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991.
高分子形状记忆材料近年来吸引了许多研究者的目光, 因其低廉的成本、优异的加工性能、良好的回复性、多变的力学和物理性能等优势迅速地发展起来. 但随着石油紧缺和全球暖化等问题, 开发绿色、可降解的生物高分子形状记忆材料成为新的发展趋势. 其中, 绿色材料聚乳酸以其优异的力学强度、生物降解性和生物相容性, 在可降解的生物高分子形状记忆材料的研究和应用方面有很大的发展前景. 本文主要就生物可降解高分子形状记忆材料的发展现状、形状记忆机理、材料选择和国内外最新研究进展等进行了介绍、评述和展望. 相似文献
992.
993.
Huang PY Ruiz-Vargas CS van der Zande AM Whitney WS Levendorf MP Kevek JW Garg S Alden JS Hustedt CJ Zhu Y Park J McEuen PL Muller DA 《Nature》2011,469(7330):389-392
The properties of polycrystalline materials are often dominated by the size of their grains and by the atomic structure of their grain boundaries. These effects should be especially pronounced in two-dimensional materials, where even a line defect can divide and disrupt a crystal. These issues take on practical significance in graphene, which is a hexagonal, two-dimensional crystal of carbon atoms. Single-atom-thick graphene sheets can now be produced by chemical vapour deposition on scales of up to metres, making their polycrystallinity almost unavoidable. Theoretically, graphene grain boundaries are predicted to have distinct electronic, magnetic, chemical and mechanical properties that strongly depend on their atomic arrangement. Yet because of the five-order-of-magnitude size difference between grains and the atoms at grain boundaries, few experiments have fully explored the graphene grain structure. Here we use a combination of old and new transmission electron microscopy techniques to bridge these length scales. Using atomic-resolution imaging, we determine the location and identity of every atom at a grain boundary and find that different grains stitch together predominantly through pentagon-heptagon pairs. Rather than individually imaging the several billion atoms in each grain, we use diffraction-filtered imaging to rapidly map the location, orientation and shape of several hundred grains and boundaries, where only a handful have been previously reported. The resulting images reveal an unexpectedly small and intricate patchwork of grains connected by tilt boundaries. By correlating grain imaging with scanning probe and transport measurements, we show that these grain boundaries severely weaken the mechanical strength of graphene membranes but do not as drastically alter their electrical properties. These techniques open a new window for studies on the structure, properties and control of grains and grain boundaries in graphene and other two-dimensional materials. 相似文献
994.
Owing to its high carrier mobility and saturation velocity, graphene has attracted enormous attention in recent years. In particular, high-performance graphene transistors for radio-frequency (r.f.) applications are of great interest. Synthesis of large-scale graphene sheets of high quality and at low cost has been demonstrated using chemical vapour deposition (CVD) methods. However, very few studies have been performed on the scaling behaviour of transistors made from CVD graphene for r.f. applications, which hold great potential for commercialization. Here we report the systematic study of top-gated CVD-graphene r.f. transistors with gate lengths scaled down to 40 nm, the shortest gate length demonstrated on graphene r.f. devices. The CVD graphene was grown on copper film and transferred to a wafer of diamond-like carbon. Cut-off frequencies as high as 155 GHz have been obtained for the 40-nm transistors, and the cut-off frequency was found to scale as 1/(gate length). Furthermore, we studied graphene r.f. transistors at cryogenic temperatures. Unlike conventional semiconductor devices where low-temperature performance is hampered by carrier freeze-out effects, the r.f. performance of our graphene devices exhibits little temperature dependence down to 4.3 K, providing a much larger operation window than is available for conventional devices. 相似文献
995.
考虑了一类非齐次热传导方程的逆时问题,它是个典型的不适定问题.通过将方程的非齐次项和T时刻的温度场u(x,T)作Fourier展开,构造出正则化的近似问题,从而获得原逆时问题的正则化解,并给出了正则化解的稳定性估计和收敛性估计.最后,用数值例子说明该正则化方法是可行的. 相似文献
996.
Spatio-temporal transcriptome of the human brain 总被引:1,自引:0,他引:1
997.
以2-乙炔基吡啶、苄基氯、叠氮化钠为原料通过Click Chemistry的方法,合成一种新的"Click"配体,并对其结构进行X-射线单晶衍射表征.再与后过渡金属NiBr2(DME)作用,合成其金属配合物,对其结构进行红外光谱、元素分析表征. 相似文献
998.
Mg、Cd掺杂AlN电子结构的第一性原理计算 总被引:2,自引:0,他引:2
基于密度泛函理论(DFT)框架下的第一性原理的平面波超软赝势方法(USPP),对Mg,Cd掺杂AlN的32原子超原胞体系进行了几何结构优化,从理论上给出了掺杂和 掺杂体系的晶体结构参数.计算了掺杂AlN晶体的结合能,电子态密度,差分电荷密度,并对计算结果进行了细致的分析.计算结果表明,相对于Cd,Mg在AlN晶体中可以提供更多的空穴,有利于形成更好的p型电导. 相似文献
999.
在无水无氧条件下利用金属钾在乙醚中还原LNiCl2得到了单核镍化合物NiL2(L=2,4,6-三甲基苯基丁二亚胺),用X-射线单晶衍射仪测定了该化合物与LNiCl2-THF的晶体结构.结果表明:NiL2中镍原子与2个配体中的4个氮原子形成4配位结构,该结构中配体化合价为-1价. 相似文献