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排序方式: 共有558条查询结果,搜索用时 31 毫秒
1.
氮化镓材料中的位错对材料物理性能的影响 总被引:4,自引:0,他引:4
氮化镓材料中的位错是制约GaN发光器件及电子器件的性能的一个关键因素。目前对于氮化镓材料中的位错的研究是一大热点。扼要综述了位错对于材料及器件的物理性能的影响:非辐射复合作用、造成器件的漏电流、缩短器件的寿命。并简要介绍了减少GaN外延层中的位错密度的几种方法。 相似文献
2.
Sumathy Arumuganathar Scott Irvine Jean R. McEwan Suwan N. Jayasinghe 《应用聚合物科学杂志》2008,107(2):1215-1225
In a recent discovery, coaxial electrospinning was explored to encapsulate living organisms within a continuous bio‐polymeric microthread from which active biological scaffolds were fabricated (Townsend‐Nicholson and Jayasinghe, Biomacromolecules 2006, 7, 3364). The cells were demonstrated to have gone through all expected cellular activity without their viability being compromised. These biologically active threads and scaffolds have direct and tremendous applicability from regenerative to therapeutic medicine. Currently these post‐processed cells as composite threads and scaffolds are being investigated in‐depth at a cellular level to establish if the processing methodology has any affect on the cellular make‐up. We now demonstrate a competing non‐electric field driven approach for fabricating composite threads and scaffolds influenced only by a differential pressure. We refer to this novel composite thread to scaffold fabrication methodology as coaxial aerodynamically assisted bio‐threading (CAABT). Our investigations firstly, demonstrate that this technique can process handle living organisms without biologically perturbing them in anyway. Secondly the process is elucidated as possessing the ability to form composite active threads from which biologically viable scaffolds are formed. Finally our study employs florescent activated cell sorting (FACScan), a method by which the cellular dynamics and viability are quantified on control and threaded cellular samples at two prescribed time points. In parallel with FACScan, optical comparison of cellular morphology at three time points within a period of three weeks is carried out to photographically observe any changes in the post‐processed cellular phenotype. Our developmental investigations into this novel aerodynamically assisted threading methodology has unearthed a unique biomicrofabrication approach, which joins cell electrospinning in the cell threading to scaffold fabrication endeavor. © 2007 Wiley Periodicals, Inc. J Appl Polym Sci, 2008 相似文献
3.
R. C. Ecob 《Journal of microscopy》1985,137(3):313-337
Whereas models of the structure and periodicity of CSL or near CSL high angle grain boundaries are relatively well developed and have been to some extent verified by experiment, the nature of more general (e.g. fcc/bcc) interfaces is very imperfectly understood. One of the major differences between homo- and heterophase boundaries is the occurrence, in the latter case, of reproducible orientation relationships due to the crystallographic requirements of phase transformation (e.g. a glissile interface for martensitic growth, low energy interfaces for classical nucleation). A review is given of the relationships commonly observed to obtain between fcc and bcc crystals (as well as ordered phases based upon these structures). Within a given relationship, it is possible to use models of the available interfaces to predict their dislocation content; the success of the various approaches is considered. It is shown that the dislocation arrays identified on fcc/bcc boundaries are consistent with the occurrence of a primary structural relaxation, but that the latter does not appear to be complete. Possible reasons for this are discussed, together with the extent to which secondary structural models may be applicable. Attempts to rationalize the observed orientation relationships and morphologies are discussed, and it is pointed out that these all rely on searches for situations of minimum primary misfit, but that the way in which the latter is quantified determines the results of the analyses. Nevertheless, the implication is that the primary misfit does appear to determine the actual behaviour, though no structural or mechanistic conclusions can safely be drawn from this observation. 相似文献
4.
The hot deformation of an Al-Cu-Mg alloy was studied in the two temperature ranges (room temperature-300℃ and 400℃-480℃). The rate-independent flow curves are typical of elasto-plastic response with significant work hardening followed by strain softening below 300℃. Similar dislocation structures with high density tangled into grain interiors were observed by TEM, which suggests that the process of obstacles arresting mobile dislocations results in this macroscopically rate-independence. At 400-480℃, all rate dependent flow behaviors characterized by a continuous softening after an initial work hardening at a small plastic strain show large tensile elongations. Long dislocation segments around the second phases infer their good mobility to climb across obstacles. Grain boundary morphology observed by TEM suggests that the capacity of the grain boundaries to absorb the dislocations sensitively accounts for the rate-dependent mechanical properties. 相似文献
5.
Recent progress in the understanding of high-temperature creep of alloys is discussed in the context of theoretical modeling
and its application to alloy development. Emphasis is placed upon those engineering alloys specifically designed for high-temperature
applications, such as precipitation and dispersion-strengthened (DS) alloys and metal-matrix composites (MMCs). Currently,
these theoretical models use one of two different approaches, (a) a phenomenological approach, which is used in such models
as those based on the internal stress concept, and those based on empirical creep equations; and (b) micromechanical models
that are based on dislocation mechanisms and the interactions of dislocations with solute atoms, second-phase particles, and
other reinforcements such as fibers. All these theoretical models have a common goal, namely, the understanding of high-temperature
strengthening mechanisms and the relationship between high-temperature strength and the micromechanical mechanisms during
high-temperature plastic deformation of the alloys. These theoretical studies can provide information that is useful in alloy
design and processing, such as the selection of alloy chemistry, and the optimization of phase microstructural features (e.g.,
reinforcement amount, shape, size, and distribution; matrix grain size; and matrix and reinforcement interfaces) by optimization
of processing methods.
L. Shi, Department of Materials Science and Engineering, University of Virginia, Charlottesville, VA 22901, USA 相似文献
6.
The mechanism of low-temperature plastic deformation is controlled by thermally activated dislocation movements. An evolutionary
constitutive law based on the principles of deformation kinetics is described in this article. The constitutive law is expressed
with a sinh function designed for computational efficiency. It is derived from rigorously defined kinetics principles. The
approximation involved in the sinh function is defined so that in applications an exact evaluation can be made of the validity
limits. The system of the constitutive law and the external constraints lead to the operational equations. Applications are
developed for constant strain-rate loading, constant stress-rate loading, stress relaxation, creep, and ratchetting processes.
The analysis provides a unified treatment for low-temperature plastic deformation. 相似文献
7.
The biological function of proteins is dependent, to a large extent, on their native three dimensional conformation. Thus, it is important to know the structure of as many proteins as possible. Since experimental methods for structure determination are very tedious, there is a significant effort to calculate the structure of a protein from its linear sequence. Direct methods of calculating structure from sequence are not available yet. Thus, an indirect approach to predict the conformation of protein, called threading, is discussed. In this approach, known structures are used as constraints, to restrict the search for the native conformation. Threading requires finding good alignments between a sequence and a structure, which is a major computational challenge and a practical bottleneck in applying threading procedures. The Genetic Algorithm paradigm, an efficient search method that is based on evolutionary ideas, is used to perform sequence to structure alignments. A proper representation is discussed in which genetic operators can be effectively implemented. The algorithm performance is tested for a set of six sequence/structure pairs. The effects of changing operators and parameters are explored and analyzed. 相似文献
8.
Symmetric multiprocessor systems are increasingly common, not only as high-throughput servers, but as a vehicle for executing
a single application in parallel in order to reduce its execution latency. This article presents Pedigree, a compilation tool
that employs a new partitioning heuristic based on the program dependence graph (PDG). Pedigree creates overlapping, potentially
interdependent threads, each executing on a subset of the SMP processors that matches the thread’s available parallelism.
A unified framework is used to build threads from procedures, loop nests, loop iterations, and smaller constructs. Pedigree
does not require any parallel language support; it is post-compilation tool that reads in object code. The SDIO Signal and
Data Processing Benchmark Suite has been selected as an example of real-time, latency-sensitive code. Its coarse-grained data
flow parallelism is naturally exploited by Pedigree to achieve speedups of 1.63×/2.13× (mean/max) and 1.71×/2.41× on two and
four processors, respectively. There is roughly a 20% improvement over existing techniques that exploit only data parallelism.
By exploiting the unidirectional flow of data for coarse-grained pipelining, the synchronization overhead is typically limited
to less than 6% for synchronization latency of 100 cycles, and less than 2% for 10 cycles.
This research was supported by ONR contract numbers N00014-91-J-1518 and N00014-96-1-0347. We would like to thank the Pittsburgh
Supercomputing Center for use of their Alpha systems. 相似文献
9.
The propagation of short cracks in the neighbourhood of grain boundaries have been investigated using a technique were the crack is modelled by distributed dislocation dipoles and the plastic deformation is represented by discrete dislocations. Discrete dislocations are emitted from the crack tip as the crack grows. Dislocations can also nucleate at the grain boundaries. The influence on crack growth characteristics of the distance between the initial crack tip and the grain boundary has been studied. It was found that crack growth rate is strongly correlated to the dislocation pile-ups at the grain boundaries. 相似文献
10.
Ali Nazari 《Computational Materials Science》2012,51(1):225-232
In the present work, Charpy impact energy of functionally graded steels produced by electroslag remelting composed of graded ferritic or austenitic layers in both crack divider and crack arrester configurations has been modeled by finite element method. The yield stress of each layer was related to the density of the statistically stored dislocations of that layer and assuming by Holloman relation for the corresponding stress-strain curves, tensile strengths of the constituent layers were determined via numerical method. By using load-displacement curves acquired from instrumented Charpy impact tests on primary specimens, the obtained stress-strain curves from uniaxial tensile tests were modified. The data used for each layer in finite element modeling were predicted modified stress-strain curves obtained from strain gradient plasticity theory. A relatively good agreement between experimental results and those obtained from simulation was observed. 相似文献