全文获取类型
收费全文 | 14579篇 |
免费 | 1396篇 |
国内免费 | 1434篇 |
专业分类
化学 | 3549篇 |
晶体学 | 226篇 |
力学 | 1913篇 |
综合类 | 238篇 |
数学 | 9067篇 |
物理学 | 2416篇 |
出版年
2024年 | 34篇 |
2023年 | 145篇 |
2022年 | 169篇 |
2021年 | 242篇 |
2020年 | 369篇 |
2019年 | 332篇 |
2018年 | 315篇 |
2017年 | 381篇 |
2016年 | 414篇 |
2015年 | 314篇 |
2014年 | 590篇 |
2013年 | 1022篇 |
2012年 | 745篇 |
2011年 | 868篇 |
2010年 | 852篇 |
2009年 | 924篇 |
2008年 | 994篇 |
2007年 | 1064篇 |
2006年 | 872篇 |
2005年 | 805篇 |
2004年 | 727篇 |
2003年 | 765篇 |
2002年 | 639篇 |
2001年 | 501篇 |
2000年 | 471篇 |
1999年 | 421篇 |
1998年 | 379篇 |
1997年 | 309篇 |
1996年 | 262篇 |
1995年 | 235篇 |
1994年 | 197篇 |
1993年 | 137篇 |
1992年 | 133篇 |
1991年 | 107篇 |
1990年 | 91篇 |
1989年 | 67篇 |
1988年 | 67篇 |
1987年 | 52篇 |
1986年 | 57篇 |
1985年 | 59篇 |
1984年 | 47篇 |
1983年 | 24篇 |
1982年 | 55篇 |
1981年 | 42篇 |
1980年 | 29篇 |
1979年 | 23篇 |
1978年 | 15篇 |
1977年 | 9篇 |
1976年 | 11篇 |
1972年 | 6篇 |
排序方式: 共有10000条查询结果,搜索用时 15 毫秒
71.
本文利用Liapunov-Schmidt方法获得了高维自治系统在共振情况下决定周期解个数的分支函数,并通过计算分支函数的主项,分析分支函数的零点,研究了具两对共轭特征根的四维系统多个周期解的共振分支问题. 相似文献
72.
G. N. Merrill 《Journal of Physical Organic Chemistry》2007,20(1):19-29
Ab initio molecular orbital calculations were carried out on epibromohydrin (EBH) and epichlorohydrin (ECH) in an attempt to elucidate their reactivity with respect to a hard nucleophile, hydroxide. These systems were modeled in both the gas phase and a polar solvent under basic conditions. In the gas phase, it was determined that a direct displacement mechanism (nucleophilic attack at the C1 position) was operative for EBH, while an indirect pathway (nucleophilic attack at the C3 position and subsequent intramolecular displacement) was followed for ECH. In an acetone solution, only the indirect displacement mechanism was found to occur. An electrostatic argument is advanced to account for this behavior in polar solution. Copyright © 2007 John Wiley & Sons, Ltd. 相似文献
73.
Small‐Angle Neutron Scattering has been performed from poly(ethylene oxide) in deuterated water at temperature ranging from 10 to 80 °C. A simple fitting model was used to obtain a correlation length and a Porod exponent. The correlation length L characterizes the average distance between entanglements in the semidilute region and is proportional to the individual coil sizes in the dilute region. L was found to increase with temperature in the semidilute region but it decreases with temperature in the dilute region. This decrease is the precursor to the single‐chain collapse which applies to very dilute polymer solutions. © 2007 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 45: 2196–2200, 2007 相似文献
74.
By using the theory of coincidence degree, we study a kind of periodic solutions to p-Laplacian neutral functional differential equation with multiple deviating arguments, a new result on the existence of periodic solutions is obtained. 相似文献
75.
用Langer变换和Olver变换求得一类具有转向点问题的n阶近似解:y(x)=v(x)ψ(x),其中ψ=λ12-14×(x2-1)14,2332=-λx∫11-τ2dτ,v(z)=A(z,λ)ξ(λ23z)+B(z,λ)'ζ(λ23z).并探讨了其特征值问题,得到λn=4n+1112,n=0,1,2….由此给出了该类问题的解的一般性结论. 相似文献
76.
利用微分方程的级数求解方法,分析了两端简支的有限长功能梯度圆筒的轴对称稳态热弹性问题,推导出了稳态温度场与应力场的解析解。分析中采用指数函数模型来描述FGM圆筒中材料性能在厚度方向的连续变化,同时忽略温度对材料性能的影响。另外,论文以金属钼和多铝红柱石制成的功能梯度圆筒为例,给出了稳态温度场和应力场的数值结果。 相似文献
77.
An inverse problem of the reconstruction of the right-hand side of the Euler-Darboux equation is studied. This problem is equivalent to the Volterra integral equation of the third kind with the operator of multiplication by a smooth nonincreasing function. Numerical solution of this problem is constructed using an integral representation of the solution of the inverse problem, the regularization method, and the method of quadratures. The convergence and stability of the numerical method is proved. 相似文献
78.
How can the basic compatibility of theory and observations be investigated for nonlinear processes without requiring stochastic characterizations for residual error terms? The present paper proposes a flexible least-cost approach. For each possible estimatex for the sequence of process states, letc D (x) andx M(x) denote the costs incurred for deviations away from the prior dynamic specifications and prior measurement specifications, respectively. Define the cost-efficiency frontier to be the greatest lower bound for the set of all possible cost pairs [c D (x),c M(x)], conditional on the given observations. State sequence estimatesx that attain the cost-efficiency frontier indicate the possible ways that the actual process could have developed over time in a manner minimally incompatible with the prior dynamic and measurement specifications. An algorithm is developed for the exact sequential updating of the cost-efficient state sequence estimates as the duration of the process increases and additional observations are obtained. 相似文献
79.
80.
We report viscometric data collected in a Couette rheometry on dilute, single‐solvent polystyrene (PS)/dioctyl phthalate (DOP) solutions over a variety of polymer molecular weights (5.5 × 105 ≤ Mw ≤ 3.0 × 106 Da) and system temperatures (288 K ≤ T ≤ 318 K). In view of the essential viscometric features, the current data may be classified into three categories: The first concerns all the investigated solutions at low shear rates, where the solution properties are found to agree excellently with the Zimm model predictions. The second includes all sample solutions, except for high‐molecular‐weight PS samples (Mw ≥ 2.0 × 106 Da), where excellent time–temperature superposition is observed for the steady‐state polymer viscosity at constant polymer molecular weights. No similar superposition applies at a constant temperature but varied polymer molecular weights, however. The third appears to be characteristic of dilute high‐molecular‐weight polymer solutions, for which the effects of temperature on the viscosity curve are further complicated at high shear rates. The implications concerning the relative importance of hydrodynamic interactions, segmental interactions, and chain extensibility with increasing polymer molecular weight, system temperature, and shear rate are discussed. © 2006 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 44: 787–794, 2006 相似文献