全文获取类型
收费全文 | 6934篇 |
免费 | 258篇 |
国内免费 | 4篇 |
专业分类
电工技术 | 130篇 |
综合类 | 31篇 |
化学工业 | 1496篇 |
金属工艺 | 225篇 |
机械仪表 | 142篇 |
建筑科学 | 348篇 |
矿业工程 | 22篇 |
能源动力 | 151篇 |
轻工业 | 655篇 |
水利工程 | 25篇 |
石油天然气 | 13篇 |
无线电 | 493篇 |
一般工业技术 | 1201篇 |
冶金工业 | 1355篇 |
原子能技术 | 108篇 |
自动化技术 | 801篇 |
出版年
2023年 | 41篇 |
2021年 | 91篇 |
2020年 | 58篇 |
2019年 | 97篇 |
2018年 | 122篇 |
2017年 | 91篇 |
2016年 | 149篇 |
2015年 | 135篇 |
2014年 | 172篇 |
2013年 | 288篇 |
2012年 | 247篇 |
2011年 | 310篇 |
2010年 | 251篇 |
2009年 | 232篇 |
2008年 | 252篇 |
2007年 | 271篇 |
2006年 | 216篇 |
2005年 | 193篇 |
2004年 | 198篇 |
2003年 | 155篇 |
2002年 | 142篇 |
2001年 | 103篇 |
2000年 | 118篇 |
1999年 | 153篇 |
1998年 | 299篇 |
1997年 | 224篇 |
1996年 | 200篇 |
1995年 | 135篇 |
1994年 | 122篇 |
1993年 | 118篇 |
1992年 | 90篇 |
1991年 | 103篇 |
1990年 | 93篇 |
1989年 | 105篇 |
1988年 | 74篇 |
1987年 | 87篇 |
1986年 | 59篇 |
1985年 | 69篇 |
1984年 | 67篇 |
1983年 | 73篇 |
1982年 | 66篇 |
1981年 | 50篇 |
1980年 | 70篇 |
1979年 | 63篇 |
1978年 | 69篇 |
1977年 | 90篇 |
1976年 | 109篇 |
1975年 | 66篇 |
1974年 | 65篇 |
1973年 | 66篇 |
排序方式: 共有7196条查询结果,搜索用时 15 毫秒
1.
Oxana Kazakova Irina Smirnova Elena Tretyakova Ren Csuk Sophie Hoenke Lucie Fischer 《International journal of molecular sciences》2021,22(4)
Semi-synthetic triterpenoids, holding an amino substituted seven-membered A-ring (azepano-ring), which could be synthesized from triterpenic oximes through a Beckmann type rearrangement followed by a reduction of lactame fragment, are considered to be novel promising agents exhibiting anti-microbial, alpha-glucosidase, and butyrylcholinesterase inhibitory activities. In this study, in an attempt to develop new antitumor candidates, a series of A-ring azepano- and 3-amino-3,4-seco-derivatives of betulin, oleanolic, ursolic, and glycyrrhetinic acids were evaluated for their cytotoxic activity against five human cancer cell lines and non-malignant mouse fibroblasts by means of a colorimetric sulforhodamine assay. Azepanoallobetulinic acid amide derivative 11 was the most cytotoxic compound of this series but showed little selectivity between the different human tumor cell lines. Flow cytometry experiments showed compound 11 to act mainly by apoptosis (44.3%) and late apoptosis (21.4%). The compounds were further screened at the National Cancer Institute towards a panel of 60 cancer cell lines. It was found that compounds 3, 4, 7, 8, 9, 11, 15, 16, 19, and 20 showed growth inhibitory (GI50) against the most sensitive cell lines at submicromolar concentrations (0.20–0.94 μM), and their cytotoxic activity (LC50) was also high (1–6 μM). Derivatives 3, 8, 11, 15, and 16 demonstrated a certain selectivity profile at GI50 level from 5.16 to 9.56 towards K-562, CCRF-CEM, HL-60(TB), and RPMI-8226 (Leukemia), HT29 (Colon cancer), and OVCAR-4 (Ovarian cancer) cell lines. Selectivity indexes of azepanoerythrodiol 3 at TGI level ranged from 5.93 (CNS cancer cell lines SF-539, SNB-19 and SNB-75) to 14.89 for HCT-116 (colon cancer) with SI 9.56 at GI50 level for the leukemia cell line K-562. The present study highlighted the importance of A-azepano-ring in the triterpenic core for the development of novel antitumor agents, and a future aim to increase the selectivity profile will thus lie in the area of modifications of azepano-triterpenic acids at their carboxyl group. 相似文献
2.
Gheorghita Menghiu Vasile Ostafe Radivoje Prodanovi Rainer Fischer Raluca Ostafe 《International journal of molecular sciences》2021,22(6)
Chitinases catalyze the degradation of chitin, a polymer of N-acetylglucosamine found in crustacean shells, insect cuticles, and fungal cell walls. There is great interest in the development of improved chitinases to address the environmental burden of chitin waste from the food processing industry as well as the potential medical, agricultural, and industrial uses of partially deacetylated chitin (chitosan) and its products (chito-oligosaccharides). The depolymerization of chitin can be achieved using chemical and physical treatments, but an enzymatic process would be more environmentally friendly and more sustainable. However, chitinases are slow-acting enzymes, limiting their biotechnological exploitation, although this can be overcome by molecular evolution approaches to enhance the features required for specific applications. The two main goals of this study were the development of a high-throughput screening system for chitinase activity (which could be extrapolated to other hydrolytic enzymes), and the deployment of this new method to select improved chitinase variants. We therefore cloned and expressed the Bacillus licheniformis DSM8785 chitinase A (chiA) gene in Escherichia coli BL21 (DE3) cells and generated a mutant library by error-prone PCR. We then developed a screening method based on fluorescence-activated cell sorting (FACS) using the model substrate 4-methylumbelliferyl β-d-N,N′,N″-triacetyl chitotrioside to identify improved enzymes. We prevented cross-talk between emulsion compartments caused by the hydrophobicity of 4-methylumbelliferone, the fluorescent product of the enzymatic reaction, by incorporating cyclodextrins into the aqueous phases. We also addressed the toxicity of long-term chiA expression in E. coli by limiting the reaction time. We identified 12 mutants containing 2–8 mutations per gene resulting in up to twofold higher activity than wild-type ChiA. 相似文献
3.
Stephan Altmann Jürgen Mut Natalia Wolf Jutta Meißner-Weigl Maximilian Rudert Franz Jakob Marcus Gutmann Tessa Lühmann Jürgen Seibel Regina Ebert 《International journal of molecular sciences》2021,22(6)
Metabolic glycoengineering enables a directed modification of cell surfaces by introducing target molecules to surface proteins displaying new features. Biochemical pathways involving glycans differ in dependence on the cell type; therefore, this technique should be tailored for the best results. We characterized metabolic glycoengineering in telomerase-immortalized human mesenchymal stromal cells (hMSC-TERT) as a model for primary hMSC, to investigate its applicability in TERT-modified cell lines. The metabolic incorporation of N-azidoacetylmannosamine (Ac4ManNAz) and N-alkyneacetylmannosamine (Ac4ManNAl) into the glycocalyx as a first step in the glycoengineering process revealed no adverse effects on cell viability or gene expression, and the in vitro multipotency (osteogenic and adipogenic differentiation potential) was maintained under these adapted culture conditions. In the second step, glycoengineered cells were modified with fluorescent dyes using Cu-mediated click chemistry. In these analyses, the two mannose derivatives showed superior incorporation efficiencies compared to glucose and galactose isomers. In time-dependent experiments, the incorporation of Ac4ManNAz was detectable for up to six days while Ac4ManNAl-derived metabolites were absent after two days. Taken together, these findings demonstrate the successful metabolic glycoengineering of immortalized hMSC resulting in transient cell surface modifications, and thus present a useful model to address different scientific questions regarding glycosylation processes in skeletal precursors. 相似文献
4.
Kowsik Ghosh M. Mangir Murshed Michael Fischer Thorsten M. Gesing 《Journal of the American Ceramic Society》2022,105(1):728-741
Mullite-type RMn2O5 (R = Y, rare-earth element) ceramics are of ongoing research attentions because of their interesting crystal-chemical, physical, and thermal properties. We report a detailed structural, spectroscopic and thermal analysis of the series of mullite-type RAlGeO5 (R = Y, Sm-Lu) phases. Polycrystalline samples are prepared by solid-state synthesis methods. Each sample is characterized by X-ray powder diffraction followed by Rietveld refinements, showing that they are isotypic and crystallize in the space group Pbam. The change of the metric parameters is explained in term of the lanthanide contraction effect. A rare inversion of Al/Ge between octahedral and pyramidal sites have been observed for these mullite-type so called O10 compounds, and the inversion parameter found to be between 0.22(1) and 0.30(1) for different R-cations. The <Al/Ge–O> bond distances and their bond valence sums (BVSs) support the respective inversions. Density functional theory (DFT) calculated phonon density of states (PDOS) and electronic band structures are compared for the vibrational and electronic band gap features respectively. Analysis of UV/Vis absorption spectra using both derivation of absorption spectra fitting (DASF) and Tauc's methods demonstrates that each of the RAlGeO5 O10 compounds is high bandgap semiconductor, possessing direct transition between 4.1(1) and 5.4(1) eV. Both Raman and Fourier transform infrared spectra show clear red shift (quasi-harmonic) of the vibrational wavenumbers with respect to the ionic radii of the R-cations. Selective Raman bands at higher wavenumber region further complement the inversion of Al/Ge between two coordination sites. The higher decomposition temperature of the RAlGeO5 compounds, compared to those of RMn2O5 phases, is explained in terms of higher bond strength of Al/Ge-O than those of Mn-O. Irrespective to the inversion between Al- and Ge-sites, the decomposition temperature also depends on the type of R-cation in RAlGeO5. 相似文献
6.
正Axel酒店坐落于马德里文学街区中心的一栋19世纪的大楼内,在西班牙"黄金世纪"时期,这一地区曾是一个重要的文化中心。经过岁月的洗礼,大楼至今仍保留着橙色的外墙和典雅的阳台。接手此次酒店设计的是西班牙室内设计工作室EL EQUIPO CREATIVO,设计师以西班牙著名导演佩德罗·阿莫多瓦(Pedro Almodóvar)的电影为灵感,在酒店内融入了马德里非常传统并广受欢迎的特色元素和一些著名的视觉元素。 相似文献
7.
Annika Jahn Anton Hoffmann Luisa Blaesing Fabian Kunde Martin Bertau Martina Bremer Steffen Fischer 《化学,工程师,技术》2020,92(11):1733-1740
The enzymatic conversion of lignins, possibly in combination with electrochemical oxidation, makes aromatics such as syringol, guaiacol, vanillin and catechol available in the qualities required by the fragrance industry. The lignins were obtained by soda digestion from wheat straw and Miscanthus, characterized and then converted with laccases. The overall yield amounted up to 9 wt % with a product spectrum confined to four substances. Catechol was the major product, with a fraction of ≈75 %. It can easily be isolated by extraction with acetone. 相似文献
8.
9.
10.