Computational uncertainty analysis in multiresolution materials via stochastic constitutive theory

A stochastic constitutive theory is proposed in this work to propagate microstructure uncertainties in computational multiscale continuum models to bulk multiresolution material behavior. Ubiquitous fine resolution uncertainty sources influencing prediction of material properties based on their structures are categorized in detail, and this research transmits these uncertainties to coarser material resolutions by introducing a stochastic constitutive theory deduced from volume element simulations. To implement the stochastic upscaling process, two advanced uncertainty quantification methods are examined: statistical copula functions and random process polynomial chaos expansion. Both methods confront the mathematical difficulty in randomizing constitutive laws by capturing the marked correlation among constitutive parameters seen in complex materials, thus the results proffer a more accurate probabilistic estimation of constitutive material behavior. The contribution of this work is twofold: uncertainty is propagated from heterogeneous material “structure” to material “property” via the stochastic constitutive theory, and rigorous, data-driven mathematics are formalized to represent complicated dependence structures in multivariate statistical distributions. To the authors’ knowledge, this is the first work in multiresolution mechanics that presents an approach to computationally derive correlation functions from numerical experiments, as opposed, for instance, to assuming one a priori. The method put forth in this research, though quite general, is applied to a mathematical example and plastic, high strength steel alloy for demonstration. Results include stochastic constitutive curve confidence intervals for the material stress–strain response and qualitative comparisons of the two stochastic methods detailed herein.     

A noncooperative power control game for multirate CDMA data networks

The authors consider a multirate code-division multiple acess system, in which all users have the same chip rate and vary their data rate by adjusting the processing gain. The receivers are assumed to be implemented using conventional matched filters, whose performance is sensitive to the received power levels. The authors' goal is to maximize the total system throughput by means of power control. A game theoretic approach is adopted. It is shown that for a certain type of pricing function, a unique Nash equilibrium solution exists and it possesses nice global properties. For example, it can be shown that for the optimal solution a high-rate connection should maintain a higher energy per bit than low-rate ones. The asymptotic spectral efficiency is also derived.     

Solution‐Processed Nickel Oxide Hole Transport Layers in High Efficiency Polymer Photovoltaic Cells

The detailed characterization of solution‐derived nickel (II) oxide (NiO) hole‐transporting layer (HTL) films and their application in high efficiency organic photovoltaic (OPV) cells is reported. The NiO precursor solution is examined in situ to determine the chemical species present. Coordination complexes of monoethanolamine (MEA) with Ni in ethanol thermally decompose to form non‐stoichiometric NiO. Specifically, the [Ni(MEA)₂(OAc)]⁺ ion is found to be the most prevalent species in the precursor solution. The defect‐induced Ni³⁺ ion, which is present in non‐stoichiometric NiO and signifies the p‐type conduction of NiO, as well as the dipolar nickel oxyhydroxide (NiOOH) species are confirmed using X‐ray photoelectron spectroscopy. Bulk heterojunction (BHJ) solar cells with a polymer/fullerene photoactive layer blend composed of poly‐dithienogermole‐thienopyrrolodione (pDTG‐TPD) and [6,6]‐phenyl‐C71‐butyric acid methyl ester (PC₇₁BM) are fabricated using these solution‐processed NiO films. The resulting devices show an average power conversion efficiency (PCE) of 7.8%, which is a 15% improvement over devices utilizing a poly(3,4‐ethylenedioxythiophene):poly(styrenesulfonate) (PEDOT:PSS) HTL. The enhancement is due to the optical resonance in the solar cell and the hydrophobicity of NiO, which promotes a more homogeneous donor/acceptor morphology in the active layer at the NiO/BHJ interface. Finally, devices incorporating NiO as a HTL are more stable in air than devices using PEDOT:PSS.     

香港空气污染成份的激光光声检测

我们建立了一套激光光声光谱检测装置,以可调谐CO激光器作为光源,成功地检测出香港几个典型交通道口空气中的若干污染气体成份:甲醛、乙醋醛、丙烯醛、乙烯、丙烯、甲苯等有毒气体。     

Fast radix-3/9 discrete Hartley transform

An efficient algorithm for computing radix-3/9 discrete Hartley transforms (DHTs) is presented. It is shown that the radix-3/9 fast Hartley transform (FHT) algorithm reduces the number of multiplications required by a radix-3 FHT algorithm for nearly 50%. For the computation of real-valued discrete Fourier transforms (DFTs) with sequence lengths that are powers of 3, it is shown that the radix-3/9 FHT algorithm reduces the number of multiplications by 16.2% over the fastest real-valued radix-3/9 fast Fourier transform (FFT) algorithm     

Tunneling and thermal emission of electrons from a distribution ofdeep traps in SiO2 [nMOSFET]

Both field-induced, or tunneling, and thermal emission of electrons from deep traps in the gate oxides on n-channel LDD CMOS devices have been observed and characterized. Experimental results show that the deep trapping effects at room temperature are similar to the shallow-level trapping effects observed by others below room temperature. In this case, however, the time constants involved are very long. This model and physical mechanisms can explain the apparent saturation observed under AC stress conditions, and also the differences observed between AC use conditions and DC stress     

Sequential packing algorithm for channel assignment under cochanneland adjacent-channel interference constraint

Generally, the channel-assignment problem (CAP) for mobile cellular systems is solved by graph-coloring algorithms. These algorithms, though sometimes can yield an optimal solution, do not supply any information on whether an optimal solution has been found or bow far away it is from the optimum. In view of these undesirable features, two relevant results are presented. First, a lower bound for the minimum number of channels required to satisfy a given call-traffic demand is derived. This lower bound is tighter than the existing ones under certain conditions and can be used as a supplement for other approximate algorithms. Second, we propose an efficient heuristic algorithm to solve this problem. Although the CAP is nondeterministic polynomial (NP) complete in general, our algorithm provides an optimal solution for a special class of network topologies. For the general case, promising results are obtained, and numerical examples show that our algorithm has a better performance than many existing algorithms     

A technique for extracting physiological parameters and therequired input function simultaneously from PET image measurements:theory and simulation study

Positron emission tomography (PET) is an important tool for enabling quantification of human brain function. However, quantitative studies using tracer kinetic modeling require the measurement of the tracer time-activity curve in plasma (PTAC) as the model input function. It is widely believed that the insertion of arterial lines and the subsequent collection and processing of the biomedical signal sampled from the arterial blood are not compatible with the practice of clinical PET, as it is invasive and exposes personnel to the risks associated with the handling of patient blood and radiation dose. Therefore, it is of interest to develop practical noninvasive measurement techniques for tracer kinetic modeling with PET. In this paper, a technique is proposed to extract the input function together with the physiological parameters from the brain dynamic images alone. The identifiability of this method is tested rigorously by using Monte Carlo simulation. The results show that the proposed method is able to quantify all the required parameters by using the information obtained from two or more regions of interest (ROIs) with very different dynamics in the PET dynamic images. There is no significant improvement in parameter estimation for the local cerebral metabolic rate of glucose (LCMRGlc) if there are more than three ROIs. The proposed method can provide very reliable estimation of LCMRGlc, which is our primary interest in this study     

Error-rate analysis for multirate DS-CDMA transmission schemes

We analyze and compare the error performance of a dual-rate direct-sequence code-division multiple-access (DS-CDMA) system using multicode (MCD) and variable-spreading gain (VSG) transmission in the uplink. Specifically, we present two sets of results. First, we consider an ideal additive white Gaussian noise channel. We show that the bit-error rate (BER) of VSG users is slightly lower than that of MCD users if the number of low-rate interferers is smaller than a specific threshold. Otherwise, they exhibit similar error performance. Second, we look at multipath fading channels. We show that with diversity RAKE reception, the VSG user suffers from a larger interference power than the MCD user if the channel delay spread is small. The reverse is true for a large delay spread. However, a larger interference power in this case does not necessarily lead to higher error probability. Essentially, our results for both cases show that: 1) in addition to the signal-to-interference ratio (SIR), the difference in error performance between the two systems strongly depends on the distributions of multiple-access and multipath interference; 2) for practical cellular communications, performances for both systems are expected to be similar most of the time.