全文获取类型
收费全文 | 824篇 |
免费 | 29篇 |
国内免费 | 16篇 |
专业分类
化学 | 555篇 |
晶体学 | 4篇 |
力学 | 14篇 |
数学 | 123篇 |
物理学 | 173篇 |
出版年
2022年 | 8篇 |
2021年 | 8篇 |
2020年 | 10篇 |
2019年 | 14篇 |
2018年 | 13篇 |
2017年 | 6篇 |
2016年 | 16篇 |
2015年 | 24篇 |
2014年 | 21篇 |
2013年 | 29篇 |
2012年 | 49篇 |
2011年 | 66篇 |
2010年 | 32篇 |
2009年 | 25篇 |
2008年 | 40篇 |
2007年 | 42篇 |
2006年 | 50篇 |
2005年 | 40篇 |
2004年 | 24篇 |
2003年 | 30篇 |
2002年 | 33篇 |
2001年 | 19篇 |
2000年 | 23篇 |
1999年 | 8篇 |
1998年 | 10篇 |
1997年 | 5篇 |
1996年 | 12篇 |
1995年 | 6篇 |
1994年 | 7篇 |
1993年 | 9篇 |
1992年 | 10篇 |
1991年 | 5篇 |
1990年 | 9篇 |
1989年 | 9篇 |
1988年 | 7篇 |
1986年 | 14篇 |
1985年 | 18篇 |
1984年 | 7篇 |
1983年 | 8篇 |
1982年 | 12篇 |
1981年 | 6篇 |
1980年 | 6篇 |
1979年 | 5篇 |
1978年 | 6篇 |
1976年 | 5篇 |
1975年 | 6篇 |
1974年 | 4篇 |
1973年 | 10篇 |
1969年 | 7篇 |
1968年 | 4篇 |
排序方式: 共有869条查询结果,搜索用时 15 毫秒
31.
Prof. Dr. Steven Stevenson Amanda J. Rothgeb Katelyn R. Tepper Prof. Dr. James Duchamp Prof. Dr. Harry C. Dorn Xian B. Powers Mrittika Roy Prof. Dr. Marilyn M. Olmstead Prof. Dr. Alan L. Balch 《Chemistry (Weinheim an der Bergstrasse, Germany)》2019,25(54):12545-12551
Purified samples of Ho3N@C2(22010)-C78 and Tb3N@C2(22010)-C78 have been isolated by two distinct processes from the rich array of fullerenes and endohedral fullerenes present in carbon soot from graphite rods doped with Ho2O3 or Tb4O7. Crystallographic analysis of the endohedral fullerenes as cocrystals with Ni(OEP) (in which OEP is the dianion of octaethylporphyrin) shows that both molecules contain the chiral C2(22010)-C78 cage. This cage does not obey the isolated pentagon rule (IPR) but has two sites where two pentagons share a common C−C bond. These pentalene units bind two of the metal ions, whereas the third metal resides near a hexagon of the cage. Inside the cages, the Ho3N or Tb3N unit is planar. Ho3N@C2(22010)-C78 and Tb3N@C2(22010)-C78 use the same cage previously found for Gd3N@C2(22010)-C78 rather than the IPR-obeying cage found in Sc3N@D3h-C78. 相似文献
32.
Exhaustive enumeration of Steiner Triple Systems is not feasible, due to the combinatorial explosion of instances. The next‐best hope is to quickly find a sample that is representative of isomorphism classes. Stinson's Hill‐Climbing algorithm [ 20 ] is widely used to produce random Steiner Triple Systems, and certainly finds a sample of systems quickly, but the sample is not uniformly distributed with respect to the isomorphism classes of STS with ν ≤ 19, and, in particular, we find that isomorphism classes with a large number of Pasch configurations are under‐represented. No analysis of the non‐uniformity of the distribution with respect to isomorphism classes or the intractability of obtaining a representative sample for ν > 19 is known. We also exhibit a modification to hill‐climbing that makes the sample if finds closer to the uniform distribution over isomorphism classes in return for a modest increase in running time. © 2007 Wiley Periodicals, Inc. J Combin Designs 15: 405–419, 2007 相似文献
33.
J A Simmons E G Freedman S B Stevenson L Chen T J Wohlgenant 《The Journal of the Acoustical Society of America》1989,86(4):1318-1332
The ability of the echolocating bat, Eptesicus fuscus, to detect a sonar target is affected by the presence of other targets along the same axis at slightly different ranges. If echoes from one target arrive at about the same delay as echoes from another target, clutter interference occurs and one set of echoes masks the other. Although the bat's sonar emissions and the echoes themselves are 2 to 5 ms long, echoes (of approximately equal sensation levels--around 15 dB SL) only interfere with each other if they arrive within 200 to 400 microseconds of the same arrival time. This figure is an estimate of the integration time of the bat's sonar receiver for echoes. The fine structure of the clutter-interference data reflects the reinforcement and cancellation of echoes according to their time separation. When clutter interference first occurs, the waveforms of test and cluttering echoes already overlap for much of their duration. The masking effect underlying clutter interference appears specifically due to overlap, not between raw echo waveforms, but between the patterns of mechanical excitation created when echoes pass through bandpass filters equivalent to auditory-nerve tuning curves. While the time scale of clutter interference is substantially shorter than the duration of echo waveforms, it still is much longer than the eventual width of a target's range-axis image expressed in terms of echo delay. 相似文献
34.
35.
Öznur Yaşar Danny Dyer David A. Pike Margo Kondratieva 《Discrete Applied Mathematics》2009,157(8):1913-1923
In traditional edge searching one tries to clean all of the edges in a graph employing the least number of searchers. It is assumed that each edge of the graph initially has a weight equal to one. In this paper we modify the problem and introduce the Weighted Edge Searching Problem by considering graphs with arbitrary positive integer weights assigned to its edges. We give bounds on the weighted search number in terms of related graph parameters including pathwidth. We characterize the graphs for which two searchers are sufficient to clear all edges. We show that for every weighted graph the minimum number of searchers needed for a not-necessarily-monotonic weighted edge search strategy is enough for a monotonic weighted edge search strategy, where each edge is cleaned only once. This result proves the NP-completeness of the problem. 相似文献
36.
Kenneth L. Stevenson Pamela B. Bell Ravi S. Dhawale Ott Horvth Attila Horvth 《Radiation Physics and Chemistry》1999,55(5-6):489-496
The ejection of hydrated electrons from 266-nm laser-photoexcited solutions containing Cu(NH3)+3, CuCl2−3, or CuBr2−3 occurs through two pathways on the nanosecond time scale: a prompt ejection (ττ>laser pulsewidth) which follows a first-order rate law. This behavior is consistent with electron ejection from two excited states: the primary CTTS state, and longer-lived triplet species consisting of an exciplex and its precursor. The quantum yields for both prompt and delayed ejection are quite high, in the 0.15–0.4 range. 相似文献
37.
Calorimetric techniques have revealed that the enthalpy of reaction with water is more exothermic by about 2.2 kcal/mol, for the perdeuteriated naphthalene anion radical (K+C10D8*-(s) + H2O(liq) --> 1/2C10D8H2(s) + 1/2C10D8(s) + KOH(aq)) than it is for the perprotiated system. These results, when coupled with the known enthalpy of electron transfer between naphthalene and its perdeuteriated analogue imply that the heat of hydrogenation of naphthalene decreases by about 1.8 kcal/mol upon perdeuteriation of the naphthalene. 相似文献
38.
Summary The analysis of several phenylurea herbicides in different waters is described using an immunoaffinity column clean-up and
determination by high performance liquid chromatography. The pesticides were selectively retained on a solid phase extraction
column containing antibodies to chlortoluron and to isoproturon immobilised onto silica. Selected phenylureas were eluted
using a simple phosphate buffered saline/ethanol mixture at low pH. The immunoaffinity approach gave extracts free of interfering
substances and allowed detection limits comparable to those required for pesticides in water in the European Community. The
proposed method was successfully applied to the determination of chlortoluron, isoproturon, linuron and chlorbromuron in tap
and river water.
Presented at the 21st ISC held in Stuttgart, Germany, 15th–20th September, 1996 相似文献
39.
Toh YC Zhang C Zhang J Khong YM Chang S Samper VD van Noort D Hutmacher DW Yu H 《Lab on a chip》2007,7(3):302-309
Mammalian cells cultured on 2D surfaces in microfluidic channels are increasingly used in drug development and biological research applications. These systems would have more biological or clinical relevance if the cells exhibit 3D phenotypes similar to the cells in vivo. We have developed a microfluidic channel based system that allows cells to be perfusion-cultured in 3D by supporting them with adequate 3D cell-cell and cell-matrix interactions. The maximal cell-cell interaction was achieved by perfusion-seeding cells through an array of micropillars; and 3D cell-matrix interactions were achieved by a polyelectrolyte complex coacervation process to form a thin layer of matrix conforming to the 3D cell shapes. Carcinoma cell lines (HepG2, MCF7), primary differentiated (hepatocytes) and primary progenitor cells (bone marrow mesenchymal stem cells) were perfusion-cultured for 72 hours to 1 week in the microfluidic channel, which preserved their 3D cyto-architecture and cell-specific functions or differentiation competence. This transparent 3D microfluidic channel-based cell culture system also allows direct optical monitoring of cellular events for a wide range of applications. 相似文献
40.
An interlaboratory study was performed on behalf of the UK Food Standards Agency to evaluate the effectiveness of an immunoaffinity column cleanup liquid chromatographic (LC) method for the determination of deoxynivalenol in a variety of cereals and cereal products at proposed European regulatory limits. The test portion was extracted with water. The sample extract was filtered a applied to an immunoaffinity column. After being washed with water, the deoxynivalenol was eluted with acetonitrile or methanol. Deoxynivalenol was quantitated by reversed-phase LC with UV determination. Samples of artificially contaminated wheat-flour, rice flour, oat flour, polenta, and wheat based breakfast cereal, naturally contaminated wheat flour, and blank (very low level) samples of each matrix were sent to 13 collaborators in 7 European countries. Participants were asked to spike test portions of all samples at a range of deoxynivalenol concentrations equivalent to 200-2000 ng/g deoxynivalenol. Average recoveries ranged from 78 to 87%. Based on results for 6 artificially contaminated samples (blind duplicates), the relative standard deviation for repeatability (RSDr) ranged from 3.1 to 14.1%, and the relative standard deviation for reproducibility (RSDR) ranged from 11.5 to 26.3%. The method showed acceptable within-laboratory and between-laboratory precision for all 5 matrixes, as evidenced by HorRat values < 1.3. 相似文献