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21.
The effects of dopant (Ga) concentration, post-heat treatment temperatures, and different heat treatment environments on the morphology, electrical and optical properties of ZnO films were studied. ZnO films doped with Ga are derived from non-alkoxide zinc acetate via the alcoholic route by a sol–gel dip-coating technique. Introduction of Ga, as a dopant, can reduce crystallite size, which is attributed to the increased number of nucleation centers. Grain growth was also observed at high post-heat treatment temperature. It was discovered that different post-heat treatment environments (air or reduced atmosphere) did not change the structure orientation, microstructure shapes and size. It was found that the sheet resistance, Rs could be reduced by an order of magnitude, using post-heat treatment in reduced atmosphere (4%H2–96%N2). 相似文献
22.
23.
Nickel molybdate catalysts used in several different upgrading reactions of coal-derived materials from low-rank Australian coals have been regenerated and their activity assessed using a number of physical and chemical techniques. Regeneration of spent coked catalysts for 20 hours in air at 450°C was sufficient to restore chemical hydrogenation activity to catalyst used for pre-asphaltene, asphaltene and oil upgrading, provided poisoning of the catalyst surface by other impurities, principally chlorine-containing molecules, had not occurred. 相似文献
24.
In this paper we discuss the different models proposed to explain the visible luminescence in porous silicon (PS). We review
our recent photoluminescence and Raman studies on PS as a function of different preparation conditions and isochronal thermal
annealing. Our results can be explained by a hybrid model which incorporates both nanostructures for quantum confinement and
silicon complexes (such as SiH
x
and siloxene) and defects at Si/SiO2 interfaces as luminescent centres. 相似文献
25.
On joint transmitter and receiver optimization formultiple-input-multiple-output (MIMO) transmission systems 总被引:1,自引:0,他引:1
The problem of joint transmitter and receiver optimization for MIMO systems under the mean-squared error (MSE) criterion is revisited. We address the general problem of N (number of users) ≠M (number of channel inputs) ≠P (number of channel outputs) when the system is Nyquist bandlimited and obtain analytical solutions for the optimal transmit-receive pair. Next, we demonstrate how the above result is directly applicable to the problem where the system has excess bandwidth, thereby generalizing the results of Salz (1985). In conclusion, several numerical examples are included to demonstrate the performance gains obtainable with jointly optimized MIMO systems vis-a-vis systems based only on receiver optimization 相似文献
26.
The application of the stochastic gradient (least mean square) algorithm, the design of linear phase finite impulse response (FIR) filters, is discussed. Analytical results are presented and supported by simulation experiments to demonstrate the superior performance of the LMS algorithm equipped with the linear phase constraint as compared to the standard LMS algorithm 相似文献
27.
28.
In reliability theory, lifetimes of systems and components are frequently studied through univariate concepts of ageing. By considering suitable multivariate generalizations of the univariate ageing properties, some multivariate ageing classes of life distributions are defined. Properties of these classes with their equivalent definitions and chain of implications are presented along with a few characterizing properties. These results are useful for obtaining reliability bounds (when component lives are independent) at the early stage of product design. Also for model selection, characterization results can be important. Moreover, the underlying mathematical treatment is univariate when viewed through conditional distributions 相似文献
29.
30.
Reliable channel estimation is indispensable for orthogonal frequency-division multiplexing (OFDM) systems employing coherent detection and adaptive loading in order to achieve high data rate communications. Several options exist in practical OFDM systems-including training symbols, cyclic prefix, virtual carriers, pilot tones, and receiver diversity-to facilitate channel estimation. In this paper, a subspace blind channel estimation method based on exploiting the presence of virtual carriers is proposed for OFDM systems over a time-dispersive channel. The method can be applied to conventional OFDM systems with cyclic prefix as well as OFDM systems with no cyclic prefix. The reduction/elimination of cyclic prefix thereby provides the OFDM systems the potential to achieve higher channel utilization than most previously reported cyclic prefix based estimators. Sufficient channel identifiability condition is developed as well. Comparison with two other recently reported subspace methods is presented via computer simulations to support the effectiveness of the proposed method. 相似文献