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   2871篇
  免费   245篇
  国内免费   8篇
  2021年   42篇
  2020年   23篇
  2019年   31篇
  2018年   42篇
  2017年   23篇
  2016年   53篇
  2015年   125篇
  2014年   132篇
  2013年   151篇
  2012年   196篇
  2011年   206篇
  2010年   120篇
  2009年   107篇
  2008年   148篇
  2007年   139篇
  2006年   158篇
  2005年   152篇
  2004年   144篇
  2003年   110篇
  2002年   81篇
  2001年   84篇
  2000年   86篇
  1999年   69篇
  1998年   33篇
  1997年   30篇
  1996年   23篇
  1995年   24篇
  1994年   22篇
  1993年   19篇
  1992年   58篇
  1991年   37篇
  1990年   38篇
  1989年   44篇
  1988年   29篇
  1987年   29篇
  1986年   31篇
  1985年   36篇
  1984年   20篇
  1983年   31篇
  1982年   13篇
  1981年   13篇
  1980年   9篇
  1979年   21篇
  1978年   11篇
  1977年   18篇
  1976年   12篇
  1975年   11篇
  1974年   10篇
  1972年   21篇
  1971年   16篇
排序方式: 共有3124条查询结果,搜索用时 15 毫秒
991.
On irradiation with UV light the fungicide isoprothiolane (diisopropyl 1,3-dithiolan-2-ylidenemalonate) decomposed rapidly on the silica gel surface. The degradation pathways involved dithiolane ring cleavage, ester hydrolysis, decarboxylation, heterocycles formation such as dithietane and trithiolane, and sulfur liberation. The photoproducts confirmed were oxalic acid, dithiolanylidenemalonic acid, dithiolanylideneacetic acid, 2,4-bis[bis(isopropoxycar-bonyl)methylene]-1, 3-dithietane, 3, 5-bis[bis(isopropoxy-carbonyl)methylene]-1,2,4-trithiolane and sulfur. The methyl and ethyl homologs of isoprothiolane similarly gave the corresponding photoproducts. The surface area where isoprothiolane was placed appeared to be related closely with the photolysis rate. Isoprothiolane decomposed much more rapidly on sand than on a glass plate. This surface effect was greatly depressed under nitrogen atmosphere. Similar phenomena were observed with some other pesticides, with particularly those containing sulfur atoms in the molecule.  相似文献   
992.
In designing HIV protease inhibitors as potential drugs for AIDS therapy, knowledge about what peptide sequences in polyproteins are cleavable by HIV proteases is very useful. In this article, based on the formulation that any octapeptide can be uniquely expressed as a 160-dimensional vector and the principle that the similarity of any two such vectors is associated with their correlation angle, a new method is proposed to predict the cleavability of a peptide sequence by HIV-1 and HIV-2 proteases. The average predicted accuracy the new method for the 105 peptide sequences whose cleavability by HIV-1 protease is known is 96/105=9.14%, which is about 8% higher than that by the existing method for the same set of data. A considerably high rate of correct prediction was also obtained when the new method was used to predict the HIV-2 protease-cleaved sites in some proteins.  相似文献   
993.
994.

Purpose

The purpose of this study was to investigate the association between apnea-hypopnea index (AHI) and metabolic markers and whether the elevated risk of Metabolic Syndrome (MetS) is related to Obstructive Sleep Apnea (OSA).

Methods

This cross-sectional study recruited 246 male bus drivers from one transportation company in Taiwan. Each participant was evaluated by a polysomnography (PSG) test and by blood lipids examination. Severity of OSA was categorized according to the apnea-hypopnea index (AHI).

Results

The results showed that a 73.3% prevalence of MetS in OSA (AHI > 15) and a 80.0% prevalence of MetS in severe OSA (AHI > 30) were found. After adjusting for confounding variables, an increased level of Body-Mass Index (BMI) and two non-MetS cardiovascular risk factors, total cholesterol/HDL-C ratio and TG/HDL-C ratio was significantly associated with AHI in subjects with severe OSA. MetS was about three times to be present in subjects with severe OSA, even adjusted for BMI.

Conclusions

The findings showed a high prevalence of MetS in OSA among professional drivers, especially in the severe group category. BMI was the major contributing factor to OSA. However, the present study did not find a sensitive clinical marker of a detrimental metabolic profile in OSA patients.  相似文献   
995.
Environmentally extended input-output analysis (EEIOA) has long been used to quantify global and regional environmental impacts and to clarify emission transfers. Structural path analysis (SPA), a technique based on EEIOA, is especially useful for measuring significant flows in this environmental-economic system. This paper constructs an imports-adjusted single-region input-output (SRIO) model considering only domestic final use elements, and it uses the SPA technique to highlight crucial routes along the production chain in both final use and sectoral perspectives. The results indicate that future mitigation policies on household consumption should change direct energy use structures in rural areas, cut unreasonable demand for power and chemical products, and focus on urban areas due to their consistently higher magnitudes than rural areas in the structural routes. Impacts originating from government spending should be tackled by managing onsite energy use in 3 major service sectors and promoting cleaner fuels and energy-saving techniques in the transport sector. Policies on investment should concentrate on sectoral interrelationships along the production chain by setting up standards to regulate upstream industries, especially for the services, construction and equipment manufacturing sectors, which have high demand pulling effects. Apart from the similar methods above, mitigating policies in exports should also consider improving embodied technology and quality in manufactured products to achieve sustainable development. Additionally, detailed sectoral results in the coal extraction industry highlight the onsite energy use management in large domestic companies, emphasize energy structure rearrangement, and indicate resources and energy safety issues. Conclusions based on the construction and public administration sectors reveal that future mitigation in secondary and tertiary industries should be combined with upstream emission intensive industries in a systematic viewpoint to achieve sustainable development. Overall, SPA is a useful tool in empirical studies, and it can be used to analyze national environmental impacts and guide future mitigation policies.  相似文献   
996.
Ixodes pacificus ticks can harbor a wide range of human and animal pathogens. To survey the prevalence of tick-borne known and putative pathogens, we tested 982 individual adult and nymphal I. pacificus ticks collected throughout California between 2007 and 2009 using a broad-range PCR and electrospray ionization mass spectrometry (PCR/ESI-MS) assay designed to detect a wide range of tick-borne microorganisms. Overall, 1.4% of the ticks were found to be infected with Borrelia burgdorferi, 2.0% were infected with Borrelia miyamotoi and 0.3% were infected with Anaplasma phagocytophilum. In addition, 3.0% were infected with Babesia odocoilei. About 1.2% of the ticks were co-infected with more than one pathogen or putative pathogen. In addition, we identified a novel Anaplasmataceae species that we characterized by sequencing of its 16S rRNA, groEL, gltA, and rpoB genes. Sequence analysis indicated that this organism is phylogenetically distinct from known Anaplasma species with its closest genetic near neighbors coming from Asia. The prevalence of this novel Anaplasmataceae species was as high as 21% at one site, and it was detected in 4.9% of ticks tested statewide. Based upon this genetic characterization we propose that this organism be called ‘Candidatus Cryptoplasma californiense’. Knowledge of this novel microbe will provide awareness for the community about the breadth of the I. pacificus microbiome, the concept that this bacterium could be more widely spread; and an opportunity to explore whether this bacterium also contributes to human or animal disease burden.  相似文献   
997.
Even though plant cells are highly plastic, plants only develop hyperplasia under very specific abiotic and biotic stresses, such as when exposed to pathogens like Beet curly top virus (BCTV). The C4 protein of BCTV is sufficient to induce hyperplasia and alter Arabidopsis development. It was previously shown that C4 interacts with two Arabidopsis Shaggy-like protein kinases, AtSK21 and 23, which are negative regulators of brassinosteroid (BR) hormone signaling. Here we show that the C4 protein interacts with five additional AtSK family members. Bikinin, a competitive inhibitor of the seven AtSK family members that interact with C4, induced hyperplasia similar to that induced by the C4 protein. The Ser49 residue of C4 was found to be critical for C4 function, since: 1) mutagenesis of Ser49 to Ala abolished the C4-induced phenotype, abolished C4/AtSK interactions, and resulted in a mutant protein that failed to induce changes in the BR signaling pathway; 2) Ser49 is phosphorylated in planta; and 3) plant-encoded AtSKs must be catalytically active to interact with C4. A C4 N-myristoylation site mutant that does not localize to the plasma membrane and does not induce a phenotype, retained the ability to bind AtSKs. Taken together, these results suggest that plasma membrane associated C4 interacts with and co-opts multiple AtSKs to promote its own phosphorylation and activation to subsequently compromise cell cycle control.  相似文献   
998.
Previous studies have rarely examined how temporal dynamic patterns, event-related coherence, and phase-locking are related to each other. This study assessed reaction-time-sorted spectral perturbation and event-related spectral perturbation in order to examine the temporal dynamic patterns in the frontal midline (F), central parietal (CP), and occipital (O) regions during a chemistry working memory task at theta, alpha, and beta frequencies. Furthermore, the functional connectivity between F-CP, CP-O, and F-O were assessed by component event-related coherence (ERCoh) and component phase-locking (PL) at different frequency bands. In addition, this study examined whether the temporal dynamic patterns are consistent with the functional connectivity patterns across different frequencies and time courses. Component ERCoh/PL measured the interactions between different independent components decomposed from the scalp EEG, mixtures of time courses of activities arising from different brain, and artifactual sources. The results indicate that the O and CP regions’ temporal dynamic patterns are similar to each other. Furthermore, pronounced component ERCoh/PL patterns were found to exist between the O and CP regions across each stimulus and probe presentation, in both theta and alpha frequencies. The consistent theta component ERCoh/PL between the F and O regions was found at the first stimulus and after probe presentation. These findings demonstrate that temporal dynamic patterns at different regions are in accordance with the functional connectivity patterns. Such coordinated and robust EEG temporal dynamics and component ERCoh/PL patterns suggest that these brain regions’ neurons work together both to induce similar event-related spectral perturbation and to synchronize or desynchronize simultaneously in order to swiftly accomplish a particular goal. The possible mechanisms for such distinct component phase-locking and coherence patterns were also further discussed.  相似文献   
999.

Background

Clinical outcomes of the drug-coated balloon (DCB) procedure in high-risk patients with femoropopliteal (FP) disease have not been investigated sufficiently.

Methods

This retrospective, single-center study analyzed 87 patients (39% dialysis) and 97 affected legs (64% critical limb ischemia [CLI]) that underwent DCB for symptomatic FP disease from March 2013 to September 2014. Risk stratification was based on FeDCLIP (female, diabetes, dialysis, CLI, lesion length >150 mm and poor runoff) score. The DCB outcomes among the different risk groups were compared and factors predicting restenosis were analyzed during follow-up.

Results

Most of study participants (84%) were moderate to high-risk patients. The procedural success rate was 100% and the 30-day major adverse vascular event rate was 2.1%. The mean lesion length was 178 ± 106 mm and the mean follow-up time was 428 ± 145 (range 50–782) days. The binary restenosis-free and clinically driven target lesion revascularization (CD-TLR)-free rates at 12 months were 77.5% and 84.3%, respectively, for all participants. No significant differences were observed in 1-year binary restenosis and CD-TLR rates in the low-, moderate-, and high-risk groups (60%, 84%, and 73%: p = 0.396; 78%, 89%, and 80%: p = 0.635, respectively). In multivariate analysis, lesion length >150 mm (Hazard ratio [HR]: 8.00, 95% confidence interval (CI) 1.12 to 55.6, p = 0.038) and Rutherford class 6 (HR: 7.09, 95% CI, 1.15 to 43.5, p = 0.034) were identified as independent predictors of binary restenosis.

Conclusions

Despite general comorbidities and advanced limb ischemia, 1-year outcomes of DCB in high-risk patients with FP disease were effective. The DCB procedure holds promise to improve vessel patency; however, lesion length >150 mm and major tissue loss were independent predictors for binary restenosis after the treatment.  相似文献   
1000.

Background

The risk of periodontitis (PD) is increased in the patient group of rheumatoid arthritis (RA). RA and PD also shared some pathological mechanism. The aim of this study is to investigate the risk of RA associated with PD exposure.

Methods and Findings

This study identified 3 mutually exclusive cohorts using the 1999–2010 Taiwanese National Health Insurance Research Database (NHIRD) to investigate the association between PD and the risk of incident RA. All patients with PD in 2000 were identified from the database of all enrollees as the PD cohort. From the representative database of 1,000,000 enrollees randomly selected in 2010 (LHID2010), individuals without any periodontal disease (PO) during 1999–2010 were selected as the non-PO cohort. Individuals who were not included in the non-PO cohort and received dental scaling (DS) no more than two times per year during 1999–2010 were selected as the DS cohort from LHID2010. Using cox proportional regression analysis, hazard ratios (HRs) with 95% confidence intervals (Cis) were calculated to quantify the association between PD exposure and RA development. In the three-group comparison using the non-PO cohort as reference, we found that the risk of RA was higher in the PD and DS cohorts (HRs, 1.89 and 1.43; 95% CIs, 1.56–2.29 and 1.09–1.87, respectively). For comparisons between two cohorts, the PD cohort had a higher risk of RA than the non-PO and DS cohorts (HRs, 1.91 and 1.35; 95% CIs, 1.57–2.30 and 1.09–1.67, respectively).

Conclusion

PD was associated with an increased risk of RA development.  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