全文获取类型
收费全文 | 550989篇 |
免费 | 6476篇 |
国内免费 | 1842篇 |
专业分类
电工技术 | 10510篇 |
综合类 | 416篇 |
化学工业 | 80426篇 |
金属工艺 | 21885篇 |
机械仪表 | 17494篇 |
建筑科学 | 11604篇 |
矿业工程 | 2706篇 |
能源动力 | 14972篇 |
轻工业 | 39359篇 |
水利工程 | 5524篇 |
石油天然气 | 9851篇 |
武器工业 | 24篇 |
无线电 | 69038篇 |
一般工业技术 | 110397篇 |
冶金工业 | 108588篇 |
原子能技术 | 11498篇 |
自动化技术 | 45015篇 |
出版年
2021年 | 4845篇 |
2020年 | 3685篇 |
2019年 | 4768篇 |
2018年 | 8007篇 |
2017年 | 7959篇 |
2016年 | 8495篇 |
2015年 | 5461篇 |
2014年 | 9080篇 |
2013年 | 26014篇 |
2012年 | 14685篇 |
2011年 | 19815篇 |
2010年 | 15808篇 |
2009年 | 17959篇 |
2008年 | 18538篇 |
2007年 | 18300篇 |
2006年 | 16534篇 |
2005年 | 14951篇 |
2004年 | 14348篇 |
2003年 | 14121篇 |
2002年 | 13480篇 |
2001年 | 13604篇 |
2000年 | 12720篇 |
1999年 | 13497篇 |
1998年 | 35324篇 |
1997年 | 24151篇 |
1996年 | 18635篇 |
1995年 | 13810篇 |
1994年 | 12136篇 |
1993年 | 12079篇 |
1992年 | 8582篇 |
1991年 | 8144篇 |
1990年 | 8042篇 |
1989年 | 7653篇 |
1988年 | 7166篇 |
1987年 | 6141篇 |
1986年 | 6124篇 |
1985年 | 6858篇 |
1984年 | 6218篇 |
1983年 | 5628篇 |
1982年 | 5142篇 |
1981年 | 5340篇 |
1980年 | 4817篇 |
1979年 | 4652篇 |
1978年 | 4609篇 |
1977年 | 5375篇 |
1976年 | 7029篇 |
1975年 | 3968篇 |
1974年 | 3765篇 |
1973年 | 3781篇 |
1972年 | 3208篇 |
排序方式: 共有10000条查询结果,搜索用时 15 毫秒
951.
Here we are presenting the case of a 70-years-old woman who has hepatic cirrhosis anti-HCV and insulin-dependent diabetes mellitus, without relevant epidemiologic ascendants or previous transfusions and HBV, HIV negatives. On admission to our hospital she showed signs of autoimmune hemolytic anaemia (AHA) which was confirmed by positive direct Coombs test and an improvement in blood test after corticoid treatment. Having discarded other possible causes such as drugs infectious diseases or essential mixed cryoglobulinemia (CME), we put forward the possible association between AHA and infection by HCV, where AHA was an extrahepatic immunological manifestation of HCV. This fact has never been brought to light in previous medical literature. 相似文献
952.
953.
Interactions in aqueous solution between pilocarpine hydrochloride (P-HCl), a rather hydrophilic drug with good water solubility, and various cyclodextrins (CDs) were described recently. To assess the influence of CDs on the diffusion behavior of pilocarpine, in vitro studies were performed using porcine or bovine corneas as diffusion barriers. The affinity of P-HCl for porcine cornea in the presence of alpha-cyclodextrin (alpha-CD) and (hydroxyethyl)-beta-cyclodextrin (HE-beta-CD) was determined by drug uptake experiments. Additionally, in vitro permeation experiments through bovine corneas were conducted with a modified diffusion device optimized for corneal perfusion studies. The results obtained from the corneal uptake studies indicate that the addition of alpha-CD led to increased tissue drug levels. The increase in permeability of pilocarpine in the presence of alpha-CD was approximately 10-fold (log Papp = -4.87 +/- 0.03) in comparison with plain P-HCl solution (log Papp = -5.89 +/- 0.06). Permeation studies with corneas pretreated with alpha-CD solution revealed enhanced corneal permeability of pilocarpine due to alpha-CD induced membrane effects. The hydroxyalkylated beta-CD derivatives HE-beta-CD (log Papp = -6.27 +/- 0.09) and (hydroxypropyl)-beta-cyclodextrin (HP-beta-CD; log Papp = -6.40 +/- 0.03), however, seemed to cause slightly decreased permeation rates, supporting the concept of an interaction between pilocarpine and the hydroxyalkylated-beta-CD derivatives. Considering physiological compatibility, the addition of CDs seems to be an effective tool to modify and optimize the ocular availability of pilocarpine. 相似文献
954.
I Tamai T Nakanishi K Hayashi T Terao Y Sai T Shiraga K Miyamoto E Takeda H Higashida A Tsuji 《Canadian Metallurgical Quarterly》1997,49(8):796-801
Although recent evidence suggests that certain beta-lactam antibiotics are absorbed via a specific transport mechanism, its nature is unclear. To confirm whether peptide transport in the rat can be largely ascribed to the intestinal oligopeptide transporter PepT1, the transporter has been functionally characterized and its significance in the intestinal absorption of beta-lactam antibiotics was evaluated. For evaluation of transport activity complementary RNA (cRNA) of rat PepT1 was synthesized in-vitro and expressed in Xenopus laevis oocytes. cRNA induced uptake of several beta-lactam antibiotics and the dipeptide [14C]glycylsarcosine; this was specifically inhibited by various dipeptides and tripeptides but not by their constituent amino acids or by tetra- or pentapeptides. The transport activity of PepT1 for beta-lactam antibiotics correlated well with their in-vivo intestinal transport and absorption. Furthermore, mutual inhibitory effects on uptake were observed between glyclsarcosine and beta-lactam antibiotics. Hybrid depletion of the functional expression of rat PepT1 in oocytes injected with rat intestinal epithelial total mRNA was studied using an antisense oligonucleotide corresponding to the 5'-coding region of PepT1. In oocytes injected with rat mRNA pre-hybridized with the antisense oligonucleotide against rat PepT1, the uptake of [14C]glycylsarcosine was almost completely abolished, whereas its uptake was not influenced by a sense oligonucleotide for the same region of PepT1. Similarly, the uptake of beta-lactam antibiotics was also reduced by the antisense oligonucleotide against rat PepT1. These results demonstrate that the intestinal proton-coupled oligopeptide transporter PepT1 plays a predominant role in the carrier-mediated intestinal absorption of beta-lactam antibiotics and native oligopeptides in the rat. 相似文献
955.
956.
To investigate the molecular basis of altered expression of the H-cadherin gene, we used polymerase chain reaction-single strand conformation polymorphism and DNA sequencing to examine the H-cadherin gene in 48 primary breast cancers in which loss of the long arm of chromosome 16 had been detected. We identified no mutations other than somatic 5-bp deletion within the coding region in a single tumor. The very low frequency of mutation found in these experiments suggests that H-cadherin is usually not a primary target for carcinogenesis in human breast cancers, and that reduction of its expression is likely to be a consequence of some other genetic event(s). 相似文献
957.
Reveliotis S.A. Lawley M.A. Ferreira P.M. 《Automatic Control, IEEE Transactions on》1997,42(10):1344-1357
The development of efficient deadlock avoidance policies (DAPs) for sequential resource allocation systems (RASs) is a problem of increasing interest in the scientific community, largely because of its relevance to the design of large-scale flexibly automated manufacturing systems. Much of the work on this problem existing in the literature is focused on the so-called single-unit RAS model, which is the simplest model in the considered class of RASs. Furthermore, due to a well-established result stating that, even for single-unit RASs, the computation of the maximally permissive DAP is intractable (NP-hard), many researchers (including our group) have focused on obtaining good suboptimal policies which are computationally tractable (scalable) and provably correct. In the first part of the paper, it is shown, however, that for a large subset (in fact, a majority) of single-unit RASs, the optimal DAP can be obtained in real-time with a computational cost which is a polynomial function of the system size (i.e., the number of resource types and the distinct route stages of the processes running through the system). The implications of this result for the entire class of single-unit RASs are also explored. With a result on the design of optimal DAPs for single-unit RASs, the second part of the paper concentrates on the development of scalable and provably correct DAPs for the more general case of conjunctive RASs 相似文献
958.
959.
In an attempt to identify genes that are involved in Drosophila embryonic cardiac development, we have cloned and characterized a gene whose function is required late in embryogenesis to control heart rate and muscular activity. This gene has been named held out wings (how) because hypomorphic mutant alleles produce adult animals that have lost their ability to fly and that keep their wings horizontal at a 90 degree angle from the body axis. In contrast to the late phenotype observed in null mutants, the How protein is expressed early in the invaginating mesoderm and this expression is apparently under the control of twist. When the different mesodermal lineages segregate, the expression of How becomes restricted to the myogenic lineage, including the cardioblasts and probably all the myoblasts. Antibodies directed against the protein demonstrate that How is localized to the nucleus. how encodes a protein containing one KH-domain which has been implicated in binding RNA. how is highly related to the mouse quaking gene which plays a role at least in myelination and that could serve to link a signal transduction pathway to the control of mRNA metabolism. The properties of the how gene described herein suggest that this gene participates in the control of expression of as yet unidentified target mRNAs coding for proteins essential to cardiac and muscular activity. 相似文献
960.
S Abu-Rabia 《Canadian Metallurgical Quarterly》1997,26(4):465-482
This study investigated the effect of vowels and context on the reading accuracy of poor and skilled native Arabic readers in reading paragraphs, sentences, and words. Central to this study is the belief that reading theory today should consider additional variables, especially when explaining the reading process in Arabic orthography among poor and normal/skilled readers. This orthography has not been studied. Reading theory today is the sum of conclusions from studies conducted in Latin orthography. The subjects were 77 native Arabic speakers, 34 of them poor readers and 44 normal/skilled readers. The subjects had to read in Arabic 15 paragraphs, 60 sentences, and 210 words. There were three reading conditions: fully vowelized, partially vowelized, and unvowelized texts. The results showed that vowels and contexts were important variables to facilitate word recognition in poor and normal/skilled readers in Arabic orthography. 相似文献