全文获取类型
收费全文 | 1083篇 |
免费 | 147篇 |
国内免费 | 143篇 |
专业分类
化学 | 387篇 |
晶体学 | 3篇 |
力学 | 246篇 |
综合类 | 4篇 |
数学 | 118篇 |
物理学 | 615篇 |
出版年
2024年 | 1篇 |
2023年 | 17篇 |
2022年 | 12篇 |
2021年 | 25篇 |
2020年 | 44篇 |
2019年 | 38篇 |
2018年 | 18篇 |
2017年 | 57篇 |
2016年 | 53篇 |
2015年 | 29篇 |
2014年 | 74篇 |
2013年 | 81篇 |
2012年 | 50篇 |
2011年 | 91篇 |
2010年 | 65篇 |
2009年 | 70篇 |
2008年 | 79篇 |
2007年 | 72篇 |
2006年 | 66篇 |
2005年 | 69篇 |
2004年 | 55篇 |
2003年 | 33篇 |
2002年 | 31篇 |
2001年 | 19篇 |
2000年 | 38篇 |
1999年 | 35篇 |
1998年 | 14篇 |
1997年 | 14篇 |
1996年 | 23篇 |
1995年 | 18篇 |
1994年 | 11篇 |
1993年 | 13篇 |
1992年 | 9篇 |
1991年 | 7篇 |
1990年 | 10篇 |
1989年 | 7篇 |
1988年 | 6篇 |
1987年 | 7篇 |
1986年 | 2篇 |
1985年 | 1篇 |
1984年 | 4篇 |
1982年 | 3篇 |
1981年 | 1篇 |
1978年 | 1篇 |
排序方式: 共有1373条查询结果,搜索用时 546 毫秒
961.
Abstract The noise figure and sub-octave spurious-free dynamic range of a carrier-suppressed analog RF photonic link are examined. Expressions for noise figure and dynamic range are derived as a function of the modulator bias angle. A closed form expression for the bias angle that optimizes link noise figure and spurious-free dynamic range is also derived. Experimental results are presented and shown to agree with calculations. These experimental results include some of the lowest noise figure and highest dynamic range results published to date for an RF photonic link incorporating a standard Mach-Zehnder modulator: 6 dB and 122 dB·Hz2/3, respectively. 相似文献
962.
Sebastian Meier Dr. Andrew J. Benie Dr. Jens Ø. Duus Prof. Ole W. Sørensen Prof. 《Chemphyschem》2009,10(6):893-895
NMR artifact purging: Modern NMR experiments depend on efficient coherence transfer pathways for their sensitivity and on suppression of undesired pathways leading to artifacts for their spectral clarity. A novel robust adiabatic element suppresses hard‐to‐get‐at artifacts (see picture).
963.
K. Deventer O.J. PozoP. Van Eenoo F.T. Delbeke 《Journal of chromatography. A》2009,1216(31):5819-5827
Direct injection of urine has gained interest in the field of analytical toxicology, including doping control analysis. However, implementation of a direct urinalysis method for the LC–MS/MS detection of 34 diuretics and 9 other doping agents yielded several analytical problems, which were not observed using a traditional liquid–liquid extraction. Therefore a comparative study was made between liquid–liquid extraction and direct injection. Comparison of validation results showed that the liquid–liquid extraction at pH 7 allows to analyze samples without major drawbacks regarding matrix effects. Hence, good sensitivity was observed and detection limits ranged between 1 and 250 ng/mL for all compounds. In the direct injection approach shifted retention times were observed for several acidic and basic compounds due to unwanted matrix effects. This shift was reduced by a 25-fold dilution of the urine samples. Besides the improved retention time stability the diluted samples also exhibited lower ion suppression than the undiluted ones. After 25-fold dilution, detection limits ranged between 10 and 250 ng/mL for all compounds. Since these detection limits are at or below the minimum required performance level, imposed by the World Anti-Doping Agency, the method could be applied to routine anti-doping analysis. Samples, previously declared positive, were reanalysed using both the liquid–liquid extraction and direct injection. With both techniques all 26 samples were found to be positive, showing the applicability of direct injection for the analysis of diuretics. 相似文献
964.
Na Yan Zaifang Zhu Ning Ding Lei Zhou Yalei Dong Xingguo Chen 《Journal of chromatography. A》2009,1216(49):8665-8670
Transient isotachophoresis (tITP) can improve the sensitivity of capillary electrophoresis (CE). In general, it was carried out under the condition of suppressed electroosmotic flow (EOF). However, some special conditions, such as extreme low pH background electrolyte and coating were needed to achieve the requirements of suppressed EOF. In this work, an approach of tITP under the strong counter-EOF in open system (counter-EOF-tITP) is presented for the rapid and sensitive preconcentrating the reduced glutathione (GSH) and the oxidized glutathione (GSSG) without modifying the capillary and the commercial CE instrument. The parameters of the experimental system, such as the concentration of leading electrolyte, the injected amount of terminating electrolyte and the injected pressure of sample were investigated in detail to understand the mechanism of counter-EOF-tITP. The sensitivity enhancement factors were of 320 for GSH and 280 for GSSG. In addition, the detection limit of 23.4 and 18.0 μg L−1 for GSH and GSSG was achieved, respectively. The method's applicability was demonstrated by determining GSH and GSSG in tomato and human serum. 相似文献
965.
A novel preconcentration/separation approach, which online combined CZE with CD‐modified MEKC, was developed for simultaneous enhancing resolving power and detection sensitivity. CZE with cation‐selective exhaustive injection and transient ITP preconcentration was used as the first dimension, from which the effluent fractions were further analyzed by CD‐modified MEKC acting as the second dimension. As the key to successful integration of CZE with MEKC, a new interface was designed and electroaccumulation focusing strategy was employed to avoid analyte band diffusion at the interface. The comprehensive 2‐D system was successfully established with only one high voltage and four electrodes. The grouping of two orthogonal separation techniques, together with analytes preconcentration techniques, significantly enhanced resolution and sensitivity for 2‐D separation of cationic compounds. The resulting electrophoregram was quite different from that of either single CZE or MEKC. Up to 14 000‐ to 35 000‐fold improvement in sensitivity was obtained relative to conventional electrokinetic injection method. The limits of detection (S/N=3) were in the range of 0.03–0.1 μg/L. The number of theoretical plates was in the range of 103 000–184 000. This method was successfully applied to the analysis of trace cationic cardiovascular drugs in wastewater. 相似文献
966.
We utilize time‐resolved electrokinetic measurements in order to study the electrokinetic properties of silica and TOPAS microfluidic channels as a function of the time history of the fluid–solid interface. In pressure‐driven flow through TOPAS microchannels, the ζ‐potential as inferred from streaming potential measurements decays exponentially by a factor of 1.5 with a characteristic decay time of 3 h after the initial formation of the fluid–solid interface. A similar exponential decay is observed immediately after water is exchanged for ethanol as the solvent in the system. In electroosmotically driven flow through TOPAS microchannels, the ζ‐potential as inferred through current monitoring experiments was constant in time. No electrokinetic transients were observed in silica microchannels under these flow conditions. 相似文献
968.
Suppression of methane formation during Fisher-Tropsch synthesis using manganese-cobalt oxide supported on H-5A zeolite as a catalyst 下载免费PDF全文
In Fischer-Tropsch synthesis reaction, methane formation is one of the side reactions which must be suppressed in order to get better catalytic selectivity for light olefins. In the present study, we have modified cobalt based Fischer-Tropsch catalyst and developed a process to minimize methane production, consequently to produce maximum yield of light olefins. Manganese-cobalt oxide supported on H-5A zeolite catalyst was synthesized using modified H-5A zeolite, to increase its surface acid sites. Increased acidity of zeolite plays a major part in the suppression of methane formation during the Fischer-Tropsch reaction. The modified zeolite results in the electronic modification of catalyst surface by creating new active catalytic sites. The results are compared with other supported catalysts along with unmodified zeolite. Appreciable reduction in methane formation is achieved on modified zeolite supported catalyst in comparison with unsupported catalyst. 相似文献
969.
To avoid disclosure of individual establishment information, data records may have to be suppressed in regional economic databases, with values represented by flags. This paper investigates this suppression process and presents a goal-programming optimization approach to estimate these flagged data, using the 2000 County Business Patterns (CBP) database as a case study. The approach minimizes the sum of weighted deviations between the estimates and target values, subject to constraints related to county and sector total employment, as well as to flag and establishment size intervals. The model is tested using Ohio and Arizona data, for both sources of inconsistencies and parameter selection. A decision-theoretic analysis of the test results points to specific strategies that yield the best estimates of the suppressed data. 相似文献
970.
Z. Tomkowicz M. Ba
anda A. J. Zaleski 《Physica C: Superconductivity and its Applications》2002,370(4):259-268
AC magnetic susceptibility measurements were performed for ceramics of the Ho1−xPrxBa2Cu3O7−δ system as a function of temperature, frequency, field amplitude and static magnetic field. From the measurements, temperature dependences of intergranular critical currents and corresponding pinning energies at grain boundaries were determined for different Pr concentrations. These critical currents strongly decrease with Pr doping. They are limited by superconductor–insulator–superconductor or superconductor–normal metal–superconductor junctions and for higher Pr concentrations additionally suppressed by flux creep. Also intragranular critical current at 4.3 K was determined as a function of Pr concentration from magnetic hysteresis loops. It is a monotonically decreasing function of Pr doping. 相似文献