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   5031篇
  免费   317篇
  国内免费   19篇
  2024年   6篇
  2023年   16篇
  2022年   69篇
  2021年   86篇
  2020年   55篇
  2019年   72篇
  2018年   100篇
  2017年   87篇
  2016年   161篇
  2015年   267篇
  2014年   322篇
  2013年   340篇
  2012年   495篇
  2011年   432篇
  2010年   277篇
  2009年   251篇
  2008年   341篇
  2007年   272篇
  2006年   260篇
  2005年   229篇
  2004年   191篇
  2003年   175篇
  2002年   151篇
  2001年   131篇
  2000年   130篇
  1999年   116篇
  1998年   45篇
  1997年   31篇
  1996年   22篇
  1995年   22篇
  1994年   21篇
  1993年   19篇
  1992年   26篇
  1991年   23篇
  1990年   14篇
  1989年   12篇
  1988年   9篇
  1987年   5篇
  1986年   10篇
  1984年   4篇
  1982年   5篇
  1981年   4篇
  1977年   8篇
  1976年   6篇
  1974年   7篇
  1973年   3篇
  1972年   3篇
  1969年   4篇
  1967年   5篇
  1966年   6篇
排序方式: 共有5367条查询结果,搜索用时 15 毫秒
991.
992.
报道了内蒙古清水河黄土丘陵地区发现的紫萼藓科1新变种——缨齿藓菱形变种[Jaffueliobryum wrightii(Sull.)Thér.var.rhombicumX.L.BaiSarula],该变种与干旱山地岩面生境中的原变种缨齿藓[Jaffueliobryum wrightii(Sull.)Thér.]相似,生境的变化导致其形态发生变化,主要表现在上部细胞菱形和细胞壁背部强烈加厚,未分化的叶上部边缘细胞、中肋横切面细胞不分化,叶片长0.7~0.8mm,毛尖长0.8~1.3mm。文中对缨齿藓及其新变种的形态学特征,分布和生境进行了描述,并提供了显微照片,另外,列出了缨齿藓属5个种的检索表。  相似文献   
993.
Soybean (Glycine max L.) leaves have unique nutraceutical and pharmacological benefits, and have been widely used as a source of healthy and functional food stuffs in Korea. In this study, we investigated the phytochemical metabolomic changes of soybean leaves depending on growth stages (maturation period) assessed based on UPLC–QTOF–MS analysis. Principal component analysis was carried out to trace the metabolite profiles of the phytochemicals from the vegetable stage (1D) through the seven reproductive stages (R1–R7). On the loading plot, significant changes in the contents of metabolites were found during the growth, and eight flavonoid kaempferol glycosides (2, 3, 6, 8, and 10), daidzein (14), genistein (17), and coumestrol (19) were evaluated as growth markers among the 19 isolated metabolites. The kaempferol glycosides were increasingly synthesized from the 1D to the R6 stage but decreased rapidly at stages R7–R8. The extensively synthesized daidzein and genistein were shown during seed growth in the pod (R5–R6), while coumestrol was increased significantly at stages R7–R8 (maturity period). The synthetic pathway of the flavonoids could be elucidated based on the concentration of the individual metabolites. These results demonstrate that the metabolite production changed depending on the growth stage; a possible pathway could be deduced using metabolomic analysis to provide information regarding physiological characterization and optimal harvesting time for crops.  相似文献   
994.
Among the various Ni‐based layered oxide systems in the form of LiNi1‐yzCoyAlzO2 (NCA), the compostions between y = 0.1–0.15, z = 0.05 are the most successful and commercialized cathodes used in electric vehicles (EVs) and hybrid electric vehicles (HEVs). However, tremendous research effort has been dedicted to searching for better composition in NCA systems to overcome the limitations of these cathodes, particularly those that arise when they are used use at high discharge/charge rates (>5C) and in high temperature (60 °C) environments. In addition, improving the thermal stability at 4.5 V is also very important in terms of the total amount of heat generated and the onset temperature. Here, a new NCA composition in the form of LiNi0.81Co0.1Al0.09O2 (y = 0.1, z = 0.09) is reported for the first time. Compared to the LiNi0.85Co0.1Al0.05O2 cathode, LiNi0.81Co0.1Al0.09O2 exhibits an excellent rate capability of 155 mAh g?1 at 10 C with a cut‐off voltage range between 3 and 4.5 V, corresponding to 562 Wh kg?1 at 24 °C. It additionally provides significantly improved thermal stability and electrochemical performance at the high temperature of 60 °C, with a discharge capacity of 122 mAh g?1 after 200 cycles with capacity retention of 59%.  相似文献   
995.
Global warming has shortened mosquitoes’ lifecycle period and increased the disease transmission rates by mosquito vectors. We reviewed only three mosquito‐borne diseases: malaria, dengue fever, and the Japanese encephalitis. Billions of people get infected with those diseases and millions of people die every year. Although we struggle to find the most effective way to control mosquitoes using various methods (including pesticides), mosquito‐borne diseases are still among the most serious problems being faced. This paper, therefore, reviews the strategies for controlling mosquitoes. The use of pesticides to control mosquitoes might have more negative effects on humans and environments than benefits. Although the development of genetically modified (GM) mosquitoes raises new hopes for effective mosquito control, it will take longer to assess the risks to humans and environments. Furthermore, there has been concern about the possible adverse effects from the release of GM mosquitoes into the environment. The various mosquito traps may not be as effective at controlling only female mosquito populations. Therefore, new strategies for the control of mosquitoes are vital. The smart mosquito counter device was developed by Korean Centre for Disease Control (KCDC) in 2013. The mosquito pest control office is able to set up the appropriate mosquito control strategies by using quantitative mosquito information. The smart device will bring mosquito control in line with modern smart generation technology and the device will also soon be able to identify different mosquito species. This new strategy will change the methods of mosquito control and will provide beneficial effects toward sustainable nature and human health.  相似文献   
996.
Species of Orchidaceae are under severe threat of extinction mainly due to overcollection and habitat destruction; accurate identification of orchid species is critical in conservation biology and sustainable utilization of orchids as plant resources. We examined 647 sequences of the cpDNA regions rbcL, matK, atpFatpH IGS, psbKpsbI IGS and trnHpsbA IGS from 89 orchid species (95 taxa) and four outgroup taxa to develop an efficient DNA barcode for Orchidaceae in Korea. The five cpDNA barcode regions were successfully amplified and sequenced for all chlorophyllous taxa, but the amplification and sequencing of the same regions in achlorophyllous taxa produced variable results. psbKpsbI IGS showed the highest mean interspecific K2P distance (0.1192), followed by matK (0.0803), atpFatpH IGS (0.0648), trnHpsbA IGS (0.0460) and rbcL (0.0248). The degree of species resolution for individual barcode regions ranged from 60.5% (rbcL) to 83.5% (trnH‐psbA IGS). The degree of species resolution was significantly enhanced in multiregion combinations of the five barcode regions. Of the 26 possible combinations of the five regions, six provided the highest degree of species resolution (98.8%). Among these, a combination of atpF‐atpH IGS, psbK‐psbI IGS and trnH‐psbA IGS, which comprises the least number of DNA regions, is the best option for barcoding of the Korean orchid species.  相似文献   
997.
Alternative splicing plays an important role in gene expression by producing different proteins from a gene. Caspase-2 pre-mRNA produces anti-apoptotic Casp-2S and pro-apoptotic Casp-2L proteins through exon 9 inclusion or skipping. However, the molecular mechanisms of exon 9 splicing are not well understood. Here we show that knockdown of SRSF3 (also known as SRp20) with siRNA induced significant increase of endogenous exon 9 inclusion. In addition, overexpression of SRSF3 promoted exon 9 skipping. Thus we conclude that SRSF3 promotes exon 9 skipping. In order to understand the functional target of SRSF3 on caspase-2 pre-mRNA, we performed substitution and deletion mutagenesis on the potential SRSF3 binding sites that were predicted from previous reports. We demonstrate that substitution mutagenesis of the potential SRSF3 binding site on exon 8 severely disrupted the effects of SRSF3 on exon 9 skipping. Furthermore, with the approach of RNA pulldown and immunoblotting analysis we show that SRSF3 interacts with the potential SRSF3 binding RNA sequence on exon 8 but not with the mutant RNA sequence. In addition, we show that a deletion of 26 nt RNA from 5′ end of exon 8, a 33 nt RNA from 3′ end of exon 10 and a 2225 nt RNA from intron 9 did not compromise the function of SRSF3 on exon 9 splicing. Therefore we conclude that SRSF3 promotes exon 9 skipping of caspase-2 pre-mRNA by interacting with exon 8. Our results reveal a novel mechanism of caspase-2 pre-mRNA splicing.  相似文献   
998.
999.

Objectives

The Seoul National University Renal Stone Complexity (S-ReSC) scoring system was developed to predict the stone-free rate (SFR) after single-tract percutaneous nephrolithotomy (PCNL). This study is an external validation of this scoring system.

Materials and methods

A retrospective review included 327 patients who underwent PCNL at 2 tertiary referral centers. The S-ReSC score was assigned from 1 to 9 based on the number of sites involved. The stone free status was defined as either complete clearance or clinically insignificant residual fragments <4 mm in size at 1 month follow-up imaging. Inter-observer and test-retest reliabilities were evaluated. The statistical performance of the prediction model was assessed by its predictive accuracy, predictive probability, and clinical usefulness.

Results

The overall SFR was 65.4%. SFRs were 83.9%, 47.6%, and 21.4% in low (1–2), intermediate (3–4), and high (5–9) score groups, respectively, with significant differences (P<0.001). Inter-observer and test-retest reliabilities revealed almost perfect agreements. External validation of the S-ReSC scoring system revealed an AUC of 0.731 (95% CI 0.675–0.788). The AUC of 3-titered S-ReSC score groups was 0.691 (95% CI, 0.629–0.753). The calibration plot showed that the predicted probability of SFR had a concordance comparable to that of the observed frequency. The Hosmer–Lemeshow goodness-of-fit statistic revealed an adequate performance of the predictive model (P = 0.10). Inter-observer and test-retest reliability showed a good level of agreement.

Conclusions

The S-ReSC scoring system is useful in predicting the post-PCNL SFR and in describing the complexity of renal stones.  相似文献   
1000.
PurposeThis prospective study aimed to examine the combined effect of viral load and alcohol consumption on the risk of persistent high-risk (HR) human papillomavirus (HPV) infection.MethodsAmong women undergoing health screening between 2002 and 2011 at the National Cancer Center, 284 and 122 women with HR-HPV infection and cytological findings of low-grade squamous intraepithelial or lower-grade lesions were followed up for 1 and 2 years, respectively. Multivariate logistic regression analysis was performed, and the relative excess risk due to interaction (RERI) and synergy index (S) were calculated.ResultsAmong drinkers, the risks of 1-year (odds ratio [OR] 4.09, 95% confidence interval [CI] 2.05–8.18) and 2-year persistence (OR 8.08, CI 2.36–27.6) were significantly higher for high HPV loads than for low HPV loads; this association was not seen for non-drinkers. The risks for 1-year (OR 4.14, CI 1.89–9.05) and 2-year persistence (OR 6.61, CI 2.09–20.9) were significantly higher in subjects with a high HPV load who were also drinkers than in those who were non-drinkers. A high HPV load together with a longer drinking duration or higher alcohol consumption was associated with increased risks of 1-year (OR 3.07, CI 1.40–6.75 or OR 2.05, CI 0.87–4.83) and 2-year persistence (OR 6.40, CI 1.72–23.8 or OR 4.14, CI 1.18–14.6). The synergistic effect of alcohol consumption and HR-HPV load was stronger on the risk of 2-year persistence (RERI = 3.26, S = 2.38) than on the risk of 1-year persistence (RERI = 1.21, S = 1.63).ConclusionsThe synergistic effect of HR-HPV load and alcohol consumption was associated with the risk of HR-HPV persistence and was stronger for longer-term HR-HPV infection. Limiting alcohol consumption might be an important measure to prevent the development of cervical cancer in women with a high HR-HPV load.  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