全文获取类型
收费全文 | 748743篇 |
免费 | 9611篇 |
国内免费 | 1678篇 |
专业分类
电工技术 | 13673篇 |
综合类 | 602篇 |
化学工业 | 118249篇 |
金属工艺 | 30624篇 |
机械仪表 | 23442篇 |
建筑科学 | 17516篇 |
矿业工程 | 5251篇 |
能源动力 | 19179篇 |
轻工业 | 62478篇 |
水利工程 | 8739篇 |
石油天然气 | 18158篇 |
武器工业 | 48篇 |
无线电 | 80504篇 |
一般工业技术 | 151121篇 |
冶金工业 | 130750篇 |
原子能技术 | 18232篇 |
自动化技术 | 61466篇 |
出版年
2021年 | 6699篇 |
2019年 | 6362篇 |
2018年 | 11154篇 |
2017年 | 11260篇 |
2016年 | 11902篇 |
2015年 | 7540篇 |
2014年 | 12702篇 |
2013年 | 34152篇 |
2012年 | 19770篇 |
2011年 | 26676篇 |
2010年 | 21572篇 |
2009年 | 24070篇 |
2008年 | 24551篇 |
2007年 | 24163篇 |
2006年 | 21048篇 |
2005年 | 19301篇 |
2004年 | 18285篇 |
2003年 | 17845篇 |
2002年 | 17448篇 |
2001年 | 16979篇 |
2000年 | 16233篇 |
1999年 | 16116篇 |
1998年 | 37785篇 |
1997年 | 27598篇 |
1996年 | 21487篇 |
1995年 | 16497篇 |
1994年 | 14921篇 |
1993年 | 14560篇 |
1992年 | 11249篇 |
1991年 | 10974篇 |
1990年 | 10784篇 |
1989年 | 10580篇 |
1988年 | 10167篇 |
1987年 | 9071篇 |
1986年 | 8851篇 |
1985年 | 10135篇 |
1984年 | 9357篇 |
1983年 | 8898篇 |
1982年 | 8032篇 |
1981年 | 8279篇 |
1980年 | 7868篇 |
1979年 | 7927篇 |
1978年 | 7818篇 |
1977年 | 8868篇 |
1976年 | 11242篇 |
1975年 | 7080篇 |
1974年 | 6790篇 |
1973年 | 6876篇 |
1972年 | 5902篇 |
1971年 | 5523篇 |
排序方式: 共有10000条查询结果,搜索用时 15 毫秒
991.
A Azzi D Boscoboinik S Clément D Marilley NK Ozer R Ricciarelli A Tasinato 《Canadian Metallurgical Quarterly》1997,57(4-5):507-514
The effects of alpha-tocopherol and beta-tocopherol have been studied in rat and human aortic smooth muscle cells. Alpha-tocopherol, but not beta-tocopherol, inhibited smooth muscle cell proliferation and protein kinase C in a dose-dependent manner, at concentrations ranging from 10 to 50 microM. Beta-tocopherol added simultaneously with alpha-tocopherol prevented both proliferation and protein kinase C inhibition. Protein kinase C inhibition was cell cycle-dependent and it was prevented by okadaic acid, a protein phosphatase inhibitor. Protein kinase C activity measured from aortas of cholesterol-fed rabbits was also inhibited by alpha-tocopherol. By using protein kinase C (PKC) isoform-specific inhibitors and immunoprecipitation reactions it was found that PKC-alpha was selectively inhibited by alpha-tocopherol. Further, an activation of protein phosphatase 2A by alpha-tocopherol was found, which caused PKC-alpha dephosphorylation and inhibition. Ultimately, this cascade of events at the level of cell signal transduction leads to the inhibition of smooth muscle cell proliferation. 相似文献
992.
993.
In a healthy 49-year-old man, a decrease in job efficiency was noticed along with bizarre behavior. On admission, he was euphoric, childish, superficial and had increased libido. Neurological findings were normal. There were no abnormal findings on routine blood tests, hematochemistry or urine analysis. MRI showed no abnormal findings. However, single photon emission CT (SPECT) showed diffuse hypoaccummulation of tracer from the temporal to frontal regions. Lumbar puncture showed clear cerebrospinal fluid (CSF) with pleocytosis and an elevated protein level. Moreover, antibody IgG titers to herpes simplex virus (HSV) and measles virus were elevated, according to EIA [serum HSV -1,202.2x, measles virus 47.1x: CSF HSV-116.1x, measles virus 9.9x]. The ratio of serum to CSF antibody titers of HSV and measles virus were 12.5 and 4.75, respectively. The antibody index values of HSV and measles virus IgG titers were 8.42 and 22.22. The ratio of albumin was 105.7. Chronic, progressive HSV encephalitis is rare, and there have been very few reports of encephalitis due to double infection by HSV and another virus. Our patient was diagnosed as having encephalitis due to double infection with HSV and measles virus, because the ratio of serum to CSF antibody titers was less than 20 and the antibody index values were over 1.91. Moreover, since the IgG index was elevated and the ratio of albumin was not low, it was suggested that the blood-brain-barrier had not been disrupted, and antibodies were being produced chronically in the medullary cavity. Hyperaccummulation of tracer on SPECT studies has been reported in the early stages of HSV encephalitis. In our case, while CT and MRI showed no abnormal findings, SPECT showed diffuse hypoaccummulation. SPECT appears to be a useful tool in the diagnosis of this disorder. In case of chronic, progressive personality change in middle-aged adults, we must be aware of double virus infection of the brain as a possible causal factor. 相似文献
994.
The importance of regulating the cellular concentrations of the myristoylated alanine-rich C kinase substrate (MARCKS), a major cellular substrate of protein kinase C, is indicated by the fact that mice lacking MARCKS exhibit gross abnormalities of central nervous system development and die shortly after birth. We previously identified a novel means of regulating cellular MARCKS concentrations that involved a specific proteolytic cleavage of the protein and implicated a cysteine protease in this process (Spizz, G., and Blackshear, P. J. (1996) J. Biol. Chem. 271, 553-562). Here we show that p40, the carboxyl-terminal fragment resulting from this cleavage of MARCKS, was associated with the mitochondrial/lysosomal pellet fraction of human diploid fibroblasts and that its generation in cells was sensitive to treatment with NH4Cl. These data suggest the involvement of lysosomes in the generation and/or stability of p40. The MARCKS-cleaving enzyme (MCE) activity was peripherally associated with a 10,000 x g pellet fraction from bovine liver, and it co-purified with the activity and immunoreactivity of a lysosomal protease, cathepsin B. Cathepsin B catalyzed the generation of p40 from MARCKS in a cell-free system and behaved similarly to the MCE with respect to mutants of MARCKS previously shown to be poor substrates for the MCE. Treatment of fibroblasts with a cell-permeable, specific inhibitor of cathepsin B, CA074-Me, resulted in parallel time- and concentration-dependent inhibition of cathepsin B and MCE activity. Incubation of a synthetic MARCKS phosphorylation site domain peptide with purified cathepsin B resulted in cleavage of the peptide at sites consistent with preferred cathepsin B substrate sites. These data provide evidence for the identity of the MCE as cathepsin B and suggest that this cleavage most likely takes place within lysosomes, perhaps as a result of specific lysosomal targeting sequences within the MARCKS primary sequence. The data also suggest a direct interaction between MARCKS and cathepsin B in cells and leave open the possibility that MARCKS may in some way regulate the protease for which it is a substrate. 相似文献
995.
996.
L Crippa SV D'Angelo L Tomassini B Rizzi G D'Alessandro A D'Angelo 《Canadian Metallurgical Quarterly》1997,82(4):446-451
BACKGROUND AND OBJECTIVE: The potential utility of D-dimer measurements for the diagnosis of deep vein thrombosis became evident soon after the development of reliable commercial assays. The purpose of this review is to outline some critical aspects affecting cost-effectiveness of D-dimer measurements in the diagnosis of deep vein thrombosis (DVT). METHODS: The authors have been working in this field contributing original papers whose data have been used for this study. In addition, the material analyzed in this article includes papers published in the journals covered by the Science Citation Index and Medline. RESULTS: D-dimer levels are very sensitive to the process of fibrin formation/dissolution occurring with ongoing thrombosis. However, they may not be highly specific for venous thromboembolism as they are influenced by the presence of comorbid conditions potentially elevating plasma D-dimer (cancer, surgery, infectious diseases). In addition, commercially available ELISA assays, although quantitative and reproducible, cannot be used under emergency conditions because they are time-consuming and suited for batch-processing of plasma samples. Recently, new assays have been introduced which permit fast and quantitative D-dimer estimations in individual patients. We have evaluated the utility of two new rapid assays (LPIA D-dimer. Mitsubishi, and VIDAS D-DIMER, bio-Merieux) in combination with compression real-time-B-mode ultrasonography for the detection of deep vein thrombosis in asymptomatic patients following elective hip replacement and in patients with clinically suspected deep vein thrombosis. In both settings, we identified cut-off values with optimal sensitivity which allow exclusion of deep vein thrombosis in a considerable percentage of patients, with substantial sparing of economic resources. In fact, based on a cost-effectiveness analysis, a diagnostic algorithm combining D-dimers measurement and compression ultrasonography would result in cost-savings ranging from 5% to 55% in patients with high or low clinical pretest probability respectively. However, the specificity of D-dimer measurements for deep vein thrombosis was much higher in symptomatic than in asymptomatic patients. Choice of the cut-off value proved to be dependent on the method as well as on the patient populations studied. CONCLUSIONS: The cost-effectiveness of D-dimers measurement in the diagnosis of asymptomatic DVT remains questionable. Conversely, our data strongly support the utility of D-dimers determinations in the diagnosis of symptomatic DVT. In terms of sparing economic resources, the introduction in the clinical laboratory of the rapid quantitative assays would be highly convenient, because they avoid a source of bias in the interpretation of D-dimers results, are easy to perform and do not require dedicated personnel or instrumentation. Prospective management studies validating the utility of D-dimer measurement in the diagnosis of deep vein thrombosis are urgently needed. 相似文献
997.
G Armengol M Tarkkanen M Virolainen A Forus J Valle T B?hling S Asko-Seljavaara C Blomqvist I Elomaa E Karaharju AH Kivioja MA Siimes E Tukiainen MR Caballín O Myklebost S Knuutila 《Canadian Metallurgical Quarterly》1997,75(10):1403-1409
Comparative genomic hybridization (CGH) was used to detect copy number changes of DNA sequences in the Ewing family of tumours (ET). We analysed 20 samples from 17 patients. Fifteen tumours (75%) showed copy number changes. Gains of DNA sequences were much more frequent than losses, the majority of the gains affecting whole chromosomes or whole chromosome arms. Recurrent findings included copy number increases for chromosomes 8 (seven out of 20 samples; 35%), 1q (five samples; 25%) and 12 (five samples; 25%). The minimal common regions of these gains were the whole chromosomes 8 and 12, and 1q21-22. High-level amplifications affected 8q13-24, 1q and 1q21-22, each once. Southern blot analysis of the specimen with high-level amplification at 1q21-22 showed an amplification of FLG and SPRR3, both mapped to this region. All cases with a gain of chromosome 12 simultaneously showed a gain of chromosome 8. Comparison of CGH findings with cytogenetic analysis of the same tumours and previous cytogenetic reports of ET showed, in general, concordant results. In conclusion, our findings confirm that secondary changes, which may have prognostic significance in ET, are trisomy 8, trisomy 12 and a gain of DNA sequences in 1q. 相似文献
998.
Several different lines of evidence have demonstrated that inherited susceptibility to rheumatoid arthritis (RA) is associated with the DRB1 genes encoding the HLA-DR4 and HLA-DR1 molecules. A contrasting hypothesis has recently been proposed, suggesting that, in general, the DRB1 locus is associated with protection to RA and that the RA-associated DRB1 alleles are not responsible for the primary disease association but merely permissive for the susceptibility conferred by the HLA-DQ alleles with which they are in linkage disequilibrium. We have performed a critical review of the literature on the HLA association in RA with special emphasis on studies in which both an HLA-DR and -DQ association has been investigated. Our analyses provide strong evidence against the hypothesis that HLA-DQ molecules play a major role in the general susceptibility to RA. Thus, the strongest association in rheumatoid arthritis is with DRB1 genes rather than DQB1 genes. 相似文献
999.
A King 《Canadian Metallurgical Quarterly》1997,48(4):576-593
By using a 'cultural' definition of 'postmodernism' (derived from Jameson and Martin) in which postmodernism is regarded as the transgression of modern boundaries, this article traces the emergence of postmodern aspects to violent male fandom at football games since the 1960s. It is argued that at games, male fans have created imaginary masculine and national boundaries by which they have affirmed their identities but that in fighting they have sought to breach these boundaries in postmodern fashion. 相似文献
1000.
The mycobacterial embCAB operon encodes arabinosyl transferases, putative targets of the antimycobacterial agent ethambutol (EMB). Mutations in embB lead to resistance to EMB in Mycobacterium tuberculosis. The basis for natural, intrinsic resistance to EMB in nontuberculous mycobacteria (NTM) is not known; neither is the practical implication of resistance to EMB in the absence of embB mutations in M. tuberculosis well understood. The conserved embB resistance-determining region (ERDR) of a collection of 13 strains of NTM and 12 EMB-resistant strains of M. tuberculosis was investigated. Genotypes were correlated with drug susceptibility phenotypes. High-level natural resistance to EMB (MIC, . or =64 microg/ml) was associated with a variant amino acid motif in the ERDR of M. abscessus, M. chelonae, and M. leprae. Transfer of the M. abscessus emb allele to M. smegmatis resulted in a 500-fold increase in the MICs. In M. tuberculosis, embB mutations were associated with MICs of > or =20 microg/ml while resistance not associated with an ERDR mutation generally resulted in MICs of < or =10 microg/ml. These data further support the notion that the emb region determines intrinsic and acquired resistance to EMB and might help in the reassessment of the current recommendations for the screening and treatment of infections with EMB-resistant M. tuberculosis and NTM. 相似文献