全文获取类型
收费全文 | 66646篇 |
免费 | 10066篇 |
国内免费 | 7751篇 |
专业分类
化学 | 34724篇 |
晶体学 | 699篇 |
力学 | 3137篇 |
综合类 | 520篇 |
数学 | 5073篇 |
物理学 | 18079篇 |
无线电 | 22231篇 |
出版年
2024年 | 243篇 |
2023年 | 1446篇 |
2022年 | 1913篇 |
2021年 | 2509篇 |
2020年 | 2410篇 |
2019年 | 2276篇 |
2018年 | 1987篇 |
2017年 | 2002篇 |
2016年 | 2843篇 |
2015年 | 3129篇 |
2014年 | 3685篇 |
2013年 | 4785篇 |
2012年 | 5621篇 |
2011年 | 5945篇 |
2010年 | 4250篇 |
2009年 | 4111篇 |
2008年 | 4365篇 |
2007年 | 4018篇 |
2006年 | 3897篇 |
2005年 | 3380篇 |
2004年 | 2626篇 |
2003年 | 2156篇 |
2002年 | 2167篇 |
2001年 | 1744篇 |
2000年 | 1583篇 |
1999年 | 1350篇 |
1998年 | 1151篇 |
1997年 | 974篇 |
1996年 | 927篇 |
1995年 | 841篇 |
1994年 | 713篇 |
1993年 | 577篇 |
1992年 | 575篇 |
1991年 | 469篇 |
1990年 | 378篇 |
1989年 | 297篇 |
1988年 | 214篇 |
1987年 | 165篇 |
1986年 | 154篇 |
1985年 | 160篇 |
1984年 | 97篇 |
1983年 | 81篇 |
1982年 | 68篇 |
1981年 | 51篇 |
1980年 | 28篇 |
1979年 | 23篇 |
1977年 | 7篇 |
1975年 | 8篇 |
1971年 | 7篇 |
1957年 | 14篇 |
排序方式: 共有10000条查询结果,搜索用时 15 毫秒
341.
The Knoevenagel condensation of Meldrum's acid with aromatic aldehydes proceeded efficiently in a reusable ionic liquid, ethylammonium nitrate, at room temperature in the absence of any catalyst with high yields. 相似文献
342.
A series of new dispiropyrrolidine derivatives were synthesized via the three‐component 1,3‐dipolar cycloaddition reaction of azomethine ylides generated in situ by the decarboxylative condensation of acenaphthenequinone and sarcosine or l ‐thioproline with 5‐benzylidene‐1,3‐dimethylpyrimidine‐2,4,6‐trione. The structures of the products were identified by IR, 1H‐NMR, and HRMS spectra. 相似文献
343.
Nitriles were prepared from alkyl and aryl carboxylic acids in dry media conditions, under microwave irradiation. Heating of the carboxylic acid, urea and amidosulfonic acid adsorbed on alumina support in a microwave oven affords nitriles in 20–93% yields. 相似文献
344.
Aluminium powder has been used to reduce titanium tetrachloride to generate the low valent titanium (LVT)reagent which can induce the reductive coupling of aldehydes and ketones to form the corresponding pinacols or olefins. However, arylsulfonyl chlorides will give diaryldisulfides under the same condition. 相似文献
345.
Potassium tert-butoxide (0.1 mol%) catalyzed a vinylogous Mukaiyama aldol reaction between aromatic and aliphatic aldehydes with 2-(trimethylsilyloxy)furan. The corresponding γ-butenolides were obtained in good yields with good diastereoselectivities. 相似文献
346.
Ya Gao Nan Chen Weiwu Hu Chuanping Feng Baogang Zhang Qian Ning Bin Xu 《Journal of solution chemistry》2013,42(4):691-704
Adsorption is an effective treatment process for removing phosphorus and thus controlling eutrophication. In this study, a clay composite material called Al–dolomite–montmorillonite (Al–DM) was prepared and characterized. Al–DM performed well with respect to phosphate removal, with its performance depending on the Al–DM loading, contact time, initial phosphorus concentration and initial solution pH. Adsorption mechanisms were investigated by conducting batch tests on phosphate adsorption using the Al–DM. The adsorption process fitted both the pseudo-second-order kinetics model and the intra-particle diffusion model. The Langmuir, Freundlich and BET models all adequately described the adsorption isotherm data. Thermodynamic studies showed that the adsorption process was endothermic and spontaneous in nature. Al–DM is an effective adsorbent for phosphate removal mainly due to its hierarchical porous structures as shown by characterization with SEM and EDS. Chemical changes occurring before and after adsorption in a water environment indicated that Al–DM had little negative effect on water quality. 相似文献
347.
Melt/solid polycondensation (MP/SSP) is deemed as an alternative synthetic route besides ring-opening polymerization (ROP) in synthesizing poly(l-lactic acid) (PLLA). However, it is found that PLLA synthesized by MP/SSP has much poorer thermal stability than that by ROP due to more residual Sn(II) metallic catalyst in the former, but sulfonic acids does not show any detrimental effect on the thermal stability of PLLA. To synthesizing PLLA with good thermal stability by MP/SSP, a variety of commercially available sulfonic acids were screened as catalysts in MP/SSP of PLLA. Among these nonmetallic catalysts, it was found that 1,3-propanedisulfonic acid (PSA) and 1,5-naphthalene disulfonic acid (NSA) exhibited satisfactory catalytic reactivity and PLLAs with excellent thermal stability, high molecular weight, little coloration and good optical purity were successfully synthesized by MP/SSP. The decomposition temperature was increased by 80–100 °C in comparison to SnCl2-catalyzed PLLA, and the thermal stability is comparable to commercial PLLA produced by ROP. 相似文献
348.
Wei Chang Lijuan Liu Jin Zhang Qiwei Pan 《Journal of Dispersion Science and Technology》2013,34(5):639-642
A kind of emulsifier-free latex (FL) was successfully synthesized from styrene (St) and butyl acrylate (BA) with 2-acrylamido-2-methyl propane sulfonic acid (AMPS) as a reactive emulsifier. The particle size of latex particles, stability against electrolytes, minimum film forming temperature (MFT) and water contact angle (CA) were evaluated and compared with a conventional latex (CL). Test results show that FL has larger particle size, better stability against electrolytes and lower MFT value compared with CL; higher AMPS content leads to smaller particle size and smaller water CA. 相似文献
349.
Chi Fang Ting Xiong Jie Hou Jing Wei Gu Mei Hu Sheng 《Journal of Dispersion Science and Technology》2013,34(4):604-610
Amidoxime (AO)/amine co-functionalized polypropylene fiber adsorbents were prepared. The all-polymeric structures were characterized by using Fourier transform infrared spectroscopy (FTIR), optical microscope, contact angle meter and electron spin resonance (ESR) analysis methods, confirming the grafting, modification, and amidoximation stages gravimetrically, spectroscopically, and visually. The properties for the removal of uranyl(VI) from aqueous solutions were investigated. For amidoxime (AO) fiber, high adsorption rate was observed within the first 30 minutes and the plateau value of 40.6% uranium loading (0.0812 mg/g) was reached at around 30 minutes. The adsorption equilibrium for AO/amine fiber was attained within 20 minutes, resulting in the adsorption of 92.6% uranium loading (0.185 mg/g). The percentage of adsorption increases with increasing pH value (2–6), reaches a maximum at pH 6.0 and then remains almost constant for AO/amine fiber, whereas reduces slightly for AO fiber. 相似文献
350.
Fu-Qiang Jin Jian-Chun Zhang Min Li Xin-Min Hao Hua Zhang 《Journal of Dispersion Science and Technology》2013,34(8):1148-1151
NaOH/poly(sodium acrylate) composites were prepared by in situ polymerization of acrylic acid with an overneutralization level by adding excess NaOH. The composites were studied by XRD, IR and 23Na MAS NMR spectroscopy. The results showed that the high neutralization degree (>100%) may lead to a complete polymerization. Both XRD and 23Na MAS NMR spectra did not show any peaks of phase-separated NaOH or Na2CO3 until the neutralization degree was up to 217.5%. It can be presumed that the aggregates of Na+ ions can contain approximately two Na+ units for every carboxyl group before the phase separation. 相似文献