全文获取类型
收费全文 | 3542篇 |
免费 | 618篇 |
国内免费 | 505篇 |
学科分类
数理化 | 4665篇 |
出版年
2024年 | 9篇 |
2023年 | 79篇 |
2022年 | 81篇 |
2021年 | 90篇 |
2020年 | 139篇 |
2019年 | 148篇 |
2018年 | 147篇 |
2017年 | 124篇 |
2016年 | 186篇 |
2015年 | 165篇 |
2014年 | 232篇 |
2013年 | 263篇 |
2012年 | 275篇 |
2011年 | 300篇 |
2010年 | 214篇 |
2009年 | 211篇 |
2008年 | 250篇 |
2007年 | 211篇 |
2006年 | 214篇 |
2005年 | 204篇 |
2004年 | 143篇 |
2003年 | 132篇 |
2002年 | 140篇 |
2001年 | 100篇 |
2000年 | 82篇 |
1999年 | 78篇 |
1998年 | 64篇 |
1997年 | 42篇 |
1996年 | 44篇 |
1995年 | 48篇 |
1994年 | 43篇 |
1993年 | 42篇 |
1992年 | 37篇 |
1991年 | 28篇 |
1990年 | 23篇 |
1989年 | 14篇 |
1988年 | 10篇 |
1987年 | 13篇 |
1986年 | 15篇 |
1985年 | 6篇 |
1984年 | 8篇 |
1983年 | 3篇 |
1982年 | 1篇 |
1981年 | 3篇 |
1980年 | 1篇 |
1971年 | 1篇 |
1957年 | 1篇 |
1936年 | 1篇 |
排序方式: 共有4665条查询结果,搜索用时 15 毫秒
991.
本文采用相对灵敏度因子,研究了99.999%-99.9999%的Ho2O3和Lu2O3中40余种杂质元素的火花源定量分析方法,讨论了谱线干扰。做了加入回收及生理性实验,大多数元素的相对标准偏差在30%以内,检测下限为0.00xμg/g。 相似文献
992.
A slow‐release and superabsorbent nitrogen fertilizer (SSNF) was synthesized by aqueous solution copolymerization of acrylic acid and maleic anhydride using N,N′‐methylene bisacrylamide as a crosslinker, ammonium persulfate and sodium sulfite as an initiator in the presence of urea. The effects on water absorbency such as amount of crosslinker, initiator, urea and maleic acid, degree of neutralization, etc. were investigated. To improve slow release and water retention properties, the product was surface‐crosslinked with trihydroxymethyl propane glycidol ether (6360). The nitrogen content of the SSNF synthesized under optimal conditions was 28.5%, and the water absorbencies were about 900 (g/g) in distilled water and 180 (g/g) in tap water. The nitrogen slow release behavior of the SSNF in water and water retention capacity of the soil with SSNF were also investigated. The results showed that the surface‐crosslinked product not only had good slow release property but also excellent soil moisture preservation capacity, which could effectively improve the utilization of fertilizer and water resources simultaneously. The results of the present work indicated that the SSNF could be found an application in agriculture and horticulture, especially in drought‐prone areas where the availability of water is insufficient. Copyright © 2006 John Wiley & Sons, Ltd. 相似文献
993.
枸杞子中红色素的提取及稳定性 总被引:10,自引:0,他引:10
枸杞子中红色素的提取及稳定性伍明,王杰,闪春霞,任仲皎,宋锡瑾,詹国庆(中南民族学院化学系武汉430074)关键词枸杞,色素,液泛提取法枸杞子为茄科枸杞属植物的果实,具有补肾养肝,润肺明目以及调节机体免疫力,抑制肿瘤生长和细胞突变,延缓衰老及抗脂肪肝... 相似文献
994.
According to the formulas obtained in the preceding paper, it may be used for all types of the hybridization with any set of azimuthal quantum numbers l, l = 0 through l = 5, and a complete theoretical data of bond angles and bond strengths are shown in this paper. 相似文献
995.
The peroxotitanium-substituted heteropolytungstate -K7[GaW11Ti(O2)O39]·11H2O has been prepared and characterized by elemental analysis, i.r., u.v., 183W-n.m.r. and by electrochemistry. The 183W-n.m.r. spectrum consists of six lines with intensity ratio 2:2:1:2:2:2, indicating that the TiO2 occupies the empty octahedral site of the -K9[GaW11O39]·xH2O anion forming a polyanion with C
s symmetry. The characteristic charge-transfer absorption band O2–
2 Ti at 385 nm occurs in the u.v. spectrum. The polargraphic reduction potential of O2–
2 at ca. + 0.95 V has been determined. 相似文献
996.
通过三(邻甲苯基)氢化锡与1-乙炔基环己醇和1-乙炔基环戊醇发生加成反应,得到两个加成物:(Z)-1-[2-(三-邻甲苯基锡基)乙烯基]环戊醇(1)和(Z)-1-[2-(三-邻甲苯基锡基)乙烯基]环己醇(2).通过元素分析、红外光谱和核磁共振氢谱等手段对其结构进行了表征.用X射线单晶衍射测定了加成物1和2的晶体结构.二者均为具有分子内O→Sn弱配位的扭曲四面体结构.1属于单斜晶系,P21/n空间群,晶胞参数:a=1.0496(2)nm,b=1.7658(4)nm,c=1.3193(3)nm,β=90.60(3)°,V=2.4451(8)nm3,Z=4,R=0.0317,wR=0.0820.2属于三斜晶系,空间群P1,晶胞参数:a=O.85592(17)nm,b=0.93016(19)nm,c=1.6368(3)nm;α=83.76(3)°,β=82.69(3)°,γ=78.05(3)°;V=12600(4)nm3,Z=2,R=0.0377,wR=O.1030. 相似文献
997.
Spin-lattice relaxation time, spin-spin relaxation time and two-dimensional nuclear Overhauser enhancement spectroscopy (2D NOESY) experiments of polyoxyethylene lauryl ether (Brij-35) micelles in aqueous solutions at a concentration of 100 times the critical micellar concentration (cmc) give direct evidence that the hydrophilic polyoxyethylene chains, staying in the exterior of the micellar core, are coiled, bent and aligned around the micellar core with a certain number of water molecules included. This hydrophilic layer is in contact with the solvent, water, keeping the micellar solution stable. 1H NMR relaxation time measurements show that the first oxyethylene group next to the alkyl chain participates in the formation of the surface area of the micellar core. The motion of the hydrophilic polyoxyethylene chains is less restricted as compared with the hy-drophobic alkyl chains. 相似文献
998.
单分散交联聚苯乙烯微球HPLC固定相的合成 总被引:3,自引:0,他引:3
用分散聚合物合成了单分散聚苯乙烯微球种子,然后用种子聚合法合成了交联的聚苯乙烯向球,其粒径范围可控制在2μm~4μm,考察了反应条件对微球粒径和粒径分布的影响,确定了最佳合成条件,初步考察了交联微球的HPLC性能。 相似文献
1000.
Lipid monolayer at the air/water interface, as half a membrane, was used here to investigate the interaction between trichosanthin (TCS), a ribosome inactivating protein, and phospholipid membrane. First, the protein adsorption experiments showed that the negatively charged DPPG caused obvious enrichment of TCS beneath the monolayer, indicating electrostatic attraction between TCS and the negatively charged phospholipid. Second, when TCS was incorporated into the phospholipid monolayer, it could not be completely squeezed out until the monolayer collapsed. The results were demonstrated to be irrelative with the phospholipid headgroup, suggesting a strong hydrophobic force between TCS and phospholipid hydrocarbon chain was involved in the interaction. Third, the protein/membrane interaction was further studied with fluorescence microscope. The results showed that TCS could penetrate into both the condensed and the fluid phase of the DPPG monolayer under low pH condition and eventually resulted in a homogeneous phospholipid phase. The breakage of ordered packing of phospholipid by TCS may be responsible for this homogenizing effect. 相似文献