全文获取类型
收费全文 | 25050篇 |
免费 | 2257篇 |
国内免费 | 787篇 |
专业分类
电工技术 | 230篇 |
综合类 | 1252篇 |
化学工业 | 7033篇 |
金属工艺 | 608篇 |
机械仪表 | 236篇 |
建筑科学 | 729篇 |
矿业工程 | 293篇 |
能源动力 | 481篇 |
轻工业 | 11949篇 |
水利工程 | 177篇 |
石油天然气 | 888篇 |
武器工业 | 28篇 |
无线电 | 523篇 |
一般工业技术 | 1359篇 |
冶金工业 | 921篇 |
原子能技术 | 268篇 |
自动化技术 | 1119篇 |
出版年
2024年 | 205篇 |
2023年 | 588篇 |
2022年 | 1074篇 |
2021年 | 1241篇 |
2020年 | 1138篇 |
2019年 | 1049篇 |
2018年 | 1040篇 |
2017年 | 1131篇 |
2016年 | 1194篇 |
2015年 | 1098篇 |
2014年 | 1443篇 |
2013年 | 1816篇 |
2012年 | 1844篇 |
2011年 | 1926篇 |
2010年 | 1326篇 |
2009年 | 1261篇 |
2008年 | 1096篇 |
2007年 | 1348篇 |
2006年 | 1101篇 |
2005年 | 796篇 |
2004年 | 681篇 |
2003年 | 615篇 |
2002年 | 489篇 |
2001年 | 410篇 |
2000年 | 339篇 |
1999年 | 261篇 |
1998年 | 211篇 |
1997年 | 191篇 |
1996年 | 163篇 |
1995年 | 151篇 |
1994年 | 146篇 |
1993年 | 121篇 |
1992年 | 110篇 |
1991年 | 78篇 |
1990年 | 72篇 |
1989年 | 52篇 |
1988年 | 49篇 |
1987年 | 35篇 |
1986年 | 36篇 |
1985年 | 27篇 |
1984年 | 21篇 |
1983年 | 17篇 |
1982年 | 11篇 |
1980年 | 21篇 |
1966年 | 6篇 |
1961年 | 5篇 |
1959年 | 8篇 |
1958年 | 5篇 |
1956年 | 5篇 |
1951年 | 5篇 |
排序方式: 共有10000条查询结果,搜索用时 15 毫秒
991.
Pipecolic Acid Hydroxylases: A Monophyletic Clade among cis‐Selective Bacterial Proline Hydroxylases that Discriminates l‐Proline 下载免费PDF全文
Johanna Mattay Dr. Wolfgang Hüttel 《Chembiochem : a European journal of chemical biology》2017,18(15):1523-1528
Proline hydroxylases are iron(II)/2‐oxoglutarate‐dependent enzymes that hydroxylate l ‐proline and derivatives, such as l pipecolic acid, which is the six‐membered‐ring homologue of l ‐proline. It has been established that there is a distinct group of conserved bacterial enzymes that hydroxylate l ‐pipecolic acid and trans‐3‐ and trans‐4‐methyl‐l ‐proline, but virtually no l ‐proline. This allows the organism to produce hydroxyproline congeners without hydroxylation of the physiologically omnipresent l ‐proline. In vitro conversions showed that the substrate spectrum of the pipecolic acid hydroxylases GetF (from a Streptomyces sp.; producer of the tetrapeptide antibiotic GE81112) and PiFa (from Frankia alni) overlaps that of proline hydroxylases, except for the nonacceptance of l ‐proline and smaller homologues. Distinct and conserved residues were determined for both types of enzymes. However, site‐directed mutagenesis in GetF did not yield variants that accepted l ‐proline; this suggested a complex interaction of several residues around the active site, which resulted in delicate changes in substrate specificity. This is supported by substrate docking in a homology model of GetF, which revealed an altered orientation for l ‐proline relative to that of preferred substrates. 相似文献
992.
993.
The effect of macadamia oil intake on muscular inflammation and oxidative profile kinetics after exhaustive exercise 下载免费PDF全文
994.
Synthesis and evaluation of cytotoxic activity of conjugated linoleic acid derivatives (esters,alcohols, and their acetates) toward cancer cell lines 下载免费PDF全文
995.
The response surface methodology was used to evaluate the effects of extraction time, power of ultrasound, liquid to solid ratio, and solvent composition on the quantity and quality (from aspect of antioxidant activity) phenolics of Stachys lavandulifolia. The best extraction time, power of ultrasound, liquid to solid ratio, and solvent composition for both the quality and quantity of phenolics were 14 min, 300 W, 40 (v/w), and 57% methanol, respectively. Only the liquid to solid ratio was effective on the quality of phenolics. Also, the comparison between the ultrasound-assisted extraction and maceration methods showed the suitability of ultrasound-assisted extraction for extracting phenolics from this plant. 相似文献
996.
Chin-San Wu 《Polymer-Plastics Technology and Engineering》2017,56(11):1177-1187
The structural and thermal properties, antibacterial activity, and biocompatibility of composite materials containing polylactide and curcumin powder were evaluated. Glycidyl methacrylate-grafted polylactide and curcumin powder were used to enhance the desired characteristics of these composites. The water resistance of glycidyl methacrylate-grafted polylactide/curcumin powder composites was greater than that of the polylactide/curcumin powder composites. A biocompatibility evaluation with tail skin fibroblasts indicated that both materials were nontoxic for biomedical material applications. Moreover, curcumin powder enhanced the Escherichia coli(BCRC10239) andStaphylococcus aureus (BCRC107801) antibacterial activities of the membranes. Membranes of glycidyl methacrylate-grafted polylactide- or polylactide-containing curcumin powder had better antibacterial activity. 相似文献
997.
Effect of air drying and freeze drying on polysaccharides of lychee with three cultivars was determined in terms of polysaccharides yield, antioxidant capacity and hypoglycemic action. The results show that Guiwei lychee pulp had the highest crude polysaccharide yield among three lychees. However, Feizixiao lychee had the lowest EC50 concentration and the highest FRAP (Ferric reducing antioxidant power) value. Moreover, Guiwei lychees dried by air drying had the best effect for improving glucose tolerance of normal mice and diabetic mice. Three freeze dried lychees had no significant effect. The effect of air dried Guiwei samples on decreasing TG was the best and it was even better than dapagliflozin group. Only freeze-dried Guiwei lychee could significant decrease the TC content in blood among all lychee samples. In a word, Feizixiao lychees had the best antioxidant capacity and Guiwei lychees had the best hypoglycemic effect. Air drying was better drying method for lychee polysaccharides than freeze drying in this study. 相似文献
998.
Poly(lactic acid)/poly(vinyl pyrrolidone) membranes produced by solution blow spinning: Structure,thermal, spectroscopic,and microbial barrier properties 下载免费PDF全文
Roberta F. Bonan Paulo R. F. Bonan André U. D. Batista Danyel E. C. Perez Lúcio R. C. Castellano Juliano E. Oliveira Eliton S. Medeiros 《应用聚合物科学杂志》2017,134(19)
Submicrometric and nanometric poly(lactic acid)/poly(vinyl pyrrolidone) (PLA/PVP) fibrous membranes containing 0, 5, 10, 15, and 20 wt % PVP, with or without 20 wt % Copaiba oil (Copaifera sp.), were produced by solution blow spinning (SBS), using polymer injection rate of 120 μL min?1, gas pressure of 2.4 kPa, working distance of 20 cm, and collector rotation of 200 rpm. The morphological, thermal, and spectroscopic properties of these membranes were characterized by scanning electron microscopy (SEM), differential scanning calorimetry (DSC), thermogravimetry (TG), and Fourier transform infrared spectroscopy (FTIR). A method for the evaluation of membrane microbial barrier properties based on resazurin colorimetric method was proposed. Results showed that the addition of both PVP and Copaiba oil produced thicker fibers; otherwise, there was no effect on morphology. Thermal analyses (TG and DSC) indicated the immiscible nature of polymer blends produced, also confirmed by the spectroscopic studies. Antimicrobial barrier properties were related to the antimicrobial effect of Copaiba oil, combined with it hydrophobic nature. The hydrophilic nature of PVP favored degradation of fiber mats, impairing barrier property when higher concentrations of PVP were added. Results indicate that produced spun mats can potentially be used in applications such as wound dressing. © 2017 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2017 , 134 , 44802. 相似文献
999.
Mahnaz Saraei Gholamreza Zarrini Moshgan Esmati Leila Ahmadzadeh 《Designed Monomers and Polymers》2017,20(1):325-331
Two novel acrylate monomers, [5-(benzyloxy)-4-oxo-4H-pyran-2-yl]methyl acrylate and {1-[(5-(benzyloxy)-4-oxo-4H-pyran-2-yl)methyl]-1,2,3-triazol-4-yl}methyl acrylate were synthesized by the reaction of 5-benzyloxy-2-(hydroxymethyl)-4H-pyran-4-one and 5-(benzyloxy)-2-{[4-(hydroxymethyl)-1,2,3-triazol-1-yl]methyl}-4H-pyran-4-one with acryloyl chloride in the presence of triethylamine, respectively. These monomers were polymerized using 2,2′-azobisisobutyronitrile (AIBN) as the initiator in N,N-dimethylformamide:14-dioxane (10:1) solution. The thermal behavior of the polymers was investigated by thermogravimetric analysis (TGA) and differential scanning calorimetry (DSC). The synthesized compounds were evaluated for their antibacterial and antifungal activites aganist bacteria and fungi using the disk diffusion method. The results indicated that some of these compounds demonstrated moderate to good antibacterial and antifungal activities. 相似文献
1000.
MnOx promotional effects on olefins synthesis directly from syngas over bimetallic Fe‐MnOx/SiO2 catalysts 下载免费PDF全文
Zhengpai Zhang Weiwei Dai Xin‐Chao Xu Jun Zhang Bianfang Shi Jing Xu Weifeng Tu Yi‐Fan Han 《American Institute of Chemical Engineers》2017,63(10):4451-4464
The direct synthesis of lower olefins via the Fischer‐Tropsch reaction (FTO) has been performed over a series of Fe‐MnOx/SiO2 catalysts. The addition of MnOx could improve the dispersion of iron species, and promote the reduction of iron oxide during the activation and subsequent carburization. Moreover, the results of characterization demonstrated that MnOx could enhance the surface basicity of the catalysts due to electronic effects and promote the formation of iron carbides. For the first time, the intrinsic power‐law kinetics for FTO was obtained for both Fe20/SiO2 and Fe20‐Mn1/SiO2 catalysts. Kinetic parameters and structure characterizations indicated that MnOx could facilitate the CO dissociation on the catalyst surface, thus enhancing the adsorption strength and capacity of surface carbonaceous intermediates. The weak hydrogenation of carbonaceous species would boost the selectivities toward lower olefins. Finally, a plausible mechanism for FTO, involving the promotional effects of MnOx on Fe, has been proposed. © 2017 American Institute of Chemical Engineers AIChE J, 63: 4451–4464, 2017 相似文献