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   9782篇
  免费   1462篇
  国内免费   1184篇
化学   7034篇
晶体学   87篇
力学   602篇
综合类   94篇
数学   983篇
物理学   3628篇
  2024年   34篇
  2023年   179篇
  2022年   311篇
  2021年   298篇
  2020年   337篇
  2019年   336篇
  2018年   258篇
  2017年   252篇
  2016年   417篇
  2015年   371篇
  2014年   554篇
  2013年   650篇
  2012年   785篇
  2011年   838篇
  2010年   574篇
  2009年   558篇
  2008年   658篇
  2007年   558篇
  2006年   493篇
  2005年   511篇
  2004年   364篇
  2003年   355篇
  2002年   373篇
  2001年   311篇
  2000年   256篇
  1999年   233篇
  1998年   207篇
  1997年   171篇
  1996年   172篇
  1995年   131篇
  1994年   149篇
  1993年   102篇
  1992年   120篇
  1991年   100篇
  1990年   77篇
  1989年   79篇
  1988年   45篇
  1987年   26篇
  1986年   37篇
  1985年   26篇
  1984年   16篇
  1983年   18篇
  1982年   14篇
  1981年   8篇
  1980年   8篇
  1978年   8篇
  1977年   7篇
  1976年   7篇
  1975年   5篇
  1974年   9篇
排序方式: 共有10000条查询结果,搜索用时 78 毫秒
291.
A method of determination of trace gold and iridium in steel is presented. The samples were preconcentrated with an anion exchanger. The chemical recoveries were determined by an isotope tracer technique using198Au and192Ir.  相似文献   
292.
链酚胺型配体锌(Ⅱ)配合物模拟碳酸酐酶研究   总被引:1,自引:2,他引:1  
合成了酚胺型链状配体,N,N'-二(2-羟基苄基)丙二胺(H~2L)及其Zn(Ⅱ)配合物(ZnL),通过元素分析、IR和^1HNRM等手段进行了表征。采用pH电位滴定法,在25℃±0.1℃,I=0.1mol·dm^-^3(KNO~3)条件下,测定了配体的质子化常数以及配体与金属离子Zn(Ⅱ)配位反应平衡常数。讨论了配体与金属离子Zn(Ⅱ)的配位情况,得到了配位酚羟基的解离常数。运用分光光度法,在25℃±0.1℃,I=0.1mol·dm^-^3(KNO~3)条件下,在pH=5.5~9.0(50mmol·dm^-^3缓冲溶液)范围内,研究了配合物作为碳酸酐酶模拟物催化对-硝基苯酚乙酸酯(NA)水解动力学,得到了NA酯水解的配合物催化速率常数k~N~P。实验结果表明,ZnHL^+的配位酚羟基的解离常数pK~a为6.83;催化速率常数k~N~P与pH之间不存在Sigmoidal型曲线关系,而是在pH值中性附近有最大值,ZnHL^+对NA酯水解有很好的催化效果,并且采取双重催化机理,是碳酸酐酶很好的模拟物。  相似文献   
293.
介绍便携式温度计校验仪的工作原理及研制过程。对该仪器的恒温槽体机械结构、制冷器和散热器的设计、恒温槽体温度测量及控制等进行了详细叙述。该仪器提供的温度场稳定、均匀,在-20~100℃范围内可任意设定温度。解决了温度计现场快速校准的问题。  相似文献   
294.
Black lead, Ti-Ru and Ti-PbO2 were used as anode and stainless steel was used as cathode The electrolytic process of producing hypophosphorous acid with four-compartment electrodialytic cell was studied. The comparison of some factors, such as anodic voltage, product concentration and current efficiency, of black lead, Ti-Ru, and Ti-PbO2 electrodes was conducted. As a result, theTi-PbO2 electrode is the optimal anode material used, it can be in electrolytic process for producing hypophosphorous acid.  相似文献   
295.
Eight benzotriazoles and (4-methoxy)benzotriazoles, mono- or di-substituted derivatives of 2,4-dihydroxyaceto(or benzo)phenone were synthesized by azo coupling of (4-methoxy)2-nitrobenzenediazonium chloride with 2,4-dihydroxyaceto(or benzo)phenone followed by reductive cyclization. Pure mono-substituted compounds were very difficult to prepare. Careful selection of thepH for the azo coupling and selection of the proper reagents for the reductive cyclization were essential. All compounds were characterized by their ultraviolet, infrared1H and13C NMR spectra and their elemental analysis. These compounds have both the 2(2-hydroxyphenyl)2H-benzotriazole unit and a 2-hydroxyaceto(or benzo)phenone unit in the same molecule and are effective and useful ultraviolet absorbers.Dedicated to Prof. Dr.K. Komarek, Vienna, on the occasion of his 60th birthday with best wishes.  相似文献   
296.
The weak hydrogen bonded systems H2CO ?HX (X = F, Cl, Br, and I) have been studied by means of ab initio MO method with pseudopotential approximation. The stabilization energies of these hydrogen bonds are 8.96, 4.12, 3.00, and 2.21 kcal/mol, respectively. The interaction eneraction energies are farther decomposed according to Morokuma's energy decomposing scheme. It is found that the title complexes are mainly electrostatic, although the contribution of charge transfer is also significant.  相似文献   
297.
[Chemical reaction: See text] We have developed a general, efficient, and inexpensive catalyst system for arylation of amines by using 10 mol % of CuI as the copper source, 20 mol % of diphenyl pyrrolidine-2-phosphonate (DPP) as the ligand, K3PO4 as the base, and DMF containing 2% water (v/v) as the solvent.  相似文献   
298.
A hydrophilic polymer, poly(N-vinyl-2-pyrrolidone), was tethered on the surface of polypropylene microfiltration membrane (PPMM) by UV photo-assisted and γ-ray pre-irradiation induced graft polymerizations. Results revealed that the γ-ray pre-irradiation graft polymerization was more efficient in view of the grafting degree. The chemical changes of the membrane surface were confirmed by Fourier transform infrared spectroscopy and X-ray photoelectron spectroscopy. Pure water contact angle on poly(N-vinyl-2-pyrrolidone)-grafted PPMM decreased with the increase of grafting degree, which indicated an enhanced hydrophilicity for the modified membrane. Both bovine serum albumin adsorption and static platelets adhesion were measured to evaluate the bio-compatibility of the poly(N-vinyl-2-pyrrolidone)-modified PPMM. The statistical amounts of adsorbed bovine serum albumin and adhered platelets on unit membrane area decreased significantly, which to a certain degree demonstrated that the hemocompatibility of PPMM was improved. The N2 permeability and the mean pore diameters of different PPMMs increased at first, then decreased after certain grafting degree. The changes of water flux followed a similar tendency. These indicated that at low grafting degree pore degradation induced by γ-ray irradiation had a major impact on permeability, while this was overcompensated by the grafted polymer at high grafting degrees.  相似文献   
299.
苯胺及其衍生物的毛细管电泳行为研究   总被引:3,自引:1,他引:3  
采用紫外吸收检测,毛细管电泳分离,研究了九种苯胺及其衍生物在毛细管区带电泳(CZE)体系和胶束电动毛细管色谱(MECC)体系中的行为特征。讨论了缓冲溶液的浓度与pH、胶束浓度及混合胶束等在不同体系中对分离组分的影响,发现在CZE体系中,控制分离的主要因素是pKb值;在MECC体系中,控制分离的主要因素是溶质分子中碳原子数。建立了一种分离测定九种苯胺及其衍生物的高效毛细管电泳方法。  相似文献   
300.
A new silver organodiphosphonate, [Ag4(O3PCH2CH2PO3)] (1), has been synthesized and characterized by X-ray diffraction, IR, TGA-DSC, electricity measurement and element analysis. 1 crystallizes in the monoclinic space group P2(1)/n (a=6.0115(16) Å, b=8.630(2) Å, c=8.462(2) Å, β=97.693(4)°, Z=2, R1=0.0604, wR2=0.1450). 1 contains one-dimensional channels and a three-dimensional Ag?Ag interacted substructural net. TGA and XRD indicate little weight loss up to 300 °C and little structure change after heated at 170 °C for 2 h, respectively. The grain interior conductivity of 1 increases continuously from 50 to 170 °C. Results of EHT calculations show that under thermal or optical excitation the conductivity of 1 is mainly due to transfer of π antibonding electrons of -PO3 group through O atom to Ag 5s orbital, which also leads to enhancement of Ag?Ag interactions and promotes formation of Ag?Ag substructural net.  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