Fabrication of Al2O3/(ZrB2 + TiB2) Composite Using MACS and Microwaves

Because of the excellent thermal and mechanical properties of engineering ceramics, they have been used as structural materials or composite matrixes and reinforcements in recent years. Alumina, titanium diboride, and zirconium diboride have found important uses in the past two decades. In this study, Al₂O₃/(ZrB₂ + TiB₂) ceramic composite powders were fabricated in situ and mechanical activation by milling was used to assist combustion synthesis (CS). A mixture of Al, ZrO₂, TiO₂, and B₂O₃ powders were used as raw materials. Mechanical activation was done using ball milling of different durations. Afterward, combustion was initiated using microwaves on the activated mixtures. X-ray diffraction (XRD) and scanning electron microscopy were used to investigate the purity and microstructure of the products. XRD analysis of the samples in the final stages of the process revealed that Al₂O₃/(ZrB₂ + TiB₂) composite powder was successfully fabricated using mechanical activation and CS, but that the CS reaction did not occur in unmilled samples. It was shown that increasing milling time from 3 to 10 hours increased purity and homogeneity of the products to the point that no noticeable impurity existed in the samples milled for 10 hours.     

Residence Time and Concentration Distribution in a Kenics Static Mixer

Static mixers, often referred to as motionless mixers, are in-line mixing devices that consist of mixing elements inserted into a length of pipe. Most of the experimental works in this field have concentrated on establishing design guidelines and pressure drop correlations. Due to experimental difficulties, few articles have been published on the investigation of the flow and mixing mechanisms. In this work, a Kenics KMX static mixer was utilized to study concentration and residence time distribution (RTD) and effect of Reynolds number on mixing. The static mixer had six mixing elements arranged in-line along the length of the tube, and the angle between two neighboring elements was 90°. The length of the mixer was 0.98 m with internal and external diameters of 5.0 cm and 6.0 cm, respectively. The main continuous fluid was water, and NaCl solution was used as a tracer. All experiments were conducted with three replications at three Reynolds numbers, Re = 1188.71, 1584.95, and 1981.19. A dispersion model was used to model the RTD data. The experimental results were compared with the model results and reasonable agreement was achieved.     

Behaviors of capacitive and Pirani vacuum gauges

Knowing the exact pressure of the process has a great impact on the validity of the results, product quality, energy efficiency, and in some processes, on the security of work with the system. Hence, the calibration of the barometers and the accuracy of the readings should be taken seriously. Choosing the appropriate time interval for re-calibration is done according to the extent and conditions of use, uncertainty, and the inaccuracy allowed in measurement, constructive suggestion, and some other things. Failure to pay attention to the importance of periodic calibration in vacuum gauges leads to some irreparable losses in the research project and the vacuum generator system. In this study, using McLeod's barometer, the deviation of capacitive and Pirani vacuum gauges is investigated at different time intervals in the middle vacuum range, and it is determined that the vacuum gauge faces a serious deviation from the actual calibrated amount for upper and lower ranges of middle vacuum in the same working pressure range over time.     

Analyzing the fractal feature of nickel thin films surfaces modified by low energy nitrogen ion

Fractal concepts are used to explore how different energies (10, 20 and 50 keV) and fluence of 5 × 10¹⁷ N⁺ cm^?2 affect the morphology of nickel thin film. The nickel thin film with thickness of 100 nm is prepared by electron beam evaporation technique at room temperature on stainless steel (AISI 316) substrates. The nanoscale three‐dimensional (3‐D) surface micro‐morphologies are investigated by atomic force microscopy (AFM). Interface width is used to describe the surface height fluctuations. The autocorrelation function with height‐height correlation function give the quantitative data about the morphology of surface. The value of roughness exponent and fractal dimension is computed by height‐height correlation function. Fractal measure is an important analysis which provides fundamental insights into the texture characteristics and a direct way of testing their functional role.     

Mono and co-substitution of Sr2+ and Ca2+ on the structural,electrical and optical properties of barium titanate ceramics

In this work, Ba_0.9Sr_0.1TiO₃, Ba_0.7Sr_0.3TiO₃, Ba_0.5Sr_0.5TiO₃, Ba_0.5Ca_0.25Sr_0.25TiO₃ and Ba_0.5Ca_0.5TiO₃ have been synthesized to evaluate the influence of mono and co-substitution of A-site dopants (Sr²⁺ and Ca²⁺) on the structural, electrical and optical properties of BaTiO₃ ceramics. Sr²⁺ added samples showed a tetragonal structure which became slightly distorted with increasing Sr²⁺ concentration and finally achieved a cubic structure for x?=?0.50. Ba_0.5Ca_0.5TiO₃ also retained their tetragonality with limited solubility. Presence of second phase, CaTiO₃ demonstrated the fact of restricted solubility. The concurrent effect of Sr²⁺ and Ca²⁺ didn't alter the tetragonal structure. Sr²⁺ substitution enhanced the apparent density as well as grain size which stimulated the domain wall motion and improved dielectric properties. However, the ferroelectric nature of Ba_1-xSr_xTiO₃ was poor due to the redistribution of point defect at grain boundary. The optical band gap was found to be reduced from 3.48?eV to 3.28?eV with increasing Sr²⁺ content. Co-substitution of cations improved the electrical property significantly. The highest value of dielectric constant was found to be ～547 for Ba_0.5Ca_0.25Sr_0.25TiO₃ ceramics. Both Ba_0.5Ca_0.25Sr_0.25TiO₃ and Ba_0.5Ca_0.5TiO₃ had developed P-E loop having lower coercive field and moderate optical band gap energy. Co-doping with Sr²⁺ and Ca²⁺ was a good approach enhancing materials electrical as well as optical property.     

Effect of Ti3SiC2 on the ablation behavior and mechanism of Cf-C-SiC-Ti3SiC2 composites under oxyacetylene torch at 3000 °C

In this research, ablation resistance of C_f-C-SiC and C_f-C-SiC-Ti₃SiC₂ composites, fabricated by liquid silicon infiltration (LSI) method were investigated. The infiltration process was conducted at 1500?°C for 30?min and then the samples were annealed at 1350?°C. X-ray diffraction (XRD) technique and scanning electron microscopy (SEM) were utilized in order to investigate the phase composition and microstructure of the ablated samples, respectively. When compared with C_f-C-SiC composite, results showed that mass and linear ablation rates of C_f-C-SiC-Ti₃SiC₂ composite have been improved by 50% and 37.5%, respectively. The mass and linear ablations rates of C_f-C-SiC composite were reached to 23.8?mg/s and 0.096?mm/s, respectively, while these values for C_f-C-SiC-Ti₃SiC₂ were reached to 11.8?mg/s and 0.06?mm/s, respectively. Microscopic investigations showed that formation of protective oxide layer and its stability on the surface of MAX-containing composite are the main reasons for improvement of ablation properties. While the oxide film formed on C_f-C-SiC composite has been blown away by flame.     

Production of Ultrafine Grained AlMnFe Samples by Confined Channel Die Pressing as Compared to Equal Channel Angular Pressing

Ultrafine grained material was produced by equal channel angular pressing and confined channel die pressing. Microstructure and mechanical properties of the obtained samples were compared. The different effects on microstructure evolution and mechanical properties from both processes are discussed.     

Effects of ZrC content on the synthesis of MAX phase and mechanical properties of Cf-C-SiC-Ti3SiC2-ZrC composites

In this research synthesis of Ti₃SiC₂ nano-laminate, effects of Al and ZrC on the amount and morphology of the synthesized MAX phase and mechanical properties of the C_f-C-SiC, C_f-C-SiC-Ti₃SiC₂ and C_f-C-SiC-Ti₃SiC₂-ZrC composites, fabricated by LSI method, were investigated. The infiltration process was conducted at 1500?°C for 30?min and then the samples were annealed at 1350?°C. X-ray diffraction (XRD) technique and scanning electron microscopy (SEM) were utilized in order to investigate the phase composition and microstructure of the samples, respectively. The results showed that the sample containing Al, had the largest amount of synthesized MAX phase and also addition of ZrC led to the decrease of intensities of MAX phase peaks. Among the samples, C_f-C-SiC-Ti₃Si(Al)C₂ had the best mechanical properties compared to the others. Bending strength, interlaminar shear strength and fracture toughness of this sample were 505?MPa, 34?MPa and 19.1?MPa?m^1/2 respectively. The results confirmed that the mechanical properties were decreased by addition of ZrC. Among ZrC-containing samples, the sample containing 10?vol% ZrC has shown the least decrease properties including the bending strength of 369.11?MPa, interlaminar shear strength of 26?MPa and fracture toughness of 16.9?MPa?m^1/2. Addition of ZrC phase caused pseudo-plastic behavior appearance in the force-displacement curve and led to fibers pull-out and also displacement enhancement. Microstructural observations confirmed the plate-like morphology of synthesized MAX phases. Furthermore, the distance between layers decreased and MAX phase size increased respectively by addition of Al. Also MAX phase size decreased by increasing the ZrC content. It was confirmed that the MAX phase-containing samples can tolerate various micro-deformation mechanisms including: crack deflection, bending and delamination of lamellae, kink boundary and laminate fracture. These mechanisms led to the toughening of the composites.     

Kinetic and CFD Modeling of Exhaust Gas Reforming of Natural Gas in a Catalytic Fixed-Bed Reactor for Spark Ignition Engines

Fuel reforming is an attractive method for performance enhancement of internal combustion engines fueled by natural gas, since the syngas can be generated inline from the reforming process. In this study, 1D and 2D steady-state modeling of exhaust gas reforming of natural gas in a catalytic fixed-bed reactor were conducted under different conditions. With increasing engine speed, methane conversion and hydrogen production increased. Similarly, increasing the fraction of recirculated exhaust gas resulted in higher consumption of methane and generation of H₂ and CO. Steam addition enhanced methane conversion. However, when the amount of steam exceeded that of methane, less hydrogen was produced. Increasing the wall temperature increased the methane conversion and reduced the H₂/CO ratio.