全文获取类型
收费全文 | 2068篇 |
免费 | 416篇 |
国内免费 | 424篇 |
专业分类
测绘学 | 109篇 |
大气科学 | 367篇 |
地球物理 | 484篇 |
地质学 | 1132篇 |
海洋学 | 160篇 |
天文学 | 219篇 |
综合类 | 228篇 |
自然地理 | 209篇 |
出版年
2024年 | 9篇 |
2023年 | 34篇 |
2022年 | 105篇 |
2021年 | 121篇 |
2020年 | 71篇 |
2019年 | 98篇 |
2018年 | 124篇 |
2017年 | 93篇 |
2016年 | 92篇 |
2015年 | 104篇 |
2014年 | 118篇 |
2013年 | 139篇 |
2012年 | 117篇 |
2011年 | 104篇 |
2010年 | 96篇 |
2009年 | 110篇 |
2008年 | 86篇 |
2007年 | 99篇 |
2006年 | 74篇 |
2005年 | 79篇 |
2004年 | 52篇 |
2003年 | 48篇 |
2002年 | 57篇 |
2001年 | 76篇 |
2000年 | 92篇 |
1999年 | 107篇 |
1998年 | 90篇 |
1997年 | 88篇 |
1996年 | 68篇 |
1995年 | 57篇 |
1994年 | 48篇 |
1993年 | 55篇 |
1992年 | 29篇 |
1991年 | 31篇 |
1990年 | 28篇 |
1989年 | 18篇 |
1988年 | 17篇 |
1987年 | 18篇 |
1986年 | 19篇 |
1985年 | 13篇 |
1984年 | 6篇 |
1983年 | 3篇 |
1982年 | 5篇 |
1981年 | 2篇 |
1980年 | 1篇 |
1979年 | 1篇 |
1976年 | 2篇 |
1958年 | 4篇 |
排序方式: 共有2908条查询结果,搜索用时 31 毫秒
831.
Preparation by enzymolysis and bioactivity of iron complex of fish protein hydrolysate (Fe-FPH)from low value fish 总被引:6,自引:0,他引:6
Preparation of Fe^2+ chelate of fish protein hydrolysate (Fe-FPH) obtained from low value fish proteins was introduced and its bioactivity was studied by compound enzymolysis. The optimum conditions for hydrolysate chelating Fe^2+ are DH (degree of hydrolysis) at 5%, pH 7.0, 20℃ and 15 min chelating time for FM (material not being defatted). Four types of Fe-FPH including CA (deposit after chelating), CB (deposit in 50% of absolute ethanol solution), CC (suspended deposit in 80% of absolute ethanol solution), and CD (bottom deposit in 80% of absolute ethanol solution) were fractionated with absolute ethanol from FM. Structural analysis through infra-red spectrum revealed that Fe^2+ was combined strongly with amino-group and carboxyl-group in each chelate and each Fe^2+ could form two five-member ring structures. All of the four chelates were shown more significant antioxidative activity and can be used as natural hydrophobic and hydrophilic antioxidant. Among all the chelates, the CB possesses the most effective antioxidative activity at 92% as high as that of a-tocopherol. Among all Fe-FPHs, only CD showed the most effective antibacterial activity against Escherichia coli, Staphylococcus aureus, Salmonella typhi, and Bacillus subtilis and can be used as natural antibacterial. It provides a more effective way for utilization of low value fish proteins and key information of Fe-FPH as additive in food industry. 相似文献
832.
针对近海盆地低渗砂岩气藏含气量较低,物性下限不明显的难题,本文进行了莺歌海盆地LD10气田低渗砂岩储层流体可动性、天然气充注下限及充注过程的研究。结果表明:研究区发育3种类型储层,分别为:(1)强溶蚀-大孔粗喉可动型Ⅰ类储层,质量最好,大孔喉为优势运移通道;(2)弱溶蚀-中孔细喉部分可动型Ⅱ类储层,较高动力下储层可动流体占比高;(3)致密-微孔管束喉道束缚型Ⅲ类储层,发育极少大孔喉,含气饱和度差异较大。大孔喉分布、储层物性是控制低渗储层流体可动性的主要因素。综合考虑储层流体可动性的微观与宏观因素,认为LD10气田低渗砂岩储层气体可动性的孔喉半径下限为0.1μm,渗透率下限为0.125×10-3μm2,Ⅲ类储层致密,可作为遮挡层。天然气充注动力较低或充注初期,Ⅰ类储层为气藏聚集的有利区;当天然气充注动力充足时,Ⅰ类储层既是优势运移通道,也是气藏的优势聚集区,Ⅱ类储层为有效储层。研究成果对低渗砂岩储层有效性的判别和气藏有利区的预测具有借鉴意义。 相似文献
833.
834.
835.
In this empirical study, we compare high-resolution observations obtained with the 65-cm vacuum reflector at Big Bear Solar
Observatory (BBSO) in 2005 and with the Dunn Solar Telescope (DST) at the National Solar Observatory/Sacramento Peak (NSO/SP)
in 2006. We measure the correction of the high-order adaptive optics (AO) systems across the field of view (FOV) using the
spectral ratio technique, which is commonly employed in speckle masking imaging, and differential image motion measurements.
The AO correction is typically much larger (10′′ to 25′′) than the isoplanatic angle and can be described by a radially symmetric function with a central core and extended wings.
The full-width at half-maximum (FWHM) of the core represents a measure of the AO correction. The average FWHM values for BBSO
and NSO/SP are 23.5′′ and 18.2′′, respectively. The extended wings of the function show that the AO systems still contribute to an improved speckle reconstruction
at the periphery of the 80′′×80′′ FOV. The major differences in the level of AO correction between BBSO and NSO/SP can be explained by different contributions
of ground-layer- and free-atmosphere-dominated seeing, as well as different FOVs of the wavefront sensors. In addition, we
find an anisotropic spectral ratio in sunspot penumbrae caused by the quasi-one-dimensional nature of penumbral filaments,
which introduces a significant error in the estimation of the Fourier amplitudes during the image restoration process. 相似文献
836.
Zhu Xiaobin Xu Weihai Wang Xinting Huang Xinping Deng Liping Kang Xinglun Jiang Zhigang Ma Xuli 《中国海洋湖沼学报》2005,23(1):39-42
Heavy metals pollution in Jiaozhou Bay was studied inRuditapes. philippinarum, a bioindicator of seawater pollution. Heavy metals in soda industry wastes “white mud” were also studied. Comparison of
microwave digestion method with general digestion method revealed that microwave digestion is superior to general digestion
in precision, recovery, digestion speed and efficiency, etc. Cd concentration in HongdaoR. philippinarum samples exceeded the national standard by 0.046 mg/kg, that of Yinghai sample by 0.02 mg/kg, and that of Hongshiya sample
by 0.22 mg/kg. Sample Pb concentration in Hongshiya was found to exceed the national standard by 0.02 mg/kg. However the heavy
metals concentration inR. philippinarum near the Qingdao Alkaline Factory was complied with the standard. This was proved byPenaeus chinensis culture experiment. Therefore, the possible contamination source may come from other land areas.
This study is supported by Project 973 (NO.2002CB 412402), and key project of the Chinese Academy of Sciences (Grant NO.KJCX315W-215). 相似文献
837.
Flow direction and specific catchment area were calculated for different flow‐routing algorithms using TAPES‐G and TauDEM. A fuzzy classification was used along with eight topo–climatic attributes to delineate six landscape classes from a 10‐m USGS DEM. A series of maps and tabular outputs were produced to compare flow‐routing predictions in different parts of the study area in the Santa Monica Mountains of southern California. The matched pair t‐test was used to compare the performance of pairs of specific catchment area grids across six user‐defined fuzzy landscape classes. The results show that (1) the ‘source’ cells predicted with the D∞, DEMON, and FD8 algorithms were confined to hilltops; (2) two single flow‐routing algorithms (Rho8, D8) produced poor results; and (3) the choice of flow‐routing algorithm has potentially important consequences for the calculation of upslope contributing areas, sediment transport capacity, topographic wetness, and several other topographic indices. Copyright © 2007 John Wiley & Sons, Ltd. 相似文献
838.
中国大陆科钻一井经过1 353天的施工, 穿越地层5 158 m, 超额完成预定5 000 m的钻探目标, 于2005年5月18日园满结束. 科钻一井在中国西部"阿尔金-柴北缘-东秦岭"超高压变质带、深地幔新矿物、极端条件微生物、在线流体4个方面有重大发现, 并在印度尼西亚大地震发生前1小时, 就捕捉到异常流体信息, 一度引起地球科学工作者的特别关注和浓厚的兴趣. 相似文献
839.
尖紫蛤分布于我国东南沿海,在广东鉴江它的栖息密度为3.5~7.0个/m~2。生活在比重1.0006~1.0007,水温19.2~30.8℃,pH 值6.9~7.2.流速0.243~0.704m/s,透明度为0.38~0.60m 的咸淡水交汇处,从低潮区至水深3m 的沙滩均有分布,栖息深度一般为0.30~0.50m,最深可达0.70m。食料以底栖硅藻和有机碎屑为主。雌雄异体,繁殖期在9~10月,一周龄个体平均大小为37.5mm×16.5mm×8.2mm。在鉴江尖紫蛤的年产量约5万kg。 相似文献
840.
TEXTUAL RESEARCH ON THE MAIN SOURCE OF THE CHANGJIANG RIVER 总被引:2,自引:1,他引:1
The Chinese people began to research the main source of the Changjiang (Yangtze) River 2,400 years ago. Limited by the scientific level, they did not discover it.The Tuotuo River was determined as the main source of the Changjiang River in the 1970s. However, this was not correct, because when comparing the length of the Tuotuo River with the Dam River, the glacier length at the headwaters was added to the Tuotuo River, resulting in that the Tuotuo River is 1 km longer than the Dam River, keeping in mind that the glacier can not be regarded as part of the river.In the summer of 1986, we investigated the source of the Changjiang River, we accurately measured the length of both the Tuotuo and Dam rivers, we discovered that the Dam River was 353.1 km long, and the Tuotuo River was 346.3 km long, the Dam River thus being 6.8 km longer than the Tuotuo River. The discharge of the Dam River is 196.18 m3/ sec., 2.6 times as large as that of the Tuotuo River, that of the Tuotuo River is 75.10 m3/sec. The drainag 相似文献