全文获取类型
收费全文 | 62328篇 |
免费 | 3852篇 |
国内免费 | 1877篇 |
专业分类
化学 | 36553篇 |
晶体学 | 432篇 |
力学 | 2145篇 |
综合类 | 78篇 |
数学 | 9619篇 |
物理学 | 19230篇 |
出版年
2023年 | 608篇 |
2022年 | 868篇 |
2021年 | 1015篇 |
2020年 | 1233篇 |
2019年 | 1150篇 |
2018年 | 1233篇 |
2017年 | 1133篇 |
2016年 | 2005篇 |
2015年 | 1799篇 |
2014年 | 2118篇 |
2013年 | 3659篇 |
2012年 | 3617篇 |
2011年 | 3875篇 |
2010年 | 2650篇 |
2009年 | 2259篇 |
2008年 | 3183篇 |
2007年 | 2983篇 |
2006年 | 2800篇 |
2005年 | 2492篇 |
2004年 | 2072篇 |
2003年 | 1718篇 |
2002年 | 1522篇 |
2001年 | 1264篇 |
2000年 | 1082篇 |
1999年 | 819篇 |
1998年 | 713篇 |
1997年 | 678篇 |
1996年 | 761篇 |
1995年 | 654篇 |
1994年 | 739篇 |
1993年 | 671篇 |
1992年 | 723篇 |
1991年 | 529篇 |
1990年 | 536篇 |
1989年 | 493篇 |
1988年 | 426篇 |
1987年 | 413篇 |
1986年 | 374篇 |
1985年 | 594篇 |
1984年 | 573篇 |
1983年 | 452篇 |
1982年 | 488篇 |
1981年 | 472篇 |
1980年 | 480篇 |
1979年 | 449篇 |
1978年 | 434篇 |
1977年 | 443篇 |
1976年 | 409篇 |
1975年 | 400篇 |
1974年 | 388篇 |
排序方式: 共有10000条查询结果,搜索用时 31 毫秒
81.
The effect of substituted hydroxybenzophenone, benzotriazole and organic metal complexes on the photooxidation of PP films has been studied. It has been established that NiDBTC added in 1.0 wt. % is the most effective photostabilizer.While in the case of other stabilizers there is a slight increase in the Co content even during the induction period, with the application of NiDBTC practically no oxidation can be observed before the stabilizer completely disappears. This means that NiDBTC inhibits the photooxidation of the polymer even in low concentration.
, - . , Ni, 1,0 . %. . , Ni , . , Ni .相似文献
82.
Türkan Gürkan 《Mikrochimica acta》1976,65(2-3):165-171
Summary The condensation products of 4 sympathomimetic amines with 1,2-naphthoquinone-4-sulphonic acid have been prepared and examined. A spectrophotometric procedure based on the chloroform-soluble orange-red condensation product is described. The same products were investigated fluorimetrically after reduction with sodium borohydride. The results obtained are satisfactory.
Zusammenfassung Die Kondensationsprodukte vier sympathicomimetischer Amine mit 1,2-Naphthochinon-4-sulfonsäure wurden dargestellt und untersucht. Ein spektralphotometrisches Verfahren auf der Grundlage des chloroformlöslichen, orangeroten Kondensationsproduktes wurde beschrieben. Die gleichen Produkte wurden auch fluoreszenzanalytisch nach ihrer Reduktion mit Natriumborhydrid untersucht. Die Ergebnisse sind befriedigend.相似文献
83.
The successive reaction of (CO)6M with Na[NCR21] and [Et3O]BF4 yields (CO)5M[C(NCR21)OEt] (II: M = Cr; III: M = W; CR21 = C(C6H4Br-p)2 (a), CPh2 (b), C(C6H4OMe-p)2 (c), C(C6H4)2O (d), CBu2tt (e)). Hexacarbonyltungsten, (CO)6W, reacts with Na[NCPh2] and MeOSO2F to give (CO)5W[C(NCPh2) OMe] (IV). X-Ray analysis of IIe shows that: (1) the CNC fragment is almost linear (171.7°); (2) the two NC bond lengths are equal within experimental error; and (3) the O,C,Cr,N plane is perpendicular to the C(Me3),C,N,C(Me3) plane (90.0°). Therefore compounds II–IV are best described as 1-alkoxy-2-azaallenyl complexes. 相似文献
84.
Yanning Song 《Journal of solid state chemistry》2003,175(1):63-71
A new two-dimensional (2d) iron phosphate, (C2N2H10)Fe2O(PO4)2, has been synthesized under hydrothermal conditions in the system of FeCl3-H3PO4-C2N2H8-H2O. The crystal data is: space group P21/c, a=10.670(1) Å, b=10.897(1) Å, c=9.918(1) Å, β=105.632(1)°, Z=4. The layered structure consists of double sheet layers, of composition Fe2O(PO4)2, built from FeO5 trigonal bipyramids and PO4 tetrahedra. The amine holds the layers together via H-bonding. The study of the magnetic properties reveals two magnetic transitions at 160 and 30 K with spin-glass-like behavior below 160 K. By varying the hydrothermal conditions, three other iron phosphates were synthesized: the one-dimensional (1d) (C2N2H10)Fe(HPO4)2(OH)·H2O, the 2d (C2N2H10)Fe2(PO4)2(OH)2, and the three-dimensional (3d) (C2N2H10)2Fe4O(PO4)4·H2O. The 1d compound can be used as the starting reagent in the synthesis of both the 2d compound and the 3d lipscombite Fe3(PO4)2(OH)2 due to the similar building blocks in their structures. In the 3d phosphate (C2N2H10)2Fe4O(PO4)4·H2O, manganese can substitute for half of the iron atoms. Magnetic study shows ordering transitions at about 30 K, however, manganese substitution depresses the magnetic ordering temperature. 相似文献
85.
Bernd G. Müller 《Journal of fluorine chemistry》1982,20(3):291-299
Single crystals of CaPdF4 (purple) and CdPdF4 (dark blue) have been obtained by heating CaF2 (CdF2) with PdF2 at 820°C (900°C) for ca. 20–30 d in sealed Pd (Pt) tubes. CaPdF4 crystallizes tetragonal (KBrF4-Type) with a = 5.521, c = 10.570 Å (space group I4/mcm ? D184h, No. 140) Z = 4, 283 unique reflexions, R = 0.083; Rw = 0.079, CdPdF4 cubic (CaF2-variant) with a = 5.403 Å (space group Pa3 ? T6h, No. 205) Z = 4, 165 unique reflexions, R = 0.047, Rw = 0.032. The high pressure modification of PdF2, obtained by heating PdF2 in closed Pt-tubes under a pressure of ≈ 60 kbar and ‘HgPdF4’, both black, a = 5.327 Å and a = 5.43 Å are isotypic. CaPdF4 is diamagnetic, CdPdF4 and HPPdF2 are antiferromegnetic. 相似文献
86.
J. Sztatisz S. Gál L. Fodor E. Pungor 《Journal of Thermal Analysis and Calorimetry》1977,12(3):351-360
The melting and crystallization of sorbitol were investigated with the DSC method and thermal microscopy. Sorbitol was found to have two crystalline modifications (confirmed by X-ray diffraction) with different melting points, while rapid cooling of molten sorbitol resulted in an amorphous form. The effect of inoculation on the crystallization of the melt was studied too. Powders of both crystalline modifications were used for this purpose. A new technological process for rapid crystallization of molten sorbitol has been worked out on the basis of the thermal analysis results. 相似文献
87.
Syntheses of New N-Vinylpyrroles The reactions of pyrrolyl potassium ( 1 ) with (ethoxymethylene)malonic acid derivatives 2a–e yielded the carbanions 3a–e , which could be hydrolyzed to 4a–e , but with the exception of 4b they were not isolated, because a transformation to the N-vinylpyrroles 5a , c–e by elimination of ethanol took place; 1 reacted with 2b at 80°C to give 4b and 6 . Hydrolysis of 4b with KOH yielded 4g , which eliminated 1 mol of ethanol to form 5f , decarboxylation of which led to N-vinylpyrrole 7 . By cyclization of 5e under various conditions the pyrrolizines 8a , b are obtained, the hydrolysis of which did not give ketone 9 but only amino alcohol 10 . Some other cyclizations of 11a–c and 13 yielded the 3H-pyrrolizine derivatives 12a–c and 14 , respectively. 相似文献
88.
The free NH3 molecule and the [Zn(NH3)4]2+ ion were studied by the kinematic coupling approach. The pure effects of this coupling were found to be small, and some modifications had to be introduced in order to get a reasonable force field. The force constants deduced for the skeletal vibrations are comparable with those of a quasi-exact force field. Calculated frequencies for [68Zn(NH3)4]2+ and [64Zn(ND3)4]2+ are reported in addition to those of [64Zn(NH3)4]2+. Mean amplitudes of vibration for [64Zn(NH3)4]2+ are given. 相似文献
89.
The kinetics of complexation reaction of Cu(II) with 9-ethyl-3-carbazolecarboxaldehyde-thiosemicarbazone (ECCAT) has been
examined spectrophotometrically. The ligand was synthesized for the first time. The complexation reaction was carried out
in a DMF-water medium at 35°C. The complex has maximum absorbance at 393 nm. Kinetic and activation parameters of the complexation
reaction were calculated by the Arrhenius and Eyring equations using the data obtained from investigating the effect of temperature
on reaction rates under the specified conditions. We also proposed reaction rate equations. Based on the studied complexation
reaction, a simple kinetic method for the spectrophotometric determination of copper(II) has been developed. The calibration
graphs are linear in the concentration range 0.2–1.9 μg/mL. The species that caused interference were investigated.
The text was submitted by the authors in English. 相似文献
90.
The basic obstacles for the general use of laser-ablation(LA)-ICP-MS in analytical laboratories are connected with its reproducibility and calibration. A mathematical relation deduced from the correlation function of the analytical signals allows the estimation of the number of craters needed for representative analyses. The procedure was applied to different samples such as manganese crusts and soils. The ion intensities of the major elements in the manganese crusts and nodules were used as internal standards, improving relative standard deviations by factors between 2 and 3. Selected samples of wood and manganese crusts were analyzed by LA-ICP-MS and the results compared with those obtained by solution ICP-MS. The agreement of the values is within the 95% confidence limits. Powdered reference materials and, in the case of wood analysis, cellulose doped with standard solutions were used for the calibration. 相似文献