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   3036篇
  免费   510篇
  国内免费   494篇
化学   2390篇
晶体学   55篇
力学   211篇
综合类   64篇
数学   297篇
物理学   1023篇
  2024年   10篇
  2023年   60篇
  2022年   122篇
  2021年   144篇
  2020年   144篇
  2019年   154篇
  2018年   121篇
  2017年   145篇
  2016年   159篇
  2015年   198篇
  2014年   202篇
  2013年   251篇
  2012年   296篇
  2011年   276篇
  2010年   216篇
  2009年   222篇
  2008年   216篇
  2007年   190篇
  2006年   160篇
  2005年   138篇
  2004年   77篇
  2003年   69篇
  2002年   75篇
  2001年   59篇
  2000年   63篇
  1999年   39篇
  1998年   34篇
  1997年   27篇
  1996年   23篇
  1995年   27篇
  1994年   19篇
  1993年   17篇
  1992年   18篇
  1991年   15篇
  1990年   20篇
  1989年   8篇
  1988年   1篇
  1987年   1篇
  1986年   2篇
  1985年   8篇
  1984年   1篇
  1983年   7篇
  1982年   1篇
  1979年   3篇
  1973年   1篇
  1957年   1篇
排序方式: 共有4040条查询结果,搜索用时 15 毫秒
121.
A multi‐channel microchip electrophoresis using a programmed step electric field strength (PSEFS) method was investigated for fast parallel detection of feline panleukopenia virus (FPV) DNA. An expanded laser beam, a 10× objective lens, and a charge‐coupled device camera were used to simultaneously detect the separations in three parallel channels using laser‐induced fluorescence detection. The parallel separations of a 100‐bp DNA ladder were demonstrated on the system using a sieving gel matrix of 0.5% poly(ethylene oxide) (Mr = 8 000 000) in the individual channels. In addition, the PSEFS method was also applied for faster DNA separation without loss of resolving power. A DNA size marker, FPV DNA sample, and a negative control were simultaneously analyzed with single‐run and one‐step detection. The FPV DNA was clearly distinguished within 30 s, which was more than 100 times faster than with conventional slab gel electrophoresis. The proposed multi‐channel microchip electrophoresis with PSEFS was demonstrated to be a simple and powerful diagnostic method to analyze multiple disease‐related DNA fragments in parallel with high speed, throughput, and accuracy.  相似文献   
122.
123.
124.
We have developed a stable and sensitive nonenzymatic glucose sensor by modifying a glassy carbon electrode (GCE) with a composite incorporating nickel(II) oxides and reduced graphene. The oxides were generated by directly electrodepositing nickel on the GCE with a graphene modifier using a multi-potential pulse process, and then oxidizing nickel to nickel(II) oxides by potential cycling. In comparison to the conventional nickel(II) oxides-modified GCE, this new nickel(II) oxides-graphene modified GCE (NiO-GR/GCE) has an about 1.5 times larger current response toward the nonenzymatic oxidation of glucose in alkaline media. The response to glucose is linear in the 20 μM to 4.5 mM concentration range. The limit of detection is 5 μM (at a S/N of 3), and the response time is very short (<3 s). Other beneficial features include selectivity, reproducibility and stability. A comparison was performed on the determination of glucose in commercial red wines by high-performance liquid chromatography (HPLC) and revealed the promising aspects of this sensor with respect to the determination of glucose in real samples.
Figure
A stable and sensitive nonenzymatic glucose sensor is developed by preparing the nickel(II) oxides-reduced graphene nanocomposite modified glassy carbon electrode (NiO-GR/GCE), and then used to detect the glucose contents in the commercial red wines. This NiO-GR/GCE also has a high selectivity  相似文献   
125.
The nitrogen-based nucleophile generated from azodicarboxylate and triphenylphosphine displayed good reactivity toward benzofuran-2,3-diones to generate a variety of spirocyclic benzofuran-2-one derivatives. The reactions accommodate a number of benzofuran-2,3-diones and different dialkylazodicarboxylates to give the enriched functionalized 3-spirooxadiazole benzofuran-2-ones with moderate to good yields (up to 93%).  相似文献   
126.
The solubility of icariin in the binary solvent system of ethanol and water was measured by UV–Vis spectrophotometry from 288.2 to 328.2 K. The solubility of icariin in the system increased with increasing temperature. A synergistic effect appeared at x 2 = 0.4 (equivalent to 68.34 % ethanol, v/v) binary mixture. Solubility data were correlated with the modified Apelblat equation. The enthalpy and entropy of solution were evaluated using van’t Hoff plots.  相似文献   
127.
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   
128.
建立了一种分散固相萃取(d-SPE)净化、气相色谱-质谱联用法(GC-MS)检测纸质包装材料中18种光引发剂(PIs)的分析方法。样品经水浸润后,用乙腈对样品进行超声提取,提取液经正己烷-乙酸乙酯溶液(3∶7,V/V)液液萃取后,用无水MgSO4、N-丙基乙二胺(PSA)和C18粉末分散固相萃取净化,以GC-MS法测定,内标法定量。18种PIs在0.01~0.2 mg/L内线性关系良好(R2>0.997),3个加标水平(0.1,0.4和2 mg/m2)的回收率在81.6%~123.8%之间,相对标准偏差(RSD)为0.2%~9.6%,检出限(LOD)为0.007~0.023 mg/m2。结果表明:本方法简便、测定结果准确,可用于纸质包装材料中18种光引发剂残留的高通量检测。  相似文献   
129.

New retinoyl sugar derivatives of 13‐cis‐retinoic acid were synthesized in three ways in this paper in order to enhance pharmacal effects, especially antiproliferative activities of 13‐cis‐retinoic acid. Their structures were confirmed by IR, 1H‐NMR, 13C‐NMR, and MS spectra and their antiproliferative activities were determined in vitro using human cancer lines. Results showed that some compounds possessed potential antitumor activities.  相似文献   
130.
研究了支化侧链型偶氮无规共聚物(PMAPB6P-AA)在THF/H2O混合溶液中的自组装行为.研究发现,通过缓慢增加体系的水含量,可以制备出具有中空结构的非球形聚集体.调节聚合物的初始浓度,可以得到不同粒径的聚集体.聚集体中偶氮生色团的光致异构化速率与异构化程度随聚合物初始浓度的增大而减小.在此基础上,采用更加缓慢的增加水含量的方法,使聚合物分子进行充分的疏水聚集与H-聚集,制备出类囊泡状聚集体.在紫外光照射条件下,观察到类囊泡聚集体发生了光致解聚集.  相似文献   
[首页] « 上一页 [8] [9] [10] [11] [12] 13 [14] [15] [16] [17] [18] 下一页 » 末  页»
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