全文获取类型
收费全文 | 369篇 |
免费 | 13篇 |
国内免费 | 1篇 |
专业分类
化学 | 291篇 |
晶体学 | 11篇 |
力学 | 2篇 |
数学 | 24篇 |
物理学 | 55篇 |
出版年
2023年 | 4篇 |
2022年 | 5篇 |
2021年 | 6篇 |
2020年 | 16篇 |
2019年 | 7篇 |
2018年 | 5篇 |
2016年 | 5篇 |
2015年 | 7篇 |
2014年 | 3篇 |
2013年 | 37篇 |
2012年 | 22篇 |
2011年 | 20篇 |
2010年 | 12篇 |
2009年 | 7篇 |
2008年 | 14篇 |
2007年 | 23篇 |
2006年 | 15篇 |
2005年 | 24篇 |
2004年 | 19篇 |
2003年 | 17篇 |
2002年 | 18篇 |
2001年 | 4篇 |
2000年 | 11篇 |
1999年 | 10篇 |
1998年 | 4篇 |
1997年 | 3篇 |
1996年 | 3篇 |
1995年 | 6篇 |
1994年 | 3篇 |
1993年 | 10篇 |
1989年 | 2篇 |
1988年 | 2篇 |
1986年 | 1篇 |
1985年 | 1篇 |
1984年 | 2篇 |
1983年 | 1篇 |
1982年 | 3篇 |
1981年 | 2篇 |
1980年 | 5篇 |
1977年 | 3篇 |
1976年 | 4篇 |
1975年 | 1篇 |
1973年 | 3篇 |
1969年 | 2篇 |
1968年 | 3篇 |
1967年 | 1篇 |
1966年 | 1篇 |
1962年 | 1篇 |
1954年 | 1篇 |
1927年 | 1篇 |
排序方式: 共有383条查询结果,搜索用时 15 毫秒
31.
32.
On atomically rough areas of a thermally cleaned rhenium field emitter, adsorbed gold behaves like it does on tungsten. The average work function gf increases at low average gold coverage gq due to formation of gold-rhenium dipoles, and at high coverage a structural transformation in the gold layer leads to a gq-independent work function. Broadly similar behaviour is found for gold on the low-index planes of tungsten, but on low-index rhenium planes gold behaves rather differently. When thermally cleaned at > 2200 K and annealed below 800 K, the work function, φ(clean), of (101̄1̄) takes one of two values 5.25 ± 0.04 eV, and 5.36 ± 0.04 eV, which are tentatively attributed to the two possible structures of this plane. Similar behaviour is expected and observed for (101̄0),but the values taken by φ(clean) are not well defined. Both forms of (101̄1̄) are thought to undergo reconstruction above 800 K forming a single structure with φ(clean) = 5.55 ± 0.03 eV. (112̄0) and (112̄2̄) each have only one possible structure, and in keeping with this, φ(clean) has a single well-defined value for each plane. The flatness of (101̄1̄) and (101̄0) leads to field reduction at their centres which produces an increase in their measured work functions by up to 10%. The initial increase in φ produced by gold condensed at 78 K and spread at low equilibration temperatures Ts on (112̄2̄), (101̄1̄) and (112̄0) is attributed to gold-rhenium dipoles, which, on the latter two planes approximate to the Topping model, giving dipoles characterised by μ0(1011) = 0.1 × 10?30 C-m with α = 10 Å3 and μ0(112̄0) = 0.32 × 10?30 C-m with α = 22 Å3, where μ0 is the zero-coverage dipole moment and α its polarizability. Failure of the Topping model on (112̄2̄) is attributed to its atomically rough structure. No dipole effect is seen on (101̄0). Energy spectroscopy of electrons field emitted at (202̄1̄) and (101̄1̄) demonstrates the non-free character of electrons in rhenium, while the small effect of adsorbed gold strengthens the belief that gold is bound through a greatly broadened 6s level centred 5.6 eV below the Fermi level and the dipolar nature of the bond supports this model. At higher values of Ts and gq gold appears to form states which are well-characterised by a coverage-independent work function. (101̄0), (101̄1̄) and (112̄0) each form two such states, one in the range 2 < gq < 4 (state 1), and the second at gq > 4 (state 2). The atomic radii of gold and rhenium are thought to be sufficiently similar to allow the possibility that state 1 is a replication of the Re plane structure by gold. The high work function and thermal stability of state 2, taken together with the observed temperature dependence of the transformation of state 1 to state 2, encourages the belief that state 2 results from atomic rearrangement of state 1 into a close-packed Au(111) structure. State 2 also forms on (112̄2̄) and the absence of state 1 on this plane suggests some surface alloying at coverages below 4 gq. 相似文献
33.
The second and third order non-linear optical susceptibilities of several donoracceptor substituted side chain polymers, some of which exhibit mesophases, will be reported. The susceptibilities were measured by harmonic generation from thin films, typically 0·5 μm thick, at a fundamental wavelength of 1064 nm and 1579 nm. For second harmonic generation, the spin coated films were contact poled on a glass substrate with an interdigited electrode pattern. Typical values obtained were χ(2)31 = 1 × 10-9 esu and χ(2)33 = 3 × 10-9 esu, the largest value was χ(2)33 = 6·3 × 10-9 esu. The difficulty in deriving reliable second order structure-property relations because of the large effect which the contact poling process has on the non-linear optical susceptibility of the films will be highlighted. The highest χ(3) value of 5·8 × 10-12 esu obtained from an amorphous film of a polymer with an asymmetrically substituted azo side group is comparable with the susceptibility of more typical third order non-linear materials such as the main chain polymers polyphenylacetylene and poly-p-phenylenebenzobisthiazole. 相似文献
34.
W. J. Coles 《Annali di Matematica Pura ed Applicata》1969,82(1):123-133
Summary A necessary and a sufficient condition are given for oscillation of all solutions of y″+f(t, y)=0. We sequire that a(t)α(y)≤f(t,
y) if y>0, and f(t, y)<-b(t)β(y) if y<0, together with continuity and integrability assumptions on a, b, α, and β. Of speciat
interest here is the relaxing of conditions a≥0, b≥0 in Machi - Wong [6].
Entrata in Redazione il 9 ottobre 1968.
Supported by NASA Research Grant NGR-45-003-038. 相似文献
35.
B. Niczyporuk Z. Jakubowski T. Zełudziewicz G. Folger B. Lurz H. Vogel U. Volland H. Wegener J. K. Bienlein R. Graumann H. -J. Trost M. Schmitz F. H. Heimlich R. Nernst A. Schwarz U. Strohbusch P. Zschorsch K. W. Chen A. C. König D. J. Schotanus M. Coles A. Engler R. W. Kraemer D. Marlow F. Messing C. Rippich S. Youssef A. Fridman G. Alexander A. Av-Shalom G. Bella J. Grunhaus W. Langguth M. Scheer LENA Collaboration 《Zeitschrift fur Physik C Particles and Fields》1983,17(3):197-202
A search for the decays γ→ρπ, γ→J/ψX and γ→γa (whereX is undetermined and a is an axion) has been completed using the LENA detector at the DORIS storage ring. No evidence for any of these processes was found. For these decay modes we set branching fraction upper limits (90% C.L.) of 2.1×10?3, 2.0×10?2 and 9.1×10?4, respectively. 相似文献
36.
We report new photomechanical effects in the ferroelectric liquid crystal SCE13 doped with a photoisomerizing guest azo dye. Low concentrations of dye (∼5 per cent wt:wt) are shown to cause an isothermal, reversible disruption of smectic phases when the system is illuminated with low power density (∼ 1 mW cm-2) UV light. In the case of a sample initially in the S*c phase, this results in a fall in the magnitude of spontaneous electrical polarization (Ps) and changes in electro-optic switching characteristics. If the sample is illuminated in the SA phase, the electroclinic switching decreases. In contrast to this, when systems containing higher concentrations of dye (≥ 10 per cent wt: wt) are UV illuminated in the SA phase, a reversible, isothermal transition to a biphasic S*c/isotropic state occurs. In this case, the Ps is seen to rise from zero in the SA phase to a finite value(∼2 nC cm-2) in the biphasic mixture and hysteresis occurs in the electro-optic switching. When these higher dye concentration mixtures are held initially in the S*c phase and UV illuminated, a more complicated variation of Ps occurs with the sample again undergoing a transition to a biphasic S*c/isotropic state. Possible mechanisms for the transition are discussed. 相似文献
37.
38.
Douglas McHugh Sherry SJ Hu Neta Rimmerman Ana Juknat Zvi Vogel J Michael Walker Heather B Bradshaw 《BMC neuroscience》2010,11(1):44
Background
Microglia provide continuous immune surveillance of the CNS and upon activation rapidly change phenotype to express receptors that respond to chemoattractants during CNS damage or infection. These activated microglia undergo directed migration towards affected tissue. Importantly, the molecular species of chemoattractant encountered determines if microglia respond with pro- or anti-inflammatory behaviour, yet the signaling molecules that trigger migration remain poorly understood. The endogenous cannabinoid system regulates microglial migration via CB2 receptors and an as yet unidentified GPCR termed the 'abnormal cannabidiol' (Abn-CBD) receptor. Abn-CBD is a synthetic isomer of the phytocannabinoid cannabidiol (CBD) and is inactive at CB1 or CB2 receptors, but functions as a selective agonist at this Gi/o-coupled GPCR. N-arachidonoyl glycine (NAGly) is an endogenous metabolite of the endocannabinoid anandamide and acts as an efficacious agonist at GPR18. Here, we investigate the relationship between NAGly, Abn-CBD, the unidentified 'Abn-CBD' receptor, GPR18, and BV-2 microglial migration. 相似文献39.
Bo Li Patrick Coles Jeffrey A. Reimer Philip Dawson Carlos A. Meriles 《Solid State Communications》2010,150(9-10):450-453
We report the use of time-resolved Faraday rotation to induce and probe the polarization of nuclear spins within a set of quantum wells with varying background electron density. The electron density was controlled over a broad range by making use of structures of mixed type-I/type-II GaAs/AlAs quantum wells that spatially separate photoexcited electron–hole pairs. We find that the optically detected nuclear magnetic field decreases quasi-monotonically with increasing electron density. The likely factors responsible for this behavior are increased electron spin-lattice relaxation, increased electron spin delocalization, and dilution of the electron spin polarization. 相似文献
40.
The pretransitional behaviour of a homologous series of mesomorphic compounds, the alkyl cyanobiphenyls (CNρρC n H 2n + 1) for n = 5–12, has been studied as a function of temperature by light scattering in the isotropic phase. The higher homologues, n = 8–12, exhibit a smectic A phase that becomes increasingly important at the expense of the nematic phase for increasing n and for n = 10 and 12 no nematic phase is observed. We have shown from light scattering and differential scanning calorimetry measurements that the presence of the smectic A phase changes markedly the pretransitional behaviour, and gives rise to two distinct pretransitional regions. The first region is for temperatures greater than ~3°C above the clearing temperature Tc where the systems exhibit a typical pretransitional behaviour adequately described by the phenomenological Landau-de Gennes model of a second order phase transition with an intervering first order transition at Tc . The constants of this model for this region are given and show a distinct odd-even effect. The second region is close to Tc where a strong divergence from the already critical behaviour is observed. This secondary divergence is observed to be a function of the alkyl chain length and its conformation, and is attributed to coupling between smectic A like layers and the orientational order parameter in the isotropic phase. 相似文献