全文获取类型
收费全文 | 4651篇 |
免费 | 202篇 |
国内免费 | 11篇 |
专业分类
化学 | 3588篇 |
晶体学 | 23篇 |
力学 | 47篇 |
数学 | 410篇 |
物理学 | 796篇 |
出版年
2023年 | 34篇 |
2022年 | 28篇 |
2021年 | 51篇 |
2020年 | 71篇 |
2019年 | 57篇 |
2018年 | 36篇 |
2017年 | 31篇 |
2016年 | 133篇 |
2015年 | 157篇 |
2014年 | 130篇 |
2013年 | 226篇 |
2012年 | 289篇 |
2011年 | 325篇 |
2010年 | 200篇 |
2009年 | 128篇 |
2008年 | 265篇 |
2007年 | 250篇 |
2006年 | 211篇 |
2005年 | 236篇 |
2004年 | 220篇 |
2003年 | 180篇 |
2002年 | 157篇 |
2001年 | 109篇 |
2000年 | 91篇 |
1999年 | 64篇 |
1998年 | 70篇 |
1997年 | 60篇 |
1996年 | 68篇 |
1995年 | 57篇 |
1994年 | 54篇 |
1993年 | 58篇 |
1992年 | 51篇 |
1991年 | 28篇 |
1990年 | 32篇 |
1989年 | 25篇 |
1988年 | 42篇 |
1987年 | 36篇 |
1986年 | 25篇 |
1985年 | 40篇 |
1984年 | 54篇 |
1983年 | 21篇 |
1982年 | 31篇 |
1981年 | 29篇 |
1980年 | 33篇 |
1979年 | 25篇 |
1978年 | 35篇 |
1977年 | 39篇 |
1976年 | 18篇 |
1974年 | 23篇 |
1968年 | 15篇 |
排序方式: 共有4864条查询结果,搜索用时 125 毫秒
991.
Z. Bryknar I.P. Bykov M.D. Glinchuk V.V. Laguta Y.L. Maximenko Z. Potuček L. Jastrabík H.-J. Schulz 《Applied Physics A: Materials Science & Processing》1998,66(5):555-559
2+ spectra in axial and cubic crystal fields. Cu2+ substitutes for Ti4+ and the excess charge can be compensated by La3+ on a nearest-neighbor site, thus creating axial symmetry. The centers of cubic symmetry are those where the charge is compensated
in distant spheres.
In contrast to pure PLZT, PLZT:Cu exhibits a new luminescence band peaking at 1.18 eV. This emission is ascribed to the 2T2(D)→2E(D) transition of Cu2+(3d9) which can be excited either in the resonant 1.87 eV band or via charge-transfer excitation bands at 2.40, 2.57, and 3.03 eV.
The absorption band at 1.45 eV is assumed to be that of Cu+ ions. Annealing in hydrogen and in oxygen atmospheres caused decrease and restoration, respectively, of the ESR and luminescence
intensities as a consequence of Cu2+ conversion into Cu1+ and vice versa.
Received: 14 November 1997/Accepted: 8 December 1997 相似文献
992.
Christine M. Sapienza Elaine T. Stathopoulos W.S. Brown Jr. Ph.D. 《Journal of voice》1997,11(2):195-201
This study examined speech breathing patterns during reading bywomen with bilateral vocal fold nodules judged as mildly dysphonic and by women without vocal nodules. Although it might be predictable that the speech breathing patterns of individuals with laryngeal dysfunction will differ from those without laryngeal dysfunction, there is a lack of empirical data to support such assumptions.The results of the current study indicated that glottal airflow was greaterduring reading for the women with vocal nodules and that a larger volume of air was expended both per syllable and per breath group during reading. The rate of speech did not significantly differ between the two groups of women. There was no significant difference for the average duration of the breath groups and no significant difference for the number of syllables spoken per breath group. Additionally, both groups of women demonstrated a similar pattern of inspiratory pause location during the reading. The results suggest that speech breathing patterns associated with dysphonia be examined independently to distinguish specifically the nature of the interaction between the laryngeal dysfunction and the speech breathing pattern. Certainly, more information on how the severity of a voice disorder influences speech breathing is necessary. 相似文献
993.
994.
A photoluminescence band with zero-phonon transitions at (this one subject to self-absorption) and at 12259 cm-1 has been detected with ZnS:Ni crystals. Based on the appearance of these two lines, on the similarity of the whole spectrum with the 3T2(F) → 3T1(F) Ni2+ luminescence, and on preparation evidence, this new emission is interpreted as the 3T1(P) → 3T1(F) transition of substitutional Ni2+(3d8) ions in a tetrahedral crystal field. 相似文献
995.
We report on the synthesis and characterization of new sila‐ and germa‐thiazolidines derived from 2‐[1‐(1‐naphthyl)ethyl]‐2‐imidazoline. The radioprotective activity was evaluated in mice by intraperitoneal injection. A notable diminution in the toxicity and increase in radioprotective efficacy were shown for the organometallic derivatives compared with the unsubstituted organic precursors. For some of the cyclic compounds, a delay effect was observed tranducing a slow opening in vivo of the thiazolidine ring. Copyright © 2004 John Wiley & Sons, Ltd. 相似文献
996.
997.
998.
The metabolic transformation of methandienone (I) in the horse was investigated. After administration of a commercial drug preparation to a female horse (0.5 mg/kg), urine samples were collected up to 96 h and processed without enzymic hydrolysis. Extraction was performed by a series of solid-liquid and liquid-liquid extractions, thus avoiding laborious purification techniques. For analysis by gas chromatography-mass spectrometry, the extracts were trimethylsilylated. Besides the parent compound I and its C-17 epimer II, three monohydroxylated metabolites were identified: 6 beta-hydroxymethandienone (III), its C-17 epimer (IV) and 16 beta-hydroxymethandienone (V). In addition, three isomers of 6 beta,16-dihydroxymethandienone (VIa-c) were discovered. Apparently, reduction of the delta 4 double bond of 16 beta-hydroxymethandienone (V) in the horse yields 16 beta,17 beta-dihydroxy-17 alpha-methyl-5 beta-androst-1-en-3-one (VII). Reduction of the isomers VIa-c results in the corresponding 6 beta,16,17-trihydroxy-17-methyl-5 beta-androst-1-en-3-ones (VIIIa-c). The data presented here suggest that screening for the isomers of VI and VIII, applying the selected-ion monitoring technique, will be the most successful way of proving methandienone administration to a horse. 相似文献
999.
Syntheses for 9-β-D-ribofuranosyluric acid (16) and its 5′-monophosphate 14 starting from guanosine and by applying the p-nitrophenylethyl blocking group are described. 相似文献
1000.
We investigate a mechanical system (forced nonlinear torsion pendulum). The state diagram is given as function of both the external driving frequency and the damping parameter. A bifurcation diagram is measured showing period doubling, chaos and periodic windows. The results are in qualitative agreement with the recent theory. 相似文献