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   4588篇
  免费   61篇
  国内免费   10篇
电工技术   56篇
综合类   3篇
化学工业   411篇
金属工艺   83篇
机械仪表   167篇
建筑科学   56篇
矿业工程   3篇
能源动力   108篇
轻工业   83篇
水利工程   28篇
石油天然气   8篇
无线电   510篇
一般工业技术   491篇
冶金工业   2111篇
原子能技术   18篇
自动化技术   523篇
  2023年   26篇
  2022年   57篇
  2021年   59篇
  2020年   50篇
  2019年   53篇
  2018年   64篇
  2017年   58篇
  2016年   81篇
  2015年   56篇
  2014年   66篇
  2013年   172篇
  2012年   88篇
  2011年   131篇
  2010年   110篇
  2009年   99篇
  2008年   101篇
  2007年   96篇
  2006年   89篇
  2005年   80篇
  2004年   60篇
  2003年   64篇
  2002年   74篇
  2001年   59篇
  2000年   63篇
  1999年   103篇
  1998年   709篇
  1997年   395篇
  1996年   288篇
  1995年   176篇
  1994年   124篇
  1993年   145篇
  1992年   58篇
  1991年   52篇
  1990年   62篇
  1989年   58篇
  1988年   42篇
  1987年   45篇
  1986年   35篇
  1985年   55篇
  1984年   22篇
  1983年   33篇
  1982年   39篇
  1981年   36篇
  1980年   39篇
  1979年   34篇
  1978年   21篇
  1977年   64篇
  1976年   111篇
  1975年   11篇
  1973年   11篇
排序方式: 共有4659条查询结果,搜索用时 984 毫秒
991.
PROBLEM: We assessed neonatal outcome of normal diamniotic twin pregnancies with known amniotic fluid (AF) volume. METHOD: The AF volume was empirically determined in 39 diamniotic twin gestations. The neonates were stratified by AF volume and evaluated for clinical outcome, gestational age, lecithin-sphingomyelin (L/S) ratio, and birth weight. RESULTS: Neonatal complications did not significantly differ among infants delivered from pregnancies with low (155 to 404 mL), moderate (405 to 807 mL), or high (808 to 5,430 mL) volumes of AF. Relatively constant volumes of AF were maintained throughout the 27 to 38 week range of gestational age, with no apparent correlation between AF volume and gestational age. Variations in the L/S ratio and newborn birth weights were also independent of AF volume. Gestational age was the only significant determinant of the frequency of neonatal complications. CONCLUSION: Amniotic fluid volume does not greatly affect neonatal outcome in normal diamniotic twin pregnancies.  相似文献   
992.
BACKGROUND: The Texas Medication Algorithm Project (TMAP), a public-academic collaborative effort, is a 3-phase project to develop, implement, and evaluate medication treatment algorithms for public sector patients with schizophrenia, major depressive disorders, or bipolar disorders. DISCUSSION: This paper, the first in a series describing the activities of the TMAP, focuses on the various definitions and reasons why guidelines have gained popularity. Also discussed are their strengths, the limitations of the various methods used to develop them, and potential barriers to their implementation.  相似文献   
993.
To investigate a possible association between G-proteins and presenilin-1 (PS-1), a series of glutathione S-transferase-fusion proteins containing portions of PS-1 were prepared and used in vitro in binding experiments with tissue and recombinant G-proteins. The results demonstrate that the 39 C-terminal amino acids of PS-1 selectively bind the brain G-protein, Go. Addition of guanosine 5'-3-O-(thio)triphosphate promoted Go dissociation from PS-1, indicating that this domain mimics the function of G-protein-coupling domains found in receptors. The 39-amino acid synthetic polypeptide activated Go in a magnesium ion-dependent manner. Physical interaction of full-length PS-1 and Go was also demonstrated. Following transfection of Goalpha and N-terminally FLAG-tagged PS-1 in COS-7 cells, Go was immunoprecipitated by FLAG antibodies. In addition, endogenous PS-1 and Goalpha were colocalized immunocytochemically in human glioma cell lines. The results indicate that PS-1 regulates Go activities in living cells.  相似文献   
994.
OBJECTIVES: To determine the age- and race-specific prostate-specific antigen (PSA) distributions in healthy men in central South Carolina and to compare these to data from other studies. METHODS: Two thousand ninety-two black men aged 40 to 69 years and white men aged 50 to 69 years from the general population in 11 counties of central South Carolina participated in a prostate cancer educational program. Seventy-two percent of the participants were black-about double the proportion in the general population-and 63% of the men (1319 of 2092) subsequently obtained a PSA determination from their own physician. The distribution of serum PSA was compared with distributions from the Olmsted County study and from the Walter Reed Army Medical Center/Center for Prostate Disease Research study. RESULTS: Older men without cancer had higher PSA levels. Regression analyses yielded an associated increase of about 3.3% per year. Reference ranges for normal PSA in men without cancer (based on their sample 95th percentiles) were zero to 1.9, 3.8, and 5.7 ng/mL in black men aged 40 to 49, 50 to 59, and 60 to 69 years, and zero to 2.7 and 4.9 mg/mL in white men aged 50 to 59 and 60 to 69 years, respectively. CONCLUSIONS: Reference ranges for normal serum PSA levels should take into account the population from which they are derived and to which they will be applied. Reference ranges that are useful in the general population can differ from those that are appropriate in a hospital setting. For the general population in central South Carolina, reference ranges for serum PSA levels are lower than previously published reference ranges, particularly among black men.  相似文献   
995.
996.
The effects of 1,25-dihydroxyvitamin D3 (1,25(OH)2D3) and four novel synthetic analogues (EB1089, KH1060, KH1230 and CB1093) on IGF-I-stimulated growth of MCF-7 human breast cancer cells have been determined. A significant time- and dose-dependent inhibition of IGF-I-stimulated cell growth was seen with EB1089, such that after 7 days of treatment with 10(-8) M EB1089, the mitogenic effect of IGF-I (30 ng/ml) was negated. Comparison with 1,25(OH)2D3 showed the synthetic analogues to be more potent. The anti-oestrogen ICI 182,780 similarly inhibited IGF-I-stimulated growth of these cells and in combination with EB1089 exerted additional inhibitory effects. Retinoids (all-trans-retinoic acid or the isomer 9-cis-retinoic acid) were less effective in limiting MCF-7 cell responsiveness to IGF-I but, in combination with EB1089, a co-operative effect was achieved. Using radioligand-binding techniques, we observed that 1,25(OH)2D3 and EB1089 down-regulated the levels of 125I-IGF-I binding to MCF-7 cell membranes. Scatchard analysis showed that EB1089 decreased maximal binding approximately 2-fold. Vitamin D derivatives were also demonstrated to reduce IGF-I receptor expression in MCF-7 cells by Western analysis. Our findings demonstrate that vitamin D derivatives limit responsiveness of MCF-7 cells to the mitogenic effects of IGF-I, which may be mediated by reduction of IGF-I receptor expression.  相似文献   
997.
Depth recovery from gray-scale images is an important topic in the field of computer and robot vision. Intensity gradient analysis (IGA) is a robust technique for inferring depth information from a sequence of images acquired by a sensor undergoing translational motion. IGA obviates the need for explicitly solving the correspondence problem and hence is an efficient technique for range estimation. Many applications require real time processing at very high frame rates. The design of special purpose hardware could significantly speed up the computations in IGA. In this paper, we propose two VLSI architectures for high-speed range estimation based on IGA. The architectures fully utilize the principles of pipelining and parallelism in order to obtain high speed and throughput. The designs are conceptually simple and suitable for implementation in VLSI  相似文献   
998.
999.
This article describes an overall video network architecture with primary focus on the ATM subnetwork. The ATM subnetwork provides efficient switching capability for providing constant bit rate and variable bit rate video communication services. The ATM subnetwork can support multiple access networks like hybrid fiber coax (HFC), asymmetric digital subscriber line (ADSL), and fiber to the curb (FTTC). A broadband network controller is presented as the external controller for the ATM subnetwork which performs the functions of the session/network manager and the ATM-based connection management. The initial deployment of video is likely to be permanent virtual connection (PVC)-based, so a dynamic PVC-based scenario is described. The ATM switch architecture presented here has been optimized to support video applications. An evolution to the switched virtual connection environment and support of multiple services over the ATM subnetwork is also addressed. Traffic management schemes are discussed which provide the negotiated quality of service to the connections  相似文献   
1000.
Rattlesnake venoms typically contain several different metalloproteases, some of which are hemorrhagic toxins. Metalloproteases contribute significantly to the often severe necrotic changes in tissues following envenomation, and these prominent components are important to the predigestive role of venoms. Venom of the northern Pacific rattlesnake (Crotalus viridis oreganus) contains at least five distinct metalloproteases, and the dominant protease (trivial name, CVO protease V) has been isolated and characterized as being a single polypeptide chain acidic protein with a molecular mass of 61 kDa and a pH optimum of approximately 9.0. It catalyzes the hydrolysis of several protein substrates, including casein, and is inhibited by metal chelators such as EDTA, EGTA and 1,10-phenanthroline but not by serine protease inhibitors such as PMSF. Calcium is present at a molar ratio of approximately 1:1, but, unlike other described venom metalloproteases, this protease does not appear to contain zinc. Caseinolytic activity is not significantly inhibited by citrate (at pH 9.0) at levels up to 2.0 mM; at 100 mM citrate (at pH 9.0) more than 65% of activity is retained. It is partially inhibited by nanomolar concentrations of ATP, but higher amounts (micromolar) do not result in further inhibition of activity. The protease shows fibrinolytic and fibrinogenolytic activity, but is only weakly hemorrhagic in rats. When stored in solution for long periods it undergoes autolytic degradation. This protease or a homolog appears to be present in venoms from several rattlesnake species but is not present in venoms from juvenile C.v. oreganus. The presence of this component in venoms from adult Pacific rattlesnakes is responsible for the age-related increase in metalloprotease activity of the crude venom.  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