全文获取类型
收费全文 | 1585篇 |
免费 | 133篇 |
国内免费 | 8篇 |
专业分类
耳鼻咽喉 | 12篇 |
儿科学 | 66篇 |
妇产科学 | 33篇 |
基础医学 | 227篇 |
口腔科学 | 12篇 |
临床医学 | 237篇 |
内科学 | 280篇 |
皮肤病学 | 27篇 |
神经病学 | 132篇 |
特种医学 | 50篇 |
外科学 | 139篇 |
综合类 | 20篇 |
一般理论 | 1篇 |
预防医学 | 253篇 |
眼科学 | 59篇 |
药学 | 87篇 |
中国医学 | 3篇 |
肿瘤学 | 88篇 |
出版年
2024年 | 3篇 |
2023年 | 20篇 |
2022年 | 26篇 |
2021年 | 59篇 |
2020年 | 34篇 |
2019年 | 50篇 |
2018年 | 41篇 |
2017年 | 43篇 |
2016年 | 60篇 |
2015年 | 48篇 |
2014年 | 59篇 |
2013年 | 89篇 |
2012年 | 149篇 |
2011年 | 139篇 |
2010年 | 75篇 |
2009年 | 64篇 |
2008年 | 114篇 |
2007年 | 107篇 |
2006年 | 90篇 |
2005年 | 77篇 |
2004年 | 76篇 |
2003年 | 85篇 |
2002年 | 58篇 |
2001年 | 11篇 |
2000年 | 3篇 |
1999年 | 7篇 |
1998年 | 16篇 |
1997年 | 10篇 |
1996年 | 8篇 |
1995年 | 11篇 |
1994年 | 6篇 |
1993年 | 6篇 |
1992年 | 5篇 |
1991年 | 5篇 |
1989年 | 3篇 |
1988年 | 7篇 |
1986年 | 4篇 |
1984年 | 3篇 |
1983年 | 3篇 |
1982年 | 4篇 |
1981年 | 4篇 |
1980年 | 3篇 |
1973年 | 2篇 |
1972年 | 5篇 |
1967年 | 2篇 |
1966年 | 3篇 |
1964年 | 4篇 |
1962年 | 2篇 |
1960年 | 2篇 |
1941年 | 2篇 |
排序方式: 共有1726条查询结果,搜索用时 15 毫秒
11.
Edward T. Tipper Emily I. Stevenson Victoria Alcock Alasdair C. G. Knight J. Jotautas Baronas Robert G. Hilton Mike J. Bickle Christina S. Larkin Linshu Feng Katy E. Relph Genevieve Hughes 《Proceedings of the National Academy of Sciences of the United States of America》2021,118(1)
Rivers carry the dissolved and solid products of silicate mineral weathering, a process that removes from the atmosphere and provides a key negative climate feedback over geological timescales. Here we show that, in some river systems, a reactive exchange pool on river suspended particulate matter, bonded weakly to mineral surfaces, increases the mobile cation flux by 50%. The chemistry of both river waters and the exchange pool demonstrates exchange equilibrium, confirmed by Sr isotopes. Global silicate weathering fluxes are calculated based on riverine dissolved sodium (Na+) from silicate minerals. The large exchange pool supplies Na+ of nonsilicate origin to the dissolved load, especially in catchments with widespread marine sediments, or where rocks have equilibrated with saline basement fluids. We quantify this by comparing the riverine sediment exchange pool and river water chemistry. In some basins, cation exchange could account for the majority of sodium in the river water, significantly reducing estimates of silicate weathering. At a global scale, we demonstrate that silicate weathering fluxes are overestimated by 12 to 28%. This overestimation is greatest in regions of high erosion and high sediment loads where the negative climate feedback has a maximum sensitivity to chemical weathering reactions. In the context of other recent findings that reduce the net consumption through chemical weathering, the magnitude of the continental silicate weathering fluxes and its implications for solid Earth degassing fluxes need to be further investigated.For decades, silicate weathering has been postulated to provide the negative climate feedback on Earth that prevents a runaway greenhouse climate like on Venus (1). Silicate mineral dissolution with carbonic acid converts atmospheric into carbonate, and releases essential nutrients to the terrestrial and marine biosphere (2). There have been many attempts to quantify the silicate weathering flux (3), mostly assuming that riverine dissolved sodium () is derived only from silicate minerals and rock salt. Here we show that there is a major addition of nonsilicate to the critical zone from ancient seawater, weakly bonded to sedimentary rocks and supplied to waters via the cation exchange process. The implication is not only that the silicate weathering flux is overestimated at a global scale, but that this nonsilicate is most important in regions previously thought to have the highest silicate weathering fluxes (so called weathering-limited regions) and greatest climate sensitivity.Cation exchange is a rapid chemical reaction between cations in the dissolved phase and mineral surfaces, particularly clays (4). Major and trace cations such as calcium (), magnesium (), sodium (), potassium (), and strontium () form the cation exchange pool, which balances negative charges on river-borne clay particle surfaces. This exchange takes place on interlayer sites, between the tetrahedral and octahedral layers, or on exposed surfaces (4). The importance of the cation exchange pool is well recognized in soils and aquifers (4, 5), has significant implications for enhanced weathering (6), and has been proposed as an important mechanism for buffering the composition of river waters (7–9). However, data on the riverine exchange pool are only available for two large river systems [Amazon and Ganges-Brahmaputra (10, 11)], despite its significance in providing a source of elements that are immediately bioavailable (12), and their potential for biasing the quantification of silicate weathering (9).It is increasingly recognized that rapidly reactive phases have a strong influence on the chemistry of river waters (13, 14). Cation exchange is a rapid reaction occurring continuously in soils, as riverine freshwaters evolve downstream interacting with particulate matter, and when they mix with seawater (15, 16). Important examples of cation exchange are the “swapping” of divalent cations and with , in particular when there is a major change in water composition such as when fluvial clays reach the ocean,[1]As a result, marine sediments have an exchange pool that is dominated by (17). Subsequently, these marine sediments are uplifted and emplaced on the continents where in the exchange pool is released by cation exchange with Ca-rich fresh waters (9). This has major implications for estimates of silicate weathering fluxes and associated consumption, because they are calculated using the content of rivers (3). Cerling et al. (9) proposed that the -rich exchange pool exerts an important control on natural waters, based on charge balance arguments from river water chemistry, but this hypothesis has never been rigorously tested (18) by determining the flux and composition of the exchange pool of rivers around the world.In this contribution, we present a large dataset of fluvial sediment cation exchange capacity (CEC) and composition in several of the world’s largest river basins. By comparing with the concomitant dissolved load chemistry, we demonstrate that 1) the exchange pool in river sediments is in equilibrium with the river water; 2) the fraction of mobile elements in the exchange pool relative to the dissolved pool can be significant, particularly in rapidly eroding, weathering-limited catchments; and 3) given reasonable inferences on the composition of old marine sedimentary rocks, modern-day silicate weathering has been overestimated and carbonate weathering has been underestimated. The results reduce the estimated magnitude of the silicate weathering flux, but increase the supply of base cations (e.g., , which can be a limiting nutrient) to the biosphere, suggesting a greater role of organic carbon burial compared with silicate weathering for the long-term atmospheric sink. 相似文献
12.
Kumar D Rotstein C Miyata G Arlen D Humar A 《The Journal of infectious diseases》2003,187(10):1639-1645
Renal transplant recipients are at increased risk for developing invasive pneumococcal disease but may have a poor response to pneumococcal polysaccharide vaccine (PPV23). For them, pneumococcal conjugate vaccine (PCV7) may be more immunogenic. Patients were given a single dose of PPV23 or PCV7 in our randomized, controlled, double-blind trial. Immunogenicity was assessed 8 weeks after vaccination by serotype-specific enzyme-linked immunosorbent assay (ELISA) and opsonophagocytic assay (OPA). Baseline demographics, renal function, time since transplantation, and immunosuppression were comparable. In the PCV7 group, the vaccine response rate was improved for serotypes 23F (P=.046) and 6B (P=.067), and mean fold increases in antibody titer were higher for serotypes 23F (P=.046) and 9V (P=.09). The response rate and mean fold increase in OPA titers were not significantly different between groups. There was a trend toward enhanced immunogenicity for PCV7 by ELISA. However, functional antibody responses were not different. 相似文献
13.
14.
15.
Danielle Hitch Michelle Taylor Genevieve Pepin 《Scandinavian journal of occupational therapy》2015,22(3):216-225
Aims. This aim of this study was to obtain a consensus from clinicians regarding occupational therapy for people with depression, for the assessments and practices they use that are not currently supported by research evidence directly related to functional performance. The study also aimed to discover how many of these assessments and practices were currently supported by research evidence. Methods. Following a previously reported systematic review of assessments and practices used in occupational therapy for people with depression, a modified nominal group technique was used to discover which assessments and practices occupational therapists currently utilize. Three online surveys gathered initial data on therapeutic options (survey 1), which were then ranked (survey 2) and re-ranked (survey 3) to gain the final consensus. Twelve therapists completed the first survey, whilst 10 clinicians completed both the second and third surveys. Major findings. Only 30% of the assessments and practices identified by the clinicians were supported by research evidence. A consensus was obtained on a total of 35 other assessments and interventions. These included both occupational-therapy-specific and generic assessments and interventions. Principle conclusion. Very few of the assessments and interventions identified were supported by research evidence directly related to functional performance. While a large number of options were generated, the majority of these were not occupational therapy specific. 相似文献
16.
17.
Charles N. S. Chan Jacques Berland Alain Cribier Paolo Rocha Gunter Stix Genevieve Derumeaux Brice Letac 《Catheterization and cardiovascular interventions》1994,32(3):223-230
Patients with mitral stenosis in Western countries are relatively old. It is anticipated that percutaneous transseptal mitral commissurotomy (PTMC) may have more complications and may not be as effective in this group of patient as in younger patients due to more calcification and fibrosis of the mitral valve. We analysed the clinical, hemodynamic, echocardiographic data in 296 consecutive patients divided prospectively into two groups; group 1 consisted of 184 patients ≥ 40 years and group 2 of 112 patients < 40 years coming mostly from developing countries. The immediate gain in valve area was 2.18 ± 0.61 cm2 in group 1 vs. 2.31 ± 0.65 cm2 in group 2 (P = ns). The incidence of acute regurgitation requiring surgical intervention was similar in both groups. Follow-up data up to 5 years after PTMC was available in 170 patients (92.4%) in group 1 (mean 20 ± 13 months) and 83 patients (74.1%) in group 2 (mean 29 ± 17 months). Restenosis by Doppler method (valve area less than 1.5 cm2 with loss of at least 50% initial gain in valve area) was found in 33 patients in group 1 (29.2%) vs. 11 (14.9%) in group 2 (P < 0.05). Events free from death, need for mitral valve replacement or repeat PTMC at 5 year follow-up was 76% in group 1 vs. 87% in group 2 (P < 0.05). We conclude that the immediate effectiveness and acute complications of PTMC in patients 40 years and above are comparable to younger patients. Restenosis is clearly higher and there is a trend towards need for mitral valve replacement in patients 40 years and above at follow-up. However, the continuing benefit for the majority of the patients 40 years and above (76% free from adverse events) would suggest that PTMC is an appropriate treatment modality even in the older patients. © 1994 Wiley-Liss,Inc.. 相似文献
18.
19.
Ter Schiphorst Adrien Gaillard Nicolas Dargazanli Cyril Mourand Isabelle Corti Lucas Charif Mahmoud Ayrignac Xavier Lippi Anaïs Bouly Stéphane Thibault Lalu Sablot Denis Blanchet-Fourcade Genevieve Landragin Nicolas Costalat Vincent Duflos Claire Arquizan Caroline 《Journal of neurology》2021,268(1):346-355
Journal of Neurology - Symptomatic isolated carotid artery occlusions (ICAO) can lead to disability, recurrent stroke, and mortality, but natural history and best therapeutic management remain... 相似文献