全文获取类型
收费全文 | 69530篇 |
免费 | 6315篇 |
国内免费 | 7244篇 |
专业分类
化学 | 60458篇 |
晶体学 | 890篇 |
力学 | 3220篇 |
综合类 | 494篇 |
数学 | 4392篇 |
物理学 | 13635篇 |
出版年
2024年 | 33篇 |
2023年 | 766篇 |
2022年 | 1266篇 |
2021年 | 1577篇 |
2020年 | 2276篇 |
2019年 | 2121篇 |
2018年 | 2034篇 |
2017年 | 2652篇 |
2016年 | 3069篇 |
2015年 | 2678篇 |
2014年 | 3275篇 |
2013年 | 5817篇 |
2012年 | 4406篇 |
2011年 | 4549篇 |
2010年 | 3799篇 |
2009年 | 4671篇 |
2008年 | 4077篇 |
2007年 | 4348篇 |
2006年 | 3761篇 |
2005年 | 3397篇 |
2004年 | 3245篇 |
2003年 | 2575篇 |
2002年 | 2100篇 |
2001年 | 1674篇 |
2000年 | 1478篇 |
1999年 | 1252篇 |
1998年 | 1116篇 |
1997年 | 1009篇 |
1996年 | 1029篇 |
1995年 | 1060篇 |
1994年 | 941篇 |
1993年 | 709篇 |
1992年 | 660篇 |
1991年 | 534篇 |
1990年 | 406篇 |
1989年 | 359篇 |
1988年 | 367篇 |
1987年 | 334篇 |
1986年 | 221篇 |
1985年 | 199篇 |
1984年 | 201篇 |
1983年 | 67篇 |
1982年 | 155篇 |
1981年 | 145篇 |
1980年 | 141篇 |
1979年 | 148篇 |
1978年 | 105篇 |
1977年 | 76篇 |
1976年 | 71篇 |
1973年 | 39篇 |
排序方式: 共有10000条查询结果,搜索用时 15 毫秒
81.
Chunlai Wu Chuanyun Peng 《International journal of environmental analytical chemistry》2019,99(7):670-682
To develop an accurate and precise method for separation and pre-concentration of Hg(II), a novel thionin functionalised core shell structure magnetic material has been prepared and characterised. The extraction ability of the material was evaluated by magnetic solid-phase extraction coupled with inductively coupled plasma mass spectrometry determination of Hg(II) in food and water samples. Combining the advantages of magnetic separation with selective extraction of thionin towards Hg(II), the material exhibits enhanced enrich selectivity and efficiency for Hg(II). The experimental parameters influencing Hg(II) extraction efficiency, including pH of the aqueous solution, the dosage of the adsorbent, extraction time and sample volume, were systematically investigated. Under the optimised conditions, concentration of Hg(II) at 1.0 μg L?1 can be successfully enriched by the material without the interference of the common co-existing ions. The enrichment factor and adsorption capacity were 250 and 75.2 mg g?1, and precise of the method was confirmed by analysing the spiked food, water samples and standard water reference samples with the recoveries of 92.5–101.8%. 相似文献
82.
Teng GUO Zhen PENG Hui ZHU Li XU Jun-Guo DONG Zheng-Xu HUANG Ping CHENG Zhen ZHOU 《分析化学》2019,47(1):13-22
Ion funnel is a new-style ion guider which can reduce spatial divergence and energy dispersity of the transmission ions by using radio frequency (RF) electric field to confine the ions radially and the direct current (DC) axial electric field to move the ions toward the exit, and thus it can greatly increase the ion transmission efficiency and improve the sensitivity of the mass spectrometry. Since ion funnel was invented in 1997, it has attracted a close attention of mass spectrometry scientists all over the world. Ion funnel has been used in various kinds of mass spectrometry, and built a bridge with high efficiency ion transmission between low vacuum ionization source and high vacuum mass analyzer. In this paper, the principle, technology development, and application progress of ion funnel are reviewed, and the future prospects are prospected. 相似文献
83.
设计了以柴油中饱和烃含量及碳数分布测定、聚丙烯酸高级醇酯(PTA)的合成与纯化、PTA的红外表征与分子量测定、PTA降凝性能评价等4个模块组成的综合实验,作为化工专业实验的考核及成绩评定平台。这种考核方式将过程考核与结果考核融合,将笔试、操作、设计融合,可以全面准确地考核学生的有机合成、仪器分析等基础实验技能,同时考核综述、数据处理、实验设计等综合实验能力和实验室工作素质。此外,还能够有效训练学生的学习能力、解决问题能力、实践能力和创新能力,并促进了实验教学水平提高。 相似文献
84.
《大分子反应工程》2018,12(1)
A model is used to simulate batch copolymerization of caprolactam with hexamethylene diamine (HMD) and adipic acid (ADA) to produce nylon 6/6,6. Four different recipes are considered: a recipe containing caprolactam and an aqueous solution of HMD and ADA, a recipe containing caprolactam and dry HMD/ADA salt, and two recipes with a portion of the caprolactam replaced by nylon 6 cyclic dimer (CD). Consuming CD would be advantageous because CD is an undesirable side product from nylon 6 production. Simulation results lead to three important findings: (i) operation using dry salt rather than aqueous salt solution leads to higher degree of polymerization, (ii) substantial quantities of CD can be consumed to produce nylon 6/6,6 copolymer, and (iii) including water in the recipe is beneficial for achieving improved consumption of CD. The results of this study will be helpful in designing experiments aimed at improving industrial nylon 6/6,6 copolymerization processes. 相似文献
85.
Xiaofeng Zheng Xinyu Zhao Jie Huang Hao Yang Qinghui Wang Yuhang Liu Peiwei Gong Laijin Tian Hongping Xiao Zhe Liu 《应用有机金属化学》2020,34(12):e5981
The low-cost, high specific surface area and porosity, controlled pore size, and chemical properties of metal–organic framework (MOF) materials have attracted much attention in the exploration of proton conduction. The method of chemically modifying MOF structures or introducing conductive medium into the holes can effectively improve the proton conductivities of the materials. Here, the structural tunability of ionic liquid (IL) and flexible MOF (fle-MOF) materials are matched to give full play to the conductivity of IL, the framework support, and the microporous effect of MOFs, which achieves the synergistic effect of performance and expands the temperature range of proton transfer. Three kinds of CS/IL@fle-MOF membranes were prepared by combining three fle-MOFs with 1-carboxymethyl-3-methylimidazole (CMMIM) in different proportions to obtain 15 pieces of membranes. The comparative analyses show that CS/IL@fle-MOF membranes have excellent proton conduction performance at a wider temperature range (263–353 K) and lower relative humidity (75% RH). Among them, the proton conductivities of CS/CMMIM@MIL-88A-25% and CS/CMMIM@MIL-88B-125% are up to 1.33 and 1.42 S cm−1 at 75% RH and 353 K, respectively; whereas those of CS/CMMIM@MIL-53(Fe)-75% and CS/CMMIM@MIL-88B-125% reach up to 2.1 × 10−3 and 1.28 × 10−3 S cm−1 at 75% RH and 263 K, respectively. The Ea of CS/CMMIM@fle-MOFs is in the range of 0.1–0.5 eV, suggesting that the proton transport follows predominantly the typical Grotthuss transfer mechanism. The results of this study indicate that the CS/CMMIM@fle-MOF membranes combinations offer great potential for the design of composite porous proton-conducting materials. 相似文献
86.
A novel metal-doped metal–organic framework (MOF) was developed by incorporating salen–Mg into NH2–MIL-101(Cr) structure under ambient conditions. The Schiff base complex was successfully prepared by condensing salicylaldehyde with a free amino group and then coordinating metal ions. Such a structure can endow the sample with higher CO2 adsorption performance. At 0°C and 1 bar, the salen–Mg-modified sample achieves the maximum adsorption capacity of 2.18 mmol g−1 for CO2, which was 5.8% higher than the pristine salen–MOF under the same conditions. Notably, the Freundlich model indicates that the CO2 adsorption process of all samples conforms to reversible adsorption. However, the correlation coefficients (R2) of the Mg-doped sample are lower than that of the pristine sample. Besides, the CO2/N2 adsorption selectivity and isosteric heat also show a similar trend. These results indicate that the salen–Mg can enhance the interaction between the material and CO2 molecules. 相似文献
87.
Álvaro Santana-Mayor Ruth Rodríguez-Ramos Bárbara Socas-Rodríguez Miguel Ángel Rodríguez-Delgado Giovanni D'Orazio 《Electrophoresis》2020,41(20):1768-1775
The separation of 11 phthalic acid ester (PAEs) was carried out by nano-liquid chromatography coupled to ultraviolet and MS detection. Preliminary experiments were achieved in order to select suitable stationary phases and chromatographic conditions. The baseline separation was obtained, for all compounds, with an XBridgeTM C18 column in less than 15 min, working in step gradient mode. The sensitivity of the method was improved by on-column focusing. PAEs were extracted from alcoholic and nonalcoholic beverages using vortex-assisted emulsification dispersive liquid–liquid microextration and natural deep eutectic solvents. The whole method was validated in terms of linearity, sensitivity, precision, recovery, and repeatability. Combination of both off-line sample preparation preconcentration and large injection volume led to obtain LOQs in the range 5–47 ng/mL. The developed nano-LC-UV method was extended to MS detection to confirm the presence of PAEs in some beverages commercialized in different types of packaging. 相似文献
88.
Determination of sugars and cyclitols isolated from various morphological parts of Medicago sativa L
《Journal of separation science》2018,41(5):1118-1128
Plant research interest has increased all over the world, and a large body of evidence has been collected to show the huge potential of medicinal plants in various disease treatments. Medicago sativa L., known as alfalfa, is a rich source of biologically active components and secondary metabolites and was frequently used from the ancient times both as fodder crop and as a traditional medicine in the treatment of various diseases. Cyclitols, naturally occurring in this plant, have a particular interest for us due to their significant anti‐diabetic, antioxidant, anti‐inflammatory, and anti‐cancer properties. In the present study we revealed the isolation, the identification, and the quantification of some cyclitols and sugars extracted from different morphological parts of alfalfa plant. Soxhlet extraction and solid phase extraction were used as extraction and purification methods, while for the analyses derivatization followed by gas chromatography with mass spectrometry was involved. The obtained results showed significant differences in the quantities of cyclitols and sugars found in the investigated morphological parts, ranging between 0.02 and 13.86 mg/g of plant in case of cyclitols, and in the range of 0.09 and 40.09 mg/g of plant for sugars. However, roots have the richest part of cyclitols and sugars in contrast to the leaves. 相似文献
89.
《Journal of separation science》2018,41(5):1083-1090
A method based on ultra‐high performance liquid chromatography was developed and validated to detect six thyreostatic compounds: tapazole, thiouracil, methylthiouracil, dimethylthiouracil, propylthiouracil, and phenylthiouracil in faeces of bovine. Thyreostats were extracted from the matrix with a mixture of methanol and buffer (pH = 8). Next step was derivatization of analytes with 3‐iodobenzylbromide. The liquid chromatographic separation of derivatives was obtained on a SB‐C18 column (50 × 2.1 mm; 1.8 μm, Agilent) with gradient elution using a mobile phase consisting of acetonitrile/0.1% acetic acid within 7.5 min. The analysis was performed on a Shimadzu NEXERA X2 ultra‐high performance liquid chromatograph with triple quadrupole MS 8050 instrument operating in positive electrospray ionization mode. Depending on the target compound, two or three diagnostic signals (selected reaction monitoring transitions) were monitored. The procedure was validated according to the Commission Decision 2002/657/EC. Recovery and repeatability met the performance criteria specified by this document for banned compounds. The recovery ranged from 97.5 to 110.5%, and repeatability did not exceed 14.1%. Decision limits and detection capabilities were below 10 μg/kg. The highest decision limits and detection capabilities concentrations were observed for phenylthiouracil of 3.48 and 6.96 μg/kg, respectively. 相似文献
90.
基于1980年以来青海省和甘肃省区域地震台网的定位地震目录,分析了研究区最小完备震级,选取空间网格尺度为0.2°×0.2°、地震活动异常学习时间段和预测时间段均为3 a,应用图像信息(PI)法,分析了门源M S6.4地震发生前震中附近地震热点图像的异常变化过程。结果表明,在2013年1月1日至2016年1月1日的预测时间窗内,冷龙岭断裂和祁连山北缘断裂附近存在明显的地震热点,且地震热点图像颜色偏深,发震概率较高,向前滑动至2014年1月1日至2017年1月1日,2015年1月1日至2018年1月1日和2016年1月1日至2019年1月1日3个预测时间窗,此两断裂附近仍存在地震热点,地震热点图像颜色逐渐由深变浅,而门源M S6.4地震就落在PI法得到的地震热点内。门源M S6.4地震前存在明显的地震热点,且地震热点相对集中,地震热点图像颜色由深变浅,震后消失;在祁连山北缘断裂附近,地震热点持续存在,分布较集中,地震热点图像颜色也呈由深变浅的过程。未来需关注祁连山北缘断裂,附近有发生强震的可能。 相似文献