全文获取类型
收费全文 | 53378篇 |
免费 | 858篇 |
国内免费 | 627篇 |
专业分类
测绘学 | 1373篇 |
大气科学 | 4447篇 |
地球物理 | 11292篇 |
地质学 | 18764篇 |
海洋学 | 4181篇 |
天文学 | 11237篇 |
综合类 | 114篇 |
自然地理 | 3455篇 |
出版年
2020年 | 319篇 |
2019年 | 329篇 |
2018年 | 781篇 |
2017年 | 733篇 |
2016年 | 1067篇 |
2015年 | 738篇 |
2014年 | 1077篇 |
2013年 | 2525篇 |
2012年 | 1200篇 |
2011年 | 1740篇 |
2010年 | 1489篇 |
2009年 | 2080篇 |
2008年 | 1825篇 |
2007年 | 1676篇 |
2006年 | 1774篇 |
2005年 | 1510篇 |
2004年 | 1552篇 |
2003年 | 1488篇 |
2002年 | 1499篇 |
2001年 | 1248篇 |
2000年 | 1278篇 |
1999年 | 1154篇 |
1998年 | 1096篇 |
1997年 | 1143篇 |
1996年 | 987篇 |
1995年 | 955篇 |
1994年 | 869篇 |
1993年 | 771篇 |
1992年 | 768篇 |
1991年 | 691篇 |
1990年 | 737篇 |
1989年 | 659篇 |
1988年 | 670篇 |
1987年 | 788篇 |
1986年 | 696篇 |
1985年 | 874篇 |
1984年 | 1003篇 |
1983年 | 1018篇 |
1982年 | 875篇 |
1981年 | 842篇 |
1980年 | 821篇 |
1979年 | 764篇 |
1978年 | 793篇 |
1977年 | 705篇 |
1976年 | 683篇 |
1975年 | 659篇 |
1974年 | 727篇 |
1973年 | 707篇 |
1972年 | 452篇 |
1971年 | 383篇 |
排序方式: 共有10000条查询结果,搜索用时 15 毫秒
201.
中国汽车工业──改革开放后的重整与国际化 总被引:4,自引:0,他引:4
大规模的重整和广泛的国际化是改革开放后中国汽车工业发展的两个主要特征。作者首先从企业组织结构、生产能力与品种结构、规模经济和空间分布等四个方面分析了近二十年来中国汽车工业重整的效果及所形成的行业特征;其次根据利用的外资和技术的来源以及涉及的技术形态分析了其国际化的程度及在世界汽车工业中的位置;最后总结了中国汽车工业面临的若干问题,并提出在散乱的格局和跨国公司竞争的双重压力下,中国汽车工业正处于成长的关键时期。 相似文献
202.
203.
Amphibole zonation in metabasites as a guide to the evolution of metamorphic conditions 总被引:2,自引:0,他引:2
Timothy J. B. Holland Stephen W. Richardson 《Contributions to Mineralogy and Petrology》1979,70(2):143-148
In low grade metabasites the amphibole components tremolite, glaucophane, edenite and tschermakite have their activities controlled
by interactions with the excess components albite, clinozoisite, chlorite, quartz and H2O vapor. Three types of reaction are involved, (i) Those in which only components of condensed phases take part: isopleths
of equilibrium constant are straight lines in the P-T plane. (ii) Dehydration reactions in which entropy change due to change in Al coordination is of the same sign as that due
to dehydration: isopleths of constant K are positive at low pressure and negative at high pressure. (iii) Dehydration reactions
in which entropy change due to Al coordination change is opposite in sign to that of dehydration: isopleths of constant K
loop in the P-T plane with positive slopes at low and at high pressure.
Zonation in naturally occurring amphiboles records the evolution of metamorphic conditions in particular rocks. In an example
from the eastern Alps (Austria) early conditions calculated as 15 kb, 200 ° C evolve upgrade to 6 kb, 525 ° C implying concurrent
heating and erosion. The record of evolving conditions may span some 30 Ma of geological history. 相似文献
204.
Viking images of Martian craters with rampart-bordered ejecta deposits reveal distinct impact ejecta morphology when compared to that associated with similar-sized craters on the Moon and Mercury. Topographic control of distribution, lobate and terraced margins, cross-cutting relationships, and multiple stratigraphic units are evidence for ejecta emplacement by surface flowage. It is suggested that target water explosively vaporized during impact alters initial ballistic trajectories of ejecta and produces surging flow emplacement. The dispersal of particulates during a series of controlled steam explosions generated by interaction of a thermite melt with water has been experimentally modeled. Preliminary results indicate that the mass ratio of water to melt and confining pressure control the degree of melt fragmentation (ejecta particle size) and the energy and mode of melt-ejecta dispersal. Study of terrestrial, lobate, volcanic ejecta produced by steam-blast explosions reveals that particle size and vapor to clast volume ratio are primary parameters characterizing the emplacement mechanism and deposit morphology. Martian crater ramparts are formed when ejecta surges lose fluidizing vapors and transported particles are deposited en masse. This deposition results from flow yield strength increasing above shear stress due to interparticle friction. 相似文献
205.
Artificial light at night: implications for early life stages development in four temperate freshwater fish species 总被引:1,自引:0,他引:1
Anika Brüning Franz Hölker Christian Wolter 《Aquatic Sciences - Research Across Boundaries》2011,73(1):143-152
Flora and fauna have both evolved under a natural cycle of light and dark. But especially in urban areas, the night is now
increasingly disturbed by artificial light. Many traits and behaviours in fish are triggered by a circadian clock, for example
hatching and swim bladder inflation, which predominantly take place at dusk or night. As lighting becomes brighter and extends
farther into rural areas, the distinction between day and night becomes increasingly blurred. Therefore, the loss of diurnal
trigger by artificial light at night was hypothesized having deleterious effects on these traits and impact fish reproduction.
To assess these effects, eggs of four native freshwater fishes, Eurasian perch Perca fluviatilis, roach Rutilus rutilus, bleak Alburnus alburnus and chub Leuciscus cephalus, were incubated under two different light conditions: a photoperiod of 14 h light:10 h darkness (LD) and continuous illumination
(LL). The time to hatch and swim bladder inflation was recorded. The species showed inconsistent reactions to the light treatments.
In roach and bleak, the time to 50% hatch was longer in LL, whereas continuous lighting had an accelerating effect in chub.
Incubation in LL elongated the hatching period in perch and roach and, in perch, the onset of darkness seemed to trigger hatching.
The swim bladder inflation was significantly promoted by continuous light in chub and bleak but was not affected in roach.
In conclusion, nocturnal artificial illumination could have an effect on hatching and initial swim bladder filling by masking
the day–night-change and thereby diminish the trigger effect. However, the reactions were species specific and the increase
in variation indicated a lack of diurnal triggering, whilst a general deleterious effect of artificial light at night has
not been identified on early life stages. 相似文献
206.
207.
土地利用变化空间模拟的进展--CLUE-S模型及其应用 总被引:24,自引:1,他引:24
土地利用变化的空间模拟是进行土地利用情景分析的重要基础。本文在介绍了国际上常用的细胞自控模型(CA)、土地利用变化及效应模型(CLUE)的基础上,重点分析了小尺度土地利用变化及效应模型(CLUE-S)的方法,并以邯郸地区为例进行了案例研究。认为CLUE-S模型采取经验模型的方法,通过建立土地利用空间分配和驱动因子之间的统计关系模拟近期土地利用变化的情景。同时也考虑了不同土地利用方式之间的竞争关系,因此可以较好地模拟小尺度地区的近期土地利用变化情景;考虑到短期或近期土地利用变化的因子主要与人类的社会经济活动有关,而社会经济因子的空间化尚存在一定难度。因此,突破这一瓶颈成为CLUE-S模型发展和应用的关键;CLUE-S模型主要解决的是不同空间尺度上的土地利用空间分配问题,在土地生产潜力评价、土地利用规划等方面具有广阔的应用前景。 相似文献
208.
G. R. Huguenin A. E. Lilley W. H. McDonough M. D. Papagiannis 《Planetary and Space Science》1964,12(12):1157-1167
Radio noise observations at frequencies of 0·700 Mc and 2·200 Mc were made at altitudes between 3000 and 11,000 km from a Blue Scout Jr. high-altitude rocket probe on 30 July 1963. A steady background flux of (7·5−3+6) × 10−19 W m−2)(c/s)−1 at 0·700 Mc and (1·8+1.0−0.5 × 10−19 W m−2 (c/s)−1 at 2·200 Mc was observed. Assuming a galactic origin of the observed fluxes at both frequencies, the averaged sky brightnesses are b(0·700 Mc) = (6−3+5) × 10−20 W m−2 (c/s)−1 sr−1b(2·200 Mc) = (1.4+1.0−0.5 × 10−20 W m−2 (c/s)−1 sr−1 The observed brightness at 2·200 Mc is in reasonable agreement with the results of other observers. The apparent brightness at 0·700 Mc is, however, greater than was expected from previous observations. An alternative source of the 0·700 Mc flux in the terrestrial exosphere, as well as characteristics of several noise bursts observed during the flight, is briefly discussed. 相似文献
209.
210.