全文获取类型
收费全文 | 793篇 |
免费 | 28篇 |
国内免费 | 17篇 |
专业分类
化学 | 466篇 |
晶体学 | 20篇 |
力学 | 24篇 |
数学 | 169篇 |
物理学 | 159篇 |
出版年
2022年 | 7篇 |
2021年 | 8篇 |
2020年 | 12篇 |
2019年 | 6篇 |
2018年 | 9篇 |
2017年 | 6篇 |
2016年 | 13篇 |
2015年 | 15篇 |
2014年 | 15篇 |
2013年 | 31篇 |
2012年 | 42篇 |
2011年 | 57篇 |
2010年 | 28篇 |
2009年 | 36篇 |
2008年 | 35篇 |
2007年 | 40篇 |
2006年 | 36篇 |
2005年 | 40篇 |
2004年 | 22篇 |
2003年 | 27篇 |
2002年 | 39篇 |
2001年 | 8篇 |
2000年 | 15篇 |
1999年 | 9篇 |
1998年 | 8篇 |
1997年 | 6篇 |
1996年 | 15篇 |
1995年 | 17篇 |
1994年 | 13篇 |
1993年 | 7篇 |
1992年 | 5篇 |
1991年 | 12篇 |
1990年 | 8篇 |
1989年 | 10篇 |
1988年 | 8篇 |
1987年 | 12篇 |
1986年 | 8篇 |
1985年 | 10篇 |
1983年 | 8篇 |
1982年 | 11篇 |
1981年 | 13篇 |
1980年 | 13篇 |
1979年 | 7篇 |
1978年 | 15篇 |
1977年 | 7篇 |
1976年 | 5篇 |
1975年 | 11篇 |
1974年 | 13篇 |
1973年 | 7篇 |
1972年 | 6篇 |
排序方式: 共有838条查询结果,搜索用时 15 毫秒
101.
We report the successful application of low-melting inorganic salts with protonated cations (e.g. ammonium) as electrolytes in fuel cells operating in the temperature range 100-200 degrees C, where even with unoptimized electrodes, cell performance is comparable to that of the phosphoric acid fuel cell operating with optimized electrodes in the same temperature range, while open circuit voltages, and efficiencies at low current densities, can be much better--and there is no need for humidification or pressure to sustain performance. 相似文献
102.
Don Zagier 《Proceedings Mathematical Sciences》1994,104(1):57-75
In 1956, Rankin described which polynomials in the derivatives of modular forms are again modular forms, and in 1977, H Cohen
defined for eachn ≥ 0 a bilinear operation which assigns to two modular formsf andg of weightk andl a modular form [f, g]n of weightk +l + 2n. In the present paper we study these “Rankin-Cohen brackets” from two points of view. On the one hand we give various explanations
of their modularity and various algebraic relations among them by relating the modular form theory to the theories of theta
series, of Jacobi forms, and of pseudodifferential operators. In a different direction, we study the abstract algebraic structure
(“RC algebra”) consisting of a graded vector space together with a collection of bilinear operations [,]n of degree + 2n satisfying all of the axioms of the Rankin-Cohen brackets. Under certain hypotheses, these turn out to be
equivalent to commutative graded algebras together with a derivationS of degree 2 and an element Φ of degree 4, up to the equivalence relation (∂,Φ) ~ (∂ - ϕE, Φ - ϕ2 + ∂(ϕ)) where ϕ is an element of degree 2 andE is the Fuler operator (= multiplication by the degree).
Dedicated to the memory of Professor K G Ramanathan 相似文献
103.
Previous research on adaptive harvest strategies has suggested that it should rarely be optimal to introduce deliberate, probing variations in harvest rates in order to reduce uncertainty about production parameters, when there is strong aversion to the risk of low catches. Here we show that this suggestion is incorrect for cases where the stock dynamics can be approximated by a simple power model and the objective is to maximize the sum over time of discounted logarithms of catches. In such cases, the objective function has a simple form that permits easy comparison between passively adaptive (certainty equivalent or cautious) versus actively adaptive policies involving probing variation in harvest rates. Probing is favored when there is high uncertainty about the power parameter that represents density dependence in the stock dynamics. 相似文献
104.
105.
CdS半导体纳米晶体高强度激发下光谱特性研究 总被引:4,自引:2,他引:2
CdS半导体微晶作为代表性介观材料(mesoscopic material)其光学吸收和发光与量子尺寸效应的关系已经得到广泛研究[1-4],发现随着CdS微晶尺寸减小,CdS本征吸收和发射带呈现显著蓝移.Rossetti等人[3]和Y.Wang等人[4]分别通过对溶胶、沸石、聚合物和玻璃中CdS纳米晶体的光致发光测量研究了发光来源以及发光与尺寸的关系,确定了两个宽带发光分别属于带隙发光(350-500nm)和表面态或缺陷发光(500-700nm).本文首次报道了利用溶胶凝胶方法制备的钠硼硅中纳米尺寸CdS晶体高激发功率条件下的发光光谱测量结果,观察到随激发功率增加发光光谱兰移和线宽明显宽化,讨论了其物理机制. 相似文献
106.
混合卟啉体系中的光化学烧孔 总被引:1,自引:0,他引:1
在有机电子转移型烧孔体系中,混杂两种电子给体-苯并卟啉锌衍生物分子,可增加吸收带的非均匀宽度。本文通过对光谱孔的宽度和烧孔效率的讨论,证明了混杂电子给体的方法对孔宽和烧孔效率无明显影响,可以达到增加此类材料体系的烧孔数目,提高信息存储密度的目的。 相似文献
107.
新型永久光谱烧孔材料及机理研究 总被引:1,自引:0,他引:1
利用永久光谱烧孔(Persistent spectral hole burning(PSHB))可使光学信息存储密度提高三个数量级以上[1].这一重要应用前景激励科学家探索和寻求具有较高的非均匀(inhomogeneous)线宽与均匀(homogeneous)线宽之比(决定存储密度)和可擦除性(erasable)新型光谱烧孔材料.有机材料易于进行分子设计和组装,是较理想的光存储(optical storage)材料. 相似文献
108.
Chung‐Yang Chuang Trong‐Ming Don Wen‐Yen Chiu 《Journal of polymer science. Part A, Polymer chemistry》2009,47(11):2798-2810
In this research, thermo‐ and pH‐responsive nanoparticles with an average diameter of about 50–200 nm were synthesized via the surfactant‐free emulsion polymerization. The thermal/pH dual responsive properties of these nanoparticles were designed by the addition of a pH sensitive monomer, acrylic acid (AA), to be copolymerized with N‐isopropylacrylamide (NIPAAm) in a chitosan (CS) solution. The molar ratio of CS/AA/NIPAAm in the feed was changed to investigate its effect on structure, morphology, thermal‐ and pH‐responsive properties of the nanoparticles. It was found that CS‐PAA‐PNIPAAm nanoparticles could be well dispersed in the aqueous solution and carried positive charges on the surface. The addition of thermal‐sensitive NIPAAm monomer affected the polymerization mechanism and interactions between CS and AA. The particle size of the nanoparticles was found to be varied with the composition of NIPAAm monomer in the feed. The synthesized nanoparticles exhibited stimuli‐responsive properties, and their mean diameter thus could be manipulated by changing pH value and temperature of the environment. The nanoparticles showed a continuous release of the encapsulated doxycycline hyclate up to 10 days during an in vitro release experiment. The environmentally responsive nanoparticles are expected to be used in many fields such as drug delivery system. © 2009 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 47: 2798–2810, 2009 相似文献
109.
Using classical Monte Carlo trajectory methods, we have fitted a new kind of empirical potential energy hypersurface to combined experimental observations on the tritium-methane hot atom reactions. All channels (including Walden inversion) are now present and the degree of uniqueness is apparently high. We present our predicted cross sections as a function of tritium energy for the abstraction, inverting substituting, and total substitution reactions. 相似文献
110.