全文获取类型
收费全文 | 1268篇 |
免费 | 180篇 |
国内免费 | 181篇 |
专业分类
化学 | 922篇 |
晶体学 | 22篇 |
力学 | 92篇 |
综合类 | 2篇 |
数学 | 139篇 |
物理学 | 452篇 |
出版年
2023年 | 24篇 |
2022年 | 41篇 |
2021年 | 56篇 |
2020年 | 49篇 |
2019年 | 58篇 |
2018年 | 38篇 |
2017年 | 41篇 |
2016年 | 57篇 |
2015年 | 67篇 |
2014年 | 81篇 |
2013年 | 84篇 |
2012年 | 116篇 |
2011年 | 138篇 |
2010年 | 69篇 |
2009年 | 76篇 |
2008年 | 61篇 |
2007年 | 60篇 |
2006年 | 77篇 |
2005年 | 53篇 |
2004年 | 58篇 |
2003年 | 39篇 |
2002年 | 30篇 |
2001年 | 20篇 |
2000年 | 25篇 |
1999年 | 39篇 |
1998年 | 23篇 |
1997年 | 23篇 |
1996年 | 20篇 |
1995年 | 22篇 |
1994年 | 14篇 |
1993年 | 15篇 |
1992年 | 12篇 |
1991年 | 8篇 |
1990年 | 8篇 |
1989年 | 4篇 |
1988年 | 2篇 |
1987年 | 8篇 |
1986年 | 8篇 |
1985年 | 1篇 |
1983年 | 1篇 |
1982年 | 2篇 |
1957年 | 1篇 |
排序方式: 共有1629条查询结果,搜索用时 46 毫秒
991.
利用阶梯形变幅杆的应变放大作用,构造了磁致伸缩/阶梯形弹性基底/压电复合结构. 采用等效电路法分析了沿长度方向振动复合结构的一阶磁电响应. 计算了Terfenol-D/阶梯形铍青铜基底/PZT-5H复合结构的磁电响应,并与实际结构的磁电响应进行了比较,由于理论分析中忽略了胶层产生的损耗,理论值和实验结果的变化规律相似,但是谐振频率点和磁电电压转换系数有一定的差异. 同时比较了阶梯形基底和等截面杆基底复合结构,分析表明前者具有更高的磁电电压转换系数. 研究了阶梯形弹性基底长度比及层厚比对复合结构纵振动一阶模
关键词:
磁电效应
磁致伸缩/压电复合结构
磁电响应 相似文献
992.
将基本尺度熵的方法在时间上做多尺度化的扩展,并将其应用到心跳间隔序列的分析研究中.研究发现,健康人的心率变异性是小时间尺度下的模式特定性与大时间尺度下的模式丰富性相结合的,而充盈性心衰患者则正好相反.这说明充盈性心衰患者在小时间尺度下心脏动力系统的控制不力,导致随机性增加,而在大时间尺度下对外界环境变化反应又不够丰富,从而导致生命更容易受威胁.据此提出了以小时间尺度下的基本尺度熵值相对于大时间尺度下平台区基本尺度熵值的变异参数δ作为区分健康人和充盈性心衰患者的诊断依据.通过对72例健康人和44例充盈性心衰患者的计算,发现两组样本差异显著,证明了参数δ的有效性.
关键词:
多尺度化的基本尺度熵
心跳间隔序列
心率变异性
充盈性心衰 相似文献
993.
The enhanced patch test proposed by Chen W J (2006) can be used to assess the convergence of the problem with non-homogeneous
differential equations. Based on this theory, we establish the patch test function for axisymmetric elements of conventional
and couple stress theories, and reach an important conclusion that the patch test function for axisymmetric elements cannot
contain non-zero constant shear.
Supported by the National Natural Science Foundation of China (Grant No. 10672032) 相似文献
994.
Phosphorus-doped ZnO nanoneedle arrays were prepared by phosphorus diffusion from InP substrate using a pulsed laser deposition (PLD) technique. The optical properties of ZnO nanoneedle were investigated by photoluminescence (PL) spectroscopy. Low-temperature photoluminescence spectrum measurements exhibited five acceptor-related emission peaks. The excitation intensity and temperature dependent photoluminescence spectra confirmed that the emission peaks corresponded to neutral-acceptor bound exciton, free electron to acceptor, donor-acceptor pairs, and their first and second photon replicas transitions. Acceptor-binding energy was determined to be 135-167 meV, which agrees well with the best-fitting result of the temperature dependent photoluminescence measurements and is reasonable in terms of theoretic prediction in ZnO. 相似文献
995.
J.X. Guo L. Guan F. Bian Q. Li B. Geng Y.L. Wang Q.X. Zhao B.T. Liu 《Applied Surface Science》2009,255(17):7512-7516
Adsorption of H2 molecule on the Ti (0 0 0 1)-(2 × 1) surface was studied by density functional theory with generalized gradient approximation (GGA). The parallel and vertical absorption cases were investigated in detail by adsorption energy and electronic structure analysis, we obtained three stable configurations of FCC-FCC (the two H atoms adsorption on the two adjacent fcc sites of Ti (0 0 0 1) surface, respectively), HCP-HCP (the two H atoms adsorption on the two adjacent hcp sites of Ti (0 0 0 1) surface, respectively) and FCC-HCP (the one H atom adsorption on the fcc site and the other adsorption on the near hcp site) based on the six different parallel adsorption sites after the H2 molecule dissociates. However, all the end configurations of four vertical adsorption sites were unstable, H2 molecule was very easy to desorb from Ti surface. The H-H bond breaking and Ti-H bond forming result from the H2 molecule dissociation. H-H bond breaking length ranges from 1.9 Å to 2.3 Å for different adsorption configurations due to the strong Ti-H bond forming. The H2 dissociative approach and the end stable configurations formation in parallel adsorption processes are attributed to the quantum mechanics steering effects. 相似文献
996.
This study examined the interaction of indirubin with bovine serum albumin (BSA) at three temperatures (286, 297, 308 K) at pH 7.40. In the presence of indirubin, the drug-BSA binding mode, binding constant and the protein structure changes in aqueous solution were determined by fluorescence quenching methods including Fourier transform infrared (FT-IR) spectroscopy and UV-Vis spectroscopy. The FT-IR change indicates that indirubin binds to BSA. The change in protein secondary structure accompanying ligand binding has been proved by fluorescence spectra data. The thermodynamic parameters, the enthalpy change (DeltaH), and the entropy change (DeltaS) calculated by the van't Hoff equation possess small negative (-2.744 kJ.mol(-1)) and positive values (112.756 J.mol(-1).K(-1)), respectively, which indicated that hydrophobic interactions play the main role in the binding of indirubin to BSA. Furthermore, the displacement experiment shows that indirubin can bind to the subdomain IIA and the distance between the tryptophan residues in BSA and indirubin bound to site I was estimated to be 2.24 nm according to F?ster's equation on the basis of fluorescence energy transfer. 相似文献
997.
998.
Q.J. Han D.H. JiG.D. Tang Z.Z. LiX. Hou W.H. QiS.R. Liu R.R. Bian 《Journal of magnetism and magnetic materials》2012
Ferrite samples of the composition Cu0.5−xNi0.5ZnxFe2O4 (0.0≤x≤0.5) were synthesized by chemical co-precipitation. The samples exhibited a single phase cubic spinel structure, and the saturation magnetization of the samples was found to increase with increasing Zn content. Using a quantum mechanical method proposed by our group, the cation distributions in the samples were estimated. Estimated cation distributions obtained by fitting the magnetic moments of the samples were then used to perform Rietveld fitting for X-ray diffraction patterns. The acceptable error parameters in the Rietveld fitting indicate that the estimated cation distributions in the samples are reasonable. 相似文献
999.
Lin Bian Lianzhi Li Qingfu Zhang Jianfang Dong Tao Xu Jinghong Li Jinming Kong 《Transition Metal Chemistry》2012,37(8):783-790
Two new V(IV) complexes, [VO(Naph?Ctrp)(phen)]·CH3OH (1) and [VO(o-Van?Ctrp)(phen)]·CH3OH·H2O (2) (Naph?CTrp?=?Schiff base derived from 2-hydroxy-1-naphthaldehyde and l-tryptophan, o-Van?Ctrp?=?Schiff base derived from o-vanillin and l-tryptophan, phen?=?1,10-phenanthroline), have been synthesized and characterized by physicochemical methods. The V(IV) atoms in both complexes are six-coordinated in a distorted octahedral environment. In the crystals of complex 1, the C?CH···?? and ?ШC?? stacking interactions form a 1D chain structure, whereas for complex 2, hydrogen bonds connect the molecular units into a 2D plane structure. The DNA binding properties and cleavage efficiencies of the complexes have been investigated by spectroscopic methods, viscosity measurements and agarose gel electrophoresis. The results suggest that both complexes can bind to CT-DNA in an intercalative mode and can also cleave pBR322 DNA. 相似文献
1000.
The binding of kaempferol‐3,7‐α‐L‐rhamnopyranoside (KRR) with bovine serum albumin (BSA) was investigated by different spectroscopic methods under simulative physiological conditions. Analysis of ?uorescence quenching data of BSA by KRR at different temperatures using Stern‐Volmer methods revealed the formation of a ground state KRR‐BSA complex with moderate binding constant of the order 104 L·mol?1. The existence of some metal ions could weaken the binding of KRR on BSA. The changes in the van't Hoff enthalpy (ΔH0) and entropy (ΔS0) of the interaction were estimated to be ?26.53 kJ·mol?1 and 3.33 J·mol?1·K?1 and both hydrophobic and electrostatic forces contributed to stabilizing the BSA‐KRR complex. According to the F?ster theory of non‐radiation energy transfer, the distance r between the donor (BSA) and the acceptor (KRR) was obtained (r=2.83 nm). Site marker competitive experiments showed that KRR could bind to Site I of BSA. In addition, synchronous fluorescence, UV‐Vis absorption and circular dichroism (CD) results indicated that the KRR binding could cause conformational changes of BSA. 相似文献