Bibliothek

Notizen: Neutron diffraction has been used to investigate the rare-earth effect on the site occupations of Al and C atoms in the combined substitutional–interstitial compounds, R2Fe15Al2Cy (R=Pr, Tb, Ho). All three samples have the rhombohedral Th2Zn17-type structure. As the composition changes from light rare-earth to heavy rare-earth, the site occupations of the substitutional Al atoms on the 6c and 18f increase while that on the 18h site decreases. However, the 18h site still has the highest Al occupation for all samples. The site occupation of the C atoms on the octahedral interstitial site (9e) is much higher in the heavy rare-earth (Ho) sample than in the light rare-earth (Pr) sample. However, the tetrahedral interstitial 18g site shows a reverse behavior. This 18g site occupation of C atoms has not been observed before except in gas phase interstitial compounds. The Al site occupation is determined by the crystal environment, while the C site occupation appears to be determined predominantly by site size. © 1997 American Institute of Physics.

Notizen: The effect of the externally introduced rf magnetron plasma on the particle size distribution on films deposited by pulsed laser ablation is investigated. A cw low energy magnetron rf plasma is sustained between the target and the substrate during the laser ablation deposition process. The ablated droplets can be negatively charged and filtered by the plasma. For Si and Al target ablation, the filtering efficiency of the inert Ar plasma is about 20%, and is independent of the ejected particle size in our system. If oxygen is introduced into the plasma to deposit oxide thin films, the oxidation of the target surface in the reactive plasma changes the size distribution of the ejected particles, and largely reduces the average size of the ejected particles. © 1995 American Institute of Physics.

Notizen: The signals from buried layer in Si-δ-doped GaAs of different undoped cap thickness have been studied by differential photoreflectance. The first-derivative-like line shape of differential photoreflectance is attributed to the energy transitions related to the modulations of two-dimensional electron gas density. We have observed a change of line shape at low temperatures. This change of line shape is probably due to the change of potential distribution in the conduction band, which is caused by the surface Fermi-level pinning and surface photovoltaic effect.

Notizen: Si-δ-doped GaAs (N2D ≈ 1011 cm−2) samples grown by molecular-beam epitaxy are investigated by using photoreflectance spectroscopy. The oscillations observed above the GaAs fundamental band gap are attributed to the Franz–Keldysh effect in the region between the δ-doped layer and the crystal surface. This ascription is confirmed by detailed studies through varying the cap thickness (250–2500 A(ring)), temperature (10–450 K), and laser pump power (0.05–7 mW/cm2). The surface potential deduced from the Franz–Keldysh oscillations is found to be temperature and laser pump power dependent, which is explained by taking the surface photovoltaic effect into account. The surface Fermi level has been measured by this method and is found to have the value 0.73±0.02 V.

Notizen: Analytical equilibria for axisymmetric plasmas with an anisotropic energetic-particle component are obtained by expansions in the inverse aspect ratio and core- and hot-plasma betas. Upon the analytical equilibria, the energetic-particle-modified Mercier criterion [L.-J. Zheng and S.-T. Tsai, Phys. Fluids B 4, 1416 (1992)] is evaluated. A drift-nonreversed energetic-particle component is shown to make stabilizing terms to the criterion, which originate from the additional Shafranov shift of magnetic surfaces and the stabilizing kinetic energy contributed by the energetic particles. The results confirm that interchange modes in tokamaks can be stabilized by properly introducing drift-nonreversed energetic particles.

Notizen: The ELMO snaky torus concept [Furth and Boozer (private communication, 1983)] is investigated in a helically symmetric model with low beta and large aspect ratio (the curvature radius of the magnetic axis over the plasma radius). The equilibria are obtained by expansions in the inverse aspect ratio and plasma beta. Application of the energetic-particle- modified Mercier criterion [Zheng and Tsai, Phys. Fluids B 4, 1416 (1992)] to the analytical equilibria confirms that the interchange mode can be stabilized in helically symmetric systems in the low-beta regime by suitably introducing drift-nonreverse energetic particles. The results show that, for effective stabilization, it is better that the pressure profile of the energetic component depend on the radius quadratically, and peak at midplane in the poloidal angle, and the volume occupied by the energetic particles, the aspect ratio, and the ratio of the curvature to the torsion of the magnetic axis should be large when the energetic particles are drift nonreverse.

Notizen: Vertically aligned carbon nitride nanotubes with a uniform diameter of about 250 nm have been synthesized on a porous alumina membrane template (50–80 μm thick) in a microwave excited plasma of C2H2 and N2 using an electron cyclotron resonance chemical vapor deposition system. A negative dc bias voltage was applied to the substrate holder of graphite to promote the flow of ionic fluxes through the nanochannels of the alumina template. This allowed the physical, and subsequent chemical, absorption of species on the walls of the nanochannels that resulted in the formation of the carbon nitride nanotubes. The hollow structure and vertically aligned properties of the nanotubes have been clearly verified by field-emission scanning electron microscope images. The absorption band between 1250 and 1750 cm−1 in the Fourier transform infrared spectroscopy spectrum proves that nitrogen atoms have been incorporated into an amorphous network of carbon. © 1999 American Institute of Physics.

Notizen: Aligned carbon nanotubes with open ends have been fabricated on silicon wafer in one step using a microwave plasma enhanced chemical vapor deposition system with a mixture of methane and hydrogen as precursors. High concentration hydrogen plasma and high negative bias voltage to the substrate induce anisotropic etching of carbon nanotubes and can effectively reduce the randomly oriented carbon nanotubes. The mechanism of aligned carbon nanotubes with open ends is proposed in this letter. © 1999 American Institute of Physics.

Notizen: This paper presents new findings on ultrasonic spray pyrolysis of zirconium hydroxyl acetate precursor drops whose sizes were precisely measured using laser light diffraction technique. Precursor concentration plays a predominant role in determination of product particle size. At 0.01 wt% precursor concentration, conventional spray pyrolysis at 750°C using precursor drops 5–8 μm in diameter, generated by an ultrasonic nebulizer at 2.66 MHz, yielded uniform spherical yttria-stabilized zirconia (YSZ) particles 73 nm in diameter measured by scanning electron microscopy. The YSZ particle diameters were much smaller than those predicted by the one-particle-per-drop mechanism. Under similar reaction conditions, the high-throughput ultrasound-modulated two-fluid (UMTF) spray pyrolysis of larger precursor drops (28-μm peak diameter) also yielded spherical dense particles; they were significantly smaller in size than those produced by the low-throughput conventional ultrasonic spray pyrolysis of smaller drops (6.8-μm peak diameter).

Notizen: Background  There is a high incidence of primary cutaneous amyloidosis (PCA) in South America, South-east Asia and Taiwan. To date, the aetiology of PCA remains unknown, but it is believed to be multifactorial. Although most cases are sporadic, some patients have a family history. Familial aggregation and different susceptibility to PCA among ethnic groups suggest that genetic factors may play an important role in its pathogenesis. However, no genetic loci for familial PCA (FPCA) have been identified so far.Objectives  In order to identify the susceptibility gene of FPCA, we took a candidate gene approach and performed linkage analysis on chromosome 1q21.3–24.2, including the 1q23.2 region where the gene encoding serum amyloid P component (APCS) is located.Patients and methods  Nine FPCA families including 29 individuals affected with PCA were recruited for this linkage study. Initially, 11 highly polymorphic microsatellite markers spanning the region from 1q21.3 to 1q24.2 were genotyped and revealed a suggestive linkage region. This region was further fine-mapped with seven additional markers. We also re-sequenced the 2·5-kb genomic region of the APCS gene in 29 affected and 42 control individuals. Two-point and multipoint linkage analyses were performed using the LINKAGE program. Nonparametric linkage (NPL) analysis and reconstruction of haplotypes were performed with the GENEHUNTER program.Results  Both two-point and multipoint linkage analysis for all 11 markers generated negative or small positive total lod scores for all nine families. However, when we considered only three families, a maximum two-point total lod score of 2·09 was obtained for the marker D1S2844 at θ = 0·01. A plateau of multipoint total lod score between D1S2768 and D1S2878 with a maximum of 2·48 at the marker D1S2844 was observed. A maximum NPL score of 3·11 (P = 0·008) was also obtained for the marker D1S2878. However, re-sequencing of the APCS gene identified no functional mutation.Conclusions  Both parametric and nonparametric linkage evidence suggested that a possible susceptibility locus for a subset of FPCA might exist on chromosome 1q23. This is the first report demonstrating suggestive evidence of linkage of FPCA to a locus in this candidate region. No functional sequence variations of the APCS gene were found to be associated with this disease among the study families. Our data imply the existence of at least one additional locus responsible for FPCA in these families, confirming genetic heterogeneity of this skin disorder.