全文获取类型
收费全文 | 83474篇 |
免费 | 3726篇 |
国内免费 | 257篇 |
专业分类
电工技术 | 948篇 |
综合类 | 734篇 |
化学工业 | 13174篇 |
金属工艺 | 1420篇 |
机械仪表 | 1830篇 |
建筑科学 | 2182篇 |
矿业工程 | 503篇 |
能源动力 | 1383篇 |
轻工业 | 8856篇 |
水利工程 | 999篇 |
石油天然气 | 245篇 |
武器工业 | 9篇 |
无线电 | 3845篇 |
一般工业技术 | 10396篇 |
冶金工业 | 28951篇 |
原子能技术 | 377篇 |
自动化技术 | 11605篇 |
出版年
2024年 | 157篇 |
2023年 | 603篇 |
2022年 | 716篇 |
2021年 | 1295篇 |
2020年 | 991篇 |
2019年 | 1157篇 |
2018年 | 2159篇 |
2017年 | 2328篇 |
2016年 | 2753篇 |
2015年 | 2062篇 |
2014年 | 2057篇 |
2013年 | 3686篇 |
2012年 | 4483篇 |
2011年 | 4860篇 |
2010年 | 2564篇 |
2009年 | 2571篇 |
2008年 | 2320篇 |
2007年 | 2201篇 |
2006年 | 1704篇 |
2005年 | 4194篇 |
2004年 | 3338篇 |
2003年 | 2822篇 |
2002年 | 1652篇 |
2001年 | 1349篇 |
2000年 | 786篇 |
1999年 | 1132篇 |
1998年 | 8334篇 |
1997年 | 5287篇 |
1996年 | 3483篇 |
1995年 | 2102篇 |
1994年 | 1553篇 |
1993年 | 1657篇 |
1992年 | 490篇 |
1991年 | 534篇 |
1990年 | 483篇 |
1989年 | 460篇 |
1988年 | 478篇 |
1987年 | 377篇 |
1986年 | 340篇 |
1985年 | 339篇 |
1984年 | 209篇 |
1983年 | 192篇 |
1982年 | 280篇 |
1981年 | 333篇 |
1980年 | 356篇 |
1979年 | 155篇 |
1978年 | 176篇 |
1977年 | 894篇 |
1976年 | 1911篇 |
1975年 | 160篇 |
排序方式: 共有10000条查询结果,搜索用时 15 毫秒
911.
C Casali JF Obadia E Canet K Bendjelid X André-Fou?t D Revel MF Janier 《Canadian Metallurgical Quarterly》1997,32(11):713-720
RATIONALE AND OBJECTIVES: Validation of new positron emission tomography (PET) tracers or magnetic (MR) imaging contrast agents is based on isolated rodent heart preparations. The use of larger animals could provide a more direct validation using the devices used for humans. METHODS: An isolated pig heart preparation has been developed and adapted to the technical constraints of whole body PET and MR imaging. This preparation could be used either in the Langendorff or working mode after selective cannulation of both coronary arteries. RESULTS: The authors showed that quantification of regional kinetics of PET tracers was possible using this preparation by measuring fluorine-18-labeled deoxyglycose (18FDG) kinetics in remote and ischemic territories. Experiments using MR imaging contrast agents, for myocardial perfusion, demonstrated the ability of this preparation to accurately validate these contrast agents over a wide range of flow rates. CONCLUSIONS: An isolated pig heart preparation could be developed to fulfill the constraints of PET and MR imaging, and proved useful for the study of the distribution of different tracers or contrast media developed for functional cardiac imaging in humans. 相似文献
912.
G Martín Carballo R Codoceo Alquila G Fernández Cano F Hawkins Carranza C Grande Aragón A Velasco Hernando R Gracia Bouthelier 《Canadian Metallurgical Quarterly》1997,47(3):302-310
OBJECTIVE: The objective of this study was to determine fibronectin levels in umbilical cord blood of infants of diabetic mothers (IDM) and evaluate a possible correlation with perinatal pathology. PATIENTS AND METHODS: A prospective study of 58 IDM (33 males and 25 females) and 58 control newborns (NB) (33 males and 25 females) was carried out. RESULTS: There were no differences in fibronectin levels between the two groups nor between the sexes. Perinatal morbidity was higher in the IDM group, but there was no correlation between fibronectin levels and the presence of perinatal pathology. CONCLUSIONS: Fibronectin levels are not useful in the perinatal evaluation of infants of diabetic mothers. 相似文献
913.
The measurement of CK-MB remains the test of choice for confirmation or exclusion of AMI and probably will remain the test of choice for routine diagnosis in the near future. Nowadays determination of cardiac troponin T (cTnT) and cardiac troponin I (cTnI) as a method relatively expensive and time-consuming should be restricted to clinical settings that really require their high specificity. 相似文献
914.
Z Popovi? Z Jovanovi? M Todori? I Butkovi? D Nikoli? 《Canadian Metallurgical Quarterly》1997,54(5):425-435
To clarify the histogenesis and differentiation potential of central neurocytoma, a pathological investigation of seven tumors from three patients was conducted using immunohistochemistry and ultrastructural analysis in addition to systematic in vitro studies. Six tumors were studied immunohistochemically and five were examined ultrastructurally. All cases that were immunostained were positive for synaptophysin in nuclear-free neuropil islands. In five tumors, a few tumor cells, in addition to reactive astrocytes, were positive for glial fibrillary acidic protein (GFAP). Vimentin staining was also positive in a few tumor cells of five specimens. Neurofilament staining was always negative. All cases for which ultrastructure was examined showed various synaptic abnormalities. Cultured cells were subdivided into three distinct tumor cell types: neuronal cells which stained for neurofilament proteins with neurosecretory granules; small flat undifferentiated cells with a high nuclear-cytoplasmic ratio and scant cytoplasmic organelles; and small round or multipolar astrocytic cells with 10-nm intermediate filaments which stained for GFAP. Our tissue culture studies disclosed that cultured neurocytoma cells form a cellular mosaic similar to subependymal plate layers that are composed of mitotically active cells, neurons and glia. 相似文献
915.
916.
Catalytic chlorination of methane was studied over SO
4
2–
/ZrO2, Pt/SO
4
2–
/ZrO2, and Fe/Mn/SO
4
2–
/ZrO2 solid superacid catalysts. The reactions were carried out in a continuous flow reactor under atmospheric pressure, at temperatures below 240°C, with a gaseous hourly space velocity of 1000 ml/g h and a methane to chlorine ratio of 4 to 1. At 200°C with 30% chlorine converted the selectivity in methyl chloride exceeds 90%. At more elevated temperatures, the selectivity decreases but stays above 80% in methyl chloride at 225°C using the sulfated zirconia catalysts. The selectivity can be enhanced by adding platinum to sulfated zirconia catalysts. An iron and manganese-doped catalyst exhibited excellent selectivities at somewhat lower conversions. Methyl chloride is obtained at 235°C in selectivities greater than 85%. No chloroform or carbon tetrachloride is formed. The electrophilic insertion involves electron-deficient metal-coordinated chlorine into the methane C-H bond.Catalysis by solid superacids, 29. For part 28 see ref. [14]. 相似文献
917.
In this work, (a) complexation reaction of zirconium tetra-n-butylate, Zr(OBu
n
)4, with MAc and different organic acids, (b) the hydrolysis reaction of modified Zr species, and (c) the polymerization reaction of complex products are studied. Zr(OBu
n
)4 was reacted with different mole ratios of methacrylic acid (MAc) at room temperature and the maximum combination ratio was found to be 1:2 [Zr(OBu
n
)4:MAc] by FT-IR. The modification of zirconium tetra-n-butylate with the acid mixtures [methacrylic acid-acetic acid (MeCOOH), methacrylic acid-propionic acid (EtCOOH), methacrylic acid-butyric acid (PrCOOH)] was made for a combination ratio of 1:1:1 [MAc:RCOOH:Zr(OBu
n
)4; R: Me, Et, Pr] and the products were characterized by1H-NMR, FT-IR, and UV spectroscopies. Following their synthesis, hydrolysis of the complexes with various amounts of water and polymerization with benzoyl peroxide were realized. The hydrolysis and polymerization products of the complexes were studied by Karl-Fischer coulometric titration and thermal analysis, respectively. Methyl ethyl ketone(MEK) and chloroform were chosen as solvents. 相似文献
918.
O Gorbatyuk M Landry P Emson I Akmayev T H?kfelt 《Canadian Metallurgical Quarterly》1997,15(8):931-938
Using immunohistochemistry and in situ hybridization the distribution of nitric oxide synthase (NOS) was investigated in the rat brain during pre- and postnatal development. At E15 weak NOS-like immunoreactivity (NOS-LI) could be seen in the differentiation field of the anterior hypothalamus. At E17 strong NOS-LI was observed in the developing neurons of the hypothalamic paraventricular nucleus, supraoptic nucleus, anterodorsal nucleus and lateral hypothalamic areas. In the thalamic paratenial nucleus a strong NOS-LI was observed in these neurons at E17, E18 and P1 with a weaker intensity at P3, P7, P9 and P15, whereas at P30 and in adult rats no NOS-positive neurons could be detected. NOS expression at E17 and P3 was verified by in situ hybridization. These results suggest that NO may have a developmental role at least in one of the regions studied, the thalamic paratenial nucleus. 相似文献
919.
920.
M Warmuth M Bergmann A Priess K H?uslmann B Emmerich M Hallek 《Canadian Metallurgical Quarterly》1997,272(52):33260-33270
bcr-abl, the oncogene causing chronic myeloid leukemia, encodes a fusion protein with constitutively active tyrosine kinase and transforming capacity in hematopoietic cells. Various intracellular signaling intermediates become activated and/or associate by/with Bcr-Abl, including the Src family kinase Hck. To elucidate some of the structural requirements and functional consequences of the association of Bcr-Abl with Hck, their interaction was investigated in transiently transfected COS7 cells. Neither the complex formation of Hck kinase with Bcr-Abl nor the activation of Hck by Bcr-Abl was dependent on the Abl kinase activity. Both inactivating point mutations of Hck and dephosphorylation of Hck enhanced its complex formation with Bcr-Abl, indicating that their physical interaction was negatively regulated by Hck (auto)phosphorylation. Finally, experiments with a series of kinase negative Bcr-Abl mutants showed that Hck phosphorylated Bcr-Abl and induced the binding of Grb2 to Tyr177 of Bcr-Abl. Taken together, our results suggest that Bcr-Abl preferentially binds inactive forms of Hck by an Abl kinase-independent mechanism. This physical interaction stimulates the Hck tyrosine kinase, which may then phosphorylate the Grb2-binding site in Bcr-Abl. 相似文献