首页 | 本学科首页   官方微博 | 高级检索  
文章检索
  按 检索   检索词:      
出版年份:   被引次数:   他引次数: 提示:输入*表示无穷大
  收费全文   794篇
  免费   9篇
  国内免费   8篇
化学   674篇
晶体学   7篇
力学   5篇
数学   36篇
物理学   89篇
  2023年   4篇
  2021年   2篇
  2020年   6篇
  2018年   2篇
  2017年   2篇
  2016年   6篇
  2015年   6篇
  2014年   25篇
  2013年   46篇
  2012年   43篇
  2011年   38篇
  2010年   32篇
  2009年   19篇
  2008年   42篇
  2007年   44篇
  2006年   56篇
  2005年   47篇
  2004年   40篇
  2003年   49篇
  2002年   45篇
  2001年   17篇
  2000年   15篇
  1999年   14篇
  1998年   8篇
  1997年   11篇
  1996年   11篇
  1995年   13篇
  1994年   12篇
  1993年   6篇
  1992年   4篇
  1991年   7篇
  1990年   7篇
  1989年   5篇
  1988年   2篇
  1987年   8篇
  1986年   5篇
  1985年   10篇
  1984年   8篇
  1983年   4篇
  1982年   12篇
  1981年   5篇
  1980年   8篇
  1979年   8篇
  1978年   13篇
  1977年   8篇
  1976年   5篇
  1974年   11篇
  1973年   7篇
  1965年   3篇
  1936年   2篇
排序方式: 共有811条查询结果,搜索用时 15 毫秒
71.
Let p be an odd prime number such that p − 1 = 2em for some odd m and e ≥ 2. In this article, by using the special linear fractional group PSL(2, p), for each i, 1 ≤ ie, except particular cases, we construct a 2-design with parameters v = p + 1, k = (p − 1)/2i + 1 and λ = ((p − 1)/2i+1)(p − 1)/2 = k(p − 1)/2, and in the case i = e we show that some of these 2-designs are 3-designs. Likewise, by using the linear fractional group PGL(2,p) we construct an infinite family of 3-designs with the same v k and λ = k(k − 2). These supplement a part of [4], in which we gave an infinite family of 3-designs with parameters v = q + 1, k = (q + 1)/2 = (q − 1)/2 + 1 and λ = (q + 1)(q − 3)/8 = k(k − 2)/2, where q is a prime power such that q − 1 = 2m for some odd m and q > 7. Some of the designs given in this article and in [4] fill in a few blanks in the table of Chee, Colbourn, and Kreher [2]. © 1997 John Wiley & Sons, Inc.  相似文献   
72.
73.
Strong interaction of a system of quantum emitters (e.g., two-level atoms) with electromagnetic field induces specific correlations in the system accompanied by a drastic increase of emitted radiation (superradiation or superfluorescence). Despite the fact that since its prediction this phenomenon was subject to a vigorous experimental and theoretical research, there remain open question, in particular, concerning the possibility of a first order phase transition to the superradiant state from the vacuum state. In systems of natural and charge-based artificial atom this transition is prohibited by “no-go” theorems. Here we demonstrate numerically and confirm analytically a similar transition in a one-dimensional quantum metamaterial – a chain of artificial atoms (qubits) strongly interacting with classical electromagnetic fields in a transmission line. The system switches from vacuum state to the quasi-superradiant (QS) phase with one or several magnetic solitons and finite average occupation of qubit excited states along the transmission line. A quantum metamaterial in the QS phase circumvents the “no-go” restrictions by considerably decreasing its total energy relative to the vacuum state by exciting nonlinear electromagnetic solitons.  相似文献   
74.
Let R be a commutative Noetherian ring. Let P(R) (respectively,I(R)) be the category of all finite R-modules of finite projective(respectively, injective) dimension. Sharp [9] constructed acategory equivalence between I(R) and P(R) for certain Cohen–Macaulaylocal rings R. Thus many properties about finite modules offinite projective dimension can be connected with those of finiteinjective dimension through this equivalence.  相似文献   
75.

Purpose

The purposes of this study were to evaluate the reproducibility for measuring the cold pressor test (CPT)-induced myocardial blood flow (MBF) alteration using phase-contrast (PC) cine MRI, and to determine if this approach could detect altered MBF response to CPT in smokers.

Materials and methods

After obtaining informed consent, ten healthy male non-smokers (mean age: 28 ± 5 years) and ten age-matched male smokers (smoking duration ≥ 5 years, mean age: 28 ± 3 years) were examined in this institutional review board approved study. Breath-hold PC cine MR images of the coronary sinus were obtained with a 3 T MR imager with 32 channel coils at rest and during a CPT performed after immersing one foot in ice water. MBF was calculated as coronary sinus flow divided by the left ventricular (LV) mass which was given as a total LV myocardial volume measured on cine MRI multiplied by the specific gravity (1.05 g/mL).

Results

In non-smokers, MBF was 0.86 ± 0.25 mL/min/g at rest, with a significant increase to 1.20 ± 0.36 mL/min/g seen during CPT (percentage change of MBF (?MBF (%)); 39.2% ± 14.4%, p < 0.001). Inter-study reproducibility for ?MBF (%) measurements by different MR technologist was good, as indicated by the intraclass correlation coefficient of 0.93 and reproducibility coefficient of 10.5%. There was no significant difference between smokers and non-smokers for resting MBF (0.85 ± 0.32 mL/min/g, p = 0.91). However, ?MBF (%) in smokers was significantly reduced (-4.0 ± 32.2% vs. 39.2 ± 14.4%, p = 0.011).

Conclusion

PC cine MRI can be used to reproducibly quantify MBF response to CPT and to detect impaired flow response in smokers. This MR approach may be useful for monitoring the sequential change of coronary blood flow in various potentially pathologic conditions and for investigating its relationship with cardiovascular risk.  相似文献   
76.
A relationship between physical properties and local structure of 20Li2O·10Fe2O3·xWO3·(70–x)V2O5 glass, abbreviated as xLFWV glass (x?=?0???25 in mol%), was investigated by 57Fe-Mössbauer spectroscopy, Fourier transform infrared spectroscopy (FT-IR), differential thermal analysis (DTA), leaching test using 20 vol% HCl and DC two- or four-probe method. 57Fe-Mössbauer spectra of xLFWV glass showed an increase of quadrupole splitting (Δ) from 0.67 to 0.73±0.02 mm s???1 and a constant isomer shift (δ) of 0.39±0.01 mm s???1 with an increase of ‘x’ from 0 to 25. This suggests that FeIIIO4 tetrahedra gradually increase their local distortion along with a substitution of WO3 for V2O5. DTA of xLFWV glass showed an increase in glass transition temperature (T g) from 252 to 298 $_{\pm 5}^{\circ}$ C with an increase of ‘x’. Composition dependency of T g and Δ indicates that FeIII atoms occupy substitutional sites of WO6 octahedra as network former (NWF), since a large slope of 680 K (mm s???1)???1 was obtained in T g ? Δ plot. Comparable electrical conductivities (σ) of 2.5 × 10???6, 1.9 × 10???6, 8.4 × 10???7 and 2.9 × 10???6 S cm???1 obtained for xLFWV glasses with ‘x’ of 0, 10, 20 and 25, respectively increased to 2.4 × 10???2, 2.4 × 10???3, 3.5 × 10???4 and 8.8 × 10???5 S cm???1 after annealing at 400 °C for 100 min. Smaller Δ values of 0.58 and 0.67±0.02 mm s???1 obtained in annealed xLFWV glasses with ‘x’ of 0 and 10, respectively indicate that structural relaxation occurs in VO4 units of vanadate glass units, as had been observed in other vanadate glasses.  相似文献   
77.
Nanoparticles (NPs) of Fe3O4 and γFe2O3 synthesized by hydrothermal reaction were characterized by X-ray diffractometry (XRD), 57Fe-Mössbauer spectroscopy and field emission scanning electron microscopy (FE-SEM). A decrease in concentration of methylene blue (MB) aqueous solution due to bulk Fe0-NP γFe2O3 mixture with the mass ratio of 3:7 was measured by ultraviolet-visible light absorption spectroscopy (UV-Vis). The Mössbauer spectrum of NP Fe3O4 prepared from hydrothermal reaction was composed of two sextets with absorption area (A), isomer shift (δ) and internal magnetic field (H int) of 56.3 %, 0.34±0.03 mm s???1 and 49.0±0.30 T for tetrahedral (T d) FeIII, and 43.7 %, 0.66±0.11 mm s???1 and 44.0±0.71 T for octahedral (O h) FeII?+?III. The FeII/FeIII ratio was determined to be 0.280 for NP Fe3O4, giving ‘x’ of 0.124 in Fe3???xO4. These results show that NP Fe3O4 prepared by hydrothermal reaction was not regular but nonstoichiometric Fe3O4. Consistent results were observed for XRD patterns of NP Fe3???xO4 indicating sharp intense peaks at 2Θ of 30.2, 35.7 and 43.3° with a large linewidth of 0.44°, yielding the crystallite size of 29–37 nm from the Scherrer’s equation. Iso-thermal annealing of NP Fe3???xO4 at 250 °C for 30 min resulted in the precipitation of NP γFe2O3 with δ of 0.33±0.03 mm s???1 and H intof 46.4±0.27 T due to magnetic tetrahedral FeIII. The Debye temperature of NP Fe3???xO4 was respectively estimated to be 267±5.45 K for Fe $^{\mathrm{III}}(T_{\mathrm{d}})$ and 282±7.17 K for Fe $^{\mathrm{II+III}}(O_{\mathrm{h}})$ , both of which were smaller than that obtained for bulk Fe3O4 of 280±4.15 K and 307±5.70 K, indicating that the chemical environment of iron of NPs is less rigid than that of the bulk compounds. A leaching test using methylene blue (MB) and mixture of bulk Fe0-NP γFe2O3 (3:7) showed a remarkable decrease in MB concentration from 1.90 × 10???2 to 9.49 × 10???4 mM for 24 h with the first order rate constant (k MB) of 2.1 × 10???3 min???1. This result verifies that MB decomposing ability is enhanced by using NP γFe2O3 compared with the k MB of 1.1 × 10???4 min???1 previously obtained from the leaching test using MB and bulk mixture of Fe0???γFe2O3 (3:7).  相似文献   
78.
Charge-discharge capacity and cyclicity of lithium ion battery (LIB) was evaluated in which 15Li2O·10Fe2O3·xSnO2·5P2O5·(70–x)V2O5 glass (x?=?0 and 20 in mol%, abbreviated as xLFSPV) was used as a cathode. A local structure of xLFSPV glass before and after charging was investigated by 57Fe- and 119Sn-Mössbauer spectroscopies. 57Fe-Mössbauer spectrum of xLFSPV glass with ‘x’ of 20 was composed of a doublet with isomer shift (δ) of 0.35±0.02 mm s???1 and quadrupole splitting (Δ) of 0.88±0.03 mm s???1 due to distorted FeIIIO4 tetrahedra. 119Sn-Mössbauer spectrum of this glass consisted of a doublet with δ of 0.08±0.01 and Δ of 0.52±0.01 mms???1 due to distorted SnVIO6 octahedra. After discharging the battery from 4.5 to 1.0 V, larger δ of 0.40±0.03 mm s???1 and Δ of 0.94±0.04 mm s???1 were obtained, indicating that both iconicity of Fe-O bonds and local distortion of FeIIIO4 tetrahedra were increased. On the contrary, identical δ of 0.09±0.01 mm s???1 and Δ of 0.50±0.01 mm s???1 were observed in the 119Sn-Mössbauer spectrum of 20LFSPV glass after the discharge, indicating that chemical environment of SnIVO6 octahedra was not affected after the discharge. Charge-discharge curve of LIB containing 20LFSPV glass as a cathode active material recorded under the current density of 8.3 mA g???1 (0.011 mA cm???2) between 1.0 and 4.5 V showed a large initial charge capacity of 431.1 mAh g???1 and discharge capacity of 382.3 mAh g???1, respectively. These results indicate that 20LFSPV glass could be a new cathode active material for LIB.  相似文献   
79.
An insoluble and assembled catalyst of palladium and a non-cross-linked amphiphilic polymer were developed. In the presence of 50-500 ppm mol equiv of catalyst, the Suzuki-Miyaura reaction proceeded efficiently under organic solvent-free conditions. The catalyst was reused 10 times without any decrease in activity and was recycled without any special treatment.[structure: see text]  相似文献   
80.
[reaction: see text] D-Mannono-1,4-lactone was efficiently converted into L-ribose in eight steps. A key step of this synthesis is the cyclization of a gamma-hydroxyalkoxamate under Mitsunobu conditions. It is noteworthy that the O-alkylation product was obtained in 94% yield and that none of the N-alkylation product was detected in this cyclization.  相似文献   
设为首页 | 免责声明 | 关于勤云 | 加入收藏

Copyright©北京勤云科技发展有限公司  京ICP备09084417号