全文获取类型
收费全文 | 2474篇 |
免费 | 33篇 |
国内免费 | 14篇 |
专业分类
化学 | 1495篇 |
晶体学 | 28篇 |
力学 | 85篇 |
数学 | 435篇 |
物理学 | 478篇 |
出版年
2020年 | 18篇 |
2019年 | 18篇 |
2018年 | 20篇 |
2017年 | 21篇 |
2016年 | 32篇 |
2015年 | 29篇 |
2014年 | 28篇 |
2013年 | 113篇 |
2012年 | 94篇 |
2011年 | 101篇 |
2010年 | 59篇 |
2009年 | 68篇 |
2008年 | 105篇 |
2007年 | 114篇 |
2006年 | 105篇 |
2005年 | 107篇 |
2004年 | 113篇 |
2003年 | 81篇 |
2002年 | 83篇 |
2001年 | 50篇 |
2000年 | 49篇 |
1999年 | 55篇 |
1998年 | 30篇 |
1997年 | 33篇 |
1996年 | 36篇 |
1995年 | 27篇 |
1994年 | 28篇 |
1993年 | 25篇 |
1992年 | 34篇 |
1991年 | 21篇 |
1990年 | 28篇 |
1989年 | 40篇 |
1988年 | 18篇 |
1987年 | 25篇 |
1986年 | 29篇 |
1985年 | 46篇 |
1984年 | 50篇 |
1983年 | 38篇 |
1982年 | 39篇 |
1981年 | 59篇 |
1980年 | 37篇 |
1979年 | 45篇 |
1978年 | 38篇 |
1977年 | 47篇 |
1976年 | 26篇 |
1975年 | 34篇 |
1974年 | 39篇 |
1973年 | 38篇 |
1972年 | 27篇 |
1970年 | 18篇 |
排序方式: 共有2521条查询结果,搜索用时 15 毫秒
51.
52.
Rhodium trichloride supported on Ti-hexagonal mesoporous silica (Ti-HMS), via a bipyridyl group, is an efficient catalyst for the hydroformylation of olefins at 120 °C and 40.8 atm of CO/H2 (CO/H2=2/1). The catalyst is selective leading to high ratios of linear or branched aldehydes from functionalized olefins, and high activity in the case of propene which gave a turnover frequency of 6209 mol/mol(Rh)/h. 相似文献
53.
Money VA Elhaïk J Radosavljevic Evans I Halcrow MA Howard JA 《Dalton transactions (Cambridge, England : 2003)》2004,(1):65-69
The spin crossover compounds [FeL2](BF4)2, L=2,6-di(3-methylpyrazol-1-yl)pyrazine and [FeL2](ClO4)2 have very unusual two stage spin transitions which are initially steep and then become more gradual. A detailed variable temperature single crystal X-ray diffraction study has shown that the course of the spin transition is controlled by an order-disorder transition in the counter anions. The high and low spin states both crystallise in the tetragonal space group I4, the structures of the high and low spin states are presented at 290 and 30 K, respectively. The title compounds are shown to undergo LIESST (Light Induced Excited Spin State Trapping) under irradiation with either red or green laser light with wavelengths of 632.8 and 532.06 nm, respectively, at 30 K. The cell parameters for the tetragonal photo-induced metastable high spin state at this temperature are a= 9.169(6), c= 17.77(1) A for [FeL2](ClO4)2 with an increase in unit cell volume of 21 A3, and a= 9.11(1), c= 17.75(2) A and an increase in volume of 42.8 A3 for [FeL2](BF4)2. 相似文献
54.
Y. Kuwahara H. Fukami R. Howard S. Ishii F. Matsumura W.E. Burkholder 《Tetrahedron》1978,34(12):1769-1774
Chemical an spectroscopic evidence is presented to show that 2,3-dihydro-2,3,5-trimethyl-6-(1-methyl-2-oxobutyl)-4H-pyran-4-one (10) is the sex pheromone produced by the female drugstore beetle, Stegobium paniceum L. 相似文献
55.
We prove, that in the world of constructible sets, there does not exist a spaceX withH″ (X,Z) isomorphic to the rational numbers. The proof requires a result about the growth of Ext z Emphasis>/i (-, Z) inside of Gödel’s constructible universeL. 相似文献
56.
Michael Woolman Jimmy Qiu Claudia M. Kuzan-Fischer Isabelle Ferry Delaram Dara Lauren Katz Fowad Daud Megan Wu Manuela Ventura Nicholas Bernards Harley Chan Inga Fricke Mark Zaidi Brad G. Wouters James T. Rutka Sunit Das Jonathan Irish Robert Weersink Howard J. Ginsberg David A. Jaffray Arash Zarrine-Afsar 《Chemical science》2020,11(33):8723
Integration between a hand-held mass spectrometry desorption probe based on picosecond infrared laser technology (PIRL-MS) and an optical surgical tracking system demonstrates in situ tissue pathology from point-sampled mass spectrometry data. Spatially encoded pathology classifications are displayed at the site of laser sampling as color-coded pixels in an augmented reality video feed of the surgical field of view. This is enabled by two-way communication between surgical navigation and mass spectrometry data analysis platforms through a custom-built interface. Performance of the system was evaluated using murine models of human cancers sampled in situ in the presence of body fluids with a technical pixel error of 1.0 ± 0.2 mm, suggesting a 84% or 92% (excluding one outlier) cancer type classification rate across different molecular models that distinguish cell-lines of each class of breast, brain, head and neck murine models. Further, through end-point immunohistochemical staining for DNA damage, cell death and neuronal viability, spatially encoded PIRL-MS sampling is shown to produce classifiable mass spectral data from living murine brain tissue, with levels of neuronal damage that are comparable to those induced by a surgical scalpel. This highlights the potential of spatially encoded PIRL-MS analysis for in vivo use during neurosurgical applications of cancer type determination or point-sampling in vivo tissue during tumor bed examination to assess cancer removal. The interface developed herein for the analysis and the display of spatially encoded PIRL-MS data can be adapted to other hand-held mass spectrometry analysis probes currently available.Integration between a hand-held mass spectrometry desorption probe based on picosecond infrared laser technology (PIRL-MS) and an optical surgical tracking system demonstrates in situ tissue pathology from point-sampled mass spectrometry data. 相似文献
57.
Summary High molecular weight poly 2- and 4-vinyl pyridines were synthesized by ionic polymerisation and their flocculation efficiency tested against kaolin and silica dispersions at pH 3. Low additions of the polymers, around 3 parts per 103 parts solid, destabilize the dispersions and clarify silica dispersions to a remarkable extent; however, over-addition of polymer leads to partial restabilization. The flocculation efficiency improves with increase of solids content, especially with clay dispersions. Flocculation efficiency increases with polymer molecular weight, especially over the lower part of the range covered, but is relatively independent of pH in the range 2–4. Quaternization of poly(4-vinyl pyridine) with bromobutane produces a good flocculant for aqueous silica dispersions in which the degree of restabilization on over-dosing depends on the KCI content. TheN-oxide derivatives of the vinyl pyridine polymers do not affect the colloidal stability of silica dispersions. The observations are interpreted in terms of bridging flocculation by the cationic polymer molecules between anionic surface sites on the solid particles. 相似文献
58.
An investigation of heparinase immobilization 总被引:1,自引:0,他引:1
Bernstein Howard Yang Victor C. Langer Robert 《Applied biochemistry and biotechnology》1987,16(1):129-143
A systematic investigation of the parameters that affect the efficiency of immobilizing heparinase onto cyanogen bromide activated
crosslinked 8% agarose beads was conducted. Two experimental measures, the “fraction bound” and the “fraction retained,” were
used to monitor the coupling efficiency. The fraction bound is the portion of the total initial enzyme that is bound to the
agarose gel. The fraction retained is the fraction of bound enzyme that is active. The product of the two measures indicates
the coupling efficiency. The activity of the immobilized heparinase was measured under conditions free of both internal and
external mass transfer limitations, and thus, the fraction retained represents the true immobilized enzyme activity.
Increasing the degree of activation of the beads results in an increase in the fraction bound, the fraction retained, and
consequently, the coupling efficiency. As the ratio of enzyme solution to gel volume increases from 1.5 to 2.2, the fraction
bound remains constant but the fraction retained decreases (heparinase concentration; 0.15 mg/mL and degree of activation;
9.5 μmol of cyanate esters/g of gel). At volume ratios greater than 2.2, both the fraction bound and the fraction retained
decline continuously. Changing the heparinase concentration in the coupling solution changes the coupling efficiency in a
manner similar to that of the volume ratio change.
When heparin is added during the coupling process, the fraction bound declines as the heparin concentration increases, whereas
the fraction retained increases up to a heparin concentration of 12 mg/mL and decreases thereafter. When arginine, lysine,
and glycine are used to block the unreacted cyanate ester groups after the coupling process, the immobilized heparinase shows
different pH optima of 6.5, 6.9, and 7.2, respectively. Based upon these findings, a protocol to optimize heparinase immobilization
is developed. 相似文献
59.
McMillan RA Howard J Zaluzec NJ Kagawa HK Mogul R Li YF Paavola CD Trent JD 《Journal of the American Chemical Society》2005,127(9):2800-2801
Self-assembling biomolecules that form highly ordered structures have attracted interest as potential alternatives to conventional lithographic processes for patterning materials. Here, we introduce a general technique for patterning nanoparticle arrays using two-dimensional crystals of genetically modified hollow protein structures called chaperonins. Constrained chemical synthesis of transition metal nanoparticles is initiated using templates functionalized with polyhistidine sequences. These nanoparticles are ordered into arrays because the template-driven synthesis is constrained by the nanoscale structure of the crystallized protein. We anticipate that this system may be used to pattern different classes of nanoparticles based on the growing library of sequences shown to specifically bind or direct the growth of materials. 相似文献
60.
Umar Hayat Philip N. Bartlett George H. Dodd Michael H. Lewis 《Journal of polymer science. Part A, Polymer chemistry》1988,26(1):201-206
The surface compositions of various polymeric films, grown electrochemically on platinum foils, have been investigated by energy-dispersive x-ray analysis in conjunction with scanning electron microscopy (SEM/EDS). Comparison of the relative area ratios of peaks for the C and N Kemission lines show that the EDS may be used to study the surface composition of polymers. The evidence presented strongly suggests that there is limited structural degradation and the elemental composition is not changed under the electron beam at relatively low accelerating voltages. This technique statistically samples the repeat units of the polymer. For samples grown in both aqueous and nonaqueous solutions. SEM/EDS provides evidence for extensive contamination with oxygen. 相似文献