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961.
Well-aligned, high-purity carbon nanotube (CNT) stacks of up to 10 layers fabricated in one batch process have been formed by water-assisted selective etching of carbon atoms. Etching takes place at the CNT caps as well as at the interface between CNTs and metal catalyst particles. This simple process generates high-purity CNTs and opens the CNT ends by removing the nanotube caps. High-resolution transmission electron microscopy indicates that the process does not damage CNT wall structures. A mechanism for stacked growth of CNT layers is proposed. 相似文献
962.
963.
Spatial and vertical distribution of polycyclic aromatic hydrocarbons in mangrove sediments 总被引:2,自引:0,他引:2
The distribution and historical changes of polycyclic aromatic hydrocarbons (PAHs) contamination in mangrove sediments in Hong Kong SAR were investigated. Surface sediments (2-3 cm) collected from four mangrove swamps exhibited significant spatial variations in concentrations of total PAH (with SigmaPAHs ranging from 56 to 3758 ng g(-1) dry wt), as well as the composition of 16 USEPA priority PAH compounds. Within a small swamp with an area of 0.68 ha, the total PAH concentrations also differed from sampling site to site, indicating that the PAH contamination is localized and confined to a very small area within the same swamp. Discharges from municipal and industrial wastewater, urban runoff, oil leakage from boats and ships, and accidental oil spill are possible sources of the PAH contamination. The sediment depth profiles reveal that the surface sediment layer (0-5 cm) had lower total PAH concentrations than that in the bottom layer (15-20 cm), and PAH composition also changed with the sediment layers. Based on the estimated annual sediment deposition rate in Hong Kong SAR of around 0.4-0.5 cm, the present findings suggest that the PAH contamination was most serious between 1958 and 1979 but started to decline thereafter. Such decline was probably due to changes in petroleum usage in urban areas and a better control of wastewater discharges from 1980 onwards in this region. 相似文献
964.
965.
Joe Wong 《Journal of Electronic Materials》1976,5(2):113-160
Vibrational studies of a variety of thin dielectric glass films relevant to the silicon technology are reviewed. Attention
is focused on those results and structural infor-mation obtained by direct transmission of materials vapor-deposited on single-crystal
silicon substrates in the infra-red region 250 to 4000 cm-1. In particular, the structure and bonding in some simple CVD binary silicate films are discussed in some detail in light
of the recent vibrational analyses performed on the borosilicate and phosphosilicate systems. 相似文献
966.
Tetramethyllead was found very toxic especially to juvenile rainbow trout (Salmo gairdneri). It was accumulated rapidly from the water by the fish and the highest concentration was found in the lipid layer of the intestine. Depuration of Me4Pb from the fish organs was initially quite rapid, followed by a slower decrease and eventually reaching residual levels. Environmental concentrations of Me4Pb detected in the fishery products thus represent a balance between uptake and depuration of this compound in fish.In conjunction with this study, a dosing system for exposing rainbow trout to Me4Pb was developed. 相似文献
967.
A sealed-tube type 14 MeV neutron generator with maximum neutron output of 1011 n/sec, incorporating a pneumatic sample transfer system of single-tube type and with a single rotation of the sample during neutron irradiation, is used to develop a method for determining oxygen content in steel in the ppm range with the best precision, and at the same time, to make the process suitable for routine work in industrial applications. The pneumatic sample transfer system is made to incline at an angle of about 20° towards the horizontal at the irradiation station. Together with a constant pressure gas reservoir for providing a constant optimum gas pressure in the transfer tube, the system gives a result of nearly perfect reproducibility in the operation.A pulse shape analyser system incorporating an organic scintillation detector is used for monitoring neutron flux level during the neutron irradiation of the sample. The percentage standard deviation of the neutron counts by the present monitoring system ranges from 0.9 to 2.7% with 0.5% as the percentage statistical deviation alone.Polyethylene, of oxygen content 163 ppm determined by comparison with lucite, is used as the steel sample carrier. A 3 × 3 in. NaI(Tl) crystal is used with a single channel analyser to count the 6.1 and 7.1 MeV gamma rays emitted from 16N as a result of the reaction 16O(n, p)16N. An optimum combination for the time of irradiation, delay and counting of the induced activity; of 30, 0.1 and 30 sec, respectively, is chosen in the present experiment. Thus, for a 100 g steel sample with an oxygen concentration of 170 ppm, the percentage standard deviation is about 4.4% which is, in fact, the counting statistic itself, resulting from a neutron flux level of 1.3 × 108 n/sec cm2 at the sample. As the present activation analysis makes use of the comparison method, a steel-mylar standard made of layered steel and mylar discs is prepared and a calibration curve constructed. A method of correcting the oxygen contribution in the polyethylene sample carrier is devised and the content of oxygen in the steel standard is determined.A survey of neutron flux distribution is also attempted and it is found that nearly symmetrical distribution of the flux, about the centre of the sample carrier which is placed with its axis in parallel to the plane of the disc-shaped target of the neutron-generating tube, is far from being flat. 相似文献
968.
969.
970.
A new diversity technique is proposed to combat Rayleigh fading in digital mobile radio systems transmitting speech signals. The speech signals are μ-law PCM encoded (mu = 255 , 8 kHz sampling, 8 bits/code word, 64 kbit/s data rate), and alternate data words are used to form two streams called "odd" and "even." The even stream is delayed by τ seconds and the streams are interleaved prior to radio transmission using two-level PSK modulation. At the receiver the odd data stream is delayed by τ and interleaved with the even stream. Consequently, if an error burst occurs, the effect of the reshuffling of the data stream is, in general, to place words with bit errors in juxtaposition to those correctly received. After μ-law PCM decoding of the words, a statistical error detection strategy is evoked to identify the erroneous samples. These samples are replaced by adjacent sample interpolation to give the recovered speech sequence. No recourse to channel protection coding is made. In our experiments a Rayleigh fading envelope was generated from a hardware simulator and stored in a computer, along with four sentences of speech. The system was then simulated and the recovered speech perceived. The objective performance measures were segmental SNR for the audio signal, and BER. Different error detection strategies were examined and restrictions on τ investigated. For a mobile speed of 30 mph, SNR values of 32, 21, and 16 dB were obtained for BER values of 0.1, 1, and 2 percent, corresponding to SNR gains over an uncorrected system of 3, 9, and 11 dB, respectively. 相似文献