全文获取类型
收费全文 | 4844篇 |
免费 | 123篇 |
国内免费 | 6篇 |
专业分类
电工技术 | 35篇 |
综合类 | 4篇 |
化学工业 | 918篇 |
金属工艺 | 98篇 |
机械仪表 | 94篇 |
建筑科学 | 182篇 |
矿业工程 | 12篇 |
能源动力 | 182篇 |
轻工业 | 420篇 |
水利工程 | 32篇 |
石油天然气 | 30篇 |
无线电 | 208篇 |
一般工业技术 | 741篇 |
冶金工业 | 1343篇 |
原子能技术 | 46篇 |
自动化技术 | 628篇 |
出版年
2023年 | 22篇 |
2022年 | 63篇 |
2021年 | 71篇 |
2020年 | 60篇 |
2019年 | 58篇 |
2018年 | 80篇 |
2017年 | 112篇 |
2016年 | 114篇 |
2015年 | 68篇 |
2014年 | 212篇 |
2013年 | 277篇 |
2012年 | 155篇 |
2011年 | 209篇 |
2010年 | 166篇 |
2009年 | 177篇 |
2008年 | 171篇 |
2007年 | 156篇 |
2006年 | 169篇 |
2005年 | 141篇 |
2004年 | 148篇 |
2003年 | 123篇 |
2002年 | 100篇 |
2001年 | 86篇 |
2000年 | 91篇 |
1999年 | 76篇 |
1998年 | 438篇 |
1997年 | 241篇 |
1996年 | 179篇 |
1995年 | 122篇 |
1994年 | 89篇 |
1993年 | 98篇 |
1992年 | 41篇 |
1991年 | 44篇 |
1990年 | 38篇 |
1989年 | 31篇 |
1988年 | 38篇 |
1987年 | 20篇 |
1986年 | 21篇 |
1985年 | 54篇 |
1984年 | 36篇 |
1983年 | 25篇 |
1982年 | 26篇 |
1981年 | 37篇 |
1980年 | 29篇 |
1979年 | 24篇 |
1978年 | 18篇 |
1977年 | 45篇 |
1976年 | 83篇 |
1975年 | 15篇 |
1974年 | 18篇 |
排序方式: 共有4973条查询结果,搜索用时 0 毫秒
1.
Kurjenniemi Janne Hämäläinen Seppo Ristaniemi Tapani 《Wireless Personal Communications》2003,27(4):337-351
In this article we consider the performance of the 3.84 Mcpstime-division duplex (TDD) mode of UTRA (Universal TerrestrialRadio Access) network. We emphasize two of the radio resourcemanagement algorithms, handover and uplink power control, whoserole in the overall system performance is studied extensively.First, a handover algorithm used in WCDMA (Wideband Code DivisionMultiple Access) standard is considered in a TDD-mode operation.This gives rise to a careful setting of different handoverparameters, and the evaluation of the effects to the systemperformance. Secondly, the specified uplink power controlalgorithm is considered. Since it is based on several user-mademeasurements which may involve both random and systematic errors acareful study about the suitability of the power control scheme iscarried out. 相似文献
2.
An increase in hydrogen evolution from the hydrogen-evolving enzyme in the actinomycete m>Frankia m> was recorded in the presence of nickel. Immunogold localisation analysis of the intracellular distribution of hydrogenase proteins indicated that they were evenly distributed in the membranes and cytosol of both hyphae and vesicles. In addition, molecular characterisation of the hydrogen-evolving enzyme at the proteomic level, using two-dimensional gel electrophoresis combined with mass spectrometry, confirmed that the m>Frankia m> hydrogen-evolving enzyme is similar to the cyanobacterial bidirectional hydrogenase of m>Anabena siamensis m>. 相似文献
3.
A Method to Measure Emissions from Dryers with Diffuse Leakages, Using Evaporated Water as a Tracer 总被引:2,自引:0,他引:2
This paper describes a novel method to measure emission from dryers. The method resolves the known difficulties caused by diffuse emissions, and also solves the problems associated with high moisture content of the drying medium. The basic idea is to use water vapor to determine the exhaust flow, while a dry ice trap is used both to preconcentrate emitted VOCs and to determine the moisture content of the drying medium. 相似文献
4.
On the effect of spatial variances in historical rainfall time series to CSO performance evaluation.
Historical, high-resolution rain series are the backbone of modern combined sewer overflow (CSO) structure design. These rain series are the input to the computational estimation of the performance of the measures with respect to CSO pollution abatement. However, those historical precipitation measurements are available at only a few locations. Frequently rain series have to be used from gauging stations at a significant distance. In order to judge and to compensate for this influence an estimate between rain characteristics and combined sewer outflow (CSO) performance indicators would be useful. In this paper such correlations have been sought for a collection of 37 rain series covering large areas of Europe. It was found that the mean annual rain volume can explain most of the variances for the performance indicators Number of overflows and CSO volume. For explaining the spatial differences in the efficiency of the CSO structure another rain characteristic, i.e. the maximum event with a return period of one year, is to be used. 相似文献
5.
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
6.
Torsten Söderström 《Circuits, Systems, and Signal Processing》2002,21(1):83-90
This paper gives a tutorial overview of basic approaches for model validation and model structure determination.Work partially supported by the Swedish Research Council for Engineering Sciences under contract 98-654. 相似文献
7.
8.
In an attempt to identify genes that are involved in Drosophila embryonic cardiac development, we have cloned and characterized a gene whose function is required late in embryogenesis to control heart rate and muscular activity. This gene has been named held out wings (how) because hypomorphic mutant alleles produce adult animals that have lost their ability to fly and that keep their wings horizontal at a 90 degree angle from the body axis. In contrast to the late phenotype observed in null mutants, the How protein is expressed early in the invaginating mesoderm and this expression is apparently under the control of twist. When the different mesodermal lineages segregate, the expression of How becomes restricted to the myogenic lineage, including the cardioblasts and probably all the myoblasts. Antibodies directed against the protein demonstrate that How is localized to the nucleus. how encodes a protein containing one KH-domain which has been implicated in binding RNA. how is highly related to the mouse quaking gene which plays a role at least in myelination and that could serve to link a signal transduction pathway to the control of mRNA metabolism. The properties of the how gene described herein suggest that this gene participates in the control of expression of as yet unidentified target mRNAs coding for proteins essential to cardiac and muscular activity. 相似文献
9.
The birthweight is the most important determinant of mortality and morbidity in the neonatal period and may have an influence on health in adult life. The high rate of low birthweight in developing countries is therefore a major health problem. Maternal malnutrition is usually assumed to be a causal factor but other environmental factors are also involved. In this study we analysed maternal nutritional and socio-economic factors as determinants of birthweight in term infants from a rural African society characterised by a high rate of chronic malnutrition. Relations of maternal weight, gestational weight gain, parity, socio-economic status and infant sex with birthweight were analysed in 1,477 women and child pairs. The selected women were followed from early pregnancy and had an uncomplicated delivery at term of a living singleton child. The gestational weight gain was 5.6 (SD 6.0) kg and the mean birthweight 2.933 kg (SD 408). Maternal weight, representing the maternal long-term nutritional situation, was the most important independent determinant of birthweight, accounting for 13.0% of the variance in birthweight. The weight gain, representing the short-term nutritional situation, explained only 5.6% of the variance. Birthweight increased by 20 g (CI 18-23) for each kg maternal weight and by 15 g (CI 12-18) for each kg gestational weight gained. The socio-economic difference in birth-weight was 153 g (CI 109-196) 88 of which (CI 48-128) remained unexplained after adjustment for differences in maternal weight, parity and gender. Improved long-term nutritional situation and living conditions seems to be the most important prerequisites to counteract low birthweight in developing countries. 相似文献
10.
Two Ghanaian women, aged 21 and 29 years, who were involved in road traffic accidents sustaining blunt trauma to the chest and who had delayed diagnosis of their total bronchial rupture and underwent successful surgical repair 18 and 29 months, respectively after injury are reported with a review of the literature. We believe these to be the first reported cases of delayed diagnosis and successful repair of total bronchial rupture in Tropical Africa. 相似文献