全文获取类型
收费全文 | 16825篇 |
免费 | 314篇 |
国内免费 | 439篇 |
专业分类
电工技术 | 346篇 |
综合类 | 441篇 |
化学工业 | 2472篇 |
金属工艺 | 936篇 |
机械仪表 | 1793篇 |
建筑科学 | 292篇 |
矿业工程 | 120篇 |
能源动力 | 1109篇 |
轻工业 | 263篇 |
水利工程 | 137篇 |
石油天然气 | 267篇 |
武器工业 | 59篇 |
无线电 | 2754篇 |
一般工业技术 | 3816篇 |
冶金工业 | 440篇 |
原子能技术 | 306篇 |
自动化技术 | 2027篇 |
出版年
2024年 | 12篇 |
2023年 | 206篇 |
2022年 | 304篇 |
2021年 | 372篇 |
2020年 | 324篇 |
2019年 | 341篇 |
2018年 | 278篇 |
2017年 | 368篇 |
2016年 | 444篇 |
2015年 | 446篇 |
2014年 | 803篇 |
2013年 | 829篇 |
2012年 | 774篇 |
2011年 | 1455篇 |
2010年 | 842篇 |
2009年 | 1003篇 |
2008年 | 939篇 |
2007年 | 896篇 |
2006年 | 876篇 |
2005年 | 757篇 |
2004年 | 634篇 |
2003年 | 618篇 |
2002年 | 572篇 |
2001年 | 379篇 |
2000年 | 360篇 |
1999年 | 416篇 |
1998年 | 395篇 |
1997年 | 354篇 |
1996年 | 343篇 |
1995年 | 244篇 |
1994年 | 207篇 |
1993年 | 151篇 |
1992年 | 136篇 |
1991年 | 131篇 |
1990年 | 96篇 |
1989年 | 104篇 |
1988年 | 72篇 |
1987年 | 13篇 |
1986年 | 14篇 |
1985年 | 7篇 |
1984年 | 7篇 |
1983年 | 9篇 |
1982年 | 12篇 |
1981年 | 10篇 |
1980年 | 4篇 |
1977年 | 2篇 |
1976年 | 3篇 |
1975年 | 5篇 |
1974年 | 3篇 |
1973年 | 6篇 |
排序方式: 共有10000条查询结果,搜索用时 15 毫秒
81.
This paper presents a new technique, called velocity modulation, for measuring the mass center location of a sphere, improving upon the accuracy of previous methods by at least a factor of 4. The sphere is rolled down a set of parallel rails. The offset of the sphere’s mass center with respect to its geometric center modulates the sphere’s velocity at the rolling frequency, which is detected by a series of differential optical shadow sensors. The measured times that the sphere crosses the sensors is compared with a simplified mechanics model in order to recover the sphere’s mass center location. The precision of the technique is demonstrated by the repeatability of independent measurements in one plane of a 50.8 mm (2 in.) stainless steel gage sphere. The best standard deviation achieved is 145 nm (95% confidence interval = 227 nm). 相似文献
82.
The synthesis and characterization of polyaniline (Pani) films embedded with PbS/CoS core–shell, having semiconductor characteristics are reported in this study. Mn2+ doped, PVA capped PbS/CoS core-shell particles dispersed in Pani matrix served as photoluminescent boosters. The absorption intensity of the core-shell particles–Pani hybrid increased significantly with wide spectrum response and hypsochromic effect. The Mott–Schottky plot showed a negative slope for PbS/CoS-Pani film, indicating typical p-type semiconductivity. The Tauc plot for PbS/CoS-Pani film revealed the direct transition type with a band gap of 2.36 eV. Solid state photovoltaic cells have been fabricated with PbS/CoS-Pani as the hole conductor. The cells show photocurrent of 0.93 mA/cm2, voltage of 432 mV and energy conversion efficiency of 0.93%. The scanning electron micrograph revealed periodic arrangement in the polymer matrix. This arrangement of the core-shell particles in the continuous polymer matrix with high carrier density promises this material for the photoelectrochemical applications. 相似文献
83.
Sunlight irradiating the surface of the Earth represents a maximum input available for a solar catalytic process of 50 WUV m−2. We propose using high-porosity, metallic, reticulated foams as the support medium for the photocatalyst in order to improve the apparent quantum yield. The layer of TiO2 was applied by dip-coating. The measurement of the degradation kinetics was carried out on a model target molecule, 2,4 dichlorophenol, at an initial concentration of 10 mg l−1. The aim was to assess the efficiency of the foams as a photocatalytic media compared to that of a suspension of catalytic powder (Degussa P25) and the flat 2D support (Ahlstrom cellulose media). The apparent quantum yield of the foam scaffold carrying the TiO2 was high, showing that, as with the powder suspension, foam makes good use of the UV rays to break down molecules. It is noteworthy that the apparent quantum yield of the foam tended towards that observed for suspensions which form the ideal support thanks to their optimal ability to harness the light. 相似文献
84.
In this study we use a combination of variable stripe and shifting excitation spot methods to evaluate linear low temperature (4.2 K) optical properties of Si/Si:Er multinanolayer grown on SOI substrate. In particular, Er-1 luminescence at 1.54 μm under continuous wave excitation was examined. Absorption coefficients of 22.4 ± 4.2 and 45.1 ± 4.2 cm−1 at photon fluxes of 2.4 × 1019 and 2.4 × 1020 cm2 s−1, respectively, have been established. These are approximately three times higher than those found earlier for a similar structure grown on Si substrate. 相似文献
85.
For the first time, Ca5(BO3)3F (CBF) fiber crystals were grown by the micro-pulling down technique from LiF-based flux. Regular and transparent fibers were obtained with good optical quality. Characterization by Raman spectroscopy and EDX microprobe showed fibers compositionally homogeneous. The specific heat capacity of the fibers was measured indicating a laser damage threshold slightly lower, at room temperature, than for crystals belonging to the related rare earth calcium oxoborate family. 相似文献
86.
L. JyothiV. Venkatramu P. BabuC.K. Jayasankar M. BettinelliG. Mariotto A. Speghini 《Optical Materials》2011,33(6):928-936
The spectroscopic properties of tellurite glasses of composition (in mol%) TNKNd: (70 − x)TeO2-15Nb2O5-15K2O-xNd2O3 (x = 0.1, 1.0, 1.5, 2.0 and 2.5) and TNLNd10: 69TeO2-15Nb2O5-15Li2O-1.0Nd2O3 and lithium metaborate glass of composition LBNNd10: 89LiBO2-10Nb2O5-1.0Nd2O3 have been investigated using absorption and emission spectra and decay curve analysis. An energy level analysis has been carried out considering the experimental energy positions of the absorption and emission bands, using the free-ion Hamiltonian model. The spectral intensities have been calculated by using the Judd-Ofelt theory and in turn the radiative properties such as radiative transition probabilities, emission cross-sections, branching ratios and radiative lifetimes have been estimated. The decay curves at the lower concentrations are exponential while they show a non-exponential behavior at higher concentrations (?1.0 mol%) due to energy transfer processes. The effective lifetimes for the 4F3/2 level are found to decrease with increase in Nd2O3 concentration for all the glasses under investigation. The non-exponential decay curves have been well-fitted to the Yokota-Tanimoto model with S = 6, indicating that the nature of energy transfer is of dipole-dipole type and energy migration also plays an important role. The results obtained have been compared with Nd3+-doped phosphate, fluorophosphate, lead borate, tellurite, germanate and silicate glasses and Nd3+-doped YAG ceramic and Ca2Nb2O7 crystals. 相似文献
87.
Brecht Verstichel Helen van Aggelen Dimitri Van Neck Paul W. Ayers Patrick Bultinck 《Computer Physics Communications》2011,(9):2025-2028
We discuss how semidefinite programming can be used to determine the second-order density matrix directly through a variational optimization. We show how the problem of characterizing a physical or N-representable density matrix leads to matrix-positivity constraints on the density matrix. We then formulate this in a standard semidefinite programming form, after which two interior point methods are discussed to solve the SDP. As an example we show the results of an application of the method on the isoelectronic series of Beryllium. 相似文献
88.
Lutz Prager Luise WennrichWolfgang Knolle Sergej NaumovAndrea Prager 《Materials Chemistry and Physics》2012
A simple but reliable method was developed for the determination of wavelength-dependent absorption coefficients in the vacuum-UV (VUV) spectral range 160 nm < λ < 195 nm. 相似文献
89.
Amit Belwalkar Hongsheng Xiao Wojciech Z. Misiolek Jean Toulouse 《Journal of Materials Processing Technology》2010,210(14):2016-2022
The extrusion behavior of tellurite glass in the supercooled liquid region was investigated. Good extrusion formability was observed under low strain rates at various temperatures in the glass transformation region investigated. Tube and holey fiber (HF) preforms were fabricated from tellurite glass billets using a laboratory press. In particular, the results for three-spoke HF design and round tube preforms with composition 75TeO2·20ZnO·5Na2O (TZN) are presented. The extruded preforms with precise geometrical features, an excellent surface quality and no crystallization were achieved in the temperatures range from 344 to 360 °C and at ram speeds ranging from 0.002 to 0.01 mm/s. Discrete shear bands were observed in the preforms, increasing in number and/or becoming better defined with increasing load and ram speed. Fewer shear bands were present when increasing the extrusion temperature from 344 to 360 °C. Thus, subsequent extrudates were successfully fabricated free of shear bands, providing good optical homogeneity that yielded solid and holey fibers that could provide much improved optical performance. 相似文献
90.
水基恒流变钻井液是一种适用于深水钻井作业的新型工作流体,目前关于该体系的报道较少。通过对一定温度压力下钻井液性能的检测以及流变模型分析,研究了水基恒流变钻井液的流变行为,并初步探索了恒流变机理。结果表明,在0.1~35.4 MPa范围内,当温度从4℃升高到65℃,黏度计读数φ6/φ3、动切力、塑性黏度等流变参数的变化幅度较小,分别在10~13、9~12、13~18 Pa及15~22 mPa·s范围内,且φ6与φ3读数随温度呈“U”型分布;在温度压力组合条件下,拟合经验流变方程的相关性排序为:宾汉塑性≈幂律<卡森≈赫-巴≈罗-斯模型,其中双参数卡森模型的相关系数较高,且表达式简洁,适于描述水基钻井液的恒流变特性;以卡森模型为初始方程,引入T/P因子建立了高预测精度的动力学流变方程f(T,P,γ),相对误差平均值为7.19%±4.07%,偏差极大值集中在100( r/min)/65℃;分析了关键处理剂的分子形貌、结构及其与黏土片层的缔合作用,提出了基于分子形态的定性构效假设,揭示水基钻井液的流变稳定性本质。 相似文献