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   3183篇
  免费   320篇
  国内免费   110篇
化学   829篇
晶体学   1篇
力学   180篇
综合类   109篇
数学   1602篇
物理学   892篇
  2024年   14篇
  2023年   33篇
  2022年   143篇
  2021年   127篇
  2020年   90篇
  2019年   83篇
  2018年   82篇
  2017年   157篇
  2016年   169篇
  2015年   92篇
  2014年   225篇
  2013年   186篇
  2012年   196篇
  2011年   213篇
  2010年   151篇
  2009年   160篇
  2008年   176篇
  2007年   172篇
  2006年   164篇
  2005年   168篇
  2004年   140篇
  2003年   100篇
  2002年   83篇
  2001年   60篇
  2000年   48篇
  1999年   41篇
  1998年   49篇
  1997年   51篇
  1996年   30篇
  1995年   27篇
  1994年   24篇
  1993年   23篇
  1992年   11篇
  1991年   20篇
  1990年   9篇
  1989年   11篇
  1988年   9篇
  1987年   9篇
  1986年   6篇
  1985年   8篇
  1984年   5篇
  1982年   7篇
  1981年   13篇
  1980年   6篇
  1979年   6篇
  1978年   4篇
  1977年   4篇
  1976年   2篇
  1972年   2篇
  1959年   1篇
排序方式: 共有3613条查询结果,搜索用时 15 毫秒
101.
针对大学化学校外实践教学基地建设和运行过程中的基地与高校的协同管理机制,实践教学基地软硬条件建设和资源配置及实践教学与理论学习的连贯性等问题,在OBE理论指导下,探究分类考核机制,在实证中构建分层次考评模型及考评指标,并建立适合于农林高校的化学校外实践教学基地的分类考评指标体系。  相似文献   
102.
周亚南  竺丽英 《化学教育》2021,42(17):70-75
研究采用视频分析法,利用NVivo软件,以东芝杯教学技能创新大赛的化学组选手为对象,从教学理念、教学情境、实验设计和教学手段等4个维度,评价卓越职前化学教师课堂教学的创新能力。结果发现:教学理念新颖,但具体落实尚有不足;情境创设生动,但主题与学生的联系不够紧密;实验设计别致,但在水平上还有提升空间;教学技术手段先进,但还需要深度融合。研究为创新型卓越化学教师的培养提供了启示。  相似文献   
103.
Exact solutions for KdV system equations hierarchy are obtained by using the inverse scattering transform. Exact solutions of isospectral KdV hierarchy, nonisospectral KdV hierarchies and ττ-equations related to the KdV spectral problem are obtained by reduction. The interaction of two solitons is investigated.  相似文献   
104.
Counting parameters has become customary in the density functional theory community as a way to infer the transferability of popular approximations to the exchange‐correlation functionals. Recent work in data science, however, has demonstrated that the number of parameters of a fitted model is not related to the complexity of the model itself, nor to its eventual overfitting. Using similar arguments, here, we show that it is possible to represent every modern exchange‐correlation functional approximations using just one single parameter. This procedure proves the futility of the number of parameters as a measure of transferability. To counteract this shortcoming, we introduce and analyze the performance of three statistical criteria for the evaluation of the transferability of exchange‐correlation functionals. The three criteria are called Akaike information criterion, Vapnik‐Chervonenkis criterion, and cross‐validation criterion and are used in a preliminary assessment to rank 60 exchange‐correlation functional approximations using the ASCDB database of chemical data.  相似文献   
105.
A simple, highly useful synthesis of 1-(2-(4,5-dimethyl-2-phenyl-1H-imidazol-1-yl)ethyl)piperazine derivatives is achieved by a four component cyclo condensation of diacetyl, aromatic aldehyde, 2-(piperazin-1-yl)ethanamine and ammonium acetate using SO42−/Y2O3 as a catalyst in ethanol. The synthesized compounds were characterized by IR, 1H and 13C NMR and mass spectral studies. All the synthesized compounds were screened for their in vitro antimicrobial studies. Among the newly synthesized compounds 5d, 5e, 5h and 5m showed excellent antibacterial and antifungal activities when compared to the standard drugs.  相似文献   
106.
辛欣  王祖浩 《化学教育》2020,41(11):1-6
在对已有试题难度量化工具进行改进的基础上,建构了化学加试题难度评估工具。对2015年至2019年国家新高考实验区浙江省的8次化学选考加试题的难度进行了量化评价。结果表明,8次化学加试题整卷难度变化较大,但同一学年、有相同学生可能参与的2次化学选考的整体难度相近;加试题能力特征和难度差异明显,“有机化学基础”加试题(第32题)难度相对稳定,而第30题(化学反应原理)与第31题(实验化学)加试题难度却表现出一定的波动性。在对8次化学加试题的难度及相关因素进行分析的基础之上,也对如何提高加试题的编制质量,提出了可行性建议。  相似文献   
107.
To improve the oil absorbency of caster oil‐based polyurethane foam, nano‐organic‐montmorillonite (OMMT) was used for the additives. The aim of this study is to evaluate the oil diffusion mechanism and dispersion uniformity of OMMT modified caster oil‐based polyurethane (MPU) using experiments and molecular dynamic simulation. Molecule movement and molecule trajectory of oil was investigated by molecular dynamic simulation and numerical simulation. According to the quantitative analyzing results, the diffusion model was put forward. The average diffusion coefficient of crude oil in 0, 1, 2 wt%, 4, and 6 wt% MPU is 2.4 × 10?4 cm2/s, 2.6 × 10?4 cm2/s, 3.0 × 10?4 cm2/s, 3.2 × 10?4 cm2/s, and 3.3 × 10?4 cm2/s, respectively. It indicated that crude oil appeared gradient in the MPU. The optimal diffusion direction of crude oil is (0, 0, 1) crystal face, and the small particles of crude oil are easy to be adsorbed. The two‐dimensional diffusion trajectory of crude oil is nonlinear. The diffusion model includes the diffusion of crude oil at the interface of oil and polyurethane, surface diffusion and pore diffusion, and pore adsorption. Furthermore, the diffusion model showed that the van der Waals force was the main reason for crude oil diffusion or adsorption. OMMT could improve the ability of oil/water separation of polyurethane.  相似文献   
108.
Background: Element-tagged immunoassay coupled with inductively coupled plasma mass spectrometry (ICP-MS) detection has the potential to revolutionize immunoassay analysis for multiplex detection. However, a further study referring to the standard evaluation and clinical sample verification is needed to ensure its reliability for simultaneous analysis in clinical laboratories. Methods: Carcinoembryonic antigen (CEA) and α-fetoprotein (AFP) were chosen for the duplex immunoassay. The performance of the assay was evaluated according to guidelines from the Clinical and Laboratory Standards Institute (CLSI). Moreover, reference intervals (RIs) of CEA and AFP were established. At last, 329 clinical samples were analyzed by the proposed method and results were compared with those obtained with electrochemiluminescent immunoassay (ECLIA) method. Results: The measurement range of the assay was 2–940 ng/mL for CEA and 1.5–1000 ng/mL for AFP, with a detection limit of 0.94 ng/mL and 0.34 ng/mL, respectively. The inter-assay and intra-assay imprecision were all less than 6.58% and 10.62%, respectively. The RI of CEA and AFP was 0–3.84 ng/mL and 0–9.94 ng/mL, respectively. Regarding to clinical sample detection, no significant difference was observed between the proposed duplex assay and the ECLIA method. Conclusions: The ICP-MS-based duplex immunoassay was successfully developed and the analytical performance fully proved clinical applicability. Well, this could be different with other analytes.  相似文献   
109.
经典AHP常用于武器装备效能或能力评估,然而经典AHP采用线性加权模型,即通过底层指标效能线性加权得到顶层指标效能,对非线性体系往往会引入较大评估误差,易产生决策性错误。为解决这一问题,本文以底层指标效能值等于1/2为展开点,给出基于泰勒展开的二阶加权模型,对线性体系,二阶加权模型可退化为经典线性加权模型;推导出二阶AHP的数学表达式,给出二阶AHP效能评估步骤,构建二阶AHP的基本框架。算例表明,二阶AHP评估结果相比经典AHP更接近于解析解,产生决策性错误的机率远小于经典AHP。  相似文献   
110.
Primary arylsulfonamide functional groups feature prominently in diverse pharmaceuticals. However, natural arylsulfonamides are relatively infrequent. In this work, two novel arylsulfonamide natural products were first synthesized, and then a series of novel molecules derived from natural arylsulfonamides were designed and synthesized, and their in vitro cytotoxic activities against A875, HepG2, and MARC145 cell lines were systematically evaluated. The results indicate that some of these arylsulfonamide derivatives exhibit significantly good cytotoxic activity against the tested cell lines compared with the control 5-fluorouracil (5-FU), such as compounds 10l, 10p, 10q, and 10r. In particular, the potential molecule 10q, containing a carbazole moiety, exhibited the highest inhibitory activity against all tested cell lines, with IC50 values of 4.19 ± 0.78, 3.55 ± 0.63, and 2.95 ± 0.78 μg/mL, respectively. This will offer the potential to discover novel drug-like compounds from the sparsely populated area of natural products that can lead to effective anticancer agents.  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