全文获取类型
收费全文 | 7398篇 |
免费 | 656篇 |
国内免费 | 49篇 |
专业分类
电工技术 | 80篇 |
综合类 | 69篇 |
化学工业 | 1617篇 |
金属工艺 | 157篇 |
机械仪表 | 203篇 |
建筑科学 | 310篇 |
矿业工程 | 49篇 |
能源动力 | 166篇 |
轻工业 | 1785篇 |
水利工程 | 83篇 |
石油天然气 | 82篇 |
武器工业 | 2篇 |
无线电 | 516篇 |
一般工业技术 | 1523篇 |
冶金工业 | 259篇 |
原子能技术 | 24篇 |
自动化技术 | 1178篇 |
出版年
2024年 | 26篇 |
2023年 | 94篇 |
2022年 | 97篇 |
2021年 | 219篇 |
2020年 | 198篇 |
2019年 | 214篇 |
2018年 | 353篇 |
2017年 | 376篇 |
2016年 | 346篇 |
2015年 | 263篇 |
2014年 | 346篇 |
2013年 | 751篇 |
2012年 | 487篇 |
2011年 | 513篇 |
2010年 | 422篇 |
2009年 | 397篇 |
2008年 | 433篇 |
2007年 | 397篇 |
2006年 | 324篇 |
2005年 | 201篇 |
2004年 | 167篇 |
2003年 | 189篇 |
2002年 | 184篇 |
2001年 | 143篇 |
2000年 | 128篇 |
1999年 | 114篇 |
1998年 | 129篇 |
1997年 | 89篇 |
1996年 | 70篇 |
1995年 | 69篇 |
1994年 | 58篇 |
1993年 | 35篇 |
1992年 | 33篇 |
1991年 | 19篇 |
1990年 | 24篇 |
1989年 | 21篇 |
1988年 | 18篇 |
1987年 | 6篇 |
1986年 | 12篇 |
1985年 | 20篇 |
1984年 | 22篇 |
1983年 | 20篇 |
1982年 | 15篇 |
1981年 | 9篇 |
1980年 | 10篇 |
1979年 | 7篇 |
1978年 | 6篇 |
1977年 | 5篇 |
1976年 | 7篇 |
1973年 | 4篇 |
排序方式: 共有8103条查询结果,搜索用时 15 毫秒
961.
Vergara-Castañeda HA Guevara-González RG Ramos-Gómez M Reynoso-Camacho R Guzmán-Maldonado H Feregrino-Pérez AA Oomah BD Loarca-Piña G 《Food & function》2010,1(3):294-300
The non-digestible fraction (NDF) of common bean (Phaseolus vulgaris L.) cultivar Bayo Madero was evaluated for its chemopreventive effect on azoxymethane (AOM) induced aberrant crypt foci (ACF) in rats. Diets containing cooked beans (CB) or its non-digestible fraction (NDF) were fed to 72 male rats after 2 azoxymethane injections (15 mg kg(-1) of body weight once a week for 2 weeks). ACF number, short chain fatty acids (SCFA) and β-glucuronidase activity were measured in colon sections from rats sacrificed 7 weeks after the last AOM injection. Food intake and weight gain of rats were unaffected by CB and NDF. CB and NDF suppressed the AOM-induced formation of ACF (0.8 and 1.5 ACF/distal zone, respectively vs. 6.6 ACF/distal zone based on methylene blue stain) and lowered β-glucuronidase activity in cecal, colonic and fecal content compared to AOM group. SCFA production was not significantly different among fecal, cecal and colonic content. These results indicate that CB and NDF from Bayo Madero provide direct chemoprotection against early stage of azoxymethane (AOM)-induced colon cancer in rats. 相似文献
962.
The best wind sites in the United States are often located far from electricity demand centers and lack transmission access. Local sites that have lower quality wind resources but do not require as much power transmission capacity are an alternative to distant wind resources. In this paper, we explore the trade-offs between developing new wind generation at local sites and installing wind farms at remote sites. We first examine the general relationship between the high capital costs required for local wind development and the relatively lower capital costs required to install a wind farm capable of generating the same electrical output at a remote site,with the results representing the maximum amount an investor should be willing to pay for transmission access. We suggest that this analysis can be used as a first step in comparing potential wind resources to meet a state renewable portfolio standard (RPS). To illustrate, we compare the cost of local wind (~50 km from the load) to the cost of distant wind requiring new transmission (~550-750 km from the load) to meet the Illinois RPS. We find that local, lower capacity factor wind sites are the lowest cost option for meeting the Illinois RPS if new long distance transmission is required to access distant, higher capacity factor wind resources. If higher capacity wind sites can be connected to the existing grid at minimal cost, in many cases they will have lower costs. 相似文献
963.
Chromium oxide deposited on phosphorus-loaded alumina is used in the oxidative dehydrogenation (ODH) of propane to propylene.
Operando Raman-GC studies allowed evaluating the molecular structure of surface Cr oxide species, their reactivity and their changes
undergone during operation. The addition of phosphorous to the alumina substrate decreases propane conversion, mainly due
to the decrease of CO
x
, there is a concomitant increase in the selectivity to propylene formation. The addition of phosphorous affects the nature
of the Cr oxide species, which do not form of a new phase. The surface interaction between dispersed chromate species and
phosphorous weakens the Cr=O bond and retards the aggregation of surface chromate species as Cr2O3 during reaction in ca. 170 °C. 相似文献
964.
Carlos Alonso-Moreno José Sancho Fernando Carrillo-Hermosilla Antonio Otero Antonio Antiñolo Isabel López-Solera 《Inorganic chemistry communications》2009,12(2):184-186
Rac and meso ansa-zirconocenes [Zr{1-Me2Si(3-R-(η5-C9H5))(3-R′-(η5-C9H5))}Cl2] (R = Et, R′ = H; R = Pr, R′ = H; and R = Et, R′ = Pr) 4–9 have been prepared by the reaction of ZrCl4 with the corresponding dilithiated derivatives from the ligands {1-Me2Si(3-R-(η5-C9H5))(3-R′-(η5-C9H5))} (R = Et, R′ = H 1; R = Pr, R′ = H 2; and R = Et, R′ = Pr 3) in diethyl ether/toluene at ?78 °C. The molecular structure of rac [Zr{1-Me2Si(3-Et-(η5-C9H5))(3-Pr-(η5-C9H5))}Cl2] 8 has been determined by single crystal X-ray diffraction studies, which show a pseudotetrahedral environment formed by the two chlorine ligands and two η5-coordinated indenyl ligands. The activity in homogeneous and heterogeneous polymerization is compared and discussed. 相似文献
965.
The electrical transport, optical, and microstructural properties of bulk polyaniline (PANI) and nano‐PANIs were investigated. A field emission scanning electron microscopy (SEM) image of bulk PANI showed macroscopic and aggregated granular particles. A SEM image of the nanostructured PANI showed the formation of one‐dimensional nano/microstructures. The formation of nanofibers was observed from the transmission electron microscopy image. The electrical conductivities of the bulk and nanostructured PANIs increased with increasing temperature, which indicated semiconductor behavior. The electrical conductivities of the bulk and nanostructured PANIs at room temperature were found to be 2.12 × 10?5 and 1.80 × 10?2 S/cm, respectively. The electrical conductivity of the nanostructured PANI was about 850 times higher than that of the bulk PANI. The obtained band gaps of the bulk and nanostructured PANIs were determined from diffuse reflectance measurements and were found to be 3.27 and 2.41 eV, respectively. The refractive index of the PANI samples changed from 1.3 to 1.61. The obtained results indicate that the electrical and optical properties of the PANI were inherently dependent on the nanostructure. © 2009 Wiley Periodicals, Inc. J Appl Polym Sci, 2009 相似文献
966.
Catherine Fernandez Yogan Monnier Elena Ormeño Virginie Baldy Stéphane Greff Vanina Pasqualini Jean-Philippe Mévy Anne Bousquet-Mélou 《Journal of chemical ecology》2009,35(8):970-979
We investigated changes in the occurrence of allelochemicals from leachates of different Pinus halepensis organs taking into account the stages of pine stand age (i.e., young < 15-years-old, middle age ± 30-years, and old > 60-years-old).
GC-MS analysis of aqueous extracts revealed approx. 59 components from needles and roots. The major constituents were divided
into different phytochemical groups—phenolics (50%), fatty acids (44%), and terpenoids. Further analyses were carried out
to characterize the distribution of allelochemicals in different organs and P. halepensis successional stages. Roots and needles had two distinct chemical profiles, while needle leachates were composed mainly of
oxygenated terpenoids (e.g., α-eudesmol, α-cadinol, and α-terpineol). Roots mainly contained fatty acids. Needles from young
pine stands had the highest content of monoterpenes, suggesting their role as potential allelochemicals that could help young
pine stands to establish. Pooling the different functional chemical groups showed that needles and, to a lesser extent, old
roots, had higher chemical diversity than the roots of young and medium-aged pines. The highest diversity in phenolic constituents
and fatty acids was in young needles (Dchem = 2.38). Finally, caffeic acid, a compound that has allelopathic properties was found in aqueous extracts at high concentrations
in both young needles and old roots. The role of this compound in mediation of biological interactions in P. halepensis ecosystem functioning is discussed. 相似文献
967.
José Miguel Laínez Georgios Kopanos Antonio Espuña Luis Puigjaner 《American Institute of Chemical Engineers》2009,55(7):1736-1753
Nowadays market competition is essentially associated to supply chain (SC) improvement. Therefore, the locus of value creation has shifted to the chain network. The strategic decision of determining the optimal SC network structure plays a vital role in the later optimization of SC operations. This work focuses on the design and retrofit of SCs. Traditional approaches available in literature addressing this problem usually utilize as departing point a rigid predefined network structure which may restrict the opportunities of adding business value. Instead, a novel flexible formulation approach which translates a recipe representation to the SC environment is proposed to solve the challenging design‐planning problem of SC networks. The resulting mixed integer linear programming model is aimed to achieve the best NPV as key performance metric. The potential of the presented approach is highlighted through illustrative examples of increasing complexity, where results of traditional rigid approaches and those offered by the flexible framework are compared. The implications of exploiting this potential flexibility to improve the SC performance are highlighted and are the subject of our further research work. © 2009 American Institute of Chemical Engineers AIChE J, 2009 相似文献
968.
Nanoscale titanium dioxide (TiO2) films were deposited on the surface of polyester nonwovens by using direct current reactive magnetron sputtering. The effect
of oxygen flow on the surface structures and properties of the fabrics was investigated in this article. The surface morphology,
microstructure, and the chemical composition of TiO2-coated fibers were characterized by atomic force microscope, X-ray diffraction, and X-ray photoelectron spectroscopy. The
effect of oxygen flow on deposition rate, white degree, and light transmission properties of the fabrics with nanoscale TiO2 films were examined. The test results proved that the oxygen flow was a key factor in sputter processing. The deposition
speed decreased and the white degree of the fabric increased with increasing oxygen flow. The ultra-violet absorption by the
polyester nonwoven fabric with TiO2 coatings was also enhanced as the oxygen flow increased in a proper range. 相似文献
969.
Narender Singh Dr. Alfonso Dueñas‐González Dr. Frank Lyko Dr. Jose L. Medina‐Franco Dr. 《ChemMedChem》2009,4(5):792-799
A series of DNA methyltransferase 1 (DNMT1) inhibitors were modeled by docking and molecular dynamics studies to rationalize their activity. Our findings will be valuable in guiding research efforts toward the rational design and virtual screening of novel DNMT inhibitors.
970.
Tania Castaño Dr. Huanchen Wang Dr. Nuria E. Campillo Dr. Sara Ballester Dr. Coral González‐García Javier Hernández Concepción Pérez Dr. Jimena Cuenca Dr. Ana Pérez‐Castillo Prof. Ana Martínez Prof. Oscar Huertas José Luis Gelpí Prof. F. Javier Luque Prof. Hengming Ke Prof. Carmen Gil Dr. 《ChemMedChem》2009,4(5):866-876
PDE7 inhibitors regulate pro‐inflammatory and immune T‐cell functions, and are a potentially novel class of drugs particularly useful for treatment of a wide variety of immune and inflammatory disorders. Structural optimization of thioxoquinazoline derivatives led to new compounds with very interesting profiles as PDE7 or PDE7/PDE4 dual inhibitors, which may be further developed as new drugs for inflammatory and neurological diseases.