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871.
Catalytic activity and aromatic selectivity of n‐butane transformation were studied over various MFI type zeolites. From the data obtained, a reaction mechanism is suggested for different catalyst systems. It is visualized that in gallium doped catalysts, Ga3+ directly takes part both in cracking and dehydrogenation. The [Ga CH3]2+ and [GaH]2+ species formed during cracking and dehydrogenation require protonic sites for regeneration of Ga3+ species. An alternative mechanism was suggested for dehydrogenation and cracking by Ga3+ without the involvement of protonic sites. However a protonic site would be required for aromatization. In case of gallosilicates a one step mechanism is suggested for cracking and dehydrogenation reaction which does not require the presence of protonic sites in the catalyst system. 相似文献
872.
β-sialon ceramics sintered with yttria additives have been studied with the use of an electron probe X-ray analysis (EPMA).
Sialon ceramics were prepared from a carbothermally derived β-sialon powder and then sintered in a nitrogen atmosphere with
yttria admixture. The above process was followed by annealing in flowing nitrogen. Scanning electron microscope (SEM) observations
have shown that the sintered material contains a glassy phase (Y-Si-Al-O-N) on the grain boundaries. X-ray diffraction (XRD)
after annealing in nitrogen revealed the presence of a considerable amount of yttrium aluminium garnet (YAG). The higher voltage
of 30 kV was used in order to excite the yttrium Kα radiation (14.96 keV) at an appropriate overvoltage ratio because in some phases of the material, the disappearance of the
yttrium Lα line has been observed during EPMA examination at an accelerating voltage of 15 kV in energy dispersive spectra (EDS). The
intensity of the yttrium Kα line was sufficiently high, while the Y Lα line was not seen in the ED spectrum. Because the position of the yttrium Lα line (1.922 keV) is very close to the Si (K) absorption edge (1.84 keV), the strong absorption at this edge is probably responsible
for the effect. This result should be considered as a serious warning in the case of EPMA (EDS) studies on compounds or mixtures
suspected to contain both silicon and yttrium, because at electrons energies lower than 15 keV, the presence of yttrium in
materials can go unnoticed. In wavelength dispersive spectra (WDS) obtained at 15 keV the intensity of the yttrium Lα line was also very low but measurable. 相似文献
873.
Y. Komeiji 《Journal of Molecular Structure》2000,530(3):237-243
Methods by which to determine conditions for a molecular dynamics (MD) simulation of biological molecules were investigated. Derivation of the optimal parameters of the Ewald summation was described so as to give same precision to the real space, the reciprocal space summations and the van der Waals interaction. Later, the procedure by which to determine the condition of the multiple time step method by RESPA (REference System Propagator Algorithm; Tuckerman et al., 1992, J. Chem. Phys., 97, 1990) was described as exemplified by MD simulations of a solvated β-sheet peptide. The conservation of the total energy in a microcanonical ensemble was measured to investigate the stability of the simulation conditions. The most feasible respective combinations of the time steps were: 0.25 fs for bond, angle and torsion interactions; 2 fs for van der Waals interaction and Ewald real-space summation; and 4 fs for Ewald reciprocal-space summation. Though it retained an acceptable accuracy, this condition accelerated the simulation ten-fold compared to that in which a simple velocity-Verlet method with a time step of 0.25 fs was used. The update of the correction term due to excluded neighbors was then investigated. Better results were obtained when the correction was updated with the real-space than when it was updated with the reciprocal-space summation. Finally, an MD simulation as long as 50 ps performed under the optimal Ewald and RESPA parameters was thus determined. The trajectory showed a good stability, indicating the feasibility of the parameters. 相似文献
874.
用Level Set算法模拟孤立波与前台阶的相互作用 总被引:1,自引:0,他引:1
进一步研究了Level Set方法的数学基础,研究了求解有自由水面水动力学问题的具体方法。并在此基础上,对孤立波与前台阶相互作用这一问题进行了计算及研究,取得了与实验结果相符合的计算结果。 相似文献
875.
876.
自然背景下人工目标的检测与分割 总被引:8,自引:1,他引:7
在分水岭分割算法的基础上 ,提出基于模糊逻辑理论的集水盆合并方法 ,对自然背景下人工目标的检测与分割以及自然背景的抑制与消除进行了探讨和研究。 相似文献
877.
P. W. M. Tsang W. T. Lee 《Journal of Visual Communication and Image Representation》2000,11(4):343-359
A novel image compression scheme based on two-dimensional adaptive decimation is reported in this paper. In this approach, images are encoded with adaptive sampling along the horizontal and vertical directions, and decoded with an edge prediction interpolation algorithm. The method is capable of maintaining reasonable coding fidelity at low bit-rate with good visual quality. As only a small amount of computation is required in the encoding and decoding processes, the compression scheme can be implemented for real time operation with simple hardware and a small amount of memory storage. The proposed scheme had been applied in encoding images at bit-rates between 0.2 and 0.33 bpp and the results are encouraging. 相似文献
878.
发射极电流集边效应理论的SPICE模拟验证 总被引:4,自引:0,他引:4
发射极电流集边效应是由晶体管基极电阻的自偏压引起的 ,描述该效应的微分方程早已建立 ,这里设计了一种用 SPICE验证精确解 /近似解的新方法 .结果表明 :近似解是在 V( x) VT 的前提条件下求得的 ,仅在弱注入区和中等注入区适用 ,不能给出在强注入区和极强注入区的归一化电位和电流密度的分布函数 ;相反 ,基于精确解的理论却能给出从弱注入区到极强注入区的归一化电位和电流密度的分布函数 ,且与用 SPICE模拟得到的结果吻合 .结论是 :从整体来看 ,精确解的理论比近似解的理论描述该效应要稍好一些 ,因为前者的适用范围比后者宽 .应当强调指出 ,该效应 相似文献
879.
X-ray absorption near the edge structure and x-ray photoelectron spectroscopy studies on pyrite prepared by thermally sulfurizing iron films 下载免费PDF全文
This paper reports how pyrite films were prepared by thermal
sulfurization of magnetron sputtered iron films and characterized by
x-ray absorption near edge structure spectra and x-ray photoelectron
spectroscopy on a 4B9B beam line at the Beijing Synchrotron Radiation
Facility. The band gap of the pyrite agrees well with the optical
band gap obtained by a spectrophotometer. The octahedral symmetry of
pyrite leads to the splitting of the d orbit into t_{\rm 2g} and
e_{\rm g} levels. The high spin and low spin states were analysed
through the difference of electron exchange interaction and the orbital
crystal field. Only when the crystal field splitting is higher than
1.5 eV, the two weak peaks above the white lines can appear, and this
was approved by experiments in the present work. 相似文献
880.
Influence of dislocations in the GaN layer on the electrical properties of an AlGaN/GaN heterostructure 下载免费PDF全文
This paper reports on a comparative study of the spatial
distributions of the electrical, optical, and structural properties
in an AlGaN/GaN heterostructure. Edge dislocation density in the GaN
template layer is shown to decrease in the regions of the wafer
where the heterostructure sheet resistance increases and the GaN
photoluminescence band-edge energy peak shifts to a high wavelength.
This phenomenon is found to be attributed to the local compressive
strain surrounding edge dislocation, which will generate a local
piezoelectric polarization field in the GaN layer in the opposite
direction to the piezoelectric polarization field in the AlGaN layer and
thus help to increase the two-dimensional electron gas
concentration. 相似文献