全文获取类型
收费全文 | 7481篇 |
免费 | 126篇 |
国内免费 | 6篇 |
专业分类
电工技术 | 73篇 |
综合类 | 4篇 |
化学工业 | 1090篇 |
金属工艺 | 113篇 |
机械仪表 | 207篇 |
建筑科学 | 233篇 |
矿业工程 | 33篇 |
能源动力 | 188篇 |
轻工业 | 523篇 |
水利工程 | 47篇 |
石油天然气 | 11篇 |
武器工业 | 2篇 |
无线电 | 639篇 |
一般工业技术 | 1172篇 |
冶金工业 | 2527篇 |
原子能技术 | 118篇 |
自动化技术 | 633篇 |
出版年
2021年 | 64篇 |
2020年 | 49篇 |
2019年 | 52篇 |
2018年 | 59篇 |
2017年 | 69篇 |
2016年 | 71篇 |
2015年 | 59篇 |
2014年 | 105篇 |
2013年 | 236篇 |
2012年 | 160篇 |
2011年 | 254篇 |
2010年 | 180篇 |
2009年 | 185篇 |
2008年 | 212篇 |
2007年 | 207篇 |
2006年 | 216篇 |
2005年 | 152篇 |
2004年 | 166篇 |
2003年 | 156篇 |
2002年 | 140篇 |
2001年 | 144篇 |
2000年 | 135篇 |
1999年 | 176篇 |
1998年 | 731篇 |
1997年 | 400篇 |
1996年 | 338篇 |
1995年 | 199篇 |
1994年 | 188篇 |
1993年 | 217篇 |
1992年 | 113篇 |
1991年 | 112篇 |
1990年 | 107篇 |
1989年 | 112篇 |
1988年 | 88篇 |
1987年 | 95篇 |
1986年 | 101篇 |
1985年 | 103篇 |
1984年 | 104篇 |
1983年 | 85篇 |
1982年 | 82篇 |
1981年 | 114篇 |
1980年 | 101篇 |
1979年 | 93篇 |
1978年 | 82篇 |
1977年 | 110篇 |
1976年 | 183篇 |
1975年 | 76篇 |
1974年 | 47篇 |
1973年 | 68篇 |
1972年 | 46篇 |
排序方式: 共有7613条查询结果,搜索用时 0 毫秒
971.
The megaspilidDendrocerus carpenteri (Curtis) is a solitary ectohyperparasitoid of aphid primary parasitoids. It avoids superparasitism, but the avoidance rate depends on the amount of time elapsed after the first oviposition. In addition, females discriminate in a time-dependent manner between hosts parasitized by themselves and hosts parasitized by conspecifics. They recognize parasitized hosts by means of two externally perceivable chemical markers that are deposited on the mummy shell. One of the markers is highly active, short-lived, not passable through gas chromatography (GC), and freezing sensitive; the other is moderately active, persistent, GC-passable and freezing tolerant. Using two markers is believed to be advantageous for the hyperparasitoid female, as their ratio provides information on mark age and on the producer of the mark (the female herself or a conspecific female). 相似文献
972.
973.
Recent studies from this laboratory [l, 2] on the reaction mechanism of the pentose phosphate pathway in rabbit and rat liver cells have accentuated the role of enzyme-catalyzed exchange reactions as a means by which tracer carbon is relocated in the absence of a net metabolic flux. While this is an intrinsic property of the group transferring enzymes of the pathway and is seen as a deleterious property of 14C for the mapping and quantitation of metabolic pathways, it has re-emphasized the possible usefulness of isotope exchange reactions in understanding the mechanism of action of nature's catalyst. 相似文献
974.
Anthony W. Thompson James C. Williams 《Metallurgical and Materials Transactions A》1984,15(5):931-937
Age hardening in a Cu-2.5 wt pct Ti alloy has been examined as a function of aging time at 300 and 500 °C. Yield strength
results exhibited a double peak in strength at each temperature,i.e., peaks at two different times. The work-hardening rate, however, increased monotonically with aging time. Slip line measurements
showed that slip became coarser with aging time but remained planar. Lack of sufficient understanding of the details of the
pzecipitation reactions and also of hardening mechanisms means that a convincing explanation of the double peak in strength
cannot yet be made, but research needed to correct this situation was identified. 相似文献
975.
I Karacan JI Thornby M Anch CE Holzer GJ Warheit JJ Schwab RL Williams 《Canadian Metallurgical Quarterly》1976,10(5):239-244
976.
We review the principle of differential imaging and its application to scanning tunnelling microscopy (STM). It is shown that placing a lateral dither on an STM tip at high frequency provides the means for transfering topographic information to a frequency range where noise is small. Differential STM imaging on graphite and gold is demonstrated. A simple relation between the differential image and the conventional topographic image is described. 相似文献
977.
978.
The first of two years of training in case study activities, adapted from the program originally developed by Fritz Redl, Daniel A. Prescott, and others at the Universities of Chicago and Maryland in the 1940s, is described. Excerpts from the case study of a 16-year-old female provide examples of activities, such as: (1) identifying recurring patterns of behavior, (2) selecting a pattern of behavior for analysis by the multiple hypotheses method, and (3) summarizing the case study record. The significance of this process is that high school personnel can learn to see what adolescents are up against in and out of school. 相似文献
979.
Molecular events including p53 and k-ras alterations in the in vitro progression of a human colorectal adenoma cell line to an adenocarcinoma 总被引:1,自引:0,他引:1
AC Williams SJ Browne WA Yeudal IC Paterson CJ Marshall DP Lane C Paraskeva 《Canadian Metallurgical Quarterly》1993,8(11):3063-3072
The aim of the current study was to identify genetic abnormalities in human colorectal adenoma and carcinoma derived cell lines, and to determine whether the genetic changes which occur in vitro are relevant to the in vivo situation. Loss of 1p(33-35) region was shown to be the most common chromosome 1 abnormality and loss of heterozygosity (LOH) of the DCC gene and/or adjacent sequences was detected in all adenoma derived cells as well as the carcinoma cell lines. The level of p53 protein was also investigated as increased cellular p53 protein had previously been associated with mutation of the p53 gene. A further aim was to investigate genetic changes in our in vitro model of tumour progression, where the adenoma derived PC/AA cell line has previously been converted in vitro to two distinct tumorigenic phenotypes, producing either an adenocarcinoma or a mucinous carcinoma in athymic nude mice. Progression to the adenocarcinoma phenotype was shown to involve a specific chromosome 1 rearrangement, loss of both normal copies of chromosome 18 (although DCC gene sequences were retained), loss of the remaining wild type allele of k-ras resulting in homozygosity for the k-ras codon 12 mutation and increased cellular p53 protein as detected by SDS-PAGE Western blotting. The increase in p53 protein was shown not to be due to the acquisition of a mutation in the p53 gene. Interestingly, progression of the adenoma derived PC/AA cell line to the mucinous malignant phenotype did not involve any of these molecular rearrangements, suggesting that different genetically distinct pathways are involved in colorectal carcinogenesis. These studies show that the genetic changes in our in vitro model of human colorectal tumour progression are similar to those observed in in vivo studies. 相似文献
980.
We have examined in detail the principal series which have been used to represent the thermodynamcs of binary systems and have concluded that in every case the higher members suffer in some important way. The best choice for a third member is based upon a series expansion of the statistical mechanical treatment of Guggenheim. An entirely new form for the fourth term is suggested for accommodating the temperature dependent part of the asymmetry.We have also determined what constitutes proper treatment of the binary terms in representing higher order systems. Not all the previous treatments have been correct in this respect. 相似文献