共查询到20条相似文献,搜索用时 15 毫秒
1.
I. Kh. Akopyan V. V. Golubkov O. A. Dyatlova B. V. Novikov A. N. Tsagan-Mandzhiev 《Physics of the Solid State》2008,50(7):1352-1356
The crystal structure of CuCl nanocrystals with effective radii ranging from 2.4 to 18.5 nm in photochromic glasses heat treated under different conditions is investigated using exciton spectroscopy, x-ray powder diffraction, and small-angle x-ray scattering. It is revealed that the size dependence of the energy of the Z 3 exciton exhibits an anomalous behavior. The results obtained suggest that small-sized CuCl nanocrystals can crystallize in a modification different from the stable cubic modification. 相似文献
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The structure of low-temperature excitonic luminescence spectra of CuBr nanocrystals of hexagonal modification is studied.
It is shown that the observed spectral structure can be explained assuming the formation of exciton-phonon complexes with
their subsequent annihilation and radiative recombination of the exciton.
Fiz. Tverd. Tela (St. Petersburg) 39, 1657–1658 (August 1997) 相似文献
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A simple relation between the copper halide laser output and the densities of ground state (2S) and metastable state (2D) copper atoms was found. This relation was used to analyze the experimental results of Nerheim. In all the analyzed systems it was found that about four percent of the ground state copper atoms take part in the lasing process. This may have implications for the efficiency of copper halide lasers. 相似文献
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R. Yuvakkumar 《Phase Transitions》2015,88(9):897-906
Niobium oxide nanocrystals were successfully synthesized employing the green synthesis method. Phase formation, microstructure and compositional properties of 1, 4 and 7 days incubation treated samples after calcinations at 450 °C were examined using X-ray diffraction, Raman, photoluminescence (PL), infrared, X-ray photoelectron spectra and transmission electron microscopic characterizations. It was observed that phase formation of Nb2O5 nanocrystals was dependent upon the incubation period required to form stable metal oxides. The characteristic results clearly revealed that with increasing incubation and aging, the transformation of cubic, orthorhombic and monoclinic phases were observed. The uniform heating at room temperature (32 °C) and the ligation of niobium atoms due to higher phenolic constituents of utilized rambutan during aging processing plays a vital role in structural phase transitions in niobium oxide nanocrystals. The defects over a period of incubation and the intensities of the PL spectra changing over a period of aging were related to the amount of the defects induced by the phase transition. 相似文献
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Two correlated phase transitions have been observed in hexagonal RbxWO3 in the concentration range 0.16≤x≤0.33. One of these is a concentration-dependent transition which occurs near x=0.25, and precipitously affects the superconducting transition temperature. The other is a temperature-dependent transition which appears as anomalies in the resistivity, Hall coefficient, and Seebeck coefficient between 100K and 280K. The temperature at which the anomalies occur depends strongly on the x-value. 相似文献
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Fluorescence recovery after pattern photobleaching is used to measure the self-diffusion of surfactant molecules, along cylinders and perpendicular to their main axis in an oriented hexagonal lyotropic phase. Unexpectedly, while the motion along cylinders is diffusive, a superdiffusive behavior is observed in the direction perpendicular to the cylinder axis. Moreover, varying the lattice parameter, we found that the perpendicular diffusion time is governed only by the number of cylinders to cross, providing experimental evidence for superdiffusion with a bounded step length. 相似文献
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R.C. Tobin 《Optics Communications》1980,32(2):325-330
The time dependences of the populations ofthe copper metastable levels (2D, 2D have been measured in a longitudinally-excited CuBr laser of tube diameter 12 mm at a dissociation energy density of 15 mJ cm-3. The metastable populations are created within 3 μs of the dissociation discharge pulse and thereafter decay to the ground level by electron collisions. The decay of the 2D population is faster than that of the 2D population and, in the case of CuBr, both decay rates are increased significantly by the decomposition products which arise from the presence of cupric oxybromide as an impurity. Decay times for the 2D, 2D populations as short as 4, 1.8 μs respectively have been observed under optimum laser conditions. The increase in the relative intensity of the 578.2 nm line at the shortest interpulse delays (ca 10 μs) is due to the more rapid decay of the 2D population. At times up to the several microseconds after the dissociation current pulse the gas in the laser tube remains highly conducting. The rise-time of the pumping pulse is then determined by the external circuit rather than by the transit time of an ionization wave. The controlled effect of the cupric oxyhalide impurity in both CuBr and CuCl lasers offers the prospect of more efficient operation of copper halide lasers at repetition rates in excess of 20 kHz. 相似文献
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D. Ghica S. V. Nistor L. C. Nistor M. Stefan C. D. Mateescu 《Journal of nanoparticle research》2011,13(9):4325-4335
The structural changes of cubic ZnS (cZnS) nanocrystals (NCs) doped with 0.2 at.% Mn2+ pulse annealed in vacuum and in air, up to 500 °C, were investigated by multifrequency electron paramagnetic resonance (EPR),
X-ray diffraction (XRD), and transmission electron microscopy (TEM). The samples, prepared by a surfactant (Tween20)-assisted
liquid–liquid reaction at pH = 6, consist of NCs with a tight size distribution around 3 nm and high crystallinity self-assembled
into a stable mesoporous structure. The EPR spectra of the as prepared samples contain only the characteristic lines of the
substitutional Mn2+(I) centers. No spectra from Mn2+ ions localized in (hydro)oxidized regions of the NCs surface were observed. The absence of such a surface layer could explain
the stability of the cubic (sphalerite) structure observed by XRD and TEM in the investigated cZnS:Mn NCs annealed in vacuum
up to 500 °C. The observation of the cubic-hexagonal transformation for the same NCs annealed in air supports the role of
such layer in promoting this structural transformation. The narrowing of the EPR spectral lines above 200 °C with the increase
in the average size of the cZnS:Mn crystallites was observed. The effect was more pronounced for the sample annealed in air.
EPR also revealed the formation of minute amounts of substitutional Mn2+-type centers in a hexagonal ZnO structure at T ~ 300 °C, corresponding to the early stages of the thermally induced oxidation of the cZnS:Mn NCs. 相似文献
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Yu. S. Nagornov B. M. Kostishko S. N. Mikov Sh. R. Atazhanov A. V. Zolotov E. S. Pchelintseva 《Technical Physics》2007,52(8):1093-1097
Based on X-ray diffraction analysis, Auger spectroscopy, and Raman scattering, it is shown that carbonization of porous silicon at temperatures of 1200–1300°C results in formation of silicon carbide nanocrystals 5–7 nm in size. The growth of 3C-SiC nanocrystals of fixed size d proceeds as follows. Silicon nanocrystals with d = 3–7 nm pass into the liquid phase, thereby effectively participating in the growth of silicon carbide. After the size of a crystallite has achieved a critical value determined by the equality of its melting point and environmental temperature, the crystallite solidifies and virtually ceases to grow. As a result, a nanocrystalline Si-SiC-amorphous SiC heterostructure is obtained. 相似文献
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One-step green route to narrowly dispersed copper nanocrystals 总被引:1,自引:0,他引:1
We report a total “green” chemical method in aqueous solution for synthesizing stable narrowly distributed copper nanoparticles with average diameter less than 5 nm in the presence of Polyvinylpyrrolidone (PVP) as a stabilizer and without any inert gas protection. In our synthesis route, ascorbic acid, natural vitamin C (VC), an excellent oxygen scavenger, acts as both reducing agent and antioxidant, to reduce the metallic ion precursor, and to effectively prevent the common oxidation process of the newborn pure copper nanoclusters. 相似文献
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The process of decomposition of hexagonal copper hydride has been observed in situ in a diamond anvil cell (DAC) using the energy dispersive X-ray diffraction (EDXRD) method. The presence and intensity of diffraction lines of the hexagonal CuH0.8 phase have been taken as a probe for the decomposition process. The intensity of diffraction lines decreases abruptly in the vicinity of 8.4 GPa, indicating complete decomposition of the hydride. The determined value of decomposition pressure is equal to 8.4±0.6 GPa. The standard Gibbs energy of formation of 54.0±1.3 kJ mol−1 (H2) calculated for copper hydride has been compared with the result obtained from calorimetric studies. The large discrepancy between the two values suggests that the decomposition pressure does not describe ‘true’ equilibrium conditions in this system. 相似文献
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V. I. Vettegren R. I. Mamalimov G. A. Sobolev S. M. Kireenkova Yu. A. Morozov A. I. Smul’skaya 《Physics of the Solid State》2013,55(5):1063-1069
The size and concentration of α-quartz nanocrystals dispersed in samples of pseudotachylite and the internal stresses in these nanocrystals have been determined using infrared spectroscopy in the temperature range 300–800 K. Pseudotachylite is a product of intense crushing of granite that undergoes in the Earth’s crust faults. It has been found that the size of the nanocrystals is ~20 nm and does not depend on temperature. As the temperature increases, their concentration decreases monotonically and tends to zero at ~650 K. This process is paralleled by a growth of the concentration of β-quartz nanocrystals. The α-quartz nanocrystal concentration regains its initial level with decreasing temperature. Thus, the α → β phase transition in quartz nanocrystals in pseudotachylite starts at temperatures lower by ~500 K than that in the bulk of the macrocrystal (846 K), and is stretched by ~350 K. At room temperature, the unit cell of nanocrystals is compressed by surface tension forces. These forces retard the α → β phase transition. The thermal expansion coefficient of nanocrystals is larger than that of macrocrystals, which entails a decrease of compression and a monotonic decrease of the concentration of α-quartz nanocrystals with increasing temperature. 相似文献
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K. Tompa P. Bánki M. Bokor G. Lasanda L. K. Varga Y. Champion L. Takács 《Applied magnetic resonance》2004,27(1-2):93-107
Quadrupole effects in room-temperature continuous-wave (CW)63Cu nuclear magnetic resonance (NMR) spectra, “π/2” pulse length shortening and amplitudes of two-pulse generated echoes were investigated on nanocrystalline copper powders produced by cryogenic melting and by ball milling techniques. Systematic measurements on the parent polycrystalline copper and on copper-based copper-palladium dilute alloys on the basis of the same experimental techniques were also made and the results were compared to that of the nanophase samples. Fractions of Cu nuclei contributing to the specific NMR responses and average field gradients coming from noncubic neighborhoods were estimated in all the investigated cases. The satellite and/or central component origins of NMR spectra of the samples are not a priori trivial even in the simplest case. Comparative analysis of CW and pulsed experiments allowed a surprising technological conclusion to be drawn by finding a smaller chemical impurity content inside the nanograins than the value characteristic of the entire sample. The measured decrease of the impurity concentration in the inner region of the copper nanoparticles is the consequence of the applied technologies: cryogenic melting and ball milling. These preparation methods cause grain-boundary segregation and result in a cleaner inside of copper nanoparticles. 相似文献
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在卤化铜激光器脉冲放电电源系统中,为减轻闸流管的工作负担并同时保证电路的快放电激励,采用了单级磁脉冲压缩器.分析了磁脉冲压缩器的工作机理,介绍了所采用磁脉冲压缩器的设计考虑及参数选用.
关键词:
磁脉冲压缩器
闸流管
CuBr激光器 相似文献
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在15 mL的不锈钢反应釜中,利用无水三氯化铝与叠氮化钠在无溶剂的条件下直接反应,合成出了六方结构氮化铝泡沫材料,反应温度650 ℃,反应时间3 h.扫描电子显微镜测试结果显示,该试样呈现泡沫状外貌特征.X射线衍射结果表明该试样为六方结构.不同温度条件下的吸收谱表明在202 nm附近存在尖锐的吸收峰.红外吸收谱中存在1381 cm-1和730 cm-1两个吸收峰.同时,提出了六方结构氮化铝泡沫材料的合成机理.
关键词:
六方氮化铝泡沫材料
合成机理
X射线衍射 相似文献
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A. Orlová 《Czechoslovak Journal of Physics》1970,20(9):985-993
It has been shown that in the high-temperature creep test at 0·85 kp mm?2 and 0·7T m (T m is the melting temperature in °K) the dislocation substructure of copper single-slip oriented crystals is formed practically at the early stage of the test. The subsequent changes of substructure consist in the growth of subgrain misorientation and in the transformation of tangled subgrain boundaries into regular dislocation networks. There is a pronounced tendency to the formation of subboundaries in the {111} and {110} planes. 相似文献