全文获取类型
收费全文 | 5037篇 |
免费 | 106篇 |
国内免费 | 49篇 |
专业分类
化学 | 2662篇 |
晶体学 | 49篇 |
力学 | 168篇 |
数学 | 937篇 |
物理学 | 1376篇 |
出版年
2020年 | 42篇 |
2019年 | 39篇 |
2018年 | 30篇 |
2017年 | 31篇 |
2016年 | 67篇 |
2015年 | 57篇 |
2014年 | 74篇 |
2013年 | 223篇 |
2012年 | 193篇 |
2011年 | 258篇 |
2010年 | 115篇 |
2009年 | 106篇 |
2008年 | 207篇 |
2007年 | 193篇 |
2006年 | 208篇 |
2005年 | 204篇 |
2004年 | 158篇 |
2003年 | 154篇 |
2002年 | 150篇 |
2001年 | 141篇 |
2000年 | 122篇 |
1999年 | 75篇 |
1998年 | 50篇 |
1997年 | 66篇 |
1996年 | 81篇 |
1995年 | 79篇 |
1994年 | 101篇 |
1993年 | 105篇 |
1992年 | 93篇 |
1991年 | 91篇 |
1990年 | 77篇 |
1989年 | 88篇 |
1988年 | 60篇 |
1987年 | 74篇 |
1986年 | 58篇 |
1985年 | 102篇 |
1984年 | 82篇 |
1983年 | 48篇 |
1982年 | 77篇 |
1981年 | 70篇 |
1980年 | 61篇 |
1979年 | 69篇 |
1978年 | 72篇 |
1977年 | 56篇 |
1976年 | 67篇 |
1975年 | 65篇 |
1974年 | 69篇 |
1973年 | 101篇 |
1972年 | 38篇 |
1971年 | 31篇 |
排序方式: 共有5192条查询结果,搜索用时 15 毫秒
71.
The literature concerning the chemical and electrochemical reactions of nitric oxide, nitrous acid and nitrogen dioxide in aqueous solutions is reviewed briefly, with emphasis on electrochemical reductions at platinum electrodes in acidic solutions. The voltammetric behavior of NO and NO2 at a Pt electrode in perchloric acid is virtually identical to that for HNO2 and this is explained on the basis of a common electroactive precursor concluded to be NO+. Three cathodic waves are obtained for acidic solutions of NO, HNO2 and NO2. The first two waves correspond to reduction of NO+ to NO and N2O3 to NO, respectively. The presence of N2O3 results from decomposition of the parent compounds. The presence of Br- or Cl- in acidic solutions of the title compounds promotes the voltammetric reductions at lower H+ concentrations. This probably results from formation of electroactive nitrosyl halides. 相似文献
72.
A. R. Demlow D. V. Eldred D. A. Johnson E. F. Westrum Jr. 《Journal of Thermal Analysis and Calorimetry》1998,52(3):1055-1062
This paper represents a fitting (modeling) of the temperature dependence of the Komada-Westrum characteristic temperature
for those γ-, δ- and ε-phase lanthanide sesquisulfides for which the total heat capacities, including internal degrees of
freedom (e.g., Schottky and magnetic contributions), were connected to the residue of only lattice vibrations yielding lattice
heat-capacity contributions. These characteristic temperatures (θKW) at 298.15 K are seen to behave smoothly (nearly linearly) as a function of (cationic) atomic number within the region of
stability of each phase as does the density. The trends between the phases also show some consistency but not predictability
of one from the other.
This revised version was published online in July 2006 with corrections to the Cover Date. 相似文献
73.
74.
Brian F.G. Johnson Jack Lewis William J.H Nelson Maria D. Vargas Dario Braga Mary McPartlin 《Journal of organometallic chemistry》1983,246(2):c69-c73
The high nuclearity mixed metal cluster monoanions [Os10C(CO)24Cu(NCMe)]? (I) and [Os10C(CO)24AuPPh3]? (II) have been obtained by reaction of the carbido-dianion [Os10C(CO)24]2? (III) with one equivalent of [Cu(NCMe)4] [BF4] and Ph3PAuCl, respectively, in CH2Cl2. X-ray analysis of the [PPh3Me]+ salts of I and II show that the Cu and Au ligands have added to capping tetrahedra of the dianion III in μ3- and μ2-bridging positions, respectively. 相似文献
75.
76.
Cosper MM Jameson GN Davydov R Eidsness MK Hoffman BM Huynh BH Johnson MK 《Journal of the American Chemical Society》2002,124(47):14006-14007
The combination of resonance Raman, electron paramagnetic resonance and M?ssbauer spectroscopies has been used to investigate the effect of S-adenosyl-l-methionine (SAM) on the spectroscopic properties of the [4Fe-4S]2+ cluster in biotin synthase. The results indicate that SAM interacts directly at a unique iron site of the [4Fe-4S]2+ cluster in BioB and support the hypothesis of a common inner-sphere mechanism for the reductive cleavage of SAM in the radical SAM family of Fe-S enzymes. 相似文献
77.
M. D. Glascock H. Neff K. S. Stryker T. N. Johnson 《Journal of Radioanalytical and Nuclear Chemistry》1994,180(1):29-35
An abbreviated NAA procedure has been developed to fingerprint obsidian artifacts in the Mesoamerican region. Despite the large number of available sources, an NAA procedure, which relies on producing short-lived isotopes, has been applied with a success rate greater than 90 percent. The abbreviated NAA procedure is rapid and cost competitive with the XRF technique more often applied in obsidian sourcing. Results from the analysis of over 1,200 obsidian artifacts from throughout Mesoamerica are presented. 相似文献
78.
Resynthesis of NSC 341,964, which had been assigned structure 1 (1-[[3-(7-chloro-4-oxo-4H-3,1-benzoxazin-2-yl)phenyl]methyl]pyridinium chloride) was approached via 7-chloro-2-(3-methylphenyl)-4H-3,1-benzoxazin-4-one ( 5 ) obtained from 3-methylbenzoyl chloride ( 2 ) and 2-amino-4-chlorobenzoic acid ( 3 ) followed by dehydration in acetic anhydride. Radical bromination provided 6 which with pyridine afforded the bromide analog 7 of 1 . Ion exchange, however, gave ring-opened benzoic acid 8 rather than 1 . The original sample of NSC 341,964 also proved to be ring-opened material. However, 7 upon standing exhibited slow hydrolysis to 8 so that the structure of the original NSC 341,964 remains uncertain. A more direct route to compound 8 is also described. 相似文献
79.
Pierce KM Hope JL Johnson KJ Wright BW Synovec RE 《Journal of chromatography. A》2005,1096(1-2):101-110
A fast and objective chemometric classification method is developed and applied to the analysis of gas chromatography (GC) data from five commercial gasoline samples. The gasoline samples serve as model mixtures, whereas the focus is on the development and demonstration of the classification method. The method is based on objective retention time alignment (referred to as piecewise alignment) coupled with analysis of variance (ANOVA) feature selection prior to classification by principal component analysis (PCA) using optimal parameters. The degree-of-class-separation is used as a metric to objectively optimize the alignment and feature selection parameters using a suitable training set thereby reducing user subjectivity, as well as to indicate the success of the PCA clustering and classification. The degree-of-class-separation is calculated using Euclidean distances between the PCA scores of a subset of the replicate runs from two of the five fuel types, i.e., the training set. The unaligned training set that was directly submitted to PCA had a low degree-of-class-separation (0.4), and the PCA scores plot for the raw training set combined with the raw test set failed to correctly cluster the five sample types. After submitting the training set to piecewise alignment, the degree-of-class-separation increased (1.2), but when the same alignment parameters were applied to the training set combined with the test set, the scores plot clustering still did not yield five distinct groups. Applying feature selection to the unaligned training set increased the degree-of-class-separation (4.8), but chemical variations were still obscured by retention time variation and when the same feature selection conditions were used for the training set combined with the test set, only one of the five fuels was clustered correctly. However, piecewise alignment coupled with feature selection yielded a reasonably optimal degree-of-class-separation for the training set (9.2), and when the same alignment and ANOVA parameters were applied to the training set combined with the test set, the PCA scores plot correctly classified the gasoline fingerprints into five distinct clusters. 相似文献
80.
To determine the free concentration of a drug (propranolol) in the interstitial space in humans in vivo, seven male students were investigated by microdialysis of the periumbilical subcutaneous tissue. The microdialysis catheters were calibrated in vivo and the propranolol concentration was determined by high-performance liquid chromatography. Ten hours after intake of 80 mg of propranolol, the total plasma and free interstitial propranolol concentrations were 80 +/- 43 and 7 +/- 2 nM, respectively. After a second dose, maximum concentration was reached after 80 +/- 10 min and 98 +/- 12 min, in plasma, and the concentrations in the interstitial water were 594 +/- 138 and 27 +/- 7 nM, respectively. In a second study, microdialysis was performed on the left ventricular wall in six pigs receiving an intravenous injection of 5 mg of propranolol followed by a constant propranolol infusion for 40 min (5 mg propranolol per h). The maximum concentrations of propranolol were 97 +/- 29 and 6 +/- 2 nM in plasma and in interstitial water, respectively. The data suggest that microdialysis is a useful tool for recording the free concentrations of a drug in the interstitial space. 相似文献