全文获取类型
收费全文 | 53468篇 |
免费 | 12876篇 |
国内免费 | 18964篇 |
专业分类
测绘学 | 9786篇 |
大气科学 | 7260篇 |
地球物理 | 9963篇 |
地质学 | 34938篇 |
海洋学 | 9839篇 |
天文学 | 896篇 |
综合类 | 4435篇 |
自然地理 | 8191篇 |
出版年
2024年 | 453篇 |
2023年 | 1156篇 |
2022年 | 3201篇 |
2021年 | 3990篇 |
2020年 | 3096篇 |
2019年 | 3936篇 |
2018年 | 3280篇 |
2017年 | 3083篇 |
2016年 | 3151篇 |
2015年 | 3715篇 |
2014年 | 3594篇 |
2013年 | 4452篇 |
2012年 | 5054篇 |
2011年 | 5024篇 |
2010年 | 4964篇 |
2009年 | 4687篇 |
2008年 | 4567篇 |
2007年 | 4244篇 |
2006年 | 4116篇 |
2005年 | 3415篇 |
2004年 | 2534篇 |
2003年 | 1814篇 |
2002年 | 1875篇 |
2001年 | 1820篇 |
2000年 | 1424篇 |
1999年 | 713篇 |
1998年 | 310篇 |
1997年 | 262篇 |
1996年 | 192篇 |
1995年 | 153篇 |
1994年 | 114篇 |
1993年 | 121篇 |
1992年 | 143篇 |
1991年 | 53篇 |
1990年 | 74篇 |
1989年 | 47篇 |
1988年 | 55篇 |
1987年 | 38篇 |
1986年 | 55篇 |
1985年 | 37篇 |
1984年 | 47篇 |
1983年 | 24篇 |
1982年 | 23篇 |
1981年 | 14篇 |
1979年 | 36篇 |
1978年 | 15篇 |
1977年 | 11篇 |
1976年 | 11篇 |
1957年 | 20篇 |
1954年 | 29篇 |
排序方式: 共有10000条查询结果,搜索用时 31 毫秒
91.
Concentrations of trace elements and heavy metals (Al, As, Ba, Cd, Co, Cr, Cu, Fe, Hg, Mn, Ni, Pb, Sb, Se, Sr, V and Zn) in
the Danjiangkou Reservoir, the water source area of the Middle Route of China’s interbasin South to North Water Transfer Project,
were analyzed using an Inductively Coupled Plasma Atomic Emission Spectrometer (ICP-AES) and compared with the national and
international standards for drinking water. The results indicated that concentrations of As, Pb, Sb and Se in the Reservoir
exceeded the standards and they would pose health risk for residents in the region and the water receiving areas of the interbasin
water transfer project. Spatial and temporal variability of the trace elements and heavy metals in the Reservoir implies their
mixed sources of natural processing and anthropogenic activities in the upper drainage of the Reservoir. The research results
would help develop water resource management and conservation strategy for the interbasin water transfer project. 相似文献
92.
93.
A. van Geen K. Radloff Z. Aziz Z. Cheng M.R. Huq K.M. Ahmed B. Weinman S. Goodbred H.B. Jung Y. Zheng M. Berg P.T.K. Trang L. Charlet J. Metral D. Tisserand S. Guillot S. Chakraborty A.P. Gajurel B.N. Upreti 《Applied Geochemistry》2008
One of the reasons the processes resulting in As release to groundwater in southern Asia remain poorly understood is the high degree of spatial variability of physical and chemical properties in shallow aquifers. In an attempt to overcome this difficulty, a simple device that collects groundwater and sediment as a slurry from precisely the same interval was developed in Bangladesh. Recently published results from Bangladesh and India relying on the needle-sampler are augmented here with new data from 37 intervals of grey aquifer material of likely Holocene age in Vietnam and Nepal. A total of 145 samples of filtered groundwater ranging in depth from 3 to 36 m that were analyzed for As (1–1000 μg/L), Fe (0.01–40 mg/L), Mn (0.2–4 mg/L) and S (0.04–14 mg/L) are compared. The P-extractable (0.01–36 mg/kg) and HCl-extractable As (0.04–36 mg/kg) content of the particulate phase was determined in the same suite of samples, in addition to Fe(II)/Fe ratios (0.2–1.0) in the acid-leachable fraction of the particulate phase. Needle-sampler data from Bangladesh indicated a relationship between dissolved As in groundwater and P-extractable As in the particulate phase that was interpreted as an indication of adsorptive equilibrium, under sufficiently reducing conditions, across 3 orders of magnitude in concentrations according to a distribution coefficient of 4 mL/g. The more recent observations from India, Vietnam and Nepal show groundwater As concentrations that are often an order of magnitude lower at a given level of P-extractable As compared to Bangladesh, even if only the subset of particularly reducing intervals characterized by leachable Fe(II)/Fe >0.5 and dissolved Fe >0.2 mg/L are considered. Without attempting to explain why As appears to be particularly mobile in reducing aquifers of Bangladesh compared to the other regions, the consequences of increasing the distribution coefficient for As between the particulate and dissolved phase to 40 mL/g for the flushing of shallow aquifers of their initial As content are explored. 相似文献
94.
95.
储量计算与勘探线剖面计算机辅助成图一体化 总被引:1,自引:0,他引:1
引入关系教据库和GIS自动编图技术,改进传统的垂直断面法储量计算过程,可实现矿山储量计算和勘探线剖面计算机辅助成图的同步和一体化.在共用的矿山地矿信息点源数据库平台基础上.通过设置本地数据库,形成GIS操作时的外挂属性库,用来保存储量计算和机助成图过程中的动态数据.储量计算中的每一步骤中,用户以GIS为操作界面来同步实现对空间图元和外挂属性的操作,这其中对工程样段组合、矿体边界圈连和剖面制图综合等开发了大量自动化辅助功能.储量计算的各步完成后,勘探线剖面图和储量报表即同步生成.这种改进的工作方式实现了垂直断面法全过程的计算机辅助化,将提高这一传统方法的应用效率. 相似文献
96.
97.
98.
99.
100.
利用MC-ICPMS对水样中硼同位素比值的测定 总被引:2,自引:0,他引:2
采用Amberlite IRA-743硼特效树脂进行水溶液中硼的分离,对洗脱液温度、体积以及树脂动态交换容量等进行了实验.它的动态交换容量为4.2 mg/g,在室温条件下,用5 mL 2%HNO3可定量从树脂柱上洗脱被吸附的硼.采用Neptune型MC-ICPMS进行硼同位素组成的测定,1 μg/g浓度的硼标准溶液获得放大的信号强度约为0.7V,硼的化学分离过程有效避免了质量歧视效应.实验结果表明,MC-ICPMS对硼存在严重的记忆效应,通过采用5%HNO3-(1%HNO3 0.1%HF)-H2O-20%NH3H2O-H2O的交替清洗方法,可在约1 h内消除硼的记忆效应.在最佳仪器条件下,对含有250 ng硼的水样,可以快速测定硼的同位素组成,占δ11B测定的绝对偏差为0.4‰-0.5‰(SD). 相似文献