全文获取类型
收费全文 | 675篇 |
免费 | 17篇 |
国内免费 | 1篇 |
专业分类
化学 | 325篇 |
晶体学 | 2篇 |
力学 | 40篇 |
数学 | 94篇 |
物理学 | 232篇 |
出版年
2023年 | 5篇 |
2022年 | 14篇 |
2021年 | 8篇 |
2020年 | 8篇 |
2019年 | 7篇 |
2018年 | 7篇 |
2017年 | 6篇 |
2016年 | 8篇 |
2015年 | 14篇 |
2014年 | 10篇 |
2013年 | 21篇 |
2012年 | 31篇 |
2011年 | 31篇 |
2010年 | 10篇 |
2009年 | 18篇 |
2008年 | 26篇 |
2007年 | 33篇 |
2006年 | 29篇 |
2005年 | 30篇 |
2004年 | 16篇 |
2003年 | 18篇 |
2002年 | 20篇 |
2001年 | 11篇 |
2000年 | 15篇 |
1999年 | 7篇 |
1998年 | 8篇 |
1997年 | 8篇 |
1996年 | 17篇 |
1995年 | 12篇 |
1994年 | 16篇 |
1993年 | 23篇 |
1992年 | 24篇 |
1991年 | 17篇 |
1990年 | 18篇 |
1989年 | 11篇 |
1988年 | 9篇 |
1986年 | 6篇 |
1985年 | 17篇 |
1983年 | 6篇 |
1982年 | 7篇 |
1981年 | 6篇 |
1980年 | 5篇 |
1979年 | 5篇 |
1978年 | 8篇 |
1977年 | 4篇 |
1976年 | 4篇 |
1975年 | 4篇 |
1938年 | 4篇 |
1867年 | 6篇 |
1863年 | 4篇 |
排序方式: 共有693条查询结果,搜索用时 308 毫秒
31.
A two-step supercritical fluid extraction of polycyclic aromatic hydrocarbons from roadside soil samples 总被引:1,自引:0,他引:1
A two-step procedure for the supercritical fluid extraction (SFE) of polycyclic aromatic hydrocarbons from soil samples was developed. The procedure consists of a static supercritical fluid treatment in a closed extraction cell at a high temperature (T=250 or 340degreesC for 20 min) and an SFE with a solvent trapping. During the static phase, the sample is exposed to a supercritical organic solvent (methanol, toluene, dichloromethane, ACN, acetone, and hexane). The solvent penetrates particles of the matrix to substitute strongly bonded molecules and dissolves the analytes in the supercritical phase. At ambient temperature, supercritical fluids became liquid and lost their solvation abilities. Most of the analytes condense on the surface of the particles or on the extraction cell walls without forming strong bonds or penetrating deep into the matrix. Thus, the pretreatment liberates the analytes and they behave similar to those in freshly spiked samples. The common SFE with toluene-modified CO2 as an extraction fluid follows the static phase. With the use of the most suitable extraction phases (toluene, ACN), the extraction efficiency of the combined procedure is much higher (approximately100%). The results of the combined procedure are compared to the SFE procedure of the same untreated sample (difference less than 5%) and to the Soxhlet extraction. The extracts were analyzed using a GC with the flame ionization detection. 相似文献
32.
33.
Ludwin A.BASILIO-HERNáNDEZ Walter CARBALLOSA Jesús LEA?OS José M.SIGARRETA 《数学学报(英文版)》2019,35(3):338-354
We introduce the differential polynomial of a graph. The differential polynomial of a graph G of order n is the polynomial B(G; x) :=∑?(G)k=-nB_k(G) x~(n+k), where B_k(G) denotes the number of vertex subsets of G with differential equal to k. We state some properties of B(G;x) and its coefficients.In particular, we compute the differential polynomial for complete, empty, path, cycle, wheel and double star graphs. We also establish some relationships between B(G; x) and the differential polynomials of graphs which result by removing, adding, and subdividing an edge from G. 相似文献
34.
35.
Lai Hong Wong Asier Unciti-Broceta Michaela Spitzer Rachel White Mike Tyers Lea Harrington 《Chemistry & biology》2013,20(3):333-340
Highlights? Growth arrest induced by human telomerase in yeast is chemically reversible ? Readout is sensitive to telomerase catalytic activity and telomere recruitment ? Three cell-permeable compounds also inhibit purified human telomerase ? Yeast can be successfully used to screen for human telomerase inhibitors 相似文献
36.
Monosaccharides and disaccharides are important dietary components, but if insufficiently metabolized by some population subgroups, they are also linked to disease patterns. Thus, the correct analytical identification, quantification, and labeling of these food components are crucial to inform and potentially protect consumers. Enzymatic assays and high-performance anion-exchange chromatography with pulsed amperometric detection are established methods for the quantification of monosaccharides and disaccharides that, however, require long measuring times (60–180 min). Accelerated methods for the identification and quantification of the nutritionally relevant monosaccharides and disaccharides d -glucose, d -galactose, d -fructose, sucrose, lactose, and maltose were therefore developed. To realize this goal, the NMR experiments HSQC (heteronuclear single quantum coherence) and acceleration by sharing adjacent polarization (ASAP)-HSQC were applied. Measurement times were reduced to 27 and 6 min, respectively, by optimizing the interscan delay and applying non-uniform sampling. The optimized methods were used to quantify d -glucose, d -galactose, d -fructose, sucrose, and lactose in various dairy products. Results of the HSQC and ASAP-HSQC methods are equivalent to the results of the reference methods in terms of both precision and accuracy, demonstrating that these methods can be used to correctly analyze nutritionally relevant monosaccharides and disaccharides in short times. 相似文献
37.
Christian Balischewski Dr. Biswajit Bhattacharyya Dr. Eric Sperlich Dr. Christina Günter Alkit Beqiraj Prof. Dr. Tillmann Klamroth Dr. Karsten Behrens Stefan Mies Alexandra Kelling Susanne Lubahn Lea Holtzheimer Anne Nitschke Prof. Dr. Andreas Taubert 《Chemistry (Weinheim an der Bergstrasse, Germany)》2022,28(64):e202201068
Fifteen N-butylpyridinium salts – five monometallic [C4Py]2[MBr4] and ten bimetallic [C4Py]2[M0.5aM0.5bBr4] (M=Co, Cu, Mn, Ni, Zn) – were synthesized, and their structures and thermal and electrochemical properties were studied. All the compounds are ionic liquids (ILs) with melting points between 64 and 101 °C. Powder and single-crystal X-ray diffraction show that all ILs are isostructural. The electrochemical stability windows of the ILs are between 2 and 3 V. The conductivities at room temperature are between 10−5 and 10−6 S cm−1. At elevated temperatures, the conductivities reach up to 10−4 S cm−1 at 70 °C. The structures and properties of the current bromide-based ILs were also compared with those of previous examples using chloride ligands, which illustrated differences and similarities between the two groups of ILs. 相似文献
38.
Christoph M. Hendrich Sebastian Senn Lea Haas Marvin T. Hoffmann Dr. Ute Zschieschang Luca C. Greiner Dr. Frank Rominger Dr. Matthias Rudolph Dr. Hagen Klauk Prof. Dr. Andreas Dreuw Prof. Dr. A. Stephen K. Hashmi 《Chemistry (Weinheim an der Bergstrasse, Germany)》2021,27(59):14778-14784
Herein, we describe a gold-catalyzed cascade cyclization of Boc-protected benzylamines bearing two tethered alkyne moieties in a domino reaction initiated by a 6-endo-dig cyclization. The reaction was screened intensively, and the scope was explored, resulting in nine new Boc-protected dihydrobenzo[c]phenanthridines with yields of up to 98 %; even a π-extension and two bidirectional approaches were successful. Furthermore, thermal cleavage of the Boc group and subsequent oxidation gave substituted benzo[c]phenanthridines in up to quantitative yields. Two bidirectional approaches under the optimized conditions were successful, and the resulting π-extended molecules were tested as organic semiconductors in organic thin-film transistors. 相似文献
39.
Welch Nathan J. Carey J. William Frash Luke P. Hyman Jeffrey D. Hicks Wes Meng Meng Li Wenfeng Menefee Anne H. 《Transport in Porous Media》2022,141(1):17-47
Transport in Porous Media - Fluid flow in fractures has the potential to drastically change the economic, environmental, and safety risks associated with a subsurface operation. This work focuses... 相似文献
40.
A potential application of blue-light-emitting InGaN LED's as a polymerizing source for dental composite materials is described. We compared a basic LED device with a conventional curing light in vitro to determine the polymerization parameters and to examine the effect of the curing process on the physical properties of these materials. It was determined that an array of six LED's was able to set a range of composite materials more quickly than a conventional light source, with the cured compounds showing similar hardness and material shrinkage parameters but with a lower material temperature rise during the curing process using the array. These findings indicate that a device consisting of several InGaN LED's would be an effective instrument for curing certain light-sensitive materials, particularly dental composites. 相似文献