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991.
The difference in the theoretical structure between monatomic and polyatomic gases in highly nonequilibrium states is discussed from the viewpoint of molecular extended thermodynamics (MET) of rarefied gases, which is free from the local equilibrium assumption. The MET theories of these two types of gases are based on the moment balance equations with different hierarchy structures due to whether the internal degrees of freedom of a molecule are incorporated in their distribution functions or not. In particular, the number of balance equations in the MET theory of polyatomic gases is greater than the number in the corresponding theory of monatomic gases. The closure procedure for the system of balance equations of polyatomic gases obtained in a recent paper (Arima et al., 2014) is adopted. We prove that the solutions for polyatomic gases converge, in the limit where the degrees of freedom of a molecule D tend to 3, to the ones for monatomic gases provided that we impose appropriate initial conditions compatible with monatomic gases. Thus a MET theory of rarefied monatomic gases can be identified as a singular limit of the corresponding MET theory of rarefied polyatomic gases. As illustrative examples, the asymptotic behaviors when D→3 in the dispersion relation of ultrasonic waves and in the shock wave structure are shown. 相似文献
992.
Yizhak Marcus 《Journal of solution chemistry》2008,37(8):1071-1098
Property data for tetraalkylammonium cations, [H(CH2)
n
]4N+, are reviewed. They pertain to the isolated cations and their transfer from the gas phase into aqueous solutions. Various
properties of these cations in aqueous and non-aqueous solutions and data for their transfer between these are also reviewed.
Emphasis is placed on the dependence of data on the length n of the alkyl chains rather than on the absolute values. Most of the data are available only for the first four members of
the series.
The properties of the isolated ions increase linearly with the chain length. Molar enthalpies of formation of the gaseous
and aqueous cations, and absolute standard molar enthalpies of hydration, are derived. Standard molar entropies of dissolution
of several salts in water are obtained from their solubilities and enthalpies of solution. The molar entropies of the crystalline
iodides of the first four members of the series then give the standard partial molar entropies of the aqueous cations and
their molar entropies of hydration.
The standard partial molar volumes in aqueous and non-aqueous solutions are quite linear with n and in non-aqueous solutions the molar volume hardly depends on the nature of the solvent. On transfer from water to non-aqueous
solvents the volume of Me4N+ suffers some shrinkage, that of Et4N+ appears to be unaffected, but from Pr4N+ onwards an increasing expansion takes place. This unexpected result is tentatively explained by hydrophobic intra-molecular
association of pairs of alkyl chains in aqueous solutions, resulting in a tightening of the structure. The transfer of the
R4N+ cations from water into non-aqueous solvents is governed by a large positive entropy change, outweighing the smaller positive
enthalpy change.
The transport properties of the aqueous R4N+ cations are non-linear with n. A major impediment to movement is thus the sticking of the water molecules to the ice-like hydrophobic hydration sheaths
of the larger cations. The number of water molecules affected by the hydrophobic cations is open to widely differing estimates
resulting from various approaches, and constitute an open issue. 相似文献
993.
Yady J. Manrique Diana P. Pacheco Fleming Martínez 《Journal of solution chemistry》2008,37(2):165-181
By using the van’t Hoff and Gibbs equations, solubility data of ibuprofen (IBP) and naproxen (NAP) determined at several temperatures
in propylene glycol (PG) + water (W) cosolvent mixtures were used to evaluate the Gibbs energies, enthalpies, and entropies
of solution, mixing and solvation. The solubilities are greater in pure PG and lower in W in both cases at all studied temperatures.
These results clearly show that a cosolvent effect is present in these systems. The solvation of these drugs was greater in
W compared to PG, and it was lower in the cosolvent mixtures compared with the pure solvents. By means of an enthalpy-entropy
compensation analysis, non-linear ΔH
solno versus ΔG
solno compensation plots were obtained, with negative and positive slopes if all of the composition intervals are considered. Accordingly,
it follows that at low PG solvent fractions the dominant factor for the solubility of IBP and NAP in this cosolvent system
is the entropy, probably due to loosening of the water structure caused by the addition of the cosolvent. 相似文献
994.
The sorption of inorganic lead(II) on two cationic resins containing different complexing groups, the iminodiacetic Chelex
100 and the carboxylic Amberlite CG-50, was investigated. The Gibbs-Donnan model was used to describe and predict the sorption
through the determination of the intrinsic complexation constants. These quantities, even though non-thermodynamic, characterize
the sorption as being independent of experimental conditions. The sorption mechanism for metals on complexing resins was also
studied by adding a competitive soluble ligand that shifts the sorption curves to higher pH. The ligand competes with the
resin for complexation with the metal ion. Lead(II) is strongly sorbed on Chelex 100 through the formation of two complexes
in the resin phase: MHL with log 10
β
111i
=−0.3 and ML with log 10
β
111i
=−3.7. The presence of the competitive ligand in solution allows for the determination of a third complex. Furthermore, on
Amberlite CG-50 the sorption is rather strong and involves the formation of the complex ML, in more acidic solution, with
log 10
β
110i
=−2.0. In the presence of the ligand PyDA, the ML(OH) complex was characterized by log 10
β
11−1i
=−5.6. In all the experiments the hydrolysis reactions in the aqueous phase are considered quantitatively. 相似文献
995.
3,6-双(1H-1,2,3,4-四唑-5-氨基)-1,2,4,5-四嗪的合成、表征及量子化学研究 总被引:3,自引:0,他引:3
以三氨基胍硝酸盐、戊二酮为起始原料, 经缩合、氧化、取代等反应合成了3,6-双(1H-1,2,3,4-四唑-5-氨基)-1,2,4,5-四嗪(BTATz), 并通过元素分析、红外、核磁、差示扫描量热法(DSC)等分析手段对其进行了表征. 采用亚硝酸钠/乙酸代替了二氧化氮/N-甲基吡咯烷酮, 改进了氧化步骤, 降低了成本, 简化了合成工艺. 用B3LYP方法, 在6-31G(d,p)基组水平上对其性能进行了计算, 得到了其稳定的几何构型和键级; 在振动分析的基础上求得体系的振动频率、IR谱及不同温度下的热力学性质, 并得到了温度对热力学性能影响的关系式; 探讨了其热解机理, 推断出四唑环开环时的过渡态和活化能. 相似文献
996.
Desulphurization mechanism of polyethylene glycol (PEG) membranes has been investigated by the study of solubility and diffusion behavior of typical gasoline components through PEG membranes with various crosslinking degrees. The sorption, diffusion and permeation coefficients were calculated by the systematic studies of dynamic sorption curves of gasoline components such as thiophene, n-heptane, cyclohexane, cyclohexene and toluene in PEG membranes. Furthermore, the temperature dependence of diffusion and solubility coefficients and the influence of crosslinking degree on sorption and diffusion behaviors were conducted to elucidate the mass-transfer mechanism. According to the discussions on dynamic sorption curves, transport mode, activation energy and thermodynamic parameters, thiophene species were the preferential permeation components. Crosslinking is an effective modification way to improve the overall performance of PEG membranes applied in gasoline desulphurization. The pervaporation (PV) and gas chromatography (GC) experiments results corresponded to the conclusions. All these investigations will provide helpful suggestions for the newly emerged membrane desulphurization technology and complex organic mixture separation by pervaporation. 相似文献
997.
Complexation equilibria and kinetics of exchange of chloroform and dichloromethane molecules between the cavity of cryptophane-E and bulk solution were investigated using NMR methods. Using one-dimensional magnetization transfer (1D-EXSY type sequence), chemical exchange rates were measured in different temperature ranges, limited by the equilibrium constant values of the complexes and the boiling points of the guest substances. From the kinetic data, activation energies were calculated using the Arrhenius equation. From the temperature-dependence of the association constant data, the enthalpy and entropy of complexation were estimated and compared with values for similar complexes of other cryptophanes. 相似文献
998.
P. Aghamkar S. Duhan M. Singh N. Kishore P. K. Sen 《Journal of Sol-Gel Science and Technology》2008,46(1):17-22
A novel sorbent for the removal of heavy metals, hydroxyapatite/poly (vinyl alcohol) cryogel, was studied. The HAp/PVA cryogel
was characteristic of macroporous structure. The experiments for Cd2+ sorption by HAp/PVA cryogel were conducted at various operating conditions such as sonication, sorption time, Cd2+ concentration, temperature, pH, and HAp/PVA mass ratio. It was concluded that the sorption was considerably affected by sorption
time, Cd2+concentration, temperature and HAp/PVA mass ratio. Nevertheless, there was slight dependence of sorption on sonication and
pH. The influences of different parameters were discussed in detail in the paper. The main mechanism was suggested to be ion-exchange. 相似文献
999.
Macropolyhedral borane clusters are concave polyhedra constituting fused convex simple polyhedra. They are formally obtained by condensation of simple polyhedral boranes under elimination of between one and four BH(3) or isoelectronic units. The number of eliminated vertexes from simple polyhedra equals the number of shared vertexes in macropolyhedral boranes. For each of the eight classes with general formulae ranging from B(n)H(n-4) to B(n)H(n+10), more than one structure type is possible, differing in the number of shared vertexes and in the types of the two combined cluster fragments. However, only one type of "potential structures" is represented by experimentally known examples and is found to be favored by theoretical calculations. A sophisticated system exists among the favored macropolyhedral borane structures. For each class of macropolyhedral boranes, the number of skeletal electron pairs is directly related to the general formula, the number of shared vertexes and the type of fused cluster fragments. In order to predict the distribution of vertexes among the fused fragments, we propose the concept of preferred fragments. Preferred fragments are those usually present in the thermodynamically most stable structure of a given class of macropolyhedral boranes and are also frequently observed in the experimentally known structures. This allows us to completely predict the cluster framework of the thermodynamically most stable macropolyhedral borane isomers. 相似文献
1000.