全文获取类型
收费全文 | 9069篇 |
免费 | 129篇 |
国内免费 | 120篇 |
专业分类
系统科学 | 95篇 |
丛书文集 | 141篇 |
教育与普及 | 34篇 |
理论与方法论 | 30篇 |
现状及发展 | 3562篇 |
研究方法 | 442篇 |
综合类 | 4927篇 |
自然研究 | 87篇 |
出版年
2016年 | 78篇 |
2015年 | 78篇 |
2014年 | 106篇 |
2013年 | 119篇 |
2012年 | 195篇 |
2011年 | 297篇 |
2010年 | 138篇 |
2009年 | 125篇 |
2008年 | 236篇 |
2007年 | 269篇 |
2006年 | 195篇 |
2005年 | 174篇 |
2004年 | 272篇 |
2003年 | 163篇 |
2002年 | 170篇 |
2001年 | 334篇 |
2000年 | 328篇 |
1999年 | 238篇 |
1992年 | 126篇 |
1991年 | 129篇 |
1990年 | 118篇 |
1989年 | 108篇 |
1988年 | 112篇 |
1987年 | 128篇 |
1986年 | 117篇 |
1985年 | 165篇 |
1984年 | 124篇 |
1983年 | 101篇 |
1982年 | 85篇 |
1981年 | 120篇 |
1980年 | 115篇 |
1979年 | 263篇 |
1978年 | 204篇 |
1977年 | 193篇 |
1976年 | 137篇 |
1975年 | 159篇 |
1974年 | 223篇 |
1973年 | 185篇 |
1972年 | 208篇 |
1971年 | 232篇 |
1970年 | 282篇 |
1969年 | 226篇 |
1968年 | 207篇 |
1967年 | 220篇 |
1966年 | 192篇 |
1965年 | 153篇 |
1958年 | 104篇 |
1957年 | 76篇 |
1956年 | 73篇 |
1954年 | 75篇 |
排序方式: 共有9318条查询结果,搜索用时 45 毫秒
1.
Mehdi Shahedi Asl Abbas Sabahi Namini Seyed Ali Delbari Quyet Van Le Mohammadreza Shokouhimehr Mohsen Mohammadi 《自然科学进展(英文版)》2021,31(1):47-54
Sintering behavior of ZrB_2 ceramic with nano-sized SiC dopant was studied. ZrB_2-25 vol% nano-sized SiC was selected as the starting mixture to fabricate the composite. The manufacturing process was accomplished at 1800℃ for 5 min under 25 MPa via spark plasma sintering(SPS). The as-sintered sample reached a relative density of 99%. Besides the initial phases, namely ZrB_2 and SiC, the high-resolution X-ray diffraction(HRXRD) was used to study the formation of an in-situ ZrC phase. The possible chemical interactions during the ZrC phase formation were scrutinized. The microstructure of the composite was studied by the field emission scanning electron microscopy(FESEM) and transmission electron microscopy(TEM). Elemental analysis through FESEM evaluations revealed the formation of amorphous phases, rich in Zr, C, Si, B, and O elements, which was in harmony with the thermodynamical assessments. TEM studies endorsed the formation of such phases, containing a glassy bed of Si–B–O with ZrC and C islands dispersed therein. 相似文献
2.
3.
This paper undertakes a comprehensive examination of 10 measures of core inflation and evaluates which measure produces the best forecast of headline inflation out‐of‐sample. We use the Personal Consumption Expenditure Price Index as our measure of inflation. We use two sets of components (17 and 50) of the Personal Consumption Expenditure Price Index to construct these core inflation measures and evaluate these measures at the three time horizons (6, 12 and 24 months) most relevant for monetary policy decisions. The best measure of core inflation for both sets of components and over all time horizons uses weights based on the first principal component of the disaggregated (component‐level) prices. Interestingly, the results vary by the number of components used; when more components are used the weights based on the persistence of each component is statistically equivalent to the weights generated by the first principal component. However, those forecasts using the persistence of 50 components are statistically worse than those generated using the first principal component of 17 components. The statistical superiority of the principal component method is due to the fact that it extracts (in the first principal component) the common source of variation in the component level prices that accurately describes trend inflation over the next 6–24 months. 相似文献
4.
5.
A broker in an open e-marketplace enables buyers and sellers to do business with each other. Although a broker plays an important role in e-marketplaces, theory and guidelines for matching between buyers and sellers in multi-attribute exchanges are limited. Therefore, a challenge for a broker’s responsibility is how to maximize a buyer’s total satisfaction degree as its goals under the consideration of trade-off between a buyer’s buying quantity and price paid to a seller, and other attributes. To solve this challenge, this paper proposes an economic model-based matching approach between a buyer’s requirements and a seller’s offers. The major contributions of this paper are that (i) a broker can model a seller’s price policy as per a buyer’s buying quantity through communication between a broker and a seller; (ii) due to each buyer’s different quantity demand, a broker models a buyer’s satisfaction degree as per a buyer’s buying quantity based on communication between a broker and a buyer; and (iii) to carry out a broker’s matching processes, an objective function and a set of constraints are generated to help a broker to maximize a buyer’s total satisfaction degree. Experimental results demonstrate the good performance of the proposed approach. 相似文献
6.
7.
8.
In this article we consider the growing interest in recent years in the use of documentary strategies in the wold of contemporary art, film and performing arts and explore some of the central epistemological assumptions underpinning the persistent idea that the documentary should be equated with ‘non-fiction’. Following Stella Bruzzi we argue that if documentary theory maintains objectivity as the primary measure of value, it will inevitably and continuously arrive at the conclusion that the documentary genre is fundamentally flawed. Instead, we propose to move beyond the ‘realist epistemology’ of documentary theory and focus on the ‘documentary real’, i.e. the specific performativity of the reality constructed in and by the documentary genre. In the last paragraphs, we introduce the various articles that address the “documentary real” in this special issue. 相似文献
9.
Lamine Alaoui Gustavo Palomino Sandy Zurawski Gerard Zurawski Sixtine Coindre Nathalie Dereuddre-Bosquet Camille Lecuroux Cecile Goujard Bruno Vaslin Christine Bourgeois Pierre Roques Roger Le Grand Olivier Lambotte Benoit Favier 《Cellular and molecular life sciences : CMLS》2018,75(10):1871-1887
Classical dendritic cells (cDCs) play a pivotal role in the early events that tip the immune response toward persistence or viral control. In vitro studies indicate that HIV infection induces the dysregulation of cDCs through binding of the LILRB2 inhibitory receptor to its MHC-I ligands and the strength of this interaction was proposed to drive disease progression. However, the dynamics of the LILRB2/MHC-I inhibitory axis in cDCs during early immune responses against HIV are yet unknown. Here, we show that early HIV-1 infection induces a strong and simultaneous increase of LILRB2 and MHC-I expression on the surface of blood cDCs. We further characterized the early dynamics of LILRB2 and MHC-I expression by showing that SIVmac251 infection of macaques promotes coordinated up-regulation of LILRB2 and MHC-I on cDCs and monocytes/macrophages, from blood and lymph nodes. Orientation towards the LILRB2/MHC-I inhibitory axis starts from the first days of infection and is transiently induced in the entire cDC population in acute phase. Analysis of the factors involved indicates that HIV-1 replication, TLR7/8 triggering, and treatment by IL-10 or type I IFNs increase LILRB2 expression. Finally, enhancement of the LILRB2/MHC-I inhibitory axis is specific to HIV-1 and SIVmac251 infections, as expression of LILRB2 on cDCs decreased in naturally controlled chikungunya virus infection of macaques. Altogether, our data reveal a unique up-regulation of LILRB2 and its MHC-I ligands on cDCs in the early phase of SIV/HIV infection, which may account for immune dysregulation at a critical stage of the anti-viral response. 相似文献
10.