全文获取类型
收费全文 | 26786篇 |
免费 | 2675篇 |
国内免费 | 19篇 |
专业分类
29480篇 |
出版年
2022年 | 237篇 |
2021年 | 483篇 |
2020年 | 267篇 |
2019年 | 330篇 |
2018年 | 449篇 |
2017年 | 371篇 |
2016年 | 555篇 |
2015年 | 1047篇 |
2014年 | 1059篇 |
2013年 | 1474篇 |
2012年 | 1871篇 |
2011年 | 1786篇 |
2010年 | 1114篇 |
2009年 | 1053篇 |
2008年 | 1493篇 |
2007年 | 1536篇 |
2006年 | 1407篇 |
2005年 | 1443篇 |
2004年 | 1407篇 |
2003年 | 1303篇 |
2002年 | 1346篇 |
2001年 | 294篇 |
2000年 | 223篇 |
1999年 | 328篇 |
1998年 | 364篇 |
1997年 | 268篇 |
1996年 | 252篇 |
1995年 | 235篇 |
1994年 | 210篇 |
1993年 | 230篇 |
1992年 | 221篇 |
1991年 | 219篇 |
1990年 | 197篇 |
1989年 | 220篇 |
1988年 | 211篇 |
1987年 | 185篇 |
1986年 | 164篇 |
1985年 | 200篇 |
1984年 | 203篇 |
1983年 | 200篇 |
1982年 | 243篇 |
1981年 | 252篇 |
1980年 | 202篇 |
1979年 | 161篇 |
1978年 | 187篇 |
1977年 | 146篇 |
1976年 | 137篇 |
1975年 | 127篇 |
1974年 | 139篇 |
1973年 | 126篇 |
排序方式: 共有10000条查询结果,搜索用时 0 毫秒
81.
82.
Joseph B. Mazzola Alan W. Neebe Christopher V. R. Dunn 《Flexible Services and Manufacturing Journal》1989,1(2):115-142
Early flexible manufacturing system (FMS) production planning models exhibited a variety of planning objectives; typically, these objectives were independent of the overall production environment. More recently, some researchers have proposed hierarchical production planning and scheduling models for FMS. In this article, we examine production planning of FMS in a material requirements planning (MRP) environment. We propose a hierarchical structure that integrates FMS production planning into a closed-loop MRP system. This structure gives rise to the FMS/MRP rough-cut capacity planning (FMRCP) problem, the FMS/MRP grouping and loading (FMGL) problem, and the FMS/MRP detailed scheduling problem. We examine the FMRCP and FMGL problems in detail and present mathematical programming models for each of these problems. In particular, the FMRCP problem is modeled as a generalized assignment problem (GAP), and a GAP-based heuristic procedure is defined for the problem. We define a two-phase heuristic for the FMGL problem and present computational experience with both heuristics. The FMRCP heuristic is shown to solve problems that exhibit a dependent-demand relation within the FMS and with FMS capacity utilization as high as 99 percent. The FMGL heuristic requires very little CPU time and obtains solutions to the test problems that are on average within 1.5 percent of a theoretical lower bound. This FMS/MRP production planning framework, together with the resulting models, constitutes an important step in the integration of FMS technology with MRP production planning. The hierarchical planning mechanism directly provides for system-level MRP planning priorities to induce appropriate production planning and control objectives on the FMS while simultaneously allowing for necessary feedback from the FMS. Moreover, by demonstrating the tractability of the FMRCP and FMGL problems, this research establishes the necessary groundwork upon which to explore systemwide issues pertaining to the coordination of the hierarchical structure. 相似文献
83.
The effects of aminophylline (10-500 microM) on isometric twitch and tetanic forces were studied in vitro on frog semitendinosus muscle. Two hypotheses were tested: 1) that micromolar concentrations of aminophylline enhanced contractility of isolated skeletal muscle and 2) that the potentiating effect of aminophylline was dependent on the presence of extracellular calcium ions. Muscles were removed, placed in aerated Ringer solution at 20 degrees C, attached to a force transducer, and stimulated directly. Muscles in normal Ringer and aminophylline Ringer were compared throughout the frequency-force relationship from twitches to maximum tetanic force. Aminophylline increased twitch force significantly at concentrations as low as 25 microM. Over a range of stimulation frequencies, but especially at 10 and 20 Hz, aminophylline increased tetanic force. The potentiating effect of aminophylline (100 microM) was reduced or eliminated in calcium-free Ringer containing 10 mM magnesium. We conclude that aminophylline, at therapeutic concentrations, enhances muscle contractility, and the enhancement is dependent on the presence of extracellular calcium. These findings support the concept that aminophylline is effective in improving respiration in humans with airway obstruction by enhancing diaphragmatic contractility. 相似文献
84.
A strain ofLactobacillus plantarum caTC2R isolated from a meat source was resistant to chloramphenicol (30 g/ml). Resistance was mediated through an inducible chloramphenicol acetyltransferase. Plasmid analysis of this strain showed three plasmids, of which the 8.5-kb plasmid apparently encodes the gene for chloramphenicol resistance. This plasmid was lost at high frequency (25%) when theLactobacillus was subcultured at a higher than optimal temperature (40°C). 相似文献
85.
To characterize the molecular/structural requirements for activationor antagonism of the arginine taste pathways in catfish, Ictaluruspunctatus, structure/activity studies were performed using integratedmultiunit responses and cross-adaptation. Of all the guanidinium-containingcompounds tested, only L-arginine, L--amino-ß-guanidinopropionic acid (L-AGPA) and L-arginine methyl and ethyl esterswere strong stimuli. Results of functional group substitutionsand modification of the L-arginine parent molecule indicatedthat: (i) stereospecificity was observed with D-arginine beinga much less effective stimulus than L-arginine; (ii) an L-aminogroup must be present and unblocked (-chloro-guanidino-N-valericacid and N-acetyl L-arginine were weak or inactive stimuli);(iii) a free carboxylic acid group was not necessary for activity;(iv) the distance between the anomeric carbon and the guanidiniumgroup was not critical (L-AGPA, having two methylene groupsless than L-arginine was a moderately strong stimulus as wasL-canavanine) and (v) modification or substitution of the guanidinumgroup by other basic groups including amine, methyl or dimethylamineor by an isosterc (ureido) resulted in loss of stimulatory ability.In general, those stimuli and analogs that were good cross-adaptersof L-arginine stimulation were also good competitors for L-[3H]argininebinding to a partial membrane fraction (P2) from catfish tasteepithelium. On the other hand, compounds that were poor cross-adaptingstimuli were also poor binding competitors. While D-argininewas a poor stimulus, it did cross-adapt L-arginine and competedwell with L-[3H]arginine for binding to fraction P2. 相似文献
86.
Anticoagulant activity of synthetic hirudin peptides 总被引:4,自引:0,他引:4
J M Maraganore B Chao M L Joseph J Jablonski K L Ramachandran 《The Journal of biological chemistry》1989,264(15):8692-8698
Synthetic peptides based on the COOH-terminal 21 residues of hirudin were prepared in order to 1) evaluate the role of this segment in hirudin action toward thrombin, 2) define the shortest peptide derivative with anticoagulant activity, and 3) investigate the role of tyrosine sulfation in the peptides' inhibitory activities. A hirudin derivative of 20 amino acids, Hir45-64 (derived from residues 45-64 of the hirudin polypeptide), was found to effect a dose-dependent increase in the activated partial thromboplastin time (APTT) of normal human plasma but to have no measurable inhibitory activity toward thrombin cleavage of a tripeptidyl p-nitroanilide substrate. Anticoagulant activity in hirudin derivatives was comparable in peptides of 20, 16, and 12 residues truncated from the NH2 terminus. Additional truncated peptides prepared by synthesis and carboxypeptidase treatment reveal that the minimal sequence of a hirudin peptide fragment with maximal anticoagulant activity is contained within the sequence: NH2-Asn-Gly-Asp-Phe-Glu-Glu-Ile-Pro-Glu-Glu-Tyr-Leu-COOH. The 12-residue derivative thus identified was reacted with dicyclohexylcarbodiimide in the presence of sulfuric acid to yield a Tyr-sulfated peptide, S-Hir53-64. By comparison to unsulfated peptide, S-Hir53-64 was found to contain a specific inhibitory activity enhanced by one order of magnitude toward increase in APTT and to effect a dose-dependent increase in thrombin time of normal human plasma to yield a 4-fold increase in thrombin time with 2.5 micrograms/ml peptide using 0.8 units/ml alpha-thrombin. Comparison of S-Hir53-64 to hirudin in thrombin time and APTT assays reveals a 50-fold difference in molar specific activities toward inhibition of thrombin. Comparison of antithrombin activities of S-Hir53-64 using a variety of animal thrombins demonstrates greatest inhibitory activity toward murine, rat, and human enzymes and a 10-fold reduced activity toward bovine thrombin. 相似文献
87.
Exposure to low doses of high-energy iron particles can alter motor behavior. The ability of rats to hang from a wire has been reported to be significantly degraded after exposure to doses as low as 0.5 Gy. In addition, deficits in the ability of acetylcholine to regulate dopamine release in the caudate nucleus (an area in the brain important for motor function) have been found. The concentrations of 3-methoxytyramine (3-MT), a metabolite of dopamine whose concentrations reflect dopamine release in vivo, were measured after rats were exposed to different doses of high-energy iron particles to gain further information about the effect of radiation on the dopaminergic system. Concentrations of 3-MT were significantly reduced 3 days after exposure to 5 Gy but returned to control values by 8 days. After 6 months, concentrations were again less than control values. Exposure to 5 Gy of high-energy electrons or gamma photons had no effect 3 days after exposure. Very high doses of electrons were needed to alter 3-MT concentrations. One hundred grays of electrons decreased 3-MT 30 min after irradiation but levels returned to control values by 60 min. Gamma photons had no effect after doses up to 200 Gy. These results provide further evidence that exposure to heavy particles can degrade motor behavior through an action on dopaminergic mechanisms and that this can occur after doses much lower than those needed for low-LET radiation. 相似文献
88.
Ganglioside Composition of Normal and Mutant Mouse Embryos 总被引:2,自引:0,他引:2
The enrichment of gangliosides in neuronal membranes suggests that they play an important role in CNS development. We recently found a marked tetrasialoganglioside deficiency in twl/twl mutant mouse embryos at embryonic day (E)-11. The recessive twl/twl mutants die at embryonic ages E-9 to E-18 from failed neural differentiation in the ventral portion of the neural tube. In the present study, we examined the composition and distribution of gangliosides in twl/twl mutant mouse embryos at E-12. The total ganglioside sialic acid concentration was significantly lower in the mutants than in normal (+/-) embryos. The mutants also expressed significant deficiencies of gangliosides in the "b" metabolic pathway (GD3, GD1b, GT1b, and GQ1b) and elevations in levels of gangliosides in the "a" metabolic pathway (GM3, GM2, GM1, and GD1a). These findings suggest that the mutants have a partial deficiency in the activity of a specific sialyltransferase in the b pathway. Regional ganglioside distribution was also studied in E-12 normal mouse embryos. The ganglioside composition in heads and bodies was similar to each other and to whole embryos. Total ganglioside concentration and the distribution of b pathway gangliosides were significantly higher in neural tube regions than in nonneural tube regions. These findings suggest that b pathway gangliosides accumulate in differentiating neural cells and that the deficiency of these gangliosides in the twl/twl mutants is closely associated with failed neural differentiation. 相似文献
89.
90.
Summary Microbodies are ubiquitous organelles in fungal cells, occurring in both vegetative hyphae and spores. They are bounded by a single membrane and may contain a crystalloid inclusion with subunits spaced at regular intervals. Typically, they contain catalase which reacts with the cytochemical stain 3,3-diaminobenzidine to yield an electron-opaque product, urate oxidase,l--hydroxy acid oxidase andd-amino acid oxidase. Their fragility and the necessity to disrupt the tough fungal cell wall before isolating them make them difficult to isolate. Analysis of enzymes in purified or partially purified microbodies from fungi indicates that they participate in fatty acid degradation, the glyoxylate cycle, purine metabolism, methanol oxidation, assimilation of nitrogenous compounds, amine metabolism and oxalate synthesis. In organisms where microbodies are known to contain enzymes of the glyoxylate cycle, they are known as glyoxysomes; where they are known to contain peroxidatic activity, they are known as peroxisomes. In some cases microbodies contain enzymes for only a portion of a pathway or cycle. Thus, they must be involved in metabolic cooperation with other organelles, particularly mitochondria. The number, size and shape of microbodies in cells, their buoyant density and their enzyme contents may vary with the composition of the medium; their proliferation in cells is regulated by the growth environment. The isolation from the same organism of microbodies with different buoyant densities and different enzymes suggests strongly that more than one type of microbody can be formed by fungi. 相似文献