全文获取类型
收费全文 | 3120篇 |
免费 | 582篇 |
国内免费 | 250篇 |
专业分类
化学 | 1020篇 |
晶体学 | 50篇 |
力学 | 86篇 |
综合类 | 20篇 |
数学 | 293篇 |
物理学 | 1282篇 |
综合类 | 1201篇 |
出版年
2024年 | 12篇 |
2023年 | 34篇 |
2022年 | 104篇 |
2021年 | 96篇 |
2020年 | 100篇 |
2019年 | 75篇 |
2018年 | 82篇 |
2017年 | 122篇 |
2016年 | 115篇 |
2015年 | 129篇 |
2014年 | 192篇 |
2013年 | 195篇 |
2012年 | 225篇 |
2011年 | 246篇 |
2010年 | 194篇 |
2009年 | 201篇 |
2008年 | 192篇 |
2007年 | 269篇 |
2006年 | 212篇 |
2005年 | 181篇 |
2004年 | 149篇 |
2003年 | 117篇 |
2002年 | 112篇 |
2001年 | 80篇 |
2000年 | 92篇 |
1999年 | 56篇 |
1998年 | 47篇 |
1997年 | 62篇 |
1996年 | 38篇 |
1995年 | 30篇 |
1994年 | 36篇 |
1993年 | 27篇 |
1992年 | 23篇 |
1991年 | 16篇 |
1990年 | 12篇 |
1989年 | 23篇 |
1988年 | 10篇 |
1987年 | 10篇 |
1986年 | 11篇 |
1985年 | 1篇 |
1984年 | 5篇 |
1983年 | 2篇 |
1982年 | 3篇 |
1981年 | 3篇 |
1980年 | 3篇 |
1978年 | 1篇 |
1973年 | 4篇 |
1959年 | 1篇 |
1957年 | 2篇 |
排序方式: 共有3952条查询结果,搜索用时 10 毫秒
991.
建立了基于低温等离子体(Low temperature plasma)剥蚀系统将固体样品直接引入电感耦合等离子体质谱(ICP-MS)并用于电路板镀层中Au,Ni和Cu的深度分析.此实验中采用介质阻挡放电(DBD)方式产生低温等离子体探针,逐层剥蚀样品表面,由ICPMS检测元素信号.对DBD所用放电气体种类、外加电场功率、放电气体流速和采样深度等实验条件进行优化.在优化条件下,应用LTP-ICPMS在30 s内完成电路板镀层(20 μm Au/10 μm Ni/Cu基底)的逐层剥蚀和深度分析,元素种类和分层顺序与X射线光电子能谱(XpS)相吻合,镀层的分辨率可拓展至微米水平,表明此技术可直接用于固体样品的深度分析. 相似文献
992.
993.
Multi-Physics Modeling of Solid Oxide Fuel Cell Fueled by Methane and Analysis of Carbon Deposition 下载免费PDF全文
Internal reformation of low steam methane fuel is important for the high e ciency and low cost operation of solid oxide fuel cell. Understanding and overcoming carbon deposition is crucial for the technology development. Here a multi-physics model is established for the relevant experimental cells. Balance of electrochemical potentials for the electrochemical reactions, generic rate expression for the methane steam reforming, dusty gas model in a form of Fick's model for anode gas transport are used in the model. Excellent agreement between the theoretical and experimental current-voltage relations is obtained, demonstrating the validity of the proposed theoretical model. The steam reaction order in low steam methane reforming reaction is found to be 1. Detailed information about the distributions of physical quantities is obtained by the numerical simulation. Carbon deposition is analyzed in detail and the mechanism for the coking inhibition by operating current is illustrated clearly. Two expressions of carbon activity are analyzed and found to be correct qualitatively, but not quantitatively. The role of anode diffusion layer on reducing the current threshold for carbon removal is also explained. It is noted that the current threshold reduction may be explained quantitatively with the carbon activity models that are only qualitatively correct. 相似文献
994.
HOU ShangGuo LIANG Le DENG SuHui CHEN JianFang HUANG Qing CHENG Ya FAN ChunHai 《中国科学:化学(英文版)》2014,57(1):100-106
Compared with other imaging techniques,fluorescence microscopy has become an essential tool to study cell biology due to its high compatibility with living cells.Owing to the resolution limit set by the diffraction of light,fluorescence microscopy could not resolve the nanostructures in the range of<200 nm.Recently,many techniques have been emerged to overcome the diffraction barrier,providing nanometer spatial resolution.In the course of development,the progress in fluorescent probes has helped to promote the development of the high-resolution fluorescence nanoscopy.Here,we describe the contributions of the fluorescent probes to far-field super resolution imaging,focusing on concepts of the existing super-resolution nanoscopy based on the photophysics of fluorescent nanoprobes,like photoswitching,bleaching and blinking.Fluorescent probe technology is crucial in the design and implementation of super-resolution imaging methods. 相似文献
995.
本文以金属Ga和NH3为原料,Al、Ni和Fe为金属缓冲层,采用化学气相沉积法(CVD)在Si(100)衬底上合成了GaN微米薄膜。利用X射线衍射仪(XRD)、场发射扫描电子显微镜(FESEM)、能量弥散X射线谱(EDS)、光致发光光谱(PL)和霍尔效应测试仪(HMS-3000)等对GaN微米薄膜进行表征。结果表明,所有样品均为六方纤锌矿结构;样品均出现了很强的近带边紫外发射峰和半峰宽较大的中心波长为672 nm红光发射峰;不同样品的电学性能差异较大。最后对合成的GaN微米薄膜的可能形成机理进行了简单分析。 相似文献
996.
Morgan A. Priolo Kevin M. Holder Tyler Guin Jaime C. Grunlan 《Macromolecular rapid communications》2015,36(10):866-879
Layer‐by‐layer (LbL) assembly has emerged as the leading non‐vacuum technology for the fabrication of transparent, super gas barrier films. The super gas barrier performance of LbL deposited films has been demonstrated in numerous studies, with a variety of polyelectrolytes, to rival that of metal and metal oxide‐based barrier films. This Feature Article is a mini‐review of LbL‐based multilayer thin films with a ‘nanobrick wall’ microstructure comprising polymeric mortar and nanoplatelet bricks that impart high gas barrier to otherwise permeable polymer substrates. These transparent, water‐based thin films exhibit oxygen transmission rates below 5 × 10‐3 cm3 m‐2 day‐1 atm‐1 and lower permeability than any other barrier material reported. In an effort to put this technology in the proper context, incumbent technologies such as metallized plastics, metal oxides, and flake‐filled polymers are briefly reviewed.
997.
Push–Pull Buta‐1,2,3‐trienes: Exceptionally Low Rotational Barriers of Cumulenic CC Bonds and Proacetylenic Reactivity 下载免费PDF全文
Przemyslaw Gawel Dr. Yi‐Lin Wu Dr. Aaron D. Finke Dr. Nils Trapp Dr. Michal Zalibera Prof. Dr. Corinne Boudon Dr. Jean‐Paul Gisselbrecht Dr. W. Bernd Schweizer Prof. Dr. Georg Gescheidt Prof. Dr. François Diederich 《Chemistry (Weinheim an der Bergstrasse, Germany)》2015,21(16):6215-6225
A variety of asymmetrically donor–acceptor‐substituted [3]cumulenes (buta‐1,2,3‐trienes) were synthesized by developed procedures. The activation barriers to rotation ΔG≠ were measured by variable temperature NMR spectroscopy and found to be as low as 11.8 kcal mol?1, in the range of the barriers for rotation around sterically hindered single bonds. The central C?C bond of the push–pull‐substituted [3]cumulene moiety is shortened down to 1.22 Å as measured by X‐ray crystallography, leading to a substantial bond length alternation (BLA) of up to 0.17 Å. All the experimental results are supported by DFT calculations. Zwitterionic transition states (TS) of bond rotation confirm the postulated proacetylenic character of donor–acceptor [3]cumulenes. Additional support for the proacetylenic character of these chromophores is provided by their reaction with tetracyanoethene (TCNE) in a cycloaddition‐retroelectrocyclization (CA–RE) cascade characteristic of donor‐polarized acetylenes. 相似文献
998.
过氧烷基自由基分子内氢迁移是低温燃烧反应中的一类重要基元反应. 本文用等键反应方法计算了该类反应的动力学参数. 所有反应物、过渡态、产物的几何结构均在B3LYP/6-311+G(d,p)水平下优化得到. 本文提出了用过渡态反应中心几何结构守恒作为反应类判据, 并将该分子内氢迁移反应分为四类, 包括(1,3)、(1,4)、(1,5)、(1,n) (n=6, 7, 8)氢迁移类. 分别将这4 类反应类中最小反应体系作为类反应的主反应, 并分别在B3LYP/6-311+G(d,p)低水平和CBS-QB3 高水平下得到其近似能垒和精确能垒. 其余氢迁移反应作为目标反应, 在B3LYP/6-311+G(d,p)低水下计算得到其近似能垒, 再采用等键反应方法校正得到目标反应的精确反应势垒和精确速率常数. 研究表明, 采用等键反应方法只需在低水平用从头算计算就可以得到大分子反应体系的高精度能垒和速率常数值, 且本文按等键反应本质的分类方法更能揭示反应类的本质, 并对反应类的定义给出了客观标准. 本文的研究为碳氢化合物低温燃烧模拟中重要的过氧烷基分子内氢迁移反应提供了准确的动力学参数. 相似文献
999.
An ultra high performance liquid chromatography with tandem mass spectrometry method for plasma and cerebrospinal fluid pharmacokinetics of rhein in patients with traumatic brain injury after administration of rhubarb decoction 下载免费PDF全文
Yang Wang Rong Fan Jiekun Luo Tao Tang Zhihua Xing Zian Xia Weijun Peng Wenzhu Wang Huiying Lv Wei Huang Yizeng Liang Lunzhao Yi Hongmei Lu Xi Huang 《Journal of separation science》2015,38(7):1100-1108
Damage of blood–brain barrier is a common result of traumatic brain injury. This damage can open the blood–brain barrier and allow drug passage. An ultraperformance liquid chromatography with tandem mass spectrometry method was established to determine the concentration of rhein in the biofluids (plasma and cerebrospinal fluid) of patients with a compromised blood–brain barrier following traumatic brain injury after rhubarb administration. Furthermore, the pharmacokinetic profiles were analyzed. A triple‐quadruple tandem mass spectrometer with electrospray ionization was used for rhein detection. The mass transition followed was m/z 283.06→239.0. The calibration curve was linear in the concentration range of 10–8000 ng/mL for the biofluids. The intra‐ and interday precisions were less than 10%. The relative standard deviation of recovery was less than 15% in biological matrices. The pharmacokinetic data showed that rhein was rapidly transported into biofluids, and exhibited a peak concentration 1 h after rhubarb administration. The elimination rate of rhein was slow. The AUCcerebrospinal fluid/AUCplasma (AUC is area under curve) of rhein was approximately 17%, indicating that portions of rhein could pass the impaired blood–brain barrier. The method was successfully applied to quantify rhein in the biofluids of all patients. The data presented can help to guide clinical applications of rhubarb for treating traumatic brain injury. 相似文献
1000.
Yibo Dou Ting Pan Simin Xu Dr. Hong Yan Dr. Jingbin Han Prof. Min Wei Prof. David G. Evans Prof. Xue Duan 《Angewandte Chemie (International ed. in English)》2015,54(33):9673-9678
Transparent and flexible gas‐barrier materials have shown broad applications in electronics, food, and pharmaceutical preservation. Herein, we report ultrahigh‐gas‐barrier films with a brick–mortar–sand structure fabricated by layer‐by‐layer (LBL) assembly of XAl‐layered double hydroxide (LDH, X=Mg, Ni, Zn, Co) nanoplatelets and polyacrylic acid (PAA) followed by CO2 infilling, denoted as (XAl‐LDH/PAA)n‐CO2. The near‐perfectly parallel orientation of the LDH “brick” creates a long diffusion length to hinder the transmission of gas molecules in the PAA “mortar”. Most significantly, both the experimental studies and theoretical simulations reveal that the chemically adsorbed CO2 acts like “sand” to fill the free volume at the organic–inorganic interface, which further depresses the diffusion of permeating gas. The strategy presented here provides a new insight into the perception of barrier mechanism, and the (XAl‐LDH/PAA)n‐CO2 film is among the best gas barrier films ever reported. 相似文献