The impact of delayed differentiation in make-to-order environments

To cope with the challenges of product proliferation, many firms are shifting their supply chain structures from make-to-stock (MTS) to make-to-order (MTO). An MTO strategy comes at a price however, as customers must wait longer for their configured products. Incorporating delayed differentiation (DD) in an MTO environment offers the potential of reducing the customer's waiting time, since the generic part/component of the products is made available before receiving customer orders. In this paper, we quantify the trade-offs involved in implementing DD in an MTO environment using both customer waiting time and system cost as performance metrics. We show that under common conditions, the introduction of DD results in shorter waiting times and higher cost over a pure MTO strategy. These results are as expected. However, we also derive conditions where DD results not only in shorter customer waiting time but also lower cost, thus dominating a pure MTO strategy. Through a simulation experiment, we test the robustness of our results for the case where the customer arrivals and production times are generally distributed. For firms with the capability of estimating the customer waiting cost, we derive the optimal base-stock level of the generic component to minimise the total cost.     

Recovery rheology via rheo-SANS: Application to step strains under out-of-equilibrium conditions

Stress relaxation from a step strain test provides important information about constituent dynamics, but if a material has experienced a complex shear history, the underlying physics is not straightforward to access. We use recovery rheology and rheo-small-angle neutron scattering to probe the nonlinear dynamics of an entangled wormlike micelle solution by applying step strains after complex shear histories enforced by large-amplitude oscillatory shear (LAOS) flow. We show that a universal relaxation modulus can be obtained from step strain tests with complex shear histories, as long as the modulus is defined in terms of the recoverable strain. The shear and normal stresses, as well as the alignment of micellar Kuhn segments, are shown to be positively correlated with the recoverable strain. We identify re-entanglement of polymeric chains after cessation of LAOS and show that this process occurs over the same timescales as linear-regime stress relaxation. This work, therefore, lays the foundation of how to accurately probe out-of-equilibrium rheology in a consistent manner.     

Isomerization of α-Pinene Oxide Over Iron-Modified Zeolites

Fe-modified mordenite, ferrierite, Y, ZSM-5, ZSM-12 and beta zeolite catalysts were prepared by solid state ion-exchange and conventional liquid phase ion-exchange methods from aqueous solutions. Sn- modified H-beta-300 zeolite catalyst was prepared by the later method. The characterization of proton form, Fe and Sn modified zeolites was carried out using X-ray powder diffraction, scanning electron microscopy, Mössbauer spectroscopy with magnetic measurements, transmission electron microscopy, nitrogen adsorption, X-ray absorption spectroscopy, X-ray photoelectron spectroscopy, inductively coupled plasma spectroscopy, thermo-gravimetric analysis and FTIR spectroscopy using pyridine as a probe molecule. Isomerization of α-pinene oxide over the Fe and Sn modified zeolite catalysts was carried out in the liquid phase using a batch-wise glass reactor. Formation of campholenic aldehyde and fencholenic aldehyde were observed to be influenced by the structure, acidity of zeolite and contents of Fe and Sn, reaction temperature and the catalysts pretreatment.     

Micellar enhanced ultrafiltration of phenol with dodecylpyridinium chloride and sodium dodecylbenzene sulfonate

Micellar enhanced ultrafiltration is a surfactant-based separation process for wastewater treatment. The ultrafiltration of dodecyl pyridinium chloride (DPC) and sodium dodecyl benzene sulfonate (SDBS) solutions was realized with 5 kDa and 15 kDa molecular weight cut-off ceramic membranes. Surfactant concentrations under and over the micellar critical concentration at three different pressures were studied, and their effects on permeate flux and retention are reported. It was found that an increase in pressure operation, in 0.8–1.8 bar range, causes a lower surfactant concentration in permeate by the presence of a polarization layer. These systems were studied for phenol removal. DPC/phenol system (73.5 mM/0.53 mM) reaches a 61% removal of organic solute. SDBS/phenol system (14.7 mM/0.58 mM) reaches a 25% removal, both with 5 kDa membrane at 1.8 bar. In the case of the cationic surfactant the micelle's positive charge plays a preponderant role in attracting the phenol molecules, while the SDBS does not present this electrostatic interaction.     

Selective Surface Engineering of Perovskite Microwire Arrays

The surface of low-dimensional perovskites play a crucial role in determining their intrinsic property. Understanding their characteristics and the influence of certain surfaces is valuable in designing functional surface-engineered structures. Meanwhile, surface passivation can also be applied to stabilize and optimize the state-of-the-art perovskite-based optoelectronics. Herein, cesium lead bromide (CsPbBr₃) microwire parallel arrays are designed and fabricated with specific (100)-terminated crystal planes, which exhibit excellent photodetection performance with long-term environment stability >3000 h. Notably, it is uncovered experimentally and theoretically that environmental oxygen can not only passivate the Br-vacancy-related trap states on the (100) surface, but also create charge carrier nanochannels to enhance the (opto)electronic properties. The coupling effects between oxygen species and the specific terminated crystal planes of perovskites highlight the importance of surface engineering for designing and optimizing perovskite-based devices.     

Bandit algorithms to personalize educational chatbots

Machine Learning - To emulate the interactivity of in-person math instruction, we developed MathBot, a rule-based chatbot that explains math concepts, provides practice questions, and offers...     

提高微型气体传感器可靠性的有效方法-解决氧化锡敏感膜龟裂问题

本文报导了一种解决氧化锡气体敏感膜龟裂问题的有效方法.实验中发现,溅射形成的氧化锡薄膜在高温处理后或在长时间的高温工作中会发生龟裂,对器件的稳定性和可靠性带来很大影响.我们用两种方法解决了这个问题.一个是用磷硅玻璃(PSG)回流对氧化锡衬底的粗糙表面进行平坦化处理,另一个是把氧化锡薄膜图形设计成许多细长条状,以释放高温引起的应力变化.这种技术也适用于其它薄膜产生的微裂问题.     

Satellite-derived multi-year trend in primary production in the Arctic Ocean

Spaceborne one month averaged data, predominantly from the Sea-viewing Wide Field-of-view Sensor (SeaWiFS) and partly from the Moderate Resolution Imaging Spectroradiometer (MODIS), were used to investigate changes in primary production (PP) by phytoplankton in the Arctic Ocean from 1998 to 2010. Several PP retrieval algorithms were tested against the collected in situ data, and it was shown that the algorithm by Behrenfeld and Falkowski gave the best results (with the coefficient of correlation, r, equal to 0.8 and 0.75, respectively, for the pelagic and shelf zones). Based on the performed test, the Behrenfeld and Falkowski algorithm was further applied for determining both the annual PP in the Arctic and the PP trend over the above-mentioned time period. Results of our analysis indicate that PP in the Arctic has increased by ?15.9% over 13 years (1998–2010). This finding, as well as the absolute annual values of PP remotely quantified in the present study, is at odds with analogous numerical assessments by other workers. These disagreements are thought to be due to differences in the applied methodologies of satellite data processing such as cloud masking and determination of phytoplankton concentration within (1) overcast areas and (2) areas of massive growth of coccolithophores as well as (3) in the shelf zone prone to a significant influence of land and river run-off.     

Minimizing total weighted flowtime subject to minimum makespan on two identical parallel machines

We study the problem of scheduling n jobs on two identical parallel processors or machines where an optimal schedule is defined as one with the shortest total weighted flowtime (i.e., the sum of the weighted completion time of all jobs), among the set of schedules with minimum makespan (i.e., the completion time of the last job finished). We present a two phase non-linear Integer Programming formulation for its solution, admittedly not to be practical or useful in most cases, but theoretically interesting since it models the problem. Thus, as an alternative, we propose an optimization algorithm, for small problems, and a heuristic, for large problems, to find optimal or near optimal solutions. Furthermore, we perform a computational study to evaluate and compare the effectiveness of the two proposed methods.