全文获取类型
收费全文 | 9112篇 |
免费 | 707篇 |
国内免费 | 530篇 |
专业分类
化学 | 2110篇 |
晶体学 | 14篇 |
力学 | 1112篇 |
综合类 | 78篇 |
数学 | 1255篇 |
物理学 | 1428篇 |
无线电 | 4352篇 |
出版年
2024年 | 23篇 |
2023年 | 90篇 |
2022年 | 178篇 |
2021年 | 234篇 |
2020年 | 277篇 |
2019年 | 181篇 |
2018年 | 173篇 |
2017年 | 317篇 |
2016年 | 425篇 |
2015年 | 385篇 |
2014年 | 559篇 |
2013年 | 600篇 |
2012年 | 593篇 |
2011年 | 659篇 |
2010年 | 503篇 |
2009年 | 519篇 |
2008年 | 509篇 |
2007年 | 538篇 |
2006年 | 519篇 |
2005年 | 424篇 |
2004年 | 428篇 |
2003年 | 366篇 |
2002年 | 296篇 |
2001年 | 207篇 |
2000年 | 187篇 |
1999年 | 153篇 |
1998年 | 144篇 |
1997年 | 139篇 |
1996年 | 127篇 |
1995年 | 115篇 |
1994年 | 83篇 |
1993年 | 66篇 |
1992年 | 47篇 |
1991年 | 44篇 |
1990年 | 36篇 |
1989年 | 18篇 |
1988年 | 27篇 |
1987年 | 24篇 |
1986年 | 16篇 |
1985年 | 19篇 |
1984年 | 15篇 |
1982年 | 16篇 |
1981年 | 6篇 |
1980年 | 7篇 |
1979年 | 14篇 |
1978年 | 8篇 |
1977年 | 8篇 |
1976年 | 11篇 |
1974年 | 4篇 |
1973年 | 3篇 |
排序方式: 共有10000条查询结果,搜索用时 15 毫秒
101.
Qing-Ping Mei Yi-Ke Tang Tai-Fu Li Li-Zhong Yao Qiong Yang Heng-Jian Zhang 《Analytical letters》2018,51(8):1176-1193
The calibration model of near-infrared (NIR) spectra established using the Kalman filter-partial least square (partial least squares combined with a Kalman filter) method can be adapted to outdated equipment, environmental changes, external samples, and other applications. However, the variance of the measurement noise estimation for NIR spectrum measurements cannot be easily obtained using Kalman filter-partial least squares; therefore, the variance in the measurement noise is often assumed to be zero for the Kalman filter-partial least square calibration model, which affects the stability of the model. In this study, the measured input and output data were used effectively, and the gamma test method for estimating the measurement noise variance was used to improve the stability of the Kalman filter-partial least square calibration model. First, an accurate estimation of the measurement noise variance was obtained, and accurate modeling was then performed using Kalman filter-partial least squares. Finally, 600 abandoned drilling fluid samples were used to confirm the validity of the proposed method. The Kalman filter-partial least square and gamma test-Kalman filter-partial least square methods are compared. Testing of external samples 401–600 demonstrated that the stability of the Kalman filter-partial least square model decreased. The root mean square error of the prediction of the Kalman filter-partial least square model was 27.135, which was worse than that of the gamma test-Kalman filter-partial least square model (20.307). The validation results show that the proposed method has better stability in tracking the evolution of the NIR spectrometer’s measurement state. 相似文献
102.
Bar adsorptive micro-extraction using three powdered activated carbons (ACs) as adsorbent phases followed by liquid desorption and high performance liquid chromatography with diode array detection (BAμE(ACs)-LD/HPLC-DAD), was developed to monitor triazinic herbicides (atrazine, simazine and terbutylazine) in environmental water matrices. ACs used present apparent surface areas around 1000 m2 g−1 with an important mesoporous volume and distinct surface chemistry characteristics (pHPZC ranging from 6.5 to 10.4). The textural and surface chemistry properties of the ACs adsorbent phases were correlated with the analytical data for a better understanding of the overall enrichment process. Assays performed on 10 mL water samples spiked at the 10.0 μg L−1 levels under optimized experimental conditions yielded recoveries around 100% for the three herbicides under study. The analytical performance showed good precision (RSD < 15.0%), convenient detection limits (≈0.1 μg L−1) and suitable linearity (1.0-12.0 μg L−1) with good correlation coefficients (r2 > 0.9914). By using the standard addition method, the application of the present method on real water matrices, such as surface water and wastewater, allowed very good performances at the trace level. The proposed methodology proved to be a suitable sorptive extraction alternative for the analysis of priority pollutants with polar characteristics, showing to be easy to implement, reliable, sensitive and requiring a low sample volume to monitor triazinic compounds in water matrices. 相似文献
103.
Marek Trojanowicz 《Journal of separation science》2010,33(3):359-371
Microcystins are cyclic peptide hepatotoxins that are produced during blooming of cyanobacteria. In recent two decades they gained a great interest because of the related ecological and public health risks. Over 70 different analogs of microcystins have been isolated from natural blooms and laboratory cultures of cyanobacteria. Because they are inhibitors of protein phosphatases they act as tumor promoters, but on the other hand, inhibitive enzymatic and ELISA methods may be employed for their screening or quantitative determination. Much larger analytical potential for this purpose have, however high‐performance separation chromatographic and electromigration techniques. Based on about 70 references from original journal papers these methods are reviewed together with presentation of their preconcentration methods and clean‐up procedures for analyzing environmental samples. Some attention is also focused on increasing application of LC/MS methods, and comparison of separation techniques with immunochemical or enzymatic methods. 相似文献
104.
105.
Fandong Meng Genhui Xu Baowei Wang Xinbin Ma. School of Chemical Engineering Technology Tianjin University Tianjin China . Luoyang Petrochemical Engineering Corporation Luoyang ChinaManuscript received March revised Apr 《天然气化学杂志》2002,(Z1)
This article describes a process for the synthesis of diethyl oxalate by a coupling reaction of carbon monoxide, catalyzed by palladium in the presence of ethyl nitrite. The kinetics and mechanism of the coupling and regeneration reaction are also discussed. This paper presents the results of a scale-up test of the catalyst and the process based on an a priori computer simulation. 相似文献
106.
Monitoring nitrite with optical sensing films 总被引:1,自引:0,他引:1
Ali A. EnsafiA. Kazemzadeh 《Microchemical Journal》2002,72(2):193-199
A new, low-cost nitrite sensor was developed by immobilizing a direct indicator dye in an optical sensing film for food and environmental monitoring. This sensor was fabricated by binding Safranine O to a cellulose acetate film that had previously been subjected to an exhaustive base hydrolysis. The membrane has good durability (>12 months) and a short response time (<8 s). Nitrite can be determined for the range of 0.005-2.00 μg ml−1 with 3δ detection limits of 0.001 μg ml−1. The method is easy to perform and uses acetylcellulose as a carrier. The reagents used for the activity of the cellulose support are inexpensive, non-toxic and widely available. 相似文献
107.
Summary Ten pesticides have been completely separated by two-dimensional (2D) development on TLC plates coated with coupled layers
of octadecyl silica (reversed-phase, RP) and plain silica (normal-phase, NP). The binary mobile phases, aqueous-organic for
RP chromatography and nonaqueous for NP chromatography, were chosen from plots ofR
F against mobile-phase composition and graphicalR
F(RP)-R
F(NP) correlations. The different selectivity of the RP and NP systems enabled dispersion of spots over the plate area and good
separation. 相似文献
108.
Sampling and sample-preparation strategies based on solid-phase microextraction for analysis of indoor air 总被引:3,自引:0,他引:3
Applications of solid-phase microextraction (SPME) to the sampling and analysis of volatile organic compounds in indoor air are reviewed, including a summary of quantification methods, coatings, compounds, concentrations, sampling locations and times, and detection limits. Strategies for on-site and off-site sampling and analysis, advantages and challenges associated with SPME for air sampling are discussed. 相似文献
109.
110.