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Let D be an acyclic digraph. The competition graph of D is a graph which has the same vertex set as D and has an edge between u and v if and only if there exists a vertex x in D such that (u,x) and (v,x) are arcs of D. For any graph G, G together with sufficiently many isolated vertices is the competition graph of some acyclic digraph. The competition number k(G) of G is the smallest number of such isolated vertices.A hole of a graph is an induced cycle of length at least four. Kim (2005) [8] conjectured that the competition number of a graph with h holes is at most h+1. Recently, Li and Chang (2009) [11] showed that the conjecture is true when the holes are independent. In this paper, we show that the conjecture is true though the holes are not independent but mutually edge-disjoint. 相似文献
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The gallium substituted and calcium deficient variants of geometrically frustrated β-CaCr2O4, β-CaCr2-2xGa2xO4 (0.02≤x≤0.25) and β-Ca1−yCr2O4 (0.075≤y≤0.15), have been investigated by X-ray powder diffraction, magnetization and specific heat measurements. This allows for a direct comparison of the effects, in a geometrically frustrated magnet, of the static disorder that arises from non-magnetic substitution and the dynamic disorder that arises from hole doping. In both cases, disturbing the Cr3+ lattice results in a reduction in the degree of magnetic frustration. On substitution of Ga, which introduces disorder without creating holes, a gradual release of spins from ordered antiferromagnetic states is observed. In contrast, in the calcium-deficient compounds the introduction of holes induces static ferrimagnetic ordering and much stronger perturbations of the β-CaCr2O4 host. 相似文献
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Iridium(III) complexes are one of the most important electrophosphorescent dyes with tunable emissions in the range of visible and near infrared lights, high photoluminescence yields and short lifetimes for high-efficiency organic light-emitting diodes (OLED) with 100% exciton harvesting. This review summarizes the recent development of electroluminescent Ir3+ complexes functionalized with host-featured carrier-transporting groups, with emphasis on correlations between functionalization, optoelectronic properties and device performance. According to the introducing approaches, the complexes were sorted with conjugated and aliphatic linkages, as well as the types of functional groups. The modification effect on physical properties and the state-of-the-art device performances were discussed. 相似文献
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Kaizheng Zhu Zhiyuan Xie Lixiang Wang Xiabin Jing Fosong Wang 《Journal of polymer science. Part A, Polymer chemistry》2002,40(9):1321-1333
Two series of highly soluble novel nitrogen‐ and sulfur‐containing conjugated polymers were synthesized via an acid‐induced self‐polycondensation of functional monomers with methyl sulfinyl and aromatic groups. The well‐defined structures of synthesized polymers were confirmed by their NMR and IR spectra. The highest occupied molecular orbital energy values for these materials, estimated by cyclic voltammetry, showed a broad range of values from about 5.0 to 5.2 eV used as hole‐transport layers (HTL) in two‐layer light‐emitting diodes ITO/HTL/Alq3/Mg:Ag [ITO = indium tin oxide, and Alq3 = tris(8‐quinolinato) aluminum]. The typical turn‐on voltage of these diodes was about 4–5 V. The maximum brightness of the device was about 3440 cd/m2 at 20 V. The maximum efficiency was estimated to be 0.15 lm/W at 10 V. © 2002 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 40: 1321–1333, 2002 相似文献
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介绍了弹性分组环异步网络中发送速率同步功能的特点和实现同步的方式.结合弹性分组环环网中的带宽分配原则提出了一种能够适用于弹性分组环异步环网的同步传输机制:动态加权发送速率.该机制依据节点主传送队列深度动态改变接收数据流的转发权值来实现同步传输.研究结果表明该机制能够实现弹性分组环异步环网中的同步传输,节点主传送队列不会溢出. 相似文献
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近年来,由有机污染物和重金属引起的水污染对人类健康、生态系统和社会可持续发展构成了严重威胁.而光催化技术以其高效、低成本、节能、无二次污染等优点成为解决日益严重的环境污染问题的一个极具吸引力的策略.众所周知,宽光谱吸收、高效载流子分离和快速的表面反应动力学是高性能光催化剂所必备的基本条件.而多孔TiO2空心球具有以下结构优势:(1)成本低、无毒、氧化还原电位适中、物理化学性质稳定;(2)中空结构有利于入射光多重散射,而且大表面积可以暴露更多的活性位点;(3)多孔结构有利于传质过程.但是,窄光谱吸收和低的光生载流子分离效率严重阻碍了空心结构TiO2的实际应用.因此,通过耦合窄带隙半导体构建异质结光催化剂可以有效提高光吸收和促进光生电荷的分离.窄带隙(Eg=2.18–2.44 eV)半导体材料SnS2具有无毒、化学性质稳定、低成本和宽谱响应等诸多优点.若将超薄SnS2纳米片锚定生长在多孔TiO2空心球表面,将对异质结的光催化性能产生显著的影响.一方面,分级结构的空心球具有高可见光捕获率;另一方面,超薄SnS2纳米片具有更短的载流子扩散距离,从而有效地抑制光生载流子在催化剂体相内部复合.然而,由于其能带结构的限制,二元TiO2/SnS2复合材料很容易形成嵌入式I型异质结,在很大程度上降低了光催化氧化还原能力.此外,在光催化剂表面聚集的光生电子和空穴容易发生随机性复合.因此,迫切需要通过引入界面驱动力来调节表面载流子的分离和转移.众多研究表明,通过化学功能化可以实现对还原氧化石墨烯(rGO)的能带结构、功函数、电导率、亲水性和光学性质的调控.功能化的rGO可以作为优良的空穴提取材料,在rGO两侧分别耦合不同能级结构的半导体光催化材料,通过异质界面能级和功函数差异带来的界面内建电场,精确调控光生载流子在界面间的空间分离和定向迁移.基于上述分析,本文通过改进的"硅保护煅烧"方法合成了分级多孔SnS2/rGO/TiO2空心球异质结光催化剂.分级多孔空心球的结构优势不仅增强其光捕获能力,而且为光氧化还原反应提供了丰富的活性位点.特别是,在TiO2和SnS2纳米薄片之间嵌入的rGO中间层可以作为空穴注入层.由不同功函数导致的界面内建电场可以精确地调控光生空穴从SnS2纳米薄片的价带向rGO空穴注入层定向迁移,显著延长了光生载流子的寿命.在可见光照射下,负载2 wt%rGO的分级多孔SnS2/rGO/TiO2空心球异质结光催化剂对罗丹明B染料的降解率可达97.3%,对Cr(VI)的还原效率可达97.09%.此外,经过六个周期的循环实验,该异质结催化降解罗丹明B和还原Cr(VI)的效率没有明显降低,表现出较好的光催化稳定性. 相似文献