全文获取类型
收费全文 | 4212篇 |
免费 | 716篇 |
国内免费 | 405篇 |
专业分类
化学 | 2858篇 |
晶体学 | 59篇 |
力学 | 303篇 |
综合类 | 19篇 |
数学 | 440篇 |
物理学 | 1654篇 |
出版年
2024年 | 7篇 |
2023年 | 116篇 |
2022年 | 102篇 |
2021年 | 167篇 |
2020年 | 195篇 |
2019年 | 181篇 |
2018年 | 167篇 |
2017年 | 141篇 |
2016年 | 237篇 |
2015年 | 201篇 |
2014年 | 236篇 |
2013年 | 327篇 |
2012年 | 407篇 |
2011年 | 411篇 |
2010年 | 272篇 |
2009年 | 234篇 |
2008年 | 250篇 |
2007年 | 236篇 |
2006年 | 226篇 |
2005年 | 191篇 |
2004年 | 133篇 |
2003年 | 105篇 |
2002年 | 86篇 |
2001年 | 75篇 |
2000年 | 87篇 |
1999年 | 74篇 |
1998年 | 68篇 |
1997年 | 62篇 |
1996年 | 62篇 |
1995年 | 54篇 |
1994年 | 30篇 |
1993年 | 40篇 |
1992年 | 41篇 |
1991年 | 23篇 |
1990年 | 21篇 |
1989年 | 15篇 |
1988年 | 12篇 |
1987年 | 8篇 |
1986年 | 6篇 |
1985年 | 10篇 |
1984年 | 1篇 |
1983年 | 7篇 |
1981年 | 2篇 |
1980年 | 2篇 |
1979年 | 2篇 |
1978年 | 1篇 |
1971年 | 1篇 |
1957年 | 1篇 |
排序方式: 共有5333条查询结果,搜索用时 156 毫秒
991.
α-萜品烯马来酰亚胺基酰腙衍生物的合成及杀菌活性研究 总被引:1,自引:0,他引:1
以α-蒎烯为原料,在质子酸催化下发生Wagner-Meerwein重排得到α-萜品烯,再与马来酸酐发生Diels-Alder环加成反应得到α-萜品烯马来酸酐(3),然后与水合肼反应制备N-氨基-α-萜品烯马来酰亚胺(4).在冰醋酸催化下,4与各种取代苯甲醛反应,合成得到17个新型α-萜品烯马来酰亚胺基酰腙化合物5a~5q.初步探索了合成条件,并利用元素分析,1H NMR,13C NMR,LC-MS,FT-IR等多种手段对目标产物作了分析表征.初步的生物活性测试表明,大部分化合物具有一定的杀菌活性,其中4-羟基-3-甲氧基苯基-α-萜品烯马来酰亚胺基酰腙(5n)在浓度为50 mg/L时对苹果轮纹病菌、花生褐斑病菌和番茄早疫病菌的抑制率分别达91%,83.3%和76.7%. 相似文献
992.
In this study, we successfully studied water‐soluble extract from Radix isatidis. Optimized conditions of MAE were listed, the sample can be extracted completely in 10 minutes under microwave power of 400W and solid/liquid ratio of 1:80. Active compounds in water‐soluble extract from R. isatidis were identified with HPLC‐DAD/ESI‐MS, these compounds followed by cytidine, uridine, guanosine, (R,S)‐goitrin and adenosine. RODWs–HPLC as a new sensitive chromatography were also first proposed and investigated, we favoringly used this method for simultaneous determination of these active constitutents in water‐soluble R. isatidis extract. Chromatographic separation was performed on a Diamonsil C18 column (5 μm, 150 mm × 4.6 mm) with a mobile phase gradient consisting of methanol and water at a flow‐rate of 1.0 mL/min, detection wavelengths 240, 250, 260 and 270 nm, the retention times of the tested five compounds were about 4.2, 5.8, 11.1, 14.2 and 20.8 min respectively, the limits of detection were 15, 12, 20, 5.8 and 24 ng/mL for cytidine, uridine, guanosine, (R,S)‐goitrin and adenosine respectively, their linear ranges were between 0.045 and 350 μg/mL with correlation coefficient (R) of 0.9998‐0.9999. The relative standard deviations (RSDs) of intra‐day and inter‐day assays were 0.30‐2.36% and 0.86‐2.54% respectively. Extraction recoveries were 94.25‐106.21%. This novel analytical method was shown to be simple, low‐cost, sensitive and reliable for multiple components in complex or undeveloped materials via MAE, ESI‐MS and RODWs‐HPLC. 相似文献
993.
The asymmetric epoxidation of 2-cyclohexen-1-one with aqueous H(2)O(2) as oxidant, 1,2-diaminocyclohexane as catalyst, and a Br?nsted acid trifluoroacetic acid (TFA) as cocatalyst has been studied by performing density functional theory calculations. It is confirmed that the catalyzed epoxidation proceeds via sequential nucleophilic addition and ring-closure processes involving a ketiminium intermediate. Four possible pathways associated with two Z isomers and two E isomers of ketiminium have been explored in detail. Our calculation indicates that these four pathways have high barriers and a small energy gap between two more favorable R and S pathways. We have analyzed the effects of the TFA anion and H(2)O on the activity and enantioselectivity of catalytic epoxidation. It is found that the TFA anion acts as a counterion to stabilize the transition states of the catalytic epoxidation by hydrogen-bond acceptance, leading to decreases in the barriers of the nucleophilic addition and ring-closure processes. The most significant decrease occurred in the ring-closure step of the Z-R-pathway, resulting in H-bond-induced enantioselectivity. Our calculations also show that water cooperates with TFA to further increase the reaction rate significantly. 相似文献
994.
A numerical model based on a hybrid finite element method is developed that seeks to join sound pressure fields in interior and exterior regions. The hybrid method is applied to the analysis of sound radiation from open pipes, or ducts, and uses mode matching to couple a finite element discretization of the region surrounding the open end of the duct to wave based modal expansions for adjoining interior and exterior regions. The hybrid method facilitates the analysis of ducts of arbitrary but uniform cross section as well the study of conical flanges and here a modal expansion based on spherical harmonics is applied. Predictions are benchmarked against analytic solutions for the limiting cases of flanged and unflanged circular ducts and excellent agreement between the two methods is observed. Predictions are also presented for flanged and unflanged rectangular ducts, and because the hybrid method retains the sparse banded and symmetric matrices of the traditional finite element method, it is shown that predictions can be obtained within an acceptable time frame even for a three dimensional problem. 相似文献
995.
996.
997.
检验人工听骨力学性质的解析方法 总被引:2,自引:0,他引:2
传导性耳聋多由鼓膜或听骨链病变所致,人工中耳已在临床上广泛应用于传导性耳聋治疗.根据人工中耳结构试验和力学假设,用变分原理建立在任意时刻$t$点,人工鼓膜受迫振动位移与激发声压的关系,推导控制方程. 采用分离变量法及贝塞尔函数对方程求解,得到鼓膜和听骨位移的解析解. 通过与文献试验数据比较,感觉曲线形状与走势基本一致,证明了该方程是正确的,能够反映声音的传导机制. 利用方程计算得到:质量小的材料传音效果好于质量大的;顶盘面积一定时,质量为某一特定值时传音效果更佳; 该结果与相关文献的试验结果相符合. 研究表明: 该方程对于人工中耳数据的获得非常方便,并且能够优化人工听骨材料,特别是对质量的控制能达到科学的效果,避免测量因素及其他因素的干扰.该方法为耳生物力学的研究及应用提供了新的视点. 相似文献
998.
There have been extensive studies on the large time behavior of solutions to systems on gas motions, such as the Navier-Stokes
equations and the Boltzmann equation. Recently, an approach is introduced by combining the energy method and the spectral
analysis to the study of the optimal rates of convergence to the asymptotic profiles. In this paper, we will first illustrate
this method by using some simple model and then we will present some recent results on the Navier-Stokes equations and the
Boltzmann equation. Precisely, we prove the stability of the non-trivial steady state for the Navier-Stokes equations with
potential forces and also obtain the optimal rate of convergence of solutions toward the steady state. The same issue was
also studied for the Boltzmann equation in the presence of the general time-space dependent forces. It is expected that this
approach can also be applied to other dissipative systems in fluid dynamics and kinetic models such as the model system of
radiating gas and the Vlasov-Poisson-Boltzmann system.
相似文献
999.
Jinqiao DUAN 《Frontiers of Mathematics in China》2009,4(3):425-436
Model uncertainties or simulation uncertainties occur in mathematical modeling of multiscale complex systems, since some mechanisms
or scales are not represented (i.e., ‘unresolved’) due to a lack in our understanding of these mechanisms or limitations in
computational power. The impact of these unresolved scales on the resolved scales needs to be parameterized or taken into
account. A stochastic scheme is devised to take the effects of unresolved scales into account, in the context of solving nonlinear
partial differential equations. An example is presented to demonstrate this strategy.
Dedicated to Professor Peter E. Kloeden on the occasion of his 60th birthday 相似文献
1000.