全文获取类型
收费全文 | 25548篇 |
免费 | 4852篇 |
国内免费 | 5108篇 |
专业分类
化学 | 19350篇 |
晶体学 | 723篇 |
力学 | 1404篇 |
综合类 | 588篇 |
数学 | 2984篇 |
物理学 | 10459篇 |
出版年
2024年 | 35篇 |
2023年 | 341篇 |
2022年 | 578篇 |
2021年 | 765篇 |
2020年 | 920篇 |
2019年 | 1025篇 |
2018年 | 816篇 |
2017年 | 931篇 |
2016年 | 1078篇 |
2015年 | 1260篇 |
2014年 | 1424篇 |
2013年 | 1913篇 |
2012年 | 2388篇 |
2011年 | 2373篇 |
2010年 | 1987篇 |
2009年 | 1943篇 |
2008年 | 2209篇 |
2007年 | 1919篇 |
2006年 | 1901篇 |
2005年 | 1700篇 |
2004年 | 1404篇 |
2003年 | 1009篇 |
2002年 | 1109篇 |
2001年 | 904篇 |
2000年 | 840篇 |
1999年 | 566篇 |
1998年 | 325篇 |
1997年 | 247篇 |
1996年 | 215篇 |
1995年 | 208篇 |
1994年 | 220篇 |
1993年 | 179篇 |
1992年 | 117篇 |
1991年 | 118篇 |
1990年 | 101篇 |
1989年 | 64篇 |
1988年 | 65篇 |
1987年 | 32篇 |
1986年 | 50篇 |
1985年 | 41篇 |
1984年 | 26篇 |
1983年 | 27篇 |
1982年 | 18篇 |
1981年 | 25篇 |
1980年 | 20篇 |
1979年 | 18篇 |
1978年 | 13篇 |
1977年 | 7篇 |
1959年 | 5篇 |
1936年 | 4篇 |
排序方式: 共有10000条查询结果,搜索用时 343 毫秒
991.
Shenqian Ma Weixin Zhao Jun Zhou Jiaou Wang Shengqi Chu Zigeng Liu Guolei Xiang 《Chemical science》2021,12(12):4411
Noncovalent π stacking of aromatic molecules is a universal form of noncovalent interactions normally occurring on planar structures (such as aromatic molecules and graphene) based on sp2-hybridized atoms. Here we reveal a new type of noncovalent surface–π stacking unusually occurring between aromatic groups and peroxide-modified titania (PMT) nanosheets, which can drive versatile aromatic adsorptions. We experimentally explore the underlying electronic-level origin by probing the perturbed changes of unoccupied Ti 3d states with near-edge X-ray absorption fine structures (NEXAFS), and find that aromatic groups can vertically attract π electrons in the surface peroxo-Ti states and increase their delocalization regions. Our discovery updates the concept of noncovalent π-stacking interactions by extending the substrates from carbon-based structures to a transition metal oxide, and presents an approach to exploit the surface chemistry of nanomaterials based on noncovalent interactions.A new type of noncovalent surface–π stacking interaction occurring on a transition metal oxide, titania, is reported, which is different from the traditional forms on sp2-hybridized planar structures like graphene. 相似文献
992.
Xing-Zhi Liu Da-Wei Fang Jia-Zhen Yang Jun Li Shu-Liang Zang 《The Journal of chemical thermodynamics》2004,36(12):1061-1065
The equilibrium molalities In3+ in {In2(SO4)3 + Na2SO4 + HDEHMTPCA + n-C8H18 + water} were measured at ionic strength from (0.1 to 2.0) mol · kg−1 containing Na2SO4 as supporting electrolyte in aqueous phase and at constant molality extractant at temperatures from (278.15 to 303.15) K in organic phase. The standard extraction constants K0 at various temperatures were obtained by methods of extrapolation and polynomial approximation. Thermodynamic quantities for the extraction process were calculated. 相似文献
993.
聚甲基丙烯酸甲酯辐射裂解和消旋的空间立构效应 总被引:1,自引:1,他引:1
本文研究了三种不同空间立构聚甲基丙烯酸甲酯的辐射效应,提出裂解过程是一种裂解与重合的动态平衡过程。分子量降低和消旋作用对温度的依赖性,是由于分子运动和笼罩效应以及重合的空间位阻效应所致。辐照温度愈高,裂解产额愈大。相同条件下辐照,全同立构比无规立构试样的裂解产额更大。 全同立构聚甲基丙烯酸甲酯辐照后,不仅分子链断裂,而且空间立构也发生很大变化。随着辐照剂量的增加,全同立构含量逐渐减少,而无规立构含量和间同立构含量却逐渐增加。 相似文献
994.
有机硅改性丙烯酸酯共聚乳液合成方法及胶膜性能的研究 总被引:59,自引:0,他引:59
用一次投料法、单体乳液滴加法和引发剂滴加法有机硅改性丙烯酸酯共聚乳液,聚合过程、胶粒形态及乳液稳定性的观测结果表明:单体乳液滴加法是合成该类乳液的最佳方法,研究了单体乳液滴加法中有机硅含量与聚合反及胶膜性能的关系,结果表明:有机硅含量在15%以下时,聚合反应可以顺利进行,胶膜性能不仅依赖于聚合时有机硅单体的总量,而且还依赖于有机硅单体中活性硅氧烷所占的比例。 相似文献
995.
合成了一系列聚丁二酸/苯基丁二酸丁二醇共聚酯(PBSBS),利用DSC、1H-NMR和X射线等测试手段对共聚物组成、热力学性能、结晶性能、等温结晶行为进行了表征和研究.结果表明,含苯基的共聚单元的引入显著改变了聚丁二酸丁二醇酯(PBS)的热力学性能4,利用Hoffman-Week曲线得到的共聚物平衡熔点随共聚组分含量的增加显著降低,玻璃化转变温度则明显升高,结晶熔点符合无规共聚物的Flory方程.此外,利用Avrami方程对均聚物PBS以及共聚物PBSBS-10分别进行了等温结晶行为研究,结果表明共聚使结晶速率降低,PBS和PBSBS-10的Avrami指数分别介于2.8~3.0和2.7~2.9之间,结晶方式为三维生长异相成核,X射线测试结果表明共聚不影响晶体结构. 相似文献
996.
屠树江   朱晓彤   房芳   章晓镜   朱松磊   李团结   史达清 王香善   纪顺俊 《中国化学》2005,23(5):596-598
A simple effective synthesis of bis(dihydropyrimidinone-4-yl)benzene derivatives, using boric acid as catalyst,from isophthalic aldehyde or terephthalic aldehyde, 1,3-dicarbonyl compounds and urea or thiourea in glacial acetic acid was described. As the expansion of the classical Biginelli reaction, this method has the advantage of excellent yields 83%-94% and short reaction time 0.5-1.5 h. 相似文献
997.
Neonatal exposure to synthetic estrogen endocrine disruptors or estrogen-receptor inhibitors induces developmental abnormalities in the male reproductive system. To investigate whether neonatal exposure affects spermatogenesis in juvenile and pubertal testis, Sprague-Dawley rat pups were given synthetic estrogen endocrine disruptors or estrogen-receptor inhibitors by a single injection on the day of birth at concentrations ranging between 2 to 40 mm, and sacrificed on day 21 (juvenile), 35 (prepuberty) or 50 (puberty). The testes were weighed and examined histologically at each stage. Further, the metabolites of steroidogenesis were analyzed using normal-phase high performance liquid chromatography. Neonatal exposure significantly reduced testis weights and steroidogenesis to one- fifth to one-half of that of the juvenile control, and further suppressed irreversible steroidogenesis and spermatogenesis during puberty. 相似文献
998.
Jun Wang Wenzhao Li Chunying Yu Yanxin Chen Yingzhen Zhang 《Reaction Kinetics and Catalysis Letters》1997,62(2):217-224
Recucibility of Mo species in Pt/MoO3 and PtMo/Al2O3 has been investigated by temperature-programmed reduction (TPR), temperature-programmed desorption of hydrogen (H2-TPD) and temperature programmed electronic conductivity (TPEC) techniques. In Pt/MoO3 at H2 atmosphere, it was found by TPEC and TPR that, a slight amount of Pt could activate the transfer of the species and H atoms
between H2 and MoO3, and thus accelerate the reduction of MoO3. In PtMo/Al2O3, TPR and H2-TPD revealed that the reduction of surface Mo species could also be facilitated by Pt. Two kinds of hydrogen molybdenum species
were proposed on the surface of the catalyst after prereduction. 相似文献
999.
The use of a laboratory-made iodide ion-selective electrode with tubular configuration and based on a crystalline membrane (AgI/Ag2S) as the detector for ion chromatographic determination of iodide in urine and serum is described. A CIS reversed-phase column was coated withN-cetylpyridinium chloride to prepare a low-exchange-capacity analytical column and with hexadecyltrimethylammonium bromide to prepare a concentrator pre-column. A 2.0 ml min–1 flow rate of deionized water and 0.1 mol 1–1 KNO3 solution was used for the pre-concentration and for the chromatographic separation, respectively. For optimum performance of the detector a background level of iodide was added into the column effluent. A linear relationship (r = 0.9997) between tubular electrode potential (as peak height) and iodide concentration in the range 5–400 g 1–1 and a detection limit of 1.47 g 1–1 were obtained. The method shows good reproducibility for both peak height (2.2% RSD) and retention time (1.3% RSD). Recoveries on its application to the samples were 93.0–100.9% for urine and 91.4–106.0% for serum. 相似文献
1000.
ClnOPodiumpoIycePhalum(VantjCYWhetHsuancalled'DuanXielin'inChineseisusedinfolkmedicineforthetreatInentofhaemorrhagiadisease.lInordertofmdtheactivecomPounds,studiesonthechedricalconstituentsofthewholeplan,collectedatHuoShanofAnhuiprovince,werecarriedout.Fromthealcoholicextracts,wehaveisolatedanewanthraquinone,Glyceroyl-l,6,8-trihydroxy-3-methyl-9)lO-dioxo-2-anthrcenecarboxylateI(Figurel).Iwasobtainedasredneedles,mP:ll9-l2l"C.TheHR-MSdareaffordedthefor-mulaasCl9Hl6O9(Found388.O791;… 相似文献