全文获取类型
收费全文 | 457篇 |
免费 | 26篇 |
国内免费 | 22篇 |
专业分类
化学 | 91篇 |
力学 | 28篇 |
综合类 | 2篇 |
数学 | 284篇 |
物理学 | 100篇 |
出版年
2024年 | 3篇 |
2023年 | 4篇 |
2022年 | 10篇 |
2021年 | 3篇 |
2020年 | 9篇 |
2019年 | 13篇 |
2018年 | 12篇 |
2017年 | 18篇 |
2016年 | 12篇 |
2015年 | 10篇 |
2014年 | 8篇 |
2013年 | 44篇 |
2012年 | 13篇 |
2011年 | 19篇 |
2010年 | 19篇 |
2009年 | 22篇 |
2008年 | 29篇 |
2007年 | 30篇 |
2006年 | 16篇 |
2005年 | 20篇 |
2004年 | 15篇 |
2003年 | 18篇 |
2002年 | 19篇 |
2001年 | 10篇 |
2000年 | 12篇 |
1999年 | 11篇 |
1998年 | 10篇 |
1997年 | 16篇 |
1996年 | 8篇 |
1995年 | 11篇 |
1994年 | 9篇 |
1993年 | 6篇 |
1992年 | 7篇 |
1991年 | 4篇 |
1990年 | 6篇 |
1989年 | 7篇 |
1988年 | 3篇 |
1987年 | 3篇 |
1986年 | 3篇 |
1984年 | 5篇 |
1983年 | 1篇 |
1981年 | 1篇 |
1976年 | 2篇 |
1974年 | 1篇 |
1973年 | 2篇 |
1969年 | 1篇 |
排序方式: 共有505条查询结果,搜索用时 15 毫秒
151.
D. Frekers P. Puppe J.H. Thies P.P. Povinec F. Šimkovic J. Staniček I. Sýkora 《Nuclear Physics A》2011
Experimental results on the measurement of the double-electron capture (εε) decay process of 74Se are described. This decay is particularly interesting, because 74Se is almost degenerate to the second excited state at 1204 keV in the daughter nucleus 74Ge. Such a degeneracy favors a neutrinoless decay because of the large phase-space suppression for a decay with two neutrinos. Subsequent de-excitation would generate a 2γ-ray cascade of 608 and 596 keV, whose coincident detection provides a unique signature for this particular εε process. The 2γ-ray cascade has been searched for by a HPGe γ-ray detector in coincidence with a NaI detector using 3 kg of natural selenium. 相似文献
152.
P. Belli R. Bernabei F. Cappella R. Cerulli F.A. Danevich S. dʼAngelo M.L. Di Vacri A. Incicchitti M. Laubenstein S.S. Nagorny S. Nisi A.V. Tolmachev V.I. Tretyak R.P. Yavetskiy 《Nuclear Physics A》2011
A search for double β decay of dysprosium was realized for the first time with the help of an ultra-low background HP Ge γ detector. After 2512 h of data taking with a 322 g sample of dysprosium oxide limits on double beta processes in 156Dy and 158Dy have been established on the level of T1/2?1014–1016 yr. Possible resonant double electron captures in 156Dy and 158Dy were restricted on a similar level. As a by-product of the experiment we have measured the radioactive contamination of the Dy2O3 sample and set limits on the α decay of dysprosium isotopes to the excited levels of daughter nuclei as T1/2?1015–1017 yr. 相似文献
153.
《Analytical letters》2012,45(11):2503-2512
Abstract A problem arises when determining the decision and detection limits in a series of results where the variance of the error distribution of the analytical signals around their expectation is not a constant when applying a method proposed by Hubaux and Vos6. In this paper an approach is proposed to overcome this difficulty, that involves the use of a transform method to convert heteroscedastic data to homoscedastic data so that the limit of detection can be obtained by projection of the confidence band about the linear regression. This approach is applied to the case of a gravimetric determination of sulphate ion in the 1 – 5 mgL?1 SO4 2- range. The results obtained from this approach are compared from others obtained previously. 相似文献
154.
An analytical method based on emission spectrography is described for the determination of the following trace elements: Ba, Co, Cr, Mn, Ni, Sc, Ti and V in CaO-matrix. The calibration curves have been constructed using the least squares method and the data treated statistically. Finally, the limits of detection, 1imits of guarantee for purity, and the precision of the method have been calculated. 相似文献
155.
We show that there are extremely simple signal detection schemes where the finiteness of energy resources places no limit on the resolution. On the contrary, larger resolution can be obtained with lower energy. To this end the generator of the signal-dependent transformation encoding the signal information on the probe state must be different from the energy. We show that the larger the deviation of the probe state from being the minimum-uncertainty state, the better the resolution. 相似文献
156.
Fabricio Macià 《偏微分方程通讯》2013,38(6):1137-1146
We study the concentration of eigenfunctions of the Laplace–Beltrami operator on manifolds all whose geodesics are closed (the so-called Zoll manifolds). Some results on the structure of the set of invariant semiclassical measures associated to sequences of eigenfunctions are given. Among these, we show that any probability measure on the unit tangent bundle of a compact rank-one symmetric space that is invariant by the geodesic flow may be realized as the semiclassical measure of a sequence of eigenfunctions of the Laplacian. This extends a previous result of Jakobson and Zelditch on spheres. 相似文献
157.
Magnetic resonance spectroscopy (MRS) has long been considered the golden standard for non-invasive measurement of tissue fat content. With improved techniques for fat/water separation, imaging has become an alternative to MRS for fat quantification. Several imaging models have been proposed, but their performance relative to MRS at very low fat contents is yet not fully established. In this work, imaging and spectroscopy were compared at 1.5 T and 3 T in phantoms with 0-3% fat fraction (FF). We propose a multispectral model with individual a priori R2 relaxation rates for water and fat, and a common unknown R2′ relaxation. Magnitude and complex image reconstructions were also compared. Best accuracy was obtained with the imaging method at 1.5 T. At 3 T, the FFs were underestimated due to larger fat-water phase shifts. Agreement between measured and true FF was excellent for the imaging method at 1.5 T (imaging: FFmeas= 0.98 FFtrue− 0.01%, spectroscopy: FFmeas= 0.77 FFtrue+ 0.08%), and fair at 3 T (imaging: FFmeas= 0.91 FFtrue− 0.19%, spectroscopy: FFmeas= 0.79 FFtrue+ 0.02%). The imaging method was able to quantify FFs down to approx. 0.5%. We conclude that the suggested imaging model is capable of fat quantification with accuracy and precision similar to or better than spectroscopy and offers an improvement vs. a model with a common R2* relaxation only. 相似文献
158.
Stanislaw Sieniutycz 《Applied Mathematical Modelling》2012,36(5):2197-2212
We present a synthesizing thermodynamic approach to modeling and power maximization in various energy converters, such as thermal, solar and chemical engines and fuel cells. Static and dynamical systems are investigated. Thermodynamic analyses lead to converters’ efficiencies in terms of propelling fluxes. Efficiency equations are applied to find maximum power points in static systems. These efficiency equations are also applied to determine maxima of integrated power (work) in dynamical systems, which work with upgrading and downgrading of a resource medium. While optimization of static systems requires using of differential calculus and Lagrange multipliers, dynamic optimization involves variational calculus and dynamic programming. In reacting mixtures balances of mass and energy are applied to derive power yield in terms of an active part of chemical affinity. Power maximization approach is finally applied for fuel cells treated as flow engines driven by fluxes of chemical reagents and electrochemical mechanism of electric current generation. The efficiency decrease is linked with thermodynamic and electrochemical irreversibilities expressed in terms of polarizations (activation, concentration and ohmic). Maximum power data provide bounds for SOFC energy generators, which are more exact and informative than reversible bounds for electrochemical transformation. 相似文献
159.
基于四种NIR仪器类型的清开灵注射液中黄芩苷成分的多变量检测限研究 总被引:1,自引:0,他引:1
选取a,b,c和d四种类型近红外光谱仪,分别采用透射模式测定清开灵注射液近红外光谱,以高效液相色谱法测定值作为参考值,建立不同仪器类型清开灵注射液中黄芩苷偏最小二乘(PLS)和间隔偏最小二乘(iPLS)定量模型,并计算模型的多变量检测限(MDL)。四种仪器的PLS模型决定系数(R2)和预测均方差(SEP)分别为0.976 2和230.4 μg·mL-1(a),0.956 1和246.4 μg·mL-1(b),0.966 2和264.4 μg·mL-1(c),0.998 5和71.5 μg·mL-1(d),其中d型仪器较其他三种类型能获得更好的模型性能。经iPLS变量筛选后,a和b两种类型仪器得到的iPLS模型R2pre和SEP分别为0.977 1和218.4 μg·mL-1,0.975 4和219.4 μg·mL-1,相较其PLS模型预测性能未见明显提高;c和d未筛选出变量。不同仪器的MDL(Δ0.05, 0.05)均低于250 μg·mL-1,其中c和d型MDL分别低至58和2.9 μg·mL-1。表明不同类型仪器定量预测性能和MDL不同。创新性采用多变量检测限理论探讨了不同类型近红外仪器的检测性能,这一方法具有可行性。在实际应用中应根据研究载体的特征选择合适的仪器类型,以确保定量准确性。 相似文献
160.
Vera Hoferichter Christoph Hirsch Thomas Sattelmayer 《Combustion Theory and Modelling》2017,21(3):382-418
Flame flashback is a major challenge in premixed combustion. Hence, the prediction of the minimum flow velocity to prevent boundary layer flashback is of high technical interest. This paper presents an analytic approach to predicting boundary layer flashback limits for channel and tube burners. The model reflects the experimentally observed flashback mechanism and consists of a local and global analysis. Based on the local analysis, the flow velocity at flashback initiation is obtained depending on flame angle and local turbulent burning velocity. The local turbulent burning velocity is calculated in accordance with a predictive model for boundary layer flashback limits of duct-confined flames presented by the authors in an earlier publication. This ensures consistency of both models. The flame angle of the stable flame near flashback conditions can be obtained by various methods. In this study, an approach based on global mass conservation is applied and is validated using Mie-scattering images from a channel burner test rig at ambient conditions. The predicted flashback limits are compared to experimental results and to literature data from preheated tube burner experiments. Finally, a method for including the effect of burner exit temperature is demonstrated and used to explain the discrepancies in flashback limits obtained from different burner configurations reported in the literature. 相似文献