The effect of thermal vibrations on extended x-ray absorption fine structure. I

The form of the Debye–Waller factor in EXAFS is discussed in detail, and an expression is obtained for this factor in a general three‐atom system of C_s symmetry. The normal modes which contribute to the Debye–Waller factors for each scattering path are dependent on the symmetry of the system. A series of model three‐atom systems with C_(2v) symmetry are studied and the Debye–Waller factors as a function of the bridging angle are discussed for each of these systems. The temperature dependence of the Debye‐Waller factor for each scattering path is also studied. In a system of C_(2v) symmetry, approximating the double and triple scattering Debye–Waller factors by the second shell single scattering factor is shown to be strictly valid only for a system close to linearity. The error due to this approximation is dependent upon the amplitude of the individual scattering paths and is shown to increase with temperature. When the single scattering contribution is unimportant, there is shown to exist a temperature where the above approximation is exact

The Political Economy of Increasing Block Tariffs for Water in Developing Countries

Increasing block tariffs (IBTs) have become the tariff structure of choice in developing countries. Multilateral donors, international financial and engineering consultants, and water sector professionals working in developing countries all commonly presume that IBT structures are the most appropriate way to determine water users' monthly bills. Most, if not all recent water tariff studies performed for developing countries propose IBT structures. The objective of this paper is to critically examine the current fashion of using IBTs in cities in developing countries. In the second section of the paper, we review the common arguments made to justify the adoption of IBTs, and present some actual IBTs currently in use in selected cities. In the third section of the paper we discuss the objectives and considerations involved in water tariff design so as to provide a basis for judging the appropriateness of IBTs.Tariff

Data analysis in extended x-ray-absorption fine structure: Determination of the background absorption and the threshold energy

Two approaches for the determination of the background absorption (μ_0) in the extended x-ray-absorption fine structure (EXAFS) are presented. Both methods, experimental and computational, take advantage of the damping of the EXAFS amplitude resulting from the convolution with Gaussian functions of different widths. In the experimental method two or more spectra are collected with the use of different spectrometer slit widths, resulting in spectra of different resolutions for the same sample. In the computational approach the convolution is accomplished via a convolution algorithm. The intersection points of the resulting spectra are used to generate μ_0. At the absorption edge, the spectra intersect at a unique point, which is shown to be a measure of the threshold energy, E_0. Illustration of the two methods for background removal is given for a copper-foil sample. The computational approach is superior to the experimental method of damping the EXAFS spectra to give μ_0

The restructuring of grain boundaries at the surfaces of meals

Please click Additional Files below to see the full abstract

Estimating Survival in Patients with Operable Skeletal Metastases: An Application of a Bayesian Belief Network

BACKGROUND: Accurate estimations of life expectancy are important in the management of patients with metastatic cancer affecting the extremities, and help set patient, family, and physician expectations. Clinically, the decision whether to operate on patients with skeletal metastases, as well as the choice of surgical procedure, are predicated on an individual patient's estimated survival. Currently, there are no reliable methods for estimating survival in this patient population. Bayesian classification, which includes bayesian belief network (BBN) modeling, is a statistical method that explores conditional, probabilistic relationships between variables to estimate the likelihood of an outcome using observed data. Thus, BBN models are being used with increasing frequency in a variety of diagnoses to codify complex clinical data into prognostic models. The purpose of this study was to determine the feasibility of developing bayesian classifiers to estimate survival in patients undergoing surgery for metastases of the axial and appendicular skeleton. METHODS: We searched an institution-owned patient management database for all patients who underwent surgery for skeletal metastases between 1999 and 2003. We then developed and trained a machine-learned BBN model to estimate survival in months using candidate features based on historical data. Ten-fold cross-validation and receiver operating characteristic (ROC) curve analysis were performed to evaluate the BNN model's accuracy and robustness. RESULTS: A total of 189 consecutive patients were included. First-degree predictors of survival differed between the 3-month and 12-month models. Following cross validation, the area under the ROC curve was 0.85 (95% CI: 0.80-0.93) for 3-month probability of survival and 0.83 (95% CI: 0.77-0.90) for 12-month probability of survival. CONCLUSIONS: A robust, accurate, probabilistic naïve BBN model was successfully developed using observed clinical data to estimate individualized survival in patients with operable skeletal metastases. This method warrants further development and must be externally validated in other patient populations

Switching at the contacts in Ge9Sb1Te5 phase-change nanowire devices

Phase-change random access memory is a promising approach to non-volatile memory. However, the inability to secure consistent, reliable switching on a nanometre scale may limit its practical use for high density applications. Here, we report on the switching behaviour of PCRAM cells comprised of single crystalline Ge9Sb1Te5 (GST) nanowires. We show that device switching is dominated by the contacts and does not result in a resistance change within the bulk of the wire. For the devices studied, the typical contact resistance was ~30 kΩ, whereas the resistance of the GST channel was 1.8 kΩ. The applied voltage was predominately dropped across the passivating oxide on the surface of the GST nanowires, resulting in local resistive switching at the contacts and local power dissipation, which limited the endurance of the devices produced. The optimal device must balance low resistance contacts with a more resistive channel, to facilitate phase change switching within the nanowires. These results highlight the importance of contact formation on the switching properties in phase change devices and help guide the future design of more reliable neuromorphic devices

Theory of extended x-ray absorption fine structure: Single and multiple scattering formalisms

A generalized formalism for the EXAFS effect is presented. The full T operator in the Lippmann–Schwinger equation is expanded and the first term and part of the second term are seen to effectively correspond to single scattering EXAFS processes. The latter term includes secondary scattering by the absorber, and within this formalism it is this term that introduces the central atom phase shift. The expression obtained for the single scattering problem is identical with that found by other investigators. A solution to the general EXAFS problem involving an absorbing atom and two neighboring atoms is also presented. The expression differs from that previously reported [B. K. Teo, J. Am. Chem. Soc. 103, 3990 (1981)]. Multiple scattering is seen to contribute increasingly to the EXAFS as the three atoms approach colinearity. Both the amplitude and frequency of the multiple scattering components are functions of the geometry of the system. The model systems studied indicate that, in many cases, multiple scattering effects must be considered in the analysis of non-nearest-neighbor shells. The theory may readily be extended to systems and scattering paths of arbitrary complexity