全文获取类型
收费全文 | 17363篇 |
免费 | 1304篇 |
国内免费 | 115篇 |
出版年
2023年 | 150篇 |
2022年 | 278篇 |
2021年 | 618篇 |
2020年 | 411篇 |
2019年 | 539篇 |
2018年 | 627篇 |
2017年 | 539篇 |
2016年 | 734篇 |
2015年 | 992篇 |
2014年 | 1087篇 |
2013年 | 1272篇 |
2012年 | 1417篇 |
2011年 | 1249篇 |
2010年 | 809篇 |
2009年 | 684篇 |
2008年 | 846篇 |
2007年 | 802篇 |
2006年 | 738篇 |
2005年 | 668篇 |
2004年 | 634篇 |
2003年 | 595篇 |
2002年 | 517篇 |
2001年 | 319篇 |
2000年 | 273篇 |
1999年 | 250篇 |
1998年 | 128篇 |
1997年 | 113篇 |
1996年 | 88篇 |
1995年 | 75篇 |
1994年 | 65篇 |
1993年 | 65篇 |
1992年 | 114篇 |
1991年 | 84篇 |
1990年 | 68篇 |
1989年 | 81篇 |
1988年 | 69篇 |
1987年 | 80篇 |
1986年 | 59篇 |
1985年 | 65篇 |
1984年 | 61篇 |
1983年 | 38篇 |
1982年 | 37篇 |
1981年 | 28篇 |
1980年 | 27篇 |
1979年 | 50篇 |
1978年 | 26篇 |
1976年 | 28篇 |
1975年 | 34篇 |
1974年 | 29篇 |
1973年 | 37篇 |
排序方式: 共有10000条查询结果,搜索用时 15 毫秒
181.
De Andrade Antônio V. M. Da Costa Nivan B. Longo Ricardo L. Malta Oscar L. Simas Alfredo M. De Sá Gilberto F. 《Molecular Engineering》1997,7(3-4):293-308
Theoretical techniques have been developed and/or improved to predict the molecular structure of lanthanide complexes which
were used to calculate their electronic properties, in particular, their electronic spectra and energy levels necessary to
calculate the rates of energy transfer from the ligands to the metal ion. The molecular structure has been obtained by the
SMLC/AM1 (Sparkle Model for the Calculation of Lanthanide Complexes – Austin Model 1) model where the lanthanide ion is simulated
by a sparkle implemented into the AM1 Hamiltonian used to perform a HF-SCF (Hartree-Fock Self-Consistent Field) calculation.
The previous implementation of the SMLC/AM1 model (sparkle/1) involving only two parameters has been generalized to be consistent
with the AM1 Hamiltonian and the new model (sparkle/2) significantly improved the prediction of molecular structures of Eu(III)
complexes. For the electronic spectra and energy level calculations of the lanthanide complexes the model replaces the metal
ion by a point charge with the ligands held in their positions as determined by the SMLC/AM1 model, and uses a INDO/S-CI (intermediate
neglect of differential overlap/spectroscopic-configuration interaction) model. A preliminary study of the solvent effects
on the absorption spectra of the free ligand is also presented. For the ligand-lanthanide ion energy transfer Fermi's golden
rule is used with the multipolar and exchange mechanisms being implemented and tested for several complexes. These theoretical
techniques have been applied to several complexes yielding very good results when compared to experimental data as well as
predictions for the molecular and electronic structures and the relative contributions of the mechanisms for the energy transfer
rates.
This revised version was published online in June 2006 with corrections to the Cover Date. 相似文献
182.
N. Marignetti M. D. Fumi A. Silva G. Battistotti M. Pomini O. Colagrande 《Biotechnology Techniques》1997,11(1):7-8
The geometry of calcium alginate gel spheres is studied by fractal interpretation for prediction of number of cells to be
immobilized. For alginate concentrations from 1 to 5% the simulated results are of 6.65×108, 1.44×108 and 5.27×107 N cell mL−1 for respectively 1.5, 2.5 and 3.5 mm gel sphere diameters. Simulated data are compared with those resulting from literature
and particularly with experimental trials where from 107 to 108 N cell mL−1, in 2% alginate beads with a diameter of 1.5 mm, is able to fulfil mechanical and swelling characteristics of alginate gel
beads besides to supply good oxygenation. 相似文献
183.
B. Prieto B. Silva T. Rivas J. Wierzchos C. Ascaso 《International biodeterioration & biodegradation》1997,40(2-4)
Tephromela atra and Ochrolechia parella are among the most abundant lichens colonizing granitic monuments in the region of Galicia (northwest Spain). In this work, their interaction with a two-mica granite used in the construction of the Toxosoutos Monastery (Noia, Galicia) was studied, using optical microscopy, scanning electron microscopy (with back-scattered-electron and energy-dispersive X-ray detection), X-ray diffractometry and atomic absorption spectroscopy to evaluate their physical, mineralogical and chemical effects. Both lichens contributed to physical weathering by penetrating intermineral voids and mineral cleavage planes, disaggregating the rock and entrapping the loosened mineral grains in their thalli. Significant chemical and mineralogical weathering also occurred, including depletion of potassium from biotite, transformation of this mica into hydroxyaluminium-vermiculite, and neoformation of whewellite and calcite in the lichen thalli. Neoformation of these calcium minerals on a calcium-poor rock such as granite is noteworthy, and this is the first time calcium carbonate has been detected within a lichen colonizing a granitic rock. Precipitation of the calcium carbonate was attributed to the local pH in the thalli having been raised due to release of sodium from nearby plagioclase during weathering. 相似文献
184.
The phylogenetic distribution of transposable families, P, gypsy, hobo, I, and mariner has been analyzed in 33 species of
11 groups of neotropical Drosophila and a Drosophilidae species Zygotrica vittimaculosa, using squash blot and dot blot. Genomic
DNA of almost all neotropical species tested hybridized with gypsy probe and some species showed a particularly strong hybridization
signal, as D. gaucha, D. virilis, and species of flavopilosa group. The hobo element was restricted to melanogaster group
and some strains of D. willistoni. Only D. simulans DNA showed hybridization to mariner probe in all species tested and D.
simulans and D. melanogaster showed hybridization with I element probe. P element homologous sequence was present in D. melanogaster
and all species and strains of the willistoni and saltans groups tested. The presence of at least one P-homologous sequence
was detected in Drosophila mediopunctata. This one was the only P-bearing species of all six tested from the tripunctata group.
Four different pairs of primers homologous to segments of the canonical sequence of D. melanogaster's P were used to amplify
specific sequences from D. mediopunctata DNA, showing the occurrence of seemingly well-conserved P-homologous sequences.
This revised version was published online in July 2006 with corrections to the Cover Date. 相似文献
185.
阳离子诱导大叶藻叶绿体膜中激发能在PSⅡ和PSⅠ之间分配变化的机理(英文) 总被引:1,自引:0,他引:1
用Ca2+ 和胰酶处理大叶藻(Zostera m arina)叶绿体膜研究了其类囊体膜多肽成分与Mg2+ 诱导其Chla荧光和类囊体膜表面电荷变化之间的相互关系,观察到:1.在正常的叶绿体膜中,Mg2+ 诱导PSⅡ荧光强度的增高与其诱导类囊体膜表面电荷密度的降低密切相关;2.用Ca2+ 处理这种叶绿体膜,除去类囊体膜表面的32~34 kD多肽对Mg2+ 诱导的上述现象无影响;3.如果用胰酶消化Ca2+ 处理过的叶绿体膜,进一步除去膜表面的26 kD多肽,Mg2+诱导的这些现象则全部消失。这些实验结果清楚地表明,在大叶藻的叶绿体膜中,类囊体膜表面的26 kD 多肽是阳离子诱导这两种相关现象的特异性作用部位。对阳离子调节激发能在PSⅡ和PSⅠ之间分配的机理进行了讨论 相似文献
186.
Thirty-four strains belonging to various species of the genus Thermus (T. aquaticus, "T. thermophilus,"
"T. brockianus,"
T. scotoductus, and genomic species 2) isolated from hot springs of different geographical areas were examined for plasmid content and restriction
fragment length polymorphism (RFLP) of plasmid DNAs. The four strains of the numerical taxonomy cluster E of genomic species
2 did not harbor plasmid DNA. Overall examination of the HindIII-RFLP profiling of plasmid DNA showed considerable variability between and within genomic species, with the exception
of presumed clonal isolates. In spite of this heterogeneity, HindIII plasmid digests within a numerical taxonomic cluster gave a subset of restriction fragments of similar or identical length.
Strains belonging to genomic species 2 or unclassified isolates from S. Pedro do Sul that harbored plasmid DNA (7 of the 14
strains studied) exhibited strong DNA homology between plasmid regions. No homologous sequences to these plasmid regions were
found in chromosomal DNA from strains isolated from S. Pedro do Sul in which no plasmids were detected. The strains belonging
to T. scotoductus formed two plasmid DNA homology groups, as estimated by probing with a plasmid fragment that coincided with the two numerical
taxonomy clusters proposed previously. Among the other species, homology of plasmid regions was also found between some strains.
Strong homology was also found between plasmid regions from some strains of different taxonomic groups, isolated from the
same and from different sources, suggesting that these sequences are highly conserved in plasmids present in Thermus. For plasmid-containing strains, results of plasmid RFLP profiling/DNA homology appear promising for the typing of Thermus at the level of biotypes or of individual strains, namely, for monitoring the diversity and frequency of isolates from a
particular hot spring.
Received: 24 October 1994 / Accepted: 6 March 1995 相似文献
187.
Cecilia Ballaré Marcela Barrio Paula Portela Jose Mordoh 《Cancer immunology, immunotherapy : CII》1995,41(1):15-22
FC-2.15 is a murine IgM monoclonal antibody (mAb) that recognizes a cell-surface antigen (Ag2.15) expressed in most tumor-proliferating cells of human breast carcinomas and other neoplasias. In this study the cytotoxic ability of mAb FC-2.15, its cell-surface binding properties and endocytosis in Ag2.15-expressing (Ag2.15+) cells were investigated. A51Cr-release assay was used to test the FC-2.15-mediated cytotoxicity. When human serum was used as source of complement, FC-2.15 exerted a strong cytotoxic effect against human Ag2.15+ cells such as MCF-7 (breast cancer cell line), primary breast carcinoma cells, polymorphonuclear leukocytes and chronic myeloid leukemia cells. The mAb concentration range was 1–50 g/ml. Cytotoxicity was completely abolished when complement was inactivated. Only 3.8±2.9% of MCF-7 cells survived the treatment with FC-2.15 in the presence of human serum. A flow-cytometry assay was performed to study the Ag2.15 expression of the surviving cells and they were found to be Ag2.15–. FC-2.15 did not mediate antibody-dependent cell cytotoxicity when different effector cells were used. Scatchard analysis with125I-FC-2.15 on MCF-7 cells demonstrated an affinity constant of 6.9×107 M–1 and 2.8×106 antigenic sites/cell.125I-FC-2.15 was internalized to cytoplasmic vesicles reaching a maximum of 27% after 6 h incubation, followed by the release of labeled degradation products to the supernatant. FC-2.15 appears to exert its cytotoxic effect mainly in the presence of human complement, it reacts with intermediate affinity with a high-density surface antigen, and it is slowly internalized by Ag2.15+ cells. 相似文献
188.
To study the effects of prenatal cocaine-exposure on the developing retinal ganglion cell layer of the rat, female Wistar rats were administered subcutaneously (sc) cocaine hydrochloride (60 mg/kg body wt/d) or saline, or were not manipulated from gestational d 8–22. Male offspring were sacrificed at postnatal day 14 and 30. Radial semithin sections of epon-embedded flat mounts of the retinal quadrants were used to evaluate the following parameters along the centroperipheral axis:
- Thickness of ganglion cell plus nerve fiber layer;
- Nuclear size of ganglion cell layer neurons; and
- Linear density (number per unit length) of ganglion cell layer neurons.
189.
应用非结构的逻辑增殖模型研究了两种酵母的单碳源和双碳源单细胞蛋白间歇培养的动力学,用改进的逻辑增殖模型研究了双碳源流加培养过程的动力学,从实验数据拟合了动力学模型参数,模型计算值与实验数据吻合良好。 相似文献
190.
Uterine activity was measured by monitoring intrauterine pressure changes using ballon-tipped catheters placed in the lumen. An infusion rate of Na(2)EDTA of 35 mg/Kg/h gave a chelation rate equivalent to the rate of calcium mobilisation, and when infused at a level greater than this, resulted in a reduction in plasma calcium concentrations and a concomitant reduction in uterine activity. In three of the four sows infused intrapartum, there was complete cessation of uterine activity; however, plasma calcium concentrations of less than 6 mg 100 ml resulted in a reduction in uterine activity at this stage of parturition. The uterus of the sow post-partum appeared to be more sensitive to the effects of hypocalcaemia with reduced uterine activity when plasma calcium concentrations fell below 8.2 mg 100 ml and complete cessation of activity between 6 and 7 mg 100 ml . Although there was evidence of a delay in the expulsion of piglets in the hypocalcaemic sows, there was no evidence of an increased number of stillborn piglets compared with the two control sows. 相似文献