全文获取类型
收费全文 | 1269篇 |
免费 | 381篇 |
国内免费 | 453篇 |
专业分类
化学 | 928篇 |
晶体学 | 48篇 |
力学 | 104篇 |
综合类 | 87篇 |
数学 | 219篇 |
物理学 | 717篇 |
出版年
2024年 | 4篇 |
2023年 | 6篇 |
2022年 | 48篇 |
2021年 | 53篇 |
2020年 | 35篇 |
2019年 | 37篇 |
2018年 | 34篇 |
2017年 | 42篇 |
2016年 | 39篇 |
2015年 | 73篇 |
2014年 | 49篇 |
2013年 | 116篇 |
2012年 | 89篇 |
2011年 | 92篇 |
2010年 | 82篇 |
2009年 | 101篇 |
2008年 | 106篇 |
2007年 | 84篇 |
2006年 | 116篇 |
2005年 | 106篇 |
2004年 | 68篇 |
2003年 | 68篇 |
2002年 | 53篇 |
2001年 | 86篇 |
2000年 | 95篇 |
1999年 | 45篇 |
1998年 | 30篇 |
1997年 | 25篇 |
1996年 | 31篇 |
1995年 | 33篇 |
1994年 | 37篇 |
1993年 | 27篇 |
1992年 | 32篇 |
1991年 | 26篇 |
1990年 | 14篇 |
1989年 | 21篇 |
1988年 | 19篇 |
1987年 | 17篇 |
1986年 | 13篇 |
1985年 | 8篇 |
1984年 | 6篇 |
1983年 | 5篇 |
1982年 | 6篇 |
1981年 | 5篇 |
1980年 | 3篇 |
1979年 | 4篇 |
1978年 | 2篇 |
1974年 | 2篇 |
1965年 | 4篇 |
1964年 | 1篇 |
排序方式: 共有2103条查询结果,搜索用时 15 毫秒
941.
二氧化硅@聚合物同轴纳米纤维 总被引:1,自引:0,他引:1
The preparation and formation mechamsm ot silica/polyvinylpyrrolidone(PAN) coaxial nanofibers were presented in this paper. The PVP-PAN composite nanofibers were obtained via an electrospinning technique, while SiO2 nanoparticles were prepared according to a Stoeher method. The measurements of water contact angle(WCA), the compared results of silica coating PVPPAN composite nanofibers with PAN nanofibers indicate that much PVP resided on the composite nanofiber surface, which resuks in the occurrence of SiO2@polymer coaxial nanofibers due to the formation of hydrogen bonding between silica and composite nanofibers and subsequent adsorption of silica on the fiber surface. 相似文献
942.
1 INTRODUCTION The design and synthesis of metal-organic coor- dination polymers are of current interest owing to their intriguing architectures and potential applica- tions, such as catalysis, fluorescence and magne- tism[1~4]. Many transition metal-organic polymers have been synthesized and exhibit interesting magneticproperties[5~9]. Among them, coordination polymers constructed by dicarboxylate ligands have attracted considerable attention due to their rich coordination modes and di… 相似文献
943.
This paper presents all possible smooth, cusped solitary wave solutions for the variant Boussinesq equations under the inhomogeneous boundary condition. The parametric conditions for the existence of smooth, cusped solitary wave solutions are given using the phase portrait analytical technique. Asymptotic analysis and numerical simulations are provided for smooth, cusped solitary wave solutions of the variant Boussinesq equations. 相似文献
944.
The development of the photocatalytic production of hydrogen from water splitting has attracted immense attention in recent years. CdS is a potential photocatalyst with a visible light response, though it still suffers from a limited activity for hydrogen production due to the fast recombination of photo-induced electron/hole pairs and the low reaction rate of hydrogen evolution on the surface. Studies on the effect of CdS surface structure and properties on hydrogen production are still very limited. In this work, we prepared three CdS nanocrystals with different morphologies: long rod, short rod, and triangular plate. The prepared samples were well characterized by transmission electron microscopy (TEM), X-ray diffraction (XRD), Brunauer-Emmett-Teller (BET) specific surface area analysis, X-ray photoelectron spectroscopy (XPS), photoluminescence (PL) spectroscopy, and UV-Vis diffuse reflectance spectroscopy (UV-Vis DRS). From the results of TEM, XRD and XPS, we find that the three CdS nanocrystals with different morphologies were successfully synthesized. From the PL spectra, we conclude that the area of exposed nonpolar surface and degree of surface defects increase with an increase in aspect ratio. We also performed the photocatalytic hydrogen production reaction using the three CdS crystals. Long rod-like CdS (lr-CdS) exhibits the highest photocatalytic activity, with a hydrogen production rate of 482 μmol·h-1·g-1, which is 2.6 times that of short rod-like CdS (sr-CdS) (183 μmol·h-1·g-1) and 8.8 times that of triangular plate-like CdS (tp-CdS, 55 μmol h-1·g-1). It is found that lr-CdS shows a higher hydrogen production rate than sr-CdS and tp-CdS. We find that the hydrogen production rate is related to the degree of surface defects. Surface defects can trap the photo-induced electrons/holes, thus decreasing their probability of recombination. In addition, these defects can be used to anchor Pd particles to form a heterojunction structure that facilitates the separation of photo-induced charges. Therefore, we also compared three CdS/Pd nanocrystals synthesized with the three abovementioned morphologies with respect to hydrogen production. With 1% (w, mass fraction) Pd, the hydrogen production rate was greatly enhanced compared to all the CdS catalysts. Compared to the unpromoted CdS, the reaction rate is enhanced 43.1, 10.7 and 6.0 times over those of sr-CdS, lr-CdS and tp-CdS, respectively. Notably, the hydrogen production rate with short rod-like CdS/Pd reaches 7884 μmol·h-1·g-1, which can be favorably compared with the ever-increasing values reported in the literature. Hopefully, this work provides knowledge on the effect of crystal surface structure and properties on photocatalysis. 相似文献
945.
946.
It is well known that the problem on the stability of the solutions for Fredholm integral equation of the first kind is an ill-posed problem in C[a,b] or L2[a,b].In this paper,the representation of the... 相似文献
947.
图G的一个L(2.1)-标号是从顶点集V(G)到非负整数的一个函数f,使得若d(u,v)=1时,有|f(u)-f(v)|≥2;若d(u,v)=2时,有|f(u)-f(v)|≥1.图G的L(2.1)-标号数λ(G)是G的所有L(2.1)-标号下的跨度max{f(v):v∈V(G)}的最小数.图Fn+1*为扇图的路上每个顶点增加一个悬挂边得到的图.图Hn为轮图的圈上每个顶点增加一个悬挂边得到的图.本文确定了图Fn+1*与Hn的L(2.1)-标号数. 相似文献
948.
通过水热反应制备了2个中心对称的四核铜Schiff碱配合物[Cu4(H2O)2(μ-Brsth)2(μ3-Brsth)2] (1)和[Cu4(H2O)2(Brsth)2(μ-Brsth)2(μ2-4,4′-bipy)2] (2),其中H2Brsth为5-溴水杨醛缩噻吩-2-甲酰腙。2个配合物晶体属单斜晶系,空间群分别为P21/c和C2/c。2个配合物中所有的铜原子都是五配位,但每个配合物中4个Cu(Ⅱ)都处于2种类型的四方锥配位环境。在配合物1中4个铜原子被4个Schiff碱配体的酚盐氧分别以 μ3-O和 μ-O方式桥联形成椅式{Cu4O4}单元,而在配合物2中2对铜原子分别被2个Schiff碱配体的酚盐氧 μ-O和2个桥联的μ2-4,4′-联吡啶连接形成环状的四核结构。体外抗癌活性的研究结果表明,配合物1和2对人肝癌细胞HEPG2均有较强的增殖抑制作用。 相似文献
949.
950.
假定μn为Rn上的标准高斯测度,X为Rn上的随机向量,分布为μn.不相连猜测说的是:如果f与g为Rn上的两个多项式,而且f(X)与g(X)相互独立,则存在Rn上的正交变换Y = LX及整数k使得f o L-1为(y1,y2,…,yk)的函数,goL-1为(yk+1,yk+2,…,yn)的函数.此时,称f与g不相连.在这... 相似文献