全文获取类型
收费全文 | 2529篇 |
免费 | 208篇 |
国内免费 | 778篇 |
专业分类
化学 | 2858篇 |
晶体学 | 24篇 |
力学 | 105篇 |
综合类 | 21篇 |
数学 | 42篇 |
物理学 | 465篇 |
出版年
2024年 | 8篇 |
2023年 | 37篇 |
2022年 | 125篇 |
2021年 | 107篇 |
2020年 | 125篇 |
2019年 | 121篇 |
2018年 | 107篇 |
2017年 | 123篇 |
2016年 | 131篇 |
2015年 | 119篇 |
2014年 | 170篇 |
2013年 | 212篇 |
2012年 | 188篇 |
2011年 | 221篇 |
2010年 | 137篇 |
2009年 | 189篇 |
2008年 | 168篇 |
2007年 | 202篇 |
2006年 | 168篇 |
2005年 | 148篇 |
2004年 | 122篇 |
2003年 | 103篇 |
2002年 | 69篇 |
2001年 | 44篇 |
2000年 | 40篇 |
1999年 | 51篇 |
1998年 | 50篇 |
1997年 | 38篇 |
1996年 | 20篇 |
1995年 | 18篇 |
1994年 | 20篇 |
1993年 | 20篇 |
1992年 | 20篇 |
1991年 | 23篇 |
1990年 | 11篇 |
1989年 | 7篇 |
1988年 | 11篇 |
1987年 | 6篇 |
1986年 | 8篇 |
1985年 | 4篇 |
1984年 | 5篇 |
1982年 | 4篇 |
1981年 | 4篇 |
1980年 | 6篇 |
1979年 | 2篇 |
1977年 | 1篇 |
1959年 | 1篇 |
1957年 | 1篇 |
排序方式: 共有3515条查询结果,搜索用时 15 毫秒
991.
992.
合成阳离子聚电解质作为新型电极添加剂 ,考察了新型电极添加剂对离子交换膜燃料电池放电性能的影响。实验结果表明在电极中加入适量的新型电极添加剂可显著地提高燃料电池的放电性能。当电极中新型添加剂的含量为 1 .8%时 ,燃料电池的放电电压和电流密度都处于高峰值状态。实验结果还发现电荷密度为45%的聚电解质型电极添加剂对燃料电池放电性能的改性作用与价格昂贵的进口Nafion乳液添加剂的改性作用基本处于同一水平。因此在离子膜燃料电池中可采用廉价易合成的聚电解质型电极添加剂取代Nafion乳液添加剂。 相似文献
993.
Teixeira Lincoln C. Linden James C. Schroeder Herbert A. 《Applied biochemistry and biotechnology》2000,84(1-9):111-127
Previous work in our laboratories has demonstrated the effectiveness of peracetic acid for improving enzymatic digestibility
of lignocellulosic materials. The use of dilute alkali solutions as a pre-pretreatment prior to peracetic acid lignin oxidation
increased carbohydrate hydrolysis yields in a synergistic as opposed to additive manner. Deacetylation of xylan is easily
achieved using dilute alkali solutions under mild conditions. In this article, we evaluate the effectiveness of peracetic
acid combined with an alkaline pre-pretreatment through simulataneous saccharification and cofermentation (SSCF) of pretreated
hybrid poplar wood and sugar can ebagasse. Respective ethanol yields of 92.8 and 91.9% of theoretical are achieved using 6%
NaOH/15% peracetic acid-pretreated substrates and recombinant Zymomonas mobilis CP4/p ZB5. Reduction of acetyl groups of the lignocellulosic materials is demonstrated following alkaline pre-pretreatments.
Such processing may be helpful in reducing peracetic acid requirements. The influence of deacetylation is more significant
in combined pretreatments using lower peracetic acid loadings. 相似文献
994.
Thesolubilityofcompressedgasesinliquidscanbeveryhigh,whichresultsintheexpansionoftheliquids.Thes0lubilitiesofsolutesinliquidscanbereducedconsiderablybydissolutionofthegases'i.e.,thegasesactasantisolvents.Recently,somecompressedgaseshavebeenusedinmanygasantisolventpr0cesses.Understandingofthepropertiesofthecompressedgas-liquidmixtUresisofgreatimportancetoboththeoreticalstUdyandindustrialprocesses.ThetautomerismofspiropyransandtheirderivativeshasbeenstUdiedbydifferentauthors"',whichindicatedth… 相似文献
995.
996.
The Panel Method for Rigid Section Group Added Mass Coefficients and Its Application to Fuel Assemblies北大核心CSCD 下载免费PDF全文
A numerical method based on the panel method was developed to calculate the added fluid mass coefficients of the rigid section group with arbitrarily complex shapes, and was successfully applied to the PWR fuel assemblies. The variation law of added mass coefficients with position deviations was analyzed in the seismic test of 1×5 fuel assemblies. The results show that, this method is suitable for the calculation of the added mass coefficients of rigid section groups with complex continuous boundaries. Compared with the gap between assemblies, the gap between baffles and assemblies has a dominant influence on the added mass coefficient. Regardless of the position deviation, the sum of the added mass coefficients of all assemblies and baffles in the assumed motion direction is approximately equal to –1, and that in the perpendicular direction is approximately equal to 0. © 2023 Editorial Office of Applied Mathematics and Mechanics. All rights reserved. 相似文献
997.
998.
BaCe0.7Zr0.2Nd0.1O3?α ceramic was prepared by solid state reaction. Phase composition, surface and fracture morphologies of the material were characterized by using XRD and SEM, respectively. Chemical stability against carbon dioxide and water steam at the high temperature was tested. The conductivity and ionic transport number of the material were measured by ac impedance spectroscopy and gas concentration cell methods in the temperature range of 500–900°C in wet hydrogen and wet air, respectively. Using the ceramic as solid electrolyte and porous platinum as electrodes, the hydrogen‐air fuel cell was constructed, and the cell performance at the temperature from 500 to 900°C was examined. The results indicate that BaCe0.7Zr0.2Nd0.1O3?α was a single phase perovskite‐type orthorhombic system, with high density and good chemical stability in carbon dioxide and water steam atmospheres at the high temperature. The conductivity of the material in wet hydrogen and wet air was increased as the temperature rises. In wet hydrogen, the material was a pure protonic conductor with the protonic transport number of 1 from 500 to 600°C, a mixed conductor of proton and electron with the protonic transport number of 0.973–0.955 from 700 to 900°C. In wet air, the material was a mixed conductor of proton, oxide ion and electron hole. The protonic transport numbers were 0.002–0.003, and the oxide ionic transport numbers were 0.124–0.179. The fuel cell could work stably. At 900°C, the maximum short‐circuit current density and power output density were 156 mA·cm?2 and 40 mW·cm?2, respectively. 相似文献
999.
FU YuBin ZHAO ZhongKai LIU Jia LI KuiZhong XU Qian & ZHANG ShaoYun Institute of Materials Science Engineering Ocean University of China Qingdao China 《中国科学B辑(英文版)》2011,(5)
A unique sulfonated polyaniline/vanadate composite was synthesized and utilized as a composite anode in microbial fuel cells on ocean floor (BMFCs). X-ray diffraction (XRD) and thermogravimetric analysis (TGA) were employed to characterize its chemical composition and morphology. Wettability of the composite anodes decreases due to the addition of polytetrafluoroethylene (PTFE). The electrochemical behavior of the composite anodes was investigated by means of linear sweep voltammetry and Tafel plot measurem... 相似文献
1000.
提出了一种石墨化炭黑过滤吸附前处理抑制轻质燃油拉曼光谱荧光背景干扰的方法和一种改进的系统聚类分析算法,实现了39个样品的种类快速识别,即能自动将样品识别为0#车用柴油、0#普通柴油、97#车用汽油、93#车用汽油、90#车用汽油和3#喷气燃料等6种类型。过滤吸附处理方法是用定制的50 mg石墨化炭黑过滤吸附0.75 mL油样,然后对其进行拉曼光谱数据采集。试验结果证明:石墨化炭黑过滤吸附处理对无荧光背景干扰的3#喷气燃料和车用汽油样品拉曼光谱特征无明显影响,且能够有效抑制车用汽油和车用柴油样品的拉曼弱荧光背景干扰,以及车用汽油和普通柴油的强荧光背景干扰。改进的有监督系统聚类分析算法将普鲁克距离作为系统聚类分析中样本间相似度的评价方法;并将经典的系统聚类分析视为标准校正样品集的“建模”过程,通过计算未知样品与各类属中心向量之间的普鲁克距离,依据距离最小原则判断未知样品的类属。通过对39个具有不同拉曼荧光背景干扰特征油样的石墨化炭黑前处理和“留一法”交互验证分类识别,分析结果证明:石墨化炭黑过滤吸附前处理抑制拉曼光谱荧光背景的方法能够有效提取轻质燃油的拉曼光谱特征并应用于定性种类识别。 相似文献