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 共查询到10条相似文献,搜索用时 78 毫秒
1.
B.B. He  W. Xu 《哲学杂志》2015,95(11):1150-1163
The effect of ausforming temperature and strain on the bainitic transformation kinetics was investigated in a low carbon boron steel. A new mechanism, which is based on the competition between the increase in nucleation rate and the decrease in average volume of bainite sheaf after deformation, is proposed. The increase in nucleation rate is due to the decrease in boron concentration at the grain boundaries after small deformation and the formation of sub-grain boundaries at the grain interior after large deformation. The decrease in average volume of bainite sheaf is ascribed to the frequent impingement of bainite sub-units after deformation. The increase in nucleation rate after deformation results in the decrease in incubation time, which is confirmed from the experiment. The increase in nucleation rate overcomes the decrease in average volume of bainite sheaf, resulting in the increase in transformation velocity and volume fraction after small deformation. On the contrary, the decrease in the average volume of bainite sheaf overcomes the increase in nucleation rate after large deformation, leading to the decrease in transformation velocity and volume fraction of bainite.  相似文献   

2.
廖天河  刘伟  高穹 《应用光学》2012,33(4):799-803
 考虑到大气中吸收系数随高度变化,通过内插法得到不同大气模型一定高度下大气的吸收系数,建立序列脉冲激光在地对空垂直传输的热晕模型。通过数值计算方法分析风场渡越时间内脉冲数为2时的序列脉冲在中纬度地区夏季和冬季、热带地区、近北极地区夏季和冬季5种大气模型下的热晕效应。结果表明:当光束孔径为0.25 m,初始功率为200 kW,波形为高斯状的激光束在近北极冬季传输10 km后沿横轴方向光强峰值偏移0.056 9 m,而热带光强峰值强度偏移最远为0.224 m,为近北极冬季时的4倍;脉冲激光在热带传输10 km后的靶面功率仅为15.06 kW,近北极冬季靶面功率为热带的10.5倍。热带地区的热晕效应最明显,中纬度地区次之,近北极地区最弱。激光在夏季传输要比冬季传输的热晕效应严重,而且近北极地区夏季要比中纬度地区冬季的热晕效应更严重。  相似文献   

3.
The differences of the pitch and duration of Chinese syllables between Putonghua (PTH) and Taiwan Mandarin (TM) were studied. The speech materials to be used are not only isolated syllables, but also sentences. The results reveal that: For the isolated syllables, T1 and T2 in TM are influenced by Minnan dialect, therefore their pitch are lower than those in PTH. T3 is fall-rise in PTH, while it is fall in TM. Moreover, the syllable duration sequence for different tone is T3〉T2〉T1〉T4 in PTH, while it is T1〉T2〉T3〉T4 in TM. For the syllables in sentences, T2 is mid-rise in PTH, while it is mid-level in TM. And the T3 is longer than T4 but shorter than T1 or T2 in PTH, while it is the shortest in TM. Furthermore the effects of prosodic phrase boundary on duration for different tones are almost the same in PTH, but the lengthening part of T1 or T2 is longer than that of T3 or T4 in TM.  相似文献   

4.
The structure and hydration of insulin-like growth factor 1 and an inactive mutant lacking the C region have been investigated in aqueous solution by molecular dynamics simulation. The overall structures of the two polypeptide resemble those determined by NMR spectroscopy. The deletion of the C region in the wild polypeptide introduces structural stability in the mutant, leading to a better definition of the secondary structure elements. A detailed hydration analysis was performed using the radial distribution functions and energy distributions. The backbone of the mutant is in general more solvent accessible than the wild polypeptide backbone. The structural rearrangements induced in the mutant led to changes in the solvent exposition of Tyr24 and Tyr60, which are residues important for ligand—receptor complex formation. Tyr24 exhibited a similar degree of solvent exposition in both IGF-I and in the mutant; however, its hydroxyl group in the wild polypeptide is better solvated than in the mutant. Tyr60 was found to be solvent exposed in the wild protein, while in the mutant the involvement of its hydroxyl group in intramolecular hydrogen bonds led to it being buried away from the solvent.  相似文献   

5.
The absorption, fluorescence, fluorescence quantum yield, and photostability of five BODIPY dyes are characterized and compared as single dyes in two environments, in 40-nm polystyrene spheres and in solution. The absorption and fluorescence spectra of the dyes in spheres are similar in profile but shifted to lower energies compared to those in solution. All the dyes are highly fluorescent, with three having fluorescence quantum yields of 1.0. For three of the five dyes, the yields were the same in spheres as in solution (1.00, 1.00, and 0.73). The high concentration of these dyes in spheres does not quench their fluorescence. For two other dyes the yields dropped, from 1.00 to 0.55 in one case and 0.83 to 0.50 in another, comparing the dyes in solution versus in spheres. The photodegradation of the dyes decreases in spheres compared to in solution in all but one case. For one dye, it decreases as much as 800-fold. Dyes overlooked because of low fluorescence or stability in solution could become useful fluorescent materials in the microsphere environment.  相似文献   

6.
赵巧华  孙绩华 《物理学报》2013,62(3):39203-039203
湖泊表层混合层深度的变化不仅影响湖泊水生生态系统的演变, 而且影响流域的局地气候、降水量的时空格局等. 基于2008年夏秋两季洱海(高原湖泊)和太湖(平原湖泊)的气象与水温廓线观测资料, 分析探讨了两湖表层混合层深度的变化特征及其机制. 结果表明: 夏季洱海能维持持续的分层现象, 秋季有明显的日分层现象, 而在相应的两季中, 太湖仅可能存在日分层现象; 洱海表层混合层深度较同期太湖更浅; 太湖两季的表层混合层深度变化较洱海频繁, 即太湖水体混合与分层的交替过程对气象条件的响应较洱海更为迅速. 太湖这类浅水湖泊, 水深是抑制其存在稳定、持续分层的关键因素, 在合适的辐射条件下, 可形成日分层现象; 而洱海这类深度的湖泊, 净热量通量是影响其是否存在持续、稳定分层的主要因子. 该研究为进一步探讨湖泊与大气两种湍流运动的耦合机制及水生生态环境演替规律等提供了有力的参考.  相似文献   

7.
J Philip  M S Kala 《Pramana》1997,49(5):555-561
This paper reports the results of the study of anisotropy in elastic wave propagation in single crystal superconducting BSCCO. The inverse and group velocities of elastic waves propagating in different directions have been computed and the corresponding slowness and ray velocity surfaces plotted, taking elastic constant data from literature. In addition, the phenomenon of phonon focussing has been investigated in this material by computing the phonon enhancement factor along different directions in spherical polar coordinates. The abnormally high values in phonon enhancement factor exhibited in certain directions for the phonon modes are interpreted as due to caustics occurring in the geometrical acoustics approximation adopted in the computational analysis. The results in LSCO and YBCO are found to be similar to those in BSCCO.  相似文献   

8.
The change in the modality of cornification from embryonic to definitive epidermis in the chick has been studied using immunocytochemistry and electron microscopy to show that the initial soft cornification based on an acidic type I alpha-keratin transits to a definitive hard cornification based on beta-proteins in the claw, scales and feathers. The first two periderm layers contain acidic keratins associated with periderm granules and participate in a mild form of cornification before shedding of the periderm. The transition from embryonic to adult cornification is best seen in the transitional layers of the claw where numerous periderm granules merge with packets or bundles of corneous beta-proteins. This process is hardly seen in scale and feathers where periderm granules remain most in the periderm or in the feather sheath. Periderm granules disappear in corneocytes generated underneath the periderm in scales or in the transitional layer in claws and are replaced by beta-proteins associated to other types of acidic alpha-keratins. This process produces a mechanically resistant corneous material underneath the softer periderm, adapted to terrestrial demand for mechanical protection in scales and in the dorsal part of the claw, the unguis. In the ventral part of the claw, the sub-unguis, scarce or no beta-proteins are accumulated resulting in a softer corneous layer. The study indicates that specific alpha-keratins form the cytoskeletal framework of definitive corneocytes in claws, scales and feathers, and that specialized corneous beta-proteins are deposited over this framework to produce epidermal layers with higher mechanical resistance.  相似文献   

9.
Water diffusion measurements were performed on rabbit Achilles tendons during static tensile loading and tendons in an unloaded state. The apparent diffusion coefficient (ADC) was measured along two directions: parallel and perpendicular to the long axis of the tendon. Tendons were studied after being prepared in two ways: (a) after being stored frozen in phosphate-buffered saline (PBS) and (b) freshly isolated. Statistically significant directional anisotropy was observed in the ADC in all tendons. The ADC was significantly greater in the direction parallel to the long axis of the tendon than in the perpendicular direction. The anisotropy is attributed to the greater restrictions seen by the water molecules in the perpendicular direction and is consistent with the known geometry of the tendon. Storage in PBS caused tendons to swell. This increased the ADC measured along both directions and reduced the anisotropy. The existence of anisotropy in the ADC was not related to the orientation of the specimen in the magnet. The ADC increased along both directions following the application of a 5-N tensile load; the increase was greatest along the perpendicular axis of the tendon. In order to determine whether load-related changes in the ADC reflected changes in interfibrilar spacing, we used electron microscopy to measure load-related changes in fibril spacing. Load-related changes in fiber spacing could not account for the observed changes in the ADC. The increase in ADC caused by loading was attributed to the extrusion of tendon water into a bulk phase along the outside surface of the tendon. In PBS-stored samples, enough fluid was extruded that it could be visualized. The transient response of the ADC to a 5-N tensile load was also studied. The absolute ADC in both directions increased with loading and recovered to baseline upon unloading. The transient changes in ADC, for both loading and unloading, had a mean time constant of approximately 15 min. The magnitude of the load-induced transient ADC changes was comparable to that seen in the static-loading experiments.  相似文献   

10.
Since the demonstration of optical gain in silicon nanocrystals, in the last few years several papers appeared in the literature reporting gain measurements in silicon nanocrystals embedded in a silica matrix produced by different techniques. However, it is still unclear which are the structural, physical and chemical factors that contribute to enhance photoluminescence and gain in this type of samples. In particular, the presence and the role of nitrogen in the SiO2 matrix are in fact supposed to be essential factors in understanding the gain mechanism.In fact it is possible to obtain similar samples with very different nitrogen content in the silica matrix changing one of the precursor gases used in the deposition process, thus evidencing the structural and chemical differences introduced by the presence of nitrogen. In this paper SIMS and XPS analysis of two series of similar samples, but with a very different nitrogen content, will be presented and compared. The data collected at different annealing temperatures, together with ellipsometric measurements, give important information on the role played by the nitrogen present in the matrix in the process of silicon nanocrystal formation. Moreover, we demonstrate that the annealing process causes always some oxidation of the sample surface and that nitrogen is incorporated in the material from the annealing atmosphere in nitrogen free samples.  相似文献   

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