全文获取类型
收费全文 | 33407篇 |
免费 | 5737篇 |
国内免费 | 6004篇 |
专业分类
化学 | 25475篇 |
晶体学 | 673篇 |
力学 | 1760篇 |
综合类 | 566篇 |
数学 | 4206篇 |
物理学 | 12468篇 |
出版年
2024年 | 33篇 |
2023年 | 565篇 |
2022年 | 850篇 |
2021年 | 1132篇 |
2020年 | 1336篇 |
2019年 | 1392篇 |
2018年 | 1199篇 |
2017年 | 1315篇 |
2016年 | 1567篇 |
2015年 | 1660篇 |
2014年 | 1991篇 |
2013年 | 2624篇 |
2012年 | 3100篇 |
2011年 | 3248篇 |
2010年 | 2525篇 |
2009年 | 2461篇 |
2008年 | 2610篇 |
2007年 | 2426篇 |
2006年 | 2279篇 |
2005年 | 1833篇 |
2004年 | 1316篇 |
2003年 | 1082篇 |
2002年 | 1038篇 |
2001年 | 925篇 |
2000年 | 814篇 |
1999年 | 663篇 |
1998年 | 449篇 |
1997年 | 371篇 |
1996年 | 329篇 |
1995年 | 308篇 |
1994年 | 299篇 |
1993年 | 220篇 |
1992年 | 191篇 |
1991年 | 177篇 |
1990年 | 159篇 |
1989年 | 119篇 |
1988年 | 99篇 |
1987年 | 69篇 |
1986年 | 84篇 |
1985年 | 65篇 |
1984年 | 44篇 |
1983年 | 37篇 |
1982年 | 33篇 |
1981年 | 17篇 |
1980年 | 13篇 |
1979年 | 14篇 |
1977年 | 8篇 |
1973年 | 9篇 |
1971年 | 8篇 |
1936年 | 5篇 |
排序方式: 共有10000条查询结果,搜索用时 125 毫秒
951.
952.
Ultrafine nylon fibers were prepared by electrospinning of nylon-6,66,1010 terpolymer solution in 2,2,2-trifluoroethanol (TFE). The morphology, crystallinity and mechanical properties of the electrospun nylon-6,66,1010 fibers were investigated by scanning electron microscope (SEM), differential scanning calorimetry (DSC), wide angle X-ray diffraction (WAXD) and tensile test. The effects of electrospun process parameters such as solution concentration, voltage and tip-to-collector distance on the morphology and the average size of the electrospun fibers were also studied. The results show that the spinnable concentration of nylon-6,66,1010/TFE solution is in the range of 6-14 wt%, and higher solution concentration favors the formation of uniform fibers without beads. The diameters of the electrospun fibers increase with increasing the solution concentration and decrease slightly with increasing the voltage and needle tip-to-collector distance. But no obvious morphology changes were found with the increase of the voltage and collection distance. DSC and WAXD results suggest that the electrospun nylon-6,66,1010 membranes have lower crystallinity than those of the corresponding casting films. The electrospun nylon-6,66,1010 membrane obtained from the 14 wt% concentration exhibits the largest tensile strength and elongation at break. 相似文献
953.
铬天青S共振光散射法测定脱氧核糖核酸 总被引:3,自引:0,他引:3
介绍在阳离子表面活性剂十六烷基三甲基溴化铵(CTMAB)存在下阴离子染料铬天青S(CAS)共振光散射(RLS)法测定脱氧核糖核酸(DNA)的方法。在pH=5.27的六次甲基四铵一盐酸缓冲溶液中,研究了CAS—CT-MAB—yDNA体系的RLS光谱特征、影响因素和最佳反应条件。在最佳条件下,体系的RLS强度增加值△I与yDNA的浓度在50~800μg/L和1000~2000μg/L呈线性关系,其线性回归方程分别为△I=0.48c 2.56和△I=0.14c 17.86,相关系数分别为0.9997和0.9991,检出限为4.3ng/mL。该方法简便、快速,应用于合成样品中yDNA的测定,测定结果的相对标准偏差为2.93%~4.71%,回收率为97.8%~105.2%。 相似文献
954.
[reaction: see text] A facile and efficient synthetic route toward highly substituted 2,3-dihydrothiopyran-4-ones 2 has been developed via a formal [5C + 1S] annulation of readily available alpha-alkenoyl ketene-(S,S)-acetals 1 with sodium sulfide nonahydrated salt (Na2S x 9H2O) and utilized in the synthesis of 2-(4-chlorophenyl)-6-(morpholin-4-yl)-4H-thiopyran-4-one 5l, an inhibitor of DNA-dependent protein kinase (DNA-PK). 相似文献
955.
倒数示波计时电位滴定新技术 总被引:2,自引:1,他引:2
倒数交流示波极谱具有抑制充电电流、突出电解电流的优点,利用(dE/dt)~(-1)-E曲线上去极剂峰的突然消失或出现指示滴定终点的方法称作倒数示波计时电位滴定法.由运算放大器组装的模拟电子线路可以对(dE/dt)~(-1)-E曲线进行反馈、反对数及微分处理,从而获得各种类型的倒数示波计时电位曲线(图1),以进一步提高滴定的灵敏度。 相似文献
956.
Electrostatic force microscopy (EFM) has become a powerful tool for investigating charges on surfaces. The use of phase measurement in EFM is a direct and fast way to detect electrostatic force gradients, but only qualitatively. With the dual‐pass scheme, the phase signal at lifted height is often assumed to exclude any influences from the topography, but it does not. We report the collection of both topography and phase data by EFM on charged, micron‐sized metal wires. In order to quantify the electrostatic force, a cone model and finite element analysis are provided to integrate the force gradient from the phase signal. Copyright © 2007 John Wiley & Sons, Ltd. 相似文献
957.
在pH 8.40的NH3-NH4Cl缓冲溶液中,钇与3-噻唑偶氮-5-氨基苯酚络合物有一灵敏的极谱吸附波,其峰电位在-0.67 V(vs.SCE),峰电流与钇的浓度在1×10-7~8×10-4mol/L之间呈线性关系,检出限为8×10-8mol/L;经多种电化学方法证明该波为络合物吸附波,其电极过程为不可逆过程,电子转移数为2,此外还试验了多种离子对峰电流的影响,所拟方法已用于茶叶中钇的测定。 相似文献
958.
Three kinds of nylon 10 14 crystals with different perfections were prepared under various crystallization conditions. The Brill transition behavior of these nylon 10 14 crystals was investigated by variable-temperature X-ray diffraction. It was found that the crystallization conditions influence the Brill transition temperature greatly. The Brill transition temperature of the lamellar crystals grown from dilute solution is so high that no Brill transition temperature can be observed before melting. However, for crystals postannealed at 125 °C, the Brill transition temperature is as low as 130 °C. The results show that the Brill transition behavior of nylons is strongly dependent on the crystallization conditions, for example, the perfections of the crystals. 相似文献
959.
960.
Substituted cyclopenta[b]indoles are selectively brominated in good yields with excess pyridine ‐ Br2 charge‐transfer complex (PyBr2) in a one‐pot reaction to provide 5 and/or 7‐bromoindoles. The mechanism involves the formation of an adduct (addition of bromine on the central double bond) which is subsequently reduced in situ with Zn and AcOH. A variety of functional groups in the cyclopentyl and the benzenoid rings are tolerated. 相似文献