全文获取类型
收费全文 | 15777篇 |
免费 | 213篇 |
国内免费 | 23篇 |
专业分类
电工技术 | 189篇 |
综合类 | 6篇 |
化学工业 | 2004篇 |
金属工艺 | 362篇 |
机械仪表 | 338篇 |
建筑科学 | 512篇 |
矿业工程 | 60篇 |
能源动力 | 308篇 |
轻工业 | 1215篇 |
水利工程 | 90篇 |
石油天然气 | 119篇 |
无线电 | 1509篇 |
一般工业技术 | 2334篇 |
冶金工业 | 5411篇 |
原子能技术 | 159篇 |
自动化技术 | 1397篇 |
出版年
2021年 | 101篇 |
2019年 | 102篇 |
2018年 | 136篇 |
2017年 | 101篇 |
2016年 | 94篇 |
2015年 | 107篇 |
2014年 | 224篇 |
2013年 | 569篇 |
2012年 | 305篇 |
2011年 | 407篇 |
2010年 | 333篇 |
2009年 | 336篇 |
2008年 | 450篇 |
2007年 | 439篇 |
2006年 | 433篇 |
2005年 | 386篇 |
2004年 | 325篇 |
2003年 | 363篇 |
2002年 | 309篇 |
2001年 | 321篇 |
2000年 | 314篇 |
1999年 | 408篇 |
1998年 | 1555篇 |
1997年 | 958篇 |
1996年 | 671篇 |
1995年 | 470篇 |
1994年 | 417篇 |
1993年 | 440篇 |
1992年 | 277篇 |
1991年 | 246篇 |
1990年 | 266篇 |
1989年 | 217篇 |
1988年 | 222篇 |
1987年 | 212篇 |
1986年 | 197篇 |
1985年 | 252篇 |
1984年 | 208篇 |
1983年 | 208篇 |
1982年 | 203篇 |
1981年 | 178篇 |
1980年 | 195篇 |
1979年 | 130篇 |
1978年 | 135篇 |
1977年 | 251篇 |
1976年 | 352篇 |
1975年 | 136篇 |
1974年 | 125篇 |
1973年 | 107篇 |
1972年 | 78篇 |
1970年 | 83篇 |
排序方式: 共有10000条查询结果,搜索用时 15 毫秒
41.
Ronald Smith 《Canadian Metallurgical Quarterly》2004,130(2):165-169
For longitudinally uniform stretches of waterways there is a mixing center for the across-channel location of a steady point source in steady flow, such that complete mixing is achieved as soon as possible and there is no concentration overshoot at either of the two shorelines. A mathematical definition of the mixing center is the zero of the first oscillatory cross-channel diffusion mode. With the shorelines plus four interior data points across the channel, the starting estimate for the mixing center suffices to keep peak shoreline concentrations to within 6% of optimal. For comparison, a source at mid flow gives 18% shoreline concentration overshoot in the test case. Should very high precision be required, the Appendix gives an iterative construction that converges to the first oscillatory diffusion mode. 相似文献
42.
Andre C.L. Carlin J.A. Boeckl J.J. Wilt D.M. Smith M.A. Pitera A.J. Lee M.L. Fitzgerald E.A. Ringel S.A. 《Electron Devices, IEEE Transactions on》2005,52(6):1055-1060
High-performance p/sup +//n GaAs solar cells were grown and processed on compositionally graded Ge-Si/sub 1-x/Ge/sub x/-Si (SiGe) substrates. Total area efficiencies of 18.1% under the AM1.5-G spectrum were measured for 0.0444 cm/sup 2/ solar cells. This high efficiency is attributed to the very high open-circuit voltages (980 mV (AM0) and 973 mV (AM1.5-G)) that were achieved by the reduction in threading dislocation density enabled by the SiGe buffers, and thus reduced carrier recombination losses. This is the highest independently confirmed efficiency and open-circuit voltage for a GaAs solar cell grown on a Si-based substrate to date. Larger area solar cells were also studied in order to examine the impact of device area on GaAs-on-SiGe solar cell performance; we found that an increase in device area from 0.36 to 4.0 cm/sup 2/ did not degrade the measured performance characteristics for cells processed on identical substrates. Moreover, the device performance uniformity for large area heteroepitaxial cells is consistent with that of homoepitaxial cells; thus, device growth and processing on SiGe substrates did not introduce added performance variations. These results demonstrate that using SiGe interlayers to produce "virtual" Ge substrates may provide a robust method for scaleable integration of high performance III-V photovoltaics devices with large area Si wafers. 相似文献
43.
44.
The author discusses the concept that data sampled beyond the Nyquist frequency is meaningless. A simple signal processing scheme to extend resolution to nearly twice the Nyquist without the problem of alias emerged. It starts by placing one-dimensional signals into a simulated sample-and-hold process in a Mathcad application 相似文献
45.
To examine the separate effects of viscosity and sweetness on astringency, aqueous solutions of grape seed tannin (GST) were thickened with carboxymethyl cellulose (CMC) from 2 to 45 cP (experiment 1) or sweetened with 0 to 1.8 g/L aspartame (experiment 2). Trained subjects continuously rated astringency and bitterness in duplicate. Subjects were categorized by the salivary flow induced by citric acid and ability to taste n-propyl thiouracil (PROP). In experiment 1, maximum intensity and total duration of astringency were significantly decreased as viscosity rose, although time to maximum intensity of astringency was not affected. Maximum intensity and total duration of bitterness were not significantly affected by increasing viscosity; however, the onset of bitterness was significantly delayed. In experiment 2, increasing sweetness had no affect on any astringency parameter, although maximum intensity of bitterness was significantly decreased. Neither PROP nor salivary flow-status had any effect on perception of bitterness or astringency in either experiment. 相似文献
46.
D. P. Papadopoulos D. V. Bandekas J. R. Smith 《Electrical Engineering (Archiv fur Elektrotechnik)》1992,75(3):215-222
Contents The control-canonical-form (CCF) method is introduced for the purpose of applying it to high-order state-space single-input single or multiple-output (SISO or SIMO) and multiple-input multiple-output (MIMO) linear time-invariant system models to determine closed-loop systems (i.e. design robust controllers) from which enchanced dynamic stability characteristics and overall performance of these systems is achieved. The method has been applied successfully to a SISO 6th-order linearized state-space open-loop model (representing a simplified hydro power system), and to a MIMO 8th-order linearized state-space open-loop model (representing an 87.5 kVA synchronous machine with conventional exciter supplying power to an electric utility system through an interconnection network)
Anwendung der kanonischen Form der Zustandsrückführung auf Erzeugersysteme zur Verbesserung der dynamischen Stabilitätseigenschaften
Übersicht Die Methode der kanonischen Form wird auf lineare zeitinvariante Systeme höherer Ordnung im Zustandsraum angewendet, die eine oder mehrere Eingangsgrößen bei einer oder mehreren Ausgangsgrößen aufweisen. Damit werden robuste Regler entworfen, welche die dynamische Stabilität und das gesamte Betriebsverhalten verbessern. Die Methode wurde erfolgreich auf ein linearisiertes Modell sechster Ordnung mit einem Eingang und einem Ausgang angewendet, das vereinfachend eine Wasserkraftanlage darstellt; außerdem auf ein linearisiertes Zustandsmodell achter Ordnung einer Synchronmaschine 87,5kVA mit konventioneller Erregung, die über ein Verbindungsnetzwerk in das öffentliche Netz einspeist.相似文献
47.
JL Alcorn ME Smith JF Smith LR Margraf CR Mendelson 《Canadian Metallurgical Quarterly》1997,17(6):672-682
Studies of the regulation of surfactant lipoprotein metabolism and secretion and surfactant protein gene expression have been hampered by the lack of a cell culture system in which the phenotypic properties of type II cells are maintained. We have developed a primary culture system that facilitates the maintenance of a number of morphologic and biochemical properties of type II pneumonocytes for up to 2 wk. Cells were isolated by collagenase digestion of midgestation human fetal lung tissue that had been maintained in organ culture in the presence of dibutyryl cyclic AMP (Bt2cAMP) for 5 days. The isolated cells were enriched for epithelial components by treatment with DEAE-dextran, plated on an extracellular matrix (ECM) derived from Madin-Darby canine kidney (MDCK) cells, and incubated at an air/liquid interface in a minimal amount of culture medium containing Bt2cAMP. The cell cultures were comprised of islands of round epithelial-like cells containing numerous dense osmiophilic granules, surrounded by sparse spindle-shaped cells with the appearance of fibroblasts. Ultrastructural examination revealed that the osmiophilic granules had the appearance of lamellar bodies, the distinguishing feature of type II pneumonocytes. Additionally, the cultures maintained elevated levels of SP-A gene expression for up to 2 wk. The expression of mRNAs encoding SP-A, SP-B, and SP-C were regulated in the cultured cells by glucocorticoids and cyclic AMP in a manner similar to that observed in fetal lung tissue in organ culture. The differentiated phenotype was most apparent when the cells were cultured at an air/liquid interface. In order to utilize the cultured type II cells for study of the effects of overexpression of various proteins and for promoter analysis, it is of essence to transfect DNA constructs into these cells with high efficiency. Unfortunately, we found the cells to be refractory to efficient transfer of DNA using conventional methods (i.e., lipofection, electroporation, or calcium phosphate-mediated transfection). However, replication-defective recombinant human adenoviruses were found to provide a highly efficient means of introducing DNA into the type II pneumonocytes. Furthermore, we observed in type II cell-enriched cultures infected with recombinant adenoviruses containing the lacZ gene under control of a cytomegalovirus promoter, that beta-galactosidase was expressed uniformly in the islands of type II cells and surrounding fibroblasts. By contrast, in cultures infected with recombinant adenoviruses containing the human growth hormone (hGH) gene under control of the SP-A gene promoter and 5'-flanking region, hGH was expressed only in the type II cells. Thus, this culture system provides an excellent means for identifying genomic elements that mediate type II cell-specific gene expression. 相似文献
48.
Estimates of loudness balance were obtained for acoustically and electrically presented 250 Hz sine signals from a patient who uses the Ineraid multichannel cochlear implant. Acoustic and electric loudness matching was possible because the patient evidenced a 25 dB HL threshold at 250 Hz in his nonimplanted ear. The level of the electrical stimulus in microamperes required for a balance of loudness grew linearly with equal increments in decibels for the acoustic stimulus. These data, in concert with the very limited data from previous studies, provide a rationale for using a logarithmic transformation of acoustic to electric intensity in signal processors for cochlear implants. 相似文献
49.
The alignment of six fluorescent materials (p-terphenyl, p-quaterphenyl, p-quinquephenyl, diphenylstilbene, 1,4-bis(2-methylstyryl)benzene, and 1,4-bis-2-(5-phenyloxazolyl)benzene) grown from the vapour phase on to friction-deposited, highly oriented poly(tetrafluoroethylene) (PTFE) films was studied. The dichroic ratio of the fluorescent films produced was determined by polarized spectroscopic fluorescence measurements, and the crystallographic orientation of these materials was determined by transmission electron microscopy. The measured dichroic ratios ranged from 1.7 for diphenylstilbene to 5.0 for the p-quaterphenyl film. Electron diffraction patterns revealed that five of the fluorescent materials grew epitaxially on the single crystal-like PTFE films, the fluorescent crystals adopting particular orientations with respect to the crystallographic planes of PTFE. On the other hand, crystals of 1,4-bis-2-(5-phenyloxazolyl) benzene aligned in a fibre pattern, apparently by a grapho-epitaxial mechanism. 相似文献
50.