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   3317篇
  免费   135篇
  国内免费   10篇
电工技术   37篇
综合类   4篇
化学工业   497篇
金属工艺   73篇
机械仪表   105篇
建筑科学   30篇
能源动力   64篇
轻工业   204篇
水利工程   6篇
石油天然气   2篇
无线电   383篇
一般工业技术   405篇
冶金工业   1431篇
原子能技术   25篇
自动化技术   196篇
  2024年   4篇
  2023年   34篇
  2022年   57篇
  2021年   75篇
  2020年   61篇
  2019年   74篇
  2018年   64篇
  2017年   70篇
  2016年   77篇
  2015年   59篇
  2014年   87篇
  2013年   132篇
  2012年   128篇
  2011年   151篇
  2010年   111篇
  2009年   105篇
  2008年   109篇
  2007年   79篇
  2006年   83篇
  2005年   72篇
  2004年   63篇
  2003年   56篇
  2002年   61篇
  2001年   51篇
  2000年   44篇
  1999年   73篇
  1998年   497篇
  1997年   304篇
  1996年   162篇
  1995年   99篇
  1994年   67篇
  1993年   87篇
  1992年   17篇
  1991年   30篇
  1990年   33篇
  1989年   26篇
  1988年   28篇
  1987年   18篇
  1986年   19篇
  1985年   12篇
  1983年   3篇
  1982年   5篇
  1981年   5篇
  1980年   11篇
  1978年   2篇
  1977年   16篇
  1976年   40篇
  1973年   1篇
排序方式: 共有3462条查询结果,搜索用时 15 毫秒
21.
We investigated clinical efficacy of green tea extracts (polyphenon E; poly E and (-)-epigallocatechin-3-gallate [EGCG]) delivered in a form of ointment or capsule in patients with human papilloma virus (HPV) infected cervical lesions. Fifty-one patients with cervical lesions (chronic cervicitis, mild dysplasia, moderate dysplasia and severe dysplasia) were divided into four groups, as compared with 39 untreated patients as a control. Poly E ointment was applied locally to 27 patients twice a week. For oral delivery, a 200 mg of poly E or EGCG capsule was taken orally every day for eight to 12 weeks. In the study, 20 out of 27 patients (74%) under poly E ointment therapy showed a response. Six out of eight patients under poly E ointment plus poly E capsule therapy (75%) showed a response, and three out of six patients (50%) under poly E capsule therapy showed a response. Six out of 10 patients (60%) under EGCG capsule therapy showed a response. Overall, a 69% response rate (35/51) was noted for treatment with green tea extracts, as compared with a 10% response rate (4/39) in untreated controls (P<0.05). Thus, the data collected here demonstrated that green tea extracts in a form of ointment and capsule are effective for treating cervical lesions, suggesting that green tea extracts can be a potential therapy regimen for patients with HPV infected cervical lesions.  相似文献   
22.
NMR structure and mutagenesis of the FADD (Mort1) death-effector domain   总被引:1,自引:0,他引:1  
When activated, membrane-bound receptors for Fas and tumour-necrosis factor initiate programmed cell death by recruiting the death domain of the adaptor protein FADD to the membrane. FADD then activates caspase 8 (also known as FLICE or MACH) through an interaction between the death-effector domains of FADD and caspase 8. This ultimately leads to the apoptotic response. Death-effector domains and homologous protein modules known as caspase-recruitment domains have been found in several proteins and are important regulators of caspase (FLICE) activity and of apoptosis. Here we describe the solution structure of a soluble, biologically active mutant of the FADD death-effector domain. The structure consists of six antiparallel, amphipathic alpha-helices and resembles the overall fold of the death domains of Fas and p75. Despite this structural similarity, mutations that inhibit protein-protein interactions involving the Fas death domain have no effect when introduced into the FADD death-effector domain. Instead, a hydrophobic region of the FADD death-effector domain that is not present in the death domains is vital for binding to FLICE and for apoptotic activity.  相似文献   
23.
The role of the external third of helix VI of the angiotensin II (AII) AT1 receptor for the interaction with its ligand and for the subsequent signal transduction was investigated by individually replacing residues 252-256 by Ala, and residues 259 or 261 by Tyr, and permanently transfecting the resulting mutants to Chinese hamster ovary (CHO) cells. Binding experiments showed no great changes in affinity of any of the mutants for AII, [Sar1]-AII, or [Sar1, Leu8]-AII, but the affinity for the nonpeptide antagonist DuP753 was significantly decreased. The inositol phosphate response to AII was remarkably decreased in mutants V254A, H256A, and F259Y. These results indicate that AT1 residues Val254, His256, and Phe259 are not involved in ligand binding but participate in signal transduction. Based in these results and in others from the literature, it is suggested that, in addition to the His256 imidazole ring, the Phe259 aromatic ring interacts with the AII's Phe8, thus contributing to the signal-triggering mechanism.  相似文献   
24.
25.
Tropical corn silage was compared with sorghum silage as a basal forage in the diets of high producing dairy cows. Sorghum and tropical corn silages were each included in place of ground corn at incremental concentrations in the experimental diets. Eight separate diets were fed, four diets containing each silage ranging in forage neutral detergent fiber (NDF) from approximately 25 to 31% and ranging in total NDF from approximately 41 to 45%. Diets were arranged in a 2 x 4 factorial design and were fed to lactating cows (n = 24; pretrial mean milk production = 39 kg/d; body weight = 656 kg; and days in milk = 81). As concentrations of dietary NDF increased, intake and milk production decreased linearly. The impact of dietary NDF on intake was greater for diets based on tropical corn silage than for diets based on sorghum silage. Energy intake and milk production were reduced, but cows consumed more fiber when challenged with higher dietary concentrations of fiber. The in vitro rate and extent of digestion of dietary samples were correlated with intake response. The rate of in vitro fiber digestion was slower for samples that contained tropical corn silage than for samples that contained sorghum silage. In vivo digestibility measurements were influenced by intake and dietary composition. Results of this trial indicated that sorghum silage can have equal or slightly greater nutritional value than tropical corn silage when these forages are fed at equal concentrations of dietary fiber.  相似文献   
26.
27.
The importance of abnormalities of function and epididymis structure in the etiology of male infertility is still not well understood. We studied 52 individuals distributed in five age groups: fetuses, children, adolescents, adults and the elderly. The region of the body of epididymis was obtained by autopsy and immediately fixed by immersion in a solution of 10% buffered formaldehyde, embedded in paraffin and histologically prepared. The samples were observed under an optic microscope. Test-points were counted in 12 random microscopic fields with the M42 test-system. The following stereological parameters were determined: ductal area, volumetric densities (Vv) of the duct, smooth muscle, connective tissue, epithelial duct and blood vessels. The main results distinguished by those whose averages were statistically significant (p < 0.05), showed that the ductal area is 9.7 times greater in the adolescent/adult/elderly group than the children's group. The Vv of the lumen of the epididymis duct occupies 11.7% of the epididymis body in the fetal period, 5.3% in the child and in individuals after puberty this figures reaches more than 15%. The Vv of smooth muscle occupies 28.3% of the body of the epididymis in the fetus and 35.9% in children, but after puberty this figures stays around 22%. The Vv of the connective tissue occupies 26% in prenatal life, 37% in children, and after puberty these figures range from 21 to 27.5%. Comparing the results of the adult group with that of the elderly group there is an increase in the volumetric density of the connective tissue by 18.1%. In conclusion, the epididymal duct area and the Vv of the ductal lumen, smooth muscle and connective tissue were significant comparing the different groups. However, the quantitative relative differences of the duct's epithelium and the blood vessels were not significant comparing these groups. The study of quantitative aspects of the normal human epididymis can increase our knowledge about male fertility.  相似文献   
28.
Eight quinolones were examined for their bacterial mutagenicity in the Ames Salmonella TA102 assay and for their effects in other bacterial genotoxicity assays. In the quantitative Ames plate incorporation assay, all eight quinolones induced His+ deletion reversion in Salmonella tester strain TA102, with maximum reversion observed at about two to eight times the MIC. The quinolones also induced the SOS response. At quinolone concentrations close to the MIC, SOS cell filamentation gene sulA was induced in sulA::lacZ fusion strain Escherichia coli PQ37. RecA-mediated cleavage of lambda repressor in lambda::lacZ fusion strain E. coli BR513 was measurable at about 10 times the MIC, though no induction occurred at 20 micrograms of nalidixic or oxolinic acid per ml. Genotoxicity of quinolones also was observed in the Bacillus subtilis DNA repair assay, in which the mutant strain M45 (recA) was more susceptible to quinolones than its parent strain, H17 (rec+). The results from these analyses indicate that quinolones induce SOS functions and are mutagenic in bacteria; these properties correspond to their antimicrobial activities.  相似文献   
29.
To determine the mechanism of measles virus-induced cell death, we studied the infection of Vero cells and monocytic cell lines with wild-type (Chicago-1) and vaccine (Edmonston) strains of measles virus. DNA fragmentation indicative of apoptosis was apparent by flow cytometry, agarose gel electrophoresis, and electron microscopy. Within syncytia, DNA strand breaks were demonstrated by end labeling with terminal transferase and then by visualization.  相似文献   
30.
Hilar cholangiocarcinoma, a highly lethal tumor, is difficult to diagnose with conventional computed tomography (CT) because of its small size. Spiral CT allows more effective evaluation of these small lesions and better demonstrates the status of the hepatic arterial or portal venous circulation. Among 27 patients with hilar cholangiocarcinoma (infiltrative in 21, exophytic in two, polypoid in one, diffuse in three), thin-section spiral CT allowed identification of each tumor as an area of focal wall thickening that obliterated the lumen. Seventeen of the infiltrative tumors (81%) showed high attenuation. Identification of the level of biliary obstruction was possible in 63% of the patients (17 of 27). The level of obstruction was underestimated in six patients and overestimated in four. Spiral CT is a valuable method for diagnosis of hilar cholangiocarcinoma; however, spiral CT is less accurate in evaluation of intraductal tumor extent because of the limited z-axis resolution.  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