全文获取类型
收费全文 | 2978篇 |
免费 | 560篇 |
国内免费 | 301篇 |
专业分类
化学 | 2294篇 |
晶体学 | 39篇 |
力学 | 162篇 |
综合类 | 14篇 |
数学 | 290篇 |
物理学 | 1040篇 |
出版年
2024年 | 6篇 |
2023年 | 61篇 |
2022年 | 101篇 |
2021年 | 119篇 |
2020年 | 160篇 |
2019年 | 160篇 |
2018年 | 143篇 |
2017年 | 121篇 |
2016年 | 177篇 |
2015年 | 181篇 |
2014年 | 178篇 |
2013年 | 266篇 |
2012年 | 283篇 |
2011年 | 287篇 |
2010年 | 180篇 |
2009年 | 150篇 |
2008年 | 219篇 |
2007年 | 158篇 |
2006年 | 154篇 |
2005年 | 106篇 |
2004年 | 98篇 |
2003年 | 82篇 |
2002年 | 60篇 |
2001年 | 51篇 |
2000年 | 42篇 |
1999年 | 49篇 |
1998年 | 35篇 |
1997年 | 24篇 |
1996年 | 30篇 |
1995年 | 28篇 |
1994年 | 27篇 |
1993年 | 17篇 |
1992年 | 15篇 |
1991年 | 9篇 |
1990年 | 20篇 |
1989年 | 11篇 |
1988年 | 10篇 |
1987年 | 4篇 |
1986年 | 5篇 |
1985年 | 2篇 |
1984年 | 4篇 |
1983年 | 3篇 |
1982年 | 2篇 |
1980年 | 1篇 |
排序方式: 共有3839条查询结果,搜索用时 15 毫秒
981.
Signomial geometric programming (SGP) has been an interesting problem for many authors recently. Many methods have been provided for finding locally optimal solutions of SGP, but little progress has been made for global optimization of SGP. In this paper we propose a new accelerating method for global optimization algorithm of SGP using a suitable deleting technique. This technique offers a possibility to cut away a large part of the currently investigated region in which the globally optimal solution of SGP does not exist, and can be seen as an accelerating device for global optimization algorithm of SGP problem. Compared with the method of Shen and Zhang [Global optimization of signomial geometric programming using linear relaxation, Appl. Math. Comput. 150 (2004) 99–114], numerical results show that the computational efficiency is improved obviously by using this new technique in the number of iterations, the required saving list length and the execution time of the algorithm. 相似文献
982.
In this paper an ionic liquid modified carbon paste electrode (CILE) was prepared and methylene blue (MB) was electropolymerized on the CILE by using the cyclic voltammetric technique in the potential range from −1.0 V to 0.8 V (vs. SCE). A stable polymer film was obtained and exhibited a pair of redox peaks. The morphology and characteristics of poly(methylene blue) (PMB) film was studied by the techniques such as scanning electron microscopy and electrochemical impedance spectroscopy. This PMB modified CILE (PMB/CILE) showed excellent electrocatalytic response to 3,4‐dihydroxybenzoic acid with the increase of the electrochemical responses. The oxidation peak current had a linear relationship with 3,4‐dihydroxybenzoic acid concentration in the range of 5.0 × 10−4 ∼ 3.0 × 10−2 mol L−1 and the detection limit was 1.72 × 10−4 Mol L−1 (3 σ). 相似文献
983.
Chen Z Jiao H Seifert G Horn AH Yu D Clark T Thiel W von Ragué Schleyer P 《Journal of computational chemistry》2003,24(8):948-953
Structural studies of fullerene-like Si(60) and Ge(60) cages using ab initio methods were augmented by density functional tight-binding molecular dynamics (DFTB-MD) simulations of finite temperature effects. Neither the perfect I(h) symmetry nor the distorted T(h) structures are true minima. The energies of both are high relative to distorted, lower symmetry minima, C(i) and T, respectively, which still preserve C(60)-type connectivity. Both Si(60) and Ge(60) favor C(i) symmetry cages in which Si and Ge vertexes exhibit either near-trigonal or pyramidal geometries. These structural variations imply significant reactivity differences between different positions. The small magnetic shielding effects (NICS) indicate that aromaticity is not important in these systems. The inorganic fullerene cages have lower stabilities compared with their carbon analogs. Si(60) is stable towards spontaneous disintegration up to 700 K according to DFTB-MD simulations, and thus has potential for experimental observation. In contrast, Ge(60) preserves its cage structure only up to 200 K. 相似文献
984.
Mengdi Liu Siyue Ma Mengyao She Jiao Chen Zhaohui Wang Ping Liu Shengyong Zhang Jianli Li 《中国化学快报》1990,30(10):1815-1824
The synthesis of biocompatibility and tissue penetrating BODIPY dyes have been summarized. 相似文献
985.
986.
在pH 1.5的B ritton-Rob inson(B-R)缓冲溶液中,藏红T(ST)在-0.35 V(vs.SCE)处有一个灵敏的线性扫描二阶导数极谱还原波。当加入一定量的透明质酸(HA)后,由于此条件下ST带正电荷,而HA带负电荷,两者之间通过静电力作用形成一种生物超分子复合物,导致溶液中游离的ST的浓度降低,相应的还原峰电流降低。实验优化了结合反应条件和电化学测定条件。在最佳条件下,还原峰电流的降低值与HA的浓度在3.0~100.0 mg/L范围内呈线性关系,线性回归方程为Δip(″nA/s-2)=27.23 15.18C(mg/L)(n=14,r=0.996),本方法成功应用于HA模拟样品的测定。 相似文献
987.
The electrooxidation of 3,3′, 5,5′-tetramethylbenzidine (TMB) is dependent on the pH value of Britton-Robinson (B-R) buffer
solution. In this work, the electrooxidation behavior of TMB was investigated with a SnO2:F film optically transparent thin-layer
spectroelectrochemical cell. TMB underwent one two-electron electrooxidation process in the pH range from 2.0 to < 4.0, and
two successive one-electron electrooxidation processes in the pH range from 4.0 to < 7.0 in the B-R buffer solution. At pH
6.5, the electrooxidative product of TMB generated a subsequent chemical reaction to yield an azo compound. Several spectroelectrochemical
techniques, such as thin-layer cyclic voltammetry, thin-layer cyclic voltabsorptometry, thin-layer potential-controlled electrolysis
absorptometry, thin-layer single-potential-step chronoabsorptometry, thin-layer double-potential-step chronoabsorptometry,
thin-layer single-potential-step open-circuit relaxation chronoabsorptometry, were applied to this investigation. The formal
potential E0′ and the electron transfer number corresponding to the electrooxidation of TMB in B-R buffer solution, and the reaction rate
constant of the subsequent chemical reaction were determined. 相似文献
988.
Wu HS Qin XF Xu XH Jiao H Schleyer PV 《Journal of the American Chemical Society》2005,127(7):2334-2338
The structures and energies of isolobal (CH)n and (BCO)n polyhedral species, computed at the B3LYP density functional theory level, reveal contrasts in behavior. The strain energies of the (BCO)n cages are much smaller. Also unlike the (CH)n cages, the most stable (BCO)n polyhedra (n > or = 10) prefer structures with the largest number of three-membered rings. The planar (or nearly planar) faces of the cage systems were modeled by computations on planar, isoelectronic (CH2)n (Dnh) and (HBCO)n (Cnv) rings. While the strain energies of all the planar carbon rings, relative to the most stable D5h (CH2)5, were large, the strain energies of all the planar (HBCO)n (Cnv) rings were small. Remarkably, the three-membered (HBCO)3 (C3v) ring was the most stable. Finally, large (BCO)n systems prefer tubelike rather than cage structures. 相似文献
989.
Xue Liang Wang Zhang Yu Yu Kui Jiao 《中国化学快报》2007,18(1):94-96
A new method to determine the concentration of amikacin(AMK)using methyl blue(MB)as electrochemical probe was developed in this paper.In pH 4.5 Britton-Robinson(B-R)buffer solution,the MB reacted with AMK to form ion association complexes,which led to the reductive peak current of MB at-0.275 V(versus SCE)to decrease,and the decreases were linear with the concentration of AMK in the range of 1.0-60.0 mg/L,the regression of equation isΔIp(nA)=-8.48 102.36c(mg/L), correlation coefficientγis 0.997.The conditions for determining the concentration of AMK using linear sweep voltammetry(SLV) were optimized.The method was used to determine the content of amikacin commercially available with satisfactory results. 相似文献
990.
为了观察宫腔粘连患者行三维经阴道超声(3D-TVS)检查时宫腔容积及血流的变化,本文纳入2016年11月~2018年11月于我院收治的68例宫腔粘连患者为观察组,另选取同期来我院体检的健康女性68例为对照组,均于月经周期中晚期行3D-TVS检查,并以病理诊断结果为金标准,观察了3D-TVS诊断宫腔粘连的灵敏度及特异度。同时,对比了两组患者3D-TVS参数如子宫内膜容积(EV)、血流指数(FI)、血管血流指数(VFI)及内膜区血管指数(VI)的变化,并分析了宫腔粘连不同分级患者上述3D-TVS参数的变化情况。经病理确诊的68例患者中,3D-TVS诊断为宫腔粘连64例,其中Ⅰ型21例,Ⅱ型31例,Ⅲ型9例,Ⅳ型3例。3D-TVS诊断宫腔粘连的灵敏度为92.65%,特异度为98.53%,准确率为95.59%,阳性预测值为98.44%,阴性预测值为93.06%,Kappa值为0.91。观察组EV、FI、VFI、VI均显著低于对照组。Ⅰ型宫腔粘连者EV显著大于Ⅱ、Ⅲ、Ⅳ型者,Ⅱ型宫腔粘连者EV显著大于Ⅲ、Ⅳ型者,Ⅲ型宫腔粘连者EV显著大于Ⅳ型者;Ⅰ型宫腔粘连者的VFI、VI显著大于Ⅲ、Ⅳ型者。上述结果证实3D-TVS对宫腔粘连患者宫腔容积及血流变化具有重要的评估价值。 相似文献