全文获取类型
收费全文 | 14488篇 |
免费 | 1950篇 |
国内免费 | 1891篇 |
专业分类
化学 | 9298篇 |
晶体学 | 52篇 |
力学 | 954篇 |
综合类 | 158篇 |
数学 | 3303篇 |
物理学 | 4564篇 |
出版年
2024年 | 37篇 |
2023年 | 231篇 |
2022年 | 349篇 |
2021年 | 472篇 |
2020年 | 733篇 |
2019年 | 510篇 |
2018年 | 500篇 |
2017年 | 567篇 |
2016年 | 740篇 |
2015年 | 592篇 |
2014年 | 998篇 |
2013年 | 1240篇 |
2012年 | 788篇 |
2011年 | 974篇 |
2010年 | 820篇 |
2009年 | 936篇 |
2008年 | 936篇 |
2007年 | 922篇 |
2006年 | 799篇 |
2005年 | 687篇 |
2004年 | 595篇 |
2003年 | 634篇 |
2002年 | 572篇 |
2001年 | 431篇 |
2000年 | 397篇 |
1999年 | 248篇 |
1998年 | 235篇 |
1997年 | 186篇 |
1996年 | 173篇 |
1995年 | 154篇 |
1994年 | 138篇 |
1993年 | 118篇 |
1992年 | 112篇 |
1991年 | 47篇 |
1990年 | 54篇 |
1989年 | 66篇 |
1988年 | 51篇 |
1987年 | 39篇 |
1986年 | 37篇 |
1985年 | 53篇 |
1984年 | 41篇 |
1983年 | 13篇 |
1982年 | 26篇 |
1981年 | 20篇 |
1980年 | 13篇 |
1979年 | 13篇 |
1978年 | 6篇 |
1977年 | 9篇 |
1976年 | 6篇 |
1971年 | 2篇 |
排序方式: 共有10000条查询结果,搜索用时 31 毫秒
71.
72.
73.
74.
Zhongxian Song Yun Xing Tingji Zhang Jinggang Zhao Junkai Wang Yanli Mao Baolin Zhao Xuejun Zhang Min Zhao Ziang Ma 《应用有机金属化学》2020,34(3):e5446
A series of Ce-Fe-Ox catalysts prepared by the different calcination temperatures (marked as CF-X, where X represented calcination temperature) were used to the selectivity catalytic reduction of NOx by NH3. The results explained the relationship between calcination temperature and the sulfate species over Ce-Fe-Ox, and then investigated the surface acidity and catalytic performance. The large amounts of sulfate species were formed over CF-450 and CF-550 while it was decomposed with further the increasing of calcination temperature, which resulted in the loss of surface acidity, causing a decrease in the catalytic activity over Ce-Fe-Ox. Thereby, the CF-450 catalyst showed the best catalytic activity and over 90% NOx conversion was obtained at 244–450 °C. Besides, the favored pore structure, more Fe3+ active species, higher Ce3+ concentration and the abundance of chemical adsorbed oxygen species, as well as the surface acid sites, would together contribute to the excellent catalytic activity of CF-450 catalyst. 相似文献
75.
The strategy of structurally integrating noble metal and metal oxides is expected to offer exceptional opportunities toward emerging functions of all. We report the creation of an efficient hetero-structured nanocatalyst consisting of Mn3O4 core, SiO2 shell impregnated with noble Ag nanoparticles. The triple nanocatalyst Mn3O4/Ag/SiO2 was synthesized by using a facile three-step approach to disperse Ag nanoparticles between the surfaces of functionalized Mn3O4 and SiO2. The physicochemical structural characterization was performed by XRD and FTIR. The surface morphologies were observed by SEM and TEM. The EDX measurements confirmed the composition of the composite. The nanocomposite has been used as a catalyst for the degradation of Direct blue 78 in the presence of sodium borohydride (NaBH4). It has a drastic catalytic effect as compared to Mn3O4/Ag and Mn3O4. The rate constant of Direct blue 78 reduction followed the order: Mn3O4/Ag/SiO2 (0.25166 min−1) > Mn3O4/Ag (0.07971 min−1) > Mn3O4 (0.00947 min−1). The effects of different reaction conditions of the catalytic reaction have been determined. The catalytic activity of the as- synthesized nanocomposite was examined for the binary dyes system by incorporation of an additional dye (Sunset yellow). Its influence on the degradation rate and efficiency of Direct blue 78 was investigated. The nanocatalyst exhibited excellent catalytic activity towards the complete degradation of both the Direct blue 78 and Sunset yellow. The degradation percentage for these dyes reached 99.33 and 94.68%, respectively. The recovery and reusability of the Mn3O4/Ag/SiO2 nanocomposite was studied in the reduction reaction of Direct blue 78. Five consecutive recovery reaction cycles were performed. They revealed high stability and constant efficiency of the catalyst for four cycles. 相似文献
76.
We theoretically investigate optomechanical force sensing via precooling and quantum noise cancellation in two coupled cavity optomechanical systems.We show that force sensing based on the reduction of noise can be used to dramatically enhance the force sensing and that the precooling process can eifectively improve the quantum noise cancellation.Specifically,we examine the effect of optomechanical cooling and noise reduction on the spectral density of the noise of the force measurement;these processes can significantly enhance the performance of optomechanical force sensing,and setting up the system in the resolved sideband regime can lead to an optimization of the cooling processes in a hybrid system.Such a scheme serves as a promising platform for quantum back-action-evading measurements of the motion and a framework for an optomechanical force sensor. 相似文献
77.
Cheng and Tang [Biometrika, 88 (2001), pp. 1169–1174] derived an upper bound on the maximum number of columns that can be accommodated in a two‐symbol supersaturated design (SSD) for a given number of rows () and a maximum in absolute value correlation between any two columns (). In particular, they proved that for (mod ) and . However, the only known SSD satisfying this upper bound is when . By utilizing a computer search, we prove that for , and . These results are obtained by proving the nonexistence of certain resolvable incomplete blocks designs. The combinatorial properties of the RIBDs are used to reduce the search space. Our results improve the lower bound for SSDs with rows and columns, for , and . Finally, we show that a skew‐type Hadamard matrix of order can be used to construct an SSD with rows and columns that proves . Hence, we establish for and for all (mod ) such that . Our result also implies that when is a prime power and (mod ). We conjecture that for all and (mod ), where is the maximum number of equiangular lines in with pairwise angle . 相似文献
78.
Cong Hu Bingyang Dong Li Liu 《Journal of polymer science. Part A, Polymer chemistry》2019,57(12):1333-1343
We report on the preparation of reduction‐responsive amphiphilic block copolymers containing pendent p‐nitrobenzyl carbamate (pNBC)‐caged primary amine moieties by reversible addition–fragmentation chain transfer (RAFT) radical polymerization using a poly(ethylene glycol)‐based macro‐RAFT agent. The block copolymers self‐assembled to form micelles or vesicles in water, depending on the length of hydrophobic block. Triggered by a chemical reductant, sodium dithionite, the pNBC moieties decomposed through a cascade 1,6‐elimination and decarboxylation reactions to liberate primary amine groups of the linkages, resulting in the disruption of the assemblies. The reduction sensitivity of assemblies was affected by the length of hydrophobic block and the structure of amino acid‐derived linkers. Using hydrophobic dye Nile red (NR) as a model drug, the polymeric assemblies were used as nanocarriers to evaluate the potential for drug delivery. The NR‐loaded nanoparticles demonstrated a reduction‐triggered release profile. Moreover, the liberation of amine groups converted the reduction‐responsive polymer into a pH‐sensitive polymer with which an accelerated release of NR was observed by simultaneous application of reduction and pH triggers. It is expected that these reduction‐responsive block copolymers can offer a new platform for intracellular drug delivery. © 2019 Wiley Periodicals, Inc. J. Polym. Sci., Part A: Polym. Chem. 2019, 57, 1333–1343 相似文献
79.
In this article, we derive expressions for conditional expectations in terms of regular expectations without conditioning but involving some weights. For this purpose, we apply two approaches: the conditional density method and the Malliavin method. We use these expressions for the numerical estimation of the price of American options and their deltas in a Lévy and jump-diffusion setting. Several examples of applications to financial and energy markets are given including numerical examples. 相似文献
80.
Necessary conditions for the existence of a super‐simple, decomposable, near‐resolvable ‐balanced incomplete block design (BIBD) whose 2‐component subdesigns are both near‐resolvable ‐BIBDs are (mod ) and . In this paper, we show that these necessary conditions are sufficient. Using these designs, we also establish that the necessary conditions for the existence of a super‐simple near‐resolvable ‐RBIBD, namely (mod ) and , are sufficient. A few new pairwise balanced designs are also given. 相似文献