全文获取类型
收费全文 | 44857篇 |
免费 | 2647篇 |
国内免费 | 39篇 |
专业分类
电工技术 | 326篇 |
综合类 | 32篇 |
化学工业 | 8599篇 |
金属工艺 | 770篇 |
机械仪表 | 910篇 |
建筑科学 | 1421篇 |
矿业工程 | 85篇 |
能源动力 | 899篇 |
轻工业 | 6267篇 |
水利工程 | 419篇 |
石油天然气 | 186篇 |
武器工业 | 4篇 |
无线电 | 2575篇 |
一般工业技术 | 7833篇 |
冶金工业 | 10627篇 |
原子能技术 | 206篇 |
自动化技术 | 6384篇 |
出版年
2024年 | 83篇 |
2023年 | 408篇 |
2022年 | 417篇 |
2021年 | 1053篇 |
2020年 | 906篇 |
2019年 | 824篇 |
2018年 | 1685篇 |
2017年 | 1696篇 |
2016年 | 1793篇 |
2015年 | 1321篇 |
2014年 | 1697篇 |
2013年 | 3292篇 |
2012年 | 2785篇 |
2011年 | 2505篇 |
2010年 | 1964篇 |
2009年 | 1788篇 |
2008年 | 1987篇 |
2007年 | 1747篇 |
2006年 | 1326篇 |
2005年 | 1066篇 |
2004年 | 997篇 |
2003年 | 870篇 |
2002年 | 798篇 |
2001年 | 517篇 |
2000年 | 495篇 |
1999年 | 648篇 |
1998年 | 3390篇 |
1997年 | 2030篇 |
1996年 | 1314篇 |
1995年 | 781篇 |
1994年 | 666篇 |
1993年 | 652篇 |
1992年 | 219篇 |
1991年 | 234篇 |
1990年 | 225篇 |
1989年 | 221篇 |
1988年 | 200篇 |
1987年 | 171篇 |
1986年 | 184篇 |
1985年 | 218篇 |
1984年 | 186篇 |
1983年 | 132篇 |
1982年 | 167篇 |
1981年 | 176篇 |
1980年 | 155篇 |
1979年 | 131篇 |
1978年 | 118篇 |
1977年 | 199篇 |
1976年 | 419篇 |
1975年 | 89篇 |
排序方式: 共有10000条查询结果,搜索用时 0 毫秒
21.
E Messou SV Sangaré R Josseran C Le Corre J Guélain 《Canadian Metallurgical Quarterly》1997,90(1):44-47
BACKGROUND: Gastric sucrose permeability is a noninvasive marker that reliably increases in association with gastrointestinal injury due to use of nonsteroidal antiinflammatory drugs. Despite the effect of Helicobacter pylori infection on the gastric mucosa, in a previous study we were unable to demonstrate that H. pylori infection was associated with abnormal gastric sucrose permeability. Our goal in this study was to explore further whether H. pylori infection changed gastric permeability; therefore, we evaluated the effect of treatment of H. pylori infection on gastric permeability to sucrose and the relation of sucrose permeability to density of polymorphonuclear leukocytes. MATERIALS AND METHODS: Five hundred milliliters of a solution containing 100 gm of sucrose was ingested by the subject at bedtime. Overnight urine was collected and assayed for sucrose by high-performance liquid chromatography. Sucrose permeability was assessed both before and approximately 4 weeks after anti-H. pylori therapy. RESULTS: Seventeen asymptomatic H. pylori-infected volunteers participated; 8 were cured. Sucrose permeability was in the range commonly found in normal controls both before and after anti-H. pylori therapy (mean excretion, 76.3 mg; range, 13-171 mg). Gastric sucrose permeability correlated with the density of polymorphonulcear cell infiltration of the mucosa. Cure of the H. pylori infection was associated with a small but significant decrease in sucrose permeability (98.8 +/- 18 mg to 51.7 +/- 9.8 mg (p = .01). Sucrose permeability was greater in those with a high density of mucosal polymorphonuclear cells compared to those with lower scores (119.5 +/- 4 vs 71.4 +/- 13 for those with scores > or = 5 compared to scores < or = 4; p = .023). Failed therapy resulted in an increase in the mucosal density of polymorphonuclear infiltration and sucrose permeability (56.4 +/- 13 mg-99.7 +/- 19 mg pretreatment vs posttreatment, respectively; p = .031). CONCLUSION: H. pylori gastritis causes a small but measurable increase in gastric permeability to sucrose that may reflect epithelial transmigration of neutrophils. 相似文献
22.
R Jover J Llach JM Bordas A Mas JM Salmerón S Navarro J Terés 《Canadian Metallurgical Quarterly》1997,20(7):344-346
There are evidence indicating that endoscopic retrograde cholangiopancreatography with sphincterotomy (ERCP+S) reduces the rate of complications in patients with severe acute biliary pancreatitis (ABP). The aim of this study was to analyze our experience in the treatment of severe ABP with ERCP+S and compare the evolution of the patients treated early (3 days or less following admission) with those treated late (after the 3rd day). A retrospective review of the clinical histories of the 16 patients with severe ABP treated with ERCP+S over the last two years is presented, analyzing their clinical features on admission and evolution. Biliary stones or biliary sludge was observed in 11 cases (69%). Nine patients had undergone early, and 7 late, ERCP+S. The patients of the latter group presented a greater number of complications during evolution than the former group (43% vs 0%; p = 0.025). The mortality of the patients of the late ERCP+S was also greater although without statistical significance (28.6% vs 0%; p = 0.15). No complication secondary to ERCP+S was detected. These findings confirm the fact that ERCP+S performed early in severe ABP significantly reduces the number of complications and is associated with a lower mortality. 相似文献
23.
JC Métraux 《Canadian Metallurgical Quarterly》1997,117(5):419-423
The purpose of this study was to assess cortical and cancellous bone responses to unilateral limb immobilization and, subsequently, to remobilization with exercise, in a young adult canine model. Right forelimbs of 14 1-2-year old mongrel dogs were immobilized in a non-weight-bearing position by a bandage for 16 weeks. Six control dogs were untreated. At 16 weeks, seven immobilized and three control dogs were euthanized. The remaining seven immobilized dogs began a recovery protocol consisting of 16 weeks of kennel confinement (without the right forelimb bandaged) followed by 16 weeks of treadmill exercise conducted three times per week. These seven dogs and three control dogs were euthanized at 48 weeks. Bone mineral density of the proximal radii was determined with dual-energy X-ray absorptiometry and humeral middiaphyseal cross-sectional areas were determined with computed tomography. Humeri were tested in cranio-caudal three-point bending to failure. Cancellous bone cores from the lateral humeral condyles had wet apparent density determined and were tested to failure in compression. Mechanical properties, bone density, and cross-sectional areas were compared between immobilized (right forelimb), contralateral weight bearing (left forelimb), and control forelimbs with Kruskal-Wallis and post hoc tests. At 16 weeks, bone mineral density, cortical load, yield, and stiffness as well as cancellous bone failure stress, yield stress, and modulus were significantly lower (p < 0.02) for immobilized limbs than control limbs. Immobilized limb cancellous bone mechanical properties were 28%-74% of control values, and cortical bone mechanical properties were 71%-98% of control values. After 32 weeks of remobilization, cortical and cancellous bone mechanical properties were not different from control values except that cortical bone failure stress and modulus were significantly higher (p < 0.01) between remobilized and control limbs. In summary, 16 weeks of forelimb immobilization was associated with significantly lower mechanical properties, and with greater differences in cancellous than cortical bone properties. Mechanical properties were not different from control values after 32 weeks of recovery that included 16 weeks of treadmill exercise. 相似文献
24.
25.
26.
Frédéric Gaschet 《Papers in Regional Science》2002,81(1):63-81
This article examines the relationships between the employment suburbanisation from central cities towards their suburbs,
and the process of intra-urban specialisation that occurred simultaneously in the fifty largest French metropolitan areas.
A methodology is proposed to identify urban subcentres and to analyse the effects of the intra-urban specialisations on suburbanisation
patterns. We conclude that the specialisation of both subcentres and central cities has a significant effect on suburbanisation
rates. Lastly, an intra-metropolitan shift/share analysis provides additional insights into the employment dynamics of central
cities and suburbs during the last twenty years.
Received: 25 July 2000 / Accepted: 29 May 2001 相似文献
27.
We compare two link analysis ranking methods of web pages in a site. The first, called Site Rank, is an adaptation of PageRank to the granularity of a web site and the second, called Popularity Rank, is based on the frequencies of user clicks on the outlinks in a page that are captured by navigation sessions of users through
the web site. We ran experiments on artificially created web sites of different sizes and on two real data sets, employing
the relative entropy to compare the distributions of the two ranking methods. For the real data sets we also employ a nonparametric
measure, called Spearman's footrule, which we use to compare the top-ten web pages ranked by the two methods. Our main result
is that the distributions of the Popularity Rank and Site Rank are surprisingly close to each other, implying that the topology
of a web site is very instrumental in guiding users through the site. Thus, in practice, the Site Rank provides a reasonable
first order approximation of the aggregate behaviour of users within a web site given by the Popularity Rank. 相似文献
28.
The extinction coefficient ? at 2.9 μm for OH in fluoride glasses is measured by determining the amount of HF evolved during heating of the glass under steam and the corresponding intensity of the OH absorption band. This coefficient is respectively equal to 31 litre mole?1 cm?1 for BTYbZ glass (15 BaF2 - 29 ThF4 - 28 YbF3 - 28 ZnF2) and 19.5 litre mole?1 cm?1 for BALLA glass (34 BaF2 - 57 ZrF4 - 4 AlF3 - 5 LaF3). 相似文献
29.
Cypriano G da Trindade Neto Ana LP Fernandes Ana IB Santos Waldenice A Morais Marcos VM Navarro Tereza NC Dantas Mrcia R Pereira Jos LC Fonseca 《Polymer International》2005,54(4):659-666
Complexation of chitosan in aqueous solutions by low molecular weight electrolytes is one of the simplest methods for the preparation of aqueous chitosan dispersions. In this work, the influence of storage time, sulfate concentration, method of preparation and surfactant content on some properties of the resultant chitosan dispersions (turbidity, viscosity and zeta potential) was analyzed. Turbidimetry was adequate to monitor the formation of particles, while viscometry was suitable to monitor changes in the dispersing phase. An analysis of the properties of these systems, mainly in terms of particle–particle and macromolecule–macromolecule interactions was carried out. Copyright © 2004 Society of Chemical Industry 相似文献
30.
Gérard Battail 《电信纪事》1987,42(9-10):499-509
We propose to redefine weighted decoding of a redundant code as consisting of reestimating a given prior probability distribution in order to take into account the code constraints. The optimum solution of this problem results from using the « minimum cross-entropy » principle and is shown to coincide with the computation of the probabilities to be compared in order to perform maximum likelihood decoding when the prior distribution is exactly known. The principle however also holds when the prior distribution is approximate or biased, which explains the robustness of decoding with respect to weighting information. Such a redefinition also enables using the intermediate steps of decoding, in the case of codes associated by concatenation or product, as a source of weighting information for the subsequent steps. One is led for simplicity’s sake to an estimate of the posterior probability in separable form with respect to the information symbols ; its definition is given and certain of its applications are discussed. 相似文献