全文获取类型
收费全文 | 272184篇 |
免费 | 2907篇 |
国内免费 | 295篇 |
专业分类
电工技术 | 4359篇 |
综合类 | 128篇 |
化学工业 | 45417篇 |
金属工艺 | 15514篇 |
机械仪表 | 10525篇 |
建筑科学 | 5457篇 |
矿业工程 | 3854篇 |
能源动力 | 4480篇 |
轻工业 | 11527篇 |
水利工程 | 4304篇 |
石油天然气 | 13072篇 |
武器工业 | 28篇 |
无线电 | 21924篇 |
一般工业技术 | 67037篇 |
冶金工业 | 36538篇 |
原子能技术 | 11325篇 |
自动化技术 | 19897篇 |
出版年
2021年 | 3292篇 |
2019年 | 3148篇 |
2018年 | 5949篇 |
2017年 | 6163篇 |
2016年 | 6656篇 |
2015年 | 3596篇 |
2014年 | 6047篇 |
2013年 | 11451篇 |
2012年 | 8435篇 |
2011年 | 10382篇 |
2010年 | 8333篇 |
2009年 | 9096篇 |
2008年 | 9287篇 |
2007年 | 8895篇 |
2006年 | 7262篇 |
2005年 | 6294篇 |
2004年 | 6199篇 |
2003年 | 5943篇 |
2002年 | 5677篇 |
2001年 | 5366篇 |
2000年 | 5286篇 |
1999年 | 4645篇 |
1998年 | 7901篇 |
1997年 | 6364篇 |
1996年 | 4843篇 |
1995年 | 4053篇 |
1994年 | 3791篇 |
1993年 | 3977篇 |
1992年 | 3475篇 |
1991年 | 3546篇 |
1990年 | 3742篇 |
1989年 | 3623篇 |
1988年 | 3472篇 |
1987年 | 3474篇 |
1986年 | 3376篇 |
1985年 | 3572篇 |
1984年 | 3469篇 |
1983年 | 3361篇 |
1982年 | 3193篇 |
1981年 | 3232篇 |
1980年 | 3233篇 |
1979年 | 3443篇 |
1978年 | 3649篇 |
1977年 | 3640篇 |
1976年 | 4135篇 |
1975年 | 3366篇 |
1974年 | 3413篇 |
1973年 | 3447篇 |
1972年 | 3078篇 |
1971年 | 2817篇 |
排序方式: 共有10000条查询结果,搜索用时 156 毫秒
981.
982.
A microdialysis flow cell has been developed for time-resolved Raman spectroscopy of biological macromolecules and their assemblies. The flow cell permits collection of Raman spectra concurrent with the efflux of small solute molecules into a solution of macromolecules and facilitates real-time spectroscopic detection of structural transitions induced by the effluent. Additionally, the flow cell is well suited to the investigation of hydrogen-isotope exchange phenomena that can be exploited as dynamic probes of viral protein folding and solvent accessibility along the assembly pathway. Here, we describe the application of the Raman dynamic probe to the maturation of the icosahedral capsid of bacteriophage P22, a double-stranded DNA virus. The P22 virion is constructed from a capsid precursor (procapsid) consisting of 420 coat subunits (gp5) in an outer shell and a few hundred scaffolding subunits (gp8) within. Capsid maturation involves expulsion of scaffolding subunits coupled with shell expansion at the time of DNA packaging. Raman static and dynamic probes reveal that the scaffolding subunit is highly alpha-helical and highly thermolabile, and lacks a typical hydrophobic core. When bound within the procapsid, the alpha-helical fold of gp8 is thermostabilized; however, this stabilization confers no apparent protection against peptide NH-->ND exchange. A molten globule model is proposed for the native scaffolding subunit that functions in procapsid assembly. Accompanying capsid expansion, a small conformational change (alpha-helix-->beta-strand) is also observed in the coat subunit. Domain movement mediated by hinge bending is proposed as the mechanism of capsid expansion. On the basis of these results, a molecular model is proposed for assembly of the P22 procapsid. 相似文献
983.
Electron transport and heating in a two-dimensional semiconductor superlattice with arbitrary orientation of the field (current)
relative to the crystallographic axes is investigated in the approximation of constant energy and velocity relaxation times
with electron-electron collisions taken into account. The nonlinear conductivity, average electron energies, and interrelationship
between field direction and the current excited by it in an unbounded superlattice, as well as the current-voltage characteristics
and the transverse electromotive force of a superlattice of finite width are found.
Fiz. Tekh. Poluprovodn. 31, 308–310 (March 1997) 相似文献
984.
The development and creating of new-generation full-scope simulator and new technology of simulation
N. N. Ponomarev-Stepnoi V. A. Lebedev M. M. Khudiykov S. D. Malkin V. V. Shalia I. D. Rakitin 《Nuclear Engineering and Design》1997,173(1-3)
Set out is a brief account of the two major accomplishments by the Russian Research Center ‘Kurchatov Institute’ in creating the full-scope simulators and mathematical modeling technologies. Presented are the basic specifications of one of the world's largest simulators—the full-scope simulator for the Leningrad NPP which is the new-generation one. Owing to the extended modeling scope accomplished is the possibility of training personnel to act in terms of not only the design-basis but rather beyond the design-basis accidents. To minimize the expenditures for creating the simulators, analyzers and other modeling and control means, the RRC ‘Kurchatov Institute' has created the unique technology of mathmodeling automation. Thanks to its versatility and application at its creation of the ELUD philosophy (easy to learn, use and develop) good use is made of this technology both in nuclear and thermal power engineering, as well as in gas industry. 相似文献
985.
A. V. Kurdyumov V. F. Britun V. B. Zelyavskii S. N. Gromyko T. S. Bartnitskaya L. A. Lyudvinskaya 《Powder Metallurgy and Metal Ceramics》1997,36(7-8):425-429
The structure of SiC–AlN powders is investigated by x-ray diffraction and transmission electron microscopy methods. The powders
were produced by joint carbon reduction and nitriding of silicon and aluminum oxide mixtures. The results show that a mixture
of solid solutions forms during joint SiC and AlN synthesis at 1700°C, with SiC forming β (3C) and α (2H) modifications with
different grain morphology. The fiber form is characteristic of β-SiC, whereas the grains of the solid solution based on SiC
have a predominantly equiaxed form. α-SiC grain dimensions are considerablys smaller than those of AlN.
Institute of Materials Science, National Academy of Sciences of Ukraine. Kiev Translated from Poroshkovaya Metallurgiya, Nos.
7–8, pp. 81–86, July–August, 1997. 相似文献
986.
Kang W.P. Wisitsora-at A. Davidson J.L. Kerns D.V. 《Electron Device Letters, IEEE》1998,19(10):379-381
A boron-doped diamond field emitter diode with ultralow turn-on voltage and high emission current is reported. The diamond field emitter diode structure with a built-in cap was fabricated using molds and electrostatic bonding techniques. The emission current versus anode voltage of the capped diamond emitter diode with boron doping, sp2 content, and vacuum thermal electric (VTE) treatment shows a very low turn-on voltage of 2 V. A high emission current of 1 μA at an anode voltage of less than 10 V can be obtained from a single diamond tip. The turn-on voltage is significantly lower than comparable silicon field emitters 相似文献
987.
V Darsonval V Berthet P Hubault C Crouzet L Fernandez 《Canadian Metallurgical Quarterly》1997,42(6):594-602
Enucleation frequently and progressively causes an enophtalmus and atrophia of the inferior eyelid, thereby leading to a height deficiency. Buccal mucous grafts give rise to phenomena such as secondary retraction. This may have complex and painful post-operative outcomes. However, when a septal chondromucous graft is performed, the height in the inferior palpebral part becomes more aesthetic, more retentive and quickly allows the wearing of a more voluminous prosthesis. Thus, the notinable enophtalmus can be corrected and the aesthetic quality of the looking is substantially restored. The authors report this surgical procedure and the results obtained with 21 patients which appear to be particularly promising. 相似文献
988.
989.
SM Kornblau E Estey T Madden HT Tran S Zhao U Consoli V Snell G Sanchez-Williams H Kantarjian M Keating RA Newman M Andreeff 《Canadian Metallurgical Quarterly》1997,15(5):1796-1802
PURPOSE: Expression of the multidrug resistance gene (MDR1) p170 protein is frequent in leukemic blasts from patients with relapsed acute myelogenous leukemia (AML). A phase I study using the nonimmunosuppressive MDR1 blocker SDZ PSC-833 (PSC) in combination with mitoxantrone (MITO) and etoposide (VP) was performed. PATIENTS AND METHODS: Starting doses (LVL0) of MITO (3.25 mg/m2/d on days 1 and 3 to 6) and VP (210 mg/m2/d on days 1 and 3 to 5) were 40% of the maximal-tolerated dose (MTD) from a prior study. A 1.5-mg/kg loading dose of PSC was followed by a 120-hour continuous infusion of 10 mg/kg/d on days 2 to 6. Blood samples for PSC, MITO, and VP pharmacokinetics (PK) were taken on days 1 and 3, and samples for MDR1 expression were taken on day 0. RESULTS: Severe mucositis developed in all patients at LVL0; therefore, MITO and VP doses were reduced to 2.5 and 170 mg/m2 (LVL-1) for the next seven patients, and this dose proved to be MTD. All LVL0 and three LVL-1 patients had transient elevations in the serum bilirubin level to > or = 4 mg/dL. Serum creatinine level increased to greater than 2 mg/dL in one case. There were no other grade 3 or 4 nonhematologic toxicities observed. The peripheral blood was cleared of leukemia in three LVL0 and four LVL-1 patients. The marrow was cleared of leukemic cells in one LVL0 and five LVL-1 patients, and a significant reduction in marrow leukemic infiltrate was observed in eight of 10. No patient achieved complete remission (CR), and all died of progressive disease (n = 8) or infection (n = 2). MDR1 expression was detected by fluorescent-activated cell sorter (FACS) analysis in five of seven cases. An elevated MDR1 mRNA level was detected by quantitative polymerase chain reaction (Q-PCR) in six of eight cases studied. Clearing of leukemia cells from the marrow occurred in four of six MDR1-positive and one of three MDR1-negative patients. Despite the fact that LVL0 doses had to be reduced due to toxicity, coadministration of PSC did not produce a consistent effect on MITO PK; however, it did repeatedly lead to increased levels of VP in the serum. CONCLUSION: We conclude that PSC-MITO-VP is a tolerable regimen with antileukemic activity. Addition of PSC necessitated a 66% reduction in MITO and VP doses from a prior study without PSC. 相似文献
990.
Jean-Louis Iehl Rodolphe Grac Luc le Gratiet Véronique Bardinal Rainer Buhleier Elena Bedel-Pereira Chantal Fontaine Michel Pugnet Jacques-Henry Collet 《电信纪事》1997,52(11-12):627-637
We study the transient gratings photogenerated in the picosecond regime in three families of structures, namely : - structures of thickness in the order of one micron, including quantum wells (GaAs/GaAlAs, CdTe/ CdZnTe). A transmission modulation due to the electric field has been observed. We show that, in accordance with our calculations, this modulation is screened faster than 10 ps at a fluence of a few µJ/cm2. - A structure including GalnAs/GalnAsP MQWS in a cavity. This structure shows a top diffraction efficiency of 2.5 × 10-2 at 1.55 µm for an energy of excitation in the order of 100 µJ/cm2. The diffraction efficiency exhibits several oscillations due to Fabry-Pårot effects. By introducing cavity effects in our model, we show that the diffraction efficiency is amplified by more than a factor 2 with respect to the no-cavity case. Calculations show that the diffraction efficiency may reach 6 × 10-2 around 1.625 µm, for a front mirror reflectivity of 90 %. - Structures including bulk GaAs microcavities. The risetime is lower or in the order of 1 ps while the diffraction efficiency attains 1 %, with an average power of 4 mW (i.e. an energy of 2 µJ/cm2/pulse), compatible with a commutation of packets at 80 MHz. 相似文献