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 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.
Significant improvement of high temperature strength of 2219 alloy was achieved by addition of equal atom percentages (at%) of Sc and Zr and using a novel three-stage heat treatment route. Processing ...of the alloy involved casting in a water-cooled copper mould with a cooling rate in the range of 102 to 103 K/s. This was followed by direct ageing of the cast alloy at 375 ℃ to form thermally stable L12 type coherent Al3(Sc, Zr) precipitates in the matrix. Subsequently, the alloy was solutionized at 535 ℃ and further aged at 200 ℃ which caused the formation of highly dense θ′′ (Al3Cu) and θ′ (Al2Cu) precipitates. From microstructural characterization it was found that the high number density of these precipitates is due to heterogeneous nucleation on pre-existing Al3(Sc, Zr) precipitates. Presence of Al3(Sc, Zr) precipitates together with high number density of θ′′ and θ′ precipitates results in remarkably high 0.2% proof stress of 456 MPa at room temperature, 295 MPa at 200 °C and 227 MPa at 250 °C along with 5% ductility. It was found that Al3(Sc, Zr) precipitates also suppress the coarsening of θ′′ and θ′ precipitates thus making them thermally stable at high temperature.
We present the first report of a tungsten-free cobalt-based superalloy having a composition Co–10Al–5Mo–2Nb. The alloy is strengthened by cuboidal precipitates of metastable Co3(Al,Mo,Nb) distributed ...throughout the microstructure. The precipitates are coherent with the face-centred cubic γ-Co matrix and possess ordered L12 structure. The microstructure is identical to the popular γ–γ’ type nickel-based superalloys and that of recently reported Co–Al–W-based alloys. Being tungsten free, the reported alloy has higher specific proof stress compared to existing cobalt-based superalloys.
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.
This investigation addresses the effect of surface nanostructure developed through ultrasonic shot peening (USSP) on corrosion behavior of 7075 aluminum alloy. USSP develops surface nanostructure, ...induces severe plastic deformation, refines grain size up to certain depth and imparts compressive residual stress in the surface region. The sample USSPed for 15 s (USSP 15) exhibited lower current density (0.564 μA/cm2) and higher corrosion potential (−0.695 V) as compared with that of the un-USSP specimen with 1.269 μA/cm2 and -0.839 V respectively. The enhancement in corrosion resistance of USSPed sample is due to rapid development of uniform, homogeneous and effective passive layer on the nano-structured surface coupled with refinement of the coarse precipitates. Also, there is optimum combination of surface roughness, compressive residual stress, and dislocation density in the surface region to produce highest corrosion resistance in the USSP 15 condition.
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•Ultrasonic shot peening successfully induces nanostructure layer in surface region.•Refinement of second phase reduced the galvanic coupling.•Nanostructure causes uniform, homogeneous and effective passive layer formation.•Increased surface roughness leads to deterioration in corrosion resistance.•Combination of several factors governs the corrosion behavior.
In this article, a new class of exact solutions for anisotropic compact objects is presented. Admitting the modified Chaplygin equation of state
p
=
H
ρ
-
K
ρ
n
, where
H
,
K
and
n
are constants with
...0
<
n
≤
1
, and employing the Buchdahl-I metric within the framework of the general relativity stellar model is obtained. Recent observations on pulsars and GW events reveal that the observed maximum mass of compact stars detected so far is approximately
2
.
59
-
0.09
+
0.08
M
⊙
. Since massive stars cannot be supported by a soft equation of state, a constraint of the equation of state must hold. The choice of a suitable equation of state for the interior matter of compact objects may predict useful information compatible with recent observations. TOV equations have been solved using the modified Chaplygin equation of state to find the maximum mass in this model. In particular, the theory can achieve
3.72
M
⊙
, when
H
=
1.0
,
K
=
10
-
7
and
n
=
1
. The model is suitable for describing the mass of pulsars PSR J2215+5135 and PSR J0952-0607 and the mass
2
.
59
-
0.09
+
0.08
M
⊙
of the companion star in the GW 190814 event. The
3.72
M
⊙
is hardly achievable theoretically in general relativity considering fast rotation effects too. To check the physical viability of this model, we have opted for the stability analysis and energy conditions. We have found that our model satisfies all the necessary criteria to be a physically realistic model.
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.
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.
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