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   6178篇
  免费   10篇
  国内免费   1篇
电工技术   4篇
综合类   1篇
化学工业   60篇
金属工艺   7篇
机械仪表   5篇
建筑科学   18篇
矿业工程   4篇
能源动力   11篇
轻工业   50篇
水利工程   1篇
无线电   24篇
一般工业技术   31篇
冶金工业   5935篇
自动化技术   38篇
  2021年   3篇
  2020年   4篇
  2019年   3篇
  2018年   6篇
  2017年   8篇
  2016年   4篇
  2014年   5篇
  2013年   8篇
  2012年   6篇
  2011年   9篇
  2010年   7篇
  2009年   4篇
  2008年   10篇
  2007年   12篇
  2006年   11篇
  2005年   17篇
  2004年   16篇
  2003年   18篇
  2002年   9篇
  2001年   4篇
  2000年   8篇
  1999年   195篇
  1998年   1938篇
  1997年   1115篇
  1996年   716篇
  1995年   391篇
  1994年   284篇
  1993年   366篇
  1992年   48篇
  1991年   46篇
  1990年   56篇
  1989年   52篇
  1988年   44篇
  1987年   44篇
  1986年   40篇
  1985年   49篇
  1984年   3篇
  1983年   10篇
  1982年   21篇
  1981年   40篇
  1980年   54篇
  1979年   5篇
  1978年   15篇
  1977年   137篇
  1976年   304篇
  1975年   14篇
  1974年   6篇
  1973年   4篇
  1972年   3篇
  1955年   4篇
排序方式: 共有6189条查询结果,搜索用时 187 毫秒
941.
BACKGROUND: Ultrasonography (US) by acknowledged experts enhances the diagnostic performance and reduces the rate of negative laparotomies in patients with suspected acute appendicitis (AA). METHODS: The diagnostic accuracy and clinical impact of routine US performed by surgical residents was prospectively studied in 504 unselected patients admitted for AA. Clinical and US findings were correlated with laparotomy findings and pathological outcome in 135 patients (113 cases with proven AA, prevalence 22.4%) and clinical as well as follow-up data were compared in the remainder. RESULTS: The overall accuracy, sensitivity, and specificity of the clinical diagnosis of AA were 84.9%, 51.3%, and 94.6% and those of US were 93. 6%, 83.1%, and 96.6%. Joint evaluation of the results from clinical evaluation and US further improved diagnostic performance (accuracy 93.4%, sensitivity 84.1%, specificity 96.2) and significantly reduced the rate of diagnostic errors to 3.4% (p < 0.001) and unnecessary laparotomies to 9.6% (p < 0.01) in patients with suspected AA. CONCLUSIONS: Ultrasonographic evaluation of the patient with suspected AA is considered to be of value in surgical practice.  相似文献   
942.
Adult varicella can be a severe illness complicated by pneumonia, encephalitis, or prolonged fever. This study measured levels of tumor necrosis factor (TNF)-alpha, interleukin-2 (IL-2), and interferon gamma (IFN-G) in a consecutive group of 31 adult varicella patients presenting within 24 hours of rash onset. All cytokines were assayed using an ELISA technique. TNF-alpha was detectable in 71% of patients with a mean level of 52 pg/ml. IL-2 was detectable in 29% with a mean level of 1040 pg/ml. IFN-gamma was detectable in only 9%. There was no correlation between TNF, IL-2, or IFN-G level and clinical severity as determined by duration and severity of cutaneous findings, duration of fever, frequency of hepatitis, or thrombocytopenia.  相似文献   
943.
944.
945.
An isoform of starch synthase from potato tubers which is present both in the stroma of the plastid and tightly bound to starch granules has been identified biochemically and a cDNA has been isolated. The protein encoded by the cDNA is 79.9 kDa and has a putative transit peptide and a distinct N-terminal domain which is predicted to be highly flexible. It is similar in both amino acid sequence and predicted structure to the granule-bound starch synthase II (GBSSII) of pea embryos. When expressed in Escherichia coli, the mature protein has starch synthase activity. The importance of the isoform has been assessed by biochemical measurements and antisense transformation experiments in which the amount of the isoform in the tuber is severely and specifically reduced. Both approaches indicate that the isoform contributes a maximum of 15% of the total starch synthase activity of the tuber. It is suggested that this isoform and the GBSSII of pea embryos represent a widely distributed class of isoforms of starch synthase. The contribution to total starch synthase activity of members of this class probably varies considerably from one type of storage organ to another.  相似文献   
946.
In order to assess environmentally and occupationally related exposures to PAH compounds it is essential to have reference or normal values in human body fluids. The establishment of reliable reference intervals is an absolute pre-requisite in determining relationships between internal PAH exposure in humans and health effects in occupationally exposed workers. In this context the estimation of the biological level of PAH metabolites in urine from reference populations has become increasingly important in the field of environmental and occupational toxicology. The present study describes the calculation of tentative reference values for urinary 1-hydroxypyrene on the basis of two reference populations and for urinary alpha-naphthol on the basis of one reference population in accordance with IFCC recommendations. The study subjects were 115 healthy male workers occupationally exposed to PAH at low levels and 121 reference subjects non-occupationally exposed to PAH. Tentative reference values for urinary 1-hydroxypyrene were estimated. In addition, 236 healthy male workers were used to estimate tentative reference values for urinary alpha-naphthol. The reference populations were described by distribution free one-sided tolerance intervals. The 95% one-sided tolerance limit calculated for 1-hydroxypyrene in urine was 0.053 mumol/mol creatinine for non-occupationally exposed individuals and 0.169 mumol/mol creatinine for low level PAH exposed workers, with the coverage interval (95 +/- 4.5) percent at a probability of 0.95. Thus, the probability was 0.975 that the tolerance interval included at least 90.5% of the distribution. In addition, the probability was 0.025 that the tolerance interval included > 99.5% of the population. The tolerance interval for alpha-naphthol in urine was 5.665 mumol/mol creatinine with the coverage interval (95 +/- 4.5) percent at a probability of 0.95.  相似文献   
947.
948.
The gene encoding the catalytic subunit of the Toxoplasma gondii DNA polymerase alpha enzyme has been isolated. The coding region is 6487 bp in length, containing three introns, and specifies a protein of 1690 aa. The seven conserved regions which characterize the polalpha polypeptide, as well as four of the five polalpha-specific aa domains, were found in the T. gondii gene. The absence of one of these domains, as well as the presence of a unique cysteine cluster between domains IV and B in the T. gondii polalpha, may result in a slight difference in the secondary or even tertiary structure compared with the human homologue and thus may be suitable for designing anti-Toxoplasma drugs. A number of amino acid differences within the seven conserved regions between the human and T. gondii polalpha, as well as variations in the spacings of these regions, were also observed.  相似文献   
949.
Cell-adhesion molecules (CAMs) are thought to play crucial roles in development and plasticity in the nervous system. This study tested for a role for cell adhesion and in particular, the recognition of two glycosyl epitopes (HNK-1 and oligomannoside) in the activity-driven sharpening of the retinotopic map formed by the regenerating retinal fibers of goldfish. HNK-1 is a prominent glycosyl epitope on many CAMs and extracellular matrix (ECM) molecules, including NCAM, L1, ependymin, and integrins, which have all been implicated in synaptic plasticity. To test for a role of HNK-1 in the sharpening process, we used osmotic minipumps to infuse HNK-1 antibodies for 7-21 days into the tectal ventricle starting at 18 days after optic nerve crush. Retinotopic maps recorded at 76-86 days postcrush showed a lack of sharpening similar to that seen previously with two antibodies to ependymin, an HNK-1-positive ECM component present in cerebrospinal fluid. The multiunit receptive fields at each point averaged 26 degrees versus 11-12 degrees in regenerates infused with control antibodies or Ringer's alone. The HNK-1 epitope also binds to the G2 domain of laminin to mediate neuron-ECM adhesion. To test for a role for laminin, a polyclonal antibody was similarly infused and also prevented sharpening to approximately the same degree. The results support a role for the HNK-1 epitope and laminin in retinotectal sharpening. The oligomannoside epitope (recognized by monoclonal antibody L3) on the CAM L1 interacts with NCAM on the same cell to promote stronger L1 homophilic interactions between cells. Both an L1-like molecule and NCAM are prominently reexpressed in the regenerating retinotectal system of fish. Infusion of oligomannosidic glycopeptides resulted in decreased sharpening, with multiunit receptive fields that averaged 22.7 degrees. Infusions of mannose-poor glycopeptides less prominently disrupted sharpening, with average multiunit receptive fields of 18 degrees. Thus, oligomannosidic glycans in particular may play a role in retinotopic sharpening. Blocking glycan-mediated interactions between CAMs and ECM molecules could decrease the extent of exploratory growth of retinal axon collaterals, preventing them from finding their retinotopic sites, or could interfere with L1 or NCAM and laminin binding at the synaptic densities preventing stabilization of retinotopically appropriate synapses. Together, these results support a prominent role for cell adhesion and glycan epitopes in visual synaptic plasticity.  相似文献   
950.
OBJECTIVE: To compare the pain relief, sedation, and common side effect profiles of ketorolac tromethamine and meperidine for the management of acute pain in the emergency department (ED). METHODS: A prospective, double-blind, randomized clinical trial was conducted over a 12-month period using consecutive adult patients presenting to a university teaching hospital ED (annual census: 32,000), who required IM analgesia for acute pain. Adult patients with acute pain of various etiologies were randomly assigned to receive a single fixed IM dose of ketorolac (60 mg) or meperidine (100 mg). RESULTS: Ninety-three patients were enrolled in the study; 46 were randomized to meperidine and 47 to ketorolac. Using a visual analog scale, there was no difference in pain relief between the ketorolac and meperidine groups even after adjusting for baseline pain level. Ketorolac caused significantly (p < 0.005) less sedation than did meperidine at one hour. Rescue analgesia was required for seven of the 46 (15.2%) patients receiving meperidine and five of the 47 (10.6%) patients receiving ketorolac (p = NS). Seventeen of 45 (38%) patients receiving meperidine experienced side effects compared with eight of the 47 (17%) patients receiving ketorolac (p = 0.0452). CONCLUSIONS: When used to treat patients who had acute pain states, 60 mg of IM ketorolac produced analgesia similar to that produced by 100 mg of IM meperidine; however, the ketorolac produced fewer subjective side effects and less sedation than did the meperidine.  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