全文获取类型
收费全文 | 1381篇 |
免费 | 239篇 |
国内免费 | 340篇 |
专业分类
化学 | 1036篇 |
晶体学 | 38篇 |
力学 | 86篇 |
综合类 | 31篇 |
数学 | 174篇 |
物理学 | 595篇 |
出版年
2024年 | 1篇 |
2023年 | 23篇 |
2022年 | 40篇 |
2021年 | 43篇 |
2020年 | 61篇 |
2019年 | 49篇 |
2018年 | 41篇 |
2017年 | 56篇 |
2016年 | 39篇 |
2015年 | 71篇 |
2014年 | 87篇 |
2013年 | 110篇 |
2012年 | 123篇 |
2011年 | 122篇 |
2010年 | 98篇 |
2009年 | 101篇 |
2008年 | 99篇 |
2007年 | 90篇 |
2006年 | 106篇 |
2005年 | 93篇 |
2004年 | 69篇 |
2003年 | 51篇 |
2002年 | 41篇 |
2001年 | 59篇 |
2000年 | 48篇 |
1999年 | 31篇 |
1998年 | 32篇 |
1997年 | 23篇 |
1996年 | 24篇 |
1995年 | 12篇 |
1994年 | 26篇 |
1993年 | 18篇 |
1992年 | 14篇 |
1991年 | 11篇 |
1990年 | 7篇 |
1989年 | 6篇 |
1988年 | 1篇 |
1987年 | 3篇 |
1986年 | 7篇 |
1985年 | 3篇 |
1984年 | 4篇 |
1983年 | 4篇 |
1982年 | 3篇 |
1981年 | 3篇 |
1980年 | 2篇 |
1976年 | 1篇 |
1974年 | 2篇 |
1973年 | 1篇 |
1957年 | 1篇 |
排序方式: 共有1960条查询结果,搜索用时 31 毫秒
891.
研究了储能电容自身损耗对氙灯放电特性的影响,对脉冲氙灯放电回路进行了分析与改进。在实际电容自身损耗不为零的情况下,将电容等效为理想电容与损耗阻抗的串联,给出了计算氙灯放电特性的改进公式,并详细计算和分析了电容自身损耗增大情况下的氙灯放电电流和放电功率变化情况,计算结果表明:损耗增大会导致氙灯放电电流、功率峰值下降,闪光时间缩短,显著影响激光泵浦效率;电容损耗增大还会导致LC放电回路出现电容反充电现象,影响氙灯正常工作并缩短灯的寿命。实际的储能电容充放电实验证明,电容损耗增大会导致氙灯闪光的波形峰值下降、闪光时间缩短和电极溅射加剧,验证了理论分析的合理性。另外,实验证明环境因素对储能电容自身损耗的增大具有非常显著的影响。 相似文献
892.
893.
Rhizophora apiculata (R. apiculata) contains an abundance of biologically active compounds due its special salt-tolerant living surroundings. In this study, the total phenolic content and antioxidant activities of various extract and fractions of stem of R. apiculata were investigated. Results indicated that butanol fraction possesses the highest total phenolic content (181.84 mg/g GAE/g dry extract) with strongest antioxidant abilities. Following in vitro antioxidant activity-guided phytochemical separation procedures, lyoniresinol-3α-O-β-arabinopyranoside (1), lyoniresinol-3α-O-β-rhamnoside (2), and afzelechin-3-O-L-rhamno-pyranoside (3) were separated from the butanol fraction. These compounds showed more noticeable antioxidant activity than a BHT standard in the DPPH, ABTS and hydroxyl radical scavenging assays. HPLC analysis results showed that among different plant parts, the highest content of 1-3 was located in the bark (0.068%, 0.066% and 0.011%, respectively). The results imply that the R. apiculata might be a potential source of natural antioxidants and 1-3 are antioxidant ingredients in R. apiculata. 相似文献
894.
895.
896.
897.
Triatomic BNB has been produced by laser ablation of a boron nitride rod in a supersonic expansion of helium carrier gas and has been investigated using resonant two-photon ionization spectroscopy in the visible region. The B 2Pi(g)-X 2Sigma(u)+ band system has been recorded near 514 nm and is dominated by a strong origin band, which has been rotationally resolved and analyzed. Both the (11)B(14)N(11)B (64% natural abundance) and the (10)B(14)N(11)B (32% natural abundance) isotopic modifications have been analyzed, leading to the spectroscopic constants (and their 1sigma error limits) of B0"(X 2Sigma(u)+)=0.466 147(70), B0'(B 2Pi(g))=0.467 255(75), and A0'(B 2Pi(g))=6.1563(38) cm(-1) for (10)B(14)N(11)B, corresponding to r(B-N)"(X 2Sigma(u)+)=1.312 47(10) A and r(B-N)'(B 2Pi(g))=1.310 92(11) A. Very similar values are obtained for the more abundant isotopomer, (11)B(14)N(11)B: B0"(X 2Sigma(u)+)=0.444 493(69), B0'(B 2Pi(g))=0.445 606(70), A0'(B 2Pi(g))=6.1455(38) cm(-1), corresponding to r(B-N)"(X 2Sigma(u)+)=1.312 41(10) A and r(B-N)'(B 2Pi(g))=1.310 77(10) A. These results are discussed as they relate to Walsh's rules and are compared to results for related molecules. 相似文献
898.
Gao P Liu Y Zhang L Xu PF Wang S Lu Y He M Zhai H 《The Journal of organic chemistry》2006,71(25):9495-9498
The first total synthesis of (+/-)-subincanadine F, a bioactive indole alkaloid structurally featuring a 1-azabicyclo[4.3.1]decane unit, has been realized from 1-(para-methoxybenzyl)tryptamine in six steps. The bridge-containing tetracyclic framework of subincanadine F was efficiently assembled by a SmI2-mediated ring opening followed by an acid-mediated Mannich reaction. In addition, the tetracyclic ketoester 6, a key intermediate potentially useful for synthesizing structurally related indole alkaloids as well, was obtained in one step from alpha,beta-diketoester 5. 相似文献
899.
Identification of phenolic constituents in Radix Salvia miltiorrhizae by liquid chromatography/electrospray ionization mass spectrometry 总被引:2,自引:0,他引:2
The phenolic components from Radix Salvia miltiorrhizae Bunge, a well-known herbal medicine (Dan-Shen in Chinese), have been investigated by high-performance liquid chromatography/electrospray ionization tandem mass spectrometry (HPLC/ESI-MS/MS). HPLC analyses were performed on a reversed-phase C18 column using gradient elution. In the ESI mass spectra a predominant [M-H]- ion was observed in negative mode and provided molecular mass information. ESI-MS/MS spectra of the [M-H]- ions were used for structural analysis, based on the spectra of standards. It was found that caffeic acid and its monomeric analogs containing a carboxyl group readily lost CO2, while dimers, trimers and tetramers of caffeic acid expelled successively danshensu or caffeic acid or their esters. Twenty-eight phenolic compounds in S. miltiorrhizae were characterized, of which eight compounds were positively identified by comparison with standards. The remaining twenty phenolics for which standards were not available were tentatively identified based on their UV spectra and MS/MS fragmentation characteristics. 相似文献
900.
An Experiment for Generating the 14-Tone Stable Carriers Using Recirculating Frequency Shifter
下载免费PDF全文
![点击此处可从《中国物理快报》网站下载免费的PDF全文](/ch/ext_images/free.gif)
In experiment, the generation of 14-tone stable carriers with recirculating frequency shifter is realized. Some factors that impact the performance of the carrier generation are discussed. It is experimentally found that the balance and the insertion loss of the inphase/quadrature (I/Q,) modulator, the accurate π/2 phase difference and equal power level of two rf signals which are applied to the two ports of the I/Q, modulator, the gain of the optical amplifier, the bandwidth and sharp window spectrum of the tunable filter and the stability of the optical signals' states of polarization fed into I/Q, modulator, play important roles in the generation of a stable multi-tone carrier source. By employing appropriate techniques we obtain 14-tone stable carriers in the experiment. 相似文献