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   2264篇
  免费   33篇
  国内免费   346篇
化学   2216篇
晶体学   23篇
力学   14篇
综合类   3篇
数学   2篇
物理学   385篇
  2023年   69篇
  2022年   36篇
  2021年   34篇
  2020年   41篇
  2019年   44篇
  2018年   43篇
  2017年   40篇
  2016年   50篇
  2015年   60篇
  2014年   87篇
  2013年   141篇
  2012年   126篇
  2011年   124篇
  2010年   125篇
  2009年   153篇
  2008年   144篇
  2007年   153篇
  2006年   134篇
  2005年   140篇
  2004年   138篇
  2003年   89篇
  2002年   71篇
  2001年   66篇
  2000年   57篇
  1999年   68篇
  1998年   44篇
  1997年   44篇
  1996年   50篇
  1995年   43篇
  1994年   35篇
  1993年   40篇
  1992年   31篇
  1991年   24篇
  1990年   18篇
  1989年   18篇
  1988年   18篇
  1987年   3篇
  1986年   11篇
  1985年   4篇
  1984年   2篇
  1983年   5篇
  1982年   4篇
  1981年   3篇
  1980年   4篇
  1979年   1篇
  1977年   1篇
  1974年   3篇
  1973年   1篇
  1972年   1篇
  1971年   1篇
排序方式: 共有2643条查询结果,搜索用时 31 毫秒
201.
Photo-induced graft copolymerization of methyl methacrylate(MMA) onto sodium salt of partially carboxymethylated guar gum(Na-PCMGG, DS = 0.291) was carried out in an aqueous medium using ceric ammonium nitrate(CAN) as photoinitiator to synthesize a novel graft copolymer, Na-PCMGG-g-PMMA, which may find its potential application as a metal adsorbent. The influences of synthesis variables such as concentrations of photoinitiator(CAN), nitric acid and monomer(MMA) as well as reaction time, temperature and amount of substrate on the grafting yields were studied and the reaction conditions for optimum photo-grafting were evaluated. At optimum concentration, the maximum values of the grafting yields achieved were G = 271.61% and GE = 63.89%. The experimental results were found to be in very good agreement with the proposed kinetic scheme. The photo-graft copolymerization of MMA onto Na-PCMGG( DS = 0.291) was also carried out in the presence and absence of ultraviolet radiation for studying the efficiency of the photoinitiator. The influence of carboxymethyl groups introduced onto the guar gum molecules with regard to its behavior towards ultra-violet radiation induced grafting with MMA was also investigated. Photo-grafting process was confirmed and the products were characterized with the help of the spectroscopic(1H-NMR and FTIR) and SEM techniques.  相似文献   
202.
Throughout this study, the effect of certain organic acids, methacrylic acid, lactic acid and tartaric acid, doped in polysulfone (PSF) casting solution onto the performance of nanofiltration (NF) membranes was investigated. Different NF membranes have been prepared from m-phenylenediamine and trimesoylchloride onto the top surface of the acid-modified PSF membranes through regulating the concentration and contact time of the conventional interfacial polymerization process. The study of scanning electron microscopy (SEM) was used to investigate the influence of acids on the morphology of membranes and cross-sectional structures. The functional groups, hydroxyl and carboxylic acid, of the acids have resulted in a significant increase in membrane thickness, porosity and hydrophilicity, with a decrease in macrovoid capacity of the PSF layer. The acid-modified PSF/TFC membranes showed higher rejection of salt, with an increment in water flux compared to the neat membrane. Water flux and salt rejection (Rs %) of the control membrane was 7.6 L/m2 h and 65.4%, whereas polysulfone/methacrylic acid (PSF/MAAc), polysulfone/tartaric acid (PSF/TAc), and polysulfone/lactic acid (PSF/LAc) were 16.8, 18.5, and 20.2 L/m2 h and 88, 88.2 and 94.1%, respectively. Efficiency of prepared NF membranes under various inlet pressures and specific salts was investigated with selectivity and salt rejection. The salt rejection of a mixed salt solution was found to meet the order of Rs % CaSO4 ≥ Rs % Na2SO4 ˃ Rs % MgSO4 ˃ Rs MgCl2 ˃ Rs % NaCl.  相似文献   
203.
Single crystals of three new strontium nitridogermanates(IV) were grown in sealed niobium ampules from sodium flux. Dark red Sr4[GeN4] crystallizes in space group P21/c with a = 9.7923(2) Å, b = 6.3990(1) Å, c = 11.6924(3) Å and β = 115.966(1)°. Black Sr8Ge2[GeN4] contains Ge4– anions coexisting with [GeIVN4]8– tetrahedra and adopts space group Cc with a = 10.1117(4) Å, b = 17.1073(7) Å, c = 10.0473(4) Å and β = 115.966(1)°. Black Sr17Ge6N14 features the same anions alongside trigonal planar [GeIVN3]5– units. It crystallizes in P1 with a = 7.5392(1) Å, b = 9.7502(2) Å, c = 11.6761(2) Å, α = 103.308(1)°, β = 94.651(1)° and γ = 110.248(1)°.  相似文献   
204.
High-perfo rmance anodes of sodium ion batteries(SIBs)largely depends on rational architecture design and binder-free smart hybridization.Herein,we report TiC/C core/shell nanowires arrays prepared by a one-step chemical vapor deposition(CVD)method and apply it as the anode of SIBs for the first time.The conductive TiC core is intimately decorated with carbon shell.The as-obtained TiC/C nanowires are homogeneously grown on the substrate and show core/shell heterostructure and porous architecture with high electronic conductivity and reinforced stability.Owing to these merits,the TiC/C electrode displays good rate performance and outstanding cycling performance with a capacity of 135.3 mAh/g at 0.1 A/g and superior capacity retention of 90.14%after 1000 cycles at 2 A/g.The reported strategy would provide a promising way to construct binder-free arrays electrodes for sodium ion storage.  相似文献   
205.
The drainage of thin liquid films between colliding bubbles is strongly influenced by the boundary conditions at the air–liquid interface. Theoretically, the interface should not resist any tangential stress (fully mobile) in a clean water system, resulting in very fast film drainage and coalescence between bubbles within milliseconds. In reality, under most experimental and industrial conditions, the presence of impurities or surfactants can immobilize the interface and significantly hinder bubble coalescence by several orders of magnitude. In this opinion, we introduce the recent progress on understanding the boundary conditions at the air–water interface, and how they may affect the outcome of bubble collisions. The transition from mobile to immobile boundary conditions in the presence of contaminations is discussed. Despite the considerable recent progress, there are still experimental and theoretical challenges remaining on this topic, for example, finding the mechanism for hindered bubble coalescence by high salt concentrations.  相似文献   
206.
An easy and delicate approach using cheap carbon source as conductive materials to construct 3D sequential porous structural Na3V2(PO4)3/C(NVP/C)with high performance for cathode materials of sodium ion battery is highly desired.In this paper,the NVP/C with 3D sequential porous structure is constructed by a delicate approach named as“cooking porridge”including evaporation and calcination stages.Especially,during evaporation,the viscosity of NVP/C precursor is optimized by controlling the adding quantity of citric acid,thus leading to a 3D sequential porous structure with a high specific surface area.Furthermore,the NVP/C with a 3D sequential porous structure enables the electrolyte to interior easily,providing more active sites for redox reaction and shortening the diffusion path of electron and sodium ion.Therefore,benefited from its unique structure,as cathode material of sodium ion batteries,the 3D sequential porous structural NVP/C exhibits high specific capacities(115.7,88.9 and 74.4 mA·h/g at current rates of 1,20 and 50 C,respectively)and excellent cycling stability(107.5 and 80.4 mA·h/g are remained at a current density of 1 C after 500 cycles and at a current density of 20 C after 2200 cycles,respectively).  相似文献   
207.
Sodium-ion batteries(SIBs)are promising for grid-scale energy storage applications due to the natural abundance and low cost of sodium.Among various Na insertion cathode materials,Na0.44MnO2 has attracted the most attention because of its cost effectiveness and structural stability.However,the low initial charge capacity for Na-poor Na0.44MnO2 hinders its practical applications.Herein,we developed a facile chemical presodiated method using sodiated biphenly to transform Na-poor Na0.44MnO2 into Na-rich Na0.66MnO2.After presodiation,the initial charge capacity of Na0.44MnO2 is greatly enhanced from 56.5 mA·h/g to 115.7 mA·h/g at 0.1 C(1 C=121 mA/g)and the excellent cycling stability(the capacity retention of 94.1%over 200 cycles at 2 C)is achieved.This presodiation strategy would open a new avenue for promoting the practical applications of Na-poor cathode materials in sodium-ion batteries.  相似文献   
208.
用溶剂浮选-气相色谱法测定铝酸钠溶液中的硬脂酸钠。选择环己酮为浮选溶剂,样品经1∶1(V/V)的盐酸酸化后,以40 mL/m in的速度通入氮气并以环己酮为浮选溶剂浮选30 m in进行富集,用CH3OH/BF3在80℃水浴中加热3 m in进行甲酯化,以毛细管色谱柱PEG-20M(30 m×0.32 mm,i.d.)分离,邻苯二甲酸二乙酯为内标,氢火焰离子化检测器检测。方法在硬脂酸与内标的质量比在0.1~10范围内呈良好的线性关系(r=0.9997);硬脂酸钠的相对标准偏差为1.35%(n=7);回收率为96.9%~98.1%。  相似文献   
209.
基于多贝斯对硫酸铈-亚硫酸钠化学发光体系的抑制作用,建立了流动注射-化学发光测定多贝斯的新方法,并且研究了各种实验条件的影响。在酸性介质中,发光强度的降低(ΔICL)与多贝斯浓度在4.0×10-6~6.0×10-5mol/L范围内呈良好线性关系,回归相关系数为0.9976,方法的检出限为2.0×10-6mol/L,相对标准偏差为1.2%(n=6)。  相似文献   
210.
本文用单扫描极谱法研究了硝普钠(SNP)-铜(Ⅱ)体系的伏安行为。在pH3.0的克拉克-鲁布斯缓冲溶液中,SNP和SNP-Cu2 体系于-0.47 V(vs.SCE)处均有极谱还原峰,但SNP-Cu2 体系的灵敏度是SNP的100倍左右,其峰高与SNP浓度在2.0×10-7~4.0×10-5mol/L范围内呈线性关系,检出限为1.0×10-7mol/L。应用本法测定静脉注射到大白鼠中SNP的含量,结果满意。  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