全文获取类型
收费全文 | 4584篇 |
免费 | 709篇 |
国内免费 | 4篇 |
专业分类
5297篇 |
出版年
2021年 | 68篇 |
2020年 | 38篇 |
2019年 | 48篇 |
2018年 | 54篇 |
2017年 | 44篇 |
2016年 | 77篇 |
2015年 | 140篇 |
2014年 | 148篇 |
2013年 | 188篇 |
2012年 | 224篇 |
2011年 | 209篇 |
2010年 | 148篇 |
2009年 | 155篇 |
2008年 | 196篇 |
2007年 | 176篇 |
2006年 | 180篇 |
2005年 | 167篇 |
2004年 | 161篇 |
2003年 | 183篇 |
2002年 | 146篇 |
2001年 | 146篇 |
2000年 | 150篇 |
1999年 | 103篇 |
1998年 | 61篇 |
1997年 | 47篇 |
1996年 | 51篇 |
1995年 | 57篇 |
1994年 | 51篇 |
1993年 | 43篇 |
1992年 | 107篇 |
1991年 | 121篇 |
1990年 | 88篇 |
1989年 | 82篇 |
1988年 | 83篇 |
1987年 | 90篇 |
1986年 | 66篇 |
1985年 | 89篇 |
1984年 | 62篇 |
1983年 | 61篇 |
1982年 | 54篇 |
1981年 | 52篇 |
1980年 | 42篇 |
1979年 | 62篇 |
1978年 | 49篇 |
1977年 | 39篇 |
1974年 | 51篇 |
1972年 | 43篇 |
1971年 | 35篇 |
1970年 | 39篇 |
1969年 | 35篇 |
排序方式: 共有5297条查询结果,搜索用时 31 毫秒
61.
62.
Michael J. Shaw 《Flexible Services and Manufacturing Journal》1989,2(2):121-144
The planning, scheduling, and control of manufacturing systems can all be viewed as problem-solving activities. In flexible manufacturing systems (FMSs), the computer program carrying out these problem-solving activities must additionally be able to handle the shorter lead time, the flexibility of job routing, the multiprocessing environment, the dynamic changing states, and the versatility of machines. This article presents an artificial intelligence (AI) method to perform manufacturing problem solving. Since the method is driven by manufacturing scenarios represented by symbolic patterns, it is referred to as pattern-directed. The method is based on three AI techniques. The first is the pattern-directed inference technique to capture the dynamic nature of FMSs. The second is the nonlinear planning technique to construct schedules and assign resources. The third is the inductive learning method to generate the pattern-directed heuristics. This article focuses on solving the FMS scheduling problem. In addition, this article reports the computation results to evaluate the utility of various heuristic functions, to identify important design parameters, and to analyze the resulting computational performance in using the pattern-directed approach for manufacturing problem-solving tasks such as scheduling. 相似文献
63.
Nucleus-associated microtubules help determine the division plane of plant epidermal cells: avoidance of four-way junctions and the role of cell geometry 总被引:3,自引:0,他引:3 下载免费PDF全文
To investigate the spatial relationship between the nucleus and the cortical division site, epidermal cells were selected in which the separation between these two areas is large. Avoiding enzyme treatment and air drying, Datura stramonium cells were labeled with antitubulin antibodies and the three-dimensional aspect of the cytoskeletons was reconstructed using computer-aided optical sectioning. In vacuolated cells preparing for division, the nucleus migrates into the center of the cell, suspended by transvacuolar strands. These strands are now shown to contain continuous bundles of microtubules which bridge the nucleus to the cortex. These nucleus-radiating microtubules adopt different configurations in cells of different shape. In elongated cells with more or less parallel side walls, oblique strands radiating from the nucleus to the long side walls are presumably unstable, for they are progressively realigned into a transverse disc (the phragmosome) as broad, cortical, preprophase bands (PPBs) become tighter. The phragmosome and the PPB are both known predictors of the division plane and our observations indicate that they align simultaneously in elongated epidermal cells. These observations suggest another hypothesis: that the PPB may contain microtubules polymerized from the nuclear surface. In elongated cells, the majority of the radiating microtubules, therefore, come to anchor the nucleus in the transverse plane, consistent with the observed tendency of such cells to divide perpendicular to the long axis. In nonrectangular isodiametric epidermal cells, which approximate regular hexagons in section, the radial microtubular strands emanating from the nucleus tend to remain associated with the middle of each subtending cell wall. The strands are not reorganized into a single dominant transverse bar, but remain as a starlike array until mitosis. PPBs in these cells are not as tight; they may only be a sparse accumulation of microtubules, even forming along non-diametrical radii. This arrangement is consistent with the irregular division patterns observed in epidermal mosaics of isodiametric D. stramonium cells. The various conformations of the radial strands can be modeled by springs held in two-dimensional hexagonal frames, and by soap bubbles in three-dimensional hexagonal frames, suggesting that the division plane may, by analogy, be selected by minimal path criteria. Such behavior offers a cytoplasmic explanation of long-standing empirically derived "rules" which state that the new cell wall tends to meet the maternal wall at right angles. The radial premitotic strands and their analogues avoid taking the longer path to the vertex of an angle where a cross wall is already present between neighboring cells.(ABSTRACT TRUNCATED AT 400 WORDS) 相似文献
64.
Feces from a specimen of Tamandua tetradactyla (Linn.) from Portel, Pará State, north Brazil, contained two different coccidial oocysts; one identified as Eimeria tamanduae Lainson 1968, and the other as a new species, described here as Eimeria corticulata n. sp. Oocysts of E. corticulata are ellipsoidal, 37.4 x 30.4 (31.2-43.7 x 23.7-35.0) microns, shape index (length/width) 1.2 (1.0-1.5). Oocyst wall 2.5-3.7 microns thick and composed of two layers; an outer thick, brown-yellow one with radial striations, and a thin inner smooth one: no visible micropyle. Oocyst residuum a large globule of about 10.7 x 10.3 microns, usually accompanied by a number of smaller attached globules. Sporocysts ellipsoidal, 21.0 x 11.0 (20.0-22.5 x 10.0-12.5) microns, with a conspicuous Stieda body; shape index 1.9 (1.6-2.2). Sporocyst residuum a small number of scattered granules: sporozoites 18.7 x 5.0 microns, with a large posterior refractile body. Eimeria zygodontomyis n. sp. is described in feces from Zygodontomys lasiurus (Lund) from the Serra dos Carajás, Pará. Oocysts ellipsoidal to cylindrical, 16.5 x 12.0 (13.7-18.7 x 11.2-12.3) microns, shape index 1.4 (1.2-1.5). Wall colorless, smooth, single-layered and about 0.6 micron thick: no micropyle. No oocyst residuum, but a polar granule of about 1.8 x 1.0 microns is sometimes present. Sporocysts ellipsoidal, 8.4 x 5.5 (7.5-8.7 x 5.0-6.2) microns, shape index 1.5 (1.4-1.7), with a thin colorless wall and a delicate Stieda body. Sporozoites enclose a compact residuum of about 2.5 x 3.7 microns. 相似文献
65.
Relationship of the env genes and the endonuclease domain of the pol genes of simian foamy virus type 1 and human foamy virus. 总被引:9,自引:7,他引:2 下载免费PDF全文
We have molecularly cloned and sequenced a portion of the simian foamy virus type 1 (SFV-1); open reading frames representing the endonuclease domain of the polymerase (pol) and the envelope (env) genes were identified by comparison with the human foamy virus (HFV). Unlike the HFV genomic organization, the SFV-1 pol gene overlaps the env gene; thus, the open reading frames reported for HFV between pol and env is not present in SFV-1. Comparisons of predicted amino acid sequences of HFV and SFV-1 reveal that the endonuclease domains of the pol genes are about 84% related. The region predicted to encode the SFV-1 extracellular env domain is 569 codons; SFV-1 and HFV have 64% amino acid similarity in this env domain. The predicted hydrophobic transmembrane env proteins of both HFV and SFV-1 show about 73% similarity. A total of 16 potential glycosylation sites are found in SFV-1 env, and 15 are found in HFV; 11 are shared. SFV-1 has 25 cysteine residues, and HFV has 23 residues; all 23 cysteine residues of HFV are conserved in SFV-1. This sequence analysis reveals that the human and simian foamy viruses are highly related. 相似文献
66.
Net increase of pluripotential hematopoietic precursors in suspension culture in response to IL-1 and IL-3 总被引:8,自引:0,他引:8
A new short-term suspension culture system is described in which pluripotential hematopoietic precursors from mouse bone marrow increase 8 to 12 times in number over a 4-day period. The increase is shown to derive from myeloid precursors undergoing repeated cell divisions prior to definitive lineage restriction. The response, which occurs in the absence of any pre-formed feeder layer, depends on dual stimulation by both IL-1 and IL-3, and the maximum effect depends on the presence of both factors together from the initiation of the cultures. The observations extend the known range of targets of IL-1 and IL-3 to include primitive pluripotential precursors capable of some degree of self-renewal, and provide a specific and relatively simple assay both for the precursors and the soluble factors which regulate them. 相似文献
67.
68.
69.
70.
Modulation of antibody-mediated glomerular injury in vivo by bacterial lipopolysaccharide, tumor necrosis factor, and IL-1 总被引:16,自引:0,他引:16
N I Tomosugi S J Cashman H Hay C D Pusey D J Evans A Shaw A J Rees 《Journal of immunology (Baltimore, Md. : 1950)》1989,142(9):3083-3090
We have investigated the effects of LPS, human rTNF (hrTNF) and human rIL-1 beta (hrIL-1 beta) pretreatment on the intensity of antibody-mediated injury in vivo by using a passive model of anti-glomerular basement membrane (GBM) antibody-mediated nephritis in rats. The experiments show that all three pretreatments exacerbate injury in this model whether judged by albuminuria or the prevalence of glomerular capillary thrombi. The effect on albuminuria was dose dependent with all three treatments. The lowest effective dose of LPS was 0.025 microgram while those for hrTNF and hrIL-1 beta were 0.4 microgram and 0.5 microgram, respectively. All three pretreatments also increased the prevalence of glomerular capillary thrombi which were rare in rats injected with anti-GBM antibodies without pretreatment. LPS pretreatment appeared to be more effective in causing glomerular capillary thrombi than hrTNF or hrIL-1 beta and this was reflected in the correlations between albuminuria and the proportion of glomeruli with capillary thrombi. This relation was linear for all three pretreatments but the slope was appreciably greater for rats pretreated with LPS (0.37) when compared with results from rats given either hrTNF (0.22) or hrIL-1 beta (0.29). Pretreatment of nephritic rats with both cytokines increased the slope to 0.42 demonstrating a synergistic effect. The synergism of hrTNF with hrIL-1 beta was also demonstrated by the effective doses needed to induce albuminuria which was evident in rats treated with 0.05 microgram of IL-1 beta and 0.4 microgram of TNF. Neither the cytokines nor LPS caused clinical, morphologic, or biochemical evidence of renal toxicity when given alone in the dose used here but they did cause a transient increase in the number of neutrophils marginated in glomerular capillaries. Pretreatment of rats with LPS or cytokines increased the glomerular neutrophil influx after anti-GBM antibodies by roughly sixfold. Our experiments show that TNF and IL-1 can increase the severity of glomerular injury in nephritis; they may be important in modulating glomerular injury clinically. 相似文献