全文获取类型
收费全文 | 4441篇 |
免费 | 567篇 |
国内免费 | 168篇 |
专业分类
化学 | 3524篇 |
晶体学 | 41篇 |
力学 | 149篇 |
综合类 | 20篇 |
数学 | 388篇 |
物理学 | 1054篇 |
出版年
2024年 | 9篇 |
2023年 | 60篇 |
2022年 | 92篇 |
2021年 | 101篇 |
2020年 | 157篇 |
2019年 | 157篇 |
2018年 | 114篇 |
2017年 | 97篇 |
2016年 | 193篇 |
2015年 | 197篇 |
2014年 | 232篇 |
2013年 | 272篇 |
2012年 | 318篇 |
2011年 | 351篇 |
2010年 | 252篇 |
2009年 | 236篇 |
2008年 | 268篇 |
2007年 | 227篇 |
2006年 | 230篇 |
2005年 | 201篇 |
2004年 | 187篇 |
2003年 | 175篇 |
2002年 | 162篇 |
2001年 | 92篇 |
2000年 | 107篇 |
1999年 | 81篇 |
1998年 | 54篇 |
1997年 | 45篇 |
1996年 | 55篇 |
1995年 | 50篇 |
1994年 | 51篇 |
1993年 | 38篇 |
1992年 | 36篇 |
1991年 | 28篇 |
1990年 | 19篇 |
1989年 | 17篇 |
1988年 | 16篇 |
1987年 | 18篇 |
1986年 | 15篇 |
1985年 | 23篇 |
1984年 | 19篇 |
1983年 | 10篇 |
1982年 | 10篇 |
1981年 | 18篇 |
1980年 | 12篇 |
1979年 | 7篇 |
1978年 | 7篇 |
1977年 | 12篇 |
1974年 | 13篇 |
1973年 | 5篇 |
排序方式: 共有5176条查询结果,搜索用时 515 毫秒
941.
Shuichi Kawashima Chi‐Kun Lin Jun‐ichi Segata 《Mathematical Methods in the Applied Sciences》2012,35(2):125-133
We consider the initial value problem for some nonlinear hyperbolic‐dispersive systems in one space dimension. Combining the classical energy method and the smoothing estimates for the Airy equation, we guarantee the time local well‐posedness for this system. We also discuss the extension of our results to more general hyperbolic‐dispersive system. Copyright © 2011 John Wiley & Sons, Ltd. 相似文献
942.
We consider two related problems, the Minimum Bounded Degree Matroid Basis problem and the Minimum Bounded Degree Submodular Flow problem. The first problem is a generalization of the Minimum Bounded Degree Spanning Tree problem: We are given a matroid and a hypergraph on its ground set with lower and upper bounds f(e)≤g(e) for each hyperedge e. The task is to find a minimum cost basis which contains at least f(e) and at most g(e) elements from each hyperedge e. In the second problem we have a submodular flow problem, a lower bound f(v) and an upper bound g(v) for each node v, and the task is to find a minimum cost 0–1 submodular flow with the additional constraint that the sum of the incoming and outgoing flow at each node v is between f(v) and g(v). Both of these problems are NP-hard (even the feasibility problems are NP-complete), but we show that they can be approximated in the following sense. Let opt be the value of the optimal solution. For the first problem we give an algorithm that finds a basis B of cost no more than opt such that f(e)?2Δ+1≤|B∩e|≤g(e)+2Δ?1 for every hyperedge e, where Δ is the maximum degree of the hypergraph. If there are only upper bounds (or only lower bounds), then the violation can be decreased to Δ?1. For the second problem we can find a 0–1 submodular flow of cost at most opt where the sum of the incoming and outgoing flow at each node v is between f(v)?1 and g(v)+1. These results can be applied to obtain approximation algorithms for several combinatorial optimization problems with degree constraints, including the Minimum Crossing Spanning Tree problem, the Minimum Bounded Degree Spanning Tree Union problem, the Minimum Bounded Degree Directed Cut Cover problem, and the Minimum Bounded Degree Graph Orientation problem. 相似文献
943.
Jaciel Robles-Nu?ez Fernando Chi?as-Castillo Manuel Sanchez-Rubio Javier Lara-Romero Rafael Huirache-Acu?a Sergio Jimenez-Sandoval Gabriel Alonso-Nu?ez 《Chemical Papers》2012,66(12):1130-1136
MoS2 sheathed carbon nanotubes have been successfully synthesized using a hydrothermal route under controlled conditions. The resultant material was studied by XRD, EDS, HRTEM, and Raman spectroscopy. Advantages of the preparation presented here compared to other methods are: a) lower reaction temperature, b) high yield of sheathed nanotubes including ends and full body, c) simple process with non-toxic materials, and d) no damage inflicted to nanotubes. 相似文献
944.
Zhang C Shephard NE Rhodes SM Chen Z 《Langmuir : the ACS journal of surfaces and colloids》2012,28(14):6052-6059
Sum frequency generation (SFG) vibrational spectroscopy was used to study the effect of silane headgroups on the molecular interactions that occur between poly(ethylene terephthalate) (PET) and various epoxy silanes at the PET/silane and PET/silicone interfaces. Three different silanes were investigated: (3-glycidoxypropyl) trimethoxysilane (γ-GPS), (3-glycidoxypropyl) methyl-dimethoxysilane (γ-GPMS), and (3-glycidoxypropyl) dimethyl-methoxysilane (γ-GPDMS). These silanes share the same backbone and epoxy end group but have different headgroups. SFG was used to examine the interfaces between PET and each of these silanes, as well as silanes mixed with methylvinylsiloxanol (MVS). We also examined the interfaces between PET and uncured or cured silicone with silanes or silane-MVS mixtures. Silanes with different headgroups were found to exhibit a variety of methoxy group interfacial segregation and ordering behaviors at various interfaces. The effect of MVS was also dependent upon silane headgroup choice, and the interfacial molecular structures of silane methoxy headgroups were found to differ at PET/silane and PET/silicone interfaces. Epoxy silanes have been widely used as adhesion promoters for polymer adhesives; therefore, the molecular structures probed using SFG were correlated to adhesion testing results to understand the molecular mechanisms of silicone-polymer adhesion. Our results demonstrated that silane methoxy headgroups play important roles in adhesion at the PET/silicone interfaces. The presence of MVS can change interfacial methoxy segregation and ordering, leading to different adhesion strengths. 相似文献
945.
电厂烟气脱硝催化剂VO-WO/TiO失活机理研究 《燃料化学学报》2012,40(11):1359-1365
分别以某电厂失活和新鲜的烟气选择性催化还原(SCR)脱硝催化剂为研究对象,模拟测试催化剂的脱硝效率,并采用扫描电子显微镜-能量色散X射线光谱(SEM-EDX)、X荧光光谱(XRF)、X光电子能谱(XPS)、氮气吸脱附、傅里叶红外光谱(FT-IR)、X射线衍射(XRD)和热重分析(TG)等手段对催化剂的微观结构和表面形态进行表征,探讨了SCR催化剂的失活机制。结果表明,380℃下失活催化剂的脱硝效率(35.0%)和比表面积(1.05 m2/g)明显低于新鲜催化剂(88.2%,72.50 m2/g)。与新鲜催化剂相比,失活催化剂中V5+的比例由17.4%升高到32.2%,并且表面出现了大量的Al2(SO4)3。另外,SEM和XRD结果表明,失活催化剂出现了烧结。催化剂表面V价态的改变、高温烧结和表面Al2(SO4)3相的大量生成是催化剂失活的主要原因。 相似文献
946.
Wang Y Corbitt TS Jett SD Tang Y Schanze KS Chi EY Whitten DG 《Langmuir : the ACS journal of surfaces and colloids》2012,28(1):65-70
The bactericidal mechanisms of poly(phenylene ethynylene) (PPE)-based cationic conjugated polyelectrolytes (CPE) and oligo-phenylene ethynylenes (OPE) were investigated using electron/optical microscopy and small-angle X-ray scattering (SAXS). The ultrastructural analysis shows that polymeric PPE-Th can significantly remodel the bacterial outer membrane and/or the peptidoglycan layer, followed by the possible collapse of the bacterial cytoplasm membrane. In contrast, oligomeric end-only OPE (EO-OPE) possesses potent bacteriolysis activity, which efficiently disintegrates the bacterial cytoplasm membrane and induces the release of bacterial cell content. Using single giant vesicles and SAXS, we demonstrated that the membrane perturbation mechanism of EO-OPE against model bacterial membranes results from a 3D membrane phase transition or perturbation. 相似文献
947.
Yan-Hung Chen Meng-Chun Chi Tzu-Fan Wang Jui-Chang Chen Long-Liu Lin 《Applied biochemistry and biotechnology》2012,166(7):1711-1722
The aim of this investigation was to synthesize the adipic acid-modified magnetic nanoparticles for the efficient immobilization
of C-terminally lysine-tagged α-amylase (BACΔNC-Lys7) from thermophilic Bacillus sp. strain TS-23. The carboxylated magnetic nanoparticles were prepared by the simple co-precipitation of Fe3+/Fe2+ in aqueous medium and then subsequently modified with adipic acid. Transmission electron microscopy micrographs showed that
the carboxylated magnetic nanoparticles remained discrete and had no significant change in size after the binding of BACΔNC-Lys7. Free enzyme was active in the temperature range of 45–70 °C and had an optimum of 60 °C, whereas the thermal stability of
BACΔNC-Lys7 was improved as a result of immobilization. The immobilized BACΔNC-Lys7 could be recycled 20 times without a significant loss of the amylase activity and had a better stability during storage with
respect to free enzyme. Taken together, the magnetic nanoparticles coated with this functional moiety can be a versatile platform
for the effective manipulation of various kinds of engineered proteins. 相似文献
948.
采用磁控溅射法制备了金属Cr膜, 并利用太赫兹时域光谱法获得了其光学参数. 利用Cr膜的光学参数计算了其相位穿透深度, 设计了基于低温GaAs 的全金属平面微腔光电导太赫兹辐射器件. 模拟结果表明: 器件的谐振频率分别为0.32, 0.65, 0.98, 1.31和1.65 THz, 与自由空间的光电导太赫兹谱相比, 在谐振频率为0.32 THz处的峰值强度提高了25倍, 光谱半高全宽压缩了50倍. 讨论了辐射偶极子与腔内驻波场之间的耦合强度对器件辐射强度的影响, 发现当辐射中心位于驻波场波腹处时, 器件辐射最强, 位于波节处时辐射被严重抑制. 太赫兹波段微腔效应的研究对于实现单色性好, 连续调谐, 高效高辐射强度的太赫兹源具有一定的理论意义. 相似文献
949.
利用混合物理化学气相沉积法(hybrid physical-chemical vapor deposition, HPCVD)可以制备出高性能的MgB2超导薄膜, 再对薄膜进行钛(Ti)离子辐照处理.经过辐照处理后的样品被掺入了Ti元素, 与未处理的干净MgB2样品相比,其超导转变温度没有出现大幅度的下降, 而在外加磁场下的临界电流密度得到了明显的提高,同时样品的上临界磁场也得到了提高. 在温度5 K, 外加垂直磁场为4 T的情况下, Ti离子辐照剂量为1× 1013/cm2的样品的临界电流密度达到了1.72× 105 A/cm2, 比干净的MgB2要高出许多,而其超导转变温度仍能维持在39.9 K的较高水平. 相似文献
950.
A novel piezochromic fluorescent (PCF) compound with aggregation-induced emission (AIE) effect and morphology-alterable emission
property was developed. The amorphous and crystalline aggregates were obtained, and their spectroscopic properties and morphological
structures were reversibly and repeatedly exhibited upon pressing (fuming) or annealing. The piezochromic fluorescent nature
was generated through crystalline-amorphous phase transformation. It was proposed that AIE compounds existing a twisted propeller-shaped
conformation will exhibit PCF activity. The common relationship betweeen AIE and PCF established will guide researchers in
identifying and synthesizing more piezochromic fluorescent materials. 相似文献