全文获取类型
收费全文 | 517758篇 |
免费 | 6205篇 |
国内免费 | 1254篇 |
专业分类
电工技术 | 9903篇 |
综合类 | 659篇 |
化学工业 | 77360篇 |
金属工艺 | 23180篇 |
机械仪表 | 18616篇 |
建筑科学 | 11850篇 |
矿业工程 | 2897篇 |
能源动力 | 11299篇 |
轻工业 | 43857篇 |
水利工程 | 5554篇 |
石油天然气 | 9042篇 |
武器工业 | 98篇 |
无线电 | 59590篇 |
一般工业技术 | 101912篇 |
冶金工业 | 90261篇 |
原子能技术 | 10880篇 |
自动化技术 | 48259篇 |
出版年
2021年 | 3745篇 |
2019年 | 3652篇 |
2018年 | 12683篇 |
2017年 | 13140篇 |
2016年 | 10422篇 |
2015年 | 4583篇 |
2014年 | 7264篇 |
2013年 | 19867篇 |
2012年 | 13591篇 |
2011年 | 22273篇 |
2010年 | 18836篇 |
2009年 | 19280篇 |
2008年 | 20091篇 |
2007年 | 21822篇 |
2006年 | 13499篇 |
2005年 | 14834篇 |
2004年 | 13202篇 |
2003年 | 12660篇 |
2002年 | 11618篇 |
2001年 | 10998篇 |
2000年 | 10675篇 |
1999年 | 10588篇 |
1998年 | 24928篇 |
1997年 | 17751篇 |
1996年 | 13939篇 |
1995年 | 10865篇 |
1994年 | 9707篇 |
1993年 | 9480篇 |
1992年 | 7519篇 |
1991年 | 6998篇 |
1990年 | 6995篇 |
1989年 | 6663篇 |
1988年 | 6424篇 |
1987年 | 5612篇 |
1986年 | 5559篇 |
1985年 | 6421篇 |
1984年 | 6085篇 |
1983年 | 5484篇 |
1982年 | 5186篇 |
1981年 | 5376篇 |
1980年 | 5080篇 |
1979年 | 4905篇 |
1978年 | 4798篇 |
1977年 | 5479篇 |
1976年 | 6876篇 |
1975年 | 4338篇 |
1974年 | 4196篇 |
1973年 | 4341篇 |
1972年 | 3549篇 |
1971年 | 3295篇 |
排序方式: 共有10000条查询结果,搜索用时 15 毫秒
991.
G. Mentzas 《欧洲信息系统杂志》1991,1(3):193-203
The purpose of this paper is to provide an analysis of the alternative modelling systems for office information systems. More specifically, we are interested in identifying the current research trends of merging office modelling ideas together with concepts that owe their origin to the object-oriented paradigm and to artificial intelligence. In this respect an office model is defined as encompassing three domains: passive office objects; dynamically defined and manipulated office procedures; and goal-directed office tasks. Following this definition we exploit the potential of using the object-oriented and knowledge representation techniques for modelling each of the three domains using evidence from real-world applications. 相似文献
992.
In this paper we suggest a new statistical method of correcting the results of hot-line experiments for the effects of background sources and we use the new method to reassess the adequacy of three probability distributions proposed in the literature for image spread from line sources. The data are from sources labelled with 125I in semi-thin resin sections 0·4-0·8 μm in thickness. The new method reveals that two of the models describe the empirical distributions more closely than earlier analysis had suggested, and it confirms an increasing relationship between half distance of image spread and the thickness of the source. However, it also confirms that considerable ‘inter hot-line’ experimental variation remains, even after background correction. This suggests that multiple experiments are needed to produce reliable estimates of half distance. 相似文献
993.
994.
995.
996.
997.
We report impedance measurements on PVC matrix membranes which contain KBPh4 with varying proportions of valinomycin. In agreement with our earlier measurements the value of the bulk membrane resistance (Rb) is much larger in the presence of valinomycin, indicating that the mobility of K+ is greatly reduced by the valinomycin. Rb shows a linear variation with valinomycin/K+ ratio between 0 and 1, but it is invariant at higher valinomycin/K+ ratios. Thus there is no evidence for a special transport mechanism for K+ in these membranes. 相似文献
998.
D. N. Mukhiddinov Kh. A. Alimov A. I. Yufa A. V. Ulubabyan V. P. Dudanov 《Fibre Chemistry》1985,16(3):204-206
Conclusions The process of drying cellulose triacetate in a vibrating bed has been studied.For final drying of cellulose triacetate before solution, it is recommended to use an industrial specimen of a unit with a vibrating bed having a capacity of 2700 kg/h.This assembly can be used to dry other fibre-forming disperse polymeric materials containing strongly bound moisture.Translated from Khimicheskie Volokna, No. 3, pp. 49–51, May–June, 1984. 相似文献
999.
Wladek T. Sobol L.John SchreinerLjubis̆a Miljković Maria E. Marcondes-HeleneLeonard W. Reeves M.Mik Pintar 《Fuel》1985
The first results of the analysis of bitumen and oil sands using the recently developed n.m.r. spingrouping technique are presented. The n.m.r. relaxation experiments were carried out on bitumen, and on natural and dried oil sands samples. The results indicate that the spin-grouping can resolve and quantify several components of the samples studied. The bitumen and bitumen fraction of the oil sands are resolved according to their spin-spin relaxation times into three major groups: solid-like (rigid), solid-like (mobile) and semi-liquid. The water in the oil sands exists in two different environments. Tentatively one environment is assigned to be the bridges between the sand grains, while the other is assigned to be the clay surface. One can conclude that with spin grouping of complex mixtures the decomposition (in which components are resolved according to their dynamic state) is possible. The accuracy of such resolution is of the order of a few per cent. 相似文献
1000.
Blended SRC-II process streams, representing a full boiling range distillate material, were fractionally distilled into non-overlapping 50 °F cuts with boiling points between 300 and 850 °F. Another set of 18 distillate cuts were obtained with boiling points ranging between 138 and 1055 °F. Distillate cuts were assayed for mutagenic activity using the histidine reversion assay with Salmonella typhimurium strains TA98, TA100, TA1535 and TA1537, as well as for mammalian-cell transformation activity in the Syrian hamster embryo test, and DNA damage in the prophage induction assay. Samples were also separated into chemical class fractions by alumina column chromatography and analysed by high resolution gas chromatography so that the chemical composition of the cuts could be related to their relative activity in the different assays. In the mammalian cell transformation and microbial mutagenicity assays, significant activity was found almost exclusively in distillate cuts with components boiling > 700 °F, with the highest activity in the transformation assay observed for cuts > 800 °F. All of the distillate cuts showed increased levels of DNA damage as expressed by lambda prophage induction in Escherichia coli 8177. However, the greatest activity was associated with distillate cuts with boiling points in the 800 °F + range. Chemical analysis of the 50 °F distillate cuts showed a variety of polycyclic aromatic hydrocarbons (PAH) and amino-PAH compounds to be present in the distillate cuts boiling > 700 °F and essentially absent from cuts boiling < 700 °F. The sample set of non-overlapping (50 °F) cuts were reblended according to the proportions of each cut found in the original blend material. These reblended composites were then assayed to compare their activity with that predicted from the activities of the component distillate cuts. The reblending experiments indicated the microbial mutagenicity response was essentially additive. Mammalian cell transformation activities were non-additive, indicating a compositional effect on the expression of transforming agents in the complex mixture. 相似文献