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   2578篇
  免费   95篇
  国内免费   50篇
电工技术   65篇
综合类   143篇
化学工业   135篇
金属工艺   58篇
机械仪表   62篇
建筑科学   54篇
矿业工程   16篇
能源动力   29篇
轻工业   154篇
水利工程   28篇
石油天然气   9篇
武器工业   4篇
无线电   99篇
一般工业技术   90篇
冶金工业   1633篇
原子能技术   9篇
自动化技术   135篇
  2025年   11篇
  2024年   20篇
  2023年   24篇
  2022年   39篇
  2021年   42篇
  2020年   35篇
  2019年   17篇
  2018年   15篇
  2017年   26篇
  2016年   26篇
  2015年   32篇
  2014年   39篇
  2013年   47篇
  2012年   71篇
  2011年   75篇
  2010年   76篇
  2009年   77篇
  2008年   71篇
  2007年   81篇
  2006年   75篇
  2005年   47篇
  2004年   41篇
  2003年   34篇
  2002年   27篇
  2001年   34篇
  2000年   18篇
  1999年   53篇
  1998年   474篇
  1997年   301篇
  1996年   179篇
  1995年   108篇
  1994年   73篇
  1993年   98篇
  1992年   16篇
  1991年   8篇
  1990年   13篇
  1989年   10篇
  1988年   12篇
  1987年   8篇
  1985年   8篇
  1984年   4篇
  1983年   6篇
  1982年   9篇
  1981年   14篇
  1980年   20篇
  1978年   9篇
  1977年   49篇
  1976年   131篇
  1975年   6篇
  1969年   3篇
排序方式: 共有2723条查询结果,搜索用时 15 毫秒
81.
Among fertile, nonpregnant, Danish women, 33% have absent or reduced iron stores; 22% have serum ferritin values above 70 micrograms/l, i.e., iron reserves of more than 530 mg, corresponding to the net iron losses during a normal pregnancy. During pregnancy, the demands for absorbed iron increase from 0.8 to 7.5 mg/day. Controlled studies show that iron-treated pregnant women have higher serum ferritin levels, i.e., larger iron stores, and higher haemoglobin levels than placebo-treated women. A supplement of 66 mg ferrous iron daily from the beginning of the 2nd trimester prevents iron deficiency anaemia. In Denmark, general iron prophylaxis with 60-70 mg ferrous iron daily from 20 weeks of gestation is recommended by the health authorities.  相似文献   
82.
A stereoselective reversed-phase liquid chromatographic method for the determination of compounds LY300502 and LY300503 (enantiomers of LY191704) in rat and dog plasma was developed. The assay involved extraction of the compounds using a strong cation-exchange solid-phase extraction column, from which the compounds are eluted with 1% of 1 M HCI in methanol. The enantiomers were separated on a Daicel Chiralcel OD-R column. The mobile phase consisted of water-acetonitrile-methanol (50:40:10, v/v) at a flow-rate of 0.3 ml/min. UV detection was achieved at 220 nm. The disposition of the enantiomers of LY191704 in rats and dogs was found to be stereoselective and species specific.  相似文献   
83.
84.
85.
The loss of immunotolerance has been implicated in the pathogenesis of both primary biliary cirrhosis (PBC) and idiopathic, immune-mediated thrombocytopenic purpura (ITP). An association between these two autoimmune diseases has been well described. We describe a 41-year-old woman in whom ITP developed 457 days after liver transplantation for PBC while receiving immunosuppressive medications sufficient to maintain allograft function. Our case report, the first to describe post-transplant ITP in association with PBC, demonstrates the persistence of the underlying immune dysregulation of PBC after transplantation. The practice of decreasing the dosage of immunosuppressive medication to maintenance levels after transplantation may unmask the effects of this defect in immunotolerance.  相似文献   
86.
CBF beta-SMMHC is expressed from the inv(16) chromosome in M4Eo AML. Mice lacking CBF subunits or expressing the CBF beta-SMMHC or AML1-ETO oncoproteins failed to develop definitive hematopoiesis. To investigate these effects on hematopoiesis, we expressed CBF beta-SMMHC from the metallothionein promoter, in both 32D cl3 myeloid cells and Ba/F3 B-lymphoid cells. Addition of zinc increased CBF beta-SMMHC levels more than tenfold, with higher levels evident in Ba/F3 lines. Levels obtained in 32D cl3 cells were similar to those of endogenous CBF beta. Indirect immunofluorescence revealed zinc-inducible speckled, nuclear staining in Ba/F3 cells and diffuse nuclear staining in 32D cl3 cells. CBF beta-SMMHC reduced endogenous CBF DNA-binding fivefold in both cell types, increased cell generation time 1.9-fold, on average, in 32D cl3 cells and 1.5-fold in Ba/ F3 cells and decreased tritiated thymidine incorporation into DNA correspondingly. CBF beta-SMMHC increased the proportion of cells in G1 1.7-fold, on average, in 32D cl3 and Ba/F3 cells, and decreased the proportion of cells in S phase by a similar degree. CBF beta-SMMHC induced a marked increase in hypophosphorylated Rb, but did not alter IL-3 Receptor alpha or beta subunit levels. Neither apoptosis nor 32D differentiation was induced by zinc in IL-3 in these lines. Induction of CBF beta-SMMHC in 32D cl3 cells did not inhibit their differentiation to neutrophils or their expression of myeloperoxidase mRNA in G-CSF, and did not produce an eosinophilic phenotype. Additional, proliferative genetic changes in M4eo AMLs might potentiate inhibition of differentiation by CBF beta-SMMHC by allowing its increased expression.  相似文献   
87.
BACKGROUND: Hospitalization may be an opportune time to change smoking behavior because it requires smokers to abstain from tobacco at the same time that illness can motivate them to quit. A hospital-based intervention may promote smoking cessation after discharge. METHODS: We tested the efficacy of a brief bedside smoking counseling program in a randomized controlled trial at Massachusetts General Hospital, Boston. The 650 adult smokers admitted to the medical and surgical services were randomly assigned to receive usual care or a hospital-based smoking intervention consisting of (1) a 15-minute bedside counseling session, (2) written self-help material, (3) a chart prompt reminding physicians to advise smoking cessation, and (4) up to 3 weekly counseling telephone calls after discharge. Smoking status was assessed 1 and 6 months after hospital discharge by self-report and validated at 6 months by measurement of saliva cotinine levels. RESULTS: One month after discharge, more intervention than control patients were not smoking (28.9% vs 18.9%; P=.003). The effect persisted after multiple logistic regression analyses adjusted for baseline group differences, length of stay, postdischarge smoking treatment, and hospital readmission (adjusted odds ratio, 2.19; 95% confidence interval, 1.34-3.57). At 6 months, the intervention and control groups did not differ in smoking cessation rate by self-report (17.3% vs 14.0%; P=.26) or biochemical validation (8.1% vs 8.7%; P=.72), although the program appeared to be effective among the 167 patients who had not previously tried to quit smoking (15.3% vs 3.7%; P=.01). CONCLUSIONS: A low-intensity, hospital-based smoking cessation program increased smoking cessation rates for 1 month after discharge but did not lead to long-term tobacco abstinence. A longer period of telephone contact after discharge might build on this initial success to produce permanent smoking cessation among hospitalized smokers.  相似文献   
88.
Nuchal screening has the following advantages: Non invasive. First trimester procedure. Results are obtained quickly. 80-90% detection rate for Down's Syndrome.  相似文献   
89.
A system that performs rapid thermal cycling of microliter and smaller liquid volumes inside glass capillary tubes that have an optically transparent thin film of indium-tin oxide (ITO) covering the exterior is described. The ITO film acts as both a heater and a temperature sensor, while cooling is accelerated with forced air. Unlike existing batch-mode thermal cycling systems, this system allows control over each sample's temperature profile. Temperature transition rates of 44 degrees Celsius per second during heating and 15 degrees Celsius per second during cooling have been achieved, allowing successful polymerase chain reaction (PCR) experiments to be performed in 20 min. Capillary external temperature can be regulated typically to within +/- 0.25 degrees Celsius, and peak temperatures more than 800 degrees Celsius have been demonstrated. Capillary internal (sample) temperatures at present are controllable typically to within 2 degrees Celsius. The resistive film can be used as a temperature sensor, and the optical transparency of the thin-film coating could permit fluorescent monitoring of the sample during thermal cycling, making this method well suited for real-time quantitative PCRs.  相似文献   
90.
Human procathepsin L has been expressed in E. coli in the form of inclusion bodies. The recombinant protein was isolated, refolded and processed at pH 5.5 by the addition of dextran sulfate which increased the overall yield of cathepsin L almost 10-fold. After the auto-activation of the 38 kDa procathepsin L at least three processing sites were determined by N-terminal amino acid sequencing. After replacing the Ala205 residue by glutamic acid, cathepsin B-like specificity was introduced into cathepsin L. This mutation resulted in a 15-fold increased activity toward the substrate Z-Arg-Arg-AMC and in a 29-fold decreased activity toward Z-Phe-Arg-AMC. Residue 205 is thereby confirmed experimentally to be critical for the specificity of cathepsins B and L.  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