全文获取类型
收费全文 | 2378篇 |
免费 | 100篇 |
国内免费 | 8篇 |
专业分类
化学 | 1620篇 |
晶体学 | 13篇 |
力学 | 103篇 |
数学 | 343篇 |
物理学 | 407篇 |
出版年
2023年 | 28篇 |
2022年 | 25篇 |
2021年 | 54篇 |
2020年 | 59篇 |
2019年 | 58篇 |
2018年 | 41篇 |
2017年 | 29篇 |
2016年 | 71篇 |
2015年 | 63篇 |
2014年 | 59篇 |
2013年 | 103篇 |
2012年 | 147篇 |
2011年 | 169篇 |
2010年 | 84篇 |
2009年 | 70篇 |
2008年 | 152篇 |
2007年 | 136篇 |
2006年 | 147篇 |
2005年 | 126篇 |
2004年 | 67篇 |
2003年 | 74篇 |
2002年 | 47篇 |
2001年 | 39篇 |
2000年 | 24篇 |
1999年 | 20篇 |
1998年 | 32篇 |
1997年 | 21篇 |
1996年 | 23篇 |
1995年 | 18篇 |
1994年 | 18篇 |
1993年 | 21篇 |
1992年 | 21篇 |
1991年 | 19篇 |
1990年 | 14篇 |
1989年 | 16篇 |
1988年 | 13篇 |
1987年 | 19篇 |
1986年 | 25篇 |
1985年 | 15篇 |
1984年 | 23篇 |
1983年 | 16篇 |
1982年 | 20篇 |
1981年 | 28篇 |
1980年 | 29篇 |
1978年 | 22篇 |
1977年 | 24篇 |
1976年 | 21篇 |
1975年 | 16篇 |
1974年 | 11篇 |
1972年 | 11篇 |
排序方式: 共有2486条查询结果,搜索用时 31 毫秒
141.
We use Monte Carlo simulations to identify the mechanism that allows for phase transitions in dipolar spin ice to occur and survive for an applied magnetic field H much larger in strength than that of the spin-spin interactions. In the most generic and highest symmetry case, the spins on one out of four sublattices of the pyrochlore decouple from the total local exchange+dipolar+applied field. In the special case where H is aligned perfectly along the [110] crystallographic direction, spin chains perpendicular to H show a transition to q=X long-range order, which proceeds via a one- to three-dimensional crossover. We propose that these transitions are relevant to the origin of specific heat features observed in powder samples of the Dy2Ti2O7 spin ice material for H above 1 Tesla. 相似文献
142.
When a nonvolatile liquid film dewets from a partly compatible liquid substrate, the advancing dewetting front leaves behind droplets formed through a Rayleigh instability mechanism at its rim. We have found that these droplets continue to move in the direction of the dewetting front for extended periods (of order one day) with an initial droplet velocity varying linearly with the droplet size, and a displacement varying logarithmically with time. We attribute this persistent motion to a transient surface tension gradient on the substrate liquid surface trailing the dewetting front. 相似文献
143.
144.
Seferos DS Banach DA Alcantar NA Israelachvili JN Bazan GC 《The Journal of organic chemistry》2004,69(4):1110-1119
The selective cleavage of arylmethyl thioethers provides a convenient protocol for the synthesis of all-E isomers of alpha, omega-bis(thioacetyl)oligophenyenevinylene molecules (OPVs). The S-methyl group is tolerant of Wittig-type and Heck-type reactions for forming OPV structures and can be converted to the S-acetyl group by treatment with sodium thiomethoxide and acetyl chloride. The thermal conditions of the deprotection/reprotection step concurrently isomerize the conjugated chromophore to the all-E isomer, regardless of the stereochemistry of the starting olefins. This approach is demonstrated for a variety of linear and [2.2]paracyclophane containing OPVs, which have been characterized by electrochemical and spectroscopic techniques. Additionally, these S-acetyl-terminated OPVs self-assemble on gold surfaces. Monolayers containing these molecules were characterized by water contact angle measurements, ellipsometry, and X-ray photoelectron spectroscopy. 相似文献
145.
Hoh's calculus for pseudo-differential operators with negative-definitesymbols is extended to the case of parameter-dependent symbols.Then this parameter-dependent calculus is applied to the studyof subordinate sub-Markovian semigroups. 相似文献
146.
The use of rotational-echo double resonance NMR to measure distances from an observed tightly coupled cluster of 13C spins to a distant 15N, 31P, or 19F is practical if 13C chemical shifts and homonuclear 13C-13C isotropic J interactions are refocused by a combination of rotor-synchronized 13C pi and pi/2 pulses. This scheme is illustrated by experiments performed on diluted and recrystallized L-[13C(3),15N]alanine and L-[13C(6),alpha-15N]histidine. 相似文献
147.
de Boer J Leonards PE Klungsøyr J McHugh B Nixon E McGovern E Rimkus GG 《Journal of AOAC International》2003,86(2):432-438
A comparison was made between electron capture negative ionization quadrupole mass spectrometry (ECNI-MS) and electron capture detection (ECD) with regard to repeatability and reproducibility for the gas chromatographic (GC) analysis of toxaphene congeners [chlorobornanes (CHBs)]. The tests, including standard solutions and several cleaned fish extracts, showed larger relative standard deviations (RSDs) for the repeatability of ECNI-MS but no differences in the reproducibility of the 2 techniques. The sensitivity of the GC-ECNI-MS was considerably better than that of GC/ECD. Four stepwise-designed comparative tests were also conducted on GC analysis, cleanup, and the complete method. The results showed that, according to the current state-of-the-art, coefficients of variation for the between-laboratory performance were not < 20% and were usually between 20 and 30%. In spite of separation problems, e.g., for CHB 26, which cannot be separated into a single-component peak, a 95% methyl 5% phenyl polysiloxane (CP Sil 8) column was preferred to more polar columns for the analysis of CHBs 26, 40, 41, 44, 50, and 62. CHB 62 was more difficult to determine than CHB 26 and 50. Addition of the CHBs 40, 41, and 44 to the standard set of 3 chlorobornanes (26, 50, and 62) resulted in more separation problems. A 3-step cleanup method was recommended. 相似文献
148.
Jacob C Giles GI Giles NM Sies H 《Angewandte Chemie (International ed. in English)》2003,42(39):4742-4758
Sulfur and selenium occur in proteins as constituents of the amino acids cysteine, methionine, selenocysteine, and selenomethionine. Recent research underscores that these amino acids are truly exceptional. Their redox activity under physiological conditions allows an amazing variety of posttranslational protein modifications, metal free redox pathways, and unusual chalcogen redox states that increasingly attract the attention of biological chemists. Unlike any other amino acid, the "redox chameleon" cysteine can participate in several distinct redox pathways, including exchange and radical reactions, as well as atom-, electron-, and hydride-transfer reactions. It occurs in various oxidation states in the human body, each of which exhibits distinctive chemical properties (e.g. redox activity, metal binding) and biological activity. The position of selenium in the periodic table between the metals and the nonmetals makes selenoproteins ideal catalysts for many biological redox transformations. It is therefore apparent that the chalcogen amino acids cysteine, methionine, selenocysteine, and selenomethionine exhibit a unique biological chemistry that is the source of exciting research opportunities. 相似文献
149.
Giles GI Fry FH Tasker KM Holme AL Peers C Green KN Klotz LO Sies H Jacob C 《Organic & biomolecular chemistry》2003,1(23):4317-4322
Oxidative stress is implicated, either directly or indirectly, in the pathology of a range of human diseases. As a consequence, the development of efficient antioxidants for medical use has become increasingly important. We have synthesised a range of structurally related organo-sulfur, -selenium and -tellurium agents and demonstrated that a combination of electrochemical methodology, in vitro assays and cell culture tests can be used to rationalise the antioxidant activity of these catalytic agents. Based on its exceptionally low anodic oxidation potential (Epa) and high activity against the representative oxidative stressors tert-butyl hydroperoxide and peroxynitrite, 4,4'-dihydroxydiphenyltelluride is predicted to be a potent antioxidant. This compound exhibits a correspondingly high activity with a remarkably low IC50 value of 20 nM, when tested in PC12 cell culture using a bioassay indicative of the early stages of Alzheimer's disease. 相似文献
150.
Cosper NJ Bienvenue DL Shokes JE Gilner DM Tsukamoto T Scott RA Holz RC 《Journal of the American Chemical Society》2003,125(48):14654-14655
The Zn K-edge extended X-ray absorption fine structure (EXAFS) spectra, of the dapE-encoded N-succinyl-l,l-diaminopimelic acid desuccinylase (DapE) from Haemophilus influenzae have been recorded in the presence of one or two equivalents of Zn(II) (i.e. [Zn_(DapE)] and [ZnZn(DapE)]). The Fourier transforms of the Zn EXAFS are dominated by a peak at ca. 2.0 A, which can be fit for both [Zn_(DapE)] and [ZnZn(DapE)], assuming ca. 5 (N,O) scatterers at 1.96 and 1.98 A, respectively. A second-shell feature at ca. 3.34 A appears in the [ZnZn(DapE)] EXAFS spectrum but is significantly diminished in [Zn_(DapE)]. These data show that DapE contains a dinuclear Zn(II) active site. Since no X-ray crystallographic data are available for any DapE enzyme, these data provide the first glimpse at the active site of DapE enzymes. In addition, the EXAFS data for DapE incubated with two competitive inhibitors, 2-carboxyethylphosphonic acid and 5-mercaptopentanoic acid, are also presented. 相似文献