全文获取类型
收费全文 | 2759篇 |
免费 | 22篇 |
国内免费 | 2篇 |
专业分类
电工技术 | 5篇 |
化学工业 | 132篇 |
金属工艺 | 11篇 |
机械仪表 | 8篇 |
建筑科学 | 30篇 |
能源动力 | 8篇 |
轻工业 | 76篇 |
水利工程 | 8篇 |
石油天然气 | 1篇 |
无线电 | 31篇 |
一般工业技术 | 107篇 |
冶金工业 | 2309篇 |
原子能技术 | 3篇 |
自动化技术 | 54篇 |
出版年
2022年 | 9篇 |
2021年 | 11篇 |
2020年 | 9篇 |
2019年 | 13篇 |
2018年 | 15篇 |
2017年 | 26篇 |
2016年 | 12篇 |
2015年 | 14篇 |
2014年 | 25篇 |
2013年 | 31篇 |
2012年 | 38篇 |
2011年 | 23篇 |
2010年 | 22篇 |
2009年 | 18篇 |
2008年 | 23篇 |
2007年 | 19篇 |
2006年 | 21篇 |
2005年 | 11篇 |
2004年 | 18篇 |
2003年 | 14篇 |
2002年 | 14篇 |
2001年 | 16篇 |
2000年 | 7篇 |
1999年 | 86篇 |
1998年 | 735篇 |
1997年 | 449篇 |
1996年 | 263篇 |
1995年 | 138篇 |
1994年 | 103篇 |
1993年 | 138篇 |
1992年 | 22篇 |
1991年 | 25篇 |
1990年 | 18篇 |
1989年 | 28篇 |
1988年 | 37篇 |
1987年 | 22篇 |
1986年 | 16篇 |
1985年 | 15篇 |
1983年 | 4篇 |
1982年 | 10篇 |
1981年 | 8篇 |
1980年 | 17篇 |
1979年 | 2篇 |
1978年 | 3篇 |
1977年 | 62篇 |
1976年 | 150篇 |
1975年 | 5篇 |
1974年 | 2篇 |
1973年 | 3篇 |
1955年 | 3篇 |
排序方式: 共有2783条查询结果,搜索用时 15 毫秒
101.
Nitrogen budget for fescue pastures fertilized with broiler litter in Major Land Resource Areas of the southeastern US 总被引:1,自引:0,他引:1
Samuel B. Marshall Michael D. Mullen Miguel L. Cabrera C. Wesley Wood Lois C. Braun Elizabeth A. Guertal 《Nutrient Cycling in Agroecosystems》2001,59(1):75-83
The southeast US produces a tremendous number of broiler chickens (Gallus gallus), which in turn produce massive quantities of litter (manure and bedding materials). In the Southeast, litter is most often disposed of via land application to pastures, however, the ultimate fate of much of the applied nitrogen (N) is not known. We have constructed N budgets for three sites across the southeastern U.S. in an effort to determine how much of the applied N is useful for plant production and how much is left to be absorbed by the environment. Study sites were located in the Coastal Plain (Alabama), Piedmont (Georgia), and Cumberland Plateau (Tennessee) Major Land Resource Areas (MLRA) of the southeastern US. Litter was applied in the Spring of two consecutive years at a rate to supply 70 kg of available N ha–1. The total amount of N applied ranged from 103 to 252 kg N ha–1 depending on site and year. Nitrogen fluxes monitored in this study were broiler litter N, ammonia (NH3) volatilization, denitrification, plant uptake, and leaching. Plant uptake represented the largest flux of applied N, averaging 43% of applied N. Losses due to NH3 volatilization and denitrification combined were only 6% of applied N on average. Loss of N due to NO3-N leaching appeared to be significant only at the Coastal Plain site where NO3-N concentrations in the groundwater peaked at 38 mg N l–1. We believe the majority of excess N shown in these budgets is likely accounted for by leaching losses and soil accumulation. Regardless of these assumptions and low gaseous losses, it is apparent that on average, 57% of applied N is destined for a fate other than plant uptake. The results of this study indicate that land-application of broiler litter at currently recommended rates has the potential for negative impacts on the environment of the southeastern U.S. in the long-term. 相似文献
102.
Eusebio Duarte Cabrera Panpan Zhang Wei‐Ching Liao Ying‐Chieh Yen Jiangfeng Yu Jose Castro L. James Lee 《Polymer Engineering and Science》2015,55(6):1374-1381
High strength light weight parts are critical for the development of new technologies, particularly electronic devices, such as laptop computers, smart phones, and tablet devices. Injection molded plastics and composites are excellent choices for mass producing such parts. As the part thickness decreases from traditional injection molding (>2 mm thickness) to thin wall molding (~1 mm thickness), and lastly, to ultra‐thin wall molding (<0.5 mm thickness), avoiding incomplete filling (short shots) becomes more challenging. Even though, methods exist today for molding thin‐wall plastic parts (i.e., fast heating/fast cooling injection molding), they require multiple steps resulting in a noncost efficient process. In this article, we demonstrate the technical feasibility of using graphene coating to facilitate flow, by promoting slip at the mold walls. We evaluate the influence of coated and uncoated mold inserts on fiber orientation. We present experimental results using un‐reinforced polypropylene and a 40% by weight carbon fiber reinforced polycarbonate/acrylonitrile butadiene styrene. POLYM. ENG. SCI., 55:1374–1381, 2015. © 2015 Society of Plastics Engineers 相似文献
103.
After hydrothermal pretreatment and enzymatic hydrolysis of wheat straw, a slurry rich in lignin but with a high content of inorganic substances, especially silica, and residual carbohydrates is produced. This slurry was used to develop an ethanol organosolv separation method to produce silica-free lignin fractions. The addition of para toluene sulphonic acid (PTSA) and the use of two alternative long-chain alcohols, oleyl alcohol or nonylphenol, were tested. In every reaction, two lignin fractions were produced and their molecular size and elemental composition were characterized. The yield of each fraction and the change in MWD were studied as a function of temperature and solid to liquid ratio. At 100, 150, and 200°C and with the use of PTSA, high-purity lignin fractions were obtained. After lignin fractionation with nonylphenol, a liquid silica-free product with high lignin content was obtained in yields between 17 and 72%. 相似文献
104.
The full characterization of chitin obtained from squid, shrimp, prawn, lobsters, and king crab is reported. Elemental analysis, including metals such as Ca, Mg, Zn, Cd, Hg, Cr, Mn, Cu, and Pb, was performed, which is quite relevant because the skeleton composition is slightly different for each species. The morphology was studied by means of TEM and their compositions were determined by energy‐dispersive X‐ray analysis. 13C cross polarization/magic angle spinning NMR was applied to determine the chemical shift of all the carbons and the difference between them. Chitin was isolated by using chemical methods, alternating hydrochloric acid and sodium hydroxide. The α‐chitin from shrimp, prawn, lobsters, and king crabs showed two signals at 73.7 and 75.6 ppm. Meanwhile, the β‐chitin from squid exhibited one signal at 75.2 ppm. FTIR studies were used to analyze α‐chitin from shrimp and β‐chitin from squid. The α‐chitin exhibited amide I vibration modes at 1660 and 1627 cm?1, whereas the β‐chitin showed one band at 1656 cm?1. X‐ray diffraction showed that α‐chitin is orthorhombic (a = 4.74 Å, b = 18.86 Å, and c = 10.32 Å) and β‐chitin had a monoclinic dihydrated form (a = 4.80 Å, b = 10.40 Å, c = 11.10 Å, and β = 97°). © 2004 Wiley Periodicals, Inc. J Appl Polym Sci 93: 1876–1885, 2004 相似文献
105.
Diez-Torrubia A Cabrera S Lambeir AM Balzarini J Camarasa MJ Velázquez S 《ChemMedChem》2012,7(4):618-628
We previously described a novel prodrug approach in which a di- or tetrapeptide moiety is linked to a wide variety of amine-containing drugs through an amide bond, which is specifically cleaved by dipeptidyl peptidase IV (DPPIV/CD26) activity. Herein we report the application of this prodrug approach to a variety of hydroxy-containing drugs (primary, secondary, tertiary, or aromatic hydroxy groups). We designed and studied tripartite prodrugs containing a dipeptide moiety (cleavable by DPPIV/CD26) and a valine as a hetero-bifunctional connector to link the dipeptide to the hydroxy group of the drug through a metabolically labile ester bond. The hydroxy-containing prodrugs showed various susceptibilities to hydrolysis by DPPIV/CD26 and serum, depending on the nature of the compound. Prodrugs of compounds containing a primary hydroxy group (as in didanosine) or a hydroxy moiety on an aromatic entity (as in acetaminophen) were most efficiently converted. In contrast, a tertiary hydroxy group was much less susceptible to conversion into its parent drug by DPPIV/CD26 or serum. A number of the prodrugs showed remarkable increases in water solubility relative to their parent drugs. 相似文献
106.
Hierarchical micro-nanostructured albite-based glass-ceramic for high dielectric strength insulators
V. Fuertes M.J. Cabrera J. Seores D. Muñoz J.F. Fernández E. Enríquez 《Journal of the European Ceramic Society》2018,38(7):2759-2766
A novel glass-ceramic material based on albite type Na-rich feldspar has been synthesized by conventional ceramic process. High crystallinity, >94%?Vol.% is obtained by fast sintering which allows energy saving processing. Albite is the main crystalline phase and tetragonal SiO2 is a secondary phase. Electrical properties were examined by complex impedance, DC measurements, and dielectric breakdown test. Dielectric characterization shows a non-Debye type dielectric behavior with low dielectric constant, 4.6 at 1?MHz, low dielectric losses, (~10?3 at 1?MHz, and a large dielectric strength, ~60?kV/mm), that it is the largest value reported in ceramic insulators. Those dielectric properties are attained by the low glassy phase content in the samples and their unique micro-nanostructure. All these properties make this novel material a very promising candidate in the market of ceramic electrical insulator, highlighting for high-voltage applications. 相似文献
107.
108.
109.
110.
Francisco A. Macías Ana M. Simonet Brigida D’Abrosca Claudia C. Maya Matías Reina Azucena González-Coloma Raimundo Cabrera Cristina Giménez Luis Villarroel 《Journal of chemical ecology》2009,35(1):39-49
The new bioactive sesquiterpenoid (3R,6E)-2,6,10-trimethyl-3-(3-p-hydroxyphenylpropanoyloxy)-dodeca-6,11-diene-2,10-diol, named megalanthine, was isolated from the resinous exudates of Heliotropium megalanthum. The degradation products of this compound were identified. Several plant-defensive properties (insecticidal, antifungal,
and phytotoxic) were evaluated after obtaining positive results in a preliminary etiolated wheat coleoptile bioassay. This
bioassay showed the need to have both the phenolic and sesquiterpene moieties of the natural product present to achieve a
biological effect. This result was confirmed in phytotoxicity bioassays. Megalanthine was ruled out as a significant plant–plant
defense agent because of its lack of stability. The positive results recorded in the antifungal and antifeedant tests suggest,
however, that this chemical is relevant in several ecological interactions involving H. megalanthum. 相似文献