全文获取类型
收费全文 | 36100篇 |
免费 | 4665篇 |
国内免费 | 4251篇 |
专业分类
化学 | 14122篇 |
晶体学 | 243篇 |
力学 | 2711篇 |
综合类 | 243篇 |
数学 | 9409篇 |
物理学 | 18288篇 |
出版年
2024年 | 92篇 |
2023年 | 345篇 |
2022年 | 789篇 |
2021年 | 837篇 |
2020年 | 1024篇 |
2019年 | 969篇 |
2018年 | 964篇 |
2017年 | 1175篇 |
2016年 | 1381篇 |
2015年 | 1227篇 |
2014年 | 1766篇 |
2013年 | 3013篇 |
2012年 | 1918篇 |
2011年 | 2356篇 |
2010年 | 1836篇 |
2009年 | 2351篇 |
2008年 | 2408篇 |
2007年 | 2538篇 |
2006年 | 2154篇 |
2005年 | 1810篇 |
2004年 | 1543篇 |
2003年 | 1478篇 |
2002年 | 1425篇 |
2001年 | 1135篇 |
2000年 | 1161篇 |
1999年 | 952篇 |
1998年 | 852篇 |
1997年 | 617篇 |
1996年 | 518篇 |
1995年 | 463篇 |
1994年 | 375篇 |
1993年 | 355篇 |
1992年 | 347篇 |
1991年 | 262篇 |
1990年 | 262篇 |
1989年 | 238篇 |
1988年 | 229篇 |
1987年 | 208篇 |
1986年 | 178篇 |
1985年 | 196篇 |
1984年 | 177篇 |
1983年 | 101篇 |
1982年 | 139篇 |
1981年 | 136篇 |
1980年 | 103篇 |
1979年 | 108篇 |
1978年 | 89篇 |
1977年 | 88篇 |
1976年 | 83篇 |
1973年 | 70篇 |
排序方式: 共有10000条查询结果,搜索用时 15 毫秒
991.
We investigated the aggregation behavior of rod-like micelles of sodium dodecyl sulfate (SDS) in concentrated NaCl solution by quasi-elastic light scattering (QLS) and viscosity measurement over a range of temperature (25 °C to 50 °C) and NaCl concentration. The reduced viscosity of aqueous SDS in the presence of NaCl has been measured by an Ubbelohde-type capillary viscometer. We show mean hydrodynamic radius of micelles can be determined from viscosity data. We also determined mean hydrodynamic radius using quasi-elastic light scattering. Micellar size decreases with increasing temperature, whereas it increases with increasing ionic strength. The results of viscosity and dynamic light-scattering measurements are interpreted as the extension of length of rod-like micelles. We compare viscosity and light scattering experimental results. 相似文献
992.
Teobald Kupka Roman Wrzalik Gra
yna Pasterna Karol Pasterny 《Journal of Molecular Structure》2002,616(1-3):17-32
The results of extended MO calculations using density functional theory (DFT) approximation supported by experimental Raman, 1H and 13C NMR studies on thiophene are reported. Raman spectra of liquid thiophene were re-examined and the performance of a hybrid B3PW91 density functional was compared with the ab initio restricted Hartree–Fock (RHF) method. With the basis sets of the 6-311++G** quality, the DFT calculated bond lengths, dipole moments and harmonic vibrations were predicted in a very good agreement with available experimental data.
Additionally, the results on thiophene were extended by calculations on 3-methylthiophene and selenophene. In this case, a significant change in geometry and charge distribution in thiophene ring due to a methyl group substituent or replacement of sulphur by selene atom was observed.
A linear correlation between the predicted harmonic vibrational frequencies (scaled using SQM method) and experimental ones for thiophene, selenophene and 3-methylthiophene was shown. The theoretically calculated spectra have satisfactorily reproduced the available experimental spectra for thiophene and selenophene. 相似文献
993.
In pH 6.0-11.2 Britton-Robinson buffer solution, binding of heparin with crystal violet (CV) can result in a significant enhancement of resonance Rayleigh scattering (RRS) and resonance non-linear scattering, such as frequency doubling scattering (FDS) and second-order scattering (SOS). Their maximum scattering wavelengths, λex/λem, appear at 492 nm/492 nm for RRS, 984 nm/492 nm for FDS and 492 nm/984 nm for SOS, respectively. The optimum conditions of the reaction, the influencing factors and the relationship between the three scattering intensities and the concentration of heparin have been investigated. New methods for the determination of trace amounts of heparin based on the RRS, FDS and SOS methods have been developed. The methods exhibit high sensitivities, the detection limit for heparin is 2.9 ng ml−1 for the RRS method, 3.5 ng ml−1 for the FDS method and 3.3 ng ml−1 for the SOS method. The methods have good selectivity and were applied to the determination of heparin in heparin sodium injection samples with satisfactory results. 相似文献
994.
Summary The electronically excited states of formaldehyde are examined by means of multiconfigurational second-order perturbation (CASPT2) theory with extended ANO-type basis sets. The calculations comprised five valence excited states plus all singlet 3s, 3p, and 3d members of the Rydberg series converging on the first ionization. The computed vertical excitation energies were found to be within 0.2 eV of the available experimental energies. Full geometry optimization has been performed for five valence excited states. Assuming a planar geometry, the 0-0 transition for the valence1A1( *) state is calculated to appear near 7.9 eV, close to the (n
y
3p) region. This state is, however, not planar and the true adiabatic energy is 7.4 eV, which is 2.3 eV below the corresponding vertical transition. 相似文献
995.
A quantitative analysis method for penicillins including ampicillin (AmP), benzyl penicillin (BP), oxacillin (OA) and amoxycillin (AmO) is proposed that makes use of the totally internally reflected resonance light scattering (TIR-RLS) signal from the penicillin at the H2O/CCl4 interface in the presence of cetyltrimethylammonium bromide (CTMAB), and enables the pharmacokinetics of penicillin taken orally and excreted through urine to be monitored. Penicillin is coadsorbed with CTMAB at the H2O/CCl4 interface in neutral solution, resulting in the formation of ion associates that display greatly enhanced TIR-RLS signals (maximum at 368–372 nm). This enhanced TIR-RLS intensity was found to be proportional to the penicillin concentration over the range 0.2×10–6 to 2.2×10–6 mol L–1, with limits of determination (3) of 5.0×10–8 to 7.0×10–8 mol L–1. Pharmacokinetics studies performed using the present method show that the excretion of orally-taken ampicillin through urine has a half-time of 1.05 h and an excremental quantum over 8 h of 49.3%, respectively. 相似文献
996.
Eugene P. Gross 《Journal of statistical physics》1981,25(4):585-604
We present a microscopic theory of the problem of finding the properties of a particle interacting with potentials located at random sites. The sites are governed by a general probability distribution. The starting point is the multiple scattering equations for the amplitude k
1|T
|k
2 in terms of the individual scattering amplitudes k
1|T
|k
2. We work with quantitiesA
defined by k
1|T
|k
2=k
1|T
|k
2exp[i(k
1–k
2)R
]. The theory is based on a splitting of the fundamental equation forA into equations for the mean A and the fluctuationsAA
. Neglect of the fluctuations yields the quasicrystalline approximation. We rearrange the equation forAA
to isolate the collective part of the fluctuations. We then make the simplest microscopic truncation which is thatAA
is a restricted two-body additive function of the site positions. With the contribution of the collective fluctuations, this yields results forA that are accurate to ordert
4.Work supported in part by the National Science Foundation under Contract No. NSF DMRWork supported in part by the National Science Foundation under Contract No. NSF DMR 相似文献
997.
C. Cammarota 《Journal of statistical physics》1981,25(1):143-152
A field theory model onR
2 in which the basic fields are Ising spins instead of Gaussian spins is examined. Using statistical mechanics techniques we discuss the ultraviolet and the infrared problems. In particular we discuss a technique yielding the asymptotic expansion in of the ground state energy, as 0, without using the cluster expansion.Supported in part by Consiglio Nazionale delle Ricerche. 相似文献
998.
This paper is mainly concerned with a theoretical model of a -size Schottky-barrier junction which could explain the responsivity roll-off and the cut-off frequencies experimentally observed in different types of detector mounts. The model assumed can also explain the observed frequency dependence of the bias current peaking the responsivity.Experimental data of resistive harmonic generation from 100 up to 600 GHz with Schottky multipliers in a wideband structure are also reported.Work supported by the Consiglio Nazionale delle Ricerche of Italy 相似文献
999.
M. Nierlich L.L.B. F. Boue L.L.B. A. Lapp L.L.B. R. Oberthür I.L.L. 《Colloid and polymer science》1985,263(12):955-964
We have studied salt free semi dilute polyelectrolyte solutions by small angle neutron scattering. Specific labelling associated with an extrapolation method has allowed the separation of the form factor of a single polyelectrolyte chainS
1(q) and the structure factorS
2(q). Two lengths are deduced from these two factors: the persistence lengthb
t which characterizes the electrostatic interactions along the chain by a fitting ofS
1(q) with calculation of the scattering function for a wormlike chain, and fromS
2(q),q
m
–1
which characterizes the interactions between chains. These two lengths vary in the same way with the concentration of polyions (b
t C
p
–1/2
,q
m
–1
C
p
–1/2
) and a constant relation exists between them: only one length is then necessary to describe the structure of polyelectrolyte soltuion on this semidilute concentration range.Laboratoire Commun CEA-CNRS. 相似文献
1000.
Parameters are developed for a practical application of the empirical van der Waals (vdW) correction infrastructure available in the CPMD density functional theory (DFT) code. The binding energy, geometry, and potential energy surface (PES) are examined for methane, ethane, ethylene, formaldehyde, ammonia, three benzene dimer geometries, and three benzene–water geometries. The vdW corrected results compare favorably with MP2 and CCSD(T) calculations near the complete basis set limits, and with experimental results where they are available. 相似文献