全文获取类型
收费全文 | 7407篇 |
免费 | 470篇 |
国内免费 | 358篇 |
专业分类
电工技术 | 334篇 |
综合类 | 330篇 |
化学工业 | 1555篇 |
金属工艺 | 750篇 |
机械仪表 | 416篇 |
建筑科学 | 202篇 |
矿业工程 | 148篇 |
能源动力 | 176篇 |
轻工业 | 874篇 |
水利工程 | 28篇 |
石油天然气 | 198篇 |
武器工业 | 85篇 |
无线电 | 642篇 |
一般工业技术 | 745篇 |
冶金工业 | 324篇 |
原子能技术 | 175篇 |
自动化技术 | 1253篇 |
出版年
2024年 | 29篇 |
2023年 | 123篇 |
2022年 | 277篇 |
2021年 | 273篇 |
2020年 | 176篇 |
2019年 | 169篇 |
2018年 | 150篇 |
2017年 | 200篇 |
2016年 | 221篇 |
2015年 | 219篇 |
2014年 | 375篇 |
2013年 | 389篇 |
2012年 | 529篇 |
2011年 | 636篇 |
2010年 | 526篇 |
2009年 | 587篇 |
2008年 | 515篇 |
2007年 | 515篇 |
2006年 | 434篇 |
2005年 | 315篇 |
2004年 | 268篇 |
2003年 | 227篇 |
2002年 | 197篇 |
2001年 | 112篇 |
2000年 | 108篇 |
1999年 | 82篇 |
1998年 | 76篇 |
1997年 | 50篇 |
1996年 | 63篇 |
1995年 | 68篇 |
1994年 | 63篇 |
1993年 | 43篇 |
1992年 | 33篇 |
1991年 | 43篇 |
1990年 | 24篇 |
1989年 | 26篇 |
1988年 | 25篇 |
1987年 | 12篇 |
1986年 | 7篇 |
1985年 | 6篇 |
1984年 | 9篇 |
1983年 | 3篇 |
1982年 | 2篇 |
1981年 | 7篇 |
1980年 | 8篇 |
1979年 | 6篇 |
1978年 | 2篇 |
1977年 | 3篇 |
1968年 | 1篇 |
1951年 | 1篇 |
排序方式: 共有8235条查询结果,搜索用时 15 毫秒
71.
付春 《精细与专用化学品》2007,15(7):13-14
6,7-二氢-5H-环戊烷并[b]吡啶主要用于药物、杀菌剂和抗菌剂的研究,也被广泛应用于制备植物保护剂和合成树脂、防老剂以及塑料制品等。目前因其被作为第四代抗生素头孢匹罗的侧链而成为研究者竞相开发的热点。6,7-二氢-5H-环戊烷并[b]吡啶有实用价值的合成方法主要有N-羟基邻苯二甲酰胺法、丙烯醛法和己二酸二乙酯法。丙烯醛法中6,7-二氢-5H-环戊烷并[b]吡啶的收率为87.4%,具有较好的开发前景。 相似文献
72.
A molecular level mechanism is proposed for the highly selective 14-e– oxidative transformation ofn-butane to maleic anhydride on the surface of vanadyl pyrophosphate. The mechanism suggests that the dimeric active sites assume at any given time, one of four possible interconvertible states which differ from each other in the number of available oxygen atoms and the formal oxidation states of the individual vanadium atoms. The relative ratios of active sites in each of the four possible states are dictated by the reaction conditions, the redox properties of the reacting gases and the structure of the vanadyl pyrophosphate active surface. A crucial feature of the mechanism is a 72k89777724v2/xxlarge8220.gif" alt="ldquo" align="MIDDLE" BORDER="0">pseudo-ozonide72k89777724v2/xxlarge8221.gif" alt="rdquo" align="MIDDLE" BORDER="0"> surface species formed by the interaction of a chemisorbed dioxygen molecule and an adjacent metal-oxo group. This unusual species is responsible for the initial activation of then-butane, which occurs when the chemisorbed dioxygen abstracts an H-atom from the alkane and the adjacent metal-oxo group reacts with the incipient alkyl radical to form an alkoxy group. The proposed mechanism is entirely consistent with literature reports describing the behaviour of (VO)2P2O7 in flow, pulse and TAP reactors. 相似文献
73.
A number of targeted cytotoxic agents have been developed that selectively kill malignant or otherwise pathological cells. These engineered proteins consist of a potent cytotoxic element connected to a ligand domain that binds to specific molecules on the surface of the target cell. Several of these agents have shown promise in clinical trials and one is currently administered to patients. A significant technical obstacle that has impeded the development of some of these toxins is the difficulty of preparing certain recombinant proteins in properly folded forms. These fusion proteins have generally been produced in bacteria requiring them to be denatured and renatured in vitro. For some proteins this is an efficient process whereas for others it is not. We describe here a system to produce fusion toxins rapidly and efficiently by engineering mammalian cells to secrete them as properly folded molecules which can be purified in native form from cell culture medium. We have used this system to produce highly active preparations of DAB(389)-IL7, a molecule consisting of the catalytic and transmembrane domains of diphtheria toxin fused to interleukin 7. This system is generalizable and can be used to produce and evaluate rapidly fusion toxins incorporating novel or uncharacterized ligands. 相似文献
74.
This paper describes the application of SwRI̵7;s cold-start PO
x
catalyst technology to reduce cold-start hydrocarbon emissions from a US Tier 2 vehicle at 722;7 76;C. A reduction in 722;7 76;C (20
76;F) cold-start hydrocarbons will help US Tier 2 vehicles meet the proposed EPA NMOG standards. Improvements in cold temperature
hydrocarbon emissions would also be beneficial in many parts of Europe during the winter months. In this work, a total hydrocarbon
reduction of 19% was realized at 24 76;C, in line with previous results, but only up to 3% at 722;7 76;C. Insufficient oxygen in
the engine-out exhaust gas at 722;7 76;C was determined to be the reason why the PO
x
catalyst failed to significantly reduce HC emissions. Addition of supplemental oxygen to the exhaust during the cold-start,
to simulate an adjustment in the engine calibration to less rich operation, resulted in a total hydrocarbon reduction of 18%
with the PO
x
catalysts in place, but no benefit when the PO
x
catalysts were removed. Hence, the PO
x
catalyst approach can be used to good effect, even under sub-ambient cold-start conditions. 相似文献
75.
76.
以4-羟基-3-硝基苯甲酸为原料,经酯化、醚化、还原合环、硝化、水解等5步反应,合成了两个未见文献报导的化合物7-硝基-3-氧-3,4-二氢-2H-[1,4]苯并噁嗪-6-羧酸和8-硝基-3-氧-3,4-二氢-2H-[1,4]苯并嗪-6-羧酸,其结构经IR、LC/MS、1HNMR和元素分析确证。 相似文献
77.
78.
Flaky monoclinic La2Ti2O7 was prepared via a hydrothermal method based on the reaction of Ti(SO4)2 and La(NO3)3. Relative to the solid-state reaction sample, the flaky La2Ti2O7 showed higher surface areas, much smaller crystal size and more efficient light absorption. All these factors led to the
higher photoactivity to produce H2 from water splitting under UV irradiation. 相似文献
79.
Stabilization of 73613r7n101/xxlarge977.gif" alt="thetav" align="BASELINE" BORDER="0">-alumina phase by silica was studied in nanocomposite (diphasic) alumina-silica gels by XRD and BET surface areas measurements. Five wt.% of silica (22 nm particles) increased the crystallization temperature of 73613r7n101/xxlarge977.gif" alt="thetav" align="BASELINE" BORDER="0"> to 73613r7n101/xxlarge945.gif" alt="agr" align="BASELINE" BORDER="0">-alumina by about 100°C from boehmite (10 nm particles) derived alumina. Stabilization of 73613r7n101/xxlarge977.gif" alt="thetav" align="BASELINE" BORDER="0">-alumina was caused by the formation of intimate contact (Al-O-Si) between components by diffusion of silica into the defect alumina structure.Also with the Department of Agronomy. 相似文献
80.
Salvia leucophylla, a shrub observed in coastal south California, produces several volatile monoterpenoids (camphor, 1,8-cineole, 765742563361g7j/xxlarge946.gif" alt="beta" align="MIDDLE" BORDER="0">-pinene, 765742563361g7j/xxlarge945.gif" alt="agr" align="BASELINE" BORDER="0">-pinene, and camphene) that potentially act as allelochemicals. The effects of these were examined using Brassica campestris as the test plant. Camphor, 1,8-cineole, and 765742563361g7j/xxlarge946.gif" alt="beta" align="MIDDLE" BORDER="0">-pinene inhibited germination of B. campestris seeds at high concentrations, whereas 765742563361g7j/xxlarge945.gif" alt="agr" align="BASELINE" BORDER="0">-pinene and camphene did not. Root growth was inhibited by all five monoterpenoids in a dose-dependent manner, but hypocotyl growth was largely unaffected. The monoterpenoids did not alter the sizes of matured cells in either hypocotyls or roots, indicating that cell expansion is relatively insensitive to these compounds. They did not decrease the mitotic index in the shoot apical region, but specifically lowered mitotic index in the root apical meristem. Moreover, morphological and biochemical analyses on the incorporation of 5-bromo-2765742563361g7j/xxlarge8242.gif" alt="prime" align="BASELINE" BORDER="0">-deoxyuridine into DNA demonstrated that the monoterpenoids inhibit both cell-nuclear and organelle DNA synthesis in the root apical meristem. These results suggest that the monoterpenoids produced by S. leucophylla could interfere with the growth of other plants in its vicinity through inhibition of cell proliferation in the root apical meristem. 相似文献