全文获取类型
收费全文 | 13595篇 |
免费 | 887篇 |
国内免费 | 3篇 |
专业分类
14485篇 |
出版年
2023年 | 80篇 |
2022年 | 160篇 |
2021年 | 271篇 |
2020年 | 202篇 |
2019年 | 272篇 |
2018年 | 329篇 |
2017年 | 303篇 |
2016年 | 472篇 |
2015年 | 691篇 |
2014年 | 748篇 |
2013年 | 953篇 |
2012年 | 1144篇 |
2011年 | 1065篇 |
2010年 | 679篇 |
2009年 | 633篇 |
2008年 | 826篇 |
2007年 | 789篇 |
2006年 | 784篇 |
2005年 | 669篇 |
2004年 | 606篇 |
2003年 | 607篇 |
2002年 | 536篇 |
2001年 | 96篇 |
2000年 | 82篇 |
1999年 | 99篇 |
1998年 | 120篇 |
1997年 | 122篇 |
1996年 | 111篇 |
1995年 | 91篇 |
1994年 | 89篇 |
1993年 | 85篇 |
1992年 | 65篇 |
1991年 | 58篇 |
1990年 | 47篇 |
1989年 | 46篇 |
1988年 | 37篇 |
1987年 | 27篇 |
1986年 | 29篇 |
1985年 | 45篇 |
1984年 | 38篇 |
1983年 | 38篇 |
1982年 | 40篇 |
1981年 | 45篇 |
1980年 | 32篇 |
1979年 | 30篇 |
1978年 | 28篇 |
1977年 | 20篇 |
1975年 | 22篇 |
1974年 | 19篇 |
1973年 | 24篇 |
排序方式: 共有10000条查询结果,搜索用时 15 毫秒
901.
Organic–inorganic perovskites have demonstrated an impressive potential for the design of the next generation of solar cells. Perovskite solar cells (PSCs) are currently considered for scaling up and commercialization. Many of the lab‐scale preparation methods are however difficult to scale up or are environmentally unfriendly. The highest efficient PSCs are currently prepared using the antisolvent method, which utilizes a significant amount of an organic solvent to induce perovskite crystallization in a thin film. An antisolvent‐free method is developed in this work using flash infrared annealing (FIRA) to prepare methylammonium lead iodide (MAPbI3) PSCs with a record stabilized power conversion efficiency of 18.3%. With an irradiation time of fewer than 2 s, FIRA enables the coating of glass and plastic substrates with pinhole‐free perovskite films that exhibit micrometer‐size crystalline domains. This work discusses the FIRA‐induced crystallization mechanism and unveils the main parameters controlling the film morphology. The replacement of the antisolvent method and the larger crystalline domains resulting from flash annealing make FIRA a highly promising method for the scale‐up of PSC manufacture. 相似文献
902.
Dipineto L Manna L Baiano A Gala M Fioretti A Gravino AE Menna LF 《Journal of wildlife diseases》2007,43(3):518-520
Skin, lymph node (popliteal), and bone marrow samples were collected from 50 red foxes (Vulpes vulpes) from May 2004 to May 2005 in southern Italy. Samples were tested for Leishmania infantum by polymerase chain reaction (PCR). The parasite was detected by PCR from 20 of 50 (40%) fox carcasses. All 20 positive cases were PCR-positive from lymph node and bone marrow samples, whereas 17 of 20 positive cases were PCR-positive from skin samples. Infection status was not related to age or sex. This is the first report of leishmaniasis in red foxes in Italy based on PCR results, and these results reinforce the assumption that this wild canid can serve as a reservoir for Leishmania. 相似文献
903.
Relatively few studies have explored sex differences in the use of foraging tools among primates other than apes. Although
male primates are thought to be more innovative, researchers have reported a female sex bias in the use of feeding tools in
wild chimpanzees. We investigate here the nature and extent of sex differences in foraging tool use over 12 mo in a free-ranging
group of bearded capuchins (2 males, 5 females, and 3 juveniles) living in the dry Caatinga forests of the Serra da Capivara
National Park, Piaui, Brazil. These capuchins used 3 major types of feeding tools: 1) tools for probing; 2) tools for pounding/cracking;
and 3) digging stones to extract tubers or roots. Adult males performed 63% (n = 134) of all events of tool use and used tools significantly more frequently than did females, although male bout lengths
across all tools (57 s ± 7.9 SE) were equivalent to those of adult females (47.3 s ± 12.6 SE). Both sexes used digging and
cracking tools, although at different rates, whereas adult males used sticks to probe for prey and other rewards far more
than females. Differential opportunities to use tools were not apparent: >71% of tool-use events occurred on the ground, and
males and females spent equal time on the ground. We suggest that sex differences in tool use may function as opportunities
for male signaling of investment quality. 相似文献
904.
Evidente A Andolfi A Abou-Donia AH Touema SM Hammoda HM Shawky E Motta A 《Phytochemistry》2004,65(14):2113-2118
A new lycorine-type alkaloid, named (-)-amarbellisine, was isolated from the bulbs of Egyptian Amaryllis belladonna L. together with the well known alkaloids (-)-lycorine, (-)-pancracine, (+)-vittatine, (+)-11-hydroxyvittatine, and (+)-hippeastrine. The new alkaloid, containing the pyrrolo[de]phenanthridine ring system, was essentially characterised by spectroscopic and optical methods, and proved to be the 2-methoxy-3a,4,5,7,11b,11c-hexahydro-1H-[1,3]dioxolo[4,5-j]pyrrolo[3,2,1-de]phenanthridinol. By using HPTLC technique we also carried out a comparative study of the relative and total alkaloidal content at two different stages of plant growth. Finally, the antimicrobial activity of the isolated alkaloids was assayed. 相似文献
905.
Pattern of expression of HtrA1 during mouse development. 总被引:1,自引:0,他引:1
Antonio De Luca Maria De Falco Luca De Luca Roberta Penta Viji Shridhar Feliciano Baldi Mara Campioni Marco G Paggi Alfonso Baldi 《The journal of histochemistry and cytochemistry》2004,52(12):1609-1617
The human HtrA family of proteases consists of four members: HtrA1, HtrA2, HtrA3, and HtrA4. In humans the four HtrA homologues appear to be involved in several important functions such as cell growth, apoptosis, and inflammatory reactions, and they control cell fate via regulated protein metabolism. In previous studies it was shown that the expression of HtrA1 was ubiquitous in normal adult human tissues. Here we examined the expression of HtrA1 protein and its corresponding mRNA during mouse embryogenesis using Northern blotting hybridization, RT-PCR, and immunohistochemical staining analyses. Our results indicate that HtrA1 is expressed in a variety of tissues in mouse embryos. Furthermore, this expression is regulated in a spatial and temporal manner. Relatively low levels of HtrA1 mRNA are detected in embryos at the beginning of organogenesis (E8), and the levels of expression increase during late organogenesis (E14-E19). Our results show that HtrA1 was expressed during embryonic development in specific areas where signaling by TGFbeta family proteins plays important regulatory roles. The expression of HtrA1, documented both at mRNA and protein levels by RT-PCR and immunohistochemistry in the developing nervous system, is consistent with a possible role of this protein both in dividing and postmitotic neurons, possibly via its documented inhibitory effects on TGFbeta proteins. An exhaustive knowledge of the different cell- and tissue-specific patterns of expression of HtrA1 in normal mouse embryos is essential for a critical evaluation of the exact role played by this protein during development. 相似文献
906.
Roberta Diavana De Souza Alinne Ulbrich Mores Laura Cavalca Rosimeire Takaki Rosa Lakshman Perera Samaranayake Edvaldo Antonio Ribeiro Rosa 《Gerodontology》2009,26(2):157-161
Background: The virulence potential of Candida albicans strains enrolled in denture‐related candidosis still remains uncertain. Candida albicans cells with higher cell surface hydrophobicity (CSH) rates, so‐called hydrophobic, present higher adhesion success in different host tissues than cells with lower rates, or even hydrophilic. Objective: The proposition of this study was to evaluate the differences in the CSH of strains isolated from denture users with and without denture‐related candidosis. Material and methods: The strains were obtained from two paired groups of patients living a same retirement house. Fungal cells were submitted to CSH evaluation by the hydrocarbon partition test using xylene. Results: The measures revealed that the yeasts from patients with candidosis had CSH values ranging from 4.52% to 12.24%, with an average of 8.22 ± 2.92%. In the countergroup, the CSH ranged from 3.86% to 14.36%, with an average of 8.38 ± 3.76%. The difference between the groups were considered not relevant (p = 0.997). Conclusion: The results let to the inference that natural populations of C. albicans from patients with and without clinical manifestation denture‐related candidosis do not differ one from the other regarding to CSH. 相似文献
907.
Jos‐Luis Glvez‐Martos Antonio Valente Mathías Martínez‐Fernndez Javier Dufour 《Journal of Industrial Ecology》2020,24(3):695-706
Calcium sulfoaluminate‐based cements (CSA) are proposed as a cement alternative with a low carbon footprint. The nature of CSA makes the manufacturing process to require lower temperature, less fuel, and less calcite. However, it requires aluminum oxide, Al2O3, which would be originated from bauxite and bauxite‐derived wastes, and sulfur, coming from calcium sulfate or elemental sulfur. An eco‐efficiency assessment of CSA cements, benchmarked against the conventional Portland cement, has been performed following the principles of ISO 14045 on eco‐efficiency for a total of 240 CSA clinker production scenarios. The eco‐efficiency indicator relates an environmental indicator with a product system value indicator, and it is calculated for each of the studied parameters: bauxite geographical origin, the fuel used for clinkering, the source of sulfur, and the composition of the clinker. Eco‐efficiency results show a strong dependence on the origin of bauxite, while other parameters, as the fuel used, its content in sulfur, or the supply of other raw materials, are of less importance. The most eco‐efficient solutions are those with certain closeness to bauxite sources. To achieve global solutions, that is, cement‐making based on CSA independently of the origin of the raw materials, the amount of bauxite needs to be minimized and CSA composition restricted. 相似文献
908.
909.
Schenker P Alfarano P Kolb P Caflisch A Baici A 《Protein science : a publication of the Protein Society》2008,17(12):2145-2155
Most synthetic inhibitors of peptidases have been targeted to the active site for inhibiting catalysis through reversible competition with the substrate or by covalent modification of catalytic groups. Cathepsin B is unique among the cysteine peptidase for the presence of a flexible segment, known as the occluding loop, which can block the primed subsites of the substrate binding cleft. With the occluding loop in the open conformation cathepsin B acts as an endopeptidase, and it acts as an exopeptidase when the loop is closed. We have targeted the occluding loop of human cathepsin B at its surface, outside the catalytic center, using a high-throughput docking procedure. The aim was to identify inhibitors that would interact with the occluding loop thereby modulating enzyme activity without the help of chemical warheads against catalytic residues. From a large library of compounds, the in silico approach identified [2-[2-(2,4-dioxo-1,3-thiazolidin-3-yl)ethylamino]-2-oxoethyl] 2-(furan-2-carbonylamino) acetate, which fulfills the working hypothesis. This molecule possesses two distinct binding moieties and behaves as a reversible, double-headed competitive inhibitor of cathepsin B by excluding synthetic and protein substrates from the active center. The kinetic mechanism of inhibition suggests that the occluding loop is stabilized in its closed conformation, mainly by hydrogen bonds with the inhibitor, thus decreasing endoproteolytic activity of the enzyme. Furthermore, the dioxothiazolidine head of the compound sterically hinders binding of the C-terminal residue of substrates resulting in inhibition of the exopeptidase activity of cathepsin B in a physiopathologically relevant pH range. 相似文献
910.