A dual band leaky wave antenna based on modified substrate integrated waveguide

In this article, a new leaky wave antenna (LWA) based on modified substrate integrated waveguide (SIW) is introduced. At first, the modified SIW structure is presented and it is shown that it supports propagation of quasi‐TEM with nearly uniform electric field distribution. Then, a new cell based on diagonal‐shaped slots embedded on top surface of the structure is introduced and its dispersion characteristics and its different radiation regions are determined. A LWA made of 15 unit cells is designed and a prototype of the antenna is fabricated. The proposed LWA is simulated using a software package and its radiation characteristics are also measured. It is shown that a good agreement is obtained between simulated and measured results and two frequency bands are obtained. In the frequency range of 7 GHz to 8.25 GHz, it radiates in forward region with maximum gain of 11.3 dB and scan angles from 54° to end‐fire. In addition, it radiates in backward region from ?70° to broadside from 14 GHz up to 20 GHz with maximum gain of 16.47 dB. High gain, compactness, and wide scan angles are the advantages of the proposed LWA.     

A heuristic algorithm for master production scheduling problem with controllable processing times and scenario-based demands

Master production scheduling (MPS) is widely used by manufacturing industries in order to handle the production scheduling decisions in the production planning hierarchy. The classical approach to MPS assumes infinite capacity, fixed (i.e. non-controllable) processing times and a single pre-determined scenario for the demand forecasts. However, the deterministic optimisation approaches are sometimes not suitable for addressing the real-world problems with high uncertainty and flexibility. Accordingly, in this paper, we propose a new practical model for designing an optimal MPS for the environments in which processing times may be controllable by allocating resources such as facilities, energy or manpower. Due to the NP-hardness of our model, an efficient heuristic algorithm using local search technique and theory of constraints is developed and analysed. The computational results especially for large-sized test problems show that the average optimality gap of proposed algorithm is four times lower than that of exact solution using GAMS while it consumes also significantly smaller run times. Also, the analysis of computational results confirms that considering the controllable processing times may improve the solution space and help to more efficiently utilise the available resources. According to the model structure and performance of the algorithm, it may be proposed for solving large and complex real-world problems particularly the machining and steel industries.     

The Relation Between Open‐Circuit Voltage and the Onset of Photocurrent Generation by Charge‐Transfer Absorption in Polymer : Fullerene Bulk Heterojunction Solar Cells

Photocurrent generation by charge‐transfer (CT) absorption is detected in a range of conjugated polymer–[6,6]‐phenyl C₆₁ butyric acid methyl ester (PCBM) based solar cells. The low intensity CT absorption bands are observed using a highly sensitive measurement of the external quantum efficiency (EQE) spectrum by means of Fourier‐transform photocurrent spectroscopy (FTPS). The presence of these CT bands implies the formation of weak ground‐state charge‐transfer complexes in the studied polymer–fullerene blends. The effective band gap (E_g) of the material blends used in these photovoltaic devices is determined from the energetic onset of the photocurrent generated by CT absorption. It is shown that for all devices, under various preparation conditions, the open‐circuit voltage (V_oc) scales linearly with E_g. The redshift of the CT band upon thermal annealing of regioregular poly(3‐hexylthiophene):PCBM and thermal aging of poly(phenylenevinylene)(PPV):PCBM photovoltaic devices correlates with the observed drop in open‐circuit voltage of high‐temperature treated versus untreated devices. Increasing the weight fraction of PCBM also results in a redshift of E_g, proportional with the observed changes in V_oc for different PPV:PCBM ratios. As E_g corresponds with the effective bandgap of the material blends, a measurement of the EQE spectrum by FTPS allows us to measure this energy directly on photovoltaic devices, and makes it a valuable technique in the study of organic bulk heterojunction solar cells.     

Effect of short carbon fiber content on the mechanical properties of TiB2-based composites prepared by spark plasma sintering

In this study, TiB₂-30 vol% SiC composites containing 0, 5, 10, and 15 vol% short carbon fibers (C_f) were produced by spark plasma sintering (SPS). The effect of carbon fiber content on microstructure, density, and mechanical properties (micro-hardness and flexural strength) of the fabricated composites was studied. Scanning electron microscopy (SEM) results indicated that the fibers were uniformly dispersed in the TiB₂–SiC matrix using wet ball milling before SPS process. Fully dense TiB₂–SiC–C_f composites were achieved by SPS process at 1900°C for 10 min under 30 MPa. With the addition of fibers, the relative density of the composites did not change considerably. Mechanical tests revealed that microhardness was reduced about 19% by the incorporation of carbon fibers, whereas the flexural strength improved significantly. However, the flexural strength diminished by adding carbon fibers above to critical value (5 vol%) due to residual thermal stresses, nonhomogeneous structure and graphitization of carbon fibers. It was found that the composite with 5 vol% C_f had the highest flexural strength (482 MPa), which was enhanced by 20% compared with the TiB₂–SiC composite.     

Fabrication of quantum dot-sensitized solar cells with multilayer TiO2/PbS(X)/CdS/CdSe/ZnS/SiO2 photoanode and optimization of the PbS nanocrystalline layer

In this work, two multilayer photoanode structures of TiO₂/PbS(X)/CdS/ZnS/SiO₂ and TiO₂/PbS(X)/CdS/CdSe/ZnS/SiO₂ were fabricated and applied in quantum dot-sensitized solar cells (QDSCs). Then, the effect of PbS QDs layer on the photovoltaic performance of corresponding cells was investigated. The sensitization was carried out by PbS and CdS QDs layers deposited on TiO₂ scaffold through successive ionic layer adsorption and reaction (SILAR) method. The CdSe QDs film was also formed by a fast, modified chemical bath deposition (CBD) approach. Two passivating ZnS and SiO₂ layers were finally deposited by SILAR and CBD methods, respectively. It was shown that the reference cell with TiO₂/CdS/ZnS/SiO₂ photoanode demonstrated a power conversion efficiency (PCE) of 3.0%. This efficiency was increased to 4.0% for the QDSC with TiO₂/PbS(2)/CdS/ZnS/SiO₂ photoelectrode. This was due to the co-absorption of incident light by low-bandgap PbS nanocrystalline film and also the CdS QDs layer and well transport of the charge carriers. For the CdSe included QDSCs, the PbS-free reference cell represented a PCE of 4.1%. This efficiency was improved to 5.1% for the optimized cell with TiO₂/PbS(2)/CdS/CdSe/ZnS/SiO₂ photoelectrode. The maximized efficiency was enhanced about 25% and 70% compared to the PbS-free reference cells with and without the CdSe QDs layer.

     

Highly refractive thiazole-containing polyimides: a structural property comparison

Highly refractive, thermally stable, and solution processable polyimides (PIs) were synthesized by the introduction of thiazole units, thioether linkages, and phenyl or nitrophenyl groups into the polymer backbones. These PIs were prepared via the polycondensation of two diamine monomers, 5,5′-thiobis(2-amino-4-phenyl-thiazole) (DA-1) or 5,5′-thiobis(2-amino-4-(3-nitrophenyl)thiazole) (DA-2), with various aromatic dianhydrides(a–d). The bulky pendant phenyl or nitrophenyl units as well as the non-coplanar conformations because of ortho-sulfide linkages endowed the resulting PIs with excellent solubility in organic solvents. These PIs showed outstanding thermal stability, with 10% weight loss temperatures exceeding 492 °C under nitrogen and 475 °C in air atmosphere, while their glass transition temperatures were in the range of 239–278 °C. In general, the synergic effects of thiazole groups, phenyl or nitrophenyl substituents, and thioether linkages provided PIs with very high refractive indices of up to 1.7646 at 632.8 nm, along with small birefringences (<0.0085) and high Abbe’s numbers. The structure–property relationships of the analogous PIs containing phenyl or nitrophenyl substituents groups were also studied in detail by comparing the results.     

Protection and Functionalizing of Stainless Steel Surface by Graphene Oxide-Polypyrrole Composite Coating

The corrosion inhibition of metallic substrates is a prime issue for many potential applications where corrosion plays a crucial role. The development of carbon based on functionalized coatings could increase the lifetime of metallic substrates by inhibiting the corrosion process. Present work is an effort to develop a corrosion inhibiting composite coating of graphene oxide and polypyrrole for AISI （American Iron and Steel Institute） type 304 stainless steel substrates. The electrochemical galvanostatic deposition process was applied for coating development. The coating morphology and ability to cover the substrate surface was analyzed with a high-resolution scanning electron microscope. The coating＇s structural and electronic properties were analyzed with Raman spectroscopy. The investigation of corrosion inhibition involved open circuit potential, Tafel, and voltammetry analysis. The standard salt test ASTM （American Society for Testing and Materials） G48A for stainless steel substrate has also been studied. Significant enhancement of corrosion potential as well as pitting potential for the composite coated substrates has been noted. Furthermore, corrosion and breakdown potential increased upon changing the material from graphene oxide to its composite coating. During the salt test analysis, the durability of the composite coating was noted up to 72 h, which is the standard time scale. Based on experimental analysis, this composite material can be used as an effective carbon based on functionalized corrosion inhibitor for stainless steel substrates to increase their lifetime.     

Conversion of CO into CO2 by high active and stable PdNi nanoparticles supported on a metal-organic framework

The solubility of Pd(NO₃)₂ in water is moderate whereas it is completely soluble in diluted HNO₃ solution. Pd/MIL-101(Cr) and Pd/MIL-101-NH₂(Cr) were synthesized by aqueous solution of Pd(NO₃)₂ and Pd(NO₃)₂ solution in dilute HNO₃ and used for CO oxidation reaction. The catalysts synthesized with Pd(NO₃)₂ solution in dilute HNO₃ showed lower activity. The aqueous solution of Pd(NO₃)₂ was used for synthesis of mono-metal Ni, Pd and bimetallic PdNi nanoparticles with various molar ratios supported on MOF. Pd₇₀Ni₃₀/MIL-101(Cr) catalyst showed higher activity than monometallic counterparts and Pd+ Ni physical mixture due to the strong synergistic effect of PdNi nanoparticles, high distribution of PdNi nanoparticles, and lower dissociation and desorption barriers. Comparison of the catalysts synthesized by MIL-101(Cr) and MIL-101-NH₂(Cr) as the supports of metals showed that Pd/MIL-101-NH₂(Cr) outperforms Pd/MIL-101-(Cr) because of the higher electron density of Pd resulting from the electron donor ability of the NH₂ functional group. However, the same activities were observed for Pd₇₀Ni₃₀/MIL-101(Cr) and Pd₇₀Ni₃₀/MIL-101-NH₂(Cr), which is due to a less uniform distribution of Pd nanoparticles in Pd₇₀Ni₃₀/MIL-101-NH₂(Cr) originated from amorphization of MIL-101-NH₂(Cr) structure during the reduction process. In contrast, Pd₇₀Ni₃₀/MIL-101(Cr) revealed the stable structure and activity during reduction and CO oxidation for a long time.     

Keys to improve the efficiency of nanoparticle-stabilized foam injection based on influential mechanisms

The effect of the existence of nanoparticles on foam stability, foamability, and the oil recovery factor (RF) has been studied experimentally, and influential phenomena and mechanisms have been examined. A sequence of experiments, including, ‘foam bulk-static experiments’, ‘surface tension (ST) measurements,’ and ‘micromodel foam flood,’ were designed and then implemented to study the foam behaviour in two foam systems: (1) anionic-nanoparticles + cationic-surfactant and (2) anionic-nanoparticles + anionic-surfactant. This study provides a comprehensive insight into the mechanisms affecting the stability of nanoparticle-stabilized foam. Also, despite previous studies, the effect of Marangoni flow on nanoparticle-stabilized foam has been discussed briefly. Results show that the interactions of effective mechanisms work differently in the two series. In the like-charge system, surfactant molecules accumulate in the interface of lamellas due to repulsive forces; therefore, stability and foamability improve as surface tension and molecular diffusion reduce. Additionally, Marangoni flow restitutes the negative impact of gravity drainage. In the unlike-charge system, observations illustrate that nanoparticles reach the interface. The presence of nanoparticles at the interface increases detachment energy significantly, and as a result, the stability is boosted. The accumulation of nanoparticles in the interface changes it to a solid-like surface with limited diffusibility and viscosity. Although Marangoni flow is lost, reducing molecular diffusion improves foam stability. Flooding tests show that foam stability increment improves sweep efficiency at near-wellbore areas even when foamability is weak. Finally, it can be claimed that in the unlike-charge system, the sweep efficiency and foam stability increase to a greater extent.