Investigation of the effect of impurity of precursor materials on Si-doped boron carbide properties

Boron carbide is a notable ceramic, with its high hardness and low density. However, it suffers a sudden loss in strength under high shear stress. Doping boron carbide with Si/B is widely used to increase its resistance to amorphization. High purity boron and silicon hexaboride precursor powders are used for doping boron carbide, but these materials have high costs and supply chain constraints. This research investigated the effect of substituting lower purity B or using pure Si powder instead of SiB₆ on materials’ properties such as elastic and mechanical properties, microstructure, and amorphization resistances. It was observed that using lower-purity boron or pure Si powder instead of SiB₆ did not significantly affect critical properties, such as fracture toughness, hardness, or amorphization resistance. However, Young's modulus values decreased as B purity decreased and as Si was used instead of SiB₆. These findings demonstrate that substituting precursor materials in Si/B co-doped B₄C is possible with little change in the material's properties. This facilitates the use of easier-to-access, cheaper production routes to be used for silicon-doped boron carbide products.     

Direct descent curvature optimal control with a modification algorithm and its application to nonlinear process

This paper presents a direct descent second order or direct descent curvature algorithm with some modifications for the optimal control computations. This algorithm is compared with Hamiltonian methods in the literature. The proposed algorithm has generated numerically robust solutions with respect to conjugate points. The weighting matrix updating scheme was developed to improve the second-order optimal control algorithm, tested the performance of the algorithm, and shown on the benchmark and industrial process. The time-varying optimal feedback (TVOFB) gains are also generated along the trajectory as byproducts. If the trajectory deviates from the optimal trajectory for any reason (i.e., changing of system parameters, step disturbance into the plant, changing of initial conditions), it is held on the optimal trajectory by means of the optimal feedback. Simulations have been given for controlling the Van der Pol and bioreactor system, which are nonlinear benchmark systems.     

Heat exchanger network design of an organic Rankine cycle integrated waste heat recovery system of a marine vessel using pinch point analysis

The coexistence of different kinds of waste heat sources on marine vessels with various temperature ranges increases the need for an optimal heat exchanger network (HEN) design for the heat collection process to reduce the unutilizable heat that needs to be discharged to overboard. The optimal HEN design has not been taken into consideration by using pinch point analysis in previous studies of marine organic Rankine cycle (ORC) systems that utilize from different kinds of waste heat sources. The objective of the study is to determine the optimal HEN design for an ORC integrated waste heat recovery system of a marine vessel by utilizing the pinch point analysis to improve the overall energy efficiency. Lubricating oil, high-temperature cooling water and scavenge air of the main engine, and the exhaust gas emitted from the boiler plant were identified as the major waste heat sources of a reference container ship. A heat collection stream, in which thermal oil is used as the heat transfer fluid that transfers the collected heat to an ORC system, was proposed. The pinch point analysis showed that the optimum waste heat recovery could be gained by separating the scavenge air cooler into three stages and the lubricating oil cooler into two stages. The results of the parametric study for the varying evaporator inlet pressure between 1000 and 3000 kPa showed that R1234ze(Z) yields the best performance among nine different organic working fluids with the thermal efficiency and exergy efficiency of 15.24% and 86.47% for the ORC system, respectively. For the proposed configuration, the unavailable waste heat that cannot be transferred to thermal oil was found as 23.71%, 16.56%, 13.17%, and 7.81% of the total waste heat produced by the heat sources, and also 8.24%, 9.80%, 11.55%, and 12.93% of the net power output produced by the main engine can be recovered for 25%, 50%, 76%, and 100% maximum continuous rating (MCR), respectively.     

Association of relative nocturnal hypertension and autonomic neuropathy in insulin-dependent diabetic children

Twenty-four hour BP and heart rate measurements were carried out in fourteen newly diagnosed-, and in twenty-eight diabetics with 5-13 years of duration; and in eight healthy control children. Mean arterial BP rose at night in five-, fell slightly (less than 10%) in five- and fall markedly (more than 10%) in eighteen diabetics with longer duration of the disease. The diurnal-nocturnal difference of mean arterial pressure was significantly lower in the groups with nocturnal BP rise and slight nocturnal BP fall, compared to the control group (< 0.001; p < 0.01, respectively). The diurnal-nocturnal differences of heart rates were significantly lower in diabetics with relative "nocturnal hypertension" compared to the control group (p < 0.05). The presence of subclinical signs of diabetic autonomic neuropathy was significantly higher in patients with nocturnal BP rise and slight nocturnal BP fall compared to patients with marked nocturnal BP fall and newly diagnosed diabetics (chi squared p = 0.02 and p = 0.01, respectively). In conclusion, the prevalence of autonomic symptoms in diabetic children could be related to change in diurnal/nocturnal arterial BP, however longitudinal studies of ABPM are needed to define, whether patients with abnormal BP profiles are candidates for the development of diabetic vascular disease.     

Polyacrylamide hydrogel as an electrolyte for oxidation‐based organic rectifiers

The effect of the internal morphology of polyacrylamide hydrogel as an electrolyte on the anodic oxidation of aluminum, which can be used for the rectification of current, was studied. Comparing with traditional electrolytes, the chemical reactions between the aluminum and the free ions (H⁺ and OH^?) occur more effectively due to the trapping of the ions in hydrogel. Oxidation efficiency, and thus the rectification, can be tuned with changing the composition of the gel. We are able to observe very high values, that is, it was around 850 at an applied potential of ±7 V, for a specific swelling degree and concentration of the hydrogel swollen in water for the rectification ratio for aluminum/hydrogel/platinum sandwich configuration. Stability of the hydrogel used as filler was studied via swelling and drying experiments. No considerable change in the performance of the gel was observed upon multiple usages. Therefore, the polyacrylamide hydrogel can be a good candidate for constructing oxidation based rectifiers. POLYM. ENG. SCI., 55:406–413, 2015. © 2014 Society of Plastics Engineers     

Colorimetric Methods for Determining Fe,V, and Ni Contents in Coke and Anodes

Prebaked carbon anodes are used in the electrolytic production of aluminum. They are made of petroleum coke, butts, recycled anodes, and coal tar pitch. The anode quality, which depends on the raw material quality and the production conditions, has an important impact on the cell performance. Metallic impurities (V, Ni, and Fe) found in cokes and anodes increase the carbon consumption by catalyzing the air and CO₂ reactivities. In turn, this increases the production cost, energy consumption, and the emission of greenhouse gases. The current methods for detecting the metallic impurities in carbon are time consuming and require intensive sample preparation, skilled personnel, and costly reagents. In this work, simple, rapid, and effective tools were developed using colorimetric methods.     

Characterization of Cracking Mechanisms of Carbon Anodes Used in Aluminum Industry by Optical Microscopy and Tomography

The objective of this work is to understand the different mechanisms of crack formation in dense anodes used in the aluminum industry. The first approach used is based on the qualitative characterization of the surface cracks and the depth of these cracks. The second approach, which constitutes a quantitative characterization, is carried out by determining the distribution of the crack width along its length as well as the percentage of the surface containing cracks. A qualitative analysis of crack formation was also carried out using 3D tomography. It was observed that mixing and forming conditions have a significant effect on crack formation in green anodes. The devolatilization of pitch during baking causes the formation and propagation of cracks in baked anodes in which large particles control the direction of crack propagation.     

Evaluation of adhesion and fatigue of MoS2-Nb solid-lubricant films deposited by pulsed-dc magnetron sputtering

The thin film deposition for tribological applications becomes more and more widespread. The tribological performance of the overlay coatings correlates with coating-substrate adhesion. Hence, it is important to measure adhesive strength. The scratch adhesion test for thin films is extensively used. In this work, MoS₂-Nb coatings deposited by closed-field unbalanced magnetron sputtering (CFUBMS) have been scratch tested in two modes. A multi-mode operation was used as sliding-fatigue, like multi-pass scratching in the same track at different fractions of critical load (unidirectional sliding) and a standard mode using progressive load operation. Failure mechanisms are discussed according to examination of response of very dense microstructure and the adhesion value. The critical load to the first failure (L_C1) was 15 N but the final adhesion value from the film and substrate interface was 120 N (L_C2) as function of the coating thinning. The coefficient of friction (COF) from the multi-scratch for MoS₂-Nb started at a very high value of around 0.067, 0.073, and 0.093 under 5, 8, and 15 N loads respectively and then drops to 0.006, 0.035, and 0.065 at the end of the 1000 cycles. The most significant finding in the test is that when the multi-scratch passes reached to 1000 cycles, micro scale fatigue failures disappeared.