全文获取类型
收费全文 | 12129篇 |
免费 | 792篇 |
国内免费 | 18篇 |
专业分类
电工技术 | 141篇 |
综合类 | 16篇 |
化学工业 | 3018篇 |
金属工艺 | 161篇 |
机械仪表 | 273篇 |
建筑科学 | 444篇 |
矿业工程 | 25篇 |
能源动力 | 387篇 |
轻工业 | 1799篇 |
水利工程 | 67篇 |
石油天然气 | 44篇 |
无线电 | 822篇 |
一般工业技术 | 1964篇 |
冶金工业 | 1628篇 |
原子能技术 | 79篇 |
自动化技术 | 2071篇 |
出版年
2023年 | 110篇 |
2022年 | 345篇 |
2021年 | 412篇 |
2020年 | 287篇 |
2019年 | 284篇 |
2018年 | 413篇 |
2017年 | 437篇 |
2016年 | 481篇 |
2015年 | 348篇 |
2014年 | 503篇 |
2013年 | 930篇 |
2012年 | 769篇 |
2011年 | 857篇 |
2010年 | 661篇 |
2009年 | 620篇 |
2008年 | 660篇 |
2007年 | 538篇 |
2006年 | 437篇 |
2005年 | 350篇 |
2004年 | 286篇 |
2003年 | 260篇 |
2002年 | 261篇 |
2001年 | 163篇 |
2000年 | 155篇 |
1999年 | 150篇 |
1998年 | 549篇 |
1997年 | 311篇 |
1996年 | 215篇 |
1995年 | 147篇 |
1994年 | 118篇 |
1993年 | 121篇 |
1992年 | 44篇 |
1991年 | 48篇 |
1990年 | 35篇 |
1989年 | 49篇 |
1988年 | 39篇 |
1987年 | 33篇 |
1986年 | 31篇 |
1985年 | 47篇 |
1984年 | 37篇 |
1983年 | 26篇 |
1982年 | 46篇 |
1981年 | 42篇 |
1980年 | 29篇 |
1979年 | 24篇 |
1978年 | 29篇 |
1977年 | 51篇 |
1976年 | 42篇 |
1975年 | 21篇 |
1973年 | 16篇 |
排序方式: 共有10000条查询结果,搜索用时 281 毫秒
91.
Zakaria Chajar Michel Primet Hélène Praliaud Michèle Chevrier Catherine Gauthier Frédéric Mathis 《Catalysis Letters》1994,28(1):33-40
The role of nitrogen dioxide in the selective reduction of NO by propane over a Cu-MFI zeolite is investigated. NO2 and NO reductions were carried out under similar conditions of reaction. In the presence of oxygen, the reduction of NO by C3H8 does not differ significantly from that of NO2. In the absence of oxygen, the reduction of NO2 by propane occurs with a partial decomposition of the nitric dioxide molecule. Such a decomposition leads to the formation of oxygen, which is responsible for the increase in catalytic activity by comparison with the same reaction performed with NO. NO2 formed and released in the gas phase during the reduction of NO by propane in the presence of oxygen does not play a predominant role in the catalytic process. 相似文献
92.
A model previously developed in our laboratory to culture rat prostate explants in serum-free chemically-defined medium was used to evaluate the direct influence of potential regulators. The aim of the present work was to verify the effects of insulin (I) and transferrin (Tr), two hormones considered as essential in other serum-free culture systems, and three androgenic hormones, since the prostate is known to be androgen-dependent. Explants of rat prostate were cultured for five days in serum-free Leibovitz's L-15 medium (37 degrees C, 95% air-5% CO2). The addition of Tr (50 micrograms/ml) had no effect, but I (5 micrograms/ml) significantly increased DNA synthesis. This influence was amplified by combination of the two hormones. However, protein synthesis was only slightly stimulated. Testosterone (T) or androstanediol significantly increased DNA synthesis when compared to corresponding control values at five days. In combination with I plus Tr, each hormone showed potentiated effects, particularly T with a twofold increase over day 0 values. When dihydrotestosterone was added singly, the incorporation of 3H-thymidine was stimulated by 300% over control values at five days, and by 100% over values in uncultured explants. This influence was maximal since it was not improved by I plus Tr. Protein synthesis was increased significantly by the triple combination. In addition, each androgen as well as the combination of I plus Tr had a positive influence on explant morphology. The above conditions optimize the present culture system and establish its usefulness as a valuable tool to study the direct influence of different effectors in prostate metabolism and to eventually identify putative cancer markers. 相似文献
93.
Benign symmetric lipomatosis (BSL), also known as Madelung's disease, is a rare condition and characterized by diffuse but painless growth of unencapsulated lipomas. A close correlation to alcohol and nicotine abuse, metabolic disturbances and malignant tumours have been observed. Surgical treatment is frequently followed by recurrence, nevertheless, it can yield satisfactory functional and cosmetic results. A case of BSL with uncommonly distributed tumors is reported. 相似文献
94.
Mario Mariani Ugo Ravasio Armando Buttafava Antonio Faucitano 《Nuclear instruments & methods in physics research. Section B, Beam interactions with materials and atoms》2007,265(1):245-250
The effect of gamma irradiation in air is investigated in two thermoplastic polyesters (PET and PEN), in order to evaluate the influence of aromatic density and the role of oxygen on radiation resistance. EPR measurements were carried out to detect radical stability against oxygen permeation and to provide radical characterization. Viscometric data reveal a different behaviour between films and thick samples. Positron annihilation spectra show a decrease of ortho-positronium intensity, which is more marked in film samples. ortho-positronium lifetime does not depend on the radiation dose. 相似文献
95.
G Giuffrè E Vitarelli G Tuccari M Ponz de Leon G Barresi 《Canadian Metallurgical Quarterly》1996,429(6):345-352
The simple mucin-type carbohydrate antigens Tn, sialosyl-Tn, T and the 'cryptic' sialylated variant of the last represent the mucin core oligosaccharide structures that are produced in the initial steps of the mucin biosynthetic pathway. Utilizing monoclonal antibodies anti-Tn antigen (HB-Tn1), anti-sialosyl-Tn antigen (HB-STn1), anti-T antigen (HB-T1) and the biotinylated Amaranthus caudatus agglutinin (ACA), we have investigated the expression of the simple mucin-type carbohydrate antigens in hereditary nonpolyposis colorectal cancer (HNPCC; 15 cases) compared with sporadic colorectal cancer (CRC; 60 cases) and normal colonic mucosa (30 cases). A variable positivity of Tn, sialosyl-Tn, T and the cryptic sialylated form of this latter antigen was encountered in both HNPCC and sporadic CRC cases; in addition, in normal colonic mucosa a constant reactivity was encountered only for Tn and the cryptic sialylated form of T, while negative results were always obtained for sialosyl-Tn and T antigens. Statistical analysis, performed using a Chi-square test, showed significantly lower (P = 0.037) expression of sialosyl-Tn and higher (P = 0.022) expression of T in HNPCC than in sporadic CRC, suggesting a greater presence of beta 1,3 galactosyltransferase activity in HNPCC than in sporadic CRC. We were unable to identify a peculiar phenotype for HNPCC with simultaneous evaluation of reactivity for HB-Tn1, HB-STn1, HB-T1 and ACA; the biological significance of the preferential expression of T antigen in HNPCC remains to be investigated. 相似文献
96.
We have tested the effect of several bile acids on the proliferation and differentiation of the HL60 human promyelocytic leukemia cell line in vitro. Deoxycholate, chenodeoxycholate and lithocholic acid caused dose-dependent inhibition of cell proliferation and induction of differentiation along the monocyte/macrophage pathway as determined by morphology, NBT test, non-specific esterase, and staining by monoclonal antibodies against specific cell-surface antigens. Optimal effects were obtained at 100, 75, and 60 microM of the 3 bile acids respectively. Cell-cycle flow-cytometric analysis showed that a substantial fraction of HL60 cells accumulated at the G0/G1 transition. Protein-kinase-C inhibitors such as sphinganine and H-7 inhibited the differentiation-inducing effect of bile acids, suggesting a possible role for PKC in this regulation. When bile acids were combined with non-effective concentrations of all-trans retinoic acid, enhancement of the monocytic differentiation of THP-1 human leukemia cells was observed. Our findings demonstrate induction of tumor-cell differentiation by bile acids, compounds that present minimal undesirable effects in humans. 相似文献
97.
98.
The degradation of interlaminar shear strength and shear fracture toughness of glass/epoxy composites due to uptake of distilled water and sea water has been studied. The composites were immersed in water for up to eight months at temperatures up to 70 °C. Unreinforced matrix resin samples were also immersed for periods up to 2 years. Sea water was absorbed less rapidly than distilled water. Weight gains below 1% did not influence the shear strength while higher weight gains reduced shear strength up to 25%. The loss in apparent interlaminar shear strength was uniquely related to specimen weight gain. Mode II fracture toughness, G
IIc, also decreased with increasing immersion time after an initial incubation period, but the accelerated tests were found to reduce G
IIc less than the room temperature tests at comparable weight gains. 相似文献
99.
Jose Miguel Leiva-Murillo Antonio Artés-Rodríguez 《Neural Networks, IEEE Transactions on》2007,18(5):1433-1441
In this paper, we present a novel scheme for linear feature extraction in classification. The method is based on the maximization of the mutual information (MI) between the features extracted and the classes. The sum of the MI corresponding to each of the features is taken as an heuristic that approximates the MI of the whole output vector. Then, a component-by-component gradient-ascent method is proposed for the maximization of the MI, similar to the gradient-based entropy optimization used in independent component analysis (ICA). The simulation results show that not only is the method competitive when compared to existing supervised feature extraction methods in all cases studied, but it also remarkably outperform them when the data are characterized by strongly nonlinear boundaries between classes. 相似文献
100.
Relaxed stability and performance conditions for Takagi-Sugeno fuzzy systems with knowledge on membership function overlap. 总被引:1,自引:0,他引:1
Antonio Sala Carlos Ari?o 《IEEE transactions on systems, man, and cybernetics. Part B, Cybernetics》2007,37(3):727-732
This correspondence presents a relaxation of some earlier linear matrix inequality (LMI) conditions, which allow setting up less conservative stability or performance conditions for Takagi-Sugeno fuzzy models. Unlike the previous literature, this correspondence takes into account the knowledge of the membership functions' shape by considering bounds on them and their cross products (interpreted as an overlap measure), introducing auxiliary LMI variables. Numerical examples illustrate the achieved improvements. 相似文献