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   10502篇
  免费   1368篇
  国内免费   1038篇
化学   7778篇
晶体学   113篇
力学   492篇
综合类   110篇
数学   1257篇
物理学   3158篇
  2024年   30篇
  2023年   176篇
  2022年   308篇
  2021年   332篇
  2020年   365篇
  2019年   360篇
  2018年   275篇
  2017年   286篇
  2016年   491篇
  2015年   454篇
  2014年   599篇
  2013年   781篇
  2012年   923篇
  2011年   945篇
  2010年   657篇
  2009年   580篇
  2008年   683篇
  2007年   583篇
  2006年   506篇
  2005年   417篇
  2004年   404篇
  2003年   358篇
  2002年   388篇
  2001年   298篇
  2000年   187篇
  1999年   229篇
  1998年   147篇
  1997年   118篇
  1996年   140篇
  1995年   103篇
  1994年   104篇
  1993年   100篇
  1992年   82篇
  1991年   82篇
  1990年   53篇
  1989年   56篇
  1988年   34篇
  1987年   36篇
  1986年   36篇
  1985年   36篇
  1984年   13篇
  1982年   8篇
  1981年   10篇
  1980年   10篇
  1979年   9篇
  1978年   8篇
  1959年   12篇
  1958年   11篇
  1957年   15篇
  1956年   10篇
排序方式: 共有10000条查询结果,搜索用时 15 毫秒
991.
焦散线法及其与应力“解冻”相结合的应用   总被引:1,自引:0,他引:1  
详细地阐述了焦散线法原理,对焦散线的形成作了说明,简述了焦散线法在平面问题中的应用,结合笔者研究工作介绍了将焦散线法拓广应用于三维问题的关键技术和实例。  相似文献   
992.
A novel C19-diterpenoid alkaloid vilmoraconitine (1) was isolated from the roots of Aconitun vilmorinianum. Its structure was mainly determined by MS, 2D NMR, and X-ray methods. This is the first aconitine-type C19-diterpenoid alkaloid with one three-membered ring at C-8, C-9, and C-10.  相似文献   
993.
Using new molecular models of ammonia and methanol and thermodynamic perturbation theory, the global phase diagrams of model mixtures of these compounds with a van der Waals fluid, representing a simple nonpolar fluid, have been calculated. The global phase diagram of these mixtures is much richer than that of corresponding aqueous mixtures. More types of critical line behavior are found, including the presence of van Laar points and a small region where the mixtures exhibit a closed liquid-liquid immiscibility loop (Type VI phase behavior). The individual mixture components are characterized by two molecular parameters, which can be adjusted to their critical temperature and critical volume; the mixture model itself contains no adjustable parameters. It is shown that the theory gives qualitatively correct predietions of mixtures with n-alkanes. This includes the prediction of Type III critical line behavior for small and large values of the ratio of the critical temperatures of the components, and Type II over a large range of conditions, including the presence or absence of absolute or limited azeotropy, and temperature and pressure extrema of critical lines and their dependence on the number of carbon atoms.  相似文献   
994.
The critical temperature and mechanism functions for thermal decomposition of ε-CL-20, RS-ε-CL-20, α-CL-20, ε-CL-20/C4, and RS-ε-CL-20/C4 were evaluated based on non-isothermal TG data. A two-step mechanism has been found for thermal decomposition of α-CL-20, ε-CL-20/C4, and RS-ε-CL-20/C4, where the initial step is partly controlled by crystal structure of CL-20. The more reasonable mean activation energies could be obtained after peak separation for each individual steps. In fact, the activation energy for the post integrated process is almost equivalent with that of the second step, indicating that the total activation energy at the main decomposition process is dominated by thermolysis of CL-20 molecular. Besides, it has been found that the decomposition of C4 matrix does not affect the decomposition of normal ε-CL-20, resulting in identical activation energy and reaction model. However, the interaction between the C4 matrix and RS-ε-CL-20 is significant especially at the initial stage, where the activation energy of RS-ε-CL-20/C4 was overestimated before peak separation, while the activation energy for the second step due to thermolysis of CL-20 molecular is underestimated. The first decomposition step for α-CL-20, ε-CL-20/C4, and RS-ε-CL-20/C4 could be considered as autocatalytic process (AC model), whereas the second as JMA model, which is also applicable to that of pure ε-CL-20 and RS-ε-CL-20. Moreover, The critical temperatures of thermal explosion (T b) are obtained as 205.6, 205.5, 209.4, 214.4, and 227.5 °C for α-CL-20, ε-CL-20, RS-ε-CL-20, ε-CL-20/C4, and RS-ε-CL-20/C4, respectively. It proves that the C4 matrix could stabilize ε-CL-20 while the crystal form of CL-20 has little effect on its thermal stability.  相似文献   
995.
碳铂类似物的合成,表征及对大鼠W—256肉瘤的抑制作用   总被引:2,自引:0,他引:2  
合成了十八种〔PtA_2X〕·yH_2O,其中A分别为NH_3、CH_3NH_2、1/2乙二胺和1/2(2,3-二甲基-2,3-丁二胺),X分别为1,1-环丙烷二羧酸根(CPrDCA)、2-甲-1,1-环丙烷二羧酸根(2-M-CPrDCA)、2-甲-1,1-环丁烷二羧酸根(2-M-CBDCA)、1,2-环戊烷二羧酸根(CPDCA)和1,1-环已烷二羧酸根(CHDCA),并进行了表征。测定了配合物抑制大鼠W-256肉瘤的活性,发现配合物〔Pt(NH_3)_2X〕系列按X不同有以下的活性次序:CPrDCA>2-M-CPrDCA>CPDCA>CBDCA(碳铂)≥2-M-CBDCA。  相似文献   
996.
We studied the growth of nanocrystalline silicon (nc-Si) thin film exhibiting a strong room temperature photoluminescence (PL) at 1.81–2.003 eV. The amorphous silicon was crystallized by Ni silicide mediated crystallization (Ni SMC) and then Secco-etched to exhibit the PL. The PL peak energy and intensity increase with increasing the metal density on the a-Si because of the reduction in the grain size down to 2 nm. The photoluminescence energy and peak intensity depend strongly on the Secco etch time because the grain size is reduced by etching the grain boundaries.  相似文献   
997.
Alginate, chitosan and gelatin were deposited on the surface of PDL‐LA films via an entrapment method. ATR‐FT‐IR, XPS and contact‐angle analyses revealed the formation of stable thin biomacromolecule layers on the PDL‐LA film, thus enhancing the hydrophilicity of the films. Confocal laser scanning microscopy showed the existence of entrapment areas of approximately 10–20 μm in depth. This simple surface‐treatment method may have the potential for many biomedical applications.  相似文献   
998.
轻工业纤维素生物质过程残渣能源化技术   总被引:1,自引:0,他引:1  
以农产品为原料的轻工业大都是典型的流程工业,在通过转化过程将原料转化为食品、饮料、添加剂、调味料、纸和中成药等产品的同时产生被称为过程残渣的固体废物与废料,如白酒糟、酒精糟、醋糟、甘蔗渣、中药渣、油粕、酱渣、菌渣和造纸黑液可熔渣等.这些残渣产生于特定的生产过程,富含纤维素、蛋白质或木质素,因此代表一种已经被集中的生物质资源.它们同时含水50%-80%、易腐烂变质、甚至呈弱酸碱性,因此是重要的环境污染源.本文着眼于轻工生物质过程残渣的高值化利用,分析指出富含纤维素的白酒糟、醋糟、甘蔗渣、中药渣、茶渣和造纸边角料等适合作为生物质能源而被转化利用,并根据资源特征提出了可能的技术路线.通过分别对热化学路线涉及的脱水干燥、燃烧发电与气化发电技术和集成乙醇发酵、沼气发酵的复合转化技术进行技术综述,最后针对不同规模的富含纤维素轻工生物质过程残渣能源化提供了技术选择建议.  相似文献   
999.
The effects of copper ion on the interaction of [Ru(bpy)2HPIP]2+(bpy = 2,2′-bipyridine, HPIP = 2-(2-hydroxyphenyl) imidazo [4,5-f] [1, 10] phenanthroline) with DNA have been investigated by electronic absorption spectroscopy and fluorescence spectroscopy. HPIP ligand of the complex with an intramolecular hydrogen bond can bind Cu2+ in the absence of DNA, as revealed by the absorbance and fluorescence decrease for [Ru(bpy)2HPIP]2+. The resultant heterometallic complex binds to DNA via intercalation of HPIP into the DNA base pairs and its DNA-binding ability is stronger than [Ru(bpy)2HPIP]2+ itself. The DNA bound [Ru(bpy)2HPIP]2+ cannot bind Cu2+ at low Cu2+ concentration and the intramolecular hydrogen bond in HPIP is located inside the DNA helix. While the Cu2+ concentration is relative high, Cu2+ can quench the fluorescence of DNA bound [Ru(bpy)2HPIP]2+. The quenching reason is proposed.  相似文献   
1000.
计算机控制光学表面成形中大规模驻留时间求解   总被引:3,自引:0,他引:3       下载免费PDF全文
采用基于稀疏矩阵的大规模非负最小二乘法,对大口径、微浮雕结构光学元件加工中的驻留时间进行了分析与求解,并对该算法开展了正则化研究。仿真结果表明:与传统非负最小二乘法相比,基于稀疏矩阵的大规模非负最小二乘法精度高、效率快。采用该算法仿真加工平均振幅为1.177 6倍波长的大口径、微浮雕结构光学元件,误差面形均方根收敛至0.067倍波长。  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