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   2518篇
  免费   15篇
电工技术   11篇
化学工业   50篇
金属工艺   4篇
机械仪表   7篇
建筑科学   11篇
能源动力   13篇
轻工业   37篇
水利工程   2篇
石油天然气   2篇
无线电   30篇
一般工业技术   45篇
冶金工业   2250篇
原子能技术   6篇
自动化技术   65篇
  2021年   3篇
  2019年   5篇
  2018年   8篇
  2017年   4篇
  2016年   6篇
  2015年   6篇
  2014年   4篇
  2013年   10篇
  2012年   6篇
  2011年   15篇
  2010年   9篇
  2009年   16篇
  2008年   5篇
  2007年   5篇
  2006年   4篇
  2005年   5篇
  2004年   7篇
  2003年   17篇
  2002年   5篇
  2001年   8篇
  2000年   8篇
  1999年   73篇
  1998年   766篇
  1997年   446篇
  1996年   276篇
  1995年   168篇
  1994年   122篇
  1993年   127篇
  1992年   21篇
  1991年   25篇
  1990年   17篇
  1989年   34篇
  1988年   20篇
  1987年   21篇
  1986年   29篇
  1985年   24篇
  1984年   8篇
  1983年   8篇
  1982年   11篇
  1981年   17篇
  1980年   14篇
  1979年   5篇
  1978年   5篇
  1977年   38篇
  1976年   67篇
  1975年   7篇
  1959年   3篇
  1948年   2篇
  1919年   2篇
  1890年   3篇
排序方式: 共有2533条查询结果,搜索用时 15 毫秒
61.
Small-cell variants of Sézary syndrome and mycosis fungoides (MF) have been described. However, in these studies the nuclear area of the small-cell variant of MF (SC-MF) as compared to histological classical MF (CL-MF) was not characterized objectively by quantitative electron microscopy. In a 14-year follow-up period, of a total of 76 patch/plaque stage MF patients seen in the Department of Dermatology of the University Hospital Utrecht, 14 (18%) had an infiltrate composed of atypical lymphocytes characterized by a distinctly smaller cell diameter and smaller, hyperchromatic, deeply indented nuclei as compared to the usual cell type of MF. The aim of the investigation was to confirm this observation objectively using quantitative electron microscopy (morphometry) and to define SC-MF as compared to CL-MF. The study was performed on the 14 patients with SC-MF, and 10 patients with clinical and histological CL-MF and 4 patients with chronic eczema. Electron micrographs of sections obtained from each biopsy were analysed by computer to produce the following data: a nuclear contour index (NCI), the mean nuclear area (MNA), the mean nuclear area of the cells above the 75th percentile (P75NA) and the percentage of cells larger than 30 microm2. The values of MNA differed significantly between patients with SC-MF and those with CL-MF (17.6 vs 23.2 microm2; P = 0.02), as did the values of P75NA (20.7 vs 27.9 microm2; P = 0.01). The NCI of the SC-MF and CL-MF patients were similar. These results are consistent with our observations that SC-MF does indeed exist.  相似文献   
62.
63.
Despite a proliferation of recent research examining childhood and adolescent depression, the area still lags behind the adult depression field, particularly in the investigation of cognitive correlates of affective psychopathology. To advance cognitive research with youth, the Children's Negative Cognitive Error Questionnaire (CNCEQ) was developed to provide a measure of cognitive distortions or errors in children and adolescents. Yet, few studies have employed the CNCEQ and no evidence exists supporting the validity of its four component cognitive error scales. The purpose of the present study was to examine the construct validity of the CNCEQ and its constituent scales through the use of factor analysis and criterion-group comparisons. Groups of adolescent psychiatric inpatients, diagnosed as affective or disruptive disordered, completed the CNCEQ following admission. Results failed to support the implicit four-factor structure of the CNCEQ, instead suggesting the appropriateness of a single-factor solution labeled "negative thinking." Despite no diagnostic group differences on the CNCEQ total or other scale scores, affective disordered patients evinced more cognitive errors on the Overgeneralizing scale. Findings suggest that the CNCEQ in its current stage of development holds promise, yet requires refinement to produce a valid measure of cognitive functioning in youth.  相似文献   
64.
A cell line that produces an autoantibody specific for DNA quadruplex structures has been isolated and cloned from a hybridoma library derived from 3-month-old nonimmunized autoimmune, immunodeficient "viable motheaten" mice. This antibody has been tested extensively in vitro and found to bind specifically to DNA quadruplex structures formed by two biologically relevant sequence motifs. Scatchard and nonlinear regression analyses using both one- and two-site models were used to derive association constants for the antibody-DNA binding reactions. In both cases, quadruplexes had higher association constants than triplex and duplex molecules. The anti-quadruplex antibody binds to the quadruplex formed by the promoter-region-derived oligonucleotide d(CGCG4GCG) (Ka = 3.3 x 10(6) M-1), and has enhanced affinity for telomere-derived quadruplexes formed by the oligonucleotides d(TG4) and d(T2G4T2G4T2G4T2G4) (Ka = 5.38 x 10(6) and 1.66 x 10(7) M-1, respectively). The antibody binds both types of quadruplexes but has preferential affinity for the parallel four-stranded structure. In vitro radioimmunofilter binding experiments demonstrated that purified anti-DNA quadruplex antibodies from anti-quadruplex antibody-producing tissue culture supernatants have at least 10-fold higher affinity for quadruplexes than for triplex and duplex DNA structures of similar base composition and length. The antibody binds intramolecular DNA triplexes formed by d(G4T3G4T3C4) and d(C4T3G4T3G4), and the duplex d(CGCGCGCGCG)2 with an affinities of 6. 76 x 10(5), 5.59 x 10(5), and 8.26 x 10(5) M-1, respectively. Competition experiments showed that melted quadruplexes are not effective competitors for antibody binding when compared to native structures, confirming that the quadruplex is bound structure-specifically. To our knowledge, this is the first immunological reagent known to specifically recognize quadruplex structures. Subsequent sequence analysis demonstrates homologies between the antibody complementarity determining regions and sequences from Myb family telomere binding proteins, which are hypothesized to control cell aging via telomeric DNA interactions. The presence of this antibody in the autoimmune repertoire suggests a possible linkage between autoimmunity, telomeric DNA binding proteins, and aging.  相似文献   
65.
Self-report questionnaires have been used to compare panic patients' fears, anxious thoughts and behaviors with those of a group of controls. Individual items revealed significant differences between groups in all three areas: factor analysis revealed group differences in the pattern of intraquestionnaire associations between items. Correlational analysis of factor scores revealed significant group differences in the pattern of interquestionnaire associations. These findings only partly support a formulation of panic based upon "catastrophic interpretations." On the other hand, they do provide evidence of cognitive rigidity among panic patients which can be interpreted as evidence of impaired effortful processing when anxious.  相似文献   
66.
The null controllable set of a system is the largest set of states that can be controlled to the origin. Control systems that have a region of attraction equal to the null controllable set are said to be maximally controllable closed-loop systems. In the case of open-loop unstable plants with amplitude constrained control it is well known that the null controllable set does not cover the entire state-space. Further the combination of input constraints and unstable system dynamics results in a set of state constraints which we call implicit constraints. It is shown that the simple inclusion of implicit constraints in a controller formulation results in a controller that achieves maximal controllability for a class of open-loop unstable systems.  相似文献   
67.
In this paper a new systematic switching control approach to adaptive stabilization of linear time-varying (LTV) discrete-time systems is presented. A feature of the localization based method is its high model falsification capability, which in the case of LTI systems is manifested as the rapid convergence of the switching controller. We believe that the proposed method may help pave the way for design of practical adaptive switching controllers applicable to a wide range of linear time-invariant and time-varying systems  相似文献   
68.
Edge detection in a hexagonal-image processing framework   总被引:2,自引:0,他引:2  
With processing power of computers and capabilities of graphics devices increasing rapidly, the time is ripe to reconsider using hexagonal sampling for computer vision in earnest. This paper reports on an investigation of edge detection in the context of hexagonally sampled images. It presents a complete framework for processing hexagonally sampled images which addresses four key aspects: conversion of square to hexagonally sampled images, storage, processing, and display of these images. Results from using edge detection on this framework show that (a) the computational requirement for processing a hexagonally sampled image is less than that for square sampled images, and (b) a better qualitative performance which is due to the compact and circular nature of the hexagonal lattice. This last point needs to be exploited in the development of edge detectors for hexagonally sampled images.  相似文献   
69.
Analysis of the S-phase fractions (SPF) measured by in vitro thymidine labeling, morphological appearances, and estrogen receptor (ER) assays of primary invasive breast carcinomas demonstrated several interrelationships. Lobular, mucinous, tubular, and adenocystic carcinomas consistently had low SPF and were usually positive for ER. The same was true for the carcinomas of no special histologic type [the not otherwise specified (NOS) group of E. R. Fisher including "infiltrating ductal" and undifferentiated carcinomas] with minimal anaplasia. Medullary, atypical medullary, and morphologically unclassifiable carcinomas with marked nuclear anaplasia nearly always had high SPF and were usually negative for ER. High SPF was associated with advanced stages of carcinoma initially or with early recurrence following mastectomy.  相似文献   
70.
During a cardiovascular survey, aimed at detecting cases of latent coronary heart disease (CHD), glucose elimination was studied after i.v. loading in 1970 presumably healthy men aged 40-59 years. The aim was to throw light on the importance of deranged glucose tolerance for the development of CHD. Of the 1970 individuals, 1798 were defined as "normals", 33 had chronic, non-anginal chest pain, 34 had slight albeit typical angina pectoris. The remaining 105 had various symptoms/signs strongly suggestive of CHD, and underwent diagnostic coronary angiography (69 angiopositive, 36 angionegative). Plasma insulin was determined in relation to the test in 249 of the subjects. The following conclusions were reached: 1) Mean k-values were similar in all subgroups (p less than 0.10). 2) Low and borderline k-values were significantly more frequent in angiographed individuals compared with the group of normals (p less than 0.025). However, an almost identical frequency was seen in angiopositive and angionegative cases. 3) K-values did not change with age between 40 and 59 years. 4) K-values were unrelated to the severity of angiographic findings in individuals with proven CHD. 5) Significantly lower k-values were found in individuals with a positive diabetic heredity, and 6) in individuals with a high insulin response. 7) The i.v. glucose loading did not influence an exercise ECG recorded in relation to a near-maximal bicycle exercise test.  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