全文获取类型
收费全文 | 923篇 |
免费 | 71篇 |
国内免费 | 56篇 |
专业分类
化学 | 569篇 |
晶体学 | 7篇 |
力学 | 28篇 |
综合类 | 5篇 |
数学 | 191篇 |
物理学 | 250篇 |
出版年
2023年 | 9篇 |
2022年 | 10篇 |
2021年 | 27篇 |
2020年 | 15篇 |
2019年 | 17篇 |
2018年 | 18篇 |
2017年 | 14篇 |
2016年 | 28篇 |
2015年 | 34篇 |
2014年 | 30篇 |
2013年 | 54篇 |
2012年 | 58篇 |
2011年 | 61篇 |
2010年 | 39篇 |
2009年 | 40篇 |
2008年 | 47篇 |
2007年 | 65篇 |
2006年 | 33篇 |
2005年 | 47篇 |
2004年 | 35篇 |
2003年 | 48篇 |
2002年 | 39篇 |
2001年 | 30篇 |
2000年 | 35篇 |
1999年 | 17篇 |
1998年 | 19篇 |
1997年 | 13篇 |
1996年 | 10篇 |
1995年 | 9篇 |
1994年 | 13篇 |
1993年 | 14篇 |
1992年 | 10篇 |
1991年 | 6篇 |
1990年 | 5篇 |
1989年 | 8篇 |
1988年 | 4篇 |
1987年 | 4篇 |
1986年 | 5篇 |
1985年 | 12篇 |
1984年 | 7篇 |
1983年 | 10篇 |
1982年 | 8篇 |
1981年 | 6篇 |
1980年 | 7篇 |
1979年 | 3篇 |
1977年 | 4篇 |
1976年 | 4篇 |
1972年 | 2篇 |
1942年 | 3篇 |
1928年 | 2篇 |
排序方式: 共有1050条查询结果,搜索用时 31 毫秒
91.
Puebla P Oshima-Franco Y Franco LM Santos MG Silva RV Rubem-Mauro L Feliciano AS 《Molecules (Basel, Switzerland)》2010,15(11):8193-8204
The effect of four sub-extracts prepared from the lyophilized hydroalcoholic bark of Dipteryx alata (Leguminosae-Papilionoideae) dissolved in a methanol-water (80:20) mixture through a liquid-liquid partition procedure has been investigated against the neuromuscular blockade of the venom of the snake Bothrops jararacussu. The active CH?Cl? sub-extract has been extensively analyzed for its chemical constituents, resulting in the isolation of four lupane-type triterpenoids: lupeol, lupenone, 28-hydroxylup-20(29)-en-3-one, betulin, nine isoflavonoids: 8-O-methylretusin, 7-hydroxy-5,6,4'-trimethoxyisoflavone, afrormosin, 7-hydroxy-8,3',4'-trimethoxyisoflavone, 7,3'-dihydroxy-8,4'-dimethoxyisoflavone, odoratin, 7,8,3'-trihydroxy-4'-methoxyisoflavone, 7,8,3'-trihydroxy-6,4'-dimethoxyisoflavone, dipteryxin, one chalcone: isoliquiritigenin, one aurone: sulfuretin and three phenolic compounds: vanillic acid, vanillin, and protocatechuic acid. Their chemical structures were elucidated on the basis of spectroscopic analysis, including HRMS, 1D- and 2D-NMR techniques. 相似文献
92.
San Martin A Rovirosa J Carrasco A Orejarena S Soto-Delgado J Contreras R Chamy MC 《Natural product communications》2010,5(12):1859-1864
The sesquiterpene pacifenol is one of the main constituents of the red alga Laurencia claviformis. Earlier work on the semisynthetic derivatives of pacifenol afforded a series of halogenated sesquiterpenes. The aim of the present work was to obtain new hydroxylated derivatives of halogenated sesquiterpenes by means of microbial transformation using Aspergillus niger, Gibberella fujikuroi and Mucor plumbeus. The best results were obtained with M. plumbeus. The microbiological transformation by M. plumbeus of pacifenol, and two semisynthetic derivatives, is described. The structures of the new compounds obtained were determined by spectroscopic means. 相似文献
93.
94.
Flores-Livas JA Debord R Botti S San Miguel A Marques MA Pailhès S 《Physical review letters》2011,106(8):087002
We present a joint experimental and theoretical study of the superconducting phase of the layered binary silicide BaSi(2). Compared with the AlB(2) structure of graphite or diboridelike superconductors, in the hexagonal structure of binary silicides the sp(3) arrangement of silicon atoms leads to corrugated sheets. Through a high-pressure synthesis procedure we are able to modify the buckling of these sheets, enhancing the superconducting transition temperature from 6 to 8.9 K when the silicon planes flatten out. By performing ab initio calculations based on density-functional theory we explain how the electronic and phonon properties are strongly affected by changes in the buckling. This mechanism is likely present in other intercalated layered superconductors, opening the way to the tuning of superconductivity through the control of internal structural parameters. 相似文献
95.
提出了一种新型光模块消光比补偿方法。该方法首先根据不同温度下驱动芯片上报电流的大小,结合光功率和消光比的计算公式,模拟出激光器光功率随电流及电压的变化曲线。然后根据要求调整激光器调制电流的大小,以保持消光比的稳定。该补偿方式有效地解决了光器件一致性较差的问题,可以将原来斜效率的指标适当放宽10%,从而提高了利用率和成品率,降低了模块生产成本。 相似文献
96.
Ramón Herrera Eugenio San Martin 《The European Physical Journal C - Particles and Fields》2011,71(7):1701
Warm-intermediate inflationary universe models in the context of braneworld cosmologies are studied. This study is done in
the weak and strong dissipative regimes. We find that the scalar potentials and dissipation coefficients in terms of the scalar
field evolve as type-power-law and powers of logarithms, respectively. General conditions required for these models to be
realizable are derived and discussed. We also study the scalar and tensor perturbations for each regime. We use recent astronomical
observations to constrain the parameters appearing in the braneworld models. 相似文献
97.
The defection or churn of customers represents an important concern for any company and a central matter of interest in customer base analysis. An additional complication arises in non-contractual settings, where the characteristics that should be observed to saying that a customer has totally or partially defected are not clearly defined. As a matter of fact, different definitions of the churn situation could be used in this context. Focusing on non-contractual settings, in this paper we propose a methodology for evaluating the short-time economic effects that using a certain definition of churn would have on a company. With this aim, we have defined two efficiency measures for the economic results of a marketing campaign implemented against churn, and these measures have been computed using a set of definitions of partial defection. Our methodology finds that definition maximizing both efficiency measures and moreover, the monetary amount that the company should invest per customer in the campaign for achieving the optimal solution. This has been modelled as a multiobjective optimization problem that we solved using compromise programming. Numerical results using real data from a Spanish retailing company are presented and discussed in order to show the performance and validity of our proposal. 相似文献
98.
In this paper, we consider the partially nonlinear errors-in-variables models when the non- parametric component is measured with additive error. The profile nonlinear least squares estimator of unknown parameter and the estimator of nonparametric component are constructed, and their asymptotic properties are derived under general assumptions. Finite sample performances of the proposed statistical inference procedures are illustrated by Monte Carlo simulation studies. 相似文献
99.
L. I. Ignat D. Pinasco J. D. Rossi A. San Antolin 《Journal d'Analyse Mathématique》2014,122(1):375-401
In this paper, we obtain bounds for the decay rate in the L r (? d )-norm for the solutions of a nonlocal and nonlinear evolution equation, namely, $$u_t \left( {x,t} \right) = \int_{\mathbb{R}^d } {K\left( {x,y} \right)\left| {u\left( {y,t} \right) - u\left( {x,t} \right)} \right|^{p - 2} \left( {u\left( {y,t} \right) - u\left( {x,t} \right)} \right)dy, x \in \mathbb{R}^d , t > 0.}$$ . We consider a kernel of the form K(x, y) = ψ(y?a(x)) + ψ(x?a(y)), where ψ is a bounded, nonnegative function supported in the unit ball and a is a linear function a(x) = Ax. To obtain the decay rates, we derive lower and upper bounds for the first eigenvalue of a nonlocal diffusion operator of the form $$T\left( u \right) = - \int_{\mathbb{R}^d } {K\left( {x,y} \right)\left| {u\left( y \right) - u\left( x \right)} \right|^{p - 2} \left( {u\left( y \right) - u\left( x \right)} \right)dy, 1 \leqslant p < \infty .}$$ . The upper and lower bounds that we obtain are sharp and provide an explicit expression for the first eigenvalue in the whole space ? d : $$\lambda _{1,p} \left( {\mathbb{R}^d } \right) = 2\left( {\int_{\mathbb{R}^d } {\psi \left( z \right)dz} } \right)\left| {\frac{1} {{\left| {\det A} \right|^{1/p} }} - 1} \right|^p .$$ Moreover, we deal with the p = ∞ eigenvalue problem, studying the limit of λ 1,p 1/p as p→∞. 相似文献
100.
The relation and transformation between hierarchical inner product encryption and spatial encryption
Hierarchical inner product encryption (HIPE) and spatial encryption (SE) are two important classes of functional encryption that have numerous applications. Although HIPE and SE both involve some notion of linear algebra, the former works in vectors while the latter is based on (affine) spaces. Moreover, they currently possess different properties in terms of security, anonymity (payload/attribute-hiding) and ciphertext sizes, for example. In this paper, we formally study the relation between HIPE and SE. In our work, we discover some interesting and novel property-preserving transformation techniques that enable generic construction of an SE scheme from an HIPE scheme, and vice versa. 相似文献