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   30415篇
  免费   4172篇
  国内免费   4645篇
化学   18436篇
晶体学   2676篇
力学   2595篇
综合类   217篇
数学   5209篇
物理学   10099篇
  2024年   38篇
  2023年   217篇
  2022年   434篇
  2021年   581篇
  2020年   817篇
  2019年   809篇
  2018年   782篇
  2017年   1026篇
  2016年   1246篇
  2015年   953篇
  2014年   1506篇
  2013年   2841篇
  2012年   1692篇
  2011年   1643篇
  2010年   1543篇
  2009年   1745篇
  2008年   1900篇
  2007年   2076篇
  2006年   2095篇
  2005年   1978篇
  2004年   1882篇
  2003年   1623篇
  2002年   1349篇
  2001年   1023篇
  2000年   986篇
  1999年   974篇
  1998年   782篇
  1997年   724篇
  1996年   682篇
  1995年   585篇
  1994年   515篇
  1993年   441篇
  1992年   387篇
  1991年   271篇
  1990年   158篇
  1989年   114篇
  1988年   114篇
  1987年   97篇
  1986年   66篇
  1985年   103篇
  1984年   78篇
  1983年   39篇
  1982年   45篇
  1981年   57篇
  1980年   33篇
  1979年   38篇
  1978年   45篇
  1977年   35篇
  1976年   26篇
  1971年   8篇
排序方式: 共有10000条查询结果,搜索用时 15 毫秒
991.
CrystalStructuresofLanthanumComplexeswithNitrobenzoicAcids¥MaJian-Fang;JinZhong-Sheng;NiJia-Zuan(LaboratoryofRareEarthChemist...  相似文献   
992.
The Crystal Structure of Na2NiAlF7 — a Contribution to the Problem of the True Space Group of Orthorhombic Weberites The X-ray single crystal structure determination of the orthorhombic weberite Na2NiAlF7 (a = 707.4(2), b = 1003.8(2), c = 731.5(1) pm; Z = 4) was performed in space group Imma, after all reflections hk0 with h(k) = 2n + 1 could be eliminated, as they proved simulated by Renninger effect and/or λ/2 reflections. The alternative space groups Imm2 resp. I212121 of former weberite structure determinations thus became obsolete. The refinement using 880 independent reflections ended at wR = 0.0232. The resulting average distances within the framework of octahedra are Ni? F = 197.3, Al? F = 180.4 pm.  相似文献   
993.
The ion-pair compound [H2(teta)]2+·[Ni(CN)4] 2-·2H2O (C20H42N8NiO2, teta = meso- 5,7,7,12,14,14-hexamethyl-1,4,8,11-tetraazacyclotetradecane) was synthesized and characterized by single-crystal X-ray diffraction. The crystal belongs to monoclinic, space group P21/n with a = 10.0784(10), b = 9.5411(7), c = 14.1010(14)(A), β= 106.752(2)o, V = 1298.4(2) (A)3, Mr = 485.33, Z = 2, Dc = 1.241 g/cm3,μ(MoKα) = 0.778 mm-1 and F(000) = 524. The structure was refined to R = 0.0391 and wR = 0.0870 for 2614 observed reflections with I > 2σ(I). The title compound contains one [Ni(CN)4]2- anion, one protonated macrocyclic tetraamine cation [H2(teta)]2+ and two water molecules. There are multiform hydrogen bonds in the compound to link the different components and stabilize the crystal structure.  相似文献   
994.
A SAPO-11 silicoaluminophosphate molecular sieve with stable crystal structure was synthesized for the first time. After removing template by calcination, its crystal space group still retains Icm2 which the as-synthesized has. The catalyst deriving from the present SAPO-I 1 materials shows higher isomerization selectivity and higher paraffin hydroisomerization yield than those reported elsewhere.  相似文献   
995.
A novel compound Ni(phen)(H2O)(V2O6) has been hydrothermally synthesized and structurally determined to be a two-dimensional compound, which contains {V2O6}n2n- chains interconnected by nickel(Ⅱ) complexes via oxygen atoms. The crystallographic data measured by single-crystal X-ray diffraction analysis are as follows: C12H10N2NiO7V2, Mr=454.81, monoclinic, space group P21/c, a=0.784 6(3), b=2.103 6(8), c=0.942 3(4) nm, β=112.872(5)°, V=1.433 0(10) nm3, Z=4, Dc=2.104 Mg·m-3, μ(Mo )=2.615 mm-1, F(000)=904, T=298(2) K, 4 480 reflections collected, 2 470 independent (Rint=0.032 2), the final R=0.058 4 and wR2=0.145 7 for 2 303 observed reflections with I>2σ(I). CCDC: 192520.  相似文献   
996.
A Contribution on the Crystal Structure of CuYW2O8, CuHoW2O8, and CuYW2O8 Single crystals of (I) CuY2O8, (II), CuHoW2O8, and (III) CuYbW2O8 were prepared and investigated by X-ray technique. (I) crystallizes with triclinic symmetry, space group C? P1 (a = 5.939 Å, b = 6.042 Å, c = 5.025 Å; α = 112.30°, β = 111.77°; Z = 1). (II) and (III) belong to monoclinic symmetry, space group C? P2/n (II) (a = 10.045 Å, b = 5.808 Å, c = 5.021 Å; β = 94.38°; z = 2 (III) a = 9.948 Å, b = 5.824 Å, c = 5.008 Å; β = 93.36°; Z = 2). The crystal structures will be discussed with respect to other to copper rare earth tungstates.  相似文献   
997.
Two polymorphs of monovalent [Ni(dmit)2] (dmit2−=2-thioxo-1,3-dithiole-4,5-dithiolate) crystals A and B, (anilinium)(18-crown-6)[Ni(dmit)2], were prepared, and the structure and magnetic properties were investigated. In these crystals, the [Ni(dmit)2] molecules form dimers, which arranged into chains between the supramolecular cation structure (anilinium)(18-crown-6). In crystal A, supramolecular cation formed a regular stack, inducing ladder structure of [Ni(dmit)2], whose magnetism had been well fitted by spin ladder equation with the spin gap of Δ=190 K. Crystal B is ca. 3% more densely packed compared to crystal A. Due to the dense packing, supramolecular cation stack is distorted, which prevented the intermolecular interaction between [Ni(dmit)2] dimers in direction corresponds to the ladder-leg direction in crystal A. Reflecting the [Ni(dmit)2] arrangement, crystal B showed a temperature dependence of magnetic susceptibility well reproduced by the singlet-triplet thermal activation model, whose antiferromagnetic exchange interaction (2J) was 140 K.  相似文献   
998.
Incomparisonwithinorganicnonlinearoptic(NLO)materials,organicpolymericNLOmaterialshavemanyadvantages,suchasliablemoleculardesignandoptimization,largernonresonanceNLOcoefficiency,lowerdielectricconstant,goodworkingability,opticaltransparencyandresistancea…  相似文献   
999.
1,5-苯并硫氮杂与苯乙酰氯反应生成1,5-苯并硫氮杂-α-苯基-β-内酰胺(Ⅱ)。X-射线单晶衍射测定结果表明,该化合物分子式为C29H22CINOS,M=467.5,晶体属单斜晶系,P21/c空间群,a=12.864(3),b=9.511(2),c=19.489(4)A,β=92.41(3)°,V=2382(1)A3,Dc=1.305g/cm3,Z=4,F(000)=976,μ=0.2636mm-1,R=0.068,Rw=0.068。该反应具有立体专一性,四员环上的2个苯基位于同一侧为顺式。产物分子中的七员环为类椅式构象。  相似文献   
1000.
Introduction As an important type of fungicides, triazole compounds are highly efficient, low poisonous and inward absorbent.1-3 At present, the studies on triazole derivatives are mainly concentrated on compounds with triazole as the only active group. The report of triazole compounds that contain both triazole group and other active group in a single molecule has rarely been found. Dialkyl-substituted dithiocarbamate salts have also shown interesting biological effects.4 N,N-Dialkyldithio-…  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