全文获取类型
收费全文 | 4407篇 |
免费 | 555篇 |
国内免费 | 328篇 |
专业分类
化学 | 1908篇 |
晶体学 | 27篇 |
力学 | 208篇 |
综合类 | 34篇 |
数学 | 322篇 |
物理学 | 753篇 |
无线电 | 2038篇 |
出版年
2024年 | 12篇 |
2023年 | 54篇 |
2022年 | 56篇 |
2021年 | 78篇 |
2020年 | 161篇 |
2019年 | 181篇 |
2018年 | 145篇 |
2017年 | 195篇 |
2016年 | 230篇 |
2015年 | 237篇 |
2014年 | 303篇 |
2013年 | 288篇 |
2012年 | 284篇 |
2011年 | 354篇 |
2010年 | 260篇 |
2009年 | 223篇 |
2008年 | 292篇 |
2007年 | 301篇 |
2006年 | 227篇 |
2005年 | 212篇 |
2004年 | 193篇 |
2003年 | 199篇 |
2002年 | 162篇 |
2001年 | 137篇 |
2000年 | 120篇 |
1999年 | 103篇 |
1998年 | 80篇 |
1997年 | 46篇 |
1996年 | 30篇 |
1995年 | 22篇 |
1994年 | 17篇 |
1993年 | 19篇 |
1992年 | 21篇 |
1991年 | 11篇 |
1990年 | 7篇 |
1989年 | 8篇 |
1988年 | 3篇 |
1987年 | 2篇 |
1986年 | 3篇 |
1985年 | 1篇 |
1984年 | 3篇 |
1983年 | 2篇 |
1981年 | 2篇 |
1977年 | 1篇 |
1973年 | 1篇 |
1968年 | 2篇 |
1966年 | 1篇 |
1959年 | 1篇 |
排序方式: 共有5290条查询结果,搜索用时 15 毫秒
991.
Computational design of tetrazolone‐based high‐energy density energetic materials: Property prediction and decomposition mechanism 下载免费PDF全文
The density functional theory methods are used to design a series of new highly energetic tetrazolone‐based molecules by the combination of the linked tetrazolone framework and versatile substitutes. The molecular and electronic structures, physicochemical, and energetic properties were analyzed and predicted. The decomposition mechanisms were computationally simulated, and 3 potential decomposition channels were proposed. These newly designed tetrazolone‐based compounds show high densities (up to 2.08 g/cm3) and highly positive heats of formation (407.0‐1377.9 kJ/mol) due to all right content of nitrogen and oxygen. Most of them exhibit good detonation velocity (8.31‐9.62 km/s) and detonation pressure (32.40‐43.86 GPa), and some are comparative to excellent explosive CL‐20. Results show that compounds 6 , 10 , 11 , 12 , 15 , 16 , 17 , 22 , 23 , and 24 own superior detonation performance than widely used explosive HMX and may be promising candidates of green high‐performance energetic materials. 相似文献
992.
The adsorption and thermal decomposition of perfluorodimethyl ether, (CF3)2O, on a high-surface-area Al2O3 surface was investigated by FTIR under both vacuum and pressure conditions. IR spectra in the 4000-1050 cm−1 range were collected and the spectral assignments were assisted by quantum chemical ab initio calculations. The spectral evidence indicated that (CF3)2O decomposed to form adsorbed fluoroformate, FCOO (ads). Increases of temperature (up to 525 K) caused the FCOO (ads) to convert to hydrogen formate, HCOO (ads). Surface hydroxyl groups participated in the decomposition of (CF3)2O and the conversion of FCOO (ads) to HCOO (ads). A decomposition mechanism is proposed. 相似文献
993.
994.
四磺酸基酞菁氯化铝复合二氧化硅干凝胶的制备及光学性能研究 总被引:1,自引:1,他引:1
采用溶胶凝胶法将不同掺杂浓度的四磺酸基酞菁氯化铝(AlClPcTS)植入二氧化硅凝胶基质,制备了均匀掺杂的复合干凝胶通过对二氧化硅凝胶基质和掺杂AlClPcTS后复合干凝胶孔结构的测定和对复合体系紫外可见吸收光谱的表征,证实了AlClPcTS在基质中的成功掺杂,并发现AlClPcTS同时以单体和二聚体的形式存在于复合体系中,且AlClPcTS二聚体所占比例随着凝胶化过程的进行而增大对不同AlClPcTS掺杂浓度复合干凝胶的光限幅性能进行了测试,并讨论了掺杂浓度对复合体系光限幅性能的影响,证明复合体系的光限幅性能随掺杂浓度的提高而增强. 相似文献
995.
小分子磷光染料掺杂的聚合物发光器件具有发光效率高、制备工艺简单等优点,但是应该注意小分子染料的聚集与相分离问题,特别是在高掺杂浓度时更应注意防止相分离现象的发生。我们的思路是通过使用聚芴(Polyfluorene,PF)改性后的母体聚合物材料PC(poly[2,7-(9,9-dihexyl fluorene)-co-alt-2,10-(cyclohex—ane-1-spiro-6/-dibenzo[d,f][1,3]dioxepin)]),由于其发光峰位蓝移到紫外区,这就与作为掺杂分子的Ir(ppy)3配合物的吸收匹配得更好,进而达到提高能量转移效率的目的。在此条件下,可以实现较低掺杂浓度的发光,这对降低小分子染料的聚集以及相分离现象的发生是有帮助的。采用的器件结构为ITO/PEDOT:PSS/polymer:Ir(ppy)3/Ba/Al.当使用传统PF材料作为母体时,Ir(ppy)3需达到4%的掺杂比例才能实现能量的完全转移,而当采用改进后的PF作为母体时,Ir(ppy)3配合物只需达到0.5%的掺杂比例就能实现能量的完全转移,改进后的器件掺杂比例大幅度降低。 相似文献
996.
测量了不同Yb3+ 离子掺杂浓度下 ,Er3+ /Yb3+ 共掺SiO2 Al2 O3 La2O3玻璃的吸收光谱、荧光光谱和Yb3+离子2 F5/ 2 的能级寿命 ,应用迈克康伯 (McCumber)理论计算了Er3+ 的受激发射截面σemi,讨论了Yb3+ 离子浓度对其自身吸收性质、Er3+ 离子发光性质 ,以及Yb3+ →Er3+ 能量传递效率 (η)的影响 ,初步探明该基质玻璃中Yb3+ 离子掺杂数浓度的最佳范围为 3 .94× 1 0 2 0 cm- 3至 5 .92× 1 0 2 0 cm- 3,在此掺杂范围内 ,Yb3+ 离子的最大吸收系数为9.8cm- 1 ,Er3+ 的峰值发射截面和Yb3+ →Er3+ 能量传递效率 (η)分别为 0 64× 1 0 - 2 4 m2 和 92 %。 相似文献
997.
The Shanghai Synchrotron Radiation Facility(SSRF)booster ring,a full energy injector for the storage ring,is deigned to accelerate the electron beam energy from 150MeV to 3.5GeV that demands high extraction efficiency at the extraction energy with low beam loss rate when electrons are ramping.Closed orbit distortion(COD)caused by bending magnet field uniformity errors which affects the machine performance harmfully could be effectively reduced by bending magnet location sorting.Considering the affections of random errors in measurement,both ideal sorting and realistic sorting are studied based on measured bending magnet field uniformity errors and one reasonable combination of bending magnets which can reduce the horizontal COD by a factor of 5is given as the final installation sequence of the booster bending magnets in this paper. 相似文献
998.
The production and transportation of fluorescent light produced in wavelength-shifting fibers (WSFs) coupled to YAP scintillation crystal is simulated using the GEANT4 codes.An advantage of the wavelength-shifting fiber readout technique over a direct readout with a position-sensitive photo-sensor is the reduced requirement for position sensitive photomultiplier tube photocathode area.With this gamma-ray detector,the gamma camera is small and flexible and has larger effective field of view and low cost.Simulation results show that a) a mean 12 of photons per 59.5 keV gamma ray interaction is produced in the WSF located nearest to the incident gamma ray,and a spatial resolution of 3.6 mm FWHM is obtained,b)a mean 27 of photons per 140 keV gamma ray interaction is produced and a spatial resolution of 3.1 mm FWHM is obtained.Results demonstrate the feasibility of this concept of a compact gamma-ray detector based on wavelength-shifting fibers readout.However,since the very low photoelectron levels,it is very important to use a photon counting device with good single photo-electron response to readout the WSFs. 相似文献
999.
LIU Yan-Ping CHEN Zhao-Yang BA Wei-Zhen FAN Yan-Wei DU Yan-Zhao PAN Shi-Lie GUO Qi 《中国物理C(英文版)》2008,32(5)
The optically stimulated luminescent (OSL) radiation dosimeter technically surveys a wide dynamic measurement range and a high sensitivity.Optical fiber dosimeters provide capability for remote monitoring of the radiation in the locations which are difficult-to-acoess and hazardous.In addition.optical fiber dosimeters are immune to electrical and radio-frequency interference.In this paper,a novel remote optical fiber radiation dosimeter is described.The optical fiber dosimeter takes advantage of the charge trapping materials CaS:Ce, Sm that exhibit OSL.The measuring range of the dosimeter is from 0.1 to 100 Gy.The equipment is relatively simple and small in size,and has low power consumption.This device is suitable for measuring the space radiation dose and also can be used in high radiation dose condition and other dangerous radiation occasions. 相似文献
1000.
Proton-detected scalar coupling based assignment strategies in MAS solid-state NMR spectroscopy applied to perdeuterated proteins 总被引:1,自引:0,他引:1
Assignment of proteins in MAS (magic angle spinning) solid-state NMR relies so far on correlations among heteronuclei. This strategy is based on well dispersed resonances in the 15N dimension. In many complex cases like membrane proteins or amyloid fibrils, an additional frequency dimension is desirable in order to spread the amide resonances. We show here that proton detected HNCO, HNCA, and HNCACB type experiments can successfully be implemented in the solid-state. Coherences are sufficiently long lived to allow pulse schemes of a duration greater than 70 ms before incrementation of the first indirect dimension. The achieved resolution is comparable to the resolution obtained in solution-state NMR experiments. We demonstrate the experiments using a triply labeled sample of the SH3 domain of chicken α-spectrin, which was re-crystallized in H2O/D2O using a ratio of 1/9. We employ paramagnetic relaxation enhancement (PRE) using EDTA chelated CuII to enable rapid data acquisition. 相似文献