مجله مواد نوین
سال یازدهم شماره 4 (پیاپی 42، زمستان 1399)

In marine diesel engines with high power to weight ratio, water-cooled exhaust manifolds are designed and manufactured to achieve a good heat transfer in exhaust system with appropriate safety and reliability in engine room. In one of high-speed marine diesel engine after 417 hours operation, water was leaked from aluminum alloy exhaust manifold due to cavity creation between coolant and gas port walls. This paper investigates the root cause analysis of this exhaust manifold failure using fishbone diagram. Results showed that film boiling was accrued on damaged surface. In this region, surface was completely covered by a continuous stable vapor film. So, heat flux of this area reached a minimum and surface temperature was increased locally. The root cause of this phenomena was decreasing or blocking of water flow due to some unwanted casting burr. Although the film boiling nucleated thermal crack on surface, but cracks and cavities propagated under an electro-chemical mechanism until coolant leakage.

In this study, NiFe2O4 nanoparticles, with respecting two variables, were prepared through co-precipitation. These two variables are the temperature gradient reaching the temperature of the heat treatment of 500 °C at 10°C/sec, 35°C/sec and 60 °C/sec and the pure oxygen pressure as the heat treatment atmosphere for one hour, at 0.5 psi, 1 psi, and 1.5 psi. So, we found 9 samples, that the sample with the best sensitivity response to formaldehyde (sample with 10°C/sec heat treatment and 1 psi pure oxygen pressure) was characterized. X-ray diffraction (XRD), scanning electron microscopy (SEM), transition electron microscopy (TEM) and X-ray fluorescence (XRF) experiments were used to study the structure of these nanoparticles. X-ray diffraction experiment has verified formation of nickel ferrite phases. Furthermore, scanning electron microscopy and transition electron microscopy experiments confirmed that the ferrite was indeed a nano-structure. Finally, NiFe2O4 formula ratio was derived by X-ray fluorescence experiment. The sensitivity property of NiFe2O4 nanoparticles for formaldehyde gas detection was studied at 100 °C. For testing the sensitivity of nanosensors, a laboratory 5 liters system was used with temperature and humidity control. This system equipped with a temperature control heater for heating sensors. Relationship between gas concentration (from 20 ppm to 200 ppm) and resistance change of nano sensor with the best response to formaldehyde was investigated.

In the present work, the effect of oblique deposition on the properties of the Fe-Co alloy thin film with the composition of Fe90Co10 was investigated in detail. The films were deposited at the incident angle of 0° and 42° using the sputtering technique. The crystalline structure of the deposited films was evaluated via the X-ray diffraction method. The static and dynamic magnetic characteristics were determined from magnetization loops and permeability spectra, respectively. The results show that although the similar diffraction patterns with (110) preferred orientation in the nanocrystalline BCC phase of Fe-Co alloy have been obtained at both deposition conditions, their lattice parameter is noticeably different. The film deposited at the incident angle of 42° shows a lower lattice parameter. In addition, this film shows obviously different easy axis and hard axis M-H loops indicating the existence of a high magnetic anisotropy field (more than 60 Oe). As a result, the ferromagnetic resonance frequency as large as 5.71 GHz is obtained in the obliquely deposited film that is significantly higher than that observed for the normally deposited film (0.843 GHz). Furthermore, good thermal stability is observed for the as-deposited films in the temperature range of 330-420 K.

Due to their potential and practical applications, magnetic nanoparticles have been the subject of many researches in recent years due to their potential and practical applications. Among which, their medical applications (e.g: hyperthermia and targeted drug delivery), defensive and communications usages (e.g: electromagnetic waves absorbents) can be mentioned. In this study, the microemulsion method as a facile and promising synthesis method was employed in order to prepare magnetic FeCo alloy nanoparticles with several Fe/Co weight ratios. For this purpose, a micellar system was applied and the reaction of Fe and Co salts reduction was proceeded in the confined spaces in liquid phase. The electromagnetic waves absorption test was conducted to determine the optimum absorbent sample. The results showed that the sample of prepared in the weight ratio of 70:30 possesses a uniform spherical morphology with an average particle size of less than 10 nm. The evaluation of magnetic property of the synthesized samples showed that Fe70Co30 nanoparticles dedicated the highest magnetic saturation of 115 emu g−1. In addition, the magnitude of electromagnetic absorption (return loss) of this sample was measured to be −8.1 dB at 16 GHz which was at the lowest level compared with other samples highlighting the significant effect of Fe/Co weight ratio on final magnetic properties of the alloy system

In this study, using Hydrothermal method, electrodes were synthesized to be used as supercapacitors. Using the alteration ratio of Al+3/Co+2/Ni+2 metal ions, Al0.5Co0.5Ni2O4 electrode with a molar ratio of 1:1:2 for Al+3/Co+2/Ni+2 ions, the highest capacitance was obtained, which was called the NiCoAl electrode. With the change of time (2, 5 and 20 hours) and the temperature of the hydrothermal process (150 and 180 °C), the performance of the manufactured electrodes changed, and as a result, the NiCoAl electrode synthesized at 150 °C for a period of 5 h, the capacity of 1473 F g-1 in the current density of 8 A g-1 was obtained as the best electrode made. By adding graphene to the electrode's constituent materials, the NiCoAl-rGO electrode, synthesized at 150 °C showed a maximum capacity of 2364 F g-1 at a current density equal to 8 A g-1. With the help of structural analysis of X-ray diffraction test (XRD) and infrared spectrum (FTIR), the presence of constituent materials in the structure was confirmed, and the presence of graphene in the NiCoAl-rGO electrode structure was observed. In order to determine how electrochemical performance works, the electrodes were subjected to cycles (CV), Galvanostatic charge and discharge (CD), and electrochemical impedance (EIS) tests.

IN738LC nickel-based superalloy is widely used in different high-stress and high-temperature applications. During the service exposure, the microstructure of this alloy undergoes various damage mechanisms such as coarsening of γ′ precipitates, formation of continuous carbide network at grain boundaries, decomposition of MC carbides, and precipitation of detrimental phases. Decomposition of primary MC carbides usually occurs during the long-term high-temperature exposure and results in the reduction of mechanical properties. In the current study, the reaction of MC carbides decomposition is investigated in the microstructure of different gas turbine blades with different heat treatment and operational history. The changes in the composition of carbide regions are analyzed for different blades and compared with their remained creep lives. Since the decomposition of MC carbides to M23C6/M6C carbides is a mostly irreversible reaction, it is less affected by heat treatment and rejuvenation processes, and thereby it provides more valuable information for lifetime assessment and damage analysis of the exposed IN738LC samples. Moreover, the results of current work suggest that analyzing the multi-phase regions around primary MC carbides can be considered as an efficient method to evaluate the microstructural and mechanical degradation of in-service components.

Vortex extrusion (VE) is a new severe plastic deformation method that simultaneous reduction in area and twisting around the longitudinal axis of the workpiece, within the fixed convergent die, leads to the accumulation of high amounts of strain on the sample. Due to the many capabilities of this process such as creating high strain in a pass, simplicity and no need for complex equipment and homogeneity of strain distribution, in this paper, the simultaneous effect of reduction in area, twist angle and curvature of slip lines on the mean effective strain, maximum damage factor and the steady state load of the process were investigated. The results showed that the mean effective strain value increased with increasing reduction in area and twist angle and there is a significant interaction between mentioned factors. The maximum damage factor was observed at the surface of work piece, which according to the values of positive mean stress in these points it can be said that the possible initiated cracks propagate under tensile state of stress. Reduction in area and twist angle are the effective factors on steady state load of process but the curvature of the slip lines does not have much effect on it. Maximum of damage factor was determined for VE die with curved streamlines, reduction in area equal to 30% and twist angle of 30 degree.

Rechargeable batteries such as lithium ion batteries have been used in various industries. In these batteries, the important issue is high energy and power density. One of the important factors affecting this issue is cathode material. LiFePO4/C has been interested as an excellent compounds in recent years. In this study, LiFePO4/C was synthesized by solid state method from raw materials (NH4) H2PO4, FeC2O4.2H2O, Li2CO3 and super pure carbon and in order to improve the weak diffusion of lithium ion and increase the battery capacity at high charge/discharge rates, yttrium and neodymium were added to the LiFePO4/C composite. In this study, TGA-DTA analysis was used to investigate the reaction temperatures of the raw materials and also to determine the synthesis temperature of the compound. SEM images were used to study the microstructure and morphology of the particles and impedance, cycling and cyclic voltammetry tests were used to investigate the electrochemical behavior of the samples. According to the results of this study, it was found that two-step synthesis of the doped samples with yttrium and neodymium ions improves ionic and electrical conductivity of the samples. From the samples synthesized with different percentages of yttrium and neodymium, LiFe99.54Y0.4Nd0.06PO4/C sample with the best electrochemical performance and maximum capacity (113 mAh/g) was selected as the optimal sample.