全文获取类型
收费全文 | 4562篇 |
免费 | 1022篇 |
国内免费 | 826篇 |
专业分类
化学 | 4484篇 |
晶体学 | 48篇 |
力学 | 31篇 |
综合类 | 37篇 |
数学 | 19篇 |
物理学 | 866篇 |
无线电 | 925篇 |
出版年
2024年 | 29篇 |
2023年 | 213篇 |
2022年 | 195篇 |
2021年 | 372篇 |
2020年 | 432篇 |
2019年 | 279篇 |
2018年 | 245篇 |
2017年 | 217篇 |
2016年 | 279篇 |
2015年 | 256篇 |
2014年 | 281篇 |
2013年 | 418篇 |
2012年 | 277篇 |
2011年 | 285篇 |
2010年 | 182篇 |
2009年 | 224篇 |
2008年 | 237篇 |
2007年 | 259篇 |
2006年 | 232篇 |
2005年 | 195篇 |
2004年 | 156篇 |
2003年 | 163篇 |
2002年 | 112篇 |
2001年 | 97篇 |
2000年 | 101篇 |
1999年 | 72篇 |
1998年 | 74篇 |
1997年 | 68篇 |
1996年 | 63篇 |
1995年 | 74篇 |
1994年 | 58篇 |
1993年 | 48篇 |
1992年 | 58篇 |
1991年 | 32篇 |
1990年 | 31篇 |
1989年 | 20篇 |
1988年 | 17篇 |
1987年 | 7篇 |
1986年 | 13篇 |
1985年 | 3篇 |
1984年 | 7篇 |
1983年 | 7篇 |
1982年 | 2篇 |
1981年 | 4篇 |
1977年 | 2篇 |
1976年 | 1篇 |
1975年 | 3篇 |
1974年 | 7篇 |
1973年 | 3篇 |
排序方式: 共有6410条查询结果,搜索用时 828 毫秒
941.
WenXing Chen Fan Liu WangYang Lü XiaoYuan Shen YuYuan Yao MinHong Xu 《中国科学B辑(英文版)》2008,51(6):570-576
A novel reactive metallophthalocyanine derivative,zinc tetra(2,4-dichloro-1,3,5-triazine)aminophthalo-cyanine(Zn-TDTAPc),was prepared and immobilized on poly(N-isopropylacrylamide)(PNIPAAm) by covalent bonding to obtain a thermosensitive polymer(Zn-TDTAPc-g-PNIPAAm).Compared with zinc tetraaminophthalocyanine(Zn-TAPc),Zn-TDTAPc-g-PNIPAAm exhibits excellent solubility in water and in most organic solvents.Furthermore,it has a special thermosensitive property in water and the lower critical solution temperature(LCST) is 34.1℃.It was found that both dissolved and precipitated Zn-TDTAPc-g-PNIPAAm present high photoactivity evidenced by the experiment of photocatalytic degra-dation of 1,3-diphenylisobenzofuran(DPBF) in the presence of Zn-TDTAPc-g-PNIPAAm.These proper-ties suggest that it can be used potentially in photodynamic therapy(PDT). 相似文献
942.
Hydrogen generation through thermal chemical water splitting technology has recently received in- creasingly international interest in the nuclear hydrogen production field. Besides the main known sulfur-iodine (S-I) cycle developed by the General Atomics Company and the UT3 cycle (iron, calcium, and bromine) developed at the University of Tokyo, the thermal cycle based on metal oxide two-step water splitting methods is also receiving research and development attention worldwide. In this work, copper ferrite was prepared by the co-precipitation method and oxygen-deficient copper ferrite was synthesized through first and second calcination steps for the application of hydrogen production by a two-step water splitting process. The crystal structure, properties, chemical composition and δ were investigated in detail by utilizing X-ray diffraction (XRD), thermogravimetry (TG) and differential thermal analysis (DTA), atomic absorption spectrometer (AAS), ultraviolet spectrophotometry (UV), gas chro- matography (GC), and so on. The experimental two-step thermal chemical cycle reactor for hydrogen generation was designed and developed in this lab. The hydrogen generation process of water splitting through CuFe2O4-δ and the cycle performance of copper ferrite regeneration were firstly studied and discussed. 相似文献
943.
In this paper, the powder microelectrode technique was employed in studying the voltametric response of the O2/2− couple, which demonstrated a nearly reversible redox process at an acetylene black powder microelectrode in N,N-dimethylformamide
(DMF). A well-developed steady state current plateau for the electrochemical reduction of oxygen was obtained in this system.
The electron transfer number (n) and heterogeneous electron transfer rate constant k
s were measured by steady-state voltametric response, and the results were 1.08, 3.4 × 10−3 cm s−1, respectively. Additionally, the scavenging activity of O2− with biological antioxidant (ascorbic acid) was evaluated by cyclic voltammetry; IC50 came to 5 × 10−4 mol/l.
Published in Russian in Elektrokhimiya, 2008, Vol. 44, No. 8, pp. 1040–1044.
The text was submitted by the authors in English. 相似文献
944.
C. A. C. Sequeira D. M. F. Santos P. S. D. Brito 《Russian Journal of Electrochemistry》2008,44(7):852-856
The electrocatalysis of the oxygen reduction reaction by lanthanum-strontium manganate La0.5Sr0.5MnO3 (LSM) has been studied by cyclic voltammetry using the rotating ring-disc electrode technique (RRDE) in alkaline medium.
From the ring-disc data and other kinetic parameters it was concluded that the oxygen reduction occurs by dissociative chemisorption
at low overpotentials. At higher overpotentials, the formation of hydrogen peroxide (HO2− in this case) on the electrocatalyst has been observed. The apparent exchange current density for oxygen reduction on LSM
has been found to be 2 × 10−7 A cm−2, while the corresponding Tafel slope is 0.100 V per decade. The possible reaction mechanism for electroreduction of oxygen
on this oxide catalyst has been discussed.
Published in Russian in Elektrokhimiya, 2008, Vol. 44, No. 7, pp. 919–923.
The text was submitted by the authors in English. 相似文献
945.
Xiaoyang Cheng Xiaotian Jiang Shuhu Yin Lifei Ji Yani Yan Guang Li Rui Huang Prof. Chongtai Wang Prof. Honggang Liao Prof. Yanxia Jiang Prof. Shigang Sun 《Angewandte Chemie (International ed. in English)》2023,62(34):e202306166
To achieve the Fe−N−C materials with both high activity and durability in proton exchange membrane fuel cells, the attack of free radicals on Fe−N4 sites must be overcome. Herein, we report a strategy to effectively eliminate radicals at the source to mitigate the degradation by anchoring CeO2 nanoparticles as radicals scavengers adjacent (Scaad-CeO2) to the Fe−N4 sites. Radicals such as ⋅OH and HO2⋅ that form at Fe−N4 sites can be instantaneously eliminated by adjacent CeO2, which shortens the survival time of radicals and the regional space of their damage. As a result, the CeO2 scavengers in Fe−NC/Scaad-CeO2 achieved ∼80 % elimination of the radicals generated at the Fe−N4 sites. A fuel cell prepared with the Fe−NC/Scaad-CeO2 showed a smaller peak power density decay after 30,000 cycles determined with US DOE PGM-relevant AST, increasing the decay of Fe−NCPhen from 69 % to 28 % decay. 相似文献
946.
Several octasubstituted zinc azaphthalocyanines (ZnAzaPcs) of the tetrapyrazinoporphyrazine type have been synthesized as potential sensitizers for photodynamic therapy (PDT). Octasubstituted complexes, with thiophen-2-yl, thiophen-3-yl or benzo[b]thiophen-3-yl peripheral groups, were synthesized and characterized. Octa(thiophen-2-yl) ZnAzaPc is a better singlet oxygen producer and has a red shifted UV absorption Q-band compared to both thiophen-3-yl and benzo[b]thiophen-3-yl substituted ZnAzaPcs. Thus, the thiophen-2-yl substituent is better suited for our purpose. Unsymmetrically substituted ZnAzaPcs were synthesized by cyclotetramerisations of pyrazine-2,3-dicarbonitriles attached to one thiophen-2-yl group and one alkylsulfanyl, thiomorpholinyl or imide group. Constitutional isomers were detected by NMR spectroscopy for some of these complexes. Compared to unsubstituted ZnAzaPc, red shifted Q-bands were observed for all these complexes, due to the presence of thiophen-2-yl groups. The least promising complexes are ZnAzaPcs with thiomorpholine or imide peripheral substituents, i.e. where the peripheral substituents are attached to the macrocycle through nitrogen atoms. Low singlet oxygen quantum yields (ΦΔ) and also low fluorescence quantum yields (ΦF) were observed for these ZnAzaPcs. In the case of combined thiophen-2-yl and alkylsulfanyl substituents, the values of ΦΔ were the highest and reached values of approximately 0.69. 相似文献
947.
Hong-Yong Zhao Yi-Ming Cao Xiao-Li Ding Mei-Qing Zhou Quan Yuan 《Journal of membrane science》2008,310(1-2):365-373
A series of copolymers containing ether oxygen groups and amino groups were prepared based on N,N-dimethylaminoethyl methacrylate (DMEMA) and polyethylene glycol methyl ether methyl acrylate (PEGMEMA). The effect of PEGMEMA content in the copolymer on density, free volume, mechanical performance, and H2, CO2, N2 and CH4 gas transport properties of the copolymer was determined. Free volume was characterized using the polymer density and group contribution theory. The permeability of the copolymer to CO2 is high, and both the CO2/N2 and CO2/H2 selectivities are high. For example, the permeability coefficient of PDMAEMA–PEGMEMA-90 (“90” represents the weight percent of PEGMEMA) to CO2 is 112 Barrer and the CO2/N2 and CO2/H2 selectivity coefficients are 31 and 7, respectively. The effect of the temperature on gas transport properties was also determined. Finally, the potential application of the copolymer membranes for CO2/light gases separation was explored. 相似文献
948.
Maria Ascensão Trancoso Sandra Catarina S. Calisto 《Accreditation and quality assurance》2008,13(1):19-28
Biochemical oxygen demand after 5 days (BOD5) is a bioassay frequently used to determine the polluting strength of wastewaters. It quantifies the mass concentration of
dissolved O2 consumed during an incubation period of 5 days at 20 °C, in the dark, by biological oxidation of organic and/or inorganic
matter in wastewater. In this work, the BOD5 measurand is reported as γ(O2). Reporting analytical results with estimation of uncertainty is an ISO/IEC 17025:2005 requirement and allows results to
be compared and their fitness for purpose to be demonstrated. The γ(O2) uncertainty was estimated using two models: the individual components’ model (model 1) and the quality-control-criteria
model in which the trueness component is calculated using the equation for recovery from CRMs (model 2.A) and the standard
deviation of all recoveries (model 2.B). Model 1 provides a realistic strategy for estimation of γ(O2) uncertainty and for assessment of performance. Models 2.A and 2.B are expeditious and simple procedures for the 100–800 mg L−1 range. When γ(O2) < 100 mg L−1 model 1 must be used because with both Models 2 uncertainty was underestimated, more drastically close to the limit of quantification.
Electronic supplementary material The online version of this article (doi:) contains supplementary material, which is available to authorized users. 相似文献
949.
Nowadays, micro-oxygenation is a very important technique used in aging wines in order to improve their characteristics. The techniques of wine tank aging imply the use of small doses of oxygen and the addition of wood pieces of oak to the wine. Considering the low dissolved oxygen (DO) levels used by micro-oxygenation technique it is necessary to choose the appropriate measurement principle to apply the precise oxygen dosage in wine at any time, in order to assure its correct assimilation. This knowledge will allow the oenologist to control and run the wine aging correctly.This work is a thorough revision of DO measurement main technologies applied to oenology. It describes the strengths and weaknesses of each of them, and draws a comparison of their workings in wine measurement. Both, the traditional systems by electrochemical probes, and the newest photoluminescence-based probes have been used. These probes adapted to red wines ageing study are then compared.This paper also details the first results of the dissolved oxygen content evolution in red wines during a traditional and alternative tank aging. Samples have been treated by three different ageing systems: oak barrels, stainless-steel tanks with small oak wood pieces (chips) and with bigger oak pieces (staves) with low micro-oxygenation levels. French and American oak barrels manufactured by the same cooperage have been used. 相似文献
950.
Orozco J Fernández-Sánchez C Mendoza E Baeza M Céspedes F Jiménez-Jorquera C 《Analytica chimica acta》2008,607(2):176-182
This work reports on the development of a graphite-polystyrene composite electrode of planar configuration, containing silver(II) oxide and copper(II) oxide catalysts (AgO-CuO), for the measurement of electrochemical oxygen demand (EOD). Optimisation studies of the composite composition as well as conditions for its processing on planar substrates and generation of an appropriate electrochemical active area resulted in the scalable fabrication of robust composite electrodes. These were evaluated with glucose as target analyte. They showed competitive low limits of detection in a linear concentration range from 5 mg L−1 to 1400 mg L−1 of O2. Besides, they were stable for at least one year. The determination of EOD in wastewater samples coming from production lines of parenteral food and winemaking was successfully carried out. 相似文献