The thermal stability and flame retardancy of polyurethanes is reviewed. Polyurethanes (PUs) are an important class of polymers that have wide application in a number of different industrial sectors. ...More than 70% of the literature that deals with PUs evaluates their thermal stability or flame retardancy and attempts to provide a structure–property correlation. The importance of studying thermal degradation, understanding the processes occurring during thermal stress as well as the parameters affecting the thermal stability of PUs are essential in order to effectively design polyurethanes having tailor-made properties suitable for the particular environment where they are to be used. A detailed description of TGA, TGA-MS and TGA-FTIR methods for studying the decomposition mechanism and kinetics is also a part of this review. In general, thermal decomposition of PUs begins with the hard segment (HS) and a number of parameters govern a polyurethane's thermal stability. Detailed description of the parameters such as HS, soft segment (SS) and chain extender (CE) structure and molecular weight, NCO:OH ratio, catalyst nature and crosslink density that affect the nature of PU degradation is given. Descriptions of approaches to improve the thermal stability in PUs such as formation of poly(urethane-isocyanurate), poly(urethane-oxazolidone) and poly(urethane-imide) in addition to other methods such as PUs with an s-triazine ring or increased aromatic ring concentration, azomethane linkages as well as use of hyperbranched polyols as crosslinking agents is given. A part of the review is also concentrated on the improvement of thermal stability via hybrid formation such as the incorporation of appropriate amounts of fillers, e.g., nano-silica; Fe
2O
3; TiO
2; silica grafting; nanocomposite formation using organically modified layered silicates; incorporation of Si–O–Si crosslinked structures via sol–gel processes; and the incorporation of polyhedral oligomeric silsesquioxane (POSS) structures into the PU backbone or side chain. Incorporation of carbon nanotubes (CNT) into PUs and the use of functionalized fullerenes in PUs are also described as these are the newest tools to obtain good thermal stability and flame retardancy. Part of the review also concentrates on the process that occurs during burning of PUs, flame retardant mechanisms and different additives or reactive type flame retardants used in the PU industry. The use and working function of expandable graphite and melamine as additive type flame retardants are shown. Description of the use of different reactive type organophosphorus compounds, cyclotriphosphazenes, aziridinyl curing agents in aqueous polyurethane dispersions (PUDs), organoboron compounds and organosilicon compounds for improving flame retardancy is also given.
The structural, optical and electrical properties of ZnO thin films (260 – 490 nm thick) deposited by direct-current sputtering technique, at a relatively low-substrate temperature (363 K), onto ...polyethylene terephthalate and glass substrates have been investigated. X-ray diffraction patterns confirm the proper phase formation of the material. Optical transmittance data show high transparency (80% to more than 98%) of the films in the visible portion of solar radiation. Slight variation in the transparency of the films is observed with a variation in the deposition time. Electrical characterizations show the room-temperature conductivity of the films deposited onto polyethylene terephthalate substrates for 4 and 5 h around 0.05 and 0.25 S cm
−
1
, respectively. On the other hand, for the films deposited on glass substrates, these values are 8.5 and 9.6 S cm
−
1
for similar variation in the deposition time. Room-temperature conductivity of the ZnO films deposited on glass substrates
is at least two orders of magnitude higher than that of ZnO films deposited onto polyethylene terephthalate substrates under identical conditions. Hall-measurements show the maximum carrier concentration of the films on PET and glass substrate around 2.8
×
10
16 and 3.1
×
10
20 cm
−
3
, respectively. This report will provide newer applications of ZnO thin films in flexible display technology.
This review addresses trends in the development of high performance polyurethane (PU) and its subclass coatings. Changes in the understanding the technical requirements and properties, along with ...novel approaches in creating high performance PU films are discussed. Some discussion of developments leading to the current status is provided, along with opportunities for the development of architectural, domestic and automotive coatings from carbamate chemistry. The review also focuses on the chemistry of PU, the importance of side products such as biuret, allophanate formation, as well as different low VOC PU coatings such as moisture-cured PU-urea, polyurea, PU-imide, UV cure and waterborne PU coatings. A brief description of functionalized dendritic/hyperbranched polymers, different monomers and synthetic approaches, and their use in the PU coating sector is addressed. In the context of nanostructuring, sol–gel chemistry to formulate ceramer coatings and use of functionalized nanomaterials in PU coatings is described. This portion also includes silica grafting and functionalization of nanosilica particles, polyhedral oligomeric silsesquioxane and the use of bridged polysilsesquioxanes in PU coatings. A wide variety of fillers, whiskers and fibers as well as clay and wollastonites with structural modification is described for use in nanocomposite PU coatings, with special emphasis on the latest development in PU-layered silicate nanocomposites.The use of different reactive-type organophosphorus compounds, aziridinyl curing agents in aqueous PU dispersions, phosphazenes, organo-boranes such as different carboranes for use in the formulation of flame retardant coatings is described. Since surface properties determine the resistivity of coatings towards corrosive chemicals, moisture and dirt repellency, a portion of the review will also be devoted towards modification of surface properties using fluoropolymer, and the synthesis and a brief description of the available fluorinated diols is given.
Type-II p–n junction three-dimensional Ag2O/TiO2 microspheres have been fabricated by assembling p-type Ag2O nanoparticle on n-type TiO2 3D microsphere. Ag2O/TiO2 microsphere nanoheterojunctions were ...obtained by hydrothermal synthesis of TiO2 microspheres at 180 °C followed by photoreduction of AgNO3. The samples were carefully characterized by X-ray diffraction (XRD), transmission electron microscopy (TEM), field-emission scanning electron microscopy (FESEM), and energy dispersive X-ray analysis (EDX). The photocatalytic activity toward degradation of methyl orange (MO) aqueous solution under UV light was investigated. The result showed that type-II p–n nanoheterojunctions Ag2O/TiO2 significantly enhanced the photocatalytic degradation compared to n-type TiO2 microsphere. It was found that the photocatalytic degradation followed the pseudo first-order reaction model. In particular, heterostructure with molar ratio of TiO2 and AgNO3 of 4:1 exhibited best photocatalytic activity and the corresponding apparent first-order rate constant of 0.138 min–1 which is 4 times than that of pure n-type microsphere.
Aluminum-doped cadmium oxide (CdO:Al) thin films are deposited on glass substrates by the sol–gel dip-coating method, taking cadmium acetate dihydrate as the precursor material. Aluminum nitrate has ...been taken as a source of Al-dopant. XRD pattern reveals the good crystallinity of CdO thin films. SEM micrograph showed the presence of faceted crystallites. Optical study shows 40–85% transparency with a bandgap value lying in the range 2.76–2.52
eV, depending upon the Al content in the films. Optimum percentage of Al was 5.22 for a maximum room temperature conductivity of 2.81×10
3 (Ω
cm)
−1. Hall measurement confirmed that the material is of n-type, with mobility and carrier concentrations lying in the range 413–14.7
cm
2/V
s, and 3.4×10
19–8.11×10
20
cm
−3, when percentage of Al varies in the range 1.32–7.24.
Evolutionary programming has emerged as a useful optimization tool for handling nonlinear programming problems. Various modifications to the basic method have been proposed with a view to enhance ...speed and robustness and these have been applied successfully on some benchmark mathematical problems. But few applications have been reported on real-world problems such as economic load dispatch (ELD). The performance of evolutionary programs on ELD problems is examined and presented in this paper in two parts. In Part I, modifications to the basic technique are proposed, where adaptation is based on scaled cost. In Part II, evolutionary programs are developed with adaptation based on an empirical learning rate. Absolute, as well as relative, performance of the algorithms are investigated on ELD problems of different size and complexity having nonconvex cost curves where conventional gradient-based methods are inapplicable.
Stoichiometric NiO, a Mott-Hubbard insulator at room temperature, shows p-type electrical conduction due to the introduction of Ni(2+) vacancies (V(Ni)('')) and self-doping of Ni(3+) ions in the ...presence of excess oxygen. The electrical conductivity of this important material is low and not sufficient for active device fabrication. Al doped NiO thin films were synthesized by radio frequency (RF) magnetron sputtering on glass substrates at a substrate temperature of 250 °C in an oxygen + argon atmosphere in order to enhance the p-type electrical conductivity. X-ray diffraction studies confirmed the correct phase formation and also oriented growth of NiO thin films. Al doping was confirmed by x-ray photoelectron spectroscopic studies. The structural, electrical and optical properties of the films were investigated as a function of Al doping (0-4 wt%) in the target. The room temperature electrical conductivity increased from 0.01-0.32 S cm (-1) for 0-4% Al doping. With increasing Al doping, above the Mott critical carrier density, energy band gap shrinkage was observed. This was explained by the shift of the band edges due to the existence of exchange and correlation energies amongst the electron-electron and hole-hole systems and also by the interaction between the impurity quasi-particle system.
Water remediation techniques like photolysis have recently piqued the interest of many researchers due to water contamination resulting from heavy industrialization and urbanization. In the current ...work, as-synthesized TiO2 nanorod decorated vertically aligned silicon nanowire (SiNW) leads to a hierarchical morphological structure formation. The photocatalytic nature of the fabricated SiNW/TiO2 nanoheterojunction is examined by the dye degradation of textile pollutants like methylene blue (MB), rhodamine B (RhB), and eosin B (EB). The catalytic dye degradation investigations revealed that 4 h hydrothermal synthesis of TiO2 on the surface of SiNW (ST4) exhibited excellent catalytic behaviour. In the presence of H2O2 and UV irradiation, the ST4 nanoheterostructure can degrade 98.89% of the model pollutant methylene blue (MB) in 15 min, demonstrating remarkable photocatalytic performance. The direct Z-scheme heterojunction exhibited by the SiNW/TiO2 structure facilitates a more efficient charge transfer mechanism with higher reducing and oxidizing ability leading to enhanced photocatalytic behaviour. The degradation pathway examined by LC-MS studies demonstrated the complete breakdown of the organic MB dye molecules ultimately mineralizing into CO2, H2O, and other inorganic substances. The photocatalyst ST4 exhibited excellent reusability and stability after multiple cycles of dye degradation enabling its use in practical water purification purposes.
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•Hierarchical SiNW/TiO2 nanorods were fabricated by MACE and hydrothermal synthesis.•Optimized SiNW/TiO2 exhibited 98.89% degradation of MB at 15 min in presence of H2O2.•SiNW/TiO2 heterojunction exhibited Z-scheme charge transfer route for photocatalysis.•Demonstrated high stability and reusability as an immobile photocatalyst under UV light.•Plausible mechanism and degradation pathway of MB dye has been proposed.
The effect of ultrasonic shot peening (USSP) was studied on microstructural modification in the surface region and low cycle fatigue (LCF) behavior of the Ti–6Al–4V alloy, at room temperature. ...Nanostructure of 17–25 nm was developed in surface region of the alloy by USSP. Fatigue life of the USSPed samples was found to increase progressively with decrease in strain amplitude to much larger extent in comparison with those of the non-USSPed ones. Fatigue life of the USSPed sample was enhanced by four times at the lowest strain amplitude of ±0.60%. There was cyclic softening from the beginning until the failure, at the high strain amplitudes (Δεt/2≥ ±0.80%) whereas cyclic hardening was exhibited during the initial 100 cycles, followed by softening at the lower strain amplitudes (Δεt/2 ≤ ±0.70%). Deformation structure of the samples, tested at different strain amplitudes was analysed using TEM. The results are discussed in terms of increase in the resistance of the material against crack initiation due to grain refinement in the surface region and the associated compressive stresses induced from USSP.
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•A nanostructure surface layer was established, up to the depth of ∼50 μm from USSP.•Low cycle fatigue life was improved ∼4 times due to USSP at the low strain amplitudes.•Grain refinement delays the process of fatigue crack initiation and propagations.•The higher degree of twinning in USSPed sample is due to severe plastic deformation.
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