compared with the laboratory results. The maximum deviation between experimental and laboratory results was within the range of ± 0.30. The percentages of the UC is more accurate when CO 2 and CO both are considered in comparison to the present existing online measurement based on CO 2 only.
The synthesis of dendritic dipeptides (4-3,4-3,5)12G2-CH2-Boc-l-Tyr-X-OMe where X = Gly, l-Val, l-Leu, l-Ile, l-Phe, and l-Pro is reported. Their self-assembly in bulk and in solution and the ...structural and retrostructural analysis of their periodic assemblies were compared to those of the previously reported and currently reinvestigated dendritic dipeptides with X = l-Ala. All dendritic dipeptides containing as X nonpolar α-amino acids self-assemble into helical porous columns. The substituent of X programs the structure of the helical pore and the resulting periodic array, in spite of the fact that its molar mass represents only between 0.05 and 4.77% from the molar mass of the dendritic dipeptide. In addition to the various 2-D columnar lattices, the dendritic dipeptides based on l-Ala, l-Leu, and l-Phe self-organize into 3-D hexagonal columnar crystals while those based on l-Val and l-Ile into an unknown columnar crystal. The principles via which the aliphatic and aromatic substituents of X program the structure of the helical pores indicate synthetic pathways to helical pores with bioinspired functions based on artificial nonpolar α-amino acids.
An experimental study on the convective heat transfer and friction factor characteristics of TiO
2
/DI water nanofluids in uniformly heated plain and helically corrugated tubes (HCT) with and without ...spiraled rod inserts (SRI) under laminar flow regime is presented in this paper. TiO
2
nanoparticles with an average size of 32 nm are dispersed in deionized (DI) water to form stable suspensions containing 0.1, 0.15, 0.2, and 0.25% volume concentrations of nanoparticles. It is found that the inclusion of nanoparticles to DI water ameliorated Nusselt number which increased with nanoparticles concentration upto 0.2%. Two spiraled rod inserts made of copper with different pitches (p
i
= 50 mm and 30 mm) are inserted in both plain and corrugated tubes and it is found that the addition of these inserts increased the Nusselt number substantially. For Helically corrugated tube with lower pitch and maximum height of corrugation (p
c
= 8 mm, h
c
= 1 mm) with 0.2% volume concentration of nanoparticles, a maximum enhancement of 15% in Nusselt number is found without insert and with insert having lower pitch (p
i
= 30 mm) the enhancement is 34% when compared to DI water in plain tube. The results on friction factor show a maximum penalty of about 53.56% for the above HCT.
PDF
