首页 | 本学科首页   官方微博 | 高级检索  
文章检索
  按 检索   检索词:      
出版年份:   被引次数:   他引次数: 提示:输入*表示无穷大
  收费全文   36533篇
  免费   6094篇
  国内免费   3583篇
化学   25616篇
晶体学   405篇
力学   2069篇
综合类   176篇
数学   3799篇
物理学   14145篇
  2024年   102篇
  2023年   732篇
  2022年   1397篇
  2021年   1411篇
  2020年   1602篇
  2019年   1529篇
  2018年   1296篇
  2017年   1150篇
  2016年   1798篇
  2015年   1696篇
  2014年   2111篇
  2013年   2722篇
  2012年   3223篇
  2011年   3355篇
  2010年   2181篇
  2009年   2087篇
  2008年   2355篇
  2007年   2089篇
  2006年   1949篇
  2005年   1554篇
  2004年   1196篇
  2003年   953篇
  2002年   918篇
  2001年   726篇
  2000年   552篇
  1999年   689篇
  1998年   567篇
  1997年   550篇
  1996年   559篇
  1995年   483篇
  1994年   413篇
  1993年   336篇
  1992年   327篇
  1991年   263篇
  1990年   242篇
  1989年   181篇
  1988年   114篇
  1987年   102篇
  1986年   122篇
  1985年   103篇
  1984年   77篇
  1983年   60篇
  1982年   63篇
  1981年   41篇
  1980年   29篇
  1979年   39篇
  1978年   26篇
  1976年   24篇
  1975年   16篇
  1974年   20篇
排序方式: 共有10000条查询结果,搜索用时 15 毫秒
961.
使用密度泛函理论B3LYP方法和6-31G(d,p)、6-31+G(d,p)、6-311G(d,p)及6-311+G(d,p)基组,分别对2-C5H10+和1-C5H10+的各种构象进行了几何构型优化,并用B3LYP/6-311G(d,p)进行了频率分析计算.计算预言1-C5H10+具有非平面构型,与以往报导的从头算计算结论相反.在两个自由基阳离子的各种构象的B3LYP几何构型上,进行了B3LYP和UMP2(full)方法的超精细偶合常数计算,得到了比以往更好的结果.  相似文献   
962.
To determine the effect of UVB exposure on the balance of type-1 or type-2 T-cells in skin, we examined the expression of key markers interferon (IFN)-gamma and interleukin (IL)-4 in cryostat sections. IFN-gamma mRNA was clearly detectable in nonirradiated control skin, and IFN-gamma protein was found in 2% of the dermal CD3pos T-cells, whereas IL-4 mRNA was hardly detectable, and no IL-4 protein was found. In contrast, IL-4 mRNA expression increased upon irradiation, and IL-4 was found in 2% of the T-cells at day 2 after UVB-exposure. Concomitantly, IFN-gamma mRNA expression decreased, and IFN-gamma protein became absent. We also analyzed T-cells present in primary dermal cell cultures, which were used as an in vitro equivalent of the in vivo situation. As compared with T-cells from control skin, T-cells in dermal cell cultures from UVB-exposed skin displayed an increased IL-4 and decreased IFN-gamma expression. No such skewing occurred when the T-cells from irradiated skin were cloned in the absence of a dermal microenvironment. Except for an occasional positive T-cell, type-1-associated cell-surface markers (CCR5, CXCR3) or type-2 markers (CCR3, CD30, CRTH2) were undetectable in situ. But these markers were expressed on cultured dermal T-cells from UVB-exposed and control skin at a comparable level, but did not correlate with the IFN-gamma and IL-4 production. Altogether, UVB-induced changes of the dermal microenvironment favor the development of type-2 T-cells.  相似文献   
963.
The photostimulated reaction of halonaphthalene with a series of carbanions derived from,propionic acid derivatives in liquid ammonia led to the naphthylation at a-carbon of the carbanions inan isolated yield ranging from 25% to 86%.In all cases,the dehalonaphthalenes were found to,be by-products and the reaction was inhibited by p-dinitrobenzere.An electron transfer from thecarbanion to the halonaphthalene followed by ejection of halogen led to naphtbyl radical,as predictedby comparing the LUMOs of the carbanions and the halonaphthalenes,was involved in the process.Absence of the alkylnaphthalene and 1,2-dinaphthylalkane in the products indicates the smoothelectron transfer between(Naph-Nu)~+ and Naph-X.C-vs.O-naphthylation is fully addressed interms of MNDO calculations and acid-base principle.  相似文献   
964.
The uniform transparent TiO2/SiO2 nanometer composite thin films were prepared via sol-gel method on the soda lime glass substrates, and were characterized by X-ray photoelectron spectroscopy (XPS), FTIR spectroscopy, UV-VIS spectroscopy, transmission electron microscopy (TEM), X-ray diffraction (XRD) and BET surface areas. It was found that the addition of SiO2 to TiO2 thin films could suppress the grain growth of TiO2 crystal and increase the hydroxyl content of the surface of TiO2 films. The photocatalytic activity of the as-prepared TiO2/SiO2 composite thin films increases for SiO2 content of less than 5 mol%.  相似文献   
965.
Massey University is a world-leading university in remote-instruction, online and mixed teaching based on the Stream teaching platform. This paper analyzed the advantages of the Stream teaching platform, resource utilization, course format, teaching team cooperation and course evaluation in Massey University. The paper also analyzed the enlightenment of teaching experience of Massey University to undergraduate teaching. During the outbreak of COVID-19 epidemic, drawing on Massey's experience, combined with the rapid development of information technology in China, the teaching team of organic chemistry adopted the multi-teaching strategy of "teacher-centered and student-centered", and the teaching mode of "teacher-led and self-taught by students". The online teaching practice of organic chemistry was carried out in agriculture university. It has been demonstrated that this model can effectively improve teaching effects.  相似文献   
966.
Uniform-sized molecularly imprinted polymer (MIP) beads were prepared using a one-step swelling and polymerization method. The obtained sulfamethazine (SMZ)-imprinted polymer showed high affinity and selectivity toward SMZ and other structurally related sulfonamides in acetonitrile or water-acetonitrile mobile phases, particularly in high aqueous systems. The column performance of the MIPs for SMZ and its analogues could be improved by elevating the column temperature and optimizing the flow rate. The hydrogen-bonding effect plays a significant role in the recognition process of SMZ-imprinted polymer systems in organic media, while the ion-exchange effect, as well as hydrophobic effect, dominates the retention mechanism in aqueous-rich media, in addition to shape recognition.  相似文献   
967.
水杨酸、甲醛和烷基伯胺(烷基:甲基,正-丙基,正-己基或正-十二烷基)通过Mannich缩合反应制得了主链上既含有水杨酸基又含有叔胺基的线型缩聚物。用这些缩聚物催化P-硝基苯酚乙酸酯(PNPA)水解反应的动力学研究结果表明,缩聚物的催化活性随N-烷基侧链的链长增加而增加。在催化剂过量的条件下,带有十二碳烷基侧链的缩聚物催化PNPA水解反应时符合简单的Michaelis-Menten动力学历程。  相似文献   
968.
用两束脉冲染料激光进行了CO的光学双共振四光子电离, 获得了转动分辩的CO A~(1Π)←X~(1Σ~+)和B~(1Σ~+)←A~(1Π)跃迁的光谱。观察到了在此2+1光子相继吸收中特有的选择定则。动力学研究表明CO基态到A~(1Π)态的双光子吸收是速率决定步骤, 其吸收截面约为1×10~(-49) cm~4 s。还测定了A~(1Π)态的单态转动传能截面。对于宇称指数e守恒、△J×±1的过程的截面约为0.05 nm; 随△J值增大而减小。对相同的△J, e正负变号的过程的截面总是小于e不变过程的截面。  相似文献   
969.
利用HZOZ和苹酚联产对苯二醋和邻苯二酚是目前二酚生产领域中比较活跃的课题.长久以来,人们曾利用酸、金属盐、金属配合物等作为催化剂,在均相体系中来研究其对苯酸羟化反应的催化活性.由平均相反应存在着明显的缺点,后来又转向了多相催化剂的开发,先后出现了TSull]TS42  相似文献   
970.
As facile,green,low-cost as possible:One more electron-deficient azaacene (TCAF) with deep LUMO (-4.52 eV), strong electronic affinity,excellent yield,and simple purification procedure was successfully created and explored as good electron injection material.It is believed TCAF would be a promising and pervasive acceptor material and bring in more significant achievements to green and sustainable organic electronics including OLEDs,OFETs,OPVs, and perovskite solar cells,etc.  相似文献   
设为首页 | 免责声明 | 关于勤云 | 加入收藏

Copyright©北京勤云科技发展有限公司  京ICP备09084417号