全文获取类型
收费全文 | 8983篇 |
免费 | 337篇 |
国内免费 | 38篇 |
专业分类
电工技术 | 121篇 |
综合类 | 22篇 |
化学工业 | 2091篇 |
金属工艺 | 167篇 |
机械仪表 | 230篇 |
建筑科学 | 573篇 |
矿业工程 | 31篇 |
能源动力 | 174篇 |
轻工业 | 772篇 |
水利工程 | 53篇 |
石油天然气 | 11篇 |
武器工业 | 1篇 |
无线电 | 596篇 |
一般工业技术 | 1442篇 |
冶金工业 | 1531篇 |
原子能技术 | 70篇 |
自动化技术 | 1473篇 |
出版年
2023年 | 51篇 |
2022年 | 117篇 |
2021年 | 142篇 |
2020年 | 105篇 |
2019年 | 119篇 |
2018年 | 168篇 |
2017年 | 110篇 |
2016年 | 191篇 |
2015年 | 191篇 |
2014年 | 249篇 |
2013年 | 562篇 |
2012年 | 397篇 |
2011年 | 545篇 |
2010年 | 410篇 |
2009年 | 389篇 |
2008年 | 434篇 |
2007年 | 420篇 |
2006年 | 364篇 |
2005年 | 312篇 |
2004年 | 291篇 |
2003年 | 222篇 |
2002年 | 228篇 |
2001年 | 175篇 |
2000年 | 168篇 |
1999年 | 158篇 |
1998年 | 263篇 |
1997年 | 196篇 |
1996年 | 159篇 |
1995年 | 141篇 |
1994年 | 123篇 |
1993年 | 137篇 |
1992年 | 130篇 |
1991年 | 75篇 |
1990年 | 92篇 |
1989年 | 79篇 |
1988年 | 94篇 |
1987年 | 66篇 |
1986年 | 54篇 |
1985年 | 79篇 |
1984年 | 94篇 |
1983年 | 60篇 |
1982年 | 66篇 |
1981年 | 83篇 |
1980年 | 68篇 |
1979年 | 85篇 |
1978年 | 68篇 |
1977年 | 68篇 |
1976年 | 56篇 |
1975年 | 58篇 |
1973年 | 47篇 |
排序方式: 共有9358条查询结果,搜索用时 15 毫秒
111.
Nitrate radicals are being recognized as key intermediates in a growing list of important chemical processes in the atmosphere. Here, the role of nitrate radicals (NO3) in tropospheric chemistry is discussed, with special emphasis on results from field measurements, most of which have been made by differential optical absorption spectroscopy (DOAS), with matrix-isolation electron spin resonance being an alternative technique. Nitrate radicals were observed in the atmosphere at peak mixing ratios of 350 ppt. Long-term observation of NO3 shows that 24-h averages in rural air masses are closer to a few ppt. Nevertheless, the NO3 radical plays an important role in the non-photochemical conversion of NOx to HNO3. Also, NO3 is a strong oxidizing agent and initiates the night-time removal of atmospheric trace species such as olefins, aromatic hydrocarbons, and organic sulfur compounds. Finally, night-time peroxy radical production and release of reactive halogen species from sea salt aerosol might be initiated by NO3 reactions. 相似文献
112.
The nucleation and growth of CO2 bubbles in non-Newtonian and Newtonian fluids that were initially supersaturated under different pressures are investigated in the present work. Quantitative information by means of two cameras reveals that at an immobile nucleation site the bubble grows rapidly followed by a linear increase in bubble diameter with time. After reaching a critical size, the bubble detaches from the stagnant site to rise in liquids with an exponential temporary increase for both the diameter and distance. A simple physical reasoning was proposed to qualitatively explain these observed phenomena. Recently, the growth rate and flow fields around a CO2 micro-bubble were measured in a microdevice by a micro-Particle Image Velocimetry in water. This information at microscale gives new insight into the complex mechanism of bubble nucleation and growth in fluids and could help to develop a rigorous theoretical modelling and numerical simulation such as the Lattice Boltzmann approach. 相似文献
113.
Frank Zok Orfeo Sbaizero Craig L. Hom Anthony G. Evans 《Journal of the American Ceramic Society》1991,74(1):187-193
The mode I fracture resistance of a ceramic matrix composite has been measured. Simultaneous observations have revealed that the resistance is dominated by frictional dissipation upon the pullout of fibers that fracture in the crack wake off the crack plane. Numerical and analytical crack growth simulations have been compared with the experimental results. One important feature in this comparison concerns the occurrence of large-scale bridging. With these effects taken into account, the simulations and the experiments are found to be in good correspondence for acceptable magnitudes of the interface sliding stress. 相似文献
114.
Willard A. Cutler Frank W. Zok Fred F. Lange 《Journal of the American Ceramic Society》1996,79(7):1825-1833
Several different hybrid laminated composites comprised of alternating layers of dense ceramic sheets (either SiC or Si3 N4 ) and fiber-reinforced ceramic-matrix-composite (CMC) layers (Nicalon fibers with either glass or glass-ceramic matrices) have been fabricated and characterized. The effects of the reinforcement architecture (unidirectional vs cross-ply) and the relative volume fractions of the phases on the tensile and flexural properties have been examined. Comparisons have been made with the properties of the constituent layers. Rudimentary models have been developed to describe the onset of cracking and for the minimum volume fraction of CMC required to develop multiple cracks and thus obtain a high failure strain. 相似文献
115.
Siebert HC André S Asensio JL Cañada FJ Dong X Espinosa JF Frank M Gilleron M Kaltner H Kozár T Bovin NV von Der Lieth CW Vliegenthart JF Jiménez-Barbero J Gabius HJ 《Chembiochem : a European journal of chemical biology》2000,1(3):181-195
This study documents the feasibility of switching to an aprotic medium in sugar receptor research. The solvent change offers additional insights into mechanistic details of receptor--carbohydrate ligand interactions. If a receptor retained binding capacity in an aprotic medium, solvent-exchangeable protons of the ligand would not undergo transfer and could act as additional sensors, thus improving the level of reliability in conformational analysis. To probe this possibility, we first focused on hevein, the smallest lectin found in nature. The NMR-spectroscopic measurements verified complexation, albeit with progressively reduced affinity by more than 1.5 orders of magnitude, in mixtures of up to 50% dimethyl sulfoxide (DMSO). Since hevein lacks the compact beta-strand arrangement of other sugar receptors, such a structural motif may confer enhanced resistance to solvent exchange. Two settings of solid-phase activity assays proved this assumption for three types of alpha- and/or beta-galactoside-binding proteins, that is, a human immunoglobulin G (IgG) subfraction, the mistletoe lectin, and a member of the galectin family of animal lectins. Computer-assisted calculations and NMR experiments also revealed no conspicuous impact of the solvent on the conformational properties of the tested ligands. To define all possible nuclear Overhauser effect (NOE) contacts in a certain conformation and to predict involvement of exchangeable protons, we established a new screening protocol applicable during a given molecular dynamics (MD) trajectory and calculated population densities of distinct contacts. Experimentally, transferred NOE (tr-NOE) experiments with IgG molecules and the disaccharide Gal'alpha1-3Galbeta1-R in DMSO as solvent disclosed that such an additional crosspeak, that is, Gal'OH2--GalOH4, was even detectable for the bound ligand under conditions in which spin diffusion effects are suppressed. Further measurements with the plant lectin and galectins confirmed line broadening of ligand signals and gave access to characteristic crosspeaks in the aprotic solvent and its mixtures with water. Our combined biochemical, computational, and NMR-spectroscopical strategy is expected to contribute notably to the precise elucidation of the geometry of ligands bound to compactly folded sugar receptors and of the role of water molecules in protein--ligand (carbohydrate) recognition, with relevance to areas beyond the glycosciences. 相似文献
116.
John Case Sanjay Jain Yuh Shin Ong Pavel Semukhin Frank Stephan 《Journal of Computer and System Sciences》2014
Automatic classes are classes of languages for which a finite automaton can decide whether a given element is in a set given by its index. The present work studies the learnability of automatic families by automatic learners which, in each round, output a hypothesis and update a long-term memory, depending on the input datum, via an automatic function. Many variants of automatic learners are investigated: where the long-term memory is restricted to be the current hypothesis whenever this exists, cannot be of length larger than the length of the longest datum seen, or has to consist of a constant number of examples seen so far. Learnability is also studied with respect to queries which reveal information about past data or past computation history; the number of queries per round is bounded by a constant. 相似文献
117.
Robert R. Mod Frank C. Magne Evald L. Skau 《Journal of the American Oil Chemists' Society》1959,36(12):616-619
Summary Eighty-eight amine salts of long-chain fatty acids have been prepared, purified by solvent crystallization, and characterized.
Forty-five of these were salts of palmitic acid. The rest included salts of capric, lauric, myristic, stearic, oleic, elaidic,
and 12-hydroxystearic acids. A variety of aliphatic, aromatic, and heterocyclic amines, including primary, secondary, and
tertiary amines, were investigated. The majority of these gave 1∶1 acid-amine compounds on solvent recrystallization of an
equimolar mixture of acid and amine. Some of the amines gave no crystallizable salt, and with others the pure salt could not
be obtained by this procedure. Under the same conditions the symmetrical alkyl, substituted alkyl, and aralkyl secondary amines
investigated gave crystallizable compounds containg two molecules of fatty acid to one of amine. Molecular compounds of 2-aminopyridine
with four molecules of saturated fatty acid were formed by recrystallization starting with a 1 to 4 amine-acid mixture. Surface-tension
measurements were made for aqueous solutions of a few of the amine salts.
One of the laboratories of the Southern Utilization Research and Development Division, Agricultural Research Service, U. S.
Department of Agriculture. 相似文献
118.
Frank C. Magne Robert R. Mod Evald L. Skau 《Journal of the American Oil Chemists' Society》1961,38(6):294-296
A systematic investigation has been made of the effect of fatty acid composition on the compatibility of the corresponding
mixed morpholides as plasticizers for vinyl chloride-vinyl acetate copolymer resins. Plasticizer evaluation data were obtained
for the morpholides of lauric, palmitic, elaidic, and epoxyoleic acids and compared with the literature values for other fatty
acid morpholides and morpholide mixtures.
Ternary compatibility-composition diagrams have been constructured for the oleic-linoleic-stearic and the oleic-linoleic-palmitic
morpholide systems. These diagrams make it possible to predict what adjustment, if any, must be made in the composition of
any mixture of these fatty acids so that the corresponding morpholide mixture will be compatible. The diagrams will be useful
in selecting the most appropriate and economical fatty acid sources from which to make compatible morpholide mixtures.
Presented at the 52nd Annual Meeting, American Oil Chemists’ Society, St. Louis, Mo., May 1–3, 1961.
One of the laboratories of the Southern Utilization Research and Development Division, Agricultural Research Service, U.S.
Department of Agriculture. 相似文献
119.
Effects of three phenolic acids on chlorophyll content and growth of soybean and grain sorghum seedlings 总被引:6,自引:0,他引:6
Experiments were designed to test the hypothesis that interference with chlorophyll metabolism may be one mechanism of inhibition of plant growth in allelopathic interactions. Effects of ferulic,p-coumaric, and vanillic acids on soybean and grain sorghum growth and chlorophyll content were quantified and compared after seedlings were treated with these compounds in a nutrient culture. Following a 6-day treatment cycle, dry weights of soybean seedlings were reduced by both 10–3 M and 5 × 10–4 M treatments of ferulic,p-coumaric and vanillic acids. Soybean weight reductions in each case were paralleled by a significant reduction in the concentration (g Chl/mg dry wt) of chlorophylls a and b and total chlorophyll in the unifoliate leaves. Sorghum seedling growth was also reduced by each of the compounds at the 5 × 10–4 M level, but leaf chlorophyll concentration was not below that of control plants. 相似文献
120.
A study has been made of the thermal-oxidative stability of several commercially available carbon fibers. Significant differences in the weight loss behavior were observed in air aging studies run on the bare fibers at 600°F (589 K). The stability of high temperature laminates, such as those based on NR-150 polyimide precursor solutions, was found to be directly affected by the stability of the reinforcing carbon fibers. In studies carried out to determine the reasons for carbon fiber instability, residual sodium sulfate was found to be the principle destabilizing factor. A mechanism whereby the sodium sulfate promotes the oxidation of carbon in the presence of air is proposed. 相似文献