全文获取类型
收费全文 | 51158篇 |
免费 | 6379篇 |
国内免费 | 4650篇 |
专业分类
化学 | 22720篇 |
晶体学 | 446篇 |
力学 | 2292篇 |
综合类 | 315篇 |
数学 | 4092篇 |
物理学 | 13568篇 |
综合类 | 18754篇 |
出版年
2024年 | 243篇 |
2023年 | 915篇 |
2022年 | 1621篇 |
2021年 | 1746篇 |
2020年 | 1653篇 |
2019年 | 1504篇 |
2018年 | 1433篇 |
2017年 | 1442篇 |
2016年 | 1958篇 |
2015年 | 2176篇 |
2014年 | 2649篇 |
2013年 | 3302篇 |
2012年 | 3823篇 |
2011年 | 3779篇 |
2010年 | 2775篇 |
2009年 | 2836篇 |
2008年 | 3068篇 |
2007年 | 2745篇 |
2006年 | 2387篇 |
2005年 | 2080篇 |
2004年 | 1604篇 |
2003年 | 1279篇 |
2002年 | 1195篇 |
2001年 | 1086篇 |
2000年 | 1033篇 |
1999年 | 1523篇 |
1998年 | 1242篇 |
1997年 | 1338篇 |
1996年 | 1205篇 |
1995年 | 1123篇 |
1994年 | 916篇 |
1993年 | 818篇 |
1992年 | 709篇 |
1991年 | 613篇 |
1990年 | 593篇 |
1989年 | 500篇 |
1988年 | 406篇 |
1987年 | 313篇 |
1986年 | 187篇 |
1985年 | 133篇 |
1984年 | 68篇 |
1983年 | 56篇 |
1982年 | 55篇 |
1981年 | 22篇 |
1980年 | 17篇 |
1979年 | 4篇 |
1959年 | 1篇 |
1957年 | 7篇 |
1955年 | 2篇 |
1936年 | 3篇 |
排序方式: 共有10000条查询结果,搜索用时 15 毫秒
81.
82.
贮氢材料电极循环寿命的定量预测定时放电半衰期法的应用韩剑文,袁满雪,周作祥,赖城明(南开大学化学系天津300071)关键词:贮氢电极,循环寿命,半衰期。前文 ̄[1]讨论了应用定终点电位放电半衰期法来预测贮氢材料电极循环寿命的问题。本文讨论如何采用定时... 相似文献
83.
用双-(2-羟乙基)二硫代甲酸铵(HEDC)在反相液相色谱中作检测某些金属离子的衍生化试剂,HEDC的金属螯合物微溶于水,可直接水样注射于C18柱中进行检测,范围为0.006~10mg/L相对偏差1%~2%,检测波长254nm,金属汞的整合物在HPLC分析前进行浓缩富集检测限可低至0.06~25μg/L,相对偏差小于2%。 相似文献
84.
Zhou HX 《Journal of the American Chemical Society》2003,125(8):2060-2061
The Gaussian-chain model for treating residual charge-charge interactions was critically tested by recent experimental pK(a) results for individual Asp, Glu, and His residues in the unfolded drkN SH3 domain. Predicted pK(a)'s were in good agreement with experiment. The clustering of Asp and Glu residues along the sequence was suggested to limit pK(a) shifts and contribute to the folding stability by destabilizing the unfolded state. 相似文献
85.
Dynamic behaviour of river colloidal and dissolved organic matter through cross-flow ultrafiltration system 总被引:1,自引:0,他引:1
Through cross-flow filtration (CFF) with a 1-kDa regenerated cellulose Pellicon 2 module, the ultrafiltration characteristics of river organic matter from Longford Stream, UK, were investigated. The concentration of organic carbon (OC) in the retentate in the Longford Stream samples increased substantially with the concentration factor (cf), reaching approximately 40 mg/L at cf 15. The results of dissolved organic carbon (DOC) and colloidal organic carbon (COC) analysis, tracking the isolation of colloids from river waters, show that 2 mg/L of COC was present in those samples and good OC mass balance (77-101%) was achieved. Fluorescence measurements were carried out for the investigation of retentate and permeate behaviour of coloured dissolved organic materials (CDOM). The concentrations of CDOM in both the retentate and permeate increased with increasing cf, although CDOM were significantly more concentrated in the retentate. The permeation model expressing the correlation between log[CDOM] in the permeate and logcf was able to describe the permeation behaviour of CDOM in the river water with regression coefficients (r(2)) of 0.94 and 0.98. Dry weight analysis indicated that the levels of organic colloidal particles were from 49 to 71%, and between 29 and 51% of colloidal particles present were inorganic. COC as a percentage of DOC was found to be 10-16% for Longford Stream samples. 相似文献
86.
The thermal degradation of acetaminophen is studied via conventional accelerated aging studies by initially thermally stressing the compound at temperatures between 160 degrees C and 190 degrees C and measuring the rate of decomposition by reversed-phase high-performance liquid chromatography. Rates of decomposition of the compound in the dry state and the activation energy for the process are determined and compared with previously published kinetic and thermodynamic data for the degradation of acetaminophen in solution. In addition, the thermal fragmentation of acetaminophen under electrospray ionization (ESI) conditions using an interface with a heated capillary inlet is studied and the apparent activation energy for this process also is characterized. A comparison of the data shows that acetaminophen is significantly more stable in the dry state than in solution. However, the gas-phase fragmentation of acetaminophen under ESI conditions occurs more readily than either dry- or solution-state degradation. Although the resulting electrospray fragmentation mimics the breakdown product that is formed when the compound undergoes either acid or base catalyzed hydrolysis in aqueous solutions, the mechanism that produces the fragment ion appears to involve a two-step process. Initially, the parent ion forms of the analyte are produced in the spray region of the interface followed by wall-catalyzed decomposition and re-ionization in the heated inlet capillary of the spectrometer. 相似文献
87.
The microstructure of water solubilized in H(2)O/surfactant/n-heptane ternary systems has been investigated by employing (1)H-NMR and FT-IR spectroscopic techniques. Two reverse micellar systems were prepared and studied, i.e., sodium bis(2-ethylhexyl) sulfosuccinate in n-heptane (H(2)O/AOT/n-heptane) and sodium bis(2-ethylhexyl) phosphate in n-heptane (H(2)O/NaDEHP/n-heptane). (1)H-NMR data showed that the chemical shift of water protons for the AOT and NaDEHP reverse micelles varied downfield and upfield, respectively, with an increase of the water content. The opposite shift directions with increasing water content are interpreted as due to a composition change of the solubilized water associated with head-groups and sodium counterions in reverse micellar systems. On the basis of deconvolution results of FT-IR spectra, a four-component model is proposed to interpret the FT-IR and (1)H-NMR results. Copyright 2000 Academic Press. 相似文献
88.
针对使用预测函数控制(Predictive Function Control, PFC)的 DC / DC 变换器在负载切换时产生的扰动对变
换器响应速度有较大影响的问题,提出通过设计观测器观测扰动来优化预测函数控制算法的控制效果;相对于其
他提高系统抗扰动的方法,通过设计观测器来提高变换器抗扰动的方法的优点是实现简单、优化后控制效果好等;
设计 Luenberger 观测器对负载切换时产生的扰动进行观测,将观测值反馈给预测函数控制算法进行最优化分析,
并结合仿真结果对目标函数进行调整,优化后的目标函数能更好地响应扰动并尽快达到稳态;最后将优化后的控
制算法与 PI 双闭环控制方法进行比较,结果显示优化后的控制算法在受到扰动后比双 PI 控制方法动态性能更
好,实现了应对负载切换时提高系统动态性能的目标,增强了 DC / DC 变换器的抗干扰能力;优化后的算法只考虑
了负载切换时的扰动对于系统其他方面的扰动没有考虑进去,可以通过设计整个系统的观测器来进一步提高 DC /
DC 变换器抗扰动的能力。 相似文献
89.
Czechoslovak Mathematical Journal - During the last decade, several research groups have published results on sufficient conditions for the hamiltonicity of graphs by using some topological... 相似文献
90.
Annals of the Institute of Statistical Mathematics - The research is about a systematic investigation on the following issues. First, we construct different outcome regression-based estimators for... 相似文献