全文获取类型
收费全文 | 10223篇 |
免费 | 1912篇 |
国内免费 | 1699篇 |
专业分类
化学 | 8039篇 |
晶体学 | 244篇 |
力学 | 538篇 |
综合类 | 103篇 |
数学 | 1204篇 |
物理学 | 3706篇 |
出版年
2024年 | 43篇 |
2023年 | 279篇 |
2022年 | 521篇 |
2021年 | 419篇 |
2020年 | 528篇 |
2019年 | 529篇 |
2018年 | 443篇 |
2017年 | 391篇 |
2016年 | 489篇 |
2015年 | 556篇 |
2014年 | 613篇 |
2013年 | 761篇 |
2012年 | 930篇 |
2011年 | 922篇 |
2010年 | 634篇 |
2009年 | 670篇 |
2008年 | 717篇 |
2007年 | 653篇 |
2006年 | 559篇 |
2005年 | 472篇 |
2004年 | 336篇 |
2003年 | 310篇 |
2002年 | 315篇 |
2001年 | 259篇 |
2000年 | 253篇 |
1999年 | 197篇 |
1998年 | 135篇 |
1997年 | 117篇 |
1996年 | 140篇 |
1995年 | 106篇 |
1994年 | 106篇 |
1993年 | 86篇 |
1992年 | 77篇 |
1991年 | 46篇 |
1990年 | 49篇 |
1989年 | 28篇 |
1988年 | 20篇 |
1987年 | 16篇 |
1986年 | 18篇 |
1985年 | 15篇 |
1984年 | 14篇 |
1983年 | 19篇 |
1982年 | 7篇 |
1981年 | 4篇 |
1979年 | 5篇 |
1978年 | 4篇 |
1977年 | 3篇 |
1976年 | 3篇 |
1973年 | 4篇 |
1957年 | 3篇 |
排序方式: 共有10000条查询结果,搜索用时 15 毫秒
181.
阿斯美胶囊中四种组分的高效液相色谱分析 总被引:2,自引:0,他引:2
建立了反相高效液相色谱法测定阿斯美胶囊中氨茶碱、盐酸甲氧非那明、马来酸氯苯那敏及那可汀四种组分的含量。色谱柱为SpherisorbC8,5μm,200cm×40mmID,流动相为体积分数05%的三乙胺溶液(磷酸调节pH55)+乙腈+甲醇(550+368+82),检测波长为264nm,线性范围分别为:氨茶碱3125~250mg/L,γ=09996,盐酸甲氧非那明15625~1250mg/L,γ=10000,马来酸氯苯那敏25~200mg/L,γ=09999,那可汀875~700mg/L,γ=09999;回收率(n=3)分别为1012%,1008%,999%,995%;精密度:日内平均RSD(n=6)分别为07%,04%,04%,06%;日间平均RSD(n=3)分别为10%,10%,11%,12%。 相似文献
182.
Zhen-Qiang Yu Xiaodong Li Wei Wan Xin-Shun Li Kuo Fu Yue Wu Alexander D. Q. Li 《Chemical science》2021,12(9):3146
Balancing the rigidity of a π-conjugated structure for strong emission and the flexibility of liquid crystals for self-assembly is the key to realizing highly emissive liquid crystals (HELCs). Here we show that (1) integrating organization-induced emission into dual molecular cooperatively-assembled liquid crystals, (2) amplifying mesogens, and (3) elongating the spacer linking the emitter and the mesogen create advanced materials with desired thermal–optical properties. Impressively, assembling the fluorescent acceptor Nile red into its host donor designed according to the aforementioned strategies results in a temperature-controlled Förster resonance energy transfer (FRET) system. Indeed, FRET exhibits strong S-curve dependence as temperature sweeps through the liquid crystal phase transformation. Such thermochromic materials, suitable for dynamic thermo-optical sensing and modulation, are anticipated to unlock new and smart approaches for controlling and directing light in stimuli-responsive devices.A temperature-sensitive Förster resonance energy transfer system was constructed using a highly emissive liquid crystal co-assembled with Nile red, enabling thermo-optical modulation for controlling and directing light in stimuli-responsive devices. 相似文献
183.
184.
On the Criteria of Instability for Electrochemical Systems 总被引:7,自引:0,他引:7
Both cyclic-voltammetry-based and impedance-based experimental criteria that have been developed recently for the oscillatory electrochemical systems are critically appraised with two typical categories of oscillators.Consistent conclusions can be drawn by the two criteria for the category of oscillators that involve the coupling of charge transfer mainly with surface steps(e.g.ad-and desorption)such as in the electrooxidation of C1 organic molecules.Whereas,impedance-based criterion is not applicable to the category of oscillators that involve the coupling of charge transfer mainly with mass transfer(e.g.diffusion and convection) such as in the Fe(CN)6^3- reduction accompanying periodic hydrogen evolution.The reason is that the negative impedance cannot include the feedback information of convection mass transfer induced by the hydrogen evolution.However,both positive and negative nonlinear feedbacks,i.e., the diffusion-limited depletion and convection-enhanced replenishment of the Fe(CN)6^3- surface concentration,that coexist between the bistability,i.e.,Fe(CN)6^3- reduction with and without hydrogen evolution at lower and higher potential sides respectively,are all reflected in the crossed cyclic voltammogram(CCV).It can be concluded that the voltammetry-based criterion(in time domain)is more intuitive,less time-consuming and has a wider range of applications than the impedancebased one (in frequency domain). 相似文献
185.
Kinetic study on the cleavage of N‐phenylphthalimide (NPhPT) in the presence of 0.05 M NH2NH2 and mixed H2O‐CH3CN solvents reveals the occurrence of reaction scheme where A, B, C, C1, An, E, and F represent NPhPT, o‐CO?2C6H4CONHC6H5, o‐CONHNH2C6H4‐ CONHC6H5, N‐aminophthalimide, aniline, o‐CO?2C6H4CONHNH2, and o‐CONHNH2C6H4‐CONHNH2, respectively. But, in the presence of either nonbuffered ?0.20 M NH2NH2 hydrazine buffer of pH ~7.30–8.26 with total buffer concentration ([Buf]T) of >0.02 M, further conversion of F to 2,3‐dihydrophthalazine‐1,4‐dione (DHPD) has been detected depending upon the length of the reaction time (t), the values of [Buf]T, and pH. It has been shown that the rate of conversion of C1 to F is much faster than that of C to C1 which is much faster than that of F to DHPD. The reaction step A → C involves general base (GB) catalysis, while step C → C1 seems to involve specific base–general acid (GA) and GB‐GB catalysis. © 2005 Wiley Periodicals, Inc. Int J Chem Kinet 37: 147–161, 2005 相似文献
186.
Preparative separation and purification of squalene from the microalga Thraustochytrium ATCC 26185 by high-speed counter-current chromatography 总被引:1,自引:0,他引:1
High-speed counter-current chromatography (HSCCC) was successfully applied to the preparative separation and purification of squalene from microalgae. Crude squalene was obtained from the microalga Thraustochytrium ATCC 26185 by extraction with organic solvents. The crude squalene was further separated using a waterless two-phase solvent system composed of n-hexane-methanol (2:1, v/v). The upper phase as the mobile phase was pumped into the column at a flow-rate of 2.0 ml min(-1) in the tail-to-head elution mode. The fractions purified and collected were analyzed by high-performance liquid chromatography. The method yielded 0.2 mg squalene at 96% purity from 150 mg of the crude squalene (0.14% squalene) with 95% recovery. The separation of squalene by HSCCC was completed in 90 min. 相似文献
187.
N-(α-Benzotriazolylalkyl)arylacetamides, readily available from an arylacetamide, an aldehyde and benzotriazole, undergo intramolecular cyclization under acidic conditions to give 1-aryl-1,4-dihydro-3(2H)-isoquinolinones in good to excellent yields. Similarly, 2-(benzotriazol-1-yl)-2-(o-hydroxyphenyl)ethanols, obtained by lithiation of 2-(benzotriazol-1-ylmethyl)phenols followed by quenching with aldehydes or ketones, eliminate a molecule of water and a molecule of benzotriazole yielding 2-substituted and 2,3-disubstituted benzofurans. 相似文献
188.
随着信息技术的高速发展,每条数据所包含的信息越来越丰富,使得数据不可避免地含有异常值,且随着维数的增加,异常值出现的可能性更大。传统的主成分聚类分析对异常值特別敏感,基于MCD估计的主成分聚类方法虽然对异常值具有防御作用,但是在高维数据下MCD估计的偏差过大,其稳健性显著降低,而且当维数大于观测值个数时MCD估计失效。为此本文提出了基于MRCD估计的稳健主成分聚类方法,数值模拟和实证分析表明,基于MRCD估计的主成分聚类分析的效果优于传统的主成分聚类分析和基于MCD估计的主成分聚类分析,尤其是在维数大于样本观测值的情况下,MRCD估计更为有效。 相似文献
189.
190.
In this paper, we aim to investigate the difference equation
\begin{align*}
\Delta^{2}y(t-1)+|y(t)|=0, \ \ \ \ \ t\in[1,T]_{\mathbb{Z}}
\end{align*}
with different boundary conditions $y(0)=0$ or $\Delta y(0)=0$ and $y(T+1)=B$ or $\Delta y(T)=B$,\ where\ $T\geq 1$ is an integer and $B\in\mathbb{R}$. We will show that how the values of $T$ and $B$ influence the existence and uniqueness of the solutions to the about problem. In details, for the different problems, the $TB$-plane explicitly divided into different parts according to the number of the solutions to the above problems. These parts of $TB$-plane for the value of $T$ and $B$ guarantee the uniqueness, the existence and the nonexistence of solutions respectively. 相似文献