全文获取类型
收费全文 | 2428篇 |
免费 | 12篇 |
国内免费 | 96篇 |
出版年
2018年 | 432篇 |
2017年 | 327篇 |
2016年 | 132篇 |
2015年 | 42篇 |
2014年 | 12篇 |
2013年 | 58篇 |
2012年 | 24篇 |
2011年 | 22篇 |
2010年 | 16篇 |
2009年 | 19篇 |
2008年 | 100篇 |
2007年 | 17篇 |
2006年 | 12篇 |
2005年 | 18篇 |
2004年 | 13篇 |
2002年 | 17篇 |
2001年 | 71篇 |
2000年 | 37篇 |
1999年 | 78篇 |
1998年 | 11篇 |
1997年 | 25篇 |
1996年 | 14篇 |
1995年 | 11篇 |
1993年 | 23篇 |
1992年 | 76篇 |
1991年 | 84篇 |
1990年 | 55篇 |
1989年 | 74篇 |
1988年 | 57篇 |
1987年 | 43篇 |
1986年 | 52篇 |
1985年 | 52篇 |
1984年 | 31篇 |
1983年 | 24篇 |
1982年 | 12篇 |
1979年 | 31篇 |
1978年 | 18篇 |
1977年 | 15篇 |
1976年 | 16篇 |
1975年 | 47篇 |
1974年 | 38篇 |
1973年 | 33篇 |
1972年 | 31篇 |
1971年 | 36篇 |
1970年 | 31篇 |
1969年 | 14篇 |
1968年 | 21篇 |
1967年 | 19篇 |
1966年 | 20篇 |
1965年 | 16篇 |
排序方式: 共有2536条查询结果,搜索用时 187 毫秒
61.
Thomas Soehl & Roger Waldinger 《Ethnic and racial studies》2013,36(9):1489-1510
This paper uses the Pew Hispanic Center's 2006 National Survey of Latinos to study the everyday, routine cross-border activities of travel, remittance sending and telephone communication among Latin American immigrants in the United States. We ask how migrants vary in the intensity of their cross-border connections, distinguishing among the transmigrants, those captured by the host-country national social field, and those who maintain some ongoing home-country tie. We then examine the characteristics associated both with variations in the intensity of connectedness and with each specific type of connection. We show that most migrants maintain some degree of home-country connectedness, with a minority severing ties and a still smaller minority maintaining ties at a high degree of intensity. Connectivity is highly responsive to the location of key social ties, acculturation, and citizenship status, as well as the costs associated with the different types of cross-border activity. 相似文献
62.
63.
Deepak K. Mishra Hridoy R. Bairagya & Bishnu P. Mukhopadhyay 《Journal of biomolecular structure & dynamics》2013,31(1)
Inosine monophosphate dehydrogenase (IMPDH) plays an important role in the Guanosine monophosphate (GMP) biosynthesis pathway. As hIMPDH-II is involved in CML-Cancer, it is thought to be an active target for leukemic drug design. The importance of conserved water molecules in the salt-bridge-mediated interdomain recognition and loop-flap recognition of hIMPDH has already been indicated in some simulation studies (Bairagya et al., 2009, 2011a, 2011b, 2012; Mishra et al., 2012). In this work, the role of conserved water molecules in the recognition of Inosine monophosphate (IMP) and NAD+ (co-factor) to active site residues of both the isoforms has been investigated by all atoms MD-Simulation studies. During 25-ns dynamics of the solvated hIMPDH-II and I (1B3O and 1JCN PDB structures), the involvement of conserved water molecular triad (W M, W L and W C) in the recognition of active site residues (Asp 274, Asn 303, Arg 322, and Asp 364), IMP and NAD+ has been observed (Figure 1). The H-bonding co-ordination of all three conserved water molecular centers is within 4–7 and their occupation frequency is 1.0. The H-bonding geometry and the electronic consequences of the water molecular interaction at the different residues (and also IMP and NAD+) may put forward some rational clues on antileukemic agent design. 相似文献
64.
L. Duchemin D. Mitton E. Jolivet J. D. Laredo Laboratoire de Radiologie Expérimentale CNRS UMR Paris France Service de Radiologie Ostéo-Articulaire H?pital Lariboisière AP-HP Paris France & W. Skalli Laboratoire de Biomécanique ENSAM CNRS UMR Paris France 《Computer methods in biomechanics and biomedical engineering》2013,16(2):105-111
In order to reduce the socio-economic burden induced by osteoporotic hip fractures, finite element models have been evaluated as an additional diagnostic tool for fracture prediction. For a future clinical application, the challenge is to reach the best compromise between model relevance and computing time. Based on this consideration, the current study focused on the development and validation of a subject-specific FE-model using an original parameterised generic model and a specific personalization method. A total of 39 human femurs were tested to failure under a quasi-static compression in stance configuration. The corresponding FE-models were generated and for each specimen the numerical fracture load (F FEM) was compared with the experimental value (F EXP), resulting in a significant correlation (F EXP = 1.006 F FEM with r 2 = 0.87 and SEE = 1220 N, p < 0.05) obtained with a reasonable computing time (30 mn). Further in vivo study should confirm the ability of this FE-model to improve the fracture risk prediction. 相似文献
65.
Michael L. Merrick & Kristen A. Fichthorn 《Molecular simulation》2013,39(5):273-279
We review recent advances made in understanding the ramifications of substrate-mediated interactions for thin-film growth. Experimental studies and first-principles calculations with density-functional theory (DFT) indicate that substrate-mediated interactions can significantly influence thin-film growth. We review the findings from our kinetic Monte Carlo simulations used to model the growth of thin films, both with and without substrate-mediated interactions. For Ag heteroepitaxy on Pt(1?1?1), the pair interaction energies and adsorbate diffusion barriers were obtained from DFT calculations. Island densities for this system show significant deviations from what is predicted by classical nucleation theory. The electronic interactions created by the adsorbed atoms lead to the formation of repulsive barriers surrounding small islands and, as a result, sharp island-size distributions are produced. The island-size distributions can be manipulated by changing the growth conditions to yield desirable island sizes and shapes. 相似文献
66.
67.
1 脱氧 D 木酮糖 5 磷酸合酶(DXS)是甲基 D 赤藓醇 4 磷酸(MEP)途径中控制影响植物萜类化合物合成的第一个限速酶。该研究对香鳞毛蕨(Dryopteris fragrans)DfDXS基因进行序列特征及生物信息学分析,并通过qRT PCR技术分析其在外源激素、干旱、盐胁迫、高温及低温处理下的表达模式,旨在探究DfDXS基因在香鳞毛蕨萜类生物合成及抗逆机制中的作用,为进一步解析香鳞毛蕨抗逆分子机制奠定基础。结果显示:(1) DfDXS1基因全长2 139 bp,编码712个氨基酸,而DfDXS2全长2 160 bp,编码719个氨基酸;结构域分析显示,其具有典型的转酮醇酶保守域,包含焦磷酸硫胺素结合位点和转酮醇酶结构域;DfDXS氨基酸序列与江南卷柏(Selaginella moellendorffii)和银杏(Ginkgo biloba)的DXS等关系较近。(2)水杨酸(SA)处理下,DfDXS基因的相对表达量先升高后降低;脱落酸(ABA)抑制DfDXS的表达;DfDXS1/2在茉莉酸甲酯(MeJA)处理下相对表达水平均显著高于对照;乙烯利(Eth)抑制DfDXS的表达,但DfDXS1处理3 h时表达水平显著高于对照。(3)聚乙二醇(PEG)、高温和低温均诱导DfDXS1上调表达。研究推测,香鳞毛蕨DXS基因在萜类物质合成与逆境胁迫机制中发挥着重要的作用。 相似文献
68.
Stefania Neiva Lavorato Policarpo Ademar Sales Júnior Silvane Maria Fonseca Murta Alvaro José Romanha Ricardo José Alves/ 《Memórias do Instituto Oswaldo Cruz》2015,110(4):566-568
We describe herein the antitrypanosomal activity of 20 novel
1,3-bis(aryloxy)propan-2-amine derivatives. Compounds 2, 4, 6, 12, 15, 16 and 19 were
significantly active against amastigote and trypomastigote forms, with half maximal
inhibitory concentrationvalues in the range of 6-18 µM. In the cytotoxicity tests
against L929 cells, only compound 4 presented selectivity index value above 10,
indicating low toxicity. 相似文献
69.
María Sol Gaspe Yael Mariana Provecho Romina Valeria Piccinali Ricardo Esteban Gürtler/ 《Memórias do Instituto Oswaldo Cruz》2015,110(3):310-318
House re-invasion by native triatomines after insecticide-based control campaigns
represents a major threat for Chagas disease vector control. We conducted a
longitudinal intervention study in a rural section (Area III, 407 houses) of Pampa
del Indio, northeastern Argentina, and used wing geometric morphometry to compare
pre-spray and post-spray (re-infestant bugs) Triatoma infestans
populations. The community-wide spraying with pyrethroids reduced the prevalence of
house infestation by T. infestans from 31.9% to < 1% during a
four-year follow-up, unlike our previous studies in the neighbouring Area I. Two
groups of bug collection sites differing in wing shape variables before interventions
(including 221 adults from 11 domiciles) were used as a reference for assigning 44
post-spray adults. Wing shape variables from post-spray, high-density bug colonies
and pre-spray groups were significantly different, suggesting that re-infestant
insects had an external origin. Insects from one house differed strongly in wing
shape variables from all other specimens. A further comparison between insects from
both areas supported the existence of independent re-infestation processes within the
same district. These results point to local heterogeneities in house re-infestation
dynamics and emphasise the need to expand the geographic coverage of vector
surveillance and control operations to the affected region. 相似文献
70.
N N Nemtsova M Ia Shvartsman A P Kashkin O V Sokovski?Nazarova E F Panarin V N Ushakova 《Antibiotiki i khimioterapii͡a》1991,36(9):27-29
To obtain diagnostic antibodies to neomycin, immunogenic properties of the neomycin conjugates with macromolecular carriers were studied. Bovine serum albumin and copolymers of N-vinylpyrrolidone with crotonic acid and N-hydroxyphthalimide ether of crotonic acid were used as carriers. It was shown that immunization of mice by the conjugates in combination with Freund's adjuvant resulted in production of neomycin antibodies, the titer being 1/80 to 1/130. When the antibiotic conjugates with the copolymers of N-vinylpyrrolidone were used and not the neomycin conjugates with the carrier of the protein nature, the neomycin antibodies were produced in the absence of Freund's full adjuvant. With the use of the isolated antibodies to neomycin a method for indirect solid phase enzyme immunoassay of neomycin was developed at the minimum detectable level of 25 ng/ml. 相似文献