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   1504篇
  免费   50篇
  国内免费   5篇
电工技术   24篇
综合类   1篇
化学工业   363篇
金属工艺   24篇
机械仪表   26篇
建筑科学   18篇
能源动力   45篇
轻工业   128篇
水利工程   9篇
石油天然气   5篇
无线电   156篇
一般工业技术   431篇
冶金工业   209篇
原子能技术   15篇
自动化技术   105篇
  2023年   7篇
  2022年   18篇
  2021年   34篇
  2020年   24篇
  2019年   9篇
  2018年   32篇
  2017年   31篇
  2016年   30篇
  2015年   24篇
  2014年   44篇
  2013年   88篇
  2012年   66篇
  2011年   62篇
  2010年   78篇
  2009年   61篇
  2008年   82篇
  2007年   68篇
  2006年   61篇
  2005年   47篇
  2004年   42篇
  2003年   42篇
  2002年   24篇
  2001年   30篇
  2000年   20篇
  1999年   32篇
  1998年   79篇
  1997年   56篇
  1996年   47篇
  1995年   35篇
  1994年   39篇
  1993年   25篇
  1992年   19篇
  1991年   13篇
  1990年   15篇
  1989年   10篇
  1988年   11篇
  1987年   14篇
  1986年   4篇
  1985年   14篇
  1984年   12篇
  1983年   9篇
  1982年   18篇
  1981年   7篇
  1980年   4篇
  1978年   10篇
  1977年   11篇
  1976年   22篇
  1973年   6篇
  1972年   3篇
  1971年   5篇
排序方式: 共有1559条查询结果,搜索用时 31 毫秒
991.
The gene (ystB) encoding the novel subtype of the heat-stable enterotoxin (Y-STb) was cloned from the chromosome of a clinical isolate of Yersinia enterocolitica 84-50 (serotype O:5, biotype 1A) and the nucleotide sequence was determined. The ystB contained 216 base pairs that encoded a protein of 71 amino acid residues. The C-terminal 30 residues of the precursor protein exactly corresponded to the amino acid sequence of the Y-STb toxin, purified from the culture supernatant of the wild strain. Homology search revealed that there are 76.9% nucleotide sequence similarity between ystB and the Yersinia kristensenii ST gene, and 73.5% with the Y. enterocolitica prototype sequence of yst (ystA). When tested with the PCR generated ystB specific probe, 36 of 304 Y. enterocolitica strains from 18 countries hybridized with the probe. All the ystB probe positive strains belonged to biotype 1A and mostly to the so-called non-pathogenic serotype O:5, O:6, O:7,8 O:7,13 and O:10, while ystA was predominantly found among the pathogenic serotypes (78.5%). Out of 36 ystB gene positive strains, 18 were clinical origin from six countries, which were also positive in the suckling mice assay suggesting that ystB may play an important role in the pathogenesis, and the so-called non-pathogenic serotypes could be virulent for human.  相似文献   
992.
The 'H2S test' is being advanced for microbiological water quality testing where conventional coliform based methods are impractical or too expensive. It involves ambient temperature incubation of water samples with nutrient formulated to generate hydrogen sulphide when 'faecal' bacteria are present. Recently a WHO review identified several concerns including the limited number of comparative studies, formulation variability, and false positives and negatives. In response we have compared the H2S test's ability to detect and quantify faecal contamination in an aquifer impacted by septic tank leachfields with measurements obtained concurrently using conventional bacterial indicators, coliphages, faecal sterol biomarkers, Cryptosporidium and Giardia. Like these other analytes, H2S testing detected a contamination gradient ranging from high (septic liquid) to moderate (exfiltration zones), to background (e.g. domestic bores), corresponding to indicator removal + dilution by factors > 10(6). Presence/absence tests could not distinguish between heavily and slightly contaminated waters, whereas multi-tube testing (e.g. 10 x 10 mL arrays) did. It was concluded that while the WHO review concerns are justified, the H2S test performance shows promise in sanitary survey work, can be improved by employing an mpn approach and has potential to aid in the protection of source water and identifying contaminated groundwater.  相似文献   
993.
Ion chromatographic (IC) methods have been developed for the assay of amylamine in BMS-181 866-02 and tert.-butylamine (TBA) in BMS-188 494-04. BMS-181 866-02 is the penultimate intermediate in the synthesis of a novel thromboxane antagonist, BMS-180 291-02, which is undergoing clinical trials. Amylamine may be present as a trace impurity in BMS-181 866-02. BMS-188 494-04 is the TBA salt of the prodrug ester of BMS-187 745, a novel oral hypocholesterolemic agent. Chromatographic separations were accomplished under isocratic conditions using a Dionex CS-14 column with conductivity detection. The methods differ only in the composition of the methanesulfonic acid-acetonitrile mobile phase. The detection limit and minimum quantifiable levels for amylamine were 0.01% and 0.02%, respectively. The method was linear over the range studied (1-12.5 micrograms/ml, n = 7, r = 0.9993). The method for TBA was linear from 5 to 30% (w/w) (50-300 micrograms/ml, n = 8, r = 0.9993) of working sample concentration (1 mg/ml BMS-188 494-04). The precision and accuracy of the methods are presented.  相似文献   
994.
In this study, solar control coatings were prepared by sequential depositions of thin films of ZnS (40 nm)–CuS (150 nm) and ZnS (40 nm)–Bi2S3 (75 nm)–CuS (150 nm) from chemical baths on 3 mm thick commercial sheet glass. These were laminated to 3 mm thick clear glass or commercially available SnO2 based heat mirror coating of sheet resistance 15 Ω on float glass of 3 mm thickness using a poly(ethylene vinyl acetate), EVA, sheet of 0.36 mm thickness in a vacuum process at 120 °C for 30 min. In total, the thickness of the glazing was 6.35 mm. The glazings possess visible transmittance, weighted for D65 solar spectra and sensitivity of the human eye for daylight vision, of 36% or 14% with solar absorptance of 71% or 78% depending on the coating type, i.e ZnS–CuS or ZnS–Bi2S3–CuS-heat mirror respectively. The solar heat gain coefficient (SHGC) was evaluated for these glazings at exterior temperatures of 15 and 32 °C for an exterior convective heat transfer coefficient (hex) of 6–100 Wm−2 K−1 using a mathematical model. The model predicts the extent of reduction in SHGC through the presence of the heat mirror coating as a function of hex and hence helps to decide on the relative benefit, which may be derived through their use in different locations. Though the deposition technique mentioned here involves longer duration compared with vacuum techniques, it may be developed into a low throughput, low-capital alternate technology for small-scale production.  相似文献   
995.
 Factors, both physical and chemical, influencing the pore-structure stability of high temperature catalysts and/or catalyst supports were discussed with a view to formulate strategies to design and develop high temperature catalyst supports. Factors influencing the sintering behavior and techniques to manipulate them are discussed. Chemical ways of modifying porosity reduction behavior can lead to changes in catalytic behavior. Therefore several physical techniques like control of packing and coordination by post precipitation treatment and nanocomposite formation, changing particle morphology and introducing rigid constraints were discussed. Received: 20 June 1998 / Accepted: 30 July 1998  相似文献   
996.
This study seeks to define the role of pretreatment of evaluation of tumour growth fraction in cervical cancer and its relationship to the clinical course of the disease. In addition, it also seeks to explain whether cell kinetics and growth factor expression have an association with tumour response to radiotherapy and hence could be of value in the management of patients. All pre-treatment biopsies were analysed for the tumour-proliferative compartment by evaluation of Ki67 antigen expression and argyrophilic nucleolar organiser region (AgNOR) counts. Growth factor analysis was done by analysing for expression of epidermal growth factor (EGF), epidermal growth factor receptor (EGF-R) and transforming growth factors alpha and beta (TGFalpha, TGFbeta). A total of 152 patients were evaluated and a correlation obtained between pre-treatment status of the tumour-growth-fraction-associated markers and clinical outcome following radiotherapy. Such patients were either disease-free (group 1, n=106) or with residual/recurrent disease (group 2, n=46) at a 16-month follow-up. Pre-treatment analysis of AgNOR significantly correlated to disease status after treatment (r=-0.517, P=0.0000). This may be due to an effect of cell proliferation. Lower AgNOR counts were significantly associated with recurrent/residual tumours, suggesting that increased proliferative activity may be a positive prognostic indicator. Similar results were also obtained for the other proliferation-associated marker Ki67 (r=-0.443, P=0.0000). Expression of EGF and EGF-R also showed significant pre-treatment correlations with the final disease outcome (r=0.248, P=0.031 and r=0.503, P=0.0000 respectively). Both these markers were expressed more by patients belonging to group 2. The opposite was the case for TGFalpha, where patients belonging to group 1 showed higher values (r=0.417, P=0.0001). The other growth factor investigated, TGFbeta, also showed a conspicuous differential expression in the two groups of patients (r=-0.604, P=0.0000). Group 1 patients showed mostly mild to moderate expression while most group 2 patients were negative for the growth factor. It therefore appears that tumours with high AgNOR counts and Ki67 index, along with expression of the two types of transforming growth factor (alpha and beta), responded better to radiotherapy.  相似文献   
997.
We report here the preparation of nanoparticles of iron oxide in the presence of polysaccharide templates. Interaction between iron (II) sulfate and template has been carried out in aqueous phase, followed by the selective and controlled removal of the template to achieve narrow distribution of particle size. Particles of iron oxide obtained have been characterized for their stability in solvent media, size, size distribution and crystallinity and found that when the negative value of the zeta potential increases, particle size decreases. A narrow particle size distribution with D 100 = 275 nm was obtained with chitosan and starch templates. SEM measurements further confirm the particle size measurement. Diffuse reflectance UV-vis spectra values show that the template is completely removed from the final iron oxide particles and powder XRD measurements show that the peaks of the diffractogram are in agreement with the theoretical data of hematite. The salient observations of our study shows that there occurs a direct correlation between zeta potential, polydispersity index, bandgap energy and particle size. The crystallite size of the particles was found to be 30–35 nm. A large negative zeta potential was found to be advantageous for achieving lower particle sizes, owing to the particles remaining discrete without agglomeration.  相似文献   
998.
999.
BACKGROUND: Patients with mesenchymal chondrosarcomas in general have a 5-year survival rate ranging from 42-54.6% and a 10-year survival rate of 28%. Nineteen cases of mesenchymal chondrosarcoma of the jaw bones were reviewed to study their clinicopathologic features and to compare their clinical behavior with that of mesenchymal chondrosarcomas of other locations. METHODS: The patients were 10 males and 9 females (age range, 2-51 years). Sixteen patients were age < 30 years, and the average age at presentation was 19 years. Eleven tumors involved the mandible and eight involved the maxilla. RESULTS: Histologically, the classic picture of a bimorphic tumor, composed of islands of well differentiated hyaline cartilage juxtaposed to a small cell undifferentiated malignancy, was present in every case. Resection, including hemimandibulectomy and hemimaxillectomy, was the main treatment in 16 patients. Seven patients had local recurrence, and five patients developed distant metastases. Six patients died of disease, and the 5-year and 10-year survival rates were 82% and 56%, respectively. CONCLUSIONS: Mesenchymal chondrosarcoma of the jaw bones appears to have a more indolent course than mesenchymal chondrosarcoma of other anatomic  相似文献   
1000.
Research on the potential ameliorating effects of social support on occupational stress produces weak, inconsistent, and even contradictory results. This study of 117 employees, mostly from a southern U.S. hospital supply company, examined potential moderators that were theorized might reduce the confusion: source congruence (congruence between sources of the stressor and of social support) and gender role. Congruence between the sources of stressors and of social support appeared to make little difference in determining the moderating or buffering effect of social support on the relationship between stressors and strain. Gender role, however, may moderate the relationship between social support and individual strains such that more feminine people react more strongly and positively to social support than more masculine people do. (PsycINFO Database Record (c) 2010 APA, all rights reserved)  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