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11.
Electrical Resistivity of Composites 总被引:23,自引:0,他引:23
David S. McLachlan Michael Blaszkiewicz Robert E. Newnham 《Journal of the American Ceramic Society》1990,73(8):2187-2203
Percolation and Bruggeman's effective media theories, as they apply to the electrical conductivity of composites, are reviewed, and a general effective media (GEM) equation, which combines most aspects of both percolation and effective media theories, is introduced. It is then shown that the GEM equation quantitatively fits electrical resistivity (conductivity) as a function of the volume fraction data for binary composites. The parameters used to fit the experimental data are the electrical resistivities of the two phases, the percolation threshold for the lower resistivity phase (ô c ), and an exponent t. Preliminary work, showing how the GEM equation can be used to model the piezoresistivity of composites by postulating that ô c is a function of the independent variable, is also presented. 相似文献
12.
Enhai Zhao John D. Cressler Monir El-Diwany Tracey L. Krakowski Alexei Sadovnikov Dimitar Kocoski 《Solid-state electronics》2006,50(11-12):1748-1755
We present an investigation of the dependence of low-frequency noise on device geometry in advanced npn silicon–germanium (SiGe) heterojunction bipolar transistors (HBTs). The devices examined in this work have fixed emitter width (WE = 0.4 μm), but varying emitter length (0.5 μm LE 20.0 μm), and thus the ratio of the emitter perimeter (PE) to the emitter area (AE) varies widely, making it ideal for examining geometrical effects. The SPICE noise parameter AF extracted from these devices decreases with increasing LE. Furthermore, the low-frequency noise measured on SiGe HBTs with significantly different PE/AE ratios suggests a possibility that the fundamental noise source for the diffusion base current may be located at the emitter periphery. Transistors with different distances between the emitter edge and the shallow trench edge (XEC), and shallow trench edge to deep trench edge (XTC), are also investigated. The SiGe HBTs with a smaller value of XEC have higher low-frequency noise, but no significant difference is found in devices with varying XTC. Explanations of the observed noise behavior are suggested. 相似文献
13.
Martyn A. McLachlan David W. McComb Mary P. Ryan Anna N. Morozovska Eugene A. Eliseev E. Andrew Payzant Stephen Jesse Katyayani Seal Arthur P. Baddorf Sergei V. Kalinin 《Advanced functional materials》2011,21(5):802-802
We describe the characterization, ferroelectric phase stability and polarization switching in strain‐free assemblies of PbZr0.3Ti0.7O3 (PZT) nanostructures. The 3‐dimensionally ordered macroporous structures present uniquely large areas and volumes of PZT where the microstructure is spatially modulated and the composition is homogeneous. Variable temperature powder X‐ray diffraction (XRD) studies show that the global structure is crystalline and tetragonal at room temperature and undergoes a reversible tetragonal to cubic phase transition on heating/cooling. The measured phase‐transition temperature is 50–60 °C lower than bulk PZT of the same composition. The local ferroelectric properties were assessed using piezoresponse force spectroscopy that reveal an enhanced piezoresponse from the nanostructured films and demonstrate that the switching polarization can be spatially mapped across these structures. An enhanced piezoresponse is observed in the nanostructured films which we attribute to the formation of strain free films, thus for the first time we are able to assess the effects of crystallite‐size independently of internal stress. Corresponding polarization distributions have been calculated for the bulk and nanostructured materials using a direct variational method and Landau‐Ginzburg‐Devonshire (LGD) theory. By correlating local and global characterization techniques we have for the first time unambiguously demonstrated the formation of tetragonal and ferroelectric PZT in large volume nanostructured architectures. With the wide range of materials available that can be formed into such controlled architectures we conclude that this study opens a pathway for the effective studies of nanoscale ferroelectrics in uniquely large volumes. 相似文献
14.
Matyas Daboczi Jinhyun Kim Jinho Lee Hongkyu Kang Iain Hamilton Chieh‐Ting Lin Stoichko D. Dimitrov Martyn A. McLachlan Kwanghee Lee James R. Durrant Ji‐Seon Kim 《Advanced functional materials》2020,30(25)
Integrated perovskite/organic bulk heterojunction (BHJ) solar cells have the potential to enhance the efficiency of perovskite solar cells by a simple one‐step deposition of an organic BHJ blend photoactive layer on top of the perovskite absorber. It is found that inverted structure integrated solar cells show significantly increased short‐circuit current (Jsc) gained from the complementary absorption of the organic BHJ layer compared to the reference perovskite‐only devices. However, this increase in Jsc is not directly reflected as an increase in power conversion efficiency of the devices due to a loss of fill factor. Herein, the origin of this efficiency loss is investigated. It is found that a significant energetic barrier (≈250 meV) exists at the perovskite/organic BHJ interface. This interfacial barrier prevents efficient transport of photogenerated charge carriers (holes) from the BHJ layer to the perovskite layer, leading to charge accumulation at the perovskite/BHJ interface. Such accumulation is found to cause undesirable recombination of charge carriers, lowering surface photovoltage of the photoactive layers and device efficiency via fill factor loss. The results highlight a critical role of the interfacial energetics in such integrated cells and provide useful guidelines for photoactive materials (both perovskite and organic semiconductors) required for high‐performance devices. 相似文献
15.
Tracey E. Swingler Lingzi Niu Matthew G. Pontifex David Vauzour Ian M. Clark 《International journal of molecular sciences》2022,23(1)
The complete molecular mechanisms underlying the pathophysiology of Alzheimer’s disease (AD) remain to be elucidated. Recently, microRNA-455-3p has been identified as a circulating biomarker of early AD, with increased expression in post-mortem brain tissue of AD patients. MicroRNA-455-3p also directly targets and down-regulates APP, with the overexpression of miR-455-3p suppressing its toxic effects. Here, we show that miR-455-3p expression decreases with age in the brains of wild-type mice. We generated a miR-455 null mouse utilising CRISPR-Cas9 to explore its function further. Loss of miR-455 resulted in increased weight gain, potentially indicative of metabolic disturbances. Furthermore, performance on the novel object recognition task diminished significantly in miR-455 null mice (p = 0.004), indicating deficits in recognition memory. A slight increase in anxiety was also captured on the open field test. BACE1 and TAU were identified as new direct targets for miR-455-3p, with overexpression of miR-455-3p leading to a reduction in the expression of APP, BACE1 and TAU in neuroblastoma cells. In the hippocampus of miR-455 null mice at 14 months of age, the levels of protein for APP, BACE1 and TAU were all increased. Such findings reinforce the involvement of miR-455 in AD progression and demonstrate its action on cognitive performance. 相似文献
16.
The treatment of acid rock drainage (ARD) places extraordinary financial burdens on governments and companies worldwide, and
an improved efficiency in treatment by as little as 1% can save many millions of dollars in rehabilitation. We investigated
a system for treating Fe-rich ARD using a three-stage reactor design. In the first reaction cell, Fe-rich ARD was partially
neutralised using rapid periodic carbonate resuspension with a rotating axial mixer. This was followed by an air-sparged oxidation
chamber and then a second reaction cell, with more carbonate periodically resuspended until a pH of 6.3 was reached, which
was followed by a settlement chamber. This reactor design has a high capacity for neutralisation, with an efficiency of ≈70%
of acidity neutralised by the acid neutralising capacity (g of CaCO3 equivalent) added to the reactor. Axial mixers were tested because of their low-energy requirements and their high reliability.
The intermediate chamber effectively removes Fe by oxidising Fe(II) to Fe(III). Given the amount of acidity neutralised, the
sludge volume produced was low compared to other technologies, providing further potential savings in sludge handling. Waste
carbonate rock proved to be an effective neutralising agent, even though it was about 60% dolomite and 40% magnesite, with
minor calcite, and despite the fact that magnesite has substantially slower dissolution kinetics compared to the more dominant
dolomite. The mixed waste carbonates were capable of raising the pH sufficiently to reduce the heavy metal loadings in Fe-rich
ARD by more than two orders of magnitude. The final settlement stage of the process was shown to be essential for metal precipitation,
for the carry-over of fine carbonates, and CO2 loss. This was associated with a rise in pH, from 6.3 to 7.5. In addition, residual slow-reacting magnesite from the mixed
carbonate remains in the sludge from the first reactor and provides acid buffering capacity within the sludge, which is commonly
lacking in the ARD neutralisation sludge of other systems. 相似文献
17.
18.
The study of numerical abilities, and how they are acquired, is being used to explore the continuity between ontogenesis and environmental learning. One technique that proves useful in this exploration is the artificial simulation of numerical abilities with neural networks, using different learning paradigms to explore development. A neural network simulation of subitization, sometimes referred to as visual enumeration, and of counting, a recurrent operation, has been developed using the so-called multi-net architecture. Our numerical ability simulations use two or more neural networks combining supervised and unsupervised learning techniques to model subitization and counting. Subitization has been simulated using networks employing unsupervised self-organizing learning, the results of which agree with infant subitization experiments and are comparable with supervised neural network simulations of subitization reported in the literature. Counting has been simulated using a multi-net system of supervised static and recurrent backpropagation networks that learn their individual tasks within an unsupervised, competitive framework. The developmental profile of the counting simulation shows similarities to that of children learning to count and demonstrates how neural networks can learn how to be combined together in a process modelling development. 相似文献
19.
Tracey M. Clarke Amy M. Ballantyne Jenny Nelson Donal D. C. Bradley James R. Durrant 《Advanced functional materials》2008,18(24):4029-4035
The function of organic solar cells is based upon charge photogeneration at donor/acceptor heterojunctions. In this paper, the origin of the improvement in short circuit current of poly(3‐hexylthiophene)/6,6‐phenyl C61‐butyric acid methyl ester (P3HT/PCBM) solar cells with thermal annealing is examined. Transient absorption spectroscopy is employed to demonstrate that thermal annealing results in an approximate two‐fold increase in the yield of dissociated charges. The enhanced charge generation is correlated with a decrease in P3HT's ionization potential upon thermal annealing. These observations are in excellent quantitative agreement with a model in which efficient dissociation of the bound radical pair into free charges is dependent upon the bound radical state being thermally hot when initially generated, enabling it to overcome its coulombic binding energy. These observations provide strong evidence that the lowest unoccupied molecular orbital (LUMO) level offset of annealed P3HT/PCBM blends may be only just sufficient to drive efficient charge generation in polythiophene‐based solar cells. This has important implications for current strategies to optimize organic photovoltaic device performance based upon the development of smaller optical bandgap polymers. 相似文献
20.