AC–DC Transfer Standard Measurements and Generalized Compensation With the AC Josephson Voltage Standard

We present AC-DC transfer standard measurements using the National Institute of Standards and Technology's pulse-driven AC Josephson voltage standard source. We have investigated the frequency dependence for several output voltages up to 200 mV for frequencies from 2.5 to 100 kHz. We found that, as the frequency increases, the ac-dc differences for the two arrays on the same chip do not agree. We explored this deviation in ac-dc difference for the two arrays by investigating different configurations of the probe cabling and wiring, chip carriers, and on-chip circuit design. We found that the circuit design produced the greatest improvement, particularly at the highest frequency (100 kHz), where the deviation in ac-dc difference was reduced by more than 60%. In this paper, we also demonstrate tenfold higher output voltages and improved operating margins for arbitrary (nonsinusoidal) waveforms. These enhancements were accomplished by implementing a more general current bias to the arrays having the same harmonic content as that of the synthesized arbitrary waveform.     

Mixed Integer Nonlinear Programming model for the optimal design of fired heaters

In this paper, a mathematical optimization model for the optimal design of industrial furnaces/fired heaters is presented. Precisely, a detailed Mixed Integer Nonlinear Programming (MINLP) model including operational and geometric constraints is developed to get an efficient furnace design. Discrete decisions connected with the geometric design such as number of tubes in convection and radiation sections, number of shield tubes, number of passes and number of tubes per pass are modelled by using integer variables. Continuous variables are used for process conditions (temperatures, flow-rates, pressure, velocities, pressure drops among others).The mathematical model and the solution procedure are implemented in General Algebraic Modelling System (GAMS), Brooke [A. Brooke, D. Kendrick, A. Meeraus, A. A. GAMS – A User’s Guide (Release 2.25), The Scientific Press, San Francisco, CA, 1992]. Based on a typical furnace configuration, several applications are successfully solved by applying the proposed MINLP model. In this paper, three case studies with increasing complexity are presented. In the first case study, the accuracy of results from the proposed model is compared satisfactorily with literature. In the second case study, the MINLP model is applied to optimize the fire heater’s efficiency. Finally, the total annual cost of the fired heater is minimized in the Case Study III. Also, a sensitive analysis of the unitary cost of fuel and capital investment is investigated. The developed model is characterized by its robustness and flexibility.     

Effect of oxygen and zirconium on the growth and superconducting properties of Nb3sn

The intermetallic compound Nb₃Sn is a type II superconductor of interest because of its high superconducting critical current density in high magnetic fields. One technique for forming Nb₃Sn is to react a molten tin alloy with a solid niobium-zirconium-oxygen alloy. It was found that the properties of Nb₃Sn are directly related to its microstructure, which is in turn directly related to the O: Zr atom ratio in the starting niobium foil. For a niobium alloy foil with an O:Zr atom ratio of 2, the resulting Nb₃Sn layer is fine grained and grows linearly with reaction anneal time until the entire Nb-Zr-O alloy core is consumed. This leads to a linear increase in critical current with time and a relatively constant critical current density. For a niobium foil without oxygen, the resulting Nb₃Sn grains are large and columnar and grow with a diffusion-limited layer growth rate. The resulting critical current density is low and decreases with reaction time. For a niobium alloy foil with an O:Zr ratio of >0 but <2, fine-grained Nb₃Sn is formed initially and grows with a linear layer growth rate, followed by a second layer of large, columnar-grained Nb₃Sn growing with a diffusion-limited rate. As a function of reaction anneal time and similar to the grain growth, the critical current initially increases linearly and then decreases. Formerly with GE Corporate Research & Development Schenectady NY     

Intrinsic stress in DC sputtered niobium

The intrinsic mechanical stress of DC-magnetron-sputtered Nb films is characterized as a function of sputtering parameters and target erosion. The zero-stress point shifts to lower cathode voltages as the target erodes. The zero-stress point is always characterized by the same cathode-current-Ar-pressure relationship     

Comparison of initial fixation strength between biodegradable and metallic interference screws and a press-fit fixation technique in a porcine model

We describe nuchal cord changes in four children with os odontoideum who had cervical spine magnetic resonance imaging (MRI) before surgery. In these four children, nuchal cord changes consistent with previous trauma were seen. The presence of the nuchal cord changes supports the concept of trauma as an etiologic factor in these four cases. This study supports trauma as an associated factor in patients with os odontoideum.     

The cell wall porin of Nocardia farcinica: biochemical identification of the channel-forming protein and biophysical characterization of the channel properties

The doctrine of sovereign or official immunity has protected medical examiners in cases alleging negligent performance of the autopsy and in cases involving negligent harvesting of organs. It has not protected examiners in cases alleging performance of autopsy without authorization. The medical examiner, therefore, is well advised to determine whether there may be any objection by the next-of-kin to an autopsy and particularly any religious objection-and if there is such an objection, to proceed only after making contemporaneous and documented decision that there is a compelling need for the autopsy.     

Intra and intermolecular relaxations 2,3-dihydroxypropyl methacrylate and 2-hydroxyethyl methacrylate hydrogels

The dynamic mechanical transitions present in the 2-hydroxyethyl methacrylate (HEMA) and 2,3-dihydroxypropyl methacrylate (DHPMA) homopolymers and two random copolymers, with one-to-one and three-to-one HEMA-to-DHPMA molar ratios, were examined by analyzing the dry xerogels and hydrogels at various states of hydration. The temperatures of the primary α transition and the secondary β and γ transitions were determined in the tension mode; the storage modulus and loss modulus as a function of temperature and frequency were recorded. The results were compared to the results obtained from dielectric analysis at low hydration using tan δ. The frequency dependence of the dispersions was calculated for the dry and hydrated states, using mechanical and dielectric data. The information obtained was used to elucidate the interaction between the polymer and the sorbed water. Analysis of the low temperature secondary γ transition and secondary transitions resulting from polymer-water interactions was emphasized. During the initial hydration, the temperature maxima of the dielectric and mechanical secondary γ transitions, and the transitions that appear in the presence of absorbed water increased as the DHPMA content increased. The apparent activation energy associated with the secondary transitions increased as well. All of this is the result of stronger intermolecular interactions due to the increased density of hydrophilic moieties in DHPMA.