全文获取类型
收费全文 | 310328篇 |
免费 | 4217篇 |
国内免费 | 712篇 |
专业分类
电工技术 | 6199篇 |
综合类 | 560篇 |
化学工业 | 46890篇 |
金属工艺 | 10333篇 |
机械仪表 | 8831篇 |
建筑科学 | 7953篇 |
矿业工程 | 863篇 |
能源动力 | 8590篇 |
轻工业 | 31156篇 |
水利工程 | 2536篇 |
石油天然气 | 3672篇 |
武器工业 | 8篇 |
无线电 | 39769篇 |
一般工业技术 | 58590篇 |
冶金工业 | 57049篇 |
原子能技术 | 5463篇 |
自动化技术 | 26795篇 |
出版年
2021年 | 2283篇 |
2019年 | 2153篇 |
2018年 | 3624篇 |
2017年 | 3459篇 |
2016年 | 3551篇 |
2015年 | 2560篇 |
2014年 | 4453篇 |
2013年 | 14329篇 |
2012年 | 7381篇 |
2011年 | 10135篇 |
2010年 | 7917篇 |
2009年 | 9151篇 |
2008年 | 9640篇 |
2007年 | 9601篇 |
2006年 | 8528篇 |
2005年 | 7913篇 |
2004年 | 7781篇 |
2003年 | 7525篇 |
2002年 | 7350篇 |
2001年 | 7556篇 |
2000年 | 7218篇 |
1999年 | 7571篇 |
1998年 | 17770篇 |
1997年 | 12914篇 |
1996年 | 10142篇 |
1995年 | 7891篇 |
1994年 | 7226篇 |
1993年 | 6937篇 |
1992年 | 5299篇 |
1991年 | 5075篇 |
1990年 | 4949篇 |
1989年 | 4795篇 |
1988年 | 4682篇 |
1987年 | 3893篇 |
1986年 | 4012篇 |
1985年 | 4728篇 |
1984年 | 4288篇 |
1983年 | 4030篇 |
1982年 | 3607篇 |
1981年 | 3770篇 |
1980年 | 3465篇 |
1979年 | 3429篇 |
1978年 | 3230篇 |
1977年 | 3832篇 |
1976年 | 4872篇 |
1975年 | 2789篇 |
1974年 | 2665篇 |
1973年 | 2686篇 |
1972年 | 2217篇 |
1971年 | 1965篇 |
排序方式: 共有10000条查询结果,搜索用时 31 毫秒
921.
R Spanagel SM H?lter K Allingham R Landgraf W Zieglg?nsberger 《Canadian Metallurgical Quarterly》1996,305(1-3):39-44
Acamprosate (calcium-acetyl homotaurinate) is a new compound in the treatment of alcoholism. Its efficacy has been proven in several clinical trials and registration is now pending in most European countries. The basic mechanisms by which acamprosate elicits its anti-craving action, thereby leading to reduced relapse rates, is not known at the moment. In the present study we describe a rat model of long-term alcohol-drinking which mimics relapse behavior in human alcoholics. The effect of acamprosate was studied in this model. Wistar rats had a free choice between water and alcohol solutions of different concentrations (5, 10, 20% v/v). After two months of continuous alcohol access, rats were deprived of alcohol for three days. Following this deprivation phase, all alcohol solutions were presented again. This procedure was repeated monthly for the following six months. The rats consumed 3.5 +/- 0.3 g/kg alcohol a day. After alcohol deprivation, alcohol intake rose to 5.2 +/- 0.3 g/kg per day resulting in blood alcohol levels of 30 +/- 6 mg/dl. Interestingly, the addition of quinine to the alcohol solutions or the additional presentation of a 5% sucrose solution did not affect the alcohol-deprivation effect after eight months of this intermittent alcohol exposure. However, when acamprosate (50-200 mg/kg i.p.) was administered twice daily, alcohol-drinking following an alcohol-deprivation phase was decreased dose dependently. Given at the highest dose alcohol intake even dropped significantly below baseline drinking. Together, these results show that acamprosate effectively diminishes the alcohol-deprivation effect. Furthermore, the described model seems to be a suitable animal model to screen compounds for their anti-relapse properties and subsequently for their anti-craving action. 相似文献
922.
923.
The constant modulus (CM) array is a blind adaptive beamformer capable of recovering a narrowband signal among several cochannel sources without using a pilot or training signal. It is a conventional weight-and-sum adaptive beamformer whose weights are updated by the constant modulus algorithm. An adaptive signal canceller follows the beamformer to remove the captured signal from the array input and to provide an estimate of its direction vector. Based on a Wiener model, we investigate the steady-state properties of the CM array and the signal canceller. For mutually uncorrelated sources and noise, it is shown that the signal canceller exactly removes the source captured by the array. Thus, identical stages of the CM array and signal canceller may be used in a multistage system to recover several cochannel sources. Computer simulations are presented to verify the analytical results and to illustrate the transient behavior of the system 相似文献
924.
Exchange and oxidation of C16O were investigated at 450°C on 18O-predosed Rh and Pt catalysts supported on A12O3, CeO2 and CeO2-Al2O3. In all cases, a rapid exchange of C16O with the surface can be observed. CO oxidation leads to C16O2, C16O18O and C18O2. Significant formation of C16O2 is due to the relatively high 16O coverage in reaction resulting from the C16O exchange and from an exchange between O surface species and 16O internal atoms. Hydrogen is also formed via a water-gas shift reaction (CO + surface OH) in higher proportion on CeO2-containing catalysts than on A12O3. Chlorine inhibits all the reactions (exchange, oxidation and WGS) and particularly the internal exchange. 相似文献
925.
V. N. Antsiferov V. G. Gilev A. G. Lanin R. M. Yakushev 《Refractories and Industrial Ceramics》1996,37(11):382-386
Results obtained in a study of factors that affect shaping and sintering of highly porous materials are presented. A technology providing highly porous silicon nitride materials that is based on shaping preliminarily fabricated pieces of thermoplastic slip makes it possible to use initial silicon powders of different dispersities. Highly porous materials can also be produced on the basis of sialons and exhibit a diminished thermal conductivity and a heat resistance at the level of similar silicon nitride materials.Translated from Ogneupory i Tekhnicheskaya Keramika, No. 11, pp. 8 – 13, November, 1996. 相似文献
926.
Wood A.C.G. O'Neill A.G. Phillips P.J. Biswas R.G. Whall T.E. Parker E.H.C. 《Electron Devices, IEEE Transactions on》1993,40(1):157-162
Delta-doped MOSFETs have been fabricated in MBE-grown silicon using for the first time boron as the dopant within the delta layer. Current-voltage characteristics have been measured, and secondary ion mass spectrometry (SIMS) is used to confirm the location of the delta layer and the extent of layer broadening by diffusion during processing. Precise threshold voltages of the devices are difficult to determine since the devices (which all operate in depletion mode) take several volts to switch off. Transconductances of the devices have been measured, and it is shown how analysis of these results can yield estimates of the carrier mobility for transport along the delta layers despite the uncertainty in the threshold voltage. A clear transition is observed in the results which is attributed to the formation of a parasitic surface-channel field-effect transistor, providing conclusive evidence that the devices are conducting along a delta channel for part of the measured range of applied gate biases 相似文献
927.
Presents a new one-memory quaternary four-state trellis code, and its four-state binary equivalent code based on a QAM constellation which is 0.52 dB better than the four-state Ungerboeck code.<> 相似文献
928.
tRNA-guanine transglycosylases (TGT) are enzymes involved in the modification of the anticodon of tRNAs specific for Asn, Asp, His and Tyr, leading to the replacement of guanine-34 at the wobble position by the hypermodified base queuine. In prokaryotes TGT catalyzes the exchange of guanine-34 with the queuine (.)precursor 7-aminomethyl-7-deazaguanine (preQ1). The crystal structure of TGT from Zymomonas mobilis was solved by multiple isomorphous replacement and refined to a crystallographic R-factor of 19% at 1.85 angstrom resolution. The structure consists of an irregular (beta/alpha)8-barrel with a tightly attached C-terminal zinc-containing subdomain. The packing of the subdomain against the barrel is mediated by an alpha-helix, located close to the C-terminus, which displaces the eighth helix of the barrel. The structure of TGT in complex with preQ1 suggests a binding mode for tRNA where the phosphate backbone interacts with the zinc subdomain and the U33G34U35 sequence is recognized by the barrel. This model for tRNA binding is consistent with a base exchange mechanism involving a covalent tRNA-enzyme intermediate. This structure is the first example of a (beta/alpha)-barrel protein interacting specifically with a nucleic acid. 相似文献
929.
Rothschild M. Forte A.R. Horn M.W. Kunz R.R. Palmateer S.C. Sedlacek J.H.C. 《IEEE journal of selected topics in quantum electronics》1995,1(3):916-923
The trend in microelectronics toward printing features 0.25 μm and below has motivated the development of lithography at the 193-nm wavelength of argon fluoride excimer lasers. This technology is in its early stages, but a picture is emerging of its strengths and limitations. The change in wavelength from 248 to 193 nm will require parallel progress in projection systems, optical materials, and photo-resist chemistries and processes. This paper reviews the current status of these various topics as they have been engineered under a multiyear program at MIT Lincoln Laboratory 相似文献
930.
D Pende R Biassoni C Cantoni S Verdiani M Falco C di Donato L Accame C Bottino A Moretta L Moretta 《Canadian Metallurgical Quarterly》1996,184(2):505-518
Human natural killer (NK) cells express inhibitory receptors that are specific for different groups of HLA-C or HLA-B alleles. The majority of these receptors belong to the immunoglobulin (Ig) superfamily and are characterized by two or three extracellular Ig-like domains. Here we describe a novel inhibitory NK receptor that is specific for a group of HLA-A alleles. The HLA-A3-specific NK cell clone DP7 has been used for mice immunization. Two mAbs, termed Q66 and Q241, bound to the immunizing clone and stained only a subset of NK cell populations or clones. Among Q66 mAb-reactive clones, we further selected those that did not express any of the previously identified HLA-class I-specific NK receptors. These clones did not lyse HLA-A3+ (or -A11+) target cells, but lysis of these targets could be detected in the presence of Q66 or Q241 mAbs. On the other hand, target cells expressing other HLA-A alleles, including -A1, -A2, and -A24, were efficiently lysed. Moreover, none of the HLA-C or HLA-B alleles that were tested exerted a protective effect. Q66+, but not Q66- NK cell clones, expressed messenger RNA coding for a novel 3 Ig domain protein homologous to the HLA-C (p58) and HLA-B (p70) receptors. The corresponding cDNA (cl.1.1) was used to generate transient and stable transfectants in COS7 and NIH3T3 cell lines, respectively. Both types of transfectants were specifically stained by Q66 and Q241 mAbs. Since the cytoplasmic tail of Q66-reactive molecules was at least 11 amino acid longer than the other known p58/p70 molecules, we could generate an antiserum specific for the COOH-terminus of Q66-reactive molecules, termed PGP-3. PGP-3 immunoprecipitated, only from Q66+ NK cells, molecules displaying a molecular mass of 140 kD, under nonreducing conditions, which resolved, under reducing conditions, in a 70-kD band. Thus, differently from the other p58/p70 receptors, Q66-reactive molecules appear to be expressed as disulphide-linked dimers and were thus termed p140. The comparative analysis of the amino acid sequences of p58, p70, and p140 molecules revealed the existence of two cysteins proximal to the transmembrane region, only in the amino acid sequence of p140 molecules. 相似文献