全文获取类型
收费全文 | 3774篇 |
免费 | 111篇 |
国内免费 | 757篇 |
专业分类
化学 | 4261篇 |
晶体学 | 69篇 |
力学 | 16篇 |
综合类 | 33篇 |
数学 | 6篇 |
物理学 | 257篇 |
出版年
2023年 | 18篇 |
2022年 | 42篇 |
2021年 | 55篇 |
2020年 | 70篇 |
2019年 | 56篇 |
2018年 | 88篇 |
2017年 | 103篇 |
2016年 | 102篇 |
2015年 | 76篇 |
2014年 | 135篇 |
2013年 | 422篇 |
2012年 | 180篇 |
2011年 | 200篇 |
2010年 | 150篇 |
2009年 | 201篇 |
2008年 | 216篇 |
2007年 | 275篇 |
2006年 | 259篇 |
2005年 | 252篇 |
2004年 | 242篇 |
2003年 | 201篇 |
2002年 | 158篇 |
2001年 | 119篇 |
2000年 | 114篇 |
1999年 | 101篇 |
1998年 | 74篇 |
1997年 | 75篇 |
1996年 | 83篇 |
1995年 | 89篇 |
1994年 | 86篇 |
1993年 | 78篇 |
1992年 | 70篇 |
1991年 | 36篇 |
1990年 | 25篇 |
1989年 | 29篇 |
1988年 | 28篇 |
1987年 | 12篇 |
1986年 | 24篇 |
1985年 | 8篇 |
1984年 | 7篇 |
1983年 | 6篇 |
1982年 | 9篇 |
1981年 | 12篇 |
1980年 | 15篇 |
1979年 | 12篇 |
1978年 | 4篇 |
1977年 | 6篇 |
1976年 | 4篇 |
1974年 | 3篇 |
1972年 | 3篇 |
排序方式: 共有4642条查询结果,搜索用时 218 毫秒
991.
V. V. Burdin V. D. Maiorov G. I. Voloshenko O. Yu. Antonova N. B. Librovich 《Russian Chemical Bulletin》1998,47(12):2415-2418
Ion-molecular interactions in the HCl−BuiOH system with different compositions (from neat isobutyl alcohol to 37 mol.% HCl) were studied by Multiple Attenuated Total
Reflectance (MATR) IR spectroscopy at 30 °C. Proton disolvates (Bui(H)O…H…O(H)Bui)+ with strong symmetrical H bonds are formed upon the addition of HCl to BuiOH. At high concentrations of HCl (C
0
HCl>33 mol.%), (Cl…H…Cl)− ions are formed along with (BuiOH)2H+. The spectra of positively and negatively charged proton disolvates were compared to those of similar ions in the HCl−PriOH and HCl−MeOH systems.
Translated fromIzvestiya Akademii Nauk. Seriya Khimicheskaya, No. 12, pp. 2496–2500, December, 1998. 相似文献
992.
K. Ogura K. Kisaka H. Furukawa 《Journal of polymer science. Part A, Polymer chemistry》1995,33(8):1375-1380
Poly(vinyl chloride) (PVC) has been converted to an electrically conductive structure by combined electrochemical and photochemical methods. PVC was cast on a polypyrrole (PPy) film electrode which had been electrochemically prepared. The PVC layer in the laminated PVC/PPy films was first dehydrochlorinated under the illumination of UV light, and the generated polyenes were subsequently doped with I2 and FeCl3. The maximum electrical conductivity achieved for such PVC film was 2.51 X 10?2 and 8.63 10?2 S cm?1 after I2 and FeCl3 doping, respectively. The temperature dependence of the electrical conductivity showed different behavior in higher and lower temperature ranges. In the former (T > 243 K), the T?1 law held, and the activation energy and bandgap were estimated as 0.25 and 0.49 eV, respectively. In the latter (T < 243 K), the conductivity mechanism followed the variable range hopping model (T?1/4 law) in which the radius of the localized state wave function and the density of the localized states at the Fermi level were 1.25 × 103 Å and 1.03 X 1015 eV?1 cm?3, respectively. © 1995 John Wiley & Sons, Inc. 相似文献
993.
ChaoYingWAN 《中国化学快报》2003,14(4):417-418
Poly(viyl chloride)/Na^ -montmorillonite(PVC/MMT)nanocomposites with different MMT contents were prepared via melt blending.Wide-angle X-ray diffraction(WAXD)and transmission electron microscopy(TEM)were used to characterize the structures.Effects of MMT content on the mechanical properties were also studied.It is found that PVC molecular chains can intercalate into the gallery of MMT layers during melt blending process,the stiffiness and toughness of the composites are inproved simultaneously within 0.5-7wt% MMT content,and the transparency and mechanical properties decrease as MMT conten further increases. 相似文献
994.
Apparent molar volumes have been determined by density measurements of aqueous solutions for a series of salts [Co(L)3]X3, where X=Cl– and Br– and L1=1,2-diaminoethane (en), L2=1,2-diaminopropane (pn) and L3=1,3-diaminopropane (tn) at 15°C and 5°C. Apparent molar volumes at infinite dilution for the complex cations at 0°C are estimated. The resulting values are related to the structure of solvent water molecules around the ions. 相似文献
995.
S. N. Bhadani Madhu Tiwari Archana Agrawal Chandra Sekhar Kavipurapu 《Mikrochimica acta》1994,117(1-2):15-22
A sensitive Spectrophotometric method for the determination of iron with tiron and a cationic surfactant, cetylpyridinium chloride, at pH 5.6 is reported. The complex is extracted into a chloroform-propan-2-ol (41) mixture and shows maximum absorbance at 520 nm. Beer's law is obeyed in the range 1–14 g/ml with an average molar absorptivity of 15800 l mol–1 cm–1. The molar ratio as determined by Job's method for Fe:tiron:CPC is 143. Interferences by various ions are examined. Zr, Ti and Mo interfere heavily. The method is applied for the determination of iron in Al-based and Cu-based alloys, using appropriate masking agents. 相似文献
996.
V. Tomašić N. Filipović-Vinceković B. Kojić-Prodić N. Kallay 《Colloid and polymer science》1991,269(12):1289-1294
The precipitation boundary in aqueous mixtures of dodecylammonium chloride (DDACl) and sodium dodecyl sulfate (NaDS) was determined in the absence and in the presence of 1 mol dm–3 NaCl. The structure and the composition of the solid phase was characterized by x-ray and chemical analysis, respectively. Activities of Cl– and Na+ ions in the aqueous phase were measured by ion-selective electrodes. As determined by calorimetry, precipitation is an exothermic process.The DDA·DS precipitate was formed in the equimolar region of the precipitation components. Its crystallographic structure is described. In an excess of sodium dodecyl sulfate, the precipitate incorporated substantial amounts of NaDS, as detected by analyses of both solid and liquid phases. X-ray analysis of the dry sample showed peaks of crystalline NaDS. According to the polarization microscopy of wet samples, one may conclude that liquid crystals, containing an excess of NaDS, are incorporated in the solid DDA·DS phase. The same was found in the case of an excess of DDACl; mixed liquid crystals with an excess of DDACl were incorporated in the solid DDA·DS.Interpretation of the solubility boundary points to the presence of DDA+·DS– ion pairs. Formation of these species at low ionic strengths is characterized by the equilibrium constantK
a
0
106. However, in the 1 mol dm–3 NaCl, the association of DDA+ and DS– ions into pairs is inhibited (K
a
0
0). This finding can be explained in terms of ionic clouds around the charged surfactant heads: if these heads are not in close contact, but separated due to structural effects of the chains, the dense distribution of counterions around them at high ionic strengths may compensate for electrostatic attraction and, thus, inhibit ion-pairing. 相似文献
997.
用苯甲酰氯(BC)/TiCl4引发异丁烯(IB)聚合及与苯乙烯(St)的共聚反应,得到分子量高、分布窄的聚异丁烯及其共聚物,并控制了BC的高活性。对IB均聚及其与ST共聚反应影响因素(体系浓度、残余水、第3组分三乙胺(TEA))进行优化,得到最佳条件为:[BC]=2.6mmol/L、[TEA]/[BC]=1.0(均聚)和n(TiCl4)/n(BC)=80、[TEA]/[BC=4.0(共聚),BC/TiCl4/TEA是最佳体系,对水不敏感,可以制备分子量高及分子量分布(MWD)为1.5(均聚)和2.0(共聚)的窄分布聚合物(GPC曲线均为单峰)。 相似文献
998.
2,4-二氯苯氧乙酰氯的气相色谱分析 总被引:5,自引:0,他引:5
建立了柱前衍生化气相色谱法测定2,4—二氯苯氧乙酰氯含量的方法;采用OV—17填充柱和火焰离子化检测器(FID),内标法定量,样品加标平均回收率96.6%,标准偏差1.48,相对标准偏差1.71%;该方法简单、快速,结果准确。 相似文献
999.
Krishnakumar V Ramasamy R 《Spectrochimica acta. Part A, Molecular and biomolecular spectroscopy》2005,61(13-14):3112-3116
The molecular structure and vibrational spectra of cyanuric chloride have been investigated by density functional theory (DFT) using standard B3LYP/6-31G* and B3LYP/6-311+G** method and basis set combinations. The DFT force field transformed to natural internal coordinates was corrected by a well-established set of scale factors that were found to be transferable to the title compound. Both the calculated structural parameters and vibrational frequencies are in good agreement with the available experimental data. 相似文献
1000.
Svante Holmberg Peter Holmlund Carl‐Eric Wiln Tanja Kallio Gran Sundholm Franciska Sundholm 《Journal of polymer science. Part A, Polymer chemistry》2002,40(4):591-600
The atom transfer radical polymerization (ATRP) of styrene onto poly(vinylidene fluoride)‐graft‐poly(vinylbenzyl chloride) (PVDF‐g‐PVBC) membranes was investigated. Novel membranes were designed for fuel‐cell applications. The benzyl chloride groups in the PVDF‐g‐PVBC membranes functioned as initiators, and a Cu‐based catalytic system with the general formula Cu(n)Xn/ligand [where X is Cl or Br and the ligand is 2,2′‐bipyridyl (bpy)] was employed for the ATRP. In addition, 10 vol % dimethylformamide was added for increased solubility of the catalyst complex in styrene. The system was homogeneous, except for the membrane, when the initiator/copper halide/ligand/monomer molar ratio was 1/1/3/500. As anticipated, the fastest polymerization rate of styrene was observed with the copper bromide/bpy‐based catalyst system. The reaction rate was strongly temperature‐dependent within the studied temperature interval of 100–130 °C. The degree of grafting increased linearly with time, thereby indicating first‐order kinetics, regardless of the polymerization temperature. Furthermore, 120 °C was the maximum polymerization temperature that could be used in practice because the membrane structure was destroyed at higher temperatures. The degree of styrene grafting reached 400% after 3 h at 120 °C. Such a high degree of grafting could not be reached with conventional uncontrolled radiation‐induced grafting methods because of termination reactions. On the basis of an Arrhenius plot, the activation energy for the homogeneous ATRP of styrene was 217 kJ/mol. The prepared membranes became proton‐conducting after sulfonation of the polystyrene grafts. The highest conductivity measured for the prepared membranes was 70 mS/cm, which is comparable to the values normally measured for commercial Nafion membranes. The scanning electron microscopy/energy‐dispersive X‐ray results showed that the membranes had to be grafted through the matrix with both PVBC and polystyrene to become proton‐conducting after sulfonation. In addition, PVDF‐g‐[PVBC‐g‐(styrene‐block‐tert‐butyl acrylate)] membranes were also synthesized by ATRP. © 2002 John Wiley & Sons, Inc. J Polym Sci Part A: Polym Chem 40: 591–600, 2002; DOI 10.1002/pola.10146 相似文献