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871.
H. Reuther O. Nikolov S. Kruijer R. A. Brand W. Keune D. Liljequist S. Weber S. Scherrer 《Hyperfine Interactions》1994,92(1):1367-1372
-Fe surfaces were implanted with a nominal dose of 5×1017 Al ions/cm2 at 50 keV and a current density of about 3.7 A/cm2. Samples of different shapes and thicknesses have been used in order to test the influence of heat flow from specimen to target holder during implantation. Integral and energy differential (depth-selective)57Fe conversion electron Mössbauer spectroscopy (CEMS and DCEMS) were employed. The spectra indicated a magnetic phase characterised by a broad hyperfine field distributionP(B
hf), a non-magnetic phase, and -Fe. The relative intensity of the non-magnetic phase was enhanced if the thermal contact during implantation became worse. An energy dependence of DCEM spectra in the L-electron range was observed. Model calculations using L-electron weight functions and experimental concentration profiles obtained by secondary neutral mass spectroscopy (SNMS) yielded fair agreement between calculated and experimental phase signals. The results demonstrate that the non-magnetic Fe-Al alloy phase with high Al concentration is located closer to the surface than the magnetic alloy phase, which extends to much larger depth than expected. 相似文献
872.
Detailed studies have been made of elastic scattering and positronium formation in low energy collisions of positrons with lithium atoms for the two partial wavesl=0,1. For this system, as for all alkali atoms, the positronium formation channel is open even at zero positron energy. A two-channel version of the Kohn variational method is used with trial functions containing many variational parameters, and reasonably well converged results are obtained. The s-wave positronium formation cross section is infinite at zero positron energy but it then falls rapidly to become several orders of magnitude smaller than the elastic scattering cross section which has a maximum value of approximately 100
0
2
at a positron energy of 0.5 eV. For p-wave scattering the positronium formation cross section rises to a value of approximately 10
0
2
at an energy of 0.1 eV, with the elastic scattering cross section rising to a maximum of approximately 60
0
2
just below the first excitation threshold at 1.84 eV. 相似文献
873.
Feng Ji Paul W. Percival Jean-Claude Brodovitch Brenda Addison-Jones Stanislaw Wlodek 《Hyperfine Interactions》1994,87(1):853-858
Muonium adds to allyl chloride, CH2=CHCH2Cl, to form two radicals: MuCH2CHCH2Cl (main product) and CH2CHMuCH2Cl (minor product). Both radicals were fully characterized bySR andLCR. In the main product, the LCR lines due to the35Cl and37Cl nuclei were observed. Also, the temperature dependence of various hyperfine coupling constants (hfc) indicates that both Mu and Cl eclipse the unpaired electronp
2-orbital in the minimum energy conformation. For the fragment-CH2Cl, the presence of Mu in the-position is found to affect significantly the hfc of Cl in the-position; an internal rotational barrier of 12 kJ mol–1 was estimated using a simpleV
2 torsional potential. 相似文献
874.
Martina Schwager Emil Roduner Ivan D. Reid Paul W. Percival Jean-Claude Brodovitch Stanislaw Wlodek Robert F. Marzke 《Hyperfine Interactions》1994,87(1):859-864
SR spectra of the ethyl radical adsorbed on porous silica were observed in transverse and in longitudinal magnetic fields in the temperature range 190–298 K. The line widths reflect the dynamic partial averaging of the hyperfine anisotropy due to reorientation and surface diffusion. 相似文献
875.
B. Hitti S. R. Kreitzman T. L. Estle R. L. Lichti K. H. Chow J. W. Schneider C. D. Lamp P. Mendels 《Hyperfine Interactions》1994,86(1):673-679
The radio frequencySR technique developed at TRIUMF was used to measure the temperature dependence of the diamagnetic muon, Mu, and Mu* amplitudes in silicon between 10 K and 500 K. Six samples doped with phosphorus (n-type) and boron (p-type) in the concentration range 1011 to 1015 cm–3 were studied. In pure Si a very good fit over the whole temperature range is obtained from a model that includes the ionization of Mu* and Mu to a bond centered
+ followed at high temperature by charge exchange involving Mu. 相似文献
876.
Richard Ley D. Hagena D. Weil G. Werth W. Arnold H. Schneider 《Hyperfine Interactions》1994,89(1):327-341
Electric dipole transitions in the microwave range have been induced between the fine-structure levels of positronium in the excited staten=2. As an indication of the transitions, we used the increase in Lyman- radiation when the metastable 23S1-level is depopulated. The results for the transitions 23S123P0,1,2 are
0=18499.65±1.20±4.00 MHz,
1=13012.42 ±0.65±1.54 MHz and
2=8624.38±0.54±1.40 MHz. The first error is statistical and the second systematic. The precision of the present measurement has improved by a factor of 3, compared to previous data. Recent bound state QED-calculations have been extended to the orderR
t8
4ln
–1. The not yet completely calculated orderR
t8
4 is estimated to contribute less than 1 MHz. Our experimental results are in good agreement with theory. By applying a weak magnetic field, we were able to observe the transition 23S121P1 which is strictly forbidden byC-invariance in zero field. Our result, corrected for Zeeman- and motional Starkeffect, is
3=11180.0±5.0±4.0 MHz. An upper limit for theC-violating matrix element of
MHz could be deduced. Our experiment used moderated slow positrons from the bremsstrahlung and pair production of a pulsed electron linear accelerator (TEPOS facility at the university of Giessen). 相似文献
877.
O. Hartmann E. Karlsson R. Wäppling L. Asch S. Henneberger G. M. Kalvius A. Kratzer H. -H. Klauß F. J. Litterst M. A. C. De Melo 《Hyperfine Interactions》1994,85(1):251-258
We have studied the muon precession frequency in a ferromagnetic single crystal of Gd metal. The overall features of our findings are compatible with earlier results on polycrystalline material. In the temperature region between 245 and 220 K where the Gd magnetization starts to turn away from the c-axis, we observe an increase in the muon depolarization rate, and a complex precession signal which can be separated into two frequency components meaning that spin turning does not occur simultaneously in different parts of the sample (domains). From these more detailed data follows that previously obtained values forB
fc andB
dip can not both be correct. Two explanations for our new result are possible: EitherB
fc undergoes a change around 230 K which is directly coupled to the spin turning angle, or the value of the dipolar field contribution used in the earlier evaluation is too low. This imposes some uncertainty as to the value of the angle at the onset of spin turning derived fromSR frequencies. 相似文献
878.
R. Kadono J. H. Brewer K. Chow S. R. Kreitzman Ch. Niedermayer T. M. Riseman J. W. Schneider T. Yamazaki 《Hyperfine Interactions》1994,85(1):259-264
The electric quadrupole interaction of55Mn nuclei was studied in the weakly ferromagnetic system MnSi using muon level-crossing resonance (LCR) technique. The temperature dependence of the electric field gradient (EFG) shows a critical behavior near the ferromagnetic transition temperature, indicating that the EFG due to the conduction electron is strongly correlated with the magnetic susceptibility in the itinerant electron magnetism. The temperature dependence of EFG is in reasonable agreement with the self-consistent renormalization theory developed by Moriya and coworkers.We gratefully acknowledge helpful discussion with Dr. N. Nishida. We also wish to thank Keith Hoyle and Curtis Ballard for technical support. 相似文献
879.
W. M. Reiff B. D. James M. Bakalova J. Liesegang A. H. White B. W. Skelton D. C. R. Hockless 《Hyperfine Interactions》1994,94(1):2081-2086
Iron-57 Mössbauer spectroscopic results for a new series of related chloroferrate salts of complex ammonium cations are presented. In particular, spectra of materials based on FeCl
4
–
, FeCl
5
2–
, FeCl
6
–
, [FeCl5(H2O)]2–, and [FeCl5(CH3OH)]2– anions are discussed relative to the systematics of their isomer shifts, coordination numbers, and low temperature 3D critical ordering temperatures. The following specific systems have been studied by Mössbauer spectroscopy and definitively characterized by single-crystal X-ray analysis: [CH3NH
3
+
]4[FeCl6]3–[Cl–](A), [Me2N(-CH2CH2)2NMe
2
+
][FeCl
5
2–
] (B), [H3NCH2CH2NH
3
2+
][FeCl5·H2O2–], (C), and [NH3(CH2)6NH
3
2+
]4[(FeCl
6
3–
)(FeCl
4
–
)ClCl
4
–
] (D). The spectral data for these complexes are considered in the light of results of previous studies for systems such as [K2FeCl5·H2O], [Co(NH3)6][FeCl6], diamethyltriethylenediammoniumpentachloroferrate (E), and the related [FeCl5·CH3OH]2– salt (F). The critical 3D ordering temperatures are 1.45, 6.80, 3.45, and 1.22K forA, B, C, andD, respectively. 相似文献
880.
C. Böttger S. J. Campbell W. Stieler F. L. Litterst J. Hesse 《Hyperfine Interactions》1994,94(1):2279-2286
A detailed analysis of room temperature57Fe Mössbauer spectra ofAuFe (5 at.% Fe) andCrFe (5, 10 at.% Fe) samples is presented. The deviation from Lorentzian line shape observed in the spectra indicates both thickness effects and structural disorder in these systems. The thickness effect has been taken into account using a new fitting technique that solves the transmission integral numerically rather than relying on the thin absorber approximation. The present results and analyses using both the thin absorber approximation and the full transmission integral methods on these alloy systems are compared with earlier results obtained on these alloy systems using only the thin absorber approximation. 相似文献