首页 | 本学科首页   官方微博 | 高级检索  
文章检索
  按 检索   检索词:      
出版年份:   被引次数:   他引次数: 提示:输入*表示无穷大
  收费全文   2215篇
  免费   18篇
电工技术   16篇
综合类   2篇
化学工业   104篇
金属工艺   9篇
机械仪表   12篇
建筑科学   13篇
能源动力   96篇
轻工业   18篇
水利工程   19篇
石油天然气   6篇
无线电   68篇
一般工业技术   116篇
冶金工业   1665篇
原子能技术   12篇
自动化技术   77篇
  2021年   6篇
  2018年   11篇
  2017年   9篇
  2016年   13篇
  2015年   15篇
  2014年   17篇
  2013年   32篇
  2012年   14篇
  2011年   28篇
  2010年   23篇
  2009年   20篇
  2008年   17篇
  2007年   25篇
  2006年   31篇
  2005年   16篇
  2004年   14篇
  2003年   17篇
  2002年   10篇
  2001年   19篇
  2000年   15篇
  1999年   64篇
  1998年   425篇
  1997年   245篇
  1996年   184篇
  1995年   90篇
  1994年   98篇
  1993年   112篇
  1992年   32篇
  1991年   36篇
  1990年   26篇
  1989年   39篇
  1988年   37篇
  1987年   41篇
  1986年   25篇
  1985年   41篇
  1984年   8篇
  1983年   8篇
  1982年   14篇
  1981年   18篇
  1980年   22篇
  1979年   8篇
  1978年   13篇
  1977年   51篇
  1976年   130篇
  1975年   26篇
  1974年   11篇
  1973年   10篇
  1972年   13篇
  1971年   7篇
  1955年   7篇
排序方式: 共有2233条查询结果,搜索用时 15 毫秒
81.
Computer-generated holograms of images reconstructed on curved surfaces   总被引:2,自引:0,他引:2  
Rosen J 《Applied optics》1999,38(29):6136-6140
When one illuminates a computer-generated hologram by a plane wave, the obtained two-dimensional image is usually displayed on a planar plane. Other possibilities for reconstructing images on arbitrary curved surfaces are discussed herein. As an example, the reconstruction of an image on a virtual spherical surface is demonstrated.  相似文献   
82.
A grip-induced muscle tension maintenance task distinguished between schizophrenic patients, regardless of medication or hospitalization status, and both normals and controls with affective (unipolar and bipolar) disorders. Unaffected 1st-degree relatives of schizophrenic patients also showed a grip deficit. Coupling the grip task with a visual discrimination task that divided attention through instruction or reinforcement contingency increased grip error times for all groups. No group differences appeared on the discrimination task, regardless of difficulty level, and the tone used to provide corrective feedback was not implicated in the grip deficit. The results suggest that the grip task is tapping, in a systematic and reliable manner, a motor-control abnormality that may be useful as a behavioral marker of schizophrenia. (PsycINFO Database Record (c) 2010 APA, all rights reserved)  相似文献   
83.
Significant progress has been made in the last 10 years on the identification of histologic parameters that are independent predictors of melanoma prognosis, immunohistochemical markers of cells of melanocytic origin and changes in adhesion molecules, cytoskeletal proteins, growth factor receptors, cell signaling, and nuclear proliferation proteins associated with tumor progression. Histologic criteria may never be completely sufficient to predict behavior accurately, because the fundamental change that renders a cell aggressive may not be morphologically reflected and may require immunohistochemical or other molecular markers to establish behavior. To date, it is humbling that no immunohistochemical or molecular marker provides a greater predictable value for aggressive behavior than does the simple calibrated ocular micrometer to measure tumor thickness. Nevertheless, development of multiple histologic parameters with the concept of nontumorigenic RGP and tumorigenic VGP provides a reliable statistical model to predict metastases. Fortunately, nontumorigenic RGP melanomas with greater than 75% regression are rare. Thus, individual patients with melanoma without regression and without the tumorigenic VGP can be given reasonable assurance of 100% survival. Nevertheless, this assurance is based on a statistical model with a finite population studied. Additional studies are needed to confirm this model, as well as more definitive markers to precisely predict outcome for those individuals with tumorigenic VGP melanoma.  相似文献   
84.
This study was undertaken to clarify several aspects of morphological and taxonomic characters of Physaloptera bispiculata Vaz and Pereira, 1935, a parasite of the water rat, Nectomys squamipes. The cephalic structures (including lips, papillae, teeth, amphids, and porous areas) and details of the posterior end of male and female adult worms were examined by scanning electron microscopy, leading to the addition of new taxonomic characters for this species. We consider P. bispiculata a valid species, based on a comparative analysis of the specific characters for P. bispiculata and P. getula Seurat, 1917, including the morphology and morphometry of body structures as well as number and disposition of caudal papillae of the males.  相似文献   
85.
Identification of enterococci species by hybridization with recently designed species-specific and group-specific 23S rDNA-targeted oligonucleotide probes was superior to results obtained with a common biochemical test panel. Considering these findings, a molecular biological procedure for the detection of enterococci in water samples was developed. A short enrichment is followed by an amplification step and a hybridization reaction in microtiter plate format. The detection limit is about 1 CFU/ml, and results are available within 26 h.  相似文献   
86.
87.
Anoikis is a form of programmed cell death induced in normal epithelial cells by detachment from the extracellular matrix [1] [2] [3]. In epithelial cells of the intestine and other organs, activated rasinduces resistance to anoikis [3] [4], but the actual molecular effectors directly involved in the apoptotic machinery that execute or block anoikis have not yet been identified. Bak, a pro-apoptotic member of the Bcl-2 family, is downregulated in a high proportion of colorectal tumours [5]. In addition, Bak is an important regulator of apoptosis in normal intestinal epithelial cells [6] [7]. Here, we show that activated rasinduces the downregulation of Bak in rat and human intestinal epithelial cells. This ras-induced downregulation of Bak expression could be suppressed by an inhibitor of phosphatidylinositol (PI) 3-kinase, an enzyme already implicated in ras-induced resistance to anoikis [8]. Ectopic expression of Bak in ras-transformed rat intestinal epithelial IEC-18 cells inhibited ras-induced resistance to anoikis and significantly reduced their tumorigenicity. We conclude, therefore, that the ability of rasto downregulate Bak, and the consequent resistance to anoikis, are essential components of the transforming capacity of this oncogene in intestinal epithelial cells.  相似文献   
88.
Missense mutations in the p53 tumor suppressor occur frequently in human breast cancer and influence both the prognosis and response to chemotherapy. Amino acid 175 (equivalent to murine 172) is the second most common site of missense mutations in p53 in human breast cancer. Over 95% of these mutations are arginine-to-histidine (R-H) substitutions resulting in a gain-of-function, and not merely a dominant-negative phenotype. Transgenic mice expressing a p53 172(R-H) construct targeted to the mammary gland by means of a whey acidic protein (WAP) promoter were characterized as a model system in order to determine the specific effects of this mutation on mammary tumorigenesis. Although transgene expression alone had no apparent effect on normal mammary development, transgenic mice treated with the chemical carcinogen dimethylbenz(a)anthracene developed tumors with much shorter latency than did control littermates and had a greater tumor burden. Tumors arising in transgenic mice did not exhibit either decreased apoptosis or increased cell proliferation relative to tumors arising in nontransgenic littermates, but did display increased genomic instability. Large pleiomorphic nuclei were visible in many tumors from transgenic mice, and DNA flow analysis confirmed the presence of significant aneuploid cell populations. Since these transgenic mice develop very few spontaneous tumors, while accelerating carcinogen-and oncogene-mediated tumorigenesis, this mouse model will, therefore, be useful in the investigation of early events in mammary tumorigenesis. It may also be used as a preclinical model to test newly developed chemotherapeutic strategies.  相似文献   
89.
Thin Au-Ni alloy (57.5 at. pct Au and 42.5 at. pct Ni) interlayer brazes between strong base metals were found to have very high ultimate tensile strengths (UTSs) as a result of the triaxial stress state within the braze (reducing the effective, or von Mises, stress) and the high strength of the braze alloy. This high strength can be rationalized by the observed very refined two-phase microstructure. The yield strength and quasi-steady-state flow stress of the braze alloy was found to be relatively strain-rate insensitive. These results explain the absence of time-dependent, or delayed, tensile failures that have been observed in other braze interlayers. The fracture process appears to be one of cavity nucleation by microplasticity, followed by cavity coalescence or impingement once the required nuclei density is achieved. Substantial cavity growth does not appear to occur. This failure process contrasts with that of other interlayer metals, in which the failure mechanism appears to be unstable cavity growth.  相似文献   
90.
Male Wistar rats were dosed with 0, 1250, 3750 or 5000 mg/l of phenylglyoxylic acid (PGA) (CAS no. 611-73-4) in the drinking water ad libitum for 3 months. During the entire treatment period, there were no gross signs of toxicity related to PGA. No changes in neurobehavior were found after using a functional observational battery or radial arm maze. An increased relative kidney weight was seen in the highest dose-group (Controls: 0.504 +/- 0.031 g/100 g b.wt.; 5000 mg PGA/l: 0.579 +/- 0.033 g/100 g b.wt.; p<0.01). No other organ weights were affected. Histopathology revealed no change in kidney structure. No changes in clinical biochemistry. In the highest dose-group three animals out of ten showed reduction in peripheral nerve myelin sheath thickness. No such changes were seen in the control group. The study revealed no changes in auditory brain stem response but minor changes in electroretinography. The noradrenaline (NA) concentration decreased in pons and thalamus whereas it increased in medulla oblongata and whole brain. The dopamine (DA) concentration increased in cerebellum, hippocampus, pons, and whole brain. The most marked DA increase was seen in hippocampus (Controls: 0.56 +/- 0.10 nmol/g tissue; 5000 mg/l: 1.04 +/- 0.11 nmol/g tissue; p<0.001). The 5-hydroxytryptamine (5-HT) concentration decreased in cerebellum, cerebral cortex, hippocampus, and medulla oblongata, whereas it increased in thalamus. The yield of synaptosomal protein, synaptosomal NA, DA, and 5-HT concentrations, and DA uptake rate were not affected. When dosed males were mated with naive females, there were no differences between groups in the pregnancy rate, number of corpora luteae, implantations, live or dead fetuses, resorptions, preimplantation loss, or postimplantation loss. It is concluded that a part of the effects on kidney, peripheral nerves, and vision, which have previously been reported after exposure to styrene, might be induced by the styrene metabolite, PGA. If PGA has ototoxic effects in rats, the dosing in the present study is not sufficient to induce the necessary ototoxic concentration in blood. Alternatively, the ototoxicity of styrene, like toluene, may be caused the parent compound itself and not by a metabolite like PGA.  相似文献   
设为首页 | 免责声明 | 关于勤云 | 加入收藏

Copyright©北京勤云科技发展有限公司  京ICP备09084417号