首页 | 本学科首页   官方微博 | 高级检索  
文章检索
  按 检索   检索词:      
出版年份:   被引次数:   他引次数: 提示:输入*表示无穷大
  收费全文   8093篇
  免费   1751篇
  国内免费   405篇
化学   1363篇
晶体学   57篇
力学   1408篇
综合类   55篇
数学   666篇
物理学   1676篇
综合类   5024篇
  2025年   89篇
  2024年   268篇
  2023年   279篇
  2022年   312篇
  2021年   319篇
  2020年   342篇
  2019年   281篇
  2018年   279篇
  2017年   321篇
  2016年   373篇
  2015年   357篇
  2014年   431篇
  2013年   510篇
  2012年   508篇
  2011年   527篇
  2010年   371篇
  2009年   448篇
  2008年   341篇
  2007年   480篇
  2006年   398篇
  2005年   345篇
  2004年   297篇
  2003年   287篇
  2002年   223篇
  2001年   191篇
  2000年   209篇
  1999年   170篇
  1998年   139篇
  1997年   179篇
  1996年   113篇
  1995年   116篇
  1994年   113篇
  1993年   114篇
  1992年   100篇
  1991年   76篇
  1990年   77篇
  1989年   64篇
  1988年   53篇
  1987年   50篇
  1986年   25篇
  1985年   19篇
  1984年   6篇
  1983年   6篇
  1982年   11篇
  1981年   3篇
  1980年   3篇
  1979年   7篇
  1978年   3篇
  1971年   4篇
  1957年   6篇
排序方式: 共有10000条查询结果,搜索用时 15 毫秒
31.
田口方法和双重曲面响应(DRSM)法   总被引:3,自引:1,他引:2  
田口参数设计在理论界遭到较多批评 ,也提出一些替代方法。双曲面响应 ( DRSM)法是结合田口参数设计的思想和统计回归模型发展起来的。本文通过正交旋转设计和计算机模拟噪声因子变化来提高双曲面响应法的精度并对两种方法应用结果进行比较 ,说明田口参数设计仍具有实践和方法论的启发意义  相似文献   
32.
氢键链中孤子对外场的响应   总被引:1,自引:1,他引:0  
以二分量孤子的振子模型为基础,讨论了质子子晶格与重离子子晶格中扭结孤子形成的孤子对的运动及其孤子对外场的响应持性,并得到了氢键链中孤子对的迁移率表达式。  相似文献   
33.
杨理  刘颂豪  廖常俊 《光学学报》1999,19(6):51-756
利用一种与Bchlund变换有关的方法严格求含非线性延时项光孤立子方程,得到了该方程的漂移型扭结孤波解,并讨论了解的极限性质和相关物理问题。最后指出了该方程在γ=0处的参数不稳定性问题  相似文献   
34.
Abstract

Effective extraction of phyto-biomolecules insures retaining maximum functionality along with higher recovery. In this study, ultrasound-solvent assisted extraction (USAE) was employed for optimal extraction of phyto-biomolecules from Sesamum indicum (sesame) leaves using the approach of Response Surface Methodology (RSM). The optimized condition of 200?W power, 59% methanol concentration with 1:14?g/mL solid–liquid ratio and 15?min of extraction time yielded 367.39?±?1.85?mg GAE/100?g of total phenolic content, 96.72?±?3.27% of free radical scavenging activity and 81.20?±?2.87% of iron chelating activity respectively. The extract consist of essential phytocomponents like gallic acid, chlorogenic acid, and quercetin with lipid peroxidation activities of >50% over incubation time of 48?h. Also, showed antimicrobial activity against various Gram’s negative and positive food borne pathogens. The results of this study implied the importance of USAE for effective and optimum recovery of phyto-biomolecules from Sesame leaves with retained functional properties.  相似文献   
35.
    
The airship structures made of multi-layer composite fabrics or membranes can offer the platform for earth observations, wireless communications and space research due to light weight and good mechanical performance. The structural safety and serviceability strongly depend on material properties and working conditions. Available studies are limited within service stress limits or are lack of suitable biaxial tensile constitutive models for understanding structural behavior. This paper thus focuses on a refined numerical model for determining inflation-burst behavior of composite airship structures considering new biaxial constitutive equations, novel failure criteria and manufacture factors.The differences between ideal and real forms of airship structures, e.g. volume difference, demonstrate the necessity for incorporating cutting-pattern effects in the initial numerical model. For structural analysis, stress distributions on real structural forms are different from those on ideal forms because of welding parts that can enhance local stiffness. The ultimate pressures are 56.7 kPa and 59.5 kPa for ideal and real structural forms. Structural breaking initiated at the maximum diameter of ideal structural forms propagates fast while welding parts can prevent breaking propagation for real structural forms. Therefore, the refined numerical model can reveal basic structural behavior and safety performance of airship structures in the inflation-burst processes.  相似文献   
36.
    
As the enantiomers of 1-phenylethanol are valuable intermediates in several industries, the lipase catalyzed kinetic resolution of (R,S) -1-phenylethanol is a relevant research topic. In this study, the goal was to determine the optimum reaction parameters to produce enantiomerically pure 1-phenylethanol by lipase (Novozyme 435) catalyzed kinetic resolution using response surface methodology (RSM). Reactions were performed with 40–400 mM (R,S)-1-phenylethanol, 120–1200 mM vinyl acetate and 2–22 mg/mL biocatalyst concentrations (BC L ), at 20–60 °C and with a stirring rate of 50–400 rpm for 5–120 min. The samples were analyzed using high performance liquid chromatography (HPLC) with a Chiralcel OB column. Optimum reaction parameters to reach 100% enantiomeric excess for the substrate ( ee s ) were determined as follows: substrate concentration (C s ): 240 mM, BC L : 11 mg/mL, at 42 °C with a reaction time of 75 min. Model validation was performed using these conditions and ee s was calculated as 100%, which indicates the predicted model was efficient and accurate. When compared to the literature, it was observed that the reaction time decreased significantly. This is an important result considering the industrial scale perspective.  相似文献   
37.
低电流密度下恒电流法制备的聚苯胺修饰电极   总被引:3,自引:1,他引:3  
研究了低电流密度下恒电流法制备的聚苯(PA)修饰电极的性质及其影响因素,探讨了低电流密度聚合的PA膜的优点。发现此种条件下聚合的PA膜具有较好的电荷传输能力,它不仅对Br^-,Tl^+/Tl等电对的氧化还原反应有更好的电催化活性,而且对H^+的Nernast响应也更接近理论值。  相似文献   
38.
从线性条件出发,推导了含n个时间常数体系恒电位阶跃时暂态响应方程,并提出一套解析形为ΔI=ΔI_∞+sum from i=1 to n k_iexp(-t/τ_i)暂态方程各未知参数的代数解法。该方法可对未知体系时间常数个数进行判定,比起一般曲线拟合更加快速简便,且抗干扰能力较强。  相似文献   
39.
In this paper we have reviewed the theoretical framework of the coupled-cluster (cc) based linear response model as a tool for directly calculating energy differences of spectroscopic interest like excitation energy (ee), ionisation potential (ip) or electron affinity (ea). In this model, the ground state of a many-electron system is described as in a coupled cluster theory for closed shells. The electronic ground state is supposed to interact with an external photon field of frequencyw, and the poles of the linear response function as a function ofw furnish with the elementary excitations of the system. Depending on the general form of the coupling term chosen, appropriate difference energies like ee, ip or EA may be generated. Pertinent derivations of the general working equations are reviewed, and specific details as well as approximations for ee, ip or ea are indicated. It is shown that the theory bears a close resemblance to the equation of motion (eom) method but is superior to the latter in that the ground state correlation is taken to all orders and may be looked upon as essentially a variant of renormalisedtda. A perturbative analysis elucidating the underlying perturbative structure of the formulation is also given which reveals that the theory has a hybrid structure: the correlation terms are treated akin to an open shellmbpt, while the relaxation terms are treated akin to a Green function theory. A critique of the methodvis-a-vis other cc-based approaches for difference energies forms the concluding part of our review.  相似文献   
40.
Summary Four most frequently used presentation forms of the ECD response are described. Equations describing the dependence of the sensitivity, linearity and minimum detectable concentration of the ECD working under constant frequency mode of operation on the pulse period are derived from Wenthworth’s kinetic model for all four forms mentioned above. The forms presented are compared on the basis of equations derived. Results of the calculations are given and some predictions are experimentally tested for sulphur hexafluoride. Good agreement was found between the predicted and observed relationships. Presented at the 15th International Symposium on Chromatography, Nürnberg, October 1984  相似文献   
设为首页 | 免责声明 | 关于勤云 | 加入收藏

Copyright©北京勤云科技发展有限公司  京ICP备09084417号