全文获取类型
收费全文 | 242682篇 |
免费 | 2412篇 |
国内免费 | 869篇 |
专业分类
化学 | 130194篇 |
晶体学 | 3688篇 |
力学 | 9889篇 |
综合类 | 2篇 |
数学 | 27045篇 |
物理学 | 75145篇 |
出版年
2020年 | 1764篇 |
2019年 | 2037篇 |
2018年 | 2469篇 |
2017年 | 2400篇 |
2016年 | 3815篇 |
2015年 | 2590篇 |
2014年 | 3889篇 |
2013年 | 11144篇 |
2012年 | 8067篇 |
2011年 | 9993篇 |
2010年 | 6563篇 |
2009年 | 6459篇 |
2008年 | 8971篇 |
2007年 | 9124篇 |
2006年 | 8367篇 |
2005年 | 7831篇 |
2004年 | 6979篇 |
2003年 | 6201篇 |
2002年 | 6129篇 |
2001年 | 7197篇 |
2000年 | 5359篇 |
1999年 | 4215篇 |
1998年 | 3547篇 |
1997年 | 3575篇 |
1996年 | 3287篇 |
1995年 | 3130篇 |
1994年 | 2944篇 |
1993年 | 2999篇 |
1992年 | 3292篇 |
1991年 | 3327篇 |
1990年 | 3143篇 |
1989年 | 3098篇 |
1988年 | 3162篇 |
1987年 | 3029篇 |
1986年 | 2923篇 |
1985年 | 3985篇 |
1984年 | 4147篇 |
1983年 | 3399篇 |
1982年 | 3749篇 |
1981年 | 3645篇 |
1980年 | 3518篇 |
1979年 | 3552篇 |
1978年 | 3777篇 |
1977年 | 3606篇 |
1976年 | 3816篇 |
1975年 | 3387篇 |
1974年 | 3516篇 |
1973年 | 3819篇 |
1972年 | 2327篇 |
1971年 | 1786篇 |
排序方式: 共有10000条查询结果,搜索用时 15 毫秒
161.
Daria E. Lonsdale Geoffrey Johnston‐Hall Amanda Fawcett Craig A. Bell Carl N. Urbani Michael R. Whittaker Michael J. Monteiro 《Journal of polymer science. Part A, Polymer chemistry》2007,45(16):3620-3625
In this work, we propose that retardation in vinyl acetate polymerization rate in the presence of toluene is due to degradative chain transfer. The transfer constant to toluene (Ctrs) determined using the Mayo method is equal to 3.8 × 10?3, which is remarkably similar to the value calculated from the rate data, assuming degradative chain transfer (2.7 × 10?3). Simulations, including chain‐length‐dependent termination, were carried out to compare our degradative chain transfer model with experimental results. The conversion–time profiles showed excellent agreement between experiment and simulation. Good agreement was found for the Mn data as a function of conversion. The experimental and simulation data strongly support the postulate that degradative chain transfer is the dominant kinetic mechanism. © 2007 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 45: 3620–3625, 2007 相似文献
162.
D. Ohlberg J.J. Blackstock R. Ragan S. Kim R. Stanley Williams 《Applied Physics A: Materials Science & Processing》2005,80(6):1327-1334
A recently demonstrated [1] in-vacuo template-stripping process is applied to the study of platinum films stripped from ultra-flat silicon-oxide surfaces. Template-stripped (TS) Pt surfaces, prepared with a range of post-deposition annealing times prior to being stripped from the templating surface in an ultra-high vacuum (UHV) environment, are examined by UHV scanning tunneling microscopy (STM). These studies reveal that without post-deposition annealing, TS Pt surfaces are largely made up of poorly-ordered, granular nanostructures undesirable for many applications. The post-deposition annealing treatments explored in the study result in the emergence and continuous growth of large smooth crystallites. Issues with crystallite orientation relative to the TS surface and artefacts arising as a result of the epoxy used in the template-stripping process are presented and discussed in relation to optimizing the template-stripping procedure for specific applications such as self-assembled monolayer (SAM) formation for molecular electronics. PACS 68.37.Ef; 68.47.De; 68.55.Jk; 81.05.Bx; 81.15.Ef 相似文献
163.
164.
S.N. Abdulmadjid M.M. Suliyanti K.H. Kurniawan T.J. Lie M. Pardede R. Hedwig K. Kagawa M.O. Tjia 《Applied physics. B, Lasers and optics》2006,82(1):161-166
We report in this paper the results of an experimental study on hydrogen analysis of solid samples in high pressure helium
ambient gas employing the basic scheme of laser induced breakdown spectroscopy (LIBS). It is shown that the metastable excited
state of helium atom can be utilized to induce delayed excitation of the ablated hydrogen atoms, and thereby avoid the Stark
broadening effect as well as overcoming the undesirable mismatch effect, which are responsible for inefficient excitation
respectively. It is further demonstrated that for samples of high boiling-point materials such as zircaloy, successful hydrogen
analysis can be achieved by a newly introduced double excitation technique employing single laser realized in a modified configuration
of the conventional LIBS method.
PACS 51-52 相似文献
165.
The magnetocrystalline anisotropy of thin magnetic wires of iron and cobalt is quite different from the bulk phases. The spin moment of monatomic Fe wire may be as high as 3.4 μB, while the orbital moment as high as 0.5 μB. The magnetocrystalline anisotropy energy (MAE) was calculated for wires up to 0.6 nm in diameter starting from monatomic wire and adding consecutive shells for thicker wires. I observe that Fe wires exhibit the change sign with the stress applied along the wire. It means that easy axis may change from the direction along the wire to perpendicular to the wire. We find that ballistic conductance of the wire depends on the direction of the applied magnetic field, i.e. shows anisotropic ballistic magnetoresistance. This effect occurs due to the symmetry dependence of the splitting of degenerate bands in the applied field which changes the number of bands crossing the Fermi level. We find that the ballistic conductance changes with applied stress. Even for thicker wires the ballistic conductance changes by factor 2 on moderate tensile stain in our 5×4 model wire. Thus, the ballistic conductance of magnetic wires changes in the applied field due to the magnetostriction. This effect can be observed as large anisotropic BMR in the experiment. 相似文献
166.
We consider queuing systems where customers are not allowed to queue, instead of that they make repeated attempts, or retrials, in order to enter service after some time. We obtain the distribution of the number of retrials produced by a tagged customer, until he finds an available server. 相似文献
167.
In cricket, a rain-affected pitch can make batting more difficult than normal. Several other conditions such as poor light or an initially lively pitch, may also result in difficulties for the batsmen. In this note, we refer to all of them as ‘sticky wickets’. On sticky wickets, lower order batsmen are often sent into ‘hold the fort’ until conditions improve. In this paper, a stochastic dynamic programming model is used to examine the appropriateness of this policy. The model suggests that the tactic is often optimal when the sticky wicket persists until the end of the day's play, but not often when the sticky wicket is transitory. In some circumstances, it is worthwhile, on a normal wicket near the end of the day, to send in a lower order batsman to hold the fort (a night watchman): when the wicket is sticky, this tactic is even more worthwhile. 相似文献
168.
169.
B. Savo P. Romano A. Nigro R. Di Leo 《Physica C: Superconductivity and its Applications》2002,370(4):275-280
The flat voltage noise component of current biased, high-transparency Nb/AlOx/Nb superconducting tunnel junctions has been investigated at frequencies up to 70 kHz. Several aspects of the analyzed phenomena suggest the presence of current noise effects induced by the discreteness of the charge carriers. At subgap voltages, where excess currents occur, a behavior coherent with a multiple Andreev reflection-assisted transport through the tunnel barrier has been found. However, the measured charge values exceed any theoretical prediction. 相似文献
170.
Potential for micromachined actuation of ultra-wide continuously tunable optoelectronic devices 总被引:1,自引:0,他引:1
H. Hillmer J. Daleiden C. Prott F. Römer S. Irmer V. Rangelov A. Tarraf S. Schüler M. Strassner 《Applied physics. B, Lasers and optics》2002,75(1):3-13
Tailored scaling represents a principle of success that, both in nature and in technology, allows the effectiveness of physical
effects to be enhanced. Mutation and selection in nature are imitated in technology, e.g. by model calculation and design.
Proper scaling of dimensions in natural photonic crystals and our fabricated artificial 1D photonic crystals (DBRs, distributed
Bragg reflectors) enable efficient diffractive interaction in a specific spectral range. For our optical microsystems we illustrate
that tailored miniaturization may also increase the mechanical stability and the effectiveness of spectral tuning by thermal
and electrostatic actuation, since the relative significance of the fundamental physical forces involved considerably changes
with scaling. These basic physical principles are rigorously applied in micromachined 1.55-μm vertical-resonator-based devices.
We modeled, implemented and characterized 1.55-μm micromachined optical filters and vertical-cavity surface-emitting laser
devices capable of wide, monotonic and kink-free tuning by a single control parameter. Tuning is achieved by mechanical actuation
of one or several air-gaps that are part of the vertical resonator including two ultra-highly reflective DBR mirrors of strong
refractive index contrast: (i) Δn=2.17 for InP/air-gap DBRs (3.5 periods) using GaInAs sacrificial layers and (ii) Δn=0.5
for Si3N4/SiO2 DBRs (12 periods) with a polymer sacrificial layer to implement the air-cavity. In semiconductor multiple air-gap filters,
a continuous tuning of >8% of the absolute wavelength is obtained. Varying the reverse voltage (U=0–5 V) between the membranes
(electrostatic actuation), a tuning range of >110 nm was obtained for a large number of devices. The correlation of the wavelength
and the applied voltage is accurately reproducible without any hysteresis. In two filters, tuning of 127 and 130 nm was observed
for about ΔU=7 V. The extremely wide tuning range and the very small voltage required are record values to the best of our
knowledge. For thermally actuated dielectric filters based on polymer sacrificial layers, Δλ/ΔU=-7 nm/V is found.
Received: 10 May 2002 / Published online: 8 August 2002 相似文献