全文获取类型
收费全文 | 28530篇 |
免费 | 79篇 |
国内免费 | 270篇 |
专业分类
化学 | 10202篇 |
晶体学 | 259篇 |
力学 | 1373篇 |
综合类 | 12篇 |
数学 | 8920篇 |
物理学 | 8113篇 |
出版年
2018年 | 1182篇 |
2017年 | 1440篇 |
2016年 | 692篇 |
2015年 | 543篇 |
2014年 | 452篇 |
2013年 | 627篇 |
2012年 | 3099篇 |
2011年 | 2270篇 |
2010年 | 1778篇 |
2009年 | 1520篇 |
2008年 | 526篇 |
2007年 | 575篇 |
2006年 | 571篇 |
2005年 | 4409篇 |
2004年 | 3888篇 |
2003年 | 2284篇 |
2002年 | 467篇 |
2001年 | 280篇 |
2000年 | 96篇 |
1999年 | 168篇 |
1998年 | 108篇 |
1997年 | 66篇 |
1996年 | 42篇 |
1995年 | 39篇 |
1994年 | 42篇 |
1993年 | 29篇 |
1992年 | 176篇 |
1991年 | 158篇 |
1990年 | 136篇 |
1989年 | 107篇 |
1988年 | 98篇 |
1987年 | 58篇 |
1986年 | 43篇 |
1985年 | 40篇 |
1984年 | 29篇 |
1983年 | 27篇 |
1979年 | 31篇 |
1978年 | 27篇 |
1976年 | 84篇 |
1975年 | 39篇 |
1974年 | 42篇 |
1973年 | 50篇 |
1972年 | 40篇 |
1971年 | 27篇 |
1970年 | 37篇 |
1969年 | 39篇 |
1968年 | 39篇 |
1967年 | 31篇 |
1966年 | 38篇 |
1923年 | 24篇 |
排序方式: 共有10000条查询结果,搜索用时 46 毫秒
991.
Yuanyuan?Zhu Rongjuan?Wang Li?Wang Yong?LiuEmail author Rui?XiongEmail author Jing?Shi 《Applied magnetic resonance》2015,46(5):505-513
We report the electron paramagnetic resonance (EPR) studies of MgTi2O4 in the 300–140 K range. Above the transition temperature T t (~258 K), the EPR results indicate that MgTi2O4 is paramagnetic. The parameters of the EPR spectra show an anomalous change at T t. The clear EPR lines can be observed in temperature between T t and 220 K. Besides that the EPR intensity, g value, and EPR linewidth increase with decreasing temperature; in temperature range below 220 K, no clear EPR line can be detected. The EPR spectra results demonstrate that magnetic spin-singlet state and the orbital density wave of MgTi2O4 system are formed gradually with decreasing temperature at low temperature range. 相似文献
992.
Camilla?Terenzi Cinzia?CasieriEmail author Francesco?De?Luca Raimondo?Quaresima Giovanni?Quarta Valentina?Tudisca 《Applied magnetic resonance》2015,46(10):1159-1178
This study deals with the application of two-dimensional proton nuclear magnetic resonance relaxometry (2D 1H NMR-R) to the characterization of porous ceramics nearly free of magnetic compounds. Different microstructural properties were obtained by firing a diamagnetic mixture of kaolin, calcium, and magnesium carbonate over a wide range of maximum temperatures (600–1100 °C) and firing times at the maximum temperature (soaking times) (0–10 h). The 2D 1H NMR-R method relies on the correlated measurement of 1H longitudinal (T 1) and transverse (T 2) relaxation times of pore-filling water by which the properties of the interconnected pore space may be investigated. In the absence of significant magnetic susceptibility effect due to para- and ferro-magnetic compounds, the 2D 1H NMR-R maps allow studying the conjoint effects on pore size distribution and inter-pore coupling due to the variations in both time and temperature of firing. The NMR experiments were performed with a low-field 1H NMR sensor, which allows non-destructive and in situ analysis. For ceramic specimens fired at 600 and 700 °C, the fraction of smallest pores increases with firing time at the expenses of those with intermediate size. The pore shrinkage occurring at this stage, and likely associated with the transformation of kaolinite in metakaolinite, is affected in a similar way by soaking time and firing temperature, in line with the concept of equivalent firing temperature. At temperatures from 800 to 1100 °C, the structural modifications involving interconnectivity and average pore size are driven primarily by firing temperature and, secondarily, by soaking time. The 2D 1H NMR-R results are confirmed by more traditional, but destructive, mineralogical, and structural analyses like X-ray powder diffraction, helium pycnometry, mercury intrusion porosimetry, and nitrogen adsorption/desorption method. 相似文献
993.
Saipeng?Huang Zhao?Chen Libo?Du Qiu?Tian Yangping?Liu Yuansuo?Zheng Yang?LiuEmail author 《Applied magnetic resonance》2015,46(5):489-504
The detection of free radicals and related species has attracted considerable attention in recent years due to their critical roles in physiological and pathological processes. Among the various methods for the detection of free radicals, electron spin resonance coupled with spin-trapping technique has been an effective approach for the characterization and quantification of free radicals due to its high specificity. In this study, we designed and synthesized a novel amphiphilic spin trap, 2-(diethoxyphosphoryl)-2-heptadecanyl-3,4-dihydro-2H-pyrrole-1-oxide (DEPHdPO), from 5-(diethoxyphosphoryl)-5-methyl-1-pyrroline-N-oxide with a long hydrocarbon chain at the C-5 position of the pyrroline ring, providing the amphiphilic character. The free-radical-trapping ability of DEPHdPO was evaluated by capturing hydroxyl radicals (·OH), superoxide anions (\( {\text{O}}_{2}^{ \cdot - } \)), and carbon-centered free radicals in a model membrane prepared from sodium dodecyl sulfate (SDS). The results indicate that the hydrophobic hydrocarbon chain of DEPHdPO can be inserted into the inner core of SDS micelles, and the hydrophilic nitronyl functional moiety is located on the surface layer. Thus, various free radicals, including ·OH radicals, \( {\text{O}}_{2}^{ \cdot - } \) anions, and carbon-centered radicals could be site-specifically detected near the membrane surface. Moreover, DEPHdPO could be successfully located on the surface of thylakoid membranes, and the nearby photo-initiated \( {\text{O}}_{2}^{ \cdot - } \) anions could be trapped site-specifically. 相似文献
994.
We discuss the discrete p-Laplacian eigenvalue problem, where T > 1 is a given positive integer and φ p (x):= |x| p?2 x, p > 1. First, the existence of an unbounded continuum C of positive solutions emanating from (λ, u) = (0, 0) is shown under suitable conditions on the nonlinearity. Then, under an additional condition, it is shown that the positive solution is unique for any λ > 0 and all solutions are ordered. Thus the continuum C is a monotone continuous curve globally defined for all λ > 0.
相似文献
$$\left\{ \begin{gathered} \Delta (\phi _p (\Delta u(k - 1))) + \lambda a(k)g(u(k)) = 0,k \in \{ 1,2,...,T\} , \hfill \\ u(0) = u(T + 1) = 0, \hfill \\ \end{gathered} \right.$$
995.
The numerical approximation of one-dimensional relativistic Dirac wave equations is considered within the recent framework consisting in deriving local scattering matrices at each interface of the uniform Cartesian computational grid. For a Courant number equal to unity, it is rigorously shown that such a discretization preserves exactly the \(L^2\) norm despite being explicit in time. This construction is well-suited for particles for which the reference velocity is of the order of \(c\), the speed of light. Moreover, when \(c\) diverges, that is to say, for slow particles (the characteristic scale of the motion is non-relativistic), Dirac equations are naturally written so as to let a “diffusive limit” emerge numerically, like for discrete 2-velocity kinetic models. It is shown that an asymptotic-preserving scheme can be deduced from the aforementioned well-balanced one, with the following properties: it yields unconditionally a classical Schrödinger equation for free particles, but it handles the more intricate case with an external potential only conditionally (the grid should be such that \(c \Delta x\rightarrow 0\)). Such a stringent restriction on the computational grid can be circumvented easily in order to derive a seemingly original Schrödinger scheme still containing tiny relativistic features. Numerical tests (on both linear and nonlinear equations) are displayed. 相似文献
996.
Our purpose is to provide an affirmative answer to Moszner’s problem [cf. Moszner (Ann Univ Paed Crac Stud Math XI:69–94, 2012), p. 93] concerning the superstability of the Cauchy equation with squaresin the class of functions mapping an Abelian semigroup into a finite-dimensional normed algebra without divisors of zero.
相似文献
$$f(x + y)^2 = (f(x) + f(y))^2$$
997.
Given a sequence A = (a 1, …, a n ) of real numbers, a block B of A is either a set B = {a i , a i+1, …, a j } where i ≤ j or the empty set. The size b of a block B is the sum of its elements. We show that when each a i ∈ [0, 1] and k is a positive integer, there is a partition of A into k blocks B 1, …, B k with |b i ?b j | ≤ 1 for every i, j. We extend this result in several directions. 相似文献
998.
Let \({\{ {f_{\lambda ;j}}\} _{\lambda \in V;1 \leqslant j \leqslant k}}\) be families of holomorphic functions in the open unit disk \({\text{D}} \subset {\Bbb C}\) ? ? depending holomorphically on a parameter λ ∈ V ? ? n . We establish a Rolle type theorem for the generalized multiplicity (called cyclicity) of zeros of the family of univariate holomorphic functions \({\left\{ {\sum\nolimits_{j = 1}^k {{f_{\lambda ;j}}} } \right\}_{\lambda \in V}}\) at 0 ∈ D. As a corollary, we estimate the cyclicity of the family of generalized exponential polynomials, that is, the family of entire functions of the form \(\sum\nolimits_{k = 1}^m {{P_k}(z){e^{{Q_k}(z)}}} \), z ∈ ?, where P k and Q k are holomorphic polynomials of degrees p and q, respectively, parameterized by vectors of coefficients of P k and Q k . 相似文献
999.
Let R be a finitely generated algebra over a field of characteristic 0 with a locally nilpotent derivation δ ≠ 0. We show that if {ie313-1}, where the invariants {ie313-2} are prime and satisfy a polynomial identity, then {ie313-3}. Furthermore, when R is a domain, the same conclusion holds without the assumption that R is finitely generated. This enables us to obtain a result on skew polynomial rings. These results extend work of Bell and Smoktunowicz on domains with GK dimension in the interval [2, 3). 相似文献
1000.
The generalized k-connectivity κ k (G) of a graph G was introduced by Chartrand et al. in 1984. As a natural counterpart of this concept, Li et al. in 2011 introduced the concept of generalized k-edge-connectivity which is defined as λ k (G) = min{λ(S): S ? V (G) and |S| = k}, where λ(S) denotes the maximum number l of pairwise edge-disjoint trees T 1, T 2, …, T l in G such that S ? V (T i ) for 1 ? i ? l. In this paper we prove that for any two connected graphs G and H we have λ 3(G □ H) ? λ 3(G) + λ 3(H), where G □ H is the Cartesian product of G and H. Moreover, the bound is sharp. We also obtain the precise values for the generalized 3-edge-connectivity of the Cartesian product of some special graph classes. 相似文献