全文获取类型
收费全文 | 23862篇 |
免费 | 4823篇 |
国内免费 | 7352篇 |
专业分类
化学 | 17646篇 |
晶体学 | 858篇 |
力学 | 1737篇 |
综合类 | 794篇 |
数学 | 3731篇 |
物理学 | 11271篇 |
出版年
2024年 | 61篇 |
2023年 | 286篇 |
2022年 | 871篇 |
2021年 | 839篇 |
2020年 | 851篇 |
2019年 | 850篇 |
2018年 | 857篇 |
2017年 | 1052篇 |
2016年 | 925篇 |
2015年 | 1177篇 |
2014年 | 1418篇 |
2013年 | 1922篇 |
2012年 | 1950篇 |
2011年 | 2084篇 |
2010年 | 1841篇 |
2009年 | 1996篇 |
2008年 | 2228篇 |
2007年 | 1956篇 |
2006年 | 1862篇 |
2005年 | 1707篇 |
2004年 | 1320篇 |
2003年 | 1011篇 |
2002年 | 985篇 |
2001年 | 1015篇 |
2000年 | 1117篇 |
1999年 | 627篇 |
1998年 | 445篇 |
1997年 | 316篇 |
1996年 | 302篇 |
1995年 | 281篇 |
1994年 | 252篇 |
1993年 | 243篇 |
1992年 | 202篇 |
1991年 | 173篇 |
1990年 | 142篇 |
1989年 | 162篇 |
1988年 | 117篇 |
1987年 | 103篇 |
1986年 | 76篇 |
1985年 | 73篇 |
1984年 | 61篇 |
1983年 | 65篇 |
1982年 | 54篇 |
1981年 | 42篇 |
1980年 | 39篇 |
1979年 | 22篇 |
1978年 | 14篇 |
1977年 | 9篇 |
1976年 | 7篇 |
1974年 | 7篇 |
排序方式: 共有10000条查询结果,搜索用时 15 毫秒
51.
Zhaoqiang Lu Lin Cheng Jun Li Kai Zhang Song Yi Jingui Qin 《Journal of polymer science. Part A, Polymer chemistry》2004,42(4):925-932
The synthesis and structural characterization of a series of novel, fluorinated poly(phthalazinone ether)s containing perfluorophenylene moieties are described. The monomers, 4‐(4′‐hydroxyaryl)phthalazin‐1(2H)‐ones ( 2a – 2d ), were conveniently and efficiently synthesized from phenols and phthalic anhydride in two steps via 2‐(4′‐hydroxybenzoyl)benzoic acids, which were first obtained by the Friedel–Crafts reaction in good yields and with high stereoselectivity and were then converted into 2a – 2d by fusion with hydrazine. All the polymers were prepared by nucleophilic aromatic substitution (SNAr) polycondensation between the compounds perfluorobiphenyl and 4‐(4′‐hydroxyaryl)phthalazin‐1(2H)‐ones ( 2a ‐ 2d ). The resulting fluorinated polymers were readily soluble in common organic solvents (e.g., CHCl3, tetrahydrofuran, dimethylformamide, dimethyl sulfoxide, N‐methylpyrrolidone, etc.) at room temperature. Their weight‐average molecular weights and the polydispersities ranged from (7.96–18.25) × 103 to 1.31–2.71, respectively. Their glass‐transition temperatures varied from 213 to 263 °C. They were all stable up to 390 °C both in air and in argon. The 5% weight‐loss temperatures of these polymers in air and argon ranged from 393–487 to 437–509 °C, respectively. Wide‐angle X‐ray diffraction studies indicated they were all amorphous and could be attributed to the presence of kink nonplanar moiety, phenyl phthalazinone along the polymer backbone. © 2004 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 42: 925–932, 2004 相似文献
52.
Hongwei Hao Minsheng Wu Yifang Chen Jiaowen Tang Qingyu Wu 《Colloids and surfaces. B, Biointerfaces》2004,33(3-4):151-156
To prevent cyanobacterial bloom in eutrophic water by ultrasonic method, ultrasonic irradiations with different parameters were tested to inhibit Spirulina platensis from growth. The experimental result based on cyanobacterial growth, chlorophyll a and photosynthetic activity showed that, the ultrasonic irradiation inhibited cyanobacterial proliferation effectively, furthermore the inhibition effectiveness increased in the order: 200 kHz>1.7 MHz>20 kHz and became saturated with the increased power. The inhibition mechanism can be mainly attributed to the mechanical damage to the cell structures caused by ultrasonic cavitation, which was confirmed by light microscopy and differential interference microscopy. The optimal frequency of 200 kHz in cavition and sonochemistry was also most effective in cyanobacterial growth inhibition. The higher frequency of 1.7 MHz is weaker than 20 kHz in cavitation, but has more effective inhibition because it is nearer to the resonance frequency of gas vesicle. The inhibition saturation with ultrasonic power was due to the ultrasonic attenuation induced by the acoustic shielding of bubbles enclosing the radiate surface of transducer. 相似文献
53.
54.
采用高速PIN光电探测器和高带宽的数字存储示波器,实时检测透射光脉冲和散射光脉冲的变化特征,并将之用作材料破坏的光学判据,测量得到K9玻璃在1.06μm纳秒脉冲激光作用下的能量损伤阈值约18mJ,相应的能量密度阈值为1.0kJ/cm2。通过分析透射光脉冲和散射光脉冲的特征,给出了材料的破坏时刻,并推断出K9玻璃所能承受的极限光强为1015W/m2。研究了能量透过率与泵浦能量的关系,并初步探讨了透明材料的破坏机理。结果表明:在多纵模激光的作用下,透明光学材料破坏是电离击穿与自聚焦效应综合作用的结果。 相似文献
55.
56.
57.
用射频等离子体方法在玻璃基底上制备的类金刚石(DLC)薄膜,采用离子注入法掺氮,并对掺氮DLC薄膜紫外(UV)辐照前后的性能变化进行了研究。研究结果表明:随氮离子注入剂量及UV辐照时间的增加,位于2 930cm-1附近的SP
3C-H吸收峰明显变小,而位于1 580cm-1附近的SP2C-H吸收峰则明显增强,薄膜的电阻率明显呈下降趋势;随UV辐照时间的增加,位于1 078cm-1附近的Si-O-Si键数量及位于786cm-1附近的Si-C键数量明显增加。即氮离子注入和UV辐照明显改变了DLC薄膜的结构与特性。 相似文献
58.
59.
60.
2H-1,2,3-Diazaphospholes 3a-3h were prepared from various ketone acylhydrazones and phosphorus trichloride in the presence of triethylamine. Compounds 3 underwent feasible hetero-Diels-Alder reactions with cyclopentadiene to afford the respective anellated [1,2,3]diazaphospholes 4a--4d as well as 4a‘--4d‘ in moderate yields. The endo rule in the reaction was observed under kinetic control conditions. 相似文献