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   3752篇
  免费   201篇
  国内免费   12篇
电工技术   63篇
综合类   3篇
化学工业   616篇
金属工艺   102篇
机械仪表   209篇
建筑科学   62篇
矿业工程   3篇
能源动力   131篇
轻工业   223篇
水利工程   10篇
石油天然气   4篇
无线电   562篇
一般工业技术   595篇
冶金工业   1030篇
原子能技术   34篇
自动化技术   318篇
  2024年   4篇
  2023年   40篇
  2022年   44篇
  2021年   83篇
  2020年   77篇
  2019年   76篇
  2018年   97篇
  2017年   100篇
  2016年   103篇
  2015年   90篇
  2014年   138篇
  2013年   147篇
  2012年   188篇
  2011年   221篇
  2010年   181篇
  2009年   192篇
  2008年   144篇
  2007年   150篇
  2006年   116篇
  2005年   114篇
  2004年   97篇
  2003年   85篇
  2002年   80篇
  2001年   65篇
  2000年   66篇
  1999年   81篇
  1998年   351篇
  1997年   230篇
  1996年   132篇
  1995年   72篇
  1994年   60篇
  1993年   79篇
  1992年   21篇
  1991年   23篇
  1990年   14篇
  1989年   18篇
  1988年   18篇
  1987年   9篇
  1986年   10篇
  1985年   17篇
  1983年   3篇
  1982年   4篇
  1981年   6篇
  1980年   13篇
  1978年   6篇
  1977年   29篇
  1976年   64篇
  1975年   3篇
  1959年   1篇
  1955年   1篇
排序方式: 共有3965条查询结果,搜索用时 15 毫秒
61.
OBJECTIVE: This investigation assessed whether current post-traumatic stress disorder (PTSD) was associated with impaired functioning in a nationally representative sample of female Vietnam veterans. METHODS: Logistic models were used to determine the association between PTSD and outcome while adjusting for demographic characteristics and medical and psychiatric co-morbidities. RESULTS: PTSD was associated with significantly elevated odds of poorer functioning in five of the six outcome domains; only the association between perpetration of violence in the past year and PTSD did not achieve statistical significance. After adjusting for demographics and medical and psychiatric co-morbidities, PTSD remained associated with significantly elevated odds of bed days, poorer physical health, and currently not working. CONCLUSIONS: Among female Vietnam veterans PTSD is associated with a broad profile of functional impairment. The significantly increased odds of impaired functioning and diminished quality of life suggest that PTSD may be the core problem of the set of problems afflicting female Vietnam veterans.  相似文献   
62.
The growth pattern of γ precipitates in the grains and at the grain boundaries has been investigated in a Ni-24Co-4Al-4Ti-5Cr-5Mo (weight percent) alloy of very small lattice misfit between the precipitate and the matrix phases under varying heat-treatment conditions. When aged at temperatures lower than the solvus temperature (T s = 1150 °C) by more than 30 °C after direct cooling from the solution-treatment temperature, the nucleation density is high. In this condition, the supersaturation is quickly removed because of the overlapping diffusion fields and the precipitates undergo Ostwald ripening from the early stage. The precipitates then have an equilibrium shape of spheres in the grains and truncated spheres at nearly straight grain boundaries. The precipitates at the grain boundaries are coherent with one of the grains, and their number density is not much larger than that in the grains, apparently because of a large contact angle (about 150 deg) with the grain boundary. Quenching the alloy after the solution treatment and aging at any temperature also produce high precipitate number density and equilibrium shapes. When aged at temperatures just belowT s (above 1140 °C), the nucleation density is low, the precipitates grow dendritically in the grains, and the grain boundaries become serrated. The observed dendritic growth characteristics do not quantitatively agree with the predictions of Mullins and Sekerka theory, but the discrepancy may be due to the uncertainties in both the observed and calculated quantities. By deeply etching the matrix, it is shown that the grain boundary serration is produced by the precipitates growing preferentially in the direction of the incoherent boundary because of the rapid solute diffusion along the grain boundary. The dendritic growth and grain boundary serration can be obtained also by slowly cooling through the temperature range just belowT s.  相似文献   
63.
64.
Independent risk factors for new coronary events in older African-American men were (1) age (risk ratio = 1.037), (2) cigarette smoking (risk ratio = 2.231), (3) hypertension (risk ratio = 2.531), (4) serum total cholesterol (risk ratio = 1.012), (5) serum high-density lipoprotein (HDL) cholesterol (inverse association) (risk ratio = 0.948), and (6) prior coronary artery disease (CAD) (risk ratio = 2.288). Independent risk factors for new coronary events in older African-American women were (1) cigarette smoking (risk ratio = 2.202), (2) hypertension (risk ratio = 2.344), (3) diabetes mellitus (risk ratio = 1.632), (4) serum total cholesterol (risk ratio = 1.008), (5) serum HDL cholesterol (inverse association) (risk ratio = 0.936), (6) age (risk ratio = 1.026), and (7) prior CAD (risk ratio = 2.368).  相似文献   
65.
Insulin-like growth factor (IGF)-I is thought to mediate a portion of the effects of bST on lactation in dairy cows. Serum concentrations of IGF-I are increased in lactating cows that were treated with bST, and IGF-I receptors are present in bovine mammary tissue. In addition, close arterial infusion of IGF-I into the mammary gland of goats increases milk yield. Little evidence exists to support a direct galactopoietic effect of IGF-I in ruminants. However, IGF-I is a potent mitogen for mammary epithelial cells and may also influence the inhibition of apoptosis of this cell type. The IGF are found in association with a family of individual binding proteins. The high affinity of the IGF for these proteins relative to the IGF receptor allows them to modulate IGF-I bioactivity in the mammary gland at the cellular level. Mammary epithelial cells synthesize multiple forms of IGF binding proteins, and one of these, IGF binding protein-3, is specifically regulated by the IGF. Stimulation of DNA synthesis by IGF-I is enhanced in bovine mammary epithelial cells that overexpress the IGF binding protein-3. These data indicate that IGF-I can stimulate the synthesis of an IGF binding protein, which enhances its own mitogenic activity. However, whether this mechanism is operative in the lactating mammary gland in vivo is unknown. Given the complexity of the interactions between the IGF and their binding proteins, more information is needed before the role of these growth factors in regulating growth, differentiation, and apoptosis of mammary epithelial cells is delineated.  相似文献   
66.
PURPOSE: The aim of this study was to determine the concentration of interleukin-1 alpha in human embryo culture medium with or without oviductal cell coculture and to correlate the interleukin-1 alpha levels with pregnancy. METHODS: Culture media from 32 in vitro fertilization and embryo transfer cycles were assayed for interleukin-1 alpha by immunoassay technique. Human embryos were cultured in Earles' balanced salt solution supplemented with 15% preovulatory serum (sEBSS) in 16 of these cycles, while embryos in the rest of the cycles were cocultured with human oviductal cells in sEBSS. RESULTS: Both sEBSS and spent sEBSS after embryo culture contained low or undetectable levels of interleukin-1 alpha in the pregnant and nonpregnant cycles. On the other hand, oviductal cells significantly increased the amount of interleukin-1 alpha immunoreactivity in the conventional culture medium or coculture medium (P < 0.001, Mann-Whitney rank sum test). The concentrations of interleukin-1 alpha in the spent sEBSS after oviductal cell culture and after coculture with human embryos were 1.5 +/- 1.0 and 1.3 +/- 0.9 pg/ml, respectively. There was no difference in the interleukin-1 alpha concentration between the pregnant and the nonpregnant coculture cycles. CONCLUSIONS: These data showed that human oviductal cells produced interleukin-1 alpha immunoreactivity in a coculture system. However, this production could not be used as a marker for successful embryo implantation.  相似文献   
67.
We investigated whether inhibition of the BCR-ABL tyrosine kinase by the CGP57418B compound would render chronic myeloid leukaemia (CML) cells susceptible to Fas (CD95, Apo-1)-mediated cell death. Only two (AR230 and SD1) out of 10 BCR-ABL positive cell lines were found to express the CD95 protein. No change in Fas expression was observed in any of the 10 cell lines after 48 h exposure to CGP57418B. AR230 cells were resistant and SD1 cells were partially resistant to Fas-mediated apoptosis induced by ligation of the Fas receptor to an anti-Fas IgM antibody. Pre-incubation with 1 microM CGP57418B did not change the susceptibility of these cell lines to Fas-mediated cell death. Similar results were observed in experiments with CD34+ cells from CML patients and from normal individuals. The data suggest that, in contrast to some cytotoxic drugs, the CGP57148B tyrosine kinase inhibitor utilizes a pathway other than the CD95 system in order to induce apoptosis in CML cells.  相似文献   
68.
69.
Ankylosing spondylitis (AS) results in disease-specific inflammation at the site of ligamentous insertion into the bone. Atlantoaxial joint subluxation and vertical subluxation of the axis may occur as a consequence of instability resulting from the inflammatory process. Spontaneous anterior atlantoaxial subluxation is a well recognized complication in about 2% of patients with AS, and presents with or without signs of spinal cord compression. Vertical subluxation may follow anterior or posterior subluxation. It was noted in 3-8% of patients with rheumatoid arthritis, but is an exceedingly rare complication of AS. Moreover, it has never been reported that multiple cerebellar infarction and bulbar symptoms developed spontaneously due to atlanto-occipital subluxation and vertical subluxation in a patient with a long [corrected] history of AS. We describe a man with AS who developed multiple cerebellar infarction due to vertebral artery obstruction and bulbar symptoms associated with atlanto-occipital subluxation and vertical subluxation.  相似文献   
70.
Many online shopping malls in which explicit rating information is not available still have difficulty in providing recommendation services using collaborative filtering (CF) techniques for their users. Applying temporal purchase patterns derived from sequential pattern analysis (SPA) for recommendation services also often makes users unhappy with the inaccurate and biased results obtained by not considering individual preferences. The objective of this research is twofold. One is to derive implicit ratings so that CF can be applied to online transaction data even when no explicit rating information is available, and the other is to integrate CF and SPA for improving recommendation quality. Based on the results of several experiments that we conducted to compare the performance between ours and others, we contend that implicit rating can successfully replace explicit rating in CF and that the hybrid approach of CF and SPA is better than the individual ones.  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