全文获取类型
收费全文 | 803篇 |
免费 | 143篇 |
国内免费 | 99篇 |
专业分类
化学 | 575篇 |
晶体学 | 8篇 |
力学 | 53篇 |
综合类 | 2篇 |
数学 | 76篇 |
物理学 | 331篇 |
出版年
2023年 | 27篇 |
2022年 | 26篇 |
2021年 | 37篇 |
2020年 | 46篇 |
2019年 | 30篇 |
2018年 | 26篇 |
2017年 | 17篇 |
2016年 | 48篇 |
2015年 | 48篇 |
2014年 | 31篇 |
2013年 | 58篇 |
2012年 | 68篇 |
2011年 | 87篇 |
2010年 | 51篇 |
2009年 | 56篇 |
2008年 | 57篇 |
2007年 | 42篇 |
2006年 | 48篇 |
2005年 | 32篇 |
2004年 | 17篇 |
2003年 | 10篇 |
2002年 | 11篇 |
2001年 | 13篇 |
2000年 | 10篇 |
1999年 | 12篇 |
1998年 | 14篇 |
1997年 | 11篇 |
1996年 | 7篇 |
1995年 | 14篇 |
1994年 | 6篇 |
1993年 | 17篇 |
1992年 | 13篇 |
1991年 | 11篇 |
1990年 | 5篇 |
1989年 | 7篇 |
1988年 | 7篇 |
1987年 | 4篇 |
1986年 | 1篇 |
1985年 | 4篇 |
1984年 | 1篇 |
1983年 | 1篇 |
1982年 | 2篇 |
1981年 | 1篇 |
1980年 | 4篇 |
1978年 | 1篇 |
1977年 | 1篇 |
1976年 | 1篇 |
1975年 | 1篇 |
1974年 | 1篇 |
1968年 | 2篇 |
排序方式: 共有1045条查询结果,搜索用时 0 毫秒
991.
采用4.6μm附近的量子级联激光器作为光源,搭建了一套二硫化碳(CS2)吸收光谱测量系统,结合可调谐二极管激光吸收光谱技术,对光谱范围为2178.99—2180.79 cm-1的CS2吸收光谱展开了深入研究,重点测量了2180.5—2180.74 cm~(-1)的四条吸收谱线,利用基于非线性最小二乘的多元线性回归算法对CS2吸收光谱进行拟合,精确得到了该范围内谱线的中心波长、线强以及空气展宽系数等光谱参数.经计算,对应谱线线强不确定度小于5%,空气展宽系数不确定度小于15%,这个结果可作为免标定CS2红外光谱探测的基础光谱参数,对痕量CS2气体传感具有重要意义.未来我们将进一步开展2170—2200 cm-1整个谱段的CS2谱线参数的测量,以期填补其在HITRAN和GEISA数据库光谱参数的空白. 相似文献
992.
一种研究生物大分子与其小分子配基相互作用的毛细管电泳新方法 总被引:3,自引:0,他引:3
建立了一种定量研究大分子与小分子间相互作用的毛细管电泳新方法,即在线微透 析三电极系统亲和毛细管电泳法。本法可测定小分子的自由浓度而不受蛋白质大分子的干 扰。详细介绍了实验装置的制作,探讨了进样机制。作为实例,研究了生理条件下pH7.4, 50mmol/L磷酸介质中,牛血清白蛋白与磺胺甲基异噁唑、L-色氨酸、D-色氨酸的亲和常数的 测定,其值分别为5.8×10~ 4L/mol, 2.3 ×10~4L/mol, 1.77×10~3L/mol。本法较简便,能在生理环 境下测定,样品用量亦少。 相似文献
993.
In this paper, we investigate the vanishing viscosity limit for solutions to the Navier–Stokes equations with a Navier slip boundary condition on general compact and smooth domains in R 3. We first obtain the higher order regularity estimates for the solutions to Prandtl’s equation boundary layers. Furthermore, we prove that the strong solution to Navier–Stokes equations converges to the Eulerian one in C([0, T]; H 1(Ω)) and ${L^\infty((0,T) \times \Omega)}$ , where T is independent of the viscosity, provided that initial velocity is regular enough. Furthermore, rates of convergence are obtained also. 相似文献
994.
Q. L. Zhang J. Zhang S. F. Du D. X. Zhang B. H. Feng J. Y. Zhang J. C. Zang 《Applied physics. B, Lasers and optics》2010,101(1-2):109-113
The characters of stimulated Raman scattering of BaWO4 crystal excited by a picoseconds laser at 1064 nm are studied based on optical parametric amplification (OPA). Up to six-order Stokes components and five-order anti-Stokes components are observed. The SRS components are amplified by an OPA and the wavelength tunable range from 411 to 2594 nm is achieved with a maximum conversion efficiency of 38% using the OPA stage. 相似文献
995.
D.?Y.?Zang H.?P.?Wang F.?P.?Dai D.?Langevin B.?WeiEmail author 《Applied Physics A: Materials Science & Processing》2011,102(1):141-145
We report a solidification mechanism transition of liquid ternary Co45Cu45Ni10 alloy when it solidifies at a critical undercooling of about 344 K. When undercooling at ΔT<344 K, the solidification process is characterized by primary S (Co) dendritic growth and a subsequent peritectic transition.
The dendritic growth velocity of S (Co) dendrite increases with the rise of undercooling. However, once ΔT>344 K, the solidification velocity decreases with the increase of undercooling. In this case, liquid/liquid phase separation
takes place prior to solidification. The minor L2 (Cu) droplets hinder the motion of the solidification front, and a monotectic transition may occur in the major L1 phase. These facts caused by metastable phase separation are responsible for the slow growth at high undercoolings. 相似文献
996.
金属卤素钙钛矿是目前最有前景的高效低成本新型太阳能电池材料,但是目前还存在环境友好性和理论效率极限较低的问题。锡钙钛矿环境友好,而且其带隙更窄理论转换效率更高,吸引了广泛的关注。锡钙钛矿太阳能电池(TPSC)近年来发展迅速,是目前效率最高的无铅钙钛矿太阳能电池。本文先介绍了锡钙钛矿的晶体结构、能带结构和光电性质,然后总结了最近在锡钙钛矿领域有代表性的工作和提高光电转化效率的策略,最后讨论了锡钙钛矿发展面临的挑战和未来的发展方向。 相似文献
997.
Li X Hihath J Chen F Masuda T Zang L Tao N 《Journal of the American Chemical Society》2007,129(37):11535-11542
We have studied electron transport through single redox molecules, perylene tetracarboxylic diimides, covalently bound to two gold electrodes via different linker groups, as a function of electrochemical gate voltage and temperature in different solvents. The conductance of these molecules is sensitive to the linker groups because of different electronic coupling strengths between the molecules and electrodes. The current through each of the molecules can be controlled reversibly over 2-3 orders of magnitude with the gate and reaches a peak near the redox potential of the molecules. The similarity in the gate effect of these molecules indicates that they share the same transport mechanism. The temperature dependence measurement indicates that the electron transport is a thermally activated process. Both the gate effect and temperature dependence can be qualitatively described by a two-step sequential electron-transfer process. 相似文献
998.
B. Z. Li C. G. Sun A. F. Zang X. H. Zhang M. Zhao X. Y. Chen J. B. Wang G. B. Chen 《Laser Physics》2011,21(5):891-893
A dual-wavelength continuous-wave (CW) diode end-pumped Nd3+:YAlO3 (Nd:YAP) laser that generates simultaneous laser action at the wavelengths 1064 and 930 nm is demonstrated. A total output
power of 2.15 W (1.57 W at 1064 nm and 0.58 W at 930 nm) for the dual-wavelength was achieved at the incident pump power of
17.8 W with optical conversion efficiency of 12.1%. The M
2 values for 930 and 1064 nm lights were found to be around 1.21 and 1.32, respectively. 相似文献
999.
Zang Q Keire DA Buhse LF Wood RD Mital DP Haque S Srinivasan S Moore CM Nasr M Al-Hakim A Trehy ML Welsh WJ 《Analytical and bioanalytical chemistry》2011,401(3):939-955
Chemometric analysis of a set of one-dimensional (1D) (1)H nuclear magnetic resonance (NMR) spectral data for heparin sodium active pharmaceutical ingredient (API) samples was employed to distinguish USP-grade heparin samples from those containing oversulfated chondroitin sulfate (OSCS) contaminant and/or unacceptable levels of dermatan sulfate (DS) impurity. Three chemometric pattern recognition approaches were implemented: classification and regression tree (CART), artificial neural network (ANN), and support vector machine (SVM). Heparin sodium samples from various manufacturers were analyzed in 2008 and 2009 by 1D (1)H NMR, strong anion-exchange high-performance liquid chromatography, and percent galactosamine in total hexosamine tests. Based on these data, the samples were divided into three groups: Heparin, DS ≤ 1.0% and OSCS = 0%; DS, DS > 1.0% and OSCS = 0%; and OSCS, OSCS > 0% with any content of DS. Three data sets corresponding to different chemical shift regions (1.95-2.20, 3.10-5.70, and 1.95-5.70 ppm) were evaluated. While all three chemometric approaches were able to effectively model the data in the 1.95-2.20 ppm region, SVM was found to substantially outperform CART and ANN for data in the 3.10-5.70 ppm region in terms of classification success rate. A 100% prediction rate was frequently achieved for discrimination between heparin and OSCS samples. The majority of classification errors between heparin and DS involved cases where the DS content was close to the 1.0% DS borderline between the two classes. When these borderline samples were removed, nearly perfect classification results were attained. Satisfactory results were achieved when the resulting models were challenged by test samples containing blends of heparin APIs spiked with non-, partially, or fully oversulfated chondroitin sulfate A, heparan sulfate, or DS at the 1.0%, 5.0%, and 10.0% (w/w) levels. This study demonstrated that the combination of 1D (1)H NMR spectroscopy with multivariate chemometric methods is a nonsubjective, statistics-based approach for heparin quality control and purity assessment that, once standardized, minimizes the need for expert analysts. 相似文献
1000.
Zang Q Keire DA Wood RD Buhse LF Moore CM Nasr M Al-Hakim A Trehy ML Welsh WJ 《Analytical and bioanalytical chemistry》2011,399(2):635-649
Heparin, a widely used anticoagulant primarily extracted from animal sources, contains varying amounts of galactosamine impurities.
Currently, the United States Pharmacopeia (USP) monograph for heparin purity specifies that the weight percent of galactosamine
(%Gal) may not exceed 1%. In the present study, multivariate regression (MVR) analysis of 1H NMR spectral data obtained from heparin samples was employed to build quantitative models for the prediction of %Gal. MVR
analysis was conducted using four separate methods: multiple linear regression, ridge regression, partial least squares regression,
and support vector regression (SVR). Genetic algorithms and stepwise selection methods were applied for variable selection.
In each case, two separate prediction models were constructed: a global model based on dataset A which contained the full range (0–10%) of galactosamine in the samples and a local model based on the subset dataset B for which the galactosamine level (0–2%) spanned the 1% USP limit. All four regression
methods performed equally well for dataset A with low prediction errors under optimal conditions, whereas SVR was clearly
superior among the four methods for dataset B. The results from this study show that 1H NMR spectroscopy, already a USP requirement for the screening of contaminants in heparin, may offer utility as a rapid method
for quantitative determination of %Gal in heparin samples when used in conjunction with MVR approaches. 相似文献