全文获取类型
收费全文 | 22320篇 |
免费 | 2277篇 |
国内免费 | 1828篇 |
专业分类
化学 | 16382篇 |
晶体学 | 275篇 |
力学 | 833篇 |
综合类 | 142篇 |
数学 | 2506篇 |
物理学 | 6287篇 |
出版年
2023年 | 228篇 |
2022年 | 386篇 |
2021年 | 534篇 |
2020年 | 586篇 |
2019年 | 626篇 |
2018年 | 491篇 |
2017年 | 535篇 |
2016年 | 857篇 |
2015年 | 853篇 |
2014年 | 1029篇 |
2013年 | 1390篇 |
2012年 | 1732篇 |
2011年 | 1948篇 |
2010年 | 1288篇 |
2009年 | 1256篇 |
2008年 | 1503篇 |
2007年 | 1376篇 |
2006年 | 1244篇 |
2005年 | 1100篇 |
2004年 | 1014篇 |
2003年 | 847篇 |
2002年 | 850篇 |
2001年 | 623篇 |
2000年 | 524篇 |
1999年 | 410篇 |
1998年 | 286篇 |
1997年 | 239篇 |
1996年 | 256篇 |
1995年 | 211篇 |
1994年 | 201篇 |
1993年 | 179篇 |
1992年 | 191篇 |
1991年 | 155篇 |
1990年 | 137篇 |
1989年 | 107篇 |
1988年 | 122篇 |
1987年 | 88篇 |
1986年 | 96篇 |
1985年 | 127篇 |
1984年 | 82篇 |
1983年 | 79篇 |
1982年 | 62篇 |
1981年 | 61篇 |
1980年 | 58篇 |
1979年 | 43篇 |
1977年 | 47篇 |
1976年 | 43篇 |
1975年 | 45篇 |
1974年 | 60篇 |
1973年 | 49篇 |
排序方式: 共有10000条查询结果,搜索用时 15 毫秒
961.
Broadband and high efficiency metal multi-layer dielectric grating based on non-quarter-wave coatings as a reflective mirror
下载免费PDF全文
![点击此处可从《中国物理 B》网站下载免费的PDF全文](/ch/ext_images/free.gif)
This article deals with designing a broadband and high efficiency metal multi-layer dielectric grating (MMDG) used to compress and stretch an ultrashort laser pulse. The diffraction characteristics of the MMDG are analysed by using the rigorous coupled-wave method. The multi-layer dielectric used as the reflective mirror is made up of non- quarter-wave coatings. Taking the diffraction efficiency of the 1 order as the value of merit function, the parameters such as groove depth, residual thickness, duty cycle, and reflective mirror are optimized to obtain broadband and high diffraction efficiency. The optimized MMDG shows an ultra-broadband working spectrum with the average efficiency exceeding 97% over 160 nm wavelength centred at 1053 nm and TE polarization. The optimized MMDG should be useful for chirped pulse amplification. 相似文献
962.
Piotr Kozlowski Silvia D. Chang Ran Meng Burkhard Mädler Robert Bell Edward C. Jones S. Larry Goldenberg 《Magnetic resonance imaging》2010
The purpose of this work was to compare diagnostic accuracy of Diffusion Tensor Imaging (DTI), dynamic contrast-enhanced magnetic resonance imaging (DCE MRI) and their combination in diagnosing prostate cancer. Twenty-five patients with clinical suspicion of prostate cancer underwent MRI, prior to transrectal ultrasound-guided biopsies. MRI data were correlated to biopsy results. Logistic regression models were constructed for the DTI parameters, DCE MRI parameters, and their combination. The areas under the receiver operator characteristic curves (AUC) were compared between the models. The nonparametric Wilcoxon signed rank test was used for statistical analysis. The sensitivity and specificity values were respectively 81% (74–87%) and 85% (79–90%) for DTI and 63% (55–70%) and 90% (85–94%) for DCE. The combination “DTI or DCE MRI” had 100% (97–100%) sensitivity and 77% (69–83%) specificity, while “DTI and DCE MRI” had 44% (37–52%) sensitivity and 98% (94–100%) specificity. The AUC for DTI+DCE parameters was significantly higher than that for either DTI (0.96 vs. 0.92, P=.0143) or DCE MRI parameters (0.96 vs. 0.87, P=.00187) alone. In conclusion, the combination of DTI and DCE MRI has significantly better accuracy in prostate cancer diagnosis than either technique alone. 相似文献
963.
964.
Residual determination and risk assessment of buprofezin in plum (Prunus domestica) grown in open‐field conditions following the application of three different formulations
下载免费PDF全文
![点击此处可从《Biomedical chromatography : BMC》网站下载免费的PDF全文](/ch/ext_images/free.gif)
Md. Humayun Kabir A.M. Abd El‐Aty Sung‐Woo Kim Han Sol Lee Md. Musfiqur Rahman Young‐Jun Lee Hyung Suk Chung Truong Lieu Jeong‐Heui Choi Ho‐Chul Shin Geon‐Jae Im Su Myeong Hong Jae‐Han Shim 《Biomedical chromatography : BMC》2016,30(11):1721-1727
This study was conducted to characterize the residual level and perform a risk assessment on buprofezin formulated as an emulsifiable concentrate, wettable powder, and suspension concentrate over various treatment schedules in plum (Prunus domestica). The samples were extracted with an AOAC quick, easy, cheap, effective, rugged, and safe, ‘QuEChERS’, method after major modifications. As intrinsic interferences were observed in blank plum samples following dispersive‐solid phase extraction (consisting of primary secondary amine and C18 sorbents), amino cartridges were used for solid‐phase extraction. Analysis was carried out using liquid chromatography with diode array detection and confirmed by liquid chromatography–tandem mass spectrometry. The method showed excellent linearity with determination coefficient (R2 = 1) and satisfactory recoveries (at two spiking levels, 0.5 and 2.5 mg/kg) between 90.98 and 94.74% with relative standard deviation (RSD) ≤8%. The limit of quantification (0.05 mg/kg) was considerably lower than the maximum residue limit (2 mg/kg) set by the Codex Alimentarius. Absolute residue levels for emulsifiable concentrates were highest, perhaps owing to the dilution rate and adjuvant. Notably, all formulation residues were lower than the maximum residue limit, and safety data proved that the fruits are safe for consumers. Copyright © 2016 John Wiley & Sons, Ltd. 相似文献
965.
966.
967.
968.
The miscibility, bioactivity, and antibacterial properties of chitosan/collagen specimens were systematically studied. The specimens were prepared by blending collagen and chitosan with varying deacetylation degrees in solutions; the collagen molecules had been extracted from pigskins using the acid swelling-pepsin digestion method. To understand the miscibility properties of collagen and chitosan molecules, the intrinsic viscosity and differential scanning calorimetry analysis of collagen, chitosan, and collagen/chitosan specimens were performed. The instrinsic viscosity measurements suggested that chitosan and collagen molecules with varying deacetylation degrees were miscible at molecular level for all compositions and degrees of deacetylation of chitosan/collagen mixture solutions prepared in this study. Fourier transform infrared analyses suggested that the percentage of preserved triple helix structures present in collagen molecules in collagen/chitosan specimens decreased with increasing chitosan contents, since the ratios of peak absorbance at 1239 cm?1 of amide III and 1455 cm?1 of C?H bending of collagen/chitosan specimens decreased significantly with increase in their chitosan contents. Abnormally high denaturation temperatures (Td) were observed as the chitosan contents of collagen/chitosan specimens reached 40 wt%, at which Td of collagen molecules was even higher than that of the corresponding pure chitosan molecules with varying deacetylation degrees. The antibacterial activity of collagen/chitosan blends increased consistently with increasing deacetylation degrees and concentrations of chitosan molecules in collagen/chitosan solutions. Possible explanations for these interesting thermal denaturation, antibacterial, and miscibility properties of chitosan/collagen specimens are reported. 相似文献
969.
Using two-colour visible (Vis)–ultraviolet (UV) photoionisation and pulsed field ionisation–photoelectron (PFI–PE) methods, we have obtained cleanly rotationally resolved photoelectron spectra for ZrO+(X 2Δ3/2,5/2; v+ = 0, 1, and 2). The rotation assignment of these state-to-state Vis–UV–PFI–PE spectra has allowed the unambiguous determination of the ground state term symmetry for ZrO+(X) to be 2Δ3/2, and the adiabatic ionisation energy of 90Zr16O, IE(90Zr16O) = 54,948.3(8) cm?1 [6.81272(10) eV]. The symmetry of the ionic ZrO+(X 2Δ3/2) ground state determined here disagrees with that reported in previous experiments. The rotational and vibrational constants determined in this experiment for the ionic 90Zr16O+(X 2Δ3/2) ground state are: Be+ = 0.4343(8) cm?1 and αe+ = 0.0019(5) cm?1, and ωe+ = 991.2(8) cm?1 and ωe+xe+ = 3.5(8) cm?1; and those for the ionic 90Zr16O+(X 2Δ5/2) excited spin-orbit state are: Be+ = 0.4357(6) cm?1 and αe+ = 0.0022(4) cm?1, and ωe+ = 991.9(8) cm?1 and ωe+xe+ = 3.6(8) cm?1, respectively. Based on the latter Be+ value, the equilibrium bond distances are determined to be re+ = 1.691(2) Å for 90Zr16O+(X 2Δ3/2) and re+ = 1.688(1) Å for 90Zr16O+(X 2Δ5/2). The IE(ZrO) along with the spectroscopic constants obtained here are valuable for benchmarking the ab initio quantum chemical calculations for energetic and structural predictions of ZrO/ZrO+. 相似文献
970.