Dense ceramic membranes for methane conversion

Dense ceramic membranes made from mixed oxygen-ionic and electronic conducting perovskite-related oxides allow separation of oxygen from an air supply at elevated temperatures (>700 °C). By combining air separation and catalytic partial oxidation of methane to syngas into a ceramic membrane reactor, this technology is expected to significantly reduce the capital costs of conversion of natural gas to liquid added-value products. The present survey is mainly concerned with the material properties that govern the performance of the mixed-conducting membranes in real operating conditions and highlights significant developments in the field.     

Sculpting nanoelectrodes with a transmission electron beam for electrical and geometrical characterization of nanoparticles

A method to produce metal electrodes with a gap of a few nanometers with a highly focused electron beam in a transmission electron microscope (TEM) is described. With this method the electrical and geometrical characterization of the same particle is possible. The I-V characteristics of a gold particle trapped between such electrodes showed the expected single-electron tunneling behavior, with a Coulomb gap corresponding to the geometry of the particle as observed with high-resolution TEM.     

Exploiting submodular value functions for scaling up active perception

In active perception tasks, an agent aims to select sensory actions that reduce its uncertainty about one or more hidden variables. For example, a mobile robot takes sensory actions to efficiently navigate in a new environment. While partially observable Markov decision processes (POMDPs) provide a natural model for such problems, reward functions that directly penalize uncertainty in the agent’s belief can remove the piecewise-linear and convex (PWLC) property of the value function required by most POMDP planners. Furthermore, as the number of sensors available to the agent grows, the computational cost of POMDP planning grows exponentially with it, making POMDP planning infeasible with traditional methods. In this article, we address a twofold challenge of modeling and planning for active perception tasks. We analyze \(\rho \)POMDP and POMDP-IR, two frameworks for modeling active perception tasks, that restore the PWLC property of the value function. We show the mathematical equivalence of these two frameworks by showing that given a \(\rho \)POMDP along with a policy, they can be reduced to a POMDP-IR and an equivalent policy (and vice-versa). We prove that the value function for the given \(\rho \)POMDP (and the given policy) and the reduced POMDP-IR (and the reduced policy) is the same. To efficiently plan for active perception tasks, we identify and exploit the independence properties of POMDP-IR to reduce the computational cost of solving POMDP-IR (and \(\rho \)POMDP). We propose greedy point-based value iteration (PBVI), a new POMDP planning method that uses greedy maximization to greatly improve scalability in the action space of an active perception POMDP. Furthermore, we show that, under certain conditions, including submodularity, the value function computed using greedy PBVI is guaranteed to have bounded error with respect to the optimal value function. We establish the conditions under which the value function of an active perception POMDP is guaranteed to be submodular. Finally, we present a detailed empirical analysis on a dataset collected from a multi-camera tracking system employed in a shopping mall. Our method achieves similar performance to existing methods but at a fraction of the computational cost leading to better scalability for solving active perception tasks.     

Constructing invariants for hybrid systems

We present a new method for generating algebraic invariants of hybrid systems. The method reduces the invariant generation problem to a constraint solving problem using techniques from the theory of ideals over polynomial rings. Starting with a template invariant—a polynomial equality over the system variables with unknown coefficients—constraints are generated on the coefficients guaranteeing that the solutions are inductive invariants. To control the complexity of the constraint solving, several stronger conditions that imply inductiveness are proposed, thus allowing a trade-off between the complexity of the invariant generation process and the strength of the resulting invariants. This research was supported in part by NSF grants CCR-01-21403, CCR-02-20134 and CCR-02-09237, by ARO grant DAAD19-01-1-0723, by ARPA/AF contracts F33615-00-C-1693 and F33615-99-C-3014, and by NAVY/ONR contract N00014-03-1-0939.     

Preparation of TEM samples for hard ceramic powders

It is challenging to prepare a good sample for high-resolution electron microscopy of polycrystalline ceramic powders containing hard particles or particles with a strong preferential cleavage. Here we demonstrate that embedding the particles in a Cu matrix in a pressed pellet allows for straightforward conventional ion milling. The method is applied to powders of Mg10Ir19B16 and Na0.5CoO2 to show its feasibility, whereby transmission electron microscopy (TEM) samples with crystalline areas thinner than 10 nm can be obtained easily.     

Applying an information transmission approach to extract valence electron information from reconstructed exit waves

The knowledge of the valence electron distribution is essential for understanding the properties of materials. However this information is difficult to obtain from HREM images because it is easily obscured by the large scattering contribution of core electrons and by the strong dynamical scattering process. In order to develop a sensitive method to extract the information of valence electrons, we have used an information transmission approach to describe the electron interaction with the object. The scattered electron wave is decomposed in a set of basic functions, which are the eigen functions of the Hamiltonian of the projected electrostatic object potential. Each basic function behaves as a communication channel that transfers the information of the object with its own transmission characteristic. By properly combining the components of the different channels, it is possible to design a scheme to extract the information of valence electron distribution from a series of exit waves. The method is described theoretically and demonstrated by means of computer simulations.     

Thermal heating within SOI

Heat diffuses from hot power devices to neighboring components such as bondpads, which shortens the lifetime of bond balls. SOI is a technology with advantages for integrated power applications. For this reason heat transport between components within one SOI die has been measured experimentally. The thermal time constants are determined. Inside an SOI transistor the heat propagates with 1 μm/μs typically until other time constants take over.     

A computational comparison of cyclical and non-cyclical control for stochastic production-inventory systems

Cyclical production planning is popular in real-life because of its organisational benefits. We study the application and performance of cyclical production planning in a complex stochastic production-inventory system with job shop routings. We propose a decision-support system that allows computing cost efficient cyclical production plans. Insights on the applicability and performance of cyclical production planning are obtained from an extensive simulation study in which the cyclical approach is compared with a state-of-the-art non-cyclical approach.     

Periodic capacity management under a lead-time performance constraint

In this paper, we study a production system that operates under a lead-time performance constraint which guarantees the completion of an order before a pre-determined lead-time with a certain probability. The demand arrival times and the service requirements for the orders are random. To reduce the capacity-related operational costs, the production system under study has the option to use flexible capacity. We focus on periodic capacity policies and model the production system as a queuing system that can change its capacity periodically and choose to operate in one of the two levels: a permanent capacity level and a permanent plus contingent capacity level. Contingent capacity is supplied if needed at the start of a period, and is available during that period, at a cost rate that is decreasing in period length in different functional forms. Next, we propose a search algorithm that finds the capacity levels and the switching point that minimizes the capacity-related costs for a given period length. The behaviour of the capacity-related costs changes drastically under different period lengths and cost structures. In our computational study, we observe that the periodic capacity flexibility can reduce the capacity-related operational costs significantly (up to 35%). However, in order to achieve these savings, the period length must be chosen carefully depending on ambition level and capacity-related costs. We also observe that the percentage savings are higher for more ambitious lead-time performance constraints. Moreover, we observe that the use of contingent capacity makes the total system costs more insensitive to the lead-time performance requirements.