全文获取类型
收费全文 | 10538篇 |
免费 | 266篇 |
国内免费 | 31篇 |
专业分类
化学 | 7871篇 |
晶体学 | 31篇 |
力学 | 162篇 |
数学 | 1782篇 |
物理学 | 989篇 |
出版年
2020年 | 138篇 |
2019年 | 106篇 |
2018年 | 77篇 |
2017年 | 81篇 |
2016年 | 189篇 |
2015年 | 198篇 |
2014年 | 188篇 |
2013年 | 352篇 |
2012年 | 392篇 |
2011年 | 454篇 |
2010年 | 336篇 |
2009年 | 279篇 |
2008年 | 455篇 |
2007年 | 446篇 |
2006年 | 469篇 |
2005年 | 440篇 |
2004年 | 378篇 |
2003年 | 312篇 |
2002年 | 327篇 |
2001年 | 225篇 |
2000年 | 157篇 |
1999年 | 145篇 |
1998年 | 180篇 |
1997年 | 148篇 |
1996年 | 150篇 |
1995年 | 150篇 |
1994年 | 143篇 |
1993年 | 191篇 |
1992年 | 127篇 |
1991年 | 127篇 |
1990年 | 114篇 |
1989年 | 135篇 |
1988年 | 131篇 |
1987年 | 167篇 |
1986年 | 157篇 |
1985年 | 174篇 |
1984年 | 186篇 |
1983年 | 155篇 |
1982年 | 144篇 |
1981年 | 151篇 |
1980年 | 153篇 |
1979年 | 147篇 |
1978年 | 138篇 |
1977年 | 119篇 |
1976年 | 135篇 |
1975年 | 91篇 |
1974年 | 89篇 |
1973年 | 88篇 |
1971年 | 80篇 |
1970年 | 71篇 |
排序方式: 共有10000条查询结果,搜索用时 15 毫秒
261.
262.
263.
Solubility measurements as a function of temperature have been shown to be a powerful tool for the determination of thermodynamic properties of sparingly-soluble transition metal carbonates. In contrast to calorimetric methods, such as solution calorimetry or drop calorimetry, the evaluation of solubility data avoids many systematic errors, yielding the enthalpy of solution at 298.15 K with an estimated uncertainty of ±3 kJ · mol−1. A comprehensive set of thermodynamic data for otavite (CdCO3), smithsonite (ZnCO3), hydrozincite (Zn5(OH)6(CO3)2), malachite (Cu2(OH)2CO3), azurite (Cu3(OH)2(CO3)2), and siderite (FeCO3) was derived. Literature values for the standard enthalpy of formation of malachite and azurite were disproved by these solubility experiments, and revised values are recommended. In the case of siderite, data for the standard enthalpy of formation given by various data bases deviate from each other by more than 10 kJ · mol−1 which can be attributed to a discrepancy in the auxiliary data for the Fe2+ ion. A critical evaluation of solubility data from various literature sources results in an optimized value for the standard enthalpy of formation for siderite. The Davies approximation, the specific ion-interaction theory, and the Pitzer concept are used for the extrapolation of the solubility constants to zero ionic strength in order to obtain standard thermodynamic properties valid at infinite dilution, T = 298.15 K, and p = 105 Pa. The application of these electrolyte models to both homogeneous and heterogeneous (solid-solute) equilibria in aqueous solution is reviewed. 相似文献
264.
The new ternary compounds Rb4Ti3S14, Cs4Zr3S14, K4Hf3Se14, and K4ZrHf2Se14 were prepared by reacting the respective transition metals in alkali metal polychalcogenide melts. Two crystallographically independent transition metal cations are present that are coordinated by eight chalcogen atoms (Q) in an irregular fashion or by seven chalcogen atoms yielding a distorted pentagonal bipyramid. The M(1)Q8 and M(2)Q7 polyhedra are connected by sharing common edges or trigonal faces leading to the formation of infinite linear one‐dimensional anionic chains running parallel to the [101] direction. The chains are separated by alkali metal cations. The optical band gaps determined are 1.59 eV for Rb4Ti3S14, 2.35 eV for Cs4Zr3S14, and 2.02 eV for K4Hf3Se14. In‐situ X‐ray powder diffractometry demonstrates that Rb4Ti3S14 decomposes at 430 °C into Rb2S5 and TiS. During the cooling cycle the re‐formation of the polysulfide is observed. According to this result the polysulfide could be prepared using TiS instead of metallic Ti as well. 相似文献
265.
Metal Complexes of Functionalized Sulfur‐containing Ligands. XVII Synthesis of S ‐Oxides of 1,2,4‐Trithiolane, 1,2,4,5‐Tetrathiane as well as 1,2,3,5,6‐Pentathiepane, and their Reactions with (Ph3P)2Pt(η2‐C2H4). X‐Ray Structure Analysis of 3,3,5,5‐Tetraphenyl‐1,2,4‐trithiolane 1‐oxide 3,3,5,5‐Tetraphenyl‐1,2,4‐trithiolan ( 1 ) was oxidized using m‐chloroperbenzoic acid to give, selectively, the 3,3,5,5‐tetraphenyl‐1,2,4‐trithiolane 1‐oxide ( 2 ). 2 was characterized structurally. The reaction of octamethyl tetrathiadispiro[3.2.3.2]dodecane‐2,9‐dione ( 3 ) with trifluoroperacetic acid at –50 °C yielded the corresponding 5‐oxide 4 . Oxidation of octamethyl pentathiadispiro[3.3.3.2]tridecane‐2,9‐dione ( 5 ) with m‐chloroperbenzoic acid at 0 °C gave the 12‐oxide 6 . Treatment of 2 with two equivalents of (Ph3P)2Pt(η2‐C2H4) ( 7 ) afforded a mixture (1 : 1) of the complexes (Ph3P)2PtSCPh2S ( 8 ) and (Ph3P)2Pt(η2‐Ph2C=S=O) ( 9 ), respectively. 相似文献
266.
267.
Lars Einfeldt Katrin Petzold Wolfgang Günther Armin Stein Manfred Kussler Dieter Klemm 《Macromolecular bioscience》2001,1(8):341-347
Reaction of starch 1 dissolved in dimethyl sulfoxide (DMSO) with bulky thexyldimethylchlorosilane (TDSCl) in the presence of pyridine leads to regioselectively functionalized silyl ethers with a degree of substitution (DS) up to 1.8. The control of the DSSi, of the regioselectivity, and of the reaction pathway is described in detail. The reaction proceeds homogeneously up to DSSi of 0.6. With ongoing silylation the polymers form a separate phase incorporating the silylating agent to form TDS‐starches with DSSi values higher than 1.0. After peracetylation of the silyl starches, the substitution pattern has been characterized not only in the anhydroglucose repeating units (AGU) but also in the non‐reducing terminal end groups (TEG) by means of two‐dimensional 1H NMR techniques. Up to DSSi 1.0, a very high regioselective functionalization of the primary 6‐OH groups in the AGU as well as in the TEG is detectable. With increasing silylation (DSSi > 1.0), the subsequent silylation takes place at the 2‐OH groups of the AGU and at the 3‐OH groups of the TEG. These results are compared with our own investigations on the silylation of starch in the reaction system N‐methylpyrrolidone (NMP)/ammonia and on the silylation of cellulose in N,N‐dimethylacetamide (DMA)/LiCl/pyridine solution. 相似文献
268.
Wolfgang Radke 《Macromolecular theory and simulations》2001,10(4):343-348
The molecular weight distribution, polydispersity and the distribution of side chains within a GPC‐slice have been calculated for coeluting comb‐shaped polymers. It is assumed that the polymers were synthesized by grafting monodisperse side chains onto a backbone having a broad molecular weight distribution. Despite the broad polydispersity of the backbone the polydispersity within a GPC‐slice is rather narrow, as is the distribution of side chains. Consequently the effect of polydispersity on properties, which can be obtained by GPC coupled with molar mass sensitive detectors is negligible. However, this result is true only for the specific branching mechanism investigated. 相似文献
269.
Mono- and Binuclear Dinitrosyl Complexes of Molybdenum and Tungsten. Crystal Structures of (PPh3Me)2[WCl4(NO)2], (PPh3Me)2[MoCl3(NO)2]2, and (PPh3Me)2[WCl3(NO)2]2 The complexes (PPh3Me)2[MCl4(NO)2] (M = Mo, W), and (PPh3Me)2[MCl3(NO)2]2, respectively, are prepared by reactions of the polymeric compounds MCl2(NO)2 with triphenylmethylphosphonium chloride in CH2Cl2, forming green crystals. According to the IR spectra the nitrosyl groups are in cis-position in all cases. The tungsten compounds as well as (PPh3Me)2[MoCl3(NO)2]2 were characterized by structure determinations with X-ray methods. (PPh3Me)2[WCl4(NO)2]: space group C2/c, Z = 4. a = 1874, b = 1046, c = 2263 pm, β = 119.99°. Structure determination with 3492 independent reflexions, R = 0.057. The compound consists of PPh3Me⊕ ions, and anions [WCl4(NO)2]2? with the nitrosyl groups in cis-position (symmetry C2v). (PPh3Me)2[WCl3(NO)2]2: Space group C2/c, Z = 4. Structure determination with 2947 independent reflexions, R = 0.059. (PPH3Me)2[MoCl3(NO)2]2: Space group P1 , Z = 1. a = 989, b = 1134, c = 1186 pm; α = 63.25°, β = 80.69°, γ = 69.94°. Structure determination with 3326 independent reflexions, R = 0.046. The compounds consist of PPh3Me⊕ ions, and centrosymmetric anions [MCl3(NO)2]22?, in which the metal atoms are associated via MCl2M bridges of slightly different lengths. One of the NO groups is in an axial position, the other one in equatorial position (symmetry C2h). 相似文献
270.
VNCl2(Pyridine)2; Synthesis, I. R. Spectrum, and Crystal Structure VNCl2(Pyridine)2 is formed from the cyclothiazeno-vanadium(IV) complex [VCl(N3S2)(Pyridine)2]2 · 2 CH2Cl2 in boiling toluene in form of brown-red, hydrolysis sensitive crystal. It was characterized by its I.R. spectrum and an X-ray crystal structure determination. VNCl2(Pyridine)2crystallizes in the orthorhombic space group Pccn with four formula units per unit cell (668 observed independent reflexions, R = 0.055). Lattice constants: a = 1550, b = 924, c = 832 pm. Monomer molecules are situated on twofold rotation axes. They are stacked along the V?N axis to form columns V?N…V. The short VN bond of 160 pm corresponds to a triple bond, whereas the V…N distance of 256 pm indicates a weak interaction. The V? N(Pyridine) and V? Cl bond lengths are 213.0 and 233.5 pm, respectively. 相似文献