In this paper, an unrelated parallel machine scheduling problem is studied, where order acceptance, sequence and machine-dependent setup times and the maximum available times of machines are additionally considered. The objective is to maximize the difference between the total revenues of accepted jobs and the makespan. A mixed integer programming (MIP) model is formulated. To tackle this problem efficiently, an exact decomposition method, which is a two-layer logic-based Benders decomposition (LBBD) based (denoted by TL-LBBD) method, is developed, where an inner LBBD is embedded into an outer LBBD. Specifically, in the outer LBBD method, the master problem is used to determine the acceptance of jobs, whereas the subproblem examines the schedule given the accepted jobs from the master problem. The subproblem could be further decomposed into an assignment master problem and a sequencing subproblem by the inner LBBD method as well. Extensive computational experiments are conducted and the results show that the developed TL-LBBD method produce better quality solutions in significantly less computation time than solving the MIP model directly. Moreover, the maximum levels of the problem instances that could be solved to optimality by the developed TL-LBBD method within 30 minutes are also evaluated.
The migration behaviors of the primary Si grains during solidification of Al-18Si alloy from mushy zone was investigated under various magnetic flux density and gradient.The effects of magnetic flux on the migration were investigated when the magnetic field gradient was set at 25 T/m,the effects of the field gradient was investigated when the flux density was set at 5 T.The results show that the migration behaviors of the primary Si grains do not take place until the magnetic flux density is over 2.3 T.The primary Si grains tend to migrate and form a remarkable Si-rich layer at magnetic flux of 6.6 T.The thickness of Si-rich layer is almost unchanged with further increase of magnetic flux density.The amount of segregated primary silicon grains increases with increasing magnetic gradient,whereas the grain size decreases.The mechanism of influence of the magnetic flux density on the migratory behavior was discussed.
Ophthalmic segment artery aneurysms (OSAs) are difficult to clip; therefore, improvement of the surgical method is of great significance to the prevention of complications, and the classification of the aneurysms is essential to formulate a reasonable surgical plan.To explore the strategies and effects of surgery for OSAs using a modified subdural Dolenc approach.The clinical data of 38 patients (12 men and 26 women, aged 48-73 years) with OSA were analyzed retrospectively. A total of 44 aneurysms were identified, 40 of which were OSAs. The 40 aneurysms were divided into types Ia1 (n = 2), Ia2 (n = 2), Ib (n = 6), IIa (n = 4), IIb (n = 4), IIIa (n = 0), IIIb (n = 4), IIIc (n = 16), and IV (n = 2) based on preoperative images. Thirty-nine OSAs were operated successfully through pterional craniotomy combined with the modified subdural Dolenc approach, and 1 aneurysm was clipped through the contralateral approach. Clinical outcomes were evaluated using the Glasgow Outcome Scale (GOS).Thirty-nine OSAs were clipped, and one was wrapped. Visual dysfunction, headache, and dizziness improved after the operation in 18 patients. One patient had new visual impairment, and there were no deaths. At discharge, the GOS score was 5 in 36 cases, 4 in 1 case, and 3 in 1 case. Thirty-seven patients had a GOS score of 5, and 1 patient had a score of 3 at 6 months after the operation.The modified subdural Dolenc approach (Zheng approach) for clipping OSAs may be associated with less trauma and good postoperative outcomes.
Textured porous Si3N4 ceramics with fine and coarse β-Si3N4 powders as the raw materials were prepared by gel casting in a strong magnetic field of 6T and subsequent pressureless sintering. These results showed that highly textured porous Si3N4 ceramics could be obtained by adjusting the ratio between fine and coarse β-Si3N4 powders in 6T. The Lotgering orientation factor in green bodies showed the value of 0.72∼0.74 with the increase of coarse powder content. The sintering process promoted texture development of the samples by Ostwald ripening owing to the big difference in the β-Si3N4 initial powder size. After sintering at 1800oC for 2 h, the degree of texture (Lotgering orientation factor) in the sample, prepared with fine and coarse powders at the ratio of 1:1, reached the maximum value (0.91), and it had relatively high density (66.91%) and low apparent porosity (34.51%). The crystallographic texture in porous Si3N4 ceramics led to the anisotropic bending strength. The method combined with gel casting in a strong magnetic field was beneficial for preparing highly textured porous ceramics with big and complex shape.
Abstract Al 2 O 3 ‐based ceramic cores with a uniform microstructure were fabricated successfully by a traditional pressing forming method, in which Al 2 O 3 powders were used as matrix and yttrium oxide as additive. The influences of yttrium oxide content and sintering temperature on properties of ceramic cores were studied carefully. Results indicated that a higher sintering temperature benefited the preparation of ceramic cores with excellent properties. As the temperature was above 1400°C, the reaction of Al 2 O 3 and yttrium oxide occurred, leading to the formation of YAG phases. And, YAG was uniformly adhered on the surface of Al 2 O 3 particles, exerting a good role in connecting Al 2 O 3 particles. Based on XRD analyses, it was found that the increase in the sintering temperature could promote the formation of more YAG phases. When sintering temperature was adjusted to 1600°C, with the increase in the yttrium oxide content, their relative density developed a trend of decreasing first and then increasing, while the apparent porosity had an opposite change tendency. With the increase in the sintering temperature, the line shrinkage and bending strength of Al 2 O 3 ‐based ceramic cores both increased gradually. In our research, their bending strength reached to 53.5 MPa and apparent porosity was 33.9% when the ceramic cores were prepared with 9 wt% yttrium oxide at 1600°C.
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