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   1481篇
  免费   48篇
  国内免费   3篇
电工技术   11篇
化学工业   283篇
金属工艺   42篇
机械仪表   19篇
建筑科学   48篇
矿业工程   3篇
能源动力   26篇
轻工业   122篇
水利工程   43篇
石油天然气   28篇
武器工业   1篇
无线电   121篇
一般工业技术   235篇
冶金工业   375篇
原子能技术   14篇
自动化技术   161篇
  2021年   20篇
  2020年   11篇
  2019年   17篇
  2018年   21篇
  2017年   24篇
  2016年   27篇
  2015年   18篇
  2014年   28篇
  2013年   99篇
  2012年   51篇
  2011年   83篇
  2010年   64篇
  2009年   49篇
  2008年   59篇
  2007年   55篇
  2006年   55篇
  2005年   46篇
  2004年   48篇
  2003年   54篇
  2002年   35篇
  2001年   24篇
  2000年   32篇
  1999年   15篇
  1998年   29篇
  1997年   44篇
  1996年   30篇
  1995年   27篇
  1994年   30篇
  1993年   23篇
  1992年   33篇
  1991年   23篇
  1990年   26篇
  1989年   24篇
  1988年   28篇
  1987年   16篇
  1986年   20篇
  1985年   14篇
  1984年   16篇
  1983年   22篇
  1982年   18篇
  1981年   19篇
  1980年   20篇
  1979年   12篇
  1978年   10篇
  1977年   12篇
  1976年   10篇
  1975年   15篇
  1974年   7篇
  1973年   13篇
  1970年   9篇
排序方式: 共有1532条查询结果,搜索用时 398 毫秒
71.
The greater wax moth, Galleria mellonella (L.), is a serious and widespread pest of the honeybee, Apis mellifera L. In contrast to most moths, for which long-range mate finding is mediated by female-produced sex pheromones, G. mellonella males attract conspecific females over long distances by emitting large amounts of a characteristic scent in combination with bursts of ultrasonic calls. The male scent for this species was previously identified as a blend of nonanal and undecanal. When these compounds were bioassayed, characteristic short-range sexual behavior, including wing fanning, was triggered in conspecific females, but the aldehyde blend failed to elicit attraction over longer distances. We identified, via analysis and synthesis, a third male-specific compound, 5,11-dimethylpentacosane. We show that it acts as a behavioral synergist to the aldehydes. In wind tunnel experiments, very few female moths responded to the aldehyde blend or to 5,11-dimethylpentacosane tested separately, but consistently showed orientation and source contact when a combination of all three compounds was applied. The level of attraction to the three-component mixture was still lower than that to male extract, indicating that the composition of compounds in the synthetic blend is suboptimal, or that additional pheromone components of G. mellonella are yet to be identified. The identification of 5,11-dimethylpentacosane is an important step for the development of an efficient long-range attractant that will be integrated with other environmentally safe strategies to reduce damage to beehives caused by wax moths.  相似文献   
72.
Alkoxide-directed metallacycle-mediated cross-coupling is a rapidly growing area of reaction methodology in organic chemistry. Over the last decade, developments have resulted in more than thirty new and highly selective intermolecular (or “convergent”) C−C bond-forming reactions that have established powerful retrosynthetic relationships in stereoselective synthesis. While early studies were focused on developing transformations that forge a single C−C bond by way of a functionalized and unsaturated metallacyclopentane intermediate, recent advances mark the ability to employ this organometallic intermediate in additional stereoselective transformations. Among these more advanced coupling processes, those that embrace the metallacycle in subsequent [4+2] chemistry have resulted in the realization of a number of highly selective annulative cross-coupling reactions that deliver densely functionalized and angularly substituted carbocycles. This review discusses the early development of this chemistry, recent advances in reaction methodology, and shares a glimpse of the power of these processes in natural product synthesis.  相似文献   
73.
Despite data that traditional laboratory-based outcome measures in dialysis are improving over time, population-based data indicate that mortality rates are not improving in parallel. With increased focus on performance measures based on laboratory-based outcomes (e.g., hematocrit, albumin, and parathyroid hormone), less emphasis has been placed on other markers, some of which may be stronger predictors of mortality. We performed a systematic review to interpret the predictive value of laboratory-based outcome measures in dialysis. We identified studies with data regarding the predictive value of laboratory-based outcomes for mortality in dialysis. We calculated the sample size-weighted pooled relative risk of death with dichotomized "high" vs. "low" levels of each measure. We rank-ordered predictors by scaling the pooled relative risk of each measure by its pooled standard deviation. There were 5171 titles, of which 128 (representing 44 laboratory-based outcomes) were selected. Nine were significantly associated with mortality, in order of decreasing scaled effect size: (1) tumor necrosis factor-α, (2) hematocrit, (3) interleukin-6, (4) troponin T, (5) Kt/Vurea, (6) prealbumin, (7) urea reduction ratio, (8) serum albumin, and (9) C-reactive protein. Other oft-cited measures such as calcium phosphate product and parathyroid hormone were not significantly associated with mortality in pooled analysis. Quality improvement efforts to improve traditional laboratory-based outcomes in end-stage renal disease are necessary, but likely insufficient, to improve overall mortality in dialysis. Renewed consideration of cardiovascular, inflammatory, and nutritional markers that are especially strong predictors of mortality may have important implications for risk stratification and targeted therapeutic interventions.  相似文献   
74.
The provision of psychological (e.g., psychotherapy, coparenting, mediation, collaborative divorce, child custody evaluation) services for families of divorce are growing specialties for many clinical and forensic psychologists. However, practice in domestic relations psychology, such as divorce and custody assessment and testimony, is a high-risk venture for incurring ethics complaints and law suits. The lead article and the three commentaries that follow enumerate the various roles practitioners might play, clarify how each role requires specific skill sets that may be outside one’s particular competence and necessitate additional training, describe how countertransferential issues arise, urge meticulous record keeping, discuss some of the subtleties of confidentiality and the releasing of information, describe how transparency in clarifying expectations leads to a lowering of contentiousness, and provide tips for divorce and forensic practice. The special challenges associated with psychologists being tempted to move beyond their role on a case and those of particularly high-risk situations (e.g., complaints of child abuse or domestic violence) are also discussed. Suggestions are offered to help psychologists better serve the public while taking steps to better inoculate themselves from complaints. (PsycINFO Database Record (c) 2010 APA, all rights reserved)  相似文献   
75.
Factor-analytic research is common in the study of constructs and measures in psychological assessment. Latent factors can represent traits as continuous underlying dimensions or as discrete categories. When examining the distributions of estimated scores on latent factors, one would expect unimodal distributions for dimensional data and bimodal or multimodal distributions for categorical data. Unfortunately, identifying modes is subjective, and the operationalization of counting local maxima has not performed very well. Rather than locating and counting modes, the authors propose performing parallel analyses of categorical and dimensional comparison data and calculating an index of the relative fit of these competing structural models. In an extensive Monte Carlo study, the authors replicated prior results for mode counting and found that trimming distributions' tails helped. However, parallel analyses of comparison data achieved much greater accuracy, improved base rate estimation, and afforded consistency checks with other taxometric procedures. Two additional studies apply this approach to empirical data either known to be categorical or presumed to be dimensional. Each study supports this new method for factor-analytic research on the latent structure of constructs and measures in psychological assessment. (PsycINFO Database Record (c) 2010 APA, all rights reserved)  相似文献   
76.
Electromagnetically assisted sheet metal stamping   总被引:3,自引:0,他引:3  
A new approach, electromagnetically assisted sheet metal stamping, has been developed to alter strain distribution and improve formability in sheet metal stamping. In this study, this new approach was applied to form a non-symmetric panel from Al 6111-T4. The results show that this new approach greatly increased the draw depth of the formed panel, compared with conventional stamping. A detailed analysis of strain distribution, stretching and draw-in shows that both a more homogeneous strain distribution and enhanced draw-in contribute to producing deeper pans in a single press operation. This work demonstrates the feasibility of electromagnetically assisted sheet metal stamping, which offers a much improved ability to make complex components in a single press stroke as compared to conventional stamping.  相似文献   
77.
In order to develop a microbiological baseline of meat produced for domestic consumption in the state of New South Wales, chilled carcases were sampled from 16 abattoirs. Aerobic Plate Counts (APCs) and Escherichia coli counts were obtained from samples taken by sponging sites specified for each species in the Microbiological Guidelines to the Australian meat standard. On beef carcases the mean log10 APC/cm2 was 2.21 and E. coli was detected on 25% of carcases (mean log positives ?0.61/cm2). For sheep carcases corresponding values were 2.4, 53% and log10 -0.06/cm2, respectively. For pig carcases values were 2.81, 63% and log10 -0.23/cm2, respectively. For skin-off goat carcases values were 1.15, 27% and log10 -0.38/cm2, respectively. In the present survey, levels of indicator bacteria on carcases processed via the co-regulatory system operated in Australia were similar to those established in surveys of abattoirs that operate the traditional system overseen by government inspectors.  相似文献   
78.
79.
In two previous studies on general and violent recidivism (Walters & Heilbrun, 2010; Walters, Knight, Grann, & Dahle, 2008), the summed composite antisocial facet of the Psychopathy Checklist displayed incremental validity relative to the other 3 facets (interpersonal, affective, lifestyle), whereas the other 3 facets generally failed to demonstrate incremental validity relative to the antisocial facet. Because summed composite scores do not account for ordinal item distributions, the 6 Walters et al. (2008) samples were reanalyzed with factor score composites derived from a 4-factor confirmatory factor analysis. The results, however, showed little change from what had been obtained earlier with summed composite scores. Two additional samples not previously included in any incremental validity analyses of the Psychopathy Checklist evidenced a 3-factor structure, with the lifestyle and antisocial facets merged into a single factor. This single factor displayed incremental validity relative to the interpersonal and affective facets, but the reverse was not true regardless of whether summed composite scores or factor score composites were used. A comparison of zero-order correlations from all 8 samples revealed that the antisocial summed composite score predicted significantly better than the summed composite scores for the other 3 facets and that a superordinate factor failed to improve on the performance of either the antisocial summed composite score or the antisocial factor score composite. (PsycINFO Database Record (c) 2011 APA, all rights reserved)  相似文献   
80.
New generations of video compression algorithms, such as those included in the under development High Efficiency Video Coding (HEVC) standard, provide substantially higher compression compared to their ancestors. The gain is achieved by improved prediction of pixels, both within a frame and between frames. Novel coding tools that contribute to the gain provide highly uncorrelated prediction residuals for which classical frequency decomposition methods, such as the discrete cosine transform, may not be able to supply a compact representation with few significant coefficients. To further increase the compression gains, this paper proposes transform skip modes which allow skipping one or both 1-D constituent transforms (i.e., vertical and horizontal), which is more suitable for sparse residuals. The proposed transform skip mode is tested in the HEVC codec and is able to provide bitrate reductions of up to 10% at the same objective quality when compared with the application of 2-D block transforms only. Moreover, the proposed transform skip mode outperforms the full transform skip currently investigated for possible adoption in the HEVC standard.  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