首页 | 本学科首页   官方微博 | 高级检索  
文章检索
  按 检索   检索词:      
出版年份:   被引次数:   他引次数: 提示:输入*表示无穷大
  收费全文   6287篇
  免费   14篇
电工技术   16篇
综合类   1篇
化学工业   249篇
金属工艺   17篇
机械仪表   29篇
建筑科学   53篇
矿业工程   6篇
能源动力   29篇
轻工业   133篇
水利工程   24篇
石油天然气   5篇
无线电   99篇
一般工业技术   147篇
冶金工业   5291篇
原子能技术   45篇
自动化技术   157篇
  2024年   8篇
  2021年   8篇
  2020年   13篇
  2019年   10篇
  2018年   8篇
  2017年   15篇
  2016年   10篇
  2015年   13篇
  2014年   22篇
  2013年   63篇
  2012年   29篇
  2011年   54篇
  2010年   50篇
  2009年   27篇
  2008年   45篇
  2007年   45篇
  2006年   47篇
  2005年   29篇
  2004年   34篇
  2003年   41篇
  2002年   37篇
  2001年   23篇
  2000年   29篇
  1999年   185篇
  1998年   1605篇
  1997年   932篇
  1996年   619篇
  1995年   371篇
  1994年   285篇
  1993年   342篇
  1992年   69篇
  1991年   78篇
  1990年   80篇
  1989年   96篇
  1988年   76篇
  1987年   85篇
  1986年   75篇
  1985年   81篇
  1984年   24篇
  1983年   28篇
  1982年   41篇
  1981年   38篇
  1980年   51篇
  1979年   21篇
  1978年   25篇
  1977年   110篇
  1976年   229篇
  1975年   17篇
  1974年   14篇
  1966年   5篇
排序方式: 共有6301条查询结果,搜索用时 15 毫秒
51.
Aliphatic compounds (alkanes, alkenes, alkanoic acids, ketones, alcohols and amines) were passed through beds of spent oil shales (Condor brown, Condor carbonaceous, Julia Creek), minerals (quartz, calcite, K-feldspar, pyrite, kaolinite) and charcoal at temperatures of 300–600 °C and the products were analysed by g.c.m.s. All the materials catalysed isomerization, aromatization and cracking to varying degrees: non-clay minerals < kaolinite ≈ spent oil shales < charcoal. Products included branched alkanes, isomeric alkenes, nitriles, ketones and alkyl-substituted benzenes, naphthalenes, pyridines, phenols, thiophenes and pyrroles. These compounds occur in shale oils and may be derived from secondary reactions of aliphatic products arising from kerogen cracking.  相似文献   
52.
The lead dioxide electrode   总被引:2,自引:0,他引:2  
The recent literature dealing with the redox mechanism of the lead-acid cell positive electrode is reviewed. The basis electrochemistry of lead dioxide in its various polymorphic modifications and states of subdivision is considered in relation to the important aspects of electrode technology of which the major industrial application of the material is the conventional lead-acid cell. The proposed mechanism of the reduction (discharge) of lead dioxide in various acidic solutions are considered in relation to the present state of electrode kinetic theory. The reverse reaction by which lead dioxide is formed and the parasitic intrusion of the self discharge are dealt with as a precursor to the total cyclic process. It is concluded that in a number of respects the mechanisms proposed do not adequately represent the totality of the experimental observations. Such shortcomings are emphasized and extensions to present research are proposed.  相似文献   
53.
The object of the investigation was to develop eutectic solvent for an enamel by the combination of several eutectics, then to add silica to increase the refractoriness of the enamel up to the limit of its capability to dissolve silica without endangering its bond to the metal body. This naturally involved a study of the entectic compositions of the RO–B2O3–SiO2 systems. Since the Na2O–B2O3-SiO2, K2O–B2O3–SiO2, and PbO–B2O3-SiO2 detormation eutectics are known a knowledge of the CaO-B2O3-SiO2 system was necessary. A study of the high B2O3 area of this system was made at approximately 4% intervals. A member high in CaO and SiO2, and which on repeated fusions showed practically 100% glass, was selected as a practical member to blend with the Na2O-B2O3–SiO3 and the PbO-B2O3–SiO2 eutedic to form a eutectic solvent for an enamel. Combinations of these eutectics and the CaO-B2O3-SiO2 mixture were made and applied to cast iron by the dry process. Additions of SiO2 were also made to these enamels up to 1.5 molecular equivalents. The physical properties of these enamels such as brilliancy, smoothness of surface, texture, bonding power, solubility in water, and resistance to acid attack were also determined.  相似文献   
54.
55.
The electro-winning of chromium from chromic oxide after chlorination in a carbon-containing fused sodium chloride flux is described. Only low values of current efficiency (based on the reaction Cr3+ + 3e ← Cr) can be attained. The effects of concentration of chromic and chromous chlorides in the melt, of current density and of melt temperature are reported. The conclusion is reached that the major factor responsible for the low current efficiency is the re-solution of deposited metal by anodic chlorine.  相似文献   
56.
Does relationship marketing exist in cyberspace?   总被引:1,自引:1,他引:0  
Abstract and Key Results
–  ▪ A central tenet of relationship marketing is that exchanges can be classified on a continuum ranging from episodic transactions to relational exchanges embedded in social bonds.
–  ▪ The aim of this study is to assess whether the unique transacting properties of the Internet (e.g., interactivity, connectivity) promote or hinder the development of exchange relationships.
–  ▪ The study reveals that exchanges conducted in unregulated cyber-markets are invested with similar expectations and norms regarding cooperative behavior as more traditional marketplace exchanges.
This project was supported by the Hong Kong University Grants Council (G-V639).  相似文献   
57.
    
Prostate development and disease are androgen dependent. However, the nature of hormonal effects on the prostate of healthy young men is not clear. We, therefore, measured prostate size in males chronically exposed to high doses of androgens (AS; habitual anabolic steroid abusers; n = 15) or estrogens (E; male to female transsexuals; n = 11) and compared the results with those in age-matched healthy eugonadal men without known prostate disorders. Prostate size was measured by planimetric ultrasound as cross-sectional areas and maximal dimensions in three orthogonal dimensions with a 7.5-megahertz B-mode sector scanner biplane in a transrectal transducer at 2.5 mm steps from the base to the apex of prostate. Total prostate volume (TPV) was reconstructed from planimetric sections, central prostate volume (CPV) was calculated by the ellipsoidal formula from the appropriate three maximum dimensions, and peripheral prostate volume was determined by the difference between TPV and CPV. Compared with age-matched controls, TPV was normal (-2%) in AS (P = 0.752) and reduced by 31% in E (P = 0.002), whereas CPV was increased by 20% in AS (P = 0.002) and reduced by 46% in E (P = 0.002), and the ratio of CPV/peripheral prostate volume was increased by 77% in AS (P < 0.001) and decreased by 33% in E (P = 0.047). Blood sex hormone-binding globulin was elevated by nearly 500% in E (P < 0.001), but was reduced by 47% in AS (P = 0.003). Prostate-specific antigen was normal (-6%) in AS (P = 0.799) and decreased by 86% in E (P = 0.002). Prostatic acid phosphatase was increased by 26% in AS (P = 0.007), but was unchanged (-28%) in E (P = 0.106). Total and free testosterone levels were reduced to castrate levels in E, whereas LH, FSH, and total testosterone levels were significantly reduced in AS. We conclude that in the human prostate of young men, CPV is more hormonally sensitive than TPV, and during high dose treatment, CPV is preferentially increased by chronic androgen treatment and decreased by chronic estrogen treatment. The reduction of TPV by estrogens was less than expected if solely attributable to inhibition of endogenous gonadotropin and testosterone secretion, suggesting that estrogens also have a positive effect on the normal human prostate. The reversibility and long term significance of androgen-induced stimulation of CPV and, in particular, its relationship to the onset and severity of benign prostatic hyperplasia remain to be clarified.  相似文献   
58.
    
  相似文献   
59.
    
We evaluated the influence of urine pH on the proportion of urinary benzidine (BZ) and N-acetylbenzidine present in the free, unconjugated state and on exfoliated urothelial cell DNA adduct levels in 32 workers exposed to BZ in India. Postworkshift urine pH was inversely correlated with the proportions of BZ (r = -0.78; P < 0.0001) and N-acetylbenzidine (r = -0.67; P < 0.0001) present as free compounds. Furthermore, the average of each subject's pre- and postworkshift urine pH was negatively associated with the predominant urothelial DNA adduct (P = 0.0037, adjusted for urinary BZ and metabolites), which has been shown to cochromatograph with a N-(3'-phosphodeoxyguanosin-8-yl)-N'-acetylbenzidine adduct standard. Controlling for internal dose, individuals with urine pH < 6 had 10-fold higher DNA adduct levels compared to subjects with urine pH > or = 7. As reported previously, polymorphisms in NAT1, NAT2, and GSTM1 had no impact on DNA adduct levels. This is the first study to demonstrate that urine pH has a strong influence on the presence of free urinary aromatic amine compounds and on urothelial cell DNA adduct levels in exposed humans. Because there is evidence that acidic urine has a similar influence on aromatic amines derived from cigarette smoke, urine pH, which is influenced by diet, may be an important susceptibility factor for bladder cancer caused by tobacco in the general population.  相似文献   
60.
    
Gorlin syndrome is an autosomal dominant disorder characterized by multiple basal cell carcinomas, medulloblastomas, ovarian fibromas, and a variety of developmental defects. All affected individuals share certain key features, but there is significant phenotypic variability within and among kindreds with respect to malformations. The gene (NBCCS) maps to chromosome 9q22, and allelic loss at this location is common in tumors from Gorlin syndrome patients. Two recessive cancer-predisposition syndromes, xeroderma pigmentosum group A (XPAC) and Fanconi anemia group C (FACC), map to the NBCCS region; and unusual, dominant mutations in these genes have been proposed as the cause of Gorlin syndrome. This study presents cytogenetic and molecular characterization of germ-line deletions in one patient with a chromosome 9q22 deletion and in a second patient with a deletion of 9q22-q3l. Both have typical features of Gorlin syndrome plus additional findings, including mental retardation, conductive hearing loss, and failure to thrive. That Gorlin syndrome can be caused by null mutations (deletions) rather than by activating mutations has several implications. First, in conjunction with previous analyses of allelic loss in tumors, this study provides evidence that associated neoplasms arise with homozygous inactivation of the gene. In addition, dominant mutations of the XPAC and FACC1 genes can be ruled out as the cause of Gorlin syndrome, since the two patients described have null mutations. Finally, phenotypic features that show variable expression must be influenced by genetic background, epigenetic effects, somatic mutations, or environmental factors, since these two patients with identical alterations (deletions) of the Gorlin syndrome gene have somewhat different manifestations of Gorlin syndrome.  相似文献   
设为首页 | 免责声明 | 关于勤云 | 加入收藏

Copyright©北京勤云科技发展有限公司  京ICP备09084417号