Finite difference frequency-domain treatment of open transmissionstructures

A full-wave solution utilizing the finite difference frequency-domain approach is applied to the computation of propagation characteristics of open transmission structures. In order to do this, one must find a way for terminating the finite difference grid at some finite distance, although the field actually extends to infinity. A method is presented based on boundary operators that terminates the grid. The boundary operators require a knowledge of the radial propagation constant which is to be solved for. This difficulty is overcome by an innovative iterative technique. Typical numerical results are presented to illustrate the accuracy of this method     

Circular expressions: elimination of static environments

Consider the connection between denotational semantics for a language with goto statements and flow diagrams for programs in such a language. The main point of interest is that the denotational semantics uses a recursively defined environment to give the meaning of labels, while a flow diagram merely has a jump to the appropriate program point. A simple reduction called “indirection elimination” strips away the environment from the denotational semantics and extracts an expression with cycles that is very close to the flow diagram of a program. The same idea applies to associating bodies with recursive procedures, or to any construct whose semantics is not wedded to the syntax. In addition to being a useful data structure and conceptual device, expressions with cycles are well defined mathematical objects—their semantics can be given by unfolding them into infinite structures that have been well studied. The practicality of the elimination of environments has been tested by constructing a trial implementation, which serves as the front end of a semantics directed compiler generator. The implementation takes a denotational semantics of a language and constructs a “black box” that maps programs in the language into an intermediate representation. The intermediate representation is a circular expression.     

Development of parthenium based activated carbon and its utilization for adsorptive removal of p-cresol from aqueous solution

The activated carbon was prepared from carbonaceous agriculture waste Parthenium hysterophorous by chemical activation using concentrated H2SO4 at 130+/-5 degrees C. The prepared activated carbon was characterized and was found as an effective adsorbent material. In order to test the efficacy of parthenium based activated carbon (PAC), batch experiments were performed to carryout the adsorption studies on PAC for the removal of highly toxic pollutant p-cresol from aqueous solution. The p-cresol adsorption studies were also carried out on commercial grade activated carbon (AC) to facilitate comparison between the adsorption capabilities of PAC and AC. For PAC and AC, the predictive capabilities of two types of kinetic models and six types of adsorption equilibrium isotherm models were examined. The effect of pH of solution, adsorbent dose and initial p-cresol concentration on adsorption behaviour was investigated, as well. The adsorption on PAC and on AC was found to follow pseudo-first order kinetics with rate constant 0.0016 min(-1) and 0.0050 min(-1), respectively. The highest adsorptive capacity of PAC and AC for p-cresol solution was attained at pH 6.0. Further, as an adsorbent PAC was found to be as good as AC for removal of p-cresol upto a concentration of 500 mg/l in aqueous solution. Freundlich, Redlich-Peterson, and Fritz-Schlunder models were found to be appropriate isotherm models for PAC while Toth, Radke-Prausnitz and Fritz-Schlunder were suitable models for AC to remove p-cresol from aqueous solution.     

Can the Observed Changes in the Size or Shape of a Colloidal Nanocatalyst Reveal the Nanocatalysis Mechanism Type: Homogeneous or Heterogeneous?

The surface energy of metallic nanocrystals is relatively high compared to bulk materials due to the metal–metal bond deficiency of the surface atoms. This results in an insufficient chemical valency. In addition, smaller nanoparticles possess a higher degree of curvature, weakening the bonding of their surface atoms. This is especially true for non-spherical shapes, which are comprised of a large number of sharp corner and edge sites. These atomic sites possess higher surface energies due to the lower number of shared bonds with the nanoparticle, resulting in instability of the surface atoms and rendering them physically unstable and chemically active. In many instances, the constant “bombardment” of these surface atoms by the solvent molecules as well as by the reactant molecules when these nanocrystals are in colloidal solution could lead to surface atom dissolution, both physically and/or chemically. This phenomenon could alter the functionality of the metallic colloidal nanoparticle from supplying catalytically active sites (in heterogeneous catalysis) to serving as a reservoir of catalytically active species to the solution (in homogeneous catalysis). In the latter type, if the atoms of the nanocatalyst appear in the products, the nanoparticle is no longer a catalyst but a reactant. In this review we attempt to answer the question raised in the title by examining our previous work on the changes in size, shape, and other physical and chemical properties of colloidal transition metal nanoparticles during the nanocatalysis of two fundamentally different and important reactions: (1) the gentle electron-transfer reaction at room temperature involving the reduction of hexacyanoferrate (III) ions with thiosulfate ions and (2) the more harsh Suzuki cross-coupling reaction between phenylboronic acid and iodobenzene that takes place at 100 °C for 12 h. Changes in the nanoparticle dimensions were followed with TEM and HRTEM. Raman and FTIR spectroscopies were used to follow the chemical changes. For each change, we will use the above definition to see if the observed change can help us determine whether the catalysis is homogeneous or heterogeneous.     

Tribology of titanium boride-coated titanium balls against alumina ceramic: Wear, friction, and micromechanisms

The tribological performance of titanium alloy (Ti–6Al–4V) balls coated with a dual boride layer comprised of titanium diboride (TiB₂) and titanium boride (TiB) whiskers mated against alumina ceramic disks has been determined using lubricated ball-on-disk wear testing. Measurements of coefficient of friction values and volumetric wear were made and electron microscopic investigation of wear spots and tracks was performed. The wear rate of the boride-coated titanium alloy balls was 40 times less than that of 97% dense alumina balls. Measurements of wear track width and depth corroborated this result. The superior wear resistance is attributed to the hardness and the unique structure of the dual (TiB₂ + TiB) whisker layer and the consequent smoothness of the wear surface created during the wear process. The material removal mechanism is abrasive in nature in the boride-coated balls compared to grain fracture and pullout in alumina.     

Design of jerk bounded feedrate with ripple effect for adaptive nurbs interpolator

A method for obtaining smooth, jerk bounded feedrate profile in high speed machining has been developed. This study proposes a NURBS interpolator based on adaptive feedrate control with a well developed lookahead algorithm. It is found that the values of the feedrate of the down stream sharp corner have profound effect on the feedrate of the upstream sharp corners causing a ripple effect (RE). By using a windowing scheme the feedrate profile obtained after lookahead is re-interpolated to obtain a continuous velocity and acceleration profiles which reduces the jerk related problems. This is compared with the adaptive NURBS interpolator to show the effectiveness of the proposed method. Simulation results indicate that the consideration of ripple effect is important in avoiding jerk and thereby increasing the machining accuracy.     

Effect of hepatitis C infection on anemia in hemodialysis patients

Hepatitis C (HCV) infection is commonly seen in dialysis patients, but its long-term deleterious effects in these patients are unknown. We evaluated the effect of HCV infection on anemia in our hemodialysis population. This retrospective case control study was carried out from January 1999 to February 2007. The HCV positive patients were assessed for a 12-month period by quarterly lab results for the prevalence of anemia, iron stores, dialysis adequacy, and alanine aminotranferase levels. Their requirements of erythropoietin (EPO) and intravenous (IV) iron were assessed during these months of clinical stability. A control group of age-matched, race-matched, and gender-matched hemodialysis patients with no history of HCV was similarly assessed for anemia, iron stores, and EPO and IV-iron requirements. Twenty-two HCV-positive patients were included for comparison analysis with 44 control patients for 1:2 matching. The mean EPO requirement for the hepatitis group was 17,307 +/- 14,708 U/month in comparison with the control group, which required 49,134 +/- 49,375 U/month (p value <0.01). The mean dose of IV-iron was 120 +/- 143 mg/month for hepatitis patients and 163 +/- 112 mg/month in the control group (p=0.07). The patients with HCV have lower requirement of exogenous EPO replacement compared with their age-matched, gender-matched, and race-matched dialysis counterparts. The IV-iron requirement was not significantly different between the 2 groups but had a suggestive lower trend in the hepatitis group. This needs to be further studied in larger trials.     

Measurements of the volatility of aerosols from alpha-pinene ozonolysis

The temperature-dependence of secondary organic aerosol (SOA) concentrations is measured using a temperature-controlled smog chamber. Aerosols are generated from reaction of alpha-pinene (14-150 ppb) and ozone at a constant temperature of 22 +/- 2 degrees C in the presence of the OH-scavenger 2-butanol. After the reactions are completed the chamber is heated or cooled in a range from 20 to 40 degrees C. SOA volume concentrations increase at temperatures below the initial formation temperature and decrease at elevated temperatures. The response to the temperature change as measured by percent mass change per degree ranged from -0.4 to -3.6% K(-1), for a total mass reduction of 5-60% upon heating from 22 to 35 degrees C. The reported range is due to two factors: (1) experimental uncertainty, arising mainly from uncertainty in evaporation and condensation behavior of particles lost to the chamber wall; (2) differences in the temperature response from experiment to experiment. Aerosol temperature sensitivity was also measured by tandem differential mobility analysis (TDMA) where similarly generated SOA were heated from 20 to 25 degrees C to 30-40 degrees C with residence times of 0.5-1.5 min, resulting in particle volume reductions of up to 20%. The TDMA experiments indicate that evaporation of the SOA particles in this system occurs with a potentially significant mass transfer limitation (e.g., accommodation coefficient <0.1).     

RF response of single-walled carbon nanotubes

We present for the first time an in-depth study of the RF response of a single-walled carbon nanotube (SWCNT) rope. Our novel electrode design, based on a tapered coplanar approach, allows for single tube measurements well into the GHz regime, minimizing substrate-related parasitics. From the analysis of the S-parameters, the ac transport mechanism in the range 30 kHz to 6 GHz is established. This work is an essential prerequisite for the fabrication of high-speed devices based on bundles of nanowires or low-dimensional structures.