全文获取类型
收费全文 | 18134篇 |
免费 | 1609篇 |
国内免费 | 900篇 |
专业分类
化学 | 17063篇 |
晶体学 | 332篇 |
力学 | 22篇 |
综合类 | 13篇 |
数学 | 6篇 |
物理学 | 1782篇 |
综合类 | 1425篇 |
出版年
2024年 | 17篇 |
2023年 | 164篇 |
2022年 | 480篇 |
2021年 | 556篇 |
2020年 | 572篇 |
2019年 | 453篇 |
2018年 | 382篇 |
2017年 | 359篇 |
2016年 | 664篇 |
2015年 | 680篇 |
2014年 | 739篇 |
2013年 | 1339篇 |
2012年 | 859篇 |
2011年 | 769篇 |
2010年 | 818篇 |
2009年 | 860篇 |
2008年 | 871篇 |
2007年 | 907篇 |
2006年 | 899篇 |
2005年 | 817篇 |
2004年 | 871篇 |
2003年 | 816篇 |
2002年 | 1517篇 |
2001年 | 472篇 |
2000年 | 410篇 |
1999年 | 390篇 |
1998年 | 342篇 |
1997年 | 296篇 |
1996年 | 476篇 |
1995年 | 454篇 |
1994年 | 201篇 |
1993年 | 167篇 |
1992年 | 189篇 |
1991年 | 108篇 |
1990年 | 90篇 |
1989年 | 106篇 |
1988年 | 93篇 |
1987年 | 61篇 |
1986年 | 49篇 |
1985年 | 58篇 |
1984年 | 41篇 |
1983年 | 27篇 |
1982年 | 21篇 |
1981年 | 23篇 |
1980年 | 14篇 |
1979年 | 10篇 |
1978年 | 14篇 |
1975年 | 9篇 |
1971年 | 48篇 |
1970年 | 25篇 |
排序方式: 共有10000条查询结果,搜索用时 31 毫秒
81.
王克强 《宁夏大学学报(自然科学版)》1992,13(2):47-51
本文提出了一种计算碱土金属化合物品格能的新方法——离子贡献法。该方法只需本文提供的离子贡献值即可计算得到晶格能,而不需要任何实验数据。对碱土金属氧化物、卤化物、氢化物、氢氧化物、硫酸盐、碳酸盐、磷酸盐等十九类75种化合物的计算结果表明,计算值与实验值之间的一致性令人满意,计算结果的平均相对误差仅0.79%,小于目前已有的所有方法的计算误差。 相似文献
82.
83.
Ivan Horvath Viera Luptakova 《Journal of inclusion phenomena and macrocyclic chemistry》1991,11(3):257-265
Benzothiazolium compounds exhibit pronounced antimicrobial activities and stimulation effects on plant growth. When applied to fields they can pollute soil colloids. In contact with the soil they can potentially interact with a clay fraction giving rise to clay organocomplexes. Model intercalation complexes were prepared using monoionic montmorillonite and a variety of water-soluble benzothia zolium salts. It was found that the adsorption into the silicate interlayer space proceeds via a cation exchange process. Substantial differences were observed in the extent of the reaction between non-substituted andN-substituted species. The sulphur atoms present in the benzothiazolium molecules repel the surface oxygen atom: this is considered to be the factor responsible for prevention of insertion of the guest cations perpendicular to the layers. 相似文献
84.
85.
86.
Meng F. Q. Lu M. K. Yang Z. H. Zeng H. 《Journal of Thermal Analysis and Calorimetry》1998,52(2):609-613
The thermal behavior of two new non-linear optical (NLO) materials, urea-(D) tartaric acid (UDT) and urea-(DL) tartaric acid
(UDLT) were studied by using DSC, TG and TMA. The results show that: 1) The two crystals have different melting points but
similar decomposition temperatures due to the influences of intermolecular forces, which is attributed to the stereo effects
of (D)-tartaric and (DL)-tartaric acid molecules; 2) There was only thermal expansion and no thermal contraction when the
UDT and UDLT crystals were heated; 3) There was no phase transition within the measured temperature range; 4) The thermal
expansion of the UDT and UDLT crystals shows a small anisotropy; 5) The specific heats of UDT and UDLT change linearly with
temperature in the measured temperature range and the value for UDT is 1.321 J g-1 K-1 at 320 K while the specific heat of
UDLT is 1.357 J g-1 K-1 at the same temperature.
This revised version was published online in July 2006 with corrections to the Cover Date. 相似文献
87.
Tertiary N-ethylamines were converted into amide derivatives by reactions with sulfur monochloride and DABCO at 0 °C. Depending on the
nature of the substituents in the amine, the reaction can be accompanied by unexpected transformations.
Published in Russian in Izvestiya Akademii Nauk. Seriya Khimicheskaya, No. 6, pp. 1135–1140, June, 2007. 相似文献
88.
The influence of humic substances on sorption and methylation processes for inorganic- and organotin species is presented. Four sediment samples from different locations of the Rivers Elbe, Mulde and Spittelwasser, Germany, with different organotin and humic contents were selected to extract the humic and fulvic acids. The various fractions—the original sediment, the humic acid, the fulvic acid and the residual sediment—were analysed for their organotin content. The individual buyltin species show quite different distribution patterns. Monobutyltin is found mostly associated with humic acids. Dibutyltin shows a nonunique behaviour. At low total organotin content, dibutyltin is found bonded to humic and fulvic acids, whereas at high organotin content dibutyltin is distributed more with the residual sediment. Most of the tributyltin remains in the sediment unextracted; only small quantities of it are in the fulvic acid fraction. Tetrabutyltin is only in the humic acid fraction when it binds to humic matter; it mostly remains in the sediment. General observations indicate that ionic butyltin species bind to fulvic acids whereas the non-polar tetrabutyltin is not found in the fulvic acid fractions in any of the samples. The appearance of monomethyl- and dimethyl-tin species in the humic and fulvic acid fractions after the alkaline extraction was surprising. There is a correlation between the humic content of the sample and the formation of methyltin species. Evidence is provided by experiments that humic substances act as methylation agents. 相似文献
89.
Summary. The photocycloaddition of aldehydes and α-ketoesters to 2,5-dimethyl-4-isobutyloxazole leads to bicyclic oxetanes with high
to moderate (exo) diastereoselectivity that can be easily ring-opened to give α-amino-β-hydroxyketones. 相似文献
90.
李兴春 《科技情报开发与经济》2005,15(19):279-280
着重从六氟化硫气体自身特性出发,结合引黄工程使用六氟化硫气体组合开关的实际情况,探讨了六氟化硫气体在引黄工程中的安全运行管理问题. 相似文献