全文获取类型
收费全文 | 2900篇 |
免费 | 49篇 |
国内免费 | 49篇 |
专业分类
测绘学 | 71篇 |
大气科学 | 357篇 |
地球物理 | 599篇 |
地质学 | 989篇 |
海洋学 | 157篇 |
天文学 | 677篇 |
综合类 | 15篇 |
自然地理 | 133篇 |
出版年
2021年 | 15篇 |
2020年 | 15篇 |
2019年 | 16篇 |
2018年 | 49篇 |
2017年 | 41篇 |
2016年 | 60篇 |
2015年 | 35篇 |
2014年 | 63篇 |
2013年 | 107篇 |
2012年 | 82篇 |
2011年 | 115篇 |
2010年 | 89篇 |
2009年 | 121篇 |
2008年 | 133篇 |
2007年 | 115篇 |
2006年 | 93篇 |
2005年 | 120篇 |
2004年 | 99篇 |
2003年 | 83篇 |
2002年 | 93篇 |
2001年 | 80篇 |
2000年 | 80篇 |
1999年 | 91篇 |
1998年 | 89篇 |
1997年 | 73篇 |
1996年 | 72篇 |
1995年 | 70篇 |
1994年 | 51篇 |
1993年 | 51篇 |
1992年 | 47篇 |
1991年 | 56篇 |
1990年 | 44篇 |
1989年 | 43篇 |
1988年 | 21篇 |
1987年 | 32篇 |
1986年 | 35篇 |
1985年 | 31篇 |
1984年 | 52篇 |
1983年 | 33篇 |
1982年 | 38篇 |
1981年 | 42篇 |
1980年 | 39篇 |
1979年 | 33篇 |
1978年 | 32篇 |
1977年 | 20篇 |
1976年 | 26篇 |
1975年 | 15篇 |
1974年 | 30篇 |
1973年 | 17篇 |
1971年 | 22篇 |
排序方式: 共有2998条查询结果,搜索用时 328 毫秒
611.
Optimum reaction time, performance and exhaust emissions of biodiesel produced by microwave irradiation 总被引:3,自引:3,他引:0
A. A. Refaat S. T. El Sheltawy K. U. Sadek 《International Journal of Environmental Science and Technology》2008,5(3):315-322
While transesterification is well established, there remain considerable inefficiencies in existing transesterification processes. In this study an alternative energy stimulant, “microwave irradiation” was used for the production of the alternative energy source, biodiesel. The optimum parametric conditions obtained from the conventional technique were applied using microwave irradiation in order to compare both systems. The results showed that application of radio frequency microwave energy offers a fast, easy route to this valuable biofuel with advantages of enhancing the reaction rate and improving the separation process. The methodology allows for the use of high free fatty acid content feedstock, including used cooking oil; hence it helps to reduce the cost of production which constitutes a major hurdle towards widespread commercialization of biodiesel. The study also showed that the optimum reaction time for microwave-enhanced biodiesel production should be highly respected. Exceeding the optimum reaction time will lead to deterioration of both biodiesel yield and purity. This paper also reported the performance and exhaust emissions from a diesel engine when fuelled with a petroleum diesel fuel and two different biodiesel fuels; one obtained by the conventional technique and the other by microwave irradiation. It was concluded that microwave-enhanced biodiesel is not, at least, inferior to that produced by the conventional technique. 相似文献
612.
L. A. K. Reddy U. C. Kulshrestha J. Satyanarayana Monika J. Kulshrestha K. Krishna Moorthy 《Journal of Earth System Science》2008,117(1):345-352
For the first time, chemical characterization of PM10 aerosols was attempted over the Bay of Bengal (BoB) and Arabian Sea (AS) during the ICARB campaign. Dominance of SO 4 2? , NH 4 + and NO 3 ? was noticed over both the regions which indicated the presence of ammonium sulphate and ammonium nitrate as major water soluble particles playing a very important role in the radiation budget. It was observed that all the chemical constituents had higher concentrations over Bay of Bengal as compared to Arabian Sea. Higher concentrations were observed near the Indian coast showing influence of landmass indicating that gaseous pollutants like SO2, NH3 and NO x are transported over to the sea regions which consequently contribute to higher SO 4 2? , NH 4 + and NO 3 ? aerosols respectively. The most polluted region over BoB was 13°?19°N and 70°?90°E while it was near 11°N and 75°E over AS. Although the concentrations were higher over Bay of Bengal for all the chemical constituents of PM10 aerosols, per cent non-sea salt (nss) fraction (with respect to Na) was higher over Arabian Sea. Very low Ca2+ concentration was observed at Arabian Sea which led to higher atmospheric acidity as compared to BoB. Nss SO 4 2? alone contributed 48% of total water soluble fraction over BoB as well as AS. Ratios SO 4 2? /NO ? 3 over both the regions (7.8 and 9 over BoB and AS respectively) were very high as compared to reported values at land sites like Allahabad (0.63) and Kanpur (0.66) which may be due to very low NO.3 over sea regions as compared to land sites. Air trajectory analysis showed four classes: (i) airmass passing through Indian land, (ii) from oceanic region, (iii) northern Arabian Sea and Middle East and (iv) African continent. The highest nss SO 4 2? was observed during airmasses coming from the Indian land side while lowest concentrations were observed when the air was coming from oceanic regions. Moderate concentrations of nss SO2. 4 were observed when air was seen moving from the Middle East and African continent. The pH of rainwater was observed to be in the range of 5.9–6.5 which is lower than the values reported over land sites. Similar feature was reported over the Indian Ocean during INDOEX indicating that marine atmosphere had more free acidity than land atmosphere. 相似文献
613.
Timing of the last deglaciation in Lithuania 总被引:1,自引:1,他引:0
VINCENT R. RINTERKNECHT ALBERTAS BITINAS PETER U. CLARK GRANT M. RAISBECK FRANÇOISE YIOU EDWARD J. BROOK 《Boreas: An International Journal of Quaternary Research》2008,37(3):426-433
Boulders from the Grūda Moraine, which is associated with the maximum extent of the Scandinavian Ice Sheet (SIS) during the last glaciation, and the Baltija (also referred to as the South Lithuanian), the Middle and North Lithuanian moraines, which are associated with recessional stages of the SIS, were sampled for surface exposure dating using 10 Be. By combining these data with existing radiocarbon ages, we developed a chronology for the retreat of the SIS margin in Lithuania. Our new 10 Be ages suggest that the SIS margin began to retreat from its maximum extent at 18.3 ± 0.8 10 Be kyr. Based on a probable correlation of the Baltija Moraine with the Pomeranian Moraine in Poland, we infer that the Baltija Moraine was formed following a re-advance of the SIS margin. The ice margin retreated from the Baltija position at 14.0 ± 0.4 10 Be kyr. The SIS-margin retreat paused at least two more times to form the Middle Lithuanian Moraine at 13.5 ± 0.6 10 Be kyr and the North Lithuanian Moraine (tentatively correlated to the Pajūris Moraine) at 13.3 ± 0.7 10 Be kyr. Subsequent ice-margin retreat from the North Lithuanian Moraine represented the final deglaciation of Lithuania. Direct dating of these moraines better constrains the relation of ice-margin positions in Lithuania to those in adjacent countries as well as the SIS response to climate change. 相似文献
614.
Monitored natural attenuation can be a viable option for remediation of groundwater contamination by BTEX compounds. Under
the field conditions, the rate of contaminant mass attenuation through natural processes, such as biodegradation, to a large
extent affected by the groundwater flow regime, which is primarily controlled by the aquifer heterogeneity. Numerical simulation
techniques were used to describe quantitatively the relationship between biodegradation rate of BTEX and aquifer heterogeneity.
Different levels of aquifer heterogeneity were described by random hydraulic conductivity fields (K) having different statistical
parameters, the coefficient of variation (CV) and the correlation length (h). The Turning Bands Algorithm was used to generate
such K fields. Visual MODFLOW/RT3D was used to simulate the fate and transport of dissolved BTEX plume within heterogeneous
aquifers. The multispecies reactive transport approach described BTEX degradation using multiple terminal electron-accepting
processes. First-order biodegradation rate constants were calculated from simulated BTEX plumes in heterogeneous flow fields.
The results showed that aquifer heterogeneity significantly affected biodegradation rate; it decreased with increasing CV
when h was in the range of up to 12 m, whereas it increased with increasing CV when h was greater than about 12 m. For well characterized aquifers, this finding could be of great value in assessing the effectiveness
of natural attenuation during feasibility studies at BTEX contaminated sites. 相似文献
615.
Salinity mapping of coastal groundwater aquifers using hydrogeochemical and geophysical methods: a case study from north Kelantan,Malaysia 总被引:2,自引:0,他引:2
Integrated hydrogeochemical and geophysical methods were used to study the salinity of groundwater aquifers along the coastal
area of north Kelantan. For the hydrogeochemical investigation, analysis of major ion contents of the groundwater was conducted,
and other chemical parameters such as pH and total dissolved solids were also determined. For the geophysical study, both
geoelectrical resistivity soundings and reflection seismic surveys were conducted to determine the characteristics of the
subsurface and groundwater contained within the aquifers. The pH values range from 6.2 to 6.8, indicating that the groundwater
in the study area is slightly acidic. Low content of chloride suggests that the groundwater in the first aquifer is fresh,
with an average concentration of about 15.8 mg/l and high geoelectrical resistivity (>45 ohm m). On the other hand, the groundwater
in the second aquifer is brackish, with chloride concentration ranging from 500 mg/l to 3,600 mg/l and very low geoelectrical
resistivity (<45 ohm m) as well as high concentration of total dissolved solids (>1,000 mg/l). The groundwater in the third
aquifer is fresh, with chloride concentrations generally ranging from 2 mg/l to 210 mg/l and geoelectrical resistivity of
greater than 45 ohm m. Fresh and saltwater interface in the first aquifer is generally located directly in the area of the
coast, but, for the second aquifer, both hydrogeochemical and geoelectrical resistivity results indicate that the fresh water
and saltwater interface is located as far as 6 km from the beach. The considerable chloride ion content initially suggests
that the salinity of the groundwater in the second aquifer is probably caused by the intrusion of seawater. However, continuous
monitoring of the chloride content of the second aquifer indicated no significant changes with time, from which it can be
inferred that the salinity of the groundwater is not affected by seasonal seawater intrusion. Schoeller diagrams illustrate
that sulphate concentrations of the groundwater of the second aquifer are relatively low compared to those of the recent seawater.
Therefore, this result suggests that the brackish water in the second aquifer is probably from ancient seawater that was trapped
within the sediments for a long period of time, rather than due to direct seawater intrusion. 相似文献
616.
G. S. Collins N. Artemieva K. Wünnemann P. A. Bland W. U. Reimold C. Koeberl 《地学学报》2008,20(2):165-168
In a provocative paper Gasperini et al. (2007) suggest that Lake Cheko, a ~300‐m‐wide lake situated a few kilometres downrange from the assumed epicentre of the 1908 Tunguska event, is an impact crater. In this response, we present several lines of observational evidence that contradicts the impact hypothesis for the lake’s origin: un‐crater‐like aspects of the lake morphology, the lack of impactor material in and around the lake, and the presence of apparently unaffected mature trees close to the lake. We also show that a tensile strength of 10–40 MPa is required for an asteroid fragment to traverse the Earth’s atmosphere and reach the surface intact and with sufficient velocity to excavate a crater the size of Lake Cheko. Inferred tensile strengths of large stony meteorites during atmospheric disruption are 10–100 times lower. We therefore conclude that Lake Cheko is highly unlikely to be an impact crater. 相似文献
617.
U. Heimhofer S. P. Hesselbo R. D. Pancost D. M. Martill P. A. Hochuli J. V. P. Guzzo 《地学学报》2008,20(5):347-354
Organic matter from two black shale units in the Santana Formation, Araripe Basin, NE Brazil, has been studied to investigate the palaeoenvironmental conditions prevailing during formation of this famous fossil lagerstätte. Optical and organic geochemical analyses indicate a predominantly autochthonous source with very limited terrestrial input. Molecular fossils of green sulphur bacteria show that the water column was stratified with euxinic conditions reaching up into the photic zone. Thus, mass‐mortality events of fish may be explained by intrusion of sulphide‐containing waters. The low quantity of material derived from land plants in this interior marine setting is interpreted to reflect a consequence of enhanced marine productivity, resulting in dilution of the continental signal. Different depositional scenarios are proposed to explain our findings including (1) coastal erosion and nutrient leaching during a transgressive event, (2) intrusion of euxinic waters during an early Albian OAE or (3) an estuarine circulation pattern associated with thermohaline stratification. 相似文献
618.
K.U. Rempel A.E. Williams-Jones A.A. Migdisov 《Geochimica et cosmochimica acta》2008,72(13):3074-3083
The solubility and speciation of the assemblage MoO2-MoO3 in water vapour were investigated in experiments conducted at 350 °C, Ptotal from 59 to 160 bar and fHCl from 0 to 3.4 bar (0-2.0 mol%). Measured solubility at these conditions ranges from 22 to 2500 ppm (∑fMo from 4.4 × 10−4 to 6.5 × 10−2 bar). The concentration of Mo in the vapour at fHCl below 0.1 bar is similar to that in pure water vapour, but increases by two orders of magnitude at fHCl above 0.1 bar. The fugacity of gaseous Mo species is independent of chloride concentration at fHCl below 0.1 bar, but increases with increasing fHCl above this pressure. The dominant Mo species at fHCl below 0.1 bar is interpreted to be the same as it is in pure water vapour, and to form as a result of the reaction
(A1) 相似文献
619.
A new method for three-carrier GNSS ambiguity resolution 总被引:2,自引:2,他引:0
U. Fernández-Plazaola T. M. Martín-Guerrero J. T. Entrambasaguas 《Journal of Geodesy》2008,82(4-5):269-278
A new method for resolving the carrier-phase integer ambiguity in Global Navigation Satellite Systems (GNSS) is presented:
the MOdified Cholesky factorization for Ambiguity (MOCA) resolution. The characteristics and features of this method are described
and results obtained using a software simulator and an emulator are presented to validate its efficiency. The results are
then compared to those obtained using another existing method and good performance of the MOCA method in new GNSS systems
is shown. Furthermore, the proposed method yields accurate results even when short time spans are used or when there are poor
estimations of measurement error, making it immune to non-ideal conditions and ultimately a practical solution for real applications. 相似文献
620.
C. Warneke R. Holzinger A. Hansel A. Jordan W. Lindinger U. Pöschl J. Williams P. Hoor H. Fischer P. J. Crutzen H. A. Scheeren J. Lelieveld 《Journal of Atmospheric Chemistry》2001,38(2):167-185
Airborne measurements of volatile organic compounds (VOC) were performed overthe tropical rainforest in Surinam (0–12 km altitude,2°–7° N, 54°–58° W) using the proton transferreaction mass spectrometry (PTR-MS) technique, which allows online monitoringof compounds like isoprene, its oxidation products methyl vinyl ketone,methacrolein, tentatively identified hydroxy-isoprene-hydroperoxides, andseveral other organic compounds. Isoprene volume mixing ratios (VMR) variedfrom below the detection limit at the highest altitudes to about 7 nmol/molin the planetary boundary layer shortly before sunset. Correlations betweenisoprene and its product compounds were made for different times of day andaltitudes, with the isoprene-hydroperoxides showing the highest correlation.Model calculated mixing ratios of the isoprene oxidation products using adetailed hydrocarbon oxidation mechanism, as well as the intercomparisonmeasurement with air samples collected during the flights in canisters andlater analysed with a GC-FID, showed good agreement with the PTR-MSmeasurements, in particular at the higher mixing ratios.Low OH concentrations in the range of 1–3 × 105molecules cm-3 averaged over 24 hours were calculated due to lossof OH and HO2 in the isoprene oxidation chain, thereby stronglyenhancing the lifetime of gases in the forest boundary layer. 相似文献