全文获取类型
收费全文 | 271030篇 |
免费 | 18370篇 |
国内免费 | 9303篇 |
专业分类
电工技术 | 12985篇 |
技术理论 | 24篇 |
综合类 | 14705篇 |
化学工业 | 43261篇 |
金属工艺 | 13713篇 |
机械仪表 | 15367篇 |
建筑科学 | 18370篇 |
矿业工程 | 7898篇 |
能源动力 | 6820篇 |
轻工业 | 15452篇 |
水利工程 | 4194篇 |
石油天然气 | 16690篇 |
武器工业 | 1652篇 |
无线电 | 26910篇 |
一般工业技术 | 30580篇 |
冶金工业 | 34133篇 |
原子能技术 | 2428篇 |
自动化技术 | 33521篇 |
出版年
2024年 | 1049篇 |
2023年 | 3991篇 |
2022年 | 6593篇 |
2021年 | 9195篇 |
2020年 | 7092篇 |
2019年 | 6057篇 |
2018年 | 7165篇 |
2017年 | 8168篇 |
2016年 | 7731篇 |
2015年 | 9618篇 |
2014年 | 11658篇 |
2013年 | 13672篇 |
2012年 | 16314篇 |
2011年 | 17831篇 |
2010年 | 14066篇 |
2009年 | 13431篇 |
2008年 | 12933篇 |
2007年 | 12463篇 |
2006年 | 12892篇 |
2005年 | 14009篇 |
2004年 | 9766篇 |
2003年 | 8223篇 |
2002年 | 6282篇 |
2001年 | 5613篇 |
2000年 | 5704篇 |
1999年 | 7049篇 |
1998年 | 11560篇 |
1997年 | 8237篇 |
1996年 | 6682篇 |
1995年 | 4938篇 |
1994年 | 3877篇 |
1993年 | 3059篇 |
1992年 | 1776篇 |
1991年 | 1527篇 |
1990年 | 1232篇 |
1989年 | 1013篇 |
1988年 | 824篇 |
1987年 | 555篇 |
1986年 | 475篇 |
1985年 | 362篇 |
1984年 | 207篇 |
1983年 | 194篇 |
1982年 | 251篇 |
1981年 | 273篇 |
1980年 | 260篇 |
1979年 | 95篇 |
1978年 | 123篇 |
1977年 | 628篇 |
1976年 | 1354篇 |
1975年 | 107篇 |
排序方式: 共有10000条查询结果,搜索用时 0 毫秒
81.
Increased concentrations of octachlorodibenzo-p-dioxin in cases with breast cancer--results from a case-control study 总被引:1,自引:0,他引:1
L Hardell G Lindstr?m G Liljegren P Dahl A Magnuson 《Canadian Metallurgical Quarterly》1996,5(5):351-357
Organochlorines are persistent and highly lipophilic environmental contaminants which bioaccumulate in the food chain. Some of these chemicals, 2,2-bis(p-chlorophenyl)-1,1,1-trichloroethane (DDT) and polychlorinated biphenyls (PCBs), have been suggested to be of significance in the aetiology of breast cancer. 2,3,7,8-Tetrachlorodibenzo-p-dioxin (TCDD) is an anti-oestrogen in animal studies and should be thus lower the risk of breast cancer. The other isomers of polychlorinated dibenzo-p-dioxins (PCDDs) or the chemically related polychlorinated dibenzofurans (PCDFs) have not been tested regarding carcinogenesis of the breast. The purpose of this study was to investigate whether PCDDs or PCDFs influence the risk for breast cancer. Consecutive patients who underwent surgery for a breast disease between 1993 and 1995 were recruited for the study. Cases were 22 patients with infiltrative breast cancer and controls were 19 patients operated for a benign breast disease during the same time period. Approximately 10 g of breast tissue free from tumour was taken from the specimen and frozen until analysis. Fat was extracted, cleaned and analysed with a high-resolution gas chromatograph coupled to a high-resolution mass spectrometer. Median concentrations of octachlorinated dibenzo-p-dioxin (OCDD) were 598 (170-14,880) and 396 (103-1,847) pg/g lipid in the cases and in the controls, respectively. In a multivariate logistic regression analysis controlling for other risk factors for breast cancer increased odds ratio (OR) was obtained for OCDD: 401-1000 pg/g lipid yielded OR 3.8, 95% confidence interval (CI) 0.4-39, > 1000 pg/g lipid gave OR 5.2, CI 0.4-72. When the lipid OCDD variable was examined as a continuous risk factor there was a 1.09 (9%), CI 0.95-1.25, increase in the adjusted OR for breast cancer per 100 unit (pg/g lipid) increase in OCDD. No differences were found between cases and controls for the other six tested PCDDs. Mean concentration of TCDD was in the cases 3.6 (1.0-7.9) and in the controls 3.3 (1.1-6.3) pg/g lipid. For PCDFs no significant differences were found between cases and controls. The results were not changed if oestrogen or progesterone receptor status, S-phase fraction and DNA ploidy were considered. Breast tissue concentration of OCDD was increased in cancer patients, whereas the concentrations of other PCDDs and PCDFs were equal in cases and controls. 相似文献
82.
Although cortico steroids are effective anti-inflammatory agents in ameliorating asthma symptoms and bronchial hperreactivity, their mechanism of action is unknown. Interleukin (IL)-5 is known to play a key role in regulating eosinophil proliferation and activation. Therefore, we examined the changes of IL-5 mRNA expressions in PBMC semi-quantitatively with RT-PCR as well as serum ECP levels and MCH-PC20 values in asthmatics before and after being treated with corticosteroids. The results revealed that there were significant decrease in the level of IL-5 mRNA and serum ECP concentration after therapy (P < 0.05) and there was remarkable improvement in the values of MCH-PC20 and FEV1% (P < 0.05). It was also found that the changes of serum ECP levels or MCH-PC20 values were accompanied by a reduction in IL-5 mRNA expression (r = 0.5426 or 0.4857, P < 0.05). These results suggested that the therapeutic effects of corticosteroids in asthma may result from modulation of IL-5 gene expression with consequent inhibition of eosinophil activation. 相似文献
83.
A distributed problem solving system can be characterized as a group of individual cooperating agents running to solve common problems. As dynamic application domains continue to grow in scale and complexity, it becomes more difficult to control the purposeful behavior of agents, especially when unexpected events may occur. This article presents an information and knowledge exchange framework to support distributed problem solving. From the application viewpoint the article concentrates on the stock trading domain; however, many presented solutions can be extended to other dynamic domains. It addresses two important issues: how individual agents should be interconnected so that their resources are efficiently used and their goals accomplished effectively; and how information and knowledge transfer should take place among the agents to allow them to respond successfully to user requests and unexpected external situations. The article introduces an architecture, the MASST system architecture, which supports dynamic information and knowledge exchange among the cooperating agents. The architecture uses a dynamic blackboard as an interagent communication paradigm to facilitate factual data, business rule, and command exchange between cooperating MASST agents. The critical components of the MASST architecture have been implemented and tested in the stock trading domain, and have proven to be a viable solution for distributed problem solving based on cooperating agents 相似文献
84.
85.
采用晶种技术,能有效地改善熟料的煅烧过程和熟料的质量。晶种和复合矿化剂的作用机理不同,在某些方面具有互补性。 相似文献
86.
87.
88.
89.
90.
腈纶纤维上油率的稳定控制 总被引:1,自引:0,他引:1
从油剂泵、皮辊和钢罗拉等设备构件运行状态考虑,探讨了腈纶纤维上油率的稳定控制问题。 相似文献