全文获取类型
收费全文 | 6993篇 |
免费 | 936篇 |
国内免费 | 256篇 |
专业分类
化学 | 4050篇 |
晶体学 | 47篇 |
力学 | 518篇 |
综合类 | 30篇 |
数学 | 364篇 |
物理学 | 3176篇 |
出版年
2024年 | 7篇 |
2023年 | 55篇 |
2022年 | 125篇 |
2021年 | 154篇 |
2020年 | 181篇 |
2019年 | 215篇 |
2018年 | 165篇 |
2017年 | 152篇 |
2016年 | 297篇 |
2015年 | 248篇 |
2014年 | 312篇 |
2013年 | 655篇 |
2012年 | 361篇 |
2011年 | 379篇 |
2010年 | 297篇 |
2009年 | 388篇 |
2008年 | 411篇 |
2007年 | 469篇 |
2006年 | 416篇 |
2005年 | 281篇 |
2004年 | 303篇 |
2003年 | 305篇 |
2002年 | 229篇 |
2001年 | 211篇 |
2000年 | 214篇 |
1999年 | 166篇 |
1998年 | 209篇 |
1997年 | 102篇 |
1996年 | 99篇 |
1995年 | 81篇 |
1994年 | 94篇 |
1993年 | 72篇 |
1992年 | 63篇 |
1991年 | 56篇 |
1990年 | 34篇 |
1989年 | 58篇 |
1988年 | 44篇 |
1987年 | 31篇 |
1986年 | 23篇 |
1985年 | 22篇 |
1984年 | 39篇 |
1983年 | 16篇 |
1982年 | 25篇 |
1981年 | 20篇 |
1980年 | 16篇 |
1979年 | 20篇 |
1978年 | 16篇 |
1977年 | 8篇 |
1976年 | 13篇 |
1973年 | 11篇 |
排序方式: 共有8185条查询结果,搜索用时 15 毫秒
951.
We measured the wavelength-dependent intensity decays of 4,9-dihydro-4,6-dimethyl-9-oxo-1H-imidazo-1,2a-purine (Yt-base) in propanol to determine the time-resolved emission spectra and rates of spectral relaxation. We found that resolution of the spectral relaxation times was dramatically improved by global analysis of the frequency-domain data with increasing amounts of the collisional quencher CCl4. Collisional quenching preferentially decreases the longer-lived relaxed component of the emission, thereby increasing the fractional contribution of the incompletely relaxed portion of the emission. The data could not be explained by a single spectral relaxation time, and at least two relaxation times are needed to describe the time-dependent emission center of gravity of Yt-base.Dedicated to Professor Stefan Paszyc on the occasion of his 70th birthday. 相似文献
952.
Summary Kubelka-Munk theory of radiation transfer in turbid media is applied to determine the influence of skin optical losses on
the efficiency of phototherapy of neonatal hyperbilirubinemia. By using a multilayer model of the skin and a rate equation
analysis of bilirubin photo-isomerization, the photon absorption rates of birilubin and of its configurational photoisomers
and, in turn, the photoequilibrium concentrations and rise time are calculated for spectrally Gaussian light sources and fluorescent
lamps used in phototherapy. It turns out that light absorption and scattering processes in the skin layers produce a 30 nm
red-shift of the optimum value of the peak excitation wave-length of bilirubin absorption and photoequilibrium rise time.
The comparative data on the phototherapeutical efficiency of different spectral regionsin vivo (Gunn rats and humans) are discussed.
Riassunto L'influenza delle perdite ottiche cutanee sul rendimento della fototerapia dell'iperblirubinemia neonatale è investigata mediante l'impiego della teoria di Kubelka-Munk sulla propagazione della luce in mezzi torbidi. L'evoluzione temporale delle concentrazioni di bilirubina e dei suoi fotoisomeri configurazionali è studiata per mezzo delle equazioni di bilancio nel caso del modello a multistrati della cute, assumendo una sorgente con distribuzione spettrale gaussiana e i profili spettrali delle principali lampade fluorescenti usate clinicamente. I risultati indicano che l'assorbimento e la diffusione della luce negli strati cutanei sposta di circa 30 nm verso lunghezze d'onda maggiori il valore ottimale della lunghezza d'onda d'assorbimento della bilirubina e del tempo di fotoequilibrio. Si discutono comparativamente i dati sul rendimento foroterapeutico di diverse regioni spettraliin vivo (su ratti Gunn e uomo).
Резюме Теория Кубелки-Мунка рационального переноса в мутной среде применяется для определения влияния оптических потерь в поверхностном слое на эффективность фототерапии. Используя многослойную модель кожи и анализ фотоизомеризации билирубина, вычисляются интенсивности поглощения фототонов билирубином и его конфигурационным фотоизомерами и, в свою очередь, фоторавновесные концентрации и времена возникновения для источников света с гауссовым спектром и для флуоресцентных ламп, используемых в фототерапии. Оказывается, что процессы поглощения и рассеяния света в поверхностных слоях кожи приводят к красному смещению 30 нм оптимальной величины длины волны для поглощения билирубином. Обсуждаются сравнительные данные об эффективности фототерапии в различных спектральных областях ?в естественных условиях?.相似文献
953.
超急速传热时球体内非稳态热传导的非傅里叶效应 总被引:13,自引:0,他引:13
本文分析了球形物体表面加有一突然温度变化这类超急速传热情形下的热波传播特性.基于超急速传热条件下,热流矢的传播和温度梯度的形成之间存在时间延迟的概念,本文采用双曲型热传导方程来描述该高度非稳态热传导问题,得到了球体内部温度分布的解析表达式.同时将所得解与抛物型热传导方程的解进行了比较,分析了其热传导的非傅里叶效应与产生条件. 相似文献
954.
Y. J. Wang H. A. Nickel B. D. McCombe F. M. Peeters J. M. Shi G. Q. Hai X. -G. Wu T. J. Eustis W. Schaff 《Physica E: Low-dimensional Systems and Nanostructures》1998,2(1-4)
A detailed experimental study of electron cyclotron resonance (CR) has been carried out at 4.2 K in three modulation-doped GaAs/Al0.3Ga0.7As multiple quantum well samples in fields up to 30 T. A strong avoided-level-crossing splitting of the CR energies due to resonant magnetopolaron effects is observed for all samples near the GaAs reststrahlen region. Resonant splittings in the region of AlAs-like interface phonon modes of the barriers are observed in two samples with narrower well width and smaller doping concentration. The interaction between electrons and the AlAs interface optical phonon modes has been calculated for our specific sample structures in the framework of the memory-function formalism. The calculated results are in good agreement with the experimental results, which confirms our assignment of the observed splitting near the AlAs-like phonon region is due to the resonant magnetopolaron interaction of electrons in the wells with AlAs-like interface phonons. 相似文献
955.
Summary The post-buckling behavior of imperfect columns made of nonlinear viscoelastic materials is investigated, taking into account
the effect of shear deformation. The material is modeled according to the Leaderman representation of nonlinear viscoelasticity.
Solutions are developed, within the elastica and the shear deformation theories, in order to calculate the growth in time
of the total deflection. The numerical results establish the importance of the shear and the nonlinear viscoelasticity effects,
and of the h/ℓ ratio in the column post-buckling behavior.
Accepted for publication 11 November 1996 相似文献
956.
D. J. Harris T. M. Teo R. J. Batt S. C. Luk 《International Journal of Infrared and Millimeter Waves》1980,1(3):339-349
Interference effects between modes in open-resonators may seriously modify the resonance characteristics. Constructive and destructive interference may take place. For multimode conditions many path lengths exhibit resonance modification.Work supported by the U. K. Science Research Council. 相似文献
957.
Summary A new method of dental fluorine prophylaxis based on the chemical reaction induced by UV irradiation in dental enamel has
been presented. Fluorine ions from a gel topic can be retained to the dental apatite in the lamp-irradiated samples at about
70% and in the laser-irradiated samples at about 80% of the maximum deposited value. The19F(p,α)16O nuclear reaction was used to measure the fluorine concentrations in the first 3 μm of the enamel of healthy teeth before
and after the gel topic applications with and without UV irradiation. This method of dental prefenction resolves the problem
of the traditional fluorine prophylaxis which brings fluorine in the enamel without binding the apatite.
To speed up publication, the authors of this paper have agreed to not receive the proofs for correction. 相似文献
958.
Technique and instrumentation to detect reliably, multiplicity of neutrons emitted in sharp bursts (≤100 μs) has been developed
where a burst of as low as 15 neutrons and continuous emission of ⋍10−1 neutron/s may be detected. Using this technique, attempts were made to detect neutron emission from various experiments in
which anomalous nuclear effects (or what is commonly referred to as cold fusion) may be expected to occur. No neutrons, above
our detection threshold, were detected in the recent series of experiments. 相似文献
959.
Summary We study the effect of temperature on the lifetime of the resonant macroscopic quantum tunnelling voltage state. In view of
experiments we consider a merit factor in order to balance between measuring extremely small signals of longer duration or
larger signals of shorter duration. 相似文献
960.
K. Totsuka 《The European Physical Journal B - Condensed Matter and Complex Systems》1998,5(3):705-717
Novel massive quantum states appearing in spin chains under a strong magnetic field are discussed. These states lead to plateaus in magnetization
curves. When the systems are axially symmetric and the field is applied parallel to the symmetry-axis, the phenomena are analogous
to metal-insulator transitions. Striking features of the plateau phenomena - exactness and rationality - are explained as
consequences the commensurability condition to the underlying lattice. The effects of the planar anisotropy are also discussed
in detail.
Received: 16 February 1998 / Revised: 20 April 1998 / Accepted: 30 April 1998 相似文献