全文获取类型
收费全文 | 2362篇 |
免费 | 84篇 |
国内免费 | 23篇 |
专业分类
化学 | 1463篇 |
晶体学 | 7篇 |
力学 | 42篇 |
数学 | 399篇 |
物理学 | 558篇 |
出版年
2023年 | 9篇 |
2022年 | 22篇 |
2021年 | 28篇 |
2020年 | 39篇 |
2019年 | 29篇 |
2018年 | 30篇 |
2017年 | 36篇 |
2016年 | 61篇 |
2015年 | 59篇 |
2014年 | 61篇 |
2013年 | 122篇 |
2012年 | 186篇 |
2011年 | 176篇 |
2010年 | 123篇 |
2009年 | 110篇 |
2008年 | 158篇 |
2007年 | 164篇 |
2006年 | 167篇 |
2005年 | 203篇 |
2004年 | 138篇 |
2003年 | 124篇 |
2002年 | 115篇 |
2001年 | 19篇 |
2000年 | 23篇 |
1999年 | 15篇 |
1998年 | 15篇 |
1997年 | 15篇 |
1996年 | 33篇 |
1995年 | 17篇 |
1994年 | 17篇 |
1993年 | 19篇 |
1992年 | 11篇 |
1991年 | 5篇 |
1990年 | 8篇 |
1989年 | 12篇 |
1988年 | 5篇 |
1987年 | 4篇 |
1986年 | 7篇 |
1985年 | 12篇 |
1984年 | 10篇 |
1983年 | 10篇 |
1982年 | 9篇 |
1981年 | 10篇 |
1980年 | 5篇 |
1979年 | 7篇 |
1977年 | 4篇 |
1976年 | 3篇 |
1973年 | 5篇 |
1972年 | 3篇 |
1970年 | 2篇 |
排序方式: 共有2469条查询结果,搜索用时 31 毫秒
971.
This paper presents a time domain formulation for the sound field radiated by moving bodies in a uniform steady flow with arbitrary orientation. The aim is to provide a formulation for prediction of noise from body so that effects of crossflow on a propeller can be modeled in the time domain. An established theory of noise generation by a moving source is combined with the moving medium Green's function for derivation of the formulation. A formula with Doppler factor is developed because it is more easily interpreted and is more helpful in examining the physic of systems. Based on the technique presented, the source of asymmetry of the sound field can be explained in terms of physics of a moving source. It is shown that the derived formulation can be interpreted as an extension of formulation 1 and 1A of Farassat based on the Ffowcs Williams and Hawkings (FW–H) equation for moving medium problems. Computational results for a stationary monopole and dipole point source in moving medium, a rotating point force in crossflow, a model of helicopter blade at incidence and a propeller case with subsonic tips at incidence verify the formulation. 相似文献
972.
Daltrop S Kotlicki A Waltham C Wolfe N Elie B Gautier F 《The Journal of the Acoustical Society of America》2012,131(1):837-843
The "gothic" style of harp was popular across most of Europe from the late medieval period to the Renaissance. These harps have a one-piece, longitudinally oriented hardwood soundboard, as opposed to the transverse-oriented bonded softwood structure on a modern instrument. In addition, the one-piece back is flexible, whereas the back of a modern instrument is a rigid molded shell. To study the gothic harp, one was constructed from plans created by the Boston Museum of Fine Arts from a late German model in their collection. The vibrational behaviors of the soundboard and soundbox were measured at various stages of construction. The completed instrument was subjected to modal analysis and radiativity measurements. The sound radiation is dominated by two breathing modes at 188 and 273 Hz, each with strong motion of the back, and modes around 350 Hz. Taken together, these modes function like the A0/T1 resonance pairs seen in the soundboxes of other instruments, and a comparison is made with the guitar. Also observed is that as the frequency increases, radiation is emitted from higher up the soundboard, and from higher soundholes. This feature has been observed in other harps, and is a consequence of the harp family's unique geometry. 相似文献
973.
Sanjeeva Witharana Chris Hodges Dan Xu Xiaojun Lai Yulong Ding 《Journal of nanoparticle research》2012,14(5):851
Nanoparticle suspensions (also called nanofluids) are often polydisperse and tend to settle with time. Settling kinetics in
these systems are known to be complex and hence challenging to understand. In this study, polydisperse spherical alumina (Al2O3) nanoparticles in the size range of ~10–100 nm were dispersed in water and examined for aggregation and settling behaviour
near its isoelectric point. A series of settling experiments were conducted and the results were analysed by photography and
by small angle X-ray scattering (SAXS). The settling curve obtained from standard bed height measurement experiments indicated
two different types of behaviour, both of which were also seen in the SAXS data. But the SAXS data were remarkably able to
pick out the rapid settling regime as a result of the high temporal resolution (10 s) used. By monitoring the SAXS intensity,
it was further possible to record the particle aggregation process for the first time. Optical microscopy images were produced
on drying and dried droplets extracted from the suspension at various times. Dried deposits showed the rapid decrease in the
number of very large particles with time which qualitatively validates the SAXS prediction, and therefore its suitability
as a tool to study unstable polydisperse colloids. 相似文献
974.
Kevin Lannon Fabrizio Margaroli Chris Neu 《The European Physical Journal C - Particles and Fields》2012,72(8):1-22
With the full Tevatron Run II and early LHC data samples, the opportunity for furthering our understanding of the properties of the top quark has never been more promising. Although the current knowledge of the top quark comes largely from Tevatron measurements, the experiments at the LHC are poised to probe top-quark production and decay in unprecedented regimes. Although no current top quark measurements conclusively contradict predictions from the standard model, the precision of most measurements remains statistically limited. Additionally, some measurements, most notably A FB in top quark pair production, show tantalizing hints of beyond-the-Standard-Model dynamics. The top quark sample is growing rapidly at the LHC, with initial results now public. This review examines the current status of top quark measurements in the particular light of searching for evidence of new physics, either through direct searches for beyond the standard model phenomena or indirectly via precise measurements of standard model top quark properties. 相似文献
975.
Flow development in the wake of a dual step cylinder has been investigated experimentally using Laser Doppler Velocimetry
and flow visualization. The dual step cylinder model is comprised of a large diameter cylinder (D) mounted at the mid-span of a small diameter cylinder (d). The experiments have been performed for a Reynolds number (Re
D
) of 1,050, a diameter ratio (D/d) of 2, and a range of large cylinder aspect ratios (L/D). The results show that the flow development is highly dependent on L/D. The following four distinct flow regimes can be identified based on vortex dynamics in the wake of the large cylinder: (1)
for L/D ≥ 15, three vortex shedding cells form in the wake of the large cylinder, one central cell bounded by two cells of lower
frequency, (2) for 8 < L/D ≤ 14, a single vortex shedding cell forms in the wake of the large cylinder, (3) for 2 < L/D ≤ 6, vortex shedding from the large cylinder is highly three-dimensional. When spanwise vortices are shed, they deform substantially
and attain a hairpin shape in the near wake, (4) for 0.2 ≤ L/D ≤ 1, the large cylinder induces vortex dislocations between small cylinder vortices. The results show that for Regimes I
to III, on the average, the frequency of vortex shedding in the large cylinder wake decreases with L/D, which is accompanied by a decrease in coherence of the shed vortices. In Regime IV, small cylinder vortices connect across
the large cylinder wake, but these connections are interrupted by vortex dislocations. With decreasing L/D, the frequency of dislocations decreases and the dominant frequency in the large cylinder wake increases toward the small
cylinder shedding frequency. 相似文献
976.
Lorenzo Grande Vishnu Teja Chundi Di Wei Chris Bower Piers Andrew Tapani Ryhänen 《Particuology》2012,10(1):1-8
Graphene-based materials are intriguing from the perspective of fundamental science and technology because they are non-toxic, chemically and thermally tolerant, and mechanically robust. Graphene exhibits superior electrical conductivity, high surface area and a broad electrochemical window that may be particularly advantageous for their applications in energy storage devices. In addition, graphene can be prepared in the form of a colloidal suspension with adjustable solubility and thus is suitable for printing applications and offers both transparency and good conductivity at the same time. In this review, applications of graphene in solar cells, batteries, supercapacitors and fuel cells are summarized with the latest developments. Furthermore, graphene as a conductive ink for printed electronics is also discussed. 相似文献
977.
Wing Yan Tam Chris S. K. Mak Alan Man Ching Ng Aleksandra B. Djurii Wai Kin Chan 《Macromolecular rapid communications》2009,30(8):622-626
The synthesis of poly(N‐vinylcarbazole)‐based block copolymers functionalized with rhenium diimine complexes or pendant terpyridine ligands is reported. The copolymers are synthesized by reversible addition–fragmentation chain transfer (RAFT) polymerization, and they exhibit interesting morphological properties as a result of the phase separation between different blocks. The rhenium complex polymer block may function as a photosensitizer, while the terpyridine‐containing polymer block can be used as the template for nanofabrication by selective deposition of zinc complexes.
978.
In this paper we consider compactifications of heterotic strings in the presence of background flux. The background metric is a T2 fibration over a K3 base times four-dimensional Minkowski space. Depending on the choice of three-form flux different amounts of supersymmetry are preserved (N=2,1,0). For supersymmetric solutions unbroken space–time supersymmetry determines all background fields except one scalar function which is related to the dilaton. The heterotic Bianchi identity gives rise to a differential equation for the dilaton which we discuss in detail for solutions preserving an N=2 supersymmetry. In this case the differential equation is of Laplace type and as a result the solvability is guaranteed. 相似文献
979.
To comply with the EU Noise Directive 2002/49/EC, Member States are required to produce strategic noise maps for designated areas, including mapping road traffic noise from major roads. These maps must be presented using the EU indicators Lden and Lnight. However, the most common noise indicator used in Ireland at present is the LA10,18h indicator arising from the use of the Calculation of Road Traffic Noise (CRTN) prediction method. Therefore, a relationship needs to be established between LA10,18h and Lden and Lnight, separately. In addition to noise mapping these indicators are used for noise abatement purposes, so the proposed relationship must be accurate and robust. In 2002, the UK’s Transport Research Laboratory (TRL) published a paper describing mathematical procedures that could be used to convert values of LA10 to Lden and Lnight. These procedures were then adopted for use in Ireland. This paper examines the suitability of the TRL conversion methods 1 and 3 for use under Irish road conditions. Method 2 was not considered in this study, as it was a methodology not applicable in an Irish scenario. Studies concluded that where hourly traffic data are available, the conversion methodology outlined in TRL Method 1 is robust and reproducible. However, in the absence of hourly traffic data where daily traffic counts are used, the relevant conversion procedures produce variable results for both Lden and Lnight when applied to Irish road conditions. To reduce the variability, new conversion procedures were developed, specifically for Irish road conditions. 相似文献
980.
Chris Heunen Nicolaas P. Landsman Bas Spitters 《Communications in Mathematical Physics》2009,291(1):63-110
The aim of this paper is to relate algebraic quantum mechanics to topos theory, so as to construct new foundations for quantum logic and quantum spaces. Motivated by Bohr’s idea that the empirical content of quantum physics is accessible only through classical physics, we show how a noncommutative C*-algebra of observables A induces a topos \({\mathcal{T}(A)}\) in which the amalgamation of all of its commutative subalgebras comprises a single commutative C*-algebra \({\underline{A}}\) . According to the constructive Gelfand duality theorem of Banaschewski and Mulvey, the latter has an internal spectrum \({\underline{\Sigma}(\underline{A})}\) in \({\mathcal{T}(A)}\) , which in our approach plays the role of the quantum phase space of the system. Thus we associate a locale (which is the topos-theoretical notion of a space and which intrinsically carries the intuitionistic logical structure of a Heyting algebra) to a C*-algebra (which is the noncommutative notion of a space). In this setting, states on A become probability measures (more precisely, valuations) on \({\underline{\Sigma}}\) , and self-adjoint elements of A define continuous functions (more precisely, locale maps) from \({\underline{\Sigma}}\) to Scott’s interval domain. Noting that open subsets of \({\underline{\Sigma}(\underline{A})}\) correspond to propositions about the system, the pairing map that assigns a (generalized) truth value to a state and a proposition assumes an extremely simple categorical form. Formulated in this way, the quantum theory defined by A is essentially turned into a classical theory, internal to the topos \({\mathcal{T}(A)}\).These results were inspired by the topos-theoretic approach to quantum physics proposed by Butterfield and Isham, as recently generalized by Döring and Isham. 相似文献