首页 | 本学科首页   官方微博 | 高级检索  
文章检索
  按 检索   检索词:      
出版年份:   被引次数:   他引次数: 提示:输入*表示无穷大
  收费全文   6381篇
  免费   514篇
  国内免费   32篇
化学   4982篇
晶体学   32篇
力学   159篇
综合类   1篇
数学   1043篇
物理学   710篇
  2022年   27篇
  2021年   54篇
  2020年   66篇
  2019年   114篇
  2018年   67篇
  2017年   63篇
  2016年   212篇
  2015年   238篇
  2014年   233篇
  2013年   394篇
  2012年   306篇
  2011年   328篇
  2010年   292篇
  2009年   289篇
  2008年   330篇
  2007年   282篇
  2006年   265篇
  2005年   236篇
  2004年   274篇
  2003年   228篇
  2002年   282篇
  2001年   154篇
  2000年   145篇
  1999年   72篇
  1998年   67篇
  1997年   80篇
  1996年   90篇
  1995年   64篇
  1994年   73篇
  1993年   72篇
  1992年   69篇
  1991年   42篇
  1990年   48篇
  1989年   49篇
  1988年   42篇
  1987年   41篇
  1986年   39篇
  1985年   49篇
  1984年   70篇
  1983年   57篇
  1982年   72篇
  1981年   75篇
  1980年   76篇
  1979年   83篇
  1978年   79篇
  1977年   87篇
  1976年   77篇
  1975年   76篇
  1974年   78篇
  1973年   54篇
排序方式: 共有6927条查询结果,搜索用时 46 毫秒
991.
992.
Abstract

We have investigated Cu, Zn Superoxide Dismutase (Cu, Zn SOD) metal sites at high pressure using X-ray absorption. XAS (X-ray Absorption Spectroscopy) gives information on local structure and it is particularly suited to metal site investigation. To the best of our knowledge, this is the first time that protein conformational states have been investigated using the high pressure XAS technique. Cu, Zn SOD catalyses the dismutation of toxic oxygen radicals produced in cells; this reaction occurs at the copper metal site. Structural changes around the copper, induced by pressure, can be directly related to protein substates. Their characterisation is thus important in the understanding of protein activity.

The high-pressure device was a Paris-Edinburgh large volume cell.

Experiments were performed on lyophilised Cu, Zn SOD between 0 and 48 kbar at the copper and zinc K-edges. The two metal local atomic environments have a different behaviour as pressure increases: copper exhibits a more flexible environment; on the contrary, zinc shows small structural modifications. We have identified a state, formed between 3 and 8 kbar, which is stable up to 48 kbar.  相似文献   
993.
Abstract

Neptunium and plutonium monosulfides were studied under high pressure up to ~60 GPa using a diamond anvil cell in an energy dispersive X-ray diffraction facility. The compounds, of cubic rock salt structure type at ambient pressure, do not show any crystallographic phase transition in the domain of investigation. From the pressure-volume relationship, we determined bulk moduli of 92 and 120 GPa with pressure derivatives of 4.6 and 4.1 for NpS and PUS respectively.  相似文献   
994.
We consider a certain class of nonsymmetric Markov chains and obtain heat kernel bounds and parabolic Harnack inequalities. Using the heat kernel estimates, we establish a sufficient condition for the family of Markov chains to converge to nonsymmetric diffusions. As an application, we approximate nonsymmetric diffusions in divergence form with bounded coefficients by nonsymmetric Markov chains. This extends the results by Stroock and Zheng to the nonsymmetric divergence forms. © 2012 Wiley Periodicals, Inc.  相似文献   
995.
This present study seeks to predict mathematical aptitude for higher education by examining the relationship between mathematics results from the Caribbean Examinations Council (CXC) general proficiency examination and the results from the General Certificate of Education (GCE) advanced level examination. This present study arose from a more extensive study that used a random sample of 177 high school students in the age cohort 17?19+ from a 7-year state-run, co-educational institution with a broad-based curriculum and a wide catchment area of student intake in The Republic of Trinidad and Tobago, West Indies. Semi-structured interviews and student academic records from the school comprised the survey instruments used for the study. Student grades were quantified using numerical scales. Findings suggest that the comprehension profile (Profile 2) of the CXC general proficiency level mathematics serves as a good predictor of mathematical aptitude for GCE advanced level mathematics. Implications for predicting mathematical aptitude for higher education and related issues are discussed.  相似文献   
996.
In recent papers, models of human locomotion by means of optimal control problems have been proposed. In this paradigm, the trajectories are assumed to be solutions of an optimal control problem whose cost has to be determined. The purpose of the present paper is to analyze the class of optimal control problems defined in this way. We prove strong convergence results for their solutions, on the one hand, for perturbations of the initial and final points (stability), and, on the other hand, for perturbations of the cost (robustness).  相似文献   
997.
Convex underestimators of a polynomial on a box. Given a non convex polynomial ${f\in \mathbb{R}[{\rm x}]}$ and a box ${{\rm B}\subset \mathbb{R}^n}$ , we construct a sequence of convex polynomials ${(f_{dk})\subset \mathbb{R}[{\rm x}]}$ , which converges in a strong sense to the “best” (convex and degree-d) polynomial underestimator ${f^{*}_{d}}$ of f. Indeed, ${f^{*}_{d}}$ minimizes the L 1-norm ${\Vert f-g\Vert_1}$ on B, over all convex degree-d polynomial underestimators g of f. On a sample of problems with non convex f, we then compare the lower bounds obtained by minimizing the convex underestimator of f computed as above and computed via the popular α BB method and some of its other refinements. In most of all examples we obtain significantly better results even with the smallest value of k.  相似文献   
998.
A type of glass modifications occurring after femto‐second laser irradiation gives rise to strong (10−2) from birefringence. This form birefringence is thought to be related to index nanostructure (called nanogratings). Analyzing induced tracks in fused silica using scanning electron microscopy (SEM) with nm resolution shows that nanostructures are porous nanoplanes with an average index lower than typical silica (Δn ∼ –0.20). Their origin is explained as arising from fast decomposition of the glass under localized, high‐intensity femtosecond laser radiation where strong nonlinear, multiphoton‐induced photoionization leads to plasma generation. Mechanistic details include Coulombic explosions characteristic of strong photoionization and the production of self‐trapped exciton (STE). Rapid relaxation of these STE prevents recombination and dissociated atomic oxygen instead recombines with each other to form molecular oxygen pointed out using Raman microscopy. Some of it is dissolved in the condensed glass whilst the rest is trapped within nanovoids. A chemical recombination can only occur at 1200 °C for many hours. This explains the thermal stability of such a nanostructure. Precise laser translation and control of these birefringent nanoporous structures allo arbitrarily tuning and positioning within the glass, an important tool for controlling optical properties for photonic applications, catalysts, molecular sieves, composites and more.  相似文献   
999.
In this paper, we show that the strict mononicity of the eigen-values of an uniformly elliptic operator of second order is equivalent to a unique continuation property.  相似文献   
1000.
Let $\mathbf{K }:=\left\{ \mathbf{x }: g(\mathbf{x })\le 1\right\} $ K : = x : g ( x ) ≤ 1 be the compact (and not necessarily convex) sub-level set of some homogeneous polynomial $g$ g . Assume that the only knowledge about $\mathbf{K }$ K is the degree of $g$ g as well as the moments of the Lebesgue measure on $\mathbf{K }$ K up to order $2d$ 2 d . Then the vector of coefficients of $g$ g is the solution of a simple linear system whose associated matrix is nonsingular. In other words, the moments up to order $2d$ 2 d of the Lebesgue measure on $\mathbf{K }$ K encode all information on the homogeneous polynomial $g$ g that defines $\mathbf{K }$ K (in fact, only moments of order $d$ d and $2d$ 2 d are needed).  相似文献   
设为首页 | 免责声明 | 关于勤云 | 加入收藏

Copyright©北京勤云科技发展有限公司  京ICP备09084417号