全文获取类型
收费全文 | 20868篇 |
免费 | 551篇 |
国内免费 | 24篇 |
专业分类
电工技术 | 218篇 |
综合类 | 18篇 |
化学工业 | 4427篇 |
金属工艺 | 492篇 |
机械仪表 | 365篇 |
建筑科学 | 925篇 |
矿业工程 | 88篇 |
能源动力 | 536篇 |
轻工业 | 1403篇 |
水利工程 | 261篇 |
石油天然气 | 205篇 |
武器工业 | 1篇 |
无线电 | 1424篇 |
一般工业技术 | 3364篇 |
冶金工业 | 4506篇 |
原子能技术 | 167篇 |
自动化技术 | 3043篇 |
出版年
2022年 | 152篇 |
2021年 | 242篇 |
2020年 | 198篇 |
2019年 | 263篇 |
2018年 | 297篇 |
2017年 | 302篇 |
2016年 | 369篇 |
2015年 | 292篇 |
2014年 | 456篇 |
2013年 | 1236篇 |
2012年 | 750篇 |
2011年 | 944篇 |
2010年 | 754篇 |
2009年 | 760篇 |
2008年 | 883篇 |
2007年 | 951篇 |
2006年 | 780篇 |
2005年 | 741篇 |
2004年 | 649篇 |
2003年 | 606篇 |
2002年 | 581篇 |
2001年 | 363篇 |
2000年 | 366篇 |
1999年 | 359篇 |
1998年 | 438篇 |
1997年 | 384篇 |
1996年 | 373篇 |
1995年 | 359篇 |
1994年 | 307篇 |
1993年 | 358篇 |
1992年 | 313篇 |
1991年 | 227篇 |
1990年 | 273篇 |
1989年 | 279篇 |
1988年 | 264篇 |
1987年 | 251篇 |
1986年 | 282篇 |
1985年 | 298篇 |
1984年 | 297篇 |
1983年 | 291篇 |
1982年 | 231篇 |
1981年 | 258篇 |
1980年 | 253篇 |
1979年 | 251篇 |
1978年 | 227篇 |
1977年 | 229篇 |
1976年 | 205篇 |
1975年 | 195篇 |
1974年 | 174篇 |
1973年 | 179篇 |
排序方式: 共有10000条查询结果,搜索用时 15 毫秒
181.
James N. Seiber Paul M. Tuskes Lincoln P. Brower Carolyn J. Nelson 《Journal of chemical ecology》1980,6(2):321-339
Adult monarch butterflies,Danaus plexippus L. (Lepidoptera: Danaidae), store only some of the cardenolides present in the larval milkweed (Asclepiadaceae) host. Feeding known doses of individual cardenolides to 4th instar monarch larvae led to more efficient larval tissue incorporation at low doses than at high ones, and favored storage of cardenolide glycosides over genins. A qualitative regulation also occurs during larval feeding; calactin and calotropin were stored as such but uscharidin was rapidly converted to a mixture of calactin and calotropin which were the forms stored by the larvae. Two genins, uzarigenin and digitoxigenin, were stored by larvae as polar cardenolide metabolites.Research supported by National Science Foundation grants DEB 7514266 and DEB 7514266-AO2 (U.C. Davis) and DEB 7514265 (Amherst College). 相似文献
182.
This paper proposes an improvement to the delay‐dependent stability of discrete systems with time‐varying delays. The approach is based on the observation that the positive definiteness of a chosen Lyapunov–Krasovskii functional does not necessarily require all the involved symmetric matrices to be positive definite, which has been overlooked in the literature. The derived delay‐dependent stability conditions are in terms of linear matrix inequalities. It is theoretically proved that our results are less conservative than the corresponding ones obtained by requiring the positive definiteness of all the symmetric matrices in a chosen Lyapunov–Krasovskii functional. The importance of the present approach is that a great number of delay‐dependent analysis and synthesis results obtained by the aforementioned requirement in the literature can be improved by the present approach without introducing any new decision variables. Copyright © 2013 John Wiley & Sons, Ltd. 相似文献
183.
An experimental investigation of the rheological properties of glass fiber-reinforced polycarbonate melts and the extrusion of such compounds through capillary and slit dies is presented. The viscosity–shear rate function seems independent of instrument for cone-plate and capillary investigations. The presence of fibers increases the level of the viscosity. Normal stresses at fixed shear stress are also increased by the presence of fibers. The extrudate swell is decreased by the presence of fibers and surface roughness is increased. Fiber orientation increases and surface roughness decreases with increasing extrusion rate. 相似文献
184.
185.
The potential hazard of a carbonate-based, phosphate-free detergent was compared with that of a variety of granular and liquid
phosphate detergents. Evaluations were made using methods prescribed by the Federal Hazardous Substances Act (FHSA) and others
designed to more closely approximate exposure in actual use. Some phosphate products produced esophageal and gastric mucosal
irritation similar to that produced by the nonphosphate detergent, while others caused varying, lesser degrees of injury.
The nonphosphate detergent, as well as several phosphate products, was seriously irritating to the unwashed eye at required
FHSA test levels. Under more realistic conditions, eye irritation was reduced significantly. Skin irritation by the nonphosphate
product was comparable to that by phosphate detergents, and in some cases was lower. In skin irritation tests with human subjects,
results with the carbonate detergent were equal to or lower than those obtained with phosphate detergents. The carbonate detergent
was found not to be phototoxic, nor a contact- nor photo-sensitizer. Total alkalinity was shown not to correlate with the
severity of tissue injury in either test animals or in man.
Presented at the AOCS Short Course, “Update on Detergents and Raw Materials,” Lake Placid, New York, June 1971. 相似文献
186.
Vasudev P. Saraf Wolfgang G. Glasser Garth L. Wilkes James E. McGrath 《应用聚合物科学杂志》1985,30(5):2207-2224
Hydroxypropyl lignin-based thermosetting polyurethanes were synthesized with excess hexamethylene diisocyanate (HDI) and tolylene diisocyanate (TDI) by solution casting. Four polyethylene glycols (PEG) of molecular weight 400, 600, 1000, and 4000 were mixed with lignin polyol to incorporate different proportions of soft segment into the network prior to crosslinking. Neither thermal nor mechanical and limited small angle x-ray scattering (SAXS) analysis provided distinct evidence for phase separation and microstructure formation. The study examines the effect of the soft segment in relation to chain length and weight contribution on the thermal and mechanical properties of the final networks. A significant sensitivity of glass transition temperature (Tg), of swelling in DMF, and of the mechanical properties to soft segment content was observed. Some of this sensitivity must, however, be attributed to differences in crosslink density since the polyol to diisocyanate weight ratio was kept constant throughout the synthesis series. The magnitude of the change of the different properties was found to be influenced by both glycol content and glycol molecular weight. The Tg of the network decreased from 105°C to as low as 38°C (HDI), and from 158°C to 70°C (TDI), with incorporation of up to 17.8% glycol, and it was greater with lower molecular weight glycols than with higher ones at any weight fraction. Swelling in DMF increased as expected with soft segment content. Mechanical properties were affected most if HDI and lower molecular weight glycols were used. The uniformity in structure, reduction in brittleness, and considerable improvement in mechanical properties with inclusion of minor PEG contents indicates that lignin-based network polyurethanes can be synthesized with controllable performance characteristics. 相似文献
187.
Phosphatidylcholine acyltransferase (lecithin:cholesterol acyltransferase or LCAT; EC 2.3.1.43) activity was found to be present
in pig ovarian follicular fluid (POFF), in addition to pig serum (PS). The cholesterol esterification rate in both POFF and
PS is linear with incubation time up to 2 hr. The mean absolute rate of POFF-cholesterol esterification was 8.1±0.4 nmoles
per ml per hr approximately one-fourth of that in PS. However, the fractional rate (percent of labeled cholesterol esterified
per hr) of POFF-cholesterol esterification was similar to that observed in PS. There was little variation of absolute rate
of cholesterol esterification in the fluid obtained from different sizes of follicles. Fatty acid or triacylglycerol did not
participate in the reaction of cholesterol esterification in POFF. No appreciable change in enzymatic activity was found from
storing POFF at 4 C for periods of time up to 24 hr or at −70 C up to 2 months, but activity was lost thereafter. On the other
hand, PS showed a much longer period of stability (5 days at 4 C and 9 months at −70 C). A discrepancy between the fatty acid
composition of cholesteryl esters formed by the LCAT reaction and the fatty acid composition at the C-2 position of phosphatidylcholine
led us to propose a two-step mechanism for the LCAT reaction. It is concluded that the LCAT of POFF, as well as that of plasma,
is specific for individual fatty acids rather than for the fatty acid composition of phosphatidylcholine. The fatty acid concentration
of lysophosphatidylcholine decreased during prolonged incubation times (6 to 21 hr) suggesting that the increased lysophosphatidylcholine
formed as a product of the LCAT reaction may be reused as substrate for the LCAT reaction or for hydrolysis by lysophosphatidylcholine
hydrolase.
Presented at the AOCS Meeting, New York, May 1977. 相似文献
188.
Rudolf H. Scheffrahn Ruei-Ching Hsu Nan-Yao Su Jacob B. Huffman Sharon L. Midland James J. Sims 《Journal of chemical ecology》1988,14(3):765-776
The heartwood of bald cypress,Taxodium distichum (L.) Rich., resisted feeding attack by the Formosan subterranean termite,Coptotermes formosanus Shiraki. Hexane-extracted heartwood, however, was consumed at > 12 times the amount of sound heartwood eaten. A bioassay usingT. distichum sapwood as a feeding substrate was employed to assess the antitermitic activity of successive hexane, acetone, and methanol extracts of heartwood shavings and isolates derived from the active hexane extract. Two fractions, eluted from the crude hexane extract by liquid chromatography, significantly reduced termite feeding compared to the parent extract, while a third fraction was less active than the original hexane extract. Each fraction contained one major component. All three components were structurally related diterpenes. The two most active heartwood constituents were identified by GC-MS and NMR as ferruginol and manool, while the third and least active, but most prevalent, compound in heartwood was identified as nezukol. Results of bioassays suggest that these allelochemicals act principally as feeding deterrents with accompanying termite mortality due to starvation. 相似文献
189.
The slope of density versus log (pressure) represents the efficiency of compaction forming. A high maximum particle packing density of powders increased the pressing efficiency and the compact density. A high granule density favors a high compact density, whereas, a low granule density favors an increase in the pressing efficiency and the elimination of the intergranular pores; an optimum clearly existed. At a very low forming pressure, the compact density using granules containing a binder was lower; but at a higher forming pressure, the binder acted as a lubricant and assisted particle sliding and rearrangement which increased the pressing efficiency and compact density. 相似文献
190.
Prior to fertilization, mammalian sperm must undergo the acrosome reaction, which involves modifications of the plasma and
outer acrosomal membranes followed by vesiculation and release of the membranes. The membrane fraction that was released from
caudal boar sperm undergoing an in vitro acrosome-like reaction was isolated and characterized with respect to density, marker
enzymes and lipid composition. This membrane had a lower phospholipid/protein ratio (mg/mg) than the sperm plasma membrane,
whereas both membranes had similar molar sterol/phospholipid ratios. The major phospholipid was sphingomyelin, followed by
phosphatidylethanolamine and phosphatidylcholine, whereas in the plasma membrane the order was reversed; the two major phosphoglycerides
contained alkylacyl and alkenylacyl species in addition to the diacyl species. The released membrane also contained lower
amounts of cholesterol sulfate and unsaturated fatty acids than the plasma membranes. These results, in combination with our
studies on the changes of the sperm membranes during maturation and acrosome reaction, will allow a better understanding of
the mechanism of the sperm acrosome reaction. 相似文献