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   16429篇
  免费   980篇
  国内免费   494篇
电工技术   670篇
综合类   683篇
化学工业   2303篇
金属工艺   686篇
机械仪表   816篇
建筑科学   910篇
矿业工程   334篇
能源动力   411篇
轻工业   747篇
水利工程   177篇
石油天然气   858篇
武器工业   72篇
无线电   1829篇
一般工业技术   1697篇
冶金工业   3874篇
原子能技术   89篇
自动化技术   1747篇
  2024年   43篇
  2023年   223篇
  2022年   355篇
  2021年   515篇
  2020年   343篇
  2019年   308篇
  2018年   345篇
  2017年   382篇
  2016年   361篇
  2015年   490篇
  2014年   665篇
  2013年   803篇
  2012年   773篇
  2011年   926篇
  2010年   780篇
  2009年   716篇
  2008年   761篇
  2007年   689篇
  2006年   746篇
  2005年   666篇
  2004年   425篇
  2003年   368篇
  2002年   296篇
  2001年   276篇
  2000年   276篇
  1999年   505篇
  1998年   1433篇
  1997年   864篇
  1996年   670篇
  1995年   389篇
  1994年   332篇
  1993年   311篇
  1992年   110篇
  1991年   106篇
  1990年   101篇
  1989年   71篇
  1988年   61篇
  1987年   39篇
  1986年   42篇
  1985年   22篇
  1984年   14篇
  1983年   10篇
  1982年   25篇
  1981年   14篇
  1980年   34篇
  1979年   7篇
  1978年   4篇
  1977年   69篇
  1976年   129篇
  1975年   5篇
排序方式: 共有10000条查询结果,搜索用时 0 毫秒
81.
STUDY DESIGN: Prospective comparative study of pain drawings with findings on lumbar spine magnetic resonance imaging. OBJECTIVES: To assess the ability of the pain drawing to predict the presence of nerve root compression. SUMMARY OF BACKGROUND DATA: Most research work has concentrated on the ability of the pain drawing to act as a screening method for psychological distress with less work directed at the influence the anatomic abnormality has on the pain drawing. METHODS: One hundred thirty-four consecutive outpatients attending for lumbar magnetic resonance imaging in the investigation of back and leg pain completed pain drawings and psychological testing immediately before the examination. The pain drawing was analyzed by previously reported criteria, and the magnetic resonance imaging was assessed independently for the presence of nerve compression by three radiologists. Multivariate stepwise discriminant analysis was used to identify patients with nerve compression on the basis of their pain drawing. RESULTS: Nerve compression was predicted by numbness in the anterolateral aspect of the foot. There was considerable overlap in the appearances of the pain drawings between patients with and without nerve compression, and the pain drawing correctly classified only 58% of patients with nerve compression. CONCLUSIONS: The pain drawing is not a good predictor of nerve compression on magnetic resonance imaging in a group of patients investigated for back and leg pain. It should be interpreted with caution and in light of the full clinical picture.  相似文献   
82.
The extensive heterogeneity of the gonadotrophin hormones, follicle stimulating hormone (FSH) and luteinizing hormone (LH), is due primarily to the heterogeneous nature of their carbohydrate side-chains, in particular sialic acid residues. In this review, we discuss the role of carbohydrate chains in receptor binding and activation, biological activity, and metabolic half-life. The synthesis and secretion of the various glycoforms of both FSH and LH appear to be under endocrine control with gonadotrophin-releasing hormone (GnRH), oestradiol and testosterone playing important roles. Evidence for different glycoforms having variable biopotency or different encoded functions is increasing, and the production and secretion of more or less acidic gonadotrophin species in different physiological states may represent an important mechanism whereby the pituitary regulates gonadal cell and organ function. This has potential importance for the development of new pharmaceutical reagents and new therapeutic regimens in assisted reproduction. It is envisaged that the use of existing and new forms of FSH/LH will allow patients to be treated in a more controlled and physiological manner, with treatment regimens individualized to the needs of the patient.  相似文献   
83.
OBJECTIVES: The authors describe the relation of provider characteristics to processes, costs, and outcomes of medical care for elderly patients hospitalized for community-acquired pneumonia. METHODS: Using Medicare claims data, Medicare beneficiaries discharged from Pennsylvania hospitals during 1990 with community-acquired pneumonia were identified. Claims data were used to ascertain mortality, readmissions, use of procedures and physician consultations, and the costs of care. The relationship of these measures to provider characteristics was analyzed using regression techniques to adjust for patient characteristics, including comorbidity and microbial etiology. RESULTS: Among 22,294 pneumonia episodes studied, 30-day mortality was 17.0%. After adjusting for patient characteristics, 30-day mortality and readmission rates were unrelated to hospital teaching status or urban location or to physician specialty. Use of procedures and physician consultations was more common and costs were 11% higher among patients discharged from teaching hospitals compared with nonteaching hospitals. Similarly, costs were 15% higher at urban hospitals compared with rural hospitals. General internists and medical subspecialists used more procedures and had higher costs than family practitioners. CONCLUSIONS: Processes and costs of care for community-acquired pneumonia varied by provider characteristics, but neither mortality nor readmission rates did. These differences cannot be explained by clinical variables in the database. Further studies should determine whether less costly patterns of care for pneumonia, and perhaps other conditions, could replace more costly ones without compromising patient outcomes.  相似文献   
84.
85.
86.
87.
BACKGROUND: Both fibroblast-mediated cytokine gene therapy and bone marrow transplantation (BMT) have proven to be efficient protocols for the recovery of bone marrow depression. In this report, the effects of fibroblast-mediated interleukin (IL)-6 gene therapy, in combination with BMT, on the recovery of irradiation-induced bone marrow depression were investigated. METHODS: NIH3T3 fibroblast cells engineered to secrete IL-6 (NIH3T3-IL-6) or NIH3T3 cells transduced with the neomycin gene (NIH3T3-Neo), in combination with 10(7), 10(6), or 10(5) syngeneic bone marrow cells, were implanted into irradiated mice. RESULTS: The platelets and white blood cells in the peripheral blood of the irradiated mice increased greatly 12 days after implantation of NIH3T3-IL-6 cells and BMT, the white blood cell counts were restored to a normal level 32 days after the combined therapy, and the platelet number was obviously higher than that in mice implanted with NIH3T3-Neo and BMT. Twenty and 25 days after the combined therapy, the mice showed accelerated recovery of colony-forming unit (CFU)-granulocyte/macrophages and CFU-megakaryocytes when compared with the mice implanted with NIH3T3-Neo cells and BMT. Ten days after lethal irradiation with gamma rays, the spleens formed more CFU-spleen in mice implanted with NIH3T3-IL-6 cells and BMT than in mice injected with phosphate-buffered saline or NIH3T3-Neo cells. Combined therapy with NIH3T3-IL-6 cell implantation and BMT delayed the survival period of the hematopoietic-depressed mice significantly when compared with therapy with NIH3T3-Neo cell implantation and BMT. CONCLUSIONS: These data demonstrated that the combined therapy of fibroblast-mediated IL-6 gene therapy and BMT could significantly promote the recovery of irradiation-induced hematopoietic depression.  相似文献   
88.
89.
本文介绍一种由机械装置,高精度圆光栅编码器,光栅位移传感器,微型计算机,接口控制电路等组成的多测头凸轮轴测量系统,该系统安装调整方便,定位准确,测量精度高,操作简单,工作可靠,能快速实现对凸轮轴的自动测量。  相似文献   
90.
In 1990-1991, a national survey was conducted to estimate the prevalence of Salmonella species among Canadian commercial turkey flocks. Two hundred and seventy flocks were randomly selected across Canada. The proportion sampled from each province was selected according to each province's share of the national turkey market. Samples, consisting of 12 pooled litter and four pooled dust samples, were used to determine the Salmonella status of the environment of each flock. Additionally, a one kilogram sample of feed was taken from each flock premise. Salmonella was recovered from environmental samples in 234/270 (86.7%) of flocks and from feed samples in 26/266 (9.8%) of flocks. Forty-eight different Salmonella serovars were isolated from flock environmental samples. The most prevalent serovars were S. anatum, S. hadar, S. agona, S. heidelberg and S. saintpaul which were isolated from 53/270 (19.6%), 49/270 (18.1%), 49/270 (18.1%), 42/270 (15.6%) and 34/270 (12.6%) flocks, respectively.  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