全文获取类型
收费全文 | 7996篇 |
免费 | 734篇 |
国内免费 | 966篇 |
专业分类
化学 | 7397篇 |
晶体学 | 70篇 |
力学 | 558篇 |
综合类 | 103篇 |
数学 | 553篇 |
物理学 | 1015篇 |
出版年
2024年 | 9篇 |
2023年 | 105篇 |
2022年 | 252篇 |
2021年 | 239篇 |
2020年 | 329篇 |
2019年 | 306篇 |
2018年 | 306篇 |
2017年 | 482篇 |
2016年 | 519篇 |
2015年 | 435篇 |
2014年 | 463篇 |
2013年 | 519篇 |
2012年 | 575篇 |
2011年 | 547篇 |
2010年 | 480篇 |
2009年 | 454篇 |
2008年 | 369篇 |
2007年 | 454篇 |
2006年 | 401篇 |
2005年 | 334篇 |
2004年 | 298篇 |
2003年 | 239篇 |
2002年 | 237篇 |
2001年 | 182篇 |
2000年 | 187篇 |
1999年 | 155篇 |
1998年 | 134篇 |
1997年 | 136篇 |
1996年 | 128篇 |
1995年 | 104篇 |
1994年 | 87篇 |
1993年 | 31篇 |
1992年 | 22篇 |
1991年 | 22篇 |
1990年 | 31篇 |
1989年 | 17篇 |
1988年 | 17篇 |
1987年 | 10篇 |
1986年 | 9篇 |
1985年 | 12篇 |
1984年 | 11篇 |
1983年 | 8篇 |
1982年 | 11篇 |
1981年 | 8篇 |
1980年 | 5篇 |
1979年 | 6篇 |
1976年 | 3篇 |
1971年 | 1篇 |
1959年 | 4篇 |
1957年 | 1篇 |
排序方式: 共有9696条查询结果,搜索用时 31 毫秒
981.
982.
为了利用吸附等温线理论模型模拟高真空多层绝热LNG槽车内吸气剂的吸附过程,对在真空技术中常用的等温式进行了分析研究。通过实验获得吸气剂在室温下的吸氢等温线,同时利用实验数据得到考虑分子间作用力的BET模型中的参数值。将模型结果与实验数据进行对比分析,结果表明:考虑分子间作用力的BET模型不能用作高真空多层绝热LNG槽车真空夹层内吸气剂吸附量的计算。利用Temkin和Freundlich等温式分段对实验数据进行拟合,对于1#吸气剂,拟合方程式计算得出的吸附量与实验数据的相对误差仅为2.6%,表明拟合方程能够对吸气剂在室温下的吸氢量进行准确预测。 相似文献
983.
V. L. Cherginets T. P. Rebrova Yu. N. Datsko T. V. Ponomarenko R. P. Yavetsky V. Yu. Pedash 《Crystal Research and Technology》2013,48(1):16-21
CsI single crystals were grown from the melt scavenged by Y3+ (YCl3) addition in 6.7·10−4–6.7·10−3 mol·kg−1 range. The addition of the scavenger amounts comparable with the total concentration of the oxygen‐containing admixtures in molten CsI results in complete destruction of the latter. Because of this, the intensity of the band with a maximum at 2.8 eV in radioluminescence spectra caused by the oxygen‐containing admixtures (anion vacancies) considerably decreases, and the fraction of the slow 2μs‐component corresponding to these admixtures becomes lower than 0.01 (0.007). The addition of larger quantities of YCl3 leads to the appearance of a wide band with a maximum at 2.8 eV caused by cation vacancies, and the intensity of the slow 2μs‐component increases to 0.02. The maximum ratio of two faster components with the decay constants equal to 7 and 30 ns reaches 0.65:0.33 at Y3+ concentration in CsI melt equal to 6.7·10‐3 mol·kg‐1, the effective luminescence time of fastest components is ca 14 ns. The dependence of the ‘Fast/Total ratio’ on Y3+ concentration passes through its maximum (0.81) corresponding to the equivalence of Y3+ and O2− concentrations in the growth CsI melt. 相似文献
984.
Arnaud METSUE Taku TSUCHIYA 《Proceedings of the Japan Academy. Series B, Physical and biological sciences》2013,89(1):51-58
We investigate the shear response of possible slip systems activated in pure and Fe-bearing MgSiO3 post-perovskite (PPv) through ab initio generalized stacking fault (GSF) energy calculations. Here we show that the [100](001) slip system has the easiest response to plastic shear among ten possible slip systems investigated. Incorporation of Fe2+ decreases the strength of all slip systems but does not change the plastic anisotropy style. Therefore, pure and Fe-bearing MgSiO3 PPv should demonstrate similar LPO patterns with a strong signature of the [100](001) slip system. An aggregate with this deformation texture is expected to produce a VSH > VSV type polarization anisotropy, being consistent with seismological observations. 相似文献
985.
《应用光谱学评论》2013,48(1):71-99
Abstract High performance liquid chromatography (HPLC) has often been employed in the separation and speciation of metal containing compounds. One of the most sensitive devices used for detecting those compounds after the separation is the inductively coupled plasma (ICP). The similar flow rates for these two techniques render the coupling of the devices trivial, usually involving only a short length of capillary tubing. Upon entering the plasma, species are typically determined either via atomic emission spectrometry or mass spectrometry. HPLC-ICP devices have been employed less frequently for the determination of non-metals. This review will describe the applications of HPLC-ICP techniques to the determination of compounds containing carbon, sulfur, phosphorus, and the halogens. The instrumentation used for each application will be described, and the performance of the systems will be summarized. 相似文献
986.
A. G. Agwu Nnanna 《实验传热》2013,26(2):157-173
This article presents an experimental study of thermo-hydrodynamic phenomena in a microchannel heat exchanger system. The aim of this investigation is to develop correlations between flow/thermal characteristics in the manifolds and the heat transfer performance of the microchannel. A rectangular microchannel fabricated by a laser-machining technique with channel width and hydraulic diameter of 87 μm and 0.17 mm, respectively, and a trapezoidal-shaped manifold are used in this study. The heat sink is subjected to iso-flux heating condition with liquid convective cooling through the channels. The temporal and spatial evolutions of temperature as well as total pressure drop across the system are monitored using appropriate sensors. Data obtained from this study were used to establish relationships between parameters such as longitudinal wall conduction factor, residence and switching time, and thermal spreading resistance with Reynolds number. Result shows that there exist an optimum Reynolds number and conditions for the microchannel heat exchanger system to result in maximum heat transfer performance. The condition in which the inlet manifold temperature surpasses the exit fluid temperature results in lower junction temperature. It further shows that for a high Reynolds number, the longitudinal wall conduction parameter is greater than unity and that the fluid has sufficient dwelling time to absorb heat from the wall of the manifold, leading to high thermal performance. 相似文献
987.
ABSTRACTPresent study investigates the heat transfer and friction characteristics of heat exchanger tube fitted with perforated twisted tape (PTT) insert having V cuts. A copper tube of 1 m length and 0.032 m inner diameter is used as test section to collect the experimental data by varying the twist ratio of PTT from 2 to 6 for the Reynolds number range of 2,700–23,400. V cuts are introduced in the PTT and the V-cut relative pitch ratio is varied from 1 to 2. The maximum thermo-hydraulic performance parameter is found to be 1.58. 相似文献
988.
Gerhard Sonnert Philip M. Sadler 《International Journal of Mathematical Education in Science & Technology》2013,44(8):1188-1207
Poor performance on placement exams keeps many US students who pursue a STEM (science, technology, engineering, mathematics) career from enrolling directly in college calculus. Instead, they must take a pre-calculus course that aims to better prepare them for later calculus coursework. In the USA, enrollment in pre-calculus courses in two- and four-year colleges continues to grow, and these courses are well-populated with students who already took pre-calculus in high school. We examine student performance in college calculus, using regression discontinuity to estimate the effects of taking college pre-calculus or not, in a national US sample of 5507 students at 132 institutions. We find that students who take college pre-calculus do not earn higher calculus grades. 相似文献
989.
Wassim M. Haddad Emmanuel G. Collins Jr. Raymond Moser 《Mathematical and Computer Modelling of Dynamical Systems: Methods, Tools and Applications in Engineering and Related Sciences》2013,19(4):299-318
Models of systems are always inexact. Hence, to better predict the performance of a system it is necessary to take into account uncertainty in a nominal model of a system. The structured singular value was developed to nonconservatively analyze robust stability and performance for systems with multiple-block uncertainty. In practice, optimization techniques are used to compute an upper bound on the structured singular value. For dynamic uncertainty with bounded magnitude and arbitrary phase (i.e., "complex uncertainty"), the standard approach to computing an upper bound involves finding diagonal scaling matrices D(jω) that minimize σmax (D(jω)G(jω)D-1(jω)) over a (theoretically) infinite number of frequencies. The order of the corresponding stable, minimum phase, rational function D(s) (if it exists) is hence arbitrary, which can lead to very high order controllers when D(s) is used for controller synthesis. This paper develops a fixed-structure approach to computing an upper bound for the complex structured singular value. In particular, by relying on results from mixed-norm H2/H∞ analysisD(s) is a priori constrained to be a rational matrix function of a chosen order and a new approach to computing an upper bound on the structured singular value is developed. The results are illustrated using two examples which clearly demonstrate the suboptimality of standard curve fitting. The proposed approach can be extended to mixed uncertainty and structured singular value controller synthesis without D — K type iteration. 相似文献
990.