全文获取类型
收费全文 | 4757篇 |
免费 | 999篇 |
国内免费 | 1700篇 |
专业分类
化学 | 3368篇 |
晶体学 | 228篇 |
力学 | 312篇 |
综合类 | 169篇 |
数学 | 756篇 |
物理学 | 2623篇 |
出版年
2024年 | 6篇 |
2023年 | 38篇 |
2022年 | 179篇 |
2021年 | 140篇 |
2020年 | 147篇 |
2019年 | 156篇 |
2018年 | 153篇 |
2017年 | 231篇 |
2016年 | 156篇 |
2015年 | 223篇 |
2014年 | 279篇 |
2013年 | 416篇 |
2012年 | 374篇 |
2011年 | 396篇 |
2010年 | 439篇 |
2009年 | 429篇 |
2008年 | 511篇 |
2007年 | 440篇 |
2006年 | 451篇 |
2005年 | 362篇 |
2004年 | 300篇 |
2003年 | 201篇 |
2002年 | 207篇 |
2001年 | 204篇 |
2000年 | 203篇 |
1999年 | 105篇 |
1998年 | 55篇 |
1997年 | 54篇 |
1996年 | 39篇 |
1995年 | 42篇 |
1994年 | 53篇 |
1993年 | 48篇 |
1992年 | 68篇 |
1991年 | 50篇 |
1990年 | 41篇 |
1989年 | 51篇 |
1988年 | 33篇 |
1987年 | 31篇 |
1986年 | 22篇 |
1985年 | 12篇 |
1984年 | 15篇 |
1983年 | 13篇 |
1982年 | 14篇 |
1981年 | 19篇 |
1980年 | 15篇 |
1979年 | 10篇 |
1978年 | 9篇 |
1977年 | 4篇 |
1965年 | 6篇 |
1963年 | 2篇 |
排序方式: 共有7456条查询结果,搜索用时 31 毫秒
931.
以密胺餐具为研究对象,模拟研究密胺餐具中重金属铅、镉的迁移规律,对密胺餐具中的铅、镉进行安全风险评估研究,并提出密胺餐具中铅、镉的安全卫生标准建议值. 相似文献
932.
阐述了研究生参与大型仪器管理的必要性、运行机制及实施环节,建立了"统一管理,专人负责,用管结合,开放使用"的管理模式.实践证明,这一措施的实施对规范大型仪器管理、提高仪器利用率以及实验室科研项目顺利进行起到了积极促进作用. 相似文献
933.
以内蒙古褐煤为原料,N-甲基吡咯烷酮为萃取剂,在不同温度下萃取制备无灰煤,进而利用KOH活化法制备活性炭,探究萃取温度对活性炭电化学性能的影响。结果表明,无灰煤萃取温度对煤基活性炭电化学性能有显著影响。对无灰煤及原料褐煤的灰分含量,表面官能团含量和对应活性炭的比表面积、孔结构及其电化学性能进行对比发现,330℃下萃取制备出的无灰煤在碱煤质量比3∶1,活化温度650℃,活化时间2h的活化过程中具备最适宜的反应性,对应活性炭比表面积高达1 252 m~2·g~(-1),表面官能团含量适中,在3 mol·L~(-1)KOH电解液中50 mA·g~(-1)电流密度下比电容高达322 F·g~(-1),2 A·g~(-1)的电流密度下比电容保持率仍可接近90%。 相似文献
934.
935.
添加Cl离子对VO-WO/TiO催化剂低温NO转化率的影响 《燃料化学学报》2018,46(11):1370-1376
考察添加不同含量Cl离子对浸渍法制备的Cl-V_2O_5-WO_3/TiO_2催化剂低温NO转化率的影响。随着Cl离子质量添加量从0增加到2.5%,Cl-V_2O_5-WO_3/TiO_2催化剂NO转化率先升高后降低,结合在含有SO_2和H2O的SCR实验结果,确定1.5%Cl-V_2O_5-WO_3/TiO_2为性能最优催化剂。在反应温度为149-362℃,NO转化率大于95%;在145-385℃,NO转化率大于90%。采用XRF、BET、XRD、TG、FT-IR和H2-TPR等方法表征了催化剂的物理化学性能和结构。结果表明,在反应气氛中加入SO_2和H2O后,催化剂比表面积和孔容均减小,副反应产物含有NH+4和SO_2-4。适量Cl离子可以抑制硫物种沉积,减少副反应产物生成,增强催化剂抗中毒能力。 相似文献
936.
Yi-Qiang FAN Hong-Liang WANG Ke-Xin GAO Jing-Ji LIU Dong-Ping CHAI Ya-Jun ZHANG 《分析化学》2018,46(12):1863-1871
Microfluidics has been widely used in the life science, analytical chemistry, environmental science and other fields in the recent years. Traditional microfluidics systems usually use a highly integrated system with multiple components for handling the fluid in the micro/nano scale. The design and fabrication of integrated microfluidics usually require highly sophisticated instruments and operation professionals. With the experience inherited from integrated circuit and micro electro mechanical system, the modular microfluidics system has been experienced a rapid development in recent years. Modular microfluidics system is a combination of a series of individual modules to achieve complicated liquid handling functions. Compared with conventional microfluidics approach, the modular microfluidics method has the potential in significantly reducing the fabrication cost by using the massive production of single chip, besides, it is easy to be operated, and the user can easily assembly the modules to obtain their customized microfluidics system. The concept of modular microfluidics also indicates the future development path for the standardization of microfluidics system and also provides a promising approach for the industrial massive production of microfluidics. However, the study of modular microfluidics is still in an early stage. Although lots of studies have been conducted with varies materials, fabrication methods and interface technologies, issues like modular interface still restricted the further development of microfluidics. In this paper, a comprehensive review for the latest research on the modular microfluidics and applications in biological and medical fields is provided, and the future research trends of modular microfluidics is also discussed. 相似文献
937.
938.
应用双晶衍射仪对MOCVD外延生长所得到的一个GaAlInP双异质结(DH)的单晶多层结构进行测试与分析。应用双晶衍射的动力学理论模拟此样品的摇摆曲线,并结合界面的融合、晶格崎变与缺陷的散射来解释测试结果。对衍射谱的展宽提供了合理的解释,比较肯定地确定了此样品的结构与晶体质量。涉及缺陷对衍射FWHM的展宽的解释与关于缺陷的半动力学衍射理论相比,简单明了,实用于实际的生产检测。并根据分析结论对晶体质量的改进提供技术方案,改进了晶体质量。 相似文献
939.
8,9,12-Trihydro-9,9-pentamethylene-12-(3-nitrophenyl)-11H-naphtho[1′,25,6] pyrano[2,3-d]pyrimidin-11(10H)-one 1, another conversion product of Friedl(a)ndler reaction, has been synthesized by the reaction of 2-amino-4-(3-nitrophenyl)-4H benzo[f]chromene-3-carbo- nitrile 2 with cyclohexanone in the presence of Lewis acid catalysts. The crystal of the title compound 1 THF solvate, C60H62N6O10, was obtained and determined by X-ray diffraction me- thod. The crystal is of triclinic, space group P with a = 11.4633(11), b = 12.6247(12), c = 19.658(2)(A), α = 72.642(8), β = 89.045(9), γ = 68.340(5)°, V = 2509.6(4), Z = 2, Dc = 1.359 g/cm3, F(000) = 1088, Mr = 1027.16, the final R = 0.0674 and wR = 0.1869 with I > 2σ(I) and the goodness-of-fit S = 1.045 on F2. The pyrimidine ring has an envelope conformation, linked with a six-membered ring through a spiro C atom. The crystal packing of 1 THF solvate is stabilized by N-H…O hydrogen bonds, and π-π stacking interaction occurs between two adjacent nearly planar molecules of 1. 相似文献
940.
Catalytic direct dehydrogenation of methanol to formaldehyde was carried out over Ag‐SiO2‐MgO‐Al2O3 catalysts prepared by sol‐gel method. The optimal preparation mass fractions were determined as 8.3% MgO, 16.5% Al2O3 and 20% silver loading. Using this optimum catalyst, excellent activity and selectivity were obtained. The conversion of methanol and the selectivity to formaldehyde both reached 100%, which were much higher than other previously reported silver supported catalysts. Based on combined characterizations, such as X‐ray diffraction (XRD), scanning electronic microscopy (SEM), diffuse reflectance ultraviolet‐visible spectroscopy (UV‐Vis, DRS), nitrogen adsorption at low temperature, temperature programmed desorption of ammonia (NH3‐TPD), desorption of CO2 (CO2‐TPD), etc., the correlation of the catalytic performance to the structural properties of the Ag‐SiO2‐ MgO‐Al2O3 catalyst was discussed in detail. This perfect catalytic performance in the direct dehydrogenation of methanol to formaldehyde without any side‐products is attributed to its unique flower‐like structure with a surface area less than 1 m2/g, and the strong interactions between neutralized support and the nano‐sized Ag particles as active centers. 相似文献