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   3473篇
  免费   687篇
  国内免费   303篇
化学   2475篇
晶体学   35篇
力学   190篇
综合类   14篇
数学   381篇
物理学   1368篇
  2024年   6篇
  2023年   81篇
  2022年   83篇
  2021年   156篇
  2020年   157篇
  2019年   142篇
  2018年   123篇
  2017年   107篇
  2016年   193篇
  2015年   187篇
  2014年   201篇
  2013年   253篇
  2012年   342篇
  2011年   354篇
  2010年   230篇
  2009年   219篇
  2008年   229篇
  2007年   198篇
  2006年   182篇
  2005年   175篇
  2004年   125篇
  2003年   90篇
  2002年   90篇
  2001年   70篇
  2000年   57篇
  1999年   51篇
  1998年   38篇
  1997年   45篇
  1996年   40篇
  1995年   38篇
  1994年   30篇
  1993年   17篇
  1992年   25篇
  1991年   33篇
  1990年   31篇
  1989年   9篇
  1988年   8篇
  1987年   13篇
  1986年   4篇
  1985年   4篇
  1984年   3篇
  1983年   6篇
  1981年   4篇
  1980年   4篇
  1978年   1篇
  1976年   2篇
  1974年   2篇
  1959年   1篇
  1947年   1篇
  1946年   1篇
排序方式: 共有4463条查询结果,搜索用时 15 毫秒
991.
采用两个易扭转异构的双三齿有机配体,双吡啶二甲基-6,6′-二酰肼-2,2′-连吡啶(H2L1)和双吡啶二乙基-6,6′-二酰肼-2,2′-连吡啶(H2L2),和金属镍离子组装得到2个金属螺旋体(helicate),Ni2(HL1)2(PF6)(BF4)(CH3OH)(H2O)2 (1)和Ni2(HL2)(H2L2)(ClO4)3(C2H5OH)(CH3OH)H2O)3 (2),并测定了它们的晶体结构。同时由配体H2L3出发,通过逐级组装的方法,得到一个镍-银杂金属的配位聚合物Ni2Ag2(HL3)2(ClO4)2(CH3CN)3 (3)。单晶结构表明,配位聚合物3中配体H2L3首先与镍离子组装成分子盒化合物(molecular box),该结构单元进一步通过Ag离子与分子盒外围N原子配位,使分子盒互相串连成一维配位聚合物3,分子盒聚集体沿c方向伸展成一维链结构,链与链之间相互平行,进一步堆积成二维孔道结构。  相似文献   
992.
Epithermal neutron activation analysis (ENAA) was applied to the determination of the contents of bromine and iodine in 40 biological and environmental standard reference materials and Chinese diets. Boron nitride (BN) for solid samples and BN+Cd for liquid samples were adopted as shield material. Irradiation was carried out in inner and outer irradiation sites in a Miniature Source Reactor (MNSR) for solid and liquid samples, respectively. The 443 keV photopeak of 128I and the 616 keV photopeak of 80Br were used. The precision of measurement (relative standard deviation) is 2∼6% for contents of iodine of more than 100 ng/g and 8∼12% in the 20∼100 ng/g range in solid samples, and 12∼18% at less than 100 ng/ml in liquid samples. For bromine, the precision of measurement is 2–8% for solid samples and lower than 13% for liquid samples. The detection limits under experimental conditions varied between 10∼30 ng/g, 55∼95 ng/g and 25∼68 ng/g for iodine and 50∼150 ng/g, 200∼450 ng/g and 100∼300 ng/g for bromine in ENAA with BN shield in inner irradiation sites, with Cd shield and BN+Cd shield in outer irradiation sites, respectively. Received: 13 June 1996 / Revised: 2 September 1996 / Accepted: 19 September 1996  相似文献   
993.
以Cu+和Zn+与CS2反应作为第一过渡金属离子与CS2反应的范例体系. 采用密度泛函UB3LYP/6-311+G*方法计算研究了第一过渡金属离子在基态和激发态与CS2反应的反应机理. 全参数优化了反应势能面上各驻点的几何构型, 用频率分析方法和内禀反应坐标(IRC)方法对过渡态进行了验证. 并用UCCSD(T)/6-311G*方法对各驻点作了单点能量校正. 在Cu+与CS2反应中, 计算了单重态初始中间体1IM1到三重态插入型中间体3IM2的反应交叉势能面. 确定了第一过渡金属离子与CS2的反应为插入-消去反应, 找到了基态和激发态金属离子与CS2反应的主要通道.  相似文献   
994.
Wang H  Liu Z  Liu C  Zhang D  Lü Z  Geng H  Shuai Z  Zhu D 《Inorganic chemistry》2004,43(13):4091-4098
Three new complexes of the formula M(2)L(2) derived from 2-(4-quinolyl)nitronyl nitroxide (4-QNNN) and M(hfac)(2) [M = Mn(II), Co(II), and Cu(II)], (4-QNNN)(2).[Mn(hfac)(2)](2) (1), (4-QNNN)(2).[Co(hfac)(2)](2).2H(2)O (2), and (4-QNNN)(2).Cu(hfac)(2).Cu'(hfac)(2) (3), were synthesized and characterized structurally as well as magnetically. Complexes 1 and 2 are four-spin complexes with quadrangle geometry, in which both the nitrogen atoms of quinoline rings and oxygen atoms of nitronyl nitroxides are involved in the formation of coordination bonds. For complex 3, however, the nitrogen atoms of quinoline rings are coordinated with Cu(II) ion to afford a three-spin complex, which is further linked to another molecule of Cu(hfac)(2) (referred to as Cu'(hfac)(2)) to form a 1D alternating chain. The magnetic behaviors of the three complexes were investigated. For complex 1, as the nitronyl nitroxides and Mn(II) ions are strongly antiferromagnetically coupled, consequently its temperature dependence of magnetic susceptibility was fitted to the model of spin-dimer with S = 2, yielding the intradimer magnetic exchange constant of J = -0.82 cm(-1). For complex 2, the temperature dependence of the magnetic susceptibility in the T > 50 K region was simulated with the model of two-spin unit with S(1) = 3/2 and S(2) = 1/2, leading to J = -321.9 cm(-1) for the magnetic interaction due to Co(II).O coordination bonding, D = -16.3 cm(-1) (the zero-field splitting parameter), g = 2.26, and zJ = -3.8 cm(-1) for the magnetic interactions between Co(II) ions and nitronyl nitroxides through quinoline rings and those between nitronyl nitroxides due to the short O.O short contacts. The temperature dependence of magnetic susceptibility of 3 was approximately fitted to a model described previously affording J(1) = -6.52 cm(-1) and J(2) = 3.64 cm(-1) for the magnetic interaction between nitronyl nitroxides and Cu(II) ions through the quinoline unit via spin polarization mechanism and the weak O.Cu coordination bonding, respectively.  相似文献   
995.
The isotopic abundance ratios of190Os/184Os and96Ru/102Ru for the metal phases of the Jilin and Taonan stone meteorites were determined by pretreatment and radiochemical neutron activation analysis. All experimental factors affecting Os and Ru isotopic ratios were discussed, including sampling, standard, irradiation, separation and counting. The statistical errors of measurement for the199Os/184Os ratio can be controlled within 1%. The experimental results indicate that the statistically significant anomalies of the190Os/184Os and96Ru/102Ru ratios have not been found relative to the terrestrial Os and Ru standards.  相似文献   
996.
本文研究以钨(Ⅵ)形成钨钼杂多酸与鲁米诺反应,产生化学发光测定钨的方法及条件。在pH1.6静置溶液,钨钼杂多酸迅速形成,具有很高的氧化活性。测定钨的灵敏度达到0.084ng/ml,钨浓度从1×10~(-10)~1×10~(-5)g/ml范围内具有线性关系。通过巯基棉分离富集,用于天然水及岩矿样品中痕量钨的测定,均获得满意结果。  相似文献   
997.
适合低PH范围测量的新型中性载体膜PH电极的研究   总被引:2,自引:0,他引:2  
柴雅琴  吴朝阳 《分析化学》1995,23(11):1252-1255
本文设计合成了一种在低PH范围对氢离子有很好Nernst响应的新型中性载体-N,N-二辛基菸酰胺,并把它制成PVC膜PH电极,测试了该电极的线性范围、选择性、稳定、重现性和内阻等各项性能参数,并试验了电极抗氢氟酸腐蚀的能力,该该电极用于氢氟酸的电离常数测定时获得了满意的结果。  相似文献   
998.
制备了由2,6-二乙酰吡啶与肼基硫代甲酯衍生的希夫碱C_5H_3N[CH=NN=C(S)XR]_2(X=S,R=CH_3,C_6H_5CH_2;X=O,R=C_6H_5CH_2)。离析出类型为MC_5H_3N[CH=NN=C(S)XR]_2(M=VO~(2 )、Mn~(2 )和Fe~(2 ))和FeC_5H_3N[CH=NN=C(S)SR]_2Cl的希夫碱配合物。希夫碱及其配合物为元素分析、红外、可见一紫外光谱以及磁化率测量所表征,结果与所提出的Mn~(2 )、Fe~(2 )及Fe~(3 )配合物的结构一致.在VO(Ⅱ)配合物的情况,则形成多聚体,其结构为: -(N_3S_2)V-O-V(N_3S_2)-O-V(N_3S_2)-.  相似文献   
999.
The distribution homogeneities of 13 elements (As, Ce, Co, Cr, Cs, Eu, Fe, Hf, La, Rb, Sc, Sm and Th) in the esturine sediment SRM1646 have been evaluated by INAA and synchronous radiation-based X-ray fluorescence spectrometry in the range of sampling amounts from 10 μg to 20 mg. The experimental results reveal that the homogeneities of the elements studied relate to their species and sampling amount. The sampling constants of these elements have been calculated.  相似文献   
1000.
A simple method for simultaneous determination of inorganic and total mercury contents in human hair by neutron activation analysis (NAA) has been developed. The method is based on the selective extraction of methylmercury from hair by hydrochloric acid. Thus, the residual phase containing inorganic mercury can be determined by NAA. Further, the methylmercury contents in hair samples are easily calculated by subtracting the inorganic mercury contribution from the total Hg simultaneously given by INAA. Several reference materials of human hair, including IAEA hair RM 085 and 086, Chinese hair RMs GBW 09101 and 07601, were analyzed by this method. Our results show that the method is reliable.  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