全文获取类型
收费全文 | 36276篇 |
免费 | 651篇 |
国内免费 | 1714篇 |
专业分类
系统科学 | 2840篇 |
丛书文集 | 960篇 |
教育与普及 | 81篇 |
理论与方法论 | 100篇 |
现状及发展 | 878篇 |
研究方法 | 4篇 |
综合类 | 33591篇 |
自然研究 | 187篇 |
出版年
2024年 | 15篇 |
2023年 | 69篇 |
2022年 | 100篇 |
2021年 | 87篇 |
2020年 | 125篇 |
2019年 | 143篇 |
2018年 | 114篇 |
2017年 | 160篇 |
2016年 | 147篇 |
2015年 | 201篇 |
2014年 | 720篇 |
2013年 | 564篇 |
2012年 | 967篇 |
2011年 | 718篇 |
2010年 | 607篇 |
2009年 | 878篇 |
2008年 | 987篇 |
2007年 | 1802篇 |
2006年 | 1912篇 |
2005年 | 1895篇 |
2004年 | 1850篇 |
2003年 | 1901篇 |
2002年 | 1427篇 |
2001年 | 1566篇 |
2000年 | 1180篇 |
1999年 | 1562篇 |
1998年 | 1140篇 |
1997年 | 1356篇 |
1996年 | 1557篇 |
1995年 | 1592篇 |
1994年 | 1711篇 |
1993年 | 1346篇 |
1992年 | 1181篇 |
1991年 | 1549篇 |
1990年 | 1626篇 |
1989年 | 1342篇 |
1988年 | 1206篇 |
1987年 | 663篇 |
1986年 | 387篇 |
1985年 | 234篇 |
1984年 | 49篇 |
1983年 | 1篇 |
1981年 | 1篇 |
1980年 | 1篇 |
1955年 | 2篇 |
排序方式: 共有10000条查询结果,搜索用时 187 毫秒
51.
相比于泥浆成槽法的传统实腹式地下连续墙,采用高频振动沉模成孔方式的格栅状地下连续墙的成墙效果和支护性能研究尚处于空白。首先,开展了软土地质条件下格栅状地下连续墙的成墙现场试验研究,继而采用有限单元方法计算得到了背景工程开挖全过程的墙体变形、地面沉降和坑底隆起等支护性能指标变化规律;分析了嵌固深度、地面超载、支撑截面等参数上述指标的影响性特征。结果表明,格栅状地下连续墙的墙身混凝土浇筑质量能够满足强度和抗渗性能要求;其支护性能与传统的钻孔桩和地下连续墙相近,仅截面刚度存在差异,因此可按照桩、墙结构进行设计;为完善结构构造和结点体系,应进一步开展试点工程的应用性研究。 相似文献
52.
喀什凹陷是塔西南坳陷油气显示丰富的地区之一,也是塔里木盆地连接中亚油气勘探的前沿阵地,确定油气显示的油源特征对油气勘探具有重要的意义。研究了克拉托背斜北翼大量存在的原油和油砂抽提物的地球化学特征,评价了凹陷北缘库孜贡苏剖面侏罗系烃源岩和南缘炮江沟剖面石炭系烃源岩,得出了两套烃源岩的分子地球化学特征,确定了原油和油砂抽提物的油源。结果显示,中、下侏罗统粉砂质泥岩和下石炭统黑色碳质泥岩是该区较好的两套烃源岩,两者存在明显的分子地球化学特征差异;通过对饱和烃馏分中甾、萜烷和芳烃馏分中三芴化合物、三芳甾烷等生物标志物特征的刻画,发现喀什凹陷原油与中、下侏罗统烃源岩具对比性,克拉托背斜油砂抽提物的分子地球化学特征明显不同于中、下侏罗统烃源岩,而与下石炭统黑色碳质泥岩具有较好的对比,三芳甾烷证据认为早期石炭系烃源贡献可能对原油和油砂都有影响。油源的新认识对喀什凹陷下一步油气勘探具有一定决策参考。 相似文献
53.
报道了吉林省堇菜属(堇菜科)一新分布记录种——阴地堇菜(Viola yezoensis Maxim.)和一种变型——白瓣紫花地丁(V.yedoensis f.candida Kitag). 相似文献
54.
冉晓莉 《重庆三峡学院学报》2015,(2)
中国抗日战争爆发之后,身处美国的赛珍珠参与和组织了大量的社会活动:创作小说、发表演讲、成立东西方协会、编纂《亚洲》杂志等。赛珍珠以支持抗战为主题的演说和文章都充分体现出她对中国的关注和热爱,对于“中国必胜”的信念,以及对中日民族精神和中美关系的独到见解,为中国赢得美国甚至世界各国人民的同情和支持,为中国抗战做出了不可忽略的贡献。 相似文献
55.
Penelope Greenslade 《Journal of Natural History》2018,52(33-34):2197-2207
A new species of Friesea was found in East Antarctica and is described here as Friesea eureka sp. nov. This is the first new collembolan species to be described from the Antarctic continent in 30 years, even though ice-free areas have been intensively surveyed over that time. It brings to five the number of described species of the genus Friesea known from the Antarctic continent and Peninsula, so is the most speciose genus of Collembola in the region. However, one of these, F. grisea Schäffer, comprises a number of cryptic species of different molecular lineages. The new species is distinguished by having 4 + 4 ocelli plus 1 + 1 hardly visible, a very reduced furca, up to four or five faintly clavate tenent hairs, four anal spines and four spinose chaetae in adult. The restricted distribution of the species emphasises the importance of protecting ice-free areas on the continent as they often contain at least one locally endemic faunal species
www.zoobank.org/urn:lsid:zoobank.org:pub:865B9AE6-0428-4364-99A4-AF38A764D6D9 相似文献
56.
Ivanklin Soares Campos-Filho Jonathas Teixeira Lisboa Giovanna Monticelli Cardoso 《Journal of Natural History》2018,52(7-8):457-482
To date, the family Pudeoniscidae includes six species and three genera, Pudeoniscus, Brasiloniscus and Iansaoniscus, with distributions restricted to Brazil. Here, we redefine the family Pudeoniscidae based on the discovery of a new genus and two new species, Oxossioniscus pataxo sp. nov. and Oxossioniscus akoko sp. nov., from the Brazilian Atlantic forest in the state of Bahia, and on the examination of Pudeoniscus birabeni and Pudeoniscus obscurus. Moreover, P. obscurus is re-described and new records are provided for P. birabeni.
http://www.zoobank.org/urn:lsid:zoobank.org:pub:A0123730-C623-49CE-8665-AE17FDDF994A. 相似文献
57.
This paper aims at closing a gap in recent Weyl research by investigating the role played by Leibniz for the development and consolidation of Weyl's notion of theoretical (symbolic) construction. For Weyl, just as for Leibniz, mathematics was not simply an accompanying tool when doing physics—for him it meant the ability to engage in well-guided speculations about a general framework of reality and experience. The paper first introduces some of the background of Weyl's notion of theoretical construction and then discusses particular Leibnizian inheritances in Weyl's ‘Philosophie der Mathematik und Naturwissenschaft’, such as the general appreciation of the principles of sufficient reason and of continuity. Afterwards the paper focuses on three themes: first, Leibniz's primary quality phenomenalism, which according to Weyl marked the decisive step in realizing that physical qualities are never apprehended directly; second, the conceptual relation between continuity and freedom; and third, Leibniz's notion of ‘expression’, which allows for a certain type of (surrogative) reasoning by structural analogy and which gave rise to Weyl's optimism regarding the scope of theoretical construction. 相似文献
58.
A small amount of cerium (below 1.0 wt%) addition to AZ91 alloy have been investigated on the hot tearing susceptibility (HTS) at a cooling rate of 1.5Ks?1. The HTS increased to a maximum value with a cerium addition of 0.6 wt% and then decreased to a stable status as cerium was gradually added. Due to high chemical activity, cerium into the melts can react with aluminum to form an Al11Ce3 intermetallic compound. X–ray diffraction results also confirmed the existence of Al11Ce3. The Al11Ce3 increased with the presence of the needle-like and block morphology. As cerium levels increased, the amount of Mg17Al12 decreased. The variations between Mg17Al12 and Al11Ce3 resulted in a similar normal distribution of the HTS in AZ91 alloy. The findings were further verified through the microstructure evolution and the surface morphology of fracture. 相似文献
59.
《Journal of Natural History》2012,46(1):173-183
The genera Bakera (Bakera), Bakera (Guinobata) and Mangganeura (Homoptera: Auchenorrhyncha: Typhlocybinae) are re-described and compared. Sandalla is considered a synonym of Bakera (Guinobata) (syn. nov.). Bakera (Bakera) nigrobilineata (Melichar) (=vittata Mahmood) is redescribed and illustrated from mango in the Philippines, on which it is frequently abundant, and found in association with Bakera (Bakera) luzonica sp. nov. which is described. Bakera (Bakera) abaca sp. nov. is described from the Philippines having been collected on abaca (Manila hemp). Bakera (Guinobata) nigroscuta Mahmood is redescribed from the Philippines and its association with coconut confirmed. Mangganeura reticulata Ghauri, described from Malaysia on mango, has been found in Thailand and the hitherto unknown female is described. 相似文献
60.
《Journal of Natural History》2012,46(3):583-596
Leucocythere helenae sp. nov. is described from a small, semi-permanent dam near Grahamstown (RSA). It belongs to Leucocythere s.s., but is at the same time characterized by a number of aberrant features, especially the shape and size of the large copulatory process on the hemipenis and the relatively short setae on the P(3) of the male. A cladistic analysis is conducted and it appears that within Leucocythere, the African species have the most plesiomorphic character states. The discovery of L. helenae sp. nov. in South Africa furthermore falsifies previous hypotheses on the age and origin of the Leucocytherini in general and Leucocythere s.s. in particular. It should now be accepted that the latter genus is not of Pleistocene age, but much older, although the lack of fossil material prevents more exact dating. Also, when the cladogram is studied in conjunction with the present day distribution map of Leucocythere, the concept of a Palaearctic origin of this genus must be rejected. The most plausible hypothesis to date on the origin of this group appears to be a generalized distribution of a leucocytherid ancestor over Europe, Asia and Africa (at least), with subsequent speciation through vicariance. 相似文献