全文获取类型
收费全文 | 5757篇 |
免费 | 536篇 |
国内免费 | 674篇 |
专业分类
化学 | 1710篇 |
晶体学 | 36篇 |
力学 | 470篇 |
综合类 | 65篇 |
数学 | 1544篇 |
物理学 | 579篇 |
综合类 | 2563篇 |
出版年
2024年 | 30篇 |
2023年 | 56篇 |
2022年 | 105篇 |
2021年 | 107篇 |
2020年 | 122篇 |
2019年 | 115篇 |
2018年 | 124篇 |
2017年 | 194篇 |
2016年 | 168篇 |
2015年 | 183篇 |
2014年 | 299篇 |
2013年 | 375篇 |
2012年 | 324篇 |
2011年 | 336篇 |
2010年 | 261篇 |
2009年 | 304篇 |
2008年 | 285篇 |
2007年 | 363篇 |
2006年 | 320篇 |
2005年 | 305篇 |
2004年 | 290篇 |
2003年 | 281篇 |
2002年 | 243篇 |
2001年 | 195篇 |
2000年 | 195篇 |
1999年 | 206篇 |
1998年 | 152篇 |
1997年 | 179篇 |
1996年 | 124篇 |
1995年 | 111篇 |
1994年 | 107篇 |
1993年 | 74篇 |
1992年 | 69篇 |
1991年 | 65篇 |
1990年 | 60篇 |
1989年 | 40篇 |
1988年 | 39篇 |
1987年 | 24篇 |
1986年 | 32篇 |
1985年 | 15篇 |
1984年 | 13篇 |
1982年 | 11篇 |
1981年 | 6篇 |
1980年 | 7篇 |
1979年 | 12篇 |
1978年 | 10篇 |
1977年 | 9篇 |
1976年 | 5篇 |
1975年 | 3篇 |
1972年 | 4篇 |
排序方式: 共有6967条查询结果,搜索用时 15 毫秒
131.
Mixed valency in the high-temperature phases of transition metal molybdates, A MoO4 ( A =Fe, Co, Ni)
Transition metal molybdates of the formulaAMoO4 whereA=Fe, Co or Ni exhibit a first-order phase transition between 670K–970K. An investigation of the lowtemperature (lt) and high-temperature (ht) phases by x-ray photoelectron spectroscopy, x-ray absorption spectroscopy, magnetic susceptibility and other physical methods
shows that the phase transition is associated with a valence change of the typeA
2++Mo6+αA
3++Mo5+ in the cases of iron and cobalt molybdates.
Contribution No. 311 from the Solid State and Structural Chemistry Unit. 相似文献
132.
Both single crystals of VCl3(THF)3 as well as isotypic cocrystals of the composition MCl3(THF)3, M=Ti/V 1/3, undergo a topotactic reversible phase transition to a hitherto unknown low-temperature modification. The close relationship between this new structure and the room-temperature phase determined by Cottonet al. is discussed from the molecular and the intermolecular point of view: Both modifications are built up by conformationally very similar molecules which change their arrangement during the phase transition. Lattice energy calculations confirm that these two alternative arrangements correspond to minima of almost the same packing energy.Dedicated to Professor Dr. G. E. Herberich on the occasion of his 60th birthday. 相似文献
133.
With the combination of the the stoichiometric displacement model for retention (SDM-R) in reversed phase liquid chromatography (RPLC) and the stoichiometric displacement model for adsorption (SDM-A) in physical chemistry,the total number of moles of the re-solvated methanol of stationary phase side.nr,and that of solute side in the mobile phase,q,corresponding the one mole of the desorbing solute,were separately determined and referred as the characterization parameters of the contributions of the adsorption mechanism and partition mechanism to the solute retention,respectively.A chromatographic system of insulin,using mobile phase consisting of the pseudo-homologue of alcohols(methanol,ethanol and 2-propanol)-water and trifluoroacetic acid was employed.The maximum number of the methanol layers on the stationary phase surface was found to be 10.6,only 3 of which being valid in usual RPLC,traditionally referred as a volume process in partition mechanism.However,it still follows the SDM-R.Both of q and nr of insulin were found not to be zero,indicating that the retention mechanism of insulin is a mixed mode of partition mechanism and adsorption mechanism.When methanol is used as the organic modifier,the ratio of q/nr was 1.13,indicating the contribution to insulin retention due to partition mechanism being a bit greater than that due to adsorption mechanism.A linear relationship between q,or nr and the carbon number of the pseudo-homologue in the mobile phase was also found.As a methodology for investigating the retention mechanism retention and behavior of biopolymers.a homologue of organic solvents as the organic modifier in mobile phase has also been explored. 相似文献
134.
Methane production from synthesis gas using a mixed culture ofR. rubrum M. barkeri,and M. formicicum
Klasson K. T. Cowger J. P. Ko C. W. Vega J. L. Clausen E. C. Gaddy J. L. 《Applied biochemistry and biotechnology》1990,(1):317-328
The components of synthesis gas, CO, H2, and CO2, may be converted into CH4 biologically through either acetate or H2/CO2 as intermediates. Of these two routes, conversion through H2/CO2 is preferred. This paper presents results of mixed-culture studies employing the photosynthetic bacteriumR. rubrum for converting CO to CO2 and H2 by the water gas shift reaction and two methanogens,M. formicicum andM. barkeri, for converting CO2 and H2 into CH4. Results are presented for triculture operation in two types of reactors, the packed bubble column and the trickle-bed reactor. 相似文献
135.
Yan-Ping Yuan De-Yuan Kong Abraham Clearfield 《Journal of solid state chemistry》2005,178(6):2030-2035
The first organically templated 3d-4f mixed metal sulfates, [H2en]2{La2M(SO4)6(H2O)2} (M=Co 1, Ni 2) have been synthesized and structurally determined from non-merohedrally twinned crystals. The two compounds are isostructural and their structures feature a three-dimensional anionic network formed by the lanthanum(III) and nickel(II) ions bridged by sulfate anions. The La(III) ions in both compounds are 10-coordinated by four sulfate anions in bidentate chelating fashion, and two sulfate anions in a unidentate fashion. The transition metal(II) ion is octahedrally coordinated by six oxygens from four sulfate anions and two aqua ligands. The doubly protonated enthylenediamine cations are located at the tunnels formed by 8-membered rings (four La and four sulfate anions). 相似文献
136.
The metal ion distribution on the two metal sites of monoclinic Mn1?xCux(HCOO)2 · 2(H,D)2O mixed crystals are studied by infrared and Raman spectroscopic methods. The spectral regions 3 200–3 400 cm?1 (vOH), 2 875–2 990 cm?1 (vCH), 2 330–2 500 cm?1 (vOD of matrix isolated HDO molecules), 1 350–1 400 cm?1 (symmetric CO2 stretching modes), 570–950 cm?1 (H2O librations), and 490 cm?1 (M? O lattice modes) are mostly sensitive to the metal ions present. The frequency shifts of these bands with increasing content of copper show that Cu2+ prefers the M(1) site, coordinated by HCOO? only. The strengths of the hydrogen bonds increase on going from manganese to copper formate, due to the increased synergetic effect of Cu2+. Solubility and X-ray data of the mixed crystals are included. Irrespective of the same crystal structure, two series of mixed crystals are formed: eutonic area at 0.65 ≥ x ≥ 0.5. 相似文献
137.
Yu. G. Yatluk N. A. Zhuravlev O. V. Koryakova L. K. Neudachina Yu. A. Skorik 《Russian Chemical Bulletin》2005,54(8):1836-1841
A series of new hybrid organo-inorganic sorbents with the 3-aminopropionate chelating group was synthesized. The synthesis
includes the copolycondensation (sol—gel method) of tetraethoxysilane, 3-aminopropyltriethoxysilane, and several modifiers
(MeSi(OEt)3, EtSi(OEt)3, Ti(OEt)4, AlONO3, ZrOCl2) followed by carboxyethylation with acrylic acid. The obtained chelating sorbents were characterized by elemental analysis,
FT-IR and 1H NMR spectroscopy, and thermogravimetry. The N-carboxylated sorbents have a higher sorption capacity with respect to metal ions (0.5–0.9 mmol g−1, pH 6.3, NH4OAc, 20 °C) than the starting sorbents with the primary amino group (0.05–0.2 mmol g−1) and manifest high selectivity for copper(II) ion extraction.
__________
Published in Russian in Izvestiya Akademii Nauk. Seriya Khimicheskaya, No. 8, pp. 1783–1788, August, 2005. 相似文献
138.
ZHOU Yin-Zhuang② LI Jin-Fang TU Shu-Jie ZHANG Min 《结构化学》2005,24(10):1193-1197
1 INTRODUCTION The chemistry of copper compounds has been extensively investigated and the relationship be- tween structure and reactivity, ranging from indus- trial catalysis to biochemistry activity, is of major importance. For binuclear copper(II) complexes equa- torially bridged by pair of hydroxide[1] or alkoxide[2, 3] groups, satisfactory linear correlation is found be- tween the Cu–O–Cu bridging angle and spin coupling between the metal centers. However, for binuclear copper(I… 相似文献
139.
The relaxation field for solutions of mixed electrolytes of any type is calculated. the calculation is based on the well-known
treatment due to Fuoss-Onsager with the same distance parameter for all the ions in solution. A general conductance-continuity
equation has been established and an improved iterative method of calculation, using Laplace transforms, is proposed. The
relaxation-field results are derived to the second iteration in the perturbation method of integration. 相似文献
140.
Mohsen M. Zareh 《Mikrochimica acta》1997,126(3-4):271-277
Mixed ion-pairs based on the use of ephedrinium (EPH)-TPB plus EPH-reineckate (II) and phenylephrine-TPB plus EPH-reineckate (III) were tried for use in plastic membranes. The results were compared to those of an EPH-reineckate (I) single ion-pair electrode. The Nernstian slopes were 50, 49 and 55 mV decade–1 for membranes I, II and III, respectively. The linear concentration ranges were 10–5–10–1, 4.0 × 10–5–10–1 and 6.3 ×–5–10–1
M ephedrine. The detection limits were 4 ×–6,10–5 and 1.2 × 10–5
M ephedrine for membranes I, II and III, respectively. The pH ranges were 4–9, 3–9 and 2–8 for I, II and III-membranes, respectively. Selectivity coefficient values for membrane II were better than those for membranes I and III. The effects of increasing KC1 concentration and temperature changes were explained for the three electrodes. The isothermal temperature coefficients were 0.00145, 0.0007 and 0.00055 V/ °C for electrodes I, II and III. Electrode III was applied for the determination of ephedrine in its pharmeaceutical preparations with an overall relative standard deviation range of 1.3–2.4% and an overall mean recovery value of 98.1%. 相似文献