全文获取类型
收费全文 | 9443篇 |
免费 | 1099篇 |
国内免费 | 325篇 |
专业分类
化学 | 4812篇 |
晶体学 | 48篇 |
力学 | 596篇 |
综合类 | 103篇 |
数学 | 1144篇 |
物理学 | 4164篇 |
出版年
2024年 | 31篇 |
2023年 | 185篇 |
2022年 | 643篇 |
2021年 | 549篇 |
2020年 | 411篇 |
2019年 | 366篇 |
2018年 | 281篇 |
2017年 | 261篇 |
2016年 | 414篇 |
2015年 | 337篇 |
2014年 | 405篇 |
2013年 | 805篇 |
2012年 | 410篇 |
2011年 | 432篇 |
2010年 | 357篇 |
2009年 | 442篇 |
2008年 | 452篇 |
2007年 | 524篇 |
2006年 | 451篇 |
2005年 | 307篇 |
2004年 | 322篇 |
2003年 | 326篇 |
2002年 | 248篇 |
2001年 | 219篇 |
2000年 | 228篇 |
1999年 | 176篇 |
1998年 | 217篇 |
1997年 | 117篇 |
1996年 | 108篇 |
1995年 | 94篇 |
1994年 | 97篇 |
1993年 | 73篇 |
1992年 | 65篇 |
1991年 | 60篇 |
1990年 | 37篇 |
1989年 | 60篇 |
1988年 | 50篇 |
1987年 | 34篇 |
1986年 | 32篇 |
1985年 | 24篇 |
1984年 | 41篇 |
1983年 | 17篇 |
1982年 | 25篇 |
1981年 | 21篇 |
1980年 | 16篇 |
1979年 | 23篇 |
1978年 | 16篇 |
1977年 | 10篇 |
1976年 | 13篇 |
1973年 | 11篇 |
排序方式: 共有10000条查询结果,搜索用时 13 毫秒
41.
Dipl.‐Chem. Alexander Zhdanko Dr. Markus Ströbele Prof. Dr. Martin E. Maier 《Chemistry (Weinheim an der Bergstrasse, Germany)》2012,18(46):14732-14744
Coordination chemistry of gold catalysts bearing eight different ligands [L=PPh3, JohnPhos (L2), Xphos (L3), DTBP, IMes, IPr, dppf, S‐tolBINAP (L8)] has been studied by NMR spectroscopy in solution at room temperature. Cationic or neutral mononuclear complexes LAuX (L=L2, L3, IMes, IPr; X=charged or neutral ligand) underwent simple ligand exchange without giving any higher coordinate complexes. For L2AuX the following ligand strength series was determined: MeOH?hex‐3‐yne <MeCN≈OTf??Me2S<2,6‐lutidine<4‐picoline<CF3CO2?≈DMAP<TMTU<PPh3<OH?≈Cl?. Some heteroligand complexes DTBPAuX exist in solution in equilibrium with the corresponding symmetrical species. Binuclear complexes dppf(AuOTf)2 and S‐tolBINAP(AuOTf)2 showed different behavior in exchange reactions with ligands depending on the ligand strength. Thus, PPh3 causes abstraction of one gold atom to give mononuclear complexes LLAuPPh3+ and (Ph3P)nAu+, but other N and S ligands give ordinary dicationic species LL(AuNu)22+. In reactions with different bases, LAu+ provided new oxonium ions whose chemistry was also studied: (DTBPAu)3O+, (L2Au)2OH+, (L2Au)3O+, (L3Au)2OH+, and (IMesAu)2OH+. Ultimately, formation of gold hydroxide LAuOH (L=L2, L3, IMes) was studied. Ligand‐ or base‐assisted interconversions between (L2Au)2OH+, (L2Au)3O+, and L2AuOH are described. Reactions of dppf(AuOTf)2 and S‐tolBINAP(AuOTf)2 with bases provided more interesting oxonium ions, whose molecular composition was found to be [dppf(Au)2]3O22+, L8(Au)2OH+, and [L8(Au)2]3O22+, but their exact structure was not established. Several reactions between different oxonium species were conducted to observe mixed heteroligand oxonium species. Reaction of L2AuNCMe+ with S2? was studied; several new complexes with sulfide are described. For many reversible reactions the corresponding equilibrium constants were determined. 相似文献
42.
XRF测定铅锌矿选矿流程中铅,锌,铜 总被引:2,自引:0,他引:2
丁雪心 《光谱学与光谱分析》1994,14(1):111-114,104
本文叙述一个XRF粉末法测定铅锌矿选矿流程中铅,锌,铜的方法,选用精,中矿和原,尾矿两个标准系列。RhKa康普顿散射线作内标克服质量吸收系数变化带来的影响,用经验系数法校正基体效应,与化学分析方法相比,本法县有测定组分含量范围广,简便,快速,准确和成本低等特点,配合选矿试验大大加快造矿周期,提高工效数倍。 相似文献
43.
H^+注入锗酸铋(Bi4Ge3O12或BGO)晶体引起某些效应,如辐射损伤,光学吸收和近表层区域的晶体分解。经H^+注入后,BGO晶体的颜色变成棕色,但实验中证实不了该变化是由色心的产生所引起。此外,实验中也示观察到H^+注入BGO晶体中有离子束诱发的光学活性变化。可见在注入过程中,未发生从Bi4Ge3O12转变到Bi12GeO20的结构相变,由此预见,注入过程中可能发生离子束引起的晶体分解。H^ 相似文献
44.
M. A. Macrì C. Del Gratta L. Di Donato S. Di Luzio G. L. Romani S. Della Penna A. Pasquarelli 《Il Nuovo Cimento D》1994,16(5):425-432
Summary A preliminary study is here reported on a new potential marker for biomagnetic measurements. The marker consists of superparamagnetic
polymer microspheres which were detected in the presence of external steady magnetic fields by means of an r.f.-SQUID magnetometer.
The particles were prepared in samples differing in the concentration value and immersed in a homogeneous magnetic field of
variable intensity. A simple model was taken into account for the distribution of the microspheres in the samples, so that
the theoretical values were compared to the marker field values measured by the biomagnetic sensor. The overall sensitivity
of the experimental apparatus and the minimum concentration value of the marker were then estimated. 相似文献
45.
陈福生 《南昌大学学报(理科版)》1988,12(2):1
本文根据含有较多磁单极的一类加速天体的引力场度?,研究加速天体的某些?力学性质,我们得出结论:(1)具有加速运动的天体,即使不?的转动,也会造成两极与?道间的温差。(2)两个极区温度不同,θ=0的极区温度高于θ?π的极区温度。磁菏q的作用能使天体表面温度降低,使引力半径减小, 相似文献
46.
Summary Frequency distributions of energy deposition in microscopic tissue volumes determined by means of a ?Rossi type? proportional
counter for 31, 12 and 8 MeV proton beams and, dose response curves for mortality and chromosome aberrations in cultured human
cells exposed to the same beams are analysed with a view to determining suitable parameters to specify radiation quality.
The behaviour of quantities LET,Z
*2/β2 and the microdosimetric parameter
as a function of energy are compared with the corresponding behaviour of the most significant radiobiological parameters
as mean lethal dose per targetD
0, mean inactivation dose
, intensity of chromosome aberrations, and dicentrics. It is shown that
is the most suitable physical parameter to characterize the relative effectiveness of protons in the (8÷31) MeV range.
Work partially supported by grant No. 8000165896 for the Finalized Project ?Tumor Growth Control?. 相似文献
47.
K. Najim L. Pibouleau M. V. Le Lann 《Journal of Optimization Theory and Applications》1990,64(2):331-347
Optimization techniques are finding increasingly numerous applications in process design, in parallel to the increase of computer sophistication. The process synthesis problem can be stated as a largescale constrained optimization problem involving numerous local optima and presenting a nonlinear and nonconvex character. To solve this kind of problem, the classical optimization methods can lead to analytical and numerical difficulties. This paper describes the feasibility of an optimization technique based on learning systems which can take into consideration all the prior information concerning the process to be optimized and improve their behavior with time. This information generally occurs in a very complex analytical, empirical, or know-how form. Computer simulations related to chemical engineering problems (benzene chlorination, distillation sequence) and numerical examples are presented. The results illustrate both the performance and the implementation simplicity of this method.Nomenclature
c
i
penalty probability
-
cp
precision parameter on constraints
-
D
variation domain of the variablex
-
f(·)
objective function
-
g(·)
constraints
-
i,j
indexes
-
k
iteration number
-
N
number of actions
-
P
probability distribution vector
-
p
i
ith component of the vectorP as iterationk
-
r
number of reactors in the flowsheet
-
u(k)
discrete value or action chosen by the algorithm at iterationk
-
u
i
discrete value of the optimization variable in [u
min,u
max]
-
u
min
lowest value of the optimization variable
-
u
max
largest value of the optimization variable
-
Z
random number
-
x
variable for the criterion function
-
xp
precision parameter on criterion function
-
W(k)
performance index unit output at iterationk
-
0, 1
reinforcement scheme parameters
-
p
sum of the probability distribution vector components 相似文献
48.
由于实际海洋环境中存在大量的非高斯噪声,一些基于高斯假设的传统去噪方法在实际海洋环境中性能下降甚至失效。针对非高斯噪声,如α稳定分布噪声、非平稳行船噪声下的脉冲信号的去噪与重构,该文提出一种基于深度学习的方法。去噪模型首先通过学习带噪信号短时傅里叶变换谱与残差谱之间的映射关系以去除环境噪声,之后对去噪信号的时频谱进行逆变换重构脉冲信号。仿真实验结果表明,深度学习模型在非高斯噪声环境下脉冲信号的去噪与重构任务中有着良好的表现,在实测样本上也表现出良好的泛化性,体现了一定的工程应用价值。 相似文献
49.
50.
Optimization theory provides a framework for determining the best decisions or actions with respect to some mathematical model of a process. This paper focuses on learning to act in a near-optimal manner through reinforcement learning for problems that either have no model or the model is too complex. One approach to solving this class of problems is via approximate dynamic programming. The application of these methods are established primarily for the case of discrete state and action spaces. In this paper we develop efficient methods of learning which act in complex systems with continuous state and action spaces. Monte-Carlo approaches are employed to estimate function values in an iterative, incremental procedure. Derivative-free line search methods are used to obtain a near-optimal action in the continuous action space for a discrete subset of the state space. This near-optimal control policy is then extended to the entire continuous state space via a fuzzy additive model. To compensate for approximation errors, a modified procedure for perturbing the generated control policy is developed. Convergence results under moderate assumptions and stopping criteria are established. 相似文献