全文获取类型
收费全文 | 548500篇 |
免费 | 6522篇 |
国内免费 | 1757篇 |
专业分类
电工技术 | 10092篇 |
综合类 | 440篇 |
化学工业 | 81666篇 |
金属工艺 | 23601篇 |
机械仪表 | 18079篇 |
建筑科学 | 11412篇 |
矿业工程 | 3796篇 |
能源动力 | 14699篇 |
轻工业 | 38071篇 |
水利工程 | 6391篇 |
石油天然气 | 13060篇 |
武器工业 | 34篇 |
无线电 | 63836篇 |
一般工业技术 | 114167篇 |
冶金工业 | 99875篇 |
原子能技术 | 13332篇 |
自动化技术 | 44228篇 |
出版年
2021年 | 5216篇 |
2020年 | 4010篇 |
2019年 | 5078篇 |
2018年 | 8712篇 |
2017年 | 8769篇 |
2016年 | 9394篇 |
2015年 | 5807篇 |
2014年 | 9636篇 |
2013年 | 25983篇 |
2012年 | 15118篇 |
2011年 | 20137篇 |
2010年 | 16017篇 |
2009年 | 18107篇 |
2008年 | 18552篇 |
2007年 | 18263篇 |
2006年 | 16196篇 |
2005年 | 14557篇 |
2004年 | 13737篇 |
2003年 | 13615篇 |
2002年 | 12910篇 |
2001年 | 12893篇 |
2000年 | 12052篇 |
1999年 | 12455篇 |
1998年 | 31651篇 |
1997年 | 21882篇 |
1996年 | 16831篇 |
1995年 | 12484篇 |
1994年 | 11010篇 |
1993年 | 11182篇 |
1992年 | 8191篇 |
1991年 | 7841篇 |
1990年 | 7688篇 |
1989年 | 7355篇 |
1988年 | 7136篇 |
1987年 | 6237篇 |
1986年 | 6152篇 |
1985年 | 6910篇 |
1984年 | 6391篇 |
1983年 | 5863篇 |
1982年 | 5324篇 |
1981年 | 5385篇 |
1980年 | 5133篇 |
1979年 | 5056篇 |
1978年 | 5079篇 |
1977年 | 5628篇 |
1976年 | 7044篇 |
1975年 | 4472篇 |
1974年 | 4231篇 |
1973年 | 4364篇 |
1972年 | 3733篇 |
排序方式: 共有10000条查询结果,搜索用时 13 毫秒
101.
In the present study the possibility of using nicotine in house dust as an index of environmental tobacco smoke (ETS) exposure was evaluated in an environmental investigation of 23 children with asthma. A standardized procedure for house dust sampling of nicotine with a filter holder connected to a vacuum cleaner, for a defined time and area was developed (F-nicotine). Also, house dust sampling was carried out from the vacuum cleaner bags of the homes (VC-nicotine). There was a larger variation in VC-nicotine (13-655, median 66 microg/g) compared with F-nicotine (15-393 median 156 microg/g). There were statistically significant associations between an inquiry data based ETS exposure index on the one hand, and urinary cotinine concentrations in children (U-cotinine), F-nicotine and VC-nicotine of their homes, on the other. The strong correlation between U-cotinine and F-nicotine (rs = 0.93; P < 0.0001) indicates that the new standardized house dust sampling method should be useful in ETS exposure assessment. However, further validation by a larger sample size with repeated measurements in the same homes is needed. 相似文献
102.
Lead is a global concern because of its ubiquity in the environment and known to be associated with abnormal neurobehavioral and cognitive development of young children. There is no study from India to describe a composite profile of blood lead and its biochemical influences in children. The present study was aimed at determining the proportion of children with >10 mug/dL blood lead levels (BLLs), association between BLLs, and sociodemographic characteristics, if any, and alterations in biochemical indices in the blood as an underlying mechanism of lead intoxication. A total of 62 children (4--12 y) of Lucknow and nearby areas were recruited to determine BLLs, delta-amimolevulinic acid dehydratase (delta-ALAD) activity, catalase (CAT) activity, and malondialdehyde (MDA) and glutathione (GSH) levels in the blood. Mean level of blood lead was 7.47+/-3.06 microg/dL (2.78--15.0) and 29%-exceeded 10 microg/dL, CDC intervention level. The BLLs were found to be significantly influenced by social status, area of residence, source of water supply, maternal educational status (p<0.001), type of house, and proximity to traffic density (p<0.01). delta-ALAD was significantly lower in the group of children with BLLs 11.39+/-1.39 microg/dL when compared to children with BLLs 7.11+/-1.25 microg/dL and 3.93+/-0.61 microg/dL (p=0.0007, 0.0005, respectively). However, CAT activity was higher in the groups of children with higher blood levels than with lower BLLs (p=0.0159, 0.0001, respectively). There was an increase in MDA level with a concomitant decrease of GSH in children with BLLs 11.39+/-1.39 microg/dL compared with those of children with BLLs 7.11+/-1.25 microg/dL and 3.93+/-0.61 microg/dL (p=0.0001, 0.0002, and p=0.0001, respectively). There was statistically significant correlation of BLLs with delta-ALAD (r=-0.44, p=0.00035), MDA (r=0.46, p=0.00018), GSH (r=-0.62, p=0.00001), and CAT (r=0.44, p=0.00035). Significantly, CAT activity, MDA, and GSH levels were in turn, found to be correlated with delta-ALAD (r=-0.45, p=0.00024; r=-0.43, p=0.00053; r=0.43, p=0.00053, respectively). Results of the present study indicate a declining trend of BLLs in children when compared with those reported from metropolitan cities of India when leaded gasoline was in practice and that the BLLs were significantly associated with biochemical indices in the blood which have the potential to be used as biomarkers of lead intoxication. 相似文献
103.
B A Marouf A K al-Hadad N A Toma N F Tawfiq J A Mahmood M A Hasoon 《The Science of the total environment》1991,106(3):191-194
Since early 1986, a monitoring program for radionuclides in imported foods has been carried out by the Iraqi Atomic Energy Commission. After the Chernobyl nuclear reactor accident in the Soviet Union, the program was expanded; our laboratory was officially designated by the Iraqi Government to measure radionuclide activity concentrations in foodstuff imported from countries known to be severely contaminated by Chernobyl radioactive fallout. Gamma-spectrometric analysis was used. Food items such as powdered milk, lamb meat, poultry, cereals and grains imported into Iraq before the Chernobyl accident did not contain any detectable fission products. However, all lamb meat, 81% of the lentil, 44% of the powdered milk and chick-pea, and 17% of the roast beef samples were contaminated with 137Cs or 134Cs and 137Cs. The highest 137Cs contamination levels found were 82, 147, 420, 6 and 4 Bq kg-1, respectively. Contamination by 134Cs was approximately 50% of the values given above. 相似文献
104.
A. N. Gavrilov E. M. Gryaznova R. R. Starkov 《Soil Mechanics and Foundation Engineering》2009,46(3):120-120
Anniversaries and Memorable Dates
Congratulations to Valerii Viktorovich Mikheev Candidate of Technical Sciences, honorable builder of the Russian Federation, and recipient of the prize awarded by the Council of Ministers of the USSR 相似文献105.
Formation of dark patinas on rocky surfaces is mainly related to the deposition of gases and particles and to sulphation mechanisms. In the present study, samples of dark patinas taken from granitic outcrops and from granitic buildings were examined in an attempt to understand the mechanisms of their formation. The outcrops are located in non-polluted areas and are characterized by the absence of any extraneous material that provides calcium, such as e.g. mortar. The buildings are located in areas with low levels of pollution. The climate in the study area favours proliferation of microorganisms. Important differences between the patinas sampled from outcrops and from buildings were observed, as the former are of biological origin and the latter of anthropogenic origin. Although the levels of pollution are low in the sampling area, sulphur was present in all of the samples from urban buildings. Sulphur was not present in patinas from outcrops or in patinas from monuments that are assumed to behave as outcrops (dolmens), although the latter are also of anthropogenic origin. Finally, the patinas were found to be formed by elements accumulated on the surface and not from elements contained within the rock itself. 相似文献
106.
A number of reports of biomagnification of trace metals by plants indicate that elements are selectively concentrated within certain tissues when plants grow on coal ash. This study determined the uptake by, and tissue bioaccumulation of, 15 chemical elements within broom sedge and nut grass growing in the drainage system of a coal ash basin. Biomagnification of these elements by the grasses was compared to concentrations of these elements found in duckweed within the same system.Aluminium, arsenic, barium, cadmium, cobalt, chromium, copper, iron, mercury, manganese, molybdenum, selenium, strontium, titanium, and zinc were measured by neutron activation analysis in roots, stems, leaves and fruit of the grasses which grew in sediments within the effluent. Mercury and zinc were biomagnified in both grasses and duckweed to a level exceeding the concentration in sediment. All elements were biomagnified above the water concentration in all parts of the plants. Bioaccumulation of elements in the leaves and fruit of these plants provides a concentrated source of potentially toxic chemical elements for passage to animals which may consume these plants. The mechanisms of movement of potentially toxic elements from coal ash or water into food webs need additional study as utilization of coal is increased. 相似文献
107.
Grattan JP Gillmore GK Gilbertson DD Pyatt FB Hunt CO McLaren SJ Phillips PS Denman A 《The Science of the total environment》2004,319(1-3):99-113
Concentrations of 222Rn were measured in ancient copper mines which exploited the Faynan Orefield in the South-Western Jordanian Desert. The concentrations of radon gas detected indicate that the ancient metal workers would have been exposed to a significant health risk and indicate that any future attempt to exploit the copper ores must deal with the hazard identified. Seasonal variations in radon concentrations are noted and these are linked to the ventilation of the mines. These modern data are used to explore the differential exposure to radon and the health of ancient mining communities. 相似文献
108.
N. A. Klimenko L. V. Nevinnaya Yu. V. Sidorenko O. G. Shvidenko Yu. O. Shvadshina 《Journal of Water Chemistry and Technology》2007,29(1):15-22
We have studied the process of a prolonged dynamic filtration on activated carbon in two biosorption systems — without preoxidation treatment of the initial matter and after ozonization. It has been shown that given the uniform load on activated carbon terms of the organic matter the efficiency of the biosorption process after preozonization is 2.3 times as small compared with that that without pretreatment, which correlates with the change of adsorption free energy. 相似文献
109.
S. R. COCHRANE MSc PhD CEng MICE MIHT N. N. J. HIGGINSON MSc PhD CEng MICE 《Water and Environment Journal》1994,8(6):680-686
A comparison is made between pre- and post-drainage scheme river flows recorded on the River Maine, Co. Antrim. A method of using five-year reference periods has been applied to the mean daily flows to smooth and eliminate, as far as possible, variations due to climatic changes.
Any loss of energy production which might have been due to changes in river flow was examined for turbine installations at a factory at Cullybackey. The quantities of usable water, and hence potential energy production, were calculated for each of the post-scheme five-year periods and compared with a pre-scheme five-year reference period. Any potential loss was then represented by a depreciation factor. It was found that no measurable loss of potential energy production has occurred on the River Maine in the post-scheme period. 相似文献
Any loss of energy production which might have been due to changes in river flow was examined for turbine installations at a factory at Cullybackey. The quantities of usable water, and hence potential energy production, were calculated for each of the post-scheme five-year periods and compared with a pre-scheme five-year reference period. Any potential loss was then represented by a depreciation factor. It was found that no measurable loss of potential energy production has occurred on the River Maine in the post-scheme period. 相似文献
110.