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   2555篇
  免费   177篇
  2732篇
  2023年   9篇
  2022年   19篇
  2021年   33篇
  2020年   27篇
  2019年   26篇
  2018年   50篇
  2017年   47篇
  2016年   76篇
  2015年   103篇
  2014年   121篇
  2013年   188篇
  2012年   192篇
  2011年   176篇
  2010年   137篇
  2009年   126篇
  2008年   172篇
  2007年   159篇
  2006年   160篇
  2005年   125篇
  2004年   152篇
  2003年   113篇
  2002年   114篇
  2001年   16篇
  2000年   12篇
  1999年   29篇
  1998年   28篇
  1997年   21篇
  1996年   19篇
  1995年   30篇
  1994年   18篇
  1993年   23篇
  1992年   20篇
  1991年   16篇
  1990年   24篇
  1989年   16篇
  1988年   10篇
  1987年   5篇
  1986年   6篇
  1985年   6篇
  1984年   7篇
  1983年   8篇
  1982年   11篇
  1981年   16篇
  1980年   8篇
  1979年   5篇
  1978年   10篇
  1977年   7篇
  1976年   8篇
  1974年   12篇
  1973年   4篇
排序方式: 共有2732条查询结果,搜索用时 0 毫秒
91.
92.
This paper presents an optimal solution, based on Markov decision theory, for the problem of optimal capacity-related reconfiguration of manufacturing systems, under stochastic market demand. Both capacity expansion and reduction are considered. The solution quantitatively takes into account the effect of the ramp-up phenomenon, following each reconfiguration, on the optimal policy. A closed-form solution is presented for when product demand is independently and generally distributed over time. A real case concerning a flexible manufacturing line in the automotive sector is shown, to prove that ignoring the ramp-up effect in the decision process can lead to significant increases in overall costs.  相似文献   
93.
Malolactic fermentation is a process that is influenced by various factors that can inhibit the growth of the malolactic bacteria. Inhibitory metabolites produced by yeast may have an important role in the correct development of malolactic fermentation. For these reasons, we have investigated the effects of such metabolites on the growth of malolactic bacteria under different environmental conditions, to aid in our understanding of the significance of these interactions in the wine-making environment. Our screening methods to detect interactions between yeast and malolactic bacteria showed a variable and wide diffusion of yeast inhibitory activity on the growth of the malolactic bacteria. However, this first approach to determine this inhibitory activity of yeast gave an overestimation when compared to the results obtained under actual wine-making conditions. The evaluation of malic acid consumption indicated that under inhibitory conditions a partial L-malic acid degradation was seen, indicating that the malolactic activity continued without bacterial growth. However, these yeast-inhibiting effects in addition to other environmental factors could cause a complete failure of malolactic fermentation.  相似文献   
94.
95.
Uterine leiom yomas are benign tumors highly prevalent in reproductive women. In thecurrent study, initially, we aimed to screen five different strawberry cultivars (Alba, Clery, Portola, Tecla, and Romina) to identify efficient cultivars in terms of phytochemical characterization and biological properties by measuring phenolic and anthocyanin content as well as antioxidant capacity, and by measuring apoptotic rate and reactive oxygen species (ROS) production in uterine leiomyoma cells. Next, we focused on the most efficient ones, cultivar Alba (A) and Romina (R) as well as Romina anthocyanin (RA) fraction for their ability to regulate oxidative phosphorylation (oxygen consumption rate [OCR]) glycolysis (extracellular acidification rate [ECAR]), and also fibrosis. Leiomyoma and myometrial cells were treated with a methanolic extract of A and R (250 μg/ml) or with RA (50 μg/ml) for 48 hr to measure OCR and ECAR, as well as gene expression associated with fibrosis. In the leiomyoma cells, RA was more effective in inducing apoptosis and increasing intracellular ROS levels, followed by R and A. In myometrial cells, all strawberry treatments increased the cellular viability and decreased ROS concentrations. Leiomyoma cells showed also a significant decrease in ECAR, especially after RA treatment, while OCR was slightly increased in both myometrial and leiomyoma cells. R and RA treatment significantly decreased collagen 1A1, fibronectin, versican, and activin A messenger RNA expression in leiomyoma cells. In conclusion, this study suggests that Romina, or its anthocyanin fraction, can be developed as a therapeutic and/or preventive agent for uterine leiomyomas, confirming the healthy effects exerted by these fruits and their bioactive compounds.  相似文献   
96.
A new chiral derivatization procedure for the HPLC resolution of chiral catecholamines and structurally related compounds is described. The homochiral reagent, (+)-(R)-1-phenylethyl isocyanate (RPEIC), was added to separate and quantitate the enantiomers of rac-5,6-dihydroxy-2-methyl-aminotetralin, the main metabolite of rac-5,6-diisobutyryl-2-methyl-aminotetralin, a potent dopamine agonist, by reversed-phase HLPC analysis. To avoid catecholamine degradation in the basic reaction medium and to obtain the selective and quantitative derivatization of the amino group of the compound, the reversible complex formation between diphenylborinic acid (DPBA) and the catechol group, in alkaline medium, was performed before homochiral isocyanate addition. The RPEIC derivatization was completed in 30 min and then the DPBA complex was dissociated by adding dilute acid. The structure of intermediates and urea derivatives was confirmed by mass spectrometry. The use of an electrochemical detector, operating in redox mode, allowed HPLC quantitation of enantiomers at the nanogram level in plasma and urine. The derivatization procedure is also suitable for other catecholamine-related compounds. © 1996 Wiley-Liss, Inc.  相似文献   
97.
98.

Background

Nonalcoholic fatty liver disease (NAFLD) and gallstone disease (GD) are both highly prevalent in the general population and associated with obesity and insulin resistance. We aimed to evaluate the prevalence of GD in a cross sectional study of NAFLD patients and to define whether the presence of GD is associated with diabetes and predicts more severe liver disease.

Methodology/Principal Findings

We merged databases of four Liver Units, comprising 524 consecutive biopsy-proven NAFLD (373 males) observed between January 2003 and June 2010. GD was diagnosed in 108 (20%), and 313 cases (60%) were classified by liver biopsy as nonalcoholic steatohepatitis (NASH). The GD subgroup was characterized by a significantly higher prevalence of females, prediabetes/diabetes, abdominal obesity and metabolic syndrome, older age, higher BMI, fasting glucose, HOMA-IR and lower ALT. The prevalence of GD progressively increased with advancing fibrosis and with the severity of necroinflammatory activity (p for trend  = 0.0001 and  = 0.01, respectively), without differences in the severity of steatosis. At multivariate analysis GD was associated with female gender (OR 1.37, 95% CI 1.04–1.8), age (OR 1.027, 95% CI1.003–1.05), fasting glucose (OR 1.21, 95% CI 1.10–1.33) and NASH (OR 1.40,95% CI 1.06–1.89), whereas ALT levels were associated with a lower GD risk (OR 0.98, 95% CI 0.97–0.99). When subjects with cirrhosis were excluded from analysis, the association between GD and fasting glucose, female gender, and NASH was maintained.

Conclusion

Patients with NAFLD have a high prevalence of GD, which characterizes subjects with altered glucose regulation and more advanced liver disease.  相似文献   
99.
PE_PGRS proteins are unique to the Mycobacterium tuberculosis complex and a number of other pathogenic mycobacteria. PE_PGRS30, which is required for the full virulence of M. tuberculosis (Mtb), has three main domains, i.e. an N-terminal PE domain, repetitive PGRS domain and the unique C-terminal domain. To investigate the role of these domains, we expressed a GFP-tagged PE_PGRS30 protein and a series of its functional deletion mutants in different mycobacterial species (Mtb, Mycobacterium bovis BCG and Mycobacterium smegmatis) and analysed protein localization by confocal microscopy. We show that PE_PGRS30 localizes at the mycobacterial cell poles in Mtb and M. bovis BCG but not in M. smegmatis and that the PGRS domain of the protein strongly contributes to protein cellular localization in Mtb. Immunofluorescence studies further showed that the unique C-terminal domain of PE_PGRS30 is not available on the surface, except when the PGRS domain is missing. Immunoblot demonstrated that the PGRS domain is required to maintain the protein strongly associated with the non-soluble cellular fraction. These results suggest that the repetitive GGA-GGN repeats of the PGRS domain contain specific sequences that contribute to protein cellular localization and that polar localization might be a key step in the PE_PGRS30-dependent virulence mechanism.  相似文献   
100.

Introduction

Polymyalgia rheumatica (PMR) is a common inflammatory disease in older people characterized by shoulder and/or pelvic girdle, and cervical and, occasionally, lumbar pain. Interspinous bursitis has been suggested as a potential cause of spinal symptoms. We evaluated, by 18 F-fluorodeoxyglucose (FDG) positron emission tomography integrated with computed tomography (PET/CT), the vertebral structures involved in PMR in a cohort of consecutive, untreated patients.

Methods

Sixty-five consecutive patients with PMR were studied. After a standardized physical examination, which included evaluation of pain and tenderness in the vertebral column, they underwent FDG-PET/CT. Sites of increased uptake and their correlation with spontaneous and provoked pain were recorded. For comparison, FDG-PET/CT was performed also in 65 age- and sex-matched controls and in 10 rheumatoid arthritis (RA) patients.

Results

The most frequent site of spontaneous and provoked pain was the cervical portion. FDG uptake was more frequent in the lumbar portion than at any other location, and in the cervical rather than in the thoracic portion (P <0.0001). No correlation was found between uptake and spontaneous or provoked pain. There was an association between presence of cervical and lumbar bursitis (r = 0.34, P = 0.007). None of the control patients and one out of ten RA patients showed interspinous bursitis.

Conclusions

Interspinous bursitis is a frequent finding in the lumbar spine of patients with PMR. However, it is not associated with clinical symptoms and can hardly explain the spinal pain reported by the patients. Cervical pain is more frequent than lumbar pain in PMR patients and may be caused by shoulder girdle involvement.  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