全文获取类型
收费全文 | 196061篇 |
免费 | 3368篇 |
国内免费 | 1019篇 |
专业分类
电工技术 | 3750篇 |
综合类 | 645篇 |
化学工业 | 31671篇 |
金属工艺 | 8257篇 |
机械仪表 | 5797篇 |
建筑科学 | 5264篇 |
矿业工程 | 1057篇 |
能源动力 | 5219篇 |
轻工业 | 18365篇 |
水利工程 | 1930篇 |
石油天然气 | 3295篇 |
武器工业 | 29篇 |
无线电 | 21981篇 |
一般工业技术 | 35935篇 |
冶金工业 | 36141篇 |
原子能技术 | 4432篇 |
自动化技术 | 16680篇 |
出版年
2021年 | 1844篇 |
2019年 | 1616篇 |
2018年 | 2506篇 |
2017年 | 2463篇 |
2016年 | 2634篇 |
2015年 | 1946篇 |
2014年 | 3289篇 |
2013年 | 8783篇 |
2012年 | 5373篇 |
2011年 | 7316篇 |
2010年 | 5797篇 |
2009年 | 6645篇 |
2008年 | 6646篇 |
2007年 | 6591篇 |
2006年 | 5668篇 |
2005年 | 5248篇 |
2004年 | 5116篇 |
2003年 | 5000篇 |
2002年 | 5218篇 |
2001年 | 5083篇 |
2000年 | 4465篇 |
1999年 | 4512篇 |
1998年 | 11416篇 |
1997年 | 8102篇 |
1996年 | 6162篇 |
1995年 | 4620篇 |
1994年 | 3874篇 |
1993年 | 3869篇 |
1992年 | 2762篇 |
1991年 | 2661篇 |
1990年 | 2544篇 |
1989年 | 2554篇 |
1988年 | 2488篇 |
1987年 | 2217篇 |
1986年 | 2164篇 |
1985年 | 2492篇 |
1984年 | 2260篇 |
1983年 | 2091篇 |
1982年 | 1954篇 |
1981年 | 2028篇 |
1980年 | 1884篇 |
1979年 | 1864篇 |
1978年 | 1821篇 |
1977年 | 2195篇 |
1976年 | 2845篇 |
1975年 | 1605篇 |
1974年 | 1573篇 |
1973年 | 1652篇 |
1972年 | 1379篇 |
1971年 | 1289篇 |
排序方式: 共有10000条查询结果,搜索用时 15 毫秒
991.
F. Barlat S. Panchanadeeswaran O. Richmond 《Metallurgical and Materials Transactions A》1991,22(7):1525-1534
A simple model of flange behavior during cup drawing was used to predict the earing profile of deep-drawn cups. The relationship
between yield surface shape and earing tendency was established, with plane stress yielding corresponding to no hold-down
pressure on the flange and plane strain corresponding to no thickening. Using the Schmid law, the earing model was applied
to the case of a single crystal in cube position and compared to Tucker’s well-known results.[6] For the plane strain case, good agreement was obtained with the experiment; but for plane stress, the predicted profile did
not agree with the experimental one. Using the Taylor/ Bishop and Hill (TBH) theory[8,9] and measured crystallite orientation distribution functions (CODF), the model was applied to the case of high-purity aluminum
sheet with various cold-rolling reductions (35, 60, 80, and 90 pct). The major experimental trends were again correctly predicted
by the plane strain case. 相似文献
992.
A diagonalized multilevel fast multipole method with spherical harmonics expansion of the k-space Integrals 总被引:3,自引:0,他引:3
Diagonalization of the fast multipole method (FMM) for the Helmholtz equation is usually achieved by expanding the multipole representation in propagating plane waves. The resulting k-space integral over the Ewald sphere is numerically evaluated. Storing the k-space quadrature samples of the method of moments (MoM) basis functions constitutes a large portion of the overall memory requirements of the resulting algorithm for solving the integral equations of scattering and radiation problems. In this paper, it is proposed to expand the k-space representation of the basis functions by spherical harmonics in order to reduce the sampling redundancy introduced by numerical quadrature rules. Aggregations, plane wave translations, and disaggregations in the realized multilevel fast multipole method (MLFMM) are carried out using the k-space samples of a numerical quadrature rule. However, the incoming plane waves on the finest MLFMM level are expanded in spherical harmonics again. Thus, due to the orthonormality of spherical harmonics, the testing integrals for the individual testing functions are simplified into series over products of spherical harmonics expansion coefficients. Overall, the resulting MLFMM can save a considerable amount of memory without compromising accuracy and numerical speed. 相似文献
993.
Business, not technology leaders should manage the change of Information Technology (IT) from a self-contained business support unit of the sixties to a distributed strategic business function of the nineties. Top management should recognize that a good business plan can only be developed and executed through the partnership between the business unit managers and the managers of the financial, the personnel, and the IT resource units. The technical problems associated with the management of IT may again be the easier part of the total management challenge. 相似文献
994.
Isotherms of neon adsorbed on compressed exfoliated graphite were measured, using a standard volumetric method, in the temperature
interval of 12–24 K for the four first layers. The critical temperatures for the first three layers were determined to be
16.0±1.0, 19.0±1.0 K, and 18.0±1.0 K, respectively. From the isotherms, the isosteric heats of adsorption were calculated
as a function of coverage, indicating some interesting features, which may be attributed to possible phase transitions. The
isosteric heat of adsorption for the fourth layer on,Q
st/R=.275 K, is comparable to the three-dimensional latent heat of sublimation 256 K at the triple point. The binding energies
for the first, second, and third layers were obtained from the isosteric heats as V → 0, yielding 302.5, 237.5, and 227.5
K, respectively. The differential molar entropies and internal molar energies were also calculated as a function of coverage.
The possibility of a coexistence region with two solids having different structures in the phase diagram for the first layer
is discussed. Whenever possible, the results are compared with theoretical evaluations and previous experimental data. 相似文献
995.
Fadwa T. Eljack Ahmed F. Abdelhady Mario R. Eden Frederico B. Gabriel Xiaoyun Qin Mahmoud M. El-Halwagi 《Computers & Chemical Engineering》2005,29(11-12):2304
Decoupling the constitutive equations from the balance and constraint equations allows for reformulating a conventional forward problem into two reverse problems. The first reverse problem is the reverse of a simulation problem, where the process model is solved in terms of the constitutive (synthesis/design) variables instead of the process variables, thus providing the synthesis/design targets. The second reverse problem (reverse property prediction) solves the constitutive equations to identify unit operations, operating conditions and/or products by matching the synthesis/design targets. Visualization of the problem is achieved by employing recently developed property clustering techniques, which allows a high-dimensional problem to be visualized in two or three dimensions. The clusters by definition satisfy intra-stream and inter-stream conservation through linear “mixing” rules, which allows for the development of consistent additive rules along with their ternary representation. 相似文献
996.
Hawkins' isothermal model developed to study noise in bipolar junction transistors (BJTs) is modified to investigate bias-dependent noise in heterojunction bipolar transistors (HBTs) by incorporating thermal effects. It is shown that the inclusion of thermal effects into the high-frequency noise model of HBTs is necessary as the temperature of the device may become very different from the ambient temperature, especially at high bias current. Calculation of the noise figure by including the thermal effect shows that the isothermal calculation may underestimate the noise figure at high bias current. It is observed that noise at low bias is ideality factor n dependent whereas high bias noise is insensitive to the variation of n. Moreover, the common base current gain plays a major role in the calculation of the minimum noise figure. The excellent fit obtained between the theoretical calculation and the measured data are attributed to the inclusion of the bias-dependent junction heating as well as C/sub De/ and C/sub bc/ into the present calculation. 相似文献
997.
The Role of Network Topologies in the Optical Core of IP-over-WDM Networks with Static Wavelength Routing 总被引:3,自引:0,他引:3
Freire Mário M. Rodrigues Joel J.P.C. Coelho Rui M.F. 《Telecommunication Systems》2003,24(2-4):111-122
In this paper, we present a performance analysis of network topologies for the optical core of IP-over-WDM networks with static wavelength routing. The performance analysis is focused on regular degree four topologies, and, for comparison purposes, degree three topologies are also considered. It is shown that the increase of the nodal degree from three (degree three topology with smallest diameter) to four (degree four topology with smallest diameter) improves the network performance if a larger number of wavelengths per link is available. However, the influence of wavelength interchange on the nodal degree gain is small. The performance of regular degree four topologies with smallest diameter is also compared with the performance of mesh–torus topologies (which are also degree four topologies), and it is shown that the blocking probability of degree four topologies with smallest diameter is about two orders of magnitude lower than the blocking probability of mesh–torus topologies. It is also presented a performance comparison of WDM-based networks with nodal degrees ranging from two to five and it is shown that the increase of the nodal degree from two to three leads to high nodal degree gains, while de increase of the nodal degree from four to five leads to low nodal degree gains. These results show that degree three and degree four topologies are very attractive for use in the optical core of IP-over-WDM networks. 相似文献
998.
N. K. Acharya P. K. Yadav S. Wate Y. K. Vijay F. Singh D. Kavasthi 《Bulletin of Materials Science》2004,27(5):417-420
Ion irradiation of Si8+ ion beam of 100 MeV was scattered by a gold foil on a Mylar membrane of 25 Μm thickness in the form of film roll (width,
12.5 cm and length, 400 cm) at the Nuclear Science Centre, New Delhi. The characterization of etched nuclear tracks was carried
out by gas permeation measurements. The samples cut from the film roll of required size for permeability measurements were
etched in a controlled manner in a constant temperature bath of 6N NaOH solution. The opening of the conical etched tracks
was characterized by hydrogen gas permeation. 相似文献
999.
The inhibition effect of three organic additives on the precipitation and polymorphism of CaCO3 deposited on gold surfaces was investigated using electrochemical and microscopic techniques. Additives, two polyacrylic acid (PAA) polymers with different molecular weights (Mw 2100, Mw 30,000), and 1,2,4,5‐benzenetetracarboxylic acid (BTCA), were either added to the solution before or during deposition. In the presence of 100 ppm of one of the three additives in solution, almost no scale was observed on the surface for at least 24 hours. In the presence of lower concentrations of PAA Mw 2100, only distorted calcite crystals were obtained while with PAA Mw 30,000 the polymorph was spherical vaterite. A mixture of calcite and vaterite was observed with the BTCA additive. Addition of the polymers inhibits further nucleation and growth even if added after partial deposition of CaCO3 while BTCA has no effect once nucleation has started. The results indicate that the inhibit ion effect of the PAA polymers is due to adsorption on the electrode surface while the effect of BTCA is related to chelation of calcium ions in solution. 相似文献
1000.
T. H. Chang K. F. Pao S. H. Chen K. R. Chu 《Journal of Infrared, Millimeter and Terahertz Waves》2003,24(9):1415-1420
Self-consistent effects on the starting current of gyrotron oscillators are examined. Field profiles in the open cavity are shown to be sensitive to the interaction dynamics. This can either significantly raise or lower the oscillation threshold, particularly for the low-Q modes. The transition from resonant-mode oscillations at the low magnetic field to backward-wave oscillations at the high magnetic field is demonstrated. 相似文献