This work presents the design and characterization of medium voltage LDMOS transistors developed for 5-6 GHz power amplifier applications. Power transistors of different size have been fabricated in ...a 0.25 μm BiCMOS technology of IHP microelectronics and were characterized using a load/source pull measurement system. The optimum load and source impedances obtained for a 1.1 mm device provide the basis for a 6 GHz power amplifier design with an output power of 27.8 dBm in saturation and 25.1 dBm at 1 dB power compression. The peak drain efficiency of this power amplifier is 25.5% with a small signal gain of 8.1 dB.
Palmyra Atoll, in the Line Islands of the equatorial Pacific, supports one of the largest remaining native stands of Pisonia grandis forest in the tropical Pacific Ocean. In 2003, we surveyed ...terrestrial arthropods in order to document extant native and introduced species richness, compare these lists to historical records, and assess potential threats to native species and ecosystem integrity. In total, 115 arthropod taxa were collected, bringing the total number of taxa recorded since 1913 to 162. Few native species were collected; most taxa were accidental introductions also recorded from the Hawaiian Islands, the presumed main source of introductions to Palmyra. The overlap with previous historical surveys in 1913 and 1948 was low (<40%) and new species continue to establish, with one species of whitefly reaching pest status between 2003 and 2005. We observed numerous dead or dying large Pisonia grandis, and the green scale Pulvinaria urbicola (Coccidae) was particularly abundant on trees of poor health. Abundant introduced ants, particularly Pheidole megacephala, tended this and other hemipterans feeding on both native and introduced plants. We hypothesize that the Pheidole--Pulvinaria facultative mutualism is causing the decline of Pisonia grandis. Because of the unique properties of Pisonia grandis forest on oceanic atolls, its importance for nesting seabirds, and its alarming global decline, immediate conservation efforts should be directed at controlling introduced Hemiptera and disrupting their mutualisms with non-native ants on Palmyra Atoll.
Theoretical studies on M\(_{13}\) (M = Fe, Co, Ni) and M\(_{13}\)Pt\(_n\) (for \(n\) = 3, 4, 5, 20) clusters including the spin-orbit coupling are done using density functional theory. The magnetic ...anisotropy energy (MAE) along with the spin and orbital moments are calculated for M\(_{13}\) icosahedral clusters. The angle-dependent energy differences are modelled using an extended classical Heisenberg model with local anisotropies. From our studies, the MAE for Jahn-Teller distorted Fe\(_{13}\), Mackay distorted Fe\(_{13}\) and nearly undistorted Co\(_{13}\) clusters are found to be 322, 60 and 5 \(\mu\)eV/atom, respectively, and are large relative to the corresponding bulk values, (which are 1.4 and 1.3 \(\mu\)eV/atom for bcc Fe and fcc Co, respectively.) However, for Ni\(_{13}\) (which practically does not show relaxation tendencies), the calculated value of MAE is found to be 0.64 \(\mu\)eV/atom, which is approximately four times smaller compared to the bulk fcc Ni (2.7 \(\mu\)eV/atom). In addition, MAE of the capped cluster (Fe\(_{13}\)Pt\(_4\)) is enhanced compared to the uncapped Jahn-Teller distorted Fe\(_{13}\) cluster.
Sequences of X-ray diffraction patterns were obtained from dehydrating, artificially oriented multilayers of isolated, bovine rod outer segment disks. A direct-phase analysis was applied to highly ...hydrated specimens to determine sequences of low resolution (approx. 30 Å) electron density profiles of the disks as dehydration proceeded. The profiles were found to evolve smoothly as the multilayer lattice simultaneously shrank and became increasingly ordered. The bilayer profiles were largely invariant under dehydration and the evolution of the diffraction consistent with simple decreases in fluid spacings. The specimens were observed to phase separate into characteristic primary and a secondary lattices when the multi-layer became too dehydrated. The small unit cell size of the secondary lattice was suggestive of a lipid phase. Large changes in the diffraction patterns from phase separated specimens were observed upon bleaching of the specimen. The changes were consistent with a reversible disordering of the primary lattice.
Abstract
The C36 Laves phase Nb
1–x
Co
2+x
has been re-investigated in order to study phase stability and structure. Constitutional data have been obtained by investigating homogenized single- and ...two-phase samples and from diffusion couples. The C36 phase Nb
1–x
Co
2+x
crystallizes with hexagonal MgNi
2
structure type (
Z
= 8, space group
P
6
3
/
mmc
,
a
= 4.7414(4) Å and
c
= 15.458(1) Å at
x
= 0.265(4)). The homogeneity range extends from 24.6(2) to 25.3(5) at% Nb. The temperature range of the phase field is limited by a eutectoid (C36 Nb
1–x
Co
2+x
= Nb
2
Co
7
+ C15 NbCo
2
) and a peritectic (L + C15 NbCo
2
= C36 Nb
1–x
Co
2+x
) reaction at ∼ 1050 °C and 1264 °C, respectively. In addition, the title phase is involved in the peritectoid reaction Co(Nb) + C36 Nb
1–x
Co
2+x
= Nb
2
Co
7
at 1086 °C and in the eutectic reaction L = Co(Nb) + C36 Nb
1–x
Co
2+x
at 1239 °C. The C36 and C15 Laves phases of the Nb—Co system are separated by an extremely small two-phase field (<0.5 at%). The crystal structure exhibits pronounced deviations from ideal parameters obtained from a hard sphere model. The Co network displays a type of distortion known from many hexagonal Laves phases. Kagom, layers display an elongation of the Co—Co edges of the basal triangles of Co
5
trigonal bipyramids and a contraction of Co—Co edges of the uncapped triangles. The Nb atoms are also displaced from their idealized sites. In the crystal structure of C36 Nb
1–x
Co
2+x
excess Co atoms randomly substitute Nb atoms — (Nb
1–x
Co
x
)Co
2
. The excess Co atoms occupy preferentially the Nb2 site approximately twice as much as Nb1. These positions differ mainly in the conformation of the corresponding Nb
6
Nb
2
fragments (Nb1—Nb1 eclipsed and Nb2–Nb2 staggered). In addition, Co atoms are displaced from the original Nb positions. The distortion of the Co and the Nb network is a consequence of the bonding situation of the defect-free crystal structure. The preferential site occupation of excess Co atoms is triggered by interactions with atoms beyond the first coordination shell. The C36 phase Nb
1–x
Co
2+x
exhibits Pauli-paramagnetic behavior (
χ
P
= +1.31 · 10
–3
emu mol
–1
). The temperature dependent part of the electrical resistivity
ρ
(300 K) –
ρ
0
is only 17 μΩ cm whereas the residual resistivity is very large with
ρ
0
= 140 μΩ cm indicating strong structural disorder.
In the past few years, the capacity of communications satellite systems has become limited by the available bandwidth. One method of overcoming this limitation is to reuse the bandwidth by employing ...spatially isolated antenna beams and/or orthogonal polarizations. This paper surveys the required antenna technology. The multiple-beam spacecraft antenna problem is addressed, and the curreent design status is summarized. The earth terminal antenna for a dual-polarized system is discussed along with polarization measurement techniques. Finally, circuits for compensating the depolarization resulting from precipitation on the propagation path are presented.
National Center for Ecological Analysis and Synthesis
Nutrient availability and consumption by herbivores control the biomass of primary producer communities to varying degrees across ecosystems. ...Ecological theory, individual experiments in many different systems, and system-specific quantitative reviews have suggested that 1) bottom-up control is pervasive but top-down control is more influential in aquatic habitats relative to terrestrial systems, and 2) bottom-up and top-down forces often interact to synergize or dampen relative influences on producer biomass. We use a simple set of dynamic models to review mechanistic hypotheses for these questions, and compare model predictions to empirical data from a comprehensive meta-analysis of 191 factorial manipulations of herbivores and nutrients from freshwater, marine and terrestrial ecosystems. Parameterized model equilibria suggest that interactive outcomes should be weak and less common than strict additivity. Producer community biomass responded positively to fertilization across all systems, although effects were most pronounced in freshwater. Herbivores suppressed producer biomass in both freshwater and marine systems, but effects were inconsistent on land. Importantly, we observed a striking absence of either synergistic or dampening interactive effects of nutrients and herbivores across ecosystem types and within most habitats. Marine temperate rocky reef systems, which showed superadditive synergism of nutrient and herbivore controls, represented an exception to this pattern. Experimental studies showed limited support for emergent interactive effects on producer community-level biomass. We suggest that compensation by multiple herbivore guilds, top-down control of herbivores, spatial and temporal heterogeneity, and herbivore-mediated nutrient recycling tend to reduce the expectation for consistent interactive effects on producer biomass. Continuing studies should expand the temporal and spatial scales of experiments, particularly in understudied terrestrial systems; broaden factorial designs to manipulate independently both multiple producer resources (e.g. nitrogen, phosphorus, light) and multiple herbivore taxa or guilds (e.g. vertebrates and invertebrates); and simultaneously assess the effects on not only producer biomass but also species diversity, community composition and structure, and nutrient status.
Gasoline direct injection (GDI) has the potential to greatly improve internal combustion engine performance through precise control of the injection rate, timing, and combustion of the fuel. A ...thorough characterization of the hydrodynamics of fuel injection has to come from a precise, quantitative analysis of the sprays, especially in the near-nozzle region. A lack of knowledge of the fuel-spray dynamics has severely limited computational modeling of the sprays and design of improved injection systems. Previously, the structure and dynamics of highly transient fuel sprays have never been visualized or reconstructed in three dimensions (3D) due to numerous technical difficulties. By using an ultrafast x-ray detector and intense monochromatic x-ray beams from synchrotron radiation, the fine structures and dynamics of 1-ms GDI fuel sprays from an outwardly opening nozzle were elucidated by a newly developed, ultrafast, microsecond computed microtomography (CT) technique. In a time-resolved manner, many detailed features associated with the transient fuel flows are readily observable in the quantitatively reconstructed 3D fuel spray density distribution as a result of the quantitative CT technique. More importantly, a computational fluid dynamics (CFD) simulation based on the Taylor analogy breakup (TAB) model has also been performed using the boundary and initial conditions obtained from the experiment data. The experimental and numerical results are in good agreement quantitatively. These results not only reveal the characteristics of the GDI fuel sprays with unprecedented detail, but will also facilitate realistic computational fluid dynamic simulations in highly transient, multiphase systems.
