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   241050篇
  免费   2960篇
  国内免费   425篇
电工技术   4287篇
综合类   257篇
化学工业   39231篇
金属工艺   9378篇
机械仪表   7396篇
建筑科学   5450篇
矿业工程   1390篇
能源动力   5174篇
轻工业   24898篇
水利工程   2473篇
石油天然气   5094篇
武器工业   32篇
无线电   25066篇
一般工业技术   44832篇
冶金工业   45393篇
原子能技术   5950篇
自动化技术   18134篇
  2021年   2110篇
  2019年   2024篇
  2018年   3497篇
  2017年   3430篇
  2016年   3705篇
  2015年   2246篇
  2014年   3907篇
  2013年   10199篇
  2012年   6169篇
  2011年   8322篇
  2010年   6561篇
  2009年   7257篇
  2008年   7604篇
  2007年   7725篇
  2006年   6900篇
  2005年   6052篇
  2004年   5824篇
  2003年   5609篇
  2002年   5678篇
  2001年   5561篇
  2000年   5388篇
  1999年   5310篇
  1998年   12591篇
  1997年   8912篇
  1996年   6865篇
  1995年   5337篇
  1994年   4741篇
  1993年   4634篇
  1992年   3667篇
  1991年   3388篇
  1990年   3595篇
  1989年   3472篇
  1988年   3304篇
  1987年   2895篇
  1986年   2933篇
  1985年   3373篇
  1984年   3219篇
  1983年   2892篇
  1982年   2738篇
  1981年   2821篇
  1980年   2719篇
  1979年   2624篇
  1978年   2684篇
  1977年   2950篇
  1976年   3805篇
  1975年   2368篇
  1974年   2243篇
  1973年   2376篇
  1972年   1970篇
  1971年   1858篇
排序方式: 共有10000条查询结果,搜索用时 312 毫秒
891.
We report a case of cervical epidural hematoma associated with anticoagulant therapy in a 68 year-old man, who presented with tetraplegia. He was operated 12 hours after the onset. Three months later, he had recovered almost completely. The etiologies and prognosis of such lesions are reviewed.  相似文献   
892.
In the present study the neurotoxic effects of a low dosage (0.5 mg/kg per day) of methylmercury (MeHg) on the developing nervous system were investigated. Pregnant rats were treated with MeHg from day 7 of pregnancy to day 7 of lactation. Locomotor activity (locomotion, rearing, and motility) and spatial learning ability were tested in the offspring at 6 months of age. The expression of tyrosine hydroxylase (TH) was examined by immunohistochemistry and in situ hybridization. A significant decrease in spontaneous motility and rearing was observed only in the MeHg-treated male rats. After administration of a low dose of d-amphetamine (0.5 mg/kg) no differences could be observed between control and MeHg-treated male rats, suggesting that changes in dopaminergic transmission were involved. However, no change in TH messenger RNA expression was observed. No changes in spatial learning acquisition or memory were shown in MeHg-treated rats. Taken together, these findings show that during development a very low dosage of MeHg exerts neurotoxic effects detectable in adulthood, and that susceptibility is gender-dependent.  相似文献   
893.
894.
The effects of the competitive antagonist of the N-methyl-D-aspartate (NMDA) receptor, LY235959, were determined on the analgesic and hypothermic effects as well as on the development of tolerance to these effects of U-50,488H, a kappa-opioid receptor agonist in mice and rats. In the mouse, a single injection of LY235959 given 10 min prior to U-50,488H did not modify the analgesic action of the latter. Similarly, chronic administration of LY235959 twice a day for 4 days did not modify U-50,488H-induced analgesia in mice. Repeated pretreatment of mice with LY235959 dose-dependently attenuated the development of tolerance to the analgesic actions of U-50,488H. In the rat, LY235959 by itself produced a significant analgesia and prior treatment of rats with LY235959 enhanced the analgesic action of U-50,488H. Similar effects were seen with the hypothermic action. Pretreatment of rats with LY235959 attenuated the development of tolerance to the analgesic but not to the hypothermic action of U-50,488H. These results provide evidence that LY235959 produces differential actions on nociception and thermic responses by itself and when given acutely with U-50,488H in mice and rats. However, when the animals are pretreated with LY235959, similar inhibitory effects are observed on the development of tolerance to the analgesic action of U-50,488H in both the species. These studies demonstrate an involvement of the NMDA receptor in the development of kappa-opioid tolerance and suggest that the biochemical consequences of an opioid's interaction with the opioid receptor are not the only factors that contribute to the acute and chronic actions of opioid analgesic drugs.  相似文献   
895.
BACKGROUND: Two recent much cited publications have raised the concern that risk associated with cigarette smoking has so far been underestimated. In this study we wish to determine whether excess all-cause mortality associated with smoking has increased during the last 20-30 years in a study population representative of the general Danish population and whether any such changes relate to changes in smoking behaviour. METHODS: Pooled data from three prospective population studies conducted in Copenhagen with detailed information on smoking habits. A total of 31,194 subjects, 17,669 males and 13,525 females, initially examined between 1964 and 1992 with examinations repeated at intervals from 1-10 years, were followed until 1995 for all-cause mortality. Relative mortality risk in smokers versus never-smokers was calculated within periods of five calendar years and compared throughout the study period. RESULTS: Male smokers' exposure did not change during the study period whereas female smokers' exposure to tobacco increased in terms of age at smoking onset, quantity smoked and depth of inhalation. During follow-up 5744 males and 2900 females died. In males, death rate ratios (comparing continuous smokers with never-smokers) did not change in the study period. In females, ratios increased from 1964-1978 to 1979-1994 by a factor of 1.3 (95% confidence interval 1.0-1.8). CONCLUSIONS: In agreement with the observed changes in smoking habits, excess mortality in male smokers did not increase whereas excess mortality in female smokers increased slightly.  相似文献   
896.
In this report we reviewed the role of Ultrasonography, Computed Tomography, Magnetic Resonance and 99Tc-Sestamibi Scintigraphy for the detection of abnormal parathyroid glands in patients with biochemical evidence of hyperparathyroidism. We also report our personal experience with CT and RM.  相似文献   
897.
Algorithm for cosine transform of Toeplitz matrices   总被引:1,自引:0,他引:1  
An algorithm for calculating the 2D cosine transform of a Toeplitz matrix is presented. The algorithm is based on the application of 1D cosine transforms. More specifically, four 1D cosine transforms of size N are needed to obtain the transform of a Toeplitz matrix of size N×N. This is an improvement over previously published algorithms. The algorithm is also simple and regular  相似文献   
898.
The pathological basis of nerve inexcitability in Guillain-Barré syndrome has not been established with certainty. We report the clinicopathological findings in a 67-year-old patient with fulminant Guillain-Barré syndrome who died 18 days after onset. Three serial electrophysiological studies revealed nerve inexcitability. Antibodies to Campylobacter jejuni were present but there was no antiganglioside reactivity. Spinal root sections revealed extensive and almost pure macrophage-associated demyelination with occasional presence of T lymphocytes and neutrophil leukocytes. Conversely, in femoral, median, and sural nerves the outstanding lesion was axonal degeneration, with some denuded axons remaining. Unmyelinated fibers, posterior root ganglia, and dorsal columns were preserved. Endoneurial postcapillary venules showed plump endothelial cells with loss of their tight junctions. We conclude that both primary demyelination and axonal degeneration secondary to inflammation account for nerve inexcitability. Our findings lend support to the hypothesis of increased endoneurial pressure as the cause of wallerian degeneration in nerve trunks.  相似文献   
899.
The purpose of this study was to determine the long-term results of allogeneic bone marrow transplantation for chronic myeloid leukemia. A retrospective analysis was carried out of the outcome of 373 consecutive transplants performed at 38 European institutions between 1980 and 1988 and reported to the registry of the European Group for Blood and Marrow Transplantation. All transplants were carried out for first chronic phase of chronic myelogenous leukemia using unmanipulated marow cells from HLA-identical sibling donors. The probability of survival and leukemia-free survival at 8 years were 54% (95% CI: 49-59) and 47% (95% CI: 41-52) respectively. The probabilities of developing acute GVHD (II-IV) at 100 days and chronic GVHD at 4 years after transplant were 47% (95% CI: 41-53) and 52% (95% CI: 46-58) respectively. The probabilities of transplant-related mortality and leukemic relapse 8 years after BMT were 41% (95% CI: 36-48) and 19% (95% CI: 14-25), respectively. Transplant within 12 months of diagnosis was associated with reduced transplant-related mortality (34 vs 45%, P = 0.013) and resulted in improved leukemia-free survival (52 vs 44%, P = 0.03). The probability of relapse was significantly reduced in patients who developed chronic GVHD (RR = 0.33, P = 0.004). The probability of relapse occurring more than 2 years after transplant was increased more than five-fold in patients transplanted from a male donor (RR = 5.5, P = 0.006). Sixty-seven patients in hematologic remission were studied for residual disease by two-step RT/PCR for BCR-ABL mRNA and 61 (91%) tested negative. We conclude that bone marrow transplantation can induce long-term survival in approximately one-half of CML patients; the majority of survivors have no evidence of residual leukemia cells when studied by molecular techniques. The probability of late relapse is increased with use of a male donor.  相似文献   
900.
A comparison of four different commercial immunometric thyrotropin (TSH) assays (Amerlite R TSH-30 Ultrasensitive assay from Kodak, BeriLux R hTSH from Behring Werke, Delfia R hTSH Ultra from Wallac and IMX R Ultrasensitive hTSH from Abbott) was made by measuring serum TSH in 81 consecutive patients referred to hospital for various reasons with a serum TSH less than 0.8 mlU/l in the IMX assay. The analytical and functional assay sensitivities of each of the assays were analysed. Even though three of the methods had a sensitivity corresponding to third generation assays, we could only demonstrate an overall agreement of serum TSH when comparing two of the kits. The measurements in Delfia Ultra and Berilux showed good agreement (P = 0.7, paired t-test and bias = 0.003 mIU/l), while the comparisons between the other assays showed different measurements (P < 0.00001, paired t-test and bias more than 0.07 mIU/l). Differences in the calibrators used in the assays might explain some of the discrepancy, although all methods were calibrated according to the same international standard. Also, differences in the specificity of the TSH monoclonal antibodies used in the assays might be an evident explanation and further studies of the specificity of the monoclonal antibodies are needed. An international collaborative study to clarify reasons for the differences between the TSH assays and to standardize the measurements is recommended.  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