全文获取类型
收费全文 | 32385篇 |
免费 | 685篇 |
国内免费 | 142篇 |
专业分类
系统科学 | 1218篇 |
丛书文集 | 321篇 |
教育与普及 | 64篇 |
理论与方法论 | 354篇 |
现状及发展 | 13000篇 |
研究方法 | 1247篇 |
综合类 | 16625篇 |
自然研究 | 383篇 |
出版年
2018年 | 763篇 |
2017年 | 785篇 |
2016年 | 509篇 |
2012年 | 707篇 |
2011年 | 1729篇 |
2010年 | 978篇 |
2009年 | 619篇 |
2008年 | 970篇 |
2007年 | 1309篇 |
2006年 | 582篇 |
2005年 | 604篇 |
2004年 | 668篇 |
2003年 | 632篇 |
2002年 | 576篇 |
2001年 | 894篇 |
2000年 | 856篇 |
1999年 | 590篇 |
1992年 | 513篇 |
1991年 | 416篇 |
1990年 | 448篇 |
1989年 | 414篇 |
1988年 | 405篇 |
1987年 | 441篇 |
1986年 | 451篇 |
1985年 | 530篇 |
1984年 | 460篇 |
1983年 | 360篇 |
1982年 | 330篇 |
1981年 | 350篇 |
1980年 | 392篇 |
1979年 | 914篇 |
1978年 | 738篇 |
1977年 | 649篇 |
1976年 | 574篇 |
1975年 | 584篇 |
1974年 | 751篇 |
1973年 | 635篇 |
1972年 | 646篇 |
1971年 | 794篇 |
1970年 | 1005篇 |
1969年 | 717篇 |
1968年 | 685篇 |
1967年 | 750篇 |
1966年 | 622篇 |
1965年 | 442篇 |
1959年 | 243篇 |
1958年 | 414篇 |
1957年 | 294篇 |
1956年 | 258篇 |
1955年 | 220篇 |
排序方式: 共有10000条查询结果,搜索用时 31 毫秒
961.
Flavin-containing monooxygenase (FMO), like cytochrome P450 (CYP), is a monooxygenase that uses the reducing equivalents of NADPH to reduce one atom of molecular oxygen to water, while the other atom is used to oxidize the substrate. Recently, it was shown that some CYP isoforms have been subject to positive selection. However, it is unknown whether the highly conserved phase I detoxification enzyme, FMO, has undergone similar positive Darwinian selection. We used maximum-likelihood models of codon substitution, evolutionary fingerprinting, and cross species comparison to investigate the occurrence of adaptive evolution in FMO sequences. We used recent genomic data from a range of species, including vertebrates and invertebrates. We present the evidence for the occurrence of adaptive evolution in mammalian FMO 3, 4, 5, and fugu FMOs but not in mammalian FMO 1, FMO 2, frog FMOs, other fish FMOs and invertebrate FMOs. The sites under adaptive evolution were significantly associated with the insertion domain in mammalian FMO 5. We identified specific amino acid sites in FMOs 3–5 that are likely targets for selection based on the patterns of parallel amino acid change. The most likely role of adaptive evolution is the repair of mutations that permitted optimal NADP+ binding and improved catalytic efficiency. The occurrence of positive selection during the evolution of phase I detoxification enzymes such as FMOs 3–5 and fugu FMO suggests the occurrence of both high selection pressure acting on species within their unique habitats and significant changes in intensity and direction (forms of xenobiotics and drugs) resulting from changes in microhabitat and food. 相似文献
962.
Gold nanoshells (GNSs), consisting of a dielectric core coated with gold, have gained extensive attention as they show readily tunable optical properties and good biocompatibility. As highly sensitive and label-free optical biosensors with wide applications, GNSs have been investigated in many fields including drug delivery, immunoassay, cancer treatment, biological sensing and imaging. Taking advantage of the adjustability of the local surface plasmon resonance (LSPR) and the sensitivity of the surfaceenhanced Raman scattering (SERS) signal of GNSs, we have developed diverse applications including plasmonic biosensors and nanoprobes based on GNSs. In this review we introduce plasmonic and electromagnetic properties and fabrication methods of GNSs. We describe research progress in recent years, and highlight several applications of GNSs developed by our group. Finally we provide a brief assessment of future development of GNSs as plasmonic materials that can be integrated with complementary analytical techniques. 相似文献
963.
Earthquake may cause severe damage to all kinds of bridge such as the falling down of the girder; therefore,effective measures should be employed to control the seismic displacement. In this paper,the method of comprehensive optimal control,com-bined with analytic hierarchy process,is employed to investigate the seismic response control of the Nanjing Jiangxinzhou Bridge,which is a single-tower self-anchored suspension bridge (SSSB). Also,3-dimensional nonlinear seismic response analyses are con-ducted. Three types of practical connection measures for seismic response control of SSSB are investigated,and the optimal pa-rameters of the connection devices are achieved by this method. Results show that both the elastic connection devices and the damp-ers with rational parameters can reduce the seismic displacement of the bridge effectively,but the elastic connection devices will in-crease the seismic force of the tower. When all factors are consid-ered,the optimal measure is by using the elastic connection devices and the dampers together. These results can provide references for seismic response control of SSSBs. 相似文献
964.
Ying Gao 《系统科学与复杂性》2011,24(3):593-603
This paper consider Hexagonal-metric codes over certain class of finite fields. The Hexagonal metric as defined by Huber is
a non-trivial metric over certain classes of finite fields. Hexagonal-metric codes are applied in coded modulation scheme
based on hexagonal-like signal constellations. Since the development of tight bounds for error correcting codes using new
distance is a research problem, the purpose of this note is to generalize the Plotkin bound for linear codes over finite fields
equipped with the Hexagonal metric. By means of a two-step method, the author presents a geometric method to construct finite
signal constellations from quotient lattices associated to the rings of Eisenstein-Jacobi (EJ) integers and their prime ideals
and thus naturally label the constellation points by elements of a finite field. The Plotkin bound is derived from simple
computing on the geometric figure of a finite field. 相似文献
965.
LI Shijun MING Ruixing HUANG Longsheng . School of Mathematics Information Sciences Jiangxi Normal University Nanchang Jiangxi China . School of Management University of Science Technology of China Hefei Anhui . School of Sciences Zhejiang Agriculture Forest University Lin’an Zhejiang 《武汉大学学报:自然科学英文版》2011,16(2):133-138
In this paper, we consider a double compound Poisson risk model involving two independent classes ofinsurance risks with a threshold dividend strategy. We derived the integro-differential equations (IDE) with certain boundary conditions for the present value of dividends until ruin. When the claims from both classes are exponentially distributed, we show that the threshold dividend strategy is an optimal dividend strategy. 相似文献
966.
The novel multi-SO3H functionalized strong Bronsted acidic ionic liquid has been prepared and its catalytic activities were investigated through the acetalization. The results showed that the novel catalyst was very efficient for the reaction with the average yield over 90% under solvent-free condition at room temperature. Operational simplicity, without need of any solvent, a small amount of usage, low cost of the catalyst used, high yields, applicability to large-scale reactions and reusability are the key features of this methodology. The novel catalyst also has great potential for the green process. 相似文献
967.
Establishing a reasonable evaluation index system is the basis and premise for the scientific evaluation of geographic national conditions. By analyzing the DPSIR model and its correlation and relation with the evaluation index system, the indexes for geographic national conditions were selected from the aspects of resources, ecology, environment, economy, society, etc to build an evaluation index system for geographic national conditions that consists of three hierarchies: index, criterion, and target levels. Furthermore, the index system was analyzed in terms of index integrity, low overlapping of elements, non-existence of isolated indexes, hierarchy of index system, etc., for the purpose of demonstrating that the DPSIR model-based evaluation index system for geographic national conditions meets the principles of index system establishment. 相似文献
968.
An inflammatory drug prospect. 总被引:2,自引:0,他引:2
969.
The abundant entities and entity-attribute relations in medical websites are important data resources for medical research. However, the medical websites are usually characterized of storing entity and attribute values in different pages. To extract those data records efficiently, we propose an automatic extraction system which is related to entity and attribute relations (attributes and values) of separate storage. Our system includes following modules: (1) rich-information interactive annotation page rendering; (2) separate storage attribute relations annotating; (3) annotated relations for pattern generating and data records extracting. This paper presents the relations about the attributes which are stored in many pages by effective annotation, then generates rules for data records extraction. The experiments show that the system can not only complete attribute relations of separate storage extraction, but also be compatible with regular relation extraction, while maintaining high accuracy. 相似文献
970.
Hydrothermal reactions of oxalic acid(H2ox) and 2-sulfobenzoic acid(H2L) with lanthanide oxide or hydroxide yielded five lanthanide oxalatosulfocarboxylates.They have a general formula [Ln2L2(ox)(H2O)6]n[Ln = La(1),Nd(2),Eu(3),Gd(4),Tb(5)].Their structures were characterized by single crystal X-ray diffraction,powder X-ray diffraction(PXRD),infrared(IR) spectroscopy,elemental analysis(EA),and thermogravimetric analysis(TGA).1-5 are isostructural coordination polymers and feature one-dimensional(1D) chains,which are extended into 3D supramolecular frameworks through inter-chain hydrogen bonds between coordinated H2O donors and oxygen acceptors from carboxylate,sulfonate and oxalate groups.In addition,the solid-state photoluminescence properties of the Nd(2),Eu(3) and Tb(5) derivatives have been measured at room temperature.The Eu and Tb-containing coordination polymers(CPs) exhibit intense red and green luminescence emissions,respectively,with lifetimes in the order of millisecond. 相似文献