A filter synthesis technique applied to the design of multistage broad-band microwave amplifiers

A method for designing multistage broad-band amplifiers based upon well-known filter synthesis techniques is presented. Common all-pole low-pass approximations are used to synthesize prototype amplifier circuits that may be scaled in frequency and impedance. All-pass filters introduced at the first stage are shown to improve input match while maintaining circuit performance less 6 dB gain. A theoretical comparison is made with the distributed amplifier and the cascaded single-stage distributed amplifier. Theoretically, a larger gain-bandwidth product is achieved using the synthesis technique. A proof-of-concept Butterworth low-pass two-stage amplifier was designed, simulated, and measured and achieved a flat gain performance of 1–4 GHz with a power gain of 14.5±1 dB close to the predicted 1–4.2 GHz, 15±1 dB

A radial mode ultrasonic horn for the inactivation of <i>Escherichia coli</i> K12

Tuned cylindrical radial mode ultrasonic horns offer advantages over ultrasonic probes in the design of flow-through devices for bacterial inactivation. This study presents a comparison of the effectiveness of a radial horn and probe in the inactivation of Escherichia coli K12. The radial horn is designed using finite element analysis and the predicted modal parameters are validated using experimental modal analysis. A validated finite element model of the probe is also presented. Visual studies of the cavitation fields produced by the radial horn and probe are carried out using luminol and also backlighting to demonstrate the advantages of radial horns in producing a more focused cavitation field with widely dispersed streamers. Microbiological studies show that, for the same power density, better inactivation of E. coli K12 is achieved using the radial horn and, also, the radial horn offers greater achievable power density resulting in further improvements in bacterial inactivation. The radial horn is shown to be more effective than the probe device and offers opportunities to design in-line flow-through devices for processing applications

Talking about My Generation

This article is a response to Fredric Jameson's criticisms of the author's 'The History of Theory' (Critical Inquiry 32/4, 2006). For Jameson's article, 'How Not to Historicise Theory', see Critical Inquiry, 34, Spring 2008. The author situates Jameson's arguments in the context of the historicisation of theory, treating them as an example of the (Marxist) theoretical program to think the historical determinations of thought. It is argued that this program is an instrument for the formation of the privileged intellectual persona of the theorist

Facing up to the challenge of behavioural observation in infant hearing assessment

The ability to assess detection and discrimination of speech by infants has proved elusive. Dr Iain Jackson and colleagues discuss how new technologies and fresh approaches might offer valuable insight into young infants’ behavioural responses to sound

EPR spectroscopy of iron- and nickel-doped [ZnAl]-layered double hydroxides: modeling active sites in heterogeneous water oxidation catalysts

Iron-doped nickel layered double hydroxides (LDHs) are among the most active heterogeneous water oxidation catalysts. Due to inter-spin interactions, however, the high density of magnetic centers results in line-broadening in magnetic resonance spectra. As a result, gaining atomic-level insight into the catalytic mechanism via electron paramagnetic resonance (EPR) is not generally possible. To circumvent spin-spin broadening, iron and nickel atoms were doped into non-magnetic [ZnAl]-LDH materials and the coordination environments of the isolated Fe(III) and Ni(II) sites were characterized. Multifrequency EPR spectroscopy identified two distinct Fe(III) sites (S = 5/2) in [Fe:ZnAl]-LDH. Changes in zero field splitting (ZFS) were induced by dehydration of the material, revealing that one of the Fe(III) sites is solvent-exposed (i.e. at an edge, corner, or defect site). These solvent-exposed sites feature an axial ZFS of 0.21 cm⁻¹ when hydrated. The ZFS increases dramatically upon dehydration (to -1.5 cm⁻¹), owing to lower symmetry and a decrease in the coordination number of iron. The ZFS of the other (“inert”) Fe(III) site maintains an axial ZFS of 0.19-0.20 cm⁻¹ under both hydrated and dehydrated conditions. We observed a similar effect in [Ni:ZnAl]-LDH materials; notably, Ni(II) (S = 1) atoms displayed a single, small ZFS (±0.30 cm⁻¹) in hydrated material, whereas two distinct Ni(II) ZFS values (±0.30 and ±1.1 cm⁻¹) were observed in the dehydrated samples. Although the magnetically-dilute materials were not active catalysts, the identification of model sites in which the coordination environments of iron and nickel were particularly labile (e.g. by simple vacuum drying) is an important step towards identifying sites in which the coordination number may drop spontaneously in water, a probable mechanism of water oxidation in functional materials

Liquid friction on charged surfaces: from hydrodynamic slippage to electrokinetics

Hydrodynamic behavior at the vicinity of a confining wall is closely related to the friction properties of the liquid/solid interface. Here we consider, using Molecular Dynamics simulations, the electric contribution to friction for charged surfaces, and the induced modification of the hydrodynamic boundary condition at the confining boundary. The consequences of liquid slippage for electrokinetic phenomena, through the coupling between hydrodynamics and electrostatics within the electric double layer, are explored. Strong amplification of electro-osmotic effects is revealed, and the non-trivial effect of surface charge is discussed. This work allows to reconsider existing experimental data, concerning Zeta potentials of hydrophobic surfaces and suggest the possibility to generate ``giant'' electro-osmotic and electrophoretic effects, with direct applications in microfluidics

A census of massive stars in NGC 346. Stellar parameters and rotational velocities

Spectroscopy for 247 stars towards the young cluster NGC 346 in the Small Magellanic Cloud has been combined with that for 116 targets from the VLT-FLAMES Survey of Massive Stars. Spectral classification yields a sample of 47 O-type and 287 B-type spectra, while radial-velocity variations and/or spectral multiplicity have been used to identify 45 candidate single-lined systems, 17 double-lined systems, and one triple-lined system. Atmospheric parameters (T$_eff$ and log$g$) and projected rotational velocities ($v_e$sin$i$) have been estimated using TLUSTY model atmospheres; independent estimates of $v_e$sin$i$ were also obtained using a Fourier Transform method. Luminosities have been inferred from stellar apparent magnitudes and used in conjunction with the T$_eff$ and $v_e$sin$i$ estimates to constrain stellar masses and ages using the BONNSAI package. We find that targets towards the inner region of NGC 346 have higher median masses and projected rotational velocities, together with smaller median ages than the rest of the sample. There appears to be a population of very young targets with ages of less than 2 Myr, which have presumably all formed within the cluster. The more massive targets are found to have lower $v_e$sin$i$ consistent with previous studies. No significant evidence is found for differences with metallicity in the stellar rotational velocities of early-type stars, although the targets in the SMC may rotate faster than those in young Galactic clusters. The rotational velocity distribution for single non-supergiant B-type stars is inferred and implies that a significant number have low rotational velocity ($\simeq$10\% with $v_e$<40 km/s), together with a peak in the probability distribution at $v_e \simeq$300 km/s. Larger projected rotational velocity estimates have been found for our Be-type sample and imply that most have rotational velocities between 200-450 km/s.Comment: Accepted by A&

The VLT-FLAMES survey of massive stars: Nitrogen abundances for Be-type stars in the Magellanic Clouds

Aims. We compare the predictions of evolutionary models for early-type stars with atmospheric parameters, projected rotational velocities and nitrogen abundances estimated for a sample of Be-type stars. Our targets are located in 4 fields centred on the Large Magellanic Cloud cluster: NGC 2004 and the N 11 region as well as the Small Magellanic Cloud clusters: NGC 330 and NGC 346. Methods. Atmospheric parameters and photospheric abundances have been determined using the non-LTE atmosphere code tlusty. Effective temperature estimates were deduced using three different methodologies depending on the spectral features observed; in general they were found to yield consistent estimates. Gravities were deduced from Balmer line profiles and microturbulences from the Si iii spectrum. Additionally the contributions of continuum emission from circumstellar discs were estimated. Given its importance in constraining stellar evolutionary models, nitrogen abundances (or upper limits) were deduced for all the stars analysed. Results. Our nitrogen abundances are inconsistent with those predicted for targets spending most of their main sequence life rotating near to the critical velocity. This is consistent with the results we obtain from modelling the inferred rotational velocity distribution of our sample and of other investigators. We consider a number of possibilities to explain the nitrogen abundances and rotational velocities of our Be-type sample.Comment: 14 pages, 9 figures, submitted to A&

The VLT-FLAMES survey of massive stars: rotation and nitrogen enrichment as the key to understanding massive star evolution

Rotation has become an important element in evolutionary models of massive stars, specifically via the prediction of rotational mixing. Here, we study a sample of stars, including rapid rotators, to constrain such models and use nitrogen enrichments as a probe of the mixing process. Chemical compositions (C, N, O, Mg and Si) have been estimated for 135 early B-type stars in the Large Magellanic Cloud with projected rotational velocities up to ~300km/s using a non-LTE TLUSTY model atmosphere grid. Evolutionary models, including rotational mixing, have been generated attempting to reproduce these observations by adjusting the overshooting and rotational mixing parameters and produce reasonable agreement with 60% of our core hydrogen burning sample. We find (excluding known binaries) a significant population of highly nitrogen enriched intrinsic slow rotators vsini less than 50km/s incompatible with our models ~20% of the sample). Furthermore, while we find fast rotators with enrichments in agreement with the models, the observation of evolved (log g less than 3.7dex) fast rotators that are relatively unenriched (a further ~20% of the sample) challenges the concept of rotational mixing. We also find that 70% of our blue supergiant sample cannot have evolved directly from the hydrogen burning main-sequence. We are left with a picture where invoking binarity and perhaps fossil magnetic fields are required to understand the surface properties of a population of massive main sequence stars.Comment: ApJL. 10 pages, 1 figure. Updated to match accepted versio