全文获取类型
收费全文 | 13982篇 |
免费 | 1416篇 |
国内免费 | 1篇 |
专业分类
15399篇 |
出版年
2022年 | 108篇 |
2021年 | 216篇 |
2020年 | 107篇 |
2019年 | 168篇 |
2018年 | 169篇 |
2017年 | 160篇 |
2016年 | 304篇 |
2015年 | 444篇 |
2014年 | 489篇 |
2013年 | 592篇 |
2012年 | 759篇 |
2011年 | 741篇 |
2010年 | 462篇 |
2009年 | 412篇 |
2008年 | 629篇 |
2007年 | 618篇 |
2006年 | 597篇 |
2005年 | 571篇 |
2004年 | 568篇 |
2003年 | 545篇 |
2002年 | 538篇 |
2001年 | 335篇 |
2000年 | 335篇 |
1999年 | 272篇 |
1998年 | 159篇 |
1997年 | 151篇 |
1996年 | 143篇 |
1995年 | 120篇 |
1994年 | 111篇 |
1993年 | 145篇 |
1992年 | 222篇 |
1991年 | 233篇 |
1990年 | 245篇 |
1989年 | 193篇 |
1988年 | 207篇 |
1987年 | 174篇 |
1986年 | 186篇 |
1985年 | 195篇 |
1984年 | 148篇 |
1983年 | 137篇 |
1982年 | 134篇 |
1981年 | 126篇 |
1980年 | 105篇 |
1979年 | 148篇 |
1978年 | 135篇 |
1977年 | 121篇 |
1976年 | 104篇 |
1975年 | 111篇 |
1973年 | 122篇 |
1971年 | 94篇 |
排序方式: 共有10000条查询结果,搜索用时 7 毫秒
1.
2.
Minh Thac Nguyen Ryan Denniston Hien Thi Thu Nguyen Tuan Anh Hoang Hana Ross Anthony D. So 《PloS one》2014,9(1)
Illicit trade carries the potential to magnify existing tobacco-related health care costs through increased availability of untaxed and inexpensive cigarettes. What is known with respect to the magnitude of illicit trade for Vietnam is produced primarily by the industry, and methodologies are typically opaque. Independent assessment of the illicit cigarette trade in Vietnam is vital to tobacco control policy. This paper measures the magnitude of illicit cigarette trade for Vietnam between 1998 and 2010 using two methods, discrepancies between legitimate domestic cigarette sales and domestic tobacco consumption estimated from surveys, and trade discrepancies as recorded by Vietnam and trade partners. The results indicate that Vietnam likely experienced net smuggling in during the period studied. With the inclusion of adjustments for survey respondent under-reporting, inward illicit trade likely occurred in three of the four years for which surveys were available. Discrepancies in trade records indicate that the value of smuggled cigarettes into Vietnam ranges from $100 million to $300 million between 2000 and 2010 and that these cigarettes primarily originate in Singapore, Hong Kong, Macao, Malaysia, and Australia. Notable differences in trends over time exist between the two methods, but by comparison, the industry estimates consistently place the magnitude of illicit trade at the upper bounds of what this study shows. The unavailability of annual, survey-based estimates of consumption may obscure the true, annual trend over time. Second, as surveys changed over time, estimates relying on them may be inconsistent with one another. Finally, these two methods measure different components of illicit trade, specifically consumption of illicit cigarettes regardless of origin and smuggling of cigarettes into a particular market. However, absent a gold standard, comparisons of different approaches to illicit trade measurement serve efforts to refine and improve measurement approaches and estimates. 相似文献
3.
A newborn child was noted to have an ulcerated lesion on the vertex of her scalp, which was diagnosed as aplasia cutis congenita. As this disorder is relatively rare and frequently misdiagnosed, this case is reported and the relevant literature reviewed. 相似文献
4.
Are we able to infer what happened to a person from a brief sample of his/her behaviour? It has been proposed that mentalising skills can be used to retrodict as well as predict behaviour, that is, to determine what mental states of a target have already occurred. The current study aimed to develop a paradigm to explore these processes, which takes into account the intricacies of real-life situations in which reasoning about mental states, as embodied in behaviour, may be utilised. A novel task was devised which involved observing subtle and naturalistic reactions of others in order to determine the event that had previously taken place. Thirty-five participants viewed videos of real individuals reacting to the researcher behaving in one of four possible ways, and were asked to judge which of the four ‘scenarios’ they thought the individual was responding to. Their eye movements were recorded to establish the visual strategies used. Participants were able to deduce successfully from a small sample of behaviour which scenario had previously occurred. Surprisingly, looking at the eye region was associated with poorer identification of the scenarios, and eye movement strategy varied depending on the event experienced by the person in the video. This suggests people flexibly deploy their attention using a retrodictive mindreading process to infer events. 相似文献
5.
Despite considerable progress in recent years in the promotion of flowering in conifers, relatively little is known about the endogenous control of sex expression or its practical manipulation to aid pollen and crop management in seed orchards. Well defined patterns of sexual zonation exist within the tree crown and shoot, and these appear to be associated with hormonal and possibly nutritional gradients. No firm conclusions, however, are possible regarding the specific role(s) of different PGRs in sex expression, this being as much as a problem of lack of critical study as the complexity of the process itself. Whether a PGR such as gibberellins or auxins, or a cultural treatment that itself may influence endogenous hormone levels, primarily promotes female or male flowering often depends on the position within the crown of the shoot being treated, and its timing in relation to stage of primordia development. Photoperiod also may be involved- short days favoring females and long days males, at least in some species- although its role in controlling sex expression is by no means clear. Practical manipulation of sex expression may be possible through selective crown pruning, different PGR combinations, treatment timing and/or photoperiod control, but the development of such techniques for operational use will require a better basic understanding of the endogenous regulatory mechanisms involved. 相似文献
6.
7.
8.
9.
J.M.M. AdamsA.B. Ross K. AnastasakisE.M. Hodgson J.A. GallagherJ.M. Jones I.S. Donnison 《Bioresource technology》2011,102(1):226-234
To avoid negative impacts on food production, novel non-food biofuel feedstocks need to be identified and utilised. One option is to utilise marine biomass, notably fast-growing, large marine ‘plants’ such as the macroalgal kelps. This paper reports on the changing composition of Laminaria digitata throughout it growth cycle as determined by new technologies. The potential of Laminaria sp. as a feedstock for biofuel production and future biorefining possibilities was assessed through proximate and ultimate analysis, initial pyrolysis rates using thermo-gravimetric analysis (TGA), metals content and pyrolysis gas chromatography-mass spectrometry.Samples harvested in March contained the lowest proportion of carbohydrate and the highest ash and alkali metal content, whereas samples harvested in July contained the highest proportions of carbohydrate, lowest alkali metals and ash content. July was therefore considered the most suitable month for harvesting kelp biomass for thermochemical conversion to biofuels. 相似文献
10.