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971.
虽然含硫碳负离子在有机合成中已有若干应用[1~4],但要有效地产生这类碳负离子,通常需用有机锂或氢化钠,这些强碱性试剂对氧和潮气都很敏感,在实际应用方面有不足之处。 相似文献
972.
973.
本文详细讨论了制约相衬显微镜衬比的各种因素,并针对不同的被观察样本提出了合适的观察方法。首先,叙述了相衬显微镜和暗场显微镜的基本原理,并根据观察时的实际情况,定义了衬比Fc.然后,分纯相位物体和既有振幅变化又有相位变化的物体两种情况,对影响衬比的各种因素进行了详细讨论,得出了相衬显微镜的衬比最大的条件,并比较了各种条件下相衬显微镜和暗场显微镜的灵敏度,从中得出:在观察弱纯相位变化的物体时,相衬显微镜要比暗场显微镜有效些;在观察既有振幅变化又有相位变化的物体时,相衬显微镜和暗场显微镜要比明场显微镜有效些,但视场中的光强变化已是物体的振幅变化和相位变化的混合效应,而不能单独反映出相位变化。 相似文献
974.
条纹图象的数字化自动分析处理技术之二:相位分析法 总被引:1,自引:0,他引:1
相位分析法是目前光学条纹图象数字化自动分析处理中的最重要方法之一。本文较全面地介绍了相位分析技术,并对相位分析的关键技术--相位的滤波和解包络方法做了重点说明。文中给出了相位解包络的实例并讨论了图象数字化自动分析处理技术的实际应用状况。 相似文献
975.
F. Decker K.-P. Holzer P. Mischo J. Petersson D. Schüßler 《The European Physical Journal B - Condensed Matter and Complex Systems》1999,8(4):507-510
The NMR satellite frequencies were measured as a function of temperature in the normal high-temperature phase for 87Rb in Rb2ZnBr4 and Rb2ZnCl4 and for 35Cl in betaine calciumchloride dihydrate. Approaching the respective normal-incommensurate phase transition an anomalous shift
of the NMR frequency is observed for the first two cases. This effect is ascribed to the increasing order parameter fluctuations.
The experimental data are compared to calculations which relate the observed behaviour of the NMR frequencies to the non-classical
critical behaviour of the substances under investigation.
Received 6 August 1998 相似文献
976.
L. Zhang W. L. Zhong C. L. Wang Y. P. Peng Y. G. Wang 《The European Physical Journal B - Condensed Matter and Complex Systems》1999,11(4):565-573
The size effect of the dielectric properties and the barrier height was investigated in the ferroelectric solid solution BaxSr1-xTiO3 system. The decrease of the grain size causes the suppression of the ferroelectricity, and the increase of the relaxation
frequency. Barrier heights increase with increasing grain size. The result is analogous to magnetic phase transitions in nanocrystals
and other solid-solid phase transitions in nanocrystals. It suggests a general rule that may be of use in the discovery of
new metastable phases. An explanation of this phenomenon was given by an electric potential model that agrees well with the
experimental results. For BaxSr1-xTiO3 system, the decrease of xcauses the decrease of the barrier height.
Received 3 August 1998 and Received in final form 22 November 1998 相似文献
977.
The mixing fraction of didodecyldimethylammonium bromide (DDAB) in dodecyltrimethylammonium bromide + DDAB to produce a lamellar
liquid crystal (L
α) abruptly decreases upon addition of a small amount of m-xylene, whereas the mixing fraction becomes constant at high m-xylene content. Similar results were obtained in saturated hydrocarbon systems. It is considered that oil molecules in the
surfactant palisade layer increases the effective cross-sectional area per surfactant head group, as, whereas as is constant if the oil molecules are solubilized in the core of the liquid crystal. The volume fraction of penetrating oil
in the total solubilized oil is defined as a penetration parameter, Pe, which is calculated from small-angle X-ray scattering
data. Pe is high in the m-xylene system, whereas it is low in the n-decane system. Even in the same oil system, Pe decreases dramatically with increasing solubilization. Hence, most of the
oil added penetrates into a palisade layer at an early stage of oil addition. This causes a change in the mixing fraction
of surfactant in the L
α phase. Thereafter the oil is solubilized in the core of the bilayer with further addition of oil.
Received: 20 April 1998 Accepted: 16 July 1998 相似文献
978.
A drift-type phase transfer catalyst, cryptand-22, adsorbed on poly(styrene/diviny benzene)-sulfonic resin was prepared and applied to catalyze the reduction of ketones, e.g., acetophenone, benzophenone and benzaldehyde with NaBH4 as a reducing agent. Before the reaction, cryptand-22 was adsorbed on the sulfonic resin with ion-pairing, resin-SO3? +NH-cryptand-22. The ion-pairs can be destroyed by adjusting the basicity of the reaction solution with NaOH and the cryptand can be released from the resin into the reaction solution as a homogeneous catalyst during the reaction period. After the reaction, the cryptand catalyst can be readsorbed on the resin by adjusting the acidity of the solution with HCl and can be readily recovered by filtration like a heterogeneous catalyst. The draft-type cryptand catalyst exhibited better catalytic ability than some common crown ethers, e.g., 15-crown-5, benzo-15-crown-5, 12-crown-4 and dibenzo-18-crown-6 for the reduction of acetophenone with NaBH4. Effects of solvents, pH of solutions, concentration of the catalyst, reducing agents and resin property on the reduction of ketones were investigated and discussed. The reaction mechanism of the cryptand catalyzed reduction was also studied. 相似文献
979.
We consider ak-out-of-n system with repair. Life times of components are independent exponentially distributed random variables with parameter λ
i
when the number of working units isi. Failed units are taken for repair to a station, manned by a single server, having no waiting room. The failed units are
brought to an orbit, if the server is found to be busy, for retrial. Reliability of the system is computed in the following
three situations: (i) Cold system (ii) Warm system and (iii) Hot system. Several other system characteristics are derived. 相似文献
980.