Implementation of Thermite Reactions in the Production of Advanced Intermetallic-Matrix Composites: The Case of Nb2O5/Al Thermite Mixture

Arman Ahmadi‑Binahri,Mandana Adeli,M. Reza Aboutalebi,Sergey Krasikov 대한금속·재료학회 2022 METALS AND MATERIALS International Vol.28 No.6

In this study, the application of thermite reactions in Nb2O5–Al system in the fabrication of novel NbAl3/Al2O3 compositeswas investigated. Combining the thermite reactions with self-propagating high-temperature synthesis (SHS) techniquein compressed powder mixtures of Nb2O5+ Al resulted in layer-by-layer progress of the exothermic reaction, controlledformation of the desired products and their coexistence in each reacted layer. Characterization of the products using X-raydiffraction (XRD), scanning electron microscopy and energy dispersive X-ray spectroscopy (SEM-EDS) techniques showedthat all samples were composed of a metallic matrix including NbAl3as the main compound, along with Al2O3as ceramicreinforcing phase. Differential thermal analysis (DTA) analysis was used to study the mechanism of reaction using bothstoichiometric and over-stoichiometric amounts of aluminum. The effect of such parameters as stoichiometry and greendensity of the samples on the combustion temperature, formed phases, microstructure, and hardness of the obtained compositeswas investigated using samples with stoichiometric as well 20 wt% and 40 wt% excess aluminum, and samples withgreen densities of 75%, 85%, and 95% of theoretical maximum density (TMD). The presence of excess aluminum resultedin prolonged reaction times, lower combustion temperatures, and lower mean values of hardness. While the variations ingreen density had no remarkable effect on the microstructural features of products within the density range under study, themost intensive reactions, highest combustion temperature, and highest mean value of hardness were encountered in sampleswith green density of 85%TMD.

Numerical study on flow field in inlet plenum of a pebble-bed modular reactor

Ahmad, I.,Kim, K.Y.,Lee, W.J.,Park, G.C. North-Holland Pub. Co 2007 Nuclear engineering and design Vol.237 No.6

Flow distribution and pressure drop analysis in the inlet plenum of a pebble-bed modular reactor (PBMR) have been performed numerically. Three-dimensional Navier-Stokes equations have been solved in conjunction with the k-@? model as a turbulence closure. Non-uniformity in the flow distribution is assessed for the reference case, and parametric studies have been performed for rising channels diameter, Reynolds number, angle between the rising channels, angle between the inlet ports, and aspect ratio of the plenum cross-section. Also, two different shapes of the inlet plenum, namely, rectangular and oval shapes, have been analyzed. The relative flow mal-distribution parameter variation shows that the flow distribution in rising channels for the reference case is strongly non-uniform. As the rising channels diameter is decreased, the flow uniformity as well as the pressure drop is found to increase. The flow distribution in the rising channels is independent of Reynolds number. Increase in the angle between the inlet ports and aspect ratio is found to increase the uniformity in flow distribution.

SYMMETRIC DUALITY FOR FRACTIONAL VARIATIONAL PROBLEMS WITH CONE CONSTRAINTS

Ahmad, I.,Yaqub, Mohd.,Ahmed, A. 한국전산응용수학회 2007 Journal of applied mathematics & informatics Vol.23 No.1

A pair of symmetric fractional variational programming problems is formulated over cones. Weak, strong, converse and self duality theorems are discussed under pseudoinvexity. Static symmetric dual fractional programs are included as special case and corresponding symmetric duality results are merely stated.

MULTIOBJECTIVE FRACTIONAL SYMMETRIC DUALITY INVOLVING CONES

Ahmad, I.,Sharma, Sarita Korean Society of Computational and Applied Mathem 2008 Journal of applied mathematics & informatics Vol.26 No.1

A pair of multiobjective fractional symmetric dual programs is formulated over arbitrary cones. Weak, strong and converse duality theorems are proved under pseudoinvexity assumptions. A self duality theorem is also discussed.

ON SYMMETRIC DUALITY IN NONDIFFERENTIABLE MATHEMATICAL PROGRAMMING WITH F-CONVEXITY

AHMAD, I.,HUSAIN, Z. 한국전산응용수학회 2005 Journal of applied mathematics & informatics Vol.19 No.1

Usual symmetric duality results are proved for Wolfe and Mond-Weir type nondifferentiable nonlinear symmetric dual programs under F-convexity F-concavity and F-pseudoconvexity F-pseudoconcavity assumptions. These duality results are then used to formulate Wolfe and Mond-Weir type nondifferentiable minimax mixed integer dual programs and symmetric duality theorems are established. Moreover, nondifferentiable fractional symmetric dual programs are studied by using the above programs.

SUFFICIENCY AND DUALITY IN MINMAX FRACTIONAL SUBSET PROGRAMMING INVOLVING GENERALIZED TYPE-I FUNCTIONS

Ahmad, I.,Sharma, Sarita Korean Society of Computational and Applied Mathem 2008 Journal of applied mathematics & informatics Vol.26 No.5

A discrete minmax fractional subset programming problem is considered. Various parametric and parameter-free global sufficient optimality conditions and duality results are discussed under generalized ($F,{\alpha},{\rho},{\theta}$)-V-type-I n-set functions.

The Effects of Hearing Aid Digital Noise Reduction and Directionality on Acceptable Noise Level

Roghayeh Ahmadi,Hamid Jalilvand,Mohammad Ebrahim Mahdavi,Fatemeh Ahmadi,Ali Reza Akbarzade Baghban 대한이비인후과학회 2018 Clinical and Experimental Otorhinolaryngology Vol.11 No.4

Objectives. Two main digital signal processing technologies inside the modern hearing aid to provide the best conditions for hearing aid users are directionality (DIR) and digital noise reduction (DNR) algorithms. There are various possible settings for these algorithms. The present study evaluates the effects of various DIR and DNR conditions (both separately and in combination) on listening comfort among hearing aid users. Methods. In 18 participants who received hearing aid fitting services from the Rehabilitation School of Shahid Beheshti University of Medical Sciences regularly, we applied acceptable noise level (ANL) as our subjective measure of listening comfort. We evaluated both of these under six different hearing aid conditions: omnidirectional-baseline, omnidirectional-broadband DNR, omnidirectional-multichannel DNR, directional, directional-broadband DNR, and directional-multichannel DNR. Results. The ANL results ranged from −3 dB to 14 dB in all conditions. The results show, among all conditions, both the omnidirectional-baseline condition and the omnidirectional-broadband DNR condition are the worst conditions for listening in noise. The DIR always reduces the amount of noise that patients received during testing. The DNR algorithm does not improve listening in noise significantly when compared with the DIR algorithms. Although both DNR and DIR algorithms yielded a lower ANL, the DIR algorithm was more effective than the DNR. Conclusion. The DIR and DNR technologies provide listening comfort in the presence of noise. Thus, user benefit depends on how the digital signal processing settings inside the hearing aid are adjusted.

Laboratory evaluation of roller compacted concrete containing RAP

Ahmadi, Amin,Gogheri, Mohammad K.,Adresi, Mostafa,Amoosoltani, Ershad Techno-Press 2020 Advances in concrete construction Vol.10 No.6

This paper investigates mechanical properties of roller compacted concrete (RCC) involving reclaimed asphalt pavement (RAP). In this way, a set of 276 cylindrical RCC specimens were prepared with different RAP sizes (i.e., fine, coarse & total) at various ratios (i.e., 10%, 20%, and 40%). Results reveal that incorporation of RAP decreases unconfined compressive strength (UCS), modulus of elasticity (E), and indirect tensile (IDT) strength of RCC. For each RAP size, a regression model was used to maximize RAP content while satisfying the UCS lower limit (27.6 Mpa) mentioned by ACI as a minimum requirement for RCC used in pavement construction. Moreover, UCS of RAP incorporated mixes, dissimilar to that of control mixes, was found to be sensitive and insensitive to the testing temperature and curing time after 7 days, respectively. The results also demonstrate that the higher amounts of RAP, the more flexibility in RCC is. This issue was also proved by the results of modulus of elasticity test. In addition, the toughness index (TI) shows that increase in RAP content leads to up to 43% increase in energy absorbance capacity of RCC.

The relationship between the level of salivary alpha amylase activity and pain severity in patients with symptomatic irreversible pulpitis

Ahmadi-Motamayel, Fatemeh,Shahriari, Shahriar,Goodarzi, Mohammad Taghi,Moghimbeigi, Abbas,Jazaeri, Mina,Babaei, Parisa The Korean Academy of Conservative Dentistry 2013 Restorative Dentistry & Endodontics Vol.38 No.3

Objectives: Assessment of dental pain severity is very challenging in dentistry. Previous studies have suggested that elevated salivary alpha amylase may contribute to increased physical stresses. There is a close association between salivary alpha amylase and plasma norepinephrine under stressful physical conditions. The aim of this study was to evaluate the relationship between pain severity and salivary alpha amylase levels in patients with symptomatic irreversible pulpitis. Materials and Methods: Thirty-six patients (20 females and 16 males) with severe tooth pain due to symptomatic irreversible pulpitis were selected. The visual analogue scale (VAS) score was used to assess the pain severity in each patient. Unstimulated whole saliva was collected, and the level of alpha amylase activity was assessed by the spectrophotometric method. Statistical analysis was performed using SPSS 13. Results: The level of alpha amylase was significantly increased in the saliva in association with pain severity assessed by VAS. The salivary alpha amylase was also elevated with increased age and in males. Conclusions: There was a significant correlation between the VAS pain scale and salivary alpha amylase level, which indicates this biomarker may be a good index for the objective assessment of pain intensity.

SCFs in offshore two-planar tubular TT-joints reinforced with internal ring stiffeners

Ahmadi, Hamid,Imani, Hossein Techno-Press 2022 Ocean systems engineering Vol.12 No.1

The majority of tubular joints commonly found in offshore jacket structures are multi-planar. Investigating the effect of loaded out-of-plane braces on the values of the stress concentration factor (SCF) in offshore tubular joints has been the objective of numerous research works. However, due to the diversity of joint types and loading conditions, a number of quite important cases still exist that have not been studied thoroughly. Among them are internally ring-stiffened two-planar TT-joints subjected to axial loading. In the present research, data extracted from the stress analysis of 243 finite element (FE) models, verified against available numerical and experimental data, was used to study the effects of geometrical parameters on the chord-side SCFs in two-planar tubular TT-joints reinforced with internal ring stiffeners subjected to two types of axial loading. Parametric FE study was followed by a set of nonlinear regression analyses to develop six new SCF parametric equations for the fatigue analysis and design of axially-loaded two-planar TT-joints reinforced with internal ring stiffeners.