全文获取类型
收费全文 | 48888篇 |
免费 | 382篇 |
国内免费 | 1231篇 |
专业分类
测绘学 | 1915篇 |
大气科学 | 3678篇 |
地球物理 | 8930篇 |
地质学 | 22177篇 |
海洋学 | 3029篇 |
天文学 | 6623篇 |
综合类 | 2198篇 |
自然地理 | 1951篇 |
出版年
2023年 | 43篇 |
2022年 | 52篇 |
2021年 | 58篇 |
2020年 | 75篇 |
2019年 | 82篇 |
2018年 | 6929篇 |
2017年 | 6230篇 |
2016年 | 3921篇 |
2015年 | 452篇 |
2014年 | 357篇 |
2013年 | 444篇 |
2012年 | 2031篇 |
2011年 | 5007篇 |
2010年 | 4335篇 |
2009年 | 4655篇 |
2008年 | 3747篇 |
2007年 | 4731篇 |
2006年 | 443篇 |
2005年 | 995篇 |
2004年 | 1006篇 |
2003年 | 1059篇 |
2002年 | 700篇 |
2001年 | 285篇 |
2000年 | 291篇 |
1999年 | 194篇 |
1998年 | 190篇 |
1997年 | 167篇 |
1996年 | 156篇 |
1995年 | 138篇 |
1994年 | 117篇 |
1993年 | 92篇 |
1992年 | 79篇 |
1991年 | 89篇 |
1990年 | 78篇 |
1989年 | 83篇 |
1988年 | 60篇 |
1987年 | 102篇 |
1986年 | 64篇 |
1985年 | 64篇 |
1984年 | 72篇 |
1983年 | 73篇 |
1982年 | 72篇 |
1981年 | 88篇 |
1980年 | 78篇 |
1979年 | 51篇 |
1977年 | 50篇 |
1976年 | 53篇 |
1975年 | 35篇 |
1974年 | 30篇 |
1973年 | 41篇 |
排序方式: 共有10000条查询结果,搜索用时 15 毫秒
941.
Helen Ruth Aspaas 《GeoJournal》2004,61(3):281-289
Women in many rural areas of the US often engage in small-scale businesses as one of several avenues for contributing to household
incomes. In those geographical areas that contain a significant minority population, many of the women's businesses display
the cultural diversity of the regions in which they live. Likewise, women's roles as economic providers as well as wives and
mothers are reflected in the manner in which they operate their businesses. Candida Brush suggests that women's strategies
for operating their businesses are highly integrative because women's business decisions are intertwined with familial responsibilities,
household economic demands and desires to contribute to their communities. Hispanic and Native American women in the Four
Corners region of southwestern US and African American women in central Virginia participated in interviews in which they
responded to numerous short answer and open-ended questions about their businesses and decision-making strategies. From these
interviews an image emerges of rural minority business women who are operating their businesses at the nexus of family obligations,
economic necessities, cultural ties and with a commitment for serving their communities.
This revised version was published online in July 2006 with corrections to the Cover Date. 相似文献
942.
S. O. Asamu 《GeoJournal》2004,61(2):183-189
There is a general shortage of potable water in Nigeria, partly through the lack of natural provision and partly because of
rapid urban growth that has largely been unplanned. The paper describes the recent redevelopment of a public water facility
in a run-down area of Ibadan which is part of the Sustainable Ibadan Project. Despite funding from national and international
bodies, the development is strongly based at community level. The local inhabitants feel they have control over their own
water supply through their representatives on the various planning and managing committees. This aims to reduce vandalism
and to increase pride and involvement in the community.
The project is a pilot for other projects around Ibadan, some of which have begun and others are being planned. This paper
describes the administrative structures and management principles that have been employed. It forcefully advocates community-based
planning and management which respects both the environment and the water-users, and involves a major “bottom-up” element.
This revised version was published online in July 2006 with corrections to the Cover Date. 相似文献
943.
944.
Z. X. Zhang 《Rock Mechanics and Rock Engineering》2004,37(3):239-248
945.
The Kundal area of Malani Igneous Suite consists of volcano-plutonic rocks. Basalt flows and gabbro intrusives are associated
with rhyolite. Both the basic rocks consist of similar mineralogy of plagioclase, clinopyroxene as essential and Fe-Ti oxides
as accessories. Basalt displays sub-ophitic and glomeroporphyritic textures whereas gabbro exhibits sub-ophitic, porphyritic
and intergrannular textures. They show comparable chemistry and are enriched in Fe, Ti and incompatible elements as compared
to MORB/CFB. Samples are enriched in LREE and slightly depleted HREE patterns with least significant positive Eu anomalies.
Petrographical study and petrogenetic modeling of [Mg]-[Fe], trace and REE suggest cogenetic origin of these basic rocks and
they probably derived from Fe-enriched source with higher Fe/Mg ratio than primitive mantle source. Thus, it is concluded
that the basic volcano-plutonic rocks of Kundal area are the result of a low to moderate degree (< 30%) partial melting of
source similar to picrite/komatiitic composition. Within plate, anorogenic setting for the basic rocks of Kundal area is suggested,
which is in conformity with the similar setting for Malani Igneous Suite. 相似文献
946.
Using observational data obtained with the Yohkoh, SOHO, and TRACE satellites, it is shown that the three-dimensional structure of the large solar flare of July 14, 2000 was determined by the topology of the large-scale magnetic field of the active region giving rise to the flare. The locations and shapes of chromospheric ribbons and brightness centers on these ribbons are explained. The observed behavior of the flare is attributed to rapid magnetic reconnection in the corona. The electric field accelerating particles in the reconnecting current sheets is estimated. 相似文献
947.
We analyze the observed parameters of massive extremely close binaries containing Wolf-Rayet stars and black holes, and identify those systems whose supernova outbursts lead to the formation of rapidly rotating Kerr black holes. It is proposed that the formation of such a black hole is accompanied by a strong gamma-ray burst. Several types of observed systems satisfy the conditions necessary for the formation of a Kerr black hole: BH+WR, BH+OB, WR+O, and BH+K,M. 相似文献
948.
We present an analysis of a high-amplitude event in the flux curve of component A of the gravitational lens QSO 2237+0305 observed by the OGLE and GLITP groups in autumn 1999. Hypothesizing this event to be associated with microlensing of a fold caustic, we analyzed the observational data using a method for the successive reconstruction of branches of the one-dimensional strip brightness distribution across the source corresponding to positive and negative arguments. The search for the branches was carried out on compact sets of nonnegative, monotonically nonincreasing, convex downward functions. The resulting shape of the strip brightness distribution for the accretion disk of the quasar is in agreement with results obtained earlier via model fitting. Features in the lensing curve that could be associated with curvature of the caustic, nearness of a cusp, or the influence of nearby caustics are noted. 相似文献
949.
KADIK A.; PINEAU F.; LITVIN Y.; JENDRZEJEWSKI N.; MARTINEZ I.; JAVOY M. 《Journal of Petrology》2004,45(7):1297-1310
Studies of iron-bearing silicate melt (ferrobasalt) + iron metallicphase + graphite + hydrogen equilibria show that carbon andhydrogen solubilities in melts are important for the evolutionof the upper mantle. In a series of experiments conducted at3·7 GPa and 15201600°C, we have characterizedthe nature (oxidized vs reduced) and quantified the abundancesof C- and H-compounds dissolved in iron-bearing silicate melts.Experiments were carried out in an anvil-with-hole apparatuspermitting the achievement of equal chemical potentials of H2in the inner Pt capsule and outer furnace assembly. The fO2for silicate meltiron equilibrium was 2·32 ±0·04 log units below ironwüstite (IW). Theferrobasalt used as starting material experienced a reductionof its iron oxides and silicate network. The counterpart wasa liberation of oxygen reacting with the hydrogen entering thecapsule. The amount of H2O dissolved in the glasses was measuredby ion microprobe and by step-heating and was found to be between1 and 2 wt %. The dissolved carbon content was found to be 1600ppm C by step-heating. The speciation of C and H componentswas determined by IR and Raman spectroscopy. It was establishedthat the main part of the liberated oxygen was used to formOH and to a much lesser extent H2O, and only traces ofH2, CO2 and 相似文献
950.
Zircon fission track dating and track length analysis in the high‐grade part of the Asemigawa region of the Sanbagawa belt demonstrates a simple cooling history passing through the partial annealing zone at 63.2 ± 5.8 (2 σ) Ma. Combining this age with previous results of phengite and amphibole K–Ar and 40Ar/39Ar dating gives a cooling rate of between 6 and 13 °C Myr?1, which can be converted to a maximum exhumation rate of 0.7 mm year?1 using the known shape of the P–T path. This is an order of magnitude lower than the early part of the exhumation history. In contrast, zircon fission track analyses in the low‐grade Oboke region show that this area has undergone a complex thermal history probably related to post‐orogenic secondary reheating younger than c. 30 Ma. This event may correlate with the widespread igneous activity in south‐west Japan around 15 Ma. The age of subduction‐related metamorphism in the Oboke area is probably considerably older than the generally accepted range of 77–70 Ma. 相似文献