全文获取类型
收费全文 | 2489篇 |
免费 | 99篇 |
国内免费 | 1篇 |
出版年
2023年 | 20篇 |
2022年 | 14篇 |
2021年 | 22篇 |
2020年 | 49篇 |
2019年 | 61篇 |
2018年 | 84篇 |
2017年 | 71篇 |
2016年 | 67篇 |
2015年 | 67篇 |
2014年 | 56篇 |
2013年 | 259篇 |
2012年 | 98篇 |
2011年 | 98篇 |
2010年 | 72篇 |
2009年 | 55篇 |
2008年 | 81篇 |
2007年 | 92篇 |
2006年 | 82篇 |
2005年 | 52篇 |
2004年 | 70篇 |
2003年 | 43篇 |
2002年 | 47篇 |
2001年 | 121篇 |
2000年 | 118篇 |
1999年 | 75篇 |
1998年 | 17篇 |
1997年 | 29篇 |
1996年 | 24篇 |
1994年 | 20篇 |
1992年 | 40篇 |
1991年 | 55篇 |
1990年 | 40篇 |
1989年 | 42篇 |
1988年 | 42篇 |
1987年 | 28篇 |
1986年 | 18篇 |
1985年 | 23篇 |
1984年 | 22篇 |
1983年 | 18篇 |
1981年 | 15篇 |
1979年 | 20篇 |
1976年 | 15篇 |
1975年 | 13篇 |
1974年 | 16篇 |
1973年 | 16篇 |
1971年 | 17篇 |
1970年 | 14篇 |
1969年 | 15篇 |
1968年 | 16篇 |
1967年 | 16篇 |
排序方式: 共有2589条查询结果,搜索用时 15 毫秒
291.
In this study, (Gd1? x Nd x )2(Zr1? x Ce x )2O7 (0 ≤ x ≤ 0.5) ceramics have been prepared by pressureless sintering at 1973 K to investigate the influence of Nd and Ce co-doping on their electrical conductivity. The electrical conductivity of the ceramics was investigated by impedance spectroscopy measurements from 723 to 1173 K over the frequency range of 20 Hz to 2 MHz in air. The measured values obey the Arrhenius relation. For each composition, the grain conductivity gradually increases with increasing temperature from 723 to 1173 K. At a given temperature, it gradually decreases with increasing neodymium and cerium contents from x = 0 to 0.3; thereafter, the grain conductivity exhibits a slight increase with further increasing neodymium and cerium contents up to x = 0.5. 相似文献
292.
H. Y. Uzunok Ertuǧrul Karaca A. Başoǧlu G. P. Srivastava 《Philosophical Magazine Letters》2013,93(9):375-391
ABSTRACTWe present results for the structural, electronic, vibrational, and electron-phonon coupling properties of LaSn3 and CaSn3 adopting the simple cubic AuCu3-type structure obtained using the the generalised gradient approximation of the density functional theory and plane wave ab initio pseudopotential method. Our electronic results show that both materials display metallic character with several bands, which have mainly Sn 5p character, crossing the Fermi level. The calculated phonon spectrum of LaSn3 accords very well with reported experimental measurements. The weights of the peaks in the Eliashberg spectral function of both compounds are enhanced with the use of experimental lattice constant in our electron-phonon calculation, increasing the value of average electron phonon coupling parameter from 0.876 to 0.937 for LaSn3 (by 7%) and from 0.642 to 0.725 for CaSn3 (by 13%). The use of experimental lattice constant also improves the agreement between theoretical and experimental values of the superconducting temperature for both compounds. 相似文献
293.
M.H. Hong P. Pirouz J. Chung S. Y. Yoon J. L. Demenet 《Philosophical Magazine Letters》2013,93(12):823-831
Single-crystal 15R-SiC boules have been successfully grown by sublimation. The Vickers hardness of a Si-terminated (0001) face has been measured in the temperature range 25-1300C. As expected, the hardness decreases with increasing temperature from about 30GPa at room temperature to about 10GPa at 1300C. The fracture toughness is estimated to be about 1.0MPam 1/2 at room temperature. Transmission electron microscopy investigation of the dislocations introduced by indentation at 900 and 1300C shows that they are activated predominantly on the basal plane. Most of them consist of a single leading partial without the corresponding trailing partial. 相似文献
294.
High-pressure X-ray diffraction studies on gadolinium sesquioxide (Gd2O3) have been carried out up to a pressure of ~25 GPa in a diamond-anvil cell at room temperature. Gadolinium oxide, which has a cubic or bixbyite structure under ambient conditions, undergoes an irreversible structural phase at around 12 GPa. The high-pressure phase has been identified as a hexagonal La2O3-type structure. The bulk modulus and its pressure derivative of this phase have been calculated. 相似文献
295.
Y. Wang S.L. Shang H. Zhang L.-Q. Chen Z.-K. Liu 《Philosophical Magazine Letters》2013,93(12):851-859
To understand the Invar anomalies, such as negative thermal expansion and spontaneous magnetization, we have applied our recently developed thermodynamic framework for a system with itinerant-electron magnetism to the ordered Fe3Pt. The framework has coherently predicted the finite temperature intermixing between the fully ferromagnetic (FM) configuration and the spin-flipping configurations (SFCs). We have also discovered a tri-critical point at which a high-temperature second-order phase transition, between the fully ordered FM phase and the paramagnetic phase which is disordered due to SFCs, becomes first order at low temperatures. 相似文献
296.
The diffusion and surface oxidation rates of Fe deposited on Si and barrier layers of Al/SiO2 and Al2O3/SiO2 have been comparatively studied and correlated with the growth of carbon nanotubes (CNTs). Initially, Fe/Si, Fe/Al/SiO2/Si and Fe/Al2O3/SiO2/Si samples were subjected to thermal chemical vapour deposition (CVD) at ~650°C for ~30?min to grow the CNTs. Scanning electron microscopy analysis showed that the height of the CNTs on the Fe/Al2O3/SiO2/Si samples was relatively high (~9.5–11?µm), as compared with the other samples. To investigate this, a few as-prepared samples were thermally annealed at ~650°C for ~30?min and characterized by dynamic secondary ion mass spectroscopy (D-SIMS) and X-ray photoelectron spectroscopy (XPS). The D-SIMS results showed that the diffusion depth, x Fe, and magnitude of the diffusivity, D Fe, of the Fe atoms are highest for the Fe/Si sample. This is attributed to vacancy-mediated migration, which leads to the formation of unstable, non-stoichiometric Fe–Si and Fe–O–Si phases. However, for the Fe/Al2O3/SiO2/Si samples, the magnitudes of x Fe and D Fe are found to be the lowest, which indicates steric hindrance to Fe by the Al2O3 layers. The XPS analysis revealed that the surface metallic state, after annealing, is almost unaffected for the Fe/Al2O3/SiO2/Si samples, whereas the majority of the Fe precipitate was observed to be oxidized in the case of the other samples. 相似文献
297.
A key challenge in the fabrication of superhard nanocomposite films is how to control the distribution of grain sizes in these materials. High-resolution transmission electron microscopy has been used to measure nanograin-size distributions in the Ti–B–N films with various B contents. The results show that the mean grain size decreases with increase of B content and the grain-size distribution conforms to a log-normal function when the hardness approaches a maximum value. The transition from normal to log-normal distributions can be determined by analysis in terms of a minimum information criterion. The origin of a log-normal size distribution probably results from heterogeneity arising from a diffusion-drift process. 相似文献
298.
One-dimensional silicon nanowires have been grown by thermal evaporation and their growth orientations determined by transmission electron microscopy studies. The nanowires, which are often highly curved in morphology and heavily twinned in microstructure, are crystallographically separated into several sections, each with a characteristic crystallographic orientation along the wire axis. Straight nanowires, or straight sections in a curved nanowire, are found to have non-unique crystallographic orientations when {111} twinning occurs. 相似文献
299.
The electrostatic potential and associated space charge across a 24°, [001] tilt boundary of SrTiO3 ceramic have been determined using electron holography. The results reveal a positive charged interface with a negative space charge on either side. The form of the double Schottky barrier and the local charge-density distribution at the interface are derived from the results. 相似文献
300.
Complete solid-state amorphization has been realized in elemental Se by means of mechanical milling of crystalline Se powder. Quantitative X-ray diffraction and thermal analyses were employed to characterize the amorphization process and indicated that the amorphization onset corresponds to a critical crystallite size and a drop in microstrain. During the major amorphization process, the remaining crystallite size remains unchanged with a constant lattice expansion. A new kinetics model of crystallite destabilization is proposed for the solid-state amorphization which satisfactorily explains the experimental observations. 相似文献