全文获取类型
收费全文 | 1448篇 |
免费 | 215篇 |
国内免费 | 429篇 |
专业分类
测绘学 | 32篇 |
大气科学 | 376篇 |
地球物理 | 283篇 |
地质学 | 581篇 |
海洋学 | 52篇 |
天文学 | 386篇 |
综合类 | 59篇 |
自然地理 | 323篇 |
出版年
2024年 | 5篇 |
2023年 | 21篇 |
2022年 | 64篇 |
2021年 | 56篇 |
2020年 | 71篇 |
2019年 | 74篇 |
2018年 | 63篇 |
2017年 | 53篇 |
2016年 | 80篇 |
2015年 | 66篇 |
2014年 | 78篇 |
2013年 | 94篇 |
2012年 | 77篇 |
2011年 | 99篇 |
2010年 | 80篇 |
2009年 | 115篇 |
2008年 | 97篇 |
2007年 | 90篇 |
2006年 | 102篇 |
2005年 | 87篇 |
2004年 | 90篇 |
2003年 | 66篇 |
2002年 | 51篇 |
2001年 | 39篇 |
2000年 | 54篇 |
1999年 | 55篇 |
1998年 | 40篇 |
1997年 | 37篇 |
1996年 | 27篇 |
1995年 | 39篇 |
1994年 | 25篇 |
1993年 | 19篇 |
1992年 | 13篇 |
1991年 | 14篇 |
1990年 | 11篇 |
1989年 | 11篇 |
1988年 | 10篇 |
1987年 | 3篇 |
1986年 | 4篇 |
1985年 | 1篇 |
1984年 | 4篇 |
1983年 | 1篇 |
1982年 | 2篇 |
1981年 | 1篇 |
1979年 | 1篇 |
1978年 | 1篇 |
1954年 | 1篇 |
排序方式: 共有2092条查询结果,搜索用时 15 毫秒
61.
Achieving long-term climate mitigation goals in Japan faces several challenges, starting with the uncertain nuclear power policy after the 2011 earthquake, the uncertain availability and progress of energy technologies, as well as energy security concerns in light of a high dependency on fuel imports. The combined weight of these challenges needs to be clarified in terms of the energy system and macroeconomic impacts. We applied a general equilibrium energy economic model to assess these impacts on an 80% emission reduction target by 2050 considering several alternative scenarios for nuclear power deployment, technology availability, end use energy efficiency, and the price of fossil fuels. We found that achieving the mitigation target was feasible for all scenarios, with considerable reductions in total energy consumption (39%–50%), higher shares of low-carbon sources (43%–72% compared to 15%), and larger shares of electricity in the final energy supply (51%–58% compared to 42%). The economic impacts of limiting nuclear power by 2050 (3.5% GDP loss) were small compared to the lack of carbon capture and storage (CCS) (6.4% GDP loss). Mitigation scenarios led to an improvement in energy security indicators (trade dependency and diversity of primary energy sources) even in the absence of nuclear power. Moreover, preliminary analysis indicates that expanding the range of renewable energy resources can lower the macroeconomic impacts of the long term target considerably, and thus further in depth analysis is needed on this aspect.
Key policy insights
For Japan, an emissions reduction target of 80% by 2050 is feasible without nuclear power or CCS.
The macroeconomic impact of such a 2050 target was largest without CCS, and smallest without nuclear power.
Energy security indicators improved in mitigation scenarios compared to the baseline.
62.
Summary This article discusses methods for providing a reliable forecast of the gas-dynamic behaviour of coal seams and adjacent strata. It shows that the ash content of carbonaceous materials determines their sorptive capacity (a universally applicable expression is presented for calculating the methane content of a sample) whilst the rate of methane desorption is a function of the degree of geological disturbance, i.e. the number and distribution of the macropores and fractures in the sample. This effect becomes less important as the sample ash content increases and effectively disappears above a certain value for high permeability materials.The quantity sorption-kinetic characteristic is shown to be a reliable means for quantifying methane emission behaviour. Large variations in coal sorption-kinetic characteristics are shown to occur over short distances as the geological structure of the seam changes, both vertically and horizontally. Consequently, sorption-kinetic characteristics may be used to predict the general level of methane emission from the seam and to quantify the risk of outburst inherent in disturbed areas of coal seams. 相似文献
63.
Emissions of N2O, CH4, and CO2 from soils at two sites in the tropical savanna of central Venezuela were determined during the dry season in February 1987. Measured arithmetic mean fluxes of N2O, CH4, and CO2 from undisturbed soil plots to the atmosphere were 2.5×109, 4.3×1010, and 3.0×1013 molecules cm-2 s-1, respectively. These fluxes were not significantly affected by burning the grass layer. Emissions of N2O increased fourfold after simulated rainfall, suggesting that production of N2O in savanna soils during the rainy season may be an important source for atmospheric N2O. The CH4 flux measurements indicate that these savanna soils were not a sink, but a small source, for atmospheric methane. Fluxes of CO2 from savanna soils increased ninefold two hours after simulated rainfall, and remained three times higher than normal after 16 hours. More research is needed to clarify the significance of savannas in the global cycles of N2O, CH4, CO2, and other trace gases, especially during the rainy season. 相似文献
64.
通过一些典型节理化岩体模型变形破坏过程中的AE事件的定位分析和破裂面形态的研究,探讨了不连续岩体变形破坏过程的空间演化特征。结果表明:单节理岩体变形破坏的空间动态特征受节理角控制,随着节理角从小变大,岩体的变形破坏方式从AE在局部集中的突发式失稳向AE弥漫于整个节理层面的类似渐进式的破坏形式发展;失稳破裂面从简单的整体状岩-膏界面变成形态复杂的层内复合破裂面;复杂节理岩体的变形破坏受一条(组)主导构造控制,强AE事件集中发生在不连续构造交汇部位,在包含多条主构造的岩体模型中最终失稳破裂面只与最后阶段AE的位置一致。 相似文献
65.
66.
给出了判断断裂之间有无相互作用的力学判据.为了研究断裂间相互作用对地球物理场的影响,对若干类常见断裂系的扩展过程及扩展过程中的声发射b 值和波速变化进行了模拟实验研究.结果表明,各种断裂系由于相互作用的不同,扩展过程,声发射b 值和波速度化均有明显的不同特征. 相似文献
67.
Jean Servant Georges Kouadio Bernard Cros Robert Delmas 《Journal of Atmospheric Chemistry》1991,12(4):367-380
In the tropical rain forests of the Congo during the dry season, from June to September 1987, carboxylic acid partial pressures (P
gas) in the air above the canopy, at ground level, and at the boundary layer, were estimated from water samples such as fog and rainwater. The concentrations of these acids were also measured in the sap of tree leaves. Tree leaves act as a sink or as a source if the acid P
gas is greater of lower than the acid concentrations in molecular form in sap. For each of these soluble gases, there is a value of P
gas where the exchange is nul. This is called the compensation point. Values of the compensation point for some tree leaves were evaluated according to Henry's law. Henry's law coefficients at ppm levels were redetermined for formic (HCOOH), acetic (CH3COOH), propionic (CH3CH2COOH), and isobutyric (CH3CH(CH3)COOH) acids.By comparison of P
gas and compensation points, it is concluded that the forest was a potential source for these acids. The soil-or the litter-acts as a significant source of a carboxylic acid of C3 or C4 atoms in the aliphatic chain. This carboxylic acid, not yet fully characterized, could play an important role in the rain acidity in forested zones of the equatorial areas.The direct emission of these carboxylic acids by vegetation was the main source in the boundary layer above the forest. The average emissions were 3.1×109, 7.8×109, and 8.4×109 molecules cm-2 s-1 for HCOOH, CH3COOH, and CH3CH2COOH, respectively. The savanna is an exogenous source of HCOOH and CH3CH2COOH during moderately rainy days for 30% of the time. The ozonolysis of isoprene seems to be a small source of HCOOH. 相似文献
68.
小试金光谱法同时测定地质样品中超痕量铂钯金 总被引:9,自引:2,他引:9
用小试金光谱法同时测定地质样品中低至0·x×10~(-9)级Pt、Pd和Au,包括先用小试金熔炼和灰吹预富集10g样品中的Pt、Pd和Au于1mg的银合粒中,然后把银合粒装入石墨电极同锇-锑合金(ω(Os)=0.06)一起激发,以发射光谱法同时测定它们的含量。小试金法类同经典火试金,但采用纯化过的2PbCO_3·Pb(OH)_2(Pt、Pd和Au的含量皆<0.05 ×10~(-9)代替商品PbO,并且用50ml高铝坩埚在950℃进行熔炼。方法的检出限是Pt0.2×10~(-9),Pd和Au均为0.1×10~(-9)。对于3×10~(-9)的含量,全过程的相对标准偏差(%)是Pt15,Pd11,Au9。此法快速、简便,适用于地质物料的分析。 相似文献
69.
70.
We began continuous monitoring of H2 concentration in soil along the San Andreas and Calaveras faults in central California in December 1980, using small H2/O2 fuel-cell sensors. Ten monitoring stations deployed to date have shown that anomalous H2 emissions take place occasionally in addition to diurnal changes. Among the ten sites, the Cienega Winery site has produced data that are characterized by very small diurnal changes, a stable baseline, and remarkably distinct spike-like H2 anomalies since its installation in July 1982. A major peak appeared on 1–10 November 1982, and another on 3 April 1983, and a medium peak on 1 November 1983. The occurrences of these peaks coincided with periods of very low seismicity within a radius of 50 km from the site. In order to methodically assess how these peaks are related to earthquakes, three H2 degassing models were examined. A plausible correlational pattern was obtained by using a model that (1) adopts a hemicircular spreading pattern of H2 along an incipient fracture plane from the hypocenter of an earthquake, (2) relies on the FeO–H2O reaction for H2 generation, and (3) relates the accumulated amount of H2 to the mass of serpentinization of underlying ophiolitic rocks; the mass was tentatively assumed to be proportional to the seismic energy of the earthquake. 相似文献