全文获取类型
收费全文 | 19359篇 |
免费 | 1704篇 |
国内免费 | 1668篇 |
出版年
2024年 | 25篇 |
2023年 | 314篇 |
2022年 | 414篇 |
2021年 | 629篇 |
2020年 | 601篇 |
2019年 | 905篇 |
2018年 | 817篇 |
2017年 | 507篇 |
2016年 | 561篇 |
2015年 | 729篇 |
2014年 | 1316篇 |
2013年 | 1434篇 |
2012年 | 947篇 |
2011年 | 1289篇 |
2010年 | 942篇 |
2009年 | 1021篇 |
2008年 | 1004篇 |
2007年 | 1149篇 |
2006年 | 1001篇 |
2005年 | 881篇 |
2004年 | 727篇 |
2003年 | 614篇 |
2002年 | 599篇 |
2001年 | 384篇 |
2000年 | 362篇 |
1999年 | 306篇 |
1998年 | 330篇 |
1997年 | 247篇 |
1996年 | 251篇 |
1995年 | 286篇 |
1994年 | 226篇 |
1993年 | 206篇 |
1992年 | 193篇 |
1991年 | 176篇 |
1990年 | 145篇 |
1989年 | 120篇 |
1988年 | 116篇 |
1987年 | 99篇 |
1986年 | 61篇 |
1985年 | 109篇 |
1984年 | 157篇 |
1983年 | 122篇 |
1982年 | 137篇 |
1981年 | 61篇 |
1980年 | 53篇 |
1979年 | 51篇 |
1978年 | 40篇 |
1977年 | 19篇 |
1976年 | 13篇 |
1974年 | 11篇 |
排序方式: 共有10000条查询结果,搜索用时 15 毫秒
1.
2.
Rapid prioritization of alien plants for eradication based on climatic suitability and eradication feasibility 下载免费PDF全文
Identifying which introduced species have the greatest potential for establishment, spread and impact is critical for prioritizing pre‐ and post‐border control. Using species distribution modelling and existing species locations we assessed the establishment risk based on the climatic suitability areas of 25 plant species listed as eradication targets under South African regulations. To improve confidence, three bioclimatic models were used to predict the potential distribution of each species. This information was combined with the number of localities and the “eradication feasibility syndromes” in a scoring‐categorical system to rank the species. Three management groups were identified. Group “A” includes species with medium‐high establishment risk and higher likelihood to be eradicated, these species should be a priority for eradication. Group “B” includes species with a medium‐low establishment risk but given the low number of known population and the species characteristics, eradication is likely to be feasible. Finally species in group “C” scored a medium‐high establishment risk but the eradication would be difficult due to the high number of known localities. This ranking provides a rapid method to prioritize the management towards the eradication of new potential invasive plant species in the country combining the establishment risk, known number of localities and the inferred eradication success. 相似文献
3.
Thiophene Rings Improve the Device Performance of Conjugated Polymers in Polymer Solar Cells with Thick Active Layers 下载免费PDF全文
Chunhui Duan Ke Gao Fallon J. M. Colberts Feng Liu Stefan C. J. Meskers Martijn M. Wienk René A. J. Janssen 《Liver Transplantation》2017,7(19)
Developing novel materials that tolerate thickness variations of the active layer is critical to further enhance the efficiency of polymer solar cells and enable large‐scale manufacturing. Presently, only a few polymers afford high efficiencies at active layer thickness exceeding 200 nm and molecular design guidelines for developing successful materials are lacking. It is thus highly desirable to identify structural factors that determine the performance of semiconducting conjugated polymers in thick‐film polymer solar cells. Here, it is demonstrated that thiophene rings, introduced in the backbone of alternating donor–acceptor type conjugated polymers, enhance the fill factor and overall efficiency for thick (>200 nm) solar cells. For a series of fluorinated semiconducting polymers derived from electron‐rich benzo[1,2‐b:4,5‐b′]dithiophene units and electron‐deficient 5,6‐difluorobenzo[2,1,3]thiazole units a steady increase of the fill factor and power conversion efficiency is found when introducing thiophene rings between the donor and acceptor units. The increased performance is a synergistic result of an enhanced hole mobility and a suppressed bimolecular charge recombination, which is attributed to more favorable polymer chain packing and finer phase separation. 相似文献
4.
5.
6.
7.
8.
Lamyai Neeratanaphan Sukanya Khamma Rachadaporn Benchawattananon Patcharin Ruchuwararak Sombat Appamaraka Somsak Intamat 《人类与生态风险评估》2017,23(5):1086-1098
This research was conducted to assess heavy metal contamination in the environment and within Oryza sativa. The translocation factors (TFs) and bioaccumulation factors (BAFs) for heavy metals in O. sativa and estimated daily intake (EDI) and health risk index (HRI) were measured. The samples were analyzed for heavy metals using inductively coupled plasma optical emission spectrometry (ICP-OES). Pb and Cr concentrations in water samples within and near the electronic-waste dumping area exceeded water quality standards for surface water sources from the Pollution Control Department in the Ministry of Natural Resources and Environment of Thailand (PCD). The Pb concentration in soil samples within the area also exceeded soil quality standards for habitat and agriculture from PCD. Most of the metals were highly concentrated in roots, except for Mn which has the highest concentration in leaves. Pb concentrations in rice grains exceeded the FAO/WHO standard (0.2 mg/kg). The average TF values for heavy metals from the soil to roots, roots to stems, stems to leaves, and stems to grains were Mn > Pb > Ni > Cr, Mn > Cr > Ni > Pb, Ni > Pb > Mn > Cr, and Pb > Ni > Cr > Mn, respectively. The average BAF values in O. sativa were Mn > Ni > Pb > Cr. The EDI for Cr, Pb, Mn, and Ni via O. sativa consumption were 6.19, 6.02, 370.57, and 3.80 µg/kg/day, respectively. The HRI for Cr, Pb, Mn, and Ni via O. sativa consumption were 0.30, 1.50, 2.60, and 0.002, respectively. 相似文献
9.
10.
摘要 目的:研究二甲双胍片联合糖脉康颗粒对2型糖尿病患者血清白介素-6(IL-6)、肿瘤坏死因子-α(TNF-α)、C反应蛋白(CRP)、脂联素(APN)水平的影响。方法:选择2013年2月至2016年10月在我院接受治疗的2型糖尿病患者130例,按照给药方法不同分为对照组和观察组,对照组采用二甲双胍片治疗,观察组以对照组为基础联合糖脉康治疗。治疗8周后,对比两组患者治疗前后血清IL-6、TNF-α、CRP、APN、总胆固醇(TC)、甘油三酯(TG)、低密度脂蛋白胆固醇(LDL-C)、高密度脂蛋白胆固醇(HDL-C),空腹血糖(FBC)、餐后2小时血糖(2hPBG)、糖化血红蛋白(HbAlc)、胰岛抵抗指数( HOMA-IR)、胰岛β细胞分泌功能指数(HOMA-IS)和胰岛素敏感指数(ISI)的变化。结果:治疗后,两组患者血清IL-6、TNF-α、CRP、TC、TG、LDL-C,FBC、2hPBC、HbAlc,HOMA-IR均较治疗前均明显降低,而血清APN、HDL-C、HOMA-IS、ISI均明显增高(P<0.05);与对照组相比,观察组患者血清IL-6、TNF-α、CRP、TC、TG、LDL-C,FBC、2hPBC、HbAlc、HOMA-IR水平均较低(P<0.05),血清APN、HDL-C,HOMA-IS、ISI水平较高(P<0.05)。结论:二甲双胍片联合糖脉康治疗2型糖尿病患者可有效稳定其血脂血糖,改善胰岛功能,可能与其降低血清IL-6、TNF-α、CRP水平,增加血清APN水平有关。 相似文献