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   435141篇
  免费   6736篇
  国内免费   1489篇
电工技术   8000篇
综合类   574篇
化学工业   66132篇
金属工艺   14744篇
机械仪表   12317篇
建筑科学   12476篇
矿业工程   949篇
能源动力   11858篇
轻工业   46854篇
水利工程   3511篇
石油天然气   1960篇
武器工业   19篇
无线电   56918篇
一般工业技术   79524篇
冶金工业   79939篇
原子能技术   5728篇
自动化技术   41863篇
  2021年   3326篇
  2020年   2262篇
  2019年   2970篇
  2018年   4439篇
  2017年   4552篇
  2016年   4753篇
  2015年   3898篇
  2014年   6427篇
  2013年   20482篇
  2012年   11172篇
  2011年   15921篇
  2010年   12192篇
  2009年   13853篇
  2008年   14744篇
  2007年   15007篇
  2006年   13392篇
  2005年   12590篇
  2004年   12266篇
  2003年   11913篇
  2002年   11550篇
  2001年   11678篇
  2000年   10639篇
  1999年   11136篇
  1998年   25525篇
  1997年   18565篇
  1996年   14601篇
  1995年   11435篇
  1994年   10107篇
  1993年   9739篇
  1992年   7276篇
  1991年   6979篇
  1990年   6584篇
  1989年   6261篇
  1988年   6015篇
  1987年   5011篇
  1986年   4962篇
  1985年   6108篇
  1984年   5690篇
  1983年   4895篇
  1982年   4593篇
  1981年   4545篇
  1980年   4296篇
  1979年   4150篇
  1978年   3843篇
  1977年   4654篇
  1976年   6271篇
  1975年   3180篇
  1974年   3040篇
  1973年   2930篇
  1972年   2344篇
排序方式: 共有10000条查询结果,搜索用时 15 毫秒
991.
Minimally invasive plastic surgery has expanded beyond the original confines of aesthetic applications to encompass all our endeavors in an attempt to restrict the size of surgical scars, limit postoperative discomfort, and hasten recovery of function. This evolution has already delineated methods to raise our workhorse muscle flaps and has negated the risks of laparotomy for various visceral flaps. It is then only a logical progression to use these endoscopic techniques to harvest fascial flaps so as to avoid the notorious donor site morbidity of the fasciocutaneous flap, which has certainly hindered the rapid acceptance of these otherwise valuable flaps. Endoscopic-facilitated elevation of a local adipofascial flap is described for which little or no additional skin incisions need ever be made.  相似文献   
992.
Algorithm for cosine transform of Toeplitz matrices   总被引:1,自引:0,他引:1  
An algorithm for calculating the 2D cosine transform of a Toeplitz matrix is presented. The algorithm is based on the application of 1D cosine transforms. More specifically, four 1D cosine transforms of size N are needed to obtain the transform of a Toeplitz matrix of size N×N. This is an improvement over previously published algorithms. The algorithm is also simple and regular  相似文献   
993.
A generator of chaos in wavelength is reported. It is formed by a wavelength-tunable laser diode with a time delayed feedback loop in which a wavelength nonlinear-linear device is introduced. The dynamical regime of wavelength emission thus obtained is ruled by a differential difference equation. Experimental results are compared with numerical simulations and with previous theoretical and experimental results  相似文献   
994.
Using high-NA step index optical fibers, we show, theoretically, that large-bandwidth second-harmonic generation and sum frequency generation can be achieved at conversion frequencies in the visible region of the spectrum  相似文献   
995.
The pathological basis of nerve inexcitability in Guillain-Barré syndrome has not been established with certainty. We report the clinicopathological findings in a 67-year-old patient with fulminant Guillain-Barré syndrome who died 18 days after onset. Three serial electrophysiological studies revealed nerve inexcitability. Antibodies to Campylobacter jejuni were present but there was no antiganglioside reactivity. Spinal root sections revealed extensive and almost pure macrophage-associated demyelination with occasional presence of T lymphocytes and neutrophil leukocytes. Conversely, in femoral, median, and sural nerves the outstanding lesion was axonal degeneration, with some denuded axons remaining. Unmyelinated fibers, posterior root ganglia, and dorsal columns were preserved. Endoneurial postcapillary venules showed plump endothelial cells with loss of their tight junctions. We conclude that both primary demyelination and axonal degeneration secondary to inflammation account for nerve inexcitability. Our findings lend support to the hypothesis of increased endoneurial pressure as the cause of wallerian degeneration in nerve trunks.  相似文献   
996.
The purpose of this study was to determine the long-term results of allogeneic bone marrow transplantation for chronic myeloid leukemia. A retrospective analysis was carried out of the outcome of 373 consecutive transplants performed at 38 European institutions between 1980 and 1988 and reported to the registry of the European Group for Blood and Marrow Transplantation. All transplants were carried out for first chronic phase of chronic myelogenous leukemia using unmanipulated marow cells from HLA-identical sibling donors. The probability of survival and leukemia-free survival at 8 years were 54% (95% CI: 49-59) and 47% (95% CI: 41-52) respectively. The probabilities of developing acute GVHD (II-IV) at 100 days and chronic GVHD at 4 years after transplant were 47% (95% CI: 41-53) and 52% (95% CI: 46-58) respectively. The probabilities of transplant-related mortality and leukemic relapse 8 years after BMT were 41% (95% CI: 36-48) and 19% (95% CI: 14-25), respectively. Transplant within 12 months of diagnosis was associated with reduced transplant-related mortality (34 vs 45%, P = 0.013) and resulted in improved leukemia-free survival (52 vs 44%, P = 0.03). The probability of relapse was significantly reduced in patients who developed chronic GVHD (RR = 0.33, P = 0.004). The probability of relapse occurring more than 2 years after transplant was increased more than five-fold in patients transplanted from a male donor (RR = 5.5, P = 0.006). Sixty-seven patients in hematologic remission were studied for residual disease by two-step RT/PCR for BCR-ABL mRNA and 61 (91%) tested negative. We conclude that bone marrow transplantation can induce long-term survival in approximately one-half of CML patients; the majority of survivors have no evidence of residual leukemia cells when studied by molecular techniques. The probability of late relapse is increased with use of a male donor.  相似文献   
997.
This paper focus on some characteristics of the qualitative methodology. Some of these methods are explored such as: participatory research, phenomenology, grounded theory and ethnography critical theory Perspectives of their utilization in nursing research are examined.  相似文献   
998.
999.
Abstract. This paper examines the capacity of the research designs of 37 empirical studies of information systems (IS) effectiveness to provide a basis for the development of theories about behaviour related to IS effectiveness. The power of each study to support causal inference was evaluated in terms of (a) its handling of the time dimension, (b) its ability to weigh differences and (c) its resistance to internal validity threats that pose alternative explanations for its reported findings. Of the reviewed studies, 29.7% could account for the time dimension, 32.4% employed a comparison group and 16.2% were not susceptible to any internal validity threats. Only 13.5% of the studies combined an accounting for the time dimension with the use of a comparison group. Of these, however, only 5.4% were also invulnerable to internal validity threats. The research designs of nearly 95% of these published studies were deficient in supporting causal inference. In those studies, suggestions that one variable was causally related to another variable could not be substantiated. Encouragement for the future capacity of IS effectiveness research to support causal inference was found in a trend towards the use of quasiexperimental designs. Recommendations are made regarding ways to increase the inferential capacity of research designs employed in the study of IS effectiveness.  相似文献   
1000.
We present a method for recovering from syntax errors encountered during parsing. The method provides a form of minimum distance repair, has linear time complexity, and is completely automatic. A formal method is presented for evaluating the performance of error recovery methods, based on global minimum-distance error correction. The minimum-distance error recovery method achieves a theoretically best performance on 80% of Pascal programs in the weighted Ripley-Druseikis collection. Comparisons of performance with other error recovery methods are given.  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