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   313895篇
  免费   3881篇
  国内免费   734篇
电工技术   6254篇
综合类   567篇
化学工业   47591篇
金属工艺   10458篇
机械仪表   8917篇
建筑科学   7942篇
矿业工程   877篇
能源动力   8841篇
轻工业   31268篇
水利工程   2562篇
石油天然气   3687篇
武器工业   8篇
无线电   40233篇
一般工业技术   59340篇
冶金工业   57240篇
原子能技术   5483篇
自动化技术   27242篇
  2021年   2439篇
  2019年   2254篇
  2018年   3763篇
  2017年   3590篇
  2016年   3704篇
  2015年   2634篇
  2014年   4591篇
  2013年   14607篇
  2012年   7546篇
  2011年   10301篇
  2010年   8067篇
  2009年   9358篇
  2008年   9739篇
  2007年   9679篇
  2006年   8604篇
  2005年   7980篇
  2004年   7840篇
  2003年   7576篇
  2002年   7408篇
  2001年   7596篇
  2000年   7255篇
  1999年   7602篇
  1998年   17781篇
  1997年   12925篇
  1996年   10193篇
  1995年   7939篇
  1994年   7268篇
  1993年   6970篇
  1992年   5326篇
  1991年   5120篇
  1990年   4986篇
  1989年   4810篇
  1988年   4707篇
  1987年   3927篇
  1986年   4043篇
  1985年   4756篇
  1984年   4300篇
  1983年   4038篇
  1982年   3623篇
  1981年   3788篇
  1980年   3474篇
  1979年   3436篇
  1978年   3249篇
  1977年   3835篇
  1976年   4874篇
  1975年   2793篇
  1974年   2669篇
  1973年   2687篇
  1972年   2219篇
  1971年   1968篇
排序方式: 共有10000条查询结果,搜索用时 10 毫秒
991.
992.
Examined the impact of increasing the percentage of hires (above the T. A. Cleary [1968] fair-hiring level) from minority groups with demonstrated lower average job performance. Increased minority hiring resulted in only a small performance loss, even when minority hiring exceeded the minority applicant representation. However, when minorities were hired at a rate equal to or greater than their applicant representation, the expected performance loss among the hired minorities was much greater than performance loss across all hires. More important, the discrepancy in performance between majority and minority hires increased as minority hiring increased. With minority hiring above the level of minority applicant representation, this discrepancy exceeded the population difference in performance between the 2 groups. These findings suggest additional considerations when raising minority hiring above minority applicant representation. (PsycINFO Database Record (c) 2010 APA, all rights reserved)  相似文献   
993.
Arylalkynes such as 4-phenyl-1-butyne (PBY), 5-phenyl-1-pentyne (PPY) and 2-ethynylnaphthalene (2-EN) are suicide inhibitors of cytochrome P450 enzymes. Arylalkyl isothiocyanates such as 6-phenylhexyl isothiocyanate (PHITC) are structurally related to arylalkynes and are known to inhibit the cytochrome P450 mediated metabolic activation and tumorigenicity of a tobacco-specific lung carcinogen, 4-(methylnitrosamino)-1-(3-pyridyl)-1-butanone (NNK). In this study, we compared the ability of PBY, PPY, 2-EN and PHITC to inhibit A/J mouse lung tumorigenesis by NNK. Groups of 20 female mice were gavaged with 5 mumol of arylalkyne or PHITC in corn oil. Two hours later they were given a single i.p. injection of 10 mumol NNK. The mice were killed 16 weeks later. PPY and PHITC were both potent inhibitors of tumorigenesis by NNK, reducing lung tumor multiplicity from 8.35 tumors per mouse to 0.40 and 0.35 respectively. PBY and 2-EN also significantly inhibited tumor multiplicity. The results of this study demonstrate that arylalkynes and PHITC are potent inhibitors of NNK induced lung tumorigenesis in A/J mice, consistent with the hypothesis that inhibition of specific cytochrome P450 enzymes is involved in inhibition of tumorigenesis.  相似文献   
994.
995.
The pattern of hepatitis C virus (HCV) viremia in blood donors who are positive for antibody to HCV (anti-HCV) according to the level of transaminase activity is unclear. A polymerase chain reaction-based HCV RNA detection method was used to study two clearly defined groups of anti-HCV-positive blood donors with repeatedly normal (n = 27) and elevated (n = 17) alanine aminotransferase (ALT) levels. HCV RNA was detected in only 4 of 27 blood donors with normal ALT values and 15 of 17 with elevated ALT values. These results indicate that anti-HCV-positive blood donors with normal ALT levels constitute a heterogeneous group, as HCV viremia is detectable in only a small proportion of cases. Polymerase chain reaction should be useful in the surveillance of anti-HCV-positive blood donors with normal ALT levels, by identifying those who might benefit from further investigation and treatment.  相似文献   
996.
The use of growth hormone (GH) as an anabolic agent is limited by its tendency to cause hyperglycemia and by its inability to reverse nitrogen wasting in some catabolic conditions. In a previous study comparing the anabolic actions of GH and IGF-I (insulin-like growth factor I), we observed that intravenous infusions of IGF-I (12 micrograms/kg ideal body wt [IBW]/h) attenuated nitrogen wasting to a degree comparable to GH given subcutaneously at a standard dose of 0.05 mg/kg IBW per d. IGF-I, however, had a tendency to cause hypoglycemia. In the present study, we treated seven calorically restricted (20 kcal/kg IBW per d) normal volunteers with a combination of GH and IGF-I (using the same doses as in the previous study) and compared its effects on anabolism and carbohydrate metabolism to treatment with IGF-I alone. The GH/IGF-I combination caused significantly greater nitrogen retention (262 +/- 43 mmol/d, mean +/- SD) compared to IGF-I alone (108 +/- 29 mmol/d; P < 0.001). GH/IGF-I treatment resulted in substantial urinary potassium conservation (34 +/- 3 mmol/d, mean +/- SE; P < 0.001), suggesting that most protein accretion occurred in muscle and connective tissue. GH attenuated the hypoglycemia induced by IGF-I as indicated by fewer hypoglycemic episodes and higher capillary blood glucose concentrations on GH/IGF-I (4.3 +/- 1.0 mmol/liter, mean +/- SD) compared to IGF-I alone (3.8 +/- 0.8 mmol/liter; P < 0.001). IGF-I caused a marked decline in C-peptide (1,165 +/- 341 pmol/liter; mean +/- SD) compared to the GH/IGF-I combination (2,280 +/- 612 pmol/liter; P < 0.001), suggesting maintenance of normal carbohydrate metabolism with the latter regimen. GH/IGF-I produced higher serum IGF-I concentrations (1,854 +/- 708 micrograms/liter; mean +/- SD) compared to IGF-I only treatment (1,092 +/- 503 micrograms/liter; P < 0.001). This observation was associated with increased concentrations of IGF binding protein 3 and acid-labile subunit on GH/IGF-I treatment and decreased concentrations on IGF-I alone. These results suggest that the combination of GH and IGF-I treatment is substantially more anabolic than either IGF-I or GH alone. GH/IGF-I treatment also attenuates the hypoglycemia caused by IGF-I alone. GH/IGF-I treatment could have important applications in diseases associated with catabolism.  相似文献   
997.
Recent data have suggested the existence of at least two major classes of calcitonin gene-related peptide (CGRP) receptors in brain and peripheral tissues [Henke et al., Brain Res., 410 (1987) 404-408; Dennis et al., J. Pharmacol. Exp. Ther., 251 (1989) 718-725; ibid, 254 (1990) 123-128; Quirion et al., Ann. NY Acad. Sci., 657 (1992) 88-105]. However, little is currently known in the structure characteristics of CGRP receptors as cloning as yet to be reported. In the present study, the sensitivity of [125I]humanCGRP alpha binding to guanine nucleotides and temperature was investigated in guinea pig atria (prototypical CGRP1 tissue) guinea pig vas deferens (prototypical CGRP2 tissue) and in the rat brain and cerebellum (mixed assay). Binding isotherms of [125I]hCGRP alpha in those four tissue preparations were curvilinear and best fitted to a two-site model under most assay conditions. The high affinity binding component was highly temperature-sensitive and accounted, under experimental conditions, for up to 18% of the total population of receptors. Moreover, these high affinity sites were also highly sensitive to guanine nucleotides (Gpp(NH)p, 100 microM) in all preparations although to a different extend depending upon assay temperatures. Taken together, this suggests that the different CGRP receptor subtypes present in these tissue all belong to a G-protein coupled receptor family.  相似文献   
998.
Contents The control-canonical-form (CCF) method is introduced for the purpose of applying it to high-order state-space single-input single or multiple-output (SISO or SIMO) and multiple-input multiple-output (MIMO) linear time-invariant system models to determine closed-loop systems (i.e. design robust controllers) from which enchanced dynamic stability characteristics and overall performance of these systems is achieved. The method has been applied successfully to a SISO 6th-order linearized state-space open-loop model (representing a simplified hydro power system), and to a MIMO 8th-order linearized state-space open-loop model (representing an 87.5 kVA synchronous machine with conventional exciter supplying power to an electric utility system through an interconnection network)
Anwendung der kanonischen Form der Zustandsrückführung auf Erzeugersysteme zur Verbesserung der dynamischen Stabilitätseigenschaften
Übersicht Die Methode der kanonischen Form wird auf lineare zeitinvariante Systeme höherer Ordnung im Zustandsraum angewendet, die eine oder mehrere Eingangsgrößen bei einer oder mehreren Ausgangsgrößen aufweisen. Damit werden robuste Regler entworfen, welche die dynamische Stabilität und das gesamte Betriebsverhalten verbessern. Die Methode wurde erfolgreich auf ein linearisiertes Modell sechster Ordnung mit einem Eingang und einem Ausgang angewendet, das vereinfachend eine Wasserkraftanlage darstellt; außerdem auf ein linearisiertes Zustandsmodell achter Ordnung einer Synchronmaschine 87,5kVA mit konventioneller Erregung, die über ein Verbindungsnetzwerk in das öffentliche Netz einspeist.
  相似文献   
999.
Systems in which semi-autonomous problem-solving agents communicate and cooperate with one another represent an exciting vision of future computing environments. However, if this vision is ever going to result in commercially viable systems, then consideration must be given to the large software base that exists within many organisations. Success requires the ability to incorporate pre-existing systems alongside purpose-built agents in a cooperating community. This requirement is vital because the former represent a substantial resource investment that companies cannot afford to consign to the scrap heap. We report on our experiences of constructing cooperating communities that contain elements that were pre-existing and some that were developed specifically for incorporation into an integrated environment. The general purpose framework of ARCHON (ARchitecture for Cooperative Heterogeneous ON-line systems) provides the underlying technology to facilitate cooperative problem solving, and the exemplar domain is the real world problem of electricity distribution management. The actual application being developed is called CIDIM (Cooperating Intelligent systems for DIstribution system Management). An evolving methodology for designing and developing a mixed system such as this is outlined, based on our experiences in CIDIM and several other real-world industrial applications. It specifies a hybrid top-down and bottom-up approach to integration, identifies the important characteristics that shape multi-agent problem analysis, and outlines key factors that impinge upon the design of the community. this methodology is then used to motivate the design decisions for the CIDIM application. Finally the process of instantiating the individual agents is discussed, some helpful guidelines on testing and evaluating future applications are given, and the implementation of one of CIDIM's cooperative scenarios is described in depth.  相似文献   
1000.
Reports an error in the original article by P. M. Lewinsohn et al ( Journal of Abnormal Psychology, 1993[Feb], Vol 102[1], 133–244). On page 140, in the Total Incidence columns in Table 4, the data for the Attention Deficit row should be switched with that for the Conduct row. (The following abstract of this article originally appeared in record 1993-25780-001.) Data were collected on the point and lifetime prevalences, 1-yr incidence, and comorbidity of depression with other Diagnostic and Statistical Manual of Mental Disorders-III-Revised (DSM-III-R) disorders in a randomly selected sample (n?=?1,710) of high school students at point of entry and at 1-yr follow-up (n?=?1,508). The Schedule for Affective Disorders and Schizophrenia for School-Age Children was used to collect diagnostic information; 9.6% met criteria for a current disorder, more than 33% had experienced a disorder over their lifetimes, and 31.7% of the latter had experienced a 2nd disorder. High relapse rates were found for all disorders, especially for unipolar depression (18.4%) and substance use (15.0%). Female Ss had significantly higher rates at all age levels for unipolar depression, anxiety disorders, eating disorders, and adjustment disorders; male Ss had higher rates of disruptive behavior disorders. (PsycINFO Database Record (c) 2010 APA, all rights reserved)  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